Molecular cloning and functional analysis of a novel P2 nucleotide receptor

Purinergic receptors in airway hydration

Airway epithelial purinergic receptors control key components of the mucociliary clearance (MCC), the dominant component of pulmonary host defense. In healthy airways, the periciliary liquid (PCL) is optimally hydrated, thus acting as an efficient lubricant layer over which the mucus layer moves by ciliary force. When the hydration of the airway surface decreases, the mucus becomes hyperconcentrated, the PCL collapses, and the "thickened" mucus layer adheres to cell surfaces, causing plaque/plug formation. Mucus accumulation is a major contributing factor to the progression of chronic obstructive lung diseases such as cystic fibrosis (CF) and chronic bronchitis (CB). Mucus hydration is regulated by finely tuned mechanisms of luminal Cl^- secretion and Na⁺ absorption with concomitant osmotically driven water flow. These activities are regulated by airway surface liquid (ASL) concentrations of adenosine and ATP, acting on airway epithelial A_2B and P2Y₂ receptors, respectively. The goal of this article is to provide an overview of our understanding of the role of purinergic receptors in the regulation of airway epithelial ion/fluid transport and the mechanisms of nucleotide release and metabolic activities that contribute to airway surface hydration in healthy and chronically obstructed airways.

P2 Receptors in Cardiac Myocyte Pathophysiology and Mechanotransduction

ATP is a major energy source in the mammalian cells, but it is an extracellular chemical messenger acting on P2 purinergic receptors. A line of evidence has shown that ATP is released from many different types of cells including neurons, endothelial cells, and muscle cells. In this review, we described the distribution of P2 receptor subtypes in the cardiac cells and their physiological and pathological roles in the heart. So far, the effects of external application of ATP or its analogues, and those of UTP on cardiac contractility and rhythm have been reported. In addition, specific genetic alterations and pharmacological agonists and antagonists have been adopted to discover specific roles of P2 receptor subtypes including P2X4-, P2X7-, P2Y2- and P2Y6-receptors in cardiac cells under physiological and pathological conditions. Accumulated data suggest that P2X4 receptors may play a beneficial role in cardiac muscle function, and that P2Y2- and P2Y6-receptors can induce cardiac fibrosis. Recent evidence further demonstrates P2Y1 receptor and P2X4 receptor as important mechanical signaling molecules to alter membrane potential and Ca²⁺ signaling in atrial myocytes and their uneven expression profile between right and left atrium.

The Role of a Selective P2Y6 Receptor Antagonist, MRS2578, on the Formation of Angiotensin II-Induced Abdominal Aortic Aneurysms

Objective

The P2Y₆ receptor has been shown to be involved in many cardiovascular diseases, including hypertension and atherosclerosis. The study is aimed at exploring the role of the P2Y₆ receptor in Ang II-induced abdominal aortic aneurysm (AAA) formation in apolipoprotein E-deficient (apoE^−/−) mice by using its selective antagonist.

Methods

Male apoE^−/− mice were fed with high-fat diet and infused with angiotensin (Ang) II (1000 ng/kg/min) for 4 weeks to induce AAA or saline as controls. Mice were divided into four groups: normal saline (NS, placebo control) group (n = 8), Ang II+vehicle (Ang II) group (n = 14), Ang II-low dose MRS2578 (Ang II+MRS-16 mg) group (n = 14), and Ang II-high dose MRS2578 (Ang II+MRS-32 mg) group (n = 14). Daily intraperitoneal injection with vehicle or MRS2578 was pretreated one week before Ang II infusion. On postoperative day 10, aorta imaging of each group was taken by ultrasonography. After 4 weeks of Ang II infusion, the excised aortas were processed for diameter measurement and quantification of aneurysm severity and tissue characteristics; the blood samples were collected for measurement of the lipid profile and levels of cytokines. Verhoeff's Van Gieson (EVG) staining and immunochemistry staining were performed to evaluate disruption of the extracellular matrix (ECM) and infiltration of macrophages. Expression and activity of matrix metalloproteinases (MMPs) was measured by gelatin zymography.

Results

Treatment with MRS2578 made no significant difference in AAA formation, and maximal aortic diameter yet caused higher AAA rupture-induced mortality from 7% (Ang II) to 21.4% (Ang II+MRS-16 mg) or 42.9% (Ang II+MRS-32 mg), respectively (p < 0.05). Consistently, the severity of aneurysm tended to be more deteriorated in MRS2578-treated groups, especially the high-dosage group. The ratios of type III and IV aneurysm were much higher in the MRS2578-coadministered groups (p < 0.05). Furthermore, histological analyses showed that administration of MRS2578 significantly increased infiltration of macrophages, expression of monocyte chemotactic protein 1 (MCP-1) and vascular cell adhesion molecule-1 (VCAM-1), and activities of MMP-2 and MMP-9 followed by aggravating degradation elastin in vivo (p < 0.05). However, the multiple effects of MRS2578 on the development of AAA are independent of changes in systolic blood pressure and lipid profiles.

Conclusions

The present study demonstrated that administration of MRS2578 exacerbated the progression and rupture of experimental AAA through promoting proinflammatory response and MMP expression and activity, which indicated a crucial role of the P2Y₆ receptor in AAA development. Clinical Relevance. Purinergic P2Y receptors have attracted much attention since the P2Y₁₂ receptor antagonist had been successfully applied in clinical practice. Elucidating the underlying mechanisms of AAA and exploring potential therapeutic strategies are essential to prevent its progression and reduce the mortality rate.

P2Y receptors in neuropathic pain

This review summarizes and evaluates the relationship between neuropathic pain and P2Y receptors from inception to 2019. Purinergic receptors have been well studied in recent years using various molecular biological methods. The main research objective of this review is to determine the association of P2Y1, P2Y2, P2Y6, P2Y12 and P2Y13 receptors with neuropathic pain. This review includes the most comprehensive subtypes of P2Y that related to neuropathic pain and the current therapeutic method of neuropathic pain. G protein-coupled P2Y receptors are located on neurons, astrocytes, oligodendrocytes and microglial cells and regulate neurotransmission. Nerve injury is the prime reason for abnormal regulation of P2Y receptor mRNA expression, subsequently, inducing neuropathic pain. Neuropathic pain is a type of chronic pain that is divided into peripheral, central and mixed. Numerous studies demonstrated a positive correlation between the expression level of P2Y receptors and neuropathic pain generation. Also, several reports showed that P2Y short hairpin RNA (shRNA) and P2Y antagonist can be used as an analgesic to relieve neuropathic pain via decreasing P2Y receptor expression level and neural cell activation. However, the transformation process from basic experiments to clinical applications is a long process. Current deficiencies and future research directions are discussed at the end of this review.

Vascular Smooth Muscle Remodeling in Conductive and Resistance Arteries in Hypertension

Cardiovascular disease is a leading cause of death worldwide and accounts for >17.3 million deaths per year, with an estimated increase in incidence to 23.6 million by 2030. ¹ Cardiovascular death represents 31% of all global deaths ² -with stroke, heart attack, and ruptured aneurysms predominantly contributing to these high mortality rates. A key risk factor for cardiovascular disease is hypertension. Although treatment or reduction in hypertension can prevent the onset of cardiovascular events, existing therapies are only partially effective. A key pathological hallmark of hypertension is increased peripheral vascular resistance because of structural and functional changes in large (conductive) and small (resistance) arteries. In this review, we discuss the clinical implications of vascular remodeling, compare the differences between vascular smooth muscle cell remodeling in conductive and resistance arteries, discuss the genetic factors associated with vascular smooth muscle cell function in hypertensive patients, and provide a prospective assessment of current and future research and pharmacological targets for the treatment of hypertension.

