Structural studies of dimerization and specificity in the fibroblast growth factor receptor family

The mammalian fibroblast growth factors (FGF1–FGF23) participate in a wide variety of biological processes. The diverse effects of FGFs are mediated by four receptor tyrosine kinases (FGFR1–FGFR4). FGFR dimerization and activation require heparin or heparan sulfate proteoglycans (HSPGs). Binding specificity between FGFs, FGFRs, and heparin or HSPGs is critical for the proper regulation of FGF signaling and is achieved by primary sequence differences among ligands and receptors, by alternative splicing of FGFR mRNA, and by differences in the sulfation pattern of heparin or HSPGs.; Besides heparin, a number of chemically diverse low molecular weight sulfated sugars, such as sucrose octasulfate (SOS), have been reported to potentiate FGF action. I present biochemical evidence that SOS potentiates FGF action by promoting FGF-FGFR dimerization and that selectively desulfated SOS analogs are unable to dimerize and thus activate FGF-FGFR complexes. In light of the difficult synthesis of homogeneously sulfated heparin oligosaccharides, SOS derivatives are attractive candidates for novel therapeutics which modulate FGF-FGFR signaling.; I also report the crystal structure of the ligand-binding portion of FGFR2b bound to FGF10, one of the most specific FGF-FGFR pairs. Unique contacts between divergent regions in FGF10 and two “b” isoform specific loops in Ig-like domain 3 (D3) of FGFR2b reveal the structural basis by which alternative splicing provides FGF10–FGFR2b specificity. Mutagenesis of FGF10 confirms the biological importance of the observed contacts. Interestingly, FGF10 binding induces a previously unobserved rotation of receptor Ig-like domain 2 (D2) to introduce specific contacts with FGF10. Hence, both D2 and D3 of FGFR2b contribute to the exceptional specificity between FGF10 and FGFR2b.; A crystallographic FGF10–FGFR2b dimer was also observed, stabilized by direct receptor-receptor contacts and by contacts between FGF10 and the adjoining receptor in the dimer. Mutagenesis of FGF10 confirms the observed mode of FGF-FGFR dimerization and supports the mechanism of FGF-FGFR binding and dimerization originally observed in the FGF2–FGFR1c crystal structure. Furthermore, the charge distribution of the heparin-binding site supports the hypothesis that different FGF-FGFR complexes might require differently sulfated heparin or HSPGs.