Extracellular ATP and P2X7 receptors in neurodegeneration

Pharmacology of P2X channels

Significant progress in understanding the pharmacological characteristics and physiological importance of homomeric and heteromeric P2X channels has been achieved in recent years. P2X channels, gated by ATP and most likely trimerically assembled from seven known P2X subunits, are present in a broad distribution of tissues and are thought to play an important role in a variety of physiological functions, including peripheral and central neuronal transmission, smooth muscle contraction, and inflammation. The known homomeric and heteromeric P2X channels can be distinguished from each other on the basis of pharmacological differences when expressed recombinantly in cell lines, but whether this pharmacological classification holds true in native cells and in vivo is less well-established. Nevertheless, several potent and selective P2X antagonists have been discovered in recent years and shown to be efficacious in various animal models including those for visceral organ function, chronic inflammatory and neuropathic pain, and inflammation. The recent advancement of drug candidates targeting P2X channels into human trials, confirms the medicinal exploitability of this novel target family and provides hope that safe and effective medicines for the treatment of disorders involving P2X channels may be identified in the near future.

P2X7 receptors mediate ATP release and amplification of astrocytic intercellular Ca2+ signaling

Modulation of synaptic transmission and brain microcirculation are new roles ascribed to astrocytes in CNS function. A mechanism by which astrocytes modify neuronal activity in the healthy brain depends on fluctuations of cytosolic Ca2+ levels, which regulate the release of "gliotransmitters" via an exocytic pathway. Under pathological conditions, however, the participation of other pathways, including connexin hemichannels and the pore-forming P2X7R, have been proposed but remain controversial. Through the use of genetically modified 1321N1 human astrocytoma cells and of spinal cord astrocytes derived from neonatal Cx43- and P2X7R-null mice, we provide strong evidence that P2X7Rs, but not Cx43 hemichannels, are sites of ATP release that promote the amplification of Ca2+ signal transmission within the astrocytic network after exposure to low divalent cation solution. Moreover, our results showing that gap junction channel blockers (heptanol, octanol, carbenoxolone, flufenamic acid, and mefloquine) are antagonists of the P2X7R indicate the inadequacy of using these compounds as evidence for the participation of connexin hemichannels as sites of gliotransmitter release.

Nucleotide signaling in nervous system development

The development of the nervous system requires complex series of cellular programming and intercellular communication events that lead from the early neural induction to the formation of a highly structured central and peripheral nervous system. Neurogenesis continuously takes place also in select regions of the adult mammalian brain. During the past years, a multiplicity of cellular control mechanisms has been identified, ranging from differential transcriptional mediators to inducers or inhibitors of cell specification or neurite outgrowth. While the identification of transcription factors typical for the stage-specific progression has been a topic of key interest for many years, less is known concerning the potential multiplicity of relevant intercellular signaling pathways and the fine tuning of epigenetic gene regulation. Nucleotide receptors can induce a multiplicity of cellular signaling pathways and are involved in multiple molecular interactions, thus opening the possibility of cross talk between several signaling pathways, including growth factors, cytokines, and extracellular matrix components. An increasing number of studies provides evidence for a role of nucleotide signaling in nervous system development. This includes progenitor cell proliferation, cell migration, neuronal and glial cellular interaction and differentiation, and synaptic network formation.

Experimental autoimmune encephalomyelitis disrupts endocannabinoid-mediated neuroprotection

Focal cerebral ischemia and traumatic brain injury induce an escalating amount of cell death because of harmful mediators diffusing from the original lesion site. Evidence suggests that healthy cells surrounding these lesions attempt to protect themselves by producing endocannabinoids (eCBs) and activating cannabinoid receptors, the molecular target for marijuana-derived compounds. Indeed, activation of cannabinoid receptors reduces the production and diffusion of harmful mediators. Here, we provide evidence that an exception to this pattern is found in experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis. We show that cell damage induced by EAE does not lead to increase in eCBs, even though cannabinoid receptors are functional because synthetic cannabinoid agonists are known to confine EAE-induced lesions. This lack of eCB increase is likely due to IFN-gamma, which is released by primed T cells invading the CNS. We show that IFN-gamma disrupts the functionality of purinergic P2X7 receptors, a key step controlling eCB production by microglia, the main source of eCBs in brain. Accordingly, induction of EAE in P2X7-/- mice results in even lower eCB levels and more pronounced cell damage than in wild-type mice. Our data suggest that the high level of CNS IFN-gamma associated with EAE disrupts eCB-mediated neuroprotection while maintaining functional cannabinoid receptors, thus providing additional support for the use of cannabinoid-based medicine to treat multiple sclerosis.

Emerging challenges of assigning P2X7 receptor function and immunoreactivity in neurons

Currently available antibodies to the P2X(7) receptor are unreliable determinants of neuronal P2X(7) immunoreactivity, owing to staining of a "P2X(7)-like" protein that is not eliminated by legitimate P2X(7) gene-knockout approaches. Despite this, compelling electrophysiological and pharmacological data strongly support a role for P2X(7) receptors in neuronal function and injury. A major priority for the field now is to identify the neuronal "P2X(7)-like" protein and develop effective antibodies selective for neuronal P2X(7) immunoreactivity. Until this occurs, we are dependent on rigorous application of multiple pharmacological criteria for attribution of neuronal function to P2X(7) receptors in non-human tissues, including greater activity in response to BzATP than to ATP, sensitivity to blockade by nanomolar concentrations of Brilliant Blue-G, irreversible antagonism by periodate-oxidized ATP, and lack of inhibition by suramin.

