Recent developments in the structural characterisation of the IR and IGF1R: implications for the design of IR-IGF1R hybrid receptor modulators

Abstract

The insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF1R) are dimeric disulfide-linked receptor tyrosine kinases, whose actions regulate metabolic and mitogenic signalling pathways inside the cell. It is well documented that in tissues co-expressing the IR and IGF1R, their respective monomers can heterodimerise to form IR-IGF1R hybrid receptors. Increased populations of the IR-IGF1R hybrid receptors are associated with several disease states, including type 2 diabetes and cancer. Recently, progress in the structural biology of IR and IGF1R has given insights into their structure-function relationships and mechanism of action. However, challenges in isolating IR-IGF1R hybrid receptors mean that their structural properties remain relatively unexplored. This review discusses the advances in the structural understanding of the IR and IGF1R, and how these discoveries can inform the design of small-molecule modulators of the IR-IGF1R hybrid receptors to understand their role in cell biology.

Fig. 1. (A) Domain organisation of the IR and IGF1R. Monomers have been denoted as monomer and monomer' for clarity. The position of the αCT helix is denoted with asterixis (*). The position of the cell membrane has been indicated with triangles (▼); (B) XRCD apo-structure of the IR ectodomain (PDB: 4ZXB). One monomer is shown with a space filling representation and the other with a ribbon representation; (C) XRCD apo-structure of the IGF1R ectodomain (PDB: 5U8R). One monomer is shown with a space filling representation and the other with a ribbon representation. Note – the ID is not resolved in the IGF1R structure.

Fig. 2. (A) XRCD structure of the apo-IR ectodomain (PDB: 4ZXB); (B) Cryo-EM structure of the Γ-shaped IR ectodomain bound to a single insulin (PDB: 6HN4 and 6HN5 (ref. 4)); (C) Cryo-EM structure of the T-shaped IR ectodomain bound to four insulin (PDB: 6SOF)). One monomer in each structure is depicted with a molecular surface and the other in cartoon format. Insulin is depicted in green.

Fig. 3. (A) XRCD structure of the apo-IGF1R ectodomain (PDB: 5U8R); (B) crystal structure of the Γ-shaped IGF1R ectodomain bound to a single IGF1 (PDB: 6PYH); (C) crystal structure of the IGF1R ectodomain bound to two IGF1 (PDB: 5U8Q). Note – this structure is unlikely to depict a physiologically relevant conformation of the receptor ectodomain. One monomer in each structure is depicted with a molecular surface and the other in cartoon format. IGF1 is depicted in green.

Fig. 4. The structures of free insulin, IGF1 and IGF2. B, C, A and D domains are depicted in purple, pink, orange and cyan respectively. Disulfide bonds are depicted in yellow; A) sequence alignment of insulin, IGF1 and IGF2 detailing the locations of their relevant domains; B) the structure of insulin (4INS); C) the structure of IGF1 (2GF1 (ref. 16)); D) the structure of IGF2 (1IGL).

Fig. 5. A comparison of insulin, IGF1 and IGF2 bound to site 1 of the IR and IGF1R. (A) Insulin (green) bound to site 1 of the IR (PDB: 6HN5 (ref. 4)); (B) IGF1 (green) bound to site 1 of the IGF1R (PDB: 6PYH); (C) IGF2 (green) bound to site 1 of the IGF1R (PDB: 6VWG).

Fig. 6. Insulin bound to site 1 and site 2 on the IR. (A) Cryo-EM structure (PDB: 6PXV) of the IR in a T-shaped conformation bound to four insulin (green). Locations of the site 1 and site 2 insulin are highlighted; (B) detailed view (PDB: 6PXV) of insulin (green) bound to the site 1a/1b epitopes, with key residues shown. The locations of the site 1a and 1b epitopes are highlighted; (C) detailed view (PDB: 6PXV) of insulin (green) bound to site 2, with key residues shown.

Fig. 7. (A) NMR structure of the IR-TMD (PDB: 2MFR). Points of membrane entry are indicated in grey; (B) crystal structure of the IR-TKD activated dimer (PDB: 4XLV). One of the TKDs is depicted in cartoon format and the other with a molecular surface representation. The αC helix (pink), nucleotide binding loop (purple), activation loop (salmon), catalytic loop (cyan) are highlighted. Key residues K1030 and E1047 (blue), aswell as phosphorylation sites Y1158, Y1162 and Y1163 (green) are shown.

Fig. 8. (A) Crystal structure of the inactivated IR kinase domain (PDB: 1IRK) with the αC helix (pink), nucleotide binding loop (purple), activation loop (salmon), catalytic loop (cyan) highlighted. Key residues K1030 and E1047 (blue), aswell as phosphorylation sites Y1158, Y1162 and Y1163 (green) are shown; (B) crystal structure of the phosphorylated (activated) IR kinase domain (PDB: 1IR3 (ref. 10)) with the αC helix (pink), nucleotide binding loop (purple), activation loop (salmon), catalytic loop (cyan) and bound peptide substrate (lime) highlighted. Key residues K1030 and E1047 (blue), aswell as phosphorylated residues pY1158, pY1162 and pY1163 (green) are shown.

Fig. 9. (A) Structures of selected small molecules which modulate the IR and/or IGF1R; (B) representative model of the full length IR/IGF1R (produced from PDB: 4ZXB,2MFR,1IRK) indicating the regions which selected small molecule IR/IGF1FR modulators are believed to bind. Point of membrane entry for the receptors is denoted in grey. Of these molecules, only NVP-AEW541 has a published protein co-ligand structure confirming its binding mode. Note – linsitinib (1), NVP-AEW541 (2) and picropodophyllin (3) are presented here as inhibitors of the IGF1R, whilst DMAQB1 (4) and chaetochromin (5) are presented as activating the IR; these are only displayed on a single model of an IR/IGF1R receptor for illustrative purposes; (C) XRCD structure (PDB: 5HZN) of NVP-AEW541 (blue) bound to the IGF1R kinase (yellow) detailing key binding interactions.