Normal thyroid thermogenesis but reduced viability and adiposity in mice lacking the mitochondrial glycerol phosphate dehydrogenase

MitoMAMMAL: a genome scale model of mammalian mitochondria predicts cardiac and BAT metabolism

Motivation

Mitochondria are essential for cellular metabolism and are inherently flexible to allow correct function in a wide range of tissues. Consequently, dysregulated mitochondrial metabolism affects different tissues in different ways leading to challenges in understanding the pathology of mitochondrial diseases. System-level metabolic modelling is useful in studying tissue-specific mitochondrial metabolism, yet despite the mouse being a common model organism in research, no mouse specific mitochondrial metabolic model is currently available.

Results

Building upon the similarity between human and mouse mitochondrial metabolism, we present mitoMammal, a genome-scale metabolic model that contains human and mouse specific gene-product reaction rules. MitoMammal is able to model mouse and human mitochondrial metabolism. To demonstrate this, using an adapted E-Flux algorithm, we integrated proteomic data from mitochondria of isolated mouse cardiomyocytes and mouse brown adipocyte tissue, as well as transcriptomic data from in vitro differentiated human brown adipocytes and modelled the context specific metabolism using flux balance analysis. In all three simulations, mitoMammal made mostly accurate, and some novel predictions relating to energy metabolism in the context of cardiomyocytes and brown adipocytes. This demonstrates its usefulness in research in cardiac disease and diabetes in both mouse and human contexts.

Availability and implementation

The MitoMammal Jupyter Notebook is available at: https://gitlab.com/habermann_lab/mitomammal.

Glycerol 3-phosphate dehydrogenases (1 and 2) in cancer and other diseases

The glycerol 3-phosphate shuttle (GPS) is composed of two different enzymes: cytosolic NAD+-linked glycerol 3-phosphate dehydrogenase 1 (GPD1) and mitochondrial FAD-linked glycerol 3-phosphate dehydrogenase 2 (GPD2). These two enzymes work together to act as an NADH shuttle for mitochondrial bioenergetics and function as an important bridge between glucose and lipid metabolism. Since these genes were discovered in the 1960s, their abnormal expression has been described in various metabolic diseases and tumors. Nevertheless, it took a long time until scientists could investigate the causal relationship of these enzymes in those pathophysiological conditions. To date, numerous studies have explored the involvement and mechanisms of GPD1 and GPD2 in cancer and other diseases, encompassing reports of controversial and non-conventional mechanisms. In this review, we summarize and update current knowledge regarding the functions and effects of GPS to provide an overview of how the enzymes influence disease conditions. The potential and challenges of developing therapeutic strategies targeting these enzymes are also discussed.

Discovery of a second citric acid cycle complex

Together, Nobel Prize honoured work, mathematics, physics and the laws of nature have drawn a concept of clockwise cycling carboxylic acids in Krebs' Citric Acid Cycle. A Citric Acid Cycle complex is defined by specific substrate, product and regulation. Recently, the Citric Acid Cycle 1.1 complex was introduced as an NAD⁺-regulated cycle with the substrate, lactic acid and the product, malic acid. Here, we introduce the concept of the Citric Acid Cycle 2.1 complex as an FAD-regulated cycle with the substrate, malic acid and the products, succinic acid or citric acid. The function of the Citric Acid Cycle 2.1 complex is to balance stress situations within the cell. We propose that the biological function of Citric Acid Cycle 2.1 in muscles is to accelerate recovery of ATP; whereas in white tissue adipocytes our testing of the theoretical concept led to the storage of energy as lipids.

The mitochondrial coenzyme Q junction and complex III: biochemistry and pathophysiology

Coenzyme Q (CoQ, ubiquinone) is the electron-carrying lipid in the mitochondrial electron transport system (ETS). In mammals, it serves as the electron acceptor for nine mitochondrial inner membrane dehydrogenases. These include the NADH dehydrogenase (complex I, CI) and succinate dehydrogenase (complex II, CII) but also several others that are often omitted in the context of respiratory enzymes: dihydroorotate dehydrogenase, choline dehydrogenase, electron-transferring flavoprotein dehydrogenase, mitochondrial glycerol-3-phosphate dehydrogenase, proline dehydrogenases 1 and 2, and sulfide:quinone oxidoreductase. The metabolic pathways these enzymes are involved in range from amino acid and fatty acid oxidation to nucleotide biosynthesis, methylation, and hydrogen sulfide detoxification, among many others. The CoQ-linked metabolism depends on CoQ reoxidation by the mitochondrial complex III (cytochrome bc₁ complex, CIII). However, the literature is surprisingly limited as for the role of the CoQ-linked metabolism in the pathogenesis of human diseases of oxidative phosphorylation (OXPHOS), in which the CoQ homeostasis is directly or indirectly affected. In this review, we give an introduction to CIII function, and an overview of the pathological consequences of CIII dysfunction in humans and mice and of the CoQ-dependent metabolic processes potentially affected in these pathological states. Finally, we discuss some experimental tools to dissect the various aspects of compromised CoQ oxidation.

Metformin's Therapeutic Efficacy in the Treatment of Diabetes Does Not Involve Inhibition of Mitochondrial Glycerol Phosphate Dehydrogenase

Mitochondrial glycerol phosphate dehydrogenase (mGPD) is the rate-limiting enzyme of the glycerol phosphate redox shuttle. It was recently claimed that metformin, a first-line drug used for the treatment of type 2 diabetes, inhibits liver mGPD 30-50%, suppressing gluconeogenesis through a redox mechanism. Various factors cast doubt on this idea. Total-body knockout of mGPD in mice has adverse effects in several tissues where the mGPD level is high but has little or no effect in liver, where the mGPD level is the lowest of 10 tissues. Metformin has beneficial effects in humans in tissues with high levels of mGPD, such as pancreatic β-cells, where the mGPD level is much higher than that in liver. Insulin secretion in mGPD knockout mouse β-cells is normal because, like liver, β-cells possess the malate aspartate redox shuttle whose redox action is redundant to the glycerol phosphate shuttle. For these and other reasons, we used four different enzyme assays to reassess whether metformin inhibited mGPD. Metformin did not inhibit mGPD in homogenates or mitochondria from insulin cells or liver cells. If metformin actually inhibited mGPD, adverse effects in tissues where the level of mGPD is much higher than that in the liver could prevent the use of metformin as a diabetes medicine.

The multiple roles of coenzyme Q in cellular homeostasis and their relevance for the pathogenesis of coenzyme Q deficiency

Coenzyme Q (CoQ) is a redox active lipid that plays a central role in cellular homeostasis. It was discovered more than 60 years ago because of its role as electron transporter in the mitochondrial respiratory chain. Since then it has become evident that CoQ has many other functions, not directly related to bioenergetics. It is a cofactor of several mitochondrial dehydrogenases involved in the metabolism of lipids, amino acids, and nucleotides, and in sulfide detoxification. It is a powerful antioxidant and it is involved in the control of programmed cell death by modulating both apoptosis and ferroptosis. CoQ deficiency is a clinically and genetically heterogeneous group of disorders characterized by the impairment of CoQ biosynthesis. CoQ deficient patients display defects in cellular bioenergetics, but also in the other pathways in which CoQ is involved. In this review we will focus on the functions of CoQ not directly related to the respiratory chain, and on how their impairment is relevant for the pathophysiology of CoQ deficiency. A better understanding of the complex set of events triggered by CoQ deficiency will allow to design novel approaches for the treatment of this condition.

