Kinetics and ion specificity of Na+/Ca2+ exchange mediated by the reconstituted beef heart mitochondrial Na+/Ca2+ antiporter

Mitochondrial membrane potential instability on reperfusion after ischemia does not depend on mitochondrial Ca2+ uptake

Physiologic Ca2+ entry via the Mitochondrial Calcium Uniporter (MCU) participates in energetic adaption to workload but may also contribute to cell death during ischemia/reperfusion (I/R) injury. The MCU has been identified as the primary mode of Ca2+ import into mitochondria. Several groups have tested the hypothesis that Ca2+ import via MCU is detrimental during I/R injury using genetically-engineered mouse models, yet the results from these studies are inconclusive. Furthermore, mitochondria exhibit unstable or oscillatory membrane potentials (ΔΨm) when subjected to stress, such as during I/R, but it is unclear if the primary trigger is an excess influx of mitochondrial Ca2+ (mCa2+), reactive oxygen species (ROS) accumulation, or other factors. Here, we critically examine whether MCU-mediated mitochondrial Ca2+ uptake during I/R is involved in ΔΨm instability, or sustained mitochondrial depolarization, during reperfusion by acutely knocking out MCU in neonatal mouse ventricular myocyte (NMVM) monolayers subjected to simulated I/R. Unexpectedly, we find that MCU knockout does not significantly alter mCa2+ import during I/R, nor does it affect ΔΨm recovery during reperfusion. In contrast, blocking the mitochondrial sodium-calcium exchanger (mNCE) suppressed the mCa2+ increase during Ischemia but did not affect ΔΨm recovery or the frequency of ΔΨm oscillations during reperfusion, indicating that mitochondrial ΔΨm instability on reperfusion is not triggered by mCa2+. Interestingly, inhibition of mitochondrial electron transport or supplementation with antioxidants stabilized I/R-induced ΔΨm oscillations. The findings are consistent with mCa2+ overload being mediated by reverse-mode mNCE activity and supporting ROS-induced ROS release as the primary trigger of ΔΨm instability during reperfusion injury.

Mitochondrial calcium exchange in physiology and disease

The uptake of calcium into and extrusion of calcium from the mitochondrial matrix is a fundamental biological process that has critical effects on cellular metabolism, signaling, and survival. Disruption of mitochondrial calcium (mCa2+) cycling is implicated in numerous acquired diseases such as heart failure, stroke, neurodegeneration, diabetes, and cancer and is genetically linked to several inherited neuromuscular disorders. Understanding the mechanisms responsible for mCa2+ exchange therefore holds great promise for the treatment of these diseases. The past decade has seen the genetic identification of many of the key proteins that mediate mitochondrial calcium uptake and efflux. Here, we present an overview of the phenomenon of mCa2+ transport and a comprehensive examination of the molecular machinery that mediates calcium flux across the inner mitochondrial membrane: the mitochondrial uniporter complex (consisting of MCU, EMRE, MICU1, MICU2, MICU3, MCUB, and MCUR1), NCLX, LETM1, the mitochondrial ryanodine receptor, and the mitochondrial permeability transition pore. We then consider the physiological implications of mCa2+ flux and evaluate how alterations in mCa2+ homeostasis contribute to human disease. This review concludes by highlighting opportunities and challenges for therapeutic intervention in pathologies characterized by aberrant mCa2+ handling and by summarizing critical unanswered questions regarding the biology of mCa2+ flux.

The Mitochondrial Na+/Ca2+ Exchanger Inhibitor CGP37157 Preserves Muscle Structure and Function to Increase Lifespan and Healthspan in Caenorhabditis elegans

We have reported recently that the mitochondrial Na⁺/Ca²⁺ exchanger inhibitor CGP37157 extends lifespan in Caenorhabditis elegans by a mechanism involving mitochondria, the TOR pathway and the insulin/IGF1 pathway. Here we show that CGP37157 significantly improved the evolution with age of the sarcomeric regular structure, delaying development of sarcopenia in C. elegans body wall muscle and increasing the average and maximum speed of the worms. Similarly, CGP37157 favored the maintenance of a regular mitochondrial structure during aging. We have also investigated further the mechanism of the effect of CGP37157 by studying its effect in mutants of aak-1;aak-2/AMP-activated kinase, sir-2.1/sirtuin, rsks-1/S6 kinase and daf-16/FOXO. We found that this compound was still effective increasing lifespan in all these mutants, indicating that these pathways are not involved in the effect. We have then monitored pharynx cytosolic and mitochondrial Ca²⁺ signalling and our results suggest that CGP37157 is probably inhibiting not only the mitochondrial Na⁺/Ca²⁺ exchanger, but also Ca²⁺ entry through the plasma membrane. Finally, a transcriptomic study detected that CGP37157 induced changes in lipid metabolism enzymes and a four-fold increase in the expression of ncx-6, one of the C. elegans mitochondrial Na⁺/Ca²⁺ exchangers. In summary, CGP37157 increases both lifespan and healthspan by a mechanism involving changes in cytosolic and mitochondrial Ca²⁺ homeostasis. Thus, Ca²⁺ signalling could be a promising target to act on aging.

Mitochondrial Calcification

One of the most fascinating aspects of mitochondria is their remarkable ability to accumulate and store large amounts of calcium in the presence of phosphate leading to mitochondrial calcification. In this paper, we briefly address the mechanisms that regulate mitochondrial calcium homeostasis followed by the extensive review on the formation and characterization of intramitochondrial calcium phosphate granules leading to mitochondrial calcification and its relevance to physiological and pathological calcifications of body tissues.

