Is there a basis for distinguishing two types of P2-purinoceptor?

Implications of COVID-19 in Parkinson's disease: the purinergic system in a therapeutic-target perspective to diminish neurodegeneration

The pathophysiology of Parkinson's disease (PD) is marked by degeneration of dopaminergic neurons in the substantia nigra. With advent of COVID-19, which is closely associated with generalized inflammation and multiple organ dysfunctions, the PD patients may develop severe conditions of disease leading to exacerbated degeneration. This condition is caused by the excessive release of pro-inflammatory markers, called cytokine storm, that is capable of triggering neurodegenerative conditions by affecting the blood-brain barrier (BBB). A possible SARS-CoV-2 infection, in serious cases, may compromise the immune system by triggering a hyperstimulation of the neuroimmune response, similar to the pathological processes found in PD. From this perspective, the inflammatory scenario triggers oxidative stress and, consequently, cellular dysfunction in the nervous tissue. The P2X7R seems to be the key mediator of the neuroinflammatory process, as it acts by increasing the concentration of ATP, allowing the influx of Ca2+ and the occurrence of mutations in the α-synuclein protein, causing activation of this receptor. Thus, modulation of the purinergic system may have therapeutic potential on the effects of PD, as well as on the damage caused by inflammation of the BBB, which may be able to mitigate the neurodegeneration caused by diseases. Considering all the processes of neuroinflammation, oxidative stress, and mitochondrial dysfunction that PD propose, we can conclude that the P2X7 antagonist acts in the prevention of viral diseases, and it also controls purinergic receptors formed by multi-target compounds directed to self-amplification circuits and, therefore, may be a viable strategy to obtain the desired disease-modifying effect. Thus, purinergic system receptor modulations have a high therapeutic potential for neurodegenerative diseases such as PD.

Overview of the role of purinergic signaling and insights into its role in cancer therapy

Innovation of cancer therapy has received a dramatic acceleration over the last fifteen years thanks to the introduction of the novel immune checkpoint inhibitors (ICI). On the other hand, the conspicuous scientific knowledge accumulated in purinergic signaling since the early seventies is finally being transferred to the clinic. Several Phase I/II clinical trials are currently underway to investigate the effect of drugs interfering with purinergic signaling as stand-alone or combination therapy in cancer. This is supporting the novel concept of "purinergic immune checkpoint" (PIC) in cancer therapy. In the present review we will address a) the basic pharmacology and cell biology of the purinergic system; b) principles of its pathophysiology in human diseases; c) implications for cell death, cell proliferation and cancer; d) novel molecular tools to investigate nucleotide homeostasis in the extracellular environment; e) recent developments in the pharmacology of P1, P2 receptors and related ecto-enzymes; f) P1 and P2 ligands as novel diagnostic tools; g) current issues in PIC-based anti-cancer therapy. This review will provide an appraisal of the current status of purinergic signaling in cancer and will help identify future avenues of development.

Discovery of a potent, Highly selective, and In vivo anti-inflammatory Efficacious, P2Y6R antagonist with a novel quinoline-pyrazole scaffold

The P2Y6 receptor (P2Y6R), as a crucial member of the purine family, is a potential therapeutic target for the treatment of intestinal inflammation, tracheal inflammation and diabetes. We first discovered the hit compound (5a, IC50 = 168.5 nM against P2Y6R) through our in-house library screening. Then, further medicinal chemistry efforts were made to optimize compound 5a, and a potent P2Y6R antagonist (5 ab) with better antagonistic activity (IC50 = 19.6 nM) was obtained. The molecular docking, CETSA, SPR and pull-down results indicated that compound 5 ab displayed strong binding to P2Y6R. Also, compound 5 ab possessed high selectivity and satisfying oral bioactivity and pharmacokinetic profiles. In experiments with LPS-induced acute lung injury in mice, after treatment with compound 5 ab, the level of inflammatory factors IL-6, TNF-α and IL-β were considerably decreased, the infiltration of immune cells was decreased. Further exploration revealed that 5 ab inhibited the expression and release of chemokines in lung tissue, suppressing the activation of the NLRP3 inflammasome. Compound 5 ab had certain anti-inflammatory abilities in vivo and in vitro. These results demonstrate that compound 5 ab is a potential P2Y6R antagonist and is worthy of further study.

Exploring P2X receptor activity: A journey from cellular impact to electrophysiological profiling

The development of in vitro pharmacological assays relies on creating genetically modified cell lines that overexpress the target protein of interest. However, the choice of the host cell line can significantly impact the experimental outcomes. This study explores the functional characterization of P2X7 and P2X4 receptor modulators through cellular assays and advanced electrophysiological techniques. The influence of different host cell lines (HEK-293, HEK-293FT, and 1321N1) on the activity of reference agonists and antagonists targeting human and murine P2X4 and P2X7 receptors was systematically investigated, highlighting the significant impact of the host cell on experimental results. The 1321N1 cell line was identified as the preferred host cell line when investigating the human P2X4 receptor due to more consistent agonist activities, antagonist potencies, and a more stable assay signal window. Furthermore, a patch-clamp protocol that allows for the repetitive recording of ATP-mediated inward currents from isolated human CD4+ T-cells was established, revealing that both P2X7 and P2X4 receptors are crucial for immune cell regulation, positioning them as promising therapeutic targets for managing inflammatory disorders.

Sex differences in the purinergic 2 receptor-mediated blood pressure response to treadmill exercise in rats with simulated peripheral artery disease

