Historical review: mitochondria and calcium: ups and downs of an unusual relationship

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.

Mitochondrial Ca2+ channels: Great unknowns with important functions

Mitochondria process local and global Ca(2+) signals. Thereby the spatiotemporal patterns of mitochondrial Ca(2+) signals determine whether the metabolism of these organelles is adjusted or cell death is executed. Mitochondrial Ca(2+) channels of the inner mitochondrial membrane (IMM) actually implement mitochondrial uptake from cytosolic Ca(2+) rises. Despite great efforts in the past, the identity of mitochondrial Ca(2+) channels is still elusive. Numerous studies aimed to characterize mitochondrial Ca(2+) uniport channels and provided a detailed profile of these great unknowns with important functions. This mini-review revisits previous research on the mechanisms of mitochondrial Ca(2+) uptake and aligns them with most recent findings.

A mechanism for functional segregation of mitochondrial and cytosolic genetic codes

The coexistence of multiple gene translation machineries is a feature of eukaryotic cells and a result of the endosymbiotic events that gave rise to mitochondria, plastids, and other organelles. The conditions required for the integration of these apparatuses within a single cell are not understood, but current evidence indicates that complete ablation of the mitochondrial protein synthesis apparatus and its substitution by its cytosolic equivalent is not possible. Why certain mitochondrial components and not others can be substituted by cytosolic equivalents is not known. In trypanosomatids this situation reaches a limit, because certain aminoacyl-tRNA synthetases are mitochondrial specific despite the fact that all tRNAs in these organisms are shared between cytosol and mitochondria. Here we report that a mitochondria-specific lysyl-tRNA synthetase in Trypanosoma has evolved a mechanism to block the activity of the enzyme during its synthesis and translocation. Only when the enzyme reaches the mitochondria is it activated through the cleavage of a C-terminal structural extension, preventing the possibility of the enzyme being active in the cytosol.

Superoxide anion regulates the mitochondrial free Ca2+ through uncoupling proteins

Mitochondrial dysfunction, which is closely related to intracellular calcium overload and excessive free radicals, is an important cause of Alzheimer's disease (AD). However, molecular mechanisms of the mitochondrial Ca(2+) disregulation induced by oxidative stress in AD are still obscure. In an effort to gain a further understanding of this problem, we investigated the effects of superoxide anion, a primary free radical, on the expression of uncoupling proteins (UCPs) and the mitochondrial free Ca(2+) levels in the neuroblastoma SH-SY5Y cell line (neo) and stably expressed wild-type human APP(APP) and APP-Swedish mutation (APPsw) SH-SY5Y cells. It was found that UCP2 and UCP4 protein levels were upregulated in neo but downregulated in APP and APPsw cells by the superoxide anion. Our results show that the superoxide anion can regulate protein levels of UCP2 and UCP4 in SH-SY5Y cells, and the mitochondrial free Ca(2+) shifted their levels, tightly coupled with the protein levels of UCPs. When UCP2 and UCP4 were knocked down by siRNA, the result was reversed. These data suggest that the superoxide anion can regulate the mitochondrial free Ca(2+) by regulating the expression of UCPs. These observations also indicate that UCPs can be potential targets in pathotherapy prevention of AD.

Philosophical basis and some historical aspects of systems biology: from Hegel to Noble - applications for bioenergetic research

We live in times of paradigmatic changes for the biological sciences. Reductionism, that for the last six decades has been the philosophical basis of biochemistry and molecular biology, is being displaced by Systems Biology, which favors the study of integrated systems. Historically, Systems Biology - defined as the higher level analysis of complex biological systems - was pioneered by Claude Bernard in physiology, Norbert Wiener with the development of cybernetics, and Erwin Schrödinger in his thermodynamic approach to the living. Systems Biology applies methods inspired by cybernetics, network analysis, and non-equilibrium dynamics of open systems. These developments follow very precisely the dialectical principles of development from thesis to antithesis to synthesis discovered by Hegel. Systems Biology opens new perspectives for studies of the integrated processes of energy metabolism in different cells. These integrated systems acquire new, system-level properties due to interaction of cellular components, such as metabolic compartmentation, channeling and functional coupling mechanisms, which are central for regulation of the energy fluxes. State of the art of these studies in the new area of Molecular System Bioenergetics is analyzed.

Mitochondria, calcium and cell death: a deadly triad in neurodegeneration

Mitochondrial Ca(2+) accumulation is a tightly controlled process, in turn regulating functions as diverse as aerobic metabolism and induction of cell death. The link between Ca(2+) (dys)regulation, mitochondria and cellular derangement is particularly evident in neurodegenerative disorders, in which genetic models and environmental factors allowed to identify common traits in the pathogenic routes. We will here summarize: i) the current view of mechanisms and functions of mitochondrial Ca(2+) homeostasis, ii) the basic principles of organelle Ca(2+) transport, iii) the role of Ca(2+) in neuronal cell death, and iv) the new information on the pathogenesis of Alzheimer's, Huntington's and Parkinson's diseases, highlighting the role of Ca(2+) and mitochondria.

Mitochondrial calcium as a key regulator of mitochondrial ATP production in mammalian cells

Mitochondrial Ca(2+) transport was initially considered important only in buffering of cytosolic Ca(2+) by acting as a "sink" under conditions of Ca(2+) overload. The main regulator of ATP production was considered to be the relative concentrations of high energy phosphates. However, work by Denton and McCormack in the 1970s and 1980s showed that free intramitochondrial Ca(2+) ([Ca(2+)](m)) activated dehydrogenase enzymes in mitochondria, leading to increased NADH and hence ATP production. This leads them to propose a scheme, subsequently termed a "parallel activation model" whereby increases in energy demand, such as hormonal stimulation or increased workload in muscle, produced an increase in cytosolic [Ca(2+)] that was relayed by the mitochondrial Ca(2+) transporters into the matrix to give an increase in [Ca(2+)](m). This then stimulated energy production to meet the increased energy demand. With the development of methods for measuring [Ca(2+)](m) in living cells that proved [Ca(2+)](m) changed over a dynamic physiological range rather than simply soaking up excess cytosolic [Ca(2+)], this model has now gained widespread acceptance. However, work by ourselves and others using targeted probes to measure changes in both [Ca(2+)] and [ATP] in different cell compartments has revealed variations in the interrelationships between these two in different tissues, suggesting that metabolic regulation by Ca(2+) is finely tuned to the demands and function of the individual organ.

