Neuronal P2X7 receptors are targeted to presynaptic terminals in the central and peripheral nervous systems

Focus on P2X7R in microglia: its mechanism of action and therapeutic prospects in various neuropathic pain models

Neuropathic pain (NP) is a common symptom of many diseases and is caused by direct or indirect damage to the nervous system. Tricyclic antidepressants and serotonin-norepinephrine reuptake inhibitors are typical drugs used in clinical practice to suppress pain. However, these drugs have drawbacks, including a short duration of action, a limited analgesic effect, and possible dependence and side effects. Therefore, developing more effective NP treatment strategies has become a priority in medical research and has attracted much research attention. P2X7 receptor (P2X7R) is a non-selective cation channel activated by adenosine triphosphate and is mainly expressed in microglia in the central nervous system. Microglial P2X7R plays an important role in pain regulation, suggesting that it could be a potential target for drug development. This review comprehensively and objectively discussed the latest research progress of P2X7R, including its structural characteristics, functional properties, relationship with microglial activation and polarization, mechanism of action, and potential therapeutic strategies in multiple NP models. This study aimed to provide in-depth insights into the association between P2X7R and NP and explore the mechanism of action of P2X7R in the pathological process of NP and the translational potential and clinical application prospects of P2X7R antagonists in pain treatment, providing a scientific basis for the precise treatment of NP.

Exploring P2X7 receptor antagonism as a therapeutic target for neuroprotection in an hiPSC motor neuron model

ATP is present in negligible concentrations in the interstitium of healthy tissues but accumulates to significantly higher concentrations in an inflammatory microenvironment. ATP binds to 2 categories of purine receptors on the surface of cells, the ionotropic P2X receptors and metabotropic P2Y receptors. Included in the family of ionotropic purine receptors is P2X7 (P2X7R), a non-specific cation channel with unique functional and structural properties that suggest it has distinct roles in pathological conditions marked by increased extracellular ATP. The role of P2X7R has previously been explored in microglia and astrocytes within the context of neuroinflammation, however the presence of P2X7R on human motor neurons and its potential role in neurodegenerative diseases has not been the focus of the current literature. We leveraged the use of human iPSC-derived spinal motor neurons (hiPSC-MN) as well as human and rodent tissue to demonstrate the expression of P2X7R on motor neurons. We extend this observation to demonstrate that these receptors are functionally active on hiPSC-MN and that ATP can directly induce death via P2X7R activation in a dose dependent manner. Finally, using a highly specific P2X7R blocker, we demonstrate how modulation of P2X7R activation on motor neurons is neuroprotective and could provide a unique pharmacologic target for ATP-induced MN death that is distinct from the role of ATP as a modulator of neuroinflammation.

Opposing effects of the purinergic P2X7 receptor on seizures in neurons and microglia in male mice

BACKGROUND

The purinergic ATP-gated P2X7 receptor (P2X7R) is increasingly recognized to contribute to pathological neuroinflammation and brain hyperexcitability. P2X7R expression has been shown to be increased in the brain, including both microglia and neurons, in experimental models of epilepsy and patients. To date, the cell type-specific downstream effects of P2X7Rs during seizures remain, however, incompletely understood.

METHODS

Effects of P2X7R signaling on seizures and epilepsy were analyzed in induced seizure models using male mice including the kainic acid model of status epilepticus and pentylenetetrazole model and in male and female mice in a genetic model of Dravet syndrome. RNA sequencing was used to analyze P2X7R downstream signaling during seizures. To investigate the cell type-specific role of the P2X7R during seizures and epilepsy, we generated mice lacking exon 2 of the P2rx7 gene in either microglia (P2rx7:Cx3cr1-Cre) or neurons (P2rx7:Thy-1-Cre). To investigate the protective potential of overexpressing P2X7R in GABAergic interneurons, P2X7Rs were overexpressed using adeno-associated virus transduction under the mDlx promoter.

RESULTS

RNA sequencing of hippocampal tissue from wild-type and P2X7R knock-out mice identified both glial and neuronal genes, in particular genes involved in GABAergic signaling, under the control of the P2X7R following seizures. Mice with deleted P2rx7 in microglia displayed less severe acute seizures and developed a milder form of epilepsy, and microglia displayed an anti-inflammatory molecular profile. In contrast, mice lacking P2rx7 in neurons showed a more severe seizure phenotype when compared to epileptic wild-type mice. Analysis of single-cell expression data revealed that human P2RX7 expression is elevated in the hippocampus of patients with temporal lobe epilepsy in excitatory and inhibitory neurons. Functional studies determined that GABAergic interneurons display increased responses to P2X7R activation in experimental epilepsy. Finally, we show that viral transduction of P2X7R in GABAergic interneurons protects against evoked and spontaneous seizures in experimental temporal lobe epilepsy and in mice lacking Scn1a, a model of Dravet syndrome.

CONCLUSIONS

Our results suggest a dual and opposing action of P2X7R in epilepsy and suggest P2X7R overexpression in GABAergic interneurons as a novel therapeutic strategy for acquired and, possibly, genetic forms of epilepsy.

Contribution of microglia to the epileptiform activity that results from neonatal hypoxia

Microglia are described as the immune cells of the brain, their immune properties have been extensively studied since first described, however, their neural functions have only been explored over the last decade. Microglia have an important role in maintaining homeostasis in the central nervous system by surveying their surroundings to detect pathogens or damage cells. While these are the classical functions described for microglia, more recently their neural functions have been defined; they are critical to the maturation of neurons during embryonic and postnatal development, phagocytic microglia remove excess synapses during development, a process called synaptic pruning, which is important to overall neural maturation. Furthermore, microglia can respond to neuronal activity and, together with astrocytes, can regulate neural activity, contributing to the equilibrium between excitation and inhibition through a feedback loop. Hypoxia at birth is a serious neurological condition that disrupts normal brain function resulting in seizures and epilepsy later in life. Evidence has shown that microglia may contribute to this hyperexcitability after neonatal hypoxia. This review will summarize the existing data on the role of microglia in the pathogenesis of neonatal hypoxia and the plausible mechanisms that contribute to the development of hyperexcitability after hypoxia in neonates. This article is part of the Special Issue on "Microglia".

Intraocular kinetics of pathological ATP after photoreceptor damage in rhegmatogenous retinal detachment

PURPOSE

Extracellular Adenosine triphosphate (ATP) released by dying cells may cause a secondary cell death in neighboring cells in retinal degeneration. We investigated intraocular ATP kinetics to gain mechanical insights into the pathology in rhegmatogenous retinal detachment (RRD).

STUDY DESIGN

Retrospective clinical study.

METHODS

Vitreous or subretinal fluids (SRF) were obtained from patients with RRD (n=75), macular hole (MH; n=20), and epiretinal membrane (ERM; n=35) during vitrectomy. ATP levels in those samples were measured by luciferase assay.

RESULTS

Mean ATP levels in the vitreous from RRD patients were significantly higher compared to those from MH and ERM patients (2.3 and 0.3 nM, respectively. P<0.01). Mean ATP levels in the SRF from RRD (11.7 nM) were higher than those in the vitreous from RRD (P<0.01). Mean ATP levels in the vitreous with short durations (1-8 days) of RRD were higher compared to those with long durations (>8 days) (3.2 and 1.4 nM, respectively. P<0.05). Similarly, ATP in SRF with short durations were higher than those with long durations (23.8 and 3.6 nM, respectively. P<0.05). Furthermore, the concentrations of ectonucleoside triphosphate diphosphohydrolase-1 (ENTPD1), a major ATP degradative enzyme, in the vitreous from RRD were higher than those from MH/ERM (1.2 and 0.2 ng/ml, respectively. P<0.01). ENTPD1 expression was localized in the cytoplasm of CD11b-positive infiltrating cells in the vitreous and retinal cells.

CONCLUSION

ATP increased in the vitreous and SRF in RRD and decreased over time with an upregulation of ENTPD1. The kinetics indicate the pathological mechanism of the excessive extracellular ATP after RRD.

