Calcium stimulates ATP-Mg/Pi carrier activity in rat liver mitochondria

Mitochondrial morphology governs ATP production rate

Life is based on energy conversion. In particular, in the nervous system, significant amounts of energy are needed to maintain synaptic transmission and homeostasis. To a large extent, neurons depend on oxidative phosphorylation in mitochondria to meet their high energy demand. For a comprehensive understanding of the metabolic demands in neuronal signaling, accurate models of ATP production in mitochondria are required. Here, we present a thermodynamically consistent model of ATP production in mitochondria based on previous work. The significant improvement of the model is that the reaction rate constants are set such that detailed balance is satisfied. Moreover, using thermodynamic considerations, the dependence of the reaction rate constants on membrane potential, pH, and substrate concentrations are explicitly provided. These constraints assure that the model is physically plausible. Furthermore, we explore different parameter regimes to understand in which conditions ATP production or its export are the limiting steps in making ATP available in the cytosol. The outcomes reveal that, under the conditions used in our simulations, ATP production is the limiting step and not its export. Finally, we performed spatial simulations with nine 3-D realistic mitochondrial reconstructions and linked the ATP production rate in the cytosol with morphological features of the organelles.

The ER-mitochondria interface, where Ca2+ and cell death meet

Endoplasmic reticulum (ER)-mitochondria contact sites are crucial to allow Ca²⁺ flux between them and a plethora of proteins participate in tethering both organelles together. Inositol 1,4,5-trisphosphate receptors (IP₃Rs) play a pivotal role at such contact sites, participating in both ER-mitochondria tethering and as Ca²⁺-transport system that delivers Ca²⁺ from the ER towards mitochondria. At the ER-mitochondria contact sites, the IP₃Rs function as a multi-protein complex linked to the voltage-dependent anion channel 1 (VDAC1) in the outer mitochondrial membrane, via the chaperone glucose-regulated protein 75 (GRP75). This IP₃R-GRP75-VDAC1 complex supports the efficient transfer of Ca²⁺ from the ER into the mitochondrial intermembrane space, from which the Ca²⁺ ions can reach the mitochondrial matrix through the mitochondrial calcium uniporter. Under physiological conditions, basal Ca²⁺ oscillations deliver Ca²⁺ to the mitochondrial matrix, thereby stimulating mitochondrial oxidative metabolism. However, when mitochondrial Ca²⁺ overload occurs, the increase in [Ca²⁺] will induce the opening of the mitochondrial permeability transition pore, thereby provoking cell death. The IP₃R-GRP75-VDAC1 complex forms a hub for several other proteins that stabilize the complex and/or regulate the complex's ability to channel Ca²⁺ into the mitochondria. These proteins and their mechanisms of action are discussed in the present review with special attention for their role in pathological conditions and potential implication for therapeutic strategies.

Balancing energy and protein homeostasis at ER-mitochondria contact sites

The endoplasmic reticulum (ER) is the largest organelle of the cell and participates in multiple essential functions, including the production of secretory proteins, lipid synthesis, and calcium storage. Sustaining proteostasis requires an intimate coupling with energy production. Mitochondrial respiration evolved to be functionally connected to ER physiology through a physical interface between both organelles known as mitochondria-associated membranes. This quasi-synaptic structure acts as a signaling hub that tunes the function of both organelles in a bidirectional manner and controls proteostasis, cell death pathways, and mitochondrial bioenergetics. Here, we discuss the main signaling mechanisms governing interorganellar communication and their putative role in diseases including cancer and neurodegeneration.

The yin and yang of mitochondrial Ca2+ signaling in cell physiology and pathology

Mitochondria are autonomous and dynamic cellular organelles orchestrating a diverse range of cellular activities. Numerous cell-signaling pathways target these organelles and Ca²⁺ is one of the most significant. Mitochondria are able to rapidly and transiently take up Ca²⁺, thanks to the mitochondrial Ca²⁺ uniporter complex, as well as to extrude it through the Na⁺/Ca²⁺ and H⁺/Ca²⁺ exchangers. The transient accumulation of Ca²⁺ in the mitochondrial matrix impacts on mitochondrial functions and cell pathophysiology. Here we summarize the role of mitochondrial Ca²⁺ signaling in both physiological (yang) and pathological (yin) processes and the methods that can be used to investigate mitochondrial Ca²⁺ homeostasis. As an example of the pivotal role of mitochondria in pathology, we described the state of the art of mitochondrial Ca²⁺ alterations in different pathological conditions, with a special focus on Alzheimer's disease.

The Two-Way Relationship Between Calcium and Metabolism in Cancer

Calcium ion (Ca²⁺) signaling is critical to many physiological processes, and its kinetics and subcellular localization are tightly regulated in all cell types. All Ca²⁺ flux perturbations impact cell function and may contribute to various diseases, including cancer. Several modulators of Ca²⁺ signaling are attractive pharmacological targets due to their accessibility at the plasma membrane. Despite this, the number of specific inhibitors is still limited, and to date there are no anticancer drugs in the clinic that target Ca²⁺ signaling. Ca²⁺ dynamics are impacted, in part, by modifications of cellular metabolic pathways. Conversely, it is well established that Ca²⁺ regulates cellular bioenergetics by allosterically activating key metabolic enzymes and metabolite shuttles or indirectly by modulating signaling cascades. A coordinated interplay between Ca²⁺ and metabolism is essential in maintaining cellular homeostasis. In this review, we provide a snapshot of the reciprocal interaction between Ca²⁺ and metabolism and discuss the potential consequences of this interplay in cancer cells. We highlight the contribution of Ca²⁺ to the metabolic reprogramming observed in cancer. We also describe how the metabolic adaptation of cancer cells influences this crosstalk to regulate protumorigenic signaling pathways. We suggest that the dual targeting of these processes might provide unprecedented opportunities for anticancer strategies. Interestingly, promising evidence for the synergistic effects of antimetabolites and Ca²⁺-modulating agents is emerging.

Regulation of permeability transition pore opening in mitochondria by external NAD(H)

BACKGROUND

The opening of the permeability transition pore (PTP) in mitochondria plays a critical role in the pathogenesis of numerous diseases. Mitochondrial matrix pyridine nucleotides are potent regulators of the PTP, but the role of extramitochondrial nucleotides is unclear.

METHODS

The PTP opening was explored in isolated mitochondria and mitochondria in permeabilized differentiated and undifferentiated cells in the presence of added NAD(P)(H) in combination with Mg²⁺, adenine nucleotides (AN), and the inhibitors of AN translocase (ANT), voltage-dependent anion channel (VDAC), and cyclophilin D.

RESULTS

Added NAD(H) and AN, but not NADP(H), inhibited the PTP opening with comparable potency. PTP suppression required neither NAD(H) oxidation nor reduction. The protective effects of NAD(H) and cyclosporin A were synergistic, and the effects of NAD(H) and millimolar AN were additive. The conformation-specific ANT inhibitors were unable to cancel the protective effect of NADH even under total ANT inhibition. Besides, NAD(H) activated the efflux of mitochondrial AN via ANT. VDAC ligand (Mg²⁺) and blockers (G3139 and 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid) potentiated and attenuated the protective effect of NAD(H), respectively. However, in embryonic and cancer (undifferentiated) cells, in contrast to isolated differentiated hepatocytes and cardiocytes, the suppression of PTP opening by NADH was negligible though all cells tested possessed a full set of VDAC isoforms.

