A genetic strategy to measure circulating Drosophila insulin reveals genes regulating insulin production and secretion

Glut1 Functions in Insulin-Producing Neurons to Regulate Lipid and Carbohydrate Storage in Drosophila

Obesity remains one of the largest health problems in the world, arising from the excess storage of triglycerides (TAGs). However, the full complement of genes that are important for regulating TAG storage is not known. The Glut1 gene encodes a Drosophila glucose transporter that has been identified as a potential obesity gene through genetic screening. Yet, the tissue-specific metabolic functions of Glut1 are not fully understood. Here, we characterized the role of Glut1 in the fly brain by decreasing neuronal Glut1 levels with RNAi and measuring glycogen and TAGs. Glut1RNAi flies had decreased TAG and glycogen levels, suggesting a nonautonomous role of Glut1 in the fly brain to regulate nutrient storage. A group of hormones that regulate metabolism and are expressed in the fly brain are Drosophila insulin-like peptides (Ilps) 2, 3, and 5. Interestingly, we observed blunted Ilp3 and Ilp5 expression in neuronal Glut1RNAi flies, suggesting Glut1 functions in insulin-producing neurons (IPCs) to regulate whole-organism TAG and glycogen storage. Consistent with this hypothesis, we also saw fewer TAGs and glycogens and decreased expression of Ilp3 and Ilp5 in flies with IPC-specific Glut1RNAi. Together, these data suggest Glut1 functions as a nutrient sensor in IPCs, controlling TAG and glycogen storage and regulating systemic energy homeostasis.

Synchronizing Drosophila larvae with the salivary gland reporter Sgs3-GFP for discovery of phenotypes in the late third instar stage

The larval stage of the Drosophila melanogaster life cycle is characterized by rapid growth and nutrient storage that occur over three instar stages separated by molts. In the third instar, the steroid hormone ecdysone drives key developmental processes and behaviors that occur in a temporally-controlled sequence and prepare the animal to undergo metamorphosis. Accurately staging Drosophila larvae within the final third instar is critical due to the rapid developmental progress at this stage, but it is challenging because the rate of development varies widely across a population of animals even if eggs are laid within a short period of time. Moreover, many methods to stage third instar larvae are cumbersome, and inherent variability in the rate of development confounds some of these approaches. Here we demonstrate the usefulness of the Sgs3-GFP transgene, a fusion of the Salivary gland secretion 3 (Sgs3) and GFP proteins, for staging third instar larvae. Sgs3-GFP is expressed in the salivary glands in an ecdysone-dependent manner from the midpoint of the third instar, and its expression pattern changes reproducibly as larvae progress through the third instar. We show that Sgs3-GFP can easily be incorporated into experiments, that it allows collection of developmentally-equivalent individuals from a mixed population of larvae, and that its use enables precise assessment of changing levels of hormones, metabolites, and gene expression during the second half of the third instar.

Drosophila neuronal Glucose-6-Phosphatase is a modulator of neuropeptide release that regulates muscle glycogen stores via FMRFamide signaling

Neuropeptides (NPs) and their cognate receptors are critical effectors of diverse physiological processes and behaviors. We recently reported of a noncanonical function of the Drosophila Glucose-6-Phosphatase (G6P) gene in a subset of neurosecretory cells in the central nervous system that governs systemic glucose homeostasis in food-deprived flies. Here, we show that G6P-expressing neurons define six groups of NP-secreting cells, four in the brain and two in the thoracic ganglion. Using the glucose homeostasis phenotype as a screening tool, we find that neurons located in the thoracic ganglion expressing FMRFamide NPs (FMRFaG6P neurons) are necessary and sufficient to maintain systemic glucose homeostasis in starved flies. We further show that G6P is essential in FMRFaG6P neurons for attaining a prominent Golgi apparatus and secreting NPs efficiently. Finally, we establish that G6P-dependent FMRFa signaling is essential for the build-up of glycogen stores in the jump muscle which expresses the receptor for FMRFamides. We propose a general model in which the main role of G6P is to counteract glycolysis in peptidergic neurons for the purpose of optimizing the intracellular environment best suited for the expansion of the Golgi apparatus, boosting release of NPs and enhancing signaling to respective target tissues expressing cognate receptors.

LGR signaling mediates muscle-adipose tissue crosstalk and protects against diet-induced insulin resistance

Obesity impairs tissue insulin sensitivity and signaling, promoting type-2 diabetes. Although improving insulin signaling is key to reversing diabetes, the multi-organ mechanisms regulating this process are poorly defined. Here, we screen the secretome and receptome in Drosophila to identify the hormonal crosstalk affecting diet-induced insulin resistance and obesity. We discover a complex interplay between muscle, neuronal, and adipose tissues, mediated by Bone Morphogenetic Protein (BMP) signaling and the hormone Bursicon, that enhances insulin signaling and sugar tolerance. Muscle-derived BMP signaling, induced by sugar, governs neuronal Bursicon signaling. Bursicon, through its receptor Rickets, a Leucine-rich-repeat-containing G-protein coupled receptor (LGR), improves insulin secretion and insulin sensitivity in adipose tissue, mitigating hyperglycemia. In mouse adipocytes, loss of the Rickets ortholog LGR4 blunts insulin responses, showing an essential role of LGR4 in adipocyte insulin sensitivity. Our findings reveal a muscle-neuronal-fat-tissue axis driving metabolic adaptation to high-sugar conditions, identifying LGR4 as a critical mediator in this regulatory network.

A comprehensive in-vitro/in-vivo screening toolbox for the elucidation of glucose homeostasis modulating properties of plant extracts (from roots) and its bioactives

Plant extracts are increasingly recognized for their potential in modulating (postprandial) blood glucose levels. In this context, root extracts are of particular interest due to their high concentrations and often unique spectrum of plant bioactives. To identify new plant species with potential glucose-lowering activity, simple and robust methodologies are often required. For this narrative review, literature was sourced from scientific databases (primarily PubMed) in the period from June 2022 to January 2024. The regulatory targets of glucose homeostasis that could be modulated by bioactive plant compounds were used as search terms, either alone or in combination with the keyword "root extract". As a result, we present a comprehensive methodological toolbox for studying the glucose homeostasis modulating properties of plant extracts and its constituents. The described assays encompass in-vitro investigations involving enzyme inhibition (α-amylase, α-glucosidase, dipeptidyl peptidase 4), assessment of sodium-dependent glucose transporter 1 activity, and evaluation of glucose transporter 4 translocation. Furthermore, we describe a patch-clamp technique to assess the impact of extracts on KATP channels. While validating in-vitro findings in living organisms is imperative, we introduce two screenable in-vivo models (the hen's egg test and Drosophila melanogaster). Given that evaluation of the bioactivity of plant extracts in rodents and humans represents the current gold standard, we include approaches addressing this aspect. In summary, this review offers a systematic guide for screening plant extracts regarding their influence on key regulatory elements of glucose homeostasis, culminating in the assessment of their potential efficacy in-vivo. Moreover, application of the presented toolbox might contribute to further close the knowledge gap on the precise mechanisms of action of plant-derived compounds.