Modulation of calcium signalling by mitochondria

In this review we will attempt to summarise the complex and sometimes contradictory effects that mitochondria have on different forms of calcium signalling. Mitochondria can influence Ca(2+) signalling indirectly by changing the concentration of ATP, NAD(P)H, pyruvate and reactive oxygen species - which in turn modulate components of the Ca(2+) signalling machinery i.e. buffering, release from internal stores, influx from the extracellular solution, uptake into cellular organelles and extrusion by plasma membrane Ca(2+) pumps. Mitochondria can directly influence the calcium concentration in the cytosol of the cell by importing Ca(2+) via the mitochondrial Ca(2+) uniporter or transporting Ca(2+) from the interior of the organelle into the cytosol by means of Na+/Ca(2+) or H+/Ca(2+) exchangers. Considerable progress in understanding the relationship between Ca(2+) signalling cascades and mitochondrial physiology has been accumulated over the last few years due to the development of more advanced optical techniques and electrophysiological approaches.

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.

Yeast mitochondria import ATP through the calcium-dependent ATP-Mg/Pi carrier Sal1p, and are ATP consumers during aerobic growth in glucose

Sal1p, a novel Ca2+-dependent ATP-Mg/Pi carrier, is essential in yeast lacking all adenine nucleotide translocases. By targeting luciferase to the mitochondrial matrix to monitor mitochondrial ATP levels, we show in isolated mitochondria that both ATP-Mg and free ADP are taken up by Sal1p with a K(m) of 0.20 +/- 0.03 mM and 0.28 +/- 0.06 mM respectively. Nucleotide transport along Sal1p is strictly Ca2+ dependent. Ca2+ increases the V(max) with a S(0.5) of 15 muM, and no changes in the K(m) for ATP-Mg. Glucose sensing in yeast generates Ca2+ transients involving Ca2+ influx from the external medium. We find that carbon-deprived cells respond to glucose with an immediate increase in mitochondrial ATP levels which is not observed in the presence of EGTA or in Sal1p-deficient cells. Moreover, we now report that during normal aerobic growth on glucose, yeast mitochondria import ATP from the cytosol and hydrolyse it through H+-ATP synthase. We identify two pathways for ATP uptake in mitochondria, the ADP/ATP carriers and Sal1p. Thus, during exponential growth on glucose, mitochondria are ATP consumers, as those from cells growing in anaerobic conditions or deprived of mitochondrial DNA which depend on cytosolic ATP and mitochondrial ATPase working in reverse to generate a mitochondrial membrane potential. In conclusion, the results show that growth on glucose requires ATP hydrolysis in mitochondria and recruits Sal1p as a Ca2+-dependent mechanism to import ATP-Mg from the cytosol. Whether this mechanism is used under similar settings in higher eukaryotes is an open question.

Huntington's disease: from pathology and genetics to potential therapies

Huntington's disease (HD) is a devastating autosomal dominant neurodegenerative disease caused by a CAG trinucleotide repeat expansion encoding an abnormally long polyglutamine tract in the huntingtin protein. Much has been learnt since the mutation was identified in 1993. We review the functions of wild-type huntingtin. Mutant huntingtin may cause toxicity via a range of different mechanisms. The primary consequence of the mutation is to confer a toxic gain of function on the mutant protein and this may be modified by certain normal activities that are impaired by the mutation. It is likely that the toxicity of mutant huntingtin is revealed after a series of cleavage events leading to the production of N-terminal huntingtin fragment(s) containing the expanded polyglutamine tract. Although aggregation of the mutant protein is a hallmark of the disease, the role of aggregation is complex and the arguments for protective roles of inclusions are discussed. Mutant huntingtin may mediate some of its toxicity in the nucleus by perturbing specific transcriptional pathways. HD may also inhibit mitochondrial function and proteasome activity. Importantly, not all of the effects of mutant huntingtin may be cell-autonomous, and it is possible that abnormalities in neighbouring neurons and glia may also have an impact on connected cells. It is likely that there is still much to learn about mutant huntingtin toxicity, and important insights have already come and may still come from chemical and genetic screens. Importantly, basic biological studies in HD have led to numerous potential therapeutic strategies.

Role of aralar, the mitochondrial transporter of aspartate-glutamate, in brain N-acetylaspartate formation and Ca(2+) signaling in neuronal mitochondria

Aralar, the Ca(2+)-dependent mitochondrial aspartate-glutamate carrier expressed in brain and skeletal muscle, is a member of the malate-aspartate NADH shuttle. Disrupting the gene for aralar, SLC25a12, in mice has enabled the discovery of two new roles of this carrier. On the one hand, it is required for synthesis of brain aspartate and N-acetylaspartate, a neuron-born metabolite that supplies acetate for myelin lipid synthesis; and on the other, it is essential for the transmission of small Ca(2+) signals to mitochondria via an increase in mitochondrial NADH.

