Mitogen-activated protein kinase and caspase signaling pathways are required for P2X7 receptor (P2X7R)-induced pore formation in human THP-1 cells

The P2X7 Receptor in Osteoarthritis

Osteoarthritis (OA) is the most common joint disease. With the increasing aging population, the associated socio-economic costs are also increasing. Analgesia and surgery are the primary treatment options in late-stage OA, with drug treatment only possible in early prevention to improve patients' quality of life. The most important structural component of the joint is cartilage, consisting solely of chondrocytes. Instability in chondrocyte balance results in phenotypic changes and cell death. Therefore, cartilage degradation is a direct consequence of chondrocyte imbalance, resulting in the degradation of the extracellular matrix and the release of pro-inflammatory factors. These factors affect the occurrence and development of OA. The P2X7 receptor (P2X7R) belongs to the purinergic receptor family and is a non-selective cation channel gated by adenosine triphosphate. It mediates Na⁺, Ca²⁺ influx, and K⁺ efflux, participates in several inflammatory reactions, and plays an important role in the different mechanisms of cell death. However, the relationship between P2X7R-mediated cell death and the progression of OA requires investigation. In this review, we correlate potential links between P2X7R, cartilage degradation, and inflammatory factor release in OA. We specifically focus on inflammation, apoptosis, pyroptosis, and autophagy. Lastly, we discuss the therapeutic potential of P2X7R as a potential drug target for OA.

New Insights into the Mechanisms of Pyroptosis and Implications for Diabetic Kidney Disease

Pyroptosis is one special type of lytic programmed cell death, featured in cell swelling, rupture, secretion of cell contents and remarkable proinflammation effect. In the process of pyroptosis, danger signalling and cellular events are detected by inflammasome, activating caspases and cleaving Gasdermin D (GSDMD), along with the secretion of IL-18 and IL-1β. Pyroptosis can be divided into canonical pathway and non-canonical pathway, and NLRP3 inflammasome is the most important initiator. Diabetic kidney disease (DKD) is one of the most serious microvascular complications in diabetes. Current evidence reported the stimulatory role of hyperglycaemia-induced cellular stress in renal cell pyroptosis, and different signalling pathways have been shown to regulate pyroptosis initiation. Additionally, the inflammation and cellular injury caused by pyroptosis are tightly implicated in DKD progression, aggravating renal fibrosis, glomerular sclerosis and tubular injury. Some registered hypoglycaemia agents exert suppressive activity in pyroptosis regulation pathway. Latest studies also reported some potential approaches to target the pyroptosis pathway, which effectively inhibits renal cell pyroptosis and alleviates DKD in in vivo or in vitro models. Therefore, comprehensively compiling the information associated with pyroptosis regulation in DKD is the main aim of this review, and we try to provide new insights for researchers to dig out more potential therapies of DKD.

A cell permeant phosphopeptide mimetic of Niban inhibits p38 MAPK and restores endothelial function after injury

Vascular injury leads to membrane disruption, ATP release, and endothelial dysfunction. Increases in the phosphorylation of p38 mitogen‐activated protein kinase (p38 MAPK) and decreases in the phosphorylation of Niban, a protein implicated in ER stress and apoptosis, are associated with vascular injury. A cell permeant phosphopeptide mimetic of Niban (NiPp) was generated. The effects of NiPp in restoring endothelial function were determined ex vivo using intact rat aortic tissue (RA) after pharmacological activation of p38 MAPK and also in multiple clinically relevant injury models. Anisomycin (Aniso) increased p38 MAPK phosphorylation and reduced endothelial‐dependent relaxation in RA. Treatment with NiPp prevented Ansio‐induced reduction in endothelial function and increases in p38 MAPK phosphorylation. NiPp treatment also restored endothelial function after stretch injury (subfailure stretch), treatment with acidic Normal Saline (NS), and P2X7R activation with 2′(3′)‐O‐(4‐Benzoylbenzoyl)adenosine 5′‐triphosphate (BzATP). Aged, diseased, human saphenous vein (HSV) remnants obtained from patients undergoing coronary bypass surgical procedures have impaired endothelial function. Treatment of these HSV segments with NiPp improved endothelial‐dependent relaxation. Kinome screening experiments indicated that NiPp inhibits p38 MAPK. These data demonstrate that p38 MAPK and Niban signaling have a role in endothelial function, particularly in response to injury. Niban may represent an endogenous regulator of p38 MAPK activation. The NiPp peptide may serve as an experimental tool to further elucidate p38 MAPK regulation and as a potential therapeutic for endothelial dysfunction.

Pleiotropic Roles of P2X7 in the Central Nervous System

The purinergic receptor P2X7 is expressed in neural and immune cells known to be involved in neurological diseases. Its ligand, ATP, is a signaling molecule that can act as a neurotransmitter in physiological conditions or as a danger signal when released in high amount by damaged/dying cells or activated glial cells. Thus, ATP is a danger-associated molecular pattern. Binding of ATP by P2X7 leads to the activation of different biochemical pathways, depending on the physiological or pathological environment. The aim of this review is to discuss various functions of P2X7 in the immune and central nervous systems. We present evidence that P2X7 may have a detrimental or beneficial role in the nervous system, in the context of neurological pathologies: epilepsy, Alzheimer’s disease, multiple sclerosis, amyotrophic lateral sclerosis, age-related macular degeneration and cerebral artery occlusion.

Normal Saline solutions cause endothelial dysfunction through loss of membrane integrity, ATP release, and inflammatory responses mediated by P2X7R/p38 MAPK/MK2 signaling pathways

Resuscitation with 0.9% Normal Saline (NS), a non-buffered acidic solution, leads to increased morbidity and mortality in the critically ill. The goal of this study was to determine the molecular mechanisms of endothelial injury after exposure to NS. The hypothesis of this investigation is that exposure of endothelium to NS would lead to loss of cell membrane integrity, resulting in release of ATP, activation of the purinergic receptor (P2X7R), and subsequent activation of stress activated signaling pathways and inflammation. Human saphenous vein endothelial cells (HSVEC) incubated in NS, but not buffered electrolyte solution (Plasma-Lyte, PL), exhibited abnormal morphology and increased release of lactate dehydrogenase (LDH), adenosine triphosphate (ATP), and decreased transendothelial resistance (TEER), suggesting loss of membrane integrity. Incubation of intact rat aorta (RA) or human saphenous vein in NS but not PL led to impaired endothelial-dependent relaxation which was ameliorated by apyrase (hydrolyzes ATP) or SB203580 (p38 MAPK inhibitor). Exposure of HSVEC to NS but not PL led to activation of p38 MAPK and its downstream substrate, MAPKAP kinase 2 (MK2). Treatment of HSVEC with exogenous ATP led to interleukin 1β (IL-1β) release and increased vascular cell adhesion molecule (VCAM) expression. Treatment of RA with IL-1β led to impaired endothelial relaxation. IL-1β treatment of HSVEC led to increases in p38 MAPK and MK2 phosphorylation, and increased levels of arginase II. Incubation of porcine saphenous vein (PSV) in PL with pH adjusted to 6.0 or less also led to impaired endothelial function, suggesting that the acidic nature of NS is what contributes to endothelial dysfunction. Volume overload resuscitation in a porcine model after hemorrhage with NS, but not PL, led to acidosis and impaired endothelial function. These data suggest that endothelial dysfunction caused by exposure to acidic, non-buffered NS is associated with loss of membrane integrity, release of ATP, and is modulated by P2X7R-mediated inflammatory responses.

