What makes the mitochondria a killer? Can we condition them to be less destructive?

Organ-Specific Mitochondrial Alterations Following Ischemia-Reperfusion Injury in Post-Cardiac Arrest Syndrome: A Comprehensive Review

BACKGROUND

Mitochondrial dysfunction, which is triggered by systemic ischemia-reperfusion (IR) injury and affects various organs, is a key factor in the development of post-cardiac arrest syndrome (PCAS). Current research on PCAS primarily addresses generalized mitochondrial responses, resulting in a knowledge gap regarding organ-specific mitochondrial dynamics. This review focuses on the organ-specific mitochondrial responses to IR injury, particularly examining the brain, heart, and kidneys, to highlight potential therapeutic strategies targeting mitochondrial dysfunction to enhance outcomes post-IR injury.

METHODS AND RESULTS

We conducted a narrative review examining recent advancements in mitochondrial research related to IR injury. Mitochondrial responses to IR injury exhibit considerable variation across different organ systems, influenced by unique mitochondrial structures, bioenergetics, and antioxidative capacities. Each organ demonstrates distinct mitochondrial behaviors that have evolved to fulfill specific metabolic and functional needs. For example, cerebral mitochondria display dynamic responses that can be both protective and detrimental to neuronal activity and function during ischemic events. Cardiac mitochondria show vulnerability to IR-induced oxidative stress, while renal mitochondria exhibit a unique pattern of fission and fusion, closely linked to their susceptibility to acute kidney injury. This organ-specific heterogeneity in mitochondrial responses requires the development of tailored interventions. Progress in mitochondrial medicine, especially in the realms of genomics and metabolomics, is paving the way for innovative strategies to combat mitochondrial dysfunction. Emerging techniques such as mitochondrial transplantation hold the potential to revolutionize the management of IR injury in resuscitation science.

CONCLUSIONS

The investigation into organ-specific mitochondrial responses to IR injury is pivotal in the realm of resuscitation research, particularly within the context of PCAS. This nuanced understanding holds the promise of revolutionizing PCAS management, addressing the unique mitochondrial dysfunctions observed in critical organs affected by IR injury.

Mesenchymal stromal cells for improvement of cardiac function following acute myocardial infarction: a matter of timing

Acute myocardial infarction (AMI) is the leading cause of cardiovascular death and remains the most common cause of heart failure. Reopening of the occluded artery, i.e., reperfusion, is the only way to save the myocardium. However, the expected benefits of reducing infarct size are disappointing due to the reperfusion paradox, which also induces specific cell death. These ischemia-reperfusion (I/R) lesions can account for up to 50% of final infarct size, a major determinant for both mortality and the risk of heart failure (morbidity). In this review, we provide a detailed description of the cell death and inflammation mechanisms as features of I/R injury and cardioprotective strategies such as ischemic postconditioning as well as their underlying mechanisms. Due to their biological properties, the use of mesenchymal stromal/stem cells (MSCs) has been considered a potential therapeutic approach in AMI. Despite promising results and evidence of safety in preclinical studies using MSCs, the effects reported in clinical trials are not conclusive and even inconsistent. These discrepancies were attributed to many parameters such as donor age, in vitro culture, and storage time as well as injection time window after AMI, which alter MSC therapeutic properties. In the context of AMI, future directions will be to generate MSCs with enhanced properties to limit cell death in myocardial tissue and thereby reduce infarct size and improve the healing phase to increase postinfarct myocardial performance.

Increased mitochondrial free Ca2+ during ischemia is suppressed, but not eliminated by, germline deletion of the mitochondrial Ca2+ uniporter

Mitochondrial Ca2+ overload is proposed to regulate cell death via opening of the mitochondrial permeability transition pore. It is hypothesized that inhibition of the mitochondrial Ca2+ uniporter (MCU) will prevent Ca2+ accumulation during ischemia/reperfusion and thereby reduce cell death. To address this, we evaluate mitochondrial Ca2+ in ex-vivo-perfused hearts from germline MCU-knockout (KO) and wild-type (WT) mice using transmural spectroscopy. Matrix Ca2+ levels are measured with a genetically encoded, red fluorescent Ca2+ indicator (R-GECO1) using an adeno-associated viral vector (AAV9) for delivery. Due to the pH sensitivity of R-GECO1 and the known fall in pH during ischemia, hearts are glycogen depleted to decrease the ischemic fall in pH. At 20 min of ischemia, there is significantly less mitochondrial Ca2+ in MCU-KO hearts compared with MCU-WT controls. However, an increase in mitochondrial Ca2+ is present in MCU-KO hearts, suggesting that mitochondrial Ca2+ overload during ischemia is not solely dependent on MCU.

Ischemic Tolerance—A Way to Reduce the Extent of Ischemia–Reperfusion Damage

Individual tissues have significantly different resistance to ischemia–reperfusion damage. There is still no adequate treatment for the consequences of ischemia–reperfusion damage. By utilizing ischemic tolerance, it is possible to achieve a significant reduction in the extent of the cell damage due to ischemia–reperfusion injury. Since ischemia–reperfusion damage usually occurs unexpectedly, the use of preconditioning is extremely limited. In contrast, postconditioning has wider possibilities for use in practice. In both cases, the activation of ischemic tolerance can also be achieved by the application of sublethal stress on a remote organ. Despite very encouraging and successful results in animal experiments, the clinical results have been disappointing so far. To avoid the factors that prevent the activation of ischemic tolerance, the solution has been to use blood plasma containing tolerance effectors. This plasma is taken from healthy donors in which, after exposure to two sublethal stresses within 48 h, effectors of ischemic tolerance occur in the plasma. Application of this activated plasma to recipient animals after the end of lethal ischemia prevents cell death and significantly reduces the consequences of ischemia–reperfusion damage. Until there is a clear chemical identification of the end products of ischemic tolerance, the simplest way of enhancing ischemic tolerance will be the preparation of activated plasma from young healthy donors with the possibility of its immediate use in recipients during the initial treatment.

