Mechanism of partial reactions in the cardiac Na(+)-Ca2+ exchange system

Neuronal and astrocyte NCX isoform/splice variants: How do they participate in Na+ and Ca2+ signalling?

NCX1, NCX2, and NCX3 gene isoforms and their splice variants are characteristically expressed in different regions of the brain. The tissue-specific splice variants of NCX1-3 isoforms show specific expression profiles in neurons and astrocytes, whereas the relevant NCX isoform/splice variants exhibit diverse allosteric modes of Na+- and Ca2+-dependent regulation. In general, overexpression of NCX1-3 genes leads to neuroprotective effects, whereas their ablation gains the opposite results. At this end, the partial contributions of NCX isoform/splice variants to neuroprotective effects remain unresolved. The glutamate-dependent Na+ entry generates Na+ transients (in response to neuronal cell activities), whereas the Na+-driven Ca2+ entry (through the reverse NCX mode) raises Ca2+ transients. This special mode of signal coupling translates Na+ transients into the Ca2+ signals while being a part of synaptic neurotransmission. This mechanism is of general interest since disease-related conditions (ischemia, metabolic stress, and stroke among many others) trigger Na+ and Ca2+ overload with deadly outcomes of downstream apoptosis and excitotoxicity. The recently discovered mechanisms of NCX allosteric regulation indicate that some NCX variants might play a critical role in the dynamic coupling of Na+-driven Ca2+ entry. In contrast, the others are less important or even could be dangerous under altered conditions (e.g., metabolic stress). This working hypothesis can be tested by applying advanced experimental approaches and highly focused computational simulations. This may allow the development of structure-based blockers/activators that can selectively modulate predefined NCX variants to lessen the life-threatening outcomes of excitotoxicity, ischemia, apoptosis, metabolic deprivation, brain injury, and stroke.

Post-translational modifications and protein quality control of mitochondrial channels and transporters

Mitochondria play a critical role in energy metabolism and signal transduction, which is tightly regulated by proteins, metabolites, and ion fluxes. Metabolites and ion homeostasis are mainly mediated by channels and transporters present on mitochondrial membranes. Mitochondria comprise two distinct compartments, the outer mitochondrial membrane (OMM) and the inner mitochondrial membrane (IMM), which have differing permeabilities to ions and metabolites. The OMM is semipermeable due to the presence of non-selective molecular pores, while the IMM is highly selective and impermeable due to the presence of specialized channels and transporters which regulate ion and metabolite fluxes. These channels and transporters are modulated by various post-translational modifications (PTMs), including phosphorylation, oxidative modifications, ions, and metabolites binding, glycosylation, acetylation, and others. Additionally, the mitochondrial protein quality control (MPQC) system plays a crucial role in ensuring efficient molecular flux through the mitochondrial membranes by selectively removing mistargeted or defective proteins. Inefficient functioning of the transporters and channels in mitochondria can disrupt cellular homeostasis, leading to the onset of various pathological conditions. In this review, we provide a comprehensive overview of the current understanding of mitochondrial channels and transporters in terms of their functions, PTMs, and quality control mechanisms.

Mitochondrial calcium at the synapse

Mitochondria are dynamic organelles, which serve various purposes, including but not limited to the production of ATP and various metabolites, buffering ions, acting as a signaling hub, etc. In recent years, mitochondria are being seen as the central regulators of cellular growth, development, and death. Since neurons are highly specialized cells with a heavy metabolic demand, it is not surprising that neurons are one of the most mitochondria-rich cells in an animal. At synapses, mitochondrial function and dynamics is tightly regulated by synaptic calcium. Calcium influx during synaptic activity causes increased mitochondrial calcium influx leading to an increased ATP production as well as buffering of synaptic calcium. While increased ATP production is required during synaptic transmission, calcium buffering by mitochondria is crucial to prevent faulty neurotransmission and excitotoxicity. Interestingly, mitochondrial calcium also regulates the mobility of mitochondria within synapses causing mitochondria to halt at the synapse during synaptic transmission. In this review, we summarize the various roles of mitochondrial calcium at the synapse.

