Activity of Novel Adenine Nucleotide Derivatives as Agonists and Antagonists at Recombinant Rat P2X Receptors

Sex-specific post-inflammatory dysbiosis mediates chronic visceral pain in colitis

BACKGROUND

Despite achieving endoscopic remission, over 20% of inflammatory bowel disease (IBD) patients experience chronic abdominal pain. Visceral pain and the microbiome exhibit sex-dependent interactions, while visceral pain in IBD shows a sex bias. Our aim was to evaluate whether post-inflammatory microbial perturbations contribute to visceral hypersensitivity in a sex-dependent manner.

METHODS

Males, cycling females, ovariectomized, and sham-operated females were given dextran sodium sulfate to induce colitis and allowed to recover. Germ-free recipients received sex-appropriate and cross-sex fecal microbial transplants (FMT) from post-inflammatory donor mice. Visceral sensitivity was assessed by recording visceromotor responses to colorectal distention. The composition of the microbiota was evaluated via 16S rRNA gene V4 amplicon sequencing, while the metabolome was assessed using targeted (short chain fatty acids - SCFA) and semi-targeted mass spectrometry.

RESULTS

Post-inflammatory cycling females developed visceral hyperalgesia when compared to males. This effect was reversed by ovariectomy. Both post-inflammatory males and females exhibited increased SCFA-producing species, but only males had elevated fecal SCFA content. FMT from post-inflammatory females transferred visceral hyperalgesia to both males and females, while FMT from post-inflammatory males could only transfer visceral hyperalgesia to males.

CONCLUSIONS

Female sex, hormonal status as well as the gut microbiota play a role in pain modulation. Our data highlight the importance of considering biological sex in the evaluation of visceral pain.

Progress of thrombus formation and research on the structure-activity relationship for antithrombotic drugs

Many populations suffer from thrombotic disorders such as stroke, myocardial infarction, unstable angina and thromboembolic disease. Thrombus is one of the major threatening factors to human health and the prevalence of cardio-cerebrovascular diseases induced by thrombus is growing worldwide, even some persons got rare and severe blood clots after receiving the AstraZeneca COVID vaccine unexpectedly. In terms of mechanism of thrombosis, antithrombotic drugs have been divided into three categories including anticoagulants, platelet inhibitors and fibrinolytics. Nowadays, a large number of new compounds possessing antithrombotic activities are emerging in an effort to remove the inevitable drawbacks of previously approved drugs such as the high risk of bleeding, a slow onset of action and a narrow therapeutic window. In this review, we describe the causes and mechanisms of thrombus formation firstly, and then summarize these reported active compounds as potential antithrombotic candidates based on their respective mechanism, hoping to promote the development of more effective bioactive molecules for treating thrombotic disorders.

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

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

P2X3 receptors participate in purinergic inhibition of gastrointestinal smooth muscle

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

Age- and Experience-Related Plasticity of ATP-Mediated Signaling in the Neocortex

There is growing recognition of the important role of interaction between neurons and glial cells for brain longevity. The extracellular ATP have been shown to bring significant contribution into bi-directional glia-neuron communications, in particular into astrocyte-driven modulation of synaptic plasticity. To elucidate a putative impact of brain aging on neuron-glia networks, we explored the aging-related plasticity of the purinoreceptors-mediated signaling in cortical neurons and astrocytes. We investigated the age- and experience-related alterations in purinergic components of neuronal synaptic currents and astroglial calcium signaling in the layer2/3 of neocortex of mice exposed to the mild caloric restriction (CR) and environmental enrichment (EE) which included ad libitum physical exercise. We observed the considerable age-related decline in the neuronal P2X receptor-mediated miniature spontaneous currents which originated from the release of ATP from both synapses and astrocytes. We also found out that purinergic astrocytic Ca²⁺-signaling underwent the substantial age-related decline but EE and CR rescued astroglial signaling, in particular mediated by P2X1, P2X1/5, and P2Y1 receptors. Our data showed that age-related attenuation in the astroglial calcium signaling caused a substantial decrease in the exocytosis of ATP leading to impairment of astroglia-derived purinergic modulation of excitatory synaptic currents and GABAergic tonic inhibitory currents. On a contrary, exposure to EE and CR, which enhanced purinergic astrocytic calcium signaling, up-regulated the excitatory and down-regulated the inhibitory currents in neurons of old mice, thus counterbalancing the impact of aging on synaptic signaling. Combined, our results strongly support the physiological importance of ATP-mediated signaling for glia-neuron interactions and brain function. Our data also show that P2 purinoreceptor-mediated communication between astrocytes and neurons in the neocortex undergoes remodeling during brain aging and decrease in the ATP release may contribute to the age-related impairment of synaptic transmission.

An autocrine purinergic signaling controls astrocyte-induced neuronal excitation

Astrocyte-derived gliotransmitters glutamate and ATP modulate neuronal activity. It remains unclear, however, how astrocytes control the release and coordinate the actions of these gliotransmitters. Using transgenic expression of the light-sensitive channelrhodopsin 2 (ChR2) in astrocytes, we observed that photostimulation reliably increases action potential firing of hippocampal pyramidal neurons. This excitation relies primarily on a calcium-dependent glutamate release by astrocytes that activates neuronal extra-synaptic NMDA receptors. Remarkably, our results show that ChR2-induced Ca²⁺ increase and subsequent glutamate release are amplified by ATP/ADP-mediated autocrine activation of P2Y1 receptors on astrocytes. Thus, neuronal excitation is promoted by a synergistic action of glutamatergic and autocrine purinergic signaling in astrocytes. This new mechanism may be particularly relevant for pathological conditions in which ATP extracellular concentration is increased and acts as a major danger signal.

Purinergic signalling in the enteric nervous system (An overview of current perspectives)

Purinergic Signalling in the Enteric Nervous System involves the regulated release of ATP (or a structurally-related nucleotide) which activates an extensive suite of membrane-inserted receptors (P2X and P2Y subtypes) on a variety of cell types in the gastrointestinal tract. P2X receptors are gated ion-channels permeable to sodium, potassium and calcium. They depolarise cells, act as a pathway for calcium influx to activate calcium-dependent processes and initiate gene transcription, interact at a molecular level as a form of self-regulation with lipids within the cell wall (e.g. PIP2) and cross-react with other membrane-inserted receptors to regulate their activity (e.g. nAChRs). P2Y receptors are metabotropic receptors that couple to G-proteins. They may release calcium ions from intracellular stores to activate calcium-dependent processes, but also may activate calcium-independent signalling pathways and influence gene transcription. Originally ATP was a candidate only for NANC neurotransmission, for inhibitory motoneurons supplying the muscularis externa of the gastrointestinal tract and bringing about the fast IJP. Purinergic signalling later included neuron-neuron signalling in the ENS, via the production of either fast or slow EPSPs. Later still, purinergic signalling included the neuro-epithelial synapse-for efferent signalling to epithelia cells participating in secretion and absorption, and afferent signalling for chemoreception and mechanoreception at the surface of the mucosa. Many aspects of purinergic signalling have since been addressed in a series of highly-focussed and authoritative reviews. In this overview however, the current focus is on key aspects of purinergic signalling where there remains uncertainty and ambiguity, with the view to stimulating further research in these areas.

