P2Y1 and P2Y12 receptors for ADP desensitize by distinct kinase-dependent mechanisms

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.

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.

Stimulation of platelet P2Y1 receptors by different endogenous nucleotides leads to functional selectivity via biased signalling

BACKGROUND AND PURPOSE

Platelet function during inflammation is dependent on activation by endogenous nucleotides. Non-canonical signalling via the P2Y1 receptor is important for these non-thrombotic functions of platelets. However, apart from ADP, the role of other endogenous nucleotides acting as agonists at P2Y1 receptors is unknown. This study compared the effects of ADP, Ap3A, NAD+ , ADP-ribose, and Up4A on platelet functions contributing to inflammation or haemostasis.

EXPERIMENTAL APPROACH

Platelets obtained from healthy human volunteers were incubated with ADP, Ap3A, NAD+ , ADP-ribose, or Up4A, with aggregation and fibrinogen binding measured (examples of function during haemostasis) or before exposure to fMLP to measure platelet chemotaxis (an inflammatory function). In silico molecular docking of these nucleotides to the binding pocket of P2Y1 receptors was then assessed.

KEY RESULTS

Platelet aggregation and binding to fibrinogen induced by ADP was not mimicked by NAD+ , ADP-ribose, and Up4A. However, these endogenous nucleotides induced P2Y1 -dependent platelet chemotaxis, an effect that required RhoA and Rac-1 activity, but not canonical PLC activity. Analysis of molecular docking of the P2Y1 receptor revealed distinct differences of amino acid interactions and depth of fit within the binding pocket for Ap3A, NAD+ , ADP-ribose, or Up4A compared with ADP.

CONCLUSION AND IMPLICATIONS

Platelet function (aggregation vs motility) can be differentially modulated by biased-agonist activation of P2Y1 receptors. This may be due to the character of the ligand-binding pocket interaction. This has implications for future therapeutic strategies aimed to suppress platelet activation during inflammation without affecting haemostasis as is the requirement of current ant-platelet drugs.

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.

Ticagrelor inverse agonist activity at the P2Y12 receptor is non-reversible versus its endogenous agonist adenosine 5´-diphosphate

BACKGROUND AND PURPOSE

Ticagrelor is labelled as a reversible, direct-acting platelet P2Y12 receptor (P2Y12 R) antagonist that is indicated clinically for the prevention of thrombotic events in patients with acute coronary syndrome (ACS). As with many antiplatelet drugs, ticagrelor therapy increases bleeding risk in patients, which may require platelet transfusion in emergency situations. The aim of this study was to further examine the reversibility of ticagrelor at the P2Y12 R.

EXPERIMENTAL APPROACH

Studies were performed in human platelets, with P2Y12 R-stimulated GTPase activity and platelet aggregation assessed. Cell-based bioluminescence resonance energy transfer (BRET) assays were undertaken to assess G protein-subunit activation downstream of P2Y12 R activation.

KEY RESULTS

Initial studies revealed that a range of P2Y12 R ligands, including ticagrelor, displayed inverse agonist activity at P2Y12 R. Only ticagrelor was resistant to washout and, in human platelet and cell-based assays, washing failed to reverse ticagrelor-dependent inhibition of ADP-stimulated P2Y12 R function. The P2Y12 R agonist 2MeSADP, which was also resistant to washout, was able to effectively compete with ticagrelor. In silico docking revealed that ticagrelor and 2MeSADP penetrated more deeply into the orthosteric binding pocket of the P2Y12 R than other P2Y12 R ligands.

CONCLUSION AND IMPLICATIONS

Ticagrelor binding to P2Y12 R is prolonged and more akin to that of an irreversible antagonist, especially versus the endogenous P2Y12 R agonist ADP. This study highlights the potential clinical need for novel ticagrelor reversal strategies in patients with spontaneous major bleeding, and for bleeding associated with urgent invasive procedures.

The Signaling Pathway of the ADP Receptor P2Y12 in the Immune System: Recent Discoveries and New Challenges

P2Y12 is a G-protein-coupled receptor that is activated upon ADP binding. Considering its well-established role in platelet activation, blocking P2Y12 has been used as a therapeutic strategy for antiplatelet aggregation in cardiovascular disease patients. However, receptor studies have shown that P2Y12 is functionally expressed not only in platelets and the microglia but also in other cells of the immune system, such as in monocytes, dendritic cells, and T lymphocytes. As a result, studies were carried out investigating whether therapies targeting P2Y12 could also ameliorate inflammatory conditions, such as sepsis, rheumatoid arthritis, neuroinflammation, cancer, COVID-19, atherosclerosis, and diabetes-associated inflammation in animal models and human subjects. This review reports what is known about the expression of P2Y12 in the cells of the immune system and the effect of P2Y12 activation and/or inhibition in inflammatory conditions. Lastly, we will discuss the major problems and challenges in studying this receptor and provide insights on how they can be overcome.

The Function and Regulation of Platelet P2Y12 Receptor

In addition to the key role in hemostasis and thrombosis, platelets have also been wildly acknowledged as immune regulatory cells and involving in the pathogenesis of inflammation-related diseases. Since purine receptor P2Y12 plays a crucial role in platelet activation, P2Y12 antagonists such as clopidogrel, prasugrel, and ticagrelor have been widely used in cardiovascular diseases worldwide in recent decades due to their potent antiplatelet and antithrombotic effects. Meanwhile, the role of P2Y12 in inflammatory diseases has also been extensively studied. Relatively, there are few studies on the regulation of P2Y12. This review first summarizes the various roles of P2Y12 in the process of platelet activation, as well as downstream effects and signaling pathways; then introduces the effects of P2Y12 in inflammatory diseases such as sepsis, atherosclerosis, cancer, autoimmune diseases, and asthma; and finally reviews the current researches on P2Y12 regulation.

Hydroxyls on the B ring and gallic acyl are essential for catechins to restrain ADP-induced thrombosis

Platelet hyperactivation could lead to various cardiovascular and cerebrovascular diseases, while epidemiological analyses have found that long-term tea drinking could prevent and restrain cardiovascular diseases. Existing studies have shown that catechins, especially epigallocatechin gallate (EGCG), are the main functional factors of tea in alleviating thrombosis, which could inhibit arterial thrombosis and platelet aggregation induced by a variety of agonists. However, their structure-activity relationship and the underlying mechanisms are still unclear. Based on the above background, this study took six typical catechins as research objects, constructed platelet activation models with different inducers, and explored the inhibitory effects and potential mechanisms of catechins with different structures on platelet aggregation through flow cytometry, immunoblotting, cell spreading, and other experiments. It was found that ester catechins could inhibit platelet aggregation induced by adenosine diphosphate (ADP), while epigallocatechin (EGC) with three hydroxyls on the B ring in non-ester catechins was also able to effectively inhibit platelet aggregation. Our data suggested that gallic acyl on the C ring and three hydroxyls on the B ring were the main functional groups affecting the antithrombotic effect of catechins, and the effect of gallic acyl on platelets was significantly stronger than that of the hydroxyl.

