Respiration-dependent removal of exogenous H2O2 in brain mitochondria: inhibition by Ca2+

Real-time imaging of mitochondrial redox reveals increased mitochondrial oxidative stress associated with amyloid β aggregates in vivo in a mouse model of Alzheimer's disease

BACKGROUND

Reactive oxidative stress is a critical player in the amyloid beta (Aβ) toxicity that contributes to neurodegeneration in Alzheimer's disease (AD). Damaged mitochondria are one of the main sources of reactive oxygen species and accumulate in Aβ plaque-associated dystrophic neurites in the AD brain. Although Aβ causes neuronal mitochondria reactive oxidative stress in vitro, this has never been directly observed in vivo in the living mouse brain. Here, we tested for the first time whether Aβ plaques and soluble Aβ oligomers induce mitochondrial oxidative stress in surrounding neurons in vivo, and whether this neurotoxic effect can be abrogated using mitochondrial-targeted antioxidants.

METHODS

We expressed a genetically encoded fluorescent ratiometric mitochondria-targeted reporter of oxidative stress in mouse models of the disease and performed intravital multiphoton microscopy of neuronal mitochondria and Aβ plaques.

RESULTS

For the first time, we demonstrated by direct observation in the living mouse brain exacerbated mitochondrial oxidative stress in neurons after both Aβ plaque deposition and direct application of soluble oligomeric Aβ onto the brain, and determined the most likely pathological sequence of events leading to oxidative stress in vivo. Oxidative stress could be inhibited by both blocking calcium influx into mitochondria and treating with the mitochondria-targeted antioxidant SS31. Remarkably, the latter ameliorated plaque-associated dystrophic neurites without impacting Aβ plaque burden.

CONCLUSIONS

Considering these results, combination of mitochondria-targeted compounds with other anti-amyloid beta or anti-tau therapies hold promise as neuroprotective drugs for the prevention and/or treatment of AD.

Mitochondrial Properties in Skeletal Muscle Fiber

Mitochondria are the primary source of energy production and are implicated in a wide range of biological processes in most eukaryotic cells. Skeletal muscle heavily relies on mitochondria for energy supplements. In addition to being a powerhouse, mitochondria evoke many functions in skeletal muscle, including regulating calcium and reactive oxygen species levels. A healthy mitochondria population is necessary for the preservation of skeletal muscle homeostasis, while mitochondria dysregulation is linked to numerous myopathies. In this review, we summarize the recent studies on mitochondria function and quality control in skeletal muscle, focusing mainly on in vivo studies of rodents and human subjects. With an emphasis on the interplay between mitochondrial functions concerning the muscle fiber type-specific phenotypes, we also discuss the effect of aging and exercise on the remodeling of skeletal muscle and mitochondria properties.

Mitochondria as a therapeutic: a potential new frontier in driving the shift from tissue repair to regeneration

Fibrosis, or scar tissue development, is associated with numerous pathologies and is often considered a worst-case scenario in terms of wound healing or the implantation of a biomaterial. All that remains is a disorganized, densely packed and poorly vascularized bundle of connective tissue, which was once functional tissue. This creates a significant obstacle to the restoration of tissue function or integration with any biomaterial. Therefore, it is of paramount importance in tissue engineering and regenerative medicine to emphasize regeneration, the successful recovery of native tissue function, as opposed to repair, the replacement of the native tissue (often with scar tissue). A technique dubbed 'mitochondrial transplantation' is a burgeoning field of research that shows promise in in vitro, in vivo and various clinical applications in preventing cell death, reducing inflammation, restoring cell metabolism and proper oxidative balance, among other reported benefits. However, there is currently a lack of research regarding the potential for mitochondrial therapies within tissue engineering and regenerative biomaterials. Thus, this review explores these promising findings and outlines the potential for mitochondrial transplantation-based therapies as a new frontier of scientific research with respect to driving regeneration in wound healing and host-biomaterial interactions, the current successes of mitochondrial transplantation that warrant this potential and the critical questions and remaining obstacles that remain in the field.

Therapeutic Efficiency of Humic Acids in Intoxications

Humins, humic and fulvic acids represent molecules with complex structures. These compounds comprising humic substances (HS) exist naturally in soil, brown coal, peat, and water. They are formed during the decomposition and transformation of organic matter (animal and plant remains) and their formation explains several theories. Within their chemical structures, there are numerous phenolic and carboxyl groups and their derivatives that affect their different properties, such as their solubility in water or their absorption of cations or mycotoxins. The manifold chemical structure of HS alters their polyelectrolyte character and thus their chelating efficiency. For many years, HS have been studied due to their detoxification, anti-, and pro-inflammatory or anticancer and antiviral ability. This article summarizes the antioxidant and adsorption properties of humic acids, highlighting their usefulness in intoxications.

Exploring Whether Iron Sequestration within the CNS of Patients with Alzheimer’s Disease Causes a Functional Iron Deficiency That Advances Neurodegeneration

The involvement of iron in the pathogenesis of Alzheimer’s disease (AD) may be multifaceted. Besides potentially inducing oxidative damage, the bioavailability of iron may be limited within the central nervous system, creating a functionally iron-deficient state. By comparing staining results from baseline and modified iron histochemical protocols, iron was found to be more tightly bound within cortical sections from patients with high levels of AD pathology compared to subjects with a diagnosis of something other than AD. To begin examining whether the bound iron could cause a functional iron deficiency, a protein-coding gene expression dataset of initial, middle, and advanced stages of AD from olfactory bulb tissue was analyzed for iron-related processes with an emphasis on anemia-related changes in initial AD to capture early pathogenic events. Indeed, anemia-related processes had statistically significant alterations, and the significance of these changes exceeded those for AD-related processes. Other changes in patients with initial AD included the expressions of transcripts with iron-responsive elements and for genes encoding proteins for iron transport and mitochondrial-related processes. In the latter category, there was a decreased expression for the gene encoding pitrilysin metallopeptidase 1 (PITRM1). Other studies have shown that PITRM1 has an altered activity in patients with AD and is associated with pathological changes in this disease. Analysis of a gene expression dataset from PITRM1-deficient or sufficient organoids also revealed statistically significant changes in anemia-like processes. These findings, together with supporting evidence from the literature, raise the possibility that a pathogenic mechanism of AD could be a functional deficiency of iron contributing to neurodegeneration.

Therapeutic Strategies Targeting Mitochondrial Calcium Signaling: A New Hope for Neurological Diseases?

Calcium (Ca²⁺) is a versatile secondary messenger involved in the regulation of a plethora of different signaling pathways for cell maintenance. Specifically, intracellular Ca²⁺ homeostasis is mainly regulated by the endoplasmic reticulum and the mitochondria, whose Ca²⁺ exchange is mediated by appositions, termed endoplasmic reticulum-mitochondria-associated membranes (MAMs), formed by proteins resident in both compartments. These tethers are essential to manage the mitochondrial Ca²⁺ influx that regulates the mitochondrial function of bioenergetics, mitochondrial dynamics, cell death, and oxidative stress. However, alterations of these pathways lead to the development of multiple human diseases, including neurological disorders, such as amyotrophic lateral sclerosis, Friedreich's ataxia, and Charcot-Marie-Tooth. A common hallmark in these disorders is mitochondrial dysfunction, associated with abnormal mitochondrial Ca²⁺ handling that contributes to neurodegeneration. In this work, we highlight the importance of Ca²⁺ signaling in mitochondria and how the mechanism of communication in MAMs is pivotal for mitochondrial maintenance and cell homeostasis. Lately, we outstand potential targets located in MAMs by addressing different therapeutic strategies focused on restoring mitochondrial Ca²⁺ uptake as an emergent approach for neurological diseases.

