Influence of NAD-linked dehydrogenase activity on flux through oxidative phosphorylation

Changes in cAMP signaling are associated with age-related downregulation of spontaneously beating atrial tissue energetic indices

The prevalence of atria-related diseases increases exponentially with age and is associated with ATP supply-to-demand imbalances. Because evidence suggests that cAMP regulates ATP supply-to-demand, we explored aged-associated alterations in atrial ATP supply-to-demand balance and its correlation with cAMP levels. Right atrial tissues driven by spontaneous sinoatrial node impulses were isolated from aged (22-26 months) and adult (3-4 months) C57/BL6 mice. ATP demand increased by addition of isoproterenol or 3-Isobutyl-1-methylxanthine (IBMX) and decreased by application of carbachol. Each drug was administrated at the dose that led to a maximal change in beating rate (X_max) and to 50% of that maximal change in adult tissue (X₅₀). cAMP, NADH, NAD + NADH, and ATP levels were measured in the same tissue. The tight correlation between cAMP levels and the beating rate (i.e., the ATP demand) demonstrated in adult atria was altered in aged atria. cAMP levels were lower in aged compared to adult atrial tissue exposed to X₅₀ of ISO or IBMX, but this difference narrowed at X_max. Neither ATP nor NADH levels correlated with ATP demand in either adult or aged atria. Baseline NADH levels were lower in aged as compared to adult atria, but were restored by drug perturbations that increased cAMP levels. Reduction in Ca²⁺-activated adenylyl cyclase-induced decreased cAMP and prolongation of the spontaneous beat interval of adult atrial tissue to their baseline levels in aged tissue, brought energetics indices to baseline levels in aged tissue. Thus, cAMP regulates right atrial ATP supply-to-demand matching and can restore age-associated ATP supply-to-demand imbalance.

Calcium dysregulation in heart diseases: Targeting calcium channels to achieve a correct calcium homeostasis

Intracellular calcium signaling is a universal language source shared by the most part of biological entities inside cells that, all together, give rise to physiological and functional anatomical units, the organ. Although preferentially recognized as signaling between cell life and death processes, in the heart it assumes additional relevance considered the importance of calcium cycling coupled to ATP consumption in excitation-contraction coupling. The concerted action of a plethora of exchangers, channels and pumps inward and outward calcium fluxes where needed, to convert energy and electric impulses in muscle contraction. All this without realizing it, thousands of times, every day. An improper function of those proteins (i.e., variation in expression, mutations onset, dysregulated channeling, differential protein-protein interactions) being part of this signaling network triggers a short circuit with severe acute and chronic pathological consequences reported as arrhythmias, cardiac remodeling, heart failure, reperfusion injury and cardiomyopathies. By acting with chemical, peptide-based and pharmacological modulators of these players, a correction of calcium homeostasis can be achieved accompanied by an amelioration of clinical symptoms. This review will focus on all those defects in calcium homeostasis which occur in the most common cardiac diseases, including myocardial infarction, arrhythmia, hypertrophy, heart failure and cardiomyopathies. This part will be introduced by the state of the art on the proteins involved in calcium homeostasis in cardiomyocytes and followed by the therapeutic treatments that to date, are able to target them and to revert the pathological phenotype.

Mitochondrial Dysfunction in Advanced Liver Disease: Emerging Concepts

Mitochondria are entrusted with the challenging task of providing energy through the generation of ATP, the universal cellular currency, thereby being highly flexible to different acute and chronic nutrient demands of the cell. The fact that mitochondrial diseases (genetic disorders caused by mutations in the nuclear or mitochondrial genome) manifest through a remarkable clinical variation of symptoms in affected individuals underlines the far-reaching implications of mitochondrial dysfunction. The study of mitochondrial function in genetic or non-genetic diseases therefore requires a multi-angled approach. Taking into account that the liver is among the organs richest in mitochondria, it stands to reason that in the process of unravelling the pathogenesis of liver-related diseases, researchers give special focus to characterizing mitochondrial function. However, mitochondrial dysfunction is not a uniformly defined term. It can refer to a decline in energy production, increase in reactive oxygen species and so forth. Therefore, any study on mitochondrial dysfunction first needs to define the dysfunction to be investigated. Here, we review the alterations of mitochondrial function in liver cirrhosis with emphasis on acutely decompensated liver cirrhosis and acute-on-chronic liver failure (ACLF), the latter being a form of acute decompensation characterized by a generalized state of systemic hyperinflammation/immunosuppression and high mortality rate. The studies that we discuss were either carried out in liver tissue itself of these patients, or in circulating leukocytes, whose mitochondrial alterations might reflect tissue and organ mitochondrial dysfunction. In addition, we present different methodological approaches that can be of utility to address the diverse aspects of hepatocyte and leukocyte mitochondrial function in liver disease. They include assays to measure metabolic fluxes using the comparatively novel Biolog’s MitoPlates in a 96-well format as well as assessment of mitochondrial respiration by high-resolution respirometry using Oroboros’ O2k-technology and Agilent Seahorse XF technology.

Restriction of an intron size en route to endothermy

Ca2+-insensitive and -sensitive E1 subunits of the 2-oxoglutarate dehydrogenase complex (OGDHC) regulate tissue-specific NADH and ATP supply by mutually exclusive OGDH exons 4a and 4b. Here we show that their splicing is enforced by distant lariat branch points (dBPs) located near the 5' splice site of the intervening intron. dBPs restrict the intron length and prevent transposon insertions, which can introduce or eliminate dBP competitors. The size restriction was imposed by a single dominant dBP in anamniotes that expanded into a conserved constellation of four dBP adenines in amniotes. The amniote clusters exhibit taxon-specific usage of individual dBPs, reflecting accessibility of their extended motifs within a stable RNA hairpin rather than U2 snRNA:dBP base-pairing. The dBP expansion took place in early terrestrial species and was followed by a uridine enrichment of large downstream polypyrimidine tracts in mammals. The dBP-protected megatracts permit reciprocal regulation of exon 4a and 4b by uridine-binding proteins, including TIA-1/TIAR and PUF60, which promote U1 and U2 snRNP recruitment to the 5' splice site and BP, respectively, but do not significantly alter the relative dBP usage. We further show that codons for residues critically contributing to protein binding sites for Ca2+ and other divalent metals confer the exon inclusion order that mirrors the Irving-Williams affinity series, linking the evolution of auxiliary splicing motifs in exons to metallome constraints. Finally, we hypothesize that the dBP-driven selection for Ca2+-dependent ATP provision by E1 facilitated evolution of endothermy by optimizing the aerobic scope in target tissues.

Calcium influx through the mitochondrial calcium uniporter holocomplex, MCUcx

Ca2+ flux into the mitochondrial matrix through the MCU holocomplex (MCUcx) has recently been measured quantitatively and with milliseconds resolution for the first time under physiological conditions in both heart and skeletal muscle. Additionally, the dynamic levels of Ca2+ in the mitochondrial matrix ([Ca2+]m) of cardiomyocytes were measured as it was controlled by the balance between influx of Ca2+ into the mitochondrial matrix through MCUcx and efflux through the mitochondrial Na+ / Ca2+ exchanger (NCLX). Under these conditions [Ca2+]m was shown to regulate ATP production by the mitochondria at only a few critical sites. Additional functions attributed to [Ca2+]m continue to be reported in the literature. Here we review the new findings attributed to MCUcx function and provide a framework for understanding and investigating mitochondrial Ca2+ influx features, many of which remain controversial. The properties and functions of the MCUcx subunits that constitute the holocomplex are challenging to tease apart. Such distinct subunits include EMRE, MCUR1, MICUx (i.e. MICU1, MICU2, MICU3), and the pore-forming subunits (MCUpore). Currently, the specific set of functions of each subunit remains non-quantitative and controversial. The more contentious issues are discussed in the context of the newly measured native MCUcx Ca2+ flux from heart and skeletal muscle. These MCUcx Ca2+ flux measurements have been shown to be a highly-regulated, tissue-specific with femto-Siemens Ca2+ conductances and with distinct extramitochondrial Ca2+ ([Ca2+]i) dependencies. These data from cardiac and skeletal muscle mitochondria have been examined quantitatively for their threshold [Ca2+]i levels and for hypothesized gatekeeping function and are discussed in the context of model cell (e.g. HeLa, MEF, HEK293, COS7 cells) measurements. Our new findings on MCUcx dependent matrix [Ca2+]m signaling provide a quantitative basis for on-going and new investigations of the roles of MCUcx in cardiac function ranging from metabolic fuel selection, capillary blood-flow control and the pathological activation of the mitochondrial permeability transition pore (mPTP). Additionally, this review presents the use of advanced new methods that can be readily adapted by any investigator to enable them to carry out quantitative Ca2+ measurements in mitochondria while controlling the inner mitochondrial membrane potential, ΔΨm.

