Adipocyte amino acid sensing controls adult germline stem cell number via the amino acid response pathway and independently of Target of Rapamycin signaling in Drosophila

Ras/MAPK signaling mediates adipose tissue control of ovarian germline survival and ovulation in Drosophila melanogaster

From insects to humans, oogenesis is tightly linked to nutritional input, yet little is known about how whole organism physiology matches dietary changes with oocyte development. Considering that diet-induced adipose tissue dysfunction is associated with an increased risk for fertility problems, and other obesity-associated pathophysiologies, it is critical to decipher the cellular and molecular mechanisms linking adipose nutrient sensing to remote control of the ovary and other tissues. Our previous studies in Drosophila melanogaster have shown that amino acid sensing, via the amino acid response pathway and mTOR-mediated signaling function within adipocytes to control germline stem cell maintenance and ovulation, respectively. Additionally, we demonstrated that insulin/insulin-like growth factor signaling within adipocytes employs distinct effector axes, PI3K/Akt1-dependent and -independent, downstream of insulin receptor activity to mediate fat-to-ovary communication. Here, we report that the Ras/MAPK signaling axis functions in adipocytes to regulate early germline cyst survival and ovulation of mature oocytes but is not important for germline stem cell maintenance or the progression through vitellogenesis. Thus, these studies uncover the complexity of signaling pathway activity that mediates inter-organ communication.

Integrated stress response signaling acts as a metabolic sensor in fat tissues to regulate oocyte maturation and ovulation

Reproduction is an energy-intensive process requiring systemic coordination. However, the inter-organ signaling mechanisms that relay nutrient status to modulate reproductive output are poorly understood. Here, we use Drosophila melanogaster as a model to establish the integrated stress response (ISR) transcription factor, Atf4, as a fat tissue metabolic sensor that instructs oogenesis. We demonstrate that Atf4 regulates lipase activity to mediate yolk lipoprotein synthesis in the fat body. Depletion of Atf4 in the fat body also blunts oogenesis recovery after amino acid deprivation and re-feeding, suggestive of a nutrient-sensing role for Atf4. We also discovered that Atf4 promotes secretion of a fat-body-derived neuropeptide, CNMamide, which modulates neural circuits that promote egg-laying behavior (ovulation). Thus, we posit that ISR signaling in fat tissue acts as a "metabolic sensor" that instructs female reproduction-directly by impacting yolk lipoprotein production and follicle maturation and systemically by regulating ovulation.

Mating-induced increase of kynurenine in Drosophila ovary enhances starvation resistance of progeny

The maternal nutritional environment can impact progeny development, stress tolerance, and longevity. Such phenotypic variation of offspring resulting from the maternal environment is often referred to as the 'maternal effect' and is observed across taxa, including in humans. While some mechanisms behind maternal effects have been revealed, such as histone modification, many studies rely on drastic genetic or nutritional manipulation in describing these mechanisms. Here we aimed to reveal how the maternal environment is regulated under physiological conditions to affect the progeny. Specifically, we detailed metabolic regulation in oocytes in response to mating using Drosophila melanogaster fruit flies. Using liquid chromatography-mass spectrometry, we found that upon mating, the ovary metabolites shifted, predominantly toward increasing amino acids and the tryptophan/kynurenine (Kyn) pathway. This mating-induced increase in ovary Kyn was driven by increased Kyn production in the fat body, a functional counterpart of the mammalian liver and white adipose tissue and the source of Kyn storage for the ovary after mating. Furthermore, we show that maternal Kyn repression decreased the starvation resistance of progeny and that administering exogenous Kyn to the maternal generation enhanced the starvation resistance of female progeny. Taken together, these findings point to a previously unidentified role of fat body Kyn distribution during reproduction on progeny survival.

Auxin exposure disrupts feeding behavior and fatty acid metabolism in adult Drosophila

The ease of genetic manipulation in Drosophila melanogaster using the Gal4/UAS system has been beneficial in addressing key biological questions. Current modifications of this methodology to temporally induce transgene expression require temperature changes or exposure to exogenous compounds, both of which have been shown to have detrimental effects on physiological processes. The recently described auxin-inducible gene expression system (AGES) utilizes the plant hormone auxin to induce transgene expression and is proposed to be the least toxic compound for genetic manipulation, with no obvious effects on Drosophila development and survival in one wild-type strain. Here, we show that auxin delays larval development in another widely used fly strain, and that short- and long-term auxin exposure in adult Drosophila induces observable changes in physiology and feeding behavior. We further reveal a dosage response to adult survival upon auxin exposure, and that the recommended auxin concentration for AGES alters feeding activity. Furthermore, auxin-fed male and female flies exhibit a significant decrease in triglyceride levels and display altered transcription of fatty acid metabolism genes. Although fatty acid metabolism is disrupted, auxin does not significantly impact adult female fecundity or progeny survival, suggesting AGES may be an ideal methodology for studying limited biological processes. These results emphasize that experiments using temporal binary systems must be carefully designed and controlled to avoid confounding effects and misinterpretation of results.

Microbes control Drosophila germline stem cell increase and egg maturation through hormonal pathways

Reproduction is highly dependent on environmental and physiological factors including nutrition, mating stimuli and microbes. Among these factors, microbes facilitate vital functions for host animals such as nutritional intake, metabolic regulation, and enhancing fertility under poor nutrition conditions. However, detailed molecular mechanisms by which microbes control germline maturation, leading to reproduction, remain largely unknown. In this study, we show that environmental microbes exert a beneficial effect on Drosophila oogenesis by promoting germline stem cell (GSC) proliferation and subsequent egg maturation via acceleration of ovarian cell division and suppression of apoptosis. Moreover, insulin-related signaling is not required; rather, the ecdysone pathway is necessary for microbe-induced increase of GSCs and promotion of egg maturation, while juvenile hormone contributes only to increasing GSC numbers, suggesting that hormonal pathways are activated at different stages of oogenesis. Our findings reveal that environmental microbes can enhance host reproductivity by modulating host hormone release and promoting oogenesis.

Auxin Exposure Disrupts Feeding Behavior and Fatty Acid Metabolism in Adult Drosophila

The ease of genetic manipulation in Drosophila melanogaster using the Gal4/UAS system has been beneficial in addressing key biological questions. Current modifications of this methodology to temporally induce transgene expression require temperature changes or exposure to exogenous compounds, both of which have been shown to have detrimental effects on physiological processes. The recently described auxin-inducible gene expression system (AGES) utilizes the plant hormone auxin to induce transgene expression and is proposed to be the least toxic compound for genetic manipulation, with no obvious effects on Drosophila development and survival in one wild-type strain. Here we show that auxin delays larval development in another widely-used fly strain, and that short- and long-term auxin exposure in adult Drosophila induces observable changes in physiology and feeding behavior. We further reveal a dosage response to adult survival upon auxin exposure, and that the recommended auxin concentration for AGES alters feeding activity. Furthermore, auxin fed male and female flies exhibit a significant decrease in triglyceride levels and display altered transcription of fatty acid metabolism genes. Although fatty acid metabolism is disrupted, auxin does not significantly impact adult female fecundity or progeny survival, suggesting AGES may be an ideal methodology for studying limited biological processes. These results emphasize that experiments using temporal binary systems must be carefully designed and controlled to avoid confounding effects and misinterpretation of results.

