Potentiation by ATP of lipopolysaccharide-stimulated nitric oxide production in cultured astrocytes

Astrocyte adaptation in Alzheimer's disease: a focus on astrocytic P2X7R

Astrocytes are key homeostatic and defensive cells of the central nervous system (CNS). They undertake numerous functions during development and in adulthood to support and protect the brain through finely regulated communication with other cellular elements of the nervous tissue. In Alzheimer's disease (AD), astrocytes undergo heterogeneous morphological, molecular and functional alterations represented by reactive remodelling, asthenia and loss of function. Reactive astrocytes closely associate with amyloid β (Aβ) plaques and neurofibrillary tangles in advanced AD. The specific contribution of astrocytes to AD could potentially evolve along the disease process and includes alterations in their signalling, interactions with pathological protein aggregates, metabolic and synaptic impairments. In this review, we focus on the purinergic receptor, P2X7R, and discuss the evidence that P2X7R activation contributes to altered astrocyte functions in AD. Expression of P2X7R is increased in AD brain relative to non-demented controls, and animal studies have shown that P2X7R antagonism improves cognitive and synaptic impairments in models of amyloidosis and tauopathy. While P2X7R activation can induce inflammatory signalling pathways, particularly in microglia, we focus here specifically on the contributions of astrocytic P2X7R to synaptic changes and protein aggregate clearance in AD, highlighting cell-specific roles of this purinoceptor activation that could be targeted to slow disease progression.

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.

Sigma-1 receptor activation ameliorates LPS-induced NO production and ROS formation through the Nrf2/HO-1 signaling pathway in cultured astrocytes

Accumulating evidence has shown that astrocytes play a critical role in neuroinflammation and protection against oxidative stress. In this study, we investigated the effects of sigma-1 receptor (Sig-1R) activation on lipopolysaccharide (LPS)-induced inflammatory reactions and oxidative/nitrosative stress in cultured astrocytes. We found that SA4503, a selective Sig-1R agonist, attenuated LPS-induced inflammatory reactions and oxidative/nitrosative stress by downregulating the expression of iNOS and tumor necrosis factor α (TNF-α) and upregulating glutathione (GSH) in cultured astrocytes. To investigate the mechanism by which SA4503 caused these effects, we then examined the expression of nuclear factor erythroid-derived 2-like 2 (Nrf2) and heme oxygenase-1 (HO-1) through western blotting. The results revealed that SA4503 treatment increased Nrf2 and HO-1 expression significantly. These results suggested that the antioxidative/nitrosative stress and anti-inflammatory effects of Sig-1R activation in astrocytes were partially mediated by Nrf2 and HO-1 activation.

RUNX2 Promotes Malignant Progression in Glioma

Glioblastoma (GBM) is the most aggressive and lethal form of brain tumor. However, therapeutic strategies against malignant gliomas have not been completely established. Runt-related transcription factor 2 (Runx2) is an essential gene for skeletal development but its regulatory role in the malignant progression of glioma remains unclear. Here we investigated expression levels of RUNX2 in glioma tissues and its regulatory effects on aberrant growth of glioma cells. RUNX2 mRNA levels were higher in GBM tissues than that of normal brains or low-grade gliomas. RUNX2 protein was detected in five out of seven human GBM cell lines and its level was positively correlated with proliferative capacity. Stable transduction of dominant-negative Runx2 in rat-derived C6 glioma cells not only inhibited the promoter activity containing Runx2 response element, but also decreased mRNA expression levels of Runx2 target genes, such as Mmp13 and Spp1, as well as the proliferative capacity. Furthermore, transient introduction of Runx2-targeted siRNAs into C6 glioma cells significantly decreased mRNA expression levels of Mmp13 and Spp1 and the proliferative capacity. Furthermore, Runx2 knockdown suppressed both Ccnd1 mRNA expression and activation of the Ccnd1 promoter by forskolin, a PKA-activating reagent, in C6 glioma cells. Our results demonstrate that cross-talk between cAMP/PKA signaling and RUNX2 promotes a malignant phenotype of glioma cells.

Extracellular ATP activates eNOS and increases intracellular NO generation in Red Blood Cells

BACKGROUND

It has been well documented that ATP activates NOS enzymes and causes increased NO production in several cell types. Although RBC known to possesses eNOS enzyme activity, it has not been investigated whether RBC eNOS could be induced by extracellular ATP.

OBJECTIVE

The aim of the present study is to evaluate extracellular ATP mediated eNOS activation and NO production in RBC.

METHODS

RBC packed were isolated from healthy volunteers and re-suspended in Hepes solution at a hematocrit of 0.01 l/l. Intracellular NO and Ca+2 levels and eNOS activation measured by flow cytometry in response to P2X receptor agonist, Bz-ATP, in the absence and presence of NOS, P2 receptors and PI3K inhibitors.

RESULTS

P2X receptor agonist Bz-ATP found to increase intracellular NO, Ca+2 and serine 1177 phosphorylated eNOS levels and these responses have shown to be suppressed by NOS enzyme, P2 receptors and PI3K inhibitors.

CONCLUSIONS

The results of the study clearly demonstrated extracellular ATP induced NO generation in RBC through intracellular Ca+2 and PI3K/Akt pathways. The mechanism we described here might be important at basal conditions and also in conditions with increased ATP release.

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.

P2X7 purinergic receptors participate in the expression and extinction processes of contextual fear conditioning memory in mice

The purinergic system consists of two large receptor families - P2X and P2Y. Both are activated by adenosine triphosphate (ATP), although presenting different functions. These receptors are present in several brain regions, including those involved in emotion and stress-related behaviors. Hence, they seem to participate in fear- and anxiety-related responses. However, few studies have investigated the purinergic system in threatening situations, as observed in contextual fear conditioning (CFC). Therefore, this study investigated the involvement of purinergic receptors in the expression and extinction of aversive memories. C57Bl/6 background mice were submitted to the CFC protocol. Wildtype (WT) mice received i.p. injection of either a nonselective P2 receptor (P2R) antagonist, P178 (10 or 30 mg/kg); a selective P2X7 receptor (P2X7R) antagonist, A438079 (10 mg/kg); a selective P2Y1 receptor (P2Y1R) antagonist, MRS2179 (10 mg/kg); or vehicle 10 min prior to or immediately after the extinction session. Additionally, P2X7R KO mice were tested in the CFC protocol. After P2R antagonist treatment, contextual fear recall increased, while acquisition of extinction was impaired. Similar results were observed with the selective P2X7R antagonist, but not with the selective P2Y1R antagonist. Interestingly, P2X7R KO mice showed increased contextual fear recall, associated with impaired acquisition of extinction, in accordance with pharmacologic P2X7R antagonism. Our results suggest that specific pharmacological or genetic blockade of P2X7R promotes anxiogenic-like effects, along with deficits in extinction learning. Thus, these receptors could present an alternative treatment of stress-related psychiatric disorders.

