Xrel3/XrelA attenuates β-catenin-mediated transcription during mesoderm formation in Xenopus embryos.

In Xenopus laevis embryonic development, activation of the Wnt/beta-catenin pathway promotes mesoderm cell fate determination via Xnr (Xenopus nodal-related) expression. We have demonstrated previously that Rel/NF-kappa B (nuclear factor kappa B) proteins expressed in presumptive ectoderm limit the activity of Xnrs to the marginal zone of embryos during mesoderm induction, which assists to distinguish mesoderm from ectoderm. The mechanism of this regulation, however, is unknown. In the present study, we investigated whether Rel/NF-kappa B proteins are able to modulate mesoderm formation by mediating Wnt/beta-catenin signalling. We determined that ectopic expression of XrelA or Xrel3 in the dorsal marginal zone perturbed dorsal mesoderm formation by down-regulating multiple Wnt/beta-catenin target genes including Xnr3, Xnr5 and Xnr6. Ventral co-expression of XrelA or Xrel3 with either wild-type beta-catenin or constitutively active beta-catenins(S37A) abrogated beta-catenin-induced axis duplication and attenuated beta-catenin-stimulated reporter transcription. Lastly, we provide evidence that XreL3, but not XrelA, can interact with beta-catenin without affecting the association of beta-catenin with other transcriptional co-activators in vitro. Both Xrel3 and XrelA, however, prevented the accumulation, in nuclei, of exogenously expressed and endogenous beta-catenin in vivo. These results suggest that Rel proteins are able to bind beta-catenin and attenuate beta-catenin-mediated transcription by nuclear exclusion.