Mitochondrial Ca2+ homeostasis during Ca2+ influx and Ca2+ release in gastric myocytes from Bufo marinus

Mitochondrial Calcium Waves by Electrical Stimulation in Cultured Hippocampal Neurons

Mitochondria are critical to cellular Ca2+ homeostasis via the sequestering of cytosolic Ca2+ in the mitochondrial matrix. Mitochondrial Ca2+ buffering regulates neuronal activity and neuronal death by shaping cytosolic and presynaptic Ca2+ or controlling energy metabolism. Dysfunction in mitochondrial Ca2+ buffering has been implicated in psychological and neurological disorders. Ca2+ wave propagation refers to the spreading of Ca2+ for buffering and maintaining the associated rise in Ca2+ concentration. We investigated mitochondrial Ca2+ waves in hippocampal neurons using genetically encoded Ca2+ indicators. Neurons transfected with mito-GCaMP5G, mito-RCaMP1h, and CEPIA3mt exhibited evidence of mitochondrial Ca2+ waves with electrical stimulation. These waves were observed with 200 action potentials at 40 Hz or 20 Hz but not with lower frequencies or fewer action potentials. The application of inhibitors of mitochondrial calcium uniporter and oxidative phosphorylation suppressed mitochondrial Ca2+ waves. However, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and N-methyl-d-aspartate receptor blockade had no effect on mitochondrial Ca2+ wave were propagation. The Ca2+ waves were not observed in endoplasmic reticula, presynaptic terminals, or cytosol in association with electrical stimulation of 200 action potentials at 40 Hz. These results offer novel insights into the mechanisms underlying mitochondrial Ca2+ buffering and the molecular basis of mitochondrial Ca2+ waves in neurons in response to electrical stimulation.

Local Ca2+ coupling between mitochondria and sarcoplasmic reticulum following depolarization in guinea pig urinary bladder smooth muscle cells

Spatiotemporal changes in cytosolic Ca²⁺ concentration ([Ca²⁺]_c) trigger a number of physiological functions in smooth muscle cells (SMCs). We previously imaged Ca²⁺-induced Ca²⁺ release following membrane depolarization as local Ca²⁺ transients, Ca²⁺ hotspots, in subplasmalemmal regions. In this study, the physiological significance of mitochondria on local Ca²⁺ signaling was examined. Cytosolic and mitochondrial Ca²⁺ images following depolarization or action potentials were recorded in single SMCs from the guinea pig urinary bladder using a fast-scanning confocal fluorescent microscope. Depolarization- and action potential-induced [Ca²⁺]_c transients occurred at several discrete sites in subplasmalemmal regions, peaked within 30 ms, and then spread throughout the whole-cell. In contrast, Ca²⁺ concentration in the mitochondria matrix ([Ca²⁺]_m) increased after a delay of ~50 ms from the start of depolarization, and then peaked within 500 ms. Following repolarization, [Ca²⁺]_c returned to the resting level with a half-decay time of ~500 ms, while [Ca²⁺]_m recovered more slowly (∼1.5 s). Carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone, a mitochondrial uncoupler, abolished depolarization-induced [Ca²⁺]_m elevations and slowed [Ca²⁺]_c changes. Importantly, short depolarization-induced changes in [Ca²⁺]_m and transmembrane potential in mitochondria coupled to Ca²⁺ hotspots were significantly larger than those in other mitochondria. Total internal reflection fluorescence imaging revealed that a subset of mitochondria closely localized with ryanodine receptors and voltage-dependent Ca²⁺ channels. These results indicate that particular mitochondria are functionally coupled to ion channels and sarcoplasmic reticulum fragments within the local Ca²⁺ microdomain, and thus, strongly contribute to [Ca²⁺]_c regulation in SMCs.

Simultaneous Measurement of Mitochondrial Calcium and Mitochondrial Membrane Potential in Live Cells by Fluorescent Microscopy

Apart from their essential role in generating ATP, mitochondria also act as local calcium (Ca²⁺) buffers to tightly regulate intracellular Ca²⁺ concentration. To do this, mitochondria utilize the electrochemical potential across their inner membrane (ΔΨm) to sequester Ca²⁺. The influx of Ca²⁺ into the mitochondria stimulates three rate-limiting dehydrogenases of the citric acid cycle, increasing electron transfer through the oxidative phosphorylation (OXPHOS) complexes. This stimulation maintains ΔΨm, which is temporarily dissipated as the positive calcium ions cross the mitochondrial inner membrane into the mitochondrial matrix. We describe here a method for simultaneously measuring mitochondria Ca²⁺ uptake and ΔΨm in live cells using confocal microscopy. By permeabilizing the cells, mitochondrial Ca²⁺ can be measured using the fluorescent Ca²⁺ indicator Fluo-4, AM, with measurement of ΔΨm using the fluorescent dye tetramethylrhodamine, methyl ester, perchlorate (TMRM). The benefit of this system is that there is very little spectral overlap between the fluorescent dyes, allowing accurate measurement of mitochondrial Ca²⁺ and ΔΨm simultaneously. Using the sequential addition of Ca²⁺ aliquots, mitochondrial Ca²⁺ uptake can be monitored, and the concentration at which Ca²⁺ induces mitochondrial membrane permeability transition and the loss of ΔΨm determined.

The mitochondrial calcium uniporter: mice can live and die without it

Calcium is of critical importance to mitochondrial and cell function, and calcium signaling is highly localized in the cell. When stimulated, mitochondria are capable of rapidly taking up calcium, affecting both matrix energetics within mitochondria and shaping the amplitude and frequency of cytosolic calcium "waves". During pathological conditions a large increase in mitochondrial calcium levels is thought to activate the mitochondrial permeability transition pore, resulting in cell death. The protein responsible for mitochondrial calcium uptake, the mitochondrial calcium uniporter (MCU), was identified in 2011 and its molecular elucidation has stimulated and invigorated research in this area. MCU knockout mice have been created, a variety of other regulators have been identified, and a disease phenotype in humans has been attributed to the loss of a uniporter regulator. In the three years since its molecular elucidation, further research into the MCU has revealed a complex uniporter, and raised many questions about its physiologic and pathologic cell roles. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease".

Examining the role of mitochondria in Ca²⁺ signaling in native vascular smooth muscle

Mitochondrial Ca²⁺ uptake contributes important feedback controls to limit the time course of Ca²⁺signals. Mitochondria regulate cytosolic [Ca²⁺] over an exceptional breath of concentrations (∼200 nM to >10 μM) to provide a wide dynamic range in the control of Ca²⁺ signals. Ca²⁺ uptake is achieved by passing the ion down the electrochemical gradient, across the inner mitochondria membrane, which itself arises from the export of protons. The proton export process is efficient and on average there are less than three protons free within the mitochondrial matrix. To study mitochondrial function, the most common approaches are to alter the proton gradient and to measure the electrochemical gradient. However, drugs which alter the mitochondrial proton gradient may have substantial off target effects that necessitate careful consideration when interpreting their effect on Ca²⁺ signals. Measurement of the mitochondrial electrochemical gradient is most often performed using membrane potential sensitive fluorophores. However, the signals arising from these fluorophores have a complex relationship with the electrochemical gradient and are altered by changes in plasma membrane potential. Care is again needed in interpreting results. This review provides a brief description of some of the methods commonly used to alter and measure mitochondrial contribution to Ca²⁺ signaling in native smooth muscle.

