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

Pilot-Scale Screening of Clinically Approved Drugs to Identify Uridine Insertion/Deletion RNA Editing Inhibitors in Trypanosoma brucei

RNA editing pathway is a validated target in kinetoplastid parasites (Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp.) that cause severe diseases in humans and livestock. An essential large protein complex, the editosome, mediates uridine insertion and deletion in RNA editing through a stepwise process. This study details the discovery of editosome inhibitors by screening a library of widely used human drugs using our previously developed in vitro biochemical Ribozyme Insertion Deletion Editing (RIDE) assay. Subsequent studies on the mode of action of the identified hits and hit expansion efforts unveiled compounds that interfere with RNA-editosome interactions and novel ligase inhibitors with IC50 values in the low micromolar range. Docking studies on the ligase demonstrated similar binding characteristics for ATP and our novel epigallocatechin gallate inhibitor. The inhibitors demonstrated potent trypanocidal activity and are promising candidates for drug repurposing due to their lack of cytotoxic effects. Further studies are necessary to validate these targets using more definitive gene-editing techniques and to enhance the safety profile.

Synthesis of Fluorescent C-C Bonded Triazole-Purine Conjugates

A design toward C-C bonded 2,6-bis(1H-1,2,3-triazol-4-yl)-9H-purine and 2-piperidinyl-6-(1H-1,2,3-triazol-4-yl)-9H-purine derivatives was established using the combination of Mitsunobu, Sonogashira, copper (I) catalyzed azide-alkyne cycloaddition, and SNAr reactions. 11 examples of 2,6-bistriazolylpurine and 14 examples of 2-piperidinyl-6-triazolylpurine intermediates were obtained, in 38-86% and 41-89% yields, respectively. Obtained triazole-purine conjugates expressed good fluorescent properties which were studied in the solution and in the thin layer film for the first time. Quantum yields reached up to 49% in DMSO for bistriazolylpurines and up to 81% in DCM and up to 95% in DMSO for monotriazolylpurines. Performed biological studies in mouse embryo fibroblast, human keratinocyte, and transgenic adenocarcinoma of the mouse prostate cell lines showed that most of obtained triazole-purine conjugates are not cytotoxic. The 50% cytotoxic concentration of the tested derivatives was in the range from 59.6 to 1528.7 µM.

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.

Medicinal chemistry of P2 and adenosine receptors: Common scaffolds adapted for multiple targets

Prof. Geoffrey Burnstock originated the concept of purinergic signaling. He demonstrated the interactions and biological roles of ionotropic P2X and metabotropic P2Y receptors. This review paper traces the historical origins of many currently used antagonists and agonists for P2 receptors, as well as adenosine receptors, in early attempts to identify ligands for these receptors - prior to the use of chemical libraries for screening. Rather than presenting a general review of current purinergic ligands, we focus on common chemical scaffolds (privileged scaffolds) that can be adapted for multiple receptor targets. By carefully analyzing the structure activity relationships, one can direct the selectivity of these scaffolds toward different receptor subtypes. For example, the weak and non-selective P2 antagonist reactive blue 2 (RB-2) was derivatized using combinatorial synthetic approaches, leading to the identification of selective P2Y2, P2Y4, P2Y12 or P2X2 receptor antagonists. A P2X4 antagonist NC-2600 is in a clinical trial, and A3 adenosine agonists show promise, for chronic pain. P2X7 antagonists have been in clinical trials for depression (JNJ-54175446), inflammatory bowel disease (IBD), Crohn's disease, rheumatoid arthritis, inflammatory pain and chronic obstructive pulmonary disease (COPD). P2X3 antagonists are in clinical trials for chronic cough, and an antagonist named after Burnstock, gefapixant, is expected to be the first P2X3 antagonist filed for approval. We are seeing that the vision of Prof. Burnstock to use purinergic signaling modulators, most recently at P2XRs, for treating disease is coming to fruition.

Demystifying P2Y1 Receptor Ligand Recognition through Docking and Molecular Dynamics Analyses

We performed a molecular modeling analysis of 100 nucleotide-like bisphosphates and 46 non-nucleotide arylurea derivatives previously reported as P2Y₁R binders using the recently solved hP2Y₁R structures. We initially docked the compounds at the X-ray structures and identified the binding modes of representative compounds highlighting key patterns in the structure-activity relationship (SAR). We subsequently subjected receptor complexes with selected key agonists (2MeSADP and MRS2268) and antagonists (MRS2500 and BPTU) to membrane molecular dynamics (MD) simulations (at least 200 ns run in triplicate, simulation time 0.6-1.6 μs per ligand system) while considering alternative protonation states of nucleotides. Comparing the temporal evolution of the ligand-protein interaction patterns with available site-directed mutagenesis (SDM) data and P2Y₁R apo state simulation provided further SAR insights and suggested reasonable explanations for loss/gain of binding affinity as well as the most relevant charged species for nucleotide ligands. The MD analysis also predicted local conformational changes required for the receptor inactive state to accommodate nucleotide agonists.

Design and Synthesis of Fluorescent Acyclic Nucleoside Phosphonates as Potent Inhibitors of Bacterial Adenylate Cyclases

Bordetella pertussis adenylate cyclase toxin (ACT) and Bacillus anthracis edema factor (EF) are key virulence factors with adenylate cyclase (AC) activity that substantially contribute to the pathogenesis of whooping cough and anthrax, respectively. There is an urgent need to develop potent and selective inhibitors of bacterial ACs with prospects for the development of potential antibacterial therapeutics and to study their molecular interactions with the target enzymes. Novel fluorescent 5-chloroanthraniloyl-substituted acyclic nucleoside phosphonates (Cl-ANT-ANPs) were designed and synthesized in the form of their diphosphates (Cl-ANT-ANPpp) as competitive ACT and EF inhibitors with sub-micromolar potency (IC50 values: 11-622 nm). Fluorescence experiments indicated that Cl-ANT-ANPpp analogues bind to the ACT active site, and docking studies suggested that the Cl-ANT group interacts with Phe306 and Leu60. Interestingly, the increase in direct fluorescence with Cl-ANT-ANPpp having an ester linker was strictly calmodulin (CaM)-dependent, whereas Cl-ANT-ANPpp analogues with an amide linker, upon binding to ACT, increased the fluorescence even in the absence of CaM. Such a dependence of binding on structural modification could be exploited in the future design of potent inhibitors of bacterial ACs. Furthermore, one Cl-ANT-ANP in the form of a bisamidate prodrug was able to inhibit B. pertussis ACT activity in macrophage cells with IC50 =12 μm.

