Genesis of muscle fiber-type diversity during mouse embryogenesis relies on Six1 and Six4 gene expression.

Adult skeletal muscles in vertebrates are composed of different types of myofibers endowed with distinct metabolic and contraction speed properties. Genesis of this fiber-type heterogeneity during development remains poorly known, at least in mammals. Six1 and Six4 homeoproteins of the Six/sine oculis family are expressed throughout muscle development in mice, and Six1 protein is enriched in the nuclei of adult fast-twitch myofibers. Furthermore, Six1/Six4 proteins are known to control the early activation of fast-type muscle genes in myocytes present in the mouse somitic myotome. Using double Six1:Six4 mutants (SixdKO) to dissect in vivo the genesis of muscle fiber-type heterogeneity, we analyzed here the phenotype of the dorsal/epaxial muscles remaining in SixdKO. We show by electron microscopy analysis that the absence of these homeoproteins precludes normal sarcomeric organization of the myofiber leading to a dystrophic aspect, and by immunohistochemistry experiments a deficiency in synaptogenesis. Affymetrix transcriptome analysis of the muscles remaining in E18.5 SixdKO identifies a major role for these homeoproteins in the control of genes that are specifically activated in the adult fast/glycolytic myofibers, particularly those controlling Ca(2+) homeostasis. Absence of Six1 and Six4 leads to the development of dorsal myofibers lacking expression of fast-type muscle genes, and mainly expressing a slow-type muscle program. The absence of restriction of the slow-type program during the fetal period in SixdKO back muscles is associated with a decreased HDAC4 protein level, and subcellular relocalization of the transcription repressor Sox6. Six genes thus behave as essential global regulators of muscle gene expression, as well as a central switch to drive the skeletal muscle fast phenotype during fetal development.

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