Propensity of red blood cells to undergo P2X7 receptor-mediated phosphatidylserine exposure does not alter during in vivo or ex vivo aging

BACKGROUND

Phosphatidylserine (PS) exposure facilitates the removal of red blood cells (RBCs) from the circulation, potentially contributing to the loss of stored RBCs after transfusion, as well as senescent RBCs. Activation of the P2X7 receptor by extracellular adenosine 5'-triphosphate (ATP) can induce PS exposure on freshly isolated human RBCs, but whether this process occurs in stored RBCs or changes during RBC aging is unknown.

STUDY DESIGN AND METHODS

RBCs were processed and stored according to Australian blood banking guidelines. PS exposure was determined by annexin V binding and flow cytometry. Efficacy of P2X antagonists was assessed by flow cytometric measurements of ATP-induced ethidium+ uptake in RPMI 8226 cells. Osmotic fragility was assessed by lysis in hypotonic saline. RBCs were fractionated by discontinuous density centrifugation.

RESULTS

ATP (1 mmol/L) induced PS exposure on RBCs stored for less than 1 week. This process was near-completely inhibited by the P2X7 antagonists A438079 and AZ10606120 and the P2X1/P2X7 antagonist MRS2159 but not the P2X1 antagonist NF499. ATP-induced PS exposure on RBCs was not dependent on K+, Na+, or Cl- fluxes. ATP did not alter the osmotic fragility of stored RBCs. ATP-induced PS exposure was similar between RBCs of different densities. ATP-induced PS exposure was also similar between RBCs stored for less than 1 week or for 6 weeks.

CONCLUSION

The propensity of RBCs to undergo P2X7-mediated PS exposure does not alter during in vivo and ex vivo aging. Thus, P2X7 activation is unlikely to be involved in the removal of senescent RBCs or stored RBCs after transfusion.

Different mechanisms of hydroxyl radical production susceptible to purine P2 receptor antagonists between carbon monoxide poisoning and exogenous ATP in rat striatum

Previous studies have suggested that carbon monoxide (CO) poisoning stimulates cAMP production via purine P2Y11-like receptors in the rat striatum, activating cAMP signaling pathways, resulting in hydroxyl radical ((•)OH) production. Extracellular ATP was thought likely to trigger the cascade, but the present study has failed to demonstrate a clear increase in the extracellular ATP due to CO poisoning. The CO-induced (•)OH production was attenuated by the P2Y11 receptor antagonist NF157, in parallel with its abilities to suppress the CO-induced cAMP production. The (•)OH production was more strongly suppressed by a non-selective P2 receptor antagonist, PPADS, which had no effect on cAMP production. More selective antagonists toward the respective P2 receptors susceptible to PPADS, including NF279, had little or no effect on the CO-induced (•)OH production. The intrastriatal administration of exogenous ATP dose-dependently stimulated (•)OH production, which was dose-dependently antagonized by PPADS and NF279 but not by NF157. Exogenous GTP and CTP dose-dependently stimulated (•)OH production, though less potently. The GTP-induced (•)OH production was susceptible to both of NF279 and PPADS, but the CTP-induced (•)OH production was resistant to PPADS. The mechanism of (•)OH production may differ between CO poisoning and exogenous ATP, while multiple P2 receptors could participate in (•)OH production. The CO-induced (•)OH production was susceptible to the inhibition of NADPH oxidase, but not xanthine oxidase. Also, the NADPH oxidase inhibition suppressed (•)OH production induced by forskolin, a stimulator of intracellular cAMP formation. It is likely that (•)OH is produced by NADPH oxidase activation via cAMP signaling pathways during CO poisoning.

P2Y1 receptor inhibits GABA transport through a calcium signalling-dependent mechanism in rat cortical astrocytes

Astrocytes express a variety of purinergic (P2) receptors, involved in astrocytic communication through fast increases in [Ca(2+) ]i . Of these, the metabotropic ATP receptors (P2Y) regulate cytoplasmic Ca(2+) levels through the PLC-PKC pathway. GABA transporters are a substrate for a number of Ca(2+) -related kinases, raising the possibility that calcium signalling in astrocytes impact the control of extracellular levels of the major inhibitory transmitter in the brain. To access this possibility we tested the influence of P2Y receptors upon GABA transport into astrocytes. Mature primary cortical astroglial-enriched cultures expressed functional P2Y receptors, as evaluated through Ca(2+) imaging, being P2Y1 the predominant P2Y receptor subtype. ATP (100 μM, for 1 min) caused an inhibition of GABA transport through either GAT-1 or GAT-3 transporters, decreasing the Vmax kinetic constant. ATP-induced inhibition of GATs activity was still evident in the presence of adenosine deaminase, precluding an adenosine-mediated effect. This, was mimicked by a specific agonist for the P2Y1,12,13 receptor (2-MeSADP). The effect of 2-MeSADP on GABA transport was blocked by the P2 (PPADS) and P2Y1 selective (MRS2179) receptor antagonists, as well as by the PLC inhibitor (U73122). 2-MeSADP failed to inhibit GABA transport in astrocytes where intracellular calcium had been chelated (BAPTA-AM) or where calcium stores were depleted (α-cyclopiazonic acid, CPA). In conclusion, P2Y1 receptors in astrocytes inhibit GABA transport through a mechanism dependent of P2Y1 -mediated calcium signalling, suggesting that astrocytic calcium signalling, which occurs as a consequence of neuronal firing, may operate a negative feedback loop to enhance extracellular levels of GABA.

G-protein-coupled receptors in adult neurogenesis

The importance of adult neurogenesis has only recently been accepted, resulting in a completely new field of investigation within stem cell biology. The regulation and functional significance of adult neurogenesis is currently an area of highly active research. G-protein-coupled receptors (GPCRs) have emerged as potential modulators of adult neurogenesis. GPCRs represent a class of proteins with significant clinical importance, because approximately 30% of all modern therapeutic treatments target these receptors. GPCRs bind to a large class of neurotransmitters and neuromodulators such as norepinephrine, dopamine, and serotonin. Besides their typical role in cellular communication, GPCRs are expressed on adult neural stem cells and their progenitors that relay specific signals to regulate the neurogenic process. This review summarizes the field of adult neurogenesis and its methods and specifies the roles of various GPCRs and their signal transduction pathways that are involved in the regulation of adult neural stem cells and their progenitors. Current evidence supporting adult neurogenesis as a model for self-repair in neuropathologic conditions, adult neural stem cell therapeutic strategies, and potential avenues for GPCR-based therapeutics are also discussed.

