Coactivation of two different G protein-coupled receptors is essential for ADP-induced platelet aggregation

Design and Pharmacological Characterization of a Novel Antithrombotic P2Y1 Receptor-Based Vaccine

Platelet activation processes begin when injury to blood vessels exposes the subendothelial matrix, leading platelets to attach to it, where they become activated and exert their hemostatic function. Excessive platelet aggregation is associated with thrombotic disorders such as arterial thrombosis. To manage such diseases, medications that inhibit thrombosis are continuously sought, despite potential drawbacks that include hemorrhage. This study described the development of a novel peptide-based vaccine that targets the purinergic ADP P2Y1 receptor (abbreviated EL2Vac) and its pharmacological characterization. Thus, we designed and developed an EL2Vac that targets the ligand-binding domain of the P2Y1 receptor protein, which is located in its second extracellular loop (EL2). We then evaluated the vaccine's ability to trigger an immune response (antibody production) in immunized mice, modulate platelet function, its antithrombotic activity, and any effects on hemostasis, by employing a thrombosis model and the tail bleeding time assay. Results showed significant levels of antibody production in mice treated with EL2Vac, in comparison with the random peptide vaccine control (EL2rVac), which persisted at least up to six months post vaccination. Moreover, we observed significant inhibition of the ADP-induced aggregation response in platelets from EL2Vac-treated mice, relative to those from EL2rVac controls. This inhibition was selective for ADP, as other agonists, such as the thromboxane A2 receptor (TPR) agonist U46619 or high-dose collagen, had no detectable effect on aggregation. As for its capacity to protect against thrombosis, our data showed a significant delay in the occlusion time of the EL2Vac mice when compared with the random peptide control vaccine, which was also observed (at least) six months post vaccination. Interestingly, EL2Vac did not appear to prolong the tail bleeding time, supporting the notion that it is devoid of a bleeding diathesis. As a conclusion, this study documents the design and evaluation of a novel peptide-based vaccine, EL2Vac, which appears to selectively target the P2Y1 receptor and protect against thrombus formation without impairing hemostasis. Thus, EL2Vac may provide a promising clinical option to treat thromboembolic disorders.

Effects and molecular mechanisms of farnesyltransferase inhibitor tipifarnib on platelet activation

Tipifarnib, a farnesyltransferase inhibitor, substantially protects against cardiovascular diseases and is currently undergoing clinical trials to treat various cancers. Platelets have a well-recognized role in the progression of cancer-associated cardiovascular diseases. Nevertheless, the effect of tipifarnib on platelet function has not been studied thus far. Thus, we investigated the effect of tipifarnib and its molecular basis on the regulation of platelet activation. 2-Methylthioadenosine diphosphate (2-MeSADP)-induced secondary waves of aggregation and dense granule secretion in murine-washed platelets were completely inhibited by tipifarnib. Since 2-MeSADP-induced secretion and the resultant secondary wave of aggregation are mediated by the positive feedback effect of thromboxane A2 (TxA2) generation, it suggests the important role of tipifarnib on TxA2 generation in platelets. Consistently, tipifarnib did not affect the 2-MeSADP-induced platelet aggregation in aspirinated platelets where the contribution of TxA2 generation was blocked. In addition, platelet aggregation and secretion induced by low concentrations of AYPGKF and thrombin, which are affected by the positive feedback effect of TxA2 generation, were partially inhibited by tipifarnib. Importantly, the ELISA assay showed that 2-MeSADP- and AYPGKF-induced TxA2 generation was significantly inhibited in the presence of tipifarnib, confirming the role of tipifarnib on TxA2 generation. Finally, tipifarnib significantly inhibited 2-MeSADP-induced protein kinase B and extracellular signal-regulated kinases phosphorylation only in nonaspirinated platelets but not in aspirinated platelets, indicating the contribution of TxA2 generation. Tipifarnib plays a role in platelet function by regulating TxA2 generation, thereby indicating the possibility of using tipifarnib as a single key to treat various patients with cancer with thromboembolic complications in the future. SIGNIFICANCE STATEMENT: Farnesyltransferase inhibitor tipifarnib regulates platelet activity by inhibiting thromboxane A2 generation through the modulation of protein kinase B and extracellular signal-regulated kinase phosphorylation. Given the dual role of platelets in both thrombosis and cancer progression, tipifarnib's ability to modulate these pathways highlights its potential as a therapeutic agent in preventing thromboembolic complications in patients with cancer.

Purinergic signaling in the battlefield of viral infections

Purinergic signaling has been associated with immune defenses against pathogens such as bacteria, protozoa, fungi, and viruses, acting as a sentinel system that signals to the cells when a threat is present. This review focuses on the roles of purinergic signaling and its therapeutic potential for viral infections. In this context, the purinergic system may play potent antiviral roles by boosting interferon signaling. In other cases, though, it can contribute to a hyperinflammatory response and disease severity, resulting in poor outcomes, such as during flu and potentially COVID-19. Lastly, a third situation may occur since viruses are obligatory intracellular parasites that hijack the host cell machinery for their infection and replication. Viruses such as HIV-1 use the purinergic system to favor their infection and persistence within the host cell. Therefore, understanding the particular nuances of purinergic signaling in each viral infection may contribute to designing proper therapeutic strategies to treat viral diseases.

Purinergic P2Y1 and P2Y12 receptors control enteric nervous system activity through neuro-glia-macrophage crosstalk

Purines are important mediators of intercellular communication in the enteric nervous system (ENS) that participate in physiological gut functions and disease. Purinergic transmission is prominent in mechanisms of crosstalk between enteric neurons and glia where enteric glia exhibit high responsiveness to adenosine diphosphate (ADP) through P2Y1 receptors and neurons to adenosine triphosphate (ATP) through P2X3 receptors. Despite functional data suggesting that enteric glia are the primary site of P2Y1 expression in the ENS, gene sequencing suggests that P2Y1 expression is more enriched in neurons than glia. The reason for the mismatch between genomic and functional data is unclear but could involve co-expression of inhibitory P2Y12 receptors in neurons. We addressed this issue by studying the expression and function of P2Y1 and P2Y12 receptors in the mouse ENS using live immunolabeling and calcium imaging techniques. The data show that ADP drives activity among enteric glia and neurons in the myenteric plexus. Interestingly, inhibiting P2Y12 activity increased neuron responses to ADP and overall spontaneous activity among enteric neurons and glia while decreasing the magnitude of glial responses to ADP. Investigating the location of the receptors involved revealed P2Y1 receptor expression by both neurons and glia, while P2Y12 receptor expression was minimal in the ENS. Instead, P2Y12 expression was enriched in the surrounding muscularis macrophages. Macrophages positive for P2Y12 overlapped with CD163 positive subsets that have known inhibitory influences over myenteric neurocircuits. Together, these data suggest that macrophage P2Y12 pathways act to constrain activity in the ENS, which could have implications in mechanisms that contribute to enteric hyperexcitability following disease.

Distinct Role of GRK3 in Platelet Activation by Desensitization of G Protein-Coupled Receptors

BACKGROUND

 Many platelet agonists mediate their cellular effects through G protein-coupled receptors (GPCRs) to induce platelet activation, and GPCR kinases (GRKs) have been demonstrated to have crucial roles in most GPCR functions in other cell types. Here, we investigated the functional role of GRK3 and the molecular basis for the regulation of GPCR desensitization by GRK3 in platelets.

METHODS

 We used mice lacking GRK3 as well as β-arrestin2, which has been shown to be important in GPCR function in platelets.

RESULTS

 Platelet aggregation and dense granule secretion induced by 2-MeSADP, U46619, thrombin, and AYPGKF were significantly potentiated in both GRK3 -/- and β-arrestin2 -/- platelets compared with wild-type (WT) platelets, whereas non-GPCR agonist collagen-induced platelet aggregation and secretion were not affected. We have previously shown that GRK6 is not involved in the regulation of Gq-coupled 5HT2A and Gz-coupled α2A adrenergic receptors. Interestingly, in contrast to GRK6, platelet aggregation induced by costimulation of serotonin and epinephrine, which activate 5-HT2A and α2A adrenergic receptors, respectively, was significantly potentiated in GRK3 -/- platelets, suggesting that GRK3 is involved in general GPCR regulation. In addition, platelet aggregation in response to the second challenge of adenosine diphosphate was restored in GRK3 -/- platelets, whereas restimulation of the agonist failed to induce aggregation in WT platelets, confirming that GRK3 contributes to general GPCR desensitization. Furthermore, 2-MeSADP- and AYPGKF-induced AKT and ERK phosphorylation were significantly potentiated in GRK3 -/- platelets. Finally, GRK3 -/- mice showed shorter tail bleeding times compared with WT, indicating that GRK3 -/- mice is more susceptible to hemostasis.

CONCLUSION

 GRK3 plays a crucial role in the regulation of platelet activation through general GPCR desensitization in platelets.

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.