Skim-Sequencing Based Genotyping Reveals Genetic Divergence of the Wild and Domesticated Population of Black Tiger Shrimp (Penaeus monodon) in the Indo-Pacific Region

The domestication of a wild-caught aquatic animal is an evolutionary process, which results in genetic discrimination at the genomic level in response to strong artificial selection. Although black tiger shrimp (Penaeus monodon) is one of the most commercially important aquaculture species, a systematic assessment of genetic divergence and structure of wild-caught and domesticated broodstock populations of the species is yet to be documented. Therefore, we used skim sequencing (SkimSeq) based genotyping approach to investigate the genetic structure of 50 broodstock individuals of P. monodon species, collected from five sampling sites (n = 10 in each site) across their distribution in Indo-Pacific regions. The wild-caught P. monodon broodstock population were collected from Malaysia (MS) and Japan (MJ), while domesticated broodstock populations were collected from Madagascar (MMD), Hawaii, HI, USA (MMO), and Thailand (MT). After various filtering process, a total of 194,259 single nucleotide polymorphism (SNP) loci were identified, in which 4983 SNP loci were identified as putatively adaptive by the pcadapt approach. In both datasets, pairwise F_ST estimates high genetic divergence between wild and domesticated broodstock populations. Consistently, different spatial clustering analyses in both datasets categorized divergent genetic structure into two clusters: (1) wild-caught populations (MS and MJ), and (2) domesticated populations (MMD, MMO and MT). Among 4983 putatively adaptive SNP loci, only 50 loci were observed to be in the coding region. The gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses suggested that non-synonymous mutated genes might be associated with the energy production, metabolic functions, respiration regulation and developmental rates, which likely act to promote adaptation to the strong artificial selection during the domestication process. This study has demonstrated the applicability of SkimSeq in a highly duplicated genome of P. monodon specifically, across a range of genetic backgrounds and geographical distributions, and would be useful for future genetic improvement program of this species in aquaculture.

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.

Dominant and sensitive control of oxidative flux by the ATP-ADP carrier in human skeletal muscle mitochondria: Effect of lysine acetylation

The adenine nucleotide translocase (ANT) of the mitochondrial inner membrane exchanges ADP for ATP. Mitochondria were isolated from human vastus lateralis muscle (n = 9). Carboxyatractyloside titration of O₂ consumption rate (J_o) at clamped [ADP] of 21 μM gave ANT abundance of 0.97 ± 0.14 nmol ANT/mg and a flux control coefficient of 82% ± 6%. Flux control fell to 1% ± 1% at saturating (2 mM) [ADP]. The KmADP for J_o was 32.4 ± 1.8 μM. In terms of the free (-3) ADP anion this KmADP was 12.0 ± 0.7 μM. A novel luciferase-based assay for ATP production gave KmADP of 13.1 ± 1.9 μM in the absence of ATP competition. The free anion KmADP in this case was 2.0 ± 0.3 μM. Targeted proteomic analyses showed significant acetylation of ANT Lysine23 and that ANT1 was the most abundant isoform. Acetylation of Lysine23 correlated positively with KmADP, r = 0.74, P = 0.022. The findings underscore the central role played by ANT in the control of oxidative phosphorylation, particularly at the energy phosphate levels associated with low ATP demand. As predicted by molecular dynamic modeling, ANT Lysine23 acetylation decreased the apparent affinity of ADP for ANT binding.

Mitochondrial Bioenergetics and Dysfunction in Failing Heart

Energy insufficiency has been recognized as a key feature of systolic heart failure. Although mitochondria have long been known to sustain myocardial work energy supply, the capacity to therapeutically target mitochondrial bioenergetics dysfunction is hampered by a complex interplay of multiple perturbations that progressively compound causing myocardial failure and collapse. Compared to non-failing human donor hearts, activity rates of complexes I and IV, nicotinamide nucleotide transhydrogenase (NADPH-transhydrogenase, Nnt) and the Krebs cycle enzymes isocitrate dehydrogenase, malate dehydrogenase and aconitase are markedly decreased in end-stage heart failure. Diminished REDOX capacity with lower total glutathione and coenzyme Q₁₀ levels are also a feature of chronic left ventricular failure. Decreased enzyme activities in part relate to abundant and highly specific oxidative, nitrosylative, and hyperacetylation modifications. In this brief review we highlight that energy deficiency in end-stage failing human left ventricle predominantly involves concomitantly impaired activities of key electron transport chain and Krebs cycle enzymes rather than altered expression of respective genes or proteins. Augmented oxidative modification of these enzyme subunit structures, and the formation of highly reactive secondary metabolites, implicates dysfunction due to diminished capacity for management of mitochondrial reactive oxygen species, which contribute further to progressive decreases in bioenergetic capacity and contractile function in human heart failure.

Patterns of natural selection acting on the mitochondrial genome of a locally adapted fish species

Background

Mitochondrial DNA (mtDNA) is frequently used in population genetic studies and is usually considered as a neutral marker. However, given the functional importance of the proteins encoded by the mitochondrial genome, and the prominent role of mitochondria in cellular energy production, the assumption of neutrality is increasingly being questioned.

Results

We tested for evidence of selection on the mitochondrial genome of the Atlantic salmon, which is a locally adapted and widely farmed species and is distributed across a large latitudinal cline. We analysed 20 independent regions of the salmon mtDNA that represented nine genes (ND1, ND2, ND3, COX1, COX2, ATP6, ND4, ND5, and CYTB). These 20 mtDNA regions were sequenced using a 454 approach from samples collected across the entire European range of this species. We found evidence of positive selection at the ND1, ND3 and ND4 genes, which is supported by at least two different codon-based methods and also by differences in the chemical properties of the amino acids involved. The geographical distribution of some of the mutations indicated to be under selection was not random, and some mutations were private to artic populations. We discuss the possibility that selection acting on the Atlantic salmon mtDNA genome might be related to the need for increased metabolic efficiency at low temperatures.

Conclusions

The analysis of sequences representing nine mitochondrial genes that are involved in the OXPHOS pathway revealed signatures of positive selection in the mitochondrial genome of the Atlantic salmon. The properties of the amino acids involved suggest that some of the mutations that were identified to be under positive selection might have functional implications, possibly in relation to metabolic efficiency. Experimental evidence, and better understanding of regional phylogeographic structuring, are needed to clarify the potential role of selection acting on the mitochondrial genome of Atlantic salmon and other locally adapted fishes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12711-015-0138-0) contains supplementary material, which is available to authorized users.

Salicylic acid binding of mitochondrial alpha-ketoglutarate dehydrogenase E2 affects mitochondrial oxidative phosphorylation and electron transport chain components and plays a role in basal defense against tobacco mosaic virus in tomato

Salicylic acid (SA) plays a critical role in plant defense against pathogen invasion. SA-induced viral defense in plants is distinct from the pathways mediating bacterial and fungal defense and involves a specific pathway mediated by mitochondria; however, the underlying mechanisms remain largely unknown. The SA-binding activity of the recombinant tomato (Solanum lycopersicum) alpha-ketoglutarate dehydrogenase (Slα-kGDH) E2 subunit of the tricarboxylic acid (TCA) cycle was characterized. The biological role of this binding in plant defenses against tobacco mosaic virus (TMV) was further investigated via Slα-kGDH E2 silencing and transient overexpression in plants. Slα-kGDH E2 was found to bind SA in two independent assays. SA treatment, as well as Slα-kGDH E2 silencing, increased resistance to TMV. SA did not further enhance TMV defense in Slα-kGDH E2-silenced tomato plants but did reduce TMV susceptibility in Nicotiana benthamiana plants transiently overexpressing Slα-kGDH E2. Furthermore, Slα-kGDH E2-silencing-induced TMV resistance was fully blocked by bongkrekic acid application and alternative oxidase 1a silencing. These results indicated that binding by Slα-kGDH E2 of SA acts upstream of and affects the mitochondrial electron transport chain, which plays an important role in basal defense against TMV. The findings of this study help to elucidate the mechanisms of SA-induced viral defense.

Calcium co-regulates oxidative metabolism and ATP synthase-dependent respiration in pancreatic beta cells

Mitochondrial energy metabolism is essential for glucose-induced calcium signaling and, therefore, insulin granule exocytosis in pancreatic beta cells. Calcium signals are sensed by mitochondria acting in concert with mitochondrial substrates for the full activation of the organelle. Here we have studied glucose-induced calcium signaling and energy metabolism in INS-1E insulinoma cells and human islet beta cells. In insulin secreting cells a surprisingly large fraction of total respiration under resting conditions is ATP synthase-independent. We observe that ATP synthase-dependent respiration is markedly increased after glucose stimulation. Glucose also causes a very rapid elevation of oxidative metabolism as was followed by NAD(P)H autofluorescence. However, neither the rate of the glucose-induced increase nor the new steady-state NAD(P)H levels are significantly affected by calcium. Our findings challenge the current view, which has focused mainly on calcium-sensitive dehydrogenases as the target for the activation of mitochondrial energy metabolism. We propose a model of tight calcium-dependent regulation of oxidative metabolism and ATP synthase-dependent respiration in beta cell mitochondria. Coordinated activation of matrix dehydrogenases and respiratory chain activity by calcium allows the respiratory rate to change severalfold with only small or no alterations of the NAD(P)H/NAD(P)(+) ratio.

