Ca2+ uptake in mitochondria occurs via the reverse action of the Na+/Ca2+ exchanger in metabolically inhibited MDCK cells

Control of Ca2+ and metabolic homeostasis by the Na+/Ca2+ exchangers (NCXs) in health and disease

Spatial and temporal control of calcium (Ca²⁺) levels is essential for the background rhythms and responses of living cells to environmental stimuli. Whatever other regulators a given cellular activity may have, localized and wider scale Ca²⁺ events (sparks, transients, and waves) are hierarchical determinants of fundamental processes such as cell contraction, excitability, growth, metabolism and survival. Different cell types express specific channels, pumps and exchangers to efficiently generate and adapt Ca²⁺ patterns to cell requirements. The Na⁺/Ca²⁺ exchangers (NCXs) in particular contribute to Ca²⁺ homeostasis by buffering intracellular Ca²⁺ loads according to the electrochemical gradients of substrate ions - i.e., Ca²⁺ and sodium (Na⁺) - and under a dynamic control of redundant regulatory processes. An interesting feature of NCX emerges from the strict relationship that connects transporter activity with cell metabolism: on the one hand NCX operates under constant control of ATP-dependent regulatory processes, on the other hand the ion fluxes generated through NCX provide mechanistic support for the Na⁺-driven uptake of glutamate and Ca²⁺ influx to fuel mitochondrial respiration. Proof of concept evidence highlights therapeutic potential of preserving a timed and balanced NCX activity in a growing rate of diseases (including excitability, neurodegenerative, and proliferative disorders) because of an improved ability of stressed cells to safely maintain ion gradients and mitochondrial bioenergetics. Here, we will summarize and review recent works that have focused on the pathophysiological roles of NCXs in balancing the two-way relationship between Ca²⁺ signals and metabolism.

Preventing Axonal Sodium Overload or Mitochondrial Calcium Uptake Protects Axonal Mitochondria from Oxidative Stress-Induced Alterations

In neuroinflammatory and neurodegenerative disorders such as multiple sclerosis, mitochondrial damage caused by oxidative stress is believed to contribute to neuroaxonal damage. Previously, we demonstrated that exposure to hydrogen peroxide (H₂O₂) alters mitochondrial morphology and motility in myelinated axons and that these changes initiate at the nodes of Ranvier, where numerous sodium channels are located. Therefore, we suggested that mitochondrial damage may lead to ATP deficit, thereby affecting the efficiency of the sodium-potassium ATPase and eventually leading to sodium overload in axons. The increased intra-axonal sodium may revert the axonal sodium-calcium exchangers and thus may lead to a pathological calcium overload in the axoplasm and mitochondria. Here, we used the explanted murine ventral spinal roots to investigate whether modulation of sodium or calcium influx may prevent mitochondrial alterations in myelinated axons during exogenous application of H₂O₂ inducing oxidative stress. For that, tetrodotoxin, an inhibitor of voltage-gated sodium ion channels, and ruthenium 360, an inhibitor of the mitochondrial calcium uniporter, were applied simultaneously with hydrogen peroxide to axons. Mitochondrial shape and motility were analyzed. We showed that inhibition of axonal sodium influx prevented oxidative stress-induced morphological changes (i.e., increase in circularity and area and decrease in length) and preserved mitochondrial membrane potential, which is crucial for ATP production. Blocking mitochondrial calcium uptake prevented decrease in mitochondrial motility and also preserved membrane potential. Our findings indicate that alterations of both mitochondrial morphology and motility in the contexts of oxidative stress can be counterbalanced by modulating intramitochondrial ion concentrations pharmacologically. Moreover, motile mitochondria show preserved membrane potentials, pointing to a close association between mitochondrial motility and functionality.

Mitochondrial calcium exchange in physiology and disease

The uptake of calcium into and extrusion of calcium from the mitochondrial matrix is a fundamental biological process that has critical effects on cellular metabolism, signaling, and survival. Disruption of mitochondrial calcium (_mCa²⁺) cycling is implicated in numerous acquired diseases such as heart failure, stroke, neurodegeneration, diabetes, and cancer, and is genetically linked to several inherited neuromuscular disorders. Understanding the mechanisms responsible for _mCa²⁺ exchange therefore holds great promise for the treatment of these diseases. The past decade has seen the genetic identification of many of the key proteins that mediate mitochondrial calcium uptake and efflux. Here, we present an overview of the phenomenon of _mCa²⁺ transport, and a comprehensive examination of the molecular machinery that mediates calcium flux across the inner mitochondrial membrane: the mitochondrial uniporter complex (consisting of MCU, EMRE, MICU1, MICU2, MICU3, MCUB, and MCUR1), NCLX, LETM1, the mitochondrial ryanodine receptor, and the mitochondrial permeability transition pore. We then consider the physiological implications of _mCa²⁺ flux and evaluate how alterations in _mCa²⁺ homeostasis contribute to human disease. This review concludes by highlighting opportunities and challenges for therapeutic intervention in pathologies characterized by aberrant _mCa²⁺ handling and by summarizing critical unanswered questions regarding the biology of _mCa²⁺ flux.

Intracellular Calcium Homeostasis and Kidney Disease

Kidney disease is a serious health problem that burdens our healthcare system. It is crucial to find the accurate pathogenesis of various types of kidney disease to provide guidance for precise therapies for patients suffering from these diseases. However, the exact molecular mechanisms underlying these diseases have not been fully understood. Disturbance of calcium homeostasis in renal cells plays a fundamental role in the development of various types of kidney disease, such as primary glomerular disease, diabetic nephropathy, acute kidney injury and polycystic kidney disease, through promoting cell proliferation, stimulating extracellular matrix accumulation, aggravating podocyte injury, disrupting cellular energetics as well as dysregulating cell survival and death dynamics. As a result, preventing the disturbance of calcium homeostasis in specific renal cells (such as tubular cells, podocytes and mesangial cells) is becoming one of the most promising therapeutic strategies in the treatment of kidney disease. The endoplasmic reticulum and mitochondria are two vital organelles in this process. Calcium ions cycle between the endoplasmic reticulum and mitochondria at the conjugation of these two organelles known as the mitochondria-associated endoplasmic reticulum membrane, maintaining calcium homeostasis. The pharmacologic modulation of cellular calcium homeostasis can be viewed as a novel therapeutic method for renal diseases. Here, we will introduce calcium homeostasis under physiological conditions and the disturbance of calcium homeostasis in kidney diseases. We will focus on the calcium homeostasis regulation in renal cells (including tubular cells, podocytes and mesangial cells), especially in the mitochondria- associated endoplasmic reticulum membranes of these renal cells.