Reduced calcium-dependent mitochondrial damage underlies the reduced vulnerability of excitotoxicity-tolerant hippocampal neurons

In central neurons, over-stimulation of NMDA receptors leads to excessive mitochondrial calcium accumulation and damage, which is a critical step in excitotoxic death. This raises the possibility that low susceptibility to calcium overload-induced mitochondrial damage might characterize excitotoxicity-resistant neurons. In this study, we have exploited two complementary models of preconditioning-induced excitotoxicity resistance to demonstrate reduced calcium-dependent mitochondrial damage in NMDA-tolerant hippocampal neurons. We have further identified adaptations in mitochondrial calcium handling that account for enhanced mitochondrial integrity. In both models, enhanced tolerance was associated with improved preservation of mitochondrial membrane potential and structure. In the first model, which exhibited modest neuroprotection, mitochondria-dependent calcium deregulation was delayed, even though cytosolic and mitochondrial calcium loads were quantitatively unchanged, indicating that enhanced mitochondrial calcium capacity accounts for reduced injury. In contrast, the second model, which exhibited strong neuroprotection, displayed further delayed calcium deregulation and reduced mitochondrial damage because downregulation of NMDA receptor surface expression depressed calcium loading. Reducing calcium entry also modified the chemical composition of the calcium-buffering precipitates that form in calcium-loaded mitochondria. It thus appears that reduced mitochondrial calcium loading is a major factor underlying the robust neuroprotection seen in highly tolerant cells.

Amyloid precursor protein intracellular domain modulates cellular calcium homeostasis and ATP content

Consecutive cleavages of amyloid precursor protein (APP) generate APP intracellular domain (AICD). Its cellular function is still unclear. In this study, we investigated the functional role of AICD in cellular Ca(2+) homeostasis. We could confirm previous observations that endoplasmic reticulum Ca(2+) stores contain less calcium in cells with reduced APP gamma-secretase cleavage products, increased AICD degradation, reduced AICD expression or in cells lacking APP. In addition, we observed an enhanced resting cytosolic calcium concentration under conditions where AICD is decreased or missing. In view of the reciprocal effects of Ca(2+) on mitochondria and of mitochondria on Ca(2+) homeostasis, we analysed further the cellular ATP content and the mitochondrial membrane potential. We observed a reduced ATP content and a mitochondrial hyperpolarisation in cells with reduced amounts of AICD. Blockade of mitochondrial oxidative phosphorylation chain in control cells lead to similar alterations as in cells lacking AICD. On the other hand, substrates of Complex II rescued the alteration in Ca(2+) homeostasis in cells lacking AICD. Based on these observations, our findings indicate that alterations observed in endoplasmic reticulum Ca(2+) storage in cells with reduced amounts of AICD are reciprocally linked to mitochondrial bioenergetic function.

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).

Stimulation-induced changes in NADH fluorescence and mitochondrial membrane potential in lizard motor nerve terminals

To investigate mitochondrial responses to repetitive stimulation, we measured changes in NADH fluorescence and mitochondrial membrane potential (Psi(m)) produced by trains of action potentials (50 Hz for 10-50 s) delivered to motor nerve terminals innervating external intercostal muscles. Stimulation produced a rapid decrease in NADH fluorescence and partial depolarization of Psi(m). These changes were blocked when Ca2+ was removed from the bath or when N-type Ca2+ channels were inhibited with omega-conotoxin GVIA, but were not blocked when bath Ca2+ was replaced by Sr2+, or when vesicular release was inhibited with botulinum toxin A. When stimulation stopped, NADH fluorescence and Psi(m) returned to baseline values much faster than mitochondrial [Ca2+]. In contrast to findings in other tissues, there was usually little or no poststimulation overshoot of NADH fluorescence. These findings suggest that the major change in motor terminal mitochondrial function brought about by repetitive stimulation is a rapid acceleration of electron transport chain (ETC) activity due to the Psi(m) depolarization produced by mitochondrial Ca2+ (or Sr2+) influx. After partial inhibition of complex I of the ETC with amytal, stimulation produced greater Psi(m) depolarization and a greater elevation of cytosolic [Ca2+]. These results suggest that the ability to accelerate ETC activity is important for normal mitochondrial sequestration of stimulation-induced Ca2+ loads.

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.

Mitochondrial calcium signalling and cell death: approaches for assessing the role of mitochondrial Ca2+ uptake in apoptosis

Local Ca(2+) transfer between adjoining domains of the sarcoendoplasmic reticulum (ER/SR) and mitochondria allows ER/SR Ca(2+) release to activate mitochondrial Ca(2+) uptake and to evoke a matrix [Ca(2+)] ([Ca(2+)](m)) rise. [Ca(2+)](m) exerts control on several steps of energy metabolism to synchronize ATP generation with cell function. However, calcium signal propagation to the mitochondria may also ignite a cell death program through opening of the permeability transition pore (PTP). This occurs when the Ca(2+) release from the ER/SR is enhanced or is coincident with sensitization of the PTP. Recent studies have shown that several pro-apoptotic factors, including members of the Bcl-2 family proteins and reactive oxygen species (ROS) regulate the Ca(2+) sensitivity of both the Ca(2+) release channels in the ER and the PTP in the mitochondria. To test the relevance of the mitochondrial Ca(2+) accumulation in various apoptotic paradigms, methods are available for buffering of [Ca(2+)], for dissipation of the driving force of the mitochondrial Ca(2+) uptake and for inhibition of the mitochondrial Ca(2+) transport mechanisms. However, in intact cells, the efficacy and the specificity of these approaches have to be established. Here we discuss mechanisms that recruit the mitochondrial calcium signal to a pro-apoptotic cascade and the approaches available for assessment of the relevance of the mitochondrial Ca(2+) handling in apoptosis. We also present a systematic evaluation of the effect of ruthenium red and Ru360, two inhibitors of mitochondrial Ca(2+) uptake on cytosolic [Ca(2+)] and [Ca(2+)](m) in intact cultured cells.

