Activation ofSiamoisby theWntPathway

Abstract

TheWntpathway and the recently describedXenopushomeobox geneSiamoisshare the ability to mimic Nieuwkoop organizer signaling activity when ectopically expressed, prompting us to ask whether theWntpathway can activateSiamoisexpression. We show that activators of theWntpathway, and the dorsalizing substance LiCl, activateSiamoisexpression inXenopusanimal caps. Furthermore,Siamoisexpression is shown to be activated by β-catenin cell-autonomously. Although certain TGF-β family members, such as Vg1, are potent dorsal mesoderm inducers, the TGF-β pathway only weakly activatedSiamois.Finally, we demonstrate thatSiamoisexpression is eliminated from the marginal zone of UV-irradiatedXenopusembryos, supporting the hypothesis that cortical rotation moves aWnt-like activity into the dorsal region. These results suggest that formation of the Nieuwkoop organizer is dependent on activation ofSiamoisby a localizedWnt-like signaling activity, which may act synergistically with a vegetally localized TGF-β signal.