Mitochondrial motility and vascular smooth muscle proliferation

OBJECTIVE

Mitochondria are widely described as being highly dynamic and adaptable organelles, and their movement is thought to be vital for cell function. Yet, in various native cells, including those of heart and smooth muscle, mitochondria are stationary and rigidly structured. The significance of the differences in mitochondrial behavior to the physiological function of cells is unclear and was studied in single myocytes and intact resistance-sized cerebral arteries. We hypothesized that mitochondrial dynamics is controlled by the proliferative status of the cells.

METHODS AND RESULTS

High-speed fluorescence imaging of mitochondria in live vascular smooth muscle cells shows that the organelle undergoes significant reorganization as cells become proliferative. In nonproliferative cells, mitochondria are individual (≈ 2 μm by 0.5 μm), stationary, randomly dispersed, fixed structures. However, on entering the proliferative state, mitochondria take on a more diverse architecture and become small spheres, short rod-shaped structures, long filamentous entities, and networks. When cells proliferate, mitochondria also continuously move and change shape. In the intact pressurized resistance artery, mitochondria are largely immobile structures, except in a small number of cells in which motility occurred. When proliferation of smooth muscle was encouraged in the intact resistance artery, in organ culture, the majority of mitochondria became motile and the majority of smooth muscle cells contained moving mitochondria. Significantly, restriction of mitochondrial motility using the fission blocker mitochondrial division inhibitor prevented vascular smooth muscle proliferation in both single cells and the intact resistance artery.

CONCLUSIONS

These results show that mitochondria are adaptable and exist in intact tissue as both stationary and highly dynamic entities. This mitochondrial plasticity is an essential mechanism for the development of smooth muscle proliferation and therefore presents a novel therapeutic target against vascular disease.

Mitochondrial organization and Ca2+ uptake

Mitochondria may function as multiple separate organelles or as a single electrically coupled continuum to modulate changes in [Ca2+]c (cytoplasmic Ca2+ concentration) in various cell types. Mitochondria may also be tethered to the internal Ca2+ store or plasma membrane in particular parts of cells to facilitate the organelles modulation of local and global [Ca2+]c increases. Differences in the organization and positioning contributes significantly to the at times apparently contradictory reports on the way mitochondria modulate [Ca2+]c signals. In the present paper, we review the organization of mitochondria and the organelles role in Ca2+ signalling.

Mitochondrial regulation of cytosolic Ca²⁺ signals in smooth muscle

The cytosolic Ca²⁺ concentration ([Ca²⁺]c) controls virtually every activity of smooth muscle, including contraction, migration, transcription, division and apoptosis. These processes may be activated by large (>10 μM) amplitude [Ca²⁺]c increases, which occur in small restricted regions of the cell or by smaller (<1 μM) amplitude changes throughout the bulk cytoplasm. Mitochondria contribute to the regulation of these signals by taking up Ca²⁺. However, mitochondria's reported low affinity for Ca²⁺ is thought to require the organelle to be positioned close to ion channels and within a microdomain of high [Ca²⁺]. In cultured smooth muscle, mitochondria are highly dynamic structures but in native smooth muscle mitochondria are immobile, apparently strategically positioned organelles that regulate the upstroke and amplitude of IP₃-evoked Ca²⁺ signals and IP₃ receptor (IP₃R) cluster activity. These observations suggest mitochondria are positioned within the high [Ca²⁺] microdomain arising from an IP₃R cluster to exert significant local control of channel activity. On the other hand, neither the upstroke nor amplitude of voltage-dependent Ca²⁺ entry is modulated by mitochondria; rather, it is the declining phase of the transient that is regulated by the organelle. Control of the declining phase of the transient requires a high mitochondrial affinity for Ca²⁺ to enable uptake to occur over the normal physiological Ca²⁺ range (<1 μM). Thus, in smooth muscle, mitochondria regulate Ca²⁺ signals exerting effects over a large range of [Ca²⁺] (∼200 nM to at least tens of micromolar) to provide a wide dynamic range in the control of Ca²⁺ signals.

Quantitative measurement of mitochondrial membrane potential in cultured cells: calcium-induced de- and hyperpolarization of neuronal mitochondria

Mitochondrial membrane potential (ΔΨM) is a central intermediate in oxidative energy metabolism. Although ΔΨM is routinely measured qualitatively or semi-quantitatively using fluorescent probes, its quantitative assay in intact cells has been limited mostly to slow, bulk-scale radioisotope distribution methods. Here we derive and verify a biophysical model of fluorescent potentiometric probe compartmentation and dynamics using a bis-oxonol-type indicator of plasma membrane potential (ΔΨP) and the ΔΨM probe tetramethylrhodamine methyl ester (TMRM) using fluorescence imaging and voltage clamp. Using this model we introduce a purely fluorescence-based quantitative assay to measure absolute values of ΔΨM in millivolts as they vary in time in individual cells in monolayer culture. The ΔΨP-dependent distribution of the probes is modelled by Eyring rate theory. Solutions of the model are used to deconvolute ΔΨP and ΔΨM in time from the probe fluorescence intensities, taking into account their slow, ΔΨP-dependent redistribution and Nernstian behaviour. The calibration accounts for matrix:cell volume ratio, high- and low-affinity binding, activity coefficients, background fluorescence and optical dilution, allowing comparisons of potentials in cells or cell types differing in these properties. In cultured rat cortical neurons, ΔΨM is −139 mV at rest, and is regulated between −108 mV and −158 mV by concerted increases in ATP demand and Ca2+-dependent metabolic activation. Sensitivity analysis showed that the standard error of the mean in the absolute calibrated values of resting ΔΨM including all biological and systematic measurement errors introduced by the calibration parameters is less than 11 mV. Between samples treated in different ways, the typical equivalent error is ∼5 mV.

