Role of Na+-K+-Cl−cotransport and Na+/Ca2+exchange in mitochondrial dysfunction in astrocytes following in vitro ischemia

Effect of abamectin on osmoregulation in red swamp crayfish (Procambarus clarkii)

As a widely used pesticide, abamectin could be a threat to nontarget organisms. In this study, the toxic mechanism of abamectin on osmoregulation in Procambarus clarkii was explored for the first time. The results of this study showed that with increasing abamectin concentration, the membrane structures of gill filaments were damaged, with changes in ATPase activities, transporter contents, biogenic amine contents, and gene expression levels. The results of this study indicated that at 0.2 mg/L abamectin, ion diffusion could maintain osmoregulation. At 0.4 mg/L abamectin, passive transport was inhibited due to damage to the membrane structures of gill filaments, and active transport needed to be enhanced for osmoregulation. At 0.6 mg/L abamectin, the membrane structures of gill filaments were seriously damaged, and the expression level of osmoregulation-related genes decreased, but the organisms were still mobilizing various transporters, ATPases, and biogenic amines to address abamectin stress. This study provided a theoretical basis for further study of the effects of contaminations in aquatic environment on the health of crustaceans.

GABAergic system and chloride cotransporters as potential therapeutic targets to mitigate cell death in ischemia

Gamma aminobutyric acid (GABA) is a critical inhibitory neurotransmitter in the central nervous system that plays a vital role in modulating neuronal excitability. Dysregulation of GABAergic signaling, particularly involving the cotransporters NKCC1 and KCC2, has been implicated in various pathologies, including epilepsy, schizophrenia, autism spectrum disorder, Down syndrome, and ischemia. NKCC1 facilitates chloride influx, whereas KCC2 mediates chloride efflux via potassium gradient. Altered expression and function of these cotransporters have been associated with excitotoxicity, inflammation, and cellular death in ischemic events characterized by reduced cerebral blood flow, leading to compromised tissue metabolism and subsequent cell death. NKCC1 inhibition has emerged as a potential therapeutic approach to attenuate intracellular chloride accumulation and mitigate neuronal damage during ischemic events. Similarly, targeting KCC2, which regulates chloride efflux, holds promise for improving outcomes and reducing neuronal damage under ischemic conditions. This review emphasizes the critical roles of GABA, NKCC1, and KCC2 in ischemic pathologies and their potential as therapeutic targets. Inhibiting or modulating the activity of these cotransporters represents a promising strategy for reducing neuronal damage, preventing excitotoxicity, and improving neurological outcomes following ischemic events. Furthermore, exploring the interactions between natural compounds and NKCC1/KCC2 provides additional avenues for potential therapeutic interventions for ischemic injury.

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.

Gypenoside Pretreatment Alleviates the Cerebral Ischemia Injury via Inhibiting the Microglia-Mediated Neuroinflammation

Neuroinflammation is closely related to prognosis in ischemic stroke. Microglia are the main immune cells in the nervous system. Under physiological conditions, microglia participate in clearance of dead cells, synapse pruning and regulation of neuronal circuits to maintain the overall health of the nervous system. Once ischemic stroke occurs, microglia function in the occurrence and progression of neuroinflammation. Therefore, the regulation of microglia-mediated neuroinflammation is a potential therapeutic strategy for ischemic stroke. The anti-inflammatory activity of gypenosides (GPs) has been confirmed to be related to the activity of microglia in other neurological diseases. However, the role of GPs in neuroinflammation after ischemic stroke has not been studied. In this study, we investigated whether GPs could reduce neuroinflammation by regulating microglia and the underlying mechanism through qRT-PCR and western blot. Results showed that GPs pretreatment mitigated blood-brain barrier (BBB) damage in the mice subjected to middle cerebral artery occlusion (MCAO) and improved motor function. According to the results of immunofluorescence staining, GPs pretreatment alleviated neuroinflammation in MCAO mice by reducing the number of microglia and promoting their phenotypic transformation from M1 to M2. Furthermore, GPs pretreatment reduced the number of astrocytes in the penumbra and inhibited their polarization into the A1 type. We applied oxygen and glucose deprivation (OGD) on BV2 cells to mimic ischemic conditions in vitro and found similar effect as that in vivo. At the molecular level, the STAT-3/HIF1-α and TLR-4/NF-κB/HIF1-α pathways were involved in the anti-inflammatory effects of GPs in vitro and in vivo. Overall, this research indicates that GPs are potential therapeutic agents for ischemic stroke and has important reference significance to further explore the possibility of GPs application in ischemic stroke.

Bidirectional Regulation of GABAA Reversal Potential in the Adult Brain: Physiological and Pathological Implications

In physiological conditions, the intracellular chloride concentration is much lower than the extracellular. As GABAA channels are permeable to anions, the reversal potential of GABAA is very close to that of Cl-, which is the most abundant free anion in the intra- and extracellular spaces. Intracellular chloride is regulated by the activity ratio of NKCC1 and KCC2, two chloride-cation cotransporters that import and export Cl-, respectively. Due to the closeness between GABAA reversal potential and the value of the resting membrane potential in most neurons, small changes in intracellular chloride have a major functional impact, which makes GABAA a uniquely flexible signaling system. In most neurons of the adult brain, the GABAA reversal potential is slightly more negative than the resting membrane potential, which makes GABAA hyperpolarizing. Alterations in GABAA reversal potential are a common feature in numerous conditions as they are the consequence of an imbalance in the NKCC1-KCC2 activity ratio. In most conditions (including Alzheimer's disease, schizophrenia, and Down's syndrome), GABAA becomes depolarizing, which causes network desynchronization and behavioral impairment. In other conditions (neonatal inflammation and neuropathic pain), however, GABAA reversal potential becomes hypernegative, which affects behavior through a potent circuit deactivation.

Mitochondrial Effects of Common Cardiovascular Medications: The Good, the Bad and the Mixed

Mitochondria are central organelles in the homeostasis of the cardiovascular system via the integration of several physiological processes, such as ATP generation via oxidative phosphorylation, synthesis/exchange of metabolites, calcium sequestration, reactive oxygen species (ROS) production/buffering and control of cellular survival/death. Mitochondrial impairment has been widely recognized as a central pathomechanism of almost all cardiovascular diseases, rendering these organelles important therapeutic targets. Mitochondrial dysfunction has been reported to occur in the setting of drug-induced toxicity in several tissues and organs, including the heart. Members of the drug classes currently used in the therapeutics of cardiovascular pathologies have been reported to both support and undermine mitochondrial function. For the latter case, mitochondrial toxicity is the consequence of drug interference (direct or off-target effects) with mitochondrial respiration/energy conversion, DNA replication, ROS production and detoxification, cell death signaling and mitochondrial dynamics. The present narrative review aims to summarize the beneficial and deleterious mitochondrial effects of common cardiovascular medications as described in various experimental models and identify those for which evidence for both types of effects is available in the literature.

Role of SPAK-NKCC1 signaling cascade in the choroid plexus blood-CSF barrier damage after stroke

BACKGROUND

The mechanisms underlying dysfunction of choroid plexus (ChP) blood-cerebrospinal fluid (CSF) barrier and lymphocyte invasion in neuroinflammatory responses to stroke are not well understood. In this study, we investigated whether stroke damaged the blood-CSF barrier integrity due to dysregulation of major ChP ion transport system, Na⁺-K⁺-Cl^- cotransporter 1 (NKCC1), and regulatory Ste20-related proline-alanine-rich kinase (SPAK).

