Wnt signaling in Xenopus embryos inhibits Bmp4 expression and activates neural development

We report a new role for Wnt signaling in the vertebrate embryo: the induct ion of neural tissue from ectoderm. Early expression of mouse wnt8, Xwnt8, beta-catenin, or dominant-negative GSK3 induces the expression of neural-sp ecific markers and inhibits the expression of Bmp4 in Xenopus ectoderm. We show that Wnt8, but not the BMP antagonist Noggin, can inhibit Bmp4 express ion at early gastrula stages. Furthermore, inhibition of beta-catenin activ ity in the neural ectoderm of whole embryos by a truncated TCE results in a decrease in neural development. Therefore, we suggest that a cleavage-stag e Wnt signal normally contributes to an early repression of Bmp4 on the dor sal side of the embryo and sensitizes the ectoderm to respond to neural ind ucing signals from the organizer. The Wnt targets Xnr3 and siamois have bee n shown previously to have neuralizing activity when overexpressed. However , antagonists of Wnt signaling, dnXwnt8 and Nxfrz8, inhibit Wnt-mediated Xn r3 and siamois induction, but not neural induction, suggesting an alternati ve mechanism for Bmp repression and neuralization. Conversely, dnTCF blocks both Wnt-mediated Xnr3 and neural induction, suggesting that both pathways require this transcription factor.