Mitochondrial Management of Reactive Oxygen Species

Mitochondria in aerobic eukaryotic cells are both the site of energy production and the formation of harmful species, such as radicals and other reactive oxygen species, known as ROS. They contain an efficient antioxidant system, including low-molecular-mass molecules and enzymes that specialize in removing various types of ROS or repairing the oxidative damage of biological molecules. Under normal conditions, ROS production is low, and mitochondria, which are their primary target, are slightly damaged in a similar way to other cellular compartments, since the ROS released by the mitochondria into the cytosol are negligible. As the mitochondrial generation of ROS increases, they can deactivate components of the respiratory chain and enzymes of the Krebs cycle, and mitochondria release a high amount of ROS that damage cellular structures. More recently, the feature of the mitochondrial antioxidant system, which does not specifically deal with intramitochondrial ROS, was discovered. Indeed, the mitochondrial antioxidant system detoxifies exogenous ROS species at the expense of reducing the equivalents generated in mitochondria. Thus, mitochondria are also a sink of ROS. These observations highlight the importance of the mitochondrial antioxidant system, which should be considered in our understanding of ROS-regulated processes. These processes include cell signaling and the progression of metabolic and neurodegenerative disease.

An update on methods and approaches for interrogating mitochondrial reactive oxygen species production

The chief ROS formed by mitochondria are superoxide (O2·−) and hydrogen peroxide (H₂O₂). Superoxide is converted rapidly to H₂O₂ and therefore the latter is the chief ROS emitted by mitochondria into the cell. Once considered an unavoidable by-product of aerobic respiration, H₂O₂ is now regarded as a central mitokine used in mitochondrial redox signaling. However, it has been postulated that O2·− can also serve as a signal in mammalian cells. Progress in understanding the role of mitochondrial H₂O₂ in signaling is due to significant advances in the development of methods and technologies for its detection. Unfortunately, the development of techniques to selectively measure basal O2·− changes has been met with more significant hurdles due to its short half-life and the lack of specific probes. The development of sensitive techniques for the selective and real time measure of O2·− and H₂O₂ has come on two fronts: development of genetically encoded fluorescent proteins and small molecule reporters. In 2015, I published a detailed comprehensive review on the state of knowledge for mitochondrial ROS production and how it is controlled, which included an in-depth discussion of the up-to-date methods utilized for the detection of both superoxide (O2·−) and H₂O₂. In the article, I presented the challenges associated with utilizing these probes and their significance in advancing our collective understanding of ROS signaling. Since then, many other authors in the field of Redox Biology have published articles on the challenges and developments detecting O2·− and H₂O₂ in various organisms [[1], [2], [3]]. There has been significant advances in this state of knowledge, including the development of novel genetically encoded fluorescent H₂O₂ probes, several O2·− sensors, and the establishment of a toolkit of inhibitors and substrates for the interrogation of mitochondrial H₂O₂ production and the antioxidant defenses utilized to maintain the cellular H₂O₂ steady-state. Here, I provide an update on these methods and their implementation in furthering our understanding of how mitochondria serve as cell ROS stabilizing devices for H₂O₂ signaling.

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Details on the toolkit for interrogating the 12 sites for mitochondrial ROS production.

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Approaches to assess mitochondrial ROS clearance.

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Novel genetically encoded H₂O₂ sensors.

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Small chemical probes for sensitive detection of O2·−.

Review: Using isolated mitochondria to investigate mitochondrial hydrogen peroxide metabolism

Mitochondria are recognized as centrally important to cellular reactive oxygen species (ROS), both as a potential source and due to their substantial antioxidant capacity. While much of the initial ROS formed by mitochondria is superoxide, this is rapidly converted to hydrogen peroxide (H₂O₂) which more readily crosses membranes making H₂O₂ important in both redox signalling mechanisms and conditions of oxidative stress. Here I outline our studies on mitochondrial H₂O₂ metabolism with a focus on some of the challenges and strategies involved with developing an integrated model of mitochondria being intrinsic regulators of H₂O₂. This view of mitochondria as regulators of H₂O₂ goes beyond the simpler contention of them being net producers or consumers. Moreover, the integration of both consumption and production can then be tied to a putative mechanism linking energy sensing at the level of the mitochondrial protonmotive force. This mechanism would provide a means of mitochondria communicating their energetic status the extramitochondrial compartment via local H₂O₂ concentrations. I conclude by explaining how these concepts developed using rodent muscle as a model have high relevance and applicability to comparative studies.

Temperature rise and copper exposure reduce heart mitochondrial reactive oxygen species scavenging capacity

Mitochondria produce and scavenge reactive oxygen species (ROS); however, whether oxidative distress due to exogenous stress arises from excessive production or impaired scavenging remains unclear. We assessed the effect of copper (Cu) and thermal stress on kinetics of ROS (H₂O₂) consumption in mitochondria isolated from fish heart. Mitochondria were energized with succinate, glutamate-malate or palmitoylcarnitine (PC) and incubated with 1-25 μM Cu at 11 (control) and 23 °C. We found that H₂O₂ consumption capacity of heart mitochondria varies with substrate and is additively reduced by temperature rise and Cu. While Cu is a potent inhibitor of H₂O₂ consumption in mitochondria oxidizing glutamate-malate and succinate, mitochondria oxidizing PC are resistant to the inhibitory effect of the metal. Moreover, the sensitivity of H₂O₂ consumption pathways to Cu depend on the substrate and are greatly impaired during oxidation of glutamate-malate. Pharmacological manipulation of mitochondrial antioxidant systems revealed that NADPH-dependent peroxidase systems are the centerpieces of ROS scavenging in heart mitochondria, with the glutathione-dependent pathway being the most prominent while catalase played a minimal role. Surprisingly, Cu is as efficacious in inhibiting thioredoxin-dependent peroxidase pathway as auranofin, a selective inhibitor of thioredoxin reductase. Taken together, our study uncovered unique mechanisms by which Cu alters mitochondrial H₂O₂ homeostasis including its ability to inhibit specific mitochondrial ROS scavenging pathways on a par with conventional inhibitors. Importantly, because of additive inhibitory effect on mitochondrial ROS removal mechanisms, hearts of organisms jointly exposed to Cu and thermal stress are likely at increased risk of oxidative distress.

Mitochondrial Membrane Intracellular Communication in Healthy and Diseased Myocardium

Research efforts in the twenty-first century have been paramount to the discovery and development of novel pharmacological treatments in a variety of diseases resulting in improved life expectancy. Yet, cardiac disease remains a leading cause of morbidity and mortality worldwide. Over time, there has been an expansion in conditions such as atrial fibrillation (AF) and heart failure (HF). Although past research has elucidated specific pathways that participate in the development of distinct cardiac pathologies, the exact mechanisms of action leading to disease remain to be fully characterized. Protein turnover and cellular bioenergetics are integral components of cardiac diseases, highlighting the importance of mitochondria and endoplasmic reticulum (ER) in driving cellular homeostasis. More specifically, the interactions between mitochondria and ER are crucial to calcium signaling, apoptosis induction, autophagy, and lipid biosynthesis. Here, we summarize mitochondrial and ER functions and physical interactions in healthy physiological states. We then transition to perturbations that occur in response to pathophysiological challenges and how this alters mitochondrial–ER and other intracellular organelle interactions. Finally, we discuss lifestyle interventions and innovative therapeutic targets that may be used to restore beneficial mitochondrial and ER interactions, thereby improving cardiac function.