Mitochondrial Calcium Uniporter (MCU) deficiency reveals an alternate path for Ca2+ uptake in photoreceptor mitochondria

Rods and cones use intracellular Ca2+ to regulate many functions, including phototransduction and neurotransmission. The Mitochondrial Calcium Uniporter (MCU) complex is thought to be the primary pathway for Ca2+ entry into mitochondria in eukaryotes. We investigate the hypothesis that mitochondrial Ca2+ uptake via MCU influences phototransduction and energy metabolism in photoreceptors using a mcu-/- zebrafish and a rod photoreceptor-specific Mcu-/- mouse. Using genetically encoded Ca2+ sensors to directly examine Ca2+ uptake in zebrafish cone mitochondria, we found that loss of MCU reduces but does not eliminate mitochondrial Ca2+ uptake. Loss of MCU does not lead to photoreceptor degeneration, mildly affects mitochondrial metabolism, and does not alter physiological responses to light, even in the absence of the Na+/Ca2+, K+ exchanger. Our results reveal that MCU is dispensable for vertebrate photoreceptor function, consistent with its low expression and the presence of an alternative pathway for Ca2+ uptake into photoreceptor mitochondria.

Membrane current evoked by mitochondrial Na+-Ca2+ exchange in mouse heart

The electrogenicity of mitochondrial Na+-Ca2+ exchange (NCXm) had been controversial and no membrane current through it had been reported. We succeeded for the first time in recording NCXm-mediated currents using mitoplasts derived from mouse ventricle. Under conditions that K+, Cl-, and Ca2+ uniporter currents were inhibited, extra-mitochondrial Na+ induced inward currents with 1 μM Ca2+ in the pipette. The half-maximum concentration of Na+ was 35.6 mM. The inward current was diminished without Ca2+ in the pipette, and was augmented with 10 μM Ca2+. The Na+-induced inward currents were largely inhibited by CGP-37157, an NCXm blocker. However, the reverse mode of NCXm, which should be detected as an outward current, was hardly induced by extra-mitochondrial application of Ca2+ with Na+ in the pipette. It was concluded that NCXm is electrogenic. This property may be advantageous for facilitating Ca2+ extrusion from mitochondria, which has large negative membrane potential.

Amyloid-β Causes Mitochondrial Dysfunction via a Ca2+-Driven Upregulation of Oxidative Phosphorylation and Superoxide Production in Cerebrovascular Endothelial Cells

Cerebrovascular pathology is pervasive in Alzheimer's disease (AD), yet it is unknown whether cerebrovascular dysfunction contributes to the progression or etiology of AD. In human subjects and in animal models of AD, cerebral hypoperfusion and hypometabolism are reported to manifest during the early stages of the disease and persist for its duration. Amyloid-β is known to cause cellular injury in both neurons and endothelial cells by inducing the production of reactive oxygen species and disrupting intracellular Ca2+ homeostasis. We present a mechanism for mitochondrial degeneration caused by the production of mitochondrial superoxide, which is driven by increased mitochondrial Ca2+ uptake. We found that persistent superoxide production injures mitochondria and disrupts electron transport in cerebrovascular endothelial cells. These observations provide a mechanism for the mitochondrial deficits that contribute to cerebrovascular dysfunction in patients with AD.

Pathophysiology of Calcium Mediated Ventricular Arrhythmias and Novel Therapeutic Options with Focus on Gene Therapy

Cardiac arrhythmias constitute a major health problem with a huge impact on mortality rates and health care costs. Despite ongoing research efforts, the understanding of the molecular mechanisms and processes responsible for arrhythmogenesis remains incomplete. Given the crucial role of Ca²⁺-handling in action potential generation and cardiac contraction, Ca²⁺ channels and Ca²⁺ handling proteins represent promising targets for suppression of ventricular arrhythmias. Accordingly, we report the different roles of Ca²⁺-handling in the development of congenital as well as acquired ventricular arrhythmia syndromes. We highlight the therapeutic potential of gene therapy as a novel and innovative approach for future arrhythmia therapy. Furthermore, we discuss various promising cellular and mitochondrial targets for therapeutic gene transfer currently under investigation.

Deletion of mitochondrial calcium uniporter incompletely inhibits calcium uptake and induction of the permeability transition pore in brain mitochondria

Ca²⁺ influx into mitochondria is mediated by the mitochondrial calcium uniporter (MCU), whose identity was recently revealed as a 40-kDa protein that along with other proteins forms the mitochondrial Ca²⁺ uptake machinery. The MCU is a Ca²⁺-conducting channel spanning the inner mitochondrial membrane. Here, deletion of the MCU completely inhibited Ca²⁺ uptake in liver, heart, and skeletal muscle mitochondria. However, in brain nonsynaptic and synaptic mitochondria from neuronal somata/glial cells and nerve terminals, respectively, the MCU deletion slowed, but did not completely block, Ca²⁺ uptake. Under resting conditions, brain MCU-KO mitochondria remained polarized, and in brain MCU-KO mitochondria, the electrophoretic Ca²⁺ ionophore ETH129 significantly accelerated Ca²⁺ uptake. The residual Ca²⁺ uptake in brain MCU-KO mitochondria was insensitive to inhibitors of mitochondrial Na⁺/Ca²⁺ exchanger and ryanodine receptor (CGP37157 and dantrolene, respectively), but was blocked by the MCU inhibitor Ru360. Respiration of WT and MCU-KO brain mitochondria was similar except that for mitochondria that oxidized pyruvate and malate, Ca²⁺ more strongly inhibited respiration in WT than in MCU-KO mitochondria. Of note, the MCU deletion significantly attenuated but did not completely prevent induction of the permeability transition pore (PTP) in brain mitochondria. Expression level of cyclophilin D and ATP content in mitochondria, two factors that modulate PTP induction, were unaffected by MCU-KO, whereas ADP was lower in MCU-KO than in WT brain mitochondria. Our results suggest the presence of an MCU-independent Ca²⁺ uptake pathway in brain mitochondria that mediates residual Ca²⁺ influx and induction of PTP in a fraction of the mitochondrial population.