Molecular machinery regulating mitochondrial calcium levels: The nuts and bolts of mitochondrial calcium dynamics

Mitochondria play vital role in regulating the cellular energetics and metabolism. Further, it is a signaling hub for cell survival and apoptotic pathways. One of the key determinants that calibrate both cellular energetics and survival functions is mitochondrial calcium (Ca2+) dynamics. Mitochondrial Ca2+ regulates three Ca2+-sensitive dehydrogenase enzymes involved in tricarboxylic acid cycle (TCA) cycle thereby directly controlling ATP synthesis. On the other hand, excessive Ca2+ concentration within the mitochondrial matrix elevates mitochondrial reactive oxygen species (mROS) levels and causes mitochondrial membrane depolarization. This leads to opening of the mitochondrial permeability transition pore (mPTP) and release of cytochrome c into cytosol eventually triggering apoptosis. Therefore, it is critical for cell to maintain mitochondrial Ca2+ concentration. Since cells can neither synthesize nor metabolize Ca2+, it is the dynamic interplay of Ca2+ handling proteins involved in mitochondrial Ca2+ influx and efflux that take the center stage. In this review we would discuss the key molecular machinery regulating mitochondrial Ca2+ concentration. We would focus on the channel complex involved in bringing Ca2+ into mitochondrial matrix i.e. Mitochondrial Ca2+ Uniporter (MCU) and its key regulators Mitochondrial Ca2+ Uptake proteins (MICU1, 2 and 3), MCU regulatory subunit b (MCUb), Essential MCU Regulator (EMRE) and Mitochondrial Ca2+ Uniporter Regulator 1 (MCUR1). Further, we would deliberate on major mitochondrial Ca2+ efflux proteins i.e. Mitochondrial Na+/Ca2+/Li+ exchanger (NCLX) and Leucine zipper EF hand-containing transmembrane1 (Letm1). Moreover, we would highlight the physiological functions of these proteins and discuss their relevance in human pathophysiology. Finally, we would highlight key outstanding questions in the field.

Functional asymmetry of bidirectional Ca2+-movements in an archaeal sodium-calcium exchanger (NCX_Mj)

Dynamic features of Ca(2+) interactions with transport and regulatory sites control the Ca(2+)-fluxes in mammalian Na(+)/Ca(2+)(NCX) exchangers bearing the Ca(2+)-binding regulatory domains on the cytosolic 5L6 loop. The crystal structure of Methanococcus jannaschii NCX (NCX_Mj) may serve as a template for studying ion-transport mechanisms since NCX_Mj does not contain the regulatory domains. The turnover rate of Na(+)/Ca(2+) exchange (kcat=0.5±0.2 s(-1)) in WT-NCX_Mj is 10(3)-10(4) times slower than in mammalian NCX. In NCX_Mj, the intrinsic equilibrium (Kint) for bidirectional Ca(2+) movements (defined as the ratio between the cytosolic and extracellular Km of Ca(2+)/Ca(2+) exchange) is asymmetric, Kint=0.15±0.5. Therefore, the Ca(2+) movement from the cytosol to the extracellular side is ∼7-times faster than in the opposite direction, thereby representing a stabilization of outward-facing (extracellular) access. This intrinsic asymmetry accounts for observed differences in the cytosolic and extracellulr Km values having a physiological relevance. Bidirectional Ca(2+) movements are also asymmetric in mammalian NCX. Thus, the stabilization of the outward-facing access along the transport cycle is a common feature among NCX orthologs despite huge differences in the ion-transport kinetics. Elongation of the cytosolic 5L6 loop in NCX_Mj by 8 or 14 residues accelerates the ion transport rates (kcat) ∼10 fold, while increasing the Kint values 100-250-fold (Kint=15-35). Therefore, 5L6 controls both the intrinsic equilibrium and rates of bidirectional Ca(2+) movements in NCX proteins. Some additional structural elements may shape the kinetic variances among phylogenetically distant NCX variants, although the intrinsic asymmetry (Kint) of bidirectional Ca(2+) movements seems to be comparable among evolutionary diverged NCX variants.

Sodium-calcium exchangers (NCX): molecular hallmarks underlying the tissue-specific and systemic functions

NCX proteins explore the electrochemical gradient of Na(+) to mediate Ca(2+)-fluxes in exchange with Na(+) either in the Ca(2+)-efflux (forward) or Ca(2+)-influx (reverse) mode, whereas the directionality depends on ionic concentrations and membrane potential. Mammalian NCX variants (NCX1-3) and their splice variants are expressed in a tissue-specific manner to modulate the heartbeat rate and contractile force, the brain's long-term potentiation and learning, blood pressure, renal Ca(2+) reabsorption, the immune response, neurotransmitter and insulin secretion, apoptosis and proliferation, mitochondrial bioenergetics, etc. Although the forward mode of NCX represents a major physiological module, a transient reversal of NCX may contribute to EC-coupling, vascular constriction, and synaptic transmission. Notably, the reverse mode of NCX becomes predominant in pathological settings. Since the expression levels of NCX variants are disease-related, the selective pharmacological targeting of tissue-specific NCX variants could be beneficial, thereby representing a challenge. Recent structural and biophysical studies revealed a common module for decoding the Ca(2+)-induced allosteric signal in eukaryotic NCX variants, although the phenotype variances in response to regulatory Ca(2+) remain unclear. The breakthrough discovery of the archaebacterial NCX structure may serve as a template for eukaryotic NCX, although the turnover rates of the transport cycle may differ ~10(3)-fold among NCX variants to fulfill the physiological demands for the Ca(2+) flux rates. Further elucidation of ion-transport and regulatory mechanisms may lead to selective pharmacological targeting of NCX variants under disease conditions.

