Partial agonists and antagonists reveal a second permeability state of human lymphocyte P2Z/P2X7 channel

High-affinity agonism at the P2X7 receptor is mediated by three residues outside the orthosteric pocket

P2X receptors are trimeric ATP-gated ion channels that activate diverse signaling cascades. Due to its role in apoptotic pathways, selective activation of P2X7 is a potential experimental tool and therapeutic approach in cancer biology. However, mechanisms of high-affinity P2X7 activation have not been defined. We report high-resolution cryo-EM structures of wild-type rat P2X7 bound to the high-affinity agonist BzATP as well as significantly improved apo receptor structures in the presence and absence of sodium. Apo structures define molecular details of pore architecture and reveal how a partially hydrated Na+ ion interacts with the conductance pathway in the closed state. Structural, electrophysiological, and direct binding data of BzATP reveal that three residues just outside the orthosteric ATP-binding site are responsible for its high-affinity agonism. This work provides insights into high-affinity agonism for any P2X receptor and lays the groundwork for development of subtype-specific agonists applicable to cancer therapeutics.

6-Furopyridine Hexamethylene Amiloride Is a Non-Selective P2X7 Receptor Antagonist

P2X7 is an extracellular adenosine 5′-triphopshate (ATP)-gated cation channel present on leukocytes, where its activation induces pro-inflammatory cytokine release and ectodomain shedding of cell surface molecules. Human P2X7 can be partially inhibited by amiloride and its derivatives at micromolar concentrations. This study aimed to screen a library of compounds derived from amiloride or its derivative 5-(N,N-hexamethylene) amiloride (HMA) to identify a potential P2X7 antagonist. 6-Furopyridine HMA (6-FPHMA) was identified as a novel P2X7 antagonist and was characterized further. 6-FPHMA impaired ATP-induced dye uptake into human RPMI8226 multiple myeloma cells and human P2X7-HEK293 cells, in a concentration-dependent, non-competitive manner. Likewise, 6-FPHMA blocked ATP-induced Ca²⁺ fluxes in human P2X7-HEK293 cells in a concentration-dependent, non-competitive manner. 6-FPHMA inhibited ATP-induced dye uptake into human T cells, and interleukin-1β release within human blood and CD23 shedding from RPMI8226 cells. 6-FPHMA also impaired ATP-induced dye uptake into murine P2X7- and canine P2X7-HEK293 cells. However, 6-FPHMA impaired ATP-induced Ca²⁺ fluxes in human P2X4-HEK293 cells and non-transfected HEK293 cells, which express native P2Y₁, P2Y₂ and P2Y₄. In conclusion, 6-FPHMA inhibits P2X7 from multiple species. Its poor selectivity excludes its use as a specific P2X7 antagonist, but further study of amiloride derivatives as P2 receptor antagonists is warranted.

Preparation of the Murine Anti-Human P2X7 Receptor Monoclonal Antibody (Clone L4)

The murine anti-human P2X7 receptor monoclonal antibody (mAb) (clone L4) has been used to study the expression and function of the P2X7 receptor on primary leukocytes, keratinocytes, osteoblasts and neuronal cells, as well as various cell lines. This antibody has also been used to characterize polymorphic variants and isoforms of the P2RX7 gene and P2X7 site-directed mutations, and to identify molecules coassociated with P2X7 in the plasma membrane. This chapter describes the maintenance and cryopreservation of the L4 hybridoma cell line, as well as the generation of tissue culture supernatant containing the anti-human P2X7 mAb, and its subsequent purification by Protein A chromatography and conjugation to DyLight™ 488. Moreover, this chapter describes flow cytometric assays to assess the blocking activity and binding of the anti-human P2X7 mAb against P2X7 on human RPMI 8226 multiple myeloma cells.

Assays to Measure Purinoceptor Pore Dilation

The P2X7 receptor is a classic purinoceptor/ion channel. After activated by ATP, it opens a cation selective channel, which dilates to a large pore over tens of seconds, allowing the entry of big molecules. This unique feature is often used to evaluate this receptor's function with DNA-binding dyes (MW 300-400 Da), such as ethidium bromide and Yo-Pro-1. Here we describe two-color flow cytometry based protocols for measuring P2X7 pore dilation. One is ATP-induced ethidium uptake by real-time multicolor flow cytometry for standardized and accurate quantitation, and the other is a quick whole blood assay which is particularly useful for ex vivo study.

The P2X7 Receptor

The P2X7 receptor is a trimeric ion channel gated by extracellular adenosine 5'-triphosphate. The receptor is present on an increasing number of different cells types including stem, blood, glial, neural, ocular, bone, dental, exocrine, endothelial, muscle, renal and skin cells. The P2X7 receptor induces various downstream events in a cell-specific manner, including inflammatory molecule release, cell proliferation and death, metabolic events, and phagocytosis. As such this receptor plays important roles in heath and disease. Increasing knowledge about the P2X7 receptor has been gained from studies of, but not limited to, protein chemistry including cloning, site-directed mutagenesis, crystal structures and atomic modeling, as well as from studies of primary tissues and transgenic mice. This chapter focuses on the P2X7 receptor itself. This includes the P2RX7 gene and its products including splice and polymorphic variants. This chapter also reviews modulators of P2X7 receptor activation and inhibition, as well as the transcriptional regulation of the P2RX7 gene via its promoter and enhancer regions, and by microRNA and long-coding RNA. Furthermore, this chapter discusses the post-translational modification of the P2X7 receptor by N-linked glycosylation, adenosine 5'-diphosphate ribosylation and palmitoylation. Finally, this chapter reviews interaction partners of the P2X7 receptor, and its cellular localisation and trafficking within cells.

Brilliant Blue Dyes in Daily Food: How Could Purinergic System Be Affected?

Dyes were first obtained from the extraction of plant sources in the Neolithic period to produce dyed clothes. At the beginning of the 19th century, synthetic dyes were produced to color clothes on a large scale. Other applications for synthetic dyes include the pharmaceutical and food industries, which are important interference factors in our lives and health. Herein, we analyzed the possible implications of some dyes that are already described as antagonists of purinergic receptors, including special Brilliant Blue G and its derivative FD&C Blue No. 1. Purinergic receptor family is widely expressed in the body and is critical to relate to much cellular homeostasis maintenance as well as inflammation and cell death. In this review, we discuss previous studies and show purinergic signaling as an important issue to be aware of in food additives development and their correlations with the physiological functions.

Inhibition of P2X Receptors Protects Human Monocytes against Damage by Leukotoxin from Aggregatibacter actinomycetemcomitans and α-Hemolysin from Escherichia coli

α-Hemolysin (HlyA) from Escherichia coli and leukotoxin A (LtxA) from Aggregatibacter actinomycetemcomitans are important virulence factors in ascending urinary tract infections and aggressive periodontitis, respectively. The extracellular signaling molecule ATP is released immediately after insertion of the toxins into plasma membranes and, via P2X receptors, is essential for the erythrocyte damage inflicted by these toxins. Moreover, ATP signaling is required for the ensuing recognition and phagocytosis of damaged erythrocytes by the monocytic cell line THP-1. Here, we investigate how these toxins affect THP-1 monocyte function. We demonstrate that both toxins trigger early ATP release and a following increase in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in THP-1 monocytes. The HlyA- and LtxA-induced [Ca²⁺]_i response is diminished by the P2 receptor antagonist in a pattern that fits the functional P2 receptor expression in these cells. Both toxins are capable of lysing THP-1 cells, with LtxA being more aggressive. Either desensitization or blockage of P2X₁, P2X₄, or P2X₇ receptors markedly reduces toxin-induced cytolysis. This pattern is paralleled in freshly isolated human monocytes from healthy volunteers. Interestingly, only a minor fraction of the toxin-damaged THP-1 monocytes eventually lyse. P2X₇ receptor inhibition generally prevents cell damage, except from a distinct cell shrinkage that prevails in response to the toxins. Moreover, we find that preexposure to HlyA preserves the capacity of THP-1 monocytes to phagocytose damaged erythrocytes and may induce readiness to discriminate between damaged and healthy erythrocytes. These findings suggest a new pharmacological target for protecting monocytes during exposure to pore-forming cytolysins during infection or injury.

