Diabetic larvae and obese flies-emerging studies of metabolism in Drosophila

Serotonin and insulin-like peptides modulate leucokinin-producing neurons that affect feeding and water homeostasis in Drosophila

Metabolic homeostasis and water balance is maintained by tight hormonal and neuronal regulation. In Drosophila, insulin-like peptides (DILPs) are key regulators of metabolism, and the neuropeptide leucokinin (LK) is a diuretic hormone that also modulates feeding. However, it is not known whether LK and DILPs act together to regulate feeding and water homeostasis. Because LK neurons express the insulin receptor (dInR), we tested functional links between DILP and LK signaling in feeding and water balance. Thus, we performed constitutive and conditional manipulations of activity in LK neurons and insulin-producing cells (IPCs) in adult flies and monitored food intake, responses to desiccation, and peptide expression levels. We also measured in vivo changes in LK and DILP levels in neurons in response to desiccation and drinking. Our data show that activated LK cells stimulate diuresis in vivo, and that LK and IPC signaling affect food intake in opposite directions. Overexpression of the dInR in LK neurons decreases the LK peptide levels, but only caused a subtle decrease in feeding, and had no effect on water balance. Next we demonstrated that LK neurons express the serotonin receptor 5-HT1B . Knockdown of this receptor in LK neurons diminished LK expression, increased desiccation resistance, and diminished food intake. Live calcium imaging indicates that serotonin inhibits spontaneous activity in abdominal LK neurons. Our results suggest that serotonin via 5-HT1B diminishes activity in the LK neurons and thereby modulates functions regulated by LK peptide, but the action of the dInR in these neurons remains less clear.

Steroid Signaling Establishes a Female Metabolic State and Regulates SREBP to Control Oocyte Lipid Accumulation

Disruptions in energy homeostasis severely affect reproduction in many organisms and are linked to several reproductive disorders in humans. As a result, understanding the mechanisms that control nutrient accumulation in the oocyte will provide valuable insights into the links between metabolic disease and reproductive dysfunction. We show that the steroid hormone ecdysone functions in Drosophila to control lipid metabolism and support oocyte production. First, local EcR-mediated signaling induces a stage-specific accumulation of lipids in stage-10 oocytes. EcR induces lipid accumulation by promoting the activation of the lipogenic transcription factor SREBP and by controlling the expression of the low-density lipoprotein (LDL) receptor homolog, LpR2. Second, global signaling via the ecdysone receptor, EcR, establishes a female metabolic state and promotes whole-body triglyceride and glycogen storage at high levels. EcR acts in the CNS to mediate these effects, in part by promoting higher levels of feeding in females. Thus, ecdysone functions at two levels to support reproduction: first by inducing lipid accumulation in the late stages of oocyte development and second by providing a signal that coordinates lipid metabolism in the germline with whole-animal lipid homeostasis. Ecdysone regulation allows females to assess the demands of oogenesis and alter their behavior and metabolic state to support the biosynthetic requirements of oocyte production.

Enhanced neuronal glucose transporter expression reveals metabolic choice in a HD Drosophila model

Huntington’s disease is a neurodegenerative disorder caused by toxic insertions of polyglutamine residues in the Huntingtin protein and characterized by progressive deterioration of cognitive and motor functions. Altered brain glucose metabolism has long been suggested and a possible link has been proposed in HD. However, the precise function of glucose transporters was not yet determined. Here, we report the effects of the specifically-neuronal human glucose transporter expression in neurons of a Drosophila model carrying the exon 1 of the human huntingtin gene with 93 glutamine repeats (HQ93). We demonstrated that overexpression of the human glucose transporter in neurons ameliorated significantly the status of HD flies by increasing their lifespan, reducing their locomotor deficits and rescuing eye neurodegeneration. Then, we investigated whether increasing the major pathways of glucose catabolism, glycolysis and pentose-phosphate pathway (PPP) impacts HD. To mimic increased glycolytic flux, we overexpressed phosphofructokinase (PFK) which catalyzes an irreversible step in glycolysis. Overexpression of PFK did not affect HQ93 fly survival, but protected from photoreceptor loss. Overexpression of glucose-6-phosphate dehydrogenase (G6PD), the key enzyme of the PPP, extended significantly the lifespan of HD flies and rescued eye neurodegeneration. Since G6PD is able to synthesize NADPH involved in cell survival by maintenance of the redox state, we showed that tolerance to experimental oxidative stress was enhanced in flies co-expressing HQ93 and G6PD. Additionally overexpressions of hGluT3, G6PD or PFK were able to circumvent mitochondrial deficits induced by specific silencing of genes necessary for mitochondrial homeostasis. Our study confirms the involvement of bioenergetic deficits in HD course; they can be rescued by specific expression of a glucose transporter in neurons. Finally, the PPP and, to a lesser extent, the glycolysis seem to mediate the hGluT3 protective effects, whereas, in addition, the PPP provides increased protection to oxidative stress.

PGC-1/Spargel Counteracts High-Fat-Diet-Induced Obesity and Cardiac Lipotoxicity Downstream of TOR and Brummer ATGL Lipase

Obesity and metabolic syndrome are associated with an increased risk for lipotoxic cardiomyopathy, which is strongly correlated with excessive accumulation of lipids in the heart. Obesity- and type-2-diabetes-related disorders have been linked to altered expression of the transcriptional cofactor PGC-1α, which regulates the expression of genes involved in energy metabolism. Using Drosophila, we identify PGC-1/spargel (PGC-1/srl) as a key antagonist of high-fat diet (HFD)-induced lipotoxic cardiomyopathy. We find that HFD-induced lipid accumulation and cardiac dysfunction are mimicked by reduced PGC-1/srl function and reversed by PGC-1/srl overexpression. Moreover, HFD feeding lowers PGC-1/srl expression by elevating TOR signaling and inhibiting expression of the Drosophila adipocyte triglyceride lipase (ATGL) (Brummer), both of which function as upstream modulators of PGC-1/srl. The lipogenic transcription factor SREBP also contributes to HFD-induced cardiac lipotoxicity, likely in parallel with PGC-1/srl. These results suggest a regulatory network of key metabolic genes that modulates lipotoxic heart dysfunction.

Fatty acid synthase cooperates with glyoxalase 1 to protect against sugar toxicity

Fatty acid (FA) metabolism is deregulated in several human diseases including metabolic syndrome, type 2 diabetes and cancers. Therefore, FA-metabolic enzymes are potential targets for drug therapy, although the consequence of these treatments must be precisely evaluated at the organismal and cellular levels. In healthy organism, synthesis of triacylglycerols (TAGs)—composed of three FA units esterified to a glycerol backbone—is increased in response to dietary sugar. Saturation in the storage and synthesis capacity of TAGs is associated with type 2 diabetes progression. Sugar toxicity likely depends on advanced-glycation-end-products (AGEs) that form through covalent bounding between amine groups and carbonyl groups of sugar or their derivatives α-oxoaldehydes. Methylglyoxal (MG) is a highly reactive α-oxoaldehyde that is derived from glycolysis through a non-enzymatic reaction. Glyoxalase 1 (Glo1) works to neutralize MG, reducing its deleterious effects. Here, we have used the power of Drosophila genetics to generate Fatty acid synthase (FASN) mutants, allowing us to investigate the consequence of this deficiency upon sugar-supplemented diets. We found that FASN mutants are lethal but can be rescued by an appropriate lipid diet. Rescued animals do not exhibit insulin resistance, are dramatically sensitive to dietary sugar and accumulate AGEs. We show that FASN and Glo1 cooperate at systemic and cell-autonomous levels to protect against sugar toxicity. We observed that the size of FASN mutant cells decreases as dietary sucrose increases. Genetic interactions at the cell-autonomous level, where glycolytic enzymes or Glo1 were manipulated in FASN mutant cells, revealed that this sugar-dependent size reduction is a direct consequence of MG-derived-AGE accumulation. In summary, our findings indicate that FASN is dispensable for cell growth if extracellular lipids are available. In contrast, FA-synthesis appears to be required to limit a cell-autonomous accumulation of MG-derived-AGEs, supporting the notion that MG is the most deleterious α-oxoaldehyde at the intracellular level.

