Affinity chromatography purification of mitochondrial inner membrane proteins with calcium transport activity

Mitochondrial Permeability Transition in Stem Cells, Development, and Disease

The mitochondrial permeability transition (mPT) is a process that permits rapid exchange of small molecules across the inner mitochondrial membrane (IMM) and thus plays a vital role in mitochondrial function and cellular signaling. Formation of the pore that mediates this flux is well-documented in injury and disease but its regulation has also emerged as critical to the fate of stem cells during embryonic development. The precise molecular composition of the mPTP has been enigmatic, with far more genetic studies eliminating molecular candidates than confirming them. Rigorous studies in the recent decade have implicated central involvement of the F₁F_o ATP synthase, or complex V of the electron transport chain, and continue to confirm a regulatory role for Cyclophilin D (CypD), encoded by Ppif, in modulating the sensitivity of the pore to opening. A host of endogenous molecules have been shown to trigger flux characteristic of mPT, including positive regulators such as calcium ions, reactive oxygen species, inorganic phosphate, and fatty acids. Conductance of the pore has been described as low or high, and reversibility of pore opening appears to correspond with the relative abundance of negative regulators of mPT such as adenine nucleotides, hydrogen ion, and divalent cations that compete for calcium-binding sites in the mPTP. Current models suggest that distinct pores could be responsible for differing reversibility and conductance depending upon cellular context. Indeed, irreversible propagation of mPT inevitably leads to collapse of transmembrane potential, arrest of ATP synthesis, mitochondrial swelling, and cell death. Future studies should clarify ambiguities in mPTP structure and reveal new roles for mPT in dictating specialized cellular functions beyond cell survival that are tied to mitochondrial fitness including stem cell self-renewal and fate. The focus of this review is to describe contemporary models of the mPTP and highlight how pore activity impacts stem cells and development.

Biochemical properties of H+-Ca2+-exchanger in the myometrium mitochondria

Some biochemical properties of the H⁺-Ca²⁺-exchanger in uterine smooth muscle mitochondria have been described. The experiments were performed on a suspension of isolated mitochondria from the myometrium of rats. Methods of confocal microscopy, spectrofluorimetry and photon correlation spectroscopy were used. Fluo-4 probe was used to record changes in ionized Ca²⁺ in the matrix and cytosol; pH changes in the matrix were evaluated with BCECF. It was experimentally proved that in the myometrium instead of Na⁺-Ca²⁺-exchanger the H⁺-Ca²⁺-exchanger functions. It was activated at a physiological pH value, was carried out in stoichiometry 1: 1 and was electrogenic. The transport system was modulated by magnesium ions and the diuretic amiloride, but was not sensitive to changes in the concentration of extra-mitochondrial potassium ions. H⁺-Ca²⁺-exchanger was suppressed by antibodies against the LETM1 protein. Calmodulin may act as a regulator of H⁺-Ca²⁺-exchanger by inhibiting it.

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It has been shown the possibility of the existence of H⁺-Ca²⁺-exchanger in the mitochondria of the myometrium.

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Functioning of H⁺-Ca²⁺-exchanger does not depend on the gradient of sodium and potassium ions; is activated at physiological pH values; is carried out in stoichiometry 1:1 and is electrogenic; inhibited by antibodies against LETM1 protein; modulated by the magnesium ions and diuretic amiloride; calmodulin may act as a regulator of H⁺-Ca²⁺-exchanger.

On the Detection and Functional Significance of the Protein-Protein Interactions of Mitochondrial Transport Proteins

Protein-protein assemblies are highly prevalent in all living cells. Considerable evidence has recently accumulated suggesting that particularly transient association/dissociation of proteins represent an important means of regulation of metabolism. This is true not only in the cytosol and organelle matrices, but also at membrane surfaces where, for example, receptor complexes, as well as those of key metabolic pathways, are common. Transporters also frequently come up in lists of interacting proteins, for example, binding proteins that catalyze the production of their substrates or that act as relays within signal transduction cascades. In this review, we provide an update of technologies that are used in the study of such interactions with mitochondrial transport proteins, highlighting the difficulties that arise in their use for membrane proteins and discussing our current understanding of the biological function of such interactions.

