Secretion of matrix metalloproteinase-9 from astrocytes by inhibition of tonic P2Y14-receptor-mediated signal(s)

Pharmacological interaction and immune response of purinergic receptors in therapeutic modulation

Nucleosides and purine nucleotides serve as transmitter and modulator agents that extend their functions beyond the cell. In this context, purinergic signaling plays a crucial role in regulating energy homeostasis and modulating metabolic alterations in tumor cells. Therefore, it is essential to consider the pharmacological targeting of purinergic receptors (PUR), which encompass the expression and inhibition of P1 receptors (metabotropic adenosine receptors) as well as P2 receptors (extracellular ATP/ADP) comprising P2X and P2Y receptors. Thus, the pharmacological interaction between inhibitors (such as RNA, monoclonal antibodies, and small molecules) and PUR represents a key aspect in facilitating the development of therapeutic interventions. Moreover, this review explores recent advancements in pharmacological inhibitors and the regulation of innate and adaptive immunity of PUR, specifically in relation to immunological and inflammatory responses. These responses encompass the release of pro-inflammatory cytokines (PIC), the production of reactive oxygen and nitrogen species (ROS and RNS), the regulation of T cells, and the activation of inflammasomes in all human leukocytes.

The bumpy road of purinergic inhibitors to clinical application in immune-mediated diseases

ABSTRACT

Purinergic signaling plays important roles throughout the body in the regulation of organ functions during and following the disruption of homeostasis. This is also reflected by the widespread expression of two families of purinergic receptors (P1 and P2) with numerous subtypes. In the last few decades, there has been increasing evidence that purinergic signaling plays an important role in the regulation of immune functions. Mainly, signals mediated by P2 receptors have been shown to contribute to immune system-mediated pathologies. Thus, interference with P2 receptors may be a promising strategy for the modulation of immune responses. Although only a few clinical studies have been conducted in isolated entities with limited success, preclinical work suggests that the use of P2 receptor inhibitors may bear some promise in various autoimmune diseases. Despite the association of P2 receptors with several disorders from this field, the use of P2 receptor antagonists in clinical therapy is still very scarce. In this narrative review, we briefly review the involvement of the purinergic system in immunological responses and clinical studies on the effect of purinergic inhibition on autoimmune processes. We then open the aperture a bit and show some preclinical studies demonstrating a potential effect of purinergic blockade on autoimmune events. Using suramin, a non-specific purinergic inhibitor, as an example, we further show that off-target effects could be responsible for observed effects in immunological settings, which may have interesting implications. Overall, we believe that it is worthwhile to further investigate this hitherto underexplored area.

Astrocytes at the intersection of ageing, obesity, and neurodegeneration

Once considered passive cells of the central nervous system (CNS), glia are now known to actively maintain the CNS parenchyma; in recent years, the evidence for glial functions in CNS physiology and pathophysiology has only grown. Astrocytes, a heterogeneous group of glial cells, play key roles in regulating the metabolic and inflammatory landscape of the CNS and have emerged as potential therapeutic targets for a variety of disorders. This review will outline astrocyte functions in the CNS in healthy ageing, obesity, and neurodegeneration, with a focus on the inflammatory responses and mitochondrial function, and will address therapeutic outlooks.

Does the functional polymorphism-1562C/T of MMP-9 gene influence brain disorders?

Metalloproteinase-9 (MMP-9) is one of the most strongly expressed matrix metalloproteinases (MMPs) in the brain. The MMP-9 activity in the brain is strictly regulated, and any disruptions in this regulation contribute to a development of many disorders of the nervous system including multiple sclerosis, brain strokes, neurodegenerative disorders, brain tumors, schizophrenia, or Guillain-Barré syndrome. This article discusses a relationship between development of the nervous system diseases and the functional single nucleotide polymorphism (SNP) at position -1562C/T within the MMP-9 gene. A pathogenic influence of MMP-9-1562C/T SNP was observed both in neurological and psychiatric disorders. The presence of the allele T often increases the activity of the MMP-9 gene promoter and consequently the expression of MMP-9 when compared to the allele C. This leads to a change in the likelihood of an occurrence of diseases and modifies the course of certain brain diseases in humans, as discussed below. The presented data indicates that the MMP-9-1562C/T functional polymorphism influences the course of many neuropsychiatric disorders in humans suggesting a significant pathological role of the MMP-9 metalloproteinase in pathologies of the human central nervous system.

