Abstract:
A stem cell niche is the microenvironment in which a stem cell receives the necessary signals to fulfill its specific roles to support homeostasis of its tissue. The niche of the colonic epithelium in both mice and humans are found within crypts of Lieberkühn, which are test-tube shaped invaginations comprised of several hundred cells which fulfill a variety of roles. A typical mouse colon is lined by hundreds of thousands of these stereotyped niche units, which replace their own equivalent in cells approximately once a week, making the colon one of the most proliferative tissues in the developed body. This makes it an excellent model for understanding epithelial stem cell maintenance in a mammalian context. The colon is also prone to inflammatory bowel diseases and cancers, making it a critical research target to unravel mechanisms that may underlie these disease states. Despite these factors, the colon remains a secondary target of research for murine gastrointestinal epithelial homeostasis, with the bulk of characterizing research being performed in the small intestine. A primary goal of my thesis is to lay out analysis methodology and then elucidate key molecular tissue patterns within colon crypt homeostasis to lay the groundwork for more comprehensive modeling in the mouse colon. There are two important characteristic cell types. The first are stem cells identified by Lgr5 expression and the second are secretory support cell population identified by Reg4 expression. I use transgenic reporter mice to characterize the cells that express these key markers. I further go on to ablate the Reg4+ lineage to identify perturbations within the niche. I identified a paradoxical shift in signaling that, which results in increased Notch pathway activation during ablation of Reg4+ cells, the primary supplier of Notch ligands in the colon. I also characterized remodeling of stromal cell populations within the mesenchyme surrounding crypts, implicating Reg4+ cell ablation as a mediator of epithelial-mesenchymal signaling. This is a critical avenue for future research, as mesenchymal remodeling is linked to chronic colon disease, yet no strong models exist to explore the role and mechanisms of epithelial-to-mesenchymal crosstalk.