Effect of magnolol on cerebral injury and blood brain barrier dysfunction induced by ischemia-reperfusion in vivo and in vitro

Magnolol, a neolignan compound isolated from traditional Chinese medicine Magnolia officinalis, has a potentially therapeutic influence on ischemic stroke. Previous studies have demonstrated that cerebral ischemia-reperfusion (I-R) and blood-brain barrier (BBB) are involved in the pathogeneses of stroke. Therefore, in vivo and in vitro studies were designed to investigate the effects of magnolol on I-R-induced neural injury and BBB dysfunction. In cerebral I-R model of mice, cerebral infarct volumes, brain water content, and the exudation of Evans blue were significantly reduced by intravenous injection with magnolol at the doses of 1.4, 7.0, and 35.0 μg/kg. When primary cultured microglial cells were treated with 1 μg/ml lipopolysaccharide (LPS) plus increasing concentrations of magnolol, ranging from 0.01 to 10 μmol/L, magnolol could statistically inhibit LPS-induced NO release, TNF-α secretion, and expression of p65 subunit of NF-κB in the nucleus of microglial cells. In the media of brain microvascular endothelial cells (BMECs), oxygen and glucose deprivation-reperfusion (OGD-R) could remarkably lead to the elevation of TNF-α and IL-1β levels, while magnolol evidently reversed these effects. In BBB model in vitro, magnolol dose- and time-dependently declined BBB hyperpermeability induced by oxygen and glucose deprivation (OGD), OGD-R, and ephrin-A1 treatment. More importantly, magnolol could obviously inhibit phosphorylation of EphA2 (p-EphA2) not only in ephrin-A1-treated BMECs but also in cerebral I-R model of mice. In contrast to p-EphA2, magnolol significantly increased ZO-1 and occludin levels in BMECs subjected to OGD. Taken together, magnolol can protect neural damage from cerebral ischemia- and OGD-reperfusion, which may be associated with suppressing cerebral inflammation and improving BBB function.

Acetate Attenuates Lipopolysaccharide-Induced Nitric Oxide Production Through an Anti-Oxidative Mechanism in Cultured Primary Rat Astrocytes

The biomolecule acetate can be utilized for energy production, lipid synthesis, and several metabolic processes. Acetate supplementation reduces neuroglial activation in a model of neuroinflammation induced by intraventricular injection of lipopolysaccharide (LPS). To investigate the mechanisms underlying the anti-inflammatory effect of acetate on glial cells, we examined the effect of acetate on nitric oxide (NO) production, which was experimentally activated by LPS, in cultured primary rat astrocytes. Acetate attenuated the LPS-induced NO production in a dose-dependent manner, although cell viability was not affected. Acetate suppressed the phosphorylation of p38-mitogen-activated protein kinase 24 h after LPS treatment. Acetate decreased the LPS-induced production of intracellular reactive oxygen species (ROS) at 4-24 h concomitant with an increase in glutathione. Acetate rescued astrocytes from the hydrogen peroxide-induced cell death by reducing ROS levels. These findings suggest that attenuation of NO production by acetate may alleviate glial cell damage during neuroinflammation. Acetate may offer a glioprotective effect through an anti-oxidative mechanism.

Co-Expression of Wild-Type P2X7R with Gln460Arg Variant Alters Receptor Function

The P2X7 receptor is a member of the P2X family of ligand-gated ion channels. A single-nucleotide polymorphism leading to a glutamine (Gln) by arginine (Arg) substitution at codon 460 of the purinergic P2X7 receptor (P2X7R) has been associated with mood disorders. No change in function (loss or gain) has been described for this SNP so far. Here we show that although the P2X7R-Gln460Arg variant per se is not compromised in its function, co-expression of wild-type P2X7R with P2X7R-Gln460Arg impairs receptor function with respect to calcium influx, channel currents and intracellular signaling in vitro. Moreover, co-immunoprecipitation and FRET studies show that the P2X7R-Gln460Arg variant physically interacts with P2X7R-WT. Specific silencing of either the normal or polymorphic variant rescues the heterozygous loss of function phenotype and restores normal function. The described loss of function due to co-expression, unique for mutations in the P2RX7 gene so far, explains the mechanism by which the P2X7R-Gln460Arg variant affects the normal function of the channel and may represent a mechanism of action for other mutations.

Extracellular ATP and other nucleotides-ubiquitous triggers of intercellular messenger release

Extracellular nucleotides, and ATP in particular, are cellular signal substances involved in the control of numerous (patho)physiological mechanisms. They provoke nucleotide receptor-mediated mechanisms in select target cells. But nucleotides can considerably expand their range of action. They function as primary messengers in intercellular communication by stimulating the release of other extracellular messenger substances. These in turn activate additional cellular mechanisms through their own receptors. While this applies also to other extracellular messengers, its omnipresence in the vertebrate organism is an outstanding feature of nucleotide signaling. Intercellular messenger substances released by nucleotides include neurotransmitters, hormones, growth factors, a considerable variety of other proteins including enzymes, numerous cytokines, lipid mediators, nitric oxide, and reactive oxygen species. Moreover, nucleotides activate or co-activate growth factor receptors. In the case of hormone release, the initially paracrine or autocrine nucleotide-mediated signal spreads through to the entire organism. The examples highlighted in this commentary suggest that acting as ubiquitous triggers of intercellular messenger release is one of the major functional roles of extracellular nucleotides. While initiation of messenger release by nucleotides has been unraveled in many contexts, it may have been overlooked in others. It can be anticipated that additional nucleotide-driven messenger functions will be uncovered with relevance for both understanding physiology and development of therapy.

Purines in neurite growth and astroglia activation

The mammalian nervous system is a complex, functional network of neurons, consisting of local and long-range connections. Neuronal growth is highly coordinated by a variety of extracellular and intracellular signaling molecules. Purines turned out to be an essential component of these processes. Here, we review the current knowledge about the involvement of purinergic signaling in the regulation of neuronal development. We particularly focus on its role in neuritogenesis: the formation and extension of neurites. In the course of maturation mammals generally lose their ability to regenerate the central nervous system (CNS) e.g. after traumatic brain injury; although, spontaneous regeneration still occurs in the peripheral nervous system (PNS). Thus, it is crucial to translate the knowledge about CNS development and PNS regeneration into novel approaches to enable neurons of the mature CNS to regenerate. In this context we give a general overview of growth-inhibitory and growth-stimulatory factors and mechanisms involved in neurite growth. With regard to neuronal growth, astrocytes are an important cell population. They provide structural and metabolic support to neurons and actively participate in brain signaling. Astrocytes respond to injury with beneficial or detrimental reactions with regard to axonal growth. In this review we present the current knowledge of purines in these glial functions. Moreover, we discuss organotypic brain slice co-cultures as a model which retains neuron-glia interactions, and further presents at once a model for CNS development and regeneration. In summary, the purinergic system is a pivotal factor in neuronal development and in the response to injury. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Detection of nitric oxide production in cell cultures by luciferin-luciferase chemiluminescence

A chemiluminescent method is proposed for quantitation of NO generation in cell cultures. The method is based on activation of soluble guanylyl cyclase by NO. The product of the guanylyl cyclase reaction, pyrophosphate, is converted to ATP by ATP sulfurylase and ATP is detected in a luciferin-luciferase system. The method has been applied to the measurement of NO generated by activated murine macrophages (RAW 264.7) and bovine aortic endothelial cells. For macrophages activated by lipopolysaccharide and γ-interferon, the rate of NO production is about 100 amol/(cell·min). The rate was confirmed by the measurements of nitrite, the product of NO oxidation. For endothelial cells, the basal rate of NO generation is 5 amol/(cell·min); the rate approximately doubles upon activation by bradykinin, Ca(2+) ionophore A23187 or mechanical stress. For both types of cells the measured rate of NO generation is strongly affected by inhibitors of NO synthase. The sensitivity of the method is about 50 pM/min, allowing the registration of NO generated by 10(2)-10(4) cells. The enzyme-linked chemiluminescent method is two orders of magnitude more sensitive than fluorescent detection using 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM).

