Advances in the purification of the mitochondrial Ca2+ uniporter using the labeled inhibitor 103Ru360

Pharmacological modulation of mitochondrial calcium homeostasis

Mitochondria are pivotal organelles in calcium (Ca²⁺ ) handling and signalling, constituting intracellular checkpoints for numerous processes that are vital for cell life. Alterations in mitochondrial Ca²⁺ homeostasis have been linked to a variety of pathological conditions and are critical in the aetiology of several human diseases. Efforts have been taken to harness mitochondrial Ca²⁺ transport mechanisms for therapeutic intervention, but pharmacological compounds that direct and selectively modulate mitochondrial Ca²⁺ homeostasis are currently lacking. New avenues have, however, emerged with the breakthrough discoveries on the genetic identification of the main players involved in mitochondrial Ca²⁺ influx and efflux pathways and with recent hints towards a deep understanding of the function of these molecular systems. Here, we review the current advances in the understanding of the mechanisms and regulation of mitochondrial Ca²⁺ homeostasis and its contribution to physiology and human disease. We also introduce and comment on the recent progress towards a systems-level pharmacological targeting of mitochondrial Ca²⁺ homeostasis.

Crosslink between calcium and sodium signalling

NEW FINDINGS

What is the topic of this review? This paper overviews the links between Ca2+ and Na+ signalling in various types of cells. What advances does it highlight? This paper highlights the general importance of ionic signalling and overviews the molecular mechanisms linking Na+ and Ca2+ dynamics. In particular, the narrative focuses on the molecular physiology of plasmalemmal and mitochondrial Na+ -Ca2+ exchangers and plasmalemmal transient receptor potential channels. Functional consequences of Ca2+ and Na+ signalling for co-ordination of neuronal activity with astroglial homeostatic pathways fundamental for synaptic transmission are discussed.

ABSTRACT

Transmembrane ionic gradients, which are an indispensable feature of life, are used for generation of cytosolic ionic signals that regulate a host of cellular functions. Intracellular signalling mediated by Ca2+ and Na+ is tightly linked through several molecular pathways that generate Ca2+ and Na+ fluxes and are in turn regulated by both ions. Transient receptor potential (TRP) channels bridge endoplasmic reticulum Ca2+ release with generation of Na+ and Ca2+ currents. The plasmalemmal Na+ -Ca2+ exchanger (NCX) flickers between forward and reverse mode to co-ordinate the influx and efflux of both ions with membrane polarization and cytosolic ion concentrations. The mitochondrial calcium uniporter channel (MCU) and mitochondrial Na+ -Ca2+ exchanger (NCLX) mediate Ca2+ entry into and release from this organelle and couple cytosolic Ca2+ and Na+ fluctuations with cellular energetics. Cellular Ca2+ and Na+ signalling controls numerous functional responses and, in the CNS, provides for fast regulation of astroglial homeostatic cascades that are crucial for maintenance of synaptic transmission.

MICU1 encodes a mitochondrial EF hand protein required for Ca(2+) uptake

Mitochondrial calcium uptake plays a central role in cell physiology by stimulating ATP production, shaping cytosolic calcium transients, and regulating cell death. The biophysical properties of mitochondrial calcium uptake have been studied in detail, but the underlying proteins remain elusive. Here, we utilize an integrative strategy to predict human genes involved in mitochondrial calcium entry based on clues from comparative physiology, evolutionary genomics, and organelle proteomics. RNA interference against 13 top candidates highlighted one gene that we now call mitochondrial calcium uptake 1 (MICU1). Silencing MICU1 does not disrupt mitochondrial respiration or membrane potential but abolishes mitochondrial calcium entry in intact and permeabilized cells, and attenuates the metabolic coupling between cytosolic calcium transients and activation of matrix dehydrogenases. MICU1 is associated with the organelle’s inner membrane and has two canonical EF hands that are essential for its activity, suggesting a role in calcium sensing. MICU1 represents the founding member of a set of proteins required for high capacity mitochondrial calcium entry. Its discovery may lead to the complete molecular characterization of mitochondrial calcium uptake pathways, and offers genetic strategies for understanding their contribution to normal physiology and disease.

