XWNT-8B - A MATERNALLY EXPRESSED XENOPUS WNT GENE WITH A POTENTIAL ROLE IN ESTABLISHING THE DORSOVENTRAL AXIS

In amphibian embryos, establishment of dorsal-ventral asymmetry is bel ieved to involve dorsal-ventral differences in vegetally derived mesod erm-inducing signals and/or differences in the competence of animal he misphere (ectodermal) cells to respond to these signals, Previous stud ies have shown that certain Wnt proteins can generate an ectopic dorsa l axis when misexpressed, and that they do so by modifying the respons e of ectodermal cells to inducers, None of these Wnt proteins are expr essed at an appropriate time to do so in vivo, In this study, we descr ibe the isolation and characterization of a full length cDNA for the X enopus Wnt gene, Xwnt-8b, whose biological activity and expression pat tern suggest that it may be involved in establishment of the dorsovent ral axis, Both maternal and zygotic Xwnt-8b transcripts undergo altern ative splicing to generate mRNAs which encode two different forms of X wnt-8b protein, During early cleavage stages Xwnt-8b transcripts are c onfined primarily to animal hemisphere blastomeres, while zygotically derived Xwnt-8b transcripts are restricted almost exclusively to a ban d of cells in the prospective forebrain of neurula and tailbud stage e mbryos, Ectopically expressed Xwnt-8b can completely rescue dorsal dev elopment of embryos ventralized by exposure to ultraviolet light, and can induce a complete secondary axis in wild-type embryos. Axis induct ion is observed only if Xwnt-8b is supplied prior to the onset of zygo tic gene transcription, This biological activity, together with the pr esence of maternal Xwnt-8b transcripts in cells that will be induced t o form the dorsal mesoderm, is consistent with the possibility that Xw nt-8b may be the endogenous agent that establishes asymmetry in the re sponse of ectodermal cells to mesoderm-inducing signals, thereby initi ating dorsal development.