Theophylline potentiates lipopolysaccharide-induced NO production in cultured astrocytes

Elucidation of the functions of astrocytes is important for understanding of the pathogenic mechanism of various neurodegenerative diseases. Theophylline is a common drug for bronchial asthma and occasionally develops side-effects, such as acute encephalopathy; although the pathogenic mechanism of the side-effects is unknown. The lipopolysaccharide (LPS)-induced nitricoxide (NO) production is generally used for an index of the activation of astrocyte in vitro. In this study, in order to elucidate the effect of theophylline on the astrocytic functions, we examined the LPS-induced NO production and the expression of iNOS in cultured rat cortex astrocytes.Theophylline alone could not induce the NO production; however, NO production induced by LPS was enhanced by theophylline in a dose-dependent manner; and by isobutylmethylxanthine, a phosphodiesterase inhibitor. The theophylline enhancement of LPS-induced NO production was further increased by dibutyryl cyclic AMP, a membrane-permeable cAMP analog; and by forskolin, an adenylate cyclase activator. When the cells were preincubated with Rp-8-Br-cAMP, an inhibitor of protein kinase A, the theophylline enhancement of LPS-induced NO production was decreased. The extent of iNOS protein expression induced by LPS was also enhanced by theophylline.It is likely that phosphodiesterase inhibition is a major action mechanism for the theophylline enhancement of LPS-induced NO production in astrocytes. Theophylline-induced acute encephalopathy might be due to the hyper-activation of astrocytes via cAMP signaling to produce excess amount of NO.

Pathological potential of astroglial purinergic receptors

Acute brain injury and neurodegenerative disorders may result in astroglial activation. Astrocytes are able to determine the progression and outcome of these neuropathologies in a beneficial or detrimental way. Nucleotides, e.g. adenosine 5'-triphosphate (ATP), released after acute or chronic neuronal injury, are important mediators of glial activation and astrogliosis.Acute injury may cause significant changes in ATP balance, resulting in (1) a decline of intracellular ATP levels and (2) an increase in extracellular ATP concentrations via efflux from the intracellular space. The released ATP may have trophic effects, but can also act as a proinflammatory mediator or cytotoxic factor, inducing necrosis/apoptosis as a universal "danger" signal. Furthermore, ATP, primarily released from astrocytes, is a means of communication between neurons, glial cells, and intracerebral blood vessels.Astrocytes express a heterogeneous battery of purinergic ionotropic and metabotropic receptors (P2XRs and P2YRs, respectively) to respond to extracellular nucleotides.In this chapter, we summarize the contemporary knowledge on the pathological potential of P2Rs in relation to changes of astrocytic functions, determined by distinct molecular signaling cascades, in a variety of diseases. We discuss specific aspects of reactive astrogliosis, with respect to the involvement of prominent receptor subtypes, such as the P2X7 and P2Y1/2Rs. Examples of purinergic signaling of microglia, oligodendrocytes, and blood vessels under pathophysiological conditions will also be presented.The understanding of the pathological potential of purinergic signaling in "controlling and fine-tuning" of astrocytic responses is important for identifying possible therapeutic principles to treat acute and chronic central nervous system diseases.

Bullatine A, a diterpenoid alkaloid of the genus Aconitum, could attenuate ATP-induced BV-2 microglia death/apoptosis via P2X receptor pathways

Bullatine A (BLA), a diterpenoid alkaloid of the genus Aconitum, possesses anti-rheumatic, anti-inflammatory and anti-nociceptive effects. The mechanism underlying the effects was examined in the present study. The effect of BLA on extracellular ATP induced cell death/apoptosis and pro-inflammatory cytokines release were investigated using BV-2 microglia cell line. The mediation/efficacy of inflammatory cytokines and P2X receptors was evaluated by detecting the mRNA levels of iNOS, IL-6, IL-1β and P2X receptors, respectively. The results demonstrated that BV-2 cells could be damaged after incubation with higher dose of ATP, leading to activation of pro-inflammatory cytokines, transcriptional activation of iNOS and overproduction of NO via activation of P2X receptor. The BLA (1-50μM) potently inhibits ATP-induced BV-2 cell death/apoptosis and P2X receptor-mediated inflammatory responses via selectively suppressing the up-regulation of P2X7 receptor mRNA. Since P2X7 receptors have an important role in immune and pain response, inflammation and inflammatory disease, this discovery of BLA as a potent P2X7 antagonist indicated that BLA may be a potential useful candidate for the treatment of neurodegenerative diseases such as arthritis.

Activation of cultured astrocytes by amphotericin B: stimulation of NO and cytokines production and changes in neurotrophic factors production

Amphotericin B (AmB) is a polyene antibiotic and reported to be one of a few reagents having therapeutic effects on prion diseases, such as the delay in the appearing of the clinical signs and the prolongation of the survival time. In prion diseases, glial cells have been suggested to play important roles by proliferating and producing various factors such as nitric oxide, proinflammatory cytokines, and neurotrophic factors. However, the therapeutic mechanism of AmB on prion diseases remains elusive. We have previously reported that AmB changed the expression of neurotoxic and neurotrophic factors in microglia (Motoyoshi et al., 2008, Neurochem. Int. 52, 1290-1296). In the present study, we examined the effects of AmB on cellular functions of rat cultured astrocytes. We found that AmB could activate astrocytes to produce nitric oxide via inducible nitric oxide synthase induction. AmB also induced mRNA expression of interleukin-1β and tumor necrosis factor-α, and productions of their proteins in astrocytes. Moreover, AmB changed levels of neurotrophic factor mRNAs and proteins. Among three neurotrophic factors examined here, neurotrophin-3 mRNA expression and its protein production in the cells were down-regulated by AmB stimulation. On the other hand, AmB significantly enhanced the amounts of glial cell line-derived neurotrophic factor and brain-derived neurotrophic factor proteins in the cells and the medium. These results suggest that AmB might show therapeutic effects on prion diseases by controlling the expression and production of such mediators in astrocytes.