Comparative studies of mitochondrial reactive oxygen species in animal longevity: Technical pitfalls and possibilities

The mitochondrial oxidative theory of aging has been repeatedly investigated over the past 30 years by comparing the efflux of hydrogen peroxide (H₂O₂) from isolated mitochondria of long‐ and short‐lived species using horseradish peroxidase‐based assays. However, a clear consensus regarding the relationship between H₂O₂ production rates and longevity has not emerged. Concomitantly, novel insights into the mechanisms of reactive oxygen species (ROS) handling by mitochondria themselves should have raised concerns about the validity of this experimental approach. Here, we review pitfalls of the horseradish peroxidase/amplex red detection system for the measurement of mitochondrial ROS formation rates, with an emphasis on longevity studies. Importantly, antioxidant systems in the mitochondrial matrix are often capable of scavenging H₂O₂ faster than mitochondria produce it. As a consequence, as much as 84% of the H₂O₂ produced by mitochondria may be consumed before it diffuses into the reaction medium, where it can be detected by the horseradish peroxidase/amplex red system, this proportion is likely not consistent across species. Furthermore, previous studies often used substrates that elicit H₂O₂ formation at a much higher rate than in physiological conditions and at sites of secondary importance in vivo. Recent evidence suggests that the activity of matrix antioxidants may correlate with longevity instead of the rate of H₂O₂ formation. We conclude that past studies have been methodologically insufficient to address the putative relationship between longevity and mitochondrial ROS. Thus, novel methodological approaches are required that more accurately encompass mitochondrial ROS metabolism.

The exceptional longevity of the naked mole-rat may be explained by mitochondrial antioxidant defenses

Naked mole-rats (NMRs) are mouse-sized mammals that exhibit an exceptionally long lifespan (>30 vs. <4 years for mice), and resist aging-related pathologies such as cardiovascular and pulmonary diseases, cancer, and neurodegeneration. However, the mechanisms underlying this exceptional longevity and disease resistance remain poorly understood. The oxidative stress theory of aging posits that (a) senescence results from the accumulation of oxidative damage inflicted by reactive oxygen species (ROS) of mitochondrial origin, and (b) mitochondria of long-lived species produce less ROS than do mitochondria of short-lived species. However, comparative studies over the past 28 years have produced equivocal results supporting this latter prediction. We hypothesized that, rather than differences in ROS generation, the capacity of mitochondria to consume ROS might distinguish long-lived species from short-lived species. To test this hypothesis, we compared mitochondrial production and consumption of hydrogen peroxide (H2 O2 ; as a proxy of overall ROS metabolism) between NMR and mouse skeletal muscle and heart. We found that the two species had comparable rates of mitochondrial H2 O2 generation in both tissues; however, the capacity of mitochondria to consume ROS was markedly greater in NMRs. Specifically, maximal observed consumption rates were approximately two and fivefold greater in NMRs than in mice, for skeletal muscle and heart, respectively. Our results indicate that differences in matrix ROS detoxification capacity between species may contribute to their divergence in lifespan.

Thyroid state affects H2O2 removal by rat heart mitochondria

We investigated the effects of thyroid state on the mechanisms underlying rat heart mitochondrial capacity to remove H₂O₂ produced by an exogenous source. The removal rates were higher in the presence of respiratory substrates independently from thyroid state and were higher in hyperthyroid than in hypothyroid preparations. The thyroid state-linked changes in H₂O₂ removal rates, mirrored those in H₂O₂ release rates, showing that endogenous and exogenous H₂O₂ do not compete for the removing system. Mitochondrial content of coenzyme Q9 and Q10 was lower in hypothyroidism and higher in hyperthyroidism suggesting that the thyroid state-linked changes in the rates of H₂O₂ production are due to changes in the ubiquinone mitochondrial content. The rates of H₂O₂ removal in the presence of antioxidant enzyme inhibitors indicated that the contribution of each antioxidant is dependent on the thyroid state. This was supported by enzymatic activity measurements. Pharmacological inhibition also showed that the overall percentage contribution of the enzymatic processes, as well as that of non-enzymatic processes, is not affected by thyroid state. Cytochrome levels, inferred by light emission measurements, and western blot determination of cytochrome c, were lower in hypothyroid and higher in hyperthyroid preparations supporting the idea that the levels of reducing compounds were modified in opposite way by the changes in thyroid state. Further support was obtained showing that the whole antioxidant capacity, which provides an evaluation of capacity of the systems, different from cytochromes, assigned to H₂O₂ scavenging, was lower in hyperthyroid than in hypothyroid state.

The LNT model for cancer induction is not supported by radiobiological data

The hallmarks of cancer have been the focus of much research and have influenced the development of risk models for radiation-induced cancer. However, natural defenses against cancer, which constitute the hallmarks of cancer prevention, have largely been neglected in developing cancer risk models. These natural defenses are enhanced by low doses and dose rates of ionizing radiation, which has aided in the continuation of human life over many generations. Our natural defenses operate at the molecular, cellular, tissue, and whole-body levels and include epigenetically regulated (epiregulated) DNA damage repair and antioxidant production, selective p53-independent apoptosis of aberrant cells (e.g. neoplastically transformed and tumor cells), suppression of cancer-promoting inflammation, and anticancer immunity (both innate and adaptive components). This publication reviews the scientific bases for the indicated cancer-preventing natural defenses and evaluates their implication for assessing cancer risk after exposure to low radiation doses and dose rates. Based on the extensive radiobiological evidence reviewed, it is concluded that the linear-no-threshold (LNT) model (which ignores natural defenses against cancer), as it relates to cancer risk from ionizing radiation, is highly implausible. Plausible models include dose-threshold and hormetic models. More research is needed to establish when a given model (threshold, hormetic, or other) applies to a given low-dose-radiation exposure scenario.

Mitochondria can act as energy-sensing regulators of hydrogen peroxide availability

Mitochondria are widely recognized as sources of reactive oxygen species in animal cells, with H₂O₂ being of particular note because it can act not only in oxidative stress but also is important to several signalling pathways. Lesser recognized is that mitochondria can have far greater capacity to consume H₂O₂ than to produce it; however, the consumption of H₂O₂ may be kinetically constrained by H₂O₂ availability especially at the low nanomolar (or lower) concentrations that occur in vivo. The production of H₂O₂ is a function of many factors, not the least of which are respiratory substrate availability and the protonmotive force (Δp). The Δp, which is predominantly membrane potential (ΔΨ), can be a strong indicator of mitochondrial energy status, particularly if respiratory substrate supply is either not meeting or exceeding demand. The notion that mitochondria may functionally act in regulating H₂O₂ concentrations may be somewhat implicit but little evidence demonstrating this is available. Here we demonstrate key assumptions that are required for mitochondria to act as regulators of H₂O₂ by an integrated system of production and concomitant consumption. In particular we show the steady-state level of H₂O₂ mitochondria approach is a function of both mitochondrial H₂O₂ consumption and production capacity, the latter of which is strongly influenced by ΔΨ. Our results are consistent with mitochondria being able to manipulate extramitochondrial H₂O₂ as a means of signalling mitochondrial energetic status, in particular the Δp or ΔΨ. Such a redox-based signal could operate with some independence from other energy sensing mechanisms such as those that transmit information using the cytosolic adenylate pool.

Significance of the mitochondrial thioredoxin reductase in cancer cells: An update on role, targets and inhibitors

Thioredoxin reductase 2 (TrxR2) is a key component of the mitochondrial thioredoxin system able to transfer electrons to peroxiredoxin 3 (Prx3) in a reaction mediated by thioredoxin 2 (Trx2). In this way, both the level of hydrogen peroxide and thiol redox state are modulated. TrxR2 is often overexpressed in cancer cells conferring apoptosis resistance. Due to their exposed flexible arm containing selenocysteine, both cytosolic and mitochondrial TrxRs are inhibited by a large number of molecules. The various classes of inhibitors are listed and the molecules acting specifically on TrxR2 are extensively described. Particular emphasis is given to gold(I/III) complexes with phosphine, carbene or other ligands and to tamoxifen-like metallocifens. Also chemically unrelated organic molecules, including natural compounds and their derivatives, are taken into account. An important feature of many TrxR2 inhibitors is provided by their nature of delocalized lipophilic cations that allows their accumulation in mitochondria exploiting the organelle membrane potential. The consequences of TrxR2 inhibition are presented focusing especially on the impact on mitochondrial pathophysiology. Inhibition of TrxR2, by hindering the activity of Trx2 and Prx3, increases the mitochondrial concentration of reactive oxygen species and shifts the thiol redox state toward a more oxidized condition. This is reflected by alterations of specific targets involved in the release of pro-apoptotic factors such as cyclophilin D which acts as a regulator of the mitochondrial permeability transition pore. Therefore, the selective inhibition of TrxR2 could be utilized to induce cancer cell apoptosis.

