An Ile-568 to Asn polymorphism prevents normal trafficking and function of the human P2X7 receptor

Association of P2X7 receptor polymorphisms with bone mineral density and osteoporosis risk in a cohort of Dutch fracture patients

The P2X7 receptor is thought to be involved in bone physiology in a pro-osteogenic manner. Therefore, we examined associations between genetic variations in the P2X7 receptor gene and bone mineral density (BMD). We found an association between four non-synonymous polymorphism of the human P2X7 receptor and the risk of osteoporosis.

INTRODUCTION

The purpose of this study was to determine whether genetic variation in the P2X7 receptor gene (P2RX7) is associated with decreased BMD and risk of osteoporosis in fracture patients.

METHODS

Six hundred ninety women and 231 men aged≥50 years were genotyped for 15 non-synonymous P2RX7 SNPs. BMD was measured at the total hip, lumbar spine and femoral neck.

RESULTS

Four non-synonymous SNPs were associated with BMD. The Ala348Thr gain-of-function polymorphism was associated with increased BMD values at the lumbar spine (p=0.012). Decreased hip BMD values were associated with two loss-of-function SNPs in the P2RX7, i.e., in subjects homozygous for the Glu496Ala polymorphism as well as in subjects carrying at least one variant allele of the Gly150Arg polymorphism (p=0.018 and p=0.011; respectively). In men, we showed that subjects either heterozygous or homozygous for the Gln460Arg gain-of-function polymorphism in the P2RX7 had a significantly 40% decrease in risk of a lower T-score value (OR=0.58 [95%CI, 0.33-1.00]).

CONCLUSION

Thus, genetic aberrations of P2X7R function are associated with lower BMD and increased osteoporosis risk. Therefore, detection of non-synonymous SNPs within the P2RX7 might be useful for osteoporosis risk estimation at an early stage, potentially enabling better osteoporosis prevention and treatment.

Functional polymorphisms in the P2X7 receptor gene are associated with osteoporosis

The P2X(7) receptor is an ATP-gated cation channel. We investigated the effect of both loss-of-function and gain-of-function polymorphisms in the P2X(7) receptor gene on BMD and risk of vertebral fractures and found that five polymorphisms and haplotypes containing three of these polymorphisms were associated with BMD and fracture risk.

INTRODUCTION

The P2X(7) receptor is an ATP-gated cation channel. P2X(7) receptor knockout mice have reduced total bone mineral content, and because several functional polymorphisms have been identified in the human P2X(7) receptor gene, we wanted to investigate the effect of these polymorphisms on BMD and risk of vertebral fractures in a case-control study including 798 individuals.

METHODS

Genotyping was carried out using TaqMan assays. BMD was measured using dual energy X-ray absorptiometry, and vertebral fractures were assessed by lateral spinal X-rays.

RESULTS

The rare allele of a splice site polymorphism, 151 + 1: G-T, was associated with increased fracture risk and reduced BMD in women. Two other loss-of-function polymorphisms, Glu496Ala and Gly150Arg, were also associated with BMD. The Glu496Ala variant allele was associated with decreased lumbar spine BMD in women and decreased total hip BMD in men. The 150Arg allele was associated with decreased total hip BMD in women and men combined. The minor allele of the gain-of-function polymorphism, Ala348Thr, was associated with reduced fracture risk and increased BMD at all sites in men. The Gln460Arg variant allele, which has been associated with increased receptor function in monocytes, was associated with increased total hip BMD in women. With the exception of His155Tyr for which we found conflicting results in men and women, our results are consistent with the phenotype of the knockout mouse. Analysis of a haplotype containing Ala348Thr, Gln460Arg, and Glu496Ala showed that the effects of the haplotypes on BMD and fracture were driven by Ala348Thr in men and by Gln460Arg and Glu496Ala in women.

CONCLUSION

In conclusion, we found that functional polymorphisms in the P2X(7) receptor gene and haplotypes containing three of these polymorphisms are associated with osteoporosis.

P2X7-regulated protection from exacerbations and loss of control is independent of asthma maintenance therapy

RATIONALE

The function of the P2X(7) nucleotide receptor protects against exacerbation in people with mild-intermittent asthma during viral illnesses, but the impact of disease severity and maintenance therapy has not been studied.

OBJECTIVES

To evaluate the association between P2X(7), asthma exacerbations, and incomplete symptom control in a more diverse population.

METHODS

A matched P2RX7 genetic case-control was performed with samples from Asthma Clinical Research Network trial participants enrolled before July 2006, and P2X(7) pore activity was determined in whole blood samples as an ancillary study to two trials completed subsequently.

MEASUREMENTS AND MAIN RESULTS

A total of 187 exacerbations were studied in 742 subjects, and the change in asthma symptom burden was studied in an additional 110 subjects during a trial of inhaled corticosteroids (ICS) dose optimization. African American carriers of the minor G allele of the rs2230911 loss-of-function single nucleotide polymorphism were more likely to have a history of prednisone use in the previous 12 months, with adjustment for ICS and long-acting β(2)-agonists use (odds ratio, 2.7; 95% confidence interval, 1.2-6.2; P = 0.018). Despite medium-dose ICS, attenuated pore function predicted earlier exacerbations in incompletely controlled patients with moderate asthma (hazard ratio, 3.2; confidence interval, 1.1-9.3; P = 0.033). After establishing control with low-dose ICS in patients with mild asthma, those with attenuated pore function had more asthma symptoms, rescue albuterol use, and FEV(1) reversal (P < 0.001, 0.03, and 0.03, respectively) during the ICS adjustment phase.

CONCLUSIONS

P2X(7) pore function protects against exacerbations of asthma and loss of control, independent of baseline severity and the maintenance therapy.

The specificity protein factor Sp1 mediates transcriptional regulation of P2X7 receptors in the nervous system

P2X7 receptors are involved not only in physiological functions but also in pathological brain processes. Although an increasing number of findings indicate that altered receptor expression has a causative role in neurodegenerative diseases and cancer, little is known about how expression of P2rx7 gene is controlled. Here we reported the first molecular and functional evidence that Specificity protein 1 (Sp1) transcription factor plays a pivotal role in the transcriptional regulation of P2X7 receptor. We delimited a minimal region in the murine P2rx7 promoter containing four SP1 sites, two of them being highly conserved in mammals. The functionality of these SP1 sites was confirmed by site-directed mutagenesis and Sp1 overexpression/down-regulation in neuroblastoma cells. Inhibition of Sp1-mediated transcriptional activation by mithramycin A reduced endogenous P2X7 receptor levels in primary cultures of cortical neurons and astrocytes. Using P2rx7-EGFP transgenic mice that express enhanced green fluorescent protein under the control of P2rx7 promoter, we found a high correlation between reporter expression and Sp1 levels in the brain, demonstrating that Sp1 is a key element in the transcriptional regulation of P2X7 receptor in the nervous system. Finally, we found that Sp1 mediates P2X7 receptor up-regulation in neuroblastoma cells cultured in the absence of serum, a condition that enhances chromatin accessibility and facilitates the exposure of SP1 binding sites.

Expression, assembly and function of novel C-terminal truncated variants of the mouse P2X7 receptor: re-evaluation of P2X7 knockouts

BACKGROUND AND PURPOSE

Splice variants of P2X7 receptor transcripts contribute to the diversity of receptor-mediated responses. Here, we investigated expression and function of C-terminal truncated (ΔC) variants of the mP2X7 receptor, which are predicted to escape inactivation in one strain of P2X7(-/-) mice (Pfizer KO).

