p38 MAP kinase regulates the expression of XMyf5 and affects distinct myogenic programs during Xenopus development.

The p38 MAPK signaling pathway is essential for skeletal muscle differentiation in tissue culture models. We demonstrate a novel role for p38 MAPK in myogenesis during early Xenopus laevis development. Interfering with p38 MAPK causes distinct defects in myogenesis. The initial expression of Myf5 is selectively blocked, while expression of MyoD is unaffected. Expression of a subset of muscle structural genes is reduced. Convergent extension movements are prevented and segmentation of the paraxial mesoderm is delayed, probably due to the failure of cells to withdraw from the cell cycle. Myotubes are properly formed; however, at later stages, they begin to degenerate, and the boundaries between somites disappear. Significant apoptotic cell death occurs in most parts of the somites. The ventral body wall muscle derived from migratory progenitor cells of the ventral somite region is poorly formed. Our data indicate that the developmental defects caused by p38alpha-knockdown were mediated by the loss of XMyf5 expression. Thus, this study identifies a specific intracellular pathway in which p38 MAPK and Myf5 proteins regulate a distinct myogenic program.