P2X7 Interactions and Signaling - Making Head or Tail of It

Immune response studies based on P2X7 receptors: A Mini-Review

Inflammation, as a complex immunopathological process, is the organism's natural defense response to the organism against harmful, foreign, and destructive immune or non-immune factors. It is the main pathological form of various diseases, such as tumors, neurodegenerative diseases, periodontitis, alcoholic steatohepatitis, asthma, and other diseases. The P2X7 receptor (P2X7R) is widely distributed in vivo and up--regulated in various inflammatory pathological states. Studies have shown that milder chronic inflammation is related to a deficiency or inhibition of P2X7R, which is an indispensable part of the pro-inflammatory mechanism in vivo. P2X7R, a unique subtype of seven purinergic P2X receptors, is an ATP-gated nonselective cationic channel. P2X7R will promote the influx of Ca2+ and the outflow of K+ after being stimulated. The influx of Ca2+ is essential for activating the body's innate immune response and inducing the production of inflammatory factors. This paper reviews the regulation of P2X7R on inflammation from the perspectives of innate immunity and adaptive immunity.

Analyzing the Role of the P2X7 Receptor in Epilepsy

Purinergic signaling is increasingly recognized to play a role during the generation of hyperexcitable networks in the brain. Among the purinergic receptors, the ionotropic ATP-gated P2X7 receptor has attracted particular attention as a possible drug target for epilepsy. P2X7 receptor expression is increased in the brain of experimental models of epilepsy and in patients and, P2X7 receptor antagonism modulates seizure severity and epilepsy development. To date, studies analyzing the role of the P2X7 receptor during epilepsy have mainly focused on temporal lobe epilepsy, the most common form of acquired epilepsy in adults which is particularly prone to drug refractoriness.Animal models of seizures and epilepsy are an essential tool in the identification of novel anticonvulsive and antiepileptogenic drug targets and much data demonstrating a role for the P2X7 receptor during epilepsy have been obtained by using these models. The aim of the present book chapter is to provide a detailed description of two commonly used mouse models of temporal lobe epilepsy, which are the intra-amygdala kainic acid model of status epilepticus and the controlled cortical impact model of traumatic brain injury. This chapter concludes with a brief description of how these models can be used to investigate the impact of targeting the P2X7 receptor on acute seizures, epilepsy development and established epilepsy .

How Structural Biology Has Directly Impacted Our Understanding of P2X Receptor Function and Gating

P2X receptors are ATP-gated ion channels expressed in a wide variety of eukaryotic cells. They play key roles in diverse processes such as platelet activation, smooth muscle contraction, synaptic transmission, nociception, cell proliferation, and inflammation making this receptor family an important pharmacological target. Structures of P2X receptors solved by X-ray crystallography have been instrumental in helping to define mechanisms of molecular P2X receptor function. In 2009, the first X-ray structure of the P2X4 receptor subtype confirmed a trimeric stoichiometry and revealed the overall architecture of the functional ion channel. Subsequent X-ray structures have provided the molecular details to define the orthosteric ATP binding pocket, the orthosteric antagonist binding pocket, an allosteric antagonist binding pocket, and the pore architecture in each of the major conformational states of the receptor gating cycle. Moreover, the unique gating mechanism by which P2X receptor subtypes desensitize at differing rates, referred to as the helical recoil model of receptor desensitization, was discovered directly from X-ray structures of the P2X3 receptor. However, structures of P2X receptors solved by X-ray crystallography have only been able to provide limited information on the cytoplasmic domain of this receptor family, as this domain was always truncated to varying degrees in order to facilitate crystallization. Because the P2X7 receptor subtype has a significantly larger cytoplasmic domain that has been shown to be necessary for its ability to initiate apoptosis, an absence of structural information on the P2X7 receptor cytoplasmic domain has limited our understanding of its complex signaling pathways as well as its unusual ability to remain open without undergoing desensitization. This absence of cytoplasmic structural information for P2X7 receptors was recently overcome when the first full-length P2X7 receptor structures were solved by single-particle cryogenic electron microscopy. These structures finally provide insight into the large and unique P2X7 receptor cytoplasmic domain and revealed two novel structural elements and several surprising findings: first, a cytoplasmic structural element called the cytoplasmic ballast was identified that contains a dinuclear zinc ion complex and a high affinity guanosine nucleotide binding site and second, a palmitoylated membrane proximal structural element called the C-cys anchor was identified which prevents P2X7 receptor desensitization. This chapter will highlight the major structural and functional aspects of P2X receptors discovered through structural biology, with a key emphasis on the most recent cryogenic electron microscopy structures of the full-length, wild-type P2X7 receptor.

Methods for Studying Cholesterol-Dependent Regulation of P2X7 Receptors

Cholesterol dynamically regulates P2X7 receptor function in both physiological and pathological conditions. Studies suggest that cholesterol suppresses P2X7 receptor activity through direct binding or through indirect effects on the biophysical properties of the membrane. Notably, the palmitoylated C-terminus seems to counteract the action of cholesterol to make it less inhibitory. However, the mechanism underlying cholesterol-dependent regulation of P2X7 receptor remains unclear. Here we describe detailed methods that facilitate the quantification of P2X7 channel activity while controlling the amount of cholesterol in the system. We will first describe the use of methyl-β-cyclodextrin (MCD), a cyclic oligosaccharide consisting of seven glucose monomers, to decrease or increase plasma membrane cholesterol levels. We will then describe protocols for the reconstitution of purified P2X7 in proteoliposomes of defined lipid composition. These methods can be combined with commonly used techniques such as dye-uptake assays or electrophysiology. We also describe a fluorescence assay to measure cholesterol-binding to P2X7. These approaches are complementary to cryo-EM or molecular dynamics simulations, which are also powerful tools for investigating cholesterol-P2X7 interactions. An improved understanding of the mechanisms of action of cholesterol on P2X7 may contribute to elucidate the roles of this receptor in ageing, inflammation, and cancer, whose progression correlates with the level of cholesterol.

IL-33 priming amplifies ATP-mediated mast cell cytokine production

Inflammatory responses are required to block pathogen infection but can also lead to hypersensitivity and chronic inflammation. Barrier tissues actively release IL-33, ATP, and other alarmins during cell stress, helping identify pathogenic stimuli. However, it is unclear how these signals are integrated. Mast cells are critical initiators of allergic inflammation and respond to IL-33 and ATP. We found that mouse mast cells had a 3-6-fold increase in ATP-induced cytokine production when pre-treated with IL-33. This effect was observed at ATP concentrations < 100 µM and required < 30-minute IL-33 exposure. ATP-induced degranulation was not enhanced by pretreatment nor was the response to several pathogen molecules. Mechanistic studies implicated the P2X7 receptor and calcineurin/NFAT pathway in the enhanced ATP response. Finally, we found that IL-33 + ATP co-stimulation enhanced peritoneal eosinophil and macrophage recruitment. These results support the hypothesis that alarmins collaborate to surpass a threshold necessary to initiate an inflammatory response.

Humanized Mouse Model to Study the P2X7 Receptor in Graft-Versus-Host Disease

Humanized mouse models of graft-versus-host disease (GVHD), where human immune cells are injected into immune deficient mice, are well established and provide opportunities to investigate pathways involved in GVHD development. This chapter provides an overview of human immune cell isolation, injection of these cells into immune deficient mice, monitoring of mice for signs of GVHD, and assessment of human cell engraftment using flow cytometry. Further, this chapter focuses on the P2X7 signaling pathway involved in GVHD, and describes a strategy to block the P2X7 receptor and examine the effect of this on GVHD development.