Potential neurotoxic activity of diverse molecules released by astrocytes

Astrocytes are the main support cells of the central nervous system. They also participate in neuroimmune reactions. In response to pathological and immune stimuli, astrocytes transform to reactive states characterized by increased release of inflammatory mediators. Some of these molecules are neuroprotective and inflammation resolving while others, including reactive oxygen species (ROS), nitric oxide (NO), matrix metalloproteinase (MMP)- 9, L-glutamate, and tumor necrosis factor α (TNF), are well-established toxins known to cause damage to surrounding cells and tissues. We hypothesized that similar to microglia, the brain immune cells, reactive astrocytes can release a broader set of diverse molecules that are potentially neurotoxic. A literature search was conducted to identify such molecules using the following two criteria: 1) evidence of their expression and secretion by astrocytes and 2) direct neurotoxic action. This review describes 14 structurally diverse molecules as less-established astrocyte neurotoxins, including C-X-C motif chemokine ligand (CXCL)10, CXCL12/CXCL12(5-67), FS-7-associated surface antigen ligand (FasL), macrophage inflammatory protein (MIP)- 2α, TNF-related apoptosis inducing ligand (TRAIL), pro-nerve growth factor (proNGF), pro-brain-derived neurotrophic factor (proBDNF), chondroitin sulfate proteoglycans (CSPGs), cathepsin (Cat)B, group IIA secretory phospholipase A₂ (sPLA₂-IIA), amyloid beta peptides (Aβ), high mobility group box (HMGB)1, ceramides, and lipocalin (LCN)2. For some of these molecules, further studies are required to establish either their direct neurotoxic effects or the full spectrum of stimuli that induce their release by astrocytes. Only limited studies with human-derived astrocytes and neurons are available for most of these potential neurotoxins, which is a knowledge gap that should be addressed in the future. We also summarize available evidence of the role these molecules play in select neuropathologies where reactive astrocytes are a key feature. A comprehensive understanding of the full spectrum of neurotoxins released by reactive astrocytes is key to understanding neuroinflammatory diseases characterized by the adverse activation of these cells and may guide the development of novel treatment strategies.

Purinergic receptors modulators: An emerging pharmacological tool for disease management

Purinergic signaling is mediated through extracellular nucleotides (adenosine 5'-triphosphate, uridine-5'-triphosphate, adenosine diphosphate, uridine-5'-diphosphate, and adenosine) that serve as signaling molecules. In the early 1990s, purines and pyrimidine receptors were cloned and characterized drawing the attention of scientists toward this aspect of cellular signaling. This signaling pathway is comprised of four subtypes of adenosine receptors (P1), eight subtypes of G-coupled protein receptors (P2YRs), and seven subtypes of ligand-gated ionotropic receptors (P2XRs). In current studies, the pathophysiology and therapeutic potentials of these receptors have been focused on. Various ligands, modulating the functions of purinergic receptors, are in current clinical practices for the treatment of various neurodegenerative disorders and cardiovascular diseases. Moreover, several purinergic receptors ligands are in advanced phases of clinical trials as a remedy for depression, epilepsy, autism, osteoporosis, atherosclerosis, myocardial infarction, diabetes, irritable bowel syndrome, and cancers. In the present study, agonists and antagonists of purinergic receptors have been summarized that may serve as pharmacological tools for drug design and development.

Naringin Relieves Diabetic Cardiac Autonomic Neuropathy Mediated by P2Y14 Receptor in Superior Cervical Ganglion