EXPERIMENTAL APPROACH

Expression in wild-type (WT) and Pfizer KO tissue was investigated by reverse transcription (RT)-PCR and Western blot analysis. ΔC variants were also cloned and expressed in HEK293 cells to investigate their assembly, trafficking and function.

KEY RESULTS

RT-PCR indicates expression of a ΔC splice variant in brain, salivary gland (SG) and spleen from WT and Pfizer KO mice. An additional ΔC hybrid transcript, containing sequences of P2X7 upstream of exon 12, part of exon 13 followed in-frame by the sequence of the vector used to disrupt the P2X7 gene, was also identified in the KO mice. By blue native (BN) PAGE analysis and the use of cross linking reagents followed by SDS-PAGE, P2X7 trimers, dimers and monomers were detected in the spleen and SG of Pfizer KO mice. The molecular mass was reduced compared with P2X7 in WT mice tissue, consistent with a ΔC variant. When expressed in HEK293 cells the ΔC variants were inefficiently trafficked to the cell surface and agonist-evoked whole cell currents were small. Co-expressed with P2X7A, the ΔC splice variant acted in a dominant negative fashion to inhibit function.

CONCLUSIONS AND IMPLICATIONS

Pfizer KO mice are not null for P2X7 receptor expression but express ΔC variants with reduced function.

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.

Single-nucleotide polymorphisms in the P2X7 receptor gene are associated with post-menopausal bone loss and vertebral fractures

The purinergic P2X7 receptor has a major role in the regulation of osteoblast and osteoclast activity and changes in receptor function may therefore affect bone mass in vivo. The aim of this study was to determine the association of non-synonymous single-nucleotide polymorphisms in the P2RX7 gene to bone mass and fracture incidence in post-menopausal women. A total of 1694 women (aged 45-58) participating in the Danish Osteoporosis Prevention Study were genotyped for 12 functional P2X7 receptor variants. Bone mineral density was determined at baseline and after 10 years. In addition, vertebral fracture incidence was documented at 10 years. We found that the rate of bone loss was clearly associated with the Arg307Gln amino acid substitution such that individuals heterozygous for this polymorphism had a 40% increased rate of bone loss. Furthermore, individuals carrying the Ile568Asn variant allele had increased bone loss. In contrast, the Gln460Arg polymorphism was associated with protection against bone loss. The Ala348Thr polymorphism was associated with a lower vertebral fracture incidence 10 years after menopause. Finally, we developed a risk model, which integrated P2RX7 genotypes. Using this model, we found a clear association between the low-risk (high-P2X7 function) alleles and low rate of bone loss. Conversely, high-risk (reduced P2X7 function) alleles were associated with a high rate of bone loss. In conclusion, an association was demonstrated between variants that reduce P2X7 receptor function and increased rate of bone loss. These data support that the P2X7 receptor is important in regulation of bone mass.

Polymorphisms in the P2X7 receptor gene are associated with low lumbar spine bone mineral density and accelerated bone loss in post-menopausal women

The P2X7 receptor gene (P2RX7) is highly polymorphic with five previously described loss-of-function (LOF) single-nucleotide polymorphisms (SNP; c.151+1G>T, c.946G>A, c.1096C>G, c.1513A>C and c.1729T>A) and one gain-of-function SNP (c.489C>T). The purpose of this study was to determine whether the functional P2RX7 SNPs are associated with lumbar spine (LS) bone mineral density (BMD), a key determinant of vertebral fracture risk, in post-menopausal women. We genotyped 506 post-menopausal women from the Aberdeen Prospective Osteoporosis Screening Study (APOSS) for the above SNPs. Lumbar spine BMD was measured at baseline and at 6–7 year follow-up. P2RX7 genotyping was performed by homogeneous mass extension. We found association of c.946A (p.Arg307Gln) with lower LS-BMD at baseline (P=0.004, β=−0.12) and follow-up (P=0.002, β=−0.13). Further analysis showed that a combined group of subjects who had LOF SNPs (n=48) had nearly ninefold greater annualised percent change in LS-BMD than subjects who were wild type at the six SNP positions (n=84; rate of loss=−0.94%/year and −0.11%/year, respectively, P=0.0005, unpaired t-test). This is the first report that describes association of the c.946A (p.Arg307Gln) LOF SNP with low LS-BMD, and that other LOF SNPs, which result in reduced or no function of the P2X7 receptor, may contribute to accelerated bone loss. Certain polymorphic variants of P2RX7 may identify women at greater risk of developing osteoporosis.

The role of the P2X₇ receptor in infectious diseases

ATP is an extracellular signal for the immune system, particularly during an inflammatory response. It is sensed by the P2X₇ receptor, the expression of which is upregulated by pro-inflammatory cytokines. Activation of the P2X₇ receptor opens a cation-specific channel that alters the ionic environment of the cell, activating several pathways, including (i) the inflammasome, leading to production of IL-1β and IL-18; (ii) the stress-activated protein kinase pathway, resulting in apoptosis; (iii) the mitogen-activated protein kinase pathway, leading to generation of reactive oxygen and nitrogen intermediates; and (iv) phospholipase D, stimulating phagosome-lysosome fusion. The P2X₇ receptor can initiate host mechanisms to remove pathogens, most particularly those that parasitise macrophages. At the same time, the P2X₇ receptor may be subverted by pathogens to modulate host responses. Moreover, recent genetic studies have demonstrated significant associations between susceptibility or resistance to parasites and bacteria, and loss-of-function or gain-of-function polymorphisms in the P2X₇ receptor, underscoring its importance in infectious disease.

P2X(7) is a scavenger receptor for apoptotic cells in the absence of its ligand, extracellular ATP

Phagocytosis of apoptotic cells is essential during development and tissue remodeling. Our previous study has shown that the P2X(7) receptor regulates phagocytosis of nonopsonized particles and bacteria. In this study, we demonstrate that P2X(7) also mediates phagocytosis of apoptotic lymphocytes and neuronal cells by human monocyte-derived macrophages under serum-free conditions. ATP inhibited this process to a similar extent as observed with cytochalasin D. P2X(7)-transfected HEK-293 cells acquired the ability to phagocytose apoptotic lymphocytes. Injection of apoptotic thymocytes into the peritoneal cavity of wild-type mice resulted in their phagocytosis by macrophages, but injection of ATP prior to thymocytes markedly decreased this uptake. In contrast, ATP failed to inhibit phagocytosis of apoptotic thymocytes in vivo by P2X(7)-deficient peritoneal macrophages. The surface expression of P2X(7) on phagocytes increased significantly during phagocytosis of either beads or apoptotic cells. A peptide screen library containing 24 biotin-conjugated peptides mimicking the extracellular domain of P2X(7) was used to evaluate the binding profile to beads, bacteria, and apoptotic cells. One peptide showed binding to all particles and cell membrane lipids. Three other cysteine-containing peptides uniquely bound the surface of apoptotic cells but not viable cells, whereas substitution of alanine for cysteine abolished peptide binding. Several thiol-reactive compounds including N-acetyl-L-cysteine abolished phagocytosis of apoptotic SH-SY5Y cells by macrophages. These data suggest that the P2X(7) receptor in its unactivated state acts like a scavenger receptor, and its extracellular disulphide bonds play an important role in direct recognition and engulfment of apoptotic cells.

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.

