Gi-dependent cell signaling responses of the human P2Y14 receptor in model cell systems

Pharmacological characterization of P2Y receptor subtypes - an update

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). The widely expressed P2Y receptors play important roles in physiology and pathophysiology. This review summarizes the use of pharmacological tools to characterize the P2Y receptor subtypes involved in these responses. MRS2500 is a potent and selective antagonist acting at the P2Y1 receptor. AR-C118925 is useful for the selective antagonism of the P2Y2 receptor. PSB16133 blocks the P2Y4 receptor, MRS2578 is an antagonist at the P2Y6 receptor and NF157 as well as NF340 block the P2Y11 receptor. ADP-induced platelet aggregation is mediated by P2Y1 and P2Y12 receptors. A number of compounds or their active metabolites reduce ADP-induced platelet aggregation by blocking the P2Y12 receptor. These include the active metabolites of the thienopyridine compounds clopidogrel and prasugrel, the nucleoside analogue ticagrelor and the nucleotide analogue cangrelor. PSB0739 is also a potent antagonist at the P2Y12 receptor useful for both in vitro and in vivo studies. MRS2211 and MRS2603 inhibit P2Y13 mediated responses. PPTN is a very potent antagonist at the P2Y14 receptor.

UDP-glucose sensing P2Y14R: A novel target for inflammation

Uridine 5'-diphosphoglucose (UDP-G) as a preferential agonist, but also other UDP-sugars, such as UDP galactose, function as extracellular signaling molecules under conditions of cell injury and apoptosis. Consequently, UDP-G is regarded to function as a damage-associated molecular pattern (DAMP), regulating immune responses. UDP-G promotes neutrophil recruitment, leading to the release of pro-inflammatory chemokines. As a potent endogenous agonist with the highest affinity for the P2Y₁₄ receptor (R), it accomplishes an exclusive relationship between P2Y₁₄Rs in regulating inflammation via cyclic adenosine monophosphate (cAMP), nod-like receptor protein 3 (NLRP3) inflammasome, mitogen-activated protein kinases (MAPKs), and signal transducer and activator of transcription 1 (STAT1) pathways. In this review, we initially present a brief introduction into the expression and function of P2Y₁₄Rs in combination with UDP-G. Subsequently, we summarize emerging roles of UDP-G/P2Y₁₄R signaling pathways that modulate inflammatory responses in diverse systems, and discuss the underlying mechanisms of P2Y₁₄R activation in inflammation-related diseases. Moreover, we also refer to the applications as well as effects of novel agonists/antagonists of P2Y₁₄Rs in inflammatory conditions. In conclusion, due to the role of the P2Y₁₄R in the immune system and inflammatory pathways, it may represent a novel target for anti-inflammatory therapy.

Pharmacological Profile of the Purinergic P2Y Receptors That Modulate, in Response to ADPβS, the Vasodepressor Sensory CGRPergic Outflow in Pithed Rats

Calcitonin gene-related peptide (CGRP), an endogenous neuropeptide released from perivascular sensory nerves, exerts a powerful vasodilatation. Interestingly, adenosine triphosphate (ATP) stimulates the release of CGRP by activation of prejunctional P2X_2/3 receptors, and adenosine 5'-O-2-thiodiphosphate (ADPβS), a stable adenosine diphosphate (ADP) analogue, produces vasodilator/vasodepressor responses by endothelial P2Y₁ receptors. Since the role of ADP in the prejunctional modulation of the vasodepressor sensory CGRPergic drive and the receptors involved remain unknown, this study investigated whether ADPβS inhibits this CGRPergic drive. Accordingly, 132 male Wistar rats were pithed and subsequently divided into two sets. In set 1, ADPβS (5.6 and 10 µg/kg·min) inhibited the vasodepressor CGRPergic responses by electrical stimulation of the spinal T₉-T₁₂ segment. This inhibition by ADPβS (5.6 µg/kg·min) was reverted after i.v. administration of the purinergic antagonists MRS2500 (300 µg/kg; P2Y₁) or MRS2211 (3000 µg/kg; P2Y₁₃), but not by PSB0739 (300 µg/kg; P2Y₁₂), MRS2211 (1000 µg/kg; P2Y₁₃) or the K_ATP blocker glibenclamide (20 mg/kg). In set 2, ADPβS (5.6 µg/kg·min) failed to modify the vasodepressor responses to exogenous α-CGRP. These results suggest that ADPβS inhibits CGRP release in perivascular sensory nerves. This inhibition, apparently unrelated to activation of ATP-sensitive K⁺ channels, involves P2Y₁ and probably P2Y₁₃, but not P2Y₁₂ receptors.

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.

Immunomodulation by foods and microbes: Unravelling the molecular tango

Metabolic health and immune function are intimately connected via diet and the microbiota. Nearly 90% of all immune cells in the body are associated with the gastrointestinal tract and these immune cells are continuously exposed to a wide range of microbes and microbial-derived compounds, with important systemic ramifications. Microbial dysbiosis has consistently been observed in patients with atopic dermatitis, food allergy and asthma and the molecular mechanisms linking changes in microbial populations with disease risk and disease endotypes are being intensively investigated. The discovery of novel bacterial metabolites that impact immune function is at the forefront of host-microbe research. Co-evolution of microbial communities within their hosts has resulted in intertwined metabolic pathways that affect physiological and pathological processes. However, recent dietary and lifestyle changes are thought to negatively influence interactions between microbes and their host. This review provides an overview of some of the critical metabolite-receptor interactions that have been recently described, which may underpin the immunomodulatory effects of the microbiota, and are of relevance for allergy, asthma and infectious diseases.

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.

GPR105-Targeted Therapy Promotes Gout Resolution as a Switch Between NETosis and Apoptosis of Neutrophils

The fate of infiltrating neutrophils in inflamed joints determines the development of acute gouty arthritis (AGA). GPR105 highly expressed in human neutrophils is sensitive to monosodium urate crystals (MSU); nevertheless, the roles of GPR105 in AGA remain unclear. Here, we show that GPR105 is significantly upregulated in peripheral polymorphonuclear neutrophils of AGA patients. GPR105 knockout (GPR105^−/−) prevented NETosis and induced apoptosis of neutrophils under MSU exposure, as well as attenuating inflammatory cascades in AGA. Mechanistically, GPR105 deletion activated cAMP-PKA signals, thereby disrupting Raf-Mek1/2-Erk1/2 pathway-mediated NADPH oxidase activation, contributing to inhibition of NETosis. Whereas, cAMP-PKA activation resulting in GPR105 deficiency modulated PI3K-Akt pathway to regulate apoptosis. More importantly, suppression of cAMP-PKA pathway by SQ22536 and H-89 restored NETosis instead of apoptosis in GPR105^−/− neutrophils, promoting MSU-induced gout flares. Interestingly, lobetyolin was screened out as a potent GPR105 antagonist using molecular docking-based virtual screening and in vitro activity test, which efficiently attenuated MSU-induced inflammatory response interacting with GPR105. Taken together, our study implicated that modulating cell death patterns between NETosis and apoptosis through targeting GPR105 could be a potential therapeutic strategy for the treatment of AGA.

