Agonist and antagonist effects of diadenosine tetraphosphate, a platelet dense granule constituent, on platelet P2Y1, P2Y12 and P2X1 receptors

Platelet Physiology

Platelets are the smallest blood cells, numbering 150 to 350 × 109/L in healthy individuals. The ability of activated platelets to adhere to an injured vessel wall and form aggregates was first described in the 19th century. Besides their long-established roles in thrombosis and hemostasis, platelets are increasingly recognized as pivotal players in numerous other pathophysiological processes including inflammation and atherogenesis, antimicrobial host defense, and tumor growth and metastasis. Consequently, profound knowledge of platelet structure and function is becoming more important in research and in many fields of modern medicine. This review provides an overview of platelet physiology focusing particularly on the structure, granules, surface glycoproteins, and activation pathways of platelets.

Migraine signaling pathways: purine metabolites that regulate migraine and predispose migraineurs to headache

Migraine is a debilitating disorder that afflicts over 1 billion people worldwide, involving attacks that result in a throbbing and pulsating headache. Migraine is thought to be a neurovascular event associated with vasoconstriction, vasodilation, and neuronal activation. Understanding signaling in migraine pathology is central to the development of therapeutics for migraine prophylaxis and for mitigation of migraine in the prodrome phase before pain sets in. The fact that both vasoactivity and neural sensitization are involved in migraine indicates that agonists which promote these phenomena may very well be involved in migraine pathology. One such group of agonists is the purines, in particular, adenosine phosphates and their metabolites. This manuscript explores what is known about the relationship between these metabolites and migraine pathology and explores the potential for such relationships through their known signaling pathways. Reported receptor involvement in vasoaction and nociception.

Effects of uridine and nucleotides on hemostasis parameters

Several purinergic receptors have been identified on platelets which are involved in hemostatic and thrombotic processes. The aim of the present study was to investigate the effects of uridine and its nucleotides on platelet aggregation and hemostasis in platelet-rich plasma (PRP) and whole blood. The effects of uridine, UMP, UDP, and UTP at different final concentrations (1 to 1000 µM) on platelet aggregation were studied using an aggregometer. In PRP samples, platelet aggregation was induced by ADP, collagen and epinephrine 3 min after addition of uridine, UMP, UDP, UTP and saline (as a control). All thromboelastogram experiments were performed at 1000 µM final concentrations of uridine and its nucleotides in whole blood. UDP and UTP were also tested in thromboelastogram with PRP. Our results showed that UDP, and especially UTP, inhibited ADP- and collagen-induced aggregation in a concentration-dependent manner. In whole blood thromboelastogram experiments, UDP stimulated clot formation while UTP suppressed clot formation. When thromboelastogram experiments were repeated with PRP, UTP's inhibitory effect on platelets was confirmed, while UDP's stimulated clot forming effect disappeared. Collectively, our data showed that UTP inhibited platelet aggregation in a concentration-dependent manner and suppressed clot formation. On the other hand, UDP exhibited distinct effects on whole blood or PRP in thromboelastogram. These data suggest that the difference on effects of UTP and UDP might have arisen from the different receptors that they stimulate and warrant further investigation with regard to their in vivo actions on platelet aggregation and hemostasis.

Synthesis and biological evaluation of N 6 derivatives of 8-azapurine as novel antiplatelet agents

Two series of novel N 6 derivatives of 8-azapurine I and II were designed as antiplatelet agents. Series I and II were N 6 amino derivatives and N 6 hydrazone derivatives of 8-azapurine, respectively. The compounds were synthesized in acceptable yields via conventional procedures, including nucleophilic substitution, diazotization, and amination or hydrazonation with amino alcohol and 4,6-dichloropyrimidine as starting materials. To assess the ability of the synthesized compounds as antiplatelet agents, the ADP-induced platelet aggregation assay of Born was performed both in vitro and in vivo using ticagrelor as a reference control substance. The analysis of the structure-activity relationship and molecular docking were also discussed in detail. The results demonstrated that series I and II compounds exhibited antiplatelet activity in vitro and IIh was the most active compound (IC50 = 0.20 μM) among the target compounds, being almost 4-fold better than ticagrelor (IC50 = 0.74 μM). For a preliminary assessment of the safety profile, a bleeding test (mouse tail) and a single-dose toxicity test were conducted. The use of compound IIh resulted in a shorter bleeding time, less blood loss and lower acute toxicity compared to ticagrelor. In addition, a molecular docking study was performed to investigate the binding capacity and binding mode between IIh and P2Y12.

