(12c) Role of the Fibronectin Synergy Site in Modulating Integrin Recruitment and Molecular-Scale Force Generation

Integrins are heterodimeric transmembrane adhesion receptors that connect the cell to the extracellular matrix, and that play central roles in transducing mechanical cues into intracellular signaling cascades that control cell migration, differentiation, and survival. Mammalian cells possess at least 26 integrin heterodimers. In most cases the molecular mechanisms by which different integrins fulfill their distinct physiological roles remain poorly understood. In particular, relatively little is known about how different integrin heterodimers may function in the context of mechanotransduction.

Here, we use single-molecule fluorescent molecular tension sensors (MTSs) to measure the forces experienced by α₅β₁ and α_v -class integrins, two classes of fibronectin-binding integrins that are thought to play distinct roles in cell adhesion and mechanotransduction. We developed MTSs that incorporate the full 9th and 10th type III fibronectin domains, here termed MTS_FN9-10. Importantly, these probes include a secondary binding site, termed the synergy site, that is specifically required for maximal binding strength and activation of α₅β₁ but not α_v-containing integrins. We observe that fibroblasts seeded on MTS_FN9-10 form large, plaque-like adhesions rich in α₅β₁ integrins, a phenotype that is lost in the absence of a functional synergy site. At the single-molecule level, preliminary data indicate that the force per integrin lies between 1 to 5 pN for cells adhering to MTS_FN9-10-coated surfaces, a range similar to that observed for cells adhering to MTSs containing only a linear RGD binding epitope. In ongoing work, we use MTS_FN9-10 and related probes to parse out the contributions of the synergy site to traction force generation and force sensing at both α₅β₁ and α_v-containing integrin complexes.