The P2X(7) receptor-pannexin connection to dye uptake and IL-1beta release

P2X4 receptor channels form large noncytolytic pores in resting and activated microglia

P2X4 ATP-gated cation channels have been shown to contribute to the microglial component of central sensitization, making their functional regulation a key element in chronic pain pathologies. Here we show that prolonged activation of native P2X4 receptor channels by ATP induces opening of a pore permeable to NMDG(+) and large fluorescent dyes in BV-2 microglial cells and primary murine microglia. This intrinsic pore formation mechanism is potentiated by LPS treatment, known to upregulate P2X4 expression in microglial cells and to mimic the microglial activation observed in neuropathic pain states. Sustained activation of the P2X7 channel subtype, also expressed in microglia, induces a pore formation that requires pannexin hemichannels and leads to plasma membrane blebbing and cytotoxicity. In contrast, P2X4 pore formation is unaffected by the pannexin blocker carbenoxolone, does not induce cytoskeletal rearrangements and does not lead to cell death. Furthermore, we show that P2X4 pore dilation is modulated by phosphoinositides (PIP(n) ) levels as it is inhibited by wortmannin, a blocker of PIP(n) synthesis, suggesting possible regulation by phospholipase C-coupled pathways. Nonlethal P2X4 pore dilation could play a role in neuropathic pain by allowing the flux of large organic molecules in microglia. Different outcomes of P2X4 and P2X7 membrane permeabilization point to subtype-specific microglial responses to ATP in normal and pathological neuro-immune crosstalks.

The participation of plasma membrane hemichannels to purinergic signaling

The field of hemichannels is closely related to the purinergic signaling and both areas have been growing in parallel. Hemichannels open in response to a wide range of stressful conditions, such as ischemia, pressure or swelling. Hemichannels represent an important mechanism for the cellular release of adenosine 5'-triphosphate (ATP), which is an agonist of the P2Y and P2X family of purinergic receptors. Therefore, hemichannels are key molecules in the regulation of purinergic receptor activation, during physiological and pathophysiological conditions. Furthermore, purinergic receptor activation can also lead to the opening of hemichannels and the subsequent amplification of purinergic signaling via a positive signaling feedback loop, giving rise to the concept of ATP-induced ATP release. Purinergic receptor signaling is involved in regulating many physiological and pathophysiological processes. P2Y receptors activate inositol trisphosphate and transiently increase intracellular calcium. This signaling opens both connexin and pannexin channels, therefore contributing to the expansion of calcium waves across astrocytes and epithelial cells. In addition, several of the P2X receptor subtypes, including the P2X2, P2X4 and P2X7 receptors, activate select cellular permeation pathways to large molecules, including the pannexin-1 channels, which are involved in the initiation of inflammatory responses and cell death. Consequently, the interplay between purinergic receptors and hemichannels could represent a novel target with substantial therapeutic implications in areas such as chronic pain, inflammation or atherosclerosis. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

P2X receptor antagonists for pain management: examination of binding and physicochemical properties

Enhanced sensitivity to noxious stimuli and the perception of non-noxious stimuli as painful are hallmark sensory perturbations associated with chronic pain. It is now appreciated that ATP, through its actions as an excitatory neurotransmitter, plays a prominent role in the initiation and maintenance of chronic pain states. Mechanistically, the ability of ATP to drive nociceptive sensitivity is mediated through direct interactions at neuronal P2X3 and P2X2/3 receptors. Extracellular ATP also activates P2X4, P2X7, and several P2Y receptors on glial cells within the spinal cord, which leads to a heightened state of neural-glial cell interaction in ongoing pain states. Following the molecular identification of the P2 receptor superfamilies, selective small molecule antagonists for several P2 receptor subtypes were identified, which have been useful for investigating the role of specific P2X receptors in preclinical chronic pain models. More recently, several P2X receptor antagonists have advanced into clinical trials for inflammation and pain. The development of orally bioavailable blockers for ion channels, including the P2X receptors, has been traditionally difficult due to the necessity of combining requirements for target potency and selectivity with suitable absorption distribution, metabolism, and elimination properties. Recent studies on the physicochemical properties of marketed orally bioavailable drugs, have identified several parameters that appear critical for increasing the probability of achieving suitable bioavailability, central nervous system exposure, and acceptable safety necessary for clinical efficacy. This review provides an overview of the antinociceptive pharmacology of P2X receptor antagonists and the chemical diversity and drug-like properties for emerging antagonists of P2X3, P2X2/3, P2X4, and P2X7 receptors.