Immunothrombosis in Acute Respiratory Dysfunction of COVID-19

COVID-19 is an acute, complex disorder that was caused by a new β-coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Based on current reports, it was surprising that the characteristics of many patients with COVID-19, who fulfil the Berlin criteria for acute respiratory distress syndrome (ARDS), are not always like those of patients with typical ARDS and can change over time. While the mechanisms of COVID-19–related respiratory dysfunction in COVID-19 have not yet been fully elucidated, pulmonary microvascular thrombosis is speculated to be involved. Considering that thrombosis is highly related to other inflammatory lung diseases, immunothrombosis, a two-way process that links coagulation and inflammation, seems to be involved in the pathophysiology of COVID-19, including respiratory dysfunction. Thus, the current manuscript will describe the proinflammatory milieu in COVID-19, summarize current evidence of thrombosis in COVID-19, and discuss possible interactions between these two.

Isolation and Functional Identification of an Antiplatelet RGD-Containing Disintegrin from Cerastes cerastes Venom

The current report focuses on purification, structural and functional characterization of Cerastategrin from Cerastes cerastes venom, a novel basic disintegrin (pI 8.36) with 128 amino acid residues and a molecular weight of 13 835.25 Da measured by MALDI-MSMS. The 3D structure of Cerastategrin is organized as α-helix (13%), β-strand (15%) and disordered structure (30%) and presents homologies with several snake venom disintegrins. Structural modeling shows that Cerastategrin presents an RGD motif that connects specifically to integrin receptors. Cerastategrin exhibits the inhibition of ADP induced platelets with an IC₅₀ of 0.88 µg/mL and shows in vivo long stable anticoagulation effect 24 h post-injection of increasing doses ranging from 0.2 to 1 mg/kg, therefore, Cerastategrin maintained irreversibly the blood incoagulable. Moreover, Cerastategrin decreases the amount of bounded αIIbβ3 and reduced significantly the quantity of externalized P-Selectin. Cerastategrin acts as a molecule targeting specifically the receptor α_IIbβ₃; therefore, it behaves as a potent platelet activation inhibitor. As a new peptide with promising pharmacological properties, Cerastategrin could have a potential therapeutical effect in the vascular pathologies and may be a new effective treatment approach.

Impact of Diabetes Mellitus and Chronic Kidney Disease on Cardiovascular Outcomes and Platelet P2Y12 Receptor Antagonist Effects in Patients With Acute Coronary Syndromes: Insights From the PLATO Trial

Background

There are limited data on how the combination of diabetes mellitus (DM) and chronic kidney disease (CKD) affects cardiovascular outcomes as well as response to different P2Y₁₂ receptor antagonists, which represented the aim of the present investigation.

Methods and Results

In this post hoc analysis of the PLATO (Platelet Inhibition and Patient Outcomes) trial, which randomized acute coronary syndrome patients to ticagrelor versus clopidogrel, patients (n=15 108) with available DM and CKD status were classified into 4 groups: DM+/CKD+ (n=1058), DM+/CKD− (n=2748), DM−/CKD+ (n=2160), and DM−/CKD− (n=9142). The primary efficacy end point was a composite of cardiovascular death, myocardial infarction, or stroke at 12 months. The primary safety end point was PLATO major bleeding. DM+/CKD+ patients had a higher incidence of the primary end point compared with DM−/CKD− patients (23.3% versus 7.1%; adjusted hazard ratio 2.22; 95% CI 1.88–2.63; P<0.001). Patients with DM+/CKD− and DM−/CKD+ had an intermediate risk profile. The same trend was shown for the individual components of the primary end point and for major bleeding. Compared with clopidogrel, ticagrelor reduced the incidence of the primary end point consistently across subgroups (P‐interaction=0.264), but with an increased absolute risk reduction in DM+/CKD+. The effects on major bleeding were also consistent across subgroups (P‐interaction=0.288).

Conclusions

In acute coronary syndrome patients, a gradient of risk was observed according to the presence or absence of DM and CKD, with patients having both risk factors at the highest risk. Although the ischemic benefit of ticagrelor over clopidogrel was consistent in all subgroups, the absolute risk reduction was greatest in patients with both DM and CKD.

