Physiological Mitochondrial Fragmentation Is a Normal Cardiac Adaptation to Increased Energy Demand

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.

Mitochondrial dysfunction: mechanisms and advances in therapy

Mitochondria, with their intricate networks of functions and information processing, are pivotal in both health regulation and disease progression. Particularly, mitochondrial dysfunctions are identified in many common pathologies, including cardiovascular diseases, neurodegeneration, metabolic syndrome, and cancer. However, the multifaceted nature and elusive phenotypic threshold of mitochondrial dysfunction complicate our understanding of their contributions to diseases. Nonetheless, these complexities do not prevent mitochondria from being among the most important therapeutic targets. In recent years, strategies targeting mitochondrial dysfunction have continuously emerged and transitioned to clinical trials. Advanced intervention such as using healthy mitochondria to replenish or replace damaged mitochondria, has shown promise in preclinical trials of various diseases. Mitochondrial components, including mtDNA, mitochondria-located microRNA, and associated proteins can be potential therapeutic agents to augment mitochondrial function in immunometabolic diseases and tissue injuries. Here, we review current knowledge of mitochondrial pathophysiology in concrete examples of common diseases. We also summarize current strategies to treat mitochondrial dysfunction from the perspective of dietary supplements and targeted therapies, as well as the clinical translational situation of related pharmacology agents. Finally, this review discusses the innovations and potential applications of mitochondrial transplantation as an advanced and promising treatment.

Inhibition of Drp1- Fis1 interaction alleviates aberrant mitochondrial fragmentation and acute kidney injury

BACKGROUND

Acute kidney injury (AKI) is a common clinical disorder with complex etiology and poor prognosis, and currently lacks specific and effective treatment options. Mitochondrial dynamics dysfunction is a prominent feature in AKI, and modulation of mitochondrial morphology may serve as a potential therapeutic approach for AKI.

METHODS

We induced ischemia-reperfusion injury (IRI) in mice (bilateral) and Bama pigs (unilateral) by occluding the renal arteries. ATP depletion and recovery (ATP-DR) was performed on proximal renal tubular cells to simulate in vitro IRI. Renal function was evaluated using creatinine and urea nitrogen levels, while renal structural damage was assessed through histopathological staining. The role of Drp1 was investigated using immunoblotting, immunohistochemistry, immunofluorescence, and immunoprecipitation techniques. Mitochondrial morphology was evaluated using confocal microscopy.

RESULTS

Renal IRI induced significant mitochondrial fragmentation, accompanied by Dynamin-related protein 1 (Drp1) translocation to the mitochondria and Drp1 phosphorylation at Ser616 in the early stages (30 min after reperfusion), when there was no apparent structural damage to the kidney. The use of the Drp1 inhibitor P110 significantly improved kidney function and structural damage. P110 reduced Drp1 mitochondrial translocation, disrupted the interaction between Drp1 and Fis1, without affecting the binding of Drp1 to other mitochondrial receptors such as MFF and Mid51. High-dose administration had no apparent toxic side effects. Furthermore, ATP-DR induced mitochondrial fission in renal tubular cells, accompanied by a decrease in mitochondrial membrane potential and an increase in the translocation of the pro-apoptotic protein Bax. This process facilitated the release of dsDNA, triggering the activation of the cGAS-STING pathway and promoting inflammation. P110 attenuated mitochondrial fission, suppressed Bax mitochondrial translocation, prevented dsDNA release, and reduced the activation of the cGAS-STING pathway. Furthermore, these protective effects of P110 were also observed renal IRI model in the Bama pig and folic acid-induced nephropathy in mice.

CONCLUSIONS

Dysfunction of mitochondrial dynamics mediated by Drp1 contributes to renal IRI. The specific inhibitor of Drp1, P110, demonstrated protective effects in both in vivo and in vitro models of AKI.

Exercise After Acute Myocarditis: When and How to Return to Sports

Myocarditis is an inflammatory disease of the myocardium secondary to infectious and noninfectious insults. The most feared consequence of myocarditis is sudden cardiac death owing to electrical instability and arrhythmia. Typical presenting symptoms include chest pain, dyspnea, palpitations and/or heart failure. Diagnosis is usually made with history, electrocardiogram, biomarkers, echocardiogram, and cardiac MRI (CMR). Application of the Lake Louise criteria to CMR results can help identify cases of myocarditis. Treatment is usually supportive with medical therapy, and patients are recommended to abstain from exercise for 3 to 6 months. Exercise restrictions may be lifted after normalization on follow-up testing.

Recent insights into the control of mitochondrial fission

Mitochondria are the powerhouse of the cell. They undergo fission and fusion to maintain cellular homeostasis. In this review, we explore the intricate regulation of mitochondrial fission at various levels, including the protein level, the post-translational modification level, and the organelle level. Malfunctions in mitochondrial fission can have detrimental effects on cells. Therefore, we also examine the association between mitochondrial fission with diseases such as breast cancer and cardiovascular disorders. We anticipate that a comprehensive investigation into the control of mitochondrial fission will pave the way for the development of innovative therapeutic strategies.

Protective Effects of Astaxanthin on Ochratoxin A-Induced Liver Injury: Effects of Endoplasmic Reticulum Stress and Mitochondrial Fission-Fusion Balance

Ochratoxin A (OTA), a common mycotoxin, can contaminate food and feed and is difficult to remove. Astaxanthin (ASTA), a natural antioxidant, can effectively protect against OTA-induced hepatotoxicity; however, its mechanism of action remains unclear. In the present study, we elucidate the protective effects of ASTA on the OTA-induced damage of the endoplasmic reticulum and mitochondria in broiler liver samples by serum biochemical analysis, antioxidant analysis, qRT-PCR, and Western blot analysis. ASTA inhibited the expressions of ahr, pxr, car, cyp1a1, cyp1a5, cyp2c18, cyp2d6, and cyp3a9 genes, and significantly alleviated OTA-induced liver oxidative damage (SOD, GSH-Px, GSH, MDA). Furthermore, it inhibited OTA-activated endoplasmic reticulum stress genes and proteins (grp94, GRP78, atf4, ATF6, perk, eif2α, ire1, CHOP). ASTA alleviated OTA-induced mitochondrial dynamic imbalance, inhibited mitochondrial division (DRP1, mff), and promoted mitochondrial fusion (OPA1, MFN1, MFN2). In conclusion, ASTA can decrease OTA-induced oxidative damage, thereby alleviating endoplasmic reticulum stress and mitochondrial dynamic imbalance.

Sports activities and cardiovascular system change

Sports activity is generally considered to be beneficial to health. The World Health Organization (WHO) recommends physical activity as part of a healthy lifestyle. Sports activities significantly affect the cardiovascular system. A number of studies show that they significantly reduce the risk of cardiovascular disease as well as decrease cardiovascular mortality. This review discusses changes in various cardiovascular parameters in athletes - vagotonia/bradycardia, hypertrophy of heart, ECG changes, blood pressure, and variability of cardiovascular parameters. Because of its relationship to the cardiovascular system, VO2max, which is widely used as an indicator of cardiorespiratory fitness, is also discussed. The review concludes with a discussion of reactive oxygen species (ROS) and oxidative stress, particularly in relation to changes in the cardiovascular system in athletes. The review appropriately summarizes the above issues and points out some new implications.

