Simultaneous measurements of mitochondrial NADH and Ca(2+) during increased work in intact rat heart trabeculae

STIM2 variants regulate Orai1/TRPC1/TRPC4-mediated store-operated Ca2+ entry and mitochondrial Ca2+ homeostasis in cardiomyocytes

The stromal interaction molecules (STIMs) are the sarcoplasmic reticulum (SR) Ca2+ sensors that trigger store-operated Ca2+ entry (SOCE) in a variety of cell types. While STIM1 isoform has been the focus of the research in cardiac pathophysiology, the function of the homolog STIM2 remains unknown. Using Ca2+ imaging and patch-clamp techniques, we showed that knockdown (KD) of STIM2 by siRNAs increased SOCE and the ISOC current in neonatal rat ventricular cardiomyocytes (NRVMs). Within this cardiomyocyte model, we identified the transcript expression of Stim2.1 and Stim2.2 splice variants, with predominance for Stim2.2. Using conventional and super-resolution confocal microscopy (STED), we found that exogenous STIM2.1 and STIM2.2 formed pre-clusters with a reticular organization at rest. Following SR Ca2+ store depletion, some STIM2.1 and STIM2.2 clusters were translocated to SR-plasma membrane (PM) junctions and co-localized with Orai1. The overexpression strategy revealed that STIM2.1 suppressed Orai1-mediated SOCE and the ISOC current while STIM2.2 enhanced SOCE. STIM2.2-enhanced SOCE was also dependent on TRPC1 and TRPC4. Even if STIM2 KD or splice variants overexpression did not affect cytosolic Ca2+ cycling, we observed, using Rhod-2/AM Ca2+ imaging, that Orai1 inhibition or STIM2.1 overexpression abolished the mitochondrial Ca2+ (mCa2+) uptake, as opposed to STIM2 KD. We also found that STIM2 was present in the mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) by interacting with the inositol trisphosphate receptors (IP3Rs), voltage-dependent anion channel (VDAC), mitochondrial Ca2+ uniporter (MCU), and mitofusin-2 (MNF2). Our results suggested that, in NRVMs, STIM2.1 constitutes the predominant functional variant that negatively regulates Orai1-generated SOCE. It participates in the control of mCa2+ uptake capacity possibly via the STIM2-IP3Rs-VDAC-MCU and MNF2 complex.

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.

Mitochondrial calcium signaling and redox homeostasis in cardiac health and disease

The energy demand of cardiomyocytes changes continuously in response to variations in cardiac workload. Cardiac excitation-contraction coupling is fueled primarily by adenosine triphosphate (ATP) production by oxidative phosphorylation in mitochondria. The rate of mitochondrial oxidative metabolism is matched to the rate of ATP consumption in the cytosol by the parallel activation of oxidative phosphorylation by calcium (Ca2+) and adenosine diphosphate (ADP). During cardiac workload transitions, Ca2+ accumulates in the mitochondrial matrix, where it stimulates the activity of the tricarboxylic acid cycle. In this review, we describe how mitochondria internalize and extrude Ca2+, the relevance of this process for ATP production and redox homeostasis in the healthy heart, and how derangements in ion handling cause mitochondrial and cardiomyocyte dysfunction in heart failure.

Ca2+ pushes and pulls energetics to maintain ATP balance in atrial cells: computational insights

To maintain atrial function, ATP supply-to-demand matching must be tightly controlled. Ca²⁺ can modulate both energy consumption and production. In light of evidence suggesting that Ca²⁺ affects energetics through "push" (activating metabolite flux and enzymes in the Krebs cycle to push the redox flux) and "pull" (acting directly on ATP synthase and driving the redox flux through the electron transport chain and increasing ATP production) pathways, we investigated whether both pathways are necessary to maintain atrial ATP supply-to-demand matching. Rabbit right atrial cells were electrically stimulated at different rates, and oxygen consumption and flavoprotein fluorescence were measured. To gain mechanistic insight into the regulators of ATP supply-to-demand matching in atrial cells, models of atrial electrophysiology, Ca²⁺ cycling and force were integrated with a model of mitochondrial Ca²⁺ and a modified model of mitochondrial energy metabolism. The experimental results showed that oxygen consumption increased in response to increases in the electrical stimulation rate. The model reproduced these findings and predicted that the increase in oxygen consumption is associated with metabolic homeostasis. The model predicted that Ca²⁺ must act both in "push" and "pull" pathways to increase oxygen consumption. In contrast to ventricular trabeculae, no rapid time-dependent changes in mitochondrial flavoprotein fluorescence were measured upon an abrupt change in workload. The model reproduced these findings and predicted that the maintenance of metabolic homeostasis is due to the effects of Ca²⁺ on ATP production. Taken together, this work provides evidence of Ca²⁺ "push" and "pull" activity to maintain metabolic homeostasis in atrial cells.

Mitochondrial calcium and reactive oxygen species in cardiovascular disease

Cardiomyocytes are one of the most mitochondria-rich cell types in the body, with ∼30-40% of the cell volume being composed of mitochondria. Mitochondria are well established as the primary site of adenosine triphosphate (ATP) generation in a beating cardiomyocyte, generating up to 90% of its ATP. Mitochondria have many functions in the cell, which could contribute to susceptibility to and development of cardiovascular disease (CVD). Mitochondria are key players in cell metabolism, ATP production, reactive oxygen species (ROS) production, and cell death. Mitochondrial calcium (Ca2+) plays a critical role in many of these pathways, and thus the dynamics of mitochondrial Ca2+ are important in regulating mitochondrial processes. Alterations in these varied and in many cases interrelated functions play an important role in CVD. This review will focus on the interrelationship of mitochondrial energetics, Ca2+, and ROS and their roles in CVD. Recent insights into the regulation and dysregulation of these pathways have led to some novel therapeutic approaches.

Mitochondrial connexin43 and mitochondrial KATP channels modulate triggered arrhythmias in mouse ventricular muscle

Connexin43 (Cx43) exits as hemichannels in the inner mitochondrial membrane. We examined how mitochondrial Cx43 and mitochondrial K_ATP channels affect the occurrence of triggered arrhythmias. To generate cardiac-specific Cx43-deficient (cCx43^-/-) mice, Cx43^flox/flox mice were crossed with α-MHC (Myh6)-cre^+/- mice. The resulting offspring, Cx43^flox/flox/Myh6-cre^+/- mice (cCx43^-/- mice) and their littermates (cCx43^+/+ mice), were used. Trabeculae were dissected from the right ventricles of mouse hearts. Cardiomyocytes were enzymatically isolated from the ventricles of mouse hearts. Force was measured with a strain gauge in trabeculae (22°C). To assess arrhythmia susceptibility, the minimal extracellular Ca²⁺ concentration ([Ca²⁺]_o,min), at which arrhythmias were induced by electrical stimulation, was determined in trabeculae. ROS production was estimated with 2',7'-dichlorofluorescein (DCF), mitochondrial membrane potential with tetramethylrhodamine methyl ester (TMRM), and Ca²⁺ spark frequency with fluo-4 and confocal microscopy in cardiomyocytes. ROS production within the mitochondria was estimated with MitoSoxRed and mitochondrial Ca²⁺ with rhod-2 in trabeculae. Diazoxide was used to activate mitochondrial K_ATP. Most of cCx43^-/- mice died suddenly within 8 weeks. Cx43 was present in the inner mitochondrial membrane in cCx43^+/+ mice but not in cCx43^-/- mice. In cCx43^-/- mice, the [Ca²⁺]_o,min was lower, and Ca²⁺ spark frequency, the slope of DCF fluorescence intensity, MitoSoxRed fluorescence, and rhod-2 fluorescence were higher. TMRM fluorescence was more decreased in cCx43^-/- mice. Most of these changes were suppressed by diazoxide. In addition, in cCx43^-/- mice, antioxidant peptide SS-31 and N-acetyl-L-cysteine increased the [Ca²⁺]_o,min. These results suggest that Cx43 deficiency activates Ca²⁺ leak from the SR, probably due to depolarization of mitochondrial membrane potential, an increase in mitochondrial Ca²⁺, and an increase in ROS production, thereby causing triggered arrhythmias, and that Cx43 hemichannel deficiency may be compensated by activation of mitochondrial K_ATP channels in mouse hearts.