Who controls the ATP supply in cancer cells? Biochemistry lessons to understand cancer energy metabolism

Applying basic biochemical principles, this review analyzes data that contrasts with the Warburg hypothesis that glycolysis is the exclusive ATP provider in cancer cells. Although disregarded for many years, there is increasing experimental evidence demonstrating that oxidative phosphorylation (OxPhos) makes a significant contribution to ATP supply in many cancer cell types and under a variety of conditions. Substrates oxidized by normal mitochondria such as amino acids and fatty acids are also avidly consumed by cancer cells. In this regard, the proposal that cancer cells metabolize glutamine for anabolic purposes without the need for a functional respiratory chain and OxPhos is analyzed considering thermodynamic and kinetic aspects for the reductive carboxylation of 2-oxoglutarate catalyzed by isocitrate dehydrogenase. In addition, metabolic control analysis (MCA) studies applied to energy metabolism of cancer cells are reevaluated. Regardless of the experimental/environmental conditions and the rate of lactate production, the flux-control of cancer glycolysis is robust in the sense that it involves the same steps: glucose transport, hexokinase, hexosephosphate isomerase and glycogen degradation, all at the beginning of the pathway; these steps together with phosphofructokinase 1 also control glycolysis in normal cells. The respiratory chain complexes exert significantly higher flux-control on OxPhos in cancer cells than in normal cells. Thus, determination of the contribution of each pathway to ATP supply and/or the flux-control distribution of both pathways in cancer cells is necessary in order to identify differences from normal cells which may lead to the design of rational alternative therapies that selectively target cancer energy metabolism.

Mechanisms of disease: is mitochondrial function altered in heart failure?

The human heart sustains an exceptional energy transfer rate, consuming more energy per gram weight than any other organ system. The healthy heart can rapidly adapt to changes in demand, while the failing heart cannot. Cardiac energy flux systems falter in the failing heart. The purpose of this review is to characterize the fundamental role of mitochondria in this energy transfer system and describe our local research on mitochondrial respiratory capacity in failing human hearts.

Glutathionylation of α-ketoglutarate dehydrogenase: the chemical nature and relative susceptibility of the cofactor lipoic acid to modification

α-Ketoglutarate dehydrogenase (KGDH) is reversibly inhibited when rat heart mitochondria are exposed to hydrogen peroxide (H2O2). H2O2-induced inhibition occurs through the formation of a mixed disulfide between a protein sulfhydryl and glutathione. Upon consumption of H2O2, glutaredoxin can rapidly remove glutathione, resulting in regeneration of enzyme activity. KGDH is a key regulatory site within the Krebs cycle. Glutathionylation of the enzyme may therefore represent an important means to control mitochondrial function in response to oxidative stress. We have previously provided indirect evidence that glutathionylation occurs on lipoic acid, a cofactor covalently bound to the E2 subunit of KGDH. However, lipoic acid contains two vicinal sulfhydryls and rapid disulfide exchange might be predicted to preclude stable glutathionylation. The current study sought conclusive identification of the site and chemistry of KGDH glutathionylation and factors that control the degree and rate of enzyme inhibition. We present evidence that, upon reaction of free lipoic acid with oxidized glutathione in solution, disulfide exchange occurs rapidly, producing oxidized lipoic acid and reduced glutathione. This prevents the stable formation of a glutathione-lipoic acid adduct. Nevertheless, 1:1 lipoic acid-glutathione adducts are formed on KGDH because the second sulfhydryl on lipoic acid is unable to participate in disulfide exchange in the enzyme's native conformation. The maximum degree of KGDH inhibition that can be achieved by treatment of mitochondria with H2O2 is 50%. Results indicate that this is not due to glutathionylation of a subpopulation of the enzyme but, rather, the unique susceptibility of lipoic acid on a subset of E2 subunits within each enzyme complex. Calcium enhances the rate of glutathionylation by increasing the half-life of reduced lipoic acid during enzyme catalysis. This does not, however, alter the maximal level of inhibition, providing further evidence that specific lipoic acid residues within the E2 complex are susceptible to glutathionylation. These findings offer chemical information necessary for the identification of mechanisms and physiological implications of KGDH glutathionylation.

Effect of calcium on the oxidative phosphorylation cascade in skeletal muscle mitochondria

Calcium is believed to regulate mitochondrial oxidative phosphorylation, thereby contributing to the maintenance of cellular energy homeostasis. Skeletal muscle, with an energy conversion dynamic range of up to 100-fold, is an extreme case for evaluating the cellular balance of ATP production and consumption. This study examined the role of Ca(2+) in the entire oxidative phosphorylation reaction network in isolated skeletal muscle mitochondria and attempted to extrapolate these results back to the muscle, in vivo. Kinetic analysis was conducted to evaluate the dose-response effect of Ca(2+) on the maximal velocity of oxidative phosphorylation (V(maxO)) and the ADP affinity. Force-flow analysis evaluated the interplay between energetic driving forces and flux to determine the conductance, or effective activity, of individual steps within oxidative phosphorylation. Measured driving forces [extramitochondrial phosphorylation potential (ΔG(ATP)), membrane potential, and redox states of NADH and cytochromes b(H), b(L), c(1), c, and a,a(3)] were compared with flux (oxygen consumption) at 37 °C; 840 nM Ca(2+) generated an ~2-fold increase in V(maxO) with no change in ADP affinity (~43 μM). Force-flow analysis revealed that Ca(2+) activation of V(maxO) was distributed throughout the oxidative phosphorylation reaction sequence. Specifically, Ca(2+) increased the conductance of Complex IV (2.3-fold), Complexes I and III (2.2-fold), ATP production/transport (2.4-fold), and fuel transport/dehydrogenases (1.7-fold). These data support the notion that Ca(2+) activates the entire muscle oxidative phosphorylation cascade, while extrapolation of these data to the exercising muscle predicts a significant role of Ca(2+) in maintaining cellular energy homeostasis.

Cofactor balance by nicotinamide nucleotide transhydrogenase (NNT) coordinates reductive carboxylation and glucose catabolism in the tricarboxylic acid (TCA) cycle

Cancer and proliferating cells exhibit an increased demand for glutamine-derived carbons to support anabolic processes. In addition, reductive carboxylation of α-ketoglutarate by isocitrate dehydrogenase 1 (IDH1) and 2 (IDH2) was recently shown to be a major source of citrate synthesis from glutamine. The role of NAD(P)H/NAD(P)(+) cofactors in coordinating glucose and glutamine utilization in the tricarboxylic acid (TCA) cycle is not well understood, with the source(s) of NADPH for the reductive carboxylation reaction remaining unexplored. Nicotinamide nucleotide transhydrogenase (NNT) is a mitochondrial enzyme that transfers reducing equivalents from NADH to NADPH. Here, we show that knockdown of NNT inhibits the contribution of glutamine to the TCA cycle and activates glucose catabolism in SkMel5 melanoma cells. The increase in glucose oxidation partially occurred through pyruvate carboxylase and rendered NNT knockdown cells more sensitive to glucose deprivation. Importantly, knocking down NNT inhibits reductive carboxylation in SkMel5 and 786-O renal carcinoma cells. Overexpression of NNT is sufficient to stimulate glutamine oxidation and reductive carboxylation, whereas it inhibits glucose catabolism in the TCA cycle. These observations are supported by an impairment of the NAD(P)H/NAD(P)(+) ratios. Our findings underscore the role of NNT in regulating central carbon metabolism via redox balance, calling for other mechanisms that coordinate substrate preference to maintain a functional TCA cycle.

Role of mitochondrial Ca2+ in the regulation of cellular energetics

Calcium is an important signaling molecule involved in the regulation of many cellular functions. The large free energy in the Ca(2+) ion membrane gradients makes Ca(2+) signaling inherently sensitive to the available cellular free energy, primarily in the form of ATP. In addition, Ca(2+) regulates many cellular ATP-consuming reactions such as muscle contraction, exocytosis, biosynthesis, and neuronal signaling. Thus, Ca(2+) becomes a logical candidate as a signaling molecule for modulating ATP hydrolysis and synthesis during changes in numerous forms of cellular work. Mitochondria are the primary source of aerobic energy production in mammalian cells and also maintain a large Ca(2+) gradient across their inner membrane, providing a signaling potential for this molecule. The demonstrated link between cytosolic and mitochondrial Ca(2+) concentrations, identification of transport mechanisms, and the proximity of mitochondria to Ca(2+) release sites further supports the notion that Ca(2+) can be an important signaling molecule in the energy metabolism interplay of the cytosol with the mitochondria. Here we review sites within the mitochondria where Ca(2+) plays a role in the regulation of ATP generation and potentially contributes to the orchestration of cellular metabolic homeostasis. Early work on isolated enzymes pointed to several matrix dehydrogenases that are stimulated by Ca(2+), which were confirmed in the intact mitochondrion as well as cellular and in vivo systems. However, studies in these intact systems suggested a more expansive influence of Ca(2+) on mitochondrial energy conversion. Numerous noninvasive approaches monitoring NADH, mitochondrial membrane potential, oxygen consumption, and workloads suggest significant effects of Ca(2+) on other elements of NADH generation as well as downstream elements of oxidative phosphorylation, including the F(1)F(O)-ATPase and the cytochrome chain. These other potential elements of Ca(2+) modification of mitochondrial energy conversion will be the focus of this review. Though most specific molecular mechanisms have yet to be elucidated, it is clear that Ca(2+) provides a balanced activation of mitochondrial energy metabolism that exceeds the alteration of dehydrogenases alone.