FUNDC1/PFKP-mediated mitophagy induced by KD025 ameliorates cartilage degeneration in osteoarthritis

Osteoarthritis (OA) is the most common joint disease, but no disease-modifying drugs have been approved for OA treatment. Mitophagy participates in mitochondrial homeostasis regulation by selectively clearing dysfunctional mitochondria, which might contribute to cartilage degeneration in OA. Here, we provide evidence of impaired mitophagy in OA chondrocytes, which exacerbates chondrocyte degeneration. Among the several classic mitophagy-regulating pathways and receptors, we found that FUNDC1 plays a key role in preserving chondrocyte homeostasis by inducing mitophagy. FUNDC1 knockdown in vitro and knockout in vivo decreased mitophagy and exacerbated mitochondrial dysfunction, exacerbating chondrocyte degeneration and OA progression. FUNDC1 overexpression via intra-articular injection of adeno-associated virus alleviated cartilage degeneration in OA. Mechanistically, our study demonstrated that PFKP interacts with and dephosphorylates FUNDC1 to induce mitophagy in chondrocytes. Further analysis identified KD025 as a candidate drug for restoring chondrocyte mitophagy by increasing the FUNDC1-PFKP interaction and thus alleviating cartilage degeneration in mice with DMM-induced OA. Our study highlights the role of the FUNDC1-PFKP interaction in chondrocyte homeostasis via mitophagy induction and identifies KD025 as a promising agent for treating OA by increasing chondrocyte mitophagy.

A Kinase Interacting Protein 1 regulates mitochondrial protein levels in energy metabolism and promotes mitochondrial turnover after exercise

A Kinase Interacting Protein 1 (AKIP1) is a signalling adaptor that promotes mitochondrial respiration and attenuates mitochondrial oxidative stress in cultured cardiomyocytes. We sought to determine whether AKIP1 influences mitochondrial function and the mitochondrial adaptation in response to exercise in vivo. We assessed mitochondrial respiratory capacity, as well as electron microscopy and mitochondrial targeted-proteomics in hearts from mice with cardiomyocyte-specific overexpression of AKIP1 (AKIP1-TG) and their wild type (WT) littermates. These parameters were also assessed after four weeks of voluntary wheel running. In contrast to our previous in vitro study, respiratory capacity measured as state 3 respiration on palmitoyl carnitine was significantly lower in AKIP1-TG compared to WT mice, whereas state 3 respiration on pyruvate remained unaltered. Similar findings were observed for maximal respiration, after addition of FCCP. Mitochondrial DNA damage and oxidative stress markers were not elevated in AKIP1-TG mice and gross mitochondrial morphology was similar. Mitochondrial targeted-proteomics did reveal reductions in mitochondrial proteins involved in energy metabolism. Exercise performance was comparable between genotypes, whereas exercise-induced cardiac hypertrophy was significantly increased in AKIP1-TG mice. After exercise, mitochondrial state 3 respiration on pyruvate substrates was significantly lower in AKIP1-TG compared with WT mice, while respiration on palmitoyl carnitine was not further decreased. This was associated with increased mitochondrial fission on electron microscopy, and the activation of pathways associated with mitochondrial fission and mitophagy. This study suggests that AKIP1 regulates the mitochondrial proteome involved in energy metabolism and promotes mitochondrial turnover after exercise. Future studies are required to unravel the mechanistic underpinnings and whether the mitochondrial changes are required for the AKIP1-induced physiological cardiac growth.

Mitophagy for cardioprotection

Mitochondrial function is maintained by several strictly coordinated mechanisms, collectively termed mitochondrial quality control mechanisms, including fusion and fission, degradation, and biogenesis. As the primary source of energy in cardiomyocytes, mitochondria are the central organelle for maintaining cardiac function. Since adult cardiomyocytes in humans rarely divide, the number of dysfunctional mitochondria cannot easily be diluted through cell division. Thus, efficient degradation of dysfunctional mitochondria is crucial to maintaining cellular function. Mitophagy, a mitochondria specific form of autophagy, is a major mechanism by which damaged or unnecessary mitochondria are targeted and eliminated. Mitophagy is active in cardiomyocytes at baseline and in response to stress, and plays an essential role in maintaining the quality of mitochondria in cardiomyocytes. Mitophagy is mediated through multiple mechanisms in the heart, and each of these mechanisms can partially compensate for the loss of another mechanism. However, insufficient levels of mitophagy eventually lead to mitochondrial dysfunction and the development of heart failure. In this review, we discuss the molecular mechanisms of mitophagy in the heart and the role of mitophagy in cardiac pathophysiology, with the focus on recent findings in the field.

Association of an Increased Abnormal Mitochondria Ratio in Cardiomyocytes with a Prolonged Oxygen Uptake Time Constant during Cardiopulmonary Exercise Testing of Patients with Non-ischemic Cardiomyopathy

Objective The cardiac function, blood distribution, and oxygen extraction in the muscles as well as the pulmonary function determine the oxygen uptake (VO2) kinetics at the onset of exercise. This factor is called the VO2 time constant, and its prolongation is associated with an unfavorable prognosis for heart failure (HF). The mitochondrial function of skeletal muscle is known to reflect exercise tolerance. Morphological changes and dysfunction in cardiac mitochondria are closely related to HF severity and its prognosis. Although mitochondria play an important role in generating energy in cardiomyocytes, the relationship between cardiac mitochondria and the VO2 time constant has not been elucidated. Methods We calculated the ratio of abnormal cardiac mitochondria in human myocardial biopsy samples using an electron microscope and measured the VO2 time constant during cardiopulmonary exercise testing. The VO2 time constant was normalized by the fat-free mass index (FFMI). Patients Fifteen patients with non-ischemic cardiomyopathy (NICM) were included. Patients were divided into two groups according to their median VO2 time constant/FFMI value. Results Patients with a low VO2 time constant/FFMI value had a lower abnormal mitochondria ratio than those with a high VO2 time constant/FFMI value. A multiple linear regression analysis revealed that the ratio of abnormal cardiac mitochondria was independently associated with a high VO2 time constant/FFMI. Conclusion An increased abnormal cardiac mitochondria ratio might be associated with a high VO2 time constant/FFMI value in patients with NICM.

Augmented Parkin-dependent mitophagy underlies the hepatoprotective effect of remote ischemic conditioning used prior to hemorrhagic shock

BACKGROUND AND AIMS

Hemorrhagic shock-resuscitation (HSR) following trauma contributes to organ dysfunction by causing ischemia-reperfusion injury (IRI). We previously showed that 'remote ischemic preconditioning' (RIPC) exerted multi-organ protection from IRI. Maintenance of mitochondrial quality by clearance of dysfunctional mitochondria via mitophagy is vital in restoring organ integrity. We hypothesized that parkin-dependent mitophagy played a role in RIPC-induced hepatoprotection following HSR.

METHODS

The hepatoprotective effect of RIPC in a murine model of HSR-IRI was investigated in wild type and parkin-/- animals. Mice were subjected to HSR ± RIPC and blood and organs were collected, followed by cytokine ELISAs, histology, qPCR, Western blots, and transmission electron microscopy.

RESULTS

HSR increased hepatocellular injury, as measured by plasma ALT and liver necrosis, while antecedent RIPC prevented this injury; in parkin^-/- mice, RIPC failed to exert hepatoprotection. The ability of RIPC to lessen HSR-induced rises in plasma IL-6 and TNFα, was lost in parkin^-/- mice. While RIPC alone did not induce mitophagy, the application of RIPC prior to HSR caused a synergistic increase in mitophagy, this increase was not observed in parkin^-/- mice. RIPC induced shifts in mitochondrial morphology favoring mitophagy in WT but not in parkin^-/- animals.