Colchicine inhibits cationic dye uptake induced by ATP in P2X2 and P2X7 receptor-expressing cells: implications for its therapeutic action

BACKGROUND AND PURPOSE

The two longest C-termini of the purinergic P2X receptors occur in the P2X2 and P2X7 receptors and are thought to interact with multiple cytoplasmic proteins, among which are members of the cytoskeleton, including microtubules. In this work we asked whether disrupting the microtubule cytoskeleton might affect the functions of these receptors.

EXPERIMENTAL APPROACH

Functions of heterologously expressed P2X2 and P2X7 receptors were evaluated with electrophysiology and dye uptake following ATP application. Permeabilization and secretion of pro-inflammatory agents were quantified from fresh or cultured peritoneal mouse macrophages, treated in vitro or in vivo with colchicine.

KEY RESULTS

Disrupting the microtubule network with colchicine did not affect currents generated by ATP in P2X2 and P2X7 receptor-expressing cells but inhibited uptake of the dye Yo-Pro-1 in Xenopus oocytes and HEK293 cells expressing these channels. Peritoneal mouse macrophages showed less ATP-induced permeabilization to ethidium bromide in the presence of colchicine, and less reactive oxygen species (ROS) formation, nitric oxide (NO) and interleukin (IL)-1β release. Colchicine treatment did not affect ATP-evoked currents in macrophages. Finally, in vivo assays with mice inoculated with lipopolysaccharide and ATP showed diminished ROS, IL-1β, interferon-γ and NO production after colchicine treatment.

CONCLUSIONS AND IMPLICATIONS

Colchicine has known anti-inflammatory actions and is used to treat several conditions involving innate immunity, including gout and familial Mediterranean fever. Here we propose a new mechanism of action - inhibition of pore formation induced by activation of P2X receptors - which could explain some of the anti-inflammatory effects of colchicine.

Novel macrophage polarization model: from gene expression to identification of new anti-inflammatory molecules

Plasticity is a well-known property of macrophages that is controlled by different changes in environmental signals. Macrophage polarization is regarded as a spectrum of activation phenotypes adjusted from one activation extreme, the classic (M1), to the other, the alternative (M2) activation. Here we show, in vitro and in vivo, that both M1 and M2 macrophage phenotypes are tightly coupled to specific patterns of gene expression. Novel M2-associated markers were characterized and identified as genes controlling the extracellular metabolism of ATP to generate pyrophosphates (PPi). Stimulation of M1 macrophages with PPi dampens both NLR and TLR signaling and thus mediates cytokine production. In this context extracellular PPi enhanced the resolution phase of a murine peritonitis model via a decrease in pro-inflammatory cytokine production. Therefore, our study reveals an additional level of plasticity modulating the resolution of inflammation.

Th1/Th17 cell induction and corresponding reduction in ATP consumption following vaccination with the novel Mycobacterium tuberculosis vaccine MVA85A

Vaccination with Bacille Calmette-Guérin (BCG) has traditionally been used for protection against disease caused by the bacterium Mycobacterium tuberculosis (M.tb). The efficacy of BCG, especially against pulmonary tuberculosis (TB) is variable. The best protection is conferred in temperate climates and there is close to zero protection in many tropical areas with a high prevalence of both tuberculous and non-tuberculous mycobacterial species. Although interferon (IFN)-γ is known to be important in protection against TB disease, data is emerging on a possible role for interleukin (IL)-17 as a key cytokine in both murine and bovine TB vaccine studies, as well as in humans. Modified Vaccinia virus Ankara expressing Antigen 85A (MVA85A) is a novel TB vaccine designed to enhance responses induced by BCG. Antigen-specific IFN-γ production has already been shown to peak one week post-MVA85A vaccination, and an inverse relationship between IL-17-producing cells and regulatory T cells expressing the ectonucleosidease CD39, which metabolises pro-inflammatory extracellular ATP has previously been described. This paper explores this relationship and finds that consumption of extracellular ATP by peripheral blood mononuclear cells from MVA85A-vaccinated subjects drops two weeks post-vaccination, corresponding to a drop in the percentage of a regulatory T cell subset expressing the ectonucleosidase CD39. Also at this time point, we report a peak in co-production of IL-17 and IFN-γ by CD4⁺ T cells. These results suggest a relationship between extracellular ATP and effector responses and unveil a possible pathway that could be targeted during vaccine design.

