Excitation-contraction coupling and mitochondrial energetics

Empagliflozin Improves Diastolic Function in HFpEF by Restabilizing the Mitochondrial Respiratory Chain

BACKGROUND

Clinical studies demonstrated beneficial effects of sodium-glucose-transporter 2 inhibitors on the risk of cardiovascular death in patients with heart failure with preserved ejection fraction (HFpEF). However, underlying processes for cardioprotection remain unclear. The present study focused on the impact of empagliflozin (Empa) on myocardial function in a rat model with established HFpEF and analyzed underlying molecular mechanisms.

METHODS

Obese ZSF1 (Zucker fatty and spontaneously hypertensive) rats were randomized to standard care (HFpEF, n=18) or Empa (HFpEF/Empa, n=18). ZSF1 lean rats (con, n=18) served as healthy controls. Echocardiography was performed at baseline and after 4 and 8 weeks, respectively. After 8 weeks of treatment, hemodynamics were measured invasively, mitochondrial function was assessed and myocardial tissue was collected for either molecular and histological analyses or transmission electron microscopy.

RESULTS

In HFpEF Empa significantly improved diastolic function (E/é: con: 17.5±2.8; HFpEF: 24.4±4.6; P<0.001 versus con; HFpEF/Empa: 19.4±3.2; P<0.001 versus HFpEF). This was accompanied by improved hemodynamics and calcium handling and by reduced inflammation, hypertrophy, and fibrosis. Proteomic analysis demonstrated major changes in proteins involved in mitochondrial oxidative phosphorylation. Cardiac mitochondrial respiration was significantly impaired in HFpEF but restored by Empa (Vmax complex IV: con: 0.18±0.07 mmol O2/s/mg; HFpEF: 0.13±0.05 mmol O2/s/mg; P<0.041 versus con; HFpEF/Empa: 0.21±0.05 mmol O2/s/mg; P=0.012 versus HFpEF) without alterations of mitochondrial content. The expression of cardiolipin, an essential stability/functionality-mediating phospholipid of the respiratory chain, was significantly decreased in HFpEF but reverted by Empa (con: 15.9±1.7 nmol/mg protein; HFpEF: 12.5±1.8 nmol/mg protein; P=0.002 versus con; HFpEF/Empa: 14.5±1.8 nmol/mg protein; P=0.03 versus HFpEF). Transmission electron microscopy revealed a reduced size of mitochondria in HFpEF, which was restored by Empa.

CONCLUSIONS

The study demonstrates beneficial effects of Empa on diastolic function, hemodynamics, inflammation, and cardiac remodeling in a rat model of HFpEF. These effects were mediated by improved mitochondrial respiratory capacity due to modulated cardiolipin and improved calcium handling.

Hyperglycemia triggers RyR2-dependent alterations of mitochondrial calcium homeostasis in response to cardiac ischemia-reperfusion: Key role of DRP1 activation

Hyperglycemia increases the heart sensitivity to ischemia-reperfusion (IR), but the underlying cellular mechanisms remain unclear. Mitochondrial dynamics (the processes that govern mitochondrial morphology and their interactions with other organelles, such as the reticulum), has emerged as a key factor in the heart vulnerability to IR. However, it is unknown whether mitochondrial dynamics contributes to hyperglycemia deleterious effect during IR. We hypothesized that (i) the higher heart vulnerability to IR in hyperglycemic conditions could be explained by hyperglycemia effect on the complex interplay between mitochondrial dynamics, Ca2+ homeostasis, and reactive oxygen species (ROS) production; and (ii) the activation of DRP1, a key regulator of mitochondrial dynamics, could play a central role. Using transmission electron microscopy and proteomic analysis, we showed that the interactions between sarcoplasmic reticulum and mitochondria and mitochondrial fission were increased during IR in isolated rat hearts perfused with a hyperglycemic buffer compared with hearts perfused with a normoglycemic buffer. In isolated mitochondria and cardiomyocytes, hyperglycemia increased mitochondrial ROS production and Ca2+ uptake. This was associated with higher RyR2 instability. These results could contribute to explain the early mPTP activation in mitochondria from isolated hearts perfused with a hyperglycemic buffer and in hearts from streptozotocin-treated rats (to increase the blood glucose). DRP1 inhibition by Mdivi-1 during the hyperglycemic phase and before IR induction, normalized Ca2+ homeostasis, ROS production, mPTP activation, and reduced the heart sensitivity to IR in streptozotocin-treated rats. In conclusion, hyperglycemia-dependent DRP1 activation results in higher reticulum-mitochondria calcium exchange that contribute to the higher heart vulnerability to IR.

So close, yet so far away: the relationship between MAM and cardiac disease

Mitochondria-associated membrane (MAM) serve as crucial contact sites between mitochondria and the endoplasmic reticulum (ER). Recent research has highlighted the significance of MAM, which serve as a platform for various protein molecules, in processes such as calcium signaling, ATP production, mitochondrial structure and function, and autophagy. Cardiac diseases caused by any reason can lead to changes in myocardial structure and function, significantly impacting human health. Notably, MAM exhibits various regulatory effects to maintain cellular balance in several cardiac diseases conditions, such as obesity, diabetes mellitus, and cardiotoxicity. MAM proteins independently or interact with their counterparts, forming essential tethers between the ER and mitochondria in cardiomyocytes. This review provides an overview of key MAM regulators, detailing their structure and functions. Additionally, it explores the connection between MAM and various cardiac injuries, suggesting that precise genetic, pharmacological, and physical regulation of MAM may be a promising strategy for preventing and treating heart failure.

Loss of mitochondrial Ca2+ uptake protein 3 impairs skeletal muscle calcium handling and exercise capacity

Mitochondrial calcium concentration ([Ca2+ ]m ) plays an essential role in bioenergetics, and loss of [Ca2+ ]m homeostasis can trigger diseases and cell death in numerous cell types. Ca2+ uptake into mitochondria occurs via the mitochondrial Ca2+ uniporter (MCU), which is regulated by three mitochondrial Ca2+ uptake (MICU) proteins localized in the intermembrane space, MICU1, 2, and 3. We generated a mouse model of systemic MICU3 ablation and examined its physiological role in skeletal muscle. We found that loss of MICU3 led to impaired exercise capacity. When the muscles were directly stimulated there was a decrease in time to fatigue. MICU3 ablation significantly increased the maximal force of the KO muscle and altered fibre type composition with an increase in the ratio of type IIb (low oxidative capacity) to type IIa (high oxidative capacity) fibres. Furthermore, MICU3-KO mitochondria have reduced uptake of Ca2+ and increased phosphorylation of pyruvate dehydrogenase, indicating that KO animals contain less Ca2+ in their mitochondria. Skeletal muscle from MICU3-KO mice exhibited lower net oxidation of NADH during electrically stimulated muscle contraction compared with wild-type. These data demonstrate that MICU3 plays a role in skeletal muscle physiology by setting the proper threshold for mitochondrial Ca2+ uptake, which is important for matching energy demand and supply in muscle. KEY POINTS: Mitochondrial calcium uptake is an important regulator of bioenergetics and cell death and is regulated by the mitochondrial calcium uniporter (MCU) and three calcium sensitive regulatory proteins (MICU1, 2 and 3). Loss of MICU3 leads to impaired exercise capacity and decreased time to skeletal muscle fatigue. Skeletal muscle from MICU3-KO mice exhibits a net oxidation of NADH during electrically stimulated muscle contractions, suggesting that MICU3 plays a role in skeletal muscle physiology by matching energy demand and supply.

Electrically stimulated in vitro heart cell mimic of acute exercise reveals novel immediate cellular responses to exercise: Reduced contractility and metabolism, but maintained calcium cycling and increased myofilament calcium sensitivity

Cardiac cellular responses to acute exercise remain undescribed. We present a model for mimicking acute aerobic endurance exercise to freshly isolated cardiomyocytes by evoking exercise-like contractions over prolonged periods of time with trains of electrical twitch stimulations. We then investigated immediate contractile, Ca2+ , and metabolic responses to acute exercise in perfused freshly isolated left ventricular rat cardiomyocytes, after a matrix-design optimized protocol and induced a mimic for acute aerobic endurance exercise by trains of prolonged field twitch stimulations. Acute exercise decreased cardiomyocyte fractional shortening 50%-80% (p < .01). This was not explained by changes to intracellular Ca2+ handling (p > .05); rather, we observed a weak insignificant Ca2+ transient increase (p = .11), while myofilament Ca2+ sensitivity increased 20%-70% (p < .05). Acidic pH 6.8 decreased fractional shortening 20%-70% (p < .05) because of 20%-30% decreased Ca2+ transients (p < .05), but no difference occurred between control and acute exercise (p > .05). Addition of 1 or 10 mM La- increased fractional shortening in control (1 mM La- : no difference, p > .05; 10 mM La- : 20%-30%, p < .05) and acute exercise (1 mM La- : 40%-90%, p < .01; 10 mM La- : 50%-100%, p < .01) and rendered acute exercise indifferent from control (p > .05). Intrinsic autofluorescence showed a resting NADstate of 0.59 ± 0.04 and FADstate of 0.17 ± 0.03, while acute exercise decreased NADH/FAD ratio 8% (p < .01), indicating intracellular oxidation. In conclusion, we show a novel approach for studying immediate acute cardiomyocyte responses to aerobic endurance exercise. We find that acute exercise in cardiomyocytes decreases contraction, but Ca2+ handling and myofilament Ca2+ sensitivity compensate for this, while acidosis and reduced energy substrate and mitochondrial ATP generation explain this.

