(en)The present invention relates to the crystal structure of the insulin receptor ectodomain, to the nature of its N-linked glycans and to methods of using the crystal and related structural information to screen for and design compounds that interact with or modulate the insulin receptor and/or the closely-related insulin-like growth factor receptors or variants thereof.

1.ApplicationNumber: US-30595107-A
1.PublishNumber: US-2010267927-A1
2.Date Publish: 20101021
3.Inventor: GARRETT THOMAS P.J.
ADMAS TIMOTHY EDWARD
LOVRECZ GEORGE
MCKERN NEIL MORETON
LAWRENCE MICHAEL COLIN
SPARROW LINDSAY GALE
WARD COLIN WESLEY
LOU MEIZHEN
STRELSTOV VICTOR

4.Inventor Harmonized: GARRETT THOMAS P J(AU)
ADMAS TIMOTHY EDWARD(AU)
LOVRECZ GEORGE(AU)
MCKERN NEIL MORETON(AU)
LAWRENCE MICHAEL COLIN(AU)
SPARROW LINDSAY GALE(AU)
WARD COLIN WESLEY(AU)
LOU MEIZHEN(AU)
STRELSTOV VICTOR(AU)

5.Country: US
6.Claims:
(en)The present invention relates to the crystal structure of the insulin receptor ectodomain, to the nature of its N-linked glycans and to methods of using the crystal and related structural information to screen for and design compounds that interact with or modulate the insulin receptor and/or the closely-related insulin-like growth factor receptors or variants thereof.

7.Description:
(en)FIELD OF THE INVENTION
The present invention relates generally to structural studies of the insulin receptor. More particularly, the present invention relates to the crystal structure of the insulin receptor ectodomain, to the nature of its N-linked glycans and to methods of using the crystal and related structural information to screen for and design compounds that interact with or modulate the insulin receptor and/or the closely-related insulin-like growth factor receptors or variants thereof.
BACKGROUND TO THE INVENTION
The insulin receptor (IR) and its homologue the type 1 insulin-like growth factor 1 receptor (IGF-1R), are closely related members of the tyrosine kinase receptor family and are large, transmembrane, glycoprotein dimers consisting of several structural domains.
The key role of the insulin receptor (IR) is in glucose uptake and metabolism by muscle and fat. Mouse knockout studies have also shown IR to be important in adipogenesis, neovascularization, the regulation of hepatic glucose synthesis and glucose-induced pancreatic insulin secretion (Kitamura et al., 2003). IR signalling is also important in the brain, being involved in the regulation of food intake, peripheral fat deposition and the reproductive endocrine axis as well as in learning and memory (Wada et al., 2005). Dysfunctional IR signalling has been implicated in diseases including type I and type II diabetes, dementia and cancer.
IR exists as two splice variant isoforms, IR-A and IR-B, which respectively lack or contain the 12 amino acids coded by exon 11. The longer variant, IR-B, is the isoform responsible for signalling metabolic responses. In contrast, IR-A signals predominantly mitogenic responses, is the preferentially expressed isoform in several cancers (Denley et al., 2003) and is capable of binding insulin-like growth factor 2 (IGF-II) with high affinity (Denley et al., 2004).
The sequence of IR is highly homologous to the sequence of IGF-1R, indicating that the three-dimensional structures of both receptors may be similar. The mature human IR and IGF-1R molecules are each homodimers comprising two α-chains and two β-chains, the α- and β-chains arising from the post-translational cleavage at the furin cleavage site at residues 720-723 (IR-A numbering with the mature N-terminal residue numbered 1) or 707-710 (IGF-1R). The structural organization of IR and IGF-1R has been reviewed extensively (Adams et al., 2000; De Meyts & Whittaker, 2002; Ward et al., 2003) and is summarized schematically in FIG. 1
Each IR or IGF-1R monomer contains a leucine-rich repeat domain (L1), a cysteine-rich region (CR) and a second leucine-rich repeat domain (L2), followed by three fibronectin type III domains, (FnIII-1, -2 and -3). The FnIII-2 domain contains a large insert domain (ID) of approximately 120 residues, within which lies the α-β cleavage site. Intracellularly, each monomer contains a tyrosine kinase catalytic domain flanked by two regulatory regions that contain the phosphotyrosine binding sites for signalling molecules. Each α-chain is linked to its partner β-chain via a disulphide bond between residues Cys647 and Cys860 (Sparrow et al., 1997) in the case of IR and/or Cys633-Cys849 in the case of IGF-1R. The α-chains of both IR and IGF-1R are cross-linked by disulphide bonds in two places. The first is at Cys524 (IR) or Cys514 (IGF-1R) in the FnIII-1 domain, cross-linked to its counterpart in the opposite monomer, and the second involves one or more of the residues Cys682, Cys683 and Cys685 (IR) or Cys669, Cys670 and Cys672 (IGF-1R) in the insert region of each FnIII-2 domain, cross-linked to their counterparts in the opposite monomer (Sparrow et al., 1997).
The domains of IR and IGF-1R exhibit high (47-67%) amino acid identity indicative of high conservation of three-dimensional structure. The crystal structure of the first three domains of IGF-1R (L1-CR-L2) has been determined (Garrett et al., 1998) and revealed that the L domains consist of a single-stranded right-handed β-helix (a helical arrangement of β-strands), while the cysteine-rich region is composed of eight related disulfide-bonded modules. The structures of IR and IGF-1R are known to be very similar from: (i) electron microscopic analyses (Tulloch et al., 1999), (ii) the fact that hybrid receptors (heterodimers of one IR monomer disulphide-bonded to one of IGF-1R monomer) exist naturally and are commonly found in tissues expressing both receptors (Bailyes et al., 1997), and (iii) the fact that receptor chimeras can be constructed which have whole domains or smaller segments of polypeptide from one receptor replaced by the corresponding domain or sequence from the other (reviewed in Adams et al., 2000).
The current model for insulin binding proposes that, in the basal state, the IR homodimer contains two identical pairs of binding sites (referred to as Site 1 and Site 2) on each monomer (De Meyts & Whittaker, 2002; Schaffer, 1994; De Meyts, 1994; De Meyts 2004). Binding of insulin to a low affinity site (Site 1) on one α-subunit is followed by a second binding event between the bound insulin and a different region of the second IR α-subunit (Site 2). This ligand-mediated bridging between the two α-subunits generates the high affinity state that results in signal transduction. In contrast, soluble IR ectodomain, which is not tethered at its C-terminus, cannot generate the high affinity receptor-ligand complex. It can bind two molecules of insulin simultaneously at its two Site 1s, but only with low affinity (Adams et al., 2000). The model for IGF-I or IGF-II binding to IGF-1R is the same as that just described for insulin binding to IR and involves a low affinity site (Site 1) and a high affinity site (Site 2) on each monomer as described for the IR.
While similar in structure, IGF-1R and IR serve different physiological functions. IGF-1R is expressed in almost all normal adult tissue except for liver, which is itself the major site of IGF-I production (Buttel et al., 1999). A variety of signalling pathways are activated following binding of IGF-I or IGF-II to IGF-1R, including Src and Ras, as well as downstream pathways, such as the MAP kinase cascade and the P13K/AKT axis (Chow et al., 1998). IR is primarily involved in metabolic functions whereas IGF-1R mediates growth and differentiation. Consistent with this, ablation of IGF-I (i.e. in IGF-I knock-out mice) results in embryonic growth deficiency, impaired postnatal growth, and infertility. In addition, IGF-1R knock-out mice were only 45% of normal size and died of respiratory failure at birth (Liu et al., 1993). However, both insulin and IGF-I can induce both mitogenic and metabolic effects.
Various 3-D structural analyses of the IR and the interaction of insulin with the IR have been undertaken (Luo et al., 1999; Ottensmeyer et al., 2000, 2001; Yip & Ottensmeyer, 2001). However, these have all been based on non-crystallographic electron microscopic techniques of low resolution (>20 angstrom) and their conclusions have been questioned (De Meyts & Whittaker, 2002). The absence of high resolution data on the IR ectodomain and insulin/IR complex has hindered elucidation of the nature of ligand/IR interactions.
SUMMARY OF THE INVENTION
We have now determined the crystal structure of the first three domains (L1-CR-L2) of human IR. We have also successfully produced crystals of the intact homodimeric ectodomain fragment of human IR and elucidated the three dimensional (3-D) structure of this homodimer using X-ray crystallography.
This data allows direct comparison, for the first time, of the regions controlling ligand specificity in IR and the closely related homologue IGF-1R, and identifies critical regions of IR involved in the initial binding of insulin and in the subsequent formation of the high affinity insulin-IR complex that leads to signal transduction. Such information also indicates, by analogy, the corresponding regions in the closely related IGF-1R that are involved in insulin growth factor (IGF) binding and in the formation of the high affinity IGF/IGF-1R signalling complex.
The identification of molecular structures having a high degree of specificity for one or the other receptor is important in the development of efficacious and safe therapeutics. For example, a molecule developed as an insulin agonist should have little or no IGF-I activity in order to avoid the mitogenic activity of IGF-I and a potential for facilitating neoplastic growth. The determination of which regions of IR and IGF-1R have sufficient differences to confer selectivity for their respective ligands or for therapeutic molecules such as chemical entities or biological reagents is therefore an important and significant advancement. Similarly, it is believed that the ability to be able to identify molecular structures that mimic the active binding regions of insulin and/or IGF-I and which impart selective agonist or antagonist activity will also aid and advance the development of new drugs.
The region of IR ectodomain consisting essentially of the first three domains of the ectodomain is referred to herein as “LCL IR ectodomain”.
Accordingly, the present invention provides a method of structure-based identification or design of a compound that interacts with IR, comprising performing structure-based identification or design of a compound that interacts with IR using the structure defined by the atomic coordinates of Appendix I and/or Appendix II.
To further assist in the design of such a compound, we have also used these structures to place a model of a modified insulin molecule in the binding site. This model, with coordinates in Appendix III, is oriented relative to atomic coordinates found in Appendix II and may be used in conjunction with atomic coordinates of Appendix I and/or Appendix II to design a compound which binds to IR.
In one embodiment the present invention provides a method for identifying or designing a compound that can potentially interact with IR, the method comprising designing or screening for a compound which interacts with the three-dimensional structure of; (i) IR ectodomain, the structure being defined by the atomic coordinates shown in Appendix I, a subset of the atomic coordinates shown in Appendix I in combination with the atomic coordinates shown in Appendix II or a subset of the atomic coordinates shown in Appendix I in combination with a subset of the atomic coordinates shown in Appendix II; (ii) a region of IR ectodomain, the structure being defined by the atomic coordinates shown in Appendix II, a subset of the atomic coordinates shown in Appendix I in combination with a subset of the atomic coordinates shown in Appendix II or a subset of the atomic coordinates shown in Appendix I or Appendix II; (iii) IR ectodomain as defined in (i) in combination with the atomic coordinates given in Appendix III and/or Appendix IV, or in combination with a subset of the atomic coordinates given in Appendix III and/or Appendix IV; or (iv) a region of IR ectodomain as defined in (ii) in combination with the atomic coordinates given in Appendix III and/or Appendix IV, or in combination with a subset of the atomic coordinates given in Appendix III and/or Appendix IV; wherein interaction of the compound with the structure is favoured energetically.
In a preferred embodiment, the method further comprises synthesising or obtaining a candidate compound designed or screened for in the first step and determining the ability of the candidate compound to interact with IR.
With regards to defining structures by combining subsets of coordinates from Appendix I and Appendix II, such combinations may be achieved by methods such as assembling combinations of complete domains from each set, assembling combinations of complete domains from each set wherein the coordinates and corresponding amino acid sequence from one structure are transposed onto those of the other, refining less resolved regions of one crystal using the corresponding coordinates of the other.
In a further embodiment, the invention provides a method of identifying or designing a compound which interacts with a target binding site of IR ectodomain.
In a preferred embodiment, the target binding site comprises one or more regions of IR ectodomain that governs specificity, i.e. one or more regions defining the low affinity binding site for insulin.
In a further embodiment, van der Waals and/or hydrophobic interactions account for the major portion of the binding energy between a compound and a low affinity binding site of IR.
In another preferred embodiment, the target binding site comprises one or more regions of IR ectodomain defining the high affinity binding site for insulin (which leads to the initiation of IR autophosphorylation).
The present invention also provides compounds that bind to IR ectodomain identified or designed using any suitable method of invention. In particular, the present invention provides compounds which bind to one or more regions of the low and/or high affinity binding site(s) for insulin identified or designed as described above.
The crystal structure of the first three domains of the ectodomain of IGF-1R has been previously reported (WO 99/028347). As will be evident to the skilled person, the structural data presented herein has now enabled, for the first time, direct comparison of the regions controlling ligand specificity in the closely related IGF-1R and IR. The crystal structure of the remaining regions of the IGF-1R ectodomain have yet to be elucidated. However, as will be evident to the skilled person, the findings presented herein on IR ectodomain structure, shape and orientation can be transposed onto the IGF-1R ectodomain structure, shape and orientation.
The present invention has also identified the critical regions of IR involved in the initial binding of insulin and in the subsequent formation of the high affinity insulin-IR complex that leads to signal transduction. Once again, it will be evident to the skilled person that these findings can be transposed onto IGF-1R.
The present invention is therefore also useful in the identification and/or design of compounds which bind to IGF-1R.
The present invention has enabled the identification of previously unrecognised target binding sites in IR ectodomain, i.e. regions of IR ectodomain involved in the initial docking of insulin and the subsequent high affinity binding leading to signal transduction. By analogy, the present invention also identifies the equivalent regions in the IGF-1R, given the structural organisation of domains in the two receptors is the same.
The present invention has also enabled the N-linked glycosylation sites of IR and IGF-1R ectodomains to be mapped onto the 3D structure of these molecules and for the glycan moieties at each site to be modeled based on the chemical compositions determined in this invention and the known structures of N-linked glycans from the protein data bank. Such mapping and modeling enables the identification of regions on the surface of these receptors that are devoid of such carbohydrate and as such more amenable to the selection and design of potential binding molecules, particularly antibody and nonimmunoglobulin binding proteins and aptamers. This allows compounds that specifically target these regions to be designed and manufactured.
Candidate compounds and compounds identified or designed using a method of the present invention may be any suitable compound, including naturally occurring compounds, de novo designed compounds, library generated compounds (chemically or recombinantly generated), mimetics etc., and include organic compounds, new chemical entities, antibodies, binding proteins other than antibody-based molecules (nonimmunoglobulin proteins) including, for example, protein scaffolds such as lipocalins, designed ankyrin repeat proteins (DARPins) and protein A domains (reviewed in Binz H K et al, 2005), avimers (Silverman et al., 2005), and other new biological entities such as nucleic acid aptamers (reviewed in Ulrich, 2006).
In another preferred embodiment, a compound is a new chemical entity.
The present invention provides a compound capable of binding to at least one target binding site and/or specific target molecule as defined by the present invention. Preferably, such compounds have an affinity (K d ) for IR of less than 10 −5 , preferably less than 10 −7 M and more preferably less than 10 −10 M.
Particularly preferred compounds are those capable of binding to a target binding site involved in the high affinity binding of insulin and/or a specific target molecule indicative of the capacity to bind to a target binding site.
In a preferred embodiment, such a compound is an antibody or antibody fragment.
In further embodiments, such a compound is a novel binding protein comprising nonimmunoglobulin domains such as avimers (Silverman et al., 2005), DARPins (Binz et al., 2005) or lipocalins (Schlehuber & Skerra, 2005) or a nucleic acid based binding entity such as an aptamer (reviewed in Ulrich, 2006).
The present invention is also useful in the identification and/or design of compounds which bind to IGF-1R. For example, known IGF-1R binding compounds can be screened against the 3D structure of IR ectodomain or a region of IR ectodomain based on the coordinates given in Appendix I and/or II or a portion thereof (optionally utilising the atomic coordinates given in Appendix III to further refine the screen and/or the assessment of the potential for binding to IR), and an assessment made of their potential for binding to IR. Alternatively, existing IGF-1R ligands which are known to bind to IR (i.e. which are non- or insufficiently selective) can be redesigned so as to be more selective for IGF-1R. For example, such ligands can be screened against a 3D structure of the IR ectodomain, those areas of energetically favoured interaction of the ligand with the IR ectodomain identified, and the ligands redesigned (i.e. modified) so as to reduce the potential to energetically interact with IR.
The present invention is also useful for improving the properties of known ligands for both IR and IGF-1R. For example, existing IR or IGF-1R ligands can be screened against the 3D structure of IR ectodomain or a region of IR ectodomain defined by the atomic coordinates of Appendix I and/or Appendix II or a portion thereof (optionally utilising the atomic coordinates given in Appendix III to further refine the screen and/or the assessment of the potential to energetically interact with IR), and an assessment made of the potential to energetically interact with IR. In the case of IR, the compound could be redesigned (e.g. chemically modified) so as to improve and/or impart one or more properties such as, for example: (i) improved affinity for the low affinity binding site of IR (i.e. the binding site governing selectivity), (ii) improved affinity for the high affinity binding site for IR (i.e. the binding site governing signal transduction) and (iii) lowered affinity for binding to IGF-1R. In the case of IGF-1R, the compound could be redesigned (i.e. chemically modified) so as to improve and/or impart one or more of the properties such as, for example: (i) reduced affinity for the low affinity binding site of IR (i.e. the binding site governing selectivity) and (ii) improved affinity for binding to IGF-1R.
When screening existing ligands or potential compounds for selectivity for binding to IR or IGF-1R, it will be important to concentrate on those areas of difference in the 3D structure between the ectodomains of IR and IGF-1R. Such areas are identified and described herein. In particular, it will be important to concentrate on those areas of difference which are identified as being potentially important in the binding of the respective ligands to the receptors.
The methods of the present invention may be used for either targeted or broad screening. Targeted screening involves the design and synthesis of chemical compounds that are analogs of some active compounds or that can specifically act with the biological target under investigation. Broad screening involves the design and synthesis of a large array of maximally diverse chemical compounds, leading to diverse libraries that are tested against a variety of biological targets.
The present invention also provides isolated molecules which represent or mimic regions of IR ectodomain to be targeted for the selection of binding molecules (“specific target molecules”), and which are identified and/or designed based on the 3D structure of a region of IR ectodomain defined by the atomic coordinates of Appendix I and/or Appendix II, or a subset thereof of both or either, optionally in combination with the atomic coordinates of Appendix IV or a subset of the atomic coordinates of Appendix IV. The specific target molecules may be conformationally constrained, for example, as in constrained peptide sequences, or may comprise domains or segments in an IR/IGF-1R chimera, or alternatively molecules which are not conformationally constrained such as, for example, non-constrained peptide sequences.
In a preferred embodiment, a molecule mimics a region of IR ectodomain which is involved in the binding of insulin, i.e. it mimics a target binding site. Those preferred regions described above in relation to the screening methods are equally applicable to the design of specific target molecules (i.e. target site binding comprising/mimicking molecules).
Molecules which mimic or represent a region of IR ectodomain involved in binding insulin can be used to screen for ligands, binding proteins or other binding entities which bind to IR. Additionally, such mimetics can be used to screen for ligands which selectively bind IR over IGF-1R.
In a preferred embodiment, the mimetic mimics, at least in part, the high affinity binding site of IR for insulin.
In a preferred embodiment, the mimetic is an isolated conformationally constrained peptide or peptidomimetic. In a preferred embodiment, the conformationally constrained peptide or peptidomimetic comprises at least one of the following: (i) the L1 binding face in general and the region governing specificity around Phe39 or the loop comprising residues 85-91 in the fourth LRR rung of L1 in particular, (ii) module 6 of the CR domain, (iii) the AB loop of FnIII-1, the CC′ loop of FnIII-1 and the EF loop of FnIII-1 which comprise the high affinity binding site for ligand, and (iv) any other regions of the receptor implicated in ligand binding and signal transduction.
Preferably, the compound is a peptidomimetic that has fewer than 30 amino acids, more preferably fewer than 25 amino acids.
In one embodiment, the specific target molecule comprises at least one region from IR and at least one region from IGF-1R, i.e. it is a IR/IGF-1R chimera. Such chimeras are capable of presenting a target binding site in its native conformation. Furthermore, such chimeric specific target molecules may represent or mimic a first target binding site specific or at least more selective for binding of insulin and a second target binding site specific or more selective for binding insulin growth factor.
The present method further provides a method for preparing a pharmaceutical composition for treating diseases associated with aberrant IR or IGF-1R signalling in which the compound identified or designed according to the invention is incorporated into a pharmaceutical composition.
The present invention also provides a crystal of IR ectodomain polypeptide. In particular, the present invention provides a crystal of IR ectodomain polypeptide having a space group C222 1 with unit cell dimensions of a=123.0 Å, b=319.7 Å and c=204.9 Å with up to about 2% variation in any cell dimension.
The present invention also provides a crystal of LCL IR ectodomain polypeptide. In particular, the present invention provides a crystal of LCL IR ectodomain polypeptide having a space group P2 1 2 1 2 1 with unit cell dimensions a=103.86 Å, b=130.24 Å, c=160.92 Å with up to about 2% variation in any cell dimension.
The present invention also provides a crystalline composition comprising a crystal of IR ectodomain or LCL IR ectodomain. Such crystals may be used in X-ray diffraction studies for the screening of chemical fragments which may bind to the ectodomain of either receptor, such fragments then being assembled into compounds which may bind to the receptor with an affinity higher than that of the individual fragments.
The present invention also provides a computer system for identifying one or more compounds that can potentially interact with IR, the system containing data representing the structure of (i) IR ectodomain, the structure being defined by the atomic coordinates shown in Appendix I, a subset of the atomic coordinates shown in Appendix I in combination with the atomic coordinates shown in Appendix II or a subset of the atomic coordinates shown in Appendix I in combination with a subset of the atomic coordinates shown in Appendix II; (ii) a region of IR ectodomain, the structure being defined by the atomic coordinates shown in Appendix II, a subset of the atomic coordinates shown in Appendix I in combination with a subset of the atomic coordinates shown in Appendix II or a subset of the atomic coordinates shown in Appendix I or Appendix II; (iii) IR ectodomain as defined in (i) in combination with the atomic coordinates given in Appendix III and/or Appendix IV, or in combination with a subset of the atomic coordinates given in Appendix III and/or Appendix IV; or (iv) a region of IR ectodomain as defined in (ii) in combination with the atomic coordinates given in Appendix III and/or Appendix IV, or in combination with a subset of the atomic coordinates given in Appendix III and/or Appendix IV.
The present invention also provides a computer-readable media having recorded thereon data representing a model and/or the atomic coordinates of (i) an IR ectodomain crystal or (ii) an LCL IR ectodomain crystal.
Also provided is a computer-readable media having recorded thereon coordinate data according to Appendix I or Appendix II, or a subset thereof of either, where the coordinate data defines a three dimensional structure of IR ectodomain or a region of IR ectodomain.
Also provided is a computer-readable media having recorded thereon coordinate data according to Appendix III, or a subset thereof, where the coordinate data defines a three dimensional structure for insulin as it associates with IR as represented by atomic coordinates found in Appendix II.
The present invention also provides a set of coordinates as shown in Appendix I or Appendix II, or a subset thereof both or either, in which the coordinates define a three dimensional structure of IR ectodomain or a region of IR ectodomain.
The three-dimensional structure of IR ectodomain or a region thereof may be used to develop models useful for drug design and in silico screening of candidate compounds that interact with and/or modulate IR. Other physicochemical characteristics may also be used in developing the model, e.g. bonding, electrostatics etc.
Generally the term “in silico” refers to the creation in a computer memory, i.e., on a silicon or other like chip. Stated otherwise “in silico” means “virtual”. When used herein the term “in silico” is intended to refer to screening methods based on the use of computer models rather than in vitro or in vivo experiments.
The present invention further provides a computer-based method of identifying a compound that potentially interacts with IR, which method comprises fitting the structure of (i) IR ectodomain having the atomic coordinates shown in Appendix I, a subset of the atomic coordinates shown in Appendix I in combination with the atomic coordinates shown in Appendix II or a subset of the atomic coordinates shown in Appendix I in combination with a subset of the atomic coordinates shown in Appendix II; (ii) a region of IR ectodomain having the atomic coordinates shown in Appendix II, a subset of the atomic coordinates shown in Appendix I in combination with a subset of the atomic coordinates shown in Appendix II or a subset of the atomic coordinates shown in Appendix I or Appendix II; (iii) IR ectodomain as defined in (i) in combination with the atomic coordinates given in Appendix III and/or Appendix IV, or in combination with a subset of the atomic coordinates given in Appendix III and/or Appendix IV; or (iv) a region of IR ectodomain as defined in (ii) in combination with the atomic coordinates given in Appendix III and/or Appendix IV, or in combination with a subset of the atomic coordinates given in Appendix III and/or Appendix IV; to the structure of a candidate compound.
The present invention further provides a computer-assisted method for identifying a compound able to interact with IR using a programmed computer comprising a processor, which method comprises the steps of: (a) generating, using computer methods, a set of atomic coordinates of a structure that possesses energetically favourable interactions with the atomic coordinates of: (i) IR ectodomain corresponding to the atomic coordinates shown in Appendix I, a subset of the atomic coordinates shown in Appendix I in combination with the atomic coordinates shown in Appendix II or a subset of the atomic coordinates shown in Appendix I in combination with a subset of the atomic coordinates shown in Appendix II; (ii) a region of IR ectodomain corresponding to the atomic coordinates shown in Appendix II, a subset of the atomic coordinates shown in Appendix I in combination with a subset of the atomic coordinates shown in Appendix II or a subset of the atomic coordinates shown in Appendix I or Appendix II; (iii) IR ectodomain as defined in (i) in combination with the atomic coordinates given in Appendix III and/or Appendix IV, or in combination with a subset of the atomic coordinates given in Appendix III and/or Appendix IV; or (iv) a region of IR ectodomain as defined in (ii) in combination with the atomic coordinates given in Appendix III and/or Appendix IV, or in combination with a subset of the atomic coordinates given in Appendix III and/or Appendix IV; which coordinates are entered into the computer, thereby generating a criteria data set; (b) comparing, using the processor, the criteria data set to a computer database of chemical structures; (c) selecting from the database, using computer methods, chemical structures which are complementary or similar to a region of the criteria data set; and, optionally, (d) outputting, to an output device, the selected chemical structures which are complementary to or similar to a region of the criteria data set.
Furthermore, the present invention provides a method for evaluating the ability of a chemical entity to interact with IR, the method comprising the steps of: (a) employing computational means to perform a fitting operation between the chemical entity and the binding surface of a computer model of at least one region of IR ectodomain using structure coordinates wherein the root mean square deviation between the structure coordinates and the structure coordinates set forth in Appendix I and/or Appendix II, or a subset thereof of both or either, is not more than 1.5 Å; and (b) analysing the results of the fitting operation to quantify the association between the chemical entity and the binding surface model.
Optionally, the atomic coordinates given in Appendix III and/or Appendix IV, or a subset of the atomic coordinates given in Appendix III and/or Appendix IV, may also be used to help define the binding surface of the computer model in step (a) and/or to analyse the fitting operation in step (b).
The model may be adaptive in a sense that it allows for slight surface changes to improve the fit between the candidate compound and the protein, e.g. by small movements in side chains or the main chain.
The IR ectodomain crystal structure and/or the LCL IR ectodomain crystal provided herein may also be used to model/solve the structure of a new crystal using molecular replacement.
Accordingly, the present invention provides a method of using molecular replacement to obtain structural information about a molecule or a molecular complex of unknown structure, comprising the steps of: (i) generating an X-ray diffraction pattern of the crystallized molecule or molecular complex; and (ii) applying at least a portion of the structure coordinates set forth in Appendix I and/or Appendix II to the X-ray diffraction pattern to generate a three-dimensional electron density map of at least a region of the molecule or molecular complex whose structure is unknown.
Preferably the molecule of unknown structure is IR or variant thereof.
In one embodiment, the molecular complex of unknown structure is a complex of IR, or variant thereof, and a ligand or candidate ligand.
In another embodiment the molecular complex of unknown structure is a complex of IR or IGF-1R and a monoclonal antibody or antibody fragment (Fab).
In another embodiment the molecular complex of unknown structure is a complex of IR or IGF-1R and a nonimmunoglobulin binding protein such as an avimer, DARPin or lipocalin or an aptamer etc.
The present invention also provides a computer-assisted method for identifying potential mimetics of IR using a programmed computer comprising a processor, the method comprising the steps of: (a) generating a critria data set from a set of atomic coordinates of: (i) IR ectodomain corresponding to the atomic coordinates shown in Appendix I, a subset of the atomic coordinates shown in Appendix I in combination with the atomic coordinates shown in Appendix II or a subset of the atomic coordinates shown in Appendix I in combination with a subset of the atomic coordinates shown in Appendix II, or any of these preceding combinations in combination with the atomic coordinates shown in Appendix IV or a subset of the atomic coordinates shown in Appendix IV; or (ii) a region of IR ectodomain corresponding to the atomic coordinates shown in Appendix II, a subset of the atomic coordinates shown in Appendix I in combination with a subset of the atomic coordinates shown in Appendix II or a subset of the atomic coordinates shown in Appendix I or Appendix II, or any of these preceding combinations in combination with the atomic coordinates shown in Appendix IV or a subset of the atomic coordinates shown in Appendix IV; which coordinates are entered into the computer; (b) (i) comparing, using the processor, the criteria data set to a computer database of chemical structures stored in a computer data storage system and selecting from the database, using computer methods, chemical structures having a region that is structurally similar to the criteria data set; or (ii) constructing, using computer methods, a model of a chemical structure having a region that is structurally similar to the criteria data set; and, optionally, (c) outputting to the output device: (i) the selected chemical structures from step (b)(i) having a region similar to the criteria data set; or (ii) the constructed model from step (b)(ii).
A region of an IR ectodomain as referred to herein may be defined by a single amino acid (or side-chain thereof), by a continuous amino acid sequence or by two or more separate amino acids and/or stretches of amino acids. Such separate amino acids and/or stretches of amino acids may exist in close spatial proximity to one another in the three dimensional structure or may have the potential to be brought into close spatial proximity, for example, upon the binding of a suitable ligand. Suitably, regions of IR ectodomains comprise amino acid sequences involved in the binding of insulin, both the initial selective low affinity binding and the subsequent high affinity binding to the other monomer in the IR dimer.
Accordingly, the present invention provides a method for preventing or treating a disease associated with aberrant IR function which method comprises administering to a subject in need thereof a compound identified by a screening method of the invention.
Particular diseases associated with dysfunctional signalling by IR include obesity, type I and type II diabetes, cardiovascular disease, osteoporosis, dementia and cancer.
The present invention also provides a pharmaceutical composition comprising a compound identified by a screening method of the invention, which compound is able to interact (e.g. bind) with the ectodomain of IR and modulate an activity of the receptor, as well as a method of preventing or treating a disease associated with aberrant IR function (e.g. IR signalling) which method comprises administering to a subject in need thereof a composition of the invention.
The term “IR” as used herein includes wild-type IR and variants thereof including allelic variants and naturally occurring mutations and genetically engineered variants. The amino acid sequence of human IR is shown in Table 1. Reference to specific amino acid residue numbers or sequence numbering is with respect to the amino acid sequence as numbered in FIG. 2 .
It will be readily apparent to the skilled person that IR ectodomains may be derived from other species not specifically disclosed herein. Furthermore, the skilled person will have no difficulties identifying such other suitable IR ectodomains or LCL IR ectodomains given the known conservation of IR sequences from primitive organisms through to mammals and humans. It will also be apparent to the skilled person through sequence alignment analysis of the respective amino acid sequences how the amino acid numbering system used herein can be applied to other IR ectodomains and where regions corresponding to those identified in the human IR ectodomain are found in other IR ectodomains.
As will be readily understood by those skilled in this field the methods of the present invention provide a rational method for designing and selecting compounds including antibodies and nonimmunoglobulin binding proteins which interact with IR ectodomain. In the majority of cases these compounds will require further development in order to increase activity. Such further development is routine in this field and will be assisted by the structural information provided in this application. It is intended that in particular embodiments the methods of the present invention includes such further developmental steps.
It is also intended that embodiments of the present invention include manufacturing steps such as incorporating the compound into a pharmaceutical composition in the manufacture of a medicament.
Throughout this specification, preferred aspects and embodiments apply, as appropriate, separately, or in combination, to other aspects and embodiments, mutatis mutandis, whether or not explicitly stated as such.



BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 : Cartoon of the insulin receptor (IR) dimer showing the distribution of domains across the α- and β-chains and the approximate location of the α-β disulfides and the α-α dimer disulfide bonds. The leucine-rich repeat domains (L1 and L2), the cysteine-rich region (CR), the three fibronectin type III domains (FnIII-1, FnIII-2a and FnIII-2b, FnIII-3) and the insert region in the CC′ loop of FnIII-2 (ID) are depicted. Inter-chain α-α disulphide bonds occur in two places (Cys524-Cys524 and at the triplet of Cys residues at 682, 683 and 685 in ID). There is only a single α-β disulphide (Cys647-Cys860) in each IR monomer.
FIG. 2 : Shows the sequence alignment of the ectodomains of human insulin receptor (1R, exon 11-isoform) and human IGF1 receptor (IGF-1R). Residues conserved between the sequences are indicated by vertical bars and potential N-linked glycosylation sites are indicated by shading. The location of secondary structure elements of the L1, CR and L2 domains of IR are indicated above the alignment. The approximate location of secondary structure elements within the FnIII domains of the IR structure are indicated and labelled according to the canonical strand nomenclature for such domains. Disulphide links are indicated by square braces above the alignment. The sites of glycosylation as well as the residues that are absent from the model, either because they were not included in the construct or because they were not discerned in the crystallographic electron density maps, are shaded. Sequence sources were: IR (Ullrich et al., 1985), human type 1 IGF receptor (Ullrich et al., 1986).
FIG. 3 : Comparison of the structures of the L1-CR-L2 domain fragments of IR-1 ( FIG. 3 a ), IR-2 ( FIG. 3 b ) and IGF-1R ( FIG. 3 c ). The L1 domains are depicted in the same orientation. Helices are indicated by curled ribbons and β-strands by broad arrows. The side-chains of disulfide-linked cysteine residues are depicted as sticks. The structure source for the L1-CR-L2 domains of IGF-1R is Protein Data Bank entry 1IGF (Garrett et al., 1998).
FIG. 4 : Comparison of main chain traces for the L1 domains of IR (dark grey) and IGF-1R (light grey). Cα atoms from L1 domains have been superposed showing the major differences in the second half of the second leucine rich repeat of the L1 domain (left hand side). The side chains of key IR residues involved in insulin binding (Ile13, Argl4, Asn15, Gln34, Phe39 and Phe64) are depicted as sticks as is the side chain of IGF-1R residue Ser35, the homologue residue of Phe39 in IR.
FIG. 5 : Electrostatic potentials calculated for the L1, CR and L2 domains of IR and IGF-1R. Panels A and E: L1 of IR viewed into the face of sheet 2. Panels B and F: L1-CR-L2 of IR. Panels C and G: L1 of IGF-1R viewed into the face of sheet 2. Panels D and H: L 1-CR-L2 of IGF-1R. In Panels A to D the dark greyed surface indicates positive electrostatic potential >10 kT/e and in Panels E to H the dark greyed surface indicates negative electrostatic potential <−10 kT/e, with other shades of grey in both panels indicating either electrostatic potential closer to zero or molecular surface normals directed significantly away from the viewer, where k is the Boltzmann constant, T the temperature and e the magnitude of the electron charge. The atom charges used in the calculation were −0.5e for Asp OD1, Asp OD2, Glu OE1 and Glu OE2; +0.5e for Arg NE1 and NE2; and +1.0e for Lys NZ. The electrostatic potentials were calculated using GRASP (Nicholls et al., 1991).
FIG. 6 : The ligand binding face of the L1 domain of IR. (A) Cartoon showing the main chain trace and the location of key residues. The hydrophobic patch is indicated by the dashed trace when viewed in the same direction as panel B. (B) Stereo view of the L1 ligand-binding surface. (C) View of the surface of insulin that interacts with the binding face of the L1 domain of IR, rotated 180 degrees about a vertical axis relative to D. (D) Surface diagram of the L1 domain of IR. Residues are shaded according to relative affinity of insulin for alanine substitutions; in three grades: <5%; 5-15%; 15-50% and no expression (see Table 2). The potential location of bound insulin is depicted in white, with residues B1-B8 dotted, A1-A20 and B9-B21 as a thin trace and an arrow to indicate the approximate position of residues B22-B30. The view direction is the same direction as in panel B and part of the Cys-rich domain trace is in purple. (E) Surface conservation based on sequence alignment of vertebrate insulin receptors. The most conserved surface region in the L1-CR-L2 fragment is the putative ligand-binding site on L1, with strictly conserved residues lightly shaded. Thirteen sequences were used from mammals, birds, amphibians and fish (NCBI accession numbers: P06213, AAR04440, P15208, P15127, XP — 542108, XP — 418250, Q9PVZ4, XP — 690534, XP — 691069, BAB836677, BAB83668, CAG083667, CAG08022). View as in panel B.
FIG. 7 : Illustration of the polypeptide fold for the IRΔβ monomer showing the relative arrangement of the L1, CR, L2 and the three FnIII domains. Intra-monomer disulphides are shown in black stick representation, whilst the location of the inter-monomer disulphide at residue 524 is explicitly labelled. The observed termini of the α- and β-chains are indicated by residue numbers. Electron density was not seen for α-chain residues 1-3 and 656-719 or for β-chain residues 724-734, 754 and 910-916.
FIG. 8 : Schematic diagram of the IRΔβ homodimer showing the juxtaposition of domains between the monomers. One monomer is shown in tube representation whilst the other is shown in CPK representation. The location of potential N-linked glycosylation sites are shown in black with the residue numbers indicated.
FIG. 9 : Nucleotide and derived amino acid sequences of the light and heavy chain variable regions of the Fab from the anti-insulin receptor monoclonal antibody 83-7. The translated amino acid sequence for the variable regions of the heavy and light chains of 83-7 was deduced from cDNA clones amplified from mRNA isolated from 83-7 hybridoma cells. For the light chain, the amplified region spanned the signal peptide (underlined), the variable region (residues 1-112) and proximal amino acids from the CH1 domain (113-121). For the heavy chain, the amplified region included the signal peptide (underlined), variable region (1-118) and proximal CH1 residues (119-138). The nucleotide sequence of the amplified cDNA is shown below the derived amino acid sequence. The three CDRs in each chain are indicated above the sequence. The additional CH1 amino acid sequences for both chains, together with hinge region residues down to the first cysteine, were obtained from published records.
FIG. 10 : Nucleotide and derived amino acid sequences of the light and heavy chain variable regions of the Fab from the anti-insulin receptor monoclonal antibody 83-14. The translated amino acid sequence for the variable regions of the heavy and light chains of 83-14 was deduced from cDNA clones amplified from mRNA isolated from 83-14 hybridoma cells. For the light chain, the amplified region spanned the signal peptide (underlined), the variable region (residues 1-108) and proximal amino acids from the CH1 domain (109-117). For the heavy chain, the amplified region included the signal peptide (underlined), variable region (1-113) and proximal CH1 residues (113-127). The nucleotide sequence for the variable domains are shown below the derived amino acid sequence. The three CDRs in each chain are indicated above the sequence. The additional CH1 amino acid sequences for both chains, together with hinge region residues down to the first cysteine, were obtained from published records.
FIG. 11 : Orthogonal views of the IRΔβ homodimer with the attached pairs of Fabs. The IRΔβ homodimer is represented and shaded as in FIG. 7 . The Fab fragments are represented and coloured as follows:— 83-7 heavy chain: thin dark shaded tube, 83-7 light chain: thin light tube, 83-14 heavy chain: thin dark tube, and 83-14 light chain: thin light tube. (a) viewed orthogonal to the homodimer axis, (b) viewed down the homodimer axis (with the viewing direction of panel (a) being indicated by the arrow), (c) detailed zoom-in on the circled area of panel (b), showing the cluster of charged residues around the homodimer axis.
FIG. 12 : (a) (a) Additional electron density on the face of the L1 domains of the IRΔβ ectodomain homodimer. This density is tentatively interpreted to correspond to a segment of the insert domain ID, most likely to the C-terminal region of the receptor α-chain (residues 704-719). (b) Schematic diagram of possible binding location of insulin bridging two monomers in the IRΔβ homodimer. Note that in this diagram the L1, CR and L2 domains are from one monomer within the homodimer, whilst the FnIII domains are from the other monomer. Domains are shaded as in FIG. 7 , whilst insulin is shown in grey CPK representation. The positioning of insulin is based on the model of insulin bound to L1-CR-L2 (described in Example 4).
FIG. 13 : Summary of the scheme for the isolation of N-linked glycopeptides from hIR ectodomain expressed in CHO-K1 cells. Enzyme digestions were carried out as described in the Experimental Procedures section described herein. The conditions for reversed-phase chromatographic isolation of each specific peptide are summarized in Table 4.
FIG. 14 : Schematic diagram of the IRΔβ homodimer showing the distribution of the N-linked glycans modelled at their respective locations on the surface of the human IRAΔβ ectodomain dimer: (a) viewed orthogonal to the homodimer two-fold axis, (b) viewed along the homodimer two-fold axis towards the apex of the inverted “V”-shaped molecule (i.e. away from the membrane), (c) viewed along the homodimer two-fold axis towards the base of the inverted “V”-shaped molecule (i.e. towards the membrane). The protein molecular surface is shown in light grey atomic sphere representation whilst the glycan atoms are shown as black atomic sphere representation.
FIG. 15 : A comparison of simulated projection images with previously determined negative stain images. (a) Simulated projection images at ˜20 Å resolution obtained from the crystal structure of IRΔβ in isolation and in complex with 83-7 Fab and/or 83-14 Fab. (b) Negative stain images obtained from the same complexes by Tulloch et al. (1999), their FIGS. 2-4 , used by permission. (c) Orthogonal views of the particles observed in (b) as proposed by Tulloch et al. (1999). Projection images in (a) were obtained using SPIDER (Frank et al. 1999).
FIG. 16 : Cartoon of insulin binding to membrane anchored IR. The symmetrical alignment of the ectodomains of the two monomers in the IR dimer are shown when viewed down the Y-axis. The shading for the domains are the same as in FIG. 7 . The approximate locations of Sites 1 and 2 on each monomer are indicated by the numbered circles. In the basal state both low affinity sites are equally accessible. In the high affinity state, one insulin molecule crosslinks Sites 1 and 2 on one side of the dimer causing the two monomers to close up on that side and open up on the opposite side. Negative co-operativity is explained by the formation of the alternate cross-link involving the two left over binding sites and the disruption of the first bridging contacts. The monomers in the dimer can be viewed as “see-sawing” between these two alternatives.