Modification of Ischemia/Reperfusion-Induced Alterations in Subcellular Organelles by Ischemic Preconditioning

It is now well established that ischemia/reperfusion (I/R) injury is associated with the compromised recovery of cardiac contractile function. Such an adverse effect of I/R injury in the heart is attributed to the development of oxidative stress and intracellular Ca²⁺-overload, which are known to induce remodeling of subcellular organelles such as sarcolemma, sarcoplasmic reticulum, mitochondria and myofibrils. However, repeated episodes of brief periods of ischemia followed by reperfusion or ischemic preconditioning (IP) have been shown to improve cardiac function and exert cardioprotective actions against the adverse effects of prolonged I/R injury. This protective action of IP in attenuating myocardial damage and subcellular remodeling is likely to be due to marked reductions in the occurrence of oxidative stress and intracellular Ca²⁺-overload in cardiomyocytes. In addition, the beneficial actions of IP have been attributed to the depression of proteolytic activities and inflammatory levels of cytokines as well as the activation of the nuclear factor erythroid factor 2-mediated signal transduction pathway. Accordingly, this review is intended to describe some of the changes in subcellular organelles, which are induced in cardiomyocytes by I/R for the occurrence of oxidative stress and intracellular Ca²⁺-overload and highlight some of the mechanisms for explaining the cardioprotective effects of IP.

Therapeutic Peptides to Treat Myocardial Ischemia-Reperfusion Injury

Cardiovascular diseases (CVD) including acute myocardial infarction (AMI) rank first in worldwide mortality and according to the World Health Organization (WHO), they will stay at this rank until 2030. Prompt revascularization of the occluded artery to reperfuse the myocardium is the only recommended treatment (by angioplasty or thrombolysis) to decrease infarct size (IS). However, despite beneficial effects on ischemic lesions, reperfusion leads to ischemia-reperfusion (IR) injury related mainly to apoptosis. Improvement of revascularization techniques and patient care has decreased myocardial infarction (MI) mortality however heart failure (HF) morbidity is increasing, contributing to the cost-intense worldwide HF epidemic. Currently, there is no treatment for reperfusion injury despite promising results in animal models. There is now an obvious need to develop new cardioprotective strategies to decrease morbidity/mortality of CVD, which is increasing due to the aging of the population and the rising prevalence rates of diabetes and obesity. In this review, we will summarize the different therapeutic peptides developed or used focused on the treatment of myocardial IR injury (MIRI). Therapeutic peptides will be presented depending on their interacting mechanisms (apoptosis, necroptosis, and inflammation) reported as playing an important role in reperfusion injury following myocardial ischemia. The search and development of therapeutic peptides have become very active, with increasing numbers of candidates entering clinical trials. Their optimization and their potential application in the treatment of patients with AMI will be discussed.

Regulation of Mitochondrial Ca2+ Uptake

Mitochondria are responsible for ATP production but are also known as regulators of cell death, and mitochondrial matrix Ca2+ is a key modulator of both ATP production and cell death. Although mitochondrial Ca2+ uptake and efflux have been studied for over 50 years, it is only in the past decade that the proteins responsible for mitochondrial Ca2+ uptake and efflux have been identified. The identification of the mitochondrial Ca2+ uniporter (MCU) led to an explosion of studies identifying regulators of the MCU. The levels of these regulators vary in a tissue- and disease-specific manner, providing new insight into how mitochondrial Ca2+ is regulated. This review focuses on the proteins responsible for mitochondrial transport and what we have learned from mouse studies with genetic alterations in these proteins.

Nuclear-mitochondrial communication involving miR-181c plays an important role in cardiac dysfunction during obesity

AIMS

In cardiomyocytes, there is microRNA (miR) in the mitochondria that originates from the nuclear genome and matures in the cytoplasm before translocating into the mitochondria. Overexpression of one such miR, miR-181c, can lead to heart failure by stimulating reactive oxygen species (ROS) production and increasing mitochondrial calcium level ([Ca²⁺]_m). Mitochondrial calcium uptake 1 protein (MICU1), a regulatory protein in the mitochondrial calcium uniporter complex, plays an important role in regulating [Ca²⁺]_m. Obesity results in miR-181c overexpression and a decrease in MICU1. We hypothesize that lowering miR-181c would protect against obesity-induced cardiac dysfunction.

METHODS AND RESULTS

We used an in vivo mouse model of high-fat diet (HFD) for 18 weeks and induced high lipid load in H9c2 cells with oleate-conjugated bovine serum albumin in vitro. We tested the cardioprotective role of lowering miR-181c by using miR-181c/d^-/- mice (in vivo) and AntagomiR against miR-181c (in vitro). HFD significantly upregulated heart levels of miR-181c and led to cardiac hypertrophy in wild-type mice, but not in miR-181c/d^-/- mice. HFD also increased ROS production and pyruvate dehydrogenase activity (a surrogate for [Ca²⁺]_m), but the increases were alleviated in miR-181c/d^-/- mice. Moreover, miR-181c/d^-/- mice fed a HFD had higher levels of MICU1 than did wild-type mice fed a HFD, attenuating the rise in [Ca²⁺]_m. Overexpression of miR-181c in neonatal ventricular cardiomyocytes (NMVM) caused increased ROS production, which oxidized transcription factor Sp1 and led to a loss of Sp1, thereby slowing MICU1 transcription. Hence, miR-181c increases [Ca²⁺]_m through Sp1 oxidation and downregulation of MICU1, suggesting that the cardioprotective effect of miR-181c/d^-/- results from inhibition of Sp1 oxidation.

CONCLUSION

This study has identified a unique nuclear-mitochondrial communication mechanism in the heart orchestrated by miR-181c. Obesity-induced overexpression of miR-181c increases [Ca²⁺]_m via downregulation of MICU1 and leads to cardiac injury. A strategy to inhibit miR-181c in cardiomyocytes can preserve cardiac function during obesity by improving mitochondrial function. Altering miR-181c expression may provide a pharmacologic approach to improve cardiomyopathy in individuals with obesity/type 2 diabetes.

Role of Mitochondrial Calcium and the Permeability Transition Pore in Regulating Cell Death

Adult cardiomyocytes are postmitotic cells that undergo very limited cell division. Thus, cardiomyocyte death as occurs during myocardial infarction has very detrimental consequences for the heart. Mitochondria have emerged as an important regulator of cardiovascular health and disease. Mitochondria are well established as bioenergetic hubs for generating ATP but have also been shown to regulate cell death pathways. Indeed many of the same signals used to regulate metabolism and ATP production, such as calcium and reactive oxygen species, are also key regulators of mitochondrial cell death pathways. It is widely hypothesized that an increase in calcium and reactive oxygen species activate a large conductance channel in the inner mitochondrial membrane known as the PTP (permeability transition pore) and that opening of this pore leads to necroptosis, a regulated form of necrotic cell death. Strategies to reduce PTP opening either by inhibition of PTP or inhibiting the rise in mitochondrial calcium or reactive oxygen species that activate PTP have been proposed. A major limitation of inhibiting the PTP is the lack of knowledge about the identity of the protein(s) that form the PTP and how they are activated by calcium and reactive oxygen species. This review will critically evaluate the candidates for the pore-forming unit of the PTP and discuss recent data suggesting that assumption that the PTP is formed by a single molecular identity may need to be reconsidered.