NCLX: the mitochondrial sodium calcium exchanger

The free Ca(2+) concentration within the mitochondrial matrix ([Ca(2+)]m) regulates the rate of ATP production and other [Ca(2+)]m sensitive processes. It is set by the balance between total Ca(2+) influx (through the mitochondrial Ca(2+) uniporter (MCU) and any other influx pathways) and the total Ca(2+) efflux (by the mitochondrial Na(+)/Ca(2+) exchanger and any other efflux pathways). Here we review and analyze the experimental evidence reported over the past 40years which suggest that in the heart and many other mammalian tissues a putative Na(+)/Ca(2+) exchanger is the major pathway for Ca(2+) efflux from the mitochondrial matrix. We discuss those reports with respect to a recent discovery that the protein product of the human FLJ22233 gene mediates such Na(+)/Ca(2+) exchange across the mitochondrial inner membrane. Among its many functional similarities to other Na(+)/Ca(2+) exchanger proteins is a unique feature: it efficiently mediates Li(+)/Ca(2+) exchange (as well as Na(+)/Ca(2+) exchange) and was therefore named NCLX. The discovery of NCLX provides both the identity of a novel protein and new molecular means of studying various unresolved quantitative aspects of mitochondrial Ca(2+) movement out of the matrix. Quantitative and qualitative features of NCLX are discussed as is the controversy regarding the stoichiometry of the NCLX Na(+)/Ca(2+) exchange, the electrogenicity of NCLX, the [Na(+)]i dependency of NCLX and the magnitude of NCLX Ca(2+) efflux. Metabolic features attributable to NCLX and the physiological implication of the Ca(2+) efflux rate via NCLX during systole and diastole are also briefly discussed.

Conformational changes of the 120-kDa Na+/Ca2+ exchanger protein upon ligand binding: a Fourier transform infrared spectroscopy study

The 120-kDa Na+/Ca2+ exchanger was purified and reconstituted into lipid vesicles. The secondary structure composition of the exchanger was 39% alpha-helices, 20% beta-sheets, 25% beta-turns, and 16% random coils, as analyzed by Fourier transform infrared attenuated total reflection spectroscopy. The secondary structure composition of the COOH-terminal portion of the protein was compatible with a topology model containing 4-6 transmembrane segments. Furthermore, the secondary structure of the NH2-terminal portion of the cytoplasmic loop was analyzed and found to be different from that of the COOH-terminal portion. Ca2+ and/or the exchange inhibitory peptide (XIP) failed to affect the secondary structure of the 120-kDa protein. Tertiary structure modifications induced by Ca2+ and XIP were analyzed by monitoring the hydrogen/deuterium exchange rate for the reconstituted exchanger. In the absence of ligand, 51% of the protein was accessible to solvent. Ca2+ decreased accessibility to 40%, implicating the shielding of at least 103 amino acids. When both Ca2+ and XIP were added, accessibility increased to 66%. No modification was obtained when XIP was added alone. Likewise, in the presence of Ca2+, XIP failed to modify the tertiary structure of the 70-kDa protein, suggesting that XIP acts at the level of the COOH-terminal portion of the intracellular loop. The present data describe, for the first time, conformational changes of the Na+/Ca2+ exchanger induced by Ca2+ and XIP, compatible with an interaction model where regulatory Ca2+ and inhibitory XIP bind to distinct sites, and where XIP binding requires the presence of Ca2+.