A high-sugar diet, but not obesity, reduces female fertility in Drosophila melanogaster

Obesity is linked to reduced fertility in various species, from Drosophila to humans. Considering that obesity is often induced by changes in diet or eating behavior, it remains unclear whether obesity, diet, or both reduce fertility. Here, we show that Drosophila females on a high-sugar diet become rapidly obese and less fertile as a result of increased death of early germline cysts and vitellogenic egg chambers (or follicles). They also have high glycogen, glucose and trehalose levels and develop insulin resistance in their fat bodies (but not ovaries). By contrast, females with adipocyte-specific knockdown of the anti-obesity genes brummer or adipose are obese but have normal fertility. Remarkably, females on a high-sugar diet supplemented with a separate source of water have mostly normal fertility and glucose levels, despite persistent obesity, high glycogen and trehalose levels, and fat body insulin resistance. These findings demonstrate that a high-sugar diet affects specific processes in oogenesis independently of insulin resistance, that high glucose levels correlate with reduced fertility on a high-sugar diet, and that obesity alone does not impair fertility.

The Vast Utility of Drosophila Oogenesis

In this chapter, we highlight examples of the diverse array of developmental, cellular, and biochemical insights that can be gained by using Drosophila melanogaster oogenesis as a model tissue. We begin with an overview of ovary development and adult oogenesis. Then we summarize how the adult Drosophila ovary continues to advance our understanding of stem cells, cell cycle, cell migration, cytoplasmic streaming, nurse cell dumping, and cell death. We also review emerging areas of study, including the roles of lipid droplets, ribosomes, and nuclear actin in egg development. Finally, we conclude by discussing the growing conservation of processes and signaling pathways that regulate oogenesis and female reproduction from flies to humans.

The expression of Catsup in escort cells affects Drosophila ovarian stem cell niche establishment and germline stem cells self-renewal via Notch signaling

Stem cell niche provides extrinsic signals to maintain stem cell renewal or initiate cell differentiation. Drosophila niche is composed of somatic terminal filament cells, cap cells and escort cells. However, the underlying mechanism for the development of stem cell niche remains largely unclear. Here we found that the expression of a zinc transporter Catsup is essential for ovary morphogenesis. Catsup knockdown in escort cells results in defects of niche establishment and germline stem cells self-renewal. These defects could be modified by altered expression of genes involved in zinc metabolism or intervention of dietary zinc levels. Further studies indicated that Catsup RNAi affected adult ovary morphogenesis by suppressing Notch signaling. Lastly, we demonstrated that the defects of Catsup RNAi could be restored by overexpression of heat shock cognate protein 70 (Hsc70). These findings expand our understanding of the mechanisms controlling adult oogenesis and niche establishment in Drosophila.

Analysis of Physiological Control of Adult Drosophila Oogenesis by Interorgan Communication

Tissue homeostasis is dependent on the interaction between various organs within an organism in response to physiological inputs. The adult Drosophila melanogaster ovary is sensitive to environmental challenges and has recently been shown to be regulated by signaling from peripheral organs. To dissect the intricate coordination between overall organism health and reproduction, it is necessary to meticulously characterize both experimental tools and oogenesis processes. This chapter provides a guide for the careful analysis of interorgan communication in regulating oogenesis in adult Drosophila melanogaster.

Methods to Analyze Nutritional and Inter-Organ Control of Drosophila Ovarian Germline Stem Cells

Physiological status, particularly dietary input, has major impacts on the Drosophila melanogaster ovarian germline stem cell lineage. Moreover, several studies have shed light on the role that inter-organ communication plays in coordinating whole-organism responses to changes in physiology. For example, nutrient-sensing signaling pathways function within the fat body to regulate germline stem cells and their progeny in the ovary. Together with its incredible genetic and cell biological toolkits, Drosophila serves as an amenable model organism to use for uncovering molecular mechanisms that underlie physiological control of adult stem cells. In this methods chapter, we describe a general dietary manipulation paradigm, genetic manipulation of adult adipocytes, and whole-mount ovary immunofluorescence to investigate physiological control of germline stem cells.

Antibody development to identify components of IIS and mTOR signaling pathways in lepidopteran species, a set of non-model insects

Nutritional stress impacts many insect species that have differing reproductive strategies and life histories, yet it is unclear how nutrient-sensing signaling pathways mediate tissue-specific responses to changes in dietary input. In Drosophila melanogaster , insulin/insulin-like growth factor (IIS) and mTOR-mediated signaling within adipocytes regulates oogenesis. To facilitate comparative study of nutrient-sensing pathway activity in the fat body, we developed antibodies to assess IIS (anti-FOXO) and mTOR signaling (anti-TOR) across three nymphalid species (Lepidoptera). By optimizing whole-mount fat body immunostaining, we find FOXO nuclear enrichment in adult adipocytes, like that observed in Drosophila . Additionally, we show a previously uncharacterized TOR localization pattern in the fat body.

orsai, the Drosophila homolog of human ETFRF1, links lipid catabolism to growth control

Background

Lipid homeostasis is an evolutionarily conserved process that is crucial for energy production, storage and consumption. Drosophila larvae feed continuously to achieve the roughly 200-fold increase in size and accumulate sufficient reserves to provide all energy and nutrients necessary for the development of the adult fly. The mechanisms controlling this metabolic program are poorly understood.

Results

Herein we identified a highly conserved gene, orsai (osi), as a key player in lipid metabolism in Drosophila. Lack of osi function in the larval fat body, the regulatory hub of lipid homeostasis, reduces lipid reserves and energy output, evidenced by decreased ATP production and increased ROS levels. Metabolic defects due to reduced Orsai (Osi) in time trigger defective food-seeking behavior and lethality. Further, we demonstrate that downregulation of Lipase 3, a fat body-specific lipase involved in lipid catabolism in response to starvation, rescues the reduced lipid droplet size associated with defective orsai. Finally, we show that osi-related phenotypes are rescued through the expression of its human ortholog ETFRF1/LYRm5, known to modulate the entry of β-oxidation products into the electron transport chain; moreover, knocking down electron transport flavoproteins EtfQ0 and walrus/ETFA rescues osi-related phenotypes, further supporting this mode of action.

Conclusions

These findings suggest that Osi may act in concert with the ETF complex to coordinate lipid homeostasis in the fat body in response to stage-specific demands, supporting cellular functions that in turn result in an adaptive behavioral response.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01417-w.

