Na+-Ca2+ Exchanger: Physiology and Pharmacology

Calcium Deregulation in Neurodegeneration and Neuroinflammation in Parkinson's Disease: Role of Calcium-Storing Organelles and Sodium-Calcium Exchanger

Parkinson's disease (PD) is a progressive neurodegenerative disorder that lacks effective treatment strategies to halt or delay its progression. The homeostasis of Ca2+ ions is crucial for ensuring optimal cellular functions and survival, especially for neuronal cells. In the context of PD, the systems regulating cellular Ca2+ are compromised, leading to Ca2+-dependent synaptic dysfunction, impaired neuronal plasticity, and ultimately, neuronal loss. Recent research efforts directed toward understanding the pathology of PD have yielded significant insights, particularly highlighting the close relationship between Ca2+ dysregulation, neuroinflammation, and neurodegeneration. However, the precise mechanisms driving the selective loss of dopaminergic neurons in PD remain elusive. The disruption of Ca2+ homeostasis is a key factor, engaging various neurodegenerative and neuroinflammatory pathways and affecting intracellular organelles that store Ca2+. Specifically, impaired functioning of mitochondria, lysosomes, and the endoplasmic reticulum (ER) in Ca2+ metabolism is believed to contribute to the disease's pathophysiology. The Na+-Ca2+ exchanger (NCX) is considered an important key regulator of Ca2+ homeostasis in various cell types, including neurons, astrocytes, and microglia. Alterations in NCX activity are associated with neurodegenerative processes in different models of PD. In this review, we will explore the role of Ca2+ dysregulation and neuroinflammation as primary drivers of PD-related neurodegeneration, with an emphasis on the pivotal role of NCX in the pathology of PD. Consequently, NCXs and their interplay with intracellular organelles may emerge as potentially pivotal players in the mechanisms underlying PD neurodegeneration, providing a promising avenue for therapeutic intervention aimed at halting neurodegeneration.

α7 nicotinic receptors play a role in regulation of cardiac hemodynamics

The α7 nicotinic receptors (NR) have been confirmed in the heart but their role in cardiac functions has been contradictory. To address these contradictory findings, we analyzed cardiac functions in α7 NR knockout mice (α7-/-) in vivo and ex vivo in isolated hearts. A standard limb leads electrocardiogram was used, and the pressure curves were recorded in vivo, in Arteria carotis and in the left ventricle, or ex vivo, in the left ventricle of the spontaneously beating isolated hearts perfused following Langedorff's method. Experiments were performed under basic conditions, hypercholinergic conditions, and adrenergic stress. The relative expression levels of α and β NR subunits, muscarinic receptors, β1 adrenergic receptors, and acetylcholine life cycle markers were determined using RT-qPCR. Our results revealed a prolonged QT interval in α7-/- mice. All in vivo hemodynamic parameters were preserved under all studied conditions. The only difference in ex vivo heart rate between genotypes was the loss of bradycardia in prolonged incubation of isoproterenol-pretreated hearts with high doses of acetylcholine. In contrast, left ventricular systolic pressure was lower under basal conditions and showed a significantly higher increase during adrenergic stimulation. No changes in mRNA expression were observed. In conclusion, α7 NR has no major effect on heart rate, except when stressed hearts are exposed to a prolonged hypercholinergic state, suggesting a role in acetylcholine spillover control. In the absence of extracardiac regulatory mechanisms, left ventricular systolic impairment is revealed.