Exacerbation of Experimental Autoimmune Encephalomyelitis in P2X7R−/−Mice: Evidence for Loss of Apoptotic Activity in Lymphocytes

The purinergic receptor P2X7R is a nucleotide-gated ion channel that has been proposed to function as a major regulator of inflammation. In this study we examined the role of this receptor in regulating inflammation in the CNS by determining the effects of the loss of this receptor (P2X7R-/-) on experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis. We show here that P2X7R-/- mice developed more severe clinical and pathological expression of EAE than wild type (WT) controls and that spleen and lymph node cells from P2X7R-/- mice proliferated more vigorously to Ag in vitro. Bone marrow (BM) radiation chimeras revealed that enhanced susceptibility to EAE was detected in chimeric mice of WT host engrafted with P2X7R-/- BM cells, indicating that the genotype of the BM cells regulated disease susceptibility. Coculture of P2X7R-/- macrophages with WT lymphocytes and vice versa showed that enhanced proliferative activity resided within the P2X7R-/- lymphocyte population and correlated with reduced levels of IFN-gamma and NO and apoptosis of lymphocytes. mRNA and protein for IFN-gamma were also significantly reduced in the CNS of P2X7R-/- mice with EAE. FACS analysis of cells isolated from the CNS showed significantly fewer annexin V/propidium iodide-positive lymphocytes in the CNS of P2X7R-/- mice early in the disease, and TUNEL staining of inflamed CNS tissues supported this result. From these data we conclude that enhanced susceptibility of P2X7R-/- mice to EAE reflects a loss of apoptotic activity in lymphocytes, supporting an important role for this receptor in lymphocyte homeostasis.

Involvement of Ca(2+)-induced Ca2+ releasing system in interleukin-1beta-associated adenosine release

Interleukin-1beta (IL-1beta) plays an important role in neuroprotective and neurodegenerative events in the central nervous system. To clarify the mechanism of controversial actions of IL-1beta, we determined the effect of IL-1beta, as well as the interaction between IL-1beta and Ca(2+)-induced Ca2+ releasing system (CICR), on adenosine releases in mice hippocampus using mini-slices method. Basal and K(+)-stimulated adenosine releases were regulated by two types of CICRs, including inositol-1,4,5-trisphosphate (IP3) receptor and ryanodine receptor. Lower concentration of IL-1beta increased both adenosine releases, whereas higher concentration did not affect their releases. The stimulatory effect of IL-1beta on basal adenosine release was reduced by removal of extracellular Ca2+ and IP3 receptor inhibitor, while the stimulatory effect of IL-1beta on K(+)-stimulated adenosine release was reduced by ryanodine receptor inhibitor. These results suggest that the potent effect of IL-1beta upon adenosine release might contribute to the neuroprotective action of IL-1beta, whereas IL-1beta-induced neurodegeneration might be due to the overload response of Ca2+ mobilization and the inactivation of adenosine exocytosis.

P2X7 receptor activation in rat brain cultured astrocytes increases the biosynthetic release of cysteinyl leukotrienes

Astrocytes have been recognized as important elements in controlling inflammatory as well as immune processes in the central nervous system (CNS). Recently, glial cells have been shown to produce cysteinyl leukotrienes (CysLTs) which are known lipid mediators of inflammation and whose extracellular concentrations rise under different pathological conditions in the brain. In the same conditions also extracellular concentrations of ATP dramatically increase reaching levels able to activate P2X7 ionotropic receptors for which an emerging role in neuroinflammation and neurodegeneration has been claimed. RTPCR analysis showed that primary cultures of rat brain astrocytes express P2X7 receptors. Application of the selective P2X7 agonist benzoyl benzoly ATP (BzATP) markedly increased [Ca2+]i which was mediated by a calcium influx from the extracellular milieu. The P2X7 antagonist, oATP, suppressed the BzATP-induced calcium increase. Consistent with the evidence that increased calcium levels activate the leukotriene biosynthetic pathway, challenge of astrocytes with either the calcium ionophore A23187 or BzATP significantly increased CysLT production and the cell pre-treatment with EGTA abolished these effects. Again the P2X7 antagonist prevented the BzATP-mediated CysLT efflux, whereas the astrocyte pretreatment with MK-571, a CysLT1 receptor antagonist, was ineffective. The astrocyte pre-treatment with a cocktail of inhibitors of ATP binding cassette (ABC) proteins reduced the BzATP-mediated CysLT production confirming that ABC transporters are involved in the release of CysLTs. The astrocyte P2X7- evoked rise of CysLT efflux was abolished in the presence of MK-886, an inhibitor of 5-lipoxygenase activating protein (FLAP) whose expression, along with that of 5-lipoxygenase (5-LO) was reported by Northern Blot analysis. The stimulation of P2X7 induced an up-regulation of FLAPmRNA that was reduced by the antagonist oATP. These data suggest that in rat brain cultured astrocytes P2X7ATP receptors may participate in the control of CysLT release thus further supporting a role for extracellular ATP as an integral component of the inflammatory brain response.

Modulation of ATP-mediated contractions of the rat vas deferens through presynaptic cannabinoid receptors

The effect of R-(+)-[2,3-dihydro-5-methyl-3-[(morpholiny)methyl]pyrolol[1,2,3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl)methanone mesylate (WIN 55,212-2; a cannabinoid receptor agonist) was investigated on contractions of the bisected (epididymal and prostatic portions) rat vas deferens to assess the role of cannabinoid receptors in sympathetic ATP neurotransmission. WIN 55,212-2 inhibited the electrically induced contractions in both portions of the rat vas deferens. In the presence of the alpha1-adrenoreceptor antagonist prazosin, electrical stimulation produces a contraction mediated exclusively by ATP. In this condition, WIN 55,212-2 in the prostatic portion elicited a concentration-dependent inhibition that was antagonized by N-piperidinyl-[8-chloro-1-(2,4-dichlorophenyl)-1,4,5,6-tetrahydrobenzo[6,7]cyclohepta[1,2-c]pyrazole-3-carboxamide] (NESS 0327), a selective cannabinoid CB1 receptor antagonist. NESS 0327 caused a parallel dextral displacement of the WIN 55,212-2 concentration-response curve. It is suggested that activation of pre-junctional cannabinoid receptors on sympathetic nerves of the vas deferens modulates ATP neurotransmission.