The purinergic P2Y14 receptor links hepatocyte death to hepatic stellate cell activation and fibrogenesis in the liver

Fibrosis contributes to ~45% of deaths in western countries. In chronic liver disease, fibrosis is a major factor determining outcomes, but efficient antifibrotic therapies are lacking. Although platelet-derived growth factor and transforming growth factor-β constitute key fibrogenic mediators, they do not account for the well-established link between cell death and fibrosis in the liver. Here, we hypothesized that damage-associated molecular patterns (DAMPs) may link epithelial cell death to fibrogenesis in the injured liver. DAMP receptor screening identified purinergic receptor P2Y14 among several candidates as highly enriched in hepatic stellate cells (HSCs), the main fibrogenic cell type of the liver. Conversely, P2Y14 ligands uridine 5'-diphosphate (UDP)-glucose and UDP-galactose were enriched in hepatocytes and were released upon different modes of cell death. Accordingly, ligand-receptor interaction analysis that combined proteomic and single-cell RNA sequencing data revealed P2Y14 ligands and P2Y14 receptor as a link between dying cells and HSCs, respectively. Treatment with P2Y14 ligands or coculture with dying hepatocytes promoted HSC activation in a P2Y14-dependent manner. P2Y14 ligands activated extracellular signal-regulated kinase (ERK) and Yes-associated protein (YAP) signaling in HSCs, resulting in ERK-dependent HSC activation. Global and HSC-selective P2Y14 deficiency attenuated liver fibrosis in multiple mouse models of liver injury. Functional expression of P2Y14 was confirmed in healthy and diseased human liver and human HSCs. In conclusion, P2Y14 ligands and their receptor constitute a profibrogenic DAMP pathway that directly links cell death to fibrogenesis.

Discovery and computational studies of 2-phenyl-benzoxazole acetamide derivatives as promising P2Y14R antagonists with anti-gout potential

The P2Y₁₄ nucleotide receptor, a subtype of P2Y receptors, is implicated in many human inflammatory diseases. Based on the identification of favorable residues of two screening hits in the almost symmetrical P2Y₁₄ binding domain, we describe the structural optimization of previously identified virtual screening hits 6 and 7 that result in the development of P2Y₁₄R antagonists with a novel 2-phenyl-benzoxazole acetamide chemical scaffold. Notably, compound 52 showed potent P2Y₁₄R antagonistic activity (IC₅₀ = 2 nM), and a stronger inhibitory effect on MSU-induced inflammatory in vitro, better than a previously described P2Y₁₄R antagonist PPTN. In vivo evaluation demonstrated that compound 52 also had satisfactory inhibitory activity on the inflammatory response of gout flares in mice. Moreover, P2Y₁₄R antagonist 52 decreased paw swelling and inflammatory cell infiltration through cAMP/NLRP3/GSDMD signaling pathways in MSU-induced acute gouty arthritis mice. The discussions on the binding mechanism that employ MM/GBSA free energy calculations/decompositions also provide some useful clues for further structural designing of compound 52. Taken together, 2-phenyl-benzoxazole acetamide derivative 52 with potent P2Y₁₄R antagonistic activity and in vivo potency could be a promising strategy for gout therapy and deserves further optimization.

Novel Non-Opioid Based Therapeutics for Chronic Neuropathic Pain

Chronic neuropathic pain is currently a major health issue in U.S. complicated by the lack of non-opioid analgesic alternatives. Our investigations led to the discovery of major signaling pathways involved in the transition of acute to chronic neuropathic pain and the identification of several targets for therapeutic intervention. Our translational approach has facilitated the advancement of novel medicines for chronic neuropathic pain that are in advanced clinical development and clinical trials.

Landscape of GPCR expression along the mouse nephron

Kidney transport and other renal functions are regulated by multiple G protein-coupled receptors (GPCRs) expressed along the renal tubule. The rapid, recent appearance of comprehensive unbiased gene expression data in the various renal tubule segments, chiefly RNA sequencing and protein mass spectrometry data, has provided a means of identifying patterns of GPCR expression along the renal tubule. To allow for comprehensive mapping, we first curated a comprehensive list of GPCRs in the genomes of mice, rats, and humans (https://hpcwebapps.cit.nih.gov/ESBL/Database/GPCRs/) using multiple online data sources. We used this list to mine segment-specific and cell type-specific expression data from RNA-sequencing studies in microdissected mouse tubule segments to identify GPCRs that are selectively expressed in discrete tubule segments. Comparisons of these mapped mouse GPCRs with other omics datasets as well as functional data from isolated perfused tubule and micropuncture studies confirmed patterns of expression for well-known receptors and identified poorly studied GPCRs that are likely to play roles in the regulation of renal tubule function. Thus, we provide data resources for GPCR expression across the renal tubule, highlighting both well-known GPCRs and understudied receptors to provide guidance for future studies.

P2Y14 Receptor as a Target for Neutrophilia Attenuation in Severe COVID-19 Cases: From Hematopoietic Stem Cell Recruitment and Chemotaxis to Thrombo-inflammation

The global SARS-CoV-2 pandemic starting in 2019 has already reached more than 2.3 million deaths. Despite the scientific community’s efforts to investigate the COVID-19 disease, a drug for effectively treating or curing patients yet needs to be discovered. Hematopoietic stem cells (HSC) differentiating into immune cells for defense express COVID-19 entry receptors, and COVID-19 infection hinders their differentiation. The importance of purinergic signaling in HSC differentiation and innate immunity has been recognized. The metabotropic P2Y14 receptor subtype, activated by UDP-glucose, controls HSC differentiation and mobilization. Thereon, the exacerbated activation of blood immune cells amplifies the inflammatory state observed in COVID-19 patients, specially through the continuous release of reactive oxygen species and extracellular neutrophil traps (NETs). Further, the P2Y14 subtype, robustly inhibits the infiltration of neutrophils into various epithelial tissues, including lungs and kidneys. Here we discuss findings suggesting that antagonism of the P2Y14 receptor could prevent the progression of COVID-19-induced systemic inflammation, which often leads to severe illness and death cases. Considering the modulation of neutrophil recruitment of extreme relevance for respiratory distress and lung failure prevention, we propose that P2Y14 receptor inhibition by its selective antagonist PPTN could limit neutrophil recruitment and NETosis, hence limiting excessive formation of oxygen reactive species and proteolytic activation of the kallikrein-kinin system and subsequent bradykinin storm in the alveolar septa of COVID-19 patients.

