Subcellular imaging of intramitochondrial Ca2+ with recombinant targeted aequorin: significance for the regulation of pyruvate dehydrogenase activity

Heterogeneity of mitochondrial matrix free ca2+: resolution of Ca2+ dynamics in individual mitochondria in situ

The role of mitochondria in Ca2+ homeostasis is controversial. We employed the Ca2+-sensitive dye rhod 2 with novel, high temporal and spatial resolution imaging to evaluate changes in the matrix free Ca2+ concentration of individual mitochondria ([Ca2+]m) in agonist-stimulated, primary cultured aortic myocytes. Stimulation with 10 microM serotonin (5-HT) evoked modest cytosolic Ca2+ transients [cytosolic free Ca2+ concentration ([Ca2+]cyt) <500 nM; measured with fura 2] and triggered contractions in short-term cultured myocytes. However, 5-HT triggered a large mitochondrial rhod 2 signal (indicating pronounced elevation of [Ca2+]m) in only 4% of cells. This revealed heterogeneity in the responses of individual mitochondria, all of which stained with MitoTracker Green FM. In contrast, stimulation with 100 microM ATP evoked large cytosolic Ca2+ transients (>1,000 nM) and induced pronounced, reversible elevation of [Ca2+]m (measured as rhod 2 fluorescence) in 60% of cells. This mitochondrial Ca2+ uptake usually lagged behind the cytosolic Ca2+ transient peak by 3-5 s, and [Ca2+]m declined more slowly than did bulk [Ca2+]cyt. The uptake delay may prevent mitochondria from interfering with rapid signaling events while enhancing the mitochondrial response to large, long-duration elevations of [Ca2+]cyt. The responses of arterial myocytes to modest physiological stimulation do not, however, depend on such marked changes in [Ca2+]m.

Mitochondrial regulation of the cytosolic Ca2+ concentration and the InsP3-sensitive Ca2+ store in guinea-pig colonic smooth muscle

1. Mitochondrial regulation of the cytosolic Ca2+ concentration ([Ca2+]c) in guinea-pig single colonic myocytes has been examined, using whole-cell recording, flash photolysis of caged InsP3 and microfluorimetry. 2. Depolarization increased [Ca2+]c and triggered contraction. Resting [Ca2+]c was virtually restored some 4 s after the end of depolarization, a time when the muscle had shortened to 50 % of its fully relaxed length. The muscle then slowly relaxed (t = 17 s). 3. The decline in the Ca2+ transient was monophasic but often undershot or overshot resting levels, depending on resting [Ca2+]c. The extent of the overshoot or undershoot increased with increasing peak [Ca2+]c. 4. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP; 5 microM), which dissipates the mitochondrial proton electrochemical gradient and therefore prevents mitochondrial Ca2+ accumulation, slowed Ca2+ removal at high ( > 300 nM) but not at lower [Ca2+]c and abolished [Ca2+]c overshoots. Oligomycin B (5 microM), which prevents mitchondrial ATP production, affected neither the rate of decline nor the magnitude of the overshoot. 5. During depolarization, the global rhod-2 signal (which represents the mitochondrial matrix Ca2+ concentration, [Ca2+]m) rose slowly in a CCCP-sensitive manner during and for about 3 s after depolarization had ended. [Ca2+]m then slowly decreased over tens of seconds. 6. Inhibition of sarcoplasmic reticulum Ca2+ uptake with thapsigargin (100 nM) reduced the undershoot and increased the overshoot. 7. Flash photolysis of caged InsP3 (20 microM) evoked reproducible increases in [Ca2+]c. CCCP (5 microM) reduced the magnitude of the [Ca2+]c transients evoked by flash photolysis of caged InsP3. Oligomycin B (5 microM) did not reduce the inhibition of the InsP3-induced Ca2+ transient by CCCP thus minimizing the possibility that CCCP lowered ATP levels by reversing the mitochondrial ATP synthase and so reducing SR Ca2+ refilling. 8. While CCCP reduced the magnitude of the InsP3-evoked Ca2+ signal, the internal Ca2+ store content, as assessed by the magnitude of ionomycin-evoked Ca2+ release, did not decrease significantly. 9. [Ca2+]c decline in smooth muscle, following depolarization, may involve mitochondrial Ca2+ uptake. Following InsP3-evoked Ca2+ release, mitochondrial uptake of Ca2+ may regulate the local [Ca2+]c near the InsP3 receptor so maintaining the sensitivity of the InsP3 receptor to release Ca2+ from the SR.

Release of Ca2+ from the sarcoplasmic reticulum increases mitochondrial [Ca2+] in rat pulmonary artery smooth muscle cells

1. The Ca2+-sensitive fluorescent indicator rhod-2 was used to measure mitochondrial [Ca2+] ([Ca2+]m) in single smooth muscle cells from the rat pulmonary artery, while simultaneously monitoring cytosolic [Ca2+] ([Ca2+]i) with fura-2. 2. Application of caffeine produced an increase in [Ca2+]i and also increased [Ca2+]m. The increase in [Ca2+]m occurred after the increase in [Ca2+]i, and remained elevated for a considerable time after [Ca2+]i had returned to resting values. 3. The protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), which causes the mitochondrial membrane potential to collapse, markedly attenuated the increase in [Ca2+]m following caffeine application and also increased the half-time for recovery of [Ca2+]i to resting values. 4. Activation of purinoceptors with ATP also produced increases in both [Ca2+]i and [Ca2+]m in these smooth muscle cells. In some cells, oscillations in [Ca2+]i were observed during ATP application, which produced corresponding oscillations in [Ca2+]m and membrane currents. 5. This study provides direct evidence that Ca2+ release from the sarcoplasmic reticulum, either through ryanodine or inositol 1,4, 5-trisphosphate (InsP3) receptors, increases both cytosolic and mitochondrial [Ca2+] in smooth muscle cells. These results have potential implications both for the role of mitochondria in Ca2+ regulation in smooth muscle, and for understanding how cellular metabolism is regulated.

Reversible Ca gradients between the subplasmalemma and cytosol differentially activate Ca-dependent Cl currents

Xenopus oocytes express several different Ca-activated Cl currents that have different waveforms and biophysical properties. We compared the stimulation of Ca-activated Cl currents measured by two-microelectrode voltage clamp with the Ca transients measured in the same cell by confocal microscopy and Ca-sensitive fluorophores. The purpose was to determine how the amplitude and/or spatio-temporal features of the Ca signal might explain how these different Cl currents were activated by Ca. Because Ca release from stores was voltage independent, whereas Ca influx depended upon the electrochemical driving force, we were able to separately assess the contribution of Ca from these two sources. We were surprised to find that Ca signals measured with a cytosolic Ca-sensitive dye, dextran-conjugated Ca-green-1, correlated poorly with Cl currents. This suggested that Cl channels located at the plasma membrane and the Ca-sensitive dye located in the bulk cytosol were sensing different [Ca]. This was true despite Ca measurement in a confocal slice very close to the plasma membrane. In contrast, a membrane-targeted Ca-sensitive dye (Ca-green-C18) reported a Ca signal that correlated much more closely with the Cl currents. We hypothesize that very local, transient, reversible Ca gradients develop between the subplasmalemmal space and the bulk cytosol. [Ca] is higher near the plasma membrane when Ca is provided by Ca influx, whereas the gradient is reversed when Ca is released from stores, because Ca efflux across the plasma membrane is faster than diffusion of Ca from the bulk cytosol to the subplasmalemmal space. Because dissipation of the gradients is accelerated by inhibition of Ca sequestration into the endoplasmic reticulum with thapsigargin, we conclude that [Ca] in the bulk cytosol declines slowly partly due to futile recycling of Ca through the endoplasmic reticulum.

