Effect of loss of P2Y(2) receptor gene expression on nucleotide regulation of murine epithelial Cl(-) transport

Therapeutic potential for P2Y2 receptor antagonism

G protein-coupled receptors are the target of more than 30% of all FDA-approved drug therapies. Though the purinergic P2 receptors have been an attractive target for therapeutic intervention with successes such as the P2Y₁₂ receptor antagonist, clopidogrel, P2Y₂ receptor (P2Y₂R) antagonism remains relatively unexplored as a therapeutic strategy. Due to a lack of selective antagonists to modify P2Y₂R activity, studies using primarily genetic manipulation have revealed roles for P2Y₂R in a multitude of diseases. These include inflammatory and autoimmune diseases, fibrotic diseases, renal diseases, cancer, and pathogenic infections. With the advent of AR-C118925, a selective and potent P2Y₂R antagonist that became commercially available only a few years ago, new opportunities exist to gain a more robust understanding of P2Y₂R function and assess therapeutic effects of P2Y₂R antagonism. This review discusses the characteristics of P2Y₂R that make it unique among P2 receptors, namely its involvement in five distinct signaling pathways including canonical Gα_q protein signaling. We also discuss the effects of other P2Y₂R antagonists and the pivotal development of AR-C118925. The remainder of this review concerns the mounting evidence implicating P2Y₂Rs in disease pathogenesis, focusing on those studies that have evaluated AR-C118925 in pre-clinical disease models.

Stem cell conversion to the cardiac lineage requires nucleotide signalling from apoptosing cells

Pluripotent stem cells can be driven by manipulation of Wnt signalling through a series of states similar to those that occur during early embryonic development, transitioning from an epithelial phenotype into the cardiogenic-mesoderm lineage and ultimately into functional cardiomyocytes. Strikingly, we observed that initiation of differentiation in induced pluripotent stem cells (iPSCs) and embryonic stem cells triggers widespread apoptosis, followed by a synchronous epithelial-mesenchymal transition (EMT). Apoptosis is caused by the absence of bFGF in the differentiation medium. EMT requires induction of the transcription factors SNAI1 and SNAI2 downstream of MESP1 expression, and double knockout of SNAI1 and SNAI2 or loss of MESP1 in iPSCs blocks EMT and prevents cardiac differentiation. Remarkably, blockade of early apoptosis, either chemically or by ablation of pro-apoptotic genes, also completely prevents EMT, suppressing even the earliest events in mesoderm conversion, including T/BRA, TBX6 and MESP1 induction. Conditioned medium from WNT-activated wild-type iPSCs overcomes the block to EMT by cells incapable of apoptosis, suggesting involvement of soluble factors from apoptotic cells in mesoderm conversion. Knockout of the PANX1 channel blocked EMT, whereas treatment with a purinergic P2-receptor inhibitor or addition of apyrase demonstrated a requirement for nucleotide triphosphate signalling. ATP and/or UTP was sufficient to induce a partial EMT in apoptosis-incapable cells treated with WNT activator. Notably, knockout of the ATP/UTP-specific P2Y2 receptor blocked EMT and mesoderm induction. We conclude that in addition to acting as chemo-attractants for clearance of apoptotic cells, nucleotides can function as essential paracrine signals that, with WNT signalling, create a logical AND gate for mesoderm specification.

Role of the Purinergic P2Y2 Receptor in Pulmonary Hypertension

Pulmonary arterial hypertension (PAH), group 1 pulmonary hypertension (PH), is a fatal disease that is characterized by vasoconstriction, increased pressure in the pulmonary arteries, and right heart failure. PAH can be described by abnormal vascular remodeling, hyperproliferation in the vasculature, endothelial cell dysfunction, and vascular tone dysregulation. The disease pathomechanisms, however, are as yet not fully understood at the molecular level. Purinergic receptors P2Y within the G-protein-coupled receptor family play a major role in fluid shear stress transduction, proliferation, migration, and vascular tone regulation in systemic circulation, but less is known about their contribution in PAH. Hence, studies that focus on purinergic signaling are of great importance for the identification of new therapeutic targets in PAH. Interestingly, the role of P2Y2 receptors has not yet been sufficiently studied in PAH, whereas the relevance of other P2Ys as drug targets for PAH was shown using specific agonists or antagonists. In this review, we will shed light on P2Y receptors and focus more on the P2Y2 receptor as a potential novel player in PAH and as a new therapeutic target for disease management.

P2Y2 Receptor Promotes High-Fat Diet-Induced Obesity

P2Y₂, a G protein-coupled receptor (R), is expressed in all organs involved in the development of obesity and insulin resistance. To explore the role of it in diet-induced obesity, we fed male P2Y₂-R whole body knockout (KO) and wild type (WT) mice (B6D2 genetic background) with regular diet (CNT; 10% calories as fat) or high-fat diet (HFD; 60% calories as fat) with free access to food and water for 16 weeks, and euthanized them. Adjusted for body weights (BW), KO mice consumed modestly, but significantly more HFD vs. WT mice, and excreted well-formed feces with no taint of fat or oil. Starting from the 2nd week, HFD-WT mice displayed significantly higher BW with terminal mean difference of 22% vs. HFD-KO mice. Terminal weights of white adipose tissue (WAT) were significantly lower in the HFD-KO vs. HFD-WT mice. The expression of P2Y₂-R mRNA in WAT was increased by 2-fold in HFD-fed WT mice. Serum insulin, leptin and adiponectin levels were significantly elevated in the HFD-WT mice, but not in the HFD-KO mice. When induced in vitro, preadipocytes derived from KO mice fed regular diet did not differentiate and mature as robustly as those from the WT mice, as assessed by cellular expansion and accumulation of lipid droplets. Blockade of P2Y₂-R by AR-C118925 in preadipocytes derived from WT mice prevented differentiation and maturation. Under basal conditions, KO mice had significantly higher serum triglycerides and showed slightly impaired lipid tolerance as compared to the WT mice. HFD-fed KO mice had significantly better glucose tolerance (GTT) as compared to HFD-fed WT mice. Whole body insulin sensitivity and mRNA expression of insulin receptor, IRS-1 and GLUT4 in WAT was significantly higher in HFD-fed KO mice vs. HFD-fed WT mice. On the contrary, the expression of pro-inflammatory molecules MCP-1, CCR2, CD68, and F4/80 were significantly higher in the WAT of HFD-fed WT vs. HFD-fed KO mice. These data suggest that P2Y₂-R plays a significant role in the development of diet-induced obesity by promoting adipogenesis and inflammation, and altering the production of adipokines and lipids and their metabolism in adipose tissue, and thereby facilitates HFD-induced insulin resistance.

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.

