mef2c is activated directly by myogenic basic helix-loop-helix proteins during skeletal muscle development in vivo

Skeletal muscle development requires the coordinated expression of numerous transcription factors to control the specification of mesodermal progenitor cells to a muscle fate and the differentiation of those committed myoblasts into functional, contractile muscle. Two families of transcription factors play key roles in these processes. The myogenic basic helix-loop-helix (bHLH) proteins, MyoD and Myf5, are required for myoblast specification, while two members of the same family, myogenin and MRF4, play key roles in myoblast differentiation in vivo. All four members of the myogenic bHLH family are sufficient to dominantly induce myogenesis when introduced into a variety of non-muscle cells in culture, however this function requires the activity of a second family of transcriptional regulators, the myocyte enhancer factor 2 (MEF2) family. MEF2 factors are essential for muscle differentiation, and previous studies have shown that MyoD and MEF2 family members function combinatorially to activate transcription and myogenesis. Consistent with these observations, the majority of skeletal muscle genes require both MyoD and MEF2 family members to activate their transcription. A possible exception to this combinatorial model for activation is suggested by the observation that myogenic bHLH factors may be able to independently activate the expression of MEF2. This raises the question as to how mef2 gene transcription is induced by MyoD factors without cooperative activation by MEF2. During skeletal muscle development, mef2c is the first member of the MEF2 family to be expressed. In this study, we have investigated the regulation of a skeletal muscle-specific enhancer from the mouse mef2c gene using a transgenic approach. We show that mef2c is a direct transcriptional target of the MyoD family in vivo via an essential E box in the skeletal muscle enhancer of mef2c, and we show that mef2c is not a direct target for autoregulation by MEF2.