Properties of the pore-forming P2X7 purinoceptor in mouse NTW8 microglial cells

Techniques for evaluating the ATP-gated ion channel P2X7 receptor function in macrophages and microglial cells

Resident macrophages are tissue-specific innate immune cells acting as sentinels, constantly patrolling their assigned tissue to maintain homeostasis, and quickly responding to pathogenic invaders or molecular danger signals molecules when necessary. Adenosine triphosphate (ATP), when released to the extracellular medium, acts as a danger signal through specific purinergic receptors. Interaction of ATP with the purinergic receptor P2X7 activates macrophages and microglial cells in different pathological conditions, triggering inflammation. The highly expressed P2X7 receptor in these cells induces cell membrane permeabilization, inflammasome activation, cell death, and the production of inflammatory mediators, including cytokines and nitrogen and oxygen-reactive species. This review explores the techniques to evaluate the functional and molecular aspects of the P2X7 receptor, particularly in macrophages and microglial cells. Polymerase chain reaction (PCR), Western blotting, and immunocytochemistry or immunohistochemistry are essential for assessing gene and protein expression in these cell types. Evaluation of P2X7 receptor function involves the use of ATP and selective agonists and antagonists and diverse techniques, including electrophysiology, intracellular calcium measurements, ethidium bromide uptake, and propidium iodide cell viability assays. These techniques are crucial for studying the role of P2X7 receptors in immune responses, neuroinflammation, and various pathological conditions. Therefore, a comprehensive understanding of the functional and molecular aspects of the P2X7 receptor in macrophages and microglia is vital for unraveling its involvement in immune modulation and its potential as a therapeutic target. The methodologies presented and discussed herein offer valuable tools for researchers investigating the complexities of P2X7 receptor signaling in innate immune cells in health and disease.

Modulation of Microglial Function by ATP-Gated P2X7 Receptors: Studies in Rat, Mice and Human

P2X receptors are a family of seven ATP-gated ion channels that trigger physiological and pathophysiological responses in a variety of cells. Five of the family members are sensitive to low concentrations of extracellular ATP, while the P2X6 receptor has an unknown affinity. The last subtype, the P2X7 receptor, is unique in requiring millimolar concentrations to fully activate in humans. This low sensitivity imparts the agonist with the ability to act as a damage-associated molecular pattern that triggers the innate immune response in response to the elevated levels of extracellular ATP that accompany inflammation and tissue damage. In this review, we focus on microglia because they are the primary immune cells of the central nervous system, and they activate in response to ATP or its synthetic analog, BzATP. We start by introducing purinergic receptors and then briefly consider the roles that microglia play in neurodevelopment and disease by referencing both original works and relevant reviews. Next, we move to the role of extracellular ATP and P2X receptors in initiating and/or modulating innate immunity in the central nervous system. While most of the data that we review involve work on mice and rats, we highlight human studies of P2X7R whenever possible.

Animal Models for the Investigation of P2X7 Receptors

The P2X7 receptor is a trimeric ligand-gated cation channel activated by extracellular adenosine 5'-triphosphate. The study of animals has greatly advanced the investigation of P2X7 and helped to establish the numerous physiological and pathophysiological roles of this receptor in human health and disease. Following a short overview of the P2X7 distribution, roles and functional properties, this article discusses how animal models have contributed to the generation of P2X7-specific antibodies and nanobodies (including biologics), recombinant receptors and radioligands to study P2X7 as well as to the pharmacokinetic testing of P2X7 antagonists. This article then outlines how mouse and rat models have been used to study P2X7. These sections include discussions on preclinical disease models, polymorphic P2X7 variants, P2X7 knockout mice (including bone marrow chimeras and conditional knockouts), P2X7 reporter mice, humanized P2X7 mice and P2X7 knockout rats. Finally, this article reviews the limited number of studies involving guinea pigs, rabbits, monkeys (rhesus macaques), dogs, cats, zebrafish, and other fish species (seabream, ayu sweetfish, rainbow trout and Japanese flounder) to study P2X7.

Purinoreceptors and ectonucleotidases control ATP-induced calcium waveforms and calcium-dependent responses in microglia: Roles of P2 receptors and CD39 in ATP-stimulated microglia

Adenosine triphosphate (ATP) and its metabolites drive microglia migration and cytokine production by activating P2X- and P2Y- class purinergic receptors. Purinergic receptor activation gives rise to diverse intracellular calcium (Ca2+ signals, or waveforms, that differ in amplitude, duration, and frequency. Whether and how these characteristics of diverse waveforms influence microglia function is not well-established. We developed a computational model trained with data from published primary murine microglia studies. We simulate how purinoreceptors influence Ca2+ signaling and migration, as well as, how purinoreceptor expression modifies these processes. Our simulation confirmed that P2 receptors encode the amplitude and duration of the ATP-induced Ca2+ waveforms. Our simulations also implicate CD39, an ectonucleotidase that rapidly degrades ATP, as a regulator of purinergic receptor-induced Ca2+ responses. Namely, it was necessary to account for CD39 metabolism of ATP to align the model's predicted purinoreceptor responses with published experimental data. In addition, our modeling results indicate that small Ca2+ transients accompany migration, while large and sustained transients are needed for cytokine responses. Lastly, as a proof-of-principal, we predict Ca2+ transients and cell membrane displacements in a BV2 microglia cell line using published P2 receptor mRNA data to illustrate how our computer model may be extrapolated to other microglia subtypes. These findings provide important insights into how differences in purinergic receptor expression influence microglial responses to ATP.

Using Whole-Cell Electrophysiology and Patch-Clamp Photometry to Characterize P2X7 Receptor Currents

The fundamental property of P2X7 receptor (P2X7R) channels is the transport of cations across the cell surface membrane. Electrophysiology and patch-clamp photometry are readily accessible methods of measuring this flux in a wide range of cell types. They are important tools used to characterize the functional properties of native cells studied in cell culture, in vitro tissue slices, and, in some cases, in situ single cells. Further, they are efficient methods of probing the relation of structure to function of recombinant receptors expressed in heterologous systems. Here, we provide step-by-step procedures for use of two standard recording protocols, broken-patch and perforated-patch voltage clamp. Further, we describe a third technique, called the dye-overload method, that uses simultaneous measurement of membrane current and fura-2 fluorescence to quantify the contribution of Ca²⁺ flux to the ATP-gated current.

Mathematical modelling of human P2X-mediated plasma membrane electrophysiology and calcium dynamics in microglia

Regulation of cytosolic calcium (Ca²⁺) dynamics is fundamental to microglial function. Temporal and spatial Ca²⁺ fluxes are induced from a complicated signal transduction pathway linked to brain ionic homeostasis. In this paper, we develop a novel biophysical model of Ca²⁺ and sodium (Na⁺) dynamics in human microglia and evaluate the contribution of purinergic receptors (P2XRs) to both intracellular Ca²⁺ and Na⁺ levels in response to agonist/ATP binding. This is the first comprehensive model that integrates P2XRs to predict intricate Ca²⁺ and Na⁺ transient responses in microglia. Specifically, a novel compact biophysical model is proposed for the capture of whole-cell patch-clamp currents associated with P2X₄ and P2X₇ receptors, which is composed of only four state variables. The entire model shows that intricate intracellular ion dynamics arise from the coupled interaction between P2X₄ and P2X₇ receptors, the Na⁺/Ca²⁺ exchanger (NCX), Ca²⁺ extrusion by the plasma membrane Ca²⁺ ATPase (PMCA), and Ca²⁺ and Na⁺ leak channels. Both P2XRs are modelled as two separate adenosine triphosphate (ATP) gated Ca²⁺ and Na⁺ conductance channels, where the stoichiometry is the removal of one Ca²⁺ for the hydrolysis of one ATP molecule. Two unique sets of model parameters were determined using an evolutionary algorithm to optimise fitting to experimental data for each of the receptors. This allows the proposed model to capture both human P2X₇ and P2X₄ data (hP2X₇ and hP2X₄). The model architecture enables a high degree of simplicity, accuracy and predictability of Ca²⁺ and Na⁺ dynamics thus providing quantitative insights into different behaviours of intracellular Na⁺ and Ca²⁺ which will guide future experimental research. Understanding the interactions between these receptors and other membrane-bound transporters provides a step forward in resolving the qualitative link between purinergic receptors and microglial physiology and their contribution to brain pathology.