The use of non-model Drosophila species to study natural variation in TOR pathway signaling

Nutrition and growth are strongly linked, but not much is known about how nutrition leads to growth. To understand the connection between nutrition through the diet, growth, and proliferation, we need to study the phenotypes resulting from the activation and inhibition of central metabolic pathways. One of the most highly conserved metabolic pathways across eukaryotes is the Target of Rapamycin (TOR) pathway, whose primary role is to detect the availability of nutrients and to either induce or halt cellular growth. Here we used the model organism Drosophila melanogaster (D. mel.) and three non-model Drosophila species with different dietary needs, Drosophila guttifera (D. gut.), Drosophila deflecta (D. def.), and Drosophila tripunctata (D. tri.), to study the effects of dietary amino acid availability on fecundity and longevity. In addition, we inhibited the Target of Rapamycin (TOR) pathway, using rapamycin, to test how the inhibition interplays with the nutritional stimuli in these four fruit fly species. We hypothesized that the inhibition of the TOR pathway would reverse the phenotypes observed under conditions of overfeeding. Our results show that female fecundity increased with higher yeast availability in all four species but decreased in response to TOR inhibition. The longevity data were more varied: most species experienced an increase in median lifespan in both genders with an increase in yeast availability, while the lifespan of D. mel. females decreased. When exposed to the TOR inhibitor rapamycin, the life spans of most species decreased, except for D. tri, while we observed a major reduction in fecundity across all species. The obtained data can benefit future studies on the evolution of metabolism by showing the potential of using non-model species to track changes in metabolism. Particularly, our data show the possibility to use relatively closely related Drosophila species to gain insight on the evolution of TOR signaling.

Target of Rapamycin Drives Unequal Responses to Essential Amino Acid Depletion for Egg Laying in Drosophila Melanogaster

Nutrition shapes a broad range of life-history traits, ultimately impacting animal fitness. A key fitness-related trait, female fecundity is well known to change as a function of diet. In particular, the availability of dietary protein is one of the main drivers of egg production, and in the absence of essential amino acids egg laying declines. However, it is unclear whether all essential amino acids have the same impact on phenotypes like fecundity. Using a holidic diet, we fed adult female Drosophila melanogaster diets that contained all necessary nutrients except one of the 10 essential amino acids and assessed the effects on egg production. For most essential amino acids, depleting a single amino acid induced as rapid a decline in egg production as when there were no amino acids in the diet. However, when either methionine or histidine were excluded from the diet, egg production declined more slowly. Next, we tested whether GCN2 and TOR mediated this difference in response across amino acids. While mutations in GCN2 did not eliminate the differences in the rates of decline in egg laying among amino acid drop-out diets, we found that inhibiting TOR signalling caused egg laying to decline rapidly for all drop-out diets. TOR signalling does this by regulating the yolk-forming stages of egg chamber development. Our results suggest that amino acids differ in their ability to induce signalling via the TOR pathway. This is important because if phenotypes differ in sensitivity to individual amino acids, this generates the potential for mismatches between the output of a pathway and the animal’s true nutritional status.

Warm and cold temperatures have distinct germline stem cell lineage effects during Drosophila oogenesis

Despite their medical and economic relevance, it remains largely unknown how suboptimal temperatures affect adult insect reproduction. Here, we report an in-depth analysis of how chronic adult exposure to suboptimal temperatures affects oogenesis using the model insect Drosophila melanogaster. In adult females maintained at 18°C (cold) or 29°C (warm), relative to females at the 25°C control temperature, egg production was reduced through distinct cellular mechanisms. Chronic 18°C exposure improved germline stem cell maintenance, survival of early germline cysts and oocyte quality, but reduced follicle growth with no obvious effect on vitellogenesis. By contrast, in females at 29°C, germline stem cell numbers and follicle growth were similar to those at 25°C, while early germline cyst death and degeneration of vitellogenic follicles were markedly increased and oocyte quality plummeted over time. Finally, we also show that these effects are largely independent of diet, male factors or canonical temperature sensors. These findings are relevant not only to cold-blooded organisms, which have limited thermoregulation, but also potentially to warm-blooded organisms, which are susceptible to hypothermia, heatstroke and fever.

Regulation of Mating-Induced Increase in Female Germline Stem Cells in the Fruit Fly Drosophila melanogaster

In many insect species, mating stimuli can lead to changes in various behavioral and physiological responses, including feeding, mating refusal, egg-laying behavior, energy demand, and organ remodeling, which are collectively known as the post-mating response. Recently, an increase in germline stem cells (GSCs) has been identified as a new post-mating response in both males and females of the fruit fly, Drosophila melanogaster. We have extensively studied mating-induced increase in female GSCs of D. melanogaster at the molecular, cellular, and systemic levels. After mating, the male seminal fluid peptide [e.g. sex peptide (SP)] is transferred to the female uterus. This is followed by binding to the sex peptide receptor (SPR), which evokes post-mating responses, including increase in number of female GSCs. Downstream of SP-SPR signaling, the following three hormones and neurotransmitters have been found to act on female GSC niche cells to regulate mating-induced increase in female GSCs: (1) neuropeptide F, a peptide hormone produced in enteroendocrine cells; (2) octopamine, a monoaminergic neurotransmitter synthesized in ovary-projecting neurons; and (3) ecdysone, a steroid hormone produced in ovarian follicular cells. These humoral factors are secreted from each organ and are received by ovarian somatic cells and regulate the strength of niche signaling in female GSCs. This review provides an overview of the latest findings on the inter-organ relationship to regulate mating-induced female GSC increase in D. melanogaster as a model. We also discuss the remaining issues that should be addressed in the future.

Integrated stress response regulates GDF15 secretion from adipocytes, preferentially suppresses appetite for a high-fat diet and improves obesity

The eIF2α phosphorylation-dependent integrated stress response (ISR) is a signaling pathway that maintains homeostasis in mammalian cells exposed to various stresses. Here, ISR activation in adipocytes improves obesity and diabetes by regulating appetite in a non-cell-autonomous manner. Adipocyte-specific ISR activation using transgenic mice decreases body weight and improves glucose tolerance and obesity induced by a high-fat diet (HFD) via preferential inhibition of HFD intake. The transcriptome analysis of ISR-activated adipose tissue reveals that growth differentiation factor 15 (GDF15) expression is induced by the ISR through the direct regulation of the transcription factors ATF4 and DDIT3. Deficiency in the GDF15 receptor GFRAL abolishes the adipocyte ISR-dependent preferential inhibition of HFD intake and the anti-obesity effects. Pharmacologically, 10(E), 12(Z)-octadecadienoic acid induces ISR-dependent GDF15 expression in adipocytes and decreases the intake of the HFD. Based on our findings the specific activation of the ISR in adipocytes controls the non-cell-autonomous regulation of appetite.

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Activation of ISR in adipocytes suppresses intake of high-fat diet and prevents obesity

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ATF4 and DDIT3 induced by ISR directly regulate GDF15 expression

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GDF15-GFRAL axis mediates the control of appetite for high-fat diet by ISR activation

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One of conjugated linoleic acids induces ISR and GDF15 expression in adipocytes

A Protein-trap allele reveals roles for Drosophila ATF4 in photoreceptor degeneration, oogenesis and wing development

Metazoans have evolved various quality control mechanisms to cope with cellular stress inflicted by external and physiological conditions. ATF4 is a major effector of the Integrated Stress Response (ISR), an evolutionarily conserved pathway that mediates adaptation to various cellular stressors. Loss of function of Drosophila ATF4, encoded by the gene cryptocephal (crc), results in lethality during pupal development. The roles of crc in Drosophila disease models and in adult tissue homeostasis thus remain poorly understood. Here, we report that a protein-trap MiMIC insertion in the crc locus generates a crc-GFP fusion protein that allows visualization of crc activity in vivo. This allele also acts as a hypomorphic mutant that uncovers previously unknown roles for crc. Specifically, the crc protein-trap line shows crc-GFP induction in a Drosophila model for Retinitis Pigmentosa (RP). This crc allele renders flies more vulnerable to amino acid deprivation and age-dependent retinal degeneration. These mutants also show defects in wing veins and oocyte maturation. Together, our data reveal previously unknown roles for crc in development, cellular homeostasis and photoreceptor survival.

