ES-cells carrying two inactivated myf-5 alleles form skeletal muscle cells: activation of an alternative myf-5-independent differentiation pathway.

Both alleles of the myogenic regulatory gene myf-5 have been inactivated in mouse embryonic stem cells by different strategies involving either consecutive gene targeting with neomycin and hygromycin replacement vectors or spontaneous loss of heterozygosity in cells targeted with the neomycin replacement vector alone. Both selection schemes provided homozygous myf-5 mutant ES-cells with normal developmental potential in vitro. Embryoid bodies differentiated into skeletal muscle cells as assessed by their typical morphology and skeletal muscle markers. The extent of differentiation in homozygous mutant myf-5 embryonic stem cells was virtually indistinguishable from control cultures, suggesting that myf-5 is not required for the early steps of myogenic development in vitro. While myocyte populations derived from wild-type and heterozygous myf-5 mutant ES-cells contained myoD-positive and myoD-negative cells, no myoD-negative muscle cells were found among myf-5 homozygous mutants. Differentiated myf-5 double-knockout cells also showed a premature expression of myoD. These results indicate a compensatory role of myoD and myf-5 during early myocyte development and suggest a possible down-regulation of myoD by myf-5 during early myogenesis.