The Intestinal Epithelium

The mammalian intestine is a prototype of a self-renewing organ. The rapid cellular turnover is supported by proliferating progenitors located in the intestinal crypt, which is also the stem cell location. Homeostasis of the intestinal epithelium has to be under stringent control to ensure lifelong self-renewal. Processes like proliferation and differentiation (often deregulated in cancer) seem to be controlled by a relative small number of signaling pathways, including Wnt and Notch. Notch signaling is playing a crucial role in the maintenance of the intestinal homeostasis by controlling progenitors proliferation and their cell fate. In this thesis project we studied Notch signaling and its role in the murine intestine focusing on 1) Its downstream mediators; 2) The identification of the physiological Notch ligands; 3) Its role on the stem cell compartment, and 4) the potential cross talk between Notch and Wnt. Furthermore, 5) we investigated the role of Notch signaling in Wnt induced intestinal tumorigenesis. This study evidenced that Hes1 is not the unique Notch signaling mediator in the intestinal epithelium. Moreover the transcription factor Klf4 is not required for goblet cell differentiation. The in vivo role of the Notch ligands Dll1, Dll4 and Jag1 was assessed deriving inducible intestine specific single and double gene-targeted mice. Inactivation of individual ligands did not show any loss of Notch phenotype with the exception of Dll1 mutant mice. Only by deriving double mutants we identified Dll1 and Dll4 as the physiological Notch ligands in the intestine. We provided evidence for the first time that Notch1 signaling is active in intestinal stem cell. Furthermore we demonstrated that Notch signaling is required for the maintenance of intestinal stem cells, as well as for the transit-amplifying progenitor compartment. Using different genetic gain and loss-off function approaches we investigated the relation between the Notch and Wnt cascade for the maintenance of the proliferative crypt compartment. A synergistic relation was observed between Notch and Wnt in intestinal tumorigenesis. Collectively our data highlight Notch signaling as a key player for the maintenance of intestinal homeostasis.