Cell competition is driven by Xrp1-mediated phosphorylation of eukaryotic initiation factor 2α

Cell competition is a context-dependent cell elimination via cell-cell interaction whereby unfit cells ('losers') are eliminated from the tissue when confronted with fitter cells ('winners'). Despite extensive studies, the mechanism that drives loser's death and its physiological triggers remained elusive. Here, through a genetic screen in Drosophila, we find that endoplasmic reticulum (ER) stress causes cell competition. Mechanistically, ER stress upregulates the bZIP transcription factor Xrp1, which promotes phosphorylation of the eukaryotic translation initiation factor eIF2α via the kinase PERK, leading to cell elimination. Surprisingly, our genetic data show that different cell competition triggers such as ribosomal protein mutations or RNA helicase Hel25E mutations converge on upregulation of Xrp1, which leads to phosphorylation of eIF2α and thus causes reduction in global protein synthesis and apoptosis when confronted with wild-type cells. These findings not only uncover a core pathway of cell competition but also open the way to understanding the physiological triggers of cell competition.

The Dynamic Regulation of mRNA Translation and Ribosome Biogenesis During Germ Cell Development and Reproductive Aging

The regulation of mRNA translation, both globally and at the level of individual transcripts, plays a central role in the development and function of germ cells across species. Genetic studies using flies, worms, zebrafish and mice have highlighted the importance of specific RNA binding proteins in driving various aspects of germ cell formation and function. Many of these mRNA binding proteins, including Pumilio, Nanos, Vasa and Dazl have been conserved through evolution, specifically mark germ cells, and carry out similar functions across species. These proteins typically influence mRNA translation by binding to specific elements within the 3′ untranslated region (UTR) of target messages. Emerging evidence indicates that the global regulation of mRNA translation also plays an important role in germ cell development. For example, ribosome biogenesis is often regulated in a stage specific manner during gametogenesis. Moreover, oocytes need to produce and store a sufficient number of ribosomes to support the development of the early embryo until the initiation of zygotic transcription. Accumulating evidence indicates that disruption of mRNA translation regulatory mechanisms likely contributes to infertility and reproductive aging in humans. These findings highlight the importance of gaining further insights into the mechanisms that control mRNA translation within germ cells. Future work in this area will likely have important impacts beyond germ cell biology.

Adipokine and fat body in flies: Connecting organs

Under conditions of nutritional and environmental stress, organismal homeostasis is preserved through inter-communication between multiple organs. To do so, higher organisms have developed a system of interorgan communication through which one tissue can affect the metabolism, activity or fate of remote organs, tissues or cells. In this review, we discuss the latest findings emphasizing Drosophila melanogaster as a powerful model organism to study these interactions and may constitute one of the best documented examples depicting the long-distance communication between organs. In flies, the adipose tissue appears to be one of the main organizing centers for the regulation of insect development and behavior: it senses nutritional and hormonal signals and in turn, orchestrates the release of appropriate adipokines. We discuss the nature and the role of recently uncovered adipokines, their regulations by external cues, their secretory routes and their modes of action to adjust developmental growth and timing accordingly. These findings have the potential for identification of candidate factors and signaling pathways that mediate conserved interorgan crosstalk.

Hormone receptor 4 is required in muscles and distinct ovarian cell types to regulate specific steps of Drosophila oogenesis

The conserved nuclear receptor superfamily has crucial roles in many processes, including reproduction. Nuclear receptors with known roles in oogenesis have been studied mostly in the context of their ovary-intrinsic requirement. Recent studies in Drosophila, however, have begun to reveal new roles of nuclear receptor signaling in peripheral tissues in controlling reproduction. Here, we identified Hormone receptor 4 (Hr4) as an oogenesis regulator required in the ovary and muscles. Global Hr4 knockdown leads to increased germline stem cell (GSC) loss, reduced GSC proliferation, early germline cyst death, slowed follicle growth and vitellogenic follicle degeneration. Tissue-specific knockdown experiments uncovered ovary-intrinsic and peripheral tissue requirements for Hr4 In the ovary, Hr4 is required in the niche for GSC proliferation and in the germline for GSC maintenance. Hr4 functions in muscles to promote GSC maintenance and follicle growth. The specific tissues that require Hr4 for survival of early germline cysts and vitellogenic follicles remain unidentified. These results add to the few examples of muscles controlling gametogenesis and expand our understanding of the complexity of nuclear receptor regulation of various aspects of oogenesis.

Amino Acid Sensing in Metabolic Homeostasis and Health

Sensing and responding to changes in nutrient levels, including those of glucose, lipids, and amino acids, by the body is necessary for survival. Accordingly, perturbations in nutrient sensing are tightly linked with human pathologies, particularly metabolic diseases such as obesity, type 2 diabetes mellitus, and other complications of metabolic syndromes. The conventional view is that amino acids are fundamental elements for protein and peptide synthesis, while recent studies have revealed that amino acids are also important bioactive molecules that play key roles in signaling pathways and metabolic regulation. Different pathways that sense intracellular and extracellular levels of amino acids are integrated and coordinated at the organismal level, and, together, these pathways maintain whole metabolic homeostasis. In this review, we discuss the studies describing how important sensing signals respond to amino acid availability and how these sensing mechanisms modulate metabolic processes, including energy, glucose, and lipid metabolism. We further discuss whether dysregulation of amino acid sensing signals can be targeted to promote metabolic disorders, and discuss how to translate these mechanisms to treat human diseases. This review will help to enhance our overall understanding of the correlation between amino acid sensing and metabolic homeostasis, which have important implications for human health.

Nuclear receptors linking physiology and germline stem cells in Drosophila

Maternal nutrition and physiology are intimately associated with reproductive success in diverse organisms. Despite decades of study, the molecular mechanisms linking maternal diet to the production and quality of oocytes remain poorly defined. Nuclear receptors (NRs) link nutritional signals to cellular responses and are essential for oocyte development. The fruit fly, Drosophila melanogaster, is an excellent genetically tractable model to study the relationship between NR signaling and oocyte production. In this review, we explore how NRs in Drosophila regulate the earliest stages of oocyte development. Long-recognized as an essential mediator of developmental transitions, we focus on the intrinsic roles of the Ecdysone Receptor and its ligand, ecdysone, in oogenesis. We also review recent studies suggesting broader roles for NRs as regulators of maternal physiology and their impact specifically on oocyte production. We propose that NRs form the molecular basis of a broad physiological surveillance network linking maternal diet with oocyte production. Given the functional conservation between Drosophila and humans, continued experimental investigation into the molecular mechanisms by which NRs promote oogenesis will likely aid our understanding of human fertility.