Synaptic Activity Protects Neurons Against Calcium-Mediated Oxidation and Contraction of Mitochondria During Excitotoxicity

AIMS

Excitotoxicity triggered by extrasynaptic N-methyl-d-aspartate-type glutamate receptors has been implicated in many neurodegenerative conditions, including Alzheimer's disease, Huntington's disease, amyotrophic lateral sclerosis, and stroke. Mitochondrial calcium overload leading to mitochondrial dysfunction represents an early event in excitotoxicity. Neurons are rendered resistant to excitotoxicity by previous periods of synaptic activity that activates a nuclear calcium-driven neuroprotective gene program. This process, termed acquired neuroprotection, involves transcriptional repression of the mitochondrial calcium uniporter leading to a reduction in excitotoxcity-associated mitochondrial calcium load. As mitochondrial calcium and the production of reactive oxygen species may be linked, we monitored excitotoxicity-associated changes in the mitochondrial redox status using the ratiometric glutathione redox potential indicator, glutaredoxin 1 (GRX1)-redox-sensitive green fluorescent protein (roGFP)2, targeted to the mitochondrial matrix. Aim of this study was to investigate if suppression of oxidative stress underlies mitoprotection afforded by synaptic activity.

RESULTS

We found that synaptic activity protects primary rat hippocampal neurons against acute excitotoxicity-induced mitochondrial oxidative stress and mitochondrial contraction associated with it. Downregulation of the mitochondrial uniporter by genetic means mimics the protective effect of synaptic activity on mitochondrial redox status. These findings indicate that oxidative stress acts downstream of mitochondrial calcium overload in excitotoxicity. Innovation and Conclusion: We established mito-GRX1-roGFP2 as a reliable and sensitive tool to monitor rapid redox changes in mitochondria during excitotoxicity. Our results highlight the importance of developing means of blocking mitochondrial calcium overload for therapeutic targeting of oxidative stress and mitochondrial dysfunction in neurodegenerative diseases. Antioxid. Redox. Signal. 29, 1109-1124.

Intracellular calcium and NF-kB regulate hypoxia-induced leptin, VEGF, IL-6 and adiponectin secretion in human adipocytes

AIMS

Hypoxia-induced adipokine release has been attributed mainly to HIF-1α. Here we investigate the role of intracellular calcium and NF-kB in the hypoxia-dependent release of leptin, VEGF, IL-6 and the hypoxia-induced inhibition of adiponectin release in human adipocytes.

MAIN METHODS

We used intracellular calcium imaging to compare calcium status in preadipocytes and in adipocytes. We subjected both cell types to hypoxic conditions and measured the release of adipokines induced by hypoxia in the presence and absence of HIF-1α inhibitor YC-1, NF-_κB inhibitor SN50 and intracellular calcium chelator BAPTA-AM.

KEY FINDINGS

We demonstrate reduced intracellular calcium oscillations and increased oxidative stress as the cells transitioned from preadipocytes to adipocytes. We show that differentiation of preadipocytes to adipocytes is associated with distinct morphological changes in the mitochondria. We also show that hypoxia-induced secretion of leptin, VEGF, IL-6 and hypoxia-induced inhibition of adiponectin secretion are independent of HIF-1α expression. The hypoxia-induced leptin, VEGF and IL-6 release are [Ca⁺⁺]_i dependent whereas adiponectin is NF-_kB dependent.

SIGNIFICANCE

Our work suggests a major role for [Ca⁺⁺]_i in preadipocyte differentiation to adipocytes and that changes in mitochondrial morphology in the adipocytes might underlie the reduced calcium oscillations observed in the adipocytes. It also demonstrates that multiple signaling pathways are associated with the hypoxia-induced adipokine secretion.

H2O2 metabolism in liver and heart mitochondria: Low emitting-high scavenging and high emitting-low scavenging systems

Although mitochondria are presumed to emit and consume reactive oxygen species (ROS), the quantitative interplay between the two processes in ROS regulation is not well understood. Here, we probed the role of mitochondrial bioenergetics in H₂O₂ metabolism using rainbow trout liver and heart mitochondria. Both liver and heart mitochondria emitted H₂O₂ at rates that depended on their metabolic state, with the emission rates (free radical leak) constituting 0.8-2.9% and 0.2-2.5% of the respiration rate in liver and heart mitochondria, respectively. When presented with exogenous H₂O₂, liver and heart mitochondria consumed it by first order reactions with half-lives (s) of 117 and 210, and rate constants of 5.96 and 3.37 (× 10^-3 s^-1), respectively. The mitochondrial bioenergetic status greatly affected the rate of H₂O₂ consumption in heart but not liver mitochondria. Moreover, the activities and contribution of H₂O₂ scavenging systems varied between liver and heart mitochondria. The significance of the scavenging systems ranked by the magnitude (%) of inhibition of H₂O₂ removal after correcting for emission were, liver (un-energized and energized): catalase > glutathione (GSH) ≥ thioredoxin reductase (TrxR); un-energized heart mitochondria: catalase > TrxR > GSH and energized heart mitochondria: GSH > TrxR > catalase. Notably, depletion of GSH evoked a massive surge in H₂O₂ emission that grossly masked the contribution of this pathway to H₂O₂ scavenging in heart mitochondria. Irrespective of the organ of their origin, mitochondria behaved as H₂O₂ regulators that emitted or consumed it depending on the ambient H₂O₂ concentration, mitochondrial bioenergetic state and activity of the scavenging enzyme systems. Indeed, manipulation of mitochondrial bioenergetics and H₂O₂ scavenging systems caused mitochondria to switch from being net consumers to net emitters of H₂O₂. Overall, our data suggest that the low levels of H₂O₂ typically present in cells would favor emission of this metabolite but the scavenging systems would prevent its accumulation.

Membrane potential and delta pH dependency of reverse electron transport-associated hydrogen peroxide production in brain and heart mitochondria

Succinate-driven reverse electron transport (RET) is one of the main sources of mitochondrial reactive oxygen species (mtROS) in ischemia-reperfusion injury. RET is dependent on mitochondrial membrane potential (Δψ_m) and transmembrane pH difference (ΔpH), components of the proton motive force (pmf); a decrease in Δψ_m and/or ΔpH inhibits RET. In this study we aimed to determine which component of the pmf displays the more dominant effect on RET-provoked ROS generation in isolated guinea pig brain and heart mitochondria respiring on succinate or α-glycerophosphate (α-GP). Δψ_m was detected via safranin fluorescence and a TPP⁺ electrode, the rate of H₂O₂ formation was measured by Amplex UltraRed, the intramitochondrial pH (pH_in) was assessed via BCECF fluorescence. Ionophores were used to dissect the effects of the two components of pmf. The K⁺/H⁺ exchanger, nigericin lowered pH_in and ΔpH, followed by a compensatory increase in Δψ_m that led to an augmented H₂O₂ production. Valinomycin, a K⁺ ionophore, at low [K⁺] increased ΔpH and pH_in, decreased Δψ_m, which resulted in a decline in H₂O₂ formation. It was concluded that Δψ_m is dominant over ∆pH in modulating the succinate- and α-GP-evoked RET. The elevation of extramitochondrial pH was accompanied by an enhanced H₂O₂ release and a decreased ∆pH. This phenomenon reveals that from the pH component not ∆pH, but rather absolute value of pH has higher impact on the rate of mtROS formation. Minor decrease of Δψ_m might be applied as a therapeutic strategy to attenuate RET-driven ROS generation in ischemia-reperfusion injury.