Consumption of sugar and lipid (fat) enriched food increases the risk of developing metabolic diseases and cancers. However, lipids are essential molecules for life, as they are the major components of cell membranes. Metabolism refers to biochemical reactions that transform nutrients into molecules required by an organism, although toxic by-products can also formed. Sugars or their derivatives are likely to induce toxic effects by forming stable conjugates with proteins. To neutralize their toxic potential, sugars are metabolized and stored as fat. Here, we have used the fruitfly model to investigate the consequences of lipogenesis deficiency upon ingestion of sugar-enriched diets. We show that lipogenesis deficient animals are dramatically sensitive to dietary sugar. Further, we have identified the sugar by-product responsible for intracellular toxicity, in the context of lipogenesis inhibition. Our study reveals that inhibiting lipogenesis does not disrupt cellular growth if extracellular lipids are available. In contrast lipogenesis inhibition may have deleterious consequences due to accumulation of toxic by-products. The efficacy of lipogenic inhibitors in fighting cancers and metabolic diseases is currently under investigation. Therefore, to evaluate the clinical benefit of these inhibitors, accumulation of the toxic molecules should be monitored in both sick and healthy cells.

Suppression of insulin production and secretion by a decretin hormone

Decretins, hormones induced by fasting that suppress insulin production and secretion, have been postulated from classical human metabolic studies. From genetic screens, we identified Drosophila Limostatin (Lst), a peptide hormone that suppresses insulin secretion. Lst is induced by nutrient restriction in gut-associated endocrine cells. limostatin deficiency led to hyperinsulinemia, hypoglycemia, and excess adiposity. A conserved 15-residue polypeptide encoded by limostatin suppressed secretion by insulin-producing cells. Targeted knockdown of CG9918, a Drosophila ortholog of Neuromedin U receptors (NMURs), in insulin-producing cells phenocopied limostatin deficiency and attenuated insulin suppression by purified Lst, suggesting CG9918 encodes an Lst receptor. NMUR1 is expressed in islet β cells, and purified NMU suppresses insulin secretion from human islets. A human mutant NMU variant that co-segregates with familial early-onset obesity and hyperinsulinemia fails to suppress insulin secretion. We propose Lst as an index member of an ancient hormone class called decretins, which suppress insulin output.

Modeling metabolic homeostasis and nutrient sensing in Drosophila: implications for aging and metabolic diseases

Over the past decade, numerous reports have underscored the similarities between the metabolism of Drosophila and vertebrates, with the identification of evolutionarily conserved enzymes and analogous organs that regulate carbohydrate and lipid metabolism. It is now well established that the major metabolic, energy-sensing and endocrine signaling networks of vertebrate systems are also conserved in flies. Accordingly, studies in Drosophila are beginning to unravel how perturbed energy balance impinges on lifespan and on the ensuing diseases when energy homeostasis goes awry. Here, we highlight several emerging concepts that are at the nexus between obesity, nutrient sensing, metabolic homeostasis and aging. Specifically, we summarize the endocrine mechanisms that regulate carbohydrate and lipid metabolism, and provide an overview of the neuropeptides that regulate feeding behavior. We further describe the various efforts at modeling the effects of high-fat or -sugar diets in Drosophila and the signaling mechanisms involved in integrating organ function. Finally, we draw attention to some of the cardinal discoveries made with these disease models and how these could spur new research questions in vertebrate systems.

Sodium chromate demonstrates some insulin-mimetic properties in the fruit fly Drosophila melanogaster

The effects of food supplementation with sodium chromate at concentrations of 1-500 μM on development of Drosophila melanogaster larvae and food intake, carbohydrate and lipid pools in adult fruit flies were investigated. Food supplementation with hexavalent chromium (Na2CrO4) at high concentrations delayed larval development and decreased the percentage of larvae that pupated which indicated a relatively low toxicity. The supplement decreased glucose levels in fly hemolymph, but at concentrations of 5-25 μM increased fly carbohydrate reserves: hemolymph trehalose and whole body trehalose and glycogen. The data on parameters of carbohydrate metabolism show that chromate possesses some insulin-mimetic properties. The changes in metabolism of carbohydrates under chromate exposure were also accompanied by an increase in total lipid levels and in the portion of triacylglycerides among all lipids. Chromate addition to fly food did not affect male or female body mass, but reduced food consumption by females at all concentrations used, whereas in males only 500 μM chromate decreased food consumption. The data show that: (1) Cr(6+) has many of the same effects as Cr(3+) suggesting that it might be just as effective to treat diabetic states, likely as a result of intracellular reduction of Cr(6+) ions, and (2) the Drosophila model can be used to develop new approaches to investigate the molecular mechanisms of chromium as an insulin-mimetic. Although it is usually believed that hexavalent chromium possesses higher toxicity than the trivalent ion, due to its easier penetration into the cell, application of hexavalent chromium may substantially decrease the chromium doses needed to get the desired effects.

Use of Drosophila as an evaluation method reveals imp as a candidate gene for type 2 diabetes in rat locus Niddm22

Type 2 diabetes (T2D) is one of the most common human diseases. QTL analysis of the diabetic Otsuka Long-Evans Tokushima Fatty (OLETF) rats has identified numerous hyperglycemic loci. However, molecular characterization and/or gene identification largely remains to be elucidated due mostly to the weak genetic variances contributed by each locus. Here we utilized Drosophila melanogaster as a secondary model organism for functional evaluation of the candidate gene. We demonstrate that the tissue specific knockdown of a homologue of igf2bp2 RNA binding protein leads to increased sugar levels similar to that found in the OLETF rat. In the mutant, the expression of two of the insulin-like peptides encoded in the fly genome, dilp2 and dilp3, were found to be downregulated. Consistent with previous reports of dilp mutants, the imp mutant flies exhibited an extension of life span; in contrast, starvation tolerance was reduced. These results further reinforce the possibility that imp is involved in sugar metabolism by modulating insulin expression.

