Physiological and stem cell compartmentalization in the adult Drosophila midgut
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The precise and systematic digestion and subsequent absorption of nutrients in food is vitally important for the life of all metazoans. The intestine is the organ primarily responsible for absorbing, processing, transporting, and excreting the vast variety of nutrients found in an organism’s diet, and the tissue is maintained throughout its length by multipotent intestinal stem cells. Although previous work identified regions of preferential copper and iron absorption, only recent work is revealing the degree to which regions of the tissue can be specialized. The work presented in this thesis categorizes the extent of physiological, morphological, organizational and molecular heterogeneity along the intestinal anterior-posterior axis. This work is also the first to test stem cells of specific intestinal regions for their tumorigenic capacity and their ability to generate progeny of neighboring regions. I analyzed the properties of intestinal cells along the anterior-posterior axis and found that the tissue contains more specialized regions than the medial zones specialized for iron and copper absorption. By analyzing nutrient staining, cell morphology, and gene expression, I identified at least 10 distinct midgut subregions, and developed a system by which they may be easily identified. Gut regions were precisely isolated using lines with region specific expression that we recovered from the Janelia GAL4 collection, and I performed RNAseq to identify localized candidate digestive pathways. The existence of significant physiological diversity along the length of the midgut raised the question of whether ISCs can support all midgut cells or if they are region specific. Like the differentiated cells of the intestinal epithelium, stem cells also vary regionally in behavior and gene expression, suggesting that they contribute to midgut sub-specialization. Using lineage tracing in regionally restricted GAL4 and protein trap lines I show that stem cells generate progeny located outside their own subregion at only one of six borders tested, suggesting that midgut subregions resemble cellular compartments involved in tissue development. Additionally, tumors generated by disrupting stem cell mediated Notch signaling arose preferentially in three subregions and tumor cells also appeared to respect regional borders. Thus, apparently similar intestinal stem cells differ regionally in cell production, gene expression and in the ability to spawn tumors. This work not only provides new tools for studying the drosophila intestine, but also suggests that heterogeneity and compartmentalization may also exist in mammalian tissues containing many apparently similar stem cells. Such work may enable us to acquire the knowledge needed to better treat digestive diseases, as well as approach similar stem cell potency studies in other stem cell based tissues.