Determination of oxidized and reduced nicotinamide adenine dinucleotide in cell monolayers using a single extraction procedure and a spectrophotometric assay

ATP Restoration by ATP-Deprived Cultured Primary Astrocytes

A high cellular concentration of adenosine triphosphate (ATP) is essential to fuel many important functions of brain astrocytes. Although cellular ATP depletion has frequently been reported for astrocytes, little is known on the metabolic pathways that contribute to ATP restoration by ATP-depleted astrocytes. Incubation of cultured primary rat astrocytes in glucose-free buffer for 60 min with the mitochondrial uncoupler BAM15 lowered the cellular ATP content by around 70%, the total amount of adenosine phosphates by around 50% and the adenylate energy charge (AEC) from 0.9 to 0.6. Testing for ATP restoration after removal of the uncoupler revealed that the presence of glucose as exclusive substrate allowed the cells to restore within 6 h around 80% of the initial ATP content, while coapplication of adenosine plus glucose enabled the cells to fully restore their initial ATP content within 60 min. A rapid but incomplete and transient ATP restoration was found for astrocytes that had been exposed to adenosine alone. This restoration was completely prevented by application of the pyruvate uptake inhibitor UK5099, the respiratory chain inhibitor antimycin A or by the continuous presence of BAM15. However, the presence of these compounds strongly accelerated the release of lactate from the cells, suggesting that the ribose moiety of adenosine can serve as substrate to fuel some ATP restoration via mitochondrial metabolism. Finally, the adenosine-accelerated ATP restoration in glucose-fed astrocytes was inhibited by the presence of the adenosine kinase inhibitor ABT-702. These data demonstrate that astrocytes require for a rapid and complete ATP restoration the presence of both glucose as substrate and adenosine as AMP precursor.

A guide to genetically-encoded redox biosensors: State of the art and opportunities

Genetically-encoded redox biosensors have become invaluable tools for monitoring cellular redox processes with high spatiotemporal resolution, coupling the presence of the redox-active analyte with a change in fluorescence signal that can be easily recorded. This review summarizes the available fluorescence recording methods and presents an in-depth classification of the redox biosensors, organized by the analytes they respond to. In addition to the fluorescent protein-based architectures, this review also describes the recent advances on fluorescent, chemigenetic-based redox biosensors and other emerging chemigenetic strategies. This review examines how these biosensors are designed, the biosensors sensing mechanism, and their practical advantages and disadvantages.

Enhanced Expression of Catalase in Mitochondria Modulates NF-κB-Dependent Lung Inflammation through Alteration of Metabolic Activity in Macrophages

NF-κB is a reduction-oxidation-sensitive transcription factor that plays a key role in regulating the immune response. In these studies, we intended to investigate the role of mitochondrial-derived reactive oxygen species in regulating NF-κB activation by studying transgenic mice that overexpress mitochondrial-targeted human catalase (mCAT). We treated wild-type (WT) and mCAT mice with intratracheal instillation of Escherichia coli LPS and found that mCAT mice had exaggerated NF-κB activation in the lungs, increased neutrophilic alveolitis, and greater lung inflammation/injury compared with WT mice. Additional studies using bone marrow chimeras revealed that this hyperinflammatory phenotype was mediated by immune/inflammatory cells. Mechanistic studies using bone marrow-derived macrophages (BMDMs) showed that LPS treatment induced a sustained increase in NF-κB activation and expression of NF-κB-dependent inflammatory mediators in mCAT BMDMs compared with WT BMDMs. Further investigations showed that cytoplasmic, but not mitochondrial, hydrogen peroxide levels were reduced in LPS-treated mCAT BMDMs. However, mCAT macrophages exhibited increased glycolytic and oxidative metabolism, coupled with increased ATP production and an increased intracellular NADH/NAD+ ratio compared with BMDMs from WT mice. Treatment of BMDMs with lactate increased the intracellular NADH/NAD+ ratio and upregulated NF-κB activation after LPS treatment, whereas treatment with a potent inhibitor of the mitochondrial pyruvate carrier (UK5099) decreased the NADH/NAD+ ratio and reduced NF-κB activation. Taken together, these findings point to an increased availability of reducing equivalents in the form of NADH as an important mechanism by which metabolic activity modulates inflammatory signaling through the NF-κB pathway.