Pharmacochemistry of the platelet purinergic receptors

Platelets contain at least five purinergic G protein-coupled receptors, e.g., the pro-aggregatory P2Y(1) and P2Y(12) receptors, a P2Y(14) receptor (GPR105) of unknown function, and anti-aggregatory A(2A) and A(2B) adenosine receptor (ARs), in addition to the ligand-gated P2X1 ion channel. Probing the structure-activity relationships (SARs) of the P2X and P2Y receptors for extracellular nucleotides has resulted in numerous new agonist and antagonist ligands. Selective agents derived from known ligands and novel chemotypes can be used to help define the subtypes pharmacologically. Some of these agents have entered into clinical trials in spite of the challenges of drug development for these classes of receptors. The functional architecture of P2 receptors was extensively explored using mutagenesis and molecular modeling, which are useful tools in drug discovery. In general, novel drug delivery methods, prodrug approaches, allosteric modulation, and biased agonism would be desirable to overcome side effects that tend to occur even with receptor subtype-selective ligands. Detailed SAR analyses have been constructed for nucleotide and non-nucleotide ligands at the P2Y(1), P2Y(12), and P2Y(14) receptors. The thienopyridine antithrombotic drugs Clopidogrel and Prasugrel require enzymatic pre-activation in vivo and react irreversibly with the P2Y(12) receptor. There is much pharmaceutical development activity aimed at identifying reversible P2Y(12) receptor antagonists. The screening of chemically diverse compound libraries has identified novel chemotypes that act as competitive, non-nucleotide antagonists of the P2Y(1) receptor or the P2Y(12) receptor, and antithrombotic properties of the structurally optimized analogues were demonstrated. In silico screening at the A(2A) AR has identified antagonist molecules having novel chemotypes. Fluorescent and other reporter groups incorporated into ligands can enable new technology for receptor assays and imaging. The A(2A) agonist CGS21680 and the P2Y(1) receptor antagonist MRS2500 were derivatized for covalent attachment to polyamidoamine dendrimeric carriers of MW 20,000, and the resulting multivalent conjugates inhibited ADP-promoted platelet aggregation. In conclusion, a wide range of new pharmacological tools is available to control platelet function by interacting with cell surface purine receptors.

Characterization of P2Y receptor subtypes functionally expressed on neonatal rat cardiac myofibroblasts

BACKGROUND AND PURPOSE

Little is known about P2Y receptors in cardiac fibroblasts, which represent the predominant cell type in the heart and differentiate into myofibroblasts under certain conditions. Therefore, we have characterized the phenotype of the cells and the different P2Y receptors at the expression and functional levels in neonatal rat non-cardiomyocytes.

EXPERIMENTAL APPROACH

Non-cardiomyocyte phenotype was determined by confocal microscopy by using discoidin domain receptor 2, alpha-actin and desmin antibodies. P2Y receptor expression was investigated by reverse transcription-polymerase chain reaction and immunocytochemistry, and receptor function by cAMP and inositol phosphate (IP) accumulation induced by adenine or uracil nucleotides in the presence or absence of selective antagonists of P2Y(1) (MRS 2179, 2-deoxy-N(6)-methyl adenosine 3',5'-diphosphate diammonium salt), P2Y(6) (MRS 2578) and P2Y(11) (NF 157, 8,8'-[carbonylbis[imino-3,1-phenylenecarbonylimino(4-fluoro-3,1-phenylene)carbonylimino]]bis-1,3,5-naphthalene trisulphonic acid hexasodium salt) receptors. G(i/o) and G(q/11) pathways were evaluated by using Pertussis toxin and YM-254890 respectively.

KEY RESULTS

The cells (>95%) were alpha-actin and discoidin domain receptor 2-positive and desmin-negative. P2Y(1), P2Y(2), P2Y(4), P2Y(6) were detected by reverse transcription-polymerase chain reaction and immunocytochemistry, and P2Y(11)-like receptors at protein level. All di- or tri-phosphate nucleotides stimulated IP production in an YM-254890-sensitive manner. AMP, ADPbetaS, ATP and ATPgammaS increased cAMP accumulation, whereas UDP and UTP inhibited cAMP response, which was abolished by Pertussis toxin. MRS 2179 and NF 157 inhibited ADPbetaS-induced IP production. MRS 2578 blocked UDP- and UTP-mediated IP responses.

CONCLUSION AND IMPLICATIONS

P2Y(1)-, P2Y(2)-, P2Y(4)-, P2Y(6)-, P2Y(11)-like receptors were co-expressed and induced function through G(q/11) protein coupling in myofibroblasts. Furthermore, P2Y(2) and P2Y(4) receptor subtypes were also coupled to G(i/o). The G(s) response to adenine nucleotides suggests a possible expression of a new P2Y receptor subtype.

Development of selective agonists and antagonists of P2Y receptors

Although elucidation of the medicinal chemistry of agonists and antagonists of the P2Y receptors has lagged behind that of many other members of group A G protein-coupled receptors, detailed qualitative and quantitative structure–activity relationships (SARs) were recently constructed for several of the subtypes. Agonists selective for P2Y₁, P2Y₂, and P2Y₆ receptors and nucleotide antagonists selective for P2Y₁ and P2Y₁₂ receptors are now known. Selective nonnucleotide antagonists were reported for P2Y₁, P2Y₂, P2Y₆, P2Y₁₁, P2Y₁₂, and P2Y₁₃ receptors. At the P2Y₁ and P2Y₁₂ receptors, nucleotide agonists (5′-diphosphate derivatives) were converted into antagonists of nanomolar affinity by altering the phosphate moieties, with a focus particularly on the ribose conformation and substitution pattern. Nucleotide analogues with conformationally constrained ribose-like rings were introduced as selective receptor probes for P2Y₁ and P2Y₆ receptors. Screening chemically diverse compound libraries has begun to yield new lead compounds for the development of P2Y receptor antagonists, such as competitive P2Y₁₂ receptor antagonists with antithrombotic activity. Selective agonists for the P2Y₄, P2Y₁₁, and P2Y₁₃ receptors and selective antagonists for P2Y₄ and P2Y₁₄ receptors have not yet been identified. The P2Y₁₄ receptor appears to be the most restrictive of the class with respect to modification of the nucleobase, ribose, and phosphate moieties. The continuing process of ligand design for the P2Y receptors will aid in the identification of new clinical targets.

Pulses of somatostatin release are slightly delayed compared with insulin and antisynchronous to glucagon

It was early proposed that somatostatin-producing delta-cells in pancreatic islets have local inhibitory effects on the release of insulin and glucagon. Recent observations that pulses of insulin and glucagon are antisynchronous make it important to examine the temporal characteristics of glucose-induced somatostatin release. Analysis of 30 s fractions from the perfused rat pancreas indicated that increase of glucose from 3 to 20 mmol/l results in initial suppression of somatostatin release followed by regular 4-5 min pulses. During continued exposure to 20 mmol/l glucose, the pulses of somatostatin overlapped those of insulin with a delay of 30 s. Somatostatin and glucagon pulses were coupled in antisynchronous fashion (phase shift 2.4+/-0.2 min), supporting the idea that the delta-cells have a local inhibitory effect on glucagon release. It was possible to remove the pulses of somatostatin and glucagon with maintenance of the insulin rhythmicity by addition of 1 micromol/l of the P2Y(1) receptor antagonist MRS 2179.

