Mapping of the cardiac sodium-calcium exchanger with monoclonal antibodies

Cardiac function is regulated by the sodium-dependent inhibition of the sodium-calcium exchanger NCX1

The Na+-Ca2+ exchanger (NCX1) is the dominant Ca2+ extrusion mechanism in cardiac myocytes. NCX1 activity is inhibited by intracellular Na+ via a process known as Na+-dependent inactivation. A central question is whether this inactivation plays a physiological role in heart function. Using CRISPR/Cas9, we inserted the K229Q mutation in the gene (Slc8a1) encoding for NCX1. This mutation removes the Na+-dependent inactivation while preserving transport properties and other allosteric regulations. NCX1 mRNA levels, protein expression, and protein localization are unchanged in K229Q male mice. However, they exhibit reduced left ventricular ejection fraction and fractional shortening, while displaying a prolonged QT interval. K229Q ventricular myocytes show enhanced NCX1 activity, resulting in action potential prolongation, higher incidence of aberrant action potentials, a faster decline of Ca2+ transients, and depressed cell shortening. The results demonstrate that NCX1 Na+-dependent inactivation plays an essential role in heart function by affecting both cardiac excitability and contractility.

Hamster Sperm Possess Functional Na+/Ca2+-Exchanger 1: Its Implication in Hyperactivation

Previous studies demonstrated that hamster sperm hyperactivation is suppressed by extracellular Na⁺ by lowering intracellular Ca²⁺ levels, and Na⁺/Ca²⁺-exchanger (NCX) specific inhibitors canceled the suppressive effects of extracellular Na⁺. These results suggest the involvement of NCX in the regulation of hyperactivation. However, direct evidence of the presence and functionality of NCX in hamster spermatozoa is still lacking. This study aimed to reveal that NCX is present and is functional in hamster spermatozoa. First, NCX1 and NCX2 transcripts were detected via RNA-seq analyses of hamster testis mRNAs, but only the NCX1 protein was detected. Next, NCX activity was determined by measuring the Na⁺-dependent Ca²⁺ influx using the Ca²⁺ indicator Fura-2. The Na⁺-dependent Ca²⁺ influx was detected in hamster spermatozoa, notably in the tail region. The Na⁺-dependent Ca²⁺ influx was inhibited by the NCX inhibitor SEA0400 at NCX1-specific concentrations. NCX1 activity was reduced after 3 h of incubation in capacitating conditions. These results, together with authors' previous study, showed that hamster spermatozoa possesses functional NCX1 and that its activity was downregulated upon capacitation to trigger hyperactivation. This is the first study to successfully reveal the presence of NCX1 and its physiological function as a hyperactivation brake.

The Cardiac Na+ -Ca2+ Exchanger: From Structure to Function

Ca²⁺ homeostasis is essential for cell function and survival. As such, the cytosolic Ca²⁺ concentration is tightly controlled by a wide number of specialized Ca²⁺ handling proteins. One among them is the Na⁺ -Ca²⁺ exchanger (NCX), a ubiquitous plasma membrane transporter that exploits the electrochemical gradient of Na⁺ to drive Ca²⁺ out of the cell, against its concentration gradient. In this critical role, this secondary transporter guides vital physiological processes such as Ca²⁺ homeostasis, muscle contraction, bone formation, and memory to name a few. Herein, we review the progress made in recent years about the structure of the mammalian NCX and how it relates to function. Particular emphasis will be given to the mammalian cardiac isoform, NCX1.1, due to the extensive studies conducted on this protein. Given the degree of conservation among the eukaryotic exchangers, the information highlighted herein will provide a foundation for our understanding of this transporter family. We will discuss gene structure, alternative splicing, topology, regulatory mechanisms, and NCX's functional role on cardiac physiology. Throughout this article, we will attempt to highlight important milestones in the field and controversial topics where future studies are required. © 2021 American Physiological Society. Compr Physiol 12:1-37, 2021.

Distinct features of calcium handling and β-adrenergic sensitivity in heart failure with preserved versus reduced ejection fraction

Key points

Heart failure (HF), the leading cause of death in developed countries, occurs in the setting of reduced (HFrEF) or preserved (HFpEF) ejection fraction. Unlike HFrEF, there are no effective treatments for HFpEF, which accounts for ∼50% of heart failure.

Abnormal intracellular calcium dynamics in cardiomyocytes have major implications for contractility and rhythm, but compared to HFrEF, very little is known about calcium cycling in HFpEF.

We used rat models of HFpEF and HFrEF to reveal distinct differences in intracellular calcium regulation and excitation‐contraction (EC) coupling.

While HFrEF is characterized by defective EC coupling at baseline, HFpEF exhibits enhanced coupling fidelity, further aggravated by a reduction in β‐adrenergic sensitivity.

These differences in EC coupling and β‐adrenergic sensitivity may help explain why therapies that work in HFrEF are ineffective in HFpEF.

Abstract

Heart failure with reduced or preserved ejection fraction (respectively, HFrEF and HFpEF) is the leading cause of death in developed countries. Although numerous therapies improve outcomes in HFrEF, there are no effective treatments for HFpEF. We studied phenotypically verified rat models of HFrEF and HFpEF to compare excitation‐contraction (EC) coupling and protein expression in these two forms of heart failure. Dahl salt‐sensitive rats were fed a high‐salt diet (8% NaCl) from 7 weeks of age to induce HFpEF. Impaired diastolic relaxation and preserved ejection fraction were confirmed in each animal echocardiographically, and clinical signs of heart failure were documented. To generate HFrEF, Sprague‐Dawley (SD) rats underwent permanent left anterior descending coronary artery ligation which, 8–10 weeks later, led to systolic dysfunction (verified echocardiographically) and clinical signs of heart failure. Calcium (Ca²⁺) transients were measured in isolated cardiomyocytes under field stimulation or patch clamp. Ultra‐high‐speed laser scanning confocal imaging captured Ca²⁺ sparks evoked by voltage steps. Western blotting and PCR were used to assay changes in EC coupling protein and RNA expression. Cardiomyocytes from rats with HFrEF exhibited impaired EC coupling, including decreased Ca²⁺ transient (CaT) amplitude and defective couplon recruitment, associated with transverse (t)‐tubule disruption. In stark contrast, HFpEF cardiomyocytes showed saturated EC coupling (increased I_Ca, high probability of couplon recruitment with greater Ca²⁺ release synchrony, increased CaT) and preserved t‐tubule integrity. β‐Adrenergic stimulation of HFpEF myocytes with isoprenaline (isoproterenol) failed to elicit robust increases in I_Ca or CaT and relaxation kinetics. Fundamental differences in EC coupling distinguish HFrEF from HFpEF.

Heart failure (HF), the leading cause of death in developed countries, occurs in the setting of reduced (HFrEF) or preserved (HFpEF) ejection fraction. Unlike HFrEF, there are no effective treatments for HFpEF, which accounts for ∼50% of heart failure.

Abnormal intracellular calcium dynamics in cardiomyocytes have major implications for contractility and rhythm, but compared to HFrEF, very little is known about calcium cycling in HFpEF.

We used rat models of HFpEF and HFrEF to reveal distinct differences in intracellular calcium regulation and excitation‐contraction (EC) coupling.

While HFrEF is characterized by defective EC coupling at baseline, HFpEF exhibits enhanced coupling fidelity, further aggravated by a reduction in β‐adrenergic sensitivity.

These differences in EC coupling and β‐adrenergic sensitivity may help explain why therapies that work in HFrEF are ineffective in HFpEF.

SLC2A9 (GLUT9) mediates urate reabsorption in the mouse kidney

Uric acid (UA) is a metabolite of purine degradation and is involved in gout flairs and kidney stones formation. GLUT9 (SLC2A9) was previously shown to be a urate transporter in vitro. In vivo, humans carrying GLUT9 loss-of-function mutations have familial renal hypouricemia type 2, a condition characterized by hypouricemia, UA renal wasting associated with kidney stones, and an increased propensity to acute renal failure during strenuous exercise. Mice carrying a deletion of GLUT9 in the whole body are hyperuricemic and display a severe nephropathy due to intratubular uric acid precipitation. However, the precise role of GLUT9 in the kidney remains poorly characterized. We developed a mouse model in which GLUT9 was deleted specifically along the whole nephron in a tetracycline-inducible manner (subsequently called kidney-inducible KO or kiKO). The urate/creatinine ratio was increased as early as 4 days after induction of the KO and no GLUT9 protein was visible on kidney extracts. kiKO mice are morphologically identical to their wild-type littermates and had no spontaneous kidney stones. Twenty-four-hour urine collection revealed a major increase of urate urinary excretion rate and of the fractional excretion of urate, with no difference in urate concentration in the plasma. Polyuria was observed, but kiKO mice were still able to concentrate urine after water restriction. KiKO mice displayed lower blood pressure accompanied by an increased heart rate. Overall, these results indicate that GLUT9 is a crucial player in renal handling of urate in vivo and a putative target for uricosuric drugs.