Mathematical modelling and computer simulation are powerful tools by which we can analyse complex biological systems, particularly, neural phenomena involved in brain dysfunction. In this research, we develop a theoretical foundation for studying P2X-mediated calcium and sodium signalling in human microglial cells. Microglia, which are brain-resident macrophages, restructure their intracellular actin cytoskeleton to enable motility; this restructuring requires a complex molecular cascade involving a set of ionic channels, membrane-coupled receptors and cytosolic components. Recent studies highlight the importance for increasing our understanding of microglia physiology, since their functions play critical roles in both normal physiological and pathological dynamics of the brain. There is a need to develop reliable human cellular models to investigate the biology of microglia aimed at understanding the influence of purinergic signalling in brain dysfunction to provide novel drug discovery targets. In this work, a detailed mathematical model is built for the dynamics of human P2XRs in microglia. Subsequently, experimental whole-cell currents are used to derive P2X-mediated electrophysiology of human microglia (i.e. sodium and calcium dynamics, and membrane potential). Our predictions reveal new quantitative insights into P2XRs on how they regulate ionic concentrations in terms of physiological interactions and transient responses.

Structural basis for the functional properties of the P2X7 receptor for extracellular ATP

The P2X7 receptor, originally known as the P2Z receptor due to its distinctive functional properties, has a structure characteristic of the ATP-gated ion channel P2X receptor family. The P2X7 receptor is an important mediator of ATP-induced purinergic signalling and is involved the pathogenesis of numerous conditions as well as in the regulation of diverse physiological functions. Functional characterisations, in conjunction with site-directed mutagenesis, molecular modelling, and, recently, structural determination, have provided significant insights into the structure–function relationships of the P2X7 receptor. This review discusses the current understanding of the structural basis for the functional properties of the P2X7 receptor.

Hyperactivation of P2X7 receptors as a culprit of COVID-19 neuropathology

Scientists and health professionals are exhaustively trying to contain the coronavirus disease 2019 (COVID-19) pandemic by elucidating viral invasion mechanisms, possible drugs to prevent viral infection/replication, and health cares to minimize individual exposure. Although neurological symptoms are being reported worldwide, neural acute and long-term consequences of SARS-CoV-2 are still unknown. COVID-19 complications are associated with exacerbated immunoinflammatory responses to SARS-CoV-2 invasion. In this scenario, pro-inflammatory factors are intensely released into the bloodstream, causing the so-called "cytokine storm". Both pro-inflammatory factors and viruses may cross the blood-brain barrier and enter the central nervous system, activating neuroinflammatory responses accompanied by hemorrhagic lesions and neuronal impairment, which are largely described processes in psychiatric disorders and neurodegenerative diseases. Therefore, SARS-CoV-2 infection could trigger and/or worse brain diseases. Moreover, patients with central nervous system disorders associated to neuroimmune activation (e.g. depression, Parkinson's and Alzheimer's disease) may present increased susceptibility to SARS-CoV-2 infection and/or achieve severe conditions. Elevated levels of extracellular ATP induced by SARS-CoV-2 infection may trigger hyperactivation of P2X7 receptors leading to NLRP3 inflammasome stimulation as a key mediator of neuroinvasion and consequent neuroinflammatory processes, as observed in psychiatric disorders and neurodegenerative diseases. In this context, P2X7 receptor antagonism could be a promising strategy to prevent or treat neurological complications in COVID-19 patients.

Positron Emission Tomography in the Inflamed Cerebellum: Addressing Novel Targets among G Protein-Coupled Receptors and Immune Receptors

Inflammatory processes preceding clinical manifestation of brain diseases are moving increasingly into the focus of positron emission tomographic (PET) investigations. A key role in inflammation and as a target of PET imaging efforts is attributed to microglia. Cerebellar microglia, with a predominant ameboid and activated subtype, is of special interest also regarding improved and changing knowledge on functional involvement of the cerebellum in mental activities in addition to its regulatory role in motor function. The present contribution considers small molecule ligands as potential PET tools for the visualization of several receptors recognized to be overexpressed in microglia and which can potentially serve as indicators of inflammatory processes in the cerebellum. The sphingosine 1 phosphate receptor 1 (S1P1), neuropeptide Y receptor 2 (NPY2) and purinoceptor Y12 (P2Y12) cannabinoid receptors and the chemokine receptor CX3CR1 as G-protein-coupled receptors and the ionotropic purinoceptor P2X7 provide structures with rather classical binding behavior, while the immune receptor for advanced glycation end products (RAGE) and the triggering receptor expressed on myeloid cells 2 (TREM2) might depend for instance on further accessory proteins. Improvement in differentiation between microglial functional subtypes in comparison to the presently used 18 kDa translocator protein ligands as well as of the knowledge on the role of polymorphisms are special challenges in such developments.

Revisiting the Idea That Amyloid-β Peptide Acts as an Agonist for P2X7

The P2X7 receptor (P2X7) is a cell surface ligand-gated ion channel, activated by its physiological nucleotide agonist ATP and a synthetic analog (BzATP). However, it has also been suggested that there may be structurally unrelated, non-nucleotide agonists such as the amyloidogenic β peptide. Here we aimed to reassess the effect of amyloid β peptides in various in vitro cell models, namely HEK293 overexpressing human P2X7, the microglial BV-2 cell line, and BV-2 cells lacking P2X7. We measured YO-PRO-1 dye uptake in response to full-length amyloid β peptide (1-42) or the shorter amyloid β peptide (25-35) and there was a concentration-dependent increase in YO-PRO-1 dye uptake in HEK-hP2X7 cells. However, these amyloid β peptide-induced increases in YO-PRO-1 dye uptake were also identical in non-transfected HEK-293 cells. We could observe small transient increases in [Ca²⁺] _i induced by amyloid β peptides in BV-2 cells, however these were identical in BV-2 cells lacking P2X7. Furthermore, our metabolic viability and LDH release experiments suggest no significant change in viability or cell membrane damage in HEK-hP2X7 cells. In the BV-2 cells we found that high concentrations of amyloid β peptides (1-42) and (25-35) could reduce cell viability by up to 35% but this was also seen in BV-2 cells lacking P2X7. We found no evidence of LDH release by amyloid β peptides. In summary, we found no evidence that amyloid β peptides act as agonists of P2X7 in our in vitro models. Our study raises the possibility that amyloid β peptides simply mimic features of P2X7 activation.

Functional expression of P2X1, P2X4 and P2X7 purinergic receptors in human monocyte-derived macrophages

This study sought to examine the co-expression of the following purinergic receptor subunits: P2X1, P2X1del, P2X4, and P2X7 and characterize the P2X response in human monocyte-derived macrophages (MDMs). Single-cell RT-PCR shows the presence of P2X1, P2X1del, P2X4, and P2X7 mRNA in 40%, 5%, 20%, and 90% of human MDMs, respectively. Of the studied human MDMs, 25% co-expressed P2X1 and P2X7 mRNA; 5% co-expressed P2X4 and P2X7; and 15% co-expressed P2X1, P2X4, and P2X7 mRNA. In whole-cell patch clamp recordings of human MDMs, rapid application of ATP (0.01 mM) evoked fast current activation and two different desensitization kinetics: 1. a rapid desensitizing current antagonized by PPADS (1 μM), reminiscent of the P2X1 receptor's current; 2. a slow desensitizing current, insensitive to PPADS but potentiated by ivermectin (3 μM), similar to the P2X4 receptor's current. Application of 5 mM ATP induced three current modalities: 1. slow current activation with no desensitization, similar to the P2X7 receptor current, present in 69% of human macrophages and antagonized by A-804598 (0.1 μM); 2. fast current activation and fast desensitization, present in 15% of human MDMs; 3. fast activation current followed by biphasic desensitization, observed in 15% of human MDMs. Both rapid and biphasic desensitization kinetics resemble those observed for the recombinant human P2X1 receptor expressed in oocytes. These data demonstrate, for the first time, the co-expression of P2X1, P2X4, and P2X7 transcripts and confirm the presence of functional P2X1, P2X4, and P2X7 receptors in human macrophages.

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

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

Simulation of P2X-mediated calcium signalling in microglia

KEY POINTS

A computational model of P2X channel activation in microglia was developed that includes downfield Ca²⁺ -dependent signalling pathways. This model provides quantitative insights into how diverse signalling pathways in microglia converge to control microglial function.