Inhibitory purinergic P2 receptor characterisation in rat distal colon

The aim of this study was to characterise the P2 receptors involved in purinergic relaxant responses in rat distal colon circular muscle. Concentration-response curves for purinergic agonists were constructed on methacholine-precontracted circular muscle strips of rat distal colon in the absence and presence of the nerve blocker TTX and the ecto-nucleotidase inhibitor ARL67156. The effects of the P2 receptor antagonists RB2, PPADS, suramin, MRS2179 and NF279, the NO-synthase inhibitor L-NAME and the small conductance K(+) channel blocker apamin were investigated. The localisation of the different P2 receptors was examined immunocytochemically. Immunocytochemistry demonstrated the expression of P2Y(1), P2Y(6) and P2X(1) receptors on smooth muscle cells and P2Y(2), P2Y(12), P2X(2) and P2X(3) receptors in the myenteric plexus; almost a quarter of the P2Y(2)-immunopositive neurons co-expressed nNOS. The P2X-selective agonist alphabetameATP and the P2Y-selective agonist ADPbetaS were the most potent relaxants; their effects were abolished by apamin. The effect of ADPbetaS was antagonised by the P2Y(1)-selective antagonist MRS2179 pointing to interaction with the muscular P2Y(1)-receptors. The relaxant effect of alphabetameATP was partially reduced by TTX and concentration-dependently antagonised by PPADS, suramin, RB2 and the P2X(1)-selective antagonist NF279; this correlates with an interaction with neuronal P2X(3) and muscular P2X(1) receptors. UTP was the least potent agonist; its effect was markedly increased by ARL67156, nearly abolished by TTX and reduced by L-NAME. This points to interaction with the neuronal P2Y(2)-receptors inducing relaxation, at least partially, by NO release.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

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.

Endothelium-dependent relaxation evoked by ATP and UTP in the aorta of P2Y2-deficient mice

Based on pharmacological criteria, we previously suggested that in the mouse aorta, endothelium-dependent relaxation by nucleotides is mediated by P2Y1 (adenosine diphosphate (ADP)), P2Y2 (adenosine triphosphate (ATP)) and P2Y6 (uridine diphosphate (UDP)) receptors. For UTP, it was unclear whether P2Y2, P2Y6 or yet another subtype was involved. Therefore, in view of the lack of selective purinergic agonists and antagonists, we used P2Y2-deficient mice to clarify the action of UTP. Thoracic aorta segments (width 2 mm) of P2Y2-deficient and wild-type (WT) mice were mounted in organ baths to measure isometric force development and intracellular calcium signalling. Relaxations evoked by ADP, UDP and acetylcholine were identical in knockout and WT mice, indicating that the receptors for these agonists function normally. P2Y2-deficient mice showed impaired ATP- and adenosine 5'[gamma-thio] triphosphate (ATPgammaS)-evoked relaxation, suggesting that in WT mice, ATP and ATPgammaS activate predominantly the P2Y2 subtype. The ATP/ATPgammaS-evoked relaxation and calcium signals in the knockout mice were partially rescued by P2Y1, as they were sensitive to 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate (MRS2179), a P2Y1-selective antagonist. In contrast to ATP, the UTP-evoked relaxation was not different between knockout and WT mice. Moreover, the action of UTP was not sensitive to MRS2179. Therefore, the action of UTP is probably mediated mainly by a P2Y6(like) receptor subtype. In conclusion, we demonstrated that ATP-evoked relaxation of the murine aorta is mainly mediated by P2Y2. But this P2Y2 receptor has apparently no major role in UTP-evoked relaxation. The vasodilator effect of UTP is probably mediated mainly by a P2Y6(like) receptor.

Antagonism of ATP responses at P2X receptor subtypes by the pH indicator dye, Phenol red

1 Many types of culture media contain a pH-sensitive dye. One commonly occurring dye, Phenol red sodium (Na(+)) salt, was tested for blocking activity at rat P2X(1-4) receptors (P2X(1-4)Rs) expressed in Xenopus oocytes. 2 Phenol red Na(+)-salt antagonised adenosine 5'-triphosphate (ATP) responses at P2X(1)R (IC(50), 3 microM) and, at higher concentrations, also blocked P2X(2)R and P2X(3)R. Phenol red Na(+)-salt, purified of lipophilic contaminants, blocked P2X(1)R and P2X(3)R by acting as an insurmountable antagonist. 3 Two lipophilic extracts of Phenol red antagonised ATP responses at P2XRs. Extract A was a potent antagonist at P2X(1)R (IC(50), 1.4 microM), whereas extract B was a potent antagonist at P2X(3)R (IC(50), 4.1 microM). A bisphenolic compound (RS151030) found in these extracts was a potent antagonist at P2X(1)R (IC(50), 0.3 microM) and at P2X(3)R (IC(50), 2.4 microM). 4 Phenolphthalein base was a potent irreversible antagonist at P2X(1)R (IC(50), 1 microM), whereas Phenolphthalein K(+)-salt was 25-fold less potent here. 5 Phenolphthalein base was a reversible antagonist of ATP responses at rat P2X(4)R (IC(50), 26 microM), whereas Phenolphthalein K(+)-salt was inactive. 6 Dimethyl sulphoxide (DMSO), used to dissolve lipophilic extracts, showed pharmacological activity by itself at rat P2X(1)R and P2X(4)R. 7 Thus, Phenol red and related compounds are antagonists at rat P2X(1)R, but are also active at other rat P2XRs. Phenolphthalein base is a newly identified, low potency antagonist of ATP responses at P2X(4)R. Culture media containing these red dyes should be used cautiously in future pharmacological studies of P2XRs. Also, wherever possible, the solvent DMSO should be used with caution.

Molecular recognition at adenine nucleotide (P2) receptors in platelets

Transmembrane signaling through P2Y receptors for extracellular nucleotides controls a diverse array of cellular processes, including thrombosis. Selective agonists and antagonists of the two P2Y receptors present on the platelet surface-the G (q)-coupled P2Y (1) subtype and the G (i)-coupled P2Y (12) subtype-are now known. High-affinity antagonists of each have been developed from nucleotide structures. The (N)-methanocarba bisphosphate derivatives MRS2279 and MRS2500 are potent and selective P2Y (1) receptor antagonists. The carbocyclic nucleoside AZD6140 is an uncharged, orally active P2Y (12) receptor antagonist of nM affinity. Another nucleotide receptor on the platelet surface, the P2X (1) receptor, the activation of which may also be proaggregatory, especially under conditions of high shear stress, has high-affinity ligands, although high selectivity has not yet been achieved. Although alpha,beta-methylene-adenosine triphosphate (ATP) is the classic agonist for the P2X (1) receptor, where it causes rapid desensitization, the agonist BzATP is among the most potent in activating this subtype. The aromatic sulfonates NF279 and NF449 are potent antagonists of the P2X (1) receptor. The structures of the two platelet P2Y receptors have been modeled, based on a rhodopsin template, to explain the basis for nucleotide recognition within the putative transmembrane binding sites. The P2Y (1) receptor model, especially, has been exploited in the design and optimization of antagonists targeted to interact selectively with that subtype.

