Genetically dissecting P2rx7 expression within the central nervous system using conditional humanized mice

Structure, function and techniques of investigation of the P2X7 receptor (P2X7R) in mammalian cells

The P2X7 receptor [P2X7R or P2RX7 in National Center for Biotechnology Information (NCBI) gene nomenclature] is a member of the P2X receptor (P2XR) subfamily of P2 receptors (P2Rs). The P2X7R is an extracellular ATP-gated ion channel with peculiar permeability properties expressed by most cell types, mainly in the immune system, where it has a leading role in cytokine release, oxygen radical generation, T lymphocyte differentiation and proliferation. A role in cancer cell growth and tumor progression has also been demonstrated. These features make the P2X7R an appealing target for drug development in inflammation and cancer. The functional P2X7R, recently (partially) crystallized and 3-D solved, is formed by the assembly of three identical subunits (homotrimer). The P2X7R is preferentially permeable to small cations (Ca2+, Na+, K+), and in most (but not all) cell types also to large positively charged molecules of molecular mass up to 900Da. Permeability to negatively charged species of comparable molecular mass (e.g., Lucifer yellow) is debated. Several highly selective P2X7R pharmacological blockers have been developed over the years, thus providing powerful tools for P2X7R studies. Biophysical properties and coupling to several different physiological responses make the P2X7R amenable to investigation by electrophysiology and cell biology techniques, which allow its identification and characterization in many different cell types and tissues. A careful description of the physiological features of the P2X7R is a prerequisite for an effective therapeutic development. Here we describe the most common techniques to asses P2X7R functions, including patch-clamp, intracellular calcium measurements, and membrane permeabilization to large fluorescent dyes in a selection of different cell types. In addition, we also describe common toxicity assays used to verify the effects of P2X7R stimulation on cell viability.

P2X7 Receptor Signaling in Stress and Depression

Stress exposure is considered to be the main environmental cause associated with the development of depression. Due to the limitations of currently available antidepressants, a search for new pharmacological targets for treatment of depression is required. Recent studies suggest that adenosine triphosphate (ATP)-mediated signaling through the P2X7 receptor (P2X7R) might play a prominent role in regulating depression-related pathology, such as synaptic plasticity, neuronal degeneration, as well as changes in cognitive and behavioral functions. P2X7R is an ATP-gated cation channel localized in different cell types in the central nervous system (CNS), playing a crucial role in neuron-glia signaling. P2X7R may modulate the release of several neurotransmitters, including monoamines, nitric oxide (NO) and glutamate. Moreover, P2X7R stimulation in microglia modulates the innate immune response by activating the NLR family pyrin domain containing 3 (NLRP3) inflammasome, consistent with the neuroimmune hypothesis of MDD. Importantly, blockade of P2X7R leads to antidepressant-like effects in different animal models, which corroborates the findings that the gene encoding for the P2X7R is located in a susceptibility locus of relevance to depression in humans. This review will discuss recent findings linked to the P2X7R involvement in stress and MDD neuropathophysiology, with special emphasis on neurochemical, neuroimmune, and neuroplastic mechanisms.

Alternative splicing of P2RX7 pre-messenger RNA in health and diseases: Myth or reality?

Alternative splicing (AS) tremendously increases the use of genetic information by generating protein isoforms that differ in protein-protein interactions, catalytic activity and/or subcellular localization. This review is not dedicated to AS in general, but rather we focus our attention on AS of P2RX7 pre-mRNA. Whereas P2RX7 mRNA is expressed by virtually all eukaryotic mammalian cells, the expression of this channel receptor is restrained to certain cells. When expressed at the cell membrane, P2RX7 controls downstream events including release of inflammatory molecules, phagocytosis, cell proliferation and death and metabolic events. Therefore, P2RX7 is an important actor of health and diseases. In this review, we summarize the general mechanisms leading to AS. Further, we recapitulate our current knowledge concerning the functional regions in P2RX7, identified at the genetic or exonic levels, and how AS may affect the expression of these regions. Finally, the potential of P2RX7 splice variants to control the fate of cancer cells is discussed.

