Astrocyte cultures exhibit P2X7 receptor channel opening in the absence of exogenous ligands

Progress of Astrocyte-Neuron Crosstalk in Central Nervous System Diseases

Neurons are the primary cells responsible for information processing in the central nervous system (CNS). However, they are vulnerable to damage and insult in a variety of neurological disorders. As the most abundant glial cells in the brain, astrocytes provide crucial support to neurons and participate in synapse formation, synaptic transmission, neurotransmitter recycling, regulation of metabolic processes, and the maintenance of the blood-brain barrier integrity. Though astrocytes play a significant role in the manifestation of injury and disease, they do not work in isolation. Cellular interactions between astrocytes and neurons are essential for maintaining the homeostasis of the CNS under both physiological and pathological conditions. In this review, we explore the diverse interactions between astrocytes and neurons under physiological conditions, including the exchange of neurotrophic factors, gliotransmitters, and energy substrates, and different CNS diseases such as Alzheimer's disease, Parkinson's disease, stroke, traumatic brain injury, and multiple sclerosis. This review sheds light on the contribution of astrocyte-neuron crosstalk to the progression of neurological diseases to provide potential therapeutic targets for the treatment of neurological diseases.

Astroglial and Microglial Purinergic P2X7 Receptor as a Major Contributor to Neuroinflammation during the Course of Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune inflammatory disease of the central nervous system that leads to the progressive disability of patients. A characteristic feature of the disease is the presence of focal demyelinating lesions accompanied by an inflammatory reaction. Interactions between autoreactive immune cells and glia cells are considered as a central mechanism underlying the pathology of MS. A glia-mediated inflammatory reaction followed by overproduction of free radicals and generation of glutamate-induced excitotoxicity promotes oligodendrocyte injury, contributing to demyelination and subsequent neurodegeneration. Activation of purinergic signaling, in particular P2X7 receptor-mediated signaling, in astrocytes and microglia is an important causative factor in these pathological processes. This review discusses the role of astroglial and microglial cells, and in particular glial P2X7 receptors, in inducing MS-related neuroinflammatory events, highlighting the importance of P2X7R-mediated molecular pathways in MS pathology and identifying these receptors as a potential therapeutic target.

Mechanisms underlying sensitization of P2X7 receptors in astrocytes for induction of ischemic tolerance

We previously showed that noninvasive mild ischemia (preconditioning; PC) induced ischemic tolerance by upregulation of P2X7 receptors in astrocytes via a hypoxia inducible factor-1α (HIF-1α)-dependent mechanism. The P2X7 receptor is known as a low-sensitivity P2 receptor that requires a high extracellular ATP (eATP) concentration for activation. PC increased the eATP level but was not sufficient to activate P2X7 receptors. Here, we show that astrocytes possess an elaborate mechanism for activation of P2X7 receptors, thus contributing to ischemic tolerance. Nicotinamide adenine dinucleotide (NAD⁺ ) was shown to increase the sensitivity of P2X7 receptors to eATP via ecto-ADP-ribosyltransferase 2 (ARTC2)-catalyzed ADP-ribosylation in peripheral immune cells. Although ARTC2-positive signals were mostly absent in the naïve brain, they were selectively increased in astrocytes by PC. The spatiotemporal pattern of PC-evoked ARTC2 was well associated with that of P2X7 receptors. In the in vitro experiments, NAD⁺ increased the sensitivity of P2X7 receptors to ATP, and at higher concentrations, NAD⁺ itself activated P2X7 receptors without eATP in cultured astrocytes. In the in vivo experiments using middle cerebral artery occlusion model mice, the PC-evoked increase in HIF-1α in astrocytes was abolished by the ARTC2 inhibitor S + 16a. S + 16a also abolished PC-evoked ischemic tolerance. Taken together, the results suggested that P2X7 receptors can be sensitized to ATP by NAD⁺ /ARTC2-catalyzed ADP-ribosylation, which allows astrocytes to drive P2X7 receptor-mediated ischemic tolerance even though PC only slightly increases the amount of eATP.

Elevation of the Blood Glucose Level is Involved in an Increase in Expression of Sweet Taste Receptors in Taste Buds of Rat Circumvallate Papillae

The gustation system for sweeteners is well-known to be regulated by nutritional and metabolic conditions, but there is no or little information on the underlying mechanism. Here, we examined whether elevation of the blood glucose level was involved in alteration of the expression of sweet taste receptors in circumvallate papillae (CP) and sweet taste sensitivity in male Sprague-Dawley rats. Rats under 4 h-fed conditions following 18 h-fasting exhibited elevated blood glucose levels and decreased pancreatic T1R3 expression, compared to rats after 18 h-fasting treatment, and they exhibited increased protein expression of sweet taste receptors T1R2 and T1R3 in CP. Under streptozotocin (STZ)-induced diabetes mellites (DM) conditions, the protein expression levels of T1R2 and T1R3 in CP were higher than those under control conditions, and these DM rats exhibited increased lick ratios in a low sucrose concentration range in a brief access test with a mixture of sucrose and quinine hydrochloride (QHCl). These findings indicate that the elevation of blood glucose level is a regulator for an increase in sweet taste receptor protein expression in rat CP, and such alteration in STZ-induced DM rats is involved in enhancement of their sweet taste sensitivity.

Reactive oxygen species play a role in P2X7 receptor-mediated IL-6 production in spinal astrocytes

Astrocytes mediate a remarkable variety of cellular functions, including gliotransmitter release. Under pathological conditions, high concentrations of the purinergic receptor agonist adenosine triphosphate (ATP) are released into the extracellular space leading to the activation of the purinergic P2X7 receptor, which in turn can initiate signaling cascades. It is well-established that reactive oxygen species (ROS) increase in macrophages and microglia following P2X7 receptor activation. However, direct evidence that activation of P2X7 receptor leads to ROS production in astrocytes is lacking to date. While it is known that P2X7R activation induces cytokine production, the mechanism involved in this process is unclear. In the present study, we demonstrated that P2X7 receptor activation induced ROS production in spinal astrocytes in a concentration-dependent manner. We also found that P2X7R-mediated ROS production is at least partially through NADPH oxidase. In addition, our ELISA data show that P2X7R-induced IL-6 release was dependent on NADPH oxidase-mediated production of ROS. Collectively, these results reveal that activation of the P2X7 receptor on spinal astrocytes increases ROS production through NADPH oxidase, subsequently leading to IL-6 release. Our results reveal a role of ROS in the P2X7 signaling pathway in mouse spinal cord astrocytes and may indicate a potential mechanism for the astrocytic P2X7 receptor in chronic pain.

Nicotinamide phosphoribosyltransferase secreted from microglia via exosome during ischemic injury

Nicotinamide phosphoribosyltransferase (NAMPT) is the key enzyme of the salvage pathway of nicotinamide adenine dinucleotide synthesis. NAMPT can also be secreted and functions as a cytokine. We have previously shown that in the brain, NAMPT expression and secretion can be induced in microglia upon neuroinflammation and injury. Yet the mechanism for NAMPT secretion remains unclear. Here we show that NAMPT can be actively secreted from microglia upon the treatment of ischemia-like injury - oxygen-glucose deprivation and recovery (OGD/R). We confirmed that classical ER-Golgi pathway is not involved in NAMPT secretion. NAMPT secretion was further enhanced by ATP, and the secretion was mediated by P2X₇ receptor and by intracellular Ca²⁺ . Importantly, we found that phospholipase D inhibitor, n-butanol, phospholipase D siRNA, and wortmannin significantly decreased OGD/R-induced and ATP-enhanced release of NAMPT in microglia. After excluding the mechanisms of involving secretory autophagy, endosomes, and secretory lysosome, we have concluded that microglial NAMPT is secreted mainly via exosome. Immune-electron microscopy identifies NAMPT in extracellular vesicles with the size and morphology characteristic of exosome. With the vesicles harvested by ultra-centrifugation, exosomal NAMPT is further confirmed by Western blotting analysis. Intriguingly, the amount of NAMPT relative to exosomal protein markers remains unchanged upon the treatment of OGD/R, suggesting a constant load of exosomal NAMPT in microglia. Taken together, we have identified NAMPT is actively secreted via exosome from microglia during neuroinflammation of ischemic injury.