Potential role of P2X7 receptor in neurodegenerative processes in a murine model of glaucoma

Glaucoma is a common cause of visual impairment and blindness, characterized by retinal ganglion cell (RGC) death. The mechanisms that trigger the development of glaucoma remain unknown and have gained significant relevance in the study of this neurodegenerative disease. P2X7 purinergic receptors (P2X7R) could be involved in the regulation of the synaptic transmission and neuronal death in the retina through different pathways. The aim of this study was to characterize the molecular signals underlying glaucomatous retinal injury. The time-course of functional, morphological, and molecular changes in the glaucomatous retina of the DBA/2J mice were investigated. The expression and localization of P2X7R was analysed in relation with retinal markers. Caspase-3, JNK, and p38 were evaluated in control and glaucomatous mice by immunohistochemical and western-blot analysis. Furthermore, electroretinogram recordings (ERG) were performed to assess inner retina dysfunction. Glaucomatous mice exhibited changes in P2X7R expression as long as the pathology progressed. There was P2X7R overexpression in RGCs, the primary injured neurons, which correlated with the loss of function through ERG measurements. All analyzed MAPK and caspase-3 proteins were upregulated in the DBA/2J retinas suggesting a pro-apoptotic cell death. The increase in P2X7Rs presence may contribute, together with other factors, to the changes in retinal functionality and the concomitant death of RGCs. These findings provide evidence of possible intracellular pathways responsible for apoptosis regulation during glaucomatous degeneration.

Eliminating the contribution of lipopolysaccharide to protein allergenicity in the human cell-line activation test (h-CLAT)

We previously developed a test for detecting naturally occurring protein-induced skin sensitization based on the markers and criteria of the human cell-line activation test (h-CLAT) and showed that the h-CLAT was useful for assessing the allergenic potency of proteins. However, test proteins were contaminated with varying amounts of lipopolysaccharide (LPS), which might have contributed to the stimulation of CD86 and CD54 expression. In this study, we developed a method to exclude the effects of LPS in the assessment of skin sensitization by naturally occurring proteins. We tested two inhibitors [the caspase-1 inhibitor acetyl-Tyr-Val-Ala-Asp-chloromethylketone (Ac-YVAD-cmk; hereafter referred to as YVAD), which can mitigate the LPS-induced increases in CD54 expression, and polymyxin B (PMB), which suppresses the effect of LPS by binding to its lipid moiety (i.e., the toxic component of LPS)]. After a 24 hr exposure, YVAD and PMB reduced LPS-induced CD86 and CD54 expression. In particular, the effect of PMB was dependent upon pre-incubation time and temperature, with the most potent effect observed following pre-incubation at 37°C for 24 hr. Moreover, only pre-incubation with cell-culture medium (CCM) at 37°C for 24 hr showed an inhibitory effect similar to that of PMB, with this result possibly caused by components of CCM binding to LPS. Similar effects were observed in the presence of ovalbumin (with 1070 EU/mg LPS) and ovomucoid, and lysozyme (with 2.82 and 0.234 EU/mg LPS, respectively) in CCM. These results indicated that PMB and CCM effectively eliminated the effects of LPS during assessment of protein allergenicity, thereby allowing a more accurate evaluation of the potential of proteins to induce skin sensitization.

ATP-Gated P2X7 Receptors Require Chloride Channels To Promote Inflammation in Human Macrophages

Immune cells of myeloid origin show robust expression of ATP-gated P2X7 receptors, two-transmembrane ion channels permeable to Na⁺, K⁺, and Ca²⁺ Receptor activation promotes inflammasome activation and release of the proinflammatory cytokines IL-1β and IL-18. In this study, we show that ATP generates facilitating cationic currents in monocyte-derived human macrophages and permeabilizes the plasma membrane to polyatomic cationic dyes. We find that antagonists of PLA₂ and Cl^- channels abolish P2X7 receptor-mediated current facilitation, membrane permeabilization, blebbing, phospholipid scrambling, inflammasome activation, and IL-1β release. Our data demonstrate significant differences in the actions of ATP in murine and human macrophages and suggest that PLA₂ and Cl^- channels mediate innate immunity downstream of P2X7 receptors in human macrophages.

ATP as a Pathophysiologic Mediator of Bacteria-Host Crosstalk in the Gastrointestinal Tract

Extracellular nucleotides, such as adenosine triphosphate (ATP), are released from host cells including nerve termini, immune cells, injured or dead cells, and the commensal bacteria that reside in the gut lumen. Extracellular ATP interacts with the host through purinergic receptors, and promotes intercellular and bacteria-host communication to maintain the tissue homeostasis. However, the release of massive concentrations of ATP into extracellular compartments initiates acute and chronic inflammatory responses through the activation of immunocompetent cells (e.g., T cells, macrophages, and mast cells). In this review, we focus on the functions of ATP as a pathophysiologic mediator that is required for the induction and resolution of inflammation and inter-species communication.

P2X7 Receptor-Associated Programmed Cell Death in the Pathophysiology of Hemorrhagic Stroke

Hemorrhagic stroke is a life-threatening disease characterized by a sudden rupture of cerebral blood vessels, and cell death is widely believed to occur after exposure to blood metabolites or subsequently damaged cells. Recently, programmed cell death, such as apoptosis, autophagy, necroptosis, pyroptosis, and ferroptosis, has been demonstrated to play crucial roles in the pathophysiology of stroke. However, the detailed mechanisms of these novel kinds of cell death are still unclear. The P2X7 receptor, previously known for its cytotoxic activity, is an ATP-gated, nonselective cation channel that belongs to the family of ionotropic P2X receptors. Evolving evidence indicates that the P2X7 receptor plays a pivotal role in central nervous system pathology; genetic deletion and pharmacological blockade of the P2X7 receptor provide neuroprotection in various neurological disorders, including intracerebral hemorrhage and subarachnoid hemorrhage. The P2X7 receptor may regulate programmed cell death via (I) exocytosis of secretory lysosomes, (II) exocytosis of autophagosomes or autophagolysosomes during formation of the initial autophagic isolation membrane or omegasome, and (III) direct release of cytosolic IL-1β secondary to regulated cell death by pyroptosis or necroptosis. In this review, we present an overview of P2X7 receptor- associated programmed cell death for further understanding of hemorrhagic stroke pathophysiology, as well as potential therapeutic targets for its treatment.