METHODS

Sham or ischemic stroke was induced in C57Bl/6J mice. Changes on the SPAK-NKCC1 complex and tight junction proteins (TJs) in the ChP were quantified by immunofluorescence staining and immunoblotting. Immune cell infiltration in the ChP was assessed by flow cytometry and immunostaining. Cultured ChP epithelium cells (CPECs) and cortical neurons were used to evaluate H₂O₂-mediated oxidative stress in stimulating the SPAK-NKCC1 complex and cellular damage. In vivo or in vitro pharmacological blockade of the ChP SPAK-NKCC1 cascade with SPAK inhibitor ZT-1a or NKCC1 inhibitor bumetanide were examined.

RESULTS

Ischemic stroke stimulated activation of the CPECs apical membrane SPAK-NKCC1 complex, NF-κB, and MMP9, which was associated with loss of the blood-CSF barrier integrity and increased immune cell infiltration into the ChP. Oxidative stress directly activated the SPAK-NKCC1 pathway and resulted in apoptosis, neurodegeneration, and NKCC1-mediated ion influx. Pharmacological blockade of the SPAK-NKCC1 pathway protected the ChP barrier integrity, attenuated ChP immune cell infiltration or neuronal death.

CONCLUSION

Stroke-induced pathological stimulation of the SPAK-NKCC1 cascade caused CPECs damage and disruption of TJs at the blood-CSF barrier. The ChP SPAK-NKCC1 complex emerged as a therapeutic target for attenuating ChP dysfunction and lymphocyte invasion after stroke.

Loop Diuretics Inhibit Ischemia-Induced Intracellular Ca2+ Overload in Neurons via the Inhibition of Voltage-Gated Ca2+ and Na+ Channels

One consequence of ischemic stroke is disruption of intracellular ionic homeostasis. Intracellular overload of both Na⁺ and Ca²⁺ has been linked to neuronal death in this pathophysiological state. The etiology of ionic imbalances resulting from stroke-induced ischemia and acidosis includes the dysregulation of multiple plasma membrane transport proteins, such as increased activity of sodium-potassium-chloride cotransporter-1 (NKCC-1). Experiments using NKCC1 antagonists, bumetanide (BMN) and ethacrynic acid (EA), were carried out to determine if inhibition of this cotransporter affects Na⁺ and Ca²⁺ overload observed following in vitro ischemia-acidosis. Fluorometric Ca²⁺ and Na⁺ measurements were performed using cultured cortical neurons, and measurements of whole-cell membrane currents were used to determine target(s) of BMN and EA, other than the electroneutral NKCC-1. Both BMN and EA depressed ischemia-acidosis induced [Ca²⁺]_i overload without appreciably reducing [Na⁺]_i increases. Voltage-gated Ca²⁺ channels were inhibited by both BMN and EA with half-maximal inhibitory concentration (IC₅₀) values of 4 and 36 μM, respectively. Similarly, voltage-gated Na⁺ channels were blocked by BMN and EA with IC₅₀ values of 13 and 30 μM, respectively. However, neither BMN nor EA affected currents mediated by acid-sensing ion channels or ionotropic glutamatergic receptors, both of which are known to produce [Ca²⁺]_i overload following ischemia. Data suggest that loop diuretics effectively inhibit voltage-gated Ca²⁺ and Na⁺ channels at clinically relevant concentrations, and block of these channels by these compounds likely contributes to their clinical effects. Importantly, inhibition of these channels, and not NKCC1, by loop diuretics reduces [Ca²⁺]_i overload in neurons during ischemia-acidosis, and thus BMN and EA could potentially be used therapeutically to lessen injury following ischemic stroke.

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.

Acetylpuerarin inhibits oxygen-glucose deprivation-induced neuroinflammation of rat primary astrocytes via the suppression of HIF-1 signaling

In the central nervous system (CNS), ischemic injury induced by inflammation associated with astrocytes serves an important role in physiological and pathological processes. Neuroinflammation leads to the release of pro-inflammatory cytokines, including tumor necrosis factor-α and interleukin-1β. The aim of the present study was to investigate whether acetylpuerarin attenuates oxygen-glucose deprivation (OGD)-induced astrocyte inflammation and secretion of pro-inflammatory cytokines via inhibiting hypoxia-inducible factor-1 (HIF-1) activation and suppressing downstream primary astrocyte signaling in rats. The results demonstrated that acetylpuerarin attenuates astrocyte viability and induces morphological changes following OGD stress. Furthermore, acetylpuerarin suppresses the stimulation of HIF-1α and nuclear factor (NF)-κB signaling pathways, while attenuating the expression and secretion of pro-inflammatory cytokines via HIF-1 suppression in OGD-induced astrocytes. These findings indicate that acetylpuerarin may attenuate OGD-induced astrocyte damage and inflammation in rat primary astrocytes via suppressing HIF-1 activation and NF-κB signaling. These results suggest that acetylpuerarin regulates inflammation associated with astrocytes and may represent a novel therapeutic agent for the treatment of neuroinflammation in the CNS.

Ionic stress induces fusion of mitochondria to 3-D networks: An electron tomography study

Mitochondria are central organelles for energy supply of cells and play an important role in maintenance of ionic balance. Consequently mitochondria are highly sensitive to any kind of stress to which they mainly response by disturbance of respiration, ROS production and release of cytochrome c into the cytoplasm. Many of the physiological and molecular stress reactions of mitochondria are well known, yet there is a lack of information on corresponding stress induced structural changes. 3-D visualization of high-pressure frozen cells by FIB-SEM tomography and TEM tomography as used for the present investigation provide an excellent tool for studying structure related mitochondrial stress reactions. In the present study it is shown that mitochondria in the unicellular fresh-water algal model system Micrasterias as well as in the closely related aquatic higher plant Lemna fuse to local networks as a consequence of exposure to ionic stress induced by addition of KCl, NaCl and CoCl₂. In dependence on concentration and duration of the treatment, fusion of mitochondria occurs either by formation of protuberances arising from the outer mitochondrial membrane, or by direct contact of the surface of elongated mitochondria. As our results show that respiration is maintained in both model systems during ionic stress and mitochondrial fusion, as well as formation of protuberances are reversible, we assume that mitochondrial fusion is a ubiquitous process that may help the cells to cope with stress. This may occur by interconnecting the respiratory chains of the individual mitochondria and by enhancing the buffer capacity against stress induced ionic imbalance.

Cerebral Dopamine Neurotrophic Factor (CDNF) Has Neuroprotective Effects against Cerebral Ischemia That May Occur through the Endoplasmic Reticulum Stress Pathway

Cerebral dopamine neurotrophic factor (CDNF), previously known as the conserved dopamine neurotrophic factor, belongs to the evolutionarily conserved CDNF/mesencephalic astrocyte-derived neurotrophic factor MANF family of neurotrophic factors that demonstrate neurotrophic activities in dopaminergic neurons. The function of CDNF during brain ischemia is still not known. MANF is identified as an endoplasmic reticulum (ER) stress protein; however, the role of CDNF in ER stress remains to be fully elucidated. Here, we test the neuroprotective effect of CDNF on middle cerebral artery occlusion (MCAO) rats and neurons and astrocytes treated with oxygen–glucose depletion (OGD). We also investigate the expression of CDNF in cerebral ischemia and in primary neurons treated with ER stress-inducing agents. Our results show that CDNF can significantly reduce infarct volume, reduce apoptotic cells and improve motor function in MCAO rats, while CDNF can increase the cell viability of neurons and astrocytes treated by OGD. The expression of CDNF was upregulated in the peri-infarct tissue at 2 h of ischemia/24 h reperfusion. ER stress inducer can induce CDNF expression in primary cultured neurons. Our data indicate that CDNF has neuroprotective effects on cerebral ischemia and the OGD cell model and the protective mechanism of CDNF may occur through ER stress pathways.

