A calcineurin- and NFAT-dependent pathway regulates Myf5 gene expression in skeletal muscle reserve cells

Myf5 is a member of the muscle regulatory factor family of transcription fa ctors and plays an important role in the determination, development, and di fferentiation of skeletal muscle. However, factors that regulate the expres sion and activity of Myf5 itself are not well understood. Recently, a role for the calcium-dependent phosphatase calcineurin was suggested in three di stinct pathways in skeletal muscle: differentiation, hypertrophy, and fiber -type determination, We propose that one downstream target of calcineurin a nd the calcineurin substrate NFAT in skeletal muscle is regulation of Myf5 gene expression, For these studies, we used myotube cultures that contain b oth multinucleated myotubes and quiescent, mononucleated cells termed 'rese rve' cells, which share many characteristics with satellite cells. Treatmen t of such myotube cultures,vith the calcium ionophore ionomycin results in an approximate to4-fold increase in Myf5 mRNA levels, but similar effects a re not observed in proliferating myoblast cultures indicating that Myf5 is regulated by different pathways in different cell populations. The increase in Myf5 mRNA levels in myotube cultures requires the activity of calcineur in and NFAT, and can be specifically enhanced by overexpressing the NFATc i soform, We used immunohistochemical analyses and fractionation of the cell populations to demonstrate that the calcium regulated expression of Myf5 oc curs in the mononucleated reserve cells, We conclude that Myf5 gene express ion is regulated by a calcineurin- and NFAT-dependent pathway in the reserv e cell population of myotube cultures, These results may provide important insights into the molecular mechanisms responsible for satellite cell activ ation and/or the renewal of the satellite cell pool following activation an d proliferation.