Manipulation of Dietary Amino Acids Prevents and Reverses Obesity in Mice Through Multiple Mechanisms That Modulate Energy Homeostasis

Reduced activation of energy metabolism increases adiposity in humans and other mammals. Thus, exploring dietary and molecular mechanisms able to improve energy metabolism is of paramount medical importance because such mechanisms can be leveraged as a therapy for obesity and related disorders. Here, we show that a designer protein-deprived diet enriched in free essential amino acids can 1) promote the brown fat thermogenic program and fatty acid oxidation, 2) stimulate uncoupling protein 1 (UCP1)-independent respiration in subcutaneous white fat, 3) change the gut microbiota composition, and 4) prevent and reverse obesity and dysregulated glucose homeostasis in multiple mouse models, prolonging the healthy life span. These effects are independent of unbalanced amino acid ratio, energy consumption, and intestinal calorie absorption. A brown fat-specific activation of the mechanistic target of rapamycin complex 1 seems involved in the diet-induced beneficial effects, as also strengthened by in vitro experiments. Hence, our results suggest that brown and white fat may be targets of specific amino acids to control UCP1-dependent and -independent thermogenesis, thereby contributing to the improvement of metabolic health.

Mechanisms of action of metformin in type 2 diabetes: Effects on mitochondria and leukocyte-endothelium interactions

Type 2 diabetes (T2D) is a very prevalent, multisystemic, chronic metabolic disorder closely related to atherosclerosis and cardiovascular diseases. It is characterised by mitochondrial dysfunction and the presence of oxidative stress. Metformin is one of the safest and most effective anti-hyperglycaemic agents currently employed as first-line oral therapy for T2D. It has demonstrated additional beneficial effects, unrelated to its hypoglycaemic action, on weight loss and several diseases, such as cancer, cardiovascular disorders and metabolic diseases, including thyroid diseases. Despite the vast clinical experience gained over several decades of use, the mechanism of action of metformin is still not fully understood. This review provides an overview of the existing literature concerning the beneficial mitochondrial and vascular effects of metformin, which it exerts by diminishing oxidative stress and reducing leukocyte-endothelium interactions. Specifically, we describe the molecular mechanisms involved in metformin's effect on gluconeogenesis, its capacity to interfere with major metabolic pathways (AMPK and mTORC1), its action on mitochondria and its antioxidant effects. We also discuss potential targets for therapeutic intervention based on these molecular actions.

Low metformin causes a more oxidized mitochondrial NADH/NAD redox state in hepatocytes and inhibits gluconeogenesis by a redox-independent mechanism

The mechanisms by which metformin (dimethylbiguanide) inhibits hepatic gluconeogenesis at concentrations relevant for type 2 diabetes therapy remain debated. Two proposed mechanisms are 1) inhibition of mitochondrial Complex 1 with consequent compromised ATP and AMP homeostasis or 2) inhibition of mitochondrial glycerophosphate dehydrogenase (mGPDH) and thereby attenuated transfer of reducing equivalents from the cytoplasm to mitochondria, resulting in a raised lactate/pyruvate ratio and redox-dependent inhibition of gluconeogenesis from reduced but not oxidized substrates. Here, we show that metformin has a biphasic effect on the mitochondrial NADH/NAD redox state in mouse hepatocytes. A low cell dose of metformin (therapeutic equivalent: <2 nmol/mg) caused a more oxidized mitochondrial NADH/NAD state and an increase in lactate/pyruvate ratio, whereas a higher metformin dose (≥5 nmol/mg) caused a more reduced mitochondrial NADH/NAD state similar to Complex 1 inhibition by rotenone. The low metformin dose inhibited gluconeogenesis from both oxidized (dihydroxyacetone) and reduced (xylitol) substrates by preferential partitioning of substrate toward glycolysis by a redox-independent mechanism that is best explained by allosteric regulation at phosphofructokinase-1 (PFK1) and/or fructose 1,6-bisphosphatase (FBP1) in association with a decrease in cell glycerol 3-phosphate, an inhibitor of PFK1, rather than by inhibition of transfer of reducing equivalents. We conclude that at a low pharmacological load, the metformin effects on the lactate/pyruvate ratio and glucose production are explained by attenuation of transmitochondrial electrogenic transport mechanisms with consequent compromised malate-aspartate shuttle and changes in allosteric effectors of PFK1 and FBP1.

The Importance of Calcium Ions for Determining Mitochondrial Glycerol-3-Phosphate Dehydrogenase Activity When Measuring Uncoupling Protein 1 (UCP1) Function in Mitochondria Isolated from Brown Adipose Tissue

Glycerol-3-phosphate is an excellent substrate for FAD-linked mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) in brown adipose tissue mitochondria and is regularly used as the primary substrate to measure oxygen consumption and reactive oxygen consumption by these mitochondria. mGPDH converts cytosolic glycerol-3-phosphate to dihydroxyacetone phosphate, feeding electrons directly from the cytosolic side of the mitochondrial inner membrane to the CoQ-pool within the inner membrane. mGPDH activity is allosterically activated by calcium, and when calcium chelators are present in the mitochondrial preparation medium and/or experimental incubation medium, calcium must be added to insure maximal mGPDH activity. It was demonstrated that in isolated brown adipose tissue mitochondria (1) mGPDH enzyme activity is maximal at free calcium ion concentrations in the 350 nM-1 μM range, (2) that ROS production also peaks in the 10-100 nM range in the presence of a UCP1 inhibitory ligand (GDP) but wanes with further increasing calcium concentration, and (3) that oxygen consumption rates peak in the 10-100 nM range with rates being maintained at higher calcium concentrations. This article provides easy-to-follow protocols to facilitate the measurement of mGPDH-dependent UCP1 activity in the presence of calcium for isolated brown adipose tissue mitochondria.

Integration of genome wide association studies and whole genome sequencing provides novel insights into fat deposition in chicken

Excessive fat deposition is a negative factor for poultry production because it reduces feed efficiency, increases the cost of meat production and is a health concern for consumers. We genotyped 497 birds from a Brazilian F₂ Chicken Resource Population, using a high-density SNP array (600 K), to estimate the genomic heritability of fat deposition related traits and to identify genomic regions and positional candidate genes (PCGs) associated with these traits. Selection signature regions, haplotype blocks and SNP data from a previous whole genome sequencing study in the founders of this chicken F2 population were used to refine the list of PCGs and to identify potential causative SNPs. We obtained high genomic heritabilities (0.43-0.56) and identified 22 unique QTLs for abdominal fat and carcass fat content traits. These QTLs harbored 26 PCGs involved in biological processes such as fat cell differentiation, insulin and triglyceride levels, and lipid biosynthetic process. Three of these 26 PCGs were located within haplotype blocks there were associated with fat traits, five overlapped with selection signature regions, and 12 contained predicted deleterious variants. The identified QTLs, PCGs and potentially causative SNPs provide new insights into the genetic control of fat deposition and can lead to improved accuracy of selection to reduce excessive fat deposition in chickens.

Targeting thermogenesis in brown fat and muscle to treat obesity and metabolic disease

Brown fat is emerging as an interesting and promising target for therapeutic intervention in obesity and metabolic disease. Activation of brown fat in humans is associated with marked improvement in metabolic parameters such as levels of free fatty acids and insulin sensitivity. Skeletal muscle is another important organ for thermogenesis, with the capacity to induce energy-consuming futile cycles. In this Review, we focus on how these two major thermogenic organs - brown fat and muscle - act and cooperate to maintain normal body temperature. Moreover, in the light of disease-relevant mechanisms, we explore the molecular pathways that regulate thermogenesis in brown fat and muscle. Brown adipocytes possess a unique cellular mechanism to convert chemical energy into heat: uncoupling protein 1 (UCP1), which can short-circuit the mitochondrial proton gradient. However, recent research demonstrates the existence of several other energy-expending 'futile' cycles in both adipocytes and muscle, such as creatine and calcium cycling. These mechanisms can complement or even substitute for UCP1-mediated thermogenesis. Moreover, they expand our view of cold-induced thermogenesis from a special feature of brown adipocytes to a more general physiological principle. Finally, we discuss how thermogenic mechanisms can be exploited to expend energy and hence offer new therapeutic opportunities.

