Sodium-calcium exchange: a molecular perspective

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

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

Mechanism of asynchronous Ca(2+) waves underlying agonist-induced contraction in the rat basilar artery

BACKGROUND AND PURPOSE

Uridine 5'-triphosphate (UTP) is a potent vasoconstrictor of cerebral arteries and induces Ca(2+) waves in vascular smooth muscle cells (VSMCs). This study aimed to determine the mechanisms underlying UTP-induced Ca(2+) waves in VSMCs of the rat basilar artery.

EXPERIMENTAL APPROACH

Isometric force and intracellular Ca(2+) ([Ca(2+)](i)) were measured in endothelium-denuded rat basilar artery using wire myography and confocal microscopy respectively.

KEY RESULTS

Uridine 5'-triphosphate (0.1-1000 micromol.L(-1)) concentration-dependently induced tonic contraction (pEC(50) = 4.34 +/- 0.13), associated with sustained repetitive oscillations in [Ca(2+)](i) propagating along the length of the VSMCs as asynchronized Ca(2+) waves. Inhibition of Ca(2+) reuptake in sarcoplasmic reticulum (SR) by cyclopiazonic acid abolished the Ca(2+) waves and resulted in a dramatic drop in tonic contraction. Nifedipine reduced the frequency of Ca(2+) waves by 40% and tonic contraction by 52%, and the nifedipine-insensitive component was abolished by SKF-96365, an inhibitor of receptor- and store-operated channels, and KB-R7943, an inhibitor of reverse-mode Na(+)/Ca(2+) exchange. Ongoing Ca(2+) waves and tonic contraction were also abolished after blockade of inositol-1,4,5-triphosphate-sensitive receptors by 2-aminoethoxydiphenylborate, but not by high concentrations of ryanodine or tetracaine. However, depletion of ryanodine-sensitive SR Ca(2+) stores prior to UTP stimulation prevented Ca(2+) waves.

CONCLUSIONS AND IMPLICATIONS

Uridine 5'-triphosphate-induced Ca(2+) waves may underlie tonic contraction and appear to be produced by repetitive cycles of regenerative Ca(2+) release from the SR through inositol-1,4,5-triphosphate-sensitive receptors. Maintenance of Ca(2+) waves requires SR Ca(2+) reuptake from Ca(2+) entry across the plasma membrane via L-type Ca(2+) channels, receptor- and store-operated channels, and reverse-mode Na(+)/Ca(2+) exchange.

The positive force-frequency relationship is maintained in absence of sarcoplasmic reticulum function in rabbit, but not in rat myocardium

Myocardial calcium handling differs between species, mainly in the relative contribution between the sources for activator calcium. To investigate the role of the myofilaments and intracellular calcium decline in governing the relaxation phase of cardiac muscle, and to elucidate additional determinants of relaxation other than the sarcoplasmic reticulum (SR) at various frequencies within the in vivo range, the present study was performed by altering the calcium handling in rat and rabbit. Trabeculae, iontophoretically loaded with bis-fura-2 to monitor cytoplasmic calcium levels, were subjected to ryanodine and cyclopiazonic acid to inhibit SR function. Simultaneous force and [Ca(2+)](i) measurements were obtained at 1-4 Hz in rabbit and at 4-8 Hz in rat before and after SR inhibition. Inhibition of the SR resulted in increased diastolic and peak calcium levels as well as decreased developed force in both species. Calcium transient amplitude decreased in rat, but increased in rabbit after SR inhibition. Time to peak tension, time from peak tension to 50% relaxation, time to peak calcium, and time from peak calcium to 50% calcium decline were all prolonged. Results suggest that L-type calcium channel current is responsible for increases in calcium with increasing frequency, and that the SR amplifies this effect in response to increased L-type current. The response of the myofilaments to alterations in calcium handling plays a critical role in the final determination of force, and may differ between species. These results imply the balance between force relaxation and calcium decline is significantly different in larger mammals, necessitating a critical re-evaluation of how myocardial relaxation is governed, specifically regarding frequency-dependent activation.

Role of calcium in regulating primary sensory neuronal excitability

The fundamental role of calcium ions (Ca(2+)) in an excitable tissue, the frog heart, was first demonstrated in a series of classical reports by Sydney Ringer in the latter part of the nineteenth century (1882a, b; 1893a, b). Even so, nearly a century elapsed before it was proven that Ca(2+) regulated the excitability of primary sensory neurons. In this chapter we review the sites and mechanisms whereby internal and external Ca(2+) can directly or indirectly alter the excitability of primary sensory neurons: excitability changes being manifested typically by variations in shape of the action potential or the pattern of its discharge.

Crystallization and preliminary X-ray crystallographic analysis of Ca2+-free primary Ca2+-sensor of Na+/Ca2+ exchanger

The plasma-membrane Na(+)/Ca(2+) exchanger (NCX) regulates intracellular Ca(2+) levels in cardiac myocytes. Two Ca(2+)-binding domains (CBD1 and CBD2) exist in the large cytosolic loop of NCX. The binding of Ca(2+) to CBD1 results in conformational changes that stimulate exchange to exclude Ca(2+) ions, whereas CBD2 maintains the structure, suggesting that CBD1 is the primary Ca(2+)-sensor. In order to clarify the structural scaffold for the Ca(2+)-induced conformational transition of CBD1 at the atomic level, X-ray structural analysis of its Ca(2+)-free form was attempted; the structure of the Ca(2+)-bound form is already available. Recombinant CBD1 (NCX1 372-508) with a molecular weight of 16 kDa was crystallized by the sitting-drop vapour-diffusion method at 293 K. The crystals belonged to the hexagonal space group P6(2)22 or P6(4)22, with unit-cell parameters a = b = 56.99, c = 153.86 A, beta = 120 degrees , and contained one molecule per asymmetric unit (V(M) = 2.25 A(3) Da(-1)) with a solvent content of about 55% (V(S) = 45.57%). Diffraction data were collected within the resolution range 27.72-3.00 A using an R-AXIS detector and gave a data set with an overall R(merge) of 10.8% and a completeness of 92.8%.

Functional linkage of Na+-Ca2+-exchanger to sarco/endoplasmic reticulum Ca2+ pump in coronary artery: comparison of smooth muscle and endothelial cells

An increase in cytosolic Ca(2+) concentration in coronary artery smooth muscle causes a contraction but in endothelium it causes relaxation. Na(+)-Ca(2+)-exchanger (NCX) may play a role in Ca(2+) dynamics in both the cell types. Here, the NCX-mediated (45)Ca(2+) uptake was compared in Na(+)-loaded pig coronary artery smooth muscle and endothelial cells. In both the cell types, this uptake was inhibited by KB-R7943, SEA 0400 and by monensin, but not by cariporide. Prior loading of the cells with the Ca(2+) chelator BAPTA increased the NCX-mediated (45)Ca(2+) uptake in smooth muscle but not in endothelial cells. In the presence or absence of BAPTA loading, the Na(+)-mediated (45)Ca(2+) uptake was greater in endothelial than in smooth muscle cells. In smooth muscle cells without BAPTA loading, thapsigargin diminished the NCX-mediated (45)Ca(2+) entry. This effect was not observed in endothelial cells or in either cell type after BAPTA loading. The results in the smooth muscle cells are consistent with a limited diffusional space model in which the NCX-mediated (45)Ca(2+) uptake was enhanced by chelation of cytosolic Ca(2+) or by its sequestration by the sarco/endoplasmic reticulum Ca(2+) pump (SERCA). They suggest a functional linkage between NCX and SERCA in the smooth muscle but not in the endothelial cells. The concept of a linkage between NCX and SERCA in smooth muscle was also confirmed by similar distribution of NCX and SERCA2 proteins when detergent-treated microsomes were fractionated by flotation on sucrose density gradients. Thus, the coronary artery smooth muscle and endothelial cells differ not only in the relative activities of NCX but also in its functional linkage to SERCA.