Na+ -K+ -2Cl- Cotransporter (NKCC) Physiological Function in Nonpolarized Cells and Transporting Epithelia

Two genes encode the Na+ -K+ -2Cl- cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na+ , 1K+ , and 2Cl- across the plasma membrane of cells. Na+ -K+ -2Cl- cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na+ /K+ pump. In many cells, NKCC1 accumulates Cl- above its electrochemical potential equilibrium, thereby facilitating Cl- channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca2+ channels, leading to Ca2+ influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl- conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl- . In combination with the Na+ /K+ pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl- -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K+ -driven fluid secretion in inner ear, and possibly in Na+ -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na+ /K+ pump participates in reabsorbing 30% of the glomerular-filtered Na+ . Overall, many critical physiological functions are maintained by the activity of the two Na+ -K+ -2Cl- cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Glial Na(+) -dependent ion transporters in pathophysiological conditions

Sodium dynamics are essential for regulating functional processes in glial cells. Indeed, glial Na(+) signaling influences and regulates important glial activities, and plays a role in neuron-glia interaction under physiological conditions or in response to injury of the central nervous system (CNS). Emerging studies indicate that Na(+) pumps and Na(+) -dependent ion transporters in astrocytes, microglia, and oligodendrocytes regulate Na(+) homeostasis and play a fundamental role in modulating glial activities in neurological diseases. In this review, we first briefly introduced the emerging roles of each glial cell type in the pathophysiology of cerebral ischemia, Alzheimer's disease, epilepsy, Parkinson's disease, Amyotrophic Lateral Sclerosis, and myelin diseases. Then, we discussed the current knowledge on the main roles played by the different glial Na(+) -dependent ion transporters, including Na(+) /K(+) ATPase, Na(+) /Ca(2+) exchangers, Na(+) /H(+) exchangers, Na(+) -K(+) -Cl(-) cotransporters, and Na(+) - HCO3- cotransporter in the pathophysiology of the diverse CNS diseases. We highlighted their contributions in cell survival, synaptic pathology, gliotransmission, pH homeostasis, and their role in glial activation, migration, gliosis, inflammation, and tissue repair processes. Therefore, this review summarizes the foundation work for targeting Na(+) -dependent ion transporters in glia as a novel strategy to control important glial activities associated with Na(+) dynamics in different neurological disorders. GLIA 2016;64:1677-1697.

Transcriptome profile of the early stages of breast cancer tumoral spheroids

Oxygen or nutrient deprivation of early stage tumoral spheroids can be used to reliably mimic the initial growth of primary and metastatic cancer cells. However, cancer cell growth during the initial stages has not been fully explored using a genome-wide approach. Thus, in the present study, we investigated the transcriptome of breast cancer cells during the initial stages of tumoral growth using RNAseq in a model of Multicellular Tumor Spheroids (MTS). Network analyses showed that a metastatic signature was enriched as several adhesion molecules were deregulated, including EPCAM, E-cadherin, integrins and syndecans, which were further supported by an increase in cell migration. Interestingly, we also found that the cancer cells at this stage of growth exhibited a paradoxical hyperactivation of oxidative mitochondrial metabolism. In addition, we found a large number of regulated (long non coding RNA) lncRNAs, several of which were co-regulated with neighboring genes. The regulatory role of some of these lncRNAs on mRNA expression was demonstrated with gain of function assays. This is the first report of an early-stage MTS transcriptome, which not only reveals a complex expression landscape, but points toward an important contribution of long non-coding RNAs in the final phenotype of three-dimensional cellular models.

Mitochondrial dynamics in astrocytes

Astrocytes exhibit cellular excitability through variations in their intracellular calcium (Ca²⁺) levels in response to synaptic activity. Astrocyte Ca²⁺ elevations can trigger the release of neuroactive substances that can modulate synaptic transmission and plasticity, hence promoting bidirectional communication with neurons. Intracellular Ca²⁺ dynamics can be regulated by several proteins located in the plasma membrane, within the cytosol and by intracellular organelles such as mitochondria. Spatial dynamics and strategic positioning of mitochondria are important for matching local energy provision and Ca²⁺ buffering requirements to the demands of neuronal signalling. Although relatively unresolved in astrocytes, further understanding the role of mitochondria in astrocytes may reveal more about the complex bidirectional relationship between astrocytes and neurons in health and disease. In the present review, we discuss some recent insights regarding mitochondrial function, transport and turnover in astrocytes and highlight some important questions that remain to be answered.

Astrocytic mitochondrial membrane hyperpolarization following extended oxygen and glucose deprivation

Astrocytes can tolerate longer periods of oxygen and glucose deprivation (OGD) as compared to neurons. The reasons for this reduced vulnerability are not well understood. Particularly, changes in mitochondrial membrane potential (Δψ_m) in astrocytes, an indicator of the cellular redox state, have not been investigated during reperfusion after extended OGD exposure. Here, we subjected primary mouse astrocytes to glucose deprivation (GD), OGD and combinations of both conditions varying in duration and sequence. Changes in Δψ_m, visualized by change in the fluorescence of JC-1, were investigated within one hour after reconstitution of oxygen and glucose supply, intended to model in vivo reperfusion. In all experiments, astrocytes showed resilience to extended periods of OGD, which had little effect on Δψ_m during reperfusion, whereas GD caused a robust Δψ_m negativation. In case no Δψ_m negativation was observed after OGD, subsequent chemical oxygen deprivation (OD) induced by sodium azide caused depolarization, which, however, was significantly delayed as compared to normoxic group. When GD preceded OD for 12 h, Δψ_m hyperpolarization was induced by both GD and subsequent OD, but significant interaction between these conditions was not detected. However, when GD was extended to 48 h preceding OGD, hyperpolarization enhanced during reperfusion. This implicates synergistic effects of both conditions in that sequence. These findings provide novel information regarding the role of the two main substrates of electron transport chain (glucose and oxygen) and their hyperpolarizing effect on Δψ_m during substrate deprivation, thus shedding new light on mechanisms of astrocyte resilience to prolonged ischemic injury.