The mitochondrial energy transduction system and the aging process

Aged mammalian tissues show a decreased capacity to produce ATP by oxidative phosphorylation due to dysfunctional mitochondria. The mitochondrial content of rat brain and liver is not reduced in aging and the impairment of mitochondrial function is due to decreased rates of electron transfer by the selectively diminished activities of complexes I and IV. Inner membrane H(+) impermeability and F(1)-ATP synthase activity are only slightly affected by aging. Dysfunctional mitochondria in aged rodents are characterized, besides decreased electron transfer and O(2) uptake, by an increased content of oxidation products of phospholipids, proteins and DNA, a decreased membrane potential, and increased size and fragility. Free radical-mediated oxidations are determining factors of mitochondrial dysfunction and turnover, cell apoptosis, tissue function, and lifespan. Inner membrane enzyme activities, such as those of complexes I and IV and mitochondrial nitric oxide synthase, decrease upon aging and afford aging markers. The activities of these three enzymes in mice brain are linearly correlated with neurological performance, as determined by the tightrope and the T-maze tests. The same enzymatic activities correlated positively with mice survival and negatively with the mitochondrial content of lipid and protein oxidation products. Conditions that increase survival, as vitamin E dietary supplementation, caloric restriction, high spontaneous neurological activity, and moderate physical exercise, ameliorate mitochondrial dysfunction in aged brain and liver. The pleiotropic signaling of mitochondrial H(2)O(2) and nitric oxide diffusion to the cytosol seems modified in aged animals and to contribute to the decreased mitochondrial biogenesis in old animals.

Integration of calcium with the signaling network in cardiac myocytes

Calcium has evolved as global intracellular messenger for signal transduction in the millisecond time range by reversibly binding to calcium-sensing proteins. In the cardiomyocyte, ion pumps, ion exchangers and channels keep the cytoplasmic calcium level at rest around approximately 100 nM which is more than 10,000-fold lower than outside the cell. Intracellularly, calcium is mainly stored in the sarcoplasmic reticulum, which comprises the bulk of calcium available for the heartbeat. Regulation of cardiac function including contractility and energy production relies on a three-tiered control system, (i) immediate and fast feedback in response to mechanical load on a beat-to-beat basis (Frank-Starling relation), (ii) more sustained regulation involving transmitters and hormones as primary messengers, and (iii) long-term adaptation by changes in the gene expression profile. Calcium signaling over largely different time scales requires its integration with the protein kinase signaling network which is governed by G-protein-coupled receptors, growth factor and cytokine receptors at the surface membrane. Short-term regulation is dominated by the beta-adrenergic system, while long-term regulation with phenotypic remodeling depends on sustained signaling by growth factors, cytokines and calcium. Mechanisms and new developments in intracellular calcium handling and its interrelation with the MAPK signaling pathways are discussed in detail.

Extrusion of Ca2+ from mouse motor terminal mitochondria via a Na+-Ca2+ exchanger increases post-tetanic evoked release

Mitochondria sequester much of the Ca2+ that enters motor nerve terminals during repetitive stimulation at frequencies exceeding 10-20 Hz. We studied the post-stimulation extrusion of Ca2+ from mitochondria by measuring changes in matrix [Ca2+] with fluorescent indicators loaded into motor terminal mitochondria in the mouse levator auris longus muscle. Trains of action potentials at 50 Hz produced a rapid increase in mitochondrial [Ca2+] followed by a plateau, which was usually maintained after the end of the stimulus train and then slowly decayed back to baseline. Increasing the Ca2+ load delivered to the terminal by increasing the number of stimuli (from 500 to 2000) or the stimulation frequency (from 50 to 100 Hz), by increasing bath [Ca2+], or by prolonging the action potential with 3,4-diaminopyridine (100 microM) prolonged the post-stimulation decay of mitochondrial [Ca2+] without increasing the amplitude of the plateau during stimulation. Inhibiting the opening of the mitochondrial permeability transition pore with cyclosporin A (5 microM) had no significant effect on the decay of mitochondrial [Ca2+]. Inhibition of the mitochondrial Na+-Ca2+ exchanger with CGP-37157 (50 microM) dramatically prolonged the post-stimulation decay of mitochondrial [Ca2+], reduced post-stimulation residual cytosolic [Ca2+], and reduced the amplitude of endplate potentials evoked after the end of a stimulus train in the presence of both low and normal bath [Ca2+]. These findings suggest that Ca2+ extrusion from motor terminal mitochondria occurs primarily via the mitochondrial Na+-Ca2+ exchanger and helps to sustain post-tetanic transmitter release at mouse neuromuscular junctions.

The calcium-dependent ATP-Mg/Pi mitochondrial carrier is a target of glucose-induced calcium signalling in Saccharomyces cerevisiae

Sal1p is a mitochondrial protein that belongs to the SCaMC (short calcium-binding mitochondrial carrier) subfamily of mitochondrial carriers. The presence of calcium-binding motifs facing the extramitochondrial space allows the regulation of the transport activity of these carriers by cytosolic calcium and provides a new mechanism to transduce calcium signals in mitochondria without the requirement of calcium entry in the organelle. We have studied its transport activity, finding that it is a carboxyatractyloside-resistant ATP-Mg carrier. Mitochondria from a disruption mutant of SAL1 have a 50% reduction in the uptake of ATP. We have also found a clear stimulation of ATP-transport activity by calcium, with an S(0.5) of approx. 30 microM. Our results also suggest that Sal1p is a target of the glucose-induced calcium signal which is non-essential in wild-type cells, but becomes essential for transport of ATP into mitochondria in yeast lacking ADP/ATP translocases.

