Diversity of cell death pathways: insight from the fly ovary

Abstract

Multiple types of cell death exist including necrosis, apoptosis, and autophagic cell death. The Drosophila ovary provides a valuable model to study the diversity of cell death modalities, and we review recent progress to elucidate these pathways. At least five distinct types of cell death occur in the ovary, and we focus on two that have been studied extensively. Cell death of mid-stage egg chambers occurs through a novel caspase-dependent pathway that involves autophagy and triggers phagocytosis by surrounding somatic epithelial cells. For every egg, 15 germline nurse cells undergo developmental programmed cell death, which occurs independently of most known cell death genes. These forms of cell death are strikingly similar to cell death observed in the germlines of other organisms.

Keywords: Drosophila; apoptosis; autophagy; oogenesis; ovary; programmed cell death.

Figure 1. Structure of the fly ovary

A) Fly ovaries contain strings of ovarioles (arrow). B) Two ovarioles stained with DAPI to label DNA (cyan) and anti-Discs large (Dlg, red) to label membranes. Each ovariole contains progressively developing egg chambers, which mature as they are pushed towards the posterior of the ovary. G=Germarium, NC=Nurse cell, FC=Follicle cell, O=oocyte. C) Schematic showing loss of polar cells (PC, yellow cytoplasm) between stage 3 (left) and stage 5 (right). D) Schematic showing stretch follicle cells (SFC) extending over the nurse cells in a stage 10 egg chamber.

Figure 2. Overview of starvation induced cell death during mid-oogenesis

Mid-stage egg chambers from starved flies stained with DAPI (cyan) to label DNA and anti-Discs large (Dlg, red) to mark the cell membranes. A–C) WT egg chambers. A) Healthy stage 8 egg chamber has large NC nuclei surrounded by a thin layer of FCs. Arrowhead indicates FC layer in all panels. B) Dying egg chamber has condensed and fragmented NC DNA, and the surrounding layer of FCs has enlarged and begun to engulf germline material. C) Late dying egg chamber has few NC nuclear fragments remaining and the FCs have completed engulfment. D) An undead egg chamber (arrowhead) where the NC nuclei have failed to condense and fragment, and many of the surrounding FCs have disappeared (arrow), resulting from overexpression of DIAP1 in the NCs. E) draper^−/− mid-dying egg chamber contains a thin layer of FCs (arrowhead) that have failed to enlarge (compare to WT in B). F) Late dying draper^−/− egg chamber (arrowhead) has lingering germline material and pyknotic FCs (arrow). Scale bar = 50 µm. G) Model of mid-stage death, showing suppression of caspases by DIAP1 (in gray) and potential regulatory mechanisms for mitochondria and nutritional deprivation (dotted lines). H) Model of mid-stage engulfment, showing activation of Draper-Rac1-JNK pathway by an unknown signal.

Figure 3. Overview of developmental programmed cell death during late oogenesis

Egg chambers from well-fed flies stained with DAPI (cyan) to mark the DNA and anti-Discs large (Dlg, red) to mark the cell membranes. A–D) WT egg chambers. A) Stage 11 egg chamber has several NCs that still contain cytoplasm. B) Stage 12 egg chamber has completed dumping and retains NC nuclei. C) Stage 13 egg chamber has begun to form dorsal appendages and has a few NC nuclei remaining. D) Stage 14 egg chamber has fully formed dorsal appendages and no longer contains any NCs. E) Overexpression of DIAP1 in the NCs leads to a weak persisting nuclei phenotype where stage 14 egg chambers still contain a few NC nuclei (arrow). F) deep orange transheterozygous (dor⁸/dor⁴) stage 14 egg chamber has a strong persisting nuclei phenotype with many NC nuclei (arrow).