The molecular basis of skeletal muscle differentiation.

In recent years, significant advances have been made in understanding the molecular mechanisms involved in the regulation of skeletal-muscle differentiation. This review focuses on the role of the MyoD family of myogenic transcription factors that includes MyoD, myf-5, myogenin, and MRF4 (herculin or myf-6) in myogenesis. Members of this family share sequence homology for the basic-helix-loop-helix (bHLH) regulatory motif. The basic domain is required for DNA binding, whereas the HLH domain is required for dimerization. The bHLH motif confers both properties of transcriptional activation of muscle specific genes and inhibition of cell growth through collaboration with the E2A gene products (E12 and E47) and the retinoblastoma gene product (pRB). The functions of the MyoD family can be suppressed through inhibition of their expression or activity by various factors. These include peptide growth factors (FGF and TGF-beta), immediate early gene products (Fos, Jun and Myc), other oncogene products (Ras, Src), the Id protein, and innervation. The use of gene-knockout animal models has shown that there is some degree of functional redundancy in that inactivation of either MyoD or myf-5 has no effect on muscle development, whereas inactivation of both genes results in an absolute lack of muscle cells. In contrast, the inactivation of myogenin alone results in mice with gross deficiency of mature muscle.