A brain-derived insulin signal encodes protein satiety for nutrient-specific feeding inhibition

The suppressive effect of insulin on food intake has been documented for decades. However, whether insulin signals can encode a certain type of nutrients to regulate nutrient-specific feeding behavior remains elusive. Here, we show that in female Drosophila, a pair of dopaminergic neurons, tritocerebrum 1-dopaminergic neurons (T1-DANs), are directly activated by a protein-intake-induced insulin signal from insulin-producing cells (IPCs). Intriguingly, opto-activating IPCs elicits feeding inhibition for both protein and sugar, while silencing T1-DANs blocks this inhibition only for protein food. Elevating insulin signaling in T1-DANs or opto-activating these neurons is sufficient to mimic protein satiety. Furthermore, this signal is conveyed to local neurons of the protocerebral bridge (PB-LNs) and specifically suppresses protein intake. Therefore, our findings reveal that a brain-derived insulin signal encodes protein satiety and suppresses feeding behavior in a nutrient-specific manner, shedding light on the functional specificity of brain insulin signals in regulating behaviors.

A nutrient responsive lipase mediates gut-brain communication to regulate insulin secretion in Drosophila

Pancreatic β cells secrete insulin in response to glucose elevation to maintain glucose homeostasis. A complex network of inter-organ communication operates to modulate insulin secretion and regulate glucose levels after a meal. Lipids obtained from diet or generated intracellularly are known to amplify glucose-stimulated insulin secretion, however, the underlying mechanisms are not completely understood. Here, we show that a Drosophila secretory lipase, Vaha (CG8093), is synthesized in the midgut and moves to the brain where it concentrates in the insulin-producing cells in a process requiring Lipid Transfer Particle, a lipoprotein originating in the fat body. In response to dietary fat, Vaha stimulates insulin-like peptide release (ILP), and Vaha deficiency results in reduced circulatory ILP and diabetic features including hyperglycemia and hyperlipidemia. Our findings suggest Vaha functions as a diacylglycerol lipase physiologically, by being a molecular link between dietary fat and lipid amplified insulin secretion in a gut-brain axis.

Dynamics of cuticle-associated transcript profiles during moulting of the bed bug Cimexlectularius

The bed bug Cimex lectularius is a worldwide human pest. The sequenced genome allows molecular analyses of all aspects of bed bug biology. The present work was conducted to contribute to bed bug cuticle biology. As in other insect species, the C. lectularius cuticle consists of the three horizontal layers procuticle, epicuticle and envelope. To analyse the genes needed for the establishment of the stratified cuticle, we studied the expression pattern of 42 key cuticle-related genes at the transition of the penultimate nymphal stage to adult animals when a new cuticle is formed. Based on gene expression dynamics, in simplified model, we distinguish two key events during cuticle renewal in C. lectularius. First, upon blood feeding, modulation of ecdysone signalling culminates in the transcriptional activation of the transcription factor Clec-Ftz-F1 that possibly controls the expression of 32 of the 42 genes tested. Second, timed expression of Clec-Ftz-F1 seems to depend also on the insulin signalling pathway as RNA interference against transcripts of the insulin receptor delays Clec-Ftz-F1 expression and stage transition. An important observation of our transcript survey is that genes needed for the construction of the three cuticle layers are largely expressed simultaneously. Based on these data, we hypothesise a considerable synchronous mechanism of layer formation rather than a strictly sequential one. Together, this work provides a basis for functional analyses of cuticle formation in C. lectularius.

Drosophila Neuronal Glucose 6 Phosphatase is a Modulator of Neuropeptide Release that Regulates Muscle Glycogen Stores via FMRFamide Signaling

Neuropeptides (NPs) and their cognate receptors are critical effectors of diverse physiological processes and behaviors. We recently reported of a non-canonical function of the Drosophila Glucose-6-Phosphatase ( G6P ) gene in a subset of neurosecretory cells in the CNS that governs systemic glucose homeostasis in food deprived flies. Here, we show that G6P expressing neurons define 6 groups of neuropeptide secreting cells, 4 in the brain and 2 in the thoracic ganglion. Using the glucose homeostasis phenotype as a screening tool, we find that neurons located in the thoracic ganglion expressing FMRFamide neuropeptides ( FMRFa G6P neurons) are necessary and sufficient to maintain systemic glucose homeostasis in starved flies. We further show that G6P is essential in FMRFa G6P neurons for attaining a prominent Golgi apparatus and secreting neuropeptides efficiently. Finally, we establish that G6P dependent FMRFa signaling is essential for the build-up of glycogen stores in the jump muscle which expresses the receptor for FMRFamides. We propose a general model in which the main role of G6P is to counteract glycolysis in peptidergic neurons for the purpose of optimizing the intracellular environment best suited for the expansion of the Golgi apparatus, boosting release of neuropeptides and enhancing signaling to respective target tissues expressing cognate receptors.

SIGNIFICANCE STATEMENT

Glucose-6-phosphtase (G6P) is a critical enzyme in sugar synthesis and catalyzes the final step in glucose production. In Drosophila - and insects in general - where trehalose is the circulating sugar and Trehalose phosphate synthase, and not G6P, is used for sugar production, G6P has adopted a novel and unique role in peptidergic neurons in the CNS. Interestingly, flies lacking G6P show diminished Neuropeptide secretions and have a smaller Golgi apparatus in peptidergic neurons. It is hypothesized that the role of G6P is to counteract glycolysis, thereby creating a cellular environment that is more amenable to efficient neuropeptide secretion.

Dietary lithium stimulates female fecundity in Drosophila melanogaster

The trace element lithium exerts a versatile bioactivity in humans, to some extend overlapping with in vivo findings in the model organism Drosophila melanogaster. A potentially essential function of lithium in reproduction has been suggested since the 1980s and multiple studies have since been published postulating a regulatory role of lithium in female gametogenesis. However, the impact of lithium on fruit fly egg production has not been at the center of attention to date. In the present study, we report that dietary lithium (0.1-5.0 mM LiCl) substantially improved life time egg production in D. melanogaster w1118 females, with a maximum increase of plus 45% when supplementing 1.0 mM LiCl. This phenomenon was not observed in the insulin receptor mutant InRE19, indicating a potential involvement of insulin-like signaling in the lithium-mediated fecundity boost. Analysis of the whole-body and ovarian transcriptome revealed that dietary lithium affects the mRNA levels of genes encoding proteins related to processes of follicular maturation. To the best of our knowledge, this is the first report on dietary lithium acting as an in vivo fecundity stimulant in D. melanogaster, further supporting the suggested benefit of the trace element in female reproduction.

Ecdysone-induced microRNA miR-276a-3p controls developmental growth by targeting the insulin-like receptor in Drosophila

Animal growth is controlled by a variety of external and internal factors during development. The steroid hormone ecdysone plays a critical role in insect development by regulating the expression of various genes. In this study, we found that fat body-specific expression of miR-276a, an ecdysone-responsive microRNA (miRNA), led to a decrease in the total mass of the larval fat body, resulting in significant growth reduction in Drosophila. Changes in miR-276a expression also affected the proliferation of Drosophila S2 cells. Furthermore, we found that the insulin-like receptor (InR) is a biologically relevant target gene regulated by miR-276a-3p. In addition, we found that miR-276a-3p is upregulated by the canonical ecdysone signalling pathway involving the ecdysone receptor and broad complex. A reduction in cell proliferation caused by ecdysone was compromised by blocking miR-276a-3p activity. Thus, our results suggest that miR-276a-3p is involved in ecdysone-mediated growth reduction by controlling InR expression in the insulin signalling pathway.