Calcium signaling and cytotoxicity

The divalent calcium cation Ca(2+) is used as a major signaling molecule during cell signal transduction to regulate energy output, cellular metabolism, and phenotype. The basis to the signaling role of Ca(2+) is an intricate network of cellular channels and transporters that allow a low resting concentration of Ca(2+) in the cytosol of the cell ([Ca(2+)]i) but that are also coupled to major dynamic and rapidly exchanging stores. This enables extracellular signals from hormones and growth factors to be transduced as [Ca(2+)]i spikes that are amplitude and frequency encoded. There is considerable evidence that a number of toxic environmental chemicals target these Ca(2+) signaling processes, alter them, and induce cell death by apoptosis. Two major pathways for apoptosis will be considered. The first one involves Ca(2+)-mediated expression of ligands that bind to and activate death receptors such as CD95 (Fas, APO-1). In the second pathway, Ca(2+) has a direct toxic effect and its primary targets include the mitochondria and the endoplasmic reticulum (ER). Mitochondria may respond to an apoptotic Ca(2+) signal by the selective release of cytochrome c or through enhanced production of reactive oxygen species and opening of an inner mitochondrial membrane pore. Toxic agents such as the environmental pollutant tributyltin or the natural plant product thapsigargin, which deplete the ER Ca(2+) stores, will induce as a direct result of this effect the opening of plasma membrane Ca(2+) channels and an ER stress response. In contrast, under some conditions, Ca(2+) signals may be cytoprotective and antagonize the apoptotic machinery.

Transient and long-lasting openings of the mitochondrial permeability transition pore can be monitored directly in intact cells by changes in mitochondrial calcein fluorescence

The occurrence and the mode of opening of the mitochondrial permeability transition pore (MTP) were investigated directly in intact cells by monitoring the fluorescence of mitochondrial entrapped calcein. When MH1C1 cells and hepatocytes were loaded with calcein AM, calcein was also present within mitochondria, because (i) its mitochondrial signal was quenched by the addition of tetramethylrhodamine methyl ester and (ii) calcein-loaded mitochondria could be visualized after digitonin permeabilization. Under the latter condition, the addition of Ca2+ induced a prompt and massive release of the accumulated calcein, which was prevented by CsA, indicating that calcein release could, in principle, probe MTP opening in intact cells as well. To study this process, we developed a procedure by which the cytosolic calcein signal was quenched by Co2+. In hepatocytes and MH1C1 cells coloaded with Co2+ and calcein AM, treatment with MTP inducers caused a rapid, though limited, decrease in mitochondrial calcein fluorescence, which was significantly reduced by CsA. We also observed a constant and spontaneous decrease in mitochondrial calcein fluorescence, which was completely prevented by CsA. Thus MTP likely fluctuates rapidly between open and closed states in intact cells.

Quasi-synaptic calcium signal transmission between endoplasmic reticulum and mitochondria

Transmission of cytosolic [Ca2+] ([Ca2+]c) oscillations into the mitochondrial matrix is thought to be supported by local calcium control between IP3 receptor Ca2+ channels (IP3R) and mitochondria, but study of the coupling mechanisms has been difficult. We established a permeabilized cell model in which the Ca2+ coupling between endoplasmic reticulum (ER) and mitochondria is retained, and mitochondrial [Ca2+] ([Ca2+]m) can be monitored by fluorescence imaging. We demonstrate that maximal activation of mitochondrial Ca2+ uptake is evoked by IP3-induced perimitochondrial [Ca2+] elevations, which appear to reach values >20-fold higher than the global increases of [Ca2+]c. Incremental doses of IP3 elicited [Ca2+]m elevations that followed the quantal pattern of Ca2+ mobilization, even at the level of individual mitochondria. In contrast, gradual increases of IP3 evoked relatively small [Ca2+]m responses despite eliciting similar [Ca2+]c increases. We conclude that each mitochondrial Ca2+ uptake site faces multiple IP3R, a concurrent activation of which is required for optimal activation of mitochondrial Ca2+ uptake. This architecture explains why calcium oscillations evoked by synchronized periodic activation of IP3R are particularly effective in establishing dynamic control over mitochondrial metabolism. Furthermore, our data reveal fundamental functional similarities between ER-mitochondrial Ca2+ coupling and synaptic transmission.

Simultaneous measurements of cytosolic and mitochondrial Ca2+ transients in HT29 cells

Loading of HT29 cells with the Ca2+ dye fura-2/AM resulted in an nonhomogeneous intracellular distribution of the dye. Cellular compartments with high fura-2 concentrations were identified by correlation with mitochondrial markers, cellular autofluorescence induced by UV, and dynamic measurement of autofluorescence after inhibition of oxidative phosphorylation. Stimulation with carbachol (10(-4) mol/liter) increased cytosolic, nuclear, and mitochondrial Ca2+ activity ([Ca2+]c, [Ca2+]n, and [Ca2+]m, respectively) measured by UV confocal and conventional imaging. Similar results were obtained with a prototype two-photon microscope (Zeiss, Jena, Germany) allowing for fura-2 excitation. The increase of [Ca2+]m lagged behind that of [Ca2+]c and [Ca2+]n by 10-20 s, and after removing the agonist, [Ca2+]m also decreased with a delay. A strong increase of [Ca2+]m occurred only when a certain threshold of [Ca2+]c (around 1 micromol/liter) was exceeded. In a very similar way, ATP, neurotensin, and thapsigargin increased [Ca2+]c and [Ca2+]m. Carbonyl cyanide p-trifluoromethoxyphenylhyrdrazone reversibly reduced the increase of [Ca2+]m. The source of the mitochondrial Ca2+ increase had intra- and extracellular components, as revealed by experiments in low extracellular Ca2+. We conclude that agonist-induced Ca2+ signals are transduced into mitochondria. 1) Mitochondria could serve as a Ca2+ sink, 2) mitochondria could allow the modulation of [Ca2+]c and [Ca2+]n signals, and 3) [Ca2+]m may serve as a stimulatory metabolic signal when a cell is highly stimulated.

Relations between intracellular Ca2+ stores and store-operated Ca2+ entry in primary cultured human glioblastoma cells

1. In primary cultured human glioblastoma cells extracellular application of ATP triggered elevation in cytoplasmic calcium concentration ([Ca2+]i) mediated entirely by generation of inositol 1,4,5-trisphosphate (InsP3)-dependent Ca2+ release from endoplasmic reticulum Ca2+ stores followed by the activation of store-operated Ca2+ entry into the cells. 2. Sensitivity of P2Y purinoceptors to extracellular ATP was regulated by extracellular Ca2+: in Ca2+-free extracellular solution the threshold concentration of ATP that induced an increase in [Ca2+]i was reduced by one order of magnitude. 3. Activation of Ca2+ release and store-operated Ca2+ entry was dissociated: low concentrations of ATP induced substantial Ca2+ release without activation of Ca2+ entry; activation of the latter required higher ATP concentrations. 4. Mitochondria participated in buffering Ca2+ loads that resulted from store-operated Ca2+ influx; in contrast Ca2+ released from intracellular stores was not accumulated by the mitochondrial depot. 5. We conclude that ATP-induced Ca2+ responses are governed by several pathways with different sensitivities to the agonist. This enables cells to respond either with pure Ca2+ release from intracellular stores (at low ATP concentrations) or (at high ATP concentrations) the response is amplified by plasmalemmal Ca2+ influx. Store-operated Ca2+ entry increases mitochondrial Ca2+ content providing a link between cellular activation and mitochondrial function.