Interaction between nitric oxide signaling and gap junctions: effects on vascular function

Nitric oxide signaling, through eNOS (or possibly nNOS), and gap junction communication are essential for normal vascular function. While each component controls specific aspects of vascular function, there is substantial evidence for cross-talk between nitric oxide signaling and the gap junction proteins (connexins), and more recently, protein-protein association between eNOS and connexins. This review will examine the evidence for interaction between these pathways in normal and diseased arteries, highlight the questions that remain about the mechanisms of their interaction, and explore the possible interaction between nitric oxide signaling and the newly discovered pannexin channels. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

A potential therapeutic role for P2X7 receptor (P2X7R) antagonists in the treatment of inflammatory diseases

INTRODUCTION

The P2X7 receptor (P2X7R) has an important role in inflammation and immunity, but until recently, clinical application has been limited by a lack of specific antagonists. Recent studies using P2X7R knockout mice and specific receptor antagonists have shown that the P2X7R is an important therapeutic target in inflammatory diseases.

AREAS COVERED

We have reviewed the current literature on the role of the P2X7R in inflammatory diseases, focusing on potential therapeutic applications of selective P2X7R antagonists as anti-inflammatory agents. Particular emphasis has been placed on the potential role of P2X7R in common inflammatory diseases. The latest developments in Phase I and II clinical trials of P2X7R antagonists are covered.

EXPERT OPINION

Recent studies using gene knockout mice and selective P2X7R antagonists suggest that P2X7R is a viable therapeutic target for inflammatory diseases. However, efficacious P2X7R antagonists for use in clinical studies are still at an early stage of development. Future challenges include: identifying potential toxicity and side effects of treatment, timing of treatment initiation and its duration in chronic inflammatory conditions, optimum dosage and development of a functional assay for P2X7R that would help to guide treatment.

Many ways to dilate the P2X7 receptor pore

The P2X7 receptor is associated with two different membrane permeabilities: a small cation conductance which opens within milliseconds, followed by the appearance of a second channel carrying higher molecular weight compounds (including organic dyes) after prolonged agonist stimulation. This activation profile has also been found in cells expressing P2X2 and P2X4 receptors; however, the P2X7 receptor-dependent pathway has the unique ability to activate pro-inflammatory signalling in macrophages. In this issue of the BJP, Marques-da-Silva et al. demonstrate that colchicine is a potent inhibitor of both P2X7 and P2X2 receptor-dependent dye uptake, without affecting the ion channels. Colchicine also blocked the pro-inflammatory signalling downstream of P2X7 receptor activation, both in vitro and in vivo. This report suggests that the dye uptake associated with activation of P2X7 receptors is distinct from the P2X7 receptor ion channel and could be a therapeutic target for the treatment of chronic inflammation.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

P2X7 receptor signaling contributes to tissue factor-dependent thrombosis in mice

Thrombosis is initiated by tissue factor (TF), a coagulation cofactor/receptor expressed in the vessel wall, on myeloid cells, and on microparticles (MPs) with variable procoagulant activity. However, the molecular pathways that generate prothrombotic TF in vivo are poorly defined. The oxidoreductase protein disulfide isomerase (PDI) is thought to be involved in the activation of TF. Here, we found that in mouse myeloid cells, ATP-triggered signaling through purinergic receptor P2X, ligand-gated ion channel, 7 (P2X7 receptor; encoded by P2rx7) induced activation (decryption) of TF procoagulant activity and promoted release of TF+ MPs from macrophages and SMCs. The generation of prothrombotic MPs required P2X7 receptor-dependent production of ROS leading to increased availability of solvent-accessible extracellular thiols. An antibody to PDI with antithrombotic activity in vivo attenuated the release of procoagulant MPs. In addition, P2rx7-/- mice were protected from TF-dependent FeCl3-induced carotid artery thrombosis. BM chimeras revealed that P2X7 receptor prothrombotic function was present in both hematopoietic and vessel wall compartments. In contrast, an alternative anti-PDI antibody showed activities consistent with cellular activation typically induced by P2X7 receptor signaling. This anti-PDI antibody restored TF-dependent thrombosis in P2rx7-/- mice. These data suggest that PDI regulates a critical P2X7 receptor-dependent signaling pathway that generates prothrombotic TF, defining a link between inflammation and thrombosis with potential implications for antithrombotic therapy.