ABSTRACT

Microglia function is orchestrated through highly coupled signalling pathways that depend on calcium (Ca²⁺ ). In response to extracellular ATP, transient increases in intracellular Ca²⁺ driven through the activation of purinergic receptors, P2X and P2Y, are sufficient to promote cytokine synthesis. Although the steps comprising the pathways bridging purinergic receptor activation with transcriptional responses have been probed in great detail, a quantitative model for how these steps collectively control cytokine production has not been established. Here we developed a minimal computational model that quantitatively links extracellular stimulation of two prominent ionotropic purinergic receptors, P2X4 and P2X7, with the graded production of a gene product, namely the tumour necrosis factor α (TNFα) cytokine. In addition to Ca²⁺ handling mechanisms common to eukaryotic cells, our model includes microglia-specific processes including ATP-dependent P2X4 and P2X7 activation, activation of nuclear factor of activated T-cells (NFAT) transcription factors, and TNFα production. Parameters for this model were optimized to reproduce published data for these processes, where available. With this model, we determined the propensity for TNFα production in microglia, subject to a wide range of ATP exposure amplitudes, frequencies and durations that the cells could encounter in vivo. Furthermore, we have investigated the extent to which modulation of the signal transduction pathways influence TNFα production. Our results suggest that pulsatile stimulation of P2X4 via micromolar ATP may be sufficient to promote TNFα production, whereas high-amplitude ATP exposure is necessary for production via P2X7. Furthermore, under conditions that increase P2X4 expression, for instance, following activation by pathogen-associated molecular factors, P2X4-associated TNFα production is greatly enhanced. Given that Ca²⁺ homeostasis in microglia is profoundly important to its function, this computational model provides a quantitative framework to explore hypotheses pertaining to microglial physiology.

6-Substituted Hexamethylene Amiloride (HMA) Derivatives as Potent and Selective Inhibitors of the Human Urokinase Plasminogen Activator for Use in Cancer

Metastasis is the cause of death in the majority (∼90%) of malignant cancers. The oral potassium-sparing diuretic amiloride and its 5-substituted derivative 5 -N, N-(hexamethylene)amiloride (HMA) reportedly show robust antitumor/metastasis effects in multiple in vitro and animal models. These effects are likely due, at least in part, to inhibition of the urokinase plasminogen activator (uPA), a key protease determinant of cell invasiveness and metastasis. This study reports the discovery of 6-substituted HMA analogs that show nanomolar potency against uPA, high selectivity over related trypsin-like serine proteases, and minimal inhibitory effects against epithelial sodium channels (ENaC), the diuretic and antikaliuretic target of amiloride. Reductions in lung metastases were demonstrated for two analogs in a late-stage experimental mouse metastasis model, and one analog completely inhibited formation of liver metastases in an orthotopic xenograft mouse model of pancreatic cancer. The results support further evaluation of 6-substituted HMA derivatives as uPA-targeting anticancer drugs.

Synthesis and in vitro characterization of a P2X7 radioligand [123I]TZ6019 and its response to neuroinflammation in a mouse model of Alzheimer disease

The purinergic receptor P2X ligand-gated ion channel 7 (P2X7 receptor) is a promising imaging target to detect neuroinflammation. Herein, we report development of a potent iodinated radiotracer for P2X7 receptor, [¹²³I]TZ6019. The radiosynthesis of [¹²³I]TZ6019 was accomplished by allylic-tin precursor iodination using [¹²³I]NaI with good radiochemical yield of 85% and high radiochemical purity of > 99%. Human embryonic kidney 293 (HEK-293) cell line stably transfected with the human P2X7 receptor was used to characterize the binding affinity of TZ6019 by fluorescence, radioactive competitive, and saturation binding assays. A radioligand competitive binding assay with [¹²³I]TZ6019 demonstrated that the nonradioactive compound TZ6019 has an IC₅₀ value of 9.49 ± 1.4nM, and the known P2X7 receptor compound GSK1482160 has an IC₅₀ value of 4.30 ± 0.86nM, consistent with previous reports. The radioligand saturation binding assay and competitive assay revealed that [¹²³I]TZ6019 specifically bound to the human P2X7 receptor with high affinity (K_i = 6.3 ± 0.9nM). In vitro autoradiography quantification with brain slices collected from 9-month old P301S tau transgenic mice along with wild type controls, revealed higher binding of [¹²³I]TZ6019 (35% increase) in the brain of P301S transgenic mice (n = 3, p = 0.04) compared to wild type controls. The immunofluorescence microscopy confirmed that expression of P2X7 receptor was colocalized with astrocytes in the tauopathy P301S transgenic mice. [¹²³I]TZ6019 has specific binding for P2X7 receptor and has great potential to be a radiotracer for screening new compounds and quantifying expression of P2X7 receptor in neuroinflammation related diseases.

The P2X7 receptor forms a dye-permeable pore independent of its intracellular domain but dependent on membrane lipid composition

The P2X7 receptor mediates extracellular ATP signaling implicated in the development of devastating diseases such as chronic pain and cancer. Activation of the P2X7 receptor leads to opening of the characteristic dye-permeable membrane pore for molecules up to ~900 Da. However, it remains controversial what constitutes this peculiar pore and how it opens. Here we show that the panda receptor, when purified and reconstituted into liposomes, forms an intrinsic dye-permeable pore in the absence of other cellular components. Unexpectedly, we found that this pore opens independent of its unique C-terminal domain. We also found that P2X7 channel activity is facilitated by phosphatidylglycerol and sphingomyelin, but dominantly inhibited by cholesterol through direct interactions with the transmembrane domain. In combination with cell-based functional studies, our data suggest that the P2X7 receptor itself constitutes a lipid-composition dependent dye-permeable pore, whose opening is facilitated by palmitoylated cysteines near the pore-lining helix.

Store-Operated Ca2+ Entry (SOCE) and Purinergic Receptor-Mediated Ca2+ Homeostasis in Murine bv2 Microglia Cells: Early Cellular Responses to ATP-Mediated Microglia Activation

Microglia activation is a neuroinflammatory response to parenchymal damage with release of intracellular metabolites, e.g., purines, and signaling molecules from damaged cells. Extracellular purines can elicit Ca²⁺-mediated microglia activation involving P2X/P2Y receptors with metabotropic (P2Y) and ionotropic (P2X) cell signaling in target cells. Such microglia activation results in increased phagocytic activity, activation of their inflammasome and release of cytokines to sustain neuroinflammatory (so-called M1/M2 polarization). ATP-induced activation of ionotropic P2X4 and P2X7 receptors differentially induces receptor-operated Ca²⁺ entry (ROCE). Although store-operated Ca²⁺ entry (SOCE) was identified to modulate ROCE in primary microglia, its existence and role in one of the most common murine microglia cell line, BV2, is unknown. To dissect SOCE from ROCE in BV2 cells, we applied high-resolution multiphoton Ca²⁺ imaging. After depleting internal Ca²⁺ stores, SOCE was clearly detectable. High ATP concentrations (1 mM) elicited sustained increases in intracellular [Ca²⁺]_i whereas lower concentrations (≤100 μM) also induced Ca²⁺ oscillations. These differential responses were assigned to P2X7 and P2X4 activation, respectively. Pharmacologically inhibiting P2Y and P2X responses did not affect SOCE, and in fact, P2Y-responses were barely detectable in BV2 cells. STIM1S content was significantly upregulated by 1 mM ATP. As P2X-mediated Ca²⁺ oscillations were rare events in single cells, we implemented a high-content screening approach that allows to record Ca²⁺ signal patterns from a large number of individual cells at lower optical resolution. Using automated classifier analysis, several drugs (minocycline, U73122, U73343, wortmannin, LY294002, AZ10606120) were tested on their profile to act on Ca²⁺ oscillations (P2X4) and sustained [Ca²⁺]_i increases. We demonstrate specific drug effects on purinergic Ca²⁺ pathways and provide new pharmacological insights into Ca²⁺ oscillations in BV2 cells. For example, minocycline inhibits both P2X7- and P2X4-mediated Ca²⁺-responses, and this may explain its anti-inflammatory action in neuroinflammatory disease. As a technical result, our novel automated bio-screening approach provides a biomedical engineering platform to allow high-content drug library screens to study neuro-inflammation in vitro.