Cardiac system bioenergetics: metabolic basis of the Frank-Starling law

The fundamental principle of cardiac behaviour is described by the Frank-Starling law relating force of contraction during systole with end-diastolic volume. While both work and respiration rates increase linearly with imposed load, the basis of mechano-energetic coupling in heart muscle has remained a long-standing enigma. Here, we highlight advances made in understanding of complex cellular and molecular mechanisms that orchestrate coupling of mitochondrial oxidative phosphorylation with ATP utilization for muscle contraction. Cardiac system bioenergetics critically depends on an interrelated metabolic infrastructure regulating mitochondrial respiration and energy fluxes throughout cellular compartments. The data reviewed indicate the significance of two interrelated systems regulating mitochondrial respiration and energy fluxes in cells: (1) the creatine kinase, adenylate kinase and glycolytic pathways that communicate flux changes generated by cellular ATPases within structurally organized enzymatic modules and networks; and (2) a secondary system based on mitochondrial participation in cellular calcium cycle, which adjusts substrate oxidation and energy-transducing processes to meet increasing cellular energy demands. By conveying energetic signals to metabolic sensors, coupled phosphotransfer reactions provide a high-fidelity regulation of the excitation-contraction cycle. Such integration of energetics with calcium signalling systems provides the basis for 'metabolic pacing', synchronizing the cellular electrical and mechanical activities with energy supply processes.

Dynamics of Mitochondria and Mitochondrial Ca2+near the Plasma Membrane of PC12 Cells: A Study by Multimode Microscopy

The goal of this study is to examine whether there is a difference in the regulation of Ca2+ between mitochondria near the cell surface and mitochondria in the cytosol. Total internal reflection fluorescence and epifluorescence microscopy were used to monitor changes in the mitochondrial Ca2+ ([Ca2+]mt) between the mitochondria near the plasma membrane and those in the cytosol. The results show that [Ca2+]mt near the plasma membrane increased earlier and decayed slower after high K+ stimulation than average mitochondria in the cytosol. In addition, the changes in [Ca2+]mt in the mitochondria near the cell surface after a second stimulation were larger than those induced by the first stimulation. The results provide direct evidence to support the hypothesis that mitochondria in different subcellular localization show differential responses to the influx of extracellular Ca2+.

An evanescent approach for mitochondrial function assay of living cells

Mitochondria are now known to function physiologically not only in the production of ATP as the major cellular energy source, but also in the regulation of intracellular signaling, in, for example, stress-induced apoptosis and buffering of cytosolic calcium. It should be noted, when interpreting mitochondrial studies in situ, that mitochondria within cells show heterogeneity in both function and location. We applied both conventional epifluorescence microscopy (EPIFM) and total-internal-reflection fluorescence microscopy (TIRFM) in this study. Image data taken from TIRFM are excellent and markedly different from those taken from EPIFM. We further investigated the physiological variations of mitochondrial functions using an EPIFM/TIRFM dual-imaging system. This system permits further analysis of functions of mitochondria and other organelles with more precision than is possible using a traditional platform.

Identification of a conserved calmodulin-binding motif in the sequence of F0F1 ATPsynthase inhibitor protein

The natural inhibitor proteins IF1 regulate mitochondrial F0F1 ATPsynthase in a wide range of species. We characterized the interaction of CaM with purified bovine IF1, two bovine IF1 synthetic peptides, as well as two homologous proteins from yeast, namely IF1 and STF1. Fluorometric analyses showed that bovine and yeast inhibitors bind CaM with a 1:1 stoichiometry in the pH range between 5 and 8 and that CaM-IF1 interaction is Ca2+-dependent. Bovine and yeast IF1 have intermediate binding affinity for CaM, while the Kd (dissociation constant) of the STF1-CaM interaction is slightly higher. Binding studies of CaM with bovine IF1 synthetic peptides allowed us to identify bovine IF1 sequence 33-42 as the putative CaM-binding region. Sequence alignment revealed that this region contains a hydrophobic motif for CaM binding, highly conserved in both yeast IF1 and STF1 sequences. In addition, the same region in bovine IF1 has an IQ motif for CaM binding, conserved as an IQ-like motif in yeast IF1 but not in STF1. Based on the pH and Ca2+ dependence of IF1 interaction with CaM, we suggest that the complex can be formed outside mitochondria, where CaM could regulate IF1 trafficking or additional IF1 roles not yet clarified.

Mitochondrial calcium signalling in cell death

The development of targeted probes (based on the molecular engineering of luminescent or fluorescent proteins) has allowed the specific measurement of [Ca2+] in intracellular organelles or cytoplasmic subdomains. This approach gave novel information on different aspects of cellular Ca2+ homeostasis. Regarding mitochondria, it was possible to demonstrate that, upon physiological stimulation of cells, Ca2+ is rapidly accumulated in the matrix. We will discuss the basic characteristics of this process, its role in modulating physiological and pathological events, such as the regulation of aerobic metabolism and the induction of cell death, and new insight into the regulatory mechanisms operating in vivo.

Calcium-induced contraction of sarcomeres changes the regulation of mitochondrial respiration in permeabilized cardiac cells

The relationships between cardiac cell structure and the regulation of mitochondrial respiration were studied by applying fluorescent confocal microscopy and analysing the kinetics of mitochondrial ADP-stimulated respiration, during calcium-induced contraction in permeabilized cardiomyocytes and myocardial fibers, and in their 'ghost' preparations (after selective myosin extraction). Up to 3 microm free calcium, in the presence of ATP, induced strong contraction of permeabilized cardiomyocytes with intact sarcomeres, accompanied by alterations in mitochondrial arrangement and a significant decrease in the apparent K(m) for exogenous ADP and ATP in the kinetics of mitochondrial respiration. The V(max) of respiration showed a moderate (50%) increase, with an optimum at 0.4 microm free calcium and a decrease at higher calcium concentrations. At high free-calcium concentrations, the direct flux of ADP from ATPases to mitochondria was diminished compared to that at low calcium levels. All of these effects were unrelated either to mitochondrial calcium overload or to mitochondrial permeability transition and were not observed in 'ghost' preparations after the selective extraction of myosin. Our results suggest that the structural changes transmitted from contractile apparatus to mitochondria modify localized restrictions of the diffusion of adenine nucleotides and thus may actively participate in the regulation of mitochondrial function, in addition to the metabolic signalling via the creatine kinase system.

