Xbra3 induces mesoderm and neural tissue in Xenopus laevis

Homologues of the murine Brachyury gene have been shown to be involved in mesoderm formation in several vertebrate species. In frogs, the Xenopus Brachyury homologue, Xbra, is required for normal formation of posterior mesoderm. We report the characterisation of a second Brachyury homologue from Xenopus, Xbra3, which has levels of identity with mouse Brachyury similar to those of Xbra. Xbra3 encodes a nuclear protein expressed in mesoderm in a temporal and spatial manner distinct from that observed for Xbra. Xbra3 expression is induced by mesoderm-inducing factors and overexpression of Xbra3 can induce mesoderm formation in animal caps. In contrast to Xbra, Xbra3 is also able to cause the formation of neural tissue in animal caps. Xbra3 overexpression induces both geminin and Xngnr-1, suggesting that Xbra3 can play a role in the earliest stages of neural induction. Xbra3 induces posterior nervous tissue by an FGF-dependent pathway; a complete switch to anterior neural tissue can be effected by the inhibition of FGF signalling. Neither noggin, chordin, follistatin, nor Xnr3 is induced by Xbra3 to an extent different from their induction by Xbra nor is BMP4 expression differentially affected.