Single Allele of MyoD or Myf5 is Necessary for Adult Myogenesis

MyoD or Myf5’s requirement for developmental myogenesis has been well established. However, their roles in adult myogenesis, regeneration after injury, were not precisely known due to the perinatal lethality of MyoD-null and Myf5-null mice. To circumvent that we created a conditional floxed allele of MyoD, MyoDcKO, that together with CreER allows temporal control of MyoD deletion. This combined with other mutant alleles of MyoD and Myf5, as well as Pax7CreER, allows us to efficiently delete MyoD and Myf5 in satellite cells, the indispensable adult skeletal muscle stem cell. Using this system, we show that at least a single allele of MyoD of Myf5 is necessary for adult myogenesis in vivo and in vitro. No other genetic or cellular factor can compensate for their loss. Satellite cells that lack both myogenic factors cannot upregulate Myog, a skeletal muscle differentiation gene, and they cannot fuse to form multinucleated myotubes. We also show that ~10% of double-null satellite cells differentiate into lipid droplet containing, mature adipocytes. The remaining cells take retain some myogenic characteristics, but also express fibroblastic markers. Finally, we reaffirm that MyoD is a more potent driver of adult myogenesis than Myf5, with satellite cells that contain only a single allele of Myf5 being more open to adipogenic differentiation, though not to as great of an extent as double-null satellite cells.