ATP-induced IL-1β secretion is selectively impaired in microglia as compared to hematopoietic macrophages

Under stressful conditions nucleotides are released from dying cells into the extracellular space, where they can bind to purinergic P2X and P2Y receptors. High concentrations of extracellular ATP in particular induce P2X7-mediated signaling, which leads to inflammasome activation. This in turn leads to the processing and secretion of pro-inflammatory cytokines, like interleukin (IL)-1β. During neurodegenerative diseases, innate immune responses are shaped by microglia and we have previously identified microglia-specific features of inflammasome-mediated responses. Here, we compared ATP-induced IL-1β secretion in primary rhesus macaque microglia and bone marrow-derived macrophages (BMDM). We assessed the full expression profile of P2 receptors and characterized the induction and modulation of IL-1β secretion by extracellular nucleotides. Microglia secreted significantly lower levels of IL-1β in response to ATP when compared to BMDM. We demonstrate that this is not due to differences in sensitivity, kinetics or expression of ATP-processing enzymes, but rather to differences in purinergic receptor expression levels and usage. Using a combined approach of purinergic receptor agonists and antagonists, we demonstrate that ATP-induced IL-1β secretion in BMDM was fully dependent on P2X7 signaling, whereas in microglia multiple purinergic receptors were involved, including P2X7 and P2X4. These cell type-specific features of conserved innate immune responses may reflect adaptations to the vulnerable CNS microenvironment. GLIA 2016;64:2231-2246.

The Impact of Vitamin D3 Supplementation on Mechanisms of Cell Calcium Signaling in Chronic Kidney Disease

Intracellular calcium concentration in peripheral blood mononuclear cells (PBMCs) of patients with chronic kidney disease (CKD) is significantly increased, and the regulatory mechanisms maintaining cellular calcium homeostasis are impaired. The purpose of this study was to examine the effect of vitamin D3 on predominant regulatory mechanisms of cell calcium homeostasis. The study involved 16 CKD stages 2-3 patients with vitamin D deficiency treated with cholecalciferol 7000-14000 IU/week for 6 months. The regulatory mechanisms of calcium signaling were studied in PBMCs and red blood cells. After vitamin D3 supplementation, serum concentration of 25(OH)D3 increased (P < 0.001) and [Ca(2+)]i decreased (P < 0.001). The differences in [Ca(2+)]i were inversely related to differences in 25(OH)D3 concentration (P < 0.01). Vitamin D3 supplementation decreased the calcium entry through calcium release activated calcium (CRAC) channels and purinergic P2X7 channels. The function of P2X7 receptors was changed in comparison with their baseline status, and the expression of these receptors was reduced. There was no effect of vitamin D3 on P2X7 pores and activity of plasma membrane Ca(2+)-ATPases. Vitamin D3 supplementation had a beneficial effect on [Ca(2+)]i decreasing calcium entry via CRAC and P2X7 channels and reducing P2X7 receptors expression.

Mechanisms of cytolysin-induced cell damage -- a role for auto- and paracrine signalling

Cytolysins inflict cell damage by forming pores in the plasma membrane. The Na(+) conductivity of these pores results in an ion influx that exceeds the capacity of the Na(+) /K(+) -pump to extrude Na(+) . This net load of intracellular osmolytes results in swelling and eventual lysis of the attacked cell. Many nucleated cells have the capacity to reduce the potential damage of pore-forming proteins, whereas erythrocytes have been regarded as essentially defenceless against cytolysin-induced cell damage. This review addresses how autocrine/paracrine signalling and the cells intrinsic volume regulation markedly influence the fate of the cell after membrane insertion of cytolysins. Moreover, it regards the various steps that may explain the relative large degree of diversity between cell types and species as well as highlights some of the current gaps in the mechanistic understanding of cytolysin-induced cell injury.

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.

Mitochondrial superoxide generation enhances P2X7R-mediated loss of cell surface CD62L on naive human CD4+ T lymphocytes

Migration of naive CD4(+) T lymphocytes into lymphoid tissue is essential for their activation and subsequent roles in adaptive immunity. The adhesion molecule L-selectin (CD62L), critical for this process, is highly expressed on naive CD4(+) T lymphocytes and is downregulated upon T lymphocyte activation. We demonstrate protein expression of P2X7R on naive CD4(+) T lymphocytes and show functional channel activity in whole-cell patch clamp recordings. CD62L downregulation occurs rapidly in response to extracellular ATP, a process that is blocked by selective antagonists of P2X7R. This loss of surface CD62L expression was not associated with externalization of phosphatidylserine. While investigating the mechanisms for this process, we revealed that pharmacological modulation of mitochondrial complex I or III, but not inhibition of NADPH oxidase, enhanced P2X7R-dependent CD62L downregulation by increasing ATP potency. Enhanced superoxide generation in the mitochondria of rotenone- and antimycin A-treated cells was observed and may contribute to the enhanced sensitivity of P2X7R to ATP. P2X7R-dependent exposure of phosphatidylserine was also revealed by preincubation with mitochondrial uncouplers prior to ATP treatment. This may present a novel mechanism whereby P2X7R-dependent phosphatidylserine exposure occurs only when cells have enhanced mitochondrial reactive oxygen species generation. The clearance of apoptotic cells may therefore be enhanced by this mechanism which requires functional P2X7R expression.

The P2X(7) loss-of-function Glu496Ala polymorphism affects ex vivo cytokine release and protects against the cytotoxic effects of high ATP-levels

BACKGROUND

The P2X(7) receptor plays an important role in cytokine release during the inflammatory response in vivo. Polymorphisms within the P2X(7) receptor gene that lead to loss of receptor function may contribute to impaired cytokine release by immune cells. Therefore, we investigated whether a known loss-of-function polymorphism (Glu496Ala) in the P2X(7) receptor gene leads to alterations in cytokine release in response to ATP.

RESULTS

An ex vivo whole blood model was used to induce an inflammatory reaction with the pro-inflammatory stimuli LPS and PHA (phytohemagglutinin). Blood from n=9 subjects with the Glu496Ala P2X7 SNP (P2X7MUT) and n=7 'wild-type' subjects (no P2X7 SNP; P2X7WT) was used.Addition of ATP (0.9-3 mM) to LPS/PHA-stimulated whole blood induced an increase in IL-1β release in P2X7MUT subjects, whereas decreased release was observed in P2X7WT subjects. Decreased levels of IL-6 and TNF-α in response to ATP were shown in both P2X7MUT and P2X7WT subjects, which was less pronounced in P2X7MUT subjects. ATP at 3 mM also significantly decreased levels of lactate dehydrogenase (LDH) in P2X7MUT subjects compared to P2X7WT subjects.