ADP receptor P2Y12 is the capstone of the cross-talk between Ca2+ mobilization pathways dependent on Ca2+ ATPases sarcoplasmic/endoplasmic reticulum type 3 and type 2b in platelets

Background

Blood platelet Ca²⁺ stores are regulated by 2 Ca²⁺-ATPases (SERCA2b and SERCA3). On thrombin stimulation, nicotinic acid adenosine dinucleotide phosphate mobilizes SERCA3-dependent stores, inducing early adenosine 5'-diphosphate (ADP) secretion, potentiating later SERCA2b-dependent secretion.

Objectives

The aim of this study was to identify which ADP P2 purinergic receptor (P2Y1 and/or P2Y12) is(are) involved in the amplification of platelet secretion dependent on the SERCA3-dependent Ca²⁺ mobilization pathway (SERCA3 stores mobilization) as triggered by low concentration of thrombin.

Methods

The study used the pharmacologic antagonists MRS2719 and AR-C69931MX, of the P2Y1 and P2Y12, respectively, as well as Serca3 ^-/- mice and mice exhibiting platelet lineage-specific inactivation of the P2Y1 or P2Y12 genes.

Results

We found that in mouse platelets, pharmacological blockade or gene inactivation of P2Y12 but not of P2Y1 led to a marked inhibition of ADP secretion after platelet stimulation with low concentration of thrombin. Likewise, in human platelets, pharmacological inhibition of P2Y12 but not of P2Y1 alters amplification of thrombin-elicited secretion through SERCA2b stores mobilization. Finally, we show that early SERCA3 stores secretion of ADP is a dense granule secretion, based on parallel adenosine triphosphate and serotonin early secretion. Furthermore, early secretion involves a single granule, based on the amount of adenosine triphosphate released.

Conclusion

Altogether, these results show that at low concentrations of thrombin, SERCA3- and SERCA2b-dependent Ca²⁺ mobilization pathways cross-talk via ADP and activation of the P2Y12, and not the P2Y1 ADP receptor. The relevance in hemostasis of the coupling of the SERCA3 and the SERCA2b pathways is reviewed.

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.

G protein-coupled receptor kinase 5 regulates thrombin signaling in platelets via PAR-1

The interindividual variation in the functional response of platelets to activation by agonists is heritable. Genome-wide association studies (GWASs) of quantitative measures of platelet function have identified fewer than 20 distinctly associated variants, some with unknown mechanisms. Here, we report GWASs of pathway-specific functional responses to agonism by adenosine 5'-diphosphate, a glycoprotein VI-specific collagen mimetic, and thrombin receptor-agonist peptides, each specific to 1 of the G protein-coupled receptors PAR-1 and PAR-4, in subsets of 1562 individuals. We identified an association (P = 2.75 × 10-40) between a common intronic variant, rs10886430, in the G protein-coupled receptor kinase 5 gene (GRK5) and the sensitivity of platelets to activate through PAR-1. The variant resides in a megakaryocyte-specific enhancer that is bound by the transcription factors GATA1 and MEIS1. The minor allele (G) is associated with fewer GRK5 transcripts in platelets and the greater sensitivity of platelets to activate through PAR-1. We show that thrombin-mediated activation of human platelets causes binding of GRK5 to PAR-1 and that deletion of the mouse homolog Grk5 enhances thrombin-induced platelet activation sensitivity and increases platelet accumulation at the site of vascular injury. This corroborates evidence that the human G allele of rs10886430 is associated with a greater risk for cardiovascular disease. In summary, by combining the results of pathway-specific GWASs and expression quantitative trait locus studies in humans with the results from platelet function studies in Grk5-/- mice, we obtain evidence that GRK5 regulates the human platelet response to thrombin via the PAR-1 pathway.

Platelets after burn injury - hemostasis and beyond

Burn injuries are common and often life-threatening trauma. With this trauma comes an interruption of normal hemostasis, with distinct impacts on platelets. Our interest in the relationships between burn injury and platelet function stems from two key perspectives: platelet function is a vital component of acute responses to injury, and furthermore the incidence of cardiovascular disease (CVD) is higher in burn survivors compared to the general population. This review explores the impact of burn injury on coagulation, platelet function, and the participation of platelets in immunopathology. Potential avenues of further research are explored, and consideration is given to what therapies may be appropriate for mediating post-burn thrombopathology.

The Predominant Role of Arrestin3 in General GPCR Desensitization in Platelets

Arrestins in concert with GPCR kinases (GRKs) function in G protein-coupled receptor (GPCR) desensitization in various cells. Therefore, we characterized the functional differences of arrestin3 versus arrestin2 in the regulation of GPCR signaling and its desensitization in platelets using mice lacking arrestin3 and arrestin2. In contrast to arrestin2, platelet aggregation and dense granule secretion induced by 2-MeSADP, U46619, thrombin, and AYPGKF were significantly potentiated in arrestin3-deficient platelets compared to wild-type (WT) platelets, while non-GPCR agonist CRP-induced platelet aggregation and secretion were not affected. Surprisingly, in contrast to GRK6, platelet aggregation induced by the co-stimulation of serotonin and epinephrine was significantly potentiated in arrestin3-deficient platelets, suggesting the central role of arrestin3 in general GPCR desensitization in platelets. In addition, the second challenge of ADP and AYPGKF restored platelet aggregation in arrestin3-deficient platelets but failed to do so in WT and arrestin2-deficient platelets, confirming that arrestin3 contributes to GPCR desensitization. Furthermore, ADP- and AYPGKF-induced Akt and ERK phosphorylation were significantly increased in arrestin3-deficient platelets. Finally, we found that arrestin3 is critical for thrombus formation in vivo. In conclusion, arrestin3, not arrestin2, plays a central role in the regulation of platelet functional responses and thrombus formation through general GPCR desensitization in platelets.

A review of non-prostanoid, eicosanoid receptors: expression, characterization, regulation, and mechanism of action

Eicosanoid signaling controls a wide range of biological processes from blood pressure homeostasis to inflammation and resolution thereof to the perception of pain and to cell survival itself. Disruption of normal eicosanoid signaling is implicated in numerous disease states. Eicosanoid signaling is facilitated by G-protein-coupled, eicosanoid-specific receptors and the array of associated G-proteins. This review focuses on the expression, characterization, regulation, and mechanism of action of non-prostanoid, eicosanoid receptors.