DETAILED DESCRIPTION OF THE INVENTION
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art (e.g. in molecular biology, biochemistry, structural biology, and computational biology). Standard techniques are used for molecular and biochemical methods (see generally, Sambrook et al., 2001, and Ausubel et al., 1999, which are incorporated herein by reference) and chemical methods.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
IR Ectodomain Crystals and Crystal Structures
The present invention provides a crystal comprising an IR ectodomain based on the IRΔβ construct (see Examples). Such crystals preferably are of the space group C222 1 with unit cell dimensions of a=123.0 Å, b=319.7 Å and c=204.9 Å.
The present invention also provides a crystal comprising an LCL IR ectodomain based on the IR485 construct (see Examples). Such crystals preferably are of space group P212121 with unit cell dimensions a=103.86 Å, b=130.24 Å, c=160.92 Å.
As used herein, the term “crystal” means a structure (such as a three dimensional (3D) solid aggregate) in which the plane faces intersect at definite angles and in which there is a regular structure (such as internal structure) of the constituent chemical species. The term “crystal” refers in particular to a solid physical crystal form such as an experimentally prepared crystal.
Crystals according to the invention may be prepared using any IR ectodomain, i.e. the IR polypeptide containing the extracellular domain and lacking the transmembrane domain and the intracellular tyrosine kinase domain. Typically, the extracellular domain comprises residues 1 to 917 (mature receptor numbering) of human IR, or the equivalent thereof together with any post-translational modifications of these residues such as N- or O-linked glycosylation.
In a preferred embodiment the IR polypeptide is human IR (GENBANK reference number NM — 000208) or its exon 11-isoform. However, the IR polypeptide may also be obtained from other species, such as other mammalian, vertebrate or invertebrate species.
Crystals may be constructed with wild-type IR polypeptide ectodomain sequences or variants thereof, including allelic variants and naturally occurring mutations as well as genetically engineered variants. Typically, variants have at least 95 or 98% sequence identity with a corresponding wild-type IR ectodomain polypeptide.
Optionally, the crystal of IR ectodomain may comprise one or more molecules which bind to the ectodomain, or otherwise soaked into the crystal or cocrystallised with IR ectodomain. Such molecules include ligands or small molecules, which may be candidate pharmaceutical agents intended to modulate the interaction between IR and its biological targets. The crystal of IR ectodomain may also be a molecular complex with other receptors of the IGF receptor family such as IGF-1R. The complex may also comprise additional molecules such as the ligands to these receptors.
The production of IR ectodomain crystals is described below.
In a preferred embodiment, an IR ectodomain crystal of the invention has the atomic coordinates set forth in Appendix I. As used herein, the term “atomic coordinates” refer to a set of values which define the position of one or more atoms with reference to a system of axes. It will be understood by those skilled in the art that atomic coordinates may be varied, without affecting significantly the accuracy of models derived therefrom. Thus, although the invention provides a very precise definition of a preferred atomic structure, it will be understood that minor variations are envisaged and the claims are intended to encompass such variations.
It will be understood that any reference herein to the atomic coordinates or subset of the atomic coordinates shown in Appendix I or Appendix II shall include, unless specified otherwise, atomic coordinates having a root mean square deviation of backbone atoms of not more than 1.5 Å, preferably not more than 1 Å, when superimposed on the corresponding backbone atoms described by the atomic coordinates shown in Appendix I or Appendix II. Also, any reference to the atomic coordinates or subset of the atomic coordinates shown in Appendix III shall include, unless specified otherwise, atomic coordinates having a root mean square deviation of backbone atoms of not more than 2.5 Å when superimposed on the corresponding backbone atoms described by the atomic coordinates shown in Appendix III.
Preferred variants are those in which the root mean square deviation (RMSD) of the x, y and z co-ordinates for all backbone atoms other than hydrogen is less than 1.5 Å (preferably less than 1 Å, 0.7 Å or less than 0.3 Å) compared with the coordinates given in Appendix I. It will be readily appreciated by those skilled in the art that a 3D rigid body rotation and/or translation of the atomic coordinates does not alter the structure of the molecule concerned.
In a highly preferred embodiment, the crystal has the atomic coordinates as shown in Appendix I.
The present invention also provides a crystal structure of an IR ectodomain polypeptide, or a region thereof.
The atomic coordinates obtained experimentally for amino acids 4 to 655 and 755 to 909 (mature receptor numbering) of human IR ectodomain are shown in Appendix I. However, a person skilled in the art will appreciate that a set of atomic coordinates determined by X-ray crystallography is not without standard error. Accordingly, any set of structure coordinates for an IR ectodomain polypeptide that has a root mean square deviation of protein backbone atoms of less than 0.75 Å when superimposed (using backbone atoms) on the atomic coordinates listed in Appendix I shall be considered identical.
The present invention also comprises the atomic coordinates of an IR ectodomain region or polypeptide that substantially conforms to the atomic coordinates listed in Appendix I and/or Appendix II.
A structure that “substantially conforms” to a given set of atomic coordinates is a structure wherein at least about 50% of such structure has an RMSD of less than about 1.5 Å for the backbone atoms in secondary structure elements in each domain, and more preferably, less than about 1.3 Å for the backbone atoms in secondary structure elements in each domain, and, in increasing preference, less than about 1.0 Å, less than about 0.7 Å, less than about 0.5 Å, and most preferably, less than about 0.3 Å for the backbone atoms in secondary structure elements in each domain.
In a more preferred embodiment, a structure that substantially conforms to a given set of atomic coordinates is a structure wherein at least about 75% of such structure has the recited RMSD value, and more preferably, at least about 90% of such structure has the recited RMSD value, and most preferably, about 100% of such structure has the recited RMSD value.
In an even more preferred embodiment, the above definition of “substantially conforms” can be extended to include atoms of amino acid side chains. As used herein, the phrase “common amino acid side chains” refers to amino acid side chains that are common to both the structure which substantially conforms to a given set of atomic coordinates and the structure that is actually represented by such atomic coordinates.
The present invention also provides subsets of the atomic coordinates listed in Appendix I and/or Appendix II, preferably Appendix I, and subsets that conform substantially thereto. Preferred subsets define one or more regions of the human IR ectodomain selected from (i) the central β-sheet of L1, (ii) the central modules of CR, (iii) the AB loop of FnIII-1, (iv) the CC′ loop of FnIII-1, (v) the EF loop of FnIII-1 and (vi) FnIII-2. Of the central β-sheet of L1, the region preferably defines at least the region including Phe39 in the 2 nd rung of the L1 face and the inserted loop comprising residues 86-91 in the 4th rung of the L1 face. Of the central modules of CR, the region preferably defines at least module 6.
The present invention also provides subsets of the atomic coordinates listed in Appendix III.
It will be appreciated that a set of structure coordinates for a polypeptide is a relative set of points that define a shape in three dimensions. Thus, it is possible that an entirely different set of coordinates could define a similar or identical shape. Moreover, slight variations in the individual coordinates will have little effect on overall shape.
The variations in coordinates may be generated due to mathematical manipulations of the structure coordinates. For example, the structure coordinates set forth in Appendix I could be manipulated by crystallographic permutations of the structure coordinates, fractionalisation of the structure coordinates, integer additions or subtractions to sets of the structure coordinates, inversion of the structure coordinates, or any combination thereof.
Alternatively, modification in the crystal structure due to mutations, additions, substitutions, and/or deletions of amino acids, or other changes in any of the components that make up the crystal could also account for variations in structure coordinates.
Various computational analyses are used to determine whether a molecular complex or a portion thereof is sufficiently similar to all or parts of the structure of the extracellular domain of IR described above. Such analyses may be carried out in current software applications, such as the Molecular Similarity program of QUANTA (Molecular Simulations Inc., San Diego, Calif.) version 4.1.
The Molecular Similarity program permits comparisons between different structures, different conformations of the same structure, and different parts of the same structure.
Comparisons typically involve calculation of the optimum translations and rotations required such that the root mean square difference of the fit over the specified pairs of equivalent atoms is an absolute minimum. This number is given in angstroms.
Accordingly, structural coordinates of an IR ectodomain within the scope of the present invention include structural coordinates related to the atomic coordinates listed in Appendix I by whole body translations and/or rotations. Accordingly, RMSD values listed above assume that at least the backbone atoms of the structures are optimally superimposed which may require translation and/or rotation to achieve the required optimal fit from which to calculate the RMSD value.
A three dimensional structure of an IR ectodomain polypeptide or region thereof which substantially conforms to a specified set of atomic coordinates can be modelled by a suitable modeling computer program such as MODELER (SalI & Blundell, 1993), as implemented in the Insight II Homology software package (Insight II (97.0), MSI, San Diego), using information, for example, derived from the following data: (1) the amino acid sequence of the human IR ectodomain polypeptide; (2) the amino acid sequence of the related portion(s) of the protein represented by the specified set of atomic coordinates having a three dimensional configuration; and, (3) the atomic coordinates of the specified three dimensional configuration. A three dimensional structure of an IR ectodomain polypeptide which substantially conforms to a specified set of atomic coordinates can also be calculated by a method such as molecular replacement, which is described in detail below.
Structure coordinates/atomic coordinates are typically loaded onto a machine readable-medium for subsequent computational manipulation. Thus models and/or atomic coordinates are advantageously stored on machine-readable media, such as magnetic or optical media and random-access or read-only memory, including tapes, diskettes, hard disks, CD-ROMs and DVDs, flash memory cards or chips, servers and the internet. The machine is typically a computer.
The structure coordinates/atomic coordinates may be used in a computer to generate a representation, e.g. an image, of the three-dimensional structure of the IR ectodomain crystal which can be displayed by the computer and/or represented in an electronic file.
The structure coordinates/atomic coordinates and models derived therefrom may also be used for a variety of purposes such as drug discovery, biological reagent (binding protein) selection and X-ray crystallographic analysis of other protein crystals.
“Target Binding Sites” for Compounds which Interact with IR
The three-dimensional structure of IR ectodomain and/or LCL IR ectodomain provided by the present invention can be used to identify potential target binding sites on IR ectodomain (i.e. to identify those regions of IR ectodomain involved in and important to the binding of insulin and signal transduction) as well as in methods for identifying or designing compounds which interact with potential target binding sites of IR ectodomain, e.g. potential modulators of IR.
Preferred target binding sites are those governing specificity, i.e. those regions of IR ectodomain involved in the initial “low affinity” binding of insulin (i.e. the initial docking of insulin to IR) and those governing signal transduction, i.e. those regions of the IR ectodomain involved in the “high affinity” binding of insulin which leads to the initiation of IR autophosphorylation.
In one embodiment, the target binding site is a “low affinity” binding site, i.e. a region of IR ectodomain involved in insulin docking to the receptor. Preferred low affinity target binding sites comprise one or more regions from the following: the L1 domain, the CT peptide and the CR domain of IR ectodomain. With regards to the L1 domain, the target binding site preferably comprises portions of the molecular surface of the central β-sheet of L1 and portions of the molecular surface of the second LRR which contain Phe39 or the loop in the fourth LRR rung of L1, or preferably both. With regards the CR domain, the target binding site preferably comprises module 6 of the CR domain.
Alternatively, the low affinity target binding site may comprise one or more amino acids from one or more of the following amino acid sequences: (i) amino acids 1-156; (ii) amino acids 704-719; and (iii) amino acids 157-310.
With regards to amino acids 1-156, the target binding site preferably comprises at least one amino acid from the amino acid sequence 1-68, preferably 1-55, and more preferably amino acid sequence 27-55. The target binding site preferably comprises at least one amino acid selected from Argl4, Asnl5, Gln34, Leu36, Leu37, Phe39, Pro43-Phe46, Phe64, Leu87, Phe88, Asn90 and Phe89, more preferably at least one amino acid selected from Argl4, Asn15, Gln34, Leu37, Phe39, Pro43-Phe46, Phe64, yet more preferably at least one amino acid selected from Phe39 and Pro43-Phe46, and most preferably at least Phe39.
With regards to amino acids 157-310, the target binding site preferably comprises at least one amino acid from the amino acid sequence 192-310, more preferably at least one amino acid from the sequence 227-303, yet more preferably least one amino acid selected from the sequence 259-284.
In another embodiment, the target binding site is a “high affinity” binding site, i.e. a region of the IR ectodomain involved in binding to insulin already bound to one IR monomer and leading to signal transduction. Preferred high affinity target binding sites comprise one or more regions from the FnIII-1 domain. In particular, high affinity target binding sites comprise one or more of the AB loop of FnIII-1, the CC′ loop of FnIII-1 and the EF loop of FnIII-1.
Alternatively, the high affinity binding site may comprise one or more amino acids from one or more of the following amino acid sequences: (i) 472-594; (ii)475-489; (iii) 508-536, and (iv) 550-569.
With regards to amino acids 475-489, the target binding site preferably comprises at least one amino acid from the amino acid sequence 478-486, and more preferably from amino acid sequence 481-483.
With regards to amino acids 508-536, the target binding site preferably comprises at least one amino acid selected from 517-534 and more preferably at least one amino acid selected from 525-534.
With regards to amino acids 550-569, the target binding site preferably comprises at least one amino acid selected from 552-562 and more preferably at least one amino acid selected from 555-558.
In a preferred embodiment, van der Waals and/or hydrophobic interactions account for the major portion of the binding energy between a compound and a low affinity binding site of IR.
The findings resulting from the three-dimensional structure of IR ectodomain and/or LCL IR ectodomain provided by the present invention can also be used to identify or more clearly elucidate potential target binding sites on IGF-1R ectodomain (i.e. to identify those regions, or at least more accurately elucidate those regions, of IGF-1R ectodomain involved in and important to the binding of IGF and signal transduction) as well as in methods used for identifying or designing compounds which interact with potential target binding sites of IGF-1R ectodomain, e.g. potential modulators of IGF-1R.
Preferred target binding sites are those governing specificity, i.e. those regions of IGF-1R ectodomain involved in the initial “low affinity” binding of IGF (i.e. the initial docking of IGF to IGF-1R) and those governing signal transduction, i.e. those regions of the IGF-1R ectodomain involved in the “high affinity” binding of IGF which leads to signal transduction.
In one embodiment, the target binding site is a “low affinity” binding site, i.e. a region of IGF-1R ectodomain involved in IGF-I docking to the receptor. Preferred low affinity target binding sites comprise one or more regions from the following: the L1 domain, the CT peptide and/or the CR domain of IGF-1R ectodomain. With regards to the L1 domain, the target binding site preferably comprises the central β-sheet of L1 in general and the top left hand corner of the repeat containing Ser35 or the loop in the fourth LRR rung of L1, or preferably both. With regards the CR domain, the target binding site preferably comprises module 6 of the CR domain.
Alternatively, the “low affinity” target binding site may comprise one or more amino acids from one or more of the following amino acid sequences: (i) amino acids 1-149; (ii) amino acids 691-706; and (iii) amino acids 150-298.
With regards to amino acids 1-149, the target binding site preferably comprises at least one amino acid from the amino acid sequence 1-62, preferably 1-49, and more preferably amino acid sequence 23-49. The target binding site preferably comprises at least one amino acid selected from Argl0, Asnl1, Tyr28, His30, Leu32, Leu33, Ile34, Ser35, Glu38, Leu56, Phe58, Arg59, Tyr61, Trp79, Leu81, Phe82, Tyr83, Asn84, Tyr85, Phe90, Glu91, more preferably at least one amino acid selected from Leu33, His20, Leu32, Asn84, Glu38, Tyr61, Tyr28, Leu56, Arg59 Trp79 and Phe90, yet more preferably at least one amino acid selected from Leu33, His 30, Tyr28, Leu56, Arg59, Trp79 and Phe 90, and most preferably at least Phe 90.
With regards to amino acids 1-149, the target binding site preferably comprises at least one amino acid from the amino acid sequence 185-298, more preferably at least one amino acid from the sequence 220-294, yet more preferably least one amino acid selected from the sequence 252-273.
In another embodiment, the target binding site is a “high affinity” binding site, i.e. a region of the IGF-1R ectodomain involved in binding to IGF already bound to one IGF-1R monomer and leading to signal transduction. Preferred “high affinity” target binding sites comprise one or more regions of the FnIII-1 domain. In particular, “high affinity target” binding sites comprise one or more of the AB loop of FnIII-1, the CC′ loop of FnIII-1 and the EF loop of FnIII-1.
Alternatively, the “high affinity” binding site may comprise one or more amino acids from one or more of the following amino acid sequences: (i) 462-580, (ii) 465-479, (iii) 498-526 and (iv) 526-543.
With regards to amino acids 465-479, the target binding site preferably comprises at least one amino acid from the amino acid sequence 468-478 and more preferably from amino acid sequence 471-473.
With regards to amino acids 498-526, the target binding site preferably comprises at least one amino acid selected from 507-524 and more preferably at least one amino acid selected from 515-524.
With regards to amino acids 526-543, the target binding site preferably comprises at least one amino acid selected from 527-537 and more preferably at least one amino acid selected from 530-533.
In a preferred embodiment, van der Waals and/or hydrophobic interactions account for the major portion of the binding energy between a compound and a low affinity binding site of IGF-1R.
Additional preferred binding sites in the case of both IR and IGF-1R, particularly for biological macromolecules such as proteins or aptamers, are those that are devoid of glycosylation or devoid of steric hindrance from glycan covalently attached to the polypeptide at sites in the spatial vicinity.
Compounds According to the Invention
Compounds of the present invention include both those designed or identified using a screening method of the invention and those which are capable of recognising and binding to a target binding site as defined above. Compounds capable of recognising and binding to a target binding site may be produced using a screening method based on use of the atomic coordinates corresponding to the 3D structure of IR ectodomain or LCL IR ectodomain, or alternatively may be identified by screening against a specific target molecule which is indicative of the capacity to bind to a target binding site. Such a specific target molecule may, for example, be a short polypeptide defining an epitope (e.g. corresponding to a loop structure identified above as a target binding site) or a mimetic, e.g. a peptidomimetic, mimicking a target binding site.
The candidate compounds and/or compounds identified or designed using a method of the present invention may be any suitable compound, synthetic or naturally occurring, preferably synthetic. In one embodiment, a synthetic compound selected or designed by the methods of the invention preferably has a molecular weight equal to or less than about 5000, 4000, 3000, 2000 or 1000 daltons. A compound of the present invention is preferably soluble under physiological conditions.
The compounds (ligands) may encompass numerous chemical classes, though typically they are organic molecules, preferably small organic compounds having a molecular weight of more than 50 and less than about 2,500 daltons, preferably less than 1500, more preferably less than 1000 and yet more preferably less than 500. Such ligands can comprise functional groups necessary for structural interaction with proteins, particularly hydrogen bonding, and typically include at least an amine, carbonyl, hydroxyl or carboxyl group, preferably at least two of the functional chemical groups. Ligands often comprise cyclical carbon or heterocyclic structures and/or aromatic or polyaromatic structures substituted with one or more of the above functional groups. Ligands can also comprise biomolecules including peptides, saccharides, fatty acids, steroids, purines, pyrimidines, derivatives, structural analogs, or combinations thereof.
Ligands may include, for example: (1) Peptides such as soluble peptides, including Ig-tailed fusion peptides and members of random peptide libraries (see, e.g., Lam et al., 1991; Houghten et al., 1991) and combinatorial chemistry-derived molecular libraries made of D- and/or L-configuration amino acids; (2) Phosphopeptides (e.g., members of random and partially degenerate, directed phosphopeptide libraries, see, e.g., Songyang et al., 1993); (3) Antibodies (e.g., polyclonal, monoclonal, humanized, anti-idiotypic, chimeric, and single chain antibodies as well as Fab, F(ab′) 2 , Fab expression library fragments, and epitope-binding fragments of antibodies); (4) Nonimmunoglobulin binding proteins such as but not restricted to avimers, DARPins and lipocalins; (5) Nucleic acid-based aptamers; and (6) Small organic and inorganic molecules.
Ligands can be obtained from a wide variety of sources including libraries of synthetic or natural compounds. Synthetic compound libraries are commercially available from, for example, Maybridge Chemical Co. (Trevillet, Cornwall, UK), Comgenex (Princeton, N.J.), Brandon Associates (Merrimack, N.H.), and Microsource (New Milord, Conn.).
Natural compound libraries comprising bacterial, fungal, plant or animal extracts are available from, for example, Pan Laboratories (Bothell, Wash.). In addition, numerous means are available for random and directed synthesis of a wide variety of organic compounds and biomolecules, including expression of randomized oligonucleotides.
Alternatively, libraries of natural compounds in the form of bacterial, fungal, plant and animal extracts can be readily produced. Methods for the synthesis of molecular libraries are readily available (see, e.g., DeWitt et al., 1993; Erb et al., 1994; Zuckermann et al., 1994; Cho et al., 1993; Carell et al., 1994a; Carell et al., 1994b; and Gallop et al., 1994). In addition, natural or synthetic compound libraries and compounds can be readily modified through conventional chemical, physical and biochemical means (see, e.g., Blondelle et al., 1996), and may be used to produce combinatorial libraries. In another approach, previously identified pharmacological agents can be subjected to directed or random chemical modifications, such as acylation, alkylation, esterification, amidification, and the analogs can be screened for IR-modulating activity.
Numerous methods for producing combinatorial libraries are known in the art, including those involving biological libraries; spatially addressable parallel solid phase or solution phase libraries; synthetic library methods requiring deconvolution; the “one-bead one-compound” library method; and synthetic library methods using affinity chromatography selection. The biological library approach is limited to polypeptide or peptide libraries, while the other four approaches are applicable to polypeptide, peptide, nonpeptide oligomer, or small molecule libraries of compounds (Lam, 1997).
Compounds also include compounds that may be synthesized from leads generated by fragment-based drug design, wherein the binding of such chemical fragments is assessed by soaking or co-crystallizing such screen fragments into crystals provided by the invention and then subjecting these to an X-ray beam and obtaining diffraction data. Difference Fourier techniques are readily applied by those skilled in the art to determine the location within the IR ectodomain structure at which these fragments bind, and such fragments can then be assembled by synthetic chemistry into larger compounds with increased affinity for the receptor.
Interaction of Compounds with IR and/or IGF-1R
A compound may interact with a specified region of IR or IGF-1R ectodomain (e.g. a target binding site) by binding either directly or indirectly to that region. A compound which binds directly, binds to the specified region. A compound which binds indirectly, binds to a region in close proximity to or adjacent to the specified region with the result that it interferes with the ability of the specified region to bind to insulin, either antagonistically or agonistically. Such interference may be steric, electrostatic, or allosteric. For example, compounds which bind to FnIII-2 can potentially sterically interfere with the binding of insulin to FnIII-1 (i.e. the high affinity binding site of IR), thus interfering with signal transduction. Preferably, a compound interacts with a specified region of the IR or IGF-1R ectodomain by binding directly to the specified region. In the case of compounds that bind to specific target molecules, such compounds bind directly to the specific target molecule.
Binding can be either by covalent or non-covalent interactions, or both. Examples of non-covalent interactions include electrostatic interactions, van der Waals interactions, hydrophobic interactions and hydrophilic interactions.
When a compound interacts with IR or IGF-1R, it preferably “modulates” IR or IGF-1R, respectively. By “modulate” we mean that the compound changes an activity of IR or IGF-1R by at least 10%. Suitably, a compound modulates IR or IGF-1R by increasing or decreasing signal transduction via IR or IGF-1R, respectively. The phrase “decreases signal transduction” is intended to encompass partial or complete inhibition of signal transduction via IR or IGF-1R. The ability of a candidate compound to increase or decrease signal transduction via IR or IGF-1R can be assessed by any one of the IR or IGF-1R cell-based assays described herein.
Compounds may act as antagonists or agonists for insulin binding to IR or as antagonists or agonists for IGF binding to IGF-1R.
Compounds of the present invention preferably have an affinity for IR or IGF-1R sufficient to provide adequate binding for the intended purpose. Suitably, such compounds and compounds which bind to specific target molecules of IR or IGF-1R have an affinity (K d ) of from 10 −5 to 10 −15 M. For use as a therapeutic, the compound suitably has an affinity (K d ) of from 10 −7 to 10 −15 M, preferably from 10 −8 to 10 −12 M and more preferably from 10 −1 ° to 10 −12 M. Where a compound is to be used as a reagent in a competitive assay to identify other ligands, the compound suitably has an affinity (K d ) of from 10 −5 to 10 −12 M.
As will be evident to the skilled person, the crystal structures presented herein have enabled, for the first time, direct comparison of the regions controlling ligand specificity in the closely related IGF-1R and IR, and the identification of critical regions of IR involved in the initial binding of insulin and in the subsequent formation of the high affinity insulin-IR complex that leads to signal transduction.
In one preferred embodiment, a compound has a high specificity for IR and/or a specific target molecule of IR but not for IGF-1R, i.e. a compound selectively binds to IR or has enhanced selectivity for IR over IGF-1R. In this respect, a compound suitably has an affinity (K d ) for IR and/or a specific target molecule of IR of no more than 10 −5 M, preferably no more than 10 −7 M, and an affinity for IGF-1R of at least 10 −5 M, preferably at least 10 −3 M. Such compounds are desirable as, for example, IR agonists where there propensity to interact with IGF-1R and thus, for example, promote undesirable cell proliferation, is reduced.
In a preferred embodiment, the (IR or specific target molecule of IR)/IGF-1R binding affinity ratio for a compound is at least 10 and preferably at least 100.
In another preferred embodiment, a compound has a high specificity for IGF-1R and/or a specific target molecule of IGF-1R but not for IR, i.e. a compound selectively binds to IGF-1R or has enhanced selectivity for IGF-1R over IR. In this respect, a compound suitably has an affinity (K d ) for IGF-1R and/or a specific target molecule of IGF-1R of no more than 10 −5 M, preferably no more than 10 −7 M, and an affinity for IR of at least 10 −5 M, preferably at least 10 −3 M. Such compounds are desirable as, for example, IGF-1R agonists where there propensity to interact with IR and thus, for example, promote glucose uptake and metabolism, is reduced.
In a preferred embodiment, the (IGF-1R or specific target molecule of IGF-1R)/(IR) binding affinity ratio for a compound is at least 10 and preferably at least 100.
Design, Selection, Fitting and Assessment of Chemical Entities that Bind IR Ectodomain
Using a variety of known modelling techniques, the crystal structure of the present invention can be used to produce a model for at least part of IR ectodomain.
As used herein, the term “modelling” includes the quantitative and qualitative analysis of molecular structure and/or function based on atomic structural information and interaction models. The term “modelling” includes conventional numeric-based molecular dynamic and energy minimisation models, interactive computer graphic models, modified molecular mechanics models, distance geometry and other structure-based constraint models.
Molecular modelling techniques can be applied to the atomic coordinates of the IR ectodomain or a region thereof to derive a range of 3D models and to investigate the structure of binding sites, such as the binding sites of monoclonal antibodies, nonimmunoglobulin binding proteins and inhibitory peptides.
These techniques may also be used to screen for or design small and large chemical entities which are capable of binding IR and modulating the ability of IR to interact with extracellular biological targets, such as insulin or members of the IGF receptor family e.g. which modulate the ability of IR to heterodimerise. The screen may employ a solid 3D screening system or a computational screening system.
Such modelling methods are to design or select chemical entities that possess stereochemical complementary to particular regions of IR ectodomain. By “stereochemical complementarity” we mean that the compound or a portion thereof makes a sufficient number of energetically favourable contacts with the receptor as to have a net reduction of free energy on binding to the receptor.
Such stereochemical complementarity is characteristic of a molecule that matches intra-site surface residues lining the groove of the receptor site as enumerated by the coordinates set out in Appendix I and/or Appendix II, optionally also utilising the coodinates set out in Appendix III. By “match” we mean that the identified portions interact with the surface residues, for example, via hydrogen bonding or by non-covalent Van der Waals and Coulomb interactions (with surface or residue) which promote desolvation of the molecule within the site, in such a way that retention of the molecule at the binding site is favoured energetically.
It is preferred that the stereochemical complementarity is such that the compound has a K d for the receptor site of less than 10 −4 M, more preferably less than 10 −5 M and more preferably 10 −6 M. In a most preferred embodiment, the K d value is less than 10 −8 M and more preferably less than 10 −9 M.
Chemical entities which are complementary to the shape and electrostatics or chemistry of the receptor site characterised by amino acids positioned at atomic coordinates set out in Appendix I and/or Appendix II will be able to bind to the receptor, and when the binding is sufficiently strong, substantially prohibit the interaction of the IR ectodomain with biological target molecules such as insulin.
It will be appreciated that it is not necessary that the complementarity between chemical entities and the receptor site extend over all residues of the receptor site in order to inhibit binding of a molecule or complex that naturally interacts with IR ectodomain.
A number of methods may be used to identify chemical entities possessing stereochemical complementarity to a region of the extracellular domain of IR. For instance, the process may begin by visual inspection of potential binding sites, for example, the binding sites for anti-IR antibodies, on the computer screen based on the IR ectodomain, or region thereof, coordinates in Appendix I and/or Appendix II generated from the machine-readable storage medium. Alternatively, selected fragments or chemical entities may then be positioned in a variety of orientations, or docked, within an individual binding site of IR ectodomain, as defined supra. Modelling software that is well known and available in the art may be used (Guida, 1994). These include QUANTA and InsightII [Molecular Simulations, Inc., San Diego, Calif., a division of Pharmacopiea, Inc., Princeton, N.J., 1992], SYBYL [Molecular Modeling Software, Tripos Associates, Inc., St. Louis, Mo., 1992]. This modelling step may be followed by energy minimization with standard molecular mechanics force fields such as AMBER (Weiner et al., 1984), and CHARMM (Brooks et al., 1983). In addition, there are a number of more specialized computer programs to assist in the process of selecting the binding moieties of this invention.
Specialised computer programs may also assist in the process of selecting fragments or chemical entities. These include, inter alia:
1. GRID (Goodford, 1985). GRID is available from Oxford University, Oxford, UK.
2. MCSS (Miranker & Karplus, 1991). MCSS is available from Molecular Simulations, Burlington, Mass.
3. AUTODOCK (Goodsell & Olsen, 1990). AUTODOCK is available from Scripps Research Institute, La Jolla, Calif.
4. DOCK (Kuntz et al., 1982). DOCK is available from University of California, San Francisco, Calif.
Once suitable chemical entities or fragments have been selected, they can be assembled into a single compound. In one embodiment, assembly may proceed by visual inspection of the relationship of the fragments to each other on the three-dimensional image displayed on a computer screen in relation to the structure coordinates of IR ectodomain. This is followed by manual model building using software such as Quanta or Sybyl. Alternatively, fragments may be joined to additional atoms using standard chemical geometry.
The above-described evaluation process for chemical entities may be performed in a similar fashion for chemical compounds.
Useful programs to aid one of skilled in the art in connecting the individual chemical entities or fragments include:
1. CAVEAT (Bartlett et al., 1989). CAVEAT is available from the University of California, Berkeley, Calif.