Intracellular labile zinc is a determinant of vulnerability of cultured astrocytes to oxidative stress

Recently, we found that treatment of cultured mouse astrocytes of ddY-strain mice (ddY-astrocytes) with 400 μM H₂O₂ for 24 h increased the intracellular labile zinc level without cell toxicity. In contrast, 170 μM H₂O₂ for 12 h is reported to kill mouse astrocytes obtained from C57BL/6-strain mice (C57BL/6-astrocytes) with an increase in intracellular labile zinc. To clarify this discrepancy, we compared the intracellular zinc levels and cell toxicity in H₂O₂-treated ddY- and C57BL/6-astrocytes. Exposure of C57BL/6-astrocytes to 170 or 400 μM H₂O₂ for 12 h dose-dependently decreased the cell viability and administration of plasma membrane-permeable zinc chelator TPEN prevented the 170 μM H₂O₂-induced astrocyte death, while neither concentration of H₂O₂ killed ddY-astrocytes. The intracellular zinc level in H₂O₂-treated C57BL/6-astrocytes was higher than that in H₂O₂-treated ddY-astrocytes, and this increase was suppressed by TPEN. There was no apparent difference in the expression levels of zinc transporters ZIPs and ZnTs between the two types of astrocytes, while expression of zinc releasable channel TRPM7 was found on the plasma membrane in ddY-astrocytes, but not in C57BL/6-astrocytes, although the total cellular expression levels were almost the same. In addition, a TRPM7 blocker, 2-aminoethoxydiphenyl borate, increased the intracellular zinc level in H₂O₂-treated ddY-, but not C57BL/6-astrocytes. Collectively, it is suggested that vulnerability of astrocytes to oxidative stress depends on an increased level of intracellular labile zinc, and TRPM7 localized on the plasma membrane contributes, at least in part, to ameliorate the cell injury by decreasing the zinc level.

Modulation of P2X7 purinergic receptor activity by extracellular Zn2+ in cultured mouse hippocampal astroglia

The P2X7R protein, a P2 type purinergic receptor functioning as a non-selective cation channel, is expressed in different cell types of the central nervous system in several regions of the brain. The activation of the P2X7R protein by ATP modulates excitatory neurotransmission and contributes to microglial activation, apoptosis and neuron-glia communication. Zinc is an essential micronutrient that is highly concentrated in the synaptic vesicles of glutamatergic hippocampal neurons where free zinc ions released into the synaptic cleft alter glutamatergic signal transmission. Changes in both P2X7R-mediated signaling and brain zinc homeostasis have been implicated in the pathogenesis of mood disorders. Here, we tested the hypothesis that extracellular zinc regulates P2X7R activity in the hippocampus. We observed that P2X7R is expressed in both neurons and glial cells in primary mouse hippocampal neuron-glia culture. Propidium iodide (PI) uptake through large pores formed by pannexins and P2X7R was dose-dependently inhibited by extracellular zinc ions. Calcium influx mediated by P2X7R in glial cells was also reduced by free zinc ions. Interestingly, no calcium influx was detected in response to ATP or 3'-O-(4-Benzoyl) benzoyl ATP (BzATP) in neurons despite the expression of P2X7R at the plasma membrane. Our results show that free zinc ions can modulate hippocampal glial purinergic signaling, and changes in the activity of P2X7R may contribute to the development of depression-like behaviors associated with zinc deficiency.

Stretch-induced Ca2+ independent ATP release in hippocampal astrocytes

KEY POINTS

Similar to neurons, astrocytes actively participate in synaptic transmission via releasing gliotransmitters. The Ca²⁺ -dependent release of gliotransmitters includes glutamate and ATP. Following an 'on-cell-like' mechanical stimulus to a single astrocyte, Ca²⁺ independent single, large, non-quantal, ATP release occurs. Astrocytic ATP release is inhibited by either selective antagonist treatment or genetic knockdown of P2X7 receptor channels. Our work suggests that ATP can be released from astrocytes via two independent pathways in hippocampal astrocytes; in addition to the known Ca²⁺ -dependent vesicular release, larger non-quantal ATP release depends on P2X7 channels following mechanical stretch.

ABSTRACT

Astrocytic ATP release is essential for brain functions such as synaptic long-term potentiation for learning and memory. However, whether and how ATP is released via exocytosis remains hotly debated. All previous studies of non-vesicular ATP release have used indirect assays. By contrast, two recent studies report vesicular ATP release using more direct assays. In the present study, using patch clamped 'ATP-sniffer cells', we re-investigated astrocytic ATP release at single-vesicle resolution in hippocampal astrocytes. Following an 'on-cell-like' mechanical stimulus of a single astrocyte, a Ca²⁺ independent single large non-quantal ATP release occurred, in contrast to the Ca²⁺ -dependent multiple small quantal ATP release in a chromaffin cell. The mechanical stimulation-induced ATP release from an astrocyte was inhibited by either exposure to a selective antagonist or genetic knockdown of P2X7 receptor channels. Functional P2X7 channels were expressed in astrocytes in hippocampal brain slices. Thus, in addition to small quantal ATP release, larger non-quantal ATP release depends on P2X7 channels in astrocytes.

Evidence that activation of P2X7R does not exacerbate neuronal death after optic nerve transection and focal cerebral ischemia in mice

Conflicting data in the literature about the function of P2X7R in survival following ischemia necessitates the conductance of in-depth studies. To investigate the impacts of activation vs inhibition of the receptor on neuronal survival as well as the downstream signaling cascades, in addition to optic nerve transection (ONT), 30min and 90min of middle cerebral artery occlusion (MCAo) models were performed in mice. Intracellular calcium levels were assessed in primary cortical neuron cultures. Here, we show that P2X7R antagonist Brilliant Blue G (BBG) decreased DNA fragmentation, infarct volume, brain swelling, neurological deficit scores and activation of microglial cells after focal cerebral ischemia. BBG also significantly increased the number of surviving retinal ganglion cells (RGCs) after ONT and the number of surviving neurons following MCAo. Importantly, receptor agonist BzATP resulted in increased activation of microglial cells and induced phosphorylation of ERK, AKT and JNK. These results indicated that inhibition of P2X7R with BBG promoted neuronal survival, not through the activation of survival kinase pathways, but possibly by improved intracellular Ca²⁺ overload and decreased the levels of Caspase 1, IL-1β and Bax proteins. On the other hand, BzATP-mediated increased number of activated microglia and increased survival kinase levels in addition to increased caspase-1 and IL-1β levels indicate the complex nature of the P2X7 receptor-mediated signaling in neuronal injury.

Targeting the P2X7 Receptor in Age-Related Macular Degeneration

The P2X7 receptor (P2X7R) is a membrane receptor for the extracellular adenosine triphosphate (ATP). It functions as a ligand-gated non-selective cation channel and can mediate formation of a large non-selective membrane pore. Activation of the P2X7R induces multiple downstream events, including oxidative stress, inflammatory responses and cell death. Although the P2X7R has been identified in the retinal pigment epithelium (RPE) and different layers of retina, its biological and pathological functions as well as its downstream signaling pathways in the RPE and retina are not yet fully understood. Better understanding of the function of P2X7R in the RPE and retina under normal and disease states might lead to novel therapeutic targets in retinal diseases, including age-related macular degeneration (AMD). This brief review will mainly focus on recent findings on in vitro and in vivo evidence for the role of the P2X7R in the RPE and AMD.