We investigated the role played by ATP-sensitive purinergic 2 (P2) receptors in evoking the pressor response to treadmill exercise in male and female rats with and without femoral arteries that were ligated for ∼72 h to induce simulated peripheral artery disease (PAD). We hypothesized that PPADS (P2 receptor antagonist, 10 mg iv) would reduce the pressor response to 4 min of treadmill exercise (15 m·min-1, 1° incline) and steady-state exercise plasma norepinephrine (NE) values in male and female rats, and that the magnitude of effect of PPADS would be greater in rats with simulated PAD ("ligated") than in sham-operated rats. In males, PPADS significantly reduced the difference between steady-state exercise and baseline mean arterial pressure (ΔMAP) response to treadmill exercise in sham (n = 8; pre-PPADS: 12 ± 2, post-PPADS: 1 ± 5 mmHg; P = 0.037) and ligated (n = 4; pre-PPADS: 20 ± 2, post-PPADS: 11 ± 3 mmHg; P = 0.028) rats with a similar magnitude of effect observed between groups (P = 0.720). In females, PPADS had no effect on the ΔMAP response to treadmill exercise in sham (n = 6; pre-PPADS: 9 ± 2, post-PPADS: 7 ± 2 mmHg; P = 0.448) or ligated (n = 6; pre-PPADS: 15 ± 2, post-PPADS: 16 ± 3 mmHg; P = 0.684) rats. When NE values were grouped by sex independent of ligation/sham status, PPADS significantly reduced plasma NE in male (P = 0.016) and female (P = 0.027) rats. The data indicate that P2 receptors contribute to the sympathetic response to exercise in both male and female rats but that the sympathoexcitatory role for P2 receptors translates into an obligatory role in the blood pressure response to exercise in male but not in female rats.NEW & NOTEWORTHY Here, we demonstrate that purinergic 2 (P2) receptors contribute significantly to the blood pressure response to treadmill exercise in male rats both with and without simulated PAD induced by femoral artery ligation. We found no role for P2 receptors in the blood pressure response to treadmill exercise in female rats, thus revealing clear sex differences in P2 receptor-mediated blood pressure control during exercise.

Extracellular ATP Is a Homeostatic Messenger That Mediates Cell-Cell Communication in Physiological Processes and Psychiatric Diseases

Neuronal activity is the basis of information encoding and processing in the brain. During neuronal activation, intracellular ATP (adenosine triphosphate) is generated to meet the high-energy demands. Simultaneously, ATP is secreted, increasing the extracellular ATP concentration and acting as a homeostatic messenger that mediates cell-cell communication to prevent aberrant hyperexcitability of the nervous system. In addition to the confined release and fast synaptic signaling of classic neurotransmitters within synaptic clefts, ATP can be released by all brain cells, diffuses widely, and targets different types of purinergic receptors on neurons and glial cells, making it possible to orchestrate brain neuronal activity and participate in various physiological processes, such as sleep and wakefulness, learning and memory, and feeding. Dysregulation of extracellular ATP leads to a destabilizing effect on the neural network, as found in the etiopathology of many psychiatric diseases, including depression, anxiety, schizophrenia, and autism spectrum disorder. In this review, we summarize advances in the understanding of the mechanisms by which extracellular ATP serves as an intercellular signaling molecule to regulate neural activity, with a focus on how it maintains the homeostasis of neural networks. In particular, we also focus on neural activity issues that result from dysregulation of extracellular ATP and propose that aberrant levels of extracellular ATP may play a role in the etiopathology of some psychiatric diseases, highlighting the potential therapeutic targets of ATP signaling in the treatment of these psychiatric diseases. Finally, we suggest potential avenues to further elucidate the role of extracellular ATP in intercellular communication and psychiatric diseases.

Interstitial Cells of Cajal and P2X3 Receptors at Ureteropelvic Junction Obstruction and Their Relationship with Pain Response

Introduction: Etiopathogenesis and the symptomatology of ureteropelvic junction obstruction (UPJO) in the pediatric population has not yet been definitely clarified, suggesting a multifactorial nature of the condition. The aim was to analyze the association between the number of Interstitial Cells of Cajal (ICCs), as well as P2X3 receptors in ureteropelvic junction (UPJ) and the pain response in pediatric patients with hydronephrosis. Methods: 50 patients with congenital hydronephrosis underwent open or laparoscopic pyeloplasty at one of two departments of pediatric surgery and urology in Poland. Patients were divided into two groups according to the pain symptoms before surgery. A total of 50 samples of UPJ were obtained intraoperatively and underwent histopathological and immunohistochemical (IHC) analysis. Quantitative assessment of ICCs was based on the number of CD117(+) cells of adequate morphology in the subepithelial layer and the muscularis propria. Expression of P2X3 receptors was evaluated as the intensity of IHC staining. Results: Patients with hydronephrosis and accompanying pain were on average 60 months older (77 vs. 17 months) than children with asymptomatic hydronephrosis (p = 0.017). Symptomatic children revealed higher numbers of ICCs in both the subepithelial layer and in the lamina muscularis propria. In particular, symptomatic patients aged 2 years or more exhibited significantly higher numbers of ICCs in the subepithelial layer. Significant differences in the distribution of ICCs between the subepithelial layer and the lamina muscularis propria were observed in both groups. Expression of P2X3 receptors was limited to the urothelium and the muscle layer and correlated between these structures. There was no relationship between pain response and the expression of P2X3 receptors. Conclusions: ICCs and P2X3 receptors may participate in the pathogenesis of UPJO and in the modulation of pain response to a dilatation of the pyelocaliceal system. Explanation of the role of ICCs and P2X3 receptors in propagation of ureteral peristaltic wave and the modulation of pain stimuli requires further studies.

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

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

Purinergic Signaling and its Role in the Stem Cell Differentiation

Purinergic signaling is a mechanism in which extracellular purines and pyrimidines interact with specialized cell surface receptors known as purinergic receptors. These receptors are divided into two families of P1 and P2 receptors, each responding to different nucleosides and nucleotides. P1 receptors are activated by adenosine, while P2 receptors are activated by pyrimidine and purines. P2X receptors are ligand-gated ion channels, including seven subunits (P2X1-7). However, P2Y receptors are the G-protein coupled receptors comprising eight subtypes (P2Y1/2/4/6/11/12/13/14). The disorder in purinergic signaling leads to various health-related issues and diseases. In various aspects, it influences the activity of non-neuronal cells and neurons. The molecular mechanism of purinergic signaling provides insight into treating various human diseases. On the contrary, stem cells have been investigated for therapeutic applications. Purinergic signaling has shown promising effect in stem cell engraftment. The immune system promotes the autocrine and paracrine mechanisms and releases the significant factors essential for successful stem cell therapy. Each subtype of purinergic receptor exerts a beneficial effect on the damaged tissue. The most common effect caused by purinergic signaling is the proliferation and differentiation that treat different health-related conditions.