The RyR2-R2474S Mutation Sensitizes Cardiomyocytes and Hearts to Catecholaminergic Stress-Induced Oxidation of the Mitochondrial Glutathione Pool

Missense mutations in the cardiac ryanodine receptor type 2 (RyR2) characteristically cause catecholaminergic arrhythmias. Reminiscent of the phenotype in patients, RyR2-R2474S knockin mice develop exercise-induced ventricular tachyarrhythmias. In cardiomyocytes, increased mitochondrial matrix Ca²⁺ uptake was recently linked to non-linearly enhanced ATP synthesis with important implications for cardiac redox metabolism. We hypothesize that catecholaminergic stimulation and contractile activity amplify mitochondrial oxidation pathologically in RyR2-R2474S cardiomyocytes. To investigate this question, we generated double transgenic RyR2-R2474S mice expressing a mitochondria-restricted fluorescent biosensor to monitor the glutathione redox potential (E_GSH). Electrical field pacing-evoked RyR2-WT and RyR2-R2474S cardiomyocyte contractions resulted in a small but significant baseline E_GSH increase. Importantly, β-adrenergic stimulation resulted in excessive E_GSH oxidization of the mitochondrial matrix in RyR2-R2474S cardiomyocytes compared to baseline and RyR2-WT control. Physiologically β-adrenergic stimulation significantly increased mitochondrial E_GSH further in intact beating RyR2-R2474S but not in RyR2-WT control Langendorff perfused hearts. Finally, this catecholaminergic E_GSH increase was significantly attenuated following treatment with the RyR2 channel blocker dantrolene. Together, catecholaminergic stimulation and increased diastolic Ca²⁺ leak induce a strong, but dantrolene-inhibited mitochondrial E_GSH oxidization in RyR2-R2474S cardiomyocytes.

Monitoring mitochondrial calcium and metabolism in the beating MCU-KO heart

Optical methods for measuring intracellular ions including Ca2+ revolutionized our understanding of signal transduction. However, these methods are not extensively applied to intact organs due to issues including inner filter effects, motion, and available probes. Mitochondrial Ca2+ is postulated to regulate cell energetics and death pathways that are best studied in an intact organ. Here, we develop a method to optically measure mitochondrial Ca2+ and demonstrate its validity for mitochondrial Ca2+ and metabolism using hearts from wild-type mice and mice with germline knockout of the mitochondria calcium uniporter (MCU-KO). We previously reported that germline MCU-KO hearts do not show an impaired response to adrenergic stimulation. We find that these MCU-KO hearts do not take up Ca2+, consistent with no alternative Ca2+ uptake mechanisms in the absence of MCU. This approach can address the role of mitochondrial Ca2+ to the myriad of functions attributed to alterations in mitochondrial Ca2+.

Deletion of Neuronal CuZnSOD Accelerates Age-Associated Muscle Mitochondria and Calcium Handling Dysfunction That Is Independent of Denervation and Precedes Sarcopenia

Skeletal muscle suffers atrophy and weakness with aging. Denervation, oxidative stress, and mitochondrial dysfunction are all proposed as contributors to age-associated muscle loss, but connections between these factors have not been established. We examined contractility, mitochondrial function, and intracellular calcium transients (ICTs) in muscles of mice throughout the life span to define their sequential relationships. We performed these same measures and analyzed neuromuscular junction (NMJ) morphology in mice with postnatal deletion of neuronal Sod1 (i-mn-Sod1^-/- mice), previously shown to display accelerated age-associated muscle loss and exacerbation of denervation in old age, to test relationships between neuronal redox homeostasis, NMJ degeneration and mitochondrial function. In control mice, the amount and rate of the decrease in mitochondrial NADH during contraction was greater in middle than young age although force was not reduced, suggesting decreased efficiency of NADH utilization prior to the onset of weakness. Declines in both the peak of the ICT and force were observed in old age. Muscles of i-mn-Sod1^-/- mice showed degeneration of mitochondrial and calcium handling functions in middle-age and a decline in force generation to a level not different from the old control mice, with maintenance of NMJ morphology. Together, the findings support the conclusion that muscle mitochondrial function decreases during aging and in response to altered neuronal redox status prior to NMJ deterioration or loss of mass and force suggesting mitochondrial defects contribute to sarcopenia independent of denervation.

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.

Transgenic expression of SOD1 specifically in neurons of Sod1 deficient mice prevents defects in muscle mitochondrial function and calcium handling

Aging is accompanied by loss of muscle mass and force, known as sarcopenia. Muscle atrophy, weakness, and neuromuscular junction (NMJ) degeneration reminiscent of normal muscle aging are observed early in adulthood for mice deficient in Cu, Zn-superoxide dismutase (SOD, Sod1^-/-). Muscles of Sod1^-/- mice also display impaired mitochondrial ATP production and increased mitochondrial reactive oxygen species (ROS) generation implicating oxidative stress in sarcopenia. Restoration of CuZnSOD specifically in neurons of Sod1^-/- mice (SynTgSod1^-/-) prevents muscle atrophy and loss of force, but whether muscle mitochondrial function is preserved is not known. To establish links among CuZnSOD expression, mitochondrial function, and sarcopenia, we examined contractile properties, mitochondrial function and ROS production, intracellular calcium transients (ICT), and NMJ morphology in lumbrical muscles of 7-9 month wild type (WT), Sod1^-/-, and SynTgSod1^-/- mice. Compared with WT values, mitochondrial ROS production was increased 2.9-fold under basal conditions and 2.2-fold with addition of glutamate and malate in Sod1^-/- muscle fibers while oxygen consumption was not significantly altered. In addition, NADH recovery was blunted following contraction and the peak of the ICT was decreased by 25%. Mitochondrial function, ROS generation and calcium handling were restored to WT values in SynTgSod1^-/- mice, despite continued lack of CuZnSOD in muscle. NMJ denervation and fragmentation were also fully rescued in SynTgSod1^-/- mice suggesting that muscle mitochondrial and calcium handling defects in Sod1^-/- mice are secondary to neuronal oxidative stress and its effects on the NMJ rather than the lack of muscle CuZnSOD. We conclude that intact neuronal function and innervation are key to maintaining excitation-contraction coupling and muscle mitochondrial function.

Regulation of Mitochondrial Ca2+ Uptake

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

Mitochondrial Ca2+ in heart failure: Not enough or too much?

Ca²⁺ serves as a ubiquitous second messenger mediating a variety of cellular processes including electrical excitation, contraction, gene expression, secretion, cell death and others. The identification of the molecular components of the mitochondrial Ca²⁺ influx and efflux pathways has created a resurgent interest in the regulation of mitochondrial Ca²⁺ balance and its physiological and pathophysiological roles. While the pace of discovery has quickened with the availability of new cellular and animal models, many fundamental questions remain to be answered regarding the regulation and functional impact of mitochondrial Ca²⁺ in health and disease. This review highlights several experimental observations pertaining to key aspects of mitochondrial Ca²⁺ homeostasis that remain enigmatic, particularly whether mitochondrial Ca²⁺ signaling is depressed or excessive in heart failure, which will determine the optimal approach to therapeutic intervention.

The yin and yang of mitochondrial Ca2+ signaling in cell physiology and pathology

Mitochondria are autonomous and dynamic cellular organelles orchestrating a diverse range of cellular activities. Numerous cell-signaling pathways target these organelles and Ca²⁺ is one of the most significant. Mitochondria are able to rapidly and transiently take up Ca²⁺, thanks to the mitochondrial Ca²⁺ uniporter complex, as well as to extrude it through the Na⁺/Ca²⁺ and H⁺/Ca²⁺ exchangers. The transient accumulation of Ca²⁺ in the mitochondrial matrix impacts on mitochondrial functions and cell pathophysiology. Here we summarize the role of mitochondrial Ca²⁺ signaling in both physiological (yang) and pathological (yin) processes and the methods that can be used to investigate mitochondrial Ca²⁺ homeostasis. As an example of the pivotal role of mitochondria in pathology, we described the state of the art of mitochondrial Ca²⁺ alterations in different pathological conditions, with a special focus on Alzheimer's disease.