Flow cytometric analysis of ca-induced membrane permeability transition of isolated rat liver mitochondria

The membrane permeability transition (MPT) of mitochondria plays an important role in the mechanism of apoptotic cell death in various cells. Classic type MPT is induced by Ca²⁺ in the presence of inorganic phosphate and respiratory substrate, and is characterized by various events including generation of reactive oxygen species (ROS), membrane depolarization, swelling, release of Ca²⁺ and high sensitivity to cyclosporine A. However, the sequence of these events and the effect of antioxidants on their events remain obscure. Flow cytometry is a convenient method to investigate the order of events among various functions occurring in MPT using a limited amount of mitochondria (200 µl of 0.02 mg protein/ml) without contamination by other organelles. Flow cytometric analysis revealed that Ca²⁺ sequentially induced ROS generation, depolarization, swelling and Ca²⁺ release in mitochondria by a cyclosporine A-inhibitable mechanism. These results were supported by the finding that Ca²⁺-induced MPT was inhibited by antioxidants, such as glutathione and N-acetylcysteine. It was also revealed that various inhibitors of Ca²⁺-induced phospholipase A₂ suppressed all of the events associated with Ca²⁺-induced MPT. These results suggested that ROS generation and phospholipase A₂ activation by Ca²⁺ underlie the mechanism of the initiation of MPT.

Anticancer effect of (E)-2-hydroxy-3',4,5'-trimethoxystilbene on breast cancer cells by mitochondrial depolarization

BACKGROUND

TMS (2,3',4,5'-tetramethoxystilbene), a stilbene analog derived from rhapontigenin, was previously demonstrated to induce apoptosis in hormone-resistant breast cancer cells. Therefore, this study investigated the anticancer effect of a new stilbene analog, HTMS ((E)-2-hydroxy-3',4,5'-trimethoxystilbene), and its mechanism in various breast cancer cell lines.

MATERIALS AND METHODS

The effect of HTMS on cell proliferation of MDA-MB-231, MCF-7, and LTED cells was evaluated using MTT assays. Cell apoptosis was detected by FITC-annexin V staining and flow cytometry analysis, changes in mitochondrial potential were determined by fluorescence microscopy using TMRE staining, and the expression of cleaved PARP and release of cytochrome c were assessed by Western blot analysis.

RESULTS

HTMS significantly decreased the cell viability of various types of breast cancer cells in a dose- and time-dependent manner, characterized by G2/M arrest of the cell cycle and the induction of apoptosis. In particular, HTMS disturbed the mitochondrial membrane potential, causing a release of cytochrome c during apoptosis. Furthermore, HTMS was superior to TMS in inhibiting cancer cell growth in a pilot comparison study.

CONCLUSION

HTMS is an effective apoptotic agent for breast cancer cells, making it a candidate therapeutic agent for the treatment of breast cancer.

Structural and functional link between the mitochondrial network and the endoplasmic reticulum

Mitochondrial and endoplasmic reticulum (ER) networks are fundamental for the maintenance of cellular homeostasis and for determination of cell fate under stress conditions. Recent structural and functional studies revealed the interaction of these networks. These zones of close contact between ER and mitochondria called MAM (mitochondria associated membranes) support communication between the two organelles including bioenergetics and cell survival. The existence of macromolecular complexes in these contact sites has also been revealed. In this contribution, we will review: (i) the ER and mitochondria structure and their dynamics, (ii) the basic principles of ER mitochondrial Ca(2+) transport, (iii) the physiological/pathological role of this cross-talk.

High- and low-calcium-dependent mechanisms of mitochondrial calcium signalling

The Ca(2+) coupling between endoplasmic reticulum (ER) and mitochondria is central to multiple cell survival and cell death mechanisms. Cytoplasmic [Ca(2+)] ([Ca(2+)](c)) spikes and oscillations produced by ER Ca(2+) release are effectively delivered to the mitochondria. Propagation of [Ca(2+)](c) signals to the mitochondria requires the passage of Ca(2+) across three membranes, namely the ER membrane, the outer mitochondrial membrane (OMM) and the inner mitochondrial membrane (IMM). Strategic positioning of the mitochondria by cytoskeletal transport and interorganellar tethers provides a means to promote the local transfer of Ca(2+) between the ER membrane and OMM. In this setting, even >100 microM [Ca(2+)] may be attained to activate the low affinity mitochondrial Ca(2+) uptake. However, a mitochondrial [Ca(2+)] rise has also been documented during submicromolar [Ca(2+)](c) elevations. Evidence has been emerging that Ca(2+) exerts allosteric control on the Ca(2+) transport sites at each membrane, providing mechanisms that may facilitate the Ca(2+) delivery to the mitochondria. Here we discuss the fundamental mechanisms of ER and mitochondrial Ca(2+) transport, particularly the control of their activity by Ca(2+) and evaluate both high- and low-[Ca(2+)]-activated mitochondrial calcium signals in the context of cell physiology.

Role of mitochondria in spontaneous rhythmic activity and intracellular calcium waves in the guinea pig gallbladder smooth muscle

Mitochondrial Ca(2+) handling has been implicated in spontaneous rhythmic activity in smooth muscle and interstitial cells of Cajal. In this investigation we evaluated the effect of mitochondrial inhibitors on spontaneous action potentials (APs), Ca(2+) flashes, and Ca(2+) waves in gallbladder smooth muscle (GBSM). Disruption of the mitochondrial membrane potential with carbonyl cyanide 3-chlorophenylhydrazone, carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone, rotenone, and antimycin A significantly reduced or eliminated APs, Ca(2+) flashes, and Ca(2+) waves in GBSM. Blockade of ATP production with oligomycin did not alter APs or Ca(2+) flashes but significantly reduced Ca(2+) wave frequency. Inhibition of mitochondrial Ca(2+) uptake and Ca(2+) release with Ru360 and CGP-37157, respectively, reduced the frequency of Ca(2+) flashes and Ca(2+) waves in GBSM. Similar to oligomycin, cyclosporin A did not alter AP and Ca(2+) flash frequency but significantly reduced Ca(2+) wave activity. These data suggest that mitochondrial Ca(2+) handling is necessary for the generation of spontaneous electrical activity and may therefore play an important role in gallbladder tone and motility.

The mitochondrial membrane potential and Ca2+ oscillations in smooth muscle

Ca2+ uptake by mitochondria might both modulate the cytosolic Ca2+ concentration ([Ca2+]c) and depolarize the mitochondrial membrane potential (delta Psi m) to limit ATP production. To investigate how physiological Ca2+ signaling might affect energy production, delta Psi m was examined during Ca2+ oscillations in smooth muscle cells. In single, voltage-clamped smooth muscle cells, inhibition of mitochondrial Ca2+ accumulation inhibited inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3]-evoked Ca2+ release and prolonged the time required for restoration of [Ca2+]c following activation of plasmalemmal Ca2+ currents (ICa). Ca2+ could be released from mitochondria immediately (within 15 seconds) after a [Ca2+]c rise evoked by Ins(1,4,5)P3 or ICa. Despite this evidence of mitochondrial Ca2+ accumulation, no change in delta Psi m was observed during single or repetitive [Ca2+]c oscillations evoked by these conditions. Occasionally, spontaneous, repetitive, persistent Ca 2+ oscillations were observed. In these cases, mitochondria displayed stochastic delta Psi m depolarizations, which were independent both of events in neighboring mitochondria and of the timing of the [Ca 2+]c oscillations themselves. Such delta Psi m depolarizations could be mimicked by increased exposure to either fluorescence excitation light or the delta Psi m-sensitive dye tetramethylrhodamine ethyl ester (TMRE) and were inhibited by antioxidants (ascorbic acid, catalase, Trolox and TEMPO) or the mitochondrial permeability transition pore (mPTP)-inhibitor cyclosporin A (CsA). Individual mitochondria within smooth muscle cells might depolarize during repetitive Ca2+ oscillations or during oxidative stress but not during the course of single [Ca2+]c transients evoked by Ca2+ influx or store release.