Calpain cleavage and inactivation of the sodium calcium exchanger-3 occur downstream of Aβ in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by pathological deposits of β-amyloid (Aβ) in senile plaques, intracellular neurofibrillary tangles (NFTs) comprising hyperphosphorylated aggregated tau, synaptic dysfunction and neuronal death. Substantial evidence indicates that disrupted neuronal calcium homeostasis is an early event in AD that could mediate synaptic dysfunction and neuronal toxicity. Sodium calcium exchangers (NCXs) play important roles in regulating intracellular calcium, and accumulating data suggests that reduced NCX function, following aberrant proteolytic cleavage of these exchangers, may contribute to neurodegeneration. Here, we show that elevated calpain, but not caspase-3, activity is a prominent feature of AD brain. In addition, we observe increased calpain-mediated cleavage of NCX3, but not a related family member NCX1, in AD brain relative to unaffected tissue and that from other neurodegenerative conditions. Moreover, the extent of NCX3 proteolysis correlated significantly with amounts of Aβ1-42. We also show that exposure of primary cortical neurons to oligomeric Aβ1-42 results in calpain-dependent cleavage of NCX3, and we demonstrate that loss of NCX3 function is associated with Aβ toxicity. Our findings suggest that Aβ mediates calpain cleavage of NCX3 in AD brain and therefore that reduced NCX3 activity could contribute to the sustained increases in intraneuronal calcium concentrations that are associated with synaptic and neuronal dysfunction in AD.

Complete atrial-specific knockout of sodium-calcium exchange eliminates sinoatrial node pacemaker activity

The origin of sinoatrial node (SAN) pacemaker activity in the heart is controversial. The leading candidates are diastolic depolarization by "funny" current (If) through HCN4 channels (the "Membrane Clock" hypothesis), depolarization by cardiac Na-Ca exchange (NCX1) in response to intracellular Ca cycling (the "Calcium Clock" hypothesis), and a combination of the two ("Coupled Clock"). To address this controversy, we used Cre/loxP technology to generate atrial-specific NCX1 KO mice. NCX1 protein was undetectable in KO atrial tissue, including the SAN. Surface ECG and intracardiac electrograms showed no atrial depolarization and a slow junctional escape rhythm in KO that responded appropriately to β-adrenergic and muscarinic stimulation. Although KO atria were quiescent they could be stimulated by external pacing suggesting that electrical coupling between cells remained intact. Despite normal electrophysiological properties of If in isolated patch clamped KO SAN cells, pacemaker activity was absent. Recurring Ca sparks were present in all KO SAN cells, suggesting that Ca cycling persists but is uncoupled from the sarcolemma. We conclude that NCX1 is required for normal pacemaker activity in murine SAN.

Transmembrane segment packing of the Na(+)/Ca(2+) exchanger investigated with chemical cross-linkers

The Na(+)/Ca(2+) exchanger (NCX1) is a plasma membrane protein important in regulating Ca(2+) in cardiac myocytes. The topological model is comprised of nine transmembrane segments (TMSs). To gain insights into the TMS packing arrangement of NCX1, we performed cysteine cross-linking experiments. Pairs of amino acids in different TMSs were mutated to cysteine on the backbone of a cysteineless NCX1. The mutated exchangers were expressed in an insect cell line and treated with cysteine-specific chemical cross-linkers followed by SDS-PAGE to determine the proximity of the introduced cysteines. Previously, we showed that TMSs 2, 3, 7, and 8 are near one another and that residues in TMSs 1 and 2 are close to TMS 6. In this report, we use the same approach to provide evidence for the arrangement of the remaining three TMSs (4, 5, and 9). We present a computer-generated two-dimensional model of transmembrane packing that minimizes the lengths of all cross-links.

Chronic administration of KB-R7943 induces up-regulation of cardiac NCX1

The NCX1 (sodium-calcium exchanger) is up-regulated in human heart failure and in many animal models of heart failure. The potential benefits and risks of therapeutically blocking NCX1 in heart failure and during ischemia-reperfusion are being actively investigated. In this study, we demonstrate that prolonged administration of the NCX1 inhibitor KB-R7943 resulted in the up-regulation of Ncx1 gene expression in both isolated adult cardiomyocytes and intact mouse hearts. Ncx1 up-regulation is mediated by the activation of p38. Importantly, p38 is not activated by KB-R7943 treatment in heart tubes from Ncx1(-/-) mice at 9.5 days postcoitum but is activated in heart tubes from Ncx1(+/+) mice. p38 activation does not appear to be in response to changes in cytosolic calcium concentration, [Ca(2+)](i). Interestingly, chronic KB-R7943 treatment in mice leads to the formation of an NCX1-p38 complex. Our study demonstrates for the first time that the electrogenic sarcolemma membrane cardiac NCX1 can act as a regulator of "activity-dependent signal transduction" leading to changes in gene expression.

Phospholemman regulates cardiac Na+/Ca2+ exchanger by interacting with the exchanger's proximal linker domain

Phospholemman (PLM) belongs to the FXYD family of small ion transport regulators. When phosphorylated at Ser(68), PLM inhibits cardiac Na(+)/Ca(2+) exchanger (NCX1). We previously demonstrated that the cytoplasmic tail of PLM interacts with the proximal intracellular loop (residues 218-358), but not the transmembrane (residues 1-217 and 765-938) or Ca(2+)-binding (residues 371-508) domains, of NCX1. In this study, we used intact Na(+)/Ca(2+) exchanger with various deletions in the intracellular loop to map the interaction sites with PLM. We first demonstrated by Western blotting and confocal immunofluorescence microscopy that wild-type (WT) NCX1 and its deletion mutants were expressed in transfected HEK-293 cells. Cotransfection with PLM and NCX1 (or its deletion mutants) in HEK-293 cells did not decrease expression of NCX1 (or its deletion mutants). Coexpression of PLM with WT NCX1 inhibited NCX1 current (I(NaCa)). Deletion of residues 240-679, 265-373, 250-300, or 300-373 from WT NCX1 resulted in loss of inhibition of I(NaCa) by PLM. Inhibition of I(NaCa) by PLM was preserved when residues 229-237, 270-300, 328-330, or 330-373 were deleted from the intracellular loop of NCX1. These results suggest that PLM mediated inhibition of I(NaCa) by interacting with two distinct regions (residues 238-270 and 300-328) of NCX1. Indeed, I(NaCa) measured in mutants lacking residues 238-270, 300-328, or 238-270 + 300-328 was not affected by PLM. Glutathione S-transferase pull-down assays confirmed that PLM bound to fragments corresponding to residues 218-371, 218-320, 218-270, 238-371, and 300-373, but not to fragments encompassing residues 250-300 and 371-508 of NCX1, indicating that residues 218-270 and 300-373 physically associated with PLM. Finally, acute regulation of I(NaCa) by PLM phosphorylation observed with WT NCX1 was absent in 250-300 deletion mutant but preserved in 229-237 deletion mutant. We conclude that PLM mediates its inhibition of NCX1 by interacting with residues 238-270 and 300-328.

Localization of sarcolemmal proteins to lipid rafts in the myocardium

The localization of sarcolemmal proteins within the membrane can have a dramatic effect on excitation-contraction coupling. We examine the localization of the Na+-Ca2+ exchanger, the dihydropyridine receptor, and other proteins involved in excitation-contraction coupling in rat heart using biochemical and immunolocalization techniques. Specifically, we assess the distribution of proteins within the lipid raft fraction of the sarcolemma. We find that the distribution of proteins in lipid raft fractions is very dependent on the solubilization technique. A common technique using sodium carbonate/pH 11 to solubilize non-lipid raft proteins was inappropriate for use with sarcolemmal membranes. Use of Triton X-100 was more efficacious as a solubilization agent. A large majority of the Na+-Ca2+ exchanger, Na+/K+-ATPase, and plasma membrane Ca2+ pump are not present in lipid rafts. In contrast, most adenosine A1 receptors and dihydropyridine receptors were in lipid raft fractions. Most of the adenosine A1 receptors could be co-immunoprecipitated with caveolin indicating a localization to caveolae (a subclass of lipid rafts). In contrast, the dihydropyridine receptors could not be co-immunoprecipitated with caveolin. Most biochemical data were confirmed by high resolution immunolocalization studies. Using correlation analysis, only a small fraction of the Na+-Ca2+ exchangers colocalized with caveolin whereas a substantial fraction of dihydropyridine and adenosine A1 receptors did colocalize with caveolin. The most pertinent findings are that the Na+-Ca2+ exchanger and the dihydropyridine receptor are in separate sarcolemmal subcompartments. These spatial relationships may be relevant for understanding excitation-contraction coupling.