Genetic Deletion of P2Y2 Receptor Offers Long-Term (5 Months) Protection Against Lithium-Induced Polyuria, Natriuresis, Kaliuresis, and Collecting Duct Remodeling and Cell Proliferation

Chronic lithium administration for the treatment of bipolar disorder leads to nephrogenic diabetes insipidus (NDI), characterized by polyuria, natriuresis, kaliuresis, and collecting duct remodeling and cell proliferation among other features. Previously, using a 2-week lithium-induced NDI model, we reported that P2Y2 receptor (R) knockout mice are significantly resistant to polyuria, natriuresis, kaliuresis, and decrease in AQP2 protein abundance in the kidney relative to wild type mice. Here we show this protection is long-lasting, and is also associated with significant amelioration of lithium-induced collecting duct remodeling and cell proliferation. Age-matched wild type and knockout mice were fed regular (n = 5/genotype) or lithium-added (40 mmol/kg chow; n = 10/genotype) diet for 5 months and euthanized. Water intake, urine output and osmolality were monitored once in every month. Salt blocks were provided to mice on lithium-diet to prevent sodium loss. At the end of 5 months mice were euthanized and serum and kidney samples were analyzed. There was a steady increase in lithium-induced polyuria, natriuresis and kaliuresis in wild type mice over the 5-month period. Increases in these urinary parameters were very low in lithium-fed knockout mice, resulting in significantly widening differences between the wild type and knockout mice. Terminal AQP2 and NKCC2 protein abundances in the kidney were significantly higher in lithium-fed knockout vs. wild type mice. There were no significant differences in terminal serum lithium or sodium levels between the wild type and knockout mice. Confocal immunofluorescence microscopy revealed that lithium-induced marked remodeling of collecting duct with significantly increased proportion of [H+]-ATPase-positive intercalated cells and decreased proportion of AQP2-positive principal cells in the wild type, but not in knockout mice. Lithium-induced collecting duct cell proliferation (indicated by Ki67 labeling), was significantly lower in knockout vs. wild type mice. This is the first piece of evidence that purinergic signaling is potentially involved in lithium-induced collecting duct remodeling and cell proliferation. Our results demonstrate that genetic deletion of P2Y2-R protects against the key structural and functional alterations in Li-induced NDI, and underscore the potential utility of targeting this receptor for the treatment of NDI in bipolar patients on chronic lithium therapy.

Purine and purinergic receptors

Adenosine 5′-triphosphate acts as an extracellular signalling molecule (purinergic signalling), as well as an intracellular energy source. Adenosine 5′-triphosphate receptors have been cloned and characterised. P1 receptors are selective for adenosine, a breakdown product of adenosine 5′-triphosphate after degradation by ectonucleotidases. Four subtypes are recognised, A₁, A_2A, A_2B and A₃ receptors. P2 receptors are activated by purine and by pyrimidine nucleotides. P2X receptors are ligand-gated ion channel receptors (seven subunits (P2X1-7)), which form trimers as both homomultimers and heteromultimers. P2Y receptors are G protein-coupled receptors (eight subtypes (P2Y_{1/2/4/6/11/12/13/14})). There is both purinergic short-term signalling and long-term (trophic) signalling. The cloning of P2X-like receptors in primitive invertebrates suggests that adenosine 5′-triphosphate is an early evolutionary extracellular signalling molecule. Selective purinoceptor agonists and antagonists with therapeutic potential have been developed for a wide range of diseases, including thrombosis and stroke, dry eye, atherosclerosis, kidney failure, osteoporosis, bladder incontinence, colitis, neurodegenerative diseases and cancer.

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.

Molecular Recognition of Agonists and Antagonists by the Nucleotide-Activated G Protein-Coupled P2Y2 Receptor

A homology model of the nucleotide-activated P2Y₂R was created based on the X-ray structures of the P2Y₁ receptor. Docking studies were performed, and receptor mutants were created to probe the identified binding interactions. Mutation of residues predicted to interact with the ribose (Arg110) and the phosphates of the nucleotide agonists (Arg265, Arg292) or that contribute indirectly to binding (Tyr288) abolished activity. The Y114F, R194A, and F261A mutations led to inactivity of diadenosine tetraphosphate and to a reduced response of UTP. Significant reduction in agonist potency was observed for all other receptor mutants (Phe111, His184, Ser193, Phe261, Tyr268, Tyr269) predicted to be involved in agonist recognition. An ionic lock between Asp185 and Arg292 that is probably involved in receptor activation interacts with the phosphate groups. The antagonist AR-C118925 and anthraquinones likely bind to the orthosteric site. The updated homology models will be useful for virtual screening and drug design.

From UTP to AR-C118925, the discovery of a potent non nucleotide antagonist of the P2Y2 receptor

The G protein-coupled P2Y₂ receptor, activated by ATP and UTP has been reported as a potential drug target for a wide range of important clinical conditions, such as tumor metastasis, kidney disorders, and in the treatment of inflammatory conditions. However, pharmacological studies on this receptor have been impeded by the limited reported availability of stable, potent and selective P2Y₂R antagonists. This article describes the design and synthesis of AR-C118925, a potent and selective non-nucleotide antagonist of the P2Y₂ receptor discovered using the endogenous P2Y₂R agonist UTP as the chemical starting point.

P2Y Receptors in Immune Response and Inflammation

Metabotropic pyrimidine and purine nucleotide receptors (P2Y receptors) are expressed in virtually all cells with implications in very diverse biological functions, including the well-established platelet aggregation (P2Y12), but also immune regulation and inflammation. The classical P2Y receptors bind nucleotides and are encoded by eight genes with limited sequence homology, while phylogenetically related receptors (e.g., P2Y12-like) recognize lipids and peptides, but also nucleotide derivatives. Growing lines of evidence suggest an important function of P2Y receptors in immune cell differentiation and maturation, migration, and cell apoptosis. Here, we give a perspective on the P2Y receptors' molecular structure and physiological importance in immune cells, as well as the related diseases and P2Y-targeting therapies. Extensive research is being undertaken to find modulators of P2Y receptors and uncover their physiological roles. We anticipate the medical applications of P2Y modulators and their immune relevance.

Activation of the P2Y2 receptor regulates bone cell function by enhancing ATP release

Bone cells constitutively release ATP into the extracellular environment where it acts locally via P2 receptors to regulate bone cell function. Whilst P2Y2 receptor stimulation regulates bone mineralisation, the functional effects of this receptor in osteoclasts remain unknown. This investigation used the P2Y2 receptor knockout (P2Y2R-/- ) mouse model to investigate the role of this receptor in bone. MicroCT analysis of P2Y2R-/- mice demonstrated age-related increases in trabecular bone volume (≤48%), number (≤30%) and thickness (≤17%). In vitro P2Y2R-/- osteoblasts displayed a 3-fold increase in bone formation and alkaline phosphatase activity, whilst P2Y2R-/- osteoclasts exhibited a 65% reduction in resorptive activity. Serum cross-linked C-telopeptide levels (CTX, resorption marker) were also decreased (≤35%). The resorption defect in P2Y2R-/- osteoclasts was rescued by the addition of exogenous ATP, suggesting that an ATP deficit could be a key factor in the reduced function of these cells. In agreement, we found that basal ATP release was reduced up to 53% in P2Y2R-/- osteoclasts. The P2Y2 receptor agonists, UTP and 2-thioUTP, increased osteoclast activity and ATP release in wild-type but not in P2Y2R-/- cells. This indicates that the P2Y2 receptor may regulate osteoclast function indirectly by promoting ATP release. UTP and 2-thioUTP also stimulate ATP release from osteoblasts suggesting that the P2Y2 receptor exerts a similar function in these cells. Taken together, our findings are consistent with the notion that the primary action of P2Y2 receptor signalling in bone is to regulate extracellular ATP levels.