CONCLUSIONS

The presence of the non-synonymous Glu496Ala loss-of-function polymorphism within the P2X(7) receptor gene is likely to be of importance in the release of cytokines during inflammation. Furthermore, this study suggests that carriers of the Glu496Ala loss-of-function polymorphism are protected against the cytotoxic effects of high ATP-levels.

Defective T-lymphocyte migration to muscles in dystrophin-deficient mice

Duchenne muscular dystrophy (DMD), an X-linked recessive disorder affecting 1 in 3500 males, is caused by mutations in the dystrophin gene. DMD leads to degeneration of skeletal and cardiac muscles and to chronic inflammation. The mdx/mdx mouse has been widely used to study DMD; this model mimics most characteristics of the disease, including low numbers of T cells in damaged muscles. In this study, we aimed to assess migration of T cells to the heart and to identify any alterations in adhesion molecules that could possibly modulate this process. In 6-week-old mdx/mdx mice, blood leukocytes, including T cells, were CD62L(+), but by 12 weeks of age down-modulation was evident, with only approximately 40% of T cells retaining this molecule. Our in vitro and in vivo results point to a P2X7-dependent shedding of CD62L (with high levels in the serum), which in 12-week-old mdx/mdx mice reduces blood T cell competence to adhere to cardiac vessels in vitro and to reach cardiac tissue in vivo, even after Trypanosoma cruzi infection, a known inducer of lymphoid myocarditis. In mdx/mdx mice treated with Brilliant Blue G, a P2X7 blocker, these blood lymphocytes retained CD62L and were capable of migrating to the heart. These results provide new insights into the mechanisms of inflammatory infiltration and immune regulation in DMD.

Corneal epithelium expresses a variant of P2X(7) receptor in health and disease

Improper wound repair of the corneal epithelium can alter refraction of light resulting in impaired vision. We have shown that ATP is released after injury, activates purinergic receptor signaling pathways and plays a major role in wound closure. In many cells or tissues, ATP activates P2X(7) receptors leading to cation fluxes and cytotoxicity. The corneal epithelium is an excellent model to study the expression of both the full-length P2X(7) form (defined as the canonical receptor) and its truncated forms. When Ca(2+) mobilization is induced by BzATP, a P2X(7) agonist, it is attenuated in the presence of extracellular Mg(2+) or Zn(2+), negligible in the absence of extracellular Ca(2+), and inhibited by the competitive P2X7 receptor inhibitor, A438079. BzATP enhanced phosphorylation of ERK. Together these responses indicate the presence of a canonical or full-length P2X(7) receptor. In addition BzATP enhanced epithelial cell migration, and transfection with siRNA to the P2X(7) receptor reduced cell migration. Furthermore, sustained activation did not induce dye uptake indicating the presence of truncated or variant forms that lack the ability to form large pores. Reverse transcription-polymerase chain reaction and Northern blot analysis revealed a P2X(7) splice variant. Western blots identified a full-length and truncated form, and the expression pattern changed as cultures progressed from monolayer to stratified. Cross-linking gels demonstrated the presence of homo- and heterotrimers. We examined epithelium from age matched diabetic and non-diabetic corneas patients and detected a 4-fold increase in P2X(7) mRNA from diabetic corneal epithelium compared to non-diabetic controls and an increased trend in expression of P2X(7)variant mRNA. Taken together, these data indicate that corneal epithelial cells express full-length and truncated forms of P2X(7), which ultimately allows P2X(7) to function as a multifaceted receptor that can mediate cell proliferation and migration or cell death.

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.

The inhibitor profiling of the caspase family of proteases using substrate-derived peptide glyoxals

A series of substrate-based α-keto-β-aldehyde (glyoxal) sequences have been synthesised and evaluated as inhibitors of the caspase family of cysteine proteases. A number of potent inhibitor sequences have been identified. For example, a palmitic acid containing sequence pal-Tyr-Val-Ala-Asp-glyoxal was demonstrated to be an extremely effective inhibitor of caspase-1, inhibiting not only the action of the protease against synthetic fluorogenic substrates (K(i)=0.3 nM) but also blocking its processing of pro-interleukin-1beta (pro-IL-1β). In addition, the peptide Ac-Asp-Glu-Val-Asp-glyoxal, which is based on the consensus cleavage sequence for caspase-3, is a potent inhibitor of this protease (K(i)=0.26 nM) yet only functions as a comparatively modest inhibitor of caspase-1 (K(i)=451 nM). Potent inhibitor sequences were also identified for caspases-6 and -8. However, the degree of discrimination between the family members is limited. The ability of Ac-Asp-Glu-Val-Asp-glyoxal to block caspase-3 like activity in whole cells and to delay the development of apoptosis was assessed. When tested against caspase-3 like activity in cell lysates, Ac-Asp-Glu-Val-Asp-glyoxal displayed effective inhibition similar to that observed against recombinant caspase-3. Treatment of whole cells with this potent caspase-3 inhibitor was however, not sufficient to significantly stall the development of apoptosis in-vitro.

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.

Inhibition of neuronal cell death after retinoic acid-induced down-regulation of P2X7 nucleotide receptor expression

Apoptosis is a major mechanism for cell death in the nervous system during development. P2X(7) nucleotide receptors are ionotropic ATP receptors that mediate cell death under pathological conditions. We developed an in vitro protocol to investigate the expression and functional responses of P2X(7) nucleotide receptors during retinoic acid (RA)-induced neuronal differentiation of human SH-SY5Y neuroblastoma cells. Neuronal differentiation was examined measuring cellular growth arrest and neuritic processes elongation. We found that SH-SY5Y cells treated for 5 days with RA under low serum content exhibited a neuron-like phenotype with neurites extending more than twice the length of the cell body and cell growth arrest. Concurrently, we detected the abolishment of intracellular-free calcium mobilization and the down-regulation of P2X(7) nucleotide receptor protein expression that protected differentiated cells from neuronal cell death and reduced caspase-3 cleavage-induced by P2X(7) nucleotide receptor agonist. The role of P2X(7) nucleotide receptors in neuronal death was established by selectively antagonizing the receptor with KN-62 prior to its activation. We assessed the involvement of protein kinases and found that p38 signaling was activated in undifferentiated after nucleotide stimulation, but abolished by the differentiating RA pretreatment. Importantly, P2X(7) receptor-induced caspase-3 cleavage was blocked by the p38 protein kinase specific inhibitor PD169316. Taken together, our results suggest that RA treatment of human SH-SY5Y cells leads to decreased P2X(7) nucleotide receptor protein expression thus protecting differentiated cells from extracellular nucleotide-induced neuronal death, and p38 signaling pathway is critically involved in this protection of RA-differentiated cells.

Purinergic P2X7 receptors regulate secretion of interleukin-1 receptor antagonist and beta cell function and survival

AIMS/HYPOTHESIS

In obesity, beta cells activate compensatory mechanisms to adapt to the higher insulin demand. Interleukin-1 receptor antagonist (IL-1Ra) prevents obesity-induced hyperglycaemia and is a potent target for the treatment of diabetes, but the mechanisms of its secretion and regulation in obesity are unknown. In the present study, we hypothesise the regulation of IL-1Ra secretion by purinergic P2X(7) receptors in islets.

