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

The Role of Astrocytic Mitochondria in the Pathogenesis of Brain Ischemia

The morbidity rate of ischemic stroke is increasing annually with the growing aging population in China. Astrocytes are ubiquitous glial cells in the brain and play a crucial role in supporting neuronal function and metabolism. Increasing evidence shows that the impairment or loss of astrocytes contributes to neuronal dysfunction during cerebral ischemic injury. The mitochondrion is increasingly recognized as a key player in regulating astrocyte function. Changes in astrocytic mitochondrial function appear to be closely linked to the homeostasis imbalance defects in glutamate metabolism, Ca2+ regulation, fatty acid metabolism, reactive oxygen species, inflammation, and copper regulation. Here, we discuss the role of astrocytic mitochondria in the pathogenesis of brain ischemic injury and their potential as a therapeutic target.

On the special role of NCX in astrocytes: Translating Na+-transients into intracellular Ca2+ signals

As a solute carrier electrogenic transporter, the sodium/calcium exchanger (NCX1-3/SLC8A1-A3) links the trans-plasmalemmal gradients of sodium and calcium ions (Na⁺, Ca²⁺) to the membrane potential of astrocytes. Classically, NCX is considered to serve the export of Ca²⁺ at the expense of the Na⁺ gradient, defined as a "forward mode" operation. Forward mode NCX activity contributes to Ca²⁺ extrusion and thus to the recovery from intracellular Ca²⁺ signals in astrocytes. The reversal potential of the NCX, owing to its transport stoichiometry of 3 Na⁺ to 1 Ca²⁺, is, however, close to the astrocytes' membrane potential and hence even small elevations in the astrocytic Na⁺ concentration or minor depolarisations switch it into the "reverse mode" (Ca²⁺ import/Na⁺ export). Notably, transient Na⁺ elevations in the millimolar range are induced by uptake of glutamate or GABA into astrocytes and/or by the opening of Na⁺-permeable ion channels in response to neuronal activity. Activity-related Na⁺ transients result in NCX reversal, which mediates Ca²⁺ influx from the extracellular space, thereby generating astrocyte Ca²⁺ signalling independent from InsP₃-mediated release from intracellular stores. Under pathological conditions, reverse NCX promotes cytosolic Ca²⁺ overload, while dampening Na⁺ elevations of astrocytes. This review provides an overview on our current knowledge about this fascinating transporter and its special functional role in astrocytes. We shall delineate that Na⁺-driven, reverse NCX-mediated astrocyte Ca²⁺ signals are involved neurone-glia interaction. Na⁺ transients, translated by the NCX into Ca²⁺ elevations, thereby emerge as a new signalling pathway in astrocytes.

Roles of glial ion transporters in brain diseases

Glial ion transporters are important in regulation of ionic homeostasis, cell volume, and cellular signal transduction under physiological conditions of the central nervous system (CNS). In response to acute or chronic brain injuries, these ion transporters can be activated and differentially regulate glial functions, which has subsequent impact on brain injury or tissue repair and functional recovery. In this review, we summarized the current knowledge about major glial ion transporters, including Na⁺ /H⁺ exchangers (NHE), Na⁺ /Ca²⁺ exchangers (NCX), Na⁺ -K⁺ -Cl^- cotransporters (NKCC), and Na⁺ -HCO₃ ^- cotransporters (NBC). In acute neurological diseases, such as ischemic stroke and traumatic brain injury (TBI), these ion transporters are rapidly activated and play significant roles in regulation of the intra- and extracellular pH, Na⁺ , K⁺ , and Ca²⁺ homeostasis, synaptic plasticity, and myelin formation. However, overstimulation of these ion transporters can contribute to glial apoptosis, demyelination, inflammation, and excitotoxicity. In chronic brain diseases, such as glioma, Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS), glial ion transporters are involved in the glioma Warburg effect, glial activation, neuroinflammation, and neuronal damages. These findings suggest that glial ion transporters are involved in tissue structural and functional restoration, or brain injury and neurological disease development and progression. A better understanding of these ion transporters in acute and chronic neurological diseases will provide insights for their potential as therapeutic targets.