Regulation of hepatic glucose metabolism in health and disease

The liver is crucial for the maintenance of normal glucose homeostasis - it produces glucose during fasting and stores glucose postprandially. However, these hepatic processes are dysregulated in type 1 and type 2 diabetes mellitus, and this imbalance contributes to hyperglycaemia in the fasted and postprandial states. Net hepatic glucose production is the summation of glucose fluxes from gluconeogenesis, glycogenolysis, glycogen synthesis, glycolysis and other pathways. In this Review, we discuss the in vivo regulation of these hepatic glucose fluxes. In particular, we highlight the importance of indirect (extrahepatic) control of hepatic gluconeogenesis and direct (hepatic) control of hepatic glycogen metabolism. We also propose a mechanism for the progression of subclinical hepatic insulin resistance to overt fasting hyperglycaemia in type 2 diabetes mellitus. Insights into the control of hepatic gluconeogenesis by metformin and insulin and into the role of lipid-induced hepatic insulin resistance in modifying gluconeogenic and net hepatic glycogen synthetic flux are also discussed. Finally, we consider the therapeutic potential of strategies that target hepatosteatosis, hyperglucagonaemia and adipose lipolysis.

Implications of glycerol metabolism for lipid production

Triacylglycerol (TAG) is an important product in oil-producing organisms. Biosynthesis of TAG can be completed through either esterification of fatty acids to glycerol backbone, or through esterification of 2-monoacylglycerol. This review will focus on the former pathway in which two precursors, fatty acid and glycerol-3-phosphate (G3P), are required for TAG formation. Tremendous progress has been made about the enzymes or genes that regulate the biosynthetic pathway of TAG. However, much attention has been paid to the fatty acid provision and the esterification process, while the possible role of G3P is largely neglected. Glycerol is extensively studied on its usage as carbon source for value-added products, but the modification of glycerol metabolism, which is directly associated with G3P synthesis, is seldom recognized in lipid investigations. The relevance among glycerol metabolism, G3P synthesis and lipid production is described, and the role of G3P in glycerol metabolism and lipid production are discussed in detail with an emphasis on how G3P affects lipid production through the modulation of glycerol metabolism. Observations of lipid metabolic changes due to glycerol related disruption in mammals, plants, and microorganisms are introduced. Altering glycerol metabolism results in the changes of final lipid content. Possible regulatory mechanisms concerning the relationship between glycerol metabolism and lipid production are summarized.

Regulation of organelle function by metformin

Metformin ameliorates hyperglycemia without the side effects of lactic acidosis or hypoglycemia. Metformin lowers the blood glucose level by decreasing hepatic glucose production in the liver and by increasing glucose uptake in the muscle. Recent studies show that metformin induces cell death in certain cancer cell lines by interfering with the metabolism of the cancer cells. Therefore, understanding the mechanisms of action for metformin will provide insights into how to better treat diabetes and other metabolic disorders and also into the development of new therapeutic drugs. One of the best understood molecular targets of metformin is the mitochondrial complex I. However, given metformin's broad effects on metabolism, it could act on multiple targets. In this review, we summarize current findings in metformin's mechanisms of action regarding its known targets in mitochondria and known effects in cancer cell lines. Then, we introduce endosomal Na⁺ /H⁺ exchangers and the V-ATPase as new potential targets of metformin's action. Finally, we will discuss the hypothesis that metformin directly acts on endosome/lysosome regulation so as to regulate metabolism and ultimately alleviate type 2 diabetes. © 2017 IUBMB Life, 69(7):459-469, 2017.

Effects of postnatal bromocriptine injection on thyroid function and prolactinemia of rats at adulthood

Previously, we demonstrated that maternal prolactin inhibition at the end of lactation, using bromocriptine (BRO), leads to an increase in leptin transfer via milk and induces the adult progeny to present hypothyroidism, leptin resistance and metabolic syndrome (obesity, hyperglycemia, hypertriglyceridemia, lower HDL). To test if these alterations are due to direct BRO action on the pups, in the present study we evaluated the long-term effects of direct injection of BRO (0.1μg/once daily) in male Wistar rats from postnatal (PN) day 1 to 10 (early treatment) or from PN11 to 20 (late treatment) on: food intake, body mass, cardiovascular parameters, hormone profile, hypothalamic leptin signaling, glucose homeostasis and thyroid hormone-dependent proteins. The respective controls were injected with methanol-saline. Offspring were killed at adulthood (PN180). Adult PN1-10 BRO-treated animals had lower food intake, hypoprolactinemia, lower leptin action (lower OBR-b, STAT-3 and SOCS-3 mRNA levels in the arcuate nucleus), lower TRH-TSH-thyroid axis as well as lower thyroid hormone markers. On the other hand, adult animals that were BRO-treated during the PN11-20 period showed hyperphagia, higher blood pressure, higher prolactinemia and OBR-b, higher TRH and plasma T3, hypercorticosteronemia as well as higher Dio2 and UCP1 mRNA expression in the brown adipose tissue. Glucose homeostasis was not changed treatment in either period. Our data show that early and late dopamine overexposure during lactation induces diverse metabolic disturbances later in life, increasing the risk of thyroid dysfunction and, consequently, changes in prolactinemia.

Metformin suppresses gluconeogenesis by inhibiting mitochondrial glycerophosphate dehydrogenase

Metformin is considered to be one of the most effective therapeutics for the treatment of type 2 diabetes (T2D) since it specifically reduces hepatic gluconeogenesis without increasing insulin secretion, inducing weight gain, or posing a risk of hypoglycemia^1,2. For over half a century, this agent has been prescribed to T2D patients worldwide, yet the underlying mechanism by which metformin inhibits hepatic gluconeogenesis remains unknown. Here we show that metformin non-competitively inhibits the redox shuttle enzyme mitochondrial glycerophosphate dehydrogenase (mGPD), resulting in an altered hepatocellular redox state, reduced conversion of lactate and glycerol to glucose, and decreased hepatic gluconeogenesis. Acute and chronic low-dose metformin treatment effectively reduced endogenous glucose production (EGP), while increasing cytosolic redox and decreasing mitochondrial redox states. Antisense oligonucleotide (ASO) knockdown of hepatic mGPD in rats resulted in a phenotype akin to chronic metformin treatment, and abrogated metformin-mediated increases in cytosolic redox state, decrease in plasma glucose concentrations and inhibition of EGP. These findings were replicated in whole-body mGPD knockout mice. These results have significant implications for understanding the mechanism of metformin’s blood glucose lowering effects and provide a novel therapeutic target for T2D.

Mining the genome for lipid genes

Mining of the genome for lipid genes has since the early 1970s helped to shape our understanding of how triglycerides are packaged (in chylomicrons), repackaged (in very low density lipoproteins; VLDL), and hydrolyzed, and also how remnant and low-density lipoproteins (LDL) are cleared from the circulation. Gene discoveries have also provided insights into high-density lipoprotein (HDL) biogenesis and remodeling. Interestingly, at least half of these key molecular genetic studies were initiated with the benefit of prior knowledge of relevant proteins. In addition, multiple important findings originated from studies in mouse, and from other types of non-genetic approaches. Although it appears by now that the main lipid pathways have been uncovered, and that only modulators or adaptor proteins such as those encoded by LDLRAP1, APOA5, ANGPLT3/4, and PCSK9 are currently being discovered, genome wide association studies (GWAS) in particular have implicated many new loci based on statistical analyses; these may prove to have equally large impacts on lipoprotein traits as gene products that are already known. On the other hand, since 2004 - and particularly since 2010 when massively parallel sequencing has become de rigeur - no major new insights into genes governing lipid metabolism have been reported. This is probably because the etiologies of true Mendelian lipid disorders with overt clinical complications have been largely resolved. In the meantime, it has become clear that proving the importance of new candidate genes is challenging. This could be due to very low frequencies of large impact variants in the population. It must further be emphasized that functional genetic studies, while necessary, are often difficult to accomplish, making it hazardous to upgrade a variant that is simply associated to being definitively causative. Also, it is clear that applying a monogenic approach to dissect complex lipid traits that are mostly of polygenic origin is the wrong way to proceed. The hope is that large-scale data acquisition combined with sophisticated computerized analyses will help to prioritize and select the most promising candidate genes for future research. We suggest that at this point in time, investment in sequence technology driven candidate gene discovery could be recalibrated by refocusing efforts on direct functional analysis of the genes that have already been discovered. This article is part of a Special Issue entitled: From Genome to Function.