2. 3D Database systems such as MACCS-3D (MDL Information Systems, San Leandro, Calif.). This area is reviewed in Martin (1992).

3. HOOK (available from Molecular Simulations, Burlington, Mass.).

Other molecular modeling techniques may also be employed in accordance with this invention, see, e.g., Cohen et al. (1990) and Navia & Murcko (1992).
There are two preferred approaches to designing a molecule, according to the present invention, that complement the stereochemistry of IR. The first approach is to in silico directly dock molecules from a three-dimensional structural database, to the receptor site, using mostly, but not exclusively, geometric criteria to assess the goodness-of-fit of a particular molecule to the site. In this approach, the number of internal degrees of freedom (and the corresponding local minima in the molecular conformation space) is reduced by considering only the geometric (hard-sphere) interactions of two rigid bodies, where one body (the active site) contains “pockets” or “grooves” that form binding sites for the second body (the complementing molecule).
This approach is illustrated by Kuntz et al. (1982) and Ewing et al. (2001), the contents of which are hereby incorporated by reference, whose algorithm for ligand design is implemented in a commercial software package, DOCK version 4.0, distributed by the Regents of the University of California and further described in a document, provided by the distributor, which is entitled “Overview of the DOCK program suite” the contents of which are hereby incorporated by reference. Pursuant to the Kuntz algorithm, the shape of the cavity represented by a site on the IR or IGF-1R is defined as a series of overlapping spheres of different radii. One or more extant databases of crystallographic data, such as the Cambridge Structural Database System maintained by Cambridge University (University Chemical Laboratory, Lensfield Road, Cambridge, U.K.), the Protein Data Bank maintained by the Research Collaboratory for Structural Bioinformatics (Rutgers University, N.J., U.S.A.), LeadQuest (Tripos Associates, Inc., St. Louis, Mo.), Available Chemicals Directory (Molecular Design Ltd., San Leandro, Calif.), and the NCl database (National Cancer Institute, U.S.A) is then searched for molecules which approximate the shape thus defined.
Molecules identified on the basis of geometric parameters, can then be modified to satisfy criteria associated with chemical complementarity, such as hydrogen bonding, ionic interactions and Van der Waals interactions. Different scoring functions can be employed to rank and select the best molecule from a database. See for example Bohm & Stahl (1999). The software package FlexX, marketed by Tripos Associates, Inc. (St. Louis, Mo.) is another program that can be used in this direct docking approach (see Rarey et al., 1996).
The second preferred approach entails an assessment of the interaction of respective chemical groups (“probes”) with the active site at sample positions within and around the site, resulting in an array of energy values from which three-dimensional contour surfaces at selected energy levels can be generated. The chemical-probe approach to ligand design is described, for example, by Goodford, (1985), the contents of which are hereby incorporated by reference, and is implemented in several commercial software packages, such as GRID (product of Molecular Discovery Ltd., West Way House, Elms Parade, Oxford OX2 9LL, U.K.).
Pursuant to this approach, the chemical prerequisites for a site-complementing molecule are identified at the outset, by probing the active site with different chemical probes, e.g., water, a methyl group, an amine nitrogen, a carboxyloxygen, or a hydroxyl. Favoured sites for interaction between the active site and each probe are thus determined, and from the resulting three-dimensional pattern of such sites a putative complementary molecule can be generated. This may be done either by programs that can search three-dimensional databases to identify molecules incorporating desired pharmacophore patterns or by programs which use the favoured sites and probes as input to perform de novo design. Suitable programs for determining and designing pharmacophores include CATALYST (including HypoGen or HipHop) (Molecular Simulations, Inc), and CERIUS2, DISCO (Abbott Laboratories, Abbott Park, Ill.) and ChemDBS-3D (Chemical Design Ltd., Oxford, U.K.).
The pharmacophore can be used to screen in silico compound libraries/three-dimensional databases, using a program such as CATALYST (Molecular Simulations, Inc); MACCS-3D and ISIS/3D (Molecular Design Ltd., San Leandro, Calif.), ChemDBS-3D (Chemical Design Ltd., Oxford, U.K.), and Sybyl/3 DB Unity (Tripos Associates, Inc., St. Louis, Mo.).
Databases of chemical structures are available from a number of sources including Cambridge Crystallographic Data Centre (Cambridge, U.K.), Molecular Design, Ltd., (San Leandro, Calif.), Tripos Associates, Inc. (St. Louis, Mo.), Chemical Abstracts Service (Columbus, Ohio), the Available Chemical Directory (MDL Inc), the Derwent World Drug Index (WDI), BioByteMasterFile, the National Cancer Institute database (NCl), and the Maybridge catalogue.
De novo design programs include LUDI (Biosym Technologies Inc., San Diego, Calif.), Leapfrog (Tripos Associates, Inc.), Aladdin (Daylight Chemical Information Systems, Irvine, Calif.), and LigBuilder (Peking University, China).
Once an entity or compound has been designed or selected by the above methods, the efficiency with which that entity or compound may bind to IR can be tested and optimised by computational evaluation. For example, a compound that has been designed or selected to function as an IR binding compound must also preferably traverse a volume not overlapping that occupied by the binding site when it is bound to the native IR. An effective IR binding compound must preferably demonstrate a relatively small difference in energy between its bound and free states (i.e., a small deformation energy of binding). Thus, the most efficient IR binding compound should preferably be designed with a deformation energy of binding of not greater than about 10 kcal/mole, preferably, not greater than 7 kcal/mole. IR binding compounds may interact with IR in more than one conformation that are similar in overall binding energy. In those cases, the deformation energy of binding is taken to be the difference between the energy of the free compound and the average energy of the conformations observed when the compound binds to the protein.
A compound designed or selected as binding to IR may be further computationally optimised so that in its bound state it would preferably lack repulsive electrostatic interaction with the target protein.
Such non-complementary (e.g., electrostatic) interactions include repulsive charge-charge, dipole-dipole and charge-dipole interactions. Specifically, the sum of all electrostatic interactions between the compound and the protein when the compound is bound to IR, preferably make a neutral or favourable contribution to the enthalpy of binding.
Once an IR-binding compound has been optimally selected or designed, as described above, substitutions may then be made in some of its atoms or side groups to improve or modify its binding properties. Generally, initial substitutions are conservative, i.e., the replacement group will have approximately the same size, shape, hydrophobicity and charge as the original group. It should, of course, be understood that components known in the art to alter conformation should be avoided. Such substituted chemical compounds may then be analysed for efficiency of fit to IR by the same computer methods described in detail above.
Specific computer software is available in the art to evaluate compound deformation energy and electrostatic interaction. Examples of programs designed for such uses include: Gaussian 92, revision C (Frisch, Gaussian, Inc., Pittsburgh, Pa.); AMBER, version 4.0 (Kollman, University of California at San Francisco); QUANTA/CHARMM (Molecular Simulations, Inc., Burlington, Mass.); and Insight II/Discover (Biosysm Technologies Inc., San Diego, Calif.).
The screening/design methods may be implemented in hardware or software, or a combination of both. However, preferably, the methods are implemented in computer programs executing on programmable computers each comprising a processor, a data storage system (including volatile and non-volatile memory and/or storage elements), at least one input device, and at least one output device. Program code is applied to input data to perform the functions described above and generate output information. The output information is applied to one or more output devices, in known fashion. The computer may be, for example, a personal computer, microcomputer, or workstation of conventional design.
Each program is preferably implemented in a high level procedural or object-oriented programming language to communicate with a computer system. However, the programs can be implemented in assembly or machine language, if desired. In any case, the language may be compiled or interpreted language.
Each such computer program is preferably stored on a storage medium or device (e.g., ROM or magnetic diskette) readable by a general or special purpose programmable computer, for configuring and operating the computer when the storage media or device is read by the computer to perform the procedures described herein. The system may also be considered to be implemented as a computer-readable storage medium, configured with a computer program, where the storage medium so configured causes a computer to operate in a specific and predefined manner to perform the functions described herein.
Screening Assays and Confirmation of Binding and Biological Activity
Compounds selected or designed in accordance with the in silico methods of the invention may be subjected to further confirmation of binding to IR by cocrystallization of the compound with IR and structural determination, as described herein.
Compounds designed or selected according to the methods of the present invention are preferably assessed by a number of in vitro and in vivo assays of IR and/or IGF-1R function to confirm their ability to interact with and modulate IR and/or IGF-IR activity. For example, compounds may be tested for their ability to bind to IR and/or IGF-1R and/or for their ability to modulate e.g. disrupt, IR and/or IGF-1R signal transduction.
Libraries may be screened in solution by methods generally known in the art for determining whether ligands competitively bind at a common binding site. Such methods may include screening libraries in solution (e.g., Houghten, 1992), or on beads (Lam, 1991), chips (Fodor, 1993), bacteria or spores (U.S. Pat. No. 5,223,409: Lardner), plasmids (Cull et al., 1992), or on phage (Scott & Smith, 1990; Devlin, 1990; Cwirla et al., 1990; Felici, 1991; U.S. Pat. No. 5,223,409: Lardner).
Where the screening assay is a binding assay, IR or IGF-1R may be joined to a label, where the label can directly or indirectly provide a detectable signal. Various labels include radioisotopes, fluorescent molecules, chemiluminescent molecules, enzymes, specific binding molecules, particles, e.g., magnetic particles, and the like. Specific binding molecules include pairs, such as biotin and streptavidin, digoxin and antidigoxin, etc. For the specific binding members, the complementary member would normally be labeled with a molecule that provides for detection, in accordance with known procedures.
A variety of other reagents may be included in the screening assay. These include reagents like salts, neutral proteins, e.g., albumin, detergents, etc., which are used to facilitate optimal protein-protein binding and/or reduce non-specific or background interactions. Reagents that improve the efficiency of the assay, such as protease inhibitors, nuclease inhibitors, antimicrobial agents, etc., may be used. The components are added in any order that produces the requisite binding. Incubations are performed at any temperature that facilitates optimal activity, typically between 4 and 40° C.
Direct binding of compounds to IR or IGF-1R can also be done by Surface Plasmon Resonance (BIAcore) (reviewed in Morton & Myszka, 1998). Here the receptor is immobilized on a CM5 or other sensor chip by either direct chemical coupling using amine or thiol-disulphide exchange coupling (Nice & Catimel, 1999) or by capturing the receptor ectodomain as an Fc fusion protein to an appropriately derivatised sensor surface (Morten & Myszka, 1998). The potential binding molecule (called an analyte) is passed over the sensor surface at an appropriate flow rate and a range of concentrations. The classical method of analysis is to collect responses for a wide range of analyte concentrations. A range of concentrations provides sufficient information about the reaction, and by using a fitting algorithm such as CLAMP (see Morton & Myszka, 1998), rate constants can be determined (Morton & Myszka, 1998; Nice & Catimel, 1999). Normally, the ligand surface is regenerated at the end of each analyte binding cycle. Surface regeneration ensures that the same number of ligand binding sites is accessible to the analyte at the beginning of each cycle.
Incubation periods are selected for optimum activity, but may also be optimized to facilitate rapid high-throughput screening. Normally, between 0.1 and 1 hour will be sufficient. In general, a plurality of assay mixtures is run in parallel with different test agent concentrations to obtain a differential response to these concentrations. Typically, one of these concentrations serves as a negative control, i.e. at zero concentration or below the level of detection.
The basic format of an in vitro competitive receptor binding assay as the basis of a heterogeneous screen for small organic molecular replacements for insulin may be as follows: occupation of the active site of IR or IGF-1R is quantified by time-resolved fluorometric detection (TRFD) as described by Denley et al. (2004). R − IR-A, R − IR-B and P6 cells are used as sources of IR-A, IR-B and IGF-1R respectively. Cells are lysed with lysis buffer (20 mM HEPES, 150 mM NaCl, 1.5 mM MgCl 2 , 10% (v/v) glycerol, 1% (v/v) Triton X-100, 1 mM EGTA pH 7.5) for 1 hour at 4° C. Lysates are centrifuged for 10 minutes at 3500 rpm and then 100 μl is added per well to a white Greiner Lumitrac 600 plate previously coated with anti-insulin receptor antibody 83-7 or anti-IGF-1R antibody 24-31. Neither capture antibody interferes with receptor binding by insulin, IGF-I or IGF-II. Approximately 100,000 fluorescent counts of europium-labelled insulin or europium-labelled IGF-I are added to each well along with various amounts of unlabelled competitor and incubated for 16 hours at 4° C. Wells are washed with 20 mM Tris, 150 mM NaCl, 0.05% (v/v) Tween 20 (TBST) and DELFIA enhancement solution (100 μl/well) is added. Time-resolved fluorescence is measured using 340 nm excitation and 612 nm emission filters with a BMG Lab Technologies Polarstar™ Fluorimeter or a Wallac Victor II (EG & G Wallac, Inc.).
Examples of other suitable assays which may be employed to assess the binding and biological activity of compounds to and on IR are well known in the art. For example, suitable assays may be found in PCT International Publication Number WO 03/027246. Examples of suitable assays include the following:
(i) Receptor autophosphorylation (as described by Denley et al. (2004). R − IR-A, R − IR-B cells or P6 cells are plated in a Falcon 96 well flat bottom plate at 2.5×10 4 cells/well and grown overnight at 37° C., 5% CO 2 . Cells are washed for 4 hours in serum-free medium before treating with one of either insulin, IGF-I or IGF-II in 100 μl DMEM with 1% BSA for 10 minutes at 37° C., 5% CO 2 . Lysis buffer containing 2 mM Na 3 VO 4 and 1 mg/ml NaF is added to cells and receptors from lysates are captured on 96 well plates precoated with antibody 83-7 or 24-31 and blocked with 1×TBST/0.5% BSA. After overnight incubation at 4° C., the plates are washed with 1×TBST. Phosphorylated receptor is detected with europium-labelled antiphosphotyrosine antibody PY20 (130 ng/well, room temperature, 2 hours). DELFIA enhancement solution (100 μl/well) is added and time resolved fluorescence detected as described above.
(ii) Glucose uptake using 2-deoxy-[U-14C] glucose (as described by Olefsky, 1978). Adipocytes between days 8-12 post-differentation in 24-well plates are washed twice in Krebs-Ringer Bicarbonate Buffer (25 mM Hepes, pH 7.4 containing 130 mM NaCl, 5 mM KCl, KH 2 PO 4 , 1.3 mM MgSO 4 .7H 2 O, 25 mM NaHCO 3 and 1.15 mM CaCl 2 ) supplemented with 1% (w/v) RIA-grade BSA and 2 mM sodium pyruvate. Adipocytes are equilibrated for 90 min at 37° C. prior to insulin addition, or for 30 min prior to agonist or antagonist addition. Insulin (Actrapid, Novogen) is added over a concentration range of 0.7 to 70 nM for 30 min at 37° C. Agonist or antagonist (0 to 500 μM) is added to adipocytes for 90 min followed by the addition of insulin in the case of antagonists. Uptake of 50 μM 2-deoxy glucose and 0.5 μCi 2-deoxy-[U- 14 C] glucose (NEN, PerkinElmer Life Sciences) per well is measured over the final 10 min of agonist stimulation by scintillation counting.
(iii) Glucose transoporter GLUT4 translocation using plasma membrane lawns (as described by Robinson & James (1992) and Marsh et al. (1995)).
(iv) GLUT4 translocation using plasma membrane lawns (as described by Marsh et al., 1995). 3T3-L1 fibroblasts are grown on glass coverslips in 6-well plates and differentiated into adipocytes. After 8-12 days post-differentiation, adipocytes are serum-starved for 18 hrs in DMEM containing 0.5% FBS. Cells are washed twice in Krebs-Ringer Bicarbonate Buffer, pH 7.4 and equilibrated for 90 min at 37° C. prior to insulin (100 nM) addition, or for 30 min prior to compound (100 μM) addition. After treatments, adipocytes are washed in 0.5 mg/ml poly-L-lysine in PBS, shocked hypotonically by three washes in 1:3 (v/v) membrane buffer (30 mM Hepes, pH 7.2 containing 70 mM KCl, 5 mM MgCl 2 , 3 mM EGTA and freshly added 1 mM DTT and 2 mM PMSF) on ice. The washed cells are then sonicated using a probe sonicator (Microson) at setting 0 in 1:1 (v/v) membrane buffer on ice, to generate a lawn of plasma membrane fragments that remain attached to the coverslip. The fragments are fixed in 2% (w/v) paraformaldehyde in membrane buffer for 20 min at 22° C. and the fixative quenched by 100 mM glycine in PBS. The plasma membrane fragments are then blocked in 1% (w/v) Blotto in membrane buffer for 60 min at 22° C. and immunolabelled with an in-house rabbit affinity purified anti-GLUT4 polyclonal antibody (clone R10, generated against a peptide encompassing the C-terminal 19 amino acids of GLUT4) and Alexa 488 goat anti-rabbit secondary antibody (Molecular Probes; 1:200). Coverslips are mounted onto slides using FluoroSave reagent (Calbiochem), and imaged using an OptiScan confocal laser scanning immunofluoroscence microscope (Optiscan, VIC., Australia). Data are analysed using ImageJ (NIH) imaging software. At least six fields are examined within each experiment for each condition, and the confocal microscope gain settings over the period of experiments are maintained to minimise between-experiment variability.
Insulin agonist activity may be determined using an adipocyte assay. Insulin increases uptake of 3 H glucose into adipocytes and its conversion into lipid. Incorporation of 3 H into a lipid phase is determined by partitioning of lipid phase into a scintillant mixture, which excludes water-soluble 3 H products. The effect of compounds on the incorporation of 3 H glucose at a sub-maximal insulin dose is determined, and the results expressed as increase relative to full insulin response. The method is adapted from Moody et al., (1974). Mouse epididymal fat pads are dissected out, minced into digestion buffer (Krebs-Ringer 25 mM HEPES, 4% HSA, 1.1 mM glucose, 0.4 mg/ml Collagenase Type 1, pH 7.4), and digested for up to 1.5 hours at 36.5 C. After filtration, washing (Krebs-Ringer HEPES, 1% HSA) and resuspension in assay buffer (Krebs-Ringer HEPES, 1% HSA), free fat cells are pipetted into 96-well Picoplates containing test solution and approximately an ED 20 insulin.
The assay is started by addition of 3 H glucose (e.g. ex. Amersham TRK 239), in a final concentration of 0.45 mM glucose. The assay is incubated for 2 hours at 36.5° C., in a Labshaker incubation tower, 400 rpm, then terminated by the addition of Permablend/Toluene scintillant (or equivalent), and the plates sealed before standing for at least 1 hour and detection in a Packard Top Counter or equivalent. A full insulin standard curve (8 dose) is run as control on each plate.
Data are presented graphically, as the effect of the compound on an (approximate) ED 20 insulin response, with data normalized to a full insulin response. The assay can also be run at basal or maximal insulin concentration.
To test the in vivo activity of a compound, an intravenous blood glucose test may be carried out on Wistar rats as follows. Male Mol:Wistar rats, weighing about 300 g, are divided into two groups. A 10 μl sample of blood is taken from the tail vein for determination of blood glucose concentration. The rats are then anaesthetized (e.g. with Hypnorm/Dormicum) at t=30 min and blood glucose measured again at t=−20 mM and at t=0 mM. After the t=0 sample is taken, the rats are injected into the tail vein with vehicle or test substance in an isotonic aqueous buffer at a concentration corresponding to a 1 ml/kg volume of injection. Blood glucose is measured at times 10, 20, 30, 40, 60, 80, 120 and 180 mM. The anaesthetic administration is repeated at 20 min intervals.
Additional assays to determine the effect of binding molecules on IGF-1R activity are as follows:
(i) Cell Viability Assay on HT29 cells with induction of Apoptosis: The ability of compounds to inhibit IGF-mediated rescue from apoptosis is measured using the colorectal cell line HT29 cells (ATCC: HTB 38) after induction with Na Butyrate. The HT29 cells are plated out onto white Fluoronunc 96 well plates (Nunc) at 12,000 cells/ml and incubated at 37° C., 5% CO 2 for 48 hours. Media is aspirated and 100 μl/well of serum free DMEM/F12 is added for 2 hours to serum starve cells. IGF (100 ul/well 0.05-50 nM dilutions) in the presence and the absence of inhibitory compound is added in 0.1% BSA solution (Sigma) in DMEM/F12 (Gibco) in triplicate. A final concentration of 5 mM Butyrate (Sigma) is added to each well. Plates are incubated at 37° C., 5% CO 2 for a further 48 hours. Plates are brought to room temperature and developed (as per instructions for CTG Assay (Promega)). Luminescence signal is measured on the Polarstar plate reader and data is evaluated using table curve to obtain the specific ED50.