Diabetes mellitus (DM), an emerging chronic epidemic, contributes to mortality and morbidity around the world. Diabetic cardiac autonomic neuropathy (DCAN) is one of the most common complications associated with DM. Previous studies have shown that satellite glial cells (SGCs) in the superior cervical ganglia (SCG) play an indispensable role in DCAN progression. In addition, it has been shown that purinergic neurotransmitters, as well as metabotropic GPCRs, are involved in the pathophysiological process of DCAN. Furthermore, one traditional Chinese medicine, naringin may potently alleviate the effects of DCAN. Ferroptosis may be involved in DCAN progression. However, the role of naringin in DCAN as well as its detailed mechanism requires further investigation. In this research, we attempted to identify the effect and relevant mechanism of naringin in DCAN mitigation. We observed that compared with those of normal subjects, there were significantly elevated expression levels of P2Y₁₄ and IL-1β in diabetic rats, both of which were remarkably diminished by treatment with either P2Y₁₄ shRNA or naringin. In addition, abnormalities in blood pressure (BP), heart rate (HR), heart rate variability (HRV), sympathetic nerve discharge (SND), and cardiac structure in the diabetic model can also be partially returned to normal through the use of those treatments. Furthermore, a reduced expression of NRF2 and GPX4, as well as an elevated level of ROS, were detected in diabetic cases, which can also be improved with those treatments. Our results showed that naringin can effectively relieve DCAN mediated by the P2Y₁₄ receptor of SGCs in the SCG. Moreover, the NRF2/GPX4 pathway involved in ferroptosis may become one of the principal mechanisms participating in DCAN progression, which can be modulated by P2Y₁₄-targeted naringin and thus relieve DCAN. Hopefully, our research can supply one novel therapeutic target and provide a brilliant perspective for the treatment of DCAN.

Chemogenetic stimulation of the Gi pathway in astrocytes suppresses neuroinflammation

Engineered G protein-coupled receptors (GPCRs) are commonly used in chemogenetics as designer receptors exclusively activated by designer drugs (DREADDs). Although several GPCRs have been studied in astrocytes using a chemogenetic approach, the functional role of the astrocytic Gi pathway is not clear, as the literature is conflicting depending on the brain regions or behaviors investigated. In this study, we evaluated the role of the astrocytic Gi pathway in neuroinflammation using a Gi -coupled DREADD (hM4Di). Gi -DREADD was expressed in hippocampal astrocytes of a lipopolysaccharide (LPS)-induced neuroinflammation mouse model using adeno-associated viruses. We found that astrocyte Gi -DREADD stimulation using clozapine N-oxide (CNO) inhibits neuroinflammation, as characterized by decreased levels of proinflammatory cytokines, glial activation, and cognitive impairment in mice. Subsequent experiments using primary astrocyte cultures revealed that Gi -DREADD stimulation significantly downregulated LPS-induced expression of Nos2 mRNA and nitric oxide production. Similarly, in vitro calcium imaging showed that activation of the astrocytic Gi pathway attenuated intracellular calcium transients triggered by LPS treatment, suggesting a positive correlation between enhanced calcium transients and the inflammatory phenotype of astrocytes observed in the inflamed brain. Taken together, our results indicate that the astrocytic Gi pathway plays an inhibitory role in neuroinflammation, providing an opportunity to identify potential cellular and molecular targets to control neuroinflammation.

Purinergic Receptors of the Central Nervous System: Biology, PET Ligands, and Their Applications

Purinergic receptors play important roles in central nervous system (CNS). These receptors are involved in cellular neuroinflammatory responses that regulate functions of neurons, microglial and astrocytes. Based on their endogenous ligands, purinergic receptors are classified into P1 or adenosine, P2X and P2Y receptors. During brain injury or under pathological conditions, rapid diffusion of extracellular adenosine triphosphate (ATP) or uridine triphosphate (UTP) from the damaged cells, promote microglial activation that result in the changes in expression of several of these receptors in the brain. Imaging of the purinergic receptors with selective Positron Emission Tomography (PET) radioligands has advanced our understanding of the functional roles of some of these receptors in healthy and diseased brains. In this review, we have accumulated a list of currently available PET radioligands of the purinergic receptors that are used to elucidate the receptor functions and participations in CNS disorders. We have also reviewed receptors lacking radiotracer, laying the foundation for future discoveries of novel PET radioligands to reveal these receptors roles in CNS disorders.

