What are the sources of hydrogen peroxide production by heart mitochondria?

Cardiac arrhythmogenesis: roles of ion channels and their functional modification

Cardiac arrhythmias cause significant morbidity and mortality and pose a major public health problem. They arise from disruptions in the normally orderly propagation of cardiac electrophysiological activation and recovery through successive cardiomyocytes in the heart. They reflect abnormalities in automaticity, initiation, conduction, or recovery in cardiomyocyte excitation. The latter properties are dependent on surface membrane electrophysiological mechanisms underlying the cardiac action potential. Their disruption results from spatial or temporal instabilities and heterogeneities in the generation and propagation of cellular excitation. These arise from abnormal function in their underlying surface membrane, ion channels, and transporters, as well as the interactions between them. The latter, in turn, form common regulatory targets for the hierarchical network of diverse signaling mechanisms reviewed here. In addition to direct molecular-level pharmacological or physiological actions on these surface membrane biomolecules, accessory, adhesion, signal transduction, and cytoskeletal anchoring proteins modify both their properties and localization. At the cellular level of excitation-contraction coupling processes, Ca2+ homeostatic and phosphorylation processes affect channel activity and membrane excitability directly or through intermediate signaling. Systems-level autonomic cellular signaling exerts both acute channel and longer-term actions on channel expression. Further upstream intermediaries from metabolic changes modulate the channels both themselves and through modifying Ca2+ homeostasis. Finally, longer-term organ-level inflammatory and structural changes, such as fibrotic and hypertrophic remodeling, similarly can influence all these physiological processes with potential pro-arrhythmic consequences. These normal physiological processes may target either individual or groups of ionic channel species and alter with particular pathological conditions. They are also potentially alterable by direct pharmacological action, or effects on longer-term targets modifying protein or cofactor structure, expression, or localization. Their participating specific biomolecules, often clarified in experimental genetically modified models, thus constitute potential therapeutic targets. The insights clarified by the physiological and pharmacological framework outlined here provide a basis for a recent modernized drug classification. Together, they offer a translational framework for current drug understanding. This would facilitate future mechanistically directed therapeutic advances, for which a number of examples are considered here. The latter are potentially useful for treating cardiac, in particular arrhythmic, disease.

Cardiovascular aging: spotlight on mitochondria

Mitochondria are cellular organelles critical for ATP production and are particularly relevant to cardiovascular diseases including heart failure, atherosclerosis, ischemia-reperfusion injury, and cardiomyopathies. With advancing age, even in the absence of clinical disease, mitochondrial homeostasis becomes disrupted (e.g., redox balance, mitochondrial DNA damage, oxidative metabolism, and mitochondrial quality control). Mitochondrial dysregulation leads to the accumulation of damaged and dysfunctional mitochondria, producing excessive reactive oxygen species and perpetuating mitochondrial dysfunction. In addition, mitochondrial DNA, cardiolipin, and N-formyl peptides are potent activators of cell-intrinsic and -extrinsic inflammatory pathways. These age-related mitochondrial changes contribute to the development of cardiovascular diseases. This review covers the impact of aging on mitochondria and links these mechanisms to therapeutic implications for age-associated cardiovascular diseases.

Molecular mechanisms of cardiotoxicity induced by acetamide and its chiral isomers

Acetamide (ACT) is used in a racemic form, and the considerable residues of this compound in the environment raise potential safety concerns for human health. We investigated the toxicity of ACT and its chiral isomers on human cardiomyocyte (AC16) cell line and zebrafish embryonic heart, and found that (+)-S-ACT was the main component causing cardiac toxicity. Our findings indicate that the IC50 of (±)-Rac-ACT on AC16 cells was 20.19 μg/mL. (-)-R-ACT, (±)-Rac-ACT, and (+)-S-ACT caused DNA damage and apoptosis in AC16 cells at this concentration. The underlying molecular mechanism may involve the induction of reactive oxygen species (ROS). The accumulation of ROS results in a decline in mitochondrial membrane potential (MMP) and prompts the release of cytochrome c (cyt c) from the mitochondria. This cascade of events ultimately activates the caspase-3 and caspase-9 signaling pathways, resulting in apoptosis. Furthermore, in vivo observations in zebrafish hearts demonstrated caspase-3 activation and the presence of the DNA damage marker (γH2AX), indicating that (+)-S-ACT is more toxic to cardiomyocytes than (-)-R-ACT and (±)-Rac-ACT. These findings suggest that (+)-S-ACT may be the primary component responsible for the toxicity of (±)-Rac-ACT in AC16 cells. Overall, these findings raise public awareness regarding the risks associated with chiral isomeric pesticides and provide a scientific foundation for their appropriate use.

Reactive nitrogen species as therapeutic targets for autophagy/mitophagy modulation to relieve neurodegeneration in multiple sclerosis: Potential application for drug discovery

Multiple sclerosis (MS) is a neuroinflammatory disease with limited therapeutic effects, eventually developing into handicap. Seeking novel therapeutic strategies for MS is timely important. Active autophagy/mitophagy could mediate neurodegeneration, while its roles in MS remain controversial. To elucidate the exact roles of autophagy/mitophagy and reveal its in-depth regulatory mechanisms, we conduct a systematic literature study and analyze the factors that might be responsible for divergent results obtained. The dynamic change levels of autophagy/mitophagy appear to be a determining factor for final neuron fate during MS pathology. Excessive neuronal autophagy/mitophagy contributes to neurodegeneration after disease onset at the active MS phase. Reactive nitrogen species (RNS) serve as key regulators for redox-related modifications and participate in autophagy/mitophagy modulation in MS. Nitric oxide (•NO) and peroxynitrite (ONOO-), two representative RNS, could nitrate or nitrosate Drp1/parkin/PINK1 pathway, activating excessive mitophagy and aggravating neuronal injury. Targeting RNS-mediated excessive autophagy/mitophagy could be a promising strategy for developing novel anti-MS drugs. In this review, we highlight the important roles of RNS-mediated autophagy/mitophagy in neuronal injury and review the potential therapeutic compounds with the bioactivities of inhibiting RNS-mediated autophagy/mitophagy activation and attenuating MS progression. Overall, we conclude that reactive nitrogen species could be promising therapeutic targets to regulate autophagy/mitophagy for multiple sclerosis treatment.

Cardiomyocyte electrophysiology and its modulation: current views and future prospects

Normal and abnormal cardiac rhythms are of key physiological and clinical interest. This introductory article begins from Sylvio Weidmann's key historic 1950s microelectrode measurements of cardiac electrophysiological activity and Singh & Vaughan Williams's classification of cardiotropic targets. It then proceeds to introduce the insights into cardiomyocyte function and its regulation that subsequently emerged and their therapeutic implications. We recapitulate the resulting view that surface membrane electrophysiological events underlying cardiac excitation and its initiation, conduction and recovery constitute the final common path for the cellular mechanisms that impinge upon this normal or abnormal cardiac electrophysiological activity. We then consider progress in the more recently characterized successive regulatory hierarchies involving Ca2+ homeostasis, excitation-contraction coupling and autonomic G-protein signalling and their often reciprocal interactions with the surface membrane events, and their circadian rhythms. Then follow accounts of longer-term upstream modulation processes involving altered channel expression, cardiomyocyte energetics and hypertrophic and fibrotic cardiac remodelling. Consideration of these developments introduces each of the articles in this Phil. Trans. B theme issue. The findings contained in these articles translate naturally into recent classifications of cardiac electrophysiological targets and drug actions, thereby encouraging future iterations of experimental cardiac electrophysiological discovery, and testing directed towards clinical management. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Mitophagy initiates retrograde mitochondrial-nuclear signaling to guide retinal pigment cell heterogeneity