METHODS

Production and regulation of P2X(7) were studied in pancreatic sections from lean and obese diabetic patients, non-diabetic controls and in isolated islets. IL-1Ra, IL-1beta and insulin secretion, glucose tolerance and beta cell mass were studied in P2x7 (also known as P2Rx7)-knockout mice.

RESULTS

P2X(7) levels were elevated in beta cells of obese patients, but downregulated in patients with type 2 diabetes mellitus. Elevated glucose and non-esterified fatty acids rapidly activated P2X(7) and IL-1Ra secretion in human islets, and this was inhibited by P2X(7) blockade. In line with our results in vitro, P2x7-knockout mice had a lower capacity to secrete IL-1Ra. They exhibited severe and rapid hyperglycaemia, glucose intolerance and impaired beta cell function in response to a high-fat/high-sucrose diet, were unable to compensate by increasing their beta cell mass in response to the diet and showed increased beta cell apoptosis.

CONCLUSIONS/INTERPRETATION

Our study shows a tight correlation of P2X(7) activation, IL-1Ra secretion and regulation of beta cell mass and function. The increase in P2X(7) production is one mechanism that may explain how beta cells compensate by adapting to the higher insulin demand. Disturbances within that system may result in the progression of diabetes.

Direct observation of ATP-induced conformational changes in single P2X(4) receptors

The ATP-gated P2X₄ receptor is a cation channel, which is important in various pathophysiological events. The architecture of the P2X₄ receptor in the activated state and how to change its structure in response to ATP binding are not fully understood. Here, we analyze the architecture and ATP-induced structural changes in P2X₄ receptors using fast-scanning atomic force microscopy (AFM). AFM images of the membrane-dissociated and membrane-inserted forms of P2X₄ receptors and a functional analysis revealed that P2X₄ receptors have an upward orientation on mica but lean to one side. Time-lapse imaging of the ATP-induced structural changes in P2X₄ receptors revealed two different forms of activated structures under 0 Ca²⁺ conditions, namely a trimer structure and a pore dilation-like tripartite structure. A dye uptake measurement demonstrated that ATP-activated P2X₄ receptors display pore dilation in the absence of Ca²⁺. With Ca²⁺, the P2X₄ receptors exhibited only a disengaged trimer and no dye uptake was observed. Thus our data provide a new insight into ATP-induced structural changes in P2X₄ receptors that correlate with pore dynamics.

ATP is not only a source of intracellular energy but can act as an intercellular signal by binding membrane receptors. Purinergic receptors, which bind with nucleotides including ATP are known as P2 receptors and are divided into two types: ion channel-type P2X receptors and metabotropic-type P2Y receptors. P2X receptors are thought to undergo conformational changes in response to ATP binding, leading to the opening of transmembrane channels, through which cations enter the cells. A growing body of evidence shows that P2X receptors control various physiological and pathophysiological cellular responses. However, the receptor structure and the conformational changes it experiences upon stimulation remained to be clarified. Here, we employed an atomic force microscope (AFM) to observe P2X receptor behavior at the single channel level. We chose to analyze the P2X4 receptor, because it is known to increase the transmembrane pore size (i.e., pore dilation) in the absence of extracellular calcium. Activated P2X4 receptor exhibited a trimeric topology with a pore-like structure in the center. When calcium was present the receptor exhibited a trimer without a pore structure at its center. These structural changes corresponded well with the changes of ion permeability of P2X4 receptor.

Fast-scanning atomic force microscopy reveals the topology, ATP-induced conformational changes, and Ca²⁺ regulation of the pore-opening in P2X₄ receptors.

Species and agonist dependent zinc modulation of endogenous and recombinant ATP-gated P2X7 receptors

Zinc (Zn2+) and copper (Cu2+) are key signalling molecules in the immune system and regulate the activity of many ion channels. Both Zn2+ and Cu2+ potently inhibit rat P2X7 receptors via a binding site identified by mutagenesis. Here we show that extracellular Cu2+ also potently inhibits mouse P2X7 receptors. By contrast, the receptor expression system and agonist strongly influence the action of extracellular Zn2+ at mouse P2X7 receptors. Consistent with previous reports, Zn2+ inhibits recombinant rat P2X7 receptors. However, recombinant mouse P2X7 receptors are potentiated by Zn2+ when activated by ATP4- but inhibited when stimulated with the ATP analogue BzATP4-. Endogenous murine macrophage P2X7 receptors are not modulated by Zn2+ when stimulated by ATP4- however Zn2+ inhibits BzATP4- mediated responses. In summary, these findings provide a fundamental insight into the differential actions of Zn2+ and Cu2+ between different P2X7 receptor species.

Negative and positive allosteric modulators of the P2X(7) receptor

BACKGROUND AND PURPOSE

Antagonist effects at the P2X(7) receptor are complex with many behaving in a non-competitive manner. In this study, the effects of N-[2-({2-[(2-hydroxyethyl)amino]ethyl}amino)-5-quinolinyl]-2-tricyclo[3.3.1.1(3,7)]dec-1-ylacetamide (compound-17) and N (2)-(3,4-difluorophenyl)-N (1)-[2-methyl-5-(1-piperazinylmethyl)phenyl]glycinamide dihydrochloride (GW791343) on P2X(7) receptors were examined and their mechanism of action explored.

EXPERIMENTAL APPROACH

Antagonist effects were studied by measuring agonist-stimulated ethidium accumulation in cells expressing human or rat recombinant P2X(7) receptors and in radioligand binding studies.

KEY RESULTS

Compound-17 and GW791343 were non-competitive inhibitors of human P2X(7) receptors. Receptor protection studies using decavanadate and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) showed that neither compound-17 nor GW791343 competitively interacted at the ATP binding site and so were probably negative allosteric modulators of the P2X(7) receptor. GW791343 prevented the slowly reversible blockade of the human P2X(7) receptor produced by compound-17 and inhibited [(3)H]-compound-17 binding to the P2X(7) receptor suggesting they may bind to similar or interacting sites. At rat P2X(7) receptors, compound-17 was a negative allosteric modulator but the predominant effect of GW791343 was to increase agonist responses. Antagonist interaction and radioligand binding studies revealed that GW791343 did not interact at the ATP binding site but did interact with the compound-17 binding site suggesting that GW791343 is a positive allosteric modulator of the rat P2X(7) receptor.

CONCLUSIONS

Compound-17 was a negative allosteric modulator of human and rat P2X(7) receptors. GW791343 was a negative allosteric modulator of the human P2X(7) receptor but at the rat P2X(7) receptor its predominant effect was positive allosteric modulation. These compounds should provide valuable tools for mechanistic studies on P2X(7) receptors.

A comparative study of interleukin-1beta production and p2x7 expression after ATP stimulation by peripheral blood mononuclear cells isolated from rheumatoid arthritis patients and normal healthy controls

Interleukin 1 beta (IL-1beta) is a proinflammatory cytokine that is considered to play an important role in the progression of rheumatoid arthritis (RA). A stimulus such as ATP is necessary to cause the release of mature IL-1beta, via activation of the P2X(7) receptor on monocytes. In this study, the production of IL-1beta in whole blood after ATP stimulation and expression of P2X(7) receptors in RA and healthy subjects were examined. Blood samples from RA patients or healthy controls were stimulated with ATP in the presence of lipopolysaccharide (LPS). Supernatants were harvested and IL-1beta levels were measured by enzyme-linked immunosorbent assay (ELISA). Expression of P2X(7) receptors was measured using flow cytometry. ATP induced significantly higher levels of IL-1beta in LPS-activated RA blood samples compared to controls. A significant up-regulation of P2X(7) receptor expression on mononuclear cells was observed after overnight incubation with ATP without any significant differences between RA patients and normals. These data suggest that RA patient mononuclear cells are more sensitive to ATP stimulation than healthy individuals perhaps due to genetic polymorphism in the P2X(7) gene.