Sodium-calcium exchangers in rat trigeminal ganglion neurons

Background

Noxious stimulation and nerve injury induce an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) via various receptors or ionic channels. While an increase in [Ca²⁺]_i excites neurons, [Ca²⁺]_i overload elicits cytotoxicity, resulting in cell death. Intracellular Ca²⁺ is essential for many signal transduction mechanisms, and its level is precisely regulated by the Ca²⁺ extrusion system in the plasma membrane, which includes the Na⁺-Ca²⁺ exchanger (NCX). It has been demonstrated that Ca²⁺-ATPase is the primary mechanism for removing [Ca²⁺]_i following excitatory activity in trigeminal ganglion (TG) neurons; however, the role of NCXs in this process has yet to be clarified. The goal of this study was to examine the expression/localization of NCXs in TG neurons and to evaluate their functional properties.

Results

NCX isoforms (NCX1, NCX2, and NCX3) were expressed in primary cultured rat TG neurons. All the NCX isoforms were also expressed in A-, peptidergic C-, and non-peptidergic C-neurons, and located not only in the somata, dendrites, axons and perinuclear region, but also in axons innervating the dental pulp. Reverse NCX activity was clearly observed in TG neurons. The inactivation kinetics of voltage-dependent Na⁺ channels were prolonged by NCX inhibitors when [Ca²⁺]_i in TG neurons was elevated beyond physiological levels.

Conclusions

Our results suggest that NCXs in TG neurons play an important role in regulating Ca²⁺-homeostasis and somatosensory information processing by functionally coupling with voltage-dependent Na⁺ channels.

Transcriptional regulation of ncx1 gene in the brain

The ubiquitous sodium-calcium exchanger isoform 1 (NCX1) is a -bidirectional transporter that plays a relevant role under physiological and pathophysiological conditions including brain ischemia by regulating intraneuronal Ca(2+) and Na(+) homeostasis. Although changes in ncx1 protein and transcript expression have been detected during stroke, its transcriptional regulation is still largely unexplored. Here, we reviewed our recent findings on several transcription factors including cAMP response element-binding protein (CREB), nuclear factor kappa B (NF-κB), and hypoxia-inducible factor-1 (HIF-1) in the control of the ncx1 gene expression in neuronal cells.

The Na+/Ca2+ exchanger-mediated Ca2+ influx triggers nitric oxide-induced cytotoxicity in cultured astrocytes

Nitric oxide (NO) is involved in many pathological conditions including neurodegenerative disorders. We have previously found that sodium nitroprusside (SNP), an NO donor, stimulates mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulating kinase (ERK), c-jun N-terminal protein kinase (JNK) and p38 MAPK, leading to caspase-independent apoptosis in cultured astrocytes. In view of the previous observation that NO stimulates the activity of the Na(+)/Ca(2+) exchanger (NCX), this study examines the involvement of NCX in cytotoxicity. The specific NCX inhibitor SEA0400 blocked SNP-induced phosphorylation of ERK, JNK and p38 MAPK, and decrease in cell viability. SNP-induced phosphorylation of ERK, JNK and p38 MAPK was blocked by removal of external Ca(2+), and SNP treatment caused an increase in (45)Ca(2+) influx. This increase in (45)Ca(2+) influx was blocked by SEA0400, but not the Ca(2+) channel blocker nifedipine. In addition, SNP-induced (45)Ca(2+) influx and cytotoxicity were reduced in NCX1-deficient cells which were transfected with NCX1 siRNA. Inhibitors of intracellular Ca(2+)-dependent proteins such as calpain and calmodulin blocked SNP-induced ERK phosphorylation and decrease in cell viability. Furthermore, the guanylate cyclase inhibitor LY83583 and the cGMP-dependent protein kinase inhibitor KT5823 blocked SNP-induced cytotoxicity. These findings suggest that NCX-mediated Ca(2+) influx triggers SNP-induced apoptosis in astrocytes, which may be mediated by a cGMP-dependent pathway.