Systematic P2Y receptor survey identifies P2Y11 as modulator of immune responses and virus replication in macrophages

The immune system is in place to assist in ensuring tissue homeostasis, which can be easily perturbed by invading pathogens or nonpathogenic stressors causing tissue damage. Extracellular nucleotides are well known to contribute to innate immune signaling specificity and strength, but how their signaling is relayed downstream of cell surface receptors and how this translates into antiviral immunity is only partially understood. Here, we systematically investigated the responses of human macrophages to extracellular nucleotides, focusing on the nucleotide-sensing GPRC receptors of the P2Y family. Time-resolved transcriptomic analysis showed that adenine- and uridine-based nucleotides induce a specific, immediate, and transient cytokine response through the MAPK signaling pathway that regulates transcriptional activation by AP-1. Using receptor trans-complementation, we identified a subset of P2Ys (P2Y1, P2Y2, P2Y6, and P2Y11) that govern inflammatory responses via cytokine induction, while others (P2Y4, P2Y11, P2Y12, P2Y13, and P2Y14) directly induce antiviral responses. Notably, P2Y11 combined both activities, and depletion or inhibition of this receptor in macrophages impaired both inflammatory and antiviral responses. Collectively, these results highlight the underappreciated functions of P2Y receptors in innate immune processes.

Purinergic receptors in cognitive disturbances

Purinergic receptors (Rs) of the ATP/ADP, UTP/UDP (P2X, P2Y) and adenosine (A1, A2A)-sensitive classes broadly interfere with cognitive processes both under quasi normal and disease conditions. During neurodegenerative illnesses, high concentrations of ATP are released from the damaged neuronal and non-neuronal cells of the brain; then, this ATP is enzymatically degraded to adenosine. Thus, the primary injury in neurodegenerative diseases appears to be caused by various protein aggregates on which a superimposed damage mediated by especially P2X7 and A2AR activation develops; this can be efficiently prevented by small molecular antagonists in animal models of the above diseases, or are mitigated in the respective knockout mice. Dementia is a leading symptom in Alzheimer's disease (AD), and accompanies Parkinson's disease (PD) and Huntington's disease (HD), especially in the advanced states of these illnesses. Animal experimentation suggests that P2X7 and A2ARs are also involved in a number of psychiatric diseases, such as major depressive disorder (MDD), obsessive compulsive behavior, and attention deficit hyperactivity disorder. In conclusion, small molecular antagonists of purinergic receptors are expected to supply us in the future with pharmaceuticals which are able to combat in a range of neurological/psychiatric diseases the accompanying cognitive deterioration.

Channel-mediated ATP release in the nervous system

ATP is well established as a transmitter and modulator in the peripheral and central nervous system. While conventional exocytotic release of ATP at synapses occurs, this transmitter is unusual in also being released into the extracellular space via large-pored plasma membrane channels. This review considers the channels that are known to be permeable to ATP and some of the functions of channel-mediated ATP release. While the possibility of ATP release via channels mediating volume transmission has been known for some time, localised ATP release via channels at specialised synapses made by taste cells to the afferent nerve has recently been documented in taste buds. This raises the prospect that "channel synapses" may occur in other contexts. However, volume transmission and channel synapses are not necessarily mutually exclusive. We suggest that certain glial cells in the brain stem and hypothalamus, which possess long processes and are known to release ATP, may be candidates for both modes of ATP release -channel-mediated volume transmission in the region of their somata and more localised transmission possibly via either conventional or channel synapses from their processes at distal targets. Finally, we consider the different characteristics of vesicular and channel synapses and suggest that channel synapses may be advantageous in requiring less energy than their conventional vesicular counterparts. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

The circadian regulation of extracellular ATP

Extracellular ATP is a potent signaling molecule released from various cells throughout the body and is intimately involved in the pathophysiological functions of the nervous system and immune system by activating P2 purinergic receptors. Recent increasingly studies showed that extracellular ATP exhibits circadian oscillation with an approximately 24-h periodicity, which participates in regulatory pathways of central oscillator suprachiasmatic nucleus and peripheral oscillator bladder, respectively. Oscillators modulate the protein expression of ATP release channels and ectonucleotidase activity through clock genes; indeed, real-time alterations of ATP release and degradation determine outcomes of temporal character on extracellular ATP rhythm. The regulatory pathways on extracellular ATP rhythm are different in central and peripheral systems. In this review, we summarize the circadian rhythm of extracellular ATP and discuss several circadian regulatory pathways in different organs via ATP release and degradation, to provide a new understanding for purinergic signaling in the regulatory mechanism of circadian rhythm and a potential target to research the circadian regulation of extracellular ATP in other circadian oscillators.

P2X receptors: Insights from the study of the domestic dog

Fifty years ago, the late Geoffrey Burnstock described the concept of purinergic nerves and transmission bringing into existence the broader concepts of purinergic signaling including P2X receptors. These receptors are trimeric ligand-gated cation channels activated by extracellular adenosine 5'-triphosphate (ATP). P2X receptors have important roles in health and disease and continue to gain interest as potential therapeutic targets in inflammatory, neurological, cardiovascular and many other disorders including cancer. Current understanding of P2X receptors has largely arisen from the study of these receptors in humans and rodents, but additional insights have been obtained from the study of P2X receptors in the domestic dog, Canis familiaris. This review article will briefly introduce purinergic signaling and P2X receptors, before detailing the pharmacological profiles of the two recombinant canine P2X receptors studied to date, P2X7 and P2X4. The article will then describe the current state of knowledge concerning the distribution and function of the P2X receptor family in dogs. The article will also discuss the characterization of single nucleotide polymorphisms in the canine P2RX7 gene, and contrast this variation to the canine P2RX4 gene, which is largely conserved between dogs. Finally, this article will outline published examples of the use of dogs to study the pharmacokinetics of P2X7 and P2X3 antagonists, and how they have contributed to the preclinical testing of antagonists to human P2X7, CE-224,535, and human P2X3, Gefapixant (AF-219, MK-7264) and Eliapixant (BAY, 1817080), with Gefapixant gaining recent approval for use in the treatment of refractory chronic cough in humans. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

The Neuronal Cotransmission: Mechanistic Insights From the Autonomic Nervous System

It is now scientifically accepted that neurons have the ability to release multiple transmitter substances simultaneously, yet, cotransmission's functionality is still limited to the scientific community. Acetylcholine is released by the noradrenergic neurons, and then the acetylcholine works prejunctionally in the promotion of the noradrenaline release. This hypothesis significantly challenged the previous idea of autonomic transmission as being a simple process that had a single transmitter. Norepinephrine was thought to be the single transmitter at the sympathetic neurovascular junction according to "Dale's principle". However, more evidence of the involvement of other neurotransmitters has been shown by many researchers in conjunction with Dale's principle and established terms such as adrenergic, purinergic, and peptidergic nerves. With the discovery of cotransmission, we now understand the existence of more than one neurotransmitter at a sympathetic neurovascular junction.