Insulin-like peptides and ovary ecdysteroidogenic hormone differentially stimulate physiological processes regulating egg formation in the mosquito Aedes aegypti

Mosquitoes including Aedes aegypti are human disease vectors because females must blood feed to produce and lay eggs. Blood feeding triggers insulin-insulin growth factor signaling (IIS) which regulates several physiological processes required for egg development. A. aegypti encodes 8 insulin-like peptides (ILPs) and one insulin-like receptor (IR) plus ovary ecdysteroidogenic hormone (OEH) that also activates IIS through the OEH receptor (OEHR). In this study, we assessed the expression of A. aegypti ILPs and OEH during a gonadotrophic cycle and produced each that were functionally characterized to further understand their roles in regulating egg formation. All A. aegypti ILPs and OEH were expressed during a gonadotrophic cycle. Five ILPs (1, 3, 4, 7, 8) and OEH were specifically expressed in the head, while antibodies to ILP3 and OEH indicated each was released after blood feeding from ventricular axons that terminate on the anterior midgut. A subset of ILP family members and OEH stimulated nutrient storage in previtellogenic females before blood feeding, whereas most IIS-dependent processes after blood feeding were activated by one or more of the brain-specific ILPs and/or OEH. ILPs and OEH with different biological activities also exhibited differences in IIS as measured by phosphorylation of the IR, phosphoinositide 3-kinase/Akt kinase (AKT) and mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK). Altogether, our results provide the first results that compare the functional activities of all ILP family members and OEH produced by an insect.

Insulin signaling in development

Nutrient intake is obligatory for animal growth and development, but nutrients alone are not sufficient. Indeed, insulin and homologous hormones are required for normal growth even in the presence of nutrients. These hormones communicate nutrient status between organs, allowing animals to coordinate growth and metabolism with nutrient supply. Insulin and related hormones, such as insulin-like growth factors and insulin-like peptides, play important roles in development and metabolism, with defects in insulin production and signaling leading to hyperglycemia and diabetes. Here, we describe the insulin hormone family and the signal transduction pathways activated by these hormones. We highlight the roles of insulin signaling in coordinating maternal and fetal metabolism and growth during pregnancy, and we describe how secretion of insulin is regulated at different life stages. Additionally, we discuss the roles of insulin signaling in cell growth, stem cell proliferation and cell differentiation. We provide examples of the role of insulin in development across multiple model organisms: Caenorhabditis elegans, Drosophila, zebrafish, mouse and human.

Identifying target ion channel-related genes to construct a diagnosis model for insulinoma

Background: Insulinoma is the most common functional pancreatic neuroendocrine tumor (PNET) with abnormal insulin hypersecretion. The etiopathogenesis of insulinoma remains indefinable. Based on multiple bioinformatics methods and machine learning algorithms, this study proposed exploring the molecular mechanism from ion channel-related genes to establish a genetic diagnosis model for insulinoma. Methods: The mRNA expression profile dataset of GSE73338 was applied to the analysis, which contains 17 insulinoma samples, 63 nonfunctional PNET (NFPNET) samples, and four normal islet samples. Differently expressed ion channel-related genes (DEICRGs) enrichment analyses were performed. We utilized the protein-protein interaction (PPI) analysis and machine learning of LASSO and support vector machine-recursive feature elimination (SVM-RFE) to identify the target genes. Based on these target genes, a nomogram diagnostic model was constructed and verified by a receiver operating characteristic (ROC) curve. Moreover, immune infiltration analysis, single-gene gene set enrichment analysis (GSEA), and gene set variation analysis (GSVA) were executed. Finally, a drug-gene interaction network was constructed. Results: We identified 29 DEICRGs, and enrichment analyses indicated they were primarily enriched in ion transport, cellular ion homeostasis, pancreatic secretion, and lysosome. Moreover, the PPI network and machine learning recognized three target genes (MCOLN1, ATP6V0E1, and ATP4A). Based on these target genes, we constructed an efficiently predictable diagnosis model for identifying insulinomas with a nomogram and validated it with the ROC curve (AUC = 0.801, 95% CI 0.674-0.898). Then, single-gene GSEA analysis revealed that these target genes had a significantly positive correlation with insulin secretion and lysosome. In contrast, the TGF-beta signaling pathway was negatively associated with them. Furthermore, statistically significant discrepancies in immune infiltration were revealed. Conclusion: We identified three ion channel-related genes and constructed an efficiently predictable diagnosis model to offer a novel approach for diagnosing insulinoma.

A genetic strategy to measure insulin signaling regulation and physiology in Drosophila

Insulin regulation is a hallmark of health, and impaired insulin signaling promotes metabolic diseases like diabetes mellitus. However, current assays for measuring insulin signaling in all animals remain semi-quantitative and lack the sensitivity, tissue-specificity or temporal resolution needed to quantify in vivo physiological signaling dynamics. Insulin signal transduction is remarkably conserved across metazoans, including insulin-dependent phosphorylation and regulation of Akt/Protein kinase B. Here, we generated transgenic fruit flies permitting tissue-specific expression of an immunoepitope-labelled Akt (AktHF). We developed enzyme-linked immunosorption assays (ELISA) to quantify picomolar levels of phosphorylated (pAktHF) and total AktHF in single flies, revealing dynamic tissue-specific physiological regulation of pAktHF in response to fasting and re-feeding, exogenous insulin, or targeted genetic suppression of established insulin signaling regulators. Genetic screening revealed Pp1-87B as an unrecognized regulator of Akt and insulin signaling. Tools and concepts here provide opportunities to discover tissue-specific regulators of in vivo insulin signaling responses.

RPS6 transcriptional modulation in neural tissues of Nauphoeta cinerea during streptozotocin-associated sugar metabolism impairment

The use of insects to model molecular events that characterize degenerative conditions was originally met with scepticism. However, the discovery of insect insulin-like peptides in the 1970's and the demonstration of evolutionary conservation of insulin-related signalling from insects to mammals have highlighted the importance and reduced cost of insect models in biomedical research. Here, we expand on our earlier described modelling of streptozotocin-induced brain glucose metabolic disruption in Nauphoeta cinerea, using RNA-sequencing analysis to study the transcriptional and genetic signatures of degeneration and stress signalling when glucose levels are elevated in the brain of the lobster cockroach. Nymphs were randomly divided into three groups: Control (0.8% NaCl), and two single streptozotocin injection doses (74 nmol and 740 nmol). The transcriptional analyses featured a dysregulation of 226 genes at high dose STZ treatment and 278 genes at the low dose. Our mRNA-sequencing data showed that ribosomal protein genes were the most upregulated genes at both 74 and 740 nmol STZ treatment. We therefore used RT-qPCR and relative transcriptional methods to validate our proposed mechanism of brain glucose toxicity-induced degeneration in Nauphoeta cinerea, which involved the upregulation of ribosomal proteins and rpS6 regulators (mTORC1, protein kinases, casein kinase 1 and Death-associated protein kinase), the upregulation of MAPK cascades (RAS, ERK, P38 and JNK), alongside the downregulation of the PI3K/AKT cascade. Taken together, this study highlights the remarkable opportunity for Nauphoeta cinerea use as an experimental organism in hyperglycaemia, degeneration, and stress signalling.

Isolation of a novel missense mutation in insulin receptor as a spontaneous revertant in ImpL2 mutants in Drosophila

Evolutionarily conserved insulin/insulin-like growth factor (IGF) signaling (IIS) correlates nutrient levels to metabolism and growth, thereby playing crucial roles in development and adult fitness. In the fruit fly Drosophila, ImpL2, an ortholog of IGFBP7, binds to and inhibits the function of Drosophila insulin-like peptides. In this study, we isolated a temperature-sensitive mutation in the insulin receptor (InR) gene as a spontaneous revertant in ImpL2 null mutants. The p.Y902C missense mutation is located at the functionally conserved amino acid residue of the first fibronectin type III domain of InR. The hypomorphic InR mutant animals showed a temperature-dependent reduction in IIS and body size. The mutant animals also exhibited metabolic defects, such as increased triglyceride and carbohydrate levels. Metabolomic analysis further revealed that defects in InR caused dysregulation of amino acid and ribonucleotide metabolism. We also observed that InR mutant females produced tiny irregular-shaped embryos with reduced fecundity. In summary, this novel allele of InR is a valuable tool for the Drosophila genetic model of insulin resistance and type 2 diabetes.

