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

Pharmacological Modulation by Low Molecular Weight Heparin of Purinergic Signaling in Cardiac Cells Prevents Arrhythmia and Lethality Induced by Myocardial Infarction

Background: Although several studies suggest that heparins prevent arrhythmias caused by acute myocardial infarction (AMI), the molecular mechanisms involved remain unclear. To investigate the involvement of pharmacological modulation of adenosine (ADO) signaling in cardiac cells by a low-molecular weight heparin (enoxaparin; ENOX) used in AMI therapy, the effects of ENOX on the incidences of ventricular arrhythmias (VA), atrioventricular block (AVB), and lethality (LET) induced by cardiac ischemia and reperfusion (CIR) were evaluated, with or without ADO signaling blockers. Methods: To induce CIR, adult male Wistar rats were anesthetized and subjected to CIR. Electrocardiogram (ECG) analysis was used to evaluate CIR-induced VA, AVB, and LET incidence, after treatment with ENOX. ENOX effects were evaluated in the absence or presence of an ADO A1-receptor antagonist (DPCPX) and/or an inhibitor of ABC transporter-mediated cAMP efflux (probenecid, PROB). Results: VA incidence was similar between ENOX-treated (66%) and control rats (83%), but AVB (from 83% to 33%) and LET (from 75% to 25%) incidences were significantly lower in rats treated with ENOX. These cardioprotective effects were blocked by either PROB or DPCPX. Conclusion: These results indicate that ENOX was effective in preventing severe and lethal arrhythmias induced by CIR due to pharmacological modulation of ADO signaling in cardiac cells, suggesting that this cardioprotective strategy could be promising in AMI therapy.

Bifurcations and Proarrhythmic Behaviors in Cardiac Electrical Excitations

The heart is a hierarchical dynamic system consisting of molecules, cells, and tissues, and acts as a pump for blood circulation. The pumping function depends critically on the preceding electrical activity, and disturbances in the pattern of excitation propagation lead to cardiac arrhythmia and pump failure. Excitation phenomena in cardiomyocytes have been modeled as a nonlinear dynamical system. Because of the nonlinearity of excitation phenomena, the system dynamics could be complex, and various analyses have been performed to understand the complex dynamics. Understanding the mechanisms underlying proarrhythmic responses in the heart is crucial for developing new ways to prevent and control cardiac arrhythmias and resulting contractile dysfunction. When the heart changes to a pathological state over time, the action potential (AP) in cardiomyocytes may also change to a different state in shape and duration, often undergoing a qualitative change in behavior. Such a dynamic change is called bifurcation. In this review, we first summarize the contribution of ion channels and transporters to AP formation and our knowledge of ion-transport molecules, then briefly describe bifurcation theory for nonlinear dynamical systems, and finally detail its recent progress, focusing on the research that attempts to understand the developing mechanisms of abnormal excitations in cardiomyocytes from the perspective of bifurcation phenomena.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Nucleoporins in cardiovascular disease

Cardiovascular disease is a pressing health problem with significant global health, societal, and financial burdens. Understanding the molecular basis of polygenic cardiac pathology is thus essential to devising novel approaches for management and treatment. Recent identification of uncharacterized regulatory functions for a class of nuclear envelope proteins called nucleoporins offers the opportunity to understand novel putative mechanisms of cardiac disease development and progression. Consistent reports of nucleoporin deregulation associated with ischemic and dilated cardiomyopathies, arrhythmias and valvular disorders suggests that nucleoporin impairment may be a significant but understudied variable in cardiopathologic disorders. This review discusses and converges existing literature regarding nuclear pore complex proteins and their association with cardiac pathologies, and proposes a role for nucleoporins as facilitators of cardiac disease.

RBM20 Mutations Induce an Arrhythmogenic Dilated Cardiomyopathy Related to Disturbed Calcium Handling

Background:

Mutations in RBM20 (RNA-binding motif protein 20) cause a clinically aggressive form of dilated cardiomyopathy, with an increased risk of malignant ventricular arrhythmias. RBM20 is a splicing factor that targets multiple pivotal cardiac genes, such as Titin (TTN) and CAMK2D (calcium/calmodulin-dependent kinase II delta). Aberrant TTN splicing is thought to be the main determinant of RBM20-induced dilated cardiomyopathy, but is not likely to explain the increased risk of arrhythmias. Here, we investigated the extent to which RBM20 mutation carriers have an increased risk of arrhythmias and explore the underlying molecular mechanism.

Methods:

We compared clinical characteristics of RBM20 and TTN mutation carriers and used our previously generated Rbm20 knockout (KO) mice to investigate downstream effects of Rbm20-dependent splicing. Cellular electrophysiology and Ca 2+ measurements were performed on isolated cardiomyocytes from Rbm20 KO mice to determine the intracellular consequences of reduced Rbm20 levels.

