Guardian of the Gut: The intestine is able to tolerate continual exposure to large amounts of commensal bacteria and foreign food antigens without triggering an inappropriate inflammatory immune response. In the large intestine, this immunological tolerance is thought to occur via a physical separation between environment and host imposed by a continuous mucous layer built up from the secreted mucin protein, MUC2. However, in the small intestine, this mucous layer is porous, necessitating an additional layer of immune control. Shan et al. (p. 447 , published online 26 September; see the Perspective by Belkaid and Grainger ) now report that in the small intestine, MUC2 plays an active role in immunological tolerance by activating a transcription factor in resident dendritic cells, thereby selectively blocking their ability to launch an inflammatory response. This work identifies MUC2 as a central mediator of immune tolerance to maintain homeostasis in the gut and possibly at other mucosal surfaces in the body.

Notch has been linked to β-catenin-dependent tumorigenesis; however, the mechanisms leading to Notch activation and the contribution of the Notch pathway to colorectal cancer is not yet understood. By microarray analysis, we have identified a group of genes downstream of Wnt/β-catenin (down-regulated when blocking Wnt/β-catenin) that are directly regulated by Notch (repressed by γ-secretase inhibitors and up-regulated by active Notch1 in the absence of β-catenin signaling). We demonstrate that Notch is downstream of Wnt in colorectal cancer cells through β-catenin-mediated transcriptional activation of the Notch-ligand Jagged1. Consistently, expression of activated Notch1 partially reverts the effects of blocking Wnt/β-catenin pathway in tumors implanted s.c. in nude mice. Crossing APC ^{Min /+} with Jagged1 ^+/Δ mice is sufficient to significantly reduce the size of the polyps arising in the APC mutant background indicating that Notch is an essential modulator of tumorigenesis induced by nuclear β-catenin. We show that this mechanism is operating in human tumors from Familial Adenomatous Polyposis patients. We conclude that Notch activation, accomplished by β-catenin-mediated up-regulation of Jagged1, is required for tumorigenesis in the intestine. The Notch-specific genetic signature is sufficient to block differentiation and promote vasculogenesis in tumors whereas proliferation depends on both pathways.

The generation of new neurons from neural stem cells in the adult hippocampal dentate gyrus contributes to learning and mood regulation. To sustain hippocampal neurogenesis throughout life, maintenance of the neural stem cell pool has to be tightly controlled. We found that the Notch/RBPJκ-signaling pathway is highly active in neural stem cells of the adult mouse hippocampus. Conditional inactivation of RBPJκ in neural stem cellsin vivoresulted in increased neuronal differentiation of neural stem cells in the adult hippocampus at an early time point and depletion of the Sox2-positive neural stem cell pool and suppression of hippocampal neurogenesis at a later time point. Moreover, RBPJκ-deficient neural stem cells displayed impaired self-renewalin vitroand loss of expression of the transcription factor Sox2. Interestingly, we found that Notch signaling increases Sox2 promoter activity and Sox2 expression in adult neural stem cells. In addition, activated Notch and RBPJκ were highly enriched on the Sox2 promoter in adult hippocampal neural stem cells, thus identifying Sox2 as a direct target of Notch/RBPJκ signaling. Finally, we found that overexpression of Sox2 can rescue the self-renewal defect in RBPJκ-deficient neural stem cells. These results identify RBPJκ-dependent pathways as essential regulators of adult neural stem cell maintenance and suggest that the actions of RBPJκ are, at least in part, mediated by control of Sox2 expression.

Phosphorylation of Notch proteins has been indirectly correlated with Notch activation and nuclear translocation as well as cellular transformation. There is evidence that the Wnt signaling pathway, which results in glycogen synthase kinase-3β (GSK-3β) inhibition, cross-talks with the Notch pathway. In this study, we show that GSK-3β is able to bind and phosphorylate Notch2 in vitro and in vivo. We identify three specific phosphorylation sites in the Notch2 serine/threoninerich domain that are dependent on GSK-3β activity. Phosphorylation of the serine/threonine-rich domain has been shown previously to be crucial in regulating cytokine-specific cell differentiation. Coimmunoprecipitation experiments show that full-length Notch2 binds more efficiently than intracellular Notch2 to GSK-3β. Nevertheless, only the processed Notch2 is a substrate for the kinase, thus suggesting that GSK-3β-dependent phosphorylation may be specifically regulating the activated Notch molecule. Consistent with this, GSK-3β inhibits the transcriptional activation of Notch target genes both in vitro and in vivo, whereas lithium chloride treatment or Wnt-1 overexpression that results in GSK-3β inhibition leads to the up-regulation of the Hes-1 promoter. Together, our results suggest that cross-talk between Notch and Wnt pathways may be partially mediated by specific regulation of GSK-3β-dependent Notch phosphorylation.

Definitive hematopoiesis in the mouse embryo originates from the aortic floor in the P-Sp/AGM region in close association with endothelial cells. An important role for Notch1 in the control of hematopoietic ontogeny has been recently established, although its mechanism of action is poorly understood. Here, we show detailed analysis of Notch family gene expression in the aorta endothelium between embryonic day (E) 9.5 and E10.5. Since Notch requires binding to RBPjκ transcription factor to activate transcription, we analyzed the aorta of the para-aortic splanchnopleura/AGM in RBPjκ mutant embryos. We found specific patterns of expression of Notch receptors, ligands and Hes genes that were lost in RBPjκ mutants. Analysis of these mutants revealed the absence of hematopoietic progenitors, accompanied by the lack of expression of the hematopoietic transcription factors Aml1/Runx1, Gata2 and Scl/Tal1. We show that in wild-type embryos, a few cells lining the aorta endothelium at E9.5 simultaneously expressed Notch1 and Gata2, and demonstrate by chromatin immunoprecipitation that Notch1 specifically associated with the Gata2 promoter in E9.5 wild-type embryos and 32D myeloid cells, an interaction lost in RBPjκ mutants. Consistent with a role for Notch1 in regulating Gata2, we observe increased expression of this gene in 32D cells expressing activated Notch1. Taken together, these data strongly suggest that activation of Gata2 expression by Notch1/RBPjκ is a crucial event for the onset of definitive hematopoiesis in the embryo.