P2X7 receptors and pannexin1 hemichannels shape presynaptic transmission

Over the last decades, since the discovery of ATP as a transmitter, accumulating evidence has been reported about the role of this nucleotide and purinergic receptors, in particular P2X7 receptors, in the modulation of synaptic strength and plasticity. Purinergic signaling has emerged as a crucial player in orchestrating the molecular interaction between the components of the tripartite synapse, and much progress has been made in how this neuron-glia interaction impacts neuronal physiology under basal and pathological conditions. On the other hand, pannexin1 hemichannels, which are functionally linked to P2X7 receptors, have appeared more recently as important modulators of excitatory synaptic function and plasticity under diverse contexts. In this review, we will discuss the contribution of ATP, P2X7 receptors, and pannexin hemichannels to the modulation of presynaptic strength and its impact on motor function, sensory processing, synaptic plasticity, and neuroglial communication, with special focus on the P2X7 receptor/pannexin hemichannel interplay. We also address major hypotheses about the role of this interaction in physiological and pathological circumstances.

P2X7 Receptor: an Emerging Target in Alzheimer's Disease

Alzheimer's disease (AD) is a major cause of age-related dementia, which is becoming a global health crisis. However, the pathogenesis and etiology of AD are still not fully understood. And there are no valid treatment methods or precise diagnostic tools for AD. There is increasing evidence that P2X7R expression is upregulated in AD and is involved in multiple related pathological processes such as Aβ plaques, neurogenic fiber tangles, oxidative stress, and chronic neuroinflammation. This suggests that P2X7R may be a key player in the development of AD. P2X7R is a member of the ligand-gated purinergic receptor (P2X) family. It has received attention in neuroscience due to its role in a wide range of aging and age-related neurological disorders. In this review, we summarize current information on the roles of P2X7R in AD and suggest potential pharmacological interventions to slow down AD progression.

DUSP1/MKP-1 represents another piece in the P2X7R intracellular signaling puzzle in cerebellar cells: our last journey with Mª Teresa along the purinergic pathways of Eden

The P2X7 receptor (P2X7R) stands out within the purinergic family as it has exclusive pharmacological and regulatory features, and it fulfills distinct roles depending on the type of stimulation and cellular environment. Tonic activation of P2X7R promotes cell proliferation, whereas sustained activation is associated with cell death. Yet strikingly, prolonged P2X7R activation in rat cerebellar granule neurons and astrocytes does not affect cell survival. The intracellular pathways activated by P2X7Rs involve proteins like MAPKs, ERK1/2 and p38, and interactions with growth factor receptors could explain their behavior in populations of rat cerebellar cells. In this study, we set out to characterize the intracellular mechanisms through which P2X7Rs and Trk receptors, EGFR (epidermal growth factor receptor) and BDNFR (brain-derived neurotrophic factor receptor), regulate the dual-specificity phosphatase DUSP1. In cerebellar astrocytes, the regulation of DUSP1 expression by P2X7R depends on ERK and p38 activation. EGFR stimulation can also induce DUSP1 expression, albeit less strongly than P2X7R. Conversely, EGF was virtually ineffective in regulating DUSP1 in granule neurons, a cell type in which BDNF is the main regulator of DUSP1 expression and P2X7R only induces a mild response. Indeed, the regulation of DUSP1 elicited by BDNF reflects the balance between both transcriptional and post-transcriptional mechanisms. Importantly, when the regulation of DUSP1 expression is compromised, the viability of both astrocytes and neurons is impaired, suggesting this phosphatase is essential to maintain proper cell cytoarchitecture and functioning.

Activation of Intra-nodose Ganglion P2X7 Receptors Elicit Increases in Neuronal Activity

Vagus nerve innervates several organs including the heart, stomach, and pancreas among others. Somas of sensory neurons that project through the vagal nerve are located in the nodose ganglion. The presence of purinergic receptors has been reported in neurons and satellite glial cells in several sensory ganglia. In the nodose ganglion, calcium depletion-induced increases in neuron activity can be partly reversed by P2X7 blockers applied directly into the ganglion. The later suggest a possible role of P2X7 receptors in the modulation of neuronal activity within this sensory ganglion. We aimed to characterize the response to P2X7 activation in nodose ganglion neurons under physiological conditions. Using an ex vivo preparation for electrophysiological recordings of the neural discharges of nodose ganglion neurons, we found that treatments with ATP induce transient neuronal activity increases. Also, we found a concentration-dependent increase in neural activity in response to Bz-ATP (ED50 = 0.62 mM, a selective P2X7 receptor agonist), with a clear desensitization pattern when applied every ~ 30 s. Electrophysiological recordings from isolated nodose ganglion neurons reveal no differences in the responses to Bz-ATP and ATP. Finally, we showed that the P2X7 receptor was expressed in the rat nodose ganglion, both in neurons and satellite glial cells. Additionally, a P2X7 receptor negative allosteric modulator decreased the duration of Bz-ATP-induced maximal responses without affecting their amplitude. Our results show the presence of functional P2X7 receptors under physiological conditions within the nodose ganglion of the rat, and suggest that ATP modulation of nodose ganglion activity may be in part mediated by the activation of P2X7 receptors.

Alterations of Purinergic Receptors Levels and Their Involvement in the Glial Cell Morphology in a Pre-Clinical Model of Autism Spectrum Disorders

Recent data suggest that defects in purinergic signalling are a common denominator of autism spectrum disorders (ASDs), though nothing is known about whether the disorder-related imbalance occurs at the receptor level. In this study, we investigated whether prenatal exposure to valproic acid (VPA) induces changes in purinergic receptor expression in adolescence and whether it corresponds to glial cell activation. Pregnant dams were subjected to an intraperitoneal injection of VPA at embryonic day 12.5. In the hippocampi of adolescent male VPA offspring, we observed an increase in the level of P2X1, with concomitant decreases in P2X7 and P2Y1 receptors. In contrast, in the cortex, the level of P2X1 was significantly reduced. Also, significant increases in cortical P2Y1 and P2Y12 receptors were detected. Additionally, we observed profound alterations in microglial cell numbers and morphology in the cortex of VPA animals, leading to the elevation of pro-inflammatory cytokine expression. The changes in glial cells were partially reduced via a single administration of a non-selective P2 receptor antagonist. These studies show the involvement of purinergic signalling imbalance in the modulation of brain inflammatory response induced via prenatal VPA exposure and may indicate that purinergic receptors are a novel target for pharmacological intervention in ASDs.

The P2X7 receptor contributes to seizures and inflammation-driven long-lasting brain hyperexcitability following hypoxia in neonatal mice

BACKGROUND AND PURPOSE

Neonatal seizures represent a clinical emergency. However, current anti-seizure medications fail to resolve seizures in ~50% of infants. The P2X7 receptor (P2X7R) is an important driver of inflammation, and evidence suggests that P2X7R contributes to seizures and epilepsy in adults. However, no genetic proof has yet been provided to determine what contribution P2X7R makes to neonatal seizures, its effects on inflammatory signalling during neonatal seizures, and the therapeutic potential of P2X7R-based treatments on long-lasting brain excitability.

EXPERIMENTAL APPROACH

Neonatal seizures were induced by global hypoxia in 7-day-old mouse pups (P7). The role of P2X7Rs during seizures was analysed in P2X7R-overexpressing and knockout mice. Treatment of wild-type mice after hypoxia with the P2X7R antagonist JNJ-47965567 was used to determine the effects of the P2X7R on long-lasting brain hyperexcitability. Cell type-specific P2X7R expression was analysed in P2X7R-EGFP reporter mice. RNA sequencing was used to monitor P2X7R-dependent hippocampal downstream signalling.

KEY RESULTS

P2X7R deletion reduced seizure severity, whereas P2X7R overexpression exacerbated seizure severity and reduced responsiveness to anti-seizure medication. P2X7R deficiency led to an anti-inflammatory phenotype in microglia, and treatment of mice with a P2X7R antagonist reduced long-lasting brain hyperexcitability. RNA sequencing identified several pathways altered in P2X7R knockout mice after neonatal hypoxia, including a down-regulation of genes implicated in inflammation and glutamatergic signalling.

