AbstractsBiology & Animal Science

Wnt/??-catenin signaling is highly conserved in metazoans and plays a variety of essential roles during embryonic development and adult homeostasis. Wnt signaling causes nuclear translocation of ??-catenin, which then complexes with DNA-binding transcription factors (TFs) to regulate target gene expression. Members of the T-Cell Factor (TCF) family are the best studied TFs of the Wnt pathway. TCFs regulate Wnt targets through a transcriptional switch, repressing transcription in the absence of signaling, while activating target gene expression when bound with ??-catenin. Vertebrates have several TCFs, which are specialized for either basal repression (e.g., TCF3) or ??-catenin-dependent activation (e.g, TCF1) of Wnt-dependent cis-regulatory modules (W-CRMs). It has been suggested that Wnt/??-catenin signaling promotes an exchange of repressive and activating TCFs on W-CRMs. This model sheds light on how specialized TCFs coordinate to regulate transcription. However, how this switch operates is not fully understood. In my thesis work, I have attempted to advance our understanding of the TCF switch by examining the novel roles of a T-box protein, VegT, in the TCF transcriptional switch, using a W-CRM controlling the dorsal organizer gene Siamois in Xenopus embryos. We demonstrated VegT directly binds to the Siamois W-CRM via several T-box sites, which mediate repression of the Siamois reporter. VegT interacts with TCF3 specifically, and cooperates with TCF3 to repress Siamois. Knocking down VegT using morpholinos resulted in vegetal expansion of the organizer genes, suggesting a repressive role of VegT. The VegT-TCF3 interaction is inhibited by homeodomain interacting protein kinase 2 (HIPK2), a kinase shown to decrease TCF3 chromatin binding. In addition to their repressive role, the T-box sites also contribute to activation, and VegT remains bound to Wnt target gene chromatin after HIPK2 activation, suggesting an activating role of VegT. T-box sites are also observed in the Vent2 W-CRM. VegT also binds to this locus, suggesting a more general role of VegT in regulating Wnt targets. Together, my findings suggest VegT is a new factor in the TCF transcriptional switch, which plays a dual role in both repression and activation. This adds a new level to our understanding of TCF transcriptional regulation in vertebrates.