ATP induces NO production in hippocampal neurons by P2X(7) receptor activation independent of glutamate signaling

To assess the putative role of adenosine triphosphate (ATP) upon nitric oxide (NO) production in the hippocampus, we used as a model both rat hippocampal slices and isolated hippocampal neurons in culture, lacking glial cells. In hippocampal slices, additions of exogenous ATP or 2′(3′)-O-(4-Benzoylbenzoyl) ATP (Bz-ATP) elicited concentration-dependent NO production, which increased linearly within the first 15 min and plateaued thereafter; agonist EC₅₀ values were 50 and 15 µM, respectively. The NO increase evoked by ATP was antagonized in a concentration-dependent manner by Coomassie brilliant blue G (BBG) or by N^ω-propyl-L-arginine, suggesting the involvement of P2X₇Rs and neuronal NOS, respectively. The ATP induced NO production was independent of N-methyl-D-aspartic acid (NMDA) receptor activity as effects were not alleviated by DL-2-Amino-5-phosphonopentanoic acid (APV), but antagonized by BBG. In sum, exogenous ATP elicited NO production in hippocampal neurons independently of NMDA receptor activity.

Honokiol inhibits the inflammatory reaction during cerebral ischemia reperfusion by suppressing NF-κB activation and cytokine production of glial cells

This study was designed to investigate the effects of honokiol, a neuroprotective agent, on cerebral edema in cerebral ischemia reperfusion (IR) mice and its mechanism of anti-inflammation. Honokiol (0.7-70μg/kg) significantly reduced brain water contents and decreased the exudation of Evans blue dye from brain capillaries in cerebral IR mice. Honokiol (0.1-10μM) significantly reduced the p65 subunit level of NF-κB in the nucleus of primary culture-microglia. It (0.01-10μM) evidently reduced nitric oxide (NO) level in the microglia culture medium and in the microglia and astrocytes coculture medium. Honokiol (0.01-10μM) significantly decreased the level of TNF-α in the microglia medium or coculture cell medium. Honokiol (10μM) decreased the level of Regulated upon Activation Normal T-cell Expressed and Secreted (RANTES/CCL5) protein in medium of microglia or astrocytes. In conclusion, Honokiol has a potent anti-inflammatory effect in cerebral ischemia-reperfusion mice and this effect might be attributed to its inhibition ability on the NF-κB activation, consequently blocking the production of inflammatory factors including: NO, tumor necrosis factor-α (TNF-α) and RANTES/CCL5 in glial cells. These results provide evidence for the anti-inflammatory effect of honokiol for the potential treatment of ischemic stroke.

Reactive oxygen species contribute to the promotion of the ATP-mediated proliferation of mouse skeletal myoblasts

Reactive oxygen species (ROS) and extracellular adenosine 5'-triphosphate (ATP) participate in autocrine and paracrine regulation in skeletal muscle. However, the link between these two signaling systems is not well established. Here, we studied cell proliferation as a possible consequence of the trophic effect of ATP in cultured skeletal mouse myoblasts and we tested the possibility that low concentrations of ROS represent the intermediate signaling molecule mediating this effect. Exposure to 10 μM ATP increased proliferation of mouse myoblasts by ~20%. ATP also induced intracellular Ca(2+) oscillations, which were independent of extracellular Ca(2+). Both effects of ATP were prevented by suramin, a broad-spectrum purinergic P2 receptor antagonist. In contrast, the adenosine receptor blocker CGS-15943 did not modify the ATP-mediated effects. Consistent with this, adenosine per se did not change myoblast growth, indicating the direct action of ATP via P2 receptor activation. The proliferative effect of ATP was prevented after depletion of hydrogen peroxide (H(2)O(2)) by the peroxidase enzyme catalase. Low-micromolar concentrations of exogenous H(2)O(2) mimicked the stimulatory effect of ATP on myoblast growth. DCF imaging revealed ATP-induced catalase and DPI-sensitive ROS production in myoblasts. In conclusion, our results indicate that extracellular ATP controls mouse myoblast proliferation via induction of ROS generation.

Pathophysiology of astroglial purinergic signalling

Astrocytes are fundamental for central nervous system (CNS) physiology and are the fulcrum of neurological diseases. Astroglial cells control development of the nervous system, regulate synaptogenesis, maturation, maintenance and plasticity of synapses and are central for nervous system homeostasis. Astroglial reactions determine progression and outcome of many neuropathologies and are critical for regeneration and remodelling of neural circuits following trauma, stroke, ischaemia or neurodegenerative disorders. They secrete multiple neurotransmitters and neurohormones to communicate with neurones, microglia and the vascular walls of capillaries. Signalling through release of ATP is the most widespread mean of communication between astrocytes and other types of neural cells. ATP serves as a fast excitatory neurotransmitter and has pronounced long-term (trophic) roles in cell proliferation, growth, and development. During pathology, ATP is released from damaged cells and acts both as a cytotoxic factor and a proinflammatory mediator, being a universal "danger" signal. In this review, we summarise contemporary knowledge on the role of purinergic receptors (P2Rs) in a variety of diseases in relation to changes of astrocytic functions and nucleotide signalling. We have focussed on the role of the ionotropic P2X and metabotropic P2YRs working alone or in concert to modify the release of neurotransmitters, to activate signalling cascades and to change the expression levels of ion channels and protein kinases. All these effects are of great importance for the initiation, progression and maintenance of astrogliosis-the conserved and ubiquitous glial defensive reaction to CNS pathologies. We highlighted specific aspects of reactive astrogliosis, especially with respect to the involvement of the P2X(7) and P2Y(1)R subtypes. Reactive astrogliosis exerts both beneficial and detrimental effects in a context-specific manner determined by distinct molecular signalling cascades. Understanding the role of purinergic signalling in astrocytes is critical to identifying new therapeutic principles to treat acute and chronic neurological diseases.

Changes in cytosolic glucose level in ATP stimulated live astrocytes

Astrocytes which lie between brain capillaries and neuronal terminals are the primary site of glucose uptake and have a key role in coupling synaptic activity to glucose utilization in the central nervous system (CNS). We used a fluorescence resonance energy transfer (FRET) based approach to monitor cytosolic glucose in astrocytes. We determined the effect of increasing extracellular glucose concentrations on FRET ratio as a measure of increased cytosolic glucose in astrocytes. By briefly raising extracellular glucose concentration, astrocytes responded promptly by increased cytosolic glucose levels, which was manifested by decreased time-dependent FRET ratio. The FRET ratio fall-time recorded at low extracellular D-glucose concentration change (from 0 to 0.5 mM) was 53 s, whereas 17 s was recorded by raising extracellular concentration of D-glucose from 0 to 10 mM, which is likely due to facilitated d-glucose entry along the increased D-glucose gradient across the plasmalemma. The relationship between the extracellular glucose concentration and the FRET ratio change is limited to the maximal ratio change, where the D-glucose plasma membrane permeability is balanced by the cytosolic utilization. We measured the effect of extracellular ATP, an important extracellular messenger for astrocyte-to-astrocyte communication, on intracellular glucose concentration. The results show that stimulation of astrocytes with ATP (1 mM) decreases cytosolic glucose concentration with a time constant of ∼145 s. The mechanism of this change is discussed.

