Cytosolic and mitochondrial isoforms of NADP+-dependent isocitrate dehydrogenases are expressed in cultured rat neurons, astrocytes, oligodendrocytes and microglial cells

G6PDi-1 is a Potent Inhibitor of G6PDH and of Pentose Phosphate pathway-dependent Metabolic Processes in Cultured Primary Astrocytes

Glucose-6-phosphate dehydrogenase (G6PDH) catalyses the rate limiting first step of the oxidative part of the pentose phosphate pathway (PPP), which has a crucial function in providing NADPH for antioxidative defence and reductive biosyntheses. To explore the potential of the new G6PDH inhibitor G6PDi-1 to affect astrocytic metabolism, we investigated the consequences of an application of G6PDi-1 to cultured primary rat astrocytes. G6PDi-1 efficiently inhibited G6PDH activity in lysates of astrocyte cultures. Half-maximal inhibition was observed for 100 nM G6PDi-1, while presence of almost 10 µM of the frequently used G6PDH inhibitor dehydroepiandrosterone was needed to inhibit G6PDH in cell lysates by 50%. Application of G6PDi-1 in concentrations of up to 100 µM to astrocytes in culture for up to 6 h did not affect cell viability nor cellular glucose consumption, lactate production, basal glutathione (GSH) export or the high basal cellular ratio of GSH to glutathione disulfide (GSSG). In contrast, G6PDi-1 drastically affected astrocytic pathways that depend on the PPP-mediated supply of NADPH, such as the NAD(P)H quinone oxidoreductase (NQO1)-mediated WST1 reduction and the glutathione reductase-mediated regeneration of GSH from GSSG. These metabolic pathways were lowered by G6PDi-1 in a concentration-dependent manner in viable astrocytes with half-maximal effects observed for concentrations between 3 and 6 µM. The data presented demonstrate that G6PDi-1 efficiently inhibits the activity of astrocytic G6PDH and impairs specifically those metabolic processes that depend on the PPP-mediated regeneration of NADPH in cultured astrocytes.

Nrf2 regulates glucose uptake and metabolism in neurons and astrocytes

The transcription factor Nrf2 and its repressor Keap1 mediate cell stress adaptation by inducing expression of genes regulating cellular detoxification, antioxidant defence and energy metabolism. Energy production and antioxidant defence employ NADH and NADPH respectively as essential metabolic cofactors; both are generated in distinct pathways of glucose metabolism, and both pathways are enhanced by Nrf2 activation. Here, we examined the role of Nrf2 on glucose distribution and the interrelation between NADH production in energy metabolism and NADPH homeostasis using glio-neuronal cultures isolated from wild-type, Nrf2-knockout and Keap1-knockdown mice. Employing advanced microscopy imaging of single live cells, including multiphoton fluorescence lifetime imaging microscopy (FLIM) to discriminate between NADH and NADPH, we found that Nrf2 activation increases glucose uptake into neurons and astrocytes. Glucose consumption is prioritized in brain cells for mitochondrial NADH and energy production, with a smaller contribution to NADPH synthesis in the pentose phosphate pathway for redox reactions. As Nrf2 is suppressed during neuronal development, this strategy leaves neurons reliant on astrocytic Nrf2 to maintain redox balance and energy homeostasis.

NADPH and Mitochondrial Quality Control as Targets for a Circadian-Based Fasting and Exercise Therapy for the Treatment of Parkinson's Disease

Dysfunctional mitochondrial quality control (MQC) is implicated in the pathogenesis of Parkinson's disease (PD). The improper selection of mitochondria for mitophagy increases reactive oxygen species (ROS) levels and lowers ATP levels. The downstream effects include oxidative damage, failure to maintain proteostasis and ion gradients, and decreased NAD⁺ and NADPH levels, resulting in insufficient energy metabolism and neurotransmitter synthesis. A ketosis-based metabolic therapy that increases the levels of (R)-3-hydroxybutyrate (BHB) may reverse the dysfunctional MQC by partially replacing glucose as an energy source, by stimulating mitophagy, and by decreasing inflammation. Fasting can potentially raise cytoplasmic NADPH levels by increasing the mitochondrial export and cytoplasmic metabolism of ketone body-derived citrate that increases flux through isocitrate dehydrogenase 1 (IDH1). NADPH is an essential cofactor for nitric oxide synthase, and the nitric oxide synthesized can diffuse into the mitochondrial matrix and react with electron transport chain-synthesized superoxide to form peroxynitrite. Excessive superoxide and peroxynitrite production can cause the opening of the mitochondrial permeability transition pore (mPTP) to depolarize the mitochondria and activate PINK1-dependent mitophagy. Both fasting and exercise increase ketogenesis and increase the cellular NAD⁺/NADH ratio, both of which are beneficial for neuronal metabolism. In addition, both fasting and exercise engage the adaptive cellular stress response signaling pathways that protect neurons against the oxidative and proteotoxic stress implicated in PD. Here, we discuss how intermittent fasting from the evening meal through to the next-day lunch together with morning exercise, when circadian NAD⁺/NADH is most oxidized, circadian NADP⁺/NADPH is most reduced, and circadian mitophagy gene expression is high, may slow the progression of PD.

Reduced nicotinamide adenine dinucleotide phosphate in redox balance and diseases: a friend or foe?

The nicotinamide adenine dinucleotide (NAD⁺/NADH) and nicotinamide adenine dinucleotide phosphate (NADP⁺/NADPH) redox couples function as cofactors or/and substrates for numerous enzymes to retain cellular redox balance and energy metabolism. Thus, maintaining cellular NADH and NADPH balance is critical for sustaining cellular homeostasis. The sources of NADPH generation might determine its biological effects. Newly-recognized biosynthetic enzymes and genetically encoded biosensors help us better understand how cells maintain biosynthesis and distribution of compartmentalized NAD(H) and NADP(H) pools. It is essential but challenging to distinguish how cells sustain redox couple pools to perform their integral functions and escape redox stress. However, it is still obscure whether NADPH is detrimental or beneficial as either deficiency or excess in cellular NADPH levels disturbs cellular redox state and metabolic homeostasis leading to redox stress, energy stress, and eventually, to the disease state. Additional study of the pathways and regulatory mechanisms of NADPH generation in different compartments, and the means by which NADPH plays a role in various diseases, will provide innovative insights into its roles in human health and may find a value of NADPH for the treatment of certain diseases including aging, Alzheimer's disease, Parkinson's disease, cardiovascular diseases, ischemic stroke, diabetes, obesity, cancer, etc.