Differential distribution of NCX1 contributes to spine-dendrite compartmentalization in CA1 pyramidal cells

Compartmentalization of Ca(2+) between dendritic spines and shafts is governed by diffusion barriers and a range of Ca(2+) extrusion mechanisms. The distinct contribution of different Ca(2+) clearance systems to Ca(2+) compartmentalization in dendritic spines versus shafts remains elusive. We applied a combination of ultrastructural and functional imaging methods to assess the subcellular distribution and role of NCX1 in rat CA1 pyramidal cells. Quantitative electron microscopic analysis of preembedding immunogold reactions revealed uniform densities of NCX1 along the shafts of apical and basal dendrites, but densities in dendritic shafts were approximately seven times higher than in dendritic spines. In line with these results, two-photon imaging of synaptically activated Ca(2+) transients during NCX blockade showed preferential action localized to the dendritic shafts for NCXs in regulating spine-dendrite coupling.

Expression of a sorcin missense mutation in the heart modulates excitation‐contraction coupling

Sorcin is a Ca2+ binding protein implicated in the regulation of intracellular Ca2+ cycling and cardiac excitation-contraction coupling. Structural and human genetic studies suggest that a naturally occurring sequence variant encoding L112-sorcin disrupts an E-F hand Ca2+ binding domain and may be responsible for a heritable form of hypertension and hypertrophic heart disease. We generated transgenic mice overexpressing L112-sorcin in the heart and characterized the effects on Ca2+ regulation and cardiac function both in vivo and in dissociated cardiomyocytes. Hearts of sorcin(F112L) transgenic mice were mildly dilated but ventricular function was preserved and systemic blood pressure was normal. Sorcin(F112L) myocytes were smaller than control cells and displayed complex alterations in Ca2+ regulation and contractility, including a slowed inactivation of L-type Ca2+ current, enhanced Ca2+ spark width, duration, and frequency, and increased Na+-Ca2+ exchange activity. In contrast, mice with cardiac-specific overexpression of wild-type sorcin displayed directionally opposite effects on L-type Ca2+ channel function and Ca2+ spark behavior. These data further define the role of sorcin in cardiac excitation-contraction coupling and highlight its negative regulation of SR calcium release. Our results also suggest that additional factors may be responsible for the development of cardiac hypertrophy and hypertension in humans expressing the L112-sorcin sequence variant.

Cytoplasmic tail of phospholemman interacts with the intracellular loop of the cardiac Na+/Ca2+ exchanger

Phospholemman (PLM), a member of the FXYD family of small ion transport regulators, inhibits cardiac Na+/Ca2+ exchanger (NCX1). NCX1 is made up of N-terminal domain consisting of the first five transmembrane segments (residues 1-217), a large intracellular loop (residues 218-764), and a C-terminal domain comprising the last four transmembrane segments (residues 765-938). Using glutathione S-transferase (GST) pull-down assay, we demonstrated that the intracellular loop, but not the N- or C-terminal transmembrane domains of NCX1, was associated with PLM. Further analysis using protein constructs of GST fused to various segments of the intracellular loop of NCX1 suggest that PLM bound to residues 218-371 and 508-764 but not 371-508. Split Na+/Ca2+ exchangers consisting of N- or C-terminal domains with different lengths of the intracellular loop were co-expressed with PLM in HEK293 cells that are devoid of endogenous PLM and NCX1. Although expression of N-terminal but not C-terminal domain alone resulted in correct membrane targeting, co-expression of both N- and C-terminal domains was required for correct membrane targeting and functional exchange activity. NCX1 current measurements indicate that PLM decreased NCX1 current only when the split exchangers contained residues 218-358 of the intracellular loop. Co-immunoprecipitation experiments with PLM and split exchangers suggest that PLM associated with the N-terminal domain of NCX1 when it contained intracellular loop residues 218-358. TM43, a PLM mutant with its cytoplasmic tail truncated, did not co-immunoprecipitate with wild-type NCX1 when co-expressed in HEK293 cells, confirming little to no interaction between the transmembrane domains of PLM and NCX1. We conclude that PLM interacted with the intracellular loop of NCX1, most likely at residues 218-358.

Cells expressing unique Na+/Ca2+exchange (NCX1) splice variants exhibit different susceptibilities to Ca2+overload

The Na+/Ca2+exchanger (NCX) NCX1 exhibits tissue-specific alternative splicing. Such NCX splice variants as NCX1.1 and NCX1.3 are also differentially regulated by Na+and Ca2+, although the physiological implications of these regulatory characteristics are unclear. On the basis of their distinct regulatory profiles, we hypothesized that cells expressing these different splice variants might exhibit unique responses to conditions promoting Ca2+overload, such as during exposure to cardiac glycosides or simulated ischemia. NCX1.1 or NCX1.3 was expressed in human embryonic kidney (HEK)-293 cells or rat neonatal ventricular cardiomyocytes (NVC), and expression was confirmed by Western blotting and immunocytochemical analyses. HEK-293 cells lacked NCX1 protein before transfection. With use of adenoviral vectors, neonatal cardiomyocytes were induced to overexpress the NCX1.1 splice variant by nearly twofold, whereas the NCX1.3 isoform was expressed on the endogenous NCX1.1 background. Total expression was comparable for NCX1.1 and NCX1.3. Exposure of NVC to ouabain induced a significant increase in cellular Ca2+, an effect that was exaggerated in cells overexpressing NCX1.1, but not NCX1.3. The increase in intracellular Ca2+was inhibited by 5 μM KB-R7943. Cardiomyocytes overexpressing NCX1.1 also exhibited a greater accumulation of intracellular Ca2+in response to simulated ischemia than did cells expressing NCX1.3. Similar responses were observed in HEK-293 cells where NCX1.1 was expressed. We conclude that expression of the NCX1.3 splice variant protects against severe Ca2+overload, whereas NCX1.1 promotes Ca2+overload in response to cardiac glycosides and ischemic challenges. These results highlight the importance of ionic regulation in controlling NCX1 activity under conditions that promote Ca2+overload.

Role of plasma membrane Ca2+-ATPase in contraction-relaxation processes of the bladder: evidence from PMCA gene-ablated mice

We investigated the roles and relationships of plasma membrane Ca2+-ATPase (PMCA), sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)2, and Na+/Ca2+ exchanger (NCX) in bladder smooth muscle contractility in Pmca-ablated mice: Pmca4-null mutant ( Pmca4 −/−) and heterozygous Pmca1 and homozygous Pmca4 double gene-targeted ( Pmca1 +/− Pmca4 −/−) mice. Gene manipulation did not alter the amounts of PMCA1, SERCA2, and NCX. To study the role of each Ca2+ transport system, contraction of circular ring preparations was elicited with KCl (80 mM) plus atropine, and then the muscle was relaxed with Ca2+-free physiological salt solution containing EGTA. We measured the contributions of Ca2+ clearance components by inhibiting SERCA2 (with 10 μM cyclopiazonic acid) and/or NCX (by replacing NaCl with N-methyl-d-glucamine/HCl plus 10 μM KB-R7943). Contraction half-time (time to 50% of maximum tension) was prolonged in the gene-targeted muscles but marginally shortened when SERCA2 or NCX was inhibited. The inhibition of NCX significantly inhibited this prolongation, suggesting that NCX activity might be augmented to compensate for PMCA4 function in the gene-targeted muscles under nonstimulated conditions. Inhibition of SERCA2 and NCX as well as gene targeting all prolonged the relaxation half-time. The contribution of PMCA to relaxation was calculated to be ∼25–30%, with that of SERCA2 being 20% and that of NCX being 70%. PMCA and SERCA2 appeared to function additively, but the function of NCX might overlap with those of other components. In summary, gene manipulation of PMCA indicates that PMCA, in addition to SERCA2 and NCX, plays a significant role in both excitation-contraction coupling and the Ca2+ extrusion-relaxation relationship, i.e., Ca2+ homeostasis, of bladder smooth muscle.

Importance of K+-dependent Na+/Ca2+-exchanger 2, NCKX2, in motor learning and memory

Plasma membrane Na+/Ca2+-exchangers play a predominant role in Ca2+ extrusion in brain. Neurons express several different Na+/Ca2+-exchangers belonging to both the K+-independent NCX family and the K+-dependent NCKX family. The unique contributions of each of these proteins to neuronal Ca2+ homeostasis and/or physiology remain largely unexplored. To address this question, we generated mice in which the gene encoding the abundant neuronal K+ -dependent Na+/Ca2+-exchanger protein, NCKX2, was knocked out. Analysis of these animals revealed a significant reduction in Ca2+ flux in cortical neurons, a profound loss of long term potentiation and an increase in long term depression at hippocampal Schaffer/CA1 synapses, and clear deficits in specific tests of motor learning and spatial working memory. Surprisingly, there was no obvious loss of photoreceptor function in cones, where expression of the NCKX2 protein had been reported previously. These data emphasize the critical and non-redundant role of NCKX2 in the local control of neuronal [Ca2+] that is essential for the development of synaptic plasticity associated with learning and memory.

