PROJECT SUMMARY/ABSTRACT Adult stem cells are crucial for regeneration, tissue repair after injury, and developmental processes such as spermatogenesis. These cells ensure both tissue growth and renewal by dividing asymmetrically, producing one daughter stem cell and one daughter that differentiates 1,2. Stem cells exist in a dynamic microenvironment termed the niche that provides signals to ensure the maintenance and self-renewal of the adult stem cell population1–5. An appreciation of the dynamic communication between stem cells and their niche is vital to understand processes such as reproduction, oncogenesis, aging, development, and regeneration. Much is known about signals sent from niche cells and received by stem cells3,6–13; however, little is known about signaling in the opposite direction: from stem cells back to their niche. Here, I use the testis stem cell niche of Drosophila melanogaster to explore signaling from stem cells back to their niche as well as the role of endocytic tumor suppressor genes in this signaling. Endocytosis regulates a myriad of signaling pathways14–18. My preliminary data indicate that knockdown of an endocytic gene in somatic stem cells greatly increases the size of adjacent niche cells and, consequently, the niche itself. Since the way that endocytic genes non-autonomously regulate niche size is unknown, but may apply widely to other niches. In Aim 1, I further characterize this phenotype by assessing the role of endocytosis in mediating signaling within the Drosophila testis stem cell niche. I will specifically knock down genes regulating endocytosis from cyst stem cells and characterize the resultant changes in neighboring niche cells, focusing on niche cell size, morphology, and ploidy. I will also examine the activity of a small set of well-characterized signaling pathways involved in cell growth in wild-type and endocytic mutant testes. I will follow up with genetic loss and gain-of- funditon analysis for pathways with changes such as the JAK-STAT pathway (preliminary data). In Aim 2, I more broadly investigate signaling from other cells to the niche cells by performing a screen of candidate niche cell signaling pathway receptors identified in our recent single-cell transcriptomic datasets. This will greatly expand our understanding of the signaling pathways requred for niche cell fate, function, size and/or quiescence. Overall, these proposed experiments will further illuminate cell signaling dynamics within the Drosophila testis stem cell niche, with implications for niches in other tissues and organisms. I will conduct this work in the lab of Dr. Erika Matunis at the Johns Hopkins School of Medicine, where I have access to over 50 departmental and core facilities and resources including Drosophila media, flow cytometry, microscopy, genetics and genomics, and a supportive mentoring team. I will supplement my research training by strengthening my quantitative skills through workshops and courses, and my...