Structural mechanisms of the human cardiac sodium-calcium exchanger NCX1

Na+/Ca2+ exchangers (NCX) transport Ca2+ in or out of cells in exchange for Na+. They are ubiquitously expressed and play an essential role in maintaining cytosolic Ca2+ homeostasis. Although extensively studied, little is known about the global structural arrangement of eukaryotic NCXs and the structural mechanisms underlying their regulation by various cellular cues including cytosolic Na+ and Ca2+. Here we present the cryo-EM structures of human cardiac NCX1 in both inactivated and activated states, elucidating key structural elements important for NCX ion exchange function and its modulation by cytosolic Ca2+ and Na+. We demonstrate that the interactions between the ion-transporting transmembrane (TM) domain and the cytosolic regulatory domain define the activity of NCX. In the inward-facing state with low cytosolic [Ca2+], a TM-associated four-stranded β-hub mediates a tight packing between the TM and cytosolic domains, resulting in the formation of a stable inactivation assembly that blocks the TM movement required for ion exchange function. Ca2+ binding to the cytosolic second Ca2+-binding domain (CBD2) disrupts this inactivation assembly which releases its constraint on the TM domain, yielding an active exchanger. Thus, the current NCX1 structures provide an essential framework for the mechanistic understanding of the ion transport and cellular regulation of NCX family proteins.

Space Environment Impacts Homeostasis: Exposure to Spaceflight Alters Mammary Gland Transportome Genes

Membrane transporters and ion channels that play an indispensable role in metabolite trafficking have evolved to operate in Earth's gravity. Dysregulation of the transportome expression profile at normogravity not only affects homeostasis along with drug uptake and distribution but also plays a key role in the pathogenesis of diverse localized to systemic diseases including cancer. The profound physiological and biochemical perturbations experienced by astronauts during space expeditions are well-documented. However, there is a paucity of information on the effect of the space environment on the transportome profile at an organ level. Thus, the goal of this study was to analyze the effect of spaceflight on ion channels and membrane substrate transporter genes in the periparturient rat mammary gland. Comparative gene expression analysis revealed an upregulation (p < 0.01) of amino acid, Ca²⁺, K⁺, Na⁺, Zn²⁺, Cl^-, PO₄^3-, glucose, citrate, pyruvate, succinate, cholesterol, and water transporter genes in rats exposed to spaceflight. Genes associated with the trafficking of proton-coupled amino acids, Mg²⁺, Fe²⁺, voltage-gated K⁺-Na⁺, cation-coupled chloride, as well as Na⁺/Ca²⁺ and ATP-Mg/Pi exchangers were suppressed (p < 0.01) in these spaceflight-exposed rats. These findings suggest that an altered transportome profile contributes to the metabolic modulations observed in the rats exposed to the space environment.

Restoration of metal homeostasis: a potential strategy against neurodegenerative diseases

Metal homeostasis is critical to normal neurophysiological activity. Metal ions are involved in the development, metabolism, redox and neurotransmitter transmission of the central nervous system (CNS). Thus, disturbance of homeostasis (such as metal deficiency or excess) can result in serious consequences, including neurooxidative stress, excitotoxicity, neuroinflammation, and nerve cell death. The uptake, transport and metabolism of metal ions are highly regulated by ion channels. There is growing evidence that metal ion disorders and/or the dysfunction of ion channels contribute to the progression of neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and multiple sclerosis (MS). Therefore, metal homeostasis-related signaling pathways are emerging as promising therapeutic targets for diverse neurological diseases. This review summarizes recent advances in the studies regarding the physiological and pathophysiological functions of metal ions and their channels, as well as their role in neurodegenerative diseases. In addition, currently available metal ion modulators and in vivo quantitative metal ion imaging methods are also discussed. Current work provides certain recommendations based on literatures and in-depth reflections to improve neurodegenerative diseases. Future studies should turn to crosstalk and interactions between different metal ions and their channels. Concomitant pharmacological interventions for two or more metal signaling pathways may offer clinical advantages in treating the neurodegenerative diseases.

Myo-D-inositol Trisphosphate Signalling in Oomycetes

Oomycetes are pathogens of plants and animals, which cause billions of dollars of global losses to the agriculture, aquaculture and forestry sectors each year. These organisms superficially resemble fungi, with an archetype being Phytophthora infestans, the cause of late blight of tomatoes and potatoes. Comparison of the physiology of oomycetes with that of other organisms, such as plants and animals, may provide new routes to selectively combat these pathogens. In most eukaryotes, myo-inositol 1,4,5 trisphosphate is a key second messenger that links extracellular stimuli to increases in cytoplasmic Ca2+, to regulate cellular activities. In the work presented in this study, investigation of the molecular components of myo-inositol 1,4,5 trisphosphate signaling in oomycetes has unveiled similarities and differences with that in other eukaryotes. Most striking is that several oomycete species lack detectable phosphoinositide-selective phospholipase C homologues, the enzyme family that generates this second messenger, but still possess relatives of myo-inositol 1,4,5 trisphosphate-gated Ca2+-channels.