Real-time monitoring of NADPH levels in living mammalian cells using fluorescence-enhancing protein bound to NADPHs

Nicotinamide adenine nucleotide phosphate (NADPH) has been known to be involved in the multiple pathways of cell metabolism. However, conventional quantification assays for NADPH have required breaking down the cell membranes of around one million cells per assay, and monitoring NADPH flux in living cells has been limited by a few available tools. Here, we visualized NADPH levels in human cervical cancer cells HeLa using metagenome-derived blue fluorescent protein (mBFP), which specifically binds to NADPH and enhances the intrinsic fluorescence of NADPH up to 10-fold when imaged by two-photon microscopy to reduce photodamage. Adding an oxidizing agent such as diamide to HeLa cells that expressed mBFP led to an immediate decrease of intracellular NADPH depending on glucose availability in culture media. Furthermore, inhibiting glucose-6-phosphate dehydrogenase (G6PD) in the pentose phosphate pathway with dehydroandrosterone (DHEA) and knockdown of G6PD transcripts gradually decreased NADPH when diamide was added to living cells. These results demonstrate that introducing a bacterial mBFP gene into mammalian cells is a straightforward approach to monitoring intracellular NADPH flux in real time at the single-cell level. Moreover, this strategy can be expanded to tracking the spatio-temporal changes in NADPH even in single-cell organelles such as mitochondria and chloroplasts, which will allow us to more precisely assess the efficacy of biochemically or biophysically metabolic perturbations in animal and plant cells.

Genetically encoded probes for NAD+/NADH monitoring

NAD⁺ and NADH participate in many metabolic reactions. The NAD⁺/NADH ratio is an important parameter reflecting the general metabolic and redox state of different types of cells. For a long time, in situ and in vivo NAD⁺/NADH monitoring has been hampered by the lack of suitable tools. The recent development of genetically encoded indicators based on fluorescent proteins linked to specific nucleotide-binding domains has already helped to address this monitoring problem. In this review, we will focus on four available indicators: Peredox, Frex family probes, RexYFP and SoNar. Each indicator has advantages and limitations. We will also discuss the most important points that should be considered when selecting a suitable indicator for certain experimental conditions.

Influence of Starvation on Respiratory Metabolism and Pyridine Nucleotide Levels in the Marine Dinoflagellate Oxyrrhis marina

Respiratory oxygen consumption rate (RO2) and potential respiration (Φ) has been monitored during a food deprivation period in the heterotrophic dinoflagellate Oxyrrhis marina. Φ was determined by measuring the activity of the enzymes from the electron transport system (ETS), the major contributor to the oxygen consumption in the cells. Additionally, we have quantified for the first time the concentration of pyridine nucleotides in this organism, both in their oxidized (NAD(P)(+)) and reduced forms (NAD(P)H). These molecules are the main electron donors at the beginning of the ETS. We observed a dramatic decrease in RO2 within the first days, whereas Φ steadily, but more gradually declined during the entire experiment. This led to a decrease of the RO2 /Φ with time. The intracellular total pool of NAD and NADP concentration, in turn, dropped exponentially in a manner parallel to the RO2. This strong decrease was mainly driven by a reduction in the concentration of the oxidized forms. The present work constitutes a first step in clarifying the role of intracellular NAD and NADP concentrations and the redox status in the control of in vivo RO2 in marine organisms.