Ca(2+)-dependent regulation of the Ca(2+) concentration in the myometrium mitochondria. I. Trifluoperazine effects on mitochondria membranes polarization and [Ca(2+)](m)

Са2+-dependent regulation of Ca2+ exchange in mitochondria is carried out with participation of calmodulin. We have shown previously that calmodulin antagonists reduced the level of mitochondrial membrane polarization and induced increase of the ionized Са concentration in both the mitochondrial matrix and cell cytoplasm. The concentration-dependent influence of trifluoperazine on the level of polarization of mitochondrial membranes has been shown in this work. The coordinates of the Hill graphs were used to calculate the constant K0.5 and the Hill coefficient. K0.5 was 24.4 ± 5 μM (n = 10). The Hill coefficient was 2.0 ± 0.2, indicating the presence of two centers of the trifluoperazine binding. We have also studied [Ca2+]m changes, when incubating mitochondria in mediums of different composition: without ATP and ions of Mg (0-medium), in the presence of 3 mM Mg (Mg-medium) and 3 mM Mg + 3 mM ATP (Mg,ATP-medium). It was shown that the composition of the incubation medium affected the [Ca2+]m values in the absence of exogenous Ca2+ and did not affect them in the presence of the latter. Preincubation of mitochondria in mediums of different composition with 25 μM trifluoperazine did not affect the [Ca2+]m values both before and after the addition of 100 µМ Са2+ to the incubation medium. It was concluded, that trifluoperazine depolarized myometrial mitochondria membranes in concentration-dependent manner. However, mitochondria preincubation with 25 μM trifluope­razine accompanied by 50% decrease in membrane polarization did not affect the [Ca2+]m values.

Different approaches to modeling analysis of mitochondrial swelling

Mitochondria are critical players involved in both cell life and death through multiple pathways. Structural integrity, metabolism and function of mitochondria are regulated by matrix volume due to physiological changes of ion homeostasis in cellular cytoplasm and mitochondria. Ca²⁺ and K⁺ presumably play a critical role in physiological and pathological swelling of mitochondria when increased uptake (influx)/decreased release (efflux) of these ions enhances osmotic pressure accompanied by high water accumulation in the matrix. Changes in the matrix volume in the physiological range have a stimulatory effect on electron transfer chain and oxidative phosphorylation to satisfy metabolic requirements of the cell. However, excessive matrix swelling associated with the sustained opening of mitochondrial permeability transition pores (PTP) and other PTP-independent mechanisms compromises mitochondrial function and integrity leading to cell death. The mechanisms of transition from reversible (physiological) to irreversible (pathological) swelling of mitochondria remain unknown. Mitochondrial swelling is involved in the pathogenesis of many human diseases such as neurodegenerative and cardiovascular diseases. Therefore, modeling analysis of the swelling process is important for understanding the mechanisms of cell dysfunction. This review attempts to describe the role of mitochondrial swelling in cell life and death and the main mechanisms involved in the maintenance of ion homeostasis and swelling. The review also summarizes and discusses different kinetic models and approaches that can be useful for the development of new models for better simulation and prediction of in vivo mitochondrial swelling.

Traditional and novel tools to probe the mitochondrial metabolism in health and disease

Mitochondrial metabolism links energy production to other essential cellular processes such as signaling, cellular differentiation, and apoptosis. In addition to producing adenosine triphosphate (ATP) as an energy source, mitochondria are responsible for the synthesis of a myriad of important metabolites and cofactors such as tetrahydrofolate, α-ketoacids, steroids, aminolevulinic acid, biotin, lipoic acid, acetyl-CoA, iron-sulfur clusters, heme, and ubiquinone. Furthermore, mitochondria and their metabolism have been implicated in aging and several human diseases, including inherited mitochondrial disorders, cardiac dysfunction, heart failure, neurodegenerative diseases, diabetes, and cancer. Therefore, there is great interest in understanding mitochondrial metabolism and the complex relationship it has with other cellular processes. A large number of studies on mitochondrial metabolism have been conducted in the last 50 years, taking a broad range of approaches. In this review, we summarize and discuss the most commonly used tools that have been used to study different aspects of the metabolism of mitochondria: ranging from dyes that monitor changes in the mitochondrial membrane potential and pharmacological tools to study respiration or ATP synthesis, to more modern tools such as genetically encoded biosensors and trans-omic approaches enabled by recent advances in mass spectrometry, computation, and other technologies. These tools have allowed the large number of studies that have shaped our current understanding of mitochondrial metabolism. WIREs Syst Biol Med 2017, 9:e1373. doi: 10.1002/wsbm.1373 For further resources related to this article, please visit the WIREs website.

Mitochondrial remodeling: Rearranging, recycling, and reprogramming

Mitochondria are highly dynamic and responsive organelles that respond to environmental cues with fission and fusion. They undergo mitophagy and biogenesis, and are subject to extensive post-translational modifications. Calcium plays an important role in regulating mitochondrial functions. Mitochondria play a central role in metabolism of glucose, fatty acids, and amino acids, and generate ATP with effects on redox poise, oxidative stress, pH, and other metabolites including acetyl-CoA and NAD(+) which in turn have effects on chromatin remodeling. The complex interplay of mitochondria, cytosolic factors, and the nucleus ensure a well-coordinated response to environmental stresses.

Intracellular Calcium Dysregulation: Implications for Alzheimer's Disease

Alzheimer's Disease (AD) is a neurodegenerative disorder characterized by progressive neuronal loss. AD is associated with aberrant processing of the amyloid precursor protein, which leads to the deposition of amyloid-β plaques within the brain. Together with plaques deposition, the hyperphosphorylation of the microtubules associated protein tau and the formation of intraneuronal neurofibrillary tangles are a typical neuropathological feature in AD brains. Cellular dysfunctions involving specific subcellular compartments, such as mitochondria and endoplasmic reticulum (ER), are emerging as crucial players in the pathogenesis of AD, as well as increased oxidative stress and dysregulation of calcium homeostasis. Specifically, dysregulation of intracellular calcium homeostasis has been suggested as a common proximal cause of neural dysfunction in AD. Aberrant calcium signaling has been considered a phenomenon mainly related to the dysfunction of intracellular calcium stores, which can occur in both neuronal and nonneuronal cells. This review reports the most recent findings on cellular mechanisms involved in the pathogenesis of AD, with main focus on the control of calcium homeostasis at both cytosolic and mitochondrial level.