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.

Contribution of two ionotropic purinergic receptors to ATP responses in submandibular gland ductal cells

The effect of extracellular ATP on salivary gland function was compared in wild-type (WT) and P2X(7) knockout (KO) mice. The increase in the intracellular concentration of calcium ([Ca(2+)](i)) in response to carbachol was similar in submandibular ductal cells of WT and KO mice. ATP and its analog, benzoyl-ATP, induced a sustained increase in the [Ca(2+)](i) in WT animals. In KO mice, ATP slightly and transiently increased the [Ca(2+)](i) and benzoyl-ATP had no effect. The response to ATP of WT but not KO mice was blocked by KN-62, Coomassie blue and magnesium. The small response of ATP observed in KO mice was completely blocked in the absence of extracellular calcium, unchanged by U73122 and potentiated by ivermectin indicating the probable involvement of a P2X(4) receptor. A RT-PCR and a Western blot confirmed the presence of these receptors in ducts of both WT and KO mice. ATP increased the permeability of the cells to ethidium bromide and stimulated a phospholipase A(2) activity in WT but not KO mice. Mice submandibular gland cells secreted IL-1beta but this secretion was not modified by ATP and was similar in both groups of animals. The volume of saliva provoked by pilocarpine and the concentration of proteins, sodium and chloride in this saliva was similar in both groups of animals. The concentration of potassium was higher in KO mice. We can conclude that the major purinergic receptors expressed in mice submandibular ductal cells are P2X(7) receptors but that P2X(4) receptors are also involved in some ATP effects.

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.

Pannexin1 is part of the pore forming unit of the P2X(7) receptor death complex

The purinergic receptor P2X(7) is part of a complex signaling mechanism participating in a variety of physiological and pathological processes. Depending on the activation scheme, P2X(7) receptors in vivo are non-selective cation channels or form large pores that can mediate apoptotic cell death. Expression of P2X(7)R in Xenopus oocytes results exclusively in formation of a non-selective cation channel. However, here we show that co-expression of P2X(7)R with pannexin1 in oocytes leads to the complex response seen in many mammalian cells, including cell death with prolonged ATP application. While the cation channel activity is resistant to carbenoxolone treatment, this gap junction and hemichannel blocking drug suppressed the currents induced by ATP in pannexin1/P2X(7)R co-expressing cells. Thus, pannexin1 appears to be the molecular substrate for the permeabilization pore (or death receptor channel) recruited into the P2X(7)R signaling complex.

Caspase-containing complexes in the regulation of cell death and inflammation

Caspases are a family of cysteine proteases that are essential in the initiation and execution of apoptosis and the proteolytic maturation of inflammatory cytokines such as IL-1beta and IL-18. Caspases can be subdivided into those that have a large prodomain and those that have not. In general, apoptotic and inflammatory signalling pathways are initiated when large-prodomain caspases are recruited to large protein complexes via homotypic interactions involving death domain folds. The formation of these specialised multimeric platforms involves three major functions: (1) the sensing of cellular stress, damage, infection or inflammation; (2) multimerisation of the platform; and (3) recruitment and conformational activation of caspases. In this overview we discuss the complexes implicated in the regulation of cell death and inflammatory processes such as the death-inducing signalling complex (DISC), the apoptosome, the inflammasomes and the PIDDosome. We describe their sensing functions, compositions and functional outcomes. Inhibitory protein families such as FLIPs and CARD-only proteins prevent the recruitment of caspases in these sensing complexes, avoiding inappropriate initiation of cell death or inflammation.

A polymorphism in the P2X7 gene increases susceptibility to extrapulmonary tuberculosis

RATIONALE

Genetic variation influences susceptibility to clinical tuberculosis (TB). Activation of the P2X(7) receptor on human macrophages induces killing of mycobacteria. We have identified polymorphisms in the P2X(7) gene that markedly reduce this killing.

OBJECTIVE

To determine if polymorphisms in P2X7 are associated with increased risk of TB, the prevalence of four polymorphisms was assessed in individuals from Southeast Asia, where the majority of patients with TB in our study originate. The association of these polymorphisms with clinical TB was subsequently investigated in two separate case-control cohorts and the function of P2X(7) was assessed in subjects from one cohort.

METHODS

Genotyping of P2X7 polymorphisms was performed from subjects in a nested case-control study of a longitudinal refugee cohort and a separate case-control study. The functional capacity of P2X(7) was investigated by measuring ATP-mediated mycobacterial killing and apoptosis.

RESULTS

Only the 1513A-C polymorphism was present in Southeast Asians and the allele was associated with reduced killing of Mycobacterium tuberculosis. The 1513C allele was strongly associated with extrapulmonary, but not pulmonary, TB in the first (odds ratio, 3.8; 95% confidence interval, 1.6-9.0) and second cohorts (odds ratio, 3.7; 95% confidence interval, 1.7-8.0). ATP-mediated killing of mycobacteria was ablated in macrophages from subjects homozygous for the 1513C allele and significantly impaired in macrophages from heterozygous subjects. There was strong correlation between the degree of mycobacterial killing and ATP-induced apoptosis.

CONCLUSIONS

The 1513C allele increases susceptibility to extrapulmonary TB, and this defect is associated with the reduction in the capacity of macrophages to kill M. tuberculosis.

Amino acid residues in the P2X7 receptor that mediate differential sensitivity to ATP and BzATP

Agonist properties of the P2X7 receptor (P2X7R) differ strikingly from other P2X receptors in two main ways: high concentrations of ATP (> 100 microM) are required to activate the receptor, and the ATP analog 2',3'-O-(4-benzoyl-benzoyl)ATP (BzATP) is both more potent than ATP and evokes a higher maximum current. However, there are striking species differences in these properties. We sought to exploit the large differences in ATP and BzATP responses between rat and mouse P2X7R to delineate regions or specific residues that may be responsible for the unique actions of these agonists at the P2X7R. We measured membrane currents in response to ATP and BzATP at wild-type rat and mouse P2X7R, at chimeric P2X7Rs, and at mouse P2X7Rs bearing point mutations. Wild-type rat P2X7R was 10 times more sensitive to ATP and 100 times more sensitive to BzATP than wild-type mouse P2X7R. We found that agonist EC50 values were determined solely by the ectodomain of the P2X7R. Two segments (residues 115-136 and 282-288), when transposed together, converted mouse sensitivities to those of rat. Point mutations through these regions revealed a single residue, asparagine284, in the rat P2X7R that fully accounted for the 10-fold difference in ATP sensitivity, whereas the 100-fold difference in BzATP sensitivity required the transfer of both Lys127 and Asn284 from rat to mouse. Thus, single amino acid differences between species can account for large changes in agonist effectiveness and differentiate between the two widely used agonists at P2X7 receptors.