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.

Inherent P2X7 Receptors Regulate Macrophage Functions during Inflammatory Diseases

Macrophages are mononuclear phagocytes which derive either from blood-borne monocytes or reside as resident macrophages in peripheral (Kupffer cells of the liver, marginal zone macrophages of the spleen, alveolar macrophages of the lung) and central tissue (microglia). They occur as M1 (pro-inflammatory; classic) or M2 (anti-inflammatory; alternatively activated) phenotypes. Macrophages possess P2X7 receptors (Rs) which respond to high concentrations of extracellular ATP under pathological conditions by allowing the non-selective fluxes of cations (Na⁺, Ca²⁺, K⁺). Activation of P2X7Rs by still higher concentrations of ATP, especially after repetitive agonist application, leads to the opening of membrane pores permeable to ~900 Da molecules. For this effect an interaction of the P2X7R with a range of other membrane channels (e.g., P2X4R, transient receptor potential A1 [TRPA1], pannexin-1 hemichannel, ANO6 chloride channel) is required. Macrophage-localized P2X7Rs have to be co-activated with the lipopolysaccharide-sensitive toll-like receptor 4 (TLR4) in order to induce the formation of the inflammasome 3 (NLRP3), which then activates the pro-interleukin-1β (pro-IL-1β)-degrading caspase-1 to lead to IL-1β release. Moreover, inflammatory diseases (e.g., rheumatoid arthritis, Crohn’s disease, sepsis, etc.) are generated downstream of the P2X7R-induced upregulation of intracellular second messengers (e.g., phospholipase A2, p38 mitogen-activated kinase, and rho G proteins). In conclusion, P2X7Rs at macrophages appear to be important targets to preserve immune homeostasis with possible therapeutic consequences.

Understanding the Role of Purinergic P2X7 Receptors in the Gastrointestinal System: A Systematic Review

Background: The role of purinergic P2X7 receptor (P2X7R) is of interest due to its involvement in inflammation and mediating immune cell responses. P2X7R is particularly implicated in the development of inflammatory bowel disease (IBD). However, the extent of the actions of P2X7R in the gastrointestinal (GI) system under physiological and pathophysiological conditions remains to be elucidated. This systematic review aimed to identify, summarize and evaluate the evidence for a critical role of P2X7R in the GI system. Methods: We searched PubMed, Embase and Scopus with search terms pertained to P2X7R in the GI system in disease or physiological state, including "P2X7 or P2X7 receptor or purinergic signaling" in combination with any of the terms "intestine or colon or gut or gastrointestinal," "pathology or inflammation or disease or disorder," and "physiology or expression." Titles and abstracts were screened for potentially eligible full texts, and animal and human studies published in English were included in this study. Data were extracted from papers meeting inclusion criteria. Meta-analysis was not feasible given the study diversity. Results: There were 48 papers included in this review. We identified 14 experimental colitis models, three sepsis models and one ischemia-reperfusion injury model. Among them, 11 studies examined P2X7R in GI infections, six studies on immune cell regulation, four studies on GI inflammation, two studies on GI malignancies, three studies involving intestinal injury due to various causes, two studies on ATP-activated P2X7R in the GI system and two studies on metabolic regulation. Conclusion: Evidence supports P2X7R mediating inflammation and immune cell responses in GI inflammation, infections and injury due to IBD and other challenges to the intestinal wall. P2X7R inhibition by gene knockout or by application of P2X7R antagonists can reduce tissue damage by suppressing inflammation. P2X7R is also implicated in GI malignancies and glucose and lipid homeostasis. P2X7R blockade, however, did not always lead to beneficial outcomes in the various pathological models of study.

P2X7 receptor is essential for cross-dressing of bone marrow-derived dendritic cells

T cell activation requires the processing and presentation of antigenic peptides in the context of a major histocompatibility complex (MHC complex). Cross-dressing is a non-conventional antigen presentation mechanism, involving the transfer of preformed peptide/MHC complexes from whole cells, such as apoptotic cells (ACs) to the cell membrane of professional antigen-presenting cells (APCs), such as dendritic cells (DCs). This is an essential mechanism for the induction of immune response against viral antigens, tumors, and graft rejection, which until now has not been clarified. Here we show for first time that the P2X7 receptor (P2X7R) is crucial to induce cross-dressing between ACs and Bone-Marrow DCs (BMDCs). In controlled ex vivo assays, we found that the P2X7R in both ACs and BMDCs is required to induce membrane and fully functional peptide/MHC complex transfer to BMDCs. These findings show that acquisition of ACs-derived preformed antigen/MHC-I complexes by BMDCs requires P2X7R expression.

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Cross-dressing of antigens to Dendritic Cells (DCs) is dependent of P2X7 receptor

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The P2X7 receptor must be present in both Dendritic Cells and antigen source

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The transfer of antigen/MHC-I complexes to DCs is functional and activates T CD8 cells

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The P2X7 receptor allows Cross-Dressing possibly through a membrane fusion process

Enteric nervous system and inflammatory bowel diseases: Correlated impacts and therapeutic approaches through the P2X7 receptor

The enteric nervous system (ENS) consists of thousands of small ganglia arranged in the submucosal and myenteric plexuses, which can be negatively affected by Crohn's disease and ulcerative colitis - inflammatory bowel diseases (IBDs). IBDs are complex and multifactorial disorders characterized by chronic and recurrent inflammation of the intestine, and the symptoms of IBDs may include abdominal pain, diarrhea, rectal bleeding, and weight loss. The P2X7 receptor has become a promising therapeutic target for IBDs, especially owing to its wide expression and, in the case of other purinergic receptors, in both human and model animal enteric cells. However, little is known about the actual involvement between the activation of the P2X7 receptor and the cascade of subsequent events and how all these activities associated with chemical signals interfere with the functionality of the affected or treated intestine. In this review, an integrated view is provided, correlating the structural organization of the ENS and the effects of IBDs, focusing on cellular constituents and how therapeutic approaches through the P2X7 receptor can assist in both protection from damage and tissue preservation.

ATP transporters in the joints

Extracellular adenosine triphosphate (ATP) plays a central role in a wide variety of joint diseases. ATP is generated intracellularly, and the concentration of the extracellular ATP pool is determined by the regulation of its transport out of the cell. A variety of ATP transporters have been described, with connexins and pannexins the most commonly cited. Both form intercellular channels, known as gap junctions, that facilitate the transport of various small molecules between cells and mediate cell-cell communication. Connexins and pannexins also form pores, or hemichannels, that are permeable to certain molecules, including ATP. All joint tissues express one or more connexins and pannexins, and their expression is altered in some pathological conditions, such as osteoarthritis (OA) and rheumatoid arthritis (RA), indicating that they may be involved in the onset and progression of these pathologies. The aging of the global population, along with increases in the prevalence of obesity and metabolic dysfunction, is associated with a rising frequency of joint diseases along with the increased costs and burden of related illness. The modulation of connexins and pannexins represents an attractive therapeutic target in joint disease, but their complex regulation, their combination of gap-junction-dependent and -independent functions, and their interplay between gap junction and hemichannel formation are not yet fully elucidated. In this review, we try to shed light on the regulation of these proteins and their roles in ATP transport to the extracellular space in the context of joint disease, and specifically OA and RA.