Antagonistic Regulation of Parvalbumin Expression and Mitochondrial Calcium Handling Capacity in Renal Epithelial Cells

Parvalbumin (PV) is a cytosolic Ca²⁺-binding protein acting as a slow-onset Ca²⁺ buffer modulating the shape of Ca²⁺ transients in fast-twitch muscles and a subpopulation of neurons. PV is also expressed in non-excitable cells including distal convoluted tubule (DCT) cells of the kidney, where it might act as an intracellular Ca²⁺ shuttle facilitating transcellular Ca²⁺ resorption. In excitable cells, upregulation of mitochondria in “PV-ergic” cells in PV-/- mice appears to be a general hallmark, evidenced in fast-twitch muscles and cerebellar Purkinje cells. Using Gene Chip Arrays and qRT-PCR, we identified differentially expressed genes in the DCT of PV-/- mice. With a focus on genes implicated in mitochondrial Ca²⁺ transport and membrane potential, uncoupling protein 2 (Ucp2), mitocalcin (Efhd1), mitochondrial calcium uptake 1 (Micu1), mitochondrial calcium uniporter (Mcu), mitochondrial calcium uniporter regulator 1 (Mcur1), cytochrome c oxidase subunit 1 (COX1), and ATP synthase subunit β (Atp5b) were found to be up-upregulated. At the protein level, COX1 was increased by 31 ± 7%, while ATP-synthase subunit β was unchanged. This suggested that these mitochondria were better suited to uphold the electrochemical potential across the mitochondrial membrane, necessary for mitochondrial Ca²⁺ uptake. Ectopic expression of PV in PV-negative Madin-Darby canine kidney (MDCK) cells decreased COX1 and concomitantly mitochondrial volume, while ATP synthase subunit β levels remained unaffected. Suppression of PV by shRNA in PV-expressing MDCK cells led subsequently to an increase in COX1 expression. The collapsing of the mitochondrial membrane potential by the uncoupler CCCP occurred at lower concentrations in PV-expressing MDCK cells than in control cells. In support, a reduction of the relative mitochondrial mass was observed in PV-expressing MDCK cells. Deregulation of the cytoplasmic Ca²⁺ buffer PV in kidney cells was counterbalanced in vivo and in vitro by adjusting the relative mitochondrial volume and modifying the mitochondrial protein composition conceivably to increase their Ca²⁺-buffering/sequestration capacity.

The excitation-contraction coupling mechanism in skeletal muscle

First coined by Alexander Sandow in 1952, the term excitation-contraction coupling (ECC) describes the rapid communication between electrical events occurring in the plasma membrane of skeletal muscle fibres and Ca2+ release from the SR, which leads to contraction. The sequence of events in twitch skeletal muscle involves: (1) initiation and propagation of an action potential along the plasma membrane, (2) spread of the potential throughout the transverse tubule system (T-tubule system), (3) dihydropyridine receptors (DHPR)-mediated detection of changes in membrane potential, (4) allosteric interaction between DHPR and sarcoplasmic reticulum (SR) ryanodine receptors (RyR), (5) release of Ca2+ from the SR and transient increase of Ca2+ concentration in the myoplasm, (6) activation of the myoplasmic Ca2+ buffering system and the contractile apparatus, followed by (7) Ca2+ disappearance from the myoplasm mediated mainly by its reuptake by the SR through the SR Ca2+ adenosine triphosphatase (SERCA), and under several conditions movement to the mitochondria and extrusion by the Na+/Ca2+ exchanger (NCX). In this text, we review the basics of ECC in skeletal muscle and the techniques used to study it. Moreover, we highlight some recent advances and point out gaps in knowledge on particular issues related to ECC such as (1) DHPR-RyR molecular interaction, (2) differences regarding fibre types, (3) its alteration during muscle fatigue, (4) the role of mitochondria and store-operated Ca2+ entry in the general ECC sequence, (5) contractile potentiators, and (6) Ca2+ sparks.

Fluorescence imaging of intracellular calcium signals in intact kidney tissue

BACKGROUND

Intracellular calcium (Ca(2+)) plays an important role in normal renal physiology and in the pathogenesis of various kidney diseases; however, the study of Ca(2+) signals in intact tissue has been limited by technical difficulties, including achieving adequate loading of Ca(2+)-sensitive fluorescent dyes. The kidney slice preparation represents a model whereby three-dimensional tissue architecture is preserved and structures in both the cortex and medulla can be imaged using confocal or multiphoton microscopy.

METHODS

Ca(2+)-sensitive dyes Rhod-2, Fura-red and Fluo-4 were loaded into tubular and vascular cells in rat kidney slices using a re-circulating perfusion system and real-time imaging of Ca(2+) signals was recorded by confocal microscopy. Kidney slices were also obtained from transgenic mice expressing the GCaMP2 Ca(2+)-sensor in their endothelial cells and real time Ca(2+) transients stimulated by physiological stimuli.

RESULTS

Wide spread loading of Ca(2+) indicators was achieved in the tubular and vascular structures of both the medulla and cortex. Real time Ca(2+) signals were successfully recorded in different intracellular compartments of both rat and mouse cortical and medullary tubules in response to physiological stimuli (ATP and angiotensin II). Glomerular Ca(2+) transients were similarly recorded in kidney slices taken from the transgenic mouse expressing the GCaMP2 Ca(2+)-sensor.

CONCLUSION

We present new approaches that can be adopted to image cytosolic and mitochondrial Ca(2+) signals within various cell types in intact kidney tissue. Moreover, techniques described in this study can be used to facilitate future detailed investigations of intracellular Ca(2+) homeostasis in renal health and disease.