CONCLUSIONS

RIPC was hepatoprotective in WT mice following HSR but not in parkin^-/- mice. Loss of protection in parkin^-/- mice corresponded with the failure of RIPC plus HSR to upregulate the mitophagic process. Improving mitochondrial quality by modulating mitophagy, may prove to be an attractive therapeutic target in disease processes caused by IRI.

Exercise After Acute Myocarditis: When and How to Return to Sports

Myocarditis is an inflammatory disease of the myocardium secondary to infectious and noninfectious insults. The most feared consequence of myocarditis is sudden cardiac death owing to electrical instability and arrhythmia. Typical presenting symptoms include chest pain, dyspnea, palpitations and/or heart failure. Diagnosis is usually made with history, electrocardiogram, biomarkers, echocardiogram, and cardiac MRI (CMR). Application of the Lake Louise criteria to CMR results can help identify cases of myocarditis. Treatment is usually supportive with medical therapy, and patients are recommended to abstain from exercise for 3 to 6 months. Exercise restrictions may be lifted after normalization on follow-up testing.

Human ApoE2 Endows Stronger Contractility in Rat Cardiomyocytes Enhancing Heart Function

(1) Background: Apolipoprotein E (ApoE) is a critical plasma apolipoprotein for lipid transport and nonlipid-related functions. Humans possess three isoforms of ApoE (2, 3, and 4). ApoE2, which exhibits beneficial effects on cardiac health, has not been adequately studied. (2) Methods: We investigated the cardiac phenotypes of the humanized ApoE knock-in (hApoE KI) rats and compared to wild-type (WT) and ApoE knock-out (ApoE KO) rats using echocardiography, ultrasound, blood pressure measurements, histology strategies, cell culture, Seahorse XF, cardiomyocyte contractility and intracellular Ca²⁺ tests, and Western blotting; (3) Results: hApoE2 rats exhibited enhanced heart contractile function without signs of detrimental remodeling. Isolated adult hApoE2 cardiomyocytes had faster and stronger sarcomere contractility because of more mitochondrial energy generation and stimulation-induced fast and elevated intracellular Ca²⁺ transient. The abundant energy is a result of elevated mitochondrial function via fatty acid β-oxidation. The fast and elevated Ca²⁺ transient is associated with decreased sarcoplasmic reticulum (SR) Ca²⁺ ATPase (SERCA2) and increased expression of cardiac ryanodine receptor 2 (RyR2) conducting a potent Ca²⁺ release from SR.; (4) Conclusions: Our studies validated the association of polymorphic ApoEs with cardiac health in the rat model, and revealed the possible mechanisms of the protective effect of ApoE2 against heart diseases.

Exercise preserves physical fitness during aging through AMPK and mitochondrial dynamics

Exercise is a nonpharmacological intervention that improves health during aging and a valuable tool in the diagnostics of aging-related diseases. In muscle, exercise transiently alters mitochondrial functionality and metabolism. Mitochondrial fission and fusion are critical effectors of mitochondrial plasticity, which allows a fine-tuned regulation of organelle connectiveness, size, and function. Here we have investigated the role of mitochondrial dynamics during exercise in the model organism Caenorhabditis elegans. We show that in body-wall muscle, a single exercise session induces a cycle of mitochondrial fragmentation followed by fusion after a recovery period, and that daily exercise sessions delay the mitochondrial fragmentation and physical fitness decline that occur with aging. Maintenance of proper mitochondrial dynamics is essential for physical fitness, its enhancement by exercise training, and exercise-induced remodeling of the proteome. Surprisingly, among the long-lived genotypes we analyzed (isp-1,nuo-6, daf-2, eat-2, and CA-AAK-2), constitutive activation of AMP-activated protein kinase (AMPK) uniquely preserves physical fitness during aging, a benefit that is abolished by impairment of mitochondrial fission or fusion. AMPK is also required for physical fitness to be enhanced by exercise, with our findings together suggesting that exercise may enhance muscle function through AMPK regulation of mitochondrial dynamics. Our results indicate that mitochondrial connectivity and the mitochondrial dynamics cycle are essential for maintaining physical fitness and exercise responsiveness during aging and suggest that AMPK activation may recapitulate some exercise benefits. Targeting mechanisms to optimize mitochondrial fission and fusion, as well as AMPK activation, may represent promising strategies for promoting muscle function during aging.

ALKBH5 attenuates mitochondrial fission and ameliorates liver fibrosis by reducing Drp1 methylation

Mitochondrial metabolism plays a pivotal role in various cellular processes and fibrosis. However, the mechanism underlying mitochondrial metabolic function and liver fibrosis remains poorly understood. In this study, we determined whether mitochondrial metabolism mediates liver fibrosis using cells, animal models, and clinical samples to elucidate the potential effects and underlying mechanism of mitochondrial metabolism in liver fibrosis. We report that AlkB Homolog 5 (ALKBH5) decreases mitochondrial membrane potential (MMP) and oxygen consumption rate (OCR), suppresses mitochondrial fission and hepatic stellate cell (HSC) proliferation and migration and ameliorates liver fibrosis. Enhancement of mitochondrial fission, an essential event during HSC proliferation and migration, is dependent on decreased ALKBH5 expression. Furthermore, we reveal that low ALKBH5 expression is associated with elevated N6-methyladenosine (m⁶A) mRNA levels. Mechanistically, ALKBH5 mediates m⁶A demethylation in the 3'UTR of Drp1 mRNA and induces its translation in a YTH domain family proteins 1 (YTHDF1)-independent manner. Subsequently, in transforming growth factor-β1 (TGF-β1) induced HSC, Dynamin-related protein 1 (Drp1) mediates mitochondrial fission and increases cell proliferation and migration. Decreased Drp1 expression inhibits mitochondrial fission and suppresses HSC proliferation and migration. Notably, human fibrotic liver and heart tissue exhibited enhanced mitochondrial fission; increased YTHDF1, Drp1, alpha-smooth muscle actin (α-SMA) and collagen I expression; decreased ALKBH5 expression and increased liver fibrosis. Our results highlight a novel mechanism by which ALKBH5 suppresses mitochondrial fission and HSC proliferation and migration by reducing Drp1 methylation in an m⁶A-YTHDF1-dependent manner, which may indicate a demethylation-based approach for liver fibrosis diagnosis and therapy.

Ubiquitin-mediated mitochondrial regulation by MITOL/MARCHF5 at a glance

Mitochondria are involved in various cellular processes, such as energy production, inflammatory responses and cell death. Mitochondrial dysfunction is associated with many age-related diseases, including neurological disorders and heart failure. Mitochondrial quality is strictly maintained by mitochondrial dynamics linked to an adequate supply of phospholipids and other substances from the endoplasmic reticulum (ER). The outer mitochondrial membrane-localized E3 ubiquitin ligase MITOL/MARCHF5 is responsible for mitochondrial quality control through the regulation of mitochondrial dynamics, formation of mitochondria-ER contacts and mitophagy. MITOL deficiency has been shown to impair mitochondrial function, cause an excessive inflammatory response and increase vulnerability to stress, resulting in the exacerbation of the disease. In this study, we overview the ubiquitin-mediated regulation of mitochondrial function by MITOL and the relationship between MITOL and diseases.