The P2X7 purinergic receptor in intervertebral disc degeneration

Mechanisms involved in the development of intervertebral disc (IVD) degeneration are only partially known, thus making the implementation of effective therapies very difficult. In this study, we investigated P2X7 purinergic receptor (P2X7R), NLRP3 inflammasome, and interleukin (IL)-1β expression in IVD specimens at different stages of disease progression, and during the in vitro dedifferentiation process of the primary cells derived thereof. We found that P2X7R, NLRP3, and IL-1β expression was higher in the IVD samples at a more advanced stage of degeneration and in the expanded IVD cells in culture which partially recapitulated the in vivo degeneration process. In IVD cells, the P2X7R showed a striking nuclear localization, while NLRP3 was mainly cytoplasmic. Stimulation with the semiselective P2X7R agonist benzoyl ATP together with lipopolysaccharide treatment triggered P2X7R transfer to the cytoplasm and P2X7R/NLRP3 colocalization. Taken together, these findings support pathophysiological evidence that the degenerated disc is a highly inflamed microenvironment and highlight the P2X7R/NLRP3 axis as a suitable therapeutic target. The immunohistochemical analysis and the assessment of subcellular localization revealed a substantial expression of P2X7R also in normal disc tissue. This gives us the opportunity to contribute to the few studies performed in natively expressed human P2X7R so far, and to understand the possible physiological ATP-mediated P2X7R homeostasis signaling. Therefore, collectively, our findings may offer a new perspective and pave the way for the exploration of a role of P2X7R-mediated purinergic signaling in IVD metabolism that goes beyond its involvement in inflammation.

Extracellular Vesicles from Human Teeth Stem Cells Trigger ATP Release and Promote Migration of Human Microglia through P2X4 Receptor/MFG-E8-Dependent Mechanisms

Extracellular vesicles (EVs) effectively suppress neuroinflammation and induce neuroprotective effects in different disease models. However, the mechanisms by which EVs regulate the neuroinflammatory response of microglia remains largely unexplored. Here, we addressed this issue by testing the action of EVs derived from human exfoliated deciduous teeth stem cells (SHEDs) on immortalized human microglial cells. We found that EVs induced a rapid increase in intracellular Ca²⁺ and promoted significant ATP release in microglial cells after 20 min of treatment. Boyden chamber assays revealed that EVs promoted microglial migration by 20%. Pharmacological inhibition of different subtypes of purinergic receptors demonstrated that EVs activated microglial migration preferentially through the P2X4 receptor (P2X4R) pathway. Proximity ligation and co-immunoprecipitation assays revealed that EVs promote association between milk fat globule-epidermal growth factor-factor VIII (MFG-E8) and P2X4R proteins. Furthermore, pharmacological inhibition of αVβ3/αVβ5 integrin suppressed EV-induced cell migration and formation of lipid rafts in microglia. These results demonstrate that EVs promote microglial motility through P2X4R/MFG-E8-dependent mechanisms. Our findings provide novel insights into the molecular mechanisms through which EVs target human microglia that may be exploited for the development of new therapeutic strategies targeting disease-associated neuroinflammation.

Circulating P2X7 Receptor Signaling Components as Diagnostic Biomarkers for Temporal Lobe Epilepsy

Circulating molecules have potential as biomarkers to support the diagnosis of epilepsy and to assist with differential diagnosis, for example, in conditions resembling epilepsy, such as in psychogenic non-epileptic seizures (PNES). The P2X7 receptor (P2X7R) is an important regulator of inflammation and mounting evidence supports its activation in the brain during epilepsy. Whether the P2X7R or P2X7R-dependent signaling molecules can be used as biomarkers of epilepsy has not been reported. P2X7R levels were analyzed by quantitative ELISA using plasma samples from controls and patients with temporal lobe epilepsy (TLE) or PNES. Moreover, blood cell P2X7R expression and P2X7R-dependent cytokine signature was measured following status epilepticus in P2X7R-EGFP reporter, wildtype, and P2X7R-knockout mice. P2X7R plasma levels were higher in TLE patients when compared with controls and patients with PNES. Plasma levels of the broad inflammatory marker protein C-Reactive protein (CRP) were similar between the three groups. Using P2X7R-EGFP reporter mice, we identified monocytes as the main blood cell type expressing P2X7R after experimentally evoked seizures. Finally, cytokine array analysis in P2X7R-deficient mice identified KC/GRO as a potential P2X7R-dependent plasma biomarker following status epilepticus and during epilepsy. Our data suggest that P2X7R signaling components may be a promising subclass of circulating biomarkers to support the diagnosis of epilepsy.

P2X Receptor-Dependent Modulation of Mast Cell and Glial Cell Activities in Neuroinflammation

Localisation of mast cells (MCs) at the abluminal side of blood vessels in the brain favours their interaction with glial cells, neurons, and endothelial cells, resulting in the activation of these cells and the release of pro-inflammatory mediators. In turn, stimulation of glial cells, such as microglia, astrocytes, and oligodendrocytes may result in the modulation of MC activities. MCs, microglia, astrocytes, and oligodendrocytes all express P2X receptors (P2XRs) family members that are selectively engaged by ATP. As increased concentrations of extracellular adenosine 5′-triphosphate (ATP) are present in the brain in neuropathological conditions, P2XR activation in MCs and glial cells contributes to the control of their communication and amplification of the inflammatory response. In this review we discuss P2XR-mediated MC activation, its bi-directional effect on microglia, astrocytes and oligodendrocytes and role in neuroinflammation.

To inhibit or to boost the ATP/P2RX7 pathway to fight cancer-that is the question

Despite new biological insights and recent therapeutic advances, many tumors remain at baseline during treatments. Therefore, there is an urgent need to find new therapeutic strategies to improve the care of patients with solid tumors. P2RX7 receptor (P2XR7), an ATP-gated ion channel characterized by its ability to form large pore within the cell membrane, is described by most of the investigators as a "chef d'orchestre" of the antitumor immune response. The purpose of this review is to detail the recent information concerning different cellular mechanisms linking P2RX7 to hallmarks of cancer and to discuss different progresses in elucidating how activation of the ATP/P2RX7/NLRP3/IL-18 pathway is a very promising approach to fight cancer progression by increasing antitumor immune responses.

The P2X7 Receptor Stimulates IL-6 Release from Pancreatic Stellate Cells and Tocilizumab Prevents Activation of STAT3 in Pancreatic Cancer Cells

Pancreatic stellate cells (PSCs) are important pancreatic fibrogenic cells that interact with pancreatic cancer cells to promote the progression of pancreatic ductal adenocarcinoma (PDAC). In the tumor microenvironment (TME), several factors such as cytokines and nucleotides contribute to this interplay. Our aim was to investigate whether there is an interaction between IL-6 and nucleotide signaling, in particular, that mediated by the ATP-sensing P2X7 receptor (P2X7R). Using human cell lines of PSCs and cancer cells, as well as primary PSCs from mice, we show that ATP is released from both PSCs and cancer cells in response to mechanical and metabolic cues that may occur in the TME, and thus activate the P2X7R. Functional studies using P2X7R agonists and inhibitors show that the receptor is involved in PSC proliferation, collagen secretion and IL-6 secretion and it promotes cancer cell migration in a human PSC-cancer cell co-culture. Moreover, conditioned media from P2X7R-stimulated PSCs activated the JAK/STAT3 signaling pathway in cancer cells. The monoclonal antibody inhibiting the IL-6 receptor, Tocilizumab, inhibited this signaling. In conclusion, we show an important mechanism between PSC-cancer cell interaction involving ATP and IL-6, activating P2X7 and IL-6 receptors, respectively, both potential therapeutic targets in PDAC.