Functional expression of P2 purinoceptors in a primary neuroglial cell culture of the rat arcuate nucleus

The arcuate nucleus (ARC) plays an important role in the hypothalamic control of energy homeostasis. Expression of various purinoceptor subtypes in the rat ARC and physiological studies suggest a modulatory function of P2 receptors within the neuroglial ARC circuitry. A differentiated mixed neuronal and glial microculture was therefore established from postnatal rat ARC, revealing neuronal expression of ARC-specific transmitters involved in food intake regulation (neuropeptide Y (NPY), proopiomelanocortin (POMC), tyrosine hydroxylase (TH)). Some NPYergic neurons cosynthesized TH, while POMC and TH expression proved to be mutually exclusive. Stimulation with the general purinoceptor agonists 2-methylthioadenosine-5'triphosphate (2-MeSATP) and ATP but not the P2X1/P2X3 receptor subtype agonist α,β-methyleneadenosine-5'triphosphate (α,β-meATP) induced intracellular calcium signals in ARC neurons and astrocytes. Some 5-10% each of 2-MeSATP responsive neurons expressed POMC, NYP or TH. Supporting the calcium imaging data, radioligand binding studies to hypothalamic membranes showed high affinity for 2-MeSATP, ATP but not α,β-meATP to displace [α-(35)S]deoxyadenosine-5'thiotriphosphate ([(35)S]dATPαS) from P2 receptors. Repetitive superfusion with equimolar 2-MeSATP allowed categorization of ARC cells into groups with a high or low (LDD) degree of purinoceptor desensitization, the latter allowing further receptor characterization. Calcium imaging experiments performed at 37°C vs. room temperature showed further reduction of desensitization. Agonist-mediated intracellular calcium signals were suppressed in all LDD neurons but only 25% of astrocytes in the absence of extracellular calcium, suggestive of metabotropic P2Y receptor expression in the majority of ARC astrocytes. The highly P2Y1-selective receptor agonists MRS2365 and 2-methylthioadenosine-5'diphosphate (2-MeSADP) activated 75-85% of all 2-MeSATP-responsive ARC astrocytes. Taking into consideration the high potency to dose-dependently stimulate ARC cells of the LDD group, the high affinity for rat P2X(1-3) and low affinity for rat P2X4, P2X7 and P2Y receptor subtypes except P2Y1 and P2Y13, the agonist 2-MeSATP primarily acted upon P2X2 and P2Y1 purinoceptors to trigger intracellular calcium signaling in ARC neurons and astrocytes.

Purinergic Mechanisms and Pain

There is a brief introductory summary of purinergic signaling involving ATP storage, release, and ectoenzymatic breakdown, and the current classification of receptor subtypes for purines and pyrimidines. The review then describes purinergic mechanosensory transduction involved in visceral, cutaneous, and musculoskeletal nociception and on the roles played by receptor subtypes in neuropathic and inflammatory pain. Multiple purinoceptor subtypes are involved in pain pathways both as an initiator and modulator. Activation of homomeric P2X3 receptors contributes to acute nociception and activation of heteromeric P2X2/3 receptors appears to modulate longer-lasting nociceptive sensitivity associated with nerve injury or chronic inflammation. In neuropathic pain activation of P2X4, P2X7, and P2Y12 receptors on microglia may serve to maintain nociceptive sensitivity through complex neural-glial cell interactions and antagonists to these receptors reduce neuropathic pain. Potential therapeutic approaches involving purinergic mechanisms will be discussed.

Expression of P2X7 ATP receptor mediating the IL8 and CCL20 release in human periodontal ligament stem cells

ATP is released by human periodontal ligament cells (hPDLCs) and has been shown to regulate PDL regeneration and responses to mechanical stress through activation of P2Y receptors. This nucleotide, however, has also been reported to trigger the pro-inflammatory cascade by inducing the maturation and/or release of chemokines/cytokines from various cell types mainly via P2X7 receptors. Much less is known on the possible role of ATP in stem cells deriving from PDL (hPDLSCs) which are considered to be a promising tool for cell-based therapy to restore lesions. Given the role played by P2X7 in pathophysiological conditions, in this study we investigated the expression of P2X7 ATP receptors in hPDLSCs. The results obtained showed that hPDLSCs express P2X7 receptors evaluated by means of cytofluorimetric, immunohistochemistry, reverse transcriptase-PCR, and Western blot analyses. P2X7 ligation by 2',3'-(benzoyl-4-benzoyl)-ATP (BzATP), a specific receptor agonist, was followed by an increase in intracellular Ca2+ and in the uptake of ethidium bromide. These effects were dramatically reduced by oxidized ATP (oATP), the P2X7 irreversible inhibitor, suggesting that the P2X7 is the functional receptor involved. At 24 h treatment of hPDLSCs with BzATP it enhanced the release of the pro-inflammatory agents IL8 and CCL20, without influencing cell viability. These effects were counteracted by pre-treating the cells with oATP or with A-740003, a selective and potent P2X7 competitive antagonist. Collectively, these results indicated that extracellular ATP mediate a pro-inflammatory response via P2X7 receptors in hPDLSCs opening a further approach to control hPDLSCs behavior in their possible application as therapeutic tool.

The phenothiazine-class antipsychotic drugs prochlorperazine and trifluoperazine are potent allosteric modulators of the human P2X7 receptor

P2X7, an ATP-gated cation channel, is involved in immune cell activation, hyperalgesia and neuropathic pain. By regulating cytokine release in the brain, P2X7 has been linked to the pathophysiology of mood disorders and schizophrenia. We here assess the impact of 123 drugs that act in the central nervous system on human P2X7. Most prominently, the tricyclic antipsychotics prochlorperazine (PCP) and trifluoperazine (TFP) potently inhibited P2X7-mediated Ca2+ entry, dye permeation and ionic currents. In divalent cation-containing bath solutions or after prolonged incubation, ATP-evoked P2X7 currents were inhibited by 10 μM PCP. This effect was not related to dopamine receptor antagonism. Surprisingly, PCP co-applied with ATP enhanced inward currents in bath solutions with low divalent cation concentrations. Intracellular perfusion with PCP did not substitute for the extracellularly applied drug, indicating that its binding sites are accessible from the extracellular space. Since P2X7 current potentiation by PCP was voltage-dependent, at least one site may be located within the electrical field of the membrane. While the channel opening and closure kinetic was altered by PCP, the apparent affinity of ATP remained unchanged (potentiation) or changed slightly (inhibition). Measurements in human monocyte-derived macrophages confirmed the PCP-induced inhibition of ATP-evoked Ca2+ influx, Yo-Pro-1 permeability, and whole cell currents. Interestingly, neither heterologously expressed rat or mouse P2X7 nor native P2X7 in rat astrocyte cultures or in mouse bone marrow-derived macrophages were inhibited by perazines with a similar potency. We conclude that perazine-type neuroleptics are potent, but species-selective allosteric modulators of human but not murine P2X7 receptors.

Hypoxia Attenuates Purinergic P2X Receptor-Induced Inflammatory Gene Expression in Brainstem Microglia

Hypoxia and increased extracellular nucleotides are frequently coincident in the brainstem. Extracellular nucleotides are potent modulators of microglial inflammatory gene expression via P2X purinergic receptor activation. Although hypoxia is also known to modulate inflammatory gene expression, little is known about how hypoxia or P2X receptor activation alone affects inflammatory molecule production in brainstem microglia, nor how hypoxia and P2X receptor signaling interact when they occur together. In the study reported here, we investigated the ability of a brief episode of hypoxia (2 hours) in the presence and absence of the nonselective P2X receptor agonist 2′(3′)-O-(4-benzoylbenzoyl)adenosine-5′-triphosphate (BzATP) to promote inflammatory gene expression in brainstem microglia in adult rats. We evaluated inducible nitric oxide synthase (iNOS), tumor necrosis factor alpha (TNFα), and interleukin (IL)-6 messenger RNA levels in immunomagnetically isolated brainstem microglia. While iNOS and IL-6 gene expression increased with hypoxia and BzATP alone, TNFα expression was unaffected. Surprisingly, BzATP-induced inflammatory effects were lost after hypoxia, suggesting that hypoxia impairs proinflammatory P2X-receptor signaling. We also evaluated the expression of key P2X receptors activated by BzATP, namely P2X1, P2X4, and P2X7. While hypoxia did not alter their expression, BzATP upregulated P2X4 and P2X7 mRNAs; these effects were ablated in hypoxia. Although both P2X4 and P2X7 receptor expression correlated with increased microglial iNOS and IL-6 levels in microglia from normoxic rats, in hypoxia, P2X7 only correlated with IL-6, and P2X4 correlated only with iNOS. In addition, correlations between P2X7 and P2X4 were lost following hypoxia, suggesting that P2X4 and P2X7 receptor signaling differs in normoxia and hypoxia. Together, these data suggest that hypoxia suppresses P2X receptor-induced inflammatory gene expression, indicating a potentially immunosuppressive role of extracellular nucleotides in brainstem microglia following exposure to hypoxia.

Insights into the Molecular Mechanisms Underlying Mammalian P2X7 Receptor Functions and Contributions in Diseases, Revealed by Structural Modeling and Single Nucleotide Polymorphisms

The mammalian P2X7 receptors (P2X7Rs), a member of the ionotropic P2X receptor family with distinctive functional properties, play an important part in mediating extracellular ATP signaling in health and disease. A clear delineation of the molecular mechanisms underlying the key receptor properties, such as ATP-binding, ion permeation, and large pore formation of the mammalian P2X7Rs, is still lacking, but such knowledge is crucial for a better understanding of their physiological functions and contributions in diseases and for development of therapeutics. The recent breakthroughs in determining the atomic structures of the zebrafish P2X4.1R in the closed and ATP-bound open states have provided the long-awaited structural information. The human P2RX7 gene is abundant with non-synonymous single nucleotide polymorphisms (NS-SNPs), which generate a repertoire of human P2X7Rs with point mutations. Characterizations of the NS-SNPs identified in patients of various disease conditions and the resulting mutations have informed previously unknown molecular mechanisms determining the mammalian P2X7R functions and diseases. In this review, we will discuss the new insights into such mechanisms provided by structural modeling and recent functional and genetic linkage studies of NS-SNPs.

