Abstract
Formation of skeletal muscle occurs at the stage of embryonic development as well as growth and regeneration in the adult. A skeletal muscle cell forms an elongated fiber containing multiple nuclei that are originated by fusion of precursor myoblasts. Recently, skeletal muscle-specific fusogenic transmembrane proteins Minion and Myomaker were identified and the mice lacking these factors showed severe muscle loss phenotype. In this study, we aimed to investigate the regulatory mechanism of the fusogenic proteins. C2C12 myoblast cell line was used as a model for in vitro myogenesis. The cells became elongated then fused to form multinucleated myotubes within 72 hours after induction of differentiation. By challenging kinase inhibitors previously known to affect muscle development, we found that activity of p38 MAPK but not ERK, JNK, ERK5 is required for induction of both Minion and Myomaker. Actually, the activity of p38 MAPK was continuously increased during C2C12 differentiation, and application of p38 inhibitor abrogated cell-cell fusion. Knockout of minion gene in C2C12 abrogated efficient fusion, but these cells were capable of inducing Tn-T, suggesting that myoblast fusion can be regulated independently of its differentiation. As p38 MAPK is a key mediator of stress and inflammatory signaling, it is possible that p38 mediated fusion process contributes to muscle regeneration after injury or might be involved in the pathology of abnormal muscle loss with increased inflammatory cytokines in cachexia.