Ca2+ microdomains in smooth muscle

In smooth muscle, Ca(2+) controls diverse activities including cell division, contraction and cell death. Of particular significance in enabling Ca(2+) to perform these multiple functions is the cell's ability to localize Ca(2+) signals to certain regions by creating high local concentrations of Ca(2+) (microdomains), which differ from the cytoplasmic average. Microdomains arise from Ca(2+) influx across the plasma membrane or release from the sarcoplasmic reticulum (SR) Ca(2+) store. A single Ca(2+) channel can create a microdomain of several micromolar near (approximately 200 nm) the channel. This concentration declines quickly with peak rates of several thousand micromolar per second when influx ends. The high [Ca(2+)] and the rapid rates of decline target Ca(2+) signals to effectors in the microdomain with rapid kinetics and enable the selective activation of cellular processes. Several elements within the cell combine to enable microdomains to develop. These include the brief open time of ion channels, localization of Ca(2+) by buffering, the clustering of ion channels to certain regions of the cell and the presence of membrane barriers, which restrict the free diffusion of Ca(2+). In this review, the generation of microdomains arising from Ca(2+) influx across the plasma membrane and the release of the ion from the SR Ca(2+) store will be discussed and the contribution of mitochondria and the Golgi apparatus as well as endogenous modulators (e.g. cADPR and channel binding proteins) will be considered.

Synaptic mitochondria are more susceptible to Ca2+overload than nonsynaptic mitochondria

Mitochondria in nerve terminals are subjected to extensive Ca2+ fluxes and high energy demands, but the extent to which the synaptic mitochondria buffer Ca2+ is unclear. In this study, we identified a difference in the Ca2+ clearance ability of nonsynaptic versus synaptic mitochondrial populations enriched from rat cerebral cortex. Mitochondria were isolated using Percoll discontinuous gradients in combination with high pressure nitrogen cell disruption. Mitochondria in the nonsynaptic fraction originate from neurons and other cell types including glia, whereas mitochondria enriched from a synaptosomal fraction are predominantly neuronal and presynaptic in origin. There were no differences in respiration or initial Ca2+ loads between nonsynaptic and synaptic mitochondrial populations. Following both bolus and infusion Ca2+ addition, nonsynaptic mitochondria were able to accumulate significantly more exogenously added Ca2+ than the synaptic mitochondria before undergoing mitochondrial permeability transition, observed as a loss in mitochondrial membrane potential and decreased Ca2+ uptake. The limited ability of synaptic mitochondria to accumulate Ca2+ could result from several factors including a primary function of ATP production to support the high energy demand of presynaptic terminals, their relative isolation in comparison with the threads or clusters of mitochondria found in the soma of neurons and glia, or the older age and increased exposure to oxidative damage of synaptic versus nonsynaptic mitochondria. By more readily undergoing permeability transition, synaptic mitochondria may initiate neuron death in response to insults that elevate synaptic levels of intracellular Ca2+, consistent with the early degeneration of distal axon segments in neurodegenerative disorders.

Microdomains of intracellular Ca2+: molecular determinants and functional consequences

Calcium ions are ubiquitous and versatile signaling molecules, capable of decoding a variety of extracellular stimuli (hormones, neurotransmitters, growth factors, etc.) into markedly different intracellular actions, ranging from contraction to secretion, from proliferation to cell death. The key to this pleiotropic role is the complex spatiotemporal organization of the [Ca(2+)] rise evoked by extracellular agonists, which allows selected effectors to be recruited and specific actions to be initiated. In this review, we discuss the structural and functional bases that generate the subcellular heterogeneity in cellular Ca(2+) levels at rest and under stimulation. This complex choreography requires the concerted action of many different players; the central role is, of course, that of the calcium ion, with the main supporting characters being all the entities responsible for moving Ca(2+) between different compartments, while the cellular architecture provides a determining framework within which all the players have their exits and their entrances. In particular, we concentrate on the molecular mechanisms that lead to the generation of cytoplasmic Ca(2+) microdomains, focusing on their different subcellular location, mechanism of generation, and functional role.

Rearrangement of the close contact between the mitochondria and the sarcoplasmic reticulum in airway smooth muscle

The mitochondria and the sarcoplasmic reticulum (SR) are two major intracellular calcium-storing organelles that exhibit close functional interaction with each other. Close spatial association is believed to be important for their functional interaction. In this study, we have characterized the spatial relationship between the SR and the mitochondria in porcine tracheal smooth muscle cells (TSMC) under different conditions. By examining the cross-section of unstimulated TSMC with electron microscopy, we found that 99.4 +/- 0.5% of the mitochondria seen on random cross-sections were situated within 30 nm of the SR and that 82.2 +/- 6.7% of the mitochondria were completely enveloped by the SR network. Overall, 48.0 +/- 3.5% of the mitochondrial outer membrane was within 30 nm with the SR. After stimulation of the TSMC with acetylcholine (ACh) or 80 mM [K(+)] solution 97.0 +/- 2.1% and 98.6 +/- 1.4% of the mitochondria observed were situated within 30 nm of the SR, respectively. However, the proportion of the mitochondria that was completely enveloped by the SR was significantly reduced to 12.2 +/- 5.9% in ACh-stimulated cells and 9.7 +/- 6.6% in 80 mM [K(+)] stimulated cells. The percentage of mitochondrial membrane closely associated with the SR was correspondingly lower at 10.1 +/- 1.0% during ACh stimulation and 10.8 +/- 0.9% during 80 mM [K(+)] stimulation. During smooth muscle cell stimulation, the SR appears to unwrap from the mitochondria and extend into the cytoplasm while maintaining close contact with the mitochondria over a smaller area. Such static and dynamic components of the close spatial association between the mitochondria and the SR may serve as a structural basis for the selective and efficient Ca(2+) trafficking between the two organelles in TSMC.