Rapid turnover of the "functional" Na(+)-Ca2+ exchanger in cardiac myocytes revealed by an antisense oligodeoxynucleotide approach

Antisense oligodeoxynucleotides (AS-ODNs) were used in combination with transient functional expression of the cardiac Na(+)-Ca2+ exchanger (NCX1) to correlate suppression of the Na(+)-Ca2+ exchange function with down-regulation of NCX1 protein expression. In a de-novo expression system (Sf9 cells), a decrease in both, NCX1 mRNA and protein after AS-ODN application was paralleled by diminished NCX1 activity, a typical hallmark of a true "antisense effect". Although AS-ODN uptake was also efficient in rat neonatal cardiac myocytes, in whole-cell extracts of these cells treated with AS-ODNs, the amount of NCX1 protein determined in a quantitative binding assay remained almost unchanged, despite a prompt loss of NCX1 function. Immunocytochemical staining of myocytes revealed that most of the immunoreactivity was not localized in the plasma membrane, but in intracellular compartments and was barely affected by AS-ODN treatment. These results indicate that the "functional half-life" of the NCX1 protein in the plasma membrane of neonatal cardiac myocytes is surprisingly short, much shorter than reported half-lifes of about 30 h for other membrane proteins.

Subunit composition and role of Na+,K+-ATPases in adrenal chromaffin cells

Adrenal medullary (AM) cells are exposed to high concentrations of cortical hormones, one of which is a ouabain-like substance. Thus, the effects of ouabain on catecholamine secretion and distribution of Na+,K+-ATPase alpha and beta subunits in rat and guinea-pig AM cells were examined using amperometry and immunological techniques. While exposure to 1 microm ouabain did not have a marked effect on resting secretion, it induced an increase in secretion due to mobilization of Ca2+ ions that were stored during a 4 min interval between muscarine applications. Immunocytochemistry revealed that Na+,K+-ATPase alpha1 subunit-like and beta3 subunit-like immunoreactive (IR) materials were distributed ubiquitously at the cell periphery, whereas alpha2- and beta2-like IR materials were present in restricted parts of the cell periphery. The alpha1 and alpha2 subunits were mainly immunoprecipitated from AM preparations by anti-beta3 and anti-beta2 antisera, respectively. Peripheral BODIPY-FL-InsP3 binding sites were localized below membrane domains with alpha2- and beta2-like IR materials. The results indicate that in AM cells, alpha1beta3 isozymes of Na+,K+-ATPase were present ubiquitously in the plasma membrane, while alpha2beta2 isozymes were in the membrane domain closely associated with peripheral Ca2+ store sites. This close association of the alpha2beta2 isozyme with peripheral Ca2+ store sites may account for the facilitation of mobilization-dependent secretion in the presence of 1 microm ouabain.

Calcium extrusion protein expression in the hippocampal formation of chronic epileptic rats after kainate-induced status epilepticus

PURPOSE

The plasma membrane Ca2+ -adenosine triphosphatase (ATPase) (PMCA) and (potassium-dependent) sodium-calcium exchange [NC(K)X] represent two main calcium-extrusion mechanisms that are important for the restoration of [Ca2+]i levels after electrical activity. We investigated whether the expression of these calcium-extrusion proteins is altered in the course of epileptogenesis.

METHODS

Hippocampal-parahippocampal protein expression of NCX1, 2, and 3, PMCA1-4, and NCKX2 at an early and late stage after kainate-induced status epilepticus (SE) was compared with that in control rats by using immunocytochemistry.

RESULTS

Several alterations were found in chronic epileptic rats: (a) NCX1 expression was permanently decreased in the inner molecular layer (IML) of the dentate gyrus (DG) and entorhinal cortex layer III (ECIII), related to neuronal loss in hilus and ECIII, respectively; (b) PMCA and NCKX2 expression was transiently upregulated in the IML, and decreased in several areas where cell loss had occurred, (c) NCX3 expression, which in control rats is abundant in presynaptic terminals of mossy fibers (MF), was extensively and permanently decreased in stratum lucidum and hilar region. In addition, newly formed MF sprouts that project to the DG iml did not noticeably express NCX3; (d) NCX2 and NCKX2 were (transiently) upregulated in astrocytes of epileptic rats throughout the hippocampal formation, including ECIII.

CONCLUSIONS

These region-specific changes in calcium-extrusion proteins reflect a change in calcium regulation. Whether these regional-specific changes of calcium-extrusion proteins are associated with an abnormal calcium homeostasis must be determined. Because some alterations of calcium-extrusion protein expression are already present at an early stage of epileptogenesis, they could be involved in this process.

Cardiac excitation-contraction coupling in the absence of Na(+) - Ca2+ exchange

We investigate cardiac excitation-contraction coupling in the absence of sarcolemmal Na(+) - Ca(2+) exchange using NCX1 knock out mice. Knock out of NCX1 is embryonic lethal, and we measure Ca(2+) transients and contractions in heart tubes from embryos at day 9.5 post coitum. Immunoblot and electron microscopy both indicate that sarcoplasmic reticular membranes are diminished in the knock out (NCX(-/-)) heart tubes. Both Ni(2+) and nifedipine block excitation-contraction coupling in NCX-containing (NCX+) and NCX(-/-) heart tubes indicating an essential role for the L-type Ca(2+) current. Under basal conditions (1Hz stimulation), the NCX(-/-) heart tubes have normal Ca(2+) transients but are unable to maintain homeostasis when Ca(2+) fluxes are increased by various interventions (increased stimulation frequency, caffeine, isoproterenol). In each case, the NCX(-/-) heart tubes respond to the intervention in a more deleterious manner (increased diastolic Ca(2+), decreased Ca(2+) transient) than the NCX+ heart tubes. Expression of the sarcolemmal Ca(2+) pump was not upregulated. The sarcolemmal Ca(2+) pump, however, was able to compensate surprisingly well for the absence of Na(+) - Ca(2+) exchange under basal conditions.

Differential expression of the Na+-Ca2+ exchanger transcripts and proteins in rat brain regions

In the central nervous system (CNS), the Na(+)-Ca(2+) exchanger plays a fundamental role in controlling the changes in the intracellular concentrations of Na(+) and Ca(2+) ions. These cations are known to regulate neurotransmitter release, cell migration and differentiation, gene expression, and neurodegenerative processes. In the present study, nonradioactive in situ hybridization and light immunohistochemistry were carried out to map the regional and cellular distribution for both transcripts and proteins encoded by the three known Na(+)-Ca(2+) exchanger genes NCX1, NCX2, and NCX3. NCX1 transcripts were particularly expressed in layers III-V of the motor cortex, in the thalamus, in CA3 and the dentate gyrus of the hippocampus, in several hypothalamic nuclei, and in the cerebellum. NCX2 transcripts were strongly expressed in all hippocampal subregions, in the striatum, and in the paraventricular thalamic nucleus. NCX3 mRNAs were mainly detected in the hippocampus, in the thalamus, in the amygdala, and in the cerebellum. Immunohistochemical analysis revealed that NCX1 protein was mainly expressed in the supragranular layers of the cerebral cortex, in the hippocampus, in the hypothalamus, in the substantia nigra and ventral tegmental area, and in the granular layer of the cerebellum. The NCX2 protein was predominantly expressed in the hippocampus, in the striatum, in the thalamus, and in the hypothalamus. The NCX3 protein was particularly found in the CA3 subregion, and in the oriens, radiatum, and lacunoso-moleculare layers of the hippocampus, in the ventral striatum, and in the cerebellar molecular layer. Collectively, these results suggest that the different Na(+)-Ca(2+) exchanger isoforms appear to be selectively expressed in several CNS regions where they might underlie different functional roles.