Physical and functional interaction of NCX1 and EAAC1 transporters leading to glutamate-enhanced ATP production in brain mitochondria

Glutamate is emerging as a major factor stimulating energy production in CNS. Brain mitochondria can utilize this neurotransmitter as respiratory substrate and specific transporters are required to mediate the glutamate entry into the mitochondrial matrix. Glutamate transporters of the Excitatory Amino Acid Transporters (EAATs) family have been previously well characterized on the cell surface of neuronal and glial cells, representing the primary players for glutamate uptake in mammalian brain. Here, by using western blot, confocal microscopy and immunoelectron microscopy, we report for the first time that the Excitatory Amino Acid Carrier 1 (EAAC1), an EAATs member, is expressed in neuronal and glial mitochondria where it participates in glutamate-stimulated ATP production, evaluated by a luciferase-luciferin system. Mitochondrial metabolic response is counteracted when different EAATs pharmacological blockers or selective EAAC1 antisense oligonucleotides were used. Since EAATs are Na⁺-dependent proteins, this raised the possibility that other transporters regulating ion gradients across mitochondrial membrane were required for glutamate response. We describe colocalization, mutual activity dependency, physical interaction between EAAC1 and the sodium/calcium exchanger 1 (NCX1) both in neuronal and glial mitochondria, and that NCX1 is an essential modulator of this glutamate transporter. Only NCX1 activity is crucial for such glutamate-stimulated ATP synthesis, as demonstrated by pharmacological blockade and selective knock-down with antisense oligonucleotides. The EAAC1/NCX1-dependent mitochondrial response to glutamate may be a general and alternative mechanism whereby this neurotransmitter sustains ATP production, since we have documented such metabolic response also in mitochondria isolated from heart. The data reported here disclose a new physiological role for mitochondrial NCX1 as the key player in glutamate-induced energy production.

Hyperuricemia induces endothelial dysfunction via mitochondrial Na+/Ca2+ exchanger-mediated mitochondrial calcium overload

BACKGROUND

Uric acid (UA) has proven to be a causal agent in endothelial dysfunction in which ROS production plays an important role. Calcium overload in mitochondria can promote the mitochondrial production of ROS. We hypothesize that calcium transduction in mitochondria contributes to UA-induced endothelial dysfunction.

METHODS AND RESULTS

We first demonstrated that high concentrations of UA cause endothelial dysfunction, marked by a reduction in eNOS protein expression and NO release in vitro. We further found that a high concentration of UA increased levels of [Ca2+]mito, total intracellular ROS, H2O2, and mitochondrial O2·-, and Δψmito but not the [Ca2+]cyt level. When the mitochondrial calcium channels NCXmito and MCU were blocked by CGP-37157 and Ru360, respectively, the UA-induced increases in the levels of [Ca2+]mito and total intracellular ROS were significantly reduced. Mitochondrial levels of O2·- and Δψmito were reduced by inhibition of NCXmito but not of MCU. Moreover, inhibition of NCXmito, but not of MCU, blocked the UA-induced reductions in eNOS protein expression and NO release.

CONCLUSIONS

The increased generation of mitochondrial O2·- induced by a high concentration of UA is triggered by mitochondrial calcium overload and ultimately leads to endothelial dysfunction. In this process, the activation of NCXmito is the major cause of the influx of calcium into mitochondria. Our results provide a new pathophysiological mechanism for UA-induced endothelial dysfunction and may offer a new therapeutic target for clinicians.

Mitochondria and heart disease

Mitochondria play a key role in the normal functioning of the heart, and in the pathogenesis and development of various types of heart disease. Physiologically, mitochondrial ATP supply needs to be matched to the often sudden changes in ATP demand of the heart, and this is mediated to a large extent by the mitochondrial Ca(2+) transport pathways allowing elevation of mitochondrial [Ca(2+)] ([Ca(2+)](m)). In turn this activates dehydrogenase enzymes to increase NADH and hence ATP supply. Pathologically, [Ca(2+)](m) is also important in generation of reactive oxygen species, and in opening of the mitochondrial permeability transition pore (MPTP); factors involved in both ischaemia-reperfusion injury and in heart failure. The MPTP has proved a promising target for protective strategies, with inhibitors widely used to show cardioprotection in experimental, and very recently human, studies. Similarly mitochondrially-targeted antioxidants have proved protective in various animal models of disease and await clinical trials. The mitochondrial Ca(2+) transport pathways, although in theory promising therapeutic targets, cannot yet be targeted in human studies due to non-specific effects of drugs used experimentally to inhibit them. Finally, specific mitochondrial cardiomyopathies due to mutations in mtDNA have been identified, usually in a gene for a tRNA, which, although rare, are almost always very severe once the mutation has exceeded its threshold.

The effect of OPA1 on mitochondrial Ca²⁺ signaling

The dynamin-related GTPase protein OPA1, localized in the intermembrane space and tethered to the inner membrane of mitochondria, participates in the fusion of these organelles. Its mutation is the most prevalent cause of Autosomal Dominant Optic Atrophy. OPA1 controls the diameter of the junctions between the boundary part of the inner membrane and the membrane of cristae and reduces the diffusibility of cytochrome c through these junctions. We postulated that if significant Ca²⁺ uptake into the matrix occurs from the lumen of the cristae, reduced expression of OPA1 would increase the access of Ca²⁺ to the transporters in the crista membrane and thus would enhance Ca²⁺ uptake. In intact H295R adrenocortical and HeLa cells cytosolic Ca²⁺ signals evoked with K⁺ and histamine, respectively, were transferred into the mitochondria. The rate and amplitude of mitochondrial [Ca²⁺] rise (followed with confocal laser scanning microscopy and FRET measurements with fluorescent wide-field microscopy) were increased after knockdown of OPA1, as compared with cells transfected with control RNA or mitofusin1 siRNA. Ca²⁺ uptake was enhanced despite reduced mitochondrial membrane potential. In permeabilized cells the rate of Ca²⁺ uptake by depolarized mitochondria was also increased in OPA1-silenced cells. The participation of Na⁺/Ca²⁺ and Ca²⁺/H⁺ antiporters in this transport process is indicated by pharmacological data. Altogether, our observations reveal the significance of OPA1 in the control of mitochondrial Ca²⁺ metabolism.

