COMBINATORIAL SIGNALING BY XWNT-11 AND XNR3 IN THE ORGANIZER EPITHELIUM

The epithelium of the Spemann organizer plays an important role in emb ryonic axis formation and transplantation experiments have shown that epithelial organizer cells have potent axis-inducing potential. Known axis-inducing molecules like noggin and chordin are not expressed in t he epithelium and cannot account for its inductive properties. Xwnt-11 is expressed in the epithelium but has only poor dorsalizing activity . In an expression screen for genes that are able to functionally coop erate with Xwnt-11 we have identified a cDNA encoding Xenopus nodal-re lated 3 (Xnr3), a member of the TGF-beta family, coexpressed with Xwnt -11 in the organizer epithelium. Xwnt-11 and Xnr3 act highly cooperati vely in inducing secondary embryonic axes and dorsalizing ventral meso derm. Xwnt-11/Xnr3 interfere with BMP signalling without themselves in ducing chordin or noggin. The results indicate that induction by the o rganizer epithelium may result from the combinatorial action of instru ctive Xnr3 and permissive Xwnt-11 signalling.