Rapid downregulation of the Ca2+-signal after exocytosis stimulation in Paramecium cells: essential role of a centrin-rich filamentous cortical network, the infraciliary lattice

We analysed in Paramecium tetraurelia cells the role of the infraciliary lattice, a cytoskeletal network containing numerous centrin isoforms tightly bound to large binding proteins, in the re-establishment of Ca2+ homeostasis following exocytosis stimulation. The wild type strain d4-2 has been compared with the mutant cell line Delta-PtCenBP1 which is devoid of the infraciliary lattice ("Delta-PtCenBP1" cells). Exocytosis is known to involve the mobilization of cortical Ca2+-stores and a superimposed Ca2+-influx and was analysed using Fura Red ratio imaging. No difference in the initial signal generation was found between wild type and Delta-PtCenBP1 cells. In contrast, decay time was greatly increased in Delta-PtCenBP1 cells particularly when stimulated, e.g., in presence of 1mM extracellular Ca2+, [Ca2+]o. Apparent halftimes of f/f0 decrease were 8.5 s in wild type and approximately 125 s in Delta-PtCenBP1 cells, requiring approximately 30 s and approximately 180 s, respectively, to re-establish intracellular [Ca2+] homeostasis. Lowering [Ca2+]o to 0.1 and 0.01 mM caused an acceleration of intracellular [Ca2+] decay to t(1/2)=33 s and 28 s, respectively, in Delta-PtCenBP1 cells as compared to 8.1 and 5.6, respectively, for wild type cells. We conclude that, in Paramecium cells, the infraciliary lattice is the most efficient endogenous Ca2+ buffering system allowing the rapid downregulation of Ca2+ signals after exocytosis stimulation.

Regulation of cardiac Na+-Ca2+ exchanger activity by protein kinase phosphorylation--still a paradox?

The cardiac Na+-Ca2+ exchanger (NCX) is an important regulator of intracellular ion homeostasis and cardiac function. Gaining insight into modulation of the NCX is therefore important in order to understand ion handling in the heart under physiological and pathological conditions. Typically, the functional contribution of the NCX is often regarded as "secondary" to the changes in luminal Na+ and Ca2+. Whilst it is well accepted that the NCX can be regulated by various factors, including the concentrations of transported ions, direct receptor-mediated modulation of the cardiac NCX is more controversial. Evidence from several different laboratories supports the notion that the cardiac NCX is a direct target of neurotransmitters and hormones and their downstream signalling pathways; however, the issue remains unresolved due to conflicting data showing a lack of direct modulation. The present review summarizes overall findings regarding the modulation of the cardiac NCX, in particular on molecular mechanisms of direct phosphorylation of NCX by beta-adrenergic/adenylate cyclase/protein kinase A and (for comparative purposes) on endothelin-1/protein kinase C signalling pathways. It also aims to consider whether it is currently possible to reconcile discrepancies between studies in the interpretation of the regulation of the cardiac NCX by agents stimulating the beta-adrenoceptor/PKA pathway.

Direct Loading of the purified endogenous inhibitor into the cytoplasm of patched cardiomyocytes blocks the ion currents and calcium transport through the NCX1 protein

The Na(+)-Ca(2+) exchanger in mammalian heart muscle (NCX1) is the central transporter protein that regulates extrusion of Ca(2+) from the heart cell. However, the functional biochemistry and physiology of NCX1 have been severely hampered by the absence of any specific high-affinity inhibitor. Here we describe advanced procedures for purifying a candidate inhibitor, previously called endogenous inhibitor factor (NCX(IF)), and demonstrate its direct actions on NCX1 activities in the single-cell system. A combination of advanced HILIC (hydrophilic interaction liquid chromatography) procedures with analytical tests suggests that the properties of NCX(IF) resemble those of a small (disaccharide size) polar molecule lacking any aromatic rings, conjugated bonds, or a primary amino group. The effects of NCX(IF) on the NCX1-mediated ion currents (I(NCX)) and cytosolic Ca(2+) extrusion were detected by a combination of patch-clamp and confocal microscopy under conditions in which the purified NCX(IF) was directly loaded into the cytoplasm of patched cardiomyocytes. It was demonstrated that cytosolic NCX(IF) blocks the Ca(2+)-activated NCX1 inward current and the accompanying extrusion of Ca(2+) from the cell with high efficacy. A constant fraction of NCX1 inhibition was observed under conditions in which the cytosolic [Ca(2+)](i) was varied at fixed doses of NCX(IF), suggesting that the degree of inhibition is controlled by NCX(IF) dose and not by cytosolic Ca(2+) concentration. NCX(IF) blocks equally well both the Ca(2+) extrusion and Ca(2+) entry modes of NCX1, consistent with thermodynamic principles expected for the functioning of a bidirectional "carrier-type" transport system. We concluded that NCX(IF) interacts with a putative regulatory domain from the cytosolic side and, thus, may play an important regulatory role in controlling Ca(2+) signaling in the heart. This may represent a new potential tool for developing novel treatments for cardiac Ca(2+) signaling dysfunction.

NCX3 is a major functional isoform of the sodium-calcium exchanger in osteoblasts

The calcium phosphate-based skeleton of vertebrates serves as the major reservoir for metabolically available calcium ions. The skeleton is formed by osteoblasts which first secrete a proteinaceous matrix and then provide Ca++ for the calcification process. The two calcium efflux ports found in most cells are the plasma membrane Ca-ATPase (PMCA) and the sodium-calcium exchanger (NCX). In osteoblasts, PMCA and NCX are located on opposing sides of the cell with NCX facing the mineralizing bone surface. Two isoforms of NCX have been identified in osteoblasts NCX1, and NCX3. The purpose of this study was to determine the extent to which each of the two NCX isoforms support delivery of Ca++ into sites of calcification and to discern if one could compensate for the other. SiRNA technology was used to knockdown each isoform separately in MC3T3-E1 osteoblasts. Osteoblasts in which either NCX1 or NCX3 was impaired were tested for Ca++ efflux using the Ca++ specific fluorophore, fluo-4, in a sodium-dependent calcium uptake assay adapted for image analysis. NCX3 was found to serve as a major contributor of Ca++ translocation out of osteoblasts into calcifying bone matrix. NCX1 had little to no involvement.

