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

Transcriptional Control of Lipid Metabolism

Transcriptional control of lipid metabolism uses a framework that parallels the control of lipid metabolism at the protein or enzyme level, via feedback and feed-forward mechanisms. Increasing the substrates for an enzyme often increases enzyme gene expression, for example. A paucity of product can likewise potentiate transcription or stability of the mRNA encoding the enzyme or enzymes needed to produce it. In addition, changes in second messengers or cellular energy charge can act as on/off switches for transcriptional regulators to control transcript (and protein) abundance. Insects use a wide range of DNA-binding transcription factors (TFs) that sense changes in the cell and its environment to produce the appropriate change in transcription at gene promoters. These TFs work together with histones, spliceosomes, and additional RNA processing factors to ultimately regulate lipid metabolism. In this chapter, we will first focus on the important TFs that control lipid metabolism in insects. Next, we will describe non-TF regulators of insect lipid metabolism such as enzymes that modify acetylation and methylation status, transcriptional coactivators, splicing factors, and microRNAs. To conclude, we consider future goals for studying the mechanisms underlying the control of lipid metabolism in insects.

How does a fly die? Insights into ageing from the pathophysiology of Drosophila mortality

The fruit fly Drosophila melanogaster is a common animal model in ageing research. Large populations of flies are used to study the impact of genetic, nutritional and pharmacological interventions on survival. However, the processes through which flies die and their relative prevalence in Drosophila populations are still comparatively unknown. Understanding the causes of death in an animal model is essential to dissect the lifespan-extending interventions that are organism- or disease-specific from those broadly applicable to ageing. Here, we review the pathophysiological processes that can lead to fly death and discuss their relation to ageing.

Lipotoxicity as a therapeutic target in obesity and diabetic cardiomyopathy

Unhealthy sources of fats, ultra-processed foods with added sugars, and a sedentary lifestyle make humans more susceptible to developing overweight and obesity. While lipids constitute an integral component of the organism, excessive and abnormal lipid accumulation that exceeds the storage capacity of lipid droplets disrupts the intracellular composition of fatty acids and results in the release of deleterious lipid species, thereby giving rise to a pathological state termed lipotoxicity. This condition induces endoplasmic reticulum stress, mitochondrial dysfunction, inflammatory responses, and cell death. Recent advances in omics technologies and analytical methodologies and clinical research have provided novel insights into the mechanisms of lipotoxicity, including gut dysbiosis, epigenetic and epitranscriptomic modifications, dysfunction of lipid droplets, post-translational modifications, and altered membrane lipid composition. In this review, we discuss the recent knowledge on the mechanisms underlying the development of lipotoxicity and lipotoxic cardiometabolic disease in obesity, with a particular focus on lipotoxic and diabetic cardiomyopathy.

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) family in physiological and pathophysiological process and diseases

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) family (PGC-1s), consisting of three members encompassing PGC-1α, PGC-1β, and PGC-1-related coactivator (PRC), was discovered more than a quarter-century ago. PGC-1s are essential coordinators of many vital cellular events, including mitochondrial functions, oxidative stress, endoplasmic reticulum homeostasis, and inflammation. Accumulating evidence has shown that PGC-1s are implicated in many diseases, such as cancers, cardiac diseases and cardiovascular diseases, neurological disorders, kidney diseases, motor system diseases, and metabolic disorders. Examining the upstream modulators and co-activated partners of PGC-1s and identifying critical biological events modulated by downstream effectors of PGC-1s contribute to the presentation of the elaborate network of PGC-1s. Furthermore, discussing the correlation between PGC-1s and diseases as well as summarizing the therapy targeting PGC-1s helps make individualized and precise intervention methods. In this review, we summarize basic knowledge regarding the PGC-1s family as well as the molecular regulatory network, discuss the physio-pathological roles of PGC-1s in human diseases, review the application of PGC-1s, including the diagnostic and prognostic value of PGC-1s and several therapies in pre-clinical studies, and suggest several directions for future investigations. This review presents the immense potential of targeting PGC-1s in the treatment of diseases and hopefully facilitates the promotion of PGC-1s as new therapeutic targets.

PGC-1α-Coordinated Hypothalamic Antioxidant Defense Is Linked to SP1-LanCL1 Axis during High-Fat-Diet-Induced Obesity in Male Mice

High-fat-diet (HFD)-induced obesity parallels hypothalamic inflammation and oxidative stress, but the correlations between them are not well-defined. Here, with mouse models targeting the antioxidant gene LanCL1 in the hypothalamus, we demonstrate that impaired hypothalamic antioxidant defense aggravates HFD-induced hypothalamic inflammation and obesity progress, and these could be improved in mice with elevated hypothalamic antioxidant defense. We also show that peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), a critical transcriptional coactivator, is implicated in regulating hypothalamic LanCL1 transcription, in collaboration with SP1 through a direct interaction, in response to HFD-induced palmitic acid (PA) accumulation. According to our results, when exposed to HFD, mice undergo a process of overwhelming hypothalamic antioxidant defense; short-time HFD exposure induces ROS production to activate PGC-1α and elevate LanCL1-mediated antioxidant defense, while long-time exposure promotes ubiquitin-mediated PGC-1α degradation and suppresses LanCL1 expression. Our findings show the critical importance of the hypothalamic PGC-1α-SP1-LanCL1 axis in regulating HFD-induced obesity, and provide new insights describing the correlations of hypothalamic inflammation and oxidative stress during this process.

Physical exercise ameliorates age-related deterioration of skeletal muscle and mortality by activating Pten-related pathways in Drosophila on a high-salt diet

The phosphatase and tensin congeners (Pten) gene affects cell growth, cell proliferation, and rearrangement of connections, and it is closely related to cellular senescence, but it remains unclear the role of muscle-Pten gene in exercise against age-related deterioration in skeletal muscle and mortality induced by a high-salt diet (HSD). In here, overexpression and knockdown of muscle Pten gene were constructed by building MhcGAL4 /PtenUAS-overexpression and MhcGAL4 /PtenUAS-RNAi system in flies, and flies were given exercise training and a HSD for 2 weeks. The results showed that muscle Pten knockdown significantly reduced the climbing speed, climbing endurance, GPX activity, and the expression of Pten, Sirt1, PGC-1α genes, and it significantly increased the expression of Akt and ROS level, and impaired myofibril and mitochondria of aged skeletal muscle. Pten knockdown prevented exercise from countering the HSD-induced age-related deterioration of skeletal muscle. Pten overexpression has the opposite effect on skeletal muscle aging when compared to it knockdown, and it promoted exercise against HSD-induced age-related deterioration of skeletal muscle. Pten overexpression significantly increased lifespan, but its knockdown significantly decreased lifespan of flies. Thus, current results confirmed that differential expression of muscle Pten gene played an important role in regulating skeletal muscle aging and lifespan, and it also affected the adaptability of aging skeletal muscle to physical exercise since it determined the activity of muscle Pten/Akt pathway and Pten/Sirt1/PGC-1α pathway.