Mitochondrial contribution to the anoxic Ca2+ signal in maize suspension-cultured cells

Anoxia induces a rapid elevation of the cytosolic Ca2+ concentration ([Ca2+]cyt) in maize (Zea mays L.) cells, which is caused by the release of the ion from intracellular stores. This anoxic Ca2+ release is important for gene activation and survival in O2-deprived maize seedlings and cells. In this study we examined the contribution of mitochondrial Ca2+ to the anoxic [Ca2+]cyt elevation in maize cells. Imaging of intramitochondrial Ca2+ levels showed that a majority of mitochondria released their Ca2+ in response to anoxia and took up Ca2+ upon reoxygenation. We also investigated whether the mitochondrial Ca2+ release contributed to the increase in [Ca2+]cyt under anoxia. Analysis of the spatial association between anoxic [Ca2+]cyt changes and the distribution of mitochondrial and other intracellular Ca2+ stores revealed that the largest [Ca2+]cyt increases occurred close to mitochondria and away from the tonoplast. In addition, carbonylcyanide p-trifluoromethoxyphenyl hydrazone treatment depolarized mitochondria and caused a mild elevation of [Ca2+]cyt under aerobic conditions but prevented a [Ca2+]cyt increase in response to a subsequent anoxic pulse. These results suggest that mitochondria play an important role in the anoxic elevation of [Ca2+]cyt and participate in the signaling of O2 deprivation.

Ca2+ homeostasis in the agonist-sensitive internal store: functional interactions between mitochondria and the ER measured In situ in intact cells

Mitochondria have a well-established capacity to detect cytoplasmic Ca2+ signals resulting from the discharge of ER Ca2+ stores. Conversely, both the buffering of released Ca2+ and ATP production by mitochondria are predicted to influence ER Ca2+ handling, but this complex exchange has been difficult to assess in situ using conventional measurement techniques. Here we have examined this interaction in single intact BHK-21 cells by monitoring intraluminal ER [Ca2+] directly using trapped fluorescent low-affinity Ca2+ indicators. Treatment with mitochondrial inhibitors (FCCP, antimycin A, oligomycin, and rotenone) dramatically prolonged the refilling of stores after release with bradykinin. This effect was largely due to inhibition of Ca2+ entry pathways at the plasma membrane, but a significant component appears to arise from reduction of SERCA-mediated Ca2+ uptake, possibly as a consequence of ATP depletions in a localized subcellular domain. The rate of bradykinin-induced Ca2+ release was reduced to 51% of control by FCCP. This effect was largely overcome by loading cells with BAPTA-AM, highlighting the importance of mitochondrial Ca2+ buffering in shaping the release kinetics. However, mitochondria-specific ATP production was also a significant determinant of the release dynamic. Our data emphasize the localized nature of the interaction between these organelles, and show that competent mitochondria are essential for generating explosive Ca2+ signals.

Glucose-dependent translocation of insulin promoter factor-1 (IPF-1) between the nuclear periphery and the nucleoplasm of single MIN6 beta-cells

Using laser-scanning confocal microscopy, we have monitored glucose-induced changes in the subcellular localization of insulin promoter factor-1 (IPF-1) labeled with a c-myc epitope tag. This construct trans-activated the insulin promoter in single living MIN6-beta-cells as assessed by luciferase-based promoter analysis. IPF-1.c-myc expression also enhanced the response of the insulin promoter to elevations in extracellular glucose concentration. In the majority (148/235, 63%) of cells maintained at low (3 mM) extracellular glucose concentration, IPF-1.c-myc immunoreactivity was confined to the nuclear periphery. Incubation of cells at stimulatory (30 mM) glucose concentrations caused a rapid redistribution of the chimera to the nucleoplasm (775/958, 81% of cells). By contrast, the irrelevant transcription factor c-Fos, tagged with either c-myc or as a chimera with luciferase, was localized exclusively to the nucleoplasm irrespective of the glucose concentration. Furthermore, IPF-1 extended with the bulky (27 kDa) enhanced green fluorescent protein (EGFP) group was confined largely to the nucleoplasm at all glucose concentrations tested and did not support trans-activation of the insulin promoter by glucose. Movement of endogenous IPF-1 from the nuclear periphery to the nucleoplasm may therefore increase the trans-activational capacity of this factor in native beta-cells exposed to high extracellular glucose concentrations.

Integrating cytosolic calcium signals into mitochondrial metabolic responses

Stimulation of hepatocytes with vasopressin evokes increases in cytosolic free Ca2+ ([Ca2+]c) that are relayed into the mitochondria, where the resulting mitochondrial Ca2+ ([Ca2+]m) increase regulates intramitochondrial Ca2+-sensitive targets. To understand how mitochondria integrate the [Ca2+]c signals into a final metabolic response, we stimulated hepatocytes with high vasopressin doses that generate a sustained increase in [Ca2+]c. This elicited a synchronous, single spike of [Ca2+]m and consequent NAD(P)H formation, which could be related to changes in the activity state of pyruvate dehydrogenase (PDH) measured in parallel. The vasopressin-induced [Ca2+]m spike evoked a transient increase in NAD(P)H that persisted longer than the [Ca2+]m increase. In contrast, PDH activity increased biphasically, with an initial rapid phase accompanying the rise in [Ca2+]m, followed by a sustained secondary activation phase associated with a decline in cellular ATP. The decline of NAD(P)H in the face of elevated PDH activity occurred as a result of respiratory chain activation, which was also manifest in a calcium-dependent increase in the membrane potential and pH gradient components of the proton motive force (PMF). This is the first direct demonstration that Ca2+-mobilizing hormones increase the PMF in intact cells. Thus, Ca2+ plays an important role in signal transduction from cytosol to mitochondria, with a single [Ca2+]m spike evoking a complex series of changes to activate mitochondrial oxidative metabolism.

Desensitization of mitochondrial Ca2+ and insulin secretion responses in the beta cell

The role of mitochondria in the desensitization of insulin secretion was investigated. In rat pancreatic beta cells, both insulin secretion and mitochondrial [Ca2+] increases were desensitized following two challenges with the mitochondrial substrate methyl succinate. In the beta cell line INS-1, similar results were observed when a 5-min interval separated two 5-min pulses. In contrast, ATP generation monitored in luciferase-expressing INS-1 cells was stimulated to the same extent during both exposures to methyl succinate. Succinate, like alpha-glycerophosphate, activates the electron transport chain at complex II. As a consequence, the mitochondrial membrane hyperpolarizes, promoting ATP synthesis and Ca2+ influx into the mitochondria through the uniporter. The mitochondrial desensitization was further studied in permeabilized INS-1 cells. Increasing extramitochondrial [Ca2+] from 100 to 500 nM enhanced succinate oxidation 4-fold. At 500 nM Ca2+, 1 mM succinate caused a blunted mitochondrial [Ca2+] increase upon the second, compared with the first, stimulation. These effects were mimicked by alpha-glycerophosphate, and there was cross-desensitization between the two compounds. Succinate hyperpolarized the mitochondrial membrane during both the first and second applications. This suggests that the uniporter itself, rather than the respiratory chain, is desensitized. These results emphasize the key role of the mitochondria not only in the stimulation of insulin secretion, but also in its desensitization.