Non-targeted Metabolomics Analysis Based on LC–MS to Assess the Effects of Different Cold Exposure Times on Piglets

Pigs are susceptible to low temperature conditions, and cold stress causes metabolic changes in the body to increase heat production as an adaption to adverse environments. To characterize and validate different metabolites in piglet livers at different cold exposure times, sixteen 30-day-old male weaned piglets with similar weights were randomly divided into four groups: the normal temperature group (24 ± 2°C, NT) and cold exposure (4 ± 2°C) 2-h group (CS2), 6-h group (CS6), and 12-h group (CS12). At the end of the experiment, the liver samples were analyzed using systemic non-targeted metabolomics. Eight known differentially abundant metabolites (farnesyl pyrophosphate, isocitrate, triethanolamine, phenylethylamine, deoxynosine, citric acid, maltotriose, and epinephrine) were observed between the CS groups and the control group in positive and negative ion modes. The eight main differentially abundant metabolites involved in seven metabolite classifications. Metabolic pathways and enrichment analyses revealed that the pathways involved three KEGG pathway classifications. Most of the pathways were related to amino acid or energy metabolism. Moreover, the metabolic pathways were not identical under different cold exposure times, with those following 2 and 6 h of cold exposure more related to carbohydrates and energy production and those following 12 h of cold exposure more related to the metabolism connected with epinephrine. Thus, under different cold exposure times, the metabolite profiles and metabolic pathways differed.

Energy transfer between the nicotinamide nucleotide transhydrogenase and ATP synthase of Escherichia coli

Membrane bound nicotinamide nucleotide transhydrogenase (TH) catalyses the hydride transfer from NADH to NADP⁺. Under physiological conditions, this reaction is endergonic and must be energized by the pmf, coupled to transmembrane proton transport. Recent structures of transhydrogenase holoenzymes suggest new mechanistic details, how the long-distance coupling between hydride transfer in the peripheral nucleotide binding sites and the membrane-localized proton transfer occurs that now must be tested experimentally. Here, we provide protocols for the efficient expression and purification of the Escherichia coli transhydrogenase and its reconstitution into liposomes, alone or together with the Escherichia coli F₁F₀ ATP synthase. We show that E. coli transhydrogenase is a reversible enzyme that can also work as a NADPH-driven proton pump. In liposomes containing both enzymes, NADPH driven H⁺-transport by TH is sufficient to instantly fuel ATP synthesis, which adds TH to the pool of pmf generating enzymes. If the same liposomes are energized with ATP, NADPH production by TH is stimulated > sixfold both by a pH gradient or a membrane potential. The presented protocols and results reinforce the tight coupling between hydride transfer in the peripheral nucleotide binding sites and transmembrane proton transport and provide powerful tools to investigate their coupling mechanism.

Identification of regulated proteins by resveratrol in glutamate-induced cortical injury of newborn rats

Glutamate induces neuronal damage by generating oxidative stress and neurotoxicities. The neurological damage caused by glutamate is more severe during brain development in newborns than in adults. Resveratrol is naturally present in a variety of fruits and medicinal plants and exerts a neuroprotective effect against brain damage. The goal of this study was to evaluate the neuroprotective effects of resveratrol and to identify changed proteins in response to resveratrol treatment during glutamate-induced neonatal cortical damage. Sprague-Dawley rat pups (7 days old) were randomly divided into vehicle, resveratrol, glutamate, and glutamate and resveratrol groups. The animals were intraperitoneally injected with glutamate (10 mg/kg) and/or resveratrol (20 mg/kg) and their brain tissue was collected 4 hr after drug administration. Glutamate exposure caused severe histopathological changes, while resveratrol attenuated this damage. We identified regulated proteins by resveratrol in glutamate-induced cortical damaged tissue using two-dimensional gel electrophoresis and mass spectrometry. Among identified proteins, we focused on eukaryotic initiation factor 4A2, γ-enolase, protein phosphatase 2A subunit B, and isocitrate dehydrogenase. These proteins decreased in the glutamate-treated group, whereas the combination treatment of glutamate and resveratrol attenuated these protein reductions. These proteins are anti-oxidant proteins and anti-apoptotic proteins. These results suggest that glutamate induces brain cortical damage in newborns; resveratrol exerts a neuroprotective effect by controlling expression of various proteins with anti-oxidant and anti-apoptotic functions.

The Menadione-Mediated WST1 Reduction by Cultured Astrocytes Depends on NQO1 Activity and Cytosolic Glucose Metabolism

The reduction of water-soluble tetrazolium salts (WSTs) is frequently used to determine the metabolic integrity and the viability of cultured cells. Recently, we have reported that the electron cycler menadione can efficiently connect intracellular oxidation reactions in cultured astrocytes with the extracellular reduction of WST1 and that this menadione cycling reaction involves an enzyme. The enzymatic reaction involved in the menadione-dependent WST1 reduction was found strongly enriched in the cytosolic fraction of cultured astrocytes and is able to efficiently use both NADH and NADPH as electron donors. In addition, the reaction was highly sensitive towards dicoumarol with Kic values in the low nanomolar range, suggesting that the NAD(P)H:quinone oxidoreductase 1 (NQO1) catalyzes the menadione-dependent WST1 reduction in astrocytes. Also, in intact astrocytes, dicoumarol inhibited the menadione-dependent WST1 reduction in a concentration-dependent manner with half-maximal inhibition observed at around 50 nM. Moreover, the menadione-dependent WST1 reduction by viable astrocytes was strongly affected by the availability of glucose. In the absence of glucose only residual WST1 reduction was observed, while a concentration-dependent increase in WST1 reduction was found during a 30 min incubation with maximal WST1 reduction already determined in the presence of 0.5 mM glucose. Mannose could fully replace glucose as substrate for astrocytic WST1 reduction, while other hexoses, lactate and the mitochondrial substrate β-hydroxybutyrate failed to provide electrons for the cell-dependent WST1 reduction. These results demonstrate that the menadione-mediated WST1 reduction involves cytosolic NQO1 activity and that this process is strongly affected by the availability of glucose as metabolic substrate.

Mitochondrial Metabolism in Astrocytes Regulates Brain Bioenergetics, Neurotransmission and Redox Balance

In the brain, mitochondrial metabolism has been largely associated with energy production, and its dysfunction is linked to neuronal cell loss. However, the functional role of mitochondria in glial cells has been poorly studied. Recent reports have demonstrated unequivocally that astrocytes do not require mitochondria to meet their bioenergetics demands. Then, the question remaining is, what is the functional role of mitochondria in astrocytes? In this work, we review current evidence demonstrating that mitochondrial central carbon metabolism in astrocytes regulates overall brain bioenergetics, neurotransmitter homeostasis and redox balance. Emphasis is placed in detailing carbon source utilization (glucose and fatty acids), anaplerotic inputs and cataplerotic outputs, as well as carbon shuttles to neurons, which highlight the metabolic specialization of astrocytic mitochondria and its relevance to brain function.