Clinical Trial Registration

URL: http://www.clinicatrials.gov. Unique identifier: NCT00391872.

Recombinant expression of ecto-nucleotide pyrophosphatase/phosphodiesterase 4 (NPP4) and development of a luminescence-based assay to identify inhibitors

Nucleotide pyrophosphatase/phosphodiesterase 4 (NPP4) is a membrane-bound enzyme that hydrolyzes extracellular diadenosine polyphosphates such as diadenosine triphosphate (Ap₃A) and diadenosine tetraphosphate (Ap₄A) yielding mononucleotides. NPP4 on the surface of endothelial cells was reported to promote platelet aggregation by hydrolyzing Ap₃A to ADP, which activates pro-thrombotic G protein-coupled P2Y₁ and P2Y₁₂ receptors. Thus, NPP4 inhibitors have potential as novel antithrombotic drugs. In the present study we expressed soluble human NPP4 in Sf9 insect cells and established an enzyme assay using diadenosine tetraphosphate (Ap₄A) as a substrate. The reaction product ATP was quantified by luciferin-luciferase reaction in a 96-well plate format. The sensitive method displayed a limit of detection (LOD) of 14.6 nM, and a Z'-factor of 0.68 indicating its suitability for high-throughput screening. The new assay was applied for studying enzyme kinetics and led to the identification of the first NPP4 inhibitors.

Impact of chronic kidney disease on platelet P2Y12 receptor signalling in patients with type 2 diabetes mellitus

Supplementary Material to this article is available online at www.thrombosis-online.com.

Incomplete reversibility of platelet inhibition following prolonged exposure to ticagrelor

Essentials Irreversible platelet inhibition persists after reversibly-binding ticagrelor is discontinued. Reversibility of platelet inhibition by ticagrelor and its active metabolite was assessed. Incomplete recovery was observed after prolonged exposure to ticagrelor. Activated GPIIb-IIIa and P-selectin, not platelet reactivity index, showed irreversibility.

SUMMARY

Introduction Ticagrelor is described as a reversible P2Y12 antagonist. However, residual platelet inhibition persists after discontinuation of ticagrelor when plasma levels are undetectable. We assessed the reversibility of platelet inhibition by ticagrelor and its active metabolite (T-AM) in comparison with cangrelor and prasugrel's active metabolite (P-AM). Methods Whole blood was treated in vitro with ~ 50% inhibitory concentrations of ticagrelor, T-AM, cangrelor, P-AM and assessed for ADP-stimulated activated GPIIb-IIIa and P-selectin and vasodilator-stimulated phosphoprotein (VASP) platelet reactivity index (PRI) before and after 100-fold dilution. Results Platelets exposed for 30 min to ticagrelor, T-AM or cangrelor showed full recovery of activated GPIIb-IIIa but only partial recovery of P-selectin. Longer exposure (24 h) to the drug decreased reversibility of activated GPIIb-IIIa by ticagrelor (65.1% [49.5-80.6], % of vehicle with 95% confidence interval [CI]) and T-AM (88.8% [79.2-98.3]), but not by cangrelor (101.4% [96.4-106.4]). Compared with 30 min exposure, the reversibility of P-selectin further decreased after 24 h exposure to ticagrelor (from 91.8% [82.1-101.5] to 51.8% [45.5-85.0]), but not T-AM (from 79.0% [67.8-90.3] to 77.4% [61.8-93.1]) or cangrelor (from 76.0% [67.6-84.4] to 76.2% [70.6-81.8]). In contrast, 24 h exposure to ticagrelor, T-AM and cangrelor resulted in full recovery of platelet reactivity as measured by PRI. Platelets exposed to P-AM showed no recovery of ADP reactivity. Conclusions Incomplete recovery after prolonged exposure to ticagrelor, observed by activated GPIIb-IIIa and P-selectin but not upstream VASP signaling, suggests that P2Y12 regains functionality and irreversible changes occur independent of VASP signaling.

