Control of non-apoptotic nurse cell death by engulfment genes in Drosophila

Programmed cell death occurs as a normal part of oocyte development in Drosophila. For each egg that is formed, 15 germline-derived nurse cells transfer their cytoplasmic contents into the oocyte and die. Disruption of apoptosis or autophagy only partially inhibits the death of the nurse cells, indicating that other mechanisms significantly contribute to nurse cell death. Recently, we demonstrated that the surrounding stretch follicle cells non-autonomously promote nurse cell death during late oogenesis and that phagocytosis genes including draper, ced-12, and the JNK pathway are crucial for this process. When phagocytosis genes are inhibited in the follicle cells, events specifically associated with death of the nurse cells are impaired. Death of the nurse cells is not completely blocked in draper mutants, suggesting that other engulfment receptors are involved. Indeed, we found that the integrin subunit, αPS3, is enriched on stretch follicle cells during late oogenesis and is required for elimination of the nurse cells. Moreover, double mutant analysis revealed that integrins act in parallel to draper. Death of nurse cells in the Drosophila ovary is a unique example of programmed cell death that is both non-apoptotic and non-cell autonomously controlled.

Components of the Engulfment Machinery Have Distinct Roles in Corpse Processing

Billions of cells die in our bodies on a daily basis and are engulfed by phagocytes. Engulfment, or phagocytosis, can be broken down into five basic steps: attraction of the phagocyte, recognition of the dying cell, internalization, phagosome maturation, and acidification. In this study, we focus on the last two steps, which can collectively be considered corpse processing, in which the engulfed material is degraded. We use the Drosophila ovarian follicle cells as a model for engulfment of apoptotic cells by epithelial cells. We show that engulfed material is processed using the canonical corpse processing pathway involving the small GTPases Rab5 and Rab7. The phagocytic receptor Draper is present on the phagocytic cup and on nascent, phosphatidylinositol 3-phosphate (PI(3)P)- and Rab7-positive phagosomes, whereas integrins are maintained on the cell surface during engulfment. Due to the difference in subcellular localization, we investigated the role of Draper, integrins, and downstream signaling components in corpse processing. We found that some proteins were required for internalization only, while others had defects in corpse processing as well. This suggests that several of the core engulfment proteins are required for distinct steps of engulfment. We also performed double mutant analysis and found that combined loss of draper and αPS3 still resulted in a small number of engulfed vesicles. Therefore, we investigated another known engulfment receptor, Crq. We found that loss of all three receptors did not inhibit engulfment any further, suggesting that Crq does not play a role in engulfment by the follicle cells. A more complete understanding of how the engulfment and corpse processing machinery interact may enable better understanding and treatment of diseases associated with defects in engulfment by epithelial cells.

Polarization of the epithelial layer and apical localization of integrins are required for engulfment of apoptotic cells in the Drosophila ovary

Inefficient clearance of dead cells or debris by epithelial cells can lead to or exacerbate debilitating conditions such as retinitis pigmentosa, macular degeneration, chronic obstructive pulmonary disease and asthma. Despite the importance of engulfment by epithelial cells, little is known about the molecular changes that are required within these cells. The misregulation of integrins has previously been associated with disease states, suggesting that a better understanding of the regulation of receptor trafficking could be key to treating diseases caused by defects in phagocytosis. Here, we demonstrate that the integrin heterodimer αPS3/βPS becomes apically enriched and is required for engulfment by the epithelial follicle cells of the Drosophila ovary. We found that integrin heterodimer localization and function is largely directed by the α-subunit. Moreover, proper cell polarity promotes asymmetric integrin enrichment, suggesting that αPS3/βPS trafficking occurs in a polarized fashion. We show that several genes previously known for their roles in trafficking and cell migration are also required for engulfment. Moreover, as in mammals, the same α-integrin subunit is required by professional and non-professional phagocytes and migrating cells in Drosophila. Our findings suggest that migrating and engulfing cells use common machinery, and demonstrate a crucial role for integrin function and polarized trafficking of integrin subunits during engulfment. This study also establishes the epithelial follicle cells of the Drosophila ovary as a powerful model for understanding the molecular changes required for engulfment by a polarized epithelium.