Recent studies on NCLX in health and diseases

The mitochondria is a major hub for cellular Ca 2+ signaling. The identification of MCU, the mitochondrial Ca 2+ influx mediator, and the mitochondrial Ca 2+ extruder NCLX, were major breakthroughs in this field. Their identification provided novel molecular tools and animal models to interrogate their physiological function and mode of regulation. Here we will focus on the mitochondrial Na + / Ca 2+ exchanger NCLX that plays a dual role in mitochondrial Na + and Ca 2+ signaling. We will discuss recent advances in NCLX mods of regulation by kinases and mitochondrial ΔΨ. We will also focus on the heterogeneity of its expression in distinct mitochondrial populations and the pathophysiological implication of its excessive degradation. We will describe the ongoing debate on the stoichiometry of Na + to Ca 2+ transport, mediated by NCLX, and its physiological implication. We will focus on the major effects of mitochondrial Na + signaling by NCLX on mitochondrial metabolism in health; and finally, we will discuss the role NCLX plays in a wide range of health disorders, from heart failure and cancer to Parkinson and Alzheimer disease, making it a prime candidate for therapeutic targeting.

Ca2+ signaling in T lymphocytes: the interplay of the endoplasmic reticulum, mitochondria, membrane potential, and CRAC channels on transcription factor activation

T cell receptor stimulation initiates a cascade of reactions that cause an increase in intracellular calcium (Ca2+) concentration mediated through inositol 1,4,5-trisphosphate (IP3). To understand the basic mechanisms by which the immune response in T cells is activated, it is useful to understand the signaling pathways that contain important targets for drugs in a quantitative fashion. A computational model helps us to understand how the selected elements in the pathways interact with each other, and which component plays the crucial role in systems. We have developed a mathematical model to explore the mechanism for controlling transcription factor activity, which regulates gene expression, by the modulation of calcium signaling triggered during T cell activation. The model simulates the activation and modulation of Ca2+ release-activated Ca2+ (CRAC) channels by mitochondrial dynamics and depletion of endoplasmic reticulum (ER) store, and also includes membrane potential in T-cells. The model simulates the experimental finding that increases in Ca2+ current enhances the activation of transcription factors and the Ca2+ influx through CRAC is also essential for the NFAT and NFκB activation. The model also suggests that plasma membrane Ca2+-ATPase (PMCA) controls a majority of the extrusion of Ca2+ and modulates the activation of CRAC channels. Furthermore, the model simulations explain how the complex interaction of the endoplasmic reticulum, membrane potential, mitochondria, and ion channels such as CRAC channels control T cell activation.

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.

Functional properties and mode of regulation of the mitochondrial Na+/Ca2+ exchanger, NCLX

Mitochondrial Ca²⁺ transient is the earliest discovered organellar Ca²⁺ signaling pathway. It consist of a Ca²⁺ influx, mediated by mitochondrial Ca²⁺ uniporter (MCU), and mitochondrial Ca²⁺ efflux mediated by a Na⁺/Ca²⁺ exchanger (NCLX). Mitochondrial Ca²⁺ signaling machinery plays a fundamental role in linking metabolic activity to cellular Ca²⁺ signaling, and in controlling local Ca²⁺ concertation in distinct cellular compartments. Impaired balance between mitochondrial Ca²⁺ influx and efflux leads to mitochondrial Ca²⁺ overload, an early and key event in ischemic or neurodegenerative syndromes. Molecular identification of NCLX and MCU happened only recently. Surprisingly, MCU knockout yielded a relatively mild phenotype while conditional knockout of NCLX led to a rapid fatal heart failure. Here we will focus on recent functional and molecular studies on NCLX structure and its mode of regulation. We will describe the unique crosstalk of this exchanger with Na⁺ and Ca²⁺ signaling pathways in the cell membrane and the endoplasmic reticulum, and with protein kinases that posttranslationally modulate NCLX activity. We will critically compare selectivity of pharmacological blockers versus molecular control of NCLX expression and activity. Finally we will discuss why this exchanger is essential for survival and can serve as an attractive therapeutic target.

Mitochondrial Calcium Handling in Physiology and Disease

Calcium (Ca²⁺) accumulation inside mitochondria represents a pleiotropic signal controlling a wide range of cellular functions, including key metabolic pathways and life/death decisions. This phenomenon has been first described in the 1960s, but the identity of the molecules controlling this process remained a mystery until just few years ago, when both mitochondrial Ca²⁺ uptake and release systems were genetically dissected. This finally opened the possibility to develop genetic models to directly test the contribution of mitochondrial Ca²⁺ homeostasis to cellular functions. Here we summarize our current understanding of the molecular machinery that controls mitochondrial Ca²⁺ handling and critically evaluate the physiopathological role of mitochondrial Ca²⁺ signaling, based on recent evidences obtained through in vitro and in vivo models.

Enjoy the Trip: Calcium in Mitochondria Back and Forth

In the last 5 years, most of the molecules that control mitochondrial Ca(2+) homeostasis have been finally identified. Mitochondrial Ca(2+) uptake is mediated by the Mitochondrial Calcium Uniporter (MCU) complex, a macromolecular structure that guarantees Ca(2+) accumulation inside mitochondrial matrix upon increases in cytosolic Ca(2+). Conversely, Ca(2+) release is under the control of the Na(+)/Ca(2+) exchanger, encoded by the NCLX gene, and of a H(+)/Ca(2+) antiporter, whose identity is still debated. The low affinity of the MCU complex, coupled to the activity of the efflux systems, protects cells from continuous futile cycles of Ca(2+) across the inner mitochondrial membrane and consequent massive energy dissipation. In this review, we discuss the basic principles that govern mitochondrial Ca(2+) homeostasis and the methods used to investigate the dynamics of Ca(2+) concentration within the organelles. We discuss the functional and structural role of the different molecules involved in mitochondrial Ca(2+) handling and their pathophysiological role.

