A protein phosphorylation-based assay for screening and monitoring of drugs modulating cyclic nucleotide pathways

High-on-treatment platelet reactivity predicts adverse outcome after carotid artery stenting: A prospective study

BACKGROUND AND PURPOSE

High-on-treatment platelet reactivity (HTPR) has been established as a predictor of major adverse cardiovascular events (MACE) in patients undergoing percutaneous coronary interventions on dual antiplatelet therapy (DAPT), but no data are available on its predictive value in patients on DAPT after carotid artery stenting (CAS). We aimed to evaluate the possible association between HTPR in patients on aspirin plus clopidogrel therapy after CAS and subsequent MACE.

METHODS

All consecutive patients treated with CAS in a single institution were enrolled in a prospective clinical study. HTPR was evaluated with 5 different laboratory assays carried out just before CAS. MACE incidence (cerebral ischemia, myocardial infarction, stent thrombosis, acute limb ischemia and vascular death) was evaluated at 30 days and thereafter at yearly visits.

RESULTS

A total of 300 patients were enrolled in the study, and eight were then excluded because blood samples resulted unsuitable for the laboratory testing or CAS aborted for technical problems. Median follow-up was 5.8 years and during this period 47 MACE occurred. HTPR detected by multiplate electronic aggregometry (MEA) and the VASP phosphorylation assay (VASP) were associated with a significantly enhanced risk of MACE (p = 0.048 and p = 0.038, respectively). However, HTPR to three tests (HTPR3) was more strongly predictive of increased risk of a vascular event at follow up (p = 0.005) at bivariate analysis and also at Cox regression multivariate analysis (p = 0.002).

CONCLUSIONS

HTPR to three different assays (mainly to VASP + PFA P2Y+ VerifyNow) in patients on DAPT after CAS has predictive value for subsequent MACE. Prospective studies to assess whether platelet function testing-guided antiplatelet therapy is superior to standard DAPT in patient undergoing CAS should be considered.

Cyclic GMP modulating drugs in cardiovascular diseases: mechanism-based network pharmacology

Mechanism-based therapy centred on the molecular understanding of disease-causing pathways in a given patient is still the exception rather than the rule in medicine, even in cardiology. However, recent successful drug developments centred around the second messenger cyclic guanosine-3'-5'-monophosphate (cGMP), which is regulating a number of cardiovascular disease modulating pathways, are about to provide novel targets for such a personalized cardiovascular therapy. Whether cGMP breakdown is inhibited or cGMP synthesis is stimulated via guanylyl cyclases or their upstream regulators in different cardiovascular disease phenotypes, the outcomes seem to be so far uniformly protective. Thus, a network of cGMP-modulating drugs has evolved that act in a mechanism-based, possibly causal manner in a number of cardiac conditions. What remains a challenge is the detection of cGMPopathy endotypes amongst cardiovascular disease phenotypes. Here, we review the growing clinical relevance of cGMP and provide a glimpse into the future on how drugs interfering with this pathway may change how we treat and diagnose cardiovascular diseases altogether.

Multifaceted effects of arachidonic acid and interaction with cyclic nucleotides in human platelets

INTRODUCTION

Arachidonic acid induced aggregation is a generally accepted test for aspirin resistance. However, doubts have been raised that arachidonic acid stimulated aggregation can be regarded as reliable testing for aspirin resistance. Arachidonic acid, in addition to platelet activation, can induce phosphatidylserine translocation on the outer surface of platelet membrane which could be mediated by apoptosis pathways or transformation of platelets to the procoagulant state.

MATERIALS AND METHODS

We explored effects of arachidonic acid over a vast range of concentrations and a wide range of read-outs for human platelet activation, procoagulant activity, and platelet viability. Additionally we tested whether cAMP- or cGMP-dependent protein kinase activation can inhibit procoagulant activity or platelet viability.

RESULTS

Arachidonic acid-induced washed platelet activation was detected at low micromolar concentrations during the first 2 min of stimulation. After longer incubation and/or at higher concentrations arachidonic acid triggered platelet procoagulant activity and reduced platelet viability. At the same time, arachidonic acid stimulated adenylate cyclase mediated protein phosphorylation which correlated with reduced platelet activation. Moreover, additional stimulation of cAMP- or cGMP-dependent protein kinase inhibited only platelet activation, but did not prevent pro-coagulant activity and platelet death.

