Mechanism of niche formation, and impact of niche position on tissue function

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Project Summary/Abstract Understanding how niches regulate stem cells is critical to human health because aberrant regulation by the niche can cause tumor formation or tissue atrophy. Well-studied niches form in predictable structures following reproducible morphogenetic changes, suggesting niche structure is regulated and functionally important. This has not been directly studied. Additionally, the mechanisms that determine the initial positioning of niches during development remain understudied. To address these deficits, we study the Drosophila posterior signaling center (PSC)—the niche of the larval lymph gland. The PSC maintains hematopoietic progenitors, and it induces differentiation of a special immune cell upon immune challenge. The PSC forms during embryogenesis: its cells migrate dorsally, where they ultimately reside, coalesced at the lymph gland posterior. Preliminary data indicates that mutants without visceral mesoderm (vm) have dispersed PSCs and fewer PSC cells. As such, the overall hypothesis is that the vm guides PSC positioning to the lymph gland posterior. We further hypothesize that a coalesced PSC is required for its optimal function as a niche. Aim 1 will identify the role of vm and its mechanism of action in instructing PSC formation via live-imaging of vm mutants to identify the timing and type of the PSC defect, and then testing vm candidate cues with vm-specific RNAi knockdown of the cue followed by analysis of PSC positioning. The PSC signal transducer will be identified by PSC-specific RNAi knockdown of candidate transducers. Ectopic expression of the vm cue in a nearby tissue will reveal if the cue is a true guidance cue sufficient to guide PSC positioning, or whether it confers competency to respond to other positional cues. Aim 2 will investigate how PSC coalescence contributes to niche function by causing PSC dispersion and then assessing PSC functions. Ability of the dispersed PSC to maintain progenitors will be tested by quantitating PSC- dependent progenitors, and ability to generate an immune response by challenging larvae with parasitoid wasp infection, and then quantitating a specialized immune cell type. The PSC can sense the organism’s nutrient and immune environment, but it is unknown if it senses feedback from the cells it regulates. This will be tested with lineage-specific ablation of progenitors or mature hemocytes in the lymph gland with Gal4/UAS-driven apoptosis. Then the level of PSC maintenance and differentiation signals will be measured for comparison to controls. Accomplishing these aims will reveal principles of niche formation, how niche structure impacts its function, and whether a niche receives feedback from the tissue it supports. Research training will take place at the University of Pennsylvania under the advisement of the Sponsor, the PI’s thesis committee, and Penn faculty as needed. The training plan consists of an integrated and creatively unique sequence of mentorship experiences to prepare the PI for their career. Training includes several opportunities to improve written and verbal communication, and professional development activities (course, conferences, seminars) for the PI to transition to a new research topic for an academic postdoctoral fellowship.