Mitochondrial free calcium transients during excitation-contraction coupling in rabbit cardiac myocytes

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

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

Neurohormonal Connections with Mitochondria in Cardiomyopathy and Other Diseases

Neurohormonal signaling and mitochondrial dynamism are seemingly distinct processes that are almost ubiquitous among multicellular organisms. Both of these processes are regulated by GTPases, and disturbances in either can provoke disease. Here, inconspicuous pathophysiological connectivity between neurohormonal signaling and mitochondrial dynamism is reviewed in the context of cardiac and neurological syndromes. For both processes, greater understanding of basic mechanisms has evoked a reversal of conventional pathophysiological concepts. Thus, neurohormonal systems induced in, and previously thought to be critical for, cardiac functioning in heart failure are now pharmaceutically interrupted as modern standard of care. And, mitochondrial abnormalities in neuropathies that were originally attributed to an imbalance between mitochondrial fusion and fission are increasingly recognized as an interruption of axonal mitochondrial transport. The data are presented in a historical context to provided insight into how scientific thought has evolved and to foster an appreciation for how seemingly different areas of investigation can converge. Finally, some theoretical notions are presented to explain how different molecular and functional defects can evoke tissue-specific disease.

Pharmacological inhibition of the mitochondrial Ca2+ uniporter: Relevance for pathophysiology and human therapy

Mitochondrial Ca2+ uptake has long been considered crucial for meeting the fluctuating energy demands of cells in the heart and other tissues. Increases in mitochondrial matrix [Ca2+] drive mitochondrial ATP production via stimulation of Ca2+-sensitive dehydrogenases. Mitochondria-targeted sensors have revealed mitochondrial matrix [Ca2+] rises that closely follow the cytoplasmic [Ca2+] signals in many paradigms. Mitochondrial Ca2+ uptake is mediated by the Ca2+ uniporter (mtCU). Pharmacological manipulation of the mtCU is potentially key to understanding its physiological significance, but no specific, cell-permeable inhibitors were identified. In the past decade, as the molecular identity of the mtCU was brought to light, efforts have focused on genetic targeting. However, in the cells/animals that are able to survive impaired mtCU function, robust compensatory changes were found in the mtCU as well as other mechanisms. Thus, the discovery, through chemical library screens on normal and mtCU-deficient cells, of new small-molecule inhibitors with improved cell permeability and specificity might offer a better chance to test the relevance of mitochondrial Ca2+ uptake. Success with the development of small molecule mtCU inhibitors is also expected to have clinical impact, considering the growing evidence for the role of mitochondrial Ca2+ uptake in a variety of diseases, including heart attack, stroke and various neurodegenerative disorders. Here, we review the progress in pharmacological targeting of mtCU and illustrate the challenges in this field using data obtained with MCU-i11, a new small molecule inhibitor.

Sarcoplasmic Reticulum-Mitochondria Kissing in Cardiomyocytes: Ca2+, ATP, and Undisclosed Secrets

In cardiomyocytes, to carry out cell contraction, the distribution, morphology, and dynamic interaction of different cellular organelles are tightly regulated. For instance, the repetitive close apposition between junctional sarcoplasmic reticulum (jSR) and specialized sarcolemma invaginations, called transverse-tubules (TTs), is essential for an efficient excitation-contraction coupling (ECC). Upon an action potential, Ca²⁺ microdomains, generated in synchrony at the interface between TTs and jSR, underlie the prompt increase in cytosolic Ca²⁺ concentration, ultimately responsible for cell contraction during systole. This process requires a considerable amount of energy and the active participation of mitochondria, which encompass ∼30% of the cell volume and represent the major source of ATP in the heart. Importantly, in adult cardiomyocytes, mitochondria are distributed in a highly orderly fashion and strategically juxtaposed with SR. By taking advantage of the vicinity to Ca²⁺ releasing sites, they take up Ca²⁺ and modulate ATP synthesis according to the specific cardiac workload. Interestingly, with respect to SR, a biased, polarized positioning of mitochondrial Ca²⁺ uptake/efflux machineries has been reported, hinting the importance of a strictly regulated mitochondrial Ca²⁺ handling for heart activity. This notion, however, has been questioned by the observation that, in some mouse models, the deficiency of specific molecules, modulating mitochondrial Ca²⁺ dynamics, triggers non-obvious cardiac phenotypes. This review will briefly summarize the physiological significance of SR-mitochondria apposition in cardiomyocytes, as well as the pathological consequences of an altered organelle communication, focusing on Ca²⁺ signaling. We will discuss ongoing debates and propose future research directions.

Sarcoplasmic reticulum and calcium signaling in muscle cells: Homeostasis and disease

The sarco/endoplasmic reticulum is an extensive, dynamic and heterogeneous membranous network that fulfills multiple homeostatic functions. Among them, it compartmentalizes, stores and releases calcium within the intracellular space. In the case of muscle cells, calcium released from the sarco/endoplasmic reticulum in the vicinity of the contractile machinery induces cell contraction. Furthermore, sarco/endoplasmic reticulum-derived calcium also regulates gene transcription in the nucleus, energy metabolism in mitochondria and cytosolic signaling pathways. These diverse and overlapping processes require a highly complex fine-tuning that the sarco/endoplasmic reticulum provides by means of its numerous tubules and cisternae, specialized domains and contacts with other organelles. The sarco/endoplasmic reticulum also possesses a rich calcium-handling machinery, functionally coupled to both contraction-inducing stimuli and the contractile apparatus. Such is the importance of the sarco/endoplasmic reticulum for muscle cell physiology, that alterations in its structure, function or its calcium-handling machinery are intimately associated with the development of cardiometabolic diseases. Cardiac hypertrophy, insulin resistance and arterial hypertension are age-related pathologies with a common mechanism at the muscle cell level: the accumulation of damaged proteins at the sarco/endoplasmic reticulum induces a stress response condition termed endoplasmic reticulum stress, which impairs proper organelle function, ultimately leading to pathogenesis.

Ca2+ Channels Mediate Bidirectional Signaling between Sarcolemma and Sarcoplasmic Reticulum in Muscle Cells

The skeletal muscle and myocardial cells present highly specialized structures; for example, the close interaction between the sarcoplasmic reticulum (SR) and mitochondria—responsible for excitation-metabolism coupling—and the junction that connects the SR with T-tubules, critical for excitation-contraction (EC) coupling. The mechanisms that underlie EC coupling in these two cell types, however, are fundamentally distinct. They involve the differential expression of Ca²⁺ channel subtypes: Ca_V1.1 and RyR1 (skeletal), vs. Ca_V1.2 and RyR2 (cardiac). The Ca_V channels transform action potentials into elevations of cytosolic Ca²⁺, by activating RyRs and thus promoting SR Ca²⁺ release. The high levels of Ca²⁺, in turn, stimulate not only the contractile machinery but also the generation of mitochondrial reactive oxygen species (ROS). This forward signaling is reciprocally regulated by the following feedback mechanisms: Ca²⁺-dependent inactivation (of Ca²⁺ channels), the recruitment of Na⁺/Ca²⁺ exchanger activity, and oxidative changes in ion channels and transporters. Here, we summarize both well-established concepts and recent advances that have contributed to a better understanding of the molecular mechanisms involved in this bidirectional signaling.

SR-mitochondria communication in adult cardiomyocytes: A close relationship where the Ca2+ has a lot to say

In adult cardiomyocytes, T-tubules, junctional sarcoplasmic reticulum (jSR), and mitochondria juxtapose each other and form a unique and highly repetitive functional structure along the cell. The close apposition between jSR and mitochondria creates high Ca2+ microdomains at the contact sites, increasing the efficiency of the excitation-contraction-bioenergetics coupling, where the Ca2+ transfer from SR to mitochondria plays a critical role. The SR-mitochondria contacts are established through protein tethers, with mitofusin 2 the most studied SR-mitochondrial "bridge", albeit controversial. Mitochondrial Ca2+ uptake is further optimized with the mitochondrial Ca2+ uniporter preferentially localized in the jSR-mitochondria contact sites and the mitochondrial Na+/Ca2+ exchanger localized away from these sites. Despite all these unique features facilitating the privileged transport of Ca2+ from SR to mitochondria in adult cardiomyocytes, the question remains whether mitochondrial Ca2+ concentrations oscillate in synchronicity with cytosolic Ca2+ transients during heartbeats. Proper Ca2+ transfer controls not only the process of mitochondrial bioenergetics, but also of mitochondria-mediated cell death, autophagy/mitophagy, mitochondrial fusion/fission dynamics, reactive oxygen species generation, and redox signaling, among others. Our review focuses specifically on Ca2+ signaling between SR and mitochondria in adult cardiomyocytes. We discuss the physiological and pathological implications of this SR-mitochondrial Ca2+ signaling, research gaps, and future trends.

