Divergent mitochondrial biogenesis responses in human cardiomyopathy

Krüppel like factor 7 regulates mitochondrial dynamics balance in myocardial infarction

Targeting the balance of mitochondrial fission and fusion can effectively alleviate the cardiac energy supply efficiency, to restore cardiac systolic dysfunction and reduce mortality. We previously found that Klf7 is closely related to cardiac energy metabolism. Here we generated cardiomyocyte-specific Klf7 knockout and overexpression mice that underwent myocardial infarction (MI) surgery. Klf7 expression increased in the ischemic myocardium of mice, and cardiomyocyte-specific knockout Klf7 significantly lowered the mortality of MI-inflicted mice and improved ATP insufficiency in MI. Subsequently, Klf7 overexpression aggravated adverse cardiac remodeling and mitochondrial fission and fusion imbalance after MI. Our results also demonstrated that Klf7 inhibited mitochondrial fusion and promoted mitochondrial fission by targeting prohibitin 2 (Phb2) and mitofusin 2 (Mfn2). Our study revealed a crucial role in upholding the overall balance of mitochondrial fission and fusion during MI. Furthermore, our findings indicated that the Klf7/Mfn2/Phb2 axis holds promise as a potential target for therapeutic interventions of MI.

Myocardial inflammation is associated with impaired mitochondrial oxidative capacity in ischaemic cardiomyopathy

AIMS

Myocardial inflammation and impaired mitochondrial oxidative capacity are hallmarks of heart failure (HF) pathophysiology. The extent of myocardial inflammation in patients suffering from ischaemic cardiomyopathy (ICM) or dilated cardiomyopathy (DCM) and its association with mitochondrial energy metabolism are unknown. We aimed at establishing a relevant role of cardiac inflammation in the impairment of mitochondrial energy production in advanced ischaemic and non-ischaemic HF.

METHODS

We included 81 patients with stage D HF (ICM, n = 44; DCM, n = 37) undergoing left ventricular assist device implantation (n = 59) or heart transplantation (n = 22) and obtained left ventricular tissue samples during open heart surgery. We quantified mitochondrial oxidative capacity, citrate synthase activity (CSA) and fibrosis and lymphocytic infiltration. We considered infiltration of >14 CD3+ cells/mm2 relevant inflammation.

RESULTS

Patients with ICM or DCM did not differ regarding age (61.5 ± 5.7 vs. 56.5 ± 12.7 years, P = 0.164), sex (86% vs. 84% male, P = 0.725), type 2 diabetes mellitus (34% vs. 18%, P = 0.126) or chronic kidney disease (8% vs. 11%, P = 0.994). ICM exhibited oxidative capacity reduced by 23% compared to DCM (108.6 ± 41.4 vs. 141.9 ± 59.9 pmol/(s*mg), P = 0.006). Maximum production of reactive oxygen species was not significantly different between ICM and DCM (0.59 ± 0.28 vs. 0.69 ± 0.36 pmol/(s*ml), P = 0.196). Mitochondrial content, detected by CSA, was lower in ICM (359.6 ± 164.1 vs. 503.0 ± 198.5 nmol/min/mg protein, P = 0.002). Notably, relevant inflammation was more common in ICM (27% vs. 6%, P = 0.024), and the absolute number of infiltrating leucocytes correlated with lower oxidative capacity (r = -0.296, P = 0.019). Fibrosis was more prevalent in ICM (20.9 ± 21.2 vs. 7.2 ± 5.6% of area, P = 0.002), but not associated with oxidative capacity (r = -0.13, P = 0.327).

CONCLUSIONS

More than every fourth ICM patient with advanced HF displays myocardial inflammation in the range of inflammatory cardiomyopathy associated with reduced mitochondrial oxidative capacity. Future studies may evaluate inflammation in ICM at earlier stages in standardised fashion to explore the therapeutic potential of immunosuppression to influence trajectories of HF in ICM.

Ultrastructure analysis of mitochondria, lipid droplet and sarcoplasmic reticulum apposition in human heart failure

Background

Cardiomyocyte structural remodeling is reported as a causal contributor to heart failure (HF) development and progression. Growing evidence highlights the role of organelle apposition in cardiomyocyte function and homeostasis. Disruptions in organelle crosstalk, such as that between the sarcoplasmic reticulum (SR) and mitochondria, are thought to impact numerous cellular processes such as calcium handling and cellular bioenergetics; two processes that are disrupted and implicated in cardiac pathophysiology. While the physical distance between organelles is thought to be essential for homeostatic cardiomyocyte function, whether the interactions and coupling of organelles are altered in human heart failure remains unclear.

Methods

Here, we utilized transmission electron microscopy and careful quantification of ultrastructure to characterize the changes in organelle apposition in cardiomyocytes isolated from the hearts of patients diagnosed with various types of HF. Subsequently we employed molecular approaches to examine the expression of proposed organelle tethers.

Results

We demonstrate that cardiomyocytes isolated from dilated cardiomyopathy, hypertrophic cardiomyopathy and ischemic cardiomyopathy hearts display smaller, more rounded mitochondria, as compared to nonfailing controls. Failing cardiomyocytes also exhibited disrupted SR-mitochondria juxtaposition and changes in the expression of proposed molecular tethers. Further analysis revealed alterations in lipid droplet dynamics including decreased lipid droplet content and less lipid droplets in association with mitochondria in failing cardiomyocytes.

Conclusion

Here we observed changes in organelle dynamics in cardiomyocytes isolated from heart failure patients diagnosed with differing etiologies. Our results suggest that organelle structure and apposition may be a ubiquitous contributor to human HF progression.

The Unexplored Role of Mitochondria-Related Oxidative Stress in Diverticular Disease

The pathophysiology of diverticular disease (DD) is not well outlined. Recent studies performed on the DD human ex vivo model have shown the presence of a predominant transmural oxidative imbalance whose origin remains unknown. Considering the central role of mitochondria in oxidative stress, the present study evaluates their involvement in the alterations of DD clinical phenotypes. Colonic surgical samples of patients with asymptomatic diverticulosis, complicated DD, and controls were analyzed. Electron microscopy, protein expression, and cytofluorimetric analyses were performed to assess the contribution of mitochondrial oxidative stress. Functional muscle activity was tested on cells in response to contractile and relaxant agents. To assess the possibility of reverting oxidative damages, N-acetylcysteine was tested on an in vitro model. Compared with the controls, DD tissues showed a marketed increase in mitochondrial number and fusion accompanied by the altered mitochondrial electron transport chain complexes. In SMCs, the mitochondrial mass increase was accompanied by altered mitochondrial metabolic activity supported by a membrane potential decrease. Ulteriorly, a decrease in antioxidant content and altered contraction-relaxation dynamics reverted by N-acetylcysteine were observed. Therefore, the oxidative stress-driven alterations resulted in mitochondrial impairment. The beneficial effects of antioxidant treatments open new possibilities for tailored therapeutic strategies that have not been tested for this disease.

Quantitative 3D electron microscopy characterization of mitochondrial structure, mitophagy, and organelle interactions in murine atrial fibrillation

Atrial fibrillation (AF) is the most common clinical arrhythmia, however there is limited understanding of its pathophysiology including the cellular and ultrastructural changes rendered by the irregular rhythm, which limits pharmacological therapy development. Prior work has demonstrated the importance of reactive oxygen species (ROS) and mitochondrial dysfunction in the development of AF. Mitochondrial structure, interactions with other organelles such as sarcoplasmic reticulum (SR) and T-tubules (TT), and degradation of dysfunctional mitochondria via mitophagy are important processes to understand ultrastructural changes due to AF. However, most analysis of mitochondrial structure and interactome in AF has been limited to two-dimensional (2D) modalities such as transmission electron microscopy (EM), which does not fully visualize the morphological evolution of the mitochondria during mitophagy. Herein, we utilize focused ion beam-scanning electron microscopy (FIB-SEM) and perform reconstruction of three-dimensional (3D) EM from murine left atrial samples and measure the interactions of mitochondria with SR and TT. We developed a novel 3D quantitative analysis of FIB-SEM in a murine model of AF to quantify mitophagy stage, mitophagosome size in cardiomyocytes, and mitochondrial structural remodeling when compared with control mice. We show that in our murine model of spontaneous and continuous AF due to persistent late sodium current, left atrial cardiomyocytes have heterogenous mitochondria, with a significant number which are enlarged with increased elongation and structural complexity. Mitophagosomes in AF cardiomyocytes are located at Z-lines where they neighbor large, elongated mitochondria. Mitochondria in AF cardiomyocytes show increased organelle interaction, with 5X greater contact area with SR and are 4X as likely to interact with TT when compared to control. We show that mitophagy in AF cardiomyocytes involves 2.5X larger mitophagosomes that carry increased organelle contents. In conclusion, when oxidative stress overcomes compensatory mechanisms, mitophagy in AF faces a challenge of degrading bulky complex mitochondria, which may result in increased SR and TT contacts, perhaps allowing for mitochondrial Ca2+ maintenance and antioxidant production.