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.

Characterization of UDP-Activated Purinergic Receptor P2Y₆ Involved in Japanese Flounder Paralichthys olivaceus Innate Immunity

Uridine 5’-diphosphate (UDP)-activated purinergic receptor P2Y₆ is a member of a G-protein-coupled purinergic receptor family that plays an important role in mammalian innate immunity. However, the role of the P2Y₆ receptor (P2Y₆R) in fish immunity has not been investigated. In this report, we characterized a P2Y₆R gene from Japanese flounder (Paralichthys olivaceus) and examined its role in fish innate immunity. Sequence analysis reveals that the Japanese flounder P2Y₆R protein is conserved and possesses four potential glycosylation sites. Quantitative real-time RT-PCR analysis shows that P2Y₆R is broadly distributed in all examined Japanese flounder tissues with dominant expression in the liver. In addition, P2Y₆R gene expression was up-regulated in head kidney macrophages (HKMs) upon lipopolysaccharides (LPS) and poly(I:C) stimulations but down-regulated by LPS challenge in peripheral blood leukocytes (PBLs). Furthermore, pharmacological inhibition of the endogenous P2Y₆ receptor activity by the potently selective P2Y₆R antagonist, MRS 2578, greatly up-regulated pro-inflammatory cytokine interleukin (IL)-1β, IL-6 and TNF-α gene expression in PBL cells treated with UDP. Moreover, LPS- and poly(I:C)-induced gene expression of IL-1β and TNF-α in Japanese flounder PBL cells was attenuated significantly by inhibition of P2Y₆R activity with antagonist MRS 2578. Collectively, we, for the first time, showed the involvement of functional purinergic P2Y₆R in fish innate immunity.

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.

Signaling mechanism for modulation by ATP of glycine receptors on rat retinal ganglion cells

ATP modulates voltage- and ligand-gated channels in the CNS via the activation of ionotropic P2X and metabotropic P2Y receptors. While P2Y receptors are expressed in retinal neurons, the function of these receptors in the retina is largely unknown. Using whole-cell patch-clamp techniques in rat retinal slice preparations, we demonstrated that ATP suppressed glycine receptor-mediated currents of OFF type ganglion cells (OFF-GCs) dose-dependently, and the effect was in part mediated by P2Y1 and P2Y11, but not by P2X. The ATP effect was abolished by intracellular dialysis of a Gq/11 protein inhibitor and phosphatidylinositol (PI)-phospholipase C (PLC) inhibitor, but not phosphatidylcholine (PC)-PLC inhibitor. The ATP effect was accompanied by an increase in [Ca(2+)]i through the IP3-sensitive pathway and was blocked by intracellular Ca(2+)-free solution. Furthermore, the ATP effect was eliminated in the presence of PKC inhibitors. Neither PKA nor PKG system was involved. These results suggest that the ATP-induced suppression may be mediated by a distinct Gq/11/PI-PLC/IP3/Ca(2+)/PKC signaling pathway, following the activation of P2Y1,11 and other P2Y subtypes. Consistently, ATP suppressed glycine receptor-mediated light-evoked inhibitory postsynaptic currents of OFF-GCs. These results suggest that ATP may modify the ON-to-OFF crossover inhibition, thus changing action potential patterns of OFF-GCs.

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'.

The Signaling Mechanism of Contraction Induced by ATP and UTP in Feline Esophageal Smooth Muscle Cells

P2 receptors are membrane-bound receptors for extracellular nucleotides such as ATP and UTP. P2 receptors have been classified as ligand-gated ion channels or P2X receptors and G protein-coupled P2Y receptors. Recently, purinergic signaling has begun to attract attention as a potential therapeutic target for a variety of diseases especially associated with gastroenterology. This study determined the ATP and UTP-induced receptor signaling mechanism in feline esophageal contraction. Contraction of dispersed feline esophageal smooth muscle cells was measured by scanning micrometry. Phosphorylation of MLC20 was determined by western blot analysis. ATP and UTP elicited maximum esophageal contraction at 30 s and 10 μM concentration. Contraction of dispersed cells treated with 10 μM ATP was inhibited by nifedipine. However, contraction induced by 0.1 μM ATP, 0.1 μM UTP and 10 μM UTP was decreased by U73122, chelerythrine, ML-9, PTX and GDPβS. Contraction induced by 0.1 μM ATP and UTP was inhibited by Gαi3 or Gαq antibodies and by PLCβ1 or PLCβ3 antibodies. Phosphorylated MLC20 was increased by ATP and UTP treatment. In conclusion, esophageal contraction induced by ATP and UTP was preferentially mediated by P2Y receptors coupled to Gαi3 and G q proteins, which activate PLCβ1 and PLCβ3. Subsequently, increased intracellular Ca(2+) and activated PKC triggered stimulation of MLC kinase and inhibition of MLC phosphatase. Finally, increased pMLC20 generated esophageal contraction.

Inhibition of UDP/P2Y6 purinergic signaling prevents phagocytosis of viable neurons by activated microglia in vitro and in vivo

Microglia activated through Toll-like receptor (TLR)-2 or -4 can cause neuronal death by phagocytosing otherwise-viable neurons—a form of cell death called “phagoptosis.” UDP release from neurons has been shown to provoke microglial phagocytosis of neurons via microglial P2Y₆ receptors, but whether inhibition of this process affects neuronal survival is unknown. We tested here whether inhibition of P2Y₆ signaling could prevent neuronal death in inflammatory conditions, and whether UDP signaling can induce phagoptosis of stressed but viable neurons. We find that delayed neuronal loss and death in mixed neuronal/glial cultures induced by the TLR ligands lipopolysaccharide (LPS) or lipoteichoic acid was prevented by: apyrase (to degrade nucleotides), Reactive Blue 2 (to inhibit purinergic signaling), or MRS2578 (to specifically block P2Y₆ receptors). In each case, inflammatory activation of microglia was not affected, and the rescued neurons remained viable for at least 7 days. Blocking P2Y₆ receptors with MRS2578 also prevented phagoptosis of neurons induced by 250 nM amyloid beta 1–42, 5 μM peroxynitrite, or 50 μM 3-morpholinosydnonimine (which releases reactive oxygen and nitrogen species). Furthermore, the P2Y₆ receptor agonist UDP by itself was sufficient to stimulate microglial phagocytosis and to induce rapid neuronal loss that was prevented by eliminating microglia or inhibiting phagocytosis. In vivo, injection of LPS into rat striatum induced microglial activation and delayed neuronal loss and blocking P2Y₆ receptors with MRS2578 prevented this neuronal loss. Thus, blocking UDP/P2Y₆ signaling is sufficient to prevent neuronal loss and death induced by a wide range of stimuli that activate microglial phagocytosis of neurons.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

P2X and P2Y nucleotide receptors as targets in cardiovascular disease

Endogenous nucleotides have widespread actions in the cardiovascular system, but it is only recently that the P2X and P2Y receptor subtypes, at which they act, have been identified and subtype-selective agonists and antagonists developed. These advances have greatly increased our understanding of the physiological and pathophysiological functions of P2X and P2Y receptors, but investigation of the clinical usefulness of selective ligands is at an early stage. Nonetheless, the evidence considered in this review demonstrates clearly that various cardiovascular disorders, including vasospasm, hypertension, congestive heart failure and cardiac damage during ischemic episodes, may be viable targets. With further development of novel, selective agonists and antagonists, our understanding will continue to improve and further therapeutic applications are likely to be discovered.

