MYF5, MYOD, MYOGENIN AND MRF4 MYOGENIC DERIVATIVES OF THE EMBRYONIC MESENCHYMAL CELL-LINE C3H10T1/2 EXHIBIT THE SAME ADULT MUSCLE PHENOTYPE

Cells of the embryonic mesenchymal cell line C3H10T1/2 have revealed the potential that the four regulatory factors belonging to the MyoD family have to activate myogenesis. In the present study we have further investigated the myogenic phenotype of C3H10T1/2 cells stably transfected with either Myf5, MyoD, myogenin or MRF4 cDNAs. We have studied the influence of each transfected cDNA on expression of the four endogenous muscle regulatory genes and on the ability of these embryonic myogenic derivatives to express adult muscle genes. No trace of endogenous transcripts distinct from the exogenous one was found in any of the four converted populations at the myoblast stage. This indicates that cross-activation within the MyoD family does not occur at the myoblast stage in these cells. Similarly, evidence was obtained that auto- or cross-activation of the Myf5 gene occurs neither at the myoblast stage nor at the myotube stage and that no autoactivation of the MRF4 gene occurs. Our results together with previous observations indicate that in C3H10T1/2 myogenic derivatives: (1) Autoactivation at the myoblast stage is restricted to MyoD (2) Expression from each cDNA alone is sufficient to establish and maintain the myoblast phenotype (3) The endogenous Myf5 gene is not mobilized. We have also observed that endogenous transcripts for MyoD and myogenin begin to accumulate at the onset of differentiation in the four myogenic derivatives, whereas accumulation of endogenous MRF4 transcripts starts after myotubes have formed and occurs at a much lower level (100- to 500-fold lower) than in differentiated cultures of myosatellite cells. However, neither this low level of MRF4 transcripts nor higher levels from the transfected MRF4 cDNA affected (prevented or stimulated) the accumulation of dystrophin transcripts or of adult muscle-gene transcripts (e.g., myosin heavy chain IIB, acetylcholine receptor E-subunit and M form of aldolase A), which occurred at similar levels in the four myogenic derivatives. Thus, despite the fact that MRF4 gene expression is predominant in adult muscle, this factor does not appear to be crucial for expression of the adult muscle genes studied, at least in cells expressing MyoD and myogenin at high levels.