Myocardial ultrastructure of human heart failure with preserved ejection fraction

Over half of patients with heart failure have a preserved ejection fraction (>50%, called HFpEF), a syndrome with substantial morbidity/mortality and few effective therapies1. Its dominant comorbidity is now obesity, which worsens disease and prognosis1-3. Myocardial data from patients with morbid obesity and HFpEF show depressed myocyte calcium-stimulated tension4 and disrupted gene expression of mitochondrial and lipid metabolic pathways5,6, abnormalities shared by human HF with a reduced EF but less so in HFpEF without severe obesity. The impact of severe obesity on human HFpEF myocardial ultrastructure remains unexplored. Here we assessed the myocardial ultrastructure in septal biopsies from patients with HFpEF using transmission electron microscopy. We observed sarcomere disruption and sarcolysis, mitochondrial swelling with cristae separation and dissolution and lipid droplet accumulation that was more prominent in the most obese patients with HFpEF and not dependent on comorbid diabetes. Myocardial proteomics revealed associated reduction in fatty acid uptake, processing and oxidation and mitochondrial respiration proteins, particularly in very obese patients with HFpEF.

Exploring Changes in Myocyte Structure, Contractility, and Energetics From Mechanical Unloading in Patients With Heart Failure Undergoing Ventricular Assist Device Implantation: A Systematic Review and Meta-Analysis

AIMS

Recent reports of myocardial recovery after mechanical unloading with left ventricular assist devices (LVADs) have challenged the prevailing notion that end-stage heart failure (HF) is irreversible. To improve our understanding of this phenomenon, we comprehensively analysed the structural, functional, and energetic changes in failing human cardiomyocytes after LVAD implantation.

METHODS

Based on a prospectively registered protocol (PROSPERO-CRD42022380214), 30 eligible studies were identified from 940 records with a pooled population of 648 patients predominantly with non-ischaemic cardiomyopathy.

RESULTS

LVAD led to a substantial regression in myocyte size similar to that of donor hearts (standardised mean difference, -1.29; p<0.001). The meta-regression analysis revealed that HF duration was a significant modifier on the changes in myocyte size. There were some suggestions of fibrosis reversal (-5.17%; p=0.009); however, this was insignificant after sensitivity analysis. Developed force did not improve in cardiac trabeculae (n=5 studies); however, non-physiological isometric contractions were tested. At the myocyte level (n=4 studies), contractile kinetics improved where the time-to-peak force reduced by 41.7%-50.7% and time to 50% relaxation fell by 47.4%-62.1% (p<0.05). Qualitatively, LVAD enhanced substrate utilisation and mitochondrial function (n=6 studies). Most studies were at a high risk of bias.

CONCLUSION

The regression of maladaptive hypertrophy, partial fibrosis reversal, and normalisation in metabolic pathways after LVAD may be a testament to the heart's remarkable plasticity, even in the advanced stages of HF. However, inconsistencies exist in force-generating capabilities. Using more physiological force-length work-loop assays, addressing the high risks of bias and clinical heterogeneity are crucial to better understand the phenomenon of reverse remodelling.

SIRT6 in Regulation of Mitochondrial Damage and Associated Cardiac Dysfunctions: A Possible Therapeutic Target for CVDs

Cardiovascular diseases (CVDs) can be described as a global health emergency imploring possible prevention strategies. Although the pathogenesis of CVDs has been extensively studied, the role of mitochondrial dysfunction in CVD development has yet to be investigated. Diabetic cardiomyopathy, ischemic-reperfusion injury, and heart failure are some of the CVDs resulting from mitochondrial dysfunction Recent evidence from the research states that any dysfunction of mitochondria has an impact on metabolic alteration, eventually causes the death of a healthy cell and therefore, progressively directing to the predisposition of disease. Cardiovascular research investigating the targets that both protect and treat mitochondrial damage will help reduce the risk and increase the quality of life of patients suffering from various CVDs. One such target, i.e., nuclear sirtuin SIRT6 is strongly associated with cardiac function. However, the link between mitochondrial dysfunction and SIRT6 concerning cardiovascular pathologies remains poorly understood. Although the Role of SIRT6 in skeletal muscles and cardiomyocytes through mitochondrial regulation has been well understood, its specific role in mitochondrial maintenance in cardiomyocytes is poorly determined. The review aims to explore the domain-specific function of SIRT6 in cardiomyocytes and is an effort to know how SIRT6, mitochondria, and CVDs are related.

The link between obesity and aging - insights into cardiac energy metabolism

Obesity and aging are well-established risk factors for a range of diseases, including cardiovascular diseases and type 2 diabetes. Given the escalating prevalence of obesity, the aging population, and the subsequent increase in cardiovascular diseases, it is crucial to investigate the underlying mechanisms involved. Both aging and obesity have profound effects on the energy metabolism through various mechanisms, including metabolic inflexibility, altered substrate utilization for energy production, deregulated nutrient sensing, and mitochondrial dysfunction. In this review, we aim to present and discuss the hypothesis that obesity, due to its similarity in changes observed in the aging heart, may accelerate the process of cardiac aging and exacerbate the clinical outcomes of elderly individuals with obesity.

Atrial natriuretic peptide stimulates autophagy/mitophagy and improves mitochondrial function in chronic heart failure

Mitochondrial dysfunction, causing increased reactive oxygen species (ROS) production, is a molecular feature of heart failure (HF). A defective antioxidant response and mitophagic flux were reported in circulating leucocytes of patients with chronic HF and reduced ejection fraction (HFrEF). Atrial natriuretic peptide (ANP) exerts many cardiac beneficial effects, including the ability to protect cardiomyocytes by promoting autophagy. We tested the impact of ANP on autophagy/mitophagy, altered mitochondrial structure and function and increased oxidative stress in HFrEF patients by both ex vivo and in vivo approaches. The ex vivo study included thirteen HFrEF patients whose peripheral blood mononuclear cells (PBMCs) were isolated and treated with αANP (10^-11 M) for 4 h. The in vivo study included six HFrEF patients who received sacubitril/valsartan for two months. PBMCs were characterized before and after treatment. Both approaches analyzed mitochondrial structure and functionality. We found that levels of αANP increased upon sacubitril/valsartan, whereas levels of NT-proBNP decreased. Both the ex vivo direct exposure to αANP and the higher αANP level upon in vivo treatment with sacubitril/valsartan caused: (i) improvement of mitochondrial membrane potential; (ii) stimulation of the autophagic process; (iii) significant reduction of mitochondrial mass-index of mitophagy stimulation-and upregulation of mitophagy-related genes; (iv) reduction of mitochondrial damage with increased inner mitochondrial membrane (IMM)/outer mitochondrial membrane (OMM) index and reduced ROS generation. Herein we demonstrate that αANP stimulates both autophagy and mitophagy responses, counteracts mitochondrial dysfunction, and damages ultimately reducing mitochondrial oxidative stress generation in PBMCs from chronic HF patients. These properties were confirmed upon sacubitril/valsartan administration, a pivotal drug in HFrEF treatment.

Novel insights into the involvement of mitochondrial fission/fusion in heart failure: From molecular mechanisms to targeted therapies

Mitochondria are dynamic organelles that alter their morphology through fission (fragmentation) and fusion (elongation). These morphological changes correlate highly with mitochondrial functional adaptations to stressors, such as hypoxia, pressure overload, and inflammation, and are important in the setting of heart failure. Pathological mitochondrial remodeling, characterized by increased fission and reduced fusion, is associated with impaired mitochondrial respiration, increased mitochondrial oxidative stress, abnormal cytoplasmic calcium handling, and increased cardiomyocyte apoptosis. Considering the impact of the mitochondrial morphology on mitochondrial behavior and cardiomyocyte performance, altered mitochondrial dynamics could be expected to induce or exacerbate the pathogenesis and progression of heart failure. However, whether alterations in mitochondrial fission and fusion accelerate or retard the progression of heart failure has been the subject of intense debate. In this review, we first describe the physiological processes and regulatory mechanisms of mitochondrial fission and fusion. Then, we extensively discuss the pathological contributions of mitochondrial fission and fusion to heart failure. Lastly, we examine potential therapeutic approaches targeting mitochondrial fission/fusion to treat patients with heart failure.