Cardioprotective Effect against Ischemia-Reperfusion Injury of PAK-200, a Dihydropyridine Analog with an Inhibitory Effect on Cl- but Not Ca2+ Current

We examined the effects of a dihydropyridine analog, PAK-200, on guinea pig myocardium during experimental ischemia and reperfusion. In isolated ventricular cardiomyocytes, PAK-200 (1 μM) had no effect on the basal peak inward and steady-state currents but inhibited the isoprenaline-induced time-independent Cl- current. In the right atria, PAK-200 had no effect on the beating rate and the chronotropic response to isoprenaline. In an ischemia-reperfusion model with coronary-perfused right ventricular tissue, a decrease in contractile force and a rise in tension were observed during a period of 30-min no-flow ischemia. Upon reperfusion, contractile force returned to less than 50% of preischemic values. PAK-200 had no effect on the decline in contractile force during the no-flow ischemia but reduced the rise in resting tension. PAK-200 significantly improved the recovery of contractile force after reperfusion to about 70% of the preischemic value. PAK-200 was also shown to attenuate the decrease in tissue ATP during ischemia. Treatment of ventricular myocytes with an ischemia-mimetic solution resulted in depolarization of the mitochondrial membrane potential and an increase in cytoplasmic and mitochondrial Ca2+ concentrations. PAK-200 significantly delayed these changes. Thus, PAK-200 inhibits the cAMP-activated chloride current in cardiac muscle and may have protective effects against ischemia-reperfusion injury through novel mechanisms.

Calcium and bicarbonate signaling pathways have pivotal, resonating roles in matching ATP production to demand

Mitochondrial ATP production in ventricular cardiomyocytes must be continually adjusted to rapidly replenish the ATP consumed by the working heart. Two systems are known to be critical in this regulation: mitochondrial matrix Ca2+ ([Ca2+]m) and blood flow that is tuned by local cardiomyocyte metabolic signaling. However, these two regulatory systems do not fully account for the physiological range of ATP consumption observed. We report here on the identity, location, and signaling cascade of a third regulatory system -- CO2/bicarbonate. CO2 is generated in the mitochondrial matrix as a metabolic waste product of the oxidation of nutrients. It is a lipid soluble gas that rapidly permeates the inner mitochondrial membrane and produces bicarbonate in a reaction accelerated by carbonic anhydrase. The bicarbonate level is tracked physiologically by a bicarbonate-activated soluble adenylyl cyclase (sAC). Using structural Airyscan super-resolution imaging and functional measurements we find that sAC is primarily inside the mitochondria of ventricular cardiomyocytes where it generates cAMP when activated by bicarbonate. Our data strongly suggest that ATP production in these mitochondria is regulated by this cAMP signaling cascade operating within the inter-membrane space by activating local EPAC1 (Exchange Protein directly Activated by cAMP) which turns on Rap1 (Ras-related protein-1). Thus, mitochondrial ATP production is increased by bicarbonate-triggered sAC-signaling through Rap1. Additional evidence is presented indicating that the cAMP signaling itself does not occur directly in the matrix. We also show that this third signaling process involving bicarbonate and sAC activates the mitochondrial ATP production machinery by working independently of, yet in conjunction with, [Ca2+]m-dependent ATP production to meet the energy needs of cellular activity in both health and disease. We propose that the bicarbonate and calcium signaling arms function in a resonant or complementary manner to match mitochondrial ATP production to the full range of energy consumption in ventricular cardiomyocytes.

DUSP12 ameliorates myocardial ischemia-reperfusion injury through HSPB8-induced mitophagy

This study aimed to explore the role of dual specificity phosphatase 12 (DUSP12) in regulating myocardial ischemia-reperfusion (I/R) injury and the underlying mechanism. The expression of DUSP12 in myocardial tissues and heat-shock protein beta-8 (HSPB8) and mitophagy-related proteins in myocardial tissues and H9c2 cells were detected by western blot analysis. The serum creatine kinase isoenzymes (CK-MB) and lactate dehydrogenase (LDH), levels of reactive oxygen species and malondialdehyde, superoxide dismutase activity in myocardial tissues and H9c2 cells, and caspase-3 activity in H9c2 cells were analyzed by corresponding assay kits. The infarct area in the rat's heart was observed by triphenyl tetrazolium chloride staining. The apoptosis of myocardial cells in myocardial tissues and H9c2 cells was detected by terminal-deoxynucleotidyl transferase dUTP-biotin nick-end labeling assay. The interaction between DUSP12 and HSPB8 was clarified by the coimmunoprecipitation assay. The transfection efficacy of si-HSPB8#1 and si-HSPB8#2 in H9c2 cells was confirmed by real-time quantitative-polymerase chain reaction and western blot analysis. As a result, DUSP12 expression was downregulated in I/R rats, which was promoted by lentivirus-expressing DUSP12. DUSP12 overexpression reduced the serum creatine kinase isoenzymes (CK-MB) and LDH, decreased the infarct area in the rat's heart, and suppressed the apoptosis and oxidative stress in myocardial tissues. DUSP12 overexpression also upregulated the expression of HSPB8 to promote mitophagy. The coimmunoprecipitation assay indicated that DUSP12 could be combined with HSPB8. In addition, DUSP12 overexpression could inhibit hypoxia/reoxygenation-elicited apoptosis as well as oxidative stress in H9c2 cells by upregulating HSPB8 expression to promote mitophagy, which was countervailed by HSPB8 deficiency. In conclusion, DUSP12 overexpression decreased the apoptosis and oxidative stress in myocardial I/R injury through HSPB8-induced mitophagy.

Targeting the mitochondrial Ca2+ uniporter complex in cardiovascular disease

Cardiovascular diseases (CVDs), the leading cause of death worldwide, share in common mitochondrial dysfunction, in specific a dysregulation of Ca²⁺ uptake dynamics through the mitochondrial Ca²⁺ uniporter (MCU) complex. In particular, Ca²⁺ uptake regulates the mitochondrial ATP production, mitochondrial dynamics, oxidative stress, and cell death. Therefore, modulating the activity of the MCU complex to regulate Ca²⁺ uptake, has been suggested as a potential therapeutic approach for the treatment of CVDs. Here, the role and implications of the MCU complex in CVDs are presented, followed by a review of the evidence for MCU complex modulation, genetically and pharmacologically. While most approaches have aimed within the MCU complex for the modulation of the Ca²⁺ pore channel, the MCU subunit, its intra- and extra- mitochondrial implications, including Ca²⁺ dynamics, oxidative stress, post-translational modifications, and its repercussions in the cardiac function, highlight that targeting the MCU complex has the translational potential for novel CVDs therapeutics.

Increased Mitochondrial Calcium Fluxes in Hypertrophic Right Ventricular Cardiomyocytes from a Rat Model of Pulmonary Artery Hypertension

Pulmonary artery hypertension causes right ventricular hypertrophy which rapidly progresses to heart failure with underlying cardiac mitochondrial dysfunction. Prior to failure, there are alterations in cytosolic Ca²⁺ handling that might impact mitochondrial function in the compensatory phase of RV hypertrophy. Our aims, therefore, were (i) to measure beat-to-beat mitochondrial Ca²⁺ fluxes, and (ii) to determine mitochondrial abundance and function in non-failing, hypertrophic cardiomyocytes. Male Wistar rats were injected with either saline (CON) or monocrotaline (MCT) to induce pulmonary artery hypertension and RV hypertrophy after four weeks. Cytosolic Ca²⁺ ([Ca²⁺]_cyto) transients were obtained in isolated right ventricular (RV) cardiomyocytes, and mitochondrial Ca²⁺ ([Ca²⁺]_mito) was recorded in separate RV cardiomyocytes. The distribution and abundance of key proteins was determined using confocal and stimulated emission depletion (STED) microscopy. The RV mitochondrial function was also assessed in RV homogenates using oxygraphy. The MCT cardiomyocytes had increased area, larger [Ca²⁺]_cyto transients, increased Ca²⁺ store content, and faster trans-sarcolemmal Ca²⁺ extrusion relative to CON. The MCT cardiomyocytes also had larger [Ca²⁺]_mito transients. STED images detected increased mitochondrial protein abundance (TOM20 clusters per μm²) in MCT, yet no difference was found when comparing mitochondrial respiration and membrane potential between the groups. We suggest that the larger [Ca²⁺]_mito transients compensate to match ATP supply to the increased energy demands of hypertrophic cardiomyocytes.

Mitochondrial dysfunction in human hypertrophic cardiomyopathy is linked to cardiomyocyte architecture disruption and corrected by improving NADH-driven mitochondrial respiration

AIMS

Genetic hypertrophic cardiomyopathy (HCM) is caused by mutations in sarcomere protein-encoding genes (i.e. genotype-positive HCM). In an increasing number of patients, HCM occurs in the absence of a mutation (i.e. genotype-negative HCM). Mitochondrial dysfunction is thought to be a key driver of pathological remodelling in HCM. Reports of mitochondrial respiratory function and specific disease-modifying treatment options in patients with HCM are scarce.