P2X7-dependent immune pathways in retinal diseases

Adenosine triphosphate (ATP) is a signalling molecule acting as a neurotransmitter but also as a danger signal. The purinergic receptor P2X7 is the main sensor of high concentration of ATP released by damaged cells. In the eye, P2X7 is expressed by resident microglia and immune cells that infiltrate the retina in disease such as age-related macular degeneration (AMD), a degenerative retinal disease, and uveitis, an inflammatory eye disease. Activation of P2X7 is involved in several physiological and pathological processes: phagocytosis, activation of the inflammasome NLRP3, release of pro-inflammatory mediators and cell death. The aim of this review is to discuss the potential involvement of P2X7 in the development of AMD and uveitis.

Purinergic signaling: Diverse effects and therapeutic potential in cancer

Regardless of improved biological insights and therapeutic advances, cancer is consuming multiple lives worldwide. Cancer is a complex disease with diverse cellular, metabolic, and physiological parameters as its hallmarks. This instigates a need to uncover the latest therapeutic targets to advance the treatment of cancer patients. Purines are building blocks of nucleic acids but also function as metabolic intermediates and messengers, as part of a signaling pathway known as purinergic signaling. Purinergic signaling comprises primarily adenosine triphosphate (ATP) and adenosine (ADO), their analogous membrane receptors, and a set of ectonucleotidases, and has both short- and long-term (trophic) effects. Cells release ATP and ADO to modulate cellular function in an autocrine or paracrine manner by activating membrane-localized purinergic receptors (purinoceptors, P1 and P2). P1 receptors are selective for ADO and have four recognized subtypes-A1, A2A, A2B, and A3. Purines and pyrimidines activate P2 receptors, and the P2X subtype is ligand-gated ion channel receptors. P2X has seven subtypes (P2X1-7) and forms homo- and heterotrimers. The P2Y subtype is a G protein-coupled receptor with eight subtypes (P2Y1/2/4/6/11/12/13/14). ATP, its derivatives, and purinoceptors are widely distributed in all cell types for cellular communication, and any imbalance compromises the homeostasis of the cell. Neurotransmission, neuromodulation, and secretion employ fast purinergic signaling, while trophic purinergic signaling regulates cell metabolism, proliferation, differentiation, survival, migration, invasion, and immune response during tumor progression. Thus, purinergic signaling is a prospective therapeutic target in cancer and therapy resistance.

Role of ATP in migraine mechanisms: focus on P2X3 receptors

Migraine is a major health burden worldwide with complex pathophysiology and multifarious underlying mechanisms. One poorly understood issue concerns the early steps in the generation of migraine pain. To elucidate the basic process of migraine pain further, it seems useful to consider key molecular players that may operate synergistically to evoke headache. While the neuropeptide CGRP is an important contributor, we propose that extracellular ATP (that generally plays a powerful nociceptive role) is also a major component of migraine headache, acting in concert with CGRP to stimulate trigeminal nociceptive neurons. The aim of the present focused review is to highlight the role of ATP activating its P2X3 membrane receptors selectively expressed by sensory neurons including their nerve fiber terminals in the meninges. Specifically, we present data on the homeostasis of ATP and related purines in the trigeminovascular system and in the CNS; the basic properties of ATP signalling at peripheral and central nerve terminals; the characteristics of P2X3 and related receptors in trigeminal neurons; the critical speed and persistence of P2X3 receptor activity; their cohabitation at the so-called meningeal neuro-immune synapse; the identity of certain endogenous agents cooperating with ATP to induce neuronal sensitization in the trigeminal sensory system; the role of P2X3 receptors in familial type migraine; the current state of P2X3 receptor antagonists and their pharmacological perspectives in migraine. It is proposed that the unique kinetic properties of P2X3 receptors activated by ATP offer an interesting translational value to stimulate future studies for innovative treatments of migraine pain.

More than a drug target: Purinergic signalling as a source for diagnostic tools in epilepsy

Epilepsy is one of the most common and disabling chronic neurological diseases affecting people of all ages. Major challenges of epilepsy management include the persistently high percentage of drug-refractoriness among patients, the absence of disease-modifying treatments, and its diagnosis and prognosis. To date, long-term video-electroencephalogram (EEG) recordings remain the gold standard for an epilepsy diagnosis. However, this is very costly, has low throughput, and in some instances has very limited availability. Therefore, much effort is put into the search for non-invasive diagnostic tests. Purinergic signalling, via extracellularly released adenosine triphosphate (ATP), is gaining increasing traction as a therapeutic strategy for epilepsy treatment which is supported by evidence from both experimental models and patients. This includes in particular the ionotropic P2X7 receptor. Besides that, other components from the ATPergic signalling cascade such as the metabotropic P2Y receptors (e.g., P2Y₁ receptor) and ATP-release channels (e.g., pannexin-1), have also been shown to contribute to seizures and epilepsy. In addition to the therapeutic potential of purinergic signalling, emerging evidence has also shown its potential as a diagnostic tool. Following seizures and epilepsy, the concentration of purines in the blood and the expression of different compounds of the purinergic signalling cascade are significantly altered. Herein, this review will provide a detailed discussion of recent findings on the diagnostic potential of purinergic signalling for epilepsy management and the prospect of translating it for clinical application. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

P2X and P2Y receptor antagonists reduce inflammation in ATP-induced microglia

Background

P2 receptors have been implicated in the release of neurotransmitter and pro-inflammatory cytokines due to their response to neuro-excitatory substances in the microglia. The P2X4, P2X7 and P2Y12 receptors are involved in the development of pain behavior induced by peripheral nerve injury. However, it is not known if blocking P2X4, P2X7 and P2Y12 receptors is associated with the expression and the release of interleukin-1B (IL-1β), interleukin-6 (IL-6), or tumor necrosis factor-α (TNF-α) in cultured neonatal spinal cord microglia.

Objective

For this reason, we examined the effects of P2X4, P2X7 and P2Y12 antagonists on the expression and the release of IL-1β, IL-6, and TNF-α in ATP-stimulated microglia.

Methods

In this study, we observed the effect of A-740003, PSB-12062 and MRS 2395 (P2X4, P2X7 and P2Y12 receptors antagonist, respectively), on the expression and release of IL-1β, IL-6 and TNF-α by using real-time fluorescence quantitative polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA).