P2X7R antagonism after subfailure overstretch injury of blood vessels reverses vasomotor dysfunction and prevents apoptosis

Human saphenous vein (HSV) is harvested and prepared prior to implantation as an arterial bypass graft. Injury and the response to injury from surgical harvest and preparation trigger cascades of molecular events and contribute to graft remodeling and intimal hyperplasia. Apoptosis is an early response after implantation that contributes the development of neointimal lesions. Here, we showed that surgical harvest and preparation of HSV leads to vasomotor dysfunction, increased apoptosis and downregulation of the phosphorylation of the anti-apoptotic protein, Niban. A model of subfailure overstretch injury in rat aorta (RA) was used to demonstrate impaired vasomotor function, increased extracellular ATP (eATP) release, and increased apoptosis following pathological vascular injury. The subfailure overstretch injury was associated with activation of p38 MAPK stress pathway and decreases in the phosphorylation of the anti-apoptotic protein Niban. Treatment of RA after overstretch injury with antagonists to purinergic P2X7 receptor (P2X7R) antagonists or P2X7R/pannexin (PanX1) complex, but not PanX1 alone, restored vasomotor function. Inhibitors to P2X7R and PanX1 reduced stretch-induced eATP release. P2X7R/PanX1 antagonism led to decrease in p38 MAPK phosphorylation, restoration of Niban phosphorylation and increases in the phosphorylation of the anti-apoptotic protein Akt in RA and reduced TNFα-stimulated caspase 3/7 activity in cultured rat vascular smooth muscle cells. In conclusion, inhibition of P2X7R after overstretch injury restored vasomotor function and inhibited apoptosis. Treatment with P2X7R/PanX1 complex inhibitors after harvest and preparation injury of blood vessels used for bypass conduits may prevent the subsequent response to injury that lead to apoptosis and represents a novel therapeutic approach to prevent graft failure.

P2X7 Receptor Induces Tumor Necrosis Factor-α Converting Enzyme Activation and Release to Boost TNF-α Production

Tumor necrosis factor (TNF)-α is a major pro-inflammatory cytokine produced in response to toll-like receptor stimulation. TNF-α release is controlled by the activity of TNF-α converting enzyme (TACE) that cut membrane-bound TNF-α to shed its ectodomain as a soluble cytokine. The purinergic receptor P2X ligand-gated ion channel 7 (P2X7) is activated in response to elevated concentrations of extracellular ATP and induces different pro-inflammatory pathways in macrophages to establish an inflammatory response. P2X7 receptor promotes the activation of the inflammasome and the release of interleukin-1β, the production of inflammatory lipids, and the generation of reactive oxygen species. In this study, we analyzed the mechanism of P2X7 receptor responsible of TNF-α release after priming macrophages with LPS doses ≤100 ng/ml. We found that P2X7 receptor increases the extracellular activity of TACE through the release of the mature form of TACE in exosomes. This effect was blocked using P2X7 receptor inhibitors or in macrophages obtained from P2X7 receptor-deficient mice. Elevation of intracellular Ca²⁺ and p38 mitogen-activated protein kinase after P2X7 receptor activation were involved in the release of TACE, which was able to process TNF-α on nearby expressing cells. Finally, we observed an increase of TNF-α in the peritoneal lavage of mice treated with LPS and ATP. In conclusion, P2X7 receptor induces the release of TACE in exosomes to the extracellular compartment that could amplify the pro-inflammatory signal associated to this receptor. These results are important for the development of therapeutics targeting P2X7 receptor.

Use of Brilliant Blue FCF during vein graft preparation inhibits intimal hyperplasia

BACKGROUND

Intimal hyperplasia remains the primary cause of vein graft failure for the 1 million yearly bypass procedures performed using human saphenous vein (HSV) grafts. This response to injury is caused in part by the harvest and preparation of the conduit. The use of Brilliant Blue FCF (FCF) restores injury-induced loss of function in vascular tissues possibly via inhibition of purinergic receptor signaling. This study investigated whether pretreatment of the vein graft with FCF prevents intimal hyperplasia.

METHODS

Cultured rat aortic smooth muscle cells (A7r5) were used to determine the effect of FCF on platelet-derived growth factor-mediated migration and proliferation, cellular processes that contribute to intimal hyperplasia. The effectiveness of FCF treatment during the time of explantation on preventing intimal hyperplasia was evaluated in a rabbit jugular-carotid interposition model and in an organ culture model using HSV.

RESULTS

FCF inhibited platelet-derived growth factor-induced migration and proliferation of A7r5 cells. Treatment with FCF at the time of vein graft explantation inhibited the subsequent development of intimal thickening in the rabbit model. Pretreatment with FCF also prevented intimal thickening of HSV in organ culture.

CONCLUSIONS

Incorporation of FCF as a component of vein graft preparation at the time of explantation represents a potential therapeutic approach to mitigate intimal hyperplasia, reduce vein graft failure, and improve outcome of the autologous transplantation of HSV.

Adenosine triphosphate as a molecular mediator of the vascular response to injury

BACKGROUND

Human saphenous veins used for arterial bypass undergo stretch injury at the time of harvest and preimplant preparation. Vascular injury promotes intimal hyperplasia, the leading cause of graft failure, but the molecular events leading to this response are largely unknown. This study investigated adenosine triphosphate (ATP) as a potential molecular mediator in the vascular response to stretch injury, and the downstream effects of the purinergic receptor, P2X7R, and p38 MAPK activation.

MATERIALS AND METHODS

A subfailure stretch rat aorta model was used to determine the effect of stretch injury on release of ATP and vasomotor responses. Stretch-injured tissues were treated with apyrase, the P2X7R antagonist, A438079, or the p38 MAPK inhibitor, SB203580, and subsequent contractile forces were measured using a muscle bath. An exogenous ATP (eATP) injury model was developed and the experiment repeated. Change in p38 MAPK phosphorylation after stretch and eATP tissue injury was determined using Western blotting. Noninjured tissue was incubated in the p38 MAPK activator, anisomycin, and subsequent contractile function and p38 MAPK phosphorylation were analyzed.

RESULTS

Stretch injury was associated with release of ATP. Contractile function was decreased in tissue subjected to subfailure stretch, eATP, and anisomycin. Contractile function was restored by apyrase, P2X7R antagonism, and p38-MAPK inhibition. Stretch, eATP, and anisomycin-injured tissue demonstrated increased phosphorylation of p38 MAPK.

CONCLUSIONS

Taken together, these data suggest that the vascular response to stretch injury is associated with release of ATP and activation of the P2X7R/P38 MAPK pathway, resulting in contractile dysfunction. Modulation of this pathway in vein grafts after harvest and before implantation may reduce the vascular response to injury.

Roles of the NLRP3 inflammasome in the pathogenesis of diabetic nephropathy

Diabetic nephropathy (DN) is a serious complication of diabetes mellitus, and persistent inflammation in circulatory and renal tissues is an important pathophysiological basis for DN. The essence of the microinflammatory state is the innate immune response, which is central to the occurrence and development of DN. Members of the inflammasome family, including both "receptors" and "regulators", are key to the inflammatory immune response. Nucleotide binding and oligomerization domain-like receptor family pyrin domain-containing 3 (NLRP3) and other inflammasome components are able to detect endogenous danger signals, resulting in activation of caspase-1 as well as interleukin (IL)-1β, IL-18 and other cytokines; these events stimulate the inflammatory cascade reaction, which is crucial for DN. Hyperglycaemia, hyperlipidaemia and hyperuricaemia can activate the NLRP3 inflammasome, which then mediates the occurrence and development of DN through the K⁺ channel model, the lysosomal damage model and the active oxygen cluster model. In this review, we survey the involvement of the NLRP3 inflammasome in various signalling pathways and highlight different aspects of their influence on DN. We also explore the important effects of the NLRP3 inflammasome on kidney function and structural changes that occur during DN development and progression. It is becoming more evident that NLRP3 inflammasome targeting has therapeutic potential for the treatment of DN.