Purinergic mechanisms and pain--an update

There is a brief summary of the background literature about purinergic signalling. The review then considers purinergic mechanosensory transduction involved in visceral, cutaneous and musculoskeletal nociception and on the roles played by P2X3, P2X2/3, P2X4, P2X7 and P2Y₁₂ receptors in neuropathic and inflammatory pain. Current developments of compounds for the therapeutic treatment of both visceral and neuropathic pain are discussed.

P2X7 receptor activation induces reactive oxygen species formation and cell death in murine EOC13 microglia

The P2X7 purinergic receptor is a ligand-gated cation channel expressed on leukocytes including microglia. This study aimed to determine if P2X7 activation induces the uptake of organic cations, reactive oxygen species (ROS) formation, and death in the murine microglial EOC13 cell line. Using the murine macrophage J774 cell line as a positive control, RT-PCR, immunoblotting, and immunolabelling established the presence of P2X7 in EOC13 cells. A cytofluorometric assay demonstrated that the P2X7 agonists adenosine-5′-triphosphate (ATP) and 2′(3′)-O-(4-benzoylbenzoyl) ATP induced ethidium⁺ or YO-PRO-1²⁺ uptake into both cell lines. ATP induced ethidium⁺ uptake into EOC13 cells in a concentration-dependent manner, with an EC₅₀ of ~130 μM. The P2X7 antagonists Brilliant Blue G, A438079, AZ10606120, and AZ11645373 inhibited ATP-induced cation uptake into EOC13 cells by 75–100%. A cytofluorometric assay demonstrated that P2X7 activation induced ROS formation in EOC13 cells, via a mechanism independent of Ca²⁺ influx and K⁺ efflux. Cytofluorometric measurements of Annexin-V binding and 7AAD uptake demonstrated that P2X7 activation induced EOC13 cell death. The ROS scavenger N-acetyl-L-cysteine impaired both P2X7-induced EOC13 ROS formation and cell death, suggesting that ROS mediate P2X7-induced EOC13 death. In conclusion, P2X7 activation induces the uptake of organic cations, ROS formation, and death in EOC13 microglia.

Calcium-dependent block of P2X7 receptor channel function is allosteric

Among purinergic P2X receptor (P2XR) channels, the P2X7R exhibits the most complex gating kinetics; the binding of orthosteric agonists at the ectodomain induces a conformational change in the receptor complex that favors a gating transition from closed to open and dilated states. Bath Ca(2+) affects P2X7R gating through a still uncharacterized mechanism: it could act by reducing the adenosine triphosphate(4-) (ATP(4-)) concentration (a form proposed to be the P2X7R orthosteric agonist), as an allosteric modulator, and/or by directly altering the selectivity of pore to cations. In this study, we combined biophysical and mathematical approaches to clarify the role of calcium in P2X7R gating. In naive receptors, bath calcium affected the activation permeability dynamics indirectly by decreasing the potency of orthosteric agonists in a concentration-dependent manner and independently of the concentrations of the free acid form of agonists and status of pannexin-1 (Panx1) channels. Bath calcium also facilitated the rates of receptor deactivation in a concentration-dependent manner but did not affect a progressive delay in receptor deactivation caused by repetitive agonist application. The effects of calcium on the kinetics of receptor deactivation were rapid and reversible. A438079, a potent orthosteric competitive antagonist, protected the rebinding effect of 2'(3')-O-4-benzoylbenzoyl)ATP on the kinetics of current decay during the washout period, but in the presence of A438079, calcium also increased the rate of receptor deactivation. The corresponding kinetic (Markov state) model indicated that the decrease in binding affinity leads to a decrease in current amplitudes and facilitation of receptor deactivation, both in an extracellular calcium concentration-dependent manner expressed as a Hill function. The results indicate that calcium in physiological concentrations acts as a negative allosteric modulator of P2X7R by decreasing the affinity of receptors for orthosteric ligand agonists, but not antagonists, and not by affecting the permeability dynamics directly or indirectly through Panx1 channels. We expect these results to generalize to other P2XRs.

Purinergic signaling in the cerebellum: Bergmann glial cells express functional ionotropic P2X7 receptors

Astrocytes constitute active networks of intercommunicating cells that support the metabolism and the development of neurons and affect synaptic functions via multiple pathways. ATP is one of the major neurotransmitters mediating signaling between neurons and astrocytes. Potentially acting through both purinergic metabotropic P2Y receptors (P2YRs) and ionotropic P2X receptors (P2XRs), up until now ATP has only been shown to activate P2YRs in Bergmann cells, the radial glia of the cerebellar cortex that envelopes Purkinje cell afferent synapses. In this study, using multiple experimental approaches in acute cerebellar slices we demonstrate the existence of functional P2XRs on Bergmann cells. In particular, we show here that Bergmann cells express uniquely P2X7R subtypes: (i) immunohistochemical analysis revealed the presence of P2X7Rs on Bergmann cell processes, (ii) in whole cell recordings P2XR pharmacological agonists induced depolarizing currents that were blocked by specific antagonists of P2X7Rs, and could not be elicited in slices from P2X₇R-deficient mice and finally, (iii) calcium imaging experiments revealed two distinct calcium signals triggered by application of exogenous ATP: a transient signal deriving from release of calcium from intracellular stores, and a persistent one following activation of P2X7Rs. Our data thus reveal a new pathway by which extracellular ATP may affect glial cell function, thus broadening our knowledge on purinergic signaling in the cerebellum.

Activation and regulation of purinergic P2X receptor channels

Mammalian ATP-gated nonselective cation channels (P2XRs) can be composed of seven possible subunits, denoted P2X1 to P2X7. Each subunit contains a large ectodomain, two transmembrane domains, and intracellular N and C termini. Functional P2XRs are organized as homomeric and heteromeric trimers. This review focuses on the binding sites involved in the activation (orthosteric) and regulation (allosteric) of P2XRs. The ectodomains contain three ATP binding sites, presumably located between neighboring subunits and formed by highly conserved residues. The detection and coordination of three ATP phosphate residues by positively charged amino acids are likely to play a dominant role in determining agonist potency, whereas an AsnPheArg motif may contribute to binding by coordinating the adenine ring. Nonconserved ectodomain histidines provide the binding sites for trace metals, divalent cations, and protons. The transmembrane domains account not only for the formation of the channel pore but also for the binding of ivermectin (a specific P2X4R allosteric regulator) and alcohols. The N- and C- domains provide the structures that determine the kinetics of receptor desensitization and/or pore dilation and are critical for the regulation of receptor functions by intracellular messengers, kinases, reactive oxygen species and mercury. The recent publication of the crystal structure of the zebrafish P2X4.1R in a closed state provides a major advance in the understanding of this family of receptor channels. We will discuss data obtained from numerous site-directed mutagenesis experiments accumulated during the last 15 years with reference to the crystal structure, allowing a structural interpretation of the molecular basis of orthosteric and allosteric ligand actions.

Physiology of microglia

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

Clemastine potentiates the human P2X7 receptor by sensitizing it to lower ATP concentrations

P2X7 receptors have emerged as potential drug targets for the treatment of medical conditions such as e.g. rheumatoid arthritis and neuropathic pain. To assess the impact of pharmaceuticals on P2X7, we screened a compound library comprising approved or clinically tested drugs and identified several compounds that augment the ATP-triggered P2X7 activity in a stably transfected HEK293 cell line. Of these, clemastine markedly sensitized Ca(2+) entry through P2X7 to lower ATP concentrations. Extracellularly but not intracellularly applied clemastine rapidly and reversibly augmented P2X7-mediated whole-cell currents evoked by non-saturating ATP concentrations. Clemastine also accelerated the ATP-induced pore formation and Yo-Pro-1 uptake, increased the fractional NMDG(+) permeability, and stabilized the open channel conformation of P2X7. Thus, clemastine is an extracellularly binding allosteric modulator of P2X7 that sensitizes P2X7 to lower ATP concentrations and facilitates its pore dilation. The activity of clemastine on native P2X7 receptors, Ca(2+) entry, and whole-cell currents was confirmed in human monocyte-derived macrophages. Similar effects were observed in murine bone marrow-derived macrophages. Consistent with the data on recombinant P2X7, clemastine augmented the ATP-induced cation entry and Yo-Pro-1 uptake. In accordance with the observation that P2X7 controls the cytokine release from LPS-primed macrophages, we found that clemastine augmented the IL-1β release from LPS-primed human macrophages. Collectively, these data point to a sensitization of the recombinantly or natively expressed human P2X7 receptor toward its physiological activator, ATP, possibly leading to a modulation of macrophage-dependent immune responses.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

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.

