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.

Obesity and nutrient sensing TOR pathway in flies and vertebrates: Functional conservation of genetic mechanisms.

The global prevalence of obesity has grown to epidemic proportions, and 400 million people are now considered to be obese. Excessive accumulation of dietary lipids (obesity) is a known risk factor for the development of deleterious metabolic conditions and has been strongly linked to the progression of heart disease and type 2 diabetes. Investigating the origin and effects of high-fat diet (HFD)-induced obesity and its genetic mediators is an important step in understanding the mechanisms that contribute to obesity. However, the mechanisms that underlie HFD pathophysiology have yet to be elucidated fully. Here we describe recent work in a Drosophila model to investigate the origin and genetic mechanisms that could underlie HFD-induced obesity, type 2 diabetes and cardiac dysfunction.

High-fat-diet-induced obesity and heart dysfunction are regulated by the TOR pathway in Drosophila.

High-fat-diet (HFD)-induced obesity is a major contributor to diabetes and cardiovascular disease, but the underlying genetic mechanisms are poorly understood. Here, we use Drosophila to test the hypothesis that HFD-induced obesity and associated cardiac complications have early evolutionary origins involving nutrient-sensing signal transduction pathways. We find that HFD-fed flies exhibit increased triglyceride (TG) fat and alterations in insulin/glucose homeostasis, similar to mammalian responses. A HFD also causes cardiac lipid accumulation, reduced cardiac contractility, conduction blocks, and severe structural pathologies, reminiscent of diabetic cardiomyopathies. Remarkably, these metabolic and cardiotoxic phenotypes elicited by HFD are blocked by inhibiting insulin-TOR signaling. Moreover, reducing insulin-TOR activity (by expressing TSC1-2, 4EBP or FOXO), or increasing lipase expression-only within the myocardium-suffices to efficiently alleviate cardiac fat accumulation and dysfunction induced by HFD. We conclude that deregulation of insulin-TOR signaling due to a HFD is responsible for mediating the detrimental effects on metabolism and heart function.

d4eBP acts downstream of both dTOR and dFoxo to modulate cardiac functional aging in Drosophila.

dTOR (target of rapamycin) and dFoxo respond to changes in the nutritional environment to induce a broad range of responses in multiple tissue types. Both dTOR and dFoxo have been demonstrated to control the rate of age-related decline in cardiac function. Here, we show that the Eif4e-binding protein (d4eBP) is sufficient to protect long-term cardiac function against age-related decline and that up-regulation of dEif4e is sufficient to recapitulate the effects of high dTOR or insulin signaling. We also provide evidence that d4eBP acts tissue-autonomously and downstream of dTOR and dFoxo in the myocardium, where it enhances cardiac stress resistance and maintains normal heart rate and myogenic rhythm. Another effector of dTOR and insulin signaling, dS6K, may influence cardiac aging nonautonomously through its activity in the insulin-producing cells, possibly by regulating dilp2 expression. Thus, elevating d4eBP activity in cardiac tissue represents an effective organ-specific means for slowing or reversing cardiac functional changes brought about by normal aging.

Bunched, the Drosophila homolog of the mammalian tumor suppressor TSC-22, promotes cellular growth.

BACKGROUND: Transforming Growth Factor-beta1 stimulated clone-22 (TSC-22) is assumed to act as a negative growth regulator and tumor suppressor. TSC-22 belongs to a family of putative transcription factors encoded by four distinct loci in mammals. Possible redundancy among the members of the TSC-22/Dip/Bun protein family complicates a genetic analysis. In Drosophila, all proteins homologous to the TSC-22/Dip/Bun family members are derived from a single locus called bunched (bun). RESULTS: We have identified bun in an unbiased genetic screen for growth regulators in Drosophila. Rather unexpectedly, bun mutations result in a growth deficit. Under standard conditions, only the long protein isoform BunA - but not the short isoforms BunB and BunC - is essential and affects growth. Whereas reducing bunA function diminishes cell number and cell size, overexpression of the short isoforms BunB and BunC antagonizes bunA function. CONCLUSION: Our findings establish a growth-promoting function of Drosophila BunA. Since the published studies on mammalian systems have largely neglected the long TSC-22 protein version, we hypothesize that the long TSC-22 protein is a functional homolog of BunA in growth regulation, and that it is antagonized by the short TSC-22 protein.