P2X7 receptor antagonists: a patent review (2010-2015)

INTRODUCTION

The P2X7 receptor (P2X7R) is a unique subtype among the family of seven purinergic P2X receptors, which are ATP-gated non-selective cation channels. P2X7R has been reported to have pathological roles in various diseases, including autoimmune diseases such as arthritis and inflammatory bowel disease, neurodegenerative diseases, chronic pain, mood disorders and cancers. Therefore, many pharmaceutical companies have endeavored to develop a clinical candidate targeting P2X7R. Areas covered: This review provides a summary of various patents on chemicals and biologics and their clinical use published between 2010 and 2015. The reader will gain information on structural claims, representative structures and biological activities of recently developed P2X7R antagonists. Expert opinion: P2X7R is a fascinating therapeutic target and potential biomarker for inflammation, pain disorders and cancers. Research on the development of P2X7R antagonists has continually increased despite the failure of AstraZeneca and Merck's compounds in phase II clinical trials. Various scaffolds have been disclosed by several pharmaceutical industries, and some compounds are currently under investigation in clinical trials.

Subfailure Overstretch Injury Leads to Reversible Functional Impairment and Purinergic P2X7 Receptor Activation in Intact Vascular Tissue

Vascular stretch injury is associated with blunt trauma, vascular surgical procedures, and harvest of human saphenous vein for use in vascular bypass grafting. A model of subfailure overstretch in rat abdominal aorta was developed to characterize surgical vascular stretch injury. Longitudinal stretch of rat aorta was characterized ex vivo. Stretch to the haptic endpoint, where the tissues would no longer lengthen, occurred at twice the resting length. The stress produced at this length was greater than physiologic mechanical forces but well below the level of mechanical disruption. Functional responses were determined in a muscle bath, and this subfailure overstretch injury led to impaired smooth muscle function that was partially reversed by treatment with purinergic receptor (P2X7R) antagonists. These data suggest that vasomotor dysfunction caused by subfailure overstretch injury may be due to the activation of P2X7R. These studies have implications for our understanding of mechanical stretch injury of blood vessels and offer novel therapeutic opportunities.

p38δ MAPK: A Novel Regulator of NLRP3 Inflammasome Activation With Increased Expression in Coronary Atherogenesis

OBJECTIVE

Activation of the inflammasome pathway in macrophages results in the secretion of 2 potent proinflammatory and proatherogenic cytokines, interleukin (IL)-1β, and IL-18. Atherosclerotic lesions are characterized by the presence of various endogenous activators of the NLR family pyrin domain containing 3 (NLRP3) inflammasome, including cholesterol crystals and extracellular ATP. The aim of this study was to comprehensively characterize the expression of inflammasome pathway components and regulators in human atherosclerotic lesions.

APPROACH AND RESULTS

Twenty human coronary artery RNA samples from 10 explanted hearts were analyzed using an inflammasome pathway-focused quantitative polymerase chain reaction array. Advanced atherosclerotic plaques, when compared with early-to-intermediate lesions from the same coronary trees, displayed significant upregulation of 12 target genes, including the key inflammasome components apoptosis-associated speck-like protein containing a CARD domain, caspase-1, and IL-18. Immunohistochemical stainings of the advanced plaques revealed macrophage foam cells positive for NLRP3 inflammasome components around the necrotic lipid cores. The polymerase chain reaction array target p38δ mitogen-activated protein kinase was upregulated in advanced plaques and strongly expressed by lesional macrophage foam cells. In cultured human monocyte-derived macrophages, the p38δ mitogen-activated protein kinase was activated by intracellular stress signals triggered during ATP- and cholesterol crystal-induced NLRP3 inflammasome activation and was required for NLRP3-mediated IL-1β secretion.

CONCLUSIONS

Increased expression of the key inflammasome components in advanced coronary lesions implies enhanced activity of the inflammasome pathway in progression of coronary atherosclerosis. The p38δ mitogen-activated protein kinase was identified as a novel regulator of NLRP3 inflammasome activation in primary human macrophages, and thus, represents a potential target for modulation of atherosclerotic inflammation.

P2X7-pannexin-1 and amyloid β-induced oxysterol input in human retinal cell: Role in age-related macular degeneration?

Age-related macular degeneration (AMD) is the most common cause of severe vision loss worldwide. Amyloid β involvement in degenerative diseases such as AMD is well known and its toxicity has been related to P2X7 receptor-pannexin-1. Recently, oxysterols (oxidized derivatives of cholesterol) have been implicated in AMD pathogenesis. The aim of our study was to highlight amyloid β/oxysterols relationship and to describe P2X7 receptor-pannexin-1 role in oxysterols toxicity. Using retinal epithelial cells, we first quantified sterols levels after amyloid β incubation and second we investigated the cytotoxic effects induced by oxysterols. For the first time, our results showed that amyloid β induced oxysterols formation in human retinal pigmented epithelial cells. We showed that oxysterol toxicity is mediated by P2X7 receptor activation. This activation was dependent on pannexin-1 with 25-hydroxycholesterol whereas P2X7 receptor signaling pathway was pannexin-1-independent for 7-ketocholesterol. Taken together our data suggest a pivotal role of P2X7 receptor-pannexin-1 in oxysterols toxicity in retinal cells which could be an important target to develop new treatments for AMD.

P2X7 receptor antagonist activity of the anti-allergic agent oxatomide

Activation of the P2X7 receptor by extracellular ATP is associated with various immune responses including allergic inflammation. Anti-allergic agents, such as H1-antihistamines, are known to inhibit the effects of different chemical mediators such as acetylcholine and platelet-activating factor. Therefore, we hypothesized that some anti-allergic agents might affect P2X7 receptor function. Using N18TG2 and J774 cells, which express functional P2X7 receptors, the effects of several anti-allergic agents on P2X7 receptor function were investigated by monitoring the ATP-induced increase in intracellular Ca(2+) concentrations ([Ca(2+)]i). Among the various agents tested, oxatomide significantly inhibited P2X7 receptor-mediated [Ca(2+)]i elevation in a concentration-dependent manner without affecting the P2Y2 receptor-mediated response in both N18TG2 and J774 cells. Consistently, oxatomide inhibited P2X7 receptor-mediated membrane current and downstream responses such as mitogen-activated protein kinase activation, inflammation-related gene induction, and cell death. In addition, oxatomide inhibited P2X7 receptor-mediated degranulation in mouse bone marrow-derived mast cells. Whole cell patch clamp analyses in HEK293 cells expressing human, mouse, and rat P2X7 receptors revealed that the inhibitory effect of oxatomide on ATP-induced current was most prominent for the human P2X7 receptor and almost non-existent for the rat P2X7 receptor. The potent inhibitory effects of oxatomide on human P2X7 receptor-mediated function were confirmed in RPMI8226 human B cell-like myeloma cells, which endogenously express the P2X7 receptor. Our results demonstrated that the antihistamine oxatomide also acts as a P2X7 receptor antagonist. Future studies should thus evaluate whether P2X7 receptor antagonism contributes to the anti-allergic effects of oxatomide.