New role of P2X7 receptor in an Alzheimer's disease mouse model

Extracellular aggregates of amyloid β (Aβ) peptides, which are characteristic of Alzheimer's disease (AD), act as an essential trigger for glial cell activation and the release of ATP, leading to the stimulation of purinergic receptors, especially the P2X7 receptor (P2X7R). However, the involvement of P2X7R in the development of AD is still ill-defined regarding the dual properties of this receptor. Particularly, P2X7R activates the NLRP3 inflammasome leading to the release of the pro-inflammatory cytokine, IL-1β; however, P2X7R also induces cleavage of the amyloid precursor protein generating Aβ peptides or the neuroprotective fragment sAPPα. We thus explored in detail the functions of P2X7R in AD transgenic mice. Here, we show that P2X7R deficiency reduced Aβ lesions, rescued cognitive deficits and improved synaptic plasticity in AD mice. However, the lack of P2X7R did not significantly affect the release of IL-1β or the levels of non-amyloidogenic fragment, sAPPα, in AD mice. Instead, our results show that P2X7R plays a critical role in Aβ peptide-mediated release of chemokines, particularly CCL3, which is associated with pathogenic CD8⁺ T cell recruitment. In conclusion, our study highlights a novel detrimental function of P2X7R in chemokine release and supports the notion that P2X7R may be a promising therapeutic target for AD.

P2 receptor interaction and signalling cascades in neuroprotection

Nucleotides can contribute to the survival of different glial and neuronal models at the nervous system via activation of purinergic P2X and P2Y receptors. Their activation counteracts different proapoptotic events, such as excitotoxicity, mitochondrial impairment, oxidative stress and DNA damage, which concur to elicit cell loss in different processes of neurodegeneration and brain injury. Thus, it is frequent to find that different neuroprotective mediators converge in the activation of the same intracellular survival pathways to protect cells from death. The present review focuses on the role of P2Y₁ and P2Y₁₃ metabotropic receptors, and P2X7 ionotropic receptors to regulate the balance between survival and apoptosis. In particular, we analyze the intracellular pathways involved in the signaling of these nucleotide receptors to elicit survival, including calcium/PLC, PI3K/Akt/GSK3, MAPK cascades, and the expression of antioxidant and antiapoptotic genes. This review emphasizes the novel contribution of nucleotide receptors to maintain cell homeostasis through the regulation of MAP kinases and phosphatases. Unraveling the different roles found for nucleotide receptors in different models and cellular contexts may be crucial to delineate future therapeutic applications based on targeting nucleotide receptors for neuroprotection.

Astrocytic rather than neuronal P2X7 receptors modulate the function of the tri-synaptic network in the rodent hippocampus

Whole-cell patch clamp recordings demonstrated that in the dentate gyrus (DG) as well as in the CA3 area of mouse hippocampal slices the prototypic P2X7 receptor (R) agonist dibenzoyl-ATP (Bz-ATP) induced inward current responses both in neurons and astrocytes. Whereas the selective P2X7R antagonist A438079 strongly inhibited both neuronal and astrocytic currents, a combination of ionotropic glutamate receptor (CNQX, AP-5) and GABA_A-R (gabazine) antagonists depressed the Bz-ATP-induced current responses in the DG (granule cells) and CA3 neurons only. It was concluded that Bz-ATP activated astrocytic P2X7Rs and thereby released glutamate and GABA to stimulate nearby neurons. The residual A438079-resistant current response of astrocytes was suggested to be due to the stimulation of P2XRs of the non-P2X7-type. Further, we searched for presynaptic P2X7Rs at the axon terminals of DG and CA3 pyramidal neurons innervating CA3 and CA1 cells, respectively. Bz-ATP potentiated the frequency of spontaneous postsynaptic currents (sPSCs) in CA1 but not CA3 pyramidal cells. However, the Bz-ATP effect in CA1 cells was inhibited by gabazine or the astrocytic toxin fluorocitrate suggesting stimulation of P2X7Rs at stratum radiatum astrocytes located near to interneurons and synapsing onto CA1 neurons. Our data suggest that functional P2X7Rs are missing at neurons in the tri-synaptic network of the rodent hippocampus, but are present at nearby astrocytes indirectly regulating network activity.

