Expression of the muscle regulatory factor MRF4 during somite and skeletal myofiber development.

The muscle regulatory factors MRF4, myogenin, myf-5, and MyoD constitute a family of proteins that can function as muscle-specific transcriptional activators. Although this gene family has been extensively studied, a specific role for each factor during myogenesis remains to be determined. Understanding how these factors function requires a detailed analysis of their expression patterns during development. Toward this goal, we examined the temporal pattern of expression of MRF4 and the other factors in the rat myogenic cell line L6J1-C, in newborn rat primary muscle cell cultures and in fetal and postnatal rat limb muscle. Our results demonstrate that MyoD, myogenin, and myf-5 transcripts accumulate maximally at various stages of myoblast differentiation and decline to low expression levels in adult muscle tissue. In contrast, MRF4 transcript accumulation is restricted to cell cultures containing multinucleate myofibers, and its expression in vivo increases sharply during late fetal muscle development. This level of MRF4 expression is maintained in the adult which, together with decreased expression of the other three muscle regulatory factors, makes MRF4 the predominant factor in adult muscle. In situ hybridization of mouse embryo tissue sections indicates that MRF4 transcripts accumulate in the limb beginning 13.5 days post coitum, which is 2 days later than the initial appearance of myogenin and MyoD transcripts. Hybridization to earlier stages of development reveals, however, that MRF4 mRNA initially is present in the myotomal compartment of the somites, just after myogenin but 2 days prior to MyoD expression. Unlike myogenin and MyoD, MRF4 expression declines in the myotomes at the time that multinucleate axial muscles begin to form in this region, although during later development MRF4 is expressed in the myofibers of axial muscles at levels comparable to those in the limb. Differences in the expression patterns for MRF4, myogenin, myf-5 and MyoD between myotomal and other skeletal muscle development suggest that the relative timing of expression for each muscle regulatory factor may control the distinct phenotypes associated with myotomal myocytes and multinucleate myofibers.