Investigation of non-autonomous control of cell death and corpse clearance in the ovary of Drosophila melanogaster

Abstract

Cell death is a fundamental aspect of development and homeostasis; its dysregulation is commonly associated with disease. Historically, apoptosis has been the most heavily studied type of cell death, but there are many other non-apoptotic forms of cell death. The Drosophila ovary provides a powerful in vivo model to study non-apoptotic cell death. Each egg chamber in the ovary contains 15 nurse cells that support an oocyte throughout development, and at the end of oogenesis the nurse cells are surrounded by stretch follicle cells and undergo non-apoptotic cell death. The work in this dissertation investigated the role of stretch follicle cells in nurse cell death. Genetic ablation of the stretch follicle cells revealed that they are required for multiple nurse cell death events including the transport of cytoplasm to the oocyte and DNA fragmentation. We found that phagocytic machinery is required in the stretch follicle cells for the acidification and elimination of nurse cells, suggesting nurse cells die by phagoptosis. Furthermore, live imaging and a transgenic engulfment detector demonstrated that nurse cells are not engulfed piece-wise despite the requirement of phagocytosis machinery, but are instead surrounded and acidified extracellularly. To determine the mechanism driving nurse cell acidification, we performed a targeted RNAi screen against lysosome-associated genes. Using tissue-specific RNAi, we demonstrated that the V-ATPase proton pump is required in the stretch follicle cells for nurse cell acidification. GFP fusion proteins and antibody staining revealed that V-ATPases become enriched and localize to the stretch follicle cell plasma membranes to acidify the nurse cells that they surround. Following acidification, the stretch follicle cells were found to release cathepsins, lysosomal proteases, to break down and degrade the nurse cell. To uncover novel pro-death proteins that mediate signaling between the stretch follicle cells and nurse cells, we utilized proximity-dependent protein labeling and identified proteins enriched in the stretch follicle cells. Altogether this work uncovers a new role for lysosomal machinery acting at the plasma membrane of stretch follicle cells to drive nurse cell death, and identifies pro-death proteins in the stretch follicle cells that promote nurse cell death.