| According to the dominant model describing development in Xenopus laevis, a maternal Wnt pathway is activated in the dorso-vegetal part of the embryo, the Nieuwkoop center. The dorsal axis develops from the Spemann organizer, induced by an extracellular signal from the Nieuwkoop center. Later, zygotic Wnt pathways are involved in the differentiation of specific types of mesoderm, but the accepted view is that Wnts are not mesoderm inducers, like the TGFbeta and FGF pathways.; I addressed the mechanism and effects of Wnt activation in the early Xenopus laevis embryo. The effector of the canonical Wnt pathway is the beta-catenin - Tcf complex. I demonstrate that beta-catenin mutants deleted of the C-terminal activation domain are inactive, but can indirectly activate the pathway through endogenous beta-catenin. The negative and positive effects can be simultaneously visualized in in situ hybridization of injected embryos. Exclusion of the Wnt-inhibited territory from organizer and axial tissue suggested an autonomous role for the Wnt pathway, in contradiction to the Nieuwkoop center model. I show with localized injections of activators and inhibitors of the Wnt pathway that activity of the pathway is autonomously necessary and sufficient for organizer formation, which makes the Nieuwkoop center concept redundant.; Experiments with dominant negative and positive mutants of Xenopus Tcf in the sea urchin revealed an essential role for the Wnt pathway in specifying endomesoderm. Using RT-PCR, in situ hybridization, and transcription assays, I similarly find that two general mesodermal markers, Xbra (Brachyury) and eFGF, depend on a zygotic Wnt activity. This implies the presence of an unknown zygotic Wnt pathway on the dorsal side of the embryo.; In conclusion, my work demonstrates the cell autonomous requirement for various Wnt pathways in establishing the Spemann organizer and the mesodermal layer in early Xenopus embryos, with potentially general implications for understanding early vertebrate development. |