Molecular Perspectives of Mitochondrial Adaptations and Their Role in Cardiac Proteostasis

Mitochondria are the key to properly functioning energy generation in the metabolically demanding cardiomyocytes and thus essential to healthy heart contractility on a beat-to-beat basis. Mitochondria being the central organelle for cellular metabolism and signaling in the heart, its dysfunction leads to cardiovascular disease. The healthy mitochondrial functioning critical to maintaining cardiomyocyte viability and contractility is accomplished by adaptive changes in the dynamics, biogenesis, and degradation of the mitochondria to ensure cellular proteostasis. Recent compelling evidence suggests that the classical protein quality control system in cardiomyocytes is also under constant mitochondrial control, either directly or indirectly. Impairment of cytosolic protein quality control may affect the position of the mitochondria in relation to other organelles, as well as mitochondrial morphology and function, and could also activate mitochondrial proteostasis. Despite a growing interest in the mitochondrial quality control system, very little information is available about the molecular function of mitochondria in cardiac proteostasis. In this review, we bring together current understanding of the adaptations and role of the mitochondria in cardiac proteostasis and describe the adaptive/maladaptive changes observed in the mitochondrial network required to maintain proteomic integrity. We also highlight the key mitochondrial signaling pathways activated in response to proteotoxic stress as a cellular mechanism to protect the heart from proteotoxicity. A deeper understanding of the molecular mechanisms of mitochondrial adaptations and their role in cardiac proteostasis will help to develop future therapeutics to protect the heart from cardiovascular diseases.

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.

Calcium, mitochondria and cell metabolism: A functional triangle in bioenergetics

The versatility of mitochondrial metabolism and its fine adjustments to specific physiological or pathological conditions regulate fundamental cell pathways, ranging from proliferation to apoptosis. In particular, Ca²⁺ signalling has emerged as a key player exploited by mitochondria to tune their activity according with cell demand. The functional interaction between mitochondria and endoplasmic reticulum (ER) deeply impacts on the correct mitochondrial Ca²⁺ signal, thus modulating cell bioenergetics and functionality. Indeed, Ca²⁺ released by the ER is taken up by mitochondria where, both in the intermembrane space and in the matrix, it regulates the activity of transporters, enzymes and proteins involved in organelles' metabolism. In this review, we will briefly summarize Ca²⁺-dependent mechanisms involved in the regulation of mitochondrial activity. Moreover, we will discuss some recent reports, in which alterations in mitochondrial Ca²⁺ signalling have been associated with specific pathological conditions, such as neurodegeneration and cancer.

Rapamycin ameliorates chitosan nanoparticle-induced developmental defects of preimplantation embryos in mice

Chitosan nanoparticles (CSNPs) are used as drug or gene delivery vehicles. However, a detailed understanding of the effects of CSNPs on embryonic development remains obscure. Here, we show that CSNPs can be internalized into mouse blastocysts, such as the zona pellucida, the perivitelline space, and the cytoplasm. Consequently, CSNPs-induced endoplasmic reticulum (ER) stress increases both of Bip/Grp78, Chop, Atf4, Perk, and Ire1a mRNAs expression levels, and reactive oxygen species. Moreover, CSNPs show double- and multi-membraned autophagic vesicles, and lead to cell death of blastocoels. Conversely, treatment with rapamycin, which plays an important role as a central regulator of cellular proliferation and stress responses, decreased CSNPs-induced mitochondrial Ca⁺² overloading, apoptosis, oxidative stress, ER stress, and autophagy. In vivo studies demonstrated that CSNPs injection has significant toxic effect on primordial and developing follicles. Notably, rapamycin rescued oxidative stress-induced embryonic defects via modulating gene expression of sirtuin and mammalian target of rapamycin. Interestingly, CSNPs treatment alters epigenetic reprogramming in mouse embryos. Overall, these observations suggest that rapamycin treatment could ameliorate CSNPs-induced developmental defects in preimplantation embryos. The data from this study would facilitate to understand the toxicity of these CSNPs, and enable the engineering of safer nanomaterials for therapeutic applications.

Successive contractile periods activate mitochondria at the onset of contractions in intact rat cardiac trabeculae

The rate of oxidative phosphorylation depends on the contractile activity of the heart. Cardiac mitochondrial oxidative phosphorylation is determined by free ADP concentration, mitochondrial Ca²⁺ accumulation, mitochondrial enzyme activities, and Krebs cycle intermediates. The purpose of the present study was to examine the factors that limit oxidative phosphorylation upon rapid changes in contractile activity in cardiac muscle. We tested the hypotheses that prior contractile performance enhances the changes in NAD(P)H and FAD concentration upon an increase in contractile activity and that this mitochondrial "priming" depends on pyruvate dehydrogenase activity. Intact rat cardiac trabeculae were electrically stimulated at 0.5 Hz for at least 30 min. Thereafter, two equal bouts at elevated stimulation frequency of 1, 2, or 3 Hz were applied for 3 min with 3 min of 0.5-Hz stimulation in between. No discernible time delay was observed in the changes in NAD(P)H and FAD fluorescence upon rapid changes in contractile activity. The amplitudes of the rapid changes in fluorescence upon an increase in stimulation frequency (the on-transients) were smaller than upon a decrease in stimulation frequency (the off-transients). A first bout in glucose-containing superfusion solution resulted, during the second bout, in an increase in the amplitudes of the on-transients, but the off-transients remained the same. No such priming effect was observed after addition of 10 mM pyruvate. These results indicate that mitochondrial priming can be observed in cardiac muscle in situ and that pyruvate dehydrogenase activity is critically involved in the mitochondrial adaptation to increases in contractile performance. NEW & NOTEWORTHY Mitochondrial respiration increases with increased cardiac contractile activity. Similar to mitochondrial "priming" in skeletal muscle, we hypothesized that cardiac mitochondrial activity is altered upon successive bouts of contractions and depends on pyruvate dehydrogenase activity. We found altered bioenergetics upon repeated contractile periods, indicative of mitochondrial priming in rat myocardium. No effect was seen when pyruvate was added to the perfusate. As such, pyruvate dehydrogenase activity is involved in the mitochondrial adaptation to increased contractile performance.

Linking metabolic and contractile dysfunction in aged cardiac myocytes

Aging is associated with declining cardiac contractile function as well as changes in metabolism and mitochondrial function. The relationship between age‐related changes in cardiac metabolism and declining cardiac contractile function has not been determined. In order to define the role energetics play in changes in contractile function, we measured mitochondrial NADH, [NADH]_m, during continuous contractions of isolated left ventricular myocytes from young (Y) and old (O) FBN rats. Second, we explored the role of metabolic disruption with rotenone and increased workload with isoproterenol (ISO) had on age‐related changes in myocytes shortening. Single, intact myocytes were stimulated for 10 min of continuous contraction at either 2 Hz or 4 Hz while being perfused with Ringer's solution. Properties of shortening (peak shortening and rate of shortening) were measured at the onset (T0) and after 10 min (T10) of continuous contraction, and the decline in shortening over time (T10/T0) was determined. Although young and old myocytes had similar contractile function under resting conditions, old myocytes demonstrated decrements in [NADH]_m during continuous stimulation, while young myocytes maintained constant [NADH]m over this time. In addition, old myocytes exhibited impaired contractile function to a workload (ISO) and metabolic (rotenone) stress compared to young myocytes. Taken together, these results demonstrated that old myocytes are susceptible to stress‐induced contractile dysfunction which may be related to altered cellular energetics.

A simulation study on the constancy of cardiac energy metabolites during workload transition

KEY POINTS

The cardiac energy metabolites such as ATP, phosphocreatine, ADP and NADH are kept relatively constant during physiological cardiac workload transition. How this is accomplished is not yet clarified, though Ca²⁺ has been suggested to be one of the possible mechanisms. We constructed a detailed mathematical model of cardiac mitochondria based on experimental data and studied whether known Ca²⁺ -dependent regulation mechanisms play roles in the metabolite constancy. Model simulations revealed that the Ca²⁺ -dependent regulation mechanisms have important roles under the in vitro condition of isolated mitochondria where malate and glutamate were mitochondrial substrates, while they have only a minor role and the composition of substrates has marked influence on the metabolite constancy during workload transition under the simulated in vivo condition where many substrates exist. These results help us understand the regulation mechanisms of cardiac energy metabolism during physiological cardiac workload transition.