CONCLUSION AND IMPLICATIONS

Treatments based on targeting the P2X7R may represent a novel therapeutic strategy for neonatal seizures with P2X7Rs contributing to the generation of neonatal seizures, driving inflammatory processes and long-term hyperexcitability states.

The Pannexin 1 Channel and the P2X7 Receptor Are in Complex Interplay to Regulate the Release of Soluble Ectonucleotidases in the Murine Bladder Lamina Propria

The bladder urothelium releases ATP into the lamina propria (LP) during filling, which can activate P2X receptors on afferent neurons and trigger the micturition reflex. Effective ATP concentrations are largely dependent on metabolism by membrane-bound and soluble ectonucleotidases (s-ENTDs), and the latter are released in the LP in a mechanosensitive manner. Pannexin 1 (PANX1) channel and P2X7 receptor (P2X7R) participate in urothelial ATP release and are physically and functionally coupled, hence we investigated whether they modulate s-ENTDs release. Using ultrasensitive HPLC-FLD, we evaluated the degradation of 1,N6-etheno-ATP (eATP, substrate) to eADP, eAMP, and e-adenosine (e-ADO) in extraluminal solutions that were in contact with the LP of mouse detrusor-free bladders during filling prior to substrate addition, as an indirect measure of s-ENDTS release. Deletion of Panx1 increased the distention-induced, but not the spontaneous, release of s-ENTDs, whereas activation of P2X7R by BzATP or high concentration of ATP in WT bladders increased both. In Panx1-/- bladders or WT bladders treated with the PANX1 inhibitory peptide 10Panx, however, BzATP had no effect on s-ENTDS release, suggesting that P2X7R activity depends on PANX1 channel opening. We concluded, therefore, that P2X7R and PANX1 are in complex interaction to regulate s-ENTDs release and maintain suitable ATP concentrations in the LP. Thus, while stretch-activated PANX1 hinders s-ENTDS release possibly to preserve effective ATP concentration at the end of bladder filling, P2X7R activation, presumably in cystitis, would facilitate s-ENTDs-mediated ATP degradation to counteract excessive bladder excitability.

Purinergic Tuning of the Tripartite Neuromuscular Synapse

The vertebrate neuromuscular junction (NMJ) is a specialised chemical synapse involved in the transmission of bioelectric signals between a motor neuron and a skeletal muscle fiber, leading to muscle contraction. Typically, the NMJ is a tripartite synapse comprising (a) a presynaptic region represented by the motor nerve ending, (b) a postsynaptic skeletal motor endplate area, and (c) perisynaptic Schwann cells (PSCs) that shield the motor nerve terminal. Increasing evidence points towards the role of PSCs in the maintenance and control of neuromuscular integrity, transmission, and plasticity. Acetylcholine (ACh) is the main neurotransmitter at the vertebrate skeletal NMJ, and its role is fine-tuned by co-released purinergic neuromodulators, like adenosine 5'-triphosphate (ATP) and its metabolite adenosine (ADO). Adenine nucleotides modulate transmitter release and expression of postsynaptic ACh receptors at motor synapses via the activation of P2Y and P2X receptors. Endogenously generated ADO modulates ACh release by acting via co-localised inhibitory A₁ and facilitatory A_2A receptors on motor nerve terminals, whose tonic activation depends on the neuronal firing pattern and their interplay with cholinergic receptors and neuropeptides. Thus, the concerted action of adenine nucleotides, ADO, and ACh/neuropeptide co-transmitters is paramount to adapting the neuromuscular transmission to the working load under pathological conditions, like Myasthenia gravis. Unravelling these functional complexities prompted us to review our knowledge about the way purines orchestrate neuromuscular transmission and plasticity in light of the tripartite synapse concept, emphasising the often-forgotten role of PSCs in this context.

Purinergic signaling in cognitive impairment and neuropsychiatric symptoms of Alzheimer's disease

About 10 million new cases of dementia develop worldwide each year, of which up to 70% are attributable to Alzheimer's disease (AD). In addition to the widely known symptoms of memory loss and cognitive impairment, AD patients frequently develop non-cognitive symptoms, referred to as behavioral and psychological symptoms of dementia (BPSDs). Sleep disorders are often associated with AD, but mood alterations, notably depression and apathy, comprise the most frequent class of BPSDs. BPSDs negatively affect the lives of AD patients and their caregivers, and have a significant impact on public health systems and the economy. Because treatments currently available for AD are not disease-modifying and mainly aim to ameliorate some of the cognitive symptoms, elucidating the mechanisms underlying mood alterations and other BPSDs in AD may reveal novel avenues for progress in AD therapy. Purinergic signaling is implicated in the pathophysiology of several central nervous system (CNS) disorders, such as AD, depression and sleep disorders. Here, we review recent findings indicating that purinergic receptors, mainly the A₁, A_2A, and P2X7 subtypes, are associated with the development/progression of AD. Current evidence suggests that targeting purinergic signaling may represent a promising therapeutic approach in AD and related conditions. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Impact of schizophrenia GWAS loci converge onto distinct pathways in cortical interneurons vs glutamatergic neurons during development

Remarkable advances have been made in schizophrenia (SCZ) GWAS, but gleaning biological insight from these loci is challenging. Genetic influences on gene expression (e.g., eQTLs) are cell type-specific, but most studies that attempt to clarify GWAS loci's influence on gene expression have employed tissues with mixed cell compositions that can obscure cell-specific effects. Furthermore, enriched SCZ heritability in the fetal brain underscores the need to study the impact of SCZ risk loci in specific developing neurons. MGE-derived cortical interneurons (cINs) are consistently affected in SCZ brains and show enriched SCZ heritability in human fetal brains. We identified SCZ GWAS risk genes that are dysregulated in iPSC-derived homogeneous populations of developing SCZ cINs. These SCZ GWAS loci differential expression (DE) genes converge on the PKC pathway. Their disruption results in PKC hyperactivity in developing cINs, leading to arborization deficits. We show that the fine-mapped GWAS locus in the ATP2A2 gene of the PKC pathway harbors enhancer marks by ATACseq and ChIPseq, and regulates ATP2A2 expression. We also generated developing glutamatergic neurons (GNs), another population with enriched SCZ heritability, and confirmed their functionality after transplantation into the mouse brain. Then, we identified SCZ GWAS risk genes that are dysregulated in developing SCZ GNs. GN-specific SCZ GWAS loci DE genes converge on the ion transporter pathway, distinct from those for cINs. Disruption of the pathway gene CACNA1D resulted in deficits of Ca2+ currents in developing GNs, suggesting compromised neuronal function by GWAS loci pathway deficits during development. This study allows us to identify cell type-specific and developmental stage-specific mechanisms of SCZ risk gene function, and may aid in identifying mechanism-based novel therapeutic targets.

Effect of endogenous purines on electrically evoked ACh release at the mouse neuromuscular junction

At the mouse neuromuscular junction, adenosine triphosphate (ATP), which is co-released with the neurotransmitter acetylcholine (ACh), and its metabolite adenosine, modulate neurotransmitter release by activating presynaptic inhibitory P2Y₁₃ receptors (a subtype of ATP/adenosine diphosphate [ADP] receptor), inhibitory A₁ and A₃ adenosine receptors, and excitatory A_2A adenosine receptors. To study the effect of endogenous purines, when phrenic-diaphragm preparations are depolarized by different nerve stimulation patterns, we analyzed the effect of the antagonists for P2Y₁₃ , A₁ , A₃ , and A_2A receptors (AR-C69931MX, 8-cyclopentyl-1,3-dipropylxanthine, MRS-1191, and SCH-58261, respectively) on the amplitude of the end-plate potentials of the trains, and contrasted these results with those obtained with the selective agonists of these receptors (2-methylthioadenosine 5'-diphosphate trisodium salt hydrate, 2-chloro-N⁶ -cyclopentyl-adenosine, inosine, and PSB-0777, respectively). During continuous 0.5-Hz stimulation, the amount of endogenous purines was not enough to activate purinergic receptors, while at continuous 5-Hz stimulation, an incipient action of endogenous purines on P2Y₁₃ , A₁ and A3 receptors might be evident just at the end of the trains. During continuous 50-Hz stimulation, the concentration of endogenous ATP/ADP and adenosine exerted an inhibitory action on ACh release after of the initial phase of the train, but when the nerve was stimulated at intermittent 50 Hz (5 bursts), this behavior was not observed. Excitatory A_2A receptors were only activated when continuous 100-Hz stimulation was applied. In conclusion, when motor nerve terminals are depolarized by repetitive stimulation of the phrenic nerve, endogenous ATP/ADP and adenosine are able to fine-tune neurosecretion depending on the frequency and pattern of stimulation.