CONCLUSIONS

The study revealed a novel mechanism of PTP regulation by external (cytosolic) NAD(H) through the allosteric site in the OM or the intermembrane space.

GENERAL SIGNIFICANCE

The mechanism might contribute to the resistance of differentiated cells under different pathological conditions including ischemia/reperfusion.

Calcium-regulated mitochondrial ATP-Mg/Pi carriers evolved from a fusion of an EF-hand regulatory domain with a mitochondrial ADP/ATP carrier-like domain

The mitochondrial ATP-Mg/Pi carrier is responsible for the calcium-dependent regulation of adenosine nucleotide concentrations in the mitochondrial matrix, which allows mitochondria to respond to changing energy requirements of the cell. The carrier is expressed in mitochondria of fungi, plants and animals and belongs to the family of mitochondrial carriers. The carrier is unusual as it consists of three separate domains: (i) an N-terminal regulatory domain with four calcium-binding EF-hands similar to calmodulin, (ii) a loop domain containing an amphipathic α-helix and (iii) a mitochondrial carrier domain related to the mitochondrial ADP/ATP carrier. This striking example of three domains coming together from different origins to provide new functions represents an interesting quirk of evolution. In this review, we outline how the carrier was identified and how its physiological role was established with a focus on human isoforms. We exploit the sequence and structural information of the domains to explore the similarities and differences to their closest counterparts; mitochondrial ADP/ATP carriers and proteins with four EF-hands. We discuss how their combined function has led to a mechanism for calcium-regulated transport of adenosine nucleotides. Finally, we compare the ATP-Mg/Pi carrier with the mitochondrial aspartate/glutamate carrier, the only other mitochondrial carrier regulated by calcium, and we will argue that they have arisen by convergent rather than divergent evolution. © 2018 The Authors. IUBMB Life published by Wiley Periodicals, Inc. on behalf of International Union of Biochemistry and Molecular Biology, 70(12):1222-1232, 2018.

Calcium, mitochondria and cell metabolism: A functional triangle in bioenergetics

The versatility of mitochondrial metabolism and its fine adjustments to specific physiological or pathological conditions regulate fundamental cell pathways, ranging from proliferation to apoptosis. In particular, Ca²⁺ signalling has emerged as a key player exploited by mitochondria to tune their activity according with cell demand. The functional interaction between mitochondria and endoplasmic reticulum (ER) deeply impacts on the correct mitochondrial Ca²⁺ signal, thus modulating cell bioenergetics and functionality. Indeed, Ca²⁺ released by the ER is taken up by mitochondria where, both in the intermembrane space and in the matrix, it regulates the activity of transporters, enzymes and proteins involved in organelles' metabolism. In this review, we will briefly summarize Ca²⁺-dependent mechanisms involved in the regulation of mitochondrial activity. Moreover, we will discuss some recent reports, in which alterations in mitochondrial Ca²⁺ signalling have been associated with specific pathological conditions, such as neurodegeneration and cancer.

Magnesium Extravaganza: A Critical Compendium of Current Research into Cellular Mg2+ Transporters Other than TRPM6/7

Magnesium research has boomed within the last 20 years. The real breakthrough came at the start of the new millennium with the discovery of a plethora of possible Mg homeostatic factors that, in particular, included putative Mg²⁺ transporters. Until that point, Mg research was limited to biochemical and physiological work, as no target molecular entities were known that could be used to explore the molecular biology of Mg homeostasis at the level of the cell, tissue, organ, or organism and to translate such knowledge into the field of clinical medicine and pharmacology. Because of the aforementioned, Mg²⁺ and Mg homeostasis, both of which had been heavily marginalized within the biomedical field in the twentieth century, have become overnight a focal point of many studies ranging from primary biomedical research to translational medicine.The amount of literature concerning cellular Mg²⁺ transport and cellular Mg homeostasis is increasing, together with a certain amount of confusion, especially about the function(s) of the newly discovered and, in the majority of instances, still only putative Mg²⁺ transporters/Mg²⁺ homeostatic factors. Newcomers to the field of Mg research will thus find it particularly difficult to orient themselves.Here, we briefly but critically summarize the status quo of the current understanding of the molecular entities behind cellular Mg²⁺ homeostasis in mammalian/human cells other than TRPM6/7 chanzymes, which have been universally accepted as being unspecific cation channel kinases allowing the flux of Mg²⁺ while constituting the major gateway for Mg²⁺ to enter the cell.

Calcium regulation of the human mitochondrial ATP-Mg/Pi carrier SLC25A24 uses a locking pin mechanism

Mitochondrial ATP-Mg/Pi carriers import adenine nucleotides into the mitochondrial matrix and export phosphate to the cytosol. They are calcium-regulated to control the size of the matrix adenine nucleotide pool in response to cellular energetic demands. They consist of three domains: an N-terminal regulatory domain containing four calcium-binding EF-hands, a linker loop domain with an amphipathic α-helix and a C-terminal mitochondrial carrier domain for the transport of substrates. Here, we use thermostability assays to demonstrate that the carrier is regulated by calcium via a locking pin mechanism involving the amphipathic α-helix. When calcium levels in the intermembrane space are high, the N-terminus of the amphipathic α-helix is bound to a cleft in the regulatory domain, leading to substrate transport by the carrier domain. When calcium levels drop, the cleft closes, and the amphipathic α-helix is released to bind to the carrier domain via its C-terminus, locking the carrier in an inhibited state.

Regulation of mitochondrial calcium in plants versus animals

Ca(2+) acts as an important cellular second messenger in eukaryotes. In both plants and animals, a wide variety of environmental and developmental stimuli trigger Ca(2+) transients of a specific signature that can modulate gene expression and metabolism. In animals, mitochondrial energy metabolism has long been considered a hotspot of Ca(2+) regulation, with a range of pathophysiology linked to altered Ca(2+) control. Recently, several molecular players involved in mitochondrial Ca(2+) signalling have been identified, including those of the mitochondrial Ca(2+) uniporter. Despite strong evidence for sophisticated Ca(2+) regulation in plant mitochondria, the picture has remained much less clear. This is currently changing aided by live imaging and genetic approaches which allow dissection of subcellular Ca(2+) dynamics and identification of the proteins involved. We provide an update on our current understanding in the regulation of mitochondrial Ca(2+) and signalling by comparing work in plants and animals. The significance of mitochondrial Ca(2+) control is discussed in the light of the specific metabolic and energetic needs of plant and animal cells.