The molecular identity of the mitochondrial Ca2+ sequestration system

There is ample evidence to suggest that a dramatic decrease in mitochondrial Ca(2+) retention may contribute to the cell death associated with stroke, excitotoxicity, ischemia and reperfusion, and neurodegenerative diseases. Mitochondria from all studied tissues can accumulate and store Ca(2+) , but the maximum Ca(2+) storage capacity varies widely and exhibits striking tissue specificity. There is currently no explanation for this fact. Precipitation of Ca(2+) and phosphate in the mitochondrial matrix has been suggested to be the major form of storage of accumulated Ca(2+) in mitochondria. How this precipitate is formed is not known. The molecular identity of almost all proteins involved in Ca(2+) transport, storage and formation of the permeability transition pore is also unknown. This review summarizes studies aimed at identifying these proteins, and describes the properties of a known mitochondrial protein that may be involved in Ca(2+) transport and the structure of the permeability transition pore.

The fateful encounter of mitochondria with calcium: how did it happen?

A number of findings in the 1950s had offered indirect indications that mitochondria could accumulate Ca2+. In 1961, the phenomenon was directly demonstrated using isolated mitochondria: the uptake process was driven by respiratory chain activity or by the hydrolysis of added ATP. It could be accompanied by the simultaneous uptake of inorganic phosphate, in which case precipitates of hydroxyapatite were formed in the matrix, buffering its free Ca2+ concentration. The properties of the uptake process were established in the 1960s and 1970s: the uptake of Ca2+ occurred electrophoretically on a carrier that has not yet been molecularly identified, and was released from mitochondria via a Na+/Ca2+ antiporter. A H+/Ca2+ release exchanger was also found to operate in some mitochondrial types. The permeability transition pore was later also found to mediate the efflux of Ca2+ from mitochondria. In the mitochondrial matrix two TCA cycle dehydrogenases and pyruvate dehydrogenase phosphate phosphatase were found to be regulated in the matrix by the cycling of Ca2+ across the inner membrane. In conditions of cytoplasmic Ca2+ overload mitochondria could store for a time large amounts of precipitated Ca2+-phosphate, thus permitting cells to survive situations of Ca2+ emergency. The uptake process was found to have very low affinity for Ca2+: since the bulk concentration of Ca2+ in the cytoplasm is in the low to mid-nM range, it became increasingly difficult to postulate a role of mitochondria in the regulation of cytoplsmic Ca2+. A number of findings had nevertheless shown that energy linked Ca2+ transport occurred efficiently in mitochondria of various tissues in situ. The paradox was only solved in the 1990s, when it was found that the concentration of Ca2+ in the cytoplasm is not uniform: perimitochondrial micropools are created by the agonist-promoted discharge of Ca2+ from vicinal stores in which the concentration of Ca2+ is high enough to activate the low affinity mitochondrial uniporter. Mitochondria thus regained center stage as important regulators of cytoplasmic Ca2+ (not only of their own internal Ca2+). Their Ca2+ uptake systems was found to react very rapidly to cytoplasmic Ca2+ demands, even in the 150-200 msec time scale of processes like the contraction and relaxation of heart. An important recent development in the area of mitochondrial Ca2+ transport is its involvement in the disease process. Ca2+ signaling defects are now gaining increasing importance in the pathogenesis of diseases, e.g., neurodegenerative diseases. Since mitochondria have now regained a central role in the regulation of cytoplasmic Ca2+, dysfunctions of their Ca2+ controlling systems have expectedly been found to be involved in the pathogenesis of numerous disease processes.

Mitochondrial ryanodine receptors and other mitochondrial Ca2+ permeable channels

Ca(2+) channels that underlie mitochondrial Ca(2+) transport first reported decades ago have now just recently been precisely characterized electrophysiologically. Numerous data indicate that mitochondrial Ca(2+) uptake via these channels regulates multiple intracellular processes by shaping cytosolic and mitochondrial Ca(2+) transients, as well as altering the cellular metabolic and redox state. On the other hand, mitochondrial Ca(2+) overload also initiates a cascade of events that leads to cell death. Thus, characterization of mitochondrial Ca(2+) channels is central to a comprehensive understanding of cell signaling. Here, we discuss recent progresses in the biophysical and electrophysiological characterization of several distinct mitochondrial Ca(2+) channels.

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.