A Drosophila ABC transporter regulates lifespan

MRP4 (multidrug resistance-associated protein 4) is a member of the MRP/ABCC subfamily of ATP-binding cassette (ABC) transporters that are essential for many cellular processes requiring the transport of substrates across cell membranes. Although MRP4 has been implicated as a detoxification protein by transport of structurally diverse endogenous and xenobiotic compounds, including antivirus and anticancer drugs, that usually induce oxidative stress in cells, its in vivo biological function remains unknown. In this study, we investigate the biological functions of a Drosophila homolog of human MRP4, dMRP4. We show that dMRP4 expression is elevated in response to oxidative stress (paraquat, hydrogen peroxide and hyperoxia) in Drosophila. Flies lacking dMRP4 have a shortened lifespan under both oxidative and normal conditions. Overexpression of dMRP4, on the other hand, is sufficient to increase oxidative stress resistance and extend lifespan. By genetic manipulations, we demonstrate that dMRP4 is required for JNK (c-Jun NH₂-terminal kinase) activation during paraquat challenge and for basal transcription of some JNK target genes under normal condition. We show that impaired JNK signaling is an important cause for major defects associated with dMRP4 mutations, suggesting that dMRP4 regulates lifespan by modulating the expression of a set of genes related to both oxidative resistance and aging, at least in part, through JNK signaling.

The drug transporters are often known for their ability to transport different physiological-related compounds across cell membranes. Although the abnormal up-regulation of some these transporters is believed to be the common cause of the clinic problem called drug resistance, the biological functions of these transporters remain largely unknown. Here we show that a Drosophila homolog of the mammalian drug transporter plays a role in lifespan regulation. Mutations of this gene increase the sensitivity to oxidative stress and reduce lifespan, while overexpression of this gene increases resistance to oxidative stress and extends lifespan. By molecular and genetic analyses, we have linked functions of this gene to a key signaling transduction pathway that has been known to be important in lifespan regulation.

Immunity in Drosophila melanogaster--from microbial recognition to whole-organism physiology

Since the discovery of antimicrobial peptide responses 40 years ago, the fruit fly Drosophila melanogaster has proven to be a powerful model for the study of innate immunity. Early work focused on innate immune mechanisms of microbial recognition and subsequent nuclear factor-κB signal transduction. More recently, D. melanogaster has been used to understand how the immune response is regulated and coordinated at the level of the whole organism. For example, researchers have used this model in studies investigating interactions between the microbiota and the immune system at barrier epithelial surfaces that ensure proper nutritional and immune homeostasis both locally and systemically. In addition, studies in D. melanogaster have been pivotal in uncovering how the immune response is regulated by both endocrine and metabolic signalling systems, and how the immune response modifies these systems as part of a homeostatic circuit. In this Review, we briefly summarize microbial recognition and antiviral immunity in D. melanogaster, and we highlight recent studies that have explored the effects of organism-wide regulation of the immune response and, conversely, the effects of the immune response on organism physiology.

High consumption of fructose rather than glucose promotes a diet-induced obese phenotype in Drosophila melanogaster

During the last 20 years, there has been a considerable scientific debate about the possible mechanisms of induction of metabolic disorders by reducing monosaccharides such as glucose or fructose. In this study, we report the metabolic rearrangement in response to consumption of these monosaccharides at concentrations ranging from 0.25% to 20% in a Drosophila model. Flies raised on high-glucose diet displayed delay in pupation and increased developmental mortality compared with fructose consumers. Both monosaccharides at high concentrations promoted an obese-like phenotype indicated by increased fly body mass, levels of uric acid, and circulating and stored carbohydrates and lipids; and decreased percentage of water in the body. However, flies raised on fructose showed lower levels of circulating glucose and higher concentrations of stored carbohydrates, lipids, and uric acid. The preferential induction of obesity caused by fructose in Drosophila was associated with increased food consumption and reduced mRNA levels of DILP2 and DILP5 in the brain of adult flies. Our data show that glucose and fructose differently affect carbohydrate and lipid metabolism in Drosophila in part by modulation of insulin/insulin-like growth factor signaling. Some reported similarities with effects observed in mammals make Drosophila as a useful model to study carbohydrate influence on metabolism and development of metabolic disorders.

The sleeping beauty: how reproductive diapause affects hormone signaling, metabolism, immune response and somatic maintenance in Drosophila melanogaster

Some organisms can adapt to seasonal and other environmental challenges by entering a state of dormancy, diapause. Thus, insects exposed to decreased temperature and short photoperiod enter a state of arrested development, lowered metabolism, and increased stress resistance. Drosophila melanogaster females can enter a shallow reproductive diapause in the adult stage, which drastically reduces organismal senescence, but little is known about the physiology and endocrinology associated with this dormancy, and the genes involved in its regulation. We induced diapause in D. melanogaster and monitored effects over 12 weeks on dynamics of ovary development, carbohydrate and lipid metabolism, as well as expression of genes involved in endocrine signaling, metabolism and innate immunity. During diapause food intake diminishes drastically, but circulating and stored carbohydrates and lipids are elevated. Gene transcripts of glucagon- and insulin-like peptides increase, and expression of several target genes of these peptides also change. Four key genes in innate immunity can be induced by infection in diapausing flies, and two of these, drosomycin and cecropin A1, are upregulated by diapause independently of infection. Diapausing flies display very low mortality, extended lifespan and decreased aging of the intestinal epithelium. Many phenotypes induced by diapause are reversed after one week of recovery from diapause conditions. Furthermore, mutant flies lacking specific insulin-like peptides (dilp5 and dilp2-3) display increased diapause incidence. Our study provides a first comprehensive characterization of reproductive diapause in D. melanogaster, and evidence that glucagon- and insulin-like signaling are among the key regulators of the altered physiology during this dormancy.

Strategies for gene disruption in Drosophila

Drosophila melanogaster has been a classic model organism for the studies of genetics. More than 15,000 Drosophila genes have been annotated since the entire genome was sequenced; however, many of them still lack functional characterization. Various gene-manipulating approaches in Drosophila have been developed for the function analysis of genes. Here, we summarize some representative strategies utilized for Drosophila gene targeting, from the unbiased ethyl methanesulfonate (EMS) mutagenesis and transposable element insertion, to insertional/replacement homologous recombination and site-specific nucleases such as the zinc-finger nuclease (ZFN), the transcription activator-like effector nuclease (TALEN) and the CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 system. Specifically, we evaluate the pros and cons of each technique in a historical perspective. This review discuss important factors that should be taken into consideration for the selection of a strategy that best fits the specific needs of a gene knockout project.

Drosophila gains traction as a repurposed tool to investigate metabolism

The use of fruit flies has recently emerged as a powerful experimental paradigm to study the core aspects of energy metabolism. The fundamental need for lipid and carbohydrate processing and storage across species dictates that the central regulators that control metabolism are highly conserved through evolution. Accordingly, the Drosophila system is being used to identify human disease genes and has the potential to model successfully human disorders that center on excessive caloric intake and metabolic dysfunction, including diet-induced lipotoxicity and type 2 diabetes. We review here recent progress on this front and contend that increasing such efforts will yield unexpectedly high rates of experimental return, thereby leading to novel approaches in the treatment of obesity and its comorbidities.