Mitochondrial Ca2+ dynamics reveals limited intramitochondrial Ca2+ diffusion

To reveal heterogeneity of mitochondrial function on the single-mitochondrion level we have studied the spatiotemporal dynamics of the mitochondrial Ca2+ signaling and the mitochondrial membrane potential using wide-field fluorescence imaging and digital image processing techniques. Here we demonstrate first-time discrete sites--intramitochondrial hotspots--of Ca2+ uptake after Ca2+ release from intracellular stores, and spreading of Ca2+ rise within the mitochondria. The phenomenon was characterized by comparison of observations in intact cells stimulated by ATP and in plasma membrane permeabilized or in ionophore-treated cells exposed to elevated buffer [Ca2+]. The findings indicate that Ca2+ diffuses laterally within the mitochondria, and that the diffusion is limited for shorter segments of the mitochondrial network. These observations were supported by mathematical simulation of buffered diffusion. The mitochondrial membrane potential was investigated using the potentiometric dye TMRM. Irradiation-induced fluctuations (flickering) of TMRM fluorescence showed synchronicity over large regions of the mitochondrial network, indicating that certain parts of this network form electrical syncytia. The spatial extension of these syncytia was decreased by 2-aminoethoxydiphenyl borate (2-APB) or by propranolol (blockers of nonclassical mitochondrial permeabilities). Our data suggest that mitochondria form syncytia of electrical conductance whereas the passage of Ca2+ is restricted to the individual organelle.

Quantitative analysis of spontaneous mitochondrial depolarizations

Spontaneous transient depolarizations in mitochondrial membrane potential (DeltaPsi(m)), mitochondrial flickers, have been observed in isolated mitochondria and intact cells using the fluorescent probe, tetramethylrhodamine ethyl ester (TMRE). In theory, the ratio of [TMRE] in cytosol and mitochondrion allows DeltaPsi(m) to be calculated with the Nernst equation, but this has proven difficult in practice due to fluorescence quenching and binding of dye to mitochondrial membranes. We developed a new method to determine the amplitude of flickers in terms of millivolts of depolarization. TMRE fluorescence was monitored using high-speed, high-sensitivity three-dimensional imaging to track individual mitochondria in freshly dissociated smooth muscle cells. Resting mitochondrial fluorescence, an exponential function of resting DeltaPsi(m), varied among mitochondria and was approximately normally distributed. Spontaneous changes in mitochondrial fluorescence, indicating depolarizations and repolarizations in DeltaPsi(m), were observed. The depolarizations were reversible and did not result in permanent depolarization of the mitochondria. The magnitude of the flickers ranged from <10 mV to >100 mV with a mean of 17.6 +/- 1.0 mV (n = 360) and a distribution skewed to smaller values. Nearly all mitochondria flickered, and they did so independently of one another, indicating that mitochondria function as independent units in the myocytes employed here.

Smooth muscle cells and interstitial cells of blood vessels

A rise in intracellular ionised calcium concentration ([Ca(2+)](i)) at sites adjacent to the contractile proteins is a primary signal for contraction in all types of muscles. Recent progress in the development of imaging techniques with special accent on the fluorescence confocal microscopy and new achievements in the synthesis of organelle- and ion-specific fluorochromes provide an experimental basis for study of the relationship between the structural organisation of the living smooth muscle myocyte and the features of calcium signalling at subcellular level. Applying fluorescent confocal microscopy and tight-seal recording of transmembrane ion currents to freshly isolated vascular myocytes we have demonstrated that: (1) Ca(2+) sparks originate from clustered opening of ryanodine receptors (RyRs) and build up a cell-wide increase in [Ca(2+)](i) upon myocyte excitation; (2) spontaneous Ca(2+) sparks occurred at the highest rate at certain preferred locations, frequent discharge sites (FDS), which are associated with a prominent portion of the sarcoplasmic reticulum (SR) located close to the cell membrane; (3) Ca(2+)-dependent K(+) and Cl(-) channels sense the local changes in [Ca(2+)](i) during a calcium spark and thereby couple changes in [Ca(2+)](i) within a microdomain to changes in the membrane potential, thus affecting excitability of the cell; (4) an intercommunication between RyRs and inositol trisphosphate receptors (IP(3)Rs) is one of the important determinants of intracellular calcium dynamics that, in turn, can modulate the cell membrane potential through differential targeting of calcium dependent membrane ion channels. Furthermore, using immunohystochemical approaches in combination with confocal imaging we identified non-contractile cells closely resembling interstitial cells (ICs) of Cajal (which are considered to be pacemaker cells in the gut) in the wall of portal vein and mesenteric artery. Using electron microscopy, tight-seal recording and fluorescence confocal imaging we obtained information on the morphology of ICs and their possible coupling to smooth muscle cells (SMCs), calcium signalling in ICs and their electrophysiological properties. The functions of these cells are not yet fully understood; in portal vein they may act as pacemakers driving the spontaneous activity of the muscle; in artery they may have other a yet unsuspected functions.

Ca2+ syntillas, miniature Ca2+ release events in terminals of hypothalamic neurons, are increased in frequency by depolarization in the absence of Ca2+ influx

Localized, brief Ca2+ transients (Ca2+ syntillas) caused by release from intracellular stores were found in isolated nerve terminals from magnocellular hypothalamic neurons and examined quantitatively using a signal mass approach to Ca2+ imaging. Ca2+ syntillas (scintilla, L., spark, from a synaptic structure, a nerve terminal) are caused by release of approximately 250,000 Ca ions on average by a Ca2+ flux lasting on the order of tens of milliseconds and occur spontaneously at a membrane potential of -80 mV. Syntillas are unaffected by removal of extracellular Ca2+, are mediated by ryanodine receptors (RyRs) and are increased in frequency, in the absence of extracellular Ca2+, by physiological levels of depolarization. This represents the first direct demonstration of mobilization of Ca2+ from intracellular stores in neurons by depolarization without Ca2+ influx. The regulation of syntillas by depolarization provides a new link between neuronal activity and cytosolic [Ca2+] in nerve terminals.

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.

Mitochondrial Ca2+ uptake requires sustained Ca2+ release from the endoplasmic reticulum

We analyzed the role of inositol 1,4,5-trisphosphate-induced Ca(2+) release from the endoplasmic reticulum (ER) (i) in powering mitochondrial Ca(2+) uptake and (ii) in maintaining a sustained elevation of cytosolic Ca(2+) concentration ([Ca(2+)](c)). For this purpose, we expressed in HeLa cells aequorin-based Ca(2+)-sensitive probes targeted to different intracellular compartments and studied the effect of two agonists: histamine, acting on endogenous H(1) receptors, and glutamate, acting on co-transfected metabotropic glutamate receptor (mGluR1a), which rapidly inactivates through protein kinase C-dependent phosphorylation and thus causes transient inositol 1,4,5-trisphosphate production. Glutamate induced a transient [Ca(2+)](c) rise and drop in ER luminal [Ca(2+)] ([Ca(2+)](er)), and then the ER refilled with [Ca(2+)](c) at resting values. With histamine, [Ca(2+)](c) after the initial peak stabilized at a sustained plateau, and [Ca(2+)](er) decreased to a low steady-state value. In mitochondria, histamine evoked a much larger mitochondrial Ca(2+) response than glutamate ( approximately 15 versus approximately 65 microm). Protein kinase C inhibition, partly relieving mGluR1a desensitization, reestablished both the [Ca(2+)](c) plateau and the sustained ER Ca(2+) release and markedly increased the mitochondrial Ca(2+) response. Conversely, mitochondrial Ca(2+) uptake evoked by histamine was drastically reduced by very transient ( approximately 2-s) agonist applications. These data indicate that efficient mitochondrial Ca(2+) uptake depends on the preservation of high Ca(2+) microdomains at the mouth of ER Ca(2+) release sites close to mitochondria. This in turn depends on continuous Ca(2+) release balanced by Ca(2+) reuptake into the ER and maintained by Ca(2+) influx from the extracellular space.

