Purification of the cardiac Na+-Ca2+ exchange protein

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.

Ca2+ entry through reverse Na+/Ca2+ exchanger in NCI-H716, glucagon-like peptide-1 secreting cells

Glucagon like peptide-1 (GLP-1) released from enteroendocine L-cells in the intestine has incretin effects due to its ability to amplify glucose-dependent insulin secretion. Promotion of an endogenous release of GLP-1 is one of therapeutic targets for type 2 diabetes mellitus. Although the secretion of GLP-1 in response to nutrient or neural stimuli can be triggered by cytosolic Ca²⁺ elevation, the stimulus-secretion pathway is not completely understood yet. Therefore, the aim of this study was to investigate the role of reverse Na⁺/Ca²⁺ exchanger (rNCX) in Ca²⁺ entry induced by muscarinic stimulation in NCI-H716 cells, a human enteroendocrine GLP-1 secreting cell line. Intracellular Ca²⁺ was repetitively oscillated by the perfusion of carbamylcholine (CCh), a muscarinic agonist. The oscillation of cytosolic Ca²⁺ was ceased by substituting extracellular Na⁺ with Li⁺ or NMG⁺. KB-R7943, a specific rNCX blocker, completely diminished CCh-induced cytosolic Ca²⁺ oscillation. Type 1 Na⁺/Ca²⁺ exchanger (NCX₁) proteins were expressed in NCI-H716 cells. These results suggest that rNCX might play a crucial role in Ca²⁺ entry induced by cholinergic stimulation in NCI-H716 cells, a GLP-1 secreting cell line.

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.

Basolateral Na+/Ca2+ exchanger 1 and Na+/K+-ATPase, which display light-enhanced gene and protein expression levels, could be involved in the absorption of exogenous Ca2+ through the ctenidium of the giant clam, Tridacna squamosa

Giant clams perform light-enhanced shell formation (calcification) and therefore need to increase the uptake of exogenous Ca²⁺ during illumination. The ctenidium of the fluted giant clam, Tridacna squamosa, is involved in light-enhanced Ca²⁺ uptake. It expresses the pore-forming voltage-gated calcium channel (VGCC) subunit alpha 1 (CACNA1) in the apical membrane of the epithelial cells, and the protein expression level of CACNA1 is upregulated in the ctenidium during illumination. This study aimed to elucidate the mechanism involved in the transport of the absorbed Ca²⁺ across the basolateral membrane of the ctenidial epithelial cells into the hemolymph. We obtained a homolog of Na⁺/Ca²⁺exchanger 1 (NCX1-like) from the ctenidium of T. squamosa, which comprised 2418 bp, encoding a protein of 806 amino acids (88.9 kDa). NCX1-like had a basolateral localization in the epithelial cells of the ctenidial filaments and tertiary water channels. Illumination resulted in significant increases in the transcript and protein levels of NCX1-like/NCX1-like in the ctenidium. Hence, NCX1-like could operate in conjunction with VGCC to increase the transport of Ca²⁺ from the ambient seawater into the hemolymph during illumination. Illumination also resulted in the upregulation of the gene and protein expression levels of Na⁺/K⁺-ATPase (NKA) α-subunit (NKAα/NKAα) in the ctenidium of T. squamosa. As light-enhanced extrusion of Ca²⁺ into the hemolymph through NCX1-like would lead to a greater influx of extracellular Na⁺, the increased expression of the basolateral NKA was required to augment the capacity of intracellular Na⁺ homeostasis.

Dynamic Palmitoylation of the Sodium-Calcium Exchanger Modulates Its Structure, Affinity for Lipid-Ordered Domains, and Inhibition by XIP

The transmembrane sodium-calcium (Na-Ca) exchanger 1 (NCX1) regulates cytoplasmic Ca levels by facilitating electrogenic exchange of Ca for Na. Palmitoylation, the only reversible post-translational modification known to modulate NCX1 activity, controls NCX1 inactivation. Here, we show that palmitoylation of NCX1 modifies the structural arrangement of the NCX1 dimer and controls its affinity for lipid-ordered membrane domains. NCX1 palmitoylation occurs dynamically at the cell surface under the control of the enzymes zDHHC5 and APT1. We identify the position of the endogenous exchange inhibitory peptide (XIP) binding site within the NCX1 regulatory intracellular loop and demonstrate that palmitoylation controls the ability of XIP to bind this site. We also show that changes in NCX1 palmitoylation change cytosolic Ca. Our results thus demonstrate the broad molecular consequences of NCX1 palmitoylation and highlight a means to manipulate the inactivation of this ubiquitous ion transporter that could ameliorate pathologies linked to Ca overload via NCX1.

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NCX1 is dynamically palmitoylated at the cell surface by zDHHC5 and APT1

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Palmitoylation modifies the NCX1 dimer’s structure and affinity for lipid rafts

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We identify the binding site of the endogenous XIP domain in NCX1’s regulatory loop

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Palmitoylation modifies NCX1 XIP affinity and hence regulates intracellular Ca

Role of Sodium/Calcium Exchangers in Tumors

The sodium/calcium exchanger (NCX) is a unique calcium transport system, generally transporting calcium ions out of the cell in exchange for sodium ions. Nevertheless, under special conditions this transporter can also work in a reverse mode, in which direction of the ion transport is inverted—calcium ions are transported inside the cell and sodium ions are transported out of the cell. To date, three isoforms of the NCX have been identified and characterized in humans. Majority of information about the NCX function comes from isoform 1 (NCX1). Although knowledge about NCX function has evolved rapidly in recent years, little is known about these transport systems in cancer cells. This review aims to summarize current knowledge about NCX functions in individual types of cancer cells.

Na/Ca exchange in the atrium: Role in sinoatrial node pacemaking and excitation-contraction coupling

Na/Ca exchange is the dominant calcium (Ca) efflux mechanism in cardiac myocytes. Although our knowledge of exchanger function (NCX1 in the heart) was originally established using biochemical and electrophysiological tools such as cardiac sarcolemmal vesicles and the giant patch technique [1-4], many advances in our understanding of the physiological/pathophysiological roles of NCX1 in the heart have been obtained using a suite of genetically modified mice. Early mouse studies focused on modification of expression levels of NCX1 in the ventricles, with transgenic overexpressors, global NCX1 knockout (KO) mice (which were embryonic lethal if homozygous), and finally ventricular-specific NCX1 KO [5-12]. We found, to our surprise, that ventricular cardiomyocytes lacking NCX1 can survive and function by engaging a clever set of adaptations to minimize Ca entry, while maintaining contractile function through an increase in excitation-contraction (EC) coupling gain [5,6,13]. Having studied ventricular NCX1 ablation in detail, we more recently focused on elucidating the role of NCX1 in the atria through altering NCX1 expression. Using a novel atrial-specific NCX1 KO mouse, we found unexpected changes in atrial cell morphology and calcium handling, together with dramatic alterations in the function of sinoatrial node (SAN) pacemaker activity. In this review, we will discuss these findings and their implications for cardiac disease.

Central and peripheral slow-pressor mechanisms contributing to Angiotensin II-salt hypertension in rats

Aims

High salt intake markedly enhances hypertension induced by angiotensin II (Ang II). We explored central and peripheral slow-pressor mechanisms which may be activated by Ang II and salt.