Lack of specificity shown by P2Y6 receptor antibodies

P2Y6 receptor in bladder smooth muscle responds to UDP by increasing muscle tone and augmenting bladder contractions. The exact cellular location of the receptor is however unknown. Three commercially available antibodies to P2Y6 receptor gave clean bands on Western blot which were eliminated by specific peptide competition. Two of the three also immunostained bladder smooth muscle cells while leaving adjacent interstitial cells of Cajal unstained. However, attempts to validate the specificity of these antibodies by performing the same assays on bladders from P2Y6 knockout mice were unsuccessful. In Western blots, all three antibodies bound similar proteins in both wild type and P2Y6 knockout tissue. Immunostaining of knockout tissue sections also showed no difference in staining patterns or intensity. We conclude that rigorous controls are required when using commercial reagents to this G-protein coupled receptor and perhaps to other members of the P2Y receptor family.

P2Y receptors in the mammalian nervous system: pharmacology, ligands and therapeutic potential

P2Y receptors for extracellular nucleotides are coupled to activation of a variety of G proteins and stimulate diverse intracellular signaling pathways that regulate functions of cell types that comprise the central nervous system (CNS). There are 8 different subtypes of P2Y receptor expressed in cells of the CNS that are activated by a select group of nucleotide agonists. Here, the agonist selectivity of these 8 P2Y receptor subtypes is reviewed with an emphasis on synthetic agonists with high potency and resistance to degradation by extracellular nucleotidases that have potential applications as therapeutic agents. In addition, the recent identification of a wide variety of subtype-selective antagonists is discussed, since these compounds are critical for discerning cellular responses mediated by activation of individual P2Y receptor subtypes. The functional expression of P2Y receptor subtypes in cells that comprise the CNS is also reviewed and the role of each subtype in the regulation of physiological and pathophysiological responses is considered. Other topics include the role of P2Y receptors in the regulation of blood-brain barrier integrity and potential interactions between different P2Y receptor subtypes that likely impact tissue responses to extracellular nucleotides in the CNS. Overall, current research suggests that P2Y receptors in the CNS regulate repair mechanisms that are triggered by tissue damage, inflammation and disease and thus P2Y receptors represent promising targets for the treatment of neurodegenerative diseases.

Cellular localization of P2Y₆ receptor in rat retina

Extracellular nucleotides exert their actions via two subfamilies of purinoceptors: P2X and P2Y. Eight mammalian P2Y receptor subtypes (P2Y(1,2,4,6,11,12,13,14)) have been identified. In this work, the localization of P2Y(6) was studied in rat retina using double immunofluorescence labeling and confocal scanning microscopy. Immunostaining for P2Y(6) was strong in the outer plexiform layer and was diffusely distributed throughout the full thickness of the inner plexiform layer. In addition, P2Y(6) immunoreactivity was clearly observed in many cells in the inner nuclear layer and the ganglion cell layer. In the outer retina photoreceptor terminals, labeled by VGluT1, and horizontal cells, labeled by calbindin, were P2Y(6)-positive. However, no P2Y(6) immunostaining was detected in bipolar cells, labeled by homeobox protein Chx10. In the inner retina P2Y(6) was localized to most of GABAergic amacrine cells, including dopaminergic and cholinergic ones, stained by tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT) respectively. Some of glycinergic amacrine cells, but not glycinergic AII amacrine cells, were also labeled by P2Y(6). Moreover, P2Y(6) immunoreactivity was seen in almost all ganglion cells, labeled by Brn3a. In Müller glial cells, stained by cellular retinaldehyde binding protein (CRALBP), however, no P2Y(6) expression was found in both somata and processes. We speculate that P2Y(6) may be involved in retinal information processing in different ways, probably by regulating the release of transmitters and/or modulating the radial flow of visual signals and lateral interaction mediated by horizontal and amacrine cells.

Involvement of protein kinase D in uridine diphosphate-induced microglial macropinocytosis and phagocytosis

The clearance of tissue debris by microglia is a crucial component of maintaining brain homeostasis. Microglia continuously survey the brain parenchyma and utilize extracellular nucleotides to trigger the initiation of their dynamic responses. Extracellular uridine diphosphate (UDP), which leaks or is released from damaged neurons, has been reported to stimulate the phagocytotic activity of microglia through P2Y(6) receptor activation. However, the intracellular mechanisms underlying microglial P2Y(6) receptor signals have not been identified. In this study, we demonstrated that UDP stimulation induced immediate and long-lasting dynamic movements in the cell membrane. After 60 min of UDP stimulation, there was an upregulation in the number of large vacuoles formed in the cell that incorporate extracellular fluorescent-labeled dextran, which indicates microglial macropinocytosis. In addition, UDP-induced vacuole formation and continuous membrane motility were suppressed by the protein kinase D (PKD) inhibitors, Gö6976 and CID755673, unlike Gö6983, which is far less sensitive to PKD. The inhibition of PKD also reduced UDP-induced incorporation of fluorescent-labeled dextran and soluble β-amyloid and phagocytosis of microspheres. UDP induced rapid phosphorylation and membrane translocation of PKD, which was abrogated by the inhibition of protein kinase C (PKC) with Gö6983. However, Gö6983 failed to suppress UDP-induced incorporation of microspheres. Finally, we found that inhibition of PKD by CID755673 significantly suppressed UDP-induced engulfment of IgG-opsonized microspheres. These data suggest that a PKC-independent function of PKD regulates UDP-induced membrane movement and contributes to the increased uptake of extracellular fluid and microspheres in microglia.

The P2Y(6) receptor stimulates bone resorption by osteoclasts

Accumulating evidence indicates that extracellular nucleotides, signaling through P2 receptors, play a significant role in bone remodeling. Osteoclasts (the bone-resorbing cell) and osteoblasts (the bone-forming cell) display expression of the G protein-coupled P2Y(6) receptor, but the role of this receptor in modulating cell function is unclear. Here, we demonstrate that extracellular UDP, acting via P2Y(6) receptors, stimulates the formation of osteoclasts from precursor cells, while also enhancing the resorptive activity of mature osteoclasts. Furthermore, osteoclasts derived from P2Y(6) receptor-deficient (P2Y(6)R(-/-)) animals displayed defective function in vitro. Using dual energy x-ray absorptiometry scanning and microcomputed tomographic analysis we showed that P2Y(6)R(-/-) mice have increased bone mineral content, cortical bone volume, and cortical thickness in the long bones and spine, whereas trabecular bone parameters were unaffected. Histomorphometric analysis showed the perimeter of the bone occupied by osteoclasts on the endocortical and trabecular surfaces was decreased in P2Y(6)R(-/-) mice. Taken together these results show the P2Y(6) receptor may play an important role in the regulation of bone cell function in vivo.

The purinergic G protein-coupled receptor 6 inhibits effector T cell activation in allergic pulmonary inflammation

We show that the P2Y(6) receptor, a purinergic G protein-coupled receptor with a high affinity for the nucleotide uridine diphosphate, is an important endogenous inhibitor of T cell function in allergic pulmonary inflammation. Mice conditionally deficient in P2Y(6) receptors [p2ry6 (flox/flox);cre/+ mice] exhibited severe airway and tissue pathology relative to P2Y(6)-sufficient [p2ry6 (flox/flox)] littermates (+/+ mice) when treated intranasally with an extract of the dust mite Dermatophagoides farinae (Df). P2Y(6) receptors were inducibly expressed by lung, lymph node, and splenic CD4(+) and CD8(+) T cells of Df-treated +/+ mice. Df-restimulated P2Y(6)-deficient lymph node cells produced higher levels of Th1 and Th2 cytokines, and polyclonally stimulated P2Y(6)-deficient CD4(+) T cells proliferated faster than comparably stimulated P2Y(6)-sufficient cells. The absence of P2Y(6) receptors on CD4(+) cells, but not APCs, was sufficient to amplify cytokine generation. Thus, P2Y(6) receptors protect the lung against exuberant allergen-induced pulmonary inflammation by inhibiting the activation of effector T cells.