Diadenosine tetraphosphate (Ap4A) inhibits ATP-induced excitotoxicity: a neuroprotective strategy for traumatic spinal cord injury treatment

Reducing cell death during the secondary injury is a major priority in the development of a cure for traumatic spinal cord injury (SCI). One of the earliest processes that follow SCI is the excitotoxicity resulting from the massive release of excitotoxicity mediators, including ATP, which induce an excessive and/or prolonged activation of their receptors and a deregulation of the calcium homeostasis. Diadenosine tetraphosphate (Ap4A) is an endogenous purinergic agonist, present in both extracellular and intracellular fluids, with promising cytoprotective effects in different diseases including neurodegenerative processes. In a search for efficient neuroprotective strategies for SCI, we have tested the capability of Ap4A to reduce the excitotoxic death mediated by the ATP-induced deregulation of calcium homeostasis and its consequences on tissue preservation and functional recovery in a mouse model of moderate contusive SCI. Our analyses with the murine neural cell line Neuro2a demonstrate that treatment with Ap4A reduces ATP-dependent excitotoxic death by both lowering the intracellular calcium response and decreasing the expression of specific purinergic receptors. Follow-up analyses in a mouse model of contusive SCI showed that acute administration of Ap4A following SCI reduces tissue damage and improves motor function recovery. These results suggest that Ap4A cytoprotection results from a decrease of the purinergic tone preventing the effects of a massive release of ATP after SCI, probably together with a direct induction of anti-apoptotic and pro-survival pathways via activation of P2Y2 proposed in previous studies. In conclusion, Ap4A may be a good candidate for an SCI therapy, particularly to reduce excitotoxicity in combination with other modulators and/or inhibitors of the excitotoxic process that are being tested.

Current knowledge on the role of P2Y receptors in cardioprotection against ischemia-reperfusion

During ischemia, numerous effective endogenous extracellular mediators have been identified, particularly, nucleosides such as adenosine as well as purinergic and pyrimidinergic nucleotides. They may play important regulatory roles within the cardiovascular system and notably as cardio-protectants. Indeed, the distribution of the P2Y receptors in mammalian heart includes several cellular constituents relevant for the pathophysiology of myocardial ischemia. Beside the well-known cardioprotective effect of adenosine, the additional protective role of P2Y receptors has emerged. However, interpretation of experimental results may be sometimes perplexing. This is due to the variability of: the experimental models, the endpoints criteria, the chemical structure of agonist and antagonist ligands and their concentrations, the sequences of drug administration with respect to the model used (before and/or during and/or after ischemia). The net effect may be in the opposite direction after a transient or a prolonged stimulation. Nevertheless, the overall reading of published data highlights the beneficial role of the P2Y_2/4 receptor stimulation, the useful and synergistic role of P2Y_6/11 receptor activation and even of the P2Y₁₁ receptor alone in cardioprotection. More, the P2Y₁₁ receptor could be involved in counter-regulation of profibrotic processes. Paradoxically, transient P2X₇ receptor stimulation could contribute to the net cardioprotective effect of ATP. Recently, experimental data have shown that blocking the P2Y₁₂ receptor after ischemia confers cardioprotection independently of platelet antiaggregatory effect. This suggests for P2Y receptors an important role in primary prevention and as a therapeutic target in myocardial protection during ischemia and reperfusion.

Synergistic Inhibition of Both P2Y1 and P2Y12 Adenosine Diphosphate Receptors As Novel Approach to Rapidly Attenuate Platelet-Mediated Thrombosis

OBJECTIVE

Unlike currently approved adenosine diphosphate receptor antagonists, the new diadenosine tetraphosphate derivative GLS-409 targets not only P2Y12 but also the second human platelet adenosine diphosphate receptor P2Y1 and may, therefore, be a promising antiplatelet drug candidate. The current study is the first to investigate the in vivo antithrombotic effects of GLS-409.

APPROACH AND RESULTS

We studied (1) the in vivo effects of GLS-409 on agonist-stimulated platelet aggregation in anesthetized rats, (2) the antithrombotic activity of GLS-409 and the associated effect on the bleeding time in a canine model of platelet-mediated coronary artery thrombosis, and (3) the inhibition of agonist-stimulated platelet aggregation by GLS-409 versus selective P2Y1 and P2Y12 inhibition in vitro in samples from healthy human subjects before and 2 hours after aspirin intake. In vivo treatment with GLS-409 significantly inhibited adenosine diphosphate- and collagen-stimulated platelet aggregation in rats. Further, GLS-409 attenuated cyclic flow variation, that is, platelet-mediated thrombosis, in vivo in our canine model of unstable angina. The improvement in coronary patency was accompanied by a nonsignificant 30% increase in bleeding time. Of note, GLS-409 exerted its effects without affecting rat and canine hemodynamics. Finally, in vitro treatment with GLS-409 showed effects similar to that of cangrelor and the combination of cangrelor with the selective P2Y1 inhibitor MRS 2179 on agonist-stimulated platelet aggregation in human platelet-rich plasma and whole blood before and 2 hours after aspirin intake.