The mitochondrial permeability transition pore is a dispensable element for mitochondrial calcium efflux

The mitochondrial permeability transition pore (mPTP) has long been known to have a role in mitochondrial calcium (Ca(2+)) homeostasis under pathological conditions as a mediator of the mitochondrial permeability transition and the activation of the consequent cell death mechanism. However, its role in the context of mitochondrial Ca(2+) homeostasis is not yet clear. Several studies that were based on PPIF inhibition or knock out suggested that mPTP is involved in the Ca(2+) efflux mechanism, while other observations have revealed the opposite result. The c subunit of the mitochondrial F1/FO ATP synthase has been recently found to be a fundamental component of the mPTP. In this work, we focused on the contribution of the mPTP in the Ca(2+) efflux mechanism by modulating the expression of the c subunit. We observed that forcing mPTP opening or closing did not impair mitochondrial Ca(2+) efflux. Therefore, our results strongly suggest that the mPTP does not participate in mitochondrial Ca(2+) homeostasis in a physiological context in HeLa cells.

How to win ATP and influence Ca(2+) signaling

This brief review discusses recent advances in studies of mitochondrial Ca(2+) signaling and considers how the relationships between mitochondria and Ca(2+) responses are shaped in secretory epithelial cells. Perhaps the more precise title of this review could have been "How to win ATP and influence Ca(2+) signaling in secretory epithelium with emphasis on exocrine secretory cells and specific focus on pancreatic acinar cells". But "brevity is a virtue" and the authors hope that many of the mechanisms discussed are general and applicable to other tissues and cell types. Among these mechanisms are mitochondrial regulation of Ca(2+) entry and the role of mitochondria in the formation of localized Ca(2+) responses. The roles of Ca(2+) signaling in the physiological adjustment of bioenergetics and in mitochondrial damage are also briefly discussed.

IP3, a small molecule with a powerful message

Research conducted over the past two decades has provided convincing evidence that cell death, and more specifically apoptosis, can exceed single cell boundaries and can be strongly influenced by intercellular communication networks. We recently reported that gap junctions (i.e. channels directly connecting the cytoplasm of neighboring cells) composed of connexin43 or connexin26 provide a direct pathway to promote and expand cell death, and that inositol 1,4,5-trisphosphate (IP3) diffusion via these channels is crucial to provoke apoptosis in adjacent healthy cells. However, IP3 itself is not sufficient to induce cell death and additional factors appear to be necessary to create conditions in which IP3 will exert proapoptotic effects. Although IP3-evoked Ca(2+) signaling is known to be required for normal cell survival, it is also actively involved in apoptosis induction and progression. As such, it is evident that an accurate fine-tuning of this signaling mechanism is crucial for normal cell physiology, while a malfunction can lead to cell death. Here, we review the role of IP3 as an intracellular and intercellular cell death messenger, focusing on the endoplasmic reticulum-mitochondrial synapse, followed by a discussion of plausible elements that can convert IP3 from a physiological molecule to a killer substance. Finally, we highlight several pathological conditions in which anomalous intercellular IP3/Ca(2+) signaling might play a role. This article is part of a Special Issue entitled:12th European Symposium on Calcium.

Mitochondrial Ca²⁺ homeostasis: mechanism, role, and tissue specificities

Mitochondria from every tissue are quite similar in their capability to accumulate Ca²⁺ in a process that depends on the electrical potential across the inner membrane; it is catalyzed by a gated channel (named mitochondrial Ca²⁺ uniporter), the molecular identity of which has only recently been unraveled. The release of accumulated Ca²⁺ in mitochondria from different tissues is, on the contrary, quite variable, both in terms of speed and mechanism: a Na⁺-dependent efflux in excitable cells (catalyzed by NCLX) and a H⁺/Ca²⁺ exchanger in other cells. The efficacy of mitochondrial Ca²⁺ uptake in living cells is strictly dependent on the topological arrangement of the organelles with respect to the source of Ca²⁺ flowing into the cytoplasm, i.e., plasma membrane or intracellular channels. In turn, the structural and functional relationships between mitochondria and other cellular membranes are dictated by the specific architecture of different cells. Mitochondria not only modulate the amplitude and the kinetics of local and bulk cytoplasmic Ca²⁺ changes but also depend on the Ca²⁺ signal for their own functionality, in particular for their capacity to produce ATP. In this review, we summarize the processes involved in mitochondrial Ca²⁺ handling and its integration in cell physiology, highlighting the main common characteristics as well as key differences, in different tissues.