Methods and results

In protocol I, Wistar rats were infused subcutaneously with low-dose Ang II (150 ng/kg/min) and fed regular (0.4%) or high salt (2%) diet for 14 days. In protocol II, Ang II-high salt was combined with intracerebroventricular infusion of mineralocorticoid receptor (MR) blockers (eplerenone, spironolactone), epithelial sodium channel (ENaC) blocker (benzamil), angiotensin II type 1 receptor (AT1R) blocker (losartan) or vehicles. Ang II alone raised mean arterial pressure (MAP) ∼10 mmHg, but Ang II-high salt increased MAP ∼50 mmHg. Ang II-high salt elevated plasma corticosterone, aldosterone and endogenous ouabain but not Ang II alone. Both Ang II alone and Ang II-high salt increased mRNA and protein expression of CYP11B2 (aldosterone synthase gene) in the adrenal cortex but not of CYP11B1 (11-β-hydroxylase gene). In the aorta, Ang II-high salt increased sodium-calcium exchanger-1 (NCX1) protein. The Ang II-high salt induced increase in MAP was largely prevented by central infusion of MR blockers, benzamil or losartan. Central blockades significantly lowered plasma aldosterone and endogenous ouabain and markedly decreased Ang II-high salt induced CYP11B2 mRNA expression in the adrenal cortex and NCX1 protein in the aorta.

Conclusion

These results suggest that in Ang II-high salt hypertension, MR-ENaC-AT1R signalling in the brain increases circulating aldosterone and endogenous ouabain, and arterial NCX1. These factors can amplify blood pressure responses to centrally-induced sympatho-excitation and thereby contribute to severe hypertension.

A vesicular Na+/Ca2+ exchanger in coral calcifying cells

The calcium carbonate skeletons of corals provide the underlying structure of coral reefs; however, the cellular mechanisms responsible for coral calcification remain poorly understood. In osteoblasts from vertebrate animals, a Na+/Ca2+ exchanger (NCX) present in the plasma membrane transports Ca2+ to the site of bone formation. The aims of this study were to establish whether NCX exists in corals and its localization within coral cells, which are essential first steps to investigate its potential involvement in calcification. Data mining identified genes encoding for NCX proteins in multiple coral species, a subset of which were more closely related to NCXs from vertebrates (NCXA). We cloned NCXA from Acropora yongei (AyNCXA), which, unexpectedly, contained a peptide signal that targets proteins to vesicles from the secretory pathway. AyNCXA subcellular localization was confirmed by heterologous expression of fluorescently tagged AyNCXA protein in sea urchin embryos, which localized together with known markers of intracellular vesicles. Finally, immunolabeling of coral tissues with specific antibodies revealed AyNCXA was present throughout coral tissue. AyNCXA was especially abundant in calcifying cells, where it exhibited a subcellular localization pattern consistent with intracellular vesicles. Altogether, our results demonstrate AyNCXA is present in vesicles in coral calcifying cells, where potential functions include intracellular Ca2+ homeostasis and Ca2+ transport to the growing skeleton as part of an intracellular calcification mechanism.

Molecular determinants of pH regulation in the cardiac Na+-Ca2+ exchanger

The cardiac Na⁺-Ca²⁺ exchanger (NCX) plays a critical role in the heart by extruding Ca²⁺ after each contraction and thus regulates cardiac contractility. The activity of NCX is strongly inhibited by cytosolic protons, which suggests that intracellular acidification will have important effects on heart contractility. However, the mechanisms underlying this inhibition remain elusive. It has been suggested that pH regulation originates from the competitive binding of protons to two Ca²⁺-binding domains within the large cytoplasmic loop of NCX and requires inactivation by intracellular Na⁺ to fully develop. By combining mutagenesis and electrophysiology, we demonstrate that NCX pH modulation is an allosteric mechanism distinct from Na⁺ and Ca²⁺ regulation, and we show that cytoplasmic Na⁺ can affect the sensitivity of NCX to protons. We further identify two histidines (His 124 and His 165) that are important for NCX proton sensitivity and show that His 165 plays the dominant role. Our results reveal a complex interplay between the different allosteric mechanisms that regulate the activity of NCX. Because of the central role of NCX in cardiac function, these findings are important for our understanding of heart pathophysiology.

Species-Dependent Mechanisms of Cardiac Arrhythmia: A Cellular Focus

Although ventricular arrhythmia remains a leading cause of morbidity and mortality, available antiarrhythmic drugs have limited efficacy. Disappointing progress in the development of novel, clinically relevant antiarrhythmic agents may partly be attributed to discrepancies between humans and animal models used in preclinical testing. However, such differences are at present difficult to predict, requiring improved understanding of arrhythmia mechanisms across species. To this end, we presently review interspecies similarities and differences in fundamental cardiomyocyte electrophysiology and current understanding of the mechanisms underlying the generation of afterdepolarizations and reentry. We specifically highlight patent shortcomings in small rodents to reproduce cellular and tissue-level arrhythmia substrate believed to be critical in human ventricle. Despite greater ease of translation from larger animal models, discrepancies remain and interpretation can be complicated by incomplete knowledge of human ventricular physiology due to low availability of explanted tissue. We therefore point to the benefits of mathematical modeling as a translational bridge to understanding and treating human arrhythmia.

The role of Na+/Ca2+ exchanger subtypes in neuronal ischemic injury

The Na(+)/Ca(2+) exchanger (NCX) plays an important role in the maintenance of Na(+) and Ca(2+) homeostasis in most cells including neurons under physiological and pathological conditions. It exists in three subtypes (NCX1-3) with different tissue distributions but all of them are present in the brain. NCX transports Na(+) and Ca(2+) in either Ca(2+)-efflux (forward) or Ca(2+)-influx (reverse) mode, depending on membrane potential and transmembrane ion gradients. During neuronal ischemia, Na(+) and Ca(2+) ionic disturbances favor NCX to work in reverse mode, giving rise to increased intracellular Ca(2+) levels, while it may regain its forward mode activity on reperfusion. The exact significance of NCX in neuronal ischemic and reperfusion states remains unclear. The differential role of NCX subtypes in ischemic neuronal injury has been extensively investigated using various pharmacological tools as well as genetic models. This review discusses the mode of action of NCX in ischemic and reperfusion states, the differential roles played by NCX subtypes in these states as well as the role of NCX in pre- and postconditioning. NCX subtypes carry variable roles in ischemic injury. Furthermore, the mode of action of each subtype varies in ischemia and reperfusion states. Thus, therapeutic targeting of NCX in stroke should be based on appropriate timing of the administration of NCX subtype-specific strategies.

Calcium handling proteins: structure, function, and modulation by exercise

Heart failure is a serious public health issue with a growing prevalence, and it is related with the aging of the population. Hypertension is identified as the main precursor of left ventricular hypertrophy and therefore can lead to diastolic dysfunction and heart failure. Scientific studies have confirmed the beneficial effects of the physical exercise by reducing the blood pressure and improving the functional status of the heart in hypertension. Several proteins are involved in the mobilization of calcium during the coupling excitation-contraction process in the heart among those are sarcoplasmic reticulum Ca(2+)-ATPase, phospholamban, calsequestrin, sodium-calcium exchanger, L-type calcium's channel, and ryanodine receptors. Our goal is to address the beneficial effects of exercise on the calcium handling proteins in a heart with hypertension.