CONCLUSIONS

Synergistic inhibition of both P2Y1 and P2Y12 adenosine diphosphate receptors by GLS-409 immediately attenuates platelet-mediated thrombosis and effectively blocks agonist-stimulated platelet aggregation irrespective of concomitant aspirin therapy.

Blood cells: an historical account of the roles of purinergic signalling

The involvement of purinergic signalling in the physiology of erythrocytes, platelets and leukocytes was recognised early. The release of ATP and the expression of purinoceptors and ectonucleotidases on erythrocytes in health and disease are reviewed. The release of ATP and ADP from platelets and the expression and roles of P1, P2Y(1), P2Y(12) and P2X1 receptors on platelets are described. P2Y(1) and P2X(1) receptors mediate changes in platelet shape, while P2Y(12) receptors mediate platelet aggregation. The changes in the role of purinergic signalling in a variety of disease conditions are considered. The successful use of P2Y(12) receptor antagonists, such as clopidogrel and ticagrelor, for the treatment of thrombosis, myocardial infarction and stroke is discussed.

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.

Modeling ligand recognition at the P2Y12 receptor in light of X-ray structural information

The G protein-coupled P2Y12 receptor (P2Y12R) is an important antithrombotic target and of great interest for pharmaceutical discovery. Its recently solved, highly divergent crystallographic structures in complex either with nucleotides (full or partial agonist) or with a nonnucleotide antagonist raise the question of which structure is more useful to understand ligand recognition. Therefore, we performed extensive molecular modeling studies based on these structures and mutagenesis, to predict the binding modes of major classes of P2Y12R ligands previously reported. Various nucleotide derivatives docked readily to the agonist-bound P2Y12R, but uncharged nucleotide-like antagonist ticagrelor required a hybrid receptor resembling the agonist-bound P2Y12R except for the top portion of TM6. Supervised molecular dynamics (SuMD) of ticagrelor binding indicated interactions with the extracellular regions of P2Y12R, defining possible meta-binding sites. Ureas, sulfonylureas, sulfonamides, anthraquinones and glutamic acid piperazines docked readily to the antagonist-bound P2Y12R. Docking dinucleotides at both agonist- and antagonist-bound structures suggested interactions with two P2Y12R pockets. Thus, our structure-based approach consistently rationalized the main structure-activity relationships within each ligand class, giving useful information for designing improved ligands.

P2Y12 receptors: structure and function

The platelet P2Y12 receptor (P2Y12R) for adenosine 5'diphosphate (ADP) plays a central role in platelet function, hemostasis, and thrombosis. Patients with inherited P2Y12R defects display mild-to-moderate bleeding diatheses. Defects of P2Y12R should be suspected when ADP, even at high concentrations (≥ 10 μm), is unable to induce full, irreversible platelet aggregation. P2Y12R also plays a role in inflammation: its role in the pathogenesis of allergic asthma has been well characterized. In addition, inhibition or genetic deficiency of P2Y12R has antitumor effects. Drugs inhibiting P2Y12R are potent antithrombotic drugs. Clopidogrel is the P2Y12R antagonist that is most widely used in the clinical setting. Its most important drawback is its inability to inhibit adequately P2Y12R-dependent platelet function in about one-third of patients. New drugs, such as prasugrel and ticagrelor, which effectively inhibit P2Y12R in the vast majority of patients, have proved to be more efficacious than clopdidogrel in preventing major adverse cardiovascular events.