Juvenile sudden death in a family with polymorphic ventricular arrhythmias caused by a novel RyR2 gene mutation: evidence of specific morphological substrates

We report on a family with a history of sudden death and effort-induced polymorphic ventricular arrhythmias. The index case was a 17-year-old boy who died suddenly and at postmortem had evidence of fibrofatty replacement in the right ventricular free wall, consistent with arrhythmogenic right ventricular cardiomyopathy, as well as calcium phosphate deposits within the myocytes. A molecular genetics investigation carried out in the paraffin-embedded myocardium of the subject and in blood samples of family members disclosed a missense mutation in exon 3 (230C-->T; A77V) of the cardiac ryanodine receptor type 2 gene. The carriers showed effort-induced polymorphic ventricular tachycardia in the setting of normal resting electrocardiogram and trivial echocardiographic abnormalities, consistent with catecholaminergic polymorphic ventricular tachycardia. The observation of both arrhythmogenic right ventricular cardiomyopathy type 2 and catecholaminergic polymorphic ventricular tachycardia in the same family suggests that the two entities might correspond to different degrees of phenotypic expression of the same disease. This experience underscores the importance of a precise autopsy diagnosis in the case of sudden cardiac death, including molecular genetics, and the mission of pathologists to guide further clinical investigation of family members.

Nitric oxide: orchestrating hypoxia regulation through mitochondrial respiration and the endoplasmic reticulum stress response

Mitochondria have long been considered to be the powerhouse of the living cell, generating energy in the form of the molecule ATP via the process of oxidative phosphorylation. In the past 20 years, it has been recognised that they also play an important role in the implementation of apoptosis, or programmed cell death. More recently it has become evident that mitochondria also participate in the orchestration of cellular defence responses. At physiological concentrations, the gaseous molecule nitric oxide (NO) inhibits the mitochondrial enzyme cytochrome c oxidase (complex IV) in competition with oxygen. This interaction underlies the mitochondrial actions of NO, which range from the physiological regulation of cell respiration, through mitochondrial signalling, to the development of "metabolic hypoxia" -- a situation in which, although oxygen is available, the cell is unable to utilise it.

Intracellular calcium involvement in pituitary adenylate cyclase-activating polypeptide stimulation of growth hormone and gonadotrophin secretion in goldfish pituitary cells

The involvement of intracellular Ca(2+) stores and their regulatory mechanisms in mediating pituitary adenylate cyclase-activating polypeptide (PACAP) stimulation of growth hormone (GH) and maturational gonadotrophin (GTH-II) secretion from goldfish pituitary cells was investigated using a cell column perifusion system. Pretreatment with caffeine abolished the GH and GTH-II responses to PACAP. Dantrolene attenuated PACAP-elicited GTH-II release but did not affect the GH response, whereas ryanodine and 8-bromo-cADP ribose did not alter PACAP-induced GH and GTH-II release. Two endoplasmic/sarcoplasmic reticulum Ca(2+) ATPase (SERCA) inhibitors, thapsigargin and cyclopiazonic acid, augmented PACAP-induced GTH-II release; similarly, thapsigargin elevated GH responses to PACAP. Treatment with carbonyl cyanide m-chlorophenylhydrazone, a mitochondrial uncoupler, reduced PACAP-stimulated GH release; however, inhibition of the mitochondrial Ca(2+) uniport by Ru360 did not affect GH and GTH-II responses. The phosphatidyl inositol (PI)-specific phospholipase C (PLC) inhibitor ET-18-OCH(3) inhibited, whereas the phosphatidyl-choline (PC)-specific PLC inhibitor D609 enhanced, PACAP-stimulated GH and GTH-II responses. On the other hand, the IP(3) receptor blocker xestospongin D had no effect on PACAP-induced GTH-II response and potentiated the GH response. These results suggest that, despite some differences between GH and GTH-II cells, PACAP actions in both cell types generally rely on a caffeine-sensitive, but a largely ryanodine receptor-independent, mechanism. PC-PLC and some SERCA negatively modulate PACAP actions but mitochondrial Ca(2+) stores per se are not important. A novel PI-PLC mechanism, which does not involve the traditional IP(3)/Ca(2+) pathway, is also suggested.

Calcium - a universal carrier of biological signals

Calcium is the most universal carrier of signals to cells. Chosen by evolution because of its peculiar flexibility as a ligand, it now regulates all important aspects of cell activity, beginning with the creation of new life at fertilization and ending with the dramatic event of apoptotic suicide at the end of the life cycle. The process of signal transduction by Ca2+ displays a number of properties that make it unique among all other carriers of signals: for instance, the ability to perform both a first messenger and a second messenger function, or the frequent activation of autoregulatory mechanisms. The aspect that distinguishes the Ca2+ signaling function most dramatically is ambivalence. Cells have an absolute dependence on the messenger function of Ca2+ in order to function properly and must control its homeostasis with precision to maintain its free concentration in their interior at the appropriate low level. Catastrophy, however, invariably follows whenever protracted failures of the control mechanisms lead to sustained Ca2+ overload.

Calcium and mitochondria: mechanisms and functions of a troubled relationship

Mitochondria promptly respond to Ca(2+)-mediated cell stimulations with a rapid accumulation of the cation into the matrix. In this article, we review (i) the basic principles of mitochondrial Ca(2+) transport, (ii) the physiological/pathological role of mitochondrial Ca(2+) uptake, (iii) the regulatory mechanisms that may operate in vivo, and (iv) the new targeted Ca(2+) probes that allowed the "rediscovery" of these organelles in calcium signalling.

