Mitochondria in the human heart

Inorganic polyphosphate and the regulation of mitochondrial physiology

Inorganic polyphosphate (polyP) is an ancient polymer that is well-conserved throughout evolution. It is formed by multiple subunits of orthophosphates linked together by phosphoanhydride bonds. The presence of these bonds, which are structurally similar to those found in ATP, and the high abundance of polyP in mammalian mitochondria, suggest that polyP could be involved in the regulation of the physiology of the organelle, especially in the energy metabolism. In fact, the scientific literature shows an unequivocal role for polyP not only in directly regulating oxidative a phosphorylation; but also in the regulation of reactive oxygen species metabolism, mitochondrial free calcium homeostasis, and the formation and opening of mitochondrial permeability transitions pore. All these processes are closely interconnected with the status of mitochondrial bioenergetics and therefore play a crucial role in maintaining mitochondrial and cell physiology. In this invited review, we discuss the main scientific literature regarding the regulatory role of polyP in mammalian mitochondrial physiology, placing a particular emphasis on its impact on energy metabolism. Although the effects of polyP on the physiology of the organelle are evident; numerous aspects, particularly within mammalian cells, remain unclear and require further investigation. These aspects encompass, for example, advancing the development of more precise analytical methods, unraveling the mechanism responsible for sensing polyP levels, and understanding the exact molecular mechanism that underlies the effects of polyP on mitochondrial physiology. By increasing our understanding of the biology of this ancient and understudied polymer, we could unravel new pharmacological targets in diseases where mitochondrial dysfunction, including energy metabolism dysregulation, has been broadly described.

Targeting mitochondrial impairment for the treatment of cardiovascular diseases: From hypertension to ischemia-reperfusion injury, searching for new pharmacological targets

Mitochondria and mitochondrial proteins represent a group of promising pharmacological target candidates in the search of new molecular targets and drugs to counteract the onset of hypertension and more in general cardiovascular diseases (CVDs). Indeed, several mitochondrial pathways result impaired in CVDs, showing ATP depletion and ROS production as common traits of cardiac tissue degeneration. Thus, targeting mitochondrial dysfunction in cardiomyocytes can represent a successful strategy to prevent heart failure. In this context, the identification of new pharmacological targets among mitochondrial proteins paves the way for the design of new selective drugs. Thanks to the advances in omics approaches, to a greater availability of mitochondrial crystallized protein structures and to the development of new computational approaches for protein 3D-modelling and drug design, it is now possible to investigate in detail impaired mitochondrial pathways in CVDs. Furthermore, it is possible to design new powerful drugs able to hit the selected pharmacological targets in a highly selective way to rescue mitochondrial dysfunction and prevent cardiac tissue degeneration. The role of mitochondrial dysfunction in the onset of CVDs appears increasingly evident, as reflected by the impairment of proteins involved in lipid peroxidation, mitochondrial dynamics, respiratory chain complexes, and membrane polarization maintenance in CVD patients. Conversely, little is known about proteins responsible for the cross-talk between mitochondria and cytoplasm in cardiomyocytes. Mitochondrial transporters of the SLC25A family, in particular, are responsible for the translocation of nucleotides (e.g., ATP), amino acids (e.g., aspartate, glutamate, ornithine), organic acids (e.g. malate and 2-oxoglutarate), and other cofactors (e.g., inorganic phosphate, NAD⁺, FAD, carnitine, CoA derivatives) between the mitochondrial and cytosolic compartments. Thus, mitochondrial transporters play a key role in the mitochondria-cytosol cross-talk by leading metabolic pathways such as the malate/aspartate shuttle, the carnitine shuttle, the ATP export from mitochondria, and the regulation of permeability transition pore opening. Since all these pathways are crucial for maintaining healthy cardiomyocytes, mitochondrial carriers emerge as an interesting class of new possible pharmacological targets for CVD treatments.

Role of Mitochondrial Iron Overload in Mediating Cell Death in H9c2 Cells

Iron overload (IO) is associated with cardiovascular diseases, including heart failure. Our study's aim was to examine the mechanism by which IO triggers cell death in H9c2 cells. IO caused accumulation of intracellular and mitochondrial iron as shown by the use of iron-binding fluorescent reporters, FerroOrange and MitoFerroFluor. Expression of cytosolic and mitochondrial isoforms of Ferritin was also induced by IO. IO-induced iron accumulation and cellular ROS was rapid and temporally linked. ROS accumulation was detected in the cytosol and mitochondrial compartments with CellROX, DCF-DA and MitoSOX fluorescent dyes and partly reversed by the general antioxidant N-acetyl cysteine or the mitochondrial antioxidant SkQ1. Antioxidants also reduced the downstream activation of apoptosis and lytic cell death quantified by Caspase 3 cleavage/activation, mitochondrial Cytochrome c release, Annexin V/Propidium iodide staining and LDH release of IO-treated cells. Finally, overexpression of MitoNEET, an outer mitochondrial membrane protein involved in the transfer of Fe-S clusters between mitochondrial and cytosol, was observed to lower iron and ROS accumulation in the mitochondria. These alterations were correlated with reduced IO-induced cell death by apoptosis in MitoNEET-overexpressing cells. In conclusion, IO mediates H9c2 cell death by causing mitochondrial iron accumulation and subsequent general and mitochondrial ROS upregulation.

Effect of exercise training on cardiac mitochondrial respiration, biogenesis, dynamics, and mitophagy in ischemic heart disease

Objective

Cardiac mitochondrial dysfunction was found in ischemic heart disease (IHD). Hence, this study determined the effects of exercise training (ET) on cardiac mitochondrial respiration and cardiac mitochondrial quality control in IHD.

Methods

A narrative synthesis was conducted after searching animal studies written in English in three databases (PubMed, Web of Science, and EMBASE) until December 2020. Studies that used aerobic exercise as an intervention for at least 3 weeks and had at least normal, negative (sedentary IHD), and positive (exercise-trained IHD) groups were included. The CAMARADES checklist was used to check the quality of the included studies.

Results

The 10 included studies (CAMARADES score: 6–7/10) used swimming or treadmill exercise for 3–8 weeks. Seven studies showed that ET ameliorated cardiac mitochondrial respiratory function as manifested by decreased reactive oxygen species (ROS) production and increased complexes I-V activity, superoxide dismutase 2 (SOD2), respiratory control ratio (RCR), NADH dehydrogenase subunits 1 and 6 (ND1/6), Cytochrome B (CytB), and adenosine triphosphate (ATP) production. Ten studies showed that ET improved cardiac mitochondrial quality control in IHD as manifested by enhanced and/or controlled mitochondrial biogenesis, dynamics, and mitophagy. Four other studies showed that ET resulted in better cardiac mitochondrial physiological characteristics.

Conclusion

Exercise training could improve cardiac mitochondrial functions, including respiration, biogenesis, dynamics, and mitophagy in IHD.

Systematic review registration

https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=226817, identifier: CRD42021226817.

Establishment of a platform for measuring mitochondrial oxygen consumption rate for cardiac mitochondrial toxicity

The heart has an abundance of mitochondria since cardiac muscles require copious amounts of energy for providing continuous blood through the circulatory system, thereby implying that myocardial function is largely reliant on mitochondrial energy. Thus, cardiomyocytes are susceptible to mitochondrial dysfunction and are likely targets of mitochondrial toxic drugs. Various methods have been developed to evaluate mitochondrial toxicity by evaluating toxicological mechanisms, but an optimized and standardized assay for cardiomyocytes remains unmet. We have therefore attempted to standardize the evaluation system for determining cardiac mitochondrial toxicity, using AC16 human and H9C2 rat cardiomyocytes. Three clinically administered drugs (acetaminophen, amiodarone, and valproic acid) and two anticancer drugs (doxorubicin and tamoxifen) which are reported to have mitochondrial effects, were applied in this study. The oxygen consumption rate (OCR), which directly reflects mitochondrial function, and changes in mRNA levels of mitochondrial respiratory complex I to complex V, were analyzed. Our results reveal that exposure to all five drugs results in a concentration-dependent decrease in the basal and maximal levels of OCR in AC16 cells and H9C2 cells. In particular, marked reduction in the OCR was observed after treatment with doxorubicin. The reduction in OCR after exposure to mitochondrial toxic drugs was found to be associated with reduced mRNA expression in the mitochondrial respiratory complexes, suggesting that the cardiac mitochondrial toxicity of drugs is majorly due to dysfunction of mitochondrial respiration. Based on the results of this study, we established and standardized a protocol to measure OCR in cardiomyocytes. We expect that this standardized evaluation system for mitochondrial toxicity can be applied as basic data for establishing a screening platform to evaluate cardiac mitochondrial toxicity of drugs, during the developmental stage of new drug discovery.