P2Y14 Receptor Antagonists Reverse Chronic Neuropathic Pain in a Mouse Model

Eight P2Y₁₄R antagonists, including three newly synthesized analogues, containing a naphthalene or phenyl-triazolyl scaffold were compared in a mouse model of chronic neuropathic pain (sciatic constriction). P2Y₁₄R antagonists rapidly (≤30 min) reversed mechano-allodynia, with maximal effects typically within 1 h after injection. Two analogues (4-[4-(4-piperidinyl)phenyl]-7-[4-(trifluoromethyl)phenyl]-2-naphthalenecarboxylic acid 1 and N-acetyl analogue 4, 10 μmol/kg, i.p.) achieved complete pain reversal (100%) at 1 to 2 h, with relief evident up to 5 h for 4 (41%). A reversed triazole analogue 7 reached 87% maximal protection. Receptor affinity was determined using a fluorescent antagonist binding assay, indicating similar mouse and human P2Y₁₄R affinity. The mP2Y₁₄R affinity was only partially predictive of in vivo efficacy, suggesting the influence of pharmacokinetic factors. Thus P2Y₁₄R is a potential therapeutic target for treating chronic pain.

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.

Cross-Talk in Nucleotide Signaling in Glioma C6 Cells

The chapter is focused on the mechanism of action of metabotropic P2Y nucleotide receptors: P2Y₁, P2Y₂, P2Y₁₂, P2Y₁₄ and the ionotropic P2X₇ receptor in glioma C6 cells. P2Y₁ and P2Y₁₂ both respond to ADP, but while P2Y₁ links to PLC and elevates cytosolic Ca²⁺ concentration, P2Y₁₂ negatively couples to adenylate cyclase, maintaining cAMP at low level. In glioma C6, these two P2Y receptors modulate activities of ERK1/2 and PI3K/Akt signaling and the effects depend on physiological conditions of the cells. During prolonged serum deprivation, cell growth is arrested, the expression of the P2Y₁ receptor strongly decreases and P2Y₁₂ becomes a major player responsible for ADP-evoked signal transduction. The P2Y₁₂ receptor activates ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulates Akt activity, contributing to glioma invasiveness. In contrast, P2Y₁ has an inhibitory effect on Akt pathway signaling. Furthermore, the P2X₇ receptor, often responsible for apoptotic fate, is not involved in Ca²⁺elevation in C6 cells. The shift in nucleotide receptor expression from P2Y₁ to P2Y₁₂ during serum withdrawal, the cross talk between both receptors and the lack of P2X₇ activity shows the precise self-regulating mechanism, enhancing survival and preserving the neoplastic features of C6 cells.

The role of purinergic P2Y12 and P2Y13 receptors in ADPβS-induced inhibition of the cardioaccelerator sympathetic drive in pithed rats

ATP is a cotransmitter released with other neurotransmitters from sympathetic nerves, where it stimulates purinergic receptors. Purinergic adenosine P₁ receptors (coupled to G_i/o proteins) produce sympatho-inhibition in several autonomic effectors by prejunctional inhibition of neurotransmitter release. Similarly, signalling through P2Y₁₂ and P2Y₁₃ receptors coupled to G_i/o proteins is initiated by the ATP breakdown product ADP. Hence, this study has pharmacologically investigated a possible role of ADP-induced inhibition of the cardioaccelerator sympathetic drive in pithed rats, using a stable ADP analogue (ADPβS) and selective antagonists for the purinergic P2Y₁, P2Y₁₂ and P2Y₁₃ receptors. Accordingly, male Wistar rats were pithed and: (i) pretreated i.v. with gallamine (25 mg/kg) and desipramine (50 μg/kg) for preganglionic spinal (C₇-T₁) stimulation of the cardioaccelerator sympathetic drive (n = 78); or (ii) prepared for receiving i.v. injections of exogenous noradrenaline (n = 12). The i.v. continuous infusions of ADPβS (10 and 30 μg/kg/min) dose-dependently inhibited the tachycardic responses to electrical sympathetic stimulation, but not those to exogenous noradrenaline. The cardiac sympatho-inhibition produced by 30 μg/kg/min ADPβS was (after i.v. administration of compounds) (i) unchanged by 1-ml/kg bidistilled water or 300-μg/kg MRS 2500 (P2Y₁ receptor antagonist), (ii) abolished by 300-μg/kg PSB 0739 (P2Y₁₂ receptor antagonist) and (iii) partially blocked by 3000-μg/kg MRS 2211 (P2Y₁₃ receptor antagonist). Our results suggest that ADPβS induces a cardiac sympatho-inhibition that mainly involves the P2Y₁₂ receptor subtype and, probably to a lesser extent, the P2Y₁₃ receptor subtype. These receptors may represent therapeutic targets for treating cardiovascular pathologies, including stroke and myocardial infarctions.

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.

Tools and drugs for uracil nucleotide-activated P2Y receptors

P2Y receptors (P2YRs) are a family of G protein-coupled receptors activated by extracellular nucleotides. Physiological P2YR agonists include purine and pyrimidine nucleoside di- and triphosphates, such as ATP, ADP, UTP, UDP, nucleotide sugars, and dinucleotides. Eight subtypes exist, P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₂, P2Y₁₃, and P2Y₁₄, which represent current or potential future drug targets. Here we provide a comprehensive overview of ligands for the subgroup of the P2YR family that is activated by uracil nucleotides: P2Y₂ (UTP, also ATP and dinucleotides), P2Y₄ (UTP), P2Y₆ (UDP), and P2Y₁₄ (UDP, UDP-glucose, UDP-galactose). The physiological agonists are metabolically unstable due to their fast hydrolysis by ectonucleotidases. A number of agonists with increased potency, subtype-selectivity and/or enzymatic stability have been developed in recent years. Useful P2Y₂R agonists include MRS2698 (6-01, highly selective) and PSB-1114 (6-05, increased metabolic stability). A potent and selective P2Y₂R antagonist is AR-C118925 (10-01). For studies of the P2Y₄R, MRS4062 (3-15) may be used as a selective agonist, while PSB-16133 (10-06) is a selective antagonist. Several potent P2Y₆R agonists have been developed including 5-methoxyuridine 5'-O-((R_p)α-boranodiphosphate) (6-12), PSB-0474 (3-11), and MRS2693 (3-26). The isocyanate MRS2578 (10-08) is used as a selective P2Y₆R antagonist, although its reactivity and low water-solubility are limiting. With MRS2905 (6-08), a potent and metabolically stable P2Y₁₄R agonist is available, while PPTN (10-14) represents a potent and selective P2Y₁₄R antagonist. The radioligand [³H]UDP can be used to label P2Y₁₄Rs. In addition, several fluorescent probes have been developed. Uracil nucleotide-activated P2YRs show great potential as drug targets, especially in inflammation, cancer, cardiovascular and neurodegenerative diseases.