Calcium regulation of mitochondrial carriers

Mitochondrial function is regulated by calcium. In addition to the long known effects of matrix Ca(2+), regulation of metabolite transport by extramitochondrial Ca(2+) represents an alternative Ca(2+)-dependent mechanism to regulate mitochondrial function. The Ca(2+) regulated mitochondrial transporters (CaMCs) are well suited for that role, as they contain long N-terminal extensions harboring EF-hand Ca(2+) binding domains facing the intermembrane space. They fall in two groups, the aspartate/glutamate exchangers, AGCs, major components of the NADH malate aspartate shuttle (MAS) and urea cycle, and the ATP-Mg(2+)/Pi exchangers or short CaMCs (APCs or SCaMCs). The AGCs are activated by relatively low Ca(2+) levels only slightly higher than resting Ca(2+), whereas all SCaMCs studied so far require strong Ca(2+) signals, above micromolar, for activation. In addition, AGCs are not strictly Ca(2+) dependent, being active even in Ca(2+)-free conditions. Thus, AGCs are well suited to respond to small Ca(2+) signals and that do not reach mitochondria. In contrast, ATP-Mg(2+)/Pi carriers are inactive in Ca(2+) free conditions and activation requires Ca(2+) signals that will also activate the calcium uniporter (MCU). By changing the net content of adenine nucleotides of the matrix upon activation, SCaMCs regulate the activity of the permeability transition pore, and the Ca(2+) retention capacity of mitochondria (CRC), two functions synergizing with those of the MCU. The different Ca(2+) activation properties of the two CaMCs are discussed in relation to their newly obtained structures. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Calcium signaling as a mediator of cell energy demand and a trigger to cell death

Calcium signaling is pivotal to a host of physiological pathways. A rise in calcium concentration almost invariably signals an increased cellular energy demand. Consistent with this, calcium signals mediate a number of pathways that together serve to balance energy supply and demand. In pathological states, calcium signals can precipitate mitochondrial injury and cell death, especially when coupled to energy depletion and oxidative or nitrosative stress. This review explores the mechanisms that couple cell signaling pathways to metabolic regulation or to cell death. The significance of these pathways is exemplified by pathological case studies, such as those showing loss of mitochondrial calcium uptake 1 in patients and ischemia/reperfusion injury.

In vitro analyses of mitochondrial ATP/phosphate carriers from Arabidopsis thaliana revealed unexpected Ca(2+)-effects

BACKGROUND

Adenine nucleotide/phosphate carriers (APCs) from mammals and yeast are commonly known to adapt the mitochondrial adenine nucleotide pool in accordance to cellular demands. They catalyze adenine nucleotide--particularly ATP-Mg--and phosphate exchange and their activity is regulated by calcium. Our current knowledge about corresponding proteins from plants is comparably limited. Recently, the three putative APCs from Arabidopsis thaliana were shown to restore the specific growth phenotype of APC yeast loss-of-function mutants and to interact with calcium via their N-terminal EF--hand motifs in vitro. In this study, we performed biochemical characterization of all three APC isoforms from A. thaliana to gain further insights into their functional properties.

RESULTS

Recombinant plant APCs were functionally reconstituted into liposomes and their biochemical characteristics were determined by transport measurements using radiolabeled substrates. All three plant APCs were capable of ATP, ADP and phosphate exchange, however, high preference for ATP-Mg, as shown for orthologous carriers, was not detectable. By contrast, the obtained data suggest that in the liposomal system the plant APCs rather favor ATP-Ca as substrate. Moreover, investigation of a representative mutant APC protein revealed that the observed calcium effects on ATP transport did not primarily/essentially involve Ca(2+)-binding to the EF-hand motifs in the N-terminal domain of the carrier.

CONCLUSION

Biochemical characteristics suggest that plant APCs can mediate net transport of adenine nucleotides and hence, like their pendants from animals and yeast, might be involved in the alteration of the mitochondrial adenine nucleotide pool. Although, ATP-Ca was identified as an apparent import substrate of plant APCs in vitro it is arguable whether ATP-Ca formation and thus the corresponding transport can take place in vivo.

Intracellular magnesium level determines cell viability in the MPP(+) model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder resulting from mitochondrial dysfunction in dopaminergic neurons. Mitochondria are believed to be responsible for cellular Mg²⁺ homeostasis. Mg²⁺ is indispensable for maintaining ordinal cellular functions, hence perturbation of the cellular Mg²⁺ homeostasis may be responsible for the disorders of physiological functions and diseases including PD. However, the changes in intracellular Mg²⁺ concentration ([Mg²⁺]i) and the role of Mg²⁺ in PD have still been obscure. In this study, we investigated [Mg²⁺]i and its effect on neurodegeneration in the 1-methyl-4-phenylpyridinium (MPP⁺) model of PD in differentiated PC12 cells. Application of MPP⁺ induced an increase in [Mg²⁺]i immediately via two different pathways: Mg²⁺ release from mitochondria and Mg²⁺ influx across cell membrane, and the increased [Mg²⁺]i sustained for more than 16 h after MPP⁺ application. Suppression of Mg²⁺ influx decreased the viability of the cells exposed to MPP⁺. The cell viability correlated highly with [Mg²⁺]i. In the PC12 cells with suppressed Mg²⁺ influx, ATP concentration decreased and the amount of reactive oxygen species (ROS) increased after an 8h exposure to MPP⁺. Our results indicate that the increase in [Mg²⁺]i inhibited cellular ROS generation and maintained ATP production, which resulted in the protection from MPP⁺ toxicity.

Molecular Basis of MgATP Selectivity of the Mitochondrial SCaMC Carrier

The mitochondrial matrix is the supplier of cellular ATP. The short Ca(2+)-binding mitochondrial carrier (SCaMC) is one of the two mitochondrial carriers responsible for transporting ATP across the mitochondrial inner membrane. While the ADP/ATP carrier (AAC) accounts for the bulk ADP/ATP recycling in the matrix, the function of SCaMC is important for mitochondrial activities that depend on adenine nucleotides, such as gluconeogenesis and mitochondrial biogenesis. A key difference between SCaMC and AAC is that SCaMC selectively transports MgATP whereas AAC only transports free nucleotides. Here, we use a combination of nuclear magnetic resonance experiments and functional mutagenesis to investigate the structural basis of the MgATP selectivity in SCaMC. Our data revealed an MgATP binding site inside the transporter cavity, while identifying an aspartic acid residue that plays an important role in the higher selectivity for MgATP over free ATP.

Functional characterization and organ distribution of three mitochondrial ATP-Mg/Pi carriers in Arabidopsis thaliana

The Arabidopsis thaliana genome contains 58 membrane proteins belonging to the mitochondrial carrier family. Three members of this family, here named AtAPC1, AtAPC2, and AtAPC3, exhibit high structural similarities to the human mitochondrial ATP-Mg(2+)/phosphate carriers. Under normal physiological conditions the AtAPC1 gene was expressed at least five times more than the other two AtAPC genes in flower, leaf, stem, root and seedlings. However, in stress conditions the expression levels of AtAPC1 and AtAPC3 change. Direct transport assays with recombinant and reconstituted AtAPC1, AtAPC2 and AtAPC3 showed that they transport phosphate, AMP, ADP, ATP, adenosine 5'-phosphosulfate and, to a lesser extent, other nucleotides. AtAPC2 and AtAPC3 also had the ability to transport sulfate and thiosulfate. All three AtAPCs catalyzed a counter-exchange transport that was saturable and inhibited by pyridoxal-5'-phosphate. The transport activities of AtAPCs were also inhibited by the addition of EDTA or EGTA and stimulated by the addition of Ca(2+). Given that phosphate and sulfate can be recycled via their own specific carriers, these findings indicate that AtAPCs can catalyze net transfer of adenine nucleotides across the inner mitochondrial membrane in exchange for phosphate (or sulfate), and that this transport is regulated both at the transcriptional level and by Ca(2+).