Upregulation of mitochondrial ATPase inhibitory factor 1 (ATPIF1) mediates increased glycolysis in mouse hearts

In hypertrophied and failing hearts, fuel metabolism is reprogrammed to increase glucose metabolism, especially glycolysis. This metabolic shift favors biosynthetic function at the expense of ATP production. Mechanisms responsible for the switch are poorly understood. We found that inhibitory factor 1 of the mitochondrial FoF1-ATP synthase (ATPIF1), a protein known to inhibit ATP hydrolysis by the reverse function of ATP synthase during ischemia, was significantly upregulated in pathological cardiac hypertrophy induced by pressure overload, myocardial infarction, or α-adrenergic stimulation. Chemical cross-linking mass spectrometry analysis of hearts hypertrophied by pressure overload suggested that increased expression of ATPIF1 promoted the formation of FoF1-ATP synthase nonproductive tetramer. Using ATPIF1 gain- and loss-of-function cell models, we demonstrated that stalled electron flow due to impaired ATP synthase activity triggered mitochondrial ROS generation, which stabilized HIF1α, leading to transcriptional activation of glycolysis. Cardiac-specific deletion of ATPIF1 in mice prevented the metabolic switch and protected against the pathological remodeling during chronic stress. These results uncover a function of ATPIF1 in nonischemic hearts, which gives FoF1-ATP synthase a critical role in metabolic rewiring during the pathological remodeling of the heart.

Mitochondrial Ca2+ Homeostasis: Emerging Roles and Clinical Significance in Cardiac Remodeling

Mitochondria are the sites of oxidative metabolism in eukaryotes where the metabolites of sugars, fats, and amino acids are oxidized to harvest energy. Notably, mitochondria store Ca²⁺ and work in synergy with organelles such as the endoplasmic reticulum and extracellular matrix to control the dynamic balance of Ca²⁺ concentration in cells. Mitochondria are the vital organelles in heart tissue. Mitochondrial Ca²⁺ homeostasis is particularly important for maintaining the physiological and pathological mechanisms of the heart. Mitochondrial Ca²⁺ homeostasis plays a key role in the regulation of cardiac energy metabolism, mechanisms of death, oxygen free radical production, and autophagy. The imbalance of mitochondrial Ca²⁺ balance is closely associated with cardiac remodeling. The mitochondrial Ca²⁺ uniporter (mtCU) protein complex is responsible for the uptake and release of mitochondrial Ca²⁺ and regulation of Ca²⁺ homeostasis in mitochondria and consequently, in cells. This review summarizes the mechanisms of mitochondrial Ca²⁺ homeostasis in physiological and pathological cardiac remodeling and the regulatory effects of the mitochondrial calcium regulatory complex on cardiac energy metabolism, cell death, and autophagy, and also provides the theoretical basis for mitochondrial Ca²⁺ as a novel target for the treatment of cardiovascular diseases.

Differential Effects of Reperfusion on Cardiac Mitochondrial Subpopulations in a Preclinical Porcine Model of Acute Myocardial Infarction

Acute myocardial infarction (AMI) leads to localized cardiac ischemia and can be fatal if untreated. Despite being treatable, the threat of ischemia-reperfusion (IR) injury remains high. Mitochondria are central to both propagation and mitigation of IR injury, and cardiac mitochondria are categorized into two major subtypes-subsarcolemmal and interfibrillar mitochondria (SSM and IFM, respectively). We hypothesized that, in our pre-clinical porcine model of AMI, SSM and IFM are differentially affected by reperfusion. AMI was induced in female pigs by balloon occlusion of the left anterior descending artery for 45 min, followed by 4 h of reperfusion. At the end of reperfusion, animals were euthanized. Cardiac SSM and IFM from the affected ischemic area and a nearby non-ischemic area were isolated to compare mitochondrial function using substrates targeting mitochondrial electron transport chain complexes I and II. Despite detecting overall significant differences in mitochondrial function including yield, mitochondrial S3 and S4 respirations, and calcium retention, consistent individual functional differences in the two mitochondrial subpopulations were not observed, both between the two mitochondrial subtypes, as well as between the ischemic and non-ischemic tissue. Nonetheless, this study describes the mitochondrial subtype response within the initial few hours of reperfusion in a clinically relevant model of AMI, which provides valuable information needed to develop novel mitochondrially targeted therapies for AMI.

ATAD3B and SKIL polymorphisms associated with antipsychotic-induced QTc interval change in patients with schizophrenia: a genome-wide association study

QTc interval prolongation is one of the most common antipsychotic-induced side effects which could lead to ventricular tachycardia or Torsade de Pointes, even cardiac arrest. There is very limited understanding on the genetic factors that associated with antipsychotic-induced QTc interval change. We conducted a genome-wide association study (GWAS) of antipsychotic-induced QTc interval change among patients with schizophrenia. A total of 2040 patients with schizophrenia were randomly assigned to six groups (olanzapine, risperidone, quetiapine, aripiprazole, ziprasidone, and first-generation antipsychotics; first-generation antipsychotics including haloperidol or perphenazine were also assigned randomly) and received 6-week antipsychotic treatment. We identified two novel loci (rs200050752 in ATAD3B and rs186507741 in SKIL) that were associated with antipsychotic-induced QTc interval change at a genome-wide significance level. The combination of polygenic risk score (PRS), based the GWAS of myocardial infarction from BioBank Japan project, and clinical data (sex, heart rate and QTc interval at baseline) could be applied to predict whether patients with schizophrenia have QTc interval prolongation (10 ms was applied as threshold, P < 0.001, area under the curve [AUC] was 0.797), especially for the first episode patients (P < 0.001, AUC was 0.872). We identified two loci located within genes related to mitochondrial function and cell growth and differentiation, which were both associated with schizophrenia and heart function. The combination of PRS and clinical data could predict whether patients with schizophrenia have the side effect of QTc interval prolongation, which could fundamentally guide the choice of antipsychotic in patients with schizophrenia, especially for the first-episode patients.

Comparative Transcriptome Analyses of Gayal (Bos frontalis), Yak (Bos grunniens), and Cattle (Bos taurus) Reveal the High-Altitude Adaptation

Gayal and yak are well adapted to their local high-altitude environments, yet the transcriptional regulation difference of the plateau environment among them remains obscure. Herein, cross-tissue and cross-species comparative transcriptome analyses were performed for the six hypoxia-sensitive tissues from gayal, yak, and cattle. Gene expression profiles for all single-copy orthologous genes showed tissue-specific expression patterns. By differential expression analysis, we identified 3,020 and 1,995 differentially expressed genes (DEGs) in at least one tissue of gayal vs. cattle and yak vs. cattle, respectively. Notably, we found that the adaptability of the gayal to the alpine canyon environment is highly similar to the yak living in the Qinghai-Tibet Plateau, such as promoting red blood cell development, angiogenesis, reducing blood coagulation, immune system activation, and energy metabolism shifts from fatty acid β-oxidation to glycolysis. By further analyzing the common and unique DEGs in the six tissues, we also found that numerous expressed regulatory genes related to these functions are unique in the gayal and yak, which may play important roles in adapting to the corresponding high-altitude environment. Combined with WGCNA analysis, we found that UQCRC1 and COX5A are the shared differentially expressed hub genes related to the energy supply of myocardial contraction in the heart-related modules of gayal and yak, and CAPS is a shared differential hub gene among the hub genes of the lung-related module, which is related to pulmonary artery smooth muscle contraction. Additionally, EDN3 is the unique differentially expressed hub gene related to the tracheal epithelium and pulmonary vasoconstriction in the lung of gayal. CHRM2 is a unique differentially expressed hub gene that was identified in the heart of yak, which has an important role in the autonomous regulation of the heart. These results provide a basis for further understanding the complex transcriptome expression pattern and the regulatory mechanism of high-altitude domestication of gayal and yak.

PHDs/CPT1B/VDAC1 axis regulates long-chain fatty acid oxidation in cardiomyocytes

Cardiac metabolism is a high-oxygen-consuming process, showing a preference for long-chain fatty acid (LCFA) as the fuel source under physiological conditions. However, a metabolic switch (favoring glucose instead of LCFA) is commonly reported in ischemic or late-stage failing hearts. The mechanism regulating this metabolic switch remains poorly understood. Here, we report that loss of PHD2/3, the cellular oxygen sensors, blocks LCFA mitochondria uptake and β-oxidation in cardiomyocytes. In high-fat-fed mice, PHD2/3 deficiency improves glucose metabolism but exacerbates the cardiac defects. Mechanistically, we find that PHD2/3 bind to CPT1B, a key enzyme of mitochondrial LCFA uptake, promoting CPT1B-P295 hydroxylation. Further, we show that CPT1B-P295 hydroxylation is indispensable for its interaction with VDAC1 and LCFA β-oxidation. Finally, we demonstrate that a CPT1B-P295A mutant constitutively binds to VDAC1 and rescues LCFA metabolism in PHD2/3-deficient cardiomyocytes. Together, our data identify an oxygen-sensitive regulatory axis involved in cardiac metabolism.