Nucleoside 5'-O-monophosphorothioates as modulators of the P2Y14 receptor and mast cell degranulation

Mast cells (MCs) are long-lived resident cells known for their substantial role in antigen-induced anaphylaxis and other immunoglobulin E-mediated allergic reactions as well as tumor promotion. MCs' activation results in the release of pro-inflammatory factors such as histamine, tryptase, tumor necrosis factor or carboxypeptidase A stored in secretory granules. IgE-dependent hypersensitivity has been thought to be the major pathway mediating degranulation of mast cells, but the P2Y14 nucleotide receptor activated by UDP-glucose (UDPG) may also enhance this process. In this study we identified thymidine 5'-O-monophosphorothioate (TMPS) as a molecule inhibiting UDPG-induced degranulation in a rat mast cell line (RBL-2H3). Additionally, TMPS diminished UDPG-evoked intracellular calcium mobilization in a stable HEK293T cell line overexpressing the P2Y14 receptor. Therefore, we demonstrate that the use of thymidine 5'-O-monophosphorothioate might be a novel anti-inflammatory approach based on preventingmast cell activation.

Human Keratinocytes Respond to Extracellular UTP by Induction of Hyaluronan Synthase 2 Expression and Increased Hyaluronan Synthesis

The release of nucleotides into extracellular space is triggered by insults like wounding and ultraviolet radiation, resulting in stimulatory or inhibitory signals via plasma membrane nucleotide receptors. As similar insults are known to activate hyaluronan synthesis we explored the possibility that extracellular UTP or its breakdown products UDP and UMP act as mediators for hyaluronan synthase (HAS) activation in human epidermal keratinocytes. UTP increased hyaluronan both in the pericellular matrix and in the culture medium of HaCaT cells. 10-100 μm UTP strongly up-regulated HAS2 expression, although the other hyaluronan synthases (HAS1, HAS3) and hyaluronidases (HYAL1, HYAL2) were not affected. The HAS2 response was rapid and transient, with the maximum stimulation at 1.5 h. UDP exerted a similar effect, but higher concentrations were required for the response, and UMP showed no stimulation at all. Specific siRNAs against the UTP receptor P2Y₂, and inhibitors of UDP receptors P2Y₆ and P2Y₁₄, indicated that the response to UTP was mediated mainly through P2Y₂ and to a lesser extent via UDP receptors. UTP increased the phosphorylation of p38, ERK, CREB, and Ser-727 of STAT3 and induced nuclear translocation of pCaMKII. Inhibitors of PKC, p38, ERK, CaMKII, STAT3, and CREB partially blocked the activation of HAS2 expression, confirming the involvement of these pathways in the UTP-induced HAS2 response. The present data reveal a selective up-regulation of HAS2 expression by extracellular UTP, which is likely to contribute to the previously reported rapid activation of hyaluronan metabolism in response to tissue trauma or ultraviolet radiation.

UDP-Glucose: A Potential Signaling Molecule in Plants?

This perspective paper focuses on the most recent results suggesting a potential role for UDP-Glucose as a signaling molecule in plants. In animals, UDP-Glucose is well-established as an extracellular signaling molecule that is sensed by G-protein coupled receptors, activating several downstream defense mechanisms. Recent studies have shown that abnormal growth occurred in both vegetative and reproductive tissue of plants with reduced UDP-Glucose levels, and this could be rescued by exogenous UDP-Glucose. In plants with increased biomass accumulation, the genes involved in UDP-Glucose production were up-regulated. However, excessive endogenous accumulation of UDP-Glucose induced programmed cell death (PCD), and this could also be obtained by exogenous UDP-Glucose application. Plants with decreased UDP-glucose were insensitive to pathogen induced PCD. We speculate that UDP-Glucose acts as an extracellular signaling molecule in plants, and that it may be perceived as a damage-associated molecular pattern.

UDP-glucose promotes neutrophil recruitment in the lung

In addition to their role in glycosylation reactions, UDP-sugars are released from cells and activate widely distributed cell surface P2Y₁₄ receptors (P2Y₁₄R). However, the physiological/pathophysiological consequences of UDP-sugar release are incompletely defined. Here, we report that UDP-glucose levels are abnormally elevated in lung secretions from patients with cystic fibrosis (CF) as well as in a mouse model of CF-like disease, the βENaC transgenic (Tg) mouse. Instillation of UDP-glucose into wild-type mouse tracheas resulted in enhanced neutrophil lung recruitment, and this effect was nearly abolished when UDP-glucose was co-instilled with the P2Y₁₄R antagonist PPTN [4-(piperidin-4-yl)-phenyl)-7-(4-(trifluoromethyl)-phenyl-2-naphthoic acid]. Importantly, administration of PPTN to βENaC-Tg mice reduced neutrophil lung inflammation. These results suggest that UDP-glucose released into the airways acts as a local mediator of neutrophil inflammation.

Geniposide attenuates inflammatory response by suppressing P2Y14 receptor and downstream ERK1/2 signaling pathway in oxygen and glucose deprivation-induced brain microvascular endothelial cells

ETHNOPHARMACOLOGICAL RELEVANCE

Fructus gardenia is widely used for treatment of stroke and infectious diseases in Chinese medicine. Geniposide is the key bioactive compound related to the pharmacodynamic actions of gardenia on ischemic stroke. The molecular mechanism by which geniposide improves the ischemic brain injury was observed in the study.

AIM OF THE STUDY

Recent studies showed that geniposide had protective activities against the inflammatory response in ischemic stroke. However, the molecular mechanism of geniposide anti-inflammatory role has not yet been fully elucidated. In this study, we investigated the effect of geniposide on the expression of P2Y14 receptor and downstream signaling pathway in brain microvascular endothelial cells (BMECs).

MATERIALS AND METHODS

An in vitro model of cerebral ischemia in BMECs was established by oxygen-glucose-deprivation (OGD). To further confirm the specific effect of geniposide on P2Y14 receptor and downstream signaling pathways, we set up a UDP-glucose (an agonist of the P2Y14 receptor) stimulated model. After administration of geniposide, the expression of P2Y14 receptor, phosphorylation of RAF-1, mitogen activated protein kinase kinase1/2 (MEK1/2), extracellular signal-regulated kinase 1/2 (ERK1/2), level of interleukin-8 (IL-8), interleukin-1β (IL-1β), monocyte chemotactic protein 1 (MCP-1) in BMECs were determined.

RESULTS

The mRNA and protein expression of P2Y14 in the rat BMECs were up-regulated in OGD-induced injury. After administration of Geniposide, the expression of P2Y14 receptor was significantly down-regulated, the phosphorylation of RAF-1, MEK1/2, ERK1/2 were suppressed. Similar data were obtained in UDP-glc stimulated model. We also observed that geniposide markedly declined the production of IL-8, IL-1β and MCP-1 in OGD-induced BMECs.

