ACTIVATION OF SIAMOIS BY THE WNT PATHWAY

The Wnt pathway and the recently described Xenopus homeobox gene Siamo is share the ability to mimic Nieuwkoop organizer signaling activity w hen ectopically expressed, prompting us to ask whether the Wnt pathway can activate Siamois expression. We show that activators of the Wnt p athway, and the dorsalizing substance LiCl, activate Siamois expressio n in Xenopus animal caps. Furthermore, Siamois expression is shown to be activated by beta-catenin cell-autonomously. Although certain TGF-b eta family members, such as Vg1, are potent dorsal mesoderm inducers, the TGF-beta pathway only weakly activated Siamois. Finally, we demons trate that Siamois expression is eliminated from the marginal zone of UV-irradiated Xenopus embryos, supporting the hypothesis that cortical rotation moves a Wnt-like activity into the dorsal region. These resu lts suggest that formation of the Nieuwkoop organizer is dependent on activation of Siamois by a localized Wnt-like signaling activity, whic h may act synergistically with a vegetally localized TGF-beta signal. (C) 1996 Academic Press, Inc.