From calcium signaling to cell death: two conformations for the mitochondrial permeability transition pore. Switching from low- to high-conductance state

The permeability transition pore (PTP) is a channel of the inner mitochondrial membrane that appears to operate at the crossroads of two distinct physiological pathways, i.e., the Ca2+ signaling network during the life of the cell, and the effector phase of the apoptotic cascade during Ca2+-dependent cell death. Correspondingly, two open conformations of the PTP can also be observed in isolated organelles. A low-conductance state, that allows the diffusion of small ions like Ca2+, is pH-operated, promoting spontaneous closure of the channel. A high-conductance state, that allows the unselective diffusion of big molecules, stabilizes the channel in the open conformation, disrupting in turn the mitochondrial structure and causing the release of proapoptotic factors. Our current results indicate that switching from low- to high-conductance state is an irreversible process that is strictly dependent on the saturation of the internal Ca2+-binding sites of the PTP. Thus, the high-conductance state of the PTP, which was shown to play a pivotal role in the course of excitotoxic and thapsigargin-induced cell death, might result from a Ca2+-dependent conformational shift of the low-conductance state, normally participating in the regulation of cellular Ca2+ homeostasis as a pH-operated channel. These observations lead us to propose a simple biophysical model of the transition between Ca2+ signaling and Ca2+-dependent apoptosis.

The Ca2+ transport mechanisms of mitochondria and Ca2+ uptake from physiological-type Ca2+ transients

Mitochondria contain a sophisticated system for transporting Ca2+. The existence of a uniporter and of both Na+-dependent and -independent efflux mechanisms has been known for years. Recently, a new mechanism, called the RaM, which seems adapted for sequestering Ca2+ from physiological transients or pulses has been discovered. The RaM shows a conductivity at the beginning of a Ca2+ pulse that is much higher than the conductivity of the uniporter. This conductivity decreases very rapidly following the increase in [Ca2+] outside the mitochondria. This decrease in the Ca2+ conductivity of the RaM is associated with binding of Ca2+ to an external regulatory site. When liver mitochondria are exposed to a sequence of pulses, uptake of labeled Ca2+ via the RaM appears additive between pulses. Ruthenium red inhibits the RaM in liver mitochondria but much larger amounts are required than for inhibition of the mitochondrial Ca2+ uniporter. Spermine, ATP and GTP increase Ca2+ uptake via the RaM. Maximum uptake via the RaM from a single Ca2+ pulse in the physiological range has been observed to be approximately 7 nmole/mg protein, suggesting that Ca2+ uptake via the RaM and uniporter from physiological pulses may be sufficient to activate the Ca2+-sensitive metabolic reactions in the mitochondrial matrix which increase the rate of ATP production. RaM-mediated Ca2+ uptake has also been observed in heart mitochondria. Evidence for Ca2+ uptake into the mitochondria in a variety of tissues described in the literature is reviewed for evidence of participation of the RaM in this uptake. Possible ways in which the differences in transport via the RaM and the uniporter may be used to differentiate between metabolic and apoptotic signaling are discussed.

Coupling between cytosolic and mitochondrial calcium oscillations: role in the regulation of hepatic metabolism

Mitochondria are strategically localized at sites of Ca2+ release, such that increases in cytosolic free Ca2+ ([Ca2+]c) from either internal Ca2+ stores or Ca2+ influx across the plasma membrane can be rapidly transported into the mitochondrial matrix. The consequent elevation in mitochondrial Ca2+ ([Ca2+]m) stimulates the Ca2+-sensitive intramitochondrial dehydrogenases, resulting in elevation of NAD(P)H. The preferential coupling between increases in [Ca2+]c and [Ca2+]m is one proposed mechanism to coordinate mitochondrial ATP production with cellular energy demand. In liver cells, hormones that act through the second messenger inositol 1,4, 5-trisphosphate (IP3) generate oscillatory [Ca2+]c signals, which result from a periodic Ca2+- and IP3-mediated activation/deactivation of intracellular Ca2+ release channels. The [Ca2+]c spiking frequency increases with agonist dose, whereas the amplitude of each [Ca2+]c spike is constant. This frequency modulation of [Ca2+]c spiking encodes the signal from the extracellular agonist, which is then decoded by the internal Ca2+-sensitive proteins such as the Ca2+-sensitive intramitochondrial dehydrogenases. Our studies have investigated the relationship between IP3-dependent [Ca2+]c signals and [Ca2+]m in primary cultured hepatocytes. In addition, the changes in cellular [Ca2+] levels have been correlated with the regulation of intramitochondrial NAD(P)H levels, pyruvate dehydrogenase activity and the magnitude of the mitochondrial proton motive force.

Intramitochondrial [Ca2+] and membrane potential in ventricular myocytes exposed to anoxia-reoxygenation

The aim of this study was to investigate the role of mitochondrial ionic homeostasis in promoting reoxygenation-induced hypercontracture in cardiac muscle. Mitochondrial membrane potential and intramitochondrial Ca2+ concentration ([Ca2+]) were measured using confocal imaging in guinea pig ventricular myocytes exposed to anoxia and reoxygenation. Anoxia produced a variable, but often profound, mitochondrial depolarization. Some cells mounted a recovery of their mitochondrial membrane potential during reoxygenation; the depolarization was sustained in other cells. Recovery of the mitochondrial membrane potential seemed essential to avoid reoxygenation-induced hypercontracture. Reoxygenation also caused a sizable elevation in intramitochondrial [Ca2+], the amplitude of which was correlated with the likelihood of a cell undergoing hypercontracture. A sustained Ca2+ load analogous to that seen during reoxygenation was imposed on cardiac mitochondria through permeabilization of the plasma membrane. Elevation of intracellular [Ca2+] to 800 nM caused a substantial mitochondrial depolarization. We propose that the conditions seen in guinea pig ventricular myocytes during reoxygenation are well suited to produce Ca2+-dependent mitochondrial depolarization, which may play a significant role in promoting irreversible cell injury.

Secretory-granule dynamics visualized in vivo with a phogrin-green fluorescent protein chimaera

To image the behaviour in real time of single secretory granules in neuroendocrine cells we have expressed cDNA encoding a fusion construct between the dense-core secretory-granule-membrane glycoprotein, phogrin (phosphatase on the granule of insulinoma cells), and enhanced green fluorescent protein (EGFP). Expressed in INS-1 beta-cells and pheochromocytoma PC12 cells, the chimaera was localized efficiently (up to 95%) to dense-core secretory granules (diameter 200-1000 nm), identified by co-immunolocalization with anti-(pro-)insulin antibodies in INS-1 cells and dopamine beta-hydroxylase in PC12 cells. Using laser-scanning confocal microscopy and digital image analysis, we have used this chimaera to monitor the effects of secretagogues on the dynamics of secretory granules in single living cells. In unstimulated INS-1 beta-cells, granule movement was confined to oscillatory movement (dithering) with period of oscillation 5-10 s and mean displacement <1 microm. Both elevated glucose concentrations (30 mM), and depolarization of the plasma membrane with K+, provoked large (5-10 microm) saltatory excursions of granules across the cell, which were never observed in cells maintained at low glucose concentration. By contrast, long excursions of granules occurred in PC12 cells without stimulation, and occurred predominantly from the cell body towards the cell periphery and neurite extensions. Purinergic-receptor activation with ATP provoked granule movement towards the membrane of PC12 cells, resulting in the transfer of fluorescence to the plasma membrane consistent with fusion of the granule and diffusion of the chimaera in the plasma membrane. These results illustrate the potential use of phogrin-EGFP chimeras in the study of secretory-granule dynamics, the regulation of granule-cytoskeletal interactions and the trafficking of a granule-specific transmembrane protein during the cycle of exocytosis and endocytosis.