A simple model of cardiac mitochondrial respiration with experimental validation

Cardiac mitochondria are intracellular organelles that play an important role in energy metabolism and cellular calcium regulation. In particular, they influence the excitation-contraction cycle of the heart cell. A large number of mathematical models have been proposed to better understand the mitochondrial dynamics, but they generally show a high level of complexity, and their parameters are very hard to fit to experimental data. We derived a model based on historical free energy-transduction principles, and results from the literature. We proposed simple expressions that allow to reduce the number of parameters to a minimum with respect to the mitochondrial behavior of interest for us. The resulting model has thirty-two parameters, which are reduced to twenty-three after a global sensitivity analysis of its expressions based on Sobol indices. We calibrated our model to experimental data that consists of measurements of mitochondrial respiration rates controlled by external ADP additions. A sensitivity analysis of the respiration rates showed that only seven parameters can be identified using these observations. We calibrated them using a genetic algorithm, with five experimental data sets. At last, we used the calibration results to verify the ability of the model to accurately predict the values of a sixth dataset. Results show that our model is able to reproduce both respiration rates of mitochondria and transitions between those states, with very low variability of the parameters between each experiment. The same methodology may apply to recover all the parameters of the model, if corresponding experimental data were available.

Endogenous Hydrogen Sulfide Contributes to Tone Generation in Porcine Lower Esophageal Sphincter Via Na+/Ca2+ Exchanger

BACKGROUND AND AIMS

Hydrogen sulfide (H₂S) is a major physiologic gastrotransmitter. Its role in the regulation of the lower esophageal sphincter (LES) function remains unknown. The present study addresses this question.

METHODS

Isometric contraction was monitored in circular smooth muscle strips of porcine LES. Changes in cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and force were simultaneously monitored in fura-2-loaded strips with front-surface fluorometry. The contribution of endogenous H₂S to LES contractility was investigated by examining the effects of inhibitors of H₂S-generating enzymes, including cystathionine-β-synthase, cystathionine-γ-lyase, and 3-mercaptopyruvate sulfurtransferase, on the LES function.

RESULTS

Porcine LES strips myogenically maintained a tetrodotoxin-resistant basal tone. Application of AOA (cystathionine-β-synthase inhibitor) or L-aspartic acid (L-Asp; 3-mercaptopyruvate sulfurtransferase inhibitor) but not DL-PAG (cystathionine-γ-lyase inhibitor), decreased this basal tone. The relaxant effects of AOA and L-Asp were additive. Maximum relaxation was obtained by combination of 1 mM AOA and 3 mM L-Asp. Immunohistochemical analyses revealed that cystathionine-β-synthase and 3-mercaptopyruvate sulfurtransferase, but not cystathionine-γ-lyase, were expressed in porcine LES. AOA+L-Asp-induced relaxation was accompanied by a decrease in [Ca²⁺]_i and inversely correlated with the extracellular Na⁺ concentration ([Na⁺]_o) (25-137.4 mM), indicating involvement of an Na⁺/Ca²⁺ exchanger. The reduction in the basal [Ca²⁺]_i level by AOA was significantly augmented in the antral smooth muscle sheets of Na⁺/Ca²⁺ exchanger transgenic mice compared with wild-type mice.

CONCLUSIONS

Endogenous H₂S regulates the LES myogenic tone by maintaining the basal [Ca²⁺]_i via Na⁺/Ca²⁺ exchanger. H₂S-generating enzymes may be a potential therapeutic target for esophageal motility disorders, such as achalasia.

