Overlapping functions of SIX homeoproteins during embryonic myogenesis.

Four SIX homeoproteins display a combinatorial expression throughout embryonic developmental myogenesis and they modulate the expression of the myogenic regulatory factors. Here, we provide a deep characterization of their role in distinct mouse developmental territories. We showed, at the hypaxial level, that the Six1:Six4 double knockout (dKO) somitic precursor cells adopt a smooth muscle fate and lose their myogenic identity. At the epaxial level, we demonstrated by the analysis of Six quadruple KO (qKO) embryos, that SIX are required for fetal myogenesis, and for the maintenance of PAX7+ progenitor cells, which differentiated prematurely and are lost by the end of fetal development in qKO embryos. Finally, we showed that Six1 and Six2 are required to establish craniofacial myogenesis by controlling the expression of Myf5. We have thus described an unknown role for SIX proteins in the control of myogenesis at different embryonic levels and refined their involvement in the genetic cascades operating at the head level and in the genesis of myogenic stem cells. Author summary: We demonstrate with double, triple and quadruple Six KO mouse embryos that specific Six combinations are required for proper myogenesis depending on the level of the mouse embryonic body axis. We show that the Six1 and Six2 genes are required for craniofacial myogenesis by controlling the engagement of unsegmented cranial paraxial mesodermal cells in the myogenic pathway. We also show that hypaxial somitic dermomyotomal cells from embryos mutant for the Six1 and Six4 genes are unable to engage in the skeletal muscle lineage. Last, we show that embryos mutant for the four Six genes expressed in the myogenic lineage exhibit a defect in self-renewal of PAX7+ stem cells present in their residual muscle masses, and that SIX proteins interact directly with several enhancer elements at the Pax7 locus to control its expression. We have thus characterized new functions of SIX proteins in the control of myogenesis at different embryonic levels and refined their involvement in the genetic cascades that govern the genesis of myogenic stem cells. [ABSTRACT FROM AUTHOR]