Excitation of rat spinal ventral horn neurons by purinergic P2X and P2Y receptor activation

ATPgammaS, a nonhydrolyzable ATP analog, was found to dose-dependently generate an inward current at a holding potential of -70 mV (EC(50)=43 microM) in lamina IX neurons of rat spinal cord slices using the whole-cell patch-clamp technique. This inward current had an extrapolated reversal potential of -9 mV and was resistant to the Na(+)-channel blocker tetrodotoxin, glutamate-receptor antagonists or nominally Ca(2+)-free medium. ATP gamma S also increased the frequency and amplitude of glutamatergic spontaneous excitatory postsynaptic current (sEPSC); this action was dose-dependent and sensitive to tetrodotoxin. Unlike ATP gamma S, the P2X-receptor agonist, BzATP or alpha,beta-methylene ATP, did not change holding currents, but the current response produced by ATP gamma S disappeared in the presence of the P2-receptor antagonist PPADS. The sEPSC frequency and amplitude increase was observed with alpha,beta-methylene ATP, but not with the P2Y-receptor agonist, 2-methylthio ADP, UTP or UDP. The current response by ATP gamma S was suppressed by the addition of GDP beta S into the patch-pipette solution. As for ATP gamma S, 2-methylthio ADP produced an inward current, while UTP and UDP had no effect on holding currents. The P2Y(1)-receptor antagonist MRS2179 inhibited the ATP gamma S-induced inward current, but did not affect the sEPSC frequency and amplitude increase produced by ATP gamma S. These data indicate that extracellular ATP increases the excitability of lamina IX neurons by membrane depolarization (probably through non-selective cation-channel activation) and spontaneous excitatory transmission enhancement, which may be mediated by P2Y(1) and P2X receptors, respectively. This finding supports the idea that purinergic receptor antagonists provide a therapy for spinal cord injury.

The role of P2X7 receptor in ATP-mediated human leukemia cell death: calcium influx-independent

Activation of the P2X7 receptor leads to a rapid, bidirectional flux of cations, causing broad range of biological responses including cytotoxicity. However, the mechanism of P2X7-mediated cytotoxicity remains largely unexplored. In our previous study, the lack of P2X7-mediated calcium response under normal conditions was found in P2X7(+) hematopoietic cell lines. In this study, the P2X7-mediated cytotoxicity in different type of cells (P2X7(-), P2X7(+) with calcium response, and P2X7(+) without calcium response) was investigated. Our results showed that P2X7 agonists, adenosine 5'-triphosphate (ATP) or 2',3'-O-(4 benzoylbenzoyl)-ATP, dose-dependently reduced the cell viability in all P2X7(+) cells tested, including J6-1, LCL, and Namalva cells which are negative for P2X7-mediated calcium response, although these effects were lower than those observed in KG1a cells which has normal P2X7 functions. The cytotoxic effect could be blocked by P2X7 antagonists, oxidized ATP and 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine. In addition, externalization of phosphatidylserine could be detected in a time-dependent manner and apoptotic morphological changes could be observed after the activation of P2X7 receptor in J6-1 cells. Furthermore, P2X7-mediated pore formation could be detected in KG1a and J6-1 cells under low-ionic conditions, but not under low-divalent conditions. These effects could not be observed in P2X7(-) Ramos cells. These results suggested that P2X7 receptor-mediated cytotoxic effects may occur independent of calcium response.

Pharmacological properties of a pore induced by raising intracellular Ca2+

Recent studies on the P2X(7) receptor in 2BH4 cells and peritoneal macrophages have demonstrated that the raise in intracellular Ca(2+) concentration induces a pore opening similar to P2X(7) receptor pore. Herein, we have investigated whether the pore activated by the elevation of intracellular Ca(2+) concentration is associated to P2X(7) receptor. Using patch clamp in cell attached, whole cell configuration, and dye uptake, we measured the pore opening in cell types that express the P2X(7) receptor (2BH4 cells and peritoneal macrophages) and in cells that do not express this receptor (HEK-293 and IT45-RI cells). In 2BH4 cells, the stimulation with ionomycin (5-10 microM) increased intracellular free Ca(2+) concentration and induced pore formation with conductance of 421 +/- 14 pS, half-time (t(1/2)) for ethidium bromide uptake of 118 +/- 17 s, and t(1/2) for Lucifer yellow of 122 +/- 11 s. P2X(7) receptor antagonists did not block these effects. Stimulation of HEK-293 and IT45-RI cells resulted in pore formation with properties similar to those found for 2BH4 cells. Connexin hemichannel inhibitors (carbenoxolone and heptanol) also did not inhibit the pore-induced effect following the increase in intracellular Ca(2+) concentration. However, 5-(N,N-hexamethylene)-amiloride, a P2X(7) receptor pore blocker, inhibited the induced pore. Moreover, intracellular signaling modulators, such as calmodulin, phospholipase C, mitogen-activated protein kinase, and cytoskeleton components were important for the pore formation. Additionally, we confirmed the results obtained for electrophysiology by using the flow cytometry, and we discarded the possibility of cellular death induced by raising intracellular Ca(2+) at the doses used by using lactate dehydrogenase release assay. In conclusion, increased concentration in intracellular Ca(+2) induces a novel membrane pore pharmacologically different from the P2X(7) associated pore and hemigap-junction pore.

Predictions suggesting a participation of beta-sheet configuration in the M2 domain of the P2X(7) receptor: a novel conformation?

Scanning experiments have shown that the putative TM2 domain of the P2X(7) receptor (P2X(7)R) lines the ionic pore. However, none has identified an alpha-helix structure, the paradigmatic secondary structure of ion channels in mammalian cells. In addition, some researchers have suggested a beta-sheet conformation in the TM2 domain of P2X(2). These data led us to investigate a new architecture within the P2X receptor family. P2X(7)R is considered an intriguing receptor because its activation induces nonselective large pore formation, in contrast to the majority of other ionic channel proteins in mammals. This receptor has two states: a low-conductance channel (approximately 10 pS) and a large pore (> 400 pS). To our knowledge, one fundamental question remains unanswered: Are the P2X(7)R channel and the pore itself the same entity or are they different structures? There are no structural data to help solve this question. Thus, we investigated the hydrophobic M2 domain with the aim of predicting the fitted position and the secondary structure of the TM2 segment from human P2X(7)R (hP2X(7)R). We provide evidence for a beta-sheet conformation, using bioinformatics algorithms and molecular-dynamics simulation in conjunction with circular dichroism in different environments and Fourier transform infrared spectroscopy. In summary, our study suggests the possibility that a segment composed of residues from part of the M2 domain and part of the putative TM2 segment of P2X(7)R is partially folded in a beta-sheet conformation, and may play an important role in channel/pore formation associated with P2X(7)R activation. It is important to note that most nonselective large pores have a transmembrane beta-sheet conformation. Thus, this study may lead to a paradigmatic change in the P2X(7)R field and/or raise new questions about this issue.

Atypical P2X receptor pharmacology in two human osteoblast-like cell lines

BACKGROUND AND PURPOSE

The expression and function of P2X(7) receptors in osteoclasts is well established, but less is known about their role in osteoblast-like cells. A study in P2X(7) receptor knockout mice suggested the involvement of these receptors in bone formation. We have investigated the expression and pharmacology of several P2X receptors in two human osteosarcoma cell lines to see if they could be involved in bone turnover in man.

EXPERIMENTAL APPROACH

Reverse transcriptase-polymerase chain reaction and Western blotting were used to study P2X(2), P2X(4) and P2X(7) receptor expression at mRNA and protein levels, respectively, in human osteoblast-like cells. P2X(7) receptor pharmacology was studied by measuring pore formation in the presence of different agonists and antagonists using the YO-PRO 1 uptake method.

KEY RESULTS

P2X(4) and P2X(7) receptor mRNA and protein were found to be expressed by these cell lines. No evidence was found for P2X(4)/P2X(7) receptor heteropolymerization. 2'-3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (DBzATP) was equipotent to ATP and the antagonists used were either ineffective or weakly blocked pore formation.

CONCLUSIONS AND IMPLICATIONS

This study demonstrates that P2X(4) and P2X(7) receptors are expressed by human osteoblast-like cells. The affinities of the different agonists suggest that the P2X(7) receptor is mainly responsible for pore formation although P2X(4) receptors may also be involved. The low affinity of DBzATP and the weak action of the antagonists support the previously described atypical pharmacology of the P2X(7) receptor in osteoblasts. Targeting the P2X(7) receptor in osteoblasts could represent a promising new treatment for bone diseases such as osteoporosis and rheumatoid arthritis.

