Electroacupuncture pretreatment enhances the calcium efflux activity of Na+/Ca2+ exchanger to attenuate cerebral injury by PI3K/Akt-mediated NCX1 upregulation after focal cerebral ischaemia

Electroacupuncture pretreatment is considered as an optimal strategy for inducing cerebral ischaemic tolerance. However, the underlying neuroprotective mechanism of this approach has never been explored from the perspective of calcium homeostasis. Intracellular calcium overload is a key inducer of cascade neuronal injury in the early stage after cerebral ischaemia attack and the Na+/Ca2+ exchanger (NCX) is the main plasma membrane calcium extrusion pathway maintaining post-ischaemic calcium homeostasis. This study aims to investigate whether the regulation of NCX-mediated calcium transport contributes to the cerebroprotective effect of electroacupuncture pretreatment against ischaemic injury and to elucidate the underlying mechanisms involved in this process. Following five days of repeated electroacupuncture stimulation on Baihui (GV20), Neiguan (PC6), and Sanyinjiao (SP6) acupoints in rats, in vivo and in vitro models of cerebral ischaemia were induced through middle cerebral artery occlusion and oxygen/glucose deprivation (OGD), respectively. Firstly, we verified the neuroprotective effect of electroacupuncture pretreatment from the perspective of neurological score, infarct volume and neuronal apoptosis. Our findings from brain slice patch-clamp indicated that electroacupuncture pretreatment enhanced the Ca2+ efflux capacity of NCX after OGD. NCX1 expression in the ischaemic penumbra exhibited a consistent decline from 1 to 24 h in MCAO rats. Electroacupuncture pretreatment upregulated the expression of NCX1, especially at 24 h, and silencing NCX1 by short hairpin RNA (shRNA) administration reversed the protective effect of electroacupuncture pretreatment against cerebral ischaemic injury. Furthermore, we administered LY294002, a phosphatidylinositol 3 kinase (PI3K) inhibitor, prior to inducing ischaemia to investigate the upstream regulatory mechanism of electroacupuncture pretreatment on NCX1 expression. Electroacupuncture pretreatment activates PI3K/Akt pathway, leading to an increase in the expression of NCX1, which facilitates calcium extrusion and exerts a neuroprotective effect against cerebral ischaemia. These findings provided a novel insight into the prevention of ischemic stroke and other similar conditions characterized by brain ischaemia or hypoperfusion.

K+-Dependent Na+/Ca2+ Exchanger Isoform 2, Nckx2, Takes Part in the Neuroprotection Elicited by Ischemic Preconditioning in Brain Ischemia

Sodium/Calcium exchangers are neuronal plasma membrane antiporters which, by coupling Ca²⁺ and Na⁺ fluxes across neuronal membranes, play a relevant role in brain ischemia. The most brain-expressed isoform among the members of the K⁺-dependent Na⁺/Ca²⁺ exchanger family, NCKX2, is involved in the progression of the ischemic lesion, since both its knocking-down and its knocking-out worsens ischemic damage. The aim of this study was to elucidate whether NCKX2 functions as an effector in the neuroprotection evoked by ischemic preconditioning. For this purpose, we investigated: (1) brain NCKX2 expression after preconditioning and preconditioning + ischemia; (2) the contribution of AKT and calpain to modulating NCKX2 expression during preconditioning; and (3) the effect of NCKX2 knocking-out on the neuroprotection mediated by ischemic preconditioning. Our results showed that NCKX2 expression increased in those brain regions protected by ischemic preconditioning. These changes were p-AKT-mediated since its inhibition prevented NCKX2 up-regulation. More interestingly, NCKX2 knocking-out significantly prevented the protection exerted by ischemic preconditioning. Overall, our results suggest that NCKX2 plays a fundamental role in the neuroprotective effect mediated by ischemic preconditioning and support the idea that the enhancement of its expression and activity might represent a reasonable strategy to reduce infarct extension after stroke.

Na+/Ca2+ exchanger isoform 1 takes part to the Ca2+-related prosurvival pathway of SOD1 in primary motor neurons exposed to beta-methylamino-L-alanine

Background

The cycad neurotoxin beta-methylamino-l-alanine (L-BMAA), one of the environmental trigger factor for amyotrophic lateral sclerosis/Parkinson-dementia complex (ALS/PDC), may cause neurodegeneration by disrupting organellar Ca²⁺ homeostasis. Through the activation of Akt/ERK1/2 pathway, the Cu,Zn-superoxide dismutase (SOD1) and its non-metallated form, ApoSOD1, prevent endoplasmic reticulum (ER) stress-induced cell death in motor neurons exposed to L-BMAA. This occurs through the rapid increase of intracellular Ca²⁺ concentration ([Ca²⁺]_i) in part flowing from the extracellular compartment and in part released from ER. However, the molecular components of this mechanism remain uncharacterized.

Methods

By an integrated approach consisting on the use of siRNA strategy, Western blotting, confocal double- labeling immunofluorescence, patch-clamp electrophysiology, and Fura 2-/SBFI-single-cell imaging, we explored in rat motor neuron-enriched cultures the involvement of the plasma membrane proteins Na⁺/Ca²⁺ exchanger (NCX) and purinergic P₂X₇ receptor as well as that of the intracellular cADP-ribose (cADPR) pathway, in the neuroprotective mechanism of SOD1.

Results

We showed that SOD1-induced [Ca²⁺]_i rise was prevented neither by A430879, a P₂X₇ receptor specific antagonist or 8-bromo-cADPR, a cell permeant antagonist of cADP-ribose, but only by the pan inhibitor of NCX, CB-DMB. The same occurred for the ApoSOD1. Confocal double labeling immunofluorescence showed a huge expression of plasmalemmal NCX1 and intracellular NCX3 isoforms. Furthermore, we identified NCX1 reverse mode as the main mechanism responsible for the neuroprotective ER Ca²⁺ refilling elicited by SOD1 and ApoSOD1 through which they promoted translocation of active Akt in the nuclei of a subset of primary motor neurons. Finally, the activation of NCX1 by the specific agonist CN-PYB2 protected motor neurons from L-BMAA-induced cell death, mimicking the effect of SOD1.

Conclusion

Collectively, our data indicate that SOD1 and ApoSOD1 exert their neuroprotective effect by modulating ER Ca²⁺ content through the activation of NCX1 reverse mode and Akt nuclear translocation in a subset of primary motor neurons.

GATA3 (GATA-Binding Protein 3)/KMT2A (Lysine-Methyltransferase-2A) Complex by Increasing H3K4-3me (Trimethylated Lysine-4 of Histone-3) Upregulates NCX3 (Na+-Ca2+ Exchanger 3) Transcription and Contributes to Ischemic Preconditioning Neuroprotection

Background and Purpose

NCX3 (Na+-Ca2+ exchanger 3) plays a relevant role in stroke; indeed its pharmacological blockade or its genetic ablation exacerbates brain ischemic damage, whereas its upregulation takes part in the neuroprotection elicited by ischemic preconditioning. To identify an effective strategy to induce an overexpression of NCX3, we examined transcription factors and epigenetic mechanisms potentially involved in NCX3 gene regulation.

Methods

Brain ischemia and ischemic preconditioning were induced in vitro by exposure of cortical neurons to oxygen and glucose deprivation plus reoxygenation (OGD/Reoxy) and in vivo by transient middle cerebral artery occlusion. Western blot and quantitative real-time polymerase chain reaction were used to evaluate transcripts and proteins of GATA3 (GATA-binding protein 3), KMT2A (lysine-methyltransferase-2A), and NCX3. GATA3 and KMT2A binding on NCX3 gene was evaluated by chromatin immunoprecipitation and Rechromatin immunoprecipitation experiments.

Results

Among the putative transcription factors sharing a consensus sequence on the ncx3 brain promoter region, GATA3 was the only able to up-regulate ncx3. Interestingly, GATA3 physically interacted with KMT2A, and their overexpression or knocking-down increased or downregulated NCX3 mRNA and protein, respectively. Notably, site-direct mutagenesis of GATA site on ncx3 brain promoter region counteracted GATA3 and KMT2A binding on NCX3 gene. More importantly, we found that in the perischemic cortical regions of preconditioned rats GATA3 recruited KMT2A and the complex H3K4-3me (trimethylated lysine-4 of histone-3) on ncx3 brain promoter region, thus reducing transient middle cerebral artery occlusion–induced damage. Consistently, in vivo silencing of either GATA3 or KMT2A prevented NCX3 upregulation and consequently the neuroprotective effect of preconditioning stimulus. The involvement of GATA3/KMT2A complex in neuroprotection elicited by ischemic preconditioning was further confirmed by in vitro experiments in which the knocking-down of GATA3 and KMT2A reverted the neuroprotection induced by NCX3 overexpression in cortical neurons exposed to anoxic preconditioning followed by oxygen and glucose deprivation plus reoxygenation.