Presynaptic control of glycine transporter 2 (GlyT2) by physical and functional association with plasma membrane Ca2+-ATPase (PMCA) and Na+-Ca2+ exchanger (NCX)

Fast inhibitory glycinergic transmission occurs in spinal cord, brainstem, and retina to modulate the processing of motor and sensory information. After synaptic vesicle fusion, glycine is recovered back to the presynaptic terminal by the neuronal glycine transporter 2 (GlyT2) to maintain quantal glycine content in synaptic vesicles. The loss of presynaptic GlyT2 drastically impairs the refilling of glycinergic synaptic vesicles and severely disrupts neurotransmission. Indeed, mutations in the gene encoding GlyT2 are the main presynaptic cause of hyperekplexia in humans. Here, we show a novel endogenous regulatory mechanism that can modulate GlyT2 activity based on a compartmentalized interaction between GlyT2, neuronal plasma membrane Ca(2+)-ATPase (PMCA) isoforms 2 and 3, and Na(+)/Ca(2+)-exchanger 1 (NCX1). This GlyT2·PMCA2,3·NCX1 complex is found in lipid raft subdomains where GlyT2 has been previously found to be fully active. We show that endogenous PMCA and NCX activities are necessary for GlyT2 activity and that this modulation depends on lipid raft integrity. Besides, we propose a model in which GlyT2·PMCA2-3·NCX complex would help Na(+)/K(+)-ATPase in controlling local Na(+) increases derived from GlyT2 activity after neurotransmitter release.

Phylogeny and a structural model of plant MHX transporters

BACKGROUND

The Arabidopsis thaliana MHX gene (AtMHX) encodes a Mg²⁺/H⁺ exchanger. Among non-plant proteins, AtMHX showed the highest similarity to mammalian Na⁺/Ca²⁺ exchanger (NCX) transporters, which are part of the Ca²⁺/cation (CaCA) exchanger superfamily.

RESULTS

Sequences showing similarity to AtMHX were searched in the databases or sequenced from cDNA clones. Phylogenetic analysis showed that the MHX family is limited to plants, and constitutes a sixth family within the CaCA superfamily. Some plants include, besides a full MHX gene, partial MHX-related sequences. More than one full MHX gene was currently identified only in Oryza sativa and Mimulus guttatus, but an EST for more than one MHX was identified only in M. guttatus. MHX genes are not present in the currently available chlorophyte genomes. The prevalence of upstream ORFs in MHX genes is much higher than in most plant genes, and can limit their expression. A structural model of the MHXs, based on the resolved structure of NCX1, implies that the MHXs include nine transmembrane segments. The MHXs and NCXs share 32 conserved residues, including a GXG motif implicated in the formation of a tight-turn in a reentrant-loop. Three residues differ between all MHX and NCX proteins. Altered mobility under reducing and non-reducing conditions suggests the presence of an intramolecular disulfide-bond in AtMHX.

CONCLUSIONS

The absence of MHX genes in non-plant genomes and in the currently available chlorophyte genomes, and the presence of an NCX in Chlamydomonas, are consistent with the suggestion that the MHXs evolved from the NCXs after the split of the chlorophyte and streptophyte lineages of the plant kingdom. The MHXs underwent functional diploidization in most plant species. De novo duplication of MHX occurred in O. sativa before the split between the Indica and Japonica subspecies, and was apparently followed by translocation of one MHX paralog from chromosome 2 to chromosome 11 in Japonica. The structural analysis presented and the identification of elements that differ between the MHXs and the NCXs, or between the MHXs of specific plant groups, can contribute to clarification of the structural basis of the function and ion selectivity of MHX transporters.

20 years from NCX purification and cloning: milestones

The Na(+)/Ca(2+) exchanger protein was first isolated from cardiac sarcolemma in 1988 and cloned in 1990. This allowed study of Na(+)/Ca(2+) exchange at the molecular level to begin. I will review the story leading to the cloning of NCX and the research that resulted from this event. This will include structure-function studies such as determination of the numbers of transmembrane segments and topological arrangement. Information on ion transport sites has been gathered from site-directed mutagenesis. The regions involved in Ca(2+) regulation have been identified, analyzed, and crystallized.We have also generated genetically altered mice to study the role of NCX in the myocardium. Of special interest are mice with atrial- or ventricular-specific KO of NCX that reveal new information on the role of NCX in excitation-contraction coupling and in cardiac pacemaker activity.

RNA therapeutics targeting osteoclast-mediated excessive bone resorption

RNA interference (RNAi) is a sequence-specific post-transcriptional gene silencing technique developed with dramatically increasing utility for both scientific and therapeutic purposes. Short interfering RNA (siRNA) is currently exploited to regulate protein expression relevant to many therapeutic applications, and commonly used as a tool for elucidating disease-associated genes. Osteoporosis and their associated osteoporotic fragility fractures in both men and women are rapidly becoming a global healthcare crisis as average life expectancy increases worldwide. New therapeutics are needed for this increasing patient population. This review describes the diversity of molecular targets suitable for RNAi-based gene knock down in osteoclasts to control osteoclast-mediated excessive bone resorption. We identify strategies for developing targeted siRNA delivery and efficient gene silencing, and describe opportunities and challenges of introducing siRNA as a therapeutic approach to hard and connective tissue disorders.

Full-length cardiac Na+/Ca2+ exchanger 1 protein is not phosphorylated by protein kinase A

The cardiac Na(+)/Ca(2+) exchanger 1 (NCX1) is an important regulator of intracellular Ca(2+) homeostasis and cardiac function. Several studies have indicated that NCX1 is phosphorylated by the cAMP-dependent protein kinase A (PKA) in vitro, which increases its activity. However, this finding is controversial and no phosphorylation site has so far been identified. Using bioinformatic analysis and peptide arrays, we screened NCX1 for putative PKA phosphorylation sites. Although several NCX1 synthetic peptides were phosphorylated by PKA in vitro, only one PKA site (threonine 731) was identified after mutational analysis. To further examine whether NCX1 protein could be PKA phosphorylated, wild-type and alanine-substituted NCX1-green fluorescent protein (GFP)-fusion proteins expressed in human embryonic kidney (HEK)293 cells were generated. No phosphorylation of full-length or calpain- or caspase-3 digested NCX1-GFP was observed with purified PKA-C and [γ-(32)P]ATP. Immunoblotting experiments with anti-PKA substrate and phosphothreonine-specific antibodies were further performed to investigate phosphorylation of endogenous NCX1. Phospho-NCX1 levels were also not increased after forskolin or isoproterenol treatment in vivo, in isolated neonatal cardiomyocytes, or in total heart homogenate. These data indicate that the novel in vitro PKA phosphorylation site is inaccessible in full-length as well as in calpain- or caspase-3 digested NCX1 protein, suggesting that NCX1 is not a direct target for PKA phosphorylation.

m-Calpain-mediated cleavage of Na+/Ca2+ exchanger-1 in caveolae vesicles isolated from pulmonary artery smooth muscle