Bioluminescent cell-based NAD(P)/NAD(P)H assays for rapid dinucleotide measurement and inhibitor screening

Abstract The central role of nicotinamide adenine dinucleotides in cellular energy metabolism and signaling makes them important nodes that link the metabolic state of cells with energy homeostasis and gene regulation. In this study, we describe the implementation of cell-based bioluminescence assays for rapid and sensitive measurement of those important redox cofactors. We show that the sensitivity of the assays (limit of detection ∼0.5 nM) enables the selective detection of total amounts of nonphosphorylated or phosphorylated dinucleotides directly in cell lysates. The total amount of NAD+NADH or NADP+NADPH levels can be detected in as low as 300 or 600 cells/well, respectively. The signal remains linear up to 5,000 cells/well with the maximum signal-to-background ratios ranging from 100 to 200 for NAD+NADH and from 50 to 100 for NADP+NADPH detection. The assays are robust (Z' value >0.7) and the inhibitor response curves generated using a known NAD biosynthetic pathway inhibitor FK866 correlate well with the reported data. More importantly, by multiplexing the dinucleotide detection assays with a fluorescent nonmetabolic cell viability assay, we show that dinucleotide levels can be decreased dramatically (>80%) by FK866 treatment before changes in cell viability are detected. The utility of the assays to identify modulators of intracellular nicotinamide adenine dinucleotide levels was further confirmed using an oncology active compound library, where novel dinucleotide regulating compounds were identified. For example, the histone deacetylase inhibitor entinostat was a potent inhibitor of cellular nicotinamide adenine dinucleotides, whereas the selective estrogen receptor modulator raloxifene unexpectedly caused a twofold increase in cellular nicotinamide adenine dinucleotide levels.

Hypoxia-Mediated Increases in L-2-hydroxyglutarate Coordinate the Metabolic Response to Reductive Stress

Metabolic adaptation to hypoxia is critical for survival in metazoan species for which reason they have developed cellular mechanisms for mitigating its adverse consequences. Here, we have identified L-2-hydroxyglutarate (L2HG) as a universal adaptive determinant of the hypoxia response. L2HG is a metabolite of unknown function produced by the reduction of mitochondrial 2-oxoglutarate by malate dehydrogenase. L2HG accumulates in response to increases in 2-oxoglutarate, which occur as a result of tricarboxylic acid cycle dysfunction and increased mitochondrial reducing potential. These changes are closely coupled to cellular redox homeostasis, as increased cellular L2HG inhibits electron transport and glycolysis to offset the adverse consequences of mitochondrial reductive stress induced by hypoxia. Thus, L2HG couples mitochondrial and cytoplasmic energy metabolism in a model of cellular redox regulation.

Stability of energy metabolites-An often overlooked issue in metabolomics studies: A review

Recent advances in analytical chemistry have set the stage for metabolite profiling to help understand complex molecular processes in physiology. Despite ongoing efforts, there are concerns regarding metabolomics workflows, since it has been shown that internal (enzyme activity, blood contamination, and the dynamic nature of metabolite concentrations) as well as external factors (storage, handling, and analysis method) may affect the metabolome profile. Many metabolites are intrinsically instable, particularly some of those associated with central carbon metabolism. While enzymatic conversions have been studied in great detail, nonenzymatic, chemical conversions received comparatively little attention. This review aims to give an in-depth overview of nonenzymatic energy metabolite degradation/interconversion chemistry focusing on a selected range of metabolites. Special attention will be given to qualitative (degradation pathways) as well as quantitative aspects, that may affect the acquisition of accurate data in the context of metabolomics studies. Problems related to the use of isotopically labeled internal standards hindering the quantitative analysis of common metabolites will be presented with an experimental example. Finally, general conclusions and perspectives are given.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Identification of LDH-A as a therapeutic target for cancer cell killing via (i) p53/NAD(H)-dependent and (ii) p53-independent pathways