Sustained Na+/H+ exchanger activation promotes gliotransmitter release from reactive hippocampal astrocytes following oxygen-glucose deprivation

Hypoxia ischemia (HI)-related brain injury is the major cause of long-term morbidity in neonates. One characteristic hallmark of neonatal HI is the development of reactive astrogliosis in the hippocampus. However, the impact of reactive astrogliosis in hippocampal damage after neonatal HI is not fully understood. In the current study, we investigated the role of Na⁺/H⁺ exchanger isoform 1 (NHE1) protein in mouse reactive hippocampal astrocyte function in an in vitro ischemia model (oxygen/glucose deprivation and reoxygenation, OGD/REOX). 2 h OGD significantly increased NHE1 protein expression and NHE1-mediated H⁺ efflux in hippocampal astrocytes. NHE1 activity remained stimulated during 1–5 h REOX and returned to the basal level at 24 h REOX. NHE1 activation in hippocampal astrocytes resulted in intracellular Na⁺ and Ca²⁺ overload. The latter was mediated by reversal of Na⁺/Ca²⁺ exchange. Hippocampal astrocytes also exhibited a robust release of gliotransmitters (glutamate and pro-inflammatory cytokines IL-6 and TNFα) during 1–24 h REOX. Interestingly, inhibition of NHE1 activity with its potent inhibitor HOE 642 not only reduced Na⁺ overload but also gliotransmitter release from hippocampal astrocytes. The noncompetitive excitatory amino acid transporter inhibitor TBOA showed a similar effect on blocking the glutamate release. Taken together, we concluded that NHE1 plays an essential role in maintaining H⁺ homeostasis in hippocampal astrocytes. Over-stimulation of NHE1 activity following in vitro ischemia disrupts Na⁺ and Ca²⁺ homeostasis, which reduces Na⁺-dependent glutamate uptake and promotes release of glutamate and cytokines from reactive astrocytes. Therefore, blocking sustained NHE1 activation in reactive astrocytes may provide neuroprotection following HI.

Ionic transporter activity in astrocytes, microglia, and oligodendrocytes during brain ischemia

Glial cells constitute a large percentage of cells in the nervous system. During recent years, a large number of studies have critically attributed to glia a new role which no longer reflects the long-held view that glia constitute solely a silent and passive supportive scaffolding for brain cells. Indeed, it has been hypothesized that glia, partnering neurons, have a much more actively participating role in brain function. Alteration of intraglial ionic homeostasis in response to ischemic injury has a crucial role in inducing and maintaining glial responses in the ischemic brain. Therefore, glial transporters as potential candidates in stroke intervention are becoming promising targets to enhance an effective and additional therapy for brain ischemia. In this review, we will describe in detail the role played by ionic transporters in influencing astrocyte, microglia, and oligodendrocyte activity and the implications that these transporters have in the progression of ischemic lesion.

Mesencephalic astrocyte-derived neurotrophic factor inhibits oxygen-glucose deprivation-induced cell damage and inflammation by suppressing endoplasmic reticulum stress in rat primary astrocytes

Astrocyte inflammation plays important roles both in physiological and pathological processes in the central nervous system (CNS). Ischemic injury in the CNS causes damage to astrocytes and the release of proinflammatory cytokines, such as tumor necrosis factor-α, interleukin-1β, and interleukin-6. This current study investigates whether mesencephalic astrocyte-derived neurotrophic factor (MANF) inhibits oxygen-glucose deprivation (OGD)-induced cell damage and inflammatory cytokine secretion by suppressing endoplasmic reticulum stress in rat primary astrocytes. We found that MANF alleviated OGD-induced astrocyte damage and rescued the cell viability, and the upregulation of GRP78 (endoplasmic reticulum (ER) stress marker) and NF-κB p65 (one of the central mediators of proinflammatory pathways) induced by OGD were significantly reduced by preincubation of MANF. In addition, the increases of secretion and mRNA expression levels of the proinflammatory cytokines IL-1β, IL-6, and TNF-α in astrocytes induced by OGD were significantly suppressed by MANF. These findings demonstrate that MANF shows the potential to alleviate cell damage and inflammation in rat primary astrocytes by suppressing ER stress, indicating that MANF plays an important role in astrocyte inflammation and functioning and may suggest a promising strategy for neuroprotection in the CNS.

Cation-chloride cotransporters NKCC1 and KCC2 as potential targets for novel antiepileptic and antiepileptogenic treatments

In cortical and hippocampal neurons, cation-chloride cotransporters (CCCs) control the reversal potential (EGABA) of GABAA receptor-mediated current and voltage responses and, consequently, they modulate the efficacy of GABAergic inhibition. Two members of the CCC family, KCC2 (the major neuron-specific K-Cl cotransporter; KCC isoform 2) and NKCC1 (the Na-K-2Cl cotransporter isoform 1 which is expressed in both neurons and glial cells) have attracted much interest in studies on GABAergic signaling under both normal and pathophysiological conditions, such as epilepsy. There is tentative evidence that loop diuretic compounds such as furosemide and bumetanide may have clinically relevant antiepileptic actions, especially when administered in combination with conventional GABA-mimetic drugs such as phenobarbital. Furosemide is a non-selective inhibitor of CCCs while at low concentrations bumetanide is selective for NKCCs. Search for novel antiepileptic drugs (AEDs) is highly motivated especially for the treatment of neonatal seizures which are often resistant to, or even aggravated by conventional AEDs. This review shows that the antiepileptic effects of loop diuretics described in the pertinent literature are based on widely heterogeneous mechanisms ranging from actions on both neuronal NKCC1 and KCC2 to modulation of the brain extracellular volume fraction. A promising strategy for the development of novel CCC-blocking AEDs is based on prodrugs that are activated following their passage across the blood-brain barrier. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

Two sides of the same coin: sodium homeostasis and signaling in astrocytes under physiological and pathophysiological conditions

The intracellular sodium concentration of astrocytes is classically viewed as being kept under tight homeostatic control and at a relatively stable level under physiological conditions. Indeed, the steep inwardly directed electrochemical gradient for sodium, generated by the Na⁺/K⁺-ATPase, contributes to maintain the electrochemical gradient of K⁺ and the highly K⁺-based negative membrane potential, and is a central element in energizing membrane transport. As such it is tightly coupled to the homeostasis of extra- and intracellular potassium, calcium or pH and to the reuptake of transmitters such as glutamate. Recent studies, however, have demonstrated that this picture is far too simplistic. It is now firmly established that transmitters, most notably glutamate, and excitatory neuronal activity evoke long-lasting sodium transients in astrocytes, the properties of which are distinctly different from those of activity-related glial calcium signals. From these studies, it emerges that sodium homeostasis and signaling are two sides of the same coin: sodium-dependent transporters, primarily known for their role in ion regulation and homeostasis, also generate relevant ion signals during neuronal activity. The functional consequences of activity-related sodium transients are manifold and are just coming into view, enabling surprising and important new insights into astrocyte function and neuron-glia interaction in the brain. The present review will highlight current knowledge about the mechanisms that contribute to sodium homeostasis in astrocytes, present recent data on the spatial and temporal properties of activity-related glial sodium signals and discuss their functional consequences with a special emphasis on pathophysiological conditions.

