MyoD cannot compensate for the absence of myogenin during skeletal muscle differentiation in murine embryonic stem cells

Myogenin (-/-) mice display severe skeletal muscle defects despite expressi ng normal levels of MyoD. The failure of MyoD to compensate for myogenin co uld be explained by distinctions in protein function or by differences in p atterns of gene expression. To distinguish between these two possibilities, we compared the abilities of constitutively expressed myogenin and MyoD to support muscle differentiation in embryoid bodies made from myogenin (-/-) ES cells. Differentiated embryoid bodies from wild-type embryonic stem (ES ) cells made extensive skeletal muscle, but embryoid bodies from myogenin ( -/-) ES cells had greatly attenuated muscle-forming capacity. The inability of myogenin (-/-) ES cells to generate muscle was independent of endogenou s MyoD expression. Skeletal muscle was restored in myogenin (-/-) ES cells by constitutive expression of myogenin. In contrast, constitutive expressio n of (-/-) MyoD resulted in only marginal enhancement of skeletal muscle, a lthough myocyte numbers greatly increased. The results indicated that const itutive expression of MyoD led to enhanced myogenic commitment of myogenin (-/-) cells but also indicated that committed cells were impaired in their ability to form muscle sheets without myogenin. Thus, despite their related ness, myogenin's role in muscle formation is distinct from that of MyoD, an d the distinction cannot be explained merely by differences in their expres sion properties. (C) 2000 Academic Press