A large, voltage-dependent channel, isolated from mitochondria by water-free chloroform extraction

We examined ion channels derived from a chloroform extract of isolated, dehydrated rat liver mitochondria. The extraction method was previously used to isolate a channel-forming complex containing poly-3-hydroxybutyrate and calcium polyphosphate from Escherichia coli. This complex is also present in eukaryotic membranes, and is located primarily in mitochondria. Reconstituted channels showed multiple subconductance levels and were voltage-dependent, showing an increased probability of higher conductance states at voltages near zero. In symmetric 150 mM KCl, the maximal conductance of the channel ranged from 350 pS to 750 pS. For voltages >+/-60 mV, conductance fluctuated in the range of approximately 50- approximately 200 pS. In the presence of a 1:3 gradient of KCl, at pH = 7.4, selectivity periodically switched between different states ranging from weakly anion-selective (V(rev) approximately -15 mV) to ideally cation-selective (V(rev) approximately +29 mV), without a significant change in its conductance. Overall, the diverse, but highly reproducible, channel activity most closely resembled the behavior of the permeability transition pore channel seen in patch-clamp experiments on native mitoplasts. We suggest that the isolated complex may represent the ion-conducting module from the permeability transition pore.

Excitotoxic calcium overload in a subpopulation of mitochondria triggers delayed death in hippocampal neurons

In neurons, excitotoxic stimulation induces mitochondrial calcium overload and the release of pro-apoptotic proteins, which triggers delayed cell death. The precise mechanisms of apoptogen release, however, remain controversial. To characterize the linkage between mitochondrial calcium load and cell vulnerability, and to test the hypothesis that only a subpopulation of mitochondria damaged by calcium overload releases apoptogens, we have measured directly the concentrations of total Ca (free plus bound) in individual mitochondria and monitored in parallel structural changes and the subcellular localization of pro-apoptotic cytochrome c after NMDA overstimulation in cultured hippocampal neurons. Beyond transient elevation of cytosolic calcium and perturbation of Na+/K+ homeostasis, NMDA stimulation induced dramatic, but mainly reversible, changes in mitochondria, including strong calcium elevation, membrane potential depolarization, and variable swelling. Elevation of matrix Ca in the approximately one-third of mitochondria that were strongly swollen, as well as the absence of swelling when Ca2+ entry was abolished, indicate an essential role for Ca overload. Shortly after NMDA exposure, cytochrome c, normally localized to mitochondria, became diffusely distributed in the cytoplasm, coincident with the appearance of severely swollen mitochondria with ruptured outer membranes; under these conditions, cytochrome c was retained in intact mitochondria, implying that it was released mainly from damaged mitochondria. Consistent with the role of mitochondrial Ca overload, carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone decreased Ca accumulation, prevented cytochrome c release, and was neuroprotective. These results support a mechanism in which delayed excitotoxic death involves apoptogen release from a subpopulation of calcium-overloaded mitochondria, whereas other, undamaged mitochondria maintain normal function.

Deranged neuronal calcium signaling and Huntington disease

Huntington disease (HD) is an autosomal-dominant neurodegenerative disorder that primarily affects medium spiny striatal neurons (MSN). HD is caused by polyglutamine (polyQ) expansion (exp) in the amino-terminal region of a protein huntingtin (Htt). The connection between polyQ expansion in Httexp and MSN neurodegeneration remains elusive. Here we discuss recent data that link polyQ expansion in Httexp and deranged Ca2+ signaling in MSN neurons. Experimental evidence indicates that (1) Ca2+ homeostasis is abnormal in mitochondria isolated from lymphoblasts of HD patients and from brains of the YAC72 HD mouse model; (2) Httexp leads to potentiation of NR1/NR2B NMDA receptor activity in heterologous expression systems and in MSN from YAC72 HD mouse model; and (3) Httexp binds to the type 1 inositol 1,4,5-trisphosphate receptor (InsP3R1) carboxy-terminus and causes sensitization of InsP3R1 to activation by InsP3 in planar lipid bilayers and in MSN. Based on these results we propose that Httexp-induced cytosolic and mitochondrial Ca2+ overload of MSN plays an important role in the pathogenesis of HD and that Ca2+ signaling blockers may play a beneficial role in treatment of HD.

In vivo monitoring of Ca(2+) uptake into mitochondria of mouse skeletal muscle during contraction

Although the importance of mitochondria in patho-physiology has become increasingly evident, it remains unclear whether these organelles play a role in Ca(2+) handling by skeletal muscle. This undefined situation is mainly due to technical limitations in measuring Ca(2+) transients reliably during the contraction-relaxation cycle. Using two-photon microscopy and genetically expressed "cameleon" Ca(2+) sensors, we developed a robust system that enables the measurement of both cytoplasmic and mitochondrial Ca(2+) transients in vivo. We show here for the first time that, in vivo and under highly physiological conditions, mitochondria in mammalian skeletal muscle take up Ca(2+) during contraction induced by motor nerve stimulation and rapidly release it during relaxation. The mitochondrial Ca(2+) increase is delayed by a few milliseconds compared with the cytosolic Ca(2+) rise and occurs both during a single twitch and upon tetanic contraction.