METHODS AND RESULTS

Respirometry was performed on septal myectomy tissue from patients with HCM (n = 59) to evaluate oxidative phosphorylation and fatty acid oxidation. Mitochondrial dysfunction was most notably reflected by impaired NADH-linked respiration. In genotype-negative patients, but not genotype-positive patients, NADH-linked respiration was markedly depressed in patients with an indexed septal thickness ≥10 compared with <10. Mitochondrial dysfunction was not explained by reduced abundance or fragmentation of mitochondria, as evaluated by transmission electron microscopy. Rather, improper organization of mitochondria relative to myofibrils (expressed as a percentage of disorganized mitochondria) was strongly associated with mitochondrial dysfunction. Pre-incubation with the cardiolipin-stabilizing drug elamipretide and raising mitochondrial NAD+ levels both boosted NADH-linked respiration.

CONCLUSION

Mitochondrial dysfunction is explained by cardiomyocyte architecture disruption and is linked to septal hypertrophy in genotype-negative HCM. Despite severe myocardial remodelling mitochondria were responsive to treatments aimed at restoring respiratory function, eliciting the mitochondria as a drug target to prevent and ameliorate cardiac disease in HCM. Mitochondria-targeting therapy may particularly benefit genotype-negative patients with HCM, given the tight link between mitochondrial impairment and septal thickening in this subpopulation.

Myocardial Infarction as a Consequence of Mitochondrial Dysfunction

Acute myocardial infarction is an event of myocardial necrosis caused by unstable ischemic syndrome. Myocardial infarction (MI) occurs when blood stops flowing to the cardiac tissue or myocardium and the heart muscle gets damaged due to poor perfusion and reduced oxygen supply. Mitochondria can serve as the arbiter of cell fate in response to stress. Oxidative metabolism is the function of mitochondria within the cell. Cardiac cells being highly oxidative tissue generates about 90% of their energy through oxidative metabolism. In this review, we focused on the role of mitochondria in energy generation in myocytes as well as its consequences on heart cells causing cell damage. The role of mitochondrial dysfunction due to oxidative stress, production of reactive oxygen species, and anaerobic production of lactate as a failure of oxidative metabolism are also discussed.

Ryanodine receptor 1-mediated Ca2+ signaling and mitochondrial reprogramming modulate uterine serous cancer malignant phenotypes

Background

Uterine serous cancer (USC) is the most common non-endometrioid subtype of uterine cancer, and is also the most aggressive. Most patients will die of progressively chemotherapy-resistant disease, and the development of new therapies that can target USC remains a major unmet clinical need. This study sought to determine the molecular mechanism by which a novel unfavorable prognostic biomarker ryanodine receptor 1 (RYR1) identified in advanced USC confers their malignant phenotypes, and demonstrated the efficacy of targeting RYR1 by repositioned FDA-approved compounds in USC treatment.

Methods

TCGA USC dataset was analyzed to identify top genes that are associated with patient survival or disease stage, and can be targeted by FDA-approved compounds. The top gene RYR1 was selected and the functional role of RYR1 in USC progression was determined by silencing and over-expressing RYR1 in USC cells in vitro and in vivo. The molecular mechanism and signaling networks associated with the functional role of RYR1 in USC progression were determined by reverse phase protein arrays (RPPA), Western blot, and transcriptomic profiling analyses. The efficacy of the repositioned compound dantrolene on USC progression was determined using both in vitro and in vivo models.

Results

High expression level of RYR1 in the tumors is associated with advanced stage of the disease. Inhibition of RYR1 suppressed proliferation, migration and enhanced apoptosis through Ca²⁺-dependent activation of AKT/CREB/PGC-1α and AKT/HK1/2 signaling pathways, which modulate mitochondrial bioenergetics properties, including oxidative phosphorylation, ATP production, mitochondrial membrane potential, ROS production and TCA metabolites, and glycolytic activities in USC cells. Repositioned compound dantrolene suppressed USC progression and survival in mouse models.

Conclusions

These findings provided insight into the mechanism by which RYR1 modulates the malignant phenotypes of USC and could aid in the development of dantrolene as a repurposed therapeutic agent for the treatment of USC to improve patient survival.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-022-02419-w.

Impact and potential mechanism of effects of chronic moderate alcohol consumption on cardiac function in aldehyde dehydrogenase 2 gene heterozygous mice

Background

Mitochondrial aldehyde dehydrogenase 2 (ALDH2) is a key enzyme in alcohol metabolism. The ALDH2*2 mutations are found in approximately 45% of East Asians, with 40% being heterozygous (HE) ALDH2*1/*2 and 5% homozygous (HO) ALDH2*2/*2. Studies have shown that HO mice lack cardioprotective effects induced by moderate alcohol consumption. However, the impact of moderate alcohol consumption on cardiac function in HE mice is unknown.

Methods

In this study, HO, HE, and wild‐type (WT) mice were subjected to a 6‐week moderate alcohol drinking protocol, following which myocardial tissue and cardiomyocytes of the mice were extracted.

Results

We found that moderate alcohol exposure did not increase mortality, myocardial fibrosis, apoptosis, or inflammation in HE mice, which differs from the effects observed in HO mice. After exposure to the 6‐week alcohol drinking protocol, there was impaired cardiac function, cardiomyocyte contractility, and intracellular Ca²⁺ homeostasis and mitochondrial function in both HE and HO mice as compared to WT mice. Moreover, these animals showed overt oxidative stress production and increased levels of the activated forms of calmodulin‐dependent protein kinase II (CaMKII) and ryanodine receptor type 2 (RYR2) phosphorylation protein.

Conclusion

We found that moderate alcohol exposure impaired cardiac function in HE mice, possibly by increasing reactive oxygen species (ROS)/CaMKII/RYR2‐mediated Ca²⁺ handling abnormalities. Hence, we advocate that people with ALDH2*1/*2 genotypes rigorously avoid alcohol consumption to prevent potential cardiovascular harm induced by moderate alcohol consumption.

Mitochondrial Fusion, Fission, and Mitophagy in Cardiac Diseases: Challenges and Therapeutic Opportunities

Significance: Mitochondria play a critical role in the physiology of the heart by controlling cardiac metabolism, function, and remodeling. Accumulation of fragmented and damaged mitochondria is a hallmark of cardiac diseases. Recent Advances: Disruption of quality control systems that maintain mitochondrial number, size, and shape through fission/fusion balance and mitophagy results in dysfunctional mitochondria, defective mitochondrial segregation, impaired cardiac bioenergetics, and excessive oxidative stress. Critical Issues: Pharmacological tools that improve the cardiac pool of healthy mitochondria through inhibition of excessive mitochondrial fission, boosting mitochondrial fusion, or increasing the clearance of damaged mitochondria have emerged as promising approaches to improve the prognosis of heart diseases. Future Directions: There is a reasonable amount of preclinical evidence supporting the effectiveness of molecules targeting mitochondrial fission and fusion to treat cardiac diseases. The current and future challenges are turning these lead molecules into treatments. Clinical studies focusing on acute (i.e., myocardial infarction) and chronic (i.e., heart failure) cardiac diseases are needed to validate the effectiveness of such strategies in improving mitochondrial morphology, metabolism, and cardiac function. Antioxid. Redox Signal. 36, 844-863.

The Cardiomyocyte in Heart Failure with Preserved Ejection Fraction—Victim of Its Environment?

Heart failure (HF) with preserved left ventricular ejection fraction (HFpEF) is becoming the predominant form of HF. However, medical therapy that improves cardiovascular outcome in HF patients with almost normal and normal systolic left ventricular function, but diastolic dysfunction is missing. The cause of this unmet need is incomplete understanding of HFpEF pathophysiology, the heterogeneity of the patient population, and poor matching of therapeutic mechanisms and primary pathophysiological processes. Recently, animal models improved understanding of the pathophysiological role of highly prevalent and often concomitantly presenting comorbidity in HFpEF patients. Evidence from these animal models provide first insight into cellular pathophysiology not considered so far in HFpEF disease, promising that improved understanding may provide new therapeutical targets. This review merges observation from animal models and human HFpEF disease with the intention to converge cardiomyocytes pathophysiological aspects and clinical knowledge.

Visualization of Dynamic Mitochondrial Calcium Fluxes in Isolated Cardiomyocytes

Background

Cardiomyocyte contraction requires a constant supply of ATP, which varies depending on work rate. Maintaining ATP supply is particularly important during excitation-contraction coupling, where cytosolic Ca2+ fluxes drive repeated cycles of contraction and relaxation. Ca2+ is one of the key regulators of ATP production, and its uptake into the mitochondrial matrix occurs via the mitochondrial calcium uniporter. Fluorescent indicators are commonly used for detecting cytosolic Ca2+ changes. However, visualizing mitochondrial Ca2+ fluxes using similar methods is more difficult, as the fluorophore must be permeable to both the sarcolemma and the inner mitochondrial membrane. Our aim was therefore to optimize a method using the fluorescent Ca2+ indicator Rhod-2 to visualize beat-to-beat mitochondrial calcium fluxes in rat cardiomyocytes.