Conclusions

Collectively, our results revealed that GATA3/KMT2A complex epigenetically activates NCX3 gene transcription during ischemic preconditioning.

Ncx3-Induced Mitochondrial Dysfunction in Midbrain Leads to Neuroinflammation in Striatum of A53t-α-Synuclein Transgenic Old Mice

The exact mechanism underlying selective dopaminergic neurodegeneration is not completely understood. The complex interplay among toxic alpha-synuclein aggregates, oxidative stress, altered intracellular Ca²⁺-homeostasis, mitochondrial dysfunction and disruption of mitochondrial integrity is considered among the pathogenic mechanisms leading to dopaminergic neuronal loss. We herein investigated the molecular mechanisms leading to mitochondrial dysfunction and its relationship with activation of the neuroinflammatory process occurring in Parkinson’s disease. To address these issues, experiments were performed in vitro and in vivo in mice carrying the human mutation of α-synuclein A53T under the prion murine promoter. In these models, the expression and activity of NCX isoforms, a family of important transporters regulating ionic homeostasis in mammalian cells working in a bidirectional way, were evaluated in neurons and glial cells. Mitochondrial function was monitored with confocal microscopy and fluorescent dyes to measure mitochondrial calcium content and mitochondrial membrane potential. Parallel experiments were performed in 4 and 16-month-old A53T-α-synuclein Tg mice to correlate the functional data obtained in vitro with mitochondrial dysfunction and neuroinflammation through biochemical analysis. The results obtained demonstrated: 1. in A53T mice mitochondrial dysfunction occurs early in midbrain and later in striatum; 2. mitochondrial dysfunction occurring in the midbrain is mediated by the impairment of NCX3 protein expression in neurons and astrocytes; 3. mitochondrial dysfunction occurring early in midbrain triggers neuroinflammation later into the striatum, thus contributing to PD progression during mice aging.

HDAC4 and HDAC5 form a complex with DREAM that epigenetically down-regulates NCX3 gene and its pharmacological inhibition reduces neuronal stroke damage

The histone deacetylases (HDACs)-dependent mechanisms regulating gene transcription of the Na⁺/Ca⁺ exchanger isoform 3 (ncx3) after stroke are still unknown. Overexpression or knocking-down of HDAC4/HDAC5 down-regulates or increases, respectively, NCX3 mRNA and protein. Likewise, MC1568 (class IIa HDACs inhibitor), but not MS-275 (class I HDACs inhibitor) increased NCX3 promoter activity, gene and protein expression. Furthermore, HDAC4 and HDAC5 physically interacted with the transcription factor downstream regulatory element antagonist modulator (DREAM). As MC1568, DREAM knocking-down prevented HDAC4 and HDAC5 recruitment to the ncx3 promoter. Importantly, DREAM, HDAC4, and HDAC5 recruitment to the ncx3 gene was increased in the temporoparietal cortex of rats subjected to transient middle cerebral artery occlusion (tMCAO), with a consequent histone-deacetylation of ncx3 promoter. Conversely, the tMCAO-induced NCX3 reduction was prevented by intracerebroventricular injection of siDREAM, siHDAC4, and siHDAC5. Notably, MC1568 prevented oxygen glucose deprivation plus reoxygenation and tMCAO-induced neuronal damage, whereas its neuroprotective effect was abolished by ncx3 knockdown. Collectively, we found that: (1) DREAM/HDAC4/HDAC5 complex epigenetically down-regulates ncx3 gene transcription after stroke, and (2) pharmacological inhibition of class IIa HDACs reduces stroke-induced neurodetrimental effects.

TGF-β Signaling Regulates SLC8A3 Expression and Prevents Oxidative Stress in Developing Midbrain Dopaminergic and Dorsal Raphe Serotonergic Neurons

Calcium homeostasis is a cellular process required for proper cell function and survival, maintained by the coordinated action of several transporters, among them members of the Na⁺/Ca²⁺-exchanger family, such as SLC8A3. Transforming growth factor beta (TGF-β) signaling defines neuronal development and survival and may regulate the expression of channels and transporters. We investigated the regulation of SLC8A3 by TGF-β in a conditional knockout mouse with deletion of TGF-β signaling from Engrailed 1-expressing cells, i.e., in cells from the midbrain and rhombomere 1, and elucidated the underlying molecular mechanisms. The results show that SLC8A3 is significantly downregulated in developing dopaminergic and dorsal raphe serotonergic neurons in mutants and that low SLC8A3 abundance prevents the expression of the anti-apoptotic protein Bcl-xL. TGF-β signaling affects SLC8A3 via the canonical and p38 signaling pathway and may increase the binding of Smad4 to the Slc8a3 promoter. Expression of the lipid peroxidation marker malondialdehyde (MDA) was increased following knockdown of Slc8a3 expression in vitro. In neurons lacking TGF-β signaling, the number of MDA- and 4-hydroxynonenal (4-HNE)-positive cells was significantly increased, accompanied with increased cellular 4-HNE abundance. These results suggest that TGF-β contributes to the regulation of SLC8A3 expression in developing dopaminergic and dorsal raphe serotonergic neurons, thereby preventing oxidative stress.

Transcriptional and epigenetic regulation of ncx1 and ncx3 in the brain

Sodium-calcium exchanger (NCX) 1 and 3, have been demonstrated to play a relevant role in controlling the intracellular homeostasis of sodium and calcium ions in physiological and patho-physiological conditions. While NCX1 and NCX3 knocking-down have been both implicated in brain ischemia, several aspects of the epigenetic regulation of these two antiporters transcription were not yet well characterized. In response to stroke, NCX1 and NCX3 transcriptional regulation occurs from specific promoter sequences. Several evidences have shown that the expression of NCX1 and NCX3 can be determined by epigenetic modifications, consisting in changes of the histone acetylation levels on their promoter sequences. An interesting issue is that histone modifications at the NCX1 and NCX3 promoters could be linked to neurodegeneration occurring after stroke. Therefore, identifying the epigenetic regulation at the NCX1 and NCX3 promoters could permit to identify new molecular targets that can open new strategies for stroke treatment. The current review reassumes the recent knowledge of histone modifications of NCX1 and NCX3 genes in brain in physiological and patho-physiological conditions.

Genetic Up-Regulation or Pharmacological Activation of the Na+/Ca2+ Exchanger 1 (NCX1) Enhances Hippocampal-Dependent Contextual and Spatial Learning and Memory

The Na⁺/Ca²⁺ exchanger 1 (NCX1) participates in the maintenance of neuronal Na⁺ and Ca²⁺ homeostasis, and it is highly expressed at synapse level of some brain areas involved in learning and memory processes, including the hippocampus, cortex, and amygdala. Furthermore, NCX1 increases Akt1 phosphorylation and enhances glutamate-mediated Ca²⁺ influx during depolarization in hippocampal and cortical neurons, two processes involved in learning and memory mechanisms. We investigated whether the modulation of NCX1 expression/activity might influence learning and memory processes. To this aim, we used a knock-in mouse overexpressing NCX1 in hippocampal, cortical, and amygdala neurons (ncx1.4^over) and a newly synthesized selective NCX1 stimulating compound, named CN-PYB2. Both ncx1.4^over and CN-PYB2-treated mice showed an amelioration in spatial learning performance in Barnes maze task, and in context-dependent memory consolidation after trace fear conditioning. On the other hand, these mice showed no improvement in novel object recognition task which is mainly dependent on non-spatial memory and displayed an increase in the active phosphorylated CaMKIIα levels in the hippocampus. Interestingly, both of these mice showed an increased level of context-dependent anxiety.Altogether, these results demonstrate that neuronal NCX1 participates in spatial-dependent hippocampal learning and memory processes.

New perspectives for selective NCX activators in neurodegenerative diseases

The Na⁺/Ca²⁺ exchanger plays a relevant role in several neurological disorders, thus the pharmacological modulation of its isoforms might represent a promising strategy to ameliorate the course of some neurological pathologies including stroke, neonatal hypoxia, multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), Alzheimer Disease (AD), and spinal muscular atrophy (SMA). This review will summarize heterocyclic, peptidergic, genetic and epigenetic compounds activating or inhibiting the expression/activity of each NCX isoform. In addition, we will focus our attention on the development of new strategies aimed to ameliorate the pathophysiological conditions in which NCX isoform changes are found.