Age-related macular degeneration (AMD), the leading cause of blindness among the elderly, is without treatment for early disease. Degenerative retinal pigment epithelial (RPE) cell heterogeneity is a well-recognized but understudied pathogenic factor. Due to the daily phagocytosis of photoreceptor outer segments, unique photo-oxidative stress, and high metabolism for maintaining vision, the RPE has robust macroautophagy/autophagy, and mitochondrial and antioxidant networks. However, the autophagy subtype, mitophagy, in the RPE and AMD is understudied. Here, we found decreased PINK1 (PTEN induced kinase 1) in perifoveal RPE of early AMD eyes. PINK1-deficient RPE have impaired mitophagy and mitochondrial function that triggers death-resistant epithelial-mesenchymal transition (EMT). This reprogramming is mediated by novel retrograde mitochondrial-nuclear signaling (RMNS) through superoxide, NFE2L2 (NFE2 like bZIP transcription factor 2), TXNRD1 (thioredoxin reductase 1), and phosphoinositide 3-kinase (PI3K)-AKT (AKT serine/threonine kinase) that induced canonical transcription factors ZEB1 (zinc finger E-box binding homeobox 1) and SNAI1 (Snail family transcriptional repressor 1) and an EMT transcriptome. NFE2L2 deficiency disrupted RMNS that paradoxically normalized morphology but decreased function and viability. Thus, RPE heterogeneity is defined by the interaction of two cytoprotective pathways that is triggered by mitophagy function. By neutralizing the consequences of impaired mitophagy, an antioxidant dendrimer tropic for the RPE and mitochondria, EMT (a recognized AMD alteration) was abrogated to offer potential therapy for early AMD, a stage without treatment.Abbreviations: ACTB: actin beta; AKT: AKT serine/threonine kinase; AMD: age-related macular degeneration; CCCP: cyanide m-chlorophenyl hydrazone; CDH1: cadherin 1; DAVID: Database for Annotation, Visualization and Integrated Discovery; DHE: dihydroethidium; D-NAC: N-acetyl-l-cysteine conjugated to a poly(amido amine) dendrimer; ECAR: extracellular acidification rate; EMT: epithelial-mesenchymal transition; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GSEA: Gene Set Enrichment Analysis; HSPD1: heat shock protein family D (Hsp60) member 1; IVT: intravitreal; KD: knockdown; LMNA, lamin A/C; MAP1LC3B: microtubule associated protein 1 light chain 3 beta; MMP: mitochondrial membrane potential; NAC: N-acetyl-l-cysteine; NQO1: NAD(P)H quinone dehydrogenase 1; NFE2L2: NFE2 like bZIP transcription factor 2; O2-: superoxide anion; OCR: oxygen consumption rate; PI3K: phosphoinositide 3-kinase; PINK1: PTEN induced kinase 1; RMNS: retrograde mitochondrial-nuclear signaling; ROS: reactive oxygen species; RPE: retinal pigment epithelium; SNAI1: snail family transcriptional repressor 1; TJP1: tight junction protein 1; TPP-D-NAC: triphenyl phosphinium and N-acetyl-l-cysteine conjugated to a poly(amido amine) dendrimer; TIMM23: translocase of inner mitochondrial membrane 23; TOMM20: translocase of outer mitochondrial membrane 20; Trig: trigonelline; TXNRD1: thioredoxin reductase 1; VIM: vimentin; WT: wild-type; ZEB1: zinc finger E-box binding homeobox 1.

Revealing the Impact of Mitochondrial Fitness During Early Neural Development Using Human Brain Organoids

Mitochondrial homeostasis -including function, morphology, and inter-organelle communication- provides guidance to the intrinsic developmental programs of corticogenesis, while also being responsive to environmental and intercellular signals. Two- and three-dimensional platforms have become useful tools to interrogate the capacity of cells to generate neuronal and glia progeny in a background of metabolic dysregulation, but the mechanistic underpinnings underlying the role of mitochondria during human neurogenesis remain unexplored. Here we provide a concise overview of cortical development and the use of pluripotent stem cell models that have contributed to our understanding of mitochondrial and metabolic regulation of early human brain development. We finally discuss the effects of mitochondrial fitness dysregulation seen under stress conditions such as metabolic dysregulation, absence of developmental apoptosis, and hypoxia; and the avenues of research that can be explored with the use of brain organoids.

Copper modulates heart mitochondrial H2O2 emission differently during fatty acid and pyruvate oxidation

Although the preferred cardiac metabolic fuels are fatty acids, glucose metabolism also plays an important role. However, irrespective of substrate type, energy generation results in mitochondrial reactive oxygen species (ROS) formation. To determine if the preference of fat over carbohydrates predisposes cardiomyocytes to oxidant production, we measured total and site-specific H₂O₂ emission in heart mitochondria oxidizing palmitoylcarnitine or pyruvate during copper (Cu) exposure. H₂O₂ emission was higher during oxidation of palmitoylcarnitine compared with pyruvate. Moreover, the bulk of the H₂O₂ emitted during palmitoylcarnitine oxidation originated from the outer ubiquinone binding site of complex III (site III_Qo) and the flavin site of electron transfer flavoprotein (site E_F). We found no evidence of ROS production from complex I ubiquinone-binding site (site I_Q) by reverse electron transport during oxidation of palmitoylcarnitine. Pyruvate oxidation also drove H₂O₂ emission primarily from sites III_Qo; however, the flavin sites of pyruvate dehydrogenase (site P_F) and complex II (site II_F) contributed substantially. The effect of Cu depended on substrate and redox site, with effects at sites O_F and III_Qo being more pronounced in mitochondria oxidizing pyruvate compared with palmitoylcarnitine. Cu imposed a concentration-saturable effect at site P_F but concentration-dependently stimulated H₂O₂ emission at site E_F. The substrate-dependent differences in H₂O₂ emission and effects of Cu suggest that fuel type and points of entry of electrons into the mitochondrial electron transport system determine the mitochondrial ROS production rate. Importantly, knowledge of sites of mitochondrial ROS production is crucial to the understanding of cardiac dysfunction associated with impaired substrate metabolism.

Fundamentals of Membrane Lipid Replacement: A Natural Medicine Approach to Repairing Cellular Membranes and Reducing Fatigue, Pain, and Other Symptoms While Restoring Function in Chronic Illnesses and Aging

Membrane Lipid Replacement (MLR) uses natural membrane lipid supplements to safely replace damaged, oxidized lipids in membranes in order to restore membrane function, decrease symptoms and improve health. Oral MLR supplements contain mixtures of cell membrane glycerolphospholipids, fatty acids, and other lipids, and can be used to replace and remove damaged cellular and intracellular membrane lipids. Membrane injury, caused mainly by oxidative damage, occurs in essentially all chronic and acute medical conditions, including cancer and degenerative diseases, and in normal processes, such as aging and development. After ingestion, the protected MLR glycerolphospholipids and other lipids are dispersed, absorbed, and internalized in the small intestines, where they can be partitioned into circulating lipoproteins, globules, liposomes, micelles, membranes, and other carriers and transported in the lymphatics and blood circulation to tissues and cellular sites where they are taken in by cells and partitioned into various cellular membranes. Once inside cells, the glycerolphospholipids and other lipids are transferred to various intracellular membranes by lipid carriers, globules, liposomes, chylomicrons, or by direct membrane-membrane interactions. The entire process appears to be driven by 'bulk flow' or mass action principles, where surplus concentrations of replacement lipids can stimulate the natural exchange and removal of damaged membrane lipids while the replacement lipids undergo further enzymatic alterations. Clinical studies have demonstrated the advantages of MLR in restoring membrane and organelle function and reducing fatigue, pain, and other symptoms in chronic illness and aging patients.

Nrf2 deficiency decreases NADPH from impaired IDH shuttle and pentose phosphate pathway in retinal pigmented epithelial cells to magnify oxidative stress-induced mitochondrial dysfunction

The nuclear factor‐erythroid 2‐related factor‐2 (Nrf2), a major antioxidant transcription factor, is decreased in several age‐related diseases including age‐related macular degeneration (AMD), the most common cause of blindness among the elderly in western society. Since Nrf2’s mito‐protective response is understudied, we investigated its antioxidant response on mitochondria. Control and Nrf2‐deficient retinal pigmented epithelial (RPE) cells were compared after treating with cigarette smoke extract (CSE). Mitochondrial antioxidant abundance and reactive oxygen species (ROS) were quantified. Mitochondrial function was assessed by TMRM assay, NADPH, electron transport chain activity, and Seahorse. Results were corroborated in Nrf2^−/− mice and relevance to AMD was provided by immunohistochemistry of human globes. CSE induced mitochondrial ROS to impair mitochondrial function. H₂O₂ increase in particular, was magnified by Nrf2 deficiency, and corresponded with exaggerated mitochondrial dysfunction. While Nrf2 did not affect mitochondrial antioxidant abundance, oxidized PRX3 was magnified by Nrf2 deficiency due to decreased NADPH from decreased expression of IDH2 and pentose phosphate pathway (PPP) genes. With severe CSE stress, intrinsic apoptosis was activated to increase cell death. PPP component TALDO1 immunolabeling was decreased in dysmorphic RPE of human AMD globes. Despite limited regulation of mitochondrial antioxidant expression, Nrf2 influences PPP and IDH shuttle activity that indirectly supplies NADPH for the TRX2 system. These results provide insight into how Nrf2 deficiency impacts the mitochondrial antioxidant response, and its role in AMD pathobiology.