(ii) Cell Migration Assay: The migration assays are performed in the modified 96-well Boyden chamber (Neuroprobe, Bethesda, Mass.). An 8 μM polycarbonate filter, which is pre-soaked in 25 μg/ml of collagen in 10 mM acetic acid overnight at 4° C., is placed so as to divide the chamber into an upper & lower compartment. Varying concentrations of the IGF-I analogues (25 μl of 0-100 nM) diluted in RPMI (Gibco) with 0.5% BSA (Sigma) tested for their migration inducing ability, are placed in the lower compartment in quadruplicates. The wells of the upper chamber are seeded with 50 μl/well of 2×10 5 SW480 (ATCC:CCL 228) pre-incubated for 30 mins/37° C. with 1.10 of 2 μM Calcein (Molecular Probes). Cells migrate for 8 hours at 37° C., 5% CO 2 . Unmigrated cells are removed by wiping the filter. The filter is then analysed in the Polarstar for fluorescence at excitation wavelength of 485 nm and emission wavelength of 520 nm. Data is evaluated using table curve to obtain the specific ED50 value.

(iii) Mouse Xenograft studies for anti-IGF-1R antibodies: In vivo studies are performed in 56-week-old female athymic BALBc nude mice, homozygous for the nunu allele. Mice are maintained in autoclaved micro-isolator cages housed in a positive pressure containment rack (Thoren Caging Systems Inc., Hazelton, Pa., USA. To establish xenografts, mice are injected subcutaneously into the left inguinal mammary line with 3×10 6 or 5×10 6 cells in 100 μl of PBS. Tumour volume (TV) is calculated by the formula (Iength×width 2 )/2 (Clarke et al., 2000), where length is the longest axis and width the measurement at right angles to length.

Initial biodistribution of potential binding molecules are ascertained by injecting 40 BALBc nude mice with established xenografts with radiolabelled 111 In- or 125 I-anti-IGFR antibody (3 μg, 10 μCi) intravenously via the tail vein (total volume=0.1 ml). At designated time points after injection of the radioconjugates (t=4 h, days 1, 2, 3, 5 and 7), groups of mice (n=35) are killed by Ethrane anaesthesia. Mice are then exsanguinated by cardiac puncture, and tumours and organs (liver, spleen, kidney, muscle, skin, bone (femur), lungs, heart, stomach, brain, small bowel, tail and colon) are resected immediately. All samples are counted in a dual gamma scintillation counter (Packard Instruments). Triplicate standards prepared from the injected material are counted at each time point with tissue and tumour samples enabling calculations to be corrected for the physical decay of the isotopes. The tissue distribution data are calculated as the mean±s.d. percent injected dose per gram tissue (% ID g −1 ) for the candidate molecule per time point.
Pharmacokinetics for the candidate compounds are ascertained as follows: Serum obtained from mice bearing xenografts, following infusion of radiolabelled-binding molecule as described above, is aliquoted in duplicate and counted in a gamma scintillation counter (Packard Instruments, Melbourne, Australia). Triplicate standards prepared from the injected material are counted at each time point with serum samples to enable calculations to be corrected for the isotope physical decay. The results of the serum are expressed as % injected dose per litre (% ID 1 −1 ). Pharmacokinetic calculations are performed of serum data using a curve fitting program (WinNonlin, Pharsight Co., Mountain View, Calif., USA). A two-compartment model is used to calculate serum pharmacokinetic parameters of AUC (area under the serum concentration curve extrapolated to infinite time), CL (total serum clearance), T 12α and T 12β (half-lives of the initial and terminal phases of disposition) for 125 I- and 111 In-labelled molecule.
(iv) Therapeutic in vivo studies: Tumour cells (3×10 6 ) in 100 μl of media are inoculated subcutaneously into both flanks of 46-week-old female nude mice (n=5 group −1 ). Candidate molecule treatment commences day 7 post-tumour cell inoculations (mean±s.e. tumour volume-60×15 mm 3 ) and consists of six intraperitoneal injections over 2 weeks of appropriate amounts of the candidate molecule or vehicle control. Tumour volume in mm 3 is determined as described previously. Data is expressed as mean tumour volume for each treatment group. Differences in tumour size between control and test groups are tested for statistical significance (P<0.05) by t-test.
Molecular Replacement/Binding
The structure coordinates of an IR ectodomain or a region of an IR ectodomain, such as those set forth in Appendix I and/or Appendix II, can also be used for determining at least a portion of the three-dimensional structure of a molecular complex which contains at least some structural features similar to at least a portion of IR. In particular, structural information about another crystallised molecular complex may be obtained. This may be achieved by any of a number of well-known techniques, including molecular replacement.
Methods of molecular replacement are generally known by those of skill in the art (generally described in Brunger, 1997; Navaza & Saludjian, 1997; Tong & Rossmann, 1997; Bentley, 1997; Lattman, 1985; Rossmann, 1972).
Generally, X-ray diffraction data are collected from the crystal of a crystallised target structure. The X-ray diffraction data is transformed to calculate a Patterson function. The Patterson function of the crystallised target structure is compared with a Patterson function calculated from a known structure (referred to herein as a search structure). The Patterson function of the crystallised target structure is rotated on the search structure Patterson function to determine the correct orientation of the crystallised target structure in the crystal. The translation function is then calculated to determine the location of the target structure with respect to the crystal axes. Once the crystallised target structure has been correctly positioned in the unit cell, initial phases for the experimental data can be calculated. These phases are necessary for calculation of an electron density map from which structural differences can be observed and for refinement of the structure. Preferably, the structural features (e.g., amino acid sequence, conserved di-sulphide bonds, and beta-strands or beta-sheets) of the search molecule are related to the crystallised target structure.
The electron density map can, in turn, be subjected to any well-known model building and structure refinement techniques to provide a final, accurate structure of the unknown crystallised molecular complex (eg see Jones et al., 1991; Brunger et al., 1998).
Obtaining accurate values for the phases, by methods other than molecular replacement, is a time-consuming process that involves iterative cycles of approximations and refinements and greatly hinders the solution of crystal structures. However, when the crystal structure of a protein containing at least a homologous portion has been solved, the phases from the known structure provide a satisfactory estimate of the phases for the unknown structure.
By using molecular replacement, all or part of the structure coordinates of IR ectodomain provided herein (and set forth in Appendix I and/or Appendix II) can be used to determine the structure of a crystallised molecular complex whose structure is unknown more rapidly and efficiently than attempting to determine such information ab initio. This method is especially useful in determining the structure of IR mutants and homologues.
The structure of any portion of any crystallised molecular complex that is sufficiently homologous to any portion of the extracellular domain of IR can be solved by this method.
Such structure coordinates are also particularly useful to solve the structure of crystals of IR co-complexed with a variety of molecules, such as chemical entities. For example, this approach enables the determination of the optimal sites for the interaction between chemical entities, and the interaction of candidate IR agonists or antagonists.
All of the complexes referred to above may be studied using well-known X-ray diffraction techniques and may be refined versus 1.5-3.5 Å resolution X-ray data to an R value of about 0.25 or less using computer software, such as X-PLOR (Yale University, distributed by Molecular Simulations, Inc.; see Brünger, 1996). This information may thus be used to optimize known IR agonist/antagonists, such as anti-IR antibodies, and more importantly, to design new or improved IR agonists/antagonists.
Antibodies and Nonimmunoglobulin Binding Proteins.
The term “antibody” as used in this invention includes intact molecules as well as fragments thereof, such as Fab, F(ab′) 2 , and Fv which are capable of binding the epitopic determinant. These antibody fragments retain some ability to selectively bind with its antigen or receptor and are defined as follows:
(1) Fab, the fragment which contains a monovalent antigen-binding fragment of an antibody molecule can be produced by digestion of whole antibody with the enzyme papain to yield an intact light chain and a portion of one heavy chain;
(2) Fab′, the fragment of an antibody molecule can be obtained by treating whole antibody with pepsin, followed by reduction, to yield an intact light chain and a portion of the heavy chain; two Fab′ fragments are obtained per antibody molecule;
(3) (Fab′) 2 , the fragment of the antibody that can be obtained by treating whole antibody with the enzyme pepsin without subsequent reduction; F(ab) 2 is a dimer of two Fab′ fragments held together by two disulfide bonds;
(4) Fv, defined as a genetically engineered fragment containing the variable region of the light chain and the variable region of the heavy chain expressed as two chains; and
(5) Single chain antibody (“SCA”), defined as a genetically engineered molecule containing the variable region of the light chain, the variable region of the heavy chain, linked by a suitable polypeptide linker as a genetically fused single chain molecule.
Methods of making these fragments are known in the art. (See for example, Harlow and Lane, Antibodies: A Laboratory Manual, Cold Spring Harbor Laboratory, New York (1988), incorporated herein by reference).
Antibodies of the present invention may be produced, for example, by immunizing mice with IR fragments, preferably purified fragments. After determining that the mice are producing anti-IR antibodies, hybridomas may be prepared and antibody specificity assayed by ELISA or Flow Cytometry.
The IR fragment suitably comprises at least one target binding site as defined herein. Such a target binding site may define a linear or non-linear epitope. In the case of a non-linear epitope, it is desirable to impart appropriate secondary and/or tertiary structure on the epitope. This may be achieved by using larger fragments, for example, of sufficient length to enable secondary structure or entire domains to form. Alternatively, in raising antibodies of the invention, it may be desirable to use derivatives of the peptides or loop structures which are conformationally constrained.
Conformational constraint refers to the stability and preferred conformation of the three-dimensional shape assumed by a peptide. Conformational constraints include local constraints, involving restricting the conformational mobility of a single residue in a peptide; regional constraints, involving restricting the conformational mobility of a group of residues, which residues may form some secondary structural unit; and global constraints, involving the entire peptide structure. For example, amino acids adjacent to or flanking the IR loop structures identified herein may be included in the construct to maintain conformation of the peptide used to raise antibodies.
In addition, the active conformation of the peptide may be stabilized by a covalent modification, such as cyclization or by incorporation of gamma-lactam or other types of bridges. For example, side chains can be cyclized to the backbone so as create a L-gamma-lactam moiety on each side of the interaction site, see, generally, Hruby et al. (1992). Cyclization also can be achieved, for example, by formation of cystine bridges, coupling of amino and carboxy terminal groups of respective terminal amino acids, or coupling of the amino group of a Lys residue or a related homolog with a carboxy group of Asp, Glu or a related homolog. Coupling of the alpha-amino group of a polypeptide with the epsilon-amino group of a lysine residue, using iodoacetic anhydride, can be also undertaken (see Wood & Wetzel, 1992).
Further the conformation of the peptide analogues may be stabilised by including amino acids modified at the alpha carbon atom (eg. α-amino-150-butyric acid) (Burgess & Leach, 1973a; Burgess & Leach, 1973b) or amino acids which lead to modifications on the peptide nitrogen atom (eg. sarcosine or N-methylalanine) (O'Donohue et al., 1995).
Another approach (described in U.S. Pat. No. 5,891,418) is to include a metal-ion complexing backbone in the peptide structure. Typically, the preferred metal-peptide backbone is based on the requisite number of particular coordinating groups required by the coordination sphere of a given complexing metal ion. In general, most of the metal ions that may prove useful have a coordination number of four to six. The nature of the coordinating groups in the peptide chain includes nitrogen atoms with amine, amide, imidazole, or guanidino functionalities; sulfur atoms of thiols or disulfides; and oxygen atoms of hydroxy, phenolic, carbonyl, or carboxyl functionalities. In addition, the peptide chain or individual amino acids can be chemically altered to include a coordinating group, such as for example oxime, hydrazino, sulfhydryl, phosphate, cyano, pyridino, piperidino, or morpholino. The peptide construct can be either linear or cyclic, however a linear construct is typically preferred.
Mimetics, Including Peptidomimetics
The present invention also provides isolated molecules which mimic one or more regions of an IR ectodomain (“mimetics”), which mimetics are identified and/or designed based on the 3D structure of a region of an IR ectodomain defined by the atomic coordinates of Appendix I and/or Appendix II, or portion thereof.
The present invention also provides isolated molecules which mimic one or more regions of an IGF-1R ectodomain, which mimetics are identified and/or designed using structural data from the 3D structure of the IR ectodomain defined by the atomic coordinates of Appendix I and/or Appendix II, or portion thereof.
The present invention also provides isolated molecules which mimic one or more regions of both an IR ectodomain and an IGF-1R ectodomain, i.e. chimeric mimetics.
The mimetics may be conformationally constrained molecules or alternatively molecules which are not conformationally constrained such as, for example, non-constrained peptide sequences.
In a preferred embodiment, a molecule mimics a region of an IR and/or IGF-1R ectodomain which is involved in the binding of insulin and/or IGF-1R, respectively, i.e. it mimics a target binding site. Molecules which mimic a region of an IR and/or IGF-1R ectodomain involved in binding insulin and/or IGF can be used to screen for ligands which bind to IR and/or IGF-1R, as the case may be. In other words, they can act as specific target molecules. Additionally, such mimetics can be used to screen for ligands which selectively bind IR over IGF-1R or vice versa.
In a preferred embodiment, the mimetic mimics, at least in part, the high affinity binding site of IR for insulin or IGF-1R for IGF.
The term “conformationally constrained molecules” means conformationally constrained peptides and conformationally constrained peptide analogues and derivatives.
The term “analogues” refers to molecules having a chemically analogous structure to the naturally occurring alpha-amino acids present in IR and/or IGF-1R. Examples include molecules containing gem-diaminoalkyl groups or alklylmalonyl groups.
The term “derivatives” includes alpha amino acids wherein one or more side groups found in the naturally occurring alpha-amino acids present in IR and/or IGF-1R have been modified. Thus, for example the naturally-occurring amino acids present in IR and/or IGF-1R may be replaced with a variety of uncoded or modified amino acids such as the corresponding D-amino acid or N-methyl amino acid. Other modifications include substitution of hydroxyl, thiol, amino and carboxyl functional groups with chemically similar groups.
The present invention encompasses the use of conformationally constrained peptidomimetics of fragments of IR and/or IGF-1R ectodomain that mimic the target binding sites defined herein (such as amino acid residues defining the second LRR rung of L1 and the fourth LRR rung of L1, module 6 of the CR domain, the AB loop of FnIII-1, the CC′ loop of FnIII-1 and the EF loop of FnIII-1), i.e. analogues and derivatives which mimic the activity of IR (or one or more regions of IR) and/or IGF-1R (or one or more regions of IGF-1R) involved in binding insulin or IGF, respectively, and are therefore capable of modulating IR and/or IGF-1R activity in vivo. These peptidomimetics are preferably substantially similar in three-dimensional shape to the peptide structures (for example, loop structures) as they exist in native IR and/or IGF-1R. Substantial similarity means that the geometric relationship of groups in the IR and/or IGF-1R peptide fragment is preserved such that the peptidomimetic will mimic the activity of IR and/or IGF-1R in vivo.
A “peptidomimetic” is a molecule that mimics the biological activity of a peptide but is no longer peptidic in chemical nature. By strict definition, a peptidomimetic is a molecule that no longer contains any peptide bonds (that is, amide bonds between amino acids). However, the term peptide mimetic is sometimes used to describe molecules that are no longer completely peptidic in nature, such as pseudo-peptides, semi-peptides and peptoids. Whether completely or partially non-peptide, peptidomimetics for use in the methods of the invention provide a spatial arrangement of reactive chemical moieties that closely resembles the three-dimensional arrangement of active groups in the peptide on which the peptidomimetic is based. As a result of this similar active-site geometry, the peptidomimetic has effects on biological systems which are similar to the biological activity of the peptide.
There are clear advantages for using a mimetic of a given peptide rather than the peptide itself, because peptides commonly exhibit two undesirable properties: (1) poor bioavailability; and (2) short duration of action. Peptide mimetics offer an obvious route around these two major obstacles, since the molecules concerned are small enough to be both orally active and have a long duration of action. There are also considerable cost savings and improved patient compliance associated with peptide mimetics, since they can be administered orally compared with parenteral administration for peptides. Furthermore, peptide mimetics are generally cheaper to produce than peptides.
Suitable peptidomimetics based on, for example, IR residues 27-55 or 259-284, can be developed using readily available techniques. Thus, for example, peptide bonds can be replaced by non-peptide bonds that allow the peptidomimetic to adopt a similar structure, and therefore biological activity, to the original peptide. Further modifications can also be made by replacing chemical groups of the amino acids with other chemical groups of similar structure. The development of peptidomimetics derived from IR peptides based on residues 27-55 or 259-284 can be aided by reference to the three dimensional structure of these residues as provided in Appendix I and/or Appendix II. This structural information can be used to search three-dimensional databases to identify molecules having a similar structure, using programs such as MACCS-3D and ISIS/3D (Molecular Design Ltd., San Leandro, Calif.), ChemDBS-3D (Chemical Design Ltd., Oxford, U.K.), and Sybyl/3 DB Unity (Tripos Associates, St. Louis, Mo.).
Those skilled in the art will recognize that the design of a peptidomimetic may require slight structural alteration or adjustment of a chemical structure designed or identified using the methods of the invention. In general, chemical compounds identified or designed using the methods of the invention can be synthesized chemically and then tested for ability to modulate IR and/or IGF-1R activity using any of the methods described herein. The methods of the invention are particularly useful because they can be used to greatly decrease the number potential mimetics which must be screened for their ability to modulate IR and/or IGF-1R activity.
The peptides or peptidomimetics of the present invention can be used in assays for screening for candidate compounds which bind to regions of IR and/or IGF-1R and potentially interfere with the binding of insulin to IR and/or signal transduction and/or the binding of IGF to IGF-1R and/or signal transduction. Peptides or peptidomimetics which mimic target binding sites are particularly useful as specific target molecules for identifying potentially useful ligands for IR and/or IGF-1R.
Standard solid-phase ELISA assay formats are particularly useful for identifying compounds that bind to the receptor. In accordance with this embodiment, the peptide or peptidomimetic of the invention is immobilized on a solid matrix, such as, for example an array of polymeric pins or a glass support. Conveniently, the immobilized peptide or peptidomimetic is a fusion polypeptide comprising Glutathione-S-transferase (GST; e.g. a CAP-ERK fusion), wherein the GST moiety facilitates immobilization of the protein to the solid phase support. This assay format can then be used to screen for candidate compounds that bind to the immobilised peptide or peptidomimetic and/or interefere with binding of a natural binding partner of IR to the immobilised peptide or peptidomimetic.
Uses of Compounds which Interact with IR or IGF-1R
Compounds/chemical entities designed or selected by the methods of the invention described above may be used to modulate IR activity in cells, i.e. activate or inhibit IR activity. They may also be used to modulate homodimerisation of IR.
Modulation of homodimerisation of IR may be achieved by direct binding of the chemical entity to a homodimerisation surface of IR and/or by an allosteric interaction elsewhere in the IR extracellular domain.
Given that aberrant IR activity is implicated in a range of disorders, the compounds described above may also be used to treat, ameliorate or prevent disorders characterised by abnormal IR signalling. Examples of such disorders include malignant conditions including tumours of the brain, head and neck, prostate, ovary, breast, cervix, lung, pancreas and colon; and melanoma, rhabdomyosarcoma, mesothelioma, squamous carcinomas of the skin and glioblastoma.
The compounds designed to interact or identified as interacting with the extracellular domain of IR, and in particular to interact with the target binding sites, are useful as agonists or antagonists against the action of insulin on IR. The compounds are useful as assay reagents for identifying other useful ligands by, for example, competition assays, as research tools for further analysis of IR (and IGF-1R for that matter) and as potential therapeutics in pharmaceutical compositions.
Compounds provided by this invention are also useful as lead compounds for identifying other more potent or selective compounds. The mimetic compounds of the present invention are also potentially useful as inhibitors of the action of insulin and in the design of assay kits directed at identifying compounds capable of binding to the low affinity and/or high affinity binding site for insulin on IR. In particular, it is envisaged that compounds of the present invention will prove particularly useful in selecting/designing ligands which are specific for IR or IGF-1R.
In one embodiment, one or more of the compounds can be provided as components in a kit for identifying other ligands (e.g., small, organic molecules) that bind to IR or IGF-1R. Such kits may also comprise IR or IGF-1R, or functional fragments thereof. The compound and receptor components of the kit may be labeled (e.g., by radioisotopes, fluorescent molecules, chemiluminescent molecules, enzymes or other labels), or may be unlabeled and labelling reagents may be provided. The kits may also contain peripheral reagents such as buffers, stabilizers, etc. Instructions for use can also be provided.
IR and IGF-1R agonists and antagonists, and in particular antagonists, provided by this invention are potentially useful as therapeutics. For example, compounds are potentially useful as treatments for cancers, including, but not limited to, breast, prostate, colorectal, and ovarian cancers. Human and breast cancers are responsible for over 40,000 deaths per year, as present treatments such as surgery, chemotherapy, radiation therapy, and immunotherapy show limited success. Recent reports have shown that a previously identified IGF-1R antagonist can suppress retinal neovascularization, which causes diabetic retinopathy (Smith et al., 1999). IGF-1R agonist compounds (i.e. existing IGF-1R compounds which have been modified employing methods of the present invention) are useful for development as treatments for neurological disorders, including stroke and diabetic neuropathy. Reports of several different groups implicate IGF-1R in the reduction of global brain ischemia, and support the use of IGF-I for the treatment of diabetic neuropathy (reviewed in Auer et al., 1998; Apfel, 1999).
The IGF-1R agonist peptides of the invention may be useful for enhancing the survival of cells and/or blocking apoptosis in cells.
Administration
Compounds of the invention, i.e. ligands of the invention or modulators of IR or IGF-1R identified or identifiable by the screening methods of the invention, may preferably be combined with various components to produce compositions of the invention. Preferably the compositions are combined with a pharmaceutically acceptable carrier or diluent to produce a pharmaceutical composition (which may be for human or animal use).
The formulation will depend upon the nature of the compound and the route of administration but typically they can be formulated for topical, parenteral, intramuscular, oral, intravenous, intra-peritoneal, intranasal inhalation, lung inhalation, intradermal or intra-articular administration. The compound may be used in an injectable form. It may therefore be mixed with any vehicle which is pharmaceutically acceptable for an injectable formulation, preferably for a direct injection at the site to be treated, although it may be administered systemically.
The pharmaceutically acceptable carrier or diluent may be, for example, sterile isotonic saline solutions, or other isotonic solutions such as phosphate-buffered saline. The compounds of the present invention may be admixed with any suitable binder(s), lubricant(s), suspending agent(s), coating agent(s), solubilising agent(s). It is also preferred to formulate the compound in an orally active form.
In general, a therapeutically effective daily oral or intravenous dose of the compounds of the invention, including compounds of the invention and their salts, is likely to range from 0.01 to 50 mg/kg body weight of the subject to be treated, preferably 0.1 to 20 mg/kg. The compounds of the invention and their salts may also be administered by intravenous infusion, at a dose which is likely to range from 0.001-10 mg/kg/hr.
Tablets or capsules of the compounds may be administered singly or two or more at a time, as appropriate. It is also possible to administer the compounds in sustained release formulations.
Typically, the physician will determine the actual dosage which will be most suitable for an individual patient and it will vary with the age, weight and response of the particular patient. The above dosages are exemplary of the average case. There can, of course, be individual instances where higher or lower dosage ranges are merited, and such are within the scope of this invention.
For some applications, preferably the compositions are administered orally in the form of tablets containing excipients such as starch or lactose, or in capsules or ovules either alone or in admixture with excipients, or in the form of elixirs, solutions or suspensions containing flavouring or colouring agents.
The compositions (as well as the compounds alone) can also be injected parenterally, for example intravenously, intramuscularly or subcutaneously. In this case, the compositions will comprise a suitable carrier or diluent.
For parenteral administration, the compositions are best used in the form of a sterile aqueous solution which may contain other substances, for example enough salts or monosaccharides to make the solution isotonic with blood.
For buccal or sublingual administration the compositions may be administered in the form of tablets or lozenges which can be formulated in a conventional manner.
For oral, parenteral, buccal and sublingual administration to subjects (such as patients), the daily dosage level of the compounds of the present invention and their pharmaceutically acceptable salts and solvates may typically be from 10 to 500 mg (in single or divided doses). Thus, and by way of example, tablets or capsules may contain from 5 to 100 mg of active compound for administration singly, or two or more at a time, as appropriate. As indicated above, the physician will determine the actual dosage which will be most suitable for an individual patient and it will vary with the age, weight and response of the particular patient.
The routes of administration and dosages described are intended only as a guide since a skilled practitioner will be able to determine readily the optimum route of administration and dosage for any particular patient depending on, for example, the age, weight and condition of the patient.
The present invention will now be described further with reference to the following examples, which are illustrative only and non-limiting. The examples refer to the figures:
EXAMPLES
Experimental Procedures
Crystallisation, Structure Solution and Refinement of IR485
Construction of the IR485 Expression Vector
The expression plasmid pEE14/IR485 was constructed by inserting the double-stranded deoxyribonucleic cassette:
Bgl II Xba I 5′ GATCTCCGACGATGACGATAAGGAACAAAAACTCATCTCAGAAGAGGATCTGAATTAGT.... 3′ 3′ ....AGGCTGCTACTGCTATTCCTTGTTTTTGAGTAGAGTCTTCTCCTACACTTAATCAGATC 5′ I (S)(D D D D K)(E Q K L I S E E D L N)*
encoding Ile485, a single Ser to reconstruct the BglII site, a five residue enterokinase cleavage site, the 11 residue c-myc epitope tag (McKern et al., 1997) and a stop codon into the larger Bgl II/Xba I fragment generated by digestion of the mammalian expression plasmid vector pEE14 (Bebbington & Hentschel, 1987) containing human insulin receptor cDNA. The Bgl II site of insulin receptor cDNA lies within codon 484 of the mature insulin receptor sequence.