CONCLUSIONS

While arachidonic acid induces platelet activation at low concentrations and during short incubation time, higher concentrations and lasting incubation evokes adenylate cyclase activation and subsequent protein phosphorylation corresponding to reduced platelet activation, but also enhanced pro-coagulant activity and reduced viability. Our observations provide further proof for the complex fine tuning of platelet responses in a time and agonist concentration dependent manner.

Temporal quantitative phosphoproteomics of ADP stimulation reveals novel central nodes in platelet activation and inhibition

Adenosine diphosphate (ADP) enhances platelet activation by virtually any other stimulant to complete aggregation. It binds specifically to the G-protein-coupled membrane receptors P2Y1 and P2Y12, stimulating intracellular signaling cascades, leading to integrin αIIbβ3 activation, a process antagonized by endothelial prostacyclin. P2Y12 inhibitors are among the most successful antiplatelet drugs, however, show remarkable variability in efficacy. We reasoned whether a more detailed molecular understanding of ADP-induced protein phosphorylation could identify (1) critical hubs in platelet signaling toward aggregation and (2) novel molecular targets for antiplatelet treatment strategies. We applied quantitative temporal phosphoproteomics to study ADP-mediated signaling at unprecedented molecular resolution. Furthermore, to mimic the antagonistic efficacy of endothelial-derived prostacyclin, we determined how Iloprost reverses ADP-mediated signaling events. We provide temporal profiles of 4797 phosphopeptides, 608 of which showed significant regulation. Regulated proteins are implicated in well-known activating functions such as degranulation and cytoskeletal reorganization, but also in less well-understood pathways, involving ubiquitin ligases and GTPase exchange factors/GTPase-activating proteins (GEF/GAP). Our data demonstrate that ADP-triggered phosphorylation occurs predominantly within the first 10 seconds, with many short rather than sustained changes. For a set of phosphorylation sites (eg, PDE3A^Ser312, CALDAG-GEFI^Ser587, ENSA^Ser109), we demonstrate an inverse regulation by ADP and Iloprost, suggesting that these are central modulators of platelet homeostasis. This study demonstrates an extensive spectrum of human platelet protein phosphorylation in response to ADP and Iloprost, which inversely overlap and represent major activating and inhibitory pathways.

Reciprocal regulation of human platelet function by endogenous prostanoids and through multiple prostanoid receptors

Platelets are permanently exposed to a variety of prostanoids formed by blood cells or the vessel wall. The two major prostanoids, prostacyclin and thromboxane act through well established pathways mediated by their respective G-protein coupled receptors inhibiting or promoting platelet aggregation accordingly. Yet the role of other prostanoids and prostanoid receptors for platelet function regulation has not been thoroughly investigated. We aimed at a comprehensive analysis of prostanoid effects on platelets, the receptors and pathways involved and functional consequences. We analyzed cAMP formation and phosphorylation of proteins pivotal to platelet function as well as functional platelet responses such as secretion, aggregation and phosphorylation. The types of prostanoid receptors contributing and their individual share in signaling pathways were analyzed and indicated a major role for prostanoid IP1 and DP1 receptors followed by prostanoid EP4 and EP3 receptors while prostanoid EP2 receptors appear less relevant. We could show for the first time the reciprocal action of the endogenous prostaglandin PGE2 on platelets by functional responses and phosphorylation events. PGE2 evokes stimulatory as well as inhibitory effects in a concentration dependent manner in platelets via prostanoid EP3 or EP4 and prostanoid DP1 receptors. A mathematical model integrating the pathway components was established which successfully reproduces the observed platelet responses. Additionally we could show that human platelets themselves produce sufficient PGE2 to act in an autocrine or paracrine fashion. These mechanisms may provide a fine tuning of platelet responses in the circulating blood by either promoting or limiting endogenous platelet activation.

