Characterization of proliferating human skeletal muscle-derived cells in vitro: Differential modulation of myoblast markers by TGF-?2

Adult human skeletal muscle-derived cells (HuSkMC) propagated in vitro are under investigation as a cell-based therapy for the treatment of myocardial infarction. We have characterized HuSkMC with respect to cell identity and state of differentiation as a prerequisite to their clinical use. Flow cytometric analysis of propagated HuSkMC revealed a population of cells that expressed the myoblast markers CD56 and desmin. The presence of myoblasts in these cultures was further confirmed by their capacity to form myotubes and increase creatine kinase activity when cultured in low serum conditions. The non-myoblast fraction of these propagated cells expressed TE7, a marker associated with the fibroblast phenotype. Spontaneous differentiation of myoblasts occurred during serial propagation of HuSkMC, as judged by myotube formation, thereby reducing the myoblast representative fraction with continued cell expansion. We examined transforming growth factor beta2 (TGF-beta2) for its utility in controlling this spontaneous differentiation of adult human myoblasts in vitro. Propagation of HuSkMC in the presence of 1 ng/ml TGF-beta2 for 5 days decreased desmin expression within the myoblast population and caused a parallel reduction of creatine kinase activity. CD56 expression was unaffected, indicating a differential regulation of these myoblast markers. The reduction in desmin expression and creatine kinase activity was, however, reversible upon the removal of TGF-beta. These data collectively indicate that TGF-beta2 restrained differentiation of adult human skeletal myoblasts during propagation without causing irreversible loss of the myoblast phenotype, demonstrating the potential utility of using TGF-beta2 during cultivation and expansion of HuSkMC intended for therapeutic implantation.