BPTU, an allosteric antagonist of P2Y1 receptor, blocks nerve mediated inhibitory neuromuscular responses in the gastrointestinal tract of rodents

P2Y1 receptors mediate nerve mediated purinergic inhibitory junction potentials (IJP) and relaxations in the gastrointestinal (GI) tract in a wide range of species including rodents and humans. A new P2Y1 antagonist, with a non-nucleotide structure, BPTU, has recently been described using X-ray crystallography as the first allosteric G-protein-coupled receptor antagonist located entirely outside of the helical bundle. In this study, we tested its effect on purinergic responses in the gastrointestinal tract of rodents using electrophysiological and myographic techniques. BPTU concentration dependently inhibited purinergic inhibitory junction potentials and inhibition of spontaneous motility induced by electrical field stimulation in the colon of rats (EC50 = 0.3 μM) and mice (EC50 = 0.06 μM). Mechanical inhibitory responses were also concentration-dependently blocked in the stomach of both species. Compared to MRS2500, BPTU displays a lower potency. In the rat colon nicotine induced relaxation was also blocked by BPTU. BPTU also blocked the cessation of spontaneous contractility elicited by ADPβS and the P2Y1 agonist MRS2365. We conclude that BPTU is a novel antagonist with different structural and functional properties than nucleotidic antagonists that is able to block the P2Y1 receptor located at the neuromuscular junction of the GI tract.

New highly active antiplatelet agents with dual specificity for platelet P2Y1 and P2Y12 adenosine diphosphate receptors

Currently approved platelet adenosine diphosphate (ADP) receptor antagonists target only the platelet P2Y12 receptor. Moreover, especially in patients with acute coronary syndromes, there is a strong need for rapidly acting and reversible antiplatelet agents in order to minimize the risk of thrombotic events and bleeding complications. In this study, a series of new P(1),P(4)-di(adenosine-5') tetraphosphate (Ap4A) derivatives with modifications in the base and in the tetraphosphate chain were synthesized and evaluated with respect to their effects on platelet aggregation and function of the platelet P2Y1, P2Y12, and P2X1 receptors. The resulting structure-activity relationships were used to design Ap4A analogs which inhibit human platelet aggregation by simultaneously antagonizing both P2Y1 and P2Y12 platelet receptors. Unlike Ap4A, the analogs do not activate platelet P2X1 receptors. Furthermore, the new compounds exhibit fast onset and offset of action and are significantly more stable than Ap4A to degradation in plasma, thus presenting a new promising class of antiplatelet agents.

Synthesis by microwave-assisted 1,3-dipolar cycloaddition of 1,2,3-triazole 1'-homo-3'-isoazanucleosides and evaluation of their anticancer activity

Racemic 1'-homo-3'-isoazanucleosides have been obtained by microwave-assisted 1,3-dipolar cycloaddition of 3,5-disubstituted proline derivative (±)-2 with different alkynes. The compounds obtained were evaluated for their cytotoxic activities in vitro against human breast carcinoma cell lines (MCF-7), human ovary carcinoma cell lines (A2780) and human lung carcinoma cell lines (NCI-H460).

Molecular modeling of the human P2Y14 receptor: A template for structure-based design of selective agonist ligands

The P2Y14 receptor (P2Y14R) is a Gi protein-coupled receptor that is activated by uracil nucleotides UDP and UDP-glucose. The P2Y14R structure has yet to be solved through X-ray crystallography, but the recent agonist-bound crystal structure of the P2Y12R provides a potentially suitable template for its homology modeling for rational structure-based design of selective and high-affinity ligands. In this study, we applied ligand docking and molecular dynamics refinement to a P2Y14R homology model to qualitatively explain structure-activity relationships of previously published synthetic nucleotide analogues and to probe the quality of P2Y14R homology modeling as a template for structure-based design. The P2Y14R model supports the hypothesis of a conserved binding mode of nucleotides in the three P2Y12-like receptors involving functionally conserved residues. We predict phosphate group interactions with R253(6.55), K277(7.35), Y256(6.58) and Q260(6.62), nucleobase (anti-conformation) π-π stacking with Y102(3.33) and the role of F191(5.42) as a means for selectivity among P2Y12-like receptors. The glucose moiety of UDP-glucose docked in a secondary subpocket at the P2Y14R homology model. Thus, P2Y14R homology modeling may allow detailed prediction of interactions to facilitate the design of high affinity, selective agonists as pharmacological tools to study the P2Y14R.

Conformationally constrained ortho-anilino diaryl ureas: discovery of 1-(2-(1'-neopentylspiro[indoline-3,4'-piperidine]-1-yl)phenyl)-3-(4-(trifluoromethoxy)phenyl)urea, a potent, selective, and bioavailable P2Y1 antagonist

Preclinical antithrombotic efficacy and bleeding models have demonstrated that P2Y1 antagonists are efficacious as antiplatelet agents and may offer a safety advantage over P2Y12 antagonists in terms of reduced bleeding liabilities. In this article, we describe the structural modification of the tert-butyl phenoxy portion of lead compound 1 and the subsequent discovery of a novel series of conformationally constrained ortho-anilino diaryl ureas. In particular, spiropiperidine indoline-substituted diaryl ureas are described as potent, orally bioavailable small-molecule P2Y1 antagonists with improved activity in functional assays and improved oral bioavailability in rats. Homology modeling and rat PK/PD studies on benchmark compound 3l will also be presented. Compound 3l was our first P2Y1 antagonist to demonstrate a robust oral antithrombotic effect with mild bleeding liability in the rat thrombosis and hemostasis models.