Novel inhibitors of mitochondrial sn-glycerol 3-phosphate dehydrogenase

Mitochondrial sn-glycerol 3-phosphate dehydrogenase (mGPDH) is a ubiquinone-linked enzyme in the mitochondrial inner membrane best characterized as part of the glycerol phosphate shuttle that transfers reducing equivalents from cytosolic NADH into the mitochondrial electron transport chain. Despite the widespread expression of mGPDH and the availability of mGPDH-null mice, the physiological role of this enzyme remains poorly defined in many tissues, likely because of compensatory pathways for cytosolic regeneration of NAD⁺ and mechanisms for glycerol phosphate metabolism. Here we describe a novel class of cell-permeant small-molecule inhibitors of mGPDH (iGP) discovered through small-molecule screening. Structure-activity analysis identified a core benzimidazole-phenyl-succinamide structure as being essential to inhibition of mGPDH while modifications to the benzimidazole ring system modulated both potency and off-target effects. Live-cell imaging provided evidence that iGPs penetrate cellular membranes. Two compounds (iGP-1 and iGP-5) were characterized further to determine potency and selectivity and found to be mixed inhibitors with IC₅₀ and K_i values between ∼1–15 µM. These novel mGPDH inhibitors are unique tools to investigate the role of glycerol 3-phosphate metabolism in both isolated and intact systems.

Mitochondrial FAD-linked Glycerol-3-phosphate Dehydrogenase: A Target for Cancer Therapeutics

Imbalances in cellular redox state are frequently observed in cancer cells, and contribute significantly to cancer progression and apoptotic resistance. Hydrogen peroxide (H₂O₂) is one reactive oxygen species (ROS) that is produced in excess within cancer cells. In this study, we investigated the mitochondrial glycerol-3-phosphate-dependent (GPD2) ROS production in PC-3 cells and demonstrated the importance of excessive H₂O₂ production on their survival. By exploiting the abnormal H₂O₂ production of PC-3 cells, we initiated a high-throughput screening of the Canadian Compound Collection, composed of 29,586 small molecules, targeting the glycerophosphate-dependent H₂O₂ formation in PC-3 cells. Eighteen compounds were identified to have significant inhibitory activity. These compounds have not been previously characterized as inhibitors of the enzyme. Six of these compounds were further analyzed in PC-3 cells and dose response studies displayed an inhibitory and anti-oxidative potency that ranged from 1 µM to 30 µM. The results presented here demonstrate that inhibitors of mitochondrial GPD2 activity elicit anti-proliferative effects on cancer cells.

The function and the role of the mitochondrial glycerol-3-phosphate dehydrogenase in mammalian tissues

Mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) is not included in the traditional textbook schemes of the respiratory chain, reflecting the fact that it is a non-standard, tissue-specific component of mammalian mitochondria. But despite its very simple structure, mGPDH is a very important enzyme of intermediary metabolism and as a component of glycerophosphate shuttle it functions at the crossroads of glycolysis, oxidative phosphorylation and fatty acid metabolism. In this review we summarize the present knowledge on the structure and regulation of mGPDH and discuss its metabolic functions, reactive oxygen species production and tissue and organ specific roles in mammalian mitochondria at physiological and pathological conditions.

The Human Obesity Gene Map: The 2004 Update

This paper presents the eleventh update of the human obesity gene map, which incorporates published results up to the end of October 2004. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTLs) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2004, 173 human obesity cases due to single-gene mutations in 10 different genes have been reported, and 49 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 166 genes which, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 221. The number of human obesity QTLs derived from genome scans continues to grow, and we have now 204 QTLs for obesity-related phenotypes from 50 genome-wide scans. A total of 38 genomic regions harbor QTLs replicated among two to four studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably with 358 findings of positive associations with 113 candidate genes. Among them, 18 genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, >600 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful publications and genomic and other relevant sites can be found at http://obesitygene.pbrc.edu.

Identification of NO induced and capacitation associated tyrosine phosphoproteins in buffalo (Bubalus bubalis) spermatozoa

To acquire the fertilizing competence, spermatozoa must undergo a cascade of physiological and biochemical changes collectively defined as capacitation. Compelling evidence signifies that the global increase in protein tyrosine phosphorylation is the driving factor for capacitation. In our laboratory, we previously demonstrated that nitric oxide (NO) induces capacitation in buffalo sperm and is associated with an increase in protein tyrosine phosphorylation. The aim of the present study is to identify the proteins undergo tyrosine phosphorylation during NO induced buffalo sperm capacitation using 2-D immunoblotting and mass spectrometry. The percentage of progressively motile and capacitated sperm was more in presence of l-arginine. Along with known tyrosine phosphoproteins like ATP synthase subunit beta, pyruvate dehydrogenase E1 component subunit beta, GST mu 3, F-actin capping protein subunit beta 2, GPD2 and VDAC2, interestingly novel tyrosine phosphoprotein substrates such as actin, serine/threonine-protein phosphatase PP1-gamma catalytic subunit, and glutamine synthetase were also identified which might be specific to the NO induced signaling and also emphasizes the species specificity with respect to tyrosine phosphorylation of proteins during capacitation. In conclusion, this study forms an essential step in delineating the proteins undergo tyrosine phosphorylation in response to NO induced signaling pathways during capacitation of buffalo sperm.

Inactivation of the mitochondrial carrier SLC25A25 (ATP-Mg2+/Pi transporter) reduces physical endurance and metabolic efficiency in mice

An ATP-Mg(2+/)P(i) inner mitochondrial membrane solute transporter (SLC25A25), which is induced during adaptation to cold stress in the skeletal muscle of mice with defective UCP1/brown adipose tissue thermogenesis, has been evaluated for its role in metabolic efficiency. SLC25A25 is thought to control ATP homeostasis by functioning as a Ca(2+)-regulated shuttle of ATP-Mg(2+) and P(i) across the inner mitochondrial membrane. Mice with an inactivated Slc25a25 gene have reduced metabolic efficiency as evidenced by enhanced resistance to diet-induced obesity and impaired exercise performance on a treadmill. Mouse embryo fibroblasts from Slc25a25(-/-) mice have reduced Ca(2+) flux across the endoplasmic reticulum, basal mitochondrial respiration, and ATP content. Although Slc25a25(-/-) mice are metabolically inefficient, the source of the inefficiency is not from a primary function in thermogenesis, because Slc25a25(-/-) mice maintain body temperature upon acute exposure to the cold (4 °C). Rather, the role of SLC25A25 in metabolic efficiency is most likely linked to muscle function as evidenced from the physical endurance test of mutant mice on a treadmill. Consequently, in the absence of SLC25A25 the efficiency of ATP production required for skeletal muscle function is diminished with secondary effects on adiposity. However, in the absence of UCP1-based thermogenesis, induction of Slc25a25 in mice with an intact gene may contribute to an alternative thermogenic pathway for the maintenance of body temperature during cold stress.

Role of glycerol-3-phosphate dehydrogenase 2 in mouse sperm capacitation

A tyrosine phosphoproteome study of hamster spermatozoa indicated that glycerol-3-phosphate dehydrogenase 2 (GPD2), is one of the proteins that enables tyrosine phosphorylation during sperm capacitation. Further, enzymatic activity of GPD2 correlated positively with sperm capacitation [Kota et al., 2009; Proteomics 9:1809-1826]. Therefore, understanding the function of GPD2 would help to unravel the molecular mechanism of sperm capacitation. In this study, involving the use of spermatozoa from Gpd2(+/+) and Gpd2(-/-) mice, it has been demonstrated that in the absence of Gpd2, hyperactivation and acrosome reaction were significantly altered, and a few changes in protein tyrosine phosphorylation were also observed during capacitation. Evidence is provided to demonstrate that GPD2 activity is required for ROS generation in mouse spermatozoa during capacitation, failing which, capacitation is impaired. These results imply that GPD2 is involved in sperm capacitation.

