Xbra3 induces mesoderm and neural tissue in Xenopus laevis

Homologues of the murine Brachyury gene have been shown to be involved in m esoderm formation in several vertebrate species. In frogs, the Xenopus Brac hyury homologue, Xbra, is required for normal formation of posterior mesode rm. We report the characterisation of a second Brachyury homologue from Xen opus, Xbra3, which has levels of identity with mouse Brachyury similar to t hose of Xbra. Xbra3 encodes a nuclear protein expressed in mesoderm in a te mporal and spatial manner distinct from that observed for Xbra. Xbra3 expre ssion is induced by mesoderm-inducing factors and overexpression of Xbra3 c an 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 ove rexpression induces both geminin and Xngnr-1, suggesting that Xbra3 can pla y a role in the earliest stages of neural induction. Xbra3 induces posterio r 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 di fferent from their induction by Xbra nor is BMP4 expression differentially affected. (C) 2000 Academic Press.