Results:

Sustained ventricular arrhythmias were more frequent in human RBM20 mutation carriers than in TTN mutation carriers (44% versus 5%, respectively, P =0.006). Splicing events that affected Ca 2+ - and ion-handling genes were enriched in Rbm20 KO mice, most notably in the genes CamkIIδ and RyR2. Aberrant splicing of CamkIIδ in Rbm20 KO mice resulted in a remarkable shift of CamkIIδ toward the δ-A isoform that is known to activate the L-type Ca 2+ current ( I Ca,L ). In line with this, we found an increased I Ca,L , intracellular Ca 2+ overload and increased sarcoplasmic reticulum Ca 2+ content in Rbm20 KO myocytes. In addition, not only complete loss of Rbm20, but also heterozygous loss of Rbm20 increased spontaneous sarcoplasmic reticulum Ca 2+ releases, which could be attenuated by treatment with the I Ca,L antagonist verapamil.

Conclusions:

We show that loss of Rbm20 disturbs Ca 2+ handling and leads to more proarrhythmic Ca 2+ releases from the sarcoplasmic reticulum. Patients that carry a pathogenic RBM20 mutation have more ventricular arrhythmias despite a similar left ventricular function, in comparison with patients with a TTN mutation. Our experimental data suggest that RBM20 mutation carriers may benefit from treatment with an I Ca,L blocker to reduce their arrhythmia burden.

Calcium- and ATP-dependent regulation of Na/Ca exchange function in BHK cells: Comparison of NCX1 and NCX3 exchangers

Na⁺/Ca²⁺ exchangers (NCX) mediate bidirectional Ca²⁺ fluxes across cell membranes and contribute to Ca²⁺ homeostasis in many cell types. Exchangers are regulated by gating reactions that depend on Na⁺ and Ca²⁺ binding to transport and regulatory sites. A Na⁺_i-dependent inactivation is prominent in all isoforms, whereas Ca²⁺_i-dependent regulation varies among isoforms. Here we characterize new details of NCX operation and describe differences and similarities between NCX3 and NCX1 regulation by intracellular Ca²⁺ and ATP. To compare isoforms, we employed BHK cells expressing NCX3 or NCX1 constitutively and exchange activity was analysed in whole-cell and excised patch recordings under "zero-trans" conditions (i.e., with only one transported ion species on each side). Using BHK cells with low cytoplasmic Ca²⁺ buffering, outward (reverse) currents, reflecting Ca²⁺ influx, are activated by applying extracellular Ca²⁺ (Ca_o) in the presence of Na⁺ on the cytoplasmic side. When firstly activated, peak outward NCX3 currents rapidly decay over seconds and then typically develop a secondary transient peak with slower kinetics, until Ca_o removal abolishes all outward current. The delayed rise of outward current is the signature of an activating process since peak outward NCX3 currents elicited at subsequent Ca_o bouts remain stimulated for minutes and slower decline towards a non-zero level during continued Ca_o application. Secondary transient peaks and current stimulation are suppressed by increasing the intracellular Ca²⁺ buffer capacity or by replacing cytoplasmic ATP with the analogues AMP-PNP or ATPγS. In BHK cells expressing NCX1, outward currents activated under identical settings decay to a steady-state level during single Ca_o application and are significantly larger, causing strong and long-lived run down of subsequent outward currents. NCX1 current run down is not prevented by increasing cytoplasmic Ca²⁺ buffering but secondary transient peaks in the outward current profile can be resolved in the presence of ATP. Finally, inward currents recorded in patches excised from NCX3-expressing cells reveal a proteolysis-sensitive, Ca-dependent inactivation process that is unusual for NCX1 forward activity. Together, our results suggest that NCX function is regulated more richly than appreciated heretofore, possibly including processes that are lost in excised membrane patches.

Calpain-3-mediated regulation of the Na⁺-Ca²⁺ exchanger isoform 3

Ca²⁺ disturbances are observed when Ca²⁺-dependent cysteine proteases malfunction, causing muscle weakness and wasting. For example, loss of calpain-3 (CAPN3) activity leads to limb-girdle muscular dystrophy 2A (LGMD2A). In neuronal excitotoxicity, the cleavage of the Na⁺-Ca²⁺ exchanger isoform 3 (NCX3) has been associated with an increase in activity and elevation of the Ca²⁺ content in the endoplasmic reticulum (ER). Since NCX3 is expressed in skeletal muscle, we evaluated the cleavage of different NCX3 splice variants by CAPN1 and CAPN3. Using Fura-2-based cellular Ca²⁺ imaging, we showed for the first time that CAPN3 increases NCX3 activity and that only NCX3-AC, the variant predominantly expressed in skeletal muscle, is sensitive to calpain. The silencing of the endogenous CAPN1 and the expression of the inactive form of CAPN3 (C129S CAPN3) confirmed the specificity for CAPN1 and CAPN3. Functional studies revealed that cellular Ca²⁺ uptake through the reverse mode of NCX3 was significantly increased independently of the mode of activation of the exchanger by either a rise in intracellular Ca²⁺ ([Ca²⁺]_i) or Na⁺ ([Na⁺]_i). Subsequently, the sensitivity to CAPN1 and CAPN3 could be abrogated by removal of the six residues coded in exon C of NCX3-AC. Additionally, mutation of the Leu-600 and Leu-601 suggested the presence of a cleavage site at Leu-602. The increased Ca²⁺ uptake of NCX3 might participate in the Ca²⁺ refilling of the sarcoplasmic reticulum (SR) after the excitation-contraction uncoupling following exercise and therefore be implicated in the impaired reticular Ca²⁺ storage observed in LGMD2A.