Ca2+ clearance and contractility in vascular smooth muscle: evidence from gene-altered murine models

The central importance of calcium clearance proteins, and their regulators, in the modulation of myocardial contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) has long been established. Key players identified include the Na(+)-Ca(2+) exchanger, the Na(+)-K(+) ATPase, the sarco(endo)plasmic reticulum Ca(2+)-ATPase and associated phospholamban. Gene-targeted and transgenic murine models have been critical in the elucidation of their function. The study of these proteins in the regulation of contractile parameters in vascular smooth muscle, on the other hand, is less well studied. More recently, gene-targeted and transgenic models have expanded our knowledge of Ca(2+) clearance proteins and their role in both tonic and phasic smooth muscle contractility. In this review, we will briefly treat the mechanisms which underlie Ca(2+) clearance in smooth muscle. These will be addressed in light of studies using gene-modified mouse models, the results of which will be compared and contrasted with those in the cardiomyocyte. The recently identified human mutations in phospholamban, which lead to dilated cardiomyopathy, are also present in vascular and other smooth muscle. Given the importance of these Ca(2+) clearance systems to modulation of smooth muscle, it is likely that mutations will also lead to smooth muscle pathology.

Calcium cycling and signaling in cardiac myocytes

Calcium (Ca) is a universal intracellular second messenger. In muscle, Ca is best known for its role in contractile activation. However, in recent years the critical role of Ca in other myocyte processes has become increasingly clear. This review focuses on Ca signaling in cardiac myocytes as pertaining to electrophysiology (including action potentials and arrhythmias), excitation-contraction coupling, modulation of contractile function, energy supply-demand balance (including mitochondrial function), cell death, and transcription regulation. Importantly, although such diverse Ca-dependent regulations occur simultaneously in a cell, the cell can distinguish distinct signals by local Ca or protein complexes and differential Ca signal integration.

Na+/Ca2+ exchanger 2 is neuroprotective by exporting Ca2+ during a transient focal cerebral ischemia in the mouse

Na(+)/Ca(2+) exchanger (NCX), by mediating Na(+) and Ca(2+) fluxes bi-directionally, assumes a role in controlling the Ca(2+) homeostasis in the ischemic brain. It has been suggested that the three isoforms of NCX (NCX1, 2 and 3) may be differentially involved in permanent cerebral ischemia. However, the role of NCX2 has not been defined in ischemic reperfusion injury after a transient focal cerebral ischemia. Furthermore, it is not known whether NCX2 imports or exports intracellular Ca(2+) ([Ca(2+)](i)) following ischemia and reperfusion. To define the role of NCX2 in ischemia and reperfusion, we examined mice lacking NCX2, in vivo and in vitro. After an in vitro ischemia, a significantly slower recovery in population spike amplitudes, a sustained elevation of [Ca(2+)](i) and an increased membrane depolarization were developed in the NCX2-deficient hippocampus. Moreover, a transient focal cerebral ischemia in vivo produced a larger infarction and more cell death in the NCX2-deficient mouse brain. In particular, in the wild type brain, NCX2-expressing neurons were largely spared from cell death after ischemia. Our results suggest that NCX2 exports Ca(2+) in ischemia and thus protects neuronal cells from death by reducing [Ca(2+)](i) in the adult mouse brain.

Insulin effects on cardiac Na+/Ca2+ exchanger activity: role of the cytoplasmic regulatory loop

Insulin can alter myocardial contractility, in part through an effect on the cardiac sarcolemmal Na(+)/Ca(2+) exchanger (NCX), but little is known about its mechanism of action. The large cytoplasmic domain (f-loop) of NCX is required for regulation by various intracellular factors, and we have shown previously that residues 562-679 are determinants of NCX inhibition by exchanger inhibitory peptide (XIP). Here we show that the same f-loop deletion eliminates the enhancement of NCX current by insulin, and we examine the signal pathways involved in the insulin response. NCX current (I(NCX)) was measured in freshly isolated or cultured (up to 48 h) adult guinea pig myocytes and in myocytes expressing canine NCX1.1 with the 562-679 f-loop deletion (NCX-(Delta562-679)) via adenoviral gene transfer. I(NCX) was recorded by whole-cell patch clamp as the Ni(2+)-sensitive current at 37 degrees C with intracellular Ca(2+) buffered. Insulin (1 microm) increased I(NCX) (at +80 mV) by 110 and 83% in fresh and cultured myocytes, respectively, whereas in myocytes expressing NCX-(Delta562-679) the response was eliminated (with 100 microm XIP included to suppress any native guinea pig I(NCX)). The insulin effect on I(NCX) was not inhibited by wortmannin, a nitric-oxide synthase inhibitor, or disruption of caveolae but was blocked by chelerythrine, implicating protein kinase C, but not phosphatidylinositol-3-kinase, in the mechanism. The insulin effect was also not additive with phosphatidylinositol-4,5-bisphosphate-induced activation of I(NCX). The finding that the 562-670 f-loop domain is implicated in both XIP and receptor-mediated modulation of NCX highlights its important role in acute physiological or pathophysiological regulation of Ca(2+) balance in the heart.

Nuclear sphingolipids: metabolism and signaling

Sphingolipids are most prominently expressed in the plasma membrane, but recent studies have pointed to important signaling and regulatory roles in the nucleus. The most abundant nuclear sphingolipid is sphingomyelin (SM), which occurs in the nuclear envelope (NE) as well as intranuclear sites. The major metabolic product of SM is ceramide, which is generated by nuclear sphingomyelinase and triggers apoptosis and other metabolic changes. Ceramide is further hydrolyzed to free fatty acid and sphingosine, the latter undergoing conversion to sphingosine phosphate by action of a specific nuclear kinase. Gangliosides are another type of sphingolipid found in the nucleus, members of the a-series of gangliotetraose gangliosides (GM1, GD1a) occurring in the NE and endonuclear compartments. GM1 in the inner membrane of the NE is tightly associated with a Na(+)/Ca(2+) exchanger whose activity it potentiates, thereby contributing to regulation of Ca(2+) homeostasis in the nucleus. This was shown to exert a cytoprotective role as absence or inactivation of this nuclear complex rendered cells vulnerable to apoptosis. This was demonstrated in the greatly enhanced kainite-induced seizure activity in knockout mice lacking gangliotetraose gangliosides. The pathology included apoptotic destruction of neurons in the CA3 region of the hippocampus. Ca(2+) homeostasis was restored in these animals with LIGA-20, a membrane-permeant derivative of GM1 that entered the NE and activated the nuclear Na(+)/Ca(2+) exchanger. Some evidence suggests the presence of uncharged glycosphingolipids in the nucleus.

Dual effect of Nai+ on Ca2+ influx through the Na+/Ca2+ exchanger in dialyzed squid axons. Experimental data confirming the validity of the squid axon kinetic model

We propose a steady-state kinetic model for the squid Na(+)/Ca(2+) exchanger that differs from other current models of regulation in that it takes into account, within a single kinetic scheme, all ionic [intracellular Ca(2+) (Ca(i)(2+))-intracellular Na(+) (Na(i)(+))-intracellular H(i)(+)] and metabolic (ATP) regulations of the exchanger in which the Ca(i)(2+)-regulatory pathway plays the central role in regulation. Although the integrated ionic-metabolic model predicts all squid steady-state experimental data on exchange regulation, a critical test for the validity of it is the predicted dual effect of Na(i)(+) on steady-state Ca(2+) influx through the exchanger. To test this prediction, an improved technique for the estimation of isotope fluxes in squid axons was developed, which allows sequential measurements of ion influx and effluxes. With this method, we report here two novel observations of the squid axon Na(+)/Ca(2+) exchanger. First, at intracellular pH (7.0) and in the absence of MgATP, Na(i)(+) has a dual effect on Ca(2+) influx: inhibition at low concentrations followed by stimulation at high Na(i)(+) concentrations, reaching levels higher than those seen without Na(i)(+). Second, in the presence of MgATP, the biphasic response to Na(i)(+) disappears and is replaced by a sigmoid activation. Furthermore, the model predicts that Ca(2+) efflux is monotonically inhibited by Na(i)(+), more pronouncedly without than with MgATP. These results are predicted by the proposed kinetic model. Although not fully applicable to all exchangers, this scheme might provide some insights on expected net Ca(2+) movements in other tissues under a variety of intracellular ionic and metabolic conditions.