Using m-calpain antibody, we have identified two major bands corresponding to the 80 kDa large and the 28 kDa small subunit of m-calpain in caveolae vesicles isolated from bovine pulmonary artery smooth muscle plasma membrane. In addition, 78, 35, and 18 kDa immunoreactive bands of m-calpain have also been detected. Casein zymogram studies also revealed the presence of m-calpain in the caveolae vesicles. We have also identified Na(+)/Ca(2+) exchanger-1 (NCX1) in the caveolae vesicles. Purification and N-terminal sequence analyses of these two proteins confirmed their identities as m-calpain and NCX1, respectively. We further sought to determine the role of m-calpain on calcium-dependent proteolytic cleavage of NCX1 in the caveolae vesicles. Treatment of the caveolae vesicles with the calcium ionophore, A23187 (1 microM) in presence of CaCl(2) (1 mM) appears to cleave NCX1 (120 kDa) to an 82 kDa fragment as revealed by immunoblot study using NCX1 monoclonal antibody; while pretreatment with the calpain inhibitors, calpeptin or MDL28170; or the Ca(2+) chelator, BAPTA-AM did not cause a discernible change in the NCX protein profile. In vitro cleavage of the purified NCX1 by the purified m-calpain supports this finding. The cleavage of NCX1 by m-calpain in the caveolae vesicles may be interpreted as an important mechanism of Ca(2+) overload, which could arise due to inhibition of Ca(2+) efflux by the forward-mode NCX and that could lead to sustained Ca(2+) overload in the smooth muscle leading to pulmonary hypertension.

Calcium-dependent cleavage of the Na(+)/Ca(2+) exchanger by m-calpain in isolated endoplasmic reticulum

We have recently demonstrated the localization of associated m-calpain and calpastatin in the endoplasmic reticulum (ER) of bovine pulmonary artery smooth muscle. Herein, we sought to determine the role of m-calpain on calcium-dependent proteolytic cleavage of Na(+)/Ca(2+) exchanger (NCX) in the ER. Treatment of the ER with Ca(2+) (5 mM) dissociates m-calpain-calpastatin association leading to the activation of m-calpain, which subsequently cleaves the ER integral transmembrane protein NCX1 (116 kDa) to an 82 kDa fragment. Pre-treatment of the ER with calpain inhibitors, calpeptin (10 microM) or MDL28170 (10 microM), or Ca(2+) chelator, EGTA (10 mM) does not cleave NCX1. In vitro cleavage of the ER purified NCX1 by the ER purified m-calpain also supports our finding. Cleavage of NCX1 by m-calpain in the ER may be interpreted as the main cause of intracellular Ca(2+) overload in the smooth muscle, which could be important for the manifestation of pulmonary hypertension.

Involvement of Na+/Ca2+ exchanger in migration and contraction of rat cultured tendon fibroblasts

In response to injury and inflammation of tendons, tendon fibroblasts are activated, migrate to the wound, and eventually induce contraction of the extracellular matrices to repair the tissue. Under such conditions, Ca(2+) signalling is involved in motility and contractility of tendon fibroblasts. Using cultured tendon fibroblasts isolated from rat Achilles tendons, we investigated functional expression of Na(+)/Ca(2+) exchangers (NCX). The fluorometric study showed that the intracellular Ca(2+) concentration ([Ca(2+)](i)) was increased by reducing extracellular Na(+) concentration ([Na(+)](o)) in tendon fibroblasts. Selective NCX inhibitors, KB-R7943 and SEA0400, both attenuated [Na(+)](o)-dependent [Ca(2+)](i) elevation and the resting [Ca(2+)](i) in tendon fibroblasts. RT-PCR, Western blots and sequence analyses revealed that NCX1.3 and NCX1.7 were expressed in cultured tendon fibroblasts. NCX2 mRNA was undetected. NCX3 expression was negligibly low. Immunofluorescence microscopy indicated that NCX1 protein localized in the plasma membrane especially at the microspikes of tendon fibroblasts. In the wound-healing scratch assay, the cells migrated toward the space created by a scratch and almost completely filled the space within 48 h. This phenomenon was significantly suppressed by KB-R7943 and SEA0400. Furthermore, the NCX inhibitors abrogated the tendon fibroblast-mediated collagen-matrix contractions. Two types of siRNAs for NCX1 also suppressed the migration and contraction of tendon fibroblasts. We conclude that NCX is expressed and mediates Ca(2+) influx in cultured tendon fibroblasts. Since the pharmacological inhibitors and siRNA for NCX1 suppressed motility and contractility of tendon fibroblasts, NCX may play an important role in the function of tendon fibroblasts in the wound healing.

Ca extrusion by NCX is compromised in olfactory sensory neurons of OMP mice

BACKGROUND

The role of olfactory marker protein (OMP), a hallmark of mature olfactory sensory neurons (OSNs), has been poorly understood since its discovery. The electrophysiological and behavioral phenotypes of OMP knockout mice indicated that OMP influences olfactory signal transduction. However, the mechanism by which this occurs remained unknown.

PRINCIPAL FINDINGS

We used intact olfactory epithelium obtained from WT and OMP(-/-) mice to monitor the Ca(2+) dynamics induced by the activation of cyclic nucleotide-gated channels, voltage-operated Ca(2+) channels, or Ca(2+) stores in single dendritic knobs of OSNs. Our data suggested that OMP could act to modulate the Ca(2+)-homeostasis in these neurons by influencing the activity of the plasma membrane Na(+)/Ca(2+)-exchanger (NCX). Immunohistochemistry verifies colocalization of NCX1 and OMP in the cilia and knobs of OSNs. To test the role of NCX activity, we compared the kinetics of Ca(2+) elevation by stimulating the reverse mode of NCX in both WT and OMP(-/-) mice. The resulting Ca(2+) responses indicate that OMP facilitates NCX activity and allows rapid Ca(2+) extrusion from OSN knobs. To address the mechanism by which OMP influences NCX activity in OSNs we studied protein-peptide interactions in real-time using surface plasmon resonance technology. We demonstrate the direct interaction of the XIP regulatory-peptide of NCX with calmodulin (CaM).

CONCLUSIONS

Since CaM also binds to the Bex protein, an interacting protein partner of OMP, these observations strongly suggest that OMP can influence CaM efficacy and thus alters NCX activity by a series of protein-protein interactions.

Sodium-calcium exchangers in the nucleus: an unexpected locus and an unusual regulatory mechanism

Whereas sodium-calcium exchangers (NCXs) have long been recognized as plasma membrane constituents that serve to maintain homeostatic concentrations of Ca2+ in the cytoplasm, they were recently shown to also occur in the nuclear envelope (NE) of neural and other cells where they function to regulate nuclear Ca2+. A unique feature of NCXs in the NE is their high-affinity binding to GM1 ganglioside, this association being required for optimal exchanger activity. The NCX-GM1 complex occurs in the inner membrane of the NE and transfers Ca2+ from the nucleoplasm to the NE lumen. In neuronal cells, nuclear GM1 levels are low prior to differentiation but increase rapidly as axonal outgrowth progresses. Cells from genetically altered mice lacking GM1 have limited ability to regulate nuclear Ca2+, and the mice themselves showed similar deficit as seen in their high susceptibility to kainite-induced seizures. These are attenuated by LIGA-20, a derivative of GM1 that enters the nuclear membrane and restores nuclear NCX activity to normal level.