Function and regulation of the Na+-Ca2+ exchanger NCX3 splice variants in brain and skeletal muscle

Isoform 3 of the Na(+)-Ca(2+) exchanger (NCX3) is crucial for maintaining intracellular calcium ([Ca(2+)]i) homeostasis in excitable tissues. In this sense NCX3 plays a key role in neuronal excitotoxicity and Ca(2+) extrusion during skeletal muscle relaxation. Alternative splicing generates two variants (NCX3-AC and NCX3-B). Here, we demonstrated that NCX3 variants display a tissue-specific distribution in mice, with NCX3-B as mostly expressed in brain and NCX-AC as predominant in skeletal muscle. Using Fura-2-based Ca(2+) imaging, we measured the capacity and regulation of the two variants during Ca(2+) extrusion and uptake in different conditions. Functional studies revealed that, although both variants are activated by intracellular sodium ([Na(+)]i), NCX3-AC has a higher [Na(+)]i sensitivity, as Ca(2+) influx is observed in the presence of extracellular Na(+). This effect could be partially mimicked for NCX3-B by mutating several glutamate residues in its cytoplasmic loop. In addition, NCX3-AC displayed a higher capacity of both Ca(2+) extrusion and uptake compared with NCX3-B, together with an increased sensitivity to intracellular Ca(2+). Strikingly, substitution of Glu(580) in NCX3-B with its NCX3-AC equivalent Lys(580) recapitulated the functional properties of NCX3-AC regarding Ca(2+) sensitivity, Lys(580) presumably acting through a structure stabilization of the Ca(2+) binding site. The higher Ca(2+) uptake capacity of NCX3-AC compared with NCX3-B is in line with the necessity to restore Ca(2+) levels in the sarcoplasmic reticulum during prolonged exercise. The latter result, consistent with the high expression in the slow-twitch muscle, suggests that this variant may contribute to the Ca(2+) handling beyond that of extruding Ca(2+).

KB-R7943, a plasma membrane Na(+)/Ca(2+) exchanger inhibitor, blocks opening of the mitochondrial permeability transition pore

The isothiourea derivative, KB-R7943, inhibits the reverse-mode of the plasma membrane sodium/calcium exchanger and protects against ischemia/reperfusion injury. The mechanism through which KB-R7943 confers protection, however, remains controversial. Recently, KB-R7943 has been shown to inhibit mitochondrial calcium uptake and matrix overload, which may contribute to its protective effects. While using KB-R7943 for this purpose, we find here no evidence that KB-R7943 directly blocks mitochondrial calcium uptake. Rather, we find that KB-R7943 inhibits opening of the mitochondrial permeability transition pore in permeabilized cells and isolated liver mitochondria. Furthermore, we find that this observation correlates with protection against calcium ionophore-induced mitochondrial membrane potential depolarization and cell death, without detrimental effects to basal mitochondrial membrane potential or complex I-dependent mitochondrial respiration. Our data reveal another mechanism through which KB-R7943 may protect against calcium-induced injury, as well as a novel means to inhibit the mitochondrial permeability transition pore.

Ca(2+) regulation in the Na(+)/Ca (2+) exchanger features a dual electrostatic switch mechanism

Ion transport performed by the Na(+)/Ca(2+) exchanger (NCX) is regulated via its cytosolic Ca(2+)-binding domains, CBD1 and CBD2, which act as sensors for intracellular Ca(2+). Striking differences in the electrostatic potential of the Ca(2+)-bound and Ca(2+)-free forms turn the CBD1 and CBD2 Ca(2+)-binding sites into electrostatic switches similar to those of C(2) domains. Binding of Ca(2+) with high affinity to CBD1 induces a conformational change that is relayed to the transmembrane domain and thereby initiates Na(+)/Ca(2+) exchange. The Ca(2+) concentration at which this conformational change occurs is determined by the Ca(2+) affinities of the strictly conserved CBD1 Ca(2+)-binding sites that are modulated by an adjacent, variable region of CBD2. In contrast, the Ca(2+)-binding properties of CBD2 depend on the isoform and the type of residues in the Ca(2+)-binding sites, encoded by a mutually exclusive exon. This second electrostatic switch, formed by CBD2, appears to be required for sustained Na(+)/Ca(2+) exchange and may allow tailored, tissue-specific exchange activities.