Inhibitors of succinate: quinone reductase/Complex II regulate production of mitochondrial reactive oxygen species and protect normal cells from ischemic damage but induce specific cancer cell death

Succinate:quinone reductase (SQR) of Complex II occupies a unique central point in the mitochondrial respiratory system as a major source of electrons driving reactive oxygen species (ROS) production. It is an ideal pharmaceutical target for modulating ROS levels in normal cells to prevent oxidative stress-induced damage or alternatively,increase ROS in cancer cells, inducing cell death.The value of drugs like diazoxide to prevent ROS production,protecting normal cells, whereas vitamin E analogues promote ROS in cancer cells to kill them is highlighted. As pharmaceuticals these agents may prevent degenerative disease and their modes of action are presently being fully explored. The evidence that SDH/Complex II is tightly coupled to the NADH/NAD+ ratio in all cells,impacted by the available supplies of Krebs cycle intermediates as essential NAD-linked substrates, and the NAD+-dependent regulation of SDH/Complex II are reviewed, as are links to the NAD+-dependent dehydrogenases, Complex I and the E3 dihiydrolipoamide dehydrogenase to produce ROS. This review collates and discusses diverse sources of information relating to ROS production in different biological systems, focussing on evidence for SQR as the main source of ROS production in mitochondria, particularly its relevance to protection from oxidative stress and to the mitochondrial-targeted anti cancer drugs (mitocans) as novel cancer therapies [corrected].

α-Ketoglutarate dehydrogenase: a mitochondrial redox sensor

α-Ketoglutarate dehydrogenase (KGDH), a key regulatory enzyme within the Krebs cycle, is sensitive to mitochondrial redox status. Treatment of mitochondria with H₂O₂ results in reversible inhibition of KGDH due to glutathionylation of the cofactor, lipoic acid. Upon consumption of H₂O₂, glutathione is removed by glutaredoxin restoring KGDH activity. Glutathionylation appears to be enzymatically catalysed or require a unique microenvironment. This may represent an antioxidant response, diminishing the flow of electrons to the respiratory chain and protecting sulphydryl residues from oxidative damage. KGDH is, however, also susceptible to oxidative damage. 4-Hydroxy-2-nonenal (HNE), a lipid peroxidation product, reacts with lipoic acid resulting in enzyme inactivation. Evidence indicates that HNE modified lipoic acid is cleaved from KGDH, potentially the first step of a repair process. KGDH is therefore a likely redox sensor, reversibly altering metabolism to reduce oxidative damage and, under severe oxidative stress, acting as a sentinel of mitochondrial viability.

Differential effects of melatonin on amyloid-beta peptide 25-35-induced mitochondrial dysfunction in hippocampal neurons at different stages of culture

beta-Amyloid (Abeta) is strongly involved in the pathogenesis of Alzheimer's disease (AD), and mitochondria play an important role in neurodegenerative disorders. To determine whether any different effect of melatonin on cultured neurons treated with Abeta in vitro and which may be produced through its different action on mitochondria at different stages of culture, we investigated the damage of cultured rat hippocampal neurons mitochondrial function induced by Abeta in young neurons [days in vitro 10 (DIV 10)] and senescent neurons (DIV 25) and the protective effect of melatonin. Rat hippocampal neurons were incubated with amyloid-beta peptide 25-35 (Abeta25-35) alone or pretreatment with melatonin. Cell viability, mitochondrial membrane potential (Deltapsim), ATP and the activity of the respiratory chain complexes were measured. Data showed that Abeta25-35 caused a reduction in Deltapsim, inhibited the activity of the respiratory chain complexes and led to ATP depletion, melatonin attenuated Abeta25-35-induced mitochondrial impairment in young neurons, whereas melatonin had no effect on Abeta25-35-induced mitochondrial damage in senescent neurons. These results demonstrate that melatonin has differential effect on Abeta25-35-induced mitochondrial dysfunction at different stages of culture and suggest that melatonin is useful for the prevention of AD, rather than treatment.

Matching ATP supply and demand in mammalian heart: in vivo, in vitro, and in silico perspectives

Although the heart rapidly adapts cardiac output to match the body's circulatory demands, the regulatory mechanisms ensuring that sufficient ATP is available to perform the required cardiac work are not completely understood. Two mechanisms have been suggested to serve as key regulators: (1) ADP and Pi concentrations--ATP utilization/hydrolysis in the cytosol increases ADP and Pi fluxes to mitochondria and hence the amount of available substrates for ATP production increases; and (2) Ca2+ concentration--ATP utilization/hydrolysis is coupled to changes in free cytosolic calcium and mitochondrial calcium, the latter controlling Ca2+-dependent activation of mitochondrial enzymes taking part in ATP production. Here we discuss the evolving perspectives of each of the putative regulatory mechanisms and the precise molecular targets (dehydrogenase enzymes, ATP synthase) based on existing experimental and theoretical evidence. The data synthesis can generate novel hypotheses and experimental designs to solve the ongoing enigma of energy supply-demand matching in the heart.

Mitochondrial dysfunction contributes to alveolar developmental arrest in hyperoxia-exposed mice

This study investigated whether mitochondrial dysfunction contributes to alveolar developmental arrest in a mouse model of bronchopulmonary dysplasia (BPD). To induce BPD, 3-day-old mice were exposed to 75% O2. Mice were studied at two time points of hyperoxia (72 h or 2 wk) and after 3 weeks of recovery in room air (RA). A separate cohort of mice was exposed to pyridaben, a complex-I (C-I) inhibitor, for 72 hours or 2 weeks. Alveolarization was quantified by radial alveolar count and mean linear intercept methods. Pulmonary mitochondrial function was defined by respiration rates, ATP-production rate, and C-I activity. At 72 hours, hyperoxic mice demonstrated significant inhibition of C-I activity, reduced respiration and ATP production rates, and significantly decreased radial alveolar count compared with controls. Exposure to pyridaben for 72 hours, as expected, caused significant inhibition of C-I and ADP-phosphorylating respiration. Similar to hyperoxic littermates, these pyridaben-exposed mice exhibited significantly delayed alveolarization compared with controls. At 2 weeks of exposure to hyperoxia or pyridaben, mitochondrial respiration was inhibited and associated with alveolar developmental arrest. However, after 3 weeks of recovery from hyperoxia or 2 weeks after 72 hours of exposure to pyridaben alveolarization significantly improved. In addition, there was marked normalization of C-I and mitochondrial respiration. The degree of hyperoxia-induced pulmonary simplification and recovery strongly (r(2) = 0.76) correlated with C-I activity in lung mitochondria. Thus, the arrest of alveolar development induced by either hyperoxia or direct inhibition of mitochondrial oxidative phosphorylation indicates that bioenergetic failure to maintain normal alveolar development is one of the fundamental mechanisms responsible for BPD.

A construct with fluorescent indicators for conditional expression of miRNA

BACKGROUND

Transgenic RNAi holds promise as a simple, low-cost, and fast method for reverse genetics in mammals. It may be particularly useful for producing animal models for hypomorphic gene function. Inducible RNAi that permits spatially and temporally controllable gene silencing in vivo will enhance the power of transgenic RNAi approach. Furthermore, because microRNA (miRNA) targeting specific genes can be expressed simultaneously with protein coding genes, incorporation of fluorescent marker proteins can simplify the screening and analysis of transgenic RNAi animals.

RESULTS

We sought to optimally express a miRNA simultaneously with a fluorescent marker. We compared two construct designs. One expressed a red fluorescent protein (RFP) and a miRNA placed in its 3' untranslated region (UTR). The other expressed the same RFP and miRNA, but the precursor miRNA (pre-miRNA) coding sequence was placed in an intron that was inserted into the 3'-UTR. We found that the two constructs expressed comparable levels of miRNA. However, the intron-containing construct expressed a significantly higher level of RFP than the intron-less construct. Further experiments indicate that the 3'-UTR intron enhances RFP expression by its intrinsic gene-expression-enhancing activity and by eliminating the inhibitory effect of the pre-miRNA on the expression of RFP. Based on these findings, we incorporated the intron-embedded pre-miRNA design into a conditional expression construct that employed the Cre-loxP system. This construct initially expressed EGFP gene, which was flanked by loxP sites. After exposure to Cre recombinase, the transgene stopped EGFP expression and began expression of RFP and a miRNA, which silenced the expression of specific cellular genes.

