Neural induction in the absence of mesoderm: beta-catenin-dependent expression of secreted BMP antagonists at the blastula stage in Xenopus.

A growing body of work indicates that neural induction may be initiated prior to the establishment of the gastrula mesodermal organizer. Here, we examine neural induction in Xenopus embryos in which mesoderm induction has been blocked by Cerberus-short, a reagent that specifically inhibits Nodal-related (Xnr) signals. We find that extensive neural structures with cyclopic eyes and brain tissue are formed despite the absence of mesoderm. This neural induction correlates with the expression of chordin and other BMP inhibitors-such as noggin, follistatin, and Xnr3-at the blastula stage, and requires beta-Catenin signaling. Activation of the beta-Catenin pathway by mRNA microinjections or by treatment with LiCl leads to differentiation of neurons, as well as neural crest, in ectodermal explants. Xnr signals are required for the maintenance, but not for the initiation, of BMP antagonist expression. Recent work has demonstrated a role for beta-Catenin signaling in neural induction mediated by the transcriptional down-regulation of BMP-4 expression. The present results suggest an additional function for beta-Catenin, the early activation of expression of secreted BMP antagonists, such as Chordin, in a preorganizer region in the dorsal side of the Xenopus blastula.