Comprehensive Analysis of Mitochondrial Dynamics Alterations in Heart Diseases

The most common alterations affecting mitochondria, and associated with cardiac pathological conditions, implicate a long list of defects. They include impairments of the mitochondrial electron transport chain activity, which is a crucial element for energy formation, and that determines the depletion of ATP generation and supply to metabolic switches, enhanced ROS generation, inflammation, as well as the dysregulation of the intracellular calcium homeostasis. All these signatures significantly concur in the impairment of cardiac electrical characteristics, loss of myocyte contractility and cardiomyocyte damage found in cardiac diseases. Mitochondrial dynamics, one of the quality control mechanisms at the basis of mitochondrial fitness, also result in being dysregulated, but the use of this knowledge for translational and therapeutic purposes is still in its infancy. In this review we tried to understand why this is, by summarizing methods, current opinions and molecular details underlying mitochondrial dynamics in cardiac diseases.

Peripheral Blood Mononuclear Cells Mitochondrial Respiration and Superoxide Anion after Heart Transplantation

INTRODUCTION

The mitochondrial function of circulating peripheral blood mononuclear cells (PBMCs) is an interesting new approach to cardiac diseases. Thus, PBMC's mitochondrial respiration decreases in relation to heart failure severity. However, no data are available on heart-transplanted patients (Htx).

POPULATION AND METHODS

We determined PBMCs mitochondrial respiration by high-resolution respirometry (Oroboros Instruments) and superoxide anion production using electron paramagnetic resonance (Bruker-Biospin) in 20 healthy subjects and 20 matched Htx and investigated clinical, biological, echocardiographic, coronarography and biopsy characteristics.

RESULTS

PBMCs mitochondrial respiratory chain complex II respiration was decreased in Htx (4.69 ± 0.84 vs. 7.69 ± 1.00 pmol/s/million cell in controls and Htx patients, respectively; p = 0.007) and complex IV respiration was increased (24.58 ± 2.57 vs. 15.68 ± 1.67 pmol/s/million cell; p = 0.0035). Superoxide anion production was also increased in Htx (1.47 ± 0.10 vs. 1.15 ± 0.10 µmol/min; p = 0.041). The leucocyte-to-lymphocyte ratio was increased in Htx, whom complex II correlated with leucocyte number (r = 0.51, p = 0.02) and with the left ventricular posterior wall peak early diastolic myocardial velocity (r = -0.62, p = 0.005). Complex IV was increased in the two patients with acute rejection and correlated negatively with Htx's isovolumetric relation time (r = -0.45, p = 0.045).

DISCUSSION

Although presenting with normal systolic function, Htx demonstrated abnormal PBMC's mitochondrial respiration. Unlike immunosuppressive therapies, subclinical diastolic dysfunction might be involved in these changes. Additionally, lymphopenia might reduce complex II, and acute rejection enhances complex IV respirations.

CONCLUSION

PBMC's mitochondrial respiration appears modified in Htx, potentially linked to cellular shift, mild diastolic dysfunction and/or acute rejection.

Inefficient Batteries in Heart Failure: Metabolic Bottlenecks Disrupting the Mitochondrial Ecosystem

Mitochondrial abnormalities have long been described in the setting of cardiomyopathies and heart failure (HF), yet the mechanisms of mitochondrial dysfunction in cardiac pathophysiology remain poorly understood. Many studies have described HF as an energy-deprived state characterized by a decline in adenosine triphosphate production, largely driven by impaired oxidative phosphorylation. However, impairments in oxidative phosphorylation extend beyond a simple decline in adenosine triphosphate production and, in fact, reflect pervasive metabolic aberrations that cannot be fully appreciated from the isolated, often siloed, interrogation of individual aspects of mitochondrial function. With the application of broader and deeper examinations into mitochondrial and metabolic systems, recent data suggest that HF with preserved ejection fraction is likely metabolically disparate from HF with reduced ejection fraction. In our review, we introduce the concept of the mitochondrial ecosystem, comprising intricate systems of metabolic pathways and dynamic changes in mitochondrial networks and subcellular locations. The mitochondrial ecosystem exists in a delicate balance, and perturbations in one component often have a ripple effect, influencing both upstream and downstream cellular pathways with effects enhanced by mitochondrial genetic variation. Expanding and deepening our vantage of the mitochondrial ecosystem in HF is critical to identifying consistent metabolic perturbations to develop therapeutics aimed at preventing and improving outcomes in HF.

Lipid-independent activation of a muscle-specific PKCα splicing variant

Protein kinase C-α (PKCα) plays a major role in a diverse range of cellular processes. Studies to date have defined the regulatory controls and function of PKCα entirely based upon the previously annotated ubiquitously expressed prototypical isoform. From RNA-seq-based transcriptome analysis in murine heart, we identified a previously unannotated PKCα variant produced by alternative RNA splicing. This PKCα transcript variant, which we named PKCα-novel exon (PKCα-NE), contains an extra exon between exon 16 and exon 17, and is specifically detected in adult mouse cardiac and skeletal muscle, but not other tissues; it is also detected in human hearts. This transcript variant yields a PKCα isoform with additional 16 amino acids inserted in its COOH-terminal variable region. Although the canonical PKCα enzyme is a lipid-dependent kinase, in vitro kinase assays show that PKCα-NE displays a high level of basal lipid-independent catalytic activity. Our unbiased proteomic analysis identified a specific interaction between PKCα-NE and eukaryotic elongation factor-1α (eEF1A1). Studies in cardiomyocytes link PKCα-NE expression to an increase in eEF1A1 phosphorylation and elevated protein synthesis. In summary, we have identified a previously uncharacterized muscle-specific PKCα splicing variant, PKCα-NE, with distinct biochemical properties that plays a unique role in the control of the protein synthesis machinery in cardiomyocytes.NEW & NOTEWORTHY PKCα is an important signaling molecule extensively studied in many cellular processes. However, no isoforms have been reported for PKCα except one prototypic isoform. Alternative mRNA splicing of Prkca gene was detected for the first time in rodent and human cardiac tissue, which can produce a previously unknown PKCα-novel exon (NE) isoform. The biochemistry and molecular effects of PKCα-NE are markedly different from PKCα wild type, suggesting potential functional diversity of PKCα signaling in muscle.

Loss of Nuclear Envelope Integrity and Increased Oxidant Production Cause DNA Damage in Adult Hearts Deficient in PKP2: A Molecular Substrate of ARVC

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is characterized by high propensity to life-threatening arrhythmias and progressive loss of heart muscle. More than 40% of reported genetic variants linked to ARVC reside in the PKP2 gene, which encodes the PKP2 protein (plakophilin-2).

METHODS

We describe a comprehensive characterization of the ARVC molecular landscape as determined by high-resolution mass spectrometry, RNA sequencing, and transmission electron microscopy of right ventricular biopsy samples obtained from patients with ARVC with PKP2 mutations and left ventricular ejection fraction >45%. Samples from healthy relatives served as controls. The observations led to experimental work using multiple imaging and biochemical techniques in mice with a cardiac-specific deletion of Pkp2 studied at a time of preserved left ventricular ejection fraction and in human induced pluripotent stem cell-derived PKP2-deficient myocytes.

RESULTS

Samples from patients with ARVC present a loss of nuclear envelope integrity, molecular signatures indicative of increased DNA damage, and a deficit in transcripts coding for proteins in the electron transport chain. Mice with a cardiac-specific deletion of Pkp2 also present a loss of nuclear envelope integrity, which leads to DNA damage and subsequent excess oxidant production (O2.- and H2O2), the latter increased further under mechanical stress (isoproterenol or exercise). Increased oxidant production and DNA damage is recapitulated in human induced pluripotent stem cell-derived PKP2-deficient myocytes. Furthermore, PKP2-deficient cells release H2O2 into the extracellular environment, causing DNA damage and increased oxidant production in neighboring myocytes in a paracrine manner. Treatment with honokiol increases SIRT3 (mitochondrial nicotinamide adenine dinucleotide-dependent protein deacetylase sirtuin-3) activity, reduces oxidant levels and DNA damage in vitro and in vivo, reduces collagen abundance in the right ventricular free wall, and has a protective effect on right ventricular function.