Role of mitochondrial calcium transport in the control of substrate oxidation

This paper reviews the model of the control of mitochondrial substrate oxidation by Ca2+ ions. The mechanism is the activation by Ca2+ of four mitochondrial dehydrogenases, viz. glycerol 3-phosphate dehydrogenase, the pyruvate dehydrogenase multienzyme complex (PDH), NAD-linked isocitrate dehydrogenase (NAD-IDH) and 2-oxoglutarate dehydrogenase (OGDH). This results in the increase, or near-maintenance, of mitochondrial NADH/NAD ratios in the activated state, depending upon the tissue and the degree of 'downstream' activation by Ca2+, likely at the level of the F1Fo ATPase. Higher values of the redox span of the respiratory chain allow for greatly increased fluxes through oxidative phosphorylation with a minimal drop in protonmotive force and phosphorylation potential. As PDH, NAD-IDH and OGDH are all located within the inner mitochondrial membrane, it is changes in matrix free Ca2+ [Ca2+]m which act as a signal to these activities. In this article, we review recent work in which [Ca2+]m is measured in cells and tissues, using different techniques, with special emphasis on the question of the degree of damping of [Ca2+]m relative to changes in cytosol free Ca2+ in cells with rapid transients in cytosol Ca2+, e.g. cardiac myocytes. Further, we put forward the point of view that the failure of mitochondrial energy transduction to keep pace with cellular energy needs in some forms of heart failure may involve a failure of [Ca2+]m to be raised adequately to allow the activation of the dehydrogenases. We present new data to show that this is so in cardiac myocytes isolated from animals suffering from chronic, streptozocin-induced diabetes. This raises the possibility of therapy based upon partial inhibition of mitochondrial Ca2+ efflux pathways, thereby raising [Ca2+]m at a given, time-average value of cytosol free Ca+2.

Rapid determination of gap junction formation using HeLa cells microinjected with cDNAs encoding wild-type and chimeric connexins

A procedure for rapidly determining the functionality of gap junctions constructed of recombinant connexins in communication-deficient HeLa cells is described. Nuclear microinjection of cDNA encoding wild-type connexins (Cx) 26, 32, 43, and a range of connexin-aequorin (Cx-Aeq) chimerase resulted in generation of gap junction intercellular communication channels. Expression of recombinant protein was detected in > 95% of cells 18-72 h following nuclear microinjection, and the functionality of the channels generated was determined according to their ability to transfer the fluorescent dye tracers Lucifer yellow and propidium iodide. The dye transfer results obtained correlated closely with other published studies using stably transfected cells and yet are obtained as rapidly as 18 h following microinjection of cDNA. Expression of a truncated form of Cx43 (Cx43 delta 244) by this new method indicated diminished intercellular transfer of both dyes and supports a channel-gating mechanism that postulates interaction between the carboxyl tail and the intracellular loop.

Close contacts with the endoplasmic reticulum as determinants of mitochondrial Ca2+ responses

The spatial relation between mitochondria and endoplasmic reticulum (ER) in living HeLa cells was analyzed at high resolution in three dimensions with two differently colored, specifically targeted green fluorescent proteins. Numerous close contacts were observed between these organelles, and mitochondria in situ formed a largely interconnected, dynamic network. A Ca2+-sensitive photoprotein targeted to the outer face of the inner mitochondrial membrane showed that, upon opening of the inositol 1,4,5-triphosphate (IP3)-gated channels of the ER, the mitochondrial surface was exposed to a higher concentration of Ca2+ than was the bulk cytosol. These results emphasize the importance of cell architecture and the distribution of organelles in regulation of Ca2+ signaling.

Aequorea victoria bioluminescence moves into an exciting new era

Bioluminescence has revolutionized research into many cellular and molecular-biological processes, ranging from intracellular signalling to gene transcription. This article focuses on the chemistry and biotechnological exploitation of the two proteins involved in bioluminescence of the jellyfish Aequorea victoria--aequorin and green fluorescent protein. Engineered recombinant aequorin has led to a novel technological approach to monitoring calcium signals in organelles and subcellular domains. A new generation of intracellular calcium indicators has been produced in which engineered variants of green fluorescent protein are used to probe their ionic environment using intramolecular fluorescence-resonance-energy transfer.

Insulin targeting to the regulated secretory pathway after fusion with green fluorescent protein and firefly luciferase

We have prepared recombinant cDNAs encoding chimaeras between human preproinsulin (sp.B.C.A., for B-, Connecting- and A-peptides) and a thermostable mutant of green fluorescent protein (GFPS65T,V163A, GFP*). The subcellular localization of the expressed chimaeras was monitored in living insulin-secreting INS-1 beta-cells by laser scanning confocal microscopy. When GFP* was fused at the immediate N-terminus of the B-chain (sp.[GFP*].B.C.A.myc) two distinct patterns of fluorescence were apparent. In 1530/1740 cells examined, fluorescence was confined to a reticular, exclusively extranuclear structure, and closely co-localized with the endoplasmic reticulum marker, calreticulin. However, 210/1740 (12.1%) of cells displayed punctate fluorescence, which partially co-localized with the trans-Golgi network marker, TGN 38, and with the dense core secretory granule marker, phogrin. Since secretion of GFP* fluorescence into the medium could not readily be measured, we prepared a chimaera in which firefly luciferase was fused at the C-terminus of proinsulin (sp.B.C.A.myc.[Luc]). This chimaera displayed a distribution closely similar to that of sp.[GFP*].B.C.A. myc, but with a lower proportion (15/310, 4.8%) of the cells showing clear punctate distribution. At substimulatory glucose concentrations (3 mM) secretion of sp.B.C.A.myc.[Luc] could not be detected (rate of release into the medium identical with that of the cytosolic Renilla reniformis luciferase), indicating that the chimaera did not enter the constitutive secretory pathway. However, elevated (30 mM) glucose stimulated the release of the sp.B.C.A.myc. [Luc] luciferase chimaera, without a detectable effect on R. reniformis luciferase release. These data suggest that fusion of insulin, and the much larger photoproteins GFP* and luciferase, leads predominantly to misfolding and retention in the endoplasmic reticulum. However, the properly folded chimaeras are apparently still correctly targeted to the regulated, rather than the constitutive, secretory pathway. These chimaeras should therefore be valuable tools to monitor the exocytosis of insulin in real time.

A phogrin-aequorin chimaera to image free Ca2+ in the vicinity of secretory granules

Microdomains of high Ca2+ concentration ([Ca2+]) may be critical to the control of intracellular processes such as secretion and metabolism without compromising other cell functions. To explore changes in [Ca2+] in the outer mantle (< 30 nm deep) that surrounds the surface of dense-core secretory granules, we have designed a recombinant chimaera between the granule protein phogrin and aequorin. When expressed in populations of insulin-secreting MIN6 or phaeochromocytoma PC12 cells, the chimaera was targeted to secretory granules as expected. The recombinant protein reported a similar [Ca2+] at the granule surface to that in the bulk cytosol, measured with untargeted aequorin. This was the case both at rest (-Ca2+- = 80-120 nM) and after stimulation with agents that provoke Ca2+ entry or Ca2+ mobilization from intracellular pools, and during activated secretion. Thus depolarization of MIN6 cell populations with high K+ increased [Ca2+] both in the bulk cytosol and close to the granules to approx. 4 microM, with near-identical kinetics of increase and recovery. Similarly, stimulation of PC12 cells with ATP provoked an increase in -Ca2+- in either domain to 1.3 microM. These data argue that, in MIN6 and PC12 neuroendocrine cells (i) significant mobilization of Ca2+ from most secretory granules probably does not occur during activated Ca2+ influx or mobilization of internal Ca2+ stores, and (ii) agonist-stimulated Ca2+-dependent secretion can occur without development of a large gradient of [Ca2+] between the surface of most secretory vesicles and the rest of the cytosol.