The link between obesity and aging - insights into cardiac energy metabolism

Obesity and aging are well-established risk factors for a range of diseases, including cardiovascular diseases and type 2 diabetes. Given the escalating prevalence of obesity, the aging population, and the subsequent increase in cardiovascular diseases, it is crucial to investigate the underlying mechanisms involved. Both aging and obesity have profound effects on the energy metabolism through various mechanisms, including metabolic inflexibility, altered substrate utilization for energy production, deregulated nutrient sensing, and mitochondrial dysfunction. In this review, we aim to present and discuss the hypothesis that obesity, due to its similarity in changes observed in the aging heart, may accelerate the process of cardiac aging and exacerbate the clinical outcomes of elderly individuals with obesity.

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

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

Regular Exercise in Drosophila Prevents Age-Related Cardiac Dysfunction Caused by High Fat and Heart-Specific Knockdown of skd

Skuld (skd) is a subunit of the Mediator complex subunit complex. In the heart, skd controls systemic obesity, is involved in systemic energy metabolism, and is closely linked to cardiac function and aging. However, it is unclear whether the effect of cardiac skd on cardiac energy metabolism affects cardiac function. We found that cardiac-specific knockdown of skd showed impaired cardiac function, metabolic impairment, and premature aging. Drosophila was subjected to an exercise and high-fat diet (HFD) intervention to explore the effects of exercise on cardiac skd expression and cardiac function in HFD Drosophila. We found that Hand-Gal4>skd RNAi (KC) Drosophila had impaired cardiac function, metabolic impairment, and premature aging. Regular exercise significantly improved cardiac function and metabolism and delayed aging in HFD KC Drosophila. Thus, our study found that the effect of skd on cardiac energy metabolism in the heart affected cardiac function. Exercise may counteract age-related cardiac dysfunction and metabolic disturbances caused by HFD and heart-specific knockdown of skd. Skd may be a potential therapeutic target for heart disease.

Deficiency of Adipose Triglyceride Lipase Induces Metabolic Syndrome and Cardiomyopathy in Zebrafish

Lipid metabolism dysfunction is related to clinical disorders including obesity, cancer, liver steatosis, and cardiomyopathy. Impaired lipolytic enzymes result in altered release of free fatty acids. The dramatic change in dyslipidemia is important in lipotoxic cardiomyopathy. Adipose triglyceride lipase (ATGL) catalyzes the lipolysis of triacylglycerol to reduce intramyocardial triglyceride levels in the heart and improve myocardial function. We examined the role of ATGL in metabolic cardiomyopathy by developing an Atgl knockout (ALKO) zebrafish model of metabolic cardiomyopathy disease by continuously expressing CRISPR/Cas9 protein and atgl gene guide RNAs (gRNAs). The expressed Cas9 protein bound to four gRNAs targeting the atgl gene locus, facilitating systemic gene KO. Ablation of Atgl interfered with lipid metabolism, which induced hyperlipidemia and hyperglycemia. ALKO adults and embryos displayed hypertrophic hearts. ALKO presented a typical dilated cardiomyopathy profile with a remarkable reduction in four sarcomere genes (myosin heavy chain 7-like, actin alpha cardiac muscle 1b, myosin binding protein C3, and troponin T type 2a) and two Ca²⁺ handling regulator genes (tropomyosin 4b and ATPase sarcoplasmic/endoplasmic reticulum Ca²⁺ transporting 2b). Immune cell infiltration in cardiac tissue of ALKO provided direct evidence of advanced metabolic cardiomyopathy. The presently described model could become a powerful tool to clarify the underlying mechanism between metabolic disorders and cardiomyopathies.

Fat Quality Impacts the Effect of a High-Fat Diet on the Fatty Acid Profile, Life History Traits and Gene Expression in Drosophila melanogaster

Feeding a high-fat diet (HFD) has been shown to alter phenotypic and metabolic parameters in Drosophila melanogaster. However, the impact of fat quantity and quality remains uncertain. We first used butterfat (BF) as an example to investigate the effects of increasing dietary fat content (3-12%) on male and female fruit flies. Although body weight and body composition were not altered by any BF concentration, health parameters, such as lifespan, fecundity and larval development, were negatively affected in a dose-dependent manner. When fruit flies were fed various 12% HFDs (BF, sunflower oil, olive oil, linseed oil, fish oil), their fatty acid profiles shifted according to the dietary fat qualities. Moreover, fat quality was found to determine the effect size of the response to an HFD for traits, such as lifespan, climbing activity, or fertility. Consistently, we also found a highly fat quality-specific transcriptional response to three exemplary HFD qualities with a small overlap of only 30 differentially expressed genes associated with the immune/stress response and fatty acid metabolism. In conclusion, our data indicate that not only the fat content but also the fat quality is a crucial factor in terms of life-history traits when applying an HFD in D. melanogaster.

Full-length transcriptomic analysis in murine and human heart reveals diversity of PGC-1α promoters and isoforms regulated distinctly in myocardial ischemia and obesity

Background

Peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) acts as a transcriptional coactivator and regulates mitochondrial function. Various isoforms are generated by alternative splicing and differentially regulated promoters. In the heart, total PGC-1α deficiency knockout leads to dilatative cardiomyopathy, but knowledge on the complexity of cardiac isoform expression of PGC-1α remains sparse. Thus, this study aims to generate a reliable dataset on cardiac isoform expression pattern by long-read mRNA sequencing, followed by investigation of differential regulation of PGC-1α isoforms under metabolic and ischemic stress, using high-fat-high-sucrose-diet-induced obesity and a murine model of myocardial infarction.

Results

Murine (C57Bl/6J) or human heart tissue (obtained during LVAD-surgery) was used for long-read mRNA sequencing, resulting in full-length transcriptomes including 58,000 mRNA isoforms with 99% sequence accuracy. Automatic bioinformatic analysis as well as manual similarity search against exonic sequences leads to identification of putative coding PGC-1α isoforms, validated by PCR and Sanger sequencing. Thereby, 12 novel transcripts generated by hitherto unknown splicing events were detected. In addition, we postulate a novel promoter with homologous and strongly conserved sequence in human heart. High-fat diet as well as ischemia/reperfusion (I/R) injury transiently reduced cardiac expression of PGC-1α isoforms, with the most pronounced effect in the infarcted area. Recovery of PGC-1α-isoform expression was even more decelerated when I/R was performed in diet-induced obese mice.

Conclusions

We deciphered for the first time a complete full-length transcriptome of the murine and human heart, identifying novel putative PGC-1α coding transcripts including a novel promoter. These transcripts are differentially regulated in I/R and obesity suggesting transcriptional regulation and alternative splicing that may modulate PGC-1α function in the injured and metabolically challenged heart.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01360-w.

mTORC2 protects the heart from high-fat diet-induced cardiomyopathy through mitochondrial fission in Drosophila

High-fat diet (HFD)-induced obesity has become the major risk factor for the development of cardiovascular diseases, but the underlying mechanisms remain poorly understood. Here, we use Drosophila as a model to study the role of mTORC2 in HFD-induced mitochondrial fission and cardiac dysfunction. We find that knockdown of mTORC2 subunit rictor blocks HFD-induced mitochondrial fragmentation and Drp1 recruitment. Knockdown of rictor further impairs cardiac contractile function under HFD treatment. Surprisingly, knockdown of Akt, the major effector of mTORC2, did not affect HFD-induced mitochondrial fission. Similar to mTORC2 inhibition, knockdown of Drp1 blocks HFD-induced mitochondrial fragmentation and induces contractile defects. Furthermore, overexpression of Drp1 restored HFD-induced mitochondrial fragmentation in rictor knockdown flies. Thus, we uncover a novel function of mTORC2 in protecting the heart from HFD treatment through Drp1-dependent mitochondrial fission.