Mechanisms of mitochondrial respiratory adaptation

Mitochondrial energetic adaptations encompass a plethora of conserved processes that maintain cell and organismal fitness and survival in the changing environment by adjusting the respiratory capacity of mitochondria. These mitochondrial responses are governed by general principles of regulatory biology exemplified by changes in gene expression, protein translation, protein complex formation, transmembrane transport, enzymatic activities and metabolite levels. These changes can promote mitochondrial biogenesis and membrane dynamics that in turn support mitochondrial respiration. The main regulatory components of mitochondrial energetic adaptation include: the transcription coactivator peroxisome proliferator-activated receptor-γ (PPARγ) coactivator 1α (PGC1α) and associated transcription factors; mTOR and endoplasmic reticulum stress signalling; TOM70-dependent mitochondrial protein import; the cristae remodelling factors, including mitochondrial contact site and cristae organizing system (MICOS) and OPA1; lipid remodelling; and the assembly and metabolite-dependent regulation of respiratory complexes. These adaptive molecular and structural mechanisms increase respiration to maintain basic processes specific to cell types and tissues. Failure to execute these regulatory responses causes cell damage and inflammation or senescence, compromising cell survival and the ability to adapt to energetically demanding conditions. Thus, mitochondrial adaptive cellular processes are important for physiological responses, including to nutrient availability, temperature and physical activity, and their failure leads to diseases associated with mitochondrial dysfunction such as metabolic and age-associated diseases and cancer.

Integrative analysis of multiomics data identifies selenium-related gene ALAD associating with keshan disease

Keshan disease is an endemic fatal dilated cardiomyopathy that can cause heart enlargement, heart failure, and cardiogenic death. Selenium deficiency is considered to be the main cause of Keshan disease. However, the molecular mechanism underlying Keshan disease remains unclear. Our whole-exome sequencing from 68 patients with Keshan disease and 100 controls found 199 candidate genes by gene-level burden tests. Interestingly, using multiomics data, the selenium-related gene ALAD (δ-aminolevulinic acid dehydratase) was the only candidate causative gene identified by three different analysis approaches. Based on single-cell transcriptome data, ALAD was highly expressed in cardiomyocytes and double mutations of human ALAD dramatically reduced its enzyme activity in vitro compared to negative control. Functional analysis of ALAD inhibition in mice resulted in a Keshan phenotype with left ventricular enlargement and cardiac dysfunction, whereas administration of sodium selenite markedly reversed the changes caused by ALAD inhibition. In addition, sodium selenite reversed Keshan phenotypes by affecting energy metabolism and mitochondrial function in mice as shown by the transcriptomic and proteomic data and the ultrastructure of cardiac myocytes. Our findings are the first to demonstrate that the selenium-related gene ALAD is essential for cardiac function by maintaining normal mitochondrial activity, providing strong molecular evidence supporting the hypothesis of selenium deficiency in Keshan disease. These results identified ALAD as a novel target for therapeutic intervention in Keshan disease and Keshan disease-related dilated cardiomyopathy.

The role of mitochondrial fission in cardiovascular health and disease

Mitochondria are organelles involved in the regulation of various important cellular processes, ranging from ATP generation to immune activation. A healthy mitochondrial network is essential for cardiovascular function and adaptation to pathological stressors. Mitochondria undergo fission or fusion in response to various environmental cues, and these dynamic changes are vital for mitochondrial function and health. In particular, mitochondrial fission is closely coordinated with the cell cycle and is linked to changes in mitochondrial respiration and membrane permeability. Another key function of fission is the segregation of damaged mitochondrial components for degradation by mitochondrial autophagy (mitophagy). Mitochondrial fission is induced by the large GTPase dynamin-related protein 1 (DRP1) and is subject to sophisticated regulation. Activation requires various post-translational modifications of DRP1, actin polymerization and the involvement of other organelles such as the endoplasmic reticulum, Golgi apparatus and lysosomes. A decrease in mitochondrial fusion can also shift the balance towards mitochondrial fission. Although mitochondrial fission is necessary for cellular homeostasis, this process is often aberrantly activated in cardiovascular disease. Indeed, strong evidence exists that abnormal mitochondrial fission directly contributes to disease development. In this Review, we compare the physiological and pathophysiological roles of mitochondrial fission and discuss the therapeutic potential of preventing excessive mitochondrial fission in the heart and vasculature.

Understanding MCL1: from cellular function and regulation to pharmacological inhibition

Myeloid cell leukemia-1 (MCL1), an antiapoptotic member of the BCL2 family characterized by a short half-life, functions as a rapid sensor that regulates cell death and other relevant processes that include cell cycle progression and mitochondrial homeostasis. In cancer, MCL1 overexpression contributes to cell survival and resistance to diverse chemotherapeutic agents; for this reason, several MCL1 inhibitors are currently under preclinical and clinical development for cancer treatment. However, the nonapoptotic functions of MCL1 may influence their therapeutic potential. Overall, the complexity of MCL1 regulation and function represent challenges to the clinical application of MCL1 inhibitors. We now summarize the current knowledge regarding MCL1 structure, regulation, and function that could impact the clinical success of MCL1 inhibitors.

Mitochondrial Genome Variants as a Cause of Mitochondrial Cardiomyopathy

Mitochondria are small double-membraned organelles responsible for the generation of energy used in the body in the form of ATP. Mitochondria are unique in that they contain their own circular mitochondrial genome termed mtDNA. mtDNA codes for 37 genes, and together with the nuclear genome (nDNA), dictate mitochondrial structure and function. Not surprisingly, pathogenic variants in the mtDNA or nDNA can result in mitochondrial disease. Mitochondrial disease primarily impacts tissues with high energy demands, including the heart. Mitochondrial cardiomyopathy is characterized by the abnormal structure or function of the myocardium secondary to genetic defects in either the nDNA or mtDNA. Mitochondrial cardiomyopathy can be isolated or part of a syndromic mitochondrial disease. Common manifestations of mitochondrial cardiomyopathy are a phenocopy of hypertrophic cardiomyopathy, dilated cardiomyopathy, and cardiac conduction defects. The underlying pathophysiology of mitochondrial cardiomyopathy is complex and likely involves multiple abnormal processes in the cell, stemming from deficient oxidative phosphorylation and ATP depletion. Possible pathophysiology includes the activation of alternative metabolic pathways, the accumulation of reactive oxygen species, dysfunctional mitochondrial dynamics, abnormal calcium homeostasis, and mitochondrial iron overload. Here, we highlight the clinical assessment of mtDNA-related mitochondrial cardiomyopathy and offer a novel hypothesis of a possible integrated, multivariable pathophysiology of disease.

Adrenergic Receptor Regulation of Mitochondrial Function in Cardiomyocytes

ABSTRACT

Adrenergic receptors (ARs) are G protein-coupled receptors that are stimulated by catecholamines to induce a wide array of physiological effects across tissue types. Both α1- and β-ARs are found on cardiomyocytes and regulate cardiac contractility and hypertrophy through diverse molecular pathways. Acute activation of cardiomyocyte β-ARs increases heart rate and contractility as an adaptive stress response. However, chronic β-AR stimulation contributes to the pathobiology of heart failure. By contrast, mounting evidence suggests that α1-ARs serve protective functions that may mitigate the deleterious effects of chronic β-AR activation. Here, we will review recent studies demonstrating that α1- and β-ARs differentially regulate mitochondrial biogenesis and dynamics, mitochondrial calcium handling, and oxidative phosphorylation in cardiomyocytes. We will identify potential mechanisms of these actions and focus on the implications of these findings for the modulation of contractile function in the uninjured and failing heart. Collectively, we hope to elucidate important physiological processes through which these well-studied and clinically relevant receptors stimulate and fuel cardiac contraction to contribute to myocardial health and disease.