Role of mitochondrial Ca2+ regulation in neuronal and glial cell signalling

It is becoming increasingly clear that mitochondrial Ca2+ uptake from and release into the cytosol has important consequences for neuronal and glial activity. Ca2+ regulates mitochondrial metabolism, and mitochondrial Ca2+ uptake and release modulate physiological and pathophysiological cytosolic responses. In glial cells, inositol 1,4,5-trisphosphate-dependent Ca2+ responses are faithfully translated into elevations in mitochondrial Ca2+ levels, which modifies cytosolic Ca2+ wave propagation and may activate mitochondrial enzymes. The location of mitochondria within neurones may partially determine their role in Ca2+ signalling. Neuronal death due to NMDA-evoked Ca2+ entry can be delayed by an inhibitor of the mitochondrial permeability transition pore, and mitochondrial dysfunction is being increasingly implicated in a number of neurodegenerative conditions. These findings are illustrative of an emerging realization by neuroscientists of the importance of mitochondrial Ca2+ regulation as a modulator of cellular energetics, endoplasmic reticulum Ca2+ release and neurotoxicity.

Mechanism of mechanically induced intercellular calcium waves in rabbit articular chondrocytes and in HIG-82 synovial cells

Intercellular communication through gap junctions allows tissue coordination of cell metabolism and sensitivity to extracellular stimuli. Intercellular Ca2+ signaling was investigated with digital fluorescence video imaging in primary cultures of articular chondrocytes and in HIG-82 synovial cells. In both cell types, mechanical stimulation of a single cell induced a wave of increased Ca2+ that was communicated to surrounding cells. Intercellular Ca2+ spreading was inhibited by 18alpha-glycyrrhetinic acid, demonstrating the involvement of gap junctions in signal propagation. In the absence of extracellular Ca2+, mechanical stimulation induced communicated Ca2+ waves similar to controls; however, the number of HIG-82 cells recruited decreased significantly. Mechanical stress induced Ca2+ influx both in the stimulated chondrocyte and HIG-82 cell, but not in the adjacent cells, as assessed by the Mn2+ quenching technique. Treatment of cells with thapsigargin and with the phospholipase C (PLC) inhibitor U73122 blocked mechanically induced signal propagation. These results provide evidence that in chondrocytes and in HIG-82 synovial cells, mechanical stimulation activates PLC, thus leading to an increase of intracellular inositol 1,4,5-trisphosphate. The second messenger, by permeating gap junctions, stimulates intracellular Ca2+ release in neighboring cells. It is concluded that intercellular Ca2+ waves may provide a mechanism to coordinate tissue responses in joint physiology.

Reduced Ca2+ uptake by mitochondria in pyruvate dehydrogenase-deficient human diploid fibroblasts

Physiological and pathological Ca2+ loads are thought to be taken up by mitochondria via a process dependent on aerobic metabolism. We sought to determine whether human diploid fibroblasts from a patient with an inherited defect in pyruvate dehydrogenase (PDH) exhibit a decreased ability to sequester cytosolic Ca2+ into mitochondria. Mobilization of Ca2+ stores with bradykinin (BK) increased the cytosolic Ca2+ concentration ([Ca2+]c) to comparable levels in control and PDH-deficient fibroblasts. In normal fibroblasts transfected with plasmid DNA encoding mitochondrion-targeted apoaequorin, BK elicited an increase in Ca2(+)-dependent aequorin luminescence corresponding to an increase in the mitochondrial Ca2+ concentration ([Ca2+]mt) of 2.0 +/- 0.2 microM. The mitochondrial uncoupling agent carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone blocked the BK-induced [Ca2+]mt increase, although it did not affect the [Ca2+]c transient. Basal [Ca2+]c and [Ca2+]mt in control and PDH-deficient cells were similar. However, confocal imaging of the potential-sensitive dye JC-1 indicated that the percentage of highly polarized mitochondria was reduced from 30 +/- 1% in normal cells to 19 +/- 2% in the PDH-deficient fibroblasts. BK-elicited [Ca2+]mt transients in PDH-deficient cells were reduced to 4% of control, indicating that PDH-deficient mitochondria have a decreased ability to take up cytosolic Ca2+. Thus cells with compromised aerobic metabolism have a reduced capacity to sequester Ca2+.

Fluorescence measurement of calcium transients in perfused rabbit heart using rhod 2

Surface fluorescence spectroscopy of the beating heart to measure cytosolic calcium has been limited by the need to use ultraviolet excitation light for many of the commonly used calcium indicators. Ultraviolet light in the heart produces a high level of background fluorescence and is highly absorbed, limiting tissue penetration. Visible wave-length fluorescence dyes such as rhod 2 are available; however, the lack of spectral shift with calcium binding precludes the use of ratio techniques to account for changes in cytosolic dye concentration. We have developed a method for in vivo quantitation of cytosolic rhod 2 concentration that in conjunction with calcium-dependent fluorescence measurements permits estimation of cytosolic calcium levels in perfused rabbit hearts. Reflective absorbance of excitation light by rhod 2 loaded into myocardium was used as an index of dye concentration and the ratio of fluorescence intensity to absorbance as a measure of cytosolic calcium concentration. Endothelial cell loading of rhod 2 was found to be minimal (< 5%), and dye leak rate out of the cytosol was slow, with approximately 5% loss of dye fluorescence occurring between 10 and 30 min after dye loading. Rhod 2 loading into subcellular compartments, determined by manganese quenching, was also minimal (< 5%). The dissociation constant of rhod 2 for calcium was measured in vitro to be 500 nM, and this value increased to 710 nM in the presence of 0.5 mM myoglobin. On the basis of this value and in vivo fluorescence measurements, cytosolic calcium concentration in the rabbit heart was found to be 229 +/- 90 nM at end diastole and 930 +/- 130 nM at peak systole, with peak fluorescence preceding peak ventricular pressure by approximately 40 ms. This technique should facilitate detailed analysis of calcium transients from the whole heart.

Role of mitochondria in metabolism-secretion coupling of insulin release in the pancreatic beta-cell

The control of insulin secretion from the pancreatic beta-cell involves a complex cascade of events in which the mitochondria play a central role. In the consensus model this role is essentially restricted to the production of ATP promoting membrane depolarisation and a rise in cytosolic [Ca2+]. Evidence for the generation of an additional mitochondrial factor implicated in metabolism-secretion coupling is provided in this review. Although not yet identified, the formation of this putative factor requires an increase of [Ca2+] in the mitochondrial matrix together with a supply of carbons to the tricarboxylic acid (TCA) cycle. In this model, calcium activates matrix dehydrogenases, in particular those of the TCA cycle. This enables the synthesis of the mitochondrial factor from the TCA cycle intermediates. Experimental evidence gathered in permeabilised cells largely supports this model.