CONCLUSION

Geniposide exerted anti-inflammatory effects by interfering with the expression of P2Y14 receptor, which subsequently inhibits the downstream ERK1/2 signaling pathways and the release of the pro-inflammatory cytokines IL-8, MCP-1, IL-1β. Therefore, this study provides the evidence for gardenia's clinical application in cerebral ischemia.

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'.

Purinergic receptors in tubulointerstitial inflammatory cells: a pathophysiological mechanism of salt-sensitive hypertension

Recent studies have suggested that both the tubulointerstitial inflammatory cells and the activation of purinergic receptors integrate common mechanisms that result in salt-sensitive hypertension. The basis of this hypothesis is that renal endothelial cells release ATP in response to shear stress in the setting of hypertension. It has been demonstrated that the over-expression and activation of the P2X7, P2Y12 and P2X1 receptors favour the elevation of blood pressure induced by high-salt intake. In addition, the release of interleukins and inflammatory mediators in the tubulointerstitial area appears to be related to the activation of these receptors. Renal vasoconstriction and tubulointerstitial injury develop as a result, which increase sodium reabsorption by epithelial cells. Consistent with these effects, the reduction of tubulointerstitial inflammation caused by immunosuppressants, such as mycophenolate mofetil, prevents the development of salt-sensitive hypertension. Also, P2X7-receptor knockout mice develop minor renal injury when hypertension is induced via the administration of deoxycorticosterone acetate and a high-salt diet. In the setting of angiotensin II-induced hypertension, which is an early stage in the development of salt-sensitive hypertension, an acute blockade with the specific, non-selective P2 antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid prevented the renal vasoconstriction induced by angiotensin II. In addition, it normalized glomerular haemodynamics and restored sodium excretion to control values. These findings suggest that chronic administration of P2 purinergic antagonists may prevent the deleterious effects of purinergic receptors during the development of salt-sensitive hypertension.

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.

Renal intercalated cells sense and mediate inflammation via the P2Y14 receptor

Uncontrolled inflammation is one of the leading causes of kidney failure. Pro-inflammatory responses can occur in the absence of infection, a process called sterile inflammation. Here we show that the purinergic receptor P2Y14 (GPR105) is specifically and highly expressed in collecting duct intercalated cells (ICs) and mediates sterile inflammation in the kidney. P2Y14 is activated by UDP-glucose, a damage-associated molecular pattern molecule (DAMP) released by injured cells. We found that UDP-glucose increases pro-inflammatory chemokine expression in ICs as well as MDCK-C11 cells, and UDP-glucose activates the MEK1/2-ERK1/2 pathway in MDCK-C11 cells. These effects were prevented following inhibition of P2Y14 with the small molecule PPTN. Tail vein injection of mice with UDP-glucose induced the recruitment of neutrophils to the renal medulla. This study identifies ICs as novel sensors, mediators and effectors of inflammation in the kidney via P2Y14.

Exploring a 2-naphthoic acid template for the structure-based design of P2Y14 receptor antagonist molecular probes

The P2Y₁₄ receptor (P2Y₁₄R), one of eight P2Y G protein-coupled receptors (GPCR), is involved in inflammatory, endocrine, and hypoxic processes and is an attractive pharmaceutical target. The goal of this research is to develop high-affinity P2Y₁₄R fluorescent probes based on the potent and highly selective antagonist 4-(4-(piperidin-4-yl)-phenyl)-7-(4-(trifluoromethyl)-phenyl)-2-naphthoic acid (6, PPTN). A model of hP2Y₁₄R based on recent hP2Y₁₂R X-ray structures together with simulated antagonist docking suggested that the piperidine ring is suitable for fluorophore conjugation while preserving affinity. Chain-elongated alkynyl or amino derivatives of 6 for click or amide coupling were synthesized, and their antagonist activities were measured in hP2Y₁₄R-expressing CHO cells. Moreover, a new Alexa Fluor 488 (AF488) containing derivative 30 (MRS4174, K_i = 80 pM) exhibited exceptionally high affinity, as compared to 13 nM for the alkyne precursor 22. A flow cytometry assay employing 30 as a fluorescent probe was used to quantify specific binding to P2Y₁₄R. Known P2Y receptor ligands inhibited binding of 30 with properties consistent with their previously established receptor selectivities and affinities. These results illustrate that potency in this series of 2-naphthoic acid derivatives can be preserved by chain functionalization, leading to highly potent fluorescent molecular probes for P2Y₁₄R. Such conjugates will be useful tools in expanding the SAR of this receptor, which still lacks chemical diversity in its collective ligands. This approach demonstrates the predictive power of GPCR homology modeling and the relevance of newly determined X-ray structures to GPCR medicinal chemistry.

Expression and characterization of purinergic receptors in rat middle meningeal artery-potential role in migraine

The dura mater and its vasculature have for decades been central in the hypothesis of migraine and headache pathophysiology. Although recent studies have questioned the role of the vasculature as the primary cause, dural vessel physiology is still relevant in understanding the complex pathophysiology of migraine. The aim of the present study was to isolate the middle meningeal artery (MMA) from rodents and characterize their purinergic receptors using a sensitive wire myograph method and RT-PCR. The data presented herein suggest that blood flow through the MMA is, at least in part, regulated by purinergic receptors. P2X1 and P2Y6 receptors are the strongest contractile receptors and, surprisingly, ADPβS caused contraction most likely via P2Y1 or P2Y13 receptors, which is not observed in other arteries. Adenosine addition, however, caused relaxation of the MMA. The adenosine relaxation could be inhibited by SCH58261 (A2A receptor antagonist) and caffeine (adenosine receptor antagonist). This gives one putative molecular mechanism for the effect of caffeine, often used as an adjuvant remedy of cranial pain. Semi-quantitative RT-PCR expression data for the receptors correlate well with the functional findings. Together these observations could be used as targets for future understanding of the in vivo role of purinergic receptors in the MMA.

Purinergic P2Y₁₄ receptor modulates stress-induced hematopoietic stem/progenitor cell senescence

Purinergic receptors of the P2Y family are G protein-coupled surface receptors that respond to extracellular nucleotides and can mediate responses to local cell damage. P2Y-dependent signaling contributes to thrombotic and/or inflammatory consequences of tissue injury by altering platelet and endothelial activation and immune cell phagocytosis. Here, we have demonstrated that P2Y14 modifies cell senescence and cell death in response to tissue stress, thereby enabling preservation of hematopoietic stem/progenitor cell function. In mice, P2Y14 deficiency had no demonstrable effect under homeostatic conditions; however, radiation stress, aging, sequential exposure to chemotherapy, and serial bone marrow transplantation increased senescence in animals lacking P2Y14. Enhanced senescence coincided with increased ROS, elevated p16(INK4a) expression, and hypophosphorylated Rb and was inhibited by treatment with a ROS scavenger or inhibition of p38/MAPK and JNK. Treatment of WT cells with pertussis toxin recapitulated the P2Y14 phenotype, suggesting that P2Y14 mediates antisenescence effects through Gi/o protein-dependent pathways. Primitive hematopoietic cells lacking P2Y14 were compromised in their ability to restore hematopoiesis in irradiated mice. Together, these data indicate that P2Y14 on stem/progenitor cells of the hematopoietic system inhibits cell senescence by monitoring and responding to the extracellular manifestations of tissue stress and suggest that P2Y14-mediated responses prevent the premature decline of regenerative capacity after injury.