Metagenome-wide association of microbial determinants of host phenotype in Drosophila melanogaster

Animal-associated bacteria (microbiota) affect host behaviors and physiological traits. To identify bacterial genetic determinants of microbiota-responsive host traits, we employed a metagenome-wide association (MGWA) approach in two steps. First, we measured two microbiota-responsive host traits, development time and triglyceride (TAG) content, in Drosophila melanogaster flies monoassociated with each of 41 bacterial strains. The effects of monoassociation on host traits were not confined to particular taxonomic groups. Second, we clustered protein-coding sequences of the bacteria by sequence similarity de novo and statistically associated the magnitude of the host trait with the bacterial gene contents. The animals had been monoassociated with genome-sequenced bacteria, so the metagenome content was unambiguous. This analysis showed significant effects of pyrroloquinoline quinone biosynthesis genes on development time, confirming the results of a published transposon mutagenesis screen, thereby validating the MGWA; it also identified multiple genes predicted to affect host TAG content, including extracellular glucose oxidation pathway components. To test the validity of the statistical associations, we expressed candidate genes in a strain that lacks them. Monoassociation with bacteria that ectopically expressed a predicted oxidoreductase or gluconate dehydrogenase conferred reduced Drosophila TAG contents relative to the TAG contents in empty vector controls. Consistent with the prediction that glucose oxidation pathway gene expression increased bacterial glucose utilization, the glucose content of the host diet was reduced when flies were exposed to these strains. Our findings indicate that microbiota affect host nutritional status through modulation of nutrient acquisition. Together, these findings demonstrate the utility of MGWA for identifying bacterial determinants of host traits and provide mechanistic insight into how gut microbiota modulate the nutritional status of a model host.

To understand how certain gut bacteria promote the health of their animal hosts, we need to identify the bacterial genes that drive these beneficial relationships. This task is challenging because the bacterial communities can vary widely among different host individuals. To overcome this difficulty, we quantified how well each of 41 bacterial species protected Drosophila fruit flies from high fat content. The genomes of the chosen bacterial strains were previously sequenced, so we could statistically associate specific bacterial genes with bacterially mediated reduction in host fat content. Bacterial genes that promote glucose utilization were strongly represented in the association, and introducing these genes into the gut bacteria was sufficient to lower the animal’s fat content. Our method is applicable to the study of many other host-microbe interactions as a way to uncover microbial genes important for host health.

Diet modification and metformin have a beneficial effect in a fly model of obesity and mucormycosis

In an experimental model of obesity and hyperglycemia in Drosophila melanogaster we studied the effect of diet modification and administration of metformin on systemic infection with Rhizopus, a common cause of mucormycosis in diabetic patients. Female Wt-type Drosophila flies were fed regular (RF) or high-fat diet (HFD; 30% coconut oil) food with or without metformin for 48 h and then injected with R. oryzae. Survival rates, glucose and triglyceride levels were compared between 1) normal-weight flies (RF), 2) obese flies (HFD), 3) obese flies fed with RF, 4) flies continuously fed on HFD + metformin, 5) flies fed on HFD + metformin, then transferred to RF, and 6) obese flies administered metformin after infection [corrected].Glucose levels were compared across groups of non-infected flies and across groups of infected flies. Survival was significantly decreased (P = 0.003) in obese flies, while post-infection glucose levels were significantly increased (P = 0.0001), compared to normal-weight flies. Diet and administration of metformin led to weight loss, normalized glucose levels during infection, and were associated with decreased mortality and tissue fungal burden. In conclusion, diet and metformin help control infection-associated hyperglycemia and improve survival in Drosophila flies with mucormycosis. Fly models of obesity bear intriguing similarities to the pathophysiology of insulin resistance and diabetes in humans, and can provide new insights into the pathogenesis and treatment of infections in obese and diabetic patients.

Four GABAergic interneurons impose feeding restraint in Drosophila

Feeding is dynamically regulated by the palatability of the food source and the physiological needs of the animal. How consumption is controlled by external sensory cues and internal metabolic state remains under intense investigation. Here, we identify four GABAergic interneurons in the Drosophila brain that establish a central feeding threshold which is required to inhibit consumption. Inactivation of these cells results in indiscriminate and excessive intake of all compounds, independent of taste quality or nutritional state. Conversely, acute activation of these neurons suppresses consumption of water and nutrients. The output from these neurons is required to gate activity in motor neurons that control meal initiation and consumption. Thus, our study reveals a layer of inhibitory control in feeding circuits that is required to suppress a latent state of unrestricted and nonselective consumption.

Molybdate partly mimics insulin-promoted metabolic effects in Drosophila melanogaster

Molybdenum-containing salts have been found to attenuate diabetes complications in mammals by affecting processes normally regulated by insulin and thus were believed to mimic insulin activity. In this study, we used a fruit fly model to test sodium molybdate, Na2MoO4, action in relation to insulin-promoted processes and toxicity. We studied how larval food supplementation with sodium molybdate affected levels of body carbohydrates and lipids in two-day old adult Drosophila melanogaster. Molybdate salt, in the concentrations used (0.025, 0.05, 0.5, 5, and 10mM), showed low toxicity to fly larvae and slightly influenced development and the percentage of pupated animals. Additionally, sodium molybdate decreased the level of hemolymph glucose in males by 30%, and increased the level of hemolymph trehalose in flies of both sexes. These changes were accompanied by an increase in whole body trehalose and glycogen of about 30-90%. Although total lipid levels in flies of both sexes were depleted by 25%, an increased amount of triacylglycerides among total lipids was observed. These effects were not related to changes in food intake. Taken together, the present data let us suggest that sodium molybdate may at least partly mimic insulin-related effects in Drosophila.

From fat fruit fly to human obesity

Obesity is a chronic metabolic disease that has become a global problem. Although a tremendous amount of effort has been spent to prevent and treat obesity, its etiology is still largely unknown and there are not yet sufficient strategies to control obesity. Recently, the fruit fly, Drosophila melanogaster, has become a useful model for studying metabolic homeostasis and obesity related disorders. The goal of this mini-review is to summarize the recent achievements of Drosophila models and to highlight the experimental protocols used in studying feeding behavior and energy homeostasis in the fly. The Drosophila models provide useful tools to understand obesity pathogenesis and to develop novel therapeutics.

Genetic Dissection of the Physiological Role of Skeletal Muscle in Metabolic Syndrome

The primary deficiency underlying metabolic syndrome is insulin resistance, in which insulin-responsive peripheral tissues fail to maintain glucose homeostasis. Because skeletal muscle is the major site for insulin-induced glucose uptake, impairments in skeletal muscle’s insulin responsiveness play a major role in the development of insulin resistance and type 2 diabetes. For example, skeletal muscle of type 2 diabetes patients and their offspring exhibit reduced ratios of slow oxidative muscle. These observations suggest the possibility of applying muscle remodeling to recover insulin sensitivity in metabolic syndrome. Skeletal muscle is highly adaptive to external stimulations such as exercise; however, in practice it is often not practical or possible to enforce the necessary intensity to obtain measurable benefits to the metabolic syndrome patient population. Therefore, identifying molecular targets for inducing muscle remodeling would provide new approaches to treat metabolic syndrome. In this review, the physiological properties of skeletal muscle, genetic analysis of metabolic syndrome in human populations and model organisms, and genetically engineered mouse models will be discussed in regard to the prospect of applying skeletal muscle remodeling as possible therapy for metabolic syndrome.