A mutation in calsequestrin, CASQ2D307H, impairs Sarcoplasmic Reticulum Ca2+ handling and causes complex ventricular arrhythmias in mice

OBJECTIVE

A naturally-occurring mutation in cardiac calsequestrin (CASQ2) at amino acid 307 was discovered in a highly inbred family and hypothesized to cause Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT). The goal of this study was to establish a causal link between CASQ2(D307H) and the CPVT phenotype using an in vivo model.

METHODS AND RESULTS

Cardiac-specific expression of the CASQ2(D307H) transgene was achieved using the alpha-MHC promoter. Multiple transgenic (TG) mouse lines expressing CASQ2(D307H) from 2- to 6-fold possess structurally normal hearts without any sign of hypertrophy. The hearts displayed normal ventricular function. Myocytes isolated from TG mice had diminished I(Ca)-induced Ca2+ transient amplitude and duration, as well as increased Ca2+ spark frequency. These myocytes, when exposed to isoproterenol and caffeine, displayed disturbances in their rhythmic Ca2+ oscillations and membrane potential, and delayed afterdepolarizations. ECG monitoring revealed that TG mice challenged with isoproterenol and caffeine developed complex ventricular arrhythmias, including non-sustained polymorphic ventricular tachycardia.

CONCLUSIONS

The findings of the present study demonstrate that expression of mutant CASQ2(D307H) in the mouse heart results in abnormal myocyte Ca2+ handling and predisposes to complex ventricular arrhythmias similar to the CPVT phenotype observed in human patients.

The influence of sodium–calcium exchange inhibitors on rabbit lens ion balance and transparency

Calcium regulation is essential to the maintenance of lens transparency. To maintain cytoplasmic calcium concentration at the required low level the lens must export calcium continuously. Here, studies were conducted to test whether sodium-calcium exchanger (NCX) inhibitors disturb calcium balance in the rabbit lens. Intact lenses were incubated up to 48 h in the presence or absence of the NCX inhibitor bepridil. Lens sodium, potassium and calcium content were determined by atomic absorption spectrophotometry. Fluo-4 was used to measure epithelial cell cytoplasmic calcium concentration in an intact lens preparation. NCX1 protein expression in lens epithelium was examined by western blot. NCX1 band density was similar in central and equatorial epithelium samples. Lenses exposed to bepridil (30 microM) lost transparency at the anterior and exhibited significant changes in electrolyte and water content. After 48 h, lens calcium content more than doubled, sodium increased four fold and potassium was significantly reduced. In contrast, lenses exposed to inhibitors of reverse mode calcium transport by NCX (KBR7943 or SN-6) remained transparent and the electrolyte and water content of the lens remained unchanged. The ability of bepridil to cause significant changes in lens transparency and electrolyte content points to an important role for NCX-meditated calcium export in the lens.

Helix Packing of the Cardiac Na+-Ca2+ Exchanger

The cardiac Na+-Ca2+ exchanger (NCX1) is a membrane protein that extrudes Ca2+ from cells using the energy of the Na+ gradient and is a key protein in regulating intracellular Ca2+ and contractility. Based on the current topological model, NCX1 consists of nine transmembrane segments (TMSs). The N-terminal five TMSs are separated from the C-terminal four TMSs by a large intracellular loop. Cysteine 768 is modeled to be in TMS 6 close to the intracellular surface. In this study, the proximity of TMS 6 to TMSs 1 and 2 was examined. Insect High Five cells were transfected with cDNAs encoding mutant NCX1 proteins. Each mutant contained cysteine 768 and an introduced cysteine in TMS 1 or 2. Cross-linking between cysteines was determined after reaction with thiol-specific cross-linkers containing spacer arms of 6.5-12 A. The data indicate that residues in TMSs 1 and 2 are close to cysteine 768 in TMS 6. Cysteine 768 cross-linked with residues at both ends of TMSs 1 and 2 and is likely located toward the middle of TMS 6. Based on these results, we present an expanded helix-packing model for NCX1.

Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions

The Na(+)/Ca(2+) exchanger's family of membrane transporters is widely distributed in cells and tissues of the animal kingdom and constitutes one of the most important mechanisms for extruding Ca(2+) from the cell. Two basic properties characterize them. 1) Their activity is not predicted by thermodynamic parameters of classical electrogenic countertransporters (dependence on ionic gradients and membrane potential), but is markedly regulated by transported (Na(+) and Ca(2+)) and nontransported ionic species (protons and other monovalent cations). These modulations take place at specific sites in the exchanger protein located at extra-, intra-, and transmembrane protein domains. 2) Exchange activity is also regulated by the metabolic state of the cell. The mammalian and invertebrate preparations share MgATP in that role; the squid has an additional compound, phosphoarginine. This review emphasizes the interrelationships between ionic and metabolic modulations of Na(+)/Ca(2+) exchange, focusing mainly in two preparations where most of the studies have been carried out: the mammalian heart and the squid giant axon. A surprising fact that emerges when comparing the MgATP-related pathways in these two systems is that although they are different (phosphatidylinositol bisphosphate in the cardiac and a soluble cytosolic regulatory protein in the squid), their final target effects are essentially similar: Na(+)-Ca(2+)-H(+) interactions with the exchanger. A model integrating both ionic and metabolic interactions in the regulation of the exchanger is discussed in detail as well as its relevance in cellular Ca(i)(2+) homeostasis.

Pharmacology of Brain Na+/Ca2+Exchanger: From Molecular Biology to Therapeutic Perspectives

In the last two decades, there has been a growing interest in unraveling the role that the Na+/Ca2+ exchanger (NCX) plays in the function and regulation of several cellular activities. Molecular biology, electrophysiology, genetically modified mice, and molecular pharmacology have helped to delve deeper and more successfully into the physiological and pathophysiological role of this exchanger. In fact, this nine-transmembrane protein, widely distributed in the brain and in the heart, works in a bidirectional way. Specifically, when it operates in the forward mode of operation, it couples the extrusion of one Ca2+ ion with the influx of three Na+ ions. In contrast, when it operates in the reverse mode of operation, while three Na+ ions are extruded, one Ca2+ enters into the cells. Different isoforms of NCX, named NCX1, NCX2, and NCX3, have been described in the brain, whereas only one, NCX1, has been found in the heart. The hypothesis that NCX can play a relevant role in several pathophysiological conditions, including hypoxia-anoxia, white matter degeneration after spinal cord injury, brain trauma and optical nerve injury, neuronal apoptosis, brain aging, and Alzheimer's disease, stems from the observation that NCX, in parallel with selective ion channels and ATP-dependent pumps, is efficient at maintaining intracellular Ca2+ and Na+ homeostasis. In conclusion, although studies concerning the involvement of NCX in the pathological mechanisms underlying brain injury during neurodegenerative diseases started later than those related to heart disease, the availability of pharmacological agents able to selectively modulate each NCX subtype activity and antiporter mode of operation will provide a better understanding of its pathophysiological role and, consequently, more promising approaches to treat these neurological disorders.

