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The molecular basis of vertebrate germ layer formation has been the focus of intense scrutiny for decades, and the inductive interactions underlying this process are well defined. Only recently, however, have studies demonstrated that the regulated inhibition of ectopic germ layer formation is also crucial for patterning the early vertebrate embryo. We report here the characterization of Xema (Xenopus Ectodermally-expressed Mesendoderm Antagonist), a novel member of the Foxi-subclass of winged-helix transcription factors that is involved in the suppression of ectopic germ layer formation in the frog, Xenopus laevis. Xema transcripts are restricted to the animal poleectoderm during early Xenopus development. Ectopic expression of Xema RNA inhibits mesoderm induction, both by growth factors and in the marginal zone, in vivo. Conversely, introduction of antisense morpholino oligonucleotides directed against the Xema transcript stimulates the expression of a broad range of mesodermal and endodermal marker genes in the animal pole. Our studies demonstrate that Xema is both necessary and sufficient for the inhibition of ectopic mesendoderm in the cells of the presumptive ectoderm, and support a model in which Fox proteins function in part to restrict inappropriate germ layer development throughout the vertebrate embryo.
Fig. 2. Expression of Xema during early development. (A) RT-PCR analysis of Xema temporal expression. The `-RT' lane contains all reagents except for reverse transcriptase and was used as a negative control. Ornithine decarboxylase (ODC) was used as a loading control (Bassez et al., 1990). Chordin expression is initiated zygotically and was used as a staging control. (B) Whole-mount in situ hybridization of early gastrula (stage 10+), midgastrula (stage 11), and early neurula (stage 15) stage embryos, using antisense Xema and Xenopus brachyury (Xbra) probes. Xema expression is seen as a blue stain throughout the animal pole of gastrula-stage albino embryos; Xema expression is excluded from the marginal zone (denoted by arrows at stage 11) and vegetal pole. Expression of the panmesodermal marker Xbra is found only in the marginal zone of gastrula stage embryos and was used as a control. Xema is expressed in the vental ectoderm (epidermis) of early neurula stage embryos (ventral view); expression is excluded from the neural plate (dorsal view). (C) RT-PCR analysis of Xema expression in early gastrula stage explants. EF1-α was used as a loading control (Krieg et al., 1989). Xbra is a panmesodermal marker at this stage (Smith et al., 1991), chordin is a dorsal endomesodermal marker (Sasai et al., 1994), and Xwnt8 is a ventrolateral marker (Christian et al., 1991; Smith and Harland, 1991).
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