Automated monitoring and quantitative analysis of feeding behaviour in Drosophila

Food ingestion is one of the defining behaviours of all animals, but its quantification and analysis remain challenging. This is especially the case for feeding behaviour in small, genetically tractable animals such as Drosophila melanogaster. Here, we present a method based on capacitive measurements, which allows the detailed, automated and high-throughput quantification of feeding behaviour. Using this method, we were able to measure the volume ingested in single sips of an individual, and monitor the absorption of food with high temporal resolution. We demonstrate that flies ingest food by rhythmically extending their proboscis with a frequency that is not modulated by the internal state of the animal. Instead, hunger and satiety homeostatically modulate the microstructure of feeding. These results highlight similarities of food intake regulation between insects, rodents, and humans, pointing to a common strategy in how the nervous systems of different animals control food intake.

Feeding is an important behaviour, but its quantification remains challenging, particularly in small animal models like Drosophila melanogaster. Here the authors describe a method which uses capacitive sensing for automated high-resolution measuring of feeding behaviour in individual flies.

Heart- and muscle-derived signaling system dependent on MED13 and Wingless controls obesity in Drosophila

Obesity develops in response to an imbalance of energy homeostasis and whole-body metabolism. Muscle plays a central role in the control of energy homeostasis through consumption of energy and signaling to adipose tissue. We reported previously that MED13, a subunit of the Mediator complex, acts in the heart to control obesity in mice. To further explore the generality and mechanistic basis of this observation, we investigated the potential influence of MED13 expression in heart and muscle on the susceptibility of Drosophila to obesity. Here, we show that heart/muscle-specific knockdown of MED13 or MED12, another Mediator subunit, increases susceptibility to obesity in adult flies. To identify possible muscle-secreted obesity regulators, we performed an RNAi-based genetic screen of 150 genes that encode secreted proteins and found that Wingless inhibition also caused obesity. Consistent with these findings, muscle-specific inhibition of Armadillo, the downstream transcriptional effector of the Wingless pathway, also evoked an obese phenotype in flies. Epistasis experiments further demonstrated that Wingless functions downstream of MED13 within a muscle-regulatory pathway. Together, these findings reveal an intertissue signaling system in which Wingless acts as an effector of MED13 in heart and muscle and suggest that Wingless-mediated cross-talk between striated muscle and adipose tissue controls obesity in Drosophila. This signaling system appears to represent an ancestral mechanism for the control of systemic energy homeostasis.

Methods for studying metabolism in Drosophila

Recent research using Drosophila melanogaster has seen a resurgence in studies of metabolism and physiology. This review focuses on major methods used to conduct this work. These include protocols for dietary interventions, measurements of triglycerides, cholesterol, glucose, trehalose, and glycogen, stains for lipid detection, and the use of gas chromatography-mass spectrometry (GC-MS) to detect major polar metabolites. It is our hope that this will provide a useful framework for both new and current researchers in the field.

Feeding regulation in Drosophila

Neuromodulators play a key role in adjusting animal behavior based on environmental cues and internal needs. Here, we review the regulation of Drosophila feeding behavior to illustrate how neuromodulators achieve behavioral plasticity. Recent studies have made rapid progress in determining molecular and cellular mechanisms that translate the metabolic needs of the fly into changes in neuroendocrine and neuromodulatory states. These neuromodulators in turn promote or inhibit discrete feeding behavioral subprograms. This review highlights the links between physiological needs, neuromodulatory states, and feeding decisions.

Insulin receptor-mediated nutritional signalling regulates juvenile hormone biosynthesis and vitellogenin production in the German cockroach

Female reproductive processes, which comprise, amongst others, the synthesis of yolk proteins and the endocrine mechanisms which regulate this synthesis, need a considerable amount of energy and resources. The role of communicating that the required nutritional status has been attained is carried out by nutritional signalling pathways and, in particular, by the insulin receptor (InR) pathway. In the present study, using the German cockroach, Blattella germanica, as a model, we analysed the role of InR in different processes, but mainly those related to juvenile hormone (JH) synthesis and vitellogenin production. We first cloned the InR cDNA from B. germanica (BgInR) and then determined that its expression levels were constant in corpora allata and fat body during the first female gonadotrophic cycle. Results showed that the observed increase in BgInR mRNA in fat body from starved compared to fed females was abolished in those females treated with systemic RNAi in vivo against the transcription factor BgFoxO. RNAi-mediated BgInR knockdown during the final two nymphal stages produced significant delays in the moults, together with smaller adult females which could not spread the fore- and hindwings properly. In addition, BgInR knockdown led to a severe inhibition of juvenile hormone synthesis in adult female corpora allata, with a concomitant reduction of mRNA levels corresponding to 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase-1, HMG-CoA synthase-2, HMG-CoA reductase and methyl farnesoate epoxidase. BgInR RNAi treatment also reduced fat body vitellogenin mRNA and oocyte growth. Our results show that BgInR knockdown produces similar phenotypes to those obtained in starved females in terms of corpora allata activity and vitellogenin synthesis, and indicate that the InR pathway mediates the activation of JH biosynthesis and vitellogenin production elicited by nutrition signalling.

The Little Fly that Could: Wizardry and Artistry of Drosophila Genomics

For more than 100 years now, the fruit fly Drosophila melanogaster has been at the forefront of our endeavors to unlock the secrets of the genome. From the pioneering studies of chromosomes and heredity by Morgan and his colleagues, to the generation of fly models for human disease, Drosophila research has been at the forefront of genetics and genomics. We present a broad overview of some of the most powerful genomics tools that keep Drosophila research at the cutting edge of modern biomedical research.

Analysis of lipid droplet dynamics and functions in Drosophila melanogaster

The lipid droplet (LD) is a unique cellular organelle containing a neutral-lipid core enclosed by a phospholipid monolayer and associated proteins. Despite the important function of LDs at the hub of cellular energy homeostasis regulation, major questions in the field of LD biology are still unanswered. Drosophila melanogaster has been used as a model organism to make fundamental discoveries in biology for over a century. In recent years, genome-wide unbiased reverse genetic screens using Drosophila cells or transgenic lines have been proven to provide valuable knowledge to the field of LD biology. Here we summarize the methods we use for functional genomic screens in Drosophila S2 cells to identify genes involved in LD biology, and the methods used for studying LD function in vivo using Drosophila as a model to combat metabolic diseases.

A small system--high-resolution study of metabolic adaptation in the central metabolic pathway to temperate climates in Drosophila melanogaster

In this article, we couple the geographic variation in 127 single-nucleotide polymorphism (SNP) frequencies in genes of 46 enzymes of central metabolism with their associated cis-expression variation to predict latitudinal or climatic-driven gene expression changes in the metabolic architecture of Drosophila melanogaster. Forty-two percent of the SNPs in 65% of the genes show statistically significant clines in frequency with latitude across the 20 local population samples collected from southern Florida to Ontario. A number of SNPs in the screened genes are also associated with significant expression variation within the Raleigh population from North Carolina. A principal component analysis of the full variance-covariance matrix of latitudinal changes in SNP-associated standardized gene expression allows us to identify those major genes in the pathway and its associated branches that are likely targets of natural selection. When embedded in a central metabolic context, we show that these apparent targets are concentrated in the genes of the upper glycolytic pathway and pentose shunt, those controlling glycerol shuttle activity, and finally those enzymes associated with the utilization of glutamate and pyruvate. These metabolites possess high connectivity and thus may be the points where flux balance can be best shifted. We also propose that these points are conserved points associated with coupling energy homeostasis and energy sensing in mammals. We speculate that the modulation of gene expression at specific points in central metabolism that are associated with shifting flux balance or possibly energy-state sensing plays a role in adaptation to climatic variation.