COMMD1 interacts with the COOH terminus of NKCC1 in Calu-3 airway epithelial cells to modulate NKCC1 ubiquitination

Mice deficient in Na-K-2Cl cotransporter (NKCC1) have been generated by targeted disruption of the gene encoding NKCC1 involving the carboxy terminus (CT-NKCC1) but not the amino terminus. We hypothesize that the resulting physiological defects are due to loss of proteins interacting with CT-NKCC1. Using a yeast two-hybrid approach, adaptor protein COMMD1 was found to bind to CT-NKCC1 (aa 1,040-1,212). Binding was verified in a yeast-independent system using GST-COMMD1 and myc-CT-NKCC1. Truncated COMMD1 and CT-NKCC1 peptides were used in binding assays to identify the site of interaction. The results demonstrate concentration-dependent binding of COMMD1 (aa 1-47) to CT-NKCC1 (aa 1,040-1,134). Endogenous COMMD1 was detected in pull downs using recombinant FLAG-CT-NKCC1; this co-pull down was blocked by COMMD1 (aa 1-47). CT-NKCC1 (aa 1,040-1,137) decreased basolateral membrane expression of NKCC1, and COMMD1 (aa 1-47) increased NKCC1 membrane expression. Downregulation of COMMD1 using silencing (si)RNA led to a transient loss of endogenous COMMD1 but did not affect activation of NKCC1 by hyperosmotic sucrose. Hyperosmolarity caused a transient increase in NKCC1 membrane expression, indicating regulated trafficking of NKCC1; downregulation of COMMD1 using siRNA reduced baseline (unstimulated) NKCC1 expression and blunted a transient elevation in NKCC1 membrane expression caused by hyperosmolarity. Constitutive downregulation of COMMD1 in HT29 engineered cells exhibited loss of COMMD1 and decreased NKCC1 membrane expression with no effect on activation of NKCC1. Loss of COMMD1 in Calu-3 cells and in HT29 cells led to reduced ubiquitinated NKCC1. The results indicate a role for COMMD1 in the regulation of NKCC1 membrane expression and ubiquitination.

A bioactive probe of the oxidative pentose phosphate cycle: novel strategy to reverse radioresistance in glucose deprived human colon cancer cells

The specific effects of glucose deprivation on oxidative pentose phosphate cycle (OPPC) function, thiol homeostasis, protein function and cell survival remain unclear due to lack of a glucose-sensitive chemical probe. Using p53 wild type and mutant human colon cells, we determined the effects of hydroxyethyl disulfide (HEDS) on NADPH, GSH, GSSG, total glutathione, total non-protein and protein thiol levels, the function of the DNA repair protein Ku, and the susceptibility to radiation-induced free radicals under normal glucose or glucose-deprived conditions. HEDS is rapidly detoxified in normal glucose but triggered a p53-independent metabolic stress in glucose depleted state that caused loss of NADPH, protein and non-protein thiol homeostasis and Ku function, and enhanced sensitivity of both p53 wild type and mutant cells to radiation induced oxidative stress. Additionally, high concentration of HEDS alone induced cell death in p53 wild type cells without significant effect on p53 mutant cells. HEDS offers a useful tool to gain insights into how glucose metabolism affects OPPC dependent stress-induced cellular functions and injury, including in tumor cells, where our findings imply a novel therapeutic approach to target glucose deprived tumor. Our work introduces a novel probe to address cancer metabolism and ischemic pathology.

DHA inhibits ER Ca2+ release and ER stress in astrocytes following in vitro ischemia

Docosahexaenoic acid (DHA) has neuroprotective effects in several neurodegenerative disease conditions. However, the underlying mechanisms are not well understood. In the present study, we investigated the effects of DHA on astrocyte Ca(2+) signaling under in vitro ischemic conditions (oxygen/glucose deprivation and reoxygenation, OGD/REOX). OGD (2h) triggered a Ca(2+) (ER) store overload (∼1.9-fold). Ca(2+) uptake by the Ca(2+) (ER) stores was further augmented during REOX and Ca(2+) (ER) was elevated by ∼4.7-fold at 90min REOX. Interestingly, Ca(2+) (ER) stores abruptly released Ca(2+) at ∼120min REOX and emptied at 160min REOX. Depletion of Ca(2+) (ER) stores led to delayed elevation of intracellular Ca(2+) concentration (Ca(2+) (cyt) ) and cell death. Activation of the purinergic receptor P2Y1 was responsible for the release of Ca(2+) (ER) . Most importantly, DHA blocked the initial Ca(2+) (ER) store overload, the delayed depletion of Ca(2+) (ER) , and rise in Ca(2+) (cyt) , which was in part via inhibiting d-myo-inositol 1,4,5-triphosphate receptors. The DHA metabolite DiHDoHE exhibited similar effects. DHA also attenuated expression of phosphorylated eukaryotic initiation factor 2α and activating transcription factor-4, two ER stress markers, following in vitro ischemia. Taken together, these findings suggest that DHA has protective effects in astrocytes following in vitro ischemia, in part, by inhibiting Ca(2+) dysregulation and ER stress.

Calcium dependence of damage to mouse motor nerve terminals following oxygen/glucose deprivation

Motor nerve terminals are especially sensitive to an ischemia/reperfusion stress. We applied an in vitro model of this stress, oxygen/glucose deprivation (OGD), to mouse neuromuscular preparations to investigate how Ca(2+) contributes to stress-induced motor terminal damage. Measurements using an ionophoretically-injected fluorescent [Ca(2+)] indicator demonstrated an increase in intra-terminal [Ca(2+)] following OGD onset. When OGD was terminated within 20-30min of the increase in resting [Ca(2+)], these changes were sometimes reversible; in other cases [Ca(2+)] remained high and the terminal degenerated. Endplate innervation was assessed morphometrically following 22min OGD and 120min reoxygenation (32.5°C). Stress-induced motor terminal degeneration was Ca(2+)-dependent. Median post-stress endplate occupancy was only 26% when the bath contained the normal 1.8mM Ca(2+), but increased to 81% when Ca(2+) was absent. Removal of Ca(2+) only during OGD was more protective than removal of Ca(2+) only during reoxygenation. Post-stress endplate occupancy was partially preserved by pharmacological inhibition of various routes of Ca(2+) entry into motor terminals, including voltage-dependent Ca(2+) channels (ω-agatoxin-IVA, nimodipine) and the plasma membrane Na(+)/Ca(2+) exchanger (KB-R7943). Inhibition of a Ca(2+)-dependent protease with calpain inhibitor VI was also protective. These results suggest that most of the OGD-induced motor terminal damage is Ca(2+)-dependent, and that inhibition of Ca(2+) entry or Ca(2+)-dependent proteolysis can reduce this damage. There was no significant difference between the response of wild-type and presymptomatic superoxide dismutase 1 G93A mutant terminals to OGD, or in their response to the protective effect of the tested drugs.

Na-K-Cl cotransporter-1 in the mechanism of cell swelling in cultured astrocytes after fluid percussion injury

Brain edema and associated increased intracranial pressure are major consequences of traumatic brain injury (TBI). An important early component of the edema associated with TBI is astrocyte swelling (cytotoxic edema). Mechanisms for such swelling, however, are poorly understood. Ion channels/transporters/exchangers play a major role in cell volume regulation, and a disturbance in one or more of these systems may result in cell swelling. To examine potential mechanisms in TBI-mediated brain edema, we employed a fluid percussion model of in vitro barotrauma and examined the role of the ion transporter Na(+)-K(+)-2Cl(-)-cotransporter 1 (NKCC1) in trauma-induced astrocyte swelling as this transporter has been strongly implicated in the mechanism of cell swelling in various neurological conditions. Cultures exposed to trauma (3, 4, 5 atm pressure) caused a significant increase in NKCC1 activity (21%, 42%, 110%, respectively) at 3 h. At 5 atm pressure, trauma significantly increased NKCC1 activity at 1 h and it remained increased for up to 3 h. Trauma also increased the phosphorylation (activation) of NKCC1 at 1 and 3 h. Inhibition of MAPKs and oxidative/nitrosative stress diminished the trauma-induced NKCC1 phosphorylation as well as its activity. Bumetanide, an inhibitor of NKCC1, significantly reduced the trauma-induced astrocyte swelling (61%). Silencing NKCC1 with siRNA led to a reduction in trauma-induced NKCC1 activity as well as in cell swelling. These findings demonstrate the critical involvement of NKCC1 in the astrocyte swelling following in vitro trauma, and suggest that blocking NKCC1 activity may represent a useful therapeutic strategy for the cytotoxic brain edema associated with the early phase of TBI.