Inhibitory Effects of Palmatine on P2X7 Receptor Expression in Trigeminal Ganglion and Facial Pain in Trigeminal Neuralgia Rats

Trigeminal Neuralgia (TN) refers to recurrent severe paroxysmal pain in the distribution area of the trigeminal nerve, which seriously affects the quality of life of patients. This research applied the chronic constriction injury of the infraorbital nerve (CCI—ION) approach to induce an animal model of TN in rats. The mechanical pain threshold of each group of rats was determined postoperatively; the expression of P2X7 receptor in trigeminal ganglion (TG) was assessed by qRT-PCR, immunofluorescence and Western blot; and the changes of the proinflammatory cytokines IL-1β and TNF-α in serum of rats were detected by ELISA. The results showed that the administration of palmatine in the TN rats could reduce the mechanical pain threshold, significantly decrease the expression of P2X7 receptor in TG, and lower the serum concentrations of IL-1β and TNF-α, compared to the sham group. In addition, the phosphorylation level of p38 in TG of TN rats was significantly decreased after treatment with palmatine. Likewise, inhibition of P2X7 expression by shRNA treatment could effectively counteract the adversary changes of pain sensitivity, IL-1β and TNF-α production, and p38 phosphorylation in TN rats. Our data suggest that palmatine may alleviate mechanical facial pain in TN rats possibly by reducing the expression of P2X7 receptor in TG of TN rats, which may be attributable to inhibiting p38 phosphorylation and reducing the release of IL-1β and TNF-α.

Pro- and anti-inflammatory macrophages express a sub-type specific purinergic receptor profile

Extracellular nucleotides act as danger signals that orchestrate inflammation by purinergic receptor activation. The expression pattern of different purinergic receptors may correlate with a pro- or anti-inflammatory phenotype. Macrophages function as pro-inflammatory M1 macrophages (M1) or anti-inflammatory M2 macrophages (M2). The present study found that murine bone marrow-derived macrophages express a unique purinergic receptor profile during in vitro polarization. As assessed by real-time polymerase chain reaction (PCR), Gαs-coupled P1 receptors A2A and A2B are upregulated in M1 and M2 compared to M0, but A2A 15 times higher in M1. The ionotropic P2 receptor P2X₅ is selectively upregulated in M1- and M2-polarized macrophages. P2X₇ is temporarily expressed in M1 macrophages. Metabotropic P2Y receptors showed a distinct expression profile in M1 and M2-polarized macrophages: Gαq coupled P2Y₁ and P2Y₆ are exclusively upregulated in M2, whereas Gαi P2Y₁₃ and P2Y₁₄ are overexpressed in M1. This consequently leads to functional differences between M1 and M2 in response to adenosine di-phosphate stimulation (ADP): In contrast to M1, M2 showed increased cytoplasmatic calcium after ADP stimulation. In the present study we show that bone marrow-derived macrophages express a unique repertoire of purinergic receptors. We show for the first time that the repertoire of purinergic receptors is highly flexible and quickly adapts upon pro- and anti-inflammatory macrophage differentiation with functional consequences to nucleotide stimulation.

ATP and adenosine-Two players in the control of seizures and epilepsy development

Despite continuous advances in understanding the underlying pathogenesis of hyperexcitable networks and lowered seizure thresholds, the treatment of epilepsy remains a clinical challenge. Over one third of patients remain resistant to current pharmacological interventions. Moreover, even when effective in suppressing seizures, current medications are merely symptomatic without significantly altering the course of the disease. Much effort is therefore invested in identifying new treatments with novel mechanisms of action, effective in drug-refractory epilepsy patients, and with the potential to modify disease progression. Compelling evidence has demonstrated that the purines, ATP and adenosine, are key mediators of the epileptogenic process. Extracellular ATP concentrations increase dramatically under pathological conditions, where it functions as a ligand at a host of purinergic receptors. ATP, however, also forms a substrate pool for the production of adenosine, via the action of an array of extracellular ATP degrading enzymes. ATP and adenosine have assumed largely opposite roles in coupling neuronal excitability to energy homeostasis in the brain. This review integrates and critically discusses novel findings regarding how ATP and adenosine control seizures and the development of epilepsy. This includes purine receptor P1 and P2-dependent mechanisms, release and reuptake mechanisms, extracellular and intracellular purine metabolism, and emerging receptor-independent effects of purines. Finally, possible purine-based therapeutic strategies for seizure suppression and disease modification are discussed.

P2X7 receptor in multifaceted cellular signalling and its relevance as a potential therapeutic target in different diseases

P2X7 receptor, a purinergic receptor family member, is abundantly expressed on many cells, including immune, muscle, bone, neuron, and glia. It acts as an ATP-activated cation channel that permits the influx of Ca²⁺, Na⁺ and efflux of K⁺ ions. The P2X7 receptor plays crucial roles in many physiological processes including cytokine and chemokine secretion, NLRP3 inflammasome activation, cellular growth and differentiation, locomotion, wound healing, transcription factors activation, cell death and T-lymphocyte survival. Past studies have demonstrated the up-regulation and direct association of this receptor in many pathophysiological conditions such as cancer, diabetics, arthritis, tuberculosis (TB) and inflammatory diseases. Hence, targeting this receptor is considered a worthwhile approach to lessen the afflictions associated with the disorders mentioned above by understanding the receptor architecture and downstream signalling processes. Here, in the present review, we have dissected the structural and functional aspects of the P2X7 receptor, emphasizing its role in various diseased conditions. This information will provide in-depth knowledge about the receptor and help to develop apt curative methodologies for the betterment of humanity in the coming years.

Antagonism of the ATP-gated P2X7 receptor: a potential therapeutic strategy for cancer

The P2X receptor 7 (P2X7R) is a plasma membrane receptor sensing extracellular ATP associated with a wide variety of cellular functions. It is most commonly expressed on immune cells and is highly upregulated in a number of human cancers where it can play a trophic role in tumorigenesis. Activation of this receptor leads to the formation of a non-selective cation channel, which has been associated with several cellular functions mediated by the PI3K/Akt pathway and protein kinases. Due to its broad range of functions, the receptor represents a potential therapeutic target for a number of cancers. This review describes the range of mechanisms associated with P2X7R activation in cancer settings and highlights the potential of targeted inhibition of P2X7R as a therapy. It also describes in detail a number of key P2X7R antagonists currently in pre-clinical and clinical development, including oxidised ATP, Brilliant Blue G (BBG), KN-62, KN-04, A740003, A438079, GSK1482160, CE-224535, JNJ-54175446, JNJ-55308942, and AZ10606120. Lastly, it summarises the in vivo studies and clinical trials associated with the use and development of these P2X7R antagonists in different disease contexts.

Having an Old Friend for Dinner: The Interplay between Apoptotic Cells and Efferocytes

Apoptosis, the programmed and intentional death of senescent, damaged, or otherwise superfluous cells, is the natural end-point for most cells within multicellular organisms. Apoptotic cells are not inherently damaging, but if left unattended, they can lyse through secondary necrosis. The resulting release of intracellular contents drives inflammation in the surrounding tissue and can lead to autoimmunity. These negative consequences of secondary necrosis are avoided by efferocytosis—the phagocytic clearance of apoptotic cells. Efferocytosis is a product of both apoptotic cells and efferocyte mechanisms, which cooperate to ensure the rapid and complete removal of apoptotic cells. Herein, we review the processes used by apoptotic cells to ensure their timely removal, and the receptors, signaling, and cellular processes used by efferocytes for efferocytosis, with a focus on the receptors and signaling driving this process.

