Dependence of myoblast fusion on a cortical actin wall and nonmuscle myosin IIA.

Cell-cell fusion is a fundamental cellular process that is essential for development as well as fertilization. Myoblast fusion to form multinucleated skeletal muscle myotubes is a well studied, yet incompletely understood example of cell-cell fusion that is essential for formation of contractile skeletal muscle tissue. Studies in this report identify several novel cytoskeletal events essential to an early phase of myoblast fusion among cultured murine myoblasts. During myoblast pairing and alignment, cortical actin filaments organize into a dense actin wall structure that parallels and extends the length of the plasma membrane of the bipolar, aligned cells. As fusion progresses, gaps appear within the actin wall at sites of vesicle accumulation, the vesicles pair across the aligned myoblasts, cell-cell contacts and fusion pores form. Inhibition of nonmuscle myosin IIA (NM-MHC-IIA) motor activity prevents formation of this cortical actin wall, as well as the appearance of vesicles at a membrane proximal location, and myoblast fusion. These results suggest that early formation of a subplasmalemmal actin wall during myoblast alignment is a critical event for myoblast fusion that supports bipolar membrane alignment and temporally regulates trafficking of vesicles to the nascent fusion sites during skeletal muscle myoblast differentiation.