ABSTRACT

The cardiac energy metabolites such as ATP, phosphocreatine, ADP and NADH are kept relatively constant over a wide range of cardiac workload, though the mechanisms are not yet clarified. One possible regulator of mitochondrial metabolism is Ca²⁺ , because it activates several mitochondrial enzymes and transporters. Here we constructed a mathematical model of cardiac mitochondria, including oxidative phosphorylation, substrate metabolism and ion/substrate transporters, based on experimental data, and studied whether the Ca²⁺ -dependent activation mechanisms play roles in metabolite constancy. Under the in vitro condition of isolated mitochondria, where malate and glutamate were used as mitochondrial substrates, the model well reproduced the Ca²⁺ and inorganic phosphate (P_i ) dependences of oxygen consumption, NADH level and mitochondrial membrane potential. The Ca²⁺ -dependent activations of the aspartate/glutamate carrier and the F₁ F_o -ATPase, and the P_i -dependent activation of Complex III were key factors in reproducing the experimental data. When the mitochondrial model was implemented in a simple cardiac cell model, simulation of workload transition revealed that cytoplasmic Ca²⁺ concentration ([Ca²⁺ ]_cyt ) within the physiological range markedly increased NADH level. However, the addition of pyruvate or citrate attenuated the Ca²⁺ dependence of NADH during the workload transition. Under the simulated in vivo condition where malate, glutamate, pyruvate, citrate and 2-oxoglutarate were used as mitochondrial substrates, the energy metabolites were more stable during the workload transition and NADH level was almost insensitive to [Ca²⁺ ]_cyt . It was revealed that mitochondrial substrates have a significant influence on metabolite constancy during cardiac workload transition, and Ca²⁺ has only a minor role under physiological conditions.

A technical review of optical mapping of intracellular calcium within myocardial tissue

Optical mapping of Ca(2+)-sensitive fluorescence probes has become an extremely useful approach and adopted by many cardiovascular research laboratories to study a spectrum of myocardial physiology and disease conditions. Optical mapping data are often displayed as detailed pseudocolor images, providing unique insight for interpreting mechanisms of ectopic activity, action potential and Ca(2+) transient alternans, tachycardia, and fibrillation. Ca(2+)-sensitive fluorescent probes and optical mapping systems continue to evolve in the ongoing effort to improve therapies that ease the growing worldwide burden of cardiovascular disease. In this technical review we provide an updated overview of conventional approaches for optical mapping of Cai (2+) within intact myocardium. In doing so, a brief history of Cai (2+) probes is provided, and nonratiometric and ratiometric Ca(2+) probes are discussed, including probes for imaging sarcoplasmic reticulum Ca(2+) and probes compatible with potentiometric dyes for dual optical mapping. Typical measurements derived from optical Cai (2+) signals are explained, and the analytics used to compute them are presented. Last, recent studies using Cai (2+) optical mapping to study arrhythmias, heart failure, and metabolic perturbations are summarized.

Distinct functional roles of cardiac mitochondrial subpopulations revealed by a 3D simulation model

Experimental characterization of two cardiac mitochondrial subpopulations, namely, subsarcolemmal mitochondria (SSM) and interfibrillar mitochondria (IFM), has been hampered by technical difficulties, and an alternative approach is eagerly awaited. We previously developed a three-dimensional computational cardiomyocyte model that integrates electrophysiology, metabolism, and mechanics with subcellular structure. In this study, we further developed our model to include intracellular oxygen diffusion, and determined whether mitochondrial localization or intrinsic properties cause functional variations. For this purpose, we created two models: one with equal SSM and IFM properties and one with IFM having higher activity levels. Using these two models to compare the SSM and IFM responses of [Ca(2+)], tricarboxylic acid cycle activity, [NADH], and mitochondrial inner membrane potential to abrupt changes in pacing frequency (0.25-2 Hz), we found that the reported functional differences between these subpopulations appear to be mostly related to local [Ca(2+)] heterogeneity, and variations in intrinsic properties only serve to augment these differences. We also examined the effect of hypoxia on mitochondrial function. Under normoxic conditions, intracellular oxygen is much higher throughout the cell than the half-saturation concentration for oxidative phosphorylation. However, under limited oxygen supply, oxygen is mostly exhausted in SSM, leaving the core region in an anoxic condition. Reflecting this heterogeneous oxygen environment, the inner membrane potential continues to decrease in IFM, whereas it is maintained to nearly normal levels in SSM, thereby ensuring ATP supply to this region. Our simulation results provide clues to understanding the origin of functional variations in two cardiac mitochondrial subpopulations and their differential roles in maintaining cardiomyocyte function as a whole.

Cellular determinants involving mitochondrial dysfunction, oxidative stress and apoptosis correlate with the synergic cytotoxicity of epigallocatechin-3-gallate and menadione in human leukemia Jurkat T cells

We have investigated the growth-suppressive action of epigallocatechin-3-gallate (EGCG) on human leukemia Jurkat T cells. Results show a strong correlation between the dose-dependent reduction of clonogenic survival following acute EGCG treatments and the EGCG-induced decline of the mitochondrial level of Ca(2+). The cell killing ability of EGCG was synergistically enhanced by menadione. In addition, the cytotoxic effect of EGCG applied alone or in combination with menadione was accompanied by apoptosis induction. We also observed that in acute treatments EGCG displays strong antioxidant properties in the intracellular milieu, but concurrently triggers some oxidative stress generating mechanisms that can fully develop on a longer timescale. In parallel, EGCG dose-dependently induced mitochondrial depolarization during exposure, but this condition was subsequently reversed to a persistent hyperpolarized mitochondrial state that was dependent on the activity of respiratory Complex I. Fluorimetric measurements suggest that EGCG is a mitochondrial Complex III inhibitor and indicate that EGCG evokes a specific cellular fluorescence with emission at 400nm and two main excitation bands (at 330nm and 350nm) that may originate from a mitochondrial supercomplex containing dimeric Complex III and dimeric ATP-synthase, and therefore could provide a valuable means to characterize the functional properties of the respiratory chain.

Modeling calcium regulation of contraction, energetics, signaling, and transcription in the cardiac myocyte

Calcium (Ca(2+)) plays many important regulatory roles in cardiac muscle cells. In the initial phase of the action potential, influx of Ca(2+) through sarcolemmal voltage-gated L-type Ca(2+) channels (LCCs) acts as a feed-forward signal that triggers a large release of Ca(2+) from the junctional sarcoplasmic reticulum (SR). This Ca(2+) drives heart muscle contraction and pumping of blood in a process known as excitation-contraction coupling (ECC). Triggered and released Ca(2+) also feed back to inactivate LCCs, attenuating the triggered Ca(2+) signal once release has been achieved. The process of ECC consumes large amounts of ATP. It is now clear that in a process known as excitation-energetics coupling, Ca(2+) signals exert beat-to-beat regulation of mitochondrial ATP production that closely couples energy production with demand. This occurs through transport of Ca(2+) into mitochondria, where it regulates enzymes of the tricarboxylic acid cycle. In excitation-signaling coupling, Ca(2+) activates a number of signaling pathways in a feed-forward manner. Through effects on their target proteins, these interconnected pathways regulate Ca(2+) signals in complex ways to control electrical excitability and contractility of heart muscle. In a process known as excitation-transcription coupling, Ca(2+) acting primarily through signal transduction pathways also regulates the process of gene transcription. Because of these diverse and complex roles, experimentally based mechanistic computational models are proving to be very useful for understanding Ca(2+) signaling in the cardiac myocyte.

Regulation of cardiac cellular bioenergetics: mechanisms and consequences

The regulation of cardiac cellular bioenergetics is critical for maintaining normal cell function, yet the nature of this regulation is not fully understood. Different mechanisms have been proposed to explain how mitochondrial ATP production is regulated to match changing cellular energy demand while metabolite concentrations are maintained. We have developed an integrated mathematical model of cardiac cellular bioenergetics, electrophysiology, and mechanics to test whether stimulation of the dehydrogenase flux by Ca²⁺ or Pi, or stimulation of complex III by Pi can increase the rate of mitochondrial ATP production above that determined by substrate availability (ADP and Pi). Using the model, we show that, under physiological conditions the rate of mitochondrial ATP production can match varying demand through substrate availability alone; that ATP production rate is not limited by the supply of reducing equivalents in the form of NADH, as a result of Ca²⁺ or Pi activation of the dehydrogenases; and that ATP production rate is sensitive to feedback activation of complex III by Pi. We then investigate the mechanistic implications on cytosolic ion homeostasis and force production by simulating the concentrations of cytosolic Ca²⁺, Na⁺ and K⁺, and activity of the key ATPases, SERCA pump, Na⁺/K⁺ pump and actin-myosin ATPase, in response to increasing cellular energy demand. We find that feedback regulation of mitochondrial complex III by Pi improves the coupling between energy demand and mitochondrial ATP production and stabilizes cytosolic ADP and Pi concentrations. This subsequently leads to stabilized cytosolic ionic concentrations and consequentially reduced energetic cost from cellular ATPases.

