Abstract
Integrin-mediated adhesions are critical for stem cell differentiation, cancer metastasis, and the immune response [Hynes RO (2009) Science 326:1216–1219]. However, the mechanisms of early adhesion formation remain unclear, especially the effects of lateral clustering of integrins and the role of the Src family kinases. Using mobile Arg–Gly–Asp (RGD) peptide ligands on lipid bilayers with nano-fabricated physical barriers [Salaita K, et al. (2010) Science 327:1380–1385], we observe surprising long-range lateral movements of ligated integrins during the process of cell spreading. Initially, RGD-activated integrin clusters stimulate actin polymerization that radiates from the clusters. Myosin II contraction of actin from adjacent clusters produces contractile pairs that move toward each other against barriers. Force generated by myosin II stimulates a Src kinase-dependent lamellipodial extension and outward movement of clusters. Subsequent retraction by myosin II causes inward movement of clusters. The final cell spread area increases with the density of periodic barriers. Early integrin clustering recruits adhesion proteins, talin, paxillin, and FAK, irrespective of force generation. However, recruitment of vinculin is only observed upon contraction. Thus, we suggest that integrin activation and early clustering are independent of lateral forces. Clustering activates Src-dependent actin polymerization from clusters. Myosin contraction of clusters to lines stimulates active spreading with outward forces from actin polymerization followed by a second wave of contraction. Many of these early mechanical steps are not evident in cells spreading on immobilized matrices perhaps because of the low forces involved. These observations can provide new targets to control integrin-dependent adhesion and motility.