UCSF

Fibrillins, large, multi-domain glycoproteins, are central organizers of elastin-containing microfibrils. They serve as scaffolds for the assembly of multi-protein complexes that contribute to the maintenance of tissue homeostasis and the regulation of growth factor signaling in the extracellular space. Fibrillin-1 is a modular glycoprotein that includes 7 latent transforming growth factor beta (TGF) binding protein-like (TB) domains and mediates cell adhesion through integrin binding to an exposed arginine, glycine, aspartic acid (RGD) motif in its 4th TB domain. A subset of missense mutations within TB4 cause Stiff Skin Syndrome (SSS), a rare autosomal dominant form of scleroderma. The fibrotic phenotype of this disease is thought to be regulated by changes in the ability of fibrillin-1 to mediate integrin binding. We characterized the ability of each RGD-binding integrin to mediate cell adhesion to fibrillin-1 or the SSS disease-causing variant. Our data show that 7 of the 8 RGD-binding integrins can mediate adhesion to fibrillin-1, four of which had not been previously described (αvβ5, αvβ1, α8β1 and αIIbβ3). A single amino acid substitution responsible for SSS (W1570C) markedly inhibited adhesion mediated by integrins α5β1, αvβ5, and αvβ6, partially inhibited adhesion mediated by αvβ1 and did not inhibit adhesion mediated by αvβ3, α8β1 or αIIbβ3. In the SSS mutant background, introduction of a new cysteine residue in place of a highly conserved tryptophan-1570 places a free sulfhydryl within range of three disulfide bonds. Introduction of this free sulfhydryl did not cause the protein to dimerize but instead induced a subtle change in the conformation of the RGD site that specifically, and differentially, mitigated interaction with only a subset of fibrillin-1-binding integrins. The goal of this work was to determine the relationship between fibrillin-1 and the RGD-binding integrins. Understanding how the mutation changes the dynamics of integrin binding could highlight a mechanism for future development of therapeutics in scleroderma.