Calcium ion transport in mitochondria

Mitochondrial Ca(2+) as a key regulator of mitochondrial activities

Mitochondria play a central role in cell biology, not only as producers of ATP but also as regulators of the Ca(2+) signal. The translocation by respiratory chain protein complexes of H(+) across the ion-impermeable inner membrane generates a very large H(+) electrochemical gradient that can be employed not only by the H(+) ATPase to run the endoergonic reaction of ADP phosphorylation, but also to accumulate cations into the matrix. Mitochondria can rapidly take up Ca(2+) through an electrogenic pathway, the uniporter, that acts to equilibrate Ca(2+) with its electrochemical gradient, and thus accumulates the cation into the matrix, and they can release it through two exchangers (with H(+) and Na(+), mostly expressed in non-excitable and excitable cells, respectively), that utilize the electrochemical gradient of the monovalent cations to prevent the attainment of electrical equilibrium.The uniporter, due to its low Ca(2+) affinity, demands high local Ca(2+) concentrations to work. In different cell systems these high Ca(2+) concentration microdomains are generated, upon cell stimulation, in proximity of the plasma membrane and the sarco/endoplasmic reticulum Ca(2+) channels.Recent work has revealed the central role of mitochondria in signal transduction pathways: evidence is accumulating that, by taking up Ca(2+), they not only modulate mitochondrial activities but also tune the cytosolic Ca(2+) signals and their related functions. This review analyses recent developments in the area of mitochondrial Ca(2+) signalling and attempts to summarize cell physiology aspects of the mitochondrial Ca(2+) transport machinery.

Mitochondrial Ca2+ channels: Great unknowns with important functions

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

Mitochondrial Ca2+, the secret behind the function of uncoupling proteins 2 and 3?

The underlying molecular action of the novel uncoupling proteins 2 and 3 (UCP2 and UCP3) is still under debate. The proteins have been implicated in many cell functions, including the regulation of insulin secretion and regulation of reactive oxygen species (ROS) generation. These effects have mainly been explained by suggesting that the proteins establish a proton leak through the inner mitochondrial membrane (IMM). However, accumulating data question this mechanism and suggest that UCP2 and UCP3 may play other roles, including carrying free fatty acids from the matrix towards the intermembrane space, or contributing to the mitochondrial Ca(2+) uniport. Accordingly, in this review we reflect on these actions of UCP2/UCP3 and discuss alternative explanations for the molecular mechanisms by which UCP2/UCP3 might contribute to aspects of cell function. Based on the potential role of UCP2/UCP3 in regulating mitochondrial Ca(2+) uptake, we propose a scheme whereby these proteins integrate Ca(2+)-dependent signal transduction and energy metabolism in order to meet the energy demand of the cell for its continuous response, adaptation, and stimulation to environmental input.

Mitochondria and Ca(2+) signaling: old guests, new functions

Mitochondria are ancient endosymbiotic guests that joined the cells in the evolution of complex life. While the unique ability of mitochondria to produce adenosine triphosphate (ATP) and their contribution to cellular nutrition metabolism received condign attention, our understanding of the organelle's contribution to Ca(2+) homeostasis was restricted to serve as passive Ca(2+) sinks that accumulate Ca(2+) along the organelle's negative membrane potential. This paradigm has changed radically. Nowadays, mitochondria are known to respond to environmental Ca(2+) and to contribute actively to the regulation of spatial and temporal patterns of intracellular Ca(2+) signaling. Accordingly, mitochondria contribute to many signal transduction pathways and are actively involved in the maintenance of capacitative Ca(2+) entry, the accomplishment of Ca(2+) refilling of the endoplasmic reticulum and Ca(2+)-dependent protein folding. Mitochondrial Ca(2+) homeostasis is complex and regulated by numerous, so far, genetically unidentified Ca(2+) channels, pumps and exchangers that concertedly accomplish the organelle's Ca(2+) demand. Notably, mitochondrial Ca(2+) homeostasis and functions are crucially influenced by the organelle's structural organization and motility that, in turn, is controlled by matrix/cytosolic Ca(2+). This review intends to provide a condensed overview on the molecular mechanisms of mitochondrial Ca(2+) homeostasis (uptake, buffering and storage, extrusion), its modulation by other ions, kinases and small molecules, and its contribution to cellular processes as fundamental basis for the organelle's contribution to signaling pathways. Hence, emphasis is given to the structure-to-function and mobility-to-function relationship of the mitochondria and, thereby, bridging our most recent knowledge on mitochondria with the best-established mitochondrial function: metabolism and ATP production.