Influence of short-term refrigeration on collagen-dependent signalling mechanisms in stored platelets

Platelet concentrates (PC) are used to treat patients with thrombocytopenia and hemorrhage, but there is still the demand to find the optimal strategy for temperature-dependent storage of PC. Recently, we could show that cold storage for 1 h (short-term refrigeration) is sufficient to induce enhanced platelet responsiveness. The aim of this study was to investigate effects of cold storage on collagen-dependent activating signalling pathways in platelets from apheresis-derived PC (APC). APC on day 1 or day 2 of storage, were either continuously kept at room temperature (RT, 22 °C), or for comparison, additionally kept at cold temperature (CT, 4 °C) for 1 h. CD62P expression was determined by flow cytometry. Western Blot technique was used to analyze collagen-induced phosphorylation of p38, ERK1/2 or Akt/PKB and its inhibition by prostaglandin E1 (PGE1) or nitric monoxide donor. Adhesion of platelets on collagen-coated surfaces and intracellular phosphorylation of vasodilator-stimulated phosphoprotein (VASP) was visualized by immune fluorescence microscopy. CD62P expression was increased after short-term refrigeration. CT exposition for 1 h induced an elevation of basal ERK1/2 phosphorylation and an alleviation of PGE1- or DEA/NO-suppressed ERK1/2 phosphorylation in APC on day 1 and 2 of storage. Similar, but more moderate effects were observable for p38 phosphorylation. Akt/PKB phosphorylation was increased only in APC on day 2. Refrigeration for 1 h promoted platelet adhesion and reduced basal VASP phosphorylation in adherent platelets. The attenuation of inhibitory signalling in short-term refrigerated stored platelets is associated with enhanced reactivity of activating signalling pathways, especially ERK1/2. Functionally, these processes correlate with increased adhesion of refrigerated platelets on collagen-coated surfaces. The results help to further optimize temperature-dependent strategies for platelet storage.

Current Strategies to Guide the Antiplatelet Therapy in Acute Coronary Syndromes

The role of antiplatelet therapy in patients with acute coronary syndromes is a moving target with considerable novelty in the last few years. The pathophysiological basis of the treatment depends on platelet biology and physiology, and the interplay between these aspects and clinical practice must guide the physician in determining the best therapeutic options for patients with acute coronary syndromes. In the present narrative review, we discuss the latest novelties in the antiplatelet therapy of patients with acute coronary syndromes. We start with a description of platelet biology and the role of the main platelet signal pathways involved in platelet aggregation during an acute coronary syndrome. Then, we present the latest evidence on the evaluation of platelet function, focusing on the strengths and weaknesses of each platelet's function test. We continue our review by describing the role of aspirin and P2Y12 inhibitors in the treatment of acute coronary syndromes, critically appraising the available evidence from clinical trials, and providing current international guidelines and recommendations. Finally, we describe alternative therapeutic regimens to standard dual antiplatelet therapy, in particular for patients at high bleeding risk. The aim of our review is to give a comprehensive representation of current data on antiplatelet therapy in patients with acute coronary syndromes that could be useful both for clinicians and basic science researchers to be up-to-date on this complex topic.

Exploring bias in platelet P2Y1 signalling: Host defence versus haemostasis

Platelets are necessary for maintaining haemostasis. Separately, platelets are important for the propagation of inflammation during the host immune response against infection. The activation of platelets also causes inappropriate inflammation in various disease pathologies, often in the absence of changes to haemostasis. The separate functions of platelets during inflammation compared with haemostasis are therefore varied and this will be reflected in distinct pathways of activation. The activation of platelets by the nucleotide adenosine diphosphate (ADP) acting on P2Y1 and P2Y12 receptors is important for the development of platelet thrombi during haemostasis. However, P2Y1 stimulation of platelets is also important during the inflammatory response and paradoxically in scenarios where no changes to haemostasis and platelet aggregation occur. In these events, Rho-GTPase signalling, rather than the canonical phospholipase Cβ (PLCβ) signalling pathway, is necessary. We describe our current understanding of these differences, reflecting on recent advances in knowledge of P2Y1 structure, and the possibility of biased agonism occurring from activation via other endogenous nucleotides compared with ADP. Knowledge arising from these different pathways of P2Y1 stimulation of platelets during inflammation compared with haemostasis may help therapeutic control of platelet function during inflammation or infection, while preserving essential haemostasis. LINKED ARTICLES: This article is part of a themed issue on Platelet purinergic receptor and non-thrombotic disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.4/issuetoc.

A novel GATA1 variant p.G229D causing the defect of procoagulant platelet formation

INTRODUCTION

GATA1 is one of the master transcription factors in hematopoietic lineages development which is crucial for megakaryocytic differentiation and maturation. Previous studies have shown that distinct GATA1 variants are associated with varying severities of macrothrombocytopenia and platelet dysfunction.

OBJECTIVE

To determine the underlying pathological mechanisms of a novel GATA1 variant (c. 686G > A, p. G229D) in a patient with recurrent traumatic muscle hematomas.

METHODS

Comprehensive phenotypic analysis of the patient platelets was performed. Procoagulant platelet formation and function were detected using flow cytometry assay and thrombin generation test (TGT), respectively. The ANO6 expression was measured by qPCR and western blot. The intracellular supramaximal calcium flux was detected by Fluo-5N fluorescent assay.

RESULTS

The patient displayed mild macrothrombocytopenia with defects of platelet granules, aggregation, and integrin αIIbβ3 activation. The percentage of the procoagulant platelet formation of the patient upon the stimulation of thrombin plus collagen was lower than that of the healthy controls (40.9 % vs 49.0 % ± 5.1 %). The patient platelets exhibited a marked reduction of thrombin generation in platelet rich plasma TGT compared to the healthy controls (peak value: ∼70 % of the healthy controls; the endogenous thrombin potential: ∼40 % of the healthy controls). The expression of ANO6 and intracellular calcium flux were impaired, which together with abnormal granules of the patient platelets might contribute to defect of procoagulant platelet function.

CONCLUSIONS

The G229D variant could lead to a novel platelet phenotype characterized by defective procoagulant platelet formation and function, which extended the range of GATA1 variants associated platelet disorders.

Blood oxygen regulation via P2Y12R expressed in the carotid body

BACKGROUND

Peripheral blood oxygen monitoring via chemoreceptors in the carotid body (CB) is an integral function of the autonomic cardiorespiratory regulation. The presence of the purinergic P2Y12 receptor (P2Y12R) has been implicated in CB; however, the exact role of the receptor in O2 sensing and signal transduction is unknown.

METHODS

The presence of P2Y12R was established by immunoblotting, RT qPCR and immunohistochemistry. Primary glomus cells were used to assess P2Y12R function during hypoxia and hypercapnia, where monoamines were measured by HPLC; calcium signal was recorded utilizing OGB-1 and N-STORM Super-Resolution System. Ingravescent hypoxia model was tested in anaesthetized mice of mixed gender and cardiorespiratory parameters were recorded in control and receptor-deficient or drug-treated experimental animals.

RESULTS

Initially, the expression of P2Y12R in adult murine CB was confirmed. Hypoxia induced a P2Y12R-dependent release of monoamine transmitters from isolated CB cells. Receptor activation with the endogenous ligand ADP promoted release of neurotransmitters under normoxic conditions, while blockade disrupted the amplitude and duration of the intracellular calcium concentration. In anaesthetised mice, blockade of P2Y12R expressed in the CB abrogated the initiation of compensatory cardiorespiratory changes in hypoxic environment, while centrally inhibited receptors (i.e. microglial receptors) or receptor-deficiency induced by platelet depletion had limited influence on the physiological adjustment to hypoxia.

CONCLUSIONS

Peripheral P2Y12R inhibition interfere with the complex mechanisms of acute oxygen sensing by influencing the calcium signalling and the release of neurotransmitter molecules to evoke compensatory response to hypoxia. Prospectively, the irreversible blockade of glomic receptors by anti-platelet drugs targeting P2Y12Rs, propose a potential, formerly unrecognized side-effect to anti-platelet medications in patients with pulmonary morbidities.

Role of Prednisolone in Platelet Activation by Inhibiting TxA2 Generation through the Regulation of cPLA2 Phosphorylation

Glucocorticoids have been commonly used in the treatment of inflammation and immune-mediated diseases in human beings and small animals such as cats and dogs. However, excessive use can lead to Cushing's syndrome along with several thrombotic and cardiovascular diseases. Although it is well-known that glucocorticoids exert a significant effect on coagulation, the effect of cortisol on platelet function is much less clear. Thus, we aimed to study the effects of prednisolone, one of the commonly used glucocorticoids, on the regulation of platelet function using murine platelets. We first evaluated the concentration-dependent effect of prednisolone on 2-MeSADP-induced platelet function and found that the 2-MeSADP-induced secondary wave of aggregation and dense granule secretion were completely inhibited from 500 nM prednisolone. Since 2-MeSADP-induced secretion and the resultant secondary wave of aggregation are mediated by TxA₂ generation, this result suggested a role of prednisolone in platelet TxA₂ generation. Consistently, prednisolone did not affect the 2-MeSADP-induced aggregation in aspirinated platelets, where the secondary wave of aggregation and secretion were blocked by eliminating the contribution of TxA₂ generation by aspirin. In addition, thrombin-induced platelet aggregation and secretion were inhibited in the presence of prednisolone by inhibiting the positive-feedback effect of TxA₂ generation on platelet function. Furthermore, prednisolone completely inhibited 2-MeSADP-induced TxA₂ generation, confirming the role of prednisolone in TxA₂ generation. Finally, Western blot analysis revealed that prednisolone significantly inhibited 2-MeSADP-induced cytosolic phospholipase A₂ (cPLA₂) and ERK phosphorylation in non-aspirinated platelets, while only cPLA₂ phosphorylation, but not ERK phosphorylation, was significantly inhibited by prednisolone in aspirinated platelets. In conclusion, prednisolone affects platelet function by the inhibition of TxA₂ generation through the regulation of cPLA₂ phosphorylation, thereby shedding light on its clinical characterization and treatment efficacy in dogs with hypercortisolism in the future.