CONCLUSION

We have designed and tested a conditional miRNA-expression construct and showed that this construct expresses both the marker genes strongly and can silence the target gene efficiently upon Cre-mediated induction of the miRNA expression. This construct can be used to increase the efficiency of making cell lines or transgenic animals that stably express miRNA targeting specific genes.

Tricarboxylic acid cycle intermediate pool size: functional importance for oxidative metabolism in exercising human skeletal muscle

The tricarboxylic acid (TCA) cycle is the major final common pathway for oxidation of carbohydrates, lipids and some amino acids, which produces reducing equivalents in the form of nicotinamide adenine dinucleotide and flavin adenine dinucleotide that result in production of large amounts of adenosine triphosphate (ATP) via oxidative phosphorylation. Although regulated primarily by the products of ATP hydrolysis, in particular adenosine diphosphate, the rate of delivery of reducing equivalents to the electron transport chain is also a potential regulatory step of oxidative phosphorylation. The TCA cycle is responsible for the generation of approximately 67% of all reducing equivalents per molecule of glucose, hence factors that influence TCA cycle flux will be of critical importance for oxidative phosphorylation. TCA cycle flux is dependent upon the supply of acetyl units, activation of the three non-equilibrium reactions within the TCA cycle, and it has been suggested that an increase in the total concentration of the TCA cycle intermediates (TCAi) is also necessary to augment and maintain TCA cycle flux during exercise. This article reviews the evidence of the functional importance of the TCAi pool size for oxidative metabolism in exercising human skeletal muscle. In parallel with increased oxidative metabolism and TCA cycle flux during exercise, there is an exercise intensity-dependent 4- to 5-fold increase in the concentration of the TCAi. TCAi concentration reaches a peak after 10-15 minutes of exercise, and thereafter tends to decline. This seems to support the suggestion that the concentration of TCAi may be of functional importance for oxidative phosphorylation. However, researchers have been able to induce dissociations between TCAi pool size and oxidative energy provision using a variety of nutritional, pharmacological and exercise interventions. Brief periods of endurance training (5 days or 7 weeks) have been found to result in reduced TCAi pool expansion at the start of exercise (same absolute work intensity) in parallel with either equivalent or increased oxidative energy provision. Cycloserine inhibits alanine aminotransferase, which catalyses the predominant anaplerotic reaction in exercising human muscle. When infused into contracting rat hindlimb muscle, TCAi pool expansion was reduced by 25% with no significant change in oxidative energy provision or power output. Glutamine supplementation has been shown to enhance TCAi pool expansion at the start of exercise with no increase in oxidative energy provision. In summary, there is a consistent dissociation between the extent of TCAi pool expansion at the onset of exercise and oxidative energy provision. At the other end of the spectrum, the parallel loss of TCAi, glycogen and adenine nucleotides and accumulation of inosine monophosphate during prolonged exercise has led to the suggestion that there is a link between muscle glycogen depletion, reduced TCA cycle flux and the development of fatigue. However, analysis of serial biopsies during prolonged exercise demonstrated dissociation between muscle TCAi content and both muscle glycogen content and muscle oxygen uptake. In addition, the delay in fatigue development achieved through increased carbohydrate availability does not attenuate TCAi reduction during prolonged exercise. Therefore, TCAi concentration in whole muscle homogenate does not seem to be of functional importance. However, TCAi content can currently only be measured in whole muscle homogenate rather than the mitochondrial subfraction where TCA cycle reactions occur. In addition, anaplerotic flux rather than TCAi content per se is likely to be of greater importance in determining TCA cycle flux, since TCAi content is probably merely reflective of anaplerotic substrate concentration. Methodological advances are required to allow researchers to address the questions of whether oxidative phosphorylation is limited by mitochondrial TCAi content and/or anaplerotic flux.

Modeling mitochondrial function

The mitochondrion represents a unique opportunity to apply mathematical modeling to a complex biological system. Understanding mitochondrial function and control is important since this organelle is critical in energy metabolism as well as playing key roles in biochemical synthesis, redox control/signaling, and apoptosis. A mathematical model, or hypothesis, provides several useful insights including a rigorous test of the consensus view of the operation of a biological process as well as providing methods of testing and creating new hypotheses. The advantages of the mitochondrial system for applying a mathematical model include the relative simplicity and understanding of the matrix reactions, the ability to study the mitochondria as a independent contained organelle, and, most importantly, one can dynamically measure many of the internal reaction intermediates, on line. The developing ability to internally monitor events within the metabolic network, rather than just the inflow and outflow, is extremely useful in creating critical bounds on complex mathematical models using the individual reaction mechanisms available. However, many serious problems remain in creating a working model of mitochondrial function including the incomplete definition of metabolic pathways, the uncertainty of using in vitro enzyme kinetics, as well as regulatory data in the intact system and the unknown chemical activities of relevant molecules in the matrix. Despite these formidable limitations, the advantages of the mitochondrial system make it one of the best defined mammalian metabolic networks that can be used as a model system for understanding the application and use of mathematical models to study biological systems.

Alpha-ketoglutarate dehydrogenase: a target and generator of oxidative stress

Alpha-ketoglutarate dehydrogenase (alpha-KGDH) is a highly regulated enzyme, which could determine the metabolic flux through the Krebs cycle. It catalyses the conversion of alpha-ketoglutarate to succinyl-CoA and produces NADH directly providing electrons for the respiratory chain. alpha-KGDH is sensitive to reactive oxygen species (ROS) and inhibition of this enzyme could be critical in the metabolic deficiency induced by oxidative stress. Aconitase in the Krebs cycle is more vulnerable than alpha-KGDH to ROS but as long as alpha-KGDH is functional NADH generation in the Krebs cycle is maintained. NADH supply to the respiratory chain is limited only when alpha-KGDH is also inhibited by ROS. In addition being a key target, alpha-KGDH is able to generate ROS during its catalytic function, which is regulated by the NADH/NAD+ ratio. The pathological relevance of these two features of alpha-KGDH is discussed in this review, particularly in relation to neurodegeneration, as an impaired function of this enzyme has been found to be characteristic for several neurodegenerative diseases.

Preconditioning prevents loss in mitochondrial function and release of cytochrome c during prolonged cardiac ischemia/reperfusion

Loss in mitochondrial function and induction of mitochondrial-mediated apoptosis occur as a result of cardiac ischemia/reperfusion. Brief and repeated cycles of ischemia/reperfusion, termed ischemic preconditioning, prevent or minimize contractile dysfunction and apoptosis associated with prolonged episodes of cardiac ischemia and reperfusion. The effects of preconditioning on various indices of ischemia/reperfusion-induced alterations in mitochondrial function and structure were therefore explored. Utilizing an in vivo rat model data is provided indicating that preconditioning completely prevents cardiac ischemia/reperfusion-induced: (1) loss in the activity of the redox sensitive Krebs cycle enzyme alpha-ketoglutarate dehydrogenase; (2) declines in NADH-linked ADP-dependent mitochondrial respiration; (3) insertion of the pro-apoptotic Bcl-2 protein Bax into the mitochondrial membrane; and (4) release of cytochrome c into the cytosol. The results of the current study indicate that preconditioning prevents specific alterations in mitochondrial structure and function that are known to impact cellular viability and provide insight into the collective benefits of preconditioning.

Distribution of mitochondrial NADH fluorescence lifetimes: steady-state kinetics of matrix NADH interactions

The lifetimes of fluorescent components of matrix NADH in isolated porcine heart mitochondria were investigated using time-resolved fluorescence spectroscopy. Three distinct lifetimes of fluorescence were resolved: 0.4 (63%), 1.8 (30%), and 5.7 (7%) ns (% total NADH). The 0.4 ns lifetime and the emission wavelength of the short component were consistent with free NADH. In addition to their longer lifetimes, the remaining pools also had a blue-shifted emission spectrum consistent with immobilized NADH. On the basis of emission frequency and lifetime data, the immobilized pools contributed >80% of NADH fluorescence. The steady-state kinetics of NADH entering the immobilized pools was measured in intact mitochondria and in isolated mitochondrial membranes. The apparent binding constants (K(D)s) for NADH in intact mitochondria, 2.8 mM (1.9 ns pool) and >3 mM (5.7 ns pool), were on the order of the estimated matrix [NADH] (approximately 3.5 mM). The affinities and fluorescence lifetimes resulted in an essentially linear relationship between matrix [NADH] and NADH fluorescence intensity. Mitochondrial membranes had shorter emission lifetimes in the immobilized poo1s [1 ns (34%) and 4.1 ns (8%)] with much higher apparent K(D)s of 100 microM and 20 microM, respectively. The source of the stronger NADH binding affinity in membranes is unknown but could be related to high order structure or other cofactors that are diluted out in the membrane preparation. In both preparations, the rate of NADH oxidation was proportional to the amount of NADH in the long lifetime pools, suggesting that a significant fraction of the bound NADH might be associated with oxidative phosphorylation, potentially in complex 1.