Expression of P2X purinoceptors during rat brain development and their inhibitory role on motor axon outgrowth in neural tube explant cultures

Extracellular ATP is well known as a neurotransmitter and neuromodulator in the CNS of adults. However, little is known about the involvement of ATP during the development of mammalian brain. In the present study, we have examined the expression pattern of P2X receptor subtype mRNA and protein during perinatal rat brain development (from embryonic day (E) 10 to postnatal day (P) 16 brain). While P2X3 receptors appeared early at E11, they declined in the stages that follow. P2X2 and P2X7 receptors were expressed from E14 onwards, while P2X4, P2X5 and P2X6 receptors were expressed from P1 onwards. P2X1 receptor expression was not observed in any of the developmental ages examined. We investigated the effect of 100 microM ATP and alpha,beta-methylene ATP (alpha,beta-meATP; selective agonist for P2X1, P2X2/3 and P2X3 receptors) on motor axon outgrowth in collagen-embedded neural tube explant cultures. Both ATP- and alpha,beta-meATP-treated neural tubes showed a significant reduction in neurite outgrowth compared with the control explants. This inhibitory effect could not be reproduced by uridine triphosphate. In conclusion, all P2X receptor subtypes, except for P2X1, were strongly represented in the developing rat brain. ATP was shown to inhibit motor axon outgrowth during early embryonic neurogenesis, most likely via the P2X3 receptor. It is speculated that P2X7 receptors may be involved in programmed cell death during embryogenesis and that P2X4, P2X(5) and P2X6 receptors might be involved in postnatal neurogenesis.

Behavioral effects and drug vulnerability in rats exposed to Pfiesteria toxin

Pfiesteria piscicida is a dinoflagellate which has a lethal effect on fish and also causes a syndrome of toxic effects in humans. Cognitive impairment is a prominent aspect of Pfiesteria's toxicity, and this neurocognitive effect resulting from toxin exposure has been demonstrated previously in a rat model. Four experiments are presented here, which replicate, confirm and extend some of the initial research and also show that similar cognitive deficits result from exposure to the toxin of another species, Pfiesteria shumwayae. Rats were given intraperitoneal injections of filtered water taken from toxic Pfiesteria cultures and tested in the radial arm maze (RAM). In two experiments, exposure to toxin from either species (piscicida or shumwayae) retarded acquisition of RAM performance in a non-interrupted win-shift RAM paradigm. A scopolamine challenge showed increased vulnerability to anticholinergic effects in exposed rats, even after nondrugged RAM performance was not different from controls. A third experiment featured a more difficult RAM test which included a 150-min interruption-delay. Toxin exposure also degraded performance in this version of the RAM, and the impairment was potentiated by the scopolamine challenge. The fourth experiment demonstrated retarded learning of the reversal of a RAM procedure which tested reference memory. In agreement with earlier research, these results indicate that Pfiesteria toxin interferes with the learning required to adapt to changing behavioral requirements. They also demonstrate that a latent toxin-produced CNS dysfunction persists after behavior appears normal, as revealed by potentiation of scopolamine's impairment of efficient RAM performance.

Activation of microglial NADPH oxidase is synergistic with glial iNOS expression in inducing neuronal death: a dual-key mechanism of inflammatory neurodegeneration

Background

Inflammation-activated glia are seen in many CNS pathologies and may kill neurons through the release of cytotoxic mediators, such as nitric oxide from inducible NO synthase (iNOS), and possibly superoxide from NADPH oxidase (NOX). We set out to determine the relative role of these species in inducing neuronal death, and to test the dual-key hypothesis that the production of both species simultaneously is required for significant neuronal death.

Methods

Primary co-cultures of cerebellar granule neurons and glia from rats were used to investigate the effect of NO (from iNOS, following lipopolysaccharide (LPS) and/or cytokine addition) or superoxide/hydrogen peroxide (from NOX, following phorbol 12-myristate 13-acetate (PMA), ATP analogue (BzATP), interleukin-1β (IL-1β) or arachidonic acid (AA) addition) on neuronal survival.

Results

Induction of glial iNOS caused little neuronal death. Similarly, activation of NOX alone resulted in little or no neuronal death. However, if NOX was activated (by PMA or BzATP) in the presence of iNOS (induced by LPS and interferon-γ) then substantial delayed neuronal death occurred over 48 hours, which was prevented by inhibitors of iNOS (1400W), NOX (apocynin) or a peroxynitrite decomposer (FeTPPS). Neurons and glia were also found to stain positive for nitrotyrosine (a putative marker of peroxynitrite) only when both iNOS and NOX were simultaneously active. If NOX was activated by weak stimulators (IL-1β, AA or the fibrillogenic prion peptide PrP106-126) in the presence of iNOS, it caused microglial proliferation and delayed neurodegeneration over 6 days, which was prevented by iNOS or NOX inhibitors, a peroxynitrite decomposer or a NMDA-receptor antagonist (MK-801).

Conclusion

These results suggest a dual-key mechanism, whereby glial iNOS or microglial NOX activation alone is relatively benign, but if activated simultaneously are synergistic in killing neurons, through generating peroxynitrite. This mechanism may mediate inflammatory neurodegeneration in response to cytokines, bacteria, ATP, arachidonate and pathological prions, in which case neurons may be protected by iNOS or NOX inhibitors, or scavengers of NO, superoxide or peroxynitrite.