Methods

Healthy, adult male Wistar rat hearts were isolated and enzymatically digested to yield rod-shaped, quiescent ventricular cardiomyocytes. The fluorescent Ca2+ indicator Rhod-2 was reduced to di-hydroRhod-2 and confocal microscopy was used to validate mitochondrial compartmentalization. Cardiomyocytes were subjected to various pharmacological interventions, including caffeine and β-adrenergic stimulation. Upon confirmation of mitochondrial Rhod-2 localization, loaded myocytes were then super-fused with 1.5 mM Ca2+ Tyrodes containing 1 μM isoproterenol and 150 μM spermine. Myocytes were externally stimulated at 0.1, 0.5 and 1 Hz and whole cell recordings of both cytosolic ([Ca2+]cyto) and mitochondrial calcium ([Ca2+]mito) transients were made.

Results

Myocytes loaded with di-hydroRhod-2 revealed a distinct mitochondrial pattern when visualized by confocal microscopy. Application of 20 mM caffeine revealed no change in fluorescence, confirming no sarcoplasmic reticulum compartmentalization. Myocytes loaded with di-hydroRhod-2 also showed a large increase in fluorescence within the mitochondria in response to β-adrenergic stimulation (P &lt; 0.05). Beat-to-beat mitochondrial Ca2+ transients were smaller in amplitude and had a slower time to peak and maximum rate of rise relative to cytosolic calcium transients at all stimulation frequencies (P &lt; 0.001).

Conclusion

Myocytes loaded with di-hydroRhod-2 revealed mitochondrial specific compartmentalization. Mitochondrial Ca2+ transients recorded from di-hydroRhod-2 loaded myocytes were distinct in comparison to the large and rapid Rhod-2 cytosolic transients, indicating different kinetics between [Ca2+]cyto and [Ca2+]mito transients. Overall, our results showed that di-hydroRhod-2 loading is a quick and suitable method for measuring beat-to-beat [Ca2+]mito transients in intact myocytes.

Prognostic impact of diabetes in chronic and acute heart failure

A strong, bidirectional relationship exists between diabetes mellitus (DM) and heart failure (HF) and DM is responsible of the activation of several molecular and pathophysiological mechanisms that may, on the long term, damage the heart. However, the prognostic role of DM in the context of chronic and acute HF is still not yet defined and there are several gaps of evidence in the literature on this topic. These gaps are related to the wide phenotypic heterogeneity of patients with chronic and acute HF and to the concept that not all diabetic patients are the same, but there is the necessity to better characterize the disease and each single patient, also considering the role of other possible comorbidities. The aim of the present review is to summarize the pathophysiological mechanisms subtending the negative effect of DM in HF and analyze the available data exploring the prognostic impact of such comorbidity in both chronic and acute HF.

A nutrient-responsive hormonal circuit mediates an inter-tissue program regulating metabolic homeostasis in adult Drosophila

Animals maintain metabolic homeostasis by modulating the activity of specialized organs that adjust internal metabolism to external conditions. However, the hormonal signals coordinating these functions are incompletely characterized. Here we show that six neurosecretory cells in the Drosophila central nervous system respond to circulating nutrient levels by releasing Capa hormones, homologs of mammalian neuromedin U, which activate the Capa receptor (CapaR) in peripheral tissues to control energy homeostasis. Loss of Capa/CapaR signaling causes intestinal hypomotility and impaired nutrient absorption, which gradually deplete internal nutrient stores and reduce organismal lifespan. Conversely, increased Capa/CapaR activity increases fluid and waste excretion. Furthermore, Capa/CapaR inhibits the release of glucagon-like adipokinetic hormone from the corpora cardiaca, which restricts energy mobilization from adipose tissue to avoid harmful hyperglycemia. Our results suggest that the Capa/CapaR circuit occupies a central node in a homeostatic program that facilitates the digestion and absorption of nutrients and regulates systemic energy balance.

Redox signaling in heart failure and therapeutic implications

Heart failure is a growing health burden worldwide characterized by alterations in excitation-contraction coupling, cardiac energetic deficit and oxidative stress. While current treatments are mostly limited to antagonization of neuroendocrine activation, more recent data suggest that also targeting metabolism may provide substantial prognostic benefit. However, although in a broad spectrum of preclinical models, oxidative stress plays a causal role for the development and progression of heart failure, no treatment that targets reactive oxygen species (ROS) directly has entered the clinical arena yet. In the heart, ROS derive from various sources, such as NADPH oxidases, xanthine oxidase, uncoupled nitric oxide synthase and mitochondria. While mitochondria are the primary source of ROS in the heart, communication between different ROS sources may be relevant for physiological signalling events as well as pathologically elevated ROS that deteriorate excitation-contraction coupling, induce hypertrophy and/or trigger cell death. Here, we review the sources of ROS in the heart, the modes of pathological activation of ROS formation as well as therapeutic approaches that may target ROS specifically in mitochondria.

Cardiolipin, Non-Bilayer Structures and Mitochondrial Bioenergetics: Relevance to Cardiovascular Disease

The present review is an attempt to conceptualize a contemporary understanding about the roles that cardiolipin, a mitochondrial specific conical phospholipid, and non-bilayer structures, predominantly found in the inner mitochondrial membrane (IMM), play in mitochondrial bioenergetics. This review outlines the link between changes in mitochondrial cardiolipin concentration and changes in mitochondrial bioenergetics, including changes in the IMM curvature and surface area, cristae density and architecture, efficiency of electron transport chain (ETC), interaction of ETC proteins, oligomerization of respiratory complexes, and mitochondrial ATP production. A relationship between cardiolipin decline in IMM and mitochondrial dysfunction leading to various diseases, including cardiovascular diseases, is thoroughly presented. Particular attention is paid to the targeting of cardiolipin by Szeto–Schiller tetrapeptides, which leads to rejuvenation of important mitochondrial activities in dysfunctional and aging mitochondria. The role of cardiolipin in triggering non-bilayer structures and the functional roles of non-bilayer structures in energy-converting membranes are reviewed. The latest studies on non-bilayer structures induced by cobra venom peptides are examined in model and mitochondrial membranes, including studies on how non-bilayer structures modulate mitochondrial activities. A mechanism by which non-bilayer compartments are formed in the apex of cristae and by which non-bilayer compartments facilitate ATP synthase dimerization and ATP production is also presented.

Implications of SGLT Inhibition on Redox Signalling in Atrial Fibrillation

Atrial fibrillation (AF) is the most common sustained (atrial) arrhythmia, a considerable global health burden and often associated with heart failure. Perturbations of redox signalling in cardiomyocytes provide a cellular substrate for the manifestation and maintenance of atrial arrhythmias. Several clinical trials have shown that treatment with sodium-glucose linked transporter inhibitors (SGLTi) improves mortality and hospitalisation in heart failure patients independent of the presence of diabetes. Post hoc analysis of the DECLARE-TIMI 58 trial showed a 19% reduction in AF in patients with diabetes mellitus (hazard ratio, 0.81 (95% confidence interval: 0.68-0.95), n = 17.160) upon treatment with SGLTi, regardless of pre-existing AF or heart failure and independent from blood pressure or renal function. Accordingly, ongoing experimental work suggests that SGLTi not only positively impact heart failure but also counteract cellular ROS production in cardiomyocytes, thereby potentially altering atrial remodelling and reducing AF burden. In this article, we review recent studies investigating the effect of SGLTi on cellular processes closely interlinked with redox balance and their potential effects on the onset and progression of AF. Despite promising insight into SGLTi effect on Ca²⁺ cycling, Na⁺ balance, inflammatory and fibrotic signalling, mitochondrial function and energy balance and their potential effect on AF, the data are not yet conclusive and the importance of individual pathways for human AF remains to be established. Lastly, an overview of clinical studies investigating SGLTi in the context of AF is provided.

Mitochondrial Dysfunction in Atrial Fibrillation-Mechanisms and Pharmacological Interventions

Despite the enormous progress in the treatment of atrial fibrillation, mainly with the use of invasive techniques, many questions remain unanswered regarding the pathomechanism of the arrhythmia and its prevention methods. The development of atrial fibrillation requires functional changes in the myocardium that result from disturbed ionic fluxes and altered electrophysiology of the cardiomyocyte. Electrical instability and electrical remodeling underlying the arrhythmia may result from a cellular energy deficit and oxidative stress, which are caused by mitochondrial dysfunction. The significance of mitochondrial dysfunction in the pathogenesis of atrial fibrillation remains not fully elucidated; however, it is emphasized by the reduction of atrial fibrillation burden after therapeutic interventions improving the mitochondrial welfare. This review summarizes the mechanisms of mitochondrial dysfunction related to atrial fibrillation and current pharmacological treatment options targeting mitochondria to prevent or improve the outcome of atrial fibrillation.

Sarcoplasmic reticulum-mitochondria communication; implications for cardiac arrhythmia

Sudden cardiac death due to ventricular tachyarrhythmias remains the major cause of mortality in the world. Heart failure, diabetic cardiomyopathy, old age-related cardiac dysfunction and inherited disorders are associated with enhanced propensity to malignant cardiac arrhythmias. Both defective mitochondrial function and abnormal intracellular Ca²⁺ homeostasis have been established as the key contributing factors in the pathophysiology and arrhythmogenesis in these conditions. This article reviews current advances in understanding of bidirectional control of ryanodine receptor-mediated sarcoplasmic reticulum Ca²⁺ release and mitochondrial function, and how defects in crosstalk between these two organelles increase arrhythmic risk in cardiac disease.