Why don't mice lacking the mitochondrial Ca2+ uniporter experience an energy crisis?

Current dogma holds that the heart balances energy demand and supply effectively and sustainably by sequestering enough Ca²⁺ into mitochondria during heartbeats to stimulate metabolic enzymes in the tricarboxylic acid (TCA) cycle and electron transport chain (ETC). This process is called excitation-contraction-bioenergetics (ECB) coupling. Recent breakthroughs in identifying the mitochondrial Ca²⁺ uniporter (MCU) and its associated proteins have opened up new windows for interrogating the molecular mechanisms of mitochondrial Ca²⁺ homeostasis regulation and its role in ECB coupling. Despite remarkable progress made in the past 7 years, it has been surprising, almost disappointing, that germline MCU deficiency in mice with certain genetic background yields viable pups, and knockout of the MCU in adult heart does not cause lethality. Moreover, MCU deficiency results in few adverse phenotypes, normal performance, and preserved bioenergetics in the heart at baseline. In this review, we briefly assess the existing literature on mitochondrial Ca²⁺ homeostasis regulation and then we consider possible explanations for why MCU-deficient mice are spared from energy crises under physiological conditions. We propose that MCU and/or mitochondrial Ca²⁺ may have limited ability to set ECB coupling, that other mitochondrial Ca²⁺ handling mechanisms may play a role, and that extra-mitochondrial Ca²⁺ may regulate ECB coupling. Since the heart needs to regenerate a significant amount of ATP to assure the perpetuation of heartbeats, multiple mechanisms are likely to work in concert to match energy supply with demand.

Spatial Separation of Mitochondrial Calcium Uptake and Extrusion for Energy-Efficient Mitochondrial Calcium Signaling in the Heart

Mitochondrial Ca2+ elevations enhance ATP production, but uptake must be balanced by efflux to avoid overload. Uptake is mediated by the mitochondrial Ca2+ uniporter channel complex (MCUC), and extrusion is controlled largely by the Na+/Ca2+ exchanger (NCLX), both driven electrogenically by the inner membrane potential (ΔΨm). MCUC forms hotspots at the cardiac mitochondria-junctional SR (jSR) association to locally receive Ca2+ signals; however, the distribution of NCLX is unknown. Our fractionation-based assays reveal that extensively jSR-associated mitochondrial segments contain a minor portion of NCLX and lack Na+-dependent Ca2+ extrusion. This pattern is retained upon in vivo NCLX overexpression, suggesting extensive targeting to non-jSR-associated submitochondrial domains and functional relevance. In cells with non-polarized MCUC distribution, upon NCLX overexpression the same given increase in matrix Ca2+ expends more ΔΨm. Thus, cardiac mitochondrial Ca2+ uptake and extrusion are reciprocally polarized, likely to optimize the energy efficiency of local calcium signaling in the beating heart.

Strategic Positioning and Biased Activity of the Mitochondrial Calcium Uniporter in Cardiac Muscle

Control of myocardial energetics by Ca2+ signal propagation to the mitochondrial matrix includes local Ca2+ delivery from sarcoplasmic reticulum (SR) ryanodine receptors (RyR2) to the inner mitochondrial membrane (IMM) Ca2+ uniporter (mtCU). mtCU activity in cardiac mitochondria is relatively low, whereas the IMM surface is large, due to extensive cristae folding. Hence, stochastically distributed mtCU may not suffice to support local Ca2+ transfer. We hypothesized that mtCU concentrated at mitochondria-SR associations would promote the effective Ca2+ transfer. mtCU distribution was determined by tracking MCU and EMRE, the proteins essential for channel formation. Both proteins were enriched in the IMM-outer mitochondrial membrane (OMM) contact point submitochondrial fraction and, as super-resolution microscopy revealed, located more to the mitochondrial periphery (inner boundary membrane) than inside the cristae, indicating high accessibility to cytosol-derived Ca2+ inputs. Furthermore, MCU immunofluorescence distribution was biased toward the mitochondria-SR interface (RyR2), and this bias was promoted by Ca2+ signaling activity in intact cardiomyocytes. The SR fraction of heart homogenate contains mitochondria with extensive SR associations, and these mitochondria are highly enriched in EMRE. Size exclusion chromatography suggested for EMRE- and MCU-containing complexes a wide size range and also revealed MCU-containing complexes devoid of EMRE (thus disabled) in the mitochondrial but not the SR fraction. Functional measurements suggested more effective mtCU-mediated Ca2+ uptake activity by the mitochondria of the SR than of the mitochondrial fraction. Thus, mtCU "hot spots" can be formed at the cardiac muscle mitochondria-SR associations via localization and assembly bias, serving local Ca2+ signaling and the excitation-energetics coupling.

The physiological role of mitochondrial calcium revealed by mice lacking the mitochondrial calcium uniporter

Mitochondrial calcium has been postulated to regulate a wide range of processes from bioenergetics to cell death. Here, we characterize a mouse model that lacks expression of the recently discovered mitochondrial calcium uniporter (MCU). Mitochondria derived from MCU^-/- mice have no apparent capacity to rapidly uptake calcium. While basal metabolism appears unaffected, the skeletal muscle of MCU^-/- mice exhibited alterations in the phosphorylation and activity of pyruvate dehydrogenase. In addition, MCU^-/- mice exhibited marked impairment in their ability to perform strenuous work. We further show that mitochondria from MCU^-/- mice lacked evidence for calcium-induced permeability transition pore (PTP) opening. The lack of PTP opening does not appear to protect MCU^-/- cells and tissues from cell death, although MCU^-/- hearts fail to respond to the PTP inhibitor cyclosporin A (CsA). Taken together, these results clarify how acute alterations in mitochondrial matrix calcium can regulate mammalian physiology.

NCLX: the mitochondrial sodium calcium exchanger

The free Ca(2+) concentration within the mitochondrial matrix ([Ca(2+)]m) regulates the rate of ATP production and other [Ca(2+)]m sensitive processes. It is set by the balance between total Ca(2+) influx (through the mitochondrial Ca(2+) uniporter (MCU) and any other influx pathways) and the total Ca(2+) efflux (by the mitochondrial Na(+)/Ca(2+) exchanger and any other efflux pathways). Here we review and analyze the experimental evidence reported over the past 40years which suggest that in the heart and many other mammalian tissues a putative Na(+)/Ca(2+) exchanger is the major pathway for Ca(2+) efflux from the mitochondrial matrix. We discuss those reports with respect to a recent discovery that the protein product of the human FLJ22233 gene mediates such Na(+)/Ca(2+) exchange across the mitochondrial inner membrane. Among its many functional similarities to other Na(+)/Ca(2+) exchanger proteins is a unique feature: it efficiently mediates Li(+)/Ca(2+) exchange (as well as Na(+)/Ca(2+) exchange) and was therefore named NCLX. The discovery of NCLX provides both the identity of a novel protein and new molecular means of studying various unresolved quantitative aspects of mitochondrial Ca(2+) movement out of the matrix. Quantitative and qualitative features of NCLX are discussed as is the controversy regarding the stoichiometry of the NCLX Na(+)/Ca(2+) exchange, the electrogenicity of NCLX, the [Na(+)]i dependency of NCLX and the magnitude of NCLX Ca(2+) efflux. Metabolic features attributable to NCLX and the physiological implication of the Ca(2+) efflux rate via NCLX during systole and diastole are also briefly discussed.