Oxidative stress-induced increase of intracellular zinc in astrocytes decreases their functional expression of P2X7 receptors and engulfing activity

Exposure of astrocytes to oxidative stress induces an increase of intracellular labile zinc and a decrease of functional expression of P2X7 receptorviaits translocation from the plasma membrane to the cytosol by altering the expression profile of P2X7 receptor and its splice variants, leading to a decrease of their engulfing activity.

Inhibitory effect of divalent metal cations on zinc uptake via mouse Zrt-/Irt-like protein 8 (ZIP8)

AIMS

There is controversy regarding the substrate specificity of ZIP8, a ZIP isoform, involved in regulation of extra- and intracellular zinc levels. Here, we investigated the inhibitory effects of divalent metal cations on zinc uptake via mouse ZIP8 (mZIP8).

MAIN METHODS

mZIP8 cDNA was transfected into HEK293T cells by a lipofection method, and its functional expression was evaluated by immunocytochemistry, Western blotting and ⁶⁵Zn (⁶⁵ZnCl₂) uptake measurement.

KEY FINDINGS

Transfection of mZIP8 cDNA into HEK293T cells induced expression of mZIP8 in the cells, and increased zinc uptake. mZIP8-mediated zinc uptake depended on extracellular bicarbonate, and the Michaelis constant for the uptake was estimated to be 8.48±2.46μM. In the inhibition study, iron and cadmium competitively, and cobalt, nickel and copper non-competitively inhibited the mZIP8-mediated zinc uptake, the inhibition constants being calculated to be 3.37, 55.5, 80.6, 198 and 48.3μM, respectively. In contrast, magnesium and manganese at concentrations of up to 1500 and 200μM, respectively, had no inhibitory effect on the zinc uptake via mZIP8.

SIGNIFICANCE

In this study, we reveal that the inhibition profiles of divalent metal cations as to zinc uptake via mZIP8 apparently differ from those for mZIP1, especially in the affinity and inhibition manner of nickel. These findings should contribute to identification of ZIP isoforms involved in total cellular zinc transport.

NLRP3 Inflammasome Activation in the Brain after Global Cerebral Ischemia and Regulation by 17β-Estradiol

17β-Estradiol (E2) is a well-known neuroprotective factor in the brain. Recently, our lab demonstrated that the neuroprotective and cognitive effects of E2 require mediation by the estrogen receptor (ER) coregulator protein and proline-, glutamic acid-, and leucine-rich protein 1 (PELP1). In the current study, we examined whether E2, acting via PELP1, can exert anti-inflammatory effects in the ovariectomized rat and mouse hippocampus to regulate NLRP3 inflammasome activation after global cerebral ischemia (GCI). Activation of the NLRP3 inflammasome pathway and expression of its downstream products, cleaved caspase-1 and IL-1β, were robustly increased in the hippocampus after GCI, with peak levels observed at 6-7 days. Expression of P2X7 receptor, an upstream regulator of NLRP3, was also increased after GCI. E2 markedly inhibited NLRP3 inflammasome pathway activation, caspase-1, and proinflammatory cytokine production, as well as P2X7 receptor expression after GCI (at both the mRNA and protein level). Intriguingly, the ability of E2 to exert these anti-inflammatory effects was lost in PELP1 forebrain-specific knockout mice, indicating a key role for PELP1 in E2 anti-inflammatory signaling. Collectively, our study demonstrates that NLRP3 inflammasome activation and proinflammatory cytokine production are markedly increased in the hippocampus after GCI, and that E2 signaling via PELP1 can profoundly inhibit these proinflammatory effects.

Oxidative stress: A major pathogenesis and potential therapeutic target of antioxidative agents in Parkinson's disease and Alzheimer's disease

Oxidative stress reflects an imbalance between the overproduction and incorporation of free radicals and the dynamic ability of a biosystem to detoxify reactive intermediates. Free radicals produced by oxidative stress are one of the common features in several experimental models of diseases. Free radicals affect both the structure and function of neural cells, and contribute to a wide range of neurodegenerative diseases, including Parkinson's disease and Alzheimer's disease. Although the precise mechanisms that result in the degeneration of neurons and the relevant pathological changes remain unclear, the crucial role of oxidative stress in the pathogenesis of neurodegenerative diseases is associated with several proteins (such as α-synuclein, DJ-1, Amyloid β and tau protein) and some signaling pathways (such as extracellular regulated protein kinases, phosphoinositide 3-kinase/Protein Kinase B pathway and extracellular signal-regulated kinases 1/2) that are tightly associated with the neural damage. In this review, we present evidence, gathered over the last decade, concerning a variety of pathogenic proteins, their important signaling pathways and pathogenic mechanisms associated with oxidative stress in Parkinson's disease and Alzheimer's disease. Proper control and regulation of these proteins' functions and the related signaling pathways may be a promising therapeutic approach to the patients. We also emphasizes antioxidative options, including some new neuroprotective agents that eliminate excess reactive oxygen species efficiently and have a certain therapeutic effect; however, controversy surrounds some of them in terms of the dose and length of therapy. These agents require further investigation by clinical application in patients suffering Parkinson's disease and Alzheimer's disease.

Hypothesizing that, A Pro-Dopamine Regulator (KB220Z) Should Optimize, but Not Hyper-Activate the Activity of Trace Amine-Associated Receptor 1 (TAAR-1) and Induce Anti-Craving of Psychostimulants in the Long-Term

Unlike other drugs of abuse such as alcohol, nicotine, opiates/opioids, the FDA has not approved any agent to treat psychostimulant dependence. Certainly, it is widely acceptable that dopaminergic signaling is a key factor in both the initiation and continued motivation to abuse this class of stimulant substances. It is also well accepted that psychostimulants such as cocaine affect not only the release of neuronal dopamine at the nucleus accumbens (NAc), but also has powerful inhibitory actions on the dopamine transporter system. Understandably, certain individuals are at high risk and very vulnerable to abuse this class of substances. Trace-amine-associated receptor 1 (TAAR1) is a G -protein coupled receptor activated by trace amines. The encoded protein responds little or not at all to dopamine, serotonin, epinephrine, or histamine, but responds well to beta-phenylethylamine, p-tyramine, octopamine, and tryptamine. This gene is thought to be intronless. TAAR1 agonists reduce the neurochemical effects of cocaine and amphetamines as well as attenuate addiction and abuse associated with these two psychostimulants. The mechanism involves blocking the firing rate of dopamine in the limbic system thereby decreasing a hyperdopaminergic trait/state, whereby the opposite is true for TAAR1 antagonists. Based on many studies, it is accepted that in Reward Deficiency Syndrome (RDS), there is weakened tonic and improved phasic dopamine discharge leading to a hypodopaminergic/glutamatergic trait. The dopamine pro-complex mixture KB220, following many clinical trials including neuroimaging studies, has been shown to enhance resting state functional connectivity in humans (abstinent heroin addicts), naïve rodent models, and regulates extensive theta action in the cingulate gyrus of abstinent psychostimulant abusers. In this article, we are hypothesizing that KB220 may induce its action on resting state functional connectivity, for example, by actually balancing (optimizing) the effects of TAAR1 on the glutamatergic system allowing for optimization of this system. This will lead to a normalized and homeostatic release of NAc dopamine. This proposed optimization, and not enhanced activation of TAAR1, should lead to well-being of the individual. Hyper-activation instead of optimizing the TAAR1 system unfortunately will lead to a prolonged hypodopaminergic state and as such, will cause enhanced craving for not only psychoactive substances, but also other drug-related and even non-drug related RDS behaviors. This hypothesis will require extensive research, which seems warranted based on the global epidemic of drug and behavioral addictions.