Protective Effects of Ginsenosides (20R)-Rg3 on H2 O2 -Induced Myocardial Cell Injury by Activating Keap-1/Nrf2/HO-1 Signaling Pathway

Ginsenosides (20S)-Rg3 and (20R)-Rg3 are famous rare ginsenosides from red ginseng, and their configurations in C-20 are different. This study aimed to investigate the protective mechanism of ginsenosides (20S)-Rg3 and (20R)-Rg3 on H₂ O₂ -induced H9C2 cells and compare their activity. The results showed that the ginsenosides (20S)-Rg3 and (20R)-Rg3 could increase the cell activity and the levels of GSH-Px, SOD and CAT, and decrease activities of LDH, MDA and ROS. Further studies showed that ginsenosides (20S)-Rg3 and (20R)-Rg3 could prevent oxidative stress injury of H9C2 cells by H₂ O₂ through the Keap-1/Nrf2/HO-1 pathway. But the ML385 counteracts these effects. Interestingly, among these results, ginsenoside (20R)-Rg3 was superior to (20S)-Rg3, indicating that ginsenoside (20R)-Rg3 have a stronger effect of antioxidative stress. This study reflected that ginsenoside (20R)-Rg3 could be used as a potential Nrf2 activator and a safe effective Chinese herbal monomer in the treatment of cardiovascular disease.

Mitochondrial and Sarcoplasmic Reticulum Interconnection in Cardiac Arrhythmia

Ca²⁺ plays a pivotal role in mitochondrial energy production, contraction, and apoptosis. Mitochondrial Ca²⁺-targeted fluorescent probes have demonstrated that mitochondria Ca²⁺ transients are synchronized with Ca²⁺ fluxes occurring in the sarcoplasmic reticulum (SR). The presence of specialized proteins tethering SR to mitochondria ensures the local Ca²⁺ flux between these organelles. Furthermore, communication between SR and mitochondria impacts their functionality in a bidirectional manner. Mitochondrial Ca²⁺ uptake through the mitochondrial Ca²⁺ uniplex is essential for ATP production and controlled reactive oxygen species levels for proper cellular signaling. Conversely, mitochondrial ATP ensures the proper functioning of SR Ca²⁺-handling proteins, which ensures that mitochondria receive an adequate supply of Ca²⁺. Recent evidence suggests that altered SR Ca²⁺ proteins, such as ryanodine receptors and the sarco/endoplasmic reticulum Ca²⁺ ATPase pump, play an important role in maintaining proper cardiac membrane excitability, which may be initiated and potentiated when mitochondria are dysfunctional. This recognized mitochondrial role offers the opportunity to develop new therapeutic approaches aimed at preventing cardiac arrhythmias in cardiac disease.

Mechanisms underlying the pathophysiology of heart failure with preserved ejection fraction: the tip of the iceberg

Heart failure with preserved ejection fraction (HFpEF) is a multifaceted syndrome with a complex aetiology often associated with several comorbidities, such as left ventricle pressure overload, diabetes mellitus, obesity, and kidney disease. Its pathophysiology remains obscure mainly due to the complex phenotype induced by all these associated comorbidities and to the scarcity of animal models that adequately mimic HFpEF. Increased oxidative stress, inflammation, and endothelial dysfunction are currently accepted as key players in HFpEF pathophysiology. However, we have just started to unveil HFpEF complexity and the role of calcium handling, energetic metabolism, and mitochondrial function remain to clarify. Indeed, the enlightenment of such cellular and molecular mechanisms represents an opportunity to develop novel therapeutic approaches and thus to improve HFpEF treatment options. In the last decades, the number of research groups dedicated to studying HFpEF has increased, denoting the importance and the magnitude achieved by this syndrome. In the current technological and web world, the amount of information is overwhelming, driving us not only to compile the most relevant information about the theme but also to explore beyond the tip of the iceberg. Thus, this review aims to encompass the most recent knowledge related to HFpEF or HFpEF-associated comorbidities, focusing mainly on myocardial metabolism, oxidative stress, and energetic pathways.

Mitochondrial depolarization promotes calcium alternans: Mechanistic insights from a ventricular myocyte model

Mitochondria are vital organelles inside the cell and contribute to intracellular calcium (Ca2+) dynamics directly and indirectly via calcium exchange, ATP generation, and production of reactive oxygen species (ROS). Arrhythmogenic Ca2+ alternans in cardiac myocytes has been observed in experiments under abnormal mitochondrial depolarization. However, complex signaling pathways and Ca2+ cycling between mitochondria and cytosol make it difficult in experiments to reveal the underlying mechanisms of Ca2+ alternans under abnormal mitochondrial depolarization. In this study, we use a newly developed spatiotemporal ventricular myocyte computer model that integrates mitochondrial Ca2+ cycling and complex signaling pathways to investigate the mechanisms of Ca2+ alternans during mitochondrial depolarization. We find that elevation of ROS in response to mitochondrial depolarization plays a critical role in promoting Ca2+ alternans. Further examination reveals that the redox effect of ROS on ryanodine receptors and sarco/endoplasmic reticulum Ca2+-ATPase synergistically promote alternans. Upregulation of mitochondrial Ca2+ uniporter promotes Ca2+ alternans via Ca2+-dependent mitochondrial permeability transition pore opening. Due to their relatively slow kinetics, oxidized Ca2+/calmodulin-dependent protein kinase II activation and ATP do not play significant roles acutely in the genesis of Ca2+ alternans after mitochondrial depolarization, but their roles can be significant in the long term, mainly through their effects on sarco/endoplasmic reticulum Ca2+-ATPase activity. In conclusion, mitochondrial depolarization promotes Ca2+ alternans acutely via the redox effect of ROS and chronically by ATP reduction. It suppresses Ca2+ alternans chronically through oxidized Ca2+/calmodulin-dependent protein kinase II activation.

TNFα induces mitochondrial fragmentation and biogenesis in human airway smooth muscle

In human airway smooth muscle (hASM), mitochondrial volume density is greater in asthmatic patients compared with normal controls. There is also an increase in mitochondrial fragmentation in hASM of moderate asthmatics associated with an increase in dynamin-related protein 1 (Drp1) and a decrease in mitofusin 2 (Mfn2) expression, mitochondrial fission, and fusion proteins, respectively. Proinflammatory cytokines such TNFα contribute to hASM hyperreactivity and cell proliferation associated with asthma. However, the involvement of proinflammatory cytokines in mitochondrial remodeling is not clearly established. In nonasthmatic hASM cells, mitochondria were labeled using MitoTracker Red and imaged in three dimensions using a confocal microscope. After 24-h TNFα exposure, mitochondria in hASM cells were more fragmented, evidenced by decreased form factor and aspect ratio and increased sphericity. Associated with increased mitochondrial fragmentation, Drp1 expression increased while Mfn2 expression was reduced. TNFα also increased mitochondrial biogenesis in hASM cells reflected by increased peroxisome proliferator-activated receptor-γ coactivator 1α expression and increased mitochondrial DNA copy number. Associated with mitochondrial biogenesis, TNFα exposure also increased mitochondrial volume density and porin expression, resulting in an increase in maximum O2 consumption rate. However, when normalized for mitochondrial volume density, O2 consumption rate per mitochondrion was reduced by TNFα exposure. Associated with mitochondrial fragmentation and biogenesis, TNFα also increased hASM cell proliferation, an effect mimicked by siRNA knockdown of Mfn2 expression and mitigated by Mfn2 overexpression. The results of this study support our hypothesis that in hASM cells exposed to TNFα mitochondria are more fragmented, with an increase in mitochondrial biogenesis and mitochondrial volume density resulting in reduced O2 consumption rate per mitochondrion.

Inflammation-Induced Protein Unfolding in Airway Smooth Muscle Triggers a Homeostatic Response in Mitochondria

The effects of airway inflammation on airway smooth muscle (ASM) are mediated by pro-inflammatory cytokines such as tumor necrosis factor alpha (TNFα). In this review article, we will provide a unifying hypothesis for a homeostatic response to airway inflammation that mitigates oxidative stress and thereby provides resilience to ASM. Previous studies have shown that acute exposure to TNFα increases ASM force generation in response to muscarinic stimulation (hyper-reactivity) resulting in increased ATP consumption and increased tension cost. To meet this increased energetic demand, mitochondrial O₂ consumption and oxidative phosphorylation increases but at the cost of increased reactive oxygen species (ROS) production (oxidative stress). TNFα-induced oxidative stress results in the accumulation of unfolded proteins in the endoplasmic reticulum (ER) and mitochondria of ASM. In the ER, TNFα selectively phosphorylates inositol-requiring enzyme 1 alpha (pIRE1α) triggering downstream splicing of the transcription factor X-box binding protein 1 (XBP1s); thus, activating the pIRE1α/XBP1s ER stress pathway. Protein unfolding in mitochondria also triggers an unfolded protein response (^mtUPR). In our conceptual framework, we hypothesize that activation of these pathways is homeostatically directed towards mitochondrial remodeling via an increase in peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC1α) expression, which in turn triggers: (1) mitochondrial fragmentation (increased dynamin-related protein-1 (Drp1) and reduced mitofusin-2 (Mfn2) expression) and mitophagy (activation of the Phosphatase and tensin homolog (PTEN)-induced putative kinase 1 (PINK1)/Parkin mitophagy pathway) to improve mitochondrial quality; (2) reduced Mfn2 also results in a disruption of mitochondrial tethering to the ER and reduced mitochondrial Ca²⁺ influx; and (3) mitochondrial biogenesis and increased mitochondrial volume density. The homeostatic remodeling of mitochondria results in more efficient O₂ consumption and oxidative phosphorylation and reduced ROS formation by individual mitochondrion, while still meeting the increased ATP demand. Thus, the energetic load of hyper-reactivity is shared across the mitochondrial pool within ASM cells.