P2X7 Receptor: A Potential Therapeutic Target for Depression?

Depression is a prime contributor to global disease burden with 300 million affected patients worldwide. The persistent lack of progress with regards to pharmacotherapy stands in stark contrast to the pandemic magnitude of the disease. Alterations of inflammatory pathways in depressed patients, including altered circulating pro-inflammatory cytokines, have been put forward as a potential pathophysiological mechanism. The P2X7 receptor (P2X7R) plays an important role regulating the release of interleukin-1β and other cytokines. Comprehensive investigation of the P2X7R Gln460Arg missense mutation (rs2230912), which has been associated with major depression and bipolar disorder, has substantially contributed to validate P2X7R as a potential genetic risk factor. We propose that P2X7R is a putative target with good prospects for therapeutic intervention in depressive disorders.

Re-evaluation of neuronal P2X7 expression using novel mouse models and a P2X7-specific nanobody

The P2X7 channel is involved in the pathogenesis of various CNS diseases. An increasing number of studies suggest its presence in neurons where its putative functions remain controversial for more than a decade. To resolve this issue and to provide a model for analysis of P2X7 functions, we generated P2X7 BAC transgenic mice that allow visualization of functional EGFP-tagged P2X7 receptors in vivo. Extensive characterization of these mice revealed dominant P2X7-EGFP protein expression in microglia, Bergmann glia, and oligodendrocytes, but not in neurons. These findings were further validated by microglia- and oligodendrocyte-specific P2X7 deletion and a novel P2X7-specific nanobody. In addition to the first quantitative analysis of P2X7 protein expression in the CNS, we show potential consequences of its overexpression in ischemic retina and post-traumatic cerebral cortex grey matter. This novel mouse model overcomes previous limitations in P2X7 research and will help to determine its physiological roles and contribution to diseases.

The human body relies on a molecule called ATP as an energy source and as a messenger. When cells die, for example if they are damaged or because of inflammation, they release large amounts of ATP into their environment. Their neighbors can detect the outpouring of ATP through specific receptors, the proteins that sit at the cell’s surface and can bind external agents.

Scientists believe that one of these ATP-binding receptors, P2X7, responds to high levels of ATP by triggering a cascade of reactions that results in inflammation and cell death. P2X7 also seems to play a role in several brain diseases such as epilepsia and Alzheimer’s, but the exact mechanisms are not known. In particular, how this receptor is involved in the death of neurons is unclear, and researchers still debate whether P2X7 is present in neurons and in other types of brain cells.

To answer this, Kaczmarek-Hájek, Zhang, Kopp et al. created genetically modified mice in which the P2X7 receptors carry a fluorescent dye. Powerful microscopes can pick up the light signal from the dye and help to reveal which cells have the receptors. These experiments show that neurons do not carry the protein; instead, P2X7 is present in certain brain cells that keep the neurons healthy. For example, it is found in the immune cells that ‘clean up’ the organ, and the cells that support and insulate neurons. Kaczmarek-Hájek et al. further provide preliminary data suggesting that, under certain conditions, if too many P2X7 receptors are present in these cells neuronal damage might be increased. It is therefore possible that the brain cells that carry P2X7 indirectly contribute to the death of neurons when large amounts of ATP are released.

The genetically engineered mouse designed for the experiments could be used in further studies to dissect the role that P2X7 plays in diseases of the nervous system. In particular, this mouse model might help to understand whether the receptor could become a drug target for neurodegenerative conditions.

P2RX7 Purinoceptor as a Therapeutic Target-The Second Coming?