A radical shift in perspective: mitochondria as regulators of reactive oxygen species

Mitochondria are widely recognized as a source of reactive oxygen species (ROS) in animal cells, where it is assumed that over-production of ROS leads to an overwhelmed antioxidant system and oxidative stress. In this Commentary, we describe a more nuanced model of mitochondrial ROS metabolism, where integration of ROS production with consumption by the mitochondrial antioxidant pathways may lead to the regulation of ROS levels. Superoxide and hydrogen peroxide (H₂O₂) are the main ROS formed by mitochondria. However, superoxide, a free radical, is converted to the non-radical, membrane-permeant H₂O₂; consequently, ROS may readily cross cellular compartments. By combining measurements of production and consumption of H₂O₂, it can be shown that isolated mitochondria can intrinsically approach a steady-state concentration of H₂O₂ in the medium. The central hypothesis here is that mitochondria regulate the concentration of H₂O₂ to a value set by the balance between production and consumption. In this context, the consumers of ROS are not simply a passive safeguard against oxidative stress; instead, they control the established steady-state concentration of H₂O₂ By considering the response of rat skeletal muscle mitochondria to high levels of ADP, we demonstrate that H₂O₂ production by mitochondria is far more sensitive to changes in mitochondrial energetics than is H₂O₂ consumption; this concept is further extended to evaluate how the muscle mitochondrial H₂O₂ balance should respond to changes in aerobic work load. We conclude by considering how differences in the ROS consumption pathways may lead to important distinctions amongst tissues, along with briefly examining implications for differing levels of activity, temperature change and metabolic depression.

Simplified qPCR method for detecting excessive mtDNA damage induced by exogenous factors

Damage to mitochondrial DNA (mtDNA) is a meaningful biomarker for evaluating genotoxicity of drugs and environmental toxins. Existing PCR methods utilize long mtDNA fragments (∼8-10kb), which complicates detecting exact sites of mtDNA damage. To identify the mtDNA regions most susceptible to damage, we have developed and validated a set of primers to amplify ∼2kb long fragments, while covering over 95% of mouse mtDNA. We have modified the detection method by greatly increasing the enrichment of mtDNA, which allows us solving the problem of non-specific primer annealing to nuclear DNA. To validate our approach, we have determined the most damage-susceptible mtDNA regions in mice treated in vivo and in vitro with rotenone and H₂O₂. The GTGR-sequence-enriched mtDNA segments located in the D-loop region were found to be especially susceptible to damage. Further, we demonstrate that H₂O₂-induced mtDNA damage facilitates the relaxation of mtDNA supercoiled conformation, making the sequences with minimal damage more accessible to DNA polymerase, which, in turn, results in a decrease in threshold cycle value. Overall, our modified PCR method is simpler and more selective to the specific sites of damage in mtDNA.

Muscles Susceptibility to Ischemia-Reperfusion Injuries Depends on Fiber Type Specific Antioxidant Level

Muscle injury resulting from ischemia-reperfusion largely aggravates patient prognosis but whether and how muscle phenotype modulates ischemia-reperfusion-induced mitochondrial dysfunction remains to be investigated. We challenged the hypothesis that glycolytic muscles are more prone to ischemia-reperfusion-induced injury than oxidative skeletal muscles. We therefore determined simultaneously the effect of 3 h of ischemia induced by aortic clamping followed by 2 h of reperfusion (IR, n = 11) on both gastrocnemius and soleus muscles, as compared to control animals (C, n = 11). Further, we investigated whether tempol, an antioxidant mimicking superoxide dismutase, might compensate a reduced defense system, likely characterizing glycolytic muscles (IR-Tempol, n = 7). In the glycolytic gastrocnemius muscle, as compared to control, ischemia-reperfusion significantly decreased mitochondrial respiration (-30.28 ± 6.16%, p = 0.003), increased reactive oxygen species production (+79.15 ± 28.72%, p = 0.04), and decreased reduced glutathione (-28.19 ± 6.80%, p = 0.011). Less deleterious effects were observed in the oxidative soleus muscle (-6.44 ± 6.30%, +4.32 ± 16.84%, and -8.07 ± 10.84%, respectively), characterized by enhanced antioxidant defenses (0.63 ± 0.05 in gastrocnemius vs. 1.24 ± 0.08 μmol L^-1 g^-1 in soleus). Further, when previously treated with tempol, glycolytic muscle was largely protected against the deleterious effects of ischemia-reperfusion. Thus, oxidative skeletal muscles are more protected than glycolytic ones against ischemia-reperfusion, thanks to their antioxidant pool. Such pivotal data support that susceptibility to ischemia-reperfusion-induced injury differs between organs, depending on their metabolic phenotypes. This suggests a need to adapt therapeutic strategies to the specific antioxidant power of the target organ to be protected.

GL-V9, a new synthetic flavonoid derivative, ameliorates DSS-induced colitis against oxidative stress by up-regulating Trx-1 expression via activation of AMPK/FOXO3a pathway

GL-V9, a new synthesized flavonoid derivative, has been reported to possess anti-cancer properties in our previous studies. Uncontrolled overproduction of reactive oxygen species (ROS) has been implicated in oxidative damage of inflammatory bowel disease (IBD). In this study, we aimed to investigate the protective effect of GL-V9 against dextran sulfate sodium (DSS)-induced colitis. GL-V9 attenuated DSS-induced body weight loss, colon length shortening and colonic pathological damage. GL-V9 also inhibited inflammatory cells infiltration and decreased myeloperoxidase (MPO) and inducible nitric oxide synthase (iNOS) activities. Moreover, GL-V9 inhibited ROS and malondialdehyde (MDA) generation, but enhanced superoxide dismutase (SOD), glutathione (GSH) and total antioxidant capacity. GL-V9 reduced pro-inflammatory cytokines production in serum and colon as well. Mechanically, GL-V9 could increase Trx-1 via activation of AMPK/FOXO3a to suppress DSS-induced colonic oxidative stress. Furthermore, GL-V9 decreased pro-inflammatory cytokines and ROS production and increased the antioxidant defenses in the mouse macrophage cells RAW264.7 by promoting Trx-1 expression. In conclusion, our study demonstrated that GL-V9 attenuated DSS-induced colitis against oxidative stress by up-regulating Trx-1 via activation of AMPK/FOXO3a pathway, suggesting that GL-V9 might be a potential effective drug for colitis.

The thioredoxin and glutathione-dependent H2O2 consumption pathways in muscle mitochondria: Involvement in H2O2 metabolism and consequence to H2O2 efflux assays

The most common methods of measuring mitochondrial hydrogen peroxide production are based on the extramitochondrial oxidation of a fluorescent probe such as amplex ultra red (AUR) by horseradish peroxidase (HRP). These traditional HRP-based assays only detect H2O2 that has escaped the matrix, raising the potential for substantial underestimation of production if H2O2 is consumed by matrix antioxidant pathways. To measure this underestimation, we characterized matrix consumers of H2O2 in rat skeletal muscle mitochondria, and developed specific means to inhibit these consumers. Mitochondria removed exogenously added H2O2 (2.5µM) at rates of 4.7 and 5.0nmol min(-1) mg protein(-1) when respiring on glutamate+malate and succinate+rotenone, respectively. In the absence of respiratory substrate, or after disrupting membranes by cycles of freeze-thaw, rates of H2O2 consumption were negligible. We concluded that matrix consumers are respiration-dependent (requiring respiratory substrates), suggesting the involvement of either the thioredoxin (Trx) and/or glutathione (GSH)-dependent enzymatic pathways. The Trx-reductase inhibitor auranofin (2µM), and a pre-treatment of mitochondria with 35µM of 1-chloro-2,4-dintrobenzene (CDNB) to deplete GSH specifically compromise these two consumption pathways. These inhibition approaches presented no undesirable "off-target" effects during extensive preliminary tests. These inhibition approaches independently and additively decreased the rate of consumption of H2O2 exogenously added to the medium (2.5µM). During traditional HRP-based H2O2 efflux assays, these inhibition approaches independently and additively increased apparent efflux rates. When used in combination (double inhibition), these inhibition approaches allowed accumulation of (endogenously produced) H2O2 in the medium at a comparable rate whether it was measured with an end point assay where 2.5µM H2O2 is initially added to the medium or with traditional HRP-based efflux assays. This finding confirms that a high degree of inhibition of all matrix consumers is attained with the double inhibition. Importantly, this double inhibition of the matrix consumers allowed revealing that a large part of the H2O2 produced in muscle mitochondria is consumed before escaping the matrix during traditional HRP-based efflux assays. The degree of this underestimation was substrate dependent, reaching >80% with malate, which complicates comparisons of substrates for their capacity to generate H2O2 in normal conditions i.e. when matrix consumers are active. Our results also urge caution in interpreting changes in H2O2 efflux in response to a treatment; when HRP-based assays are used, large changes in apparent H2O2 efflux may come from altered capacity of the matrix consumers.