P2X7 receptor gene polymorphism analysis in rheumatoid arthritis

The P2X7 receptor, a member of the P2X family of nucleotide-gated channels, is predominantly expressed by monocytic cells. The activation of this receptor has been associated with downstream-signalling cascades, resulting in the release of a number of inflammatory mediators. There are more than 815 single nucleotide polymorphisms (SNPs) that have been described in the human P2X7R gene, but only few have been functionally characterized. The main aim of this study is to determine whether P2X7R gene polymorphisms confer susceptibility to rheumatoid arthritis (RA). A total of 125 patients with RA and 158 healthy volunteers were enrolled in this study. DNA fragment was PCR amplified and sequenced on the AB 3130 Genetic Analyzer. No significant difference in allele frequencies of 489 C→T, 1096 C→G and 1513 A→C polymorphisms, among sporadic cases of RA and healthy controls was found. However, the 1513A/C genotype was significantly associated with the presence of rheumatoid factor and anti-MCV autoantibody in RA patients. Interestingly, the genotype frequency of 1068 A/A was 0.19 in the RA group and 0.09 in control group (P = 0.025). Consequently, this polymorphism (AA) is two folds greater in the RA group compared to controls. Moreover, this polymorphism was significantly associated with mean concentration of C-reactive protein in RA patients. In contrast, 946G→A and 1729 T→A were not detected in both groups. As a result, these two polymorphisms are uncommon in Omani Arab population. Polymorphism at position 1068 and 1513 in the P2X7R gene might contribute to the pathogenesis of RA. Moreover, the loss-of-function SNP at position 1096 C→G or the gain-of-function SNP at position 489 C→T of the P2X7 gene does not appear to be a susceptibility gene locus for the development of RA. Further studies are required to confirm this finding.

P2X7 receptor positively regulates MyD88-dependent NF-κB activation

Recent studies have demonstrated that P2X7 plays a critical role in the immune system. Here, our results showed that P2X7 activated a NF-κB - but not an IFN-β-dependent luciferase reporter gene in HEK293T cells. P2X7 was involved in the LPS- and ATP-induced NF-κB activation but did not significantly impact the response to Zymosan in RAW264.7 cells. The activation of NF-κB and IFN-β induced by myeloid differentiation primary-response protein 88 (MyD88) was enhanced by P2X7 co-expression. The siRNA silencing MyD88 almost abolished the NF-κB activation induced by P2X7, and co-immunoprecipitation showed that P2X7 interacted with MyD88. The amino acids in the C-terminus, especially the LPS-binding region of P2X7, were critical for the cellular localization and immune function of P2X7. P2X7ΔC (190 amino acids deleted in the C-terminus) and P2X7 G586A variants localized throughout the cytoplasma with a little aggregation, which differs from the cell membrane localization of wild type P2X7. Both of them could not localize to Golgi or endoplasmic reticulum. P2X7ΔC and P2X7 G586A had impaired proteolytic cleavage of caspase-1 into the functional p20 subunit, which can activate pro-inflammatory cytokines such as IL-1β. P2X7 G586A also showed a slight interaction with MyD88 in our co-immunoprecipitation experiment. This interaction might result in the attenuated activation of NF-κB and IFN-β induced by MyD88.

Transcriptional control mechanisms associated with the nucleotide receptor P2X7, a critical regulator of immunologic, osteogenic, and neurologic functions

The nucleotide receptor P2X(7) is an attractive therapeutic target and potential biomarker for multiple inflammatory and neurologic disorders, and it is expressed in several immune, osteogenic, and neurologic cell types. Aside from its role in the nervous system, it is activated by ATP released at sites of tissue damage, inflammation, and infection. Ligand binding to P2X(7) stimulates many cell responses, including calcium fluxes, MAPK activation, inflammatory mediator release, and apoptosis. Much work has centered on P2X(7) action in cell death and mediator processing (e.g., pro-interleukin-1 cleavage by the inflammasome), but the contribution of P2X(7) to transcriptional regulation is less well defined. This review will focus on the growing evidence for the importance of nucleotide-mediated gene expression, highlight several animal models, human genetic, and clinical studies that support P2X(7) as a therapeutic target, and discuss the latest developments in anti-P2X(7) clinical trials.

Targeting P2X₇ receptor inhibits the metastasis of murine P388D1 lymphoid neoplasm cells to lymph nodes

The P2X₇R (P2X₇ receptor) is an ATP-gated cation channel expressed in normal cells that participates in both cell proliferation and apoptosis. Here, we have confirmed P2X₇R expression on murine P388D1 lymphoid neoplasm cells. In addition, ATP-stimulated P2X₇R expression was found to trigger increased intracellular calcium flux. Furthermore, silencing with short hairpin RNA and blocking with P2X₇R antibody significantly reduced the metastasis of P388D1 cells to lymph nodes. These results indicate that inhibition of the expression and function of P2X₇R attenuates the metastatic ability of murine lymphoid neoplasm cell line P388D1, which represents a new potential target for anti-metastatic therapy.

C terminus of the P2X7 receptor: treasure hunting

P2X receptor (P2XR) is a family of the ATP-gated ion channel family and can permeabilize the plasma membrane to small cations such as potassium, sodium, and calcium, resulting in cellular depolarization. There are seven P2XR that have been described and cloned, with 45% identity in amino acid sequence. Each P2X receptors has two transmembrane domains that are separated by an extracellular loop and an intracellular N and C terminus. Unlike the other P2X receptors, the P2X7R has a larger C terminus with an extra 200 amino acid residues compared with the other receptors. The C terminus of the P2X7R has been implicated in regulating receptor function including signaling pathway activation, cellular localization, protein-protein interactions, and post-translational modification (PTM). In the present review, we discuss the role of the P2X7R C terminus in regards to receptor function, describe the specific domains and motifs found therein and compare the C terminus sequence with others proteins to discover predicted domains or sites of PTM.

Residues 155 and 348 contribute to the determination of P2X7 receptor function via distinct mechanisms revealed by single-nucleotide polymorphisms

P2X(7) receptors are important in mediating the physiological functions of extracellular ATP, and altered receptor expression and function have a causative role in the disease pathogenesis. Here, we investigated the mechanisms determining the P2X(7) receptor function by following two human single-nucleotide polymorphism (SNP) mutations that replace His-155 and Ala-348 in the human (h) P2X(7) receptor with the corresponding residues, Tyr-155 and Thr-348, in the rat (r) P2X(7) receptor. H155Y and A348T mutations in the hP2X(7) receptor increased ATP-induced currents, whereas the reciprocal mutations, Y155H and T348A, in the rP2X(7) receptor caused the opposite effects. Such a functional switch is a compelling indication that these residues are critical for P2X(7) receptor function. Additional mutations of His-155 and Ala-348 in the hP2X(7) receptor to residues with diverse side chains revealed a different dependence on the side chain properties, supporting the specificity of these two residues. Substitutions of the residues surrounding His-155 and Ala-348 in the hP2X(7) receptor with the equivalent ones in the rP2X(7) receptor also affected ATP-induced currents but were not fully reminiscent of the H155Y and A348T effects. Immunofluorescence imaging and biotin labeling assays showed that H155Y in the hP2X(7) receptor increased and Y155H in the rP2X(7) receptor decreased cell-surface expression. Such contrasting effects were not obvious with the reciprocal mutations of residue 348. Taken together, our results suggest that residues at positions 155 and 348 contribute to P2X(7) receptor function via determining the surface expression and the single-channel function, respectively. Such interpretations are consistent with the locations of the residues in the structural model of the hP2X(7) receptor.