Calpain activation and Na+/Ca2+ exchanger degradation occur downstream of calcium deregulation in hippocampal neurons exposed to excitotoxic glutamate

Delayed calcium deregulation (DCD) plays an essential role in glutamate excitotoxicity, a major detrimental factor in stroke, traumatic brain injury, and various neurodegenerations. In the present study, we examined the role of calpain activation and Na(+)/Ca(2+) exchanger (NCX) degradation in DCD and excitotoxic cell death in cultured hippocampal neurons. Exposure of neurons to glutamate caused DCD accompanied by secondary mitochondrial depolarization. Activation of calpain was evidenced by detecting NCX isoform 3 (NCX3) degradation products. Degradation of NCX isoform 1 (NCX1) was below the detection limit of Western blotting. Degradation of NCX3 was detected only after 1 hr of incubation with glutamate, whereas DCD occurred on average within 15 min after glutamate application. Calpeptin, an inhibitor of calpain, significantly attenuated NCX3 degradation but failed to inhibit DCD and excitotoxic neuronal death. Calpain inhibitors I, III, and VI also failed to influence DCD and glutamate-induced neuronal death. On the other hand, MK801, an inhibitor of the NMDA subtype of glutamate receptors, added shortly after the initial glutamate-induced jump in cytosolic Ca(2+), completely prevented DCD and activation of calpain and strongly protected neurons against excitotoxicity. Taken together, our results suggest that, in glutamate-treated hippocampal neurons, the initial increase in cytosolic Ca(2+) that precedes DCD is insufficient for sustained calpain activation, which most likely occurs downstream of DCD.

Brain development and susceptibility to damage; ion levels and movements

Responses of immature brains to physiological and pathological stimuli often differ from those in the adult. Because CNS function critically depends on ion movements, this chapter evaluates ion levels and gradients during ontogeny and their alterations in response to adverse conditions. Total brain Na(+) and Cl(-) content decreases during development, but K(+) content rises, reflecting shrinkage of the extracellular and increase in the intracellular water spaces and a reduction in total brain water volume. Unexpectedly, [K(+)](i) seems to fall during the first postnatal week, which should reduce [K(+)](i)/ [K(+)](e) and result in a lower V(m), consistent with experimental observations. Neuronal [Cl(-)](i) is high during early postnatal development, hence the opening of Cl(-) conduction pathways may lead to plasma membrane depolarization. Equivalent loss of K(+)(i) into a relatively large extracellular space leads to a smaller increase in [K(+)](e) in immature animals, while the larger reservoir of Ca(2+)(e) may result in a greater [Ca(2+)](i) rise. In vivo and in vitro studies show that compared with adult, developing brains are more resistant to hypoxic/ischemic ion leakage: increases in [K(+)](e) and decreases in [Ca(2+)](e) are slower and smaller, consistent with the known low level of energy utilization and better maintenance of [ATP]. Severe hypoxia/ischemia may, however, lead to large Ca(2+)(i) overload. Rises in [K(+)](e) during epileptogenesis in vivo are smaller and take longer to manifest themselves in immature brains, although the rate of K(+) clearance is slower. By contrast, in vitro studies suggest the existence of a period of enhanced vulnerability sometime during the developmental period. This chapter concludes that there is a great need for more information on ion changes during ontogeny and poses the question whether the rat is the most appropriate model for investigation of mechanisms of pathological changes in human neonates.

