Dissecting the Mechanisms and the Nutritional Requirements of the Expansion of Normal and Tumorigenic Intestinal Stem Cell Lineages in Drosophila melanogaster

Organisms are constantly challenged by injury and senescence. To replenish lost cells, adult tissue must contain a cellular reserve to maintain homeostasis. For example, adult stem cells exhibit two unique properties, self-renewal and potency, that make them well suited for this role. Tight regulatory control of stem cell properties is critical to maintain homeostatic balance; misregulation often leads to tumors or diseases. Recently, homeostasis of the intestinal epithelium has been shown to depend on a population of molecular defined intestinal stem cells: ISCs) in both Drosophila and mammals. However, the regulatory mechanisms controlling ISC behaviors remain poorly defined. We took advantage of the molecular genetic approaches available in Drosophila melanogaster to investigate how ISC properties are regulated by both intrinsic and extrinsic factors. In the course of the thesis, we identified two conserved signaling pathways that regulate ISC properties intrinsically. First, we demonstrated that WNT/APC signaling is an important regulator of ISC proliferation and relatively dispensable for potency. Parallel studies in mammals also show that loss of APC: APClof) in the ISC lineage leads to hyperplasia, supporting a model whereAPClofISCs can function as a cell-of-origin for intestinal tumors. In follow-up studies, we have taken a genetic approach to identify modifiers that control the expansion of tumorigenicAPCloflineages. We have now identified Insulin signaling as a modifier that is necessary and sufficient for the expansion of tumorigenicAPCloflineages. Future studies will define the precise cellular and molecular mechanisms underlying this phenomenon. A second key finding of our studies is that JAK/STAT signaling pathway coordinates ISC proliferation and regulates the competency for daughter cell differentiation. In summary, ISC properties are regulated by conserved WNT/APC and JAK/STAT signaling similar to what has been shown in other stem cell lineages. The intestinal tract is a primary site of nutrient absorption. We hypothesized that dietary nutrients can extrinsically regulate ISC properties. However, the precise requirement of individual macronutrients or micronutrients could not be investigated because a chemically defined food: CDF) consisting entirely of purified compounds was not available. We developed a new CDF media that permits CDFs with different nutrient compositions at iso-caloric density to be generated. Using this CDF recipe, we defined the individual macronutrient requirements for development, reproduction and longevity of Drosophila. Ongoing experiments are focused on determining the role of macronutrients in regulating ISC properties. We also seek to determine if nutrients are differentially required for normal and tumorigenic ISC lineages. Findings from this nutritional study may identify new therapeutic targets for regenerative medicine or cancer therapy