Na+/Ca2+ exchanger in Na+ efflux-Ca2+ influx mode of operation exerts a neuroprotective role in cellular models of in vitro anoxia and in vivo cerebral ischemia

Excitatory Synaptic Transmission in Ischemic Stroke: A New Outlet for Classical Neuroprotective Strategies

Stroke is one of the leading causes of death and disability in the world, of which ischemia accounts for the majority. There is growing evidence of changes in synaptic connections and neural network functions in the brain of stroke patients. Currently, the studies on these neurobiological alterations mainly focus on the principle of glutamate excitotoxicity, and the corresponding neuroprotective strategies are limited to blocking the overactivation of ionic glutamate receptors. Nevertheless, it is disappointing that these treatments often fail because of the unspecificity and serious side effects of the tested drugs in clinical trials. Thus, in the prevention and treatment of stroke, finding and developing new targets of neuroprotective intervention is still the focus and goal of research in this field. In this review, we focus on the whole processes of glutamatergic synaptic transmission and highlight the pathological changes underlying each link to help develop potential therapeutic strategies for ischemic brain damage. These strategies include: (1) controlling the synaptic or extra-synaptic release of glutamate, (2) selectively blocking the action of the glutamate receptor NMDAR subunit, (3) increasing glutamate metabolism, and reuptake in the brain and blood, and (4) regulating the glutamate system by GABA receptors and the microbiota–gut–brain axis. Based on these latest findings, it is expected to promote a substantial understanding of the complex glutamate signal transduction mechanism, thereby providing excellent neuroprotection research direction for human ischemic stroke (IS).

The dual face of glutamate: from a neurotoxin to a potential survival factor-metabolic implications in health and disease

Glutamate is the major excitatory neurotransmitter in the central nervous system. Beyond this function, glutamate also plays a key role in intermediary metabolism in all organs and tissues, linking carbohydrate and amino acid metabolism via the tricarboxylic acid cycle. Under both physiological and pathological conditions, we have recently found that the ability of glutamate to fuel cell metabolism selectively relies on the activity of two main transporters: the sodium-calcium exchanger (NCX) and the sodium-dependent excitatory amino-acid transporters (EAATs). In ischemic settings, when glutamate is administered at the onset of the reoxygenation phase, the coordinate activity of EAAT and NCX allows glutamate to improve cell viability by stimulating ATP production. So far, this phenomenon has been observed in both cardiac and neuronal models. In this review, we focus on the most recent findings exploring the unusual activity of glutamate as a potential survival factor in different settings.

Emerging Roles of microRNAs in Ischemic Stroke: As Possible Therapeutic Agents

Stroke is one of the leading causes of death and physical disability worldwide. The consequences of stroke injuries are profound and persistent, causing in considerable burden to both the individual patient and society. Current treatments for ischemic stroke injuries have proved inadequate, partly owing to an incomplete understanding of the cellular and molecular changes that occur following ischemic stroke. MicroRNAs (miRNA) are endogenously expressed RNA molecules that function to inhibit mRNA translation and have key roles in the pathophysiological processes contributing to ischemic stroke injuries. Potential therapeutic areas to compensate these pathogenic processes include promoting angiogenesis, neurogenesis and neuroprotection. Several miRNAs, and their target genes, are recognized to be involved in these recoveries and repair mechanisms. The capacity of miRNAs to simultaneously regulate several target genes underlies their unique importance in ischemic stroke therapeutics. In this Review, we focus on the role of miRNAs as potential diagnostic and prognostic biomarkers, as well as promising therapeutic agents in cerebral ischemic stroke.

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.