A wild derived quantitative trait locus on mouse chromosome 2 prevents obesity

Background

The genetic architecture of multifactorial traits such as obesity has been poorly understood. Quantitative trait locus (QTL) analysis is widely used to localize loci affecting multifactorial traits on chromosomal regions. However, large confidence intervals and small phenotypic effects of identified QTLs and closely linked loci are impeding the identification of causative genes that underlie the QTLs. Here we developed five subcongenic mouse strains with overlapping and non-overlapping wild-derived genomic regions from an F2 intercross of a previously developed congenic strain, B6.Cg-Pbwg1, and its genetic background strain, C57BL/6J (B6). The subcongenic strains developed were phenotyped on low-fat standard chow and a high-fat diet to fine-map a previously identified obesity QTL. Microarray analysis was performed with Affymetrix GeneChips to search for candidate genes of the QTL.

Results

The obesity QTL was physically mapped to an 8.8-Mb region of mouse chromosome 2. The wild-derived allele significantly decreased white fat pad weight, body weight and serum levels of glucose and triglyceride. It was also resistant to the high-fat diet. Among 29 genes residing within the 8.8-Mb region, Gpd2, Upp2, Acvr1c, March7 and Rbms1 showed great differential expression in livers and/or gonadal fat pads between B6.Cg-Pbwg1 and B6 mice.

Conclusions

The wild-derived QTL allele prevented obesity in both mice fed a low-fat standard diet and mice fed a high-fat diet. This finding will pave the way for identification of causative genes for obesity. A further understanding of this unique QTL effect at genetic and molecular levels may lead to the discovery of new biological and pathologic pathways associated with obesity.

Regulation of insulin secretion: role of mitochondrial signalling

Pancreatic beta cells are specialised endocrine cells that continuously sense the levels of blood sugar and other fuels and, in response, secrete insulin to maintain normal fuel homeostasis. During postprandial periods an elevated level of plasma glucose rapidly stimulates insulin secretion to decrease hepatic glucose output and promote glucose uptake into other tissues, principally muscle and adipose tissues. Beta cell mitochondria play a key role in this process, not only by providing energy in the form of ATP to support insulin secretion, but also by synthesising metabolites (anaplerosis) that can act, both intra- and extramitochondrially, as factors that couple glucose sensing to insulin granule exocytosis. ATP on its own, and possibly modulated by these coupling factors, triggers closure of the ATP-sensitive potassium channel, resulting in membrane depolarisation that increases intracellular calcium to cause insulin secretion. The metabolic imbalance caused by chronic hyperglycaemia and hyperlipidaemia severely affects mitochondrial metabolism, leading to the development of impaired glucose-induced insulin secretion in type 2 diabetes. It appears that the anaplerotic enzyme pyruvate carboxylase participates directly or indirectly in several metabolic pathways which are important for glucose-induced insulin secretion, including: the pyruvate/malate cycle, the pyruvate/citrate cycle, the pyruvate/isocitrate cycle and glutamate-dehydrogenase-catalysed alpha-ketoglutarate production. These four pathways enable 'shuttling' or 'recycling' of these intermediate(s) into and out of mitochondrion, allowing continuous production of intracellular messenger(s). The purpose of this review is to present an account of recent progress in this area of central importance in the realm of diabetes and obesity research.

Lower succinyl-CoA:3-ketoacid-CoA transferase (SCOT) and ATP citrate lyase in pancreatic islets of a rat model of type 2 diabetes: knockdown of SCOT inhibits insulin release in rat insulinoma cells

Succinyl-CoA:3-ketoacid-CoA transferase (SCOT) is a mitochondrial enzyme that catalyzes the reversible transfer of coenzyme-A from acetoacetyl-CoA to succinate to form acetoacetate and succinyl-CoA. mRNAs of SCOT and ATP citrate lyase were decreased 55% and 58% and enzyme activities were decreased >70% in pancreatic islets of the GK rat, a model of type 2 diabetes. INS-1 832/13 cells were transfected with shRNAs targeting SCOT mRNA to generate cell lines with reduced SCOT activity. Two cell lines with >70% knockdown of SCOT activity showed >70% reduction in glucose- or methyl succinate-plus-beta-hydroxybutyrate-stimulated insulin release. Less inhibition of insulin release was observed with two cell lines with less knockdown of SCOT. Previous studies showed knockdown of ATP citrate lyase in INS-1 832/13 cells does not lower insulin release. The results further support work that suggests mitochondrial pathways involving SCOT which supply acetoacetate for export to the cytosol are important for insulin secretion.

Functional cooperation between CREM and GCNF directs gene expression in haploid male germ cells

Cellular differentiation and development of germ cells critically depend on a coordinated activation and repression of specific genes. The underlying regulation mechanisms, however, still lack a lot of understanding. Here, we describe that both the testis-specific transcriptional activator CREMτ (cAMP response element modulator tau) and the repressor GCNF (germ cell nuclear factor) have an overlapping binding site which alone is sufficient to direct cell type-specific expression in vivo in a heterologous promoter context. Expression of the transgene driven by the CREM/GCNF site is detectable in spermatids, but not in any somatic tissue or at any other stages during germ cell differentiation. CREMτ acts as an activator of gene transcription whereas GCNF suppresses this activity. Both factors compete for binding to the same DNA response element. Effective binding of CREM and GCNF highly depends on composition and epigenetic modification of the binding site. We also discovered that CREM and GCNF bind to each other via their DNA binding domains, indicating a complex interaction between the two factors. There are several testis-specific target genes that are regulated by CREM and GCNF in a reciprocal manner, showing a similar activation pattern as during spermatogenesis. Our data indicate that a single common binding site for CREM and GCNF is sufficient to specifically direct gene transcription in a tissue-, cell type- and differentiation-specific manner.

The genetics of brown adipose tissue

Brown adipose tissue is highly differentiated and has evolved as a mechanism for heat production based upon uncoupling of mitochondrial oxidative phosphorylation. Additionally, large amounts of lipid can be stored in the cells to provide fuel necessary for heat production upon adrenergic stimulation from the central nervous system, and a highly developed vascular system evolved to rapidly deliver heat to vital organs. For unknown reasons, the development of brown adipocytes has two independent pathways: one originates from muscle progenitor cells in the fetus and leads to a fully functional cell at birth (interscapular-type brown fat), while the other transiently emerges in traditional white fat depots at weaning, regresses, and then can be induced in adult mice upon adrenergic stimulation. No genetic variants have been found for interscapular fat, but naturally occurring alleles at eight genetic loci in mice lead to over 100-fold variation for brown adipocytes in white fat upon adrenergic stimulation. The ability to activate this potential for energy expenditure is of great interest in obesity research.

UCP1: its involvement and utility in obesity

Energy balance to prevent the development of obesity is dependent on energy expenditure. Although physical activity is the dominant mechanism for dissipating excess energy, a system of thermogenesis that evolved to protect the body from hypothermia is based upon the uncoupling of oxidative phosphorylation in brown adipocytes by the mitochondrial uncoupling protein (UCP1). It has been shown that upregulation of UCP1 by genetic manipulations or pharmacological agents can reduce obesity and improve insulin sensitivity. Recent evidence has shown the existence of two sources for brown adipocytes, one appearing as discrete brown fat depots during fetal development and the other appears during post-natal development as diffuse populations in traditional white fat depots. The latter can be induced by adrenergic stimulation depending on the genetic background of the animals and the nutritional environment. Understanding the biological and environmental factors controlling the expression of these two brown adipocyte populations promises to provide new strategies by which enhanced thermogenesis can be used to reduce obesity.International Journal of Obesity (2008) 32, S32-S38; doi:10.1038/ijo.2008.236.