Most cancer cells use aerobic glycolysis to fuel their growth. The enzyme lactate dehydrogenase-A (LDH-A) is key to cancer's glycolytic phenotype, catalysing the regeneration of nicotinamide adenine dinucleotide (NAD(+)) from reduced nicotinamide adenine dinucleotide (NADH) necessary to sustain glycolysis. As such, LDH-A is a promising target for anticancer therapy. Here we ask if the tumour suppressor p53, a major regulator of cellular metabolism, influences the response of cancer cells to LDH-A suppression. LDH-A knockdown by RNA interference (RNAi) induced cancer cell death in p53 wild-type, mutant and p53-null human cancer cell lines, indicating that endogenous LDH-A promotes cancer cell survival irrespective of cancer cell p53 status. Unexpectedly, however, we uncovered a novel role for p53 in the regulation of cancer cell NAD(+) and its reduced form NADH. Thus, LDH-A silencing by RNAi, or its inhibition using a small-molecule inhibitor, resulted in a p53-dependent increase in the cancer cell ratio of NADH:NAD(+). This effect was specific for p53(+/+) cancer cells and correlated with (i) reduced activity of NAD(+)-dependent deacetylase sirtuin 1 (SIRT1) and (ii) an increase in acetylated p53, a known target of SIRT1 deacetylation activity. In addition, activation of the redox-sensitive anticancer drug EO9 was enhanced selectively in p53(+/+) cancer cells, attributable to increased activity of NAD(P)H-dependent oxidoreductase NQO1 (NAD(P)H quinone oxidoreductase 1). Suppressing LDH-A increased EO9-induced DNA damage in p53(+/+) cancer cells, but importantly had no additive effect in non-cancer cells. Our results identify a unique strategy by which the NADH/NAD(+) cellular redox status can be modulated in a cancer-specific, p53-dependent manner and we show that this can impact upon the activity of important NAD(H)-dependent enzymes. To summarise, this work indicates two distinct mechanisms by which suppressing LDH-A could potentially be used to kill cancer cells selectively, (i) through induction of apoptosis, irrespective of cancer cell p53 status and (ii) as a part of a combinatorial approach with redox-sensitive anticancer drugs via a novel p53/NAD(H)-dependent mechanism.

Chemical and technical challenges in the analysis of central carbon metabolites by liquid-chromatography mass spectrometry

This review deals with chemical and technical challenges in the analysis of small-molecule metabolites involved in central carbon and energy metabolism via liquid-chromatography mass-spectrometry (LC-MS). The covered analytes belong to the prominent pathways in biochemical carbon oxidation such as glycolysis or the tricarboxylic acid cycle and, for the most part, share unfavorable properties such as a high polarity, chemical instability or metal-affinity. The topic is introduced by selected examples on successful applications of metabolomics in the clinic. In the core part of the paper, the structural features of important analyte classes such as nucleotides, coenzyme A thioesters or carboxylic acids are linked to "problematic hotspots" along the analytical chain (sample preparation and-storage, separation and detection). We discuss these hotspots from a chemical point of view, covering issues such as analyte degradation or interactions with metals and other matrix components. Based on this understanding we propose solutions wherever available. A major notion derived from these considerations is that comprehensive carbon metabolomics inevitably requires multiple, complementary analytical approaches covering different chemical classes of metabolites.

Galloflavin suppresses lactate dehydrogenase activity and causes MYC downregulation in Burkitt lymphoma cells through NAD/NADH-dependent inhibition of sirtuin-1

Activation of the myc oncogene in cancer cells upregulates lactate dehydrogenase A (LDH-A) expression, leading to a sustained glycolytic flux that is needed to produce ATP under hypoxic conditions. We studied the effects of galloflavin (GF), a recently identified LDH inhibitor, on myc overexpressing Burkitt lymphoma (BL) cells. Epstein-Barr virus-infected lymphoblasts were used as a non-neoplastic control. Our results showed that myc overactivation induced a two- to seven-fold increase in LDH-A expression in BL cells compared with non-neoplastic lymphoblasts; this result is consistent with previously reported data. Moreover, GF treatment suppressed LDH activity and inhibited BL cell replication but did not affect lymphoblast viability. Surprisingly, we found that increased levels of the MYC and LDH-A proteins did not lead to a metabolic shift in BL cells toward glycolytic ATP generation. BL cells were treated with GF at doses that achieved 50% inhibition of cell growth and lactate production, and ATP levels were scarcely affected after GF treatment. The same results were also obtained by suppressing LDH activity with oxamate, an LDH specific inhibitor. Our data suggest that LDH activity is important for maintaining a correct NAD/NADH balance in BL cells. LDH inhibition led to decreased NAD cellular levels, which resulted in sirtuin-1 inhibition. Confirming previous studies, sirtuin-1 inhibition caused a reduction in MYC protein levels, depriving BL cells of their most important survival signal. This study further describes the biological functions of the LDH enzyme and suggests that LDH inhibition could be useful for the treatment of cancer.