Exercise training maintains cardiovascular health: signaling pathways involved and potential therapeutics

Exercise training has been widely recognized as a healthy lifestyle as well as an effective non-drug therapeutic strategy for cardiovascular diseases (CVD). Functional and mechanistic studies that employ animal exercise models as well as observational and interventional cohort studies with human participants, have contributed considerably in delineating the essential signaling pathways by which exercise promotes cardiovascular fitness and health. First, this review summarizes the beneficial impact of exercise on multiple aspects of cardiovascular health. We then discuss in detail the signaling pathways mediating exercise's benefits for cardiovascular health. The exercise-regulated signaling cascades have been shown to confer myocardial protection and drive systemic adaptations. The signaling molecules that are necessary for exercise-induced physiological cardiac hypertrophy have the potential to attenuate myocardial injury and reverse cardiac remodeling. Exercise-regulated noncoding RNAs and their associated signaling pathways are also discussed in detail for their roles and mechanisms in exercise-induced cardioprotective effects. Moreover, we address the exercise-mediated signaling pathways and molecules that can serve as potential therapeutic targets ranging from pharmacological approaches to gene therapies in CVD. We also discuss multiple factors that influence exercise's effect and highlight the importance and need for further investigations regarding the exercise-regulated molecules as therapeutic targets and biomarkers for CVD as well as the cross talk between the heart and other tissues or organs during exercise. We conclude that a deep understanding of the signaling pathways involved in exercise's benefits for cardiovascular health will undoubtedly contribute to the identification and development of novel therapeutic targets and strategies for CVD.

Brief exposure of neuronal cells to levels of SCFAs observed in human systemic circulation impair lipid metabolism resulting in apoptosis

Communication between gut microbiota and the brain is an enigma. Alterations in the gut microbial community affects enteric metabolite levels, such as short chain fatty acids (SCFAs). SCFAs have been proposed as a possible mechanism through which the gut microbiome modulate brain health and function. This study analyzed for the first time the effects of SCFAs at levels reported in human systemic circulation on SH-SY5Y human neuronal cell energy metabolism, viability, survival, and the brain lipidome. Cell and rat brain lipidomics was done using high resolution mass spectrometry (HRMS). Neuronal cells viability, survival and energy metabolism were analyzed via flow cytometer, immunofluorescence, and SeahorseXF platform. Lipidomics analysis demonstrated that SCFAs significantly remodeled the brain lipidome in vivo and in vitro. The most notable remodulation was observed in the metabolism of phosphatidylethanolamine plasmalogens, and mitochondrial lipids carnitine and cardiolipin. Increased mitochondrial mass, fragmentation, and hyperfusion occurred concomitant with the altered mitochondrial lipid metabolism resulting in decreased neuronal cell respiration, adenosine triphosphate (ATP) production, and increased cell death. This suggests SCFAs at levels observed in human systemic circulation can adversely alter the brain lipidome and neuronal cell function potentially negatively impacting brain health outcomes.

Influence of biological sex and exercise on murine cardiac metabolism

Although the structural and functional effects of exercise on the heart are well established, the metabolic changes that occur in the heart during and after exercise remain unclear. In this study, we used metabolomics to assess time-dependent changes in the murine cardiac metabolome following 1 session of treadmill exercise. After the exercise bout, we also recorded blood lactate, glucose, and ketone body levels and measured cardiac mitochondrial respiration. In both male and female mice, moderate- and high-intensity exercise acutely increased blood lactate levels. In both sexes, low- and moderate-intensity exercise augmented circulating 3-hydroxybutryrate levels immediately after the exercise bout; however, only in female mice did high-intensity exercise increase 3-hydroxybutyrate levels, with significant increases occurring 1 h after the exercise session. Untargeted metabolomics analyses of sedentary female and male hearts suggest considerable sex-dependent differences in basal cardiac metabolite levels, with female hearts characterized by higher levels of pantothenate, pyridoxamine, homoarginine, tryptophan, and several glycerophospholipid and sphingomyelin species and lower levels of numerous metabolites, including acetyl coenzyme A, glucuronate, gulonate, hydroxyproline, prolyl-hydroxyproline, carnosine, anserine, and carnitinylated and glycinated species, as compared with male hearts. Immediately after a bout of treadmill exercise, both male and female hearts had higher levels of corticosterone; however, female mice showed more extensive exercise-induced changes in the cardiac metabolome, characterized by significant, time-dependent changes in amino acids (e.g., serine, alanine, tyrosine, tryptophan, branched-chain amino acids) and the ketone body 3-hydroxybutyrate. Results from experiments using isolated cardiac mitochondria suggest that high-intensity treadmill exercise does not acutely affect respiration or mitochondrial coupling; however, female cardiac mitochondria demonstrate generally higher adenosine diphosphate sensitivity compared with male cardiac mitochondria. Collectively, these findings in mice reveal key sex-dependent differences in cardiac metabolism and suggest that the metabolic network in the female heart is more responsive to physiological stress caused by exercise.

Mitochondrial Quality Control in the Heart: The Balance between Physiological and Pathological Stress

Alterations in mitochondrial function and morphology are critical adaptations to cardiovascular stress, working in concert in an attempt to restore organelle-level and cellular-level homeostasis. Processes that alter mitochondrial morphology include fission, fusion, mitophagy, and biogenesis, and these interact to maintain mitochondrial quality control. Not all cardiovascular stress is pathologic (e.g., ischemia, pressure overload, cardiotoxins), despite a wealth of studies to this effect. Physiological stress, such as that induced by aerobic exercise, can induce morphologic adaptations that share many common pathways with pathological stress, but in this case result in improved mitochondrial health. Developing a better understanding of the mechanisms underlying alterations in mitochondrial quality control under diverse cardiovascular stressors will aid in the development of pharmacologic interventions aimed at restoring cellular homeostasis.

GJA1-20k and Mitochondrial Dynamics

Connexin 43 (Cx43) is the primary gap junction protein of mammalian heart ventricles and is encoded by the gene Gja1 which has a single coding exon and therefore cannot be spliced. We previously identified that Gja1 mRNA undergoes endogenous internal translation initiated at one of several internal AUG (M) start codons, generating N-terminal truncated protein isoforms that retain the C-terminus distal to the start site. GJA1-20k, whose translation initiates at mRNA M213, is usually the most abundant isoform in cells and greatly increases after ischemic and metabolic stress. GJA1-20k consists of a small segment of the last transmembrane domain and the complete C-terminus tail of Cx43, with a total size of about 20 kDa. The original role identified for GJA1-20k is as an essential subunit that facilitates the trafficking of full-length Cx43 hexameric hemichannels to cell-cell contacts, generating traditional gap junctions between adjacent cells facilitating, in cardiac muscle, efficient spread of electrical excitation. GJA1-20k deficient mice (generated by a M213L substitution in Gja1) suffer poor electrical coupling between cardiomycytes and arrhythmogenic sudden death two to 4 weeks after their birth. We recently identified that exogenous GJA1-20k expression also mimics the effect of ischemic preconditioning in mouse heart. Furthermore, GJA1-20k localizes to the mitochondrial outer membrane and induces a protective and DRP1 independent form of mitochondrial fission, preserving ATP production and generating less reactive oxygen species (ROS) under metabolic stress, providing powerful protection of myocardium to ischemic insult. In this manuscript, we focus on the detailed roles of GJA1-20k in mitochondria, and its interaction with the actin cytoskeleton.

