EXPRESSION OF A DOMINANT-NEGATIVE WNT BLOCKS INDUCTION OF MYOD IN XENOPUS EMBRYOS

During gastrulation of Xenopus embryos the prospective mesoderm is ind uced initially with domains of dorsal and ventral fate, then further p atterned to generate semitic mesoderm by signals from the gastrula org anizer. Although Xwnt-8 is expressed in future ventrolateral mesoderm and induces prospective epidermis to differentiate in vitro as ventral mesoderm, no loss-of-function studies have demonstrated a requirement for Wnt signaling for the normal expression of mesodermal genes in th e gastrula. We report development of a dominant-negative Wnt (dnXwnt-8 ) that inhibits embryonic responses to Wnt signaling in a cell-nonauto nomous fashion. By expressing dnXwnf-8 in Xenopus embryos, we uncover a requirement of Wnt signaling for localized expression in prospective mesoderm of XMyoDa and Xenopus-posterior (Xpo). Because ectopic expre ssion of functional Xwnt-8 in the dorsal marginal zone of the gastrula induces ectopic XMyoDa and Xpo, both gain-of-function and loss-of-fun ction experiments support a model in which endogenous Xwnt-8 functions to induce expression of genes involved in specification of ventral an d semitic mesoderm.