Expression level of P2X7 receptor is a determinant of ATP-induced death of mouse cultured neurons

Activation of P2X7 receptor (P2X7R), a purinergic receptor, expressed by neurons is well-known to induce their death, but whether or not their sensitivity to ATP depends on its expression levels remains unclear. Here, we examined the effect of the expression level of P2X7Rs on cell viability using pure neuron cultures, co-cultures with astrocytes derived from SJL- and ddY-strain mice, and mouse P2X7R-expressing HEK293T cell systems. Treatment of pure neuron cultures with 5mM ATP for 2h, followed by 3-h incubation in fresh medium, resulted in death of both types of neurons, and their death was prevented by administration of P2X7R-specific antagonists. In both SJL- and ddY-neurons, ATP-induced neuronal death was inhibited by a mitochondrial permeability transition pore inhibitor cyclosporine A, mitochondrial dysfunction being involved in their death. The ATP-induced neuronal death was greater for SJL-neurons than for ddY-ones, this being correlated with the expression level of P2X7R in them, and the same results were obtained for the HEK293T cell systems. Co-culture of neurons with astrocytes increased the ATP-induced neuronal death compared to the case of pure neuron cultures. Overall, we reveal that neuronal vulnerability to ATP depends on the expression level of P2X7R, and co-existence of astrocytes exacerbates ATP-induced neuronal death.

Fluorescent dyes as a reliable tool in P2X7 receptor-associated pore studies

Since the nineteenth century, a great amount of different biological structures and processes have been assessed by fluorescent dyes. Along with the uses of these compounds as vital and histological dyes, some fluorescent dyes have become valuable tools for the study of the pore phenomenon in plasma membranes. Some ion channels capable of forming large conductance channels, such as P2X7, TRPV1, VDAC-1 and the maxi-anion channels transiently alter the plasma membrane permeability, producing pores, which permit the passage of molecules of up to 1,000 Da. In this review, we discuss the uses of the fluorescent dyes chosen in diverse studies of this topic up to now.

P2X7 R-mediated Ca(2+) -independent d-serine release via pannexin-1 of the P2X7 R-pannexin-1 complex in astrocytes

D-serine is a coagonist of N-methyl-d-aspartate (NMDA) subtype of glutamate receptor and plays a role in regulating activity-dependent synaptic plasticity. In this study, we examined the mechanism by which extracellular ATP triggers the release of d-serine from astrocytes and discovered a novel Ca(2+) -independent release mechanism mediated by P2X7 receptors (P2X7 R). Using [(3) H] d-serine, which was loaded into astrocytes via the neutral amino acid transporter 2 (ASCT2), we observed that ATP and a potent P2X7 R agonist, 2'(3')-O-(4-benzoylbenzoyl)adenosine-5'-triphosphate (BzATP), stimulated [(3) H]D-serine release and that were abolished by P2X7 R selective antagonists and by shRNAs, whereas enhanced by removal of intracellular or extracellular Ca(2+) . The P2X7 R-mediated d-serine release was inhibited by pannexin-1 antagonists, such as carbenoxolone (CBX), probenecid (PBN), and (10) Panx-1 peptide, and shRNAs, and stimulation of P2X7 R induced P2X7 R-pannexin-1 complex formation. Simply incubating astrocytes in Ca(2+) /Mg(2+) -free buffer also induced the complex formation, and that enhanced basal d-serine release through pannexin-1. The P2X7 R-mediated d-serine release assayed in Ca(2+) /Mg(2+) -free buffer was enhanced as well, and that was inhibited by CBX. Treating astrocytes with general protein kinase C (PKC) inhibitors, such as chelerythrine, GF109203X, and staurosporine, but not Ca(2+) -dependent PKC inhibitor, Gö6976, inhibited the P2X7 R-mediated d-serine release. Thus, we conclude that in astrocytes, P2X7 R-pannexin-1 complex formation is crucial for P2X7 R-mediated d-serine release through pannexin-1 hemichannel. The release is Ca(2+) -independent and regulates by a Ca(2+) -independent PKC. The activated P2X7 R per se is also functioned as a permeation channel to release d-serine in part. This P2X7 R-mediated d-serine release represents an important mechanism for activity-dependent neuron-glia interaction.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

P2X7 receptor inhibition increases CNTF in the subventricular zone, but not neurogenesis or neuroprotection after stroke in adult mice

Increasing endogenous ciliary neurotrophic factor (CNTF) expression with a pharmacological agent might be beneficial after stroke as CNTF both promotes neurogenesis and, separately, is neuroprotective. P2X7 purinergic receptor inhibition is neuroprotective in rats and increases CNTF release in rat CMT1A Schwann cells. We, first, investigated the role of P2X7 in regulating CNTF and neurogenesis in adult mouse subventricular zone (SVZ). CNTF expression was increased by daily intravenous injections of the P2X7 antagonist Brilliant Blue G (BBG) in naïve C57BL/6 or Balb/c mice over 3 days. Despite the ∼40-60 % increase or decrease in CNTF with BBG or the agonist BzATP, respectively, the number of proliferated BrdU+SVZ nuclei did not change. BBG failed to increase FGF2, which is involved in CNTF-regulated neurogenesis, but induced IL-6, LIF, and EGF, which are known to reduce SVZ proliferation. Injections of IL-6 next to the SVZ induced CNTF and FGF2, but not proliferation, suggesting that IL-6 counteracts their neurogenesis-inducing effects. Following ischemic injury of the striatum by middle cerebral artery occlusion (MCAO), a 3-day BBG treatment increased CNTF in the medial penumbra containing the SVZ. BBG also induced CNTF and LIF, which are known to be protective following stroke, in the whole striatum after MCAO, but not GDNF or BDNF. However, BBG treatment did not reduce the lesion area or apoptosis in the penumbra. Even so, this study shows that P2X7 can be targeted with systemic drug treatments to differentially regulate neurotrophic factors in the brain following stroke.

Regulation of activity of P2X7 receptor by its splice variants in cultured mouse astrocytes

Of purinergic receptors, P2X7 receptor (P2X7R, defined as a full-length receptor) has unique characteristics, and its activation leads to ion channel activity and pore formation, causing cell death. Previously, we demonstrated that P2X7R expressed by nonstimulated astrocyte cultures obtained from SJL-strain mice exhibits constitutive activation, implying its role in maintenance of cellular homeostasis. To obtain novel insights into its physiological roles, we examined whether constitutive activation of P2X7R is regulated by expression of its splice variants in such resting astrocytes, and whether their distinct expression profiles in different mouse strains affect activation levels of astrocytic P2X7Rs. In SJL- and ddY-mouse astrocytes, spontaneous YO-PRO-1 uptake, an indicator of pore activity of P2X7R, was detected, but the uptake by the formers was significantly greater than that by the latter. Between the two mouse strains, there was a difference in their sensitivity of YO-PRO-1 uptake to antagonists, but not in the expression levels and sequences of P2X7R and pannexin-1. Regarding expression of splice variants of P2X7R, expression of P2X7R variant-3 (P2X7R-v3) and -4 (P2X7R-v4), but not variant-2 and -k, was lower in SJL-mouse astrocytes than in ddY-mouse ones. On transfection of P2X7R-v3 and -v4 into SJL-mouse astrocytes, the pore activity was attenuated as in the case of the HEK293T cell-expression system. These findings demonstrate that basal activity of P2X7R expressed by resting astrocytes is negatively regulated by P2X7R-v3 and -v4, and that their distinct expression profiles result in the different activation levels of astrocytic P2X7Rs in different mouse strains.