Reverse-mode Na+/Ca2+ exchange is an important mediator of venous contraction

The Na(+)/Ca(2+) exchanger (NCX) is a bi-directional regulator of cytosolic Ca(2+), causing Ca(2+) efflux in forward-mode and Ca(2+) influx in reverse-mode. We hypothesized that reverse-mode NCX is a means of Ca(2+) entry in rat aorta (RA) and vena cava (RVC). NCX protein in RA and RVC was confirmed by immunoprecipitation. To assess NCX function, isometric contraction and intracellular Ca(2+) was measured in RA and RVC rings in response to low extracellular Na(+), endothelin-1 (ET-1), and KCl, in the presence or absence of the NCX antagonist KB-R7943. In RVC, low extracellular Na(+) caused vasoconstriction and an increase in intracellular Ca(2+) that was attenuated by 10μM KB-R7943. KB-R7943 (10 μM) attenuated maximal contraction to ET-1 in RVC (53 ± 9% of control), but not RA (91±1% of control). KB-R7943 (10 μM) reduced the maximal contraction to KCl in RA (48 ± 5%) and nearly abolished it in RVC (9 ± 2%), suggesting that voltage-dependent Ca(2+) influx may be inhibited by KB-R7943 as well. However, the L-type Ca(2+) channel inhibitor nifedipine (1 μM) did not alter ET-1-induced contraction. Our findings suggest that reverse-mode NCX is an important mechanism of Ca(2+) influx in RVC but not RA, especially during ET-1-induced contraction. Also, the effects of KB-R7943 on ET-1-induced contraction of RA and RVC are predominantly mediated by reverse-mode NCX inhibition and not due to off-target inhibition of Ca(2+) channels.

Expression of alternatively spliced variants of Na-Ca-exchanger-1 in experimental colitis: role in reduced colonic contractility

Inflammation-induced colonic motility dysfunction is associated with a disturbance in Ca(2+) ion transporting mechanisms. The main objective of this study was to identify the types of Na-Ca-exchanger-1 (NCX-1) variants expressed in the rat colon, and how this was affected by colitis. In addition, the effect of colitis on the possible involvement of NCX-1 in the reduced carbachol-induced contraction of the rat colon was examined. Colitis was induced in male Sprague-Dawley rats by intra-rectal instillation of trinitrobenzenesulphonic acid (TNBS). Animals were killed on day 5. Colitis was characterized by estimating myeloperoxidase (MPO) activity, body weight, and histological scores. NCX-1 mRNA and protein variants were confirmed by RT-PCR coupled nucleotide sequencing and by Western blot analysis, respectively. Contractility of the colon segments was studied using standard procedure. There was a significant reduction in body weight of TNBS-treated rats. A significant increase in MPO activity and infiltration of inflammatory cells were observed in the inflamed rat colon. RT-PCR coupled nucleotide sequencing identified NCX-1.3 mRNA variant containing exons B and D. Western blot analysis confirmed 70 and 120 kDa molecular mass NCX-1 protein variants in rat colon. There was no significant difference (p > 0.05) in the level of NCX-1 protein variants in inflamed colon as compared to non-colitis controls. Functional experiments demonstrated that NCX in reverse mode played a role in carbachol-induced contraction of colon, and this was not affected by colitis. These findings demonstrated expression of a NCX-1.3 mRNA splice variant, and 70 and 118 kDa protein variants. Inhibition of the reverse mode of NCX-1 was not different in reduced carbachol-induced contraction between the groups. These findings are interpreted to suggest that NCX-1, though expressed did not play a role in reduced contractility in experimental colitis.

Ca2+ regulation of ion transport in the Na+/Ca2+ exchanger

The binding of Ca(2+) to two adjacent Ca(2+)-binding domains, CBD1 and CBD2, regulates ion transport in the Na(+)/Ca(2+) exchanger. As sensors for intracellular Ca(2+), the CBDs form electrostatic switches that induce the conformational changes required to initiate and sustain Na(+)/Ca(2+) exchange. Depending on the presence of a few key residues in the Ca(2+)-binding sites, zero to four Ca(2+) ions can bind with affinities between 0.1 to 20 μm. Importantly, variability in CBD2 as a consequence of alternative splicing modulates not only the number and affinities of the Ca(2+)-binding sites in CBD2 but also the Ca(2+) affinities in CBD1.

Triple threat: the Na+/Ca2+ exchanger in the pathophysiology of cardiac arrhythmia, ischemia and heart failure

The Na(+)/Ca(2+) exchanger (NCX) is the main Ca(2+) extrusion mechanism of the cardiac myocyte and thus is crucial for maintaining Ca(2+) homeostasis. It is involved in the regulation of several parameters of cardiac excitation contraction coupling, such as cytosolic Ca(2+) concentration, repolarization and contractility. Increased NCX activity has been identified as a mechanism promoting heart failure, cardiac ischemia and arrhythmia. Transgenic mice as well as pharmacological interventions have been used to support the idea of using NCX inhibition as a future pharmacological strategy to treat cardiovascular disease.

Reverse mode Na+/Ca2+ exchange mediated by STIM1 contributes to Ca2+ influx in airway smooth muscle following agonist stimulation

BACKGROUND

Agonist stimulation of airway smooth muscle (ASM) results in IP3 mediated Ca2+ release from the sarcoplasmic reticulum followed by the activation of store operated and receptor operated non-selective cation channels. Activation of these non-selective channels also results in a Na+ influx. This localised increase in Na+ levels can potentially switch the Na+/Ca2+ exchanger into reverse mode and so result in a further influx of Ca2+. The aim of this study was to characterise the expression and physiological function of the Na+/Ca2+ exchanger in cultured human bronchial smooth muscle cells and determine its contribution to agonist induced Ca2+ influx into these cells.