Calcium uptake mechanisms of mitochondria

The ability of mitochondria to capture Ca2+ ions has important functional implications for cells, because mitochondria shape cellular Ca2+ signals by acting as a Ca2+ buffer and respond to Ca2+ elevations either by increasing the cell energy supply or by triggering the cell death program of apoptosis. A mitochondrial Ca2+ channel known as the uniporter drives the rapid and massive entry of Ca2+ ions into mitochondria. The uniporter operates at high, micromolar cytosolic Ca2+ concentrations that are only reached transiently in cells, near Ca2+ release channels. Mitochondria can also take up Ca2+ at low, nanomolar concentrations, but this high affinity mode of Ca2+ uptake is not well characterized. Recently, leucine-zipper-EF hand-containing transmembrane region (Letm1) was proposed to be an electrogenic 1:1 mitochondrial Ca2+/H+ antiporter that drives the uptake of Ca2+ into mitochondria at nanomolar cytosolic Ca2+ concentrations. In this article, we will review the properties of the Ca2+ import systems of mitochondria and discuss how Ca2+ uptake via an electrogenic 1:1 Ca2+/H+ antiport challenges our current thinking of the mitochondrial Ca2+ uptake mechanism.

Analysis of mitochondrial pH and ion concentrations

Detailed practical information is provided with emphasis on mapping cytosolic and mitochondrial pH, mitochondrial Na(+), and briefly also aspects related to mitochondrial Ca(2+) measurements in living cells, as grown on (un)coated glass coverslips. This chapter lists (laser scanning confocal) microscope instrumentation and setup requirements for proper imaging conditions, cell holders, and an easy-to-use incubator stage. For the daily routine of preparing buffer and calibration solutions, extensive annotated protocols are provided. In addition, detailed measurement and image analysis protocols are given to routinely obtain optimum results with confidence, while avoiding a number of typical pitfalls.

Regulation of the Human Cardiac Mitochondrial Ca 2+ Uptake by 2 Different Voltage-Gated Ca 2+ Channels

Background— Impairment of intracellular Ca 2+ homeostasis and mitochondrial function has been implicated in the development of cardiomyopathy. Mitochondrial Ca 2+ uptake is thought to be mediated by the Ca 2+ uniporter (MCU) and a thus far speculative non-MCU pathway. However, the identity and properties of these pathways are a matter of intense debate, and possible functional alterations in diseased states have remained elusive.

Methods and Results— By patch clamping the inner membrane of mitochondria from nonfailing and failing human hearts, we have identified 2 previously unknown Ca 2+ -selective channels, referred to as mCa1 and mCa2. Both channels are voltage dependent but differ significantly in gating parameters. Compared with mCa2 channels, mCa1 channels exhibit a higher single-channel amplitude, shorter openings, a lower open probability, and 3 to 5 subconductance states. Similar to the MCU, mCa1 is inhibited by 200 nmol/L ruthenium 360, whereas mCa2 is insensitive to 200 nmol/L ruthenium 360 and reduced only by very high concentrations (10 μmol/L). Both mitochondrial Ca 2+ channels are unaffected by blockers of other possibly Ca 2+ -conducting mitochondrial pores but were activated by spermine (1 mmol/L). Notably, activity of mCa1 and mCa2 channels is decreased in failing compared with nonfailing heart conditions, making them less effective for Ca 2+ uptake and likely Ca 2+ -induced metabolism.

Conclusions— Thus, we conclude that the human mitochondrial Ca 2+ uptake is mediated by these 2 distinct Ca 2+ channels, which are functionally impaired in heart failure. Current properties reveal that the mCa1 channel underlies the human MCU and that the mCa2 channel is responsible for the ruthenium red–insensitive/low-sensitivity non-MCU–type mitochondrial Ca 2+ uptake.

Role of the mitochondrial sodium/calcium exchanger in neuronal physiology and in the pathogenesis of neurological diseases

In neurons, as in other excitable cells, mitochondria extrude Ca(2+) ions from their matrix in exchange with cytosolic Na(+) ions. This exchange is mediated by a specific transporter located in the inner mitochondrial membrane, the mitochondrial Na(+)/Ca(2+) exchanger (NCX(mito)). The stoichiometry of NCX(mito)-operated Na(+)/Ca(2+) exchange has been the subject of a long controversy, but evidence of an electrogenic 3 Na(+)/1 Ca(2+) exchange is increasing. Although the molecular identity of NCX(mito) is still undetermined, data obtained in our laboratory suggest that besides the long-sought and as yet unfound mitochondrial-specific NCX, the three isoforms of plasmamembrane NCX can contribute to NCX(mito) in neurons and astrocytes. NCX(mito) has a role in controlling neuronal Ca(2+) homeostasis and neuronal bioenergetics. Indeed, by cycling the Ca(2+) ions captured by mitochondria back to the cytosol, NCX(mito) determines a shoulder in neuronal [Ca(2+)](c) responses to neurotransmitters and depolarizing stimuli which may then outlast stimulus duration. This persistent NCX(mito)-dependent Ca(2+) release has a role in post-tetanic potentiation, a form of short-term synaptic plasticity. By controlling [Ca(2+)](m) NCX(mito) regulates the activity of the Ca(2+)-sensitive enzymes pyruvate-, alpha-ketoglutarate- and isocitrate-dehydrogenases and affects the activity of the respiratory chain. Convincing experimental evidence suggests that supraphysiological activation of NCX(mito) contributes to neuronal cell death in the ischemic brain and, in epileptic neurons coping with seizure-induced ion overload, reduces the ability to reestablish normal ionic homeostasis. These data suggest that NCX(mito) could represent an important target for the development of new neurological drugs.

Acute inhibition of the betaine transporter by ATP and adenosine in renal MDCK cells

Extracellular ATP interacts with purinergic P2 receptors to regulate a range of physiological responses, including downregulation of transport activity in the nephron. ATP is released from cells by mechanical stimuli such as cell volume changes, and autocrine signaling by extracellular ATP could occur in renal medullary cells during diuresis. This was tested in Madin-Darby canine kidney (MDCK) cells, a model used frequently to study P1 and P2 receptor activity. ATP was released within 1 min after transfer from 500 to 300 mosmol/kgH2O medium. A 30-min incubation with ATP produced dose-dependent inhibition (0.01-0.10 mM) of the renal betaine/GABA transporter (BGT1) with little effect on other osmolyte transporters. Inhibition was reproduced by specific agonists for P2X (alpha,beta-methylene-ATP) and P2Y (UTP) receptors. Adenosine, the final product of ATP hydrolysis, also inhibited BGT1 but not taurine transport. Inhibition by ATP and adenosine was blocked by pertussis toxin and A73122, suggesting involvement of inhibitory G protein and PLC in postreceptor signaling. Both ATP and adenosine (0.1 mM) produced rapid increases in intracellular Ca2+, due to the mobilization of intracellular Ca2+ stores and Ca2+ influx. Blocking these Ca2+ increases with BAPTA-AM also blocked the action of ATP and adenosine on BGT1 transport. Finally, immunohistochemical studies indicated that inhibition of BGT1 transport may be due to endocytic accumulation of BGT1 proteins from the plasma membrane. We conclude that ATP and adenosine, through stimulation of PLC and intracellular Ca2+, may be rapidly acting regulators of BGT1 transport especially in response to a fall in extracellular osmolarity.