Synthesis, characterization, and in vitro evaluation of the selective P2Y2 receptor antagonist AR-C118925

The G_q protein-coupled, ATP- and UTP-activated P2Y₂ receptor is a potential drug target for a range of different disorders, including tumor metastasis, inflammation, atherosclerosis, kidney disorders, and osteoporosis, but pharmacological studies are impeded by the limited availability of suitable antagonists. One of the most potent and selective antagonists is the thiouracil derivative AR-C118925. However, this compound was until recently not commercially available and little is known about its properties. We therefore developed an improved procedure for the synthesis of AR-C118925 and two derivatives to allow up-scaling and assessed their potency in calcium mobilization assays on the human and rat P2Y₂ receptors recombinantly expressed in 1321N1 astrocytoma cells. The compound was further evaluated for inhibition of P2Y₂ receptor-induced β-arrestin translocation. AR-C118925 behaved as a competitive antagonist with pA ₂ values of 37.2 nM (calcium assay) and 51.3 nM (β-arrestin assay). Selectivity was assessed vs. related receptors including P2X, P2Y, and adenosine receptor subtypes, as well as ectonucleotidases. AR-C118925 showed at least 50-fold selectivity against the other investigated targets, except for the P2X1 and P2X3 receptors which were blocked by AR-C118925 at concentrations of about 1 μM. AR-C118925 is soluble in buffer at pH 7.4 (124 μM) and was found to be metabolically highly stable in human and mouse liver microsomes. In Caco2 cell experiments, the compound displayed moderate permeability indicating that it may show limited peroral bioavailability. AR-C118925 appears to be a useful pharmacological tool for in vitro and in vivo studies.

Drug-like Antagonists of P2Y Receptors-From Lead Identification to Drug Development

P2Y receptors are expressed in virtually all cells and tissue types and mediate an astonishing array of biological functions, including platelet aggregation, smooth muscle cell proliferation, and immune regulation. The P2Y receptors belong to the G protein-coupled receptor superfamily and are composed of eight members encoded by distinct genes that can be subdivided into two groups on the basis of their coupling to specific G-proteins. Extensive research has been undertaken to find modulators of P2Y receptors, although to date only a limited number of small-molecule P2Y receptor antagonists have been approved by drug/medicines agencies. This Perspective reviews the known P2Y receptor antagonists, highlighting oral drug-like receptor antagonists, and considers future opportunities for the development of small molecules for clinical evaluation.

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'.

Targeting renal purinergic signalling for the treatment of lithium-induced nephrogenic diabetes insipidus

Lithium still retains its critical position in the treatment of bipolar disorder by virtue of its ability to prevent suicidal tendencies. However, chronic use of lithium is often limited by the development of nephrogenic diabetes insipidus (NDI), a debilitating condition. Lithium-induced NDI is due to resistance of the kidney to arginine vasopressin (AVP), leading to polyuria, natriuresis and kaliuresis. Purinergic signalling mediated by extracellular nucleotides (ATP/UTP), acting via P2Y receptors, opposes the action of AVP on renal collecting duct (CD) by decreasing the cellular cAMP and thus AQP2 protein levels. Taking a cue from this phenomenon, we discovered the potential involvement of ATP/UTP-activated P2Y2 receptor in lithium-induced NDI in rats and showed that P2Y2 receptor knockout mice are significantly resistant to Li-induced polyuria, natriuresis and kaliuresis. Extension of these studies revealed that ADP-activated P2Y12 receptor is expressed in the kidney, and its irreversible blockade by the administration of clopidogrel bisulphate (Plavix(®)) ameliorates Li-induced NDI in rodents. Parallel in vitro studies showed that P2Y12 receptor blockade by the reversible antagonist PSB-0739 sensitizes CD to the action of AVP. Thus, our studies unravelled the potential beneficial effects of targeting P2Y2 or P2Y12 receptors to counter AVP resistance in lithium-induced NDI. If established in further studies, our findings may pave the way for the development of better and safer methods for the treatment of NDI by bringing a paradigm shift in the approach from the current therapies that predominantly counter the anti-AVP effects to those that enhance the sensitivity of the kidney to AVP action.

Baseline Goblet Cell Mucin Secretion in the Airways Exceeds Stimulated Secretion over Extended Time Periods, and Is Sensitive to Shear Stress and Intracellular Mucin Stores

Airway mucin secretion studies have focused on goblet cell responses to exogenous agonists almost to the exclusion of baseline mucin secretion (BLMS). In human bronchial epithelial cell cultures (HBECCs), maximal agonist-stimulated secretion exceeds baseline by ~3-fold as measured over hour-long periods, but mucin stores are discharged completely and require 24 h for full restoration. Hence, over 24 h, total baseline exceeds agonist-induced secretion by several-fold. Studies with HBECCs and mouse tracheas showed that BLMS is highly sensitive to mechanical stresses. Harvesting three consecutive 1 h baseline luminal incubations with HBECCs yielded equal rates of BLMS; however, lengthening the middle period to 72 h decreased the respective rate significantly, suggesting a stimulation of BLMS by the gentle washes of HBECC luminal surfaces. BLMS declined exponentially after washing HBECCs (t_1/2 = 2.75 h), to rates approaching zero. HBECCs exposed to low perfusion rates exhibited spike-like increases in BLMS when flow was jumped 5-fold: BLMS increased >4 fold, then decreased within 5 min to a stable plateau at 1.5–2-fold over control. Higher flow jumps induced proportionally higher BLMS increases. Inducing mucous hyperplasia in HBECCs increased mucin production, BLMS and agonist-induced secretion. Mouse tracheal BLMS was ~6-fold higher during perfusion, than when flow was stopped. Munc13-2 null mouse tracheas, with their defect of accumulated cellular mucins, exhibited similar BLMS as WT, contrary to predictions of lower values. Graded mucous metaplasia induced in WT and Munc13-2 null tracheas with IL-13, caused proportional increases in BLMS, suggesting that naïve Munc13-2 mouse BLMS is elevated by increased mucin stores. We conclude that BLMS is, [i] a major component of mucin secretion in the lung, [ii] sustained by the mechanical activity of a dynamic lung, [iii] proportional to levels of mucin stores, and [iv] regulated differentially from agonist-induced mucin secretion.