Antiplatelet Effect of Daphnetin Is Regulated by cPLA2-Mediated Thromboxane A2 Generation in Mice

Coumarin derivatives have been recognized for their antithrombotic, anti-inflammatory, and antioxidant properties, and daphnetin is one of the natural coumarin derivatives isolated from Daphne Koreana Nakai. Although the pharmacological value of daphnetin is well documented in diverse biological activities, its antithrombotic effect has not been studied to date. Here, we characterized the role and underlying mechanism of daphnetin in the regulation of platelet activation using murine platelets. In order to check the effect of daphnetin on platelet function, we first measured the effect of daphnetin on platelet aggregation and secretion. Collagen-induced platelet aggregation and dense granule secretion were partially inhibited by daphnetin. Interestingly, 2-MeSADP-induced secondary waves of aggregation and secretion were completely inhibited by daphnetin. It is known that 2-MeSADP-induced secretion and the resultant secondary wave of aggregation are mediated by the positive feedback effect of thromboxane A₂ (TxA₂) generation, suggesting the important role of daphnetin on TxA₂ generation in platelets. Consistently, daphnetin did not affect the 2-MeSADP-induced platelet aggregation in aspirinated platelets where the contribution of TxA₂ generation was blocked. Additionally, platelet aggregation and secretion induced by a low concentration of thrombin, which is affected by the positive feedback effect of TxA₂ generation, were partially inhibited in the presence of daphnetin. Importantly, 2-MeSADP- and thrombin-induced TxA₂ generation was significantly inhibited in the presence of daphnetin, confirming the role of daphnetin on TxA₂ generation. Finally, daphnetin significantly inhibited 2-MeSADP-induced cytosolic phospholipase A₂ (cPLA₂) and ERK phosphorylation in non-aspirinated platelets. Only cPLA₂ phosphorylation, not ERK phosphorylation, was significantly inhibited by daphnetin in aspirinated platelets. In conclusion, daphnetin plays a critical role in platelet function by inhibiting TxA₂ generation through the regulation of cPLA₂ phosphorylation.

Platelet P2Y1 receptor exhibits constitutive G protein signaling and β-arrestin 2 recruitment

BACKGROUND

Purinergic P2Y₁ and P2Y₁₂ receptors (P2Y₁-R and P2Y₁₂-R) are G protein-coupled receptors (GPCR) activated by adenosine diphosphate (ADP) to mediate platelet activation, thereby playing a pivotal role in hemostasis and thrombosis. While P2Y₁₂-R is the major target of antiplatelet drugs, no P2Y₁-R antagonist has yet been developed for clinical use. However, accumulating data suggest that P2Y₁-R inhibition would ensure efficient platelet inhibition with minimal effects on bleeding. In this context, an accurate characterization of P2Y₁-R antagonists constitutes an important preliminary step.

RESULTS

Here, we investigated the pharmacology of P2Y₁-R signaling through Gq and β-arrestin pathways in HEK293T cells and in mouse and human platelets using highly sensitive resonance energy transfer-based technologies (BRET/HTRF). We demonstrated that at basal state, in the absence of agonist ligand, P2Y₁-R activates Gq protein signaling in HEK293T cells and in mouse and human platelets, indicating that P2Y₁-R is constitutively active in physiological conditions. We showed that P2Y₁-R also promotes constitutive recruitment of β-arrestin 2 in HEK293T cells. Moreover, the P2Y₁-R antagonists MRS2179, MRS2279 and MRS2500 abolished the receptor dependent-constitutive activation, thus behaving as inverse agonists.

CONCLUSIONS

This study sheds new light on P2Y₁-R pharmacology, highlighting for the first time the existence of a constitutively active P2Y₁-R population in human platelets. Given the recent interest of P2Y₁₂-R constitutive activity in patients with diabetes, this study suggests that modification of constitutive P2Y₁-R signaling might be involved in pathological conditions, including bleeding syndrome or high susceptibility to thrombotic risk. Thus, targeting platelet P2Y₁-R constitutive activation might be a promising and powerful strategy for future antiplatelet therapy.

Influence of genetic polymorphisms in P2Y12 receptor signaling pathway on antiplatelet response to clopidogrel in coronary heart disease

BACKGROUNDS

Remarkable interindividual variability in clopidogrel response is observed, genetic polymorphisms in P2RY12 and its signal pathway is supposed to affect clopidogrel response in CHD patients.

METHODS

539 CHD patients treated with clopidogrel were recruited. The platelet reaction index (PRI) indicated by VASP-P level were detected in 12-24 h after clopidogrel loading dose or within 5-7 days after initiation of maintain dose clopidogrel. A total of 13 SNPs in relevant genes were genotyped in sample A (239 CHD patients). The SNPs which have significant differences in PRI will be validated in another sample (sample B, 300 CHD patients).

RESULTS

CYP2C19*2 increased the risk of clopidogrel resistance significantly. When CYP2C19*2 and CYP2C19*3 were considered, CYP2C19 loss of function (LOF) alleles were associated with more obviously increased the risk of clopidogrel resistance; P2RY12 rs6809699C > A polymorphism was also associated with increased risk of clopidogrel resistance (AA vs CC: P = 0.0398). This difference still existed after stratification by CYP2C19 genotypes. It was also validated in sample B. The association was also still significant even in the case of stratification by CYP2C19 genotypes in all patients (sample A + B).

CONCLUSION

Our data suggest that P2RY12 rs6809699 is associated with clopidogrel resistance in CHD patients. Meanwhile, the rs6809699 AA genotype can increase on-treatment platelet activity independent of CYP2C19 LOF polymorphisms.

Deferiprone, an iron chelator, alleviates platelet hyperactivity in patients with β-thalassaemia/HbE

Background

Hyperfunctional platelets play important roles in thromboembolism in patients with β-thalassaemia/ haemoglobin E (β-thal/HbE). Our previous study revealed ex vivo inhibitory effects of deferiprone on normal platelets. Herein, we aimed to investigate the in vivo effects on platelets in patients with β-thal/HbE.

Methods

A prospective, self-controlled clinical study on 30 patients with β-thal/HbE who had received therapeutic deferiprone (20.8-94.5 mg/kg/day) was conducted. The study included a 4-week washout period followed by 4 and 12 weeks of deferiprone treatment. Platelet aggregation was performed by a turbidimetric method. Levels of deferiprone and soluble platelet (sP)-selectin in serum were measured by high-performance liquid chromatography (HPLC) and enzyme-linked immunosorbent assay (ELISA) kit, respectively.

Results

The washout period significantly enhanced platelet hyperactivity both in patients who had undergone splenectomy and in those who had not. At 2 hours following the administration of a single dose of deferiprone, platelet sensitivity to ADP and arachidonic acid was significantly reduced. The inhibitory effects of deferiprone were gradually increased over the period of 4 and 12 weeks. Deferiprone also depressed sP-selectin levels, but the effect was stable over longer follow-up periods. Correlation analysis demonstrated the relationship between serum levels of deferiprone, sP-selectin, and platelet activities induced by ADP and arachidonic acid.

Conclusion

We first demonstrated the in vivo antiplatelet effect and benefit of short-term treatment of deferiprone in patients with β-thal/HbE. The impact on thrombotic outcomes deserves further study.

GRK2 regulates ADP signaling in platelets via P2Y1 and P2Y12

The critical role of G protein-coupled receptor kinase 2 (GRK2) in regulating cardiac function has been well documented for >3 decades. Targeting GRK2 has therefore been extensively studied as a novel approach to treating cardiovascular disease. However, little is known about its role in hemostasis and thrombosis. We provide here the first evidence that GRK2 limits platelet activation and regulates the hemostatic response to injury. Deletion of GRK2 in mouse platelets causes increased platelet accumulation after laser-induced injury in the cremaster muscle arterioles, shortens tail bleeding time, and enhances thrombosis in adenosine 5'-diphosphate (ADP)-induced pulmonary thromboembolism and in FeCl3-induced carotid injury. GRK2-/- platelets have increased integrin activation, P-selectin exposure, and platelet aggregation in response to ADP stimulation. Furthermore, GRK2-/- platelets retain the ability to aggregate in response to ADP restimulation, indicating that GRK2 contributes to ADP receptor desensitization. Underlying these changes in GRK2-/- platelets is an increase in Ca2+ mobilization, RAS-related protein 1 activation, and Akt phosphorylation stimulated by ADP, as well as an attenuated rise of cyclic adenosine monophosphate levels in response to ADP in the presence of prostaglandin I2. P2Y12 antagonist treatment eliminates the phenotypic difference in platelet accumulation between wild-type and GRK2-/- mice at the site of injury. Pharmacologic inhibition of GRK2 activity in human platelets increases platelet activation in response to ADP. Finally, we show that GRK2 binds to endogenous Gβγ subunits during platelet activation. Collectively, these results show that GRK2 regulates ADP signaling via P2Y1 and P2Y12, interacts with Gβγ, and functions as a signaling hub in platelets for modulating the hemostatic response to injury.