Isosteviol sodium attenuates high fat/high cholesterol-induced myocardial dysfunction by regulating the Sirt1/AMPK pathway

A fat-rich diet triggers obesity, and promotes cardiomyocyte injury. Till now, no prior investigations suggested a beneficial role of Isosteviol Sodium (STVNa) in cardiac activity in high fat diet (HFD)-exposed obese rats. However, there is evidence that STVNa accelerates healing of multiple tissue injuries. Herein, we explored the underlying mechanism behind the STVNa-based protection against HFD-induced myocardial dysfunction (MCD) in a rat model of myocardial injury. We employed dosages of 1, 10, and 20 mg/kg STVNa to treat MCD in rats fed with a HFD. Based on our results, STVNa repressed MCD (as indicated by ecocardiographic analysis), myocardium function, pathological structure, and myocardial enzymes. Mechanistically, the STVNa-mediated protection against HFD-induced MCD involved inhibition of inflammation and oxidative stress. Furthermore, using Western blot analysis, we revealed that the critical members of the Sirt1/AMPK network were markedly activated in the STVNa-treated group, relative to HFD-fed controls. Collectively, these evidences suggested that the STVNa offered strong protection against HFD-induced MCD. Moreover, this effect was mediated by the activation of the Sirt1/AMPK network, which, in turn, promoted lipid metabolism.

Genetic variation of macronutrient tolerance in Drosophila melanogaster

Carbohydrates, proteins and lipids are essential nutrients to all animals; however, closely related species, populations, and individuals can display dramatic variation in diet. Here we explore the variation in macronutrient tolerance in Drosophila melanogaster using the Drosophila genetic reference panel, a collection of ~200 strains derived from a single natural population. Our study demonstrates that D. melanogaster, often considered a "dietary generalist", displays marked genetic variation in survival on different diets, notably on high-sugar diet. Our genetic analysis and functional validation identify several regulators of macronutrient tolerance, including CG10960/GLUT8, Pkn and Eip75B. We also demonstrate a role for the JNK pathway in sugar tolerance and de novo lipogenesis. Finally, we report a role for tailless, a conserved orphan nuclear hormone receptor, in regulating sugar metabolism via insulin-like peptide secretion and sugar-responsive CCHamide-2 expression. Our study provides support for the use of nutrigenomics in the development of personalized nutrition.

Embracing complexity in Drosophila cancer models

Cancer continues to be a leading cause of death worldwide, largely due to metastases and cachexia. It is a complex disease that is commonly associated with a variety of comorbidities. With global increases in ageing populations and obesity, multimorbidity is a rapidly growing clinical issue in the context of cancer. Cancer is also genetically heterogeneous, with a tumour's unique profile determining its incidence of metastasis, degree of cachexia and response to therapeutics. These complexities of human cancer are difficult to replicate in animal models and are, in part, responsible for the failures in translational cancer research. In this Perspective, we highlight the fruit fly, Drosophila melanogaster, as a powerful model organism to investigate multimorbidity and tumour diversity. We also highlight how harnessing these complexities in Drosophila can, potentially, enhance cancer research and advance therapeutic discoveries.

Summary: Comorbidities and tumour genetic diversity more accurately reflect the cancer patient landscape but are largely neglected in animal models. Drosophila holds the potential to address these complexities to better understand their impacts on cancer development.

Phenotyping of Drosophila melanogaster—A Nutritional Perspective

The model organism Drosophila melanogaster was increasingly applied in nutrition research in recent years. A range of methods are available for the phenotyping of D. melanogaster, which are outlined in the first part of this review. The methods include determinations of body weight, body composition, food intake, lifespan, locomotor activity, reproductive capacity and stress tolerance. In the second part, the practical application of the phenotyping of flies is demonstrated via a discussion of obese phenotypes in response to high-sugar diet (HSD) and high-fat diet (HFD) feeding. HSD feeding and HFD feeding are dietary interventions that lead to an increase in fat storage and affect carbohydrate-insulin homeostasis, lifespan, locomotor activity, reproductive capacity and stress tolerance. Furthermore, studies regarding the impacts of HSD and HFD on the transcriptome and metabolome of D. melanogaster are important for relating phenotypic changes to underlying molecular mechanisms. Overall, D. melanogaster was demonstrated to be a valuable model organism with which to examine the pathogeneses and underlying molecular mechanisms of common chronic metabolic diseases in a nutritional context.

Ca2+ mishandling and mitochondrial dysfunction: a converging road to prediabetic and diabetic cardiomyopathy

Diabetic cardiomyopathy is defined as the myocardial dysfunction that suffers patients with diabetes mellitus (DM) in the absence of hypertension and structural heart diseases such as valvular or coronary artery dysfunctions. Since the impact of DM on cardiac function is rather silent and slow, early stages of diabetic cardiomyopathy, known as prediabetes, are poorly recognized, and, on many occasions, cardiac illness is diagnosed only after a severe degree of dysfunction was reached. Therefore, exploration and recognition of the initial pathophysiological mechanisms that lead to cardiac dysfunction in diabetic cardiomyopathy are of vital importance for an on-time diagnosis and treatment of the malady. Among the complex and intricate mechanisms involved in diabetic cardiomyopathy, Ca²⁺ mishandling and mitochondrial dysfunction have been described as pivotal early processes. In the present review, we will focus on these two processes and the molecular pathway that relates these two alterations to the earlier stages and the development of diabetic cardiomyopathy.

Drosophila melanogaster: A Powerful Tiny Animal Model for the Study of Metabolic Hepatic Diseases

Animal experimentation is limited by unethical procedures, time-consuming protocols, and high cost. Thus, the development of innovative approaches for disease treatment based on alternative models in a fast, safe, and economic manner is an important, yet challenging goal. In this paradigm, the fruit-fly Drosophila melanogaster has become a powerful model for biomedical research, considering its short life cycle and low-cost maintenance. In addition, biological processes are conserved and homologs of ∼75% of human disease-related genes are found in the fruit-fly. Therefore, this model has been used in innovative approaches to evaluate and validate the functional activities of candidate molecules identified via in vitro large-scale analyses, as putative agents to treat or reverse pathological conditions. In this context, Drosophila offers a powerful alternative to investigate the molecular aspects of liver diseases, since no effective therapies are available for those pathologies. Non-alcoholic fatty liver disease is the most common form of chronic hepatic dysfunctions, which may progress to the development of chronic hepatitis and ultimately to cirrhosis, thereby increasing the risk for hepatocellular carcinoma (HCC). This deleterious situation reinforces the use of the Drosophila model to accelerate functional research aimed at deciphering the mechanisms that sustain the disease. In this short review, we illustrate the relevance of using the fruit-fly to address aspects of liver pathologies to contribute to the biomedical area.

