Expression of beta-catenin in the developing chick myotome is regulated bymyogenic signals

The developmental signals that govern cell specification and differentiatio n in vertebrate somites are well understood. However, little is known about the downstream signalling pathways involved. We have shown previously that a combination of Shh protein and Wnt1 or Wnt3a-expressing fibroblasts is s ufficient to activate skeletal muscle-specific gene expression in somite ex plants, Here, we have examined the molecular mechanisms by which the Wnt-me diated signal acts on myogenic precursor cells. We show that chick frizzled 1 (Fz1), beta -catenin and Lef1 are expressed during somitogenesis, Lef1 a nd beta -catenin transcripts become restricted to the developing myotome, F urthermore, beta -catenin is expressed prior to the time at which MyoD tran scripts can be detected. Expression of beta -catenin mRNA is regulated by p ositive and negative signals derived from neural tube, notochord and latera l plate mesoderm, These signals include Bmp4, Shh and Wnt1/Wnt3a itself. In somite explants, Fz1, beta -catenin and Lef1 are expressed prior to activa tion of myogenesis in response to Shh and Wnt signals. Thus, our data show that a combination of Shh and Wnt1 upregulates expression of Wnt pathway co mponents in developing somites prior to myogenesis. Thus, Wnt1 could act th rough beta -catenin on cells in the myotome.