P2Y receptors on astrocytes and microglia mediate opposite effects in astroglial proliferation

Nucleotides released upon brain injury signal to astrocytes and microglia playing an important role in astrogliosis, but the participation of microglia in the purinergic modulation of astrogliosis is still unclear. Highly enriched astroglial cultures and co-cultures of astrocytes and microglia were used to investigate the influence of microglia in the modulation of astroglial proliferation mediated by nucleotides. In highly enriched astroglial cultures, adenosine-5'-triphosphate (ATP), adenosine 5'-O-(3-thio)-triphosphate (ATPγS), adenosine 5'-O-(3-thio)-diphosphate (ADPβS; 0.01-1 mM), and adenosine-5'-diphosphate (ADP; 0.1-1 mM) increased proliferation up to 382%, an effect abolished in co-cultures containing 8% of microglia. The loss of ATP proliferative effect in co-cultures is supported by its fast metabolism and reduced ADP accumulation, an agonist of P2Y(1,12) receptors that mediate astroglial proliferation. No differences in ADPβS and ATPγS metabolism or P2Y(1,12) receptors expression were found in co-cultures that could explain the loss of their proliferative effect. However, conditioned medium from microglia cultures or co-cultures treated with ADPβS, when tested in highly enriched astroglial cultures, also prevented ADPβS proliferative effect. None of the uracil nucleotides tested had any effect in proliferation of highly enriched astroglial cultures, but uridine-5'-triphosphate (UTP; 0.1-1 mM) inhibited proliferation up to 66% in co-cultures, an effect that was dependent on uridine-5'-diphosphate (UDP) accumulation, coincident with a co-localization of P2Y(6) receptors in microglia and due to cell apoptosis. The results indicate that microglia control astroglial proliferation by preventing the proliferative response to adenine nucleotides and favouring an inhibitory effect of UTP/UDP. Several microglial P2Y receptors may be involved by inducing the release of messengers that restrain astrogliosis, a beneficial effect for neuronal repair mechanisms following brain injury.

Uridine adenosine tetraphosphate-induced contraction is increased in renal but not pulmonary arteries from DOCA-salt hypertensive rats

Uridine adenosine tetraphosphate (Up(4)A) was reported as a novel endothelium-derived contracting factor. Up(4)A contains both purine and pyrimidine moieties, which activate purinergic (P2)X and P2Y receptors. However, alterations in the vasoconstrictor responses to Up(4)A in hypertensive states remain unclear. The present study examined the effects of Up(4)A on contraction of isolated renal arteries (RA) and pulmonary arteries (PA) from DOCA-salt rats using isometric tension recording. RA from DOCA-salt rats exhibited increased contraction to Up(4)A versus arteries from control uninephrectomized rats in the absence and presence of N(G)-nitro-l-arginine (nitric oxide synthase inhibitor). On the other hand, the Up(4)A-induced contraction in PA was similar between the two groups. Up(4)A-induced contraction was inhibited by suramin (nonselective P2 antagonist) but not by diinosine pentaphosphate pentasodium salt hydrate (Ip(5)I; P2X(1) antagonist) in RA from both groups. Furthermore, 2-thiouridine 5'-triphosphate tetrasodium salt (2-ThioUTP; P2Y(2) agonist)-, uridine-5'-(γ-thio)-triphosphate trisodium salt (UTPγS; P2Y(2)/P2Y(4) agonist)-, and 5-iodouridine-5'-O-diphosphate trisodium salt (MRS 2693; P2Y(6) agonist)-induced contractions were all increased in RA from DOCA-salt rats. Protein expression of P2Y(2)-, P2Y(4)-, and P2Y(6) receptors in RA was similar between the two groups. In DOCA-salt RA, the enhanced Up(4)A-induced contraction was reduced by PD98059, an ERK pathway inhibitor, and Up(4)A-stimulated ERK activation was increased. These data are the first to indicate that Up(4)A-induced contraction is enhanced in RA from DOCA-salt rats. Enhanced P2Y receptor signaling and activation of the ERK pathway together represent a likely mechanism mediating the enhanced Up(4)A-induced contraction. Up(4)A might be of relevance in the pathophysiology of vascular tone regulation and renal dysfunction in arterial hypertension.

Expression of P2Y(6) receptors in the developing mouse skeletal muscle and after injury and repair

In this study, single and double-labeling immunofluorescence histochemistry, Western blot and real-time polymerase chain reaction were used to study the expression of P2Y(6) receptors in developing mouse skeletal muscle and during injury and repair. The results show that P2Y(6) receptor immunoreactive (ir) cells were first detected in the dermamyotome at embryonic (E) day 9. The number and immunostaining intensity of the P2Y(6) receptor-ir cells increased from E9 to E13, but decreased from E15 to postnatal day 60 in the developing skeletal muscle system. The expression levels of P2Y(6) receptor protein and mRNA increased rapidly from 1 to 5 days after skeletal muscle injury and then decreased almost to the control level from 7 to 10 days, at the beginning of regeneration. P2Y(6) receptor-immunoreactivity was mainly localized to the ends of single myoblasts and myotube processes in the developing and injury-repair skeletal muscle tissues. These data suggest that the P2Y(6) receptor may be involved in the development and regeneration of skeletal muscle, especially in the migration and extension of the myoblast and myotube in developing and regenerating skeletal muscle.

Discovery of potent competitive antagonists and positive modulators of the P2X2 receptor

Evaluation and optimization of anthraquinone derivatives related to Reactive Blue 2 at P2X2 receptors yielded the first potent and selective P2X2 receptor antagonists. The compounds were tested for inhibition of ATP (10 μM) mediated currents in Xenopus oocytes expressing the rat P2X2 receptor. The most potent antagonists were sodium 1-amino-4-[3-(4,6-dichloro[1,3,5]triazine-2-ylamino)phenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (63, PSB-10211, IC(50) 86 nM) and disodium 1-amino-4-[3-(4,6-dichloro[1,3,5]triazine-2-ylamino)-4-sulfophenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (57, PSB-1011, IC(50) 79 nM). Compound 57 exhibited a competitive mechanism of action (pA(2) 7.49). It was >100-fold selective versus P2X4, P2X7, and several investigated P2Y receptor subtypes (P2Y(2,4,6,12)); selectivity versus P2X1 and P2X3 receptors was moderate (>5-fold). Compound 57 was >13-fold more potent at the homomeric P2X2 than at the heteromeric P2X2/3 receptor. Several anthraquinone derivatives were found to act as positive modulators of ATP effects at P2X2 receptors, for example, sodium 1-amino-4-(3-phenoxyphenylamino)-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (51, PSB-10129, EC(50) 489 nM), which led to about a 3-fold increase in the ATP-elicited current.

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.

Ca(2+) signalling by P2Y receptors in cultured rat aortic smooth muscle cells

Background and purpose:

P2Y receptors evoke Ca²⁺ signals in vascular smooth muscle cells and regulate contraction and proliferation, but the roles of the different P2Y receptor subtypes are incompletely resolved.