Sodium-calcium exchanger 1 regulates epithelial cell migration via calcium-dependent extracellular signal-regulated kinase signaling

Na(+)/Ca(2+) exchanger-1 (NCX1) is a major calcium extrusion mechanism in renal epithelial cells enabling the efflux of one Ca(2+) ion and the influx of three Na(+) ions. The gradient for this exchange activity is provided by Na,K-ATPase, a hetero-oligomer consisting of a catalytic α-subunit and a regulatory β-subunit (Na,K-β) that also functions as a motility and tumor suppressor. We showed earlier that mice with heart-specific ablation (KO) of Na,K-β had a specific reduction in NCX1 protein and were ouabain-insensitive. Here, we demonstrate that Na,K-β associates with NCX1 and regulates its localization to the cell surface. Madin-Darby canine kidney cells with Na,K-β knockdown have reduced NCX1 protein and function accompanied by 2.1-fold increase in free intracellular calcium and a corresponding increase in the rate of cell migration. Increased intracellular calcium up-regulated ERK1/2 via calmodulin-dependent activation of PI3K. Both myosin light chain kinase and Rho-associated kinase acted as mediators of ERK1/2-dependent migration. Restoring NCX1 expression in β-KD cells reduced migration rate and ERK1/2 activation, suggesting that NCX1 functions downstream of Na,K-β in regulating cell migration. In parallel, inhibition of NCX1 by KB-R7943 in Madin-Darby canine kidney cells, LLC-PK1, and human primary renal epithelial cells (HREpiC) increased ERK1/2 activation and cell migration. This increased migration was associated with high myosin light chain phosphorylation by PI3K/ERK-dependent mechanism in HREpiC cells. These data confirm the role of NCX1 activity in regulating renal epithelial cell migration.

Roles of cathepsins in reperfusion-induced apoptosis in cultured astrocytes

Astrocytic apoptosis may play a role in the central nervous system injury. We previously showed that reperfusion of cultured astrocytes with normal medium after exposure to hydrogen peroxide (H(2)O(2))-containing medium causes apoptosis. This study examines the involvement of the lysosomal enzymes cathepsins B and D in the astrocytic apoptosis. Reperfusion after exposure to H(2)O(2) caused a marked increase in caspase-3 and cathepsin D activities and a marked decrease in cathepsin B activity. Pepstatin A, an inhibitor of cathepsin D, and acetyl-L-aspartyl-L-methionyl-L-glutaminyl-L-aspart-1-aldehyde (Ac-DMQD-CHO), a specific inhibitor of caspase-3, blocked the H(2)O(2)-induced decrease in cell viability and DNA ladder formation in cultured rat astrocytes. The (L-3-trans-(propylcarbamoyl)oxirane-2-carbonyl)-L-isoleucyl-L-proline methyl ester (CA074 Me), a specific inhibitor of cathepsin B, did not affect the H(2)O(2)-induced cell injury. On the other hand, CA074 Me decreased cell viability with DNA ladder formation when cultured in the presence of Ac-DMQD-CHO. This caspase-independent apoptosis was attenuated by the addition of the cathepsin D inhibitor pepstatin A. Caspase-3 like activity was markedly inhibited by Ac-DMQD-CHO and partially by pepstatin A. Pepstatin A and CA074 Me inhibited cathepsin B and cathepsin D activities, respectively, in the presence and absence of Ac-DMQD-CHO. These results suggest that cathepsins B and D are involved in astrocytic apoptosis: cathepsin D acts as a death-inducing factor upstream of caspase-3 and the caspase-independent apoptosis is regulated antagonistically by cathepsins B and D.