Calcium signaling in cardiac mitochondria

Mitochondrial Ca signaling contributes to the regulation of cellular energy metabolism, and mitochondria participate in cardiac excitation-contraction coupling (ECC) through their ability to store Ca, shape the cytosolic Ca signals and generate ATP required for contraction. The mitochondrial inner membrane is equipped with an elaborate system of channels and transporters for Ca uptake and extrusion that allows for the decoding of cytosolic Ca signals, and the storage of Ca in the mitochondrial matrix compartment. Controversy, however remains whether the fast cytosolic Ca transients underlying ECC in the beating heart are transmitted rapidly into the matrix compartment or slowly integrated by the mitochondrial Ca transport machinery. This review summarizes established and novel findings on cardiac mitochondrial Ca transport and buffering, and discusses the evidence either supporting or arguing against the idea that Ca can be taken up rapidly by mitochondria during ECC.

A minimal model for the mitochondrial rapid mode of Ca²+ uptake mechanism

Mitochondria possess a remarkable ability to rapidly accumulate and sequester Ca²⁺. One of the mechanisms responsible for this ability is believed to be the rapid mode (RaM) of Ca²⁺ uptake. Despite the existence of many models of mitochondrial Ca²⁺ dynamics, very few consider RaM as a potential mechanism that regulates mitochondrial Ca²⁺ dynamics. To fill this gap, a novel mathematical model of the RaM mechanism is developed herein. The model is able to simulate the available experimental data of rapid Ca²⁺ uptake in isolated mitochondria from both chicken heart and rat liver tissues with good fidelity. The mechanism is based on Ca²⁺ binding to an external trigger site(s) and initiating a brief transient of high Ca²⁺ conductivity. It then quickly switches to an inhibited, zero-conductive state until the external Ca²⁺ level is dropped below a critical value (∼100–150 nM). RaM's Ca²⁺- and time-dependent properties make it a unique Ca²⁺ transporter that may be an important means by which mitochondria take up Ca²⁺in situ and help enable mitochondria to decode cytosolic Ca²⁺ signals. Integrating the developed RaM model into existing models of mitochondrial Ca²⁺ dynamics will help elucidate the physiological role that this unique mechanism plays in mitochondrial Ca²⁺-homeostasis and bioenergetics.

Uncoupling protein-2 modulates myocardial excitation-contraction coupling

RATIONALE

Uncoupling protein (UCP)2 is a mitochondrial inner membrane protein that is expressed in mammalian myocardium under normal conditions and upregulated in pathological states such as heart failure. UCP2 is thought to protect cardiomyocytes against oxidative stress by dissipating the mitochondrial proton gradient and mitochondrial membrane potential (DeltaPsi(m)), thereby reducing mitochondrial reactive oxygen species generation. However, in apparent conflict with its uncoupling role, UCP2 has also been proposed to be essential for mitochondrial Ca(2+) uptake, which could have a protective action by stimulating mitochondrial ATP production.

OBJECTIVE

The goal of this study was to better understand the role of myocardial UCP2 by examining the effects of UCP2 on bioenergetics, Ca(2+) homeostasis, and excitation-contraction coupling in neonatal cardiomyocytes.

METHODS AND RESULTS

Adenoviral-mediated expression of UCP2 caused a mild depression of DeltaPsi(m) and increased the basal rate of oxygen consumption but did not affect total cellular ATP levels. Mitochondrial Ca(2+) uptake was examined in permeabilized cells loaded with the mitochondria-selective Ca(2+) probe, rhod-2. UCP2 overexpression markedly inhibited mitochondrial Ca(2+) uptake. Pretreatment with the UCP2-specific inhibitor genipin largely reversed the effects UCP2 expression on mitochondrial Ca(2+) handling, bioenergetics, and oxygen utilization. Electrically evoked cytosolic Ca(2+) transients and spontaneous cytosolic Ca(2+) sparks were examined using fluo-based probes and confocal microscopy in line scan mode. UCP2 overexpression significantly prolonged the decay phase of [Ca(2+)](c) transients in electrically paced cells, increased [Ca(2+)](c) spark activity and increased the probability that Ca(2+) sparks propagated into Ca(2+) waves. This dysregulation results from a loss of the ability of mitochondria to suppress local Ca(2+)-induced Ca(2+) release activity of the sarcoplasmic reticulum.

CONCLUSION

Increases in UCP2 expression that lower DeltaPsi(m) and contribute to protection against oxidative stress, also have deleterious effects on beat-to-beat [Ca(2+)](c) handling and excitation-contraction coupling, which may contribute to the progression of heart disease.

Simultaneous optical mapping of intracellular free calcium and action potentials from Langendorff perfused hearts

The cardiac action potential (AP) controls the rise and fall of intracellular free Ca2+ (Ca(i)), and thus the amplitude and kinetics of force generation. Besides excitation-contraction coupling, the reverse process where Ca(i) influences the AP through Ca(i)-dependent ionic currents has been implicated as the mechanism underlying QT alternans and cardiac arrhythmias in heart failure, ischemia/reperfusion, cardiac myopathy, myocardial infarction, congenital and drug-induced long QT syndrome, and ventricular fibrillation. The development of dual optical mapping at high spatial and temporal resolution provides a powerful tool to investigate the role of Ca(i) anomalies in eliciting cardiac arrhythmias. This unit describes experimental protocols to map APs and Ca(i) transients from perfused hearts by labeling the heart with two fluorescent dyes, one to measure transmembrane potential (Vm), the other Ca(i) transients. High spatial and temporal resolution is achieved by selecting Vm and Ca(i) probes with the same excitation but different emission wavelengths, to avoid cross-talk and mechanical components.

Mitochondrial calcium and the permeability transition in cell death

Dysregulation of Ca(2+) has long been implicated to be important in cell injury. A Ca(2+)-linked process important in necrosis and apoptosis (or necrapoptosis) is the mitochondrial permeability transition (MPT). In the MPT, large conductance permeability transition (PT) pores open that make the mitochondrial inner membrane abruptly permeable to solutes up to 1500 Da. The importance of Ca(2+) in MPT induction varies with circumstance. Ca(2+) overload is sufficient to induce the MPT. By contrast after ischemia-reperfusion to cardiac myocytes, Ca(2+) overload is the consequence of bioenergetic failure after the MPT rather than its cause. In other models, such as cytotoxicity from Reye-related agents and storage-reperfusion injury to liver grafts, Ca(2+) appears to be permissive to MPT onset. Lastly in oxidative stress, increased mitochondrial Ca(2+) and ROS generation act synergistically to produce the MPT and cell death. Thus, the exact role of Ca(2+) for inducing the MPT and cell death depends on the particular biologic setting.

Controlling metabolism and cell death: at the heart of mitochondrial calcium signalling

Transient increases in intracellular calcium concentration activate and coordinate a wide variety of cellular processes in virtually every cell type. This review describes the main homeostatic mechanisms that control Ca(2+) transients, focusing on the mitochondrial checkpoint. We subsequently extend this paradigm to the cardiomyocyte and to the interplay between cytosol, endoplasmic reticulum and mitochondria that occurs beat-to-beat in excitation-contraction coupling. The mechanisms whereby mitochondria decode fast cytosolic calcium spikes are discussed in the light of the results obtained with recombinant photoproteins targeted to the mitochondrial matrix of contracting cardiomyocytes. Mitochondrial calcium homeostasis is then highlighted as a crucial point of convergence of the environmental signals that mediate cardiac cell death, both by necrosis and by apoptosis. Altogether we point to a role of the mitochondrion as an integrator of calcium signalling and a fundamental decision maker in cardiomyocyte metabolism and survival.

Mitochondrial Ca2+ uptake: tortoise or hare?