A Novel Nicotinamide Adenine Dinucleotide Correction Method for Mitochondrial Ca(2+) Measurement with FURA-2-FF in Single Permeabilized Ventricular Myocytes of Rat

Fura-2 analogs are ratiometric fluoroprobes that are widely used for the quantitative measurement of [Ca(2+)]. However, the dye usage is intrinsically limited, as the dyes require ultraviolet (UV) excitation, which can also generate great interference, mainly from nicotinamide adenine dinucleotide (NADH) autofluorescence. Specifically, this limitation causes serious problems for the quantitative measurement of mitochondrial [Ca(2+)], as no available ratiometric dyes are excited in the visible range. Thus, NADH interference cannot be avoided during quantitative measurement of [Ca(2+)] because the majority of NADH is located in the mitochondria. The emission intensity ratio of two different excitation wavelengths must be constant when the fluorescent dye concentration is the same. In accordance with this principle, we developed a novel online method that corrected NADH and Fura-2-FF interference. We simultaneously measured multiple parameters, including NADH, [Ca(2+)], and pH/mitochondrial membrane potential; Fura-2-FF for mitochondrial [Ca(2+)] and TMRE for Ψm or carboxy-SNARF-1 for pH were used. With this novel method, we found that the resting mitochondrial [Ca(2+)] concentration was 1.03 µM. This 1 µM cytosolic Ca(2+) could theoretically increase to more than 100 mM in mitochondria. However, the mitochondrial [Ca(2+)] increase was limited to ~30 µM in the presence of 1 µM cytosolic Ca(2+). Our method solved the problem of NADH signal contamination during the use of Fura-2 analogs, and therefore the method may be useful when NADH interference is expected.

The Mitochondrial Calcium Uniporter Matches Energetic Supply with Cardiac Workload during Stress and Modulates Permeability Transition

Cardiac contractility is mediated by a variable flux in intracellular calcium (Ca(2+)), thought to be integrated into mitochondria via the mitochondrial calcium uniporter (MCU) channel to match energetic demand. Here, we examine a conditional, cardiomyocyte-specific, mutant mouse lacking Mcu, the pore-forming subunit of the MCU channel, in adulthood. Mcu(-/-) mice display no overt baseline phenotype and are protected against mCa(2+) overload in an in vivo myocardial ischemia-reperfusion injury model by preventing the activation of the mitochondrial permeability transition pore, decreasing infarct size, and preserving cardiac function. In addition, we find that Mcu(-/-) mice lack contractile responsiveness to acute β-adrenergic receptor stimulation and in parallel are unable to activate mitochondrial dehydrogenases and display reduced bioenergetic reserve capacity. These results support the hypothesis that MCU may be dispensable for homeostatic cardiac function but required to modulate Ca(2+)-dependent metabolism during acute stress.

Rapid changes in NADH and flavin autofluorescence in rat cardiac trabeculae reveal large mitochondrial complex II reserve capacity

KEY POINTS

A photometry-based technique was developed to measure nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) autofluorescence and contractile properties simultaneously in intact rat trabeculae at a high time resolution. This provides insight into the function of mitochondrial complex I and II. Maximal complex I and complex II activities were determined in saponin-permeabilized right ventricular tissue by respirometry. In trabeculae, complex II function was considerably smaller than the maximal complex II activity, suggesting large complex II reserve capacity. Up-down asymmetry in NADH and FAD kinetics suggests a complex interaction between mitochondrial and contractile function. These data show that simultaneous measurement of contractile properties and NADH and FAD kinetics in cardiac trabeculae provides a mean to study the differences in complex I and II function in intact preparations in health and disease.

ABSTRACT

The functional properties of cardiac mitochondria in intact preparations have been mainly studied by measurements of nicotinamide adenine dinucleotide (NADH) autofluorescence, which reflects mitochondrial complex I function. To assess complex II function, we extended this method by measuring flavin adenine dinucleotide (FAD)-related autofluorescence in electrically stimulated cardiac trabeculae isolated from the right ventricle from the rat at 27°C. NADH and FAD autofluorescence and tension responses were measured when stimulation frequency was increased from 0.5 Hz to 1, 2 or 3 Hz for 3 min, and thereafter decreased to 0.5 Hz. Maximal complex I and complex II activity in vitro were determined in saponin-permeabilized right ventricular tissue by respirometry. NADH responses upon an increase in stimulation frequency showed a rapid decline, followed by a slow recovery towards the initial level. FAD responses followed a similar time course, but in the opposite direction. The amplitudes of early rapid changes in the NADH and FAD concentration correlated well with the change in tension time integral per second (R(2)  = 0.833 and 0.660 for NADH and FAD, respectively), but with different slopes for the up and down transient. Maximal velocity of the increase in FAD concentration (16 ± 4 μm s(-1) ), measured upon an increase in stimulation frequency from 0.5 to 3 Hz was considerably smaller than that of the decrease in NADH (78 ± 13 μm s(-1) ). The respiration measurements indicated that the maximal velocity of NADH utilization (143 ± 14 μm s(-1) ) was 2 times smaller than that of FADH2 (291 ± 19 μm s(-1) ). This indicates that in cardiac mitochondria considerable complex II activity reserve is present.

Age affects the contraction-induced mitochondrial redox response in skeletal muscle

Compromised mitochondrial respiratory function is associated with advancing age. Damage due to an increase in reactive oxygen species (ROS) with age is thought to contribute to the mitochondrial deficits. The coenzyme nicotinamide adenine dinucleotide in its reduced (NADH) and oxidized (NAD⁺) forms plays an essential role in the cyclic sequence of reactions that result in the regeneration of ATP by oxidative phosphorylation in mitochondria. Monitoring mitochondrial NADH/NAD⁺ redox status during recovery from an episode of high energy demand thus allows assessment of mitochondrial function. NADH fluoresces when excited with ultraviolet light in the UV-A band and NAD⁺ does not, allowing NADH/NAD⁺ to be monitored in real time using fluorescence microscopy. Our goal was to assess mitochondrial function by monitoring the NADH fluorescence response following a brief period of high energy demand in muscle from adult and old wild-type mice. This was accomplished by isolating whole lumbrical muscles from the hind paws of 7- and 28-month-old mice and making simultaneous measurements of force and NADH fluorescence responses during and after a 5 s maximum isometric contraction. All muscles exhibited fluorescence oscillations that were qualitatively similar and consisted of a brief transient increase followed by a longer transient period of reduced fluorescence and, finally, an increase that included an overshoot before recovering to resting level. Compared with the adult mice, muscles from the 28 mo mice exhibited a delayed peak during the first fluorescence transient and an attenuated recovery following the second transient. These findings indicate an impaired mitochondrial capacity to maintain NADH/NAD⁺ redox homeostasis during contractile activity in skeletal muscles of old mice.