RNAi-based screens uncover a potential new role for the orphan neuropeptide receptor Moody in Drosophila female germline stem cell maintenance

Reproduction is highly sensitive to changes in physiology and the external environment. Neuropeptides are evolutionarily conserved signaling molecules that regulate multiple physiological processes. However, the potential reproductive roles of many neuropeptide signaling pathways remain underexplored. Here, we describe the results of RNAi-based screens in Drosophila melanogaster to identify neuropeptides/neuropeptide receptors with potential roles in oogenesis. The screen read-outs were either the number of eggs laid per female per day over time or fluorescence microscopy analysis of dissected ovaries. We found that the orphan neuropeptide receptor encoded by moody (homologous to mammalian melatonin receptors) is likely required in somatic cells for normal egg production and proper germline stem cell maintenance. However, the egg laying screens had low signal-to-noise ratio and did not lead to the identification of additional candidates. Thus, although egg count assays might be useful for large-scale screens to identify oogenesis regulators that result in dramatic changes in oogenesis, more labor-intensive microscopy-based screen are better applicable for identifying new physiological regulators of oogenesis with more subtle phenotypes.

The Nuclear Receptor Seven Up Regulates Genes Involved in Immunity and Xenobiotic Response in the Adult Drosophila Female Fat Body

The physiology of organisms depends on inter-organ communication in response to changes in the environment. Nuclear receptors are broadly expressed transcription factors that respond to circulating molecules to control many biological processes, including immunity, detoxification, and reproduction. Although the tissue-intrinsic roles of nuclear receptors in reproduction have been extensively studied, there is increasing evidence that nuclear receptor signaling in peripheral tissues can also influence oogenesis. We previously showed that the Drosophila nuclear receptor Seven up (Svp) is required in the adult fat body to regulate distinct steps of oogenesis; however, the relevant downstream targets of Svp remain unknown. Here, we took an RNA sequencing approach to identify candidate Svp targets specifically in the adult female fat body that might mediate this response. svp knockdown in the adult female fat body significantly downregulated immune genes involved in the first line of pathogen defense, suggesting a role for Svp in stimulating early immunity. In addition, we found that Svp transcriptionally regulates genes involved in each step of the xenobiotic detoxification response. Based on these findings, we propose a testable model in which Svp functions in the adult female fat body to stimulate early defense against pathogens and facilitate detoxification as part of its mechanisms to promote oogenesis.

Analysis of Gal4 Expression Patterns in Adult Drosophila Females

Precise genetic manipulation of specific cell types or tissues to pinpoint gene function requirement is a critical step in studies aimed at unraveling the intricacies of organismal physiology. Drosophila researchers heavily rely on the UAS/Gal4/Gal80 system for tissue-specific manipulations; however, it is often unclear whether the reported Gal4 expression patterns are indeed specific to the tissue of interest such that experimental results are not confounded by secondary sites of Gal4 expression. Here, we surveyed the expression patterns of commonly used Gal4 drivers in adult Drosophila female tissues under optimal conditions and found that multiple drivers have unreported secondary sites of expression beyond their published cell type/tissue expression pattern. These results underscore the importance of thoroughly characterizing Gal4 tools as part of a rigorous experimental design that avoids potential misinterpretation of results as we strive for understanding how the function of a specific gene/pathway in one tissue contributes to whole-body physiology.

Wolbachia's Deleterious Impact on Aedes aegypti Egg Development: The Potential Role of Nutritional Parasitism

Simple Summary

Mosquito-borne viral diseases such as dengue, Zika and chikungunya cause a significant global health burden and are currently increasing in outbreak frequency and geographical reach. Wolbachia pipientis, an endosymbiotic bacterium, offers a solution to this. When Wolbachia is introduced into the main mosquito vector of these viruses, Aedes aegypti, it alters the mosquito’s reproductive biology, as well as reducing the ability of the mosquitoes to transmit viruses. These traits can be leveraged to reduce virus transmission within a community by mass releasing Wolbachia-infected mosquitoes. However, Wolbachia has some negative effects on Aedes aegypti fitness, particularly egg longevity, and the reason behind this remains ambiguous. Insect fitness is very important for the success for Wolbachia-biocontrol strategies as they rely on the released insects being competitive with the wild mosquito population. This review summarises the fitness effects of Wolbachia on Aedes aegypti and investigates the possible contribution of Wolbachia as a nutritional parasite in lowering host fitness. It proposes the next stages of research that can be conducted to address nutritional parasitism to aid in the expansion of Wolbachia-based disease management programs worldwide.

Abstract

The artificial introduction of the endosymbiotic bacterium, Wolbachia pipientis, into Aedes (Ae.) aegypti mosquitoes reduces the ability of mosquitoes to transmit human pathogenic viruses and is now being developed as a biocontrol tool. Successful introgression of Wolbachia-carrying Ae. aegypti into native mosquito populations at field sites in Australia, Indonesia and Malaysia has been associated with reduced disease prevalence in the treated community. In separate field programs, Wolbachia is also being used as a mosquito population suppression tool, where the release of male only Wolbachia-infected Ae. aegypti prevents the native mosquito population from producing viable eggs, subsequently suppressing the wild population. While these technologies show great promise, they require mass rearing of mosquitoes for implementation on a scale that has not previously been done. In addition, Wolbachia induces some negative fitness effects on Ae. aegypti. While these fitness effects differ depending on the Wolbachia strain present, one of the most consistent and significant impacts is the shortened longevity and viability of eggs. This review examines the body of evidence behind Wolbachia’s negative effect on eggs, assesses nutritional parasitism as a key cause and considers how these impacts could be overcome to achieve efficient large-scale rearing of these mosquitoes.

Translational induction of ATF4 during integrated stress response requires noncanonical initiation factors eIF2D and DENR

The Integrated Stress Response (ISR) helps metazoan cells adapt to cellular stress by limiting the availability of initiator methionyl-tRNA for translation. Such limiting conditions paradoxically stimulate the translation of ATF4 mRNA through a regulatory 5' leader sequence with multiple upstream Open Reading Frames (uORFs), thereby activating stress-responsive gene expression. Here, we report the identification of two critical regulators of such ATF4 induction, the noncanonical initiation factors eIF2D and DENR. Loss of eIF2D and DENR in Drosophila results in increased vulnerability to amino acid deprivation, susceptibility to retinal degeneration caused by endoplasmic reticulum (ER) stress, and developmental defects similar to ATF4 mutants. eIF2D requires its RNA-binding motif for regulation of 5' leader-mediated ATF4 translation. Consistently, eIF2D and DENR deficient human cells show impaired ATF4 protein induction in response to ER stress. Altogether, our findings indicate that eIF2D and DENR are critical mediators of ATF4 translational induction and stress responses in vivo.