Ca(2+) regulation of mitochondrial function in neurons

Calcium is thought to regulate respiration but it is unclear whether this is dependent on the increase in ATP demand caused by any Ca(2+) signal or to Ca(2+) itself. [Na(+)]i, [Ca(2+)]i and [ATP]i dynamics in intact neurons exposed to different workloads in the absence and presence of Ca(2+) clearly showed that Ca(2+)-stimulation of coupled respiration is required to maintain [ATP]i levels. Ca(2+) may regulate respiration by activating metabolite transport in mitochondria from outer face of the inner mitochondrial membrane, or after Ca(2+) entry in mitochondria through the calcium uniporter (MCU). Two Ca(2+)-regulated mitochondrial metabolite transporters are expressed in neurons, the aspartate-glutamate exchanger ARALAR/AGC1/Slc25a12, a component of the malate-aspartate shuttle, and the ATP-Mg/Pi exchanger SCaMC-3/APC2/Slc25a23, with S0.5 for Ca(2+) of 300nM and 3.4μM, respectively. The lack of SCaMC-3 results in a smaller Ca(2+)-dependent stimulation of respiration only at high workloads, as caused by veratridine, whereas the lack of ARALAR reduced by 46% basal OCR in intact neurons using glucose as energy source and the Ca(2+)-dependent responses to all workloads: a reduction of about 65-70% in the response to the high workload imposed by veratridine, and completely suppression of the OCR responses to moderate (K(+)-depolarization) and small (carbachol) workloads, effects reverted by pyruvate supply. For K(+)-depolarization, this occurs in spite of the presence of large [Ca(2+)]mit signals and increased formation of mitochondrial NAD(P)H. These results show that ARALAR-MAS is a major contributor of Ca(2+)-stimulated respiration in neurons by providing increased pyruvate supply to mitochondria. In its absence and under moderate workloads, matrix Ca(2+) is unable to stimulate pyruvate metabolism and entry in mitochondria suggesting a limited role of MCU in these conditions. This article was invited for a Special Issue entitled: 18th European Bioenergetic Conference.

SLC25A23 augments mitochondrial Ca²⁺ uptake, interacts with MCU, and induces oxidative stress-mediated cell death

Emerging findings suggest that two lineages of mitochondrial Ca(2+) uptake participate during active and resting states: 1) the major eukaryotic membrane potential-dependent mitochondrial Ca(2+) uniporter and 2) the evolutionarily conserved exchangers and solute carriers, which are also involved in ion transport. Although the influx of Ca(2+) across the inner mitochondrial membrane maintains metabolic functions and cell death signal transduction, the mechanisms that regulate mitochondrial Ca(2+) accumulation are unclear. Solute carriers--solute carrier 25A23 (SLC25A23), SLC25A24, and SLC25A25--represent a family of EF-hand-containing mitochondrial proteins that transport Mg-ATP/Pi across the inner membrane. RNA interference-mediated knockdown of SLC25A23 but not SLC25A24 and SLC25A25 decreases mitochondrial Ca(2+) uptake and reduces cytosolic Ca(2+) clearance after histamine stimulation. Ectopic expression of SLC25A23 EF-hand-domain mutants exhibits a dominant-negative phenotype of reduced mitochondrial Ca(2+) uptake. In addition, SLC25A23 interacts with mitochondrial Ca(2+) uniporter (MCU; CCDC109A) and MICU1 (CBARA1) while also increasing IMCU. In addition, SLC25A23 knockdown lowers basal mROS accumulation, attenuates oxidant-induced ATP decline, and reduces cell death. Further, reconstitution with short hairpin RNA-insensitive SLC25A23 cDNA restores mitochondrial Ca(2+) uptake and superoxide production. These findings indicate that SLC25A23 plays an important role in mitochondrial matrix Ca(2+) influx.

Purification, crystallization and preliminary X-ray diffraction of the N-terminal calmodulin-like domain of the human mitochondrial ATP-Mg/Pi carrier SCaMC1

SCaMC is an ATP-Mg/Pi carrier protein located at the mitochondrial inner membrane. SCaMC has an unusual N-terminal Ca(2+)-binding domain (NTD) in addition to its characteristic six-helix transmembrane bundle. The NTD of human SCaMC1 (residues 1-193) was expressed and purified in order to study its role in Ca(2+)-regulated ATP-Mg/Pi transport mediated by its transmembrane domain. While Ca(2+)-bound NTD could be crystallized, the apo state resisted extensive crystallization trials. Selenomethionine-labeled Ca(2+)-bound NTD crystals, which belonged to space group P6(2)22 with one molecule per asymmetric unit, diffracted X-rays to 2.9 Å resolution.

A self-sequestered calmodulin-like Ca²⁺ sensor of mitochondrial SCaMC carrier and its implication to Ca²⁺-dependent ATP-Mg/P(i) transport

The mitochondrial carriers play essential roles in energy metabolism. The short Ca²⁺-binding mitochondrial carrier (SCaMC) transports ATP-Mg in exchange for Pi and is important for activities that depend on adenine nucleotides. SCaMC adopts, in addition to the transmembrane domain (TMD) that transports solutes, an extramembrane N-terminal domain (NTD) that regulates solute transport in a Ca²⁺-dependent manner. Crystal structure of the Ca²⁺-bound NTD reveals a compact architecture in which the functional EF hands are sequestered by an endogenous helical segment. Nuclear magnetic resonance (NMR) relaxation rates indicated that removal of Ca²⁺ from NTD results in a major conformational switch from the rigid and compact Ca²⁺-bound state to the dynamic and loose apo state. Finally, we showed using surface plasmon resonance and NMR titration experiments that free apo NTDs could specifically interact with liposome-incorporated TMD, but that Ca²⁺ binding drastically weakened the interaction. Our results together provide a molecular explanation for Ca²⁺-dependent ATP-Mg flux in mitochondria.

Glucagon regulation of oxidative phosphorylation requires an increase in matrix adenine nucleotide content through Ca2+ activation of the mitochondrial ATP-Mg/Pi carrier SCaMC-3

It has been known for a long time that mitochondria isolated from hepatocytes treated with glucagon or Ca(2+)-mobilizing agents such as phenylephrine show an increase in their adenine nucleotide (AdN) content, respiratory activity, and calcium retention capacity (CRC). Here, we have studied the role of SCaMC-3/slc25a23, the mitochondrial ATP-Mg/Pi carrier present in adult mouse liver, in the control of mitochondrial AdN levels and respiration in response to Ca(2+) signals as a candidate target of glucagon actions. With the use of SCaMC-3 knock-out (KO) mice, we have found that the carrier is responsible for the accumulation of AdNs in liver mitochondria in a strictly Ca(2+)-dependent way with an S0.5 for Ca(2+) activation of 3.3 ± 0.9 μm. Accumulation of matrix AdNs allows a SCaMC-3-dependent increase in CRC. In addition, SCaMC-3-dependent accumulation of AdNs is required to acquire a fully active state 3 respiration in AdN-depleted liver mitochondria, although further accumulation of AdNs is not followed by increases in respiration. Moreover, glucagon addition to isolated hepatocytes increases oligomycin-sensitive oxygen consumption and maximal respiratory rates in cells derived from wild type, but not SCaMC-3-KO mice and glucagon administration in vivo results in an increase in AdN content, state 3 respiration and CRC in liver mitochondria in wild type but not in SCaMC-3-KO mice. These results show that SCaMC-3 is required for the increase in oxidative phosphorylation observed in liver mitochondria in response to glucagon and Ca(2+)-mobilizing agents, possibly by allowing a Ca(2+)-dependent accumulation of mitochondrial AdNs and matrix Ca(2+), events permissive for other glucagon actions.

