Anterior endomesoderm specification in Xenopus by Wnt/beta-catenin and TGF-beta signalling pathways

In Xenopus, XHex and cerberus are early marker genes of the anterior endome soderm (AE), a subset of endoderm cells fated to form the liver and foregut and implicated in head induction. Using XHex and cerberus as markers we ha ve examined the signals underlying AE induction. We show that the AE is spe cified by the early blastula in the absence of mesodermal signals but that cell-cell contact between presumptive AE cells is required. In overexpressi on experiments maternal Wnt/beta-catenin and TGF-beta signals (Vg1, Xnr1-2) can induce ectopic XHex and cerberus. Inhibiting these pathways with domin ant interfering signalling components blocks endogenous XHex and cerberus e xpression. We assess the role of signals from the organiser and show that t he BMP antagonists noggin and chordin are important for maintaining XHex an d cerberus expression. Finally, ventral injection of XHex mRNA can induce e ctopic cerberus. Our results indicate that endodermal and mesodermal patter ning are closely coordinated and that the AE is likely to be specified by t he combined action of dorsal Wnt/beta-catenin signals and endoderm-specific factors mediated by TGF-beta signalling. These results provide a starting point for understanding the molecular events underlying the progressive det ermination of endodermally derived organs, such as the liver and foregut. ( C) 1999 Academic Press.