Can N-3 polyunsaturated fatty acids be considered a potential adjuvant therapy for COVID-19-associated cardiovascular complications?

Coronavirus disease 2019 (COVID-19), caused by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), has currently led to a global pandemic with millions of confirmed and increasing cases around the world. The novel SARS-CoV-2 not only affects the lungs causing severe acute respiratory dysfunction but also leads to significant dysfunction in multiple organs and physiological systems including the cardiovascular system. A plethora of studies have shown the viral infection triggers an exaggerated immune response, hypercoagulation and oxidative stress, which contribute significantly to poor cardiovascular outcomes observed in COVID-19 patients. To date, there are no approved vaccines or therapies for COVID-19. Accordingly, cardiovascular protective and supportive therapies are urgent and necessary to the overall prognosis of COVID-19 patients. Accumulating literature has demonstrated the beneficial effects of n-3 polyunsaturated fatty acids (n-3 PUFA) toward the cardiovascular system, which include ameliorating uncontrolled inflammatory reactions, reduced oxidative stress and mitigating coagulopathy. Moreover, it has been demonstrated the n-3 PUFAs, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are precursors to a group of potent bioactive lipid mediators, generated endogenously, which mediate many of the beneficial effects attributed to their parent compounds. Considering the favorable safety profile for n-3 PUFAs and their metabolites, it is reasonable to consider n-3 PUFAs as potential adjuvant therapies for the clinical management of COVID-19 patients. In this article, we provide an overview of the pathogenesis of cardiovascular complications secondary to COVID-19 and focus on the mechanisms that may contribute to the likely benefits of n-3 PUFAs and their metabolites.

Low-Dose Ammonium Preconditioning Enhances Endurance in Submaximal Physical Exercises

Preconditioning is often used in medicine to protect organs from ischemic damage and in athletes to enhance the performances. We tested whether low-dose ammonium preconditioning (AMP) could have a beneficial effect on physical exercises (PE). We used Cardiopulmonary Exercise Testing (CPET) on a treadmill to investigate the effects of low-dose AMP on the physical exercise capacity of professional track and field athletes and tested twenty-five athletes. Because of the individual differences between athletes, we performed a preliminary treadmill test (Pre-test) and, according to the results, the athletes were randomly allocated into the AMP and control (placebo, PL) group based on the similarity of the total distance covered on a treadmill. In the AMP group, the covered distance increased (11.3 ± 3.6%, p < 0.02) compared to Pre-test. Similarly, AMP significantly increased O₂ uptake volume—VO₂ (4.6 ± 2.3%, p < 0.03) and pulmonary CO₂ output—VCO₂ (8.7 ± 2.8%, p < 0.01). Further, the basic blood parameters (pH, pO₂, and lactate) shift was lower despite the greater physical exercise progress in the AMP group compared to Pre-test, whereas in the placebo group there were no differences between Pre-test and Load-test. Importantly, the AMP significantly increased red blood cell count (6.8 ± 2.0%, p < 0.01) and hemoglobin concentration (5.3 ± 1.9%, p < 0.01), which might explain the beneficial effects in physical exercise progress. For the first time, we showed that low-dose AMP had clear beneficial effects on submaximal PE.

Oxidative stress as a therapeutic target for the prevention and treatment of early age-related macular degeneration

Age-related macular degeneration, the leading cause of irreversible visual loss among older adults in developed countries, is a chronic, multifactorial, and progressive disease with the development of painless, central vision loss. Retinal pigment epithelial cell dysfunction is a core change in age-related macular degeneration that results from aging and the accumulated effects of genetic and environmental factors that, in part, is both caused by and leads to oxidative stress. In this review, we describe the role of oxidative stress, the cytoprotective oxidative stress pathways, and the impact of oxidative stress on critical cellular processes involved in age-related macular degeneration pathobiology. We also offer targeted therapy that may define how antioxidant therapy can either prevent or improve specific stages of age-related macular degeneration.

Update on antiarrhythmic drug pharmacology

Cardiac arrhythmias constitute a major public health problem. Pharmacological intervention remains mainstay to their clinical management. This, in turn, depends upon systematic drug classification schemes relating their molecular, cellular, and systems effects to clinical indications and therapeutic actions. This approach was first pioneered in the 1960s Vaughan-Williams classification. Subsequent progress in cardiac electrophysiological understanding led to a lag between the fundamental science and its clinical translation, partly addressed by The working group of the European Society of Cardiology (1991), which, however, did not emerge with formal classifications. We here utilize the recent Revised Oxford Classification Scheme to review antiarrhythmic drug pharmacology. We survey drugs and therapeutic targets offered by the more recently characterized ion channels, transporters, receptors, intracellular Ca²⁺ handling, and cell signaling molecules. These are organized into their strategic roles in cardiac electrophysiological function. Following analysis of the arrhythmic process itself, we consider (a) pharmacological agents directly targeting membrane function, particularly the Na⁺ and K⁺ ion channels underlying depolarizing and repolarizing events in the cardiac action potential. (b) We also consider agents that modify autonomic activity that, in turn, affects both the membrane and (c) the Ca²⁺ homeostatic and excitation-contraction coupling processes linking membrane excitation to contractile activation. Finally, we consider (d) drugs acting on more upstream energetic and structural remodeling processes currently the subject of clinical trials. Such systematic correlations of drug actions and arrhythmic mechanisms at different molecular to systems levels of cardiac function will facilitate current and future antiarrhythmic therapy.

Molecular basis of arrhythmic substrate in ageing murine peroxisome proliferator-activated receptor γ co-activator deficient hearts modelling mitochondrial dysfunction

INTRODUCTION

Ageing and chronic metabolic disorders are associated with mitochondrial dysfunction and cardiac pro-arrhythmic phenotypes which were recently attributed to slowed atrial and ventricular action potential (AP) conduction in peroxisome proliferator-activated receptor γ co-activator deficient (Pgc-1β-/-) mice.

METHODS

We compared expression levels of voltage-gated Na+ channel (NaV1.5) and gap junction channels, Connexins 40 and 43 (Cx40 and Cx43) in the hearts of young and old, and wild-type (WT) and Pgc-1β-/- mice. This employed Western blotting (WB) for NaV1.5, Cx40 and Cx43 in atrial/ventricular tissue lysates, and immunofluorescence (IF) from Cx43 was explored in tissue sections. Results were analysed using two-way analysis of variance (ANOVA) for independent/interacting effects of age and genotype.

RESULTS

In atria, increased age and Pgc-1β-/- genotype each independently decreased both Cx40 and Cx43 expression without interacting effects. In IF experiments, both age and Pgc-1β deletion independently reduced Cx43 expression. In ventricles, age and genotype exerted interacting effects in WB studies of NaV1.5 expression. Young Pgc-1β-/- then showed greater NaV1.5 expression than young WT ventricles. However, neither age nor Pgc-1β deletion affected Cx43 expression, independently or through interacting effects in both WB and IF studies.

CONCLUSION

Similar pro-arrhythmic atrial/ventricular phenotypes arise in aged/Pgc-1β-/- from differing contributions of altered protein expression and functional effects that may arise from multiple acute mechanisms.

Benzo[a]pyrene (BaP) exposure generates persistent reactive oxygen species (ROS) to inhibit the NF-κB pathway in medaka (Oryzias melastigma)

Benzo[a]pyrene (BaP), a common environmental pollutant, can modulate the immune-associated signal pathway NF-κB, which is one of the critical signal pathways involved in various immune responses. BaP exposure usually generates reactive oxygen species (ROS), but whether ROS are predominantly involved in the modulation mechanism of the NF-κB pathway has not been clearly understood. In this study, an in vivo examination of Oryzias melastigma demonstrated that BaP exposure led to a down-regulation of the NF-κB pathway and increased levels of ROS. Conversely, in vitro results using the medaka liver cell line DIT-29 and a widely applied H₂O₂ method showed the opposite: up-regulation of the NF-κB pathway. However, the down-regulation of NF-κB upon BaP exposure in vitro was inhibited by the addition of a ROS inhibitor, indicating ROS are involved in the modulation of NF-κB. The discrepancy between in vivo and in vitro results of ROS impacts on NF-κB activation might be related to the concentration and persistence of ROS. Using a modified luminol detection system, BaP was found to generate sustained physiological concentrations of ROS for 24 h, while an H₂O₂ bolus generated ROS for less than 30 min. Furthermore, a steady-state sub-micromolar H₂O₂ system (H₂O₂ss) was developed in parallel as a positive control of ROS, by which H₂O₂ could be maintained for 24 h. Comparative evaluation using H₂O₂, H₂O₂ss and BaP exposures on the medaka cell line with pGL4.32 demonstrated that the persistent physiological concentrations of ROS generated upon BaP exposure or treatment with H₂O₂ss inhibited the NF-κB pathway, but direct H₂O₂ exposure had the opposite effect. Moreover, a western-blot assay and EMSA detection further confirmed the modulation of the NF-κB pathway in DIT-29. Taken together, this study shows that BaP exposure inhibits the NF-κB pathway by generating sustained physiological concentrations of ROS.