Palytoxin-induced cell death cascade in bovine aortic endothelial cells

The plasmalemmal Na(+)-K(+)-ATPase (NKA) pump is the receptor for the potent marine toxin palytoxin (PTX). PTX binds to the NKA and converts the pump into a monovalent cation channel that exhibits a slight permeability to Ca(2+). However, the ability of PTX to directly increase cytosolic free Ca(2+) concentration ([Ca(2+)](i)) via Na(+) pump channels and to initiate Ca(2+) overload-induced oncotic cell death has not been examined. Thus the purpose of this study was to determine the effect of PTX on [Ca(2+)](i) and the downstream events associated with cell death in bovine aortic endothelial cells. PTX (3-100 nM) produced a graded increase in [Ca(2+)](i) that was dependent on extracellular Ca(2+). The increase in [Ca(2+)](i) initiated by 100 nM PTX was blocked by pretreatment with ouabain with an IC(50) < 1 microM. The elevation in [Ca(2+)](i) could be reversed by addition of ouabain at various times after PTX, but this required much higher concentrations of ouabain (0.5 mM). These results suggest that the PTX-induced rise in [Ca(2+)](i) occurs via the Na(+) pump. Subsequent to the rise in [Ca(2+)](i), PTX also caused a concentration-dependent increase in uptake of the vital dye ethidium bromide (EB) but not YO-PRO-1. EB uptake was also blocked by ouabain added either before or after PTX. Time-lapse video microscopy showed that PTX ultimately caused cell lysis as indicated by release of transiently expressed green fluorescent protein (molecular mass 27 kDa) and rapid uptake of propidium iodide. Cell lysis was 1) greatly delayed by removing extracellular Ca(2+) or by adding ouabain after PTX, 2) blocked by the cytoprotective amino acid glycine, and 3) accompanied by dramatic membrane blebbing. These results demonstrate that PTX initiates a cell death cascade characteristic of Ca(2+) overload.

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

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

A Thr357 to Ser polymorphism in homozygous and compound heterozygous subjects causes absent or reduced P2X7 function and impairs ATP-induced mycobacterial killing by macrophages

The P2X(7) receptor is a ligand-gated cation channel that is highly expressed on mononuclear leukocytes and that mediates ATP-induced apoptosis and killing of intracellular pathogens. There is a wide variation in P2X(7) receptor function between subjects, explained in part by four loss-of-function polymorphisms (R307Q, E496A, I568N, and a 5'-intronic splice site polymorphism), as well as rare mutations. In this study, we report the allele frequencies of 11 non-synonymous P2X(7) polymorphisms and describe a fifth loss-of-function polymorphism in the gene (1096C --> G), which changes Thr(357) to Ser (T357S) with an allele frequency of 0.08 in the Caucasian population. P2X(7) function was measured by ATP-induced ethidium(+) influx into peripheral blood lymphocytes and monocytes and, when compared with wild-type subjects, was reduced to 10-65% in heterozygotes, 1-18% in homozygotes, and 0-10% in compound heterozygotes carrying T357S and a second loss-of-function polymorphism. Overexpression of the T357S mutant P2X(7) in either HEK-293 cells or Xenopus oocytes gave P2X(7) function of approximately 50% that of wild-type constructs. Differentiation of monocytes to macrophages, which also up-regulates P2X(7), restored P2X(7) function to near normal in cells heterozygous for T357S and to a value 50-65% of wild-type in cells homozygous for T357S or compound heterozygous for T357S/E496A. However, macrophages from subjects that are compound heterozygous for either T357S/R307Q or T357S/stop codon had near-to-absent P2X(7) function. These functional deficits induced by T357S were paralleled by impaired ATP-induced apoptosis and mycobacteria killing in macrophages from these subjects. Lymphocytes, monocytes, and macrophages from subjects homozygous for T357S or compound heterozygous for T357S and a second loss-of-function allele have reduced or absent P2X(7) receptor function.

Acidic ATP activates lymphocyte outwardly rectifying chloride channels via a novel pathway

Using whole-cell patch-clamp techniques we found that ATP activated an outwardly rectifying current in Daudi human B lymphoma cells under acidic conditions. The substitution of Cl- for gluconate(-) shifted the reversal potential, while Cl- channel blockers, 4,4'-diisothiocyanostibene-2,2'-disulfonic acid (DIDS) and 9-anthracene carboxylic acid (9-AC), blocked the current, indicating that ATP induces this current by activating the outwardly rectifying chloride channel (ORCC). The effect of ATP on ORCC was mimicked by ADP, but not by other P2 receptor agonists such as ATPgammaS (a poorly hydrolyzable analog of ATP), 2',3'-O-benzoyl-4-benzoyl-ATP (BzATP), and UTP. The ATP-induced ORCC current was completely blocked by 100 microM suramin (a P2 receptor antagonist), and was partially blocked by 100 microM pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium (PPADS), which is another P2 receptor antagonist. Neither inactivation of G proteins nor elimination of extracellular Ca2+ affected the ATP-induced current, indicating that G protein-coupled P2Y receptors and Ca(2+)-permeable P2X receptors are not involved. Based on the pharmacological profile and the fact that acidic conditions are required for ATP to activate the ORCC, we suggest that acidic ATP activates the lymphocyte ORCC via a novel pathway, which is not associated with any previously described purinergic receptors.

Membrane phosphatidylserine distribution as a non-apoptotic signalling mechanism in lymphocytes

Phosphatidylserine (PS) exposure is normally associated with apoptosis and the removal of dying cells. We observed that PS is exposed constitutively at high levels on T lymphocytes that express low levels of the transmembrane tyrosine phosphatase CD45RB. CD45 was shown to be a negative regulator of PS translocation in response to various signals, including activation of the ATP receptor P2X(7). Changes in PS distribution were shown to modulate several membrane activities: Ca(2+) and Na(+) uptake through the P2X(7) cation channel itself; P2X(7)-stimulated shedding of the homing receptor CD62L; and reversal of activity of the multidrug transporter P-glycoprotein. The data identify a role for PS distribution changes in signal transduction, rapidly modulating the activities of several membrane proteins. This seems to be an all-or-none effect, coordinating the activity of most or all the molecules of a target protein in each cell. The data also suggest a new approach to circumventing multidrug resistance.

Human melanomas express functional P2X7 receptors

Adenosine 5'-triphosphate is known to function as a potent extracellular messenger, producing its effects via a distinct family of cell surface receptors. Different receptor subtypes have been shown to modulate different cellular functions such as proliferation, differentiation and apoptosis. We have investigated the functional expression and apoptotic action of the P2 X (7) receptor in human malignant melanoma tissue and cells. Incubation of cells with the potent P2 X (7) receptor agonist 2'-3'-O-(4-benzoyl-benzoyl) adenosine 5'-triphosphate leads to a decrease in cell number, which is dose-dependent and reversible by the antagonist 1-N,O-bis-[5-isoquinoline-sulfonyl]-N-methyl-L-tyrosyl)-4-phenyl-piperazine. Synthesis of the P2 X(7) receptor by these cells has been established by reverse transcriptase-polymerase chain reaction, immunohistochemistry, immunocytochemistry and cellular accumulation of the fluorescent DNA-binding dye YO-PRO-1. The P2 X(7) receptors have been shown to mediate apoptotic actions of extracellular nucleotides and represent a novel target for melanoma therapy.