The hyposensitive N187D P2X7 mutant promotes malignant progression in nude mice

Nucleotides are new players in the intercellular communication network. P2X7 is a member of the P2X family of receptors, which are ATP-gated plasma membrane ion channels with diverse biological functions. Abnormal expression and dysfunction of P2X7 have been reported in leukemias. Here, we report a new P2X7 mutant (an A(559)-to-G substitution causing N187D P2X7) cloned from J6-1 leukemia cells. The characteristics of N187D P2X7 were studied by establishing stably transfected K562 cell lines. Our results show that N187D P2X7 required a higher concentration of agonist for its activation, leading to Ca(2+) influx (EC(50) = 293.3 ± 6.6 μm for the mutant and 93.6 ± 2.2 μm for wild-type P2X7) and ERK phosphorylation, which were not caused by differential cell-surface expression or related to high ATPase activity on the cell surface and in the extracellular space. K562 cells expressing this N187D mutant showed a proliferative advantage and reduced pro-apoptosis effects in vitro and in vivo. Furthermore, elevated angiogenesis and CD206-positive macrophage infiltration were found in tumor tissues formed by K562-M cells. In addition, higher expression of VEGF and MCP1 could be detected in tumor tissues formed by K562-M cells. Our results suggest that N187D P2X7, representing mutants hyposensitive to agonist, might be a positive regulator in the progression of hematopoietic malignancies.

Evidence for associations between the purinergic receptor P2X(7) (P2RX7) and toxoplasmosis

Congenital Toxoplasma gondii infection can result in intracranial calcification, hydrocephalus, and retinochoroiditis. Acquired infection is commonly associated with ocular disease. Pathology is characterized by strong pro-inflammatory responses. Ligation of ATP by purinergic receptor P2X₇, encoded by P2RX7, stimulates pro-inflammatory cytokines and can lead directly to killing of intracellular pathogens. To determine whether P2X₇ plays a role in susceptibility to congenital toxoplasmosis, we examined polymorphisms at P2RX7 in 149 child/parent trios from North America. We found association (FBAT Z scores ±2.429; P= 0.015) between the derived C(+)G(−) allele (f= 0.68; OR= 2.06; 95% CI: 1.14–3.75) at SNP rs1718119 (1068T>C; Thr-348-Ala), and a second synonymous variant rs1621388 in linkage disequilibrium with it, and clinical signs of disease per se. Analysis of clinical sub-groups showed no association with hydrocephalus, with effect sizes for associations with retinal disease and brain calcifications enhanced (OR=3.0 to 4.25; 0.004<P<0.009) when hydrocephalus was removed from the analysis. Association with toxoplasmic retinochoroiditis was replicated (FBAT Z scores ±3.089; P= 0.002) in a small family-based study (60 families; 68 affected offspring) of acquired infection in Brazil, where the ancestral T(+) allele (f= 0.296) at SNP rs1718119 was strongly protective (OR= 0.27; 95% CI: 0.09–0.80). (Words 194)

Expression and function of the P2X(7) purinergic receptor in patients with systemic lupus erythematosus and rheumatoid arthritis

Because the synthesis of pro-inflammatory cytokines and apoptosis of lymphoid cells can be induced through P2X(7), we decided to study its expression, function (apoptosis, shedding of CD62L and synthesis of IL-1beta induced by ATP) and genetic polymorphisms (1513 AC and -762 T/C) in peripheral blood mononuclear cells from 101 patients with systemic lupus erythematosus (SLE), 122 with rheumatoid arthritis (RA) and 90 healthy controls. We found no significant differences in the distribution of 1513 and -762 genotypes of P2X(7) gene in SLE or RA patients compared with healthy controls. However, a diminished induction of apoptosis of CD4(+) T lymphocytes and monocytes was observed in SLE patients with the 1513 AC genotype, and the release of IL-1beta upon stimulation with ATP was significantly decreased in SLE patients. In contrast, in RA patients we detected that the release of IL-1beta was increased. In addition, in patients with SLE and RA the SNPs 1513 AC was associated with a low expression of P2X(7). These results suggest a possible involvement of P2X(7) in the pathogenesis of inflammatory autoimmune diseases.

Two haplotypes of the P2X(7) receptor containing the Ala-348 to Thr polymorphism exhibit a gain-of-function effect and enhanced interleukin-1beta secretion

The P2X(7) receptor is an ATP-gated cation channel expressed in immune cells and plays a role in proinflammatory cytokine release from monocytes and macrophages. This study investigated the coinheritance of 12 functionally relevant single nucleotide polymorphisms (SNPs) in the human P2X(7) gene (P2RX7), and the functional effect of each singly and in combination was assessed by measurements of ATP-induced currents and ethidium(+) uptake. Genotyping of 3430 Caucasian subjects identified 4 common haplotypes in addition to the common (wild-type) P2X(7)-1. Two haplotypes (denoted P2X(7)-2 and P2X(7)-4) contained various combinations of gain-of-function SNPs. P2X(7)-4 was identified uniquely by the Gln-460 to Arg polymorphism (rs2230912). When expressed in HEK-293 cells, recombinant P2X(7)-2, and P2X(7)-4 haplotypes displayed a 3-fold and 5-fold increase, respectively, in receptor function compared to the wild-type P2X(7)-1. Both P2X(7) haplotypes contained the Ala-348>Thr polymorphism (rs1718119), and this mutation was critical for the gain-of-function effect. Peripheral blood monocytes and erythrocytes from subjects homozygous for gain-of-function P2X(7) haplotypes exhibited increased ATP-induced ethidium(+) uptake and (86)Rb(+) efflux, respectively, and this correlated with increased IL-1beta secretion from LPS-primed monocytes. Inheritance of these P2X(7) haplotypes predisposing to increased proinflammatory cytokine secretion may be important in genetic association studies of inflammatory, infectious, and psychiatric disorders.

Single nucleotide polymorphisms that were identified in affective mood disorders affect ATP-activated P2X7 receptor functions

Genetic linkage studies have previously identified many single non-synonymous nucleotide polymorphisms (SNPs) in the human P2RX7 gene in individuals with affective mood disorders. The P2RX7 gene encodes the P2X(7) receptor (P2X(7)R) that operates as an ATP-activated Ca(2+)-permeable cationic channel and induces formation of a large pore, the two functional properties that are critical for the physiological and pathological roles of the receptor. The current knowledge regarding the effects of SNPs on the P2X(7)R functional properties, which is indispensable to help elucidate the disease mechanism, is limited. In this study, we introduced by site-directed mutagenesis twelve SNP mutations in the human P2X(7) receptor that were previously identified in or associated with affective mood disorders, expressed the resultant mutants in human embryonic kidney cells, and characterized their functional properties by electrophysiology. All mutations except Q460R gave rise to profound effects on the P2X(7)R function. G150R, E186K and I568N conferred complete loss of function. V76A, R117W, L191P, T357S and E496A resulted in strong impairment of, whereas H155Y and A348T caused significant increase in, both ATP-activated ion channel function and pore formation. Q521H reduced the receptor's sensitivity to extracellular Ca(2+) inhibition. An atomic structure model of the human P2X(7)R, based on the crystal structure of the zebrafish P2X(4) receptor, suggests that the SNP mutational effects may result from changes in subunit interaction, agonist binding and/or channel gating. These results provide essential knowledge for a better understanding of the relationships between human P2RX7 SNPs and associated pathologies as well as the receptor structure-function relationships.