C-5 hydroxyethyl and hydroxypropyl acyclonucleosides as substrates for thymidine kinase of herpes simplex virus type 1 (HSV-1 TK): syntheses and biological evaluation

The efficient syntheses of 5-(2-hydroxyethyl)- and 5-(3-hydroxypropyl)-substituted pyrimidine derivatives bearing 2,3-dihydroxypropyl, acyclovir-, ganciclovir- and penciclovir-like side chains are reported. A synthetic approach that included the alkylation of an N-anionic-5-substituted pyrimidine intermediate (method A) provided the target acyclonucleosides in significantly higher overall yields in comparison to those obtained by method B using sylilation reaction. The phosphorylation assays of novel compounds as potential substrates for thymidine kinase of herpes simplex virus type 1 (HSV-1 TK) showed that solely pyrimidine 5-substituted acyclonucleosides with a penciclovir-like side chain acted as a fraudulent substrates of HSV-1 TK. Moreover, the uracil derivative with penciclovir-like side chain with less bulky 2-hydroxyethyl substituent at C-5 proved to be a better substrate than the corresponding one with a 3-hydroxypropyl substituent. Therefore, this acyclonucleoside was selected as a lead compound for the development of a positron emission tomography HSV-1 TK activity imaging agent.

Discovery of 2-(phenoxypyridine)-3-phenylureas as small molecule P2Y1 antagonists

Two distinct G protein-coupled purinergic receptors, P2Y1 and P2Y12, mediate ADP-driven platelet activation. The clinical effectiveness of P2Y12 blockade is well established. Recent preclinical data suggest that P2Y1 and P2Y12 inhibition provide equivalent antithrombotic efficacy, while targeting P2Y1 has the potential for reduced bleeding liability. In this account, the discovery of a 2-(phenoxypyridine)-3-phenylurea chemotype that inhibited ADP-mediated platelet aggregation in human blood samples is described. Optimization of this series led to the identification of compound 16, 1-(2-(2-tert-butylphenoxy)pyridin-3-yl)-3-4-(trifluoromethoxy)phenylurea, which demonstrated a 68 ± 7% thrombus weight reduction in an established rat arterial thrombosis model (10 mg/kg plus 10 mg/kg/h) while only prolonging cuticle and mesenteric bleeding times by 3.3- and 3.1-fold, respectively, in provoked rat bleeding time models. These results suggest that a P2Y1 antagonist could potentially provide a safe and efficacious antithrombotic profile.

Modified diadenosine tetraphosphates with dual specificity for P2Y1 and P2Y12 are potent antagonists of ADP-induced platelet activation

BACKGROUND

Diadenosine 5',5'''-P(1),P(4)-tetraphosphate (Ap(4)A), a natural compound stored in platelet dense granules, inhibits ADP-induced platelet aggregation. Ap(4)A inhibits the platelet ADP receptors P2Y(1) and P2Y(12), is a partial agonist of P2Y(12), and is a full agonist of the platelet ATP-gated ion channel P2X1. Modification of the Ap(4)A tetraphosphate backbone enhances inhibition of ADP-induced platelet aggregation. However, the effects of these Ap(4)A analogs on human platelet P2Y(1), P2Y(12) and P2X1 are unclear.

OBJECTIVE

To determine the agonist and antagonist activities of diadenosine tetraphosphate analogs towards P2Y(1), P2Y(12), and P2X1.

METHODS

We synthesized the following Ap(4)A analogs: P(1),P(4)-dithiotetraphosphate; P(2),P(3)-chloromethylenetetraphosphate; P(1)-thio-P(2),P(3)-chloromethylenetetraphosphate; and P(1),P(4)-dithio-P(2),P(3)-chloromethylenetetraphosphate. We then measured the effects of these analogs on: (i) ADP-induced platelet aggregation; (ii) P2Y(1)-mediated changes in cytosolic Ca(2+); (iii) P2Y(12)-mediated changes in vasodilator-stimulated phosphoprotein phosphorylation; and (iv) P2X1-mediated entry of extracellular Ca(2+).

RESULTS

Ap(4)A analogs with modifications in the phosphate backbone inhibited both P2Y(1) and P2Y(12), and showed no agonist activity towards these receptors. The dithio modification increased inhibition of P2Y(1), P2Y(12), and platelet aggregation, whereas the chloromethylene modification increased inhibition of P2Y(12) and platelet aggregation, but decreased P2Y(1) inhibition. Combining the dithio and chloromethylene modifications increased P2Y(1) and P2Y(12) inhibition. As compared with Ap(4)A, each modification decreased agonist activity towards P2X1, and the dual modification completely eliminated P2X1 agonist activity.

CONCLUSIONS

As compared with Ap(4)A, tetraphosphate backbone analogs of Ap(4)A have diminished activity towards P2X1 but inhibit both P2Y(1) and P2Y(12) and, with greater potency, inhibit ADP-induced platelet aggregation. Thus, diadenosine tetraphosphate analogs with dual receptor selectivity may have potential as antiplatelet drugs.

P2Y(1) receptors mediate inhibitory neuromuscular transmission in the rat colon

BACKGROUND AND PURPOSE

Inhibitory junction potentials (IJP) are responsible for smooth muscle relaxation in the gastrointestinal tract. The aim of this study was to pharmacologically characterize the neurotransmitters [nitric oxide (NO) and adenosine triphosphate (ATP)] and receptors involved at the inhibitory neuromuscular junctions in the rat colon using newly available P2Y(1) antagonists.

EXPERIMENTAL APPROACH

Organ bath and microelectrode recordings were used to evaluate the effect of drugs on spontaneous mechanical activity and resting membrane potential. IJP and mechanical relaxation were studied using electrical field stimulation (EFS).