Cardioprotective effects of melatonin and metformin against doxorubicin-induced cardiotoxicity in rats are through preserving mitochondrial function and dynamics

Doxorubicin (Dox) is widely used in chemotherapy regimens for several malignant conditions. Unfortunately, cumulative and irreversible cardiotoxicity of Dox is the most prominent adverse effect which limits its use. Several pharmacological interventions which exert antioxidant properties, including melatonin and metformin, have demonstrated beneficial effects against various cardiac pathological conditions. However, the exact molecular mechanisms underlying their cardioprotective effects are not completely understood. We hypothesized that treatment with either melatonin or metformin provides cardioprotection against Dox-induced cardiotoxicity through mitochondrial protection. Thirty-two male Wistar rats received 6 doses of either 0.9% normal saline solution (0.9% NSS, n = 8) or Dox (3 mg/kg, i.p., n = 24). The Dox-treated rats (n = 8/group) were co-treated with: 1) Vehicle (0.9% NSS), 2) Melatonin (10 mg/kg/day), and 3) Metformin (250 mg/kg/day) for 30 consecutive days via oral gavage. Following the treatment, left ventricular (LV) function, oxidative stress, inflammation, mitochondrial function, dynamics, biogenesis and bioenergetics, mitophagy, autophagy, and apoptosis were determined. Dox induced excessive oxidative stress, inflammation, autophagy, apoptosis, reduced mitochondrial function, dynamics balance, biogenesis, and bioenergetics leading to LV dysfunction. Treatment with either melatonin or metformin exerted equal measures of cardioprotection via reducing oxidative stress, inflammation, autophagy, apoptosis, and improved mitochondrial function, dynamics balance, biogenesis, and bioenergetics in the Dox-treated rats. Melatonin and metformin exerted both anti-cancer and cardioprotective properties, suggesting they have potential roles in concomitant therapy in cancer patients receiving Dox treatment.

The multiple mechanisms of MCL1 in the regulation of cell fate

MCL1 (myeloid cell leukemia-1) is a widely recognized pro-survival member of the Bcl-2 (B-cell lymphoma protein 2) family and a promising target for cancer therapy. While the role MCL1 plays in apoptosis is well defined, its participation in emerging non-apoptotic signaling pathways is only beginning to be appreciated. Here, we synthesize studies characterizing MCL1s influence on cell proliferation, DNA damage response, autophagy, calcium handling, and mitochondrial quality control to highlight the broader scope that MCL1 plays in cellular homeostasis regulation. Throughout this review, we discuss which pathways are likely to be impacted by emerging MCL1 inhibitors, as well as highlight non-cancerous disease states that could deploy Bcl-2 homology 3 (BH3)-mimetics in the future.

In this review Widden and Placzek synthesize studies characterizing the influence that myeloid cell leukemia-1 (MCL1) has on cell proliferation, DNA damage response, autophagy, calcium handling, and mitochondrial quality control to highlight the broader scope that it plays in cellular homeostasis regulation. They discuss which pathways are likely to be impacted by emerging MCL1 inhibitors, as well as highlight non-cancerous disease states that could deploy BH3-mimetics in the future.

Signal transducer and transcriptional activation 1 protects against pressure overload-induced cardiac hypertrophy

Signal transducers and transcriptional activation 1 (Stat1) is a member of the STATs family, and its role in various biological responses, including cell proliferation, differentiation, migration, apoptosis, and immune regulation has been extensively studied. We aimed to investigate its role in pathological cardiac hypertrophy, which is currently poorly understood. Experiments using H9C2 cardiomyocytes, Stat1, and IfngR cardiomyocyte-specific knockout mice revealed that Stat1 had a protective effect on cardiac hypertrophy. Using transverse aortic constriction (TAC)-induced cardiac hypertrophy in mice, we analyzed the degree of hypertrophy using echocardiography, pathology, and at the molecular level. Mice lacking Stat1 had more pronounced cardiac hypertrophy and fibrosis than wild-type TAC mice. Analysis of the molecular mechanisms suggested that Stat1 downregulated the mRNA levels of hypertrophy and fibrosis markers to inhibit cardiac hypertrophy, and promotes mitochondrial fission through the Ucp2/P-Drp1 pathway, enhancing mitochondrial function, and increasing compensatory myocardial ATP production in the compensatory phase for cardiac hypertrophy inhibition. Overall, this comprehensive analysis revealed that Stat1 inhibits cardiac hypertrophy by downregulating hypertrophic and fibrotic marker genes and enhancing the mitochondrial function to enhance cardiomyocyte function through the Ucp2/P-Drp1 signaling pathway.

Mitochondrial DNA Content Is Linked to Cardiovascular Disease Patient Phenotypes

Background We sought to determine whether mitochondrial DNA (mtDNA) content can be used as markers for 12 key phenotypes among cardiovascular disease patients, and whether these markers are valid across patients with diverse ancestries. Methods and Results DNA was collected from the peripheral blood of 996 cardiovascular disease patients at the Cleveland Clinic. The mtDNA copy number and DNA-level variation were assessed from whole-genome sequence. Patients were also ascertained retrospectively for histories of 10 clinical events, as well as for maximum stenosis and extent of disease at baseline. Self-reported race and maternal ancestry inferred from mtDNA sequence were recorded. MtDNA copy number and overall mtDNA rare variant load were significantly lower in patients with histories of various adverse clinical events, and mtDNA copy number was inversely correlated with extent of disease. Strong associations were also found between absence of rare variants in the genes MT-ATP6 and MT-COII and patient histories of hyperlipidemia and myocardial infarction, respectively. Importantly, associations were not ancestry dependent. Conclusions This study provides evidence that mtDNA copy number in circulation is associated with a variety of cardiovascular disease patient phenotypes. Results also suggest a protective role for some rare inherited mtDNA variants. Overall, the study supports the potential of mtDNA content and abundance as biomarkers in heart disease, in a manner that is valid across diverse ancestries.

Depleted Myocardial Coenzyme Q10 in Cavalier King Charles Spaniels with Congestive Heart Failure Due to Myxomatous Mitral Valve Disease

Congestive heart failure (CHF) has been associated with depleted myocardial coenzyme Q10 (Q10) concentrations in human patients. The aim of this study was to investigate associations between myocardial Q10 concentrations and myxomatous mitral valve disease (MMVD) severity in dogs. Furthermore, citrate synthase (CS) activity was analysed to determine if a reduction in myocardial Q10 was associated with mitochondrial depletion in the myocardium. Thirty Cavalier King Charles spaniels (CKCS) in MMVD stages B1 (n = 11), B2 (n = 5) and C (n = 14) according to the American College of Veterinary Internal Medicine (ACVIM) guidelines and 10 control (CON) dogs of other breeds were included. Myocardial Q10 concentration was analysed in left ventricular tissue samples using HPLC-ECD. CKCS with congestive heart failure (CHF; group C) had significantly reduced Q10 concentrations (median, 1.54 µg/mg; IQR, 1.36–1.94), compared to B1 (2.76 µg/mg; 2.10–4.81, p < 0.0018), B2 (3.85 µg/mg; 3.13–4.46, p < 0.0054) and CON dogs (2.8 µg/mg; 1.64–4.88, p < 0.0089). CS activity was comparable between disease groups. In conclusion, dogs with CHF due to MMVD had reduced myocardial Q10 concentrations. Studies evaluating antioxidant defense mechanisms as a therapeutic target for treatment of CHF in dogs are warranted.

Natural compounds against doxorubicin-induced cardiotoxicity: A review on the involvement of Nrf2/ARE signaling pathway

Cardiotoxicity is the main concern for long-term use of the doxorubicin (DOX). Reactive oxygen species (ROS) generation leads to oxidative stress that significantly contributes to the cardiac damage induced by DOX. The nuclear factor erythroid 2-related factor (Nrf2) acts as a protective player against DOX-induced myocardial oxidative stress. Several natural compounds (NCs) with anti-oxidative effects, were examined to suppress DOX cardiotoxicity such as asiatic acid, α-linolenic acid, apigenin, baicalein, β-lapachone, curdione, dioscin, ferulic acid, Ganoderma lucidum polysaccharides, genistein, ginsenoside Rg3, indole-3-carbinol, naringenin-7-O-glucoside, neferine, p-coumaric acid, pristimerin, punicalagin, quercetin, sulforaphane, and tanshinone IIA. The present article, reviews NCs that showed protective effects against DOX-induced cardiac injury through induction of Nrf2 signaling pathway.