The P2RX7 receptor is a unique member of a family of extracellular ATP (eATP)-gated ion channels expressed in immune cells, where its activation triggers the inflammatory cascade. Therefore, P2RX7 has been long investigated as a target in the treatment of infectious and inflammatory diseases. Subsequently, P2RX7 signaling has been documented in other physiological and pathological processes including pain, CNS and psychiatric disorders and cancer. As a result, a range of P2RX7 antagonists have been developed and trialed. Interestingly, the recent crystallization of mammalian and chicken receptors revealed that most widely-used antagonists may bind a unique allosteric site. The availability of crystal structures allows rational design of improved antagonists and modeling of binding sites of the known or presumed inhibitors. However, several unanswered questions limit the cogent development of P2RX7 therapies. Firstly, this receptor functions as an ion channel, but its chronic stimulation by high eATP causes opening of the non-selective large pore (LP), which can trigger cell death. Not only the molecular mechanism of LP opening is still not fully understood but its function(s) are also unclear. Furthermore, how can tumor cells take advantage of P2RX7 for growth and spread and yet survive overexpression of potentially cytotoxic LP in the eATP-rich environment? The recent discovery of the feedback loop, wherein the LP-evoked release of active MMP-2 triggers the receptor cleavage, provided one explanation. Another mechanism might be that of cancer cells expressing a structurally altered P2RX7 receptor, devoid of the LP function. Exploiting such mechanisms should lead to the development of new, less toxic anticancer treatments. Notably, targeted inhibition of P2RX7 is crucial as its global blockade reduces the immune and inflammatory responses, which have important anti-tumor effects in some types of malignancies. Therefore, another novel approach is the synthesis of tissue/cell specific P2RX7 antagonists. Progress has been aided by the development of p2rx7 knockout mice and new conditional knock-in and knock-out models are being created. In this review, we seek to summarize the recent advances in our understanding of molecular mechanisms of receptor activation and inhibition, which cause its re-emergence as an important therapeutic target. We also highlight the key difficulties affecting this development.

The potential of P2X7 receptors as a therapeutic target, including inflammation and tumour progression

Seven P2X ion channel nucleotide receptor subtypes have been cloned and characterised. P2X7 receptors (P2X7R) are unusual in that there are extra amino acids in the intracellular C terminus. Low concentrations of ATP open cation channels sometimes leading to cell proliferation, whereas high concentrations of ATP open large pores that release inflammatory cytokines and can lead to apoptotic cell death. Since many diseases involve inflammation and immune responses, and the P2X7R regulates inflammation, there has been recent interest in the pathophysiological roles of P2X7R and the potential of P2X7R antagonists to treat a variety of diseases. These include neurodegenerative diseases, psychiatric disorders, epilepsy and a number of diseases of peripheral organs, including the cardiovascular, airways, kidney, liver, bladder, skin and musculoskeletal. The potential of P2X7R drugs to treat tumour progression is discussed.

The P2X7 Receptor in Inflammatory Diseases: Angel or Demon?

Under physiological conditions, adenosine triphosphate (ATP) is present at low levels in the extracellular milieu, being massively released by stressed or dying cells. Once outside the cells, ATP and related nucleotides/nucleoside generated by ectonucleotidases mediate a high evolutionary conserved signaling system: the purinergic signaling, which is involved in a variety of pathological conditions, including inflammatory diseases. Extracellular ATP has been considered an endogenous adjuvant that can initiate inflammation by acting as a danger signal through the activation of purinergic type 2 receptors-P2 receptors (P2Y G-protein coupled receptors and P2X ligand-gated ion channels). Among the P2 receptors, the P2X7 receptor is the most extensively studied from an immunological perspective, being involved in both innate and adaptive immune responses. P2X7 receptor activation induces large-scale ATP release via its intrinsic ability to form a membrane pore or in association with pannexin hemichannels, boosting purinergic signaling. ATP acting via P2X7 receptor is the second signal to the inflammasome activation, inducing both maturation and release of pro-inflammatory cytokines, such as IL-1β and IL-18, and the production of reactive nitrogen and oxygen species. Furthermore, the P2X7 receptor is involved in caspases activation, as well as in apoptosis induction. During adaptive immune response, P2X7 receptor modulates the balance between the generation of T helper type 17 (Th17) and T regulatory (Treg) lymphocytes. Therefore, this receptor is involved in several inflammatory pathological conditions. In infectious diseases and cancer, P2X7 receptor can have different and contrasting effects, being an angel or a demon depending on its level of activation, cell studied, type of pathogen, and severity of infection. In neuroinflammatory and neurodegenerative diseases, P2X7 upregulation and function appears to contribute to disease progression. In this review, we deeply discuss P2X7 receptor dual function and its pharmacological modulation in the context of different pathologies, and we also highlight the P2X7 receptor as a potential target to treat inflammatory related diseases.