Nucleotides Acting at P2Y Receptors: Connecting Structure and Function

Eight G protein-coupled P2Y receptor (P2YR) subtypes are important physiologic mediators. The human P2YRs are fully activated by ATP (P2Y2 and P2Y11), ADP (P2Y1, P2Y12, and P2Y13), UTP (P2Y2 and P2Y4), UDP (P2Y6 and P2Y14), and UDP glucose (P2Y14). Their structural elucidation is progressing rapidly. The X-ray structures of three ligand complexes of the Gi-coupled P2Y12R and two of the Gq-coupled P2Y1Rs were recently determined and will be especially useful in structure-based ligand design at two P2YR subfamilies. These high-resolution structures, which display unusual binding site features, complement mutagenesis studies for probing ligand recognition and activation. The structural requirements for nucleotide agonist recognition at P2YRs are relatively permissive with respect to the length of the phosphate moiety, but less so with respect to base recognition. Nucleotide-like antagonists and partial agonists are also known for P2Y1, P2Y2, P2Y4, and P2Y12Rs. Each P2YR subtype has the ability to be activated by structurally bifunctional agonists, such as dinucleotides, typically, dinucleoside triphosphates or tetraphosphates, and nucleoside polyphosphate sugars (e.g., UDP glucose) as well as the more conventional mononucleotide agonists. A range of dinucleoside polyphosphates, from triphosphates to higher homologs, occurs naturally. Earlier modeling predictions of the P2YRs were not very accurate, but recent findings have provided much detailed structural insight into this receptor family to aid in the rational design of new drugs.

Agonist-bound structure of the human P2Y12 receptor

The P2Y₁₂ receptor (P2Y₁₂R), one of eight members of the P2YR family expressed in humans, has been identified as one of the most prominent clinical drug targets for inhibition of platelet aggregation. Consequently, extensive mutagenesis and modeling studies of the P2Y₁₂R have revealed many aspects of agonist/antagonist binding^1-4. However, the details of agonist and antagonist recognition and function at the P2Y₁₂R remain poorly understood at the molecular level. Here, we report the structures of the human P2Y₁₂R in complex with a full agonist 2-methylthio-adenosine-5′-diphosphate (2MeSADP, a close analogue of endogenous agonist ADP) at 2.5 Å resolution, and the corresponding ATP derivative 2-methylthio-adenosine-5′-triphosphate (2MeSATP) at 3.1 Å resolution. Analysis of these structures, together with the structure of the P2Y₁₂R with antagonist ethyl 6-(4-((benzylsulfonyl)carbamoyl)piperidin-1-yl)-5-cyano-2-methylnicotinate (AZD1283)⁵, reveals dramatic conformational changes between nucleotide and non-nucleotide ligand complexes in the extracellular regions, providing the first insight into a different ligand binding landscape in the δ-group of class A G protein-coupled receptors (GPCRs). Agonist and non-nucleotide antagonist adopt different orientations in the P2Y₁₂R, with only partially overlapped binding pockets. The agonist-bound P2Y₁₂R structure answers long-standing ambiguities surrounding P2Y₁₂R-agonist recognition, and reveals interactions with several residues that had not been reported to be involved in agonist binding. As a first example of a GPCR where agonist access to the binding pocket requires large scale rearrangements in the highly malleable extracellular region, the structural studies therefore will provide invaluable insight into the pharmacology and mechanisms of action of agonists and different classes of antagonists for the P2Y₁₂R and potentially for other closely related P2YRs.

Highly sensitive, selective and rapid LC-MS method for simultaneous quantification of diadenosine polyphosphates in human plasma

BACKGROUND

Diadenosine polyphosphates (ApnAs) are endogenous mediators involved in large number of physiologic and pathophysiologic processes. The quantification of diadenosine polyphosphates in plasma and biological matrices is still challenging. Therefore, there is an urgent need for a simple and reliable quantification method suitable for clinical studies. The classical quantification of diadenosine polyphosphates is based on chromatographic separation and UV adsorption of the resulting fractions. These procedures are associated with low selectivity due to co-eluting plasma components. Therefore, we developed and validated a highly sensitive, selective and rapid LC-ESI-MS method for simultaneous quantification of ApnAs (with n=3-6) in human plasma within this study. The identities of the endogenous ApnAs (with n=3-6) were revealed by comparison of ESI-MS/MS fragment spectra of isolated endogenous compounds with those of authentic ApnAs.

METHODS

Diadenosine polyphosphates were extracted from 100μl human plasma using weak anion-exchange extraction cartridges. The separation of ApnAs was achieved using capillary C18 columns. ESI-HCT mass spectrometer (Bruker Daltonik, Germany) operated in negative ion mode was used for detection and quantification of ApnAs.