Involvement of protein kinase C in the regulation of Na+/Ca2+ exchanger in bovine adrenal chromaffin cells

1. The Na(+)/Ca(2+) exchanger (NCX) exchanges Na+ and Ca(2+) bidirectionally through the forward mode (Ca(2+) extrusion) or the reverse mode (Ca(2+) influx). The present study was undertaken to clarify the role of protein kinase C (PKC) in the regulation of NCX in bovine adrenal chromaffin cells. The Na(+)-loaded cells were prepared by treatment with 100 micromol/L ouabain and 50 micromol/L veratridine. Incubation of Na(+)-loaded cells with Na(+)-free solution in the presence of the Ca(2+) channel blockers nicardipine (3 micromol/L) and omega-conotoxin MVIIC (0.3 micromol/L) caused Ca(2+) uptake and catecholamine release. 2. The Na(+)-dependent Ca(2+) uptake and catecholamine release were inhibited by 2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400; 1 micromol/L) and 2-[2-[4-(4-nitrobenzyloxy)phenyl]isothiourea (KB-R7943; 10 micromol/L), both NCX inhibitors. These results indicate that the Na(+)-dependent responses are mostly due to activation of the NCX working in the reverse mode. 3. In addition, we examined the effects of PKC inhibitors and an activator on the NCX-mediated Ca(2+) uptake and catecholamine release. Bisindolylmaleimide I (0.3-10 micromol/L) and chelerythrine (3-100 micromol/L), both PKC inhibitors, inhibited NCX-mediated responses. In contrast, phorbol 12,13-dibutyrate (0.1-10 micromol/L), a PKC activator, enhanced the responses. Bisindolylmaleimide I and chelerythrine, at effective concentrations for inhibition of Na(+)-dependent catecholamine release, had a little or no effect on high K(+)-induced catecholamine release in intact cells or on Ca(2+)-induced catecholamine release in beta-escin-permeabilized cells. 4. These results suggest that PKC is involved in the activation of NCX in bovine adrenal chromaffin cells.

Searching for a role of NCX/NCKX exchangers in neurodegeneration

Control of intracellular calcium signaling is essential for neuronal development and function. Maintenance of Ca2+ homeostasis depends on the functioning of specific transport systems that remove calcium from the cytosol. Na+/Ca2+ exchange is the main calcium export mechanism across the plasma membrane that restores resting levels of calcium in neurons after stimulation. Two families of Na+/Ca2+ exchangers exist, one of which requires the co-transport of K+ and Ca2+ in exchange for Na+ ions. The malfunctioning of Na+/Ca2+ exchangers has been related to the development of pathological conditions in the regulation of neuronal death after hypoxia-anoxia, brain trauma, and nerve injury. In addition, the Na+/Ca2+ exchanger function has been associated with impaired Ca2+ homeostasis during aging of the brain, as well as with a role in Alzheimer's disease by regulating beta-amyloid toxicity. In this review, we summarize the current knowledge about the Na+/Ca2+ exchanger families and their implications in neurodegenerative disorders.

[Molecular pharmacologic approaches to functional analysis of new biological target molecules for drug discovery]

This review focuses on two pharmacologic approaches to the functional evaluation of new target molecules for drug discovery. One is the development of a novel specific antagonist of the Na(+)-Ca(++) exchanger (NCX) SEA0400. The other is a comprehensive analysis of the functions of pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide ligand for G protein-coupled receptors. NCX is the one of the last target molecules regulating the cellular Ca(++) concentration. There was no efficient way to address the pathophysiologic roles of NCX until a specific antagonist, 2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400), was developed. Our recent studies using SEA0400 clearly showed the possible roles of NCX in several pathologic states of cardiovascular and nervous tissues. In our second approach including gene-targeting methods, we found new, unexpected roles of PACAP in higher brain functions, such as psychomotor, cognition, photoentrainment, and nociception. Based on these experimental findings, a genetic association study in schizophrenia patients revealed that the single-nucleotide polymorphisms of the PACAP gene are significantly associated with the hypofunction of the hippocampus. Regarding the peripheral roles of PACAP, we found that PACAP is involved not only in the regulation of insulin secretion in pancreatic islets, but also in the regulation of islet turnover. In subsequent phenotypic analysis of PACAP transgenic mice, we identified novel candidate genes that probably have promising functional roles.

