Résumé
Maternal Wnt/beta-Catenin signaling establishes a program of dorsal-specific gene expression required for axial patterning in Xenopus. We previously reported that a subset of dorsally expressed genes depends not only on Wnt/beta-Catenin stimulation, but also on a MyD88-dependent Toll-like receptor/IL1-receptor (TLR/IL1-R) signaling pathway. Here we show that these two signal transduction cascades converge in the nucleus to coactivate gene transcription in blastulae through a direct interaction between beta-Catenin and NF-kappa B proteins. A transdominant inhibitor of NF-kappa B, Delta NI kappa B alpha, phenocopies loss of MyD88 protein function, implicating Rel/NF-kappa B proteins as selective activators of dorsal-specific gene expression. Sensitive axis formation assays in the embryo demonstrate that dorsalization by Wnt/beta-Catenin requires NF-kappa B protein activity, and vice versa. Xenopus nodal-related 3 (Xnr3) is one of the genes with dual beta-Catenin/NF-kappa B input, and a proximal NF-kappa B consensus site contributes to the regional activity of its promoter. We demonstrate in vitro binding of Xenopus beta-Catenin to several XRel proteins. This interaction is observed in vivo upon Wnt-stimulation. Finally, we show that a synthetic luciferase reporter gene responds to both endogenous and exogenous beta-Catenin levels in an NF-kappa B motif dependent manner. These results suggest that beta-Catenin acts as a transcriptional co-activator of NF-kappa B-dependent transcription in frog primary embryonic cells.