Tumor necrosis factor-related apoptosis-inducing ligand reduces the expression of the neuroprotective Na+ /Ca2+ exchanger isoform NCX3 in human neuroblastoma SH-SY5Y cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), a cytokine belonging to the TNF superfamily, is regarded as a mediator of neurotoxicity. The constitutively expressed ion exchanger Na⁺ /Ca²⁺ exchanger isoform-3 (NCX3) has been shown to protect neurons from injury. Its expression is induced by nerve growth factor (NGF) through activation of its tyrosine kinase receptor trkA. The latter, in turn, activates downstream kinases, such as extracellular signal-regulated kinase (ERK) and the survival-related kinase protein kinase B (AKT). Here, we verified whether TRAIL could influence the expression of NCX3 via modulation of the NGF/trkA system. Differentiated human neuroblastoma SH-SY5Y cells were incubated with TRAIL and, subsequently, the expression of the NCX3 protein was studied at different times by means of western blot analysis. Then, the expression of the phosphorylated forms of either trkA, ERK or AKT was analyzed at identical intervals. Western blot analysis revealed that the expression of NCX3 protein decreased in a time-dependent fashion in SH-SY5Y cells treated with TRAIL, to reach its minimum at 48 h. On the other hand, p-trkA, p-ERK, and p-AKT expression was increased in cells treated with TRAIL after 6 and 16 h; then it declined to nearly undetectable levels after 48 h. Results indicate that the increase in TRAIL expression occurring during neuronal damage may be responsible of NCX3 down-regulation and weakens its neuroprotective effects. The TRAIL system could thus represent a potential target for treatment of neuronal damage characterized by NCX3 function impairment.

Resveratrol treatment reduces the vulnerability of SH-SY5Y cells and cortical neurons overexpressing SOD1-G93A to Thimerosal toxicity through SIRT1/DREAM/PDYN pathway

In humans, mutation of glycine 93 to alanine of Cu⁺⁺/Zn⁺⁺ superoxide dismutase type-1 (SOD1-G93 A) has been associated to some familial cases of Amyotrophic Lateral Sclerosis (ALS). Several evidence proposed the involvement of environmental pollutants that like mercury could accelerate ALS symptoms. SH-SY5Y cells stably transfected with SOD1 and G93 A mutant of SOD1 constructs were exposed to non-toxic concentrations (0.01 μM) of ethylmercury thiosalicylate (thimerosal) for 24 h. Interestingly, we found that thimerosal, in SOD1-G93 A cells, but not in SOD1 cells, reduced cell survival. Furthermore, thimerosal-induced cell death occurred in a concentration dependent-manner and was prevented by the Sirtuin 1 (SIRT1) activator Resveratrol (RSV). Moreover, thimerosal decreased the protein expression of transcription factor Downstream Regulatory Element Antagonist Modulator (DREAM), but not DREAM gene. Interestingly, DREAM reduction was blocked by co-treatment with RSV, suggesting the participation of SIRT1 in determining this effect. Immunoprecipitation experiments in SOD1-G93 A cells exposed to thimerosal demonstrated that RSV increased DREAM deacetylation and reduced its polyubiquitination. In addition, RSV counteracted thimerosal-enhanced prodynorphin (PDYN) mRNA, a DREAM target gene. Furthermore, cortical neurons transiently transfected with SOD1-G93 A construct and exposed to thimerosal (0.5 μM/24 h) showed a reduction of DREAM and an up-regulation of the prodynorphin gene. Importantly, both the treatment with RSV or the transfection of siRNA against prodynorphin significantly reduced thimerosal-induced neurotoxicity, while DREAM knocking-down potentiated thimerosal-reduced cell survival. These results demonstrate the particular vulnerability of SOD1-G93 A neuronal cells to thimerosal and that RSV via SIRT1 counteracts the neurodetrimental effect of this toxicant by preventing DREAM reduction and prodynorphin up-regulation.

Protective effects of echinacoside against anoxia/reperfusion injury in H9c2 cells via up-regulating p-AKT and SLC8A3

Echinacoside is a natural ingredient with various pharmacological activities. In this study, we investigated the protective effects of echinacoside on cardiomyocytes (rat H9c2 cells) in an anoxia/reperfusion (A/R) model. Further, the regulatory function of sodium-calcium exchanger protein 3 (SLC8A3/NCX3) as well as the protein kinase B (AKT) signaling were studied. The present results indicated that echinacoside protected against A/R-induced apoptosis in a dose manner, which was characterized by a decrease in the apoptosis and caspase 3 protein levels in H9c2 cells. Further, Ca²⁺ uptake were dose-dependently reduced in H9c2 cells by echinacoside under A/R conditions. Whereas, relative mRNA expression of SLC8A3 and protein levels of SLC8A3 and p-AKT showed opposite tendency. On the one hand, the A/R-induced abnormalities in H9c2 cells were remarkably ameliorated by activated p-AKT and over-expression of SLC8A3 but aggravated by inhibited p-AKT, and the aggravated effection were ameliorated by echinacoside. Moreover, protein levels of SLC8A3 were positively regulated by p-AKT signaling. On the other hand, apoptosis and Ca²⁺ uptake as well as protein levels of caspase 3 were significantly increased by SLC8A3 silencing in H9c2 cells under normoxic conditions, and this symptom was remarkably reversed by echinacoside or Nimodipine (an antagonis of Ca²⁺) treatment. Collectively, echinacoside has showed a cardioprotective effect against A/R treatment in a dose dependent manner in vitro, and this cardioprotective effect was potentially achieved via up-regulating p-AKT and SLC8A3.

Neuronal NCX1 overexpression induces stroke resistance while knockout induces vulnerability via Akt

Three different Na⁺/Ca²⁺ exchanger (NCX) isoforms, NCX1, NCX2, and NCX3, are expressed in brain where they play a relevant role in maintaining Na⁺ and Ca²⁺ homeostasis. Although the neuroprotective roles of NCX2 and NCX3 in stroke have been elucidated, the relevance of NCX1 is still unknown because of embryonic lethality of its knocking-out, heart dysfunctions when it is overexpressed, and the lack of selectivity in currently available drugs. To overcome these limitations we generated two conditional genetically modified mice that upon tamoxifen administration showed a selective decrease or increase of NCX1 in cortical and hippocampal neurons. Interestingly, in cortex and hippocampus NCX1 overexpression increased, where NCX1 knock-out reduced, both exchanger activity and Akt1 phosphorylation, a neuronal survival signaling. More important, mice overexpressing NCX1 showed a reduced ischemic volume and an amelioration of focal and general deficits when subjected to transient middle cerebral artery occlusion. Conversely, NCX1-knock-out mice displayed a worsening of brain damage, focal and neurological deficits with a decrease in Akt phosphorylation. These results support the idea that NCX1 overexpression/activation may represent a feasible therapeutic opportunity in stroke intervention.

MC1568 Inhibits Thimerosal-Induced Apoptotic Cell Death by Preventing HDAC4 Up-Regulation in Neuronal Cells and in Rat Prefrontal Cortex

Ethylmercury thiosalicylate (thimerosal) is an organic mercury-based compound commonly used as an antimicrobial preservative that has been found to be neurotoxic. In contrast, histone deacetylases (HDACs) inhibition has been found to be neuroprotective against several environmental contaminants, such as polychlorinated biphenyls, di-2-ethylhexyl phthalate, and methylmercury. The aim of this study was to investigate the effect of HDAC inhibition on thimerosal-induced neurotoxicity in neuroblastoma cells and cortical neurons. Interestingly, we found that thimerosal, at 0.5 μM in SH-SY5Y cells and at 1 μM in neurons, caused cell death by activation of apoptosis, which was prevented by the HDAC class IIA inhibitor MC1568 but not the class I inhibitor MS275. Furthermore, thimerosal specifically increased HDAC4 protein expression but not that of HDACs 5, 6, 7, and 9. Western blot analysis revealed that MC1568 prevented thimerosal-induced HDAC4 increase. In addition, both HDAC4 knocking-down and MC1568 inhibited thimerosal-induced cell death in SH-SY5Y cells and cortical neurons. Importantly, intramuscular injection of 12 μg/kg thimerosal on postnatal days 7, 9, 11, and 15 increased HDAC4 levels in the prefrontal cortex (PFC), which decreased histone H4 acetylation in infant male rats, in parallel increased motor activity changes. In addition, coadministration of 40 mg/kg MC1568 (intraperitoneal injection) moderated the HDAC4 increase which reduced histone H4 deacetylation and caspase-3 cleavage in the PFC. Finally, open-field testing showed that thimerosal-induced motor activity changes are reduced by MC1568. These findings indicate that HDAC4 regulates thimerosal-induced cell death in neurons and that treatment with MC1568 prevents thimerosal-induced activation of caspase-3 in the rat PFC.