Dynamical modelling of prostaglandin signalling in platelets reveals individual receptor contributions and feedback properties

Prostaglandins are the key-players in diminishing platelet function. They exert their effects via a variety of surface receptors that are linked to the cAMP/PKA-signalling cascade. However, less is known about the quantitative impact of the individual receptors on the underlying pathway. We present here a comprehensive ordinary differential equation-based model of the platelet cAMP pathway, including the four prostaglandin receptors IP, DP1, EP3 and EP4, the ADP receptor P2Y12, a detailed PKA-module as well as downstream-targets. Parameter estimation along with a comprehensive combination of time-course and dose-response measurements revealed the individual quantitative role of each receptor in elevating or decreasing pathway activity. A comparison of the two inhibiting receptors EP3 and P2Y12 exhibited a greater signalling strength of the EP3 receptor with implications for antithrombotic treatment. Furthermore, analysis of different model topologies revealed a direct influence of PKA on adenylate cyclase, reducing its maximum catalytic speed. Finally, we show here for the first time the dynamic behaviour of VASP-phosphorylation, which is commonly used as a marker for platelet-inhibition. We validate our model by comparing it to further experimental data.

Time-resolved characterization of cAMP/PKA-dependent signaling reveals that platelet inhibition is a concerted process involving multiple signaling pathways

One of the most important physiological platelet inhibitors is endothelium-derived prostacyclin which stimulates the platelet cyclic adenosine monophosphate/protein kinase A (cAMP/PKA)-signaling cascade and inhibits virtually all platelet-activating key mechanisms. Using quantitative mass spectrometry, we analyzed time-resolved phosphorylation patterns in human platelets after treatment with iloprost, a stable prostacyclin analog, for 0, 10, 30, and 60 seconds to characterize key mediators of platelet inhibition and activation in 3 independent biological replicates. We quantified over 2700 different phosphorylated peptides of which 360 were significantly regulated upon stimulation. This comprehensive and time-resolved analysis indicates that platelet inhibition is a multipronged process involving different kinases and phosphatases as well as many previously unanticipated proteins and pathways.

'VASPFix' for measurement of VASP phosphorylation in platelets and for monitoring effects of P2Y12 antagonists

Vasodilator-stimulated phosphoprotein (VASP) is phosphorylated and dephosphorylated consequent to increases and decreases in cyclic nucleotide levels. Monitoring changes in VASP phosphorylation is an established method for indirect measurement of cyclic nucleotides. Here we describe the use of an innovative cocktail, VASPFix, which allows sensitive and reproducible measurement of phosphorylated VASP (VASP-P) in a simple, single-step procedure using cytometric bead technology. Frozen VASPFix-treated samples are stable for at least six months prior to analysis. We successfully used VASPFix to measure VASP-P in platelets in both platelet-rich plasma and blood in response to compounds that increase (dibutyryl cAMP, adenosine, iloprost, PGE1) and decrease (ADP, PGE1) cAMP, and to determine the effects of certain receptor antagonists on the results obtained. The change in VASP-P brought about by adding ADP to PGE1-stimulated platelets is a combination of the effect of ADP at the P2Y12 receptor and of PGE1 at both IP and EP3 receptors. For iloprost-stimulated platelets EP3 receptors are not involved. A procedure in which iloprost, ADP and VASPFix were used to determine effectiveness of clopidogrel and prasugrel in patients was compared with an established commercial procedure that uses PGE1 and ADP; the latter produced higher platelet reactivity values that were the result of PGE1 interacting with platelet EP3 receptors. We conclude that VASPFix can be used both as a research tool and for clinical investigations and provides better specificity for P2Y12 receptor inhibition. The latter confers a distinct advantage over existing methods used to monitor effects of P2Y12 antagonists on platelet function.

Determination of ATP and ADP Secretion from Human and Mouse Platelets by an HPLC Assay

BACKGROUND

Secretion of ADP and ATP is an essential prerequisite for platelet aggregation. Impaired nucleotide secretion can cause aggregation defects and increased bleeding risk. Quantitative determination of platelet nucleotide content and exocytosis is thus of importance for the characterization and diagnosis of bleeding phenotypes. For transgenic animal models with hemostatic defects analysis of potential secretion defects is as well imperative.

METHODS

Supernatants of washed platelets and platelet-rich plasma were analyzed by HPLC for ADP and ATP concentration. Calibration of the HPLC data was accomplished with an internal standard compensating for loss of analyte, detection sensitivity, and interference of the biomatrix.

RESULTS

HPLC analysis of nucleotide secretion was carried out with human and mouse platelets. Detection limits were determined for washed platelet and platelet-rich plasma samples. In the physiological concentration range linearity with respect to the peak area is maintained.