Pharmacokinetic/pharmacodynamic modeling of drug interactions at the P2Y12 receptor between selatogrel and oral P2Y12 antagonists

Selatogrel is a potent and reversible P2Y₁₂ receptor antagonist developed for subcutaneous self‐administration by patients with suspected acute myocardial infarction. After single‐dose emergency treatment with selatogrel, patients are switched to long‐term treatment with oral P2Y₁₂ receptor antagonists. Selatogrel shows rapid onset and offset of inhibition of platelet aggregation (IPA) to overcome the critical initial time after acute myocardial infarction. Long‐term benefit is provided by oral P2Y₁₂ receptor antagonists such as clopidogrel, prasugrel, and ticagrelor. A population pharmacokinetic (PK)/pharmacodynamic (PD) model based on data from 545 subjects in 4 phase I and 2 phase II studies well described the effect of selatogrel on IPA alone and in combination with clopidogrel, prasugrel, and ticagrelor. The PK of selatogrel were described by a three‐compartment model. The PD model included a receptor‐pool compartment to which all drugs can bind concurrently, reversibly or irreversibly, depending on their mode of action. Furthermore, ticagrelor and its active metabolite can bind to the selatogrel‐receptor complex allosterically, releasing selatogrel from the binding site. The model provided a framework for predicting the effect on IPA of selatogrel followed by reversibly and irreversibly binding oral P2Y₁₂ receptor antagonists for sustained effects. Determining the timepoint for switching from emergency to maintenance treatment is critical to achieve sufficient IPA at all times. Simulations based on the interaction model showed that loading doses of clopidogrel and prasugrel administered 15 h and 4.5 h after selatogrel, respectively, provide sustained IPA with clinically negligible drug interaction.

GRK6 regulates the hemostatic response to injury through its rate-limiting effects on GPCR signaling in platelets

G protein-coupled receptors (GPCRs) mediate the majority of platelet activation in response to agonists. However, questions remain regarding the mechanisms that provide negative feedback toward activated GPCRs to limit platelet activation and thrombus formation. Here we provide the first evidence that GPCR kinase 6 (GRK6) serves this role in platelets, using GRK6-/- mice generated by CRISPR-Cas9 genome editing to examine the consequences of GRK6 knockout on GPCR-dependent signaling. Hemostatic thrombi formed in GRK6-/- mice are larger than in wild-type (WT) controls during the early stages of thrombus formation, with a rapid increase in platelet accumulation at the site of injury. GRK6-/- platelets have increased platelet activation, but in an agonist-selective manner. Responses to PAR4 agonist or adenosine 5'-diphosphate stimulation in GRK6-/- platelets are increased compared with WT littermates, whereas the response to thromboxane A2 (TxA2) is normal. Underlying these changes in GRK6-/- platelets is an increase in Ca2+ mobilization, Akt activation, and granule secretion. Furthermore, deletion of GRK6 in human MEG-01 cells causes an increase in Ca2+ response and PAR1 surface expression in response to thrombin. Finally, we show that human platelet activation in response to thrombin causes an increase in binding of GRK6 to PAR1, as well as an increase in the phosphorylation of PAR1. Deletion of GRK6 in MEG-01 cells causes a decrease in PAR1 phosphorylation. Taken together, these data show that GRK6 regulates the hemostatic response to injury through PAR- and P2Y12-mediated effects, helping to limit the rate of platelet activation during thrombus growth and prevent inappropriate platelet activation.

The GRKs Reactome: Role in Cell Biology and Pathology

G protein-coupled receptor kinases (GRKs) are protein kinases that function in concert with arrestins in the regulation of a diverse class of G protein-coupled receptors (GPCRs) signaling. Although GRKs and arrestins are key participants in the regulation of GPCR cascades, the complex regulatory mechanisms of GRK expression, its alternation, and their function are not thoroughly understood. Several studies together with the work from our lab in recent years have revealed the critical role of these kinases in various physiological and pathophysiological processes, including cardiovascular biology, inflammation and immunity, neurodegeneration, thrombosis, and hemostasis. A comprehensive understanding of the mechanisms underlying functional interactions with multiple receptor proteins and how these interactions take part in the development of various pathobiological processes may give rise to novel diagnostic and therapeutic strategies. In this review, we summarize the current research linking the role of GRKs to various aspects of cell biology, pathology, and therapeutics, with a particular focus on thrombosis and hemostasis.

What about Platelet Function in Platelet Concentrates?

The characterization of platelet concentrates (PCs) in transfusion medicine has been performed with different analytical methods and platelet lesions (from biochemistry to cell biology) have been documented. In routine quality assessment and validation of manufacturing processes of PCs for transfusion purposes, only basic parameters are monitored and the platelet functions are not included. However, PCs undergo several manipulations during the processing and the basic parameters do not provide sensitive analyses to properly picture out the impact of the blood component preparation and storage on platelets. To improve the transfusion supply chain and the platelet functionalities, additional parameters should be used. The present short review will focus on the different techniques to monitor ex vivo platelet lesions from phenotype characterization to advanced omic analyses. Then, the opportunities to use these methods in quality control, process validation, development, and research will be discussed. Functional markers should be considered because they would be an advantage for the future developments in transfusion medicine.

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.

Rhizoma Bletillae polysaccharide elicits hemostatic effects in platelet-rich plasma by activating adenosine diphosphate receptor signaling pathway

Rhizoma Bletillae, the tubes of Bletilla striata, has been traditionally used in China as a hemostatic agent. In preliminary studies, the major active fraction responsible for its hemostatic effect have been confirmed to be Rhizoma Bletillae polysaccharide (RBp), but the hemostatic mechanism of action of RBp is still unknown.The main aim of this study was to clarify its mechanism of hemostatic effect. RBp was prepared by 80 % ethanol precipitation of the water extract of Rhizoma Bletillae followed by the Sevag method to remove proteins. The average molecular weight (Mw) of the crude RBp maintained at a range of 30.06-200 KDa. The hemostatic effects of RBp were evaluated by testing its effect on the platelet aggregation of rat platelet-rich plasma (PRP). PRP was dealt with different concentrations of RBp and platelet aggregation was measured by the turbidimetric method. The hemostatic mechanism of RBp was investigated by examining its effect on platelet shape, platelet secretion, and activation of related receptors (P2Y₁, P2Y₁₂ and TXA2) by electron microscopy and the turbidimetric method. RBp significantly enhanced the platelet aggregations at concentrations of 50-200 mg/L in a concentration-dependent manner. The inhibitory rate of platelet aggregation was significantly increased by apyrase and Ro31-8220 in a concentration-dependent manner, while RBp-induced platelet aggregation was completely inhibited by P2Y₁, P2Y₁₂ and the PKC receptor antagonists. However, the aggregation was not sensitive to TXA2. RBp, the active ingredients of Rhizoma Bletillae responsible for its hemostatic effect, could significantly accelerate the platelet aggregation and shape change. The hemostatic mechanism may involve activation of the P2Y₁, P2Y₁₂, and PKC receptors in the adenosine diphosphate (ADP) receptor signaling pathway.

