Myosin X is Recruited to Focal Adhesion and Induces Filopodia Initiation

Abstract

Filopodia is the structure protruding from the edge of the cells that plays an important role in diverse cell motility. Myosin-X is involved in promoting filopodia formation and localizes at the tips of filopodia. However, the mechanism of myosin-X-induced filopodia formation remains largely unknown. Here we studied the mechanism of myosin-X-induced filopodia formation by directly monitoring the dynamics of myosin-X, actin and actin regulating proteins such as Arp2/3, vinculin and VASP during filopodia initiation and elongation. We found a specific local nucleation of actin and Arp2/3 at the cell's leading edge, where integrin was accumulated during filopodia initiation. Myosin X was then recruited to these sites by the lateral movement and gradual clustering along the actin nucleation sites, and initiated filopodia formation. During filopodia extension, we found the translocation of Arp2/3 and other actin regulating proteins along filopodia. Arp2/3 localized not only at the base of filopodia, but also at the middle of long filopodia, from where myosin X initiated the phased extension of filopodia, with the change of extension directions. Elimination of integrin-b by siRNA significantly attenuated myosin X induced filopodia formation and multiple phased elongations. Integrin-β is localized at the position where filopodia direction changes. Based upon present findings, we propose the following mechanism. Myosin X accumulates at focal adhesions at the cell's leading edge where integrin is localized. Myosin X promotes actin convergence to produce the base of filopodia. Myosin X transports VASP to filopodial tips to facilitate elongation of filopodia. At the tip where myosin X is accumulated, integrin is also accumulated and forms focal adhesion, from where myosin X promotes second phase elongation to further extend filopodia.