RESULTS

A calibration curve was established for diadenosine polyphosphate free plasma in the concentration range 1.9-125nM (r(2)>0.998) for all analytes. The intra- and inter-day accuracies were in the range of 91.4% and 110.9%. The intra- and inter-day precisions were determines as 0.1% and 11.4%, respectively. The mean plasma concentrations of ApnAs were quantified as 31.9±5.9nM for Ap3A, 40.4±6.6nM for Ap4A, 10.7±1.5nM for Ap5A and 10.0±18.9nM for Ap6A.

DISCUSSION

The developed and validated ESI MS-based method for quantification of diadenosine polyphosphates in human plasma was successfully evaluated within the study. Conclusion Since the quantification is based on a volume of 100μl plasma, this method is highly applicable for clinical applications aiming at the validation of the impact of highly physiological and pathophysiological active diadenosine polyphosphates.

Antiplatelet activity, P2Y₁ and P2Y₁₂ inhibition, and metabolism in plasma of stereoisomers of diadenosine 5',5'″-P¹ ,P⁴-dithio-P²,P³-chloromethylenetetraphosphate

Background

Diadenosine tetraphosphate (Ap₄A), a constituent of platelet dense granules, and its P¹,P⁴-dithio and/or P²,P³-chloromethylene analogs, inhibit adenosine diphosphate (ADP)-induced platelet aggregation. We recently reported that these compounds antagonize both platelet ADP receptors, P2Y₁ and P2Y₁₂. The most active of those analogs, diadenosine 5′,5″″-P¹,P⁴-dithio-P²,P³-chloromethylenetetraphosphate, (compound 1), exists as a mixture of 4 stereoisomers.

Objective

To separate the stereoisomers of compound 1 and determine their effects on platelet aggregation, platelet P2Y₁ and P2Y₁₂ receptor antagonism, and their metabolism in human plasma.

Methods

We separated the 4 diastereomers of compound 1 by preparative reversed-phase chromatography, and studied their effect on ADP-induced platelet aggregation, P2Y₁-mediated changes in cytosolic Ca²⁺, P2Y₁₂-mediated changes in VASP phosphorylation, and metabolism in human plasma.

Results

The inhibition of ADP-induced human platelet aggregation and human platelet P2Y₁₂ receptor, and stability in human plasma strongly depended on the stereo-configuration of the chiral P¹- and P⁴-phosphorothioate groups, the S_PS_P diastereomer being the most potent inhibitor and completely resistant to degradation in plasma, and the R_PR_P diastereomer being the least potent inhibitor and with the lowest plasma stability. The inhibitory activity of S_PR_P diastereomers depended on the configuration of the pseudo-asymmetric carbon of the P²,P³-chloromethylene group, one of the configurations being significantly more active than the other. Their plasma stability did not differ significantly, being intermediate to that of the S_PS_P and the R_PR_P diastereomers.

Conclusions

The presently-described stereoisomers have utility for structural, mechanistic, and drug development studies of dual antagonists of platelet P2Y₁ and P2Y₁₂ receptors.

Identification of a potent endothelium-derived angiogenic factor

The secretion of angiogenic factors by vascular endothelial cells is one of the key mechanisms of angiogenesis. Here we report on the isolation of a new potent angiogenic factor, diuridine tetraphosphate (Up4U) from the secretome of human endothelial cells. The angiogenic effect of the endothelial secretome was partially reduced after incubation with alkaline phosphatase and abolished in the presence of suramin. In one fraction, purified to homogeneity by reversed phase and affinity chromatography, Up4U was identified by MALDI-LIFT-fragment-mass-spectrometry, enzymatic cleavage analysis and retention-time comparison. Beside a strong angiogenic effect on the yolk sac membrane and the developing rat embryo itself, Up4U increased the proliferation rate of endothelial cells and, in the presence of PDGF, of vascular smooth muscle cells. Up4U stimulated the migration rate of endothelial cells via P2Y2-receptors, increased the ability of endothelial cells to form capillary-like tubes and acts as a potent inducer of sprouting angiogenesis originating from gel-embedded EC spheroids. Endothelial cells released Up4U after stimulation with shear stress. Mean total plasma Up4U concentrations of healthy subjects (N=6) were sufficient to induce angiogenic and proliferative effects (1.34 ± 0.26 nmol L(-1)). In conclusion, Up4U is a novel strong human endothelium-derived angiogenic factor.