Visualization of NMDA receptor-induced mitochondrial calcium accumulation in striatal neurons

Ca2+ influx through NMDA receptor-gated channels and the subsequent rise in intracellular Ca2+ concentration ([Ca2+]i) have been implicated in cytotoxic processes that lead to irreversible neuronal injury. While many studies have focused on cytosolic Ca2+ homeostasis, much less is known about Ca2+ fluxes in subcellular organelles, such as mitochondria. The mitochondria play an important role in Ca2+ homeostasis by sequestering cytosolic Ca2+ loads. However, mitochondrial Ca2+ overload can impair ATP synthesis, induce free radical formation, and lead to lipid peroxidation. Thus, it is also important to understand the mitochondrial Ca2+ fluxes induced by NMDA. In this study, changes in mitochondrial Ca2+ concentration ([Ca2+]m) in cultured striatal neurons were monitored with a Ca(2+)-binding fluorescent probe, rhod-2, and laser scanning confocal microscopy. The rhod-2 fluorescence signal was highly localized in mitochondrial areas of confocal images. A rapid increase of [Ca2+]m was observed when neurons were treated with 100 microM NMDA. The increased [Ca2+]m induced by NMDA could not be observed in the presence of ruthenium red, an inhibitor of the mitochondrial Ca2+ uniporter, or CCCP, a protonophore that breaks down the mitochondrial membrane potential necessary for Ca2+ uptake. The magnitude and reversibility of changes in [Ca2+]m induced by NMDA were variable. In neurons receiving multiple pulses of NMDA, [Ca2+]m did not return to baseline. The elevated [Ca2+]m may persist indefinitely and may rise further after successive NMDA exposures. These data demonstrate that Ca2+ accumulates in mitochondria in response to NMDA receptor activation. This Ca2+ accumulation may play a role in the excitotoxic mitochondrial dysfunction induced by NMDA.

Upstream stimulatory factor-2 (USF2) activity is required for glucose stimulation of L-pyruvate kinase promoter activity in single living islet beta-cells

Elevated glucose concentrations stimulate L-pyruvate kinase (L-PK) gene transcription in liver and islet beta-cells. A glucose response element termed the L4 box (two noncanonical E-boxes located -165 and -154 base pairs upstream of the transcriptional start point) has previously been defined within the proximal promoter region of the gene. However, the identity of the transacting factor(s) which binds to this site remains unclear. We have used photon counting digital imaging of firefly luciferase activity to monitor promoter activity continuously in single living islet beta and derived INS-1 cells, and to analyze the molecular basis of the regulation by glucose. L-PK promoter activity, normalized to cytomegalovirus promoter activity using the distinct Renilla reniformis luciferase, was >/=6-fold higher in cells cultured at 16 mM glucose or above compared with cells cultured at 3 mM glucose. Microinjection of antibodies against the ubiquitous transcription factor USF2 inhibited L-PK promoter activity in beta- and INS-1 cells incubated at 30 mM glucose by 71-87%. Anti-USF2 antibodies had a much smaller effect on promoter activity in INS-1 cells cultured at 3 mM glucose, and on the activity of a modified promoter construct lacking an L4 box. These data support the view that glucose enhances L-PK gene transcription in beta-cells by modifying the transactivational capacity of USF2 bound to the upstream L4 box.

Mitochondrial activation directly triggers the exocytosis of insulin in permeabilized pancreatic beta-cells

In the pancreatic beta-cell, insulin secretion is stimulated by glucose metabolism resulting in membrane potential-dependent elevation of cytosolic Ca2+ ([Ca2+]c). This cascade involves the mitochondrial membrane potential (delta psi[m]) hyperpolarization and elevation of mitochondrial Ca2+ ([Ca2+]m) which activates the Ca(2+)-sensitive NADH-generating dehydrogenases. Metabolism-secretion coupling requires unidentified signals, other than [Ca2+]c, possibly generated by the mitochondria through the rise in [Ca2+]m. To test this paradigm, we have established an alpha-toxin permeabilized cell preparation permitting the simultaneous monitoring of [Ca2+] with mitochondrially targeted aequorin and insulin secretion under conditions of saturating [ATP] (10 mM) and of clamped [Ca2+]c at substimulatory levels (500 nM). The tricarboxylic acid (TCA) cycle intermediate succinate hyperpolarized delta psi(m), raised [Ca2+]m up to 1.5 microM and stimulated insulin secretion 20-fold, without changing [Ca2+]c. Blockade of the uniporter-mediated Ca2+ influx into the mitochondria abolished the secretory response. Moreover, glycerophosphate, which raises [Ca2+]m by hyperpolarizing delta psi(m) without supplying carbons to the TCA cycle, failed to stimulate exocytosis. Activation of the TCA cycle with citrate evoked secretion only when combined with glycerophosphate. Thus, mitochondrially driven insulin secretion at permissive [Ca2+]c requires both a substrate for the TCA cycle and a rise in [Ca2+]m. Therefore, mitochondrial metabolism generates factors distinct from Ca2+ and ATP capable of inducing insulin exocytosis.

Measurement of mitochondrial calcium in single living cardiomyocytes by selective removal of cytosolic indo 1

The aim of the present study was to determine whether, in indo 1 acetoxymethyl ester (AM)-loaded rat cardiomyocytes, it was possible to remove cytosolic but not mitochondrial indo 1 by promoting loss of cytosolic indo 1 through plasma membrane anion pumps (which are blocked by probenecid). Isolated rat ventricular myocytes were loaded with indo 1-AM under conditions (15 min at 30 degrees C) in which about half of the dye is located within mitochondria. Cells were then maintained at 25 degrees C for 2.5 h followed by incubation at 37 degrees C for 1.5 h. After this "heat treatment," the myocyte fluorescence signal was 44% of the value of cells measured before heat treatment, and loss of fluorescence was prevented by 1 mM probenecid. The remaining fluorescence was shown to originate from mitochondria, since 1) Ca2+ uptake and efflux could be inhibited by ruthenium red and clonazepam, respectively, and 2) low concentrations of digitonin, which release only cytosolic marker enzymes, decreased fluorescence of untreated myocytes but had little effect on the fluorescence signal of heat-treated cells. We conclude that heat treatment selectively removes cytosolic indo 1, leaving a signal due to mitochondrial indo 1 only.

Mitochondrial regulation of store-operated calcium signaling in T lymphocytes

Mitochondria act as potent buffers of intracellular Ca2+ in many cells, but a more active role in modulating the generation of Ca2+ signals is not well established. We have investigated the ability of mitochondria to modulate store-operated or "capacitative" Ca2+ entry in Jurkat leukemic T cells and human T lymphocytes using fluorescence imaging techniques. Depletion of the ER Ca2+ store with thapsigargin (TG) activates Ca2+ release-activated Ca2+ (CRAC) channels in T cells, and the ensuing influx of Ca2+ loads a TG-insensitive intracellular store that by several criteria appears to be mitochondria. Loading of this store is prevented by carbonyl cyanide m-chlorophenylhydrazone or by antimycin A1 + oligomycin, agents that are known to inhibit mitochondrial Ca2+ import by dissipating the mitochondrial membrane potential. Conversely, intracellular Na+ depletion, which inhibits Na+-dependent Ca2+ export from mitochondria, enhances store loading. In addition, we find that rhod-2 labels mitochondria in T cells, and it reports changes in Ca2+ levels that are consistent with its localization in the TG-insensitive store. Ca2+ uptake by the mitochondrial store is sensitive (threshold is <400 nM cytosolic Ca2+), rapid (detectable within 8 s), and does not readily saturate. The rate of mitochondrial Ca2+ uptake is sensitive to extracellular [Ca2+], indicating that mitochondria sense Ca2+ gradients near CRAC channels. Remarkably, mitochondrial uncouplers or Na+ depletion prevent the ability of T cells to maintain a high rate of capacitative Ca2+ entry over prolonged periods of >10 min. Under these conditions, the rate of Ca2+ influx in single cells undergoes abrupt transitions from a high influx to a low influx state. These results demonstrate that mitochondria not only buffer the Ca2+ that enters T cells via store-operated Ca2+ channels, but also play an active role in modulating the rate of capacitative Ca2+ entry.