Role of the immune system in chronic pain

During the past two decades, an important focus of pain research has been the study of chronic pain mechanisms, particularly the processes that lead to the abnormal sensitivity - spontaneous pain and hyperalgesia - that is associated with these states. For some time it has been recognized that inflammatory mediators released from immune cells can contribute to these persistent pain states. However, it has only recently become clear that immune cell products might have a crucial role not just in inflammatory pain, but also in neuropathic pain caused by damage to peripheral nerves or to the CNS.

P2X(7) receptor: Death or life?

The P2X₇ plasma membrane receptor is an intriguing molecule that is endowed with the ability to kill cells, as well as to activate many responses and even stimulate proliferation. Here, the authors give an overview on the multiplicity and complexity of P2X₇-mediated responses, discussing recent information on this receptor. Particular attention has been paid to early and late signs of apoptosis and necrosis linked to activation of the receptor and to the emerging field of P2X₇ function in carcinogenesis.

Immune and glial cell factors as pain mediators and modulators

A decade ago the attention of pain scientists was focused on a small number of molecules such as prostaglandin and bradykinin as peripheral pain mediators or modulators. These factors were known to be produced by tissue damage or inflammation, and considered responsible for the activation and sensitization of peripheral pain signaling sensory neurons. A small number of molecules were also identified as central pain mediators, most notably glutamate and substance P released from central nociceptive nerve terminals, and, starting at that time, appreciation that nitric oxide might be produced by dorsal horn neurons and act as a diffusible transmitter to increase excitability of central pain circuits. During the last decade evidence has emerged for many novel pain mediators. The old ones have not disappeared, although their roles have been redefined in some cases. Prostaglandin E2 (PGE2), for instance, is now recognized as playing a prominent role in CNS as well as peripheral tissues. The newly identified mediators include a variety of factors produced and released from nonneuronal cells-predominantly immune and glial cells. The evidence is now growing apace that these are important mediators of persistent pain states and can act at a number of loci. Here we review the actions of several of these factors-the pro-inflammatory cytokines, some chemokines, and some neurotrophic factors, which, in addition to their traditionally recognized roles, are all capable of changing the response properties of peripheral and central pain signaling neurons. We review these actions, first in periphery, where a substantial literature has accumulated, and then in spinal cord, where the role of factors from nonneuronal cells has only recently been identified as of considerable importance.

Metabotropic P2 receptor activation regulates oligodendrocyte progenitor migration and development

To gain insights into the role of purinergic receptors in oligodendrocyte development, we characterized the expression and functional activity of P2 receptors in cultured rat oligodendrocyte progenitors and investigated the effects of ATP and its breakdown products on the migration and proliferation of this immature glial cell population. Using Western blot analysis, we show that oligodendrocyte progenitors express several P2X (P2X(1,2,3,4,7)) and P2Y (P2Y(1,2,4)) receptors. Intracellular Ca(2+) recording by Fura-2 video imaging allowed to determine the rank potency order of the P2 agonists tested: ADPbetaS = ADP = Benzoyl ATP > ATP > ATPgammaS > UTP, alpha,beta-meATP ineffective. Based on the above findings, on pharmacological inhibition by the antagonists oxATP and MRS2179, and on the absence of alpha,betameATP-induced inward current in whole-cell recording, P2X(7) and P2Y(1) were identified as the main ionotropic and metabotropic P2 receptors active in OPs. As a functional correlate of these findings, we show that ATP and, among metabotropic agonists, ADP and the P2Y(1)-specific agonist ADPbetaS, but not UTP, induce oligodendrocyte progenitor migration. Moreover, ATP and ADP inhibited the proliferation of oligodendrocyte progenitors induced by platelet-derived growth factor, both in purified cultures and in cerebellar tissue slices. The effects of ATP and ADP on cell migration and proliferation were prevented by the P2Y(1) antagonist MRS2179. By confocal laser scanning microscopy, P2Y(1) receptors were localized in NG2-labeled oligodendrocyte progenitors in the developing rat brain. These data indicate that ATP and ADP may regulate oligodendrocyte progenitor functions by a mechanism that involves mainly activation of P2Y(1) receptors.

ATP regulates oligodendrocyte progenitor migration, proliferation, and differentiation: involvement of metabotropic P2 receptors

Extracellular nucleotides act as potent signaling molecules in the neuron-glia and glia-glia communication, via the activation of specific ligand-gated P2X and G-protein-coupled metabotropic P2Y receptors. Most of the data available about the effects of P2 receptor activation in the CNS concern astrocytes, microglia, and neurons. To gain insights into the role of purinergic receptors in oligodendrocyte development, we characterized the expression and functional activity of P2 receptors in rat oligodendrocyte progenitors (OPs) and investigated the effects of ATP and its breakdown products on their functions. We describe here that rat OPs express different types of P2 receptors and that nucleotide-induced Ca(2+) raises in these progenitor cells are mainly due to the activation of P2X(7) ionotropic and ADP-sensitive P2Y(1) metabotropic receptors. We also show that ATP and ADP stimulate OP migration, inhibit the mitogenic response of OPs to PDGF and promote oligodendrocyte differentiation. The pharmacological profile of the nucleotide-induced effects demonstrates the important regulatory role of P2Y(1) receptor signaling in OP functions. These findings suggest that ATP, which is released in high amounts under inflammatory conditions and following cell death, might regulate remyelination processes in inflammatory demyelinating diseases of the CNS, like multiple sclerosis.

