It was observed that focal complexes appear first after cell–matrix interaction, and focal contacts evolve from them in a Rho-dependent manner ( Clark et al. Activation of Rho is required for the formation of focal contacts and the associated stress fibers, whereas formation of punctate focal complexes depends on the activity of Rac ( Nobes and Hall 1995 Rottner et al. Thus, focal contacts function as both adhesion and signal transduction organelles, informing cells about the state of the ECM.Īssembly and morphogenesis of matrix adhesions are in turn regulated by signals from small G-proteins of the Rho family. Among the components of focal contacts, several types of signaling molecules including tyrosine kinases, tyrosine phosphatases, and adaptor proteins have been identified ( Yamada and Geiger 1997 Bershadsky and Geiger 1999 Schoenwaelder and Burridge 1999). 1999), contain high levels of tyrosine-phosphorylated proteins, a hallmark of signaling molecules. In particular, focal contacts, but not fibrillar adhesions ( Zamir et al. In addition to their function as adhesion sites, matrix adhesions participate in adhesion-dependent signaling ( Yamada and Geiger 1997 Schoenwaelder and Burridge 1999). 2000), is involved in the fibronectin fibrillogenesis ( Pankov et al. An additional form of adhesion site, tensin-enriched fibrillar adhesions ( Zamir et al. Elongated (3–10 μm in length) streak-like structures associated with actin- and myosin-containing filament bundles (stress fibers) are known as focal contacts or focal adhesions ( Heath and Dunn 1978 Rottner et al. 1993) also known as focal complexes ( Nobes and Hall 1995) are localized at the edges of lamellipodia. Small (0.5–1 μm) dot-like or point contacts ( Bershadsky et al. In cultured cells, integrin-based molecular complexes form discrete morphological entities of several types. 1998 Bershadsky and Geiger 1999 Critchley 2000), which are all involved in the dynamic association with actin filaments. 1995 Burridge and Chrzanowska-Wodnicka 1996 Yamada and Geiger 1997 Small et al. Receptors for ECM proteins, transmembrane integrin molecules, are associated via their cytoplasmic domains with a complex of proteins including vinculin, talin, paxillin, tensin, and many others ( Jockusch et al. Our experiments show that integrin-containing focal complexes behave as individual mechanosensors exhibiting directional assembly in response to local force.Ĭells adhere to the extracellular matrix (ECM) via integrin-mediated adhesions that link the ECM to the actin cytoskeleton. Thus, as long as mDia1 is active, external tension force bypasses the requirement for ROCK-mediated myosin II contractility in the induction of focal contacts. Force-induced formation of the focal contacts still occurred in cells subjected to myosin II and ROCK inhibition. 1:136–143), were sufficient to restore force-induced focal contact formation in C3 transferase-treated cells. However, constitutively active mutants of another Rho target, the formin homology protein mDia1 (Watanabe, N., T. Inhibition of Rho activity by C3 transferase suppressed this force-induced focal contact formation. Local centripetal pulling led to local assembly and elongation of these structures and to their development into streak-like focal contacts, as revealed by the dynamics of green fluorescent protein–tagged vinculin or paxillin and interference reflection microscopy. To dissect the mechanism of Rho-dependent induction of focal contacts and to elucidate the role of cell contractility, we applied mechanical force to vinculin-containing dot-like adhesions at the cell edge using a micropipette.
In particular, activation of myosin II–driven contractility by a Rho target known as Rho-associated kinase (ROCK) was shown to be essential for focal contact formation.
The transition of cell–matrix adhesions from the initial punctate focal complexes into the mature elongated form, known as focal contacts, requires GTPase Rho activity.
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