The P2X7-nonmuscle myosin membrane complex regulates phagocytosis of nonopsonized particles and bacteria by a pathway attenuated by extracellular ATP

Phagocytosis of nonopsonized bacteria is central to innate immunity, but its regulation is less defined. We show that overexpression of the P2X(7) receptor greatly augments the phagocytosis of nonopsonized beads and heat-killed bacteria by transfected HEK-293 cells, whereas blocking P2X(7) expression by siRNA significantly reduces the phagocytic ability of human monocytic cells. An intact P2X(7)-nonmuscle myosin complex is required for phagocytosis of nonopsonized beads because activation of P2X(7) receptors by adenosine triphosphate (ATP), which dissociates myosin IIA from the P2X(7) complex, inhibits this phagocytic pathway. Fresh human monocytes rapidly phagocytosed live and heat-killed Staphylococcus aureus and Escherichia coli in the absence of serum, but the uptake was reduced by prior incubation with ATP, or P2X(7) monoclonal antibody, or recombinant P2X(7) extracellular domain. Injection of beads or bacteria into the peritoneal cavity of mice resulted in their brisk phagocytosis by macrophages, but injection of ATP before particles markedly decreased this uptake. These data demonstrate a novel pathway of phagocytosis of nonopsonized particles and bacteria, which operate in vivo and require an intact P2X(7)-nonmuscle myosin IIA membrane complex. The inhibitory effect of ATP on particle uptake by the macrophage is regulated by the P2X(7) receptor and defines this phagocytic pathway.

Identification and characterization of a novel variant of the human P2X(7) receptor resulting in gain of function

The P2X(7) receptor exhibits significant allelic polymorphism in humans, with both loss and gain of function variants potentially impacting on a variety of infectious and inflammatory disorders. At least five loss-of-function polymorphisms (G150R, R307Q, T357S, E496A, and I568N) and two gain-of-function polymorphisms (H155Y and Q460R) have been identified and characterized to date. In this study, we used RT-PCR cloning to isolate and characterize P2X(7) cDNA clones from human PBMCs and THP-1 cells. A previously unreported variant with substitutions of V80M and A166G was identified. When expressed in HEK293 cells, this variant exhibited heightened sensitivity to the P2X(7) agonist (BzATP) relative to the most frequent allele, as shown by pore formation measured by fluorescent dye uptake into cells. Mutational analyses showed that A166G alteration was critical for the gain-of-function change, while V80M was not. Full-length variants with multiple previously identified nonsynonymous SNPs (H155Y, H270R, A348T, and E496A) were also identified. Distinct functional phenotypes of the P2X(7) variants or mutants constructed with multiple polymorphisms were observed. Gain-of-function variations (A166G or H155Y) could not rescue the loss-of-function E496A polymorphism. Synergistic effects of the gain-of-function variations were also observed. We also identified the A348T alteration as a weak gain-of-function variant. Thus, these results identify the new gain-of-function variant A166G and demonstrate that multiple-gene polymorphisms contribute to functional phenotypes of the human P2X(7) receptor. Furthermore, the results demonstrate that the C-terminal of the cysteine-rich domain 1 of P2X(7) is critical for regulation of P2X(7)-mediated pore formation.

Cloning and pharmacological characterization of the dog P2X7 receptor

BACKGROUND AND PURPOSE

Human and rodent P2X7 receptors exhibit differences in their sensitivity to antagonists. In this study we have cloned and characterized the dog P2X7 receptor to determine if its antagonist sensitivity more closely resembles the human or rodent orthologues.

EXPERIMENTAL APPROACH

A cDNA encoding the dog P2X7 receptor was isolated from a dog heart cDNA library, expressed in U-2 OS cells using the BacMam viral expression system and characterized in electrophysiological, ethidium accumulation and radioligand binding studies. Native P2X7 receptors were examined by measuring ATP-stimulated interleukin-1beta release in dog and human whole blood.

KEY RESULTS

The dog P2X7 receptor was 595 amino acids long and exhibited high homology (>70%) to the human and rodent orthologues although it contained an additional threonine at position 284 and an amino acid deletion at position 538. ATP possessed low millimolar potency at dog P2X7 receptors. 2'-&3'-O-(4benzoylbenzoyl) ATP had slightly higher potency but was a partial agonist. Dog P2X7 receptors possessed relatively high affinity for a number of selective antagonists of the human P2X7 receptor although there were some differences in potency between the species. Compound affinities in human and dog blood exhibited a similar rank order of potency as observed in studies on the recombinant receptor although absolute potency was considerably lower.

CONCLUSIONS AND IMPLICATIONS

Dog recombinant and native P2X7 receptors display a number of pharmacological similarities to the human P2X7 receptor. Thus, dog may be a suitable species for assessing target-related toxicity of antagonists intended for evaluation in the clinic.

The P2X7 purinergic receptor: from physiology to neurological disorders

Purine nucleotides are well established as extracellular signaling molecules. P2X receptors are ATP-gated cation channels that mediate fast excitatory transmission in diverse regions of the brain and spinal cord. Several P2X receptor subtypes, including P2X(7), have the unusual property of changing their ion selectivity during prolonged exposure to ATP, which results in progressive dilation of the channel pore and the development of permeability to molecules as large as 900 Da. The P2X(7) receptor was originally described in cells of hematopoietic origin, including macrophages, microglia, and certain lymphocytes, and mediates the influx of Ca(2+) and Na(+) ions, as well as the release of proinflammatory cytokines. P2X(7) receptors may affect neuronal cell death through their ability to regulate the processing and release of interleukin-1beta, a key mediator in neurodegeneration, chronic inflammation, and chronic pain. Activation of P2X(7) receptors provides an inflammatory stimulus, and P2X(7) receptor-deficient mice have substantially attenuated inflammatory responses, including models of neuropathic and chronic inflammatory pain. Moreover, P2X(7) receptor activity, by regulating the release of proinflammatory cytokines, may be involved in the pathophysiology of depression. The P2X(7) receptor may thus represent a critical communication link between the nervous and immune systems, while providing a target for therapeutic exploitation. This review discusses the current biology and cellular signaling pathways of P2X(7) receptor function, as well as insights into the role for this receptor in neurological/psychiatric diseases, outstanding questions, and the therapeutic potential of P2X(7) receptor antagonism.

P2X(7) Receptors in Neurological and Cardiovascular Disorders

P2X receptors are ATP-gated cation channels that mediate fast excitatory transmission in diverse regions of the brain and spinal cord. Several P2X receptor subtypes, including P2X(7), have the unusual property of changing their ion selectivity during prolonged exposure to ATP, which results in a channel pore permeable to molecules as large as 900 daltons. The P2X(7) receptor was originally described in cells of hematopoietic origin, and mediates the influx of Ca(2+) and Na(+) and Ca(2+) and Na(+) ions as well as the release of proinflammatory cytokines. P2X(7) receptors may affect neuronal cell death through their ability to regulate the processing and release of interleukin-1beta, a key mediator in neurodegeneration, chronic inflammation, and chronic pain. Activation of P2X(7), a key mediator in neurodegeneration, chronic inflammation, and chronic pain. Activation of P2X(7) receptors provides an inflammatory stimulus, and P2X(7) receptor-deficient mice have substantially attenuated inflammatory responses, including models of neuropathic and chronic inflammatory pain. Moreover, P2X(7) receptor activity, by regulating the release of proinflammatory cytokines, may be involved in the pathophysiology of depression. Apoptotic cell death occurs in a number of vascular diseases, including atherosclerosis, restenosis, and hypertension, and may be linked to the release of ATP from endothelial cells, P2X(7) receptor activation, proinflammatory cytokine production, and endothelial cell apoptosis. In this context, the P2X(7) receptor may be viewed as a gateway of communication between the nervous, immune, and cardiovascular systems.