Acute effects of pulsed microwaves and 3-nitropropionic acid on neuronal ultrastructure in the rat caudate-putamen

Ultrastructure of the medium sized "spiny" neuron in rat dorsal-lateral caudate-putamen was assessed after administration of 3-nitropropionic acid (3-NP) and exposure to pulsed microwaves. Sprague-Dawley male rats were given two daily intraperitoneal doses of 0 or 10 mg/kg 3-NP and 1.5 h after each dose were exposed to microwave radiation at a whole body averaged specific absorption rate (SAR) of 0 (sham exposure), 0.6, or 6 W/kg for 30 min. Microwave exposure consisted of 1.25 GHz radiation delivered as 5.9 micros pulses with repetition frequency 10 Hz. Tissue samples taken 2-3 h after the second sham or microwave exposure showed no injury with light microscope methods. Blinded qualitative assessment of ultrastructure of randomly selected neurons from the same samples did reveal differences. Subsequent detailed, quantitative measurements showed that, when followed by sham exposure, administration of 3-NP significantly increased endoplasmic reticulum (ER) intracisternal width, ER area density, and nuclear envelope thickness. Microwave exposure at 6 W/kg alone also significantly increased these measures. Exposure of 3-NP treated animals at 6 W/kg significantly increased effects of 3-NP on ultrastructure. Although exposure at 0.6 W/kg alone did not affect ultrastructure measures, exposure of 3-NP treated animals at 0.6 W/kg reduced the effects of 3-NP. We concluded that 3-NP changed neuronal ultrastructure and that the microwave exposures used here changed neuronal ultrastructure in ways that depended on microwave SAR and neuron metabolic status. The apparent cancellation of 3-NP induced changes by exposure to pulsed microwaves at 0.6 W/kg indicated the possibility that such exposure can protect against the effects of mitochondrial toxins on the nervous system.

Regulation of P2X7-induced pore formation and cell death in pericyte-containing retinal microvessels

The purpose if this study was to elucidate how extracellular ATP causes cell death in the retinal microvasculature. Although ATP appears to serve as a vasoactive signal acting via P2X(7) and P2Y(4) purinoceptors, this nucleotide can kill microvascular cells of the retina. Because P2X(7) receptor activation causes transmembrane pores to form and microvascular cells to die, we initially surmised that pore formation accounted for ATP's lethality. To test this hypothesis, we isolated pericyte-containing microvessels from rat retinas, assessed cell viability using Trypan blue dye exclusion, detected pores by determining the uptake of the fluorescent dye YO-PRO-1, measured intracellular Ca(2+) with the use of fura-2, and monitored ionic currents via perforated patch pipettes. As predicted, ATP-induced cell death required P2X(7) receptor activation. However, we found that pore formation was minimal because ATP's activation of P2Y(4) receptors prevented P2X(7) pores from forming. Rather than opening lethal pores, ATP kills via a mechanism involving voltage-dependent Ca(2+) channels (VDCC). Our experiments suggest that when high concentrations of ATP caused nearly all microvascular P2X(7) receptor channels to open, the resulting profound depolarization opened VDCC. Consistent with lethal Ca(2+) influx via VDCC, ATP-induced cell death was markedly diminished by the VDCC blocker nifedipine or a nitric oxide (NO) donor that inhibited microvascular VDCC. We propose that purinergic vasotoxicity is normally prevented in the retina by NO-mediated inhibition of VDCC and P2Y(4)-mediated inhibition of P2X(7) pore formation. Conversely, dysfunction of these protective mechanisms may be a previously unrecognized cause of cell death within the retinal microvasculature.

P2Y2 receptors activate neuroprotective mechanisms in astrocytic cells

Mechanical or ischemic trauma to the CNS causes the release of nucleotides and other neurotransmitters into the extracellular space. Nucleotides can activate nucleotide receptors that modulate the expression of genes implicated in cellular adaptive responses. In this investigation, we used human 1321N1 astrocytoma cells expressing a recombinant P2Y2 receptor to assess the role of this receptor in the regulation of anti-apoptotic (bcl-2 and bcl-xl) and pro-apoptotic (bax) gene expression. Acute treatment with the P2Y2 receptor agonist UTP up-regulated bcl-2 and bcl-xl, and down-regulated bax, gene expression. Activation of P2Y2 receptors was also coupled to the phosphorylation of cyclic AMP responsive element binding protein that positively regulates bcl-2 and bcl-xl gene expression. Cyclic AMP responsive element decoy oligonucleotides markedly attenuated the UTP-induced increase in bcl-2 and bcl-xl mRNA levels. Activation of P2Y2 receptors induced the phosphorylation of the pro-apoptotic factor Bad and caused a reduction in bax/bcl-2 mRNA expression ratio. All these signaling pathways are known to be involved in cell survival mechanisms. Using cDNA microarray analysis and RT-PCR, P2Y2 receptors were found to up-regulate the expression of genes for neurotrophins, neuropeptides and growth factors including nerve growth factor 2; neurotrophin 3; glia-derived neurite-promoting factor, as well as extracellular matrix proteins CD44 and fibronectin precursor--genes known to regulate neuroprotection. Consistent with this observation, conditioned media from UTP-treated 1321N1 cells expressing P2Y2 receptors stimulated the outgrowth of neurites in PC-12 cells. Taken together, our results suggest an important novel role for the P2Y2 receptor in survival and neuroprotective mechanisms under pathological conditions.