Postnatal developmental changes in Ca2+ homeostasis in supraoptic magnocellular neurons

Supraoptic magnocellular neurons (SMNs) undergo dramatic changes in morphological and electrical properties during postnatal development. We investigated the developmental change in Ca2+ homeostasis in SMNs. The decay rate of Ca2+ transients markedly increased during the third postnatal week (PW3) to an adult level. This increase in the Ca2+ decay rate was paralleled by hypertrophy of the SMN somata. Activity of Na+/Ca2+ exchanger (Na/CaX) and sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) was quantified as a decrement in the Ca2+ decay rate caused by extracellular [Na+] reduction and that by thapsigargin, respectively. SERCA activity was negligible during PW2, and markedly increased during PW3. SERCA activity and soma size remained stable thereafter. Na/CaX activity was a major Ca2+-clearance mechanism (CCM) during PW2, increased further during PW3, but was negligible in mature SMNs (PW10). In parallel with the decrease in Na/CaX activity, endogenous Ca2+ buffering capacity declined, resulting that the apparent Ca2+ decay rate remained relatively constant between PW4 and PW10. Replacement of intracellular K+ with Li+ had no effect on Na/CaX activity, suggesting that NCX rather than NCKX comprises Na/CaX. These findings indicate a developmental shift in the balance of CCMs from Ca2+ extrusion via NCX toward Ca2+ sequestration into endoplasmic reticulum via SERCA.

Sodium/calcium exchanger expression in the mouse and rat olfactory systems

Sodium/calcium (Na(+)/Ca(2+)) exchangers are membrane transport systems that regulate Ca(2+)-homeostasis in many eukaryotic cells. In olfactory and vomeronasal sensory neurons ligand-induced olfactory signal transduction is associated with influx and elevation of intracellular Ca(2+), [Ca(2+)](i). While much effort has been devoted to the characterization of Ca(2+)-related excitation and adaptation events of olfactory chemosensory neurons (OSNs), much less is known about mechanisms that return [Ca(2+)](i) to the resting state. To identify proteins participating in the poststimulus Ca(2+)-clearance of mouse OSNs, we analyzed the expression of three potassium (K(+))-independent (NCX1, 2, 3) and three K(+)-dependent (NCKX1, 2, 3) Na(+)/Ca(2+) exchangers. In situ hybridization showed that mRNAs of all six Na(+)/Ca(2+) exchangers coexist in neurons of the olfactory and vomeronasal systems, and that some are already detectable in the embryo. Of these, NCX1 and NCKX1 represent the most and least abundant mRNAs, respectively. Moreover, immunohistochemistry revealed that the NCX1, 2, and 3 proteins are expressed in nearly all neurons of the olfactory epithelium, the vomeronasal organ, the septal organ of Masera, and the Grueneberg ganglion. These three exchanger proteins display different expression profiles in dendrites, knobs, and plasma membranes of OSNs and in sustentacular cells. Furthermore, we show that NCX1 mRNA in rat olfactory mucosa is expressed as 8 alternative splice variants. This is the first comprehensive analysis of Na(+)/Ca(2+) exchanger expression in the mammalian olfactory system. Our results suggest that Ca(2+)-extrusion by OSNs utilizes multiple different Na(+)/Ca(2+) exchangers and that different subtypes are targeted to different subcellular compartments.

Expression of prokineticin receptors in mouse cultured astrocytes and involvement in cell proliferation

Effects of prokineticins (PKs), a novel family of bioactive peptides with a mitogenic action to endothelial cells of the endocrine gland and testis, on astrocytic functions were examined. Mouse cultured astrocytes expressed PK-R1 type PK receptors, while there was little expression of the PK-R2 type. PKs caused increases in astrocytic cytosolic Ca2+ levels and BrdU incorporation. Increases in Ca2+ levels by PK-2 were diminished by U73122 (a phospholipase C inhibitor). PK-induced BrdU incorporation was inhibited by U73122, GF109203 (a protein kinase C inhibitor) and PD98059 (a MEK/ERK inhibitor). These results indicate that PK receptors are expressed in astrocytes and regulate astrocytic proliferation.

