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 αv -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 MTSFN9-10. 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 αv-containing integrins. We observe that fibroblasts seeded on MTSFN9-10 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 MTSFN9-10-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 MTSFN9-10 and related probes to parse out the contributions of the synergy site to traction force generation and force sensing at both α5β1 and αv-containing integrin complexes.