Interorgan communication through peripherally derived peptide hormones in Drosophila

In multicellular organisms, endocrine factors such as hormones and cytokines regulate development and homoeostasis through communication between different organs. For understanding such interorgan communications through endocrine factors, the fruit fly Drosophila melanogaster serves as an excellent model system due to conservation of essential endocrine systems between flies and mammals and availability of powerful genetic tools. In Drosophila and other insects, functions of neuropeptides or peptide hormones from the central nervous system have been extensively studied. However, a series of recent studies conducted in Drosophila revealed that peptide hormones derived from peripheral tissues also play critical roles in regulating multiple biological processes, including growth, metabolism, reproduction, and behaviour. Here, we summarise recent advances in understanding target organs/tissues and functions of peripherally derived peptide hormones in Drosophila and describe how these hormones contribute to various biological events through interorgan communications.

Genome-wide analysis identifies Homothorax and Extradenticle as regulators of insulin in Drosophila Insulin-Producing cells

Drosophila Insulin-Producing Cells (IPCs) are the main production site of the Drosophila Insulin-like peptides or dilps which have key roles in regulating growth, development, reproduction, lifespan and metabolism. To better understand the signalling pathways and transcriptional networks that are active in the IPCs we queried publicly available transcriptome data of over 180 highly inbred fly lines for dilp expression and used dilp expression as the input for a Genome-wide association study (GWAS). This resulted in the identification of variants in 125 genes that were associated with variation in dilp expression. The function of 57 of these genes in the IPCs was tested using an RNAi-based approach. We found that IPC-specific depletion of most genes resulted in differences in expression of one or more of the dilps. We then elaborated further on one of the candidate genes with the strongest effect on dilp expression, Homothorax, a transcription factor known for its role in eye development. We found that Homothorax and its binding partner Extradenticle are involved in regulating dilp2, -3 and -5 expression and that genetic depletion of both TFs shows phenotypes associated with reduced insulin signalling. Furthermore, we provide evidence that other transcription factors involved in eye development are also functional in the IPCs. In conclusion, we showed that this expression level-based GWAS approach identified genetic regulators implicated in IPC function and dilp expression.

Insulin signalling has a central and evolutionarily conserved role in many processes including growth, development, reproduction, lifespan, stress resistance and metabolic homeostasis. In the fruitfly Drosophila melanogaster insulin-producing cells in the brain are the main source of three insulin-like peptides, Dilp2, -3 and -5. How the production and secretion of these three insulin-like peptides are regulated remains incompletely understood. In the current study, genome-wide association studies were used to identify 50 novel regulators of Dilp2, -3 and -5. We show that one of the top candidate regulators, Homothorax, is an important regulator of dilp2, -3 and –5 expression in the IPCs and is necessary for normal systemic insulin signalling and regulates adult size and developmental timing. We also show that the Hth interactor Extradenticle (Exd) is equally required in the adult but not in the larval IPCs. Finally, we show that most genes of the so-called retinal determination gene network are expressed in the IPCs and regulate normal dilp2 and -5 expression. Together, these results identify further regulatory levels active in the IPCs and implicate a reshuffled version of a previously identified gene regulatory network therein.

An improved organ explant culture method reveals stem cell lineage dynamics in the adult Drosophila intestine

In recent years, live-imaging techniques have been developed for the adult midgut of Drosophila melanogaster that allow temporal characterization of key processes involved in stem cell and tissue homeostasis. However, these organ culture techniques have been limited to imaging sessions of <16 hours, an interval too short to track dynamic processes such as damage responses and regeneration, which can unfold over several days. Therefore, we developed an organ explant culture protocol capable of sustaining midguts ex vivo for up to 3 days. This was made possible by the formulation of a culture medium specifically designed for adult Drosophila tissues with an increased Na+/K+ ratio and trehalose concentration, and by placing midguts at an air-liquid interface for enhanced oxygenation. We show that midgut progenitor cells can respond to gut epithelial damage ex vivo, proliferating and differentiating to replace lost cells, but are quiescent in healthy intestines. Using ex vivo gene induction to promote stem cell proliferation using RasG12V or string and Cyclin E overexpression, we demonstrate that progenitor cell lineages can be traced through multiple cell divisions using live imaging. We show that the same culture set-up is useful for imaging adult renal tubules and ovaries for up to 3 days and hearts for up to 10 days. By enabling both long-term imaging and real-time ex vivo gene manipulation, our simple culture protocol provides a powerful tool for studies of epithelial biology and cell lineage behavior.

Drosophila melanogaster diabetes models and its usage in the research of anti-diabetes management with traditional Chinese medicines

The prevalence of diabetes mellitus (DM) is increasing rapidly worldwide, but the underlying molecular mechanisms of disease development have not been elucidated, and the current popular anti-diabetic approaches still have non-negligible limitations. In the last decades, several different DM models were established on the classic model animal, the fruit fly (Drosophila melanogaster), which provided a convenient way to study the mechanisms underlying diabetes and to discover and evaluate new anti-diabetic compounds. In this article, we introduce the Drosophila Diabetes model from three aspects, including signal pathways, established methods, and pharmacodynamic evaluations. As a highlight, the progress in the treatments and experimental studies of diabetes with Traditional Chinese Medicine (TCM) based on the Drosophila Diabetes model is reviewed. We believe that the values of TCMs are underrated in DM management, and the Drosophila Diabetes models can provide a much more efficient tool to explore its values of it.

Adipose mitochondrial metabolism controls body growth by modulating systemic cytokine and insulin signaling

Animals must adapt their growth to fluctuations in nutrient availability to ensure proper development. These adaptations often rely on specific nutrient-sensing tissues that control whole-body physiology through inter-organ communication. While the signaling mechanisms that underlie this communication are well studied, the contributions of metabolic alterations in nutrient-sensing tissues are less clear. Here, we show how the reprogramming of adipose mitochondria controls whole-body growth in Drosophila larvae. We find that dietary nutrients alter fat-body mitochondrial morphology to lower their bioenergetic activity, leading to rewiring of fat-body glucose metabolism. Strikingly, we find that genetic reduction of mitochondrial bioenergetics just in the fat body is sufficient to accelerate body growth and development. These growth effects are caused by inhibition of the fat-derived secreted peptides ImpL2 and tumor necrosis factor alpha (TNF-α)/Eiger, leading to enhanced systemic insulin signaling. Our work reveals how reprogramming of mitochondrial metabolism in one nutrient-sensing tissue can couple nutrient availability to whole-body growth.

WAKE-mediated modulation of cVA perception via a hierarchical neuro-endocrine axis in Drosophila male-male courtship behaviour

The nervous and endocrine systems coordinate with each other to closely influence physiological and behavioural responses in animals. Here we show that WAKE (encoded by wide awake, also known as wake) modulates membrane levels of GABA_A receptor Resistance to Dieldrin (Rdl), in insulin-producing cells of adult male Drosophila melanogaster. This results in changes to secretion of insulin-like peptides which is associated with changes in juvenile hormone biosynthesis in the corpus allatum, which in turn leads to a decrease in 20-hydroxyecdysone levels. A reduction in ecdysone signalling changes neural architecture and lowers the perception of the male-specific sex pheromone 11-cis-vaccenyl acetate by odorant receptor 67d olfactory neurons. These finding explain why WAKE-deficient in Drosophila elicits significant male-male courtship behaviour.