A crosstalk between Na⁺ channels, Na⁺/K⁺ pump and mitochondrial Na⁺ transporters controls glucose-dependent cytosolic and mitochondrial Na⁺ signals

Glucose-dependent cytosolic Na(+) influx in pancreatic islet β cells is mediated by TTX-sensitive Na(+) channels and is propagated into the mitochondria through the mitochondrial Na(+)/Ca(2+) exchanger, NCLX. Mitochondrial Na(+) transients are also controlled by the mitochondrial Na(+)/H(+) exchanger, NHE, while cytosolic Na(+) changes are governed by Na(+)/K(+) ATPase pump. The functional interaction between the Na(+) channels, Na(+)/K(+) ATPase pump and mitochondrial Na(+) transporters, NCLX and NHE, in mediating Na(+) signaling is poorly understood. Here, we combine fluorescent Na(+) imaging, pharmacological inhibition by TTX, ouabain and EIPA, with molecular control of NCLX expression, so as to investigate the crosstalk between Na(+) transporters on both the plasma membrane and the mitochondria. According to our results, glucose-dependent cytosolic Na(+) response was enhanced by ouabain and was followed by a rise in mitochondrial Na(+) signal. Silencing of NCLX expression using siNCLX, did not affect the glucose- or ouabain-dependent cytosolic rise in Na(+). In contrast, the ouabain-dependent rise in mitochondrial Na(+) was strongly suppressed by siNCLX. Furthermore, mitochondrial Na(+) influx rates were accelerated in cells treated with the Na(+)/H(+) exchanger inhibitor, EIPA or by combination of EIPA and ouabain. Similarly, TTX blocked the cytosolic and mitochondrial Na(+) responses, which were enhanced by ouabain or EIPA, respectively. Our results suggest that Na(+)/K(+) ATPase pump controls cytosolic glucose-dependent Na(+) rise, in a manner that is mediated by TTX-sensitive Na(+) channels and subsequent mitochondrial Na(+) uptake via NCLX. Furthermore, these results indicate that mitochondrial Na(+) influx via NCLX is antagonized by Na(+) efflux, which is mediated by the mitochondrial NHE; thus, the duration of mitochondrial Na(+) transients is set by the interplay between these pivotal transporters.

Pancreatic β-cell Na+ channels control global Ca2+ signaling and oxidative metabolism by inducing Na+ and Ca2+ responses that are propagated into mitochondria

Communication between the plasma membrane and mitochondria is essential for initiating the Ca(2+) and metabolic signals required for secretion in β cells. Although voltage-dependent Na(+) channels are abundantly expressed in β cells and activated by glucose, their role in communicating with mitochondria is unresolved. Here, we combined fluorescent Na(+), Ca(2+), and ATP imaging, electrophysiological analysis with tetrodotoxin (TTX)-dependent block of the Na(+) channel, and molecular manipulation of mitochondrial Ca(2+) transporters to study the communication between Na(+) channels and mitochondria. We show that TTX inhibits glucose-dependent depolarization and blocks cytosolic Na(+) and Ca(2+) responses and their propagation into mitochondria. TTX-sensitive mitochondrial Ca(2+) influx was largely blocked by knockdown of the mitochondrial Ca(2+) uniporter (MCU) expression. Knockdown of the mitochondrial Na(+)/Ca(2+) exchanger (NCLX) and Na(+) dose response analysis demonstrated that NCLX mediates the mitochondrial Na(+) influx and is tuned to sense the TTX-sensitive cytosolic Na(+) responses. Finally, TTX blocked glucose-dependent mitochondrial Ca(2+) rise, mitochondrial metabolic activity, and ATP production. Our results show that communication of the Na(+) channels with mitochondria shape both global Ca(2+) and metabolism signals linked to insulin secretion in β cells.- Nita, I. I., Hershfinkel, M., Kantor, C., Rutter, G. A., Lewis, E. C., Sekler, I. Pancreatic β-cell Na(+) channels control global Ca(2+) signaling and oxidative metabolism by inducing Na(+) and Ca(2+) responses that are propagated into mitochondria.

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.

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".

Mitochondria Na(+)-Ca (2+) exchange in cardiomyocytes and lymphocytes

Mitochondria Na(+)-Ca(2+) exchange (NCX(mit)) was first discovered by Carafoli et al. in 1974. Thereafter, the mechanisms and roles of NCX(mit) have been extensively studied. We review NCX(mit) in cardiomyocytes and lymphocytes by presenting our recent studies on it. Studies of NCX(mit) in rat ventricular cells demonstrated that NCX(mit) is voltage dependent and electrogenic. A targeted knockdown and knockout of NCLX in HL-1 cardiomyocytes and B lymphocytes, respectively, significantly reduced the NCX(mit) activity, indicating that NCLX is a major component of NCX(mit) in these cells. The store-operated Ca(2+) entry was greatly attenuated in NCLX knockout lymphocytes, suggesting that substantial amount of Ca(2+) enters into mitochondria and is released to cytosol via NCX(mit). NCX(mit) or NCLX has pivotal roles in Ca(2+) handling in mitochondria and cytoplasm.