Pathways regulating the trafficking and turnover of pannexin1 protein and the role of the C-terminal domain

Pannexin1 (Panx1) is an integral membrane protein comprised of three species as follows: an unglycosylated core-Gly0, a high mannose-Gly1, and a complex glycosylated Gly2 species. Although Panx1 channels mediate several cellular responses, the domain regulating its oligomerization and cell surface trafficking and the mechanisms governing its internalization and degradation have not been identified. This study characterizes the role of the Panx1 C-tail domain by truncating the polypeptide at residue 307 and expressing the mutant in BICR-M1R(k) and HEK-293T cells. Enzymatic digestion and immunolabeling assays revealed that the Panx1(T307)-RFP was glycosylated primarily to the high mannose species consistent with its retention in the endoplasmic reticulum. Co-expression of Panx1(T307)-RFP with Panx1 followed by co-immunoprecipitation assays revealed that the mutant and Panx1 could interact, whereas biotinylation assays showed that this interaction inhibited Panx1 from maturing into the Gly2 species and reaching the cell surface. Additional inhibitor studies indicated that the degradation of the mutant was via proteasomes, whereas Panx1 was degraded by lysosomes. Analysis of the pathways important in Panx1 internalization revealed partial co-distribution of Panx1 with many molecular constituents of the endocytic machinery that include clathrin, AP2, dynamin II, caveolin-1, and caveolin-2. However, co-immunoprecipitation assays together with the disruption of lipid rafts by methyl-β-cyclodextrin suggest that Panx1 does not engage this endocytic machinery. Furthermore, dominant-negative and pharmacological studies revealed that Panx1 internalization was dynamin II-independent. Collectively, these results indicate that the oligomerization and trafficking of Panx1 are regulated by the C-terminal domain, whereas internalization of long lived Panx1 channels occurs in a manner that is distinct from classical endocytic pathways.

Expression and roles of pannexins in ATP release in the pituitary gland

Pannexins are a newly discovered three-member family of proteins expressed in the brain and peripheral tissues that belong to the superfamily of gap junction proteins. However, in mammals pannexins do not form gap junctions, and their expression and function in the pituitary gland have not been studied. Here we show that the rat pituitary gland expresses mRNA and protein transcripts of pannexins 1 and 2 but not pannexin 3. Pannexin 1 was more abundantly expressed in the anterior lobe, whereas pannexin 2 was more abundantly expressed in the intermediate and posterior pituitary. Pannexin 1 was identified in corticotrophs and a fraction of somatotrophs, the S100-positive pituicytes of the posterior pituitary and AtT-20 (mouse pituitary adrenocorticotropin-secreting cells) and rat immortalized pituitary cells secreting prolactin, whereas pannexin 2 was detected in the S100-positive folliculostellate cells of the anterior pituitary, melanotrophs of the intermediate lobe, and vasopressin-containing axons and nerve endings in the posterior lobe. Overexpression of pannexins 1 and 2 in AtT-20 pituitary cells enhanced the release of ATP in the extracellular medium, which was blocked by the gap junction inhibitor carbenoxolone. Basal ATP release in At-T20 cells was also suppressed by down-regulating the expression of endogenous pannexin 1 but not pannexin 2 with their short interfering RNAs. These results indicate that pannexins may provide a pathway for delivery of ATP, which is a native agonist for numerous P2X cationic channels and G protein-coupled P2Y receptors endogenously expressed in the pituitary gland.