Mitochondria As A Target For Early Detection and Diagnosis of Cancer

Mitochondrial dysfunction and mutations in mitochondrial DNA (mtDNA) have been frequently reported in cancer, neurodegenerative diseases, diabetes, and aging syndromes. The mitochondrion genome (16.5 Kb) codes only for a small fraction (estimated to be 1%) of the proteins housed within this organelle. The other proteins are encoded by the nuclear DNA (nDNA) and transported into the mitochondria. The identification of mitochondrial proteins that are aberrantly expressed in cancer cells and other diseases is now possible through recent developments in proteomic and bioinformatic technologies. These developments set the stage for a comprehensive organelle-based proteomic approach for the identification of new markers for the early detection, risk assessment, and diagnosis of cancer, and other diseases and for the identification of new targets for therapeutic prevention and intervention.

Mitochondrial ion channels

In work spanning more than a century, mitochondria have been recognized for their multifunctional roles in metabolism, energy transduction, ion transport, inheritance, signaling, and cell death. Foremost among these tasks is the continuous production of ATP through oxidative phosphorylation, which requires a large electrochemical driving force for protons across the mitochondrial inner membrane. This process requires a membrane with relatively low permeability to ions to minimize energy dissipation. However, a wealth of evidence now indicates that both selective and nonselective ion channels are present in the mitochondrial inner membrane, along with several known channels on the outer membrane. Some of these channels are active under physiological conditions, and others may be activated under pathophysiological conditions to act as the major determinants of cell life and death. This review summarizes research on mitochondrial ion channels and efforts to identify their molecular correlates. Except in a few cases, our understanding of the structure of mitochondrial ion channels is limited, indicating the need for focused discovery in this area.

Mitochondrial channelopathies in aging

Defects in ion channels (channelopathies) are increasingly found in a large spectrum of human pathologies including aging. Mutations in genes encoding ion channel proteins, which disrupt channel function, are the most commonly identified cause of channelopathies. Mutations in associated proteins, alterations in the expression of ion channels, or changes in the activity of non-mutated channel genes or associated proteins can also produce acquired channelopathies. Mitochondria, the powerhouse of the cells, are considered to be the most important cellular organelles to contribute to aging mainly because of their role in the production of reactive oxygen species in the initiation of apoptotic cell remodeling and in efficient ATP synthesis. During the past 50 years, multiple ion channels or transporters have been found in mitochondria, and the relationship between the activity of these channels and cellular aging, as well as the overall cellular biological function, has been intensively studied in a number of cell types and animal models. In this review, we discuss the better characterized mitochondrial ion channels whose dysfunction (mitochondrial channelopathies) may affect or accelerate the aging processes. These channels include the mitochondrial ATP-sensitive potassium channel (mitoK(ATP)), Ca(2+) transporters, voltage-dependent anion channel, and the mitochondrial permeability transition pore (mitoPTP).

A Photoactivable Probe for Calcium Binding Proteins

Ca2+ as a signaling molecule carries information pivotal to cell life and death via its reversible interaction with a specific site in a protein. Although numerous Ca2+-dependent activities are known, the proteins responsible for some of these activities remain unidentified. We synthesized and characterized a photoreactive reagent, azido ruthenium (AzRu), which interacts specifically with Ca2+ binding proteins and strongly inhibits their Ca2+-dependent activities, regardless of their catalytic mechanisms or functional state as purified proteins, embedded in the membrane or in intact cells. As expected from a Ca2+ binding protein-specific reagent, AzRu had no effect on Ca2+-independent and Mg2+-dependent activities. Az103Ru covalently bound, and specifically labeled, known Ca2+ binding proteins. AzRu is a photoreactive reagent that provides an approach for identification of Ca2+ binding proteins, characterization of their binding sites, and exploration of new Ca2+-dependent processes.

Mitochondrial glycosidic residues contribute to the interaction between ruthenium amine complexes and the calcium uniporter

The role of glycosidic residues in the inhibitory properties of ruthenium complexes on mitochondrial calcium uptake was determined in mitoplasts. Our results showed that the binding and inhibitory properties of ruthenium amine complexes were modified when mitoplasts were exposed to N-glycosidase F action, but calcium uptake was not altered. N-linked proteins of the mitochondrial inner membrane were identified. We detected an 18-kDa protein that binds labeled Ru360 under control conditions, but failed to bind the inhibitor after deglycosilation. A relationship between this protein and the action of ruthenium amine inhibitors of the mitochondrial uniporter is proposed.