CONCLUSIONS

Loss of nuclear envelope integrity and subsequent DNA damage is a key substrate in the molecular pathology of ARVC. We show transcriptional downregulation of proteins of the electron transcript chain as an early event in the molecular pathophysiology of the disease (before loss of left ventricular ejection fraction <45%), which associates with increased oxidant production (O2.- and H2O2). We propose therapies that limit oxidant formation as a possible intervention to restrict DNA damage in ARVC.

Energetic metabolic reprogramming in Jurkat DFF40-deficient cancer cells

DNA fragmentation factor 40 (DFF40), or the caspase-activated DNase (CAD), is an endonuclease specific for double-stranded DNA. Alterations in its function and expression have been linked to apoptosis resistance, a mechanism likely used by cancer cells. However, how the DFF40-related apoptosis resistance pathway occurs remains unclear. Here, we sought to determine if DFF40 expression could be linked to cell metabolism through the regulation of mitochondrial integrity and function. We demonstrated that DFF40-deficient cells are more resistant to staurosporine and tributyltin (TBT)-induced apoptosis, and express higher levels of Mcl-1 at basal state. Treatment with TBT induces higher Bcl-2 and caspase-9 mRNA transcripts in DFF40 KO Jurkat cells, as well as enhanced Bcl-2 phosphorylation. A loss of DFF40 expression induces a higher mitochondrial mass, mtDNA copy number, mitochondrial membrane potential, and glycolysis rates in resting T cells. DFF40-deficient cells exhibit the Warburg effect phenotype, where they rely significantly more on glycolysis than oxidative phosphorylation and have a higher proliferative state, demonstrated by a higher Ki-67 transcription factor expression and AKT phosphorylation. Finally, we demonstrated with cell fractioning that DFF40 can translocate to the mitochondria following apoptosis induction. Our study reveals that DFF40 may act as a regulator of mitochondria during cell death and its loss could compromise mitochondrial integrity and cause an energetic reprogramming in pathologies such as cancer.

Peripheral Blood Mitochondrial DNA Levels Were Modulated by SARS-CoV-2 Infection Severity and Its Lessening Was Associated With Mortality Among Hospitalized Patients With COVID-19

Introduction

During severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, the virus hijacks the mitochondria causing damage of its membrane and release of mt-DNA into the circulation which can trigger innate immunity and generate an inflammatory state. In this study, we explored the importance of peripheral blood mt-DNA as an early predictor of evolution in patients with COVID-19 and to evaluate the association between the concentration of mt-DNA and the severity of the disease and the patient’s outcome.

Methods

A total 102 patients (51 COVID-19 cases and 51 controls) were included in the study. mt-DNA obtained from peripheral blood was quantified by qRT-PCR using the NADH mitochondrial gene.

Results

There were differences in peripheral blood mt-DNA between patients with COVID-19 (4.25 ng/μl ± 0.30) and controls (3.3 ng/μl ± 0.16) (p = 0.007). Lower mt-DNA concentrations were observed in patients with severe COVID-19 when compared with mild (p= 0.005) and moderate (p= 0.011) cases of COVID-19. In comparison with patients with severe COVID-19 who survived (3.74 ± 0.26 ng/μl) decreased levels of mt-DNA in patients with severe COVID-19 who died (2.4 ± 0.65 ng/μl) were also observed (p = 0.037).

Conclusion

High levels of mt-DNA were associated with COVID-19 and its decrease could be used as a potential biomarker to establish a prognosis of severity and mortality of patients with COVID-19.

Role of mitochondrial DNA copy number in incident cardiovascular diseases and the association between cardiovascular disease and type 2 diabetes: A follow-up study on middle-aged women

BACKGROUND AND AIMS

Mitochondrial DNA copy number (mtDNA-CN) is a surrogate biomarker of mitochondrial dysfunction and is associated with type 2 diabetes (T2D) and cardiovascular disease (CVD). However, despite being associated with both CVD and T2D, it is not known what role mtDNA-CN has in the association between T2D and CVD. Our aims were to investigate whether, (1) baseline mtDNA-CN is associated with CVD incidence and (2) mtDNA-CN has a role as a mediator between T2D and CVD.

METHOD

We quantified absolute mtDNA-CN by droplet digital PCR method in a population-based follow-up study of middle aged (52-65 years) women (n = 3062). The median follow-up period was 17 years.

RESULTS

Our results show that low baseline levels of mtDNA-CN (<111 copies/μL) were associated with an increased risk of CVD (HR = 1.32, 95% CI = 1.08; 1.63) as well as with specific CVDs: coronary heart disease (HR = 1.28, 95% CI = 0.99; 1.66), stroke (HR = 1.26, 95% CI = 0.87; 1.84) and abdominal aortic aneurysm (HR = 2.61, 95% CI = 1.03; 6.62). The associations decreased but persisted even after adjustment for potential confounders. Furthermore, our results show that the total effect of T2D on future risk of CVD was reduced after controlling for mtDNA-CN and the proportion mediated by mtDNA-CN was estimated to be 4.9%.

CONCLUSIONS

Lower baseline mtDNA-CN is associated with incident CVD and may have a mediating effect on the association between T2D and CVD; however, this novel observation needs to be confirmed in future studies.

Biomarkers in the clinical management of patients with atrial fibrillation and heart failure

Atrial fibrillation (AF) and heart failure (HF) are two cardiovascular diseases with an increasing prevalence worldwide. These conditions share common pathophysiologiesand frequently co-exit. In fact, the occurrence of either condition can 'cause' the development of the other, creating a new patient group that demands different management strategies to that if they occur in isolation. Regardless of the temproral association of the two conditions, their presence is linked with adverse cardiovascular outcomes, increased rate of hospitalizations, and increased economic burden on healthcare systems. The use of low-cost, easily accessible and applicable biomarkers may hasten the correct diagnosis and the effective treatment of AF and HF. Both AF and HF effect multiple physiological pathways and thus a great number of biomarkers can be measured that potentially give the clinician important diagnostic and prognostic information. These will then guide patient centred therapeutic management. The current biomarkers that offer potential for guiding therapy, focus on the physiological pathways of miRNA, myocardial stretch and injury, oxidative stress, inflammation, fibrosis, coagulation and renal impairment. Each of these has different utility in current clinincal practice.

Mitochondrial function, dynamics and quality control in the pathophysiology of HFpEF

Heart failure (HF) is one of the leading causes of hospitalization for the adult population and a major cause of mortality worldwide. The HF syndrome is characterized by the heart's inability to supply the cardiac output required to meet the body's metabolic requirements or only at the expense of elevated filling pressures. HF without overt impairment of left ventricular ejection fraction (LVEF) was initially labeled as "diastolic HF" until recognizing the coexistence of both systolic and diastolic abnormalities in most cases. Acknowledging these findings, the preferred nomenclature is HF with preserved EF (HFpEF). This syndrome primarily affects the elderly population and is associated with a heterogeneous overlapping of comorbidities that makes its diagnosis challenging. Despite extensive research, there is still no evidence-based therapy for HFpEF, reinforcing the need for a thorough understanding of the pathophysiology underlying its onset and progression. The role of mitochondrial dysfunction in developing the pathophysiological changes that accompany HFpEF onset and progression (low-grade systemic inflammation, oxidative stress, endothelial dysfunction, and myocardial remodeling) has just begun to be acknowledged. This review summarizes our current understanding of the participation of the mitochondrial network in the pathogenesis of HFpEF, with particular emphasis on the signaling pathways involved, which may provide future therapeutic targets.