Inhibition of microglial activation contributes to propofol-induced protection against post-cardiac arrest brain injury in rats

It has been suggested that propofol can modulate microglial activity and hence may have potential roles against neuroinflammation following brain ischemic insult. However, whether and how propofol can inhibit post-cardiac arrest brain injury via inhibition of microglia activation remains unclear. A rat model of asphyxia cardiac arrest (CA) was created followed by cardiopulmonary resuscitation. CA induced marked microglial activation in the hippocampal CA1 region, revealed by increased OX42 and P2 class of purinoceptor 7 (P2X7R) expression, as well as p38 MAPK phosphorylation. Morris water maze showed that learning and memory deficits following CA could be inhibited or alleviated by pre-treatment with the microglial inhibitor minocycline or propofol. Microglial activation was significantly suppressed likely via the P2X7R/p-p38 pathway by propofol. Moreover, hippocampal neuronal injuries after CA were remarkably attenuated by propofol. In vitro experiment showed that propofol pre-treatment inhibited ATP-induced microglial activation and release of tumor necrosis factor-α and interleukin-1β. In addition, propofol protected neurons from injury when co-culturing with ATP-treated microglia. Our data suggest that propofol pre-treatment inhibits CA-induced microglial activation and neuronal injury in the hippocampus and ultimately improves cognitive function. We proposed a possible mechanism of propofol-mediated brain protection after cardiac arrest (CA). CA induces P2X7R upregulation and p38 phosphorylation in microglia, which induces release of TNF-α and IL-1β and consequent neuronal injury. Propofol could inhibit microglial activation and alleviate neuronal damage. Our results suggest propofol-induced anti-inflammatory treatment as a plausible strategy for therapeutic intervention in post-CA brain injury.

Selective permeabilization of cervical cancer cells to an ionic DNA-binding cytotoxin by activation of P2Y receptors

Extracellular ATP is known to permeabilize certain cell types to polyatomic cations like YO-PRO1. Here, we report that extracellularly applied ATP stimulated rapid uptake and accumulation of an otherwise weakly membrane permeable fluorescent DNA-binding cytotoxin, Hoechst 33258, into cervical cancer cells. While ATP stimulated Hoechst 33258 uptake in 20-70% of cells from seven cervical cancer cell lines, it stimulated uptake in less than 8% of cervical epithelial cells obtained from the normal transformation zone and ectocervix tissue of 11 patients. ATP-evoked Hoechst 33258 uptake was independent of ionotropic P2X receptors, but dependent on activation of P2Y receptors. Thus, we show here that cervical cancer cells can be selectively induced to take up and accumulate an ionic cytotoxin by exposure to extracellular ATP.

The Concise Guide to PHARMACOLOGY 2013/14: ligand-gated ion channels

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full.

Ligand-gated ion channels are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets.

It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

The P2X7 receptor channel: recent developments and the use of P2X7 antagonists in models of disease

The P2X7 receptor is a trimeric ATP-gated cation channel found predominantly, but not exclusively, on immune cells. P2X7 activation results in a number of downstream events, including the release of proinflammatory mediators and cell death and proliferation. As such, P2X7 plays important roles in various inflammatory, immune, neurologic and musculoskeletal disorders. This review focuses on the use of P2X7 antagonists in rodent models of neurologic disease and injury, inflammation, and musculoskeletal and other disorders. The cloning and characterization of human, rat, mouse, guinea pig, dog, and Rhesus macaque P2X7, as well as recent observations regarding the gating and permeability of P2X7, are discussed. Furthermore, this review discusses polymorphic and splice variants of P2X7, as well as the generation and use of P2X7 knockout mice. Recent evidence for emerging signaling pathways downstream of P2X7 activation and the growing list of negative and positive modulators of P2X7 activation and expression are also described. In addition, the use of P2X7 antagonists in numerous rodent models of disease is extensively summarized. Finally, the use of P2X7 antagonists in clinical trials in humans and future directions exploring P2X7 as a therapeutic target are described.

Principles and properties of ion flow in P2X receptors

P2X receptors are a family of trimeric ion channels that are gated by extracellular adenosine 5′-triphosphate (ATP). These receptors have long been a subject of intense research interest by virtue of their vital role in mediating the rapid and direct effects of extracellular ATP on membrane potential and cytosolic Ca²⁺ concentration, which in turn underpin the ability of ATP to regulate a diverse range of clinically significant physiological functions, including those associated with the cardiovascular, sensory, and immune systems. An important aspect of an ion channel's function is, of course, the means by which it transports ions across the biological membrane. A concerted effort by investigators over the last two decades has culminated in significant advances in our understanding of how P2X receptors conduct the inward flux of Na⁺ and Ca²⁺ in response to binding by ATP. However, this work has relied heavily on results from current recordings of P2X receptors altered by site-directed mutagenesis. In the absence of a 3-dimensional channel structure, this prior work provided only a vague and indirect appreciation of the relationship between structure, ion selectivity and flux. The recent publication of the crystal structures for both the closed and open channel conformations of the zebrafish P2X4 receptor has thus proved a significant boon, and has provided an important opportunity to overview the amassed functional data in the context of a working 3-dimensional model of a P2X receptor. In this paper, we will attempt to reconcile the existing functional data regarding ion permeation through P2X receptors with the available crystal structure data, highlighting areas of concordance and discordance as appropriate.

Purinergic receptor P2X₇: a novel target for anti-inflammatory therapy

Purinergic receptors, also known as purinoceptors, are ligand gated membrane ion channels involved in many cellular functions. Among all identified purinergic receptors, P2X₇ subform is unique since it induces the caspase activity, cytokine secretion, and apoptosis. The distribution of P2X₇ receptors, and the need of high concentration of ATP required to activate this receptor exhibited its ability to function as 'danger' sensor associated with tissue inflammation and damage. Further, the modulation of other signalling pathways associated with P2X₇ has also been proposed to play an important role in the control of macrophage functions and inflammatory responses, especially towards lipopolysaccharides. Experimentally, researchers have also observed the decreased severity of inflammatory responses in P2X₇ receptor expressing gene (P2RX₇) knockout (KO) phenotypes. Therefore, newly developed potent antagonists of P2X₇ receptor would serve as novel therapeutic agents to combat various inflammatory conditions. In this review article, we tried to explore various aspects of P2X₇ receptors including therapeutic potential, and recent discoveries and developments of P2X₇ receptor antagonists.

CAY10593 inhibits the human P2X7 receptor independently of phospholipase D1 stimulation

The P2X7 receptor is a trimeric ATP-gated cation channel important in health and disease. We have observed that the specific phospholipase D (PLD)1 antagonist, CAY10593 impairs P2X7-induced shedding of the 'low affinity' IgE receptor, CD23. The current study investigated the mode of action of this compound on P2X7 activation. Measurements of ATP-induced ethidium(+) uptake revealed that CAY10593 impaired P2X7-induced pore formation in human RPMI 8226 B cells, P2X7-transfected HEK-293 cells and peripheral blood mononuclear cells. Concentration response curves demonstrated that CAY10593 impaired P2X7-induced pore formation in RPMI 8226 cells more potently than the PLD2 antagonist CAY10594 and the non-specific PLD antagonist halopemide. Electrophysiology measurements demonstrated that CAY10593 also inhibited P2X7-induced inward currents. Notably, RT-PCR demonstrated that PLD1 was absent in RPMI 8226 cells, while choline-Cl medium or 1-butanol, which block PLD stimulation and signalling respectively did not impair P2X7 activation in these cells. This data indicates that CAY10593 impairs human P2X7 independently of PLD1 stimulation and highlights the importance of ensuring that compounds used in signalling studies downstream of P2X7 activation do not affect the receptor itself.