Experimental approach:

Quantitative PCR was used to define expression of mRNA encoding P2Y receptor subtypes in freshly isolated and cultured rat aortic smooth muscle cells (ASMC). Fluorescent indicators in combination with selective ligands were used to measure the changes in cytosolic free [Ca²⁺] in cultured ASMC evoked by each P2Y receptor subtype.

Key results:

The mRNA for all rat P2Y receptor subtypes are expressed at various levels in cultured ASMC. Four P2Y receptor subtypes (P2Y1, P2Y2, P2Y4 and P2Y6) evoke Ca²⁺ signals that require activation of phospholipase C and comprise both release of Ca²⁺ from stores and Ca²⁺ entry across the plasma membrane.

Conclusions and implications:

Combining analysis of P2Y receptor expression with functional analyses using selective agonists and antagonists, we isolated the Ca²⁺ signals evoked in ASMC by activation of P2Y1, P2Y2, P2Y4 and P2Y6 receptors.

Adrenergic receptor activation involves ATP release and feedback through purinergic receptors

Formyl peptide receptor-induced chemotaxis of neutrophils depends on the release of ATP and autocrine feedback through purinergic receptors. Here, we show that adrenergic receptor signaling requires similar purinergic feedback mechanisms. Real-time RT-PCR analysis revealed that human embryonic kidney (HEK)-293 cells express several subtypes of adrenergic (α(1)-, α(2)-, and β-receptors), adenosine (P1), and nucleotide receptors (P2). Stimulation of G(q)-coupled α(1)-receptors caused release of cellular ATP and MAPK activation, which was blocked by inhibiting P2 receptors with suramin. Stimulation of G(i)-coupled α(2)-receptors induced weak ATP release, while G(s)-coupled β-receptors caused accumulation of extracellular ADP and adenosine. β-Receptors triggered intracellular cAMP signaling, which was blocked by scavenging extracellular adenosine with adenosine deaminase or by inhibiting A2a adenosine receptors with SCH58261. These findings suggest that adrenergic receptors require purinergic receptors to elicit downstream signaling responses in HEK-293 cells. We evaluated the physiological relevance of these findings using mouse aorta tissue rings. Stimulation of α(1)-receptors induced ATP release and tissue contraction, which was reduced by removing extracellular ATP with apyrase or in the absence of P2Y(2) receptors in aorta rings from P2Y(2) receptor knockout mice. We conclude that, like formyl peptide receptors, adrenergic receptors require purinergic feedback mechanisms to control complex physiological processes such as smooth muscle contraction and regulation of vascular tone.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

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.

Development of potent and selective inhibitors of ecto-5'-nucleotidase based on an anthraquinone scaffold

ecto-5'-Nucleotidase (eN, CD73) plays a major role in controlling extracellular adenosine levels. eN inhibitors have potential as novel drugs, for example, for the treatment of cancer. In the present study, we synthesized and investigated a series of 55 anthraquinone derivatives as potential inhibitors of eN, 11 of which are novel compounds and another 11 of which had previously been described but have now been synthesized by an improved method. We identified several potent inhibitors of rat eN. The most potent compounds were 1-amino-4-[4-fluoro-2-carboxyphenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (45, PSB-0952, K(i) = 260 nM) and 1-amino-4-[2-anthracenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (52, PSB-0963, 150 nM), with 52 being the most potent eN inhibitor described to date. Selected compounds were further characterized and found to exhibit a competitive mechanism of inhibition. Investigations of ecto-nucleoside triphosphate diphosphohydrolases (NTPDases) and the P2Y receptor subtypes P2Y(2), P2Y(4), P2Y(6), and P2Y(12) showed that compound 45 exhibited the highest degree of selectivity (>150-fold).

P2Y6 receptor and immunoinflammation

The immunocytes microglia in the central nervous system (CNS) were reported to play a crucial role in neurodegeneration. As a member of P2 receptors family, purinoceptor P2Y6 has attracted much attention recently. Previous studies showed that purinoceptor P2Y6 mainly contributed to microglia activation and their later phagocytosis in CNS, while in immune system, it participated in the secretion of interleukin (IL)-8 from monocytes and macrocytes. So there raises a question: whether purinoceptor P2Y6 also takes part in neuroinflammation? Thus, this review mainly concerns about the properties and roles of purinoceptor P2Y6, including (1) structure of purinoceptor P2Y6; (2) distribution and properties of purinoceptor P2Y6; (3) relationships between purinoceptor P2Y6 and microglia; (4) relationships between purinoceptor P2Y6 and immunoinflammation. Itos proposed that purinoceptor P2Y6 may play a role in neuroinflammation in CNS, although further research is still required.

Therapeutic potential of extracellular ATP and P2 receptors in nervous system diseases

Extracellular adenosine 5 inch-triphosphate (ATP) is a key signaling molecule present in the central nervous system (CNS), and now is receiving greater attention due to its role as a messenger in the CNS during different physiological and pathological events. ATP is released into the extracellular space through vesicular exocytosis or from damaged and dying cells. Once in the extracellular environment, ATP binds to the specific receptors termed P2, which mediate ATP effects and are present broadly in both neurons and glial cells. There are P2X, the ligand-gated ionotropic receptors, possessing low affinity for ATP and responsible for fast excitatory neurotransmission, and P2Y, the metabotropic G-protein-coupled receptors, possessing high affinity for ATP. Since massive extracellular release of ATP often occurs after stress, brain ischemia and trauma, the extracellular ATP is considered relating to or involving in the pathological processes of many nervous system diseases. Conversely, the trophic functions have also been extensively described for the extracellular ATP. Therefore, extracellular ATP plays a very complex role in the CNS and its binding to P2 receptors can be related to toxic and/or beneficial effects. In this review, we described the extracellular ATP acting via P2 receptors as a potent therapeutic target for treatment of nervous system diseases.

P2Y6 receptors require an intact cysteinyl leukotriene synthetic and signaling system to induce survival and activation of mast cells

Cysteinyl leukotrienes (cys-LTs) induce inflammatory responses through type 1 (CysLT1R) and type 2 (CysLT2R) cys-LT receptors and activate mast cells in vitro. We previously demonstrated that cys-LTs cross-desensitized IL-4-primed primary human mast cells (hMCs) to stimulation with the nucleotide uridine diphosphate (UDP). We now report that hMCs, mouse bone marrow-derived mast cells (mBMMCs), and the human MC line LAD2 all express UDP-selective P2Y6 receptors that cooperate with CysLT1R to promote cell survival and chemokine generation by a pathway involving reciprocal ligand-mediated cross-talk. Leukotriene (LT) D4, the most potent CysLT1R ligand, and UDP both induced phosphorylation of ERK and prolonged the survival of cytokine-starved hMCs and mBMMCs. ERK activation and cytoprotection in response to either ligand were attenuated by treatment of the cells with a selective P2Y6 receptor antagonist (MRS2578), which did not interfere with signaling through recombinant CysLT1R. Surprisingly, both UDP and LTD4-mediated ERK activation and cytoprotection were absent in mBMMCs lacking CysLT1R and the biosynthetic enzyme LTC4 synthase, implying a requirement for a cys-LT-mediated autocrine loop. In IL-4-primed LAD2 cells, LTD4 induced the generation of MIP-1beta, a response blocked by short hairpin RNA-mediated knockdown of CysLT1R or P2Y6 receptors, but not of CysLT2R. Thus, CysLT1R and P2Y6 receptors, which are coexpressed on many cell types of innate immunity, reciprocally amplify one another's function in mast cells through endogenous ligands.