Ionic homeostasis in brain conditioning

Most of the current focus on developing neuroprotective therapies is aimed at preventing neuronal death. However, these approaches have not been successful despite many years of clinical trials mainly because the numerous side effects observed in humans and absent in animals used at preclinical level. Recently, the research in this field aims to overcome this problem by developing strategies which induce, mimic, or boost endogenous protective responses and thus do not interfere with physiological neurotransmission. Preconditioning is a protective strategy in which a subliminal stimulus is applied before a subsequent harmful stimulus, thus inducing a state of tolerance in which the injury inflicted by the challenge is mitigated. Tolerance may be observed in ischemia, seizure, and infection. Since it requires protein synthesis, it confers delayed and temporary neuroprotection, taking hours to develop, with a pick at 1–3 days. A new promising approach for neuroprotection derives from post-conditioning, in which neuroprotection is achieved by a modified reperfusion subsequent to a prolonged ischemic episode. Many pathways have been proposed as plausible mechanisms to explain the neuroprotection offered by preconditioning and post-conditioning. Although the mechanisms through which these two endogenous protective strategies exert their effects are not yet fully understood, recent evidence highlights that the maintenance of ionic homeostasis plays a key role in propagating these neuroprotective phenomena. The present article will review the role of protein transporters and ionic channels involved in the control of ionic homeostasis in the neuroprotective effect of ischemic preconditioning and post-conditioning in adult brain, with particular regards to the Na⁺/Ca2⁺ exchangers (NCX), the plasma membrane Ca2⁺-ATPase (PMCA), the Na⁺/H⁺ exchange (NHE), the Na⁺/K⁺/2Cl⁻ cotransport (NKCC) and the acid-sensing cation channels (ASIC). Ischemic stroke is the third leading cause of death and disability. Up until now, all clinical trials testing potential stroke neuroprotectants failed. For this reason attention of researchers has been focusing on the identification of brain endogenous neuroprotective mechanisms activated after cerebral ischemia. In this context, ischemic preconditioning and ischemic post-conditioning represent two neuroprotecive strategies to investigate in order to identify new molecular target to reduce the ischemic damage.

Sp3/REST/HDAC1/HDAC2 Complex Represses and Sp1/HIF-1/p300 Complex Activates ncx1 Gene Transcription, in Brain Ischemia and in Ischemic Brain Preconditioning, by Epigenetic Mechanism

The Na(+)-Ca(2+) exchanger 1 (NCX1) is reduced in stroke by the RE1-silencing transcription factor (REST), whereas it is increased in ischemic brain preconditioning (PC) by hypoxia-inducible factor 1 (HIF-1). Because ncx1 brain promoter (ncx1-Br) has five putative consensus sequences, named Sp1A-E, for the specificity protein (Sp) family of transcription factors (Sp1-4), we investigated the role of this family in regulating ncx1 transcription in rat cortical neurons. Here we found that Sp1 is a transcriptional activator, whereas Sp3 is a transcriptional repressor of ncx1, and that both bind ncx1-Br in a sequence-specific manner, modulating ncx1 transcription through the Sp1 sites C-E. Furthermore, by transient middle cerebral artery occlusion (tMCAO) in rats, the transcriptional repressors Sp3 and REST colocalized with the two histone-deacetylases (HDACs) HDAC1 and HDAC2 on the ncx1-Br, with a consequent hypoacetylation. Contrarily, in PC+tMCAO the transcriptional activators Sp1 and HIF-1 colocalized with histone acetyltransferase p300 on ncx1-Br with a consequent hyperacetylation. In addition, in neurons silenced with siRNA of NCX1 and subjected to oxygen and glucose deprivation (OGD) (3 h) plus reoxygenation (RX) (24 h), the neuroprotection of Class I HDAC inhibitor MS-275 was counteracted, whereas in neurons overexpressing NCX1 and subjected to ischemic preconditioning (PC+OGD/RX), the neurotoxic effect of p300 inhibitor C646 was prevented. Collectively, these results demonstrate that NCX1 expression is regulated by the Sp3/REST/HDAC1/HDAC2 complex in tMCAO and by the Sp1/HIF-1/p300 complex in PC+tMCAO and that epigenetic intervention, by modulating the acetylation of ncx1-Br, may be a strategy for the development of innovative therapeutic intervention in stroke.

A new cell-penetrating peptide that blocks the autoinhibitory XIP domain of NCX1 and enhances antiporter activity

The plasma membrane Na(+)/Ca(2+) exchanger (NCX) is a high-capacity ionic transporter that exchanges 3Na(+) ions for 1Ca(2+) ion. The first 20 amino acids of the f-loop, named exchanger inhibitory peptide (XIP(NCX1)), represent an autoinhibitory region involved in the Na(+)-dependent inactivation of the exchanger. Previous research has shown that an exogenous peptide having the same amino acid sequence as the XIP(NCX1) region exerts an inhibitory effect on NCX activity. In this study, we identified another regulatory peptide, named P1, which corresponds to the 562-688aa region of the exchanger. Patch-clamp analysis revealed that P1 increased the activity of the exchanger, whereas the XIP inhibited it. Furthermore, P1 colocalized with NCX1 thus suggesting a direct binding interaction. In addition, site-directed mutagenesis experiments revealed that the binding and the stimulatory effect of P1 requires a functional XIP(NCX1) domain on NCX1 thereby suggesting that P1 increases the exchanger activity by counteracting the action of this autoinhibitory sequence. Taken together, these results open a new strategy for developing peptidomimetic compounds that, by mimicking the functional pharmacophore of P1, might increase NCX1 activity and thus exert a therapeutic action in those diseases in which an increase in NCX1 activity might be helpful.

Involvement of the Na+/Ca2+ exchanger isoform 1 (NCX1) in neuronal growth factor (NGF)-induced neuronal differentiation through Ca2+-dependent Akt phosphorylation

NGF induces neuronal differentiation by modulating [Ca(2+)]i. However, the role of the three isoforms of the main Ca(2+)-extruding system, the Na(+)/Ca(2+) exchanger (NCX), in NGF-induced differentiation remains unexplored. We investigated whether NCX1, NCX2, and NCX3 isoforms could play a relevant role in neuronal differentiation through the modulation of [Ca(2+)]i and the Akt pathway. NGF caused progressive neurite elongation; a significant increase of the well known marker of growth cones, GAP-43; and an enhancement of endoplasmic reticulum (ER) Ca(2+) content and of Akt phosphorylation through an early activation of ERK1/2. Interestingly, during NGF-induced differentiation, the NCX1 protein level increased, NCX3 decreased, and NCX2 remained unaffected. At the same time, NCX total activity increased. Moreover, NCX1 colocalized and coimmunoprecipitated with GAP-43, and NCX1 silencing prevented NGF-induced effects on GAP-43 expression, Akt phosphorylation, and neurite outgrowth. On the other hand, the overexpression of its neuronal splicing isoform, NCX1.4, even in the absence of NGF, induced an increase in Akt phosphorylation and GAP-43 protein expression. Interestingly, tetrodotoxin-sensitive Na(+) currents and 1,3-benzenedicarboxylic acid, 4,4'-[1,4,10-trioxa-7,13-diazacyclopentadecane-7,13-diylbis(5-methoxy-6,12-benzofurandiyl)]bis-, tetrakis[(acetyloxy)methyl] ester-detected [Na(+)]i significantly increased in cells overexpressing NCX1.4 as well as ER Ca(2+) content. This latter effect was prevented by tetrodotoxin. Furthermore, either the [Ca(2+)]i chelator(1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) (BAPTA-AM) or the PI3K inhibitor LY 294002 prevented Akt phosphorylation and GAP-43 protein expression rise in NCX1.4 overexpressing cells. Moreover, in primary cortical neurons, NCX1 silencing prevented Akt phosphorylation, GAP-43 and MAP2 overexpression, and neurite elongation. Collectively, these data show that NCX1 participates in neuronal differentiation through the modulation of ER Ca(2+) content and PI3K signaling.