A nutritional conditional lethal mutant due to pyridoxine 5'-phosphate oxidase deficiency in Drosophila melanogaster

The concept of auxotrophic complementation has been proposed as an approach to identify genes in essential metabolic pathways in Drosophila melanogaster. However, it has achieved limited success to date, possibly due to the low probability of finding mutations fit with the chemically defined profile. Instead of using the chemically defined culture media lacking specific nutrients, we used bare minimum culture medium, i.e., 4% sucrose, for adult Drosophila. We identified a nutritional conditional lethal mutant and localized a c.95C > A mutation in the Drosophila pyridoxine 5'-phosphate oxidase gene [dPNPO or sugarlethal (sgll)] using meiotic recombination mapping, deficiency mapping, and whole genome sequencing. PNPO converts dietary vitamin B6 such as pyridoxine to its active form pyridoxal 5'-phosphate (PLP). The missense mutation (sgll(95)) results in the substitution of alanine to aspartate (p.Ala32Asp). The sgll(95) flies survive well on complete medium but all die within 6 d on 4% sucrose only diet, which can be rescued by pyridoxine or PLP supplement, suggesting that the mutation does not cause the complete loss of PNPO activity. The sgll knockdown further confirms its function as the Drosophila PNPO. Because better tools for positional cloning and cheaper whole genome sequencing have made the identification of point mutations much easier than before, alleviating the necessity to pinpoint specific metabolic pathways before gene identification, we propose that nutritional conditional screens based on bare minimum growth media like ours represent promising approaches for discovering important genes and mutations in metabolic pathways, thereby accelerating the establishment of in vivo models that recapitulate human metabolic diseases.

Identification of conserved and novel microRNAs in Manduca sexta and their possible roles in the expression regulation of immunity-related genes

The tobacco hornworm Manduca sexta has served as a model for insect biochemical and physiological research for decades. However, knowledge of the posttranscriptional regulation of gene expression by microRNAs is still rudimentary in this species. Our previous study (Zhang et al., 2012) identified 163 conserved and 13 novel microRNAs in M. sexta, most of which were present at low levels in pupae. To identify additional M. sexta microRNAs and more importantly to examine their possible roles in the expression regulation of immunity-related genes, we constructed four small RNA libraries using fat body and hemocytes from naïve or bacteria-injected larvae and obtained 32.9 million reads of 18-31 nucleotides by Illumina sequencing. Mse-miR-929 and mse-miR-1b (antisense microRNA of mse-miR-1) were predicted in the previous study and now found to be conserved microRNAs in the tissue samples. We also found four novel microRNAs, two of which result from a gene cluster. Mse-miR-281-star, mse-miR-965-star, mse-miR-31-star, and mse-miR-9b-star were present at higher levels than their respective mature strands. Abundance changes of microRNAs were observed after the immune challenge. Based on the quantitative data of mRNA levels in control and induced fat body and hemocytes as well as the results of microRNA target site prediction, we suggest that certain microRNAs and microRNA*s regulate gene expression for pattern recognition, prophenoloxidase activation, cellular responses, antimicrobial peptide synthesis, and conserved intracellular signal transduction (Toll, IMD, JAK-STAT, MAPK-JNK-p38, and small interfering RNA pathways). In summary, this study has enriched our knowledge on M. sexta microRNAs and how some of them may participate in the expression regulation of immunity-related genes.

Systemic Activin signaling independently regulates sugar homeostasis, cellular metabolism, and pH balance in Drosophila melanogaster

The ability to maintain cellular and physiological metabolic homeostasis is key for the survival of multicellular organisms in changing environmental conditions. However, our understanding of extracellular signaling pathways that modulate metabolic processes remains limited. In this study we show that the Activin-like ligand Dawdle (Daw) is a major regulator of systemic metabolic homeostasis and cellular metabolism in Drosophila. We find that loss of canonical Smad signaling downstream of Daw leads to defects in sugar and systemic pH homeostasis. Although Daw regulates sugar homeostasis by positively influencing insulin release, we find that the effect of Daw on pH balance is independent of its role in insulin signaling and is caused by accumulation of organic acids that are primarily tricarboxylic acid (TCA) cycle intermediates. RNA sequencing reveals that a number of TCA cycle enzymes and nuclear-encoded mitochondrial genes including genes involved in oxidative phosphorylation and β-oxidation are up-regulated in the daw mutants, indicating either a direct or indirect role of Daw in regulating these genes. These findings establish Activin signaling as a major metabolic regulator and uncover a functional link between TGF-β signaling, insulin signaling, and metabolism in Drosophila.

Identification of neurons responsible for feeding behavior in the Drosophila brain

Drosophila melanogaster feeds mainly on rotten fruits, which contain many kinds of sugar. Thus, the sense of sweet taste has evolved to serve as a dominant regulator and driver of feeding behavior. Although several sugar receptors have been described, it remains poorly understood how the sensory input is transformed into an appetitive behavior. Here, we used a neural silencing approach to screen brain circuits, and identified neurons labeled by three Gal4 lines that modulate Drosophila feeding behavior. These three Gal4 lines labeled neurons mainly in the suboesophageal ganglia (SOG), which is considered to be the fly's primary taste center. When we blocked the activity of these neurons, flies decreased their sugar consumption significantly. In contrast, activation of these neurons resulted in enhanced feeding behavior and increased food consumption not only towards sugar, but to an array of food sources. Moreover, upon neuronal activation, the flies demonstrated feeding behavior even in the absence of food, which suggests that neuronal activation can replace food as a stimulus for feeding behavior. These findings indicate that these Gal4-labeled neurons, which function downstream of sensory neurons and regulate feeding behavior towards different food sources is necessary in Drosophila feeding control.

Sugar and chromosome stability: clastogenic effects of sugars in vitamin B6-deficient cells

Pyridoxal 5′-phosphate (PLP), the active form of vitamin B6, has been implicated in preventing human pathologies, such as diabetes and cancer. However, the mechanisms underlying the beneficial effects of PLP are still unclear. Using Drosophila as a model system, we show that PLP deficiency, caused either by mutations in the pyridoxal kinase-coding gene (dPdxk) or by vitamin B6 antagonists, results in chromosome aberrations (CABs). The CAB frequency in PLP-depleted cells was strongly enhanced by sucrose, glucose or fructose treatments, and dPdxk mutant cells consistently displayed higher glucose contents than their wild type counterparts, an effect that is at least in part a consequence of an acquired insulin resistance. Together, our results indicate that a high intracellular level of glucose has a dramatic clastogenic effect if combined with PLP deficiency. This is likely due to an elevated level of Advanced Glycation End-products (AGE) formation. Treatment of dPdxk mutant cells with α-lipoic acid (ALA) lowered both AGE formation and CAB frequency, suggesting a possible AGE-CAB cause-effect relationship. The clastogenic effect of glucose in PLP-depleted cells is evolutionarily conserved. RNAi-mediated silencing of PDXK in human cells or treatments with PLP inhibitors resulted in chromosome breakage, which was potentiated by glucose and reduced by ALA. These results suggest that patients with concomitant hyperglycemia and vitamin B6 deficiency may suffer chromosome damage. This might impact cancer risk, as CABs are a well-known tumorigenic factor.