Results

ATP induced the increased expression of IL-1β, IL-6 and TNF-α at the level of messenger RNA (mRNA). ATP-evoked increase in IL-1β, IL-6 and TNF-α mRNA expression was inhibited by the P2X4 receptor antagonist A-740003 or P2X7 receptor antagonist PSB-12062, respectively. Similarly, ATP-evoked release of IL-1β, IL-6 and TNF-α was inhibited by A-740003 and PSB-12062. Furthermore, ATP-evoked increased expression of Iba-1, IL-1β, IL-6 and TNF-α mRNA, and release of IL-1β, IL-6 and TNF-α were nearly all blocked after co-administration of A-740003 plus PSB-12062. Finally, ATP-evoked increased gene expression and release of IL-1β, IL-6 and TNF-α were also inhibited by MRS 2395 (P2Y12 antagonist).

Conclusion

These observations suggest a new clue for therapeutic strategies to treat the neuro-inflammation.

Discovery of a Series of 5-Amide-1H-pyrazole-3-carboxyl Derivatives as Potent P2Y14R Antagonists with Anti-Inflammatory Characters

UDPG/P2Y₁₄R signaling pathway has been considered as a potential therapeutic target for innate immune system diseases. Based on the scaffold hopping strategy, a series of pyrazole analogues were designed and synthesized as novel P2Y₁₄R antagonists with improved physicochemical properties, together with potential anti-inflammatory activities. Additionally, we designed and synthesized a fluorescent probe based on highly selective and potent PPTN to study the affinity of synthesized compounds. The optimized compound 16 (1-(4-fluorobenzyl)-5-(4-methylbenzamido)-1H-pyrazole-3-carboxylic acid, P2Y₁₄R IC₅₀ = 1.93 nM) showed strong binding ability to P2Y₁₄R, high selectivity, notably improved solubility, and more favorable pharmacokinetic profiles. Moreover, compound 16 possessed extremely low cytotoxicity and anti-inflammatory effect in vitro. In an acute peritonitis model, compound 16 could effectively reduce the levels of inflammatory factor IL-6, IL-1β, and TNF-α of mice induced by LPS. Compound 16, with potent in vitro and in vivo efficacy and favorable druggability, can be a promising candidate for further research.

Purinergic signaling in the male reproductive tract

Purinergic receptors are ubiquitously expressed throughout the body and they participate in the autocrine and paracrine regulation of cell function during normal physiological and pathophysiological conditions. Extracellular nucleotides activate several types of plasma membrane purinergic receptors that form three distinct families: P1 receptors are activated by adenosine, P2X receptors are activated by ATP, and P2Y receptors are activated by nucleotides including ATP, ADP, UTP, UDP, and UDP-glucose. These specific pharmacological fingerprints and the distinct intracellular signaling pathways they trigger govern a large variety of cellular responses in an organ-specific manner. As such, purinergic signaling regulates several physiological cell functions, including cell proliferation, differentiation and death, smooth muscle contraction, vasodilatation, and transepithelial transport of water, solute, and protons, as well as pathological pathways such as inflammation. While purinergic signaling was first discovered more than 90 years ago, we are just starting to understand how deleterious signals mediated through purinergic receptors may be involved in male infertility. A large fraction of male infertility remains unexplained illustrating our poor understanding of male reproductive health. Purinergic signaling plays a variety of physiological and pathophysiological roles in the male reproductive system, but our knowledge in this context remains limited. This review focuses on the distribution of purinergic receptors in the testis, epididymis, and vas deferens, and their role in the establishment and maintenance of male fertility.

Extracellular Vesicle-Mediated Metastasis Suppressors NME1 and NME2 Modify Lipid Metabolism in Fibroblasts

Simple Summary

Communication between cancer and stromal cells involves paracrine signalling mediated by extracellular vesicles (EVs). EVs transmit essential factors among cells of the tumour microenvironment. EVs derived from both cancer and stromal cells have been implicated in tumour progression. In this study, we focused on the first identified metastasis suppressor NME1, and on its close homolog NME2, and investigated their function in EVs in the interplay between cancer and stromal cells.

Abstract

Nowadays, extracellular vesicles (EVs) raise a great interest as they are implicated in intercellular communication between cancer and stromal cells. Our aim was to understand how vesicular NME1 and NME2 released by breast cancer cells influence the tumour microenvironment. As a model, we used human invasive breast carcinoma cells overexpressing NME1 or NME2, and first analysed in detail the presence of both isoforms in EV subtypes by capillary Western immunoassay (WES) and immunoelectron microscopy. Data obtained by both methods showed that NME1 was present in medium-sized EVs or microvesicles, whereas NME2 was abundant in both microvesicles and small-sized EVs or exosomes. Next, human skin-derived fibroblasts were treated with NME1 or NME2 containing EVs, and subsequently mRNA expression changes in fibroblasts were examined. RNAseq results showed that the expression of fatty acid and cholesterol metabolism-related genes was decreased significantly in response to NME1 or NME2 containing EV treatment. We found that FASN (fatty acid synthase) and ACSS2 (acyl-coenzyme A synthetase short-chain family member 2), related to fatty acid synthesis and oxidation, were underexpressed in NME1/2-EV-treated fibroblasts. Our data show an emerging link between NME-containing EVs and regulation of tumour metabolism.

The purinergic signalling and inflammation in the pathogenesis and progression of diabetes: key factors and therapeutic targets

Type 2 diabetes mellitus (T2DM) is an important chronic disease around the world, and according to the World Health Organization, it is the 9th principal cause of global death. This pathology is characterized by high levels of circulating glucose as a result of insulin resistance, and it is well stated that inflammation related to obesity is directly associated with the development of the disease. The purinergic signalling is involved in both pancreatic destruction, which impairs insulin secretion, and the cytokine production that favors insulin resistance in T2DM. In this review, the purinergic signalling aspects will be discussed, showing the impact of the enzymes, nucleotides, nucleosides, and receptors of this system and the cytokines that result in inflammation, in the development and progression of T2DM, besides, pointing the purinergic receptors as a possible therapeutic approach.

Diet, herbs and erectile function: A good friendship!

Plants and plant materials have been used for thousands of years to treat and control erectile dysfunction in men. This practice has spanned many cultures and traditions around the world, with the therapeutic effects of many plants attributed to their phytochemical constituents. This review explains how polyphenols (including phenolic acids, flavonoids, terpenoids, carotenoids, alkaloids and polyunsaturated fatty acids) in plants and plant food products interact with key enzymes (phosphodiesterase-5 [PDE-5], angiotensin-converting enzyme [ACE], acetylcholinesterase [AChE], adenosine deaminase [ADA] and arginase) associated with erectile dysfunction. By modulating or altering the activity of these physiologically important enzymes, various bioactive compounds from plants or plant products can synergistically or additively provide tremendous protection against male erectile problems.

