MYOGENINS FUNCTIONS DO NOT OVERLAP WITH THOSE OF MYOD OR MYF-5 DURINGMOUSE EMBRYOGENESIS

The four myogenic basic helix-loop-helix proteins, MyoD, myogenin, Myf -5, and MRF4, can each activate skeletal muscle differentiation when i ntroduced into nonmuscle cells. During embryogenesis, each of these ge nes is expressed in a unique but overlapping pattern in skeletal muscl e precursors and their descendants. Gene knockout experiments have sho wn that MyoD and Myf-5 play seemingly redundant roles in the generatio n of myoblasts. However, the role of either of these genes during diff erentiation in vivo has not been determined. In contrast, a myogenin-n ull mutation blocks differentiation and results in a dramatic decrease in muscle fiber formation, yet the role of myogenin in the generation or maintenance of myoblast populations is not known. Because myogenin possesses the same myogenic activity as MyoD and Myf-5 in vitro and t he expression patterns of these three genes overlap in vivo, we sought to determine if myogenin shares certain functions with either MyoD or Myf-5 in vivo. We therefore generated mice with double homozygous nul l mutations in the genes encoding MyoD and myogenin or Myf-5 and myoge nin. These mice showed embryonic and perinatal phenotypes characterist ic of the combined defects observed in mice mutant for each gene alone . As shown by histological analysis and expression of muscle-specific genes, the numbers of undifferentiated myoblasts and residual myofiber s were comparable between myogenin-mutant homozygotes and the double-m utant homozygotes. Myoblasts isolated from neonates of the combined mu tant genotypes underwent myogenesis in tissue culture, indicating that no more than two of the four myogenic factors are required to support muscle differentiation. These results demonstrate that the functions of myogenin do not overlap with those of MyoD or Myf-5 and support the view that myogenin acts in a genetic pathway downstream of MyoD and M yf-5. (C) 1995 Academic Press, Inc.