Recombinant expression of the voltage-dependent anion channel enhances the transfer of Ca2+ microdomains to mitochondria

Although the physiological relevance of mitochondrial Ca2+ homeostasis is widely accepted, no information is yet available on the molecular identity of the proteins involved in this process. Here we analyzed the role of the voltage-dependent anion channel (VDAC) of the outer mitochondrial membrane in the transmission of Ca2+ signals between the ER and mitochondria by measuring cytosolic and organelle [Ca2+] with targeted aequorins and Ca2+-sensitive GFPs. In HeLa cells and skeletal myotubes, the transient expression of VDAC enhanced the amplitude of the agonist-dependent increases in mitochondrial matrix Ca2+ concentration by allowing the fast diffusion of Ca2+ from ER release sites to the inner mitochondrial membrane. Indeed, high speed imaging of mitochondrial and cytosolic [Ca2+] changes showed that the delay between the rises occurring in the two compartments is significantly shorter in VDAC-overexpressing cells. As to the functional consequences, VDAC-overexpressing cells are more susceptible to ceramide-induced cell death, thus confirming that mitochondrial Ca2+ uptake plays a key role in the process of apoptosis. These results reveal a novel function for the widely expressed VDAC channel, identifying it as a molecular component of the routes for Ca2+ transport across the mitochondrial membranes.

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

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

The influence of different InsP(3) receptor isoforms on Ca(2+) signaling in tracheal smooth muscle cells

In airway myocytes, like in many cells, Ca(2+) signaling is controlled by inositol 1,4,5-trisphosphate (InsP(3)) via InsP(3) receptors (InsP(3)R) located in the sarco-endoplasmic reticulum. Three types of InsP(3)R exist, labeled Types 1, 2, and 3, which differ in their gating kinetics. We analyze a possible impact of the different gating kinetics of Type 1 and Type 3 InsP(3)R on the time course of cytosolic Ca(2+) concentration in tracheal smooth muscle cells upon agonist stimulation. Previous experimental data in rat tracheal myocytes showed that upon gradually increased stimulation with acetylcholine (ACh), a contractile agonist that acts via InsP(3) production, signal spikes, several spikes with declining maxima, and sustained oscillations appear. Our model reproduces the time courses of cytosolic Ca(2+) measured in tracheal myocytes. Moreover, by postulating slight variations in the model parameters which determine the total number of receptors expressed and the ratio between Type 1 and Type 3 InsP(3)R, it offers an explanation to the experimental observation of qualitatively different responses of cells within a presumably homogeneous tissue.

Presynaptic mitochondrial calcium sequestration influences transmission at mammalian central synapses

Beyond their role in generating ATP, mitochondria have a high capacity to sequester calcium. The interdependence of these functions and limited access to presynaptic compartments makes it difficult to assess the role of sequestration in synaptic transmission. We addressed this important question using the calyx of Held as a model glutamatergic synapse by combining patch-clamp with a novel mitochondrial imaging method. Presynaptic calcium current, mitochondrial calcium concentration ([Ca(2+)](mito), measured using rhod-2 or rhod-FF), cytoplasmic calcium concentration ([Ca(2+)](cyto), measured using fura-FF), and the postsynaptic current were monitored during synaptic transmission. Presynaptic [Ca(2+)](cyto) rose to 8.5 +/- 1.1 microM and decayed rapidly with a time constant of 45 +/- 3 msec; presynaptic [Ca(2+)](mito) also rose rapidly to >5 microM but decayed slowly with a half-time of 1.5 +/- 0.4 sec. Mitochondrial depolarization with rotenone and carbonyl cyanide p-trifluoromethoxyphenylhydrazone abolished mitochondrial calcium rises and slowed the removal of [Ca(2+)](cyto) by 239 +/- 22%. Using simultaneous presynaptic and postsynaptic patch clamp, combined with presynaptic mitochondrial and cytoplasmic imaging, we investigated the influence of mitochondrial calcium sequestration on transmitter release. Depletion of ATP to maintain mitochondrial membrane potential was blocked with oligomycin, and ATP was provided in the patch pipette. Mitochondrial depolarization raised [Ca(2+)](cyto) and reduced transmitter release after short EPSC trains (100 msec, 200 Hz); this effect was reversed by raising mobile calcium buffering with EGTA. Our results suggest a new role for presynaptic mitochondria in maintaining transmission by accelerating recovery from synaptic depression after periods of moderate activity. Without detectable thapsigargin-sensitive presynaptic calcium stores, we conclude that mitochondria are the major organelle regulating presynaptic calcium at central glutamatergic terminals.

Structural and functional relationships between Ca2+ puffs and mitochondria in Xenopus oocytes

Ca2+ uptake and release from endoplasmic reticulum (ER) and mitochondrial Ca2+ stores play important physiological and pathological roles, and these processes are shaped by interactions that depend on the structural intimacy between these organelles. Here we investigate the morphological and functional relationships between mitochondria, ER, and the sites of intracellular Ca2+ release in Xenopus laevis oocytes by combining confocal imaging of local Ca2+ release events ("Ca2+ puffs") with mitochondrial localization visualized using vital dyes and subcellularly targeted fluorescent proteins. Mitochondria and ER are localized in cortical bands approximately 6-8 microm wide, with the mitochondria arranged as densely packed "islands" interconnected by discrete strands. The ER is concentrated more superficially than mitochondria, and the mean separation between Ca2+ puff sites and mitochondria is approximately 2.3 microm. However, a subpopulation of Ca2+ puff sites is intimately associated with mitochondria (approximately 28% within <600 nm), a greater number than expected if Ca2+ puff sites were randomly distributed. Ca2+ release sites close to mitochondria exhibit lower Ca2+ puff activity than Ca2+ puff sites in regions with lower mitochondrial density. Furthermore, Ca2+ puff sites in close association with mitochondria rarely serve as the sites for Ca2+ wave initiation. We conclude that mitochondria play important roles in regulating local ER excitability, Ca2+ wave initiation, and, thereby, spatial patterning of global Ca2+ signals.