Inhibition of mitochondrial Na+-Ca2+ exchanger increases mitochondrial metabolism and potentiates glucose-stimulated insulin secretion in rat pancreatic islets

The mitochondrial Na(+)-Ca(2+) exchanger (mNCE) mediates efflux of Ca(2+) from mitochondria in exchange for influx of Na(+). We show that inhibition of the mNCE enhances mitochondrial oxidative metabolism and increases glucose-stimulated insulin secretion in rat islets and INS-1 cells. The benzothiazepine CGP37157 inhibited mNCE activity in INS-1 cells (50% inhibition at IC(50) = 1.5 micro mol/l) and increased the glucose-induced rise in mitochondrial Ca(2+) ([Ca(2+)](m)) 2.1 times. Cellular ATP content was increased by 13% in INS-1 cells and by 49% in rat islets by CGP37157 (1 micro mol/l). Krebs cycle flux was also stimulated by CGP37157 when glucose was present. Insulin secretion was increased in a glucose-dependent manner by CGP37157 in both INS-1 cells and islets. In islets, CGP37157 increased insulin secretion dose dependently (half-maximal efficacy at EC(50) = 0.06 micro mol/l) at 8 mmol/l glucose and shifted the glucose dose response curve to the left. In perifused islets, mNCE inhibition had no effect on insulin secretion at 2.8 mmol/l glucose but increased insulin secretion by 46% at 11 mmol/l glucose. The effects of CGP37157 could not be attributed to interactions with the plasma membrane sodium calcium exchanger, L-type calcium channels, ATP-sensitive K(+) channels, or [Ca(2+)](m) uniporter. In hyperglycemic clamp studies of Wistar rats, CGP37157 increased plasma insulin and C-peptide levels only during the hyperglycemic phase of the study. These results illustrate the potential utility of agents that affect mitochondrial metabolism as novel insulin secretagogues.

Alterations in mitochondrial function in a mouse model of hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (HCM) is an autosomal dominant disease characterized by varying degrees of ventricular hypertrophy and myofibrillar disarray. Mutations in cardiac contractile proteins cause HCM. However, there is an unexplained wide variability in the clinical phenotype, and it is likely that there are multiple contributing factors. Because mitochondrial dysfunction has been described in heart disease, we tested the hypothesis that mitochondrial dysfunction contributes to the varying HCM phenotypes. Mitochondrial function was assessed in two transgenic models of HCM: mice with a mutant myosin heavy chain gene (MyHC) or with a mutant cardiac troponin T (R92Q) gene. Despite mitochondrial ultrastructural abnormalities in both models, the rate of state 3 respiration was significantly decreased only in the mutant MyHC mice by approximately 23%. Notably, this decrease in state 3 respiration preceded hemodynamic dysfunction. The maximum activity of alpha-ketogutarate dehydrogenase as assayed in isolated disrupted mitochondria was decreased by 28% compared with isolated control mitochondria. In addition, complexes I and IV were decreased in mutant MyHC transgenic mice. Inhibition of beta-adrenergic receptor kinase, which is elevated in mutant MyHC mouse hearts, can prevent mitochondrial respiratory impairment in mutant MyHC mice. Thus our results suggest that mitochondria may contribute to the hemodynamic dysfunction seen in some forms of HCM and offer a plausible mechanism responsible for some of the heterogeneity of the disease phenotypes.

Selective inactivation of redox-sensitive mitochondrial enzymes during cardiac reperfusion

Reperfusion of ischemic myocardial tissue results in an increase in mitochondrial free radical production and declines in respiratory activity. The effects of ischemia and reperfusion on the activities of Krebs cycle enzymes, as well as enzymes involved in electron transport, were evaluated to provide insight into whether free radical events are likely to affect enzymatic and mitochondrial function(s). An in vivo rat model was utilized in which ischemia is induced by ligating the left anterior descending coronary artery. Reperfusion, initiated by release of the ligature, resulted in a significant decline in NADH-linked ADP-dependent mitochondrial respiration as assessed in isolated cardiac mitochondria. Assays of respiratory chain complexes revealed reduction in the activities of complex I and, to a lesser extent, complex IV exclusively during reperfusion, with no alterations in the activities of complexes II and III. Moreover, Krebs cycle enzymes alpha-ketoglutarate dehydrogenase and aconitase were susceptible to reperfusion-induced inactivation with no decline in the activities of other Krebs cycle enzymes. The decline in alpha-ketoglutarate dehydrogenase activity during reperfusion was associated with a loss in native lipoic acid on the E2 subunit, suggesting oxidative inactivation. Inhibition of complex I in vitro promotes free radical generation. alpha-Ketoglutarate dehydrogenase and aconitase are uniquely susceptible to in vitro oxidative inactivation. Thus, our results suggest a scenario in which inhibition of complex I promotes free radical production leading to oxidative inactivation of alpha-ketoglutarate dehydrogenase and aconitase.

Cardiac energy metabolism homeostasis: role of cytosolic calcium

The heart is capable of dramatically altering its overall energy flux with minimal changes in the concentrations of metabolites that are associated with energy metabolism. This cardiac energy metabolism homeostasis is discussed with regard to the potential cytosolic control network responsible for controlling the major energy conversion pathway, oxidative phosphorylation in mitochondria. Several models for this cytosolic control network have been proposed, but a cytosolic Ca(2+) dependent parallel activation scheme for metabolism and work is consistent with most of the experimental results. That model proposes that cytosolic Ca(2+) regulates both the utilization of ATP by the work producing ATPases as well as the mitochondrial production of ATP. Recent studies have provided evidence supporting this role of cytosolic Ca(2+). These data include the demonstration that mitochondrial [Ca(2+)] can track cytosolic [Ca(2+)] and that the compartmentation of cytosolic [Ca(2+)] can facilitate this process. On the metabolic side, Ca(2+) has been shown to rapidly activate several steps in oxidative phosphorylation, including F(1)F(0)-ATPase ATP production as well as several dehydrogenases, which results in a homeostasis of mitochondrial metabolites similar to that observed in the cytosol. Numerous problems with the Ca(2+) parallel activation hypothesis remain including the lack of specific mechanisms of mitochondrial Ca(2+) transport and regulation of F(1)F(0)-ATPase, the time dependence of Ca(2+) activation of cytosolic ATPases as well as oxidative phosphorylation, and the role of cytosolic compartmentation. In addition, the lack of cytosolic or mitochondrial [Ca(2+)] measurements under in vivo conditions is problematic. Several lines of investigation to address these issues are suggested. A model of the cardiac energy metabolism control network is proposed that includes a Ca(2+) parallel activation component together with more classical elements including metabolite feedback and cytosolic compartmentation.

Melatonin increases the activity of the oxidative phosphorylation enzymes and the production of ATP in rat brain and liver mitochondria

We recently showed that melatonin counteracted mitochondrial oxidative stress and increased the activity of the mitochondrial oxidative phosphorylation (OXPHOS) enzymes both in vivo and in vitro. To further clarify these effects, we studied here the activity of OXPHOS enzymes and the synthesis of ATP in rat liver and brain mitochondria in vitro. In sub-mitochondrial particles, melatonin increases the activity of the complexes I and IV dose-dependently, the effect being significant between 1 and 10nM. Blue native-PAGE followed by histochemical analysis of the OXPHOS enzymes further showed the melatonin-induced increase of complex I activity. Titration studies show that melatonin counteracts the partial inhibition of complex IV induced by 5 microM potassium cyanide. However, melatonin (up to 5mM) was unable to recover the activity of complex IV when it was completely blocked by 100 microM cyanide. These data suggest that the indoleamine could stimulate the activity of the non-inhibited part of the complex IV. Melatonin also increases the production of ATP in control mitochondria and counteracts the cyanide-induced inhibition of ATP synthesis. These results provide new hormonal mechanism regulating mitochondrial homeostasis and may explain, at least in part, the anti-aging and neuroprotective properties of melatonin.