Heat shock inhibits hydrogen peroxide-induced apoptosis in cultured astrocytes

Heat shock proteins (HSPs) have been shown to act as inhibitors of apoptosis, but this anti-apoptotic effect is not known in the central nervous system. Prior heat shock has been demonstrated to protect astrocytes from cell death in a model of reperfusion injury (Brain Res. 735 (1996) 265). The present study examines the mechanism underlying the protective effect of the heat shock. Preincubation of astrocytes at 40 degrees C for 10 min attenuated the hydrogen peroxide (H(2)O(2))-induced decrease in cell viability, DNA ladder formation and nuclear condensation, and these effects were blocked by the protein synthesis inhibitor cycloheximide. The thermal stress inhibited the H(2)O(2)-induced increase in caspase-3 like protease activity, but it did not affect the H(2)O(2)-induced loss of mitochondrial membrane potential. The cytosol prepared from preheated cells did not affect Ca(2+)-induced swelling of mitochondria, a marker of the permeable transition pore. The protective effect of the thermal stress on the H(2)O(2)-induced decrease in cell viability was not affected by the mitogen-activated protein/extracellular signal-regulated kinase kinase inhibitor 2'-amino-3'-methoxyflavone, the phosphatidylinositol-3 kinase inhibitor wortmannin and the NF-kappaB inhibitor pyrrolidinedithiocarbamate. These findings suggest that HSPs inhibit apoptosis via an inhibition of caspase-3 activation without effect on mitochondrial dysfunction.

Ionic ligand interactions with the intracellular loop of the sodium-calcium exchanger. Modulation by ATP

In the last decade, there has been a large increase in the study of the Na(+)/Ca(2+) exchanger due to its implications in physiological and pathophysiological processes at the cell and organ levels. Key areas of these studies have been molecular biology, regulation and physiology-pathophysiology of the exchanger. There are three main types of regulation that take place at the large intracellular loop of the Na(+)/Ca(2+) exchanger: (i) ionic (sodium inactivation, calcium regulation and proton inhibition), (ii) metabolic (ATP as phosphoryl group donor), and (iii) genetic (alternative splicing). This review analyzes the most recent data on the mutual interactions of regulatory ionic ligands (Ca(2+), Na(+), H(+)) and how they are secondarily modulated by MgATP, emphasizing the importance of the binding of Ca(2+) to its regulatory site as an essential requirement for the exchange function. Intracellular protons and sodium inhibit the Na(+)/Ca(2+) exchanger by reducing the apparent affinity of the Ca(i)-regulatory site for Ca(2+). Although the metabolic pathways are different in the mammalian heart (membrane lipids) and squid nerve cells (soluble cytosolic regulatory protein), the final mechanism for the protective effect of MgATP is the same: a reduction of Na(i)(+)-H(i)(+) binding affinities facilitating the attachment of Ca(2+) to its regulatory site. Kinetic models, which partially analyzed some of these ionic and metabolic interactions, can be integrated into a single scheme where the Ca(i)-regulatory site plays a central role.

Anti-apoptotic effect of cGMP in cultured astrocytes: inhibition by cGMP-dependent protein kinase of mitochondrial permeable transition pore

Reperfusion of cultured astrocytes with normal medium after exposure to H(2)O(2)-containing medium causes apoptosis. We have recently shown that ibudilast, which has been used for bronchial asthma and cerebrovascular disorders, attenuated the H(2)O(2)-induced apoptosis of astrocytes via the cGMP signaling pathway. This study examines the mechanism underlying the protective effect of cGMP. The membrane-permeable cGMP analog dibutyryl-cGMP attenuated the H(2)O(2)-induced decrease in cell viability, DNA ladder formation, nuclear condensation, reduction of the mitochondrial membrane potential, cytochrome c release from mitochondria, and caspase-3 activation in cultured astrocytes. These effects of dibutyryl-cGMP were almost completely inhibited by the cGMP-dependent protein kinase (PKG) inhibitor KT5823. In isolated rat brain mitochondria, cGMP in the presence of cytosolic extract from astrocytes inhibited the mitochondrial permeability transition pore (PTP) as determined by monitoring Ca(2+)-induced mitochondrial swelling. This ability of the cytosolic extract was inactivated by heat treatment and was mimicked by exogenous PKG. The effect of cGMP on the mitochondrial swelling was blocked by KT5823. The PTP inhibitors cyclosporin A and bongkrekic acid prevented the H(2)O(2)-induced decrease in cell viability and caspase-3 activation. These findings demonstrate that cGMP inhibits the mitochondrial PTP via the activation of PKG, and the prevention of mitochondrial dysfunction contributes to its anti-apoptotic effect.