The Roles of GRKs in Hemostasis and Thrombosis

Along with cancer, cardiovascular and cerebrovascular diseases remain by far the most common causes of death. Heart attacks and strokes are diseases in which platelets play a role, through activation on ruptured plaques and subsequent thrombus formation. Most platelet agonists activate platelets via G protein-coupled receptors (GPCRs), which make these receptors ideal targets for many antiplatelet drugs. However, little is known about the mechanisms that provide feedback regulation on GPCRs to limit platelet activation. Emerging evidence from our group and others strongly suggests that GPCR kinases (GRKs) are critical negative regulators during platelet activation and thrombus formation. In this review, we will summarize recent findings on the role of GRKs in platelet biology and how one specific GRK, GRK6, regulates the hemostatic response to vascular injury. Furthermore, we will discuss the potential role of GRKs in thrombotic disorders, such as thrombotic events in COVID-19 patients. Studies on the function of GRKs during platelet activation and thrombus formation have just recently begun, and a better understanding of the role of GRKs in hemostasis and thrombosis will provide a fruitful avenue for understanding the hemostatic response to injury. It may also lead to new therapeutic options for the treatment of thrombotic and cardiovascular disorders.

Effects of bacterial lipopolysaccharides on platelet function: inhibition of weak platelet activation

Platelets are anucleate blood cells with reported roles in hemostasis and immune responses, which possess a functional receptor for bacterial lipopolysaccharides (LPSs), the well-known inducers of inflammation. However, LPSs effects on platelets are contradictory. Here we aim to investigate mechanisms of platelet functioning in the presence of LPS and to find the cause of the discrepancy in the previously published data. Cell activity was analyzed by flow cytometry, western blotting, and aggregometry. Thrombus growth was assessed by fluorescent microscopy. LPS' activity was checked by their capability to induce PMN activation. However, LPSs did not substantially affect either thrombus growth in flow chambers, irreversible platelet aggregation, or platelet responses to strong activation. Platelet aggregation in response to 1 μM of ADP was significantly inhibited by LPSs. Flow cytometry analysis revealed that platelet activation responses to weak stimulation were also diminished by LPSs, while VASP phosphorylation was weakly increased. Additionally, LPSs were capable of inhibition of ADP-induced P2-receptor desensitization. Incubation of platelets with a pan-PDE inhibitor IBMX significantly enhanced the LPSs-induced platelet inhibition, implying cAMP/cGMP dependent mechanism. The discrepancy in the previously published data could be explained by LPS-induced weak inhibition of platelet activation and the prevention of platelet desensitization.

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.

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.

ADP exerts P2Y12 -dependent and P2Y12 -independent effects on primary human T cell responses to stimulation

Purinergic signaling plays a complex role in inflammation. Nucleotides released by T lymphocytes, endothelial cells, and platelets during inflammation induce cellular responses by binding to receptors that regulate intracellular signaling pathways. Previous studies have found that purinergic signaling can have both proinflammatory and anti-inflammatory effects, but the roles of specific pathways in specific cell types are poorly understood. We investigated the role of the P2Y₁₂ signaling pathway in the activation of T lymphocytes in vitro. We isolated peripheral blood mononuclear cells (PBMCs) from healthy donors and pretreated them with ADP (a P2Y₁₂ agonist), AR-C69931MX (a P2Y₁₂ antagonist), or both. We then stimulated PBMC using phytohemagglutinin (PHA) or anti-CD3/CD28 antibodies. We found that ADP affects T cell responses in term of cell activity and receptor expression through both P2Y₁₂-dependent and P2Y₁₂-independent pathways and other responses (cytokine secretion) primarily through P2Y₁₂ -independent pathways. The ADP-mediated effect changed over time and was stimulus-specific.

Arrestin-3 differentially regulates platelet GPCR subsets

The principal demonstrated role of the nonvisual arrestins in vivo is to limit G protein-coupled receptor (GPCR) signaling. Nonetheless, a direct demonstration of this fundamental ability in platelets remains lacking, despite the prominent role played by GPCRs in platelet activation. This paper describes the basic characterization of the activatory responses of platelets from mice lacking arrestin-3 (arr3-/-), revealing pleiotropic roles dependent on GPCR ligand. Functionally, arrestin-3 acts as a brake on platelet aggregation regardless of ligand tested. Downstream of P2Y receptors, arr3-/- mice show increased secretion and integrin activation mirrored by enhanced intracellular calcium signaling and global PKC-dependent phosphorylation. Furthermore, P2Y12 receptor (P2Y12R) activity as assessed by ADP-mediated reduction of VASP phosphorylation is enhanced in arr3-/-mice. Downstream of PAR receptors there are similar increases in secretion and integrin activation in arr3-/- mice, together with enhanced PKC activity. Last, in arr3-/- mice the TP receptor displays unaltered PKC activity but markedly reduced calcium responses, which together with the kinetics of the aggregation response suggested a unique positive regulatory role for arrestin-3 in TP signaling. Overall, this paper reveals pleiotropic roles for arrestin-3 dependent on GPCR ligand describing for the first time a negative regulatory function for arrestin-3 in platelets.

The effect of mild agonist stimulation on the platelet reactivity in patients with type 2 diabetes mellitus

BACKGROUND

Patients with type 2 diabetes mellitus (T2DM) have accelerated atherosclerosis as a pro thrombotic state that is associated with the platelet activation priming. Platelets, which undergo the continuous mild stimulation, may lose their sensitivity to react to a strong stimulation. The present study aimed to investigate activation responses of platelets to mild and subsequent strong stimulations in patients with T2DM and healthy individuals.

METHODS

Blood samples, which were taken from 40 patients with T2DM and 35 healthy individuals, were collected into the citrate containing tubes. The samples were subjected to the soft centrifugation to prepare the platelet rich plasma (PRP). Platelets in PRP samples were treated at a low (1 μM) concentration and then at a high (10 μM) concentration of ADP. Before and after stimulation with different doses of ADP, levels of CD62P expression and formation of platelet micro particles (PMPs) were measured using a flow cytometry method.