The authors show that the Drosophila master regulator WAKE modulates the secretion of insulin-like peptides, triggering a decrease in 20-hydroxyecdysone levels. This lowers the perception of a male-specific sex pheromone and explains why WAKE-deficient Drosophila flies show male-male courtship behaviour.

Suppression of intestinal dysfunction in a Drosophila model of Parkinson's disease is neuroprotective

The innate immune response mounts a defense against foreign invaders and declines with age. An inappropriate induction of this response can cause diseases. Previous studies showed that mitochondria can be repurposed to promote inflammatory signaling. Damaged mitochondria can also trigger inflammation and promote diseases. Mutations in pink1, a gene required for mitochondrial health, cause Parkinson's disease, and Drosophila melanogaster pink1 mutants accumulate damaged mitochondria. Here, we show that defective mitochondria in pink1 mutants activate Relish targets and demonstrate that inflammatory signaling causes age-dependent intestinal dysfunction in pink1-mutant flies. These effects result in the death of intestinal cells, metabolic reprogramming and neurotoxicity. We found that Relish signaling is activated downstream of a pathway stimulated by cytosolic DNA. Suppression of Relish in the intestinal midgut of pink1-mutant flies restores mitochondrial function and is neuroprotective. We thus conclude that gut-brain communication modulates neurotoxicity in a fly model of Parkinson's disease through a mechanism involving mitochondrial dysfunction.

A local insulin reservoir in Drosophila alpha cell homologs ensures developmental progression under nutrient shortage

Insulin/insulin-like growth factor (IGF) signaling (IIS) controls many aspects of development and physiology. In Drosophila, a conserved family of insulin-like peptides called Dilps is produced by brain neurosecretory cells, and it regulates organismal growth and developmental timing. To accomplish these systemic functions, the Dilps are secreted into the general circulation, and they signal to peripheral tissues in an endocrine fashion. Here, we describe the local uptake and storage of Dilps in the corpora cardiaca (CC), an endocrine organ composed of alpha cell homologs known to produce the glucagon-like adipokinetic hormone (AKH). We show that Dilp uptake by the CC relies on the expression of an IGF-binding protein called ImpL2. Following their uptake, immunogold staining demonstrates that Dilps are co-packaged with AKH in dense-core vesicles for secretion. In response to nutrient shortage, this specific Dilp reservoir is released and activates IIS in a paracrine manner in the prothoracic gland. This stimulates the production of the steroid hormone ecdysone and initiates entry into pupal development. We therefore uncover a sparing mechanism whereby insulin stores in CC serve to locally activate IIS and the production of ecdysone in the PG, accelerating developmental progression in adverse food conditions.

Cholecystokinin/sulfakinin peptide signaling: conserved roles at the intersection between feeding, mating and aggression

Neuropeptides are the most diverse messenger molecules in metazoans and are involved in regulation of daily physiology and a wide array of behaviors. Some neuropeptides and their cognate receptors are structurally and functionally well conserved over evolution in bilaterian animals. Among these are peptides related to gastrin and cholecystokinin (CCK). In mammals, CCK is produced by intestinal endocrine cells and brain neurons, and regulates gall bladder contractions, pancreatic enzyme secretion, gut functions, satiety and food intake. Additionally, CCK plays important roles in neuromodulation in several brain circuits that regulate reward, anxiety, aggression and sexual behavior. In invertebrates, CCK-type peptides (sulfakinins, SKs) are, with a few exceptions, produced by brain neurons only. Common among invertebrates is that SKs mediate satiety and regulate food ingestion by a variety of mechanisms. Also regulation of secretion of digestive enzymes has been reported. Studies of the genetically tractable fly Drosophila have advanced our understanding of SK signaling mechanisms in regulation of satiety and feeding, but also in gustatory sensitivity, locomotor activity, aggression and reproductive behavior. A set of eight SK-expressing brain neurons plays important roles in regulation of these competing behaviors. In males, they integrate internal state and external stimuli to diminish sex drive and increase aggression. The same neurons also diminish sugar gustation, induce satiety and reduce feeding. Although several functional roles of CCK/SK signaling appear conserved between Drosophila and mammals, available data suggest that the underlying mechanisms differ.

Nutrients and pheromones promote insulin release to inhibit courtship drive

Food and reproduction are the fundamental needs for all animals. However, the neural mechanisms that orchestrate nutrient intake and sexual behaviors are not well understood. Here, we find that sugar feeding immediately suppresses sexual drive of male Drosophila, a regulation mediated by insulin that acts on insulin receptors on the courtship-promoting P1 neurons. The same pathway was co-opted by anaphrodisiac pheromones to suppress sexual hyperactivity to suboptimal mates. Activated by repulsive pheromones, male-specific PPK23 neurons on the leg tarsus release crustacean cardioactive peptide (CCAP) that acts on CCAP receptor on the insulin-producing cells in the brain to trigger insulin release, which then inhibits P1 neurons. Our results reveal how male flies avoid promiscuity by balancing the weight between aphrodisiac and anaphrodisiac inputs from multiple peripheral sensory pathways and nutritional states. Such a regulation enables male animals to make an appropriate mating decision under fluctuating feeding conditions.

Transgenic Drosophila lines for LexA-dependent gene and growth regulation

Conditional expression of short hairpin RNAs with binary genetic systems is an indispensable tool for studying gene function. Addressing mechanisms underlying cell-cell communication in vivo benefits from simultaneous use of 2 independent gene expression systems. To complement the abundance of existing Gal4/UAS-based resources in Drosophila, we and others have developed LexA/LexAop-based genetic tools. Here, we describe experimental and pedagogical advances that promote the efficient conversion of Drosophila Gal4 lines to LexA lines, and the generation of LexAop-short hairpin RNA lines to suppress gene function. We developed a CRISPR/Cas9-based knock-in system to replace Gal4 coding sequences with LexA, and a LexAop-based short hairpin RNA expression vector to achieve short hairpin RNA-mediated gene silencing. We demonstrate the use of these approaches to achieve targeted genetic loss-of-function in multiple tissues. We also detail our development of secondary school curricula that enable students to create transgenic flies, thereby magnifying the production of well-characterized LexA/LexAop lines for the scientific community. The genetic tools and teaching methods presented here provide LexA/LexAop resources that complement existing resources to study intercellular communication coordinating metazoan physiology and development.

Nutrient Sensing via Gut in Drosophila melanogaster

Nutrient-sensing mechanisms in animals' sense available nutrients to generate a physiological regulatory response involving absorption, digestion, and regulation of food intake and to maintain glucose and energy homeostasis. During nutrient sensing via the gastrointestinal tract, nutrients interact with receptors on the enteroendocrine cells in the gut, which in return respond by secreting various hormones. Sensing of nutrients by the gut plays a critical role in transmitting food-related signals to the brain and other tissues informing the composition of ingested food to digestive processes. These signals modulate feeding behaviors, food intake, metabolism, insulin secretion, and energy balance. The increasing significance of fly genetics with the availability of a vast toolbox for studying physiological function, expression of chemosensory receptors, and monitoring the gene expression in specific cells of the intestine makes the fly gut the most useful tissue for studying the nutrient-sensing mechanisms. In this review, we emphasize on the role of Drosophila gut in nutrient-sensing to maintain metabolic homeostasis and gut-brain cross talk using endocrine and neuronal signaling pathways stimulated by internal state or the consumption of various dietary nutrients. Overall, this review will be useful in understanding the post-ingestive nutrient-sensing mechanisms having a physiological and pathological impact on health and diseases.