KEY RESULTS

N(omega)-nitro-L-arginine (L-NNA) inhibited the slow component of the IJP and partially inhibited the mechanical relaxation induced by EFS. MRS2179, MRS2500 and MRS2279, all selective P2Y(1) receptor antagonists, inhibited the fast component of the IJP without having a major effect on the relaxation induced by EFS. The combination of both L-NNA and P2Y(1) antagonists inhibited the fast and the slow components of the IJP and completely blocked the mechanical relaxation induced by EFS. Sodium nitroprusside caused smooth muscle hyperpolarization and cessation of spontaneous motility that was prevented by oxadiazolo[4,3-alpha]quinoxalin-1-one. Adenosine 5'-O-2-thiodiphosphate, a preferential P2Y agonist, hyperpolarized smooth muscle cells and decreased spontaneous motility. This effect was inhibited by P2Y(1) antagonists.

CONCLUSIONS AND IMPLICATIONS

The co-transmission process in the rat colon involves ATP and NO. P2Y(1) receptors mediate the fast IJP and NO the slow IJP. The rank order of potency of the P2Y(1) receptor antagonists is MRS2500 greater than MRS2279 greater than MRS2179. P2Y(1) receptors might be potential pharmacological targets for the regulation of gastrointestinal motility.

Adenine-based acyclic nucleotides as novel P2X3 receptor ligands

A new series of acyclic nucleotides based on the adenine skeleton and bearing in 9-position a phosphorylated four carbon chain has been synthesized. Various substituents were introduced in 2-position of the adenine core. The new compounds were evaluated on rat P2X3 receptors, using patch clamp recording from HEK transfected cells and the full P2X3 agonist alpha,beta-meATP as reference compound. The results suggest that certain acyclic nucleotides, in particular compounds 28 and 29, are endowed with modest partial agonism on P2X3 receptors. This is an interesting property that can depress the function of P2X3 receptors, whose activation is believed to be involved in a number of chronic pain conditions including neuropathic pain and migraine. In fact, the new acyclic nucleotides are able to persistently block (by desensitization) P2X3 receptor activity after a brief, modest activation, yet leaving the ability of sensory neurons to mediate responses to standard painful stimuli via a lower level of signaling.

Distinct spatio-temporal Ca2+ signaling elicited by integrin alpha2beta1 and glycoprotein VI under flow

We studied how integrin alpha2beta1 and glycoprotein VI (GPVI) contribute to collagen-induced platelet activation under flow conditions by evaluating stable adhesion and intracellular Ca(2+) concentration ([Ca(2+)](i)) of FLUO 3-AM-labeled platelets perfused over acid-soluble type I or microfibrillar type VI collagen. Adhering platelets displayed 2 kinds of [Ca(2+)](i) oscillations. Rapid alpha-like peaks were unaffected by the membrane-impermeable Ca(2+) chelator ethyleneglycoltetraacetic acid but abolished by membrane-permeable BAPTA-AM. Longer-lasting gamma-like peaks were always preceded by at least one alpha-like peak and abolished by intracellular or extracellular Ca(2+) chelation. Inhibition of phosphatidylinositol 3-kinase or phospholipase C and modulation of cyclic nucleotides, but not blockage of adenosine diphosphate receptors, prevented both Ca(2+) responses. Human or mouse platelets lacking GPVI function exhibited alpha-like but not gamma-like Ca(2+) peaks, whereas those lacking alpha2beta1 showed markedly reduced to absent alpha-like and no gamma-like Ca(2+) peaks. Specific alpha2beta1 ligation induced alpha-like but not gamma-like peaks. Thus, alpha2beta1 may generate Ca(2+) signals that are reinforced by GPVI and required for subsequent longer-lasting Ca(2+) oscillation mediated by GPVI through transmembrane ion flux. Our results delineate a GPVI-independent signaling role of alpha2beta1 in response to collagen stimulation.

Third-generation immucillins: syntheses and bioactivities of acyclic immucillin inhibitors of human purine nucleoside phosphorylase

ImmH (1) and DADMe-ImmH (2) are potent inhibitors of human purine nucleoside phoshorylase (PNP), developed by us and currently in clinical trials for the treatment of a variety of T-cell related diseases. Compounds 1 and 2 were used as templates for the design and synthesis of a series of acyclic immucillin analogues (8-38) in order to identify simplified alternatives to 1 and 2. SerMe-ImmG (8) and DATMe-ImmG (9) displayed the lowest inhibition constants of 2.1 and 3.4 pM, respectively, vs PNP. It was postulated that the flexible natures of 8 and 9 enabled them to adopt conformations resembling those of 1 and 2 within the active site of PNP and that the positioning of two hydroxyl groups was critical for picomolar activity. SerMe-ImmH (10, K(d) = 5.2 pM) was shown to be orally available in mice with a long biological residence time on blood PNP.

Synthesis of uracil nucleotide analogs with a modified, acyclic ribose moiety as P2Y(2) receptor antagonists

A series of new uracil nucleotide analogs (monophosphates, triphosphates, and phosphonates) was synthesized, in which the ribose moiety was replaced by acyclic chains, including branched or linear alkyl or dialkylether linkers. 1-omega-Bromoalkyluracil derivatives (2) were converted to the corresponding alcohols by treatment with sodium hydroxide and subsequently phosphorylated using phosphorus oxychloride followed by hydrolysis to yield the monophosphates, or by coupling with diphosphate to form the triphosphates. Reaction of 2 with triethyl phosphite followed by deprotection with trimethylsilyl bromide led to the omega-phosphonylalkyluracil derivatives. These products could be further phosphorylated by converting them into their imidazolides and subsequent treatment with diphosphate yielding the corresponding UTP analogs. Nucleoside analogs with an oxygen atom in the 2'-position, which are more similar to the natural ribosides, were synthesized from silylated uracil and trimethylsilyl iodide-treated 1,3-dioxolane, or 1,3-dioxane, respectively, and subsequently phosphorylated by standard procedures. The nucleotide analogs were investigated in a functional assay at NG108-15 cells, a neuroblastomaxglioma hybrid cell line which expresses the UTP- and ATP-activated nucleotide receptor subtype P2Y(2). The acyclic nucleotide analogs were generally weaker ligands than UTP, and-in contrast to UTP-they were antagonistic. The most potent compound was diphosphoric 5-(2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl)pentylphosphonic anhydride (5c) with an IC(50) value of 92microM showing that the replacement of the alpha-phosphate by phosphonate, which leads to enhanced stability, was well tolerated.