Oxygen-dependence of mitochondrial ROS production as detected by Amplex Red assay

The initial rates of superoxide plus hydrogen peroxide (ROS) generation by intact or permeabilized rat heart mitochondria and coupled inside-out bovine heart submitochondrial particles (SMP) oxidizing NAD-dependent substrates, NADH, and succinate were measured by detecting resorufin formation in the Amplex Red assay at various oxygen concentrations. Linear dependences of the initial rates on oxygen concentration within the range of ~125-750 μM were found for all significant mitochondrial generators, i.e. the respiratory complexes and ammonium-stimulated dihydrolipoamide dehydrogenase. At lower oxygen concentrations upon its decrease from air saturation level to zero, the time-course of resorufin formation by SMP catalyzing coupled oxidation of succinate (the total ROS production by respiratory complexes II and III and by the reverse electron transfer (RET)-mediated by complex I) also corresponds to the linear dependence on oxygen with the same first-order rate constant determined in the initial rate studies. Prolonged incubation of SMP generating succinate-supported complex I-mediated ROS affected neither their NADH oxidase nor ROS generating activity. In contrast to SMP significant deviation from the first-order oxygen dependence in the time-course kinetics during coupled oxidation of succinate by intact mitochondria was evident. Complex I catalyzes the NADH:resorufin oxidoreductase reaction resulting in formation of colorless reduced resorufin. Hydrogen peroxide oxidizes reduced resorufin in the presence of peroxidase, thus showing its dihydroresorufin peroxidase activity. Combined NADH:resorufin reductase and dihydroresorufin peroxidase activities result in underestimation of the amount of hydrogen peroxide generated by mitochondria. We conclude that only initial rates of the mitochondrial ROS production, not the amount of resorufin accumulated, should be taken as the reliable measure of the mitochondrial ROS-generating activity, because of the cycling of the oxidized and reduced resorufin during Amplex Red assays fed by NADH and other possible reductant(s) present in mitochondria.

Aquaporin Membrane Channels in Oxidative Stress, Cell Signaling, and Aging: Recent Advances and Research Trends

Reactive oxygen species (ROS) are produced as a result of aerobic metabolism and as by-products through numerous physiological and biochemical processes. While ROS-dependent modifications are fundamental in transducing intracellular signals controlling pleiotropic functions, imbalanced ROS can cause oxidative damage, eventually leading to many chronic diseases. Moreover, increased ROS and reduced nitric oxide (NO) bioavailability are main key factors in dysfunctions underlying aging, frailty, hypertension, and atherosclerosis. Extensive investigation aims to elucidate the beneficial effects of ROS and NO, providing novel insights into the current medical treatment of oxidative stress-related diseases of high epidemiological impact. This review focuses on emerging topics encompassing the functional involvement of aquaporin channel proteins (AQPs) and membrane transport systems, also allowing permeation of NO and hydrogen peroxide, a major ROS, in oxidative stress physiology and pathophysiology. The most recent advances regarding the modulation exerted by food phytocompounds with antioxidant action on AQPs are also reviewed.

Hepatitis E Virus Induces Hepatocyte Apoptosis via Mitochondrial Pathway in Mongolian Gerbils

Previous studies demonstrated that Mongolian gerbils can be infected by hepatitis E virus (HEV), which induces the hepatic injury. Here, the mitochondria in hepatocytes from HEV-infected gerbils were considerably swollen, thin cristae. After HEV infection, the activity of superoxide dismutase significantly decreased (p < 0.01), while malondialdehyde concentrations significantly increased, compared with those in the control group (p < 0.01). Adenosine triphosphatase levels decreased significantly in the hepatocyte of the inoculated groups, compared with those in control group (p < 0.05) at days 21, 28, 42 post-inoculation (dpi) as well. Furthermore, the levels of ATP synthetase ATP5A1 significantly decreased during HEV infection, compared with those in the control group (p < 0.05). According to the TdT mediated dUTP nick end labeling (TUNEL) detection, TUNEL positive hepatocytes increased in the inoculated group, compared with that in the control group (p < 0.05). Up-regulation of the mitochondrion-mediated apoptosis regulating proteins, Bax and Bcl-2, in the HEV-infected gerbils (p < 0.05) was observed. However, cytochrome c levels in mitochondria decreased, while this molecule was detected in the cytoplasm of the infected animals, in contrast to that in the control group. Apaf-1, and active caspase-9 and -3 levels were shown to be significantly higher in the inoculated group compared with those in the control group (p < 0.05). Taken together, our results demonstrated that HEV infection induces hepatocyte injuries and activity of the mitochondrial apoptotic pathway, which trigger the hepatocyte apoptosis in Mongolian gerbils.

Pro-arrhythmic atrial phenotypes in incrementally paced murine Pgc1β-/- hearts: effects of age

New Findings

What is the central question of this study?

Can we experimentally replicate atrial pro‐arrhythmic phenotypes associated with important chronic clinical conditions, including physical inactivity, obesity, diabetes mellitus and metabolic syndrome, compromising mitochondrial function, and clarify their electrophysiological basis?

What is the main finding and its importance?

Electrocardiographic and intracellular cardiomyocyte recording at progressively incremented pacing rates demonstrated age‐dependent atrial arrhythmic phenotypes in Langendorff‐perfused murine Pgc1β^−/− hearts for the first time. We attributed these to compromised action potential conduction and excitation wavefronts, whilst excluding alterations in recovery properties or temporal electrophysiological instabilities, clarifying these pro‐arrhythmic changes in chronic metabolic disease.

Atrial arrhythmias, most commonly manifesting as atrial fibrillation, represent a major clinical problem. The incidence of atrial fibrillation increases with both age and conditions associated with energetic dysfunction. Atrial arrhythmic phenotypes were compared in young (12–16 week) and aged (>52 week) wild‐type (WT) and peroxisome proliferative activated receptor, gamma, coactivator 1 beta (Ppargc1b)‐deficient (Pgc1β^−/−) Langendorff‐perfused hearts, previously used to model mitochondrial energetic disorder. Electrophysiological explorations were performed using simultaneous whole‐heart ECG and intracellular atrial action potential (AP) recordings. Two stimulation protocols were used: an S1S2 protocol, which imposed extrasystolic stimuli at successively decremented intervals following regular pulse trains; and a regular pacing protocol at successively incremented frequencies. Aged Pgc1β^−/− hearts showed greater atrial arrhythmogenicity, presenting as atrial tachycardia and ectopic activity. Maximal rates of AP depolarization (dV/dt_max) were reduced in Pgc1β^−/− hearts. Action potential latencies were increased by the Pgc1β^−/− genotype, with an added interactive effect of age. In contrast, AP durations to 90% recovery (APD₉₀) were shorter in Pgc1β^−/− hearts despite similar atrial effective recovery periods amongst the different groups. These findings accompanied paradoxical decreases in the incidence and duration of alternans in the aged and Pgc1β^−/− hearts. Limiting slopes of restitution curves of APD₉₀ against diastolic interval were correspondingly reduced interactively by Pgc1β^−/− genotype and age. In contrast, reduced AP wavelengths were associated with Pgc1β^−/− genotype, both independently and interacting with age, through the basic cycle lengths explored, with the aged Pgc1β^−/− hearts showing the shortest wavelengths. These findings thus implicate AP wavelength in possible mechanisms for the atrial arrhythmic changes reported here.