Infection by Toxoplasma gondii, a severe parasite in neonates and AIDS patients, causes impaired anion secretion in airway epithelia

The airway epithelia initiate and modulate the inflammatory responses to various pathogens. The cystic fibrosis transmembrane conductance regulator-mediated Cl(-) secretion system plays a key role in mucociliary clearance of inhaled pathogens. We have explored the effects of Toxoplasma gondii, an opportunistic intracellular protozoan parasite, on Cl(-) secretion of the mouse tracheal epithelia. In this study, ATP-induced Cl(-) secretion indicated the presence of a biphasic short-circuit current (Isc) response, which was mediated by a Ca(2+)-activated Cl(-) channel (CaCC) and the cystic fibrosis transmembrane conductance regulator. However, the ATP-evoked Cl(-) secretion in T. gondii-infected mouse tracheal epithelia and the elevation of [Ca(2+)]i in T. gondii-infected human airway epithelial cells were suppressed. Quantitative reverse transcription-PCR revealed that the mRNA expression level of the P2Y2 receptor (P2Y2-R) increased significantly in T. gondii-infected mouse tracheal cells. This revealed the influence that pathological changes in P2Y2-R had on the downstream signal, suggesting that P2Y2-R was involved in the mechanism underlying T. gondii infection in airways. These results link T. gondii infection as well as other pathogen infections to Cl(-) secretion, via P2Y2-R, which may provide new insights for the treatment of pneumonia caused by pathogens including T. gondii.

Close association of B2 bradykinin receptors with P2Y2 ATP receptors

Two G-protein-coupled receptors (GPCRs) that couple with Gαq/11, B2 bradykinin (BK) receptor (B2R) and ATP/UTP receptor P2Y2 (P2Y2R), are ubiquitously expressed and responsible for vascular tone, inflammation, and pain. We analysed the cellular signalling of P2Y2Rs in cells that express B2Rs. B2R desensitization induced by BK or B2R internalization-inducing glycans cross-desensitized the P2Y2R response to ATP/UTP. Fluorescence resonance energy transfer from P2Y2R-AcGFP to B2R-DsRed was detected in the cells and on the cell surfaces, showing the close association of these GPCRs. BK- and ATP-induced cross-internalization of P2Y2R and B2R, respectively, was shown in a β-galactosidase complementation assay using P2Y2R or B2R fused to the H31R substituted α donor peptide of a β-galactosidase reporter enzyme (P2Y2R-α or B2R-α) with coexpression of the FYVE domain of endofin, an early endosome protein, fused to the M15 acceptor deletion mutant of β-galactosidase (the ω peptide, FYVE-ω). Arrestin recruitment to the GPCRs by cross-activation was also shown with the similar way. Coimmunoprecipitation showed that B2R and P2Y2R were closely associated in the cotransfected cells. These results indicate that B2R couples with P2Y2R and that these GPCRs act together to fine-tune cellular responsiveness. The collaboration between these receptors may permit rapid onset and turning off of biological events.

Potential for developing purinergic drugs for gastrointestinal diseases

Treatments for inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), functional dyspepsia, or motility disorders are not adequate, and purinergic drugs offer exciting new possibilities. Gastrointestinal symptoms that could be targeted for therapy include visceral pain, inflammatory pain, dysmotility, constipation, and diarrhea. The focus of this review is on the potential for developing purinergic drugs for clinical trials to treat gastrointestinal symptoms. Purinergic receptors are divided into adenosine P1 (A(1), A(2A), A(2B), A(3)), ionotropic ATP-gated P2X ion channel (P2X(1-7)), or metabotropic P2Y(1,2,4,6,11-14) receptors. There is good experimental evidence for targeting A(2A), A(2B), A(3), P2X(7), and P2X(3) receptors or increasing endogenous adenosine levels to treat IBD, inflammatory pain, IBS/visceral pain, inflammatory diarrhea, and motility disorders. Purine genes are also potential biomarkers of disease. Advances in medicinal chemistry have an accelerated pace toward clinical trials: Methotrexate and sulfasalazine, used to treat IBD, act by stimulating CD73-dependent adenosine production. ATP protects against NSAID-induced enteropathy and has pain-relieving properties in humans. A P2X(7)R antagonist AZD9056 is in clinical trials for Crohn's disease. A(3) adenosine receptor drugs target inflammatory diseases (e.g., CF101, CF102). Dipyridamole, a nucleoside uptake inhibitor, is in trials for endotoxemia. Drugs for pain in clinical trials include P2X(3)/P2X(2/3) (AF-219) and P2X(7) (GSK1482160) antagonists and A(1) (GW493838) or A(2A) (BVT.115959) agonists. Iberogast is a phytopharmacon targeting purine mechanisms with efficacy in IBS and functional dyspepsia. Purinergic drugs have excellent safety/efficacy profile for prospective clinical trials in IBD, IBS, functional dyspepsia, and inflammatory diarrhea. Genetic polymorphisms and caffeine consumption may affect susceptibility to treatment. Further studies in animals can clarify mechanisms and test new generation drugs. Finally, there is still a huge gap in our knowledge of human pathophysiology of purinergic signaling.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

The role of purinergic pathways in the pathophysiology of gut diseases: pharmacological modulation and potential therapeutic applications

Gut homeostasis results from complex neuro-immune interactions aimed at triggering stereotypical and specific programs of coordinated mucosal secretion and powerful motor propulsion. A prominent role in the regulation of this highly integrated network, comprising a variety of immune/inflammatory cells and the enteric nervous system, is played by purinergic mediators. The cells of the digestive tract are literally plunged into a "biological sea" of functionally active nucleotides and nucleosides, which carry out the critical task of driving regulatory interventions on cellular functions through the activation of P1 and P2 receptors. Intensive research efforts are being made to achieve an integrated view of the purinergic system, since it is emerging that the various components of purinergic pathways (i.e., enzymes, transporters, mediators and receptors) are mutually linked entities, deputed to finely modulating the magnitude and the duration of purinergic signaling, and that alterations occurring in this balanced network could be intimately involved in the pathophysiology of several gut disorders. This review article intends to provide a critical appraisal of current knowledge on the purinergic system role in the regulation of gastrointestinal functions, considering these pathways as a whole integrated network, which is capable of finely controlling the levels of bioactive nucleotides and nucleosides in the biophase of their respective receptors. Special attention is paid to the mechanisms through which alterations in the various compartments of the purinergic system could contribute to the pathophysiology of gut disorders, and to the possibility of counteracting such dysfunctions by means of pharmacological interventions on purinergic molecular targets.

Inhibition of extracellular nucleotides hydrolysis intensifies the allergic bronchospasm. A novel protective role of ectonucleotidases

BACKGROUND

Nucleotides released to the extracellular space stimulate purinergic receptors, and their effects are modulated by ectonucleotidases. The role of ATP in the allergic bronchospasm has been scantly studied.

METHODS

We used several techniques (plethysmography, organ baths, confocal microscopy, RT-PCR, ATP measurement) to explore the role of nucleotides and ectonucleotidases in the allergic bronchospasm in guinea pigs.