Ischemia Reperfusion Injury: Mechanisms of Damage/Protection and Novel Strategies for Cardiac Recovery/Regeneration

Ischemic diseases in an aging population pose a heavy social encumbrance. Moreover, current therapeutic approaches, which aimed to prevent or minimize ischemia-induced damage, are associated with relevant costs for healthcare systems. Early reperfusion by primary percutaneous coronary intervention (PPCI) has undoubtedly improved patient's outcomes; however, the prevention of long-term complications is still an unmet need. To face these hurdles and improve patient's outcomes, novel pharmacological and interventional approaches, alone or in combination, reducing myocardium oxygen consumption or supplying blood flow via collateral vessels have been proposed. A number of clinical trials are ongoing to validate their efficacy on patient's outcomes. Alternative options, including stem cell-based therapies, have been evaluated to improve cardiac regeneration and prevent scar formation. However, due to the lack of long-term engraftment, more recently, great attention has been devoted to their paracrine mediators, including exosomes (Exo) and microvesicles (MV). Indeed, Exo and MV are both currently considered to be one of the most promising therapeutic strategies in regenerative medicine. As a matter of fact, MV and Exo that are released from stem cells of different origin have been evaluated for their healing properties in ischemia reperfusion (I/R) settings. Therefore, this review will first summarize mechanisms of cardiac damage and protection after I/R damage to track the paths through which more appropriate interventional and/or molecular-based targeted therapies should be addressed. Moreover, it will provide insights on novel non-invasive/invasive interventional strategies and on Exo-based therapies as a challenge for improving patient's long-term complications. Finally, approaches for improving Exo healing properties, and topics still unsolved to move towards Exo clinical application will be discussed.

Fmr1 protects cardiomyocytes against lipopolysaccharide-induced myocardial injury

The present study explored the mechanisms by which fragile X mental retardation 1 (fmr1) overexpression inhibits lipopolysaccharide (LPS)-induced cardiomyocyte injury. Factors including oxidative stress reaction, mitochondrial membrane potential variation and cell apoptosis were evaluated. The viability of H9c2 cells was evaluated with a Cell Counting Kit-8 assay after cells were treated with LPS at different concentrations (0, 1, 3, 6 and 9 µg/ml) for various durations (4, 12 and 24 h). Flow cytometry was used to determine variations in reactive oxygen species (ROS), mitochondrial membrane potential and cell apoptosis. Reverse transcription-quantitative polymerase chain reaction and western blot analysis were performed to detect the levels of apoptosis-associated factors, and western blot analysis was used to determine the phosphorylation levels of phosphoinositide-3 kinase (PI3K), Akt and forkhead box (Fox)O3a. The results indicated that LPS decreased the viability of H9c2 cells in a dose- and time-dependent manner. Overexpression of fmr1 inhibited the LPS-induced decrease in the mitochondrial membrane potential and the production of ROS as well as apoptosis in H9c2 cells. Fmr1 also inhibited LPS-induced reductions in antioxidant enzyme activities, including those of superoxide dismutase and reduced/oxidized glutathione ratio, and decreased LPS-associated increases in the lipid peroxidation product malondialdehyde. Apoptosis-associated factors were identified to be involved in the effects of Fmr1. Overexpression of Fmr1 attenuated LPS-associated increases in the apoptosis-activating factors B-cell lymphoma 2 (Bcl-2)-associated X protein and caspase-3 and decreases in apoptosis inhibitors, including Bcl-2 and X-linked inhibitor of apoptosis protein. Fmr1 overexpression also reduced LPS-induced increases in the phosphorylation levels of PI3K, Akt and FoxO3a. In conclusion, fmr1 overexpression alleviated oxidative stress and apoptosis in H9c2 cardiomyocytes injured by LPS via regulating oxidative stress and apoptosis-associated factors, as well as the PI3K/Akt pathway. This information may provide a novel and effective therapeutic strategy for heart diseases.

Mitochondria in Cardiac Postconditioning

Mitochondria play a pivotal role in cardioprotection. Here we report some fundamental studies which considered the role of mitochondrial components (connexin 43, mitochondrial KATP channels and mitochondrial permeability transition pore) in postconditioning cardioprotection. We briefly discuss the role of mitochondria, reactive oxygen species and gaseous molecules in postconditioning. Also the effects of anesthetics-used as cardioprotective substances-is briefly considered in the context of postconditioning. The role of mitochondrial postconditioning signaling in determining the limitation of cell death is underpinned. Issues in clinical translation are briefly considered. The aim of the present mini-review is to discuss in a historical perspective the role of main mitochondria mechanisms in cardiac postconditioning.

Does p53 Inhibition Suppress Myocardial Ischemia-Reperfusion Injury?

p53 is well known as a regulator of apoptosis and autophagy. In addition, a recent study showed that p53 is a modulator of the opening of the mitochondrial permeability transition pore (mPTP), a trigger event of necrosis, but the role of p53 in necrosis induced by myocardial ischemia-reperfusion (I/R) remains unclear. The aim of this study was to determine the role of p53 in acute myocardial I/R injury in perfused mouse hearts. In male C57BL6 mice between 12 and 15 weeks of age, 2 types of p53 inhibitors were used to suppress p53 function during I/R: pifithrin-α, an inhibitor of transcriptional functions of p53, and pifithrin-μ, an inhibitor of p53 translocation from the cytosol to mitochondria. Neither infusion of these inhibitors before ischemia nor infusion for the first 30-minute period of reperfusion reduced infarct size after 20-minute ischemia/120-minute reperfusion. Infarct sizes were similar in p53 heterozygous knockout mice (p53^+/-) and wild-type mice (WT), but recovery of rate pressure product (RRP) 120 minutes after reperfusion was higher in p53^+/- than in WT. The protein expression of p53 in WT was negligible under baseline conditions, during ischemia, and at 10 minutes after the start of reperfusion, but it became detectable at 120 minutes after reperfusion. In conclusion, upregulation of p53 during the late phase of reperfusion plays a significant role in contractile dysfunction after reperfusion, although p53 is not involved in cardiomyocyte necrosis during ischemia or in the early phase of reperfusion.