Chronic hypoxia inhibits Na+/Ca2+ exchanger expression in cortical astrocytes

Ca signalling is central to many diverse functions of astrocytes. Of the numerous proteins involved in Ca homeostasis, the Na(+)/Ca(2+) exchanger is of particular importance in signalling regulation. We have shown that Ca signaling is dramatically remodelled in astrocytes by periods of chronic hypoxia, in part by inhibition of Na(+)/Ca(2+) exchanger. Here, we demonstrate that bepridil-sensitive Ca extrusion (indicative of Na(+)/Ca(2+) exchanger activity) is suppressed following 24 h hypoxia (2.5 or 1% O2) owing to a loss of Na(+)/Ca(2+) exchanger expression, as determined using immunocytochemistry and Western blots. Hypoxic Na(+)/Ca(2+) exchanger 1 inhibition occurs at the level of transcription, as mRNA for Na(+)/Ca(2+) exchanger 1 was significantly suppressed by hypoxia. Our results show hypoxia perturbs Ca homeostasis in astrocytes via the suppression of Na(+)/Ca(2+) exchanger 1 expression.

Functional proteins involved in regulation of intracellular Ca(2+) for drug development: pharmacology of SEA0400, a specific inhibitor of the Na(+)-Ca(2+) exchanger

The Na(+)-Ca(2+) exchanger (NCX) is involved in regulation of intracellular Ca(2+) concentration. A specific inhibitor of NCX has been required for clarification of the physiological and pathological roles of NCX. We have developed 2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400), a highly potent and selective inhibitor of NCX. SEA0400 in the concentration range that inhibits NCX exhibits negligible affinities for the Ca(2+) channels, Na(+) channels, K(+) channels, noradrenaline transporter, and 14 receptors; and it does not affect the activities of the store-operated Ca(2+) channel, Na(+)-H(+) exchanger, and several enzymes including Na(+),K(+)-ATPase and Ca(2+)-ATPase. Furthermore, recent studies show that SEA0400 attenuates ischemia-reperfusion injury in the brain, heart, and kidney and radiofrequency lesion-induced edema in rat brain. These findings suggest that NCX plays a key role in ischemia-reperfusion injury and may be a target molecule for treatment of reperfusion injury-related diseases.

Energy metabolism in mammalian brain during development

Production of energy for the maintenance of ionic disequilibria necessary for generation and transmission of nerve impulses is one of the primary functions of the brain. This review attempts to link the plethora of information on the maturation of the central nervous system with the ontogeny of ATP metabolism, placing special emphasis on variations that occur during development in different brain regions and across the mammalian species. It correlates morphological events and markers with biochemical changes in activities of enzymes and pathways that participate in the production of ATP. The paper also evaluates alterations in energy levels as a function of age and, based on the tenet that ATP synthesis and utilization cannot be considered in isolation, investigates maturational profiles of the key processes that utilize energy. Finally, an attempt is made to assess the relevance of currently available animal models to improvement of our understanding of the etiopathology of various disease states in the human infant. This is deemed essential for the development and testing of novel strategies for prevention and treatment of several severe neurological deficits.

Differential contribution of plasmalemmal Na/Ca exchange isoforms to sodium-dependent calcium influx and NMDA excitotoxicity in depolarized neurons