Role of mitochondria in liver metabolic health and diseases

The liver is a major organ that coordinates the metabolic flexibility of the whole body, which is characterized by the ability to adapt dynamically in response to fluctuations in energy needs and supplies. In this context, hepatocyte mitochondria are key partners in fine-tuning metabolic flexibility. Here we review the metabolic and signalling pathways carried by mitochondria in the liver, the major pathways that regulate mitochondrial function and how they function in health and metabolic disorders associated to obesity, i.e. insulin resistance, non-alcoholic steatosis and steatohepatitis and hepatocellular carcinoma. Finally, strategies targeting mitochondria to counteract liver disorders are discussed.

Cellular and mitochondrial mechanisms of atrial fibrillation

The molecular mechanisms underlying atrial fibrillation (AF), the most common form of arrhythmia, are poorly understood and therefore target-specific treatment options remain an unmet clinical need. Excitation–contraction coupling in cardiac myocytes requires high amounts of adenosine triphosphate (ATP), which is replenished by oxidative phosphorylation in mitochondria. Calcium (Ca²⁺) is a key regulator of mitochondrial function by stimulating the Krebs cycle, which produces nicotinamide adenine dinucleotide for ATP production at the electron transport chain and nicotinamide adenine dinucleotide phosphate for the elimination of reactive oxygen species (ROS). While it is now well established that mitochondrial dysfunction plays an important role in the pathophysiology of heart failure, this has been less investigated in atrial myocytes in AF. Considering the high prevalence of AF, investigating the role of mitochondria in this disease may guide the path towards new therapeutic targets. In this review, we discuss the importance of mitochondrial Ca²⁺ handling in regulating ATP production and mitochondrial ROS emission and how alterations, particularly in these aspects of mitochondrial activity, may play a role in AF. In addition to describing research advances, we highlight areas in which further studies are required to elucidate the role of mitochondria in AF.

Effect of norepinephrine combined with sodium phosphocreatine on cardiac function and prognosis of patients with septic shock

Septic shock (SS) leads to a high mortality rate for sepsis patients. Norepinephrine (NE) is a preferred vasoactive agent in SS treatment. This study aimed to assess the effects of NE at different administration time and NE combined with SP treatment on the cardiac function and prognosis of SS.

SS patients received NE treatment at different administration time and NE combined with SP treatment were enrolled in this study. The serum levels of cardiac troponin I (cTnI) and B-type natriuretic peptide (BNP), ejection fraction (EF), and pressure-adjusted heart rate (PAR) value were analyzed to evaluate cardiac function. The 28-day survival information was collected and assessed using the Kaplan-Meier method and log-rank test.

The cardiac function of SS patients was improved significantly by NE treatment, especially in the patients received NE at 2 h after fluid infusion, which evidenced by the increased BNP and cTnI levels and EF% and the decreased RAP. In the NE-2 h group, SS patients had a better 28-day survival rate compared with those patients in NE-1 h and -3 h groups. Furthermore, the significantly improved cardiac function and survival outcomes were found in patients received NE combined SP treatment.

Taken together, this study results show that NE administration at 2 h after fluid infusion may be the optimal time point for the treatment of SS and NE combined with SP treatment can improve early cardiac dysfunction and 28-day survival outcomes in patients with SS.

Ginsenoside Rb2 alleviates myocardial ischemia/reperfusion injury in rats through SIRT1 activation

The cardioprotective effects of ginsenoside Rb2 on oxidative stress, which is induced by hydrogen peroxide and myocardial ischemia/reperfusion (MI/R) injury, have been studied. The mechanisms were associated with the inhibition of cardiomyocyte apoptosis, a high concentration of antioxidant defense enzymes, and scavenging oxidative stress products. Because of the association with oxidative reaction and cardioprotection, sirtuin-1 (SIRT1) was selected as a promising target for investigating whether MI/R injury can be alleviated by ginsenoside Rb2 pretreatment through SIRT1 activation. The rats were exposed to ginsenoside Rb2 with or without SIRT1 inhibitor EX527 before ligation of coronary artery. Ginsenoside Rb2 reduced myocardial superoxide generation; downregulated gp91^phox expression; and decreased the mRNA expression levels and activities of interleukin-1β, interleukin-6, and tumor necrosis factor-α. The results demonstrated that ginsenoside Rb2 significantly attenuated oxidative stress and inflammation induced by MI/R injury. In addition, ginsenoside Rb2 upregulated SIRT1 expression and downregulated Ac-p53 expression. However, EX527 blocked the protective effects, indicating that the pharmacological action of ginsenoside Rb2 involves SIRT1. Our results thus revealed that ginsenoside Rb2 alleviated MI/R injury in rats by inhibiting oxidative stress and inflammatory response through SIRT1 activation. PRACTICAL APPLICATION: Ginsenoside Rb2 has a protective effect on MI/R injury by activating SIRT1 expression, reducing myocardium inflammation, and alleviating oxidative stress. Thus, ginsenoside Rb2 is a promising novel agent for ameliorating MI/R injury in ischemic heart diseases and cardiac surgery.

Specific cyprinid HIF isoforms contribute to cellular mitochondrial regulation

Hypoxia-inducible factor 1 (HIF-1) functions as a master regulator of the cellular response to hypoxic stress. Two HIF-1α paralogs, HIF-1αA and HIF-1αB, were generated in euteleosts by the specific, third round of genome duplication, but one paralog was later lost in most families with the exception of cyprinid fish. How these duplicates function in mitochondrial regulation and whether their preservation contributes to the hypoxia tolerance demonstrated by cyprinid fish in freshwater environments is not clear. Here we demonstrated the divergent function of these two zebrafish Hif-1a paralogs through cellular approaches. The results showed that Hif-1aa played a role in tricarboxylic acid cycle by increasing the expression of Citrate synthase and the activity of mitochondrial complex II, and it also enhanced mitochondrial membrane potential and ROS production by reducing free Ca²⁺ in the cytosol. Hif-1ab promoted intracellular ATP content by up-regulating the activity of mitochondrial complexes I, III and IV and the expression of related genes. Furthermore, both the two zebrafish Hif-1a paralogs promoted mitochondrial mass and the expression level of mtDNA, contributing to mitochondrial biogenesis. Our study reveals the divergent functions of Hif-1aa and Hif-1ab in cellular mitochondrial regulation.

Mitochondrial CaMKII causes adverse metabolic reprogramming and dilated cardiomyopathy

Despite the clear association between myocardial injury, heart failure and depressed myocardial energetics, little is known about upstream signals responsible for remodeling myocardial metabolism after pathological stress. Here, we report increased mitochondrial calmodulin kinase II (CaMKII) activation and left ventricular dilation in mice one week after myocardial infarction (MI) surgery. By contrast, mice with genetic mitochondrial CaMKII inhibition are protected from left ventricular dilation and dysfunction after MI. Mice with myocardial and mitochondrial CaMKII overexpression (mtCaMKII) have severe dilated cardiomyopathy and decreased ATP that causes elevated cytoplasmic resting (diastolic) Ca2+ concentration and reduced mechanical performance. We map a metabolic pathway that rescues disease phenotypes in mtCaMKII mice, providing insights into physiological and pathological metabolic consequences of CaMKII signaling in mitochondria. Our findings suggest myocardial dilation, a disease phenotype lacking specific therapies, can be prevented by targeted replacement of mitochondrial creatine kinase or mitochondrial-targeted CaMKII inhibition.

Effect of delay in transportation of extracellular glucose into cardiomyocytes under diabetic condition: a study through mathematical model

A four-dimensional model was built to mimic the cross-talk among plasma glucose, plasma insulin, intracellular glucose and cytoplasmic calcium of a cardiomyocyte. A time delay was considered to represent the time required for performing various cellular mechanisms between activation of insulin receptor and subsequent glucose entry from extracellular region into intracellular region of a cardiac cell. We analysed the delay-induced model and deciphered conditions for stability and bifurcation. Extensive numerical computations were performed to validate the analytical results and give further insights. Sensitivity study of the system parameters using LHS-PRCC method reveals that some rate parameters, which represent the input of plasma glucose, absorption of glucose by noncardiac cells and insulin production, are sensitive and may cause significant change in the system dynamics. It was observed that the time taken for transportation of extracellular glucose into the cell through GLUT4 plays an important role in maintaining physiological oscillations of the state variables. Parameter recalibration exercise showed that reduced input rate of glucose in the blood plasma or an alteration in transportation delay may be used for therapeutic targets in diabetic-like condition for maintaining normal cardiac function.

