Role of H2 Calponin in Myoblast Differentiation, Fusion and Myogenesis

Abstract

Actin cytoskeleton plays a pivotal role in myoblast motility, adhesion, fusion and differentiation into myotubes. However, how the actin cytoskeleton is regulated during myogenesis is largely unknown. H2-calponin is a regulatory protein that stabilizes the actin cytoskeleton and regulates cell motility and adhesion. In the present study, we investigated the role of h2 calponin in the regulation of myoblast migration, differentiation and fusion. The results showed that h2-calponin is expressed at significant levels in undifferentiated mouse primary satellite cells and C2C12 myoblasts. In migrating satellite cells and C2C12 myoblasts, h2-calponin co-localizes with actin stress fibers at both the leading edge and the tail regions. In newly fused myotubes derived from primary satellite cells, h2-calponin is mainly present at the ends of the myotubes. The level of h2-calponin significantly decreases during satellite cell and myoblast differentiation and fusion, and becomes undetectable in mature myocytes. Primary satellite cells isolated from h2-calponin gene knockout mice had significantly decreased cell spreading area indicating reduced substrate adhesion and increased rate of cell proliferation as compared with wild type controls. Biochemical and histological studies revealed that skeletal muscles of h2-calponin knockout mice retain normal muscle weight and myofilament protein compositions. Myotubes derived from h2-calponin-null satellite cells showed apparently normal sarcomere striations and expression of myofilament proteins. Therefore, deletion of h2-calponin does not abolish the maximum capacity of myoblast function and myogenesis. However, in vitro differentiation studies revealed that h2-calponin-hull myoblasts had higher efficiency of fusion, as indicated by increased fusion index, than wild type controls. The data suggest that the function of h2-calponin provides a novel mechanism in regulating actin cytoskeleton activity during myoblast differentiation, fusion and myogenesis.