Insulin signaling couples growth and early maturation to cholesterol intake in Drosophila

Nutrition is one of the most important influences on growth and the timing of maturational transitions including mammalian puberty and insect metamorphosis. Childhood obesity is associated with precocious puberty, but the assessment mechanism that links body fat to early maturation is unknown. During development, the intake of nutrients promotes signaling through insulin-like systems that govern the growth of cells and tissues and also regulates the timely production of the steroid hormones that initiate the juvenile-adult transition. We show here that the dietary lipid cholesterol, which is required as a component of cell membranes and as a substrate for steroid biosynthesis, also governs body growth and maturation in Drosophila via promoting the expression and release of insulin-like peptides. This nutritional input acts via the nutrient sensor TOR, which is regulated by the Niemann-Pick-type-C 1 (Npc1) cholesterol transporter, in the glia of the blood-brain barrier and cells of the adipose tissue to remotely drive systemic insulin signaling and body growth. Furthermore, increasing intracellular cholesterol levels in the steroid-producing prothoracic gland strongly promotes endoreduplication, leading to an accelerated attainment of a nutritional checkpoint that normally ensures that animals do not initiate maturation prematurely. These findings, therefore, show that a Npc1-TOR signaling system couples the sensing of the lipid cholesterol with cellular and systemic growth control and maturational timing, which may help explain both the link between cholesterol and cancer as well as the connection between body fat (obesity) and early puberty.

Endocrine Regulation of Lifespan in Insect Diapause

Diapause is a physiological adaptation to conditions that are unfavorable for growth or reproduction. During diapause, animals become long-lived, stress-resistant, developmentally static, and non-reproductive, in the case of diapausing adults. Diapause has been observed at all developmental stages in both vertebrates and invertebrates. In adults, diapause traits weaken into adaptations such as hibernation, estivation, dormancy, or torpor, which represent evolutionarily diverse versions of the traditional diapause traits. These traits are regulated through modifications of the endocrine program guiding development. In insects, this typically includes changes in molting hormones, as well as metabolic signals that limit growth while skewing the organism's energetic demands toward conservation. While much work has been done to characterize these modifications, the interactions between hormones and their downstream consequences are incompletely understood. The current state of diapause endocrinology is reviewed here to highlight the relevance of diapause beyond its use as a model to study seasonality and development. Specifically, insect diapause is an emerging model to study mechanisms that determine lifespan. The induction of diapause represents a dramatic change in the normal progression of age. Hormones such as juvenile hormone, 20-hydroxyecdysone, and prothoracicotropic hormone are well-known to modulate this plasticity. The induction of diapause-and by extension, the cessation of normal aging-is coordinated by interactions between these pathways. However, research directly connecting diapause endocrinology to the biology of aging is lacking. This review explores connections between diapause and aging through the perspective of endocrine signaling. The current state of research in both fields suggests appreciable overlap that will greatly contribute to our understanding of diapause and lifespan determination.

Protein visualization and manipulation in Drosophila through the use of epitope tags recognized by nanobodies

Expansion of the available repertoire of reagents for visualization and manipulation of proteins will help understand their function. Short epitope tags linked to proteins of interest and recognized by existing binders such as nanobodies facilitate protein studies by obviating the need to isolate new antibodies directed against them. Nanobodies have several advantages over conventional antibodies, as they can be expressed and used as tools for visualization and manipulation of proteins in vivo. Here, we characterize two short (<15aa) NanoTag epitopes, 127D01 and VHH05, and their corresponding high-affinity nanobodies. We demonstrate their use in Drosophila for in vivo protein detection and re-localization, direct and indirect immunofluorescence, immunoblotting, and immunoprecipitation. We further show that CRISPR-mediated gene targeting provides a straightforward approach to tagging endogenous proteins with the NanoTags. Single copies of the NanoTags, regardless of their location, suffice for detection. This versatile and validated toolbox of tags and nanobodies will serve as a resource for a wide array of applications, including functional studies in Drosophila and beyond.

The Glucagon-Like Adipokinetic Hormone in Drosophila melanogaster - Biosynthesis and Secretion

Metabolic homeostasis requires the precise regulation of circulating sugar titers. In mammals, homeostatic control of circulating sugar titers requires the coordinated secretion and systemic activities of glucagon and insulin. Metabolic homeostasis is similarly regulated in Drosophila melanogaster through the glucagon-like adipokinetic hormone (AKH) and the Drosophila insulin-like peptides (DILPs). In flies and mammals, glucagon and AKH are biosynthesized in and secreted from specialized endocrine cells. KATP channels borne on these cells respond to fluctuations in circulating glucose titers and thereby regulate glucagon secretion. The influence of glucagon in the pathogenesis of type 2 diabetes mellitus is now recognized, and a crucial mechanism that regulates glucagon secretion was reported nearly a decade ago. Ongoing efforts to develop D. melanogaster models for metabolic syndrome must build upon this seminal work. These efforts make a critical review of AKH physiology timely. This review focuses on AKH biosynthesis and the regulation of glucose-responsive AKH secretion through changes in CC cell electrical activity. Future directions for AKH research in flies are discussed, including the development of models for hyperglucagonemia and epigenetic inheritance of acquired metabolic traits. Many avenues of AKH physiology remain to be explored and thus present great potential for improving the utility of D. melanogaster in metabolic research.

The Genetics of Diabetes: What We Can Learn from Drosophila

Diabetes mellitus is a heterogeneous disease characterized by hyperglycemia due to impaired insulin secretion and/or action. All diabetes types have a strong genetic component. The most frequent forms, type 1 diabetes (T1D), type 2 diabetes (T2D) and gestational diabetes mellitus (GDM), are multifactorial syndromes associated with several genes’ effects together with environmental factors. Conversely, rare forms, neonatal diabetes mellitus (NDM) and maturity onset diabetes of the young (MODY), are caused by mutations in single genes. Large scale genome screenings led to the identification of hundreds of putative causative genes for multigenic diabetes, but all the loci identified so far explain only a small proportion of heritability. Nevertheless, several recent studies allowed not only the identification of some genes as causative, but also as putative targets of new drugs. Although monogenic forms of diabetes are the most suited to perform a precision approach and allow an accurate diagnosis, at least 80% of all monogenic cases remain still undiagnosed. The knowledge acquired so far addresses the future work towards a study more focused on the identification of diabetes causal variants; this aim will be reached only by combining expertise from different areas. In this perspective, model organism research is crucial. This review traces an overview of the genetics of diabetes and mainly focuses on Drosophila as a model system, describing how flies can contribute to diabetes knowledge advancement.

Constraints and Opportunities for the Evolution of Metamorphic Organisms in a Changing Climate

We argue that developmental hormones facilitate the evolution of novel phenotypic innovations and timing of life history events by genetic accommodation. Within an individual’s life cycle, metamorphic hormones respond readily to environmental conditions and alter adult phenotypes. Across generations, the many effects of hormones can bias and at times constrain the evolution of traits during metamorphosis; yet, hormonal systems can overcome constraints through shifts in timing of, and acquisition of tissue specific responses to, endocrine regulation. Because of these actions of hormones, metamorphic hormones can shape the evolution of metamorphic organisms. We present a model called a developmental goblet, which provides a visual representation of how metamorphic organisms might evolve. In addition, because developmental hormones often respond to environmental changes, we discuss how endocrine regulation of postembryonic development may impact how organisms evolve in response to climate change. Thus, we propose that developmental hormones may provide a mechanistic link between climate change and organismal adaptation.