Enlightening activation gating in P2X receptors

P2X receptors are trimeric nonselective cation channels gated by ATP. They assemble from seven distinct subunit isoforms as either homo- or heteromeric complexes and contain three extracellularly located binding sites for ATP. P2X receptors are expressed in nearly all tissues and are there involved in physiological processes like synaptic transmission, pain, and inflammation. Thus, they are a challenging pharmacological target. The determination of crystal and cryo-EM structures of several isoforms in the last decade in closed, open, and desensitized states has provided a firm basis for interpreting the huge amount of functional and biochemical data. Electrophysiological characterization in conjugation with optical approaches has generated significant insights into structure–function relationships of P2X receptors. This review focuses on novel optical and related approaches to better understand the conformational changes underlying the activation of these receptors.

Recent progress in the structural biology of P2X receptors

P2X receptors are ATP-gated trimeric nonselective cation channels that are important for various physiological and pathological processes, including synaptic transmission, pain perception, immune regulation, and apoptosis. Accordingly, they attract a wide range of interest as drug targets, such as those for chronic cough, neuropathic pain, and depression. After the zebrafish P2X4 receptor structure was reported in 2009, various other P2X receptor structures have been reported, extending our understanding of the molecular mechanisms of P2X receptors. This review article describes the recent progress on understanding the structures and mechanisms of P2X receptors, especially of the mechanisms underlying ATP binding and conformational changes during the gating cycle. In addition, since several antagonists for different P2X subtypes have entered into clinical trials, this review also summarizes the binding sites and regulatory mechanisms of these antagonists, which may contribute to new strategies of targeting P2X receptors for drug discovery.

Chronic cough: new insights and future prospects

Chronic cough is defined in adults as a cough that lasts for ≥8 weeks. When it proves intractable to standard-of-care treatment, it can be referred to as refractory chronic cough (RCC). Chronic cough is now understood to be a condition of neural dysregulation. Chronic cough and RCC result in a serious, often unrecognized, disease burden, which forms the focus of the current review.The estimated global prevalence of chronic cough is 2-18%. Patients with chronic cough and RCC report many physical and psychological effects, which impair their quality of life. Chronic cough also has a significant economic burden for the patient and healthcare systems. RCC diagnosis and treatment are often delayed for many years as potential treatable triggers must be excluded first and a stepwise empirical therapeutic regimen is recommended.Evidence supporting most currently recommended treatments is limited. Many treatments do not address the underlying pathology, are used off-label, have limited efficacy and produce significant side-effects. There is therefore a significant unmet need for alternative therapies for RCC that target the underlying disease mechanisms. Early clinical data suggest that antagonists of the purinergic P2X3 receptor, an important mediator of RCC, are promising, though more evidence is needed.

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

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

Effects of a P2X7 receptor antagonist on myenteric neurons in the distal colon of an experimental rat model of ulcerative colitis

Inflammatory bowel diseases (IBDs) are chronic diseases of the gastrointestinal tract that include ulcerative colitis and Crohn's disease and affect enteric neurons. Research has shown that Brilliant Blue G (BBG), a P2X7 receptor antagonist, restores enteric neurons following ischemia and reperfusion. This study aimed to evaluate the effect of BBG on myenteric neurons of the distal colon in an experimental rat model of ulcerative colitis. Colitis was induced by injection of 2,4,6-trinitrobenzene sulfonic acid (TNBS) into the large intestine. BBG was administered 1 h after colitis induction and for five consecutive days thereafter. Distal colons were collected 24 h or 7 days after TNBS injection. The animals were divided into 24-h and 7-day sham (vehicle injection rather than colitis induction), 24-h colitis, 24-h BBG, 7-day colitis and 7-day BBG groups. The disease activity index (DAI), neuronal density and profile of neuronal nitric oxide synthase (nNOS)-, choline acetyltransferase (ChAT)- and P2X7 receptor-immunoreactive enteric neurons were analyzed, and histological analysis was performed. The results showed recovery of the DAI and histological tissue integrity in the BBG groups compared to those in the colitis groups. In addition, the numbers of neurons positive for nNOS, ChAT and the P2X7 receptor per area were decreased in the colitis groups, and these measures were recovered in the BBG groups. Neuronal size was increased in the colitis groups and restored in the BBG groups. In conclusion, BBG is effective in improving experimental ulcerative colitis, and the P2X7 receptor may be a therapeutic target.

Targeting purinergic receptors to suppress the cytokine storm induced by SARS-CoV-2 infection in pulmonary tissue

The etiological agent of coronavirus disease (COVID-19) is the new member of the Coronaviridae family, a severe acute respiratory syndrome coronavirus 2 virus (SARS-CoV-2), responsible for the pandemic that is plaguing the world. The single-stranded RNA virus is capable of infecting the respiratory tract, by binding the spike (S) protein on its viral surface to receptors for the angiotensin II-converting enzyme (ACE2), highly expressed in the pulmonary tissue, enabling the interaction of the virus with alveolar epithelial cells promoting endocytosis and replication of viral material. The infection triggers the activation of the immune system, increased purinergic signaling, and the release of cytokines as a defense mechanism, but the response can become exaggerated and prompt the so-called “cytokine storm”, developing cases such as severe acute respiratory syndrome (SARS). This is characterized by fever, cough, and difficulty breathing, which can progress to pneumonia, failure of different organs and death. Thus, the present review aims to compile and correlate the mechanisms involved between the immune and purinergic systems with COVID-19, since the modulation of purinergic receptors, such as A2A, A2B, and P2X7 expressed by immune cells, seems to be effective as a promising therapy, to reduce the severity of the disease, as well as aid in the treatment of acute lung diseases and other cases of generalized inflammation.