Solanum anguivi Lam. fruit preparations counteract the negative effects of a high-sugar diet on the glucose metabolism in Drosophila melanogaster

Solanum anguivi Lam. fruits (SALF) are traditionally consumed as a remedy for type 2 diabetes mellitus (T2DM). However, data regarding the potential anti-diabetic effect of SALF and its underlying mechanisms are scarce. As the fruit fly's energy metabolism has been suggested to be comparable with mammals including the secretion of insulin-like peptides, we fed Drosophila melanogaster a high-sugar diet (HSD) to induce a T2DM-like phenotype and subsequently exposed them to a HSD supplemented with SALF. Following, flies were analyzed for various biomarkers in relation to energy metabolism. The HSD-induced glucose levels were significantly down-regulated in flies exposed to a HSD supplemented with SALF. In addition, flies exposed to SALF-supplemented HSD exhibited a better survival in comparison to HSD-fed counterparts. Other parameters of the energy metabolism such as triglyceride levels, weights, and fitness were not affected by SALF supplementation. This was also true for the expression levels of the insulin-like-peptides 3 and 6 as well as for spargel, the Drosophila homolog of PPARγ-co-activator 1α, a central player in mitochondrial biogenesis. Overall, the present study shows that SALF significantly lowered the HSD-induced glucose levels and increased the survival while the biomarkers of the energy metabolism were not affected.

Loss of Stearoyl-CoA desaturase 1 leads to cardiac dysfunction and lipotoxicity

Diets high in carbohydrates are associated with type 2 diabetes and its co-morbidities, including hyperglycemia, hyperlipidemia, obesity, hepatic steatosis and cardiovascular disease. We used a high-sugar diet to study the pathophysiology of diet-induced metabolic disease in Drosophila melanogaster. High-sugar diets produce hyperglycemia, obesity, insulin resistance and cardiomyopathy in flies, along with ectopic accumulation of toxic lipids, or lipotoxicity. Stearoyl-CoA desaturase 1 is an enzyme that contributes to long-chain fatty acid metabolism by introducing a double bond into the acyl chain. Knockdown of stearoyl-CoA desaturase 1 in the fat body reduced lipogenesis and exacerbated pathophysiology in flies reared on high-sucrose diets. These flies exhibited dyslipidemia and growth deficiency in addition to defects in cardiac and gut function. We assessed the lipidome of these flies using tandem mass spectrometry to provide insight into the relationship between potentially lipotoxic species and type 2 diabetes-like pathophysiology. Oleic acid supplementation is able to rescue a variety of phenotypes produced by stearoyl-CoA desaturase 1 RNAi, including fly mass, triglyceride storage, gut development and cardiac failure. Taken together, these data suggest a protective role for monounsaturated fatty acids in diet-induced metabolic disease phenotypes.

Activation of cardiac Nmnat/NAD+/SIR2 pathways mediates endurance exercise resistance to lipotoxic cardiomyopathy in aging Drosophila

Endurance exercise is an important way to resist and treat high-fat diet (HFD)-induced lipotoxic cardiomyopathy, but the underlying molecular mechanisms are poorly understood. Here, we used Drosophila to identify whether cardiac Nmnat/NAD+/SIR2 pathway activation mediates endurance exercise-induced resistance to lipotoxic cardiomyopathy. The results showed that endurance exercise activated the cardiac Nmnat/NAD+/SIR2/FOXO pathway and the Nmnat/NAD+/SIR2/PGC-1α pathway, including up-regulating cardiac Nmnat, SIR2, FOXO and PGC-1α expression, superoxide dismutase (SOD) activity and NAD+ levels, and it prevented HFD-induced or cardiac Nmnat knockdown-induced cardiac lipid accumulation, malondialdehyde (MDA) content and fibrillation increase, and fractional shortening decrease. Cardiac Nmnat overexpression also activated heart Nmnat/NAD+/SIR2 pathways and resisted HFD-induced cardiac malfunction, but it could not protect against HFD-induced lifespan reduction and locomotor impairment. Exercise improved lifespan and mobility in cardiac Nmnat knockdown flies. Therefore, the current results confirm that cardiac Nmnat/NAD+/SIR2 pathways are important antagonists of HFD-induced lipotoxic cardiomyopathy. Cardiac Nmnat/NAD+/SIR2 pathway activation is an important underlying molecular mechanism by which endurance exercise and cardiac Nmnat overexpression give protection against lipotoxic cardiomyopathy in Drosophila.

Physical exercise prevents age-related heart dysfunction induced by high-salt intake and heart salt-specific overexpression in Drosophila

A long-term high-salt intake (HSI) seems to accelerate cardiac aging and age-related diseases, but the molecular mechanism is still not entirely clear. Exercise is an effective way to delay cardiac aging. However, it remains unclear whether long-term exercise (LTE) can protect heart from aging induced by high-salt stress. In this study, heart CG2196(salt) specific overexpression (HSSO) and RNAi (HSSR) was constructed by using the UAS/hand-Gal4 system in Drosophila. Flies were given exercise and a high-salt diet intervention from 1 to 5 weeks of age. Results showed that HSSR and LTE remarkably prevented heart from accelerated age-related defects caused by HSI and HSSO, and these defects included a marked increase in heart period, arrhythmia index, malondialdehyde (MDA) level, salt expression, and dTOR expression, and a marked decrease in fractional shortening, SOD activity level, dFOXO expression, PGC-1α expression, and the number of mitochondria and myofibrils. The combination of HSSR and LTE could better protect the aging heart from the damage of HSI. Therefore, current evidences suggested that LTE resisted HSI-induced heart presenility via blocking CG2196(salt)/TOR/oxidative stress and activating dFOXO/PGC-1α. LTE also reversed heart presenility induced by cardiac-salt overexpression via activating dFOXO/PGC-1α and blocking TOR/oxidative stress.

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.

Systemic and mitochondrial effects of metabolic inflexibility induced by high fat diet in Drosophila melanogaster

Metabolic inflexibility is a condition that occurs following a nutritional stress which causes blunted fuel switching at the mitochondrial level in response to hormonal and cellular signalling. Linked to obesity and obesity related disorders, chronic exposure to a high-fat diet (HFD) in animal models has been extensively used to induce metabolic inflexibility and investigate the development of various metabolic diseases. However, many questions concerning the systemic and mitochondrial responses to metabolic inflexibility remain. In this study, we investigated the global and mitochondrial variations following a 10-day exposure to a HFD in adult Drosophila melanogaster. Our results show that following 10-day exposure to the HFD, mitochondrial respiration rates measured in isolated mitochondria at the level of complex I were decreased. This was associated with increased contributions of non-classical providers of electrons to the electron transport system (ETS) such as the proline dehydrogenase (ProDH) and the mitochondrial glycerol-3-phosphate dehydrogenase (mtG3PDH) alleviating complex I dysfunctions, as well as with increased ROS production per molecule of oxygen consumed. Our results also show an accumulation of metabolites from multiple different metabolic pathways in whole adult Drosophila and a drastic shift in the lipid profile which translated into decreased proportion of saturated and monounsaturated fatty acids combined with an increased proportion of polyunsaturated fatty acids. Thus, our results demonstrate the various responses to the HFD treatment in adult Drosophila melanogaster that are hallmarks of the development of metabolic inflexibility and reinforce this organism as a suitable model for the study of metabolic disorders.

