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The progression from the undifferentiated state to the fully differentiated skeletal myocyte is orchestrated by a highly complex network of transcription factors whose expression is regulated by several families of molecules. However, not much is known about the early events that lead to the expression of muscle progenitor markers. This series of studies aims to further understand the mechanisms which regulate commitment to the myogenic lineage. In Chapter 2, Sox 15 was found to be sufficient for the expression of pre-skeletal mesodermal markers, but failed to upregulate the expression of the myogenic regulatory factors while Sox7 was able to induce the entire myogenic program. Cells overexpressing Sox15 show elevated levels of genes that inhibit differentiation and promote proliferation, which coincides with the lack of progression past the muscle precursor stage. These results demonstrate that Sox15 and Sox7 have differential roles in regulating skeletal myogenesis. Chapter 3 demonstrates that the Wnt signaling pathway positively regulates the expression of Foxc1/2. This result was further extended by the demonstration that beta-catenin occupies the chromatin upstream of Foxc1 during differentiation. Loss of functional Gli2 and Meox1 led to the downregulation of Foxc1/2, while misexpression of Gli2 upregulated the expression of these two genes. These studies have helped us define the pathways that regulate Foxc1/2 expression, and more specifically that functional beta-catenin is required for Foxc1/2 expression. Chapter 4 investigates the contribution of soluble signaling molecules to the regulation of Sox7 expression. Treatment with retinoic acid accelerated Sox7 and Wnt3a expression, and activation of the Wnt pathway led to upregulation of Sox7, most likely by direct interaction of beta-catenin with the promoter, as demonstrated by chromatin immunoprecipitation. It is likely that retinoic acid acts upstream of the Wnt signaling pathway, since it was unable to overcome the repression of Sox7 expression by a non-functional beta-catenin. Therefore, Sox7 can be placed downstream of both the Wnt pathway and retinoic acid signaling during myogenesis in P19 cells. Theses studies have provided a clearer understanding of the pathways that regulate the expression of genes involved in the regulation muscle precursor cell fate. |