Protein Stores Regulate When Reproductive Displays Begin in the Male Caribbean Fruit Fly

Many animals exhibit reproductive behavior that requires expenditure of valuable nutrients. In males of many species, competitive energetically demanding displays and the development of sexual ornaments require prior accumulation of nutrient stores. Males must coordinate nutrient stores with ornament development and reproductive displays or they risk depleting their resources mid-development or mid-display, reducing their chance of mating. Males may use nutrient stores to regulate their reproductive behavior. Amino acid reserves may be important for reproduction, but the roles of amino acid stores in initiating maturation and reproductive behavior are less studied than fat stores. Insects store amino acids as hexamerin storage proteins. Many fly species use a specific hexamerin, larval serum protein 2 (LSP-2), as both a juvenile storage medium and to store protein consumed after adult eclosion. Protein stored as LSP-2 has previously been suggested to regulate reproduction in females, but no role has been proposed for LSP-2 in regulating male maturation. We use males of the Caribbean fruit fly, Anastrepha suspensa, a species with nutrient-intensive male sexual displays to test whether LSP-2 stores regulate male reproductive displays. We fed adult A. suspensa males a diet with or without protein, then assayed these males for lsp-2 transcript abundance via qRT-PCR, LSP-2 protein abundance via Western blot, and reproductive display behavior via observation. We found that adult males with ad libitum dietary protein had greater lsp-2 transcript and protein abundance, earlier sexual display behavior, and were more likely to exhibit sexual display behavior than protein-deprived adult males. We show that lsp-2 knockdown via RNAi decreases the proportion of males exhibiting reproductive displays, particularly early in the onset of reproductive behavior. Our results suggest circulating LSP-2 protein stores regulate reproductive behavior in A. suspensa males, consistent with protein stores modulating reproduction in males with expensive reproductive strategies. Our results are consistent with hexamerin storage proteins performing dual roles of protein storage and protein signaling. Our work also has substantial practical applications because tephritid flies are a pest group and the timing and expression of male reproductive displays in this group are important for control efforts using the sterile insect technique.

Regulation of adult female germline stem cells by nutrient-responsive signaling

Insect oogenesis is greatly affected by nutrient availability. When nutrients are abundant, oocytes are rapidly generated, but the process is slowed to conserve energy under nutrient-deficient conditions. To properly allocate limited resources toward oogenesis, systemic factors coordinate the behavioral response of ovarian germline stem cells (GSCs) to nutritional inputs by acting on the GSC itself, GSC supporting cells (the niche), or the adipose tissue surrounding the ovary. In this review, we describe current knowledge of the Drosophila ovarian GSC-niche-adipocyte system and major nutrient sensing pathways (insulin/IGF signaling, TOR signaling, and GCN2-dependent amino acid sensing) that intrinsically or extrinsically regulate GSC responses to nutrient signals.

The nuclear receptor seven up functions in adipocytes and oenocytes to control distinct steps of Drosophila oogenesis

Reproduction is intimately linked to the physiology of an organism. Nuclear receptors are widely expressed transcription factors that mediate the effects of many circulating molecules on physiology and reproduction. While multiple studies have focused on the roles of nuclear receptors intrinsically in the ovary, it remains largely unknown how the actions of nuclear receptors in peripheral tissues influence oogenesis. We identified the nuclear receptor encoded by svp as a novel regulator of oogenesis in adult Drosophila. Global somatic knockdown of svp reduces egg production by increasing GSC loss, death of early germline cysts, and degeneration of vitellogenic follicles. Tissue-specific knockdown experiments revealed that svp remotely controls these different steps of oogenesis through separate mechanisms involving distinct tissues. Specifically, adipocyte-specific svp knockdown impairs GSC maintenance and early germline cyst survival, whereas oenocyte-specific svp knockdown increases the death of vitellogenic follicles without any effects on GSCs or early cysts. These results illustrate that nuclear receptors can control reproduction through a variety of mechanisms involving peripheral tissues.

Local and Physiological Control of Germline Stem Cell Lineages in Drosophila melanogaster

The long-term survival of any multicellular species depends on the success of its germline in producing high-quality gametes and maximizing survival of the offspring. Studies in Drosophila melanogaster have led our growing understanding of how germline stem cell (GSC) lineages maintain their function and adjust their behavior according to varying environmental and/or physiological conditions. This review compares and contrasts the local regulation of GSCs by their specialized microenvironments, or niches; discusses how diet and diet-dependent factors, mating, and microorganisms modulate GSCs and their developing progeny; and briefly describes the tie between physiology and development during the larval phase of the germline cycle. Finally, it concludes with broad comparisons with other organisms and some future directions for further investigation.

Turning food into eggs: insights from nutritional biology and developmental physiology of Drosophila

Nutrition plays a central role in fecundity, regulating the onset of reproductive maturity, egg production, and the survival and health of offspring from insects to humans. Although decades of research have worked to uncover how nutrition mediates these effects, it has proven difficult to disentangle the relative role of nutrients as the raw material for egg and offspring development versus their role in stimulating endocrine cascades necessary to drive development. This has been further complicated by the fact that both nutrients and the signalling cascades they regulate interact in complex ways to control fecundity. Separating the two effects becomes important when trying to understand how fecundity is regulated, and in devising strategies to offset the negative effects of nutrition on reproductive health. In this review, we use the extensive literature on egg development in the fruit fly Drosophila melanogaster to explore how the nutrients from food provide the building blocks and stimulate signalling cascades necessary for making an egg.

Circadian clock genes' overexpression in Drosophila alters diet impact on lifespan

Diet restriction is one of the most accurately confirmed interventions which extend lifespan. Genes coding circadian core clock elements are known to be the key controllers of cell metabolism especially in aging aspect. The molecular mechanisms standing behind the phenomenon of diet-restriction-mediated life extension are connected to circadian clock either. Here we investigate the effects of protein-rich and low-protein diets on lifespan observed in fruit flies overexpressing core clock genes (cry, per, Clk, cyc and tim). The majority of core clock genes being upregulated in peripheral tissues (muscles and fat body) on protein-rich diet significantly decrease the lifespan of male fruit flies from 5 to 61%. Nevertheless, positive increments of median lifespan were observed in both sexes, males overexpressing cry in fat body lived 20% longer on poor diet. Overexpression of per also on poor medium resulted in life extension in female fruit flies. Diet restriction reduces mortality caused by overexpression of core clock genes. Cox-regression model revealed that diet restriction seriously decreases mortality risks of flies which overexpress core clock genes. The hazard ratios are lower for flies overexpressing clock genes in fat body relatively to muscle-specific overexpression. The present work suggests a phenomenological view of how two peripheral circadian oscillators modify effects of rich and poor diets on lifespan and hazard ratios.