Changes in the control of gastric motor activity during metamorphosis in the amphibian Xenopus laevis, with special emphasis on purinergic mechanisms

The stomach of the amphibian Xenopus laevis is subject to extensive remodelling during metamorphosis. We investigated the changes in gastric activity control during this period using in vitro circular smooth muscle preparations mounted in organ baths. The nitric oxide synthase inhibitor L-NAME increased mean force in metamorphic and juvenile frogs but not in prometamorphic tadpoles. Serotonin (5-HT) relaxed stomach muscle prior to metamorphosis but elicited a biphasic response in juveniles consisting of contraction at low concentrations and relaxation at high concentrations. The effects of 5-HT were blocked by methysergide. In the prometamorphic tadpole, ATP elicited relaxation that was blocked by the ectonucleotidase inhibitor ARL67156 and the adenosine A(1) receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), suggesting adenosine as the mediator. Exogenous adenosine and the A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) induced relaxation at all stages. After metamorphosis, the potency of ATP decreased and neither DPCPX nor ARL67156 could block ATP-induced relaxation. Uridine 5'-triphosphate (UTP) induced relaxation prior to metamorphosis, but caused contraction of muscle strips from metamorphosing tadpoles. Single doses of UTP blocked phasic contractions in juveniles in a tetrodotoxin (TTX)-sensitive manner while the simultaneous increase in muscle tension was TTX insensitive. The P2X(1)/P2X(3) receptor agonist alpha-beta-MeATP elicited pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS)-sensitive contractions at all stages investigated. These results indicate the development of an inhibitory nitrergic tonus during metamorphosis and a 5-HT receptor involved in muscle contraction. Also, the development of UTP receptors mediating increased tension and neural UTP receptors decreasing contraction frequency in juveniles is indicated. An adenosine A(1)-like receptor mediating relaxation and a P2X-like receptor mediating contraction is demonstrated at all stages.

Knockout mice reveal a role for P2Y6 receptor in macrophages, endothelial cells, and vascular smooth muscle cells

P2Y receptors are G-protein-coupled receptors activated by extracellular nucleotides. The P2Y(6) receptor is selectively activated by UDP, and its transcript has been detected in numerous organs, including the spleen, thymus, intestine, blood leukocytes, and aorta. To investigate the biological functions of this receptor, we generated P2Y(6)-null mice by gene targeting. The P2Y(6) knockout (KO) mice are viable and are not distinguishable from the wild-type (WT) mice in terms of growth or fertility. In thioglycollate-elicited macrophages, the production of inositol phosphate in response to UDP stimulation was lost, indicating that P2Y(6) is the unique UDP-responsive receptor expressed by mouse macrophages. Furthermore, the amount of interleukin-6 and macrophage-inflammatory protein-2, but not tumor necrosis factor-alpha, released in response to lipopolysaccharide stimulation was significantly enhanced in the presence of UDP, and this effect was lost in the P2Y(6) KO macrophages. The endothelium-dependent relaxation of the aorta by UDP was abolished in KO P2Y(6) mice. The contractile effect of UDP on the aorta, observed when endothelial nitric-oxide synthase is blocked, was also abolished in P2Y(6)-null mice. In conclusion, we generated P2Y(6)-deficient mice and have shown that these mice have a defective response to UDP in macrophages, endothelial cells, and vascular smooth muscle cells. These observations might be relevant to several physiopathological conditions such as atherosclerosis or hypertension.

Up4A stimulates endothelium-independent contraction of isolated rat pulmonary artery

Extracellular nucleotides, such as ATP, UDP, and UTP, regulate pulmonary vascular tone through P2X and P2Y receptors. Recently, uridine adenosine tetraphosphate (Up(4)A) was reported as a novel endothelium-derived vasoconstrictive factor. Up(4)A contains both purine and pyrimidine moieties, which potentially activate P2X and P2Y receptors. The present study examined the effect of Up(4)A on contractility of isolated rat pulmonary artery. Up(4)A at 1-100 microM stimulated contraction in a concentration-dependent manner. Up(4)A was equipotent as UTP and UDP in the endothelium-denuded artery while much more effective than UTP and UDP in endothelium-intact preparations. The vasoconstrictor effect of Up(4)A was inhibited by suramin but not IP(5)I or desensitization of P2X receptors with alpha,beta-methylene-ATP (alpha,beta-Me-ATP). Up(4)A-induced contraction was also inhibited by pretreatment with thapsigargin, nitrendipine, or EGTA but unaffected by H1152. Furthermore, unlike ATP and UTP, Up(4)A did not induce relaxation of endothelium-intact preparations precontracted with phenylephrine. These results suggest that Up(4)A is a potent vasoconstrictor, but not a vasodilator, of the rat pulmonary artery. Up(4)A likely acts through a suramin-sensitive P2Y receptor. The contractile effect of Up(4)A involves the entry of extracellular Ca(2+) and release of Ca(2+) from intracellular stores but not Ca(2+) sensitization via the RhoA/Rho kinase pathway. Up(4)A, therefore, potentially plays an important role in the regulation of pulmonary vascular tone.

P2 purinoceptors: historical perspective and classification

This article presents an overview that gives some historical perspective to the detailed papers at the cutting edge of P2 purinoceptor research that follow. I consider the proposal, first put forward by Abbracchio & Burnstock (Pharmacol Ther 64:445-475, 1994), that P2 purinoceptors should be regarded as members of two main families: a P2X purinoceptor family consisting of ligand-gated ion channels, and a P2Y purinoceptor family consisting of G protein-coupled receptors. The latest subclasses of these two families (P2X1-4 and P2Y1-5), identified largely on the basis of molecular cloning and expression, are tabled. Finally, I suggest some future directions for P2 purinoceptor research, including studies of the long-term (trophic) actions of purines, the evolution and development of purinoceptors and therapeutic applications.

Structure-activity relationship of uridine 5'-diphosphoglucose analogues as agonists of the human P2Y14 receptor

UDP-glucose (UDPG) and derivatives are naturally occurring agonists of the Gi protein-coupled P2Y14 receptor, which occurs in the immune system. We synthesized and characterized pharmacologically novel analogues of UDPG modified on the nucleobase, ribose, and glucose moieties, as the basis for designing novel ligands in conjunction with modeling. The recombinant human P2Y14 receptor expressed in COS-7 cells was coupled to phospholipase C through an engineered Galpha-q/i protein. Most modifications of the uracil or ribose moieties abolished activity; this is among the least permissive P2Y receptors. However, a 2-thiouracil modification in 15 (EC50 49 +/- 2 nM) enhanced the potency of UDPG (but not UDP-glucuronic acid) by 7-fold. 4-Thio analogue 13 was equipotent to UDPG, but S-alkylation was detrimental. Compound 15 was docked in a rhodposin-based receptor homology model, which correctly predicted potent agonism of UDP-fructose, UDP-mannose, and UDP-inositol. The hexose moiety of UDPG interacts with multiple H-bonding and charged residues and provides a fertile region for agonist modification.

UDP acting at P2Y6 receptors is a mediator of microglial phagocytosis

Microglia, brain immune cells, engage in the clearance of dead cells or dangerous debris, which is crucial to the maintenance of brain functions. When a neighbouring cell is injured, microglia move rapidly towards it or extend a process to engulf the injured cell. Because cells release or leak ATP when they are stimulated or injured, extracellular nucleotides are thought to be involved in these events. In fact, ATP triggers a dynamic change in the motility of microglia in vitro and in vivo, a previously unrecognized mechanism underlying microglial chemotaxis; in contrast, microglial phagocytosis has received only limited attention. Here we show that microglia express the metabotropic P2Y6 receptor whose activation by endogenous agonist UDP triggers microglial phagocytosis. UDP facilitated the uptake of microspheres in a P2Y6-receptor-dependent manner, which was mimicked by the leakage of endogenous UDP when hippocampal neurons were damaged by kainic acid in vivo and in vitro. In addition, systemic administration of kainic acid in rats resulted in neuronal cell death in the hippocampal CA1 and CA3 regions, where increases in messenger RNA encoding P2Y6 receptors that colocalized with activated microglia were observed. Thus, the P2Y6 receptor is upregulated when neurons are damaged, and could function as a sensor for phagocytosis by sensing diffusible UDP signals, which is a previously unknown pathophysiological function of P2 receptors in microglia.