Structure-Activity Relationship of Heterocyclic P2Y14 Receptor Antagonists: Removal of the Zwitterionic Character with Piperidine Bioisosteres

A known zwitterionic, heterocyclic P2Y14R antagonist 3a was substituted with diverse groups on the central phenyl and terminal piperidine moieties, following a computational selection process. The most potent analogues contained an uncharged piperidine bioisostere, prescreened in silico, while an aza-scan (central phenyl ring) reduced P2Y14R affinity. Piperidine amide 11, 3-aminopropynyl 19, and 5-(hydroxymethyl)isoxazol-3-yl) 29 congeners in the triazole series maintained moderate receptor affinity. Adaption of 5-(hydroxymethyl)isoxazol-3-yl gave the most potent naphthalene-containing (32; MRS4654; IC50, 15 nM) and less active phenylamide-containing (33) scaffolds. Thus, a zwitterion was nonessential for receptor binding, and molecular docking and dynamics probed the hydroxymethylisoxazole interaction with extracellular loops. Also, amidomethyl ester prodrugs were explored to reversibly block the conserved carboxylate group to provide neutral analogues, which were cleavable by liver esterase, and in vivo efficacy demonstrated. We have, in stages, converted zwitterionic antagonists into neutral molecules designed to produce potent P2Y14R antagonists for in vivo application.

Design, synthesis and evaluation of 3-amide-5-aryl benzoic acid derivatives as novel P2Y14R antagonists with potential high efficiency against acute gouty arthritis

P2Y₁₄ nucleotide receptor plays important roles in series of physiological and pathologic events especially associated with immune and inflammation. Based on the 3-amide benzoic acid scaffold reported by our group previously, a series of 5-aryl-3-amide benzoic acid derivatives were designed as novel P2Y₁₄ antagonists with improved pharmacokinetic properties. Among which compound 11m showed most potent P2Y₁₄ antagonizing activity with an IC₅₀ value of 2.18 nM, furnishing greatly improved water solubility and bioavailability compared with PPTN. In MSU-induced acute gouty arthritis model in mice, 11m exerted promising in vivo efficacy in alleviating mice paw swelling and inflammatory infiltration. Mechanistically, compound 11m notably blocked pyroptosis of macrophages through inhibiting NLRP3 inflammasome activation. This work may contribute to the identification of potential therapeutic agents to intervene in acute gouty arthritis.

From astrocytes to satellite glial cells and back: A 25 year-long journey through the purinergic modulation of glial functions in pain and more

Fundamental progresses have been made in pain research with a comprehensive understanding of the neuronal pathways which convey painful sensations from the periphery and viscera to the central nervous system and of the descending modulating pathways. Nevertheless, many patients still suffer from various painful conditions, which are often associated to other primary pathologies, and get no or poor relief from available painkillers. Thus, the interest of many researchers has concentrated on new and promising cellular targets and biochemical pathways. This is the case of glia cells, both in the peripheral and in the central nervous system, and of purinergic receptors. Starting from many intuitions and hypotheses raised by Prof. Geoffrey Burnstock, data have accumulated which clearly highlight the fundamental role exerted by several nucleotide and nucleoside receptors in the modulation of glial cell reaction to pain triggers and of their cross-talk with sensory neurons which significantly contributes to the transition from acute to chronic pain. The purinergic system has therefore become an appealing pharmacological target in pain research, also based on the quite unexpected discovery that purines are involved in ancient analgesic techniques such as acupuncture. A more in-depth understanding of the complex and intricated purine-orchestrated scenario in pain conditions will hopefully lead to the identification and clinical development of new and effective analgesics.

Connexin Hemichannel Mimetic Peptide Attenuates Cortical Interneuron Loss and Perineuronal Net Disruption Following Cerebral Ischemia in Near-Term Fetal Sheep

Perinatal hypoxia-ischemia is associated with disruption of cortical gamma-aminobutyric acid (GABA)ergic interneurons and their surrounding perineuronal nets, which may contribute to persisting neurological deficits. Blockade of connexin43 hemichannels using a mimetic peptide can alleviate seizures and injury after hypoxia-ischemia. In this study, we tested the hypothesis that connexin43 hemichannel blockade improves the integrity of cortical interneurons and perineuronal nets. Term-equivalent fetal sheep received 30 min of bilateral carotid artery occlusion, recovery for 90 min, followed by a 25-h intracerebroventricular infusion of vehicle or a mimetic peptide that blocks connexin hemichannels or by a sham ischemia + vehicle infusion. Brain tissues were stained for interneuronal markers or perineuronal nets. Cerebral ischemia was associated with loss of cortical interneurons and perineuronal nets. The mimetic peptide infusion reduced loss of glutamic acid decarboxylase-, calretinin-, and parvalbumin-expressing interneurons and perineuronal nets. The interneuron and perineuronal net densities were negatively correlated with total seizure burden after ischemia. These data suggest that the opening of connexin43 hemichannels after perinatal hypoxia-ischemia causes loss of cortical interneurons and perineuronal nets and that this exacerbates seizures. Connexin43 hemichannel blockade may be an effective strategy to attenuate seizures and may improve long-term neurological outcomes after perinatal hypoxia-ischemia.