Detection of human P2X7 nucleotide receptor polymorphisms by a novel monocyte pore assay predictive of alterations in lipopolysaccharide-induced cytokine production

The nucleotide receptor P2X(7) is expressed by most leukocytes and initiates signaling events that amplify numerous LPS responses. We tested the hypothesis that loss-of-function polymorphisms in the human P2X(7) gene predispose to the production of an anti-inflammatory mediator balance. Accordingly, we developed a novel P2X(7) pore assay in whole blood that magnifies the activity from wild-type alleles and preserves the gene dosage effect for the 1513 C polymorphism (AA, 69 +/- 4; AC, 42 +/- 4; and CC, 6 +/- 1-fold stimulation). Thirty of 200 healthy individuals were identified as having low P2X(7) pore activity. Seven low pore subjects were 1513 CC, 3 and 11 participants had the other known variants 946 GA and 1729 TA respectively; the remaining 9 volunteers likely have novel polymorphisms. Because platelets are a large source of extracellular ATP during inflammation, whole blood was treated ex vivo with Salmonella typhimurium LPS in the absence of exogenous nucleotides. LPS-stimulated whole blood from individuals in the low pore activity group generated reduced plasma levels of TNF-alpha (p = 0.036) and higher amounts of IL-10 (p < 0.001) relative to the high pore controls. This reduction in the TNF-alpha to IL-10 ratio persisted to at least 24 h and is further decreased by cotreatment with 2-methylthio-ATP. The ability of P2X(7) polymorphisms to regulate the LPS-induced TNF-alpha to IL-10 ratio suggests that 15% of healthy adults may exhibit anti-inflammatory mediator responses during major infectious perturbations of the immune system, which can be predicted by P2X(7) pore activity.

A 5′ intronic splice site polymorphism leads to a null allele of the P2X7gene in 1-2% of the Caucasian population

The P2X(7) gene is important for the innate immune response but known polymorphisms do not explain all subjects with loss of P2X(7) function. A splice site mutation (g-->t) was found at position +1 of the first intron of the P2X(7) gene in 7 of 336 Caucasians and 1 of 39 subjects of Indian ethnicity. All eight subjects were heterozygous for the uncommon 1513A-->C polymorphism of the P2X(7) gene. RT-PCR and sequencing showed the splice site mutation was on the 1513C allele in the Caucasians and on the 1513A allele in the Indian subject. The splice site mutation is an inherited polymorphism and gives rise to a P2X(7) null allele in 1-2% of the Caucasian population.

The cytokine IL-1beta transiently enhances P2X7 receptor expression and function in human astrocytes

Extracellular nucleotide di- and triphosphates such as ATP and ADP mediate their effects through purinergic P2 receptors belonging to either the metabotropic P2Y or the ionotropic P2X receptor family. The P2X7R is a unique member of the P2X family, which forms a pore in response to ligand stimulation, regulating cell permeability, cytokine release, and/or apoptosis. This receptor is also unique in that its affinity for the ligand benzoyl-benzoyl ATP (BzATP) is at least 10-fold greater than that of ATP. Primary human fetal astrocytes in culture express low-levels of P2X7R mRNA and protein, and BzATP induces only a slight influx in intracellular calcium [Ca2+]i, with little demonstrable effect on gene expression or pore formation in these cells. We now show that, following treatment with the proinflammatory cytokine IL-1beta, BzATP induces a robust rise in [Ca2+]i with agonist and antagonist profiles indicative of the P2X7R. IL-1beta also induced the formation of membrane pores as evidenced by the uptake of YO-PRO-1 (375 Da). Quantitative real-time PCR demonstrated transient upregulation of P2X7R mRNA in IL-1beta-treated cells, while FACS analysis indicated a similar upregulation of P2X7R protein at the cell membrane. In multiple sclerosis lesions, immunoreactivity for the P2X7R was demonstrated on reactive astrocytes in autopsy brain tissues. In turn, P2X7R stimulation increased the production of IL-1-induced nitric oxide synthase activity by astrocytes in culture. These studies suggest that signaling via the P2X7R may modulate the astrocytic response to inflammation in the human central nervous system.

Are second messengers crucial for opening the pore associated with P2X7 receptor?

Stimulation of the P2X7 receptor by ATP induces cell membrane depolarization, increase in intracellular Ca2+ concentration, and, in most cases, permeabilization of the cell membrane to molecules up to 900 Da. After the activation of P2X7, at least two phenomena occur: the opening of low-conductance (8 pS) cationic channels and pore formation. At least two conflicting hypotheses have been postulated to reconcile these findings: 1) the P2X7 pore is formed as a result of gradual permeability increase (dilation) of cationic channels, and 2) the P2X7 pore represents a distinct channel, possibly activated by a second messenger and not directly by extracellular nucleotides. In this study, we investigated whether second messengers are necessary to open the pore associated with the P2X7 receptor in cells that expressed the pore activity by using the patch-clamp technique in whole cell and cell-attached configurations in conjunction with fluorescent imaging. In peritoneal macrophages and 2BH4 cells, we detected permeabilization and single-channel currents in the cell-attached configuration when ATP was applied outside the membrane patch in a condition in which oxidized ATP and Lucifer yellow were maintained within the pipette. Our data support Ca2+ as a second messenger associated with pore formation because the permeabilization depended on the presence of intracellular Ca2+ and was blocked by BAPTA-AM. In addition, MAPK inhibitors (SB-203580 and PD-98059) blocked the permeabilization and single-channel currents in these cells. Together our data indicate that the P2X7 pore depends on second messengers such as Ca2+ and MAP kinases.

Extracellular ATP increases cation fluxes in human erythrocytes by activation of the P2X7 receptor

Canine erythrocytes are known to undergo a reversible increase in cation permeability when incubated with extracellular ATP. We have examined the expression and function of P2X receptors on human erythrocytes using confocal microscopy and a panel of anti-P2X(1-7) antibodies and have measured monovalent cation fluxes in the presence of various nucleotide agonists. Human erythrocytes expressed P2X7 receptors on all cells examined from eight of eight subjects, as well as P2X2 at a far lower staining intensity in six of eight subjects. ATP stimulated the efflux of 86Rb+ (K+) from human erythrocytes in a dose-dependent fashion with an EC50 of approximately 95 microM. Other nucleotides also induced an efflux of 86Rb+ from erythrocytes with an order of agonist potency of 2'- and 3'-O(4-benzoylbenzoyl) ATP (BzATP) > ATP > 2-methylthio-ATP (2MeSATP) > adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), whereas ADP or UTP had no effect. ATP-induced efflux of 86Rb+ from erythrocytes was inhibited by extracellular Na+ and oxidized ATP, as well as by KN-62, an antagonist specific for the human P2X7 receptor. When erythrocytes were incubated in isotonic KCl medium, the addition of ATP stimulated an 86Rb+ influx approximately equal in magnitude to ATP-stimulated 86Rb+ efflux from the same cells. BzATP also stimulated the influx of 22Na+ into erythrocytes incubated in isotonic NaCl medium. Both ATP-induced efflux and influx of 86Rb+ and 22Na+ were impaired in erythrocytes from subjects who had inherited loss-of-function polymorphisms in the P2X7 receptor. These results suggest that the reversible permeabilization of erythrocytes by extracellular ATP is mediated by the P2X7 receptor.

