Senescence is a form of cell-cycle arrest linked to tumor suppression and aging. However, it remains controversial and has not been documented in nonpathologic states. Here we describe senescence as a normal developmental mechanism found throughout the embryo, including the apical ectodermal ridge (AER) and the neural roof plate, two signaling centers in embryonic patterning. Embryonic senescent cells are nonproliferative and share features with oncogene-induced senescence (OIS), including expression of p21, p15, and mediators of the senescence-associated secretory phenotype (SASP). Interestingly, mice deficient in p21 have defects in embryonic senescence, AER maintenance, and patterning. Surprisingly, the underlying mesenchyme was identified as a source for senescence instruction in the AER, whereas the ultimate fate of these senescent cells is apoptosis and macrophage-mediated clearance. We propose that senescence is a normal programmed mechanism that plays instructive roles in development, and that OIS is an evolutionarily adapted reactivation of a developmental process.

Notch has been linked to ␤-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/␤-catenin (down-regulated when blocking Wnt/␤-catenin) that are directly regulated by Notch (repressed by ␥-secretase inhibitors and up-regulated by active Notch1 in the absence of ␤-catenin signaling). We demonstrate that Notch is downstream of Wnt in colorectal cancer cells through ␤-catenin-mediated transcriptional activation of the Notch-ligand Jagged1. Consistently, expression of activated Notch1 partially reverts the effects of blocking Wnt/␤-catenin pathway in tumors implanted s.c. in nude mice. Crossing APC Min/؉ with Jagged1 ؉/⌬ 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 ␤-catenin. We show that this mechanism is operating in human tumors from Familial Adenomatous Polyposis patients. We conclude that Notch activation, accomplished by ␤-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. beta-catenin ͉ APC ͉ intestine ͉ crosstalk

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.

Specific deletion of Notch1 and RBPjκ in the mouse results in abrogation of definitive haematopoiesis concomitant with the loss of arterial identity at embryonic stage. As prior arterial determination is likely to be required for the generation of embryonic haematopoiesis, it is difficult to establish the specific haematopoietic role of Notch in these mutants. By analysing different Notch‐ligand‐null embryos, we now show that Jagged1 is not required for the establishment of the arterial fate but it is required for the correct execution of the definitive haematopoietic programme, including expression of GATA2 in the dorsal aorta. Moreover, successful haematopoietic rescue of the Jagged1‐null AGM cells was obtained by culturing them with Jagged1‐expressing stromal cells or by lentiviral‐mediated transduction of the GATA2 gene. Taken together, our results indicate that Jagged1‐mediated activation of Notch1 is responsible for regulating GATA2 expression in the AGM, which in turn is essential for definitive haematopoiesis in the mouse.