Role of UDP-Sugar Receptor P2Y14 in Murine Osteoblasts

The purinergic (P2) receptor P2Y₁₄ is the only P2 receptor that is stimulated by uridine diphosphate (UDP)-sugars and its role in bone formation is unknown. We confirmed P2Y₁₄ expression in primary murine osteoblasts (CB-Ob) and the C2C12-BMP2 osteoblastic cell line (C2-Ob). UDP-glucose (UDPG) had undiscernible effects on cAMP levels, however, induced dose-dependent elevations in the cytosolic free calcium concentration ([Ca²⁺]_i) in CB-Ob, but not C2-Ob cells. To antagonize the P2Y₁₄ function, we used the P2Y₁₄ inhibitor PPTN or generated CRISPR-Cas9-mediated P2Y₁₄ knockout C2-Ob clones (Y14_KO). P2Y₁₄ inhibition facilitated calcium signalling and altered basal cAMP levels in both models of osteoblasts. Importantly, P2Y₁₄ inhibition augmented Ca²⁺ signalling in response to ATP, ADP and mechanical stimulation. P2Y₁₄ knockout or inhibition reduced osteoblast proliferation and decreased ERK1/2 phosphorylation and increased AMPKα phosphorylation. During in vitro osteogenic differentiation, P2Y₁₄ inhibition modulated the timing of osteogenic gene expression, collagen deposition, and mineralization, but did not significantly affect differentiation status by day 28. Of interest, while P2ry14^-/- mice from the International Mouse Phenotyping Consortium were similar to wild-type controls in bone mineral density, their tibia length was significantly increased. We conclude that P2Y₁₄ in osteoblasts reduces cell responsiveness to mechanical stimulation and mechanotransductive signalling and modulates osteoblast differentiation.

A conceptualized model linking matrix metalloproteinase-9 to schizophrenia pathogenesis

Matrix metalloproteinase 9 (MMP-9) is an extracellularly operating zinc-dependent endopeptidase that is commonly expressed in the brain, other tissues. It is synthesized in a latent zymogen form known as pro-MMP-9 that is subsequently converted to the active MMP-9 enzyme following cleavage of the pro-domain. Within the central nervous system, MMP-9 is localized and released from neurons, astrocytes and microglia where its expression levels are modulated by cytokines and growth factors during both normal and pathological conditions as well as by reactive oxygen species generated during oxidative stress. MMP-9 is involved in a number of key neurodevelopmental processes that are thought to be affected in schizophrenia, including maturation of the inhibitory neurons that contain the calcium-binding protein parvalbumin, developmental formation of the specialized extracellular matrix structure perineuronal net, synaptic pruning, and myelination. In this context, the present article provides a narrative synthesis of the existing evidence linking MMP-9 dysregulation to schizophrenia pathogenesis. We start by providing an overview of MMP-9 involvement in brain development and physiology. We then discuss the potential mechanisms through which MMP-9 dysregulation may affect neural circuitry maturation as well as how these anomalies may contribute to the disease process of schizophrenia. We conclude by articulating a comprehensive, cogent, and experimentally testable hypothesis linking MMP-9 to the developmental pathophysiologic cascade that triggers the onset and sustains the chronicity of the illness.

The P2Y14 receptor in the trigeminal ganglion contributes to the maintenance of inflammatory pain