Tumor Cell-Induced Platelet Aggregation as an Emerging Therapeutic Target for Cancer Therapy

Tumor cells have the ability to induce platelet activation and aggregation. This has been documented to be involved in tumor progression in several types of cancers, such as lung, colon, breast, pancreatic, ovarian, and brain. During the process, platelets protect circulating tumor cells from the deleterious effects of shear forces, shield tumor cells from the immune system, and provide growth factors, facilitating metastatic spread and tumor growth at the original site as well as at the site of metastasis. Herein, we present a wider view on the induction of platelet aggregation by specific factors primarily developed by cancer, including coagulation factors, adhesion receptors, growth factors, cysteine proteases, matrix metalloproteinases, glycoproteins, soluble mediators, and selectins. These factors may be presented on the surface of tumor cells as well as in their microenvironment, and some may trigger more than just one simple receptor-ligand mechanism. For a better understanding, we briefly discuss the physiological role of the factors in the platelet activation process, and subsequently, we provide scientific evidence and discuss their potential role in the progression of specific cancers. Targeting tumor cell-induced platelet aggregation (TCIPA) by antiplatelet drugs may open ways to develop new treatment modalities. On the one hand, it may affect patients' prognosis by enhancing known therapies in advanced-stage tumors. On the other hand, the use of drugs that are mostly easily accessible and widely used in general practice may be an opportunity to propose an unparalleled antitumor prophylaxis. In this review, we present the recent discoveries of mechanisms by which cancer cells activate platelets, and discuss new platelet-targeted therapeutic strategies.

Antiplatelet Agents Affecting GPCR Signaling Implicated in Tumor Metastasis

Metastasis requires that cancer cells survive in the circulation, colonize distant organs, and grow. Despite platelets being central contributors to hemostasis, leukocyte trafficking during inflammation, and vessel stability maintenance, there is significant evidence to support their essential role in supporting metastasis through different mechanisms. In addition to their direct interaction with cancer cells, thus forming heteroaggregates such as leukocytes, platelets release molecules that are necessary to promote a disseminating phenotype in cancer cells via the induction of an epithelial-mesenchymal-like transition. Therefore, agents that affect platelet activation can potentially restrain these prometastatic mechanisms. Although the primary adhesion of platelets to cancer cells is mainly independent of G protein-mediated signaling, soluble mediators released from platelets, such as ADP, thromboxane (TX) A2, and prostaglandin (PG) E2, act through G protein-coupled receptors (GPCRs) to cause the activation of more additional platelets and drive metastatic signaling pathways in cancer cells. In this review, we examine the contribution of the GPCRs of platelets and cancer cells in the development of cancer metastasis. Finally, the possible use of agents affecting GPCR signaling pathways as antimetastatic agents is discussed.

The antiplatelet effect of mirtazapine is mediated by co-blocking 5-HT2A and α2-adrenergic receptors on platelets: An in vitro human plasma-based study

Mirtazapine (MTZ) is a noradrenergic and specific serotonergic antidepressant that has been associated with an increased risk of bleeding. However, there is insufficient evidence confirming this association. We hypothesised that 5-HT2A and α2 receptor-mediated inhibitory effects of MTZ on platelets suppress platelet aggregation and increase the risk of bleeding. In this study, we examined the antiplatelet effect of MTZ on human platelets to test our hypothesis. Blood samples for platelet aggregation tests were obtained from 14 healthy volunteers. The antiplatelet effect of MTZ was evaluated using light transmission aggregometry. MTZ significantly suppressed platelet aggregation mediated both by the synergistic interaction of serotonin (5-HT) and adrenaline and the synergistic interaction of ADP and 5-HT or adrenaline. In conclusion, MTZ exerts its antiplatelet effects by co-blocking the 5-HT2A and α2-adrenergic receptors on platelets and also suppresses platelet aggregation induced by ADP and 5-HT or adrenaline. Therefore, when MTZ is used, especially for patients with a high risk of bleeding, the significance of its use must be considered carefully. In addition, the platelet aggregation pattern by adrenaline + 5-HT, ADP + adrenaline, and ADP + 5-HT was similar between humans and mice; however, this study did not directly compare the effects of MTZ on human and murine platelets. Therefore, under the conditions for inducing platelet aggregation using adrenaline + 5-HT, ADP + adrenaline, and ADP + 5-HT, mouse platelets can be used in the evaluation of the efficacy of antiplatelet drugs in humans.

Inhibitory Mechanisms of Lusianthridin on Human Platelet Aggregation

Lusianthridin is a phenanthrene derivative isolated from Dendrobium venustum. Some phenanthrene compounds have antiplatelet aggregation activities via undefined pathways. This study aims to determine the inhibitory effects and potential mechanisms of lusianthridin on platelet aggregation. The results indicated that lusianthridin inhibited arachidonic acid, collagen, and adenosine diphosphate (ADP)-stimulated platelet aggregation (IC₅₀ of 0.02 ± 0.001 mM, 0.14 ± 0.018 mM, and 0.22 ± 0.046 mM, respectively). Lusianthridin also increased the delaying time of arachidonic acid-stimulated and the lag time of collagen-stimulated and showed a more selective effect on the secondary wave of ADP-stimulated aggregations. Molecular docking studies revealed that lusianthridin bound to the entrance site of the cyclooxygenase-1 (COX-1) enzyme and probably the active region of the cyclooxygenase-2 (COX-2) enzyme. In addition, lusianthridin showed inhibitory effects on both COX-1 and COX-2 enzymatic activities (IC₅₀ value of 10.81 ± 1.12 µM and 0.17 ± 1.62 µM, respectively). Furthermore, lusianthridin significantly inhibited ADP-induced suppression of cAMP formation in platelets at 0.4 mM concentration (p < 0.05). These findings suggested that possible mechanisms of lusianthridin on the antiplatelet effects might act via arachidonic acid-thromboxane and adenylate cyclase pathways.

Mechanisms of thrombosis and research progress on targeted antithrombotic drugs

Globally, the incidence of thromboembolic diseases has increased in recent years, accompanied by an increase in patient mortality. Currently, several targeting delivery strategies have been developed to treat thromboembolic diseases. In this review, we discuss the mechanisms of thrombolysis and current anticoagulant drugs, particularly those with targeting capability, highlighting advances in the accurate treatment of thrombolysis with fewer adverse effects. Such approaches include magnetic drug-loading systems combined with molecular imaging to recanalize blood vessels and systems based on chimeric Arg-Gly-Asp (RGD) sequences that can target platelet glycoprotein receptor. With such progress in targeted antithrombotic drugs, targeted thrombolysis treatment shows significant potential benefit for patients.

Current Understanding of the Relationship between Blood Donor Variability and Blood Component Quality

While differences among donors has long challenged meeting quality standards for the production of blood components for transfusion, only recently has the molecular basis for many of these differences become understood. This review article will examine our current understanding of the molecular differences that impact the quality of red blood cells (RBC), platelets, and plasma components. Factors affecting RBC quality include cytoskeletal elements and membrane proteins associated with the oxidative response as well as known enzyme polymorphisms and hemoglobin variants. Donor age and health status may also be important. Platelet quality is impacted by variables that are less well understood, but that include platelet storage sensitive metabolic parameters, responsiveness to agonists accumulating in storage containers and factors affecting the maintenance of pH. An increased understanding of these variables can be used to improve the quality of blood components for transfusion by using donor management algorithms based on a donors individual molecular and genetic profile.

A Three-Dimensional Microfluidic Device for Monitoring Cancer and Chemotherapy-Associated Platelet Activation

Platelet activation and the risk of thrombosis are increased in cancer patients, especially after chemotherapy. Our previous studies indicated that chemotherapy-induced platelet activation is largely due to endothelial cell damage. Thus, simple in vitro tests, such as aggregometry, are not desirable tests to predict platelet responsiveness to different chemotherapeutic agents because other contributory factors, such as tumor cells, endothelial cells, and the flow rate of platelets, also contribute to the formation of cancer-associated thrombosis. Therefore, developing a platelet detection system, which includes all possible risk parameters, is necessary. In the present study, we described a microengineered microfluidic system that contained a drug concentration generator, cancer cell culture chip, and three-dimensional (3D) circular microvascular model covered with a confluent endothelial layer and perfused with human platelets at a stable flow rate. Doxorubicin was injected through two injection sites. Endothelial cell injury was evaluated by counting, cell cytoskeleton observation, and the level of IACM1 and ET-1 in endothelial cells or a culture medium. Prestained platelets were perfused into the artificial blood vessel, and platelet-endothelial cell adhesion was measured. We found that (i) MCF7 cell-released factors had a cytotoxicity effect on both endothelial cells and platelets. (ii) We confirmed that doxorubicin-induced platelet activation was endothelial cell-dependent. (iii) A lower dosage of doxorubicin (0-2.0 μM) induced platelet activation, while a higher dosage of doxorubicin (2.0-4.0 μM) led to platelet death. Our findings indicated that platelet-endothelial cell adhesion could be used as a diagnostic marker of platelet activation, providing a simple and rapid detective way to predict platelet responsiveness before or during chemotherapy.