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.

Macrophage Response Driven by Extracellular ATP

The purine nucleotide ATP is a fundamental unit in cellular energy metabolism. Extracellular ATP and its metabolites are also ligands for a family of receptors, known as purinergic receptors, which are expressed ubiquitously in almost every cell type. In the immune system, extracellular ATP and its signals regulate the migration and activation of immune cells to orchestrate the induction and resolution of inflammation. In this review, we provide an overview of purinergic receptors and their downstream signaling related to macrophage activation. We also discuss the roles of purinergic signaling for macrophage functions in physiological and pathological conditions.

P2X7 promotes the progression of MLL-AF9 induced acute myeloid leukemia by upregulation of Pbx3

Nucleotides mediate intercellular communication by activating purinergic receptors and take part in various physiological and pathological processes. Abnormal purinergic signaling plays important roles in malignant progression. P2X7, which belongs to the P2X family of purinergic receptors, is abnormally expressed in various types of malignancies including leukemia. However, its role and molecular mechanism in leukemia have not been elucidated. Here, we analyzed the correlation between P2X7 expression and AML clinical outcome; explored the role and mechanism of P2X7 in AML progression by using mouse acute myeloid leukemia (AML), nude mouse xenograft and patient-derived xenograft models. High levels of P2X7 expression were correlated with worse survival in AML. P2X7 was highly expressed in MLL-rearranged AML. Furthermore, P2X7 accelerated the progression of MLL-rearranged AML by both promoting cell proliferation and increasing leukemia stem cell (LSC) levels. Moreover, P2X7 caused upregulation of Pbx3 accounts for its pro-leukemic effects. The P2X7-Pbx3 pathway might also contribute to the progression of other types of leukemia as well as solid tumors with high levels of P2X7 expression. Our study provides new insights into the malignant progression caused by abnormal purinergic signaling.

Medicinal chemistry of P2 and adenosine receptors: Common scaffolds adapted for multiple targets

Prof. Geoffrey Burnstock originated the concept of purinergic signaling. He demonstrated the interactions and biological roles of ionotropic P2X and metabotropic P2Y receptors. This review paper traces the historical origins of many currently used antagonists and agonists for P2 receptors, as well as adenosine receptors, in early attempts to identify ligands for these receptors - prior to the use of chemical libraries for screening. Rather than presenting a general review of current purinergic ligands, we focus on common chemical scaffolds (privileged scaffolds) that can be adapted for multiple receptor targets. By carefully analyzing the structure activity relationships, one can direct the selectivity of these scaffolds toward different receptor subtypes. For example, the weak and non-selective P2 antagonist reactive blue 2 (RB-2) was derivatized using combinatorial synthetic approaches, leading to the identification of selective P2Y2, P2Y4, P2Y12 or P2X2 receptor antagonists. A P2X4 antagonist NC-2600 is in a clinical trial, and A3 adenosine agonists show promise, for chronic pain. P2X7 antagonists have been in clinical trials for depression (JNJ-54175446), inflammatory bowel disease (IBD), Crohn's disease, rheumatoid arthritis, inflammatory pain and chronic obstructive pulmonary disease (COPD). P2X3 antagonists are in clinical trials for chronic cough, and an antagonist named after Burnstock, gefapixant, is expected to be the first P2X3 antagonist filed for approval. We are seeing that the vision of Prof. Burnstock to use purinergic signaling modulators, most recently at P2XRs, for treating disease is coming to fruition.

P2X7 Receptors in Neurodegeneration: Potential Therapeutic Applications From Basic to Clinical Approaches

Purinergic receptors play important roles in central nervous system (CNS), where the bulk of these receptors are implicated in neuroinflammatory responses and regulation of cellular function of neurons, microglial and astrocytes. Within the P2X receptor family, P2X₇ receptor is generally known for its inactivity in normal conditions and activation by moderately high concentrations (>100 μM) of extracellular adenosine 5′-triphosphate (ATP) released from injured cells as a result of brain injury or pathological conditions. Activation of P2X₇R contributes to the activation and proliferation of microglia and directly contribute to neurodegeneration by provoking microglia-mediated neuronal death, glutamate-mediated excitotoxicity, and NLRP3 inflammasome activation that results in initiation, maturity and release of the pro-inflammatory cytokines and generation of reactive oxygen and nitrogen species. These components of the inflammatory response play important roles in many neural pathologies and neurodegeneration disorders. In CNS, expression of P2X₇R on microglia, astrocytes, and oligodendrocytes are upregulated under neuroinflammatory conditions. Several in vivo studies have demonstrated beneficial effects of the P2X₇ receptor antagonists in animal model systems of neurodegenerative diseases. A number of specific and selective P2X₇ receptor antagonists have been developed, but only few of them have shown efficient brain permeability. Finding potent and selective P2X₇ receptor inhibitors which are also CNS penetrable and display acceptable pharmacokinetics (PK) has presented challenges for both academic researchers and pharmaceutical companies. In this review, we discuss the role of P2X₇ receptor function in neurodegenerative diseases, the pharmacological inhibition of the receptor, and PET radiopharmaceuticals which permit non-invasive monitoring of the P2X₇ receptor contribution to neuroinflammation associated with neurodegeneration.

Role of the purinergic signaling network in lung ischemia-reperfusion injury

PURPOSE OF REVIEW

Primary graft dysfunction (PGD) is the leading cause of early mortality following lung transplantation and is typically caused by lung ischemia-reperfusion injury (IRI). Current management of PGD is largely supportive and there are no approved therapies to prevent lung IRI after transplantation. The purinergic signaling network plays an important role in this sterile inflammatory process, and pharmacologic manipulation of said network is a promising therapeutic strategy. This review will summarize recent findings in this area.

RECENT FINDINGS

In the past 18 months, our understanding of lung IRI has improved, and it is becoming clear that the purinergic signaling network plays a vital role. Recent works have identified critical components of the purinergic signaling network (Pannexin-1 channels, ectonucleotidases, purinergic P1 and P2 receptors) involved in inflammation in a number of pathologic states including lung IRI. In addition, a functionally-related calcium channel, the transient receptor potential vanilloid type 4 (TRPV4) channel, has recently been linked to purinergic signaling and has also been shown to mediate lung IRI.

SUMMARY

Agents targeting components of the purinergic signaling network are promising potential therapeutics to limit inflammation associated with lung IRI and thus decrease the risk of developing PGD.

Structural and Functional Features of the P2X4 Receptor: An Immunological Perspective

Extracellular nucleotides are important mediators of activation, triggering various responses through plasma membrane P2 and P1 receptors. P2 receptors are further subdivided into ionotropic P2X receptors and G protein-coupled P2Y receptors. P2X4 is an ATP-gated cation channel broadly expressed in most tissues of the body. Within the P2X family, P2X4 has a unique subcellular distribution, being preferentially localized in lysosomes. In these organelles, high ATP concentrations do not trigger P2X4 because of the low pH. However, when the pH increases to 7.4, P2X4 can be stimulated by intra-lysosomal ATP, which is in its active, tetra-anionic form. Elucidation of P2X4, P2X3 and P2X7 structures has shed some light on the functional differences between these purinergic receptors. The potential interaction between P2X4 and P2X7 has been extensively studied. Despite intensive effort, it has not been possible yet to determine whether P2X4 and P2X7 interact as heterotrimers or homotrimers at the plasma membrane. However, several publications have shown that functional interactions between P2X4 and P2X7 do occur. Importantly, these studies indicate that P2X4 potentiates P2X7-dependent activation of inflammasomes, leading to increased release of IL-1β and IL-18. The role of P2X4 in various diseases could be beneficial or deleterious even though the pathophysiological mechanisms involved are still poorly defined. However, in diseases whose physiopathology involves activation of the NLRP3 inflammasome, P2X4 was found to exacerbate severity of disease. The recent production of monoclonal antibodies specific for the human and mouse P2X4, some of which are endowed with agonist or antagonist properties, raises the possibility that they could be used therapeutically. Analysis of single nucleotide polymorphisms of the human P2RX4 gene has uncovered the association of P2RX4 gene variants with susceptibility to several human diseases.