Cytoplasmic Na+-dependent modulation of mitochondrial Ca2+ via electrogenic mitochondrial Na+-Ca2+ exchange

To clarify the role of mitochondrial Na(+)-Ca(2+) exchange (NCX(mito)) in regulating mitochondrial Ca(2+) (Ca(2+)(mito)) concentration at intact and depolarized mitochondrial membrane potential (DeltaPsi(mito)), we measured Ca(2+)(mito) and DeltaPsi(mito) using fluorescence probes Rhod-2 and TMRE, respectively, in the permeabilized rat ventricular cells. Applying 300 nm cytoplasmic Ca(2+) (Ca(2+)(c)) increased Ca(2+)(mito) and this increase was attenuated by cytoplasmic Na(+) (Na(+)(c)) with an IC(50) of 2.4 mm. To the contrary, when DeltaPsi(mito) was depolarized by FCCP, a mitochondrial uncoupler, Na(+)(c) enhanced the Ca(2+)(c)-induced increase in Ca(2+)(mito) with an EC(50) of about 4 mm. This increase was not significantly affected by ruthenium red or cyclosporin A. The inhibition of NCX(mito) by CGP-37157 further increased Ca(2+)(mito) when DeltaPsi(mito) was intact, while it suppressed the Ca(2+)(mito) increase when DeltaPsi(mito) was depolarized, suggesting that DeltaPsi(mito) depolarization changed the exchange mode from forward to reverse. Furthermore, DeltaPsi(mito) depolarization significantly reduced the Ca(2+)(mito) decrease via forward mode, and augmented the Ca(2+)(mito) increase via reverse mode. When the respiratory chain was attenuated, the induction of the reverse mode of NCX(mito) hyperpolarized DeltaPsi(mito), while DeltaPsi(mito) depolarized upon inducing the forward mode of NCX(mito). Both changes in DeltaPsi(mito) were remarkably inhibited by CGP-37157. The above experimental data indicated that NCX(mito) is voltage dependent and electrogenic. This notion was supported theoretically by computer simulation studies with an NCX(mito) model constructed based on present and previous studies, presuming a consecutive and electrogenic Na(+)-Ca(2+) exchange and a depolarization-induced increase in Na(+) flux. It is concluded that Ca(2+)(mito) concentration is dynamically modulated by Na(+)(c) and DeltaPsi(mito) via electrogenic NCX(mito).

Calcium in cell injury and death

Loss of Ca(2+) homeostasis, often in the form of cytoplasmic increases, leads to cell injury. Depending upon cell type and the intensity of Ca(2+) toxicity, the ensuing pathology can be reversible or irreversible. Although multiple destructive processes are activated by Ca(2+), lethal outcomes are determined largely by Ca(2+)-induced mitochondrial permeability transition. This form of damage is primarily dependent upon mitochondrial Ca(2+) accumulation, which is regulated by the mitochondrial membrane potential. Retention of the mitochondrial membrane potential during Ca(2+) increases favors mitochondrial Ca(2+) uptake and overload, resulting in mitochondrial permeability transition and cell death. In contrast, dissipation of mitochondrial membrane potential reduces mitochondrial Ca(2+) uptake, retards mitochondrial permeability transition, and delays death, even in cells with large Ca(2+) increases. The rates of mitochondrial membrane potential dissipation and mitochondrial Ca(2+) uptake may determine cellular sensitivity to Ca(2+) toxicity under pathological conditions, including ischemic injury.

Excitation-contraction coupling and mitochondrial energetics

Cardiac excitation-contraction (EC) coupling consumes vast amounts of cellular energy, most of which is produced in mitochondria by oxidative phosphorylation. In order to adapt the constantly varying workload of the heart to energy supply, tight coupling mechanisms are essential to maintain cellular pools of ATP, phosphocreatine and NADH. To our current knowledge, the most important regulators of oxidative phosphorylation are ADP, Pi, and Ca2+. However, the kinetics of mitochondrial Ca2+-uptake during EC coupling are currently a matter of intense debate. Recent experimental findings suggest the existence of a mitochondrial Ca2+ microdomain in cardiac myocytes, justified by the close proximity of mitochondria to the sites of cellular Ca2+ release, i. e., the ryanodine receptors of the sarcoplasmic reticulum. Such a Ca2+ microdomain could explain seemingly controversial results on mitochondrial Ca2+ uptake kinetics in isolated mitochondria versus whole cardiac myocytes. Another important consideration is that rapid mitochondrial Ca2+ uptake facilitated by microdomains may shape cytosolic Ca2+ signals in cardiac myocytes and have an impact on energy supply and demand matching. Defects in EC coupling in chronic heart failure may adversely affect mitochondrial Ca2+ uptake and energetics, initiating a vicious cycle of contractile dysfunction and energy depletion. Future therapeutic approaches in the treatment of heart failure could be aimed at interrupting this vicious cycle.