A biophysical model of the mitochondrial ATP-Mg/P(i) carrier

Mitochondrial adenine nucleotide (AdN) content is regulated through the Ca(2+)-activated, electroneutral ATP-Mg/P(i) carrier (APC). The APC is a protein in the mitochondrial carrier super family that localizes to the inner mitochondrial membrane (IMM). It is known to modulate a number of processes that depend on mitochondrial AdN content, such as gluconeogenesis, protein synthesis, and citrulline synthesis. Despite this critical role, a kinetic model of the underlying mechanism has not been developed and validated. Here, a biophysical model of the APC is developed that is thermodynamically balanced and accurately reproduces a number of reported data sets from isolated rat liver and rat kidney mitochondria. The model is based on an ordered bi-bi mechanism for heteroexchange of ATP and P(i) and includes homoexchanges of ATP and P(i) to explain both the initial rate and time course data on ATP and P(i) transport via the APC. The model invokes seven kinetic parameters regarding the APC mechanism and three parameters related to matrix pH regulation by external P(i). These parameters are estimated based on 19 independent data curves; the estimated parameters are validated using six additional data curves. The model takes into account the effects of pH, Mg(2+), and Ca(2+) on ATP and P(i) transport via the APC, and supports the conclusion that the pH gradient across the IMM serves as the primary driving force for AdN uptake or efflux. Moreover, computer simulations demonstrate that extramatrix Ca(2+) modulates the turnover rate of the APC and not the binding affinity of ATP, as previously suggested.

Role of mitochondrial Ca2+ in the regulation of cellular energetics

Calcium is an important signaling molecule involved in the regulation of many cellular functions. The large free energy in the Ca(2+) ion membrane gradients makes Ca(2+) signaling inherently sensitive to the available cellular free energy, primarily in the form of ATP. In addition, Ca(2+) regulates many cellular ATP-consuming reactions such as muscle contraction, exocytosis, biosynthesis, and neuronal signaling. Thus, Ca(2+) becomes a logical candidate as a signaling molecule for modulating ATP hydrolysis and synthesis during changes in numerous forms of cellular work. Mitochondria are the primary source of aerobic energy production in mammalian cells and also maintain a large Ca(2+) gradient across their inner membrane, providing a signaling potential for this molecule. The demonstrated link between cytosolic and mitochondrial Ca(2+) concentrations, identification of transport mechanisms, and the proximity of mitochondria to Ca(2+) release sites further supports the notion that Ca(2+) can be an important signaling molecule in the energy metabolism interplay of the cytosol with the mitochondria. Here we review sites within the mitochondria where Ca(2+) plays a role in the regulation of ATP generation and potentially contributes to the orchestration of cellular metabolic homeostasis. Early work on isolated enzymes pointed to several matrix dehydrogenases that are stimulated by Ca(2+), which were confirmed in the intact mitochondrion as well as cellular and in vivo systems. However, studies in these intact systems suggested a more expansive influence of Ca(2+) on mitochondrial energy conversion. Numerous noninvasive approaches monitoring NADH, mitochondrial membrane potential, oxygen consumption, and workloads suggest significant effects of Ca(2+) on other elements of NADH generation as well as downstream elements of oxidative phosphorylation, including the F(1)F(O)-ATPase and the cytochrome chain. These other potential elements of Ca(2+) modification of mitochondrial energy conversion will be the focus of this review. Though most specific molecular mechanisms have yet to be elucidated, it is clear that Ca(2+) provides a balanced activation of mitochondrial energy metabolism that exceeds the alteration of dehydrogenases alone.

Inactivation of the mitochondrial carrier SLC25A25 (ATP-Mg2+/Pi transporter) reduces physical endurance and metabolic efficiency in mice

An ATP-Mg(2+/)P(i) inner mitochondrial membrane solute transporter (SLC25A25), which is induced during adaptation to cold stress in the skeletal muscle of mice with defective UCP1/brown adipose tissue thermogenesis, has been evaluated for its role in metabolic efficiency. SLC25A25 is thought to control ATP homeostasis by functioning as a Ca(2+)-regulated shuttle of ATP-Mg(2+) and P(i) across the inner mitochondrial membrane. Mice with an inactivated Slc25a25 gene have reduced metabolic efficiency as evidenced by enhanced resistance to diet-induced obesity and impaired exercise performance on a treadmill. Mouse embryo fibroblasts from Slc25a25(-/-) mice have reduced Ca(2+) flux across the endoplasmic reticulum, basal mitochondrial respiration, and ATP content. Although Slc25a25(-/-) mice are metabolically inefficient, the source of the inefficiency is not from a primary function in thermogenesis, because Slc25a25(-/-) mice maintain body temperature upon acute exposure to the cold (4 °C). Rather, the role of SLC25A25 in metabolic efficiency is most likely linked to muscle function as evidenced from the physical endurance test of mutant mice on a treadmill. Consequently, in the absence of SLC25A25 the efficiency of ATP production required for skeletal muscle function is diminished with secondary effects on adiposity. However, in the absence of UCP1-based thermogenesis, induction of Slc25a25 in mice with an intact gene may contribute to an alternative thermogenic pathway for the maintenance of body temperature during cold stress.

The regulation of OXPHOS by extramitochondrial calcium

Despite extensive research, the regulation of mitochondrial function is still not understood completely. Ample evidence shows that cytosolic Ca2+ has a strategic task in co-ordinating the cellular work load and the regeneration of ATP by mitochondria. Currently, the paradigmatic view is that Cacyt2+ taken up by the Ca2+ uniporter activates the matrix enzymes pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase and isocitrate dehydrogenase. However, we have recently found that Ca2+ regulates the glutamate-dependent state 3 respiration by the supply of glutamate to mitochondria via aralar, a mitochondrial glutamate/aspartate carrier. Since this activation is not affected by ruthenium red, glutamate transport into mitochondria is controlled exclusively by extramitochondrial Ca2+. Therefore, this discovery shows that besides intramitochondrial also extramitochondrial Ca2+ regulates oxidative phosphorylation. This new mechanism acts as a mitochondrial "gas pedal", supplying the OXPHOS with substrate on demand. These results are in line with recent findings of Satrustegui and Palmieri showing that aralar as part of the malate-aspartate shuttle is involved in the Ca2+-dependent transport of reducing hydrogen equivalents (from NADH) into mitochondria. This review summarises results and evidence as well as hypothetical interpretations of data supporting the view that at the surface of mitochondria different regulatory Ca2+-binding sites exist and can contribute to cellular energy homeostasis. Moreover, on the basis of our own data, we propose that these surface Ca2+-binding sites may act as targets for neurotoxic proteins such as mutated huntingtin and others. The binding of these proteins to Ca2+-binding sites can impair the regulation by Ca2+, causing energetic depression and neurodegeneration.

The role of Ca(2+) signaling in the coordination of mitochondrial ATP production with cardiac work

The heart is capable of balancing the rate of mitochondrial ATP production with utilization continuously over a wide range of activity. This results in a constant phosphorylation potential despite a large change in metabolite turnover. The molecular mechanisms responsible for generating this energy homeostasis are poorly understood. The best candidate for a cytosolic signaling molecule reflecting ATP hydrolysis is Ca(2+). Since Ca(2+) initiates and powers muscle contraction as well as serves as the primary substrate for SERCA, Ca(2+) is an ideal feed-forward signal for priming ATP production. With the sarcoplasmic reticulum to cytosolic Ca(2+) gradient near equilibrium with the free energy of ATP, cytosolic Ca(2+) release is exquisitely sensitive to the cellular energy state providing a feedback signal. Thus, Ca(2+) can serve as a feed-forward and feedback regulator of ATP production. Consistent with this notion is the correlation of cytosolic and mitochondrial Ca(2+) with work in numerous preparations as well as the localization of mitochondria near Ca(2+) release sites. How cytosolic Ca(2+) signaling might regulate oxidative phosphorylation is a focus of this review. The relevant Ca(2+) sensitive sites include several dehydrogenases and substrate transporters together with a post-translational modification of F1-FO-ATPase and cytochrome oxidase. Thus, Ca(2+) apparently activates both the generation of the mitochondrial membrane potential as well as utilization to produce ATP. This balanced activation extends the energy homeostasis observed in the cytosol into the mitochondria matrix in the never resting heart.