A hydrazine coupled cycling assay validates the decrease in redox ratio under starvation in Drosophila

A commonly used enzymatic recycling assay for pyridine nucleotides has been adapted to directly measure the NAD⁺/NADH redox ratio in Drosophila melanogaster. This method is also suitable for quantification of NADP⁺ and NADPH. The addition of a coupling reaction removing acetaldehyde produced from the alcohol dehydrogenase (ADH) reaction was shown to improve the linearity of NAD(H) assay. The advantages of this assay method are that it allows the determination of both NAD⁺ and NADH simultaneously while keeping enzymatic degradation of pyridine nucleotides minimal and also achieving better sensitivity. This method was used to determine the redox ratio of D. melanogaster and validated substantial decrease of redox ratio during starvation.

Whole-genome microarray and gene deletion studies reveal regulation of the polyhydroxyalkanoate production cycle by the stringent response in Ralstonia eutropha H16

Poly(3-hydroxybutyrate) (PHB) production and mobilization in Ralstonia eutropha are well studied, but in only a few instances has PHB production been explored in relation to other cellular processes. We examined the global gene expression of wild-type R. eutropha throughout the PHB cycle: growth on fructose, PHB production using fructose following ammonium depletion, and PHB utilization in the absence of exogenous carbon after ammonium was resupplied. Our results confirm or lend support to previously reported results regarding the expression of PHB-related genes and enzymes. Additionally, genes for many different cellular processes, such as DNA replication, cell division, and translation, are selectively repressed during PHB production. In contrast, the expression levels of genes under the control of the alternative sigma factor σ(54) increase sharply during PHB production and are repressed again during PHB utilization. Global gene regulation during PHB production is strongly reminiscent of the gene expression pattern observed during the stringent response in other species. Furthermore, a ppGpp synthase deletion mutant did not show an accumulation of PHB, and the chemical induction of the stringent response with DL-norvaline caused an increased accumulation of PHB in the presence of ammonium. These results indicate that the stringent response is required for PHB accumulation in R. eutropha, helping to elucidate a thus-far-unknown physiological basis for this process.

The c-MYC oncoprotein, the NAMPT enzyme, the SIRT1-inhibitor DBC1, and the SIRT1 deacetylase form a positive feedback loop

Silent information regulator 1 (SIRT1) represents an NAD(+)-dependent deacetylase that inhibits proapoptotic factors including p53. Here we determined whether SIRT1 is downstream of the prototypic c-MYC oncogene, which is activated in the majority of tumors. Elevated expression of c-MYC in human colorectal cancer correlated with increased SIRT1 protein levels. Activation of a conditional c-MYC allele induced increased levels of SIRT1 protein, NAD(+), and nicotinamide-phosphoribosyltransferase (NAMPT) mRNA in several cell types. This increase in SIRT1 required the induction of the NAMPT gene by c-MYC. NAMPT is the rate-limiting enzyme of the NAD(+) salvage pathway and enhances SIRT1 activity by increasing the amount of NAD(+). c-MYC also contributed to SIRT1 activation by sequestering the SIRT1 inhibitor deleted in breast cancer 1 (DBC1) from the SIRT1 protein. In primary human fibroblasts previously immortalized by introduction of c-MYC, down-regulation of SIRT1 induced senescence and apoptosis. In various cell lines inactivation of SIRT1 by RNA interference, chemical inhibitors, or ectopic DBC1 enhanced c-MYC-induced apoptosis. Furthermore, SIRT1 directly bound to and deacetylated c-MYC. Enforced SIRT1 expression increased and depletion/inhibition of SIRT1 reduced c-MYC stability. Depletion/inhibition of SIRT1 correlated with reduced lysine 63-linked polyubiquitination of c-Myc, which presumably destabilizes c-MYC by supporting degradative lysine 48-linked polyubiquitination. Moreover, SIRT1 enhanced the transcriptional activity of c-MYC. Taken together, these results show that c-MYC activates SIRT1, which in turn promotes c-MYC function. Furthermore, SIRT1 suppressed cellular senescence in cells with deregulated c-MYC expression and also inhibited c-MYC-induced apoptosis. Constitutive activation of this positive feedback loop may contribute to the development and maintenance of tumors in the context of deregulated c-MYC.

