Role of the insulin/Tor signaling network in starvation-induced programmed cell death in Drosophila oogenesis

Abstract

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 ...

A visual screen for diet-regulated proteins in the Drosophila ovary using GFP protein trap lines

“…Vitellogenic egg chambers undergo apoptosis when nutrients are limiting (Drummond-Barbosa and Spradling, 2001), and insulin, TOR and ecdysone signaling are involved in this process (Drummond-Barbosa and Spradling, 2001; Hsu et al, 2008; LaFever and Drummond-Barbosa, 2005; LaFever et al, 2010; Pritchett and McCall, 2012; Sieber and Spradling, 2015). We identified 17 GFP-trap lines corresponding to 14 proteins that were expressed in the germ cells or follicle cells of vitellogenic egg chambers and whose expression levels change in response to diet (Fig.…”

Ecdysone response gene E78 controls ovarian germline stem cell niche formation and follicle survival in Drosophila

“…Two recent large-scale screens for regulators of ecdysone-regulated cell death in a haemocyte cell line and in salivary glands (Chittaranjan et al, 2009; Wang et al, 2008) may prove useful for identifying targets involved in the decision between cell death and survival. Interestingly, the stage 4/5 cyst death we observed in E78 Δ31 mutants is phenocopied by mutations in insulin and target of rapamycin (TOR) signaling pathway components, including InR , chico , TOR , and S6 kinase (Pritchett and McCall, 2012). Since EcR and E78 appear to functionally cooperate, future studies should test whether EcR and E78 regulate members of the insulin/TOR signaling pathways (or vice-versa) to control cyst viability.…”

The Histone Deacetylase Gene Rpd3 Is Required for Starvation Stress Resistance

“…Although the role of Rpd3 located in the nucleoplasm is still unclear, it may relate to the regulation of genes involved in a well-known stress responding signaling pathway, such as the mTOR pathway [34]. Further analyses are necessary to clarify this point.…”