RESULTS

While allergenic challenge with a low-dose ovalbumin (OVA) only produced a small bronchospasm (~2-fold the basal lung resistance), previous inhibition of ectonucleotidases by ARL-67156 greatly intensified this response (~11-fold the basal lung resistance, with 44% mortality). Bronchoalveolar lavage fluid obtained during this bronchospasm contained increased ATP concentration. This potentiation was abolished by antagonism of purinergic receptors (suramin+RB2) or TXA2 receptor (SQ29548), or by intratracheal apyrase. In tracheal rings and lung parenchyma strips, OVA caused a concentration-dependent contraction. Suramin+RB2 or levamisole produced a significant rightward displacement of this response, and ARL-67156 did not modify it. Platelets stimulated with OVA released ATP. Confocal images of nonsensitized tracheas showed slight fluorescence for P2Y6 receptors in epithelium and none for P2Y4 . Sensitized animals showed strong fluorescence to both receptors and to alkaline phosphatase in the airway epithelium. This correlated with a large increment in mRNA for P2Y4 and P2Y6 receptors in sensitized animals.

CONCLUSIONS

Nucleotides greatly potentiate the allergic bronchospasm when ectonucleotidases activity is diminished, and this effect is probably favored by the upregulation of P2Y4 and P2Y6 receptors in airway epithelium during sensitization. These results prompt for further research on these mechanisms in human asthma.

Adenosine increases nasal mucociliary clearance rate in mice through A2A and A2B adenosine receptors

OBJECTIVES/HYPOTHESIS

Mucociliary clearance (MCC) is an important mechanism of host defense in the upper and lower respiratory tract. Impaired MCC plays a critical role in the development and perpetuation of chronic rhinosinusitis (CRS). The aim of this investigation was to determine the influence of adenosine on nasal MCC, and to determine the receptors mediating this physiology in vivo.

STUDY DESIGN

Prospective study using an animal model.

METHODS

Nasal MCC was measured by whole-nose scintigraphic acquisition in vivo. The effects of both endogenous and exogenous adenosine were investigated in wild-type and adenosine receptor knockout (A(2A)(-/-), A(2B)(-/-), A(2A)(-/-)A(2B)(-/-), and A(1)(-/- )A(3)(-/-)) mice.

RESULTS

Exogenous adenosine aerosol robustly enhanced nasal MCC. The augmentation of MCC by adenosine was abolished in mice lacking both A(2A) and A(2B) receptors, but remained robust in mice lacking either A(2A) or A(2B) . Likewise, basal nasal MCC was reduced in mice lacking both the A(2A) and A(2B) receptors, but was statistically identical among wild-type mice and mice lacking either A(2A) or A(2B) .

CONCLUSIONS

These findings indicate that activation of both G(s) -coupled adenosine receptors can accelerate nasal MCC. Targeting these receptors may represent a novel therapeutic approach for enhancing MCC in CRS.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

Coupling of P2Y receptors to G proteins and other signaling pathways

P2Y receptors are G protein-coupled receptors (GPCRs) that are activated by adenine and uridine nucleotides and nucleotide sugars. There are eight subtypes of P2Y receptors (P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₂, P2Y₁₃, and P2Y₁₄), which activate intracellular signaling cascades to regulate a variety of cellular processes, including proliferation, differentiation, phagocytosis, secretion, nociception, cell adhesion, and cell migration. These signaling cascades operate mainly by the sequential activation or deactivation of heterotrimeric and monomeric G proteins, phospholipases, adenylyl and guanylyl cyclases, protein kinases, and phosphodiesterases. In addition, there are numerous ion channels, cell adhesion molecules, and receptor tyrosine kinases that are modulated by P2Y receptors and operate to transmit an extracellular signal to an intracellular response.

Molecular modulation of airway epithelial ciliary response to sneezing

Our purpose was to evaluate the effect of the mechanical force of a sneeze on sinonasal cilia function and determine the molecular mechanism responsible for eliciting the ciliary response to a sneeze. A novel model was developed to deliver a stimulation simulating a sneeze (55 mmHg for 50 ms) at 26°C to the apical surface of mouse and human nasal epithelial cells. Ciliary beating was visualized, and changes in ciliary beat frequency (CBF) were determined. To interrogate the molecular cascades driving sneeze-induced changes of CBF, pharmacologic manipulation of intra- and extracellular calcium, purinergic, PKA, and nitric oxide (NO) signaling were performed. CBF rapidly increases by ≥150% in response to a sneeze, which is dependent on the release of adenosine triphosphate (ATP), calcium influx, and PKA activation. Furthermore, apical release of ATP is independent of calcium influx, but calcium influx and subsequent increase in CBF are dependent on the ATP release. Lastly, we observed a blunted ciliary response in surgical specimens derived from patients with chronic rhinosinusitis compared to control patients. Apical ATP release with subsequent calcium mobilization and PKA activation are involved in sinonasal ciliary response to sneezing, which is blunted in patients with upper-airway disease.

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.

Purines as potential morphogens during embryonic development

Components of purinergic signalling are expressed in the early embryo raising the possibility that ATP, ADP and adenosine may contribute to the mechanisms of embryonic development. We summarize the available data from four developmental models—mouse, chick, Xenopus and zebrafish. While there are some notable examples where purinergic signalling is indeed important during development, e.g. development of the eye in the frog, it is puzzling that deletion of single components of purinergic signalling often results in rather minor developmental phenotypes. We suggest that a key step in further analysis is to perform combinatorial alterations of expression of purinergic signalling components to uncover their roles in development. We introduce the concept that purinergic signalling could create novel morphogenetic fields to encode spatial location via the concentration of ATP, ADP and adenosine. We show that using minimal assumptions and the known properties of the ectonucleotidases, complex spatial patterns of ATP and adenosine can be set up. These patterns may provide a new way to assess the potential of purinergic signalling in developmental processes.

Genetic deletion of the P2Y2 receptor offers significant resistance to development of lithium-induced polyuria accompanied by alterations in PGE2 signaling

Lithium (Li)-induced polyuria is due to resistance of the medullary collecting duct (mCD) to the action of arginine vasopressin (AVP), apparently mediated by increased production of PGE(2). We previously reported that the P2Y(2) receptor (P2Y(2)-R) antagonizes the action of AVP on the mCD and may play a role in Li-induced polyuria by enhancing the production of PGE(2) in mCD. Hence, we hypothesized that genetic deletion of P2Y(2)-R should ameliorate Li-induced polyuria. Wild-type (WT) or P2Y(2)-R knockout (KO) mice were fed normal or Li-added diets for 14 days and euthanized. Li-induced polyuria, and decreases in urine osmolality and AQP2 protein abundance in the renal medulla, were significantly less compared with WT mice despite the lack of differences in Li intake or terminal serum or inner medullary tissue Li levels. Li-induced increased urinary excretion of PGE(2) was not affected in KO mice. However, prostanoid EP(3) receptor (EP3-R) protein abundance in the renal medulla of KO mice was markedly lower vs. WT mice, irrespective of the dietary regimen. The protein abundances of other EP-Rs were not altered across the groups irrespective of the dietary regimen. Ex vivo stimulation of mCD with PGE(2) generated significantly more cAMP in Li-fed KO mice (130%) vs. Li-fed WT mice (100%). Taken together, these data suggest 1) genetic deletion of P2Y(2)-R offers significant resistance to the development of Li-induced polyuria; and 2) this resistance is apparently due to altered PGE(2) signaling mediated by a marked decrease in EP3-R protein abundance in the medulla, thus attenuating the EP3-mediated decrease in cAMP levels in mCD.

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.