Potent effect of interleukin-1 beta to evoke ATP and adenosine release from rat hippocampal slices

In this study the effect of IL-1 beta on [(3)H]purine release from rat hippocampal slices was explored. IL-1 beta (3 x 10(-18)-3 x 10(-14) M) concentration-dependently elevated the basal [(3)H]purine efflux, and this effect was reversed by the selective IL-1RI receptor antagonist IL-1ra (10(-12) M). HPLC analysis revealed that the amount of [(3)H]ATP and [(3)H]adenosine significantly increased in the effluent in response to IL-1 beta. The sodium channel inhibitor tetrodotoxin, the NMDA and non-NMDA receptor antagonists d(-)-2-amino-5-phosphonopentanoic acid (AP-5) plus 6-cyano-7-nitroquinoxaline-2,3-dione-disodium (CNQX) almost completely abolished IL-1 beta-evoked [(3)H]purine release. The effect of IL-1 beta on [(3)H]purine efflux was also prevented by the p38 MAP kinase inhibitor SB 203580, by the nucleoside transport inhibitor nitrobenzyl-thioinosine (NBTI) and by low temperature (4 degrees C). In summary IL-1 beta triggers a transporter mediated [(3)H]purine efflux in the hippocampus which is conveyed by glutamate receptor activation and the p38 MAP kinase pathway, and could serve as a mediator of IL-1 beta-induced synaptic depression.

ATP-induced inhibition of gap junctional communication is enhanced by interleukin-1 beta treatment in cultured astrocytes

Nucleotides are signaling molecules involved in variety of interactions between neurons, between glial cells as well as between neurons and glial cells. In addition, ATP and other nucleotides are massively released following brain insults, including inflammation, and may thereby be involved in mechanisms of cerebral injury. Recent concepts have shown that in astrocytes intercellular communication through gap junctions may play an important role in neuroprotection. Therefore, we have studied the effects of nucleotides on gap junction communication in astrocytes. Based on measurement of intercellular dye coupling and recording of junctional currents, the present study shows that ATP (10-100 microM) induces a rapid and a concentration-dependent inhibition of gap junction communication in cultured cortical astrocytes from newborn mice. Effects of agonists and antagonists of purinergic receptors indicate that the inhibition of gap junctional communication by ATP mainly involves the stimulation of metabotropic purinergic 1 (P2Y(1)) receptors. Pretreatment with the pro-inflammatory cytokine interleukin-1beta (10 ng/ml, 24 h), which has no effect by itself on gap junctional communication, increases the inhibitory effect of ATP and astrocytes become sensitive to uridine 5'-triphosphate (UTP). As indicated by the enhanced expression of P2Y(2) receptor mRNA, P2Y(2) receptors are responsible for the increased responses evoked by ATP and UTP in interleukin-1beta-treated cells. In addition, the effect of endothelin-1, a well-known inhibitor of gap junctional communication in astrocytes was also exacerbated following interleukin-1beta treatment. We conclude that ATP decreases intercellular communication through gap junctions in astrocytes and that the increased sensitivity of gap junction channels to nucleotides and endothelin-1 is a characteristic feature of astrocytes exposed to pro-inflammatory treatments.

P2X7 receptor inhibition improves recovery after spinal cord injury

Secondary injury exacerbates the extent of spinal cord insults, yet the mechanistic basis of this phenomenon has largely been unexplored. Here we report that broad regions of the peritraumatic zone are characterized by a sustained process of pathologic, high ATP release. Spinal cord neurons expressed P2X7 purine receptors (P2X7R), and exposure to ATP led to high-frequency spiking, irreversible increases in cytosolic calcium and cell death. To assess the potential effect of P2X7R blockade in ameliorating acute spinal cord injury (SCI), we delivered P2X7R antagonists OxATP or PPADS to rats after acute impact injury. We found that both OxATP and PPADS significantly improved functional recovery and diminished cell death in the peritraumatic zone. These observations demonstrate that SCI is associated with prolonged purinergic receptor activation, which results in excitotoxicity-based neuronal degeneration. P2X7R antagonists inhibit this process, reducing both the histological extent and functional sequelae of acute SCI.

Secretion of Intracellular IL-1 Receptor Antagonist (Type 1) Is Dependent on P2X7 Receptor Activation

Inflammatory mechanisms are critical in the arterial response to injury. Both IL-1 and the naturally occurring inhibitor of IL-1, IL-1R antagonist (IL-1ra), are expressed in the arterial wall, and in particular in the endothelium. Previous studies suggest that endothelial cells only make the intracellular type I isoform of IL-1ra (icIL-1ra1), an isoform known to lack a secretory signal peptide. It is unclear how icIL-1ra is released from the endothelial cell to act as an antagonist on cell surface IL-1 type I receptors. IL-1beta, which also lacks a secretory signal peptide, may be released by ATP stimulation of the P2X(7)R. Therefore, we examined whether icIL-1ra1 release occurs in an analogous manner, using both the mouse macrophage cell line RAW264.7 and HUVECs. P2X(7)R activation caused icIL-1ra1 release from LPS-primed RAW264.7 macrophages and from HUVECs. This release was inhibited in the absence of extracellular calcium, and attenuated by preincubation with oxidized ATP, KN62, and apyrase. Endogenous ATP release, which also facilitated release of icIL-1ra1, was detected during LPS treatment of both RAW264.7 macrophages and HUVECs. Annexin V assays showed that ATP stimulation resulted in a rapid phosphatidylserine (PS) exposure on the cell surface of RAW264.7 macrophages, and that PS-exposed microvesicles contained icIL-1ra1. However, PS flip and microvesicle shedding was not apparent in ATP-treated HUVECs. These data support a general role for the P2X(7)R in the release of leaderless cytokines into the extracellular medium, and indicate how icIL-1ra1 may act upon its extracellular target, the IL-1R.