Inhibition of purinoceptors amplifies glucose-stimulated insulin release with removal of its pulsatility

External ATP has been proposed to be an autocrine regulator of glucose-stimulated insulin secretion and responsible for the synchronization of the Ca2+ rhythmicity in the beta-cells required for a pulsatile release of insulin from the pancreas. The importance of external ATP for glucose-stimulated insulin release was evaluated in rats with the aid of 2-deoxy-N-methyladenosine-3,5-bisphosphate (MRS 2179), an inhibitor of the purinoceptors known to affect the Ca2+ signaling in beta-cells. The concentration of cytoplasmic Ca2+ was measured in single beta-cells and small aggregates with ratiometric fura-2 technique and the release of insulin recorded from isolated islets and the perfused pancreas. Addition of 1 micromol/l ATP induced premature cytoplasmic Ca2+ concentration ([Ca2+]i) oscillations similar to those found in beta-cells exposed to 20 mmol/l glucose. In most experiments, the presence of 10 micromol/l MRS 2179 did not remove the glucose-induced [Ca2+]i rhythmicity in single beta-cells or the synchronization seen in coupled cells. Nevertheless, the same concentration of MRS 2179 promptly interrupted the pulsatility (frequency 0.22 +/- 0.01/min) of insulin secretion, raising the total amounts released from the pancreas. Prolonged exposure of islets to 1 and 10 micromol/l MRS 2179 enhanced insulin secretion at 20 mmol/l glucose 33% (P < 0.05) and 63% (P < 0.01), respectively, without affecting the release at 3 mmol/l glucose. The results support the idea that neural ATP signals entrain the islets into a common rhythm resulting in pulsatile release of insulin and that glucose stimulation of the secretory activity is counteracted by accumulation of inhibitory ATP around the beta-cells.

Antiaggregatory activity in human platelets of potent antagonists of the P2Y 1 receptor

Activation of the P2Y(1) nucleotide receptor in platelets by ADP causes changes in shape and aggregation, mediated by activation of phospholipase C (PLC). Recently, MRS2500(2-iodo-N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate) was introduced as a highly potent and selective antagonist for this receptor. We have studied the actions of MRS2500 in human platelets and compared these effects with the effects of two acyclic nucleotide analogues, a bisphosphate MRS2298 and a bisphosphonate derivative MRS2496, which act as P2Y(1) receptor antagonists, although less potently than MRS2500. Improved synthetic methods for MRS2500 and MRS2496 were devised. The bisphosphonate is predicted to be more stable in general in biological systems than phosphate antagonists due to the non-hydrolyzable CP bond. MRS2500 inhibited the ADP-induced aggregation of human platelets with an IC(50) value of 0.95 nM. MRS2298 and MRS2496 also both inhibited the ADP-induced aggregation of human platelets with IC(50) values of 62.8 nM and 1.5 microM, respectively. A similar order of potency was observed for the three antagonists in binding to the recombinant human P2Y(1) receptor and in inhibition of ADP-induced shape change and ADP-induced rise in intracellular Ca(2+). No substantial antagonism of the pathway linked to the inhibition of cyclic AMP was observed for the nucleotide derivatives, indicating no interaction of these three P2Y(1) receptor antagonists with the proaggregatory P2Y(12) receptor, which is also activated by ADP. Thus, all three of the bisphosphate derivatives are highly selective antagonists of the platelet P2Y(1) receptor, and MRS2500 is the most potent such antagonist yet reported.

Investigation of the effects of P2 purinoceptor ligands on the micturition reflex in female urethane-anaesthetized rats

1 The effects of purinoceptor ligands for P2X1 and/or P2X3 receptors (alpha,beta-meATP, IP(5)I, TNP-ATP, MRS 2179, PPADS, Phenol red and RO116-6446/008; i.v., n=4-5) and for P2Y1 receptors (PPADS, MRS 2179 and MRS 2269; i.v., n=3-5) were investigated on the distension-evoked 'micturition reflex' in the urethane-anaesthetized female rat. 2 Alpha,beta-meATP (180 nmol kg(-1) min(-1)), IP5I (10, 30 and 100 nmol kg(-1)), TNP-ATP (1 micromol kg(-1)), MRS 2179 (1 micromol kg(-1)) and PPADS (17 micromol kg(-1)) each caused maintained bladder contractions to occur during the infusion of saline into the bladder. PPADS (17 micromol kg(-1) min(-1)) had a similar effect when infused intravesicularly. Regular bladder contractions were not observed until the infusion of saline was halted. For IP5I, TNP-ATP, MRS 2179 and PPADS, the magnitude of postinfusion isovolumetric contractions was significantly reduced and, for IP5I, this action was also associated with a significant reduction in urethral relaxation. Additionally, TNP-ATP caused a significant increase in the pressure and volume thresholds required to initiate a reflex. 3 Phenol red (a P2X1/P2X3 antagonist; 0.1 and 1 micromol kg(-1)) caused a significant increase in the pressure and volume thresholds required to initiate a reflex and, at the higher dose, also caused a reduction in postinfusion isovolumetric contractions. 4 RO116-6446/008 (a P2X1-selective antagonist; 1 and 10 micromol kg(-1)) only caused a reduction in postinfusion isovolumetric contractions. 5 It is concluded that P2X1 and P2X3 receptors play a fundamental role in the micturition reflex in urethane-anesthetized female rats. P2X3 receptor blockade raised the pressure and volume thresholds for the reflex, whereas P2X1 receptor blockade diminished motor activity associated with voiding. P2Y1 receptors may be involved in inhibition of rat detrusor tone.

Molecular recognition at purine and pyrimidine nucleotide (P2) receptors

In comparison to other classes of cell surface receptors, the medicinal chemistry at P2X (ligand-gated ion channels) and P2Y (G protein-coupled) nucleotide receptors has been relatively slow to develop. Recent effort to design selective agonists and antagonists based on a combination of library screening, empirical modification of known ligands, and rational design have led to the introduction of potent antagonists of the P2X(1) (derivatives of pyridoxal phosphates and suramin), P2X(3)(A-317491), P2X(7) (derivatives of the isoquinoline KN-62), P2Y(1)(nucleotide analogues MRS 2179 and MRS 2279), P2Y(2)(thiouracil derivatives such as AR-C126313), and P2Y(12)(nucleotide/nucleoside analogues AR-C69931X and AZD6140) receptors. A variety of native agonist ligands (ATP, ADP, UTP, UDP, and UDP-glucose) are currently the subject of structural modification efforts to improve selectivity. MRS2365 is a selective agonist for P2Y(1)receptors. The dinucleotide INS 37217 potently activates the P2Y(2)receptor. UTP-gamma-S and UDP-beta-S are selective agonists for P2Y(2)/P2Y(4)and P2Y(6)receptors, respectively. The current knowledge of the structures of P2X and P2Y receptors, is derived mainly from mutagenesis studies. Site-directed mutagenesis has shown that ligand recognition in the human P2Y(1)receptor involves individual residues of both the TMs (3, 5, 6, and 7), as well as EL 2 and 3. The binding of the negatively-charged phosphate moiety is dependent on positively charged lysine and arginine residues near the exofacial side of TMs 3 and 7.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