Mitochondria are equipped with an efficient machinery for Ca(2+) uptake and extrusion and are capable of storing large amounts of Ca(2+). Furthermore, key steps of mitochondrial metabolism (ATP production) are Ca(2+)-dependent. In the field of cardiac physiology and pathophysiology, two main questions have dominated the thinking about mitochondrial function in the heart: 1) how does mitochondrial Ca(2+) buffering shape cytosolic Ca(2+) levels and affect excitation-contraction coupling, particularly the Ca(2+) transient, on a beat-to-beat basis, and 2) how does mitochondrial Ca(2+) homeostasis influence cardiac energy metabolism. To answer these questions, a thorough understanding of the kinetics of mitochondrial Ca(2+) transport and buffer capacity is required. Here, we summarize the role of mitochondrial Ca(2+) signaling in the heart, discuss the evidence either supporting or arguing against the idea that Ca(2+) can be taken up rapidly by mitochondria during excitation-contraction coupling and highlight some interesting new areas for further investigation.

The mitochondrion as janus bifrons

The signaling function of mitochondria is considered with a special emphasis on their role in the regulation of redox status of the cell, possibly determining a number of pathologies including cancer and aging. The review summarizes the transport role of mitochondria in energy supply to all cellular compartments (mitochondria as an electric cable in the cell), the role of mitochondria in plastic metabolism of the cell including synthesis of heme, steroids, iron-sulfur clusters, and reactive oxygen and nitrogen species. Mitochondria also play an important role in the Ca(2+)-signaling and the regulation of apoptotic cell death. Knowledge of mechanisms responsible for apoptotic cell death is important for the strategy for prevention of unwanted degradation of postmitotic cells such as cardiomyocytes and neurons.

Adriamycin-induced interference with cardiac mitochondrial calcium homeostasis

Adriamycin (doxorubicin) is a potent and broad-spectrum antineoplastic agent, the clinical utility of which is limited by the development of a cumulative and irreversible cardiomyopathy. Although the drug affects numerous structures in different cell types, the mitochondrion appears to a principal subcellular target for the development of cardiomyopathy. This review describes evidence demonstrating that adriamycin redox cycles on complex I of the mitochondrial electron transport chain to liberate highly reactive free radical species of molecular oxygen. The primary effect of adriamycin on mitochondrial performance is the interference with oxidative phosphorylation and inhibition of ATP synthesis. Free radicals liberated from adriamycin redox cycling are thought to be responsible for many of the secondary effects of adriamycin, including lipid peroxidation, the oxidation of both proteins and DNA, and the depletion of glutathione and pyridine nucleotide reducing equivalents in the cell. It is this altered redox status that is believed to cause assorted changes in intracellular regulation, including the induction of the mitochondrial permeability transition and complete loss of mitochondrial integrity and function. Associated with this is the interference with mitochondrial-mediated cell calcium signaling, which is implicated as essential to the capacity of mitochondria to participate in bioenergetic regulation in response to external signals reflecting changes in metabolic demand. If taken to an extreme, this loss of mitochondrial plasticity may manifest in the liberation of signals mediating either oncotic or necrotic cell death, further perpetuating the cardiac failure associated with adriamycin-induced mitochondrial cardiomyopathy.

Excitation-contraction coupling and mitochondrial energetics

Cardiac excitation-contraction (EC) coupling consumes vast amounts of cellular energy, most of which is produced in mitochondria by oxidative phosphorylation. In order to adapt the constantly varying workload of the heart to energy supply, tight coupling mechanisms are essential to maintain cellular pools of ATP, phosphocreatine and NADH. To our current knowledge, the most important regulators of oxidative phosphorylation are ADP, Pi, and Ca2+. However, the kinetics of mitochondrial Ca2+-uptake during EC coupling are currently a matter of intense debate. Recent experimental findings suggest the existence of a mitochondrial Ca2+ microdomain in cardiac myocytes, justified by the close proximity of mitochondria to the sites of cellular Ca2+ release, i. e., the ryanodine receptors of the sarcoplasmic reticulum. Such a Ca2+ microdomain could explain seemingly controversial results on mitochondrial Ca2+ uptake kinetics in isolated mitochondria versus whole cardiac myocytes. Another important consideration is that rapid mitochondrial Ca2+ uptake facilitated by microdomains may shape cytosolic Ca2+ signals in cardiac myocytes and have an impact on energy supply and demand matching. Defects in EC coupling in chronic heart failure may adversely affect mitochondrial Ca2+ uptake and energetics, initiating a vicious cycle of contractile dysfunction and energy depletion. Future therapeutic approaches in the treatment of heart failure could be aimed at interrupting this vicious cycle.

The mitochondrial ryanodine receptor in rat heart: A pharmaco-kinetic profile

A protein discovered within inner mitochondrial membranes (IMM), designated as the mitochondrial ryanodine receptor (mRyR), has been recognized recently as a modulator of Ca(2+) fluxes in mitochondria. The present study provides fundamental pharmacological and electrophysiological properties of this mRyR. Rat cardiac IMM fused to lipid bilayers revealed the presence of a mitochondrial channel with gating characteristics similar to those of classical sarcoplasmic reticulum RyR (SR-RyR), but a variety of other mitochondrial channels obstructed clean recordings. Mitochondrial vesicles were thus solubilized and subjected to sucrose sedimentation to obtain mRyR-enriched fractions. Reconstitution of sucrose-purified fractions into lipid bilayers yielded Cs(+)-conducting, Ca(2+)-sensitive, large conductance (500-800 pS) channels with signature properties of SR-RyRs. Cytosolic Ca(2+) increased the bursting frequency and mean open time of the channel. Micromolar concentrations of ryanodine induced the appearance of subconductance states or inhibited channel activity altogether, while Imperatoxin A (IpTx(a)), a specific activator of RyRs, reversibly induced the appearance of distinct subconductance states. Remarkably, the cardiac mRyR displayed a Ca(2+) dependence of [(3)H]ryanodine binding curve similar to skeletal RyR (RyR1), not cardiac RyR (RyR2). Overall, the mRyR displayed elemental attributes that are present in single channel lipid bilayer recordings of SR-RyRs, although some exquisite differences were also noted. These results therefore provide the first direct evidence that a unique RyR occurs in mitochondrial membranes.

Mitochondrial channelopathies in aging

Defects in ion channels (channelopathies) are increasingly found in a large spectrum of human pathologies including aging. Mutations in genes encoding ion channel proteins, which disrupt channel function, are the most commonly identified cause of channelopathies. Mutations in associated proteins, alterations in the expression of ion channels, or changes in the activity of non-mutated channel genes or associated proteins can also produce acquired channelopathies. Mitochondria, the powerhouse of the cells, are considered to be the most important cellular organelles to contribute to aging mainly because of their role in the production of reactive oxygen species in the initiation of apoptotic cell remodeling and in efficient ATP synthesis. During the past 50 years, multiple ion channels or transporters have been found in mitochondria, and the relationship between the activity of these channels and cellular aging, as well as the overall cellular biological function, has been intensively studied in a number of cell types and animal models. In this review, we discuss the better characterized mitochondrial ion channels whose dysfunction (mitochondrial channelopathies) may affect or accelerate the aging processes. These channels include the mitochondrial ATP-sensitive potassium channel (mitoK(ATP)), Ca(2+) transporters, voltage-dependent anion channel, and the mitochondrial permeability transition pore (mitoPTP).

Mechanisms of reduced mitochondrial Ca2+ accumulation in failing hamster heart

Mitochondrial Ca(2+) plays important roles in the regulation of energy metabolism and cellular Ca(2+) homeostasis. In this study, we characterized mitochondrial Ca(2+) accumulation in Syrian hamster hearts with hereditary cardiomyopathy (strain BIO 14.6). Exposure of isolated mitochondria from 70 nM to 30 microM Ca(2+) ([Ca(2+)](o)) caused a concentration-dependent increase in intramitochondrial Ca(2+) concentrations ([Ca(2+)](m)). The [Ca(2+)](m) was significantly lower in cardiomyopathic (CMP) hamsters than in healthy hamsters when [Ca(2+)](o) was higher than 1 microM and a decrease of about 52% was detected at [Ca(2+)](o) of 30 microM (916 +/- 67 nM vs 1,932 +/- 132 nM in control). A possible mechanism responsible for the decreased mitochondrial Ca(2+) uptake in CMP hamsters is the depolarization of mitochondrial membrane potential (Delta psi (m)). Using a tetraphenylphosphonium (TPP(+)) electrode, the measured Delta psi (m) in failing heart mitochondria was -136 +/- 1.5 mV compared with -159 +/- 1.3 mV in controls. Analyses of mitochondrial respiratory chain demonstrated a significant impairment of complex I and complex IV activities in failing heart mitochondria. In summary, a less negative Delta psi (m) resulting from defects in the respiratory chain may lead to attenuated mitochondrial Ca(2+) accumulation, which in turn may contribute to the depressed energy production and myocardial contractility in this model of heart failure. In addition to other known impairments of ion transport in sarcoplasmic reticulum and plasma membrane, results from this paper on mitochondrial dysfunctions expand our understanding of the molecular mechanisms leading to heart failure.