Purinergic P2X7 receptors mediate cell death in mouse cerebellar astrocytes in culture

The brain distribution and functional role of glial P2X7 receptors are broader and more complex than initially anticipated. We characterized P2X7 receptors from cerebellar astrocytes at the molecular, immunocytochemical, biophysical, and cell physiologic levels. Mouse cerebellar astrocytes in culture express mRNA coding for P2X7 receptors, which is translated into P2X7 receptor protein as proven by Western blot analysis and immunocytochemistry. Fura-2 imaging showed cytosolic calcium responses to ATP and the synthetic analog 3'-O-(4-benzoyl)benzoyl-ATP (BzATP) exhibited two components, namely an initial transient and metabotropic component followed by a sustained one that depended on extracellular calcium. This latter component, which was absent in astrocytes from P2X7 receptor knockout mice (P2X7 KO), was modulated by extracellular Mg(2+), and was sensitive to Brilliant Blue G (BBG) and 3-(5-(2,3-dichlorophenyl)-1H-tetrazol-1-yl)methyl pyridine (A438079) antagonism. BzATP also elicited inwardly directed nondesensitizing whole-cell ionic currents that were reduced by extracellular Mg(2+) and P2X7 antagonists (BBG and calmidazolium). In contrast to that previously reported in rat cerebellar astrocytes, sustained BzATP application induced a gradual increase in membrane permeability to large cations, such as N-methyl-d-glucamine and 4-[3-methyl-2(3H)-benzoxazolylidene)-methyl]-1-[3-(triethylammonio)propyl]diiodide, which ultimately led to the death of mouse astrocytes. Cerebellar astrocyte cell death was prevented by BBG but not by calmidazolium, removal of extracellular calcium, or treatment with the caspase-3 inhibitor, benzyloxycarbonyl-Asp(OMe)-Glu(OMe)-Val-Asp(OMe)-fluoromethylketone, thus suggesting a necrotic-type mechanism of cell death. Since this cellular response was not observed in astrocytes from P2X7 KO mice, this study suggests that stimulation of P2X7 receptor may convey a cell death signal to cerebellar astrocytes in a species-specific manner.

Neuronal and glial purinergic receptors functions in neuron development and brain disease

Brain development requires the interaction of complex signaling pathways, involving different cell types and molecules. For a long time, most attention has focused on neurons in a neuronocentric conceptualization of central nervous system development, these cells fulfilling an intrinsic program that establishes the brain’s morphology and function. By contrast, glia have mainly been studied as support cells, offering guidance or as the cells that react to brain injury. However, new evidence is appearing that demonstrates a more fundamental role of glial cells in the control of different aspects of neuronal development and function, events in which the influence of neurons is at best weak. Moreover, it is becoming clear that the function and organization of the nervous system depends heavily on reciprocal neuron–glia interactions. During development, neurons are often generated far from their final destination and while intrinsic mechanisms are responsible for neuronal migration and growth, they need support and regulatory influences from glial cells in order to migrate correctly. Similarly, the axons emitted by neurons often have to reach faraway targets and in this sense, glia help define the way that axons grow. Moreover, oligodendrocytes and Schwann cells ultimately envelop axons, contributing to the generation of nodes of Ranvier. Finally, recent publications show that astrocytes contribute to the modulation of synaptic transmission. In this sense, purinergic receptors are expressed widely by glial cells and neurons, and recent evidence points to multiple roles of purines and purinergic receptors in neuronal development and function, from neurogenesis to axon growth and functional axonal maturation, as well as in pathological conditions in the brain. This review will focus on the role of glial and neuronal secreted purines, and on the purinergic receptors, fundamentally in the control of neuronal development and function, as well as in diseases of the nervous system.

P2X7 receptors regulate engulfing activity of non-stimulated resting astrocytes

We previously demonstrated that P2X7 receptors (P2X7Rs) expressed by cultured mouse astrocytes were activated without any exogenous stimuli, but its roles in non-stimulated resting astrocytes remained unknown. It has been reported that astrocytes exhibit engulfing activity, and that the basal activity of P2X7Rs regulates the phagocytic activity of macrophages. In this study, therefore, we investigated whether P2X7Rs regulate the engulfing activity of mouse astrocytes. Uptake of non-opsonized beads by resting astrocytes derived from ddY-mouse cortex time-dependently increased, and the uptaken beads were detected in the intracellular space. The bead uptake was inhibited by cytochalasin D (CytD), an F-actin polymerization inhibitor, and agonists and antagonists of P2X7Rs apparently decreased the uptake. Spontaneous YO-PRO-1 uptake by ddY-mouse astrocytes was reduced by the agonists and antagonists of P2X7Rs, but not by CytD. Down-regulation of P2X7Rs using siRNA decreased the bead uptake by ddY-mouse astrocytes. In addition, compared to in the case of ddY-mouse astrocytes, SJL-mouse astrocytes exhibited higher YO-PRO-1 uptake activity, and their bead uptake was significantly greater. These findings suggest that resting astrocytes exhibit engulfing activity and that the activity is regulated, at least in part, by their P2X7Rs.

Mitochondrial dysfunction and NAD(+) metabolism alterations in the pathophysiology of acute brain injury

Mitochondrial dysfunction is commonly believed to be one of the major players in mechanisms of brain injury. For several decades, pathologic mitochondrial calcium overload and associated opening of the mitochondrial permeability transition (MPT) pore were considered a detrimental factor causing mitochondrial damage and bioenergetics failure. Mitochondrial and cellular bioenergetic metabolism depends on the enzymatic reactions that require NAD(+) or its reduced form NADH as cofactors. Recently, it was shown that NAD(+) also has an important function as a substrate for several NAD(+) glycohydrolases whose overactivation can contribute to cell death mechanisms. Furthermore, downstream metabolites of NAD(+) catabolism can also adversely affect cell viability. In contrast to the negative effects of NAD(+)-catabolizing enzymes, enzymes that constitute the NAD(+) biosynthesis pathway possess neuroprotective properties. In the first part of this review, we discuss the role of MPT in acute brain injury and its role in mitochondrial NAD(+) metabolism. Next, we focus on individual NAD(+) glycohydrolases, both cytosolic and mitochondrial, and their role in NAD(+) catabolism and brain damage. Finally, we discuss the potential effects of downstream products of NAD(+) degradation and associated enzymes as well as the role of NAD(+) resynthesis enzymes as potential therapeutic targets.

Is pannexin the pore associated with the P2X7 receptor?

The P2X7 receptor (P2X7R), an ATP-gated cation channel, is expressed predominantly in leukocytes. Activation of P2X7R has been implicated in the formation of a cytolytic pore (i.e., a large conductance channel) that allows the passage of molecules up to 900 Da in macrophages. At least two hypotheses have been presented to explain the conversion of a nonselective cation channel to a cytolytic pore. One hypothesis suggests that the pore is a separate molecular structure activated by P2X7R, and the second asserts that this is an intrinsic property of P2X7R (pore dilation). Based on connexin knockout and hemichannel antagonist studies, some groups have concluded that connexins and pannexins, the hemichannel-forming proteins in vertebrates, are fundamental components of the large conductance channel associated with P2X7R. Dye uptake and electrophysiology experiments were used to evaluate the efficacy and specificity of some hemichannel antagonists under conditions known to open the large conductance channel associated with P2X7R. Hemichannel antagonists and interference RNA (RNAi) targeting pannexin-1 did not affect P2X7R macroscopic currents [ATP, 1,570±189 pA; ATP+100 μM carbenoxolone (CBX), 1,498±100 pA; ATP+1 mM probenecid (Prob), 1,522±9 pA] or dye uptake in a FACS assay (ATP, 63±5%; ATP+100 μM CBX, 51.51±8.4%; ATP+1 mM Prob, 57.7±4.3%) in mouse macrophages. These findings strongly suggest that the high-permeability pore evident after prolonged P2X7R activation does not occur through connexin or pannexin hemichannels in murine macrophages. Another membrane protein may be involved in P2X7R pore formation.