Na(+)-Ca(2+) exchanger isoforms in rat neuronal preparations: different changes in their expression during postnatal development

We examined the relative amounts of Na(+)-Ca(2+) exchanger (NCX) isoform mRNAs in cultured neurons, astrocytes and developmental rat brain. NCX1 transcript was predominant in neurons and astrocytes, but NCX2 transcript was about four-fold higher than NCX1 or NCX3 transcript in adult rat cortex. NCX2 transcript in the cortex increased markedly during postnatal development, whereas NCX1 and NCX3 transcripts decreased. Na(+)-dependent 45Ca(2+) uptake in the cortical homogenate increased significantly during postnatal development.

CV-2619 protects cultured astrocytes against reperfusion injury via nerve growth factor production

In this study, we examined the effect of the neuroprotective agent 2, 3-dimethoxy-5-methyl-6-(10-hydroxydecyl)-1,4-benzoquinone (CV-2619) on reperfusion injury in cultured rat astrocytes after exposure to hydrogen peroxide (H(2)O(2))-containing medium. CV-2619 (10 nM to 10 microM) significantly attenuated the reperfusion-induced decrease in cell viability. The compound showed an anti-apoptotic effect in this astrocyte injury model. Antioxidants such as ascorbic acid, alpha-tocopherol and reduced glutathione also inhibited H(2)O(2) exposure-induced cytotoxicity. CV-2619 did not affect the levels of reactive oxygen species, but it increased nerve growth factor (NGF) production. The effect of CV-2619 on H(2)O(2) exposure-induced cytotoxicity was blocked by cycloheximide and anti-NGF antibody. The protective effect of CV-2619 was antagonized by the mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK) kinase inhibitor 2'-amino-3'-methoxyflavone and the phosphatidylinositol-3 kinase inhibitor wortmannin. These findings suggest that the effect of CV-2619 is mediated at least partly by NGF production in astrocytes and that ERK and phosphatidylinositol-3 kinases play a role in the downstream mechanism.

T-588 inhibits astrocyte apoptosis via mitogen-activated protein kinase signal pathway

The effect of (1R)-1-benzo[b]thiophen-5-yl-2-[2-(diethylamino)ethoxy]ethan -1-ol hydrochloride (T-588), a cognition enhancer, on reperfusion injury was studied in cultured rat astrocytes. T-588 at 1-10 microM partially protected astrocytes against reperfusion injury after exposure to Ca(2+)-free medium or hydrogen peroxide. Nerve growth factor (NGF) had a similar protective effect. Addition of both T-588 and NGF resulted in complete protection against Ca(2+) reperfusion injury. T-588 did not stimulate NGF production in astrocytes. The effect of T-588 on Ca(2+) reperfusion injury including apoptosis was inhibited by the mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK) kinase inhibitor 2'-amino-3'-methoxyflavone (PD98059), but not by the phosphoinositide 3-kinase inhibitor wortmannin. The effect of NGF was inhibited by PD98059 and wortmannin. T-588 stimulated rapidly the phosphorylation of ERK, but did not affect that of Akt in astrocytes. These findings suggest that the ERK MAP kinase pathway has a role in the protective effects of T-588 and NGF.

SM-20220, a potent Na+/H+ exchange inhibitor, improves consciousness recovery and neurological outcome following transient cerebral ischaemia in gerbils

We studied the cerebroprotective effect of SM-20220 (N-(aminoiminomethyl)-1-methyl-1H-indole-2-carboxamide methanesulphonate), a newly synthesized Na+/H+ exchanger (NHE) inhibitor, in Mongolian gerbil global ischaemia. Transient cerebral ischaemia was induced by clipping both common carotid arteries for 30 min followed by 24h reperfusion. Intravenous administration of SM-20220 (0.3 or 1.0 mg kg(-1)) immediately after reperfusion significantly shortened the consciousness recovery time (P < 0.01). SM-20220 also improved the neurological outcome (McGraw's scale) after reperfusion. At the dose of 1.0 mg kg(-1), the mortality rate was significantly reduced at 24 h after reperfusion (P < 0.01). This study shows that NHE is involved in the aggravation of cerebral function, represented by consciousness recovery, and neurological outcome following transient forebrain ischaemia, and that its inhibitor may exert protective effects on post-ischaemic brain damage.