Canonical and Non-Canonical Roles of Connexin43 in Cardioprotection

Since the mid-20th century, ischemic heart disease has been the world’s leading cause of death. Developing effective clinical cardioprotection strategies would make a significant impact in improving both quality of life and longevity in the worldwide population. Both ex vivo and in vivo animal models of cardiac ischemia/reperfusion (I/R) injury are robustly used in research. Connexin43 (Cx43), the predominant gap junction channel-forming protein in cardiomyocytes, has emerged as a cardioprotective target. Cx43 posttranslational modifications as well as cellular distribution are altered during cardiac reperfusion injury, inducing phosphorylation states and localization detrimental to maintaining intercellular communication and cardiac conduction. Pre- (before ischemia) and post- (after ischemia but before reperfusion) conditioning can abrogate this injury process, preserving Cx43 and reducing cell death. Pre-/post-conditioning has been shown to largely rely on the presence of Cx43, including mitochondrial Cx43, which is implicated to play a major role in pre-conditioning. Posttranslational modifications of Cx43 after injury alter the protein interactome, inducing negative protein cascades and altering protein trafficking, which then causes further damage post-I/R injury. Recently, several peptides based on the Cx43 sequence have been found to successfully diminish cardiac injury in pre-clinical studies.

The roles of resveratrol on cardiac mitochondrial function in cardiac diseases

Left ventricular (LV) dysfunction is commonly associated with a variety of health conditions including acute myocardial infarction and obesity/diabetes. In addition, administration of several pharmacological agents such as anticancer, antiviral, and immunosuppressive drugs has been shown to be related with LV dysfunction. The molecular mechanism responsible for LV dysfunction has been extensively studied, and it has been proposed that the overproduction of reactive oxygen species (ROS) plays a crucial role in the regulation of this function. Mitochondria require the balance between ROS production and antioxidants to maintain their appropriate function and to prevent excessive ROS production. Thus, the excessive production of ROS and the reduced scavenging process under any pathological conditions could disrupt mitochondrial function, leading to energy depletion with subsequent cell death. Therefore, maintenance of the balance between oxidative stress and antioxidants is essential. Resveratrol, a stilbene, has been investigated extensively, and potentially used to treat or prevent various cardiovascular diseases. Resveratrol directly upregulates antioxidative capacity by increasing antioxidant genes such as heme oxygenase-1, superoxide dismutase, catalase, and glutathione. In this review, accumulated data from in vitro, ex vivo, and in vivo studies regarding the effects of resveratrol on cardiac mitochondrial function in cardiac pathologies are comprehensively summarized and discussed. Since there is no conclusive available clinical study regarding the effects of resveratrol on cardiac mitochondrial function, this review also aims to encourage more clinical investigations to confirm findings from basic research. This comprehensive review will provide insight regarding the potential mechanistic roles of resveratrol in preventing and/or treating patients with cardiovascular diseases to improve LV function and their health status.

Cardiolipin remodeling in Barth syndrome and other hereditary cardiomyopathies

Mitochondria play a prominent role in cardiac energy metabolism, and their function is critically dependent on the integrity of mitochondrial membranes. Disorders characterized by mitochondrial dysfunction are commonly associated with cardiac disease. The mitochondrial phospholipid cardiolipin directly interacts with a number of essential protein complexes in the mitochondrial membranes including the respiratory chain, mitochondrial metabolite carriers, and proteins critical for mitochondrial morphology. Barth syndrome is an X-linked disorder caused by an inherited defect in the biogenesis of the mitochondrial phospholipid cardiolipin. How cardiolipin deficiency impacts on mitochondrial function and how mitochondrial dysfunction causes cardiomyopathy has been intensively studied in cellular and animal models of Barth syndrome. These findings may also have implications for the molecular mechanisms underlying other inherited disorders associated with defects in cardiolipin, such as Sengers syndrome and dilated cardiomyopathy with ataxia (DCMA).

Experimental oxygen concentration influences rates of mitochondrial hydrogen peroxide release from cardiac and skeletal muscle preparations

Mitochondria utilize the majority of oxygen (O₂) consumed by aerobic organisms as the final electron acceptor for oxidative phosphorylation (OXPHOS) but also to generate reactive oxygen species (mtROS) that participate in cell signaling, physiological hormesis, and disease pathogenesis. Simultaneous monitoring of mtROS production and oxygen consumption (Jo₂) from tissue mitochondrial preparations is an attractive investigative approach, but it introduces dynamic changes in media O₂ concentration ([O₂]) that can confound experimental results and interpretation. We utilized high-resolution fluorespirometry to evaluate Jo₂ and hydrogen peroxide release (Jh₂o₂) from isolated mitochondria (Mt), permeabilized fibers (Pf), and tissue homogenates (Hm) prepared from murine heart and skeletal muscle across a range of experimental [O₂]s typically encountered during respirometry protocols (400-50 µM). Results demonstrate notable variations in Jh₂o₂ across tissues and sample preparations during nonphosphorylating (LEAK) and OXPHOS-linked respiration states at 250 µM [O₂] but a linear decline in Jh₂o₂ of 5-15% per 50-µM decrease in chamber [O₂] in all samples. Jo₂ was generally stable in Mt and Hm across [O₂]s above 50 µM but tended to decline below 250 µM in Pf, leading to wide variations in assayed rates of Jh₂o₂/O₂ across chamber [O₂]s and sample preparations. Development of chemical background fluorescence from the H₂O₂ probe (Amplex Red) was also O₂ sensitive, emphasizing relevant calibration considerations. This study highlights the importance of monitoring and reporting the chamber [O₂] at which Jo₂ and Jh₂o₂ are recorded during fluorespirometry experiments and provides a basis for selecting sample preparations for studies addressing the role of mtROS in physiology and disease.

High resolution respirometry to assess function of mitochondria in native homogenates of human heart muscle

Impaired myocardial bioenergetics is a hallmark of many cardiac diseases. There is a need of a simple and reproducible method of assessment of mitochondrial function from small human myocardial tissue samples. In this study we adopted high-resolution respirometry to homogenates of fresh human cardiac muscle and compare it with isolated mitochondria. We used atria resected during cardiac surgery (n = 18) and atria and left ventricles from brain-dead organ donors (n = 12). The protocol we developed consisting of two-step homogenization and exposure of 2.5% homogenate in a respirometer to sequential addition of 2.5 mM malate, 15 mM glutamate, 2.5 mM ADP, 10 μM cytochrome c, 10 mM succinate, 2.5 μM oligomycin, 1.5 μM FCCP, 3.5 μM rotenone, 4 μM antimycin and 1 mM KCN or 100 mM Sodium Azide. We found a linear dependency of oxygen consumption on oxygen concentration. This technique requires < 20 mg of myocardium and the preparation of the sample takes <20 min. Mitochondria in the homogenate, as compared to subsarcolemmal and interfibrillar isolated mitochondria, have comparable or better preserved integrity of outer mitochondrial membrane (increase of respiration after addition of cytochrome c is up to 11.7±1.8% vs. 15.7±3.1%, p˂0.05 and 11.7±3.5%, p = 0.99, resp.) and better efficiency of oxidative phosphorylation (Respiratory Control Ratio = 3.65±0.5 vs. 3.04±0.27, p˂0.01 and 2.65±0.17, p˂0.0001, resp.). Results are reproducible with coefficient of variation between two duplicate measurements ≤8% for all indices. We found that whereas atrial myocardium contains less mitochondria than the ventricle, atrial bioenergetic profiles are comparable to left ventricle. In conclusion, high resolution respirometry has been adapted to homogenates of human cardiac muscle and shown to be reliable and reproducible.

Mitochondrial DNA Sequence Variants Associated With Blood Pressure Among 2 Cohorts of Older Adults

Background

Age‐related changes in blood pressure are associated with a variety of poor health outcomes. Genetic factors are proposed contributors to age‐related increases in blood pressure, but few genetic loci have been identified. We examined the role of mitochondrial genomic variation in blood pressure by sequencing the mitochondrial genome.