The P2X7 receptor is not essential for development of imiquimod-induced psoriasis-like inflammation in mice

Psoriasis is a chronic inflammatory skin disorder, characterised by epidermal hyperplasia (acanthosis) and leukocyte infiltration of the skin. Current therapies are inadequate, highlighting the need for new therapeutic targets. The P2X7 receptor is implicated in the pathogenesis of psoriasis. This study investigated the role of P2X7 in imiquimod (IMQ)-induced psoriasis-like inflammation. Topically applied IMQ caused twofold greater ear swelling in BALB/c mice compared to C57BL/6 mice, which encode a partial loss-of-function missense mutation in the P2RX7 gene. However, there was no difference in histological skin pathology (acanthosis and leukocyte infiltration) between the two strains. IMQ treatment up-regulated P2X7 expression in skin from both mouse strains. Additionally, IMQ induced ATP release from cultured human keratinocytes, a process independent of cell death. Injection of the P2X7 antagonist Brilliant Blue G (BBG) but not A-804598 partly reduced ear swelling compared to vehicle-injected control mice. Neither antagonist altered skin pathology. Moreover, no difference in ear swelling or skin pathology was observed between C57BL/6 and P2X7 knock-out (KO) mice. Flow cytometric analysis of IMQ-treated skin from C57BL/6 and P2X7 KO mice demonstrated similar leukocyte infiltration, including neutrophils, macrophages and T cells. In conclusion, this study demonstrates that P2X7 is not essential for development of IMQ-induced psoriasis-like inflammation but does not exclude a role for this receptor in psoriasis development in humans or other mouse models of this disease.

Heterozygosity for the Mood Disorder-Associated Variant Gln460Arg Alters P2X7 Receptor Function and Sleep Quality

A single nucleotide polymorphism substitution from glutamine (Gln, Q) to arginine (Arg, R) at codon 460 of the purinergic P2X7 receptor (P2X7R) has repeatedly been associated with mood disorders. The P2X7R-Gln460Arg variant per se is not compromised in its function. However, heterologous expression of P2X7R-Gln460Arg together with wild-type P2X7R has recently been demonstrated to impair receptor function. Here we show that this also applies to humanized mice coexpressing both human P2X7R variants. Primary hippocampal cells derived from heterozygous mice showed an attenuated calcium uptake upon agonist stimulation. While humanized mice were unaffected in their behavioral repertoire under basal housing conditions, mice that harbor both P2X7R variants showed alterations in their sleep quality resembling signs of a prodromal disease stage. Also healthy heterozygous human subjects showed mild changes in sleep parameters. These results indicate that heterozygosity for the wild-type P2X7R and its mood disorder-associated variant P2X7R-Gln460Arg represents a genetic risk factor, which is potentially able to convey susceptibility to mood disorders.SIGNIFICANCE STATEMENT Depression and bipolar disorder are the most common mood disorders. The P2X7 receptor (P2X7R) regulates many cellular functions. Its polymorphic variant Gln460Arg has repeatedly been associated with mood disorders. Genetically engineered mice, with human P2X7R, revealed that heterozygous mice (i.e., they coexpress the disease-associated Gln460Arg variant together with its normal version) have impaired receptor function and showed sleep disturbances. Human participants with the heterozygote genotype also had subtle alterations in their sleep profile. Our findings suggest that altered P2X7R function in heterozygote individuals disturbs sleep and might increase the risk for developing mood disorders.