Dynamic buffering of mitochondrial Ca2+ during Ca2+ uptake and Na+-induced Ca2+ release

In cardiac mitochondria, matrix free Ca(2+) ([Ca(2+)]m) is primarily regulated by Ca(2+) uptake and release via the Ca(2+) uniporter (CU) and Na(+)/Ca(2+) exchanger (NCE) as well as by Ca(2+) buffering. Although experimental and computational studies on the CU and NCE dynamics exist, it is not well understood how matrix Ca(2+) buffering affects these dynamics under various Ca(2+) uptake and release conditions, and whether this influences the stoichiometry of the NCE. To elucidate the role of matrix Ca(2+) buffering on the uptake and release of Ca(2+), we monitored Ca(2+) dynamics in isolated mitochondria by measuring both the extra-matrix free [Ca(2+)] ([Ca(2+)]e) and [Ca(2+)]m. A detailed protocol was developed and freshly isolated mitochondria from guinea pig hearts were exposed to five different [CaCl2] followed by ruthenium red and six different [NaCl]. By using the fluorescent probe indo-1, [Ca(2+)]e and [Ca(2+)]m were spectrofluorometrically quantified, and the stoichiometry of the NCE was determined. In addition, we measured NADH, membrane potential, matrix volume and matrix pH to monitor Ca(2+)-induced changes in mitochondrial bioenergetics. Our [Ca(2+)]e and [Ca(2+)]m measurements demonstrate that Ca(2+) uptake and release do not show reciprocal Ca(2+) dynamics in the extra-matrix and matrix compartments. This salient finding is likely caused by a dynamic Ca(2+) buffering system in the matrix compartment. The Na(+)- induced Ca(2+) release demonstrates an electrogenic exchange via the NCE by excluding an electroneutral exchange. Mitochondrial bioenergetics were only transiently affected by Ca(2+) uptake in the presence of large amounts of CaCl2, but not by Na(+)- induced Ca(2+) release.

Modeling the calcium sequestration system in isolated guinea pig cardiac mitochondria

Under high Ca(2+) load conditions, Ca(2+) concentrations in the extra-mitochondrial and mitochondrial compartments do not display reciprocal dynamics. This is due to a paradoxical increase in the mitochondrial Ca(2+) buffering power as the Ca(2+) load increases. Here we develop and characterize a mechanism of the mitochondrial Ca(2+) sequestration system using an experimental data set from isolated guinea pig cardiac mitochondria. The proposed mechanism elucidates this phenomenon and others in a mathematical framework and is integrated into a previously corroborated model of oxidative phosphorylation including the Na(+)/Ca(2+) cycle. The integrated model reproduces the Ca(2+) dynamics observed in both compartments of the isolated mitochondria respiring on pyruvate after a bolus of CaCl2 followed by ruthenium red and a bolus of NaCl. The model reveals why changes in mitochondrial Ca(2+) concentration of Ca(2+) loaded mitochondria appear significantly mitigated relative to the corresponding extra-mitochondrial Ca(2+) concentration changes after Ca(2+) efflux is initiated. The integrated model was corroborated by simulating the set-point phenomenon. The computational results support the conclusion that the Ca(2+) sequestration system is composed of at least two classes of Ca(2+) buffers. The first class represents prototypical Ca(2+) buffering, and the second class encompasses the complex binding events associated with the formation of amorphous calcium phosphate. With the Ca(2+) sequestration system in mitochondria more precisely defined, computer simulations can aid in the development of innovative therapeutics aimed at addressing the myriad of complications that arise due to mitochondrial Ca(2+) overload.

The mitochondrial Na+/Ca2+ exchanger upregulates glucose dependent Ca2+ signalling linked to insulin secretion

Mitochondria mediate dual metabolic and Ca²⁺ shuttling activities. While the former is required for Ca²⁺ signalling linked to insulin secretion, the role of the latter in β cell function has not been well understood, primarily because the molecular identity of the mitochondrial Ca²⁺ transporters were elusive and the selectivity of their inhibitors was questionable. This study focuses on NCLX, the recently discovered mitochondrial Na⁺/Ca²⁺ exchanger that is linked to Ca²⁺ signalling in MIN6 and primary β cells. Suppression either of NCLX expression, using a siRNA construct (siNCLX) or of its activity, by a dominant negative construct (dnNCLX), enhanced mitochondrial Ca²⁺ influx and blocked efflux induced by glucose or by cell depolarization. In addition, NCLX regulated basal, but not glucose-dependent changes, in metabolic rate, mitochondrial membrane potential and mitochondrial resting Ca²⁺. Importantly, NCLX controlled the rate and amplitude of cytosolic Ca²⁺ changes induced by depolarization or high glucose, indicating that NCLX is a critical and rate limiting component in the cross talk between mitochondrial and plasma membrane Ca²⁺ signalling. Finally, knockdown of NCLX expression was followed by a delay in glucose-dependent insulin secretion. These findings suggest that the mitochondrial Na⁺/Ca²⁺ exchanger, NCLX, shapes glucose-dependent mitochondrial and cytosolic Ca²⁺ signals thereby regulating the temporal pattern of insulin secretion in β cells.

Molecular identity and functional properties of the mitochondrial Na+/Ca2+ exchanger

The mitochondrial membrane potential that powers the generation of ATP also facilitates mitochondrial Ca(2+) shuttling. This process is fundamental to a wide range of cellular activities, as it regulates ATP production, shapes cytosolic and endoplasmic recticulum Ca(2+) signaling, and determines cell fate. Mitochondrial Ca(2+) transport is mediated primarily by two major transporters: a Ca(2+) uniporter that mediates Ca(2+) uptake and a Na(+)/Ca(2+) exchanger that subsequently extrudes mitochondrial Ca(2+). In this minireview, we focus on the specific role of the mitochondrial Na(+)/Ca(2+) exchanger and describe its ion exchange mechanism, regulation by ions, and putative partner proteins. We discuss the recent molecular identification of the mitochondrial exchanger and how its activity is linked to physiological and pathophysiological processes.