Mitochondrial ion channels as targets for cardioprotection

Acute myocardial infarction (AMI) and the heart failure (HF) that often result remain the leading causes of death and disability worldwide. As such, new therapeutic targets need to be discovered to protect the myocardium against acute ischaemia/reperfusion (I/R) injury in order to reduce myocardial infarct (MI) size, preserve left ventricular function and prevent the onset of HF. Mitochondrial dysfunction during acute I/R injury is a critical determinant of cell death following AMI, and therefore, ion channels in the inner mitochondrial membrane, which are known to influence cell death and survival, provide potential therapeutic targets for cardioprotection. In this article, we review the role of mitochondrial ion channels, which are known to modulate susceptibility to acute myocardial I/R injury, and we explore their potential roles as therapeutic targets for reducing MI size and preventing HF following AMI.

A Comparative Study of Rat Urine 1H-NMR Metabolome Changes Presumably Arising from Isoproterenol-Induced Heart Necrosis Versus Clarithromycin-Induced QT Interval Prolongation

Cardiotoxicity remains a challenging concern both in drug development and in the management of various clinical situations. There are a lot of examples of drugs withdrawn from the market or stopped during clinical trials due to unpredicted cardiac adverse events. Obviously, current conventional methods for cardiotoxicity assessment suffer from a lack of predictivity and sensitivity. Therefore, there is a need for developing new tools to better identify and characterize any cardiotoxicity that can occur during the pre-clinical and clinical phases of drug development as well as after marketing in exposed patients. In this study, isoproterenol and clarithromycin were used as prototypical cardiotoxic agents in rats in order to evaluate potential biomarkers of heart toxicity at very early stages using ¹H-NMR-based metabonomics. While isoproterenol is known to cause heart necrosis, clarithromycin may induce QT interval prolongation. Heart necrosis and QT prolongation were validated by histological analysis, serum measurement of lactate dehydrogenase/creatine phosphate kinase and QTc measurement by electrocardiogram (ECG). Urine samples were collected before and repeatedly during daily exposure to the drugs for ¹H-NMR based-metabonomics investigations. Specific metabolic signatures, characteristic of each tested drug, were obtained from which potential predictive biomarkers for drug-induced heart necrosis and drug-induced QT prolongation were retrieved. Isoproterenol-induced heart necrosis was characterized by higher levels of taurine, creatine, glucose and by lower levels of Krebs cycle intermediates, creatinine, betaine/trimethylamine N-oxide (TMAO), dimethylamine (DMA)/sarcosine. Clarithromycin-induced QT prolongation was characterized by higher levels of creatinine, taurine, betaine/TMAO and DMA/sarcosine and by lower levels of Krebs cycle intermediates, glucose and hippurate.

May the Force Not Be With You During Culture: Eliminating Mechano-Associated Feedback During Culture Preserves Cultured Atrial and Pacemaker Cell Functions

Cultured cardiomyocytes have been shown to possess significant potential as a model for characterization of mechano-Ca²⁺, mechano-electric, and mechano-metabolic feedbacks in the heart. However, the majority of cultured cardiomyocytes exhibit impaired electrical, mechanical, biochemical, and metabolic functions. More specifically, the cells do not beat spontaneously (pacemaker cells) or beat at a rate far lower than their physiological counterparts and self-oscillate (atrial and ventricular cells) in culture. Thus, efforts are being invested in ensuring that cultured cardiomyocytes maintain the shape and function of freshly isolated cells. Elimination of contraction during culture has been shown to preserve the mechano-Ca²⁺, mechano-electric, and mechano-metabolic feedback loops of cultured cells. This review focuses on pacemaker cells, which reside in the sinoatrial node (SAN) and generate regular heartbeat through the initiation of the heart’s electrical, metabolic, and biochemical activities. In parallel, it places emphasis on atrial cells, which are responsible for bridging the electrical conductance from the SAN to the ventricle. The review provides a summary of the main mechanisms responsible for mechano-electrical, Ca²⁺, and metabolic feedback in pacemaker and atrial cells and of culture methods existing for both cell types. The work concludes with an explanation of how the elimination of mechano-electrical, mechano-Ca²⁺, and mechano-metabolic feedbacks during culture results in sustained cultured cell function.

Disturbed cardiac mitochondrial and cytosolic calcium handling in a metabolic risk-related rat model of heart failure with preserved ejection fraction

AIM

Calcium ions play a pivotal role in matching energy supply and demand in cardiac muscle. Mitochondrial calcium concentration is lower in animal models of heart failure with reduced ejection fraction (HFrEF), but limited information is available about mitochondrial calcium handling in heart failure with preserved ejection fraction (HFpEF).

METHODS

We assessed mitochondrial Ca²⁺ handling in intact cardiomyocytes from Zucker/fatty Spontaneously hypertensive F1 hybrid (ZSF1)-lean (control) and ZSF1-obese rats, a metabolic risk-related model of HFpEF. A mitochondrially targeted Ca²⁺ indicator (MitoCam) was expressed in cultured adult rat cardiomyocytes. Cytosolic and mitochondrial Ca²⁺ transients were measured at different stimulation frequencies. Mitochondrial respiration and swelling, and expression of key proteins were determined ex vivo.

RESULTS

At rest, mitochondrial Ca²⁺ concentration in ZSF1-obese was larger than in ZSF1-lean. The diastolic and systolic mitochondrial Ca²⁺ concentrations increased with stimulation frequency, but the steady-state levels were larger in ZSF1-obese. The half-widths of the contractile responses, the resting cytosolic Ca²⁺ concentration and the decay half-times of the cytosolic Ca²⁺ transients were higher in ZSF1-obese, likely because of a lower SERCA2a/phospholamban ratio. Mitochondrial respiration was lower, particularly with nicotinamide adenine dinucleotide (NADH) (complex I) substrates, and mitochondrial swelling was larger in ZSF1-obese.

CONCLUSION

The free mitochondrial calcium concentration is higher in HFpEF owing to alterations in mitochondrial and cytosolic Ca²⁺ handling. This coupling between cytosolic and mitochondrial Ca²⁺ levels may compensate for myocardial ATP supply in vivo under conditions of mild mitochondrial dysfunction. However, if mitochondrial Ca²⁺ concentration is sustainedly increased, it might trigger mitochondrial permeability transition pore opening.

Endoplasmic Reticulum Stress and Mitochondrial Function in Airway Smooth Muscle

Inflammatory airway diseases such as asthma affect more than 300 million people world-wide. Inflammation triggers pathophysiology via such as tumor necrosis factor α (TNFα) and interleukins (e.g., IL-13). Hypercontraction of airway smooth muscle (ASM) and ASM cell proliferation are major contributors to the exaggerated airway narrowing that occurs during agonist stimulation. An emergent theme in this context is the role of inflammation-induced endoplasmic reticulum (ER) stress and altered mitochondrial function including an increase in the formation of reactive oxygen species (ROS). This may establish a vicious cycle as excess ROS generation leads to further ER stress. Yet, it is unclear whether inflammation-induced ROS is the major mechanism leading to ER stress or the consequence of ER stress. In various diseases, inflammation leads to an increase in mitochondrial fission (fragmentation), associated with reduced levels of mitochondrial fusion proteins, such as mitofusin 2 (Mfn2). Mitochondrial fragmentation may be a homeostatic response since it is generally coupled with mitochondrial biogenesis and mitochondrial volume density thereby reducing demand on individual mitochondrion. ER stress is triggered by the accumulation of unfolded proteins, which induces a homeostatic response to alter protein balance via effects on protein synthesis and degradation. In addition, the ER stress response promotes protein folding via increased expression of molecular chaperone proteins. Reduced Mfn2 and altered mitochondrial dynamics may not only be downstream to ER stress but also upstream such that a reduction in Mfn2 triggers further ER stress. In this review, we summarize the current understanding of the link between inflammation-induced ER stress and mitochondrial function and the role played in the pathophysiology of inflammatory airway diseases.

SGLT2 Inhibitors Play a Salutary Role in Heart Failure via Modulation of the Mitochondrial Function

Three cardiovascular outcome trials of sodium glucose cotransporter 2 (SGLT2) inhibitors, including the EMPA-REG OUTCOME trial, CANVAS Program, and DECLARE TIMI 58 trial, revealed that SGLT2 inhibitors were superior to a matching placebo in reducing cardiovascular events, including mortality and hospitalization for heart failure, in patients with type 2 diabetes. However, the detailed mechanism underlying the beneficial effects that SGLT2 inhibitors exert on cardiovascular diseases remains to be elucidated. We herein review the latest findings of the salutary mechanisms of SGLT2 inhibitors in cardiomyocytes, especially focusing on their mitochondrial function-mediated beneficial effects. The administration of SGLT2 inhibitors leads to the elevation of plasma levels of ketone bodies, which are an efficient energy source in the failing heart, by promoting oxidation of the mitochondrial coenzyme Q couple and enhancing the free energy of cytosolic ATP hydrolysis. SGLT2 inhibitors also promote sodium metabolism-mediated cardioprotective effects. These compounds could reduce the intracellular sodium overload to improve mitochondrial energetics and oxidative defense in the heart through binding with NHE and/or SMIT1. Furthermore, SGLT2 inhibitors could modulate mitochondrial dynamics by regulating the fusion and fission of mitochondria. Together with ongoing large-scale clinical trials to evaluate the efficacy of SGLT2 inhibitors in patients with heart failure, intensive investigations regarding the mechanism through which SGLT2 inhibitors promote the restoration in cases of heart failure would lead to the establishment of these drugs as potent anti-heart failure drugs.