Brain adiponectin signaling controls peripheral insulin response in Drosophila

The brain plays a key role in energy homeostasis, detecting nutrients, metabolites and circulating hormones from peripheral organs and integrating this information to control food intake and energy expenditure. Here, we show that a group of neurons in the Drosophila larval brain expresses the adiponectin receptor (AdipoR) and controls systemic growth and metabolism through insulin signaling. We identify glucose-regulated protein 78 (Grp78) as a circulating antagonist of AdipoR function produced by fat cells in response to dietary sugar. We further show that central AdipoR signaling inhibits peripheral Juvenile Hormone (JH) response, promoting insulin signaling. In conclusion, we identify a neuroendocrine axis whereby AdipoR-positive neurons control systemic insulin response.

Circulating adiponectin controls sensitivity to insulin in tissues. Here, Arquier et al. show that adiponectin receptor activity in neurons of the Drosophila brain controls insulin response in peripheral tissues via juvenile hormone signaling.

The sugar-responsive enteroendocrine neuropeptide F regulates lipid metabolism through glucagon-like and insulin-like hormones in Drosophila melanogaster

The enteroendocrine cell (EEC)-derived incretins play a pivotal role in regulating the secretion of glucagon and insulins in mammals. Although glucagon-like and insulin-like hormones have been found across animal phyla, incretin-like EEC-derived hormones have not yet been characterised in invertebrates. Here, we show that the midgut-derived hormone, neuropeptide F (NPF), acts as the sugar-responsive, incretin-like hormone in the fruit fly, Drosophila melanogaster. Secreted NPF is received by NPF receptor in the corpora cardiaca and in insulin-producing cells. NPF-NPFR signalling resulted in the suppression of the glucagon-like hormone production and the enhancement of the insulin-like peptide secretion, eventually promoting lipid anabolism. Similar to the loss of incretin function in mammals, loss of midgut NPF led to significant metabolic dysfunction, accompanied by lipodystrophy, hyperphagia, and hypoglycaemia. These results suggest that enteroendocrine hormones regulate sugar-dependent metabolism through glucagon-like and insulin-like hormones not only in mammals but also in insects.

Factors that regulate expression patterns of insulin-like peptides and their association with physiological and metabolic traits in Drosophila

Insulin-like peptides (ILPs) and components of the insulin signaling pathway are conserved across different animal phyla. Eight ILPs (called DILPs) and two receptors, dInR and Lgr3, have been described in Drosophila. DILPs regulate varied physiological traits including lifespan, reproduction, development, feeding behavior, stress resistance and metabolism. At the same time, different conditions such as nutrition, dietary supplements and environmental factors affect the expression of DILPs. This review focuses primarily on DILP2, DILP3, and DILP5 which are produced by insulin-producing cells in the brain of Drosophila. Although they are produced by the same cells and can potentially compensate for each other, DILP2, DILP3, and DILP5 expression may be differentially regulated at the mRNA level. Thus, we summarized available data on the conditions affecting the expression profiles of these DILPs in adult Drosophila. The accumulated data indicate that transcript levels of DILPs are determined by (a) nutritional conditions such as the protein-to-carbohydrate ratio, (b) carbohydrate type within the diet, (c) malnutrition or complete starvation; (d) environmental factors such as stress or temperature; (e) mutations of single peptides that induce changes in the expression of the other peptides; and (f) dietary supplements of drugs or natural substances. Furthermore, manipulation of specific genes in a cell- and tissue-specific manner affects mRNA levels for DILPs and, thereby, modulates various physiological traits and metabolism in Drosophila.

Discovering signaling mechanisms governing metabolism and metabolic diseases with Drosophila

There has been rapid growth in the use of Drosophila and other invertebrate systems to dissect mechanisms governing metabolism. New assays and approaches to physiology have aligned with superlative genetic tools in fruit flies to provide a powerful platform for posing new questions, or dissecting classical problems in metabolism and disease genetics. In multiple examples, these discoveries exploit experimental advantages as-yet unavailable in mammalian systems. Here, we illustrate how fly studies have addressed long-standing questions in three broad areas-inter-organ signaling through hormonal or neural mechanisms governing metabolism, intestinal interoception and feeding, and the cellular and signaling basis of sexually dimorphic metabolism and physiology-and how these findings relate to human (patho)physiology. The imaginative application of integrative physiology and related approaches in flies to questions in metabolism is expanding, and will be an engine of discovery, revealing paradigmatic features of metabolism underlying human diseases and physiological equipoise in health.

Insulin-Like Peptides and Cross-Talk With Other Factors in the Regulation of Insect Metabolism

The insulin-like peptide (ILP) and insulin-like growth factor (IGF) signalling pathways play a crucial role in the regulation of metabolism, growth and development, fecundity, stress resistance, and lifespan. ILPs are encoded by multigene families that are expressed in nervous and non-nervous organs, including the midgut, salivary glands, and fat body, in a tissue- and stage-specific manner. Thus, more multidirectional and more complex control of insect metabolism can occur. ILPs are not the only factors that regulate metabolism. ILPs interact in many cross-talk interactions of different factors, for example, hormones (peptide and nonpeptide), neurotransmitters and growth factors. These interactions are observed at different levels, and three interactions appear to be the most prominent/significant: (1) coinfluence of ILPs and other factors on the same target cells, (2) influence of ILPs on synthesis/secretion of other factors regulating metabolism, and (3) regulation of activity of cells producing/secreting ILPs by various factors. For example, brain insulin-producing cells co-express sulfakinins (SKs), which are cholecystokinin-like peptides, another key regulator of metabolism, and express receptors for tachykinin-related peptides, the next peptide hormones involved in the control of metabolism. It was also shown that ILPs in Drosophila melanogaster can directly and indirectly regulate AKH. This review presents an overview of the regulatory role of insulin-like peptides in insect metabolism and how these factors interact with other players involved in its regulation.

What fuels the fly: Energy metabolism in Drosophila and its application to the study of obesity and diabetes

The organs and metabolic pathways involved in energy metabolism, and the process of ATP production from nutrients, are comparable between humans and Drosophila melanogaster This level of conservation, together with the power of Drosophila genetics, makes the fly a very useful model system to study energy homeostasis. Here, we discuss the major organs involved in energy metabolism in Drosophila and how they metabolize different dietary nutrients to generate adenosine triphosphate. Energy metabolism in these organs is controlled by cell-intrinsic, paracrine, and endocrine signals that are similar between Drosophila and mammals. We describe how these signaling pathways are regulated by several physiological and environmental cues to accommodate tissue-, age-, and environment-specific differences in energy demand. Last, we discuss several genetic and diet-induced fly models of obesity and diabetes that can be leveraged to better understand the molecular basis of these metabolic diseases and thereby promote the development of novel therapies.

A novel satiety sensor detects circulating glucose and suppresses food consumption via insulin-producing cells in Drosophila

Sensing satiety is a crucial survival skill for all animal species including human. Despite the discovery of numerous neuromodulators that regulate food intake in Drosophila, the mechanism of satiety sensing remains largely elusive. Here, we investigated how neuropeptidergic circuitry conveyed satiety state to influence flies' food consumption. Drosophila tackykinin (DTK) and its receptor TAKR99D were identified in an RNAi screening as feeding suppressors. Two pairs of DTK⁺ neurons in the fly brain could be activated by elevated D-glucose in the hemolymph and imposed a suppressive effect on feeding. These DTK⁺ neurons formed a two-synapse circuitry targeting insulin-producing cells, a well-known feeding suppressor, via TAKR99D⁺ neurons, and this circuitry could be rapidly activated during food ingestion and cease feeding. Taken together, we identified a novel satiety sensor in the fly brain that could detect specific circulating nutrients and in turn modulate feeding, shedding light on the neural regulation of energy homeostasis.