Targeted disruption of Na+/Ca2+ exchanger 3 (NCX3) gene leads to a worsening of ischemic brain damage

Na+/Ca+ exchanger 3 (NCX3), one of the three isoforms of the NCX family, is highly expressed in the brain and is involved in the maintenance of intracellular Na+ and Ca2+ homeostasis. Interestingly, whereas the function of NCX3 under physiological conditions has been determined, its role under anoxia is still unknown. To assess NCX3 role in cerebral ischemia, we exposed ncx3-/- mice to transient middle cerebral artery occlusion followed by reperfusion. In addition, to evaluate the effect of ncx3 ablation on neuronal survival, organotypic hippocampal cultures and primary cortical neurons from ncx3-/- mice were subjected to oxygen glucose deprivation (OGD) plus reoxygenation. Here we report that ncx3 gene suppression leads to a worsening of brain damage after focal ischemia and to a massive neuronal death in all the hippocampal fields of organotypic cultures as well as in cortical neurons from ncx3-/- mice exposed to OGD plus reoxygenation. In addition, in ncx3-/- cortical neurons exposed to hypoxia, NCX currents, recorded in the reverse mode of operation, were significantly lower than those detected in ncx3+/+. From these results, NCX3 protein emerges as a new molecular target that may have a potential therapeutic value in modulating cerebral ischemia.

Structure and dynamics of Ca2+-binding domain 1 of the Na+/Ca2+ exchanger in the presence and in the absence of Ca2+

The Na(+)/Ca(2+) exchanger is the major exporter of Ca(2+) across the cell membrane of cardiomyocytes. The activity of the exchanger is regulated by a large intracellular loop that contains two Ca(2+)-binding domains, calcium-binding domain (CBD) 1 and CBD2. CBD1 binds Ca(2+) with much higher affinity than CBD2 and is considered to be the primary Ca(2+) sensor. The effect of Ca(2+) on the structure and dynamics of CBD1 has been characterized by NMR spectroscopy using chemical shifts, residual dipolar couplings, and spin relaxation. Residual dipolar couplings are used in a new way for residue selection in the determination of the anisotropic rotational diffusion tensor from spin relaxation data. The results provide a highly consistent description across these complementary data sets and show that Ca(2+) binding is accompanied by a selective conformational change among the binding site residues. Residues that exhibit a significant conformational change are also sites of altered dynamics. In particular, Ca(2+) binding restricts the mobility of the major acidic segment and affects the dynamics of several nearby binding loops. These observations indicate that Ca(2+) elicits a local transition to a well-ordered coordination geometry in the CBD1-binding site.

The participation of the Na+-Ca2+ exchanger in primary cardiac myofibroblast migration, contraction, and proliferation

Cardiac ventricular myofibroblast motility, proliferation, and contraction contribute to post-myocardial infarct wound healing, infarct scar formation, and remodeling of the ventricle remote to the site of infarction. The Na+-Ca2+ exchanger (NCX1) is involved in altered calcium handling in cardiac myocytes during cardiac remodeling associated with heart failure, however, its role in cardiac myofibroblast cell function is unexplored. In this study we investigated the involvement of NCX1 as well as the role of non-selective-cation channels (NSCC) in cardiac myofibroblast cell function in vitro. Immunofluorescence and Western blots revealed that P1 cells upregulate alpha-smooth muscle actin (alphaSMA) and embryonic smooth muscle myosin heavy chain (SMemb) expression. NCX1 mRNA and proteins as well as Ca(v)1.2a protein are also expressed in P1 myofibroblasts. Myofibroblast motility in the presence of 50 ng/ml PDGF-BB was blocked with AG1296. Myofibroblast motility, contraction, and proliferation were sensitive to KB-R7943, a specific NCX1 reverse-mode inhibitor. In contrast, only proliferation and contraction, but not motility were sensitive to nifedipine, while gadolinium (NSCC blocker) was only associated with decreased motility. ML-7 treatment was associated with inhibition of the chemotactic response and contraction. Thus cardiac myofibroblast chemotaxis, contraction, and proliferation were sensitive to different pharmacologic treatments suggesting that regulation of transplasmalemmal calcium movements may be important in growth factor receptor-mediated processes. NCX1 may represent an important moiety in suppression of myofibroblast functions.

GM1 in the nuclear envelope regulates nuclear calcium through association with a nuclear sodium-calcium exchanger

The inner membrane of the nuclear envelope (NE) of neurons and other cells has been shown to contain GM1 tightly associated with a Na(+)/Ca(2+) exchanger (NCX) whose activity it potentiates in mediating transfer of Ca(2+) from nucleoplasm to the NE lumen. This is consistent with localization of the NCX/GM1 complex in the inner membrane of the NE. NCXs of the plasma membrane, in contrast, appear to bind GM1 much less avidly. This is believed due to different isoforms of NCX in the two membranes, and a difference in topology of NCX relative to GM1. A cytoprotective function for nuclear NCX/GM1 was suggested in the observation that cultured neurons from mice lacking GM1 (GM2/GD2 synthase knockout) were vulnerable to Ca(2+)-induced apoptosis. These neurons in culture were rescued to some extent by GM1 but more effectively by LIGA-20, a membrane-permeant derivative of GM1 that entered the NE. Further indication came from a study of the mutant mice, which were highly susceptible to kainate-induced seizures and could be rescued by LIGA-20. This correlated with the ability of LIGA-20 to cross the blood-brain barrier, enter brain cells, insert into the NE, and potentiate nuclear NCX.

In the squid axon Na+/Ca2+ exchanger the state of the Ca i-regulatory site influences the affinities of the intra- and extracellular transport sites for Na+ and Ca2+

In squid axons, intracellular Mg2+ reduces the activity of the Na+/Ca2+ exchanger by competing with Ca2+ i for its regulatory site. The state of the Ca i-regulatory site (active-inactive) also alters the apparent affinity of intra- and extracellular transport sites. Conditions that hinder the binding of Ca2+ i (low pH i, low [Ca2+]i, high [Mg2+]i) diminish the apparent affinity of intracellular transport sites, in particular for Na i due to its synergism with H+ inhibition, but less noticeably for Ca2+ i because of its antagonism towards (Ha i + Na+ i) and Mg2+ i inhibitions. These are kinetic effects unrelated to the true affinity of the sites. With the Ca i-regulatory site saturated, the intracellular transporting sites are insensitive to [H+]i and to ATP. Likewise, the state of the Ca i-regulatory site (activated or inactivated) influences the affinity of the extracellular Ca o and Na o-transport sites (trans effects). In this case, the effects are opposite to those predicted by any of the transport schemes proposed for the Na+/Ca2+exchanger; i.e. its mechanism remains unexplained. In addition to their intrinsic importance for a full understanding of the properties of the Na+/Ca2+ exchanger, these findings show a new way by which the state of the Ca i-regulatory site may determine net movements of Ca2+ through this system.

