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 pri or to the establishment of the gastrula mesodermal organizer. Here, we exam ine neural induction in Xenopus embryos in which mesoderm induction has bee n blocked by Cerberus-short, a reagent that specifically inhibits Nodal-rel ated (Xnr) signals. We find that extensive neural structures with cyclopic eyes and brain tissue are formed despite the absence of mesoderm. This neur al induction correlates with the expression of chordin and other BMP inhibi tors-such as noggin, follistatin, and Xnr3-at the blastula stage, and requi res beta -Catenin signaling. Activation of the beta -Catenin pathway by mRN A microinjections or by treatment with LiCl leads to differentiation of neu rons, as well as neural crest, in ectodermal explants. Xnr signals are requ ired for the maintenance, but not for the initiation, of BMP antagonist exp ression. 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 -C atenin, the early activation of expression of secreted BMP antagonists, suc h as Chordin, in a preorganizer region in the dorsal side of the Xenopus bl astula. (C) 2001 Academic Press.