After half a century mitochondrial calcium in- and efflux machineries reveal themselves

Mitochondrial Ca(2+) uptake and release play a fundamental role in the control of different physiological processes, such as cytoplasmic Ca(2+) signalling, ATP production and hormone metabolism, while dysregulation of mitochondrial Ca(2+) handling triggers the cascade of events that lead to cell death. The basic mechanisms of mitochondrial Ca(2+) homeostasis have been firmly established for decades, but the molecular identities of the channels and transporters responsible for Ca(2+) uptake and release have remained mysterious until very recently. Here, we briefly review the main findings that have led to our present understanding of mitochondrial Ca(2+) homeostasis and its integration in cell physiology. We will then discuss the recent work that has unravelled the biochemical identity of three key molecules: NCLX, the mitochondrial Na(+)/Ca(2+) antiporter, MCU, the pore-forming subunit of the mitochondrial Ca(2+) uptake channel, and MICU1, one of its regulatory subunits.

Plant cation/H+ exchangers (CAXs): biological functions and genetic manipulations

Inorganic cations play decisive roles in many cellular and physiological processes and are essential components of plant nutrition. Therefore, the uptake of cations and their redistribution must be precisely controlled. Vacuolar antiporters are important elements in mediating the intracellular sequestration of these cations. These antiporters are energized by the proton gradient across the vacuolar membrane and allow the rapid transport of cations into the vacuole. CAXs (for CAtion eXchanger) are members of a multigene family and appear to predominately reside on vacuoles. Defining CAX regulation and substrate specificity have been aided by utilising yeast as an experimental tool. Studies in plants suggest CAXs regulate apoplastic Ca(2+) levels in order to optimise cell wall expansion, photosynthesis, transpiration and plant productivity. CAX studies provide the basis for making designer transporters that have been used to develop nutrient enhanced crops and plants for remediating toxic soils.

PfCHA is a mitochondrial divalent cation/H+ antiporter in Plasmodium falciparum

The human malaria parasite Plasmodium falciparum is capable of adapting to vastly different extracellular Ca(2+) environments while maintaining tight control of its intracellular Ca(2+) concentration. The mechanisms underpinning Ca(2+) homeostasis in this important pathogen are only partly understood. Here we have functionally expressed the putative Ca(2+)/H(+) antiporter PfCHA in Xenopus laevis oocytes. Our data suggest that PfCHA mediates H(+)-coupled Ca(2+) and Mn(2+) exchange. The apparent dissociation constant K(M) for Ca(2+) of 2.2 +/- 0.7 mM and the maximal velocity V(max) of 0.6 +/- 0.1 nmol per oocyte per hour are consistent with PfCHA being a low-affinity high-capacity Ca(2+) carrier. In the parasite, PfCHA was found to localize to the mitochondrion. Physiological studies conducted with live parasitized erythrocytes, and using Fluo-4 and Rhod-2 to monitor cytoplasmic and mitochondrial Ca(2+) dynamics, suggest that the mitochondrion serves as a dynamic Ca(2+) store and that PfCHA functions as a Ca(2+) efflux system expelling excess Ca(2+) from the mitochondrion. PfCHA lacks appreciable homologies to the human mitochondrial Ca(2+)/H(+) exchanger and might represent an evolutionary divergent class of mitochondrial cation antiporter, which, in turn, might provide novel opportunities for intervention.

Calcium signaling in brain mitochondria: interplay of malate aspartate NADH shuttle and calcium uniporter/mitochondrial dehydrogenase pathways

Ca2+ signaling in mitochondria has been mainly attributed to Ca2+ entry to the matrix through the Ca2+ uniporter and activation of mitochondrial matrix dehydrogenases. However, mitochondria can also sense increases in cytosolic Ca2+ through a mechanism that involves the aspartate-glutamate carriers, extramitochondrial Ca2+ activation of the NADH malate-aspartate shuttle (MAS). Both pathways are linked through the shared substrate alpha-ketoglutarate (alphaKG). Here we have studied the interplay between the two pathways under conditions of Ca2+ activation. We show that alphaKG becomes limiting when Ca2+ enters in brain or heart mitochondria, but not liver mitochondria, resulting in a drop in alphaKG efflux through the oxoglutarate carrier and in a drop in MAS activity. Inhibition of alphaKG efflux and MAS activity by matrix Ca2+ in brain mitochondria was fully reversible upon Ca2+ efflux. Because of their differences in cytosolic calcium concentration requirements, the MAS and Ca2+ uniporter-mitochondrial dehydrogenase pathways are probably sequentially activated during a Ca2+ transient, and the inhibition of MAS at the center of the transient may provide an explanation for part of the increase in lactate observed in the stimulated brain in vivo.