Poly(ADP-ribose) polymerase-1-induced NAD(+) depletion promotes nuclear factor-κB transcriptional activity by preventing p65 de-acetylation

NF-κB is a transcription factor that integrates pro-inflammatory and pro-survival responses in diverse cell types. The activity of NF-κB is regulated in part by acetylation of its p65 subunit at lysine 310, which is required for transcription complex formation. De-acetylation at this site is performed by sirtuin 1(SIRT1) and possibly other sirtuins in an NAD(+) dependent manner, such that SIRT1 inhibition promotes NF-κB transcriptional activity. It is unknown, however, whether changes in NAD(+) levels can influence p65 acetylation and cellular inflammatory responses. Poly(ADP-ribose)-1 (PARP-1) is an abundant nuclear enzyme that consumes NAD(+) in the process of forming (ADP-ribose)polymers on target proteins, and extensive PARP-1 activation can reduce intracellular NAD(+) concentrations. Here we tested the idea that PARP-1 activation can regulate NF-κB transcriptional activity by reducing NAD(+) concentrations and thereby inhibiting de-acetylation of p65. Primary astrocyte cultures were treated with the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) to induce PARP-1 activation. This resulted in sustained acetylation of p65 and increased NF-κB transcriptional activity as monitored by a κB-driven eGFP reporter gene. These effects of MNNG were negated by a PARP-1 inhibitor, in PARP-1(-/-) cells, and in PARP-1(-/-) cells transfected with a catalytically inactive PARP-1 construct, thus confirming that these effects are mediated by PARP-1 catalytic activity. The effects of PARP-1 activation were replicated by a SIRT1 inhibitor, EX-527, and were reversed by exogenous NAD(+). These findings demonstrate that PARP-1-induced changes in NAD(+) levels can modulate NF-κB transcriptional activity through effects on p65 acetylation.

NAD(+) influx through connexin hemichannels prevents poly(ADP-ribose) polymerase-mediated astrocyte death

AIM

Cell death induced by excessive activation of poly(ADP-ribose) polymerase (PARP) is inhibited by administration of NAD(+) extracellularly, but its preventive mechanism remains unclear. Here we investigated the involvement of NAD(+) and/or its metabolites, adenosine and nicotinamide, in the rescue of PARP-mediated astrocyte death by NAD(+) and explored the pathway through which intact NAD(+) could enter cells.

MAIN METHODS

PARP activation was induced by treatment with N-methyl-N'-nitro-N-nitrosoguanidine, a DNA-alkylating agent. The cellular NAD(+) content was determined by an enzymatic recycling assay, and cell viability was determined by measuring intracellular LDH activity.

KEY FINDINGS

NAD(+), but not adenosine and nicotinamide, could restore the cellular NAD(+) levels decreased by PARP activation. Pharmacological inhibition of the uptake of adenosine and nicotinamide had no effect on the prevention of PARP-triggered cell death by NAD(+), suggesting that unmetabolized NAD(+) remaining in the extracellular milieu might prevent PARP-mediated NAD(+) consumption and cell death. The increase in the cellular NAD(+) level caused by NAD(+) administration to PARP-activated cells was significantly inhibited by a connexin hemichannel blocker, carbenoxolone, but not by P2X7 receptor inhibition with selective antagonists and siRNA, or pannexin-selective blockers. Finally, pharmacological blockade of connexin hemichannels with 18β-glycyrrhetinic acid, octanol and carbenoxolone inhibited the NAD(+)-mediated cell rescue of PARP-triggered cell death.

SIGNIFICANCE

These findings suggested that intact NAD(+) could get into astrocytes through connexin hemichannels, and that this process should play a key role in NAD(+)-mediated prevention of PARP-triggered astrocyte death.

Mitochondrial dysfunction is involved in P2X7 receptor-mediated neuronal cell death

P2X7 receptor (P2X7R) is known to be a 'death receptor' in immune cells, but its functional expression in non-immune cells such as neurons is controversial. Here, we examined the involvement of P2X7R activation and mitochondrial dysfunction in ATP-induced neuronal death in cultured cortical neurons. In P2X7R- and pannexin-1-expressing neuron cultures, 5 or more mM ATP or 0.1 or more mM BzATP induced neuronal death including apoptosis, and cell death was prevented by oxATP, P2X7R-selective antagonists. ATP-treated neurons exhibited Ca(2+) entry and YO-PRO-1 uptake, the former being inhibited by oxATP and A438079, and the latter by oxATP and carbenoxolone, while P2X7R antagonism with oxATP, but not pannexin-1 blocking with carbenoxolone, prevented the ATP-induced neuronal death. The ATP treatment induced reactive oxygen species generation through activation of NADPH oxidase and activated poly(ADP-ribose) polymerase, but both of them made no or negligible contribution to the neuronal death. Rhodamine123 efflux from neuronal mitochondria was increased by the ATP-treatment and was inhibited by oxATP, and a mitochondrial permeability transition pore inhibitor, cyclosporine A, significantly decreased the ATP-induced neuronal death. In ATP-treated neurons, the cleavage of pro-caspase-3 was increased, and caspase inhibitors, Q-VD-OPh and Z-DEVD-FMK, inhibited the neuronal death. The cleavage of apoptosis-inducing factor was increased, and calpain inhibitors, MDL28170 and PD151746, inhibited the neuronal death. These findings suggested that P2X7R was functionally expressed by cortical neuron cultures, and its activation-triggered Ca(2+) entry and mitochondrial dysfunction played important roles in the ATP-induced neuronal death.

NAD induces astrocyte calcium flux and cell death by ART2 and P2X7 pathway

Mono-ADP-ribosyltransferase 2 (ART2) is found in mouse T cells and has mediated NAD-induced cell death (NICD) alongside the P2X7 pathway. We determined whether ART2 was expressed in mouse brain astrocytes and the possible function of the NAD-ART2-P2X7 pathway in astrocytes. Our results demonstrate that ART2 existed both in cultured mouse astrocytes and mouse brain slices. Exposure of astrocytes to the ART2 substrate, NAD, induced calcium elevation, which was blocked by ART2 and P2X7 inhibitors. ATP and NAD had an additive effect on calcium elevation. NICD in low-calcium conditions was blocked by ART2 and P2X7 inhibitors. The harmful effect of ATP on astrocytes was inhibited by P2X7 and ART2 inhibitors, meaning that endogenous NAD release may occur. Both NICD function and oxygen-glucose deprivation injury in mouse brain slices were also involved in the ART2-P2X7 pathway. Collectively, to our knowledge, our study provides the first evidence that ART2 exists in mouse brain astrocytes and NAD induces calcium elevation and astrocyte death by an ART2 and P2X7-mediated mechanism. The results suggest a novel approach for manipulating astrocyte death.

