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 t o the acquisition of new target genes and novel regulatory circuits for ske letal muscle formation. One unresolved question is the extent to which each factor has evolved specialized functions for regulating muscle gene transc ription. We have developed a system using differentiated myogenin (-/-) emb ryonic stem (ES) cells to determine whether other myogenic factors can repl ace myogenin's function in muscle differentiation. Previously, we showed th at constitutively expressed myogenin restores myofiber formation in differe ntiated 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 myof iber formation in myogenin's absence. Our analysis reveals a correlation be tween the levels of myogenin plus MRF4 and the extent of myofiber formation , suggesting a synergy between these factors. The results indicate that unl ike MyoD, MRF4 plays a role similar to myogenin in skeletal muscle differen tiation. (C) 2001 Wiley-Liss, Inc.