Inhibition of human SERCA3 by PL/IM430. Molecular analysis of the interaction

The monoclonal antibody PL/IM430 has previously been reported to uncouple Ca(2+) transport from ATP hydrolysis in platelet membranes (Hack, N., Wilkinson, J. M., and Crawford, N. (1988) Biochem. J. 250, 355-361). More recently, we have demonstrated that this antibody is specific for human SERCA3 (Poch, E., Leach, S., Snape, S., Cacic, T., MacLennan, D. H., and Lytton, J. (1998) Am. J. Physiol. 275, C1449-C1458). In this paper, we have extended the analysis of the PL/IM430-SERCA3 interaction. Using HEK293 cells to express human SERCA3a, we were able to measure both ATP-mediated, oxalate-dependent (45)Ca(2+) uptake and Ca(2+)-dependent ATP hydrolysis activities due exclusively to SERCA3. Treatment with PL/IM430 inhibited both activities almost identically, with a maximal inhibition of 81 and 73% and a half-maximal concentration of 8.3 and 5.9 microg/ml, for Ca(2+) uptake and ATP hydrolysis, respectively. We conclude that PL/IM430 does inhibit SERCA3 activity but does not uncouple Ca(2+) transport from ATP hydrolysis. Using a combination of partial proteolysis, GST fusion protein expression, and mutation of residues that differ between rat and human SERCA3, we have identified human SERCA3 amino acids Pro(8) and Glu(192) as essential to forming the PL/IM430 epitope. PL/IM430 thus recognizes a linearly noncontiguous set of amino acids within the actuator domain of human SERCA3. We propose that PL/IM430 inhibits SERCA3 activity by sterically preventing movement of the actuator domain into a catalytically critical position in the E2 conformation of the enzyme.

Phospholemman modulates Na+/Ca2+ exchange in adult rat cardiac myocytes

Previous studies have shown that overexpression of phospholemman (PLM) affected contractile function and Ca(2+) homeostasis in adult rat myocytes. We tested the hypothesis that PLM modulated Na(+)/Ca(2+) exchanger (NCX1) activity. PLM was overexpressed in adult rat myocytes by adenovirus-mediated gene transfer. After 72 h, the half-time of relaxation from caffeine-induced contracture, an estimate of forward NCX1 activity, was prolonged 1.8-fold (P < 0.003) in myocytes overexpressing PLM compared with control myocytes overexpressing green fluorescent protein alone. Reverse NCX1 current (3 Na(+) out:1 Ca(2+) in) was significantly (P < 0.0001) lower in PLM myocytes, especially at more positive voltages. Immunofluorescence demonstrated colocalization of PLM and NCX1 to the plasma membrane and t-tubules. Resting membrane potential, action potential amplitude and duration, myocyte size, and NCX1 and calsequestrin protein levels were not affected by PLM overexpression. At 5 mM extracellular [Ca(2+)] ([Ca(2+)](o)), the depressed contraction amplitudes in PLM myocytes were increased towards normal by cooverexpression with NCX1. At 0.6 mM [Ca(2+)](o), the supranormal contraction amplitudes in PLM myocytes were reduced by cooverexpression with NCX1. We conclude that PLM modulated myocyte contractility partly by inhibiting Na(+)/Ca(2+) exchange.

Sodium/calcium exchanger subtypes NCX1, NCX2 and NCX3 show cell-specific expression in rat hippocampus cultures

Na(+)/Ca(2+) exchange activity is known to be expressed throughout the brain in both glial and neuronal tissue. mRNA of all three major subtypes of the mammalian Na(+)/Ca(2+) exchanger protein (NCX1, NCX2, NCX3) has been detected in most brain areas, albeit at varying densities. [The term 'subtype' is used for exchangers that are products of different genes (NCX1, NCX2, NCX3); 'isoform' is used for splice variants of a single gene product]. However, for lack of subtype specific labels, the cellular expression pattern of this transport protein has remained largely unknown. We have now used three subtype-specific antibodies, two monoclonal and one polyclonal, to identify the cellular distribution of the exchanger subtypes in rat hippocampus cell cultures. Surprisingly, we found little overlap for the expression of this membrane protein in different cell types. NCX1 labeled mainly the membranes of neuronal cells and their associated dendritic network. It was found in nearly all neuronal cells of the population growing in culture. In cultures maintained for more than 3 weeks, NCX1 was increasingly detected in the membrane of glia cells. NCX2 immunoreactivity was predominantly localized in various types of glia cells. It was also detected in the membranes of a few neuronal cell bodies but never in the dendritic network. In addition to labeling membranes, the NCX2 antibody strongly cross-reacted with an unidentified glial fibrillar protein. NCX3 expression appeared very low in hippocampus cultures and was restricted to a small subpopulation of neuronal cells. It was never detected in glia cells. Our results provide novel information on the cell-specific expression of the three Na(+)/Ca(2+) exchanger subtypes (NCX1, NCX2 and NCX3) in mammalian brain. These data may reflect functional differences among the subtypes that are not obvious from studies in recombinant cell lines and hence, may help to understand the functional role of specific glia- or neuron-associated Ca(2+) transport systems.

Toward a topological model of the NCX1 exchanger

The Na(+)/Ca(2+) exchanger (NCX1) catalyzes the counter-transport of sodium and calcium ions. Understanding how this is accomplished requires knowledge of the structure of NCX1 and identifying amino acid residues involved in binding and transport of ions. The amino acid sequence of NCX1 has been known for more than a decade. Based on hydropathy analysis, NCX1 was modeled to contain 12 transmembrane segments. In this model, the alpha-repeat regions, which are the result of a gene duplication event (see below), are oriented on the extracellular face of NCX1. In the years since NCX1 was sequenced, a considerable amount of effort has gone into testing the initial 12-transmembrane-segment model. Immunologic and protein-processing studies as well as functional analyses of mutants have determined the location of the amino and carboxy termini and several intracellular regions. However, disulfide bond analysis and cysteine mutagenesis coupled with accessibility studies indicate that the structure of NCX1 diverges from a simple membrane protein consisting only of transmembrane alpha-helical segments. These recent data support a model containing 9 transmembrane alpha-helices with the alpha-repeat regions forming nonhelical re-entrant loops. A bacterial protein containing a pair of alpha-repeat regions but of unknown function has also been shown to have oppositely oriented alpha-repeats.

Immunohistochemical detection of the sodium-calcium exchanger in rat hippocampus cultures using subtype-specific antibodies

All of the known Na+/Ca2+ exchanger subtypes, NCX1-3, are expressed in the brain, albeit with marked regional differences. On the mRNA level, overall expression seems most prominent for NCX2, intermediate for NCX1, and, except for a few regions, low for NCX3. Using three subtype-specific antibodies, we have now studied the cellular expression of the NCX subtypes in rat hippocampus cultures by immunohistochemical techniques. Our results provide evidence for a highly cell-specific expression pattern of NCX subtypes and show surprisingly little colocalization. NCX1 and NCX3 are both primarily expressed in neuronal cells. While NCX1 is found in the large majority of neurons, NCX3 expression was restricted to a small minority of cells. By contrast, NCX2 was almost exclusively present in glial cells. The NCX2 antibody, a IgM, stained glial cell membranes as well as an intermediate fibrillar system. In spite of extensive screening, the nature of this fiber system has not yet been identified.

The human SLC8A3 gene and the tissue-specific Na+/Ca2+ exchanger 3 isoforms

We have identified the human gene for member 3 of Solute Carrier family 8 (SLC8A3) by bioinformatic analysis of human genomic sequences. The gene is located on chromosome 14q24.2, and spans a region of about 150 kb. The full-length DNA complementary to RNA encoding the Na(+)/Ca(2+) exchanger isoform 3 (NCX3), amplified by reverse transcriptase-polymerase chain reaction (RT-PCR) from the human neuroblastoma SH-SY5Y RNA, includes seven exons and encodes a protein of about 100 kDa. RT-PCR analysis was performed in different tissues to determine the exon composition in the region encoding the large intracellular loop of the protein. The region underwent modifications by alternative tissue-specific splicing. NCX3.2, including exon 4 but not exon 5, was found in human brain and in the neuroblastoma cell line. In human skeletal muscle two additional isoforms were identified: NCX3.3, including exons 4 and 5, and a truncated isoform (NCX3.4) produced by the skipping of both exons 3 and 4. The skipping causes a frame shift downstream of the exon 2 sequence. The new coding sequence of 25 amino acids terminates with a stop codon in exon 6. The NCX3.4 isoform (68 kDa) is truncated in the C-terminal portion of the domain first found in Drosophila Na(+)/Ca(2+) exchanger domain (Calxbeta) and lacks the C-terminal hydrophobic segments.

Increased exchange current but normal Ca2+ transport via Na+-Ca2+ exchange during cardiac hypertrophy after myocardial infarction

Hypertrophied and failing cardiac myocytes generally show alterations in intracellular Ca2+ handling associated with changes in the contractile function and arrhythmogenicity. The cardiac Na+-Ca2+ exchange (NCX) is an important mechanism for Ca2+ extrusion and cell relaxation. Its possible involvement in changes of excitation-contraction coupling (EC-coupling) with disease remains uncertain. We analyzed the NCX function in rat ventricular myocytes 5 to 6 months after experimental myocardial infarction (PMI) produced by left coronary artery ligation and from sham-operated (SO) hearts. Caged Ca2+ was dialyzed into the cytoplasm via a patch-clamp pipette and Ca2+ was released by flash photolysis to activate NCX and measure the associated currents (I(NaCa)), whereas [Ca2+]i changes were simultaneously recorded with a confocal microscope. I(NaCa) density normalized to the [Ca2+]i jumps was 2.6-fold higher in myocytes from PMI rats. The level of total NCX protein expression in PMI myocytes was also increased. Interestingly, although the I(NaCa) density in PMI cells was larger, PMI and SO myocytes presented virtually identical Ca2+ transport via the NCX. This discrepancy was explained by a reduced surface/volume ratio (34.8%) observed in PMI cells. We conclude that the increase in NCX density may be a mechanism to maintain the required Ca2+ extrusion from a larger cell to allow adequate relaxation.