Impact of manganese on and transfer across blood-brain and blood-cerebrospinal fluid barrier in vitro

Manganese occupational and dietary overexposure has been shown to result in specific clinical central nervous system syndromes, which are similar to those observed in Parkinson disease. To date, modes of neurotoxic action of Mn are still to be elucidated but are thought to be strongly related to Mn accumulation in brain and oxidative stress. However, the pathway and the exact process of Mn uptake in the brain are yet not fully understood. Here, two well characterized primary porcine in vitro models of the blood-brain and the blood-cerebrospinal fluid (CSF) barrier were applied to assess the transfer of Mn in the brain while monitoring its effect on the barrier properties. Thus, for the first time effects of MnCl(2) on the integrity of these two barriers as well as Mn transfer across the respective barriers are compared in one study. The data reveal a stronger Mn sensitivity of the in vitro blood-CSF barrier compared with the blood-brain barrier. Very interestingly, the negative effects of Mn on the structural and functional properties of the highly Mn-sensitive blood-CSF barrier were partly reversible after incubation with calcium. In summary, both the observed stronger Mn sensitivity of the in vitro blood-CSF barrier and the observed site-directed, most probably active, Mn transport toward the brain facing compartment, reveal that, in contrast to the general assumption in literature, after oral Mn intake the blood-CSF barrier might be the major route for Mn into the brain.

Critical involvement of extracellular ATP acting on P2RX7 purinergic receptors in photoreceptor cell death

Stressed cells release ATP, which participates in neurodegenerative processes through the specific ligation of P2RX7 purinergic receptors. Here, we demonstrate that extracellular ATP and the more specific P2RX7 agonist, 2'- and 3'-O-(4-benzoylbenzoyl)-ATP, both induce photoreceptor cell death when added to primary retinal cell cultures or when injected into the eyes from wild-type mice, but not into the eyes from P2RX7(-/-) mice. Photoreceptor cell death was accompanied by the activation of caspase-8 and -9, translocation of apoptosis-inducing factor from mitochondria to nuclei, and TUNEL-detectable chromatin fragmentation. All hallmarks of photoreceptor apoptosis were prevented by premedication or co-application of Brilliant Blue G, a selective P2RX7 antagonist that is already approved for the staining of internal limiting membranes during ocular surgery. ATP release is up-regulated by nutrient starvation in primary retinal cell cultures and seems to be an initializing event that triggers primary and/or secondary cell death via the positive feedback loop on P2RX7. Our results encourage the potential application of Brilliant Blue G as a novel neuroprotective agent in retinal diseases or similar neurodegenerative pathologies linked to excessive extracellular ATP.

Contribution of P2X7 receptors to adenosine uptake by cultured mouse astrocytes

Nucleotides and nucleosides play important roles by maintaining brain homeostasis, and their extracellular concentrations are mainly regulated by ectonucleotidases and nucleoside transporters expressed by astrocytes. Extracellularly applied NAD(+) prevents astrocyte death caused by excessive activation of poly(ADP-ribose) polymerase-1, of which the molecular mechanism has not been fully elucidated. Recently, exogenous NAD(+) was reported to enter astrocytes via the P2X7 receptor (P2X7R)-associated channel/pore. In this study, we examined whether the intact form of NAD(+) is incorporated into astrocytes. A large portion of extracellularly added NAD(+) was degraded into metabolites such as AMP and adenosine in the extracellular space. The uptake of adenine ring-labeled [(14)C]NAD(+), but not nicotinamide moiety-labeled [(3)H]NAD(+), showed time- and temperature-dependency, and was significantly enhanced on addition of apyrase, and was reduced by 8-Br-cADPR and ARL67156, inhibitors of CD38 and ectoapyrase, respectively, and P2X7R knockdown, suggesting that the detected uptake of [(14)C]NAD(+) resulted from [(14)C]adenosine acting as a metabolite of [(14)C]NAD(+). Pharmacological and genetic inhibition of P2X7R with brilliant blue G, KN-62, oxATP, and siRNA transfection resulted in a decrease of [(3)H]adenosine uptake, and the uptake was also reduced by low concentration of carbenoxolone and pannexin1 selective peptide blocker (10)panx. Taken together, these results indicate that exogenous NAD(+) is degraded by ectonucleotidases and that adenosine, as its metabolite, is taken up into astrocytes via the P2X7R-associated channel/pore.

ATP: a ubiquitous gliotransmitter integrating neuron-glial networks

Astrocytes are ideally situated to integrate glial and neuronal functions and neurovascular coupling by way of their multiple contacts with neurons, glia and blood vessels. There is a high degree of specialisation of astroglial membranes at the different sites of contact, including the expression of neurotransmitter receptors, ion channels, transporters and gap junctional proteins. An apparently universal property of astrocytes throughout the CNS is their responsiveness to ATP acting via metabotropic P2Y receptors, with a prominent role for the P2Y1 receptor subtype. Activation of astroglial P2Y receptors triggers a rise in intracellular calcium, which is the substrate for astroglial excitability and intercellular communication. In addition, astrocytes have a number of mechanisms for the release of ATP, which can be considered a 'gliotransmitter'. Astrocytes may be the most widespread source of ATP release in the CNS, and astroglial ATP and its metabolite adenosine activate purine receptors on neurons, microglia, oligodendrocytes and blood vessels. There is compelling evidence that astroglial ATP and adenosine regulate neuronal synaptic strength, although the physiological significance of this astrocyte-to-neuron signalling is questioned. A less appreciated aspect of astrocyte signalling is that they also release neurotransmitters onto other glia. Notably, both ATP and adenosine control microglial behaviour and regulate oligodendrocyte differentiation and myelination. P2 receptors also mediate injury responses in all glial cell types, with a prominent role for the P2X7 receptor subtype. In addition, ATP is a potent vasoconstrictor and astrocytes provide a route for coupling blood flow to neuronal activity by way of their synaptic and perivascular connections. Thus, astrocytes are the fulcrum of neuron-glial-vascular networks and purinergic signalling is the primary mechanism by which astrocytes can integrate the functions of these diverse elements.

Involvement of nerve injury and activation of peripheral glial cells in tetanic sciatic stimulation-induced persistent pain in rats

Tetanic stimulation of the sciatic nerve (TSS) produces long-lasting pain hypersensitivity in rats. Long-term potentiation (LTP) of C- and A-fiber-evoked field potentials in the spinal cord has been explored as contributing to central sensitization in pain pathways. However, the peripheral mechanism underlying TSS-induced pain hypersensitivity remains largely unknown. We investigated the effect of TSS on peripheral nerve and the expression of activating transcription factor 3 (ATF3) in dorsal root ganglion (DRG) as a marker of neuronal injury. TSS induced a mechanical allodynia for at least 35 days and induced ATF3 expression in the ipsilateral DRG. ATF3 is colocalized with NF200-labeled myelinated DRG neurons or CGRP- and IB4-labeled unmyelinated ones. Furthermore, we found that TSS induced Wallerian degeneration of sciatic nerve at the level of myelinisation by S100 protein (to label Schwann cells) immunohistochemistry, luxol fast blue staining, and electron microscopy. TSS also elicited the activation of satellite glial cells (SGCs) and enhanced the colocalization of GFAP and P2X7 receptors. Repeated local treatment with tetrodotoxin decreased GFAP expression in SGCs and behavioral allodynia induced by TSS. Furthermore, reactive microglia and astrocytes were found in the spinal dorsal horn after TSS. These results suggest that TSS-induced nerve injury and glial activation in the DRG and spinal dorsal horn may be involved in cellular mechanisms underlying the development of persistent pain after TSS and that TSS-induced nerve injury may be used as a novel neuropathic pain model.