Succinate, an intermediate in metabolism, signal transduction, ROS, hypoxia, and tumorigenesis

Succinate is an important metabolite at the cross-road of several metabolic pathways, also involved in the formation and elimination of reactive oxygen species. However, it is becoming increasingly apparent that its realm extends to epigenetics, tumorigenesis, signal transduction, endo- and paracrine modulation and inflammation. Here we review the pathways encompassing succinate as a metabolite or a signal and how these may interact in normal and pathological conditions.(1).

The Response to Oxidative DNA Damage in Neurons: Mechanisms and Disease

There is a growing body of evidence indicating that the mechanisms that control genome stability are of key importance in the development and function of the nervous system. The major threat for neurons is oxidative DNA damage, which is repaired by the base excision repair (BER) pathway. Functional mutations of enzymes that are involved in the processing of single-strand breaks (SSB) that are generated during BER have been causally associated with syndromes that present important neurological alterations and cognitive decline. In this review, the plasticity of BER during neurogenesis and the importance of an efficient BER for correct brain function will be specifically addressed paying particular attention to the brain region and neuron-selectivity in SSB repair-associated neurological syndromes and age-related neurodegenerative diseases.

Differentiating between apparent and actual rates of H2O2 metabolism by isolated rat muscle mitochondria to test a simple model of mitochondria as regulators of H2O2 concentration

Mitochondria are often regarded as a major source of reactive oxygen species (ROS) in animal cells, with H2O2 being the predominant ROS released from mitochondria; however, it has been recently demonstrated that energized brain mitochondria may act as stabilizers of H2O2 concentration (Starkov et al. [1]) based on the balance between production and the consumption of H2O2, the later of which is a function of [H2O2] and follows first order kinetics. Here we test the hypothesis that isolated skeletal muscle mitochondria, from the rat, are able to modulate [H2O2] based upon the interaction between the production of ROS, as superoxide/H2O2, and the H2O2 decomposition capacity. The compartmentalization of detection systems for H2O2 and the intramitochondrial metabolism of H2O2 leads to spacial separation between these two components of the assay system. This results in an underestimation of rates when relying solely on extramitochondrial H2O2 detection. We find that differentiating between these apparent rates found when using extramitochondrial H2O2 detection and the actual rates of metabolism is important to determining the rate constant for H2O2 consumption by mitochondria in kinetic experiments. Using the high rate of ROS production by mitochondria respiring on succinate, we demonstrate that net H2O2 metabolism by mitochondria can approach a stable steady-state of extramitochondrial [H2O2]. Importantly, the rate constant determined by extrapolation of kinetic experiments is similar to the rate constant determined as the [H2O2] approaches a steady state.

Effect of thyroid state on enzymatic and non-enzymatic processes in H2O2 removal by liver mitochondria of male rats

We investigated thyroid state effect on capacity of rat liver mitochondria to remove exogenously produced H2O2, determining their ability to decrease fluorescence generated by an H2O2 detector system. The rate of H2O2 removal by both non respiring and respiring mitochondria was increased by hyperthyroidism and decreased by hypothyroidism. However, the rate was higher in the presence of respiratory substrates, in particular pyruvate/malate, indicating a respiration-dependent process. Generally, the changes in H2O2 removal rates mirrored those in H2O2 release rates excluding the possibility that endogenous and exogenous H2O2 competed for the removing system. Pharmacological inhibition revealed thyroid state-linked differences in antioxidant enzyme contribution to H2O2 removal which were consistent with those in antioxidant system activities. The H2O2 removal was only in part due to enzymatic systems and that imputable to non-enzymatic processes was higher in hyperthyroid and lower in hypothyroid mitochondria. The levels of cytochrome c and the light emissions, due to luminol oxidation catalyzed by cytochrome/H2O2, exhibited similar changes with thyroid state supporting the idea that non-enzymatic scavenging was mainly due to hemoprotein action, which produces hydroxyl radicals. Further support was obtained showing that the whole antioxidant capacity, which provides an evaluation of capacity of the systems, different from cytochromes, assigned to H2O2 scavenging, was lower in hyperthyroid than in hypothyroid state. In conclusion, our results show that mitochondria from hyperthyroid liver have a high capacity for H2O2 removal, which, however, leading in great part to more reactive oxygen species, results harmful for such organelles.

Enhanced hydrogen peroxide generation accompanies the beneficial bioenergetic effects of methylene blue in isolated brain mitochondria

The redox dye methylene blue (MB) is proven to have beneficial effects in various models of neurodegenerative diseases. Here we investigated the effects of MB (100 nM, 300 nM, and 1 μM) on key bioenergetic parameters and on H2O2 production/elimination in isolated guinea pig brain mitochondria under normal as well as respiration-impaired conditions. As measured by high-resolution Oxygraph the rate of resting oxygen consumption was increased, but the ADP-stimulated respiration was unaffected by MB with any of the substrates (glutamate malate, succinate, or α-glycerophosphate) used for supporting mitochondrial respiration. In mitochondria treated with inhibitors of complex I or complex III MB moderately but significantly increased the rate of ATP production, restored ΔΨm, and increased the rate of Ca(2+) uptake. The effects of MB are consistent with transferring electrons from upstream components of the electron transport chain to cytochrome c, which is energetically favorable when the flow of electrons in the respiratory chain is compromised. On the other hand, MB significantly increased the production of H2O2 measured by Amplex UltraRed fluorimetry under all conditions, in resting, ATP-synthesizing, and respiration-impaired mitochondria, with each substrate combination supporting respiration. Furthermore, it also decreased the elimination of H2O2. Generation of H2O2 without superoxide formation, observed in the presence of MB, is interpreted as a result of reduction of molecular oxygen to H2O2 by the reduced MB. The elevated generation and impaired elimination of H2O2 should be considered for the overall oxidative state of mitochondria treated with MB.