Reduction of prefrontal purinergic signaling is necessary for the analgesic effect of morphine

Morphine is commonly used to relieve moderate to severe pain, but repeated doses cause opioid tolerance. Here, we used ATP sensor and fiber photometry to detect prefrontal ATP level. It showed that prefrontal ATP level decreased after morphine injection and the event amplitude tended to decrease with continuous morphine exposure. Morphine had little effect on prefrontal ATP due to its tolerance. Therefore, we hypothesized that the analgesic effect of morphine might be related to ATP in the medial prefrontal cortex (mPFC). Moreover, local infusion of ATP partially antagonized morphine analgesia. Then we found that inhibiting P2X7R in the mPFC mimicked morphine analgesia. In morphine-tolerant mice, pretreatment with P2X4R or P2X7R antagonists in the mPFC enhanced analgesic effect. Our findings suggest that reduction of prefrontal purinergic signaling is necessary for the morphine analgesia, which help elucidate the mechanism of morphine analgesia and may lead to the development of new clinical treatments for neuropathic pain.

The P2X7 Receptor in Microglial Cells Modulates the Endolysosomal Axis, Autophagy, and Phagocytosis

Microglial cells regulate neural homeostasis by coordinating both immune responses and clearance of debris, and the P2X₇ receptor for extracellular ATP plays a central role in both functions. The P2X₇ receptor is primarily known in microglial cells for its immune signaling and NLRP3 inflammasome activation. However, the receptor also affects the clearance of extracellular and intracellular debris through modifications of lysosomal function, phagocytosis, and autophagy. In the absence of an agonist, the P2X₇ receptor acts as a scavenger receptor to phagocytose material. Transient receptor stimulation induces autophagy and increases LC3-II levels, likely through calcium-dependent phosphorylation of AMPK, and activates microglia to an M1 or mixed M1/M2 state. We show an increased expression of Nos2 and Tnfa and a decreased expression of Chil3 (YM1) from primary cultures of brain microglia exposed to high levels of ATP. Sustained stimulation can reduce lysosomal function in microglia by increasing lysosomal pH and slowing autophagosome-lysosome fusion. P2X₇ receptor stimulation can also cause lysosomal leakage, and the subsequent rise in cytoplasmic cathepsin B activates the NLRP3 inflammasome leading to caspase-1 cleavage and IL-1β maturation and release. Support for P2X₇ receptor activation of the inflammasome following lysosomal leakage comes from data on primary microglia showing IL-1β release following receptor stimulation is inhibited by cathepsin B blocker CA-074. This pathway bridges endolysosomal and inflammatory roles and may provide a key mechanism for the increased inflammation found in age-dependent neurodegenerations characterized by excessive lysosomal accumulations. Regardless of whether the inflammasome is activated via this lysosomal leakage or the better-known K⁺-efflux pathway, the inflammatory impact of P2X₇ receptor stimulation is balanced between the autophagic reduction of inflammasome components and their increase following P2X₇-mediated priming. In summary, the P2X₇ receptor modulates clearance of extracellular debris by microglial cells and mediates lysosomal damage that can activate the NLRP3 inflammasome. A better understanding of how the P2X₇ receptor alters phagocytosis, lysosomal health, inflammation, and autophagy can lead to therapies that balance the inflammatory and clearance roles of microglial cells.

The Purinergic P2X7 Receptor-NLRP3 Inflammasome Pathway: A New Target in Alcoholic Liver Disease?

The World Health Organization has estimated that approximately 3 million deaths are attributable to alcohol consumption each year. Alcohol consumption is notably associated with the development and/or progression of many non-communicable inflammatory diseases-particularly in the liver. Although these alcoholic liver diseases were initially thought to be caused by the toxicity of ethanol on hepatocytes, the latest research indicates Kupffer cells (the liver macrophages) are at the heart of this "inflammatory shift". Purinergic signaling (notably through P2X7 receptors and the NLRP3 inflammasome) by Kupffer cells appears to be a decisive factor in the pathophysiology of alcoholic liver disease. Hence, the modulation of purinergic signaling might represent a new means of treating alcoholic liver disease. Here, we review current knowledge on the pathophysiology of alcoholic liver diseases and therapeutic perspectives for targeting these inflammatory pathways.

The role of P2X7R in neuroinflammation and implications in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia and is set to rise in prevalence as the global trends in population aging. The extracellular deposition of amyloid protein (Aβ) and the intracellular formation of neurofibrillary tangles in the brain have been recognized as the two core pathologies of AD. Over the past decades, the presence of neuroinflammation in the brain has been documented as the third core pathology of AD. In recent years, emerging evidence demonstrated that the purinergic receptor P2X7 (P2X7R) serves a critical role in microglia responses and neuroinflammation. Besides, targeting P2X7R by genetic or pharmacological strategies attenuates the symptoms and pathological changes of AD models, and P2X7R has been recognized as a promising therapeutic target for AD. In this review, we summarized the recent evidence concerning the roles of P2X7R in neuroinflammation and implications in AD pathogenesis.

P2X7 Variants in Oncogenesis

The P2X7 receptor for extracellular ATP is a well-established mediator of tumoral development and progression both in solid cancers and hematological malignancies. The human P2X7 gene is highly polymorphic, and several splice variants of the receptor have been identified in time. P2X7 single-nucleotide polymorphisms (SNPs) have been broadly analyzed by studies relating them to pathologies as different as infectious, inflammatory, nervous, and bone diseases, among which cancer is included. Moreover, in the last years, an increasing number of reports concentrated on P2X7 splice variants’ different roles and their implications in pathological conditions, including oncogenesis. Here, we give an overview of established and recent literature demonstrating a role for human P2X7 gene products in oncological conditions, mainly focusing on current data emerging on P2X7 isoform B and nfP2X7. We explored the role of these and other genetic variants of P2X7 in cancer insurgence, dissemination, and progression, as well as the effect of chemotherapy on isoforms expression. The described literature strongly suggests that P2X7 variants are potential new biomarkers and therapeutical targets in oncological conditions and that their study in carcinogenesis deserves to be further pursued.

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.

Stress-Related Brain Neuroinflammation Impact in Depression: Role of the Corticotropin-Releasing Hormone System and P2X7 Receptor

Depression and other psychiatric stress-related disorders are leading causes of disability worldwide. Up to date, treatments of mood disorders have limited success, most likely due to the multifactorial etiology of these conditions. Alterations in inflammatory processes have been identified as possible pathophysiological mechanisms in psychiatric conditions. Here, we review the main features of 2 systems involved in the control of these inflammatory pathways: the CRH system as a key regulator of the stress response and the ATP-gated ion-channel P2X7 receptor (P2X7R) involved in the control of immune functions. The pathophysiology of depression as a stress-related psychiatric disorder is depicted in terms of the impact of CRH and P2X7R function on inflammatory pathways in the brain. Understanding pathogenesis of affective disorders will lead to the development of therapies for treatment of depression and other stress-related diseases.