The regulatory function of plasma-membrane Ca2+-ATPase (PMCA) in the heart

The PMCA (plasma-membrane Ca2+-ATPase) is a ubiquitously expressed calcium-extruding enzymatic pump important in the control of intracellular calcium concentration. Unlike in non-excitable cells, where PMCA is the only system for calcium extrusion, in excitable cells, such as cardiomyocytes, PMCA has been shown to play only a minor role in calcium homoeostasis compared with the NCX (sodium/calcium exchanger), another system of calcium extrusion. However, increasing evidence points to an important role for PMCA in signal transduction; of particular interest in cardiac physiology is the modulation of nNOS (neuronal nitric oxide synthase) by isoform 4b of PMCA. In the present paper, we will discuss recent advances that support a key role for PMCA4 in modulating the nitric oxide signalling pathway in the heart.

Expression and water calcium dependence of calcium transporter isoforms in zebrafish gill mitochondrion-rich cells

Background

Freshwater fish absorb Ca²⁺ predominantly from ambient water, and more than 97% of Ca²⁺ uptake is achieved by active transport through gill mitochondrion-rich (MR) cells. In the current model for Ca²⁺ uptake in gill MR cells, Ca²⁺ passively enters the cytosol via the epithelium Ca²⁺ channel (ECaC), and then is extruded into the plasma through the basolateral Na⁺/Ca²⁺ exchanger (NCX) and plasma membrane Ca²⁺-ATPase (PMCA). However, no convincing molecular or cellular evidence has been available to support the role of specific PMCA and/or NCX isoforms in this model. Zebrafish (Danio rerio) is a good model for analyzing isoforms of a gene because of the plentiful genomic databases and expression sequence tag (EST) data.

Results

Using a strategy of BLAST from the zebrafish genome database (Sanger Institute), 6 isoforms of PMCAs (PMCA1a, PMCA1b, PMCA2, PMCA3a, PMCA3b, and PMCA4) and 7 isoforms of NCXs (NCX1a, NCX1b, NCX2a, NCX2b, NCX3, NCX4a, and NCX4b) were identified. In the reverse-transcriptase polymerase chain reaction (RT-PCR) analysis, 5 PMCAs and 2 NCXs were ubiquitously expressed in various tissues including gills. Triple fluorescence in situ hybridization and immunocytochemistry showed the colocalization of zecac, zpmca2, and zncx1b mRNAs in a portion of gill MR cells (using Na⁺-K⁺-ATPase as the marker), implying a subset of ionocytes specifically responsible for the transepithelial Ca²⁺ uptake in zebrafish gills. The gene expressions in gills of high- or low-Ca²⁺-acclimated zebrafish by quantitative real-time PCR analysis showed that zecac was the only gene regulated in response to environmental Ca²⁺ levels, while zpmcas and zncxs remained steady.

Conclusion

The present study provides molecular evidence for the specific isoforms of Ca²⁺ transporters, zECaC, zPMCA2, and zNCX1b, supporting the current Ca²⁺ uptake model, in which ECaC may play a role as the major regulatory target for this mechanism during environmental challenge.

Hypertrophy and heart failure in mice overexpressing the cardiac sodium-calcium exchanger

BACKGROUND

The cardiac sodium-calcium exchanger (NCX1) is a key sarcolemmal protein for the maintenance of calcium homeostasis in the heart. Because heart failure is associated with increased expression of NCX1, heterozygous (HET) and homozygous (HOM) transgenic mice overexpressing NCX1 were developed and evaluated.

METHODS AND RESULTS

The NCX1 transgenic mice display 2.3-fold (HET) and 3.1-fold (HOM) increases in exchanger activity from wild-type (WT) mice. Functional information was obtained by echocardiography and catheterizations before and after hemodynamic stress from pregnancy, treadmill exercise or transaortic constriction (TAC). HET and HOM mice exhibited hypertrophy and blunted responses with beta-adrenergic stimulation. Postpartum mice from all groups were hypertrophied, but only the HOM mice exhibited premature death from heart failure. HOM mice became exercise intolerant after 6 weeks of daily treadmill running. After 21 days TAC, HET, and HOM mice exhibited significant contractile dysfunction and 15% to 40% mortality with clinical evidence of heart failure.

CONCLUSIONS

Hemodynamic stress results in a compensated hypertrophy in WT mice, but NCX1 transgenic mice exhibit decreased contractile function and heart failure in proportion to their level of NCX1 expression. Thus exchanger overexpression in mice leads to abnormal calcium handling and a decompensatory transition to heart failure with stress.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Calcium and cancer: targeting Ca2+ transport

Ca2+ is a ubiquitous cellular signal. Altered expression of specific Ca2+ channels and pumps are characterizing features of some cancers. The ability of Ca2+ to regulate both cell death and proliferation, combined with the potential for pharmacological modulation, offers the opportunity for a set of new drug targets in cancer. However, the ubiquity of the Ca2+ signal is often mistakenly presumed to thwart the specific therapeutic targeting of proteins that transport Ca2+. This Review presents evidence to the contrary and addresses the question: which Ca2+ channels and pumps should be targeted?

Novel stimulus-induced calcium efflux in Drosophila mushroom bodies

The mushroom body (MB) is an important part of the Drosophila brain, and is involved in many behaviors, including olfactory learning and memory and some visual cognition. However, the physiological properties of MB neurons remain elusive. Here we used a calcium-imaging technique to study calcium signals in Drosophila MB. We found that, rather than increasing calcium spread, electrical stimuli dramatically decreased calcium signals in the terminals of MB fibers. This novel calcium decrease occurred at all developmental stages from larvae to adults, but was specific for certain regions of the MB neurons. GABA receptor blockade promoted calcium propagation through the MB fibers, but did not disrupt the stimulus-induced decrease in calcium. Furthermore, this decrease in calcium was independent of extracellular calcium concentration and was not due to altered uptake by intracellular calcium stores and mitochondria. Rather, we found that inhibition of sodium-calcium exchangers significantly attenuated the stimulus-induced decrease in calcium, whereas the decrease persisted when membrane calcium pumps were blocked. Our findings indicate that MB neurons exhibit a novel stimulus-induced calcium efflux, which may be importantly regulated by sodium-calcium exchangers in the Drosophila MB.

Tissue-specific mRNA expression profiling in grape berry tissues

Background

Berries of grape (Vitis vinifera) contain three major tissue types (skin, pulp and seed) all of which contribute to the aroma, color, and flavor characters of wine. The pericarp, which is composed of the exocarp (skin) and mesocarp (pulp), not only functions to protect and feed the developing seed, but also to assist in the dispersal of the mature seed by avian and mammalian vectors. The skin provides volatile and nonvolatile aroma and color compounds, the pulp contributes organic acids and sugars, and the seeds provide condensed tannins, all of which are important to the formation of organoleptic characteristics of wine. In order to understand the transcriptional network responsible for controlling tissue-specific mRNA expression patterns, mRNA expression profiling was conducted on each tissue of mature berries of V. vinifera Cabernet Sauvignon using the Affymetrix GeneChip^® Vitis oligonucleotide microarray ver. 1.0. In order to monitor the influence of water-deficit stress on tissue-specific expression patterns, mRNA expression profiles were also compared from mature berries harvested from vines subjected to well-watered or water-deficit conditions.