Effects of ageing on pro-arrhythmic ventricular phenotypes in incrementally paced murine Pgc-1β -/- hearts

A range of chronic clinical conditions accompany cardiomyocyte energetic dysfunction and constitute independent risk factors for cardiac arrhythmia. We investigated pro-arrhythmic and arrhythmic phenotypes in energetically deficient C57BL mice with genetic ablation of the mitochondrial promoter peroxisome proliferator-activated receptor-γ coactivator-1β (Pgc-1β), a known model of ventricular arrhythmia. Pro-arrhythmic and cellular action potential (AP) characteristics were compared in intact Langendorff-perfused hearts from young (12–16 week) and aged (> 52 week), wild-type (WT) and Pgc-1β^−/− mice. Simultaneous electrocardiographic and intracellular microelectrode recordings were made through successive trains of 100 regular stimuli at progressively incremented heart rates. Aged Pgc-1β^−/− hearts displayed an increased incidence of arrhythmia compared to other groups. Young and aged Pgc-1β^−/− hearts showed higher incidences of alternans in both AP activation (maximum AP upshoot velocity (dV/dt)_max and latency), recovery (action potential duration (APD₉₀) and resting membrane potential (RMP) characteristics compared to WT hearts. This was particularly apparent at lower pacing frequencies. These findings accompanied reduced (dV/dt)_max and increased AP latency values in the Pgc-1β^−/− hearts. APs observed prior to termination of the protocol showed lower (dV/dt)_max and longer AP latencies, but indistinguishable APD₉₀ and RMPs in arrhythmic compared to those in non-arrhythmic hearts. APD restitution analysis showed that Pgc-1β^−/− and WT hearts showed similar limiting gradients. However, Pgc-1β^−/− hearts had shortened plateau AP wavelengths, particularly in aged Pgc-1β^−/− hearts. Pgc-1β^−/− hearts therefore show pro-arrhythmic instabilities attributable to altered AP conduction and activation rather than recovery characteristics.

Mitochondria, Bioenergetics and Excitotoxicity: New Therapeutic Targets in Perinatal Brain Injury

Injury to the fragile immature brain is implicated in the manifestation of long-term neurological disorders, including childhood disability such as cerebral palsy, learning disability and behavioral disorders. Advancements in perinatal practice and improved care mean the majority of infants suffering from perinatal brain injury will survive, with many subtle clinical symptoms going undiagnosed until later in life. Hypoxic-ischemia is the dominant cause of perinatal brain injury, and constitutes a significant socioeconomic burden to both developed and developing countries. Therapeutic hypothermia is the sole validated clinical intervention to perinatal asphyxia; however it is not always neuroprotective and its utility is limited to developed countries. There is an urgent need to better understand the molecular pathways underlying hypoxic-ischemic injury to identify new therapeutic targets in such a small but critical therapeutic window. Mitochondria are highly implicated following ischemic injury due to their roles as the powerhouse and main energy generators of the cell, as well as cell death processes. While the link between impaired mitochondrial bioenergetics and secondary energy failure following loss of high-energy phosphates is well established after hypoxia-ischemia (HI), there is emerging evidence that the roles of mitochondria in disease extend far beyond this. Indeed, mitochondrial turnover, including processes such as mitochondrial biogenesis, fusion, fission and mitophagy, affect recovery of neurons after injury and mitochondria are involved in the regulation of the innate immune response to inflammation. This review article will explore these mitochondrial pathways, and finally will summarize past and current efforts in targeting these pathways after hypoxic-ischemic injury, as a means of identifying new avenues for clinical intervention.

Reactive oxygen species production induced by pore opening in cardiac mitochondria: The role of complex II

Succinate-driven reverse electron transport (RET) through complex I is hypothesized to be a major source of reactive oxygen species (ROS) that induces permeability transition pore (PTP) opening and damages the heart during ischemia/reperfusion. Because RET can only generate ROS when mitochondria are fully polarized, this mechanism is self-limiting once PTP opens during reperfusion. In the accompanying article (Korge, P., Calmettes, G., John, S. A., and Weiss, J. N. (2017) J. Biol. Chem. 292, 9882-9895), we showed that ROS production after PTP opening can be sustained when complex III is damaged (simulated by antimycin). Here we show that complex II can also contribute to sustained ROS production in isolated rabbit cardiac mitochondria following inner membrane pore formation induced by either alamethicin or calcium-induced PTP opening. Two conditions are required to maximize malonate-sensitive ROS production by complex II in isolated mitochondria: (a) complex II inhibition by atpenin A5 or complex III inhibition by stigmatellin that results in succinate-dependent reduction of the dicarboxylate-binding site of complex II (site II_f); (b) pore opening in the inner membrane resulting in rapid efflux of succinate/fumarate and other dicarboxylates capable of competitively binding to site II_f The decrease in matrix [dicarboxylate] allows O₂ access to reduced site II_f, thereby making electron donation to O₂ possible, explaining the rapid increase in ROS production provided that site II_f is reduced. Because ischemia is known to inhibit complexes II and III and increase matrix succinate/fumarate levels, we hypothesize that by allowing dicarboxylate efflux from the matrix, PTP opening during reperfusion may activate sustained ROS production by this mechanism after RET-driven ROS production has ceased.

Metabolic analysis of radioresistant medulloblastoma stem-like clones and potential therapeutic targets

Medulloblastoma is a fatal brain tumor in children, primarily due to the presence of treatment-resistant medulloblastoma stem cells. The energy metabolic pathway is a potential target of cancer therapy because it is often different between cancer cells and normal cells. However, the metabolic properties of medulloblastoma stem cells, and whether specific metabolic pathways are essential for sustaining their stem cell-like phenotype and radioresistance, remain unclear. We have established radioresistant medulloblastoma stem-like clones (rMSLCs) by irradiation of the human medulloblastoma cell line ONS-76. Here, we assessed reactive oxygen species (ROS) production, mitochondria function, oxygen consumption rate (OCR), energy state, and metabolites of glycolysis and tricarboxylic acid cycle in rMSLCs and parental cells. rMSLCs showed higher lactate production and lower oxygen consumption rate than parental cells. Additionally, rMSLCs had low mitochondria mass, low endogenous ROS production, and existed in a low-energy state. Treatment with the metabolic modifier dichloroacetate (DCA) resulted in mitochondria dysfunction, glycolysis inhibition, elongated mitochondria morphology, and increased ROS production. DCA also increased radiosensitivity by suppression of the DNA repair capacity through nuclear oxidization and accelerated the generation of acetyl CoA to compensate for the lack of ATP. Moreover, treatment with DCA decreased cancer stem cell-like characters (e.g., CD133 positivity and sphere-forming ability) in rMSLCs. Together, our findings provide insights into the specific metabolism of rMSLCs and illuminate potential metabolic targets that might be exploited for therapeutic benefit in medulloblastoma.

Membrane Lipid Replacement for chronic illnesses, aging and cancer using oral glycerolphospholipid formulations with fructooligosaccharides to restore phospholipid function in cellular membranes, organelles, cells and tissues

Membrane Lipid Replacement is the use of functional, oral supplements containing mixtures of cell membrane glycerolphospholipids, plus fructooligosaccharides (for protection against oxidative, bile acid and enzymatic damage) and antioxidants, in order to safely replace damaged, oxidized, membrane phospholipids and restore membrane, organelle, cellular and organ function. Defects in cellular and intracellular membranes are characteristic of all chronic medical conditions, including cancer, and normal processes, such as aging. Once the replacement glycerolphospholipids have been ingested, dispersed, complexed and transported, while being protected by fructooligosaccharides and several natural mechanisms, they can be inserted into cell membranes, lipoproteins, lipid globules, lipid droplets, liposomes and other carriers. They are conveyed by the lymphatics and blood circulation to cellular sites where they are endocytosed or incorporated into or transported by cell membranes. Inside cells the glycerolphospholipids can be transferred to various intracellular membranes by lipid globules, liposomes, membrane-membrane contact or by lipid carrier transfer. Eventually they arrive at their membrane destinations due to 'bulk flow' principles, and there they can stimulate the natural removal and replacement of damaged membrane lipids while undergoing further enzymatic alterations. Clinical trials have shown the benefits of Membrane Lipid Replacement in restoring mitochondrial function and reducing fatigue in aged subjects and chronically ill patients. Recently Membrane Lipid Replacement has been used to reduce pain and other symptoms as well as removing hydrophobic chemical contaminants, suggesting that there are additional new uses for this safe, natural medicine supplement. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Tetrahydrocurcumin reduces oxidative stress-induced apoptosis via the mitochondrial apoptotic pathway by modulating autophagy in rats after traumatic brain injury