Characterization of the contractile P2Y14 receptor in mouse coronary and cerebral arteries

Extracellular UDP-glucose can activate the purinergic P2Y14 receptor. The aim of the present study was to examine the physiological importance of P2Y14 receptors in the vasculature. The data presented herein show that UDP-glucose causes contraction in mouse coronary and basilar arteries. The EC50 values and immunohistochemistry illustrated the strongest P2Y14 receptor expression in the basilar artery. In the presence of pertussis toxin, UDP-glucose inhibited contraction in coronary arteries and in the basilar artery it surprisingly caused relaxation. After organ culture of the coronary artery, the EC50 value decreased and an increased staining for the P2Y14 receptor was observed, showing receptor plasticity.

Extracellular UDP-glucose activates P2Y14 Receptor and Induces Signal Transducer and Activator of Transcription 3 (STAT3) Tyr705 phosphorylation and binding to hyaluronan synthase 2 (HAS2) promoter, stimulating hyaluronan synthesis of keratinocytes

Hyaluronan, a major matrix molecule in epidermis, is often increased by stimuli that enhance keratinocyte proliferation and migration. We found that small amounts of UDP-sugars were released from keratinocytes and that UDP-glucose (UDP-Glc) added into keratinocyte cultures induced a specific, rapid induction of hyaluronan synthase 2 (HAS2), and an increase of hyaluronan synthesis. The up-regulation of HAS2 was associated with JAK2 and ERK1/2 activation, and specific Tyr(705) phosphorylation of transcription factor STAT3. Inhibition of JAK2, STAT3, or Gi-coupled receptors blocked the induction of HAS2 expression by UDP-Glc, the latter inhibitor suggesting that the signaling was triggered by the UDP-sugar receptor P2Y14. Chromatin immunoprecipitations demonstrated increased promoter binding of Tyr(P)(705)-STAT3 at the time of HAS2 induction. Interestingly, at the same time Ser(P)(727)-STAT3 binding to its response element regions in the HAS2 promoter was unchanged or decreased. UDP-Glc also stimulated keratinocyte migration, proliferation, and IL-8 expression, supporting a notion that UDP-Glc signals for epidermal inflammation, enhanced hyaluronan synthesis as an integral part of it.

A selective high-affinity antagonist of the P2Y14 receptor inhibits UDP-glucose-stimulated chemotaxis of human neutrophils

The nucleotide-sugar-activated P2Y14 receptor (P2Y14-R) is highly expressed in hematopoietic cells. Although the physiologic functions of this receptor remain undefined, it has been strongly implicated recently in immune and inflammatory responses. Lack of availability of receptor-selective high-affinity antagonists has impeded progress in studies of this and most of the eight nucleotide-activated P2Y receptors. A series of molecules recently were identified by Gauthier et al. (Gauthier et al., 2011) that exhibited antagonist activity at the P2Y14-R. We synthesized one of these molecules, a 4,7-disubstituted 2-naphthoic acid derivative (PPTN), and studied its pharmacological properties in detail. The concentration-effect curve of UDP-glucose for promoting inhibition of adenylyl cyclase in C6 glioma cells stably expressing the P2Y14-R was shifted to the right in a concentration-dependent manner by PPTN. Schild analyses revealed that PPTN-mediated inhibition followed competitive kinetics, with a KB of 434 pM observed. In contrast, 1 μM PPTN exhibited no agonist or antagonist effect at the P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, or P2Y13 receptors. UDP-glucose-promoted chemotaxis of differentiated HL-60 human promyelocytic leukemia cells was blocked by PPTN with a concentration dependence consistent with the KB determined with recombinant P2Y14-R. In contrast, the chemotactic response evoked by the chemoattractant peptide fMetLeuPhe was unaffected by PPTN. UDP-glucose-promoted chemotaxis of freshly isolated human neutrophils also was blocked by PPTN. In summary, this work establishes PPTN as a highly selective high-affinity antagonist of the P2Y14-R that is useful for interrogating the action of this receptor in physiologic systems.

Cross-talk in nucleotide signaling in glioma C6 cells

The chapter is focused on the mechanism of action of metabotropic P2Y nucleotide receptors: P2Y(1), P2Y(2), P2Y(12), P2Y(14) and the ionotropic P2X(7) receptor in glioma C6 cells. P2Y(1) and P2Y(12) both respond to ADP, but while P2Y(1) links to PLC and elevates cytosolic Ca(2+) concentration, P2Y(12) negatively couples to adenylate cyclase, maintaining cAMP at low level. In glioma C6, these two P2Y receptors modulate activities of ERK1/2 and PI3K/Akt signaling and the effects depend on physiological conditions of the cells. During prolonged serum deprivation, cell growth is arrested, the expression of the P2Y(1) receptor strongly decreases and P2Y(12) becomes a major player responsible for ADP-evoked signal transduction. The P2Y(12) receptor activates ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulates Akt activity, contributing to glioma invasiveness. In contrast, P2Y(1) has an inhibitory effect on Akt pathway signaling. Furthermore, the P2X(7) receptor, often responsible for apoptotic fate, is not involved in Ca(2+)elevation in C6 cells. The shift in nucleotide receptor expression from P2Y(1) to P2Y(12) during serum withdrawal, the cross talk between both receptors and the lack of P2X(7) activity shows the precise self-regulating mechanism, enhancing survival and preserving the neoplastic features of C6 cells.