Inhibition of Krebs cycle enzymes by hydrogen peroxide: A key role of [alpha]-ketoglutarate dehydrogenase in limiting NADH production under oxidative stress

In this study we addressed the function of the Krebs cycle to determine which enzyme(s) limits the availability of reduced nicotinamide adenine dinucleotide (NADH) for the respiratory chain under H(2)O(2)-induced oxidative stress, in intact isolated nerve terminals. The enzyme that was most vulnerable to inhibition by H(2)O(2) proved to be aconitase, being completely blocked at 50 microm H(2)O(2). alpha-Ketoglutarate dehydrogenase (alpha-KGDH) was also inhibited but only at higher H(2)O(2) concentrations (>/=100 microm), and only partial inactivation was achieved. The rotenone-induced increase in reduced nicotinamide adenine dinucleotide (phosphate) [NAD(P)H] fluorescence reflecting the amount of NADH available for the respiratory chain was also diminished by H(2)O(2), and the effect exerted at small concentrations (</=50 microm) of the oxidant was completely prevented by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), an inhibitor of glutathione reductase. BCNU-insensitive decline by H(2)O(2) in the rotenone-induced NAD(P)H fluorescence correlated with inhibition of alpha-ketoglutarate dehydrogenase. Decrease in the glutamate content of nerve terminals was induced by H(2)O(2) at concentrations inhibiting aconitase. It is concluded that (1) aconitase is the most sensitive enzyme in the Krebs cycle to inhibition by H(2)O(2), (2) at small H(2)O(2) concentrations (</=50 microm) when aconitase is inactivated, glutamate fuels the Krebs cycle and NADH generation is unaltered, (3) at higher H(2)O(2) concentrations (>/=100 microm) inhibition of alpha-ketoglutarate dehydrogenase limits the amount of NADH available for the respiratory chain, and (4) increased consumption of NADPH makes a contribution to the H(2)O(2)-induced decrease in the amount of reduced pyridine nucleotides. These results emphasize the importance of alpha-KGDH in impaired mitochondrial function under oxidative stress, with implications for neurodegenerative diseases and cell damage induced by ischemia/reperfusion.

Inhibition of Krebs cycle enzymes by hydrogen peroxide: A key role of [alpha]-ketoglutarate dehydrogenase in limiting NADH production under oxidative stress

In this study we addressed the function of the Krebs cycle to determine which enzyme(s) limits the availability of reduced nicotinamide adenine dinucleotide (NADH) for the respiratory chain under H(2)O(2)-induced oxidative stress, in intact isolated nerve terminals. The enzyme that was most vulnerable to inhibition by H(2)O(2) proved to be aconitase, being completely blocked at 50 microm H(2)O(2). alpha-Ketoglutarate dehydrogenase (alpha-KGDH) was also inhibited but only at higher H(2)O(2) concentrations (>/=100 microm), and only partial inactivation was achieved. The rotenone-induced increase in reduced nicotinamide adenine dinucleotide (phosphate) [NAD(P)H] fluorescence reflecting the amount of NADH available for the respiratory chain was also diminished by H(2)O(2), and the effect exerted at small concentrations (</=50 microm) of the oxidant was completely prevented by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), an inhibitor of glutathione reductase. BCNU-insensitive decline by H(2)O(2) in the rotenone-induced NAD(P)H fluorescence correlated with inhibition of alpha-ketoglutarate dehydrogenase. Decrease in the glutamate content of nerve terminals was induced by H(2)O(2) at concentrations inhibiting aconitase. It is concluded that (1) aconitase is the most sensitive enzyme in the Krebs cycle to inhibition by H(2)O(2), (2) at small H(2)O(2) concentrations (</=50 microm) when aconitase is inactivated, glutamate fuels the Krebs cycle and NADH generation is unaltered, (3) at higher H(2)O(2) concentrations (>/=100 microm) inhibition of alpha-ketoglutarate dehydrogenase limits the amount of NADH available for the respiratory chain, and (4) increased consumption of NADPH makes a contribution to the H(2)O(2)-induced decrease in the amount of reduced pyridine nucleotides. These results emphasize the importance of alpha-KGDH in impaired mitochondrial function under oxidative stress, with implications for neurodegenerative diseases and cell damage induced by ischemia/reperfusion.

2,3-butanedione monoxime unmasks Ca(2+)-induced NADH formation and inhibits electron transport in rat hearts

We used 2,3-butanedione monoxime (BDM) to suppress work by the perfused rat heart and to investigate the effects of calcium on NADH production and tissue energetics. Hearts were perfused with buffer containing BDM and elevated perfusate calcium to maintain the rates of cardiac work and oxygen consumption at levels similar to those of control perfused hearts. BDM plus calcium hearts displayed higher levels of NADH surface fluorescence, indicating calcium activation of mitochondrial dehydrogenases. These hearts, however, displayed 20% lower phosphocreatine levels. BDM suppressed the rates of state 3 respiration of isolated mitochondria. Uncoupled respiration was suppressed to a lesser degree, and the state 4 respiration rates were not affected. Double-inhibitor experiments with liver mitochondria using BDM and carboxyatractyloside (CAT) were used to identify the site of inhibition. BDM at low levels (0-5 mM) suppressed respiration. In the presence of CAT at levels that inhibit respiration by 60%, low levels of BDM were without effect. Because these effects were not additive, BDM does not inhibit adenine nucleotide transport. This was supported by an assay of adenine nucleotide transport in liver mitochondria. BDM did not inhibit ATP hydrolysis by submitochondrial particles but strongly suppressed reversed electron transport from succinate to NAD(+). Oxidation of NADH by submitochondrial particles was inhibited by BDM but oxidation of succinate was not. We conclude that BDM inhibits electron transport at site 1.

Substrate oxidation and ATP supply in AS-30D hepatoma cells

The oxidation of several metabolites in AS-30D tumor cells was determined. Glucose and glycogen consumption and lactic acid production showed high rates, indicating a high glycolytic activity. The utilization of ketone bodies, oxidation of endogenous glutamate, and oxidative phosphorylation were also very active: tumor cells showed a high respiration rate (100 ng atoms oxygen (min x 10(7) cells)(-1)), which was 90% oligomycin-sensitive. AS-30D tumor cells underwent significant intracellular volume changes, which preserved high concentrations of several metabolites. A high O(2) concentration, but a low glucose concentration were found in the cell-free ascites liquid. Glutamine was the oxidizable substrate found at the highest concentration in the ascites liquid. We estimated that cellular ATP was mainly provided by oxidative phosphorylation. These data indicated that AS-30D hepatoma cells had a predominantly oxidative and not a glycolytic type of metabolism. The NADH-ubiquinol oxido reductase and the enzyme block for ATP utilization were the sites that exerted most of the control of oxidative phosphorylation (flux control coefficient = 0.3-0.42).

Ca(2+) activation of heart mitochondrial oxidative phosphorylation: role of the F(0)/F(1)-ATPase

Ca(2+) has been postulated as a cytosolic second messenger in the regulation of cardiac oxidative phosphorylation. This hypothesis draws support from the well-known effects of Ca(2+) on muscle activity, which is stimulated in parallel with the Ca(2+)-sensitive dehydrogenases (CaDH). The effects of Ca(2+) on oxidative phosphorylation were further investigated in isolated porcine heart mitochondria at the level of metabolic driving force (NADH or Deltapsi) and ATP production rates (flow). The resulting force-flow (F-F) relationships permitted the analysis of Ca(2+) effects on several putative control points within oxidative phosphorylation, simultaneously. The F-F relationships resulting from additions of carbon substrates alone provided a model of pure CaDH activation. Comparing this curve with variable Ca(2+) concentration ([Ca(2+)]) effects revealed an approximate twofold higher ATP production rate than could be explained by a simple increase in NADH or Deltapsi via CaDH activation. The half-maximal effect of Ca(2+ )at state 3 was 157 nM and was completely inhibited by ruthenium red (1 microM), indicating matrix dependence of the Ca(2+) effect. Arsenate was used as a probe to differentiate between F(0)/F(1)-ATPase and adenylate translocase activity by a futile recycling of ADP-arsenate within the matrix, catalyzed by the F(0)/F(1)-ATPase. Ca(2+) increased the ADP arsenylation rate more than twofold, suggesting a direct effect on the F(0)/F(1)-ATPase. These results suggest that Ca(2+) activates cardiac aerobic respiration at the level of both the CaDH and F(0)/F(1)-ATPase. This type of parallel control of both intermediary metabolism and ATP synthesis may provide a mechanism of altering ATP production rates with minimal changes in the high-energy intermediates as observed in vivo.

Regulation of ATP supply in mammalian skeletal muscle during resting state-->intensive work transition

In the present debating paper, the problem how the rate of ATP supply by oxidative phosphorylation in mitochondria is adjusted to meet a greatly increased demand for ATP during intensive exercise of skeletal muscle is discussed. Different experimental results are collected from different positions of the literature and confronted with five conceptual models of the regulation of the oxidative phosphorylation system. The previously performed computer simulations using a dynamic model of oxidative phosphorylation are also discussed in this context. The possible regulatory mechanisms considered in the present article are: (A) output activation: an external effector activates directly only the output of the system (ATP turnover); (B) input/output activation: an external effector activates directly the output (ATP usage) and input (substrate dehydrogenation) of the system; (C) removal of substrate shortage: only ATP consumption and substrate supply by blood are directly activated; (D) removal of oxygen shortage: only ATP consumption and oxygen supply by blood are directly activated; (E) each step activation: an external effector activates both the ATP-consuming subsystem and all the steps in the ATP-producing subsystem (particular enzymes/carriers/blocks of oxidative phosphorylation, substrate supply, oxygen supply). The performed confrontation of the considered mechanisms with the presented results leads to the conclusion that only the each step activation model is quantitatively consistent with the whole set of experimental data discussed. It is therefore postulated that a universal effector/regulatory mechanism of a still unknown nature which activates all steps of oxidative phosphorylation should exist and be discovered. A possible nature of such an effector is shortly discussed.