Mitochondrial Ca²⁺ homeostasis: mechanism, role, and tissue specificities

Mitochondria from every tissue are quite similar in their capability to accumulate Ca²⁺ in a process that depends on the electrical potential across the inner membrane; it is catalyzed by a gated channel (named mitochondrial Ca²⁺ uniporter), the molecular identity of which has only recently been unraveled. The release of accumulated Ca²⁺ in mitochondria from different tissues is, on the contrary, quite variable, both in terms of speed and mechanism: a Na⁺-dependent efflux in excitable cells (catalyzed by NCLX) and a H⁺/Ca²⁺ exchanger in other cells. The efficacy of mitochondrial Ca²⁺ uptake in living cells is strictly dependent on the topological arrangement of the organelles with respect to the source of Ca²⁺ flowing into the cytoplasm, i.e., plasma membrane or intracellular channels. In turn, the structural and functional relationships between mitochondria and other cellular membranes are dictated by the specific architecture of different cells. Mitochondria not only modulate the amplitude and the kinetics of local and bulk cytoplasmic Ca²⁺ changes but also depend on the Ca²⁺ signal for their own functionality, in particular for their capacity to produce ATP. In this review, we summarize the processes involved in mitochondrial Ca²⁺ handling and its integration in cell physiology, highlighting the main common characteristics as well as key differences, in different tissues.

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.

The mitochondrial Na(+)/Ca(2+) exchanger

Powered by the steep mitochondrial membrane potential Ca(2+) permeates into the mitochondria via the Ca(2+) uniporter and is then extruded by a mitochondrial Na(+)/Ca(2+) exchanger. This mitochondrial Ca(2+) shuttling regulates the rate of ATP production and participates in cellular Ca(2+) signaling. Despite the fact that the exchanger was functionally identified 40 years ago its molecular identity remained a mystery. Early studies on isolated mitochondria and intact cells characterized the functional properties of a mitochondrial Na(+)/Ca(2+) exchanger, and showed that it possess unique functional fingerprints such as Li(+)/Ca(2+) exchange and that it is displaying selective sensitivity to inhibitors. Purification of mitochondria proteins combined with functional reconstitution led to the isolation of a polypeptide candidate of the exchanger but failed to molecularly identify it. A turning point in the search for the exchanger molecule came with the recent cloning of the last member of the Na(+)/Ca(2+) exchanger superfamily termed NCLX (Na(+)/Ca(2+)/Li(+) exchanger). NCLX is localized in the inner mitochondria membrane and its expression is linked to mitochondria Na(+)/Ca(2+) exchange matching the functional fingerprints of the putative mitochondrial Na(+)/Ca(2+) exchanger. Thus NCLX emerges as the long sought mitochondria Na(+)/Ca(2+) exchanger and provide a critical molecular handle to study mitochondrial Ca(2+) signaling and transport. Here we summarize some of the main topics related to the molecular properties of the Na(+)/Ca(2+) exchanger, beginning with the early days of its functional identification, its kinetic properties and regulation, and culminating in its molecular identification.

After half a century mitochondrial calcium in- and efflux machineries reveal themselves

Mitochondrial Ca(2+) uptake and release play a fundamental role in the control of different physiological processes, such as cytoplasmic Ca(2+) signalling, ATP production and hormone metabolism, while dysregulation of mitochondrial Ca(2+) handling triggers the cascade of events that lead to cell death. The basic mechanisms of mitochondrial Ca(2+) homeostasis have been firmly established for decades, but the molecular identities of the channels and transporters responsible for Ca(2+) uptake and release have remained mysterious until very recently. Here, we briefly review the main findings that have led to our present understanding of mitochondrial Ca(2+) homeostasis and its integration in cell physiology. We will then discuss the recent work that has unravelled the biochemical identity of three key molecules: NCLX, the mitochondrial Na(+)/Ca(2+) antiporter, MCU, the pore-forming subunit of the mitochondrial Ca(2+) uptake channel, and MICU1, one of its regulatory subunits.

Mitochondrial Ca2+ influx and efflux rates in guinea pig cardiac mitochondria: low and high affinity effects of cyclosporine A

Ca(2+) plays a central role in energy supply and demand matching in cardiomyocytes by transmitting changes in excitation-contraction coupling to mitochondrial oxidative phosphorylation. Matrix Ca(2+) is controlled primarily by the mitochondrial Ca(2+) uniporter and the mitochondrial Na(+)/Ca(2+) exchanger, influencing NADH production through Ca(2+)-sensitive dehydrogenases in the Krebs cycle. In addition to the well-accepted role of the Ca(2+)-triggered mitochondrial permeability transition pore in cell death, it has been proposed that the permeability transition pore might also contribute to physiological mitochondrial Ca(2+) release. Here we selectively measure Ca(2+) influx rate through the mitochondrial Ca(2+) uniporter and Ca(2+) efflux rates through Na(+)-dependent and Na(+)-independent pathways in isolated guinea pig heart mitochondria in the presence or absence of inhibitors of mitochondrial Na(+)/Ca(2+) exchanger (CGP 37157) or the permeability transition pore (cyclosporine A). cyclosporine A suppressed the negative bioenergetic consequences (ΔΨ(m) loss, Ca(2+) release, NADH oxidation, swelling) of high extramitochondrial Ca(2+) additions, allowing mitochondria to tolerate total mitochondrial Ca(2+) loads of >400nmol/mg protein. For Ca(2+) pulses up to 15μM, Na(+)-independent Ca(2+) efflux through the permeability transition pore accounted for ~5% of the total Ca(2+) efflux rate compared to that mediated by the mitochondrial Na(+)/Ca(2+) exchanger (in 5mM Na(+)). Unexpectedly, we also observed that cyclosporine A inhibited mitochondrial Na(+)/Ca(2+) exchanger-mediated Ca(2+) efflux at higher concentrations (IC(50)=2μM) than those required to inhibit the permeability transition pore, with a maximal inhibition of ~40% at 10μM cyclosporine A, while having no effect on the mitochondrial Ca(2+) uniporter. The results suggest a possible alternative mechanism by which cyclosporine A could affect mitochondrial Ca(2+) load in cardiomyocytes, potentially explaining the paradoxical toxic effects of cyclosporine A at high concentrations. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