The role of Na+/Ca2+ exchanger subtypes in neuronal ischemic injury

The Na(+)/Ca(2+) exchanger (NCX) plays an important role in the maintenance of Na(+) and Ca(2+) homeostasis in most cells including neurons under physiological and pathological conditions. It exists in three subtypes (NCX1-3) with different tissue distributions but all of them are present in the brain. NCX transports Na(+) and Ca(2+) in either Ca(2+)-efflux (forward) or Ca(2+)-influx (reverse) mode, depending on membrane potential and transmembrane ion gradients. During neuronal ischemia, Na(+) and Ca(2+) ionic disturbances favor NCX to work in reverse mode, giving rise to increased intracellular Ca(2+) levels, while it may regain its forward mode activity on reperfusion. The exact significance of NCX in neuronal ischemic and reperfusion states remains unclear. The differential role of NCX subtypes in ischemic neuronal injury has been extensively investigated using various pharmacological tools as well as genetic models. This review discusses the mode of action of NCX in ischemic and reperfusion states, the differential roles played by NCX subtypes in these states as well as the role of NCX in pre- and postconditioning. NCX subtypes carry variable roles in ischemic injury. Furthermore, the mode of action of each subtype varies in ischemia and reperfusion states. Thus, therapeutic targeting of NCX in stroke should be based on appropriate timing of the administration of NCX subtype-specific strategies.

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.

Effects of estradiol and IGF-1 on the sodium calcium exchanger in rat cultured cortical neurons

The Na(+)/Ca(2+) exchanger (NCX) is an important bidirectional transporter of calcium in neurons and has been shown to be involved in neuroprotection. Calcium can activate a number of cascades that can result in apoptosis and cell death, and NCX is a key factor in regulating the cytoplasmic concentration of this ion. 17-β-estradiol and insulin-like growth factor 1 (IGF-1) are known neuroprotective hormones with interacting mechanisms and effects on intracellular calcium; however, their relationship with the NCX has not been explored. In this article, the effects of these two hormones on neuronal NCX were tested using the whole-cell patch clamp technique on rat primary culture neurons. Both 17-β-estradiol and IGF-1 produced an increase in the NCX-mediated inward current and a decrease in the NCX-mediated outward current. However, the IGF-1 effect was lower than that of 17-β-estradiol, and the effect of both agents together was greater than the sum of each agent alone. Neither of the agents affected the pattern of regulation by extracellular or intrapipette calcium. Inhibitors of the IGF-1 and 17-β-estradiol receptors and inhibitors of the main signaling pathways failed to change the observed effects, indicating that these actions were not mediated by the classical receptors of these hormones. These effects on the NCX could be a mechanism explaining the neuroprotective actions of 17-β-estradiol and IGF-1, and these findings could help researchers to understand the role of the NCX in neuroprotection.

Calcium clearance and its energy requirements in cerebellar neurons

Quick cytosolic calcium clearance is essential for the effective modulation of various cellular functions. An excess of cytosolic calcium after influx is largely removed via ATP-dependent mechanisms located in the plasma membrane and the endoplasmic reticulum. Therefore, calcium clearance depends critically on the adequate supply of ATP, which may come from either glycolysis or mitochondria, or both. However, it presently remains unknown the degree to which individual ATP generating pathways - glycolysis and mitochondria power ATP-dependent calcium as well as other vital ion clearance mechanisms in neurons. In this study, we explored the relationship between the energy generating pathways and ion clearance mechanisms in neurons by characterizing the effects of glycolytic and mitochondrial inhibitors of ATP synthesis on calcium clearance kinetics in the soma, dendrites and spines. Stimulation of cultured cerebellar granule cells by brief pulses of 60mM potassium ACSF, and electrical stimulation of purkinje cells in acutely prepared slices led to a transient calcium influx, whose clearance was largely mediated by the plasma membrane Ca(2+)-ATPase pump. Inhibition of glycolysis by deoxyglucose or iodoacetic acid resulted in a marked slowing in calcium clearance in the soma, dendrites, and spines with the spines affected the most. However, inhibition of the mitochondrial citric acid cycle with fluoroacetate and arsenite, or mitochondrial ATP synthase with oligomycin did not produce any immediate effects on calcium clearance kinetics in any of those neuronal regions. Although cytosolic calcium clearance was not affected by the inhibition of mitochondria, the magnitude of the calcium clearance delay induced by glycolytic inhibitors in different neuronal compartments was related to their mitochondrial density. Conversely, the endoplasmic reticulum Ca(2+)-ATPase pump activity is fuelled by both glycolytic and mitochondrial ATP, as evidenced by a minimal change in the endoplasmic reticulum calcium contents in cells treated with either deoxyglucose supplemented with lactate or arsenite. Taken together, these data suggest that calcium clearance in cerebellar granule and purkinje cells relies on the plasma membrane Ca(2+)-ATPase, and is powered by glycolysis.