P2X7 gene polymorphisms do not appear to be a susceptibility gene locus in sporadic cases of systemic lupus erythematosus

The P2X(7) receptor is a ligand-gated cation-selective channel that mediates ATP-induced apoptosis of cells of the immune system. A loss-of-function single nucleotide polymorphism (SNP) at position 1513 (1513 A-->C) of the P2X(7) gene has recently been identified in both healthy and chronic lymphocytic leukemia (CLL) B-cells, translating into a loss of P2X(7)-mediated apoptosis in these cells. This antiapoptotic effect results in increased B-cell numbers, thereby potentially contributing to the survival of B-CLL clones. It was hypothesized that prolonged cell survival may also predispose to induction of autoimmunity. The objective of this study is to analyze the role of the P2X(7) receptor and its loss-of-function 1513 A-->C polymorphism (SNP) in the development of systemic lupus erythematosus (SLE). DNA samples obtained from patients with sporadic SLE were analyzed for the presence of the 1513 A-->C polymorphism using polymerase chain reaction (PCR) amplification and then direct sequencing. No significant difference in allele frequencies (1513 A-->C polymorphism) between sporadic cases of SLE and controls was found. A loss-of-function SNP at position 1513 (1513 A-->C) of the P2X(7) gene does not appear to be a susceptibility gene locus for the development of sporadic SLE.

Extracellular ATP dissociates nonmuscle myosin from P2X(7) complex: this dissociation regulates P2X(7) pore formation

The P2X(7) receptor is a ligand-gated cation channel that is highly expressed on monocyte-macrophages and that mediates the pro-inflammatory effects of extracellular ATP. Dilation of the P2X(7) channel and massive K(+) efflux follows initial channel opening, but the mechanism of secondary pore formation is unclear. The proteins associated with P2X(7) were isolated by using anti-P2X(7) monoclonal antibody-coated Dynabeads from both interferon-gamma plus LPS-stimulated monocytic THP-1 cells and P2X(7)-transfected HEK-293 cells. Two nonmuscle myosins, NMMHC-IIA and myosin Va, were found to associate with P2X(7) in THP-1 cells and HEK-293 cells, respectively. Activation of the P2X(7) receptor by ATP caused dissociation of P2X(7) from nonmuscle myosin in both cell types. The interaction of P2X(7) and NMMHC-IIA molecules was confirmed by fluorescent life time measurements and fluorescent resonance of energy transfer-based time-resolved flow cytometry assay. Reducing the expression of NMMHC-IIA or myosin Va by small interfering RNA or short hairpin RNA led to a significant increase of P2X(7) pore function without any increase in surface expression or ion channel function of P2X(7) receptors. S-l-blebbistatin, a specific inhibitor of NMMHC-IIA ATPase, inhibited both ATP-induced ethidium uptake and ATP-induced dissociation of P2X(7)-NMMHC-IIA complex. In both cell types nonmuscle myosin closely interacts with P2X(7) and is dissociated from the complex by extracellular ATP. Dissociation of this anchoring protein may be required for the transition of P2X(7) channel to a pore.

Genetics of the P2X7 receptor and human disease

The P2RX7 gene is highly polymorphic, and many single nucleotide polymorphisms (SNPs) underlie the wide variation observed in P2X7 receptor responses. We review the discovery of those non-synonymous SNPs that affect receptor function and compare their frequencies in different ethnic populations. Analysis of pairwise linkage disequilibrium (LD) predicts a limited range of haplotypes. The strong LD between certain functional SNPs provides insight into published studies of the association between SNPs and human disease.

Cell signaling via the P2X(7) nucleotide receptor: linkage to ROS production, gene transcription, and receptor trafficking

Extracellular nucleotides can act as important intercellular signals in diverse biological processes, including the enhanced production of factors that are key to immune response regulation. One receptor that binds extracellular adenosine triphosphate released at sites of infection and injury is P2X(7), which is an ionotrophic receptor that can also lead to the formation of a non-specific pore, activate multiple mitogen-activated protein kinases (MAPKs), and stimulate the production of immune mediators including interleukin family members and reactive oxygen species (ROS). In the present report, we have investigated the signaling mechanisms by which P2X(7) promotes monocytic cell mediator production and induces transcription factor expression/phosphorylation, as well as how receptor-associated pore activity is regulated by intracellular trafficking. We report that P2X(7) stimulates ROS production in macrophages through the MAPKs ERK1/2 and the nicotinamide adenine dinucleotide phosphate oxidase complex, activates several transcription factors including cyclic-AMP response element-binding protein and components of the activating protein-1 complex, and contains specific sequences within its intracellular C-terminus that appear critical for its activity. Altogether, these data further implicate P2X(7) activation and signaling as a fundamental modulator of macrophage immune responses.

Characterisation of the R276A gain-of-function mutation in the ectodomain of murine P2X7

The cytolytic P2X7 purinoceptor is widely expressed on leukocytes and has sparked interest because of its key role in the activation of the inflammasome, the release of the pro-inflammatory cytokine IL-1beta and cell death. We report here the functional characterisation of a R276A gain-of-function mutant analysed for its capacities to induce membrane depolarisation, calcium influx and opening of a large membrane pore permeable to YO-PRO-1. Our results highlight the particular sensitivity of R276A mutant to low micromolar adenosine triphosphate (ATP) concentrations, which possibly reflect an increased affinity for its ligands, and a slower closing kinetics of the receptor channel. Our findings support the notion that evolutionary pressures maintain the low sensitivity of P2X7 to ATP. We also believe that the R276A mutant described here may be useful for the generation of new animal models with exacerbated P2X7 functions that will serve to better characterise its role in inflammation and in immune responses.

Expression, signaling, and function of P2X7 receptors in bone

Nucleotides released from cells in response to mechanical stimulation or injury may serve as paracrine regulators of bone cell function. Extracellular nucleotides bind to multiple subtypes of P2 receptors on osteoblasts (the cells responsible for bone formation) and osteoclasts (cells with the unique ability to resorb mineralized tissues). Both cell lineages express the P2X7 receptor subtype. The skeletal phenotype of mice with targeted disruption of P2rx7 points to interesting roles for this receptor in the regulation of bone formation and resorption, as well as the response of the skeleton to mechanical stimulation. This paper reviews recent work on the expression of P2X7 receptors in bone, their associated signal transduction mechanisms and roles in regulating bone formation and resorption. Areas for future research in this field are also discussed.

Autocrine regulation of T-cell activation by ATP release and P2X7 receptors

T-cell activation requires the influx of extracellular calcium, although mechanistic details regarding such activation are not fully defined. Here, we show that P2X(7) receptors play a key role in calcium influx and downstream signaling events associated with the activation of T cells. By real-time PCR and immunohistochemistry, we find that Jurkat T cells and human CD4(+) T cells express abundant P2X(7) receptors. We show, using a novel fluorescent microscopy technique, that T-cell receptor (TCR) stimulation triggers the rapid release of ATP (<100 microM). This release of ATP is required for TCR-mediated calcium influx, NFAT activation, and interleukin-2 (IL-2) production. TCR activation up-regulates P2X(7) receptor gene expression. Removal of extracellular ATP by apyrase or alkaline phosphatase treatment, inhibition of ATP release with the maxi-anion channel blocker gadolinium chloride, or siRNA silencing of P2X(7) receptors blocks calcium entry and inhibits T-cell activation. Moreover, lymphocyte activation is impaired in C57BL/6 mice that express poorly functional P2X(7) receptors, compared to control BALB/c mice, which express fully functional P2X(7) receptors. We conclude that ATP release and autocrine, positive feedback through P2X(7) receptors is required for the effective activation of T cells.

Variation in the purinergic P2RX(7) receptor gene and schizophrenia

INTRODUCTION

The purinergic receptor gene P2RX(7) is located in a major linkage hotspot for schizophrenia and bipolar disorders, 12q21-33. It has previously been associated with bipolar disorder but has never been analysed in relation to schizophrenia, although it is involved in several neuronal processes associated with schizophrenia.

METHODS

Nine functionally characterised variants in P2RX(7) were genotyped in 389 patients diagnosed with schizophrenia, each matched on sex, birth-year and month with two healthy controls.