Modeling of ATP-ADP steady-state exchange rate mediated by the adenine nucleotide translocase in isolated mitochondria

A computational model for the ATP-ADP steady-state exchange rate mediated by adenine nucleotide translocase (ANT) versus mitochondrial membrane potential dependence in isolated rat liver mitochondria is presented. The model represents the system of three ordinary differential equations, and the basic components included are ANT, F(0)/F(1)-ATPase, and the phosphate carrier. The model reproduces quantitatively the relationship between mitochondrial membrane potential and the ATP-ADP steady-state exchange rate mediated by the ANT operating in the forward mode, with the assumption that the phosphate carrier functions under rapid equilibrium. Furthermore, the model can simulate the kinetics of experimentally measured data on mitochondrial membrane potential titrated by an uncoupler. Verified predictions imply that the ADP influx rate is highly dependent on the mitochondrial membrane potential, and in the 0-100 mV range it is close to zero, owing to extremely low matrix ATP values. In addition to providing theoretical values of free matrix ATP and ADP, the model explains the diminished ADP-ATP exchange rate in the presence of nigericin, a condition in which there is hyperpolarization of the inner mitochondrial membrane at the expense of the mitochondrial Delta pH gradient; the latter parameter influences matrix inorganic phosphate and ATP concentrations in a manner also described.

A novel kinetic assay of mitochondrial ATP-ADP exchange rate mediated by the ANT

A novel method exploiting the differential affinity of ADP and ATP to Mg(2+) was developed to measure mitochondrial ADP-ATP exchange rate. The rate of ATP appearing in the medium after addition of ADP to energized mitochondria, is calculated from the measured rate of change in free extramitochondrial [Mg(2+)] reported by the membrane-impermeable 5K(+) salt of the Mg(2+)-sensitive fluorescent indicator, Magnesium Green, using standard binding equations. The assay is designed such that the adenine nucleotide translocase (ANT) is the sole mediator of changes in [Mg(2+)] in the extramitochondrial volume, as a result of ADP-ATP exchange. We also provide data on the dependence of ATP efflux rate within the 6.8-7.8 matrix pH range as a function of membrane potential. Finally, by comparing the ATP-ADP steady-state exchange rate to the amount of the ANT in rat brain synaptic, brain nonsynaptic, heart and liver mitochondria, we provide molecular turnover numbers for the known ANT isotypes.

Mitochondria and energetic depression in cell pathophysiology

Mitochondrial dysfunction is a hallmark of almost all diseases. Acquired or inherited mutations of the mitochondrial genome DNA may give rise to mitochondrial diseases. Another class of disorders, in which mitochondrial impairments are initiated by extramitochondrial factors, includes neurodegenerative diseases and syndromes resulting from typical pathological processes, such as hypoxia/ischemia, inflammation, intoxications, and carcinogenesis. Both classes of diseases lead to cellular energetic depression (CED), which is characterized by decreased cytosolic phosphorylation potential that suppresses the cell's ability to do work and control the intracellular Ca(2+) homeostasis and its redox state. If progressing, CED leads to cell death, whose type is linked to the functional status of the mitochondria. In the case of limited deterioration, when some amounts of ATP can still be generated due to oxidative phosphorylation (OXPHOS), mitochondria launch the apoptotic cell death program by release of cytochrome c. Following pronounced CED, cytoplasmic ATP levels fall below the thresholds required for processing the ATP-dependent apoptotic cascade and the cell dies from necrosis. Both types of death can be grouped together as a mitochondrial cell death (MCD). However, there exist multiple adaptive reactions aimed at protecting cells against CED. In this context, a metabolic shift characterized by suppression of OXPHOS combined with activation of aerobic glycolysis as the main pathway for ATP synthesis (Warburg effect) is of central importance. Whereas this type of adaptation is sufficiently effective to avoid CED and to control the cellular redox state, thereby ensuring the cell survival, it also favors the avoidance of apoptotic cell death. This scenario may underlie uncontrolled cellular proliferation and growth, eventually resulting in carcinogenesis.

Impaired regulation of brain mitochondria by extramitochondrial Ca2+ in transgenic Huntington disease rats

Huntington disease (HD) is characterized by polyglutamine expansions of huntingtin (htt), but the underlying pathomechanisms have remained unclear. We studied brain mitochondria of transgenic HD rats with 51 glutamine repeats (htt(51Q)), modeling the adult form of HD. Ca(free)(2+) up to 2 mum activated state 3 respiration of wild type mitochondria with glutamate/malate or pyruvate/malate as substrates. Ca(free)(2+) above 2 mum inhibited respiration via cyclosporin A-dependent permeability transition (PT). Ruthenium red, an inhibitor of the mitochondrial Ca(2+) uniporter, did not affect the Ca(2+)-dependent activation of respiration but reduced Ca(2+)-induced inhibition. Thus, Ca(2+) activation was mediated exclusively by extramitochondrial Ca(2+), whereas inhibition was promoted also by intramitochondrial Ca(2+). In contrast, htt(51Q) mitochondria showed a deficient state 3 respiration, a lower sensitivity to Ca(2+) activation, and a higher susceptibility to Ca(2+)-dependent inhibition. Furthermore htt(51Q) mitochondria exhibited a diminished membrane potential stability in response to Ca(2+), lower capacities and rates of Ca(2+) accumulation, and a decreased Ca(2+) threshold for PT in a substrate-independent but cyclosporin A-sensitive manner. Compared with wild type, Ca(2+)-induced inhibition of respiration of htt(51Q) mitochondria was less sensitive to ruthenium red, indicating the involvement of extramitochondrial Ca(2+). In conclusion, we demonstrate a novel mechanism of mitochondrial regulation by extramitochondrial Ca(2+). We suggest that specific regulatory Ca(2+) binding sites on the mitochondrial surface, e.g. the glutamate/aspartate carrier (aralar), mediate this regulation. Interactions between htt(51Q) and distinct targets such as aralar and/or the PT pore may underlie mitochondrial dysregulation leading to energetic depression, cell death, and tissue atrophy in HD.

Ca2+ Activation kinetics of the two aspartate-glutamate mitochondrial carriers, aralar and citrin: role in the heart malate-aspartate NADH shuttle

Ca(2+) regulation of the Ca(2+) binding mitochondrial carriers for aspartate/glutamate (AGCs) is provided by their N-terminal extensions, which face the intermembrane space. The two mammalian AGCs, aralar and citrin, are members of the malate-aspartate NADH shuttle. We report that their N-terminal extensions contain up to four pairs of EF-hand motifs plus a single vestigial EF-hand, and have no known homolog. Aralar and citrin contain one fully canonical EF-hand pair and aralar two additional half-pairs, in which a single EF-hand is predicted to bind Ca(2+). Shuttle activity in brain or skeletal muscle mitochondria, which contain aralar as the major AGC, is activated by Ca(2+) with S(0.5) values of 280-350 nm; higher than those obtained in liver mitochondria (100-150 nm) that contain citrin as the major AGC. We have used aralar- and citrin-deficient mice to study the role of the two isoforms in heart, which expresses both AGCs. The S(0.5) for Ca(2+) activation of the shuttle in heart mitochondria is about 300 nm, and it remains essentially unchanged in citrin-deficient mice, although it undergoes a drastic reduction to about 100 nm in aralar-deficient mice. Therefore, aralar and citrin, when expressed as single isoforms in heart, confer differences in Ca(2+) activation of shuttle activity, probably associated with their structural differences. In addition, the results reveal that the two AGCs fully account for shuttle activity in mouse heart mitochondria and that no other glutamate transporter can replace the AGCs in this pathway.