Midgut-derived neuropeptide F controls germline stem cell proliferation in a mating-dependent manner

Stem cell maintenance is established by neighboring niche cells that promote stem cell self-renewal. However, it is poorly understood how stem cell activity is regulated by systemic, tissue-extrinsic signals in response to environmental cues and changes in physiological status. Here, we show that neuropeptide F (NPF) signaling plays an important role in the pathway regulating mating-induced germline stem cell (GSC) proliferation in the fruit fly Drosophila melanogaster. NPF expressed in enteroendocrine cells (EECs) of the midgut is released in response to the seminal-fluid protein sex peptide (SP) upon mating. This midgut-derived NPF controls mating-induced GSC proliferation via ovarian NPF receptor (NPFR) activity, which modulates bone morphogenetic protein (BMP) signaling levels in GSCs. Our study provides a molecular mechanism that describes how a gut-derived systemic factor couples stem cell behavior to physiological status, such as mating, through interorgan communication.

Communication between different organs is essential to respond quickly to environmental cues or changes in the physiological status of an organism. Recent studies have shown the existence of humoral factors or hormones, which are transported by the circulatory system to different organs and achieve coordination between them. Here, we have analyzed the communication mechanism between organs that regulates proliferation of germline stem cells (GSCs) in the ovary of the fruit fly Drosophila melanogaster. We show that a peptide hormone called neuropeptide F (NPF) is a key player in this process. This peptide is produced in both the brain and the midgut, and, remarkably, we find that only NPF released from the midgut is crucial for controlling post-mating GSC proliferation. Our data suggest that mating stimulates the release of NPF from the endocrine cells of the midgut stimulated by the presence of a seminal peptide. Midgut-derived NPF is then transduced through an NPF-specific G-protein–coupled receptor expressed in the ovary, and this triggers GSC proliferation. Our study identifies an essential interaction between the digestive system and the ovary that regulates the size of stem cell populations in flies depending on mating.

Maintenance of Proper Germline Stem Cell Number Requires Adipocyte Collagen in Adult Drosophila Females

Stem cells reside in specialized niches and are regulated by a variety of physiological inputs. Adipocytes influence whole-body physiology and stem cell lineages; however, the molecular mechanisms linking adipocytes to stem cells are poorly understood. Here, we report that collagen IV produced in adipocytes is transported to the ovary to maintain proper germline stem cell (GSC) number in adult Drosophila females. Adipocyte-derived collagen IV acts through β-integrin signaling to maintain normal levels of E-cadherin at the niche, thereby ensuring proper adhesion to GSCs. These findings demonstrate that extracellular matrix components produced in adipocytes can be transported to and incorporated into an established adult tissue to influence stem cell number.

Insulin signaling acts in adult adipocytes via GSK-3β and independently of FOXO to control Drosophila female germline stem cell numbers

Tissue-specific stem cells are tied to the nutritional and physiological environment of adult organisms. Adipocytes have key endocrine and nutrient-sensing roles and have emerged as major players in relaying dietary information to regulate other organs. For example, previous studies in Drosophila melanogaster revealed that amino acid sensing as well as diet-dependent metabolic pathways function in adipocytes to influence the maintenance of female germline stem cells (GSCs). How nutrient-sensing pathways acting within adipocytes influence adult stem cell lineages, however, is just beginning to be elucidated. Here, we report that insulin/insulin-like growth factor signaling in adipocytes promotes GSC maintenance, early germline cyst survival, and vitellogenesis. Further, adipocytes use distinct mechanisms downstream of insulin receptor activation to control these aspects of oogenesis, all of which are independent of FOXO. We find that GSC maintenance is modulated by Akt1 through GSK-3β, early germline cyst survival is downstream of adipocyte Akt1 but independent of GSK-3β, and vitellogenesis is regulated through an Akt1-independent pathway in adipocytes. These results indicate that, in addition to employing different types of nutrient sensing, adipocytes can use distinct axes of a single nutrient-sensing pathway to regulate multiple stages of the GSC lineage in the ovary.

Drosophila as a model to study obesity and metabolic disease

Excess adipose fat accumulation, or obesity, is a growing problem worldwide in terms of both the rate of incidence and the severity of obesity-associated metabolic disease. Adipose tissue evolved in animals as a specialized dynamic lipid storage depot: adipose cells synthesize fat (a process called lipogenesis) when energy is plentiful and mobilize stored fat (a process called lipolysis) when energy is needed. When a disruption of lipid homeostasis favors increased fat synthesis and storage with little turnover owing to genetic predisposition, overnutrition or sedentary living, complications such as diabetes and cardiovascular disease are more likely to arise. The vinegar fly Drosophila melanogaster (Diptera: Drosophilidae) is used as a model to better understand the mechanisms governing fat metabolism and distribution. Flies offer a wealth of paradigms with which to study the regulation and physiological effects of fat accumulation. Obese flies accumulate triacylglycerols in the fat body, an organ similar to mammalian adipose tissue, which specializes in lipid storage and catabolism. Discoveries in Drosophila have ranged from endocrine hormones that control obesity to subcellular mechanisms that regulate lipogenesis and lipolysis, many of which are evolutionarily conserved. Furthermore, obese flies exhibit pathophysiological complications, including hyperglycemia, reduced longevity and cardiovascular function - similar to those observed in obese humans. Here, we review some of the salient features of the fly that enable researchers to study the contributions of feeding, absorption, distribution and the metabolism of lipids to systemic physiology.

Drosophila as a model for ageing

Drosophila melanogaster has been a key model in developing our current understanding of the molecular mechanisms of ageing. Of particular note is its role in establishing the evolutionary conservation of reduced insulin and IGF-1-like signaling in promoting healthy ageing. Capitalizing on its many advantages for experimentation, more recent work has revealed how precise nutritional and genetic interventions can improve fly lifespan without obvious detrimental side effects. We give a brief summary of these recent findings as well as examples of how they may modify ageing via actions in the gut and muscle. These discoveries highlight how expanding our understanding of metabolic and signaling interconnections will provide even greater insight into how these benefits may be harnessed for anti-ageing interventions.

Adipocyte Metabolic Pathways Regulated by Diet Control the Female Germline Stem Cell Lineage in Drosophila melanogaster

Nutrients affect adult stem cells through complex mechanisms involving multiple organs. Adipocytes are highly sensitive to diet and have key metabolic roles, and obesity increases the risk for many cancers. How diet-regulated adipocyte metabolic pathways influence normal stem cell lineages, however, remains unclear. Drosophila melanogaster has highly conserved adipocyte metabolism and a well-characterized female germline stem cell (GSC) lineage response to diet. Here, we conducted an isobaric tags for relative and absolute quantification (iTRAQ) proteomic analysis to identify diet-regulated adipocyte metabolic pathways that control the female GSC lineage. On a rich (relative to poor) diet, adipocyte Hexokinase-C and metabolic enzymes involved in pyruvate/acetyl-CoA production are upregulated, promoting a shift of glucose metabolism toward macromolecule biosynthesis. Adipocyte-specific knockdown shows that these enzymes support early GSC progeny survival. Further, enzymes catalyzing fatty acid oxidation and phosphatidylethanolamine synthesis in adipocytes promote GSC maintenance, whereas lipid and iron transport from adipocytes controls vitellogenesis and GSC number, respectively. These results show a functional relationship between specific metabolic pathways in adipocytes and distinct processes in the GSC lineage, suggesting the adipocyte metabolism-stem cell link as an important area of investigation in other stem cell systems.