2-Substitution of adenine nucleotide analogues containing a bicyclo[3.1.0]hexane ring system locked in a northern conformation: enhanced potency as P2Y1 receptor antagonists

Preference for the northern (N) ring conformation of the ribose moiety of adenine nucleotide 3',5'-bisphosphate antagonists of P2Y(1) receptors was established by using a ring-constrained methanocarba (a bicyclo[3.1.0]hexane) ring as a ribose substitute (Nandanan et al. J. Med. Chem. 2000, 43, 829-842). We have now combined the ring-constrained (N)-methanocarba modification with other functionalities at the 2-position of the adenine moiety. A new synthetic route to this series of bisphosphate derivatives was introduced, consisting of phosphorylation of the pseudoribose moiety prior to coupling with the adenine base. The activity of the newly synthesized analogues was determined by measuring antagonism of 2-methylthio-ADP-stimulated phospholipase C (PLC) activity in 1321N1 human astrocytoma cells expressing the recombinant human P2Y(1) receptor and by using the radiolabeled antagonist [(3)H]2-chloro-N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate 5 in a newly developed binding assay in Sf9 cell membranes. Within the series of 2-halo analogues, the most potent molecule at the hP2Y(1) receptor was an (N)-methanocarba N(6)-methyl-2-iodo analogue 12, which displayed a K(i) value in competition for binding of [(3)H]5 of 0.79 nM and a K(B) value of 1.74 nM for inhibition of PLC. Thus, 12 is the most potent antagonist selective for the P2Y(1) receptor yet reported. The 2-iodo group was substituted with trimethyltin, thus providing a parallel synthetic route for the introduction of an iodo group in this high-affinity antagonist. The (N)-methanocarba-2-methylthio, 2-methylseleno, 2-hexyl, 2-(1-hexenyl), and 2-(1-hexynyl) analogues bound less well, exhibiting micromolar affinity at P2Y(1) receptors. An enzymatic method of synthesis of the 3',5'-bisphosphate from the corresponding 3'-monophosphate, suitable for the preparation of a radiophosphorylated analogue, was explored.

The P2Y(1) receptor as a target for new antithrombotic drugs: a review of the P2Y(1) antagonist MRS-2179

MRS-2179 is a selective P2Y(1) receptor antagonist, a strong inhibitor of ADP-induced platelet aggregation in vitro and ex vivo. By i.v. administration to mice MRS-2179 increases resistance to thromboembolism induced by a mixture of collagen and epinephrine or by a tissue factor. Likewise, it significantly increases the time to thrombus formation in a ferric chloride-induced model of localized arterial thrombosis. MRS-2179 also confers resistance to localized venous thrombosis, which is dependent on thrombin generation and in which platelets play a relatively minor role as compared to stasis or activation of coagulation. These data provide considerable encouragement for the development of new P2Y(1) receptor antagonists. Nevertheless, the properties of MRS-2179 indicate that new compounds should be optimized in order to increase the half-life of the molecule in vivo and its selectivity and potency at the P2Y(1) receptor. Further directions include the synthesis of molecules with modifications of the nucleotide structure which replace the fragile moiety by a stable bond and should lead to a non-hydrolysable structure. In conclusion, P2Y(1) antagonists have been shown to be efficient antithrombotic agents. MRS-2179 is the first P2Y(1) antagonist with antithrombotic action. Its effectiveness demonstrates that the P2Y(1) receptor is a potentially promising target for drugs designed to treat thrombotic syndromes.

Endothelium-dependent relaxation and endothelial hyperpolarization by P2Y receptor agonists in rat-isolated mesenteric artery

(1) Vasorelaxation and hyperpolarization of endothelial cells by adenosine 5'-[beta-thio]diphosphate (ADPbetaS) and adenosine 5'-[gamma-thio]triphosphate (ATPgammaS) were studied in rat-isolated mesenteric artery. Effects from stimulation of P2X receptors were avoided by desensitization with alpha,beta-methylene adenosine triphosphate. (2) ADPbetaS caused concentration- and endothelium-dependent relaxations of methoxamine-precontracted small (third generation) and main mesenteric artery. These were inhibited by N(omega)-nitro-L-arginine methyl ester (L-NAME) or a combination of apamin plus charybdotoxin (inhibitors of Ca(2+)-activated K(+) channels); L-NAME, apamin and charybdotoxin applied together abolished the response. (3) ATPgammaS induced limited relaxation (35% of methoxamine-induced tone at 10 micro M) of small mesenteric artery, which was sensitive to L-NAME or endothelium denudation. However, it almost completely relaxed the main mesenteric artery over an extended concentration range (>6 orders of magnitude) in an endothelium-dependent manner. This relaxation was inhibited by either L-NAME or a combination of apamin with charybdotoxin, and abolished by a combination of all the three inhibitors. (4) The P2Y(1) receptor antagonist MRS 2179 (2'-deoxy-N(6)-methyladenosine 3',5'-bisphosphate; 0.3-3 micro M) caused parallel rightward shifts of the concentration/relaxation curve to ADPbetaS (pA(2)=7.1). However, MRS 2179 did not inhibit, but potentiated, relaxant responses to ATPgammaS. MRS 2179 did not affect the contractile responses ATPgammaS in small mesenteric artery; ATPgammaS did not contract the main mesenteric artery. (5) ADPbetaS hyperpolarized the endothelium of the main mesenteric artery in a concentration-dependent manner. This was unaffected by L-NAME but antagonized by MRS 2179. ATPgammaS also hyperpolarized the mesenteric artery endothelium in a concentration-dependent manner but, when ATPgammaS was applied at 10 micro M, its effect was potentiated by MRS 2179 (3 micro M). (6) It is concluded that both relaxation and hyperpolarization to ADPbetaS are mediated by P2Y(1) receptors and that the endothelial hyperpolarization is related to the L-NAME-resistant relaxation. Relaxation to the P2Y(2) agonist ATPgammaS shows regional variation along the mesenteric vasculature. The mechanisms for potentiation of relaxation and hyperpolarization by ATPgammaS are unknown, but may indicate interactions between P2Y receptor subtypes.