Development of selective high affinity antagonists, agonists, and radioligands for the P2Y1 receptor

The P2Y(1) receptor is a member of the P2Y family of nucleotide-activated G protein-coupled receptors, and it is an important therapeutic target based on its broad tissue distribution and essential role in platelet aggregation. We have designed a set of highly selective and diverse pharmacological tools for studying the P2Y(1) receptor using a rational approach to ligand design. Based on the discovery that bisphosphate analogues of the P2Y(1) receptor agonist, ADP, are partial agonists/competitive antagonists of this receptor, an iterative approach was used to develop competitive antagonists with enhanced affinity and selectivity. Halogen substitutions of the 2-position of the adenine ring provided increased affinity while an N(6) methyl substitution eliminated partial agonist activity. Furthermore, various replacements of the ribose ring with symmetrically branched, phosphorylated acyclic structures revealed that the ribose is not necessary for recognition at the P2Y(1) receptor. Finally, replacement of the ribose ring with a five member methanocarba ring constrained in the Northern conformation conferred dramatic increases in affinity to both P2Y(1) receptor antagonists as well as agonists. These combined structural modifications have resulted in a series of selective high affinity antagonists of the P2Y(1) receptor, two broadly applicable radioligands, and a high affinity agonist capable of selectively activating the P2Y(1) receptor in human platelets. Complementary receptor modeling and computational ligand docking have provided a putative structural framework for the drug-receptor interactions. A similar rational approach is being applied to develop selective ligands for other subtypes of P2Y receptors.

On the applicability of GPCR homology models to computer-aided drug discovery: a comparison between in silico and crystal structures of the beta2-adrenergic receptor

The publication of the crystal structure of the beta2-adrenergic receptor (beta2-AR) proved that G protein-coupled receptors (GPCRs) share a structurally conserved rhodopsin-like 7TM core. Here, to probe to which extent realistic GPCR structures can be recreated through modeling, carazolol was docked at two rhodopsin-based homology models of the human beta 2-AR. The first featured a rhodopsin-like second extracellular loop, which interfered with ligand docking and with the orientation of several residues in the binding pocket. The second featured a second extracellular loop built completely de novo, which afforded a more accurate model of the binding pocket and a better docking of the ligand. Furthermore, incorporating available biochemical and computational data to the model by correcting the conformation of a single residue lining the binding pocket --Phe290(6.52)--, resulted in significantly improved docking poses. These results support the applicability of GPCR modeling to the design of site-directed mutagenesis experiments and to drug discovery.

Defining the nucleotide binding sites of P2Y receptors using rhodopsin-based homology modeling

Ongoing efforts to model P2Y receptors for extracellular nucleotides, i.e., endogenous ADP, ATP, UDP, UTP, and UDP-glucose, were summarized and correlated for the eight known subtypes. The rhodopsin-based homology modeling of the P2Y receptors is supported by a growing body of site-directed mutagenesis data, mainly for P2Y(1) receptors. By comparing molecular models of the P2Y receptors, it was concluded that nucleotide binding could occur in the upper part of the helical bundle, with the ribose moiety accommodated between transmembrane domain (TM) 3 and TM7. The nucleobase was oriented towards TM1, TM2, and TM7, in the direction of the extracellular side of the receptor. The phosphate chain was oriented towards TM6, in the direction of the extracellular loops (ELs), and was coordinated by three critical cationic residues. In particular, in the P2Y(1), P2Y(2), P2Y(4), and P2Y(6) receptors the nucleotide ligands had very similar positions. ADP in the P2Y(12) receptor was located deeper inside the receptor in comparison to other subtypes, and the uridine moiety of UDP-glucose in the P2Y(14) receptor was located even deeper and shifted toward TM7. In general, these findings are in agreement with the proposed binding site of small molecules to other class A GPCRs.

Development and validation of a platelet calcium flux assay using a fluorescent imaging plate reader

Calcium signaling in platelets is an important physiological response to various aggregation stimuli. Loading platelets with various fluorescent dyes and measuring the change in calcium concentration using a spectrofluorometer has been the traditional approach to studying calcium signaling. This method suffers from the need for large platelet samples and a decrease in total fluorescence signal with time due to photobleaching. Therefore, it is rarely used to measure the quantitative effect of an agonist or antagonist on calcium signaling. Adaptation of these measurements to a fluorescent imaging plate reader (FLIPR) format allows the sample size to be reduced by 5- to 10-fold, and the microplate format allows a significant increase in throughput. Addition of the agonists to all wells simultaneously serves to normalize the total response. This article describes the first use of a FLIPR to study the calcium flux in human platelets. The IC(50) values showed a linear correlation with the K(i) for receptor binding in washed platelets. The generality of the methodology was shown by measuring EC(50) values for agonists and IC(50) values for antagonists of the platelet G protein-coupled receptor P2Y(1) and for the ion channel P2X(1).

Synthesis of potential purinoceptor antagonists: application of P1-tBU phosphazene base for alkylation of adenine in solution and on solid phase

Alkylation of adenine in solution and on solid phase was accelerated by phosphazene base P1-tBu compared to mineral bases. The reactions in solution afforded regioselectively the appropriate N9-alkylated adenines with high preparative yields while the reaction with polystyrene resin-bound N-bromoacetylated peptides gave three regioisomers (alkylated at the N9, N7, and N3 position of adenine) in a 4:2:1 molar ratio. Ten novel nonphosphate nucleotide analogues were tested in an ADP-induced platelet aggregation assay.