Mitochondrial Bioenergetics Is Altered in Fibroblasts from Patients with Sporadic Alzheimer's Disease

The identification of an early biomarker to diagnose Alzheimer's disease (AD) remains a challenge. Neuropathological studies in animal and AD patients have shown that mitochondrial dysfunction is a hallmark of the development of the disease. Current studies suggest the use of peripheral tissues, like skin fibroblasts as a possibility to detect the early pathological alterations present in the AD brain. In this context, we studied mitochondrial function properties (bioenergetics and morphology) in cultured fibroblasts obtained from AD, aged-match and young healthy patients. We observed that AD fibroblasts presented a significant reduction in mitochondrial length with important changes in the expression of proteins that control mitochondrial fusion. Moreover, AD fibroblasts showed a distinct alteration in proteolytic processing of OPA1, a master regulator of mitochondrial fusion, compared to control fibroblasts. Complementary to these changes AD fibroblasts showed a dysfunctional mitochondrial bioenergetics profile that differentiates these cells from aged-matched and young patient fibroblasts. Our findings suggest that the human skin fibroblasts obtained from AD patients could replicate mitochondrial impairment observed in the AD brain. These promising observations suggest that the analysis of mitochondrial bioenergetics could represent a promising strategy to develop new diagnostic methods in peripheral tissues of AD patients.

Using chemical biology to assess and modulate mitochondria: progress and challenges

Our understanding of the role of mitochondria in biomedical sciences has expanded considerably over the past decade. In addition to their well-known metabolic roles, mitochondrial are also central to signalling for various processes through the generation of signals such as ROS and metabolites that affect cellular homeostasis, as well as other processes such as cell death and inflammation. Thus, mitochondrial function and dysfunction are central to the health and fate of the cell. Consequently, there is considerable interest in better understanding and assessing the many roles of mitochondria. Furthermore, there is also a growing realization that mitochondrial are a promising drug target in a wide range of pathologies. The application of interdisciplinary approaches at the interface between chemistry and biology are opening up new opportunities to understand mitochondrial function and in assessing the role of the organelle in biology. This work and the experience thus gained are leading to the development of new classes of therapies. Here, we overview the progress that has been made to date on exploring the chemical biology of the organelle and then focus on future challenges and opportunities that face this rapidly developing field.

Ischemia/Reperfusion Injury following Acute Myocardial Infarction: A Critical Issue for Clinicians and Forensic Pathologists

Acute myocardial infarction (AMI) is a leading cause of morbidity and mortality. Reperfusion strategies are the current standard therapy for AMI. However, they may result in paradoxical cardiomyocyte dysfunction, known as ischemic reperfusion injury (IRI). Different forms of IRI are recognized, of which only the first two are reversible: reperfusion-induced arrhythmias, myocardial stunning, microvascular obstruction, and lethal myocardial reperfusion injury. Sudden death is the most common pattern for ischemia-induced lethal ventricular arrhythmias during AMI. The exact mechanisms of IRI are not fully known. Molecular, cellular, and tissue alterations such as cell death, inflammation, neurohumoral activation, and oxidative stress are considered to be of paramount importance in IRI. However, comprehension of the exact pathophysiological mechanisms remains a challenge for clinicians. Furthermore, myocardial IRI is a critical issue also for forensic pathologists since sudden death may occur despite timely reperfusion following AMI, that is one of the most frequently litigated areas of cardiology practice. In this paper we explore the literature regarding the pathophysiology of myocardial IRI, focusing on the possible role of the calpain system, oxidative-nitrosative stress, and matrix metalloproteinases and aiming to foster knowledge of IRI pathophysiology also in terms of medicolegal understanding of sudden deaths following AMI.

[Skeletal muscle ischemia-reperfusion and ischemic conditioning pathophysiology-clinical applications for the vascular surgeon]

Ischemia-reperfusion, which is characterized by deficient oxygen supply and subsequent restoration of blood flow, can cause irreversible damage to tissue. The vascular surgeon is daily faced with ischemia-reperfusion situations. Indeed, arterial clamping induces ischemia, followed by reperfusion when declamping. Mechanisms underlying ischemia-reperfusion injury are complex and multifactorial. Increases in cellular calcium and reactive oxygen species, initiated during ischemia and then amplified upon reperfusion are thought to be the main mediators of reperfusion injury. Mitochondrial dysfunction also plays an important role. Extensive research has focused on increasing skeletal muscle tolerance to ischemia-reperfusion injury, especially through the use of ischemic conditioning strategies. The purpose of this review is to focus on the cellular responses associated with ischemia-reperfusion, as well as to discuss the effects of ischemic conditioning strategies. This would help the vascular surgeon in daily practice, in order to try to improve surgical outcome in the setting of ischemia-reperfusion.

Mitochondrial Dysfunction and Myocardial Ischemia-Reperfusion: Implications for Novel Therapies

Mitochondria have emerged as key participants in and regulators of myocardial injury during ischemia and reperfusion. This review examines the sites of damage to cardiac mitochondria during ischemia and focuses on the impact of these defects. The concept that mitochondrial damage during ischemia leads to cardiac injury during reperfusion is addressed. The mechanisms that translate ischemic mitochondrial injury into cellular damage, during both ischemia and early reperfusion, are examined. Next, we discuss strategies that modulate and counteract these mechanisms of mitochondrial-driven injury. The new concept that mitochondria are not merely stochastic sites of oxidative and calcium-mediated injury but that they activate cellular responses of mitochondrial remodeling and cellular reactions that modulate the balance between cell death and recovery is reviewed, and the therapeutic implications of this concept are discussed.

Understanding and preventing mitochondrial oxidative damage

Mitochondrial oxidative damage has long been known to contribute to damage in conditions such as ischaemia-reperfusion (IR) injury in heart attack. Over the past years, we have developed a series of mitochondria-targeted compounds designed to ameliorate or determine how this damage occurs. I will outline some of this work, from MitoQ to the mitochondria-targeted S-nitrosating agent, called MitoSNO, that we showed was effective in preventing reactive oxygen species (ROS) formation in IR injury with therapeutic implications. In addition, the protection by this compound suggested that ROS production in IR injury was mainly coming from complex I. This led us to investigate the mechanism of the ROS production and using a metabolomic approach, we found that the ROS production in IR injury came from the accumulation of succinate during ischaemia that then drove mitochondrial ROS production by reverse electron transport at complex I during reperfusion. This surprising mechanism led us to develop further new therapeutic approaches to have an impact on the damage that mitochondrial ROS do in pathology and also to explore how mitochondrial ROS can act as redox signals. I will discuss how these approaches have led to a better understanding of mitochondrial oxidative damage in pathology and also to the development of new therapeutic strategies.