Antiplatelet Effect of Mirtazapine via Co-blocking of the 5-HT2A and α2-Adrenergic Receptors on Platelets

Mirtazapine (MTZ) is a noradrenergic and specific serotonergic antidepressant. MTZ is reportedly associated with an increased risk of bleeding. However, the underlying mechanism remains unclear. In this study, we investigated the antiplatelet effect of MTZ in mice via light transmission aggregometry to elucidate the mechanism of MTZ-induced bleeding. The results of the ex vivo study showed that the oral administration of MTZ (20 or 100 mg/kg) significantly suppressed platelet aggregation mediated by the synergic interaction of 5-hydroxytryptamine (5-HT) and adrenaline. Additionally, MTZ significantly suppressed platelet aggregation, mediated by the synergic interaction of ADP and 5-HT or adrenaline. Similar results were obtained in vitro, under the condition of 5-HT- and adrenaline-induced platelet aggregation. Overall, the results suggest that MTZ exerts antiplatelet effect by co-blocking 5-HT_2A and α₂-adrenergic receptors on platelets and suppresses platelet aggregation mediated by ADP, increased by either 5-HT or adrenaline. Thus, a detailed monitoring of bleeding is recommended for patients taking MTZ.

ADP-Mediated Upregulation of Expression of CD62P on Human Platelets Is Critically Dependent on Co-Activation of P2Y1 and P2Y12 Receptors

This study probed the differential utilization of P2Y1 and P2Y12 receptors in mobilizing CD62P (P-selectin) from intracellular granules following activation of human platelets with adenosine 5′-diphosphate (ADP, 100 µmol·L⁻¹) Platelet-rich plasma (PRP) was prepared from the blood of adult humans. CD62P was measured by flow cytometry following activation of PRP with ADP in the absence and presence of the selective antagonists of P2Y1 and P2Y12 receptors, MRS2500 and PSB0739 (both 0.155–10 µmol·L⁻¹), respectively. Effects of the test agents on ADP-activated, CD62P-dependent formation of neutrophil:platelet (NP) aggregates were also measured by flow cytometry, while phosphatidylinositol 3-kinase (PI3K) activity was measured according to Akt1 phosphorylation in platelet lysates. Treatment with MRS2500 or PSB0739 at 10 µmol·L⁻¹ almost completely attenuated (94.6% and 86% inhibition, respectively) ADP-activated expression of CD62P and also inhibited NP aggregate formation. To probe the mechanisms involved in P2Y1/P2Y12 receptor-mediated expression of CD62P, PRP was pre-treated with U73122 (phospholipase C (PLC) inhibitor), 2-aminoethoxy-diphenyl borate (2-APB, inositol triphosphate receptor antagonist), calmidazolium chloride (calmodulin inhibitor), or wortmannin (PI3K inhibitor). U73122, 2-APB, and wortmannin caused almost complete inhibition of ADP-activated expression of CD62P, while calmidazolium chloride caused statistically significant, partial inhibition. PSB0739, but not MRS2500, caused potent inhibition of PI3K-mediated phosphorylation of Akt1. Optimal mobilization of CD62P by ADP-stimulated platelets is critically dependent on the co-activation of platelet P2Y1 and P2Y12 receptors. P2Y12 receptor activation is the key event in activation of PI3K, while activation of the P2Y1 receptor appears to create a high cytosolic Ca²⁺ environment conducive to optimum PI3K activity.

Platelets and Their Role in the Pathogenesis of Cardiovascular Events in Patients With Community-Acquired Pneumonia

Community-acquired pneumonia (CAP) remains an important cause of morbidity and mortality throughout the world with much recent and ongoing research focused on the occurrence of cardiovascular events (CVEs) during the infection, which are associated with adverse short-term and long-term survival. Much of the research directed at unraveling the pathogenesis of these events has been undertaken in the settings of experimental and clinical CAP caused by the dangerous, bacterial respiratory pathogen, Streptococcus pneumoniae (pneumococcus), which remains the most common bacterial cause of CAP. Studies of this type have revealed that although platelets play an important role in host defense against infection, there is also increasing recognition that hyperactivation of these cells contributes to a pro-inflammatory, prothrombotic systemic milieu that contributes to the etiology of CVEs. In the case of the pneumococcus, platelet-driven myocardial damage and dysfunction is exacerbated by the direct cardiotoxic actions of pneumolysin, a major pore-forming toxin of this pathogen, which also acts as potent activator of platelets. This review is focused on the role of platelets in host defense against infection, including pneumococcal infection in particular, and reviews the current literature describing the potential mechanisms by which platelet activation contributes to cardiovascular complications in CAP. This is preceded by an evaluation of the burden of pneumococcal infection in CAP, the clinical features and putative pathogenic mechanisms of the CVE, and concludes with an evaluation of the potential utility of the anti-platelet activity of macrolides and various adjunctive therapies.

PAK Membrane Translocation and Phosphorylation Regulate Platelet Aggregation Downstream of Gi and G12/13 Pathways

Platelet activation plays a pivotal role in physiological hemostasis and pathological thrombosis causing heart attack and stroke. Previous studies conclude that simultaneous activation of Gi and G_12/13 signaling pathways is sufficient to cause platelet aggregation. However, using Gq knockout mice and Gq-specific inhibitors, we here demonstrated that platelet aggregation downstream of coactivation of Gi and G_12/13 depends on agonist concentrations; coactivation of Gi and G_12/13 pathways only induces platelet aggregation under higher agonist concentrations. We confirmed Gi and G_12/13 pathway activation by showing cAMP (cyclic adenosine monophosphate) decrease and RhoA activation in platelets stimulated at both low and high agonist concentrations. Interestingly, we found that though Akt and PAK (p21-activated kinase) translocate to the platelet membrane upon both low and high agonist stimulation, membrane-translocated Akt and PAK only phosphorylate at high agonist concentrations, correlating well with platelet aggregation downstream of concomitant Gi and G_12/13 pathway activation. PAK inhibitor abolishes Akt phosphorylation, inhibits platelet aggregation in vitro and arterial thrombus formation in vivo. We propose that the PAK-PI3K/Akt pathway mediates platelet aggregation downstream of Gi and G_12/13, and PAK may represent a potential antiplatelet and antithrombotic target.

Biased signaling in platelet G-protein coupled receptors

Platelets are small megakaryocyte-derived, anucleate, disk-like structures that play an outsized role in human health and disease. Both a decrease in the number of platelets and a variety of platelet function disorders result in petechiae or bleeding that can be life threatening. Conversely, the inappropriate activation of platelets, within diseased blood vessels, remains the leading cause of death and morbidity by affecting heart attacks and stroke. The fine balance of the platelet state in healthy individuals is controlled by a number of receptor-mediated signaling pathways that allow the platelet to rapidly respond and maintain haemostasis. G-protein coupled receptors (GPCRs) are particularly important regulators of platelet function. Here we focus on the major platelet-expressed GPCRs and discuss the roles of downstream signaling pathways (e.g., different G-protein subtypes or β-arrestin) in regulating the different phases of the platelet activation. Further, we consider the potential for selectively targeting signaling pathways that may contribute to platelet responses in disease through development of biased agonists. Such selective targeting of GPCR-mediated signaling pathways by drugs, often referred to as biased signaling, holds promise in delivering therapeutic interventions that do not present significant side effects, especially in finely balanced physiological systems such as platelet activation in haemostasis.

The Impact of Cold Storage on Adenosine Diphosphate-Mediated Platelet Responsiveness

Introduction  Cold storage of platelets is considered to contribute to lower risk of bacterial growth and to more efficient hemostatic capacity. For the optimization of storage strategies, it is required to further elucidate the influence of refrigeration on platelet integrity. This study focused on adenosine diphosphate (ADP)-related platelet responsiveness.

Materials and Methods  Platelets were prepared from apheresis-derived platelet concentrates or from peripheral whole blood, stored either at room temperature or at 4°C. ADP-induced aggregation was tested with light transmission. Activation markers, purinergic receptor expression, and P2Y12 receptor function were determined by flow cytometry. P2Y1 and P2X1 function was assessed by fluorescence assays, cyclic nucleotide concentrations by immunoassays, and vasodilator-stimulated phosphoprotein (VASP)-phosphorylation levels by Western blot analysis.

Results  In contrast to room temperature, ADP-induced aggregation was maintained under cold storage for 6 days, associated with elevated activation markers like fibrinogen binding or CD62P expression. Purinergic receptor expression was not essentially different, whereas P2Y1 function deteriorated rapidly at cold storage, but not P2Y12 activity. Inhibitory pathways of cold-stored platelets were characterized by reduced responses to nitric oxide and prostaglandin E1. Refrigeration of citrated whole blood also led to the attenuation of induced inhibition of platelet aggregation, detectable within 24 hours.

Conclusion  ADP responsiveness is preserved under cold storage for 6 days due to stable P2Y12 activity and concomitant disintegration of inhibitory pathways enabling a higher reactivity of stored platelets. The ideal storage time at cold temperature for the highest hemostatic effect of platelets should be evaluated in further studies.