The electrophysiology of prefrontal serotonin systems: therapeutic implications for mood and psychosis

A newly described synaptic action of serotonin (5-HT) in the cerebral cortex is reviewed, and implications for mood and psychosis are discussed. Recordings in brain slices show that 5-HT induces a rapid increase in excitatory postsynaptic potentials/currents (EPSPs/EPSCs) in virtually all layer V pyramidal cells of neocortex. This effect is mediated by the 5-HT2A receptor, which has been linked to the action of hallucinogenic and atypical antipsychotic drugs. The increase in EPSCs is seen most prominently in medial prefrontal cortex and other frontal regions where 5-HT2A receptors are enriched. The induction of EPSCs by 5-HT appears to occur through a novel mechanism that does not depend on the activation of afferent impulse flow. Instead, 5-HT appears to act presynaptically, directly or indirectly, to induce a focal release of glutamate from a subpopulation of glutamatergic terminals impinging upon the apical (but not basilar) dendrites of layer V pyramidal cells; a working hypothesis of the transduction pathway (involving asynchronous transmitter release) for this process is presented. Consistent with a focal action upon glutamatergic nerve terminals, the 5-HT-induced EPSPs can be suppressed by presynaptic inhibitory modulators such as mu-opiate or group II/III metabotropic agonists. We suggest that the suppression of 5-HT-induced EPSCs by 5-HT2A antagonists and mu-opiate agonists may underlie certain shared clinical effects of 5-HT2A antagonists and mu-opiate agonists. We suggest further that since presynaptic group II/III metabotropic glutamate agonists suppress 5-HT-induced EPSCs, metabotropic glutamate agonists may also possess antidepressant and/or antipsychotic properties.

NCX1 Na/Ca exchanger inhibition by antisense oligonucleotides in mouse distal convoluted tubule cells

BACKGROUND

Plasma membrane NCX1 Na+/Ca2+ exchangers mediate cellular Ca2+ efflux. Renal distal convoluted tubule (DCT) cells express transcripts encoding three alternatively spliced NCX1 isoforms: NACA2 (exons B, C, D), NACA3 (exons B and D), and NACA6 (exons A, C, D). We used antisense oligodeoxynucleotides (ODNs) to determine the function of these NACA isoforms on Na+/Ca2+ exchanger activity and expression in DCT cells.

METHODS

Sense and antisense ODNs targeting exchanger transcripts were introduced into DCT cells permeabilized with streptolysin O. Na+/Ca2+ exchange activity was assessed by measuring Na+-dependent changes of free intracellular Ca2+ concentration (delta[Ca2+]i), in single cells, when the electrochemical gradient for Na+ was reversed.

RESULTS

The change of [Ca2+]i in cells treated with antisense ODNs to a downstream or upstream region common to all NCX1 isoforms was 173 nM (-66%) to the downstream region located in the putative ninth transmembrane domain, and 226 nM (-39%) with ODNs to an upstream region located 5' to the variable portion of the intracellular loop. Antisense ODNs to exon B, present in both NACA2 and NACA3, decreased delta[Ca2+]i by 209 nM (-44%), while antisense ODNs specific for NACA6 (exon A) were without effect. Antisense ODNs specific for exon C, present in NACA2 and NACA6, decreased delta[Ca2+]i by 226 nM (-39%). Northern analysis of mRNA prepared from primary cultures of distal tubule cells revealed exon B- but not exon A-containing transcripts. Immunofluorescence analysis using a polyclonal antibody that recognizes NCX1 confirmed that protein expression was inhibited after treatment with the exon B antisense ODNs.

CONCLUSION

These findings show that Na+-dependent cellular Ca2+ efflux in DCT cells is primarily mediated by NACA2 and NACA3.