Methods and Results

Mitochondrial DNA (mtDNA) data from 1755 participants from the LIFE (Lifestyle Interventions and Independence for Elders) studies and 788 participants from the Health ABC (Health, Aging, and Body Composition) study were evaluated using replication analysis followed by meta‐analysis. Participants were aged ≥69 years, of diverse racial backgrounds, and assessed for systolic blood pressure (SBP), diastolic blood pressure, and mean arterial pressure. After meta‐analysis across the LIFE and Health ABC studies, statistically significant associations of mtDNA variants with higher SBP (m.3197T>C, 16S rRNA; P=0.0005) and mean arterial pressure (m.15924A>G, t‐RNA‐thr; P=0.004) were identified in white participants. Among black participants, statistically significant associations with higher SBP (m.93A>G, HVII; m.16183A>C, HVI; both P=0.0001) and mean arterial pressure (m.16172T>C, HVI; m.16183A>C, HVI; m.16189T>C, HVI; m.12705C>T; all P's<0.0004) were observed. Significant pooled effects on SBP were observed across all transfer RNA regions (P=0.0056) in white participants. The individual and aggregate variant results are statistically significant after multiple comparisons adjustment for the number of mtDNA variants and mitochondrial regions examined.

Conclusions

These results suggest that mtDNA‐encoded variants are associated with variation in SBP and mean arterial pressure among older adults. These results may help identify mitochondrial activities to explain differences in blood pressure in older adults and generate new hypotheses surrounding mtDNA variation and the regulation of blood pressure.

Clinical Trial Registration

URL: http://www.ClinicalTrials.gov. Unique identifiers: NCT01072500 and NCT00116194.

A new insight into the molecular hydrogen effect on coenzyme Q and mitochondrial function of rats

Mitochondria are the major source of cellular energy metabolism. In the cardiac cells, mitochondria produce by way of the oxidative phosphorylation more than 90% of the energy supply in the form of ATP, which is utilized in many ATP-dependent processes, like cycling of the contractile proteins or maintaining ion gradients. Reactive oxygen species (ROS) are by-products of cellular metabolism and their levels are controlled by intracellular antioxidant systems. Imbalance between ROS and the antioxidant defense leads to oxidative stress and oxidative changes to cellular biomolecules. Molecular hydrogen (H₂) has been proved as beneficial in the prevention and therapy of various diseases including cardiovascular disorders. It selectively scavenges hydroxyl radical and peroxynitrite, reduces oxidative stress, and has anti-inflammatory and anti-apoptotic effects. The effect of H₂ on the myocardial mitochondrial function and coenzyme Q levels is not well known. In this paper, we demonstrated that consumption of H₂-rich water (HRW) resulted in stimulated rat cardiac mitochondrial electron respiratory chain function and increased levels of ATP production by Complex I and Complex II substrates. Similarly, coenzyme Q₉ levels in the rat plasma, myocardial tissue, and mitochondria were increased and malondialdehyde level in plasma was reduced after HRW administration. Based on obtained data, we hypothesize a new metabolic pathway of the H₂ effect in mitochondria on the Q-cycle and in mitochondrial respiratory chain function. The Q-cycle contains three coenzyme Q forms: coenzyme Q in oxidized form (ubiquinone), radical form (semiquinone), or reduced form (ubiquinol). H₂ may be a donor of both electron and proton in the Q-cycle and thus we can suppose stimulation of coenzyme Q production. When ubiquinone is reduced to ubiquinol, lipid peroxidation is reduced. Increased CoQ₉ concentration can stimulate electron transport from Complex I and Complex II to Complex III and increase ATP production via mitochondrial oxidative phosphorylation. Our results indicate that H₂ may function to prevent/treat disease states with disrupted myocardial mitochondrial function.

Mitochondrial genetic haplogroups and cardiovascular diseases: Data from the Osteoarthritis Initiative

Background

Some case-control studies reported that mitochondrial haplogroups could be associated with the onset of cardiovascular diseases (CVD), but the literature regarding this topic is limited. We aimed to investigate whether any mitochondrial haplogroup carried a higher or lower risk of CVD in a large cohort of North American people affected by knee osteoarthritis or at high risk for this condition.

Materials and methods

A longitudinal cohort study including individuals from the Osteoarthritis Initiative was done. Haplogroups were assigned through a combination of sequencing and PCR-RFLP techniques. All the mitochondrial haplogroups have been named following this nomenclature: HV, JT, UK, IWX, and superHV/others. The strength of the association between mitochondrial haplogroups and incident CVD was evaluated through a Cox’s regression analysis, adjusted for potential confounders, and reported as hazard ratios (HRs) with their 95% confidence intervals (CIs).

Results

Overall, 3,288 Caucasian participants (56.8% women) with a mean age of 61.3±9.2 years without CVD at baseline were included. During a median follow-up of 8 years, 322 individuals (= 9.8% of baseline population) developed a CVD. After adjusting for 11 potential confounders at baseline and taking those with the HV haplotype as reference (the most frequent), those with JT carried a significant lower risk of CVD (HR = 0.75; 95%CI: 0.54–0.96; p = 0.03). Participants with the J haplogroup had the lowest risk of CVD (HR = 0.71; 95%CI: 0.46–0.95; p = 0.02).

Conclusions

The presence of JT haplogroups (particularly J) may be associated with a reduced risk of CVD. However, this result was not based on a high level of statistical significance. Thus, future research with larger sample size is needed to assess whether our results can be corroborated.

EGCG protects cardiomyocytes against hypoxia-reperfusion injury through inhibition of OMA1 activation

Mitochondria are important for energy production and cardiomyocyte homeostasis. OMA1, a metalloendopeptidase, initiates the proteolytic process of the fusion-allowing protein OPA1, to deteriorate mitochondrial structure and function. In this study, mouse embryonic fibroblasts (MEFs) and neonatal mouse cardiomyocytes (NMCMs) subjected to hypoxia-reperfusion injury (HRI) and/or H₂O₂ were used to mimic oxidative stress in the heart following ischemia-reperfusion injury (IRI). In vitro experiments demonstrated that HRI or stimulation with H₂O₂ induced self-cleavage of OMA1 and the subsequent conversion of OPA1 from its long form to its short form, leading to mitochondrial fragmentation, cytochrome c release and apoptosis. By using Molecular Operating Environment (MOE) software to simulate the binding interaction of 2295 phytochemicals against OMA1, epigallocatechin gallate (EGCG) and betanin were selected as candidates of OMA1 inhibitor. We found that EGCG directly interacted with OMA1 and potently inhibited self-cleavage of OMA1, leading to attenuated OPA1 cleavage. This study, therefore, suggests to use OMA1 inhibition induced by EGCG to treat cardiac IRI.

Mitochondrial quality control in the cardiac system: An integrative view

Recent studies have led to the discovery of multiple mitochondrial quality control (mQC) processes that operate at various scales, ranging from the degradation of proteins by mitochondrial proteases to the degradation of selected cargos or entire organelles in lysosomes. While the mechanisms governing these mQC processes are progressively being delineated, their role and importance remain unclear. Converging evidence however point to a complex system whereby multiple and partly overlapping processes are recruited to orchestrate a cell type specific mQC response that is adapted to the physiological state and level of stress encountered. Knowledge gained from basic model systems of mQC therefore need to be integrated within organ-specific (patho)physiological frameworks. Building on this notion, this article focuses on mQC in the heart, where developmental metabolic reprogramming, sustained contraction, and multiple pathophysiological conditions pose broadly different constraints. We provide an overview of current knowledge of mQC processes, and discuss their implication in cardiac mQC under normal and diseased conditions.

Binding of FUN14 Domain Containing 1 With Inositol 1,4,5-Trisphosphate Receptor in Mitochondria-Associated Endoplasmic Reticulum Membranes Maintains Mitochondrial Dynamics and Function in Hearts in Vivo

BACKGROUND

FUN14 domain containing 1 (FUNDC1) is a highly conserved outer mitochondrial membrane protein. The aim of this study is to examine whether FUNDC1 modulates the mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs), mitochondrial morphology, and function in cardiomyocytes and intact hearts.

METHODS

The impacts of FUNDC1 on MAMs formation and cardiac functions were studied in mouse neonatal cardiomyocytes, in mice with cardiomyocyte-specific Fundc1 gene knockout (Fundc1^f/Y/Cre^αMyHC+/- ), and in the cardiac tissues of the patients with heart failure.