Mitochondrial (dys)function in adipocyte (de)differentiation and systemic metabolic alterations

In mammals, adipose tissue, composed of BAT and WAT, collaborates in energy partitioning and performs metabolic regulatory functions. It is the most flexible tissue in the body, because it is remodeled in size and shape by modifications in adipocyte cell size and/or number, depending on developmental status and energy fluxes. Although numerous reviews have focused on the differentiation program of both brown and white adipocytes as well as on the pathophysiological role of white adipose tissues, the importance of mitochondrial activity in the differentiation or the dedifferentiation programs of adipose cells and in systemic metabolic alterations has not been extensively reviewed previously. Here, we address the crucial role of mitochondrial functions during adipogenesis and in mature adipocytes and discuss the cellular responses of white adipocytes to mitochondrial activity impairment. In addition, we discuss the increase in scientific knowledge regarding mitochondrial functions in the last 10 years and the recent suspicion of mitochondrial dysfunction in several 21st century epidemics (ie, obesity and diabetes), as well as in lipodystrophy found in HIV-treated patients, which can contribute to the development of new therapeutic strategies targeting adipocyte mitochondria.

Haploinsufficiency of the GPD2 gene in a patient with nonsyndromic mental retardation

We have investigated the chromosome abnormalities in a female patient exhibiting mild nonsyndromic mental retardation. The patient carries a de novo balanced reciprocal translocation 46,XX,t(2;7)(q24.1;q36.1). Physical mapping of the breakpoints by fluorescent in situ hybridization experiments revealed the disruption of the GPD2 gene at the 2q24.1 region. This gene encodes the mitochondrial glycerophosphate dehydrogenase (mGPDH), which is located on the outer surface of the inner mitochondrial membrane, and catalyzes the unidirectional conversion of glycerol-3-phosphate (G3P) to dihydroxyacetone phosphate with concomitant reduction of the enzyme-bound FAD. Molecular and functional studies showed approximately a twofold decrease of GPD2 transcript level as well as decreased activity of the coded mGPDH protein in lymphoblastoid cell lines of the patient compared to controls. Bioinformatics analysis allowed us to confirm the existence of a novel transcript of the GPD2 gene, designated GPD2c, which is directly disrupted by the 2q breakpoint. To validate GPD2 as a new candidate gene for mental retardation, we performed mutation screening of the GPD2 gene in 100 mentally retarded patients; however, no mutations have been identified. Nevertheless, our results propose that a functional defect of the mGPDH protein could be associated with mental retardation, suggesting that GPD2 gene could be involved in mental retardation in some cases.

High efficiency of ROS production by glycerophosphate dehydrogenase in mammalian mitochondria

We investigated hydrogen peroxide production in mitochondria with low (liver, heart, brain) and high (brown adipose tissue, BAT) content of glycerophosphate dehydrogenase (mGPDH). ROS production at state 4 due to electron backflow from mGPDH was low, but after inhibition of electron transport with antimycin A high rates of mGPDH-dependent ROS production were observed in liver, heart and brain mitochondria. When this ROS production was related to activity of mGPDH, many-fold higher ROS production was found in contrast to succinate- (39-, 28-, 3-fold) or pyruvate plus malate-dependent ROS production (32-, 96-, 5-fold). This specific rate of mGPDH-dependent ROS production was also exceedingly higher (28-, 66-, 22-fold) compared to that in BAT. mGPDH-dependent ROS production was localized to the dehydrogenase+CoQ and complex III, the latter being the highest in all mitochondria but BAT. Our results demonstrate high efficiency of mGPDH-dependent ROS production in mammalian mitochondria with a low content of mGPDH and suggest its endogenous inhibition in BAT.

Inactivation of UCP1 and the glycerol phosphate cycle synergistically increases energy expenditure to resist diet-induced obesity

Our current paradigm for obesity assumes that reduced thermogenic capacity increases susceptibility to obesity, whereas enhanced thermogenic capacity protects against obesity. Here we report that elimination of two major thermogenic pathways encoded by the mitochondrial uncoupling protein (Ucp1) and mitochondrial glycerol-3-phosphate dehydrogenase (Gdm) result in mice with increased resistance to diet-induced obesity when housed at 28 degrees C, provided prior adaptation occurred at 20 degrees C. Obesity resistant Gdm(-/-).Ucp1(-/-) mice maintained at 28 degrees C have increased energy expenditure, in part through conversion of white to brown adipocytes in inguinal fat. Increased oxygen consumption in inguinal fat cell suspensions and the up-regulation of genes of mitochondrial function and fat metabolism indicated increased thermogenic activity, despite the absence of UCP1, whereas liver and skeletal muscle showed no changes in gene expression. Additionally, comparisons of energy expenditure in UCP1-deficient and wild type mice fed an obesogenic diet indicates that UCP1-based brown fat-based thermogenesis plays no role in so-called diet-induced thermogenesis. Accordingly, a new paradigm for obesity emerges in which the inactivation of major thermogenic pathways force the induction of alternative pathways that increase metabolic inefficiency.

A mutation in the inner mitochondrial membrane peptidase 2-like gene (Immp2l) affects mitochondrial function and impairs fertility in mice

The mitochondrion is involved in energy generation, apoptosis regulation, and calcium homeostasis. Mutations in genes involved in mitochondrial processes often result in a severe phenotype or embryonic lethality, making the study of mitochondrial involvement in aging, neurodegeneration, or reproduction challenging. Using a transgenic insertional mutagenesis strategy, we generated a mouse mutant, Immp2lTg(Tyr)979Ove, with a mutation in the inner mitochondrial membrane peptidase 2-like (Immp2l) gene. The mutation affected the signal peptide sequence processing of mitochondrial proteins cytochrome c1 and glycerol phosphate dehydrogenase 2. The inefficient processing of mitochondrial membrane proteins perturbed mitochondrial function so that mitochondria from mutant mice manifested hyperpolarization, higher than normal superoxide ion generation, and higher levels of ATP. Homozygous Immp2lTg(Tyr)979Ove females were infertile due to defects in folliculogenesis and ovulation, whereas mutant males were severely subfertile due to erectile dysfunction. The data suggest that the high superoxide ion levels lead to a decrease in the bioavailability of nitric oxide and an increase in reactive oxygen species stress, which underlies these reproductive defects. The results provide a novel link between mitochondrial dysfunction and infertility and suggest that superoxide ion targeting agents may prove useful for treating infertility in a subpopulation of infertile patients.

Mammalian Sir2 homolog SIRT3 regulates global mitochondrial lysine acetylation

Homologs of the Saccharomyces cerevisiae Sir2 protein, sirtuins, promote longevity in many organisms. Studies of the sirtuin SIRT3 have so far been limited to cell culture systems. Here, we investigate the localization and function of SIRT3 in vivo. We show that endogenous mouse SIRT3 is a soluble mitochondrial protein. To address the function and relevance of SIRT3 in the regulation of energy metabolism, we generated and phenotypically characterized SIRT3 knockout mice. SIRT3-deficient animals exhibit striking mitochondrial protein hyperacetylation, suggesting that SIRT3 is a major mitochondrial deacetylase. In contrast, no mitochondrial hyperacetylation was detectable in mice lacking the two other mitochondrial sirtuins, SIRT4 and SIRT5. Surprisingly, despite this biochemical phenotype, SIRT3-deficient mice are metabolically unremarkable under basal conditions and show normal adaptive thermogenesis, a process previously suggested to involve SIRT3. Overall, our results extend the recent finding of lysine acetylation of mitochondrial proteins and demonstrate that SIRT3 has evolved to control reversible lysine acetylation in this organelle.