Frataxin activates mitochondrial energy conversion and oxidative phosphorylation

Friedreich's ataxia (FA) is an autosomal recessive disease caused by decreased expression of the mitochondrial protein frataxin. The biological function of frataxin is unclear. The homologue of frataxin in yeast, YFH1, is required for cellular respiration and was suggested to regulate mitochondrial iron homeostasis. Patients suffering from FA exhibit decreased ATP production in skeletal muscle. We now demonstrate that overexpression of frataxin in mammalian cells causes a Ca(2+)-induced up-regulation of tricarboxylic acid cycle flux and respiration, which, in turn, leads to an increased mitochondrial membrane potential (delta psi(m)) and results in an elevated cellular ATP content. Thus, frataxin appears to be a key activator of mitochondrial energy conversion and oxidative phosphorylation.

Mitochondrial transport of cations: channels, exchangers, and permeability transition

This review provides a selective history of how studies of mitochondrial cation transport (K+, Na+, Ca2+) developed in relation to the major themes of research in bioenergetics. It then covers in some detail specific transport pathways for these cations, and it introduces and discusses open problems about their nature and physiological function, particularly in relation to volume regulation and Ca2+ homeostasis. The review should provide the basic elements needed to understand both earlier mitochondrial literature and current problems associated with mitochondrial transport of cations and hopefully will foster new interest in the molecular definition of mitochondrial cation channels and exchangers as well as their roles in cell physiology.

The Ca2+ threshold for the mitochondrial permeability transition and the content of proteins related to Bcl-2 in rat liver and Zajdela hepatoma mitochondria

Zajdela hepatoma mitochondria were able to accumulate two to five times more Ca2+ than rat liver mitochondria before the permeability transition was induced. Pulses of Ca2+ were given in series to determine the Ca2+ threshold by recording changes in [Ca2+] and membrane potential, the permeability transition causing the release of accumulated Ca2+ and collapse of the membrane potential. Hepatoma mitochondria had lower Ca2+ efflux rates, higher net Ca2+ uptake rates and lower phosphorylation rates than liver mitochondria. Since the differences in regard to induction of the permeability transition might be due to higher expression of the Bcl-2 protein in hepatoma cells than in hepatocytes, the transcription of Bcl-2 and the proteins reacting with a Bcl-2 polyclonal antiserum were estimated by Northern and Western blotting, respectively. Hepatoma cells had two Bcl-2 specific mRNA bands of 7 and 2.4 kb, and substantial amounts of the Bcl-2 protein, whereas in liver cells and mitochondria these were not detected. Both cell lines had a reactive band at 19-20 kDa, and hepatocytes a small band at 31-32 kDa. Bcl-2 antibodies stimulated the permeability transition potently in hepatoma mitochondria.

Direct measurement of calcium transport across chloroplast inner-envelope vesicles

The initial rate of Ca2+ movement across the inner-envelope membrane of pea (Pisum sativum L.) chloroplasts was directly measured by stopped-flow spectrofluorometry using membrane vesicles loaded with the Ca2+-sensitive fluorophore fura-2. Calibration of fura-2 fluorescence was achieved by combining a ratiometric method with Ca2+-selective minielectrodes to determine pCa values. The initial rate of Ca2+ influx in predominantly right-side-out inner-envelope membrane vesicles was greater than that in largely inside-out vesicles. Ca2+ movement was stimulated by an inwardly directed electrochemical proton gradient across the membrane vesicles, an effect that was diminished by the addition of valinomycin in the presence of K+. In addition, Ca2+ was shown to move across the membrane vesicles in the presence of a K+ diffusion potential gradient. The potential-stimulated rate of Ca2+ transport was slightly inhibited by diltiazem and greatly inhibited by ruthenium red. Other pharmacological agents such as LaCl3, verapamil, and nifedipine had little or no effect. These results indicate that Ca2+ transport across the chloroplast inner envelope can occur by a potential-stimulated uniport mechanism.

