MyoD stimulates delta-1 transcription and triggers notch signaling in the Xenopus gastrula.

The Notch signaling cascade is involved in many developmental decisions, a paradigm of which has been the selection between epidermal and neural cell fates in both invertebrates and vertebrates. Notch has also been implicated as a regulator of myogenesis, although its precise function there has remained controversial. Here we show that the muscle-determining factor MyoD is a direct, positive regulator of the Notch ligand Delta-1 in prospective myoblasts of the pre-involuted mesoderm in Xenopus gastrulae. Injection of a dominant MyoD repressor variant ablates mesodermal Delta-1 expression in vivo. Furthermore, MyoD-dependent Delta-1 induction is sufficient to activate transcription from promoters of E(spl)-related genes in a Notch-dependent manner. These results indicate that a hallmark of neural cell fate determination, i.e. the feedback loop between differentiation promoting basic helix-loop-helix proteins and the Notch regulatory circuitry, is conserved in myogenesis, supporting a direct involvement of Notch in muscle determination.