Sleeve Gastrectomy-Induced AMPK Activation Attenuates Diabetic Cardiomyopathy by Maintaining Mitochondrial Homeostasis via NR4A1 Suppression in Rats

Diabetic cardiomyopathy (DCM) is characterized by impaired diastolic and systolic myocardial performance and is a major cause of morbidity and mortality in patients with diabetes. Surgical bariatric procedures, such as sleeve gastrectomy (SG), result in remission of type 2 diabetes (T2DM) and have benefits with myocardial function. Maintaining cardiac mitochondrial homeostasis is a promising therapeutic strategy for DCM. However, whether SG surgery affects mitochondrial function and its underlying mechanism remains unclear. This study aimed to investigate the effect of SG surgery on mitochondrial homeostasis and intracellular oxidative stress in rats with DCM. We also examined cellular phenotypes and molecular mechanisms in high glucose and high fat-stimulated myocytes. The rat model of DCM was established by high-fat diet feeding and low-dose streptozotocin injection. We observed a remarkably metabolic benefit of SG, including a reduced body weight, food intake, blood glucose levels, and restored glucose tolerance and insulin sensitivity post-operatively. Also, SG ameliorated the pathological cardiac hypertrophy, myocardial fibrosis and the dysfunction of myocardial contraction and diastole, consequently delayed the progression of DCM. Also, SG restored the mitochondrial dysfunction and fragmentation through the AMPK signaling activation mediated nuclear receptor subfamily 4 group A member 1 (NR4A1)/DRP1 suppression in vivo. H9c2 cardiomyocytes showed that activation of AMPK could reverse the mitochondrial dysfunction somehow. Collectively, our study provided evidence that SG surgery could alleviate mitochondrial dysfunction in DCM. Moreover, AMPK-activated NR4A1/DRP1 repression might act as a significant reason for maintaining mitochondrial homeostasis in the myocardium, thus contributing to morphological and functional alleviation of DCM.

Regulation of Mitochondrial Dynamics by Aerobic Exercise in Cardiovascular Diseases

Mitochondrial dynamics, including continuous biogenesis, fusion, fission, and autophagy, are crucial to maintain mitochondrial integrity, distribution, size, and function, and play an important role in cardiovascular homeostasis. Cardiovascular health improves with aerobic exercise, a well-recognized non-pharmaceutical intervention for both healthy and ill individuals that reduces overall cardiovascular disease (CVD) mortality. Increasing evidence shows that aerobic exercise can effectively regulate the coordinated circulation of mitochondrial dynamics, thus inhibiting CVD development. This review aims to illustrate the benefits of aerobic exercise in prevention and treatment of cardiovascular disease by modulating mitochondrial function.

Exercise: a molecular tool to boost muscle growth and mitochondrial performance in heart failure?

Impaired exercise capacity is the key symptom of heart failure (HF) and is associated with reduced quality of life and higher mortality rates. Unfortunately, current therapies, although generally lifesaving, have only small or marginal effects on exercise capacity. Specific strategies to alleviate exercise intolerance may improve quality of life, while possibly improving prognosis as well. There is overwhelming evidence that physical exercise improves performance in cardiac and skeletal muscles in health and disease. Unravelling the mechanistic underpinnings of exercise‐induced improvements in muscle function could provide targets that will allow us to boost exercise performance in HF. With the current review we discuss: (i) recently discovered signalling pathways that govern physiological muscle growth as well as mitochondrial quality control mechanisms that underlie metabolic adaptations to exercise; (ii) the mechanistic underpinnings of exercise intolerance in HF and the benefits of exercise in HF patients on molecular, functional and prognostic levels; and (iii) potential molecular therapeutics to improve exercise performance in HF. We propose that novel molecular therapies to boost adaptive muscle growth and mitochondrial quality control in HF should always be combined with some form of exercise training.

Altered Cardiac Energetics and Mitochondrial Dysfunction in Hypertrophic Cardiomyopathy

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is a complex disease partly explained by the effects of individual gene variants on sarcomeric protein biomechanics. At the cellular level, HCM mutations most commonly enhance force production, leading to higher energy demands. Despite significant advances in elucidating sarcomeric structure-function relationships, there is still much to be learned about the mechanisms that link altered cardiac energetics to HCM phenotypes. In this work, we test the hypothesis that changes in cardiac energetics represent a common pathophysiologic pathway in HCM.

METHODS

We performed a comprehensive multiomics profile of the molecular (transcripts, metabolites, and complex lipids), ultrastructural, and functional components of HCM energetics using myocardial samples from 27 HCM patients and 13 normal controls (donor hearts).

RESULTS

Integrated omics analysis revealed alterations in a wide array of biochemical pathways with major dysregulation in fatty acid metabolism, reduction of acylcarnitines, and accumulation of free fatty acids. HCM hearts showed evidence of global energetic decompensation manifested by a decrease in high energy phosphate metabolites (ATP, ADP, and phosphocreatine) and a reduction in mitochondrial genes involved in creatine kinase and ATP synthesis. Accompanying these metabolic derangements, electron microscopy showed an increased fraction of severely damaged mitochondria with reduced cristae density, coinciding with reduced citrate synthase activity and mitochondrial oxidative respiration. These mitochondrial abnormalities were associated with elevated reactive oxygen species and reduced antioxidant defenses. However, despite significant mitochondrial injury, HCM hearts failed to upregulate mitophagic clearance.

CONCLUSIONS

Overall, our findings suggest that perturbed metabolic signaling and mitochondrial dysfunction are common pathogenic mechanisms in patients with HCM. These results highlight potential new drug targets for attenuation of the clinical disease through improving metabolic function and reducing mitochondrial injury.

Decreased dynamin-related protein 1-related mitophagy induces myocardial apoptosis in the aging heart

An increase in cardiomyocyte apoptosis is the main contributor to the observed high morbidity of cardiac disease during aging. Mitochondria play important roles in cardiac apoptosis, and dynamin-related protein 1 (Drp1) is the critical factor that participates in mitochondrial fission and induces mitophagy to maintain mitochondria quality. However, whether Drp1 is involved in the increase of apoptosis in aging heart remains unclear. The purpose of this study was to determine whether Drp1 participates in inducing the apoptosis through regulating mitophagy in aging myocardium. To explore the effect of mitophagy and apoptosis in aging heart, we detected the expression of COX IV and the co-localization of COX IV and LC3 II, which reflect mitophagy, and measured adenosine triphosphate and reactive oxygen species contents, which reflect mitochondrial injury. Cell apoptosis was detected by measuring the activity of caspase-3 and the expression of cleaved caspase-3 and further confirmed by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) assay. The results showed an increase in apoptosis and a decrease in mitophagy in aging cardiomyocytes, and apoptosis was ameliorated after the induction of mitophagy by carbonyl cyanide m-chlorophenyl hydrazone (a mitophagy activator) in D-galactose (D-gal)-induced senescence H9c2 cells. To clarify the role of Drp1 in apoptosis, we knocked down Drp1 by transfecting si-Drp1, or overexpressed Drp1 in senescent cells, and then detected mitophagy, mitochondrial injury, and apoptosis. The data showed that downregulated Drp1 induces mitochondrial damage and apoptosis. In addition, to explore the regulatory relationship between Drp1 and phosphatase and tensin homologue (PTEN)-induced putative kinase 1 (PINK1)/Parkin-mediated mitophagy, we detected the expressions of PINK1 and Parkin after the overexpression of Drp1 in the D-gal group cells and found that Drp1-mediated mitophagy inhibited the PINK1/Parkin pathway in senescent cells. Our results demonstrated that insufficient Drp1 induces cardiomyocyte apoptosis by inhibiting mitophagy, and Drp1 affects the PINK1/Parkin pathway of mitophagy in the aging heart.

