A selective role for phosphatidylinositol 3,4,5-trisphosphate in the Gi-dependent activation of platelet Rap1B

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.

The role of PI3Kγ in the immune system: new insights and translational implications

Over the past two decades, new insights have positioned phosphoinositide 3-kinase-γ (PI3Kγ) as a context-dependent modulator of immunity and inflammation. Recent advances in protein structure determination and drug development have allowed for generation of highly specific PI3Kγ inhibitors, with the first now in clinical trials for several oncology indications. Recently, a monogenic immune disorder caused by PI3Kγ deficiency was discovered in humans and modelled in mice. Human inactivated PI3Kγ syndrome confirms the immunomodulatory roles of PI3Kγ and strengthens newly defined roles of this molecule in modulating inflammatory cytokine release in macrophages. Here, we review the functions of PI3Kγ in the immune system and discuss how our understanding of its potential as a therapeutic target has evolved.

Upregulation of P2Y12 inhibits chondrocyte apoptosis in lumbar osteoarthritis through the PI3K/AKT signaling pathway

Lumbar facet osteoarthritis (FJOA) is a major cause of severe lower back pain and disability worldwide. However, the mechanism underlying cartilage degeneration in FJOA remains unclear. The purpose of this study was to investigate the regulation and mechanism of P2Y12 on chondrocyte apoptosis in FJOA. The experimental rats were randomly divided into non-operation (n = 20) and operation groups (n = 20). In the operation group, Sodium iodoacetate (MIA, Sigma, 200 mg/mL) was injected into the right L4/5 facet process using a blunt nanoneedle 26 (WPI, Sarasota, FL, USA) under the control of an injection pump. The final injection volume was 5µL and the injection rate was 2µL/min. The facet joint was removed four weeks after surgery. After the operation, samples were stored at -80 °C until further use, whereby the right facet joints in each group were tested. Hematoxylin and eosin (HE) and iron-red solid green staining were used to observe the degeneration of articular chondrocytes in rats. Immunohistochemistry and western blotting were used to observe the expressions of P2Y12, Matrix metalloproteinase 13 (MMP13), Collagen II (COL2), and other cartilage degeneration and apoptosis-related genes. Co-localization of P2Y12-cleaved caspase-3 in the apoptosis model was detected by dual-standard immunofluorescence staining. Apoptosis was also detected by flow cytometry and TUNEL assay.P2Y12 is highly expressed in OA cartilage tissue, and inhibits IL-1β -induced chondrocyte apoptosis through PI3K/AKT signaling pathway, thus playing a certain protective role on cartilage.

RasGRP2 Structure, Function and Genetic Variants in Platelet Pathophysiology

RasGRP2 is calcium and diacylglycerol-regulated guanine nucleotide exchange factor I that activates Rap1, which is an essential signaling-knot in "inside-out" αIIbβ3 integrin activation in platelets. Inherited platelet function disorder caused by variants of RASGRP2 represents a new congenital bleeding disorder referred to as platelet-type bleeding disorder-18 (BDPLT18). We review here the structure of RasGRP2 and its functions in the pathophysiology of platelets and of the other cellular types that express it. We will also examine the different pathogenic variants reported so far as well as strategies for the diagnosis and management of patients with BDPLT18.

RAP1 Downregulation by miR-320c Reduces Platelet Activation in Ex-vivo Storage

BACKGROUND

A small GTPase Protein, the Ras-related Protein 1 (RAP1), abundant in platelets is known to be activated following agonist-induced platelet activation, suggesting that RAP1 downregulation could, in turn, reduce platelet activation in storage. Our objective of this study is to identify RAP1 regulating miRNAs and their role in platelet activation during storage.

METHODS

We applied MS2-TRAP (tagged RNA affinity purification) methodology to enrich miRNAs that target the 3' untranslated region (3'UTR) of RAP1 mRNA in two mammalian cell lines followed by miRNA identification by microarray of total RNA samples enriched for miRNAs. Data analyses were done using different bioinformatics approaches. The direct miR:RAP1 3'UTR interaction was confirmed by using a dual luciferase reporter gene expression system in a mammalian cell line. Subsequently, platelets were transfected with one selected miR to evaluate RAP1 downregulation by this miRNA and its effect on platelet activation.

RESULTS

Six miRNAs (miR-320c, miR-181a, miR-3621, miR-489, miR-4791 and miR-4744) were identified to be enriched in the two cell lines tested. We randomly selected miR-320c for further evaluation. The luciferase reporter assay system confirmed the direct interaction of miR-320c with RAP1 3'UTR. Further, in platelets treated with miR-320c, RAP1 protein expression was decreased and concomitantly, platelet activation was also decreased.

CONCLUSION

Overall, the results demonstrate that miRNA-based RAP1 downregulation in ex vivo stored platelets reduces platelet activation.

Effects of the NO/soluble guanylate cyclase/cGMP system on the functions of human platelets

Platelets are circulating sentinels of vascular integrity and are activated, inhibited, or modulated by multiple hormones, vasoactive substances or drugs. Endothelium- or drug-derived NO strongly inhibits platelet activation via activation of the soluble guanylate cyclase (sGC) and cGMP elevation, often in synergy with cAMP-elevation by prostacyclin. However, the molecular mechanisms and diversity of cGMP effects in platelets are poorly understood and sometimes controversial. Recently, we established the quantitative human platelet proteome, the iloprost/prostacyclin/cAMP/protein kinase A (PKA)-regulated phosphoproteome, and the interactions of the ADP- and iloprost/prostacyclin-affected phosphoproteome. We also showed that the sGC stimulator riociguat is in vitro a highly specific inhibitor, via cGMP, of various functions of human platelets. Here, we review the regulatory role of the cGMP/protein kinase G (PKG) system in human platelet function, and our current approaches to establish and analyze the phosphoproteome after selective stimulation of the sGC/cGMP pathway by NO donors and riociguat. Present data indicate an extensive and diverse NO/riociguat/cGMP phosphoproteome, which has to be compared with the cAMP phosphoproteome. In particular, sGC/cGMP-regulated phosphorylation of many membrane proteins, G-proteins and their regulators, signaling molecules, protein kinases, and proteins involved in Ca²⁺ regulation, suggests that the sGC/cGMP system targets multiple signaling networks rather than a limited number of PKG substrate proteins.