RESULTS

The platelets from patients with T2DM had higher levels of CD62P expression before any stimulation (P = 0.003) than control samples. Platelets, which underwent the mild stimulation, indicated lower responses to CD62P expression, but higher PMPs formation after stimulation with high dose of ADP. Patients with T2DM had higher platelet micro particles in all states with the ADP stimulation. (P = 0.004, SD: ±74.52).

CONCLUSIONS

The flow cytometry data indicated that platelets were pre-active and associated with metabolic conditions in patients with type 2 diabetes mellitus. The induction of desensitization state helped platelets to reduce the platelet activation and sensitivity to ADP in a diabetic environment. Furthermore, the production of platelets micro-particles was high in the patients; and desensitized platelets were more susceptible to shedding of micro-particles.

Long-term depression of presynaptic cannabinoid receptor function at parallel fibre synapses

KEY POINTS

Inhibition of synaptic responses by activation of presynaptic cannabinoid type-1 (Cb1) receptors is reduced at parallel fibre synapses in the cerebellum following 4 Hz stimulation. Activation of adenylyl cyclase is necessary and sufficient for down-regulation of Cb1 receptors induced by 4 Hz stimulation. 4 Hz stimulation reduces Cb1 receptor function by (i) increasing the rate of endocannabinoid clearance from the synapse and (ii) decreasing expression of Cb1 receptors.

ABSTRACT

Cannabinoid type-1 receptors (Cb1R) are expressed in the presynaptic membrane of many synapses, including parallel fibre-Purkinje cell synapses in the cerebellum, where they are involved in short- and long-term plasticity of synaptic responses. We show that Cb1R expression itself is a plastic property of the synapse regulated by physiological activity patterns. We made patch clamp recordings from Purkinje cells in cerebellar slices and assessed Cb1R activity by measuring depolarization-induced suppression of excitation (DSE). We find that DSE is normally stable at parallel fibre synapses but, following 4 Hz stimulation, DSE is persistently reduced and recovers more rapidly. Using a combination of electrophysiology, pharmacology and biochemistry, we show that changes in DSE are a result of the reduced expression of Cb1Rs and increased degradation of endocannabinoids by monoacylglycerol lipase. Long-term changes in presynaptic Cb1R expression may alter other forms of Cb1R-dependent plasticity at parallel fibre synapses, priming or inhibiting the circuit for associative learning.

Pharmacological Studies Pertaining to Smooth Muscle Relaxant, Platelet Aggregation Inhibitory and Hypotensive Effects of Ailanthus altissima

OBJECTIVE

This in vitro and in vivo study was conducted to rationalize some of traditional medicinal uses of Ailanthus altissima in gastrointestinal, respiratory, and cardiovascular systems.

MATERIALS

Crude extract of Ailanthus altissima (Aa.Cr) and its fractions were prepared and utilized in in vitro and in vivo studies. For in vitro studies, Aa.Cr was investigated on isolated rabbit jejunum, isolated tracheal strip, and isolated aorta of rat suspended in tissue organ bath. Platelet rich and platelet poor plasma were used to study platelet aggregation inhibitory activity. In vivo antidiarrheal effect of Aa.Cr was investigated on balb/c mice pretreated with castor oil to induce diarrhea and SD rats were used to study hypotensive activity.

RESULTS

Concentration dependent spasmolytic effects of Aa.Cr and its DCM fraction (Aa.DCM) were observed on spontaneous and spasmogen induced contractions in jejunum isolated from rabbit, but effect against high potassium (high-K+) induced contractions was more potent. Moreover Aa.Cr showed parallel shifting of calcium response curve to the right side. While its aqueous fraction (Aa.aq) caused spasmogenesis of isolated rabbit jejunum, this effect was blocked partially with prior administration of atropine (1μM). Concentration dependent protection against castor oil induced diarrhea was also observed. Relaxant effect was observed by the application of Aa.Cr and Aa.DCM against high-K+ and carbachol (CCh) induced contractions in tracheal strips isolated from SD rats, while Aa.Aq caused partial relaxation of high-K+ induced contractions, but no effect was observed against CCh induced contractions. Relaxation of rat aorta by the application of Aa.Cr and its fractions was also observed. Inhibition of force of contraction in rabbit atrium was also observed. Inhibition of platelet aggregation was observed against epinephrine and ADP induced aggregation.

CONCLUSION

Keeping in view the observed results, it is concluded that smooth muscle relaxant, platelet aggregation inhibitory and hypotensive effect may be due to the blockage of calcium channels.

Gradient-dependent inhibition of stimulatory signaling from platelet G protein-coupled receptors

As platelet activation is an irreversible and potentially harmful event, platelet stimulatory signaling must be tightly regulated to ensure the filtering-out of inconsequential fluctuations of agonist concentrations in the vascular milieu. Herein, we show that platelet activation via G protein-coupled receptors is gradient-dependent, i.e., determined not only by agonist concentrations per se but also by how rapidly concentrations change over time. We demonstrate that gradient-dependent inhibition is a common feature of all major platelet stimulatory G protein-coupled receptors, while platelet activation via the non-G protein-coupled receptor glycoprotein VI is strictly concentration-dependent. By systematically characterizing the effects of variations in temporal agonist concentration gradients on different aspects of platelet activation, we demonstrate that gradient-dependent inhibition of protease-activated receptors exhibits different kinetics, with platelet activation occurring at lower agonist gradients for protease-activated receptor 4 than for protease-activated receptor 1, but shares a characteristic bimodal effect distribution, as gradient-dependent inhibition increases over a narrow range of gradients, below which aggregation and granule secretion is effectively shut off. In contrast, the effects of gradient-dependent inhibition on platelet activation via adenosine diphosphate and thromboxane receptors increase incrementally over a large range of gradients. Furthermore, depending on the affected activation pathway, gradient-dependent inhibition results in different degrees of refractoriness to subsequent autologous agonist stimulation. Mechanistically, our study identifies an important role for the cyclic adenosine monophosphate-dependent pathway in gradient-dependent inhibition. Together, our findings suggest that gradient-dependent inhibition may represent a new general mechanism for hemostatic regulation in platelets.

Generation of procoagulant collagen- and thrombin-activated platelets in platelet concentrates derived from buffy coat: the role of processing, pathogen inactivation, and storage

BACKGROUND

Collagen- and thrombin-activated (COAT) platelets (PLTs), generated by dual-agonist stimulation with collagen and thrombin (THR), enhance THR generation at the site of vessel wall injury. There is evidence that higher amounts of procoagulant COAT PLTs are associated with stroke, while a decreased ability to generate them is associated with bleeding diathesis. Our aim was to study PLT functions, particularly the ability to generate COAT PLTs, in PLT concentrates (PCs) from buffy coat. Thus, we investigated the effect of processing, pathogen inactivation treatment (amotosalen-UVA), and PC storage.