Expression of calcium transporters in the retina of the tiger salamander (Ambystoma tigrinum)

Changes in intracellular calcium concentration, [Ca2+]i, modulate the flow of visual signals across all stages of processing in the retina, yet the identities of Ca2+ transporters responsible for these changes are still largely unknown. In the current study, the distribution of plasma membrane and intracellular Ca2+ transporters in the retina of tiger salamander, a model system for physiological studies of retinal function, was determined. Plasma membrane calcium ATPases (PMCAs), responsible for high-affinity Ca2+ extrusion, were highly expressed in the salamander retina. PMCA isoforms 1, 2, and 4 were localized to photoreceptors, whereas the inner retina expressed all four isoforms. PMCA3 was expressed in a sparse population of amacrine and ganglion neurons, whereas PMCA2 was expressed in most amacrine and ganglion cells. Na+/Ca2+ exchangers, a high-capacity Ca2+ extrusion system, were expressed in the outer plexiform layer and in a subset of inner nuclear and ganglion layer cells. Intracellular Ca2+ store transporters were also represented prominently. SERCA2a, a splice variant of the sarcoplasmic-endoplasmic Ca2+ ATPase, was found mostly in photoreceptors, whereas SERCA2b was found in the majority of retinal neurons and in glial cells. The predominant endoplasmic reticulum (ER) Ca2+ channels in the salamander retina are represented by the isoform 2 of the IP3 receptor family and the isoform 2 of the ryanodine receptor family. These results indicate that Ca2+ transporters in the salamander retina are expressed in a cell type-specific manner.

C6 cells express a sodium-calcium exchanger/GM1 complex in the nuclear envelope but have no exchanger in the plasma membrane: comparison to astrocytes

Previous work demonstrated the presence of an isoform of Na(+)/Ca(2+) exchanger in the nuclear envelope of neurons and NG108-15 cells that is tightly associated with GM1 ganglioside and potentiated by the latter. This contrasted with the Na(+)/Ca(2+) exchanger(s) in the plasma membrane, which were suggested to associate more loosely with GM1. To study these aspects of Na(+)/Ca(2+) exchanger expression in nonneuronal neural cells, we have examined nuclear and plasma membrane exchanger patterns in astrocytes and C6 cells, a glia-derived line. We find both cell types contain the tightly associated exchanger/GM1 complex in the nuclear envelope but, surprisingly, only astrocytes possess Na(+)/Ca(2+) exchanger activity in the plasma membrane. This is the first reported example of a cell (C6) with Na(+)/Ca(2+) exchangers in the nuclear envelope but not in the plasma membrane. RT-PCR established the presence of the NCX1 subtype in C6 cells and both NCX1 and NCX2 in astrocytes. Comparison was made with NG108-15 cells, which have Na(+)/Ca(2+) exchangers in both nuclear and plasma membranes, and Jurkat cells, which have no Na(+)/Ca(2+) exchanger in either membrane. Culturing of C6 cells in the presence dibutyryl-cAMP caused upregulation of a high molecular weight isoform of the exchanger together with GM1 in the nuclear envelope, resulting in significant elevation of Na(+)/Ca(2+) exchanger activity in the latter. Application of exogenous GM1 to nuclei from non-treated cells also potentiated exchanger activity, although to a lesser degree. The Na(+)/Ca(2+) exchanger/GM1 complex occurs in the inner membrane of the nuclear envelope, suggesting a functional role in transferring Ca(2+) between nucleoplasm and the envelope lumen.

Na+/Ca2+ exchanger overexpression impairs frequency- and ouabain-dependent cell shortening in adult rat cardiomyocytes

The Na(+)/Ca(2+) exchanger (NCX) may influence cardiac function depending on its predominant mode of action, forward mode or reverse mode, during the contraction-relaxation cycle. The intracellular Na(+) concentration ([Na(+)](i)) and the duration of the action potential as well as the level of NCX protein expression regulate the mode of action of NCX. [Na(+)](i) and NCX expression have been reported to be increased in human heart failure. Nevertheless, the consequences of altered NCX expression in heart failure are still a matter of discussion. We aimed to characterize the influence of NCX expression on intracellular Ca(2+) transport in rat cardiomyocytes by adenoviral-mediated gene transfer. A five- to ninefold (dose dependent) overexpression of NCX protein was achieved after 48 h by somatic gene transfer (Ad.NCX.GFP) versus control (Ad.GFP). NCX activity, determined by Na(+) gradient-dependent (45)Ca(2+)-uptake, was significantly increased. The protein expressions of sarco(endo)plasmic reticulum Ca(2+)-ATPase, phospholamban, and calsequestrin were unaffected by NCX overexpression. Fractional shortening (FS) of isolated cardiomyocytes was significantly increased at low stimulation rates in Ad.NCX.GFP. After a step-wise enhancing frequency of stimulation to 3.0 Hz, FS remained unaffected in Ad.GFP cells but declined in Ad.NCX.GFP cells. The positive inotropic effect of the cardiac glycoside ouabain was less effective in Ad.NCX.GFP cells, whereas the positive inotropic effect of beta-adrenergic stimulation remained unchanged. In conclusion, NCX overexpression results in a reduced cell shortening at higher stimulation frequencies as well as after inhibition of sarcolemmal Na(+)-K(+)-ATPase, i.e., in conditions with enhanced [Na(+)](i). At low stimulation rates, increased NCX expression enhances both intracellular systolic Ca(2+) and contraction amplitude.

Phosphoarginine regulation of the squid nerve Na+/Ca2+ exchanger: metabolic pathway and exchanger-ligand interactions different from those seen with ATP

In squid nerves the Na(+)-Ca(2+) exchanger is up-regulated by ATP and phosphoarginine (PA). ATP regulation involves drastic alterations in the Na(+)(i), H(+)(i) and Ca(2+)(i) interactions with the large intracellular cytoplasmic loop of the exchanger protein. In this work we explored the mechanisms associated with PA regulation in intracellular dialysed squid axons and squid optic nerve membrane vesicles. Dialysed axons were used to measure the four modes of exchange fluxes (Na(+)(o)-Ca(2+)(i) or forward exchange, Ca(2+)(o)-Na(+)(i) or reverse exchange, Ca(2+)(o)-Ca(2+)(i) exchange and Na(+)(o)-Na(+)(i) exchange) under controlled intra- and extracellular conditions. Inside-out membrane vesicles allowed measurement of the Na(+)-gradient-dependent (45)Ca(2+) uptake (forward mode) as influenced by ligands and digestion with chymotrypsin from the intracellular side. The results show that, unlike ATP, PA regulation does not affect the H(+)(i), Na(+)(i) and Ca(2+)(i) interactions with the intracellular 'regulatory' loop, but increases the affinity of the intracellular transport sites, preferentially for Ca(2+)(i) (about 20-fold) over Na(+)(i) (50%); i.e. PA favours the forward mode over the other exchange modes. Intracellular chymotrypsin digestion removed ATP regulation while leaving modulation by PA unmodified. Western blot analysis suggested that chymotrypsin disrupts the large intracellular loop. Together these results indicate that ATP and PA regulations are associated with different structures inside and outside the exchanger protein. Based on these observations we expanded our previous model for metabolic regulation of the Na(+)-Ca(2+) exchanger by adding to the original 'ATP region' a new zone, the 'PA region', related to the intracellular transport sites for Na(+)(i) and Ca(2+)(i). This new model is able to explain most previous and present results.