MicroRNA-181c targets Bcl-2 and regulates mitochondrial morphology in myocardial cells

Apoptosis is an important mechanism for the development of heart failure. Mitochondria are central to the execution of apoptosis in the intrinsic pathway. The main regulator of mitochondrial pathway of apoptosis is Bcl-2 family which includes pro- and anti-apoptotic proteins. MicroRNAs are small noncoding RNA molecules that regulate gene expression by inhibiting mRNA translation and/or inducing mRNA degradation. It has been proposed that microRNAs play critical roles in the cardiovascular physiology and pathogenesis of cardiovascular diseases. Our previous study has found that microRNA-181c, a miRNA expressed in the myocardial cells, plays an important role in the development of heart failure. With bioinformatics analysis, we predicted that miR-181c could target the 3' untranslated region of Bcl-2, one of the anti-apoptotic members of the Bcl-2 family. Thus, we have suggested that miR-181c was involved in regulation of Bcl-2. In this study, we investigated this hypothesis using the Dual-Luciferase Reporter Assay System. Cultured myocardial cells were transfected with the mimic or inhibitor of miR-181c. We found that the level of miR-181c was inversely correlated with the Bcl-2 protein level and that transfection of myocardial cells with the mimic or inhibitor of miR-181c resulted in significant changes in the levels of caspases, Bcl-2 and cytochrome C in these cells. The increased level of Bcl-2 caused by the decrease in miR-181c protected mitochondrial morphology from the tumour necrosis factor alpha-induced apoptosis.

Mitochondrial targets for pharmacological intervention in human disease

Over the past several years, mitochondrial dysfunction has been linked to an increasing number of human illnesses, making mitochondrial proteins (MPs) an ever more appealing target for therapeutic intervention. With 20% of the mitochondrial proteome (312 of an estimated 1500 MPs) having known interactions with small molecules, MPs appear to be highly targetable. Yet, despite these targeted proteins functioning in a range of biological processes (including induction of apoptosis, calcium homeostasis, and metabolism), very few of the compounds targeting MPs find clinical use. Recent work has greatly expanded the number of proteins known to localize to the mitochondria and has generated a considerable increase in MP 3D structures available in public databases, allowing experimental screening and in silico prediction of mitochondrial drug targets on an unprecedented scale. Here, we summarize the current literature on clinically active drugs that target MPs, with a focus on how existing drug targets are distributed across biochemical pathways and organelle substructures. Also, we examine current strategies for mitochondrial drug discovery, focusing on genetic, proteomic, and chemogenomic assays, and relevant model systems. As cell models and screening techniques improve, MPs appear poised to emerge as relevant targets for a wide range of complex human diseases, an eventuality that can be expedited through systematic analysis of MP function.

Ischemic conditioning: the challenge of protecting the diabetic heart

The successful clinical translation of novel therapeutic strategies to attenuate lethal myocardial ischemia-reperfusion injury and limit infarct size has been identified as a major unmet need, and is of particular importance in patients with type-2 diabetes. There is a wealth of preclinical evidence that ischemic conditioning (encompassing the three paradigms of preconditioning, postconditioning and remote conditioning) is profoundly cardioprotective and, via up-regulation of endogenous signaling cascades, renders the heart resistant to infarction. However, current phase II trials aimed at exploiting ischemic conditioning for the clinical treatment of myocardial ischemia-reperfusion injury have yielded mixed results, possibly reflecting the emerging concern that the efficacy of conditioning-induced cardioprotection may be compromised in the diabetic heart. Our goal in this review is to provide a summary of our present understanding of the effect of type-2 diabetes on the infarct-sparing effect of ischemic conditioning, and the challenges of limiting ischemia-reperfusion injury in the diabetic heart.

Mitochondrial calcium and the regulation of metabolism in the heart

Consumption of adenosine triphosphate (ATP) by the heart can change dramatically as the energetic demands increase from a period of rest to strenuous activity. Mitochondrial ATP production is central to this metabolic response since the heart relies largely on oxidative phosphorylation as its source of intracellular ATP. Significant evidence has been acquired indicating that Ca(2+) plays a critical role in regulating ATP production by the mitochondria. Here the evidence that the Ca(2+) concentration in the mitochondrial matrix ([Ca(2+)]m) plays a pivotal role in regulating ATP production by the mitochondria is critically reviewed and aspects of this process that are under current active investigation are highlighted. Importantly, current quantitative information on the bidirectional Ca(2+) movement across the inner mitochondrial membrane (IMM) is examined in two parts. First, we review how Ca(2+) influx into the mitochondrial matrix depends on the mitochondrial Ca(2+) channel (i.e., the mitochondrial calcium uniporter or MCU). This discussion includes how the MCU open probability (PO) depends on the cytosolic Ca(2+) concentration ([Ca(2+)]i) and on the mitochondrial membrane potential (ΔΨm). Second, we discuss how steady-state [Ca(2+)]m is determined by the dynamic balance between this MCU-based Ca(2+) influx and mitochondrial Na(+)/Ca(2+) exchanger (NCLX) based Ca(2+) efflux. These steady-state [Ca(2+)]m levels are suggested to regulate the metabolic energy supply due to Ca(2+)-dependent regulation of mitochondrial enzymes of the tricarboxylic acid cycle (TCA), the proteins of the electron transport chain (ETC), and the F1F0 ATP synthase itself. We conclude by discussing the roles played by [Ca(2+)]m in influencing mitochondrial responses under pathological conditions. This article is part of a Special Issue entitled "Mitochondria: From BasicMitochondrial Biology to Cardiovascular Disease."