P2X7 receptors mediate ischemic damage to oligodendrocytes

Brain ischemia leading to stroke is a major cause of disability in developed countries. Therapeutic strategies have most commonly focused on protecting neurons from ischemic damage. However, ischemic damage to white matter causes oligodendrocyte death, myelin disruption, and axon dysfunction, and it is partially mediated by glutamate excitotoxicity. We have previously demonstrated that oligodendrocytes express ionotropic purinergic receptors. The objective of this study was to investigate the role of purinergic signaling in white matter ischemia. We show that, in addition to glutamate, enhanced ATP signaling during ischemia is also deleterious to oligodendrocytes and myelin, and impairs white matter function. Thus, ischemic oligodendrocytes in culture display an inward current and cytosolic Ca(2+) overload, which is partially mediated by P2X7 receptors. Indeed, oligodendrocytes release ATP after oxygen and glucose deprivation through the opening of pannexin hemichannels. Consistently, ischemia-induced mitochondrial depolarization as well as oxidative stress culminating in cell death are partially reversed by P2X7 receptor antagonists, by the ATP degrading enzyme apyrase and by blockers of pannexin hemichannels. In turn, ischemic damage in isolated optic nerves, which share the properties of brain white matter, is greatly attenuated by all these drugs. Ultrastructural analysis and electrophysiological recordings demonstrated that P2X7 antagonists prevent ischemic damage to oligodendrocytes and myelin, and improved action potential recovery after ischemia. These data indicate that ATP released during ischemia and the subsequent activation of P2X7 receptor is critical to white matter demise during stroke and point to this receptor type as a therapeutic target to limit tissue damage in cerebrovascular diseases.

Roles of P2X7 receptor in glial and neuroblastoma cells: the therapeutic potential of P2X7 receptor antagonists

Recently, one of the P2 purinergic receptors, the P2X(7) receptor, has been extensively studied in nervous system and important functions have been revealed in both astrocytes and microglia. Stimulation of the receptors induces a sustained and nondesensitized increase in intracellular Ca(2+) concentration ([Ca(2+)](i)). In astrocytes purinergic receptors primarily regulate neurotransmission by inducing gliotransmitters release whereas in microglia the receptors stimulate the processing and release of proinflammation cytokines such as interleukin-1 and are thereby involved in inflammation and neurodegeneration. Thus, P2X(7) receptors are considered not only to exert physiological functions but also mediate cell death. P2X(7) receptors have also been identified in various cancer cells and in neuroblastoma cells. In these cells, the P2X(7) receptor-mediated sustained Ca(2+) signal is important in maintaining cellular viability and growth. Accordingly, these findings not only lead to a better understanding of roles of the receptor but also prompt the development of more potent, selective and safer P2X(7) selective antagonists. These emerging antagonists bring new hope in the treatment of inflammatory-induced neurodegenerative diseases as well as neuroblastoma.

Protective effect of nicotinamide against poly(ADP-ribose) polymerase-1-mediated astrocyte death depends on its transporter-mediated uptake

AIM

Poly(ADP-ribose) polymerase-1 (PARP-1) is a DNA repair enzyme, and its excessive activation, following ischemia, trauma, etc., depletes cellular nicotinamide adenine dinucleotide (NAD(+)) as a substrate and eventually leads to brain cell death. Nicotinamide, an NAD(+) precursor and a PARP-1 inhibitor, is known to prevent PARP-1-triggered cell death, but there is no available information on the mechanisms involved in its transport. Here we clarified the transport characteristics of nicotinamide in primary cultured mouse astrocytes.

MAIN METHODS

Uptake characteristics of [(14)C]nicotinamide were assessed by a conventional method with primary cultured mouse astrocytes. Cell viability and PARP-1 activity were determined with intracellular LDH activity and immunocytochemical detection of PAR accumulation, respectively.

KEY FINDINGS

PARP-1 activation was induced by treatment of astrocytes with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), an alkylating agent. MNNG-triggered astrocyte death and PAR accumulation were completely inhibited by treatment with nicotinamide as with DPQ (3,4-dihydro-5-(4-(1-piperidinyl)butoxy)-1(2H)-isoquinolinone), a second generation PARP inhibitor. The uptake of [(14)C]nicotinamide was time-, temperature-, concentration- and pH-dependent, and was inhibited and stimulated by co- and pre-treatment with N-methylnicotinamide, a representative substrate of an organic cation transport system, respectively. Co-treatment of astrocytes with nicotinamide and N-methylnicotinamide resulted in a decrease in PAR accumulation and absolute prevention of cell death.

SIGNIFICANCE

These findings suggest that nicotinamide has a protective effect against PARP-1-induced astrocyte death and that its transporter-mediated uptake, which is extracellular pH-sensitive and common to N-methylnicotinamide, is critical for prevention of PARP-1-triggered cell death.

NAD+ depletion is necessary and sufficient for poly(ADP-ribose) polymerase-1-mediated neuronal death

Poly(ADP-ribose)-1 (PARP-1) is a key mediator of cell death in excitotoxicity, ischemia, and oxidative stress. PARP-1 activation leads to cytosolic NAD(+) depletion and mitochondrial release of apoptosis-inducing factor (AIF), but the causal relationships between these two events have been difficult to resolve. Here, we examined this issue by using extracellular NAD(+) to restore neuronal NAD(+) levels after PARP-1 activation. Exogenous NAD(+) was found to enter neurons through P2X(7)-gated channels. Restoration of cytosolic NAD(+) by this means prevented the glycolytic inhibition, mitochondrial failure, AIF translocation, and neuron death that otherwise results from extensive PARP-1 activation. Bypassing the glycolytic inhibition with the metabolic substrates pyruvate, acetoacetate, or hydroxybutyrate also prevented mitochondrial failure and neuron death. Conversely, depletion of cytosolic NAD(+) with NAD(+) glycohydrolase produced a block in glycolysis inhibition, mitochondrial depolarization, AIF translocation, and neuron death, independent of PARP-1 activation. These results establish NAD(+) depletion as a causal event in PARP-1-mediated cell death and place NAD(+) depletion and glycolytic failure upstream of mitochondrial AIF release.

Possible involvement of PPAR gamma in the regulation of basal channel opening of P2X7 receptor in cultured mouse astrocytes

AIMS

Recently, we demonstrated that cultured mouse astrocytes exhibited basal channel opening of P2X7 receptor (P2X7R) in the absence of any exogenous ligand, but the regulatory mechanism involved was not elucidated. Since our preliminary experiments suggested possible involvement of peroxisome proliferator-activated receptor (PPAR) gamma in the regulation, we examined whether PPAR gamma regulated P2X7R basal channel opening in mouse astrocytes.

MAIN METHODS

P2X7R channel opening was assessed as to the uptake of a marker dye, YO-PRO-1 (YP), in the presence or absence of agonists and antagonists for PPAR gamma under a fluorescence microscope. Expression of PPAR gamma was evaluated by Western blotting and immunocytochemistry.

KEY FINDINGS

NSAIDs such as flufenamic acid (FFA) and indomethacin, which are a cyclooxygenase inhibitor and a PPAR gamma agonist, showed enhancing and inhibiting effects on YP uptake at low and high concentrations, respectively, and the enhanced uptake was abolished by periodate-oxidized ATP (oxATP), a selective P2X7R antagonist. The PPAR gamma agonists 15-deoxy-Delta(12,14)-prostaglandin J(2) and ciglitazone decreased the basal and FFA-enhanced YP uptake, while the antagonist GW9662 increased YP uptake, this effect being blocked by the agonists and also by oxATP. PPAR gamma was distributed in the nucleus and cytosolic/membrane fraction of cultured mouse astrocytes.

SIGNIFICANCE

These findings indicate that basal channel opening of P2X7R in mouse astrocytes is at least in part regulated by PPAR gamma.