Repression of XMyoD expression and myogenesis by Xhairy-1 in Xenopus earlyembryo

Activated Notch-Delta signalling was shown to inhibit myogenesis, but wheth er and how it regulates myogenic gene expression is not clear. We analyzed the implication of Xenopus hairy-1 (Xhairy-1), a member of the hairy and en hancer-of-split (E(spl)) family that may function as nuclear effector of No tch signalling pathway, in regulating XMyoD gene expression at the initial step of myogenesis. Xhairy-1 transcripts are expressed soon after mid-blast ula transition and exhibits overlapping expression with Notch pathway genes such as Delta-1 in the posterior somitic mesoderm. We show that overexpres sion of Xhairy-1 blocks the expression of XMyoD in early gastrula ectoderma l cells treated with the mesoderm-inducing factor activin, and in the mesod erm tissues of early embryos. It inhibits myogenesis and produces trunk def ects at later stages. Xhairy-1 also inhibits the expression of the pan-meso dermal marker Xbra, but expression of other early mesoderm markers such as goosecoid and chordin is not affected. These effects require the basic heli x-loop-helix (bHLH) domain, as well as a synergy between the central Orange domain and the C-terminus WRPW-Groucho-interacting domain. Furthermore, ov erexpression in ectodermal cells of Xhairy-1/VP16, in which Xhairy-1 repres sor domain is replaced by the activator domain of the viral protein VP16, i nduces the expression of XMyoD in the absence of protein synthesis. Interes tingly, Xhairy-1/VP16 does not induce the expression of Xbra and XMyf5 in t he same condition. During neurulation, the expression of XMyoD induced by X hairy-1/VP16 declines and the expression of muscle actin gene was never det ected. These results suggest that Notch signalling through hairy-related ge nes may specifically regulate XMyoD expression at the initial step of myoge nesis in vertebrates. (C) 2001 Elsevier Science Ireland Ltd. All rights res erved.