P2Y purinergic receptors expressed in neurons and satellite glial cells (SGCs) of the trigeminal ganglion (TG) contribute to inflammatory and neuropathic pain. P2Y₁₄ receptor expression is reported in the spinal cord, dorsal root ganglion (DRG), and TG. In present study, the role of P2Y₁₄ receptor in the TG in inflammatory orofacial pain of Sprague-Dawley (SD) rats was investigated. Peripheral injection of complete Freund's adjuvant (CFA) induced mechanical hyperalgesia with the rapid upregulation of P2Y₁₄ receptor, glial fibrillary acidic protein (GFAP), interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), C-C chemokine CCL2, phosphorylated extracellular signal-regulated kinase 1/2 (p-ERK1/2), and phosphorylated p38 (p-p38) proteins in the TG. Furthermore, immunofluorescence staining confirmed the CFA-induced upregulation of P2Y₁₄ receptor. Double immunostaining showed that P2Y₁₄ receptor colocalized with glutamine synthetase (GS) and neuronal nuclei (NeuN). Finally, trigeminal injection of a selective antagonist (PPTN) of P2Y₁₄ receptor attenuated CFA-induced mechanical hyperalgesia. PPTN also decreased the upregulation of the GFAP, IL-1β, TNF-α, CCL2, p-ERK1/2, and p-p38 proteins. Our findings showed that P2Y₁₄ receptor in TG may contribute to orofacial inflammatory pain via regulating SGCs activation, releasing cytokines (IL-1β, TNF-α, and CCL2), and phosphorylating ERK1/2 and p38.

P2Y14 receptor is functionally expressed in satellite glial cells and mediates interleukin-1β and chemokine CCL2 secretion

Satellite glial cells (SGCs) activation in the trigeminal ganglia (TG) is critical in various abnormal orofacial sensation in nerve injury and inflammatory conditions. SGCs express several subtypes of P2 purinergic receptors contributing to the initiation and maintenance of neuropathic pain. The P2Y₁₄ receptor, a G-protein-coupled receptor activated by uridine diphosphate (UDP)-glucose and other UDP sugars, mediates various physiologic events such as immune, inflammation, and pain. However, the expression, distribution, and function of P2Y₁₄ receptor in SGCs remains largely unexplored. Our study reported the expression and functional identification of P2Y₁₄ receptor in SGCs. SGCs were isolated from TG of rat, and the P2Y₁₄ receptor expression was examined using immunofluorescence technique. Cell proliferation and viability were examined via cell counting kit-8 experiment. Immunofluorescence demonstrated the presence of P2Y₁₄ receptor in SGCs. Immunofluorescence and western blot showed that UDP-glucose treatment upregulated glial fibrillary acid protein, a common marker for glial activation. Extracellular UDP-glucose enhanced the phosphorylation of extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38, which were both abolished by the P2Y₁₄ receptor inhibitor (PPTN). Furthermore, quantitative reverse transcription polymerase chain reaction and enzyme-linked immunosorbent assay demonstrated that extracellular UDP-glucose significantly enhanced interleukin-1β (IL-1β) and chemokine CCL2 (CCL2) release, which was abolished by PPTN and significantly decreased by inhibitors of MEK/ERK (U0126) and p38 (SB202190). Our findings directly proved the functional presence of P2Y₁₄ receptor in SGCs. It was also verified that P2Y₁₄ receptor activation was involved in activating SGCs, phosphorylating MAPKs, and promoting the secretion of IL-1β and CCL2 via ERK and p38 pathway.

Pharmacology of P2Y receptors

P2Y receptors are G-protein-coupled receptors (GPCRs) for extracellular nucleotides. There are eight mammalian P2Y receptor subtypes divided into two subgroups (P2Y₁, P2Y₂, P2Y₄, P2Y₆, and P2Y₁₁) and (P2Y₁₂, P2Y₁₃, and P2Y₁₄). The P2Y receptors are expressed in various cell types and play important roles in physiology and pathophysiology including inflammatory responses and neuropathic pain. The antagonism of P2Y₁₂ receptors is used in pharmacotherapy for the prevention and therapy of cardiovascular events. The nucleoside analogue ticagrelor and active metabolites of the thienopyridine compounds ticlopidine, clopidogrel and prasugrel inhibit platelet P2Y₁₂ receptors and reduce thereby platelet aggregation. The P2Y₂ receptor agonist diquafosol is used for the treatment of the dry eye syndrome. The P2Y receptor subtypes differ in their amino acid sequences, their pharmacological profiles and their signaling transduction pathways. Recently, selective receptor ligands have been developed for all subtypes. The published crystal structures of the human P2Y₁ and P2Y₁₂ receptors as well as receptor models will facilitate the development of novel drugs for pharmacotherapy.