The plasma membrane Ca²+ ATPase and the plasma membrane sodium calcium exchanger cooperate in the regulation of cell calcium

Calcium is an ambivalent signal: it is essential for the correct functioning of cell life, but may also become dangerous to it. The plasma membrane Ca(2+) ATPase (PMCA) and the plasma membrane Na(+)/Ca(2+) exchanger (NCX) are the two mechanisms responsible for Ca(2+) extrusion. The NCX has low Ca(2+) affinity but high capacity for Ca(2+) transport, whereas the PMCA has a high Ca(2+) affinity but low transport capacity for it. Thus, traditionally, the PMCA pump has been attributed a housekeeping role in maintaining cytosolic Ca(2+), and the NCX the dynamic role of counteracting large cytosolic Ca(2+) variations (especially in excitable cells). This view of the roles of the two Ca(2+) extrusion systems has been recently revised, as the specific functional properties of the numerous PMCA isoforms and splicing variants suggests that they may have evolved to cover both the basal Ca(2+) regulation (in the 100 nM range) and the Ca(2+) transients generated by cell stimulation (in the μM range).

Inhibiting the Na+/H+ exchanger reduces reperfusion injury: a small animal MRI study

We used magnetic resonance imaging (MRI) to assess the efficacy of Na+/H+ exchanger isoform 1 (NHE-1) inhibition following cerebral ischemia. Transient focal cerebral ischemia was induced in wild-type controls (NHE-1(+/+)), NHE-1 genetic knockdown mice (NHE-1(+/-)), and NHE-1(+/+) mice treated with the selective NHE-1 inhibitor HOE642. Diffusion weighted imaging (DWI) revealed a brain lesion as early as 1 hour following reperfusion and illustrated significant protection in NHE-1(+/-) mice (16.2 +/- 7.9 mm3 in NHE-1(+/-) mice vs. 47.5 +/- 16.6 mm3 in NHE-1(+/+) mice). Knockdown of NHE-1 showed significantly smaller infarct at 72 hours on T2 imaging (21.2 +/- 12.6 mm3 in NHE-1(+/-) mice vs. 64.6 +/- 2.5 mm3 in NHE-1(+/+) mice). Administration of HOE642 prior to reperfusion or during early reperfusion reduced ischemic damage. Thus, high resolution T2 images can be used for consistent and precise calculation of lesion volumes, while changes of DWI are a sensitive early marker of ischemic injury. The results of this study demonstrate the therapeutic potential for inhibition of NHE-1 in treating cerebral ischemia.

Excessive Na+/H+ exchange in disruption of dendritic Na+ and Ca2+ homeostasis and mitochondrial dysfunction following in vitro ischemia

Neuronal dendrites are vulnerable to injury under diverse pathological conditions. However, the underlying mechanisms for dendritic Na(+) overload and the selective dendritic injury remain poorly understood. Our current study demonstrates that activation of NHE-1 (Na(+)/H(+) exchanger isoform 1) in dendrites presents a major pathway for Na(+) overload. Neuronal dendrites exhibited higher pH(i) regulation rates than soma as a result of a larger surface area/volume ratio. Following a 2-h oxygen glucose deprivation and a 1-h reoxygenation, NHE-1 activity was increased by ∼70-200% in dendrites. This elevation depended on activation of p90 ribosomal S6 kinase. Moreover, stimulation of NHE-1 caused dendritic Na(+)(i) accumulation, swelling, and a concurrent loss of Ca(2+)(i) homeostasis. The Ca(2+)(i) overload in dendrites preceded the changes in soma. Inhibition of NHE-1 or the reverse mode of Na(+)/Ca(2+) exchange prevented these changes. Mitochondrial membrane potential in dendrites depolarized 40 min earlier than soma following oxygen glucose deprivation/reoxygenation. Blocking NHE-1 activity not only attenuated loss of dendritic mitochondrial membrane potential and mitochondrial Ca(2+) homeostasis but also preserved dendritic membrane integrity. Taken together, our study demonstrates that NHE-1-mediated Na(+) entry and subsequent Na(+)/Ca(2+) exchange activation contribute to the selective dendritic vulnerability to in vitro ischemia.

Endoplasmic reticulum Ca2+ signaling and mitochondrial Cyt c release in astrocytes following oxygen and glucose deprivation

In the present study, we investigated changes of cytosolic Ca2+([Ca2+](cyt)), endoplasmic reticulum Ca2+([Ca2+](ER)) and mitochondrial Ca2+(Ca2+(m)) in astrocytes following oxygen/glucose deprivation and reoxygenation (OGD/REOX). Two hours OGD did not cause changes in [Ca2+](cyt), but led to a significant increase in [Ca2+](ER). The elevation in [Ca2+](ER) continued and reached a peak level (130 +/- 2 microM) by 90 min REOX. An abrupt release of Ca2+(ER) occurred during 1.5-2.5 h REOX, which was accompanied with a delayed and sustained rise in [Ca2+](cyt). Moreover, Ca2+(m) content was increased significantly within 15 min REOX followed by a secondary rise (approximately 4.5-fold) and a release of mitochondrial cytochrome c (Cyt c). Astrocytes exhibited translocation of Cyt c from mitochondria to endoplasmic reticulum (ER) and up regulation of ER stress protein p-eIF2alpha. Blocking Na+-K+-Cl(-) cotransporter isoform 1 activity, either by its potent inhibitor bumetanide or genetic ablation, abolished release of ER Ca2+, delayed rise in [Ca2+](cyt) and Ca2+(m). Inhibition of the reverse mode operation of the Na+/Ca2+ exchanger significantly attenuated OGD/REOX-mediated Cyt c release. In summary, this study illustrates that OGD/REOX triggers a time-dependent loss of Ca2+ homeostasis in cytosol and organelles (ER and mitochondria) in astrocytes. Collective stimulation of Na+-K+-Cl(-) cotransporter isoform 1 and reverse mode function of Na+/Ca2+ exchanger contributes to these changes.

The Na-K-Cl Co-transporter in astrocyte swelling

Ion channels, exchangers and transporters are known to be involved in cell volume regulation. A disturbance in one or more of these systems may result in loss of ion homeostasis and cell swelling. In particular, activation of the Na(+)-K(+)-Cl(-) cotransporters has been shown to regulate cell volume in many conditions. The Na(+)-K(+)-Cl- cotransporters (NKCC) are a class of membrane proteins that transport Na, K, and Cl ions into and out of a wide variety of epithelial and nonepithelial cells. Studies have established the role of NKCC1 in astrocyte swelling/brain edema in ischemia and trauma. Our recent studies suggest that NKCC1 activation is also involved in astrocyte swelling induced by ammonia and in the brain edema in the thioacetamide model of acute liver failure. This review will focus on mechanisms of NKCC1 activation and its contribution to astrocyte swelling/brain edema in neurological disorders, with particular emphasis on ammonia neurotoxicity and acute liver failure.