The Na+-Ca2+ exchanger is essential for the action of cardiac glycosides

The widely accepted model to explain the positive inotropic effect of cardiac glycosides invokes altered Na+-Ca2+ exchange activity secondary to Na+ pump inhibition. However, proof of this model is lacking and alternative mechanisms have been proposed. We directly tested the role of the Na+-Ca2+ exchanger in the action of the glycoside ouabain using Na+-Ca2+ exchanger knockout mice. Ablation of the exchanger is embryonic lethal, but contractility can be studied in embryonic heart tubes at day 9.5 postcoitum. Heart tubes isolated from homozygous Na+-Ca2+ exchanger knockout mice (NCX-/-) display surprisingly normal Ca2+ transients. Removal of extracellular Na+ induces Ca2+ overload in wild-type heart tubes but does not alter the Ca2+ transients of NCX-/- heart tubes. Similarly, ouabain, at levels causing Ca2+ overload in wild-type heart tubes, has no effect on NCX-/- heart tubes. We conclude that in embryonic mouse myocytes the Na+-Ca2+ exchanger is absolutely required for the effect of cardiac glycosides on Ca2+(i).

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+.

Helix packing of functionally important regions of the cardiac Na(+)-Ca(2+) exchanger

In a revised topological model of the cardiac Na(+)-Ca(2+) exchanger, there are nine transmembrane segments (TMSs) and two possible re-entrant loops (Nicoll, D. A., Ottolia, M., Lu, Y., Lu, L., and Philipson, K. D. (1999) J. Biol. Chem. 274, 910-917; Iwamoto, T., Nakamura, T. Y., Pan, Y., Uehara, A., Imanaga, I., and Shigekawa, M. (1999) FEBS Lett. 446, 264-268). The TMSs form two clusters separated by a large intracellular loop between TMS5 and TMS6. We have combined cysteine mutagenesis and oxidative cross-linking to study proximity relationships of TMSs in the exchanger. Pairs of cysteines were reintroduced into a cysteine-less exchanger, one in a TMS in the NH(2)-terminal cluster (TMSs 1-5) and the other in a TMS in the COOH-terminal cluster (TMSs 6-9). The mutant exchanger proteins were expressed in HEK293 cells, and disulfide bond formation between introduced cysteines was analyzed by gel mobility shifts. Western blots showed that S117C/V804C, A122C/Y892C, A151C/T815C, and A151C/A821C mutant proteins migrated at 120 kDa under reducing conditions and displayed a partial mobility shift to 160 kDa under nonreducing conditions. This shift indicates the formation of a disulfide bond between these paired cysteine residues. Copper phenanthroline and the cross-linker N', N'-o-phenylenedimaleimide enhanced the mobility shift to 160 kDa. Our data suggest that TMS7 is close to TMS3 near the intracellular side of the membrane and is in the vicinity of TMS2 near the extracellular surface. Also, TMS2 must adjoin TMS8. This initial packing model of the exchanger brings two functionally important domains in the exchanger, the alpha 1 and alpha 2 repeats, close to each other.

Expression of the Na(+)/Ca(2+) exchanger in skeletal muscle

The expression of the Na(+)/Ca(2+) exchanger was studied in differentiating muscle fibers in rats. NCX1 and NCX3 isoform (Na(+)/Ca(2+) exchanger isoform) expression was found to be developmentally regulated. NCX1 mRNA and protein levels peaked shortly after birth. Conversely, NCX3 isoform expression was very low in muscles of newborn rats but increased dramatically during the first 2 wk of postnatal life. Immunocytochemical analysis showed that NCX1 was uniformly distributed along the sarcolemmal membrane of undifferentiated rat muscle fibers but formed clusters in T-tubular membranes and sarcolemma of adult muscle. NCX3 appeared to be more uniformly distributed along the sarcolemma and inside myoplasm. In the adult, NCX1 was predominantly expressed in oxidative (type 1 and 2A) fibers of both slow- and fast-twitch muscles, whereas NCX3 was highly expressed in fast glycolytic (2B) fibers. NCX2 was expressed in rat brain but not in skeletal muscle. Developmental changes in NCX1 and NCX3 as well as the distribution of these isoforms at the cellular level and in different fiber types suggest that they may have different physiological roles.

Quantitative analysis of Na+-Ca2+ exchanger expression in guinea-pig heart

In previous studies, regional variations in the expression of the Na+-Ca2+ exchanger (NCX) have been examined qualitatively in human heart using the C2C12 monoclonal antibody [Wang, J., Schwinger, R.H., Frank, K., Muller-Ehmsen, J., Martin-Vasallo, P., Pressley, T.A., Xiang, A., Erdmann, E. & McDonough, A.A. (1996) J. Clin. Invest. 98, 1650-1658]. Although NCX expression was found to be significantly lower in the atria compared to the septum, no significant differences were found between atrial and ventricular tissue. NCX has been located in the general sarcolemma and t-tubules of ventricular muscle and as t-tubules are sparse in atrial tissue compared to ventricular tissue, it is surprising that NCX expression was found to be similar in both atria and ventricles [Wang et al. (1996)]. To reinvestigate this, we have used SDS/PAGE and a quantitative Western blotting technique to determine the pattern of expression of NCX in guinea-pig heart in tissue samples from left atrium, right atrium, septum, left ventricle and right ventricle. NCX protein expression was 17.5 +/- 3.9 pmol.mg-1 of protein in the left atrium and 29.2 +/- 6.1 pmol.mg-1 of protein in the right atrium, which were both significantly lower (P < 0.05) than NCX expression in the septum, left ventricle and right ventricle (64.7 +/- 15.2, 76.8 +/- 19.5 and 69.4 +/- 14.1 pmol.mg-1 of protein, respectively, n = 7). These differences in NCX expression may reflect variations in the cellular location of NCX protein in these regions. To study this, we used confocal immunofluorescence of single isolated myocytes to examine differences in the proportion of fluorescent staining on the general surface membrane compared with the interior of the cell (which presumably reflects a t-tubular location). We found that the general membrane staining was 79.0 +/- 1.2% in cells from the atria which was significantly higher (P < 0. 001) than that seen in cells from the septum, left ventricle and right ventricle, with 48.1 +/- 1.1%, 48.2 +/- 1.8% and 45.6 +/- 1.3%, respectively (n = 20). These results illustrate a similar pattern of NCX expression in guinea-pig and human, with expression in atrial tissue significantly lower than in ventricular tissue. However, the cellular location of NCX differs regionally; in atrial tissue, the majority of the NCX protein is located in the general sarcolemma whereas in ventricular and septal tissue, approximately 50% of NCX protein is located within the cell (presumably at the level of the t-tubules).

Comparative analysis of the isoform expression pattern of Ca(2+)-regulatory membrane proteins in fast-twitch, slow-twitch, cardiac, neonatal and chronic low-frequency stimulated muscle fibers

Although all muscle cells generate contractile forces by means of organized filament systems, isoform expression patterns of contractile and regulatory proteins in heart are not identical compared to developing, conditioned or mature skeletal muscles. In order to determine biochemical parameters that may reflect functional variations in the Ca(2+)-regulatory membrane systems of different muscle types, we performed a comparative immunoblot analysis of key membrane proteins involved in ion homeostasis. Cardiac isoforms of the alpha(1)-dihydropyridine receptor, Ca(2+)-ATPase and calsequestrin are also present in skeletal muscle and are up-regulated in chronic low-frequency stimulated fast muscle. In contrast, the cardiac RyR2 isoform of the Ca(2+)-release channel was not found in slow muscle but was detectable in neonatal skeletal muscle. Up-regulation of RyR2 in conditioned muscle was probably due to degeneration-regeneration processes. Fiber type-specific differences were also detected in the abundance of auxiliary subunits of the dihydropyridine receptor, the ryanodine receptor and the Ca(2+)-ATPase, as well as triad markers and various Ca(2+)-binding and ion-regulatory proteins. Hence, the variation in innervation of different types of muscle appears to have a profound influence on the levels and pattern of isoform expression of Ca(2+)-regulatory membrane proteins reflecting differences in the regulation of Ca(2+)-homeostasis. However, independent of the muscle cell type, key Ca(2+)-regulatory proteins exist as oligomeric complexes under native conditions.