METHODS

The expression profile of NCX (which encodes the Na+/Ca2+ exchanger) homologues in cultured human bronchial smooth muscle cells was determined by reverse transcriptase PCR. The functional activity of reverse mode NCX was investigated using a combination of whole cell patch clamp, intracellular Ca2+ measurements and porcine airway contractile analyses. KB-R7943 (an antagonist for reverse mode NCX) and target specific siRNA were utilised as tools to inhibit NCX function.

RESULTS

NCX1 protein was detected in cultured human bronchial smooth muscle cells (HBSMC) cells and NCX1.3 was the only mRNA transcript variant detected. A combination of intracellular Na+ loading and addition of extracellular Ca2+ induced an outwardly rectifying current which was augmented following stimulation with histamine. This outwardly rectifying current was inhibited by 10 μM KB-R7943 (an antagonist of reverse mode NCX1) and was reduced in cells incubated with siRNA against NCX1. Interestingly, this outwardly rectifying current was also inhibited following knockdown of STIM1, suggesting for the first time a link between store operated cation entry and NCX1 activation. In addition, 10 μM KB-R7943 inhibited agonist induced changes in cytosolic Ca2+ and induced relaxation of porcine peripheral airways.

CONCLUSIONS

Taken together, these data demonstrate a potentially important role for NCX1 in control of Ca2+ homeostasis and link store depletion via STIM1 directly with NCX activation.

Reduced expression of the Na+/Ca2+ exchanger in adult cardiomyocytes via adenovirally delivered shRNA results in resistance to simulated ischemic injury

The Na+/Ca2+ exchanger (NCX) is proposed to be an important protein in the regulation of Ca2+ movements in the heart. This Ca2+ regulatory action is thought to modulate contractile activity in the heart under normal physiological conditions and may contribute to the Ca2+ overload that occurs during ischemic reperfusion challenge. To evaluate these hypotheses, adult rat cardiomyocytes were exposed to an adenovirus that codes for short hairpin RNA (shRNA) targeting NCX gene expression through RNA interference. An adenovirus transcribing a short RNA with a scrambled nucleotide sequence was compared with the NCX-shRNA nucleotide sequence and used as a control. Freshly isolated rat cardiomyocytes were infected with virus for 48 h before examination. Cardiomyocytes maintained their characteristic morphological appearance during this short time period after isolation. NCX expression was inhibited by up to ∼60% by the shRNA treatment as determined by Western blot analysis. The depletion in NCX protein was accompanied by a significant depression of NCX activity in shRNA-treated cells. Ca2+ homeostasis was unaltered in the shRNA-treated cells upon electrical stimulation compared with control cells. However, when cardiomyocytes were exposed to a simulated ischemic solution, NCX-depleted cells were significantly protected from the rise in cytoplasmic Ca2+ and damage that was detected in control cells during ischemia and reperfusion. Our data support the role for NCX in ischemic injury to the heart and demonstrate the usefulness of altering gene expression with an adenoviral-delivery system of shRNA in adult cardiomyocytes.

Hypervariable intronic region in NCX1 is enriched in short insertion-deletion polymorphisms and showed association with cardiovascular traits

Background

Conserved non-coding regions (CNR) have been shown to harbor gene expression regulatory elements. Genetic variations in these regions may potentially contribute to complex disease susceptibility.

Methods

We targeted CNRs of cardiovascular disease (CVD) candidate gene, Na(+)-Ca(2+) exchanger (NCX1) with polymorphism screening among CVD patients (n = 46) using DHPLC technology. The flanking region (348 bp) of the 14 bp indel in intron 2 was further genotyped by DGGE assay in two Eastern-European CVD samples: essential hypertension (HYPEST; 470 cases, 652 controls) and coronary artery disease, CAD (CADCZ; 257 cases, controls 413). Genotype-phenotype associations were tested by regression analysis implemented in PLINK. Alignments of primate sequences were performed by ClustalW2.

Results

Nine of the identified NCX1 variants were either singletons or targeted by commercial platforms. The 14 bp intronic indel (rs11274804) was represented with substantial frequency in HYPEST (6.82%) and CADCZ (14.58%). Genotyping in Eastern-Europeans (n = 1792) revealed hypervariable nature of this locus, represented by seven alternative alleles. The alignments of human-chimpanzee-macaque sequences showed that the major human variant (allele frequency 90.45%) was actually a human-specific deletion compared to other primates. In humans, this deletion was surrounded by other short (5-43 bp) deletion variants and a duplication (40 bp) polymorphism possessing overlapping breakpoints. This indicates a potential indel hotspot, triggered by the initial deletion in human lineage. An association was detected between the carrier status of 14 bp indel ancestral allele and CAD (P = 0.0016, OR = 2.02; Bonferroni significance level alpha = 0.0045), but not with hypertension. The risk for the CAD development was even higher among the patients additionally diagnosed with metabolic syndrome (P = 0.0014, OR = 2.34). Consistent with the effect on metabolic processes, suggestive evidence for the association with heart rate, serum triglyceride and LDL levels was detected (P = 0.04).