Proteomics of protein trafficking by in vivo tissue-specific labeling

Conventional approaches to identify secreted factors that regulate homeostasis are limited in their abilities to identify the tissues/cells of origin and destination. We established a platform to identify secreted protein trafficking between organs using an engineered biotin ligase (BirA*G3) that biotinylates, promiscuously, proteins in a subcellular compartment of one tissue. Subsequently, biotinylated proteins are affinity-enriched and identified from distal organs using quantitative mass spectrometry. Applying this approach in Drosophila, we identify 51 muscle-secreted proteins from heads and 269 fat body-secreted proteins from legs/muscles, including CG2145 (human ortholog ENDOU) that binds directly to muscles and promotes activity. In addition, in mice, we identify 291 serum proteins secreted from conditional BirA*G3 embryo stem cell-derived teratomas, including low-abundance proteins with hormonal properties. Our findings indicate that the communication network of secreted proteins is vast. This approach has broad potential across different model systems to identify cell-specific secretomes and mediators of interorgan communication in health or disease.

A glucose-supplemented diet enhances gut barrier integrity in Drosophila

Dietary intervention has received considerable attention as an approach to extend lifespan and improve aging. However, questions remain regarding optimal dietary regimes and underlying mechanisms of lifespan extension. Here, we asked how an increase of glucose in a chemically defined diet extends the lifespan of adult Drosophila melanogaster We showed that glucose-dependent lifespan extension is not a result of diminished caloric intake, or changes to systemic insulin activity, two commonly studied mechanisms of lifespan extension. Instead, we found that flies raised on glucose-supplemented food increased the expression of cell-adhesion genes, delaying age-dependent loss of intestinal barrier integrity. Furthermore, we showed that chemical disruption of the gut barrier negated the lifespan extension associated with glucose treatment, suggesting that glucose-supplemented food prolongs adult viability by enhancing the intestinal barrier. We believe our data contribute to understanding intestinal homeostasis, and may assist efforts to develop preventative measures that limit effects of aging on health.

Roles of PINK1 in regulation of systemic growth inhibition induced by mutations of PTEN in Drosophila

The maintenance of mitochondrial homeostasis requires PTEN-induced kinase 1 (PINK1)-dependent mitophagy, and mutations in PINK1 are associated with Parkinson's disease (PD). PINK1 is also downregulated in tumor cells with PTEN mutations. However, there is limited information concerning the role of PINK1 in tissue growth and tumorigenesis. Here, we show that the loss of pink1 caused multiple growth defects independent of its pathological target, Parkin. Moreover, knocking down pink1 in muscle cells induced hyperglycemia and limited systemic organismal growth by the induction of Imaginal morphogenesis protein-Late 2 (ImpL2). Similarly, disrupting PTEN activity in multiple tissues impaired systemic growth by reducing pink1 expression, resembling wasting-like syndrome in cancer patients. Furthermore, the re-expression of PINK1 fully rescued defects in carbohydrate metabolism and systemic growth induced by the tissue-specific pten mutations. Our data suggest a function for PINK1 in regulating systemic growth in Drosophila and shed light on its role in wasting in the context of PTEN mutations.

Larval nutrition influences adult fat stores and starvation resistance in Drosophila

Insulin plays a major role in connecting nutrient availability to energy homeostasis by regulating metabolic pathways. Defects in insulin signalling is the primary cause for diabetes, obesity and various metabolic disorders. Nutritional status during growth and developmental stages play a crucial role in determining adult size, fecundity and ageing. However, the association between developmental nutrition and adult metabolic disorders has not been fully explored. Here, we address the effects of nutrient status during the larval growth phase on adult metabolism in Drosophila. We report that restricted food supply in larvae led to higher fat reserves and starvation resistance in mature adult flies, which we attribute to low insulin signalling. A lesser amount of stored fat was mobilised during early adult stages and during acute starvation, which accounts for the metabolic effects. Furthermore, larval diet influenced the expression of fat mobilisation genes brummer and lipid storage droplet-2 in adult flies, which led to the metabolic phenotypes reported here. Thus, the restricted nutrient environment in developing larvae led to adaptive changes that entrain the adult flies for scarce food availability.

Female-biased upregulation of insulin pathway activity mediates the sex difference in Drosophila body size plasticity

Nutrient-dependent body size plasticity differs between the sexes in most species, including mammals. Previous work in Drosophila showed that body size plasticity was higher in females, yet the mechanisms underlying increased female body size plasticity remain unclear. Here, we discover that a protein-rich diet augments body size in females and not males because of a female-biased increase in activity of the conserved insulin/insulin-like growth factor signaling pathway (IIS). This sex-biased upregulation of IIS activity was triggered by a diet-induced increase in stunted mRNA in females, and required Drosophila insulin-like peptide 2, illuminating new sex-specific roles for these genes. Importantly, we show that sex determination gene transformer promotes the diet-induced increase in stunted mRNA via transcriptional coactivator Spargel to regulate the male-female difference in body size plasticity. Together, these findings provide vital insight into conserved mechanisms underlying the sex difference in nutrient-dependent body size plasticity.

Systemic Regulation of Host Energy and Oogenesis by Microbiome-Derived Mitochondrial Coenzymes

Gut microbiota have been shown to promote oogenesis and fecundity, but the mechanistic basis of remote influence on oogenesis remained unknown. Here, we report a systemic mechanism of influence mediated by bacterial-derived supply of mitochondrial coenzymes. Removal of microbiota decreased mitochondrial activity and ATP levels in the whole-body and ovary, resulting in repressed oogenesis. Similar repression was caused by RNA-based knockdown of mitochondrial function in ovarian follicle cells. Reduced mitochondrial function in germ-free (GF) females was reversed by bacterial recolonization or supplementation of riboflavin, a precursor of FAD and FMN. Metabolomics analysis of GF females revealed a decrease in oxidative phosphorylation and FAD levels and an increase in metabolites that are degraded by FAD-dependent enzymes (e.g., amino and fatty acids). Riboflavin supplementation opposed this effect, elevating mitochondrial function, ATP, and oogenesis. These findings uncover a bacterial-mitochondrial axis of influence, linking gut bacteria with systemic regulation of host energy and reproduction.

Streptozotocin induces brain glucose metabolic changes and alters glucose transporter expression in the Lobster cockroach; Nauphoeta cinerea (Blattodea: Blaberidae)

The development of new models to study diabetes in invertebrates is important to ensure adherence to the 3R's principle and to expedite knowledge of the complex molecular events underlying glucose toxicity. Streptozotocin (STZ)-an alkylating and highly toxic agent that has tropism to mammalian beta cells-is used as a model of type 1 diabetes in rodents, but little is known about STZ effects in insects. Here, the cockroach; Nauphoeta cinerea was used to determine the acute toxicity of 74 and 740 nmol of STZ injection per cockroach. STZ increased the glucose content, mRNA expression of glucose transporter 1 (GLUT1) and markers of oxidative stress in the head. Fat body glycogen, insect survival, acetylcholinesterase activity, triglyceride content and viable cells in head homogenate were reduced, which may indicate a disruption in glucose utilization by the head and fat body of insects after injection of 74 and 740 nmol STZ per nymph. The glutathione S-transferase (GST) activity and reduced glutathione levels (GSH) were increased, possibly via activation of nuclear factor erythroid 2 related factor as a compensatory response against the increase in reactive oxygen species. Our data present the potential for metabolic disruption in N. cinerea by glucose analogues and opens paths for the study of brain energy metabolism in insects. We further phylogenetically demonstrated conservation between N. cinerea glucose transporter 1 and the GLUT of other insects in the Neoptera infra-class.