Cardioprotection of vitexin on myocardial ischemia/reperfusion injury in rat via regulating inflammatory cytokines and MAPK pathway

This study was conducted to demonstrate myocardial protective effects and possible underlying mechanisms of vitexin on myocardial ischemia/reperfusion (I/R) injury in rats. Occluding the anterior descending artery for 30 min and restoring blood perfusion for 60 min in rat established a model of myocardial I/R. The elevation of the ST segment of Electrocardiograph (ECG) was observed. The infarct size of the rat heart was assessed by triphenyltetrazolium chloride staining (TTC). LDH, CK, SOD activities and MDA content were determined. An immunohistochemical analysis was applied to measure the expression of myocardial NF-κBp65 and TNF-α. ERK/phospho-ERKand c-Jun/phospho-c-Jun protein expression was examined via Western Blot. Vitexin significantly reduced the elevation of the ST segment of ECG and myocardial infarct size. LDH and CK activities and MDA content were attenuated in serum, while SOD activity was markedly enhanced. Vitexin significantly attenuated I/R-induced increases of myocardial NF-κB and TNF-α. Moreover, Western Blot analysis presented that vitexin markedly enhanced the expression of phospho-ERK and weakened the expression of phospho-c-Jun compared to I/R group. The significant protective effect against myocardial ischemical/reperfusion injury in rat, which is exhibited by vitexin, may be related to its antioxidative and anti-inflammatory effects by regulating inflammatory cytokines and the MAPK pathway.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Beating oxygen: chronic anoxia exposure reduces mitochondrial F1FO-ATPase activity in turtle (Trachemys scripta) heart

The freshwater turtle Trachemys scripta can survive in the complete absence of O2 (anoxia) for periods lasting several months. In mammals, anoxia leads to mitochondrial dysfunction, which culminates in cellular necrosis and apoptosis. Despite the obvious clinical benefits of understanding anoxia tolerance, little is known about the effects of chronic oxygen deprivation on the function of turtle mitochondria. In this study, we compared mitochondrial function in hearts of T. scripta exposed to either normoxia or 2 weeks of complete anoxia at 5°C and during simulated acute anoxia/reoxygenation. Mitochondrial respiration, electron transport chain activities, enzyme activities, proton conductance and membrane potential were measured in permeabilised cardiac fibres and isolated mitochondria. Two weeks of anoxia exposure at 5°C resulted in an increase in lactate, and decreases in ATP, glycogen, pH and phosphocreatine in the heart. Mitochondrial proton conductance and membrane potential were similar between experimental groups, while aerobic capacity was dramatically reduced. The reduced aerobic capacity was the result of a severe downregulation of the F1FO-ATPase (Complex V), which we assessed as a decrease in enzyme activity. Furthermore, in stark contrast to mammalian paradigms, isolated turtle heart mitochondria endured 20 min of anoxia followed by reoxygenation without any impact on subsequent ADP-stimulated O2 consumption (State III respiration) or State IV respiration. Results from this study demonstrate that turtle mitochondria remodel in response to chronic anoxia exposure and a reduction in Complex V activity is a fundamental component of mitochondrial and cellular anoxia survival.

Conundrum of pathogenesis of diabetic cardiomyopathy: role of vascular endothelial dysfunction, reactive oxygen species, and mitochondria

Diabetic cardiomyopathy and heart failure have been recognized as the leading causes of mortality among diabetics. Diabetic cardiomyopathy has been characterized primarily by the manifestation of left ventricular dysfunction that is independent of coronary artery disease and hypertension among the patients affected by diabetes mellitus. A complex array of contributing factors including the hypertrophy of left ventricle, alterations of metabolism, microvascular pathology, insulin resistance, fibrosis, apoptotic cell death, and oxidative stress have been implicated in the pathogenesis of diabetic cardiomyopathy. Nevertheless, the exact mechanisms underlying the pathogenesis of diabetic cardiomyopathy are yet to be established. The critical involvement of multifarious factors including the vascular endothelial dysfunction, microangiopathy, reactive oxygen species (ROS), oxidative stress, mitochondrial dysfunction has been identified in the mechanism of pathogenesis of diabetic cardiomyopathy. Although it is difficult to establish how each factor contributes to disease, the involvement of ROS and mitochondrial dysfunction are emerging as front-runners in the mechanism of pathogenesis of diabetic cardiomyopathy. This review highlights the role of vascular endothelial dysfunction, ROS, oxidative stress, and mitochondriopathy in the pathogenesis of diabetic cardiomyopathy. Furthermore, the review emphasizes that the puzzle has to be solved to firmly establish the mitochondrial and/or ROS mechanism(s) by identifying their most critical molecular players involved at both spatial and temporal levels in diabetic cardiomyopathy as targets for specific and effective pharmacological/therapeutic interventions.

Treatment of Rats With Hypolipidemic Compound Pirinixic Acid Protects Their Hearts Against Ischemic Injury: Are Mitochondrial KATP Channels and Reactive Oxygen Species Involved?

Hypolipidemic compound pirinixic acid (WY-14643, WY) is known to exert pleiotropic (other than primary) effects, such as activation of peroxisome proliferator-activated receptors (PPAR-α), transcription factors regulating different cardiac functions. Their role in ischemia-reperfusion (I/R) injury and cardioprotection is less clear, although protective effects of PPAR agonists have been documented. This study was designed to explore the effects of WY on the I/R injury in the rat heart and potential mechanisms involved, including mitochondrial KATP channels (mitoKATP) opening and production of reactive oxygen species (ROS). Langendorff-perfused hearts of rats intragastrally treated with WY (3 mg/kg/day) for 5 days and of control animals were subjected to 30-min global ischemia and 2-h reperfusion with or without 15-min perfusion with mitoKATP blocker 5-hydroxydecanoate (5-HD) prior to I/R. Evaluation of the infarct size (IS, TTC staining) served as the main end-point of protection. Lipid peroxidation (a marker of ROS production) was determined by measurement of myocardial concentration of conjugated dienes (CD), whereas protein expression of endothelial NO synthase was analysed by Western blotting. A 2-fold increase in the cardiac protein levels of eNOS after treatment with WY was accompanied by lower post-I/R levels of CD compared with those in the hearts of untreated controls, although WY itself enhanced ROS generation prior to ischemia. IS was reduced by 47 % in the hearts of WY-treated rats (P<0.05), and this effect was reversed by 5-HD. Results suggest that PPAR-α activation may confer protection against lethal I/R injury in the rat heart that involves up-regulation of eNOS, mitoKATP opening and reduced oxidative stress during I/R.

Cardioprotective Role of Caveolae in Ischemia-Reperfusion Injury

Caveolae are flask-like invaginations of the plasma membrane enriched in cholesterol, sphingolipids, the marker protein caveolin and the coat protein cavin. In cardiomyocytes, multiple signaling molecules are concentrated and organized within the caveolae to mediate signaling transduction. Recent studies suggest that caveolae and caveolae-associated signaling molecules play an important role in protecting the myocardium against ischemia-reperfusion injury. For example, cardiac-specific overexpression of caveolin-3 has been shown to lead to protection that mimics ischemic preconditioning, while the knockout of caveolin-3 abolished ischemic preconditioning. In this review, we discuss the molecular mechanisms and signaling pathways that are involved in caveolae-mediated cardioprotection, and examine the potential for caveolae as a therapeutic target for pharmaceutical intervention to treat cardiovascular disease.

