title: The Xenopus homeobox gene twin mediates Wnt induction of goosecoid in establishment of Spemann's organizer.
abstract: We describe the isolation of the Xenopus homeobox gene twin (Xtwn), which was identified in an expression cloning screen for molecules with dorsalizing activities. Injection of synthetic Xtwn mRNA restores a complete dorsal axis in embryos lacking dorsal structures and induces a complete secondary dorsal axis when ectopically expressed in normal embryos. The sequence homology, expression pattern and gain-of-function phenotype of Xtwn is most similar to the previously isolated Xenopus homeobox gene siamois (Xsia) suggesting that Xtwn and Xsia comprise a new subclass of homeobox genes important in dorsal axis specification. We find that Xtwn is able to activate the Spemann organizer-specific gene goosecoid (gsc) via direct binding to a region of the gsc promoter previously shown to mediate Wnt induction. Since Xtwn expression is strongly induced in ectodermal (animal cap) cells in response to overexpression of a dorsalizing Wnt molecule, we examined the possibility that Xtwn might be a direct target of a Wnt signal transduction cascade. First, we demonstrate that purified LEF1 protein can interact, in vitro, with consensus LEF1/TCF3-binding sites found within the Xtwn promoter. Second, these binding sites were shown to be required for Wnt-mediated induction of a Xtwn reporter gene containing these sites. As LEF1/TCF3 family transcription factors have previously been shown to directly mediate Wnt signaling, these results suggest that Xtwn induction by Wnt may be direct. Finally, in UV-hyperventralized embryos, expression of endogenous Xtwn is confined to the vegetal pole and a Xtwn reporter gene is hyperinduced vegetally in a LEF1/TCF3-binding-site-dependent manner. These results suggest that cortical rotation distributes Wnt-like dorsal determinants to the dorsal side of the embryo, including the dorsal marginal zone, and that these determinants may directly establish Spemann's organizer in this region.
authors.affiliation: Department of Developmental and Cell Biology, and the Developmental Biology Center, University of California, Irvine 92697-2300, USA.
authors.forename: M N
authors.initials: MN
authors.lastname: Laurent
authors.forename: I L
authors.initials: IL
authors.lastname: Blitz
authors.forename: C
authors.initials: C
authors.lastname: Hashimoto
authors.forename: U
authors.initials: U
authors.lastname: Rothbächer
authors.forename: K W
authors.initials: KW
authors.lastname: Cho
mesh_terms: D028341:Activins; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D003598:Cytoskeletal Proteins / Q000235:genetics / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D004625:Embryo, Nonmammalian; D018507:Gene Expression Regulation, Developmental; D051956:Goosecoid Protein; D018398:Homeodomain Proteins / Q000235:genetics* / Q000378:metabolism; D007265:Inhibins / Q000235:genetics / Q000378:metabolism; D036341:Intercellular Signaling Peptides and Proteins; D051821:Lymphoid Enhancer-Binding Factor 1; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic; D011506:Proteins / Q000235:genetics / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D012045:Regulatory Sequences, Nucleic Acid; D012097:Repressor Proteins; D015534:Trans-Activators; D014157:Transcription Factors / Q000378:metabolism; D051153:Wnt Proteins; D014981:Xenopus / Q000196:embryology / Q000235:genetics*; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; D051176:beta Catenin
chemical_list: C495266:CTNNB1 protein, Xenopus; D003598:Cytoskeletal Proteins; D004268:DNA-Binding Proteins; D051956:Goosecoid Protein; D018398:Homeodomain Proteins; D036341:Intercellular Signaling Peptides and Proteins; D051821:Lymphoid Enhancer-Binding Factor 1; D011506:Proteins; D011518:Proto-Oncogene Proteins; D012097:Repressor Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D051153:Wnt Proteins; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; D051176:beta Catenin; C100866:cer1 protein, Xenopus; C109979:sia2 protein, Xenopus; D028341:Activins; D007265:Inhibins