Conclusions

Compared to SNPs targeted by large number of locus-specific and genome-wide assays, considerably less attention has been paid to short indel variants in the human genome. The data of genome dynamics, mutation rate and population genetics of short indels, as well as their impact on gene expressional profile and human disease susceptibility is limited. The characterization of NCX1 intronic hypervariable non-coding region enriched in human-specific indel variants contributes to this gap of knowledge.

Ca2+ regulation in the Na+/Ca2+ exchanger features a dual electrostatic switch mechanism

Regulation of ion-transport in the Na(+)/Ca(2+) exchanger (NCX) occurs via its cytoplasmic Ca(2+)-binding domains, CBD1 and CBD2. Here, we present a mechanism for NCX activation and inactivation based on data obtained using NMR, isothermal titration calorimetry (ITC) and small-angle X-ray scattering (SAXS). We initially determined the structure of the Ca(2+)-free form of CBD2-AD and the structure of CBD2-BD that represent the two major splice variant classes in NCX1. Although the apo-form of CBD2-AD displays partially disordered Ca(2+)-binding sites, those of CBD2-BD are entirely unstructured even in an excess of Ca(2+). Striking differences in the electrostatic potential between the Ca(2+)-bound and -free forms strongly suggest that Ca(2+)-binding sites in CBD1 and CBD2 form electrostatic switches analogous to C(2)-domains. SAXS analysis of a construct containing CBD1 and CBD2 reveals a conformational change mediated by Ca(2+)-binding to CBD1. We propose that the electrostatic switch in CBD1 and the associated conformational change are necessary for exchanger activation. The response of the CBD1 switch to intracellular Ca(2+) is influenced by the closely located cassette exons. We further propose that Ca(2+)-binding to CBD2 induces a second electrostatic switch, required to alleviate Na(+)-dependent inactivation of Na(+)/Ca(2+) exchange. In contrast to CBD1, the electrostatic switch in CBD2 is isoform- and splice variant-specific and allows for tailored exchange activities.

Sodium-calcium exchange mediated contraction in left anterior descending and left ventricular branch arteries

We tested the hypothesis that the de-endothelialized artery rings from the left anterior descending (LAD) coronary artery and its left ventricular branch (LVB) differ in their contractile responses to Na⁺–Ca²⁺-exchanger (NCX) mediated Ca²⁺-entry, muscarinic receptor activation with carbachol, and sarco/endoplasmic reticulum Ca²⁺ pump (SERCA) inhibition with thapsigargin. In LVB, the force of contraction (in N/g tissue) produced by the NCX mediated Ca²⁺-entry (17.5 ± 1.4) and carbachol (18 ± 1.5) was only slightly smaller than that due to membrane depolarization with KCl (24.0 ± 1.0). In contrast, in LAD the force of contraction produced with NCX (8.7 ± 0.7) and carbachol (6.1 ± 1.1) was much smaller than with KCl (15.7 ± 0.7). Thapsigargin also contracted LVB with greater force than LAD. When isolated microsomes were used, the binding to the muscarinic receptor antagonist quinuclidinyl benzilate was greater in LVB than in LAD. Microsomes were also used for Western blots. The intensities of signals for both SERCA and NCX were greater in LVB than in LAD. These biochemical observations were consistent with the contractile experiments. Thus, it appears that the differences between LAD and the resistance arteries may begin as early as LVB.

The Na+/Ca2+ exchange inhibitor 2-(2-(4-(4-nitrobenzyloxy)phenyl)ethyl)isothiourea methanesulfonate (KB-R7943) also blocks ryanodine receptors type 1 (RyR1) and type 2 (RyR2) channels

Na(+)/Ca(2+) exchanger (NCX) is a plasma membrane transporter that moves Ca(2+) in or out of the cell, depending on membrane potential and transmembrane ion gradients. NCX is the main pathway for Ca(2+) extrusion from excitable cells. NCX inhibitors can ameliorate cardiac ischemia-reperfusion injury and promote high-frequency fatigue of skeletal muscle, purportedly by inhibiting the Ca(2+) inward mode of NCX. Here we tested two known NCX inhibitors, 2-(2-(4-(4-nitrobenzyloxy)phenyl)ethyl)-isothiourea methanesulfonate (KB-R7943) and the structurally related 2-[[4-[(4-Nitrophenyl)methoxy]phenyl]methyl]-4-thiazoli dinecarboxylic acid ethyl ester (SN-6), for their influence on electrically or caffeine-evoked Ca(2+) transients in adult dissociated flexor digitorum brevis (FDB) skeletal muscle fibers and human embryonic kidney (HEK) 293 cells that have stable expression of type 1 ryanodine receptor (RyR1). KB-R7943 (< or = 10 microM) reversibly attenuates electrically evoked Ca(2+) transients in FDB and caffeine-induced Ca(2+) release in HEK 293, whereas the structurally related NCX inhibitor SN-6 does not, suggesting that KB-R7943 directly inhibits RyR1. In support of this interpretation, KB-R7943 inhibits high-affinity binding of [(3)H]ryanodine to RyR1 (IC(50) = 5.1 +/- 0.9 microM) and the cardiac isoform RyR2 (IC(50) = 13.4 +/- 1.8 microM). KB-R7943 interfered with the gating of reconstituted RyR1 and RyR2 channels, reducing open probability (P(o)), shortening mean open time, and prolonging mean closed time. KB-R7943 was more effective at blocking RyR1 with cytoplasmic conditions favoring high P(o) compared with those favoring low P(o). SN-6 has negligible activity toward altering [(3)H]ryanodine binding of RyR1 and RyR2. Our results identify that KB-R7943 is a reversible, activity-dependent blocker of the two most broadly expressed RyR channel isoforms and contributes to its pharmacological and therapeutic activities.