Inhibition of Na(+),K(+)-ATPase during NMDA applications greatly increased NMDA-induced excitotoxicity in primary cultures of forebrain neurons (FNs), but not in cerebellar granule cells (CGCs). Because Na(+),K(+)-ATPase inhibition promotes reversal of plasmalemmal Na(+)/Ca(2+) exchangers, we compared the activities of reversed K(+)-independent (NCX) and K(+)-dependent (NCKX) Na(+)/Ca(2+) exchangers in these cultures. To this end, we measured gramicidin-induced and Na(+)-dependent elevation in cytosolic [Ca(2+)] ([Ca(2+)](c)) that represents Ca(2+) influx via reversed NCX and NCKX; NCX activity was dissected out by removing external K(+). The [Ca(2+)](c) elevations mediated by NCX alone, and NCX plus NCKX combined, were 17 and 6 times more rapid in FNs than in CGCs, respectively. Northern blot analysis showed that FNs preferentially express NCX1 whereas CGCs expressed NCX3. Differences in expression of other isoforms (NCX2, NCKX2, NCKX3 and NCKX4) were less pronounced. We tested whether the NCX or NCKX family of exchangers contributes most to the toxic NMDA-induced Ca(2+) influx in depolarized neurons. We found that in FNs, inhibition of NCX alone was sufficient to significantly limit NMDA excitotoxicity, whereas in CGCs, inhibition of both NCX and NCKX was required. The data suggest that the high activity of NCX isoforms expressed in FNs, possibly NCX1, sensitizes these neurons to NMDA excitotoxicity.

Possible involvement of astrocytes in neuroprotection by the cognitive enhancer T-588

We have previously shown that the cognition enhancer (1R)-1-benzo[b]thiophen-5-yl-2-[2-(diethylamino)ethoxy]ethan-1-ol hydrochloride (T-588) protects astrocytes against hydrogen peroxide (H2O2) injury via activation of extracellular signal-regulated kinase (ERK) pathway. The present study examines whether the effect of T-588 on astrocytes contributes to neuroprotection in neuronal injury models. Astrocyte-conditioned medium (ACM) protected against neuronal injury induced by amyloid-beta protein (A beta) in cultured cortical neurons. The effect of ACM on A beta-induced injury was blocked by the ERK kinase inhibitor 2'-amino-3'-methoxyflavone. ACM stimulated ERK phosphorylation in cultured neurons. ACM derived from astrocytes exposed to H2O2 lost the activities to stimulate ERK phosphorylation and protect against neuronal injury. T-588 blocked the H2O2-induced loss of the activities of ACM. These results suggest that ACM protects against neuronal injury by an ERK-dependent mechanism, and the effect of T-588 on astrocytic injury results in neuroprotection.

Enhanced learning and memory in mice lacking Na+/Ca2+ exchanger 2

The plasma membrane Na(+)/Ca(2+) exchanger (NCX) plays a role in regulation of intracellular Ca(2+) concentration via the forward mode (Ca(2+) efflux) or the reverse mode (Ca(2+) influx). To define the physiological function of the exchanger in vivo, we generated mice deficient for NCX2, the major isoform in the brain. Mutant hippocampal neurons exhibited a significantly delayed clearance of elevated Ca(2+) following depolarization. The frequency threshold for LTP and LTD in the hippocampal CA1 region was shifted to a lowered frequency in the mutant mice, thereby favoring LTP. Behaviorally, the mutant mice exhibited enhanced performance in several hippocampus-dependent learning and memory tasks. These results demonstrate that NCX2 can be a temporal regulator of Ca(2+) homeostasis and as such is essential for the control of synaptic plasticity and cognition.

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

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

Protective effect of T-588 on toxic damage by serum deprivation and amyloid-beta protein in cultured neurons

The present study examines the effect of the cognition enhancer (1R)-1-benzo[b]thiophen-5-yl-2-[2-(diethylamino)ethoxy]ethan-1-ol hydrochloride (T-588) on neuronal injury induced by serum deprivation or amyloid-beta protein (A beta). T-588 protected partially against neuronal injury induced by serum deprivation or A beta in cultured cortical neurons. T-588 did not affect the phosphorylation of extracellular signal-regulated kinase (ERK) in cortical neurons and SH-SY5Y cells. These results suggest that T-588 has a protective effect in neuronal injury models and the effect is not mediated by an ERK signal pathway.

