The Wnt/beta-catenin pathway posteriorizes neural tissue in Xenopus by an indirect mechanism requiring FGF signalling

In order to identify factors involved in posteriorization of the central ne rvous system, we undertook a functional screen in Xenopus animal cap explan ts which involved coinjecting noggin RNA together with pools of RNA from a chick somite cDNA library. In the course of this screen, we isolated a clon e encoding a truncated form of beta -catenin, which induced posterior neura l and dorsal mesodermal markers when coinjected with noggin in animal caps. Similar results were obtained with Xwnt-8 and Xwnt-3a, suggesting that the se effects are a consequence of activating the canonical Wnt signalling pat hway. To investigate whether the activation of posterior neural markers req uires mesoderm induction, we performed experiments using a chimeric inducib le form of beta -catenin. Activation of this protein during blastula stages resulted in the induction of both posterior neural and mesodermal markers, while activation during gastrula stages induced only posterior neural mark ers. We show that this posteriorizing activity occurs by an indirect and no ncell-autonomous mechanism requiring FGF signalling. (C) 2001 Academic Pres s.