Similar myogenic functions for myogenin and MRF4 but not MyoD in differentiated murine embryonic stem cells.

The emergence of four genes encoding myogenic bHLH transcription factors in the stem vertebrate is a unique feature of vertebrate evolution that led to the acquisition of new target genes and novel regulatory circuits for skeletal muscle formation. One unresolved question is the extent to which each factor has evolved specialized functions for regulating muscle gene transcription. We have developed a system using differentiated myogenin (-/-) embryonic stem (ES) cells to determine whether other myogenic factors can replace myogenin's function in muscle differentiation. Previously, we showed that constitutively expressed myogenin restores myofiber formation in differentiated myogenin (-/-) ES cells, but constitutively expressed MyoD does not. Here, we confirm the distinction between myogenin and MyoD using another expression vector and show that constitutively expressed MRF4 leads to myofiber formation in myogenin's absence. Our analysis reveals a correlation between the levels of myogenin plus MRF4 and the extent of myofiber formation, suggesting a synergy between these factors. The results indicate that unlike MyoD, MRF4 plays a role similar to myogenin in skeletal muscle differentiation.