Design, Synthesis, and in vitro Evaluation of P2X7 Antagonists

The P2X7 receptor is a promising target for the treatment of various diseases due to its significant role in inflammation and immune cell signaling. This work describes the design, synthesis, and in vitro evaluation of a series of novel derivatives bearing diverse scaffolds as potent P2X7 antagonists. Our approach was based on structural modifications of reported (adamantan-1-yl)methylbenzamides able to inhibit the receptor activation. The adamantane moieties and the amide bond were replaced, and the replacements were evaluated by a ligand-based pharmacophore model. The antagonistic potency of the synthesized analogues was assessed by two-electrode voltage clamp experiments, using Xenopus laevis oocytes that express the human P2X7 receptor. SAR studies suggested that the replacement of the adamantane ring by an aryl-cyclohexyl moiety afforded the most potent antagonists against the activation of the P2X7 cation channel, with analogue 2-chloro-N-[1-(3-(nitrooxymethyl)phenyl)cyclohexyl)methyl]benzamide (56) exhibiting the best potency with an IC₅₀ value of 0.39 μM.

P2X receptors in cancer growth and progression

It is increasingly appreciated that ion channels have a crucial role in tumors, either as promoters of cancer cell growth, or modulators of immune cell functions, or both. Among ion channels, P2X receptors have a special status because they are gated by ATP, a common and abundant component of the tumor microenvironment. Furthermore, one P2X receptor, i.e. P2X7, may also function as a conduit for ATP release, thus fuelling the increased extracellular ATP level in the tumor interstitium. These findings show that P2X receptors and extracellular ATP are indissoluble partners and key regulators of tumor growth, and suggest the exploitation of the extracellular ATP-P2X partnership to develop innovative therapeutic approaches to cancer.

Purinergic Signaling in Pancreas-From Physiology to Therapeutic Strategies in Pancreatic Cancer

The purinergic signaling has an important role in regulating pancreatic exocrine secretion. The exocrine pancreas is also a site of one of the most serious cancer forms, the pancreatic ductal adenocarcinoma (PDAC). Here, we explore how the network of purinergic and adenosine receptors, as well as ecto-nucleotidases regulate normal pancreatic cells and various cells within the pancreatic tumor microenvironment. In particular, we focus on the P2X7 receptor, P2Y2 and P2Y12 receptors, as well as A2 receptors and ecto-nucleotidases CD39 and CD73. Recent studies indicate that targeting one or more of these candidates could present new therapeutic approaches to treat pancreatic cancer. In pancreatic cancer, as much as possible of normal pancreatic function should be preserved, and therefore physiology of purinergic signaling in pancreas needs to be considered.

Deviant reporter expression and P2X4 passenger gene overexpression in the soluble EGFP BAC transgenic P2X7 reporter mouse model

The ATP-gated P2X7 receptor is highly expressed in microglia and has been involved in diverse brain diseases. P2X7 effects were also described in neurons and astrocytes but its localisation and function in these cell types has been challenging to demonstrate in situ. BAC transgenic mouse lines have greatly advanced neuroscience research and two BAC-transgenic P2X7 reporter mouse models exist in which either a soluble EGFP (sEGFP) or an EGFP-tagged P2X7 receptor (P2X7-EGFP) is expressed under the control of a BAC-derived P2rx7 promoter. Here we evaluate both mouse models and find striking differences in both P2X expression levels and EGFP reporter expression patterns. Most remarkably, the sEGFP model overexpresses a P2X4 passenger gene and sEGFP shows clear neuronal localisation but appears to be absent in microglia. Preliminary functional analysis in a status epilepticus model suggests functional consequences of the observed P2X receptor overexpression. In summary, an aberrant EGFP reporter pattern and possible effects of P2X4 and/or P2X7 protein overexpression need to be considered when working with this model. We further discuss reasons for the observed differences and possible caveats in BAC transgenic approaches.

P2X7 Receptor-Dependent Layer-Specific Changes in Neuron-Microglia Reactivity in the Prefrontal Cortex of a Phencyclidine Induced Mouse Model of Schizophrenia

Background: It has been consistently reported that the deficiency of the adenosine triphosphate (ATP) sensitive purinergic receptor P2X7 (P2X7R) ameliorates symptoms in animal models of brain diseases.

Objective: This study aimed to investigate the role of P2X7R in rodent models of acute and subchronic schizophrenia based on phencyclidine (PCP) delivery in animals lacking or overexpressing P2X7R, and to identify the underlying mechanisms involved.

Methods: The psychotomimetic effects of acute i.p. PCP administration in C57Bl/6J wild-type, P2X7R knockout (P2rx7^−/−) and overexpressing (P2X7-EGFP) young adult mice were quantified. The medial prefrontal cortex (mPFC) of P2rx7^−/− and heterozygous P2X7-EGFP acutely treated animals was characterized through immunohistochemical staining. The prefrontal cortices of young adult P2rx7^−/− and P2rx7^tg/+ mice were examined with tritiated dopamine release experiments and the functional properties of the mPFC pyramidal neurons in layer V from P2rx7^−/− mice were assessed by patch-clamp recordings. P2rx7^−/− animals were subjected to a 7 days subchronic systemic PCP treatment. The animals working memory performance and PFC cytokine levels were assessed.

Results: Our data strengthen the hypothesis that P2X7R modulates schizophrenia-like positive and cognitive symptoms in NMDA receptor antagonist models in a receptor expression level-dependent manner. P2X7R expression leads to higher medial PFC susceptibility to PCP-induced circuit hyperactivity. The mPFC of P2X7R knockout animals displayed distinct alterations in the neuronal activation pattern, microglial organization, specifically around hyperactive neurons, and were associated with lower intrinsic excitability of mPFC neurons.

Conclusions: P2X7R expression exacerbated PCP-related effects in C57Bl/6J mice. Our findings suggest a pleiotropic role of P2X7R in the mPFC, consistent with the observed behavioral phenotype, regulating basal dopamine concentration, layer-specific neuronal activation, intrinsic excitability of neurons in the mPFC, and the interaction of microglia with hyperactive neurons. Direct measurements of P2X7R activity concerning microglial ramifications and dynamics could help to further elucidate the molecular mechanisms involved.

Role of Conserved Residues and F322 in the Extracellular Vestibule of the Rat P2X7 Receptor in Its Expression, Function and Dye Uptake Ability

Activation of the P2X7 receptor results in the opening of a large pore that plays a role in immune responses, apoptosis, and many other physiological and pathological processes. Here, we investigated the role of conserved and unique residues in the extracellular vestibule connecting the agonist-binding domain with the transmembrane domain of rat P2X7 receptor. We found that all residues that are conserved among the P2X receptor subtypes respond to alanine mutagenesis with an inhibition (Y51, Q52, and G323) or a significant decrease (K49, G326, K327, and F328) of 2',3'-O-(benzoyl-4-benzoyl)-ATP (BzATP)-induced current and permeability to ethidium bromide, while the nonconserved residue (F322), which is also present in P2X4 receptor, responds with a 10-fold higher sensitivity to BzATP, much slower deactivation kinetics, and a higher propensity to form the large dye-permeable pore. We examined the membrane expression of conserved mutants and found that Y51, Q52, G323, and F328 play a role in the trafficking of the receptor to the plasma membrane, while K49 controls receptor responsiveness to agonists. Finally, we studied the importance of the physicochemical properties of these residues and observed that the K49R, F322Y, F322W, and F322L mutants significantly reversed the receptor function, indicating that positively charged and large hydrophobic residues are important at positions 49 and 322, respectively. These results show that clusters of conserved residues above the transmembrane domain 1 (K49-Y51-Q52) and transmembrane domain 2 (G326-K327-F328) are important for receptor structure, membrane expression, and channel gating and that the nonconserved residue (F322) at the top of the extracellular vestibule is involved in hydrophobic inter-subunit interaction which stabilizes the closed state of the P2X7 receptor channel.