Hepatic Gi signaling regulates whole-body glucose homeostasis

An increase in hepatic glucose production (HGP) is a key feature of type 2 diabetes. Excessive signaling through hepatic Gs-linked glucagon receptors critically contributes to pathologically elevated HGP. Here, we tested the hypothesis that this metabolic impairment can be counteracted by enhancing hepatic Gi signaling. Specifically, we used a chemogenetic approach to selectively activate Gi-type G proteins in mouse hepatocytes in vivo. Unexpectedly, activation of hepatic Gi signaling triggered a pronounced increase in HGP and severely impaired glucose homeostasis. Moreover, increased Gi signaling stimulated glucose release in human hepatocytes. A lack of functional Gi-type G proteins in hepatocytes reduced blood glucose levels and protected mice against the metabolic deficits caused by the consumption of a high-fat diet. Additionally, we delineated a signaling cascade that links hepatic Gi signaling to ROS production, JNK activation, and a subsequent increase in HGP. Taken together, our data support the concept that drugs able to block hepatic Gi-coupled GPCRs may prove beneficial as antidiabetic drugs.

Differential proteomic analysis of platelets suggested target-related proteins in rabbit platelets treated with Rhizoma Corydalis

CONTEXT

Corydalis yanhusuo W.T. Wang (Papaveraceae) (Rhizoma Corydalis) showed inhibitory effects on rabbit platelet aggregation induced by ADP, thrombin (THR) or arachidonic acid (AA).

OBJECTIVE

This study separates and identifies the possible target-related platelet proteins and suggests possible signal cascades of RC antiplatelet aggregation.

MATERIALS AND METHODS

Based on comparative proteomics, the differentially expressed platelet proteins treated before and after with 50 mg/mL RC 90% ethanol extract (for 15 min at 37 °C) were analyzed and identified by two dimensional gel electrophoresis (2-DE) and MALDI-TOF-MS/MS. To further verify the possible signalling pathways of RC antiplatelet aggregation function, the concentration of calcium (Ca²⁺) was measured by Fura-2/AM fluorescence (Ex 340/380 nm, Em 500 nm) (RC final concentrations of 0.0156-0.1563 mg/mL), the levels of P-selectin and cyclic guanosine monophosphate (cGMP) were quantified by ELISA (OD. 450 nm) (RC final concentrations of 0.0156-1.5625 mg/mL), and the 5-hydroxytryptamine (5-HT) level was measured using ortho-phthalaldehyde (OPT) fluorescence (Ex 340 nm, Em 470 nm) (RC final concentrations of 0.3125-1.5625 mg/mL).

RESULTS

The expression of 52 proteins were altered in rabbit platelets after the treatment and the MALDI-TOF-MS analysis indicated that those proteins include 12 cytoskeleton proteins, 7 cell signalling proteins, 3 molecular chaperone proteins, 6 proteins related to platelet function, 16 enzymes and 7 other related proteins. Furthermore, RC extract could decrease the levels of 5-HT [inhibition rate of 96.80% (p < 0.05, vs. THR-activated group) treated with 0.7813 mg/mL of RC], Ca²⁺[172.73 ± 5.07 to 113.56 ± 5.46 nM (p < 0.001, vs. THR-activated group) treated with 0.0313 mg/mL of RC] and P-selectin [13.48 ± 0.96 ng/3 × 10⁸ to 11.64 ± 0.17 ng/3 × 10⁸ (p < 0.05, vs. THR-activated group) treated with 0.0156 mg/mL of RC], and increase in cGMP level [38.93 ± 0.57 to 50.26 ± 4.05 ng/3 × 10⁸ (p < 0.05, vs. THR-activated group) treated with 1.5165 mg/mL of RC] in ADP (10 μmol/L), THR (0.25 u/mL) or AA-(0.205 mmol/L) activated rabbit platelets.

DISCUSSION AND CONCLUSION

The present study indicated that P2Y12 receptor might be one of the direct target proteins of RC in platelets. The signal cascades network of RC after binding with P2Y12 receptor is mediating Gαi proteins to activate downstream signalling pathways (AC and/or PI3K signalling pathways) for the inhibition of platelet aggregation.

New Concepts and Mechanisms of Platelet Activation Signaling

Upon blood vessel injury, platelets are exposed to adhesive proteins in the vascular wall and soluble agonists, which initiate platelet activation, leading to formation of hemostatic thrombi. Pathological activation of platelets can induce occlusive thrombosis, resulting in ischemic events such as heart attack and stroke, which are leading causes of death globally. Platelet activation requires intracellular signal transduction initiated by platelet receptors for adhesion proteins and soluble agonists. Whereas many platelet activation signaling pathways have been established for many years, significant recent progress reveals much more complex and sophisticated signaling and amplification networks. With the discovery of new receptor signaling pathways and regulatory networks, some of the long-standing concepts of platelet signaling have been challenged. This review provides an overview of the new developments and concepts in platelet activation signaling.

Role of P2Y12 Receptor in Thrombosis

P2Y₁₂ receptor is a 342 amino acid Gi-coupled receptor predominantly expressed on platelets. P2Y₁₂ receptor is physiologically activated by ADP and inhibits adenyl cyclase (AC) to decrease cyclic AMP (cAMP) level, resulting in platelet aggregation. It also activates PI3 kinase (PI3K) pathway leading to fibrinogen receptor activation, and may protect platelets from apoptosis. Abnormalities of P2Y₁₂ receptor include congenital deficiencies or high activity in diseases like diabetes mellitus (DM) and chronic kidney disease (CKD), exposing such patients to a prothrombotic condition. A series of clinical antiplatelet drugs, such as clopidogrel and ticagrelor, are designed as indirect or direct antagonists of P2Y₁₂ receptor to reduce incidence of thrombosis mainly for patients of acute coronary syndrome (ACS) who are at high risk of thrombotic events. Studies on novel dual-/multi-target antiplatelet agents consider P2Y₁₂ receptor as a promising part in combined targets. However, the clinical practical phenomena, such as "clopidogrel resistance" due to gene variations of cytochrome P450 or P2Y₁₂ receptor constitutive activation, call for better antiplatelet agents. Researches also showed inverse agonist of P2Y₁₂ receptor could play a better role over neutral antagonists. Personalized antiplatelet therapy is the most ideal destination for antiplatelet therapy in ACS patients with or without other underlying diseases like DM or CKD, however, there is still a long way to go.