A biophysically based mathematical model for the kinetics of mitochondrial Na+-Ca2+ antiporter

Sodium-calcium antiporter is the primary efflux pathway for Ca(2+) in respiring mitochondria, and hence plays an important role in mitochondrial Ca(2+) homeostasis. Although experimental data on the kinetics of Na(+)-Ca(2+) antiporter are available, the structure and composition of its functional unit and kinetic mechanisms associated with the Na(+)-Ca(2+) exchange (including the stoichiometry) remains unclear. To gain a quantitative understanding of mitochondrial Ca(2+) homeostasis, a biophysical model of Na(+)-Ca(2+) antiporter is introduced that is thermodynamically balanced and satisfactorily describes a number of independent data sets under a variety of experimental conditions. The model is based on a multistate catalytic binding mechanism for carrier-mediated facilitated transport and Eyring's free energy barrier theory for interconversion and electrodiffusion. The model predicts the activating effect of membrane potential on the antiporter function for a 3Na(+):1Ca(2+) electrogenic exchange as well as the inhibitory effects of both high and low pH seen experimentally. The model is useful for further development of mechanistic integrated models of mitochondrial Ca(2+) handling and bioenergetics to understand the mechanisms by which Ca(2+) plays a role in mitochondrial signaling pathways and energy metabolism.

The fateful encounter of mitochondria with calcium: how did it happen?

A number of findings in the 1950s had offered indirect indications that mitochondria could accumulate Ca2+. In 1961, the phenomenon was directly demonstrated using isolated mitochondria: the uptake process was driven by respiratory chain activity or by the hydrolysis of added ATP. It could be accompanied by the simultaneous uptake of inorganic phosphate, in which case precipitates of hydroxyapatite were formed in the matrix, buffering its free Ca2+ concentration. The properties of the uptake process were established in the 1960s and 1970s: the uptake of Ca2+ occurred electrophoretically on a carrier that has not yet been molecularly identified, and was released from mitochondria via a Na+/Ca2+ antiporter. A H+/Ca2+ release exchanger was also found to operate in some mitochondrial types. The permeability transition pore was later also found to mediate the efflux of Ca2+ from mitochondria. In the mitochondrial matrix two TCA cycle dehydrogenases and pyruvate dehydrogenase phosphate phosphatase were found to be regulated in the matrix by the cycling of Ca2+ across the inner membrane. In conditions of cytoplasmic Ca2+ overload mitochondria could store for a time large amounts of precipitated Ca2+-phosphate, thus permitting cells to survive situations of Ca2+ emergency. The uptake process was found to have very low affinity for Ca2+: since the bulk concentration of Ca2+ in the cytoplasm is in the low to mid-nM range, it became increasingly difficult to postulate a role of mitochondria in the regulation of cytoplsmic Ca2+. A number of findings had nevertheless shown that energy linked Ca2+ transport occurred efficiently in mitochondria of various tissues in situ. The paradox was only solved in the 1990s, when it was found that the concentration of Ca2+ in the cytoplasm is not uniform: perimitochondrial micropools are created by the agonist-promoted discharge of Ca2+ from vicinal stores in which the concentration of Ca2+ is high enough to activate the low affinity mitochondrial uniporter. Mitochondria thus regained center stage as important regulators of cytoplasmic Ca2+ (not only of their own internal Ca2+). Their Ca2+ uptake systems was found to react very rapidly to cytoplasmic Ca2+ demands, even in the 150-200 msec time scale of processes like the contraction and relaxation of heart. An important recent development in the area of mitochondrial Ca2+ transport is its involvement in the disease process. Ca2+ signaling defects are now gaining increasing importance in the pathogenesis of diseases, e.g., neurodegenerative diseases. Since mitochondria have now regained a central role in the regulation of cytoplasmic Ca2+, dysfunctions of their Ca2+ controlling systems have expectedly been found to be involved in the pathogenesis of numerous disease processes.

NCLX is an essential component of mitochondrial Na+/Ca2+ exchange

Mitochondrial Ca(2+) efflux is linked to numerous cellular activities and pathophysiological processes. Although it is established that an Na(+)-dependent mechanism mediates mitochondrial Ca(2+) efflux, the molecular identity of this transporter has remained elusive. Here we show that the Na(+)/Ca(2+) exchanger NCLX is enriched in mitochondria, where it is localized to the cristae. Employing Ca(2+) and Na(+) fluorescent imaging, we demonstrate that mitochondrial Na(+)-dependent Ca(2+) efflux is enhanced upon overexpression of NCLX, is reduced by silencing of NCLX expression by siRNA, and is fully rescued by the concomitant expression of heterologous NCLX. NCLX-mediated mitochondrial Ca(2+) transport was inhibited, moreover, by CGP-37157 and exhibited Li(+) dependence, both hallmarks of mitochondrial Na(+)-dependent Ca(2+) efflux. Finally, NCLX-mediated mitochondrial Ca(2+) exchange is blocked in cells expressing a catalytically inactive NCLX mutant. Taken together, our results converge to the conclusion that NCLX is the long-sought mitochondrial Na(+)/Ca(2+) exchanger.

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.