RESULTS

In mouse neonatal cardiomyocytes and intact hearts, FUNDC1 was localized in MAMs by binding to ER-resided inositol 1,4,5-trisphosphate type 2 receptor (IP₃R2). Fundc1 ablation disrupted MAMs and reduced the levels of IP₃R2 and Ca²⁺ in both mitochondria and cytosol, whereas overexpression of Fundc1 increased the levels of IP₃R2 and Ca²⁺ in both mitochondria and cytosol. Consistently, Fundc1 ablation increased Ca²⁺ levels in ER, whereas Fundc1 overexpression lowered ER Ca²⁺ levels. Further, Fundc1 ablation in cardiomyocytes elongated mitochondria and compromised mitochondrial functions. Mechanistically, we found that Fundc1 ablation-induced reduction of intracellular Ca²⁺ levels suppressed mitochondrial fission 1 protein (Fis1) expression and mitochondrial fission by reducing the binding of the cAMP response element binding protein (CREB) in the Fis1 promoter. Fundc1^f/Y/Cre^αMyHC+/- mice but not their littermate control mice (Fundc1^wt/Y/Cre^αMyHC+/- ) exhibited cardiac dysfunction. The ligation of the left ventricle artery of Fundc1^f/Y/Cre^αMyHC+/- mice caused more severe cardiac dysfunction than those in sham-treated Fundc1^f/Y/Cre^αMyHC+/- mice. Finally, we found that the FUNDC1/MAMs/CREB/Fis1 signaling axis was significantly suppressed in patients with heart failure.

CONCLUSIONS

We conclude that FUNDC1 binds to IP₃R2 to modulate ER Ca²⁺ release into mitochondria and cytosol. Further, a disruption of the FUNDC1 and IP₃R2 interaction lowers the levels of Ca²⁺ in mitochondria and cytosol, both of which instigate aberrant mitochondrial fission, mitochondrial dysfunction, cardiac dysfunction, and heart failure.

Roles of mitochondrial dynamics modulators in cardiac ischaemia/reperfusion injury

The current therapeutic strategy for the management of acute myocardial infarction (AMI) is to return blood flow into the occluded coronary artery of the heart, a process defined as reperfusion. However, reperfusion itself can increase mortality rates in AMI patients because of cardiac tissue damage and dysfunction, which is termed ‘ischaemia/reperfusion (I/R) injury’. Mitochondria play an important role in myocardial I/R injury as disturbance of mitochondrial dynamics, especially excessive mitochondrial fission, is a predominant cause of cardiac dysfunction. Therefore, pharmacological intervention and therapeutic strategies which modulate the mitochondrial dynamics balance during I/R injury could exert great beneficial effects to the I/R heart. This review comprehensively summarizes and discusses the effects of mitochondrial fission inhibitors as well as mitochondrial fusion promoters on cardiac and mitochondrial function during myocardial I/R injury. The comparison of the effects of both compounds given at different time‐points during the course of I/R injury (i.e. prior to ischaemia, during ischaemia and at the reperfusion period) are also summarized and discussed. Finally, this review also details important information which may contribute to clinical practices using these drugs to improve the quality of life in AMI patients.

Mitochondrial Mutations in Cardiac Disorders

Mitochondria individually encapsulate their own genome, unlike other cellular organelles. Mitochondrial DNA (mtDNA) is a circular, double-stranded, 16,569-base paired DNA containing 37 genes: 13 proteins of the mitochondrial respiratory chain, two ribosomal RNAs (rRNAs; 12S and 16S), and 22 transfer RNAs (tRNAs). The mtDNA is more vulnerable to oxidative modifications compared to nuclear DNA because of its proximity to ROS-producing sites, limited presence of DNA damage repair systems, and continuous replication in the cell. mtDNA mutations can be inherited or sporadic. Simple mtDNA mutations are point mutations, which are frequently found in mitochondrial tRNA loci, causing mischarging of mitochondrial tRNAs or deletion, duplication, or reduction in mtDNA content. Because mtDNA has multiple copies and a specific replication mechanism in cells or tissues, it can be heterogenous, resulting in characteristic phenotypic presentations such as heteroplasmy, genetic drift, and threshold effects. Recent studies have increased the understanding of basic mitochondrial genetics, providing an insight into the correlations between mitochondrial mutations and cardiac manifestations including hypertrophic or dilated cardiomyopathy, arrhythmia, autonomic nervous system dysfunction, heart failure, or sudden cardiac death with a syndromic or non-syndromic phenotype. Clinical manifestations of mitochondrial mutations, which result from structural defects, functional impairment, or both, are increasingly detected but are not clear because of the complex interplay between the mitochondrial and nuclear genomes, even in homoplasmic mitochondrial populations. Additionally, various factors such as individual susceptibility, nutritional state, and exposure to chemicals can influence phenotypic presentation, even for the same mtDNA mutation.In this chapter, we summarize our current understanding of mtDNA mutations and their role in cardiac involvement. In addition, epigenetic modifications of mtDNA are briefly discussed for future elucidation of their critical role in cardiac involvement. Finally, current strategies for dealing with mitochondrial mutations in cardiac disorders are briefly stated.

Posttranslational modifications and dysfunction of mitochondrial enzymes in human heart failure

Deficiency of energy supply is a major complication contributing to the syndrome of heart failure (HF). Because the concurrent activity profile of mitochondrial bioenergetic enzymes has not been studied collectively in human HF, our aim was to examine the mitochondrial enzyme defects in left ventricular myocardium obtained from explanted end-stage failing hearts. Compared with nonfailing donor hearts, activity rates of complexes I and IV and the Krebs cycle enzymes isocitrate dehydrogenase, malate dehydrogenase, and aconitase were lower in HF, as determined spectrophotometrically. However, activity rates of complexes II and III and citrate synthase did not differ significantly between the two groups. Protein expression, determined by Western blotting, did not differ between the groups, implying posttranslational perturbation. In the face of diminished total glutathione and coenzyme Q10 levels, oxidative modification was explored as an underlying cause of enzyme dysfunction. Of the three oxidative modifications measured, protein carbonylation was increased significantly by 31% in HF (P < 0.01; n = 18), whereas levels of 4-hydroxynonenal and protein nitration, although elevated, did not differ. Isolation of complexes I and IV and F1FoATP synthase by immunocapture revealed that proteins containing iron-sulphur or heme redox centers were targets of oxidative modification. Energy deficiency in end-stage failing human left ventricle involves impaired activity of key electron transport chain and Krebs cycle enzymes without altered expression of protein levels. Augmented oxidative modification of crucial enzyme subunit structures implicates dysfunction due to diminished capacity for management of mitochondrial reactive oxygen species, thus contributing further to reduced bioenergetics in human HF.

Assessment of Mitochondrial Dysfunction and Monoamine Oxidase Contribution to Oxidative Stress in Human Diabetic Hearts

Mitochondria-related oxidative stress is a pathomechanism causally linked to coronary heart disease (CHD) and diabetes mellitus (DM). Recently, mitochondrial monoamine oxidases (MAOs) have emerged as novel sources of oxidative stress in the cardiovascular system and experimental diabetes. The present study was purported to assess the mitochondrial impairment and the contribution of MAOs-related oxidative stress to the cardiovascular dysfunction in coronary patients with/without DM. Right atrial appendages were obtained from 75 patients randomized into 3 groups: (1) Control (CTRL), valvular patients without CHD; (2) CHD, patients with confirmed CHD; and (3) CHD-DM, patients with CHD and DM. Mitochondrial respiration was measured by high-resolution respirometry and MAOs expression was evaluated by RT-PCR and immunohistochemistry. Hydrogen peroxide (H₂O₂) emission was assessed by confocal microscopy and spectrophotometrically. The impairment of mitochondrial respiration was substrate-independent in CHD-DM group. MAOs expression was comparable among the groups, with the predominance of MAO-B isoform but no significant differences regarding oxidative stress were detected by either method. Incubation of atrial samples with MAOs inhibitors significantly reduced the H₂O₂ in all groups. In conclusion, abnormal mitochondrial respiration occurs in CHD and is more severe in DM and MAOs contribute to oxidative stress in human diseased hearts with/without DM.

Contribution of inorganic polyphosphate towards regulation of mitochondrial free calcium

BACKGROUND

Calcium signaling plays a key role in the regulation of multiple processes in mammalian mitochondria, from cellular bioenergetics to the induction of stress-induced cell death. While the total concentration of calcium inside the mitochondria can increase by several orders of magnitude, the concentration of bioavailable free calcium in mitochondria is maintained within the micromolar range by the mitochondrial calcium buffering system. This calcium buffering system involves the participation of inorganic phosphate. However, the mechanisms of its function are not yet understood. Specifically, it is not clear how calcium-orthophosphate interactions, which normally lead to formation of insoluble precipitates, are capable to dynamically regulate free calcium concentration. Here we test the hypothesis that inorganic polyphosphate, which is a polymerized form of orthophosphate, is capable to from soluble complexes with calcium, playing a significant role in the regulation of the mitochondrial free calcium concentration.