Results

Overall, berry tissues were found to express approximately 76% of genes represented on the Vitis microarray. Approximately 60% of these genes exhibited significant differential expression in one or more of the three major tissue types with more than 28% of genes showing pronounced (2-fold or greater) differences in mRNA expression. The largest difference in tissue-specific expression was observed between the seed and pulp/skin. Exocarp tissue, which is involved in pathogen defense and pigment production, showed higher mRNA abundance relative to other berry tissues for genes involved with flavonoid biosynthesis, pathogen resistance, and cell wall modification. Mesocarp tissue, which is considered a nutritive tissue, exhibited a higher mRNA abundance of genes involved in cell wall function and transport processes. Seeds, which supply essential resources for embryo development, showed higher mRNA abundance of genes encoding phenylpropanoid biosynthetic enzymes, seed storage proteins, and late embryogenesis abundant proteins. Water-deficit stress affected the mRNA abundance of 13% of the genes with differential expression patterns occurring mainly in the pulp and skin. In pulp and seed tissues transcript abundance in most functional categories declined in water-deficit stressed vines relative to well-watered vines with transcripts for storage proteins and novel (no-hit) functional assignments being over represented. In the skin of berries from water-deficit stressed vines, however, transcripts from several functional categories including general phenypropanoid and ethylene metabolism, pathogenesis-related responses, energy, and interaction with the environment were significantly over-represented.

Conclusion

These results revealed novel insights into the tissue-specific expression mRNA expression patterns of an extensive repertoire of genes expressed in berry tissues. This work also establishes an extensive catalogue of gene expression patterns for future investigations aimed at the dissection of the transcriptional regulatory hierarchies that govern tissue-specific expression patterns associated with tissue differentiation within berries. These results also confirmed that water-deficit stress has a profound effect on mRNA expression patterns particularly associated with the biosynthesis of aroma and color metabolites within skin and pulp tissues that ultimately impact wine quality.

A structure-function analysis of ion transport in crustacean gills and excretory organs

Osmotic and ionic regulation in the Crustacea is mostly accomplished by the multifunctional gills, together with the excretory organs. In addition to their role in gas exchange, the gills constitute organs of active, transepithelial, ion transport, an activity of major importance that underlies many essential physiological functions like osmoregulation, calcium homeostasis, ammonium excretion and extracellular pH regulation. This review focuses on structure-function relationships in crustacean gills and excretory effectors, from the organ to molecular levels of organization. We address the diversity of structural architectures encountered in different crustacean gill types, and in constituent cell types, before examining the physiological mechanisms of Na(+), Cl(-), Ca(2+) and NH(4)(+) transport, and of acid-base equivalents, based on findings obtained over the last two decades employing advanced techniques. The antennal and maxillary glands constitute the principal crustacean excretory organs, which have received less attention in functional studies. We examine the diversity present in antennal and maxillary gland architecture, highlighting the structural similarities between both organ types, and we analyze the functions ascribed to each glandular segment. Emphasis is given to volume and osmoregulatory functions, capacity to produce dilute urine in freshwater crustaceans, and the effect of acclimation salinity on urine volume and composition. The microanatomy and diversity of function ascribed to gills and excretory organs are appraised from an evolutionary perspective, and suggestions made as to future avenues of investigation that may elucidate evolutionary and adaptive trends underpinning the invasion and exploitation of novel habitats.

Shedding light on the Na+/Ca2+ exchanger

The Na+/Ca2+ exchanger (NCX) regulates cardiac contractility by adjusting the amount of Ca2+ inside myocytes. NCX accomplishes this by using the electrochemical gradient of Na+: during each cycle three Na+ ions enter the cell and one Ca2+ ion is extruded against its gradient. In addition to being transported, cytoplasmic Na+ and Ca2+ ions also regulate exchanger activity. The physiological relevance and molecular processes underlying ionic regulation remain unclear. Also unresolved are the events that regulate NCX trafficking to the membrane and its oligomeric state. This is essential information to interpret structure-function data. The full-length exchanger was fused to both CFP and YFP, creating active fluorescent exchangers used in FRET experiments to assess both conformational changes associated with ionic regulation and the oligomeric state of NCX. Electrophysiological characterization demonstrates that these constructs behave similarly to the wild-type (WT) exchanger. We have been able for the first time to monitor conformational changes of the exchanger Ca2+-binding site in vivo. These studies provide a better understanding of the molecular properties of the NCX.

Regulation of Na+/Ca2+ exchange current in the normal and failing heart

Cardiac NCX is modulated by diverse regulatory elements. Although there is consensus about the regulatory function of Na+ and Ca2+ and other elements, for example, ATP, there is still a controversial debate about the functional role of cyclic nucleotides and protein kinases. Future studies should focus on that topic since disturbances of cAMP/cGMP concentration and kinase activity may lead to severe functional disorders in the diseased heart. S100A1 is presumably a novel regulator of NCX.

Distinct phenotypes among plasma membrane Ca2+-ATPase knockout mice

Ca2+ gradients across the plasma membrane, required for Ca2+ homeostasis and signaling, are maintained in part by plasma membrane Ca2+-ATPase (PMCA) isoforms 1-4. Gene targeting has been used to analyze the functions of PMCA1, PMCA2, and PMCA4 in mice. PMCA1 null mutant embryos die during the preimplantation stage, and loss of a single copy of the PMCA1 gene contributes to apoptosis in vascular smooth muscle. PMCA2 deficiency in sensory hair cells of the inner ear causes deafness and balance defects, most likely by affecting both intracellular Ca2+ and extracellular Ca2+ in the endolymph. PMCA2 is required for viability of certain neurons, consistent with a major role in maintenance of intracellular Ca2+. Surprisingly, loss of PMCA2 in lactating mammary glands causes a sharp reduction in milk Ca2+, consistent with a macrocalcium secretory function. Although PMCA4 is widely expressed and is the most abundant isoform in some tissues, null mutants appear healthy. However, male PMCA4 null mutants are infertile due to a failure of hyperactivated sperm motility resulting from the absence of PMCA4 in the sperm tail, and Ca2+ signaling in B lymphocytes, involving interactions between PMCA4, CD22, and the tyrosine phosphatase SHP-1, is defective. Studies of bladder smooth muscle from PMCA4 null mutants and PMCA1 heterozygous mice suggest that PMCA1 and PMCA4 play different roles in smooth muscle contractility, with PMCA1 contributing to overall Ca2+ clearance and PMCA4 being required for carbachol-stimulated contraction. These phenotypes indicate that PMCA1 serves essential housekeeping functions, whereas PMCA4 and particularly PMCA2 serve more specialized physiological functions.