Detection and Functional Evaluation of the P2X7 Receptor in hiPSC Derived Neurons and Microglia-Like Cells

A large body of evidence suggests the involvement of the ATP-gated purinergic receptor P2X7 (P2X7R) in neurodegenerative diseases, including Alzheimer's disease. While it is well-described to be present and functional on microglia cells contributing to inflammatory responses, some reports suggest a neuronal expression of the receptor as well. Here, we present experimental results showing P2X7 receptors to be expressed on human hiPSC-derived microglia-like cells, hiPSC-derived neuronal progenitors and hiPSC-derived matured neuronal cells. By applying cell surface protein detection assays, we show that P2X7R is not localized on the cell membrane, despite being detected in neuronal cells and thus may not be available for directly mediating neurotoxicity. On hiPSC-derived microglia-like cells, a clear membranous expression was detected. Additionally, we have not observed differences in P2X7R functions between control and familial Alzheimer's disease patient-derived neuronal cells. Functional assays employing a P2X7R antagonist JNJ 47965567 confirm these findings by showing P2X7R-dependent modulation of microglia-like cells viability upon treatment with P2X7R agonists ATP and BzATP, while the same effect was absent from neuronal cells. Since the majority of P2X7R research was done on rodent models, our work on human hiPSC-derived cells presents a valuable contribution to the field, extending the work on animal models to the human cellular system and toward clinical translation.

Astroglial and Microglial Purinergic P2X7 Receptor as a Major Contributor to Neuroinflammation during the Course of Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune inflammatory disease of the central nervous system that leads to the progressive disability of patients. A characteristic feature of the disease is the presence of focal demyelinating lesions accompanied by an inflammatory reaction. Interactions between autoreactive immune cells and glia cells are considered as a central mechanism underlying the pathology of MS. A glia-mediated inflammatory reaction followed by overproduction of free radicals and generation of glutamate-induced excitotoxicity promotes oligodendrocyte injury, contributing to demyelination and subsequent neurodegeneration. Activation of purinergic signaling, in particular P2X7 receptor-mediated signaling, in astrocytes and microglia is an important causative factor in these pathological processes. This review discusses the role of astroglial and microglial cells, and in particular glial P2X7 receptors, in inducing MS-related neuroinflammatory events, highlighting the importance of P2X7R-mediated molecular pathways in MS pathology and identifying these receptors as a potential therapeutic target.

P2X7 Receptor as a Potential Target for Major Depressive Disorder

Major depressive disorder (MDD) is a common mental disorder. Although the genetic, biochemical, and psychological factors have been related to the development of MDD, it is generally believed that a series of pathological changes in the brain caused by chronic stress is the main cause of MDD. However, the specific mechanisms underlying chronic stress-induced MDD are largely undermined. Recent investigations have found that increased pro-inflammatory cytokines and changes in the inflammatory pathway in the microglia cells in the brain are the potential pathophysiological mechanism of MDD. P2X7 receptor (P2X7R) and its mediated signaling pathway play a key role in microglia activation. The present review aimed to present and discuss the accumulating data on the role of P2X7R in MDD. Firstly, we summarized the research progress in the correlation between P2X7R and MDD. Subsequently, we presented the P2X7R mediated microglia activation in MDD and the role of P2X7R in increased blood-brain barrier (BBB) permeability caused by chronic stress. Lastly, we also discussed the potential mechanism underlying-P2X7R expression changes after chronic stress. In conclusion, P2X7R is a key molecule regulating the activation of microglia. Chronic stress activates microglia in the hippocampus by secreting interleukin- 1β (IL-1β) and other inflammatory cytokines, and increasing the BBB permeability, thus promoting the occurrence and development of MDD, which indicated that P2X7R might be a promising therapeutic target for MDD.

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.

Paraventricular Nucleus P2X7 Receptors Aggravate Acute Myocardial Infarction Injury via ROS-Induced Vasopressin-V1b Activation in Rats

The present study was designed to investigate the mechanisms by which P2X7 receptors (P2X7Rs) mediate the activation of vasopressinergic neurons thereby increasing sympathetic hyperactivity in the paraventricular nucleus (PVN) of the hypothalamus of rats with acute myocardial ischemia (AMI). The left anterior descending branch of the coronary artery was ligated to induce AMI in rats. The rats were pretreated with BBG (brilliant blue G, a P2X7R antagonist), nelivaptan (a vasopressin V1b receptor antagonist), or diphenyleneiodonium (DPI) [an nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor]. Hemodynamic parameters of the heart were monitored. Myocardial injury and cardiomyocyte apoptosis were assessed. In the PVN of AMI rats, P2X7R mediated microglial activation, while reactive oxygen species (ROS) and NADPH oxidase 2 (NOX2) were higher than in the sham group. Intraperitoneal injection of BBG effectively reduced ROS production and vasopressin expression in the PVN of AMI rats. Moreover, both BBG and DPI pretreatment effectively reduced sympathetic hyperactivity and ameliorated AMI injury, as represented by reduced inflammation and apoptosis of cardiomyocytes. Furthermore, microinjection of nelivaptan into the PVN improved cardiac function and reduced the norepinephrine (AE) levels in AMI rats. Collectively, the results suggest that, within the PVN of AMI rats, P2X7R upregulation mediates microglial activation and the overproduction of ROS, which in turn activates vasopressinergic neuron-V1b receptors and sympathetic hyperactivity, hence aggravating myocardial injury in the AMI setting.

P2X7 receptors in the central nervous system

For the past three decades, our laboratory has conducted pioneering research to elucidate the complexity of purinergic signaling in the CNS, alone and in collaboration with other groups, inspired by the ground-breaking efforts of Geoffrey Burnstock. This review summarizes our contribution to understand the nucleotide receptor signaling in the CNS with a special focus on the P2X7 receptor.

Predicted the P2RX7 rs3751143 polymorphism is associated with cancer risk: a meta-analysis and systematic review

BACKGROUND

Both meta-analyses and systematic reviews were used to assess the relationship between purinergic receptor P2X ligand-gated ion channel 7 (P2RX7) rs3751143 polymorphism and the risk of cancer.

MATERIALS AND METHODS

The data used in this research were collected from Google Scholar, Web of Science, CNKI, and Wan Fang Data databases. The final retrieval ended on 22 February 2019. The strength of correlation was assessed using odds ratios and 95% confidence intervals. Based on the heterogeneity test results, fixed-effect (Mantel-Haenszel) or random-effects (DerSimonian-Laird) models were selected to summarise the collective effects.

RESULTS

Eight separate studies containing 1462 cancer cases and 3037 controls were enrolled. Overall, there was no significant association between P2RX7 rs3751143 polymorphism and the risk of cancer in the allelic, homozygous, heterozygous, dominant, or recessive models.

CONCLUSIONS

Our meta-analysis indicates that there is no significant association between P2RX7 rs3751143 polymorphism and the risk of cancer in the allelic, homozygous, heterozygous, dominant, and recessive models.