Calcium overload and cardiac function

The changes in cardiac function caused by calcium overload are reviewed. Intracellular Ca(2+) may increase in different structures [e.g. sarcoplasmic reticulum (SR), cytoplasm and mitochondria] to an excessive level which induces electrical and mechanical abnormalities in cardiac tissues. The electrical manifestations of Ca(2+) overload include arrhythmias caused by oscillatory (V(os)) and non-oscillatory (V(ex)) potentials. The mechanical manifestations include a decrease in force of contraction, contracture and aftercontractions. The underlying mechanisms involve a role of Na(+) in electrical abnormalities as a charge carrier in the Na(+)-Ca(2+) exchange and a role of Ca(2+) in mechanical toxicity. Ca(2+) overload may be induced by an increase in [Na(+)](i) through the inhibition of the Na(+)-K(+) pump (e.g. toxic concentrations of digitalis) or by an increase in Ca(2+) load (e.g. catecholamines). The Ca(2+) overload is enhanced by fast rates. Purkinje fibers are more susceptible to Ca(2+) overload than myocardial fibers, possibly because of their greater Na(+) load. If the SR is predominantly Ca(2+) overloaded, V(os) and fast discharge are induced through an oscillatory release of Ca(2+) in diastole from the SR; if the cytoplasm is Ca(2+) overloaded, the non-oscillatory V(ex) tail is induced at negative potentials. The decrease in contractile force by Ca(2+) overload appears to be associated with a decrease in high energy phosphates, since it is enhanced by metabolic inhibitors and reduced by metabolic substrates. The ionic currents I(os) and I(ex) underlie V(os) and V(ex), respectively, both being due to an electrogenic extrusion of Ca(2+) through the Na(+)-Ca(2+) exchange. I(os) is an oscillatory current due to an oscillatory release of Ca(2+) in early diastole from the Ca(2+)-overloaded SR, and I(ex) is a non-oscillatory current due to the extrusion of Ca(2+) from the Ca(2+)-overloaded cytoplasm. I(os) and I(ex) can be present singly or simultaneously. An increase in [Ca(2+)](i) appears to be involved in the short- and long-term compensatory mechanisms that tend to maintain cardiac output in physiological and pathological conditions. Eventually, [Ca(2+)](i) may increase to overload levels and contribute to cardiac failure. Experimental evidence suggests that clinical concentrations of digitalis increase force in Ca(2+)-overloaded cardiac cells by decreasing the inhibition of the Na(+)-K(+) pump by Ca(2+), thereby leading to a reduction in Ca(2+) overload and to an increase in force of contraction.

Effects of polyamines on mitochondrial Ca2+ transport

Mammalian mitochondria are able to enhance Ca(2+) accumulation in the presence of polyamines by activating the saturable systems of Ca(2+) inward transport and buffering extramitochondrial Ca(2+) concentrations to levels similar to those in the cytosol of resting cells. This effect renders them responsive to regulate free Ca(2+) concentrations in the physioloical range. The mechanism involved is due to a rise in the affinity of the Ca(2+) transport system, induced by polyamines, most probably exhibiting allosteric behaviour. The regulatory site of this mechanism is the so-called S(1) binding site of polyamines, which operates in physiological conditions and is located in the energy well between the two peaks present in the energy profile of mitochondrial spermine transport. Spermine is bidirectionally transported across teh inner membrane by cycling, in which influx and efflux are driven by electrical and pH gradients, respectively. Most probably, polyamine affects the Ca(2+) transport system when it acts from the outside-that is, in the direction of its uniporter channel, in order to reach the S(1) site. Important physiological functions are related to activation of Ca(2+) transport systems by polyamines and their interactions with the S(1) site. These functions include a rise in the metabolic rate for energy supply and modulation of mitochondrial permeability transition induction, with consequent effects on the triggering of the apoptotic pathway.

Flirting in little space: the ER/mitochondria Ca2+ liaison

Mitochondria have long been known to accumulate Ca2+; the apparent inconsistency between the low affinity of mitochondrial Ca2+ uptake mechanisms, the low concentration of global Ca2+ signals observed in cytoplasm, and the efficiency in intact cells of mitochondrial Ca2+ uptake led to the formulation of the "hotspot hypothesis." This hypothesis proposes that mitochondria preferentially accumulate Ca2+ at microdomains of elevated Ca2+ concentration ([Ca2+]) that exist near endoplasmic reticulum (ER) Ca2+ release sites and other Ca2+ channels. Physiological Ca2+ signals may affect mitochondrial function--both by stimulating key metabolic enzymes and, under some conditions, by promoting apoptosis. Mitochondria in turn may affect both Ca2+ release from the ER and capacitative Ca2+ entry across the plasma membrane, thereby shaping the size and duration of the intracellular Ca2+ signal. Interactions between mitochondria and the ER are critically dependent on the spatial localization of mitochondria within the cell. The molecular mechanisms that define the organization of mitochondria with regard to the ER and other Ca2+ sources, and the extent to which mitochondrial function varies among different cell types, are open questions whose answers remain to be determined.

Spontaneous mitochondrial depolarizations are independent of SR Ca2+ release

The mitochondrial membrane potential (DeltaPsi(m)) underlies many mitochondrial functions, including Ca(2+) influx into the mitochondria, which allows them to serve as buffers of intracellular Ca(2+). Spontaneous depolarizations of DeltaPsi(m), flickers, have been observed in isolated mitochondria and intact cells using the fluorescent cationic lipophile tetramethylrhodamine ethyl ester (TMRE), which distributes across the inner mitochondrial membrane in accordance with the Nernst equation. Flickers in cardiomyocytes have been attributed to uptake of Ca(2+) released from the sarcoplasmic reticulum (SR) via ryanodine receptors in focal transients called Ca(2+) sparks. We have shown previously that an increase in global Ca(2+) in smooth muscle cells causes an increase in mitochondrial Ca(2+) and depolarization of DeltaPsi(m). Here we sought to determine whether flickers in smooth muscle cells are caused by uptake of Ca(2+) released focally in Ca(2+) sparks. High-speed three-dimensional imaging was used to monitor DeltaPsi(m) in freshly dissociated myocytes from toad stomach that were simultaneously voltage clamped at 0 mV to ensure the cytosolic TMRE concentration was constant and equal to the low level in the bath (2.5 nM). This approach allows quantitative analysis of flickers as we have previously demonstrated. Depletion of SR Ca(2+) not only failed to eliminate flickers but rather increased their magnitude and frequency somewhat. Flickers were not altered in magnitude or frequency by ryanodine or xestospongin C, inhibitors of intracellular Ca(2+) release, or by cyclosporin A, an inhibitor of the permeability transition pore. Focal Ca(2+) release from the SR does not cause flickers in the cells employed here.