2-Hydroxyglutarate in Cancer Cells

Significance: Cancer cells are stabilized in an undifferentiated state similar to stem cells. This leads to profound modifications of their metabolism, which further modifies their genetics and epigenetics as malignancy progresses. Specific metabolites and enzymes may serve as clinical markers of cancer progression. Recent Advances: Both 2-hydroxyglutarate (2HG) enantiomers are associated with reprogrammed metabolism, in grade III/IV glioma, glioblastoma, and acute myeloid leukemia cells, and numerous other cancer types, while acting also in the cross talk of tumors with immune cells. 2HG contributes to specific alternations in cancer metabolism and developed oxidative stress, while also inducing decisions on the differentiation of naive T lymphocytes, and serves as a signal messenger in immune cells. Moreover, 2HG inhibits chromatin-modifying enzymes, namely 2-oxoglutarate-dependent dioxygenases, and interferes with hypoxia-inducible factor (HIF) transcriptome reprogramming and mammalian target of rapamycin (mTOR) pathway, thus dysregulating gene expression and further promoting cancerogenesis. Critical Issues: Typically, heterozygous mutations within the active sites of isocitrate dehydrogenase isoform 1 (IDH1)^R132H and mitochondrial isocitrate dehydrogenase isoform 2 (IDH2)^R140Q provide cells with millimolar r-2-hydroxyglutarate (r-2HG) concentrations, whereas side activities of lactate and malate dehydrogenase form submillimolar s-2-hydroxyglutarate (s-2HG). However, even wild-type IDH1 and IDH2, notably under shifts toward reductive carboxylation glutaminolysis or changes in other enzymes, lead to "intermediate" 0.01-0.1 mM 2HG levels, for example, in breast carcinoma compared with 10^-8M in noncancer cells. Future Directions: Uncovering further molecular metabolism details specific for given cancer cell types and sequence-specific epigenetic alternations will lead to the design of diagnostic approaches, not only for predicting patients' prognosis or uncovering metastases and tumor remissions but also for early diagnostics.

Identification of proteins differentially expressed by glutamate treatment in cerebral cortex of neonatal rats

Glutamate leads to neuronal cell damage by generating neurotoxicity during brain development. The objective of this study is to identify proteins that differently expressed by glutamate treatment in neonatal cerebral cortex. Sprague-Dawley rat pups (post-natal day 7) were intraperitoneally injected with vehicle or glutamate (10 mg/kg). Brain tissues were isolated 4 h after drug treatment and fixed for morphological study. Moreover, cerebral cortices were collected for protein study. Two-dimensional gel electrophoresis and mass spectrometry were carried out to identify specific proteins. We observed severe histopathological changes in glutamate-exposed cerebral cortex. We identified various proteins that differentially expressed by glutamate exposure. Identified proteins were thioredoxin, peroxiredoxin 5, ubiquitin carboxy-terminal hydrolase L1, proteasome subunit alpha proteins, isocitrate dehydrogenase, and heat shock protein 60. Heat shock protein 60 was increased in glutamate exposed condition. However, other proteins were decreased in glutamate-treated animals. These proteins are related to anti-oxidant, protein degradation, metabolism, signal transduction, and anti-apoptotic function. Thus, our findings can suggest that glutamate leads to neonatal cerebral cortex damage by regulation of specific proteins that mediated with various functions.

Consequences of a Metabolic Glucose-Depletion on the Survival and the Metabolism of Cultured Rat Astrocytes

Brain astrocytes are considered to be highly glycolytic, but these cells also produce ATP via mitochondrial oxidative phosphorylation. To investigate how a metabolic depletion of glucose will affect the metabolism of astrocytes, we applied glucose at an initial concentration of 2 mM to cultured primary astrocytes and monitored the cell viability and various metabolic parameters during an incubation for up to 2 weeks. Already within 2 days of incubation the cells had completely consumed the applied glucose and lactate had accumulated in the medium to a concentration of around 3 mM. During the subsequent 10 days of incubation, the cell viability was not compromised while the extracellular lactate concentration declined to values of around 0.2 mM, before the cell viability was compromised. Application of known inhibitors of mitochondrial metabolism strongly accelerated glucose consumption and initial lactate production, while the lactate consumption was completely (antimycin A or 8-hydroxy efavirenz) and partially (efavirenz, metformin or tyrphostin 23) inhibited which caused rapid and delayed cell toxicity, respectively. The switch from glycolytic glucose metabolism to mitochondrial metabolism during the incubation was neither accompanied by alterations in the specific cytosolic lactate dehydrogenase activity or in the WST1 reduction capacity nor in the mitochondrial citrate synthase activity, but a cellular redistribution of mitochondria from a perinuclear to a more spread cytoplasmic localization was observed during the lactate consumption phase. These results demonstrate that cultured astrocytes survive a metabolism-induced glucose depletion very well by consuming lactate as fuel for mitochondrial ATP generation.

Control of the NADPH supply and GSH recycling for oxidative stress management in hepatoma and liver mitochondria

To unveil what controls mitochondrial ROS detoxification, the NADPH supply and GSH/GSSG recycling for oxidative stress management were analyzed in cancer and non-cancer mitochondria. Therefore, proteomic and kinetomic analyses were carried out of the mitochondrial (i) NADPH producing and (ii) GSH/GSSG recycling enzymes associated to oxidative stress management. The protein contents of the eight enzymes analyzed were similar or even higher in AS-30D rat hepatoma mitochondria (HepM) than in rat liver (RLM) and rat heart (RHM) mitochondria, suggesting that the NADPH/GSH/ROS pathway was fully functional in cancer mitochondria. The Vmax values of IDH-2 were much greater than those of GDH, TH and ME, suggesting that IDH-2 is the predominant NADPH producer in the three mitochondrial types; in fact, the GDH reverse reaction was favored. The Vmax values of GR and GPx were lower in HepM than in RLM, suggesting that the oxidative stress management is compromised in cancer mitochondria. The Km values of IDH-2, GR and GPx were all similar among the different mitochondrial types. Kinetic modeling revealed that the oxidative stress management was mainly controlled by GR, GPx and IDH. Modeling and experimentation also revealed that, due to their higher IDH-2 activity and lower GPx activity presumably by acetylation, HepM (i) showed higher steady-state NADPH levels; (ii) required greater peroxide concentrations to achieve reliable steady-state fluxes and metabolite concentration; and (iii) endured higher peroxide concentrations without collapsing their GSH/GSSG ratios. Then, to specifically prompt lower GSH/GSSG ratios under oxidative stress thus compromising cancer mitochondria functioning, GPx should be re-activated.