Reverse Remodeling With Left Ventricular Assist Devices

This review provides a comprehensive overview of the past 25+ years of research into the development of left ventricular assist device (LVAD) to improve clinical outcomes in patients with severe end-stage heart failure and basic insights gained into the biology of heart failure gleaned from studies of hearts and myocardium of patients undergoing LVAD support. Clinical aspects of contemporary LVAD therapy, including evolving device technology, overall mortality, and complications, are reviewed. We explain the hemodynamic effects of LVAD support and how these lead to ventricular unloading. This includes a detailed review of the structural, cellular, and molecular aspects of LVAD-associated reverse remodeling. Synergisms between LVAD support and medical therapies for heart failure related to reverse remodeling, remission, and recovery are discussed within the context of both clinical outcomes and fundamental effects on myocardial biology. The incidence, clinical implications and factors most likely to be associated with improved ventricular function and remission of the heart failure are reviewed. Finally, we discuss recognized impediments to achieving myocardial recovery in the vast majority of LVAD-supported hearts and their implications for future research aimed at improving the overall rates of recovery.

Mitochondrial Morphology and Mitophagy in Heart Diseases: Qualitative and Quantitative Analyses Using Transmission Electron Microscopy

Transmission electron microscopy (TEM) has long been an important technique, capable of high degree resolution and visualization of subcellular structures and organization. Over the last 20 years, TEM has gained popularity in the cardiovascular field to visualize changes at the nanometer scale in cardiac ultrastructure during cardiovascular development, aging, and a broad range of pathologies. Recently, the cardiovascular TEM enabled the studying of several signaling processes impacting mitochondrial function, such as mitochondrial fission/fusion, autophagy, mitophagy, lysosomal degradation, and lipophagy. The goals of this review are to provide an overview of the current usage of TEM to study cardiac ultrastructural changes; to understand how TEM aided the visualization of mitochondria, autophagy, and mitophagy under normal and cardiovascular disease conditions; and to discuss the overall advantages and disadvantages of TEM and potential future capabilities and advancements in the field.

The Power of Yeast in Modelling Human Nuclear Mutations Associated with Mitochondrial Diseases

The increasing application of next generation sequencing approaches to the analysis of human exome and whole genome data has enabled the identification of novel variants and new genes involved in mitochondrial diseases. The ability of surviving in the absence of oxidative phosphorylation (OXPHOS) and mitochondrial genome makes the yeast Saccharomyces cerevisiae an excellent model system for investigating the role of these new variants in mitochondrial-related conditions and dissecting the molecular mechanisms associated with these diseases. The aim of this review was to highlight the main advantages offered by this model for the study of mitochondrial diseases, from the validation and characterisation of novel mutations to the dissection of the role played by genes in mitochondrial functionality and the discovery of potential therapeutic molecules. The review also provides a summary of the main contributions to the understanding of mitochondrial diseases emerged from the study of this simple eukaryotic organism.

Melatonin/Nrf2/NLRP3 Connection in Mouse Heart Mitochondria during Aging

Aging is a major risk for cardiovascular diseases (CVD). Age-related disorders include oxidative stress, mitochondria dysfunction, and exacerbation of the NF-κB/NLRP3 innate immune response pathways. Some of the molecular mechanisms underlying these processes, however, remain unclear. This study tested the hypothesis that NLRP3 inflammasome plays a role in cardiac aging and melatonin is able to counteract its effects. With the aim of investigating the impact of NLRP3 inflammasome and the actions and target of melatonin in aged myocardium, we analyzed the expression of proteins implied in mitochondria dynamics, autophagy, apoptosis, Nrf2-dependent antioxidant response and mitochondria ultrastructure in heart of wild-type and NLRP3-knockout mice of 3, 12, and 24 months-old, with and without melatonin treatment. Our results showed that the absence of NLRP3 prevented age-related mitochondrial dynamic alterations in cardiac muscle with minimal effects in cardiac autophagy during aging. The deficiency of the inflammasome affected Bax/Bcl2 ratio, but not p53 or caspase 9. The Nrf2-antioxidant pathway was also unaffected by the absence of NLRP3. Furthermore, NLRP3-deficiency prevented the drop in autophagy and mice showed less mitochondrial damage than wild-type animals. Interestingly, melatonin treatment recovered mitochondrial dynamics altered by aging and had few effects on cardiac autophagy. Melatonin supplementation also had an anti-apoptotic action in addition to restoring Nrf2-antioxidant capacity and improving mitochondria ultrastructure altered by aging.

Mitochondrial unfolded protein response, mitophagy and other mitochondrial quality control mechanisms in heart disease and aged heart

Mitochondria are involved in crucial homeostatic processes in the cell: the production of adenosine triphosphate and reactive oxygen species, and the release of pro-apoptotic molecules. Thus, cell survival depends on the maintenance of proper mitochondrial function by mitochondrial quality control. The most important mitochondrial quality control mechanisms are mitochondrial unfolded protein response, mitophagy, biogenesis, and fusion-fission dynamics. This review deals with mitochondrial quality control in heart diseases, especially myocardial infarction and heart failure. Some previous studies have demonstrated that the activation of mitochondrial quality control mechanisms may be beneficial for the heart, while others have shown that it may lead to heart damage. Our aim was to describe the mechanisms by which mitochondrial quality control contributes to heart protection or damage and to provide evidence that may resolve the seemingly contradictory results from the previous studies.

Endoplasmic reticulum stress and mitochondrial biogenesis are potential therapeutic targets for abdominal aortic aneurysm

Endoplasmic reticulum (ER) and mitochondria are crucial organelles for cell homeostasis and alterations of these organelles have been implicated in cardiovascular disease. However, their roles in abdominal aortic aneurysm (AAA) pathogenesis remain largely unknown. In a recent issue of Clinical Science, Navas-Madronal et al. ((2019), 133(13), 1421-1438) reported that enhanced ER stress and dysregulation of mitochondrial biogenesis are associated with AAA pathogenesis in humans. The authors also proposed that disruption in oxysterols network such as an elevated concentration of 7-ketocholestyerol in plasma is a causative factor for AAA progression. Their findings highlight new insights into the underlying mechanism of AAA progression through ER stress and dysregulation of mitochondrial biogenesis. Here, we will discuss the background, significance of the study, and future directions.

Mitochondrial fission and fusion: A dynamic role in aging and potential target for age-related disease

The mitochondria is the major hub to convert energy for cellular processes. Dysregulation of mitochondrial function is one of the classical hallmarks of aging, and mitochondrial interventions have repeatedly been shown to improve outcomes in age-related diseases. Crucial to mitochondrial regulation is the dynamic nature of their network structure. Mitochondria separate and merge using fission and fusion processes in response to changes in energy and stress status. While many mitochondrial processes are already characterized in relation to aging, specific evidence in multicellular organisms causally linking mitochondrial dynamics to the regulation of lifespan is limited. There does exist, however, a large body of evidence connecting mitochondrial dynamics to other aging-related cellular processes and implicates them in a number of human diseases. Here, we discuss the mechanisms of mitochondrial fission and fusion, the current evidence of their role in aging of multicellular organisms, and how these connect to cell cycle regulation, quality control, and transmission of energy status. Finally, we discuss the current evidence implicating these processes in age-related human pathologies, such as neurodegenerative or cardio-metabolic diseases. We suggest that deeper understanding of the regulatory mechanisms within this system and downstream implications could benefit in understanding and intervention of these conditions.

Protective role of peroxiredoxin-4 in heart failure

Recently, we have read with great interest the article published by Ibarrola et al. (Clin. Sci. (Lond.) (2018) 132, 1471-1485), which used proteomics and immunodetection methods to show that Galectin-3 (Gal-3) down-regulated the antioxidant peroxiredoxin-4 (Prx-4) in cardiac fibroblasts. Authors concluded that 'antioxidant activity of Prx-4 had been identified as a protein down-regulated by Gal-3. Moreover, Gal-3 induced a decrease in total antioxidant capacity which resulted in a consequent increase in peroxide levels and oxidative stress markers in cardiac fibroblasts.' We would like to point out some results stated in the article that need further investigation and more detailed discussion to clarify certain factors involved in the protective role of Prx-4 in heart failure.

(Letter to the Editor) Response to: protective role of peroxiredoxin-4 in heart failure

We thank Ahmed et al. for their letter regarding our study 'Galectin-3 down-regulates antioxidant peroxiredoxin-4 in human cardiac fibroblasts' [1]. As emphasized by Ahmed et al., Prx-4 levels decrease [2] whereas MFN-2, OPA-1 and PGC-1α levels increase [3] in dilated cardiomyopathy (DCM). Moreover, Gal-3 expression is also increased in DCM [4]. In our study, we showed in vitro that Gal-3 decreased Prx-4 without modifying MFN-2 or PGC-1α levels in human cardiac fibroblasts. Although cardiac Prx-4 decrease could be a direct consequence of Gal-3 effects on cardiac fibroblasts, we cannot exclude the possibility that other factors increase MFN-2, OPA-1 and PGC-1α levels in both cardiac fibroblasts or cardiomyocytes in the context of DCM. Further studies are needed to clarify the association between Prx-4 decrease and the increase in other mitochondrial proteins in DCM.