Mitochondrial effects of triarylmethane dyes

The mitochondrial effects of submicromolar concentrations of six triarylmethane dyes, with potential applications in antioncotic photodynamic therapy, were studied. All dyes promoted an inhibition of glutamate or succinate-supported respiration in uncoupled mitochondria, in a manner stimulated photodynamically. No inhibition of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) supported respiration was observed, indicating that these dyes do not affect mitochondrial complex IV. When mitochondria were energized with TMPD in the absence of an uncoupler, treatment with victoria blue R, B, or BO, promoted a dissipation of mitochondrial membrane potential and increase of respiratory rates, compatible with mitochondrial uncoupling. This effect was observed even in the dark, and was not prevented by EGTA, Mg2+ or cyclosporin A, suggesting that it is promoted by a direct effect of the dye on inner mitochondrial membrane permeability to protons. Indeed, victoria blue R, B, and BO promoted swelling of valinomycin-treated mitochondria incubated in a hyposmotic K+-acetate-based medium, confirming that these dyes act as classic protonophores such as FCCP. On the other hand, ethyl violet, crystal violet, and malachite green promoted a dissipation of mitochondrial membrane potential, accompanied by mitochondrial swelling, which was prevented by EGTA, Mg2+, and cyclosporin A, demonstrating that these drugs induce mitochondrial permeability transition. This mitochondrial permeabilization was followed by respiratory inhibition, attributable to cytochrome c release, and was caused by the oxidation of NAD(P)H promoted by these drugs.

Depolarization of in situ mitochondria due to hydrogen peroxide-induced oxidative stress in nerve terminals: inhibition of alpha-ketoglutarate dehydrogenase

Mitochondrial membrane potential (delta psi(m)) was determined in intact isolated nerve terminals using the membrane potential-sensitive probe JC-1. Oxidative stress induced by H2O2 (0.1-1 mM) caused only a minor decrease in delta psi(m). When complex I of the respiratory chain was inhibited by rotenone (2 microM), delta psi(m) was unaltered, but on subsequent addition of H2O2, delta psi(m) started to decrease and collapsed during incubation with 0.5 mM H2O2 for 12 min. The ATP level and [ATP]/[ADP] ratio were greatly reduced in the simultaneous presence of rotenone and H2O2. H2O2 also induced a marked reduction in delta psi(m) when added after oligomycin (10 microM), an inhibitor of F0F1-ATPase. H2O2 (0.1 or 0.5 mM) inhibited alpha-ketoglutarate dehydrogenase and decreased the steady-state NAD(P)H level in nerve terminals. It is concluded that there are at least two factors that determine delta psi(m) in the presence of H2O2: (a) The NADH level reduced owing to inhibition of alpha-ketoglutarate dehydrogenase is insufficient to ensure an optimal rate of respiration, which is reflected in a fall of delta psi(m) when the F0F1-ATPase is not functional. (b) The greatly reduced ATP level in the presence of rotenone and H2O2 prevents maintenance of delta psi(m) by F0F1-ATPase. The results indicate that to maintain delta psi(m) in the nerve terminal during H2O2-induced oxidative stress, both complex I and F0F1-ATPase must be functional. Collapse of delta psi(m) could be a critical event in neuronal injury in ischemia or Parkinson's disease when H2O2 is generated in excess and complex I of the respiratory chain is simultaneously impaired.

Declines in mitochondrial respiration during cardiac reperfusion: age-dependent inactivation of alpha-ketoglutarate dehydrogenase

We previously reported that cardiac reperfusion results in declines in mitochondrial NADH-linked respiration. The degree of inactivation increased with age and was paralleled by modification of protein by the lipid peroxidation product 4-hydroxy-2-nonenal. To gain insight into potential sites of oxidative damage, the present study was undertaken to identify specific mitochondrial protein(s) inactivated during ischemia and reperfusion and to determine which of these losses in activity are responsible for observed declines in mitochondrial respiration. Using a Langendorff rat heart perfusion protocol, we observed age-dependent inactivation of complex I during ischemia and complex IV and alpha-ketoglutarate dehydrogenase during reperfusion. Although losses in complex I and IV activities were found not to be of sufficient magnitude to cause declines in mitochondrial respiration, an age-related decrease in complex I activity during ischemia may predispose old animals to more severe oxidative damage during reperfusion. It was determined that inactivation of alpha-ketoglutarate dehydrogenase is responsible, in large part, for observed reperfusion-induced declines in NADH-linked respiration. alpha-Ketoglutarate dehydrogenase is highly susceptible to 4-hydroxy-2-nonenal inactivation in vitro. Thus, our results suggest a plausible mechanism for age-dependent, reperfusion-induced declines in mitochondrial function and identify alpha-ketoglutarate dehydrogenase as a likely site of free radical-mediated damage.

Interaction of factors determining oxygen uptake at the onset of exercise

Considerable debate surrounds the issue of whether the rate of adaptation of skeletal muscle O2 consumption (QO2) at the onset of exercise is limited by 1) the inertia of intrinsic cellular metabolic signals and enzyme activation or 2) the availability of O2 to the mitochondria, as determined by an extrinsic inertia of convective and diffusive O2 transport mechanisms. This review critically examines evidence for both hypotheses and clarifies important limitations in the experimental and theoretical approaches to this issue. A review of biochemical evidence suggests that a given respiratory rate is a function of the net drive of phosphorylation potential and redox potential and cellular mitochondrial PO2 (PmitoO2). Changes in both phosphorylation and redox potential are determined by intrinsic metabolic inertia. PmitoO2 is determined by the extrinsic inertia of both convective and diffusive O2 transport mechanisms during the adaptation to exercise and the rate of mitochondrial O2 utilization. In a number of exercise conditions, PmitoO2 appears to be within a range capable of modulating muscle metabolism. Within this context, adjustments in the phosphate energy state of the cell would serve as a cytosolic "transducer," linking ATP consumption with mitochondrial ATP production and, therefore, O2 consumption. The availability of reducing equivalents and O2 would modulate the rate of adaptation of QO2.

PUFA and aging modulate cardiac mitochondrial membrane lipid composition and Ca2+ activation of PDH

Aberrations in cell Ca2+ homeostasis have been known to parallel both changes in membrane lipid composition and aging. Previous work has shown that the lowered efficiency of work performance, which occurs in isolated hearts from rats fed a diet rich in n-6 polyunsaturated fatty acids (PUFA), relative to those fed n-3 PUFA, could be raised by mitochondrial (Mito) Ca2+ transport inhibition. We tested whether, after Ca2+-dependent stress, the Ca2+-dependent activation of pyruvate dehydrogenase (PDHA/PDHTotal) and Mito Ca2+ cycling could be manipulated by varying the ratio of n-3 to n-6 PUFA in Mito membranes in young (6 mo) and aged (24 mo) isolated rat hearts treated to n-3 or n-6 PUFA-rich diet. Inotropic stimulation by 1 microM norepinephrine (NE) of 24-mo n-6 PUFA-rich hearts elevated total Mito Ca2+ content 38% more than in 6-mo hearts (P < 0. 05). However, both the NE-induced rise in Mito Ca2+ and the difference in response between 6- and 24-mo hearts were partially abolished by n-3 PUFA treatment. NE increased the fractional activation of PDH by 44% above control levels in the 6-mo group compared with 49% in the 24-mo group after n-6 PUFA diet. However, NE stimulation of PDHA was attenuated by n-3 PUFA diet, attaining values only 29 and 23% above control levels in 6- and 24-mo mitochondria, respectively (P < 0.05). Global ischemia and reperfusion (I/R) in n-6 PUFA hearts gave rise to higher levels of total Mito Ca2+ concentration (P < 0.0001) and PDHA (P < 0.0001) compared with n-3 PUFA. Ruthenium red (3.4 microM) abolished the effects of I/R in all groups. With aging, heart Mito membrane phosphatidylcholine was increased after n-6 PUFA-rich diet (by approximately 15%, P < 0.05), whereas cardiolipin and n-3 PUFA content were diminished by 31% (P < 0.05) and 73% (P < 0.05), respectively. These effects were prevented by n-3 PUFA-rich diet. The present study, by directly manipulating the cardiac Mito membrane n-3-to-n-6 PUFA ratio, shows that the activation of Ca2+-dependent PDH can be augmented when the n-3-to-n-6 PUFA ratio is low (n-6 PUFA-rich diet; 24-mo hearts) or attenuated when this ratio is relatively high (n-3 PUFA-rich diet). We propose that one of the consequences of dietary-induced manipulation of membrane phospholipids and PUFAs may be the altered flux of Ca2+ across the Mito membrane and thus altered intramitochondrial Ca2+-dependent processes.