The regulation of the Na/Ca exchanger and plasmalemmal Ca2+ ATPase by other proteins

Na/Ca exchanger (NCX) and plasma membrane Ca2+ ATPase are the Ca2+ efflux mechanisms known in mammalian cells. NCX is the main transporter to efflux intracellular Ca2+ in the heart. NCX protein contains nine putative transmembrane domains and a large intracellular loop joining two sets of the transmembrane domains. The intracellular loop regulates the activity of the NCX by interacting with other proteins and nonprotein factors, such as ions, PIP2. Several proteins that are associated with NCX have been identified recently. Similarly, plasmalemmal Ca2+ ATPase (PMCA) has 10 putative transmembrane domains, and the C-terminal intracellular region inhibits transporter activity. There are several proteins associated with PMCA, and the roles of the associated proteins of PMCA vary from specific localization to involving PMCA in signal transduction. Elucidation of structural and functional roles played by these associated proteins of NCX and PMCA will provide opportunities to develop drugs of potential therapeutic value.

Plasma membrane calcium ATPase and its relationship to nitric oxide signaling in the heart

The plasma membrane calcium/calmodulin-dependent ATPase (PMCA) is a ubiquitously expressed calcium-extruding enzymatic pump. In the majority of cells the main function of PMCA is as the only system to extrude calcium from the cytosol, however, in the excitable cells of the heart it has only a minor role in the bulk removal of calcium compared to the sodium-calcium exchanger. There is increasing evidence to suggest that PMCA has an additional role as a potential modulator of a number of signal transduction pathways. Of key interest in the heart is the functional interaction between the calcium/calmodulin-dependent enzyme neuronal nitric oxide synthase (nNOS) and isoform 4 of PMCA. Nitric oxide production from nNOS is known to be important in the regulation of excitation-contraction (EC) coupling and subsequently contractility. This article will focus on recent evidence suggesting that PMCA4 has a regulatory role in the nitric oxide signaling pathway in the heart.

Na+/Ca2+ exchanger knockout mice: plasticity of cardiac excitation-contraction coupling

The Na+/Ca2+ exchanger (NCX) is the main Ca2+ extrusion mechanism of the cardiac myocyte. Nevertheless, cardiac-specific NCX knockout (KO) mice are viable to adulthood. We have identified two adaptations of excitation-contraction coupling (ECC) to the absence of NCX in these animals: (a) a reduction of the L-type Ca2+ current (I(Ca)) with an increase in ECC gain and (b) a shortening of the action potential (AP) to further limit Ca2+ influx. Both mechanisms contribute to Ca2+ homeostasis by reducing Ca2+ influx while maintaining contractility. These adaptations may comprise important feedback mechanisms by which cardiomyocytes may be able to limit Ca2+ influx in situations of compromised Ca2+ extrusion capacity.

Regulation of cardiac Na+/Ca2+ exchanger by phospholemman

Phospholemman (PLM) is the first sequenced member of the FXYD family of regulators of ion transport. The mature protein has 72 amino acids and consists of an extracellular N terminus containing the signature FXYD motif, a single transmembrane (TM) domain, and a cytoplasmic C-terminal domain containing four potential sites for phosphorylation. PLM and other members of the FXYD family are known to regulate Na+-K+-ATPase. Using adenovirus-mediated gene transfer into adult rat cardiac myocytes, we showed that changes in contractility and intracellular Ca2+ homeostasis associated with PLM overexpression or downregulation are not consistent with the effects expected from inhibition of Na+-K+-ATPase by PLM. Additional studies with heterologous expression of PLM and cardiac Na+/Ca2+ exchanger 1 (NCX1) in HEK293 cells and cardiac myocytes isolated from PLM-deficient mice demonstrated by co-localization, co-immunoprecipitation, and electrophysiological and radioactive tracer uptake techniques that PLM associates with NCX1 in the sarcolemma and transverse tubules and that PLM inhibits NCX1, independent of its effects on Na+-K+-ATPase. Mutational analysis indicates that the cytoplasmic domain of PLM is required for its regulation of NCX1. In addition, experiments using phosphomimetic and phospho-deficient PLM mutants, as well as activators of protein kinases A and C, indicate that PLM phosphorylated at serine68 is the active form that inhibits NCX1. This is in sharp contrast to the finding that the unphosphorylated PLM form inhibits Na+-K+-ATPase. We conclude that PLM regulates cardiac contractility by modulating the activities of NCX and Na+-K+-ATPase.

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.

AMPA-mediated excitotoxicity in oligodendrocytes: role for Na(+)-K(+)-Cl(-) co-transport and reversal of Na(+)/Ca(2+) exchanger

We investigated the role of Na(+)-K(+)-Cl(-) co-transporter isoform 1 (NKCC1) and reversal of Na(+)/Ca(2+) exchanger (NCX(rev)) in glutamate-mediated excitotoxicity in oligodendrocytes obtained from rat spinal cords (postnatal day 6-8). An immunocytochemical characterization showed that these cultures express NKCC1 and Na(+)/Ca(2+) exchanger isoforms 1, 2, and 3 (NCX1, NCX2, NCX3). Exposing the cultures to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) plus cyclothiazide (CTZ) led to a transient rise in intracellular (), which was followed by a sustained overload, NKCC1 phosphorylation, and a NKCC1-mediated Na(+) influx. In the presence of a specific AMPA receptor inhibitor 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX), the AMPA/CTZ failed to elicit any changes in . The AMPA/CTZ-induced sustained rise led to mitochondrial Ca(2+) accumulation, release of cytochrome c from mitochondria, and cell death. The AMPA/CTZ-elicited increase, mitochondrial damage, and cell death were significantly reduced by inhibiting NKCC1 or NCX(rev). These data suggest that in cultured oligodendrocytes, activation of AMPA receptors leads to NKCC1 phosphorylation that enhances NKCC1-mediated Na(+) influx. The latter triggers NCX(rev) and NCX(rev)-mediated overload and compromises mitochondrial function and cellular viability.

Maximal Ca2+i stimulation of cardiac Na+/Ca2+ exchange requires simultaneous alkalinization and binding of PtdIns-4,5-P2 to the exchanger

Using bovine heart sarcolemma vesicles we studied the effects of protons and phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2) on the affinity of the mammalian Na(+)/Ca(2+) exchanger (NCX1) for intracellular Ca(2+). By following the effects of extravesicular ligands in inside-out vesicles, their interactions with sites of NCX1 facing the intracellular medium were investigated. Two Na(+)-gradient-dependent fluxes were studied: Ca(2+) uptake and Ca(2+) release. PtdIns-4,5-P2 binding to NCX1 was investigated in parallel. Without MgATP (no 'de novo' synthesis of PtdIns-4,5-P2), alkalinization increased the affinity for Ca(2+) and the PtdIns-4,5-P2 bound to NCX1. Vesicles depleted of phosphoinositides were insensitive to alkalinization, but became responsive following addition of exogenous PtdIns-4,5-P2 or PtdIns plus MgATP. Acidification reduced the affinity for Ca(2+)(ev); this was only partially reversed by MgATP, despite the increase in bound PtdIns-4,5-P2 to levels observed with alkalinization. Inhibition of Ca(2+) uptake by increasing extravesicular [Na(+)] indicates that it is related to H(+)(i) and Na(+)(i) synergistic inhibition of the Ca(2+)(i) regulatory site. Therefore, the affinity of the NCX1 Ca(2+)(i) regulatory site for Ca(2+) was maximal when both intracellular alkalinization and an increase in PtdIns-4,5-P2 bound to NCX1 (not just of the total membrane PtdIns-4,5-P2) occurred simultaneously. In addition, protons influenced the distribution, or the exposure, of PtdIns-4,5-P2 molecules in the surroundings and/or on the exchanger protein.