Different subunit location of the inhibition and transport sites in the mitochondrial calcium uniporter

The mitochondrial calcium uniporter behaves as a cooperative mechanism, where the velocity is dependent on [Ca2+]ex. Transport kinetics follows a sigmoidal behavior with a Hill coefficient near 2.0, indicating the binding of at least two calcium molecules. Calcium transport in mitochondria is dependent on a negative inner membrane potential and is inhibited by policationic ruthenium compounds. In this study, calcium uptake activity was reconstituted into cytochrome oxidase vesicles by incorporating solubilized mitochondrial proteins. Calcium accumulation plotted against increasing Ca2+ concentrations followed a sigmoidal behavior with a Hill coefficient of 1.53. The uptake was sensitive to ruthenium policationic inhibitors, e.g. ruthenium red and Ru360. After mitochondrial proteins were separated by preparative isoelectrofocusing and incorporated into cytochrome oxidase vesicles, two peaks of calcium uptake activity were recovered. One of the activities was inhibited by Ru360, while the second activity was insensitive to Ru360 and was associated with proteins focused at very acidic isoelectric points. By using a thiol-group crosslinker and radiolabeled Ru360, we proposed a scheme of partial dissociation of the uniporter inhibitor-binding subunit under acidic conditions.

A historical review of cellular calcium handling, with emphasis on mitochondria

Calcium ions are of central importance in cellular physiology, as they carry the signal activating cells to perform their programmed function. Ca(2+) is particularly suitable for this role because of its chemical properties and because its free concentration gradient between the extra-cellular and the cytosolic concentrations is very high, about four orders of magnitude. The cytosolic concentration of Ca(2+) is regulated by binding and chelation by various substances and by transport across plasma and intracellular membranes. Various channels, transport ATPases, uniporters, and antiporters in the plasma membrane, endoplasmic and sarcoplasmic reticulum, and mitochondria are responsible for the transport of Ca(2+). The regulation of these transport systems is the subject of an increasing number of studies. In this short review, we focus on the mitochondrial transporters, i.e. the calcium uniporter used for Ca(2+) uptake, and the antiporters used for the efflux, i.e. the Ca(2+)/Na(+) antiporter in mitochondria and the plasma membrane of excitable cells, and the Ca(2+)/nH(+) antiporter in liver and some other mitochondrial types. Mitochondria are of special interest in that Ca(2+) stimulates respiration and oxidative phosphorylation to meet the energy needs of activated cells. The studies on Ca(2+) and mitochondria began in the fifties, but interest in mitochondrial Ca(2+) handling faded in the late seventies since it had become apparent that mitochondria in resting cells contain very low Ca(2+). Interest increased again in the nineties also because it was discovered that mitochondria and Ca(2+) had a central role in apoptosis and necrosis. This is of special interest in calcium overload and oxidative stress conditions, when the opening of the mitochondrial permeability transition pore is stimulated.

Thyroid hormone may induce changes in the concentration of the mitochondrial calcium uniporter

We explored the possibility that the hormone 3,3',5-tri-iodothyronine can regulate the biosynthesis of the mitochondrial calcium uniporter. To meet this objective experiments on Ca(2+) transport, and binding of the specific inhibitor Ru(360) were carried out in mitochondria isolated from euthyroid, hyperthyroid and hypothyroid rats. It was found that V(max) for Ca(2+) transport increased from 11.67+/-0.8 in euthyroid to 14.36+/-0.44 in hyperthyroid, and decreased in hypothyroid mitochondria to 8.62+/-0.63 nmol Ca(2+)/mg/s. Furthermore, the K(i) for the specific inhibitor Ru(360), depends on the thyroid status, i.e. 18, 19 and 13 nM for control, hyper- and hypothyroid mitochondria, respectively. In addition, the binding of 103Ru(360) was increased in hyperthyroid and decreased in hypothyroid mitochondria. Scatchard analysis for the binding of 103Ru(360) showed the following values: 28, 40 and 23 pmol/mg for control, hyper- and hypothyroid mitochondria, respectively. The K(d) for 103Ru(360) was found to be 30.39, 37.03 and 35.71 nM for controls, hyper- and hypothyroid groups, respectively. When hypothyroid rats were treated with thyroid hormone, mitochondrial Ca(2+) transport, as well as 103Ru(360) binding, reached similar values to those found for euthyroid mitochondria.