Protective mitochondrial fission induced by stress-responsive protein GJA1-20k

The Connexin43 gap junction gene GJA1 has one coding exon, but its mRNA undergoes internal translation to generate N-terminal truncated isoforms of Connexin43 with the predominant isoform being only 20 kDa in size (GJA1-20k). Endogenous GJA1-20k protein is not membrane bound and has been found to increase in response to ischemic stress, localize to mitochondria, and mimic ischemic preconditioning protection in the heart. However, it is not known how GJA1-20k benefits mitochondria to provide this protection. Here, using human cells and mice, we identify that GJA1-20k polymerizes actin around mitochondria which induces focal constriction sites. Mitochondrial fission events occur within about 45 s of GJA1-20k recruitment of actin. Interestingly, GJA1-20k mediated fission is independent of canonical Dynamin-Related Protein 1 (DRP1). We find that GJA1-20k-induced smaller mitochondria have decreased reactive oxygen species (ROS) generation and, in hearts, provide potent protection against ischemia-reperfusion injury. The results indicate that stress responsive internally translated GJA1-20k stabilizes polymerized actin filaments to stimulate non-canonical mitochondrial fission which limits ischemic-reperfusion induced myocardial infarction.

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.

Hypothermia promotes mitochondrial elongation In cardiac cells via inhibition of Drp1

Hypothermia is a valuable clinical tool in mitigating against the consequences of ischemia in surgery, stroke, cardiac arrest and organ preservation. Protection is afforded principally by a reduction of metabolism, manifesting as reduced rates of oxygen uptake, preservation of ATP levels, and a curtailing of ischemic calcium overload. The effects of non-ischemic hypothermic stress are relatively unknown. We sought to investigate the effects of clinically mild-to-severe hypothermia on mitochondrial morphology, oxygen consumption and protein expression in normoxic hearts and cardiac cells. Normoxic perfusion of rat hearts at 28-32 °C was associated with inhibition of mitochondrial fission, evidenced by a reduced abundance of the active phosphorylated form of the fission receptor Drp1 (pDrp1^S616). Abundance of the same residue was reduced in H9c2 cells subjected to hypothermic culture (25-32 °C), in addition to a reduced abundance of the Drp1 receptor MFF. Hypothermia-treated H9c2 cardiomyocytes exhibited elongated mitochondria and depressed rates of mitochondrial-associated oxygen consumption, which persisted upon rewarming. Hypothermia also promoted a reduction in mRNA expression of the capsaicin receptor TRPV1 in H9c2 cells. When normothermic H9c2 cells were transfected with TRPV1 siRNA we observed reduced pDrp1^S616 and MFF abundance, elongated mitochondria, and reduced rates of mitochondrial-associated oxygen consumption, mimicking the effects of hypothermic culture. In conclusion hypothermia promoted elongation of cardiac mitochondria via reduced pDrp1^S616 abundance which was also associated with suppression of cellular oxygen consumption. Silencing of TRPV1 in H9c2 cardiomyocytes reproduced the morphological and respirometric phenotype of hypothermia. This report demonstrates a novel mechanism of cold-induced inhibition of mitochondrial fission.

Structural and Functional Remodeling of Mitochondria in Cardiac Diseases

Mitochondria undergo structural and functional remodeling to meet the cell demand in response to the intracellular and extracellular stimulations, playing an essential role in maintaining normal cellular function. Merging evidence demonstrated that dysregulation of mitochondrial remodeling is a fundamental driving force of complex human diseases, highlighting its crucial pathophysiological roles and therapeutic potential. In this review, we outlined the progress of the molecular basis of mitochondrial structural and functional remodeling and their regulatory network. In particular, we summarized the latest evidence of the fundamental association of impaired mitochondrial remodeling in developing diverse cardiac diseases and the underlying mechanisms. We also explored the therapeutic potential related to mitochondrial remodeling and future research direction. This updated information would improve our knowledge of mitochondrial biology and cardiac diseases’ pathogenesis, which would inspire new potential strategies for treating these diseases by targeting mitochondria remodeling.

At the heart of mitochondrial quality control: many roads to the top

Mitochondrial quality control depends upon selective elimination of damaged mitochondria, replacement by mitochondrial biogenesis, redistribution of mitochondrial components across the network by fusion, and segregation of damaged mitochondria by fission prior to mitophagy. In this review, we focus on mitochondrial dynamics (fusion/fission), mitophagy, and other mechanisms supporting mitochondrial quality control including maintenance of mtDNA and the mitochondrial unfolded protein response, particularly in the context of the heart.

FGF1ΔHBS prevents diabetic cardiomyopathy by maintaining mitochondrial homeostasis and reducing oxidative stress via AMPK/Nur77 suppression

As a classically known mitogen, fibroblast growth factor 1 (FGF1) has been found to exert other pleiotropic functions such as metabolic regulation and myocardial protection. Here, we show that serum levels of FGF1 were decreased and positively correlated with fraction shortening in diabetic cardiomyopathy (DCM) patients, indicating that FGF1 is a potential therapeutic target for DCM. We found that treatment with a FGF1 variant (FGF1^∆HBS) with reduced proliferative potency prevented diabetes-induced cardiac injury and remodeling and restored cardiac function. RNA-Seq results obtained from the cardiac tissues of db/db mice showed significant increase in the expression levels of anti-oxidative genes and decrease of Nur77 by FGF1^∆HBS treatment. Both in vivo and in vitro studies indicate that FGF1^∆HBS exerted these beneficial effects by markedly reducing mitochondrial fragmentation, reactive oxygen species (ROS) generation and cytochrome c leakage and enhancing mitochondrial respiration rate and β-oxidation in a 5' AMP-activated protein kinase (AMPK)/Nur77-dependent manner, all of which were not observed in the AMPK null mice. The favorable metabolic activity and reduced proliferative properties of FGF1^∆HBS testify to its promising potential for use in the treatment of DCM and other metabolic disorders.

Effects of hyperbaric environment on endurance and metabolism are exposure time-dependent in well-trained mice

Hyperbaric exposure (1.3 atmospheres absolute with 20.9% O2 ) for 1 h a day was shown to improve exercise capacity. The present study was designed to reveal whether the daily exposure time affects exercise performance and metabolism in skeletal and cardiac muscles. Male mice in the training group were housed in a cage with a wheel activity device for 7 weeks from 5 weeks old. Trained mice were then subjected to hybrid training (HT, endurance exercise for 30 min followed by sprint interval exercise for 30 min). Hyperbaric exposure was applied following daily HT for 15 min (15HT), 30 min (30HT), or 60 min (60HT) for 4 weeks. In the endurance capacity test, maximal work values were significantly increased by 30HT and 60HT. In the left ventricle (LV), activity levels of 3-hydroxyacyl-CoA-dehydrogenase, citrate synthase, and carnitine palmitoyl transferase (CPT) 2 were significantly increased by 60HT. CPT2 activity levels were markedly increased by hyperbaric exposure in red gastrocnemius (Gr) and plantaris muscle (PL). Pyruvate dehydrogenase complex activity values in PL were enhanced more by 30HT and 60HT than by HT. Protein levels of N-terminal isoform of PGC1α (NT-PGC1α) protein were significantly enhanced in three hyperbaric exposed groups in Gr, but not in LV. These results indicate that hyperbaric exposure for 30 min or longer has beneficial effects on endurance, and 60-min exposure has the potential to further increase performance by facilitating fatty acid metabolism in skeletal and cardiac muscles in highly trained mice. NT-PGC1α may have important roles for these adaptations in skeletal muscle.