Bioenergetic dysfunction in Huntington's disease human cybrids

In this work we studied the mitochondrial-associated metabolic pathways in Huntington's disease (HD) versus control (CTR) cybrids, a cell model in which the contribution of mitochondrial defects from patients is isolated. HD cybrids exhibited an interesting increase in ATP levels, when compared to CTR cybrids. Concomitantly, we observed increased glycolytic rate in HD cybrids, as revealed by increased lactate/pyruvate ratio, which was reverted after inhibition of glycolysis. A decrease in glucose-6-phosphate dehydrogenase activity in HD cybrids further indicated decreased rate of the pentose-phosphate pathway. ATP levels of HD cybrids were significantly decreased under glycolysis inhibition, which was accompanied by a decrease in phosphocreatine. Nevertheless, pyruvate supplementation could not recover HD cybrids' ATP or phosphocreatine levels, suggesting a dysfunction in mitochondrial use of that substrate. Oligomycin also caused a decrease in ATP levels, suggesting a partial support of ATP generation by the mitochondria. Nevertheless, mitochondrial NADH/NAD(t) levels were decreased in HD cybrids, which was correlated with a decrease in pyruvate dehydrogenase activity and protein expression, suggesting decreased tricarboxylic acid cycle (TCA) input from glycolysis. Interestingly, the activity of alpha-ketoglutarate dehydrogenase, a critical enzyme complex that links the TCA to amino acid synthesis and degradation, was increased in HD cybrids. In accordance, mitochondrial levels of glutamate were increased and alanine was decreased, whereas aspartate and glutamine levels were unchanged in HD cybrids. Conversely, malate dehydrogenase activity from total cell extracts was unchanged in HD cybrids. Our results suggest that inherent dysfunction of mitochondria from HD patients affects cellular bioenergetics in an otherwise functional nuclear background.

Disruption of pyridine nucleotide redox status during oxidative challenge at normal and low-glucose states: implications for cellular adenosine triphosphate, mitochondrial respiratory activity, and reducing capacity in colon epithelial cells

We recently demonstrated that menadione (MQ), a redox cycling quinone, mediated the loss of mitochondrial glutathione/glutathione disulfide redox balance. In this study, we showed that MQ significantly disrupted cellular pyridine nucleotide (NAD(+)/NADH, NADP(+)/NADPH) redox balance that compromised cellular ATP, mitochondrial respiratory activity, and NADPH-dependent reducing capacity in colonic epithelial cells, a scenario that was exaggerated by low glucose. In the cytosol, MQ induced NAD(+) loss concurrent with increased NADP(+) and NAD kinase activity, but decreased NADPH. In the mitochondria, NADH loss occurred in conjunction with increased nicotinamide nucleotide transhydrogenase activity and NADP(+), and decreased NADPH. These results are consistent with cytosolic NAD(+)-to-NADP(+) and mitochondrial NADH-to-NADPH shifts, but compromised NADPH availability. Thus, despite the sacrifice of NAD(+)/NADH in favor of NADPH generation, steady-state NADPH levels were not maintained during MQ challenge. Impairments of cellular bioenergetics were evidenced by ATP losses and increased mitochondrial O(2) dependence of pyridine nucleotide oxidation-reduction; half-maximal oxidation (P(50)) was 10-fold higher in low glucose, which was lowered by glutamate or succinate supplementation. This exaggerated O(2) dependence is consistent with increased O(2) diversion to nonmitochondrial O(2) consumption by MQ-semiquinone redox cycling secondary to decreased NADPH-dependent MQ detoxication at low glucose, a situation that was corrected by glucose-sparing mitochondrial substrates.