Na,K-ATPase activity regulates AMPA receptor turnover through proteasome-mediated proteolysis

Neuronal activity largely depends on two key components on the membrane: the Na,K-ATPase (NKA) that maintains the ion gradients and sets the foundation of excitability, and the ionotropic glutamatergic AMPA receptors (AMPARs) through which sodium influx forms the driving force for excitation. Because the frequent sodium transients from glutamate receptor activity need to be efficiently extruded, a functional coupling between NKA and AMPARs should be a necessary cellular device for synapse physiology. We show that NKA is enriched at synapses and associates with AMPARs. NKA dysfunction induces a rapid reduction in AMPAR cell-surface expression as well as total protein abundance, leading to a long-lasting depression in synaptic transmission. AMPAR proteolysis requires sodium influx, proteasomal activity and receptor internalization. These data elucidate a novel mechanism by which NKA regulates AMPAR turnover and thereby synaptic strength and brain function.

NCX3 knockout mice exhibit increased hippocampal CA1 and CA2 neuronal damage compared to wild-type mice following global cerebral ischemia

There is uncertainty as to whether the plasma membrane Na(+)/Ca(2+)exchanger (NCX) has a neuroprotective or neurodamaging role following cerebral ischemia. To address this issue we compared hippocampal neuronal injury in NCX3 knockout mice (Ncx3(-/-)) and wild-type mice (Ncx3(+/+)) following global cerebral ischemia. Using a bilateral common carotid artery occlusion (BCCAO) model of global ischemia we subjected NCX3 knockout and wild-type mice to 17 and 15 minutes of ischemia. Following the 17 minute period of ischemia, wild-type mice exhibited approximately 80% CA1 neuronal loss and approximately 40% CA2 neuronal loss. In contrast, NCX3 knockout mice displayed >95% CA1 neuronal loss and approximately 95% CA2 neuronal loss. Following the 15 minute period of ischemia, wild-type mice did not exhibit any significant hippocampal neuronal loss. In contrast, NCX3 knockout mice displayed approximately 45% CA1 neuronal loss and approximately 25% CA2 neuronal loss. The results clearly demonstrate that mice deficient in the NCX3 protein are more susceptible to global cerebral ischemia than wild-type mice. Our findings suggest NCX3 has a positive role in maintaining neuronal intracellular calcium homeostasis following ischemia, and that when exchanger function is compromised neurons are more susceptible to calcium deregulation and cell death.

The role of the Na(+)/Ca(2+) exchanger (NCX) in neurons following ischaemia

The Na(+)/Ca(2+) exchanger (NCX) is a bi-directional membrane ion transporter. Under normal conditions, the exchanger transports one calcium ion out of the cell and three sodium ions into the cell. This is known as the calcium exit, or "forward" mode. Under certain conditions, however, the exchanger can reverse and transport calcium ions into the cell (calcium entry mode). Because dysregulation of sodium and calcium homeostasis is an integral feature of ischaemic brain injury, the role of the NCX in neurons following ischaemia has been investigated using a number of in vitro and in vivo models. Studies using in vitro ischaemia-related models (hypoxia, glutamate) have produced conflicting results, with some showing that NCX activity is neuroprotective while others indicate that it is neurodamaging. The majority of in vivo studies using the focal cerebral ischaemia model indicate that blocking NCX activity is neurodamaging while increasing NCX activity is neuroprotective. We have reviewed the major in vitro and in vivo neuronal ischaemia-related NCX studies in an attempt to clarify the reason for the conflicting findings. The use of different ischaemia models and doubts as to the specificity of pharmacological NCX inhibitors and stimulators has contributed to the confusion over the role of the NCX in ischaemic brain injury. The development of NCX transgenic animals may help our understanding of the role of this ion exchanger in neurons following ischaemia and aid the development of an effective stroke treatment.