Stimulation of the yeast mitochondrial calcium uniporter by hypotonicity and by ruthenium red

The Ca2+ uptake by mitochondria from the yeast Endomyces magnusii has earlier been found to be driven by the membrane potential and to be stimulated by spermine. It thus functions in a similar fashion as the animal mitochondrial calcium uniporter. Here, it is shown that the uptake is stimulated, i.e., Ca2+ can be accumulated from lower [Ca2+], under hypotonic conditions. Ruthenium Red, an inhibitor of the animal uniporter, under certain conditions, stimulates the yeast uniporter. The mechanism of the stimulation by hypotonicity and Ruthenium Red is discussed.

On the mechanism of rebounding of calcium in liver mitochondria

Rat liver mitochondria are able to temporarily lower the steady state concentration of external Ca2+ after having accumulated a pulse of added Ca2+. This could be due to inhibition of efflux or/and stimulation of influx of Ca2+. This question has been addressed in mitochondria respiring on succinate +/- malonate. In the presence of malonate the depression of the membrane potential during Ca2+ uptake is more extensive and the rate of Ca2+ uptake slower. There were no discernible differences in the rates of efflux either after inhibition of the calcium uniporter by Ruthenium Red or by studying efflux of preloaded 45Ca-labeled Ca2+. The efflux was not changed by diltiazem or cyclosporin A to inhibit Ca2+ exchange on the Ca2+/nNA+ antiporter or efflux through the permeability transition pore. It is concluded that the rebounding is due mainly to stimulation of the calcium uniporter.

Effect of butylhydroxytoluene and related compounds on permeability of the inner mitochondrial membrane

Mitochondrial inner membrane contains a latent pore (PTP) that when opened uncouples mitochondrial energy transduction and allows rapid equilibration of low-molecular-weight solutes between the matrix and exterior. Based on sensitivity of the PTP to well-known free radical scavenger butylhydroxytoluene (BHT), it has been proposed that increased steady-state level of oxygen radicals, and subsequent radical attack of proteins and lipids, is a central event in activation of this pore (Novgorodov et al., J. Bioenerg. Biomembr. 19, 191-202, 1987; Carbonera and Azzone, Biochim. Biophys. Acta 943, 245-255, 1988). Present studies revealed that DBT, a derivative of BHT devoid of radical scavenging activity, exerts an analogous effect on the permeability of the inner membrane. Inhibition of the Ca2+-induced PTP opening is essentially complete at dose range of 50-60 nmol/mg protein with IC50 values of about 32 and 23 nmol/mg protein for DBT and BHT, respectively. Electron microscopy and osmotic experiments utilizing polyethylene glycols with different Stokes radii showed that the apparent lack of inhibition seen at high concentrations of these compounds results from cyclosporin A- and Ca2+-insensitive pore formation in the inner membrane. Experiments employing antioxidants with similar structure but dissimilar hydrophobicity provided evidence for localization of the antioxidant binding sites within the hydrophobic zone of the inner membrane or in the matrix space. The data obtained do not refute the notion that oxygen radicals modulate the PTP, but rather indicate that BHT operates independently of its free radical scavenging activity. Overall, the sensitivity to BHT and other antioxidants is not always a reliable criterion for the involvement of free radical reactions in the processes under study.

Microtubule-active drugs suppress the closure of the permeability transition pore in tumour mitochondria

We report the effects of anticancer drugs, inhibitors of microtubule organisation, on the mitochondrial permeability transition pore (PTP) in Ehrlich ascites tumour cells. Taxol (5-20 microM) and colchicine (100-500 microM) prevented closing of the cyclosporin A-sensitive PTP. No taxol or colchicine effects on oxidative phosphorylation were observed in the range of concentrations used. We suggest that either membrane-bound tubulin per se can be part of PTP and/or the attachment of mitochondria to the microtubular network is essential for PTP regulation. The taxol inhibition of PTP closure, mediated through interaction with the cytoskeleton, sheds new light on the cytotoxic properties of this anticancer drug.

Is Zn2+ transported by the mitochondrial calcium uniporter?

Zinc ions were found to inhibit Ca2+ uptake by rat liver mitochondria driven by succinate respiration but not that by a valinomycin-induced membrane potential. Zn2+ at 1 microM or higher concentrations induced a lowering of the membrane potential under the former but not the latter conditions. It is concluded that it is the lowered membrane potential in the presence of Zn2+ that reduces the rate of respiration-driven Ca2+. Ruthenium red was found to inhibit the uptake of Zn2+ but had no influence on its action upon the membrane potential. Zn2+ did not affect the Ruthenium red-insensitive Ca2+ efflux. Ca2+ stimulated the uptake of Zn2+. It is concluded that Zn2+ may be transported by the mitochondrial calcium uniporter but that it may have access to sites required for inhibition of respiration by other routes.