Synaptic P2X7 and oxygen/glucose deprivation in organotypic hippocampal cultures

The P2X7 receptor for extracellular ATP is the main candidate, among P2 receptors, inducing cell death in the immune system. Here, we demonstrate the direct participation of this receptor to cell damage induced by oxygen/glucose deprivation, in the ex vivo model of organotypic hippocampal cultures. By pharmacological and immunological approaches, we show that P2X7 is rapidly and transiently up regulated in hippocampal areas eliciting metabolism impairment. Moreover, the P2 antagonists 2',3',-dialdehyde ATP and reactive blue 2 prevent both up regulation of this receptor and hypoxic/hypoglycemic damage. By confocal laser microscopy, we show that P2X7 is present at the synaptic level of fibers extending from the CA1-2 pyramidal cell layer throughout the strata oriens and radiatum, but absent on oligodendrocytes, astrocytes or neuronal cell bodies. Colocalization of P2X7 is obtained with neurofilament-L protein and with synaptophysin, not with myelin basic protein, glial fibrillary acidic protein or a marker for neuronal nuclei. P2X7 up regulation and diffuse cellular damage are also induced by 3'-O-(4-benzoyl) benzoyl-ATP, an agonist selective but not exclusive for P2X7. In summary, our study demonstrates that P2X7 not only directly participates to the hypoxic/hypoglycemic process, but also owns specific phenotypic localization. We do not exclude that it might serve as a sensor of dysregulated neuronal activity and ATP release, both occurring during oxygen/glucose deprivation.

P2X7 receptors control 2-arachidonoylglycerol production by microglial cells

Endogenous cannabinoid ligands (endocannabinoids) produced by neurons, astrocytes, and microglial cells activate cannabinoid receptors, the molecular target for marijuana's bioactive ingredient Delta(9)-tetrahydrocannabinol. The molecular mechanism underlying the production of the most abundant endocannabinoid, 2-arachidonoylglycerol (2-AG), is unclear. A prevalent hypothesis proposes that activation of metabotropic receptors coupled to the phosphatidylinositol-specific phospholipase C and diacylglycerol (DG) lipase pathway will systematically lead to increases in 2-AG production. Here, we show that ATP increases 2-AG production by cultured microglial cells in a phosphatidylinositol-specific phospholipase C and DG lipase-dependent manner. However, efficacious activation of metabotropic P2Y purinergic receptors coupled to phosphatidylinositol-specific phospholipase C does not increase 2-AG production. This suggests that ionotropic, and not metabotropic, purinergic receptors control 2-AG production at an unexpected enzymatic step of its metabolic pathway. We show that activation of P2X(7) ionotropic receptors, which are highly permeable to calcium, is necessary and sufficient to increase 2-AG production in microglial cells. We also show that the sustained rise in intracellular calcium induced by activation of P2X(7) receptors directly increases DG lipase activity while inhibiting the activity of monoacylglycerol lipase, the enzyme that degrades 2-AG. This inverse sensitivity of DG lipase and monoacylglycerol lipase to calcium constitutes an original and efficient modality for sustained accumulation of 2-AG. Because prolonged increases in 2-AG amounts in brain parenchyma are thought to orchestrate neuroinflammation, the enzymatic steps involved in 2-AG synthesis and degradation by microglial cells constitute appealing targets for therapy aimed at controlling exacerbated neuroinflammation.

ATP-induced ATP release from astrocytes

Propagation of interastrocyte Ca2+ waves is mediated by diffusion of extracellular adenosine triphosphate (ATP), and may require regenerative release of ATP. The ability of ATP to initiate release of intracellular ATP was assessed by labeling adenine nucleotide pools in astrocyte cultures with 14C-adenine. The 14C-purines released during exposure to ATP were then identified by thin-layer chromatography. ATP treatment caused a five-fold increase in release of 14C-ATP but not 14C-ADP or 14C-AMP, indicating selectivity for release of ATP. Other P2 receptor agonists also caused significant 14C-ATP release, and the P2 receptor antagonists suramin, reactive blue-2 and pyridoxalphosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS) inhibited ATP-induced 14C-ATP release to varying degrees, suggesting the involvement of a P2 receptor. ATP-induced 14C-ATP release was not affected by chelation of intracellular Ca2+ with BAPTA-AM, or by blockers of Ca2+ release from intracellular stores or of extracellular Ca2+ influx, suggesting a Ca2+-independent response. ATP-induced 14C-ATP release was significantly inhibited by non-selective anion channel blockers but not by blockers of ATP-binding cassette proteins, gap junction hemichannels, or vesicular exocytosis. Release of adenine nucleotides induced by 0 Ca2+ was, in contrast, not selective for ATP, and was susceptible to inhibition by gap junction blockers. These findings indicate that astrocytes are capable of ATP-induced ATP release and support a role for regenerative ATP release in glial Ca2+ wave propagation.

ATP: a vasoactive signal in the pericyte-containing microvasculature of the rat retina

In this study we tested the hypothesis that extracellular ATP regulates the function of the pericyte-containing retinal microvessels. Pericytes, which are more numerous in the retina than in any other tissue, are abluminally located cells that may adjust capillary perfusion by contracting and relaxing. At present, knowledge of the vasoactive molecules that regulate pericyte function is limited. Here, we focused on the actions of extracellular ATP because this nucleotide is a putative glial-to-vascular signal, as well as being a substance released by activated platelets and injured cells. In microvessels freshly isolated from the adult rat retina, we monitored ionic currents via perforated-patch pipettes, measured intracellular calcium levels with the use of fura-2, and visualized microvascular contractions with the aid of time-lapse photography. We found that ATP induced depolarizing changes in the ionic currents, increased calcium levels and caused pericytes to contract. P2X7 receptors and UTP-activated receptors mediated these effects. Consistent with ATP serving as a vasoconstrictor for the pericyte-containing microvasculature of the retina, the microvascular lumen narrowed when an adjacent pericyte contracted. In addition, the sustained activation of P2X7 receptors inhibited cell-to-cell electrotonic transmission within the microvascular networks. Thus, ATP not only affects the contractility of individual pericytes, but also appears to regulate the spatial and temporal dynamics of the vasomotor response.