P2 receptor signaling in neurons and glial cells of the central nervous system

Purine and pyrimidine nucleotides are extracellular signaling molecules in the central nervous system (CNS) leaving the intracellular space of various CNS cell types via nonexocytotic mechanisms. In addition, ATP is a neuro-and gliotransmitter released by exocytosis from neurons and neuroglia. These nucleotides activate P2 receptors of the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. In mammalians, seven P2X and eight P2Y receptor subunits occur; three P2X subtypes form homomeric or heteromeric P2X receptors. P2Y subtypes may also hetero-oligomerize with each other as well as with other G protein-coupled receptors. P2X receptors are able to physically associate with various types of ligand-gated ion channels and thereby to interact with them. The P2 receptor homomers or heteromers exhibit specific sensitivities against pharmacological ligands and have preferential functional roles. They may be situated at both presynaptic (nerve terminals) and postsynaptic (somatodendritic) sites of neurons, where they modulate either transmitter release or the postsynaptic sensitivity to neurotransmitters. P2 receptors exist at neuroglia (e.g., astrocytes, oligodendrocytes) and microglia in the CNS. The neuroglial P2 receptors subserve the neuron-glia cross talk especially via their end-feets projecting to neighboring synapses. In addition, glial networks are able to communicate through coordinated oscillations of their intracellular Ca(2+) over considerable distances. P2 receptors are involved in the physiological regulation of CNS functions as well as in its pathophysiological dysregulation. Normal (motivation, reward, embryonic and postnatal development, neuroregeneration) and abnormal regulatory mechanisms (pain, neuroinflammation, neurodegeneration, epilepsy) are important examples for the significance of P2 receptor-mediated/modulated processes.

A protein-protein interaction of stress-responsive myosin VI endowed to inhibit neural progenitor self-replication with RNA binding protein, TLS, in murine hippocampus

We have shown preferential expression of both mRNA and corresponding protein for myosin VI (Myo6) in the murine hippocampus within 24 h after the extreme traumatic experience, water-immersion restraint stress (WIRS), prior to a drastic decrease in neural progenitor proliferation in the dentate gyrus. Myosin (Myo6) protein levels were significantly increased in hippocampus within 24 h after flashback experience in mice previously exposed to WIRS. Myo6 protein was ubiquitously distributed in discrete mouse brain regions with exceptionally high expression in olfactory bulb, whereas Myo6 protein was expressed in cultured rat astroglia and neurons, in addition to Myo6 mRNA expression by cultured neural progenitors. In mouse embryonal carcinoma P19 cells endowed to proliferate and differentiate, Myo6 protein was expressed in line with astroglial marker protein expression. Transient over-expression of Myo6 induced a significant decrease in the size of clustered aggregates as an index of self-replication in P19 cells. Immunoprecipitation analysis revealed the interaction between Myo6 and the RNA-binding protein, translocated in liposarcoma (TLS), while TLS was predominantly expressed by neurons in the cortex, striatum, cerebellum, and hippocampus. These results suggest that Myo6 may play a pivotal role in the mechanism underlying the suppressed adult neurogenesis after traumatic stress in association with TLS.

Nitric oxide-mediated modulation of synaptic activity by astrocytic P2Y receptors

We investigated the mechanism of synaptic suppression by P2Y receptors in mixed hippocampal cultures wherein networked neurons exhibit synchronized Ca²⁺ oscillations (SCO) due to spontaneous glutamatergic synaptic transmission. Pharmacological studies suggested that SCO suppression was mediated by P2Y2/P2Y4 receptors. Immunostaining studies and characterization of ATP/UTP-stimulated Ca²⁺ responses in solitary neurons and astrocytes revealed that the SCO attenuation was effectuated by astrocytes. We demonstrate that nitric oxide released from activated astrocytes causes synaptic suppression by inhibiting neurotransmitter release. Physiological concentrations of ATP and UTP evoked NO production in astrocytes. SCO suppression was considerably diminished by removal of extracellular NO by membrane-impermeable scavenger c-PTIO or by pretreatment of cells with nitric oxide synthase inhibitor L-NAME. The nitric oxide donor DETA/NO effectively suppressed the SCO. ATP/UTP inhibited KCl-induced exocytosis at presynaptic terminals in an NO-dependent manner. In the absence of exogenously added ATP/UTP, both the NO scavenger and NOS inhibitor enhanced the frequency of SCO, implying that astrocytes release NO during spontaneous synaptic activity and exert a suppressive effect. We report for the first time that under physiological conditions astrocytes use NO as a messenger molecule to modulate the synaptic strength in the networked neurons.

Effects of Amphotericin B on the expression of neurotoxic and neurotrophic factors in cultured microglia

Amphotericin B (AmB) is a polyene antibiotic and reported to have therapeutic effects on prion diseases, in which the microglial activation has been suggested to play important roles by proliferating and producing various factors such as nitric oxide, proinflammatory cytokines, and so on. However, the therapeutic mechanism of AmB on prion diseases remains elusive. In the present study, we investigated the effects of AmB on cellular functions of rat primary cultured microglia. We found that AmB, similarly as lipopolysaccharide (LPS), could activate microglia to produce nitric oxide via inducible nitric oxide synthase. Both AmB and LPS also induced mRNA expressions of interleukin-1beta, interleukin-6, and tumor necrosis factor-alpha in microglia. AmB also changed the expression levels of neurotrophic factors mRNAs. AmB and LPS significantly down-regulated the level of ciliary neurotrophic factor mRNA. However, AmB, but not LPS, significantly up-regulated the level of glial cell-line derived neurotrophic factor mRNA in microglia. In addition, brain-derived neurotrophic factor mRNA expression level was tending upward by treatment with AmB, but not with LPS. Taken together, these results suggest that AmB regulates the microglial activation in different manner from LPS and that microglia may participate in the therapeutic effects of AmB on prion diseases by controlling the expression and production of such mediators.