Gene structure evolution of the Na+-Ca2+ exchanger (NCX) family

Background

The Na⁺-Ca²⁺ exchanger (NCX) is an important regulator of cytosolic Ca²⁺ levels. Many of its structural features are highly conserved across a wide range of species. Invertebrates have a single NCX gene, whereas vertebrate species have multiple NCX genes as a result of at least two duplication events. To examine the molecular evolution of NCX genes and understand the role of duplicated genes in the evolution of the vertebrate NCX gene family, we carried out phylogenetic analyses of NCX genes and compared NCX gene structures from sequenced genomes and individual clones.

Results

A single NCX in invertebrates and the protochordate Ciona, and the presence of at least four NCX genes in the genomes of teleosts, an amphibian, and a reptile suggest that a four member gene family arose in a basal vertebrate. Extensive examination of mammalian and avian genomes and synteny analysis argue that NCX4 may be lost in these lineages. Duplicates for NCX1, NCX2, and NCX4 were found in all sequenced teleost genomes. The presence of seven genes encoding NCX homologs may provide teleosts with the functional specialization analogous to the alternate splicing strategy seen with the three NCX mammalian homologs.

Conclusion

We have demonstrated that NCX4 is present in teleost, amphibian and reptilian species but has been secondarily and independently lost in mammals and birds. Comparative studies on conserved vertebrate homologs have provided a possible evolutionary route taken by gene duplicates subfunctionalization by minimizing homolog number.

Inhibitive effects of enhanced lipid peroxidation on Ca(2+)-ATPase in lenses of hereditary cataract ICR/f rats

Our previous studies have demonstrated that the instillation of eye drops containing disulfiram, a radical scavenger and nitric oxide synthase inhibitor, delays cataract development in ICR/f rats, and we have suggested that the production of nitric oxide (NO) and lipid peroxide (LPO) in the lens may relate to the delay in cataract development brought about by disulfiram. However, the involvement of NO and LPO in lenses of ICR/f rats during cataract development has not yet been established. In the present study, we determined changes in NO and LPO levels in lenses of ICR/f rats during cataract development. Opacification of ICR/f rat lenses started at 77 days of age, and the lenses of 91-day-old ICR/f rats were almost entirely opaque. The Ca(2+)-ATPase activity in the lenses of ICR/f rats decreased with increasing age, and an elevation in Ca(2+) content was observed in ICR/f rat lenses with the decrease in Ca(2+)-ATPase activity. NO levels in the lenses of ICR/f rats increased from 63 to 85 days of age, reaching a maximum at 77 days of age. In addition, LPO levels in the lenses of ICR/f rats also increased with increasing age. LPO levels in the lenses of 63- to 91-day-old ICR/f rats were found to be significantly higher compared with those in 22-day-old ICR/f rats. These changes of Ca(2+), Ca(2+)-ATPase, NO and LPO were attenuated by instillation of DSF eye drops. These results suggest that excessive NO may cause enhanced lipid peroxidation resulting in the inhibition of Ca(2+)-ATPase. The decrease in Ca(2+)-ATPase activity may cause the elevation in lens Ca(2+), leading to lens opacification in ICR/f rats.

Identification and characterization of a novel alternative splice variant of mouse GMx33alpha/GPP34

We have cloned and characterized a novel splice variant of mouse GMx33alpha/Golgi-associated protein of 34 kDa (GPP34), hereby designated GMx33alphaV/GPP34V. This splice variant skips the second and third exons, and the resulting frame shift generates a stop codon in the fourth exon. GMx33alphaV/GPP34V is comprised of 81 amino acid residues derived from the N-terminal end of the full length protein and corresponds to approximately one-third of the full length GMx33alpha/GPP34 sequence with a calculated molecular mass of 8900. In contrast to GMx33alpha/GPP34 mRNA which is expressed at similar levels in various tissues, GMx33alphaV/GPP34V mRNA was differentially expressed when examined by RT-PCR. Compared to other tissues, skeletal muscle showed relatively strong expression of GMx33alphaV/GPP34V mRNA. This splice variant cDNA was also detected in a human cell line.