Intravascular hemolysis triggers ADP-mediated generation of platelet-rich thrombi in precapillary pulmonary arterioles

Patients with hereditary or acquired hemolytic anemias have a high risk of developing in situ thrombosis of the pulmonary vasculature. While pulmonary thrombosis is a major morbidity associated with hemolytic disorders, the etiological mechanism underlying hemolysis-induced pulmonary thrombosis remains largely unknown. Here, we use intravital lung microscopy in mice to assess the pathogenesis of pulmonary thrombosis following deionized water-induced acute intravascular hemolysis. Acute hemolysis triggered the development of αIIbβ3-dependent platelet-rich thrombi in precapillary pulmonary arterioles, which led to the transient impairment of pulmonary blood flow. The hemolysis-induced pulmonary thrombosis was phenocopied with intravascular ADP- but not thrombin-triggered pulmonary thrombosis. Consistent with a mechanism involving ADP release from hemolyzing erythrocytes, the inhibition of platelet P2Y12 purinergic receptor signaling attenuated pulmonary thrombosis and rescued blood flow in the pulmonary arterioles of mice following intravascular hemolysis. These findings are the first in vivo studies to our knowledge to suggest that acute intravascular hemolysis promotes ADP-dependent platelet activation, leading to thrombosis in the precapillary pulmonary arterioles, and that thrombin generation most likely does not play a significant role in the pathogenesis of acute hemolysis-triggered pulmonary thrombosis.

Synergistic platelet inhibition between Omega-3 and acetylsalicylic acid dose titration; an observational study

BACKGROUND

Omega-3 and acetylsalicylic acid (ASA) are two widely used "over-the-counter" drugs. Previous research has shown multiple electrode aggregometry (MEA) can detect ASA and varying Omega-3 platelet inhibiting effects. Synergistic platelet inhibiting effects of ASA and Omega-3 have been found using other methods than MEA. The aim of this study was to investigate the antiplatelet effects of Omega-3, and ASA synergism with MEA.

METHODS

Ten healthy male volunteers ingested Omega-3 (1260 mg/day) for 5 days. MEA was used to analyse platelet function before and after Omega-3 intake. Aggregation was initiated using three different agonists and measured as area under the curve (AUC): adenosine diphosphate (ADP), thrombin receptor activating peptide (TRAP) and arachidonic acid (ASPI). Two concentrations of ASA were dose titrated ex vivo to 2 out of 3 ASPI test cells in order to measure synergism between Omega-3 and ASA.

RESULTS

Following 5 days Omega-3 intake, ADP, TRAP and ASPI AUC did not change significantly. In vitro ASA before Omega-3 intake, reduced ASPI AUC < 30 U, indicating a strong platelet inhibiting effect. Below this AUC level, the 5 days Omega-3 intake increased ASPI-AUC with the ex vivo added low dose ASA (P = 0.02) and high dose ASA (P = 0.04).

CONCLUSIONS

No synergism between ASA and Omega-3 was found using the MEA ASPI test. The surprising increase in ASPI-AUC following Omega-3 intake and ex vivo ASA suggest that there are methodological issuses with the MEA ASPI test.

TRIAL REGISTRATION

Trial registration ISRCTN78027929 . Registered 19 May 2015.

Ticagrelor reverses the mitochondrial dysfunction through preventing accumulated autophagosomes-dependent apoptosis and ER stress in insulin-resistant H9c2 myocytes

Ticagrelor, a P2Y12-receptor inhibitor, and a non-thienopyridine agent are used to treat diabetic patients via its effects on off-target mechanisms. However, the exact sub-cellular mechanisms by which ticagrelor exerts those effects remains to be elucidated. Accordingly, the present study aimed to examine whether ticagrelor influences directly the cardiomyocytes function under insulin resistance through affecting mitochondria-sarco(endo)plasmic reticulum (SER) cross-talk. Therefore, we analyzed the function and ultrastructure of mitochondria and SER in insulin resistance-mimicked (50-μM palmitic acid for 24-h) H9c2 cardiomyocytes in the presence or absence of ticagrelor (1-µM for 24-h). We found that ticagrelor treatment significantly prevented depolarization of mitochondrial membrane potential and increases in reactive oxygen species with a marked increase in the ATP level in insulin-resistant H9c2 cells. Ticagrelor treatment also reversed the increases in the resting level of free Ca2+ and mRNA level of P2Y12 receptors as well as preserved ER stress and apoptosis in insulin-resistant H9c2 cells. Furthermore, we determined marked repression with ticagrelor treatment in the increased number of autophagosomes and degeneration of mitochondrion, including swelling and loss of crista besides recoveries in enlargement and irregularity seen in SER in insulin-resistant H9c2 cells. Moreover, ticagrelor treatment could prevent the altered mRNA levels of Becklin-1 and type 1 equilibrative nucleoside transporter (ENT1), which are parallel to the preservation of ultrastructural ones. Our overall data demonstrated that ticagrelor can directly affect cardiomyocytes and provide marked protection against ER stress and dramatic induction of autophagosomes, and therefore, can alleviate the ER stress-induced oxidative stress increase and cell apoptosis during insulin resistance.

Role of GRK6 in the Regulation of Platelet Activation through Selective G Protein-Coupled Receptor (GPCR) Desensitization

Platelet G protein-coupled receptors (GPCRs) regulate platelet function by mediating the response to various agonists, including adenosine diphosphate (ADP), thromboxane A₂, and thrombin. Although GPCR kinases (GRKs) are considered to have the crucial roles in most GPCR functions, little is known regarding the regulation of GPCR signaling and mechanisms of GPCR desensitization by GRKs in platelets. In this study, we investigated the functional role of GRK6 and the molecular basis for regulation of specific GPCR desensitization by GRK6 in platelets. We used GRK6 knockout mice to evaluate the functional role of GRK6 in platelet activation. Platelet aggregation, dense- and α-granule secretion, and fibrinogen receptor activation induced by 2-MeSADP, U46619, thrombin, and AYPGKF were significantly potentiated in GRK6^−/− platelets compared to the wild-type (WT) platelets. However, collagen-related peptide (CRP)-induced platelet aggregation and secretion were not affected in GRK6^−/− platelets. Interestingly, platelet aggregation induced by co-stimulation of serotonin and epinephrine which activate G_q-coupled 5HT_2A and G_z-coupled α_2A adrenergic receptors, respectively, was not affected in GRK6^−/− platelets, suggesting that GRK6 was involved in specific GPCR regulation. In addition, platelet aggregation in response to the second challenge of ADP and AYPGKF was restored in GRK6^−/− platelets whereas re-stimulation of the agonist failed to induce aggregation in WT platelets, indicating that GRK6 contributed to P2Y₁, P2Y₁₂, and PAR4 receptor desensitization. Furthermore, 2-MeSADP-induced Akt phosphorylation and AYPGKF-induced Akt, extracellular signal-related kinase (ERK), and protein kinase Cδ (PKCδ) phosphorylation were significantly potentiated in GRK6^−/− platelets. Finally, GRK6^−/− mice exhibited an enhanced and stable thrombus formation after FeCl₃ injury to the carotid artery and shorter tail bleeding times, indicating that GRK6^−/− mice were more susceptible to thrombosis and hemostasis. We conclude that GRK6 plays an important role in regulating platelet functional responses and thrombus formation through selective GPCR desensitization.

Effects of Antiplatelet and Nonsteroidal Anti-inflammatory Medications on Platelet-Rich Plasma: A Systematic Review

Background:

Platelet-rich plasma (PRP) has wide applications in orthopaedic care. Its beneficial effects are attributed to the growth factor profile from the platelet secretome. In theory, these effects would be diminished by medications that inhibit platelet activation and/or the subsequent release of growth factors.

Purpose:

To determine whether commonly used antiplatelets, nonsteroidal anti-inflammatory drugs (NSAIDs), or anticoagulant medications affect platelet growth factor release in PRP.

Study Design:

Systematic review; Level of evidence, 2.

Method:

A systematic review of the literature related to antiplatelet, anti-inflammatory, and anticoagulant drugs was performed following the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. We used the Downs and Black objective quality scoring system. The literature search consisted of PubMed and Cochrane Library databases. Search terms consisted of 1 item selected from “platelet-rich plasma,” “platelet-derived growth factor,” and “platelet-rich plasma AND growth factor” combined with 1 item from “antiplatelet,” “aspirin,” “anticoagulant,” and “NSAID.” Only studies published within the past 25 years were included.

Results:

A total of 15 studies met the inclusion criteria: 7 studies detected no significant decrease in growth factors or mitogenesis, whereas 6 detected a decrease with antiplatelet agents, 1 detected mixed results with an antiplatelet agent, and 1 had mixed results with an antiplatelet agent/vasodilator. In terms of PRP activation, all 3 studies assessing collagen, the 2 studies analyzing adenosine diphosphate alone, and the 1 study investigating arachidonic acid found a decrease in growth factor concentration.

Conclusion:

Antiplatelet medications may decrease the growth factor release profile in a cyclooxygenase 1– and cyclooxygenase 2–dependent manner. Eight of 15 studies found a decrease in growth factors or mitogenesis. However, more studies are needed to comprehensively understand antiplatelet effects on the PRP secretome.