STUDY DESIGN AND METHODS

Two PCs from five donors each were pooled and split in two bags; one of them was pathogen inactivated and the other one was left untreated (n = 5). Flow cytometric analyses were performed immediately after PC preparation (Day 1) and thereafter on Days 2, 5, 7, and 9 in treated and untreated PCs to measure the reactivity of PLTs (CD62P and PAC-1), the content and secretion of dense granule after stimulation with different agonists, and the percentage of COAT PLTs after dual stimulation with convulxin (agonist of the collagen receptor GPVI) and THR.

RESULTS

Preparation of PCs resulted in a significant decrease of COAT PLTs and in an impaired response to adenosine 5'-diphosphate sodium (ADP). Storage further decreased ADP response. Minor differences were observed between untreated or amotosalen-UVA-treated PCs.

CONCLUSION

Preparation of PCs from buffy coats decreased the ability to generate COAT PLTs and impaired PLT response to ADP.

Effect of Pneumatic Tubing System Transport on Platelet Apheresis Units

Platelet apheresis units are transfused into patients to mitigate or prevent bleeding. In a hospital, platelet apheresis units are transported from the transfusion service to the healthcare teams via two methods: a pneumatic tubing system (PTS) or ambulatory transport. Whether PTS transport affects the activity and utility of platelet apheresis units is unclear. We quantified the gravitational forces and transport time associated with PTS and ambulatory transport within our hospital. Washed platelets and supernatants were prepared from platelet apheresis units prior to transport as well as following ambulatory or PTS transport. For each group, we compared resting and agonist-induced platelet activity and platelet aggregate formation on collagen or von Willebrand factor (VWF) under shear, platelet VWF-receptor expression and VWF multimer levels. Subjection of platelet apheresis units to rapid acceleration/deceleration forces during PTS transport did not pre-activate platelets or their ability to activate in response to platelet agonists as compared to ambulatory transport. Platelets within platelet apheresis units transported via PTS retained their ability to adhere to surfaces of VWF and collagen under shear, although platelet aggregation on collagen and VWF was diminished as compared to ambulatory transport. VWF multimer levels and platelet GPIb receptor expression was unaffected by PTS transport as compared to ambulatory transport. Subjection of platelet apheresis units to PTS transport did not significantly affect the baseline or agonist-induced levels of platelet activation as compared to ambulatory transport. Our case study suggests that PTS transport may not significantly affect the hemostatic potential of platelets within platelet apheresis units.

Salvianolic acids from antithrombotic Traditional Chinese Medicine Danshen are antagonists of human P2Y1 and P2Y12 receptors

Many hemorheologic Traditional Chinese Medicines (TCMs) that are widely-used clinically lack molecular mechanisms of action. We hypothesized that some of the active components of hemorheologic TCMs may function through targeting prothrombotic P2Y1 and/or P2Y12 receptors. The interactions between 253 antithrombotic compounds from TCM and these two G protein-coupled P2Y receptors were evaluated using virtual screening. Eleven highly ranked hits were further tested in radioligand binding and functional assays. Among these compounds, salvianolic acid A and C antagonized the activity of both P2Y1 and P2Y12 receptors in the low µM range, while salvianolic acid B antagonized the P2Y12 receptor. These three salvianolic acids are the major active components of the broadly-used hemorheologic TCM Danshen (Salvia militorrhiza), the antithrombotic molecular mechanisms of which were largely unknown. Thus, the combination of virtual screening and experimental validation identified potential mechanisms of action of multicomponent drugs that are already employed clinically.

Restored response to ADP downstream of purinergic P2Y12 receptor in apheresis platelets after pathogen-reducing xenon flash treatment

BACKGROUND

Our previous study revealed that pathogen-reducing filtered xenon flash-treated platelets (fXe-PLTs) showed sustained aggregation in response to adenosine diphosphate (ADP), but apheresis-collected PLTs (Aph-PLTs) showed reversible aggregation.

STUDY DESIGN AND METHODS

Aph-PLTs, fXe-PLTs, and freshly prepared PLTs (PRP-PLTs) from whole blood were used to investigate the following responses to ADP: concentration response and effects of ADP receptor antagonists on aggregation, the cytosolic calcium (Ca²⁺ ) flux downstream of P2Y₁ receptor signaling, and phosphorylation of vasodilator-stimulated phosphoprotein (VASP) and signaling intermediate protein Akt downstream of the P2Y₁₂ receptor.

RESULTS

The aggregation of Aph-PLTs by ADP (10 µM) changed from reversible to sustained in an fXe flash dose-dependent manner. The concentration-response curve of Aph-PLTs showed a fivefold higher 50% effective concentration compared with PRP-PLTs, and fXe treatment decreased it to threefold. While the basal Ca²⁺ level was higher both in Aph- and fXe-PLTs than in PRP-PLTs, the increase of cytosolic Ca²⁺ by ADP remained unchanged in Aph- and PRP-PLTs, but was slightly reduced in fXe-PLTs. Although the forskolin-induced VASP phosphorylation was significantly reduced in Aph-PLTs, and partially restored by the fXe treatment, ADP stimulation attenuated this phosphorylation to an equivalent extent among the three PLT types. The ADP-stimulated time-dependent Akt phosphorylation was weak in Aph-PLTs, whereas fXe-PLTs and PRP-PLTs showed a marked increase.

CONCLUSION

These results indicate that the reversible aggregation of Aph-PLTs is the consequence of insufficient Akt phosphorylation. The fXe treatment restores the increase of phosphorylated Akt, resulting in the sustained aggregation of fXe-PLTs similar to those of PRP-PLTs.

Pin1 induces the ADP-induced migration of human dental pulp cells through P2Y1 stabilization

PIN1, which belongs to a family of prolyl isomerases, specifically binds to phosphorylated Ser/Thr-pro motifs to catalytically regulate the post-phosphorylation conformation of its substrates. This study aimed to investigate the importance of Pin1 expression in human dental pulp cells (hDPCs) to understand the involvement of Pin1 in the regulation of P2Y1 and the activation of ADP-mediated P2Y1 signaling. This study found that the protein levels of P2Y1 gradually decreased after the onset of cell recovery following heat stress. Interestedly, hDPC migration significantly decreased during the recovery period. An in vitro study revealed that the silencing of PIN1 by siRNA or the pharmacologic inhibition of its activity decreased the migration of P2Y1 and P2Y1 expression in these cells. In addition, we found that Pin1 directly interacts with S252 of P2Y1 and that its binding stabilizes the P2Y1 protein to increase migration activity. These results strongly suggest that Pin1 mediates cell migration by stabilizing P2Y1 and that the Pin1/P2Y1 signaling pathways might serve as a novel mechanism of cell migration progression in hDPCs.

