Role of Wnt4 in Chick Myogenesis

The Wnt family of signaling proteins controls cell proliferation, specification and differentiation. Evidence links one family member, Wnt4, to myogenesis. First, expression of Wnt4 in the neural tube borders the region of the somite where myogenesis is initiated and immediately precedes expression of Myf5 and MyoD, the first apparent myogenic markers. Second, ectopic Wnt4 causes differentiation of C2C12 myoblasts into muscle cells. Based on these studies, I hypothesized that Wnt4 acts to promote the differentiation of myogenic precursor cells into muscle in vivo. To test my hypothesis, I overexpressed or knocked down Wnt4 in the chick neural tube and analyzed sections immunostained with myosin heavy chain (MHC), a marker for differentiated muscle. Consistent with my hypothesis, overexpression of Wnt4 led to a significant increase in the area of MHC expression (the myotome). The increase in myotome size could be caused by cellular hypertrophy or an influx of myogenic precursor cells. To distinguish between these two possibilities, I first measured the size of myogenic cells after immunostaining for β-catenin, which outlines the cell surface. I also quantitated the total number of myogenic cells after staining nuclei. Overexpression of Wnt4 leads to a significant increase in cell size, but no change in the number of cells. Conversely, knockdown of Wnt4 caused a significant decrease in myotome size, which was accompanied by a reduction in the number of cells. I am currently exploring the mechanism underlying the decrease in muscle cell number in knockdown embryos.