Effects of partial inhibition of respiratory complex I on H2O 2 production by isolated brain mitochondria in different respiratory states

The aim of this work was to characterize the effects of partial inhibition of respiratory complex I by rotenone on H2O2 production by isolated rat brain mitochondria in different respiratory states. Flow cytometric analysis of membrane potential in isolated mitochondria indicated that rotenone leads to uniform respiratory inhibition when added to a suspension of mitochondria. When mitochondria were incubated in the presence of a low concentration of rotenone (10 nm) and NADH-linked substrates, oxygen consumption was reduced from 45.9 ± 1.0 to 26.4 ± 2.6 nmol O2 mg(-1) min(-1) and from 7.8 ± 0.3 to 6.3 ± 0.3 nmol O2 mg(-1) min(-1) in respiratory states 3 (ADP-stimulated respiration) and 4 (resting respiration), respectively. Under these conditions, mitochondrial H2O2 production was stimulated from 12.2 ± 1.1 to 21.0 ± 1.2 pmol H2O2 mg(-1) min(-1) and 56.5 ± 4.7 to 95.0 ± 11.1 pmol H2O2 mg(-1) min(-1) in respiratory states 3 and 4, respectively. Similar results were observed when comparing mitochondrial preparations enriched with synaptic or nonsynaptic mitochondria or when 1-methyl-4-phenylpyridinium ion (MPP(+)) was used as a respiratory complex I inhibitor. Rotenone-stimulated H2O2 production in respiratory states 3 and 4 was associated with a high reduction state of endogenous nicotinamide nucleotides. In succinate-supported mitochondrial respiration, where most of the mitochondrial H2O2 production relies on electron backflow from complex II to complex I, low rotenone concentrations inhibited H2O2 production. Rotenone had no effect on mitochondrial elimination of micromolar concentrations of H2O2. The present results support the conclusion that partial complex I inhibition may result in mitochondrial energy crisis and oxidative stress, the former being predominant under oxidative phosphorylation and the latter under resting respiration conditions.

Scavenging of H2O2 by mouse brain mitochondria

Mitochondrial reactive oxygen species (ROS) metabolism is unique in that mitochondria both generate and scavenge ROS. Recent estimates of ROS scavenging capacity of brain mitochondria are surprisingly high, ca. 9-12 nmol H2O2/min/mg, which is ~100 times higher than the rate of ROS generation. This raises a question whether brain mitochondria are a source or a sink of ROS. We studied the interaction between ROS generation and scavenging in mouse brain mitochondria by measuring the rate of removal of H2O2 added at a concentration of 0.4 μM, which is close to the reported physiological H2O2 concentrations in tissues, under conditions of low and high levels of mitochondrial H2O2 generation. With NAD-linked substrates, the rate of H2O2 generation by mitochondria was ~50-70 pmol/min/mg. The H2O2 scavenging dynamics was best approximated by the first order reaction equation. H2O2 scavenging was not affected by the uncoupling of mitochondria, phosphorylation of added ADP, or the genetic ablation of glutathione peroxidase 1, but decreased in the absence of respiratory substrates, in the presence of thioredoxin reductase inhibitor auranofin, or in partially disrupted mitochondria. With succinate, the rate of H2O2 generation was ~2,200-2,900 pmol/min/mg; the scavenging of added H2O2 was masked by a significant accumulation of generated H2O2 in the assay medium. The obtained data were fitted into a simple model that reasonably well described the interaction between H2O2 scavenging and production. It showed that mitochondria are neither a sink nor a source of H2O2, but can function as both at the same time, efficiently stabilizing exogenous H2O2 concentration at a level directly proportional to the ratio of the H2O2 generation rate to the rate constant of the first order scavenging reaction.

Nicotinamide nucleotide transhydrogenase (Nnt) links the substrate requirement in brain mitochondria for hydrogen peroxide removal to the thioredoxin/peroxiredoxin (Trx/Prx) system

Mitochondrial reactive oxygen species are implicated in the etiology of multiple neurodegenerative diseases, including Parkinson disease. Mitochondria are known to be net producers of ROS, but recently we have shown that brain mitochondria can consume mitochondrial hydrogen peroxide (H2O2) in a respiration-dependent manner predominantly by the thioredoxin/peroxiredoxin system. Here, we sought to determine the mechanism linking mitochondrial respiration with H2O2 catabolism in brain mitochondria and dopaminergic cells. We hypothesized that nicotinamide nucleotide transhydrogenase (Nnt), which utilizes the proton gradient to generate NADPH from NADH and NADP(+), provides the link between mitochondrial respiration and H2O2 detoxification through the thioredoxin/peroxiredoxin system. Pharmacological inhibition of Nnt in isolated brain mitochondria significantly decreased their ability to consume H2O2 in the presence, but not absence, of respiration substrates. Nnt inhibition in liver mitochondria, which do not require substrates to detoxify H2O2, had no effect. Pharmacological inhibition or lentiviral knockdown of Nnt in N27 dopaminergic cells (a) decreased H2O2 catabolism, (b) decreased NADPH and increased NADP(+) levels, and (c) decreased basal, spare, and maximal mitochondrial oxygen consumption rates. Nnt-deficient cells possessed higher levels of oxidized mitochondrial Prx, which rendered them more susceptible to steady-state increases in H2O2 and cell death following exposure to subtoxic levels of paraquat. These data implicate Nnt as the critical link between the metabolic and H2O2 antioxidant function in brain mitochondria and suggests Nnt as a potential therapeutic target to improve the redox balance in conditions of oxidative stress associated with neurodegenerative diseases.

Mitochondrial respiratory chain complexes as sources and targets of thiol-based redox-regulation

The respiratory chain of the inner mitochondrial membrane is a unique assembly of protein complexes that transfers the electrons of reducing equivalents extracted from foodstuff to molecular oxygen to generate a proton-motive force as the primary energy source for cellular ATP-synthesis. Recent evidence indicates that redox reactions are also involved in regulating mitochondrial function via redox-modification of specific cysteine-thiol groups in subunits of respiratory chain complexes. Vice versa the generation of reactive oxygen species (ROS) by respiratory chain complexes may have an impact on the mitochondrial redox balance through reversible and irreversible thiol-modification of specific target proteins involved in redox signaling, but also pathophysiological processes. Recent evidence indicates that thiol-based redox regulation of the respiratory chain activity and especially S-nitrosylation of complex I could be a strategy to prevent elevated ROS production, oxidative damage and tissue necrosis during ischemia-reperfusion injury. This review focuses on the thiol-based redox processes involving the respiratory chain as a source as well as a target, including a general overview on mitochondria as highly compartmentalized redox organelles and on methods to investigate the redox state of mitochondrial proteins. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.

A transient increase in lipid peroxidation primes preadipocytes for delayed mitochondrial inner membrane permeabilization and ATP depletion during prolonged exposure to fatty acids

Preadipocytes are periodically subjected to fatty acid (FA) concentrations that are potentially cytotoxic. We tested the hypothesis that prolonged exposure of preadipocytes of human origin to a physiologically relevant mix of FAs leads to mitochondrial inner membrane (MIM) permeabilization and ultimately to mitochondrial crisis. We found that exposure of preadipocytes to FAs led to progressive cyclosporin A-sensitive MIM permeabilization, which in turn caused a reduction in MIM potential, oxygen consumption, and ATP synthetic capacity and, ultimately, death. Additionally, we showed that FAs induce a transient increase in intramitochondrial reactive oxygen species (ROS) and lipid peroxide production, lasting roughly 30 and 120min for the ROS and lipid peroxides, respectively. MIM permeabilization and its deleterious consequences including mitochondrial crisis and cell death were prevented by treating the cells with the mitochondrial FA uptake inhibitor etomoxir, the mitochondrion-selective superoxide and lipid peroxide antioxidants MitoTempo and MitoQ, or the lipid peroxide and reactive carbonyl scavenger l-carnosine. FAs also promoted a delayed oxidative stress phase. However, the beneficial effects of etomoxir, MitoTempo, and l-carnosine were lost by delaying the treatment by 2h, suggesting that the initial phase was sufficient to prime the cells for the delayed MIM permeabilization and mitochondrial crisis. It also suggested that the second ROS production phase is a consequence of this loss in mitochondrial health. Altogether, our data suggest that approaches designed to diminish intramitochondrial ROS or lipid peroxide accumulation, as well as MIM permeabilization, are valid mechanism-based therapeutic avenues to prevent the loss in preadipocyte metabolic fitness associated with prolonged exposure to elevated FA levels.