The P2X7 receptor channel pore dilates under physiological ion conditions

Activation of the purinergic P2X₇ receptor leads to the rapid opening of an integral ion channel that is permeable to small cations. This is followed by a gradual increase in permeability to fluorescent dyes by integrating the actions of the pannexin-1 channel. Here, we show that during the prolonged agonist application a rapid current that peaked within 200 ms was accompanied with a slower current that required tens of seconds to reach its peak. The secondary rise in current was observed under different ionic conditions and temporally coincided with the development of conductivity to larger organic cations. The biphasic response was also observed in cells with blocked pannexin channels and in cells not expressing these channels endogenously. The biphasic current was preserved in N-terminal T15A, T15S, and T15V mutants that have low or no permeability to organic cations, reflecting enhanced permeability to inorganic cations. In contrast, the T15E, T15K, and T15W mutants, and the Δ18 mutant with deleted P2X₇ receptor–specific 18–amino acid C-terminal segment, were instantaneously permeable to organic cations and generated high amplitude monophasic currents. These results indicate that the P2X₇ receptor channel dilates under physiological ion conditions, leading to generation of biphasic current, and that this process is controlled by residues near the intracellular side of the channel pore.

Facilitation of P2X7 receptor currents and membrane blebbing via constitutive and dynamic calmodulin binding

The ATP-gated P2X(7) receptor (P2X(7)R) is a highly unusual calcium-permeable cationic channel in that within seconds of its activation, dramatic and reversible cytoskeletal rearrangements with prominent membrane blebbing occurs. Agonist-induced membrane currents at hyperpolarized potentials show pronounced facilitation during the initial 30-100 s of receptor activation but mechanisms responsible have not been elucidated. We measured facilitation of ATP-gated currents in HEK cells expressing rat P2X(7)R and delineated distinct calcium-dependent and independent processes. The calcium-dependent facilitation was composed of an instantaneous (millisecond time domain) and slowly developing (time constant, 20 s with maximum agonist stimulation) component. Both components were prevented when recording with a highly specific calmodulin (CaM) inhibitory peptide but only the instantaneous component was reduced by expression of the dominant-negative EF-handless CaM mutant. Coimmunoprecipitation assays detected low levels of CaM binding to unstimulated P2X(7)R, and this increased by 50% during 45 s stimulation of the receptor. We identified a novel 1-5-16 Ca(2+)-dependent CaM binding motif in the intracellular C terminus of P2X(7)R; mutations in this domain resulted in the absence of calcium-dependent facilitation and binding of CaM to unstimulated or stimulated receptor. Blockade of CaM binding also delayed membrane blebbing by threefold. Our results demonstrate that CaM binds constitutively to closed P2X(7)R channels and dynamically during channel activation to significantly enhance and prolong calcium entry. This is the first example of CaM deregulating, rather than tightly controlling, calcium entry through an ion channel.

Stimulated efflux of amino acids and glutathione from cultured hippocampal slices by omission of extracellular calcium: likely involvement of connexin hemichannels

Omission of extracellular Ca(2+) for 15 min from the incubation medium of cultured hippocampal slices stimulated the efflux of glutathione, phosphoethanolamine, hypotaurine, and taurine. The efflux was reduced by several blockers of gap junctions, i.e. carbenoxolone, flufenamic acid, and endothelin-1, and by the connexin43 hemichannel blocking peptide Gap26 but was unchanged by the P2X(7) receptor inhibitor oxidized ATP, a pannexin1 hemichannel blocking peptide and an inactive analogue of carbenoxolone. Pretreatment of the slices with the neurotoxin N-methyl-d -aspartate left the efflux by Ca(2+) omission unchanged, indicating that the stimulated efflux primarily originated from glia. Elevated glutamate efflux was detected when Ca(2+) omission was combined with the glutamate uptake blocker l-trans-pyrrolidine-2,4-dicarboxylate and when both Ca(2+) and Mg(2+) were omitted from the medium. Omission of Ca(2+) for 15 min alone did not induce delayed toxicity, but in combination with blocked glutamate uptake, significant cell death was observed 24 h later. Our results indicate that omission of extracellular Ca(2+) stimulates efflux of glutathione and specific amino acids including glutamate via opening of glial hemichannels. This type of efflux may have protective functions via glutathione efflux but can aggravate toxicity in situations when glutamate reuptake is impaired, such as following a stroke.

Activation of P2X(7) receptors stimulates the expression of P2Y(2) receptor mRNA in astrocytes cultured from rat brain

Under pathological conditions brain cells release ATP at concentrations reported to activate P2X(7) ionotropic receptor subtypes expressed in both neuronal and glial cells. In the present study we report that the most potent P2X(7) receptor agonist BzATP stimulates the expression of the metabotropic ATP receptor P2Y(2) in cultured rat brain astrocytes. In other cell types several kinds of stimulation, including stress or injury, induce P2Y(2) expression that, in turn, is involved in different cell reactions. Similarly, it has recently been found that in astrocytes and astrocytoma cells P2Y(2) sites can trigger neuroprotective pathways through the activation of several mechanisms, including the induction of genes for antiapoptotic factors, neurotrophins, growth factors and neuropeptides. Here we present evidence that P2Y(2) mRNA expression in cultured astrocytes peaks 6 h after BzATP exposure and returns to basal levels after 24 h. This effect was mimicked by high ATP concentrations (1 mM) and was abolished by P2X(7)-antagonists oATP and BBG. The BzATP-evoked P2Y(2) receptor up-regulation in cultured astrocytes was coupled to an increased UTP-mediated intracellular calcium response. This effect was inhibited by oATP and BBG and by P2Y(2)siRNA, thus supporting evidence of increased P2Y(2) activity. To further investigate the mechanisms by which P2X(7) receptors mediated the P2Y(2) mRNA up-regulation, the cells were pre-treated with the chelating agent EGTA, or with inhibitors of mitogen-activated kinase (MAPK) (PD98059) or protein kinase C, (GF109203X). Each inhibitor significantly reduced the extent to which BzATP induced P2Y(2) mRNA. Both BzATP and ATP (1 mM) increased ERK1/2 activation. P2X(7)-induced ERK1/2 phosphorylation was unaffected by pre-treatment of astrocytes with EGTA whereas it was inhibited by GF109203X. Phorbol-12-myristate-13-acetate (PMA), an activator of PKCs, rapidly increased ERK1/2 activation. We conclude that activation of P2X(7) receptors in astrocytes enhances P2Y(2) mRNA expression by a mechanism involving both calcium influx and PKC/MAPK signalling pathways.

Functional expression of ionotropic purinergic receptors on mouse taste bud cells

Neurotransmitter receptors on taste bud cells (TBCs) and taste nerve fibres are likely to contribute to taste transduction by mediating the interaction among TBCs and that between TBCs and taste nerve fibres. We investigated the functional expression of P2 receptor subtypes on TBCs of mouse fungiform papillae. Electrophysiological studies showed that 100 microm ATP applied to their basolateral membranes either depolarized or hyperpolarized a few cells per taste bud. Ca(2+) imaging showed that similarly applied 1 mum ATP, 30 microm BzATP (a P2X(7) agonist), or 1 microm 2MeSATP (a P2Y(1) and P2Y(11) agonist) increased intracellular Ca(2+) concentration, but 100 microm UTP (a P2Y(2) and P2Y(4) agonist) and alpha,beta-meATP (a P2X agonist except for P2X(2), P2X(4) and P2X(7)) did not. RT-PCR suggested the expression of P2X(2), P2X(4), P2X(7), P2Y(1), P2Y(13) and P2Y(14) among the seven P2X subtypes and seven P2Y subtypes examined. Immunohistostaining confirmed the expression of P2X(2). The exposure of the basolateral membranes to 3 mm ATP for 30 min caused the uptake of Lucifer Yellow CH in a few TBCs per taste bud. This was antagonized by 100 microm PPADS (a non-selective P2 blocker) and 1 microm KN-62 (a P2X(7) blocker). These results showed for the first time the functional expression of P2X(2) and P2X(7) on TBCs. The roles of P2 receptor subtypes in the taste transduction, and the renewal of TBCs, are discussed.

Agonist potency at P2X7 receptors is modulated by structurally diverse lipids

BACKGROUND AND PURPOSE

The P2X(7) receptor exhibits a high degree of plasticity with agonist potency increasing after prolonged receptor activation. In this study we investigated the ability of lipids to modulate agonist potency at P2X(7) receptors.