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

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

Functional regulation of alternatively spliced Na+/Ca2+ exchanger (NCX1) isoforms

Alternative splicing of RNA transcripts is a general characteristic for NCX genes in mammals, mollusks, and arthropods. Among the family of three NCX genes in mammals, the NCX1 gene contains six exons, namely, A, B, C, D, E, and F, that make up the alternatively spliced region. Studies of the NCX1 gene transcripts suggested that 16 distinct gene products can be produced from the NCX1 gene. The exons A and B are mutually exclusive when expressed. Generally, exon A-containing transcripts are predominantly found in excitable cells like cardiomyoctes and neurons, whereas exon B-containing transcripts are mostly found in nonexcitable cells like astrocytes and kidney cells. Other alternatively spliced exons (C-F) appear to be cassette-type exons and are found in various combinations. Interestingly, exon D is present in all characterized transcripts. The alternatively spliced isoforms of NCX1 show tissue-specific expression patterns, suggesting functional adaptation to tissues. To investigate functional differences among alternatively spliced isoforms of NCX1, we expressed an exon A-containing transcript present in cardiac tissue (NCX1.1) and an exon B-containing transcript found in the kidney (NCX1.3) in Xenopus oocytes. We demonstrated that the Na(+)/Ca(2+) exchangers expressed by exon A- and exon B-containing transcripts display differences in activation by PKA and by [Ca(2+)](i). We also observed that these two isoforms show differences in voltage dependence. Surprisingly, the alternatively spliced isoforms of NCX1 display greater functional differences among themselves than the products of different gene loci, NCX1, NCX2, and NCX3.

Na(+)/Ca(2+) exchanger expression in the developing rat cortex

The Na(+)/Ca(2+) exchanger (NCX) participates in the regulation of neuronal Ca(2+) homeostasis and is also believed to be involved in the neuronal responses to hypoxia. However, there are very limited data on how NCX mRNA and protein expression are regulated during brain development. In the present study, we sought to elucidate the developmental expression of NCX1 and NCX2 in the rat cortex from late fetal to adult stages using reverse transcription-polymerase chain reaction and western blot assays. The primers for NCX1 mRNA targeted the alternative splicing domain to allow differentiation between NCX1 splice variants. Our results show that: (1) only two NCX1 mRNA splice variants (NCX1.5 and NCX1.4) are present in the cortex and their expression is age-dependent; (2) total NCX1 mRNA levels are low in fetal tissue, reach maximum density at postnatal day 8 and substantially decline with further maturation; (3) NCX2 mRNA density is significantly greater than total NCX1 mRNA for all ages and increases markedly during maturation from fetus/neonate to adult; and (4) NCX1 protein expression is lowest in late fetal cortex and reaches maximum levels after 2 weeks postnatally, even though expression levels are not significantly different between newborn and adult animals. Also, we found a similar NCX1 protein trend in the subcortical and cerebellar regions during development. From these data we suggest that NCX1 and NCX2 are differentially expressed in the cortex with a predominance of NCX2 levels during postnatal development. We speculate that the developmental increase in NCX2 expression is responsible for the overall increase in Na(+)/Ca(2+) exchange capacity during maturation.

Preferential expression of plasmalemmal K-dependent Na+/Ca2+ exchangers in neurons versus astrocytes

Numerous isoforms of plasmalemmal K-dependent (NCKX) and K-independent (NCX) Na+/Ca2+ exchangers are expressed in the brain. The physiological functions of each isoform are presently unknown. Therefore, in this study, we compared expression of NCKX and NCX transcripts between primary cultures of cerebellar granule cells, and astrocytes. Northern blot analysis showed that granule cells expressed NCKX2, NCKX3, NCKX4 and NCX3, whereas astrocytes expressed primarily NCX1. Consistent with this molecular characterization, a significant fraction of 45Ca2+ accumulation in Na-loaded granule cells, but not in astrocytes, depended on external K+. This is the first demonstration of native NCKX activity in neurons derived from the central nervous system. Our data suggest that NCKX isoform expression may correspond to the unique Ca2+ homeostasis requirements of neurons.