Mitochondrial dysfunction in uremic cardiomyopathy

Uremic cardiomyopathy (UCM) is characterized by metabolic remodelling, compromised energetics, and loss of insulin-mediated cardioprotection, which result in unsustainable adaptations and heart failure. However, the role of mitochondria and the susceptibility of mitochondrial permeability transition pore (mPTP) formation in ischemia-reperfusion injury (IRI) in UCM are unknown. Using a rat model of chronic uremia, we investigated the oxidative capacity of mitochondria in UCM and their sensitivity to ischemia-reperfusion mimetic oxidant and calcium stressors to assess the susceptibility to mPTP formation. Uremic animals exhibited a 45% reduction in creatinine clearance (P < 0.01), and cardiac mitochondria demonstrated uncoupling with increased state 4 respiration. Following IRI, uremic mitochondria exhibited a 58% increase in state 4 respiration (P < 0.05), with an overall reduction in respiratory control ratio (P < 0.01). Cardiomyocytes from uremic animals displayed a 30% greater vulnerability to oxidant-induced cell death determined by FAD autofluorescence (P < 0.05) and reduced mitochondrial redox state on exposure to 200 μM H2O2 (P < 0.01). The susceptibility to calcium-induced permeability transition showed that maximum rates of depolarization were enhanced in uremia by 79%. These results demonstrate that mitochondrial respiration in the uremic heart is chronically uncoupled. Cardiomyocytes in UCM are characterized by a more oxidized mitochondrial network, with greater susceptibility to oxidant-induced cell death and enhanced vulnerability to calcium-induced mPTP formation. Collectively, these findings indicate that mitochondrial function is compromised in UCM with increased vulnerability to calcium and oxidant-induced stressors, which may underpin the enhanced predisposition to IRI in the uremic heart.

Defective sarcoplasmic reticulum-mitochondria calcium exchange in aged mouse myocardium

Mitochondrial alterations are critically involved in increased vulnerability to disease during aging. We investigated the contribution of mitochondria–sarcoplasmic reticulum (SR) communication in cardiomyocyte functional alterations during aging. Heart function (echocardiography) and ATP/phosphocreatine (NMR spectroscopy) were preserved in hearts from old mice (>20 months) with respect to young mice (5–6 months). Mitochondrial membrane potential and resting O₂ consumption were similar in mitochondria from young and old hearts. However, maximal ADP-stimulated O₂ consumption was specifically reduced in interfibrillar mitochondria from aged hearts. Second generation proteomics disclosed an increased mitochondrial protein oxidation in advanced age. Because energy production and oxidative status are regulated by mitochondrial Ca²⁺, we investigated the effect of age on mitochondrial Ca²⁺ uptake. Although no age-dependent differences were found in Ca²⁺ uptake kinetics in isolated mitochondria, mitochondrial Ca²⁺ uptake secondary to SR Ca²⁺ release was significantly reduced in cardiomyocytes from old hearts, and this effect was associated with decreased NAD(P)H regeneration and increased mitochondrial ROS upon increased contractile activity. Immunofluorescence and proximity ligation assay identified the defective communication between mitochondrial voltage-dependent anion channel and SR ryanodine receptor (RyR) in cardiomyocytes from aged hearts associated with altered Ca²⁺ handling. Age-dependent alterations in SR Ca²⁺ transfer to mitochondria and in Ca²⁺ handling could be reproduced in cardiomyoctes from young hearts after interorganelle disruption with colchicine, at concentrations that had no effect in aged cardiomyocytes or isolated mitochondria. Thus, defective SR–mitochondria communication underlies inefficient interorganelle Ca²⁺ exchange that contributes to energy demand/supply mistmach and oxidative stress in the aged heart.

Bnip3 mediates doxorubicin-induced cardiac myocyte necrosis and mortality through changes in mitochondrial signaling

Doxorubicin (DOX) is widely used for treating human cancers, but can induce heart failure through an undefined mechanism. Herein we describe a previously unidentified signaling pathway that couples DOX-induced mitochondrial respiratory chain defects and necrotic cell death to the BH3-only protein Bcl-2-like 19 kDa-interacting protein 3 (Bnip3). Cellular defects, including vacuolization and disrupted mitochondria, were observed in DOX-treated mice hearts. This coincided with mitochondrial localization of Bnip3, increased reactive oxygen species production, loss of mitochondrial membrane potential, mitochondrial permeability transition pore opening, and necrosis. Interestingly, a 3.1-fold decrease in maximal mitochondrial respiration was observed in cardiac mitochondria of mice treated with DOX. In vehicle-treated control cells undergoing normal respiration, the respiratory chain complex IV subunit 1 (COX1) was tightly bound to uncoupling protein 3 (UCP3), but this complex was disrupted in cells treated with DOX. Mitochondrial dysfunction induced by DOX was accompanied by contractile failure and necrotic cell death. Conversely, shRNA directed against Bnip3 or a mutant of Bnip3 defective for mitochondrial targeting abrogated DOX-induced loss of COX1-UCP3 complexes and respiratory chain defects. Finally, Bnip3(-/-) mice treated with DOX displayed relatively normal mitochondrial morphology, respiration, and mortality rates comparable to those of saline-treated WT mice, supporting the idea that Bnip3 underlies the cardiotoxic effects of DOX. These findings reveal a new signaling pathway in which DOX-induced mitochondrial respiratory chain defects and necrotic cell death are mutually dependent on and obligatorily linked to Bnip3 gene activation. Interventions that antagonize Bnip3 may prove beneficial in preventing mitochondrial injury and heart failure in cancer patients undergoing chemotherapy.

Mitochondrial calcium and the regulation of metabolism in the heart

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

The location of energetic compartments affects energetic communication in cardiomyocytes

The heart relies on accurate regulation of mitochondrial energy supply to match energy demand. The main regulators are Ca²⁺ and feedback of ADP and P_i. Regulation via feedback has intrigued for decades. First, the heart exhibits a remarkable metabolic stability. Second, diffusion of ADP and other molecules is restricted specifically in heart and red muscle, where a fast feedback is needed the most. To explain the regulation by feedback, compartmentalization must be taken into account. Experiments and theoretical approaches suggest that cardiomyocyte energetic compartmentalization is elaborate with barriers obstructing diffusion in the cytosol and at the level of the mitochondrial outer membrane (MOM). A recent study suggests the barriers are organized in a lattice with dimensions in agreement with those of intracellular structures. Here, we discuss the possible location of these barriers. The more plausible scenario includes a barrier at the level of MOM. Much research has focused on how the permeability of MOM itself is regulated, and the importance of the creatine kinase system to facilitate energetic communication. We hypothesize that at least part of the diffusion restriction at the MOM level is not by MOM itself, but due to the close physical association between the sarcoplasmic reticulum (SR) and mitochondria. This will explain why animals with a disabled creatine kinase system exhibit rather mild phenotype modifications. Mitochondria are hubs of energetics, but also ROS production and signaling. The close association between SR and mitochondria may form a diffusion barrier to ADP added outside a permeabilized cardiomyocyte. But in vivo, it is the structural basis for the mitochondrial-SR coupling that is crucial for the regulation of mitochondrial Ca²⁺-transients to regulate energetics, and for avoiding Ca²⁺-overload and irreversible opening of the mitochondrial permeability transition pore.

Novel insights into the antiproliferative effects and synergism of quercetin and menadione in human leukemia Jurkat T cells

The flavonoid quercetin and menadione (vitamin K3) are known as potent apoptogens in human leukemia Jurkat T cells. We explored some underlying mechanisms and the potential relevance of the combination quercetin-menadione for clinical applications. In acute treatments, quercetin manifested a strong antioxidant character, but induced a transient loss of Δψm, likely mediated by opening of the mitochondrial permeability transition pore. After removal of quercetin, persistent mitochondrial hyperpolarization was generated via stimulation of respiratory Complex I. In contrast, menadione-induced Δψm dissipation was only partially and transiently reversed after menadione removal. Results indicate that Ca(2+) release is a necessary event in quercetin-induced cell death and that the survival response to quercetin is delineated within 1h from exposure. Depending on dose, the two agents exhibited either antagonistic or synergistic effects in reducing clonogenicity of Jurkat cells. 24-h combinatorial regimens at equimolar concentrations of 10-15 μM, which are compatible with a clinically achievable (and safe) scheme, reduced cell viability at efficient rates. Altogether, these findings support the idea that the combination quercetin-menadione could improve the outcome of conventional leukemia therapies, and warrant the utility of additional studies to investigate the therapeutic effects of this combination in different cellular or animal models for leukemia.