The role of adenosine and P2Y receptors expressed by multiple cell types in pain transmission

The role of extracellular nucleotides and nucleosides as signaling molecules in cell-to-cell communication has now been clearly established. This is particularly true in the central and peripheral nervous system, where purines and pyrimidines are involved in both physiological and pathological interactions between neurons and surrounding glial cells. It can be thus foreseen that the purinergic system could represent a new potential target for the development of effective analgesics, also through the normalization of neuronal functions and the inhibition of glial cell activation. Research in the last 15 years has progressively confirmed this hypothesis, but no purinergic-based analgesics have reach the market so far; in the present review we have collected the more recent discoveries on the role of G protein-coupled P2Y nucleotide and of adenosine receptors expressed by both neurons and glial cells under painful conditions, and we have highlighted some of the challenges that must be faced to translate basic and preclinical studies to clinics.

Structure-Guided Modification of Heterocyclic Antagonists of the P2Y14 Receptor

The P2Y14 receptor (P2Y14R) mediates inflammatory activity by activating neutrophil motility, but few classes of antagonists are known. We have explored the structure-activity relationship of a 3-(4-phenyl-1 H-1,2,3-triazol-1-yl)-5-(aryl)benzoic acid antagonist scaffold, assisted by docking and molecular dynamics (MD) simulation at a P2Y14R homology model. A computational pipeline using the High Throughput MD Python environment guided the analogue design. Selection of candidates was based upon ligand-protein shape and complementarity and the persistence of ligand-protein interactions over time. Predictions of a favorable substitution of a 5-phenyl group with thiophene and an insertion of a three-methylene spacer between the 5-aromatic and alkyl amino moieties were largely consistent with empirical results. The substitution of a key carboxylate group on the core phenyl ring with tetrazole or truncation of the 5-aryl group reduced affinity. The most potent antagonists, using a fluorescent assay, were a primary 3-aminopropyl congener 20 (MRS4458) and phenyl p-carboxamide 30 (MRS4478).

Structure-Based Design of 3-(4-Aryl-1H-1,2,3-triazol-1-yl)-Biphenyl Derivatives as P2Y14 Receptor Antagonists

UDP and UDP-glucose activate the P2Y14 receptor (P2Y14R) to modulate processes related to inflammation, diabetes, and asthma. A computational pipeline suggested alternatives to naphthalene of a previously reported P2Y14R antagonist (3, PPTN) using docking and molecular dynamics simulations on a hP2Y14R homology model based on P2Y12R structures. By reevaluating the binding of 3 to P2Y14R computationally, two alternatives, i.e., alkynyl and triazolyl derivatives, were identified. Improved synthesis of fluorescent antagonist 4 enabled affinity quantification (IC50s, nM) using flow cytometry of P2Y14R-expressing CHO cells. p-F3C-phenyl-triazole 65 (32) was more potent than a corresponding alkyne 11. Thus, additional triazolyl derivatives were prepared, as guided by docking simulations, with nonpolar aryl substituents favored. Although triazoles were less potent than 3 (6), simpler synthesis facilitated further structural optimization. Additionally, relative P2Y14R affinities agreed with predicted binding of alkynyl and triazole analogues. These triazoles, designed through a structure-based approach, can be assessed in disease models.

Pharmacology and structure of P2Y receptors

P2Y receptors are G-protein-coupled receptors (GPCRs) for extracellular nucleotides. There are eight mammalian P2Y receptor subtypes (P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13, and P2Y14). P2Y receptors are widely expressed and play important roles in physiology and pathophysiology. One important example is the ADP-induced platelet aggregation mediated by P2Y1 and P2Y12 receptors. Active metabolites of the thienopyridine compounds ticlopidine, clopidogrel and prasugrel as well as the nucleoside analogue ticagrelor block P2Y12 receptors and thereby platelet aggregation. These drugs are used for the prevention and therapy of cardiovascular events. Moreover, P2Y receptors play important roles in the nervous system. Adenine nucleotides modulate neuronal activity and neuronal fibre outgrowth by activation of P2Y1 receptors and control migration of microglia by P2Y12 receptors. UDP stimulates microglial phagocytosis through activation of P2Y6 receptors. There is evidence for a role for P2Y2 receptors in Alzheimer's disease pathology. The P2Y receptor subtypes are highly diverse in both their amino acid sequences and their pharmacological profiles. Selective receptor ligands have been developed for the pharmacological characterization of the receptor subtypes. The recently published three-dimensional crystal structures of the human P2Y1 and P2Y12 receptors will facilitate the development of therapeutic agents that selectively target P2Y receptors. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