EXPERIMENTAL APPROACH

A variety of lipids, including lysophosphatidylcholine, sphingosylphosphorylcholine and hexadecylphosphorylcholine were studied for their effect on P2X(7) receptor-stimulated ethidium bromide accumulation in cells expressing human recombinant P2X(7) receptors and on P2X(7) receptor-stimulated interleukin-1 beta (IL1 beta) release from THP-1 cells. The effects of the lipids were also assessed in radioligand binding studies on human P2X(7) receptors.

KEY RESULTS

At concentrations (3-30 microM) below the threshold to cause cell lysis, the lipids increased agonist potency and/or maximal effects at P2X(7) receptors in both ethidium accumulation and IL1 beta release studies. There was little structure activity relationship (SAR) for this effect and sub-lytic concentrations of Triton X-100 partially mimicked the effects of the lipids. The lipids caused cell lysis and increased intracellular calcium at higher concentrations (30-100 microM) which complicated interpretation of their effects in functional studies. However, the lipids (3-100 microM) also increased agonist potency 30-100 fold in radioligand binding studies.

CONCLUSIONS AND IMPLICATIONS

This study demonstrates that a diverse range of lipids increase agonist potency at the P2X(7) receptor in functional and binding studies. The broad SAR, including the effect of Triton X-100, suggests this may reflect changes in membrane properties rather than a direct effect on the P2X(7) receptor. Since many of the lipids studied accumulate in disease states they may enhance P2X(7) receptor function under pathophysiological conditions.

Characteristics of P2X7-like receptor activated by adenosine triphosphate in HIT-T15 cells

OBJECTIVES

The study examined the presence of a P2X7 receptor subtype and its functional roles in pancreatic beta cells.

METHODS

In a hamster beta-cell line, HIT-T15 cells, purinergic stimulation was investigated using fluorometry, electrophysiology, flow cytometry, and electrophoresis.

RESULTS

Adenosine triphosphate (ATP) and 2'-3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) increased in the intracellular free Ca2+ concentration, with an EC50 of 398.0 and 136.6 microM, respectively. Preincubation with oxidized ATP, a P2X7 receptor antagonist, inhibited the ATP- and BzATP-induced increase in the intracellular Ca2+ level. The BzATP-induced increase in the intracellular Ca2+ level was dependent on the extracellular Ca2+ concentration. The extracellular Mg2+ had a significant effect on the ATP-induced increase in the intracellular Ca2+ level. The ATP also induced depolarization like high potassium chloride. In the voltage-clamp experiments, ATP evoked inward currents, which were reversed at almost 0 mV. The ATP stimulated the slow influx of ethidium bromide, indicating permeability to larger molecules. Flow cytometry showed that the number of hypodiploid cells (A0), which are indicative of apoptosis, increased when the cells were exposed to ATP for 24 hours. The ATP also induced DNA fragmentation.

CONCLUSIONS

These results suggest that the HIT-T15 cells have endogenous P2X7-like receptors and that purinergic stimulation increased the level of intracellular Ca2+, depolarization, inward current, permeability, and apoptosis.

Prion infection correlates with hypersensitivity of P2X7 nucleotide receptor in a mouse microglial cell line

We recently established mouse microglial cells persistently infected with mouse-adapted scrapie ME7 (ScMG20/ME7) for in vitro study of prion pathogenesis. Here, we found that ScMG20/ME7 cells were hypersensitive to P2X7 receptor agonists, as demonstrated by sustained Ca(2+) influx, membrane pore formation, cell death, and interleukin-1beta release. P2X7 mRNA expression was upregulated in these cells, and also in scrapie-infected mice brains. Treatment with pentosan polysulfate eliminated the infectivity and disease-related forms of prion protein from ScMG20/ME7 cell cultures, however, hypersensitivity of P2X7 receptors remained. These results suggest that prion infections may strongly affect the P2X7 receptor system in mouse microglial cells.

Influence of extracellular monovalent cations on pore and gating properties of P2X7 receptor-operated single-channel currents

Using the patch-clamp method, we studied the influence of external alkali and organic monovalent cations on the single-channel properties of the adenosine triphosphate (ATP)-activated recombinant human P2X(7) receptor. The slope conductance of the hP2X(7) channel decreased and the reversal potential was shifted to more negative values as the ionic diameter of the organic test cations increased. From the relationship between single-channel conductance and the dimensions of the inward current carrier, the narrowest portion of the pore was estimated to have a mean diameter of approximately 8.5 A. Single-channel kinetics and permeation properties remained unchanged during receptor activation by up to 1 mM ATP(4-) for >1 min, arguing against a molecular correlate of pore dilation at the single P2X(7) channel level. Substitution of extracellular Na(+) by any other alkali or organic cation drastically increased the open probability of the channels by prolonging the mean open time. This effect seems to be mediated allosterically through an extracellular voltage-dependent Na(+) binding site with a K(d) of approximately 5 mM Na(+) at a membrane potential of -120 mV. The modulation of the ATP-induced hP2X(7) receptor gating by extracellular Na(+) could be well described by altering the rate constant from the open to the neighboring closed state in a C-C-C-O kinetic receptor model. We suggest that P2X(7) receptor-induced depolarization and associated K(+)-efflux may reduce Na(+) occupancy of the regulatory Na(+) binding site and thus increase the efficacy of ATP(4-) in a feed-forward manner in P2X(7) receptor-expressing cells.

Lysophosphatidylcholine potentiates Ca2+ influx, pore formation and p44/42 MAP kinase phosphorylation mediated by P2X7 receptor activation in mouse microglial cells

The P2X7 receptor (P2X7R) is an ATP-gated ion channel highly expressed in microglia. P2X7R plays important roles in inflammatory responses in the brain. However, little is known about the mechanisms regulating its functions in microglia. Lysophosphatidylcholine (LPC), an inflammatory phospholipid that promotes microglial activation, may have some relevance to P2X7R signaling in terms of microglial function. In this study, we examined its effects on P2X7R signaling in a mouse microglial cell line (MG6) and primary microglia. LPC facilitated the sustained increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) through P2X7R channels activated by ATP or BzATP. The potentiated increase in [Ca(2+)](i) was actually inhibited by P2X7R antagonists, brilliant blue G and oxidized ATP. The potentiating effect of LPC was not observed with P2Y receptor systems, which are also expressed in MG6 cells. G2A, a receptor for LPC, was expressed in MG6 cells, but not involved in the facilitating effect of LPC on the P2X7R-mediated change in [Ca(2+)](i). Furthermore, LPC enhanced the P2X7R-associated formation of membrane pores and the activation of p44/42 mitogen-activated protein kinase. These results suggest that LPC may regulate microglial functions in the brain by enhancing the sensitivity of P2X7R.

P2X7-induced apoptosis decreases by aging in mice myeloblasts

In the current study, the ability of ATP to promote apoptosis in myeloblasts at different ages was investigated. We have observed that high concentration of extracellular ATP (>1mM), which activates P2X(7) receptor, produced cell shrinkage an increase in the number of events in the sub-G(0)/G(1) region of the cellular cycle and annexin-V/propidium iodide label, which characterizes the apoptotic cell death. In addition, BzATP produced apoptosis, but not ADP and UTP. Gr-1(+) cells express the P2X(7) receptor and oxidized ATP, a specific P2X(7) inhibitor, blocked the ATP-dependent apoptosis. ATP-dependent apoptosis is decreased by aging in myeloblasts of 12 and 22-month-old mice. Furthermore, P2X(7) expression decrease was observed in older mice, explaining apoptosis decrease. This decrease in apoptosis by aging may be related to some diseases in the myelocyte lineage.

Kinetics of P2X7 receptor-operated single channels currents

Human P2X7 receptors were expressed in Xenopus laevis oocytes and single channels were recorded using the patch-clamp technique in the outside-out configuration. ATP4- evoked two types of P2X7 receptor-mediated single channel currents characterized by short-lived and long-lived openings. The short- and long-lasting open states had mean open times of approximately 5 and approximately 20 ms and slope conductances near -60 mV of 9 and 13 pS, respectively. The open probabilities of the short and long openings were strongly [ATP4-]-dependent with EC50 values of approximately 0.3 mM and approximately 0.1 mM ATP4-, respectively. The channel kinetics did not change significantly during sustained P2X7 receptor activation for several minutes, as was also observed in recordings in the cell-attached patch-clamp configuration. Activation and deactivation of the short openings followed exponential time courses with time constants in the range of 20 ms, and displayed a shallow [ATP4-] dependence of the activation process. The kinetics of the short channel openings at negative membrane potentials fitted well to a linear C-C-C-O model with two ATP4- binding steps at equal binding sites with a dissociation constant Kd of 139 microM.