Mitochondrial Transporters as Novel Targets for Intracellular Calcium Signaling

Ca2+signaling in mitochondria is important to tune mitochondrial function to a variety of extracellular stimuli. The main mechanism is Ca2+entry in mitochondria via the Ca2+uniporter followed by Ca2+activation of three dehydrogenases in the mitochondrial matrix. This results in increases in mitochondrial NADH/NAD ratios and ATP levels and increased substrate uptake by mitochondria. We review evidence gathered more than 20 years ago and recent work indicating that substrate uptake, mitochondrial NADH/NAD ratios, and ATP levels may be also activated in response to cytosolic Ca2+signals via a mechanism that does not require the entry of Ca2+in mitochondria, a mechanism depending on the activity of Ca2+-dependent mitochondrial carriers (CaMC). CaMCs fall into two groups, the aspartate-glutamate carriers (AGC) and the ATP-Mg/Picarriers, also named SCaMC (for short CaMC). The two mammalian AGCs, aralar and citrin, are members of the malate-aspartate NADH shuttle, and citrin, the liver AGC, is also a member of the urea cycle. Both types of CaMCs are activated by Ca2+in the intermembrane space and function together with the Ca2+uniporter in decoding the Ca2+signal into a mitochondrial response.

Integration of rapid cytosolic Ca2+signals by mitochondria in cat ventricular myocytes

Decoding of fast cytosolic Ca2+concentration ([Ca2+]i) transients by mitochondria was studied in permeabilized cat ventricular myocytes. Mitochondrial [Ca2+] ([Ca2+]m) was measured with fluo-3 trapped inside mitochondria after removal of cytosolic indicator by plasma membrane permeabilization with digitonin. Elevation of extramitochondrial [Ca2+] ([Ca2+]em) to >0.5 μM resulted in a [Ca2+]em-dependent increase in the rate of mitochondrial Ca2+accumulation ([Ca2+]emresulting in half-maximal rate of Ca2+accumulation = 4.4 μM) via Ca2+uniporter. Ca2+uptake was sensitive to the Ca2+uniporter blocker ruthenium red and the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone and depended on inorganic phosphate concentration. The rates of [Ca2+]mincrease and recovery were dependent on the extramitochondrial [Na+] ([Na+]em) due to Ca2+extrusion via mitochondrial Na+/Ca2+exchanger. The maximal rate of Ca2+extrusion was observed with [Na+]emin the range of 20–40 mM. Rapid switching (0.25–1 Hz) of [Ca2+]embetween 0 and 100 μM simulated rapid beat-to-beat changes in [Ca2+]i(with [Ca2+]itransient duration of 100–500 ms). No [Ca2+]moscillations were observed, either under conditions of maximal rate of Ca2+uptake (100 μM [Ca2+]em, 0 [Na+]em) or with maximal rate of Ca2+removal (0 [Ca2+]em, 40 mM [Na+]em). The slow frequency-dependent increase of [Ca2+]margues against a rapid transmission of Ca2+signals between cytosol and mitochondria on a beat-to-beat basis in the heart. [Ca2+]mchanges elicited by continuous or pulsatile exposure to elevated [Ca2+]emshowed no difference in mitochondrial Ca2+uptake. Thus in cardiac myocytes fast [Ca2+]itransients are integrated by mitochondrial Ca2+transport systems, resulting in a frequency-dependent net mitochondrial Ca2+accumulation.

ATP regulation in adult rat cardiomyocytes: time-resolved decoding of rapid mitochondrial calcium spiking imaged with targeted photoproteins

The mechanisms that enable the heart to rapidly increase ATP supply in line with increased demand have not been fully elucidated. Here we used an adenoviral system to express the photoproteins luciferase and aequorin, targeted to the mitochondria or cytosol of adult cardiomyocytes, to investigate the interrelationship between ATP and Ca(2+) in these compartments. In neither compartment were changes in free [ATP] observed upon increased workload (addition of isoproterenol) in myocytes that were already beating. However, when myocytes were stimulated to beat rapidly from rest, in the presence of isoproterenol, a significant but transient drop in mitochondrial [ATP] ([ATP](m)) occurred (on average to 10% of the initial signal). Corresponding changes in cytosolic [ATP] ([ATP](c)) were much smaller (<5%), indicating that [ATP](c) was effectively buffered in this compartment. Although mitochondrial [Ca(2+)] ([Ca(2+)](m)) is an important regulator of respiratory chain activity and ATP production in other cells, the kinetics of mitochondrial Ca(2+) transport are controversial. Parallel experiments in cells expressing mitochondrial aequorin showed that the drop in [ATP](m) occurred over the same time scale as average [Ca(2+)](m) was increasing. Conversely, in the absence or presence of isoproterenol, clear beat-to-beat peaks in [Ca(2+)](m) were observed at 0.9 or 1.3 mum, respectively, concentrations similar to those observed in the cytosol. These results suggest that mitochondrial Ca(2+) transients occur during the contractile cycle and are translated into a time-averaged increase in mitochondrial ATP production that keeps pace with increased cytosolic demand.

Elevated cytosolic Na+ decreases mitochondrial Ca2+ uptake during excitation-contraction coupling and impairs energetic adaptation in cardiac myocytes

Mitochondrial Ca2+ ([Ca2+]m) regulates oxidative phosphorylation and thus contributes to energy supply and demand matching in cardiac myocytes. Mitochondria take up Ca2+ via the Ca2+ uniporter (MCU) and extrude it through the mitochondrial Na+/Ca2+ exchanger (mNCE). It is controversial whether mitochondria take up Ca2+ rapidly, on a beat-to-beat basis, or slowly, by temporally integrating cytosolic Ca2+ ([Ca2+]c) transients. Furthermore, although mitochondrial Ca2+ efflux is governed by mNCE, it is unknown whether elevated intracellular Na+ ([Na+]i) affects mitochondrial Ca2+ uptake and bioenergetics. To monitor [Ca2+]m, mitochondria of guinea pig cardiac myocytes were loaded with rhod-2-acetoxymethyl ester (rhod-2 AM), and [Ca2+]c was monitored with indo-1 after dialyzing rhod-2 out of the cytoplasm. [Ca2+]c transients, elicited by voltage-clamp depolarizations, were accompanied by fast [Ca2+]m transients, whose amplitude (delta) correlated linearly with delta[Ca2+]c. Under beta-adrenergic stimulation, [Ca2+]m decay was approximately 2.5-fold slower than that of [Ca2+]c, leading to diastolic accumulation of [Ca2+]m when amplitude or frequency of delta[Ca2+]c increased. The MCU blocker Ru360 reduced delta[Ca2+]m and increased delta[Ca2+]c, whereas the mNCE inhibitor CGP-37157 potentiated diastolic [Ca2+]m accumulation. Elevating [Na+]i from 5 to 15 mmol/L accelerated mitochondrial Ca2+ decay, thus decreasing systolic and diastolic [Ca2+]m. In response to gradual or abrupt changes of workload, reduced nicotinamide-adenine dinucleotide (NADH) levels were maintained at 5 mmol/L [Na+]i, but at 15 mmol/L, the NADH pool was partially oxidized. The results indicate that (1) mitochondria take up Ca2+ rapidly and contribute to fast buffering during a [Ca2+]c transient; and (2) elevated [Na+]i impairs mitochondrial Ca2+ uptake, with consequent effects on energy supply and demand matching. The latter effect may have implications for cardiac diseases with elevated [Na+]i.