We show that the active form of vitamin B6 (Pyridoxal 5′-phosphate, PLP) plays an important role in the maintenance of genome integrity. We found, using Drosophila as a model system, that PLP deficiency results in chromosome breaks and rearrangements (collectively dubbed chromosome aberrations, abbreviated with CABs). Most importantly, we observed that in PLP deficient cells, sucrose, glucose, or fructose strongly enhance the frequency of CABs. The mutagenic effects of sugars in the presence of PLP deficiency are evolutionarily conserved, as PLP depletion or inhibition in human cells results in CAB formation, which is potentiated by glucose or fructose. These results suggest that patients with concomitant hyperglycemic crises and vitamin B6 deficiency may suffer genetic damage, which might promote cancer and diabetes complications. Our work further suggests that patients treated with PLP antagonist drugs should keep under control the level of sugar in their blood and compensate their vitamin B6 level.

Niemann–Pick disease type C2 protein induces triglyceride accumulation in silkworm and mammalian cell lines

Niemann–Pick disease type C2 protein was identified as a triglyceride-accumulating factor in insect fluid.

High intake of dietary sugar enhances bisphenol A (BPA) disruption and reveals ribosome-mediated pathways of toxicity

Bisphenol A (BPA) is an organic compound to which human populations are ubiquitously exposed. Epidemiological data suggest BPA exposure might be associated with higher rates of diabetes and reproductive anomalies. Health concerns also include transgenerational consequences, but these mechanisms are crudely defined. Similarly, little is known about synergistic interactions between BPA and other substances. Here we show that acute and chronic exposure to BPA causes genome-wide modulation of several functionally coherent genetic pathways in the fruit fly Drosophila melanogaster. In particular, BPA exposure causes massive downregulation of testis-specific genes and upregulation of ribosome-associated genes widely expressed across tissues. In addition, it causes the modulation of transposable elements that are specific to the ribosomal DNA loci, suggesting that nucleolar stress might contribute to BPA toxicity. The upregulation of ribosome-associated genes and the impairment of testis-specific gene expression are significantly enhanced upon BPA exposure with a high-sugar diet. Our results suggest that BPA and dietary sugar might functionally interact, with consequences to regulatory programs in both reproductive and somatic tissues.

CDP-diacylglycerol synthetase coordinates cell growth and fat storage through phosphatidylinositol metabolism and the insulin pathway

During development, animals usually undergo a rapid growth phase followed by a homeostatic stage when growth has ceased. The increase in cell size and number during the growth phase requires a large amount of lipids; while in the static state, excess lipids are usually stored in adipose tissues in preparation for nutrient-limited conditions. How cells coordinate growth and fat storage is not fully understood. Through a genetic screen we identified Drosophila melanogaster CDP-diacylglycerol synthetase (CDS/CdsA), which diverts phosphatidic acid from triacylglycerol synthesis to phosphatidylinositol (PI) synthesis and coordinates cell growth and fat storage. Loss of CdsA function causes significant accumulation of neutral lipids in many tissues along with reduced cell/organ size. These phenotypes can be traced back to reduced PI levels and, subsequently, low insulin pathway activity. Overexpressing CdsA rescues the fat storage and cell growth phenotypes of insulin pathway mutants, suggesting that CdsA coordinates cell/tissue growth and lipid storage through the insulin pathway. We also revealed that a DAG-to-PE route mediated by the choline/ethanolamine phosphotransferase Bbc may contribute to the growth of fat cells in CdsA RNAi.

During development, animals undergo a rapid increase in cell size and number, which requires large amounts of lipids, in the form of phospholipids, for the expansion of cell membranes. Once the growth phase ends, excess lipids are usually stored as body fat, in the form of triacylglycerol (TAG), for use when nutrients are limited. How cells coordinate growth and fat storage is not fully understood. By screening for genes that affect lipid storage in the fruitfly Drosophila we discovered that the enzyme CDP-diacylglycerol synthetase (CdsA) coordinates cell growth and fat storage. Phospholipids and TAG have a common precursor, phosphatidic acid, which is diverted by CdsA from TAG synthesis to synthesis of the phospholipid phosphatidylinositol (PI). We also uncovered a link between CdsA and the insulin signaling pathway, which plays a major role in regulating cell and tissue growth. CdsA regulates the level of PI, which modulates insulin pathway activity; insulin pathway activity, in turn, influences the level of CdsA. The lipid metabolism pathways and the insulin signaling pathway are conserved in other animals including humans. Our findings may therefore provide further insights into clinically important imbalances in fat storage such as obesity.

Insight into insulin secretion from transcriptome and genetic analysis of insulin-producing cells of Drosophila

Insulin-producing cells (IPCs) in the Drosophila brain produce and release insulin-like peptides (ILPs) to the hemolymph. ILPs are crucial for growth and regulation of metabolic activity in flies, functions analogous to those of mammalian insulin and insulin-like growth factors (IGFs). To identify components functioning in IPCs to control ILP production, we employed genomic and candidate gene approaches. We used laser microdissection and messenger RNA sequencing to characterize the transcriptome of larval IPCs. IPCs highly express many genes homologous to genes active in insulin-producing β-cells of the mammalian pancreas. The genes in common encode ILPs and proteins that control insulin metabolism, storage, secretion, β-cell proliferation, and some not previously linked to insulin production or β-cell function. Among these novelties is unc-104, a kinesin 3 family gene, which is more highly expressed in IPCs compared to most other neurons. Knockdown of unc-104 in IPCs impaired ILP secretion and reduced peripheral insulin signaling. Unc-104 appears to transport ILPs along axons. As a complementary approach, we tested dominant-negative Rab genes to find Rab proteins required in IPCs for ILP production or secretion. Rab1 was identified as crucial for ILP trafficking in IPCs. Inhibition of Rab1 in IPCs increased circulating sugar levels, delayed development, and lowered weight and body size. Immunofluorescence labeling of Rab1 showed its tight association with ILP2 in the Golgi of IPCs. Unc-104 and Rab1 join other proteins required for ILP transport in IPCs.

Modeling obesity and its associated disorders in Drosophila

In recent years, obesity has been recognized as a major public health problem due to its increased prevalence in both children and adults and its association with numerous life-threatening complications including diabetes, heart disease, hypertension, and cancer. Obesity is a complex disorder that is the result of the interaction between predisposing genetic and environmental factors. However, the precise nature of these gene-gene and gene-environment interactions remains unclear. Here, we will describe recent studies demonstrating how fruit flies can be used to identify and characterize the mechanisms underlying obesity and to establish models of obesity-associated disorders.