Dual inhibition of PI3K and mTOR by VS-5584 suppresses thrombus formation

VS-5584 is a highly selective dual kinase inhibitor which suppresses phosphatidylinositol 3-kinase (PI3K) and mammalian target of rapamycin (mTOR) activity. Because these kinases are crucially involved in primary hemostasis, we herein investigated the effect of this compound on thrombus formation in vitro and in vivo. Pretreatment of washed platelets (WP) or platelet-rich plasma (PRP) with VS-5584 inhibited the agonist-induced activation of surface glycoprotein complex (GP)IIb/IIIa and the upregulation of P-selectin. This was associated with a significantly reduced formation of platelet-leukocyte aggregates (PLA). VS-5584 further attenuated platelet aggregation and adhesion after agonist stimulation. In contrast, endothelial expression of intercellular adhesion molecule (ICAM)-1 and vascular cellular adhesion molecule (VCAM)-1 and secretion of von Willebrand Factor (vWF) were not affected by the dual kinase inhibitor. In vivo, VS-5584 inhibited photochemically induced thrombus formation as shown by a significantly prolonged time to complete vessel occlusion when compared to vehicle-treated controls. This was associated with an elevated tail vein bleeding time, indicating a potential hemorrhagic risk in VS-5584-treated mice. Taken together, these novel findings demonstrate that VS-5584 is a potent inhibitor of primary hemostasis targeting multiple platelet functions.

Integrin-αIIbβ3-mediated outside-in signalling activates a negative feedback pathway to suppress platelet activation

Integrin-αIIbβ3-mediated outside-in signalling is widely accepted as an amplifier of platelet activation; accumulating evidence suggests that outside-in signalling can, under certain conditions, also function as an inhibitor of platelet activation. The role of integrin-αIIbβ3-mediated outside-in signalling in platelet activation is disputable. We employed flow cytometry, aggregometry, immunoprecipitation, and immunoblotting to investigate the role of integrin-αIIbβ3-mediated outside-in signalling in platelet activation. Integrin αIIbβ3 inhibition enhances agonist-induced platelet ATP secretion. Human platelets lacking expression of αIIbβ3 exhibited more platelet ATP secretion than their wild-type counterparts. Moreover, integrin-αIIbβ3-mediated outside-in signals activate SHIP-1, which in turn mediates p-Akt dephosphorylation, leading to inactivation of PI3K/Akt signalling. Furthermore, 3AC (SHIP-1 inhibitor) inhibits platelet disaggregation, and promotes platelet ATP secretion. Upon ADP stimulation, Talin is recruited to αIIbβ3, and it is dissociated from αIIbβ3 when platelets disaggregate. In addition, treatment with RUC2, an inhibitor of αIIbβ3, which blocks αIIbβ3-mediated outside-in signalling, can markedly prevent the dissociation of talin from integrin. SHIP1 Inhibitor 3AC inhibits the dissociation of talin from integrin-β3. These results suggest that integrin-αIIbβ3-mediated outside-in signalling can serve as a brake to restrict unnecessary platelet activation by activated SHIP-1, which mediated the disassociation of talin from β3, leading to integrin inactivation and blocking of PI3K/Akt signalling to restrict platelet ATP secretion.

Rap2B GTPase: structure, functions, and regulation

Rap2B GTPase, a member of Ras-related protein superfamily, was first discovered from a platelet cDNA library in the early 1990s. Since then, it has been reported to play an important role in regulating cellular processes including cytoskeletal organization, cell growth, and proliferation. It can be stimulated and suppressed by a wide range of external and internal inducers, circulating between GTP-bound active state and GDP-bound inactive state. Increasing focus on Ras signaling pathway reveals critical effects of Rap2B on tumorigenesis. In particular, Rap2B behaves in a p53-dependent manner in regulation of apoptosis and migration. Apart from being an oncogenic activator, Rap2B has been found to participate in many other physiological events via diverse downstream effectors. In this review, we present recent studies on the structure, regulation, and multiple biological functions of Rap2B, shedding light on its potential status in treatment of cancer as well as other diseases.

RAP1-GTPase signaling and platelet function

Platelets are critical for hemostasis, i.e., the body's ability to prevent blood loss at sites of vascular injury. They patrol the vasculature in a quiescent, non-adhesive state for approximately 10 days, after which they are removed from circulation by phagocytic cells of the reticulo-endothelial system. At sites of vascular injury, they promptly shift to an activated, adhesive state required for the formation of a hemostatic plug. The small GTPase RAP1 is a critical regulator of platelet adhesiveness. Our recent studies demonstrate that the antagonistic balance between the RAP1 regulators, CalDAG-GEFI and RASA3, is critical for the modulation of platelet adhesiveness, both in circulation and at sites of vascular injury. The RAP1 activator CalDAG-GEFI responds to small changes in the cytoplasmic calcium concentration and thus provides sensitivity and speed to the activation response, essential for efficient platelet adhesion under conditions of hemodynamic shear stress. The RAP1 inhibitor RASA3 ensures that circulating platelets remain quiescent by restraining CalDAG-GEFI-dependent RAP1 activation. Upon cellular stimulation, it is turned off by P2Y12 signaling to enable sustained RAP1 activation, required for the formation of a stable hemostatic plug. This review will summarize important studies that elucidated the signaling pathways that control RAP1 activation in platelets.