Alcohol and mitochondria in cardiac apoptosis: mechanisms and visualization

Apoptosis of myocytes is likely to contribute to a variety of heart conditions and could also be important in the development of alcoholic heart disease. A fundamental pathway to apoptosis is through mitochondrial membrane permeabilization and release of proapoptotic factors from the mitochondrial intermembrane space to the cytosol. The authors' results show that prolonged exposure of cultured cardiac cells to ethanol (35 mM for 48 hr) promotes Ca2+-induced activation of the mitochondrial permeability transition pore (PTP). PTP-dependent mitochondrial membrane permeabilization is followed by release of cytochrome c and execution of apoptosis. The authors propose that chronic ethanol exposure, in combination with other stress signals, may allow for activation of the PTP by physiological calcium oscillations, providing a trigger for cardiac apoptosis during chronic alcohol abuse. Coincidence of apoptosis promoting factors occurs in only a small fraction of myocytes, but because of the absence of regeneration, even a modest increase in the rate of cell death may contribute to a decrease in cardiac contractility. Detection of apoptotic changes that are present in only a few myocytes at a certain time in the heart is not feasible with most of the apoptotic assays. Fluorescence imaging is a powerful technology to visualize changes that are confined to a minor fraction of cells in a tissue, and the use of multiphoton excitation permits imaging in situ deep in the wall of the intact heart. This article discusses potential mechanisms of the effect of alcohol on mitochondrial membrane permeabilization and visualization of mitochondria-dependent apoptosis in cardiac muscle.

Mitochondrial Ca2+transport in frog early distal tubule

A global and transient rise of intracellular Ca2+ (Ca2+i) is central to the operation of pump-leak coupling in the frog early distal tubule (EDT). The endoplasmic reticulum (ER) is the site of this Ca2+ release and reuptake; however, it is likely that other intracellular pools, such as mitochondria, also contribute to cellular Ca2+ homeostasis. The present study was performed to seek evidence of mitochondrial Ca2+ transport in the frog EDT. Experiments were performed on isolated and permeabilized EDT segments from the frog kidney loaded with the low-affinity, Ca2+-sensitive fluorescent indicator, mag-fura-2. Ca2+ uptake in the absence of SarcoEndoplasmic Reticulum Calcium ATPase (SERCA) activity (inhibition by 2,5-di-t-butyl hydroquinone, TBQ) was evident at a bath [Ca2+] of 1 microm, but not at 200 nm, in the presence of ATP. This uptake was sensitive to the protonophore FCCP and the ATP-synthase inhibitor oligomycin. Ca2+ uptake was also stimulated by respiratory substrates; this uptake was enhanced by oligomycin and reversed by the application of FCCP. These findings provide the first evidence of mitochondrial Ca2+ transport in renal tubules, which appears to occur via a low-affinity pathway and which will act as a physiological Ca2+ buffer, protecting the cell from large increases in Ca2+i.

Ca transients from Ca channel activity in rat cardiac myocytes reveal dynamics of dyad cleft and troponin C Ca binding

The properties of the dyad cleft can in principle significantly impact excitation-contraction coupling, but these properties are not easily amenable to experimental investigation. We simultaneously measured the time course of the rise in integrated Ca current ( ICa) and the rise in concentration of fura 2 with Ca bound ([Ca-fura 2]) with high time resolution in rat myocytes for conditions under which Ca entry is only via L-type Ca channels and sarcoplasmic reticulum (SR) Ca release is blocked, and compared these measurements with predictions from a finite-element model of cellular Ca diffusion. We found that 1) the time course of the rise of [Ca-fura 2] follows the time course of integrated ICaplus a brief delay (1.36 ± 0.43 ms, n = 6 cells); 2) from the model, high-affinity Ca binding sites in the dyad cleft at the level previously envisioned would result in a much greater delay (≥3 ms) and are therefore unlikely to be present at that level; 3) including ATP in the model promoted Ca efflux from the dyad cleft by a factor of 1.57 when low-affinity cleft Ca binding sites were present; 4) the data could only be fit to the model if myofibrillar troponin C (TnC) Ca binding were low affinity (4.56 μM), like that of soluble troponin C, instead of the high-affinity value usually used (0.38 μM). In a “good model,” the rate constants for Ca binding and dissociation were 0.375 times the values for soluble TnC; and 5) consequently, intracellular Ca buffering at the rise of the Ca transient is inferred to be low.

Cell death induced by granzyme C

Although the functions of granzymes A and B have been defined, the functions of the other highly expressed granzymes (Gzms) of murine cytotoxic lymphocytes (C, D, and F) have not yet been evaluated. In this report, we describe the ability of murine GzmC (which is most closely related to human granzyme H) to cause cell death. The induction of death requires its protease activity and is characterized by the rapid externalization of phosphatidylserine, nuclear condensation and collapse, and single-stranded DNA nicking. The kinetics of these events are similar to those caused by granzyme B, and its potency (defined on a molar basis) is also equivalent. The induction of death did not involve the activation of caspases, the cleavage of BID, or the activation of the CAD nuclease. However, granzyme C did cause rapid mitochondrial swelling and depolarization in intact cells or in isolated mitochondria, and this mitochondrial damage was not prevented by cyclosporin A pretreatment. These results suggest that granzyme C rapidly induces target cell death by attacking nuclear and mitochondrial targets and that these targets are distinct from those used by granzyme B to cause classical apoptosis.

Old players in a new role: mitochondria-associated membranes, VDAC, and ryanodine receptors as contributors to calcium signal propagation from endoplasmic reticulum to the mitochondria

In many cell types, IP(3) and ryanodine receptor (IP(3)R/RyR)-mediated Ca(2+) mobilization from the sarcoendoplasmic reticulum (ER/SR) results in an elevation of mitochondrial matrix [Ca(2+)]. Although delivery of the released Ca(2+) to the mitochondria has been established as a fundamental signaling process, the molecular mechanism underlying mitochondrial Ca(2+) uptake remains a challenge for future studies. The Ca(2+) uptake can be divided into the following three steps: (1) Ca(2+) movement from the IP(3)R/RyR to the outer mitochondrial membrane (OMM); (2) Ca(2+) transport through the OMM; and (3) Ca(2+) transport through the inner mitochondrial membrane (IMM). Evidence has been presented that Ca(2+) delivery to the OMM is facilitated by a local coupling between closely apposed regions of the ER/SR and mitochondria. Recent studies of the dynamic changes in mitochondrial morphology and visualization of the subcellular pattern of the calcium signal provide important clues to the organization of the ER/SR-mitochondrial interface. Interestingly, key steps of phospholipid synthesis and transfer to the mitochondria have also been confined to subdomains of the ER tightly associated with the mitochondria, referred as mitochondria-associated membranes (MAMs). Through the OMM, the voltage-dependent anion channels (VDAC, porin) have been thought to permit free passage of ions and other small molecules. However, recent studies suggest that the VDAC may represent a regulated step in Ca(2+) transport from IP(3)R/RyR to the IMM. A novel proposal regarding the IMM Ca(2+) uptake site is a mitochondrial RyR that would mediate rapid Ca(2+) uptake by mitochondria in excitable cells. An overview of the progress in these directions is described in the present paper.

Ca2+ marks: miniature calcium signals in single mitochondria driven by ryanodine receptors

Propagation of cytosolic [Ca(2+)] ([Ca(2+)](c)) signals to the mitochondria is believed to be supported by a local communication between Ca(2+) release channels and adjacent mitochondrial Ca(2+) uptake sites, but the signaling machinery has not been explored at the level of elementary Ca(2+) release events. Here, we demonstrate that [Ca(2+)](c) sparks mediated by ryanodine receptors are competent to elicit miniature mitochondrial matrix [Ca(2+)] signals that we call "Ca(2+) marks." Ca(2+) marks are restricted to single mitochondria and typically last less than 500 ms. The decay of Ca(2+) marks relies on extrusion of Ca(2+) from the mitochondria through the Ca(2+) exchanger, whereas [Ca(2+)](c) sparks decline primarily by diffusion. Mitochondria also appear to have a direct effect on the properties of [Ca(2+)](c) sparks, because inhibition of mitochondrial Ca(2+) uptake results in an increase in the frequency and duration of [Ca(2+)](c) sparks. Thus, a short-lasting opening of a cluster of Ca(2+) release channels can yield activation of mitochondrial Ca(2+) uptake, and the competency of mitochondrial Ca(2+) handling may be an important determinant of cardiac excitability through local feedback control of elementary [Ca(2+)](c) signals.

