The Notch and Wnt signaling pathways are important for cell fate decisions in many contexts, including some in which the two pathways oppose one another. In some cases, Wnt signaling represses Notch signaling to result in a “Wnt-ON/Notch-OFF” state, but the mechanism by which this occurs is unclear. Upon binding to an extracellular ligand, such as Jagged or Delta, the transmembrane receptor Notch is cleaved by γ-secretase and the Notch intracellular domain (NICD) translocates into the nucleus, where it cooperates with the transcription factor RBPJκ (recombination signal binding protein for immunoglobulin κ J region) to activate transcription of target genes. Collu et al. report that Wnt-mediated inhibition of Notch signaling can occur through direct inhibition of RBPJκ by Dishevelled (Dvl). Dvl is a scaffold protein that recruits the β-catenin destruction complex to activated Wnt receptors, which inhibits the activity of the destruction complex to promote the stability and ultimately the nuclear accumulation of β-catenin. Collu et al. found that expression of a Notch signaling–responsive luciferase reporter in cultured neuroblastoma cells was reduced by expression of a transgene encoding Wnt1 or by treatment of the cells with conditioned medium from Wnt1-expressing cells, and reducing the abundance of Dvl2 by RNA interference prevented this Wnt1-mediated reduction in Notch reporter expression. Whereas overexpression of Wnt1 or Dvl2 reduced the transcriptional activity of an activated form of Notch, expression of a transgene encoding a stabilized form of β-catenin did not. In Chinese hamster ovary (CHO) cells, overexpression of Dvl2 inhibited the activity of all four human Notch proteins but did not prevent proteolytic processing of Notch or nuclear translocation of NICD. Both endogenous and tagged versions of the Dvl2 and RBPJκ proteins coimmunoprecipitated from CHO cell extracts, and overexpression of Wnt1 or Dvl2 reduced the abundance of RBPJκ in nuclear extracts. In Xenopus laevis (frog) embryos, injection of morpholino oligonucleotides targeting XDvl2 and injection of XDvl2 mRNA had opposite effects on Notch-mediated fate specification of ciliated epidermal cells. Overexpression of β-catenin had no effect on the specification of these cells. Increased abundance of XDvl2 also reduced the expression of an endogenous Notch target gene and rescued a dominant Notch gain-of-function phenotype in frog embryos. Both fly and frog orthologs of Dvl2 inhibited the ability of the corresponding species-specific RBPJκ orthologs to activate expression of a luciferase reporter in CHO cells. These results suggest a model in which Dvl inhibits Notch signaling by preventing the formation of NICD-RBPJκ transcriptional activator complexes.
G. M. Collu, A. Hidalgo-Sastre, A. Acar, L. Bayston, C. Gildea, M. K. Leverentz, C. G. Mills, T. W. Owens, O. Meurette, K. Dorey, K. Brennan, Dishevelled limits Notch signalling through inhibition of CSL. Development 139, 4405–4415 (2012). [Abstract] [Full Text]