Differential effects of selegiline on glucose synthesis in rabbit kidney-cortex tubules and hepatocytes. In vitro and in vivo studies

The action of selegiline, a selective and irreversible inhibitor of monoamine oxidase B, commonly applied in the therapy of Parkinson's disease, on glucose formation was investigated in isolated rabbit hepatocytes and kidney-cortex tubules, maintaining the whole body glucose homeostasis via gluconeogenic pathway activity. An intensive hepatic metabolism of selegiline resulted in formation of selegiline-N-oxide, desmethylselegiline, methamphetamine and amphetamine, whereas during slow degradation of the drug in freshly isolated renal tubules selegiline-N-oxide was mainly produced. At 100 microM concentration selegiline markedly diminished glucose synthesis in isolated renal tubules incubated with dihydroxyacetone or alanine+glycerol+octanoate (by about 60 and 30%, respectively), while at 5 microM concentration a similar degree of inhibition was achieved in renal tubules grown in primary culture under the same conditions (about 40 and 60%, respectively). Moreover, desmethylselegiline and selegiline-N-oxide considerably diminished glucose production in renal tubules whereas selegiline and its metabolites did not affect gluconeogenesis in hepatocytes. Contrary to control animals, following selegiline administration to alloxan-diabetic rabbits for 8 days (10 mg kg(-1) body wt. daily) the blood glucose and serum creatinine levels were significantly diminished, suggesting a decrease in renal gluconeogenesis and improvement of kidney functions. Since in renal tubules selegiline induced a decline in the intracellular levels of gluconeogenic intermediates and ATP content accompanied by a decrease in oxygen consumption in both kidney-cortex and hepatic mitochondria it seems possible that its inhibitory action on renal gluconeogenesis might result from an impairment of mitochondrial function, while an intensive selegiline metabolism in hepatocytes causes decrease of its concentration and in consequence no inhibition of gluconeogenesis. In view of these observations it is likely that an increased risk of selegiline-induced hypoglycemia might be expected particularly in patients exhibiting an impairment of liver function and following transdermal administration of this drug, i.e. under conditions of increased serum selegiline concentrations.

Citrin/Mitochondrial Glycerol-3-phosphate Dehydrogenase Double Knock-out Mice Recapitulate Features of Human Citrin Deficiency

Citrin is the liver-type mitochondrial aspartate-glutamate carrier that participates in urea, protein, and nucleotide biosynthetic pathways by supplying aspartate from mitochondria to the cytosol. Citrin also plays a role in transporting cytosolic NADH reducing equivalents into mitochondria as a component of the malate-aspartate shuttle. In humans, loss-of-function mutations in the SLC25A13 gene encoding citrin cause both adult-onset type II citrullinemia and neonatal intrahepatic cholestasis, collectively referred to as human citrin deficiency. Citrin knock-out mice fail to display features of human citrin deficiency. Based on the hypothesis that an enhanced glycerol phosphate shuttle activity may be compensating for the loss of citrin function in the mouse, we have generated mice with a combined disruption of the genes for citrin and mitochondrial glycerol 3-phosphate dehydrogenase. The resulting double knock-out mice demonstrated citrullinemia, hyperammonemia that was further elevated by oral sucrose administration, hypoglycemia, and a fatty liver, all features of human citrin deficiency. An increased hepatic lactate/pyruvate ratio in the double knock-out mice compared with controls was also further elevated by the oral sucrose administration, suggesting that an altered cytosolic NADH/NAD(+) ratio is closely associated with the hyperammonemia observed. Microarray analyses identified over 100 genes that were differentially expressed in the double knock-out mice compared with wild-type controls, revealing genes potentially involved in compensatory or downstream effects of the combined mutations. Together, our data indicate that the more severe phenotype present in the citrin/mitochondrial glycerol-3-phosphate dehydrogenase double knock-out mice represents a more accurate model of human citrin deficiency than citrin knock-out mice.

Thermogenic mechanisms and their hormonal regulation

Increased heat generation from biological processes is inherent to homeothermy. Homeothermic species produce more heat from sustaining a more active metabolism as well as from reducing fuel efficiency. This article reviews the mechanisms used by homeothermic species to generate more heat and their regulation largely by thyroid hormone (TH) and the sympathetic nervous system (SNS). Thermogenic mechanisms antecede homeothermy, but in homeothermic species they are activated and regulated. Some of these mechanisms increase ATP utilization (same amount of heat per ATP), whereas others increase the heat resulting from aerobic ATP synthesis (more heat per ATP). Among the former, ATP utilization in the maintenance of ionic gradient through membranes seems quantitatively more important, particularly in birds. Regulated reduction of the proton-motive force to produce heat, originally believed specific to brown adipose tissue, is indeed an ancient thermogenic mechanism. A regulated proton leak has been described in the mitochondria of several tissues, but its precise mechanism remains undefined. This leak is more active in homeothermic species and is regulated by TH, explaining a significant fraction of its thermogenic effect. Homeothermic species generate additional heat, in a facultative manner, when obligatory thermogenesis and heat-saving mechanisms become limiting. Facultative thermogenesis is activated by the SNS but is modulated by TH. The type II iodothyronine deiodinase plays a critical role in modulating the amount of the active TH, T(3), in BAT, thereby modulating the responses to SNS. Other hormones affect thermogenesis in an indirect or permissive manner, providing fuel and modulating thermogenesis depending on food availability, but they do not seem to have a primary role in temperature homeostasis. Thermogenesis has a very high energy cost. Cold adaptation and food availability may have been conflicting selection pressures accounting for the variability of thermogenesis in humans.

Chromosome 2 locus Nidd5 has a potent effect on adiposity in the TSOD mouse

We previously reported a quantitative trait locus for body weight, non-insulin-dependent diabetes 5 (Nidd5), on Chromosome 2 in the TSOD (Tsumura, Suzuki, Obese Diabetes) mouse, a model of polygenic obese type 2 diabetes. To find the gene responsible for a specific component of the pathogenesis, we used a marker-assisted selection protocol to produce congenic strains. These mice are designed to carry a control BALB/cA-derived genomic interval and a TSOD background to look for loss of phenotype. One of the strains with the widest congenic interval, D2Mit297-D2Mit304, showed reductions in both body weight and adiposity compared with TSOD mice. The phenotypic analyses of other congenic strains further narrowed the locus in a 9.4-Mb interval between D2Mit433 and D2Mit91, around which numerous loci for body weight and adiposity have been mapped previously. Although the locus showed a relatively modest effect on body weight, it had a major influence on fat mass that explains approximately 60% of the difference in the adipose index between parental TSOD and BALB/cA mice. Furthermore, the congenic strain with a minimal BALB/cA-derived region showed significantly smaller cell sizes of white and brown adipocytes compared with the control littermates. However, the locus did not primarily affect food consumption, general activity, or rectal temperature after cold exposure, although there are clear differences in these traits between the parental strains. The present work physically delineates the major locus for adiposity in the TSOD mouse.

The human obesity gene map: the 2005 update

This paper presents the 12th update of the human obesity gene map, which incorporates published results up to the end of October 2005. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTL) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2005, 176 human obesity cases due to single-gene mutations in 11 different genes have been reported, 50 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 244 genes that, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 408. The number of human obesity QTLs derived from genome scans continues to grow, and we now have 253 QTLs for obesity-related phenotypes from 61 genome-wide scans. A total of 52 genomic regions harbor QTLs supported by two or more studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably, with 426 findings of positive associations with 127 candidate genes. A promising observation is that 22 genes are each supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. The electronic version of the map with links to useful publications and relevant sites can be found at http://obesitygene.pbrc.edu.