Comparison of primary and secondary rat astrocyte cultures regarding glucose and glutathione metabolism and the accumulation of iron oxide nanoparticles

Astrocyte-rich primary cultures (APCs) are frequently used as a model system for the investigation of properties of brain astrocytes. However, as APCs contain a substantial number of microglial and oligodendroglial cells, biochemical parameters determined for such cultures may at least in part reflect also the presence of the contaminating cell types. To lower the potential contributions of microglial and oligodendroglial cells on properties of the astrocytes in APCs we prepared rat astrocyte-rich secondary cultures (ASCs) by subculturing of APCs and compared these ASCs with APCs regarding basal metabolic parameters, specific enzyme activities and the accumulation of iron oxide nanoparticles. Immunocytochemical characterization revealed that ASCs contained only minute amounts of microglial and oligodendroglial cells. ASCs and APCs did not significantly differ in their specific glucose consumption and lactate production rates, in their specific iron and glutathione contents, in their specific activities of various enzymes involved in glucose and glutathione metabolism nor in their accumulation of iron oxide nanoparticles. Thus, the absence or presence of some contaminating microglial and oligodendroglial cells appears not to substantially modulate the investigated metabolic parameters of astrocyte cultures.

8-Hydroxy-efavirenz, the primary metabolite of the antiretroviral drug Efavirenz, stimulates the glycolytic flux in cultured rat astrocytes

In active antiretroviral therapy antiretroviral drugs are employed for the restoration of a functional immune system in patients suffering from the acquired immunodeficiency syndrome. However, potential adverse effects of such compounds to brain cells are discussed in connection with the development of neurocognitive impairments in patients. To investigate potential effects of antiretroviral drugs on cell viability and the glycolytic flux of brain cells, astrocyte-rich primary cultures were exposed to various antiretroviral compounds, including the non-nucleoside reverse transcriptase inhibitor efavirenz. In a concentration of 10 μM, neither efavirenz nor any of the other investigated antiretroviral compounds acutely compromised the cell viability nor altered glucose consumption or lactate production. In contrast, the primary metabolite of efavirenz, 8-hydroxy-efavirenz, stimulated the glycolytic flux in viable astrocytes in a time- and concentration-dependent manner with half-maximal and maximal effects at concentrations of 5 and 10 μM, respectively. The stimulation of glycolytic flux by 8-hydroxy-efavirenz was not additive to that obtained for astrocytes that were treated with the respiratory chain inhibitor rotenone and was abolished by removal of extracellular 8-hydroxy-efavirenz. In a concentration of 10 μM, 8-hydroxy-efavirenz and efavirenz did not affect mitochondrial respiration, while both compounds lowered in a concentration of 60 μM significantly the oxygen consumption by mitochondria that had been isolated form cultured astrocytes, suggesting that the stimulation of glycolytic flux by 8-hydroxy-efavrienz is not caused by direct inhibition of respiration. The observed alteration of astrocytic glucose metabolism by 8-hydroxy-efavirenz could contribute to the adverse neurological side effects reported for patients that are chronically treated with efavirenz-containing medications.

Exploiting metabolic differences in glioma therapy

Brain function depends upon complex metabolic interactions amongst only a few different cell types, with astrocytes providing critical support for neurons. Astrocyte functions include buffering the extracellular space, providing substrates to neurons, interchanging glutamate and glutamine for synaptic transmission with neurons, and facilitating access to blood vessels. Whereas neurons possess highly oxidative metabolism and easily succumb to ischemia, astrocytes rely more on glycolysis and metabolism associated with synthesis of critical intermediates, hence are less susceptible to lack of oxygen. Astrocytoma and higher grade glioma cells demonstrate both basic metabolic mechanisms of astrocytes as well as tumors in general, e.g. they show a high glycolytic rate, lactate extrusion, ability to proliferate even under hypoxia, and opportunistic use of mechanisms to enhance metabolism and blood vessel generation, and suppression of cell death pathways. There may be differences in metabolism between neurons, normal astrocytes and astrocytoma cells, providing therapeutic opportunities against astrocytomas, including a wide range of enzyme and transporter differences, regulation of hypoxia-inducible factor (HIF), glutamate uptake transporters and glutamine utilization, differential sensitivities of monocarboxylate transporters, presence of glycogen, high interlinking with gap junctions, use of NADPH for lipid synthesis, utilizing differential regulation of synthetic enzymes (e.g. isocitrate dehydrogenase, pyruvate carboxylase, pyruvate dehydrogenase, lactate dehydrogenase, malate-aspartate NADH shuttle) and different glucose uptake mechanisms. These unique metabolic susceptibilities may augment conventional therapeutic attacks based on cell division differences and surface receptors alone, and are starting to be implemented in clinical trials.

The Role of Mitochondrial NADPH-Dependent Isocitrate Dehydrogenase in Cancer Cells

Isocitrate dehydrogenase 2 (IDH2) is located in the mitochondrial matrix. IDH2 acts in the forward Krebs cycle as an NADP⁺-consuming enzyme, providing NADPH for maintenance of the reduced glutathione and peroxiredoxin systems and for self-maintenance by reactivation of cystine-inactivated IDH2 by glutaredoxin 2. In highly respiring cells, the resulting NAD⁺ accumulation then induces sirtuin-3-mediated activating IDH2 deacetylation, thus increasing its protective function. Reductive carboxylation of 2-oxoglutarate by IDH2 (in the reverse Krebs cycle direction), which consumes NADPH, may follow glutaminolysis of glutamine to 2-oxoglutarate in cancer cells. When the reverse aconitase reaction and citrate efflux are added, this overall “anoxic” glutaminolysis mode may help highly malignant tumors survive aglycemia during hypoxia. Intermittent glycolysis would hypothetically be required to provide ATP. When oxidative phosphorylation is dormant, this mode causes substantial oxidative stress. Arg172 mutants of human IDH2—frequently found with similar mutants of cytosolic IDH1 in grade 2 and 3 gliomas, secondary glioblastomas, and acute myeloid leukemia—catalyze reductive carboxylation of 2-oxoglutarate and reduction to D-2-hydroxyglutarate, which strengthens the neoplastic phenotype by competitive inhibition of histone demethylation and 5-methylcytosine hydroxylation, leading to genome-wide histone and DNA methylation alternations. D-2-hydroxyglutarate also interferes with proline hydroxylation and thus may stabilize hypoxia-induced factor α.