[32P]2-iodo-N6-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate ([32P]MRS2500), a novel radioligand for quantification of native P2Y1 receptors

Analysis of the P2Y family of nucleotide-activated G-protein-coupled receptors has been compromised by the lack of selective high-affinity, high-specific-radioactivity radioligands. We have pursued quantification of the P2Y1 receptor through the development of a series of selective P2Y1 receptor antagonists. Recently, we synthesized 2-iodo-N6-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate (MRS2500), a selective, competitive antagonist that exhibits a Ki of 0.8 nM in competition-binding assays with [3H]MRS2279. A 3'-monophosphate precursor molecule, MRS2608, was radiolabeled at the 5' position with 32P using polynucleotide kinase and [gamma32P]ATP to yield [32P]MRS2500. [32P]MRS2500 bound selectively to Sf9 insect cell membranes expressing the human P2Y1 receptor (Sf9-P2Y1), but did not detectably bind membranes expressing other P2Y receptors. P2Y1 receptor binding to [32P]MRS2500 was saturable with a KD of 1.2 nM. Agonists and antagonists of the P2Y1 receptor inhibited [32P]MRS2500 binding in Sf9-P2Y1 membranes with values in agreement with those observed in functional assays of the P2Y1 receptor. A high-affinity binding site for [32P]MRS2500 (KD=0.33 nM) was identified in rat brain, which exhibited the pharmacological selectivity of the P2Y1 receptor. Distribution of this binding site varied among rat tissues, with the highest amount of binding appearing in lung, liver, and brain. Among brain regions, distribution of the [32P]MRS2500 binding site varied by six-fold, with the highest and lowest amounts of sites detected in cerebellum and cortex, respectively. Taken together, these data illustrate the synthesis and characterization of a novel P2Y1 receptor radioligand and its utility for examining P2Y1 receptor expression in native mammalian tissues.

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.

Design and synthesis of new bicyclic diketopiperazines as scaffolds for receptor probes of structurally diverse functionality

Diketopiperazines (DKPs) are a common motif in various biologically active natural products, and hence they may be useful scaffolds for the rational design of receptor probes and therapeutic agents. We constructed a new bicyclic scaffold that combines a DKP bridged with a 10-membered ring. In this way we obtained a three-dimensional molecular skeleton, with several amendable sites that provide a starting point to design a new combinatorial library having diverse substituent groups. Structural variation is based upon the flexibility of alkylation of the nitrogen atoms of the DKP and on the side-chain olefin. We obtained a 10-membered secondary ring through a ring-closure metathesis reaction using the second generation Grubbs catalyst. Rings containing both O-ethers and S-ethers were compared. N-Alkyl or arylalkyl groups were introduced optionally at the two Nalpha-atoms. This is a general scheme that will allow us to test rings of varying sizes, linkages, and stereochemical parameters. The DKP derivatives were tested for activity in astrocytoma cells expressing receptors coupled to phospholipase C. Inhibitory effects were observed for signaling elicited by activation of human nucleotide P2Y receptors but not m3 muscarinic receptors. Compound 20 selectively inhibited calcium mobilization (IC50 value of 486 +/- 16 nM) and phosphoinositide turnover elicited by a selective P2Y1 receptor agonist, but this compound did not compete for binding of a radiolabeled nucleotide-competitive receptor antagonist. Therefore, the new class of DKP derivatives shows utility as pharmacological tools for P2Y receptors.

Fluorescent epsilon-ATP analogues for probing physicochemical properties of proteins. Synthesis, biochemical evaluation, and sensitivity to properties of the medium

Despite the significance of the elucidation of proteins' physicochemical parameters to understand various molecular phenomena, direct methods for measuring these parameters are not readily available. Here, we propose the use of 8-[p-amino-Ph]-epsilon-ATP, 3b, as a fluorescent probe for the elucidation of physicochemical parameters of binding sites in certain proteins. We synthesized novel fluorescent nucleotide analogues based on an extension of the epsilon-ATP scaffold. These analogues bear a primary or tertiary p-amino-phenyl moiety on the etheno-bridge. We explored the recognition of the fluorescent analogues by the target proteins: P2Y(1)-receptor (P2Y(1)-R) and NTPDase1. Based on the high affinity to the P2Y(1)-R (EC(50) 100nM), 3b proved a suitable probe for the investigation of this receptor. Next, we elucidated the dependencies of the absorption and emission spectra of 3b on environmental parameters, for establishing correlation equations. These equations will help determine the properties of the ATP-binding site from the spectral data of the protein-bound 3b. For this purpose, the sensitivity of the probe to acidity, dielectricity, H-bonding, viscosity, and to correlation between these parameters was determined. Thus, the pH-dependence of 3b emission intensity is bell shaped. At pH2.8 the quantum yield (phi) is enhanced 150-fold, as compared to neutral pH. The basic nitrogen atoms of 3b were assigned and pK(a) values were determined. A linear relationship was found between log phi and log viscosity, however, emission maxima (lambda(max)) remained constant. A linear relationship was found between both phi and lambda(max) and dielectricity, as measured in protic or aprotic solvents of comparable viscosity. pK(a)-like values were measured in acid-titrated alcohols with varying dielectricity but comparable viscosity, or with varying viscosity but comparable dielectricity. An inverse relationship and a linear relationship were found between the pK(a) values of 3b and the medium dielectricity and viscosity, respectively. These correlations help the calibration of properties of a protein ATP-binding site.

[Synthesis of phosphonic acid and phosphinic acid derivatives for development of biologically active compounds]