Expression of the sodium/calcium exchanger in mammalian skeletal muscle cells in primary culture

Previous investigations have demonstrated molecular and functional expression, at early phases of development of skeletal muscle cells in primary culture, of cardiac isoforms of proteins involved in calcium transport and regulation, like the L-type calcium channel. Here the expression of the cardiac isoform of the Na(+)/Ca(2+) exchanger (NCX1) was studied in skeletal muscle cells developing in vitro, by using biochemical, immunological, and electrophysiological techniques. Northern and Western blot experiments revealed the presence of this cardiac exchanger and its increasing expression during the early phases of development. Confocal imaging of myotubes showed an NCX1 distribution that was predominantly sarcolemmal. The whole-cell patch-clamp technique allowed us to record ionic currents, the direction and the amplitude of which depended on extracellular sodium and calcium concentrations. The developmental changes of this functional expression could be correlated with the molecular NCX1 expression changes. Taken together these data demonstrate the presence of the NCX1 isoform of the Na(+)/Ca(2+) exchanger during in vitro myogenesis and reinforce the theory that significant levels of cardiac-type proteins are transiently expressed during the early phases of the skeletal muscle cell development.

Chimeric analysis of Na(+)/Ca(2+) exchangers NCX1 and NCX3 reveals structural domains important for differential sensitivity to external Ni(2+) or Li(+)

Externally applied Ni(2+), which apparently competes with Ca(2+) in all three isoforms of Na(+)/Ca(2+) exchanger, inhibits exchange activity of NCX1 or NCX2 with a 10-fold higher affinity than that of NCX3, whereas stimulation of exchange by external Li(+) is significantly greater in NCX2 and NCX3 than in NCX1 (Iwamoto, T., and Shigekawa, M. (1998) Am. J. Physiol. 275, C423-C430). Here we identified structural domains in the exchanger that confer differential sensitivity to Ni(2+) or Li(+) by measuring intracellular Na(+)-dependent (45)Ca(2+) uptake in CCL39 cells stably expressing NCX1/NCX3 chimeras or mutants. We found that two segments in the exchanger corresponding mostly to the internal alpha-1 and alpha-2 repeats are individually responsible for the alteration of Ni(2+) sensitivity, both together accounting for approximately 80% of the difference between NCX1 and NCX3. In contrast, the segment corresponding to the alpha-2 repeat fully accounts for the differential Li(+) sensitivity between the isoforms. The Ni(2+) sensitivity was mimicked, respectively, by simultaneous substitution of two amino acids in the alpha-1 repeat (N125G/T127I in NCX1 and G159N/I161T in NCX3) and substitution of one amino acid in the alpha-2 repeat (V820A in NCX1 and A809V in NCX3). On the other hand, the Li(+) sensitivity was mimicked by double substitution mutation in the alpha-2 repeat (V820A/Q826V in NCX1 and A809V/V815Q in NCX3). Single substitution mutations at Asn(125) and Val(820) of NCX1 caused significant alterations in the interactions of the exchanger with Ca(2+) and Ni(2+), and Ni(2+) and Li(+), respectively, although the extent of alteration varied depending on the nature of side chains of substituted residues. Since the above four important residues are mostly in the putative loops of the alpha repeats, these regions might form an ion interaction domain in the exchanger.

Ni2+ transport by the human Na+/Ca2+ exchanger expressed in Sf9 cells

The mechanism of Ni2+ block of the Na+/Ca2+ exchanger was examined in Sf 9 cells expressing the human heart Na+/Ca2+ exchanger (NCX1-NACA1). As predicted from the reported actions of Ni2+, its application reduced extracellular Na+-dependent changes in intracellular Ca2+ concentration (measured by fluo 3 fluorescence changes). However, contrary to expectation, the reduced fluorescence was accompanied by measured 63Ni2+ entry. The 63Ni2+ entry was observed in Sf 9 cells expressing the Na+/Ca2+ exchanger but not in control cells. The established sequential transport mechanism of the Na+/Ca2+ exchanger could be compatible with these results if one of the two ion translocation steps is blocked by Ni2+ and the other permits Ni2+ translocation. We conclude that, because Ni2+ entry was inhibited by extracellular Ca2+ and enhanced by extracellular Na+, the Ca2+ translocation step moved Ni2+, whereas the Na+ translocation step was inhibited by Ni2+. A model is presented to discuss these findings.

Reduced sodium pump alpha1, alpha3, and beta1-isoform protein levels and Na+,K+-ATPase activity but unchanged Na+-Ca2+ exchanger protein levels in human heart failure

BACKGROUND

Cardiac glycosides initiate an increase in force of contraction by inhibiting the sarcolemmal sodium pump (Na+, K+-ATPase), thereby decreasing Ca2+ extrusion by the Na+-Ca2+ exchanger, which increases the cellular content of Ca2+. In patients with heart failure the sensitivity toward cardiac glycosides is enhanced.

METHODS AND RESULTS

Because the inotropic effect of cardiac glycosides may be a function of the sodium pump and Na+-Ca2+ exchanger (NCE) expression levels, the present study aimed to investigate protein expression of both transporters (immunoblot with specific antibodies against the sodium pump catalytic alpha1-, alpha2-, alpha3-, and glycoprotein beta1-isoforms and against NCE) in left ventricle from failing (heart transplantations, New York Heart Association class IV, n=21) compared with nonfailing (donor hearts, NF, n=22) human myocardium. The density of 3H-ouabain-binding sites (Bmax) and the Na+,K+-ATPase activity were also measured. In NYHA class IV, protein levels of Na+,K+-ATPase alpha1- (0.62+/-0.06 of control), alpha3- (0.70+/-0.09), and beta1- (0.61+/-0.04) but not alpha2-isoforms were significantly reduced (P<0.01), whereas levels of NCE (0.92+/-0.13 of control) and calsequestrin (0.98+/-0.06) remained unchanged. Both Na+,K+-ATPase activity (NF: 1.9+/-0.29; NYHA class IV: 1.1+/-0.17 micromol ATP/min per milligram of protein) and the 3H-ouabain binding sites (Bmax NF: 15.9+/-1.9 pmol/mg protein; NYHA class IV: 9.7+/-1.5) were reduced in NYHA class IV and correlated significantly to each other (r2=0. 73; P<0.0001), as did beta1-subunit expression. In left ventricular papillary muscle strips from NYHA class IV compared with nonfailing tissue the Na+-channel modulator BDF 9198 exerted an increase in force of contraction with unchanged effectiveness but enhanced potency.

CONCLUSIONS

The enhanced sensitivity of failing human myocardium toward cardiac glycosides may be, at least in part, attributed to a reduced protein expression and activity of the sarcolemmal Na+,K+-ATPase without a change in Na+-Ca2+ exchanger protein expression.

A circularized sodium-calcium exchanger exon 2 transcript

Previous reports of Na/Ca exchanger gene 1 (NCX1) expression have revealed a major RNA transcript of 7 kilobase pairs (kb), minor transcripts of approximately 13 and approximately 4 kb, and a relatively abundant 1.8-kb RNA band. In the present report we demonstrate that the 1.8-kb message, which has a tissue and subcellular distribution matching that of full-length NCX1 but is not polyadenylated, corresponds to a perfectly circularized exon 2 species. The circular transcript contained the normal NCX1 start codon, a new stop codon introduced as a consequence of circularization, and encoded a protein corresponding to the NH2-terminal portion of NCX1, terminating just after amino acid 600 in the cytoplasmic loop. A linear version of the circular transcript was prepared and transfected into HEK-293 cells. A protein, matching the predicted size of approximately 70 kDa, was expressed, and the transfected cells possessed Na/Ca exchange activity. Although in native tissue we could not detect a protein corresponding exactly to that predicted from the circular transcript, a prominent band of slightly shorter size, possibly representing further proteolytic processing of circular transcript protein, was observed in membranes from LLC-MK2 cells and rat kidney.