Human P2X7 receptors - Properties of single ATP-gated ion channels

Patch clamp investigations of single ion channels give insight into the function of these proteins on the molecular level. Utilizing this technique, we performed detailed investigations of the human P2X7 receptor, which is a ligand gated ion channel opened by binding of ATP, like the other P2X receptor subtypes. P2X7 receptors become activated under pathological conditions of ATP release like hypoxia or cell destruction. They are involved in inflammatory and nociceptive reactions of the organism to these pathological events. Knowledge about the function of the P2X7 receptor might lead to a deeper insight into the signaling within these pathophysiological processes and to reveal targets of anti-inflammatory and anti-nociceptive therapies. We found that hP2X7 receptors become activated by ATP within a few milliseconds and are permeable only to cations. Their ion channel conductance remains constant across minutes of activation, which argues against dilation of the ion channel pore. Substitution of Na⁺ or Cl^- ions not only influences the ion channel current amplitude but also the channel gating. Polar residues of the second transmembrane domains of the three protein subunits are important for ion conduction, with S342 constituting the ion selectivity filter and the gate of the channel. The specific long C-terminal domains are important for hP2X7 receptor ion channel function, as their loss strongly decreases ion channel currents.

P2X7 Receptors: An Untapped Target for the Management of Cardiovascular Disease

Chronic low-grade inflammation contributes to the development of several diseases, including cardiovascular disease. Adequate strategies to target inflammation in cardiovascular disease are in their infancy and remain an avenue of great interest. The purinergic receptor P2X7 is a ubiquitously expressed receptor that predominately mediates inflammation and cellular death. P2X7 is a ligand-gated cation channel that is activated in response to high concentrations of extracellular ATP, triggering the assembly and activation of the NLRP3 (nuclear oligomerization domain like receptor family pyrin domain containing 3) inflammasome and subsequent release of proinflammatory cytokines IL (interleukin)-1β and IL-18. Increased P2X7 activation and IL-1β and IL-18 concentrations have been implicated in the development of many cardiovascular conditions including hypertension, atherosclerosis, ischemia/reperfusion injury, and heart failure. P2X7 receptor KO (knockout) mice exhibit a significant attenuation of the inflammatory response, which corresponds with reduced disease severity. P2X7 antagonism blunts blood pressure elevation in hypertension and progression of atherosclerosis in animal models. IL-1β and IL-18 inhibition has shown efficacy in clinical trials reducing major adverse cardiac events, including myocardial infarction, and heart failure. With several P2X7 antagonists available with proven safety margins, P2X7 antagonism could represent an untapped potential for therapeutic intervention in cardiovascular disorders.

Inhibition of purinergic P2X receptor 7 (P2X7R) decreases granulocyte-macrophage colony-stimulating factor (GM-CSF) expression in U251 glioblastoma cells

Glioblastoma is the most aggressive form of primary brain cancer, with a median survival of 12-15 months. The P2X receptor 7 (P2X7R) is upregulated in glioblastoma and is associated with increased tumor cell proliferation. The cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) is also upregulated in glioblastoma and has been shown to have both pro- and anti-tumor functions. This study investigates the potential mechanism linking P2X7R and GM-CSF in the U251 glioblastoma cell line and the therapeutic potential of P2X7R antagonism in this setting. P2X7R protein and mRNA was demonstrated to be expressed in the U251 cell line as assessed by immunocytochemistry and qPCR. Its channel function was intact as demonstrated by live cell confocal imaging using a calcium indicator Fluo-4 AM. Inhibition of P2X7R using antagonist AZ10606120, decreased both GM-CSF mRNA (P < 0.05) and protein (P < 0.01) measured by qPCR and ELISA respectively. Neutralization of GM-CSF with an anti-GM-CSF antibody did not alter U251 cell proliferation, however, P2X7R antagonism with AZ10606120 significantly reduced U251 glioblastoma cell numbers (P < 0.01). This study describes a novel link between P2X7R activity and GM-CSF expression in a human glioblastoma cell line and highlights the potential therapeutic benefit of P2X7R inhibition with AZ10606120 in glioblastoma.

Characterization of the Expression of the ATP-Gated P2X7 Receptor Following Status Epilepticus and during Epilepsy Using a P2X7-EGFP Reporter Mouse

Mounting evidence suggests that the ATP-gated P2X7 receptor contributes to increased hyperexcitability in the brain. While increased expression of P2X7 in the hippocampus and cortex following status epilepticus and during epilepsy has been repeatedly demonstrated, the cell type-specific expression of P2X7 and its expression in extra-hippocampal brain structures remains incompletely explored. In this study, P2X7 expression was visualized by using a transgenic mouse model overexpressing P2X7 fused to the fluorescent protein EGFP. The results showed increased P2X7-EGFP expression after status epilepticus induced by intra-amygdala kainic acid and during epilepsy in different brain regions including the hippocampus, cortex, striatum, thalamus and cerebellum, and this was most evident in microglia and oligodendrocytes. Co-localization of P2X7-EGFP with cell type-specific markers was not detected in neurons or astrocytes. These data suggest that P2X7 activation is a common pathological hallmark across different brain structures, possibly contributing to brain inflammation and neurodegeneration following acute seizures and during epilepsy.

Amyloid Beta Peptide (Aβ1-42) Reverses the Cholinergic Control of Monocytic IL-1β Release

Amyloid-β peptide (Aβ_1-42), the cleavage product of the evolutionary highly conserved amyloid precursor protein, presumably plays a pathogenic role in Alzheimer's disease. Aβ_1-42 can induce the secretion of the pro-inflammatory cytokine intereukin-1β (IL-1β) in immune cells within and out of the nervous system. Known interaction partners of Aβ_1-42 are α7 nicotinic acetylcholine receptors (nAChRs). The physiological functions of Aβ_1-42 are, however, not fully understood. Recently, we identified a cholinergic mechanism that controls monocytic release of IL-1β by canonical and non-canonical agonists of nAChRs containing subunits α7, α9, and/or α10. Here, we tested the hypothesis that Aβ_1-42 modulates this inhibitory cholinergic mechanism. Lipopolysaccharide-primed monocytic U937 cells and human mononuclear leukocytes were stimulated with the P2X7 receptor agonist 2'(3')-O-(4-benzoylbenzoyl)adenosine-5'-triphosphate triethylammonium salt (BzATP) in the presence or absence of nAChR agonists and Aβ_1-42. IL-1β concentrations were measured in the supernatant. Aβ_1-42 dose-dependently (IC₅₀ = 2.54 µM) reversed the inhibitory effect of canonical and non-canonical nicotinic agonists on BzATP-mediated IL-1β-release by monocytic cells, whereas reverse Aβ_42-1 was ineffective. In conclusion, we discovered a novel pro-inflammatory Aβ_1-42 function that enables monocytic IL-1β release in the presence of nAChR agonists. These findings provide evidence for a novel physiological function of Aβ_1-42 in the context of sterile systemic inflammation.