Preconditioning attenuates ischemia-reperfusion-induced remodeling of Na+-K+-ATPase in hearts

The aim of this study was to determine whether changes in protein content and/or gene expression of Na+-K+-ATPase subunits underlie its decreased enzyme activity during ischemia and reperfusion. We measured protein and mRNA subunit levels in isolated rat hearts subjected to 30 min of ischemia and 30 min of reperfusion (I/R). The effect of ischemic preconditioning (IP), induced by three cycles of ischemia and reperfusion (10 min each), was also assessed on the molecular changes in Na+-K+-ATPase subunit composition due to I/R. I/R reduced the protein levels of the alpha2-, alpha3-, beta1-, and beta2-isoforms by 71%, 85%, 27%, and 65%, respectively, whereas the alpha1-isoform was decreased by <15%. A similar reduction in mRNA levels also occurred for the isoforms of Na+-K+-ATPase. IP attenuated the reduction in protein levels of Na+-K+-ATPase alpha2-, alpha3-, and beta2-isoforms induced by I/R, without affecting the alpha1- and beta1-isoforms. Furthermore, IP prevented the reduction in mRNA levels of Na+-K+-ATPase alpha2-, alpha3-, and beta1-isoforms following I/R. Similar alterations in protein contents and mRNA levels for the Na+/Ca2+ exchanger were seen due to I/R as well as IP. These findings indicate that remodeling of Na+-K+-ATPase may occur because of I/R injury, and this may partly explain the reduction in enzyme activity in ischemic heart disease. Furthermore, IP may produce beneficial effects by attenuating the remodeling of Na+-K+-ATPase and changes in Na+/Ca2+ exchanger in hearts after I/R.

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.

Molecular determinants of cAMP-mediated regulation of the Na+-Ca2+ exchanger expressed in human cell lines

The cardiac Na+-Ca2+ exchanger (NCX1) is one of the major sarcolemmal Ca2+ transporters of cardiomyocytes. Structure-function studies suggest that beta-adrenergic inhibition of NCX1, as reported for frog, but not mammalian hearts, may be associated with a unique splice variant of frog cardiac NCX1 where insertion of an extra exon completes the coding of a nucleotide binding P-loop. To test the involvement of the P-loop in cAMP-mediated regulation of NCX1 we used four stably transfected human cell lines (a previously established line of baby hamster kidney (BHK) cells and three new lines of human embryonic kidney (HEK) cells) expressing: (1) wild-type dog NCX1 (dog NCX1); (2) wild-type frog NCX1 (frog NCX1); (3) chimeric frog-dog NCX1 incorporating the completed P-loop from the frog NCX1 into the dog NCX1 sequence (frog/dog NCX1); and (4) a mutated frog NCX1 where a putative protein kinase A (PKA) site was disrupted by substitution of a single serine residue with glycine (S374G frog NCX1). Structural expression of these NCX1 constructs was confirmed using Western blot analysis of extracted proteins and immunofluorescence imaging. The NCX1-generated current (INa-Ca) was reliably measured in cells expressing dog (2.0 +/- 0.15 pA pF-1), frog (0.6 +/- 0.1 pA pF-1) and frog/dog (0.6 +/- 0.1 pA pF-1) NCX1, but less so in those expressing S374G frog NCX1 (0.3 +/- 0.1 pA pF-1). Addition of 100 microM 8-bromoadenosine 3',5' cyclic monophosphate (8-Br-cAMP) suppressed INa-Ca of frog and frog/dog NCX1 by 60-80 %. The suppression of INa-Ca was smaller and transient in cells expressing S374G frog NCX1, and absent in cells expressing dog NCX1. Intracellular Ca2+ (Ca2+i)-transients, activated by rapid withdrawal of Na+, were also downregulated in the frog and frog/dog NCX1 and to a smaller and transient extent in S374G frog NCX1. Our findings suggest that the suppressive effect of beta-adrenergic agonists requires the presence of the P-loop domain of the frog NCX1, and provide evidence that the putative PKA site, present in both dog and frog NCX1, might also be critical in the cAMP-mediated regulation of the exchanger.

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.

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.

Effects of Na(+)/Ca(2+) exchanger downregulation on contractility and [Ca(2+)](i) transients in adult rat myocytes

Postmyocardial infarction (MI) rat myocytes demonstrated depressed Na(+)/Ca(2+) exchange (NCX1) activity, altered contractility, and intracellular Ca(2+) concentration ([Ca(2+)](i)) transients. We investigated whether NCX1 downregulation in normal myocytes resulted in contractility changes observed in MI myocytes. Myocytes infected with adenovirus expressing antisense (AS) oligonucleotides to NCX1 had 30% less NCX1 at 3 days and 66% less NCX1 at 6 days. The half-time of relaxation from caffeine-induced contracture was twice as long in ASNCX1 myocytes. Sarcoplasmic reticulum (SR) Ca(2+)-ATPase abundance, SR Ca(2+) uptake, resting membrane potential, action potential amplitude and duration, L-type Ca(2+) current density and cell size were not affected by ASNCX1 treatment. At extracellular Ca(2+) concentration ([Ca(2+)](o)) of 5 mM, ASNCX1 myocytes had significantly lower contraction and [Ca(2+)](i) transient amplitudes and SR Ca(2+) contents than control myocytes. At 0.6 mM [Ca(2+)](o), contraction and [Ca(2+)](i) transient amplitudes and SR Ca(2+) contents were significantly higher in ASNCX1 myocytes. At 1.8 mM [Ca(2+)](o), contraction and [Ca(2+)](i) transient amplitudes were not different between control and ASNCX1 myocytes. This pattern of contractile and [Ca(2+)](i) transient abnormalities in ASNCX1 myocytes mimics that observed in rat MI myocytes. We conclude that downregulation of NCX1 in adult rat myocytes resulted in decreases in both Ca(2+) influx and efflux during a twitch. We suggest that depressed NCX1 activity may partly account for the contractile abnormalities after MI.

Effects of sprint training on contractility and [Ca(2+)](i) transients in adult rat myocytes

The effects of 6-8 wk of high-intensity sprint training (HIST) on rat myocyte contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) transients were investigated. Compared with sedentary (Sed) myocytes, HIST induced a modest (5%) but significant (P < 0.0005) increase in cell length with no changes in cell width. In addition, the percentage of myosin heavy chain alpha-isoenzyme increased significantly (P < 0.02) from 0.566 +/- 0.077% in Sed rats to 0.871 +/- 0.006% in HIST rats. At all three (0.6, 1.8, and 5 mM) extracellular Ca(2+) concentrations ([Ca(2+)](o)) examined, maximal shortening amplitudes and maximal shortening velocities were significantly (P < 0.0001) lower and half-times of relaxation were significantly (P < 0.005) longer in HIST myocytes. HIST myocytes had significantly (P < 0.0001) higher diastolic [Ca(2+)](i) levels. Compared with Sed myocytes, systolic [Ca(2+)](i) levels in HIST myocytes were higher at 0.6 mM [Ca(2+)](o), similar at 1.8 mM [Ca(2+)](o), and lower at 5 mM [Ca(2+)](o). The amplitudes of [Ca(2+)](i) transients were significantly (P < 0.0001) lower in HIST myocytes. Half-times of [Ca(2+)](i) transient decline, an estimate of sarcoplasmic reticulum (SR) Ca(2+) uptake activity, were not different between Sed and HIST myocytes. Compared with Sed hearts, Western blots demonstrated a significant (P < 0.03) threefold decrease in Na(+)/Ca(2+) exchanger, but SR Ca(2+)-ATPase and calsequestrin protein levels were unchanged in HIST hearts. We conclude that HIST effected diminished myocyte contractile function and [Ca(2+)](i) transient amplitudes under the conditions studied. We speculate that downregulation of Na(+)/Ca(2+) exchanger may partly account for the decreased contractility in HIST myocytes.

