Notch-Regulated Mechanisms of Epithelial Cell Fate Selection in the Intestine.

Throughout the lifetime of an organism, progenitor cells in the intestine proliferate and differentiate to form cells of the secretory and absorptive lineages. Many intercellular signaling pathways, including the Notch pathway, coordinate to develop and maintain the intestine. My thesis work has investigated how Notch signaling regulates intestinal cell fate using several novel genetically engineered and pharmacological mouse models. Developmental analysis of a transgenic mouse model with forced expression of the Notch-regulated transcription factor Mouse atonal homolog 1 (Math1) showed increased numbers of all secretory cell types and loss of absorptive cells, demonstrating that Math1 is the key factor regulating intestinal secretory cell differentiation. Furthermore, these data suggest that Math1 can redirect a bipotential progenitor cell to the secretory cell fate. To study the role of Math1 in adult intestine, I describe an inducible Math1 transgenic model; however, Math1 protein was not increased subsequent to transgene activation, suggesting that there may be active degradation of Math1 protein in the intestine. Previous studies have shown that inhibition of Notch signaling resulted in decreased epithelial cell proliferation and altered cell fate, suggesting that a stem or progenitor cell is targeted by Notch signaling; however, the identity of this target was unknown. Pharmacological inhibition of Notch signaling in both fetal and adult intestine showed that expression of Olfactomedin 4 (Olfm4), a crypt base columnar stem cell gene, was markedly decreased upon Notch inhibition. Transcriptional studies in the human colon cancer cell line LS174T confirmed that Notch signaling activated OLFM4 gene expression and identified a region containing critical cis-regulatory DNA elements. Finally, I made the novel observation that a population of intermediate cells that express both goblet and Paneth cell markers emerged upon Notch inhibition in adult mouse ileum and colon. In conclusion, my thesis research has shown that Math1 is the key regulator of secretory cell differentiation in the intestine and that Notch signaling directly targets the crypt base columnar stem cell. Collectively, these studies have provided important information about Notch-regulated mechanisms of intestinal development and cell lineage determination.