Focal contacts as mechanosensors: Externally applied local mechanical force induces growth of focal contacts by an mDia1-dependent and ROCK-independent mechanism

The transition of cell-matrix adhesions from the initial punctate focal com plexes into the mature elongated form, known as focal contacts, requires GT Pase Rho activity. In particular, activation of myosin II-driven contractil ity by a Rho target known as Rho-associated kinase (ROCK) was shown to be e ssential for focal contact formation. To dissect the mechanism of Rho-depen dent induction of focal contacts and to elucidate the role of cell contract ility, we applied mechanical force to vinculin-containing dot-like adhesion s at the cell edge using a micropipette. Local centripetal pulling led to l ocal assembly and elongation of these structures and to their development i nto streak-like focal contacts, as revealed by the dynamics of green fluore scent protein-tagged vinculin or paxillin and interference reflection micro scopy. Inhibition of Rho activity by C3 transferase suppressed this force-i nduced focal contact formation. However, constitutively active mutants of a nother Rho target, the formin homology protein mDia1 (Watanabe, N., T Kato, A. Fujita, T. Ishizaki, and S. Narumiya. 1999. Nat. Cell Biol. 1:136-143), were sufficient to restore force-induced focal contact formation in C3 tra nsferase-treated cells. Force-induced formation of the focal contacts still occurred in cells subjected to myosin II and ROCK inhibition. Thus, as lon g as mDia1 is active, external tension force bypasses the requirement for R OCK-mediated myosin II contractility in the induction of focal contacts. Ou r experiments show that integrin-containing focal complexes behave as indiv idual mechanosensors exhibiting directional assembly in response to local f orce.