Organ-to-Organ Communication: A Drosophila Gastrointestinal Tract Perspective

The long-term maintenance of an organism's homeostasis and health relies on the accurate regulation of organ-organ communication. Recently, there has been growing interest in using the Drosophila gastrointestinal tract to elucidate the regulatory programs that underlie the complex interactions between organs. Data obtained in this field have dramatically improved our understanding of how organ-organ communication contributes to the regulation of various aspects of the intestine, including its metabolic and physiological status. However, although research uncovering regulatory programs associated with interorgan communication has provided key insights, the underlying mechanisms have not been extensively explored. In this review, we highlight recent findings describing gut-neighbor and neighbor-neighbor communication models in adults and larvae, respectively, with a special focus on how a range of critical strategies concerning continuous interorgan communication and adjustment can be used to manipulate different aspects of biological processes. Given the high degree of similarity between the Drosophila and mammalian intestinal epithelia, it can be anticipated that further analyses of the Drosophila gastrointestinal tract will facilitate the discovery of similar mechanisms underlying organ-organ communication in other mammalian organs, such as the human intestine.

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

The effect of diet on reproduction is well documented in a large number of organisms; however, much remains to be learned about the molecular mechanisms underlying this connection. The Drosophila ovary has a well described, fast and largely reversible response to diet. Ovarian stem cells and their progeny proliferate and grow faster on a yeast-rich diet than on a yeast-free (poor) diet, and death of early germline cysts, degeneration of early vitellogenic follicles and partial block in ovulation further contribute to the ∼60-fold decrease in egg laying observed on a poor diet. Multiple diet-dependent factors, including insulin-like peptides, the steroid ecdysone, the nutrient sensor Target of Rapamycin, AMP-dependent kinase, and adipocyte factors mediate this complex response. Here, we describe the results of a visual screen using a collection of green fluorescent protein (GFP) protein trap lines to identify additional factors potentially involved in this response. In each GFP protein trap line, an artificial GFP exon is fused in frame to an endogenous protein, such that the GFP fusion pattern parallels the levels and subcellular localization of the corresponding native protein. We identified 53 GFP-tagged proteins that exhibit changes in levels and/or subcellular localization in the ovary at 12-16 hours after switching females from rich to poor diets, suggesting them as potential candidates for future functional studies.

Control of Germline Stem Cell Lineages by Diet and Physiology

Tight coupling of reproduction to environmental factors and physiological status is key to long-term species survival. In particular, highly conserved pathways modulate germline stem cell lineages according to nutrient availability. This chapter focuses on recent in vivo studies in genetic model organisms that shed light on how diet-dependent signals control the proliferation, maintenance, and survival of adult germline stem cells and their progeny. These signaling pathways can operate intrinsically in the germ line, modulate the niche, or act through intermediate organs to influence stem cells and their differentiating progeny. In addition to illustrating the extent of dietary regulation of reproduction, findings from these studies have implications for fertility during aging or disease states.

4E-BP is a target of the GCN2-ATF4 pathway during Drosophila development and aging

Kang et al. show that the GCN2–ATF4 pathway induces 4E-BP transcription in response to amino acid deprivation and also during the development of certain Drosophila tissues. 4E-BP has selective effects on translation; therefore, this pathway helps to shift the mRNA expression profiles of cells.

Reduced amino acid availability attenuates mRNA translation in cells and helps to extend lifespan in model organisms. The amino acid deprivation–activated kinase GCN2 mediates this response in part by phosphorylating eIF2α. In addition, the cap-dependent translational inhibitor 4E-BP is transcriptionally induced to extend lifespan in Drosophila melanogaster, but through an unclear mechanism. Here, we show that GCN2 and its downstream transcription factor, ATF4, mediate 4E-BP induction, and GCN2 is required for lifespan extension in response to dietary restriction of amino acids. The 4E-BP intron contains ATF4-binding sites that not only respond to stress but also show inherent ATF4 activity during normal development. Analysis of the newly synthesized proteome through metabolic labeling combined with click chemistry shows that certain stress-responsive proteins are resistant to inhibition by 4E-BP, and gcn2 mutant flies have reduced levels of stress-responsive protein synthesis. These results indicate that GCN2 and ATF4 are important regulators of 4E-BP transcription during normal development and aging.

Interorgan Communication Pathways in Physiology: Focus on Drosophila

Studies in mammals and Drosophila have demonstrated the existence and significance of secreted factors involved in communication between distal organs. In this review, primarily focusing on Drosophila, we examine the known interorgan communication factors and their functions, physiological inducers, and integration in regulating physiology. Moreover, we describe how organ-sensing screens in Drosophila can systematically identify novel conserved interorgan communication factors. Finally, we discuss how interorgan communication enabled and evolved as a result of specialization of organs. Together, we anticipate that future studies will establish a model for metazoan interorgan communication network (ICN) and how it is deregulated in disease.

Amino acid transporters implicated in endocytosis of Buchnera during symbiont transmission in the pea aphid

Background

Many insects host their obligate, maternally transmitted symbiotic bacteria in specialized cells called bacteriocytes. One of the best-studied insect nutritional endosymbioses is that of the aphid and its endosymbiont, Buchnera aphidicola. Aphids and Buchnera are metabolically and developmentally integrated, but the molecular mechanisms underlying Buchnera transmission and coordination with aphid development remain largely unknown. Previous work using electron microscopy to study aphid asexual embryogenesis has revealed that Buchnera transmission involves exocytosis from a maternal bacteriocyte followed by endocytotic uptake by a blastula. While the importance of exo- and endocytic cellular processes for symbiont transmission is clear, the molecular mechanisms that regulate these processes are not known. Here, we shed light on the molecular mechanisms that regulate Buchnera transmission and developmental integration.

Results

We present the developmental atlas of ACYPI000536 and ACYPI008904 mRNAs during asexual embryogenesis in the pea aphid, Acyrthosiphon pisum. Immediately before Buchnera invasion, transcripts of both genes were detected by whole-mount in situ hybridization in the posterior syncytial nuclei of late blastula embryos. Following Buchnera invasion, expression of both genes was identified in the region occupied by Buchnera throughout embryogenesis. Notably during Buchnera migration, expression of both genes was not concomitant with the entirety of the bacterial mass but rather expression colocalized with Buchnera in the anterior region of the bacterial mass. In addition, we found that ACYPI000536 was expressed in nuclei at the leading edge of the bacterial mass, joining the bacterial mass in subsequent developmental stages. Finally, quantitative reverse transcription real-time PCR suggested that early in development both transcripts were maternally provisioned to embryos.

Conclusions

We venture that ACYPI000536 and ACYPI008904 function as nutrient sensors at the site of symbiont invasion to facilitate TOR-pathway-mediated endocytosis of Buchnera by the aphid blastula. Our data support earlier reports of bacteriocyte determination involving a two-step recruitment process but suggest that the second wave of recruitment occurs earlier than previously described. Finally, our work highlights that bacteriocyte-enriched amino acid transporter paralogs have additionally been retained to play novel developmental roles in both symbiont recruitment and bacteriome development.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0061-7) contains supplementary material, which is available to authorized users.