The properties, distribution and function of Na(+)-Ca(2+) exchanger isoforms in rat cutaneous sensory neurons

The Na(+)-Ca(2+) exchanger (NCX) appears to play an important role in the regulation of the high K(+)-evoked Ca(2+) transient in putative nociceptive dorsal root ganglion (DRG) neurons. The purpose of the present study was to (1) characterize the properties of NCX activity in subpopulations of DRG neurons, (2) identify the isoform(s) underlying NCX activity, and (3) begin to assess the function of the isoform(s) in vivo. In retrogradely labelled neurons from the glabrous skin of adult male Sprague-Dawley rats, NCX activity, as assessed with fura-2-based microfluorimetry, was only detected in putative nociceptive IB4+ neurons. There were two modes of NCX activity: one was evoked in response to relatively large and long lasting (∼325 nm for >12 s) increases in the concentration of intracellular Ca(2+) ([Ca(2+)]i), and a second was active at resting [Ca(2+)]i > ∼150 nm. There also were two modes of evoked activity: one that decayed relatively rapidly (<5 min) and a second that persisted (>10 min). Whereas mRNA encoding all three NCX isoforms (NCX1-3) was detected in putative nociceptive cutaneous neurons with single cell PCR, pharmacological analysis and small interfering RNA (siRNA) knockdown of each isoform in vivo suggested that NCX2 and 3 were responsible for NCX activity. Western blot analyses suggested that NCX isoforms were differentially distributed within sensory neurons. Functional assays of excitability, action potential propagation, and nociceptive behaviour suggest NCX activity has little influence on excitability per se, but instead influences axonal conduction velocity, resting membrane potential, and nociceptive threshold. Together these results indicate that the function of NCX in the regulation of [Ca(2+)]i in putative nociceptive neurons may be unique relative to other cells in which these exchanger isoforms have been characterized and it has the potential to influence sensory neuron properties at multiple levels.

MicroRNA-103-1 selectively downregulates brain NCX1 and its inhibition by anti-miRNA ameliorates stroke damage and neurological deficits

Na(+)/Ca2+ exchanger (NCX) is a plasma membrane transporter that, by regulating Ca2+ and Na(+) homeostasis, contributes to brain stroke damage. The objectives of this study were to investigate whether there might be miRNAs in the brain able to regulate NCX1 expression and, thereafter, to set up a valid therapeutic strategy able to reduce stroke-induced brain damage by regulating NCX1 expression. Thus, we tested whether miR-103-1, a microRNA belonging to the miR-103/107 family that on the basis of sequence analysis might be a potential NCX1 regulator, could control NCX1 expression. The role of miR-103-1 was assessed in a rat model of transient cerebral ischemia by evaluating the effect of the correspondent antimiRNA on both brain infarct volume and neurological deficits. NCX1 expression was dramatically reduced when cortical neurons were exposed to miR-103-1. This alleged tight regulation of NCX1 by miR-103-1 was further corroborated by luciferase assay. Notably, antimiR-103-1 prevented NCX1 protein downregulation induced by the increase in miR-103-1 after brain ischemia, thereby reducing brain damage and neurological deficits. Overall, the identification of a microRNA able to selectively regulate NCX1 in the brain clarifies a new important molecular mechanism of NCX1 regulation in the brain and offers the opportunity to develop a new therapeutic strategy for stroke.

Endoplasmic reticulum refilling and mitochondrial calcium extrusion promoted in neurons by NCX1 and NCX3 in ischemic preconditioning are determinant for neuroprotection

Ischemic preconditioning (IPC), an important endogenous adaptive mechanism of the CNS, renders the brain more tolerant to lethal cerebral ischemia. The molecular mechanisms responsible for the induction and maintenance of ischemic tolerance in the brain are complex and still remain undefined. Considering the increased expression of the two sodium calcium exchanger (NCX) isoforms, NCX1 and NCX3, during cerebral ischemia and the relevance of nitric oxide (NO) in IPC modulation, we investigated whether the activation of the NO/PI3K/Akt pathway induced by IPC could regulate calcium homeostasis through changes in NCX1 and NCX3 expression and activity, thus contributing to ischemic tolerance. To this aim, we set up an in vitro model of IPC by exposing cortical neurons to a 30-min oxygen and glucose deprivation (OGD) followed by 3-h OGD plus reoxygenation. IPC was able to stimulate NCX activity, as revealed by Fura-2AM single-cell microfluorimetry. This effect was mediated by the NO/PI3K/Akt pathway since it was blocked by the following: (a) the NOS inhibitors L-NAME and 7-Nitroindazole, (b) the IP3K/Akt inhibitors LY294002, wortmannin and the Akt-negative dominant, (c) the NCX1 and NCX3 siRNA. Intriguingly, this IPC-mediated upregulation of NCX1 and NCX3 activity may control calcium level within endoplasimc reticulum (ER) and mitochondria, respectively. In fact, IPC-induced NCX1 upregulation produced an increase in ER calcium refilling since this increase was prevented by siNCX1. Moreover, by increasing NCX3 activity, IPC reduced mitochondrial calcium concentration. Accordingly, the inhibition of NCX by CGP37157 reverted this effect, thus suggesting that IPC-induced NCX3-increased activity may improve mitochondrial function during OGD/reoxygenation. Collectively, these results indicate that IPC-induced neuroprotection may occur through the modulation of calcium homeostasis in ER and mitochondria through NO/PI3K/Akt-mediated NCX1 and NCX3 upregulation.

Loss of the AE3 Cl(-)/HCO(-) 3 exchanger in mice affects rate-dependent inotropy and stress-related AKT signaling in heart

Cl(-)/HCO(-) 3 exchangers are expressed abundantly in cardiac muscle, suggesting that HCO(-) 3 extrusion serves an important function in heart. Mice lacking Anion Exchanger Isoform 3 (AE3), a major cardiac Cl(-)/HCO(-) 3 exchanger, appear healthy, but loss of AE3 causes decompensation in a hypertrophic cardiomyopathy (HCM) model. Using intra-ventricular pressure analysis, in vivo pacing, and molecular studies we identified physiological and biochemical changes caused by loss of AE3 that may contribute to decompensation in HCM. AE3-null mice had normal cardiac contractility under basal conditions and after β-adrenergic stimulation, but pacing of hearts revealed that frequency-dependent inotropy was blunted, suggesting that AE3-mediated HCO(-) 3 extrusion is required for a robust force-frequency response (FFR) during acute biomechanical stress in vivo. Modest changes in expression of proteins that affect Ca(2+)-handling were observed, but Ca(2+)-transient analysis of AE3-null myocytes showed normal twitch-amplitude and Ca(2+)-clearance. Phosphorylation and expression of several proteins implicated in HCM and FFR, including phospholamban (PLN), myosin binding protein C, and troponin I were not altered in hearts of paced AE3-null mice; however, phosphorylation of Akt, which plays a central role in mechanosensory signaling, was significantly higher in paced AE3-null hearts than in wild-type controls and phosphorylation of AMPK, which is affected by Akt and is involved in energy metabolism and some cases of HCM, was reduced. These data show loss of AE3 leads to impaired rate-dependent inotropy, appears to affect mechanical stress-responsive signaling, and reduces activation of AMPK, which may contribute to decompensation in heart failure.

Does Na⁺/Ca²⁺ exchanger, NCX, represent a new druggable target in stroke intervention?

Stroke causes a rapid cell death in the core of the injured region and triggers mechanisms in surrounding penumbra area that leads to changes in concentrations of several ions like intracellular Ca²⁺, Na⁺, H⁺, K⁺, and radicals such as reactive oxygen species and reactive nitrogen species. When a dysregulation of homeostasis of these messengers occurs, it can trigger cell death. In particular, it is widely accepted that a critical factor in determining neuronal death during cerebral ischemia is progressive dysregulation of Ca²⁺, Na⁺, K⁺, and H⁺ homeostasis that activate several death pathways, including oxidative and nitrosative stress, mitochondrial dysfunction, protease activation, and apoptosis. In the last decade, several seminal experimental works are markedly changing the scenario of research of principal players of an ischemic event. Indeed, some plasma membrane channels and transporters, involved in the control of Ca²⁺, Na⁺, K⁺, and H⁺ ion influx or efflux and, therefore, responsible for maintaining the homeostasis of these four cations, might function as crucial players in initiation of brain ischemic process. Indeed, these proteins, by regulating ionic homeostasis, may provide the molecular basis underlying glutamate-independent Ca²⁺ and Na⁺ overload mechanisms in neuronal ischemic cell death and, most importantly, may represent more suitable molecular targets for therapeutic intervention. Recently, a great deal of interest has been devoted to clarify the role of the plasma membrane protein known as Na⁺/Ca²⁺ exchanger, a transporter able to control Na⁺ and Ca²⁺ homeostasis. In this review, the pathophysiological role of NCX and its implication as a potential target in stroke intervention will be examined.

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.