RESULTS

We did not find association between P2RX(7) and schizophrenia and stratification on gender did not change this result. The high ethnic and diagnostic homogeneity of the sample adds credibility to this finding.

CONCLUSION

P2XR(7) was not associated with schizophrenia in this study.

Assembly and trafficking of P2X purinergic receptors (Review)

P2X receptors are cation selective ion channels gated by the binding of extracellular ATP. Seven subtypes have been identified and they have widespread and overlapping distributions throughout the body. They form homo- and heterotrimeric complexes that differ in their functional properties and subcellular localization. They form part of larger signalling complexes, interacting with unrelated ion channels and other membrane and cytosolic proteins. Up- or down-regulation of their expression is associated with several disease states. This review aims to summarize recent work on the assembly and trafficking of this family of receptors.

The role of purinergic signaling in the liver and in transplantation: effects of extracellular nucleotides on hepatic graft vascular injury, rejection and metabolism

Extracellular nucleotides (e.g. ATP, UTP, ADP) are released by activated endothelium, leukocytes and platelets within the injured vasculature and bind specific cell-surface type-2 purinergic (P2) receptors. This process drives vascular inflammation and thrombosis within grafted organs. Importantly, there are also vascular ectonucleotidases i.e. ectoenzymes that hydrolyze extracellular nucleotides in the blood to generate nucleosides (viz. adenosine). Endothelial cell NTPDase1/CD39 has been shown to critically modulate levels of circulating nucleotides. This process tends to limit the activation of platelet and leukocyte expressed P2 receptors and also generates adenosine to reverse inflammatory events. This vascular protective CD39 activity is rapidly inhibited by oxidative reactions, such as is observed with liver ischemia reperfusion injury. In this review, we chiefly address the impact of these signaling cascades following liver transplantation. Interestingly, the hepatic vasculature, hepatocytes and all non-parenchymal cell types express several components co-ordinating the purinergic signaling response. With hepatic and vascular dysfunction, we note heightened P2- expression and alterations in ectonucleotidase expression and function that may predispose to progression of disease. In addition to documented impacts upon the vasculature during engraftment, extracellular nucleotides also have direct influences upon liver function and bile flow (both under physiological and pathological states). We have recently shown that alterations in purinergic signaling mediated by altered CD39 expression have major impacts upon hepatic metabolism, repair mechanisms, regeneration and associated immune responses. Future clinical applications in transplantation might involve new therapeutic modalities using soluble recombinant forms of CD39, altering expression of this ectonucleotidase by drugs and/or using small molecules to inhibit deleterious P2-mediated signaling while augmenting beneficial adenosine-mediated effects within the transplanted liver.

Cloning and pharmacological characterization of the guinea pig P2X7 receptor orthologue

BACKGROUND AND PURPOSE

The human, rat, and mouse P2X(7) receptors have been previously characterized, and in this study we report the cloning and pharmacological properties of the guinea pig orthologue.

EXPERIMENTAL APPROACH

A cDNA encoding for the guinea pig P2X(7) receptor was isolated from a guinea pig brain library. The receptor was expressed in U-2 OS cells using the BacMam viral expression system. A monoclonal antibody was used to confirm high levels of cell surface expression and the functional properties were determined in ethidium bromide accumulation studies.

KEY RESULTS

The predicted guinea pig protein is one amino acid shorter than the human and rat orthologues and over 70% identical to the rat and human receptors. In contrast to human and rat P2X(7) receptors, 2'-&3'-O-(4benzoylbenzoyl)ATP (BzATP) was a partial agonist of the guinea pig P2X(7) receptor when compared to ATP and acted as an antagonist in some assays. However, as at other species orthologues, BzATP was more potent than ATP. The guinea pig P2X(7) receptor possessed higher affinity for 1-[N,O-bis(5-isoquinoline sulphonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN62), suramin and Coomassie Brilliant Blue than human or rat P2X(7) receptors suggesting that it is pharmacologically different to other rodent or human P2X(7) receptors.

CONCLUSIONS AND IMPLICATIONS

The guinea pig recombinant P2X(7) receptor displays a number of unique properties that differentiate it from the human, rat and mouse orthologues and this structural and functional information should aid in our understanding of the interaction of agonists and antagonist with the P2X(7) receptor.

Extracellular ATP mediates necrotic cell swelling in SN4741 dopaminergic neurons through P2X7 receptors

Extracellular ATP has recently been identified as an important regulator of cell death in response to pathological insults. When SN4741 cells, which are dopaminergic neurons derived from the substantia nigra of transgenic mouse embryos, are exposed to ATP, cell death occurs. This cell death is associated with prominent cell swelling, loss of ER integrity, the formation of many large cytoplasmic vacuoles, and subsequent cytolysis and DNA release. In addition, the cleavage of caspase-3, a hallmark of apoptosis, is induced by ATP treatment. However, caspase inhibitors do not overcome ATP-induced cell death, indicating that both necrosis and apoptosis are associated with ATP-induced cell death and suggesting that a necrotic event might override the apoptotic process. In this study we also found that P2X(7) receptors (P2X(7)Rs) are abundantly expressed in SN4741 cells, and both ATP-induced swelling and cell death are reversed by pretreatment with the P2X(7)Rs antagonist, KN62, or by knock-down of P2X(7)Rs with small interfering RNAs. Therefore, extracellular ATP release from injured tissues may act as an accelerating factor in necrotic SN4741 dopaminergic cell death via P2X(7)Rs.

Canine erythrocytes express the P2X7 receptor: greatly increased function compared with human erythrocytes

Over three decades ago, Parker and Snow (Am J Physiol 223: 888-893, 1972) demonstrated that canine erythrocytes undergo an increase in cation permeability when incubated with extracellular ATP. In this study we examined the expression and function of the channel/pore-forming P2X(7) receptor on canine erythrocytes. P2X(7) receptors were detected on canine erythrocytes by immunocytochemistry and immunoblotting. Extracellular ATP induced (86)Rb(+) (K(+)) efflux from canine erythrocytes that was 20 times greater than that from human erythrocytes. The P2X(7) agonist 2'(3')-O-(4-benzoylbenzoyl)adenosine 5'-trisphosphate (BzATP) was more potent than ATP, and both stimulated (86)Rb(+) efflux from erythrocytes in a dose-dependent fashion with EC(50) values of approximately 7 and approximately 309 microM, respectively. 2-Methylthioadenosine 5'-triphosphate and adenosine 5'-O-(3-thiotriphosphate) induced a smaller (86)Rb(+) efflux from erythrocytes, whereas ADP, AMP, UTP, or adenosine had no effect. ATP-induced (86)Rb(+) efflux from erythrocytes was inhibited by oxidized ATP, KN-62, and Brilliant blue G, known P2X(7) antagonists. ATP also induced uptake of choline(+) into canine erythrocytes that was 60 times greater than that into human erythrocytes. Overnight incubation of canine erythrocytes with ATP and BzATP induced phosphatidylserine exposure in >80% of cells and caused up to 20% hemolysis. In contrast, <30% of human erythrocytes showed phosphatidylserine exposure after overnight incubation with ATP and BzATP, and hemolysis was negligible. Flow cytometric measurements of ATP-induced ethidium(+) uptake showed that P2X(7) function was three times lower in canine monocytes than in human monocytes. These data show that the massive cation permeability increase induced by extracellular ATP in canine erythrocytes results from activation and opening of the P2X(7) receptor channel/pore.

Single nucleotide polymorphisms in the P2X7 gene are associated to fracture risk and to effect of estrogen treatment

OBJECTIVES

The purinergic P2RX7 receptor (P2RX7) has been shown to play a role in the regulation of osteoblast and osteoclast activity. The aim of this study was to determine the presence of polymorphisms in exon 13 of the P2X7 gene and the association with osteoclast apoptosis in vitro and bone status in vivo.

