Bcl-2 Expression Identifies an Early Stage of Myogenesis and Promotes Clonal Expansion of Muscle Cells

We show that Bcl-2 expression in skeletal muscle cells identifies an early stage of the myogenic pathway, inhibits apoptosis, and promotes clonal expansion. Bcl-2 expression was limited to a small proportion of the mononucleate cells in muscle cell cultures, ranging from ∼1–4% of neonatal and adult mouse muscle cells to ∼5–15% of the cells from the C2C12 muscle cell line. In rapidly growing cultures, some of the Bcl-2–positive cells coexpressed markers of early stages of myogenesis, including desmin, MyoD, and Myf-5. In contrast, Bcl-2 was not expressed in multinucleate myotubes or in those mononucleate myoblasts that expressed markers of middle or late stages of myogenesis, such as myogenin, muscle regulatory factor 4 (MRF4), and myosin. The small subset of Bcl-2–positive C2C12 cells appeared to resist staurosporine-induced apoptosis. Furthermore, though myogenic cells from genetically Bcl-2–null mice formed myotubes normally, the muscle colonies produced by cloned Bcl-2–null cells contained only about half as many cells as the colonies produced by cells from wild-type mice. This result suggests that, during clonal expansion from a muscle progenitor cell, the number of progeny obtained is greater when Bcl-2 is expressed.