Rapid interaction of FRCRCFa with the cytosolic side of the cardiac sarcolemma Na(+)-Ca2+ exchanger blocks the ion transport without preventing the binding of either sodium or calcium

Positively charged cyclic hexapeptide Phe-Arg-Cys-Arg-Cys-Phe-CONH2 (FRCRCFa) represents a group of conformationally constrained peptides that block the cardiac sarcolemma Na(+)-Ca2+ exchanger [Khananshvili et al. (1995b) J. Biol. Chem. 270, 16182-16188]. Here, we study the kinetic mechanisms of FRCRCFa-induced inhibition of Na(+)-Ca2+ exchange and its partial reaction, the Ca(2+)-Ca2+ exchange. The Nai-dependent 45Ca uptake and Cai-dependent 45Ca uptake were measured by adding the EGTA quench in the semirapid mixer. The reverse mode of Na(+)-Ca2+ exchange (Nao-dependent Ca efflux) was monitored (t = 10-5000 ms) in the stopped-flow machine by measuring extravesicular free calcium with a fluorescence probe fluo-3. Saturating concentrations of FRCRCFa inhibit completely the forward and reverse modes of exchange, suggesting that the inside-out vesicles contribute to most (if not all) of the exchange activity. A short time exposure (t = 10-20 ms) of FRCRCFa with the vesicles is enough to reach a rapid equilibrium between FRCRCFa and a putative inhibitory site at the extravesicular (cytosolic) side of the membrane. A lower limit for the second-order rate constant of FRCRCFa binding can be estimated as a kon of > 10(6) M-1 s-1. A possible competition between FRCRCFa and either Na+ or Ca2+ has been tested at the extravesicular (cytosolic) side of the membrane. At different extravesicular Cao concentrations of 13-250 microM, FRCRCFa inhibits the Na(+)-Ca2+ and Ca(2+)-Ca2+ exchanges with an IC50 of 11-16 microM, suggesting no competition between FRCRCFa and Ca2+. At different extravesicular Nao concentrations of 40-160 mM, FRCRCFa inhibits Nao-dependent Ca efflux with an IC50 of 12-18 microM, suggesting that FRCRCFa and Na+ do not compete for binding at the extravesicular side. A mild proteolytic treatment of vesicles activates the Nai-dependent 45Ca uptake, but has a little effect on the FRCRCFa-induced inhibition. Thus, the "inhibitory site" is still functional after the proteolytic treatment of the inside-out vesicles. In conclusion, a rapid (< 20 ms) interaction of extravesicular (cytosolic) FRCRCFa with the exchanger prevents the ion translocation through the exchanger, while the inhibitory peptide does not interact with the ion transport sites of the exchanger at the cytosolic side of the membrane.

Rapid charge translocation by the cardiac Na(+)-Ca2+ exchanger after a Ca2+ concentration jump

The kinetics of Na(+)-Ca2+ exchange current after a cytoplasmic Ca2+ concentration jump (achieved by photolysis of DM-nitrophen) was measured in excised giant membrane patches from guinea pig or rat heart. Increasing the cytoplasmic Ca2+ concentration from 0.5 microM in the presence of 100 mM extracellular Na+ elicits an inward current that rises with a time constant tau 1 < 50 microseconds and decays to a plateau with a time constant tau 2 = 0.65 +/- 0.18 ms (n = 101) at 21 degrees C. These current signals are suppressed by Ni2+ and dichlorobenzamil. No stationary current, but a transient inward current that rises with tau 1 < 50 microseconds and decays with tau 2 = 0.28 +/- 0.06 ms (n = 53, T = 21 degrees C) is observed if the Ca2+ concentration jump is performed under conditions that promote Ca(2+)-Ca2+ exchange (i.e., no extracellular Na+, 5 mM extracellular Ca2+). The transient and stationary inward current is not observed in the absence of extracellular Ca2+ and Na+. The application of alpha-chymotrypsin reveals the influence of the cytoplasmic regulatory Ca2+ binding site on Ca(2+)-Ca2+ and forward Na(+)-Ca2+ exchange and shows that this site regulates both the transient and stationary current. The temperature dependence of the stationary current exhibits an activation energy of 70 kj/mol for temperatures between 21 degrees C and 38 degrees C, and 138 kj/mol between 10 degrees C and 21 degrees C. For the decay time constant an activation energy of 70 kj/mol is observed in the Na(+)-Ca2+ and the Ca(2+)-Ca2+ exchange mode between 13 degrees C and 35 degrees C. The data indicate that partial reactions of the Na(+)-Ca2+ exchanger associated with Ca2+ binding and translocation are very fast at 35 degrees C, with relaxation time constants of about 6700 s-1 in the forward Na(+)-Ca2+ exchange and about 12,500 s-1 in the Ca(2+)-Ca2+ exchange mode and that net negative charge is moved during Ca2+ translocation. According to model calculations, the turnover number, however, has to be at least 2-4 times smaller than the decay rate of the transient current, and Na+ inward translocation appears to be slower than Ca2+ outward movement.