Platelet activation and prothrombotic mediators at the nexus of inflammation and atherosclerosis: Potential role of antiplatelet agents

Platelets are central to inflammation-related manifestations of cardiovascular diseases (CVD) such as atherosclerosis. Platelet-activating factor (PAF), thrombin, thromboxane A₂ (TxA₂), and adenosine diphosphate (ADP) are some of the key agonists of platelet activation that are at the intersection between a plethora of inflammatory pathways that modulate pro-inflammatory and coagulation processes. The aim of this article is to review the role of platelets and the relationship between their structure, function, and the interactions of their constituents in systemic inflammation and atherosclerosis. Antiplatelet therapies are discussed with a view to primary prevention of CVD by the clinical reduction of platelet reactivity and inflammation. Current antiplatelet therapies are effective in reducing cardiovascular risk but increase bleeding risk. Novel therapeutic antiplatelet approaches beyond current pharmacological modalities that do not increase the risk of bleeding require further investigation. There is potential for specifically designed nutraceuticals that may become safer alternatives to pharmacological antiplatelet agents for the primary prevention of CVD but there is serious concern over their efficacy and regulation, which requires considerably more research.

Apyrase with anti-platelet aggregation activity from the nymph of the camel tick Hyalomma dromedarii

Apyrase is one of the essential platelet aggregation inhibitors in hematophagous arthropods due to its ability to hydrolyze ATP and ADP molecules. Here, an apyrase (TNapyrase) with antiplatelet aggregation activity was purified and characterized from the nymphs of the camel tick Hyalomma dromedarii through anion exchange and gel filtration columns. The homogeneity of TNapyrase was confirmed by native-PAGE, SDS-PAGE as well as with isoelectric focusing. Purified TNapyrase had a molecular mass of 25 kDa and a monomer structure. TNapyrase hydrolyzed various nucleotides in the order of ATP > PPi > ADP > UDP > 6GP. The K_m value was 1.25 mM ATP and its optimum activity reached at pH 8.4. The influence of various ions on TNapyrase activity showed that FeCl₂, FeCl₃ and ZnCl₂ are activators of TNapyrase. EDTA inhibited TNapyrase activity competitively with a single binding site on the molecule and K_i value of 2 mM. Finally, TNapyrase caused 70% inhibition of ADP-stimulated platelets aggregation and is a possible target for antibodies in future tick vaccine studies.

Platelets: emerging facilitators of cellular crosstalk in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease in which a variety of circulating pro-inflammatory cells and dysregulated molecules are involved in disease aetiology and progression. Platelets are an important cellular element in the circulation that can bind several dysregulated molecules (such as collagen, thrombin and fibrinogen) that are present both in the synovium and the circulation of patients with RA. Platelets not only respond to dysregulated molecules in their environment but also transport and express their own inflammatory mediators, and serve as regulators at the boundary between haemostasis and immunity. Activated platelets also produce microparticles, which further convey signalling molecules and receptors to the synovium and circulation, thereby positioning these platelet-derived particles as strategic regulators of inflammation. These diverse functions come together to make platelets facilitators of cellular crosstalk in RA. Thus, the receptor functions, ligand binding potential and dysregulated signalling pathways in platelets are becoming increasingly important for treatment in RA. This Review aims to highlight the role of platelets in RA and the need to closely examine platelets as health indicators when designing effective pharmaceutical targets in this disease.

Novel Antiplatelet Agents in Cardiovascular Disease

Antiplatelet therapy reduces atherothrombotic risk and has therefore become a cornerstone in the treatment of cardiovascular disease. Aspirin, adenosine diphosphate P2Y12 receptor antagonists, glycoprotein IIb/IIIa inhibitors, and the thrombin receptor blocker vorapaxar are effective antiplatelet agents but significantly increase the risk of bleeding. Moreover, atherothrombotic events still impair the prognosis of many patients with cardiovascular disease despite established antiplatelet therapy. Over the last years, advances in the understanding of thrombus formation and hemostasis led to the discovery of various new receptors and signaling pathways of platelet activation. As a consequence, many new antiplatelet agents with high antithrombotic efficacy and supposedly only moderate effects on regular hemostasis have been developed and yielded promising results in preclinical and early clinical studies. Although their long journey from animal studies to randomized clinical trials and finally administration in daily clinical routine has just begun, some of the new agents may in the future become meaningful additions to the pharmacological armamentarium in cardiovascular disease.

Ca2+ waves coordinate purinergic receptor-evoked integrin activation and polarization

Cells sense extracellular nucleotides through the P2Y class of purinergic G protein-coupled receptors (GPCRs), which stimulate integrin activation through signaling events, including intracellular Ca²⁺ mobilization. We investigated the relationship between P2Y-stimulated repetitive Ca²⁺ waves and fibrinogen binding to the platelet integrin α_IIbβ₃ (GPIIb/IIIa) through confocal fluorescence imaging of primary rat megakaryocytes. Costimulation of the receptors P2Y₁ and P2Y₁₂ generated a series of Ca²⁺ transients that each induced a rapid, discrete increase in fibrinogen binding. The peak and net increase of individual fibrinogen binding events correlated with the Ca²⁺ transient amplitude and frequency, respectively. Using BAPTA loading and selective receptor antagonists, we found that Ca²⁺ mobilization downstream of P2Y₁ was essential for ADP-evoked fibrinogen binding, whereas P2Y₁₂ and the kinase PI3K were also required for α_IIbβ₃ activation and enhanced the number of Ca²⁺ transients. ADP-evoked fibrinogen binding was initially uniform over the cell periphery but subsequently redistributed with a polarity that correlated with the direction of the Ca²⁺ waves. Polarization of α_IIbβ₃ may be mediated by the actin cytoskeleton, because surface-bound fibrinogen is highly immobile, and its motility was enhanced by cytoskeletal disruption. In conclusion, spatial and temporal patterns of Ca²⁺ increase enable fine control of α_IIbβ₃ activation after cellular stimulation. P2Y₁-stimulated Ca²⁺ transients coupled to α_IIbβ₃ activation only in the context of P2Y₁₂ coactivation, thereby providing an additional temporal mechanism of synergy between these Gq- and Gi-coupled GPCRs.

Dye D, Falasca M, Metharom P. Antiplatelet Drug Ticagrelor Enhances Chemotherapeutic Efficacy by Targeting the Novel P2Y12-AKT Pathway in Pancreatic Cancer Cells

Background: Extensive research has reported that extracellular ADP in the tumour microenvironment can stimulate platelets through interaction with the platelet receptor P2Y12. In turn, activated platelets release biological factors supporting cancer progression. Experimental data suggest that the tumour microenvironment components, of which platelets are integral, can promote chemotherapy resistance in pancreatic ductal adenocarcinoma (PDAC). Thus, overcoming chemoresistance requires combining multiple inhibitors that simultaneously target intrinsic pathways in cancer cells and extrinsic factors related to the tumour microenvironment. We aimed to determine whether ticagrelor, an inhibitor of the ADP–P2Y12 axis and a well-known antiplatelet drug, could be a therapeutic option for PDAC. Methods: We investigated a functional P2Y12 receptor and its downstream signalling in a panel of PDAC cell lines and non-cancer pancreatic cells termed hTERT-HPNE. We tested the synergistic effect of ticagrelor, a P2Y12 inhibitor, in combination with chemotherapeutic drugs (gemcitabine, paclitaxel and cisplatin), in vitro and in vivo. Results: Knockdown studies revealed that P2Y12 contributed to epidermal growth factor receptor (EGFR) activation and the expression of SLUG and ZEB1, which are transcriptional factors implicated in metastasis and chemoresistance. Studies using genetic and pharmacological inhibitors showed that the P2Y12–EGFR crosstalk enhanced cancer cell proliferation. Inhibition of P2Y12 signalling significantly reduced EGF-dependent AKT activation and promoted the anticancer activity of anti-EGFR treatment. Importantly, ticagrelor significantly decreased the proliferative capacity of cancer but not normal pancreatic cells. In vitro, synergism was observed when ticagrelor was combined with several chemodrugs. In vivo, a combination of ticagrelor with gemcitabine significantly reduced tumour growth, whereas gemcitabine or ticagrelor alone had a minimal effect. Conclusions: These findings uncover a novel effect and mechanism of action of the antiplatelet drug ticagrelor in PDAC cells and suggest a multi-functional role for ADP-P2Y12 signalling in the tumour microenvironment.

Heterotrimeric G Proteins as Therapeutic Targets in Drug Discovery

Heterotrimeric G proteins are molecular switches in GPCR signaling pathways and regulate a plethora of physiological and pathological processes. GPCRs are efficient drug targets, and more than 30% of the drugs in use target them. However, selectively targeting an individual GPCR may be undesirable in various multifactorial diseases in which multiple receptors are involved. In addition, abnormal activation or expression of G proteins is frequently associated with diseases. Furthermore, G proteins harboring mutations often result in malignant diseases. Thus, targeting G proteins instead of GPCRs might provide alternative approaches for combating these diseases. In this review, we discuss the biochemistry of heterotrimeric G proteins, describe the G protein-associated diseases, and summarize the currently known modulators that can regulate the activities of G proteins. The outlook for targeting G proteins to treat diverse diseases is also included in this manuscript.

Does the Lipid Bilayer Orchestrate Access and Binding of Ligands to Transmembrane Orthosteric/Allosteric Sites of G Protein-Coupled Receptors?