Advances in the role and mechanism of BAG3 in dilated cardiomyopathy

The B cell lymphoma 2-associated anthanogene (BAG3) is an anti-apoptotic co-chaperone protein. Previous reports suggest that mutations in BAG3 are associated with dilated cardiomyopathy. This review aims to summarize the current understanding of the relationship between BAG3 mutations and dilated cardiomyopathy, primarily focusing on the role and protective mechanism of BAG3 in cardiomyocytes from individuals with dilated cardiomyopathy. The results of published studies show that BAG3 is critically important for reducing cardiomyocyte apoptosis, maintaining protein homeostasis, regulating mitochondrial stability, modulating myocardial contraction, and reducing cardiac arrhythmia, which suggests an indispensable protective mechanism of BAG3 in dilated cardiomyopathy. The significant role of BAG3 in protecting cardiomyocytes provides a new direction for the diagnosis and treatment of dilated cardiomyopathy. However, further research is required to explore the molecular mechanisms that regulate BAG3 expression, to identify a novel therapy for patients with dilated cardiomyopathy.

Mitochondrial Morphology, Dynamics, and Function in Human Pressure Overload or Ischemic Heart Disease With Preserved or Reduced Ejection Fraction

BACKGROUND

The FOXO3a (forkhead box O3a)-BNIP3 (B-cell lymphoma 2/adenovirus E1B 19kDa interacting protein 3) pathway modulates mitochondrial dynamics and function and contributes to myocardial remodeling in rodent models of heart failure. We sought to investigate the expression of this pathway along with the expression of mitochondrial biogenesis (PGC-1α [peroxisome proliferator-activated receptor-γ coactivator-1α]), dynamics (DRP-1 [dynamin-related protein 1], OPA-1 [optic atrophy 1], and MFN 2 [mitofusin 2]), and oxidative phosphorylation (citrate synthase and electron transport chain complexes) markers and COX IV (cytochrome C oxidase) activity in myocardium from patients with valvular or ischemic heart disease and heart failure with preserved ejection fraction (HFpEF) or heart failure with reduced ejection fraction (HFrEF).

METHODS AND RESULTS

Subepicardial left ventricular biopsies (10×1×1 mm³) were obtained at aortic valve replacement (HFpEF_AVR, n=5; and HFrEF_AVR, n=4), coronary artery bypass grafting (HFpEF_CABG, n=5; and HFrEF_CABG, n=5), or left ventricular assist device implantation (HFrEF_LVAD, n=4). Subepicardial biopsies from patients with normal left ventricular function (n=2) and from donor hearts (n=3) served as controls (normal). Relative to normal, mitochondrial fragmentation and cristae destruction were evident, and mitochondrial area was decreased in HFpEF; 1.00±0.09 versus 0.71±0.08; P=0.016. These mitochondrial morphological changes were more pronounced in HFrEF (0.54±0.06); P=0.002 HFpEF versus HFrEF. BNIP3 (monomer+dimer) expression was increased in HFpEF (3.99±2.44) and in HFrEF (5.19±1.70) relative to normal; P=0.004 and P<0.001, respectively. However, BNIP3 monomer was increased in HFrEF (4.32±1.43) compared with normal (0.99±0.06) and HFpEF (1.97±0.90); P=0.001 and 0.004, respectively. The HFrEF group uniquely showed increase in DRP-1 expression (1.94±0.38) and decreases in PGC-1α expression (0.61±0.07) and COX IV activity (0.70±0.10) relative to normal; P=0.013, P<0.001, and P<0.001, respectively, with no significant change in electron transport chain complexes expression.

CONCLUSIONS

These findings in human myocardium confirm studies in rodents where contractile dysfunction is associated with activation of the FOXO3a-BNIP3 pathway and altered mitochondrial dynamics, biogenesis, and function.

Mitochondrial dynamics and their potential as a therapeutic target

Mitochondrial dynamics shape the mitochondrial network and contribute to mitochondrial function and quality control. Mitochondrial fusion and division are integrated into diverse cellular functions and respond to changes in cell physiology. Imbalanced mitochondrial dynamics are associated with a range of diseases that are broadly characterized by impaired mitochondrial function and increased cell death. In various disease models, modulating mitochondrial fusion and division with either small molecules or genetic approaches has improved function. Although additional mechanistic understanding of mitochondrial fusion and division will be critical to inform further therapeutic approaches, mitochondrial dynamics represent a powerful therapeutic target in a wide range of human diseases.

Association between mitochondrial DNA copy number and cardiovascular disease: Current evidence based on a systematic review and meta-analysis

Background

Mitochondria are energy-producing structure of the cell and help to maintain redox environment. In cardiovascular disease, the number of mitochondrial DNA (mtDNA) will changes accordingly compare to normal condition. Some investigators ask whether it has a clear association between mtDNA and cardiovascular disease with its adverse events. Thus, we conduct the meta-analysis to assess the role of circulating mtDNA in evaluating cardiovascular disease.

Methods

The meta-analysis was conducted in accordance with a predetermined protocol following the recommendations of Cochrane Handbook of Systematic Reviews. We searched the Pubmed, Embase, the Cochrane Central Register of Controlled Trials and World Health Organization clinical trials registry center to identify relevant studies up to the end of October 2017. Data were analyzed using STATA. Besides, publication bias and meta-regression analysis were also conducted.

Results

We collected results from 5 articles for further analyses with 8,252 cases and 20,904 control. The normalized mtDNA copy number level is lower in cardiovascular disease (CVD) than the control groups with a pooled standard mean difference (SMD) of -0.36(95%CI,-0.65 to -0.08); The pooled odds ratio (OR) for CVD proportion associated with a 1-SD (standard deviation) decrease in mtDNA copy number level is 1.23 (95% CI,1.06–1.42); The OR for CVD patients with mtDNA copy number lower than median level is 1.88(95% CI,1.65–2.13); The OR for CVD patients with mtDNA copy number located in the lowest quartile part is 2.15(95% CI, 1.46–3.18); the OR between mtDNA copy number and the risk of sudden cardiac death (SCD) is 1.83(95% CI, 1.22–2.74).

Conclusion

Although inter-study variability, the overall performance test of mtDNA for evaluating CVD and SCD revealed that the mtDNA copy number presented the potential to be a biomarker for CVD and SCD prediction. Given that, the fewer copies of mtDNA, the higher the risk of CVD.

Chronic heart failure is characterized by altered mitochondrial function and structure in circulating leucocytes

Oxidative stress is currently viewed as a key factor in the genesis and progression of Heart Failure (HF). The aim of this study was to characterize the mitochondrial changes linked to oxidative stress generation in circulating peripheral blood mononuclear cells isolated from chronic HF patients (HF_PBMCs) in order to highlight the involvement of mitochondrial dysfunction in the pathophysiology of HF.

To assess the production of reactive oxygen species (ROS), mitochondrial function and ultrastructure and the mitophagic flux in circulating PBMCs we enrolled 15 patients with HF and a control group of ten healthy subjects. The HF_PBMCs showed a mitochondrial population consisting of damaged and less functional organelles responsible of higher superoxide anion production both at baseline and under in vitro stress conditions, with evidence of cellular apoptosis. Although the mitophagic flux at baseline was enhanced in HF_PBMCs at level similar to those that could be achieved in control PBMCs only under inflammatory stress conditions, the activation of mitophagy was unable to preserve a proper mitochondrial dynamics upon stress stimuli in HF.

In summary, circulating HF_PBMCs show structural and functional derangements of mitochondria with overproduction of reactive oxidant species. This mitochondrial failure sustains a leucocyte dysfunctional status in the blood that may contribute to development and persistence of stress conditions within the cardiovascular system in HF.