Calcium binding and translocation by the voltage-dependent anion channel: a possible regulatory mechanism in mitochondrial function

Mitochondria play a central role in energy metabolism, Ca(2+) signalling, aging and cell death. To control cytosolic or mitochondrial Ca(2+) concentration, mitochondria possess several Ca(2+)-transport systems across the inner membrane. However, the pathway for Ca(2+) crossing the outer membrane has not been directly addressed. We report that purified voltage-dependent anion channel (VDAC) reconstituted into lipid bilayers or liposomes is highly permeable to Ca(2+). VDAC contains Ca(2+)-binding sites that bind Ruthenium Red (RuR), La(3+) and that RuR completely closed VDACs in single or multichannel experiments. Energized, freshly prepared mitochondria accumulate Ca(2+) (500-700 nmol/mg of protein), and subsequently released it. The release of Ca(2+) is accompanied by cyclosporin A-inhibited swelling, suggesting activation of permeability transition pore (PTP). RuR and ruthenium amine binuclear complex, when added to mitochondria after Ca(2+) accumulation has reached a maximal level and before PTP is activated, prevented the release of Ca(2+) and the accompanied mitochondrial swelling. RuR also prevented PTP opening promoted by atractyloside, an adenine nucleotide translocase inhibitor. These results suggest that VDAC, located in the mitochondrial outer membrane, controls Ca(2+) transport into and from the mitochondria, and that the inhibition of Ca(2+) uptake by RuR and La(3+) may result from their interaction with VDAC Ca(2+)-binding sites. Inhibition of PTP opening or assembly by RuR and ruthenium amine binuclear complex suggest the involvement of VDAC in PTP activity and/or regulation. The permeability of VDAC to Ca(2+) and its binding of Ca(2+), suggest that VDAC has a role in regulation of the mitochondrial Ca(2+) homoeostasis.

Mitochondrial calcium transport: mechanisms and functions

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

Characterization of a second member of the subfamily of calcium-binding mitochondrial carriers expressed in human non-excitable tissues

We have recently identified a subfamily of mitochondrial carriers that bind calcium, and cloned ARALAR1, a member of this subfamily expressed in human muscle and brain. We have now cloned a second human ARALAR gene (ARALAR2) coding for a protein 78.3% identical to Aralar1, but expressed in liver and non-excitable tissues. Aralar2 is identical to citrin, the product of the gene mutated in type-II citrullinaemia [Kobayashi, Sinasac, Iijima, Boright, Begum, Lee, Yasuda, Ikeda, Hirano, Terazono et al. (1999) Nat. Genet. 22, 159-163]. A related protein, DmAralar, 69% identical to Aralar1, was found in Drosophila melanogaster, the DMARALAR locus lying on the right arm of the third chromosome, band 99F. The N-terminal half of Aralar2/citrin is able to bind calcium and this requires the presence of the two most distal EF-hands. The localization of Aralar2/citrin expressed in human cell lines is mitochondrial, the C-terminal half containing sufficient information for import and assembly into mitochondria. The C-terminal half of Aralar proteins is related to the yeast YPR020c gene, with a very high sequence conservation (54.3% identity), suggesting that these proteins play an important role. Thus Aralar proteins are probably expressed in all tissues in an isoform-specific fashion, where they function as calcium-regulated metabolite (possibly anionic) carriers.

Inhibitory properties of ruthenium amine complexes on mitochondrial calcium uptake

The recent finding that the inhibition of Ca2+-stimulated respiration by ruthenium red is mainly due to a binuclear ruthenium complex (Ru360) present in the commercial samples of the classical inhibitor ruthenium red (Ying et. al., 1991), showed that this complex is the more potent and specific inhibitor of the mitochondrial calcium uniporter. This work was aimed to provide insights into the mechanism by which Ru360 and other ruthenium-related compounds inhibits calcium uptake. Ruthenium red and a synthesized analog (Rrphen) were compared with Ru360. The inhibition by this binuclear complex was noncompetitive, with a Ki of 9.89 nM. The number of specific binding sites for Ru360 was 6.2 pmol/mg protein. Ruthenium red and Ru360 were mutually exclusive inhibitors. Bound La3+ was not displaced by Ru360. Rrphen was the least effective for inhibiting calcium uptake. The results support the notion of a specific binding site in the uniporter for the polycationic complexes and a negative charged region from the phospholipids in the membrane, closely associated with the uniporter inhibitor-binding site.