Endothelin-1 decreases [Ca2+]i via Na+/Ca2+ exchanger in CHO cells stably expressing endothelin ETA receptor

Endothelin ET(A) receptor couples to Gq/11 protein that transduces a variety of receptor signals to modulate diverse cellular responses including Ca2+ mobilization. Stimulation of endothelin ETA receptor with endothelin-1 is generally believed to induce an increase in intracellular Ca2+ concentration ([Ca2+]i) via Gq/11 protein. Here we provide the first convincing evidence that endothelin-1 elicited Gq/11 protein-dependent and -independent 'decrease' in [Ca2+]i via Na+/Ca2+ exchanger (NCX) in Chinese hamster ovary (CHO) cells stably expressing human endothelin ETA receptor. In the cells treated with 1 microM thapsigargin, an inhibitor of endoplasmic Ca2+ pump, that induces an increase in [Ca2+]i via capacitative Ca2+ entry, endothelin-1 induced a decrease in [Ca2+]i which was partially inhibited by YM-254890, a specific inhibitor of Gq/11, indicating that Gq/11-dependent and independent pathways are involved in the decrease. The endothelin-1-induced decrease in [Ca2+]i was markedly suppressed by 3',4'-dichlorobenzamil hydrochloride, a potent NCX inhibitor, and also by a replacement of extracellular Na+ with Li+, which was not transported by NCX, indicating a major role of NCX operating in the forward mode in the endothelin-1-induced decrease in [Ca2+]i. Molecular approach with RT-PCR demonstrated the expression of mRNA for NCX1, NCX2 and NCX3. These results suggest that stimulation of endothelin ETA receptor with endothelin-1 activates the forward mode NCX through Gq/11-dependent and -independent mechanisms: the NCX exports Ca2+ out of the cell depending on Na+ gradient across the cell membrane, resulting in the decrease in [Ca2+]i.

Molecular evolution and structural analysis of the Ca(2+) release-activated Ca(2+) channel subunit, Orai

Depletion of intracellular Ca(2+) stores evokes Ca(2+) entry across the plasma membrane by inducing Ca(2+) release-activated Ca(2+) (CRAC) currents in many cell types. Recently, Orai and STIM proteins were identified as the molecular identities of the CRAC channel subunit and the endoplasmic reticulum Ca(2+) sensor, respectively. Here, extensive database searching and phylogenetic analysis revealed several lineage-specific duplication events in the Orai protein family, which may account for the evolutionary origins of distinct functional properties among mammalian Orai proteins. Based on similarity to key structural domains and essential residues for channel functions in Orai proteins, database searching also identifies a putative primordial Orai sequence in hyperthermophilic archaeons. Furthermore, modern Orai appears to acquire new structural domains as early as Urochodata, before divergence into vertebrates. The evolutionary patterns of structural domains might be related to distinct functional properties of Drosophila and mammalian CRAC currents. Interestingly, Orai proteins display two conserved internal repeats located at transmembrane segments 1 and 3, both of which contain key amino acids essential for channel function. These findings demonstrate biochemical and physiological relevance of Orai proteins in light of different evolutionary origins and will provide novel insights into future structural and functional studies of Orai proteins.

Involvement of Na+-Ca2+ exchanger on metabotropic glutamate receptor 1-mediated [Ca2+]i transients in rat cerebellar Purkinje neurons

Cerebellar Purkinje neurons have intracellular regulatory systems including Ca2+-binding proteins, intracellular Ca2+ stores, Ca2+-ATPase and Na+-Ca2+ exchanger (NCX) that keep intracellular Ca2+ concentration ([Ca2+]i) in physiological range. Among these, NCX interacts with AMPA receptors, activation of which induces cerebellar synaptic plasticity. And the activation of metabotropic glutamate receptor 1 (mGluR1) is also involved in the induction of cerebellar long-term depression. The interaction of NCX with mGluR1 is not known yet. Thus, in this study, the functional relationship between NCX and mGluR1 in modulating the [Ca2+]i in rat Purkinje neurons was investigated. The interaction between NCX and mGluR1 in Purkinje neurons was studied by measuring intracellular Ca2+ transients induced by an agonist of group I mGluRs, 3,5-dihydroxyphenylglycine (DHPG). The DHPG-induced Ca2+ transient was significantly reduced by treatments of NCX inhibitors, bepridil and KB-R7943. When cells were pretreated with antisense oligodeoxynucleotides of NCX, the DHPG-induced Ca2+ transient was also inhibited. These results suggest that NCX modulates the activity of mGluR1 in cerebellar Purkinje neurons. Therefore, NCX appears to play an important role in the physiological function of cerebellar Purkinje neurons such as synaptic plasticity.

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.

Mechanism of Na+/Ca2+ exchanger activation by hydrogen peroxide in guinea-pig ventricular myocytes

Using the whole-cell voltage clamp, we examined the mechanism of activation of the Na(+)/Ca(2+) exchanger (NCX) by hydrogen peroxide (H(2)O(2)) in isolated guinea-pig cardiac ventricular myocytes. Exposure to H(2)O(2) increased the NCX current. The effect was inhibited by cariporide, an inhibitor of the Na(+)/H(+) exchanger (NHE), suggesting that there are NHE-dependent and -independent pathways in the effect of H(2)O(2) on NCX. In addition, both pathways were blocked by edaravone, a hydroxyl radical (*OH) scavenger; pertussis toxin, a Galpha(i/o) protein inhibitor; and U0126, an inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK). On the other hand, wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, inhibited only the NHE-dependent pathway, while PP2, a Src family protein tyrosine kinase inhibitor, inhibited only the NHE-independent pathway. Taken together, our data suggest that H(2)O(2) increases the NCX current via two signal transduction pathways. The common pathway is the conversion of H(2)O(2) to *OH, which activates Galpha(i/o) protein and a mitogen-activated protein (MAP) kinase signaling pathway. Then, one pathway activates NHE with a PI3K-dependent mechanism and indirectly increases the NCX current. Another pathway involves activation of a Src family tyrosine kinase.

Homozygous Overexpression of the Na+-Ca2+ Exchanger in Mice: Evidence for Increased Transsarcolemmal Ca2+ Fluxes

Mice with homozygous overexpression of the Na+-Ca2+ exchanger (NCX) exhibit threefold levels of NCX expression and an increased Ca2+ extrusion rate. To investigate how Ca2+ homeostasis is maintained in this model, we have characterized Ca2+ influx under these conditions. We find that L-type Ca2+ currents (I(Ca)) inactivate slower due to a reduction of Ca2+-dependent inactivation. Additionally, NCX-overexpressing animals exhibit a prolongation of the action potential (AP). We conclude that transsarcolemmal Ca2+ fluxes in NCX-overexpressing myocytes are balanced by an increase in Ca2+ influx via (a) slowed inactivation of I(Ca) and (b) a prolongation of the AP to compensate for increased Ca2+ efflux.