Salient brain entities labelled in P2rx7-EGFP reporter mouse embryos include the septum, roof plate glial specializations and circumventricular ependymal organs

The purinergic system is one of the oldest cell-to-cell communication mechanisms and exhibits relevant functions in the regulation of the central nervous system (CNS) development. Amongst the components of the purinergic system, the ionotropic P2X7 receptor (P2X7R) stands out as a potential regulator of brain pathology and physiology. Thus, P2X7R is known to regulate crucial aspects of neuronal cell biology, including axonal elongation, path-finding, synapse formation and neuroprotection. Moreover, P2X7R modulates neuroinflammation and is posed as a therapeutic target in inflammatory, oncogenic and degenerative disorders. However, the lack of reliable technical and pharmacological approaches to detect this receptor represents a major hurdle in its study. Here, we took advantage of the P2rx7-EGFP reporter mouse, which expresses enhanced green fluorescence protein (EGFP) immediately downstream of the P2rx7 proximal promoter, to conduct a detailed study of its distribution. We performed a comprehensive analysis of the pattern of P2X7R expression in the brain of E18.5 mouse embryos revealing interesting areas within the CNS. Particularly, strong labelling was found in the septum, as well as along the entire neural roof plate zone of the brain, except chorioidal roof areas, but including specialized circumventricular roof formations, such as the subfornical and subcommissural organs (SFO; SCO). Moreover, our results reveal what seems a novel circumventricular organ, named by us postarcuate organ (PArcO). Furthermore, this study sheds light on the ongoing debate regarding the specific presence of P2X7R in neurons and may be of interest for the elucidation of additional roles of P2X7R in the idiosyncratic histologic development of the CNS and related systemic functions.

Purinergic Receptors in Basal Ganglia Diseases: Shared Molecular Mechanisms between Huntington's and Parkinson's Disease

Huntington's (HD) and Parkinson's diseases (PD) are neurodegenerative disorders caused by the death of GABAergic and dopaminergic neurons in the basal ganglia leading to hyperkinetic and hypokinetic symptoms, respectively. We review here the participation of purinergic receptors through intracellular Ca2+ signaling in these neurodegenerative diseases. The adenosine A2A receptor stimulates striatopallidal GABAergic neurons, resulting in inhibitory actions on GABAergic neurons of the globus pallidus. A2A and dopamine D2 receptors form functional heteromeric complexes inducing allosteric inhibition, and A2A receptor activation results in motor inhibition. Furthermore, the A2A receptor physically and functionally interacts with glutamate receptors, mainly with the mGlu5 receptor subtype. This interaction facilitates glutamate release, resulting in NMDA glutamate receptor activation and an increase of Ca2+ influx. P2X7 receptor activation also promotes glutamate release and neuronal damage. Thus, modulation of purinergic receptor activity, such as A2A and P2X7 receptors, and subsequent aberrant Ca2+ signaling, might present interesting therapeutic potential for HD and PD.

Neonatal Seizures and Purinergic Signalling

Neonatal seizures are one of the most common comorbidities of neonatal encephalopathy, with seizures aggravating acute injury and clinical outcomes. Current treatment can control early life seizures; however, a high level of pharmacoresistance remains among infants, with increasing evidence suggesting current anti-seizure medication potentiating brain damage. This emphasises the need to develop safer therapeutic strategies with a different mechanism of action. The purinergic system, characterised by the use of adenosine triphosphate and its metabolites as signalling molecules, consists of the membrane-bound P1 and P2 purinoreceptors and proteins to modulate extracellular purine nucleotides and nucleoside levels. Targeting this system is proving successful at treating many disorders and diseases of the central nervous system, including epilepsy. Mounting evidence demonstrates that drugs targeting the purinergic system provide both convulsive and anticonvulsive effects. With components of the purinergic signalling system being widely expressed during brain development, emerging evidence suggests that purinergic signalling contributes to neonatal seizures. In this review, we first provide an overview on neonatal seizure pathology and purinergic signalling during brain development. We then describe in detail recent evidence demonstrating a role for purinergic signalling during neonatal seizures and discuss possible purine-based avenues for seizure suppression in neonates.

P2X7 Receptor Upregulation in Huntington's Disease Brains

Huntington’s disease (HD) is a fatal degenerative disorder affecting the nervous system. It is characterized by motor, cognitive, and psychiatric dysfunctions, with a late onset and an autosomal dominant pattern of inheritance. HD-causing mutation consists in an expansion of repeated CAG triplets in the huntingtin gene (HTT), encoding for an expanded polyglutamine (polyQ) stretch in the huntingtin protein (htt). The mutation causes neuronal dysfunction and loss through multiple mechanisms, affecting both the nucleus and cytoplasm. P2X7 receptor (P2X7R) emerged as a major player in neuroinflammation, since ATP – its endogenous ligand – is massively released under this condition. Indeed, P2X7R stimulation in the central nervous system (CNS) is known to enhance the release of pro-inflammatory cytokines from microglia and of neurotransmitters from neuronal presynaptic terminals, as well as to promote apoptosis. Previous experiments performed with neurons expressing the mutant huntingtin and exploiting HD mouse models demonstrated a role of P2X7R in HD. On the basis of those results, here, we explore for the first time the status of P2X7R in HD patients’ brain. We report that in HD postmortem striatum, as earlier observed in HD mice, the protein levels of the full-length form of P2X7R, also named P2X7R-A, are upregulated. In addition, the exclusively human naturally occurring variant lacking the C-terminus region, P2X7R-B, is upregulated as well. As we show here, this augmented protein levels can be explained by elevated mRNA levels. Furthermore, in HD patients’ striatum, P2X7R shows not only an augmented total transcript level but also an alteration of its splicing. Remarkably, P2X7R introns 10 and 11 are more retained in HD patients when compared with controls. Taken together, our data confirm that P2X7R is altered in brains of HD subjects and strengthen the notion that P2X7R may represent a potential therapeutic target for HD.

Modulatory Roles of ATP and Adenosine in Cholinergic Neuromuscular Transmission

A review of the data on the modulatory action of adenosine 5’-triphosphate (ATP), the main co-transmitter with acetylcholine, and adenosine, the final ATP metabolite in the synaptic cleft, on neuromuscular transmission is presented. The effects of these endogenous modulators on pre- and post-synaptic processes are discussed. The contribution of purines to the processes of quantal and non-quantal secretion of acetylcholine into the synaptic cleft, as well as the influence of the postsynaptic effects of ATP and adenosine on the functioning of cholinergic receptors, are evaluated. As usual, the P2-receptor-mediated influence is minimal under physiological conditions, but it becomes very important in some pathophysiological situations such as hypothermia, stress, or ischemia. There are some data demonstrating the same in neuromuscular transmission. It is suggested that the role of endogenous purines is primarily to provide a safety factor for the efficiency of cholinergic neuromuscular transmission.

The P2X7 Receptor: Central Hub of Brain Diseases

The P2X7 receptor is a cation channel activated by high concentrations of adenosine triphosphate (ATP). Upon long-term activation, it complexes with membrane proteins forming a wide pore that leads to cell death and increased release of ATP into the extracellular milieu. The P2X7 receptor is widely expressed in the CNS, such as frontal cortex, hippocampus, amygdala and striatum, regions involved in neurodegenerative diseases and psychiatric disorders. Despite P2X7 receptor functions in glial cells have been extensively studied, the existence and roles of this receptor in neurons are still controversially discussed. Regardless, P2X7 receptors mediate several processes observed in neuropsychiatric disorders and brain tumors, such as activation of neuroinflammatory response, stimulation of glutamate release and neuroplasticity impairment. Moreover, P2X7 receptor gene polymorphisms have been associated to depression, and isoforms of P2X7 receptors are implicated in neuropsychiatric diseases. In view of that, the P2X7 receptor has been proposed to be a potential target for therapeutic intervention in brain diseases. This review discusses the molecular mechanisms underlying P2X7 receptor-mediated signaling in neurodegenerative diseases, psychiatric disorders, and brain tumors. In addition, it highlights the recent advances in the development of P2X7 receptor antagonists that are able of penetrating the central nervous system.

P2X7 receptors exert a permissive effect on the activation of presynaptic AMPA receptors in rat trigeminal caudal nucleus glutamatergic nerve terminals

Background

Purine receptors play roles in peripheral and central sensitization and are associated with migraine headache. We investigated the possibility that ATP plays a permissive role in the activation of AMPA receptors thus inducing Glu release from nerve terminals isolated from the rat trigeminal caudal nucleus (TCN).

Methods

Nerve endings isolated from the rat TCN were loaded with [³H]D-aspartic acid ([³H]D-ASP), layered into thermostated superfusion chambers, and perfused continuously with physiological medium, alone or with various test drugs. Radioactivity was measured to assess [³H]D-ASP release under different experimental conditions.