UDP-Sugars as Extracellular Signaling Molecules: Cellular and Physiologic Consequences of P2Y14 Receptor Activation

UDP-sugars, which are indispensable for protein glycosylation reactions in cellular secretory pathways, also act as important extracellular signaling molecules. We discuss here the broadly expressed P2Y14 receptor, a G-protein-coupled receptor targeted by UDP sugars, and the increasingly diverse set of physiologic responses discovered recently functioning downstream of this receptor in many epithelia as well as in immune, inflammatory, and other cells.

Evidence for the existence of pyrimidinergic transmission in rat brain

The uridine nucleotides uridine-5'-triphosphate (UTP) and uridine-5'-diphosphate (UDP) have previously been identified in media from cultured cells. However, no study to date has demonstrated their presence in brain extracellular fluid (ECF) obtained in vivo. Using a novel method, we now show that UTP and UDP, as well as uridine, are detectable in dialysates of striatal ECF obtained from freely-moving rats. Intraperitoneal (i.p.) administration of uridine or exposure of striatum to depolarizing concentrations of potassium chloride increases extracellular uridine, UTP and UDP, while tetrodotoxin (TTX) decreases their ECF levels. Uridine administration also enhances cholinergic neurotransmission which is accompanied by enhanced brain levels of diacylglycerol (DAG) and inositol trisphosphate (IP3) and blocked by suramin, but not by PPADS (pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid) or MRS2578 suggesting a possible mediation of P2Y2 receptors activated by UTP. These observations suggest that uridine, UTP and UDP may function as pyrimidinergic neurotransmitters, and that enhancement of such neurotransmission underlies pharmacologic effects of exogenous uridine on the brain.

P2Y receptors in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting more than 10% of people over the age of 65. Age is the greatest risk factor for AD, although a combination of genetic, lifestyle and environmental factors also contribute to disease development. Common features of AD are the formation of plaques composed of beta-amyloid peptides (Aβ) and neuronal death in brain regions involved in learning and memory. Although Aβ is neurotoxic, the primary mechanisms by which Aβ affects AD development remain uncertain and controversial. Mouse models overexpressing amyloid precursor protein and Aβ have revealed that Aβ has potent effects on neuroinflammation and cerebral blood flow that contribute to AD progression. Therefore, it is important to consider how endogenous signalling in the brain responds to Aβ and contributes to AD pathology. In recent years, Aβ has been shown to affect ATP release from brain and blood cells and alter the expression of G protein-coupled P2Y receptors that respond to ATP and other nucleotides. Accumulating evidence reveals a prominent role for P2Y receptors in AD pathology, including Aβ production and elimination, neuroinflammation, neuronal function and cerebral blood flow.

The two-hit hypothesis for neuroinflammation: role of exogenous ATP in modulating inflammation in the brain

Brain inflammation is a common occurrence following responses to varied insults such as bacterial infections, stroke, traumatic brain injury and neurodegenerative disorders. A common mediator for these varied inflammatory responses is prostaglandin E₂ (PGE₂), produced by the enzymatic activity of cyclooxygenases (COX) 1 and 2. Previous attempts to reduce neuronal inflammation through COX inhibition, by use of nonsteroidal anti-inflammatory drugs (NSAIDs), have met with limited success. We are proposing the two-hit model for neuronal injury—an initial localized inflammation mediated by PGE₂ (first hit) and the simultaneous release of adenosine triphosphate (ATP) by injured cells (second hit), which significantly enhances the inflammatory response through increased synthesis of PGE₂. Several evidences on the role of exogenous ATP in inflammation have been reported, including contrary instances where extracellular ATP reduces inflammatory events. In this review, we will examine the current literature on the role of P2 receptors, to which ATP binds, in modulating inflammatory reactions during neurodegeneration. Targeting the P2 receptors, therefore, provides a therapeutic alternative to reduce inflammation in the brain. P2 receptor-based anti-inflammatory drugs (PBAIDs) will retain the activities of essential COX enzymes, yet will significantly reduce neuroinflammation by decreasing the enhanced production of PGE₂ by extracellular ATP.