Neuronal-astrocyte metabolic interactions: understanding the transition into abnormal astrocytoma metabolism

Brain function depends on complex metabolic interactions among only a few different cell types, with astrocytes providing critical support for neurons. Astrocyte functions include buffering the extracellular space, providing substrates to neurons, interchanging glutamate and glutamine for synaptic transmission with neurons, and facilitating access to blood vessels. Whereas neurons possess highly oxidative metabolism and easily succumb to ischemia, astrocytes rely more on glycolytic metabolism and hence are less susceptible tolack of oxygen. Astrocytoma cells seem to retain basic metabolic mechanisms of astrocytes; for example, they show a high glycolytic rate, lactate extrusion, ability to flourish under hypoxia, and opportunistic use of mechanisms to enhance cell division and maintain growth. Differences in metabolism between neurons and astrocytes may also extend to astrocytoma cells, providing therapeutic opportunities against astrocytomas, including sensitivity to acetate, a high rate of glycolysis and lactate extrusion, glutamate uptake transporters, differential sensitivities of monocarboxylate transporters, presence of glycogen, high interlinking with gap junctions, use of nicotinamide adenine dinucleotide phosphate for lipid synthesis, using different isoforms of synthetic enzymes (e.g. isocitrate dehydrogenase, pyruvate carboxylase, pyruvate kinase, lactate dehydrogenase), and different glucose uptake mechanisms. These unique metabolic susceptibilities may augment conventional therapeutic attacks based on cell division differences and surface receptors alone.

Activation of neuronal nitric oxide synthase in cerebellum of chronic hepatic encephalopathy rats is associated with up-regulation of NADPH-producing pathway

Cerebellum-associated functions get affected during mild hepatic encephalopathy (MHE) in patients with chronic liver failure (CLF). Involvement of nitrosative and antioxidant factors in the pathogenesis of chronic hepatic encephalopathy is an evolving concept and needs to be defined in a true CLF animal model. This article describes profiles of NADPH-dependent neuronal nitric oxide synthase (nNOS) and those of glutathione peroxidase and glutathione reductase (GR) vis-a-vis regulation of NADPH-producing pathway in the cerebellum of CLF rats induced by administration of thioacetamide (100 mg kg⁻¹ b.w., i.p.) up to 10 days and confirming MHE on Morris water maze tests. Significant increases in the expression of nNOS protein and nitric oxide (NOx) level coincided with a similar increment in NADPH-diaphorase activity in the cerebellum of CLF rats. Glutathione peroxidase and GR utilize NADPH to regenerate reduced glutathione (GSH) in the cells. Both these enzymes and GSH level were found to be static and thus suggested efficient turnover of GSH in the cerebellum of MHE rats. Relative levels of glucose-6-phosphate dehydrogenase (G6PD) vs. phosphofructokinase 2 (PFK2) determine the rate of pentose phosphate pathway (PPP) responsible to synthesize NADPH. The cerebellum of CLF rats showed overactivation of G6PD with a significant decline in the expression of PFK2 and thus suggested activation of PPP in the cerebellum during MHE. It is concluded that concordant activations of PPP and nNOS in cerebellum of MHE rats could be associated with the implication of NOx in the pathogenesis of MHE.

Two hypomorphic alleles of mouse Ass1 as a new animal model of citrullinemia type I and other hyperammonemic syndromes

Citrullinemia type I (CTLN1, OMIM# 215700) is an inherited urea cycle disorder that is caused by an argininosuccinate synthetase (ASS) enzyme deficiency. In this report, we describe two spontaneous hypomorphic alleles of the mouse Ass1 gene that serve as an animal model of CTLN1. These two independent mouse mutant alleles, also described in patients affected with CTLN1, interact to produce a range of phenotypes. While some mutant mice died within the first week after birth, others survived but showed severe retardation during postnatal development as well as alopecia, lethargy, and ataxia. Notable pathological findings were similar to findings in human CTLN1 patients and included citrullinemia and hyperammonemia along with delayed cerebellar development, epidermal hyperkeratosis, and follicular dystrophy. Standard treatments for CTLN1 were effective in rescuing the phenotype of these mutant mice. Based on our studies, we propose that defective cerebellar granule cell migration secondary to disorganization of Bergmann glial cell fibers cause cerebellar developmental delay in the hyperammonemic and citrullinemic brain, pointing to a possible role for nitric oxide in these processes. These mouse mutations constitute a suitable model for both mechanistic and preclinical studies of CTLN1 and other hyperammonemic encephalopathies and, at the same time, underscore the importance of complementing knockout mutations with hypomorphic mutations for the generation of animal models of human genetic diseases.

Vitamin B3, the nicotinamide adenine dinucleotides and aging

Organism aging is a process of time and maturation culminating in senescence and death. The molecular details that define and determine aging have been intensely investigated. It has become appreciated that the process is partly an accumulation of random yet inevitable changes, but it can be strongly affected by genes that alter lifespan. In this review, we consider how NAD(+) metabolism plays important roles in the random patterns of aging, and also in the more programmatic aspects. The derivatives of NAD(+), such as reduced and oxidized forms of NAD(P)(+), play important roles in maintaining and regulating cellular redox state, Ca(2+) stores, DNA damage and repair, stress responses, cell cycle timing and lipid and energy metabolism. NAD(+) is also a substrate for signaling enzymes like the sirtuins and poly-ADP-ribosylpolymerases, members of a broad family of protein deacetylases and ADP-ribosyltransferases that regulate fundamental cellular processes such as transcription, recombination, cell division, proliferation, genome maintenance, apoptosis, stress resistance and senescence. NAD(+)-dependent enzymes are increasingly appreciated to regulate the timing of changes that lead to aging phenotypes. We consider how metabolism, specifically connected with Vitamin B3 and the nicotinamide adenine dinucleotides and their derivatives, occupies a central place in the aging processes of mammals.

The cytosolic redox state of astrocytes: Maintenance, regulation and functional implications for metabolite trafficking

Astrocytes have important functions in the metabolism of the brain. These cells provide neurons with metabolic substrates for energy production as well as with precursors for neurotransmitter and glutathione synthesis. Both the metabolism of astrocytes and the subsequent supply of metabolites from astrocytes to neurons are strongly affected by alterations in the cellular redox state. The cytosolic redox state of astrocytes depends predominantly on the ratios of the oxidised and reduced partners of the redox pairs NADH/NAD(+), NADPH/NADP(+) and GSH/GSSG. The NADH/NAD(+) pair is predominately in the oxidised state to accept electrons that are produced during glycolysis. In contrast, the redox pairs NADPH/NADP(+) and GSH/GSSG are biased towards the reduced state under unstressed conditions to provide electrons for reductive biosyntheses and antioxidative processes, respectively. In this review article we describe the metabolic processes that maintain the redox pairs in their desired redox states in the cytosol of astrocytes and discuss the consequences of alterations of the normal redox state for the regulation of cellular processes and for metabolite trafficking from astrocytes to neurons.