Drosophila aging 2006/2007.

Research on aging in Drosophila continues to provide new insights into this complex process. Drosophila is highly amenable to study aging because of its short generation time, comprehensive resources for genetic manipulation, and functionally conserved physiology. Importantly, many of these physiological processes such as heart function, sleep, and metabolism functionally senescence in older flies. As the evolutionarily conserved insulin and TOR pathways are critical regulators of aging, the influence of insulin and TOR signaling on these processes is an important area for future research. An important emerging theme is determining the age-dependent alterations that occur at the organ level and how this functional senescence is regulated by different tissues.

Activated FOXO-mediated insulin resistance is blocked by reduction of TOR activity.

Reducing insulin/IGF signaling allows for organismal survival during periods of inhospitable conditions by regulating the diapause state, whereby the organism stockpiles lipids, reduces fertility, increases stress resistance, and has an increased lifespan. The Target of Rapamycin (TOR) responds to changes in growth factors, amino acids, oxygen tension, and energy status; however, it is unclear how TOR contributes to physiological homeostasis and disease conditions. Here, we show that reducing the function of Drosophila TOR results in decreased lipid stores and glucose levels. Importantly, this reduction of dTOR activity blocks the insulin resistance and metabolic syndrome phenotypes associated with increased activity of the insulin responsive transcription factor, dFOXO. Reduction in dTOR function also protects against age-dependent decline in heart function and increases longevity. Thus, the regulation of dTOR activity may be an ancient "systems biological" means of regulating metabolism and senescence, that has important evolutionary, physiological, and clinical implications.

Antioxidants protect PINK1-dependent dopaminergic neurons in Drosophila.

Parkinson's disease (PD) is the most frequent neurodegenerative movement disorder. Mutations in the PINK1 gene are linked to the autosomal recessive early onset familial form of PD. The physiological function of PINK1 and pathological abnormality of PD-associated PINK1 mutants are largely unknown. We here show that inactivation of Drosophila PINK1 (dPINK1) using RNAi results in progressive loss of dopaminergic neurons and in ommatidial degeneration of the compound eye, which is rescued by expression of human PINK1 (hPINK1). Expression of human SOD1 suppresses neurodegeneration induced by dPINK1 inactivation. Moreover, treatment of dPINK1 RNAi flies with the antioxidants SOD and vitamin E significantly inhibits ommatidial degeneration. Thus, dPINK1 plays an essential role in maintaining neuronal survival by preventing neurons from undergoing oxidative stress, thereby suggesting a potential mechanism by which a reduction in PINK1 function leads to PD-associated neurodegeneration.

TOR-mediated cell-cycle activation causes neurodegeneration in a Drosophila tauopathy model.

BACKGROUND: Previous studies have demonstrated reexpression of cell-cycle markers within postmitotic neurons in neurodegenerative tauopathies, including Alzheimer's disease (AD). However, the critical questions of whether cell-cycle activation is causal or epiphenomenal to tau-induced neurodegeneration and which signaling pathways mediate cell-cycle activation in tauopathy remain unresolved. RESULTS: Cell-cycle activation accompanies wild-type and mutant tau-induced neurodegeneration in Drosophila, and genetically interfering with cell-cycle progression substantially reduces neurodegeneration. Our data support a role for cell-cycle activation downstream of tau phosphorylation, directly preceding apoptosis. We accordingly show that ectopic cell-cycle activation leads to apoptosis of postmitotic neurons in vivo. As in AD, TOR (target of rapamycin kinase) activity is increased in our model and is required for neurodegeneration. TOR activation enhances tau-induced neurodegeneration in a cell cycle-dependent manner and, when ectopically activated, drives cell-cycle activation and apoptosis in postmitotic neurons. CONCLUSIONS: TOR-mediated cell-cycle activation causes neurodegeneration in a Drosophila tauopathy model, identifying TOR and the cell cycle as potential therapeutic targets in tauopathies and AD.

The insulin-PI3K/TOR pathway induces a HIF-dependent transcriptional response in Drosophila by promoting nuclear localization of HIF-alpha/Sima.