Summary: Apical integrin localization, mediated by polarized and directed trafficking, is crucial for proper engulfment by epithelial cells.

Detection of Cell Death and Phagocytosis in the Drosophila Ovary

Billions of cells die and are cleared throughout the development and homeostasis of an organism. Either improper death or clearance can lead to serious illnesses. In the adult Drosophila ovary, germline cells can die by programmed cell death (PCD) at three distinct stages; here we focus on cell death that occurs in mid- and late oogenesis. In mid-oogenesis, the germline of egg chambers can undergo apoptosis in response to nutrient deprivation. In late oogenesis, the nurse cells are eliminated through a developmentally regulated, non-apoptotic cell death. In this chapter, we describe several methods to detect cell death and phagocytosis in the Drosophila ovary. DAPI stains the chromatin of all cells and can be used to detect morphological changes in cells that die by different mechanisms. TUNEL labels fragmented DNA, which can occur in both apoptotic and non-apoptotic death. LysoTracker, an acidophilic dye, marks acidic vesicles and some dying cells; therefore, it can be used to study both death and phagocytosis. We also describe several antibodies that can be used to investigate cell death and/or phagocytosis: active caspase Dcp-1, membrane markers, and lamins. Many of these antibodies can be used in combination with GFP fusion transgenes for further analysis; we show Rab5-GFP and Rab7-GFP, which can be used to study phagocytosis in further detail.

Use of necrotic markers in the Drosophila ovary

Necrosis is a form of cell death characterized by cytoplasmic and organelle swelling, compromised -membrane integrity, intracellular acidification, and increased levels of reactive oxygen species (ROS) and cytosolic Ca(2+). In the Drosophila ovary, two distinct forms of cell death occur naturally. In response to starvation, caspase-dependent cell death occurs during mid-oogenesis. Additionally, the nurse cells, which support the developing oocyte, undergo developmental programmed cell death during late oogenesis after they dump their contents into the oocyte. Evidence suggests that necrosis may be playing an important role during developmental programmed cell death of the nurse cells during late oogenesis. Here, we describe several methods to detect events associated with necrosis in the Drosophila ovary. Propidium iodide is used to detect cells with compromised membrane integrity, and H2DCFDA is used as an indicator of ROS levels in a cell. In addition, LysoTracker detects intracellular acidification and X-rhod-1 detects cytosolic Ca(2+). We also describe transgenic methods to detect Ca(2+) levels and expression patterns. These methods performed in the Drosophila ovary, as well as other tissues, may lead to a further understanding of the mechanisms of necrosis as a form of programmed cell death.

Draper acts through the JNK pathway to control synchronous engulfment of dying germline cells by follicular epithelial cells

The efficient removal of dead cells is an important process in animal development and homeostasis. Cell corpses are often engulfed by professional phagocytes such as macrophages. However, in some tissues with limited accessibility to circulating cells, engulfment is carried out by neighboring non-professional phagocytes such as epithelial cells. Here, we investigate the mechanism of corpse clearance in the Drosophila melanogaster ovary, a tissue that is closed to circulating cells. In degenerating egg chambers, dying germline cells are engulfed by the surrounding somatic follicular epithelium by unknown mechanisms. We show that the JNK pathway is activated and required in engulfing follicle cells. We find that the receptor Draper is also required in engulfing follicle cells, and activates the JNK pathway. Overexpression of Draper or the JNK pathway in follicle cells is sufficient to induce death of the underlying germline, suggesting that there is coordination between the germline and follicular epithelium to promote germline cell death. Furthermore, activation of JNK bypasses the need for Draper in engulfment. The induction of JNK and Draper in follicle cells occurs independently of caspase activity in the germline, indicating that at least two pathways are necessary to coordinate germline cell death with engulfment by the somatic epithelium.