Neuronal preconditioning with the antianginal drug, bepridil

It has recently been shown that the antianginal drug bepridil (BEP) activates mitochondrial ATP-sensitive potassium (mitoK(ATP)) channels and thus confers cardioprotection. Our aim was to investigate whether BEP could induce preconditioning in cultured rat cortical neurons. Although BEP depolarized isolated and in situ mitochondria and increased reactive oxygen species generation, no acute protection was observed. However, a 3-day BEP-treatment elicited dose-dependent delayed neuroprotection against 180 min of oxygen-glucose deprivation (cell viability: untreated, 52.5 +/- 0.85%; BEP 1 micromol/L, 59.6 +/- 1.53%*; BEP 2.5 micromol/L, 71.9 +/- 1.23%*; BEP 5 micromol/L, 95.3 +/- 0.89%*; mean +/- SEM; *p < 0.05 vs. untreated) and 60 min of glutamate excitotoxicity (200 micromol/L; cell viability: untreated, 54.1 +/- 0.69%; BEP 1 micromol/L, 61.2 +/- 1.19%*; BEP 2.5 micromol/L, 78.1 +/- 1.67%*; BEP 5 micromol/L, 91.2 +/- 1.20%*; mean +/- SEM; *p < 0.05 vs. untreated), and inhibited the reactive oxygen species surge upon glutamate exposure. The protection was antagonized with co-application of the superoxide dismutase mimetic M40401, but not with reduced glutathione, catalase, or with the mitoK(ATP) blocker 5-hydroxydecanoate. Furthermore, BEP treatment resulted in increased levels of phosphorylated protein kinase C, manganese-dependent superoxide dismutase, glutathione peroxidase, and Bcl-2. Our results indicate that BEP induces delayed neuronal preconditioning which is dependent on superoxide generation but perhaps not on direct mitoK(ATP) activation.

Selective alteration of expression of Na+/Ca2+ exchanger isoforms after transient focal cerebral ischemia in rats

The Na+/Ca2+ exchanger (NCX) is an antiporter located in the plasma membrane of many cells, which can maintain the intracellular Ca(2+) homeostasis. Some studies have shown the close relationship of NCX and cerebral ischemia. But controversial results were obtained. Three NCX isoforms, NCX1, NCX2, and NCX3 were distributed selectively in central nervous system, which suggests that each isoform may have different function in cerebral ischemia. In this study we investigated the time-related alteration of gene and protein expressions of NCX1, NCX2, and NCX3 in rat brain cortex after 2 h of transient middle cerebral artery occlusion (tMCAO). Reverse transcription-polymerase chain reaction (RT-PCR) was used to investigate the mRNA levels of each NCX isoform at 2, 6, 12, and 24 h of reperfusion, respectively. Western blot was used to measure the protein expressions of each NCX isoform at 2, 12, and 24 h of reperfusion, respectively. The results showed that NCX1 mRNA level was reduced by 42.1% and 27.8%, respectively, at 2 and 6h of reperfusion and restored to normal level at 12 and 24 h of reperfusion. NCX1 protein was decreased by 36.6% at 2 h of reperfusion and recovered at 12 and 24 h of reperfusion. The mRNA and protein levels of NCX2 and NCX3 did not change significantly over time. These results suggest that NCX1 might play an important role in transient focal cerebral ischemia.