Hypothermia-induced neuroprotection is associated with reduced mitochondrial membrane permeability in a swine model of cardiac arrest

Increasing evidence has shown that mild hypothermia is neuroprotective for comatose patients resuscitated from cardiac arrest, but the mechanism of this protection is not fully understood. The aim of this study was to determine whether prolonged whole-body mild hypothermia inhibits mitochondrial membrane permeability (MMP) in the cerebral cortex after return of spontaneous circulation (ROSC). Thirty-seven inbred Chinese Wuzhishan minipigs were successfully resuscitated after 8 minutes of untreated ventricular fibrillation (VF) and underwent recovery under normothermic (NT) or prolonged whole-body mild hypothermic (HT; 33°C) conditions for 24 or 72 hours. Cerebral samples from the frontal cortex were collected at 24 and 72 hours after ROSC. Mitochondria were isolated by differential centrifugation. At 24 hours, relative to NT, HT was associated with reductions in opening of the mitochondrial permeability transition pore, release of pro-apoptotic substances from mitochondria, caspase 3 cleavage, apoptosis, and neurologic deficit scores, as well as increases in mitochondrial membrane potential and mitochondrial respiration. Together, these findings suggest that mild hypothermia inhibits ischemia-induced increases in MMP, which may provide neuroprotection against cerebral injury after cardiac arrest.

Reduced expression of mitochondrial electron transport chain proteins from hibernating hearts relative to ischemic preconditioned hearts in the second window of protection

Although protection against necrosis has been observed in both hibernating (HIB) and ischemic preconditioned hearts in the second window of protection (SWOP), a comparison of the mitochondrial proteome between the two entities has not been previously performed. Anesthetized swine underwent instrumentation with a fixed constrictor around the LAD artery and were followed for 12 weeks (HIB; N=7). A second group of anesthetized swine underwent ischemic preconditioning by inflating a balloon within the LAD artery 10 times for 2 min, each separated by 2 min reperfusion and were sacrificed 24h later (SWOP; N=7). Myocardial blood flow and high-energy nucleotides were obtained in the LAD region and normalized to remote regions. Post-sacrifice, protein content as measured with iTRAQ was compared in isolated mitochondria from the LAD area of a Sham heart. Basal regional blood flow in the LAD region when normalized to the remote region was 0.86±0.04 in HIB and 1.02±0.02 in SWOP tissue (P<0.05). Despite reduced regional blood flows in HIB hearts, ATP content in the LAD region, when normalized to the remote region was similar in HIB versus SWOP (1.06±0.06 and 1.02±0.05 respectively; NS) as was the transmural phosphocreatine (PCr) to ATP ratio (2.1±0.2 and 2.2±0.2 respectively; NS). Using iTRAQ, 64 common proteins were identified in HIB and SWOP hearts. Compared with SWOP, the relative abundance of mitochondrial proteins involved with electron transport chain (ETC) were reduced in HIB including NADH dehydrogenase, Cytochrome c reductase and oxidase, ATP synthase, and nicotinamide nucleotide transhydrogenase. Within chronically HIB heart tissue with reduced blood flow, the relative abundance of mitochondrial ETC proteins is decreased when compared with SWOP tissue. These data support the concept that HIB heart tissue subjected to chronically reduced blood flow is associated with a down-regulation in the expression of key mitochondrial proteins involved in electron transport.

Cytotoxicity of gold nanoparticles

Nanomaterials are now routinely used in technical as well as medical applications. The very physicochemical properties that favor nanomaterial application are the prime cause that these materials cannot be considered "generally safe." We are still far from predicting the toxicological profile of new nanoparticles, despite continuous attempts to establish a structure-function relation between the physical and chemical properties of nanoparticles and their interactions with biological systems. Herein, we summarize some basic concept to assess nanoparticle toxicity, death pathways, cell cycle, and oxidative stress in response to nanoparticle exposure of cells.

Nuclear miRNA regulates the mitochondrial genome in the heart

RATIONALE

Mitochondria are semiautonomous cellular organelles with their own genome, which not only supply energy but also participate in cell death pathways. MicroRNAs (miRNAs) are usually 19 to 25 nt long, noncoding RNAs, involved in posttranscriptional gene regulation by binding to the 3'-untranslated regions of target mRNA, which impact on diverse cellular processes.

OBJECTIVE

To determine if nuclear miRNAs translocate into the mitochondria and regulate mitochondrial function with possible pathophysiological implications in cardiac myocytes.

METHODS AND RESULTS

We find that miR-181c is encoded in the nucleus, assembled in the cytoplasm, and finally translocated into the mitochondria of cardiac myocytes. Immunoprecipitation of Argonaute 2 from the mitochondrial fraction indicates binding of cytochrome c oxidase subunit 1 (mt-COX1) mRNA from the mitochondrial genome with miR-181c. Also, a luciferase reporter construct shows that mi-181c binds to the 3'UTR of mt-COX1. To study whether miR-181c regulates mt-COX1, we overexpressed precursor miR-181c (or a scrambled sequence) in primary cultures of neonatal rat ventricular myocytes. Overexpression of miR-181c did not change mt-COX1 mRNA but significantly decreased mt-COX1 protein, suggesting that miR-181c is primarily a translational regulator of mt-COX1. In addition to altering mt-COX1, overexpression of miR-181c results in increased mt-COX2 mRNA and protein content, with an increase in both mitochondrial respiration and reactive oxygen species generation in neonatal rat ventricular myocytes. Thus, our data show for the first time that miR-181c can enter and target the mitochondrial genome, ultimately causing electron transport chain complex IV remodeling and mitochondrial dysfunction.

CONCLUSIONS

Nuclear miR-181c translocates into the mitochondria and regulates mitochondrial genome expression. This unique observation may open a new dimension to our understanding of mitochondrial dynamics and the role of miRNA in mitochondrial dysfunction.

Mitochondrial pharmacology

Mitochondria are being recognized as key factors in many unexpected areas of biomedical science. In addition to their well-known roles in oxidative phosphorylation and metabolism, it is now clear that mitochondria are also central to cell death, neoplasia, cell differentiation, the innate immune system, oxygen and hypoxia sensing, and calcium metabolism. Disruption to these processes contributes to a range of human pathologies, making mitochondria a potentially important, but currently seemingly neglected, therapeutic target. Mitochondrial dysfunction is often associated with oxidative damage, calcium dyshomeostasis, defective ATP synthesis, or induction of the permeability transition pore. Consequently, therapies designed to prevent these types of damage are beneficial and can be used to treat many diverse and apparently unrelated indications. Here we outline the biological properties that make mitochondria important determinants of health and disease, and describe the pharmacological strategies being developed to address mitochondrial dysfunction.

