ECM area threshold for focal adhesion assembly

Integrin-based focal adhesion (FA) complexes – which also contain actins, cytoskeletal elements, such as talin and vinculin, and signalling molecules – form following integrin activation and ligand binding, and transmit anchorage and traction forces between the cell and the extracellular matrix (ECM). To investigate the physical parameters of the ECM that control assembly of FAs and force transduction in non-migrating cells, Andrés García and colleagues (p. 5110) have been examining stable FA assembly and force transduction in mouse fibroblasts exposed to artificially engineered ‘nanoislands’ of the integrin ligand fibronectin (FN). The authors report that the size of the individual nanoisland, and not the number of islands or total adhesive area, controls integrin–FN clustering and adhesion strength. They identify a threshold for the ECM area below which few integrin–FN clusters form, and show that this adhesive area threshold is not constant but is regulated by the recruitment of talin and vinculin to the FA complex, and by the cytoskeletal tension applied to these adhesive clusters. On the basis of their results, the authors propose a force equilibrium model for ECM area-controlled assembly of integrin–FN clusters, which provides a simple, local regulatory mechanism for the assembly and disassembly of adhesive structures and the transmission of adhesive forces.