SUMMARYThe 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.

The Drosophila melanogaster ovary is a powerful yet simple system with only a few cell types. Cell death in the ovary can be induced in response to multiple developmental and environmental signals. These cell deaths occur at distinct stages of oogenesis and involve unique mechanisms utilizing apoptotic, autophagic and perhaps necrotic processes. In this review, we summarize recent progress characterizing cell death mechanisms in the fly ovary.

Mutations of the tumor suppressor gene von Hippel-Lindau (VHL) can lead to benign and malignant tumors, including clear-cell renal cell carcinoma (ccRCC). To understand the progression of ccRCC, we generated a novel mouse Vhlh conditional knockout, using Hoxb7-driven Cre that is specific for the collecting ducts and a subset of distal tubules. These mice exhibited wide-spread epithelial disruption and interstitial inflammation as early as 2 months of age with high penetrance. Lesions are cystic, show severe fibrosis and display significant hyperplasia. An abundance of infiltrating macrophages and lymphocytes was detected. Interestingly, the Vhlh mutant lesions could be rescued when Hif-1α, but not Hif-2α, was also knocked out. In addition, administration of a JAK1/2 kinase inhibitor alleviated the Vhlh knockout phenotypes. Taken together, these results suggest that HIF-1α-dependent inflammation and fibrosis may be an early event in the development of ccRCC.

Amino-acid starvation leads to an inhibition of cellular proliferation and the induction of programmed cell death (PCD) in the Drosophila ovary. Disruption of insulin signaling has been shown to inhibit the progression of oogenesis, but it is unclear whether this phenotype mimics starvation. Here, we investigate whether the insulin-mediated phosphoinositide kinase-3 pathway regulates PCD in mid oogenesis. We reasoned that under well-fed conditions, disruption of positive components of the insulin signaling pathway within the germline would mimic starvation and produce degenerating egg chambers. Surprisingly, mutants did not mimic starvation but instead produced many abnormal egg chambers in which the somatic follicle cells disappeared and the germline persisted. These abnormal egg chambers did not show an induction of caspases and lysosomes like that observed in wild-type (WT) degenerating egg chambers. Egg chambers from insulin signaling mutants were resistant to starvation-induced PCD, indicating that a complete block in insulin-signaling prevents the proper response to starvation. However, target of rapamycin (Tor) mutants did show a phenotype that mimicked WT starvation-induced PCD, indicating an insulin independent regulation of PCD via Tor signaling. These results suggest that inhibition of the insulin signaling pathway is not sufficient to regulate starvation-induced PCD in mid oogenesis. Furthermore, starvation-induced PCD is regulated by Tor signaling converging with the canonical insulin signaling pathway. The insulin-mediated class I phosphoinositide kinase-3 (PI3K) pathway is a highly conserved nutrient-sensing pathway in diverse organisms. In mammals, insulin signaling affects oocyte maturation and fertilization. 1,2 Mutations in the insulin signaling pathway in C. elegans can lengthen life span but reduce fertility. 3,4 Improper insulin signaling in Drosophila leads to reduced body size and female sterility. 5 Thus, insulin signaling is essential for reproduction in diverse organisms, but how this pathway regulates fertility is not fully understood.Proper nutrition during Drosophila oogenesis is critical for egg chamber production. 6-8 Depriving flies of yeast as a protein source leads to programmed cell death (PCD) at two stages: early oogenesis in the germarium and mid oogenesis during stages 7-9. 7 Egg chambers undergoing PCD during mid oogenesis are characterized by nuclear condensation and fragmentation of germline-derived nurse cells (NCs), engulfment by somatic follicle cells (FCs), and ultimately FC death. 5 During PCD in mid oogenesis, dying NCs show characteristics of both apoptotic and autophagic PCD. 5 The effector caspase Death caspase-1 (Dcp-1) is essential for germline PCD in mid oogenesis. dcp-1 mutants that have been starved display mid-stage egg chambers that have a persistence of uncondensed NC nuclei but an absence of FCs. 5 This phenotype indicates that the dcp-1 mutant germline is unable to die in response to starvation, although the FCs respond and undergo PCD. dcp-1 mutants ...

In Drosophila, the checkpoint protein-2 kinase (DmChk2) and its downstream effector protein, Dmp53, are required for DNA damage-mediated cell cycle arrest, DNA repair and apoptosis. In this study we focus on understanding the function of these two apoptosis inducing factors during ovarian development. We found that expression of Dmp53, but not DmChk2, led to loss of ovarian stem cells. We demonstrate that expression of DmChk2, but not Dmp53, induced midoogenesis cell death. DmChk2 induced cell death was not suppressed by Dmp53 mutant, revealing for the first time that in Drosophila, overexpression of DmChk2 can induce cell death which is independent of Dmp53. We found that over-expression of caspase inhibitors such as DIAP1, p35 and p49 did not suppress DmChk2-and Dmp53-induced cell death. Thus, our study reveals stagespecific effects of Dmp53 and DmChk2 in oogenesis. Moreover, our results demonstrate that although DmChk2 and Dmp53 affect different stages of ovarian development, loss of ovarian stem cells by p53 expression and mid-oogenesis cell death induced by DmChk2 do not require caspase activity.