Apoptosis: opening PANdora's BoX

Extracellular nucleotides have been reported to act as a 'find-me' signal in the context of phagocyte recruitment by apoptotically dying cells. A new study now examines the mechanisms of nucleotide release during apoptosis and describes the hemichannel-forming protein pannexin 1 as a crucial player in this scenario.

Neuroinflammation leads to region-dependent alterations in astrocyte gap junction communication and hemichannel activity

Inflammation attenuates gap junction (GJ) communication in cultured astrocytes. Here we used a well-characterized model of experimental brain abscess as a tool to query effects of the CNS inflammatory milieu on astrocyte GJ communication and electrophysiological properties. Whole-cell patch-clamp recordings were performed on green fluorescent protein (GFP)-positive astrocytes in acute brain slices from glial fibrillary acidic protein-GFP mice at 3 or 7 d after Staphylococcus aureus infection in the striatum. Astrocyte GJ communication was significantly attenuated in regions immediately surrounding the abscess margins and progressively increased to levels typical of uninfected brain with increasing distance from the abscess proper. Conversely, astrocytes bordering the abscess demonstrated hemichannel activity as evident by enhanced ethidium bromide (EtBr) uptake that could be blocked by several pharmacological inhibitors, including the connexin 43 (Cx43) mimetic peptide Gap26, carbenoxolone, the pannexin1 (Panx1) mimetic peptide (10)Panx1, and probenecid. However, hemichannel opening was transient with astrocytic EtBr uptake observed near the abscess at day 3 but not day 7 after infection. The region-dependent pattern of hemichannel activity at day 3 directly correlated with increases in Cx43, Cx30, Panx1, and glutamate transporter expression (glial L-glutamate transporter and L-glutamate/L-aspartate transporter) along the abscess margins. Changes in astrocyte resting membrane potential and input conductance correlated with the observed changes in GJ communication and hemichannel activity. Collectively, these findings indicate that astrocyte coupling and electrical properties are most dramatically affected near the primary inflammatory site and reveal an opposing relationship between the open states of GJ channels versus hemichannels during acute infection. This relationship may extend to other CNS diseases typified with an inflammatory component.

Functional significance of the negative-feedback regulation of ATP release via pannexin-1 hemichannels under ischemic stress in astrocytes

The opening of pannexin-1 (Px1) hemichannels is regulated by the activity of P2X(7) receptors (P2X(7)Rs). At present, however, little is known about how extracellular ATP-sensitive P2X(7)Rs regulates the opening and closure of Px1 hemichannels. Several lines of evidence suggest that P2X(7)Rs are activated under pathological conditions such as ischemia, resulting in the opening of Px1 hemichannels responsible for the massive influx of Ca(2+) from the extracellular space and the release of ATP from the cytoplasm, leading to cell death. Here we show in cultured astrocytes that the suppression of the activity of P2X(7)Rs during simulated ischemia (oxygen/glucose deprivation, OGD) resulted in the opening of Px1 hemichannels, leading to the enhanced release of ATP. In addition, the suppression of the activity of P2X(7)Rs during OGD resulted in a significant increase in astrocytic damage. Both the P2X(7)Rs suppression-induced enhancement of the release of ATP and cell damage were reversed by co-treatment with blockers of Px1 hemichannels, suggesting that suppression of the activity of PX(7)Rs resulted in the opening of Px1 hemichannels. All these findings suggested the existence of a negative-feedback loop regulating the release of ATP via Px1 hemichannels; ATP-induced suppression of ATP release. The present study indicates that ATP, released through Px1 hemichannels, activates P2X(7)Rs, resulting in the closure of Px1 hemichannels during ischemia. This negative-feedback mechanism, suppressing the loss of cellular ATP and Ca(2+) influx, might contribute to the survival of astrocytes under ischemic stress.