Positively charged cyclic hexapeptides, novel blockers for the cardiac sarcolemma Na(+)-Ca2+ exchanger

Positively charged cyclic hexapeptides have been synthesized and tested for their effects on the cardiac sarcolemma Na(+)-Ca2+ exchange activities with a goal to identify a potent blocker. The cyclic hexapeptides, having the different amino acid sequence, contain two arginines (to retain a positive charge), two phenylalanines (to control hydrophobicity), and two cysteines (to form an intramolecular S-S bond). The effect of cyclic hexapeptides were tested on Na(+)-Ca2+ exchange and its partial reaction, the Ca(2+)-Ca2+ exchange, by measuring the 45Ca fluxes in the semi-rapid mixer or monitoring the calcium-sensitive dye Arsenazo III and voltage-sensitive dyes (Oxanol-V or Merocyanine-540). Seven cyclic hexapeptides inhibit Na(+)-Ca2+ exchange with a different potency (IC50 = 2-300 microM). Phe-Arg-Cys-Arg-Cys-Phe-CONH2 (FRCRCFa) inhibits the Na+i-dependent 45Ca uptake (Na(+)-Ca2+ exchange) and Ca2+i-dependent 45Ca uptake (Ca(2+)-Ca2+ exchange) in the isolated cardiac sarcolemma vesicles with IC50 = 10 +/- 2 microM and IC50 = 7 +/- 3 microM, respectively. Interaction of FRCRCFa with a putative inhibitory site does not involve a "slow" binding (a maximal inhibitory effect is already observed after t = 1 s of mixing). The inside positive potential, generated by Na+o-dependent Ca2+ efflux, was monitored by Oxanol-V (A635-A612) or Merocyanine-540 (A570-A500). In both assay systems, FRCRCFa inhibits the Na(+)-Ca2+ exchange with IC50 = 2-3 microM, while a complete inhibition occurs at 20 microM FRCRCFa. The forward (Na+i-dependent Ca2+ influx) and reverse (Na+o-dependent Ca2+ efflux) modes of Na(+)-Ca2+ exchange, monitored by Arsenazo III (A600-A785), are also inhibited by FRCRCFa. The L-Arg4-->D-Arg4 substitution in FRCRCFa does not alter the IC50, meaning that this structural change may increase a proteolytic resistance without a loss of inhibitory potency. At fixed [Na+]i (160 mM) or [Ca2+]i (250 microM) and varying 45Cao (2-200 microM), FRCRCFa decreases Vmax without altering the Km. Therefore, FRCRCFa is a noncompetitive inhibitor in regard to extravesicular Ca2+ either for Na(+)-Ca2+ or Ca(2+)-Ca2+ exchange. It is suggested that FRCRCFa prevents the ion movements through the exchanger rather than the ion binding.

The cardiac Na(+)-Ca2+ exchanger: relative rates of calcium and sodium movements and their modulation by protonation-deprotonation of the carrier

The exchange cycle of the cardiac Na(+)-Ca2+ exchanger can be described as separate steps of Ca2+ and Na+ transport [Khananshvili, D. (1990) Biochemistry 29, 2437-2442]. In order to determine the relative rates of Na+ and Ca2+ movement during the Na(+)-Ca2+ and Ca(2+)-Ca2+ exchange modes, the ratios (R) of Na(+)-Ca2+/Ca(2+)-Ca2+ exchanges were estimated with saturating concentrations of ions at both sides of the membrane. The effect of extravesicular pH and voltage (potassium valinomycin) on the initial rates (t = 1 s) of Na(+)-Ca2+ and Ca(2+)-Ca2+ exchange were investigated by assuming that, under the conditions tested, the intravesicular pH (pH 7.4) is not affected. Na(+)- or Ca(2+)-preloaded sarcolemma vesicles were diluted rapidly in assay medium containing 45Ca and buffer (pH 5.0-10.9), and the reaction of 45Ca uptake was quenched by using a semi-rapid-mixing device. Under conditions in which [45Ca]o = [Ca]i = 250 microM, the pH-dependent curve of Ca(2+)-Ca2+ exchange shows a bell shape in the acidic range (pKa1 = 5.1 +/- 0.1 and pKa2 = 6.5 +/- 0.2) followed by activation of the exchange in the alkaline range (pKa3 = 10.0 +/- 0.2). With [45Ca]o = 250 microM and [Na]i = 160 mM, the Na(+)-Ca2+ exchange increases monotonically from pH 5.0 to 9.5 (pKa1 = 5.1 +/- 0.1, pKa2 = 7.2 +/- 0.2, and pKa3 = 9.1 +/- 0.2). At pH < 6.1, the ratio of Na(+)-Ca2+/Ca(2+)-Ca2+ exchange is close to unity (R approximately 1), while it increases to R = 3-4 in the range of pH 7.1-9.3.(ABSTRACT TRUNCATED AT 250 WORDS)