Cardiac energy metabolism: models of cellular respiration

The heart requires a large amount of energy to sustain both ionic homeostasis and contraction. Under normal conditions, adenosine triphosphate (ATP) production meets this demand. Hence, there is a complex regulatory system that adjusts energy production to meet this demand. However, the mechanisms for this control are a topic of active debate. Energy metabolism can be divided into three main stages: substrate delivery to the tricarboxylic acid (TCA) cycle, the TCA cycle, and oxidative phosphorylation. Each of these processes has multiple control points and exerts control over the other stages. This review discusses the basic stages of energy metabolism, mechanisms of control, and the mathematical and computational models that have been used to study these mechanisms.

Participation of mitochondria in calcium signalling in the exocrine pancreas

This minireview is an attempt to put together some of the recent advances regarding the implications of mitochondria in Ca2+ homeostasis. Although the main role of this cytoplasmic organelle is ATP supply to the cell, during the past years strong evidence has been accumulated supporting an active role of these organelles in Ca2+ handling by the cell. The discovery of mitochondrial specific fluorescent dyes has permitted the study of these organelles within living cells. Due to its ubiquitous localisation within the cytosol, mitochondria would play an important role in the modulation of the subcellular patterns of Ca2+ signalling, and therefore would act as modulators of Ca2+-dependent cellular processes.

Beat-to-beat oscillations of mitochondrial [Ca2+] in cardiac cells

The Ca2+-sensitive photoprotein aequorin and the new green fluorescent protein-based fluorescent Ca2+ indicators 'ratiometric-pericam' were selectively expressed in the mitochondria, cytosol and/or nucleus of spontaneously beating ventricular myocytes from neonatal rats. This combined strategy reveals that mitochondrial [Ca2+] oscillates rapidly and in synchrony with cytosolic and nuclear [Ca2+]. The Ca2+ oscillations were reduced in frequency and/or amplitude by verapamil and carbachol and were enhanced by isoproterenol and elevation of extracellular [Ca2+]. An increased frequency and/or amplitude of cytosolic Ca2+ spikes was rapidly mirrored by similar changes in mitochondrial Ca2+ spikes and more slowly by elevations of the interspike Ca2+ levels. The present data unequivocally demonstrate that in cardiac cells mitochondrial [Ca2+] oscillates synchronously with cytosolic [Ca2+] and that mitochondrial Ca2+ handling rapidly adapts to inotropic or chronotropic inputs.

Identification of a ryanodine receptor in rat heart mitochondria

Recent studies have shown that, in a wide variety of cells, mitochondria respond dynamically to physiological changes in cytosolic Ca(2+) concentrations ([Ca(2+)](c)). Mitochondrial Ca(2+) uptake occurs via a ruthenium red-sensitive calcium uniporter and a rapid mode of Ca(2+) uptake. Surprisingly, the molecular identity of these Ca(2+) transport proteins is still unknown. Using electron microscopy and Western blotting, we identified a ryanodine receptor in the inner mitochondrial membrane with a molecular mass of approximately 600 kDa in mitochondria isolated from the rat heart. [(3)H]Ryanodine binds to this mitochondrial ryanodine receptor with high affinity. This binding is modulated by Ca(2+) but not caffeine and is inhibited by Mg(2+) and ruthenium red in the assay medium. In the presence of ryanodine, Ca(2+) uptake into isolated heart mitochondria is suppressed. In addition, ryanodine inhibited mitochondrial swelling induced by Ca(2+) overload. This swelling effect was not observed when Ca(2+) was applied to the cytosolic fraction containing sarcoplasmic reticulum. These results are the first to identify a mitochondrial Ca(2+) transport protein that has characteristics similar to the ryanodine receptor. This mitochondrial ryanodine receptor is likely to play an essential role in the dynamic uptake of Ca(2+) into mitochondria during Ca(2+) oscillations.

Calcium activation of heart mitochondrial oxidative phosphorylation: rapid kinetics of mVO2, NADH, AND light scattering

Parallel activation of heart mitochondria NADH and ATP production by Ca(2+) has been shown to involve the Ca(2+)-sensitive dehydrogenases and the F(0)F(1)-ATPase. In the current study we hypothesize that the response time of Ca(2+)-activated ATP production is rapid enough to support step changes in myocardial workload ( approximately 100 ms). To test this hypothesis, the rapid kinetics of Ca(2+) activation of mV(O(2)), [NADH], and light scattering were evaluated in isolated porcine heart mitochondria at 37 degrees C using a variety of optical techniques. The addition of Ca(2+) was associated with an initial response time (IRT) of mV(O(2)) that was dose-dependent with a minimum IRT of 0.27 +/- 0.02 s (n = 41) at 535 nm Ca(2+). The IRTs for NADH fluorescence and light scattering in response to Ca(2+) additions were similar to mV(O(2)). The Ca(2+) IRT for mV(O(2)) was significantly shorter than 1.6 mm ADP (2.36 +/- 0.47 s; p < or = 0.001, n = 13), 2.2 mm P(i) (2.32 +/- 0.29, p < or = 0.001, n = 13), or 10 mm creatine (15.6.+/-1.18 s, p < or = 0.001, n = 18) under similar experimental conditions. Calcium effects were inhibited with 8 microm ruthenium red (2.4 +/- 0.31 s; p < or = 0.001, n = 16) and reversed with EGTA (1.6 +/- 0.44; p < or = 0.01, n = 6). Estimates of Ca(2+) uptake into mitochondria using optical Ca(2+) indicators trapped in the matrix revealed a sufficiently rapid uptake to cause the metabolic effects observed. These data are consistent with the notion that extramitochondrial Ca(2+) can modify ATP production, via an increase in matrix Ca(2+) content, rapidly enough to support cardiac work transitions in vivo.

Quantification of calcium signal transmission from sarco-endoplasmic reticulum to the mitochondria

Recent studies have shown that ryanodine and IP3 receptor (RyR/IP3R)-mediated cytosolic Ca2+ signals propagate to the mitochondria, initiating chains of events vital in the regulation of different cellular functions. However, the fraction of released Ca2+ utilized by the mitochondria during these processes has not been quantified. To measure the amount of Ca2+ taken up by the mitochondria, we used a novel approach that involves simultaneous fluorescence imaging of mitochondrial and cytosolic [Ca2+] in permeabilized H9c2 myotubes and RBL-2H3 mast cells. Communication between sarco-endoplasmic reticulum (SR/ER) and mitochondria is maintained in these permeabilized cells, as evidenced by the large RyR/IP3R-driven mitochondrial matrix [Ca2+] and NAD(P)H signals and also by preservation of the morphology of the SR/ER-mitochondrial junctions. Ca2+ was released from the SR/ER by addition of saturating caffeine or IP3 and subsequently thapsigargin (Tg), an inhibitor of SR/ER Ca2+ pumps. The amount of Ca2+ transmitted to the mitochondria was determined by measuring increases of global [Ca2+] in the incubation medium (cytosolic [Ca2+] ([Ca2+]c)). Mitochondrial Ca2+ uptake was calculated from the difference between [Ca2+]c responses recorded in the absence and presence of uncoupler or from [Ca2+]c elevations evoked by uncoupler or ionophore applied after complete Ca2+ mobilization from the SR/ER. [Ca2+]c increases were calibrated by adding Ca2+ pulses to the permeabilized cells. In H9c2 cells, caffeine induced partial mobilization of SR Ca2+ and mitochondria accumulated 26% of the released Ca2+. Sequential application of caffeine and Tg elicited complete discharge of SR Ca2+ without further increase in mitochondrial Ca2+ uptake. In RBL-2H3 mast cells, IP3 by itself elicited complete discharge of the ER Ca2+ store and the increase of the ionophore-releasable mitochondrial Ca2+ content reached 50% of the Ca2+ amount mobilized by IP3 + Tg. Thus, RyR/IP3R direct a substantial fraction of released Ca2+ to the mitochondria.

