In the vertebrate embryo, skeletal muscle is derived from the myotome of the somites. Notch1-3 demonstrate overlapping and distinct expression patterns in mouse somites. Notch1 and Notch2 have been shown to be inhibitors of skeletal myogenesis. The current data demonstrate that Notch3 also is an effective inhibitor of MyoD induced myogenesis. Numb, an adaptor protein that promotes Notch degradation by recruiting the E3 ubiquitin ligase, Itch, is limited in expression to dividing cells of the dorsal medial lip of the dermomyotome and the myotome itself. Here the specificity of the four protein isoforms of Numb for the Notch receptors was examined. In transcription and myogenic differentiation assays, Notch1 was consistently negatively regulated by all four Numb isoforms, and Notch3 was not a target for Numb. Notch2 however was variably affected. Subsequent analyses showed that unlike Notch1, that Notch3 was not polyubiquitinated, nor degraded when co-expressed in cells with Numb. These data provide the first observations that Notch receptors are variably affected by Numb and will be important for the interpretation of the function of Notch and Numb interactions during the development of many different cells and tissues.
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