This paper covers recent publications from our laboratory on the synthesis of a variety of phosphonate and phosphinate derivatives. New methods for the enantioselective synthesis of alpha-hydroxyphosphonates were established by Lewis acid-mediated cleavage of homochiral 1,3-dioxaneacetals with P(OEt)(3) and chiral metal ligand-mediated hydrophosphonylation of aldehydes. Two diastereomers of HPmp derivatives were prepared by an application of these methods. The HPmp derivatives were convered to FPmp derivatives but with low diastereoselectivity. Hydrophosphonylation of alpha-aminoaldehydes afforded threo- and erythro-beta-amino-alpha-hydroxyphosphonates under chelation and nonchelation controlled conditions, respectively. The asymmetric dihydroxylation of alpha, beta-, and beta, gamma-unsaturated phosphonates with AD-mix-alpha and AD-mix-beta reagents gave alpha, beta- and beta, gamma-dihydroxyphosphonates with high enantioselectivity. The method was applied to the kinetic resolution of racemic alpha-oxygetated beta, gamma-unsaturated phosphonates. Treatment of allyloxymethylphosphonates with the base afforded alpha-hydroxyphosphonates via the [2,3]-Wittig reaction. Threo- and erythro-beta-amino-alpha-hydroxyphosphinates were obtained with high diastereoselectivity by phosphinylation of alpha-aminoaldehydes in the presence of (R)- and (S)-ALB, respectively. The phosphinylation of alpha-oxygenated aldehydes afforded the corresponding alpha, beta-dioxygenated phosphinates, but with low diastereoselectivity. Sphingomyelin analogues containing CF(2)PO(OH)(2) were synthesized starting from (S)- and (R)-Garner aldehyde for the purpose of obtaining potent sphyngomyelinase inhibitors. A useful method for the synthesis of alpha, alpha-difluorobenzylphosphonates was established based on the cross coupling reaction of an iodobenzene derivative with ZnCuBr(2)CF(2)PO(OEt)(2). The synthetic utility of ZnCuBr(2)CF(2)PO(OEt)(2) was examined to obtain alpha, alpha-difluoromethylenenphosphonates. The method was applied to a synthesis of PNP-inhibitory active compounds by combination of the purine base and alcohols containing difluoromethylenephosphonate. The methodology for the beta-selective N-glycosylation of 2,3-dideoxy glucoside was established by introducing phosphonothioates at the 3-position of glycosyl doners instead of phosphonate. Synthesis of new acylic nucleotide analogues designed based on the structural modification of ARS2267 is also described. Finally, kiral synthesis of some phosphonates was achieved using lipase through kinetic resolution.

Synthesis and antiviral activity of novel acyclic nucleosides in the 5-alkynyl- and 6-alkylfuro[2,3-d]pyrimidine series

The synthesis of novel acyclic nucleosides in the 5-alkynyl and 6-alkylfuro[2,3-d]pyrimidine series is described. These compounds were evaluated against HIV and HSV in order to determine their spectrum of antiviral activity. Their cytotoxicities against PBM, CEM and VERO cells were also determined. Compounds 21d and 24b displayed moderate EC50s of 2.7 and 4.9 microM, respectively, against HIV-1 and of 6.3 and 4.8 microM, respectively, against HSV. Nevertheless, these compounds also showed cellular toxicity, suggesting that the antiviral effects are secondary to the toxic effects.

Induction of novel agonist selectivity for the ADP-activated P2Y1 receptor versus the ADP-activated P2Y12 and P2Y13 receptors by conformational constraint of an ADP analog

ADP is the cognate agonist of the P2Y1, P2Y12, and P2Y13 receptors. With the goal of identifying a high potency agonist that selectively activates the P2Y1 receptor, we examined the pharmacological selectivity of the conformationally constrained non-nucleotide analog (N)-methanocarba-2MeSADP [(1'S,2'R, 3'S,4'R,5'S)-4-[(6-amino-2-methylthio-9H-purin-9-yl)-1-diphosphoryloxymethyl]bicyclo[3.1.0]hexane-2,3-diol] among the three ADP-activated receptors. Each P2Y receptor was expressed transiently in COS-7 cells, and inositol lipid hydrolysis was quantified as a measure of receptor activity. In the case of the Gi-linked P2Y12 and P2Y13 receptors, a chimeric G protein, Galphaq/i, was coexpressed to confer a capacity of these Gi-linked receptors to activate phospholipase C. 2MeSADP (2-methylthio-ADP) was a potent agonist at all three receptors exhibiting EC50 values in the sub to low nanomolar range. In contrast, whereas (N)-methanocarba-2MeSADP was an extremely potent (EC50=1.2 +/- 0.2 nM) agonist at the P2Y1 receptor, this non-nucleotide analog exhibited no agonist activity at the P2Y12 receptor and very low activity at the P2Y13 receptor. (N)-Methanocarba-2MeSADP also failed to block the action of 2MeSADP at the P2Y12 and P2Y13 receptors, indicating that the (N)-methanocarba analog is not an antagonist at these receptors. The P2Y1 receptor selectivity of (N)-methanocarba-2MeSADP was confirmed in human platelets where it induced the shape change promoted by P2Y1 receptor activation without inducing the sustained platelet aggregation that requires simultaneous activation of the P2Y12 receptor. These results provide the first demonstration of a high-affinity agonist that discriminates among the three ADP-activated P2Y receptors, and therefore, introduce a potentially important new pharmacological tool for delineation of the relative biological action of these three signaling proteins.

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.

Nucleotide analogues containing 2-oxa-bicyclo[2.2.1]heptane and l-alpha-threofuranosyl ring systems: interactions with P2Y receptors

The ribose moiety of adenine nucleotide 3',5'-bisphosphate antagonists of the P2Y(1) receptor has been successfully substituted with a rigid methanocarba ring system, leading to the conclusion that the North (N) ring conformation is preferred in receptor binding. Similarly, at P2Y(2) and P2Y(4) receptors, nucleotides constrained in the (N) conformation interact equipotently with the corresponding ribosides. We now have synthesized and examined as P2Y receptor ligands nucleotide analogues substituted with two novel ring systems: (1) a (N) locked-carbocyclic (cLNA) derivative containing the oxabicyclo[2.2.1]heptane ring system and (2) l-alpha-threofuranosyl derivatives. We have also compared potencies and preferred conformations of these nucleotides with the known anhydrohexitol-containing P2Y(1) receptor antagonist MRS2283. A cLNA bisphosphate derivative MRS2584 21 displayed a K(i) value of 22.5 nM in binding to the human P2Y(1) receptor, and antagonized the stimulation of PLC by the potent P2Y(1) receptor agonist 2-methylthio-ADP (30 nM) with an IC(50) of 650 nM. The parent cLNA nucleoside bound only weakly to an adenosine receptor (A(3)). Thus, this ring system afforded some P2Y receptor selectivity. A l-alpha-threofuranosyl bisphosphate derivative 9 displayed an IC(50) of 15.3 microM for inhibition of 2-methylthio-ADP-stimulated PLC activity. l-alpha-Threofuranosyl-UTP 13 was a P2Y receptor agonist with a preference for P2Y(2) (EC(50)=9.9 microM) versus P2Y(4) receptors. The P2Y(1) receptor binding modes, including rotational angles, were estimated using molecular modeling and receptor docking.

