Regulation of Intestinal Epithelial Cell Proliferation

PROJECT SUMMARY The intestinal epithelium possesses one of the highest self-renewing capacities among tissues due to the presence and functions of active intestinal stem cells (aISCs). Concurrently, aISCs are susceptible to injury due to various pathological conditions because of their high proliferative capability. Recent studies have shown that subpopulations of intestinal epithelial cells such as reserve stem cells (e.g., those marked by Bmi1, Hopx1, Lrig1, mTert), progenitors (Atoh1, Dll1), and several terminally differentiated cell types (enteroendocrine, Paneth, enterocyte) become activated in the aftermath of injury to replenish the basal crypt compartment, which then reconstitutes cell populations of the villi. Additionally, recent studies demonstrated that subpopulations of mesenchymal cells marked by Pdgfra, Gli1, or Cd81 provide essential factors to sustain the renewal of the stem cell niche. Nonetheless, there needs to be a complete understanding of the mechanisms that regulate communication between various cell types and their influence on the regenerative capacity of the intestine. Identifying the mechanisms that coordinate interactions between multiple cell populations (e.g., epithelial cells and mesenchymal cells) in the intestine during regeneration may aid in identifying targets for therapeutic interventions. The Long-Term Goal of this research proposal is to elucidate the mechanisms regulating the activation of and coordination between intestinal epithelial and stromal cells upon γ radiation-induced injury. Our exciting preliminary results showed that Sonic Hedgehog (SHH), normally absent under homeostatic conditions, increases in the intestinal epithelium shortly after irradiation. Furthermore, new preliminary results demonstrate increased levels of HIF1α and KLF4 in the intestinal epithelium upon injury and their ability to induce Shh promoter activity by direct binding to its promoter. SHH ligands then induce Gli1 expression in stromal cells and stimulate their proliferation. In turn, activated stromal cells secrete WNT ligands, promoting the expansion of Bmi1+ cells to repopulate the intestinal crypts. Inhibition of SHH and WNT signaling, in vitro and in vivo, abrogates regeneration of the intestinal epithelium post-irradiation. Based on these compelling and exciting findings, our Central Hypothesis is that precise spatiotemporal communication between intestinal epithelial and stromal cells is fundamental for regenerating the intestinal epithelium following radiation injury. To test this hypothesis, we designed three Specific Aims: (SA1) to investigate the mechanism that regulates the induction of Sonic Hedgehog (SHH) signaling in intestinal epithelial cells upon irradiation; (SA2) to identify and characterize the stromal cell population(s) promoting intestinal regeneration; and (SA3) to examine the mechanism underlying the intestinal epithelial cell activation upon stromal stimulation. The proposed studies will advance our understanding of the mechanisms regulating intestinal epithelial regeneration and will have potential therapeutic implications in diseases of the gastrointestinal tract.