Derived from Adult Skeletal Muscle

To gain insight into the regeneration deficit of MyoD 2 / 2 muscle, we investigated the growth and differentiation of cultured MyoD 2 / 2 myogenic cells. Primary MyoD 2 / 2 myogenic cells exhibited a stellate morphology distinct from the compact morphology of wild-type myoblasts, and expressed c-met, a receptor tyrosine kinase expressed in satellite cells. However, MyoD 2 / 2 myogenic cells did not express desmin, an intermediate filament protein typically expressed in cultured myoblasts in vitro and myogenic precursor cells in vivo. Northern analysis indicated that proliferating MyoD 2 / 2 myogenic cells expressed fourfold higher levels of Myf-5 and sixfold higher levels of PEA3 , an ETS-domain transcription factor expressed in newly activated satellite cells. Under conditions that normally induce differentiation, MyoD 2 / 2 cells continued to proliferate and with delayed kinetics yielded reduced numbers of predominantly mononuclear myocytes. Northern analysis revealed delayed induction of myogenin , MRF4 , and other differentiation-specific markers although p21 was upregulated normally. Expression of M-cadherin mRNA was severely decreased whereas expression of IGF-1 was markedly increased in MyoD 2 / 2 myogenic cells. Mixing of lacZ-labeled MyoD 2 / 2 cells and wild-type myoblasts revealed a strict autonomy in differentiation potential. Transfection of a MyoD -expression cassette restored cytomorphology and rescued the differentiation deficit. We interpret these data to suggest that MyoD 2 / 2 myogenic cells represent an intermediate stage between a quiescent satellite cell and a myogenic precursor cell.