Regulation of epaxial myogenesis and Myf5 expression by Shh signaling, Gli activation, and BMP antagonism

Current investigations of muscle development in vertebrate embryos are providing exciting insights into regulatory mechanisms that control the formation of anatomically defined groups of muscles, as well as fundamental knowledge of the transcriptional and signaling mechanisms that control specification of stem cell lineages. Progress in the field of skeletal myogenesis has been greatly stimulated by studies of muscle differentiation in tissue culture models, leading to the discovery of the myogenic regulatory factors (MRFs), a family of conserved bHLH transcription factors with the remarkable ability to convert non-muscle cells into muscle. Myf5 is the first of the MRFs expressed in the embryo, controlling the specification of myogenic cell lineages in the developing vertebrate embryo. A complex interplay of developmental signaling molecules have been demonstrated to control Myf5 activation in the embryo. Chief among these is Shh, a secreted protein that regulates target genes through the Gli family of transcription factors. Previous genetic studies have shown that Shh is required for Myf5 expression in the specification of epaxial muscle progenitors of the dorsal somite. Despite the studies linking Shh and epaxial myogenesis, the mechanism for epaxial Myf5 regulation by Shh remains largely unresolved. To better understand these issues we set out to investigate the transcriptional regulation of Myf5 in the epaxial progenitors of the dorsal somite. Using transgenic studies, we identified a 650bp Epaxial Somite (ES) transcription enhancer that controls the epaxial expression of the muscle regulatory gene, Myf5. Subsequent genetic and transgenic analysis have demonstrated that the Myf5 epaxial enhancer is a direct target of Shh signaling through a Gli binding site essential for enhancer activity. We have gone on to demonstrate that Gli2 and GO are redundantly required for the activation of the ES enhancer and the initiation of epaxial Myf5 expression. Finally, the BMP antagonist Noggin is required in epaxial muscle progenitors for the Shh-induced activation of Myf5. Future studies of the derivation of the embryonic musculature can be expected to lay the groundwork for an understanding of adult myogenesis and the eventual development of cell based therapies for the treatment of muscle disease and damage in humans.