Glu496 to Ala polymorphism in the P2X7 receptor impairs ATP-induced IL-1 beta release from human monocytes

Priming of monocytes with LPS produces large quantities of intracellular, biologically inactive IL-1beta that can be processed and released by subsequent activation of the P2X7 receptor by extracellular ATP. We examined whether a loss-of-function polymorphism of the human P2X7 receptor (Glu496Ala) impairs this process. Both ATP-induced ethidium+ uptake and ATP-induced shedding of L-selectin (CD62L) were nearly absent in monocytes from four subjects homozygous for Glu496Ala confirming that this polymorphism impairs P2X7 function. The level of ATP-induced IL-1beta released in 2 h from LPS-activated whole blood from homozygous subjects was 50% of that from wild-type samples. A more marked defect in IL-1beta release was observed from LPS-activated monocytes of homozygous subjects which was only 22% of that released from wild-type monocytes after a 30-min incubation with ATP. However, after a 60-min incubation with ATP, the amount of IL-1beta released from homozygous monocytes was 70% of that released from wild-type monocytes. Incubation of monocytes of either genotype with nigericin resulted in a similar release of IL-1beta. Western blotting demonstrated that ATP induced the release of mature 17-kDa IL-1beta from monocytes, and confirmed that this process was impaired in homozygous monocytes. Finally, ATP-induced 86Rb+ efflux was 9-fold lower from homozygous monocytes than from wild-type monocytes. The results indicate that ATP-induced release of IL-1beta is slower in monocytes from subjects homozygous for the Glu496Ala polymorphism in the P2X7 receptor and that this reduced rate of IL-1beta release is associated with a lower ATP-induced K+ efflux.

A P2X7 receptor stimulates plasma membrane trafficking in the FRTL rat thyrocyte cell line

Thyroid cells express a variety of P2Y and P2X purinergic receptor subtypes. G protein-coupled P2Y receptors influence a wide variety of thyrocyte-specific functions; however, functional P2X receptor-gated channels have not been observed. In this study, we used whole cell patch-clamp recording and fluorescence imaging of the plasma membrane marker FM1-43 to examine the effects of extracellular ATP on membrane permeability and trafficking in the Fisher rat thyroid cell line FRTL. We found a cation-selective current that was gated by ATP and 2',3'-O-(4-benzoylbenzoyl)-ATP but not by UTP. The ATP-evoked currents were inhibited by pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid, adenosine 5'-triphosphate-2',3'-dialdehyde, 100 microM Zn(2+), and 50 microM Cu(2+). Fluorescence imaging revealed pronounced, temperature-sensitive stimulation of exocytosis and membrane internalization by ATP with the same pharmacological profile as observed for activation of current. The EC(50) for ATP stimulation of internalization was 440 microM in saline containing 2 mM Ca(2+) and 2 mM Mg(2+), and 33 microM in low-Mg(2+), nominally Ca(2+)-free saline. Overall, the results are most consistent with activation of a P2X(7) receptor by ATP(4-). However, low permeability to N-methyl-d-glucamine(+) and the propidium cation YO-PRO-1 indicates absence of the cytolytic pore that often accompanies P2X(7) receptor activation. ATP stimulation of internalization occurs in Na(+)-free, Ca(2+)-free, or low-Mg(2+) saline and therefore does not depend on cation influx through the ATP-gated channel. We conclude that ATP activation of a P2X(7) receptor stimulates membrane internalization in FRTL cells via a transduction pathway that does not depend on cation influx.

An Arg307 to Gln polymorphism within the ATP-binding site causes loss of function of the human P2X7 receptor

The P2X(7) receptor is a ligand-gated channel that is highly expressed on mononuclear cells of the immune system and that mediates ATP-induced apoptosis. Wide variations in the function of the P2X receptor have been observed, explained in part by (7)loss-of-function polymorphisms that change Glu(496) to Ala (E496A) and Ile(568) to Asn (I568N). In this study, a third polymorphism, which substitutes an uncharged glutamine for the highly positively charged Arg(307) (R307Q), has been found in heterozygous dosage in 12 of 420 subjects studied. P2X(7) function was measured by ATP-induced fluxes of Rb(+), Ba(2+), and ethidium(+) into peripheral blood monocytes or various lymphocyte subsets and was either absent or markedly decreased. Transfection experiments showed that P2X(7) carrying the R307Q mutation lacked either channel or pore function despite robust protein synthesis and surface expression of the receptor. The monoclonal antibody (clone L4) that binds to the extracellular domain of wild type P2X(7) and blocks P2X(7) function failed to bind to the R307Q mutant receptor. Differentiation of monocytes to macrophages up-regulated P2X(7) function in cells heterozygous for the R307Q to a value 10-40% of that for wild type macrophages. However, macrophages from a subject who was double heterozygous for R307Q/I568N remained totally non-functional for P2X(7), and lymphocytes from the same subject also lacked ATP-stimulated phospholipase D activity. These data identify a third loss-of-function polymorphism affecting the human P2X(7) receptor, and since the affected Arg(307) is homologous to those amino acids essential for ATP binding to P2X(1) and P2X(2), it is likely that this polymorphism abolishes the binding of ATP to the extracellular domain of P2X(7).

P2X7 polymorphism and chronic lymphocytic leukaemia: lack of correlation with incidence, survival and abnormalities of chromosome 12

The P2X7 receptor, a plasma membrane ATP-gated ion channel that plays a role in lymphocyte apoptosis, has been suggested as an important contributory factor to the pathogenesis of chronic lymphocytic leukaemia (CLL). The P2X7 gene resides on chromosome 12 and is polymorphic in the population at large (1513A/C) with the A and C alleles encoding fully active and nonfunctional proteins, respectively. We have evaluated the significance of this polymorphism by genotyping 144 patients with CLL and 348 healthy controls using a tetraprimer ARMS assay. We found no significant difference in allele frequency between patients and controls. Although patients with the C allele (A/C heterozygotes or C/C homozygotes) had a marginally shorter survival than those who were homozygous for the A allele, this difference was not significant for either the patient group considered as a whole or for IgVH-mutated/unmutated subsets. Finally, no association was found between trisomy 12 and P2X7 genotype. We conclude that the influence, if any, of P2X7 genotype on susceptibility to CLL or clinical outcome is small.

Chicken DT40 cells stably transfected with the rat P2X7 receptor ion channel: a system suitable for the study of purine receptor-mediated cell death

We have generated and characterised a clone of chicken DT40 lymphocytes stably transfected with the rat P2X(7) receptor (rP2X(7)). Successful transfection was confirmed by Western blotting. Under voltage clamp, P2X(7)-expressing cells responded to ATP and dibenzoyl-ATP (Bz-ATP) (a more potent P2X(7) receptor agonist) with a rapidly activating and sustained inward current. The EC(50) values for these agonists were 305 and 15 microM, respectively. Bz-ATP evoked Ca(2+) and Mn(2+) influx into transfected cells as determined by Fura-2 spectrofluorimetry. Responses to Bz-ATP were inhibited by pre-treatment of cells with oxidised ATP. Treatment of cells with Bz-ATP for up to 24hr produced time- and concentration-dependent cell death. This was associated with an increase in caspase-3-like activity, exposure of phosphatidylserine on the outside of cell membrane and DNA cleavage, indicating death by apoptosis. Pre-treatment with Z-VAD-fmk, a pan-caspase inhibitor, reduced the DNA fragmentation and phosphatidylserine externalisation, but did not affect overall rates of cell death at 24hr, implicating caspase-independent mechanisms. The properties of rP2X(7) receptors expressed in DT40 cells are similar to those described for other expression systems. Because DT40 cells lack functionally detectable endogenous P2 receptors and are highly amenable to genetic manipulation, stably transfected DT40 cells provide a novel and potentially useful model system in which to investigate the intracellular signal transduction pathways associated with P2X(7) receptor stimulation, in particular those involved in induction of cell death.