The discovery and development of gefapixant

Gefapixant is the approved generic name for a compound also known as MK-7264, and prior to that AF-219 and RO-4926219. It is the first-in-class clinically developed antagonist for the P2X3 subtype of trimeric ionotropic purinergic receptors, a family of ATP-gated excitatory ion channels, showing nanomolar potency for the human P2X3 homotrimeric channel and essentially no activity at related channels devoid of P2X3 subunits. As the first P2X3 antagonist to have progressed into clinical studies it has now progressed to the point of successful completion of Phase 3 investigations for the treatment of cough, and the NDA application is under review with US FDA for treatment of refractory chronic cough or unexplained chronic cough. The molecule was discovered in the laboratories of Roche Pharmaceuticals in Palo Alto, California, but clinical development then continued with the formation of Afferent Pharmaceuticals for the purpose of identifying the optimal therapeutic indication for this novel mechanism and establishing a clinical plan for development in the optimal patient populations selected. Geoff Burnstock was a close collaborator and advisor to the P2X3 program for close to two decades of discovery and development. Progression of gefapixant through later stage clinical studies has been conducted by the research laboratories of Merck & Co., Inc., Kenilworth, NJ, USA (MRL; following acquisition of Afferent in 2016), who may commercialize the product once authorization has been granted by regulatory authorities.

Role of TRPV4-P2X7 Pathway in Neuropathic Pain in Rats with Chronic Compression of the Dorsal Root Ganglion

Transient receptor potential vanilloid 4 (TRPV4) is a Ca2+-permeable non-selective cation channel that is involved in the development of neuropathic pain. P2X7 receptor (P2X7) belongs to a class of ATP-gated nonselective cation channels that plays an important role in neuropathic pain. Nevertheless, little is known about the interaction between them for neuropathic pain. In this paper, we investigated role of TRPV4-P2X7 pathway in neuropathic pain. We evaluated the effect of TRPV4-P2X7 pathway on neuropathic pain in a chronic compression of the dorsal root ganglion (DRG) (hereafter termed CCD) model. We analyzed the effect of P2X7 on mechanical and thermal hyperalgesia mediated by TRPV4 in CCD. Furthermore, we assessed the effect of TRPV4 on the expression of P2X7 and the release of IL-1β and IL-6 in DRG after CCD. We found that intraperitoneal injection of TRPV4 agonist GSK-1016790A led to a significant increase of mechanical and thermal hyperalgesia in CCD, which was partially suppressed by P2X7 blockade with antagonist Brilliant Blue G (BBG). Then, we further noticed that GSK-1016790A injection increased the P2X7 expression of CCD, which was decreased by TRPV4 blockade with antagonist RN-1734 and HC-067047. Furthermore, we also discovered that the expressions of IL-1β and IL-6 were upregulated by GSK-1016790A injection but reduced by RN-1734 and HC-067047. Our results provide evidence that P2X7 contributes to development of neuropathic pain mediated by TRPV4 in the CCD model, which may be the basis for treatment of neuropathic pain relief.

Analysis of Spatial and Temporal Distribution of Purinergic P2 Receptors in the Mouse Hippocampus

ATP and other nucleotides are important glio-/neurotransmitters in the central nervous system. They bind to purinergic P2X and P2Y receptors that are ubiquitously expressed in various brain regions modulating various physiological and pathophysiological processes. P2X receptors are ligand-gated ion channels mediating excitatory postsynaptic responses whereas P2Y receptors are G protein-coupled receptors mediating slow synaptic transmission. A variety of P2X and P2Y subtypes with distinct neuroanatomical localization provide the basis for a high diversity in their function. There is increasing evidence that P2 receptor signaling plays a prominent role in learning and memory and thus, in hippocampal neuronal plasticity. Learning and memory are time-of-day-dependent. Moreover, extracellular ATP shows a diurnal rhythm in rodents. However, it is not known whether P2 receptors have a temporal variation in the hippocampus. This study provides a detailed systematic analysis on spatial and temporal distribution of P2 in the mouse hippocampus. We found distinct spatial and temporal distribution patterns of the P2 receptors in different hippocampal layers. The temporal distribution of P2 receptors can be segregated into two large time domains, the early to mid-day and the mid to late night. This study provides an important basis for understanding dynamic P2 purinergic signaling in the hippocampal glia/neuronal network.

Purinergic signalling in the urinary bladder - When function becomes dysfunction

Knowledge of the participation of ATP and related purines in urinary tract physiology has been established over the last five decades through the work of many independent groups, inspired by, and building on the pioneering studies of Professor Geoffrey Burnstock and his coworkers. As part of a series of reviews in this tribute edition, the present article summarises our current understanding of purines and purinergic signalling in modulating and regulating urinary tract function. Purinergic mechanisms underlying the origin of bladder pain; sensations of bladder filling and urinary tract motility; and regulation of detrusor smooth muscle contraction are described, encompassing the relevant history of discovery and consolidation of knowledge as methodologies and pharmacological tools have developed. We consider normal physiology, including development and ageing and then move to pathophysiology, discussing the causal and consequential contribution of purinergic signalling mechanism and their constituent components (receptors, signal transduction, effector molecules) to bladder dysfunction.

The Role of Purinergic Signaling in Trichomonas vaginalis Infection

Trichomoniasis, one of the most common non-viral sexually transmitted infections worldwide, is caused by the parasite Trichomonas vaginalis. The pathogen colonizes the human urogenital tract, and the infection is associated with complications such as adverse pregnancy outcomes, cervical cancer, and an increase in HIV transmission. The mechanisms of pathogenicity are multifactorial, and controlling immune responses is essential for infection maintenance. Extracellular purine nucleotides are released by cells in physiological and pathological conditions, and they are hydrolyzed by enzymes called ecto-nucleotidases. The cellular effects of nucleotides and nucleosides occur via binding to purinoceptors, or through the uptake by nucleoside transporters. Altogether, enzymes, receptors and transporters constitute the purinergic signaling, a cellular network that regulates several effects in practically all systems including mammals, helminths, protozoa, bacteria, and fungi. In this context, this review updates the data on purinergic signaling involved in T. vaginalis biology and interaction with host cells, focusing on the characterization of ecto-nucleotidases and on purine salvage pathways. The implications of the final products, the nucleosides adenosine and guanosine, for human neutrophil response and vaginal epithelial cell damage reveal the purinergic signaling as a potential new mechanism for alternative drug targets.

