Reduced differentiation potential of primary MyoD-/- myogenic cells derived from adult skeletal muscle

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