Role of the mitochondrial sodium/calcium exchanger in neuronal physiology and in the pathogenesis of neurological diseases

In neurons, as in other excitable cells, mitochondria extrude Ca(2+) ions from their matrix in exchange with cytosolic Na(+) ions. This exchange is mediated by a specific transporter located in the inner mitochondrial membrane, the mitochondrial Na(+)/Ca(2+) exchanger (NCX(mito)). The stoichiometry of NCX(mito)-operated Na(+)/Ca(2+) exchange has been the subject of a long controversy, but evidence of an electrogenic 3 Na(+)/1 Ca(2+) exchange is increasing. Although the molecular identity of NCX(mito) is still undetermined, data obtained in our laboratory suggest that besides the long-sought and as yet unfound mitochondrial-specific NCX, the three isoforms of plasmamembrane NCX can contribute to NCX(mito) in neurons and astrocytes. NCX(mito) has a role in controlling neuronal Ca(2+) homeostasis and neuronal bioenergetics. Indeed, by cycling the Ca(2+) ions captured by mitochondria back to the cytosol, NCX(mito) determines a shoulder in neuronal [Ca(2+)](c) responses to neurotransmitters and depolarizing stimuli which may then outlast stimulus duration. This persistent NCX(mito)-dependent Ca(2+) release has a role in post-tetanic potentiation, a form of short-term synaptic plasticity. By controlling [Ca(2+)](m) NCX(mito) regulates the activity of the Ca(2+)-sensitive enzymes pyruvate-, alpha-ketoglutarate- and isocitrate-dehydrogenases and affects the activity of the respiratory chain. Convincing experimental evidence suggests that supraphysiological activation of NCX(mito) contributes to neuronal cell death in the ischemic brain and, in epileptic neurons coping with seizure-induced ion overload, reduces the ability to reestablish normal ionic homeostasis. These data suggest that NCX(mito) could represent an important target for the development of new neurological drugs.

Analysis of cardiac mitochondrial Na+-Ca2+ exchanger kinetics with a biophysical model of mitochondrial Ca2+ handling suggests a 3:1 stoichiometry

Calcium is a key ion and is known to mediate signalling pathways between cytosol and mitochondria and modulate mitochondrial energy metabolism. To gain a quantitative, biophysical understanding of mitochondrial Ca(2+) regulation, we developed a thermodynamically balanced model of mitochondrial Ca(2+) handling and bioenergetics by integrating kinetic models of mitochondrial Ca(2+) uniporter (CU), Na(+)-Ca(2+) exchanger (NCE), and Na(+)-H(+) exchanger (NHE) into an existing computational model of mitochondrial oxidative phosphorylation. Kinetic flux expressions for the CU, NCE and NHE were developed and individually parameterized based on independent data sets on flux rates measured in purified mitochondria. While available data support a wide range of possible values for the overall activity of the CU in cardiac and liver mitochondria, even at the highest estimated values, the Ca(2+) current through the CU does not have a significant effect on mitochondrial membrane potential. This integrated model was then used to analyse additional data on the dynamics and steady-states of mitochondrial Ca(2+) governed by mitochondrial CU and NCE. Our analysis of the data on the time course of matrix free [Ca(2+)] in respiring mitochondria purified from rabbit heart with addition of different levels of Na(+) to the external buffer medium (with the CU blocked) with two separate models--one with a 2:1 stoichiometry and the other with a 3:1 stoichiometry for the NCE--supports the hypothesis that the NCE is electrogenic with a stoichiometry of 3:1. This hypothesis was further tested by simulating an additional independent data set on the steady-state variations of matrix free [Ca(2+)] with respect to the variations in external free [Ca(2+)] in purified respiring mitochondria from rat heart to show that only the 3:1 stoichiometry model predictions are consistent with the data. Based on these analyses, it is concluded that the mitochondrial NCE is electrogenic with a stoichiometry of 3:1.

Ca2+-induced Ca2+ release in Aplysia bag cell neurons requires interaction between mitochondrial and endoplasmic reticulum stores

Intracellular Ca2+ is influenced by both Ca2+ influx and release. We examined intracellular Ca2+ following action potential firing in the bag cell neurons of Aplysia californica. Following brief synaptic input, these neuroendocrine cells undergo an afterdischarge, resulting in elevated Ca2+ and the secretion of neuropeptides to initiate reproduction. Cultured bag cell neurons were injected with the Ca2+ indicator, fura-PE3, and subjected to simultaneous imaging and electrophysiology. Delivery of a 5-Hz, 1-min train of action potentials (mimicking the fast phase of the afterdischarge) produced a Ca2+ rise that markedly outlasted the initial influx, consistent with Ca2+-induced Ca2+ release (CICR). This response was attenuated by about half with ryanodine or depletion of the endoplasmic reticulum (ER) by cyclopiazonic acid. However, depletion of the mitochondria, with carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone, essentially eliminated CICR. Dual depletion of the ER and mitochondria did not reduce CICR further than depletion of the mitochondria alone. Moreover, tetraphenylphosphonium, a blocker of mitochondrial Ca2+ release, largely prevented CICR. The Ca2+ elevation during and subsequent to a stimulus mimicking the full afterdischarge was prominent and enhanced by protein kinase C activation. Traditionally, the ER is seen as the primary Ca2+ source for CICR. However, bag cell neuron CICR represents a departure from this view in that it relies on store interaction, where Ca2+ released from the mitochondria may in turn liberate Ca2+ from the ER. This unique form of CICR may be used by both bag cell neurons, and other neurons, to initiate secretion, activate channels, or induce gene expression.