Negative regulators of platelet activation and adhesion

Platelets are small anucleated cells that constantly patrol the cardiovascular system to preserve its integrity and prevent excessive blood loss where the vessel lining is breached. Their key challenge is to form a hemostatic plug under conditions of high shear forces. To do so, platelets have evolved a molecular machinery that enables them to sense trace amounts of signals at the site of damage and to rapidly shift from a non-adhesive to a pro-adhesive state. However, this highly efficient molecular machinery can also lead to unintended platelet activation and cause clinical complications such as thrombocytopenia and thrombosis. Thus, several checkpoints are in place to tightly control platelet activation and adhesiveness in space and time. In this review, we will discuss select negative regulators of platelet activation, which are critical to maintain patrolling platelets in a quiescent, non-adhesive state and/or to limit platelet adhesion to sites of injury.

Receptor homodimerization plays a critical role in a novel dominant negative P2RY12 variant identified in a family with severe bleeding

Essentials Three dominant variants for the autosomal recessive bleeding disorder type-8 have been described. To date, there has been no phenotype/genotype correlation explaining their dominant transmission. Proline plays an important role in P2Y12R ligand binding and signaling defects. P2Y12R homodimer formation is critical for the receptor function and signaling.

SUMMARY

Background Although inherited platelet disorders are still underdiagnosed worldwide, advances in molecular techniques are improving disease diagnosis and patient management. Objective To identify and characterize the mechanism underlying the bleeding phenotype in a Caucasian family with an autosomal dominant P2RY12 variant. Methods Full blood counts, platelet aggregometry, flow cytometry and western blotting were performed before next-generation sequencing (NGS). Detailed molecular analysis of the identified variant of the P2Y12 receptor (P2Y12R) was subsequently performed in mammalian cells overexpressing receptor constructs. Results All three referred individuals had markedly impaired ADP-induced platelet aggregation with primary wave only, despite normal total and surface P2Y12R expression. By NGS, a single P2RY12:c.G794C substitution (p.R265P) was identified in all affected individuals, and this was confirmed by Sanger sequencing. Mammalian cell experiments with the R265P-P2Y12R variant showed normal receptor surface expression versus wild-type (WT) P2Y12R. Agonist-stimulated R265P-P2Y12R function (both signaling and surface receptor loss) was reduced versus WT P2Y12R. Critically, R265P-P2Y12R acted in a dominant negative manner, with agonist-stimulated WT P2Y12R activity being reduced by variant coexpression, suggesting dramatic loss of WT homodimers. Importantly, platelet P2RY12 cDNA cloning and sequencing in two affected individuals also revealed three-fold mutant mRNA overexpression, decreasing even further the likelihood of WT homodimer formation. R265 located within extracellular loop 3 (EL3) is one of four residues that are important for receptor functional integrity, maintaining the binding pocket conformation and allowing rotation following ligand binding. Conclusion This novel dominant negative variant confirms the important role of R265 in EL3 in the functional integrity of P2Y12R, and suggests that pathologic heterodimer formation may underlie this family bleeding phenotype.

Platelets at the Vascular Interface

In this brief review paper, we will summarize the State-of-the-Art on how platelet reactivity is regulated in circulation and at sites of vascular injury. Our review discusses recent and ongoing work, presented at this year's International Society on Thrombosis and Haemostasis (ISTH) meeting, on the role of platelets in (1) classical hemostasis at sites of mechanical injury, and (2) the maintenance of vascular integrity at sites of inflammation.

Monitoring storage induced changes in the platelet proteome employing label free quantitative mass spectrometry

Shelf life of platelet concentrates is limited to 5-7 days due to loss of platelet function during storage, commonly referred to as the platelet storage lesion (PSL). To get more insight into the development of the PSL, we used label free quantitative mass spectrometry to identify changes in the platelet proteome during storage. In total 2501 proteins were accurately quantified in 3 biological replicates on at least 1 of the 7 different time-points analyzed. Significant changes in levels of 21 proteins were observed over time. Gene ontology enrichment analysis of these proteins revealed that the majority of this set was involved in platelet degranulation, secretion and regulated exocytosis. Twelve of these proteins have been shown to reside in α-granules. Upon prolonged storage (13-16 days) elevated levels of α-2-macroglobulin, glycogenin and Ig μ chain C region were identified. Taken together this study identifies novel markers for monitoring of the PSL that may potentially also be used for the detection of "young" and "old" platelets in the circulation.

ASK1 facilitates tumor metastasis through phosphorylation of an ADP receptor P2Y12 in platelets

Tumor metastasis is the major cause of deaths in cancer patients and is modulated by intertwined stress-responsive signaling cascades. Here we demonstrate that deletion of stress-responsive apoptosis signal-regulating kinase 1 (Ask1) in platelets results in unstable hemostasis and drastic attenuation of tumor lung metastasis, both of which are attributable to platelet dysfunction. Platelet-specific deletion of Ask1 in mice leads to defects in ADP-dependent platelet aggregation, unstable hemostasis and subsequent attenuation of tumor metastasis. We also revealed that activating phosphorylation of Akt is attenuated in Ask1-deficient platelets, contrary to the previous reports suggesting that Akt is negatively regulated by ASK1. Mechanistically, ASK1-JNK/p38 axis phosphorylates an ADP receptor P2Y₁₂ at Thr345, which is required for the ADP-dependent sustained Akt activity that is vital to normal platelet functions. Our findings offer insight into positive regulation of Akt signaling through P2Y₁₂ phosphorylation as well as MAPK signaling in platelets by ASK1 and suggest that ASK1-JNK/p38 axis provides a new therapeutic opportunity for tumor metastasis.

Pathophysiological consequences of receptor mistraffic: Tales from the platelet P2Y12 receptor

Genetic variations in G protein-coupled receptor (GPCR) genes can disrupt receptor function in a wide variety of human genetic diseases, including platelet bleeding disorders. Platelets are critical for haemostasis with inappropriate platelet activation leading to the development of arterial thrombosis, which can result in heart attack and stroke whilst decreased platelet activity is associated with an increased risk of bleeding. GPCRs expressed on the surface of platelets play key roles in regulating platelet activity and therefore function. Receptors include purinergic receptors (P2Y₁ and P2Y₁₂), proteinase-activated receptor (PAR1 and PAR4) and thromboxane receptors (TPα), among others. Pharmacological blockade of these receptors forms a powerful therapeutic tool in the treatment and prevention of arterial thrombosis. With the advance of genomic technologies, there has been a substantial increase in the identification of naturally occurring rare and common GPCR variants. These variants include single-nucleotide polymorphisms (SNPs) and insertion or deletions that have the potential to alter GPCR expression or function. A number of defects in platelet GPCRs that disrupt receptor function have now been characterized in patients with mild bleeding disorders. This review will focus on rare, function-disrupting variants of platelet GPCRs with particular emphasis upon mutations in the P2Y₁₂ receptor gene that affect receptor traffic to modulate platelet function. Further this review will outline how the identification and characterization of function-disrupting GPCR mutations provides an essential link in translating our detailed understanding of receptor traffic and function in cell line studies into relevant human biological systems.