Changes in mitochondrial Ca2+ detected with Rhod-2 in single frog and mouse skeletal muscle fibres during and after repeated tetanic contractions

The present study investigated mitochondrial Ca2+ uptake and release in intact living skeletal muscle fibres subjected to bouts of repetitive activity. Confocal microscopy was used in conjunction with the Ca2+-sensitive dye Rhod-2 to monitor changes in mitochondrial Ca2+ in single Xenopus or mouse muscle fibres. A marked increase in the mitochondrial Ca2+ occurred in Xenopus fibres after 10 tetani applied at 4 s intervals. The mitochondrial Ca2+ continued to increase with increasing number of tetani. After the end of tetanic stimulation, mitochondrial Ca2+ declined to 50% of the maximal increase within 10 min and thereafter took up to 60 min to return to its original value. Depolarization of the mitochondria with FCCP greatly attenuated the rise in the mitochondrial Ca2+ evoked by repetitive tetanic stimulation. In addition, FCCP slowed the rate of decay of the tetanic Ca2+ transient which in turn led to an elevation of resting cytosolic Ca2+. Accumulation of Ca2+ in the mitochondria was accompanied by a modest mitochondrial depolarization. In contrast to the situation in Xenopus fibres, mitochondria in mouse toe muscle fibres did not show any change in the mitochondrial Ca2+ during repetitive stimulation and FCCP had no effect on the rate of decay of the tetanic Ca2+ transient. It is concluded that in Xenopus fibres, mitochondria play a role in the regulation of cytosolic Ca2+ and contribute to the relaxation of tetanic Ca2+ transients. In contrast to their important role in Xenopus fibres, mitochondria in mouse fast-twitch skeletal fibres play little role in Ca2+ homeostasis.

Mechanisms underlying the activation of large conductance Ca2+-activated K+ channels by nordihydroguaiaretic acid

The mechanisms underlying the activation of large conductance Ca2+-activated K+ (BK) channel by nordihydroguaiaretic acid (NDGA) were examined in human embryonic kidney (HEK293) cells, where BK channel alpha (BKalpha) or a plus beta1 subunit (BKalphabeta1) was heterologously expressed, and also in freshly isolated porcine coronary arterial smooth muscle cells (PCASMCs). The activity of both BKalpha and BKalphabeta1 channels was increased by 10 microM NDGA in similar manners, indicating the selective action on the a subunit to increase Ca2+ sensitivity. The application of NDGA to PCASMCs induced outward current and hyperpolarization under voltage and current clamp, respectively, in a concentration-dependent manner (> or = 3 microM). These effects were blocked by 100 nM iberiotoxin. Electrical events induced by NDGA (> or = 10 microM) were, unexpectedly, associated with the increase in [Ca2+]i. After the treatment with caffeine and ryanodine, the [Ca2+]i increase by NDGA was markedly reduced and the hyperpolarization by NDGA was attenuated. The Ca2+ release by 10 microM NDGA was preceded by membrane depolarization of mitochondria. These results indicate that BK channel opening by NDGA in PCASMCs is due to the direct action on a subunit and also to Ca2+ release from sarcoplasmic reticulum, presumably via, at least in part, the inhibition of mitochondria respiration.

Ca(2+) oscillations, gradients, and homeostasis in vascular smooth muscle

Vascular smooth muscle shows both plasticity and heterogeneity with respect to Ca(2+) signaling. Physiological perturbations in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) may take the form of a uniform maintained rise, a transient uniform [Ca(2+)](i) elevation, a transient localized rise in [Ca(2+)](i) (also known as spark and puff), a transient propagated wave of localized [Ca(2+)](i) elevation (Ca(2+) wave), recurring asynchronous Ca(2+) waves, or recurring synchronized Ca(2+) waves dependent on the type of blood vessel and the nature of stimulation. In this overview, evidence is presented which demonstrates that interactions of ion transporters located in the membranes of the cell, sarcoplasmic reticulum, and mitochondria form the basis of this plasticity of Ca(2+) signaling. We focus in particular on how the junctional complexes of plasmalemma and superficial sarcoplasmic reticulum, through the generation of local cytoplasmic Ca(2+) gradients, maintain [Ca(2+)](i) oscillations, couple these to either contraction or relaxation, and promote Ca(2+) cycling during homeostasis.

Mitochondrial modulation of calcium signaling at the initiation of development

Fertilization triggers cytosolic Ca(2+) oscillations that activate mammalian eggs and initiate development. Extensive evidence demonstrates that Ca(2+) is released from endoplasmic reticulum stores; however, less is known about how the increased Ca(2+) is restored to its resting level, forming the Ca(2+) oscillations. We investigated whether mitochondria also play a role in activation-associated Ca(2+) signaling. Mitochondrial dysfunction induced by the mitochondrial uncoupler FCCP or antimycin A disrupted cytosolic Ca(2+) oscillations, resulting in sustained increase in cytosolic Ca(2+), followed by apoptotic cell death. This suggests that functional mitochondria may participate in sequestering the released Ca(2+), contributing to cytosolic Ca(2+) oscillations and preventing cell death. By centrifugation, mouse eggs were stratified and separated into fractions containing both endoplasmic reticulum and mitochondria and fractions containing endoplasmic reticulum with no mitochondria. The former showed Ca(2+) oscillations by activation, whereas the latter exhibited sustained elevation in cytosolic Ca(2+) but no Ca(2+) oscillations, suggesting that mitochondria take up released cytosolic Ca(2+). Further, using Rhod-2 for detection of mitochondrial Ca(2+), we found that mitochondria exhibited Ca(2+) oscillations, the frequency of which was not different from that of cytosolic Ca(2+) oscillations, indicating that mitochondria are involved in Ca(2+) signaling during egg activation. Therefore, we propose that mitochondria play a crucial role in Ca(2+) signaling that mediates egg activation and development, and apoptotic cell death.

Selective, high-resolution fluorescence imaging of mitochondrial Ca2+ concentration

We have developed a digital image processing technique based on highpass filtering of microfluorimetric images for selective transmission of fine image details corresponding to mitochondria. This technique enabled the detection of the mitochondrial calcium signals with high selectivity, simultaneously with the cytosolic calcium signal. The validity of this technique was supported in primary cultures of rat brain capillary endothelial cells loaded with X-rhod-1 by the results that (i) inhibition of the mitochondrial Ca2+ uptake by discharging the mitochondrial membrane potential selectively abolished the transient of the highpass filtered signal evoked by ATP, and (ii) CGP-37157, a selective blocker of the mitochondrial Na+/Ca2+ exchanger, increased the peak amplitude of highpass filtered (mitochondrial) Ca2+ transients and caused a sustained plateau. The highpass filtering technique enabled the analysis of the mitochondrial Ca2+ transients in high temporal resolution. We found a uniform and monophasic rise of [Ca2+] in the mitochondrial population of the cell, following the cytosolic [Ca2+] with a delay at onset and peak. The introduced highpass filtering technique is a powerful tool in the high spatial and temporal resolution analysis of mitochondrial calcium transients, and it could be especially important in specimens where genetically targeted probes fail.

