January 15, 2006;
Maternal XTcf1 and XTcf4 have distinct roles in regulating Wnt target genes.
Wnt signaling pathways have essential roles in developing embryos and adult tissue
, and alterations in their function are implicated in many disease processes including cancers. The major nuclear transducers of Wnt signals are the Tcf/LEF family of transcription factors, which have binding sites for both the transcriptional co-repressor groucho, and the co-activator beta-catenin. The early Xenopus embryo
expresses three maternally inherited Tcf/LEF mRNAs, and their relative roles in regulating the expression of Wnt target genes are not understood. We have addressed this by using antisense oligonucleotides to deplete maternal XTcf1
mRNAs in oocytes. We find that XTcf1
represses expression of Wnt target genes ventrally and laterally, and activates their expression dorsally. Double depletions of XTcf1
suggest that they act cooperatively to repress Wnt target genes ventrally. In contrast, XTcf4
has no repressive role but is required to activate expression of Xnr3
cells at the gastrula
stage. This work provides evidence for distinct roles for XTcfs in regulating Wnt target gene expression.
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Fig. 7. The effects of maternal XTcf4 depletion. (A) Wnt target gene expression in whole embryos at the late blastula (stage 9.5) and early gastrula stage (stage 10.5). Real time RT-PCR showing the relative expression of chordin, Xhex, siamois, and Xnr3. Embryos were uninjected controls (uninj), injected with 4 ng XTcf4.1 oligo (XTcf4–), or injected with 4 ng XTcf4.1 oligo and 48 h later with 12.5 pg XTcf4A mRNA (XTcf4–+RNA). XTcf4 depletion causes a loss of expression of Wnt target genes, particularly chordin, Xhex, and Xnr3, changes that are partially rescued by the re-introduction of XTcf4 mRNA. (B) The phenotypes of tailbud stage embryos injected as oocytes with 7 ng XTcf4.1 oligo (red) compared to uninjected control siblings (brown). The ventralized phenotype is partially rescued by the introduction of 12.5 pg XTcf4A + 4B mRNA (blue). (C) Chordin expression extends further ventrally in XTcf1-depleted embryos (center) than in uninjected controls (left). Chordin expression is greatly diminished in XTcf4-depleted embryos (right). Vegetal view of stage 10.5 embryos, dorsal at top. (D) Chordin expression is much diminished in XTcf1-depleted early gastrulae compared to uninjected controls at the early gastrula stage. (E) Vegetal view of stage 10.5 control embryos (left) and XTcf1− embryos (right), dorsal at top. β-galactosidase mRNA injected into cell B1 at the 32-cell stage marks dorsal tissue in the gastrula, visualized in red. The red stain colocalizes with the most intense expression of chordin, visualized in blue, in all the embryos, showing that there has been no reversal of dorsoventral polarity as a result of XTcf1 depletion.