P2X7 receptor channels allow direct permeation of nanometer-sized dyes

P2X receptors are widely distributed in the nervous system, and P2X7 receptors have roles in neuropathic pain and in the release of cytokines from microglia. They are trimeric membrane proteins, which open an integral ion channel when ligated by extracellular ATP. This channel is preferentially permeable to small cations (sodium, potassium, calcium) but also allows permeation of larger cations such as N-methyl-d-glucamine. ATP also leads to entry of fluorescent dyes in many cells expressing P2X7 receptors, but controversy persists as to whether such large molecules pass directly through the open ion channel or enter the cell by a different route. We measured cellular fluorescence and membrane currents in individual human embryonic kidney cells expressing rat P2X7 receptors. Introduction of positive side chains by mutagenesis into the inner half of the pore-forming second transmembrane domain of the receptor (T348K, D352N, D352K) increased relative permeability of chloride ions. It also promoted entry of the large (>1 nm) negative dye fluorescein-5-isothiocyanate while decreasing entry of the structurally similar but positive dye ethidium. Furthermore, larger cysteine-reactive methanethiosulfonates [sulforhodamine-methanethiosulfonate and 2-((biotinoyl)amino)ethyl methanethiosulfonate] reduced both ATP-evoked currents and dye entry when applied to open P2X7[G345C] receptors. The results demonstrate that the open channel of the P2X7 receptor can allow passage of molecules with sizes up to 1.4 nm.

Permeation and block of TRPV1 channels by the cationic lidocaine derivative QX-314

QX-314 (N-ethyl-lidocaine) is a cationic lidocaine derivative that blocks voltage-dependent sodium channels when applied internally to axons or neuronal cell bodies. Coapplication of external QX-314 with the transient receptor potential vanilloid 1 protein (TRPV1) agonist capsaicin produces long-lasting sodium channel inhibition in TRPV1-expressing neurons, suggestive of QX-314 entry into the neurons. We asked whether QX-314 entry occurs directly through TRPV1 channels or through a different pathway (e.g., pannexin channels) activated downstream of TRPV1 and whether QX-314 entry requires the phenomenon of "pore dilation" previously reported for TRPV1. With external solutions containing 10 or 20 mM QX-314 as the only cation, inward currents were activated by stimulation of both heterologously expressed and native TRPV1 channels in rat dorsal root ganglion neurons. QX-314-mediated inward current did not require pore dilation, as it activated within several seconds and in parallel with Cs-mediated outward current, with a reversal potential consistent with PQX-314/PCs = 0.12. QX-314-mediated current was no different when TRPV1 channels were expressed in C6 glioma cells, which lack expression of pannexin channels. Rapid addition of QX-314 to physiological external solutions produced instant partial inhibition of inward currents carried by sodium ions, suggesting that QX-314 is a permeant blocker. Maintained coapplication of QX-314 with capsaicin produced slowly developing reduction of outward currents carried by internal Cs, consistent with intracellular accumulation of QX-314 to concentrations of 50-100 μM. We conclude that QX-314 is directly permeant in the "standard" pore formed by TRPV1 channels and does not require either pore dilation or activation of additional downstream channels for entry.

Cardiomyocyte ATP release through pannexin 1 aids in early fibroblast activation

Fibrosis following myocardial infarction is associated with increases in arrhythmias and sudden cardiac death. Initial steps in the development of fibrosis are not clear; however, it is likely that cardiac fibroblasts play an important role. In immune cells, ATP release from pannexin 1 (Panx1) channels acts as a paracrine signal initiating activation of innate immunity. ATP has been shown in noncardiac systems to initiate fibroblast activation. Therefore, we propose that ATP release through Panx1 channels and subsequent fibroblast activation in the heart drives the development of fibrosis in the heart following myocardial infarction. We identified for the first time that Panx1 is localized within sarcolemmal membranes of canine cardiac myocytes where it directly interacts with the postsynaptic density 95/Drosophila disk large/zonula occludens-1-containing scaffolding protein synapse-associated protein 97 via its carboxyl terminal domain (amino acids 300-357). Induced ischemia rapidly increased glycosylation of Panx1, resulting in increased trafficking to the plasma membrane as well as increased interaction with synapse-associated protein 97. Cellular stress enhanced ATP release from myocyte Panx1 channels, which, in turn, causes fibroblast transformation to the activated myofibroblast phenotype via activation of the MAPK and p53 pathways, both of which are involved in the development of cardiac fibrosis. ATP release through Panx1 channels in cardiac myocytes during ischemia may be an early paracrine event leading to profibrotic responses to ischemic cardiac injury.

P2X7 receptor activation induces reactive oxygen species formation in erythroid cells

The presence of P2X7 on erythroid cells is well established, but its physiological role remains unclear. The current study aimed to determine if P2X7 activation induces reactive oxygen species (ROS) formation in murine erythroleukaemia (MEL) cells, a commonly used erythroid cell line. ATP induced ROS formation in a time- and concentration-dependent fashion. The most potent P2X7 agonist, 2'(3')-O-(4-benzoylbenzoyl)ATP, but not UTP or ADP, also induced ROS formation. The P2X7 antagonist, A-438079, impaired ATP-induced ROS formation. The ROS scavenger, N-acetyl-L-cysteine, and the ROS inhibitor, diphenyleneiodonium, also impaired P2X7-induced ROS formation, but use of enzyme-specific ROS inhibitors failed to identify the intracellular source of P2X7-induced ROS formation. P2X7-induced ROS formation was impaired partly by physiological concentrations of Ca(2+) and Mg(2+) and almost completely in cells in N-methyl-D-glucamine chloride medium. The p38 MAPK inhibitors SB202190 and SB203580, and the caspase inhibitor Z-VAD-FMK, but not N-acetyl-L-cysteine, impaired P2X7-induced MEL cell apoptosis. ATP also stimulated p38 MAPK and caspase activation, both of which could be impaired by A-438079. In conclusion, these findings indicate that P2X7 activation induces ROS formation in MEL cells and that this process may be involved in events downstream of P2X7 activation, other than apoptosis, in erythroid cells.

Molecular and functional properties of P2X receptors--recent progress and persisting challenges

ATP-gated P2X receptors are trimeric ion channels that assemble as homo- or heteromers from seven cloned subunits. Transcripts and/or proteins of P2X subunits have been found in most, if not all, mammalian tissues and are being discovered in an increasing number of non-vertebrates. Both the first crystal structure of a P2X receptor and the generation of knockout (KO) mice for five of the seven cloned subtypes greatly advanced our understanding of their molecular and physiological function and their validation as drug targets. This review summarizes the current understanding of the structure and function of P2X receptors and gives an update on recent developments in the search for P2X subtype-selective ligands. It also provides an overview about the current knowledge of the regulation and modulation of P2X receptors on the cellular level and finally on their physiological roles as inferred from studies on KO mice.