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

Calcium dysregulation induces apoptosis-inducing factor release: cross-talk between PARP-1- and calpain-signaling pathways

Recent discoveries show that caspase-independent cell death pathways are a pervasive mechanism in neurodegenerative diseases, and apoptosis-inducing factor (AIF) is an important effector of this mode of neuronal death. There are currently two known mechanisms underlying AIF release following excitotoxic stress, PARP-1 and calpain. To test whether there is an interaction between PARP-1 and calpain in triggering AIF release, we used the NMDA toxicity model in rat primary cortical neurons. Exposure to NMDA resulted in AIF truncation and nuclear translocation, and shRNA-mediated knockdown of AIF resulted in neuroprotection. Both calpain and PARP-1 are involved with AIF processing as AIF truncation, nuclear translocation and neuronal death were attenuated by calpain inhibition using adeno-associated virus-mediated overexpression of the endogenous calpain inhibitor, calpastatin, or treatment with the PARP-1 inhibitor 3-ABA. Activation of PARP-1 is necessary for calpain activation as PARP-1 inhibition blocked mitochondrial calpain activation. Finally, NMDA toxicity induces mitochondrial Ca(2+) dysregulation in a PARP-1 dependent manner. Thus, PARP-1 and mitochondrial calpain activation are linked via PARP-1-induced alterations in mitochondrial Ca(2+) homeostasis. Collectively, these findings link the two seemingly independent mechanisms triggering AIF-induced neuronal death.

Ion transporters and ischemic mitochondrial dysfunction

Ischemia-induced ionic imbalance leads to the activation of numerous events including mitochondrial dysfunction and eventual cell death. Dysregulation of mitochondrial Ca(2+) (Ca(2+)(m)) plays a critical role in cell damage under pathological conditions including traumatic brain injury and stroke. High Ca(2+)(m) levels can induce the persistent opening of the mitochondrial permeability transition pore and trigger mitochondrial membrane depolarization, Ca(2+) release, cessation of oxidative phosphorylation, matrix swelling and eventually outer membrane rupture with release of cytochrome c and other apoptogenic proteins. Thus, the dysregulation of mitochondrial Ca(2+) homeostasis is now recognized to play a crucial role in triggering mitochondrial dysfunction and subsequent apoptosis. Recent studies show that some secondary active transport proteins, such as Na(+)-dependent chloride transporter and Na(+)/Ca(2+) exchanger, contribute to ischemia-induced dissipation of ion homeostasis including Ca(2+)(m).

Na-K-Cl Cotransporter-1 in the mechanism of ammonia-induced astrocyte swelling

Brain edema and the consequent increase in intracranial pressure and brain herniation are major complications of acute liver failure (fulminant hepatic failure) and a major cause of death in this condition. Ammonia has been strongly implicated as an important factor, and astrocyte swelling appears to be primarily responsible for the edema. Ammonia is known to cause cell swelling in cultured astrocytes, although the means by which this occurs has not been fully elucidated. A disturbance in one or more of these systems may result in loss of ion homeostasis and cell swelling. In particular, activation of the Na-K-Cl cotransporter (NKCC1) has been shown to be involved in cell swelling in several neurological disorders. We therefore examined the effect of ammonia on NKCC activity and its potential role in the swelling of astrocytes. Cultured astrocytes were exposed to ammonia (NH(4)Cl; 5 mm), and NKCC activity was measured. Ammonia increased NKCC activity at 24 h. Inhibition of this activity by bumetanide diminished ammonia-induced astrocyte swelling. Ammonia also increased total as well as phosphorylated NKCC1. Treatment with cyclohexamide, a potent inhibitor of protein synthesis, diminished NKCC1 protein expression and NKCC activity. Since ammonia is known to induce oxidative/nitrosative stress, and antioxidants and nitric-oxide synthase inhibition diminish astrocyte swelling, we also examined whether ammonia caused oxidation and/or nitration of NKCC1. Cultures exposed to ammonia increased the state of oxidation and nitration of NKCC1, whereas the antioxidants N-nitro-l-arginine methyl ester and uric acid all significantly diminished NKCC activity. These agents also reduced phosphorylated NKCC1 expression. These results suggest that activation of NKCC1 is an important factor in the mediation of astrocyte swelling by ammonia and that such activation appears to be mediated by NKCC1 abundance as well as by its oxidation/nitration and phosphorylation.

Endoplasmic reticulum Ca2+ dysregulation and endoplasmic reticulum stress following in vitro neuronal ischemia: role of Na+-K+-Cl- cotransporter

We investigated the role of Na(+)-K(+)-Cl(-) cotransporter (NKCC1) in conjunction with Na(+)/Ca(2+) exchanger (NCX) in disruption of endoplasmic reticulum (ER) Ca(2+) homeostasis and ER stress development in primary cortical neurons following in vitro ischemia. Oxygen-glucose deprivation (OGD) and reoxygenation (REOX) caused a rise in [Na(+)](cyt) which was accompanied by an elevation in [Ca(2+)](cyt). Inhibition of NKCC1 with its potent inhibitor bumetanide abolished the OGD/REOX-induced rise in [Na(+)](cyt) and [Ca(2+)](cyt). Moreover, OGD significantly increased Ca(2+)(ER) accumulation. Following REOX, a biphasic change in Ca(2+)(ER) occurred with an initial release of Ca(2+)(ER) which was sensitive to inositol 1,4,5-trisphosphate receptor (IP(3)R) inhibition and a subsequent refilling of Ca(2+)(ER) stores. Inhibition of NKCC1 activity with its inhibitor or genetic ablation prevented the release of Ca(2+)(ER). A similar result was obtained with inhibition of reversed mode operation of NCX (NCX(rev)). OGD/REOX also triggered a transient increase of glucose regulated protein 78 (GRP78), phospho-form of the alpha subunit of eukaryotic initiation factor 2 (p-eIF2alpha), and cleaved caspase 12 proteins. Pre-treatment of neurons with NKCC1 inhibitor bumetanide inhibited upregulation of GRP78 and attenuated the level of cleaved caspase 12 and p-eIF2alpha. Inhibition of NKCC1 reduced cytochrome C release and neuronal death. Taken together, these results suggest that NKCC1 and NCX(rev) may be involved in ischemic cell damage in part via disrupting ER Ca(2+) homeostasis and ER function.

Bioenergetics of cerebral ischemia: a cellular perspective

In cerebral ischemia survival of neurons, astrocytes, oligodendrocytes and endothelial cells is threatened during energy deprivation and/or following re-supply of oxygen and glucose. After a brief summary of characteristics of different cells types, emphasizing the dependence of all on oxidative metabolism, the bioenergetics of focal and global ischemia is discussed, distinguishing between events during energy deprivation and subsequent recovery attempt after re-circulation. Gray and white matter ischemia are described separately, and distinctions are made between mature and immature brains. Next comes a description of bioenergetics in individual cell types in culture during oxygen/glucose deprivation or exposure to metabolic inhibitors and following re-establishment of normal aerated conditions. Due to their expression of NMDA and non-NMDA receptors neurons and oligodendrocytes are exquisitely sensitive to excitotoxicity by glutamate, which reaches high extracellular concentrations in ischemic brain for several reasons, including failing astrocytic uptake. Excitotoxicity kills brain cells by energetic exhaustion (due to Na(+) extrusion after channel-mediated entry) combined with mitochondrial Ca(2+)-mediated injury and formation of reactive oxygen species. Many (but not all) astrocytes survive energy deprivation for extended periods, but after return to aerated conditions they are vulnerable to mitochondrial damage by cytoplasmic/mitochondrial Ca(2+) overload and to NAD(+) deficiency. Ca(2+) overload is established by reversal of Na(+)/Ca(2+) exchangers following Na(+) accumulation during Na(+)-K(+)-Cl(-) cotransporter stimulation or pH regulation, compensating for excessive acid production. NAD(+) deficiency inhibits glycolysis and eventually oxidative metabolism, secondary to poly(ADP-ribose)polymerase (PARP) activity following DNA damage. Hyperglycemia can be beneficial for neurons but increases astrocytic death due to enhanced acidosis.