Adipose triglyceride lipase protects renal cell endocytosis in a Drosophila dietary model of chronic kidney disease

Obesity-related renal lipotoxicity and chronic kidney disease (CKD) are prevalent pathologies with complex aetiologies. One hallmark of renal lipotoxicity is the ectopic accumulation of lipid droplets in kidney podocytes and in proximal tubule cells. Renal lipid droplets are observed in human CKD patients and in high-fat diet (HFD) rodent models, but their precise role remains unclear. Here, we establish a HFD model in Drosophila that recapitulates renal lipid droplets and several other aspects of mammalian CKD. Cell type-specific genetic manipulations show that lipid can overflow from adipose tissue and is taken up by renal cells called nephrocytes. A HFD drives nephrocyte lipid uptake via the multiligand receptor Cubilin (Cubn), leading to the ectopic accumulation of lipid droplets. These nephrocyte lipid droplets correlate with endoplasmic reticulum (ER) and mitochondrial deficits, as well as with impaired macromolecular endocytosis, a key conserved function of renal cells. Nephrocyte knockdown of diglyceride acyltransferase 1 (DGAT1), overexpression of adipose triglyceride lipase (ATGL), and epistasis tests together reveal that fatty acid flux through the lipid droplet triglyceride compartment protects the ER, mitochondria, and endocytosis of renal cells. Strikingly, boosting nephrocyte expression of the lipid droplet resident enzyme ATGL is sufficient to rescue HFD-induced defects in renal endocytosis. Moreover, endocytic rescue requires a conserved mitochondrial regulator, peroxisome proliferator-activated receptor-gamma coactivator 1α (PGC1α). This study demonstrates that lipid droplet lipolysis counteracts the harmful effects of a HFD via a mitochondrial pathway that protects renal endocytosis. It also provides a genetic strategy for determining whether lipid droplets in different biological contexts function primarily to release beneficial or to sequester toxic lipids.

Drosophila as a useful model for understanding the evolutionary physiology of obesity resistance and metabolic thrift

Evolved metabolic thriftiness in humans is a proposed contributor to the obesity epidemic. Insect models have been shown to evolve both 'metabolic thrift' in response to rearing on high-protein diets that promote leanness, and 'obesity resistance' when reared on fattening high-carbohydrate, low-protein foods. Despite the hypothesis that human obesity is caused by evolved metabolic thrift, genetic contributions to this physiological trait remain elusive. Here we conducted a pilot study to determine whether thrift and obesity resistance can arise under laboratory based 'quasi-natural selection' in the genetic model organism Drosophila melanogaster. We found that both these traits can evolve within 16 generations. Contrary to predictions from the 'thrifty genotype/phenotype' hypothesis, we found that when animals from a metabolic thrift inducing high-protein environment are mismatched to fattening high-carbohydrate foods, they did not become 'obese'. Rather, they accumulate less triglyceride than control animals, not more. We speculate that this may arise through as yet un-quantified parental effects - potentially epigenetic. This study establishes that D. melanogaster could be a useful model for elucidating the role of the trans- and inter-generational effects of diet on the genetics of metabolic traits in higher animals.

The Drosophila model to interrogate triacylglycerol biology

The deposition of storage fat in the form of triacylglycerol (TAG) is an evolutionarily conserved strategy to cope with fluctuations in energy availability and metabolic stress. Organismal TAG storage in specialized adipose tissues provides animals a metabolic reserve that sustains survival during development and starvation. On the other hand, excessive accumulation of adipose TAG, defined as obesity, is associated with an increasing prevalence of human metabolic diseases. During the past decade, the fruit fly Drosophila melanogaster, traditionally used in genetics and developmental biology, has been established as a versatile model system to study TAG metabolism and the etiology of lipid-associated metabolic diseases. Similar to humans, Drosophila TAG homeostasis relies on the interplay of organ systems specialized in lipid uptake, synthesis, and processing, which are integrated by an endocrine network of hormones and messenger molecules. Enzymatic formation of TAG from sugar or dietary lipid, its storage in lipid droplets, and its mobilization by lipolysis occur via mechanisms largely conserved between Drosophila and humans. Notably, dysfunctional Drosophila TAG homeostasis occurs in the context of aging, overnutrition, or defective gene function, and entails tissue-specific and organismal pathologies that resemble human disease. In this review, we summarize the physiology and biochemistry of TAG in Drosophila and outline the potential of this organism as a model system to understand the genetic and dietary basis of TAG storage and TAG-related metabolic disorders.

Fat-body brummer lipase determines survival and cardiac function during starvation in Drosophila melanogaster

The cross talk between adipose tissue and the heart has an increasing importance for cardiac function under physiological and pathological conditions. This study characterizes the role of fat body lipolysis for cardiac function in Drosophila melanogaster. Perturbation of the function of the key lipolytic enzyme, brummer (bmm), an ortholog of the mammalian ATGL (adipose triglyceride lipase) exclusively in the fly's fat body, protected the heart against starvation-induced dysfunction. We further provide evidence that this protection is caused by the preservation of glycerolipid stores, resulting in a starvation-resistant maintenance of energy supply and adequate cardiac ATP synthesis. Finally, we suggest that alterations of lipolysis are tightly coupled to lipogenic processes, participating in the preservation of lipid energy substrates during starvation. Thus, we identified the inhibition of adipose tissue lipolysis and subsequent energy preservation as a protective mechanism against cardiac dysfunction during catabolic stress.

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A cross talk between fat body and the heart regulates cardiac function in Drosophila

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Knockdown of fat-body brummer lipase prevents starvation-induced cardiac dysfunction

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This involves preservation of lipid stores and maintenance of cardiac energy supply

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Brummer-mediated preservation of fat body lipid stores involves lipolysis and lipogenesis

A Low-Carbohydrate Ketogenic Diet and Treadmill Training Enhanced Fatty Acid Oxidation Capacity but Did Not Enhance Maximal Exercise Capacity in Mice

The low-carbohydrate ketogenic diet (LCKD) is a dietary approach characterized by the intake of high amounts of fat, a balanced amount of protein, and low carbohydrates, which is insufficient for metabolic demands. Previous studies have shown that an LCKD alone may contribute to fatty acid oxidation capacity, along with endurance. In the present study, we combined a 10-week LCKD with an 8-week forced treadmill running program to determine whether training in conjunction with LCKD enhanced fatty acid oxidation capacity, as well as whether the maximal exercise capacity would be affected by an LCKD or training in a mice model. We found that the lipid pool and fatty acid oxidation capacity were both enhanced following the 10-week LCKD. Further, key fatty acid oxidation related genes were upregulated. In contrast, the 8-week training regimen had no effect on fatty acid and ketone body oxidation. Key genes involved in carbohydrate utilization were downregulated in the LCKD groups. However, the improved fatty acid oxidation capacity did not translate into an enhanced maximal exercise capacity. In summary, while favoring the fatty acid oxidation system, an LCKD, alone or combined with training, had no beneficial effects in our intensive exercise-evaluation model. Therefore, an LCKD may be promising to improve endurance in low- to moderate-intensity exercise, and may not be an optimal choice for those partaking in high-intensity exercise.