Invited review: mechanisms of calcium handling in smooth muscles

The concentration of cytoplasmic Ca(2+) regulates the contractile state of smooth muscle cells and tissues. Elevations in global cytoplasmic Ca(2+) resulting in contraction are accomplished by Ca(2+) entry and release from intracellular stores. Pathways for Ca(2+) entry include dihydropyridine-sensitive and -insensitive Ca(2+) channels and receptor and store-operated nonselective channels permeable to Ca(2+). Intracellular release from the sarcoplasmic reticulum (SR) is accomplished by ryanodine and inositol trisphosphate receptors. The impact of Ca(2+) entry and release on cytoplasmic concentration is modulated by Ca(2+) reuptake into the SR, uptake into mitochondria, and extrusion into the extracellular solution. Highly localized Ca(2+) transients (i.e., sparks and puffs) regulate ionic conductances in the plasma membrane, which can provide feedback to cell excitability and affect Ca(2+) entry. This short review describes the major transport mechanisms and compartments that are utilized for Ca(2+) handling in smooth muscles.

Ultra rapid calcium events in electrically stimulated frog nerve terminals

Fast calcium events occurring in cytoplasmic organelles after a single electrical stimulus were investigated by electron spectroscopic imaging (an electron microscope technique that reveals total calcium with high sensitivity and spatial resolution) in quick frozen presynaptic terminals of the frog neuromuscular junction. In resting preparations synaptic vesicles showed a prominent calcium signal whereas mitochondria were mostly negative and only some of the cisternae of the endoplasmic reticulum were clearly positive. In preparations quick frozen 10 ms after the application to the nerve of a single, supramaximal electric stimulus, no obvious change was observed in synaptic vesicles, while calcium levels rose to high values in the endoplasmic reticulum cisternae and in the matrix of mitochondria. Voltage-induced influx of Ca(2+) within synaptic terminals appears therefore to induce an extremely rapid uptake into selected organelles. The possible physiological role of this response is discussed.

Total calcium ultrastructure: advances in excitable cells

This is a review which is written on the basis of a cell calcium lecture delivered on 22 July 2000 at the European Research Meeting 'Calcium as a molecule of cellular integration'.

Direct visualization of sarcoplasmic reticulum regions discharging Ca(2+)sparks in vascular myocytes

Localized Ca(2+)-release events, Ca(2+)sparks, have been suggested to be the 'elementary building blocks' of the calcium signalling system in all types of muscles. In striated muscles these occur at regular intervals along the fibre corresponding to the sarcomeric structures which do not exist in smooth muscle. We showed previously that in visceral and vascular myocytes Ca(2+)sparks occurred much more frequently at certain sites (frequent discharge sites [FDSs]). In this paper, we have related the position of FDSs to the distribution of the sarcoplasmic reticulum in the same living myocyte. The three-dimensional distribution of the SR in freshly isolated rabbit portal vein myocytes was visualized by means of high-resolution confocal imaging after staining with DiOC(6)and/or BODIPY TR-X ryanodine. Both fluorochromes revealed a similar staining pattern indicating a helical arrangement of well-developed superficial SR which occupied about 6% of the cell volume. Computing the frequency of spontaneous Ca(2+)sparks detected by means of fluo-4 fluorescence revealed that in about 70% of myocytes there was only one major FDS located on a prominent portion of superficial SR network usually within 1-2 microm of the nuclear envelope, although a few sparks occurred at other sites scattered generally in superficial locations throughout the cell. Polarized mitochondria were readily identified by accumulation of tetramethylrhodamine ethyl ester (TMRE). These were closely associated with the SR network in extra-nuclear regions. TMRE staining, however, failed to reveal any mitochondria near the FDS-related SR element. When observed, propagating [Ca(2+)](i)waves and associated myocyte contractions were initiated at FDSs. This study provide first insight into the three-dimensional arrangement of the SR in living smooth muscle cells and relates the peculiarity of the structural organization of the myocyte to the features of Ca(2+)signalling at subcellular level.

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

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

Mitochondria and calcium: from cell signalling to cell death

While a pathway for Ca2+ accumulation into mitochondria has long been established, its functional significance is only now becoming clear in relation to cell physiology and pathophysiology. The observation that mitochondria take up Ca2+ during physiological Ca2+ signalling in a variety of cell types leads to four questions: (i) 'What is the impact of mitochondrial Ca2+ uptake on mitochondrial function?' (ii) 'What is the impact of mitochondrial Ca2+ uptake on Ca2+ signalling?' (iii) 'What are the consequences of impaired mitochondrial Ca2+ uptake for cell function?' and finally (iv) 'What are the consequences of pathological [Ca2+]c signalling for mitochondrial function?' These will be addressed in turn. Thus: (i) accumulation of Ca2+ into mitochondria regulates mitochondrial metabolism and causes a transient depolarisation of mitochondrial membrane potential. (ii) Mitochondria may act as a spatial Ca2+ buffer in many cells, regulating the local Ca2+ concentration in cellular microdomains. This process regulates processes dependent on local cytoplasmic Ca2+ concentration ([Ca2+]c), particularly the flux of Ca2+ through IP3-gated channels of the endoplasmic reticulum (ER) and the channels mediating capacitative Ca2+ influx through the plasma membrane. Consequently, mitochondrial Ca2+ uptake plays a substantial role in shaping [Ca2+]c signals in many cell types. (iii) Impaired mitochondrial Ca2+ uptake alters the spatiotemporal characteristics of cellular [Ca2+]c signalling and downregulates mitochondrial metabolism. (iv) Under pathological conditions of cellular [Ca2+]c overload, particularly in association with oxidative stress, mitochondrial Ca2+ uptake may trigger pathological states that lead to cell death. In the model of glutamate excitotoxicity, microdomains of [Ca2+]c are apparently central, as the pathway to cell death seems to require the local activation of neuronal nitric oxide synthase (nNOS), itself held by scaffolding proteins in close association with the NMDA receptor. Mitochondrial Ca2+ uptake in combination with NO production triggers the collapse of mitochondrial membrane potential, culminating in delayed cell death.

Mitochondrial calcium transport: mechanisms and functions

Ca(2+)transport across the mitochondrial inner membrane is facilitated by transporters having four distinct sets of characteristics as well as through the Ca(2+)-induced mitochondrial permeability transition pore (PTP). There are two modes of inward transport, referred to as the Ca(2+)uniporter and the rapid mode or RaM. There are also two distinct mechanisms mediating outward transport, which are not associated with the PTP, referred to as the Na(+)-dependent and the Na(+)-independent Ca(2+)efflux mechanisms. Several important functions have been proposed for these mechanisms, including control of the metabolic rate for cellular energy (ATP) production, modulation of the amplitude and shape of cytosolic Ca(2+)transients, and induction of apoptosis through release of cytochrome c from the mitochondrial inter membrane space into the cytosolic space. The goals of this review are to survey the literature describing the characteristics of the mechanisms of mitochondrial Ca(2+)transport and their proposed physiological functions, emphasizing the more recent contributions, and to consider how the observed characteristics of the mitochondrial Ca(2+)transport mechanisms affect our understanding of their functions.