The consequences of a constitutive expression of MyoD1 in ES cells and mouse embryos.

A variety of differentiated cell types can be converted to skeletal muscle following transfection with the myogenic regulatory gene MyoD1. To determine whether multipotent embryonic stem (ES) cells respond similarly, cultures of two ES cell lines were electroporated with a MyoD1 cDNA driven by the beta-actin promoter. All transfected clones tested, carrying single copy of the exogenous gene, expressed high levels of MyoD1 mRNA. Surprisingly, although maintained in mitogen-rich medium, this ectopic expression was associated with a transactivation of the endogenous myogenin and myosin light chain 2 genes but not the endogenous MyoD1, MRF4, myf5, skeletal muscle actin or myosin heavy chain genes. Preferential myogenesis and the appearance of contracting skeletal muscle fibers was observed only when the transfected cells were allowed to differentiate, via embryoid bodies, in low mitogen-containing medium. Myogenesis was associated with the activation of MRF4 and myf5 genes and in a significant increase in the level of myogenin mRNA. Not all cells were converted to skeletal muscle, indicating that only a subset of stem cells can respond to MyoD1. Moreover, the continued expression of MyoD1 was not required for myogenesis. Interestingly, no preferential myogenesis was observed when the transfected ES cells were allowed to differentiate in vivo to teratocarcinomas. These results show that ES cells can respond to MyoD1, but environmental factors control the expression of its myogenic differentiation function. Second, MyoD1 function in ES cells, even under environmental conditions that favour differentiation, is not dominant (incomplete penetrance). Third, that the exogenous MyoD1 transactivates the endogenous myogenin and MLC2 genes in ES cells. No live transgenic mice could be produced following microinjection of the beta-actin/MyoD1 gene into the pronuclei of fertilized eggs. Transgenic embryos died before mid gestation. The majority of tested embryos between 7.5 and 9.5 days, although retarded compared to control litermates, differentiated into tissues representative of all three germ layers. The expression of the introduced gene was detected in all ectodermal and mesodermal tissues but was absent in all endodermal cells. These results demonstrate again that MyoD1 is not a dominant regulatory factor.