Purinergic Signalling: Therapeutic Developments

Purinergic signalling, i.e., the role of nucleotides as extracellular signalling molecules, was proposed in 1972. However, this concept was not well accepted until the early 1990's when receptor subtypes for purines and pyrimidines were cloned and characterised, which includes four subtypes of the P1 (adenosine) receptor, seven subtypes of P2X ion channel receptors and 8 subtypes of the P2Y G protein-coupled receptor. Early studies were largely concerned with the physiology, pharmacology and biochemistry of purinergic signalling. More recently, the focus has been on the pathophysiology and therapeutic potential. There was early recognition of the use of P1 receptor agonists for the treatment of supraventricular tachycardia and A2A receptor antagonists are promising for the treatment of Parkinson's disease. Clopidogrel, a P2Y12 antagonist, is widely used for the treatment of thrombosis and stroke, blocking P2Y12 receptor-mediated platelet aggregation. Diquafosol, a long acting P2Y2 receptor agonist, is being used for the treatment of dry eye. P2X3 receptor antagonists have been developed that are orally bioavailable and stable in vivo and are currently in clinical trials for the treatment of chronic cough, bladder incontinence, visceral pain and hypertension. Antagonists to P2X7 receptors are being investigated for the treatment of inflammatory disorders, including neurodegenerative diseases. Other investigations are in progress for the use of purinergic agents for the treatment of osteoporosis, myocardial infarction, irritable bowel syndrome, epilepsy, atherosclerosis, depression, autism, diabetes, and cancer.

Neuronal P2X7 Receptors Revisited: Do They Really Exist?

P2X7 receptors (Rs) constitute a subclass of ATP-sensitive ionotropic receptors (P2X1-P2X7). P2X7Rs have many distinguishing features, mostly based on their long intracellular C terminus regulating trafficking to the cell membrane, protein-protein interactions, and post-translational modification. Their C-terminal tail is especially important in enabling the transition from the nonselective ion channel mode to a membrane pore allowing the passage of large molecules. There is an ongoing dispute on the existence of neuronal P2X7Rs with consequences for our knowledge on their involvement in neuroinflammation, aggravating stroke, temporal lobe epilepsy, neuropathic pain, and various neurodegenerative diseases. Whereas early results appeared to support the operation of P2X7Rs at neurons, more recently glial P2X7Rs are increasingly considered as indirect causes of neuronal effects. Specific tools for P2X7Rs are of limited value because of the poor selectivity of agonists, and the inherent failure of antibodies to differentiate between the large number of active and inactive splice variants, or gain-of-function and loss-of-function small nucleotide polymorphisms of the receptor. Unfortunately, the available P2RX7 knock-out mice generated by pharmaceutical companies possess certain splice variants, which evade inactivation. In view of the recently discovered bidirectional dialogue between astrocytes and neurons (and even microglia and neurons), we offer an alternative explanation for previous data, which assumedly support the existence of P2X7Rs at neurons. We think that the unbiased reader will follow our argumentation on astrocytic or microglial P2X7Rs being the primary targets of pathologically high extracellular ATP concentrations, although a neuronal localization of these receptors cannot be fully excluded either.

Neuronal P2X7 Receptor: Involvement in Neuronal Physiology and Pathology

The proposed presence of P2X7 receptor (P2X7R) in neurons has been the source of some contention. Initial studies suggested an absence of P2X7R mRNA in neurons, and the apparent nonspecificity of the antibodies used to identify P2X7R raised further doubts. However, subsequent studies using new pharmacological and biomolecular tools provided conclusive evidence supporting the existence of functional P2X7Rs in neurons. The P2X7 receptor has since been shown to play a leading role in multiple aspects of neuronal physiology, including axonal elongation and branching and neurotransmitter release. P2X7R has also been implicated in neuronal pathologies, in which it may influence neuronal survival. Together, this body of research suggests that P2X7R may constitute an important therapeutic target for a variety of neurological disorders.