Experimental characterization and mathematical modeling of P2X7 receptor channel gating

The P2X7 receptor is a trimeric channel with three binding sites for ATP, but how the occupancy of these sites affects gating is still not understood. Here we show that naive receptors activated and deactivated monophasically at low and biphasically at higher agonist concentrations. Both phases of response were abolished by application of Az10606120, a P2X7R-specific antagonist. The slow secondary growth of current in the biphasic response coincided temporally with pore dilation. Repetitive stimulation with the same agonist concentration caused sensitization of receptors, which manifested as a progressive increase in the current amplitude, accompanied by a slower deactivation rate. Once a steady level of the secondary current was reached, responses at high agonist concentrations were no longer biphasic but monophasic. Sensitization of receptors was independent of Na(+) and Ca(2+) influx and ∼30 min washout was needed to reestablish the initial gating properties. T15E- and T15K-P2X7 mutants showed increased sensitivity for agonists, responded with monophasic currents at all agonist concentrations, activated immediately with dilated pores, and deactivated slowly. The complex pattern of gating exhibited by wild-type channels can be accounted for by a Markov state model that includes negative cooperativity of agonist binding to unsensitized receptors caused by the occupancy of one or two binding sites, opening of the channel pore to a low conductance state when two sites are bound, and sensitization with pore dilation to a high conductance state when three sites are occupied.

Irritating channels: the case of TRPA1

Transient receptor potential (TRP) channels have been extensively studied over the past years. Yet, in most cases, the gating mechanisms of these polymodal cation channels still remain a puzzle. Using the nociceptive channel TRPA1 as an example, we discuss the role of dynamic regulation of the pore size (pore dilatation) on channel gating. Additionally, we critically revise current knowledge of the role of intracellular domains, such as ankyrin repeats and EF hand motifs, in channel activation and function. Finally, we assess some problems inherent to activation of TRPA1 by the reaction of electrophilic compounds with the nucleophilic thiol sink of N-terminal reactive cysteines.

Neuronal soma-satellite glial cell interactions in sensory ganglia and the participation of purinergic receptors

It has been known for some time that the somata of neurons in sensory ganglia respond to electrical or chemical stimulation and release transmitters in a Ca2+-dependent manner. The function of the somatic release has not been well delineated. A unique characteristic of the ganglia is that each neuronal soma is tightly enwrapped by satellite glial cells (SGCs). The somatic membrane of a sensory neuron rarely makes synaptic contact with another neuron. As a result, the influence of somatic release on the activity of adjacent neurons is likely to be indirect and/or slow. Recent studies of neuron-SGC interactions have demonstrated that ATP released from the somata of dorsal root ganglion neurons activates SGCs. They in turn exert complex excitatory and inhibitory modulation of neuronal activity. Thus, SGCs are actively involved in the processing of afferent information. In this review, we summarize our understanding of bidirectional communication between neuronal somata and SGCs in sensory ganglia and its possible role in afferent signaling under normal and injurious conditions. The participation of purinergic receptors is emphasized because of their dominant roles in the communication.

Pannexin channels in ATP release and beyond: an unexpected rendezvous at the endoplasmic reticulum

The pannexin (Panx) family of proteins, which is co-expressed with connexins (Cxs) in vertebrates, was found to be a new GJ-forming protein family related to invertebrate innexins. During the past ten years, different studies showed that Panxs mainly form hemichannels in the plasma membrane and mediate paracrine signalling by providing a flux pathway for ions such as Ca²(+), for ATP and perhaps for other compounds, in response to physiological and pathological stimuli. Although the physiological role of Panxs as a hemichannel was questioned, there is increasing evidence that Panx play a role in vasodilatation, initiation of inflammatory responses, ischemic death of neurons, epilepsy and in tumor suppression. Moreover, it is intriguing that Panxs may also function at the endoplasmic reticulum (ER) as intracellular Ca²(+)-leak channel and may be involved in ER-related functions. Although the physiological significance and meaning of such Panx-regulated intracellular Ca²(+) leak requires further exploration, this functional property places Panx at the centre of many physiological and pathophysiological processes, given the fundamental role of intracellular Ca²(+) homeostasis and dynamics in a plethora of physiological processes. In this review, we therefore want to focus on Panx as channels at the plasma membrane and at the ER membranes with a particular emphasis on the potential implications of the latter in intracellular Ca²(+) signalling.