The UDP-sugar-sensing P2Y(14) receptor promotes Rho-mediated signaling and chemotaxis in human neutrophils

The G(i)-coupled P2Y(14) receptor (P2Y(14)-R) is potently activated by UDP-sugars and UDP. Although P2Y(14)-R mRNA is prominently expressed in circulating neutrophils, the signaling pathways and functional responses associated with this receptor are undefined. In this study, we illustrate that incubation of isolated human neutrophils with UDP-glucose resulted in cytoskeleton rearrangement, change of cell shape, and enhanced cell migration. We also demonstrate that UDP-glucose promotes rapid, robust, and concentration-dependent activation of RhoA in these cells. Ecto-nucleotidases expressed on neutrophils rapidly hydrolyzed extracellular ATP, but incubation with UDP-glucose for up to 1 h resulted in negligible metabolism of the nucleotide-sugar. HL60 human promyelocytic leukemia cells do not express the P2Y(14)-R, but neutrophil differentiation of HL60 cells with DMSO resulted in markedly enhanced P2Y(14)-R expression. Accordingly, UDP-glucose, UDP-galactose, and UDP-N-acetylglucosamine promoted Rho activation in differentiated but not in undifferentiated HL60 cells. Stable expression of recombinant human P2Y(14)-R conferred UDP-sugar-promoted responses to undifferentiated HL60 cells. UDP-glucose-promoted RhoA activation also was accompanied by enhanced cell migration in differentiated HL60 cells, and these responses were blocked by Rho kinase inhibitors. These results support the notion that UDP-glucose is a stable and potent proinflammatory mediator that promotes P2Y(14)-R-mediated neutrophil motility via Rho/Rho kinase activation.

The impact of commercially available purinergic ligands on purinergic signalling research

Due to the extremely wide-spread expression of purinergic receptors, purinergic signalling has been implicated in numerous physiological and pathophysiological areas. To better understand the involvement of purinergic receptors in such areas, the researcher's requirement for diverse and varied purinergic receptor ligands has greatly increased. This has generated increased commercial opportunities for life science suppliers, and ultimately, has led to a rapid expansion in the number of commercially available purinergic receptor ligands. The wide-spread availability of ligands to researchers has greatly benefited the scientific community, nurturing the rapid and continued expansion of the purinergic signalling field.

Discovery of Novel P2Y14 Agonist and Antagonist Using Conventional and Nonconventional Methods

P2Y14 is a member of the pyrimidinergic GPCR family. UDP-Glc has been previously shown to activate human P2Y14, whereas UDP was unable to activate the receptor. In this study, the authors used conventional and nonconventional methods to further characterize P2Y14 and its ligands. Conventional calcium mobilization and nonconventional cellular impedance functional assays revealed that UMP and UDP selectively activated HEK cells coexpressing P2Y14 and Gαqi5. In the impedance assays, the presence of exogenous Gαqi5 resulted in agonist-induced Gq signaling, whereas in the absence of exogenous Gαqi5, the signal was indicative of Gi. The authors established the first P2Y14 membrane filtration binding assay using a novel optimized expression vector and [3H]UDP as radioligand. UDP-Glc, UMP, and UDP dose dependently inhibited [3H]UDP binding in the binding assay, and saturation analysis revealed that UDP bound P2Y14 with a KD = 10 nM and a Bmax = 110 pmol/mg. The authors screened a phosphonate library and identified compound A, which inhibited UDP-Glc–mediated calcium signaling in the fluorometric imaging plate reader assay (IC50 = 2.3 µM) and competed for [3H]UDP binding in the novel binding assay with a Ki = 1280 nM.

P2Y receptors as regulators of lung endothelial barrier integrity

Endothelial cells (ECs), forming a semi-permeable barrier between the interior space of blood vessels and underlying tissues, control such diverse processes as vascular tone, homeostasis, adhesion of platelets, and leukocytes to the vascular wall and permeability of vascular wall for cells and fluids. Mechanisms which govern the highly clinically relevant process of increased EC permeability are under intense investigation. It is well known that loss of this barrier (permeability increase) results in tissue inflammation, the hall mark of inflammatory diseases such as acute lung injury and its severe form, acute respiratory distress syndrome. Little is known about processes which determine the endothelial barrier enhancement or protection against permeability increase. It is now well accepted that extracellular purines and pyrimidines are promising and physiologically relevant barrier-protective agents and their effects are mediated by interaction with cell surface P2Y receptors which belong to the superfamily of G-protein-coupled receptors. The therapeutic potential of P2Y receptors is rapidly expanding field in pharmacology and some selective agonists became recently available. Here, we present an overview of recently identified P2Y receptor agonists that enhance the pulmonary endothelial barrier and inhibit and/or reverse endothelial barrier disruption.

Nucleotide release by airway epithelia

The purinergic events regulating the airways' innate defenses are initiated by the release of purines from the epithelium, which occurs constitutively and is enhanced by chemical or mechanical stimulation. While the external triggers have been reviewed exhaustively, this chapter focuses on current knowledge of the receptors and signaling cascades mediating nucleotide release. The list of secreted purines now includes ATP, ADP, AMP and nucleotide sugars, and involves at least three distinct mechanisms reflecting the complexity of airway epithelia. First, the constitutive mechanism involves ATP translocation to the ER/Golgi complex as energy source for protein folding, and fusion of Golgi-derived vesicles with the plasma membrane. Second, goblet cells package ATP with mucins into granules, which are discharged in response to P2Y(2)R activation and Ca(2+)-dependent signaling pathways. Finally, non-mucous cells support a regulated mechanism of ATP release involving protease activated receptor (PAR)-elicited G(12/13) activation, leading to the RhoGEF-mediated exchange of GDP for GTP on RhoA, and cytoskeleton rearrangement. Together, these pathways provide fine tuning of epithelial responses regulated by purinergic signaling events.

Molecular pharmacology, physiology, and structure of the P2Y receptors

The P2Y receptors are a widely expressed group of eight nucleotide-activated G protein-coupled receptors (GPCRs). The P2Y(1)(ADP), P2Y(2)(ATP/UTP), P2Y(4)(UTP), P2Y(6)(UDP), and P2Y(11)(ATP) receptors activate G(q) and therefore robustly promote inositol lipid signaling responses. The P2Y(12)(ADP), P2Y(13)(ADP), and P2Y(14)(UDP/UDP-glucose) receptors activate G(i) leading to inhibition of adenylyl cyclase and to Gβγ-mediated activation of a range of effector proteins including phosphoinositide 3-kinase-γ, inward rectifying K(+) (GIRK) channels, phospholipase C-β2 and -β3, and G protein-receptor kinases 2 and 3. A broad range of physiological responses occur downstream of activation of these receptors ranging from Cl(-) secretion by epithelia to aggregation of platelets to neurotransmission. Useful structural models of the P2Y receptors have evolved from extensive genetic analyses coupled with molecular modeling based on three-dimensional structures obtained for rhodopsin and several other GPCRs. Selective ligands have been synthesized for most of the P2Y receptors with the most prominent successes attained with highly selective agonist and antagonist molecules for the ADP-activated P2Y(1) and P2Y(12) receptors. The widely prescribed drug, clopidogrel, which results in irreversible blockade of the platelet P2Y(12) receptor, is the most important therapeutic agent that targets a P2Y receptor.