Mitochondrial quality surveillance as a therapeutic target in myocardial infarction

Myocardial infarction (MI) is a leading cause of morbidity and mortality worldwide. As mitochondrial dysfunction critically contributes to the pathogenesis of MI, intensive research is focused on the development of therapeutic strategies targeting mitochondrial homeostasis. Mitochondria possess a quality control system which maintains and restores their structure and function by regulating mitochondrial fission, fusion, biogenesis, degradation and death. In response to slight damage such as transient hypoxia or mild oxidative stress, mitochondrial metabolism shifts from oxidative phosphorylation to glycolysis, in order to reduce oxygen consumption and maintain ATP output. Mitochondrial dynamics are also activated to modify mitochondrial shape and structure, in order to meet cardiomyocyte energy requirements through augmenting or reducing mitochondrial mass. When damaged mitochondria cannot be repaired, poorly structured mitochondria will be degraded through mitophagy, a process which is often accompanied by mitochondrial biogenesis. Once the insult is severe enough to induce lethal damage in the mitochondria and the cell, mitochondrial death pathway activation is an inevitable consequence, and the cardiomyocyte apoptosis or necrosis program will be initiated to remove damaged cells. Mitochondrial quality surveillance is a hierarchical system preserving mitochondrial function and defending cardiomyocytes against stress. A failure of this system has been regarded as one of the potential pathologies underlying MI. In this review, we discuss the recent findings focusing on the role of mitochondrial quality surveillance in MI, and highlight the available therapeutic approaches targeting mitochondrial quality surveillance during MI.

β2-Adrenergic Signaling Modulates Mitochondrial Function and Morphology in Skeletal Muscle in Response to Aerobic Exercise

The molecular mechanisms underlying skeletal muscle mitochondrial adaptations induced by aerobic exercise (AE) are not fully understood. We have previously shown that AE induces mitochondrial adaptations in cardiac muscle, mediated by sympathetic stimulation. Since direct sympathetic innervation of neuromuscular junctions influences skeletal muscle homeostasis, we tested the hypothesis that β₂-adrenergic receptor (β₂-AR)-mediated sympathetic activation induces mitochondrial adaptations to AE in skeletal muscle. Male FVB mice were subjected to a single bout of AE on a treadmill (80% Vmax, 60 min) under β₂-AR blockade with ICI 118,551 (ICI) or vehicle, and parameters of mitochondrial function and morphology/dynamics were evaluated. An acute bout of AE significantly increased maximal mitochondrial respiration in tibialis anterior (TA) isolated fiber bundles, which was prevented by β₂-AR blockade. This increased mitochondrial function after AE was accompanied by a change in mitochondrial morphology towards fusion, associated with increased Mfn1 protein expression and activity. β₂-AR blockade fully prevented the increase in Mfn1 activity and reduced mitochondrial elongation. To determine the mechanisms involved in mitochondrial modulation by β₂-AR activation in skeletal muscle during AE, we used C2C12 myotubes, treated with the non-selective β-AR agonist isoproterenol (ISO) in the presence of the specific β₂-AR antagonist ICI or during protein kinase A (PKA) and Gα_i protein blockade. Our in vitro data show that β-AR activation significantly increases mitochondrial respiration in myotubes, and this response was dependent on β₂-AR activation through a Gα_s-PKA signaling cascade. In conclusion, we provide evidence for AE-induced β₂-AR activation as a major mechanism leading to alterations in mitochondria function and morphology/dynamics. β₂-AR signaling is thus a key-signaling pathway that contributes to skeletal muscle plasticity in response to exercise.

Mitochondrial architecture in cardiac myocytes depends on cell shape and matrix rigidity

Contraction of cardiac myocytes depends on energy generated by the mitochondria. During cardiac development and disease, the structure and function of the mitochondrial network in cardiac myocytes is known to remodel in concert with many other factors, including changes in nutrient availability, hemodynamic load, extracellular matrix (ECM) rigidity, cell shape, and maturation of other intracellular structures. However, the independent role of each of these factors on mitochondrial network architecture is poorly understood. In this study, we tested the hypothesis that cell aspect ratio (AR) and ECM rigidity regulate the architecture of the mitochondrial network in cardiac myocytes. To do this, we spin-coated glass coverslips with a soft, moderate, or stiff polymer. Next, we microcontact printed cell-sized rectangles of fibronectin with AR matching cardiac myocytes at various developmental or disease states onto the polymer surface. We then cultured neonatal rat ventricular myocytes on the patterned surfaces and used confocal microscopy and image processing techniques to quantify sarcomeric α-actinin volume, nucleus volume, and mitochondrial volume, surface area, and size distribution. On some substrates, α-actinin volume increased with cell AR but was not affected by ECM rigidity. Nucleus volume was mostly uniform across all conditions. In contrast, mitochondrial volume increased with cell AR on all substrates. Furthermore, mitochondrial surface area to volume ratio decreased as AR increased on all substrates. Large mitochondria were also more prevalent in cardiac myocytes with higher AR. For select AR, mitochondria were also smaller as ECM rigidity increased. Collectively, these results suggest that mitochondrial architecture in cardiac myocytes is strongly influenced by cell shape and moderately influenced by ECM rigidity. These data have important implications for understanding the factors that impact metabolic performance during heart development and disease.

Mitochondrial Fission Protein 1: Emerging Roles in Organellar Form and Function in Health and Disease

Mitochondrial fission protein 1 (Fis1) was identified in yeast as being essential for mitochondrial division or fission and subsequently determined to mediate human mitochondrial and peroxisomal fission. Yet, its exact functions in humans, especially in regard to mitochondrial fission, remains an enigma as genetic deletion of Fis1 elongates mitochondria in some cell types, but not others. Fis1 has also been identified as an important component of apoptotic and mitophagic pathways suggesting the protein may have multiple, essential roles. This review presents current perspectives on the emerging functions of Fis1 and their implications in human health and diseases, with an emphasis on Fis1's role in both endocrine and neurological disorders.

The Molecular Mechanisms Associated with Aerobic Exercise-Induced Cardiac Regeneration

The leading cause of heart failure is cardiomyopathy and damage to the cardiomyocytes. Adult mammalian cardiomyocytes have the ability to regenerate, but this cannot wholly compensate for myocardial cell loss after myocardial injury. Studies have shown that exercise has a regulatory role in the activation and promotion of regeneration of healthy and injured adult cardiomyocytes. However, current research on the effects of aerobic exercise in myocardial regeneration is not comprehensive. This review discusses the relationships between aerobic exercise and the regeneration of cardiomyocytes with respect to complex molecular and cellular mechanisms, paracrine factors, transcriptional factors, signaling pathways, and microRNAs that induce cardiac regeneration. The topics discussed herein provide a knowledge base for physical activity-induced cardiomyocyte regeneration, in which exercise enhances overall heart function and improves the efficacy of cardiac rehabilitation.

Mitochondrial Membrane Intracellular Communication in Healthy and Diseased Myocardium

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