Drosophila as a model system for deciphering the 'host physiology-nutrition-microbiome' axis

For metazoans, nutritional stressors, such as undernutrition during growth and development, results in serious outcomes, including growth impairments and organ wasting. When undernutrition is accompanied by other complications, including chronic inflammation, a more complex pathophysiology may emerge, such as environmental enteropathy. Although nutrition is one of the most important environmental factors that influences host physiology, the mechanism by which undernutrition induces host pathophysiology is not fully understood. Recently, gut microbiome was found to alleviate undernutrition-induced pathophysiology in an insect model, revealing the importance of nutrition-microbiome interactions. Here, we discussed how nutrition-microbiome interactions influence host physiology, including growth, tissue homeostasis, immunity, and behavior, by regulating the central metabolic signaling pathways with an emphasis on findings made through Drosophila, an insect model.

PPARGC1A gene polymorphism is associated with exercise-induced fat loss

Obesity is a widespread problem within modern society, serving to increase the risk of cardiovascular, metabolic, and neurodegenerative disorders. Peroxisome proliferator-activated receptor gamma (PPARγ) and PPARγ coactivator 1 α (PGC1α) play a key role in the regulation of cellular energy metabolism and is implicated in the pathology of these diseases. This study examined the association between polymorphisms of the PPARG and PPARGC1A genes and individual variability in weight loss in response to physical activity intervention. 39 obese Ukrainian women (44.4 ± 7.5 years, BMI > 30.0 kg/m²) undertook a 3-month fitness program whilst following a hypocaloric diet (~ 1500 cal). Anthropometric and biochemical measurements took place before and after the program. Single nucleotide polymorphisms within or near PPARG (n = 94) and PPARGC1A (n = 138) were identified and expression of PPARG mRNA was measured via reverse transcription and amplification. The association between DNA polymorphisms and exercise-induced weight loss, initial body mass, biochemistry and PPARG expression was determined using one-way analysis of variance (ANOVA). The present intervention induced significant fat loss in all participants (total fat: 40.3 ± 5.3 vs 36.4 ± 5.7%; P < 0.00001). Only one polymorphism (rs17650401 C/T) within the PPARGC1A gene was found to be associated with fat loss efficiency after correction for multiple testing, with T allele carriers showing the greatest reduction in body fat percentage (2.5-fold; P = 0.00013) compared to non-carriers. PPARGC1A (rs17650401) is associated with fat loss efficiency of the fitness program in obese women. Further studies are warranted to test whether this variation is associated with fat oxidation.

Prenatal High-Salt Diet-Induced Metabolic Disorders via Decreasing Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1α in Adult Male Rat Offspring

SCOPE

Although prenatal high-salt (HS) intake leads to physiological complications in the offspring, little is known regarding its effects on the offspring's glucose metabolism. Therefore, the objectives of this study are to determine the consequences of prenatal HS diet on the offspring's metabolism and to test a potential therapy.

METHODS AND RESULTS

Pregnant rats are fed either a normal-salt (1% NaCl) or high-salt (8% NaCl) diet during the whole pregnancy. Experiments are conducted in five-month-old male offspring. It is found that the prenatal HS diet reduced the glucose tolerance and insulin sensitivity of the offspring. Additionally, there is down-regulation of peroxisome proliferator-activated receptor gamma coactivator 1 alpha (Ppargc1a/PPARGC1A) at the transcript and protein level, which leads to decreased mitochondrial biogenesis and oxidative respiration in skeletal muscle. Moreover, the down-regulation of Ppargc1a is accompanied by decreases in the expression of glucose transporter type 4 (Glut4). With endurance exercise training, these changes are mitigated, which ultimately resulted in improved insulin resistance.

CONCLUSION

These findings suggest that prenatal HS intake induces metabolic disorders via the decreased expression of Ppargc1a in the skeletal muscle of adult offspring, providing novel information concerning the mechanisms and early prevention of metabolic diseases of fetal origins.

Endurance exercise resistance to lipotoxic cardiomyopathy is associated with cardiac NAD+/dSIR2/PGC-1α pathway activation in old Drosophila

Lipotoxic cardiomyopathy is caused by excessive lipid accumulation in myocardial cells and it is a form of cardiac dysfunction. Cardiac PGC-1α overexpression prevents lipotoxic cardiomyopathy induced by a high-fat diet (HFD). The level of NAD⁺ and Sir2 expression upregulate the transcriptional activity of PGC-1α. Exercise improves cardiac NAD⁺ level and PGC-1α activity. However, the relationship between exercise, NAD⁺/dSIR2/PGC-1α pathway and lipotoxic cardiomyopathy remains unknown. In this study, flies were fed a HFD and exercised. The heart dSir2 gene was specifically expressed or knocked down by UAS/hand-Gal4 system. The results showed that either a HFD or dSir2 knockdown remarkably increased cardiac TG level and d FAS expression, reduced heart fractional shortening and diastolic diameter, increased arrhythmia index, and decreased heart NAD⁺ level, dSIR2 protein, dSir2 and PGC-1α expression levels. Contrarily, either exercise or dSir2 overexpression remarkably reduced heart TG level, dFAS expression and arrhythmia index, and notably increased heart fractional shortening, diastolic diameter, NAD⁺ level, dSIR2 level, and heart dSir2 and PGC-1α expression. Therefore, we declared that exercise training could improve lipotoxic cardiomyopathy induced by a HFD or cardiac dSir2 knockdown in old Drosophila The NAD⁺/dSIR2/PGC-1α pathway activation was an important molecular mechanism of exercise resistance against lipotoxic cardiomyopathy.

Spargel/dPGC-1 is essential for oogenesis and nutrient-mediated ovarian growth in Drosophila

Dietary proteins are crucial for oogenesis. The Target of Rapamycin (TOR) is a major nutrient sensor controlling organismal growth and fertility, but the downstream effectors of TOR signaling remain largely uncharacterized. We previously identified Drosophila Spargel/dPGC-1 as a terminal effector of the TOR-TSC pathway, and now report that Spargel connects nutrition to oogenesis. We found that Spargel is expressed predominantly in the ovaries of adult flies, and germline spargel knockdown inhibits cyst growth, ultimately leading to egg chamber degeneration and female sterility. In situ staining demonstrated nuclear localization of Spargel in the nurse cells and follicle cells of the ovariole. Furthermore, Spargel/dPGC-1 expression is influenced by dietary yeast concentration and TOR signaling, suggesting Spargel/dPGC-1 might transmit nutrient-mediated signals into ovarian growth. We propose that potentiating Spargel/dPGC-1 expression in the ovary is instrumental in nutrient-mediated regulation of oogenesis.

Metabolic regulation by the PGC-1α and PGC-1β coactivators in larval zebrafish (Danio rerio)

The peroxisome proliferator activated receptor γ coactivator-1 (PGC-1) family is composed of three coactivators whose role in regulating mammalian bioenergetics regulation is clear, but is much less certain in other vertebrates. Current evidence suggests that in fish, PGC-1α and PGC-1β may exhibit much less redundancy in the control of fatty acid oxidation and mitochondrial biogenesis compared to mammals. To assess these roles directly, we knocked down PGC-1α and PGC-1β expression with morpholinos in zebrafish embryos, and we investigated the resulting molecular and physiological phenotypes. First, we found no effects of either morpholinos on larval hatching, heart rates and oxygen consumption over the first few days of development. Next, at 3 days post fertilization (dpf), we confirmed by real time PCR a specific knock down of both coactivators, that resulted in a significant reduction in the transcript levels of citrate synthase (CS), 3-hydroxyacyl-CoA dehydrogenase (HOAD), and medium-chain acyl-coenzyme A dehydrogenase (MCAD) in both morphant groups. However, there was no effect on transcription factors' gene expression except for a marked reduction in estrogen related receptor α (ERRα) transcripts in PGC-1α morphants. Finally, we assessed whole embryonic enzyme activity for CS, cytochrome oxidase (COX), HOAD and carnitine palmitoyltransferase I (CPT-1) at 4 dpf. The only significant effect of the knockdown was a reduced CS activity in PGC-1α morphants and a counterintuitive increase of cytochrome oxidase activity in PGC-1β morphants. Overall, our results indicate that in larval zebrafish, PGC-1α and PGC-1β both play a role in regulating expression of important mitochondrial genes potentially through ERRα.