PI3K/Akt in platelet integrin signaling and implications in thrombosis

Blood platelets are anucleated circulating cells that play a critical role in hemostasis and are also implicated in arterial thrombosis, a major cause of death worldwide. The biological function of platelets strongly relies in their reactiveness to a variety of extracellular agonists that regulate their adhesion to extracellular matrix at the site of vascular injury and their ability to form rapidly growing cell aggregates. Among the membrane receptors expressed on the cell surface, integrins are crucial for both platelet activation, adhesion and aggregation. Integrin affinity for specific ligands is regulated by intracellular signaling pathways activated in stimulated platelets, and, once engaged, integrins themselves generate and propagate signals inside the cells to reinforce and consolidate platelet response and thrombus formation. Phosphatidylinositol 3-Kinases (PI3Ks) have emerged as crucial players in platelet activation, and they are directly implicated in the regulation of integrin function. This review will discuss the contribution of PI3Ks in platelet integrin signaling, focusing on the role of specific members of class I PI3Ks and their downstream effector Akt on both integrin inside-out and outside-in signaling. The contribution of the PI3K/Akt pathways stimulated by integrin engagement and platelet activation in thrombus formation and stabilization will also be discussed in order to highlight the possibility to target these enzymes in effective anti-thrombotic therapeutic strategies.

Phosphatidylinositol-3,4,5-trisphosphate stimulates Ca(2+) elevation and Akt phosphorylation to constitute a major mechanism of thromboxane A2 formation in human platelets

Phosphatidylinositol trisphosphate (PIP3) has been implicated in many platelet functions however many of the mechanisms need clarification. We have used cell permeable analogues of PIP3,1-O-(1,2-di-palmitoyl-sn-glyero-3-O-phosphoryl)-D-myo-inositol-3,4,5-trisphosphate (DiC16-PIP3) or 1-O-(1,2-di-octanoyl-sn-glyero-3-O-phosphoryl)-D-myo-inositol-3,4,5-trisphosphate (DiC8-PIP3) to study their effects on activation on washed human platelets. Addition of either DiC8- or DiC16-PIP3 to human platelets induced aggregation in the presence of extracellular Ca(2+). This was reduced by the presence of indomethacin, the phospholipase C inhibitor U73122 and apyrase. DiC8-PIP3 induced the phosphorylation of Akt-Ser(473) which was reduced by the Akt inhibitor IV, wortmannin and EGTA (suggesting a dependence on Ca(2+) entry). In Fura2 loaded platelets DiC8-PIP3 was effective at increasing intracellular Ca(2+) in a distinct and transient manner that was reduced in the presence of indomethacin, U73122 and 2-aminoethyl diphenylborinate (2APB). Ca(2+) elevation was reduced by the non-SOCE inhibitor LOE908 and also by the SOCE inhibitor BTP2. DiC8-PIP3 induced the release of Ca(2+) from stores which was not affected by the proton dissipating agent bafilomycin A1 and was more potent than the two-pore channel agonist DiC8-PI[3,5]P2 suggesting release from an endoplasmic reticulum type store. DiC8-PIP3 weakly induced the tyrosine phosphorylation of Syk but not of PLCγ2. Finally like thrombin DiC8-PIP3 induced the formation of thromboxane B2 that was inhibited by the Akt inhibitor IV. These studies suggest that PIP3 via Ca(2+) elevation and Akt phosphorylation forms a central role in thromboxane A2 formation and the amplification of platelet activation.

RASA3 is a critical inhibitor of RAP1-dependent platelet activation

The small GTPase RAP1 is critical for platelet activation and thrombus formation. RAP1 activity in platelets is controlled by the GEF CalDAG-GEFI and an unknown regulator that operates downstream of the adenosine diphosphate (ADP) receptor, P2Y12, a target of antithrombotic therapy. Here, we provide evidence that the GAP, RASA3, inhibits platelet activation and provides a link between P2Y12 and activation of the RAP1 signaling pathway. In mice, reduced expression of RASA3 led to premature platelet activation and markedly reduced the life span of circulating platelets. The increased platelet turnover and the resulting thrombocytopenia were reversed by concomitant deletion of the gene encoding CalDAG-GEFI. Rasa3 mutant platelets were hyperresponsive to agonist stimulation, both in vitro and in vivo. Moreover, activation of Rasa3 mutant platelets occurred independently of ADP feedback signaling and was insensitive to inhibitors of P2Y12 or PI3 kinase. Together, our results indicate that RASA3 ensures that circulating platelets remain quiescent by restraining CalDAG-GEFI/RAP1 signaling and suggest that P2Y12 signaling is required to inhibit RASA3 and enable sustained RAP1-dependent platelet activation and thrombus formation at sites of vascular injury. These findings provide insight into the antithrombotic effect of P2Y12 inhibitors and may lead to improved diagnosis and treatment of platelet-related disorders.

Modulation of platelet activation and thrombus formation using a pan-PI3K inhibitor S14161

The phosphatidylinositol 3–kinase (PI3K) signaling pathway is critical in modulating platelet functions. In the present study, we evaluated the effect of S14161, a recently identified pan-class I PI3K inhibitor, on platelet activation and thrombus formation. Results showed that S14161 inhibited human platelet aggregation induced by collagen, thrombin, U46619, and ADP in a dose-dependent manner. Flow cytometric studies showed that S14161 inhibited convulxin- or thrombin-induced P-selectin expression and fibrinogen binding of single platelet. S14161 also inhibited platelet spreading on fibrinogen and clot retraction, processes mediated by outside-in signaling. Using a microfluidic chamber we demonstrated that S14161 decreased platelet adhesion on collagen-coated surface by about 80%. Western blot showed that S14161 inhibited phosphorylation of Akt at both Ser473 and Thr308 sites, and GSK3β at Ser9 in response to collagen, thrombin, or U46619. Comparable studies showed that S14161 has a higher potential bioavailability than LY294002, a prototypical inhibitor of pan-class I PI3K. Finally, the effects of S14161 on thrombus formation in vivo were measured using a ferric chloride-induced carotid artery injury model in mice. The intraperitoneal injection of S14161 (2 mg/kg) to male C57BL/6 mice significantly extended the first occlusion time (5.05±0.99 min, n = 9) compared to the vehicle controls (3.72±0.95 min, n = 8) (P<0.05), but did not prolong the bleeding time (P>0.05). Taken together, our data showed that S14161 inhibits platelet activation and thrombus formation without significant bleeding tendency and toxicity, and considering its potential higher bioavailability, it may be developed as a novel therapeutic agent for the prevention of thrombotic disorders.