Complex roads from genotype to phenotype in dilated cardiomyopathy: scientific update from the Working Group of Myocardial Function of the European Society of Cardiology

Dilated cardiomyopathy (DCM) frequently affects relatively young, economically, and socially active adults, and is an important cause of heart failure and transplantation. DCM is a complex disease and its pathological architecture encounters many genetic determinants interacting with environmental factors. The old perspective that every pathogenic gene mutation would lead to a diseased heart, is now being replaced by the novel observation that the phenotype depends not only on the penetrance-malignancy of the mutated gene-but also on epigenetics, age, toxic factors, pregnancy, and a diversity of acquired diseases. This review discusses how gene mutations will result in mutation-specific molecular alterations in the heart including increased mitochondrial oxidation (sarcomeric gene e.g. TTN), decreased calcium sensitivity (sarcomeric genes), fibrosis (e.g. LMNA and TTN), or inflammation. Therefore, getting a complete picture of the DCM patient will include genomic data, molecular assessment by preference from cardiac samples, stratification according to co-morbidities, and phenotypic description. Those data will help to better guide the heart failure and anti-arrhythmic treatment, predict response to therapy, develop novel siRNA-based gene silencing for malignant gene mutations, or intervene with mutation-specific altered gene pathways in the heart.This article is part of the Mini Review Series from the Varenna 2017 meeting of the Working Group of Myocardial Function of the European Society of Cardiology.

Mitochondrial cardiomyopathies feature increased uptake and diminished efflux of mitochondrial calcium

Calcium (Ca2+) influx into the mitochondrial matrix stimulates ATP synthesis. Here, we investigate whether mitochondrial Ca2+ transport pathways are altered in the setting of deficient mitochondrial energy synthesis, as increased matrix Ca2+ may provide a stimulatory boost. We focused on mitochondrial cardiomyopathies, which feature such dysfunction of oxidative phosphorylation. We study a mouse model where the main transcription factor for mitochondrial DNA (transcription factor A, mitochondrial, Tfam) has been disrupted selectively in cardiomyocytes. By the second postnatal week (10-15day old mice), these mice have developed a dilated cardiomyopathy associated with impaired oxidative phosphorylation. We find evidence of increased mitochondrial Ca2+ during this period using imaging, electrophysiology, and biochemistry. The mitochondrial Ca2+ uniporter, the main portal for Ca2+ entry, displays enhanced activity, whereas the mitochondrial sodium-calcium (Na+-Ca2+) exchanger, the main portal for Ca2+ efflux, is inhibited. These changes in activity reflect changes in protein expression of the corresponding transporter subunits. While decreased transcription of Nclx, the gene encoding the Na+-Ca2+ exchanger, explains diminished Na+-Ca2+ exchange, the mechanism for enhanced uniporter expression appears to be post-transcriptional. Notably, such changes allow cardiac mitochondria from Tfam knockout animals to be far more sensitive to Ca2+-induced increases in respiration. In the absence of Ca2+, oxygen consumption declines to less than half of control values in these animals, but rebounds to control levels when incubated with Ca2+. Thus, we demonstrate a phenotype of enhanced mitochondrial Ca2+ in a mitochondrial cardiomyopathy model, and show that such Ca2+ accumulation is capable of rescuing deficits in energy synthesis capacity in vitro.

Correlation Between Mitochondrial DNA Content Measured in Myocardium and Peripheral Blood of Patients with Non-Ischemic Heart Failure

BACKGROUND/OBJECTIVES

Heart failure (HF) is associated with disturbances in mitochondrial energy production. This mitochondrial dysfunction is reflected by depletion of mitochondrial DNA (mtDNA) in different tissues. Our aims were to determine if there was a correlation between mtDNA content measured in myocardial tissue and the easily accessible peripheral blood cells of patients with non-ischemic HF; and to determine if there was a correlation between myocardial mtDNA and left ventricular (LV) ejection fraction.

METHODS

We prospectively collected paired myocardial tissue and peripheral blood samples from 13 consecutive end-stage non-ischemic HF patients undergoing cardiac transplantation. mtDNA content was assessed with real-time quantitative PCR by calculating the relative ratio of two specific mitochondrial sequences and one nuclear control gene sequence.

RESULTS

HF patients with lower myocardial mtDNA content had a significantly lower LV ejection fraction (r = 0.65, p = 0.016). Peripheral blood mtDNA content correlated positively with right ventricular myocardial mtDNA content (r = 0.63, p = 0.021). We also observed that averaged myocardial DNA content tended to correlate with peripheral blood mtDNA content (r = 0.53, p = 0.061).

CONCLUSIONS

In non-ischemic HF patients, myocardial mtDNA content is positively correlated with peripheral blood mtDNA content and LV function as assessed by echocardiography.

Spatial detection of fetal marker genes expressed at low level in adult human heart tissue

Heart failure is a major health problem linked to poor quality of life and high mortality rates. Hence, novel biomarkers, such as fetal marker genes with low expression levels, could potentially differentiate disease states in order to improve therapy. In many studies on heart failure, cardiac biopsies have been analyzed as uniform pieces of tissue with bulk techniques, but this homogenization approach can mask medically relevant phenotypes occurring only in isolated parts of the tissue. This study examines such spatial variations within and between regions of cardiac biopsies. In contrast to standard RNA sequencing, this approach provides a spatially resolved transcriptome- and tissue-wide perspective of the adult human heart, and enables detection of fetal marker genes expressed by minor subpopulations of cells within the tissue. Analysis of patients with heart failure, with preserved ejection fraction, demonstrated spatially divergent expression of fetal genes in cardiac biopsies.

Antihypertrophic Effects of Small Molecules that Maintain Mitochondrial ATP Levels Under Hypoxia

Since impaired mitochondrial ATP production in cardiomyocytes is thought to lead to heart failure, a drug that protects mitochondria and improves ATP production under disease conditions would be an attractive treatment option. In this study, we identified small-molecule drugs, including the anti-parasitic agent, ivermectin, that maintain mitochondrial ATP levels under hypoxia in cardiomyocytes. Mechanistically, transcriptomic analysis and gene silencing experiments revealed that ivermectin increased mitochondrial ATP production by inducing Cox6a2, a subunit of the mitochondrial respiratory chain. Furthermore, ivermectin inhibited the hypertrophic response of human induced pluripotent stem cell-derived cardiomyocytes. Pharmacological inhibition of importin β, one of the targets of ivermectin, exhibited protection against mitochondrial ATP decline and cardiomyocyte hypertrophy. These findings indicate that maintaining mitochondrial ATP under hypoxia may prevent hypertrophy and improve cardiac function, providing therapeutic options for mitochondrial dysfunction.

Mitochondrial Bioenergetics and Dysfunction in Failing Heart

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

Mitochondrial Function in Non-ischemic Heart Failure

Provision for the continuous demand for energy from the beating heart relies heavily on efficient mitochondrial activity. Non-ischemic cardiomyopathy in which oxygen supply is not limiting results from etiologies such as pressure overload. It is associated with progressive development of metabolic stress culminating in energy depletion and heart failure. The mitochondria from the ventricular walls undergoing non-ischemic cardiomyopathy are subjected to long periods of adaptation to support the changing metabolic milieu, which has been described as mal-adaptation since it ultimately results in loss of cardiac contractile function. While the chronicity of exposure to metabolic stressors, co-morbidities and thereby adaptive changes in mitochondria maybe different between ischemic and non-ischemic heart failure, the resulting pathology is very similar, especially in late stage heart failure. Understanding of the mitochondrial changes in early-stage heart failure may guide the development of mitochondrial-targeted therapeutic options to prevent progression of non-ischemic heart failure. This chapter reviews findings of mitochondrial functional changes in animal models and humans with non-ischemic heart failure. While most animal models of non-ischemic heart failure exhibit cardiac mitochondrial dysfunction, studies in humans have been inconsistent despite confirmed reduction in ATP production. This chapter also reviews the possibility of impairment of substrate supply processes upstream of the mitochondria in heart failure, and discusses potential metabolism-targeted therapeutic options.

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.

