SYNERGISTIC EFFECTS OF VG1 AND WNT SIGNALS IN THE SPECIFICATION OF DORSAL MESODERM AND ENDODERM

In amphibians, dorsoventral asymmetry is established by cortical rotat ion, a cytoplasmic rearrangement in the egg which activates a dorsal d eterminant on one side of the zygote. This determinant has been propos ed to be either Vg1, an endodermally derived molecule that can directl y induce ectoderm to form dorsal mesoderm, or a member of the Wnt fami ly, which patterns the ectoderm such that it forms dorsal mesoderm in response to ventral inductive signals. In this study, we have investig ated whether the endogenous dorsal determinant(s) functions as a direc t inducer of dorsal mesoderm (Vg1-like) or whether it acts to pattern the response of ectoderm to inductive signals (Wnt-like). We report he re that cortical rotation enhances both the dorsal-inductive activity of endodermal cells and the response of ectodermal cells to endogenous inductive signals and that both of these activities are required for notochord induction in ectoderm/endoderm recombinants. While ectopical ly expressed Xwnt-8b can substitute for the dorsalizing signals activa ted in either ectoderm or endoderm, and can allow notochord formation in recombinants, Vg1 alone is not sufficient to induce notochord in ec todermal explants in the absence of signals activated by cortical rota tion. Coexpression of Xwnt-8b along with Vg1 restores ectodermal compe tence to form notochord. Finally, in endodermal explants, ectopically expressed Xwnt-8b, but not Vg1, can divert the fate of ventral endoder mal cells along a dorsal pathway. Thus, while Vg1 is most likely requi red for induction of mesoderm in vivo, our data suggest that a materna l Wnt-like signal acts synergistically with Vg1 to specify a dorsal fa te not only in the mesoderm, but also in the endoderm. (C) 1996 Academ ic Press, Inc.