P2 receptors in atherosclerosis and postangioplasty restenosis

Atherosclerosis is an immunoinflammatory process that involves complex interactions between the vessel wall and blood components and is thought to be initiated by endothelial dysfunction [Ross (Nature 362:801–09, 1993); Fuster et al. (N Engl J Med 326:242–50, 1992); Davies and Woolf (Br Heart J 69:S3–S11, 1993)]. Extracellular nucleotides that are released from a variety of arterial and blood cells [Di Virgilio and Solini (Br J Pharmacol 135:831–42, 2002)] can bind to P2 receptors and modulate proliferation and migration of smooth muscle cells (SMC), which are known to be involved in intimal hyperplasia that accompanies atherosclerosis and postangioplasty restenosis [Lafont et al. (Circ Res 76:996–002, 1995)]. In addition, P2 receptors mediate many other functions including platelet aggregation, leukocyte adherence, and arterial vasomotricity. A direct pathological role of P2 receptors is reinforced by recent evidence showing that upregulation and activation of P2Y₂ receptors in rabbit arteries mediates intimal hyperplasia [Seye et al. (Circulation 106:2720–726, 2002)]. In addition, upregulation of functional P2Y receptors also has been demonstrated in the basilar artery of the rat double-hemorrhage model [Carpenter et al. (Stroke 32:516–22, 2001)] and in coronary artery of diabetic dyslipidemic pigs [Hill et al. (J Vasc Res 38:432–43, 2001)]. It has been proposed that upregulation of P2Y receptors may be a potential diagnostic indicator for the early stages of atherosclerosis [Elmaleh et al. (Proc Natl Acad Sci U S A 95:691–95, 1998)]. Therefore, particular effort must be made to understand the consequences of nucleotide release from cells in the cardiovascular system and the subsequent effects of P2 nucleotide receptor activation in blood vessels, which may reveal novel therapeutic strategies for atherosclerosis and restenosis after angioplasty.

Uridine and cytidine in the brain: their transport and utilization

The pyrimidines cytidine (as CTP) and uridine (which is converted to UTP and then CTP) contribute to brain phosphatidylcholine and phosphatidylethanolamine synthesis via the Kennedy pathway. Their uptake into brain from the circulation is initiated by nucleoside transporters located at the blood-brain barrier (BBB), and the rate at which uptake occurs is a major factor determining phosphatide synthesis. Two such transporters have been described: a low-affinity equilibrative system and a high-affinity concentrative system. It is unlikely that the low-affinity transporter contributes to brain uridine or cytidine uptake except when plasma concentrations of these compounds are increased several-fold experimentally. CNT2 proteins, the high-affinity transporters for purines like adenosine as well as for uridine, have been found in cells comprising the BBB of rats. However, to date, no comparable high-affinity carrier protein for cytidine, such as CNT1, has been detected at this location. Thus, uridine may be more available to brain than cytidine and may be the major precursor in brain for both the salvage pathway of pyrimidine nucleotides and the Kennedy pathway of phosphatide synthesis. This recognition may bear on the effects of cytidine or uridine sources in neurodegenerative diseases.

Structure-activity relationships of uridine 5'-diphosphate analogues at the human P2Y6 receptor

The structure-activity relationships and molecular modeling of the uracil nucleotide activated P2Y6 receptor have been studied. Uridine 5'-diphosphate (UDP) analogues bearing substitutions of the ribose moiety, the uracil ring, and the diphosphate group were synthesized and assayed for activity at the human P2Y6 receptor. The uracil ring was modified at the 4 position, with the synthesis of 4-substituted-thiouridine 5'-diphosphate analogues, as well as at positions 2, 3, and 5. The effect of modifications at the level of the phosphate chain was studied by preparing a cyclic 3',5'-diphosphate analogue, a 3'-diphosphate analogue, and several dinucleotide diphosphates. 5-Iodo-UDP 32 (EC50 = 0.15 microM) was equipotent to UDP, while substitutions of the 2'-hydroxyl (amino, azido) greatly reduce potency. The 2- and 4-thio analogues, 20 and 21, respectively, were also relatively potent in comparison to UDP. However, most other modifications greatly reduced potency. Molecular modeling indicates that the beta-phosphate of 5'-UDP and analogues is essential for the establishment of electrostatic interactions with two of the three conserved cationic residues of the receptor. Among 4-thioether derivatives, a 4-ethylthio analogue 23 displayed an EC50 of 0.28 microM, indicative of favorable interactions predicted for a small 4-alkylthio moiety with the aromatic ring of Y33 in TM1. The activity of analogue 19 in which the ribose was substituted with a 2-oxabicyclohexane ring in a rigid (S)-conformation (P = 126 degrees , 1'-exo) was consistent with molecular modeling. These results provide a better understanding of molecular recognition at the P2Y6 receptor and will be helpful in designing selective and potent P2Y6 receptor ligands.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

P2 purinergic receptors signal to STAT3 in astrocytes: Difference in STAT3 responses to P2Y and P2X receptor activation

Extracellular ATP, released upon tissue damage to the CNS, can evoke reactive astrogliosis. The released ATP activates P2 purinergic receptors associated with the proliferation of normally quiescent astrocytes, although the underlying mechanisms remain to be fully elucidated. Signal transducer and activator of transcription 3 (STAT3) has been implicated in reactive astrogliosis and plays an important role in cell cycle regulation. Therefore, we investigated whether extracellular ATP and purinergic receptors regulate STAT3 signaling. Using immunoblot analysis, we found that addition of ATP to primary cultures of rat cortical astrocytes increased Ser-727 phosphorylation of STAT3 in a time-sensitive and concentration-dependent manner. ATP-stimulated Ser-727 STAT3 phosphorylation was mediated through P2 receptor activation since suramin, an antagonist of P2 receptors, diminished this response, whereas 8-(para-sulfo-phenyl)-theophylline (8PSTP), an antagonist of P1 receptors, did not. We found that UTP, an agonist of P2Y(2/4/6) receptors, stimulated rapid and robust phosphorylation of Ser727-STAT3, whereas BzATP, an agonist for P2X receptors, exhibited a delayed and weaker response. In contrast, both P2Y and P2X agonists stimulated phosphorylation of Tyr705-STAT3 to a similar extent. P2 receptors can couple to extracellular signal-regulated protein kinases (ERK) and we found that inhibition of ERK signaling blocked phosphorylation of Ser727-STAT3. Further characterization of the Ser727-STAT3 phosphorylation response to P2Y receptor activation supported a role for P2Y2 and P2Y4, but not P2Y6, receptors as well as a partial role for P2Y1 receptors. Because phosphorylation of Ser727-STAT3 can promote DNA transcriptional activity of cell cycle regulatory genes, the differences in phosphorylation of Ser727-STAT3 may contribute to the mechanism by which P2Y receptors promote, whereas P2X receptors inhibit, astrocyte proliferation. In support of this hypothesis, inhibition of STAT3 activation by cucurbitacin I prevented ATP-stimulated mitogenesis. We conclude that P2 receptors stimulate STAT3 activation and suggest that P2 receptor/STAT3 signaling may play an important role in astrocyte proliferation and reactive astrogliosis.