Can the antiplatelet effects of cangrelor be reliably studied in mice under in vivo and in vitro conditions using flow cytometry?

BACKGROUND

The effects of blood platelet inhibitors are often not quite equivalent under in vivo and in vitro conditions. Amongst various models of human pathology using laboratory animals, mice offer several benefits that make them convenient tools for studying the putative therapeutic value of various compounds. However, despite its advantages, the mouse model has methodological limitations concerning the small amount of blood available and technical difficulties with its collection. Among the variety of available methods used to study blood platelet activation and/or reactivity, flow cytometry seems an attractive technique that largely minimizes the constraints of using small rodents and enables outcomes of laboratory research to be transferred successfully to clinical practice. In this study we aimed at a critical evaluation of the optimal discriminative flow cytometric protocol, useful for reliable studies of the effect of cangrelor, a P2Y12 receptor antagonist, on mouse platelets under in vitro and in vivo conditions.

METHODS

Blood samples were drawn from two-month-old female BALB/c mice. Protocols differing in methods of anesthesia, blood withdrawal, anticoagulation, gating antibodies, blood preparation and fixation were tested to optimize the one best suited to discrimination between resting and activated platelets. The antiplatelet capabilities of cangrelor were tested in vitro (140 μM in whole blood) and in vivo (7.8 mg/kg b.w. administered once, directly into the bloodstream through the vena cava of the anesthetized animal, 15 min prior to blood withdrawal). Expressions of P-selectin, activated α(IIb)β3 complex and GPIba were monitored using two-color flow cytometry.

RESULTS

"Washed blood" anticoagulated with low molecular weight heparin demonstrated the best discrimination between circulating (resting) platelets and upon their in vitro response to thrombin, collagen or ADP in freshly-stained unfixed cell suspensions. Cangrelor inhibited the expression of the active form of the integrin a(IIb)β3 to approximately the same extent under in vitro and in vivo conditions (84.5 ± 7.7% vs. 75.4 ± 19.5% for the in vitro and in vivo approaches, respectively, n.s.).

CONCLUSIONS

The agreement between the in vivo and in vitro approaches with respect to cangrelor-inhibited hallmarks of blood platelet activation and reactivity supports our proposal that flow cytometry is useful and reliable for determining the effects of antiplatelet agents on the activation of circulating platelets in the mouse model, as well as the in vitro response of platelets to agonists.

Platelet lipidomics: modern day perspective on lipid discovery and characterization in platelets

Lipids are diverse families of biomolecules that perform essential structural and signaling roles in platelets. Their formation and metabolism are tightly controlled by enzymes and signal transduction pathways, and their dysregulation leads to significant defects in platelet function and disease. Platelet activation is associated with significant changes to membrane lipids, and formation of diverse bioactive lipids plays essential roles in hemostasis. In recent years, new generation mass spectrometry analysis of lipids (termed lipidomics) has begun to alter our understanding of how these molecules participate in key cellular processes. Although the application of lipidomics to platelet biology is still in its infancy, seminal earlier studies have shaped our knowledge of how lipids regulate key aspects of platelet biology, including aggregation, shape change, coagulation, and degranulation, as well as how lipids generated by platelets influence other cells, such as leukocytes and the vascular wall, and thus how they regulate hemostasis, vascular integrity, and inflammation, as well as contribute to pathologies, including arterial/deep vein thrombosis and atherosclerosis. This review will provide a brief historical perspective on the characterization of lipids in platelets, then an overview of the new generation lipidomic approaches, their recent application to platelet biology, and future perspectives for research in this area. The major platelet-regulatory lipid families, their formation, metabolism, and their role in health and disease, will be summarized.

Akt signaling in platelets and thrombosis

Akt is a Ser-Thr kinase with pleiotropic effects on cell survival, growth and metabolism. Recent evidence from gene-deletion studies in mice, and analysis of human platelets treated with Akt inhibitors, suggest that Akt regulates platelet activation, with potential consequences for thrombosis. Akt activation is regulated by the level of phosphoinositide 3-phosphates, and proteins that regulate concentrations of this lipid also regulate Akt activation and platelet function. Although the effectors through which Akt contributes to platelet activation are not definitively known, several candidates are discussed, including endothelial nitric oxide synthase, glycogen synthase kinase 3β, phosphodiesterase 3A and the integrin β(3) tail. Selective inhibitors of Akt isoforms or of proteins that contribute to its activation, such as individual PI3K isoforms, may make attractive targets for antithrombotic therapy. This review summarizes the current literature describing Akt activity and its regulation in platelets, including speculation regarding the future of Akt or its regulatory pathways as targets for the development of antithrombotic therapies.

Class I PI 3-kinases signaling in platelet activation and thrombosis: PDK1/Akt/GSK3 axis and impact of PTEN and SHIP1

Class I phosphoinositide 3-kinases (PI3K) have been extensively studied in different models these last years and several isoforms are now promising drug targets to treat cancer and immune diseases. Blood platelets are non-nucleated cells critical for hemostasis and strongly involved in arterial thrombosis, a leading cause of death worldwide. Besides their role in hemostasis and thrombosis, platelets provide an interesting model to characterize the implication of the different isoforms of PI3K in signaling. They are specialized for regulated adhesion, particularly under high shear stress conditions found in arteries and use highly regulated signaling mechanisms to form and stabilize a thrombus. In this review we will highlight the role of class I PI3K in these processes and the pertinence of targeting them in the context of antithrombotic strategies but also the potential consequences on the bleeding risk of inhibiting the PI3K signaling in cancer therapy. The implication of upstream regulators of the most important isoforms of PI3K in platelets and their downstream effectors such as protein kinase B (PKB or Akt) and its target glycogen synthase kinase 3 (GSK3) will be discussed as well as the impact of PTEN and SHIP phosphatases as modulators of this pathway.