NTPDase1 governs P2X7-dependent functions in murine macrophages

P2X7 receptor is an adenosine triphosphate (ATP)-gated ion channel within the multiprotein inflammasome complex. Until now, little is known about regulation of P2X7 effector functions in macrophages. In this study, we show that nucleoside triphosphate diphosphohydrolase 1 (NTPDase1)/CD39 is the dominant ectonucleotidase expressed by murine peritoneal macrophages and that it regulates P2X7-dependent responses in these cells. Macrophages isolated from NTPDase1-null mice (Entpd1(-/-)) were devoid of all ADPase and most ATPase activities when compared with WT macrophages (Entpd1(+/+)). Entpd1(-/-) macrophages exposed to millimolar concentrations of ATP were more susceptible to cell death, released more IL-1beta and IL-18 after TLR2 or TLR4 priming, and incorporated the fluorescent dye Yo-Pro-1 more efficiently (suggestive of increased pore formation) than Entpd1(+/+) cells. Consistent with these observations, NTPDase1 regulated P2X7-associated IL-1beta release after synthesis, and this process occurred independently of, and prior to, cytokine maturation by caspase-1. NTPDase1 also inhibited IL-1beta release in vivo in the air pouch inflammatory model. Exudates of LPS-injected Entpd1(-/-) mice had significantly higher IL-1beta levels when compared with Entpd1(+/+) mice. Altogether, our studies suggest that NTPDase1/CD39 plays a key role in the control of P2X7-dependent macrophage responses.

Pannexin-I/P2X 7 purinergic receptor channels mediate the release of cardioprotectants induced by ischemic pre- and postconditioning

Ischemic pre- and postconditioning protect ex vivo rat hearts from ischemia/reperfusion injury by promoting the release of cardioprotective agents by an unknown mechanism. Because P2X( 7) purinergic receptors are known to combine with pannexin-1 to form channels that allow adenosine triphosphate (ATP) release from cells, we hypothesized that these channels have a role in the release of multiple cardioprotectants during ischemic preconditioning (IPC). Addition of either a pannexin-1 hemichannel blocker (5 micromol/L carbenoxolone [CBX] or 0.4 micromol/L mefloquine [MF]) or a selective antagonist of the rat P2X(7) purinergic receptor (2 micromol/L brilliant blue G [BBG]) blocked IPC. These antagonists also blocked ischemic postconditioning. Preconditioning by exogenous addition of either sphingosine-1-phosphate or adenosine was not blocked by either CBX or BBG, indicating that they only affected the release of endogenous mediators, not any subsequent steps. To determine if only ATP release was mediated by pannexin-1/P2X(7) channels, we added an extra cycle of IPC to release sufficient quantities of additional cardioprotectants to eliminate the dependence on adenosine derivatives. This did not override the inhibition of IPC by CBX or MF, suggesting that the channel mediates the release of multiple cardioprotectants. Inhibitors of other P2X receptors, P2Y receptors, or connexins did not affect IPC. We conclude that a pannexin-1/P2X(7) channel is responsible for the release of cardioprotectants induced by ischemic pre- and postconditioning.

P2X7-dependent release of interleukin-1beta and nociception in the spinal cord following lipopolysaccharide

The cytokine interleukin-1beta (IL-1beta) released by spinal microglia in enhanced response states contributes significantly to neuronal mechanisms of chronic pain. Here we examine the involvement of the purinergic P2X7 receptor in the release of IL-1beta following activation of Toll-like receptor-4 (TLR4) in the dorsal horn, which is associated with nociceptive behavior and microglial activation. We observed that lipopolysaccharide (LPS)-induced release of IL-1beta was prevented by pharmacological inhibition of the P2X7 receptor with A-438079, and was absent in spinal cord slices taken from P2X7 knock-out mice. Application of ATP did not evoke release of IL-1beta from the dorsal horn unless preceded by an LPS priming stimulus, and this release was dependent on P2X7 receptor activation. Extensive phosphorylation of p38 MAPK in microglial cells in the dorsal horn was found to correlate with IL-1beta secretion following both LPS and ATP. In behavioral studies, intrathecal injection of LPS in the lumbar spinal cord produced mechanical hyperalgesia in rat hindpaws, which was attenuated by concomitant injections of either a nonspecific (oxidized ATP) or a specific (A-438079) P2X7 antagonist. In addition, LPS-induced hypersensitivity was observed in wild-type but not P2X7 knock-out mice. These data suggest a critical role for the P2X7 receptor in the enhanced nociceptive transmission associated with microglial activation and secretion of IL-1beta in the dorsal horn. We suggest that CNS-penetrant P2X7 receptor antagonists, by targeting microglia in pain-enhanced response states, may be beneficial for the treatment of persistent pain.

