Wnt and FGF pathways cooperatively pattern anteroposterior neural ectoderm in Xenopus.

Previous gain-of-function assays in Xenopus have demonstrated that Xwnt-3a can pattern neural tissue by reducing the expression of anterior neural genes, and elevating the expression of posterior neural genes. To date, no loss-of-function studies have been conducted in Xenopus to show a requirement of endogenous Wnt signaling for patterning of the neural ectoderm along the anteroposterior axis. We report that expression of a dominant negative Wnt in Xenopus embryos causes a reduction in the expression of posterior neural genes, and an elevation in the expression of anterior neural genes, thereby confirming the involvement of endogenous Wnt signaling in patterning the neural axis. We further demonstrate that the ability of Xwnt-3a to decrease expression of anterior neural genes in noggin-treated explants is dependent upon a functional FGF signaling pathway, while the elevation of expression of posterior neural genes does not require FGF signaling. The previously reported ability of FGF to elevate the expression of posterior neural genes in noggin-treated explants was found to be dependent on endogenous Wnt signaling. We conclude that neural induction occurs initially in a Wnt-independent manner, but that generation of complete anteroposterior neural pattern requires the cooperative actions of Wnt and FGF pathways.