Time-resolved monitoring of electrogenic Na+-Ca2+ exchange in the isolated cardiac sarcolemma vesicles by using a rapid-response fluorescent probe

As a major Ca exit system in myocytes, the electrogenic Na+-Ca2+ exchange is exposed to rapid changes of regulatory factors (e.g., cytosolic Ca) during the excitation-contraction coupling. The dynamic aspects of the exchanger response to regulatory factors have not been resolved in the past due to technical limitations. Here, we describe stopped-flow protocols for monitoring the electrogenic activity of Na+-Ca2+ exchange in cardiac sarcolemma vesicles by using a rapid-response voltage-sensitive dye Merocyanine-540 (M540). The M540 signal of Nao-dependent Ca efflux is generated by mixing the Ca-loaded vesicles with Na buffer, yielding 160 mM extravesicular Na and 6 microM Cafree. This signal is inhibited by a cyclic peptide blocker (FRCRCFa), by a Ca ionophore (ionomycin), or by an electrogenic uncoupler (valinomycin or FCCP). The M540 signal of Nao-dependent Ca efflux shows a rapid pre-steady-state burst (210 s-1), followed by slow steady-state phase (</=5 s-1). Extravesicular (cytosolic) Ni inhibits both phases with an IC50 of 0.80 +/- 0.24 mM. At an extravesicular pH of 6.0, the Nao-dependent Ca efflux is able to generate the M540 signal, thereby supporting the idea that the stoichiometry of Na+-Ca2+ exchange is not altered at low pH [Khanashvili, D., et al. (1995) Biochemistry 34, 10290-10297]. The M540 signal of Nao-dependent Ca efflux is lost when the extravesicular Cafree concentration drops to 0.2 microM. This effect cannot be explained by a lack of Ca access to extravesicular (cytosolic) transport sites, because the reaction of Nao-dependent Ca efflux utilizes intravesicular Ca as a substrate. These data suggest that in sarcolemma vesicles a regulatory cytosolic Ca site controls the exchanger activity. The properties of this putative regulatory site do not resemble the properties of the "slow" Ca regulatory mode, observed in electrophysiological studies. Under saturating ionic conditions, the Nao-dependent Ca efflux generates the initial rates of 21 mV/ms in the vesicles with a diameter of 3000-5000 A. If a site density of 300-400 exchangers/micrometer2 and a vesicular surface of 0.5 micrometer2 are assumed, each vesicle may contain 150-200 exchanger molecules with a maximal turnover rate of 4000-5000 s-1. This upper limit for turnover (no matter what the site density is) may put considerable restrictions on the exchanger capacity to mediate Ca entry in the cell under physiologically related conditions.

A new topological model of the cardiac sarcolemmal Na+-Ca2+ exchanger

The current topological model of the Na+-Ca2+ exchanger consists of 11 transmembrane segments with extracellular loops a, c, e, g, i, and k and cytoplasmic loops b, d, f, h, and j. Cytoplasmic loop f, which plays a role in regulating the exchanger, is large and separates the first five from the last six transmembrane segments. We have tested this topological model by mutating residues near putative transmembrane segments to cysteine and then examining the effects of intracellular and extracellular applications of sulfhydryl-modifying reagents on exchanger activity. To aid in our topological studies, we also constructed a cysteineless Na+-Ca2+ exchanger. This mutant is fully functional in Na+ gradient-dependent 45Ca2+ uptake measurements and displays wild-type regulatory properties. It is concluded that the 15 endogenous cysteine residues are not essential for either activity or regulation of the exchanger. Our data support the current model by placing loops c and e at the extracellular surface and loops d, j, and l at the intracellular surface. However, the data also support placing Ser-788 of loop h at the extracellular surface and Gly-837 of loop i at the intracellular surface. To account for these data, we propose a revision of the model that places transmembrane segment 6 in cytoplasmic loop f. Additionally, we propose that putative transmembrane segment 9 does not span the membrane, but may form a "P-loop"-like structure.

Membrane topology of the rat brain Na+-Ca2+ exchanger

To provide experimental evidence for the topology of the Na+-Ca2+ exchanger protein NCX1 in the membrane, indirect immunofluorescence studies using site specific anti-peptide antibodies and Flag-epitope insertion into chosen locations of the protein were carried out. Anti-peptide antibodies AbO-6 and AbO-8 were raised against peptide segments present in a large hydrophilic loop of about 500 amino acids, which separates the hydrophobic amino terminal part of the protein from the hydrophobic carboxy terminal. AbO-10 was raised against the C-terminal tail of the protein. All three antibodies bound to the exchanger protein expressed in transfected cells, in rat brain synaptic plasma membrane and in dog sarcolemmal preparations. The antibodies bound only to those NCX1 isoforms that contained the epitope against which they were raised. Detection of the exchanger protein in transfected cells in situ required the addition of permeabilizing agents suggesting an intracellular location of the epitopes to which AbO-6, AbO-8 and AbO-10 bind. The Flag epitope was inserted into ten putative extramembraneous segments along the exchanger protein. For topology studies, only the Flag-mutants that retained Na+-Ca2+ exchange activity in whole HeLa cells, were used. Immunofluorescence studies indicated, that the N-terminal of the protein is extracellular, the first hydrophilic loop separating transmembrane helices 1 and 2 as well as the C-terminal, are intracellular.

Topology of a functionally important region of the cardiac Na+/Ca2+ exchanger

The cardiac Na+/Ca2+ exchanger, NCX1, has been modeled to consist of 11 transmembrane segments and a large cytoplasmic loop (loop f). Cysteine mutagenesis and sulfhydryl modification experiments demonstrate that the loop connecting transmembrane segments 1 and 2 (loop b) is located on the cytoplasmic side of the membrane, as previously modeled. A mutation in loop b, asparagine 101 to cysteine (N101C), renders the exchanger insensitive to regulation by cytoplasmic Na+ and Ca2+. Nearby mutations at residue threonine 103 (T103C or T103V) increase the apparent affinity of the exchanger for cytoplasmic Na+ and also produce a significant Li+ transport capacity. The evidence suggests that the region at the interface of cytoplasmic loop b and transmembrane segment 2 is important in Na+ transport and also in secondary regulation. Thus, this region may form part of the link between the ion translocation pathway formed by the transmembrane segments and regulatory sites that have previously been localized to loop f.

Sodium-calcium exchanger in cultured human retinal pigment epithelium

Regulation of intracellular free Ca2+ concentration ([Ca2+]i) by an Na+/Ca2+ exchanger was studied in cultures of human retinal pigment epithelial cells using Ca(2+)-indicator dyes (fura-2 and fluo-3) and digital fluorescence imaging. Mean resting [Ca2+]i of cultured RPE in a control Ringer solution was 189 +/- 16 nM. Replacing extracellular Na+ with N-methyl-D-glucamine elicited a two-fold rise in [Ca2+]i; the magnitude of the [Na+]o-free-induced rise in [Ca2+]i varied as a function of extracellular [Ca2+]. The [Na+]o-free response was not significantly affected by the Ca2+ channel blocker nifedipine, or by pretreatment with thapsigargin which depletes intracellular Ca2+ stores. By contrast, the [Na+]o-free-induced rise in [Ca2+]i was significantly reduced by CBDMB, an amiloride derivative that is highly selective for Na+/Ca2+ exchange inhibition. These findings indicate that removal of extracellular Na+ promotes net [Ca2+]i gain via Na+/Ca2+ exchange. Western and Northern blot analyses, respectively, confirmed the presence of Na+/Ca2+ exchanger protein and mRNA in cultures of human RPE. Specifically, Western blot analysis of whole cell lysates of cultured RPE using a polyclonal antibody made against the canine cardiac exchanger identified a major band at approximately 126 kD. Northern blot analysis of total human RPE RNA using a restriction fragment cRNA probe coding for the canine cardiac Na+/Ca2+ exchanger showed that the major exchanger-related transcript was approximately 6.8 kb. In sum, our findings demonstrate the presence of a cardiac-exchanger-related transcript was approximately 6.8 kb. In sum, our findings demonstrate the presence of a cardiac-type Na+/Ca2+ exchanger in cultures of human RPE.

Immunolocalization of the Na(+)-Ca2+ exchanger in mammalian myelinated axons

Previous studies on the pathophysiology of white matter anoxic injury have revealed that the Na(+)-Ca2+ exchanger is an important mediator of Ca2+ overload. To date, however, the localization of this key Ca2+ transporter in myelinated axons has not been demonstrated. The present study uses immunofluorescence labeling with a monoclonal antibody (R3F1) to the canine cardiac type I Na(+)-Ca2+ exchanger to localize exchanger protein to rat peripheral and central myelinated axons. The indirect immunofluorescence labeling technique was used to study paraformaldehyde fixed frozen cryostat sections of sciatic nerve, optic nerve and spinal cord. Examination of sciatic nerve sections with both conventional and confocal microscopy revealed a staining pattern which suggested both a glial and axonal localization of the exchanger. In the rat optic nerve, positive label was associated with cell bodies and their processes, likely glia, and with numerous finer processes arranged in parallel, running longitudinally. These finer processes likely represent axonal profiles. A similar staining pattern was observed in lateral and dorsal columns from spinal cord. Immunoelectron microscopy of dorsal root axons revealed gold particles associated with the paranodal and internodal myelin, in the axoplasm, and close to the nodal/paranodal axon membrane. The high density of Na(+)-Ca2+ exchanger demonstrated in central and peripheral myelinated mammalian axons supports the importance of this transporter in Ca2+ regulation in these tissues.