Maillard products as biomarkers in cancer

Because tumors exert increased glycolysis rates, a high intracellular carbonyl stress with the formation of Maillard products may evolve. Therefore, we studied the presence of N epsilon-(carboxymethyl)lysine (CML) modification in breast cancer tissues from 20 patients and found significant cytoplasmatic staining in tumor cells that was independent of the tumor stage, tumor type, and microanatomic localization. Studying breast cancer cell lines, we also found strong cytoplasmatic CML staining that was again independent of their invasive or metastatic behavior. Our results reveal that tumor cells show a strong cytoplasmatic immunoreactivity to CML without evident association with breast carcinoma type, differentiation, tumor stage, or intratumoral localization. We conclude that CML formation is a general tumor cell-associated process.

NAD+/NADH and NADP+/NADPH in cellular functions and cell death: regulation and biological consequences

Accumulating evidence has suggested that NAD (including NAD+ and NADH) and NADP (including NADP+ and NADPH) could belong to the fundamental common mediators of various biological processes, including energy metabolism, mitochondrial functions, calcium homeostasis, antioxidation/generation of oxidative stress, gene expression, immunological functions, aging, and cell death: First, it is established that NAD mediates energy metabolism and mitochondrial functions; second, NADPH is a key component in cellular antioxidation systems; and NADH-dependent reactive oxygen species (ROS) generation from mitochondria and NADPH oxidase-dependent ROS generation are two critical mechanisms of ROS generation; third, cyclic ADP-ribose and several other molecules that are generated from NAD and NADP could mediate calcium homeostasis; fourth, NAD and NADP modulate multiple key factors in cell death, such as mitochondrial permeability transition, energy state, poly(ADP-ribose) polymerase-1, and apoptosis-inducing factor; and fifth, NAD and NADP profoundly affect aging-influencing factors such as oxidative stress and mitochondrial activities, and NAD-dependent sirtuins also mediate the aging process. Moreover, many recent studies have suggested novel paradigms of NAD and NADP metabolism. Future investigation into the metabolism and biological functions of NAD and NADP may expose fundamental properties of life, and suggest new strategies for treating diseases and slowing the aging process.

Reconstruction and flux analysis of coupling between metabolic pathways of astrocytes and neurons: application to cerebral hypoxia

BACKGROUND

It is a daunting task to identify all the metabolic pathways of brain energy metabolism and develop a dynamic simulation environment that will cover a time scale ranging from seconds to hours. To simplify this task and make it more practicable, we undertook stoichiometric modeling of brain energy metabolism with the major aim of including the main interacting pathways in and between astrocytes and neurons.

MODEL

The constructed model includes central metabolism (glycolysis, pentose phosphate pathway, TCA cycle), lipid metabolism, reactive oxygen species (ROS) detoxification, amino acid metabolism (synthesis and catabolism), the well-known glutamate-glutamine cycle, other coupling reactions between astrocytes and neurons, and neurotransmitter metabolism. This is, to our knowledge, the most comprehensive attempt at stoichiometric modeling of brain metabolism to date in terms of its coverage of a wide range of metabolic pathways. We then attempted to model the basal physiological behaviour and hypoxic behaviour of the brain cells where astrocytes and neurons are tightly coupled.

RESULTS

The reconstructed stoichiometric reaction model included 217 reactions (184 internal, 33 exchange) and 216 metabolites (183 internal, 33 external) distributed in and between astrocytes and neurons. Flux balance analysis (FBA) techniques were applied to the reconstructed model to elucidate the underlying cellular principles of neuron-astrocyte coupling. Simulation of resting conditions under the constraints of maximization of glutamate/glutamine/GABA cycle fluxes between the two cell types with subsequent minimization of Euclidean norm of fluxes resulted in a flux distribution in accordance with literature-based findings. As a further validation of our model, the effect of oxygen deprivation (hypoxia) on fluxes was simulated using an FBA-derivative approach, known as minimization of metabolic adjustment (MOMA). The results show the power of the constructed model to simulate disease behaviour on the flux level, and its potential to analyze cellular metabolic behaviour in silico.

CONCLUSION

The predictive power of the constructed model for the key flux distributions, especially central carbon metabolism and glutamate-glutamine cycle fluxes, and its application to hypoxia is promising. The resultant acceptable predictions strengthen the power of such stoichiometric models in the analysis of mammalian cell metabolism.

Metabolic and antioxidant system alterations in an astrocytoma cell line challenged with mitochondrial DNA deletion

Oxidative stress can induce mitochondrial dysfunction, mitochondrial DNA (mtDNA) depletion, and neurodegeneration, although the underlying mechanisms are poorly understood. The major mitochondrial antioxidant system that protects cells consists of manganese superoxide dismutase (MnSOD), glutathione peroxidase (GPx) and glutathione (GSH). To investigate the putative adaptive changes in antioxidant enzyme protein expression and targeting to mitochondria as mtDNA depletion occurs, we progressively depleted U87 astrocytoma cells of mtDNA by chronic treatment with ethidium bromide (EB, 50 ng/ml). Cellular MnSOD protein expression was markedly increased in a time-related manner while that of GPx showed time-related decreases. The mtDNA depletion also altered targeting or subcellular distribution of GPx, suggesting the importance of intact mtDNA in mitochondrial genome-nuclear genome signaling/communication. Cellular NADP(+)-ICDH activity also showed marked, time-related increases while their GSH content decreased. Thus, our findings suggest that interventions to elevate MnSOD, GPx, NADP(+)-ICDH, and GSH levels may protect brain cells from oxidative stress.

Electrophysiological study of infant and adult rats under acute intoxication with fluoroacetamide

A study was conducted of acute intoxication of infant and adult Wistar rats with fluoroacetamide (FAA), an inhibitor of oxidative metabolism. FAA was administered orally to adult rats at 1/2 LD(50) and subcutaneously to infant rats at LD(100) or 1/10 LD(50). Electrocardiogram (ECG), respiration and motor activity were registered for 7 days. Clinical analysis of ECG and the heart rate variability (HRV) was carried out to assess the state of the vegetative nervous system. In adult rats, FAA caused marked disturbances in the activity of cardiovascular and respiratory systems, including the development of a potentially lethal acute cor pulmonale. Conversely, there were no significant changes of cardiac function and respiration in infant rats; they died because of extreme emaciation accompanied by retardation of development. In adult rats, bursts of associated cardiac and respiratory tachyarrhythmia, as well as regular high amplitude spasmodic sighs having a deca-second rhythm were observed. In both infant and adult rats, FAA caused short-term enhancement of humoral (metabolic) and sympathetic activities, followed by a gradual and stable predominance of parasympathetic influence on HRV. Under conditions of FAA inhibition of the tricarboxylic acid cycle, the observed physiological reactions may be explained by activation of alternative metabolic pathways. This is also supported by a lack of ontogenetically caused inhibition of spontaneous motor activity in infant rats poisoned with FAA, which highlights the significance of the alternative metabolic pathways for implementation of deca-second and minute rhythms and a lack of a rigid dependence of these rhythms upon activity of neuronal networks.