P2Y receptor-activating nucleotides modulate cellular reactive oxygen species production in dissociated hippocampal astrocytes and neurons in culture independent of parallel cytosolic Ca(2+) rise and change in mitochondrial potential

With mixed cultures of hippocampal astrocytes and neurons, we investigated the influence of nucleotides on cytosolic Ca(2+) level, generation of reactive oxygen species (ROS), and mitochondrial potential. We employed ATP and four purine/pyrimidine derivates, which are P2Y receptor subtype-preferring agonists. Stimulation with ATP, a P2Y(1/2/4) receptor agonist in rat, caused a large cytosolic Ca(2+) increase in astrocytes and a considerably smaller Ca(2+) response in neighboring neurons. The P2Y(1) receptor antagonist MRS2179 completely blocked the ATP-induced Ca(2+) response in astrocytes and neurons. Application of ATP significantly reduced the mitochondrial potential in neurons, which was not inhibited by MRS2179. Interestingly, MRS2179 mediated a mitochondrial depolarization without affecting the cytosolic Ca(2+) level. Stimulation with UDP, a P2Y(6) receptor agonist; UTP, a P2Y(2/4) receptor agonist; 2MeSATP, a P2Y(1) receptor agonist; or 2MeSADP, a P2Y(1/12/13) receptor agonist, evoked significant Ca(2+) responses in astrocytes but small Ca(2+) responses in neurons. In astrocytes, there was an inverse relationship between the amplitude of the cytosolic Ca(2+) peak and the rate of ROS generation in response to nucleotide application. Activation with UDP resulted in the highest ROS generation that we detected, whereas 2MeSADP and 2MeSATP reduced the ROS generation below the basal level. 2MeSADP and UDP caused mitochondrial depolarization of comparable size. Thus, neither in astrocytes nor in neurons did the degree of mitochondrial depolarization correlate with ROS generation. Nucleotides acting via P2Y receptors can modulate ROS generation of hippocampal neurons without acutely changing the cytosolic Ca(2+) level. Thus, ROS might function as a signaling molecule upon nucleotide-induced P2Y receptor activation in brain.

Do ATP and NO interact in the CNS?

Enzymatically derived NO and extracellular ATP are receiving greater attention due to their role as messengers in the CNS during different physiological and pathological processes. Ionotropic (P2XR) and metabotropic (P2YR) purinergic receptors mediate ATP effects and are present throughout the body. Particularly P2XR are crucial for brain plasticity mechanisms, and are involved in the pathogenesis of different CNS illnesses. NO does not have a specific receptor and its actions are directly dependent on the production on demand by different nitric oxide synthase isoforms. NO synthesizing enzymes are present virtually in all tissues, and NO influences multifarious physiological and pathological functions. Interestingly, various are the tissue and organs modulated by both ATP and NO, such as the immune, brain and vascular systems. Moreover, direct interactions between purinergic and nitrergic mechanisms outside the CNS are well documented, with several studies also indicating that ATP and NO do participate to the same CNS functions. In the past few years, further experimental evidence supported the physiological and pathological relevance of ATP and NO direct interactions in the CNS. The aim of the present review is to provide an account of the available information on the interplay between purinergic and nitrergic systems, focussing on the CNS. The already established relevance of ATP and NO in different pathological processes would predict that the knowledge of ATP/NO cross-talk mechanisms would support pharmacological approaches toward the development of novel ATP/NO combined pharmacological agents.

Upregulation of the glutamine transporter through transactivation mediated by cAMP/protein kinase A signals toward exacerbation of vulnerability to oxidative stress in rat neocortical astrocytes

In the present study, we have evaluated the possible functionality in astrocytes of the glutamine (Gln) transporter (GlnT) known to predominate in neurons for the neurotransmitter pool of glutamate. Sustained exposure to the adenylyl cyclase activator forskolin for 24 h led to a significant increase in mRNA expression of GlnT among different membrane transporters capable of transporting Gln, with an increase in [(3)H]Gln accumulation sensitive to a system A transporter inhibitor, in cultured rat neocortical astrocytes, but not neurons. Forskolin drastically stimulated GlnT promoter activity in a manner sensitive to a protein kinase A (PKA) inhibitor in rat astrocytic C6 glioma cells, while deletion mutation analysis revealed that the stimulation was mediated by a cAMP responsive element (CRE)/activator protein-1 (AP-1) like site located on GlnT gene promoter. Forskolin drastically stimulated the promoter activity in a fashion sensitive to a PKA inhibitor in C6 glioma cells transfected with a CRE or AP-1 reporter plasmid, in association with the phosphorylation of CRE binding protein on serine133. Transient overexpression of GlnT significantly exacerbated the cytotoxicity of hydrogen peroxide in cultured astrocytes. These results suggest that GlnT expression is upregulated by cAMP/PKA signals for subsequent exacerbation of the vulnerability to oxidative stress in astrocytes.

Cytoprotective properties of phenolic antidiarrheic ingredients in cultured astrocytes and neurons of rat brains

We have previously shown that particular phenolic antidiarrheic ingredients, including 2-methoxy-4-methylphenol (2M4MP) and 2-methoxy-4-ethyphenol (2M4EP), but not 2-methoxyphenol (2MP), significantly inhibit cellular maturation and differentiation of the bone-resorbing osteoclasts with concomitant protection of the bone-forming osteoblasts against oxidative stress by hydrogen peroxide (H(2)O(2)). In the present study, we evaluated the pharmacological actions of these three major phenolic antidiarrheic ingredients on the cellular viability in cultured astrocytes and neurons of the rat brain in vitro. Both 2M4MP and 2M4EP induced more efficient prevention of cell death induced by the brief exposure to 0.1 mM H(2)O(2) for 2 h than 2MP upon simultaneous exposure in cultured rat cortical astrocytes. Similarly, both 2M4MP and 2M4EP were more effective than 2MP in significantly protecting the cytotoxicity by brief exposure to 0.1 mM H(2)O(2) for 6 h in cultured rat hippocampal neurons, with concomitant suppression of the generation of intracellular reactive oxygen species in neurons exposed to H(2)O(2). Moreover, the three ingredients not only significantly prevented cell death in hippocampal neurons exposed to 0.1 mM glutamate for 1 h when determined 48 h after the brief exposure, but also inhibited the generation of intracellular reactive oxygen species and the elevation of intracellular Ca(2+) ions in neurons exposed to glutamate. These results suggest that particular phenolic antidiarrheic ingredients may prevent cell death through a mechanism related to diminution of the neurotoxicity of glutamate in neurons, in addition to eliciting cytoprotection against oxidative stress in astrocytes and neurons, in the rat brain.

Activator protein-1 responsive to the group II metabotropic glutamate receptor subtype in association with intracellular calcium in cultured rat cortical neurons

Activation of ionotropic glutamate (Glu) receptors, such as N-methyl-d-aspartate receptors, is shown to modulate the gene transcription mediated by the transcription factor activator protein-1 (AP1) composed of Fos and Jun family proteins in the brain, while little attention has been paid to the modulation of AP1 expression by metabotropic Glu receptors (mGluRs). In cultured rat cortical neurons, where constitutive expression was seen with all groups I, II and III mGluR subtypes, a significant and selective increase was seen in the DNA binding activity of AP1 120 min after the brief exposure to the group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) for 5 min. In cultured rat cortical astrocytes, by contrast, a significant increase was induced by a group I mGluR agonist, but not by either a group II or III mGluR agonist. The increase by DCG-IV was significantly prevented by a group II mGluR antagonist as well as by either an intracellular Ca(2+) chelator or a voltage-sensitive Ca(2+) channel blocker, but not by an intracellular Ca(2+) store inhibitor. Moreover, DCG-IV significantly prevented the increase of cAMP formation by forskolin in cultured neurons. Western blot analysis revealed differential expression profiles of Fos family members in neurons briefly exposed to DCG-IV and NMDA. Prior or simultaneous exposure to DCG-IV led to significant protection against neuronal cell death by NMDA. These results suggest that activation of the group II mGluR subtype would modulate the gene expression mediated by AP1 through increased intracellular Ca(2+) levels in cultured rat cortical neurons.