Intracellular coenzymes as natural biomarkers for metabolic activities and mitochondrial anomalies

Mitochondria play a pivotal role in energy metabolism, programmed cell death and oxidative stress. Mutated mitochondrial DNA in diseased cells compromises the structure of key enzyme complexes and, therefore, mitochondrial function, which leads to a myriad of health-related conditions such as cancer, neurodegenerative diseases, diabetes and aging. Early detection of mitochondrial and metabolic anomalies is an essential step towards effective diagnoses and therapeutic intervention. Reduced nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) play important roles in a wide range of cellular oxidation-reduction reactions. Importantly, NADH and FAD are naturally fluorescent, which allows noninvasive imaging of metabolic activities of living cells and tissues. Furthermore, NADH and FAD autofluorescence, which can be excited using distinct wavelengths for complementary imaging methods and is sensitive to protein binding and local environment. This article highlights recent developments concerning intracellular NADH and FAD as potential biomarkers for metabolic and mitochondrial activities.

Production of panic-like symptoms by lactate is associated with increased neural firing and oxidation of brain redox in the rat hippocampus

Lactate uses an unknown mechanism to induce panic attacks in people and panic-like symptoms in rodents. We tested whether intraperitoneal (IP) lactate injections act peripherally or centrally to induce panic-like symptoms in rats by examining whether IP lactate directly affects the CNS. In Long-Evans rats, IP lactate (2 mmol/kg) injection increased lactate levels in the plasma and the cerebrospinal fluid. IP lactate also induced tachycardia and behavioral freezing suggesting the production of panic-like behavior. To enter intermediate metabolism, lactate is oxidized by lactate dehydrogenase (LDH) to pyruvate with co-reduction of NAD(+) to NADH. Therefore, we measured the ratio of NADH/NAD(+) to test whether IP lactate altered lactate metabolism in the CNS. Lactate metabolism was studied in the hippocampus, a brain region believed to contribute to panic-like symptoms. IP lactate injection lowered the ratio of NADH/NAD(+) without altering the total amount of NADH and NAD(+) suggesting oxidation of hippocampal redox state. Lactate oxidized hippocampal redox since intrahippocampal injection of the LDH inhibitor, oxamate (50mM) prevented the oxidation of NADH/NAD(+) by IP lactate. In addition to oxidizing hippocampal redox, IP lactate rapidly increased the firing rate of hippocampal neurons. Similar IP pyruvate injections had no effect. Neural discharge also increased following intrahippocampal lactate injection suggesting that increased discharge was a direct action of lactate on the hippocampus. These studies show that oxidation of brain redox and increased hippocampal firing are direct actions of lactate on the CNS that may contribute to the production of lactate-induced panic.

Hydrogen sulfide enhances reducing activity in neurons: neurotrophic role of H2S in the brain?

Hydrogen sulfide (H2S) can enzymatically be produced from cysteine in the brain. H2S functions as a synaptic modulator as well as a neuroprotectant from oxidative stress in the brain. Here we show that H2S specifically enhances the reducing activity in neurons and mouse neuroblastoma Neuro2a cells. An inhibitor of protein tyrosine kinase, genistein, suppresses the effect of H2S, suggesting that tyrosine kinase may be involved in the enhancement of reducing activity by H2S. The H2S-specific enhancement of the reducing activity in neurons may lead to a neurotrophic role in the brain.

P2X7 receptors mediate NADH transport across the plasma membranes of astrocytes

NADH plays critical roles in mitochondrial functions and energy metabolism. There has been no study demonstrating that NADH can be transported across the plasma membranes of cells. In this study we tested our hypothesis that NADH can be transported across the plasma membranes of astrocytes by a P2X7 receptor (P2X7R)-mediated mechanism. We found that treatment of astrocytes with NADH led to increases in both intracellular NADH and NAD+. Three lines of studies suggest that P2X7R mediates the NADH transport into astrocytes: the P2X receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) blocked the NADH transport; RNAi knockdown of P2X7R led to decreased NADH transport; and transfection of HEK293 cells with mouse P2X7R cDNA led to increased NADH transport. Collectively, our study provides the first direct evidence demonstrating that NADH can be transported across the plasma membranes of astrocytes by a P2X7R-mediated mechanism. Our study also suggests a novel approach for manipulating intracellular NADH and NAD+ levels.