Results

Synaptosomal [³H]D-ASP spontaneous release was stimulated by ATP and to an even greater extent by the ATP analogue benzoylbenzoylATP (BzATP). The stimulation of [³H]D-ASP basal release by the purinergic agonists was prevented by the selective P2X7 receptor antagonist A438079. AMPA had no effect on basal [³H]D-ASP release, but the release observed when synaptosomes were exposed to AMPA plus a purinoceptor agonist exceeded that observed with ATP or BzATP alone. The selective AMPA receptor antagonist NBQX blocked this “excess” release. Co-exposure to AMPA and BzATP, each at a concentration with no release-stimulating effects, evoked a significant increase in [³H]D-ASP basal release, which was prevented by exposure to a selective AMPA antagonist.

Conclusions

P2X7 receptors expressed on glutamatergic nerve terminals in the rat TCN can mediate Glu release directly and indirectly by facilitating the activation of presynaptic AMPA receptors. The high level of glial ATP that occurs during chronic pain states can promote widespread release of Glu as well as can increase the function of AMPA receptors. In this manner, ATP contributes to the AMPA receptor activation involved in the onset and maintenance of the central sensitization associated with chronic pain.

Glial ATP and Large Pore Channels Modulate Synaptic Strength in Response to Chronic Inactivity

A key feature of neurotransmission is its ability to adapt to changes in neuronal environment, which is essential for many brain functions. Homeostatic synaptic plasticity (HSP) emerges as a compensatory mechanism used by neurons to adjust their excitability in response to changes in synaptic activity. Recently, glial cells emerged as modulators for neurotransmission by releasing gliotransmitters into the synaptic cleft through pathways that include P2X₇ receptors (P2X₇R), connexons, and pannexons. However, the role of gliotransmission in the activity-dependent adjustment of presynaptic strength is still an open question. Here, we investigated whether glial cells participate in HSP upon chronic inactivity and the role of adenosine triphosphate (ATP), connexin43 hemichannels (Cx43HCs), and pannexin1 (Panx1) channels in this process. We used immunocytochemistry against vesicular glutamate transporter 1 (vGlut1) to estimate changes in synaptic strength in hippocampal dissociated cultures. Pharmacological manipulations indicate that glial-derived ATP and P2X₇R are required for HSP. In addition, inhibition of Cx43 and Panx1 channels reveals a pivotal role for these channels in the compensatory adjustment of synaptic strength, emerging as new pathways for ATP release upon inactivity. The involvement of Panx1 channels was confirmed by using Panx1-deficient animals. Lacking Panx1 in neurons is sufficient to prevent the P2X₇R-dependent upregulation of presynaptic strength; however, the P2X₇R-dependent compensatory adjustment of synapse density requires both neuronal and glial Panx1. Together, our data supports an essential role for glial ATP signaling and Cx43HCs and Panx1 channels in the homeostatic adjustment of synaptic strength in hippocampal cultures upon chronic inactivity.

P2X7 activation enhances skeletal muscle metabolism and regeneration in SOD1G93A mouse model of amyotrophic lateral sclerosis

Muscle weakness plays an important role in neuromuscular disorders comprising amyotrophic lateral sclerosis (ALS). However, it is not established whether muscle denervation originates from the motor neurons, the muscles or more likely both. Previous studies have shown that the expression of the SOD1G93A mutation in skeletal muscles causes denervation of the neuromuscular junctions, inability to regenerate and consequent atrophy, all clear symptoms of ALS. In this work, we used SOD1G93A mice, a model that best mimics some pathological features of both familial and sporadic ALS, and we investigated some biological effects induced by the activation of the P2X7 receptor in the skeletal muscles. The P2X7, belonging to the ionotropic family of purinergic receptors for extracellular ATP, is abundantly expressed in the healthy skeletal muscles, where it controls cell duplication, differentiation, regeneration or death. In particular, we evaluated whether an in vivo treatment in SOD1G93A mice with the P2X7 specific agonist 2'(3')-O-(4-Benzoylbenzoyl) adenosine5'-triphosphate (BzATP) just before the onset of a pathological neuromuscular phenotype could exert beneficial effects in the skeletal muscles. Our findings indicate that stimulation of P2X7 improves the innervation and metabolism of myofibers, moreover elicits the proliferation/differentiation of satellite cells, thus preventing the denervation atrophy of skeletal muscles in SOD1G93A mice. Overall, this study suggests that a P2X7-targeted and site-specific modulation might be a strategy to interfere with the complex multifactorial and multisystem nature of ALS.

Spinal P2X7R contributes to streptozotocin-induced mechanical allodynia in mice

Painful diabetic neuropathy (PDN) is a diabetes mellitus complication. Unfortunately, the mechanisms underlying PDN are still poorly understood. Adenosine triphosphate (ATP)-gated P2X7 receptor (P2X7R) plays a pivotal role in non-diabetic neuropathic pain, but little is known about its effects on streptozotocin (STZ)-induced peripheral neuropathy. Here, we explored whether spinal cord P2X7R was correlated with the generation of mechanical allodynia (MA) in STZ-induced type 1 diabetic neuropathy in mice. MA was assessed by measuring paw withdrawal thresholds and western blotting. Immunohistochemistry was applied to analyze the protein expression levels and localization of P2X7R. STZ-induced mice expressed increased P2X7R in the dorsal horn of the lumbar spinal cord during MA. Mice injected intrathecally with a selective antagonist of P2X7R and P2X7R knockout (KO) mice both presented attenuated progression of MA. Double-immunofluorescent labeling demonstrated that P2X7R-positive cells were mostly co-expressed with Iba1 (a microglia marker). Our results suggest that P2X7R plays an important role in the development of MA and could be used as a cellular target for treating PDN.

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.

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.

Regulation of P2X7 receptor expression and function in the brain

Because of its prominent role in driving inflammatory processes, the ATP-gated purinergic P2X7 receptor has attracted much attention over the past decade as a potential therapeutic target for numerous human conditions, particularly diseases of the central nervous system, including neurodegenerative diseases (e.g. Alzheimer's and Huntington's disease), psychiatric disorders (e.g. schizophrenia and depression) and the neurological disease, epilepsy. Evidence stems from studies using experimental models and patient tissue showing changes in P2X7 expression and function under pathological conditions and beneficial effects provided by P2X7 antagonism. Apart from promoting neuroinflammation, P2X7, however, also impacts on other pathological processes in the brain, including cell death, hyperexcitability, changes in neurotransmitter release and neurogenesis. Reports also suggest a role for P2X7 in the maintenance of blood-brain-barrier integrity. It therefore comes as no surprise that the regulation of P2X7 expression and function is complex, providing tight control on P2X7 activation. Much progress has been made in understanding how P2X7 is regulated during physiological and pathological conditions and what the consequences are of pathological P2X7 expression and function. Regulatory mechanisms altering P2X7 expression include transcriptional and post-translational regulation including nucleotide polymorphisms, promoter regulation via DNA methylation, transcription factors (e.g. Sp1 and HIF-1α), the generation of different splice variants and receptor phosphorylation, glycosylation and palmitoylation. Finally, more recently, reports have also shown P2X7-targeting by microRNAs, blocking P2X7 translation into functional proteins. The present review provides a broad overview of what is known to-date about the complex regulation of P2X7 expression with a particular emphasis on the brain and how each of these regulatory mechanisms impacts on receptor function and pathology.