Extracellular nucleotides induce vasodilatation in human arteries via prostaglandins, nitric oxide and endothelium-derived hyperpolarising factor

1. The present study was aimed at examining P2 receptor-mediated vasodilatation in human vessels. The isometric tension was recorded in isolated segments of the human left internal mammary artery branches precontracted with 1 microM noradrenaline. 2. Endothelial denudation abolished the dilator responses. 3. The selective P2Y(1) agonist, 2-MeSADP, induced a potent vasodilatation (pEC(50)=6.9+/-0.1). The P2Y(1) antagonist of 10 microM, MRS 2216, shifted the 2-MeSADP concentration-response curve 1.1 log units to the right. The combined P2Y(1) and P2X agonist, 2-MeSATP, stimulated a dilatation with a potency similar to that of 2-MeSADP. Furthermore, MRS 2216 had a similar antagonistic effect on both 2-MeSATP and 2-MeSADP indicating that P2X receptors do not mediate vasodilatation. 4. Both the P2Y(2/4) agonist, UTPgammaS and the P2Y(6) agonist, UDPbetaS, stimulated potent dilatations (pEC(50)=7.8+/-0.4 for UTPgammaS and 8.4+/-0.2 for UDPbetaS). 5. The 2-MeSADP-induced nitric oxide (NO)-mediated dilatation was studied in the presence of 10 micro M indomethacin, 50 nM charybdotoxin and 1 microM apamin. The involvement of the endothelium-derived hyperpolarising factor (EDHF) was investigated in the presence of 0.1 mM L-NOARG and indomethacin. The involvement of prostaglandins was investigated in the presence of L-NOARG, charybdotoxin and apamin. Both NO, EDHF and prostaglandins mediated 2-MeSADP dilatation with similar efficacy (E(max)=25+/-5% for NO, 25+/-6% for EDHF and 27+/-5% for prostaglandins). 6. In conclusion, extracellular nucleotides induce endothelium-derived vasodilatation in human vessels by stimulating P2Y(1), P2Y(2/4) and P2Y(6) receptors, while P2X receptors are not involved. Endothelial P2Y receptors mediate dilatation by release of EDHF, NO and prostaglandins.

2-Chloro N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate is a selective high affinity P2Y(1) receptor antagonist

1. We reported previously that bisphosphate derivatives of adenosine are antagonists of the P2Y(1) receptor and that modification of the ribose in these analogues is tolerated in the P2Y(1) receptor binding pharmacophore. 2. Here we delineate the pharmacological activity of one such non-nucleotide molecule, 2-chloro N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate (MRS2279), in which the ribose is replaced by a cyclopentane ring constrained in the (N)-conformation by a cyclopropane moiety. 3. MRS2279 antagonized 2MeSADP-stimulated inositol phosphate formation in turkey erythrocyte membranes with competitive kinetics (pK(B)=7.75). High affinity competitive antagonism by MRS2279 was also observed at the human P2Y(1) receptor (pK(B)=8.10) stably expressed in 1321N1 human astrocytoma cells. Antagonism was specific for the P2Y(1) receptor since MRS2279 had no effect on activation of the human P2Y(2), P2Y(4), P2Y(6), or P2Y(11) receptors by their cognate agonists. 4. MRS2279 also did not block the capacity of ADP to act through the Gi/adenylyl cyclase linked P2Y receptor of platelets to inhibit cyclic AMP accumulation. 5. In contrast, the P2Y(1) receptor is known to be obligatory in the process of ADP-induced platelet aggregation, and MRS2279 competitively inhibited ADP-promoted platelet aggregation with an apparent affinity (pK(B)=8.05) similar to that observed at the human P2Y(1) receptor heterologously expressed in 1321N1 cells. 6. Taken together these results illustrate selective high affinity antagonism of the P2Y(1) receptor by a non-nucleotide molecule that should prove useful for pharmacological delineation of this receptor in various tissues.

Novel antagonists acting at the P2Y(1) purinergic receptor: synthesis and conformational analysis using potentiometric and nuclear magnetic resonance titration techniques

The human P2Y(1) receptor is widely distributed in many tissues and has a classical structure of a G protein-coupled receptor. Activated by adenosine-5'-diphosphate (ADP), this receptor is essential for platelet aggregation. In the present paper, we describe the synthesis of novel P2Y(1) antagonists that could be of interest at least as tools to define the physiological roles of the P2Y(1) receptor, at best as new antithrombotic agents. Thus, we prepared the 2,N(6)-dimethyl-2'-deoxyadenosine-3',5'-bisphosphate derivative, 1e. The biological activity was demonstrated by the ability of compound 1e to inhibit ADP-induced platelet aggregation, shape change, and intracellular calcium rise. This compound was a full antagonist at the P2Y(1) receptor with a pA(2) value of 7.11 +/- 0.11 and was found to be 4-fold more potent than the reference N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate (1a, pA(2) = 6.55 +/- 0.05), revealing the potency-enhancing effects of the 2-methyl group. The better activity of 1e as compared to 1a was analyzed using both potentiometric and nuclear magnetic resonance titration techniques, which highlighted specific conformational features of this compound. These results clearly indicate the preference for both compounds for an anti conformation at the N-glycosyl linkage. Furthermore, the percentage of S conformer of 1e is close to that of 1a, which is nearly 70% at pH = 2.8 and increases dramatically when pH increases. From the macroprotonation constants, it can be noted that compound 1e is significantly more basic than 1a. This is indeed expected for the N1 adenine nitrogen due to the electron-donating character of the methyl moiety. By considering the microconstants of the phosphate groups, the higher basicity of P3 and P5 for 1e may be due to the decrease in the local dielectric constant induced by the substitution of the hydrogen atom by a more lipophilic methyl group. Thus, it may be suggested that the gain in activity of 1e when compared to the reference compound 1a would result from its gain in basicity rather than steric and conformational modifications. The synthesis of the first selective radioligand acting at the P2Y(1) receptor ([(33)P]-N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate, 17) is also reported and will be used in the future for efficient screening needed for drug optimization.

EctoNucleotidase in cardiac sympathetic nerve endings modulates ATP-mediated feedback of norepinephrine release

ATP, coreleased with norepinephrine, affects adrenergic transmission by acting on purinoceptors at sympathetic nerve endings. Ectonucleotidases terminate the actions of ATP. Previously, we had preliminary evidence for ectonucleotidase activity in cardiac sympathetic nerve terminals. Therefore, we investigated whether this ectonucleotidase might influence norepinephrine release in the heart. Sympathetic nerve endings isolated from guinea pig heart (cardiac synaptosomes) were rich in Ca(2+)-dependent ectonucleotidase activity, as measured by metabolism of exogenously added radiolabeled ATP or ADP. By its inhibitor profile, ectonucleotidase resembled ectonucleoside triphosphate diphosphohydrolase 1 (E-NTPDase1). Exogenous ATP elicited concentration-dependent norepinephrine release from cardiac synaptosomes (EC(50) 0.96 microM). This release was antagonized by the P2X receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (10 microM) and potentiated by the P2Y receptor antagonist 2'-deoxy-N(6)-methyladenosine-3',5'-diphosphate (MRS 2179) (30 nM). Norepinephrine release promoted by ATP was also potentiated by the nucleotidase inhibitor 6-N,N-diethyl-beta-gamma-dibromomethylene-D-adenosine-5'-triphosphate (ARL67156) (30 microM) and blocked by a recombinant, soluble form of human E-NTPDase1 (solCD39). In contrast, ARL67156 had no effect on norepinephrine release induced by the nonhydrolyzable analog, alpha, beta-methyleneadenosine-5'-triphosphate (alpha,beta-MeATP). Depolarization of cardiac synaptosomes with K(+) elicited release of endogenous norepinephrine. This was attenuated by PPADS and solCD39 and potentiated by MRS 2179 and ARL67156. Importantly, our results demonstrate that facilitation of ATP-induced norepinephrine release from cardiac sympathetic nerves is a composite of two autocrine components: positive, mediated by P2X receptors, and negative, mediated by P2Y receptors. Modulation of norepinephrine release by coreleased ATP is terminated by endogenous as well as exogenous ectonucleotidase. We propose that ectonucleotidase control of norepinephrine release should provide cardiac protection in hyperadrenergic states such as myocardial ischemia.