The 8-oxo-deoxyguanosine glycosylase increases its migration to mitochondria in compensated cardiac hypertrophy

Cardiac hypertrophy is a compensatory mechanism maladapted because it presents an increase in the oxidative stress which could be associated with the development of the heart failure. A mechanism proposed is by mitochondrial DNA (mtDNA) oxidation, which evolved to a vicious cycle because of the synthesis of proteins encoded in the genome is committed. Therefore, the aim of the present work was to evaluate the mtDNA damage and enzyme repairing the 8-oxo-deoxyguanosine glycosylase mitochondrial isoform 1-2a (OGG1-2a) in the early stage of compensated cardiac hypertrophy induced by abdominal aortic constriction (AAC). Results showed that after 6 weeks of AAC, hearts presented a compensated hypertrophy (22%), with an increase in the cell volume (35%), mitochondrial mass (12%), and mitochondrial membrane potential (94%). However, the increase of oxidative stress did not affect mtDNA most probably because OGG1-2a was found to increase 3.2 times in the mitochondrial fraction. Besides, mitochondrial function was not altered by the cardiac hypertrophy condition but in vitro mitochondria from AAC heart showed an increased sensibility to stress induced by the high Ca²⁺ concentration. The increase in the oxidative stress in compensated cardiac hypertrophy induced the OGG1-2a migration to mitochondria to repair mtDNA oxidation, as a mechanism that allows maintaining the cardiac function in the compensatory stage.

Metabolic remodeling in hypertrophied and failing myocardium: a review

The energy starvation hypothesis proposes that maladaptive metabolic remodeling antedates, initiates, and maintains adverse contractile dysfunction in heart failure (HF). Better understanding of the cardiac metabolic phenotype and metabolic signaling could help identify the role metabolic remodeling plays within HF and the conditions known to transition toward HF, including "pathological" hypertrophy. In this review, we discuss metabolic phenotype and metabolic signaling in the contexts of pathological hypertrophy and HF. We discuss the significance of alterations in energy supply (substrate utilization, oxidative capacity, and phosphotransfer) and energy sensing using observations from human and animal disease models and models of manipulated energy supply/sensing. We aim to provide ways of thinking about metabolic remodeling that center around metabolic flexibility, capacity (reserve), and efficiency rather than around particular substrate preferences or transcriptomic profiles. We show that maladaptive metabolic remodeling takes multiple forms across multiple energy-handling domains. We suggest that lack of metabolic flexibility and reserve (substrate, oxidative, and phosphotransfer) represents a final common denominator ultimately compromising efficiency and contractile reserve in stressful contexts.

The role of mitochondria in cardiac development and protection

Mitochondria are essential for the development as well as maintenance of the myocardium, the most energy consuming tissue in the human body. Mitochondria are not only a source of ATP energy but also generators of reactive oxygen species (ROS), that cause oxidative damage, but also regulate physiological processes such as the switch from hyperplastic to hypertrophic growth after birth. As excess ROS production and oxidative damage are associated with cardiac pathology, it is not surprising that much of the research focused on the deleterious aspects of free radicals. However, cardiomyocytes are naturally highly adapted against repeating oxidative insults, with evidence suggesting that moderate and acute ROS exposure has beneficial consequences for mitochondrial maintenance and cardiac health. Antioxidant defenses, mitochondrial quality control, mtDNA maintenance mechanisms as well as mitochondrial fusion and fission improve mitochondrial function and cardiomyocyte survival under stress conditions. As these adaptive processes can be induced, promoting mitohormesis or mitochondrial biogenesis using controlled ROS exposure could provide a promising strategy to increase cardiomyocyte survival and prevent pathological remodeling of the myocardium.

Postnatal Hyperoxia Exposure Durably Impairs Right Ventricular Function and Mitochondrial Biogenesis

Prematurity complicates 12% of births, and young adults with a history of prematurity are at risk to develop right ventricular (RV) hypertrophy and impairment. The long-term risk for pulmonary vascular disease, as well as mechanisms of RV dysfunction and ventricular-vascular uncoupling after prematurity, remain poorly defined. Using an established model of prematurity-related lung disease, pups from timed-pregnant Sprague Dawley rats were randomized to normoxia or hyperoxia (fraction of inspired oxygen, 0.85) exposure for the first 14 days of life. After aging to 1 year in standard conditions, rats underwent hemodynamic assessment followed by tissue harvest for biochemical and histological evaluation. Aged hyperoxia-exposed rats developed significantly greater RV hypertrophy, associated with a 40% increase in RV systolic pressures. Although cardiac index was similar, hyperoxia-exposed rats demonstrated a reduced RV ejection fraction and significant RV-pulmonary vascular uncoupling. Hyperoxia-exposed RV cardiomyocytes demonstrated evidence of mitochondrial dysregulation and mitochondrial DNA damage, suggesting potential mitochondrial dysfunction as a cause of RV dysfunction. Aged rats exposed to postnatal hyperoxia recapitulate many features of young adults born prematurely, including increased RV hypertrophy and decreased RV ejection fraction. Our data suggest that postnatal hyperoxia exposure results in mitochondrial dysregulation that persists into adulthood with eventual RV dysfunction. Further evaluation of long-term mitochondrial function is warranted in both animal models of premature lung disease and in human adults who were born preterm.

Mitophagy and mitochondrial biogenesis in atrial tissue of patients undergoing heart surgery with cardiopulmonary bypass

Mitophagy occurs during ischemia/reperfusion (I/R) and limits oxidative stress and injury. Mitochondrial turnover was assessed in patients undergoing cardiac surgery involving cardiopulmonary bypass (CPB). Paired biopsies of right atrial appendage before initiation and after weaning from CPB were processed for protein analysis, mitochondrial DNA/nuclear DNA ratio (mtDNA:nucDNA ratio), mtDNA damage, mRNA, and polysome profiling. Mitophagy in the post-CPB samples was evidenced by decreased levels of mitophagy adapters NDP52 and optineurin in whole tissue lysate, decreased Opa1 long form, and translocation of Parkin to the mitochondrial fraction. PCR analysis of mtDNA comparing amplification of short vs. long segments of mtDNA revealed increased damage following cardiac surgery. Surprisingly, a marked increase in several mitochondria-specific protein markers and mtDNA:nucDNA ratio was observed, consistent with increased mitochondrial biogenesis. mRNA analysis suggested that mitochondrial biogenesis was traniscription independent and likely driven by increased translation of existing mRNAs. These findings demonstrate in humans that both mitophagy and mitochondrial biogenesis occur during cardiac surgery involving CPB. We suggest that mitophagy is balanced by mitochondrial biogenesis during I/R stress experienced during surgery. Mitigating mtDNA damage and elucidating mechanisms regulating mitochondrial turnover will lead to interventions to improve outcome after I/R in the setting of heart disease.

Eicosapentaenoic acid ameliorates palmitate-induced lipotoxicity via the AMP kinase/dynamin-related protein-1 signaling pathway in differentiated H9c2 myocytes

BACKGROUND

Emerging evidence suggested the preferable effects of eicosapentaenoic acid (EPA; n-3 polyunsaturated fatty acid) against cardiac lipotoxicity, which worsens cardiac function by means of excessive serum free fatty acids due to chronic adrenergic stimulation under heart failure. Nonetheless, the precise molecular mechanisms remain elusive. In this study, we focused on dynamin-related protein-1 (Drp1) as a possible modulator of the EPA-mediated cardiac protection against cardiac lipotoxicity, and investigated the causal relation between AMP-activated protein kinase (AMPK) and Drp1.

METHODS AND RESULTS

When differentiated H9c2 myocytes were exposed to palmitate (PAL; saturated fatty acid, 400µM) for 24h, these myocytes showed activation of caspases 3 and 7, enhanced caspase 3 cleavage, depolarized mitochondrial membrane potential, depleted intracellular ATP, and enhanced production of intracellular reactive oxygen species. These changes suggested lipotoxicity due to excessive PAL. PAL enhanced mitochondrial fragmentation with increased Drp1 expression, as well. EPA (50µM) restored the PAL-induced apoptosis, mitochondrial dysfunction, and mitochondrial fragmentation with increased Drp1 expression by PAL. EPA activated phosphorylation of AMPK, and pharmacological activation of AMPK by 5-aminoimidazole-4-carboxamide ribonucleotide ameliorated the PAL-induced apoptosis, mitochondrial dysfunction, and downregulated Drp1. An AMPK knockdown via RNA interference enhanced Drp1 expression and attenuated the protective effects of EPA against the PAL-induced lipotoxicity.

CONCLUSION

EPA ameliorates the PAL-induced lipotoxicity via AMPK activation, which subsequently suppresses mitochondrial fragmentation and Drp1 expression. Our findings may provide new insights into the molecular mechanisms of EPA-mediated myocardial protection in heart failure.