CONCLUSION

The method combines reasonable sensitivity with robustness. The internal standard ensures reliable quantification of nucleotide concentrations even in presence of otherwise interfering substances. The low sample consumption renders possible the application to analysis of small samples like in mouse experiments.

Deciphering of ADP-induced, phosphotyrosine-dependent signaling networks in human platelets by Src-homology 2 region (SH2)-profiling

Tyrosine phosphorylation plays a central role in signal transduction controlling many important biological processes. In platelets, the activity of several signaling proteins is controlled by tyrosine phosphorylation ensuring proper platelet activation and aggregation essential for regulation of the delicate balance between bleeding and hemostasis. Here, we applied Src-homology 2 region (SH2)-profiling for deciphering of the phosphotyrosine state of human platelets activated by adenosine diphosphate (ADP). Applying a panel of 31 SH2-domains, rapid and complex regulation of the phosphotyrosine state of platelets was observed after ADP stimulation. Specific inhibition of platelet P2Y receptors by synthetic drugs revealed a major role for the P2Y1 receptor in tyrosine phosphorylation. Concomitant activation of protein kinase A (PKA) abolished ADP-induced tyrosine phosphorylation in a time and concentration-dependent manner. Given the fact that PKA activity is negatively regulated by the P2Y12 receptor, our data provide evidence for a novel link of synergistic control of the state of tyrosine phosphorylation by both P2Y receptors. By SH2 domain pull down and MS/MS analysis, we identified distinct tyrosine phosphorylation sites in cell adhesion molecules, intracellular adapter proteins and phosphatases suggesting a major, functional role of tyrosine phosphorylation of theses candidate proteins in ADP-dependent signaling in human platelets.

Time-resolved in silico modeling of fine-tuned cAMP signaling in platelets: feedback loops, titrated phosphorylations and pharmacological modulation

Background

Hemostasis is a critical and active function of the blood mediated by platelets. Therefore, the prevention of pathological platelet aggregation is of great importance as well as of pharmaceutical and medical interest. Endogenous platelet inhibition is predominantly based on cyclic nucleotides (cAMP, cGMP) elevation and subsequent cyclic nucleotide-dependent protein kinase (PKA, PKG) activation. In turn, platelet phosphodiesterases (PDEs) and protein phosphatases counterbalance their activity. This main inhibitory pathway in human platelets is crucial for countervailing unwanted platelet activation. Consequently, the regulators of cyclic nucleotide signaling are of particular interest to pharmacology and therapeutics of atherothrombosis. Modeling of pharmacodynamics allows understanding this intricate signaling and supports the precise description of these pivotal targets for pharmacological modulation.

Results

We modeled dynamically concentration-dependent responses of pathway effectors (inhibitors, activators, drug combinations) to cyclic nucleotide signaling as well as to downstream signaling events and verified resulting model predictions by experimental data. Experiments with various cAMP affecting compounds including anti-platelet drugs and their combinations revealed a high fidelity, fine-tuned cAMP signaling in platelets without cross-talk to the cGMP pathway. The model and the data provide evidence for two independent feedback loops: PKA, which is activated by elevated cAMP levels in the platelet, subsequently inhibits adenylyl cyclase (AC) but as well activates PDE3. By multi-experiment fitting, we established a comprehensive dynamic model with one predictive, optimized and validated set of parameters. Different pharmacological conditions (inhibition, activation, drug combinations, permanent and transient perturbations) are successfully tested and simulated, including statistical validation and sensitivity analysis. Downstream cyclic nucleotide signaling events target different phosphorylation sites for cAMP- and cGMP-dependent protein kinases (PKA, PKG) in the vasodilator-stimulated phosphoprotein (VASP). VASP phosphorylation as well as cAMP levels resulting from different drug strengths and combined stimulants were quantitatively modeled. These predictions were again experimentally validated. High sensitivity of the signaling pathway at low concentrations is involved in a fine-tuned balance as well as stable activation of this inhibitory cyclic nucleotide pathway.

Conclusions

On the basis of experimental data, literature mining and database screening we established a dynamic in silico model of cyclic nucleotide signaling and probed its signaling sensitivity. Thoroughly validated, it successfully predicts drug combination effects on platelet function, including synergism, antagonism and regulatory loops.