Transdifferentiation of esophageal smooth to skeletal muscle is myogenic bHLH factor-dependent

Previously, coexpression of smooth and skeletal differentiation markers, bu t not myogenic regulatory factors (MRFs), was observed from E16.5 mouse fet uses in a small percentage of diaphragm level esophageal muscle cells, sugg esting that MRFs are not involved in the process of initiation of developme ntally programmed transdifferentiation in the esophagus, To investigate smo oth-to-skeletal esophageal muscle transition, we analyzed Myf5nlacZ knock-i n mice, MyoD-lacZ and myogenin-lacZ transgenic embryos with a panel of the antibodies reactive with myogenic regulatory factors (MRFs) and smooth and skeletal muscle markers. We observed that lacZ-expressing myogenic precurso rs were not detected in the esophagus before E15.5, arguing against the hyp othesis that muscle precursor cells populate the esophagus at an earlier st age of development. Rather, the expression of the MRFs initiated in smooth muscle cells in the upper esophagus of E15.5 mouse embryos and was immediat ely followed by the expression of skeletal muscle markers. Moreover, transd ifferentiation was markedly delayed or absent only in the absence of Myf5, suggesting that appropriate initiation and progression, of smooth-to-skelet al muscle transdifferentiation is Myf5-dependent. Accordingly, the esophagu s of Myf5(-/-):MyoD(-/-) embryos completely failed to undergo skeletal myog enesis and consisted entirely of smooth muscle. Lastly, extensive prolifera tion of muscularis precursor cells, without programmed cell death, occurred concomitantly with esophageal smooth-to-skeletal muscle transdifferentiati on. Taken together, these results indicate that transdifferentiation is the fate of all smooth muscle cells in the upper esophagus and is normally ini tiated by Myf5.