Regulation of Pannexin 1 Surface Expression by Extracellular ATP: Potential Implications for Nervous System Function in Health and Disease

Pannexin 1 (Panx1) channels are widely recognized for their role in ATP release, and as follows, their function is closely tied to that of ATP-activated P2X7 purinergic receptors (P2X7Rs). Our recent work has shown that extracellular ATP induces clustering of Panx1 with P2X7Rs and their subsequent internalization through a non-canonical cholesterol-dependent mechanism. In other words, we have demonstrated that extracellular ATP levels can regulate the cell surface expression of Panx1. Here we discuss two situations in which we hypothesize that ATP modulation of Panx1 surface expression could be relevant for central nervous system function. The first scenario involves the development of new neurons in the ventricular zone. We propose that ATP-induced Panx1 endocytosis could play an important role in regulating the balance of cell proliferation, survival, and differentiation within this neurogenic niche in the healthy brain. The second scenario relates to the spinal cord, in which we posit that an impairment of ATP-induced Panx1 endocytosis could contribute to pathological neuroplasticity. Together, the discussion of these hypotheses serves to highlight important outstanding questions regarding the interplay between extracellular ATP, Panx1, and P2X7Rs in the nervous system in health and disease.

The role of microglial P2X7: modulation of cell death and cytokine release

Background

ATP-gated P2X7 is a non-selective cation channel, which participates in a wide range of cellular functions as well as pathophysiological processes including neuropathic pain, immune response, and neuroinflammation. Despite its abundant expression in microglia, the role of P2X7 in neuroinflammation still remains unclear.

Methods

Primary microglia were isolated from cortices of P0-2 C57BL/6 wild-type or P2X7 knockout (P2X7^−/−) mouse pups. Lipopolysaccharide, lipopolysaccharide plus IFNγ, or IL4 plus IL13 were used to polarize microglia to pro-inflammatory or anti-inflammatory states. P2rx7 expression level in resting or activated mouse and human microglia was measured by RNA-sequencing and quantitative real-time PCR. Microglial cell death was measured by cell counting kit-8 and immunocytochemistry, and microglial secretion in wild-type or P2X7^−/− microglia was examined by Luminex multiplex assay or ELISA using P2X7 agonist BzATP or P2X7 antagonist A-804598. P2X7 signaling was analyzed by Western blot.

Results

First, we confirmed that P2rx7 is constitutively expressed in mouse and human primary microglia. Moreover, P2rx7 mRNA level was downregulated in mouse microglia under both pro- and anti-inflammatory conditions. Second, P2X7 agonist BzATP caused cell death of mouse microglia, while this effect was suppressed either by P2X7 knockout or by A-804598 under both basal and pro-inflammatory conditions, which suggests the mediating role of P2X7 in BzATP-induced microglial cell death. Third, BzATP-induced release of IL1 family cytokines including IL1α, IL1β, and IL18 was blocked in P2X7^−/− microglia or by A-804598 in pro-inflammatory microglia, while the release of other cytokines/chemokines was independent of P2X7 activation. These findings support the specific role of P2X7 in IL1 family cytokine release. Finally, P2X7 activation was discovered to be linked to AKT and ERK pathways, which may be the underlying mechanism of P2X7 functions in microglia.

Conclusions

These results reveal that P2X7 mediates BzATP-induced microglial cell death and specific release of IL1 family cytokines, indicating the important role of P2X7 in neuroinflammation and implying the potential of targeting P2X7 for the treatment of neuroinflammatory disorders.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0904-8) contains supplementary material, which is available to authorized users.

The P2X7 Receptor

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