Non-junction functions of pannexin-1 channels

Pannexins are large-pore ion channels with broad expression in the central nervous system (CNS). The channels function by releasing large signaling molecules, such ATP and arachidonic acid derivatives, from neurons and possibly astrocytes. They might also contribute to novel forms of non-synaptic communication in the CNS, thereby affecting synaptic function, astrocytic Ca(2+) wave propagation and possibly regulation of vascular tone in the brain. Panx1 activation in various in vitro pathological conditions implicates these channels in ischemic, excitotoxic and ATP-dependent cell death, whereas Panx coupling with purinergic receptors triggers the inflammasome. Novel functions for the pannexin channels are likely to be discovered as current understanding of how they are regulated in physiological and pathological situations improves.

The neural-glial purinergic receptor ensemble in chronic pain states

Chronic pain is characterized by enhanced sensory neurotransmission that underlies increased sensitivity to noxious stimuli and the perception of non-noxious stimuli as painful. Evidence from neurophysiological and pharmacological studies demonstrates that ATP produces pain by directly enhancing neuronal excitability via the activation of specific ligand-gated ion channels, the P2X3 and P2X2/3 receptors. In addition, ATP activates CNS glial cells (e.g. microglia) in response to persistent nociceptive stimulation. This latter effect involves several distinct receptor-mediated signaling pathways linked to the P2X4, P2X7 and P2Y(12) receptors. This review summarizes new data that places these purinergic signaling events in a mechanistic context that illustrates the ability of ATP to initiate and maintain states of heightened sensory neuron excitability associated with persistent pain.

Pannexins, distant relatives of the connexin family with specific cellular functions?

Intercellular communication (IC) is mediated by gap junctions (GJs) and hemichannels, which consist of proteins. This has been particularly well documented for the connexin (Cx) family. Initially, Cxs were thought to be the only proteins capable of GJ formation in vertebrates. About 10 years ago, however, a new GJ-forming protein family related to invertebrate innexins (Inxs) was discovered in vertebrates, and named the pannexin (Panx) family. Panxs, which are structurally similar to Cxs, but evolutionarily distinct, have been shown to be co-expressed with Cxs in vertebrates. Both protein families show distinct properties and have their own particular function. Identification of the mechanisms that control Panx channel gating is a major challenge for future work. In this review, we focus on the specific properties and role of Panxs in normal and pathological conditions.

5-Nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) stimulates cellular ATP release through exocytosis of ATP-enriched vesicles

Cells release ATP in response to physiologic stimuli. Extracellular ATP regulates a broad range of important cellular functions by activation of the purinergic receptors in the plasma membrane. The purpose of these studies was to assess the role of vesicular exocytosis in cellular ATP release. FM1-43 fluorescence was used to measure exocytosis and bioluminescence to measure ATP release in HTC rat hepatoma and Mz-Cha-1 human cholangiocarcinoma cells. Exposure to a Cl(-) channel inhibitor 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) (50-300 microM) stimulated a 5-100-fold increase in extracellular ATP levels within minutes of the exposure. This rapid response was not a result of changes in cell viability or Cl(-) channel activity. NPPB also potently stimulated ATP release in HEK293 cells and HEK293 cells expressing a rat P2X7 receptor indicating that P2X7 receptors are not involved in stimulation of ATP release by NPPB. In all cells studied, NPPB rapidly stimulated vesicular exocytosis that persisted many minutes after the exposure. The kinetics of NPPB-evoked exocytosis and ATP release were similar. Furthermore, the magnitudes of NPPB-evoked exocytosis and ATP release were correlated (correlation coefficient 0.77), indicating that NPPB may stimulate exocytosis of a pool of ATP-enriched vesicles. These findings provide further support for the concept that vesicular exocytosis plays an important role in cellular ATP release and suggest that NPPB can be used as a biochemical tool to specifically stimulate ATP release through exocytic mechanisms.