Date : 08.27.2008

Mechanical forces facilitate actin polymerization at focal adhesions in a zyxin-dependent manner

Category : Research Topic
 Division of Intracellular Metabolism
Department of Molecular Physiology
 

The transmembrane adhesive molecules integrins are clustered at focal adhesions (FAs) forming tight bindings to extracellular matrices, while they link to actin cytoskeleton at their cytoplasmic domains. FAs serve as sites for actin polymerization, and accumulated F-actin presumably strengthens the integrin-actin cytoskeleton linkage. On the other hand, the linkage between integrin and actin cytoskeleton is regulated by intra- and extracellular mechanical environments. Therefore, it is conceivable that actin polymerization at FAs is affected by mechanical forces, however, their exact role in actin polymerization remains unknown. To address this issue, we assessed actin polymerization at FAs by introducing fluorescence-labeled actin molecules into digitonin-permeabilized human skin fibroblasts cultured on fibronectin. When cell contractility was inhibited by the myosin II inhibitor blebbistatin, actin polymerization at FAs was diminished, whileα5β1 integrin remained accumulated at FAs. To examine the action of mechanical forces more directly, blebbistatin-treated cells were subjected to a sustained uniaxial stretch, which induced actin polymerization at FAs. These results demonstrate the novel role of mechanical forces in actin polymerization at FAs. To reveal the molecular mechanism underlying the force-induced actin polymerization at FAs, we examined the distribution of zyxin, a postulated actin-regulatory protein. Actin polymerizing activity was strong at FAs with large amounts of zyxin. Accumulation of zyxin at FAs was diminished by blebbistatin, while uniaxial stretching of the cells induced zyxin accumulation. When endogenous zyxin was displaced from FAs by overexpressing the C-terminal LIM region of zyxin that is responsible for targeting zyxin to FAs, the force-induced actin polymerization at FAs was decreased. These results demonstrare that zyxin plays a crucial role in the mechanical force-dependent facilitation of actin polymerization at FAs.