Characterization of mature rat oligodendrocytes: a proteomic approach

Oligodendrocytes are glial cells responsible for the synthesis and maintenance of myelin in the central nervous system (CNS). Oligodendrocytes are vulnerable to damage occurring in a variety of neurological diseases. Understanding oligodendrocyte biology is crucial for the dissemination of de- and remyelination mechanisms. The goal of the present study is the construction of a protein database of mature rat oligodendrocytes. Post-mitotic oligodendrocytes were isolated from mature Wistar rats and subjected to immunocytochemistry. Proteins were extracted and analyzed by means of two-dimensional gel electrophoresis and two-dimensional liquid chromatography, both coupled to mass spectrometry. The combination of the gel-based and gel-free approach resulted in confident identification of a total of 200 proteins. A minority of proteins were identified in both proteomic strategies. The identified proteins represent a variety of functional groups, including novel oligodendrocyte proteins. The results of this study emphasize the power of the applied proteomic strategy to study known or to reveal new proteins and to investigate their regulation in oligodendrocytes in different disease models.

A pyruvate cycling pathway involving cytosolic NADP-dependent isocitrate dehydrogenase regulates glucose-stimulated insulin secretion

Glucose-stimulated insulin secretion (GSIS) from pancreatic islet beta-cells is central to control of mammalian fuel homeostasis. Glucose metabolism mediates GSIS in part via ATP-regulated K+ (KATP) channels, but multiple lines of evidence suggest participation of other signals. Here we investigated the role of cytosolic NADP-dependent isocitrate dehydrogenase (ICDc) in control of GSIS in beta-cells. Delivery of small interfering RNAs specific for ICDc caused impairment of GSIS in two independent robustly glucose-responsive rat insulinoma (INS-1-derived) cell lines and in primary rat islets. Suppression of ICDc also attenuated the glucose-induced increments in pyruvate cycling activity and in NADPH levels, a predicted by-product of pyruvate cycling pathways, as well as the total cellular NADP(H) content. Metabolic profiling of eight organic acids in cell extracts revealed that suppression of ICDc caused increases in lactate production in both INS-1-derived cell lines and primary islets, consistent with the attenuation of pyruvate cycling, with no significant changes in other intermediates. Based on these studies, we propose that a pyruvate cycling pathway involving ICDc plays an important role in control of GSIS.

Oxidative and antioxidative potential of brain microglial cells

Microglial cells are the resident immune cells of the central nervous system. These cells defend the central nervous system against invading microorganisms and clear the debris from damaged cells. Upon activation, microglial cells produce a large number of neuroactive substances that include cytokines, proteases, and prostanoids. In addition, activated microglial cells release radicals, such as superoxide and nitric oxide, that are products of the enzymes NADPH oxidase and inducible nitric oxide synthase, respectively. Microglia-derived radicals, as well as their reactive reaction products hydrogen peroxide and peroxynitrite, have the potential to harm cells and have been implicated in contributing to oxidative damage and neuronal cell death in neurological diseases. For self-protection against oxidative damage, microglial cells are equipped with efficient antioxidative defense mechanisms. These cells contain glutathione in high concentrations, substantial activities of the antioxidative enzymes superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase, as well as NADPH-regenerating enzymes. Their good antioxidative potential protects microglial cells against oxidative damage that could impair important functions of these cells in defense and repair of the brain.

Glucose-6-phosphate dehydrogenase and NADPH-consuming enzymes in the rat olfactory bulb

The resistance to oxidative stress is a multifactorial reaction involving the clustering of transcriptionally regulated genes. Because glucose-6-phosphate dehydrogenase (G6PD), the principal enzyme responsible for reducing power, is highly expressed in the olfactory bulb (OB), it is of interest to verify whether other enzymes utilizing NADPH are also highly expressed. The level and localization of G6PD- and NADPH-consuming enzymes, such as NADPH-cytochrome P450 oxidoreductase (P450R), glutathione reductase (GR), and NADPH-diaphorase (NADPH-d), were analyzed in the rat olfactory bulb (OB) by quantitative histochemistry and immunohistochemistry. The highest concentration of G6PD, P450R, and GR was observed in the olfactory nerve layer (ONL), suggesting a correlation in the expression of these enzymes at the gene level. Correlation in staining intensity between G6PD and NADPH-d activities occurred only in part of the ONL, some glomeruli, and scattered periglomerular cells. This peculiar distribution of NADPH-d could reflect a spatial patterning of the nose to bulb projections. Taken together, these results indicate that G6PD expression in the ONL could be related to the importance of generating a substantial supply of NADPH to sustain the detoxifying systems represented by GR and P450R reactions and, only in discrete zones, by NADPH-d activity.

Expression analysis of SIX3 and SIX6 in human tissues reveals differences in expression and a novel correlation between the expression of SIX3 and the genes encoding isocitrate dehyhrogenase and cadherin 18

SIX3 and SIX6 are transcription factors expressed during early stages of eye development. Limited expression data for SIX3 and SIX6 are available in the literature but, to date, there are no reports of the relative levels of expression of these genes throughout the human body and in adult tissues in particular. In this paper, we report extensive real-time quantitative PCR analyses of SIX3 and SIX6 expression in many different tissues of the adult human body, including ocular tissues, and a comparison of expression data with that of many other genes to identify similarity in expression. Using this powerful technique, we have detected a novel statistical correlation between the spatial distribution and the quantitative expression of SIX3 and 5 other transcripts including IDH1, the gene encoding the NADP(+)-dependent enzyme isocitrate dehydrogenase, and cadherin 18, type 2 (CDH14). Our data demonstrate that this novel technique can be used to generate hypotheses by comparison of gene expression profiles to identify possible interactions between genes or gene products.

NADPH-consuming enzymes correlate with glucose-6-phosphate dehydrogenase in Purkinje cells: an immunohistochemical and enzyme histochemical study of the rat cerebellar cortex

In cerebellum of the adult rat, glucose-6-phosphate dehydrogenase (G6PD) activity is particularly localized in Purkinje cells, showing lower activity in the molecular and granule cell layers. G6PD is the first and rate-limiting step of the hexose monophosphate shunt (HMS), which has the physiological role of providing NADPH for reductive biosynthesis and detoxifying reactions. In this study, we searched for a possible correlation between G6PD and other NADPH-consuming enzymes, such as NADPH-cytochrome P450 reductase (P450R), glutathione reductase (GR) and NADPH-diaphorase (NADPH-d). This study was performed by means of immunohistochemistry and enzyme histochemistry followed by quantitative densitometric and confocal laser scanning microscopic analyses. Our results demonstrated that G6PD, P450R and GR have a similar distribution pattern characterized by the highest concentration of these enzymes in the somata of Purkinje cells, and by lower concentrations in the molecular and the granule cell layers. Moreover, in Purkinje cells, G6PD colocalized with both P450R and GR. NADPH-d activity showed a different distribution pattern when compared to the other enzymes, revealing the highest activity in the molecular layer and the lowest in Purkinje cells. Our results suggest a coordinated regulative mechanism of G6PD, P450R and GR based on the request of NADPH or on specific transcription factors.