CREB activation induced by mitochondrial dysfunction triggers triglyceride accumulation in 3T3-L1 preadipocytes

Several mitochondrial pathologies are characterized by lipid redistribution and microvesicular cell phenotypes resulting from triglyceride accumulation in lipid-metabolizing tissues. However, the molecular mechanisms underlying abnormal fat distribution induced by mitochondrial dysfunction remain poorly understood. In this study, we show that inhibition of respiratory complex III by antimycin A as well as inhibition of mitochondrial protein synthesis trigger the accumulation of triglyceride vesicles in 3T3-L1 fibroblasts. We also show that treatment with antimycin A triggers CREB activation in these cells. To better delineate how mitochondrial dysfunction induces triglyceride accumulation in preadipocytes, we developed a low-density DNA microarray containing 89 probes, which allows gene expression analysis for major effectors and/or markers of adipogenesis. We thus determined gene expression profiles in 3T3-L1 cells incubated with antimycin A and compared the patterns obtained with differentially expressed genes during the course of in vitro adipogenesis induced by a standard pro-adipogenic cocktail. After an 8-day treatment, a set of 39 genes was found to be differentially expressed in cells treated with antimycin A, among them CCAAT/enhancer-binding protein alpha (C/EBPalpha), C/EBP homologous protein-10 (CHOP-10), mitochondrial glycerol-3-phosphate dehydrogenase (GPDmit), and stearoyl-CoA desaturase 1 (SCD1). We also demonstrate that overexpression of two dominant negative mutants of the cAMP-response element-binding protein CREB (K-CREB and M1-CREB) and siRNA transfection, which disrupt the factor activity and expression, respectively, inhibit antimycin-A-induced triglyceride accumulation. Furthermore, CREB knockdown with siRNA also downregulates the expression of several genes that contain cAMP-response element (CRE) sites in their promoter, among them one that is potentially involved in synthesis of triglycerides such as SCD1. These results highlight a new role for CREB in the control of triglyceride metabolism during the adaptative response of preadipocytes to mitochondrial dysfunction.

High activity of mitochondrial glycerophosphate dehydrogenase and glycerophosphate-dependent ROS production in prostate cancer cell lines

Most malignant cells are highly glycolytic and produce high levels of reactive oxygen species (ROS) compared to normal cells. Mitochondrial glycerophosphate dehydrogenase (mGPDH) participates in the reoxidation of cytosolic NADH by delivering reducing equivalents from this molecule into the electron transport chain, thus sustaining glycolysis. Here, we investigate the role of mGPDH in maintaining an increased rate of glycolysis and evaluate glycerophosphate-dependent ROS production in prostate cancer cell lines (LNCaP, DU145, PC3, and CL1). Immunoblot, polarographic, and spectrophotometric analyses revealed that mGPDH abundance and activity was significantly elevated in prostate cancer cell lines when compared to the normal prostate epithelial cell line PNT1A. Furthermore, both the glycolytic capacity and glycerophosphate-dependent ROS production was increased 1.68- to 4.44-fold and 5- to 7-fold, respectively, in prostate cancer cell lines when compared to PNT1A cells. Overall, these data demonstrate that mGPDH is involved in maintaining a high rate of glycolysis and is an important site of electron leakage leading to ROS production in prostate cancer cells.

Differential effects of thyroid hormones on energy metabolism of rat slow- and fast-twitch muscles

Thyroid hormone (TH) is an important regulator of mitochondrial content and activity. As mitochondrial content and properties differ depending on muscle-type, we compared mitochondrial regulation and biogenesis by T3 in slow-twitch oxidative (soleus) and fast-twitch mixed muscle (plantaris). Male Wistar rats were treated for 21 to 27 days with T3 (200 microg/kg/day). Oxidative capacity, regulation of mitochondrial respiration by substrates and phosphate acceptors, and transcription factors were studied. In soleus, T3 treatment increased maximal oxygen consumption (Vmax) and the activities of citrate synthase (CS) and cytochrome oxidase (COX) by 100%, 45%, and 71%, respectively (P < 0.001), whereas in plantaris only Vmax increased, by 39% (P < 0.01). ADP-independent respiration rate was increased in soleus muscle by 216% suggesting mitochondrial uncoupling. Mitochondrial substrate utilization in soleus was also influenced by T3, as were mitochondrial enzymes. Lactate dehydrogenase (LDH) activity was elevated in soleus and plantaris by 63% and 11%, respectively (P < 0.01), and soleus creatine kinase was increased by 48% (P < 0.001). T3 increased the mRNA content of the transcriptional co-activator of mitochondrial genes, PGC-1alpha, and the I and IV COX subunits in soleus. The muscle specific response to thyroid hormones could be explained by a lower content of TH receptors in plantaris than soleus. Moreover, TRalpha mRNA level decreased further after T3 treatment. These results demonstrate that TH has a major effect on mitochondrial content, regulation and coupling in slow oxidative muscle, but to a lesser extent in fast muscle, due to the high expression of TH receptors and PGC-1alpha transcription factor.

Perspective: emerging evidence for signaling roles of mitochondrial anaplerotic products in insulin secretion

The importance of mitochondrial biosynthesis in stimulus secretion coupling in the insulin-producing beta-cell probably equals that of ATP production. In glucose-induced insulin secretion, the rate of pyruvate carboxylation is very high and correlates more strongly with the glucose concentration the beta-cell is exposed to (and thus with insulin release) than does pyruvate decarboxylation, which produces acetyl-CoA for metabolism in the citric acid cycle to produce ATP. The carboxylation pathway can increase the levels of citric acid cycle intermediates, and this indicates that anaplerosis, the net synthesis of cycle intermediates, is important for insulin secretion. Increased cycle intermediates will alter mitochondrial processes, and, therefore, the synthesized intermediates must be exported from mitochondria to the cytosol (cataplerosis). This further suggests that these intermediates have roles in signaling insulin secretion. Although evidence is quite good that all physiological fuel secretagogues stimulate insulin secretion via anaplerosis, evidence is just emerging about the possible extramitochondrial roles of exported citric acid cycle intermediates. This article speculates on their potential roles as signaling molecules themselves and as exporters of equivalents of NADPH, acetyl-CoA and malonyl-CoA, as well as alpha-ketoglutarate as a substrate for hydroxylases. We also discuss the "succinate mechanism," which hypothesizes that insulin secretagogues produce both NADPH and mevalonate. Finally, we discuss the role of mitochondria in causing oscillations in beta-cell citrate levels. These parallel oscillations in ATP and NAD(P)H. Oscillations in beta-cell plasma membrane electrical potential, ATP/ADP and NAD(P)/NAD(P)H ratios, and glycolytic flux are known to correlate with pulsatile insulin release. Citrate oscillations might synchronize oscillations of individual mitochondria with one another and mitochondrial oscillations with oscillations in glycolysis and, therefore, with flux of pyruvate into mitochondria. Thus citrate oscillations may synchronize mitochondrial ATP production and anaplerosis with other cellular oscillations.

Germ Cell Nuclear Factor Relieves cAMP-response Element Modulator τ-mediated Activation of the Testis-specific Promoter of Human Mitochondrial Glycerol-3-phosphate Dehydrogenase

Mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) is an essential component of the glycerol phosphate shuttle that transfers reduction equivalents from the cytosol into the mitochondrion. Within the testis, immunohistological analysis localized human mGPDH to late spermatids and to the midpiece of spermatozoa. The expression of human mGPDH is regulated by two somatic promoters, and here, we describe a third testis-specific promoter of human mGPDH. The usage of this testis-specific promoter correlates with the expression of a shortened mGPDH transcript of approximately 2.4 kb in length, which is solely detectable from testicular RNA. Within the testis-specific promoter, we detected a cAMP-response element (CRE) site at -51, which binds the testis-specific transcriptional activator CRE modulator tau (CREMtau) in electrophoretic mobility shift assays. This recognition site overlaps with a nuclear receptor binding half-site at -49, which binds the testis-specific transcriptional repressor germ cell nuclear factor (GCNF). Both factors compete for binding to the same DNA response element. Ectopic expression of CREMtau in HepG2 cells activated a promoter-driven luciferase construct in transient transfection experiments. Additional cotransfection of GCNF relieved this activity, suggesting a down-regulation of CREMtau-mediated activation by GCNF. This effect was preserved by introducing the CRE/nuclear receptor-binding element into a heterologous promoter context. Our data suggest a down-regulation of CREMtau-mediated gene expression by GCNF, which might be a general regulation mechanism for several postmeiotically expressed genes with a temporal expression peak during early spermatid development.