Role of enzymatic and non-enzymatic processes in H2O2 removal by rat liver and heart mitochondria

We compared the capacity of rat liver and heart mitochondria to remove exogenously produced H2O2, determining their ability to decrease fluorescence generated by H2O2 detector system. In the absence of substrates, liver and heart mitochondria removed H2O2 at similar rates. Respiratory substrate addition increased removal rates, indicating a respiration-dependent process. Moreover, the rates were higher with pyruvate/malate than with succinate and in heart than in liver mitochondria. Generally, the changes in H2O2 removal rates mirrored those of H2O2 release rates excluding the possibility that endogenous and exogenous H2O2 competed for the removing system. This idea was supported by the observation that the heaviest of three liver mitochondrial fractions exhibited the highest rates of both H2O2 release and removal. Pharmacological inhibition showed tissue-linked differences in antioxidant enzyme contribution to H2O2 removal which were consistent with the differences in antioxidant system activities. The enzymatic processes accounted only in part for net H2O2 removal and the non-enzymatic ones participated to H2O2 scavenging to a degree that was higher for heart than for liver mitochondria. The idea that non-enzymatic scavenging was due in great part to hemoproteins action was consistent with observation that the concentration of cytochromes, in particular cytochrome c, was higher in heart mitochondria. Indirect support was also obtained by a technique of enhanced luminescence, utilizing the capacity of cytochrome c/H2O2 to catalyze the luminol oxidation, which showed that luminescence response to an oxidative challenge was higher in heart mitochondria.

A single amino acid residue controls ROS production in the respiratory Complex I from Escherichia coli

Reactive oxygen species (ROS) production by respiratory Complex I from Escherichia coli was studied in bacterial membrane fragments and in the isolated and purified enzyme, either solubilized or incorporated in proteoliposomes. We found that the replacement of a single amino acid residue in close proximity to the nicotinamide adenine dinucleotide (NADH)-binding catalytic site (E95 in the NuoF subunit) dramatically increases the reactivity of Complex I towards dioxygen (O2 ). In the E95Q variant short-chain ubiquinones exhibit strong artificial one-electron reduction at the catalytic site, also leading to a stronger increase in ROS production. Two mechanisms can contribute to the observed kinetic effects: (a) a change in the reactivity of flavin mononucleotide (FMN) towards dioxygen at the catalytic site, and (b) a change in the population of the ROS-generating state. We propose the existence of two (closed and open) states of the NAD(+) -bound enzyme as one feature of the substrate-binding site of Complex I. The analysis of the kinetic model of ROS production allowed us to propose that the population of Complex I with reduced FMN is always low in the wild-type enzyme even at low ambient redox potentials, minimizing the rate of reaction with O2 in contrast to E95Q variant.

Neuroprotective Mechanisms of Taurine against Ischemic Stroke

Ischemic stroke exhibits a multiplicity of pathophysiological mechanisms. To address the diverse pathophysiological mechanisms observed in ischemic stroke investigators seek to find therapeutic strategies that are multifaceted in their action by either investigating multipotential compounds or by using a combination of compounds. Taurine, an endogenous amino acid, exhibits a plethora of physiological functions. It exhibits antioxidative properties, stabilizes membrane, functions as an osmoregulator, modulates ionic movements, reduces the level of pro-inflammators, regulates intracellular calcium concentration; all of which contributes to its neuroprotective effect. Data are accumulating that show the neuroprotective mechanisms of taurine against stroke pathophysiology. In this review, we describe the neuroprotective mechanisms employed by taurine against ischemic stroke and its use in clinical trial for ischemic stroke.

The pH sensitivity of H2O2 metabolism in skeletal muscle mitochondria

Mitochondria have the capacity to produce and consume H2O2. We examined H2O2 metabolism in isolated rat skeletal muscle mitochondria and found that the substrate-dependent capacity to consume extramitochondrial H2O2 was markedly higher than the observed rate of H2O2 efflux from mitochondria under the same conditions. The substrate-dependent capacity to consume H2O2 was sensitive to the pH of the medium and we propose that pH related differences in H2O2 consumption pathways may explain inconsistencies we observed between H2O2 efflux rate and the reduction state of the matrix NADH pool.

Principles in redox signaling: from chemistry to functional significance

Reactive oxygen and nitrogen species are currently considered not only harmful byproducts of aerobic respiration but also critical mediators of redox signaling. The molecules and the chemical principles sustaining the network of cellular redox regulated processes are described. Special emphasis is placed on hydrogen peroxide (H(2)O(2)), now considered as acting as a second messenger, and on sulfhydryl groups, which are the direct targets of the oxidant signal. Cysteine residues of some proteins, therefore, act as sensors of redox conditions and are oxidized in a reversible reaction. In particular, the formation of sulfenic acid and disulfide, the initial steps of thiol oxidation, are described in detail. The many cell pathways involved in reactive oxygen species formation are reported. Central to redox signaling processes are the glutathione and thioredoxin systems controlling H(2)O(2) levels and, hence, the thiol/disulfide balance. Lastly, some of the most important redox-regulated processes involving specific enzymes and organelles are described. The redox signaling area of research is rapidly expanding, and future work will examine new pathways and clarify their importance in cellular pathophysiology.

Mitochondrial handling of excess Ca2+ is substrate-dependent with implications for reactive oxygen species generation

The mitochondrial electron transport chain is the major source of reactive oxygen species (ROS) during cardiac ischemia. Several mechanisms modulate ROS production; one is mitochondrial Ca(2+) uptake. Here we sought to elucidate the effects of extramitochondrial Ca(2+) (e[Ca(2+)]) on ROS production (measured as H(2)O(2) release) from complexes I and III. Mitochondria isolated from guinea pig hearts were preincubated with increasing concentrations of CaCl(2) and then energized with the complex I substrate Na(+) pyruvate or the complex II substrate Na(+) succinate. Mitochondrial H(2)O(2) release rates were assessed after giving either rotenone or antimycin A to inhibit complex I or III, respectively. After pyruvate, mitochondria maintained a fully polarized membrane potential (ΔΨ; assessed using rhodamine 123) and were able to generate NADH (assessed using autofluorescence) even with excess e[Ca(2+)] (assessed using CaGreen-5N), whereas they remained partially depolarized and did not generate NADH after succinate. This partial ΔΨ depolarization with succinate was accompanied by a large release in H(2)O(2) (assessed using Amplex red/horseradish peroxidase) with later addition of antimycin A. In the presence of excess e[Ca(2+)], adding cyclosporin A to inhibit mitochondrial permeability transition pore opening restored ΔΨ and significantly decreased antimycin A-induced H(2)O(2) release. Succinate accumulates during ischemia to become the major substrate utilized by cardiac mitochondria. The inability of mitochondria to maintain a fully polarized ΔΨ under excess e[Ca(2+)] when succinate, but not pyruvate, is the substrate may indicate a permeabilization of the mitochondrial membrane, which enhances H(2)O(2) emission from complex III during ischemia.

Superoxide generation by complex III: from mechanistic rationales to functional consequences

Apart from complex I (NADH:ubiquinone oxidoreductase) the mitochondrial cytochrome bc1 complex (complex III; ubiquinol:cytochrome c oxidoreductase) has been identified as the main producer of superoxide and derived reactive oxygen species (ROS) within the mitochondrial respiratory chain. Mitochondrial ROS are generally linked to oxidative stress, aging and other pathophysiological settings like in neurodegenerative diseases. However, ROS produced at the ubiquinol oxidation center (center P, Qo site) of complex III seem to have additional physiological functions as signaling molecules during cellular processes like the adaptation to hypoxia. The molecular mechanism of superoxide production that is mechanistically linked to the electron bifurcation during ubiquinol oxidation is still a matter of debate. Some insight comes from extensive kinetic studies with mutated complexes from yeast and bacterial cytochrome bc1 complexes. This review is intended to bridge the gap between those mechanistic studies and investigations on complex III ROS in cellular signal transduction and highlights factors that impact superoxide generation. This article is part of a Special Issue entitled: Respiratory complex III and related bc complexes.