METHODS

A total of 1764 postmenopausal women were genotyped for three single nucleotide polymorphisms detected after sequencing of exon 13 of P2X7. Bone markers, bone mineral density of the hip and lumbar spine were determined at baseline and after 10 years, and vertebral fracture incidence after 10 years. In-vitro ATP-induced caspase-1 determinations were performed on osteoclasts from the different genotypes.

RESULTS

Three polymorphisms were detected (Gln460Arg, Glu496Ala, and Ile568Asn). None of the polymorphisms was related to bone mineral density or changes in bone mineral density over 10 years in hormone replacement therapy naïve women. The Ile568Asn polymorphism was however, associated with effect of hormone replacement therapy. Furthermore, the 10-year fracture incidence was significantly associated with both the Glu496Ala and the Ile568Asn. The Glu496Ala polymorphism was closely related to ATP-induced osteoclast apoptosis in vitro, as osteoclasts from individuals homozygous for the C allele had significantly decreased apoptotic activity.

CONCLUSION

The P2X7 Glu496Ala and the Ile568Asn single nucleotide polymorphisms are associated with 10-year fracture risk in postmenopausal women and response to hormone replacement therapy treatment. Further, the Glu496Ala polymorphism is strongly influencing osteoclast apoptosis in vitro, which could contribute to increased fracture risk.

Synaptic P2X7 receptor regenerative-loop hypothesis for depression

Forty-five years ago the surprising discovery was made, in a Melbourne University laboratory, that peripheral synapses exist that release neither noradrenaline nor acetylcholine. The same laboratory went on to show that one of these then novel transmitters is adenosine 5'-triphosphate (ATP), for which a class of receptors has been dubbed P2X7. Recent linkage studies have shown that the P2X7 gene is associated with major depression and bipolar disorder. This speculative paper considers possible mechanisms that could link polymorphisms in the P2X7 gene with the functioning of neural networks, especially in the hippocampus. A selective review of the neurobiological literature on the location and function of the P2X7 receptor at synapses and on astrocytes as well as microglial cells was performed in the context of determining viable hypotheses as to the function of these receptors during synaptic transmission in the neural networks of the hippocampus. It is suggested that P2X7 receptors participate in a regenerative loop at central glutamatergic synapses. In this loop glutamate-evoked release of ATP from both astrocytes and microglia cells, as well as ATP derived from an autocatalytic release from astrocytes, provides purines that can act on presynaptic P2X7 purinergic receptors. This increases glutamate release to further the amount of ATP at the synapse, leading to a new functional state of the neural network in which the synapse participates. This synaptic ATP can also act on microglia P2X7 receptors to release the cytokine tumour necrosis factor-alpha (TNF-alpha), as can glutamate, with this TNF-alpha acting on the post-synaptic neuronal membrane to increase glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors there. As synaptic ATP and glutamate are maintained by the regenerative loop they provide a sustained release of TNF-alpha, and therefore of AMPA receptor enhancement, increasing synaptic efficacy, and so contributing to the new functional state of the neural network. Infections can change this state by activating toll-like (TOL) receptors on the microglia concomitantly with their P2X7 receptor activation by the regenerative loop, thereby releasing the cytokine interleukin-1beta, which decreases the AMPA receptors in the neural membrane, so decreasing synaptic efficacy and changing the functional state of the neural network in which the synapse resides. Polymorphisms in the P2X7 gene that modify operation of the regenerative loop or the release of cytokines, as can infections, change the functional state of neural networks, which may then lead to vulnerability to mood disorders.

A quantitative method for routine measurement of cell surface P2X7 receptor function in leucocyte subsets by two-colour time-resolved flow cytometry

The P2X(7) receptor is a ligand-gated cation channel activated by extracellular ATP and highly expressed on monocytes, macrophages and lymphocytes. Activation of this receptor by exposure to extracellular ATP opens a selective cation channel that allows Ca(2+) and Ba(2+) influx, and K(+) efflux. Over the first minute the channel adopts a second and larger permeability state allowing the uptake of ethidium(+), followed by a cascade of intracellular downstream effects. Current methods used to study the P2X(7) receptor function, do not give quantitative measurement in sub-populations of a mixed cell suspension. We describe a quantitative method to determine the P2X(7) receptor function using time-resolved two-colour flow cytometry by assessing ATP-induced ethidium(+) uptake. Practical factors such as ethidium bromide concentration, agonists, temperature and buffers are also studied. Moreover, the ATP-induced ethidium(+) uptake method is compared to ATP induced barium (Ba(2+)) influx with Fura-Red. These two compatible methods can be used to screen the channel/pore function of the cell surface P2X(7) receptor among individuals and the results may be useful to estimate susceptibility of subjects to certain infectious diseases.

Oxygen/glucose deprivation increases the integration of recombinant P2X7 receptors into the plasma membrane of HEK293 cells

Recombinant human P2X(7) receptors, C-terminally labelled with enhanced green fluorescent protein (P2X(7)-EGFP), were transiently expressed in HEK293 cells. Activation of these receptors by their preferential agonist 2',3'-O-(4-benzoylbenzoyl)-ATP (BzATP) induced inward currents and propidium ion uptake indicating the opening of cationic channels and of large pores permeable for dye molecules, respectively. Two mutants of P2X(7) receptors (P2X(7)-EGFP-I568N, -E496A) representing polymorphisms in the P2X(7) gene known to interfere with normal receptor-trafficking and with optimal assembly of its subunits, responded with much lower current amplitudes to BzATP than their wild-type counterpart. Similarly, the normal propidium ion uptake induced by BzATP at the wild-type P2X(7) receptor was abolished by the two mutants. Confocal laser scanning microscopy indicated that in vitro ischemia of 12h duration increased the integration of P2X(7)-EGFP, but not of its two mutants, into the plasma membrane of HEK293 cells. Further, this ischemic stimulus facilitated the current response to BzATP in HEK293 cells permanently transfected with P2X(7) receptors. Finally, the fluorescence intensity per cell measured by flow cytometry and P2X(7) antibodies directed against an extracellular, but not an intracellular epitope of the receptor, were also increased. In conclusion, P2X(7) receptors may alter their trafficking properties during ischemia and thereby contribute to the ATP-induced damage of various cell-types including neurons.

Discovery of P2X7 receptor-selective antagonists offers new insights into P2X7 receptor function and indicates a role in chronic pain states

ATP-sensitive P2X(7) receptors are localized on cells of immunological origin including peripheral macrophages and glial cells in the CNS. Activation of P2X(7) receptors leads to rapid changes in intracellular calcium concentrations, release of the proinflammatory cytokine interleukin-1beta and following prolonged agonist exposure, the formation of cytolytic pores in plasma membranes. Both the localization and functional consequences of P2X(7) receptor activation indicate a role in inflammatory processes. The phenotype of P2X(7) receptor gene-disrupted mice also indicates that P2X(7) receptor activation contributes to ongoing inflammation. More recently, P2X(7) receptor knockout data has also suggested a specific role in inflammatory and neuropathic pain states. The recent discovery of potent and highly selective antagonists for P2X(7) receptors has helped to further clarify P2X receptor pharmacology, expanded understanding of P2X(7) receptor signaling, and offers new evidence that P2X(7) receptors play a specific role in nociceptive signaling in chronic pain states. In this review, we incorporate the recent discoveries of novel P2X(7) receptor-selective antagonists with a brief update on P2X(7) receptor pharmacology and its therapeutic potential.