Cytoplasmic Na+-dependent modulation of mitochondrial Ca2+ via electrogenic mitochondrial Na+-Ca2+ exchange

To clarify the role of mitochondrial Na(+)-Ca(2+) exchange (NCX(mito)) in regulating mitochondrial Ca(2+) (Ca(2+)(mito)) concentration at intact and depolarized mitochondrial membrane potential (DeltaPsi(mito)), we measured Ca(2+)(mito) and DeltaPsi(mito) using fluorescence probes Rhod-2 and TMRE, respectively, in the permeabilized rat ventricular cells. Applying 300 nm cytoplasmic Ca(2+) (Ca(2+)(c)) increased Ca(2+)(mito) and this increase was attenuated by cytoplasmic Na(+) (Na(+)(c)) with an IC(50) of 2.4 mm. To the contrary, when DeltaPsi(mito) was depolarized by FCCP, a mitochondrial uncoupler, Na(+)(c) enhanced the Ca(2+)(c)-induced increase in Ca(2+)(mito) with an EC(50) of about 4 mm. This increase was not significantly affected by ruthenium red or cyclosporin A. The inhibition of NCX(mito) by CGP-37157 further increased Ca(2+)(mito) when DeltaPsi(mito) was intact, while it suppressed the Ca(2+)(mito) increase when DeltaPsi(mito) was depolarized, suggesting that DeltaPsi(mito) depolarization changed the exchange mode from forward to reverse. Furthermore, DeltaPsi(mito) depolarization significantly reduced the Ca(2+)(mito) decrease via forward mode, and augmented the Ca(2+)(mito) increase via reverse mode. When the respiratory chain was attenuated, the induction of the reverse mode of NCX(mito) hyperpolarized DeltaPsi(mito), while DeltaPsi(mito) depolarized upon inducing the forward mode of NCX(mito). Both changes in DeltaPsi(mito) were remarkably inhibited by CGP-37157. The above experimental data indicated that NCX(mito) is voltage dependent and electrogenic. This notion was supported theoretically by computer simulation studies with an NCX(mito) model constructed based on present and previous studies, presuming a consecutive and electrogenic Na(+)-Ca(2+) exchange and a depolarization-induced increase in Na(+) flux. It is concluded that Ca(2+)(mito) concentration is dynamically modulated by Na(+)(c) and DeltaPsi(mito) via electrogenic NCX(mito).

The role of Na dysregulation in cardiac disease and how it impacts electrophysiology

Ca(2+) is well known as the central player in cardiac cell physiology, mediating Ca(2+) activation of myosin ATPase and contraction, the stimulation of Ca(2+)-activated signaling pathways and modulation of mitochondrial energy production. Abnormalities of Ca(2+) handling are a well-studied mechanism of decompensation in heart failure. Less appreciated is the role of cytosolic Na(+) (Na(i) (+)), which can dramatically influence the transfer rates and distribution of Ca(2+) among the intracellular compartments of the myocyte. Since Na(i) (+) can vary widely under different physiological and pathological conditions, and its effects depend on multiple ion gradients and membrane electrical potentials, unraveling the global influence of Na(i) (+) on cell function is complex, requiring an integrative view of cardiomyocyte physiology. Here, we discuss how abnormal Na(i) (+) regulation not only influences the cytosolic Ca(2+) transient and the cellular action potential but also alters mitochondrial Ca(2+) uptake and the balance of energy supply and demand of the cardiomyocyte, which may contribute to oxidative stress and cardiac decompensation. The implications for sudden cardiac death and the potential for novel therapeutic interventions are discussed.

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.

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).

Effect of Ca2+ on cardiac mitochondrial energy production is modulated by Na+ and H+ dynamics

The energy production of mitochondria in heart increases during exercise. Several works have suggested that calcium acts at multiple control points to activate net ATP production in what is termed "parallel activation". To study this, a computational model of mitochondrial energy metabolism in the heart has been developed that integrates the Dudycha-Jafri model for the tricarboxylic acid cycle with the Magnus-Keizer model for mitochondrial energy metabolism and calcium dynamics. The model improves upon the previous formulation by including an updated formulation for calcium dynamics, and new descriptions of sodium, hydrogen, phosphate, and ATP balance. To this end, it incorporates new formulations for the calcium uniporter, sodium-calcium exchange, sodium-hydrogen exchange, the F(1)F(0)-ATPase, and potassium-hydrogen exchange. The model simulates a wide range of experimental data, including steady-state and simulated pacing protocols. The model suggests that calcium is a potent activator of net ATP production and that as pacing increases energy production due to calcium goes up almost linearly. Furthermore, it suggests that during an extramitochondrial calcium transient, calcium entry and extrusion cause a transient depolarization that serve to increase NADH production by the tricarboxylic acid cycle and NADH consumption by the respiration driven proton pumps. The model suggests that activation of the F(1)F(0)-ATPase by calcium is essential to increase ATP production. In mitochondria very close to the release sites, the depolarization is more severe causing a temporary loss of ATP production. However, due to the short duration of the depolarization the net ATP production is also increased.

Mitochondrial Ca2+ Flux through Na+/Ca2+ Exchange

To clarify the property of mitochondrial Na+/Ca2+ exchange in situ, we measured mitochondrial Ca2+ using Rhod-2 in permeabilized rat ventricular myocytes. Cytoplasmic 300 nM Ca2+ (Ca2+(c)) augmented the Rhod-2 intensity by approximately ninefolds without cytoplasmic Na+ (Na+(c)). Increasing Na+(c) attenuated the maximum level of Rhod-2 fluorescence, probably due to the activation of forward mode of mitochondrial Na+/Ca2+ exchange. The Rhod-2 intensity decayed upon removing Ca2+(c). The decay was dependent on Na+(c) (K(1/2) = approximately 1 mM) and largely abolished by an inhibitor of mitochondrial Na+/Ca2+ exchange, CGP-37157. It was suggested that Na+ binding to the mitochondrial Na+/Ca2+ exchange is saturated in the physiological concentration of Na+(c).

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.