NCX1 is a new rest target gene: role in cerebral ischemia

The Na(+)-Ca(2+) exchanger 1 (NCX1), a bidirectional transporter that mediates the electrogenic exchange of one calcium ion for three sodium ions across the plasma membrane, is known to be involved in brain ischemia. Since the RE1-silencing transcription factor (REST) is a key modulator of neuronal gene expression in several neurological conditions, we studied the possible involvement of REST in regulating NCX1 gene expression and activity in stroke. We found that: (1) REST binds in a sequence specific manner and represses through H4 deacetylation, ncx1 gene in neuronal cells by recruting CoREST, but not mSin3A. (2) In neurons and in SH-SY5Y cells REST silencing by siRNA and site-direct mutagenesis of REST consensus sequence on NCX1 brain promoter determined an increase in NCX1 promoter activity. (3) By contrast, REST overexpression caused a reduction in NCX1 protein expression and activity. (4) Interestingly, in rats subjected to transient middle cerebral artery occlusion (tMCAO) and in organotypic hippocampal slices or SH-SY5Y cells exposed to oxygen and glucose deprivation (OGD) plus reoxygenation (RX), the increase in REST was associated with a decrease in NCX1. However, this reduction was reverted by REST silencing. (5) REST knocking down, along with the deriving NCX1 overexpression in the deep V and VIb cortical layers caused a marked reduction in infarct volume after tMCAO. Double silencing of REST and NCX1 completely abolished neuroprotection induced by siREST administration. Collectively, these results demonstrate that REST, by regulating NCX1 expression, may represent a potential druggable target for the treatment of brain ischemia.

Neurounina-1, a novel compound that increases Na+/Ca2+ exchanger activity, effectively protects against stroke damage

Previous studies have demonstrated that the knockdown or knockout of the three Na(+)/Ca(2+) exchanger (NCX) isoforms, NCX1, NCX2, and NCX3, worsens ischemic brain damage. This suggests that the activation of these antiporters exerts a neuroprotective action against stroke damage. However, drugs able to increase the activity of NCXs are not yet available. We have here succeeded in synthesizing a new compound, named neurounina-1 (7-nitro-5-phenyl-1-(pyrrolidin-1-ylmethyl)-1H-benzo[e][1,4]diazepin-2(3H)-one), provided with an high lipophilicity index and able to increase NCX activity. Ca(2+) radiotracer, Fura-2 microfluorimetry, and patch-clamp techniques revealed that neurounina-1 stimulated NCX1 and NCX2 activities with an EC(50) in the picomolar to low nanomolar range, whereas it did not affect NCX3 activity. Furthermore, by using chimera strategy and site-directed mutagenesis, three specific molecular determinants of NCX1 responsible for neurounina-1 activity were identified in the α-repeats. Interestingly, NCX3 became responsive to neurounina-1 when both α-repeats were replaced with the corresponding regions of NCX1. In vitro studies showed that 10 nM neurounina-1 reduced cell death of primary cortical neurons exposed to oxygen-glucose deprivation followed by reoxygenation. Moreover, in vitro, neurounina-1 also reduced γ-aminobutyric acid (GABA) release, enhanced GABA(A) currents, and inhibited both glutamate release and N-methyl-d-aspartate receptors. More important, neurounina-1 proved to have a wide therapeutic window in vivo. Indeed, when administered at doses of 0.003 to 30 μg/kg i.p., it was able to reduce the infarct volume of mice subjected to transient middle cerebral artery occlusion even up to 3 to 5 hours after stroke onset. Collectively, the present study shows that neurounina-1 exerts a remarkable neuroprotective effect during stroke and increases NCX1 and NCX2 activities.

ERK1/2, p38, and JNK regulate the expression and the activity of the three isoforms of the Na+ /Ca2+ exchanger, NCX1, NCX2, and NCX3, in neuronal PC12 cells

We evaluated whether changes in expression and activity of the three sodium/calcium exchanger isoforms, NCX1, NCX2, and NCX3 occurred in PC12 cells when the extracellular-signal-regulated kinases 1/2 (ERK1/2), c-Jun N-terminal kinases (JNK), and p38 mitogen-activated protein kinases (MAPKs) were silenced, pharmacologically blocked, or activated with nerve growth factor (NGF). Several findings suggesting that MAPKs control NCX emerged: (1) A decrease in NCX1 and NCX3 basal expression occurred when JNK or MEK1, the extracellular-signal-regulated kinases 1/2 upstream activator, were pharmacologically blocked, respectively; (2) NGF increased cAMP response element-binding 1 (CREB1) and Specificity Protein 1 (Sp1) binding to ncx1 promoter and CREB1 binding to two different sequences close to ncx2 transcription start site on genomic DNA; (3) An up-regulation of NCX1 and NCX3, abrogated upon either MEK1 or p38 blockade, and a down-regulation of NCX2, abolished upon p38 blockade, occurred upon NGF-induced MAPK activation. The NCX1 up-regulation was abolished upon either CREB1 or Sp1 silencing, whereas NCX2 down-regulation was abrogated only by CREB1 silencing. The NCX3 up-regulation was unaffected by CREB1 or Sp1 silencing and abolished upon proteasomal inhibition; (4) Whole-cell Na(+) /Ca(2+) exchange decreased when MEK1 and JNK were blocked and increased when MAPKs were activated by NGF. Collectively, these results demonstrate a MAPK-dependent regulation of NCX expression and activity which could be relevant in mediating some of the effects of MAPKs in neurons.

Intracellular and plasma membrane-initiated pathways involved in the [Ca2+]i elevations induced by iodothyronines (T3 and T2) in pituitary GH3 cells

The role of 3,5,3'-triiodo-l-thyronine (T3) and its metabolite 3,5-diiodo-l-thyronine (T2) in modulating the intracellular Ca(2+) concentration ([Ca(2+)](i)) and endogenous nitric oxide (NO) synthesis was evaluated in pituitary GH(3) cells in the absence or presence of extracellular Ca(2+). When applied in Ca(2+)-free solution, T2 and T3 increased [Ca(2+)](i), in a dose-dependent way, and NO levels. Inhibition of neuronal NO synthase by N(G)-nitro-l-arginine methyl ester and l-n(5)-(1-iminoethyl)ornithine hydrochloride significantly reduced the [Ca(2+)](i) increase induced by T2 and T3. However, while depletion of inositol trisphosphate-dependent Ca(2+) stores did not interfere with the T2- and T3-induced [Ca(2+)](i) increases, the inhibition of phosphatidylinositol 3-kinase by LY-294002 and the dominant negative form of Akt mutated at the ATP binding site prevented these effects. Furthermore, the mitochondrial protonophore carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone prevented the increases in both [Ca(2+)](i) and NO elicited by T2 or T3. Interestingly, rotenone blocked the early [Ca(2+)](i) increases elicited by T2 and T3, while antimycin prevented only that elicited by T3. Inhibition of mitochondrial Na(+)/Ca(2+) exchanger by CGP37157 significantly reduced the [Ca(2+)](i) increases induced by T2 and T3. In the presence of extracellular calcium (1.2 mM), under carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone, T2 and T3 increased both [Ca(2+)](i) and intracellular Na(+) concentration; nimodipine reduced the [Ca(2+)](i) increases elicited by T2 and T3, but inhibition of NO synthase and blockade of the Na(+)/H(+) pump by 5-(N-ethyl-N-isopropyl)amiloride prevented only that elicited by T3; and CB-DMB, bisindolylmaleimide, and LY-294002 (inhibitors of the Na(+)/Ca(2+) exchanger, PKC, and phosphatidylinositol 3-kinase, respectively) failed to modify the T2- and T3-induced effects. Collectively, the present results suggest that T2 and T3 exert short-term nongenomic effects on intracellular calcium and NO by modulating plasma membrane and mitochondrial pathways that differ between these iodothyronines.

Alternative strategies in arrhythmia therapy: evaluation of Na/Ca exchange as an anti-arrhythmic target

The search for alternative anti-arrhythmic strategies is fueled by an unmet medical need as well as by the opportunities arising from identification of novel targets and novel drugs. Na/Ca exchange is a potential target involved in several types of arrhythmias, such as those related to ischemia-reperfusion, heart failure and also some forms of genetic arrhythmias. Inhibition of Na/Ca exchange is theoretically not only anti-arrhythmic but also increases cellular Ca(2+) content. This could be an advantage in conditions of low inotropy, such as in heart failure, but may also worsen conditions such as the recovery from ischemia or relaxation abnormalities. With the available drugs such as KB-R7943 and SEA-0400 these theories have now been tested in a number of cellular and in vivo models. Experience is overall rather positive and seems less hampered by the potential drawbacks than expected. This may be because the currently available drugs are not highly selective, with additional benefit derived from concurrent effects. While this precludes a definite answer regarding the benefit of a pure NCX inhibitor, they indicate that Na/Ca exchange inhibition as part of a multi-target strategy is an avenue to be considered. Such studies will need further 'bench' work and testing in relevant preclinical models, including chronic disease.