METHODS

We used confocal fluorescence microscopy to measure the relative levels of mitochondrial free calcium in cultured hepatoma cells (HepG2) with variable levels of inorganic polyphosphate (polyP).

RESULTS

The depletion of polyP leads to the significantly lower levels of mitochondrial free calcium concentration under conditions of pathological calcium overload. These results are coherent with previous observations showing that inorganic polyphosphate (polyP) can inhibit calcium-phosphate precipitation and, thus, increase the amount of free calcium.

CONCLUSIONS

Inorganic polyphosphate plays an important role in the regulation of mitochondrial free calcium, leading to its significant increase.

GENERAL SIGNIFICANCE

Inorganic polyphosphate is a previously unrecognized integral component of the mitochondrial calcium buffering system.

Mitochondrial oxidative metabolism and uncoupling proteins in the failing heart

Despite significant progress in cardiovascular medicine, myocardial ischemia and infarction, progressing eventually to the final end point heart failure (HF), remain the leading cause of morbidity and mortality in the USA. HF is a complex syndrome that results from any structural or functional impairment in ventricular filling or blood ejection. Ultimately, the heart's inability to supply the body's tissues with enough blood may lead to death. Mechanistically, the hallmarks of the failing heart include abnormal energy metabolism, increased production of reactive oxygen species (ROS) and defects in excitation-contraction coupling. HF is a highly dynamic pathological process, and observed alterations in cardiac metabolism and function depend on the disease progression. In the early stages, cardiac remodeling characterized by normal or slightly increased fatty acid (FA) oxidation plays a compensatory, cardioprotective role. However, upon progression of HF, FA oxidation and mitochondrial oxidative activity are decreased, resulting in a significant drop in cardiac ATP levels. In HF, as a compensatory response to decreased oxidative metabolism, glucose uptake and glycolysis are upregulated, but this upregulation is not sufficient to compensate for a drop in ATP production. Elevated mitochondrial ROS generation and ROS-mediated damage, when they overwhelm the cellular antioxidant defense system, induce heart injury and contribute to the progression of HF. Mitochondrial uncoupling proteins (UCPs), which promote proton leak across the inner mitochondrial membrane, have emerged as essential regulators of mitochondrial membrane potential, respiratory activity and ROS generation. Although the physiological role of UCP2 and UCP3, expressed in the heart, has not been clearly established, increasing evidence suggests that these proteins by promoting mild uncoupling could reduce mitochondrial ROS generation and cardiomyocyte apoptosis and ameliorate thereby myocardial function. Further investigation on the alterations in cardiac UCP activity and regulation will advance our understanding of their physiological roles in the healthy and diseased heart and also may facilitate the development of novel and more efficient therapies.

Metabolic abnormalities of the heart in type II diabetes

Type 2 diabetes mellitus escalates the risk of heart failure partly via its ability to induce a cardiomyopathic state that is independent of coronary artery disease and hypertension. Although the pathogenesis of diabetic cardiomyopathy has yet to be fully elucidated, aberrations in cardiac substrate metabolism and energetics are thought to be key drivers. These aberrations include excessive fatty acid utilisation and storage, suppressed glucose oxidation and impaired mitochondrial oxidative phosphorylation. An appreciation of how these abnormalities arise and synergise to promote adverse cardiac remodelling is critical to their effective amelioration. This review focuses on disturbances in myocardial fuel (fatty acids and glucose) flux and energetics in type 2 diabetes, how these disturbances relate to the development of diabetic cardiomyopathy and the potential therapeutic agents that could be used to correct them.

Teaching the fundamentals of electron transfer reactions in mitochondria and the production and detection of reactive oxygen species

Mitochondria fulfill a number of biological functions which inherently depend on ATP and O2(-•)/H2O2 production. Both ATP and O2(-•)/H2O2 are generated by electron transfer reactions. ATP is the product of oxidative phosphorylation whereas O2(-•) is generated by singlet electron reduction of di-oxygen (O2). O2(-•) is then rapidly dismutated by superoxide dismutase (SOD) producing H2O2. O2(-•)/H2O2 were once viewed as unfortunately by-products of aerobic respiration. This characterization is fitting considering over production of O2(-•)/H2O2 by mitochondria is associated with range of pathological conditions and aging. However, O2(-•)/H2O2 are only dangerous in large quantities. If produced in a controlled fashion and maintained at a low concentration, cells can benefit greatly from the redox properties of O2(-•)/H2O2. Indeed, low rates of O2(-•)/H2O2 production are required for intrinsic mitochondrial signaling (e.g. modulation of mitochondrial processes) and communication with the rest of the cell. O2(-•)/H2O2 levels are kept in check by anti-oxidant defense systems that sequester O2(-•)/H2O2 with extreme efficiency. Given the importance of O2(-•)/H2O2 in cellular function, it is imperative to consider how mitochondria produce O2(-•)/H2O2 and how O2(-•)/H2O2 genesis is regulated in conjunction with fluctuations in nutritional and redox states. Here, I discuss the fundamentals of electron transfer reactions in mitochondria and emerging knowledge on the 11 potential sources of mitochondrial O2(-•)/H2O2 in tandem with their significance in contributing to overall O2(-•)/H2O2 emission in health and disease. The potential for classifying these different sites in isopotential groups, which is essentially defined by the redox properties of electron donator involved in O2(-•)/H2O2 production, as originally suggested by Brand and colleagues is also surveyed in detail. In addition, redox signaling mechanisms that control O2(-•)/H2O2 genesis from these sites are discussed. Finally, the current methodologies utilized for measuring O2(-•)/H2O2 in isolated mitochondria, cell culture and in vivo are reviewed.

Thiamine Deficiency-Mediated Brain Mitochondrial Pathology in Alaskan Huskies with Mutation in SLC19A3.1

Alaskan Husky encephalopathy (AHE(1) ) is a fatal brain disease associated with a mutation in SLC19A3.1 (c.624insTTGC, c.625C>A). This gene encodes for a thiamine transporter 2 with a predominately (CNS) central nervous system distribution. Considering that brain is particularly vulnerable to thiamine deficiency because of its reliance on thiamine pyrophosphate (TPP)-dependent metabolic pathways involved in energy metabolism and neurotransmitter synthesis, we characterized the impact of this mutation on thiamine status, brain bioenergetics and the contribution of oxidative stress to this phenotype. In silico modeling of the mutated transporter indicated a significant loss of alpha-helices resulting in a more open protein structure suggesting an impaired thiamine transport ability. The cerebral cortex and thalamus of affected dogs were severely deficient in TPP-dependent enzymes accompanied by decreases in mitochondrial mass and oxidative phosphorylation (OXPHOS) capacity, and increases in oxidative stress. These results along with the behavioral and pathological findings indicate that the phenotype associated with AHE is consistent with a brain-specific thiamine deficiency, leading to brain mitochondrial dysfunction and increased oxidative stress. While some of the biochemical deficits, neurobehavior and affected brain areas in AHE were shared by Wernicke's and Korsakoff's syndromes, several differences were noted likely arising from a tissue-specific vs. that from a whole-body thiamine deficiency.

Regulation of metabolism in individual mitochondria during excitation-contraction coupling

The heart is an excitable organ that undergoes spontaneous force generation and relaxation cycles driven by excitation-contraction (EC) coupling. A fraction of the oscillating cytosolic Ca(2+) during each heartbeat is taken up by mitochondria to stimulate mitochondrial metabolism, the major source of energy in the heart. Whether the mitochondrial metabolism is regulated individually during EC coupling and whether this heterogeneous regulation bears any physiological or pathological relevance have not been studied. Here, we developed a novel approach to determine the regulation of individual mitochondrial metabolism during cardiac EC coupling. Through monitoring superoxide flashes, which are stochastic and bursting superoxide production events arising from increased metabolism in individual mitochondria, we found that EC coupling stimulated the metabolism in individual mitochondria as indicated by significantly increased superoxide flash activity during electrical stimulation of the cultured intact myocytes or perfused heart. Mechanistically, cytosolic calcium transients promoted individual mitochondria to take up calcium via mitochondrial calcium uniporter, which subsequently triggered transient opening of the permeability transition pore and stimulated metabolism and bursting superoxide flash in that mitochondrion. The bursting superoxide, in turn, promoted local calcium release. In the early stage of heart failure, EC coupling regulation of superoxide flashes was compromised. This study highlights the heterogeneity in the regulation of cardiac mitochondrial metabolism, which may contribute to local redox signaling.

Cardiac, skeletal, and smooth muscle mitochondrial respiration: are all mitochondria created equal?

