No guts, no glory : EphB mediated signaling in intestinal stem and progenitor cells
Abstract: The work presented in this thesis is focused on the intestinal stem cell niche, the crypts, and the Eph tyrosine kinase receptors together with their ligands that are expressed by the crypt epithelial progenitor cells. Two fundamental cellular processes, high turnover coupled to a rapid ordered migration of the newborn cells out from the crypt, characterize the normal intestine. Transformed epithelial cells mirror these processes, gaining an advantage over the untransformed cells by the capacity of going through seemingly endless rounds of division, as well as expanding spatially into surrounding tissues by increased motility. The B class Eph receptors are involved in regulating both proliferation and migration in normal as well as transformed intestinal epithelial cells. In Paper I, we show that EphB receptors, in addition to directing cell migration, regulate proliferation in the intestine. We use both loss and gain of function experiments to correlate the EphB forward signaling to the rate of progenitor cell proliferation. EphB signaling promotes cell cycle reentry of progenitor cells and accounts for up to half the mitogenic activity in the adult intestine. We suggest a model where the restricted Wnt source plays an important role in establishing the EphBephrin- B gradients, hence extending the proliferative domain beyond the region directly influenced by the Wnt proteins themselves. In Paper II we continue to investigate the dual role of EphB receptors. We show that cell migration and proliferation are controlled independently by the EphB2 receptor. EphB2 regulated cell positioning is kinase-independent and mediated via PI3-kinase, whereas EphB2 tyrosine kinase activity regulates cell proliferation through an Abl-cyclin D1 pathway. Cyclin D1 regulation becomes uncoupled from EphB signaling during the progression from adenoma to colon carcinoma, allowing continued proliferation with invasive growth. The dissociation of EphB2 signaling pathways enables the selective inhibition of the mitogenic signaling pathway without affecting the tumor suppressor function of EphB2. In Paper III we investigate the role of the two ligands expressed in the intestinal epithelium, ephrin-B1 and ephrin-B2 and show that, although both ligands can bind EphB2 and EphB3, they have distinct functional outcomes when activating the receptors. Ephrin-B1 selectively affects migration, without influencing proliferation, whereas ephrin-B2 affects proliferation, but not migration.
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