The control of lipid metabolism by mRNA splicing in Drosophila

The storage of lipids is an evolutionarily conserved process that is important for the survival of organisms during shifts in nutrient availability. Triglycerides are stored in lipid droplets, but the mechanisms of how lipids are stored in these structures are poorly understood. Previous in vitro RNAi screens have implicated several components of the spliceosome in controlling lipid droplet formation and storage, but the in vivo relevance of these phenotypes is unclear. In this study, we identify specific members of the splicing machinery that are necessary for normal triglyceride storage in the Drosophila fat body. Decreasing the expression of the splicing factors U1-70K, U2AF38, U2AF50 in the fat body resulted in decreased triglyceride levels. Interestingly, while decreasing the SR protein 9G8 in the larval fat body yielded a similar triglyceride phenotype, its knockdown in the adult fat body resulted in a substantial increase in lipid stores. This increase in fat storage is due in part to altered splicing of the gene for the β-oxidation enzyme CPT1, producing an isoform with less enzymatic activity. Together, these data indicate a role for mRNA splicing in regulating lipid storage in Drosophila and provide a link between the regulation of gene expression and lipid homeostasis.

Regulation of insect behavior via the insulin-signaling pathway

The insulin/insulin-like growth factor signaling (IIS) pathway is well-established as a critical regulator of growth and metabolic homeostasis across the animal kingdom. Insulin-like peptides (ILPs), the functional analogs of mammalian insulin, were initially discovered in the silkmoth Bombyx mori and subsequently identified in many other insect species. Initial research focused on the role of insulin signaling in metabolism, cell proliferation, development, reproduction and aging. More recently however, increasing attention has been given to the role of insulin in the regulation of neuronal function and behavior. Here we review the role of insulin signaling in two specific insect behaviors: feeding and locomotion.

Ovary ecdysteroidogenic hormone functions independently of the insulin receptor in the yellow fever mosquito, Aedes aegypti

Most mosquito species must feed on the blood of a vertebrate host to produce eggs. In the yellow fever mosquito, Aedes aegypti, blood feeding triggers medial neurosecretory cells in the brain to release insulin-like peptides (ILPs) and ovary ecdysteroidogenic hormone (OEH). Theses hormones thereafter directly induce the ovaries to produce ecdysteroid hormone (ECD), which activates the synthesis of yolk proteins in the fat body for uptake by oocytes. ILP3 stimulates ECD production by binding to the mosquito insulin receptor (MIR). In contrast, little is known about the mode of action of OEH, which is a member of a neuropeptide family called neuroparsin. Here we report that OEH is the only neuroparsin family member present in the Ae. aegypti genome and that other mosquitoes also encode only one neuroparsin gene. Immunoblotting experiments suggested that the full-length form of the peptide, which we call long OEH (lOEH), is processed into short OEH (sOEH). The importance of processing, however, remained unclear because a recombinant form of lOEH (rlOEH) and synthetic sOEH exhibited very similar biological activity. A series of experiments indicated that neither rlOEH nor sOEH bound to ILP3 or the MIR. Signaling studies further showed that ILP3 activated the MIR but rlOEH did not, yet both neuropeptides activated Akt, which is a marker for insulin pathway signaling. Our results also indicated that activation of TOR signaling in the ovaries required co-stimulation by amino acids and either ILP3 or rlOEH. Overall, we conclude that OEH activates the insulin signaling pathway independently of the MIR, and that insulin and TOR signaling in the ovaries is coupled.

Metabolic and transcriptional response to a high-fat diet in Drosophila melanogaster

Obesity has dramatically increased in prevalence, making it essential to understand its accompanying metabolic changes. Modeling diet-induced obesity in Drosophila melanogaster (fruit flies), we elucidated transcriptional and metabolic changes in w¹¹¹⁸ flies on a high-fat diet (HFD). Mass spectrometry-based metabolomics revealed altered fatty acid, amino acid, and carbohydrate metabolism with HFD. Microarray analysis uncovered transcriptional changes in nitrogen metabolism, including CG9510, homolog of human argininosuccinate lyase (ASL). CG9510 knockdown in flies phenocopied traits observed with HFD, namely increased triglyceride levels and decreased cold tolerance. Restoration of CG9510 expression ameliorated observed negative consequences of HFD. Metabolomic analysis of CG9510 knockdown flies confirmed functional similarity to ASL, regulating the balance of carbon and nitrogen metabolism. In summary, we found that HFD suppresses CG9510 expression, a gene required for proper triglyceride storage and stress tolerance. These results draw an important link between regulation of amino acid metabolism and the response to diet-induced obesity.

In vivo fluorescent adenosine 5'-triphosphate (ATP) imaging of Drosophila melanogaster and Caenorhabditis elegans by using a genetically encoded fluorescent ATP biosensor optimized for low temperatures

Adenosine 5'-triphosphate (ATP) is the major energy currency of all living organisms. Despite its important functions, the spatiotemporal dynamics of ATP levels inside living multicellular organisms is unclear. In this study, we modified the genetically encoded Förster resonance energy transfer (FRET)-based ATP biosensor ATeam to optimize its affinity at low temperatures. This new biosensor, AT1.03NL, detected ATP changes inside Drosophila S2 cells more sensitively than the original biosensor did, at 25 °C. By expressing AT1.03NL in Drosophila melanogaster and Caenorhabditis elegans, we succeeded in imaging the in vivo ATP dynamics of these model animals at single-cell resolution.

Targeted lipidomics in Drosophila melanogaster identifies novel 2-monoacylglycerols and N-acyl amides

Lipid metabolism is critical to coordinate organ development and physiology in response to tissue-autonomous signals and environmental cues. Changes to the availability and signaling of lipid mediators can limit competitiveness, adaptation to environmental stressors, and augment pathological processes. Two classes of lipids, the N-acyl amides and the 2-acyl glycerols, have emerged as important signaling molecules in a wide range of species with important signaling properties, though most of what is known about their cellular functions is from mammalian models. Therefore, expanding available knowledge on the repertoire of these lipids in invertebrates will provide additional avenues of research aimed at elucidating biosynthetic, metabolic, and signaling properties of these molecules. Drosophila melanogaster is a commonly used organism to study intercellular communication, including the functions of bioactive lipids. However, limited information is available on the molecular identity of lipids with putative biological activities in Drosophila. Here, we used a targeted lipidomics approach to identify putative signaling lipids in third instar Drosophila larvae, possessing particularly large lipid mass in their fat body. We identified 2-linoleoyl glycerol, 2-oleoyl glycerol, and 45 N-acyl amides in larval tissues, and validated our findings by the comparative analysis of Oregon-RS, Canton-S and w1118 strains. Data here suggest that Drosophila represent another model system to use for the study of 2-acyl glycerol and N-acyl amide signaling.

Spectroscopic Imaging of Deep Tissue through Photoacoustic Detection of Molecular Vibration

The quantized vibration of chemical bonds provides a way of imaging target molecules in a complex tissue environment. Photoacoustic detection of harmonic vibrational transitions provides an approach to visualize tissue content beyond the ballistic photon regime. This method involves pulsed laser excitation of overtone transitions in target molecules inside a tissue. Fast relaxation of the vibrational energy into heat results in a local temperature rise on the order of mK and a subsequent generation of acoustic waves detectable with an ultrasonic transducer. In this perspective, we review recent advances that demonstrate the advantages of vibration-based photoacoustic imaging and illustrate its potential in diagnosing cardiovascular plaques. An outlook into future development of vibrational photoacoustic endoscopy and tomography is provided.