MyoD: A Regulatory Gene of Skeletal Myogenesis

MyoD is expressed only in skeletal muscle (1), and could be considered a master regulatory gene of myogenesis. When expressed in non-muscle cells, MyoD converts those cells to myoblasts (2). Specifically, the mouse fibroblast cell line 10T1/2 does not express either MyoD or any of several muscle specific genes. Forced expression of MyoD in l0Tl/2 cells converts this cell to a myoblast: In growth medium the cell divides and expresses MyoD and other muscle lineage markers, but does not express myosin, desmin, or other markers of muscle terminal differentiation; in differentiation medium the cell withdraws from the cell cycle and initiates expression of desmin, myosin and other proteins of terminal differentiation (1). In addition to activating muscle structural genes, MyoD also positively regulates its own transcription, as well as myogenin, a related muscle regulatory protein (3). This positive autoregulatory circuit could be a mechanism for amplifying expression of muscle regulatory genes and might serve to maintain expression of these genes once a cell has become committed to the myoblast lineage.