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

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

Metabolic inhibition-induced transient Ca2+ increase depends on mitochondria in a human proximal renal cell line

During ischemia or hypoxia an increase in intracellular cytosolic Ca(2+) induces deleterious events but is also implicated in signaling processes triggered in such conditions. In MDCK cells (distal tubular origin), it was shown that mitochondria confer protection during metabolic inhibition (MI), by buffering the Ca(2+) overload via mitochondrial Na(+)-Ca(2+) exchanger (NCX). To further assess this process in cells of human origin, human cortical renal epithelial cells (proximal tubular origin) were subjected to MI and changes in cytosolic Ca(2+) ([Ca(2+)](i)), Na(+), and ATP concentrations were monitored. MI was accomplished with both antimycin A and 2-deoxyglucose and induced a 3.5-fold increase in [Ca(2+)](i), reaching 136.5 +/- 15.8 nM in the first 3.45 min. Subsequently [Ca(2+)](i) dropped and stabilized to 62.7 +/- 7.3 nM by 30 min. The first phase of the transient increase was La(3+) sensitive, not influenced by diltiazem, and abolished when mitochondria were deenergized with the protonophore carbonylcyanide p-trifluoromethoxyphenylhydrazone. The subsequent recovery phase was impaired in a Na(+)-free medium and weakened when the mitochondrial NCX was blocked with 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP-37157). Thus Ca(2+) entry is likely mediated by store-operated Ca(2+) channels and depends on energized mitochondria, whereas [Ca(2+)](i) recovery relied partially on the activity of mitochondrial NCX. These results indicate a possible mitochondrial-mediated signaling process triggered by MI, support the hypothesis that mitochondrial NCX has an important role in the Ca(2+) clearance, and overall suggest that mitochondria play a preponderant role in the regulation of responses to MI in human renal epithelial cells.

PKD1 haploinsufficiency causes a syndrome of inappropriate antidiuresis in mice

Mutations in PKD1 are associated with autosomal dominant polycystic kidney disease. Studies in mouse models suggest that the vasopressin (AVP) V2 receptor (V2R) pathway is involved in renal cyst progression, but potential changes before cystogenesis are unknown. This study used a noncystic mouse model to investigate the effect of Pkd1 haploinsufficiency on water handling and AVP signaling in the collecting duct (CD). In comparison with wild-type littermates, Pkd1(+/-) mice showed inappropriate antidiuresis with higher urine osmolality and lower plasma osmolality at baseline, despite similar renal function and water intake. The Pkd1(+/-) mice had a decreased aquaretic response to both a water load and a selective V2R antagonist, despite similar V2R distribution and affinity. They showed an inappropriate expression of AVP in brain, irrespective of the hypo-osmolality. The cAMP levels in kidney and urine were unchanged, as were the mRNA levels of aquaporin-2 (AQP2), V2R, and cAMP-dependent mediators in kidney. However, the (Ser256) phosphorylated AQP2 was upregulated in Pkd1(+/-) kidneys, with AQP2 recruitment to the apical plasma membrane of CD principal cells. The basal intracellular Ca(2+) concentration was significantly lower in isolated Pkd1(+/-) CD, with downregulated phosphorylated extracellular signal-regulated kinase 1/2 and decreased RhoA activity. Thus, in absence of cystic changes, reduced Pkd1 gene dosage is associated with a syndrome of inappropriate antidiuresis (positive water balance) reflecting decreased intracellular Ca(2+) concentration, decreased activity of RhoA, recruitment of AQP2 in the CD, and inappropriate expression of AVP in the brain. These data give new insights in the potential roles of polycystin-1 in the AVP and Ca(2+) signaling and the trafficking of AQP2 in the CD.

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).

Inhibition of the renal betaine transporter by calcium ions

Chronic upregulation of the renal betaine/GABA transporter (BGT1) by hypertonic stress has been well documented, but it is not known whether BGT1 can be regulated acutely after insertion in the basolateral plasma membrane. Related transporters, such as the rat brain GABA transporter, can be rapidly removed from the plasma membrane through activation of G protein-coupled receptors. The goal of the present study was to determine whether acute changes in extracellular and/or intracellular Ca2+will regulate BGT1 transport activity at the plasma membrane level in Madin-Darby canine kidney cells subjected to 24-h hypertonic stress. After brief pretreatment with a Ca2+-free solution, the addition of extracellular Ca2+in the transport assay produced dose-dependent inhibition of Na+-GABA cotransport. Maximum inhibition was 49% at 2 mM Ca2+( P < 0.05). Fura 2 imaging confirmed that addition of 2 mM Ca2+produced a transient increase in intracellular Ca2+that preceded transport inhibition. Acute inhibition of Na+-GABA cotransport was reproduced by addition of thapsigargin (5 μM) and ionomycin (10 μM). Amino acid transport system A, assayed as a control, was not inhibited. Brief treatment with phorbol esters reproduced the specific inhibition of Na+-GABA cotransport, and the inhibition was blocked by staurosporine. Surface biotinylation confirmed that the response to phorbol esters was accompanied by loss of BGT1 protein from the plasma membrane, and immunohistochemistry showed a shift to an intracellular distribution. We conclude that BGT1 can be inhibited acutely by extracellular Ca2+through a mechanism involving BGT1 protein internalization, and protein kinase C may play a role.

Role of mitochondrial Na+ concentration, measured by CoroNa red, in the protection of metabolically inhibited MDCK cells

In ischemic or hypoxic tissues, elevated cytosolic calcium levels can induce lethal processes. Mitochondria, besides the endoplasmic reticulum, play a key role in clearing excessive cytosolic Ca2+. In a previous study, it was suggested that the clearance of cytosolic Ca2+, after approximately 18 min of metabolic inhibition (MI) in renal epithelial cells, occurs via the reverse action of the mitochondrial Na+/Ca2+ exchanger (NCX). For further investigating the underlying mechanism, changes in the mitochondrial Na+ concentration ([Na+](m)) were monitored in metabolically inhibited MDCK cells. CoroNa Red, a sodium-sensitive fluorescence probe, was used to monitor [Na+]m. In the first 15 min of MI, a twofold increase of [Na+]m was observed reaching 113 +/- 7 mM, whereas the cytosolic Na+ concentration ([Na+]c) elevated threefold, to a level of 65 +/- 6 mM. In the next 45 min of MI, [Na+]m dropped to 91 +/- 7 mM, whereas [Na+]c further increased to 91 +/- 4 mM. The striking rise in [Na+]m is likely sufficient to sustain the driving force for mitochondrial Ca2+ uptake via the NCX. Furthermore, when CGP-37157, a specific inhibitor of the mitochondrial NCX, was applied during MI, the second-phase drop of [Na+]m was completely abolished. The obtained results support the hypothesis that the mitochondrial NCX reverses after approximately 15 min of MI. Moreover, because the cellular homeostasis can recover after MI, the mitochondria likely protect MDCK cells from injury during MI by the reversal of the mitochondrial NCX. This study is the first to report [Na+]m measurements in nonpermeabilized living cells.