Nucleotides enhance the secretion of interleukin 7 from primary-cultured murine intestinal epithelial cells

Our previous studies showed that dietary nucleotides fed to mice enhanced the secretion of interleukin 7 (IL-7) and transforming growth factor beta (TGF-beta) from intestinal epithelial cells (IECs). To explore whether nucleotides influence IECs directly to enhance the secretion of the cytokines or not, the effects of nucleotides added in vitro on the cytokine secretion from primary-cultured murine IECs were examined. When the mixture of nucleotide 5'-monophosphates (CMP, GMP, IMP, and UMP) or individual nucleotide 5'-monophosphates were added to the primary culture of IECs derived from BALB/c mice, the secretion of IL-7, but not that of TGF-beta, was increased significantly. Addition of nucleotides to the culture did not alter the number of the IECs. Secretion of IL-6 and granulocyte-macrophage colony-stimulating factor, which are known to be secreted from IECs, was not enhanced by the addition of nucleotides. These results demonstrate that nucleotides can affect IECs directly to enhance the secretion of IL-7, and suggest that the increased secretion of TGF-beta from IECs by dietary nucleotides was due to indirect effects of the nucleotides, which may affect intestinal microflora or cells other than IECs that in turn influence the cytokine secretion of IECs.

Purinergic signalling in epithelial ion transport: regulation of secretion and absorption

Intracellular ATP, the energy source for many reactions, is crucial for the activity of plasma membrane pumps and, thus, for the maintenance of transmembrane ion gradients. Nevertheless, ATP and other nucleotides/nucleosides are also extracellular molecules that regulate diverse cellular functions, including ion transport. In this review, I will first introduce the main components of the extracellular ATP signalling, which have become known as the purinergic signalling system. With more than 50 components or processes, just at cell membranes, it ranks as one of the most versatile signalling systems. This multitude of system components may enable differentiated regulation of diverse epithelial functions. As epithelia probably face the widest variety of potential ATP-releasing stimuli, a special attention will be given to stimuli and mechanisms of ATP release with a focus on exocytosis. Subsequently, I will consider membrane transport of major ions (Cl(-) , HCO(3)(-) , K(+) and Na(+) ) and integrate possible regulatory functions of P2Y2, P2Y4, P2Y6, P2Y11, P2X4, P2X7 and adenosine receptors in some selected epithelia at the cellular level. Some purinergic receptors have noteworthy roles. For example, many studies to date indicate that the P2Y2 receptor is one common denominator in regulating ion channels on both the luminal and basolateral membranes of both secretory and absorptive epithelia. In exocrine glands though, P2X4 and P2X7 receptors act as cation channels and, possibly, as co-regulators of secretion. On an organ level, both receptor types can exert physiological functions and together with other partners in the purinergic signalling, integrated models for epithelial secretion and absorption are emerging.

Long-term (trophic) purinergic signalling: purinoceptors control cell proliferation, differentiation and death

The purinergic signalling system, which uses purines and pyrimidines as chemical transmitters, and purinoceptors as effectors, is deeply rooted in evolution and development and is a pivotal factor in cell communication. The ATP and its derivatives function as a 'danger signal' in the most primitive forms of life. Purinoceptors are extraordinarily widely distributed in all cell types and tissues and they are involved in the regulation of an even more extraordinary number of biological processes. In addition to fast purinergic signalling in neurotransmission, neuromodulation and secretion, there is long-term (trophic) purinergic signalling involving cell proliferation, differentiation, motility and death in the development and regeneration of most systems of the body. In this article, we focus on the latter in the immune/defence system, in stratified epithelia in visceral organs and skin, embryological development, bone formation and resorption, as well as in cancer.

Rho signaling regulates pannexin 1-mediated ATP release from airway epithelia

ATP released from airway epithelial cells promotes purinergic receptor-regulated mucociliary clearance activities necessary for innate lung defense. Cell swelling-induced membrane stretch/strain is a common stimulus that promotes airway epithelial ATP release, but the mechanisms transducing cell swelling into ATP release are incompletely understood. Using knockdown and knockout approaches, we tested the hypothesis that pannexin 1 mediates ATP release from hypotonically swollen airway epithelia and investigated mechanisms regulating this activity. Well differentiated primary cultures of human bronchial epithelial cells subjected to hypotonic challenge exhibited enhanced ATP release, which was paralleled by the uptake of the pannexin probe propidium iodide. Both responses were reduced by pannexin 1 inhibitors and by knocking down pannexin 1. Importantly, hypotonicity-evoked ATP release from freshly excised tracheas and dye uptake in primary tracheal epithelial cells were impaired in pannexin 1 knockout mice. Hypotonicity-promoted ATP release and dye uptake in primary well differentiated human bronchial epithelial cells was accompanied by RhoA activation and myosin light chain phosphorylation and was reduced by the RhoA dominant negative mutant RhoA(T19N) and Rho and myosin light chain kinase inhibitors. ATP release and Rho activation were reduced by highly selective inhibitors of transient receptor potential vanilloid 4 (TRPV4). Lastly, knocking down TRPV4 impaired hypotonicity-evoked airway epithelial ATP release. Our data suggest that TRPV4 and Rho transduce cell membrane stretch/strain into pannexin 1-mediated ATP release in airway epithelia.

Animal models of airway diseases

Over the past 20 years, the growing awareness that purinergic signaling events literally shape the immune and inflammatory responses to infection and allergic reactions warranted the development of animal models to assess their importance in vivo in acute lung injury and chronic airway diseases. The pioneer work conducted with the adenosine deaminase (ADA)-deficient mouse provided irrefutable evidence that excess adenosine (ADO) accumulating in the lungs of asthmatic patients, constitutes a powerful mediator of disease severity. These original studies launched the development of murine strains for the two major ectonucleotidases responsible for the generation of airway ADO from ATP release: CD39 and CD73. The dramatic acute lung injury and chronic lung complications, manifested by these knockout mice in response to allergens and endotoxin, demonstrated the critical importance of regulating the availability of ATP and ADO for their receptors. Therapeutic targets are currently evaluated using knockout mice and agonists/antagonists for each ADO receptor (A(1)R, A(2A)R, A(2B)R, and A(3)R) and the predominant ATP receptors (P2Y(2)R and P2X(7)R). This chapter provides an in-depth description of each in vivo study, and a critical view of the therapeutic potentials for the treatment of airway diseases.