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.

NPP4 is a procoagulant enzyme on the surface of vascular endothelium

Ap3A is a platelet-dense granule component released into the extracellular space during the second wave of platelet aggregation on activation. Here, we identify an uncharacterized enzyme, nucleotide pyrophosphatase/phosphodiesterase-4 (NPP4), as a potent hydrolase of Ap3A capable of stimulating platelet aggregation and secretion. We demonstrate that NPP4 is present on the surface of vascular endothelium, where it hydrolyzes Ap3A into AMP and ADP, and Ap4A into AMP and ATP. Platelet aggregation assays with citrated platelet-rich plasma reveal that the primary and secondary waves of aggregation and dense granule release are strongly induced by nanomolar NPP4 in a concentration-dependent manner in the presence of Ap3A, while Ap3A alone initiates a primary wave of aggregation followed by rapid disaggregation. NPP2 and an active site NPP4 mutant, neither of which appreciably hydrolyzes Ap3A, have no effect on platelet aggregation and secretion. Finally, by using ADP receptor blockade we confirm that NPP4 mediates platelet aggregation via release of ADP from Ap3A and activation of ADP receptors. Collectively, these studies define the biologic and enzymatic basis for NPP4 and Ap3A activity in platelet aggregation in vitro and suggest that NPP4 promotes hemostasis in vivo by augmenting ADP-mediated platelet aggregation at the site of vascular injury.

The P2X1 receptor and platelet function

Extracellular nucleotides are ubiquitous signalling molecules, acting via the P2 class of surface receptors. Platelets express three P2 receptor subtypes, ADP-dependent P2Y1 and P2Y12 G-protein-coupled receptors and the ATP-gated P2X1 non-selective cation channel. Platelet P2X1 receptors can generate significant increases in intracellular Ca(2+), leading to shape change, movement of secretory granules and low levels of α(IIb)β(3) integrin activation. P2X1 can also synergise with several other receptors to amplify signalling and functional events in the platelet. In particular, activation of P2X1 receptors by ATP released from dense granules amplifies the aggregation responses to low levels of the major agonists, collagen and thrombin. In vivo studies using transgenic murine models show that P2X1 receptors amplify localised thrombosis following damage of small arteries and arterioles and also contribute to thromboembolism induced by intravenous co-injection of collagen and adrenaline. In vitro, under flow conditions, P2X1 receptors contribute more to aggregate formation on collagen-coated surfaces as the shear rate is increased, which may explain their greater contribution to localised thrombosis in arterioles compared to venules within in vivo models. Since shear increases substantially near sites of stenosis, anti-P2X1 therapy represents a potential means of reducing thrombotic events at atherosclerotic plaques.

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.

Thrombin and collagen induce a feedback inhibitory signaling pathway in platelets involving dissociation of the catalytic subunit of protein kinase A from an NFkappaB-IkappaB complex

Protein kinase A (PKA) activation by cAMP phosphorylates multiple target proteins in numerous platelet inhibitory pathways that have a very important role in maintaining circulating platelets in a resting state. Here we show that in thrombin- and collagen-stimulated platelets, PKA is activated by cAMP-independent mechanisms involving dissociation of the catalytic subunit of PKA (PKAc) from an NFkappaB-IkappaBalpha-PKAc complex. We demonstrate mRNA and protein expression for most of the NFkappaB family members in platelets. From resting platelets, PKAc was co-immunoprecipitated with IkappaBalpha, and conversely, IkappaBalpha was also co-immunoprecipitated with PKAc. This interaction was significantly reduced in thrombin- and collagen-stimulated platelets. Stimulation of platelets with thrombin- or collagen-activated IKK, at least partly by PI3 kinase-dependent pathways, leading to phosphorylation of IkappaBalpha, disruption of an IkappaBalpha-PKAc complex, and release of free, active PKAc, which phosphorylated VASP and other PKA substrates. IKK inhibitor inhibited thrombin-stimulated IkBalpha phosphorylation, PKA-IkBalpha dissociation, and VASP phosphorylation, and potentiated integrin alphaIIbbeta3 activation and the early phase of platelet aggregation. We conclude that thrombin and collagen not only cause platelet activation but also appear to fine-tune this response by initiating downstream NFkappaB-dependent PKAc activation, as a novel feedback inhibitory signaling mechanism for preventing undesired platelet activation.