Cell Culture and Transfection
Plasmid pEE14/IR485 was transfected into Lec8 mutant CHO cells (Stanley, 1989) obtained from the American Tissue Culture Collection (CRL:1737) using LipofectAMINE (Gibco-BRL). Cell lines were maintained after transfection in glutamine-free medium (Glascow modification of Eagle's medium [GMEM], ICN Biomedicals, Australia) and 10% dialysed FCS (Sigma, Australia) containing 25 μM methionine sulphoximine (MSX; Sigma, Australia) as described (Bebbington & Hentschel, 1987).
Transfectants were screened for protein expression by a dot-blot procedure using the anti-c-myc monoclonal antibody (Mab) 9E10 (Evan et al., 1985) or the anti-IR Mab 83.7 (gift from Ken Siddle, University of Cambridge, UK) as primary antibodies and sheep anti-mouse Ig-horseradish peroxidase conjugate (Silenus Laboratories, Australia) as the secondary antibody. Immunoprecipitates, generated from positive clones by treatment of culture supernatants with a combination of Mab 83.7 and Protein A Sepharose CL-4B (Pharmacia Biotech, Sweden), were examined by sodium dodecyl sulfate polyacrylamide gel electrophoresis and autoradiography after overnight growth of cell cultures in medium supplemented with 35S-cysteine and 35S-methionine (Express; Dupont NEN, U.S.A.). These autoradiographs indicated the presence of a predominant radiolabelled product of approximately 70 kDa in culture supernatants from the positive clones initially identified by dot-blotting.
Protein Production and Purification
Large-scale cultivation of selected clones expressing IR485 was carried out in a Celligen Plus bioreactor (New Brunswick Scientific, USA) containing 70 g Fibra-Cel Disks (Sterilin, UK) as carriers in a 1.25 L working volume. Continuous perfusion culture using GMEM medium supplemented with non-essential amino acids, nucleosides, 25 μM MSX and 10% FCS was maintained for 1 to 2 weeks followed by the more enriched DMEM/F12 without glutamine, with the same supplementation, for the next 4-5 weeks. Cell growth was poor during the initial stages of the fermentation when GMEM medium was employed, but improved substantially following the switch to DMEM/F12. Target protein productivity was essentially constant during the period from ˜100 to 700 h of the 760 h fermentation, as measured by ELISA using Mab 9E10 as the capture antibody and biotinylated Mab 83-7 as the developing antibody.
Soluble IR485 protein was recovered from harvested fermentation medium by affinity chromatography on columns of Mab 9E10 coupled to agarose beads (Mini Leak; Kern En Tec, Denmark) as previously described (McKern et al., 1997). Briefly, harvested culture medium containing secreted target protein at pH 8.0 was pumped through a 50 ml Mab 9E10 antibody column at 4° C., washed with at least ten column volumes of Tris-buffered saline containing 0.02% sodium azide (TBSA) and the bound protein eluted with a solution of 0.2-0.4 mg/ml of the c-myc peptide EQKLISEEDLN in TBSA. Residual bound protein was eluted with sodium citrate buffer at pH 3.0 into sufficient 1M Tris HCl buffer (pH 8.0) to neutralize the eluant.
Further purification was effected by gel filtration on Superdex 200 HR (Pharmacia, Sweden) in TBSA, monitored at 280 nm. A dominant peak was observed together with small quantities (estimated <10% total) of aggregated protein (not shown). The apparent molecular weight of the main peak was ˜68 kDa (presumed to be monomer) when the concentration of eluted protein was ˜25 ug/ml. At higher concentrations IR485 began to form dimers and gave a single peak of ˜140 kDa when the concentration of eluted protein loaded was ˜800 ug/ml. SDS-PAGE (Laemmli, 1970) using BioRad Protean II Minigels gave a single broad monomeric band under reduced and non-reduced conditions for both the monomer and dimer fractions (not shown). An estimated overall yield of 156 mg of purified receptor protein was obtained from 150 L of harvested medium following 9E10 Mab affinity chromatography and size-exclusion chromatography.
Isoelectric focussing polyacrylamide gel electrophoresis was carried out using either Novex pH3-7 or BioRad pH5-8 isoelectric focussing minigels. Isoforms of purified IR485/myc protein were loaded onto a Uno QP anion exchange column connected to a BioLogic liquid chromatography system (BioRad). Protein was eluted in 40 mM Tris-HCl buffer at pH8.0 using either shallow gradients or specific concentrations of KCl.
IR485Crystallization Trials and Isoform Heterogeneity
Purified IR485 was shown, by isoelectric focussing, to consist of multiple bands (pI range 5.7-6.8), with the proportion of the more basic bands increasing on storage. Partial purification of the isoforms was achieved by shallow KCl elution of protein bound to two connected Uno QP columns (BioRad). N-terminal sequence analysis using a Hewlett Packard G1005A sequencer (Sparrow et al., 1997), SDS-PAGE mobilities and the ability to react with mAb 9E10 indicated that the more basic isoforms were generated by progressive proteolytic degradation at the C-terminus resulting in partial or complete removal of the acidic c-myc tag and enterokinase cleavage site.
For optimal crystallization trials, it was desirable to have a stable isoform of IR485. Enterokinase was inefficient at effecting cleavage at the engineered DDDDK site immediately upstream of the c-myc epitope and left an undesirable acidic C-terminus of —SDDDD after cleavage (not shown). An alternative partial proteolysis treatment method using endoproteinase Asp-N (Boehringer) was found to be effective for production of a stable, major isoform that could be isolated. At an enzyme:protein ratio (w/w) of 1:1000 at ambient temperature, four major bands of pI ˜6.6-7.0 were produced after two minutes, the pattern remaining largely unchanged up to the 60 minute limit of this digestion experiment (not shown). The most basic isoform (pI ˜7.0) was present in greatest amount and was sufficiently separated from other isoforms to allow substantial enrichment by ion-exchange chromatography. Following scale-up, AspN treatment at ambient temperature on 5 mg of protein at an enzyme:protein ratio of 1:2500 for 3 h, the diluted incubation mixture was loaded onto a Uno Q2 column and eluted isocratically at a concentration of KCl just sufficient to displace the basic isoform (not shown). This procedure yielded a fraction that was enriched in the most basic species, and contained a second isoform in lesser abundance (not shown). Treatment with endoproteinase Asp-N was expected to progressively cleave the X-D peptide bonds within the 17 residue unstructured affinity tag and cleavage site, SDDDDKEQKLISEEDLN, resulting in a series of protein species truncated at the C-terminus.
IR485 Crystallization and Data Collection
Crystallization trials were performed with a factorial screen (Jancarik & Kim, 1991) using the hanging drop method. Initially, small rod-shaped crystals grew within 4-5 days, which diffracted to ˜4 Å. Further crystallization trials using seeding, led to the production of large-sized crystals (0.7 mm×0.1 mm×0.1 mm) that diffracted to 3.3 Å in the laboratory and 2.3 Å at the synchrotron. The best crystallization conditions were 1.5-1.65 M (NH4) 2 SO4, 2% PEG 400, pH 8.5. Crystals were cryo-cooled to −170° C. in 20% PEG, 20% glycerol. Diffraction data to 3.31 Å resolution were initially recorded as 130 1°-exposures on a Rigaku RAXIS IV area detector using RU-300 Rigaku generator (Cu Kα radiation) equipped with ellipsoidal glass capillary optics (AXCO). Data were integrated and scaled using DENZO/SCALEPACK (Otwinowski & Minor 1997) giving mosaic spread 0.37°, Rsym=0.211, <I/sigmaI>=5.3, multiplicity 4.6, completeness 99.9%. A second set of diffraction data were recorded to 2.3 Å resolution using a Mar 345 detector at the Advanced Photon Source (APS) beamline 14-BM-D (wavelength 1.037 Å) and integrated in a similar manner, with 116 0.75°-exposures, mosaic spread 0.27°, Rsym=0.072, <I/sigmaI>=12.0, multiplicity 3.4, completeness 98%. The space group is P212121 with unit cell dimensions a=103.86 Å, b=130.24 Å, c=160.92 Å. From the apparent molecular mass of 68 kDa per monomer and two molecules per asymmetric unit, the solvent content is estimated as 69%.
IR485 Structure Solution and Refinement
The structure was solved by molecular replacement with AMORE using data (8-4 Å resolution) from the initial data set and the structure of IGF-1R 1-300 (L1-CR) and 301-459 (L2) as search models. For L1-CR, solutions corresponded to the two highest peaks from translation functions. Using these solutions, one solution was found for L2 and a second L2 domain was placed by inspection from electron density maps. Four rigid bodies were refined with XPLOR (Brunger, 1996), giving R=0.462 for data 10-4 Å resolution. Despite the low predicted protein content of the crystals (25%, VM=4.9), electron density was only observed for two molecules of IR485 per asymmetric unit. With data from the APS, structure refinement proceeded with rounds of manual refitting with O (Jones et al., 1991), alternated with energy minimization, B factor refinement and sometimes simulated annealing, using XPLOR, CNS (Brunger et al., 1998) and REFMAC (Murshudov et al., 1997). The resolution was extended in a stepwise manner and a bulk solvent correction and overall anisotropic thermal parameters were applied. The final model contains 931 amino acids, 40 carbohydrate residues 1 PEG 400 molecule and 262 solvent molecules, giving R=0.194, Rfree=0.235 (data 40-2.317 Å). For residues 1-3 of both molecules and 270-273 from IR-2, the electron density is unclear and there is no density for residues beyond residues 468 in IR-1 or 469 in IR-2.
Calculation of Electrostatic Potential
The electrostatic potential was calculated and mapped on to the molecular surface of each of the structures IGF-1R and IR-1 using PARSE charges (Sitkoff et al., 1994) for each atom, and performed with the GRASP v. 1.6 program (Nicholls et al., 1991), which computes the electrostatic potentials in a continuum representation of the electrostatics by numerically solving the finite difference Poisson-Boltzmann equation.
Crystallisation, Structure Solution and Refinement of IRΔβ.
IRA Ectodomain Protein Production and Purification
The IR-A ectodomain construct was generated as described previously (Tulloch et al., 1999) and converted to the IRA13 construct by oligonucleotide-directed in vitro mutagenesis using the USBT7 Gen™ kit. In IRΔβ, residues 731-734 (VTVA) near the start of the f3-chain are replaced with the less hydrophobic sequence AGNN and the nucleotides coding for the 19 residues from 735-753 (VPTVAAFPNTSSTSVPTSP) are removed. IRΔβ lacks the two N-linked glycosylation sites at 730 and 743, all six IR O-glycosylation sites at T732, T737, 5745, S746, T747 and T751 (Sparrow, 2006) and residue 917 (K917), the last residue of the ectodomain before the transmembrane region, which was removed in constructing the termination codon.
For mammalian cell expression, the IRΔβ cDNA was inserted into the pEE 14 vector and the resulting plasmid used to transfect Lec8 mutant CHO cells as described previously (McKern et al, 1997). IRΔβ protein was produced in spinner flasks and purified by affinity chromatography and gel-filtration chromatography as described previously (Tulloch et al., 1999). Final purification was by ion-exchange chromatography on Resource Q (Pharmacia, Sweden) using shallow salt gradients (McKern et al, 1997), which enabled the separation into three fractions corresponding to well-defined peaks.
Fab fragments for the monoclonal antibodies 83-7 and 83-14 (Soos et al, 1986) were prepared as previously described (Tulloch et al., 1999).
Cloning of Monoclonal Antibody Variable Region cDNA.
cDNA corresponding to the heavy and light chain variable regions of the monoclonal antibodies 83-7 and 83-14 (Soos et al., 1986) was prepared as described previously (Gilliland et al, 1996) and cloned using the Zero Blunt TOPO PCR Cloning Kit (Invitrogen). A minimum of three individual clones for each variable region was sequenced to establish the translated amino acid sequence.
IR-A Ectodomain Crystallization.
The complex of IRΔβ from the first fraction of the ion exchange chromatography with Fab 83-7, Fab 83-14 and the S519-N20 peptide (Pillutla et al., 2002; Schaffer et al., 2003) was prepared at a molar ratio of 3:8:8:10 respectively in 0.15M NaCl-40 mM TrisHCl (pH 7.9), 0.02% sodium azide and 10% D-trehalose. After at least 4 hours at 4° C., the complex was passed over a Superdex 200 column to remove excess reagents, and concentrated to 3 mg/ml in 10 mM HEPES (pH 7.5). Initial crystallization conditions were found by vapour diffusion using an in-house screen of 808 conditions. Subsequent refinement of these conditions led to crystals being obtained via vapour diffusion protocols with (0.24 M ammonium tartrate and 15% PEG 3350 at either 4° C. or 20° C. A di-t-iodobis(ethylenediamine) diplatinum (II) nitrate (PIP) derivatized crystal was generated by soaking a crystal in a solution consisting of the well solution plus 2 mM PIP.
IR-A Ectodomain X-Ray Data Collection
A native data set and a PIP derivative data set were collected at the IMCA-CAT 17-ID undulator beamline at the Advanced Photon Source (Argonne, USA), each from single crystals which diffracted to 4.5 Å and 5.5 Å respectively. A further native data set was collected from a single crystal up to 3.8 Å resolution at the BL5A wiggler beamline at the Photon Factory (Tsukuba, Japan). These beamlines are equipped respectively with ADSC Quantum 210 and 315 CCD detectors. Data processing was conducted with the HKL suite (Otwinowski & Minor, 1997) and with D*trek (Pflugrath, 1999). Details of collection and processing are presented in Table 3. The space group was identified as C222 1 with unit cell dimensions (a=123.0 Å, b=319.7 Å, c=204.9 Å).
IRA Ectodomain Phasing, Model Building and Refinement
The initial structure solution was by molecular replacement using PHASER (McCoy et al., 2005) to address native data set 1 (Table 3). This process succeeded to locate in turn Fab domains, the L1-CR domain and the L2 domain. The Fab search model was that of the Hy-HEL5 antibody (PDB entry 1BQL) set at 5° hinge angle increments, whilst those of the L1-CR and L2 domain were derived from the earlier structure of the L1-CR-L2 fragment (examples 1-4). These results suggested that the solvent content of the unit cell was likely high (about 75%), with the asymmetric unit containing only one IRAQ monomer and its attached pair of Fabs.
Phase improvement then followed using both RESOLVE (Terwilliger, 2000) and BUSTER-TNT (Blanc et al., 2004) with the resultant electron density map revealing the likely location of three fibronectin type-3 like domains. At this stage it was clear that the CDRs of one Fab were positioned so as to interact with the CR region, identifying the Fab as 83-7, whilst the other was positioned to interact with one of the fibronectin domains, identifying it as 83-14 and the fibronectin domain as FnIII-1 [see Adams et al., 2000]. No electron density was visible for the insert domain nor for the constant domain of Fab 83-14.
At this stage, the second set of native data became available. The molecular replacement solution was verified afresh using a combination of single isomorphous replacement phases derived from the PIP-derivative data set (Table 3) processed with SHARP (Bricogne, 1997; de la Fortelle & Bricogne, 1997) and subsequent solvent-flattening using DM (Cowtan, 1994) and SOLOMON. Models of the FnIII domains were then built directly into the density using known FnIII domains 7, 8 and 9 of the fibronectin structure (PDB code 1FNF) to guide the building. Crystallographic refinement followed using iterative cycles of BUSTER-TNT (Blanc et al., 2004) and/or REFMAC5 manual model building using XtalView/Xfit (McRee, 1999) and/or O (Jones et al., 1991). Within BUSTER-TNT scattering from the missing atoms was modelled with a low resolution homographic exponential distribution and maximum entropy density completion was used at the end of each round of refinement to recover the density for missing parts of the structure.
During this process the sequences of Fabs 83-7 and 83-14 became available and were included in the model building. The loop (518-529) region of FnIII-1 was modelled with the C524-0524 disulphide bond included between 2-fold related q chains of homodimer. There was no convincing density for residues 656 to 754 from the insert domain. Significant continuous and extended electron density lay across the ligand binding face of L1 and was initially interpreted to be the S519N20 peptide. However, crystallographic data derived subsequently derived from crystals grown (under similar conditions to that described above) of the same Fab-complexed IRΔβ ectodomain construct but without the inclusion of the S519N20 peptide in the complex revealed a similar segment of electron density extending across the binding face of the L1. With out being bound, it is believed that this segment of electron density correspondes to a part of the insert domain ID, and most likely to the CT peptide or residues immediately upstream of this segment (Kristensen et al., 1998, 1999). Concommitantly, there appears to be no electron density present that can be associated definitively with the S519N20 peptide. It appears that this peptide did not bind to the Fab-complex IRΔβ receptor ectodomain. Electron density consistent with N-linked glycan was detected at 10 of the 16 potential N-linked sites within the modelled fragment but no sugar residues were built at these sites at this stage. The final model comprises 439 residues from Fab 83-7, 429 residues from Fab 83-14 and residues 4 to 655 and 755 to 909 of IRAJ3. Final refinement statistics are shown in Table 3.
Isolation and Chemical Characterisation of the N-linked Glycans of Human IR and Their Incorporation into the 3D Crystal Structure of IRΔβ.
Materials
Human IR ectodomain was expressed in CHO-K1 cells and in Lec8 cells and purified as previously described (Sparrow et al., 1997; Tulloch et al., 1999). PNGase F was obtained from New England Biolabs; O-Glycosidase (BSA-Free) and pepsin were from Boehringer Mannheim; Staphylococcus aureus V8 protease was from ICN; Streptococcal neuraminidase was obtained from Genzyme. The sources of all other reagents are listed in (Sparrow et al., 1997).
Protein Digestions and Peptide Purification
Digestions with pepsin were carried out in 1% (v/v) formic acid at 37° C. for 18 h and the reactions stopped by freezing. The conditions for other protease digestions were as follows: trypsin and endoproteinase Lys-C, 0.1 M Tris-HCl buffer, pH 8.5 at 37° C. for 16-18 h using an enzyme to substrate ratio of approximately 1:20; endoproteinase Asp-N, 50 mM sodium phosphate buffer, pH 8.0 at 37° C. for 16-18 h with an enzyme to substrate ratio of approximately 1:3 ; S. aureus V8 protease, 0.1 M ammonium bicarbonate, pH 8.2 at 37° C. for various times using an enzyme to substrate ratio of approximately 1:10. These protease digestions were stopped by acidification with 10% (v/v) trifluoracetic acid and fractionated by RP-HPLC. The methods used for all other protein cleavages are described in (Sparrow et al., 1997). Peptides were separated by RP-HPLC and identified by N-terminal amino acid sequence analysis. The presence of N- or O-linked carbohydrate was indicated by a blank cycle during sequencing instead of the expected PTH-asparagine or serine/threonine respectively. Glycopeptides were characterised by mass spectrometry thus allowing the glycan mass to be calculated and then used to assign the carbohydrate composition. O-Glycosidase digestions were carried out in 0.1 M sodium acetate, pH 6.0, at 37° C. for 16-18 h using 2.5 mU of enzyme, following a preliminary treatment for 4 h at 37° C. with neuraminidase (50 mU).
Reduction and Carboxymethylation
Peptides or peptide mixtures were reduced with 10 mM DTT for 2 h at 37° C. under argon in 6 M guanidine hydrochloride, 0.1 M Tris, 1 mM EDTA, pH 9.0; the mixture was then made 25 mM in iodoacetate and incubated for a further 30 min. Alkylation was stopped by the addition of excess DTT and the reduced, carboxymethylated peptides isolated by RP-HPLC.
Oligosaccharide Characterization Using 1-phenyl-3-methyl-5-pyrazolone (PMP) Derivatization
The glycopeptides derived from the N-terminal tryptic peptide of CN11 (see Tables 3 and 4) contained both N- and O-linked glycans. The N-linked glycans from these peptides (sites 742 and 755) were investigated by derivatization with PMP following liberation with PNGase F; the resulting PMP derivatives were characterised by Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) (Pitt and Gorman, 1997). Glycan masses were obtained by subtraction of 349 (the mass of the derivatising reagent) from the mass of the corresponding PMP derivative.
Other Techniques
The methods used for chromatography, polyacrylamide gel electrophoresis and protein analysis are described in (Sparrow et al., 1997). The buffers employed for RP-HPLC linear gradients were buffer A, ˜0.1% (v/v) trifluoroacetic acid; buffer B, 0.1% (v/v) trifluoroacetic acid in 70% (v/v) acetonitrile. MALDI-TOF MS was performed with a Bruker Reflex mass spectrometer (Bruker-Franzen Analytik, Bremen) as described in (Sparrow et al., 1997; Pitt & Gorman, 1996; Pitt & Gorman 1997). From the masses obtained for each glycopeptide, the corresponding glycan masses were determined by subtraction of the peptide moiety. This allowed the calculation of the saccharide composition of each glycan in terms of the numbers of sialic acid, hexose, N-acetylhexosamine and deoxyhexose moieties present. Based on the known structures of N-linked glycans found on proteins expressed in CHO cells (see Spellman et al., 1989) these monosaccharides were assumed to be sialic acid, mannose, galactose, N-acetylglucosamine and fucose.
Molecular Modelling
The crystallographic electron density maps derived from the atomic model of the IR ectodomain (Appendix I) exhibits difference electron density consistent with the fourteen N-linked glycan at sites Asn16, Asn25, Asn111, Asn215, Asn255, Asn295, Asn337, Asn395, Asn418, Asn514, Asn606, Asn624, Asn881 and Asn894 (IR-A numbering).
The difference electron density discernible at the 1.0 σ level at each site was typically restricted to that associated with the first GlcNac residue of the glycan, with more diffuse and broken density extending outwards along the glycan stalk and into the volume occupied by the glycan antennae. Initial model structures for each N-linked glycan were built using the GLYPROT server (Bohne-Lang & von der Leith, 2005; Lütteke et al., 2006), followed by adjustment of the glycosidic torsion angles within the glycan so as to position the individual sugar residues as best as possible into available difference electron density. The atomic modelling of these glycans employed maps derived from preliminary phasing of data from a slightly better quality crystal of the receptor/Fab complex than that described above, the more detailed refinement of which is still in progress. In each instance, the model employed the most prevalent glycoform at each site based on the glycan composition data given in Table 5, except in the case of (i) the glycan at Asn881 (IR-A numbering), which was modelled as a complex carbohydrate, compatible with better mass spectroscopic data obtained from the same analysis of Lec8 derived protein, and (ii) the glycan at Asn894 (IR-A numbering), which was modelled as a complex carbohydrate in the absence of any data from the peptide digest. During this process reasonable stereochemistry, assessed using the CARP server (Lütteke et al., 2005), was maintained. This process was inevitably crude, given the low resolution of the diffraction data, the likely presence of more than one glycoform at each site and the torsional flexibility of the glycans themselves. Nevertheless, the stalks of the modelled glycan isoforms were directed away from the protein surface in a way that was compatible with the difference density, and that the antennae components of the model arguably lay within volume spanned by the disordered glycan species. In some instances small adjustment of the protein backbone conformation in the vicinity of the asparagine residue was needed to overcome the steric hindrance. All interactive graphics was conducted with O (Jones et al., 1991). As such these coordinates allow one to discern regions of the protein surface that should be excluded from consideration as target surface during the design of biologicals that target IR. These coordinates are listed in Appendix IV.
Examples 1-4 analyse the IR485 crystal structure, compare the structure with the corresponding structure for IGF-1R and analyse the interaction of insulin with the L1 domain. Examples 5-11 analyse the IRA43 crystal structure, compare the structure with previously predicted structures, analyse the implications for signal transduction in the IR and analyse the distribution of the N-linked glycans over the surface of IR and by analogy the closely related IGF-1R and the impact of these oligosaccharide moieties on the accessibility of various regions of the insulin and IGF-1 receptors to binding macromolecules such as monoclonal antibodies and their fragments, nonimmunoglobulin binding proteins and nucleic acid aptamers.
Example 1
IR485 Structure
The truncated IR485 comprises the L1, CR and L2 domains (residues 1-469) plus the next 16 residues that include part of the first Fn III module. This fragment was previously shown to be stably expressed in mammalian cells (Schaefer et al., 1990). Unlike IGF-1R462, the IR485 crystals contain two receptor molecules in the unit cell, whose structures are very similar to each other but not identical. The comparative structures are shown in FIG. 3 and the amino acid sequences and secondary structural assignments in FIG. 2 . Residues 470-485 at the C-terminus of the fragment, immediately after the end of the L2 domain, are disordered in both copies of IR485 in the crystal. As shown in FIG. 3 the IR fragment adopts an extended bilobal structure (40 Å×49 Å×110 Å) very similar to that seen for the corresponding IGF-1R462 fragment. Carbohydrate was detected at 8 of the 10 potential N-linked sites in the L1/CR/L2 fragment: at residues 16, 25 and 111 but not 78 in the L1 domain, 215 and 255 but not 295 in CR, and 337, 397 and 418 in the L2 domain. For the two sites with no visible carbohydrate attached, chemical analyses have shown that Asn78 is not glycosylated while Asn295 was shown to carry a complex, tetra-antennary glycan moiety (see Example 7).
The most notable differences in the overall structure are the relative orientations of the L2 domains, which are rotated 17° (molecule 1) and 32° (molecule 2) relative to IGF-1R462 ( FIG. 3 ). While crystal packing is responsible for the relative orientations observed here, the structures do reveal that the L2 domain is capable of global movement with respect to the L1/CR domains as previously suggested (Garrett et al., 1998).
Example 2
IR and IGF-1R Domain Comparisons
The largest differences seen in the individual domain comparisons of IR and IGF-1R are in the two regions governing ligand specificity, the left hand side of the ligand binding surface of the L1 domain as viewed in FIG. 4 , and the sixth module of the CR domain ( FIGS. 3 and 5 ). The comparative backbone structures of the L1 domains of IR and IGF-1R generally follow each other closely owing to their high (70%) sequence identity (rmsd between Ca atoms of IR-1 and IGF-1R is 1.2 Å; and between IR-2 and IGF-1R is 1.3 Å). There is a major difference between the structures of IR residues 39-48 and the equivalent sequence in IGF-1R (residues 35-42) in the second leucine rich repeat of the L1 domain ( FIG. 4 ), with the largest Ca deviations being near Lys40 (3.3 Å) and Pro43-Arg47 (3.5-4.2 Å). A striking feature is the disposition of the side chain of IR-Phe39 which can be seen to contribute to the ligand-binding surface (second β-sheet) of L1. This is in contrast to the corresponding residue, Ser35, at the end of the third β-sheet of IGF-1R, whose side chain extends in an orthogonal direction to that of IR-Phe39 ( FIG. 4 ) and is thus in a position that is unlikely to contribute to low affinity ligand binding by the soluble ectodomain. As shown in FIG. 2 , the IR sequence also has an insert of two amino acids in the region Lys40-Pro43 compared to IGF-1R. Phe39, and to a lesser extent Arg42 and Pro43, have been shown to be the key residues responsible for the increased insulin binding of IR/IGF-1R chimeras (Kjeldsen et al., 1994). The importance of Phe39 in insulin binding is further indicated by the data in Table 2 which show that five of the seven most important residues for insulin binding to IR, located in the region that governs specificity (first 68 residues of L1) are conserved in the IGF-1R and thus not responsible for this specificity difference. The two exceptions are Gln34 and Phe39, the importance of the latter being confirmed by site-specific mutagenesis analyses (Williams et al., 1995).
Other differences in L1 involve residues Pro43-Phe46 which form a short α-helix in IR-1 and IR-2, while the corresponding residues in IGF-1R do not ( FIGS. 2 and 3 ), and the insertion of Gly4 at the start of the L1 domain in IGF-1R causes Pro5 in that structure to extend further (Cα deviation of 2.7 Å) into the central ligand-binding cavity than does the equivalent Pro9 in the IR-1 and IR-2 structures. The contribution, if any, of this difference to receptor binding specificity is not known. There is also a backbone deviation around Asn152 (4.1 Å) at the end of the L1 domain, a region of IR that also contains a single residue insertion compared to IGF-1R (see FIG. 2 ).
The other major difference in the two structures is seen in the CR domain (sequence identity 47%), which consists of 8 compact modules, and governs IGF specificity (Gustafson & Rutter, 1990; Schumacher et al., 1991; Kjeldsen et al., 1991; Hoyne et al., 2000) through its interaction with the C-domain of IGF (Zhang et al., 1994). The backbones of the third, fourth, and fifth modules in IR-1 and IGF-1R structures follow each other quite closely, while the backbones of the other modules deviate significantly. The largest difference is seen in module 6 in the region 260-276, corresponding to 253-265 in IGF-1R ( FIG. 2 ). Here there is no sequence identity between the two receptors, and IR contains a 4 residue insertion and an extra disulfide bond ( FIG. 2 ). Although the size of the insertion is modest, the new sequence forms an α-helix which extends further into the putative binding pocket (see FIG. 3 ) with the structure being maintained by the additional disulfide bond. Temperature factors in this region are relatively high and connected electron density was observed only in IR-1 and not in IR-2. However, this region is likely to be better ordered than in IGF-1R where the loop was ordered only by making a substantial crystal contact with an adjacent molecule (Garrett et al., 1998).
In addition to these structural differences ( FIGS. 2 and 3 ), CR module 6 of IR shows very different electrostatic surface potential to IGF-1R ( FIG. 5 ), reflecting the abundance of positively charged residues in the IR sequence compared to the acidic residues in the corresponding region of IGF-1R ( FIG. 2 ). Both structures have electrostatic potentials that are dominantly negative in the L1 domains and dominantly positive in the L2 domains. However, the structures differ in the electrostatic potential in their CR domains, that of IR being predominantly positive while that of IGF-1R is overwhelmingly negative. The electronegative characteristics of the CR domain of IGF-1R are complementarity to the electropositive nature of the C-domain of IGF-I (that contains Arg36 and Arg37). The CR domain is the region in the receptor known to be important in governing IGF binding specificity (Gustafson & Ritter, 1990; Schumacher et al., 1991; Kjeldsen et al., 1991; Hoyne et al., 2000) and the region shown to be responsible for the recognition of IGF-1Residues Arg36 and Arg37 (Zhang et al., 1994). Alanine substitution of Arg36 and Arg37 leads not only to a 15-fold loss in binding potency for the IGF-1R but also to a 29-fold increase in binding potency to the IR (Zhang et al., 1994), consistent with the electrostatic differences discussed above.
In contrast to the differences seen between the L1 and CR domain structures in the two receptors, the L2 domains of IR and IGF-1R are very similar. There are no insertions or deletions in the L2 domain of IR relative to IGF-1R ( FIG. 2 ) and the sequence identity for the L2 domains is high (65%). Consequently, the backbones of the L2 domains of IR-1 (0.9 Å) and IR-2 (0.8 Å) superimpose on the L2 domain of IGF-1R better than do the L1 domain comparisons, with only two significant deviations. The first deviation is in the vicinity of IR Glu316/IGF-1R Lys306 (3.0 Å) where a strong salt bridge between Glu304 and Lys309 in the case of IGF-1R facilitates the deviation of the IGF-1R backbone in this region compared to IR (residues Leu314 to Lys319). The other deviation is near Asn348/Asn338 (4.0 Å) in the loop between the third β-strand and the α-helix in the second repeat of L2. In both receptors this Asn residue is directed towards the external solvent. The L2 domain backbones of IR-1 and IR-2 also differ from each other most at these same regions (2.6 Å near Leu315 and 1.5 Å near Asn348).
Example 3
Ligand Binding Region of L1
The central (second) β-sheet of the L1 domain is a major contributor to ligand binding, based on studies of naturally-occurring receptor mutants and receptors subjected to alanine scanning mutagenesis (Table 2). Most of the mutant receptors from patients with defects in insulin binding (Taylor et al., 1994; Rouard et al., 1999) have mutations in this face. Similarly, of the 47 single site and six double site IR alanine mutations, the 14 mutations with defects in insulin binding (Williams et al., 1995) were all in this central β-sheet.
As illustrated in FIG. 5 , the characteristic feature of this L1 face is a large hydrophobic patch with a cavity at the centre, formed by Q34, L36, L62, F64, F88, F89, V94, F96, R118 and E120 ( FIG. 6B ). Clearly this cavity can bind hydrophobic residues and two instances of this phenomenon have been observed. In the structure of IGF-1R, two residues, (L and I), from the c-myc purification tag sit in the L1 cavity on an adjacent molecule and in this study the side-chain of F89 in IR-2 is swung out and sits in the cavity of IR-1. The hydrophobic patch is ringed by hydrophilic amino acids and notably, charged residues, some of which are particularly important for insulin binding ( FIG. 6D ).
This L1 surface is highly conserved in IRs and is the largest conserved patch of surface in the L1-CR-L2 fragment in vertebrates ( FIG. 6E ). The most important residues for insulin binding in both the A and β isoforms of human IR are Argl4, Asn15 and Phe64, which, when replaced by alanine, resulted in receptors with negligible insulin binding ability (Table 2). The next most important set of L1 residues are Leu37 (20-fold reduction), Phe39 (10- to 25-fold) and Ile13, Gln34 and Lys121 (all 11- to 12-fold) followed by Asp12, Leu36, Leu87, Glu97 and Asn90 (6- to 9-fold reduction in affinity). Other residues Phe89, Tyr91, Met38, Glu44, Glu120, Tyr67 and Phe88 showed reductions in affinity ranging from 3- to 5-fold (Table 2). The mutation Leu87Ile increased binding four-fold (Nakae et al., 1995) while the Phe89 to Leu, Ile, Ser, Pro, Trp or H is mutations all abolished insulin binding (De Meyts et al., 1990). As shown in Table 2, the seven most important residues for insulin binding—Ile13, Argl4, Asn15, Gln34, Leu37, Phe39 and Phe64 plus five of the less important ones (Asp12, Leu36, Met38, Glu44, Tyr67) occur in the first 68 residues of IR, the region found to confer insulin binding specificity in IR/IGF-1R chimeras (Kjeldsen et al., 1991; Kjeldsen et al., 1994; Andersen et al., 1992). The distribution of these residues over the central (second) β-sheet of the L1 domain of IR is shown in FIG. 6D and can be seen to be a subset of the highly conserved residues found in the vertebrate IRs ( FIG. 6C ).
Example 4
Insulin-L1 Domain Binding Model
A possible model for the L1 domain-insulin interaction is shown in FIGS. 6E 6 E and the atomic coordinates given in Appendix III and is based on fitting a hydrophobic surface of insulin (comprising residues from its dimer surface) onto the hydrophobic patch of the receptor described above. In the model presented here we have represented insulin in the R-state, (where the B-chain helix extends from B1 to B19), the more active conformation suggested to be adopted by insulin on binding to the receptor (Nakagawa et al., 2005). In addition, the B-chain has been truncated after residue GluB21, given the known mobility of the B-chain C-terminus (Hua et al., 1991; Ludvigsen et al., 1998; Wan et al., 2005), the increased binding affinity of B-chain despenta-amide insulins (Schaffer, 1994) and the low activity of the single-chain, B29-A 1 peptide-linked insulin despite its near native crystal structure (Derewenda et al., 1991). The B-chain C-terminus appears to not be essential for binding and is believed to rotate away from its close contact with residues A1 and A2 during the process of receptor interaction. This movement exposes the hydrophobic surface comprised of A1, A2, A3, A19, B24 B25 and B26 (Ludvigsen et al., 1998; Xu et al., 2004; Wan et al., 2005).
From the shape and size of these hydrophobic surfaces on the L1 face and the insulin dimer surface, the favoured orientation is with the B-chain helix running along the length of L1 from the bottom to the top, as viewed in FIG. 6E , with the critical insulin residue TyrA19 (Pullen et al., 1976; Kristensen et al., 1997) buried deep into the central hydrophobic cavity formed by Gln34, Leu36, Leu62, Phe64, Phe88 and Va194. The now-exposed GlyA1-ValA3 is placed up against residues Phe88 and Phe89 which are in the hydrophobic loop in the fourth LRR rung at the left hand edge of the L1 binding face (see FIGS. 2 and 3 ). Other key residues used to position insulin in the model are ValB12 and TyrB16, the contiguous residues from the B-chain helix, and PheB24 (Pullen et al., 1976; Kurose et al., 1994; Huang et al., 2004, Xu et al., 2004). Photoactivatable derivatives of TyrB 16 and TyrB24 have been shown to cross-link to the L1 domain of IR while ValB 12 could not be derivatised without destroying its receptor binding ability (Huang et al., 2004; Xu et al., 2004). In this model, each of these three residues contacts the L1 binding face. ValB12 is buried in the interface and sits directly over Phe64 and the lower end of Arg65, with Phe96 underneath and Leu37 above, while TyrB 16, sits directly between Leu37 and Phe39, two of the residues which are highly sensitive to Ala substitution (Table 2). The residue equivalent to TyrB16 in IGF is Gln 15 and it is interesting to note that swapping IGF-1Residues Gln15 and Phe 16 with their counterparts TyrB16 and LeuB17 from insulin, increased the IR binding of the mutant IGF-I ten-fold but had no effect on its binding affinity for IGF-1R (Bayne et al., 1988). The atomic coordinates of the model are provided as Appendix III.
While not shown in the model, the mobile C-terminal tail of the insulin B-chain (residues 22-30) is envisaged to have moved away from the core of the ligand, to a position approximately 90° to the axis of the B-chain helix (see Hua et al., 1991). This movement places PheB24 in a position where it can still contact the top rungs of the L1 binding face to which it can be cross-linked (Xu et al., 2004); it places B25 in an exposed position to interact with the IR α-chain C-terminal 16 amino acids to which it can be cross-linked (Kurose et al., 1994) and has LysB29 extending over to the cleft between L1 and CR, a position consistent with the ability to label either of these receptor regions depending on the nature of the LysB29 side-chain derivative (Yip et al., 1988; Wedekind et al., 1989). Finally, in the model described, ValA3 and ThrA8, derivatives of which, like PheB25, can be cross-linked to the 13 kDa C-terminal fragment of the IR α-chain, are positioned on the bottom left hand surface of insulin ( FIG. 6E ) in a semi-exposed position but well away from PheB25. This supports the suggestion (Wan et al., 2004) that ValA3 and ThrA8 may contact a region of the insert domain that is peripheral to the critical site involved in contacting PheB25. Such disparate contact points by the IR insert domain are possible given it appears to be intrinsically disordered based on predictions with the DisProt web server (Peng et al., 2005). Such an orientation of insulin on the L1 surface results in LeuB17 and LeuA 13 (Schaffer, 1994) and other residues from the hexamer face, the so-called second binding region (De Meyts, 2004), being positioned where they can readily interact with other parts of the receptor dimer and induced IR signaling.
As shown in FIG. 6 , the hydrophobic patch on IR is ringed with charged residues as is the hydrophobic patch on insulin. A number of these hydrophilic residues—GluA4, AsnA21 and GluB 13 on the periphery of the insulin hydrophobic patch could form favourable interactions with Argl18, Gln34 and Arg65 on the IR L1 binding surface and the A chain N-terminus could salt link to Glu120. Clearly the central cavity in L1 binds hydrophobic residues and two instances of this phenomenon have been seen previously. In the structure of IGF-1R (Garrett et al., 1998), two residues, (Leu and Ile), from the c-myc purification tag sit in the L1 cavity on an adjacent molecule and in this study the side-chain of Phe89 in IR-2 is swung out and sits in the cavity of IR-1.
In the model described above for insulin binding, van der Waals or hydrophobic interactions contribute the major portion of the ligand-receptor binding energy. However, as discussed earlier with IGF-I binding to the IGF-1R, electrostatic complementarity between the electropositive C-domain and the electronegative sixth module of the CR domain would have an additional orientational effect and could be the driving force in binding in the IGF-1/IGF-1R complex. Binding of insulin to the insulin receptor appears to be less electrostatically driven.
Both IGF-I and IGF-II both can compete with insulin on either IR or IGF-1R (Adams et al., 2000) and, as receptors and ligands have homologous sequences, the preferred orientations of these ligands on IR and IGF-1R receptors is the same. A model for IGF-I or IGF II binding to IR can therefore be generated from the model of insulin binding to IR by taking the structures of IGF-I or IGF-II and superimposing C-alpha coordintes of identical residues as defined by the alignment in FIG. 11 of Adams et al. (2000). These models can then be used the same manner as that for insulin in the design of molecules which interact with IR or IGF-1R to modulate their actions.
Example 5
Quaternary Structure of the IR-A Ectodomain
Domain arrangements. The IR construct used in these studies comprises all of the residues within the mature IR-A polypeptide ectodomain except for the region of O-glycosylation, residues 735-753, in the insert domain, ID, near the N-terminal end of the β-chain. Also deleted is residue 917, the last residue before the transmembrane region, while residues 731-734 have been mutated. The construct lacks the N-linked sites at 730 and 743 and the six O-linked glycan sites at Thr732, Thr737, Thr745, Ser746, Thr747 and Thr751 (Sparrow, 2006).
The crystallographic asymmetric unit structure comprised one IR ectodomain α-β-chain pair (i.e. the ectodomain monomer), one 83-7 Fab and one 83-14 Fab. The structure of the IR monomer is shown in FIG. 7 and the comparative sequences and secondary structure assignments in FIG. 2 . The consecutive domains of each α-β-chain monomer have an “inverted V” layout with respect to the cell membrane. One leg of the V is formed by L1-CR-L2 and the other by an extended linear arrangement of FnIII-1, FnIII-2 and FnIII-3. The ID is mostly disordered in this structure with electron density visible only for the first 20 and the last 4 of the 104 residues in the construct. This is consistent with our predictions of secondary structure using the method of Peng et al. (2005), that the ID region is largely disordered. At the apex of the inverted V lies the connection between L2 and FnIII-1: these domains do not appear to be in extensive contact. The relative arrangement of the L1, CR and L2 domains observed here is similar, though not identical, to that observed for these domains in isolation for both human IR (see examples 1-4) and human IGFR-1 (Garrett et al., 1998). In particular, the centre-of-mass of the L2 domain is positioned 5 Å to 10 Å closer to the L1 domain here than is observed in the L1-CR-L2 fragment structures.
The above monomers pair unambiguously within the unit cell to form the IR ectodomain homodimer, with alternative pairings showing negligible interaction (see below). The homodimer arises from a two-fold rotation of the monomer about an axis running approximately parallel to, but displaced about 10 Å from, the axis of the inverted V ( FIG. 8 ). The homodimer thus retains the inverted V conformation with the L2 domain of the first monomer in contact with the FnIII-1 domain of the second monomer at the apex and the L1 domain of the first monomer in contact with the FnIII-2 domain of the second monomer at the approximate midpoint of one of the two legs of the inverted V. The ligand-binding faces of the two L1 domains are thus located on opposite sides of the dimer some 65 Å from each other, a distance that is too great to allow insulin to contact both L1 domains simultaneously as previously suggested for the high affinity state of the insulin/IR complex (Luo et al., 1999; Ottensmeyer et al., 2000, 2001; Yip & Ottensmeyer, 1999). The C-termini of the two FnIII-3 domains are at the base of the inverted V structure where, in the intact IR, they would extend through the cell membrane to the tyrosine kinase domains ( FIG. 1 )
Located at the apex of the IR dimer, in close proximity to each other, is the pair of Lys460 residues in the last leucine-rich repeat in L2, residues that have been implicated in the regulation of co-operative interactions between the two monomers in the IR dimer and suggested to be the sites for chemical cross-linking of IR monomers with disuccinimidyl suberate (Kadowaki et al., 1990). The Lys460Glu mutation was found in a patient (leprechaun/Ark-1) with insulin resistance resulting from the effect of this mutation on increased stability of insulin binding at acid pH, impaired dissociation of the insulin-IR complex following internalization, impaired IR recycling and accelerated IR degradation (Kadowaki et al., 1990). The side chain of Lys460 is surrounded by three acidic residues Asp464, Asp574 and Glu575. While the Lys460Glu mutation would make this region considerably more electronegative, it is not clear why this leads to the phenotype described, nor why the Lys460Arg mutation markedly diminishes the ability of the mutant IR to display negative co-operativity (Kadowaki et al., 1990). The importance of the L2 domain in mediating signal transduction is further supported by the properties of two mutant receptors, Phe382Val (Accili et al., 1991) and Trp412Ser (van der Vorm et al., 1994) from human patients. These residues occur in equivalent positions in the third and fourth repeats of L2 ( FIG. 2 ). Neither has a significant effect on insulin binding, but both inhibit the capacity of the IR to undergo insulin-mediated autophosphorylation, presumably by distorting the structure of the L2 domain and its interactions with the FnIII-1 domain of the other monomer.
Fab contacts. Previous studies with receptor chimeras have established that the epitope for the mouse monoclonal antibody 83-7 lies in the residue segment 191-297 (Adams et al., 2000) and the epitope for mouse monoclonal antibody 83-14 lies in the residue segment 469 to 592 (Prigent et al., 1990). The sequences for these Fabs are provided in FIGS. 9 and 10 ). Our structure supports these results ( FIG. 11 ).
The 83-7 Fab is observed to interact via its CDRs with residues within the segment 233 to 281 of the CR domain. This epitope includes primarily the fifth module of the CR domain and also parts of the fourth and sixth CR modules. Of the eight sequence differences (Ser210Lys, Gln218Glu, Arg236Gln, His264Phe, Lys267Arg, Arg271Lys, Gln272Pro and Leu300Met) between the CR domains of human and mouse IR only 8236 is located in the 83-7 epitope.
The 83-14 Fab in our structure interacts solely with FnIII-1 ( FIG. 11 ). Portions of the A strand (residues 476-481) of FnIII-1 lie parallel to and on top of the cleft formed by the 83-14 heavy and light chain CDR3s, whilst portions of the B strand (residues 484 to 490) and E strand (residues 546 to 554) of FnIII-1 form contacts with the remaining molecular surface of the heavy chain CDRs. Of the five sequence differences (Tyr477Phe, Leu538Gln, a two residue insertion between 546-547 and N547S) between human and mouse IRs only Tyr477 is buried in the epitope. Residues 546 and 547 are on the periphery. The constant domain of the 83-14 Fab is somewhat disordered in our structure, lying in very poorly defined electron density and forming no crystal contact with any other domains within the crystal structure.
The psuedo two-fold axis of the variable domains of both Fabs lie approximately 45° to the two-fold axis of the IR ectodomain homodimer and protrude as extended “handles” to the IR ectodomain structure ( FIG. 11 ). Interestingly, the overwhelming majority of the inter-molecular contacts within the crystal (i.e. between one Fab-complexed IR ectodomain and its neighbour) are Fab-to-Fab. The Fabs thus “shield” the heavily glycosylated IR ectodomain from direct involvement in crystal contacts and arguably play a key role in the success of crystallization. The only exception is the involvement of the FnIII-3 domain of the receptor in a crystal contact with the constant domain of the 83-7 Fab attached to a neighbouring receptor.
The segment of electron density extending across the ligand-binding face of the L1 domain. The different electron density maps revealed a tube of density lying across the ligand-binding face of L1 at an angle of approximately 45° to the β-strands ( FIG. 12( a )). Without being bound, it is believed that this density may arise from the C-terminal region of the IR ectodomain (residues 704-719, the “CT peptide”) or residues immediately upstream thereof. The CT peptide is known to be required for ligand binding (Kristensen et al., 1998, 1999) and lies in close juxtaposition to the L1 domain (Xu et al., 2004). In principle there are two possible connections for this putative CT segment in the dimer. It may belong to the same IR monomer as the L1 domain against which it is positioned, or it may belong to the ID of the second monomer. Without being bound, it is believed the latter arrangement in wild type receptor is more likely, given that pro-receptor cleavage occurs after receptor dimerisation (for a review of receptor biosynthesis see Adams et al., 2000) and that the crystal structure shows that dimerisation of the uncleaved IR would position the CT region of one monomer in the immediate vicinity of the ligand-binding surface of the L1 domain of the other. This arrangement has now been experimentally confirmed by complementation analysis (Chan et al., 2007).
Example 6
Secondary Structure of the Fibronectin Domains and Location of the Inter-Chain Disulphide Bonds
The structure provides experimental confirmation of three fibronectin Type III domains in the IR. Whilst their existence has been predicted previously on the basis of sequence analysis (O'Bryan et al., 1991; Schaefer et al., 1992; Mulhem, et al., 1998; Marino-Buslje et al., 1999; Ward, 1999), the details of the location of the individual strands within these domains and the location of the insert domain (ID) within FnIII-2 have been unclear. These details are now presented in FIGS. 2 and 7 .
Cys524 which forms one of the inter α-chain disulphide bonds with its corresponding Cys524 partner, lies within a large, 28 residue loop (Lys508-Asp535) linking the C and C′ strands of the FnIII-1 domain. While the electron density associated with this segment is relatively weak and disjointed, the loop was built in to the density to show that the reported inter α-chain disulphide bond (Sparrow et al., 1997) is possible. The α-13 disulphide Cys647-Cys860 is observed, with Cys647 lying in the ID in a long extension of the C strand of FnIII-2 that runs parallel to FnIII-3. Its partner, Cys860, is located close to the C-terminus of the C′ strand of FnIII-3. An internal disulphide bridge links Cys786 and Cys795 at the base of the F-G loop in FnIII-2. The unpaired C872 (Sparrow et al., 1997) has its side chain partially buried in FnIII-3. It has no counterpart in the IGF-1R ( FIG. 2 ) or the insulin receptor related receptor (Sparrow et al., 1997).
The structure reported here also reveals that the insert domain (ID), which contains the α-β cleavage site at Arg720-Arg723, is located in the C—C′ loop of FnIII-2 and consists of 123 residues (A1a636 to Pro758). No convincing electron density is seen for 80 (Arg656-Ala734//Glu754) of the 104 residues that make up the truncated ID in our construct, implying that these segments are significantly disordered in our crystal. This disordered region contains the last 64 residues (Arg656-Ser719) of the IR α-chain including the three cysteine residues (Cys682, Cys683 and Cys685) involved in the second set of α-α disulphide bonds (Sparrow et al., 1997). This disordered region also contains the modified (see methods) N-terminus of the β-chain (Ser724 to 7Asn34//Glu754). As shown in FIG. 8 , the N-terminal region of the α-chain ID region can be seen to extend down on the inside of the inverted V, almost to the bottom of the last FnIII domain, with the pair of Ser655 residues at the end of the visible density approximately 60 Å apart, sufficiently close to allow the cysteine triplet (Cys682, Cys683 and Cys685) to form the observed inter-chain disulphide bonds (Sparrow et al., 1997).
The previous predictions of the FnIII boundaries and strand locations (O'Bryan et al., 1991; Schaefer et al., 1992; Mulhern, et al., 1998; Marino-Buslje et al., 1999; Ward, 1999) are shown to be mostly correct. However, none positioned the FnIII-1 C′ strand or the FnIII-2 C′ strand correctly, and in each case only one of the predictions was correct for the E strands of FnIII-1 (Marino-Buslje et al., 1999) and FnIII-3 (Schaefer et al., 1992). None positioned the ID in the C—C′ loop of FnIII-2.
Example 7
N-Linked Glycosylation
The IR ectodomain homodimer contains a total of 36 potential N-linked glycosylation sites distributed across all of the extracellular domains (Adams et al., 2000). There are 14 predicted sites for N-glycosylation in the hIR α-chain (residues 16, 25, 78, 111, 215, 255, 295, 337, 397, 418, 514, 606, 624, 671) and four in the β-chain (residues 742, 755, 893, 906, IR-B numbering). The presence of such a high degree of glycosylation (about one site per fifty residues) is likely to be one of the factors that has hindered attempts to crystallize the intact IR ectodomain. Electron density corresponding to glycan was seen at residues 16, 25, 111, 215, 255, 295 (poor), 337, 397 and 418, but not at Asn78, as previously reported in the structure of the L1-CR-L2 fragment of IR (see examples 1-4) and at five (514, 606, 624, 881 and 894, IR-A numbering) of the six N-linked sites that are present in the insert domain and the three FnIII domains of our construct. These findings are consistent with our chemical analyses of isolated IR glycopeptides summarised in Table 5. As shown in FIGS. 8 and 11 , the N-linked glycosylation sites in the L1-CR-L2 half of the monomer are distributed over the outer surface of these domains, away from the surfaces involved in ligand binding or monomer-monomer interactions in the dimer.
To elucidate the structural consequences of N-glycosylation on the accessibility of different domains for protein-protein, protein-macromolecular or protein-small molecule interactions we determined the chemical composition of the oligosaccharides in the exon 11 plus (or B isoform), of the hIR ectodomain expressed and secreted from the CHO-K1 cells. The enzyme digests and the chromatographic procedures used to isolate each glycopeptide are summarized in FIG. 13 and Table 4 and the glycan compositions for the isolated glycopeptides are summarized in Table 5. Mass spectrometry showed that eleven sites had multiple species of complex glycans (residues 16, 25, 255, 295, 418, 606, 624, 742 and 755, IR-B numbering) or high mannose glycans (residues 111, 514) while only single glycans were identified at Asn671 (GlcNAc2Man3Gal4Fuc) and Asn215 and Asn893 (both GlcNAc2Man6, IR-B numbering). The N-linked sites at Asn397 and Asn906 (IR-B numbering) were shown to be glycosylated by amino acid sequencing but were not characterized by mass spectrometry due to insufficient material. The N-linked site at Asn78, and the atypical site Asn282LysCys, were not glycosylated. The peptide containing the final site, Asn337, was not recovered.
The structure of the IR ectodomain with the glycan structures incorporated is shown in FIG. 14 and the coordinates for the glycosylated IR are listed in Appendix IV.
Example 8
Comparison with Electron Microscopy Data
Low-resolution projection images generated from our structure both with and without the 83-7 and 83-14 Fabs attached, can be compared with negative stain electron micrograph images obtained previously (Tulloch et al., 1999) ( FIG. 15 ). The correspondence of our projected structure with these images is striking, suggesting that the three-dimensional structure observed here in the crystal is closely similar in conformation to the particles observed in our EM studies (Tulloch et al., 1999). However, the current structure reveals that the original interpretation of the EM images in terms of the arrangement of domains was incorrect. The L1 and L2 domains of one monomer are not in an upper layer at the ends of the U or V in contact with the L2 and L1 domains of the other monomer respectively, nor do the FnIII domains form a second layer at the bottom of the U (Tulloch et al., 1999). Rather, the U-shaped EM model should be inverted with the apex being formed by the L2 and FnIII-1 domains and the two legs by L1-CR-L2 and FnIII-1-FnIII-2-FnIII-3 respectively as described above.
We are unable to correlate our model with that proposed for the whole receptor in complex with insulin based of a 3D reconstruction of approximately 700 STEM dark-field images (Luo et al., 1999; Ottensmeyer et al., 2000, 2001; Yip & Ottensmeyer, 1999). Our data are incompatible with their suggested overall shape of the particle and with their proposed arrangement of domains. In particular, their EM model has the L1 domains protruding at the membrane distal end of the molecule and the FnIII-2 and FnIII-3 domains lying co-planar to the membrane, whereas our structure has the L1 domains lying towards the centre of the molecule at the start of one leg of the inverted V and the FnIII domains arranged in a linear fashion forming the other leg ( FIGS. 7 and 8 ). These differences may be a consequence of structural transitions upon insulin binding, similar to that seen with the EGFR family (Burgess et al., 2003), given their EM data is on the detergent solubilised, high-affinity, whole receptor/insulin complex compared to our crystal structure on low affinity IR ectodomain. However this appears to us to be unlikely given the observation that ligand binding causes the receptor to become more compact rather than more extended (Lee et al., 1997; Florke et al., 2001).
Example 9
Implications for Signalling
Residues implicated in insulin binding by IR have been identified by examining patients' receptors, site-directed mutants, chimeric receptors and chemical cross-linking (reviewed in—Adams et al., 2000; De Meyts & Whittaker, 2002; Ward et al., 2003).
This data reveals the importance of residues in three regions of the receptor:
(i) the central β-sheet of the L1 domain; (ii) the last 16 residues of the α-chain (the so-called CT peptide) (Surinya et al., 2002); and (iii) the central modules of the CR region.