Sodium-glucose co-transporter 2 inhibition as a mitochondrial therapy for atrial fibrillation in patients with diabetes?

While patients with type 2 diabetes mellitus (T2DM) are at increased risk to develop atrial fibrillation (AF), the mechanistic link between T2DM and AF-susceptibility remains unclear. Common co-morbidities of T2DM, particularly hypertension, may drive AF in the setting of T2DM. But direct mechanisms may also explain this relation, at least in part. In this regard, recent evidence suggests that mitochondrial dysfunction drives structural, electrical and contractile remodelling of atrial tissue in patients T2DM. Mitochondrial dysfunction may therefore be the mechanistic link between T2DM and AF and could also serve as a therapeutic target. An elegant series of experiments published in Cardiovascular Diabetology provide compelling new evidence to support this hypothesis. Using a model of high fat diet (HFD) and low-dose streptozotocin (STZ) injection, Shao et al. provide data that demonstrate a direct association between mitochondrial dysfunction and the susceptibility to develop AF. But the authors also demonstrated that the sodium-glucose co-transporter 2 inhibitors (SGLT2i) empagliflozin has the capacity to restore mitochondrial function, ameliorate electrical and structural remodelling and prevent AF. These findings provide a new horizon in which mitochondrial targeted therapies could serve as a new class of antiarrhythmic drugs.

TRPV4 Complexes With the Na+/Ca2+ Exchanger and IP3 Receptor 1 to Regulate Local Intracellular Calcium and Tracheal Tension in Mice

Intracellular Ca²⁺ is critical for regulating airway smooth muscle (ASM) tension. A rapid rise in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) of ASM cells is crucial for modulating the intensity and length of the ASM contraction. Because this rapid increase in [Ca²⁺]_i largely depends on the balance between Ca²⁺ released from intracellular Ca²⁺ stores and extracellular Ca²⁺ entry, exploring the mechanisms mediating Ca²⁺ transport is critical for understanding ASM contractility and the pathogenesis of bronchial contraction disorders. Transient receptor potential vanilloid 4 (TRPV4) is a highly Ca²⁺-permeable non-selective cation channel that mediates Ca²⁺ influx to increase [Ca²⁺]_i, which then directly or indirectly regulates the contraction and relaxation of ASM. The [Ca²⁺]_i returns to basal levels through several uptake and extrusion pumps, such as the sarco(endo)plasmic reticulum Ca²⁺ ATPase and inositol 1,4,5-trisphosphate receptors (IP₃Rs), the plasmalemmal Ca²⁺ ATPase, and the plasma membrane Na⁺/Ca²⁺ exchanger (NCX). Thus, to further understand ASM tension regulation in normal and diseased tissue, the present study examined whether an interaction exists among TRPV4, IP₃Rs, and NCX. The TRPV4-specific and potent agonist GSK1016790A increased [Ca²⁺]_i in mouse ASM cells, an effect that was completely blocked by the TRPV4-specific antagonist HC067047. However, GSK1016790A induced relaxation in mouse tracheal rings precontracted with carbachol in vitro. To determine the mechanism underlying this TRPV4-induced relaxation of ASM, we blocked specific downstream molecules. We found that the GSK1016790A-induced relaxation was abolished by the NCX inhibitors KB-R7943 and LiCl but not by specific inhibitors of the Ca²⁺-activated large-, intermediate-, or small-conductance K⁺ channels (BK_Ca, IK, and SK₃, respectively). The results of co-immunoprecipitation (co-IP) assays showed an interaction of TRPV4 and IP₃R₁ with NCX_s. Taken together, these findings support a physical and functional interaction of TRPV4 and IP₃R₁ with NCXs as a novel TRPV4-mediated Ca²⁺ signaling mechanism and suggest a potential target for regulation of ASM tension and treatment of respiratory diseases, especially tracheal spasm.

Ryanodine Receptor Glycation Favors Mitochondrial Damage in the Senescent Heart

BACKGROUND

Senescent cardiomyocytes exhibit a mismatch between energy demand and supply that facilitates their transition toward failing cells. Altered calcium transfer from sarcoplasmic reticulum (SR) to mitochondria has been causally linked to the pathophysiology of aging and heart failure.

METHODS

Because advanced glycation-end products accumulate throughout life, we investigated whether intracellular glycation occurs in aged cardiomyocytes and its impact on SR and mitochondria.

RESULTS

Quantitative proteomics, Western blot and immunofluorescence demonstrated a significant increase in advanced glycation-end product-modified proteins in the myocardium of old mice (≥20months) compared with young ones (4-6months). Glyoxalase-1 activity (responsible for detoxification of dicarbonyl intermediates) and its cofactor glutathione were decreased in aged hearts. Immunolabeling and proximity ligation assay identified the ryanodine receptor (RyR2) in the SR as prominent target of glycation in aged mice, and the sites of glycation were characterized by quantitative mass spectrometry. RyR2 glycation was associated with more pronounced calcium leak, determined by confocal microscopy in cardiomyocytes and SR vesicles. Interfibrillar mitochondria-directly exposed to SR calcium release-from aged mice had increased calcium content compared with those from young ones. Higher levels of advanced glycation-end products and reduced glyoxalase-1 activity and glutathione were also present in atrial appendages from surgical patients ≥75 years as compared with the younger ones. Elderly patients also exhibited RyR2 hyperglycation and increased mitochondrial calcium content that was associated with reduced myocardial aerobic capacity (mitochondrial O₂ consumption/g) attributable to less respiring mitochondria. In contracting HL-1 cardiomyocytes, pharmacological glyoxalase-1 inhibition recapitulated RyR2 glycation and defective SR-mitochondria calcium exchange of aging.

CONCLUSIONS

Mitochondria from aging hearts develop calcium overload secondary to SR calcium leak. Glycative damage of RyR2, favored by deficient dicarbonyl detoxification capacity, contributes to calcium leak and mitochondrial damage in the senescent myocardium.

Simultaneous assessment of calcium handling and contractility dynamics in isolated ventricular myocytes of a rat model of post-acute isoproterenol-induced cardiomyopathy

Stress-induced cardiomyopathy (SIC) results from a profound catecholaminergic surge during strong emotional or physical stress. SIC is characterized by acute left ventricular apex hypokinesia, in the absence of coronary arteries occlusion, and can lead to arrhythmias and acute heart failure. Although, most SIC patients recover, the process could be slow, and recurrence or death may occur. Despite that the SIC common denominator is a large catecholamine discharge, the pathophysiological mechanism is incompletely understood. It is thought that catecholamines have direct cytotoxicity on apical ventricular myocytes (VM), which have the highest β-adrenergic receptors density, and whose overstimulation might cause acute Ca²⁺ overload and oxidative stress, causing death in some VM and stunning others. Rodents receiving acute isoproterenol (ISO) overdose (OV) mimic SIC development, however, they have not been used to simultaneously assess Ca²⁺ handling and contractility status in isolated VM, which might explain ventricular hypokinesia. Therefore, treating rats with a single ISO-OV (67 mg/kg body weight), we sought out to characterize, with confocal imaging, Ca²⁺ and shortening dynamics in Fluo-4-loaded VM, during the early (1-5 days) and late post-acute phases (15 days). We found that ISO-OV VM showed contractile dysfunction; blunted shortening with slower force development and relaxation. These correlated with Ca²⁺ mishandling; blunted Ca²⁺ transient, with slower time to peak and SR Ca²⁺ recovery. SR Ca²⁺ content was low, nevertheless, diastolic Ca²⁺ sparks were more frequent, and their duration increased. Contractility and Ca²⁺ dysfunction aggravated or remained altered over time, explaining slow recovery. We conclude that diminished VM contractility is the main determinant of ISO-OV hypokinesia and is mostly related to Ca²⁺ mishandling.

Calcium Signaling and Reactive Oxygen Species in Mitochondria

In heart failure, alterations of Na⁺ and Ca²⁺ handling, energetic deficit, and oxidative stress in cardiac myocytes are important pathophysiological hallmarks. Mitochondria are central to these processes because they are the main source for ATP, but also reactive oxygen species (ROS), and their function is critically controlled by Ca²⁺ During physiological variations of workload, mitochondrial Ca²⁺ uptake is required to match energy supply to demand but also to keep the antioxidative capacity in a reduced state to prevent excessive emission of ROS. Mitochondria take up Ca²⁺ via the mitochondrial Ca²⁺ uniporter, which exists in a multiprotein complex whose molecular components were identified only recently. In heart failure, deterioration of cytosolic Ca²⁺ and Na⁺ handling hampers mitochondrial Ca²⁺ uptake and the ensuing Krebs cycle-induced regeneration of the reduced forms of NADH (nicotinamide adenine dinucleotide) and NADPH (nicotinamide adenine dinucleotide phosphate), giving rise to energetic deficit and oxidative stress. ROS emission from mitochondria can trigger further ROS release from neighboring mitochondria termed ROS-induced ROS release, and cross talk between different ROS sources provides a spatially confined cellular network of redox signaling. Although low levels of ROS may serve physiological roles, higher levels interfere with excitation-contraction coupling, induce maladaptive cardiac remodeling through redox-sensitive kinases, and cell death through mitochondrial permeability transition. Targeting the dysregulated interplay between excitation-contraction coupling and mitochondrial energetics may ameliorate the progression of heart failure.