Mechanism of P2X7 receptor-dependent enhancement of neuromuscular transmission in pannexin 1 knockout mice

P2X7 receptors are present in presynaptic membranes of motor synapses, but their regulatory role in modulation of neurotransmitter release remains poorly understood. P2X7 receptors may interact with pannexin 1 channels to form a purinergic signaling unit. The potential mechanism of P2X7 receptor-dependent modulation of acetylcholine (ACh) release was investigated by recording miniature endplate potentials (MEPPs) and evoked endplate potentials (EPPs) in neuromuscular junctions of wild-type (WT) and pannexin 1 knockout (Panx1^-/-) mice. Modulation of P2X7 receptors with the selective inhibitor A740003 or the selective agonist BzATP did not alter the parameters of either spontaneous or evoked ACh release in WT mice. In Panx1^-/- mice, BzATP-induced activation of P2X7 receptors resulted in a uniformly increased quantal content of EPPs during a short stimulation train. This effect was accompanied by an increase in the size of the readily releasable pool, while the release probability did not change. Inhibition of calmodulin by W-7 or of calcium/calmodulin-dependent kinase II (CaMKII) by KN-93 completely prevented the potentiating effect of BzATP on the EPP quantal content. The blockade of L-type calcium channels also prevented BzATP action on evoked synaptic activity. Thus, the activation of presynaptic P2X7 receptors in mice lacking pannexin 1 resulted in enhanced evoked ACh release. Such enhanced release was provoked by triggering the calmodulin- and CaMKII-dependent signaling pathway, followed by activation of presynaptic L-type calcium channels. We suggest that in WT mice, this pathway is downregulated due to pannexin 1-dependent tonic activation of inhibitory presynaptic purinergic receptors, which overcomes P2X7-mediated effects.

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.

Purinergic Receptors in Neurological Diseases With Motor Symptoms: Targets for Therapy

Since proving adenosine triphosphate (ATP) functions as a neurotransmitter in neuron/glia interactions, the purinergic system has been more intensely studied within the scope of the central nervous system. In neurological disorders with associated motor symptoms, including Parkinson's disease (PD), motor neuron diseases (MND), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), Huntington's Disease (HD), restless leg syndrome (RLS), and ataxias, alterations in purinergic receptor expression and activity have been noted, indicating a potential role for this system in disease etiology and progression. In neurodegenerative conditions, neural cell death provokes extensive ATP release and alters calcium signaling through purinergic receptor modulation. Consequently, neuroinflammatory responses, excitotoxicity and apoptosis are directly or indirectly induced. This review analyzes currently available data, which suggests involvement of the purinergic system in neuro-associated motor dysfunctions and underlying mechanisms. Possible targets for pharmacological interventions are also discussed.

The P2RX7 polymorphism rs2230912 is associated with depression: A meta-analysis

Various studies have investigated whether single nucleotide polymorphisms (SNPs) in the gene purinergic receptor P2X7 (P2RX7), and rs2230912 specifically, were associated with mood disorders. While some studies found positive evidence, a large number of studies reported no significant associations. In a previously published meta-analysis, Feng et al. did not find a significant association and only moderate odds ratios (ORs) in case-control studies. They reported significant findings only for family-based studies. We revisited this finding and conducted a meta-analysis including 8,652 cases and 11,153 controls, adding unpublished results from the Munich Antidepressant Response Signature (MARS) study. We found a significant association between rs2230912 and combined mood disorders (major depressive disorder (MDD) or bipolar disorder (BD)) for the allelic, dominant and heterozygous-disadvantage model, all withstanding the threshold of correction for multiple testing. Stratifying by disorder revealed significant findings for the MDD-subgroup (OR of 1.12 for the allelic model), while the BD-subgroup presented with a lower effect size (OR of 1.05) and no significance. P2RX7 encodes a purinergic receptor which is expressed in the brain and also localized in immune cells. Animal studies and functional studies will be necessary to enlighten its involvement in the etiology of mood disorders and its applicability for pharmacological purposes.

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

The purinergic P2X7 receptor (P2X7R) has attracted considerable interest as a potential target for various central nervous system (CNS) pathologies including affective and neurodegenerative disorders. To date, the distribution and cellular localization of the P2X7R in the brain are not fully resolved and a matter of debate mainly due to the limitations of existing tools. However, this knowledge should be a prerequisite for understanding the contribution of the P2X7R to brain disease. Here, we generated a genetic mouse model by humanizing the P2X7R in the mouse as mammalian model organism. We demonstrated its functionality and revealed species-specific characteristics of the humanized receptor, compared to the murine ortholog, regarding its receptivity to activation and modulation by 2',3'-O-(benzoyl-4-benzoyl)-adenosine 5'-triphosphate (BzATP) and trifluoperazine (TFP). This humanized P2rx7 allele is accessible to spatially and temporally controlled Cre recombinase-mediated inactivation. In contrast to previously generated knockout (KO) mice, none of the described P2rx7 splice variants evade this null allele. By selective disruption and assessment of human P2RX7 expression in different brain regions and cell types, we were able to demonstrate that the P2X7R is specifically expressed in glutamatergic pyramidal neurons of the hippocampus. Also, P2X7R is expressed in major non-neuronal lineages throughout the brain, i.e., astrocytes, oligodendrocytes, and microglia. In conclusion, this humanized mouse model provides the means for detailed assessment of human P2X7R function in vivo including evaluation of agonists or antagonists. In addition, this conditional allele will enable future loss-of-function studies in conjunction with mouse models for CNS disorders.

The Role of the P2X7 Receptor in Ocular Stresses: A Potential Therapeutic Target

The P2X7 receptor is expressed in both anterior and posterior segments of the eyeball. In the ocular surface, the P2X7 receptor is activated in case of external aggressions: preservatives and surfactants induce the activation of P2X7 receptors, leading to either apoptosis, inflammation, or cell proliferation. In the retina, the key endogenous actors of age-related macular degeneration, diabetic retinopathy, and glaucoma act through P2X7 receptors’ activation and/or upregulation of P2X7 receptors’ expression. Different therapeutic strategies aimed at the P2X7 receptor exist. P2X7 receptor antagonists, such as divalent cations and Brilliant Blue G (BBG) could be used to target either the ocular surface or the retina, as long as polyunsaturated fatty acids may exert their effects through the disruption of plasma membrane lipid rafts or saffron that reduces the response evoked by P2X7 receptor stimulation. Treatments against P2X7 receptor activation are proposed by using either eye drops or food supplements.

Neonatal Maternal Deprivation Enhances Presynaptic P2X7 Receptor Transmission in Insular Cortex in an Adult Rat Model of Visceral Hypersensitivity

AIMS

Insular cortex (IC) is involved in processing the information of pain. The aim of this study was to investigate roles and mechanisms of P2X7 receptors (P2X7Rs) in IC in development of visceral hypersensitivity of adult rats with neonatal maternal deprivation (NMD).

METHODS

Visceral hypersensitivity was quantified by abdominal withdrawal reflex threshold to colorectal distension (CRD). Expression of P2X7Rs was determined by qPCR and Western blot. Synaptic transmission in IC was recorded by patch-clamp recording.

RESULTS

The expression of P2X7Rs and glutamatergic neurotransmission in IC was significantly increased in NMD rats when compared with age-matched controls. Application of BzATP (P2X7R agonist) enhanced the frequency of spontaneous excitatory postsynaptic currents (sEPSC) and miniature excitatory postsynaptic currents (mEPSC) in IC slices of control rats. Application of BBG (P2X7R antagonist) suppressed the frequencies of sEPSC and mEPSC in IC slices of NMD rats. Microinjection of BzATP into right IC significantly decreased CRD threshold in control rats while microinjection of BBG or A438079 into right IC greatly increased CRD threshold in NMD rats.

CONCLUSION

Data suggested that the enhanced activities of P2X7Rs in IC, likely through a presynaptic mechanism, contributed to visceral hypersensitivity of adult rats with NMD.

Neurodevelopmental alterations and seizures developed by mouse model of infantile hypophosphatasia are associated with purinergic signalling deregulation

Hypomorphic mutations in the gene encoding the tissue-nonspecific alkaline phosphatase (TNAP) enzyme, ALPL in human or Akp2 in mice, cause hypophosphatasia (HPP), an inherited metabolic bone disease also characterized by spontaneous seizures. Initially, these seizures were attributed to the impairment of GABAergic neurotransmission caused by altered vitamin B6 (vit-B6) metabolism. However, clinical cases in human newborns and adults whose convulsions are refractory to pro-GABAergic drugs but controlled by the vit-B6 administration, suggest that other factors are involved. Here, to evaluate whether neurodevelopmental alterations are underlying the seizures associated to HPP, we performed morphological and functional characterization of postnatal homozygous TNAP null mice, a model of HPP. These analyses revealed that TNAP deficient mice present an increased proliferation of neural precursors, an altered neuronal morphology, and an augmented neuronal activity. We found that these alterations were associated with a partial downregulation of the purinergic P2X7 receptor (P2X7R). Even though deficient P2X7R mice present similar neurodevelopmental alterations, they do not develop neonatal seizures. Accordingly, we found that the additional blockage of P2X7R prevent convulsions and extend the lifespan of mice lacking TNAP. In agreement with these findings, we also found that exogenous administration of ATP or TNAP antagonists induced seizures in adult wild-type mice by activating P2X7R. Finally, our results also indicate that the anticonvulsive effects attributed to vit-B6 may be due to its capacity to block P2X7R. Altogether, these findings suggest that the purinergic signalling regulates the neurodevelopmental alteration and the neonatal seizures associated to HPP.