The proline-rich tyrosine kinase Pyk2 regulates platelet integrin αIIbβ3 outside-in signaling

BACKGROUND

The proline-rich tyrosine kinase Pyk2 is a focal adhesion kinase expressed in blood platelets, and is activated downstream of G-protein coupled receptors as well as integrin α2β1.

OBJECTIVE

In this study we have investigated the involvement of Pyk2 in integrin αIIbβ3 outside-in signaling in human and murine platelets.

METHODS

We analyzed the stimulation of intracellular signaling pathways in platelets from Pyk2 knockout mice adherent to immobilized fibrinogen.

RESULTS

Pyk2 was rapidly phosphorylated and activated in human and murine platelets adherent to fibrinogen through integrin αIIbβ3. Activation of Pyk2 was Src-dependent, but did not require phospholipase Cγ2 activity. Platelets from Pyk2 knockout mice showed a defective ability to adhere and spread on fibrinogen, in association with a dramatic reduction of phosphatidylinositol 3-kinase (PI3K) activation and Akt phosphorylation. Pharmacological and genetic analysis demonstrated that integrin αIIbβ3 engagement selectively stimulated the β-isoform of PI3K (PI3Kβ), and that, as for Pyk2, PI3Kβ activation required Src family kinases activity, but not phospholipase Cγ2. In fibrinogen-adherent platelets, both Pyk2 and PI3Kβ were necessary for stimulation of the small GTPase Rap1b, a regulator of cell adhesion and spreading. Integrin αIIbβ3 engagement triggered the association of the PI3Kβ regulatory subunit p85 with the adaptor protein c-Cbl, which was mediated by the p85 SH3 domain, and was independent of c-Cbl tyrosine phosphorylation. However, p85-associated c-Cbl was tyrosine phosphorylated by activated Pyk2 in fibrinogen adherent platelets.

CONCLUSIONS

These results identify a novel pathway of integrin αIIbβ3 outside-in signaling and recognize the tyrosine kinase Pyk2 as a major regulator of platelet adhesion and spreading on fibrinogen.

Platelet function in health and disease: from molecular mechanisms, redox considerations to novel therapeutic opportunities

Increased oxidative stress appears to be of fundamental importance in the pathogenesis and development of several disease processes. Indeed, it is well known that reactive oxygen species (ROS) exert critical regulatory functions within the vascular wall, and it is, therefore, plausible that platelets represent a relevant target for their action. Platelet activation cascade (including receptor-mediated tethering to the endothelium, rolling, firm adhesion, aggregation, and thrombus formation) is tightly regulated. In addition to already well-defined platelet regulatory factors, ROS may participate in the regulation of platelet activation. It is already established that enhanced ROS release from the vascular wall can indirectly affect platelet activity by scavenging nitric oxide (NO), thereby decreasing the antiplatelet properties of endothelium. On the other hand, recent data suggest that platelets themselves generate ROS, which may evoke pro-thrombotic responses, triggering many biological processes participating in atherosclerosis initiation, progression, and complication. That oxidative stress may alter platelet function is conceivable when considering that antioxidants play a role in the prevention of cardiovascular disease, although the precise mechanism accounting for changes attributable to antioxidants in atherosclerosis remains unknown. It is possible that the effects of antioxidants may be a consequence of their enhancing or promoting the antiplatelet effects of NO derived from both endothelial cells and platelets. This review focuses on current knowledge regarding ROS-dependent regulation of platelet function in health and disease, and summarizes in vitro and in vivo evidence for their physiological and potential therapeutic relevance.

P2Y12 receptor: platelet thrombus formation and medical interventions

Platelets express a wide range of receptors and proteins that play essential roles in thrombus formation. Among these, the P2Y(12) receptor, a member of the G protein-coupled receptor family, has attracted a significant amount of attention. Stimulation of the P2Y(12) receptor by ADP results in activation of various signaling pathways involved in amplification of platelet activation and aggregation. There have been extensive attempts to design an ideal antithrombotic agent to block P2Y(12), which shows selective expression, as an intervention for cardiovascular disease. Current inhibitors of the P2Y(12) receptor include indirect inhibitor members of the thienopyridine family (ticlopidine, clopidogrel, and prasugrel), and direct P2Y(12) inhibitors (ticagrelor, cangrelor and elinogrel). Of these, clopidogrel is the most commonly prescribed P2Y(12) blocker; however, this product does not fulfill the ideal therapeutic requirements. The main limitations of clopidogrel administration include slow onset, prevention of recovery of platelet functions, and interindividual variability. Hence, advanced studies have been carried out to achieve more efficient and safer P2Y(12) blockade. In this review, we provide a brief but comprehensive report on P2Y(12), its role on platelet thrombus formation, and the targeting of this receptor as an intervention for cardiovascular disease, for the benefit of basic science and clinical researchers.

The Small GTPase Rap1b: A Bidirectional Regulator of Platelet Adhesion Receptors

Integrins and other families of cell adhesion receptors are responsible for platelet adhesion and aggregation, which are essential steps for physiological haemostasis, as well as for the development of thrombosis. The modulation of platelet adhesive properties is the result of a complex pattern of inside-out and outside-in signaling pathways, in which the members of the Rap family of small GTPases are bidirectionally involved. This paper focuses on the regulation of the main Rap GTPase expressed in circulating platelets, Rap1b, downstream of adhesion receptors, and summarizes the most recent achievements in the investigation of the function of this protein as regulator of platelet adhesion and thrombus formation.