P2Y(6) agonist uridine 5'-diphosphate promotes host defense against bacterial infection via monocyte chemoattractant protein-1-mediated monocytes/macrophages recruitment

Extracellular nucleotides are important messengers involved in series crucial physiological functions through the activation of P2 purinergic receptors. The detailed function and mechanism of the P2Y family in regulating immune response against invaded pathogens still remains unknown. In this study, the activation of purinoreceptor P2Y(6) by UDP was found to play a crucial role in promoting host defense against invaded bacteria through monocytes/macrophages recruitment. The expression level of P2Y(6) was much higher than other purinoreceptors in RAW264.7 cells, bone marrow macrophages, and peritoneal macrophages determined by real-time PCR. The supernatant of UDP (P2Y(6)-specific agonist)-treated RAW264.7 cells exhibited direct chemotaxis to monocytes/macrophages in vitro through Boyden Chambers assay. Meanwhile, the releasing of MCP-1 (MCP-1/CCL2) was enhanced obviously by UDP both in mRNA and protein level. Furthermore, the activation of P2Y(6) receptor by UDP also promotes ERK phosphorylation and AP-1 activation in a concentration- and time-dependent manner in RAW264.7 cells. This UDP-induced activation could be inhibited by P2Y(6) selectivity antagonist (MRS2578), MEK inhibitor (U0126), and MCP-1 blocking Ab, respectively. Moreover, i.p. injection with UDP resulted in a more efficacious clearance of invaded Escherichia coli and lower mortality in peritonitis mouse model. Together, our studies demonstrate that P2Y(6) receptor could be a novel mediator in upregulating innate immune response against the invaded pathogens through recruiting monocytes/macrophages.

Renal sodium transporter/channel expression and sodium excretion in P2Y2 receptor knockout mice fed a high-NaCl diet with/without aldosterone infusion

The P2Y(2) receptor (P2Y2-R) antagonizes sodium reabsorption in the kidney. Apart from its effect in distal nephron, hypothetically, P2Y(2)-R may modulate activity/abundances of sodium transporters/channel subunits along the nephron via antagonism of aldosterone or vasopressin or interaction with mediators such as nitric oxide (NO), and prostaglandin E(2) (PGE(2)) or oxidative stress (OS). To determine the extent of the regulatory role of P2Y(2)-R in renal sodium reabsorption, in study 1, we fed P2Y(2)-R knockout (KO; n = 5) and wild-type (WT; n = 5) mice a high (3.15%)-sodium diet (HSD) for 14 days. Western blotting revealed significantly higher protein abundances for cortical and medullary bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2), medullary α-1-subunit of Na-K-ATPase, and medullary α-subunit of the epithelial sodium channel (ENaC) in KO vs. WT mice. Molecular analysis of urine showed increased excretion of nitrates plus nitrites (NOx), PGE(2), and 8-isoprostane in the KO, relative to WT mice, supporting a putative role for these molecules in determining alterations of proteins involved in sodium transport along the nephron. To determine whether genotype differences in response to aldosterone might have played a role in these differences due to HSD, in study 2 aldosterone levels were clamped (by osmotic minipump infusion). Clamping aldosterone (with HSD) led to significantly impaired natriuresis with elevated Na/H exchanger isoform 3 in the cortex, and NKCC2 in the medulla, and modest but significantly lower levels of NKCC2, and α- and β-ENaC in the cortex of KO vs. WT mice. This was associated with significantly reduced urinary NOx in the KO, although PGE(2) and 8-isoprostane remained significantly elevated vs. WT mice. Taken together, our results suggest that P2Y(2)-R is an important regulator of sodium transporters along the nephron. Pre- or postreceptor differences in the response to aldosterone, perhaps mediated via prostaglandins or changes in NOS activity or OS, likely play a role.

Formation of cysts by principal-like MDCK cells depends on the synergy of cAMP- and ATP-mediated fluid secretion

It has been suggested that more than 70% of the renal cysts in patients with autosomal dominant polycystic kidney disease (ADPKD) arise from the collecting duct and that within this segment cysts originate almost exclusively from principal rather than intercalated cells. The mechanisms for this predisposition of principal cells have so far remained elusive. We, therefore, used Madin-Darby canine kidney (MDCK) subclones resembling principal cells and alpha-intercalated cells in a three-dimensional in vitro model to determine differences in cystogenesis and cyst growth, including the response to cyclic adenosine monophosphate (cAMP) elevation and the dependence on ATP signaling. We found that in vitro cysts developed only from principal-like but not from intercalated-like MDCK cell clones. This specificity could be verified in mixed MDCK cultures enriched for principal- or intercalated-like cells. In vitro cyst growth upon elevation of intracellular cAMP was mainly driven by fluid secretion, rather than increased cell proliferation. The cAMP-dependent fluid secretion was found to depend on extracellular adenosine-5'-triphosphate (ATP) and to act synergistically with purinergic signaling, as the use of the ATP scavenger apyrase, as well as the P2 receptor inhibitor suramin, reduced cAMP-driven fluid secretion, while increasing extracellular ATP potentiated cAMP-mediated cyst growth. In conclusion, we provide in vitro evidence for the ability of principal rather than intercalated cells to form cysts, based on a synergism of cAMP and ATP signaling in enhancing apical fluid secretion.

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.

The purinergic receptor P2Y, G-protein coupled, 2 (P2RY2) gene associated with essential hypertension in Japanese men

P2RY2 has an important function in the regulation of blood pressure by activating adenosine triphosphate (ATP). The aim of this study was to investigate the association between the human P2RY2 gene and essential hypertension (EH) through a haplotype-based case-control study that included two gender groups. The 273 EH patients and 255 age-matched controls were genotyped for five single-nucleotide polymorphisms (SNPs) of the human P2RY2 gene (rs4944831, rs1783596, rs4944832, rs4382936 and rs10898909). Data were analysed for men and women separately and then as a combined total group. For the total and the men only groups, the genotype distribution of the T allele of rs4944831 and the recessive model (GG vs TG+TT) of rs4944831 differed significantly between the EH patients and controls (P=0.028 and 0.019; P=0.009 and 0.008, respectively). Logistic regression showed that for the total and men groups, the TG+TT genotype of rs4944831 was more prevalent in EH patients than in the controls (P=0.026 and 0.011, respectively). For men, the overall distribution of the haplotype (SNP2-SNP4-SNP5) was significantly different between the EH patients and the controls (P=0.006). As compared with controls, the frequency of the T-A-G haplotype was significantly higher, whereas the T-C-G haplotype was significantly lower for the EH patients (P=0.001 and 0.014, respectively). In conclusion, the present results indicate that rs4944831 and the T-A-G haplotype of the human P2RY2 gene might be genetic markers for EH in Japanese men.

Key determinants of nucleotide-activated G protein-coupled P2Y(2) receptor function revealed by chemical and pharmacological experiments, mutagenesis and homology modeling

The P2Y(2) receptor, which is activated by UTP, ATP, and dinucleotides, was studied as a prototypical nucleotide-activated GPCR. A combination of receptor mutagenesis, determination of its effects on potency and efficacy of agonists and antagonists, homology modeling, and chemical experiments was applied. R272 (extracellular loop EL3) was found to play a gatekeeper role, presumably responsible for recognition and orientation of the nucleotides. R272 is also directly involved in binding of dinucleotides, which behaved as partial agonists. Y118A (3.37) mutation led to dramatically reduced efficacy of agonists; it is part of the entry channel as well as the triphosphate binding site. While the Y114A (3.33) mutation did not have any effect on agonist activities, the antagonist Reactive Blue 2 (6) was completely inactive at that mutant. The disulfide bridge Cys25-Cys278 was found to be important for agonist potency but neither for agonist efficacy nor for antagonist potency.

