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 t he potential that the four regulatory factors belonging to the MyoD fa mily have to activate myogenesis. In the present study we have further investigated the myogenic phenotype of C3H10T1/2 cells stably transfe cted with either Myf5, MyoD, myogenin or MRF4 cDNAs. We have studied t he influence of each transfected cDNA on expression of the four endoge nous muscle regulatory genes and on the ability of these embryonic myo genic derivatives to express adult muscle genes. No trace of endogenou s 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 suf ficient to establish and maintain the myoblast phenotype (3) The endog enous Myf5 gene is not mobilized. We have also observed that endogenou s transcripts for MyoD and myogenin begin to accumulate at the onset o f differentiation in the four myogenic derivatives, whereas accumulati on of endogenous MRF4 transcripts starts after myotubes have formed an d occurs at a much lower level (100- to 500-fold lower) than in differ entiated cultures of myosatellite cells. However, neither this low lev el of MRF4 transcripts nor higher levels from the transfected MRF4 cDN A affected (prevented or stimulated) the accumulation of dystrophin tr anscripts or of adult muscle-gene transcripts (e.g., myosin heavy chai n IIB, acetylcholine receptor E-subunit and M form of aldolase A), whi ch occurred at similar levels in the four myogenic derivatives. Thus, despite the fact that MRF4 gene expression is predominant in adult mus cle, this factor does not appear to be crucial for expression of the a dult muscle genes studied, at least in cells expressing MyoD and myoge nin at high levels.