OPA1, associated with autosomal dominant optic atrophy, is widely expressed in the human brain

Autosomal dominant optic atrophy (adOA) is the most prevalent hereditary optic neuropathy with moderate to severe visual field loss and loss of retinal ganglion cells. The majority of cases of adOA is associated with mutations in the OPA1 gene. Northern blot analyses showed that OPA1 is expressed in all tissues examined, with the highest transcript level in the retina and in the brain. Here we addressed the cell type-specific expression of the OPA1 protein in human brain sections using immunohistochemical techniques and Western blotting. We studied OPA1 expression in normal cerebellum and various cerebral CNS tissue specimen of different areas obtained at autopsy from patients with no reported neurological symptoms or diseases and no neuropathological alterations using a polyclonal antibody raised against a C-terminal peptide of OPA1. We found OPA1 expression in somata and dendrites of neurons of the layers II-VI of the motor cortex and frontal brain. In the cerebellar cortex, OPA1 expression was detected in the Purkinje cell layer, in the granule cell layer and in the molecular layer. Double-labeling experiments showed also OPA1 expression in GFAP-positive astrocytes. Since mutations in the OPA1 gene specifically causes optic atrophy and occurrence of cerebral anomalies in adOA patients is not characteristic, this finding may suggest different cellular susceptibility of OPA1 in brain and retinal tissues.

Peroxide detoxification by brain cells

Peroxides are generated continuously in cells that consume oxygen. Among the different peroxides, hydrogen peroxide is the molecule that is formed in highest quantities. In addition, organic hydroperoxides are synthesized as products of cellular metabolism. Generation and disposal of peroxides is a very important process in the human brain, because cells of this organ consume 20% of the oxygen used by the body. To prevent cellular accumulation of peroxides and damage generated by peroxide-derived radicals, brain cells contain efficient antioxidative defense mechanisms that dispose of peroxides and protect against oxidative damage. Cultured brain cells have been used frequently to investigate peroxide metabolism of neural cells. Efficient disposal of exogenous hydrogen peroxide was found for cultured astrocytes, oligodendrocytes, microglial cells, and neurons. Comparison of specific peroxide clearance rates revealed that cultured oligodendrocytes dispose of the peroxide quicker than the other neural cell cultures. Both catalase and the glutathione system contribute to the clearance of hydrogen peroxide by brain cells. For efficient glutathione-dependent reduction of peroxides, neural cells contain glutathione in high concentration and have substantial activity of glutathione peroxidase, glutathione reductase, and enzymes that supply the NADPH required for the glutathione reductase reaction. This article gives an overview on the mechanisms involved in peroxide detoxification in brain cells and on the capacity of the different types of neural cells to dispose of peroxides.

Iron alters glutamate secretion by regulating cytosolic aconitase activity

Glutamate has many important physiological functions, including its role as a neurotransmitter in the retina and the central nervous system. We have made the novel observations that retinal pigment epithelial cells underlying and intimately interacting with the retina secrete glutamate and that this secretion is significantly affected by iron. In addition, iron increased secretion of glutamate in cultured lens and neuronal cells, indicating that this may be a common mechanism for the regulation of glutamate production in many cell types. The activity of the iron-dependent enzyme cytosolic aconitase (c-aconitase) is increased by iron. The conversion of citrate to isocitrate by c-aconitase is the first step in a three-step process leading to glutamate formation. In the present study, iron increased c-aconitase activity, and this increase was associated with an increase in glutamate secretion. Inhibition of c-aconitase by oxalomalate decreased glutamate secretion and completely inhibited the iron-induced increase in glutamate secretion. Derangements in both glutamate secretion and iron metabolism have been noted in neurological diseases and retinal degeneration. Our results are the first to provide a functional link between these two physiologically important substances by demonstrating a significant role for iron in the regulation of glutamate production and secretion in mammalian cells resulting from iron regulation of aconitase activity. Glutamatergic systems are found in many nonneuronal tissues. We provide the first evidence that, in addition to secreting glutamate, retinal pigment epithelial cells express the vesicular glutamate transporter VGLUT1 and that regulated vesicular release of glutamate from these cells can be inhibited by riluzole.

Decreased cardiac mitochondrial NADP+-isocitrate dehydrogenase activity and expression: a marker of oxidative stress in hypertrophy development

Mitochondrial dysfunction subsequent to increased oxidative stress and alterations in energy metabolism is considered to play a role in the development of cardiac hypertrophy and its progression to failure, although the sequence of events remains to be elucidated. This study aimed at characterizing the impact of hypertrophy development on the activity and expression of mitochondrial NADP+-isocitrate dehydrogenase (mNADP+-ICDH), a metabolic enzyme that controls redox and energy status. We expanded on our previous finding of its inactivation through posttranslational modification by the lipid peroxidation product 4-hydroxynonenal (HNE) in 7-wk-old spontaneously hypertensive rat (SHR) hearts before hypertrophy development (Benderdour et al. J Biol Chem 278: 45154-45159, 2003). In this study, we used 7-, 15-, and 30-wk-old SHR and Sprague-Dawley (SD) rats with abdominal aortic coarctation. Compared with age-matched control Wistar-Kyoto (WKY) rats, SHR hearts showed a significant 25% decrease of mNADP+-ICDH activity, which preceded in time 1) the decline in its protein and mRNA expression levels (between 10% and 35%) and 2) the increase in hypertrophy markers. The chronic and persistent loss of mNADP+-ICDH activity in SHR was associated with enhanced tissue accumulation of HNE-mNADP+-ICDH and total HNE-protein adducts at all ages and contrasted with the profile of changes in the activity of other mitochondrial enzymes involved in antioxidant or energy metabolism. Two-way ANOVA of the data also revealed a significant effect of age on most parameters measured in SHR and WKY hearts. The mNADP+-ICDH activity, protein, and mRNA expression were reduced between 25% and 35% in coarctated SD rats and were normalized by treatment of SHR or coarctated SD rats with renin-angiotensin system inhibitors, which prevented or attenuated hypertrophy. Altogether, our data show that cardiac mNADP+-ICDH activity and expression are differentially and sequentially affected in hypertrophy development and, to a lesser extent, with aging. Decreased cardiac mNADP+-ICDH activity, which is attributed at least in part to HNE adduct formation, appears to be a relevant early and persistent marker of mitochondrial oxidative stress-related alterations in hypertrophy development. Potentially, this could also contribute to the aetiology of cardiomyopathy.