Potentiation of a sodium-calcium exchanger in the nuclear envelope by nuclear GM1 ganglioside

Calcium is recognized as an important intracellular messenger with a pivotal role in the regulation of many cytosolic and nuclear processes. Gangliosides of various types, especially GM1, are known to have a role in some aspects of Ca2+ regulation, operating through a variety of mechanisms that are gradually coming to light. The present study provides evidence for a sodium-calcium exchanger in the nuclear envelope of NG108-15 neuroblastoma cells that is potently and specifically activated by GM1. Immunoblot analysis revealed an unusually tight association of GM1 with the exchanger in the nuclear envelope but not with that in the plasma membrane. Exchanger and associated GM1 were located in the inner membrane of the nuclear envelope, suggesting this system could function to transfer Ca2+ between nucleoplasm and the envelope lumen. The GM1-enhanced exchange was blocked by cholera toxin B subunit while C2-ceramide, a recently discovered inhibitor of the exchanger, blocked all transfer. Exchanger activity was significantly elevated in nuclei isolated from cells that were induced to differentiate by KCl + dibutyryl-cAMP, a treatment previously shown to promote up-regulation of nuclear GM1 in conjunction with axonogenesis. Similar enhancement was achieved by addition of exogenous GM1 to nuclei from undifferentiated cells. These results suggest a prominent role for nuclear GM1 in regulation of nuclear Ca2+ homeostasis.

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).

Overexpression of the Na/Ca exchanger shapes stimulus-induced cytosolic Ca(2+) oscillations in insulin-producing BRIN-BD11 cells

In response to glucose, mouse beta-cells display slow oscillations of the membrane potential and cytosolic free Ca(2+) concentration ([Ca(2+)](i)), whereas rat beta-cells display a staircase increase in these parameters. Mouse and rat islet cells differ also by their level of Na/Ca exchanger (NCX) activity. The view that the inward current generated by Na/Ca exchange shapes stimulus-induced electrical activity and [Ca(2+)](i) oscillations in pancreatic beta-cells was examined in insulin-producing BRIN-BD11 cells overexpressing the Na/Ca exchanger. BRIN-BD11 cells were stably transfected with NCX1.7, one of the exchanger isoforms identified in the beta-cell. Overexpression could be assessed at the mRNA and protein level. Appropriate targeting to the plasma membrane could be assessed by microfluorescence and the increase in Na/Ca exchange activity. In response to K(+), overexpressing cells showed a more rapid increase in [Ca(2+)](i) on membrane depolarization as well as a more rapid decrease of [Ca(2+)](i) on membrane repolarization. In response to glucose and tolbutamide, control BRIN cells showed large amplitude [Ca(2+)](i) oscillations. In contrast, overexpressing cells showed a staircase increase in [Ca(2+)](i) without such large oscillations. Diazoxide-induced membrane hyperpolarization restored large amplitude [Ca(2+)](i) oscillations in overexpressing cells. The present data confirm that Na/Ca exchange plays a significant role in the rat beta-cell [Ca(2+)](i) homeostasis, the exchanger being a versatile system allowing both Ca(2+) entry and outflow. Our data suggest that the current generated by the exchanger shapes stimulus-induced membrane potential and [Ca(2+)](i) oscillations in insulin-secreting cells, with the difference in electrical activity and [Ca(2+)](i) behavior seen in mouse and rat beta-cells resulting in part from a difference in Na/Ca exchange activity between these two cells.

Overexpression of Na+/Ca2+ exchanger alters contractility and SR Ca2+ content in adult rat myocytes

The functional consequences of overexpression of rat heart Na+/Ca2+ exchanger (NCX1) were investigated in adult rat myocytes in primary culture. When maintained under continued electrical field stimulation conditions, cultured adult rat myocytes retained normal contractile function compared with freshly isolated myocytes for at least 48 h. Infection of myocytes by adenovirus expressing green fluorescent protein (GFP) resulted in >95% infection as ascertained by GFP fluorescence, but contraction amplitude at 6-, 24-, and 48-h postinfection was not affected. When they were examined 48 h after infection, myocytes infected by adenovirus expressing both GFP and NCX1 had similar cell sizes but exhibited significantly altered contraction amplitudes and intracellular Ca2+ concentration ([Ca2+]i) transients, and lower resting and diastolic [Ca2+]i when compared with myocytes infected by the adenovirus expressing GFP alone. The effects of NCX1 overexpression on sarcoplasmic reticulum (SR) Ca2+ content depended on extracellular Ca2+ concentration ([Ca2+]o), with a decrease at low [Ca2+]o and an increase at high [Ca2+]o. The half-times for [Ca2+]i transient decline were similar, suggesting little to no changes in SR Ca2+-ATPase activity. Western blots demonstrated a significant (P < or = 0.02) threefold increase in NCX1 but no changes in SR Ca2+-ATPase and calsequestrin abundance in myocytes 48 h after infection by adenovirus expressing both GFP and NCX1 compared with those infected by adenovirus expressing GFP alone. We conclude that overexpression of NCX1 in adult rat myocytes incubated at high [Ca2+]o resulted in enhanced Ca2+ influx via reverse NCX1 function, as evidenced by greater SR Ca2+ content, larger twitch, and [Ca2+]i transient amplitudes. Forward NCX1 function was also increased, as indicated by lower resting and diastolic [Ca2+]i.

A disulfide bond is required for functional assembly of NCX1 from complementary fragments

The cardiac Na(+)-Ca(2+) exchanger consists of a single polypeptide with two transmembrane segment (TMS) clusters separated by a large intracellular loop between TMS5 and TMS6 (Nicoll et al. (1999) J. Biol. Chem. 274, 910-917; Iwamoto et al. (1999) FEBS Lett. 446, 264-268). A "split" exchanger can be expressed by dividing the exchanger cDNA into two fragments so that the NH(2)- and CO(2)H-terminal portions of the protein are expressed as separate polypeptides in HEK293 cells. Expression of partial exchanger molecules did not result in detectable exchanger activity. Cells coexpressing both portions of the exchanger, however, displayed between 30 and 50% of the activity of the intact wild-type exchanger. The full-length exchanger contains a disulfide bond between residues 14 or 20 and 792. We examined the role of this disulfide bond in the split exchanger by mutagenesis and expression studies. Our results indicate that the function of the exchanger requires both TMS clusters and that the C(14 or 20)/C792 disulfide bond is essential for expression of active exchangers from half molecules.