The machinery of local Ca2+ signalling between sarco-endoplasmic reticulum and mitochondria

Growing evidence suggests that propagation of cytosolic [Ca2+] ([Ca2+]c) spikes and oscillations to the mitochondria is important for the control of fundamental cellular functions. Delivery of [Ca2+]c spikes to the mitochondria may utilize activation of the mitochondrial Ca2+ uptake sites by the large local [Ca2+]c rise occurring in the vicinity of activated sarco-endoplasmic reticulum (SR/ER) Ca2+ release channels. Although direct measurement of the local [Ca2+]c sensed by the mitochondria has been difficult, recent studies shed some light onto the molecular mechanism of local Ca2+ communication between SR/ER and mitochondria. Subdomains of the SR/ER are in close contact with mitochondria and display a concentration of Ca2+ release sites, providing the conditions for an effective delivery of released Ca2+ to the mitochondrial targets. Furthermore, many functional properties of the signalling between SR/ER Ca2+ release sites and mitochondrial Ca2+ uptake sites, including transient microdomains of high [Ca2+], saturation of mitochondrial Ca2+ uptake sites by released Ca2+, connection of multiple release sites to each uptake site and quantal transmission, are analogous to the features of the coupling between neurotransmitter release sites and postsynaptic receptors in synaptic transmission. As such, Ca2+ signal transmission between SR/ER and mitochondria may utilize discrete communication sites and a closely related functional architecture to that used for synaptic signal propagation between cells.

Elevation of mitochondrial calcium by ryanodine-sensitive calcium-induced calcium release

Calcium is an important regulator of mitochondrial function. Since there can be tight coupling between inositol 1,4, 5-trisphosphate-sensitive Ca(2+) release and elevation of mitochondrial calcium concentration, we have investigated whether a similar relationship exists between the release of Ca(2+) from the ryanodine receptor and the elevation of mitochondrial Ca(2+). Perfusion of permeabilized A10 cells with inositol 1,4, 5-trisphosphate resulted in a large transient elevation of mitochondrial Ca(2+) to about 8 microm. The response was inhibited by heparin but not ryanodine. Perfusion of the cells with Ca(2+) buffers in excess of 1 microm leads to large increases in mitochondrial Ca(2+) that are much greater than the perfused Ca(2+). These increases, which average around 10 microm, are enhanced by caffeine and inhibited by ryanodine and depletion of the intracellular stores with either orthovanadate or thapsigargin. We conclude that Ca(2+)-induced Ca(2+) release at the ryanodine receptor generates microdomains of elevated Ca(2+) that are sensed by adjacent mitochondria. In addition to ryanodine-sensitive stores acting as a source of Ca(2+), Ca(2+)-induced Ca(2+) release is required to generate efficient elevation of mitochondrial Ca(2+).

Mitochondrial calcium transients in adult rabbit cardiac myocytes: inhibition by ruthenium red and artifacts caused by lysosomal loading of Ca(2+)-indicating fluorophores

A cold/warm loading protocol was used to ester-load Rhod 2 into mitochondria and other organelles and Fluo 3 into the cytosol of adult rabbit cardiac myocytes for confocal fluorescence imaging. Transient increases in both cytosolic Fluo 3 and mitochondrial Rhod 2 fluorescence occurred after electrical stimulation. Ruthenium red, a blocker of the mitochondrial Ca(2+) uniporter, inhibited mitochondrial Rhod 2 fluorescence transients but not cytosolic Fluo 3 transients. Thus the ruthenium red-sensitive mitochondrial Ca(2+) uniporter catalyzes Ca(2+) uptake during beat-to-beat transients of mitochondrial free Ca(2+), which in turn may help match mitochondrial ATP production to myocardial ATP demand. After ester loading, substantial amounts of Ca(2+)-indicating fluorophores localized into an acidic lysosomal/endosomal compartment. This lysosomal fluorescence did not respond to electrical stimulation. Because fluorescence arose predominantly from lysosomes after the cold loading/warm incubation procedure, total cellular fluorescence failed to track beat-to-beat changes of mitochondrial fluorescence. Only three-dimensionally resolved confocal imaging distinguished the relatively weak mitochondrial signal from the bright lysosomal fluorescence.

Calcium signal transmission between ryanodine receptors and mitochondria

Control of energy metabolism by increases of mitochondrial matrix [Ca(2+)] ([Ca(2+)](m)) may represent a fundamental mechanism to meet the ATP demand imposed by heart contractions, but the machinery underlying propagation of [Ca(2+)] signals from ryanodine receptor Ca(2+) release channels (RyR) to the mitochondria remains elusive. Using permeabilized cardiac (H9c2) cells we investigated the cytosolic [Ca(2+)] ([Ca(2+)](c)) and [Ca(2+)](m) signals elicited by activation of RyR. Caffeine, Ca(2+), and ryanodine evoked [Ca(2+)](c) spikes that often appeared as frequency-modulated [Ca(2+)](c) oscillations in these permeabilized cells. Rapid increases in [Ca(2+)](m) and activation of the Ca(2+)-sensitive mitochondrial dehydrogenases were synchronized to the rising phase of the [Ca(2+)](c) spikes. The RyR-mediated elevations of global [Ca(2+)](c) were in the submicromolar range, but the rate of [Ca(2+)](m) increases was as large as it was in the presence of 30 microm global [Ca(2+)](c). Furthermore, RyR-dependent increases of [Ca(2+)](m) were relatively insensitive to buffering of [Ca(2+)](c) by EGTA. Therefore, RyR-driven rises of [Ca(2+)](m) appear to result from large and rapid increases of perimitochondrial [Ca(2+)]. The falling phase of [Ca(2+)](c) spikes was followed by a rapid decay of [Ca(2+)](m). CGP37157 slowed down relaxation of [Ca(2+)](m) spikes, whereas cyclosporin A had no effect, suggesting that activation of the mitochondrial Ca(2+) exchangers accounts for rapid reversal of the [Ca(2+)](m) response with little contribution from the permeability transition pore. Thus, rapid activation of Ca(2+) uptake sites and Ca(2+) exchangers evoked by RyR-mediated local [Ca(2+)](c) signals allow mitochondria to respond rapidly to single [Ca(2+)](c) spikes in cardiac cells.

Inositol 1,4,5-trisphosphate directs Ca(2+) flow between mitochondria and the Endoplasmic/Sarcoplasmic reticulum: a role in regulating cardiac autonomic Ca(2+) spiking

The signaling role of the Ca(2+) releaser inositol 1,4, 5-trisphosphate (IP(3)) has been associated with diverse cell functions. Yet, the physiological significance of IP(3) in tissues that feature a ryanodine-sensitive sarcoplasmic reticulum has remained elusive. IP(3) generated by photolysis of caged IP(3) or by purinergic activation of phospholipase Cgamma slowed down or abolished autonomic Ca(2+) spiking in neonatal rat cardiomyocytes. Microinjection of heparin, blocking dominant-negative fusion protein, or anti-phospholipase Cgamma antibody prevented the IP(3)-mediated purinergic effect. IP(3) triggered a ryanodine- and caffeine-insensitive Ca(2+) release restricted to the perinuclear region. In cells loaded with Rhod2 or expressing a mitochondria-targeted cameleon and TMRM to monitor mitochondrial Ca(2+) and potential, IP(3) induced transient Ca(2+) loading and depolarization of the organelles. These mitochondrial changes were associated with Ca(2+) depletion of the sarcoplasmic reticulum and preceded the arrest of cellular Ca(2+) spiking. Thus, IP(3) acting within a restricted cellular region regulates the dynamic of calcium flow between mitochondria and the endoplasmic/sarcoplasmic reticulum. We have thus uncovered a novel role for IP(3) in excitable cells, the regulation of cardiac autonomic activity.