C3G transgenic mouse models with specific expression in platelets reveal a new role for C3G in platelet clotting through its GEF activity

We have generated mouse transgenic lineages for C3G (tgC3G) and C3GΔCat (tgC3GΔCat, C3G mutant lacking the GEF domain), where the transgenes are expressed under the control of the megakaryocyte and platelet specific PF4 (platelet factor 4) gene promoter. Transgenic platelet activity has been analyzed through in vivo and in vitro approaches, including bleeding time, aggregation assays and flow cytometry. Both transgenes are expressed (RNA and protein) in purified platelets and megakaryocytes and do not modify the number of platelets in peripheral blood. Transgenic C3G animals showed bleeding times significantly shorter than control animals, while tgC3GΔCat mice presented a remarkable bleeding diathesis as compared to their control siblings. Accordingly, platelets from tgC3G mice showed stronger activation in response to platelet agonists such as thrombin, PMA, ADP or collagen than control platelets, while those from tgC3GΔCat animals had a lower response. In addition, we present data indicating that C3G is a mediator in the PKC pathway leading to Rap1 activation. Remarkably, a significant percentage of tgC3G mice presented a higher level of neutrophils than their control siblings. These results indicate that C3G plays an important role in platelet clotting through a mechanism involving its GEF activity and suggest that it might be also involved in neutrophil development.

P2Y(12) receptors in platelets and other hematopoietic and non-hematopoietic cells

The P2Y(12) receptor is a Gi-coupled ADP receptor first described in blood platelets where it plays a central role in the complex processes of activation and aggregation. Platelet granules store important amounts of ADP which are released upon stimulation by interaction of platelets with the damaged vessel wall. Therefore, the P2Y(12) receptor is a key player in primary hemostasis and in arterial thrombosis and is an established target of antithrombotic drugs like the thienopyridine compounds ticlopidine, clopidogrel, and prasugrel or the direct, reversible antagonists ticagrelor and cangrelor. Beyond the platelet physiology and pharmacology, recent studies have revealed the expression of the P2Y(12) receptor in other hematopoietic cells including leukocyte subtypes and microglia in the central nervous system as well as in vascular smooth muscle cells. These studies indicate putative roles of the P2Y(12) receptor in inflammatory states and diseases of the brain, lung, and blood vessels. The selective role of P2Y(12) among other P2 receptors as well as the possible impact of P2Y(12) targeting drugs in these processes remain to be evaluated.

Role and regulation of phosphatidylinositol 3-kinase β in platelet integrin α2β1 signaling

Integrin α2β1-mediated adhesion of human platelets to monomeric type I collagen or to the GFOGER peptide caused a time-dependent activation of PI3K and Akt phosphorylation. This process was abrogated by pharmacologic inhibition of PI3Kβ, but not of PI3Kγ or PI3Kα. Moreover, Akt phosphorylation was undetectable in murine platelets expressing a kinase-dead mutant of PI3Kβ (PI3Kβ(KD)), but occurred normally in PI3Kγ(KD) platelets. Integrin α2β1 failed to stimulate PI3Kβ in platelets from phospholipase Cγ2 (PLCγ2)-knockout mice, and we found that intracellular Ca(2+) linked PLCγ2 to PI3Kβ activation. Integrin α2β1 also caused a time-dependent stimulation of the focal kinase Pyk2 downstream of PLCγ2 and intracellular Ca(2+). Whereas activation of Pyk2 occurred normally in PI3Kβ(KD) platelets, stimulation of PI3Kβ was strongly reduced in Pyk2-knockout mice. Neither Pyk2 nor PI3Kβ was required for α2β1-mediated adhesion and spreading. However, activation of Rap1b and inside-out stimulation of integrin αIIbβ3 were reduced after inhibition of PI3Kβ and were significantly impaired in Pyk2-deficient platelets. Finally, both PI3Kβ and Pyk2 significantly contributed to thrombus formation under flow. These results demonstrate that Pyk2 regulates PI3Kβ downstream of integrin α2β1, and document a novel role for Pyk2 and PI3Kβ in integrin α2β1 promoted inside-out activation of integrin αIIbβ3 and thrombus formation.

Streptococcus sanguinis-induced cytokine release from platelets

BACKGROUND

There is increasing evidence that both chronic and acute infections play a role in the development and progression of atherothrombotic disorders. One potential mechanism is the direct activation of platelets by bacteria. A wide range of bacterial species activate platelets through heterogeneous mechanisms. The oral micro-organism S. sanguinis stimulates platelet aggregation in vitro in a strain-dependent manner, although there are no reports of associated cytokine production.

OBJECTIVE

The aim of the present study was to determine whether platelet activation by S. sanguinis involved the release of pro-inflammatory and immune modulating factors, and whether activation was enhanced by epinephrine.

METHODS AND RESULTS

Four strains of S. sanguinis and one of S. gordonii stimulated the release of RANTES, PF4, sCD40L and PDGF-AB, whereas only one S. sanguinis strain caused the release of sCD62p. Epinephrine enhanced S. sanguinis-induced platelet aggregation and phosphorylation of phospholipase Cγ2 and Erk, but inhibited RANTES, PF4, sCD40L and PDGF-AB release. Wortmannin inhibited S. sanguinis-induced aggregation and release; however, only aggregation was partially reversed by epinephrine.

CONCLUSIONS

The present study demonstrates that platelets respond to S. sanguinis with both prothrombotic and pro-inflammatory/immune-modulating responses. Epinephrine, potentially released in response to infection and/or stress, can significantly enhance the prothrombotic response, thereby providing a putative link between bacteraemia and acute coronary events during stress. In contrast, epinephrine inhibited the pro-inflammatory/immune-modulating response by an undetermined mechanism.

