A role for Zic1 and Zic2 in Myf5 regulation and somite myogenesis

Zic genes encode a conserved family of zinc finger proteins with essential functions in neural development and axial skeletal patterning in the vertebrate embryo. Zic proteins also function as Gli co-factors in Hedgehog signaling. Here, we report that Zic genes have a role in Myf5 regulation for epaxial somite myogenesis in the mouse embryo. In situ hybridization studies show that Zic1, 2, and 3 transcripts are expressed in Myf5-expressing epaxial myogenic progenitors in the dorsal medial dermomyotome of newly forming somites, and immunohistological studies show that Zic2 protein is co-localized with Myf5 and Pax3 in the dorsal medial lip of the dermomyotome, but is not expressed in the forming myotome. In functional reporter assays, Zic1 and Zic2, but not Zic3, potentiate the transactivation of Gli-dependent Myf5 epaxial somite-specific (ES) enhancer activity in 3T3 cells, and Zic1 activates endogenous Myf5 expression in 10T1/2 cells and in presomitic mesoderm explants. Zic2 also co-immunoprecipitates with Gli2, indicating that Zic2 forms complexes with Gli2 to promote Myf5 expression. Genetic studies show that, although Zic2 and Zic1 are activated normally in sonic hedgehog(-/-) mutant embryos, Myf5 expression in newly forming somites is deficient in both sonic hedgehog(-/-) and in Zic2(kd/kd) mutant mouse embryos, providing further evidence that these Zic genes are upstream regulators of Hedgehog-mediated Myf5 activation. Myf5 activation in newly forming somites is delayed in Zic2 mutant embryos until the time of Zic1 activation, and both Zic2 and Myf5 require noggin for their activation.