A concerted role of Na+ -K+ -Cl- cotransporter and Na+/Ca2+ exchanger in ischemic damage

Na+-K+-Cl(-) cotransporter isoform 1 (NKCC1) and Na+/Ca2+ exchanger isoform 1 (NCX1) were expressed in cortical neurons. Three hours of oxygen and glucose deprivation (OGD) significantly increased expression of full-length NCX1 protein ( approximately 116 kDa), which remained elevated during 1 to 21 h reoxygenation (REOX) and was accompanied with concurrent cleavage of NCX1. Na+/Ca2+ exchanger isoform 1 heterozygous (NCX1+/-) neurons with approximately 50% less of NCX1 protein exhibited approximately 64% reduction in NCX-mediated Ca2+ influx. Expression of NCX1 and NKCC1 proteins was reduced in double heterozygous (NCX1+/-/NKCC1+/-) neurons. NCX-mediated Ca2+ influx was nearly abolished in these neurons. Three-hour OGD and 21-h REOX caused approximately 80% mortality rate in NCX1+/+ neurons and in NCX1+/- neurons. In contrast, NKCC1+/- neurons exhibited approximately 45% less cell death. The lowest mortality rate was found in NCX1+/-/NKCC1+/- neurons ( approximately 65% less neuronal death). The increased tolerance to ischemic damage was also observed in NCX1+/-/NKCC1+/- brains after transient cerebral ischemia. NCX1+/-/NKCC1+/- mice had a significantly reduced infarct volume at 24 and 72 h reperfusion. In conclusion, these data suggest that NKCC1 in conjunction with NCX1 plays a role in reperfusion-induced brain injury after ischemia.

Cerebral microvascular nNOS responds to lowered oxygen tension through a bumetanide-sensitive cotransporter and sodium-calcium exchanger

Na(+) cotransporters have a substantial role in neuronal damage during brain hypoxia. We proposed these cotransporters have beneficial roles in oxygen-sensing mechanisms that increase periarteriolar nitric oxide (NO) concentration ([NO]) during mild to moderate oxygen deprivation. Our prior studies have shown that cerebral neuronal NO synthase (nNOS) is essential for [NO] responses to decreased oxygen tension and that endothelial NO synthase (eNOS) is of little consequence. In this study, we explored the mechanisms of three specific cotransporters known to play a role in the hypoxic state: KB-R7943 for blockade of the Na(+)/Ca(2+) exchanger, bumetanide for the Na(+)-K(+)-2Cl(-) cotransporter, and amiloride for Na(+)/H(+) cotransporters. In vivo measurements of arteriolar diameter and [NO] at normal and locally reduced oxygen tension in the rat parietal cortex provided the functional analysis. As previously found for intestinal arterioles, bumetanide-sensitive cotransporters are primarily responsible for sensing reduced oxygen because the increased [NO] and dilation were suppressed. The Na(+)/Ca(2+) exchanger facilitated increased NO formation because blockade also suppressed [NO] and dilatory responses to decreased oxygen. Amiloride-sensitive Na(+)/H(+) cotransporters did not significantly contribute to the microvascular regulation. To confirm that nNOS rather than eNOS was primarily responsible for NO generation, eNOS was suppressed with the fusion protein cavtratin for the caveolae domain of eNOS. Although the resting [NO] decreased and arterioles constricted as eNOS was suppressed, most of the increased NO and dilatory response to oxygen were preserved because nNOS was functional. Therefore, nNOS activation secondary to Na(+)-K(+)-2Cl(-) cotransporter and Na(+)/Ca(2+) exchanger functions are key to cerebral vascular oxygen responses.

Prothymosin alpha plays a key role in cell death mode-switch, a new concept for neuroprotective mechanisms in stroke

After stroke or traumatic damages, both necrotic and apoptotic neuronal death cause a loss of functions including memory, sensory perception, and motor skills. From the fact that necrosis has a nature to expand, while apoptosis to cease the cell death cascade in the brain, it is considered that the promising target for the rapid treatment for stroke is the necrosis. In this study, I introduce the discovery of prothymosin alpha (ProTalpha), which inhibits neuronal necrosis, and propose its potentiality of clinical use for stroke. First of all, it should be noted that ProTalpha inhibits the neuronal necrosis induced by serum-free starvation or ischemia-reperfusion stress, which causes a rapid internalization of GLUT1/4, leading a decrease in glucose uptake and cellular ATP levels. Underlying mechanisms are determined to be through an activation of Gi/o, phospholipase C and PKCbetaII. ProTalpha also causes apoptosis later through a similar mechanism. However, we found that ProTalpha-induced apoptosis is completely inhibited by the concomitant treatment with neurotrophins, which are up-regulated by ischemic stress in the brain. Of most importance is the finding that the systemic injection of ProTalpha completely inhibits the brain damages, motor dysfunction and learning memory defect induced by cerebral ischemia-reperfusion stress. As ProTalpha almost entirely prevents the focal ischemia-induced motor dysfunction 4 h after the start of ischemia, this protein seems to have a promising potentiality for clinical use.

Role of maximum rate of depolarization in predicting action potential duration during ventricular fibrillation

During ventricular fibrillation (VF) only 39% of the variation in action potential duration (APD) is accounted for by the previous diastolic interval [DI((n-1))], i.e., restitution, and the previous APD [APD((n-1))], i.e., memory. We tested the hypothesis that a characteristic of the AP upstroke, the maximum rate of depolarization (V(max)), also helps account for its APD. A floating microelectrode was used to make transmembrane recordings at 16,000 samples/s from the anterior left ventricular wall during four 20-s episodes of VF in each of six pigs. V(max), time from V(max) to 60% repolarization (APD(60)), and DI were calculated throughout all episodes. Stepwise linear regression was used to determine how well each APD(60) (APD(60n)) was predicted by V(max) of that AP, the four previous DIs (n-1, n - 2, n - 3, n - 4), and the three previous APD(60)s (n-1, n - 2, n - 3). V(max) entered in the regression equation significantly more often (86% of VF episodes) than either APD((n-1)) (47% of episodes) or DI((n-1)) (58% of episodes). When these three variables entered first or second, their coefficients were almost always positive, consistent with a longer APD associated with 1) a larger V(max), 2) a longer APD((n-1)), and 3) a longer DI((n-1)). R(2) of the regression for all entered variables was 0.51 +/- 0.01 (mean +/- SD). During the first 20 s of VF in swine, V(max) is a more important determinant of APD than the previous DI (restitution) or the previous APD (memory). All variables together account for only one-half of APD variation during VF.

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

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