Role of Mitogen-Activated Protein Kinases in Myocardial Ischemia-Reperfusion Injury during Heart Transplantation

In solid organ transplantation, ischemia/reperfusion (IR) injury during organ procurement, storage and reperfusion is an unavoidable detrimental event for the graft, as it amplifies graft inflammation and rejection. Intracellular mitogen-activated protein kinase (MAPK) signaling pathways regulate inflammation and cell survival during IR injury. The four best-characterized MAPK subfamilies are the c-Jun NH2-terminal kinase (JNK), extracellular signal- regulated kinase-1/2 (ERK1/2), p38 MAPK, and big MAPK-1 (BMK1/ERK5). Here, we review the role of MAPK activation during myocardial IR injury as it occurs during heart transplantation. Most of our current knowledge regarding MAPK activation and cardioprotection comes from studies of preconditioning and postconditioning in nontransplanted hearts. JNK and p38 MAPK activation contributes to myocardial IR injury after prolonged hypothermic storage. p38 MAPK inhibition improves cardiac function after cold storage, rewarming and reperfusion. Small-molecule p38 MAPK inhibitors have been tested clinically in patients with chronic inflammatory diseases, but not in transplanted patients, so far. Organ transplantation offers the opportunity of starting a preconditioning treatment before organ procurement or during cold storage, thus modulating early events in IR injury. Future studies will need to evaluate combined strategies including p38 MAPK and/or JNK inhibition, ERK1/2 activation, pre- or postconditioning protocols, new storage solutions, and gentle reperfusion.

Altered expression of mitochondrial electron transport chain proteins and improved myocardial energetic state during late ischemic preconditioning

Altered expression of mitochondrial electron transport proteins has been shown in early preconditioned myocardial tissue. We wished to determine whether these alterations persist in the Second Window of Protection (SWOP) and if so, whether a favorable energetic state is facilitated during subsequent ischemia. Fourteen pigs underwent a SWOP protocol with ten 2-minute balloon inflations in the LAD artery, each separated by 2 minutes reperfusion. Twenty-four hours later, mitochondria were isolated from SWOP and SHAM pig hearts and analyzed for uncoupling protein (UCP)-2 content by western blot analysis, proteomic changes by iTRAQ(®) and respiration by an oxygen electrode. In parallel in vivo studies, high-energy nucleotides were obtained by transmural biopsy from anesthetized SWOP and SHAM pigs at baseline and during sustained low-flow ischemia. Compared with SHAM mitochondria, ex vivo SWOP heart tissue demonstrated increased expression of UCP-2, Complex IV (cytochrome c oxidase) and Complex V (ATPase) proteins. In comparison with SHAM pigs during in vivo conditions, transmural energetics in SWOP hearts, as estimated by the free energy of ATP hydrolysis (ΔG(0)), were similar at baseline but had decreased by the end of low-flow ischemia (-57.0 ± 2.1 versus -51.1 ± 1.4 kJ/mol; P < 0.05). In conclusion, within isolated mitochondria from preconditioned SWOP hearts, UCP-2 is increased and in concert with enhanced Complex IV and V proteins, imparts a favorable energetic state during low-flow ischemia. These data support the notion that mitochondrial adaptations that may reduce oxidant damage do not reduce the overall efficiency of energetics during sustained oxygen deprivation.

Efficacy of cardioprotective 'conditioning' strategies in aging and diabetic cohorts: the co-morbidity conundrum

Evidence obtained in multiple experimental models has revealed that cardiac 'conditioning' strategies--including ischaemic preconditioning, postconditioning, remote conditioning and administration of pharmacological conditioning mimetics--are profoundly protective and significantly attenuate myocardial ischaemia-reperfusion injury. As a result, there is considerable interest in translating these cardioprotective paradigms from the laboratory to patients. However, the majority of studies investigating conditioning-induced cardioprotection have utilized healthy adult animals devoid of the risk factors and co-morbidities associated with cardiovascular disease and acute myocardial infarction. The aim of this article is to summarize the growing consensus that two well established risk factors, aging and diabetes mellitus, may render the heart refractory to the favourable effects of myocardial conditioning, and discuss the clinical implications of a loss in efficacy of cardiac conditioning paradigms in these patient populations.

The role of comorbidities in cardioprotection

Cardioprotective strategies such as pre- and postconditioning result in a robust reduction in infarct size in young, healthy male animals. However, there are data suggesting that the protection is diminished in animals with comorbidities such as hypertension, hypercholesterolemia, and diabetes. It is important to understand at a mechanistic level the reasons for these differences. The effects of sex and diseases need to be considered in design of cardioprotective interventions in animal studies and clinical trials.

Regulation of mitochondrial processes by protein S-nitrosylation

BACKGROUND

Nitric oxide (NO) exerts powerful physiological effects through guanylate cyclase (GC), a non-mitochondrial enzyme, and through the generation of protein cysteinyl-NO (SNO) adducts-a post-translational modification relevant to mitochondrial biology. A small number of SNO proteins, generated by various mechanisms, are characteristically found in mammalian mitochondria and influence the regulation of oxidative phosphorylation and other aspects of mitochondrial function.

SCOPE OF REVIEW

The principles by which mitochondrial SNO proteins are formed and their actions, independently or collectively with NO binding to heme, iron-sulfur centers, or to glutathione (GSH) are reviewed on a molecular background of SNO-based signal transduction.

MAJOR CONCLUSIONS

Mitochondrial SNO-proteins have been demonstrated to inhibit Complex I of the electron transport chain, to modulate mitochondrial reactive oxygen species (ROS) production, influence calcium-dependent opening of the mitochondrial permeability transition pore (MPTP), promote selective importation of mitochondrial protein, and stimulate mitochondrial fission. The ease of reversibility and the affirmation of regulated S-nitros(yl)ating and denitros(yl)ating enzymatic reactions support hypotheses that SNO regulates the mitochondrion through redox mechanisms. SNO modification of mitochondrial proteins, whether homeostatic or adaptive (physiological), or pathogenic, is an area of active investigation.

GENERAL SIGNIFICANCE

Mitochondrial SNO proteins are associated with mainly protective, bur some pathological effects; the former mainly in inflammatory and ischemia/reperfusion syndromes and the latter in neurodegenerative diseases. Experimentally, mitochondrial SNO delivery is also emerging as a potential new area of therapeutics. This article is part of a Special Issue entitled: Regulation of cellular processes by S-nitrosylation.