Minimal effects of spargel (PGC-1) overexpression in a Drosophila mitochondrial disease model

PGC-1α and its homologues have been proposed to act as master regulators of mitochondrial biogenesis in animals. Most relevant studies have been conducted in mammals, where interpretation is complicated by the fact that there are three partially redundant members of the gene family. In Drosophila, only a single PGC-1 homologue, spargel (srl), is present in the genome. Here, we analyzed the effects of srl overexpression on phenotype and on gene expression in tko^25t, a recessive bang-sensitive mutant with a global defect in oxidative phosphorylation, resulting from a deficiency of mitochondrial protein synthesis. In contrast to previous reports, we found that substantial overexpression of srl throughout development had only minimal effects on the tko^25t mutant phenotype. Copy number of mtDNA was unaltered and srl overexpression produced no systematic effects on a representative set of transcripts related to mitochondrial OXPHOS and other metabolic enzymes, although these were influenced by sex and genetic background. This study provides no support to the concept of Spargel as a global regulator of mitochondrial biogenesis, at least in the context of the tko^25t model.

Summary: Overexpression of spargel, the fly PGC-1 homologue proposed as a mitochondrial biogenesis regulator, has minimal effects on the phenotype of tko^25t, considered a fly model for mitochondrial disease.

As time flies by: Investigating cardiac aging in the short-lived Drosophila model

Aging is associated with a decline in heart function across the tissue, cellular, and molecular levels. The risk of cardiovascular disease grows significantly over time, and as developed countries continue to see an increase in lifespan, the cost of cardiovascular healthcare for the elderly will undoubtedly rise. The molecular basis for cardiac function deterioration with age is multifaceted and not entirely clear, and there is a limit to what investigations can be performed on human subjects or mammalian models. Drosophila melanogaster has emerged as a useful model organism for studying aging in a short timeframe, benefitting from a suite of molecular and genetic tools and displaying highly conserved traits of cardiac senescence. Here, we discuss recent advances in our understanding of cardiac aging and how the fruit fly has aided in these developments.

Mito-nuclear interactions modify Drosophila exercise performance

Endurance exercise has received increasing attention as a broadly preventative measure against age-related disease and dysfunction. Improvement of mitochondrial quality by enhancement of mitochondrial turnover is thought to be among the important molecular mechanisms underpinning the benefits of exercise. Interactions between the mitochondrial and nuclear genomes are important components of the genetic basis for variation in longevity, fitness and the incidence of disease. Here, we examine the effects of replacing the mitochondrial genome (mtDNA) of several Drosophila strains with mtDNA from other strains, or from closely related species, on exercise performance. We find that mitochondria from flies selected for longevity increase the performance of flies from a parental strain. We also find evidence that mitochondria from other strains or species alter exercise performance, with examples of both beneficial and deleterious effects. These findings suggest that both the mitochondrial and nuclear genomes, as well as interactions between the two, contribute significantly to exercise capacity.

Ceramide-Protein Interactions Modulate Ceramide-Associated Lipotoxic Cardiomyopathy

Lipotoxic cardiomyopathy (LCM) is characterized by abnormal myocardial accumulation of lipids, including ceramide; however, the contribution of ceramide to the etiology of LCM is unclear. Here, we investigated the association of ceramide metabolism and ceramide-interacting proteins (CIPs) in LCM in the Drosophila heart model. We find that ceramide feeding or ceramide-elevating genetic manipulations are strongly associated with cardiac dilation and defects in contractility. High ceramide-associated LCM is prevented by inhibiting ceramide synthesis, establishing a robust model of direct ceramide-associated LCM, corroborating previous indirect evidence in mammals. We identified several CIPs from mouse heart and Drosophila extracts, including caspase activator Annexin-X, myosin chaperone Unc-45, and lipogenic enzyme FASN1, and remarkably, their cardiac-specific manipulation can prevent LCM. Collectively, these data suggest that high ceramide-associated lipotoxicity is mediated, in part, through altering caspase activation, sarcomeric maintenance, and lipogenesis, thus providing evidence for conserved mechanisms in LCM pathogenesis in mammals.

Dynamic mitochondrial responses to a high-fat diet in Drosophila melanogaster

Mitochondria can utilize different fuels according to physiological and nutritional conditions to promote cellular homeostasis. However, during nutrient overload metabolic inflexibility can occur, resulting in mitochondrial dysfunctions. High-fat diets (HFDs) are usually used to mimic this metabolic inflexibility in different animal models. However, how mitochondria respond to the duration of a HFD exposure is still under debate. In this study, we investigated the dynamic of the mitochondrial and physiological functions in Drosophila melanogaster at several time points following an exposure to a HFD. Our results showed that after two days on the HFD, mitochondrial respiration as well as ATP content of thorax muscles are increased, likely due to the utilization of carbohydrates. However, after four days on the HFD, impairment of mitochondrial respiration at the level of complex I, as well as decreased ATP content were observed. This was associated with an increased contribution of complex II and, most notably of the mitochondrial glycerol-3-phosphate dehydrogenase (mG3PDH) to mitochondrial respiration. We suggest that this increased mG3PDH capacity reflects the occurrence of metabolic inflexibility, leading to a loss of homeostasis and alteration of the cellular redox status, which results in senescence characterized by decreased climbing ability and premature death.

Intergenerational inheritance of high fat diet-induced cardiac lipotoxicity in Drosophila

Obesity is strongly correlated with lipotoxic cardiomyopathy, heart failure and thus mortality. The incidence of obesity has reached alarming proportions worldwide, and increasing evidence suggests that the parents' nutritional status may predispose their offspring to lipotoxic cardiomyopathy. However, to date, mechanisms underlying intergenerational heart disease risks have yet to be elucidated. Here we report that cardiac dysfunction induced by high-fat-diet (HFD) persists for two subsequent generations in Drosophila and is associated with reduced expression of two key metabolic regulators, adipose triglyceride lipase (ATGL/bmm) and transcriptional cofactor PGC-1. We provide evidence that targeted expression of ATGL/bmm in the offspring of HFD-fed parents protects them, and the subsequent generation, from cardio-lipotoxicity. Furthermore, we find that intergenerational inheritance of lipotoxic cardiomyopathy correlates with elevated systemic H3K27 trimethylation. Lowering H3K27 trimethylation genetically or pharmacologically in the offspring of HFD-fed parents prevents cardiac pathology. This suggests that metabolic homeostasis is epigenetically regulated across generations.