Signalling and pharmacological properties of the P2Y receptor

The P2Y(14) receptor is a relatively broadly expressed G protein-coupled receptor that is prominently associated with immune and inflammatory cells as well as with many epithelia. This receptor historically was thought to be activated selectively by UDP-glucose and other UDP-sugars. However, UDP is also a very potent agonist of this receptor, and may prove to be one of its most important cognate activators.

UDP-glucose acting at P2Y14 receptors is a mediator of mast cell degranulation

UDP-glucose (UDPG), a glycosyl donor in the biosynthesis of carbohydrates, is an endogenous agonist of the G protein-coupled P2Y(14) receptor. RBL-2H3 mast cells endogenously express a P2Y(14) receptor at which UDPG mediates degranulation as indicated by beta-hexosaminidase (HEX) release. Both UDPG and a more potent, selective 2-thio-modified UDPG analog, MRS2690 (diphosphoric acid 1-alpha-d-glucopyranosyl ester 2-[(2-thio)uridin-5''-yl] ester), caused a substantial calcium transient in RBL-2H3 cells, which was blocked by pertussis toxin, indicating the presence of the G(i)-coupled P2Y(14) receptor, supported also by quantitative detection of abundant mRNA. Expression of the closely related P2Y(6) receptor was over 100 times lower than the P2Y(14) receptor, and the P2Y(6) agonist 3-phenacyl-UDP was inactive in RBL-2H3 cells. P2Y(14) receptor agonists also induced [(35)S]GTPgammaS binding to RBL-2H3 cell membranes, and phosphorylation of ERK1/2, P38 and JNK. UDPG and MRS2690 concentration-dependently enhanced HEX release with EC(50) values of 1150+/-320 and 103+/-18nM, respectively. The enhancement was completely blocked by pertussis toxin and significantly diminished by P2Y(14) receptor-specific siRNA. Thus, mast cells express an endogenous P2Y(14) receptor, which mediates G(i)-dependent degranulation and is therefore a potential novel therapeutic target for allergic conditions.

Quantification of Gi-mediated inhibition of adenylyl cyclase activity reveals that UDP is a potent agonist of the human P2Y14 receptor

The P2Y14 receptor was initially identified as a G protein-coupled receptor activated by UDP-glucose and other nucleotide sugars. We have developed several cell lines that stably express the human P2Y14 receptor, allowing facile examination of its coupling to native Gi family G proteins and their associated downstream signaling pathways (J Pharmacol Exp Ther 330:162-168, 2009). In the current study, we examined P2Y14 receptor-dependent inhibition of cyclic AMP accumulation in human embryonic kidney (HEK) 293, C6 glioma, and Chinese hamster ovary (CHO) cells stably expressing this receptor. Not only was the human P2Y14 receptor activated by UDP-glucose, but it also was activated by UDP. The apparent efficacies of UDP and UDP-glucose were similar, and the EC50 values (74, 33, and 29 nM) for UDP-dependent activation of the P2Y14 receptor in HEK293, CHO, and C6 glioma cells, respectively, were similar to the EC50 values (323, 132, and 72 nM) observed for UDP-glucose. UDP and UDP-glucose also stimulated extracellular signal-regulated kinase (ERK) 1/2 phosphorylation in P2Y14 receptor-expressing HEK293 cells but not in wild-type HEK293 cells. A series of analogs of UDP were potent P2Y14 receptor agonists, but the naturally occurring nucleoside diphosphates, CDP, GDP, and ADP exhibited agonist potencies over 100-fold less than that observed with UDP. Two UDP analogs were identified that selectively activate the P2Y14 receptor over the UDP-activated P2Y6 receptor, and these molecules stimulated phosphorylation of ERK1/2 in differentiated human HL-60 promyeloleukemia cells, which natively express the P2Y14 receptor but had no effect in wild-type HL-60 cells, which do not express the receptor. We conclude that UDP is an important cognate agonist of the human P2Y14 receptor.

Quantification of extracellular UDP-galactose

The human P2Y(14) receptor is potently activated by UDP-glucose (UDP-Glc), UDP-galactose (UDP-Gal), UDP-N-acetylglucosamine (UDP-GlcNAc), and UDP-glucuronic acid. Recently, cellular release of UDP-Glc and UDP-GlcNAc has been reported, but whether additional UDP-sugars are endogenous agonists for the P2Y(14) receptor remains poorly defined. In the present study, we describe an assay for the quantification of UDP-Gal with subnanomolar sensitivity. This assay is based on the enzymatic conversion of UDP-Gal to UDP, using 1-4-beta-galactosyltransferase. UDP is subsequently phosphorylated by nucleoside diphosphokinase in the presence of [gamma-(32)P]ATP and the formation of [gamma-(32)P]UTP is monitored by high-performance liquid chromatography. The overall conversion of UDP-Gal to [gamma-(32)P]UTP was linear between 0.5 and 30 nM UDP-Gal. Extracellular UDP-Gal was detected on resting cultures of various cell types, and increased release of UDP-Gal was observed in 1321N1 human astrocytoma cells stimulated with the protease-activated receptor agonist thrombin. The occurrence of regulated release of UDP-Gal suggests that, in addition to its role in glycosylation reactions, UDP-Gal is an important extracellular signaling molecule.

Enhanced potency of nucleotide-dendrimer conjugates as agonists of the P2Y14 receptor: multivalent effect in G protein-coupled receptor recognition

The P2Y(14) receptor is a G protein-coupled receptor activated by uridine-5'-diphosphoglucose and other nucleotide sugars that modulates immune function. Covalent conjugation of P2Y(14) receptor agonists to PAMAM (polyamidoamine) dendrimers enhanced pharmacological activity. Uridine-5'-diphosphoglucuronic acid (UDPGA) and its ethylenediamine adduct were suitable functionalized congeners for coupling to several generations (G2.5-6) of dendrimers (both terminal carboxy and amino). Prosthetic groups, including biotin for avidin complexation, a chelating group for metal complexation (and eventual magnetic resonance imaging), and a fluorescent moiety, also were attached with the eventual goals of molecular detection and characterization of the P2Y(14) receptor. The activities of conjugates were assayed in HEK293 cells stably expressing the human P2Y(14) receptor. A G3 PAMAM conjugate containing 20 bound nucleotide moieties (UDPGA) was 100-fold more potent (EC(50) 2.4 nM) than the native agonist uridine-5'-diphosphoglucose. A molecular model of this conjugate docked in the human P2Y(14) receptor showed that the nucleotide-substituted branches could extend far beyond the dimensions of the receptor and be available for multivalent docking to receptor aggregates. Larger dendrimer carriers and greater loading favored higher potency. A similar conjugate of G6 with 147 out of 256 amino groups substituted with UDPGA displayed an EC(50) value of 0.8 nM. Thus, biological activity was either retained or dramatically enhanced in the multivalent dendrimer conjugates in comparison with monomeric P2Y(14) receptor agonists, depending on size, degree of substitution, terminal functionality, and attached prosthetic groups.