Akirin1 (Mighty), a novel promyogenic factor regulates muscle regeneration and cell chemotaxis

Akirin1 (Mighty) is a downstream target gene of myostatin and has been shown to be a promyogenic factor. Although expressed in many tissues, akirin1 is negatively regulated by myostatin specifically in skeletal muscle tissue. In this manuscript we have characterized the possible function of akirin1 in postnatal muscle growth. Molecular and immunohistological analyses indicated that while low levels of akirin1 are associated with quiescent satellite cells (SC), higher levels of akirin1 are detected in activated proliferating SC indicating that akirin1 could be associated with satellite cell activation. In addition to SC, macrophages also express akirin1, and increased expression of akirin1 resulted in more efficient chemotaxis of both macrophages and myoblasts. Akirin1 appears to regulate chemotaxis of both macrophages and myoblasts by reorganising actin cytoskeleton, leading to more efficient lamellipodia formation via a PI3 kinase dependent pathway. Expression analysis during muscle regeneration also indicated that akirin1 expression is detected very early (day 2) in regenerating muscle, and expression gradually peaks to coincide the nascent myotube formation stage of muscle regeneration. Based on these results we propose that akirin1 could be acting as a transducer of early signals of muscle regeneration. Thus, we speculate that myostatin regulates key steps of muscle regeneration including chemotaxis of inflammatory cells, SC activation and migration through akirin1.