The CR region appears to be involved in restricting IGF-I binding to IR and promoting IGF binding to the IGF-1R. The electrostatic potential properties of the CR region differs markedly between the two receptors with that of IR being predominantly positive while that of IGF-1R is overwhelmingly negative as discussed in Example 2. These three ligand contact regions (L1, CT and CR) are all important for ligand binding to soluble ectodomain and account for the low affinity site that governs specificity. The details of the low affinity site and the potential interactions with ligand that govern specificity are discussed in detail in Examples 1-4.
The generation of the high affinity binding state leading to the initiation of IR autophosphorylation requires additional receptor contacts that induce a change in the conformation of the IR dimer (Schaffer, 1994). The domain arrangements seen in the present structure indicate how such high affinity binding may occur. Our structure reveals how an insulin molecule bound to the L1 and α-chain CT binding site of one receptor monomer could make contact with the AB, CC′ and EF loops of FnIII-1 from the other to generate the high affinity state ( FIG. 12 b ). Evidence supporting the involvement of these portions of FnIII-1 in high affinity binding is:
(i) the demonstrated importance of the region 325-524 in insulin binding to IR/IGF-1R chimeras (Schumacher et al., 1993); (ii) the labeling, with a photoreactive PheB1 insulin derivative (Fabry et al., 1992), of an 18-kDa fragment (extending from Gly390 to either Arg498 or Lys508 allowing for the two N-linked glycans at 397 and 418) and (iii) the differential binding properties of a series of truncated IR ectodomains (Kristensen et al., 1999; Surinya et al., 2002).

