Analysis of engulfment and cell corpse processing by epithelial cells in the Drosophila ovary

Abstract

Engulfment of dead cells by epithelial cells is crucial for the health of an organism. Defective engulfment can lead to serious conditions such as retinitis pigmentosa and asthma. In the Drosophila melanogaster ovary, protein starvation induces apoptotic cell death of germline cells, which are then engulfed by adjacent epithelial follicle cells. The follicle cells synchronously enlarge approximately four-to-five fold as they engulf the dying germline, suggesting that significant changes are required. However, the molecular changes needed to drive enlargement and engulfment by epithelial cells are still poorly understood. In this dissertation, I determined the role of integrins in engulfment, the interactions between the core engulfment machinery and the corpse processing pathway, and the roles of GTPases in engulfment by epithelial cells. First, I found that the integrin heterodimer, αPS3/βPS, becomes apically enriched and is required in the epithelial follicle cells for engulfment. αPS3/βPS is trafficked in a polarized fashion using much of the same machinery as migrating cells, suggesting similarities between engulfing and migrating cells. The canonical corpse processing pathway has been well-characterized, however, little is known about how the core engulfment machinery interacts with the corpse processing pathway. I found that the phagocytic receptor Draper is present on the phagocytic cup and early phagosomes, whereas integrins are maintained on the cell surface. Engulfment mutants fell into three distinct categories based on their specific effects on internalization and phagosome maturation, suggesting that components of the core engulfment machinery are required for distinct steps of corpse processing. Last, I investigated the roles of the Rho family GTPases during engulfment. Strikingly, Rac2 becomes induced during engulfment whereas Rac1 does not change its expression pattern. Both Rac1 and Rac2 are required for engulfment, however, Rac1 has defects in both enlargement and vesicle uptake whereas Rac2 only has defects in enlargement. Furthermore, I found that Rac1 and Rac2 may have differential effects on the Jun kinase signaling pathway, which suggests complexity in the regulatory network controlling engulfment in epithelial cells. Together, this work has provided a greater understanding of the molecular changes required within epithelial cells for proper engulfment.