Maternal XTcf1 and XTcf4 have distinct roles in regulating Wnt target genes

AbstractWnt 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 β-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 and XTcf4 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 and XTcf3 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 and chordin in organizer cells at the gastrula stage. This work provides evidence for distinct roles for XTcfs in regulating Wnt target gene expression

VegT initiates endoderm formation

During cleavage stages, maternal VegT mRNA and protein are localized to the Xenopus embryo's vegetal region from which the endoderm will arise and where several zygotic gene transcripts will be localized. Previous loss-of-function experiments on this T-box transcription factor suggested a role for VegT in Xenopus endoderm formation. Here, we test whether VegT is required to initiate endoderm formation using a loss of function approach. We find that the endodermal genes, Bix1, Bix3, Bix4, Milk (Bix2), Mix.1, Mix.2, Mixer, Xsox17α, Gata4, Gata5, Gata6 and endodermin, as well as the anterior endodermal genes Xhex and cerberus, and the organizer specific gene, Xlim1, are downstream of maternal VegT. We also find that the TGFβs, Xnr1, Xnr2, Xnr4 and derrière rescue expression of these genes, supporting the idea that cell interactions are critical for proper endoderm formation. Additionally, inhibitory forms of Xnr2 and Derrière blocked the ability of VegT mRNA injection to rescue VegT-depleted embryos. Furthermore, a subset of endodermal genes was rescued in VegT-depleted vegetal masses by induction from an uninjected vegetal mass. Finally, we begin to establish a gene hierarchy downstream of VegT by testing the ability of Mixer and Gata5 to rescue the expression of other endodermal genes. These results identify VegT as the maternal regulator of endoderm initiation and illustrate the complexity of zygotic pathways activated by VegT in the embryo's vegetal region