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.

Role of aralar, the mitochondrial transporter of aspartate-glutamate, in brain N-acetylaspartate formation and Ca(2+) signaling in neuronal mitochondria

Aralar, the Ca(2+)-dependent mitochondrial aspartate-glutamate carrier expressed in brain and skeletal muscle, is a member of the malate-aspartate NADH shuttle. Disrupting the gene for aralar, SLC25a12, in mice has enabled the discovery of two new roles of this carrier. On the one hand, it is required for synthesis of brain aspartate and N-acetylaspartate, a neuron-born metabolite that supplies acetate for myelin lipid synthesis; and on the other, it is essential for the transmission of small Ca(2+) signals to mitochondria via an increase in mitochondrial NADH.

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).

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.

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.

Developmental changes in the Ca2+-regulated mitochondrial aspartate-glutamate carrier aralar1 in brain and prominent expression in the spinal cord

Aralar1 and citrin are two isoforms of the mitochondrial carrier of aspartate-glutamate (AGC), a calcium regulated carrier, which is important in the malate-aspartate NADH shuttle. The expression and cell distribution of aralar1 and citrin in brain cells has been studied during development in vitro and in vivo. Aralar1 is the only isoform expressed in neurons and its levels undergo a marked increase during in vitro maturation, which is higher than the increase in mitochondrial DNA in the same time window. The enrichment in aralar1 per mitochondria during neuronal maturation is associated with a prominent rise in the function of the malate-aspartate NADH shuttle. Paradoxically, during in vivo development of rat or mouse brain there is very little postnatal increase in total aralar1 levels per mitochondria. This is explained by the fact that astrocytes develop postnatally, have aralar1 levels much lower than neurons, and their increase masks that of aralar1. Aralar1 mRNA and protein are widely expressed throughout neuron-rich areas in adult mouse CNS with clear enrichments in sets of neuronal nuclei in the brainstem and, particularly, in the ventral horn of the spinal cord. These aralar1-rich neurons represent a subset of the cytochrome oxidase-rich neurons in the same areas. The presence of aralar1 could reflect a tonic activity of these neurons, which is met by the combination of high malate-aspartate NADH shuttle and respiratory chain activities.

Mechanism of N-terminal autoinhibition in the Arabidopsis Ca(2+)/H(+) antiporter CAX1

Regulation of Ca(2+)/H(+) antiporters may be an important function in determining the duration and amplitude of cytosolic Ca(2+) oscillations. Previously the Arabidopsis Ca(2+)/H(+) transporter, CAX1 (cation exchanger 1), was identified by its ability to suppress yeast mutants defective in vacuolar Ca(2+) transport. Recently, a 36-amino acid N-terminal regulatory region on CAX1 has been identified that inhibits CAX1-mediated Ca(2+)/H(+) antiport. Here we show that a synthetic peptide designed against the CAX1 36 amino acids inhibited Ca(2+)/H(+) transport mediated by an N-terminal-truncated CAX1 but did not inhibit Ca(2+) transport by other Ca(2+)/H(+) antiporters. Ca(2+)/H(+) antiport activity measured from vacuolar-enriched membranes of Arabidopsis root was also inhibited by the CAX1 peptide. Through analyzing CAX chimeric constructs the region of interaction of the N-terminal regulatory region was mapped to include 7 amino acids (residues 56-62) within CAX1. The CAX1 N-terminal regulatory region was shown to physically interact with this 7-amino acid region by yeast two-hybrid analysis. Mutagenesis of amino acids within the N-terminal regulatory region implicated several residues as being essential for regulation. These findings describe a unique mode of antiporter autoinhibition and demonstrate the first detailed mechanisms for the regulation of a Ca(2+)/H(+) antiporter from any organism.

Quantification of Ca2+/H+ antiporter VCAX1p in vacuolar membranes and its absence in roots of mung bean

Vacuolar Ca2+/H+ antiporter VCAX1p, which contributes to the Ca2+ accumulation into vacuoles, was quantified by immunochemistry. The antiporter content in vacuolar membranes was 0.14 and 1.1 microg mg(-1) of membrane protein for hypocotyls and epicotyls, respectively. The calculated turnover number was 120 s(-1). Roots lacked the antiporter protein and the transcript.

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.