P2X7 receptor-stimulation causes fever via PGE2 and IL-1β release

Prostaglandins (PGs) are important lipid mediators involved in the development of inflammatory associated pain and fever. PGE2 is a well-established endogenous pyrogen activated by proinflammatory cytokine interleukin (IL)-1β. P2X7 receptors (P2X7Rs) expressed by inflammatory cells are stimulated by the danger signal extracellular ATP to activate the inflammasome and release IL-1β. Here we show that P2X7R activation is required for the release of PGE2 and other autacoids independent of inflammasome activation, with an ATP EC(50) for PGE2 and IL-1β release of 1.58 and 1.23 mM, respectively. Furthermore, lack of P2X7R or specific antagonism of P2X7R decreased the febrile response in mice triggered after intraperitoneal LPS or IL-1β inoculation. Accordingly, LPS inoculation caused intraperitoneal ATP accumulation. Therefore, P2X7R antagonists emerge as novel therapeutics for the treatment for acute inflammation, pain and fever, with wider anti-inflammatory activity than currently used cyclooxygenase inhibitors.-Barberà-Cremades, M., Baroja-Mazo, A., Gomez, A. I., Machado, F., Di Virgilio, F., Pelegrín, P. P2X7 receptor-stimulation causes fever via PGE2 and IL-1β release.

A novel role for P2X7 receptor signalling in the survival of mouse embryonic stem cells

The growth of a pluripotent embryonic stem (ES) cell population is dependent on cell survival, proliferation and self-renewal. The nucleotide ATP represents an important extracellular signalling molecule that regulates the survival of differentiated cells, however, its role is largely undefined in embryonic stem cells. Here we report a role for ATP-gated P2X7 receptors in ES cell survival. The functional expression of P2X7 receptors in undifferentiated mouse ES cells is demonstrated using a selective P2X7 antagonist and small interfering RNA knockdown of these receptors. Our data illustrate a key role for the P2X7 receptor as an essential pro-survival signal required for optimal ES cell colony growth in the presence of leukemia inhibitor factor (LIF). However, chronic exposure to exogenous ATP leads to rapid P2X7-dependent cell death via necrosis. Together, these data demonstrate a novel role for P2X7 receptors in regulation of ES cell behaviour where they can mediate either a pro-survival or pro-death signal depending on the mode of activation.

ATP-P2X7 receptor signaling controls basal and TNFα-stimulated glial cell proliferation

Activation and proliferation of glial cells and their progenitors is a key process of neuroinflammation associated with many neurodegenerative disorders. Under neuropathological conditions where glial cell activation and proliferation is evident, controlling the population of glia might be of therapeutic importance. The proliferative action of the cytokine tumor necrosis factor alpha (TNFα) on microglia has been reported, but the molecular mechanism of TNFα regulation of glial cell proliferation is largely unknown. Using a model of organotypic hippocampal-entorhinal cortex (HEC) slice culture, we investigated the role of ATP-P2X(7) receptor signaling in glial proliferation by TNFα. Populations of proliferating cells in HEC culture were labeled with 5-bromo-2'-deoxyuridine (BrdU). Treatment with TNFα induced strong expression of P2X(7) receptor mRNA and immunoreactivity in BrdU+ cells while markedly increasing proliferation of BrdU+ cells. In addition, TNFα increased aquaporin 4 (AQP4) expression, an ion channel involved in glial proliferation. The proliferative action of TNFα was attenuated by blocking the P2X(7) receptors with the specific antagonists oxATP, BBG, and KN62, or by lowering extracellular ATP with ATP hydrolysis apyrase. Basal proliferation of BrdU+ cells was also sensitive to blockade of ATP-P2X(7) signaling. Furthermore, TNFα activation of P2X(7) receptors appear to regulate AQP4 expression through protein kinase C cascade and down regulation of AQP4 expression can reduce TNFα-stimulated BrdU+ cell proliferation. Taken together, these novel findings demonstrate the importance of ATP-P2X(7) signaling in controlling proliferation of glial progenitors under the pathological conditions associated with increased TNFα.

Lipopolysaccharide inhibits the channel activity of the P2X7 receptor

The purinergic P2X7 receptor (P2X7R) plays an important role during the immune response, participating in several events such as cytokine release, apoptosis, and necrosis. The bacterial endotoxin lipopolysaccharide (LPS) is one of the strongest stimuli of the immune response, and it has been shown that P2X7R activation can modulate LPS-induced responses. Moreover, a C-terminal binding site for LPS has been proposed. In order to evaluate if LPS can directly modulate the activity of the P2X7R, we tested several signaling pathways associated with P2X7R activation in HEK293 cells that do not express the TLR-4 receptor. We found that LPS alone was unable to induce any P2X7R-related activity, suggesting that the P2X7R is not directly activated by the endotoxin. On the other hand, preapplication of LPS inhibited ATP-induced currents, intracellular calcium increase, and ethidium bromide uptake and had no effect on ERK activation in HEK293 cells. In splenocytes-derived T-regulatory cells, in which ATP-induced apoptosis is driven by the P2X7R, LPS inhibited ATP-induced apoptosis. Altogether, these results demonstrate that LPS modulates the activity of the P2X7R and suggest that this effect could be of physiological relevance.

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.

Involvement of microglial P2X7 receptors and downstream signaling pathways in long-term potentiation of spinal nociceptive responses

Tetanic stimulation of the sciatic nerve (TSS) produces long-term potentiation (LTP) of C-fiber-evoked field potentials in the spinal cord. This potentiation is considered to be a substrate for long-lasting sensitization in the spinal pain pathway. Because microglia have previously been shown to regulate the induction of spinal LTP, we hypothesize that P2X7 receptors (P2X7R), which are predominantly expressed in microglia and participate in the communication between microglia and neurons, may play a role in this induction. This study investigated the potential roles of P2X7Rs in spinal LTP and persistent pain induced by TSS in rats. OxATP or BBG, a P2X7R antagonist, prevented the induction of spinal LTP both in vivo and in spinal cord slices in vitro and alleviated mechanical allodynia. Down-regulation of P2X7Rs with P2X7-siRNA blocked the induction of spinal LTP and inhibited mechanical allodynia. Double immunofluorescence showed colocalization of P2X7Rs with the microglial marker OX-42, but not with the astrocytic marker GFAP or the neuronal marker NeuN. Intrathecal injection of BBG suppressed the up-regulation of microglial P2X7Rs and increased expression of Fos in the spinal superficial dorsal horn. Further, pre-administration of BBG inhibited increased expression of the microglial marker Iba-1, phosphorylated p38 (p-p38), interleukin 1β (IL-1β) and GluR1 following TSS. Pre-administration of the IL-1 receptor antagonist (IL-1ra) blocked both the induction of spinal LTP and the up-regulation of GluR1. These results suggest that microglial P2X7Rs and its downstream signaling pathways play a pivotal role in the induction of spinal LTP and persistent pain induced by TSS.