The ligand-binding sites of many G protein-coupled receptors (GPCRs) are situated around and deeply embedded within the central pocket formed by their seven transmembrane-spanning α-helical domains. Generally, these binding sites are assumed accessible to endogenous ligands from the aqueous phase. Recent advances in the structural biology of GPCRs, along with biophysical and computational studies, suggest that amphiphilic and lipophilic molecules may gain access to these receptors by first partitioning into the membrane and then reaching the binding site via lateral diffusion through the lipid bilayer. In addition, several crystal structures of class A and class B GPCRs bound to their ligands offer unprecedented details on the existence of lipid-facing allosteric binding sites outside the transmembrane helices that can only be reached via lipid pathways. The highly organized structure of the lipid bilayer may direct lipophilic or amphiphilic drugs to a specific depth within the bilayer, changing local concentration of the drug near the binding site and affecting its binding kinetics. Additionally, the constraints of the lipid bilayer, including its composition and biophysical properties, may play a critical role in "pre-organizing" ligand molecules in an optimal orientation and conformation to facilitate receptor binding. Despite its clear involvement in molecular recognition processes, the critical role of the membrane in binding ligands to lipid-exposed transmembrane binding sites remains poorly understood and warrants comprehensive investigation. Understanding the mechanistic basis of the structure-membrane interaction relationship of drugs will not only provide useful insights about receptor binding kinetics but will also enhance our ability to take advantage of the apparent membrane contributions when designing drugs that target transmembrane proteins with improved efficacy and safety. In this minireview, we summarize recent structural and computational studies on membrane contributions to binding processes, elucidating both lipid pathways of ligand access and binding mechanisms for several orthosteric and allosteric ligands of class A and class B GPCRs.

Disrupted hippocampal growth hormone secretagogue receptor 1α interaction with dopamine receptor D1 plays a role in Alzheimer's disease

Hippocampal lesions are a defining pathology of Alzheimer's disease (AD). However, the molecular mechanisms that underlie hippocampal synaptic injury in AD have not been fully elucidated. Current therapeutic efforts for AD treatment are not effective in correcting hippocampal synaptic deficits. Growth hormone secretagogue receptor 1α (GHSR1α) is critical for hippocampal synaptic physiology. Here, we report that GHSR1α interaction with β-amyloid (Aβ) suppresses GHSR1α activation, leading to compromised GHSR1α regulation of dopamine receptor D1 (DRD1) in the hippocampus from patients with AD. The simultaneous application of the selective GHSR1α agonist MK0677 with the selective DRD1 agonist SKF81297 rescued Ghsr1α function from Aβ inhibition, mitigating hippocampal synaptic injury and improving spatial memory in an AD mouse model. Our data reveal a mechanism of hippocampal vulnerability in AD and suggest that a combined activation of GHSR1α and DRD1 may be a promising approach for treating AD.

Antiplatelet activity of deferiprone through cyclooxygenase-1 inhibition

Thalassemia patients are susceptible to both iron overload and thromboembolism. Deferiprone is an iron chelator that shows an antiplatelet activity and thus may alleviate platelet hyperactivation in thalassemia. Therefore, this study aimed to characterize the inhibitory effects and mechanisms of deferiprone on normal human platelets. The results illustrated that deferiprone inhibited platelet aggregation at the iron chelating concentrations (0.08-0.25 mmol/l). Deferiprone inhibited human platelet aggregation stimulated by arachidonic acid and ADP more potently than epinephrine and collagen, with the IC₅₀ of 0.24 mmol/l and 0.25 mmol/l vs. 3.36 mmol/l and 3.73 mmol/l, respectively. Interestingly, deferiprone significantly inhibited COX-1 activity, with the IC₅₀ of 0.33 mmol/l, and slightly increased cAMP level at the high concentration of 4 mmol/l. Moreover, the results from molecular docking showed that deferiprone interacted closely with key residues in the peroxidase active site of COX-1. These results suggested that deferiprone possessed antiplatelet activity mainly through the inhibition of COX-1 activity.

Emotional Blunting, Cognitive Impairment, Bone Fractures, and Bleeding as Possible Side Effects of Long-Term Use of SSRIs

OBJECTIVE

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are amongst the most prescribed drugs worldwide not only for psychiatric conditions, but also for medical purposes. Converging data gathered throughout the decades following their development would indicate that SSRIs have a broader side effect profile than previously assumed. Therefore, the aim of the present paper was to to review available literature highlighting less common side effects emerging with their long-term use.

METHOD

This systematic review, carried out according to PRISMA guidelines, was performed through searching electronic databases of PubMed, Google Scholar, Cochrane Library, Embase, MEDLINE, PsycINFO and Scopus. The keyword used was "SSRIs" combined with the following: "Side effects", or "Emotional blunting or flattening", or "Cognition", or "Neuroimaging", or "Bone", "or "Platelet aggregation", or "Bleeding".

RESULTS

The most common side effects, besides the classical ones described in the literature are represented by decreased emotional response to both adversive and pleasurable events, some cognitive impairments, bone fractures and prolonged overall bleeding time.

CONCLUSIONS

After analyzing critically the available findings, it should be noted that only the so-called "emotional blunting" is supported by converging data, while results on cognitive impairment are extremely controversial, given some evidence showing that SSRIs may improve cognition. Similarly, no agreement exists on the detrimental effects of SSRIs on bone metabolism and coagulation.Large, prospective and long-term studies are needed to clarify the possible impact of SSRIs on emotions, cognitive functions, bone fractures and coagulation, as well to detect other possible still neglected side effects.

Adrenoceptor α2A signalling countervails the taming effects of synchronous cyclic nucleotide-elevation on thrombin-induced human platelet activation and aggregation

The healthy vascular endothelium constantly releases autacoids which cause an increase of intracellular cyclic nucleotides to tame platelets from inappropriate activation. Elevating cGMP and cAMP, in line with previous reports, cooperated in the inhibition of isolated human platelet intracellular calcium-mobilization, dense granules secretion, and aggregation provoked by thrombin. Further, platelet alpha granules secretion and, most relevant, integrin α_IIaβ₃ activation in response to thrombin are shown to be prominently affected by the combined elevation of cGMP and cAMP. Since stress-related sympathetic nervous activity is associated with an increase in thrombotic events, we investigated the impact of epinephrine in this setting. We found that the assessed signalling events and functional consequences were to various extents restored by epinephrine, resulting in full and sustained aggregation of isolated platelets. The restoring effects of epinephrine were abolished by either interfering with intracellular calcium-elevation or with PI3-K signalling. Finally, we show that in our experimental setting epinephrine likewise reconstitutes platelet aggregation in heparinized whole blood, which may indicate that this mechanism could also apply in vivo.

Responses of Plasma Catecholamine, Serotonin, and the Platelet Serotonin Transporter to Cigarette Smoking

Cigarette smoking is one of the major causes of coronary heart disease with a thirty percent mortality rate in the United States. Cigarette smoking acting on the central nervous system (CNS) to stimulate the sympathetic nervous system (SNS) through, which facilitates the secretion of serotonin (5-HT) and catecholamines to supraphysiological levels in blood. The enhanced levels of 5-HT and catecholamines in smokers’ blood are associated with increases in G protein-coupled receptor signaling and serotonylation of small GTPases, which in turn lead to remodeling of cytoskeletal elements to enhance granule secretion and promote unique expression of sialylated N-glycan structures on smokers’ platelets. These mechanisms enhance aggregation and adhesion of smokers’ platelets relative to those of non-smokers. This review focuses on the known mechanisms by which 5-HT and SERT, in coordinated signaling with catecholamines, impacts cigarette smokers’ platelet biology.

Characterization of the distinct mechanism of agonist-induced canine platelet activation

Platelet activation has a major role in hemostasis and thrombosis. Various agonists including adenosine diphosphate (ADP) and thrombin interact with G protein-coupled receptors (GPCRs) which transduce signals through various G proteins. Recent studies have elucidated the role of GPCRs and their corresponding G proteins in the regulation of events involved in platelet activation. However, agonist-induced platelet activation in companion animals has not been elucidated. This study was designed to characterize the platelet response to various agonists in dog platelets. We found that 2-methylthio-ADP-induced dog platelet aggregation was blocked in the presence of either P2Y₁ receptor antagonist MRS2179 or P2Y₁₂ receptor antagonist AR-C69931MX, suggesting that co-activation of both the P2Y₁ and P2Y₁₂ receptors is required for ADP-induced platelet aggregation. Thrombin-induced dog platelet aggregation was inhibited in the presence of either AR-C69931MX or the PKC inhibitor GF109203X, suggesting that thrombin requires secreted ADP to induce platelet aggregation in dog platelets. In addition, thrombin-mediated Akt phosphorylation was inhibited in the presence of GF109203X or AR-C69931MX, indicating that thrombin causes Gi stimulation through the P2Y₁₂ receptor by secreted ADP in dog platelets. Unlike human and murine platelets, protease-activated receptor 4 (PAR4)-activating peptide AYPGKF failed to cause dog platelet aggregation. Moreover, PAR1-activating peptide SFLLRN or co-stimulation of SFLLRN and AYPGKF failed to induce dog platelet aggregation. We conclude that ADP induces platelet aggregation through the P2Y₁ and P2Y₁₂ receptors in dogs. Unlike human and murine platelets, selective activation of the PAR4 receptor may be insufficient to cause platelet aggregation in dog platelets.