Metabolic regulation of the squid nerve Na⁺/Ca²⁺ exchanger: recent kinetic, biochemical and structural developments

The Na⁺/Ca²⁺ exchangers are structural membrane proteins, essential for the extrusion of Ca²⁺ from most animal cells. Apart from the transport sites, they have several interacting ionic and metabolic sites located at the intracellular loop of the exchanger protein. One of these, the intracellular Ca²⁺ regulatory sites, are essential and must be occupied by Ca²⁺ to allow any type of ion (Na⁺ or Ca²⁺) translocation. Intracellular protons and Na⁺ are inhibitory by reducing the affinity of the regulatory sites for Ca²⁺; MgATP stimulates by antagonizing H⁺ and Na⁺. We have proposed a kinetic scheme to explain all ionic and metabolic regulation of the squid nerve Na⁺/Ca²⁺ exchanger. This model uniquely accounts for most of the new kinetic data provided here; however, none of the existing models can explain the trans effects of the Ca(i)²⁺-regulatory sites on external cation transport sites; i.e. all models are incomplete. MgATP up-regulation of the squid Na⁺/Ca²⁺ exchanger requires a cytosolic protein, which has been recently identified as a member of the lipocalin super family of Lipid Binding Proteins (LBP or FABP) of 132 amino acids (ReP1-NCXSQ, access to GenBank EU981897). This protein was cloned, expressed and purified. To be active, ReP1-NCXSQ must be phosphorylated from MgATP by a kinase present in the plasma membrane. Phosphorylated ReP1-NCXSQ can stimulate the exchanger in the absence of ATP. Experiments with proteoliposomes proved that this up-regulation can take place just with the lipid membrane and the exchanger protein. The structure of ReP1-NCXSQ predicted from the amino acid sequence has been confirmed by X-ray crystal analysis; it has a "barrel" formed by ten beta sheets and two alpha helices, with a lipid coordinated by hydrogen bonds with Arg 126 and Tyr 128.

In dialyzed squid axons oxidative stress inhibits the Na+/Ca2+ exchanger by impairing the Cai2+-regulatory site

The Na(+)/Ca(2+) exchanger, a major mechanism by which cells extrude calcium, is involved in several physiological and physiopathological interactions. In this work we have used the dialyzed squid giant axon to study the effects of two oxidants, SIN-1-buffered peroxynitrite and hydrogen peroxide (H(2)O(2)), on the Na(+)/Ca(2+) exchanger in the absence and presence of MgATP upregulation. The results show that oxidative stress induced by peroxynitrite and hydrogen peroxide inhibits the Na(+)/Ca(2+) exchanger by impairing the intracellular Ca(2+) (Ca(i)(2+))-regulatory sites, leaving unharmed the intracellular Na(+)- and Ca(2+)-transporting sites. This effect is efficiently counteracted by the presence of MgATP and by intracellular alkalinization, conditions that also protect H(i)(+) and (H(i)(+) + Na(i)(+)) inhibition of Ca(i)(2+)-regulatory sites. In addition, 1 mM intracellular EGTA reduces oxidant inhibition. However, once the effects of oxidants are installed they cannot be reversed by either MgATP or EGTA. These results have significant implications regarding the role of the Na(+)/Ca(2+) exchanger in response to pathological conditions leading to tissue ischemia-reperfusion and anoxia/reoxygenation; they concur with a marked reduction in ATP concentration, an increase in oxidant production, and a rise in intracellular Ca(2+) concentration that seems to be the main factor responsible for cell damage.

Intestinal Na(+)/Ca(2+) exchanger protein and gene expression are regulated by 1,25(OH)(2)D(3) in vitamin D-deficient chicks

The role of 1,25(OH)(2)D(3) on the intestinal NCX activity was studied in vitamin D-deficient chicks (-D) as well as the hormone effect on NCX1 protein and gene expression and the potential molecular mechanisms underlying the responses. Normal, -D and -D chicks treated with cholecalciferol or 1,25(OH)(2)D(3) were employed. In some experiments, -D chicks were injected with cycloheximide or with cycloheximide and 1,25(OH)(2)D(3) simultaneously. NCX activity was decreased by -D diet, returning to normal values after 50 IU daily of cholecalciferol/10 days or a dose of 1μg calcitriol/kg of b.w. for 15 h. Cycloheximide blocked NCX activity enhancement produced by 1,25(OH)(2)D(3). NCX1 protein and gene expression were diminished by -D diet and enhanced by 1,25(OH)(2)D(3). Vitamin D receptor expression was decreased by -D diet, effect that disappeared after 1,25(OH)(2)D(3) treatment. Rapid effects of 1,25(OH)(2)D(3) on intestinal NCX activity were also demonstrated. The abolition of the rapid effects through addition of Rp-cAMPS and staurosporine suggests that non genomic effects of 1,25(OH)(2)D(3) on NCX activity are mediated by activation of PKA and PKC pathways. In conclusion, 1,25(OH)(2)D(3) enhances the intestinal NCX activity in -D chicks through genomic and non genomic mechanisms.

High levels of synaptosomal Na(+)-Ca(2+) exchangers (NCX1, NCX2, NCX3) co-localized with amyloid-beta in human cerebral cortex affected by Alzheimer's disease

Synaptosomal expression of NCX1, NCX2, and NCX3, the three variants of the Na(+)-Ca(2+) exchanger (NCX), was investigated in Alzheimer's disease parietal cortex. Flow cytometry and immunoblotting techniques were used to analyze synaptosomes prepared from cryopreserved brain of cognitively normal aged controls and late stage Alzheimer's disease patients. Major findings that emerged from this study are: (1) NCX1 was the most abundant NCX isoform in nerve terminals of cognitively normal patients; (2) NCX2 and NCX3 protein levels were modulated in parietal cortex of late stage Alzheimer's disease: NCX2 positive terminals were increased in the Alzheimer's disease cohort while counts of NCX3 positive terminals were reduced; (3) NCX1, NCX2 and NCX3 isoforms co-localized with amyloid-beta in synaptic terminals and all three variants are up-regulated in nerve terminals containing amyloid-beta. Taken together, these data indicate that NCX isoforms are selectively regulated in pathological terminals, suggesting different roles of each NCX isoform in Alzheimer's disease terminals.

NCX1 is a novel target gene for hypoxia-inducible factor-1 in ischemic brain preconditioning

BACKGROUND AND PURPOSE

The sodium-calcium exchanger-1 (NCX1) represents a key mediator for maintaining [Na(+)](i) and [Ca(2+)](i) homeostasis. Although changes in NCX1 protein and transcript expression have been detected during stroke, its transcriptional regulation is still unknown. Thus far, however, there is evidence that hypoxia-inducible factor-1 (HIF-1) is a nuclear factor required for transcriptional activation of several genes implicated in stroke. The main objective of this study was to investigate whether NCX1 gene might be a novel target of HIF-1 in the brain.

METHODS

Here we report that: (1) in neuronal cells, NCX1 increased expression after oxygen and glucose deprivation or cobalt-induced HIF-1 activation was prevented by silencing HIF-1; (2) the brain NCX1 promoter cloned upstream of the firefly-luciferase gene contained 2 regions of HIF-1 target genes called hypoxia-responsive elements that are sensitive to oxygen and glucose deprivation or cobalt chloride; (3) HIF-1 specifically bound hypoxia-responsive elements on brain NCX1, as demonstrated by band-shift and chromatin immunoprecipitation assays; (4) HIF-1α silencing prevented NCX1 upregulation and neuroprotection induced by ischemic preconditioning; and (5) NCX1 silencing partially reverted the preconditioning-induced neuroprotection in rats.

CONCLUSIONS

NCX1 gene is a novel HIF-1 target, and HIF-1 exerts its prosurvival role through NCX1 upregulation during brain preconditioning.