Acyclic and cyclopropyl analogues of adenosine bisphosphate antagonists of the P2Y1 receptor: structure-activity relationships and receptor docking

The activation of P2Y1 receptors in platelets contributes to platelet aggregation, and selective antagonists are sought as potential antithrombotic agents. We reported (Kim et al. J. Med. Chem. 2000, 43, 746-755) that acyclic analogues of adenine nucleotides, containing two phosphate groups on a symmetrically branched aliphatic chain, attached at the 9-position of adenine, are moderately potent P2Y1 receptor antagonists. In this study we have varied the chain structure, to include asymmetric substitution, olefinic, and cyclopropyl groups. These antagonists inhibited the stimulation of phospholipase C in turkey erythrocyte membranes induced by 30 nM 2-MeS-ADP in the micromolar range. In the series of symmetrically branched aliphatic groups substituted with two phosphate groups, the optimal antagonist potency occurred with the 2-methylpropyl group. A 2-chloro-N(6)-methyladenine derivative, 2-[2-(2-chloro-6-methylaminopurin-9-yl)methyl]propane-1,3-bisoxy(diammoniumphosphate) (7), was a full antagonist at the P2Y1 receptor with an IC(50) value of 0.48 microM. Esterification of one of the phosphate groups or substitution with O-acetyl greatly reduced the antagonist potency at the P2Y1 receptor. Removal of a methylene group of 7 or inclusion of an olefinic or cyclopropyl group also reduced potency. A pair of enantiomeric glycerol derivatives demonstrated a 5-fold stereoselectivity for the S-isomer. Stereoisomerically defined analogues of 7 containing a cyclopropyl group in place of the branched carbon were less potent than 7 as antagonists, with IC(50) values of 2-3 microM. No agonist activity was observed for these analogues. A new rhodopsin-based molecular model of the P2Y1 receptor indicated that the optimal docked orientation of the two monophosphate moieties relative to the adenine N(6) (compared to a rigid, bicyclic analogue) was consistent with the dependence of antagonist potency on chain length. The 3'-phosphate was predicted to occupy a restricted space, deeper in the binding cleft than the 5'-phosphate location. In summary, modification of the flexible spacer chain linking bisphosphate groups to the adenine moiety provided many moderately potent antagonists.

Structurally related nucleotides as selective agonists and antagonists at P2Y1 receptors

The P2Y1 receptor responds to adenine nucleotides and is present in platelets, heart, smooth muscles prostate, ovary, and brain. A selective antagonist may be useful as an antithrombotic agent. We have analyzed the binding site of this G protein-coupled receptor using ligand design, site-directed mutagenesis, and homology modeling based on rhodopsin. We have designed and synthesized a series of deoxyadenosine 3',5'-bisphosphate derivatives that act as antagonists, or, in some cases with small structural changes, as agonists or partial agonists. The 2-position accommodates Cl or thioethers, whereas the N6-position is limited to Me or Et. 2'-Substitution with OH or OMe increases agonist efficacy over 2'-H. Using molecular modeling of the binding site, the oxygen atoms of the ribose moiety were predicted to be non-essential, i.e. no specific H-bonds with the receptor protein appear in the model. We have, therefore, substituted this moiety with carbocylics, smaller and larger rings, conformationally constrained rings, and acyclics, with retention of affinity for the receptor. With simplified pharmacophores we are exploring the steric and electronic requirements of the receptor binding site, and the structural basis of receptor activation.

Probing Adenosine and P2 Receptors: Design of Novel Purines and Nonpurines as Selective Ligands

Recent approaches to the design of selective agonists and antagonists at adenosine (AR) and P2 receptors include both modifying known receptor ligands and searching for structurally diverse antagonists. The ribose-like moiety of nucleoside/nucleotide derivatives was rigidified with a methanocarba (mc) modification, to constrain the ring in a conformation that was favored in binding to ARs or P2Y receptors. (N)-mc analogs of various N6-substituted adenosine derivatives, including cyclopentyl and 3-iodobenzyl, in which the parent compounds are potent agonists at either A1 or A3ARs, respectively, retained high receptor affinity and selectivity. For nucleotides acting as P2Y1 receptor antagonists, the (N)-mc analog MRS 2279 ((1R,2S,4S,5S)-1-[(phosphato)methyl]-4-(2-chloro-6-methylaminopurin-9-yl) bicyclo [3.1.0]-hexane-2-phosphate) proved to be a selective antagonist, with an IC50 of 52 nM. Other ribose substitutions possible in P2Y1 receptor antagonists were 4- and 6-membered rings and acyclic derivatives. High affinity for the A2BAR was achieved through the formation of anilides and benzylamides of XCC (8-[4-[[[carboxy]methyl]oxy]phenyl]-1,3-dipropylxanthine). A p-cyanoaniline derivative (MRS 1754, Ki value 1.97 nM) was 205-, 255-, and 289-fold selective for the human A2BARs vs. human A1/A2A/A3 ARs, respectively. A template approach based on the pyridine family, i.e., 1,4-dihydropyridine nucleus and the corresponding 3,5-diacylpyridines, was used for the design of novel adenosine antagonists. The pyridine derivative MRS 1523 (5-propyl-2-ethyl-4-propyl-3-(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate) was shown to be a selective antagonist at the rat A3AR as well as the human A3AR. Chemical libraries were screened computationally and using binding assays to identify novel AR antagonists. Molecular modeling of ARs and P2Y receptors provided hypotheses for ligand docking. Drug Dev. Res. 52:178-186, 2001.

Ribose modified nucleosides and nucleotides as ligands for purine receptors

Molecular modeling of receptors for adenosine and nucleotide (P2) receptors with docked ligand, based on mutagenesis, was carried out. Adenosine 3',5'-bisphosphate derivatives act as selective P2Y1 antagonists/partial agonists. The ribose moiety was replaced with carbocyclics, smaller and larger rings, conformationally constrained rings, and acyclics, producing compounds that retained receptor affinity. Conformational constraints were built into the ribose rings of nucleoside and nucleotide ligands using the methanocarba approach, i.e. fused cyclopropane and cyclopentane rings in place of ribose, suggesting a preference for the Northern (N) conformation among ligands for P2Y1 and A1 and A3ARs.