Tetrahydrocurcumin (THC) has been identified as a multi-functional neuroprotective agent in numerous neurological disorders. Oxidative stress as a result of injury may induce neuronal apoptosis after traumatic brain injury (TBI). Treatment with THC may improve neurological function following TBI by attenuating oxidative stress and apoptosis and by enhancing autophagy. The purpose of this study was to investigate the mechanism of neuroprotection by THC against oxidative stress-induced neuronal apoptosis after TBI. We hypothesized that neuroprotection by THC may involve modulation of autophagy and the mitochondria apoptotic pathway. We used western blot analysis to evaluate the effect of THC on proteins involved in mitochondrial autophagy and apoptosis after TBI. The terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) assay and immunofluorescence staining were used to confirm the role of THC in apoptosis and autophagy, respectively. THC-induced neuroprotection was assessed by neurological severity scoring (NSS) and by measuring the brain water content. We demonstrated that treatment with THC increased expression of autophagy-associated proteins LC3-II and Beclin-1 at 24 h post-TBI. Treatment with THC also reduced expression of malondialdehyde (MDA) and increased glutathione peroxidase (GPx) activity. Further, treatment with THC attenuated apoptosis by modulating mitochondrial apoptosis and reducing oxidative stress. Treatment with 3-methyladenine (3-MA) mitigated autophagy activation and reversed the inhibitory effect of THC on the translocation of Bax to the mitochondrial membrane. Moreover, treatment with THC improved neurological function and reduced the brain water content in rats after TBI. We concluded that the neuroprotective effects of THC are mediated by enhancing autophagy activation and by attenuation of oxidative stress and apoptosis after TBI, probably by modulating the mitochondrial apoptotic pathway. We suggest that THC may be an effective therapeutic agent to treat TBI.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Respiratory complex II: ROS production and the kinetics of ubiquinone reduction

Bovine heart mitochondrial respiratory complex II generates ROS, mostly as superoxide, at the rate of about 20% of that detected during simultaneous operation of complex I and II when oxidation of ubiquinol is prevented by myxothiazol. ROS generating activity at different fumarate/succinate concentrations ratio implies that an enzyme component with a midpoint potential 40mV more positive than that of fumarate/succinate couple is the donor for one-electron reduction of oxygen. This suggests that the iron-sulfur cluster(s) is(are) involved in superoxide formation. Complex II-mediated ROS production exhibits a maximum at low (about 50μM) succinate concentration and gradually declines to zero activity upon further increase of the substrate. This phenomenology is explained and kinetically modeled to suggest a ping-pong mechanism of ROS generating activity where only dicarboxylate free reduced enzyme is oxidized by oxygen. The succinate:quinone reductase activity catalyzed by purified succinate:ubiquinone reductase also exhibits a ping-pong mechanism where only dicarboxylate free enzyme is oxidized by added quinone. Together these data suggest long distance interaction between the succinate (fumarate) binding and ubiquinone (ubiquinol) reactive sites.

Migraine Triggers and Oxidative Stress: A Narrative Review and Synthesis

BACKGROUND

Blau theorized that migraine triggers are exposures that in higher amounts would damage the brain. The recent discovery that the TRPA1 ion channel transduces oxidative stress and triggers neurogenic inflammation suggests that oxidative stress may be the common denominator underlying migraine triggers.

OBJECTIVE

The aim of this review is to present and discuss the available literature on the capacity of common migraine triggers to generate oxidative stress in the brain.

METHODS

A Medline search was conducted crossing the terms "oxidative stress" and "brain" with "alcohol," "dehydration," "water deprivation," "monosodium glutamate," "aspartame," "tyramine," "phenylethylamine," "dietary nitrates," "nitrosamines," "noise," "weather," "air pollutants," "hypoglycemia," "hypoxia," "infection," "estrogen," "circadian," "sleep deprivation," "information processing," "psychosocial stress," or "nitroglycerin and tolerance." "Flavonoids" was crossed with "prooxidant." The reference lists of the resulting articles were examined for further relevant studies. The focus was on empirical studies, in vitro and of animals, of individual triggers, indicating whether and/or by what mechanism they can generate oxidative stress.

RESULTS

In all cases except pericranial pain, common migraine triggers are capable of generating oxidative stress. Depending on the trigger, mechanisms include a high rate of energy production by the mitochondria, toxicity or altered membrane properties of the mitochondria, calcium overload and excitotoxicity, neuroinflammation and activation of microglia, and activation of neuronal nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. For some triggers, oxidants also arise as a byproduct of monoamine oxidase or cytochrome P450 processing, or from uncoupling of nitric oxide synthase.

CONCLUSIONS

Oxidative stress is a plausible unifying principle behind the types of migraine triggers encountered in clinical practice. The possible implications for prevention and for understanding the nature of the migraine attack are discussed.

Integrating mitochondriomics in children's environmental health

The amount of scientific research linking environmental exposures and childhood health outcomes continues to grow; yet few studies have teased out the mechanisms involved in environmentally-induced diseases. Cells can respond to environmental stressors in many ways: inducing oxidative stress/inflammation, changes in energy production and epigenetic alterations. Mitochondria, tiny organelles that each retains their own DNA, are exquisitely sensitive to environmental insults and are thought to be central players in these pathways. While it is intuitive that mitochondria play an important role in disease processes, given that every cell of our body is dependent on energy metabolism, it is less clear how environmental exposures impact mitochondrial mechanisms that may lead to enhanced risk of disease. Many of the effects of the environment are initiated in utero and integrating mitochondriomics into children's environmental health studies is a critical priority. This review will highlight (i) the importance of exploring environmental mitochondriomics in children's environmental health, (ii) why environmental mitochondriomics is well suited to biomarker development in this context, and (iii) how molecular and epigenetic changes in mitochondria and mitochondrial DNA (mtDNA) may reflect exposures linked to childhood health outcomes.

Calcium and ROS: A mutual interplay

Calcium is an important second messenger involved in intra- and extracellular signaling cascades and plays an essential role in cell life and death decisions. The Ca²⁺ signaling network works in many different ways to regulate cellular processes that function over a wide dynamic range due to the action of buffers, pumps and exchangers on the plasma membrane as well as in internal stores. Calcium signaling pathways interact with other cellular signaling systems such as reactive oxygen species (ROS). Although initially considered to be potentially detrimental byproducts of aerobic metabolism, it is now clear that ROS generated in sub-toxic levels by different intracellular systems act as signaling molecules involved in various cellular processes including growth and cell death. Increasing evidence suggests a mutual interplay between calcium and ROS signaling systems which seems to have important implications for fine tuning cellular signaling networks. However, dysfunction in either of the systems might affect the other system thus potentiating harmful effects which might contribute to the pathogenesis of various disorders.

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Calcium and ROS act as signaling molecules inside the cell and their pathways can interact.

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The mutual interplay of calcium and ROS is required for the fine tuning of signaling.

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Failure in the interplay results in dysfunction and pathologies.

Mitophagy and heart failure

Cardiac mitochondria are responsible for generating energy in the form of ATP through oxidative phosphorylation and are crucial for cardiac function. Mitochondrial dysfunction is a major contributor to loss of myocytes and development of heart failure. Myocytes have quality control mechanisms in place to ensure a network of functional mitochondria. Damaged mitochondria are degraded by a process called mitochondrial autophagy, or mitophagy, where the organelle is engulfed by an autophagosome and subsequently delivered to a lysosome for degradation. Evidence suggests that mitophagy is important for cellular homeostasis, and reduced mitophagy leads to inadequate removal of dysfunctional mitochondria. In this review, we discuss the regulation of mitophagy and the emerging evidence of the cardioprotective role of mitophagy. We also address the prospect of therapeutically targeting mitophagy to treat patients with cardiovascular disease.

Mitochondrial production of reactive oxygen species

Numerous biochemical studies are aimed at elucidating the sources and mechanisms of formation of reactive oxygen species (ROS) because they are involved in cellular, organ-, and tissue-specific physiology. Mitochondria along with other cellular organelles of eukaryotes contribute significantly to ROS formation and utilization. This review is a critical account of the mitochondrial ROS production and methods for their registration. The physiological and pathophysiological significance of the mitochondrially produced ROS are discussed.