The NCX3 isoform of the Na+/Ca2+ exchanger contributes to neuroprotection elicited by ischemic postconditioning

It has been recently shown that a short sublethal brain ischemia subsequent to a prolonged harmful ischemic episode may confer ischemic neuroprotection, a phenomenon termed ischemic postconditioning. Na(+)/Ca(2+) exchanger (NCX) isoforms, NCX1, NCX2, and NCX3, are plasma membrane ionic transporters widely distributed in the brain and involved in the control of Na(+) and Ca(2+) homeostasis and in the progression of stroke damage. The objective of this study was to evaluate the role of these three proteins in the postconditioning-induced neuroprotection. The NCX protein and mRNA expression was evaluated at different time points in the ischemic temporoparietal cortex of rats subjected to tMCAO alone or to tMCAO plus ischemic postconditioning. The results of this study showed that NCX3 protein and ncx3 mRNA were upregulated in those brain regions protected by postconditioning treatment. These changes in NCX3 expression were mediated by the phosphorylated form of the ubiquitously expressed serine/threonine protein kinase p-AKT, as the p-AKT inhibition prevented NCX3 upregulation. The relevant role of NCX3 during postconditioning was further confirmed by results showing that NCX3 silencing, induced by intracerebroventricular infusion of small interfering RNA (siRNA), partially reverted the postconditioning-induced neuroprotection. The results of this study support the idea that the enhancement of NCX3 expression and activity might represent a reasonable strategy to reduce the infarct extension after stroke.

Nuclear and mitochondrial signalling Akts in cardiomyocytes

Biological actions resulting from phosphoinositide synthesis trigger multiple downstream signalling cascades by recruiting proteins with pleckstrin homology domains, including phosphoinositide-dependent kinase-1 and protein kinase B (also known as Akt). Retrospectively, more attention has been focused on the plasma membrane-associated interactions of these molecules and resulting cytoplasmic target activation. The complex biological activities exerted by Akt activation suggest, however, that more subtle and complex subcellular control mechanisms are involved. This review examines the regulation of Akt activity from the perspective of subcellular compartmentalization and focuses specifically upon the actions of Akt activation downstream from phosphoinositide synthesis that influence cell biology by altering nuclear signalling leading to Pim-1 kinase induction as well as hexokinase phosphorylation that, together with Akt, serves to preserve mitochondrial integrity.

Akt mediated mitochondrial protection in the heart: metabolic and survival pathways to the rescue

Cardiomyocyte death is now recognized as a critical factor in the development of heart disease. Mitochondria are not only responsible for energy production to ensure that cardiac output meets the body's energy demands, but they serve as critical integrators of cell survival signals. Numerous stressors are known to induce cell death by necrosis and/or apoptosis mediated through mitochondrial dysregulation. Anti- and pro-apoptotic Bcl-2 family proteins regulate apoptosis by controlling mitochondrial outer membrane permeability, whereas opening of the mitochondrial permeability transition pore (PT-pore) induces large amplitude permeability of the inner membrane and consequent rupture of the outer membrane. Akt is one of the best described survival kinases activated by receptor ligands and its activation preserves mitochondrial integrity and protects cardiomyocytes against necrotic and apoptotic death. The mechanisms responsible for Akt-mediated mitochondrial protection have not been fully elucidated. There is, however, accumulating evidence that multiple Akt target molecules, recruited through both transcriptional and post-transcriptional mechanisms, directly impinge upon and protect mitochondria. In this review we discuss mechanisms by which Akt activation can effect changes at the mitochondria that protect cardiomyocytes and attenuate pathophysiological responses of the heart.

Cellular oxygen sensing, signalling and how to survive translational arrest in hypoxia

Hypoxia is a consequence of inadequate oxygen availability. At the cellular level, lowered oxygen concentration activates signal cascades including numerous receptors, ion channels, second messengers, as well as several protein kinases and phosphatases. This, in turn, activates trans-factors like transcription factors, RNA-binding proteins and miRNAs, mediating an alteration in gene expression control. Each cell type has its unique constellation of oxygen sensors, couplers and effectors that determine the activation and predominance of several independent hypoxia-sensitive pathways. Hence, altered gene expression patterns in hypoxia result from a complex regulatory network with multiple divergences and convergences. Although hundreds of genes are activated by transcriptional control in hypoxia, metabolic rate depression, as a consequence of reduced ATP level, causes inhibition of mRNA translation. In a multi-phase response to hypoxia, global protein synthesis is suppressed, mainly by phosphorylation of eIF2-alpha by PERK and inhibition of mTOR, causing suppression of 5'-cap-dependent mRNA translation. Growing evidence suggests that mRNAs undergo sorting at stress granules, which determines the fate of mRNA as to whether being translated, stored, or degraded. Data indicate that translation is suppressed only at 'free' polysomes, but is active at subsets of membrane-bound ribosomes. The recruitment of specific mRNAs into subcellular compartments seems to be crucial for local mRNA translation in prolonged hypoxia. Furthermore, ribosomes themselves may play a significant role in targeting mRNAs for translation. This review summarizes the multiple facets of the cellular adaptation to hypoxia observed in mammals.

Anoxia-induced NF-kappaB-dependent upregulation of NCX1 contributes to Ca2+ refilling into endoplasmic reticulum in cortical neurons

BACKGROUND AND PURPOSE

The 3 gene products of the Na(+)/Ca(2+) exchanger (NCX), viz, NCX1, NCX2, and NCX3, may play a pivotal role in the pathophysiology of brain ischemia. The aim of this study was to investigate the transductional and posttranslational mechanisms involved in the expression of these isoforms during oxygen and glucose deprivation and their role in endoplasmic reticulum Ca(2+) refilling in cortical neurons.

METHODS

NCX1, NCX2, and NCX3 transcript and protein expression was evaluated in primary cortical neurons by reverse transcriptase-polymerase chain reaction and Western blot. NCX currents (I(NCX)) and cytosolic Ca(2+) concentrations ([Ca(2+)](i)) were monitored by means of patch-clamp in whole-cell configuration and Fura-2AM single-cell video imaging, respectively.

RESULTS

Exposure of cortical neurons to 3 hours of oxygen and glucose deprivation yielded dissimilar effects on the 3 isoforms. First, it induced an upregulation in NCX1 transcript and protein expression. This change was exerted at the transcriptional level because the inhibition of nuclear factor kappa B translocation by small interfering RNA against p65 and SN-50 prevented oxygen and glucose deprivation-induced NCX1 upregulation. Second, it elicited a downregulation of NCX3 protein expression. This change, unlike NCX1, was exerted at the posttranscriptional level because it was prevented by the proteasome inhibitor MG-132. Finally, we found that it significantly increased I(NCX) both in the forward and reverse modes of operation and promoted an increase in ER Ca(2+) accumulation. Interestingly, such accumulation was prevented by the silencing of NCX1 and the NCX inhibitor CB-DMB that triggered caspase-12 activation.

CONCLUSIONS

These results suggest that nuclear factor kappa B-dependent NCX1 upregulation may play a fundamental role in Ca(2+) refilling in the endoplasmic reticulum, thus helping neurons to prevent endoplasmic reticulum stress during oxygen and glucose deprivation.

Amiloride inhibits hydrogen peroxide-induced Ca2+ responses in human CNS pericytes

The aims of the present study were to investigate the mechanisms of Ca(2+) signaling caused by hydrogen peroxide in CNS pericytes. In cultured human brain microvascular pericytes, cytosolic Ca(2+) concentration was measured by means of fura-2 fluorescence. Reverse transcription and polymerase chain reaction was performed to examine the expression of mRNA. Knockdown of Na(+)/H(+) exchanger (NHE) was done by transfecting the cells with specific double-strand siRNAs for NHE. Externally applied hydrogen peroxide dose-dependently (100 microM-10 mM) increased cytosolic Ca(2+) in human CNS pericytes. Cytosolic Ca(2+) remained high after wash-out of hydrogen peroxide. However, the addition of dithiothreitol rapidly reversed cytosolic Ca(2+) to the resting level. The hydrogen peroxide-induced Ca(2+) increase was not inhibited by nicardipine, Gd(3+), La(3+), or omission of external Ca(2+). Neither thapsigargin nor carbonyl cyanide 4-trifluoromethoxyphenylhydrazone attenuated the hydrogen peroxide-induced Ca(2+) rise. Amiloride and its derivatives, benzamil and hexamethylene amiloride reversed the hydrogen peroxide-induced Ca(2+) increase. Human CNS pericytes expressed acid sensing ion channel (ASIC) 1a, Na(+)/Ca(2+) exchanger (NCX) 1, Na(+)/H(+) exchanger (NHE) 1, and NHE7. However, the removal of external Na(+), treatment with KB-R 7943 and mibefradil, or knockdown of NHE1 and NHE7 did not affect the hydrogen peroxide-induced Ca(2+) increase. Hydrogen peroxide releases Ca(2+) from intracellular Ca(2+) pool via an amiloride-sensitive protein, which is controlled by oxidation of thiol group in human CNS pericytes.