The only differences between the IR593.CT fragment, which shows high-affinity binding (Surinya et al., 2002), and the IR mini-receptor (Kristensen et al., 1999) which binds insulin with 1000-fold lower affinity, are the presence of the first fibronectin domain (FnIII-1) between L2 and the CT peptide and the assembly of IR593.CT into a disulphide-linked dimer.


Examples 1-4 present a model for insulin binding to the L1 face of IR. The model is consistent with the above suggestions for the formation of the high affinity state and has the residues corresponding to the hexamer face of insulin (Schaffer, 1994; De Meyts, 1994; De Meyts, 2004), the so-called second site, fully exposed. The structure of the IR ectodomain dimer, described in Examples 5-10 suggests that this second region of insulin binds to one or more of the three loops AB, CC′ and EF in FnIII-1 on the other monomer to generate the high affinity, signalling complex. Molecular modelling based on the wild-type receptor-like properties of the IR ectodomain-GCN4 leucine zipper chimera (Hoyne et al., 2000) suggests that the distance between the pair of FnIII-3 domains, observed here with the soluble ectodomain, could be halved in the membrane-anchored wild type receptor dimer. This would result in an increase in the distance between the L1 domain ligand-binding face and the loops of FnIII-1 adjacent to FnIII-2. High affinity binding would thus require some movement of the L1-CR module of one monomer towards the bottom of the FnIII-1 domain of the other. Such closure between the two monomers on one side of the dimer may open up space between the equivalent contact sites on the other side as first suggested by De Meyts (De Meyts, 1994). Such a “see-saw” model ( FIG. 15 ) would explain the phenomenon of negative co-operativity and the ability of the IR dimer to bind simultaneously one molecule of insulin with high affinity and a second molecule of insulin with low affinity (Schaffer, 1994). Such a model is also consistent with the observation that a hybrid receptor comprising a normal monomer and a binding defective mutant could still create one normal, high affinity complex, but could not exhibit negative co-operativity (Chakravarty et al., 2005) because the alternate combination of potential ligand binding sites cannot bind ligand because of the mutation. Such arguments remain speculative and high resolution structures of insulin/IR complexes will be required to establish the details of ligand/receptor binding and the domain re-arrangements that accompany signalling.
Example 10
Models for Insulin Binding and Negative Co-Operativity
The current view of insulin binding to the IR has been reviewed extensively (Schaffer 1994; De Meyts 1994, 2004; De Meyts & Whittaker 2002) and is as follows. The soluble ectodomain dimer can bind two molecules of insulin, but only with low affinity. It shows linear Scatchard plots and fast dissociation rates which are not accelerated by the presence of unlabelled insulin (does not exhibit negative co-operativity). Isolated half-receptors (IR monomers) are similar and display low-affinity binding, fast dissociation rates and do not exhibit negative co-operativity. In contrast, the membrane-anchored receptor dimer exhibits negative co-operativity and curvilinear Scatchard plots indicating that it binds one insulin molecule with high affinity and a second insulin molecule with low affinity. In the high-affinity state, the insulin molecule makes a bridging contact between the two monomers in the IR dimer.
Schaffer (1994) was the first to propose a cross-linking model for insulin/IR binding that accounted for the ability of one insulin molecule to bridge two distinct sites, Site 1 and Site 2, on the two monomers in the IR dimer as well as for a second and possibly a third insulin molecule to bind with low affinity to the remaining two unoccupied Sites 1 and 2 in the dimer. However, the explanation for the mechanism of negative co-operativity was less convincing and required negative interactions between the bound tracer and the unlabelled insulin at the left-over binding Site 1. The inhibition of accelerated dissociation by high levels of unlabelled insulin was suggested to result from the binding of a third insulin molecule at the left-over Site2 which in effect put a “lid” on the complex.
De Meyts (1994) overcame these latter difficulties simply by recognizing the fact that the IR dimer would have internal symmetry, as subsequently demonstrated (Tulloch et al., 1999), which would allow alternative cross-linking at either of the two sets of α-chain binding sites. In this revised model, Site 1 on each monomer is positioned near the Site 2 of the other. In the high-affinity state one molecule of insulin simultaneously contacts Site 1 of one monomer and Site 2 of the other. Negative co-operativity occurs because high-affinity binding can only occur with either the tracer on one side or the unlabelled insulin on the alternate side of the dimer; .i.e. the ligand/receptor bridging can oscillate from Sites 1 and 2′ on one side of the receptor dimer to Sites 1′ and 2 on the other side. The 1:2 stoichiometry indicates that two insulin molecules cannot bridge a Site 1-Site 2 pair simultaneously. This model elegantly showed how a second insulin molecule could bind the vacant Site 1 in a high-affinity insulin/IR complex to trigger the alternate bridging associated with negative co-operativity. It also showed how at very high ligand concentrations, a third insulin might bind the left-over Site 2 and thus prevent the switch to the alternate high-affinity configuration given both left-over sites are now occupied and the high affinity state is capped as Schaffer (1994) suggested.
Our data is consistent with the key features of the De Meyts model (De Meyts 1994, 2004; De Meyts & Whittaker 2002) and allow descriptions of the two binding sites to be made. We suggest that Site 1 corresponds to the low-affinity site which controls ligand binding specificity and includes contributions from several distinct regions of the receptor: the L1 domain binding face; the C-terminus of the α-chain (referred to as CT); possibly an additional peripheral portion of the insert region (ID) (Wan et al., 2004), and in the case of IGF binding, the CR region. All of these regions are important for low-affinity ligand-binding to the soluble ectodomain. Our structure, described in Examples 5-9 suggests that Site 2 corresponds to one or more of the AB, CC′ and EF loops at the C-terminal end of the first FnIII domain, FnIII-1.
The features of the model are summarized in FIG. 15 . We envisage that high-affinity binding is associated with some movement of the L1-CR module of one monomer towards the bottom of the FnIII-1 domain of the other. Such closure between the two monomers on one side of the dimer would open up the space between the equivalent contact sites on the other side, as first suggested by De Meyts (1994). Such a “see-saw” model would explain the phenomenon of negative co-operativity and the ability of the IR dimer to bind simultaneously one molecule of insulin with high affinity and a second molecule of insulin with low affinity (De Meyts 1994; 2004). Such a model is also consistent with the properties of a hybrid IGF-1R dimer comprising a normal monomer disulphide bonded to a monomer with a mutation (in Site 1) that abolishes binding (Chakravarty et al., 2005). This hybrid receptor showed wild-type binding since it could still create one normal, high-affinity binding site with Site 1 from the wild-type monomer and the non-mutated Site 2 of its partner. However, it could not exhibit negative co-operativity (Chakravarty et al., 2005) because the alternate combination of the mutated (defective) Site 1 and wild-type Site 2 is unable to bind ligand and thus is unable to form the alternate high affinity cross-link.
In this model we propose that the classical binding surface of insulin contacts Site 1 on the receptor, while the second binding site of insulin, involving residues from its hexamer forming surface, contacts Site 2. We further propose that the order of binding is Site 1 which induces the known conformational changes at the N- and C-terminal ends of insulin (Wan et al., 2004), followed by Site 2. We base this suggestion on the fact that the insulin residues involved in binding to our suggested Site 1 are equally important for low-affinity binding to soluble ectodomain and to half-receptors, whereas mutations in the residues in the hexamer face selectively impair only the formation of the high-affinity state and signaling. This is opposite to the order suggested by De Meyts (De Meyts 1994; 2004).
The disclosure of all publications referred to in this application are incorporated herein by reference.
It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.
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