Mitochondrial reactive oxygen species (ROS) and ROS-induced ROS release

Byproducts of normal mitochondrial metabolism and homeostasis include the buildup of potentially damaging levels of reactive oxygen species (ROS), Ca(2+), etc., which must be normalized. Evidence suggests that brief mitochondrial permeability transition pore (mPTP) openings play an important physiological role maintaining healthy mitochondria homeostasis. Adaptive and maladaptive responses to redox stress may involve mitochondrial channels such as mPTP and inner membrane anion channel (IMAC). Their activation causes intra- and intermitochondrial redox-environment changes leading to ROS release. This regenerative cycle of mitochondrial ROS formation and release was named ROS-induced ROS release (RIRR). Brief, reversible mPTP opening-associated ROS release apparently constitutes an adaptive housekeeping function by the timely release from mitochondria of accumulated potentially toxic levels of ROS (and Ca(2+)). At higher ROS levels, longer mPTP openings may release a ROS burst leading to destruction of mitochondria, and if propagated from mitochondrion to mitochondrion, of the cell itself. The destructive function of RIRR may serve a physiological role by removal of unwanted cells or damaged mitochondria, or cause the pathological elimination of vital and essential mitochondria and cells. The adaptive release of sufficient ROS into the vicinity of mitochondria may also activate local pools of redox-sensitive enzymes involved in protective signaling pathways that limit ischemic damage to mitochondria and cells in that area. Maladaptive mPTP- or IMAC-related RIRR may also be playing a role in aging. Because the mechanism of mitochondrial RIRR highlights the central role of mitochondria-formed ROS, we discuss all of the known ROS-producing sites (shown in vitro) and their relevance to the mitochondrial ROS production in vivo.

Oxidative stress induces mitochondrial dysfunction and a protective unfolded protein response in RPE cells

How cells degenerate from oxidative stress in aging-related disease is incompletely understood. This study's intent was to identify key cytoprotective pathways activated by oxidative stress and determine the extent of their protection. Using an unbiased strategy with microarray analysis, we found that retinal pigmented epithelial (RPE) cells treated with cigarette smoke extract (CSE) had overrepresented genes involved in the antioxidant and unfolded protein response (UPR). Differentially expressed antioxidant genes were predominantly located in the cytoplasm, with no induction of genes that neutralize superoxide and H2O2 in the mitochondria, resulting in accumulation of superoxide and decreased ATP production. Simultaneously, CSE induced the UPR sensors IRE1α, p-PERK, and ATP6, including CHOP, which was cytoprotective because CHOP knockdown decreased cell viability. In mice given intravitreal CSE, the RPE had increased IRE1α and decreased ATP and developed epithelial-mesenchymal transition, as suggested by decreased LRAT abundance, altered ZO-1 immunolabeling, and dysmorphic cell shape. Mildly degenerated RPE from early age-related macular degeneration (AMD) samples had prominent IRE1α, but minimal mitochondrial TOM20 immunolabeling. Although oxidative stress is thought to induce an antioxidant response with cooperation between the mitochondria and the ER, herein we show that mitochondria become impaired sufficiently to induce epithelial-mesenchymal transition despite a protective UPR. With similar responses in early AMD samples, these results suggest that mitochondria are vulnerable to oxidative stress despite a protective UPR during the early phases of aging-related disease.

Mitochondria: hub of injury responses in the developing brain

Progress in the field of mitochondrial biology in the past few years has shown that mitochondrial activities go beyond bioenergetics. These new aspects of mitochondrial physiology and pathophysiology have important implications for the immature brain. A picture emerges in which mitochondrial biogenesis, mitophagy, migration, and morphogenesis are crucial for brain development and synaptic pruning, and play a part in recovery after acute insults. Mitochondria also affect brain susceptibility to injury, and mitochondria-directed interventions can make the immature brain highly resistant to acute injury. Finally, the mitochondrion is a platform for innate immunity, contributes to inflammation in response to infection and acute damage, and participates in antiviral and antibacterial defence. Understanding of these new aspects of mitochondrial function will provide insights into brain development and neurological disease, and enable discovery and development of new strategies for treatment.

The role of mitochondrial function and cellular bioenergetics in ageing and disease

Mitochondria constitute an important topic of biomedical enquiry (one paper in every 154 indexed in PubMed since 1998 is retrieved by the keyword 'mitochondria') because of widespread recognition of their importance in cell physiology and pathology. Mitochondrial dysfunction is widely implicated in ageing and in the diseases of ageing, through dysfunction in adenosine triphosphate (ATP) synthesis, Ca(2+) homeostasis, central metabolic pathways or radical production. Nonetheless, the mechanisms and regulation of superoxide and hydrogen peroxide formation by mitochondria remain poorly described. Measurement of the capacities of different sites of superoxide and hydrogen peroxide production in isolated skeletal muscle mitochondria show that the maximum capacities of sites in complexes I, II and III and in several associated redox enzymes greatly exceed the native rates observed in the absence of respiratory chain inhibitors. In vitro, the native rates and the relative importance of different sites both depend on the substrate being oxidized, with sites IQ, IIF, GPDH, IF and IIIQo each being important with particular substrates. The techniques involved in measuring rates from each site should become applicable to cell cultures and in vivo in the future.

Lipid Replacement Therapy: a natural medicine approach to replacing damaged lipids in cellular membranes and organelles and restoring function

Lipid Replacement Therapy, the use of functional oral supplements containing cell membrane phospholipids and antioxidants, has been used to replace damaged, usually oxidized, membrane glycerophospholipids that accumulate during aging and in various clinical conditions in order to restore cellular function. This approach differs from other dietary and intravenous phospholipid interventions in the composition of phospholipids and their defense against oxidation during storage, ingestion, digestion and uptake as well as the use of protective molecules that noncovalently complex with phospholipid micelles and prevent their enzymatic and bile disruption. Once the phospholipids have been taken in by transport processes, they are protected by several natural mechanisms involving lipid receptors, transport and carrier molecules and circulating cells and lipoproteins until their delivery to tissues and cells where they can again be transferred to intracellular membranes by specific and nonspecific transport systems. Once delivered to membrane sites, they naturally replace and stimulate removal of damaged membrane lipids. Various chronic clinical conditions are characterized by membrane damage, mainly oxidative but also enzymatic, resulting in loss of cellular function. This is readily apparent in mitochondrial inner membranes where oxidative damage to phospholipids like cardiolipin and other molecules results in loss of trans-membrane potential, electron transport function and generation of high-energy molecules. Recent clinical trials have shown the benefits of Lipid Replacement Therapy in restoring mitochondrial function and reducing fatigue in aged subjects and patients with a variety of clinical diagnoses that are characterized by loss of mitochondrial function and include fatigue as a major symptom. This Article is Part of a Special Issue Entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Redox modification of ryanodine receptors by mitochondria-derived reactive oxygen species contributes to aberrant Ca2+ handling in ageing rabbit hearts

Ageing is associated with a blunted response to sympathetic stimulation and an increased risk of arrhythmia and sudden cardiac death. Aberrant calcium (Ca(2+)) handling is an important contributor to the electrical and contractile dysfunction associated with ageing. Yet, the specific molecular mechanisms underlying abnormal Ca(2+) handling in ageing heart remain poorly understood. In this study, we used ventricular myocytes isolated from young (5-9 months) and old (4-6 years) rabbit hearts to test the hypothesis that changes in Ca(2+) homeostasis are caused by post-translational modification of ryanodine receptors (RyRs) by mitochondria-derived reactive oxygen species (ROS) generated in the ageing heart. Changes in parameters of Ca(2+) handling were determined by measuring cytosolic and intra-sarcoplasmic reticulum (SR) Ca(2+) dynamics in intact and permeabilized ventricular myocytes using confocal microscopy. We also measured age-related changes in ROS production and mitochondria membrane potential using a ROS-sensitive dye and a mitochondrial voltage-sensitive fluorescent indicator, respectively. In permeablized myocytes, ageing did not change SERCA activity and spark frequency but decreased spark amplitude and SR Ca(2+) load suggesting increased RyR activity. Treatment with the antioxidant dithiothreitol reduced RyR-mediated SR Ca(2+) leak in permeabilized myocytes from old rabbit hearts to the level comparable to young. Moreover, myocytes from old rabbits had more depolarized mitochondria membrane potential and increased rate of ROS production. Under β-adrenergic stimulation, Ca(2+) transient amplitude, SR Ca(2+) load, and latency of pro-arrhythmic spontaneous Ca(2+) waves (SCWs) were decreased while RyR-mediated SR Ca(2+) leak was increased in cardiomyocytes from old rabbits. Additionally, with β-adrenergic stimulation, scavenging of mitochondrial ROS in myocytes from old rabbit hearts restored redox status of RyRs, which reduced SR Ca(2+) leak, ablated most SCWs, and increased latency to levels comparable to young. These data indicate that an age-associated increase of ROS production by mitochondria leads to the thiol-oxidation of RyRs, which underlies the hyperactivity of RyRs and thereby shortened refractoriness of Ca(2+) release in cardiomyocytes from the ageing heart. This mechanism probably plays an important role in the increased incidence of arrhythmia and sudden death in the ageing population.