The hypoxia-inducible factor (HIF) is a heterodimeric transcription factor composed of a constitutively expressed HIF-beta subunit and an oxygen-regulated HIF-alpha subunit. We have previously defined a hypoxia-inducible transcriptional response in Drosophila melanogaster that is homologous to the mammalian HIF-dependent response. In Drosophila, the bHLH-PAS proteins Similar (Sima) and Tango (Tgo) are the functional homologues of the mammalian HIF-alpha and HIF-beta subunits, respectively. HIF-alpha/Sima is regulated by oxygen at several different levels that include protein stability and subcellular localization. We show here for the first time that insulin can activate HIF-dependent transcription, both in Drosophila S2 cells and in living Drosophila embryos. Using a pharmacological approach as well as RNA interference, we determined that the effect of insulin on HIF-dependent transcriptional induction is mediated by PI3K-AKT and TOR pathways. We demonstrate that stimulation of the transcriptional response involves upregulation of Sima protein but not sima mRNA. Finally, we have analyzed in vivo the effect of the activation of the PI3K-AKT pathway on the subcellular localization of Sima protein. Overexpression of dAKT and dPDK1 in normoxic embryos provoked a major increase in Sima nuclear localization, mimicking the effect of a hypoxic treatment. A similar increase in Sima nuclear localization was observed in dPTEN homozygous mutant embryos, confirming that activation of the PI3K-AKT pathway promotes nuclear accumulation of Sima protein. We conclude that regulation of HIF-alpha/Sima by the PI3K-AKT-TOR pathway is a major conserved mode of regulation of the HIF-dependent transcriptional response in Drosophila.

Insulin/IGF and target of rapamycin signaling: a TOR de force in growth control.

'They come in all sizes.' Apart from its origin and use in the clothing industry, this saying reflects the fact that the size of organisms spans an enormous range. Whether destined to be large or small, species grow in an organized fashion to reach their final specified size. For growth to proceed, food must be metabolized to liberate energy in the form of adenosine triphosphate (ATP) and protein building blocks in the form of amino acids. One major orchestrator of this complex growth process in diverse metazoan species is the insulin/insulin-like growth factor (IGF) system. This review summarizes current studies primarily from Drosophila regarding the function of the insulin/IGF system in the control of growth.

The Drosophila insulin/IGF receptor controls growth and size by modulating PtdInsP(3) levels.

Understanding the control of size is of fundamental biological and clinical importance. Insulin/IGF signaling during development controls growth and size, possibly by coordinating the activities of the Ras and PI 3-kinase signaling pathways. We show that in Drosophila mutating the consensus binding site for the Ras pathway adaptor Drk/Grb2 in Chico/IRS does not interfere with growth whereas mutating the binding sites of the PI 3-kinase adaptor p60 completely abrogates Chico function. Furthermore, we present biochemical and genetic evidence that loss of the homolog of the tumor suppressor gene, Pten, results in increased PtdInsP(3) levels and that these increased levels are sufficient to compensate for the complete loss of the Insulin/insulin-like growth factor receptor function. This reduction of Pten activity is also sufficient to vastly increase organism size. These results suggest that PtdInsP(3) is a second messenger for growth and that levels of PtdInsP(3) during development regulate organismal size.

Living with lethal PIP3 levels: viability of flies lacking PTEN restored by a PH domain mutation in Akt/PKB.

The phosphoinositide phosphatase PTEN is mutated in many human cancers. Although the role of PTEN has been studied extensively, the relative contributions of its numerous potential downstream effectors to deregulated growth and tumorigenesis remain uncertain. We provide genetic evidence in Drosophila melanogaster for the paramount importance of the protein kinase Akt [also called protein kinase B (PKB)] in mediating the effects of increased phosphatidylinositol 3,4,5-trisphosphate (PIP3) concentrations that are caused by the loss of PTEN function. A mutation in the pleckstrin homology (PH) domain of Akt that reduces its affinity for PIP3 sufficed to rescue the lethality of flies devoid of PTEN activity. Thus, Akt appears to be the only critical target activated by increased PIP3 concentrations in Drosophila.