Inhibiting Na+/K+ ATPase can impair mitochondrial energetics and induce abnormal Ca2+ cycling and automaticity in guinea pig cardiomyocytes

Cardiac glycosides have been used for the treatment of heart failure because of their capabilities of inhibiting Na⁺/K⁺ ATPase (NKA), which raises [Na⁺]_i and attenuates Ca²⁺ extrusion via the Na⁺/Ca²⁺ exchanger (NCX), causing [Ca²⁺]_i elevation. The resulting [Ca²⁺]_i accumulation further enhances Ca²⁺-induced Ca²⁺ release, generating the positive inotropic effect. However, cardiac glycosides have some toxic and side effects such as arrhythmogenesis, confining their extensive clinical applications. The mechanisms underlying the proarrhythmic effect of glycosides are not fully understood. Here we investigated the mechanisms by which glycosides could cause cardiac arrhythmias via impairing mitochondrial energetics using an integrative computational cardiomyocyte model. In the simulations, the effect of glycosides was mimicked by blocking NKA activity. Results showed that inhibiting NKA not only impaired mitochondrial Ca²⁺ retention (thus suppressed reactive oxygen species (ROS) scavenging) but also enhanced oxidative phosphorylation (thus increased ROS production) during the transition of increasing workload, causing oxidative stress. Moreover, concurrent blocking of mitochondrial Na⁺/Ca²⁺ exchanger, but not enhancing of Ca²⁺ uniporter, alleviated the adverse effects of NKA inhibition. Intriguingly, NKA inhibition elicited Ca²⁺ transient and action potential alternans under more stressed conditions such as severe ATP depletion, augmenting its proarrhythmic effect. This computational study provides new insights into the mechanisms underlying cardiac glycoside-induced arrhythmogenesis. The findings suggest that targeting both ion handling and mitochondria could be a very promising strategy to develop new glycoside-based therapies in the treatment of heart failure.

NADH changes during hypoxia, ischemia, and increased work differ between isolated heart preparations

Langendorff-perfused hearts and working hearts are established isolated heart preparation techniques that are advantageous for studying cardiac physiology and function, especially when fluorescence imaging is a key component. However, oxygen and energy requirements vary widely between isolated heart preparations. When energy supply and demand are not in harmony, such as when oxygen is not adequately available, the imbalance is reflected in NADH fluctuations. As such, NADH imaging can provide insight into the metabolic state of tissue. Hearts from New Zealand white rabbits were prepared as mechanically silenced Langendorff-perfused hearts, Langendorff-perfused hearts, or biventricular working hearts and subjected to sudden changes in workload, instantaneous global ischemia, and gradual hypoxia while heart rate, aortic pressure, and epicardial NADH fluorescence were monitored. Fast pacing resulted in a dip in NADH upon initiation and a spike in NADH when pacing was terminated in biventricular working hearts only, with the magnitude of the changes greatest at the fastest pacing rate. Working hearts were also most susceptible to changes in oxygen supply; NADH was at half-maximum value when perfusate oxygen was at 67.8 ± 13.7%. Langendorff-perfused and mechanically arrested hearts were the least affected by low oxygen supply, with half-maximum NADH occurring at 42.5 ± 5.0% and 23.7 ± 4.6% perfusate oxygen, respectively. Although the biventricular working heart preparation can provide a useful representation of mechanical in vivo heart function, it is not without limitations. Understanding the limitations of isolated heart preparations is crucial when studying cardiac function in the context of energy supply and demand.

Age-related changes of myocardial ATP supply and demand mechanisms

In advanced age, the resting myocardial oxygen consumption rate (MVO2) and cardiac work (CW) in the rat remain intact. However, MVO2, CW and cardiac efficiency achieved at high demand are decreased with age, compared to maximal values in the young. Whether this deterioration is due to decrease in myocardial ATP demand, ATP supply, or the control mechanisms that match them remains controversial. Here we discuss evolving perspectives of age-related changes of myocardial ATP supply and demand mechanisms, and critique experimental models used to investigate aging. Specifically, we evaluate experimental data collected at the level of isolated mitochondria, tissue, or organism, and discuss how mitochondrial energetic mechanisms change in advanced age, both at basal and high energy-demand levels.

Mitochondrial colocalization with Ca2+ release sites is crucial to cardiac metabolism

In cardiomyocyte subcellular structures, colocalization of mitochondria with Ca2+ release sites is implicated in regulation of cardiac energetics by facilitating Ca2+ influx into mitochondria to modulate the tricarboxylic acid (TCA) cycle. However, current experimental techniques limit detailed examination of this regulatory mechanism. Earlier, we developed a three-dimensional (3D) finite-element cardiomyocyte model featuring a subcellular structure that integrates excitation-contraction coupling and energy metabolism. Here, using this model, we examined the influence of distance between mitochondria and Ca2+ release sites by comparing a normal (50-nm) distance model and a large (200-nm) distance model (LD). The influence of distance was minimal under a low pacing rate (0.25 Hz), but under a higher pacing rate (2 Hz), lower levels of mitochondrial Ca2+ and NADH, elevated phosphate, and suppressed force generation became apparent in the LD model. Such differences became greater when functional impairments (reduced TCA cycle activity, uncoupling effect, and failing excitation-contraction coupling) were additionally imposed. We concluded that juxtaposition of the mitochondria and the Ca2+ release sites is crucial for rapid signal transmission to maintain cardiac-energy balance. The idealized 3D model of cardiac excitation-contraction and metabolism is a powerful tool to study cardiac energetics.

Intracellular Na⁺ and cardiac metabolism

In heart failure, alterations of excitation-contraction underlie contractile dysfunction. One important defect is an elevation of the intracellular Na(+) concentration in cardiac myocytes ([Na(+)]i), which has an important impact on cytosolic and mitochondrial Ca(2+) homeostasis. While elevated [Na(+)]i is thought to compensate for decreased Ca(2+) load of the sarcoplasmic reticulum (SR), it yet negatively affects energy supply-and-demand matching and can even induce mitochondrial oxidative stress. Here, we review the mechanisms underlying these pathophysiological changes. The chain of events may constitute a vicious cycle of ion dysregulation, oxidative stress and energetic deficit, resembling characteristic cellular deficits that are considered key hallmarks of the failing heart. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Sorcin modulates mitochondrial Ca(2+) handling and reduces apoptosis in neonatal rat cardiac myocytes

Sorcin localizes in cellular membranes and has been demonstrated to modulate cytosolic Ca(2+) handling in cardiac myocytes. Sorcin also localizes in mitochondria; however, the effect of sorcin on mitochondrial Ca(2+) handling is unknown. Using mitochondrial pericam, we measured mitochondrial Ca(2+) concentration and fluxes in intact neonatal cardiac myocytes overexpressing sorcin. Our results showed that sorcin increases basal and caffeine-stimulated mitochondrial Ca(2+) concentration. This effect was associated with faster Ca(2+) uptake and release. The effect of sorcin was specific for mitochondria, since similar results were obtained with digitonin-permeabilized cells, where cytosolic Ca(2+) flux was disrupted. Furthermore, mitochondria of cardiac myocytes in which sorcin was overexpressed were more Ca(2+)-tolerant. Experiments analyzing apoptotic signaling demonstrated that sorcin prevented 2-deoxyglucose-induced cytochrome c release. Furthermore, sorcin prevented hyperglycemia-induced cytochrome c release and caspase activation. In contrast, antisense sorcin induced caspase-3 activation. Thus, sorcin antiapoptotic properties may be due to modulation of mitochondrial Ca(2+) handling in cardiac myocytes.

A three-dimensional simulation model of cardiomyocyte integrating excitation-contraction coupling and metabolism

Recent studies have revealed that Ca(2+) not only regulates the contraction of cardiomyocytes, but can also function as a signaling agent to stimulate ATP production by the mitochondria. However, the spatiotemporal resolution of current experimental techniques limits our investigative capacity to understand this phenomenon. Here, we created a detailed three-dimensional (3D) cardiomyocyte model to study the subcellular regulatory mechanisms of myocardial energetics. The 3D cardiomyocyte model was based on the finite-element method, with detailed subcellular structures reproduced, and it included all elementary processes involved in cardiomyocyte electrophysiology, contraction, and ATP metabolism localized to specific loci. The simulation results were found to be reproducible and consistent with experimental data regarding the spatiotemporal pattern of cytosolic, intrasarcoplasmic-reticulum, and mitochondrial changes in Ca(2+); as well as changes in metabolite levels. Detailed analysis suggested that although the observed large cytosolic Ca(2+) gradient facilitated uptake by the mitochondrial Ca(2+) uniporter to produce cyclic changes in mitochondrial Ca(2+) near the Z-line region, the average mitochondrial Ca(2+) changes slowly. We also confirmed the importance of the creatine phosphate shuttle in cardiac energy regulation. In summary, our 3D model provides a powerful tool for the study of cardiac function by overcoming some of the spatiotemporal limitations of current experimental approaches.

