THE MUSCLE REGULATORY FACTORS MYOD AND MYF-5 UNDERGO DISTINCT CELL CYCLE-SPECIFIC EXPRESSION IN MUSCLE-CELLS

The muscle regulators MyoD and Myf-5 control cell cycle withdrawal and induction of differentiation in skeletal muscle cells. By immunofluor escence analysis, we show that MyoD and Myf-5 expression patterns beco me mutually exclusive when C2 cells are induced to differentiate with Myf-5 staining present in cells which fail to differentiate. Isolation of these undifferentiated cells reveals that upon serum stimulation t hey reenter the cell cycle, express MyoD and downregulate Myf-5. Simil ar regulations of MyoD and Myf-5 were observed using cultured primary myoblasts derived from satellite cells. To further analyze these regul ations of MyoD and Myf-5 expression, we synchronized proliferating myo blasts, Analysis of MyoD and Myf-5 expression during cell cycle progre ssion revealed distinct and contrasting profiles of expression. MyoD i s absent in G0 peaks in mid-G1, falls to its minimum level at G1/S and reaugments from S to M. In contrast, Myf-5 protein is high in G0, dec reases during G1 and reappears at the end of G1 to remain stable until mitosis. These data demonstrate that the two myogenic factors MyoD an d Myf-5 undergo specific and distinct cell cycle-dependent regulation, thus establishing a correlation between the cell cycle-specific ratio s of MyoD and Myf-5 and the capacity of cells to differentiate: (a) in G1, when cells express high levels of MyoD and enter differentiation; (b) in G0, when cells express high levels of Myf-5 and fail to differ entiate.