Amplification loop of the inflammatory process is induced by P2X7R activation in intestinal epithelial cells in response to neutrophil transepithelial migration

Inflammatory bowel diseases (IBD) are characterized during their active phase by polymorphonuclear leukocyte (PMNL) transepithelial migration. The efflux of PMNL into the mucosa is associated with the production of proinflammatory cytokines and the release of ATP from damaged and necrotic cells. The expression and function of purinergic P2X(7) receptor (P2X(7)R) in intestinal epithelial cells (IEC) and its potential role in the "cross talk" between IEC and PMNL have not been explored. The aims of the present study were 1) to examine P2X(7)R expression in IEC (T84 cells) and in human intestinal biopsies; 2) to detect any changes in P2X(7)R expression in T84 cells during PMNL transepithelial migration, and during the active and quiescent phases of IBD; and 3) to test whether P2X(7)R stimulation in T84 monolayers can induce caspase-1 activation and IL-1beta release by IEC. We found that a functional ATP-sensitive P2X(7)R is constitutively expressed at the apical surface of IEC T84 cells. PMNL transmigration regulates dynamically P2X(7)R expression and alters its distribution from the apical to basolateral surface of IEC during the early phase of PMNL transepithelial migration in vitro. P2X(7)R expression was weak in intestinal biopsies obtained during the active phase of IBD. We show that activation of epithelial P2X(7)R is mandatory for PMNL-induced caspase-1 activation and IL-1beta release by IEC. Overall, these changes in P2X(7)R function may serve to tailor the intensity of the inflammatory response and to prevent IL-1beta overproduction and inflammatory disease.

Large-conductance channel formation mediated by P2X7 receptor activation is regulated through distinct intracellular signaling pathways in peritoneal macrophages and 2BH4 cells

The P2X(7) receptor (P2X7R) is a ligand-gated ATP receptor that acts as a low- and large-conductance channel (pore) and is known to be coupled to several downstream effectors. Recently, we demonstrated that the formation of a large-conductance channel associated with the P2X(7) receptor is induced by increasing the intracellular Ca(2+) concentration (Faria et al., Am J Physiol Cell Physiol 297:C28-C42, 2005). Here, we investigated the intracellular signaling pathways associated with P2X(7) large-conductance channel formation using the patch clamp technique in conjunction with fluorescent imaging and flow cytometry assays in 2BH4 cells and peritoneal macrophages. Different antagonists were applied to investigate the following pathways: Ca(2+)-calmodulin, phospholipase A, phospholipase D, phospholipase C, protein kinase C (PKC), mitogen-activated protein kinase (MAPK), MAPK/extracellular signal-regulated kinase, phosphoinositide 3-kinase (PI3K), and cytoskeletal proteins. Macroscopic ionic currents induced by 1 mM ATP were reduced by 85% in the presence of PKC antagonists. The addition of antagonists for MAPK, PI3K, and the cytoskeleton (actin, intermediary filament, and microtubule) blocked 92%, 83%, and 95% of the ionic currents induced by 1 mM ATP, respectively. Our results show that PKC, MAPK, PI3K, and cytoskeletal components are involved in P2X(7) receptor large-channel formation in 2BH4 cells and peritoneal macrophages.

P2X receptor ligands and pain

P2X receptors belong to a superfamily of ligand-gated ion channels that conduct the influx of Ca(2+), Na(+) and K(+) cations following activation by extracellular nucleotides such as ATP. Molecular cloning studies have identified seven subunits, namely P2X(1-7), that share approximately 40 - 50% identity in amino acid sequences within the subfamily. Using gene-silencing, pharmacological and electrophysiological approaches, recent studies have revealed roles for P2X(2), P2X(3), P2X(4) and P2X(7) receptors in nociceptive signalling. Homomeric P2X(3) and heteromeric P2X(2/3) receptors are highly localised in the peripheral sensory afferent neurons that conduct nociceptive sensory information to the spinal chord and brain. The discovery of A-317491, a selective and potent non-nucleotide P2X(3) antagonist, provided a pharmacological tool to determine the site and mode of action of P2X(3)-containing receptors in different pain behaviours, including neuropathic, inflammatory and visceral pain. Other P2X receptors (P2X(4) and P2X(7)) that are predominantly expressed in microglia, macrophages and cells of immune origin can trigger the release of cytokines, such as IL-1-beta and TNF-alpha. Genetic disruption of P2X(4) and P2X(7) signalling has been demonstrated to reduce inflammatory and neuropathic pain, suggesting that these two receptors might serve as integrators of neuroinflammation and pain. This article provides an overview of recent scientific literature and patents focusing on P2X(3), P2X(4) and P2X(7) receptors, and the identification of small molecule ligands for the potential treatment of neuropathic and inflammatory pain.

Dynamic changes in the TRPA1 selectivity filter lead to progressive but reversible pore dilation

TRPA1 is a nonselective cation channel belonging to the transient receptor potential (TRP) family that is expressed in peripheral sensory neurons and may play important roles in pain perception and inflammation. We found that agonist stimulation of TRPA1, along with other members of the TRP family (TRPV1-4 and TRPM8), can induce the appearance of a large pore permeable to large organic cations such as Yo-Pro (YP) and N-methyl-d-glucamine, in an agonist and divalent cation-dependent manner. YP uptake was not inhibited by a panel of putative gap junction/pannexin blockers, suggesting that gap junction proteins are not required in this process. Our data suggest that changes in the TRP channel selectivity filter itself result in a progressive but reversible pore dilation process, a process that is under strong regulation by external calcium ions. Our data suggest that calcium plays a novel role in setting the amount of time TRPA1 channels spend in a dilated state providing a mechanism that may limit sensory neuron activation by painful or irritating substances.

Corneal protection with high-molecular-weight hyaluronan against in vitro and in vivo sodium lauryl sulfate-induced toxic effects

PURPOSE

The aim of this study was to investigate high-molecular-weight hyaluronan (HA-HMW) corneal protection against sodium lauryl sulfate (SLS)-induced toxic effects with in vitro and in vivo experimental approaches.

METHODS

In vitro experiments consisted of a human corneal epithelial cell line incubated with HA-HMW, rinsed, and incubated with SLS. Cell viability, oxidative stress, chromatin condensation, caspase-3, -8, -9, and P2X7 cell death receptor activation, interleukin-6, and interleukin-8 production were investigated. In vivo experiments consisted of 36 New Zealand white rabbits treated for 3 days, 3 times per day, with HA-HMW or phosphate-buffered salt solution. At day 4, eyes were treated with SLS. Clinical observation and in vivo confocal microscopy using the Rostock Cornea Module of the Heidelberg Retina Tomograph-II were performed to evaluate and to compare SLS-induced toxicity between eyes treated with HA-HMW and eyes treated with phosphate-buffered salt solution.

RESULTS

In vitro data indicate that exposure of human corneal epithelial cells to HA-HMW significantly decreased SLS-induced oxidative stress, apoptosis, and inflammation cytokine production. In vivo data indicate that SLS cornea injuries, characterized by damaged corneal epithelium, damaged anterior stroma, and inflammatory infiltrations, were attenuated with HA-HMW treatment.

CONCLUSIONS

A good correlation was seen between in vitro and in vivo findings showing that HA-HMW decreases SLS-induced toxic effects and protects cornea.