Static stretch promotes in a calcineurin and p38 dependent manner MEF2A nuclear translocation and expression of neonatal Myosin Heavy Chain in C2C12 myocytes

Although the effects of mechanical stimuli have been extensively studied in fully differentiated skeletal muscle and have been shown to promote changes in phenotype, including altered myosin heavy chain isoforms expression, the effects of a change in mechanical environment have been poorly studied at earlier stages of skeletal muscle differentiation. In particular the early events elicited by mechanical stimuli upon differentiating myocytes have not been investigated. In this study the effect of static stretch on the activation of transcriptional factors MEF2A and NFATc1, which have been shown to be involved in the differentiation and phenotype regulation of skeletal muscle, have been examined. Furthermore, putative second messenger signalling pathways that could be involved in the de-phosphorylation and hence activation of these factors were also examined. We show in this study that static stretch application produces a robust increase of p38 phosphorylation preceding MEF2A, but not NFATc1, nuclear translocation as well as deactivation of GSK3 via its phosphorylation. Using SB203580 and cyclosporine A drugs, to inhibit both p38 or/and calcineurin dependent signals respectively, we show that MEF2A phosphorylation and subsequent nuclear translocation is regulated by p38 and calcineurin in a biphasic manner time-dependent. Moreover, we also present evidence for another kinase involved in the stretch-related signal triggering MEF2A hyper-phosphorylation, impairing its nuclear translocation, and related to p38. Finally we show that static stretch application overnight promotes neonatal myosin heavy chain expression, which is inhibited by an inactivation of both p38 and calcineurin.