Modular long-range regulation of Myf5 reveals unexpected heterogeneity between skeletal muscles in the mouse embryo

The myogenic factor Myf5 plays a key role in muscle cell determination, in response to signalling cascades that lead to the specification of muscle pr ogenitor cells. We have adopted a YAC transgenic approach to identify regul atory sequences that direct the complex spatiotemporal expression of this g ene during myogenesis in the mouse embryo, Important regulatory regions wit h distinct properties are distributed over 96 kb upstream of the Myf5 gene. The proximal 23 kb region directs early expression in the branchial arches , epaxial dermomyotome and in a central part of the myotome, the epaxial in tercalated domain. Robust expression at most sites in the embryo where skel etal muscle forms depends on an enhancer-like sequence located between -58 and -48 kb from the Myf5 gene, This element is active in the epaxial and hy paxial myotome, in limb muscles, in the hypoglossal chord and also at the s ites of Myf5 transcription in prosomeres p1 and p4 of the brain, However la ter expression of Myf5 depends on a more distal region between -96 and -63 kb, which does not behave as an enhancer. This element is necessary for exp ression in head muscles but strikingly only plays a role in a subset of tru nk muscles, notably the hypaxially derived ventral body muscles and also th ose of the diaphragm and tongue, Transgene expression in limb muscle masses is not affected by removal of the -96/-63 region. Epaxially derived muscle s and some hypaxial muscles, such as the intercostals and those of the limb girdles, are also unaffected. This region therefore reveals unexpected het erogeneity between muscle masses, which may be related to different facets of myogenesis at these sites. Such regulatory heterogeneity may underlie th e observed restriction of myopathies to particular muscle subgroups.