Tolerability and efficacy study of P2X7 inhibition in experimental Charcot-Marie-Tooth type 1A (CMT1A) neuropathy

Charcot-Marie-Tooth 1A (CMT1A) is a demyelinating hereditary neuropathy for which pharmacological treatments are not yet available. An abnormally high intracellular Ca(2+) concentration was observed in Schwann cells (SC) from CMT1A rats, caused by the PMP22-mediated overexpression of the P2X7 purinoceptor. The purpose of this study was to investigate the tolerability and therapeutic potential of a pharmacological antagonist of the P2X7 receptor (A438079) in CMT1A. A438079 ameliorated in vitro myelination of organotypic DRG cultures from CMT1A rats. Furthermore, we performed an experimental therapeutic trial in PMP22 transgenic and in wild-type rats. A preliminary dose-escalation trial showed that 3mg/kg A438079 administered via intraperitoneal injection every 24h for four weeks was well tolerated by wild type and CMT1A rats. Affected rats treated with 3mg/kg A438079 revealed a significant improvement of the muscle strength, when compared to placebo controls. Importantly, histologic analysis revealed a significant increase of the total number of myelinated axons in tibial nerves. Moreover, a significant decrease of the hypermyelination of small caliber axons and a significant increase of the frequency and diameter of large caliber myelinated axons was highlighted. An improved distal motor latencies was recorded, whereas compound muscle action potentials (CMAP) remained unaltered. A438079 reduced the SC differentiation defect in CMT1A rats. These results show that pharmacological inhibition of the P2X7 receptor is well tolerated in CMT1A rats and represents a proof-of-principle that antagonizing this pathway may correct the molecular derangements and improve the clinical phenotype in the CMT1A neuropathy.

P2X7R modulation of visually evoked synaptic responses in the retina

P2X₇Rs are distributed throughout all layers of the retina, and thus, their localisation on various cell types puts into question their specific site(s) of action. Using a dark-adapted, ex vivo mouse retinal whole mount preparation, the present study aimed to characterise the effect of P2X₇R activation on light-evoked, excitatory RGC ON-field excitatory post-synaptic potentials (fEPSPs) and on outer retinal electroretinogram (ERG) responses under comparable conditions. The pharmacologically isolated NMDA receptor-mediated RGC ON-fEPSP was reduced in the presence of BzATP, an effect which was significantly attenuated by A438079 and other selective P2X₇R antagonists A804598 or AF27139. In physiological Krebs medium, BzATP induced a significant potentiation of the ERG a-wave, with a concomitant reduction in the b-wave and the power of the oscillatory potentials. Conversely, in the pharmacologically modified Mg²⁺-free perfusate, BzATP reduced both the a-wave and b-wave. The effects of BzATP on the ERG components were suppressed by A438079. A role for P2X₇R function in visual processing in both the inner and outer retina under physiological conditions remains controversial. The ON-fEPSP was significantly reduced in the presence of A804598 but not by A438079 or AF27139. Furthermore, A438079 did not have any effect on the ERG components in physiological Krebs but potentiated and reduced the a-wave and b-wave, respectively, when applied to the pharmacologically modified medium. Therefore, activation of P2X₇Rs affects the function in the retinal ON pathway. The presence of a high concentration of extracellular ATP would most likely contribute to the modulation of visual transmission in the retina in the pathophysiological microenvironment.

P2X Receptor-Mediated Modulation of Sensory Transmission to the Spinal Cord Dorsal Horn

In the somatosensory system, P2X receptors are expressed on both peripheral and central terminals of primary afferent neurons. Those expressed on peripheral terminals are activated in response to both nociceptive and innocuous stimuli, whereas those at central terminals (“central terminal P2X receptors”) play an important role in modulating sensory transmission to the spinal cord dorsal horn. The author reviews recent studies on the central terminal P2X receptors. It is proposed that central terminal P2X receptors, once activated, may be involved in both central sensitization and initiation of pain. Thus, these receptors may repesent a promising target for therapeutic management of pathological pain.

P2Y13 receptors mediate presynaptic inhibition of acetylcholine release induced by adenine nucleotides at the mouse neuromuscular junction

It is known that adenosine 5'-triphosphate (ATP) is released along with the neurotransmitter acetylcholine (ACh) from motor nerve terminals. At mammalian neuromuscular junctions (NMJs), we have previously demonstrated that ATP is able to decrease ACh secretion by activation of P2Y receptors coupled to pertussis toxin-sensitive Gi/o protein. In this group, the receptor subtypes activated by adenine nucleotides are P2Y12 and P2Y13. Here, we investigated, by means of pharmacological and immunohistochemical assays, the P2Y receptor subtype that mediates the modulation of spontaneous and evoked ACh release in mouse phrenic nerve-diaphragm preparations. First, we confirmed that the preferential agonist for P2Y12-13 receptors, 2-methylthioadenosine 5'-diphosphate trisodium salt hydrate (2-MeSADP), reduced MEPP frequency without affecting MEPP amplitude as well as the amplitude and quantal content of end-plate potentials (EPPs). The effect on spontaneous secretion disappeared after the application of the selective P2Y12-13 antagonists AR-C69931MX or 2-methylthioadenosine 5'-monophosphate triethylammonium salt hydrate (2-MeSAMP). 2-MeSADP was more potent than ADP and ATP in reducing MEPP frequency. Then we demonstrated that the selective P2Y13 antagonist MRS-2211 completely prevented the inhibitory effect of 2-MeSADP on MEPP frequency and EPP amplitude, whereas the P2Y12 antagonist MRS-2395 failed to do this. The preferential agonist for P2Y13 receptors inosine 5'-diphosphate sodium salt (IDP) reduced spontaneous and evoked ACh secretion and MRS-2211 abolished IDP-mediated modulation. Immunohistochemical studies confirmed the presence of P2Y13 but not P2Y12 receptors at the end-plate region. Disappearance of P2Y13 receptors after denervation suggests the presynaptic localization of the receptors. We conclude that, at motor nerve terminals, the Gi/o protein-coupled P2Y receptors implicated in presynaptic inhibition of spontaneous and evoked ACh release are of the subtype P2Y13. This study provides new insights into the types of purinergic receptors that contribute to the fine-tuning of cholinergic transmission at mammalian neuromuscular junction.

Co-Expression of Wild-Type P2X7R with Gln460Arg Variant Alters Receptor Function

The P2X7 receptor is a member of the P2X family of ligand-gated ion channels. A single-nucleotide polymorphism leading to a glutamine (Gln) by arginine (Arg) substitution at codon 460 of the purinergic P2X7 receptor (P2X7R) has been associated with mood disorders. No change in function (loss or gain) has been described for this SNP so far. Here we show that although the P2X7R-Gln460Arg variant per se is not compromised in its function, co-expression of wild-type P2X7R with P2X7R-Gln460Arg impairs receptor function with respect to calcium influx, channel currents and intracellular signaling in vitro. Moreover, co-immunoprecipitation and FRET studies show that the P2X7R-Gln460Arg variant physically interacts with P2X7R-WT. Specific silencing of either the normal or polymorphic variant rescues the heterozygous loss of function phenotype and restores normal function. The described loss of function due to co-expression, unique for mutations in the P2RX7 gene so far, explains the mechanism by which the P2X7R-Gln460Arg variant affects the normal function of the channel and may represent a mechanism of action for other mutations.