Mitochondrial Transporters as Novel Targets for Intracellular Calcium Signaling

Ca2+signaling in mitochondria is important to tune mitochondrial function to a variety of extracellular stimuli. The main mechanism is Ca2+entry in mitochondria via the Ca2+uniporter followed by Ca2+activation of three dehydrogenases in the mitochondrial matrix. This results in increases in mitochondrial NADH/NAD ratios and ATP levels and increased substrate uptake by mitochondria. We review evidence gathered more than 20 years ago and recent work indicating that substrate uptake, mitochondrial NADH/NAD ratios, and ATP levels may be also activated in response to cytosolic Ca2+signals via a mechanism that does not require the entry of Ca2+in mitochondria, a mechanism depending on the activity of Ca2+-dependent mitochondrial carriers (CaMC). CaMCs fall into two groups, the aspartate-glutamate carriers (AGC) and the ATP-Mg/Picarriers, also named SCaMC (for short CaMC). The two mammalian AGCs, aralar and citrin, are members of the malate-aspartate NADH shuttle, and citrin, the liver AGC, is also a member of the urea cycle. Both types of CaMCs are activated by Ca2+in the intermembrane space and function together with the Ca2+uniporter in decoding the Ca2+signal into a mitochondrial response.

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

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

Novel variants of human SCaMC-3, an isoform of the ATP-Mg/P(i) mitochondrial carrier, generated by alternative splicing from 3'-flanking transposable elements

CaMCs (calcium-dependent mitochondrial carriers) represent a novel subfamily of metabolite carriers of mitochondria. The ATP-Mg/P(i) co-transporter, functionally characterized more than 20 years ago, has been identified to be a CaMC member. There are three isoforms of the ATP-Mg/P(i) carrier in mammals, SCaMC-1 (short CaMC-1), -2 and -3 (or APC-1, -3 and -2 respectively), corresponding to the genes SLC25A24, SLC25A25 and SLC25A23 respectively, as well as six N-terminal variants generated by alternative splicing for SCaMC-1 and -2 isoforms. In the present study, we describe four new variants of human SCaMC-3 generated by alternative splicing. The new mRNAs use the exon 9 3'-donor site and distinct 5'-acceptor sites from repetitive elements, in regions downstream of exon 10, the last exon in all SCaMCs. Transcripts lacking exon 10 (SCaMC-3b, -3b', -3c and -3d) code for shortened proteins lacking the last transmembrane domain of 422, 456 and 435 amino acids, and were found in human tissues and HEK-293T cells. Mitochondrial targeting of overexpressed SCaMC-3 variants is incomplete. Surprisingly, the import impairment is overcome by removing the N-terminal extension of these proteins, suggesting that the hydrophilic N-terminal domain also participates in the mitochondrial import process, as shown for the CaMC members aralar and citrin [Roesch, Hynds, Varga, Tranebjaerg and Koehler (2004) Hum. Mol. Genet. 13, 2101-2111].

Identification of the mitochondrial ATP-Mg/Pi transporter. Bacterial expression, reconstitution, functional characterization, and tissue distribution

The mitochondrial carriers are a family of transport proteins that, with a few exceptions, are found in the inner membranes of mitochondria. They shuttle metabolites, nucleotides, and cofactors through this membrane and thereby connect and/or regulate cytoplasm and matrix functions. ATP-Mg is transported in exchange for phosphate, but no protein has ever been associated with this activity. We have isolated three human cDNAs that encode proteins of 458, 468, and 489 amino acids with 66-75% similarity and with the characteristic features of the mitochondrial carrier family in their C-terminal domains and three EF-hand Ca(2+)-binding motifs in their N-terminal domains. These proteins have been overexpressed in Escherichia coli and reconstituted into phospholipid vesicles. Their transport properties and their targeting to mitochondria demonstrate that they are isoforms of the ATP-Mg/Pi carrier described in the past in whole mitochondria. The tissue specificity of the three isoforms shows that at least one isoform was present in all of the tissues investigated. Because phosphate recycles via the phosphate carrier in mitochondria, the three isoforms of the ATP-Mg/Pi carrier are most likely responsible for the net uptake or efflux of adenine nucleotides into or from the mitochondria and hence for the variation in the matrix adenine nucleotide content, which has been found to change in many physiopathological situations.

Identification of a novel human subfamily of mitochondrial carriers with calcium-binding domains

Aralar1 and citrin were identified as calcium binding aspartate/glutamate carriers (AGC) in mitochondria. The presence of calcium binding motifs facing the extramitochondrial space allows the regulation of the transport activity of these carriers by cytosolic calcium and provides a new mechanism to transduce calcium signals in mitochondria without the requirement of calcium entry in the organelle. We now report the complete characterization of a second subfamily of human calcium binding mitochondrial carriers named SCaMC (short calcium-binding mitochondrial carriers). We have identified three SCaMC genes in the human genome. All code for highly conserved proteins (about 70-80% identity), of about 500 amino acids with a characteristic mitochondrial carrier domain at the C terminus, and an N-terminal extension harboring four EF-hand binding motifs with high similarity to calmodulin. All SCaMC proteins were found to be located exclusively in mitochondria, and their N-terminal extensions were dispensable for the correct mitochondrial targeting of the polypeptides. SCaMC-1 is the human orthologue of the rabbit Efinal protein, which was reported to be located in peroxisomes, and SCaMC-2 is the human orthologue of the rat MCSC protein, described as up-regulated by dexamethasone in AR42J cells. One of the SCaMC genes, SCaMC-2, has four variants generated by alternative splicing, resulting in proteins with a common C terminus but with variations in their N-terminal halves, including the loss of one to three EF-hand motifs. These results make SCaMC one of most complex subfamilies of mitochondrial carriers and suggest that the large number of isoforms and splice variants may confer different calcium sensitivity to the transport activity of these carriers.

Effect of adenine nucleotide pool size in mitochondria on intramitochondrial ATP levels

Net adenine nucleotide transport into and out of the mitochondrial matrix via the ATP-Mg/Pi carrier is activated by micromolar calcium concentrations in rat liver mitochondria. The purpose of this study was to induce net adenine nucleotide transport by varying the substrate supply and/or extramitochondrial ATP consumption in order to evaluate the effect of the mitochondrial adenine nucleotide pool size on intramitochondrial adenine nucleotide patterns under phosphorylating conditions. Above 12 nmol/mg protein, intramitochondrial ATP/ADP increased with an increase in the mitochondrial adenine nucleotide pool. The relationship between the rate of respiration and the mitochondrial ADP concentration did not depend on the mitochondrial adenine nucleotide pool size up to 9 nmol ADP/mg mitochondrial protein. The results are compatible with the notion that net uptake of adenine nucleotides at low energy states supports intramitochondrial ATP consuming processes and energized mitochondria may lose adenine nucleotides. The decrease of the mitochondrial adenine nucleotide content below 9 nmol/mg protein inhibits oxidative phosphorylation. In particular, this could be the case within the postischemic phase which is characterized by low cytosolic adenine nucleotide concentrations and energized mitochondria.