Role of mitochondrial Ca2+ in the regulation of cellular energetics

Calcium is an important signaling molecule involved in the regulation of many cellular functions. The large free energy in the Ca(2+) ion membrane gradients makes Ca(2+) signaling inherently sensitive to the available cellular free energy, primarily in the form of ATP. In addition, Ca(2+) regulates many cellular ATP-consuming reactions such as muscle contraction, exocytosis, biosynthesis, and neuronal signaling. Thus, Ca(2+) becomes a logical candidate as a signaling molecule for modulating ATP hydrolysis and synthesis during changes in numerous forms of cellular work. Mitochondria are the primary source of aerobic energy production in mammalian cells and also maintain a large Ca(2+) gradient across their inner membrane, providing a signaling potential for this molecule. The demonstrated link between cytosolic and mitochondrial Ca(2+) concentrations, identification of transport mechanisms, and the proximity of mitochondria to Ca(2+) release sites further supports the notion that Ca(2+) can be an important signaling molecule in the energy metabolism interplay of the cytosol with the mitochondria. Here we review sites within the mitochondria where Ca(2+) plays a role in the regulation of ATP generation and potentially contributes to the orchestration of cellular metabolic homeostasis. Early work on isolated enzymes pointed to several matrix dehydrogenases that are stimulated by Ca(2+), which were confirmed in the intact mitochondrion as well as cellular and in vivo systems. However, studies in these intact systems suggested a more expansive influence of Ca(2+) on mitochondrial energy conversion. Numerous noninvasive approaches monitoring NADH, mitochondrial membrane potential, oxygen consumption, and workloads suggest significant effects of Ca(2+) on other elements of NADH generation as well as downstream elements of oxidative phosphorylation, including the F(1)F(O)-ATPase and the cytochrome chain. These other potential elements of Ca(2+) modification of mitochondrial energy conversion will be the focus of this review. Though most specific molecular mechanisms have yet to be elucidated, it is clear that Ca(2+) provides a balanced activation of mitochondrial energy metabolism that exceeds the alteration of dehydrogenases alone.

Computational modeling of mitochondrial function

The advent of techniques with the ability to scan massive changes in cellular makeup (genomics, proteomics, etc.) has revealed the compelling need for analytical methods to interpret and make sense of those changes. Computational models built on sound physico-chemical mechanistic basis are unavoidable at the time of integrating, interpreting, and simulating high-throughput experimental data. Another powerful role of computational models is predicting new behavior provided they are adequately validated.Mitochondrial energy transduction has been traditionally studied with thermodynamic models. More recently, kinetic or thermo-kinetic models have been proposed, leading the path toward an understanding of the control and regulation of mitochondrial energy metabolism and its interaction with cytoplasmic and other compartments. In this work, we outline the methods, step-by-step, that should be followed to build a computational model of mitochondrial energetics in isolation or integrated to a network of cellular processes. Depending on the question addressed by the modeler, the methodology explained herein can be applied with different levels of detail, from the mitochondrial energy producing machinery in a network of cellular processes to the dynamics of a single enzyme during its catalytic cycle.

A computational model of cytosolic and mitochondrial [ca] in paced rat ventricular myocytes

We carried out a series of experiment demonstrating the role of mitochondria in the cytosolic and mitochondrial Ca(2+) transients and compared the results with those from computer simulation. In rat ventricular myocytes, increasing the rate of stimulation (1~3 Hz) made both the diastolic and systolic [Ca(2+)] bigger in mitochondria as well as in cytosol. As L-type Ca(2+) channel has key influence on the amplitude of Ca(2+)-induced Ca(2+) release, the relation between stimulus frequency and the amplitude of Ca(2+) transients was examined under the low density (1/10 of control) of L-type Ca(2+) channel in model simulation, where the relation was reversed. In experiment, block of Ca(2+) uniporter on mitochondrial inner membrane significantly reduced the amplitude of mitochondrial Ca(2+) transients, while it failed to affect the cytosolic Ca(2+) transients. In computer simulation, the amplitude of cytosolic Ca(2+) transients was not affected by removal of Ca(2+) uniporter. The application of carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP) known as a protonophore on mitochondrial membrane to rat ventricular myocytes gradually increased the diastolic [Ca(2+)] in cytosol and eventually abolished the Ca(2+) transients, which was similarly reproduced in computer simulation. The model study suggests that the relative contribution of L-type Ca(2+) channel to total transsarcolemmal Ca(2+) flux could determine whether the cytosolic Ca(2+) transients become bigger or smaller with higher stimulus frequency. The present study also suggests that cytosolic Ca(2+) affects mitochondrial Ca(2+) in a beat-to-beat manner, however, removal of Ca(2+) influx mechanism into mitochondria does not affect the amplitude of cytosolic Ca(2+) transients.

Mitochondrial free [Ca2+] increases during ATP/ADP antiport and ADP phosphorylation: exploration of mechanisms

ADP influx and ADP phosphorylation may alter mitochondrial free [Ca2+] ([Ca2+](m)) and consequently mitochondrial bioenergetics by several postulated mechanisms. We tested how [Ca2+](m) is affected by H2PO4(-) (P(i)), Mg2+, calcium uniporter activity, matrix volume changes, and the bioenergetic state. We measured [Ca2+](m), membrane potential, redox state, matrix volume, pH(m), and O2 consumption in guinea pig heart mitochondria with or without ruthenium red, carboxyatractyloside, or oligomycin, and at several levels of Mg2+ and P(i). Energized mitochondria showed a dose-dependent increase in [Ca2+](m) after adding CaCl2 equivalent to 20, 114, and 485 nM extramatrix free [Ca2+] ([Ca2+](e)); this uptake was attenuated at higher buffer Mg2+. Adding ADP transiently increased [Ca2+](m) up to twofold. The ADP effect on increasing [Ca2+](m) could be partially attributed to matrix contraction, but was little affected by ruthenium red or changes in Mg2+ or P(i). Oligomycin largely reduced the increase in [Ca2+](m) by ADP compared to control, and [Ca2+](m) did not return to baseline. Carboxyatractyloside prevented the ADP-induced [Ca2+](m) increase. Adding CaCl2 had no effect on bioenergetics, except for a small increase in state 2 and state 4 respiration at 485 nM [Ca2+](e). These data suggest that matrix ADP influx and subsequent phosphorylation increase [Ca2+](m) largely due to the interaction of matrix Ca2+ with ATP, ADP, P(i), and cation buffering proteins in the matrix.

Cyclophilin D controls mitochondrial pore-dependent Ca(2+) exchange, metabolic flexibility, and propensity for heart failure in mice

Cyclophilin D (which is encoded by the Ppif gene) is a mitochondrial matrix peptidyl-prolyl isomerase known to modulate opening of the mitochondrial permeability transition pore (MPTP). Apart from regulating necrotic cell death, the physiologic function of the MPTP is largely unknown. Here we have shown that Ppif(-/-) mice exhibit substantially greater cardiac hypertrophy, fibrosis, and reduction in myocardial function in response to pressure overload stimulation than control mice. In addition, Ppif(-/-) mice showed greater hypertrophy and lung edema as well as reduced survival in response to sustained exercise stimulation. Cardiomyocyte-specific transgene expression of cyclophilin D in Ppif(-/-) mice rescued the enhanced hypertrophy, reduction in cardiac function, and rapid onset of heart failure following pressure overload stimulation. Mechanistically, the maladaptive phenotype in the hearts of Ppif(-/-) mice was associated with an alteration in MPTP-mediated Ca(2+) efflux resulting in elevated levels of mitochondrial matrix Ca(2+) and enhanced activation of Ca(2+)-dependent dehydrogenases. Elevated matrix Ca(2+) led to increased glucose oxidation relative to fatty acids, thereby limiting the metabolic flexibility of the heart that is critically involved in compensation during stress. These findings suggest that the MPTP maintains homeostatic mitochondrial Ca(2+) levels to match metabolism with alterations in myocardial workload, thereby suggesting a physiologic function for the MPTP.