Ca2+-pumps and Na2+-Ca2+-exchangers in coronary artery endothelium versus smooth muscle

Vascular endothelial cells (EC) and smooth muscle cells (SMC) require a decrease in cytoplasmic Ca²⁺ concentration after activation. This can be achieved by Ca²⁺ sequestration by the sarco-/endoplasmic reticulum Ca²⁺ pumps (SERCA) and Ca²⁺ extrusion by plasma membrane Ca²⁺ pumps (PMCA) and Na⁺–Ca²⁺-exchangers (NCX). Since the two cell types differ in their structure and function, we compared the activities of PMCA, NCX and SERCA in pig coronary artery EC and SMC, the types of isoforms expressed using RT-PCR, and their protein abundance using Western blots. The activity of NCX is higher in EC than in SMC but those of PMCA and SERCA is lower. Consistently, the protein abundance for NCX protein is higher in EC than in SMC and those of PMCA and SERCA is lower. Based on RT-PCR experiments, the types of RNA present are as follows: EC for PMCA1 while SMC for PMCA4 and PMCA1; EC for SERCA2 and SERCA3 and SMC for SERCA2. Both EC and SMC express NCX1 (mainly NCX1.3). PMCA, SERCA and NCX differ in their affinities for Ca²⁺ and regulation. Based on these observations and the literature, we conclude that the tightly regulated Ca²⁺ removal systems in SMC are consistent with the cyclical control of contractility of the filaments and those in EC are consistent with Ca²⁺ regulation of the endothelial nitric oxide synthase near the cell surface. The differences between EC and SMC should be considered in therapeutic interventions of cardiovascular diseases.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

BHK cells transfected with NCX3 are more resistant to hypoxia followed by reoxygenation than those transfected with NCX1 and NCX2: Possible relationship with mitochondrial membrane potential

The specific role played by NCX1, NCX2, and NCX3, the three isoforms of the Na+/Ca2+ exchanger (NCX), has been explored during hypoxic conditions in BHK cells stably transfected with each of these isoforms. Six major findings emerged from the present study: (1) all the three isoforms were highly expressed on the plasma membranes of BHK cells; (2) under physiological conditions, the three NCX isoforms showed similar functional activity; (3) hypoxia plus reoxygenation induced a lower increase of [Ca2+]i in BHK-NCX3-transfected cells than in BHK-NCX1- and BHK-NCX2-transfected cells; (4) NCX3-transfected cells were more resistant to chemical hypoxia plus reoxygenation than NCX1- and NCX2-transfected cells. Interestingly, such augmented resistance was eliminated by CBDMD (10 microM), an inhibitor of NCX and by the specific silencing of the NCX3 isoform; (5) chemical hypoxia plus reoxygenation produced a loss of mitochondrial membrane potential in NCX1- and NCX2-transfected cells, but not in NCX3-transfected cells; (6) the forward mode of operation in NCX3-transfected cells was not affected by ATP depletion, as it occurred in NCX1- and NCX2-transfected cells. Altogether, these results indicate that the brain specifically expressed NCX3 isoform more significantly contributes to the maintenance of [Ca2+]i homeostasis during experimental conditions mimicking ischemia, thereby preventing mitochondrial delta psi collapses and cell death.

Changes in regulation of sodium/calcium exchanger of avian ventricular heart cells during embryonic development

It has been suggested that the sodium/calcium exchanger NCX1 may have a more important physiological role in embryonic and neonatal hearts than in adult hearts. However, in chick heart sarcolemmal vesicles, sodium-dependent calcium transport is reported to be small and, moreover, to be 3-12 times smaller in hearts at embryonic day (ED) 4-5 than at ED18, the opposite of what would be expected of a transporter that is more important in early development. To better assess the role of NCX1 in calcium regulation in the chick embryonic heart, we measured the activity of NCX1 in chick embryonic hearts as extracellular calcium-activated exchanger current (I(NCX)) under controlled ionic conditions. With intracellular calcium concentration ([Ca(2+)](i)) = 47 nM, I(NCX) density increased from 1.34 +/- 0.28 pA/pF at ED2 to 3.22 +/- 0.55 pA/pF at ED11 (P = 0.006); however, with [Ca(2+)](i) = 481 nM, the increase was small and statistically insignificant, from 4.54 +/- 0.77 to 5.88 +/- 0.73 pA/pF (P = 0.20, membrane potential = 0 mV, extracellular calcium concentration = 2 mM). Plots of I(NCX) density against [Ca(2+)](i) were well fitted by the Michaelis-Menton equation and extrapolated to identical maximal currents for ED2 and ED11 cells (extracellular calcium concentration = 1, 2, or 4 mM). Thus the increase in I(NCX) at low [Ca(2+)](i) appeared to reflect a developmental change in allosteric regulation of the exchanger by intracellular calcium rather than an increase in the membrane density of NCX1. Supporting this conclusion, RT-PCR demonstrated little change in the amount of mRNA encoding NCX1 expression from ED2 through ED18.