Cytoplasmic Ca2+ signalling and reduction of mitochondrial pyridine nucleotides in adrenal glomerulosa cells in response to K+, angiotensin II and vasopressin

We have examined the mitochondrial formation of NAD(P)H in rat adrenal glomerulosa cells. A short-term elevation of the K+ concentration from 3.6 to 8.4 mM induced a reversible increase in the formation of reduced pyridine nucleotides. Potassium applied after the addition of rotenone had no further effect, confirming that the redox signal was of mitochondrial origin. Inhibition of aldosterone synthesis by aminoglutethimide in K+-stimulated cells decreased the rate of decay of the NAD(P)H signal upon the termination of stimulation, indicating that the NADPH formed was consumed in aldosterone synthesis. When the NAD(P)H signal was measured simultaneously with the cytoplasmic free Ca2+ concentration ([Ca2+]i), elevation of the K+ concentration to 6.6 or 8.4 mM induced parallel increases in [Ca2+]i and NAD(P)H formation. The rates of increase and decrease of NAD(P)H were lower than for [Ca2+]i, confirming that the redox signal was secondary to the Ca2+ signal. Angiotensin II (100 pM-1 nM) induced an oscillatory NAD(P)H signal which usually returned to a lower baseline concentration, while a sustained signal with superimposed oscillations was observed at higher concentrations. Simultaneous measurements showed that NAD(P)H levels followed the [Ca2+]i pattern evoked by angiotensin II. Vasopressin (100 nM) also induced parallel oscillations of [Ca2+]i and NAD(P)H. A sustained rise in the extramitochondrial Ca2+ concentration to 1 microM induced a sustained elevation of the intramitochondrial Ca2+ concentration in permeabilized cells, as measured with rhod-2. A sustained rise in [Ca2+]i evoked by long-term stimulation with 8.4 mM K+ or 2.5 nM angiotensin II resulted in sustained NAD(P)H production. These Ca2+-dependent changes in the mitochondrial redox state support the biological response, i.e. aldosterone secretion by glomerulosa cells.

Mitochondrial participation in the intracellular Ca2+ network

Calcium can activate mitochondrial metabolism, and the possibility that mitochondrial Ca2+ uptake and extrusion modulate free cytosolic [Ca2+] (Cac) now has renewed interest. We use whole-cell and perforated patch clamp methods together with rapid local perfusion to introduce probes and inhibitors to rat chromaffin cells, to evoke Ca2+ entry, and to monitor Ca2+-activated currents that report near-surface [Ca2+]. We show that rapid recovery from elevations of Cac requires both the mitochondrial Ca2+ uniporter and the mitochondrial energization that drives Ca2+ uptake through it. Applying imaging and single-cell photometric methods, we find that the probe rhod-2 selectively localizes to mitochondria and uses its responses to quantify mitochondrial free [Ca2+] (Cam). The indicated resting Cam of 100-200 nM is similar to the resting Cac reported by the probes indo-1 and Calcium Green, or its dextran conjugate in the cytoplasm. Simultaneous monitoring of Cam and Cac at high temporal resolution shows that, although Cam increases less than Cac, mitochondrial sequestration of Ca2+ is fast and has high capacity. We find that mitochondrial Ca2+ uptake limits the rise and underlies the rapid decay of Cac excursions produced by Ca2+ entry or by mobilization of reticular stores. We also find that subsequent export of Ca2+ from mitochondria, seen as declining Cam, prolongs complete Cac recovery and that suppressing export of Ca2+, by inhibition of the mitochondrial Na+/ Ca2+ exchanger, reversibly hastens final recovery of Cac. We conclude that mitochondria are active participants in cellular Ca2+ signaling, whose unique role is determined by their ability to rapidly accumulate and then release large quantities of Ca2+.

Subcellular analysis of Ca2+ homeostasis in primary cultures of skeletal muscle myotubes

Specifically targeted aequorin chimeras were used for studying the dynamic changes of Ca2+ concentration in different subcellular compartments of differentiated skeletal muscle myotubes. For the cytosol, mitochondria, and nucleus, the previously described chimeric aequorins were utilized; for the sarcoplasmic reticulum (SR), a new chimera (srAEQ) was developed by fusing an aequorin mutant with low Ca2+ affinity to the resident protein calsequestrin. By using an appropriate transfection procedure, the expression of the recombinant proteins was restricted, within the culture, to the differentiated myotubes, and the correct sorting of the various chimeras was verified with immunocytochemical techniques. Single-cell analysis of cytosolic Ca2+ concentration ([Ca2+]c) with fura-2 showed that the myotubes responded, as predicted, to stimuli known to be characteristic of skeletal muscle fibers, i.e., KCl-induced depolarization, caffeine, and carbamylcholine. Using these stimuli in cultures transfected with the various aequorin chimeras, we show that: 1) the nucleoplasmic Ca2+ concentration ([Ca2+]n) closely mimics the [Ca2+]c, at rest and after stimulation, indicating a rapid equilibration of the two compartments also in this cell type; 2) on the contrary, mitochondria amplify 4-6-fold the [Ca2+]c increases; and 3) the lumenal concentration of Ca2+ within the SR ([Ca2+]sr) is much higher than in the other compartments (> 100 microM), too high to be accurately measured also with the aequorin mutant with low Ca2+ affinity. An indirect estimate of the resting value (approximately 1-2 mM) was obtained using Sr2+, a surrogate of Ca2+ which, because of the lower affinity of the photoprotein for this cation, elicits a lower rate of aequorin consumption. With Sr2+, the kinetics and amplitudes of the changes in [cation2+]sr evoked by the various stimuli could also be directly analyzed.

Glucose-stimulated insulin secretion correlates with changes in mitochondrial and cytosolic Ca2+ in aequorin-expressing INS-1 cells

Nutrient-stimulated insulin secretion is dependent upon the generation of metabolic coupling factors in the mitochondria of the pancreatic B cell. To investigate the role of Ca2+ in mitochondrial function, insulin secretion from INS-1 cells stably expressing the Ca2+-sensitive photoprotein aequorin in the appropriate compartments was correlated with changes in cytosolic calcium ([Ca2+]c) and mitochondrial calcium ([Ca2+]m). Glucose and KCl, which depolarize the cell membrane, as well as the Ca2+-mobilizing agonist, carbachol (CCh), cause substantial increases in [Ca2+]m which are associated with smaller rises in [Ca2+]c. The L-type Ca2+-channel blocker, SR7037, abolished the effects of glucose and KCl while attenuating the CCh response. Glucose-induced increases in [Ca2+]m, [Ca2+]c, and insulin secretion all demonstrate a pronounced initial peak followed by a sustained plateau. All three parameters are increased synergistically when glucose and CCh are combined. Finally, [Ca2+]m, [Ca2+]c, and insulin secretion also display desensitization phenomena following repeated additions of the three stimuli. The high sensitivity of [Ca2+]m to Ca2+ influx and the desensitization-resensitization effects can be explained by a model in which the mitochondria of INS-1 cells are strategically located to sense Ca2+ influx through plasma membrane Ca2+ channels. In conclusion, the correlation of [Ca2+]m and [Ca2+]c with insulin secretion may indicate a fundamental role for Ca2+ in the adaptation of oxidative metabolism to the generation of metabolic coupling factors and the energy requirements of exocytosis.