P2X3 receptors participate in purinergic inhibition of gastrointestinal smooth muscle

The ATP analogue α,β-meATP is a potent relaxant of gastrointestinal smooth muscle, but its molecular target is uncertain inside the gut. α,β-meATP relaxed the carbachol-precontracted guinea-pig taenia coli in a concentration-dependent manner (EC₅₀, 2.0 ± 0.1 μM). A luciferase-based assay confirmed that α,β-meATP solutions were minimally contaminated with ATP. α,β-meATP-evoked relaxations were inhibited by the competitive P2Y1 antagonist MRS2179 (pA₂ = 5.36), but also by the competitive P2X3 antagonist, A-317491 (pA₂ = 5.51). When MRS2179 and A-317491 were applied together, residual α,β-meATP responses converted from brief to prolonged relaxations. Sodium nitroprusside (a nitric oxide donor) also caused prolonged relaxations. Immunohistochemistry revealed that P2X3 receptors were present in myenteric ganglion cells and their varicose nerve terminals. The amplitude of α,β-meATP responses was not inhibited by TTX (NaV channel blocker) and ωCgTx (N-type CaV channel blocker). However, responses to α,β-meATP were inhibited by TEA (non-selective K⁺-channel blocker), indicating that relaxations involved opening K⁺-channels. The findings of this study are consistent with the conclusion that α,β-meATP stimulates Ca²⁺-permeable P2X3 receptors on varicose nerve terminals to release inhibitory nucleotides: 1) ATP and β-NAD release results in P2Y1-mediated brief relaxations; 2) another released transmitter (possibly NO) results in prolonged relaxations. Prejunctional P2X3 receptors represent a purinergic feed-forward mechanism to augment the action of inhibitory nerves on gut motility. This positive feed-forward mechanism may counter-balance the known negative feedback mechanism caused by adenosine and prejunctional A1 receptors on inhibitory motor nerves.

Neurotransmitters responsible for purinergic motor neurotransmission and regulation of GI motility

Classical concepts of peripheral neurotransmission were insufficient to explain enteric inhibitory neurotransmission. Geoffrey Burnstock and colleagues developed the idea that ATP or a related purine satisfies the criteria for a neurotransmitter and serves as an enteric inhibitory neurotransmitter in GI muscles. Cloning of purinergic receptors and development of specific drugs and transgenic mice have shown that enteric inhibitory responses depend upon P2Y₁ receptors in post-junctional cells. The post-junctional cells that transduce purinergic neurotransmitters in the GI tract are PDGFRα⁺ cells and not smooth muscle cells (SMCs). PDGFRα⁺ cells express P2Y₁ receptors, are activated by enteric inhibitory nerve stimulation and generate Ca²⁺ oscillations, express small-conductance Ca²⁺-activated K⁺ channels (SK3), and generate outward currents when exposed to P2Y₁ agonists. These properties are consistent with post-junctional purinergic responses, and similar responses and effectors are not functional in SMCs. Refinements in methodologies to measure purines in tissue superfusates, such as high-performance liquid chromatography (HPLC) coupled with etheno-derivatization of purines and fluorescence detection, revealed that multiple purines are released during stimulation of intrinsic nerves. β-NAD⁺ and other purines, better satisfy criteria for the purinergic neurotransmitter than ATP. HPLC has also allowed better detection of purine metabolites, and coupled with isolation of specific types of post-junctional cells, has provided new concepts about deactivation of purine neurotransmitters. In spite of steady progress, many unknowns about purinergic neurotransmission remain and require additional investigation to understand this important regulatory mechanism in GI motility.

Isolation of Arabidopsis extracellular ATP binding proteins by affinity proteomics and identification of PHOSPHOLIPASE C-LIKE 1 as an extracellular protein essential for fumonisin B1 toxicity

ATP is secreted to the extracellular matrix, where it activates plasma membrane receptors for controlling plant growth and stress-adaptive processes. DOES NOT RESPOND TO NUCLEOTIDES 1 (DORN1), was the first plant ATP receptor to be identified but key downstream proteins remain sought after. Here, we identified 120 proteins secreted by Arabidopsis cell cultures and screened them for putative stress-responsive proteins using ATP-affinity purification. We report three Arabidopsis proteins isolated by ATP-affinity: PEROXIDASE 52, SUBTILASE-LIKE SERINE PROTEASE 1.7 and PHOSPHOLIPASE C-LIKE 1. In wild-type Arabidopsis, the expression of genes encoding all three proteins responded to fumonisin B1, a cell death-activating mycotoxin. The expression of PEROXIDASE 52 and PHOSPHOLIPASE C-LIKE 1 was altered in fumonisin B1-resistant salicylic acid induction-deficient (sid2) mutants. Exposure to fumonisin B1 suppressed PHOSPHOLIPASE C-LIKE 1 expression in sid2 mutants, suggesting that the inactivation of this gene might provide mycotoxin tolerance. Accordingly, gene knockout mutants of PHOSPHOLIPASE C-LIKE 1 were resistant to fumonisin B1-induced death. The activation of PHOSPHOLIPASE C-LIKE 1 gene expression by exogenous ATP was not blocked in dorn1 loss-of-function mutants, indicating that DORN1 is not required. Furthermore, exogenous ATP rescued both the wild type and the dorn1 mutants from fumonisin-B1 toxicity, suggesting that different ATP receptor(s) are operational in this process. Our results point to the existence of additional plant ATP receptor(s) and provide crucial downstream targets for use in designing screens to identify these receptors. Finally, PHOSPHOLIPASE C-LIKE 1 serves as a convergence point for fumonisin B1 and extracellular ATP signalling, and functions in the Arabidopsis stress response to fumonisin B1.

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

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

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

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

Biological insights from the direct measurement of purine release

Although purinergic signalling has been a well-established and accepted mechanism of chemical communication for many years, it remains important to measure the extracellular concentration of ATP and adenosine in real time. In this review I summarize the reasons why such measurements are still needed, how they provide additional mechanistic insight and give an overview of the techniques currently available to make spatially localised measurements of ATP and adenosine in real time. To illustrate the impact of direct real-time measurements, I explore CO2 and nutrient sensing in the medulla oblongata and hypothalamus. In both of these examples, the sensing involves hemichannel mediated ATP release from glial cells. For CO2 the hemichannels involved, connexin26, are directly CO2-sensitive. This mechanism contributes to the chemosensory control of breathing. In the hypothamalus, specialised glial cells, tanycytes, directly contact the cerebrospinal fluid in the 3rd ventricle and sense nutrients via sweet and umami taste receptors. Nutrient sensing by tanycytes is likely to contribute to the control of body weight as their selective stimulation alters food intake. To illustrate the importance of direct adenosine measurements, I consider the complex and multiple mechanisms of activity-dependent adenosine release in different brain regions. This activity dependent release of adenosine is likely to mediate important feedback regulation and may also be involved in controlling the sleep-wake state. I finish by briefly considering the potential of whole blood purine measurements in clinical practice.

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

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

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

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