Inverse agonism at the P2Y12 receptor and ENT1 transporter blockade contribute to platelet inhibition by ticagrelor

Ticagrelor is a potent antagonist of the P2Y₁₂ receptor (P2Y₁₂R) and consequently an inhibitor of platelet activity effective in the treatment of atherothrombosis. Here, we sought to further characterize its molecular mechanism of action. Initial studies showed that ticagrelor promoted a greater inhibition of adenosine 5'-diphosphate (ADP)-induced Ca²⁺ release in washed platelets vs other P2Y₁₂R antagonists. This additional effect of ticagrelor beyond P2Y₁₂R antagonism was in part as a consequence of ticagrelor inhibiting the equilibrative nucleoside transporter 1 (ENT1) on platelets, leading to accumulation of extracellular adenosine and activation of G_s-coupled adenosine A_2A receptors. This contributed to an increase in basal cyclic adenosine monophosphate (cAMP) and vasodilator-stimulated phosphoprotein phosphorylation (VASP-P). In addition, ticagrelor increased platelet cAMP and VASP-P in the absence of ADP in an adenosine receptor-independent manner. We hypothesized that this increase originated from a direct effect on basal agonist-independent P2Y₁₂R signaling, and this was validated in 1321N1 cells stably transfected with human P2Y₁₂R. In these cells, ticagrelor blocked the constitutive agonist-independent activity of the P2Y₁₂R, limiting basal G_i-coupled signaling and thereby increasing cAMP levels. These data suggest that ticagrelor has the pharmacological profile of an inverse agonist. Based on our results showing insurmountable inhibition of ADP-induced Ca²⁺ release and forskolin-induced cAMP, the mode of antagonism of ticagrelor also appears noncompetitive, at least functionally. In summary, our studies describe 2 novel modes of action of ticagrelor, inhibition of platelet ENT1 and inverse agonism at the P2Y₁₂R that contribute to its effective inhibition of platelet activation.

Evaluation of the role of the GPIb-IX-V receptor complex in development of the platelet storage lesion

BACKGROUND AND OBJECTIVES

In mice, loss of sialic acid resulting in shedding of glycoprotein (GP) Ibα and GPV has been linked to platelet survival. The aim of this study was to determine whether loss of sialic acid and the GPIb-IX-V complex contributes to development of the platelet storage lesion (PSL) in human platelet concentrates (PCs).

MATERIALS AND METHODS

PCs (stored in plasma (with or without Mirasol treatment); PAS-C or PAS-E) were stored at room temperature. Flow cytometry was used to monitor membrane expression of the GPIb-IX-V complex, CD62P, surface glycans and PS exposure. The functionality of stored platelets was determined employing aggregometry and ristocetin-induced VWF binding.

RESULTS

Storage time of PCs in blood banks is limited to 7 days. During this time period, a minor but gradually increasing subpopulation of GPIbα-negative platelets was observed. Also, ristocetin-induced VWF binding was impaired in a small population of platelets. Mean surface expression of GPIbα and GPV remained stable until day 9, whereas CD62P expression increased; also a rapid decrease in ADP-induced aggregation was observed for PAS-C, PAS-E and Mirasol-treated PCs. Upon prolonged storage (>9 days), a slow decline in surface expression of GPIbα and GPV was observed; no major changes were observed in surface sialylation with the exception of Mirasol-treated platelets.

CONCLUSION

In a small population of stored platelets, changes in GPIbα occur from day 2 onwards. Loss of sialic acid and subsequent shedding of GPIbα and GPV is not an early event during the development of the PSL.

Platelet signaling: a complex interplay between inhibitory and activatory networks

The role of platelets in hemostasis and thrombosis is dependent on a complex balance of activatory and inhibitory signaling pathways. Inhibitory signals released from the healthy vasculature suppress platelet activation in the absence of platelet receptor agonists. Activatory signals present at a site of injury initiate platelet activation and thrombus formation; subsequently, endogenous negative signaling regulators dampen activatory signals to control thrombus growth. Understanding the complex interplay between activatory and inhibitory signaling networks is an emerging challenge in the study of platelet biology, and necessitates a systematic approach to utilize experimental data effectively. In this review, we will explore the key points of platelet regulation and signaling that maintain platelets in a resting state, mediate activation to elicit thrombus formation, or provide negative feedback. Platelet signaling will be described in terms of key signaling molecules that are common to the pathways activated by platelet agonists and can be described as regulatory nodes for both positive and negative regulators.

P2Y(1) receptors mediate purinergic relaxation in the equine pelvic flexure

In the equine large intestine, the knowledge of the basic mechanisms underlying motility function is crucial to properly treat motility disorders. P2Y1 receptors are responsible for mediating purinergic colonic relaxation in several species. In vitro experimental studies of the circular muscle from the equine pelvic flexure (n = 6) were performed to characterize inhibitory and excitatory neuromuscular transmission. Electrophysiological studies showed that electrical field stimulation (EFS) evoked biphasic inhibitory junction potentials (IJPs) in smooth muscle cells: a fast IJP (IJPf) followed by a sustained IJP (IJPs). IJPs was sensitive to L-NNA 1 mM (a nitric oxide synthase inhibitor) (P <0.01), while IJPf was abolished by MRS2500 1 µM (a P2Y1 receptor antagonist) (P <0.001). EFS (5 Hz for 2 min) in the organ bath inhibited rhythmic contractions to 3.0 ± 2.5% of basal area under the curve (P <0.0001). EFS under MRS2500 1 µM or L-NNA 1 mM incubation inhibited contractions to 6.0 ± 2.8% (P <0.05) and 24.4 ± 11.3% respectively (P <0.05). Combination of MRS2500 1 µM and L-NNA 1 mM completely reversed the EFS-induced inhibition of colonic motility. Non-nitrergic, non-purinergic conditions were used to reveal voltage-dependent EFS-induced contractions sensitive to atropine 1 µM (P <0.001) and, therefore, cholinergic. In conclusion, nerve-mediated relaxation and contraction in the equine pelvic flexure involve the same mechanisms as those observed in the human colon. P2Y1 receptors mediate purinergic relaxations and are potential targets for the treatment of equine colonic motor disorders.