Thrombin promotes release of ATP from lung epithelial cells through coordinated activation of rho- and Ca2+-dependent signaling pathways

Extracellular ATP controls key aspects of lung function via activation of epithelial cell purinergic receptors, but how ATP is released from cells remains poorly understood. To identify mechanistic components upstream of ATP release, we examined the effect of selected G protein coupled-receptor activation on ATP release from lung epithelial cells. The protease-activated receptor (PAR) agonist thrombin elicited a rapid Ca(2+)-dependent release of ATP from A549 cells. In contrast, the P2Y(2) receptor agonist UTP caused negligible ATP release, despite promoting a robust Ca(2+) response. Agonist-elicited ATP release was associated with Rho activation and was reduced in cells transfected with dominant negative mutants of p115-Rho GEF or RhoA, and by inhibitors of Rho kinase (ROCK). However, RhoA activation alone did not promote ATP release if temporally separated from Ca(2+) mobilization. PAR3 was the only PAR subtype detected in A549 cells by reverse transcription-PCR. Transfection of cells with human PAR3 cDNA increased thrombin-promoted ATP release, inositol phosphate formation, and RhoA activation. Conversely, small interference RNA against PAR3 diminished thrombin-evoked responses. Thrombin-elicited ATP release was accompanied by an enhanced cellular uptake of propidium iodide in a Ca(2+)- and ROCK-dependent manner and was inhibited by connexin/pannexin hemichannel blockers. Our data suggest that thrombin promotes ATP release from A549 cells via Rho- and Ca(2+)-dependent activation of connexin/pannexin hemichannels. The relevance of these findings is highlighted by the observation that exposure of primary cultures of well differentiated human bronchial epithelial cells to thrombin resulted in robust ATP release, which was inhibited by ROCK inhibitors and by connexin/pannexin hemichannel blockers.

P2Y(2) receptors and water transport in the kidney

The kidneys play a critical role in the maintenance of water homeostasis. This is achieved by the inherent architecture of the nephron along with the expression of various membrane transporters and channels that are responsible for the vectorial transport of salt and water. The collecting duct has become a focus of attention by virtue of its ability to transport water independent of solutes (free-water transport), and its apparent involvement in various water balance disorders. It was originally believed that the water transport capability of the collecting duct was solely under the influence of the circulating hormone, arginine vasopressin (AVP). However, during the past decade, locally produced autocrine and/or paracrine factors have emerged as potent modulators of transport of water by the collecting duct. Recently, much attention has been focused on the purinergic regulation of renal water transport. This review focuses on the role of the P2Y(2) receptor, the predominant purinergic receptor expressed in the collecting duct, in the modulation of water transport in physiological and pathophysiological conditions, and its therapeutic potential as a drug target to treat water balance disorders in the clinic. Studies carried out by us and other investigators are unravelling potent interactions among AVP, prostanoid and purinergic systems in the medullary collecting duct, and the perturbations of these interactions in water balance disorders such as acquired nephrogenic diabetes insipidus. Future studies should address the potential therapeutic benefits of modulators of P2Y(2) receptor signalling in water balance disorders, which are extremely prevalent in hospitalised patients irrespective of the underlying pathology.

Development of a novel transgenic rat overexpressing the P2Y(2) nucleotide receptor using a lentiviral vector

The G protein-coupled P2Y(2) nucleotide receptor (P2Y(2)R) is upregulated in response to stress and tissue injury and has been postulated to play a role in chronic inflammation seen in atherosclerosis, Alzheimer's disease and Sjogren's syndrome. The role of P2Y(2)R upregulation in vivo is poorly understood, in part due to the lack of a P2Y(2)R overexpressing animal model. The P2Y(2)R overexpressing transgenic rat was generated using a lentiviral vector. Rats overexpressing P2Y(2)R showed a significant increase in P2Y(2)R mRNA levels in all tissues screened as compared to nontransgenic rats. Fura 2 imaging of smooth muscle cells (SMCs) isolated from aorta indicated that the percentage of cells exhibiting increases in the intracellular free calcium concentration in response to P2Y(2)R agonists was significantly greater in freshly isolated SMCs from transgenic rats than wild-type controls. Histopathological examination of tissues revealed that P2Y(2)R overexpressing rats develop lymphocytic infiltration in lacrimal glands and kidneys as early as at 3 months of age. These rats show similarities to patients with Sjogren's syndrome who display lymphocyte-mediated tissue damage. This transgenic rat model of P2Y(2)R overexpression may prove useful for linking P2Y(2)R upregulation with chronic inflammatory diseases, neurodegenerative diseases and Sjogren's syndrome.

Membrane compartments and purinergic signalling: occurrence and function of P2X receptors in lung

P2X receptors are cation-selective ion channels activated by extracellular ATP. They form homo- and heterotrimeric complexes that differ in their functional properties and subcellular localization. These membrane ion channels are also expressed in pulmonary epithelial cells. Recent work indicates that alveolar epithelial type I cells selectively express P2X(4) and P2X(7) receptor subtypes in addition to a large number of other ion channels present in the alveolar epithelium. Up- or downregulation of their expression is associated with several disease states. This minireview analyses the role of P2X receptors and of extracellular ATP and adenosine in lung disease, the relationship of P2X receptors to other ion channels in the alveolar epithelium and their distribution in lipid rafts/caveolae.

Regulated catalysis of extracellular nucleotides by vascular CD39/ENTPD1 is required for liver regeneration

BACKGROUND & AIMS

Little is known about how endothelial cells respond to injury, regulate hepatocyte turnover and reconstitute the hepatic vasculature. We aimed to determine the effects of the vascular ectonucleotidase CD39 on sinusoidal endothelial cell responses following partial hepatectomy and to dissect purinergic and growth factor interactions in this model.

METHODS

Parameters of liver injury and regeneration, as well as the kinetics of hepatocellular and sinusoidal endothelial cell proliferation, were assessed following partial hepatectomy in mice that do not express CD39, that do not express ATP/UTP receptor P2Y2, and in controls. The effects of extracellular ATP on vascular endothelial growth factor (VEGF), hepatocyte growth factor (HGF), and interleukin-6 responses were determined in vivo and in vitro. Phosphorylation of the endothelial VEGF receptor in response to extracellular nucleotides and growth factors was assessed in vitro.

RESULTS

After partial hepatectomy, expression of the vascular ectonucleotidase CD39 increased on sinusoidal endothelial cells. Targeted disruption of CD39 impaired hepatocellular regeneration, reduced angiogenesis, and increased hepatic injury, resulting in pronounced vascular endothelial apoptosis, and decreased survival. Decreased HGF release by sinusoidal endothelial cells, despite high levels of VEGF, reduced paracrine stimulation of hepatocytes. Failure of VEGF receptor-2/KDR transactivation by extracellular nucleotides on CD39-null endothelial cells was associated with P2Y2 receptor desensitization.

CONCLUSIONS

Regulated phosphohydrolysis of extracellular nucleotides by CD39 coordinates both hepatocyte and endothelial cell proliferation following partial hepatectomy. Lack of CD39 activity is associated with decreased hepatic regeneration and failure of vascular reconstitution.