Nucleotide receptor signalling and the generation of reactive oxygen species

Elevated levels of extracellular nucleotides are present at sites of inflammation, platelet degranulation and cellular damage or lysis. These extracellular nucleotides can lead to the activation of purinergic (nucleotide) receptors on various leukocytes, including monocytes, macrophages, eosinophils, and neutrophils. In turn, nucleotide receptor activation has been linked to increased cellular production and release of multiple inflammatory mediators, including superoxide anion, nitric oxide and other reactive oxygen species (ROS). In the present review, we will summarize the evidence that extracellular nucleotides can facilitate the generation of multiple ROS by leukocytes. In addition, we will discuss several potential mechanisms by which nucleotide-enhanced ROS production may occur. Delineation of these mechanisms is important for understanding the processes associated with nucleotide-induced antimicrobial activities, cell signalling, apoptosis, and pathology.

Maturation-dependent reduced responsiveness of intracellular free Ca2+ ions to repeated stimulation by N-methyl-d-aspartate in cultured rat cortical neurons

In contrast to other ionotropic glutamate receptors, N-methyl-d-aspartate (NMDA) receptor channels are rather stable after the simulation. Brief exposure to NMDA at 50 microM rapidly increased the fluorescence intensity for increased intracellular free Ca(2+) levels in a reversible- and concentration-dependent manner in rat cortical neurons cultured for 3-15 days in vitro (DIV), while EC(50) values were significantly decreased in proportion to cellular maturation from 3 to 15 DIV. Although a constant increase was persistently seen in the fluorescence throughout the sustained exposure to NMDA for 60 min irrespective of the cell maturation from 3 to 15 DIV, the second brief exposure for 5 min resulted in a less efficient increase in the fluorescence than that found after the first brief exposure for 5 min in a manner dependent on intervals between the two repetitive brief exposures. In vitro maturation significantly shortened the interval required for the reduced responsiveness to the second brief exposure, while in immature neurons prolonged intervals were required for the reduced responsiveness to the second brief exposure to NMDA. Moreover, brief exposure to NMDA led to a marked decrease in immunoreactivity to extracellular loop of NR1 subunit in cultured neurons not permeabilized in proportion to the time after washing. These results suggest that cellular maturation would facilitate the desensitization process to repeated stimulation by NMDA, without markedly affecting that to sustained stimulation, through a mechanism related to the decreased number of NMDA receptors expressed at cell surfaces in cultured rat cortical neurons.

Reactive oxygen species mediate the potentiating effects of ATP on GABAergic synaptic transmission in the immature hippocampus

Reactive oxygen species (ROS) constitute important signaling molecules in the central nervous system. They regulate a number of different functions both under physiological conditions and under pathological conditions. Here we tested the hypothesis that in the immature hippocampus ATP, the most diffuse neurotransmitter in the brain, modulates synaptic transmission via ROS. We show that ATP, acting on metabotropic P2Y1 receptors, increased the frequency of GABA(A)-mediated spontaneous postsynaptic currents (SPSCs) in CA3 principal cells, an effect that was prevented by the antioxidant N-acetyl-cysteine or by catalase, an enzyme that breaks down H2O2. The effect of ATP on SPSCs was mimicked by H2O2 or by the pro-oxidant, Fe2+, which, through the Fentol reaction, catalyzes the conversion of H2O2 into highly reactive hydroxyl radicals. MRS-2179, a P2Y1 receptor antagonist, removed the facilitatory action of Fe2+ on SPSCs, suggesting that endogenous ATP acting on P2Y1 receptors is involved in Fe2+-induced modulation of synaptic transmission. Imaging ROS with the H2O2-sensitive dye DCF revealed that ATP induces generation of peroxide in astrocytes via activation of P2Y1 receptors coupled to intracellular calcium rise. Neither N-acetyl-cysteine nor catalase prevented Ca2+ transients induced by ATP in astrocytes. Since a single hippocampal astrocyte can contact many neurons, ATP-induced ROS signaling may control thousands of synapses. This may be crucial for information processing in the immature brain when GABAergic activity is essential for the proper wiring of the hippocampal network.

Functional expression of A glutamine transporter responsive to down-regulation by lipopolysaccharide through reduced promoter activity in cultured rat neocortical astrocytes

The prevailing view is that the glutamine (Gln) transporter (GlnT/ATA1/SAT1/SNAT1) is a member of the system A transporter superfamily with the ability to fuel the glutamate/Gln cycle at nerve terminals in glutamatergic neurons. Semiquantitative reverse transcription-polymerase chain reaction revealed similarly high expression of mRNA for GlnT by rat brain neocortical astrocytes as well as neurons, with progressively lower expression by cerebellar astrocytes, hippocampal astrocytes, and whole-brain microglia in culture. [(3)H]Gln was accumulated in a temperature-dependent manner with a saturable profile in both cultured neocortical neurons and astrocytes, whereas biochemical and pharmacological analyses on [(3)H]Gln accumulation revealed the expression of both system A and system L transporters by cultured neocortical neurons and astrocytes. Exposure to lipopolysaccharide (LPS) for 24 hr resulted in a significant decrease in both GlnT mRNA expression and [(3)H]Gln accumulation, with a concomitant drastic increase in nitrite formation in cultured neocortical astrocytes. Moreover, LPS significantly inhibited the promoter activity of GlnT in the astrocytic cell line C6 glioma cells as well as primary rat neocortical astrocytes in culture. These results suggest that activation by LPS would lead to down-regulation of the expression of GlnT responsible for the incorporation of extracellular Gln into intracellular spaces across plasma membranes through the inhibition of its promoter activity in cultured rat neocortical astrocytes.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

Potentiation by high potassium of lipopolysaccharide-induced nitric oxide production from cultured astrocytes

Uptake of K+ is an important role of astrocytes to maintain physiological lower extracellular K+ concentration in the CNS. In this study, the effect of high K+ concentration was examined on the cellular function of astrocytes from embryonic rat brain in primary culture. Nitric oxide (NO) production induced by lipopolysaccharide (LPS) was measured as an index of cellular function of astrocytes. Increasing KCl concentration to about 40 mM did not directly evoke NO production, but doubled the level of LPS (1 ng/ml)-induced NO production. K-gluconate showed a similar enhancing effect although the degree of enhancement was about half of that of KCl. Neither NaCl nor Na-gluconate showed any effect. The K(+)-channel blocker, 4-aminopyridine, but not tetraethylammonium or apamin, inhibited the enhancing effect of KCl. The LPS-induced iNOS protein expression determined by immunoblotting analysis was enhanced by high K+ treatment. The level of iNOS mRNA determined by real-time RT-PCR technique was also augmented by the presence of 40 mM KCl. These results indicate that the elevation of extracellular K+ concentration regulates astrocytic cell functions through a mechanism involving K(A)-type K(+)-channels and that potentiation of NO production by high K+ is due to the augmentation of iNOS mRNA and iNOS protein levels.