Learning impairment caused by a toxin produced by Pfiesteria piscicida infused into the hippocampus of rats

Pfiesteria piscicida, an estuarine dinoflagellate, which has been shown to kill fish, has also been associated with neurocognitive deficits in humans. With a rat model, we have demonstrated the cause-and-effect relationship between Pfiesteria exposure and learning impairment. In several studies, we have replicated the finding in Sprague-Dawley rats that exposure to fixed acute doses of Pfiesteria cells or filtrates caused radial-arm maze learning impairment. Recently, this finding of Pfiesteria-induced learning impairment in rats has been independently replicated in another laboratory as well. We have demonstrated significant Pfiesteria-induced learning impairment in both the win-shift and repeated-acquisition tasks in the radial-arm maze and in reversal learning in a visual operant signal detection task. These learning impairments have been seen as long as 10 weeks after a single acute exposure to Pfiesteria. In the current study, we used a hydrophilic toxin isolated from clonal P. piscicida cultures (PfTx) and tested its effect when applied locally to the ventral hippocampus on repeated acquisition of rats in the radial-arm maze. Toxin exposure impaired choice accuracy in the radial-arm maze repeated acquisition procedure. The PfTx-induced impairment was seen at the beginning of the session and the early learning deficit was persistent across 6 weeks of testing after a single administration of the toxin. Eventually, with enough practice, in each session, the PfTx-exposed rats did learn that session's problem as did control rats. This model has demonstrated the cause-and-effect relationship between exposure to a hydrophilic toxin produced by P. piscicida and learning impairment, and specifically that the ventral hippocampus was critically involved.

Interleukin-18 induces expression and release of cytokines from murine glial cells: interactions with interleukin-1 beta

Interleukin (IL)-18, a member of the IL-1 cytokine family, is an important mediator of peripheral inflammation and host defence responses. IL-1 is a key proinflammatory cytokine in the brain, but the role of IL-18 in the CNS is not yet clear. The objective of this study was to investigate the actions of IL-18 on mouse glial cells. IL-18 induced intracellular expression of IL-1 alpha and proIL-1 beta, and release of IL-6 from mixed glia. Treatment of lipopolysaccharide-primed microglia with adenosine triphosphate (ATP), an endogenous secondary stimulus, induced IL-1 beta and IL-18 release. Although deletion of the IL-18 gene did not affect IL-1 beta expression or release in this experimental paradigm, IL-1 beta knockout microglia released significantly less IL-18 compared to wild-type microglia. In addition, ATP induced release of mature IL-1 beta from IL-18-primed microglia. These data suggest that IL-18 may contribute to inflammatory responses in the brain, and demonstrate that, in spite of several common features, IL-18 and IL-1 beta differ in their regulation and actions.

Glial cells, but not interstitial cells, express P2X7, an ionotropic purinergic receptor, in rat gastrointestinal musculature

Purinergic (ATP) neurotransmission is a component of the inhibitory response of the musculature in various regions of the gastrointestinal tract. So far, seven ionotropic purinergic receptors (P2X1-7) have been cloned. As specific antibodies become available, their respective distribution in the gastrointestinal tract can be elucidated. Here, we used high-resolution tricolor confocal microscopy, to study the distribution of P2X7-immunoreactive (-ir) cells in the muscularis propria of the rat stomach, small intestine, and colon. Smooth muscle cells, KIT-ir interstitial cells of Cajal, and CD34/SK3-ir fibroblastlike cells were P2X7-negative, whereas P2X7 immunoreactivity was observed in nerves and S100-ir glial cells. In all regions studied, P2X7 immunoreactivity was also observed in myenteric and submucosal ganglia, where perineuronal nerve endings appeared brightly labeled. Our observations suggest that purinergic signaling could influence the enteric glia through P2X7 receptors.

P2X7 mediates superoxide production in primary microglia and is up-regulated in a transgenic mouse model of Alzheimer's disease

Primary rat microglia stimulated with either ATP or 2'- and 3'-O-(4-benzoylbenzoyl)-ATP (BzATP) release copious amounts of superoxide (O(2)(-)*). ATP and BzATP stimulate O(2)(-)* production through purinergic receptors, primarily the P2X(7) receptor. O(2)(-)* is produced through the activation of the NADPH oxidase. Although both p42/44 MAPK and p38 MAPK were activated rapidly in cells stimulated with BzATP, only pharmacological inhibition of p38 MAPK attenuated O(2)(-)* production. Furthermore, an inhibitor of phosphatidylinositol 3-kinase attenuated O(2)(-)* production to a greater extent than an inhibitor of p38 MAPK. Both ATP and BzATP stimulated microglia-induced cortical cell death indicating this pathway may contribute to neurodegeneration. Consistent with this hypothesis, P2X(7) receptor was specifically up-regulated around beta-amyloid plaques in a mouse model of Alzheimer's disease (Tg2576).

Role of P2X7 receptors in ischemic and excitotoxic brain injury in vivo

Purinergic P2X7 receptors may affect neuronal cell death through their ability to regulate the processing and release of interleukin-1beta (IL-1beta), a key mediator in neurodegeneration. The authors tested the hypothesis that ATP, acting at P2X7 receptors, contributes to experimentally induced neuronal death in rodents in vivo. Deletion of P2X7 receptors (P2X7 knockout mice) did not affect cell death induced by temporary cerebral ischemia, which was reduced by treatment with IL-1 receptor antagonist (IL-1RA). Treatment of mice with P2X7 antagonists did not affect ischemic or excitotoxic cell death, suggesting that P2X7 receptors are not primary mediators of experimentally induced neuronal death.