Unlike cardiac and skeletal muscle, little is known about vascular smooth muscle mitochondrial respiration. Therefore, the present study examined mitochondrial respiratory rates in smooth muscle of healthy human feed arteries and compared with that of healthy cardiac and skeletal muscles. Cardiac, skeletal, and smooth muscles were harvested from a total of 22 subjects (53 ± 6 yr), and mitochondrial respiration was assessed in permeabilized fibers. Complex I + II, state 3 respiration, an index of oxidative phosphorylation capacity, fell progressively from cardiac to skeletal to smooth muscles (54 ± 1, 39 ± 4, and 15 ± 1 pmol·s(-1)·mg(-1), P < 0.05, respectively). Citrate synthase (CS) activity, an index of mitochondrial density, also fell progressively from cardiac to skeletal to smooth muscles (222 ± 13, 115 ± 2, and 48 ± 2 μmol·g(-1)·min(-1), P < 0.05, respectively). Thus, when respiration rates were normalized by CS (respiration per mitochondrial content), oxidative phosphorylation capacity was no longer different between the three muscle types. Interestingly, complex I state 2 normalized for CS activity, an index of nonphosphorylating respiration per mitochondrial content, increased progressively from cardiac to skeletal to smooth muscles, such that the respiratory control ratio, state 3/state 2 respiration, fell progressively from cardiac to skeletal to smooth muscles (5.3 ± 0.7, 3.2 ± 0.4, and 1.6 ± 0.3 pmol·s(-1)·mg(-1), P < 0.05, respectively). Thus, although oxidative phosphorylation capacity per mitochondrial content in cardiac, skeletal, and smooth muscles suggest all mitochondria are created equal, the contrasting respiratory control ratio and nonphosphorylating respiration highlight the existence of intrinsic functional differences between these muscle mitochondria. This likely influences the efficiency of oxidative phosphorylation and could potentially alter ROS production.

(-)-Epigallocatechin-3-gallate attenuated myocardial mitochondrial dysfunction and autophagy in diabetic Goto-Kakizaki rats

Type 2 diabetes mellitus (T2DM) is a risk factor for heart disease. However, the mechanisms of T2DM involvement in cardiac complications are still unclear. In the present study, we investigated mitochondria-related mechanisms underlying the pathogenesis of myocardial disorders in diabetic Goto-Kakizaki (GK) rats. We found that remarkable myocardial mitochondrial deficiency and dysfunction as well as oxidative stress occurred in the heart of GK rats. In addition, our results suggested that the loss of mitochondria was in response to elevated autophagy and upstream FoxO factors in diabetic myocardium. More importantly, (-)-epigallocatechin-3-gallate (EGCG), a polyphenol derived from green tea, successfully improved mitochondrial function and autophagy in the heart of GK rats. Our findings revealed that diabetes-associated myocardial mitochondrial deficiency and dysfunction was associated with enhanced autophagy in myocardium, and EGCG might be a potential agent in preventing and treating myocardial disorders involved in diabetes.

Profiling of Nutrient Transporter Expression in Human Stem Cell–Derived Cardiomyocytes Exposed to Tyrosine Kinase Inhibitor Anticancer Drugs Using RBD Ligands

We applied a novel profiling approach using receptor binding domain (RBD) ligands to cell surface domains of a panel of nutrient transporters to characterize the impact of a number of tyrosine kinase inhibitor anticancer drugs on human stem cell–derived cardiomyocytes. High-content screening and flow cytometry analysis showed diagnostic changes in nutrient transporter expression correlating with glycolysis and oxidative phosphorylation–based cell metabolism in glucose and galactose media. Cluster analysis of RBD binding signatures of drug-treated cells cultured in glucose medium showed good correlation with sensitization of mitochondrial toxicity in cells undergoing oxidative phosphorylation in galactose medium. These data demonstrate the potential for RBD ligands as profiling tools to improve the clinical predictivity of in vitro cell assays for drug toxicity.

Formation and repair of oxidative damage in the mitochondrial DNA

The mitochondrial DNA (mtDNA) encodes for only 13 polypeptides, components of 4 of the 5 oxidative phosphorylation complexes. But despite this apparently small numeric contribution, all 13 subunits are essential for the proper functioning of the oxidative phosphorylation circuit. Thus, accumulation of lesions, mutations and deletions/insertions in the mtDNA could have severe functional consequences, including mitochondrial diseases, aging and age-related diseases. The DNA is a chemically unstable molecule, which can be easily oxidized, alkylated, deaminated and suffer other types of chemical modifications, throughout evolution the organisms that survived were those who developed efficient DNA repair processes. In the last two decades, it has become clear that mitochondria have DNA repair pathways, which operate, at least for some types of lesions, as efficiently as the nuclear DNA repair pathways. The mtDNA is localized in a particularly oxidizing environment, making it prone to accumulate oxidatively generated DNA modifications (ODMs). In this article, we: i) review the major types of ODMs formed in mtDNA and the known repair pathways that remove them; ii) discuss the possible involvement of other repair pathways, just recently characterized in mitochondria, in the repair of these modifications; and iii) address the role of DNA repair in mitochondrial function and a possible cross-talk with other pathways that may potentially participate in mitochondrial genomic stability, such as mitochondrial dynamics and nuclear-mitochondrial signaling. Oxidative stress and ODMs have been increasingly implicated in disease and aging, and thus we discuss how variations in DNA repair efficiency may contribute to the etiology of such conditions or even modulate their clinical outcomes.

Freshly isolated mitochondria from failing human hearts exhibit preserved respiratory function

In heart failure mitochondrial dysfunction is thought to be responsible for energy depletion and contractile dysfunction. The difficulties in procuring fresh left ventricular (LV) myocardium from humans for assessment of mitochondrial function have resulted in the reliance on surrogate markers of mitochondrial function and limited our understanding of cardiac energetics. We isolated mitochondria from fresh LV wall tissue of patients with heart failure and reduced systolic function undergoing heart transplant or left ventricular assist device placement, and compared their function to mitochondria isolated from the non-failing LV (NFLV) wall tissue with normal systolic function from patients with pulmonary hypertension undergoing heart-lung transplant. We performed detailed mitochondrial functional analyses using 4 substrates: glutamate-malate (GM), pyruvate-malate (PM) palmitoyl carnitine-malate (PC) and succinate. NFLV mitochondria showed preserved respiratory control ratios and electron chain integrity with only few differences for the 4 substrates. In contrast, HF mitochondria had greater respiration with GM, PM and PC substrates and higher electron chain capacity for PM than for PC. Surprisingly, HF mitochondria had greater respiratory control ratios and lower ADP-independent state 4 rates than NFLV mitochondria for GM, PM and PC substrates demonstrating that HF mitochondria are capable of coupled respiration ex vivo. Gene expression studies revealed decreased expression of key genes in pathways for oxidation of both fatty acids and glucose. Our results suggest that mitochondria from the failing LV myocardium are capable of tightly coupled respiration when isolated and supplied with ample substrates. Thus energy starvation in the failing heart may be the result of dysregulation of metabolic pathways, impaired substrate supply or reduced mitochondrial number but not the result of reduced mitochondrial electron transport capacity.

Confocal imaging of single mitochondrial superoxide flashes in intact heart or in vivo

Mitochondrion is a critical intracellular organelle responsible for energy production and intracellular signaling in eukaryotic systems. Mitochondrial dysfunction often accompanies and contributes to human disease. Majority of the approaches that have been developed to evaluate mitochondrial function and dysfunction are based on in vitro or ex vivo measurements. Results from these experiments have limited ability in determining mitochondrial function in vivo. Here, we describe a novel approach that utilizes confocal scanning microscopy for the imaging of intact tissues in live aminals, which allows the evaluation of single mitochondrial function in a real-time manner in vivo. First, we generate transgenic mice expressing the mitochondrial targeted superoxide indicator, circularly permuted yellow fluorescent protein (mt-cpYFP). Anesthetized mt-cpYFP mouse is fixed on a custom-made stage adaptor and time-lapse images are taken from the exposed skeletal muscles of the hindlimb. The mouse is subsequently sacrificed and the heart is set up for Langendorff perfusion with physiological solutions at 37 °C. The perfused heart is positioned in a special chamber on the confocal microscope stage and gentle pressure is applied to immobilize the heart and suppress heart beat induced motion artifact. Superoxide flashes are detected by real-time 2D confocal imaging at a frequency of one frame per second. The perfusion solution can be modified to contain different respiration substrates or other fluorescent indicators. The perfusion can also be adjusted to produce disease models such as ischemia and reperfusion. This technique is a unique approach for determining the function of single mitochondrion in intact tissues and in vivo.