Role of mitochondrial dysfunction and altered autophagy in cardiovascular aging and disease: from mechanisms to therapeutics

Advanced age is associated with a disproportionate prevalence of cardiovascular disease (CVD). Intrinsic alterations in the heart and the vasculature occurring over the life course render the cardiovascular system more vulnerable to various stressors in late life, ultimately favoring the development of CVD. Several lines of evidence indicate mitochondrial dysfunction as a major contributor to cardiovascular senescence. Besides being less bioenergetically efficient, damaged mitochondria also produce increased amounts of reactive oxygen species, with detrimental structural and functional consequences for the cardiovascular system. The age-related accumulation of dysfunctional mitochondrial likely results from the combination of impaired clearance of damaged organelles by autophagy and inadequate replenishment of the cellular mitochondrial pool by mitochondriogenesis. In this review, we summarize the current knowledge about relevant mechanisms and consequences of age-related mitochondrial decay and alterations in mitochondrial quality control in the cardiovascular system. The involvement of mitochondrial dysfunction in the pathogenesis of cardiovascular conditions especially prevalent in late life and the emerging connections with neurodegeneration are also illustrated. Special emphasis is placed on recent discoveries on the role played by alterations in mitochondrial dynamics (fusion and fission), mitophagy, and their interconnections in the context of age-related CVD and endothelial dysfunction. Finally, we discuss pharmacological interventions targeting mitochondrial dysfunction to delay cardiovascular aging and manage CVD.

Partitioning of superoxide and hydrogen peroxide production by mitochondrial respiratory complex I

Membrane-bound respiratory complex I in inside-out submitochondrial particles (SMP) catalyzes both superoxide and hydrogen peroxide formation in NADH- and/or succinate-supported reactions. At optimal NADH concentration (50μM), the complex I-mediated process results in a formation of two superoxide anions and H(2)O(2) as the reaction products in approximately 0.7 ratio. Almost the same ratio is found for purified complex I (0.6) and for the aerobic succinate-supported reverse electron transfer reaction. Superoxide production is depressed at high, more physiologically relevant NADH concentrations, whereas hydrogen peroxide formation is insensitive to the elevated level of NADH. The rates of H(2)O(2) formation at variable NAD(+)/NADH ratios satisfactorily fit the Nernst equation for a single reactive two-electron donor component equilibrated with ambient midpoint redox potential of -347mV (0.13 NAD(+)/NADH ratio, pH 8.0). Half-maximal superoxide production rate proceeds at significantly higher NAD(+)/NADH ratio (0.33). Guanidine strongly stimulates NADH-supported hydrogen peroxide and superoxide production at any NADH concentration and activates NADH:ferricyanide and inhibits NADH:hexaammineruthenium (III) reductase activities while showing no effects on NADH oxidase of SMP. In the low range of NADH concentration, superoxide production rate shows a simple hyperbolic dependence on NADH with apparent K(m)(NADH) of 0.5μM, whereas sigmoidal dependence of hydrogen peroxide production is seen with half-maximal rate at 25μM NADH. We interpret the data as to suggest that at least two sites participate in complex I-mediated ROS generation: FMNH(-) that produces hydrogen peroxide, and an iron-sulfur center (likely N-2) that produces superoxide anion.

Upregulated autophagy protects cardiomyocytes from oxidative stress-induced toxicity

Autophagy is a cellular self-digestion process that mediates protein quality control and serves to protect against neurodegenerative disorders, infections, inflammatory diseases and cancer. Current evidence suggests that autophagy can selectively remove damaged organelles such as the mitochondria. Mitochondria-induced oxidative stress has been shown to play a major role in a wide range of pathologies in several organs, including the heart. Few studies have investigated whether enhanced autophagy can offer protection against mitochondrially-generated oxidative stress. We induced mitochondrial stress in cardiomyocytes using antimycin A (AMA), which increased mitochondrial superoxide generation, decreased mitochondrial membrane potential and depressed cellular respiration. In addition, AMA augmented nuclear DNA oxidation and cell death in cardiomyocytes. Interestingly, although oxidative stress has been proposed to induce autophagy, treatment with AMA did not result in stimulation of autophagy or mitophagy in cardiomyocytes. Our results showed that the MTOR inhibitor rapamycin induced autophagy, promoted mitochondrial clearance and protected cardiomyocytes from the cytotoxic effects of AMA, as assessed by apoptotic marker activation and viability assays in both mouse atrial HL-1 cardiomyocytes and human ventricular AC16 cells. Importantly, rapamycin improved mitochondrial function, as determined by cellular respiration, mitochondrial membrane potential and morphology analysis. Furthermore, autophagy induction by rapamycin suppressed the accumulation of ubiquitinylated proteins induced by AMA. Inhibition of rapamycin-induced autophagy by pharmacological or genetic interventions attenuated the cytoprotective effects of rapamycin against AMA. We propose that rapamycin offers cytoprotection against oxidative stress by a combined approach of removing dysfunctional mitochondria as well as by degrading damaged, ubiquitinated proteins. We conclude that autophagy induction by rapamycin could be utilized as a potential therapeutic strategy against oxidative stress-mediated damage in cardiomyocytes.

Mitochondria and the aging heart

The average human life span has markedly increased in modern society largely attributed to advances in medical and therapeutic sciences that have successfully reduced important health risks. However, advanced age results in numerous alterations to cellular and subcellular components that can impact the overall health and function of an individual. Not surprisingly, advanced age is a major risk factor for the development of heart disease in which elderly populations observe increased morbidity and mortality. Even healthy individuals that appear to have normal heart function under resting conditions, actually have an increased susceptibility and vulnerability to stress. This is confounded by the impact that stress and disease can have over time to both the heart and vessels. Although, there is a rapidly growing body of literature investigating the effects of aging on the heart and how age-related alterations affect cardiac function, the biology of aging and underlying mechanisms remain unclear. In this review, we summarize effects of aging on the heart and discuss potential theories of cellular aging with special emphasis on mitochondrial dysfunction.

Mitochondrial aquaporin-8 knockdown in human hepatoma HepG2 cells causes ROS-induced mitochondrial depolarization and loss of viability

Human aquaporin-8 (AQP8) channels facilitate the diffusional transport of H(2)O(2) across membranes. Since AQP8 is expressed in hepatic inner mitochondrial membranes, we studied whether mitochondrial AQP8 (mtAQP8) knockdown in human hepatoma HepG2 cells impairs mitochondrial H(2)O(2) release, which may lead to organelle dysfunction and cell death. We confirmed AQP8 expression in HepG2 inner mitochondrial membranes and found that 72h after cell transfection with siRNAs targeting two different regions of the human AQP8 molecule, mtAQP8 protein specifically decreased by around 60% (p<0.05). Studies in isolated mtAQP8-knockdown mitochondria showed that H(2)O(2) release, assessed by Amplex Red, was reduced by about 45% (p<0.05), an effect not observed in digitonin-permeabilized mitochondria. mtAQP8-knockdown cells showed an increase in mitochondrial ROS, assessed by dichlorodihydrofluorescein diacetate (+120%, p<0.05) and loss of mitochondrial membrane potential (-80%, p<0.05), assessed by tetramethylrhodamine-coupled quantitative fluorescence microscopy. The mitochondria-targeted antioxidant MitoTempol prevented ROS accumulation and dissipation of mitochondrial membrane potential. Cyclosporin A, a mitochondrial permeability transition pore blocker, also abolished the mtAQP8 knockdown-induced mitochondrial depolarization. Besides, the loss of viability in mtAQP8 knockdown cells verified by MTT assay, LDH leakage, and trypan blue exclusion test could be prevented by cyclosporin A. Our data on human hepatoma HepG2 cells suggest that mtAQP8 facilitates mitochondrial H(2)O(2) release and that its defective expression causes ROS-induced mitochondrial depolarization via the mitochondrial permeability transition mechanism, and cell death.