Bi-functional effects of ATP/P2 receptor activation on tumor necrosis factor-alpha release in lipopolysaccharide-stimulated astrocytes

Neuroinflammation is associated with a variety of CNS pathologies. Levels of tumor necrosis factor-alpha (TNF-alpha), a major proinflammatory cytokine, as well as extracellular ATP, are increased following various CNS insults. Here we report on the relationship between ATP/P2 purinergic receptor activation and lipopolysaccharide (LPS)-induced TNF-alpha release from primary cultures of rat cortical astrocytes. Using ELISA, we confirmed that treatment with LPS stimulated the release of TNF-alpha in a concentration and time dependent manner. ATP treatment alone had no effect on TNF-alpha release. LPS-induced TNF-alpha release was attenuated by 1 mm ATP, a concentration known to activate P2X7 receptors. Consistent with this, 3'-O-(4-Benzoyl)benzoyl-ATP (BzATP), a P2X7 receptor agonist, also attenuated LPS-induced TNF-alpha release. This reduction in TNF-alpha release was not due to loss of cell viability. Adenosine and 2-chloroadenosine were ineffective, suggesting that attenuation of LPS-induced TNF-alpha release by ATP was not due to ATP breakdown and subsequent activation of adenosine/P1 receptors. Interestingly, treatment of astrocyte cultures with 10 microm or 100 microm ATP potentiated TNF-alpha release induced by a submaximal concentration of LPS. UTP and 2methylthioADP (2-MeSADP), P2Y receptor agonists, also enhanced this LPS-induced TNF-alpha release. Our observations demonstrate opposing effects of ATP/P2 receptor activation on TNF-alpha release, i.e. P2X receptor activation attenuates, whereas P2Y receptor activation potentiates TNF-alpha release in LPS-stimulated astrocytes. These observations suggest a mechanism whereby astrocytes can sense the severity of damage in the CNS via ATP release from damaged cells and can modulate the TNF-alpha mediated inflammatory response depending on the extracellular ATP concentration and corresponding type of astrocyte ATP/P2 receptor activated.

An increase in intracellular free calcium ions by nicotinic acetylcholine receptors in a single cultured rat cortical astrocyte

Neuronal nicotinic acetylcholine receptors (nAChRs) are composed of an assembly between at least seven alpha (alpha2-alpha7, alpha9) and three beta (beta2-beta4) subunits in mammals. The addition of 50 mM KCl or 1 mM nicotine immediately increased the number of cells with high fluorescence intensity in rat cortical astrocytes on fluo-3 fluorescence measurement. Nicotine was effective at increasing the fluorescence intensity in astrocytes cultured for 2 days after replating, but not in those used 1 or 5 days after replating, without markedly affecting the cellular viability irrespective of the exposure period. Nicotine markedly increased the fluorescence intensity in a concentration-dependent manner at a concentration range of 10-100 microM in cultured astrocytes when analyzed on a responsive single cell. In these responsive single cells, the increase by nicotine was significantly prevented by the heteromeric alpha4/beta2 subtype antagonist dihydro-beta-erythroidine and the homomeric alpha7 subtype antagonist methyllycaconitine, as well as by nifedipine and EGTA but not thapsigargin. Methyllycaconitine failed to inhibit further the increase by nicotine in the presence of nifedipine, however, whereas the expression of mRNA was seen for all mammalian neuronal nAChR subunits in cultured rat cortical astrocytes as well as neurons. These results suggest that nicotine may increase intracellular free Ca2+ through the influx of extracellular Ca2+ across L-type voltage-gated Ca2+ channels rather than Ca2+ release from intracellular stores, in a manner related to the alpha4/beta2 and/or alpha7 nAChR channels functionally expressed in cultured rat cortical astrocytes.

Reactive oxygen species contribute to the presynaptic action of extracellular ATP at the frog neuromuscular junction

During normal cell metabolism the production of intracellular ATP is associated with the generation of reactive oxygen species (ROS), which appear to be important signalling molecules. Both ATP and ROS can be released extracellularly by skeletal muscle during intense activity. Using voltage clamp recording combined with imaging and biochemical assay of ROS, we tested the hypothesis that at the neuromuscular junction extracellular ATP generates ROS to inhibit transmitter release from motor nerve endings. We found that ATP produced the presynaptic inhibitory action on multiquantal end-plate currents. The inhibitory action of ATP (but not that of adenosine) was significantly reduced by several antioxidants or extracellular catalase, which breaks down H2O2. Consistent with these data, the depressant effect of ATP was dramatically potentiated by the pro-oxidant Fe2+. Exogenous H2O2 reproduced the depressant effects of ATP and showed similar sensitivity to anti- and pro-oxidants. While NO also inhibited synaptic transmission, inhibitors of the NO-producing cascade did not prevent the depressant action of ATP. The ferrous oxidation in xylenol orange assay showed the increase of ROS production by ATP and 2-MeSADP but not by adenosine. Suramin, a non-selective antagonist of P2 receptors, and pertussis toxin prevented the action of ATP on ROS production. Likewise, imaging with the ROS-sensitive dye carboxy-2',7'-dichlorodihydrofluorescein revealed increased production of ROS in the muscle treated with ATP or ADP while UTP or adenosine had no effect. Thus, generation of ROS contributed to the ATP-mediated negative feedback mechanism controlling quantal secretion of ACh from the motor nerve endings.

Oxidative metabolites are involved in polyamine-induced microglial cell death

Pathological activation of microglia, which reside quiescently in physiological CNS, is associated with various neurodegenerative diseases. Endogenous polyamines, spermidine and spermine, are known to be activators of cell proliferation and differentiation. We previously reported that both spermidine and spermine induce dose-dependent cell death in cultured rat brain microglia at a submicromolar concentration range via apoptotic process, whereas cultured astrocytes were less sensitive to these polyamines [Neuroscience 120 (2003) 961]. These polyamine effects were observed only in the presence of fetal bovine serum. In the present study we examined further the mechanism of polyamine-induced microglial cell death. Amine oxidase in fetal bovine serum produces hydrogen peroxide and an aminoaldehyde from spermine, and the latter generates acrolein spontaneously. Acrolein was found to be much more toxic to microglia than to astrocytes and the effective concentration of acrolein was similar to that of spermine, whereas hydrogen peroxide was marginally toxic. Aminoguanidine, an inhibitor of amine oxidase, blocked the toxic effects of spermine on microglia. Spermine cytotoxicity was also prevented by antioxidant reagents; glutathione (reduced form), cysteine, and N-acetylcysteine. These results suggest that polyamine-induced apoptotic cell death of microglia is triggered by an oxidative stress with acrolein, which is produced by amine oxidase from polyamine. The different toxicities of polyamine between two glial cells may regulate the balance of glial activation in some pathological conditions of CNS.