Magnesium transport by mitochondria

The pathways for the uptake and extrusion of Mg2+ by mitochondria are now well defined, the present evidence suggests that uptake occurs by nonspecific diffusive pathways in response to elevated membrane potential. There is disagreement as to some of the properties of Mg2+ efflux from mitochondria, but the reaction resembles K+ efflux in many ways and may occur in exchange for H+. Matrix free magnesium ion concentration, [Mg2+], can be measured using fluorescent probes and is set very close to cytosol [Mg2+] by a balance between influx and efflux and by the availability of ligands, such as Pi. There are indications that matrix [Mg2+] may be under hormonal control and that it contributes to the regulation of mitochondrial metabolism and transport reactions.

Mechanism and regulation of the mitochondrial ATP-Mg/P(i) carrier

The mitochondrial ATP-Mg/P(i) carrier functions to modulate the matrix adenine nucleotide pool size (ATP + ADP + AMP). Micromolar Ca2+ is required to activate the carrier. Net adenine nucleotide transport occurs as an electroneutral divalent exchange of ATP-Mg2- for HPO4(2-). A steady-state adenine nucleotide pool size is attained when the HPO4(2-) and ATP-Mg2- matrix/cytoplasm concentration ratios are the same. This means that ATP-Mg2- can be accumulated against a concentration gradient in proportion to the [HPO4(2-)] gradient that is normally maintained by the P(i)/OH- carrier. In liver, changes in matrix adenine nucleotide concentrations that are brought about by the ATP-Mg/P(i) carrier can affect the activity of adenine nucleotide-dependent enzymes that are in the mitochondrial compartment. These enzymes in turn contribute to the overall regulation of bioenergetic function, flux through the gluconeogenesis and urea synthesis pathways, and organelle biogenesis. The ATP-Mg/P(i) carrier is distinct from other mitochondrial transport systems with respect to kinetics and to substrate and inhibitor sensitivity. It is the only carrier regulated by Ca2+. This carrier is present in kidney and liver mitochondria, but not in heart.

Regulation of the mitochondrial ATP-Mg/Pi carrier in isolated hepatocytes

This study investigated the cellular regulation of net adenine nucleotide movements between the cytoplasm and mitochondria in intact cells. Such movements are presumed to occur primarily by ATP-Mg exchange with Pi via the mitochondrial ATP-Mg/Pi carrier. Vasopressin, A23187, and thapsigargin all elevate intracellular free [Ca2+] and all caused dose-dependent increases in the mitochondrial adenine nucleotide content (29, 63, and 39%, respectively). Phorbol 12-myristate 13-acetate had no effect. The effect of vasopressin was abolished when cytoplasmic [ATP] was decreased (by 43%) and [Pi] was increased (3-fold) by addition of carboxyatractyloside. The effect of thapsigargin was abolished by addition of xylulose to deplete cytoplasmic [ATP] (by 50%) and [Pi] (> 4-fold). The results indicate that in intact cells Ca2+ activates the mitochondrial ATP-Mg/Pi carrier to enable changes in the subcellular distribution of adenine nucleotides and that the relative [ATP] and [Pi] gradients govern the direction and magnitude of net adenine nucleotide movements between the cytoplasm and mitochondria.

ATP-Mg/Pi carrier activity in rat liver mitochondria

The ATP-Mg/Pi carrier in liver mitochondria is activated by micromolar Ca2+ and mediates net adenine nucleotide transport into and out of the mitochondrial matrix. The purpose of this study was to characterize certain features of ATP-Mg/Pi carrier activity that are essential for understanding how the mitochondrial adenine nucleotide content is regulated. The relative importance of ATP and ADP as transport substrates was investigated using specific trap assays to measure their separate rates of carrier-mediated efflux with Pi as the external counterion. Under energized conditions ATP efflux accounted for 88% of total ATP+ADP efflux. With oligomycin present to lower the matrix ATP/ADP ratio, ATP efflux was eliminated and ADP efflux was relatively unaffected. Mg2+ was stoichiometrically required for ATP influx and is probably transported simultaneously with ATP. Ca2+ and Mn2+ could substitute for the stoichiometric Mg2+ requirement. ADP influx and Pi-induced adenine nucleotide efflux were unaffected by external Mg2+. Experiments with Pi analogues suggested that Pi is transported as the divalent anion, HPO4(2-). The results show that ATP-Mg and divalent Pi are the major transport substrates; the most probable transport mechanism for the ATP-Mg/Pi carrier is an electroneutral exchange. The results are consistent with the hypothesis that the direction and magnitude of net adenine nucleotide movements are determined mainly by the (ATP-Mg)2- and HPO4(2-) concentration gradients across the inner mitochondrial membrane.

Cyclosporine A inhibits ATP net uptake of rat kidney mitochondria

The adenine nucleotide content of mitochondria varies at several physiological and pathological situations. Both net transport and intramitochondrial catabolism of adenine nucleotides has been suggested to be responsible for these changes. Here, the influence of cyclosporine A on the ATP net uptake of isolated rat kidney mitochondria was examined. The ATP net uptake of mitochondria depleted of matrix adenine nucleotides by pyrophosphate treatment was inhibited by cyclosporine A showing a I50 value of about 4 nmol/mg mitochondrial protein. Because intramitochondrial adenine nucleotide content is important for several mitochondrial functions such as oxidative phosphorylation, Ca2+ homoeostasis and mitochondrial biogenesis, it is concluded that the inhibition of adenine nucleotide net transport and a decrease of adenine nucleotide content may be involved in the immunosuppressive and nephrotoxic effects of cyclosporine A.

The contribution of adenine nucleotide loss to ischemia-induced impairment of rat kidney cortex mitochondria

Adenine nucleotides and respiration were assayed with rat kidney mitochondria depleted of adenine nucleotides by pyrophosphate treatment and by normothermic ischemia, respectively, with the aim of identifying net uptake of ATP as well as elucidating the contribution of adenine nucleotide loss to the ischemic impairment of oxidative phosphorylation. Treatment of rat kidney mitochondria with pyrophosphate caused a loss of adenine nucleotides as well as a decrease of state 3 respiration. After incubation of pyrophosphate-treated mitochondria with ATP, Mg2+ and phosphate, the content of adenine nucleotides increased. We propose that kidney mitochondria possess a mechanism for net uptake of ATP. Restoration of a normal content of matrix adenine nucleotides was related to full recovery of the rate of state 3 respiration. A hyperbolic relationship between the matrix content of adenine nucleotides and the rate of state 3 respiration was observed. Mitochondria isolated from kidneys exposed to normothermic ischemia were characterized by a decrease in the content of adenine nucleotides as well as in state 3 respiration. Incubation of ischemic mitochondria with ATP, Mg2+ and phosphate restored the content of adenine nucleotides to values measured in freshly-isolated mitochondria. State 3 respiration of ischemic mitochondria reloaded with ATP recovered only partially. The rate of state 3 respiration increased by ATP-reloading approached that of uncoupler-stimulated respiration measured with ischemic mitochondria. These findings suggest that the decrease of matrix adenine nucleotides contributes to the impairment of ischemic mitochondria as well as underlining the occurrence of additional molecular changes of respiratory chain limiting the oxidative phosphorylation.