Architecture of P2Y nucleotide receptors: structural comparison based on sequence analysis, mutagenesis, and homology modeling

Human P2Y receptors encompass at least eight subtypes of Class A G protein-coupled receptors (GPCRs), responding to adenine and/or uracil nucleotides. Using a BLAST search against the Homo sapiens subset of the SWISS-PROT and TrEMBL databases, we identified 68 proteins showing high similarity to P2Y receptors. To address the problem of low sequence identity between rhodopsin and the P2Y receptors, we performed a multiple-sequence alignment of the retrieved proteins and the template bovine rhodopsin, combining manual identification of the transmembrane domains (TMs) with automatic techniques. The resulting phylogenetic tree delineated two distinct subgroups of P2Y receptors: Gq-coupled subtypes (e.g., P2Y1) and those coupled to Gi (e.g., P2Y12). On the basis of sequence comparison we mutated three Tyr residues of the putative P2Y1 binding pocket to Ala and Phe and characterized pharmacologically the mutant receptors expressed in COS-7 cells. The mutation of Y306 (7.35, site of a cationic residue in P2Y12) or Y203 in the second extracellular loop selectively decreased the affinity of the agonist 2-MeSADP, and the Y306F mutation also reduced antagonist (MRS2179) affinity by 5-fold. The Y273A (6.48) mutation precluded the receptor activation without a major effect on the ligand-binding affinities, but the Y273F mutant receptor still activated G proteins with full agonist affinity. Thus, we have identified new recognition elements to further define the P2Y1 binding site and related these to other P2Y receptor subtypes. Following sequence-based secondary-structure prediction, we constructed complete models of all the human P2Y receptors by homology to rhodopsin. Ligand docking on P2Y1 and P2Y12 receptor models was guided by mutagenesis results, to identify the residues implicated in the binding process. Different sets of cationic residues in the two subgroups appeared to coordinate phosphate-bearing ligands. Within the P2Y1 subgroup these residues are R3.29, K/R6.55, and R7.39. Within the P2Y12 subgroup, the only residue in common with P2Y1 is R6.55, and the role of R3.29 in TM3 seems to be fulfilled by a Lys residue in EL2, whereas the R7.39 in TM7 seems to be substituted by K7.35. Thus, we have identified common and distinguishing features of P2Y receptor structure and have proposed modes of ligand binding for the two representative subtypes that already have well-developed ligands.

Distinct roles of ADP receptors in von Willebrand factor-mediated platelet signaling and activation under high flow

We have investigated the role of adenosine diphosphate (ADP) receptors in the adhesion, activation, and aggregation of platelets perfused over immobilized von Willebrand factor (VWF) under high shear stress. Blocking P2Y(1) prevented stable platelet adhesion and aggregation, indicative of a complete inhibition of alpha(IIb)beta(3) activation, and decreased the duration of transient arrests from 5.9 seconds +/- 2.8 seconds to 1.2 seconds +/- 0.8 seconds; in contrast, blocking P2Y(12) inhibited only the formation of larger aggregates. Moreover, blocking P2Y(1) decreased the proportion of platelets showing early intracytoplasmic Ca(++) elevations (alpha/beta peaks) from 20.6% +/- 1.6% to 14.6% +/- 1.5% (P < .01), and the corresponding peak ion concentration from 1543 nM +/- 312 nM to 1037 nM +/- 322 nM (P < .05); it also abolished the Ca(++) elevations seen in firmly attached platelets (gamma peaks). Blocking P2Y(12) had no effect on these parameters, and did not enhance the effect of inhibiting P2Y(1). Inhibition of phospholipase C had similar consequences as the blocking of P2Y(1), whereas inhibition of Src family kinases abolished both type alpha/beta and gamma Ca(++) oscillations, although the former effect required a higher inhibitor concentration. Our results demonstrate that, under elevated shear stress conditions, ADP signaling through P2Y(1) may contribute to the initial stages of platelet adhesion and activation mediated by immobilized VWF, and through P2Y(12) to sustained thrombus formation.

Signaling through GP Ib-IX-V activates alpha IIb beta 3 independently of other receptors

Platelet adhesion to von Willebrand factor (VWF) activates alpha IIb beta 3, a prerequisite for thrombus formation. However, it is unclear whether the primary VWF receptor, glycoprotein (GP) Ib-IX-V, mediates alpha IIb beta 3 activation directly or through other signaling proteins physically associated with it (eg, FcR gamma-chain), possibly with the contribution of other agonist receptors and of VWF signaling through alpha IIb beta 3. To resolve this question, human and GP Ibalpha transgenic mouse platelets were plated on dimeric VWF A1 domain (dA1VWF), which engages only GP Ib-IX-V, in the presence of inhibitors of other agonist receptors. Platelet adhesion to dA1VWF induced Src kinase-dependent tyrosine phosphorylation of the FcR gamma-chain and the adapter molecule, ADAP, and triggered intracellular Ca(2+) oscillations and alpha IIb beta 3 activation. Inhibition of Ca(2+) oscillations with BAPTA-AM prevented alpha IIb beta 3 activation but not tyrosine phosphorylation. Pharmacologic inhibition of protein kinase C (PKC) or phosphatidylinositol 3-kinase (PI 3-kinase) prevented alpha IIb beta 3 activation but not Ca(2+) oscillations. Inhibition of Src with 2 distinct compounds blocked all responses downstream of GP Ib-IX-V under static or flow conditions. However, dA1VWF-induced responses were reduced only slightly in GP Ibalpha transgenic platelets lacking FcR gamma-chain. These data establish that GP Ib-IX-V itself can signal to activate alpha IIb beta 3, through sequential actions of Src kinases, Ca(2+) oscillations, and PI 3-kinase/PKC.