Purification of the channel component of the mitochondrial calcium uniporter and its reconstitution into planar lipid bilayers

The purification of the channel-forming component of the mitochondrial calcium uniporter and its channel properties are described. After ethanol and 50% ethanol-water extraction of mitochondria from beef heart or perfused rat liver, the extract was passed through thiopropyl-Sepharose 6B column, and absorbed components were eluted with 2-mercaptoethanol, followed by gel-filtration on Sephadex G-15. The last fraction eluted (M(r) about 2000) was then subjected to reverse-phase high-performance liquid chromatography. Of the more than 10 distinct peaks, only one showed specific Ca(2+)-channel activity in BLM with properties similar to earlier, less extensively purified preparations, i.e., conductance of 20 pS and multiples thereof, clustering of channels, participation of 2 or more subunits in channel formation, and sensitivity to 1 microM ruthenium red. Voltage sensitivity and cooperativity between channels are described. The Ca(2+)-binding glycoprotein with which the peptide was associated was found to have high homology with human acid alpha 1-glycoprotein (orosomucoid) and to show identity with beef plasma orosomucoid in the Ouchterlony immunodiffusion test.

The Ca(2+)-induced permeability transition pore is involved in Ca(2+)-induced mitochondrial oscillations. A study on permeabilised Ehrlich ascites tumour cells

The Ca(2+)-induced permeability transition of the mitochondrial inner membrane was studied in digitonin-permeabilized Ehrlich ascites tumour cells respiring on succinate in an isotonic medium. Addition of a sufficient amount of Ca2+ to induce an efflux of accumulated Ca2+ from mitochondria produced an oscillatory state with periodically changing rates of respiration, transmembrane potential, delta pH and direction of Ca2+ fluxes. This contrasts with liver mitochondria in which only a Ca2+ efflux is induced under these conditions. Addition of traces of cyclosporin A (approximately 0.1 nM) damped the oscillations by inhibiting the phase in which Ca2+ efflux occurs and promoting the reestablishment of a higher transmembrane potential. Efflux was also prevented by addition of ATP or ADP, ATP being more potent. Efflux was also inhibited by low concentrations of spermine. It is concluded that Ca(2+)-induced oscillations involve the cyclosporin A-sensitive pore and that the Ehrlich ascites tumour cell mitochondria differ from liver mitochondria in being far more sensitive to cyclosporin A and ATP. The possible physiological role of the oscillatory state is discussed.

Inhibition of the mitochondrial calcium uniporter by antibodies against a 40-kDa glycoproteinT

Polyclonal rabbit antibodies against a Ca(2+)-binding mitochondrial glycoprotein were found to inhibit the uniporter-mediated transport of Ca2+ in mitoplasts prepared from rat liver mitochondria. Spermine, a modulator of the uniporter, decreased the inhibition. This glycoprotein of M(r) 40,000, isolated from beef heart mitochondria and earlier shown to form Ca(2+)-conducting channels in black-lipid membranes, thus is a good candidate for being a component of the uniporter. Antibody-IgG was found to specifically bind to mitochondria in human fibroblasts.

Characterization of cellular and elevated serum phospholipase A2 activities with a comparison of two methods

The phospholipase A assay of Hoffmann et al based on the enzymatic photometric determination of the fatty acids liberated from soy-bean phospholipids was compared with the fluorometric assay of Thurén et al. where a synthetic pyrene-labelled substrate is used. Sera from patients with suspected pancreatitis or sepsis were studied. High values compared well while the Hoffmann method was not sensitive enough to detect slightly elevated values in sera from patients with suspected pancreatitis. The phospholipase A2 activities from enzymes purified from human duodenal juice, human sera from patients with sepsis and rat liver mitochondria were characterized in regard to activity towards several synthetic pyrene-labelled substrates, activation by Ca2+ and inhibition by Sr2+ and Mg2+. The enzyme from serum was distinctly different from both the pancreatic secretory and the mitochondrial ones, both in its substrate specificity pattern and in being most strongly inhibited by Mg2+.