Fat Body Biology in the Last Decade

The insect fat body is analogous to vertebrate adipose tissue and liver. In this review, the new and exciting advancements made in fat body biology in the last decade are summarized. Controlled by hormonal and nutritional signals, insect fat body cells undergo mitosis during embryogenesis, endoreplication during the larval stages, and remodeling during metamorphosis and regulate reproduction in adults. Fat body tissues are major sites for nutrient storage, energy metabolism, innate immunity, and detoxification. Recent studies have revealed that the fat body plays a central role in the integration of hormonal and nutritional signals to regulate larval growth, body size, circadian clock, pupal diapause, longevity, feeding behavior, and courtship behavior, partially by releasing fat body signals to remotely control the brain. In addition, the fat body has emerged as a fascinating model for studying metabolic disorders and immune diseases. Potential future directions for fat body biology are also proposed herein.

RNF34 modulates the mitochondrial biogenesis and exercise capacity in muscle and lipid metabolism through ubiquitination of PGC-1 in Drosophila

The transcriptional co-activator PGC-1α is a key regulator of mitochondrial function and muscle fiber specification in the skeletal muscle. The E3 ubiquitin ligase RNF34 ubiquitinates PGC-1α and negatively regulates mammalian brown fat cell metabolism. However, the functional importance of RNF34 in the skeletal muscle and its impact on energy metabolism remain unknown. The Drosophila PGC-1 homolog dPGC-1 and its mammalian counterparts have conserved functions in mitochondria and insulin signaling. Here, we showed that the Drosophila RNF34 (dRNF34) ubiquitinates the Drosophila PGC-1α (dPGC-1) and promotes its degradation in HEK293T cells by immunoprecipitation and western blot analysis. This allows us to use Drosophila as a powerful model system to study the physiological role of RNF34 in mitochondrial function and metabolism. In the in vivo studies, by separately expressing two independent UAS-dRNF34 RNAi transgenes driven by the muscle-specific 24B-Gal4 driver, we found that knockdown of dRNF34 specifically in muscle promotes mitochondrial biogenesis, improves negative geotaxis, extends climbing time to exhaustion in moderate aged flies and counteracts high-fat-diet-induced high triglyceride content. Furthermore, we showed that knockdown of dPGC-1 reversed the effects of the dRNF34 knockdown phenotypes described above. Our results reveal that dRNF34 plays an important role in regulating mitochondrial biogenesis in muscle and lipid metabolism through dPGC-1. Thus, inhibition of RNF34 activity provides a potential novel therapeutic strategy for the treatment of age-related muscle dysfunction.

Reduced expression of Twist 1 is protective against insulin resistance of adipocytes and involves mitochondrial dysfunction

Insulin resistance (IR) has become a global epidemic that represents a serious hazard to public health. However, the precise mechanisms modulating IR have not been fully elucidated. The present study aimed to investigate the role of transcriptional factor Twist 1 in adipocyte IR and to further explore the molecular mechanism. An in vitro IR model based on cultured 3T3-L1 adipocytes was established under high glucose/insulin stimulation and an in vivo IR model in C57/BL6J mice induced by a high fat diet (HFD) was also developed. Lentivirus targeting Twist 1 silencing was introduced. The relationships between Twist 1 expression and IR state, mitochondrial dysfunction and the downstream insulin signaling pathway were assayed. Our results firstly showed the elevation of Twist 1 in IR adipocytes, and Twist 1 silencing attenuated IR. Then mitochondrial ultra-structural damage, elevated ROS, decreased MMP and ATP, and changes in mitochondrial biosynthesis-related genes in IR group indicated mitochondrial dysfunction. Further, the downstream IRS/PI3K/AKT/GluT4 pathway was showed involved in Twist 1-mediated IR. In total, we provide evidence of a protective role of Twist 1 silencing in relieving the IR state of adipocytes. Mitochondrial dysfunction and the downstream IRS/PI3K/AKT/GluT4 pathway were involved in this Twist 1-mediated IR.

IL-6: A Potential Role in Cardiac Metabolic Homeostasis

Interleukin-6 (IL-6) is implicated in multiple biological functions including immunity, neural development, and haematopoiesis. Recently, mounting evidence indicates that IL-6 plays a key role in metabolism, especially lipid metabolic homeostasis. A working heart requires a high and constant energy input which is largely generated by fatty acid (FA) β-oxidation. Under pathological conditions, the precise balance between cardiac FA uptake and metabolism is perturbed so that excessive FA is accumulated, thereby predisposing to myocardial dysfunction (cardiac lipotoxicity). In this review, we summarize the current evidence that suggests the involvement of IL-6 in lipid metabolism. Cardiac metabolic features and consequences of myocardial lipotoxicity are also briefly analyzed. Finally, the roles of IL-6 in cardiac FA uptake (i.e., serum lipid profile and myocardial FA transporters) and FA metabolism (namely, β-oxidation, mitochondrial function, biogenesis, and FA de novo synthesis) are discussed. Overall, understanding how IL-6 transmits signals to affect lipid metabolism in the heart might allow for development of better clinical therapies for obesity-associated cardiac lipotoxicity.

Obesity and Aging in the Drosophila Model

Being overweight increases the risk of many metabolic disorders, but how it affects lifespan is not completely clear. Not all obese people become ill, and the exact mechanism that turns excessive fat storage into a health-threatening state remains unknown. Drosophila melanogaster has served as an excellent model for many diseases, including obesity, diabetes, and hyperglycemia-associated disorders, such as cardiomyopathy or nephropathy. Here, we review the connections between fat storage and aging in different types of fly obesity. Whereas obesity induced by high-fat or high-sugar diet is associated with hyperglycemia, cardiomyopathy, and in some cases, shortening of lifespan, there are also examples in which obesity correlates with longevity. Transgenic lines with downregulations of the insulin/insulin-like growth factor (IIS) and target of rapamycin (TOR) signaling pathways, flies reared under dietary restriction, and even certain longevity selection lines are obese, yet long-lived. The mechanisms that underlie the differential lifespans in distinct types of obesity remain to be elucidated, but fat turnover, inflammatory pathways, and dysregulations of glucose metabolism may play key roles. Altogether, Drosophila is an excellent model to study the physiology of adiposity in both health and disease.

High fat diet alters Drosophila melanogaster sexual behavior and traits: decreased attractiveness and changes in pheromone profiles

Sexual traits convey information about individual quality to potential mates. Environmental and genetic factors affect sexual trait expression and perception via effects on animal condition and health. High fat diet (HFD) is one environmental factor that adversely affects Drosophila melanogaster health, and its effects on animal health are mediated through conserved metabolic signaling pathways. HFD decreases female attractiveness, resulting in reduced male mating behaviors toward HFD females. HFD also affects the ability of males to judge mate attractiveness and likely alters fly condition and sexual traits to impact mating behavior. Here we show that HFD affects both visual (body size) and non-visual (pheromone profiles) sexual traits, which likely contribute to decreased fly attractiveness. We also demonstrate that adult-specific HFD effects on male mate preference can be rescued by changing metabolic signaling. These results demonstrate that HFD alters Drosophila sexual cues to reflect concurrent effects on condition and that less severe behavioral defects can be reversed by genetic manipulations that rescue fly health. This work expands on current knowledge of the role that metabolic signaling pathways play in linking animal health, sexual traits, and mating behavior, and provides a robust assay in a genetically tractable system to continue examining these processes.