Purinergic signalling and intercellular Ca2+ wave propagation in the organ of Corti

Extracellular ATP is a key neuromodulator of visual and auditory sensory epithelia. In the rat cochlea, pharmacological dissection indicates that ATP, acting through a highly sensitive purinergic/IP(3)-mediated signaling pathway with (little or) no involvement of ryanodine receptors, is the principal paracrine mediator implicated in the propagation of calcium waves through supporting and epithelial cells. Measurement of sensitivity to UTP and other purinergic agonists implicate P2Y(2) and P2Y(4) as the main P2Y receptor isoforms involved in these responses. Ca2+ waves, elicited under highly reproducible conditions by carefully controlling dose (1 microM) and timing of focal agonist application (0.2s), extended over radial distance greater than 160 microm from the source, identical to those activated by damaging single outer hair cells. Altogether, these results indicate that intercellular calcium waves are a robust phenomenon that confers a significant ability for cell-cell communication in the mammalian cochlea. Further ongoing research will reveal the roles that such Ca2+ waves play in the inner ear.

P2 receptors in atherosclerosis and postangioplasty restenosis

Atherosclerosis is an immunoinflammatory process that involves complex interactions between the vessel wall and blood components and is thought to be initiated by endothelial dysfunction [1-3]. Extracellular nucleotides that are released from a variety of arterial and blood cells [4] can bind to P2 receptors and modulate proliferation and migration of smooth muscle cells (SMC), which is known to be involved in intimal hyperplasia that accompanies atherosclerosis and postangioplasty restenosis [5]. In addition, P2 receptors mediate many other functions, including platelet aggregation, leukocyte adherence, and arterial vasomotoricity. A direct pathological role of P2 receptors is reinforced by recent evidence showing that up-regulation and activation of P2Y(2) receptors in rabbit arteries mediates intimal hyperplasia [6]. In addition, up-regulation of functional P2Y receptors also has been demonstrated in the basilar artery of the rat double-hemorrhage model [7] and in coronary arteries of diabetic dyslipidemic pigs [8]. It has been proposed that up-regulation of P2Y receptors may be a potential diagnostic indicator for the early stages of atherosclerosis [9]. Therefore, particular effort must be made to understand the consequences of nucleotide release from cells in the cardiovascular system and the subsequent effects of P2 nucleotide receptor activation in blood vessels, which may reveal novel therapeutic strategies for atherosclerosis and restenosis after angioplasty.

Characterization of the purinergic receptor subtype on guinea-pig suburothelial myofibroblasts

OBJECTIVE

To identify particular purinoceptor subtypes by immunohistochemical labelling, as a layer of suburothelial myofibroblasts has been identified in the urinary bladder, and these cells respond to exogenous ATP by generating an intracellular Ca2+ transient, but the particular purinoceptor that responds to ATP is unclear.

MATERIALS AND METHODS

Tissue sections and isolated cells from the urothelial layer of the guinea-pig bladder were used. Preparations were labelled with primary antibodies to the intermediate-filament protein, vimentin, or the purinoceptors P2X3, P2Y1, P2Y2, P2Y4 and P2Y6. For single-labelling we used a secondary antibody tagged with the fluorescent marker Cy3, and for double-labelling also a secondary antibody tagged with fluorescein isothiocyanate or Cy2. Images were examined using a confocal microscope, with an argon (488 nm) or helium-neon (543 nm) laser.

RESULTS

Vimentin-labelling was confined to the suburothelial layer and appeared as discrete signals. Isolated cells labelled with vimentin and strongly for the P2Y6 antibody. There was weaker staining for P2X3, P2Y2 and P2Y4, but none to P2Y1. With frozen sections there was P2Y6 labelling in the urothelial and suburothelial layer.

CONCLUSION

The predominant purinoceptor in suburothelial myofibroblasts, from these labelling studies, is the P2Y6 subtype. However, there was weaker labelling to other subtypes, suggesting multiple receptor subtypes or heterogeneity of receptor subunits. The consequences of there being multiple purinoceptor subtypes in the suburothelial space with respect to sensory signalling are discussed.

Structure activity and molecular modeling analyses of ribose- and base-modified uridine 5'-triphosphate analogues at the human P2Y2 and P2Y4 receptors

With the long-term goal of developing receptor subtype-selective high affinity agonists for the uracil nucleotide-activated P2Y receptors we have carried out a series of structure activity and molecular modeling studies of the human P2Y2 and P2Y4 receptors. UTP analogues with substitutions in the 2'-position of the ribose moiety retained capacity to activate both P2Y2 and P2Y4 receptors. Certain of these analogues were equieffective for activation of both receptors whereas 2'-amino-2'-deoxy-UTP exhibited higher potency for the P2Y2 receptor and 2'-azido-UTP exhibited higher potency for the P2Y4 receptor. 4-Thio substitution of the uracil base resulted in a UTP analogue with increased potency relative to UTP for activation of both the P2Y2 and P2Y4 receptors. In contrast, 2-thio substitution and halo- or alkyl substitution in the 5-position of the uracil base resulted in molecules that were 3-30-fold more potent at the P2Y2 receptor than P2Y4 receptor. 6-Aza-UTP was a P2Y2 receptor agonist that exhibited no activity at the P2Y4 receptor. Stereoisomers of UTPalphaS and 2'-deoxy-UTPalphaS were more potent at the P2Y2 than P2Y4 receptor, and the R-configuration was favored at both receptors. Molecular docking studies revealed that the binding mode of UTP is similar for both the P2Y2 and P2Y4 receptor binding pockets with the most prominent dissimilarities of the two receptors located in the second transmembrane domain (V90 in the P2Y2 receptor and I92 in the P2Y4 receptor) and the second extracellular loop (T182 in the P2Y2 receptor and L184 in the P2Y4 receptor). In summary, this work reveals substitutions in UTP that differentially affect agonist activity at P2Y2 versus P2Y4 receptors and in combination with molecular modeling studies should lead to chemical synthesis of new receptor subtype-selective drugs.

Neurons and glial cells differentially express P2Y receptor mRNAs in the rat dorsal root ganglion and spinal cord

We examined the precise distribution of mRNAs for six cloned rat P2Y receptor subtypes, P2Y1, P2Y2, P2Y4, P2Y6, P2Y12, and P2Y14, in the dorsal root ganglion (DRG) and spinal cord by in situ hybridization histochemistry (ISHH) with 35S-labeled riboprobes. In the DRG, P2Y1 and P2Y2 mRNAs were expressed by 15% and 24% of all neurons, respectively. Although each receptor was evenly distributed between neurofilament-positive and -negative neurons, P2Y2 was rather selectively expressed by TrkA-positive neurons. Schwann cells expressed P2Y2 mRNA, and the nonneuronal cells around the DRG neurons, perhaps the satellite cells, expressed P2Y12 and P2Y14 mRNAs. No ISHH signals for P2Y4 or P2Y6 were seen in any cellular components of the DRG. In the spinal cord, P2Y1 and P2Y4 mRNAs were expressed by some of the dorsal horn neurons, whereas the motor neurons in the ventral horn had P2Y4 and P2Y6 mRNAs. In addition, astrocytes in the gray matter had P2Y1 mRNA, and the microglia throughout the spinal cord expressed P2Y12 mRNA. P2Y14 mRNA was weakly expressed by putative microglia. These findings should provide useful information in interpreting pharmacological and electrophysiological studies in this field given the lack of highly selective antagonists for each P2Y receptor subtype.