Distinct roles for Rap1b protein in platelet secretion and integrin αIIbβ3 outside-in signaling

Rap1b is activated by platelet agonists and plays a critical role in integrin α(IIb)β(3) inside-out signaling and platelet aggregation. Here we show that agonist-induced Rap1b activation plays an important role in stimulating secretion of platelet granules. We also show that α(IIb)β(3) outside-in signaling can activate Rap1b, and integrin outside-in signaling-mediated Rap1b activation is important in facilitating platelet spreading on fibrinogen and clot retraction. Rap1b-deficient platelets had diminished ATP secretion and P-selectin expression induced by thrombin or collagen. Importantly, addition of low doses of ADP and/or fibrinogen restored aggregation of Rap1b-deficient platelets. Furthermore, we found that Rap1b was activated by platelet spreading on immobilized fibrinogen, a process that was not affected by P2Y(12) or TXA(2) receptor deficiency, but was inhibited by the selective Src inhibitor PP2, the PKC inhibitor Ro-31-8220, or the calcium chelator demethyl-1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis. Clot retraction was abolished, and platelet spreading on fibrinogen was diminished in Rap1b-deficient platelets compared with wild-type controls. The defects in clot retraction and spreading on fibrinogen of Rap1b-deficient platelets were not rescued by addition of MnCl(2), which elicits α(IIb)β(3) outside-in signaling in the absence of inside-out signaling. Thus, our results reveal two different activation mechanisms of Rap1b as well as novel functions of Rap1b in platelet secretion and in integrin α(IIb)β(3) outside-in signaling.

P2 receptors and platelet function

Following vessel wall injury, platelets adhere to the exposed subendothelium, become activated and release mediators such as TXA(2) and nucleotides stored at very high concentration in the so-called dense granules. Released nucleotides and other soluble agents act in a positive feedback mechanism to cause further platelet activation and amplify platelet responses induced by agents such as thrombin or collagen. Adenine nucleotides act on platelets through three distinct P2 receptors: two are G protein-coupled ADP receptors, namely the P2Y(1) and P2Y(12) receptor subtypes, while the P2X(1) receptor ligand-gated cation channel is activated by ATP. The P2Y(1) receptor initiates platelet aggregation but is not sufficient for a full platelet aggregation in response to ADP, while the P2Y(12) receptor is responsible for completion of the aggregation to ADP. The latter receptor, the molecular target of the antithrombotic drugs clopidogrel, prasugrel and ticagrelor, is responsible for most of the potentiating effects of ADP when platelets are stimulated by agents such as thrombin, collagen or immune complexes. The P2X(1) receptor is involved in platelet shape change and in activation by collagen under shear conditions. Each of these receptors is coupled to specific signal transduction pathways in response to ADP or ATP and is differentially involved in all the sequential events involved in platelet function and haemostasis. As such, they represent potential targets for antithrombotic drugs.

Signaling during platelet adhesion and activation

Upon vascular injury, platelets are activated by adhesion to adhesive proteins, such as von Willebrand factor and collagen, or by soluble platelet agonists, such as ADP, thrombin, and thromboxane A(2). These adhesive proteins and soluble agonists induce signal transduction via their respective receptors. The various receptor-specific platelet activation signaling pathways converge into common signaling events that stimulate platelet shape change and granule secretion and ultimately induce the "inside-out" signaling process leading to activation of the ligand-binding function of integrin α(IIb)β(3). Ligand binding to integrin α(IIb)β(3) mediates platelet adhesion and aggregation and triggers "outside-in" signaling, resulting in platelet spreading, additional granule secretion, stabilization of platelet adhesion and aggregation, and clot retraction. It has become increasingly evident that agonist-induced platelet activation signals also cross talk with integrin outside-in signals to regulate platelet responses. Platelet activation involves a series of rapid positive feedback loops that greatly amplify initial activation signals and enable robust platelet recruitment and thrombus stabilization. Recent studies have provided novel insight into the molecular mechanisms of these processes.

Phosphoinositide 3-kinase induced activation and cytoskeletal translocation of protein kinase CK2 in protease activated receptor 1-stimulated platelets

CK2 is a highly conserved protein kinase involved in several cellular events. CK2 is expressed in platelets but its role in platelet activation remains poorly understood. In the present study, we tested the hypothesis that CK2 plays a role in platelet activation, particularly in the PAR1-dependent signal transduction pathway. The effect of CK2 and PI 3-kinase inhibitors on aggregation of platelets, activation of GPIIb/IIIa, activation and translocation of CK2 was examined. Platelets were incubated with the cell permeable CK2 inhibitors, DRB, DMAT and TBB and stimulated with the PAR1-AP (SFLLRNP). CK2 inhibitors showed the specific inhibitory pattern of platelet aggregation, characterized by a primary phase of aggregation followed by progressive disaggregation. CK2 inhibitors suppressed the activation of GPIIb/IIIa. PAR1-AP induced two-fold increase in CK2 activity and stimulated the translocation of CK2 from Triton X-100-soluble to -insoluble fraction. Preincubation of platelets with the PI 3-kinase inhibitor, wortmannin or LY294002, impaired PAR1-AP-induced aggregation of platelets. PAR1-AP-induced increase in CK2 activity and translocation of CK2 were inhibited by these treatments. Taken together, the present study demonstrated, for the first time, that PI 3-kinase-CK2 pathway plays an important role in the mechanism of PAR1-dependent platelet aggregation.

Role of phosphoinositide 3-kinase beta in platelet aggregation and thromboxane A2 generation mediated by Gi signalling pathways

PI3Ks (phosphoinositide 3-kinases) play a critical role in platelet functional responses. PI3Ks are activated upon P2Y12 receptor stimulation and generate pro-aggregatory signals. P2Y12 receptor has been shown to play a key role in the platelet aggregation and thromboxane A2 generation caused by co-stimulation with Gq or Gz, or super-stimulation of Gi pathways. In the present study, we evaluated the role of specific PI3K isoforms alpha, beta, gamma and delta in platelet aggregation, thromboxane A2 generation and ERK (extracellular-signal-regulated kinase) activation. Our results show that loss of the PI3K signal impaired the ability of ADP to induce platelet aggregation, ERK phosphorylation and thromboxane A2 generation. We also show that Gq plus Gi- or Gi plus Gz-mediated platelet aggregation, ERK phosphorylation and thromboxane A2 generation in human platelets was inhibited by TGX-221, a PI3Kbeta-selective inhibitor, but not by PIK75 (a PI3Kalpha inhibitor), AS252424 (a PI3Kgamma inhibitor) or IC87114 (a PI3Kdelta inhibitor). TGX-221 also showed a similar inhibitory effect on the Gi plus Gz-mediated platelet responses in platelets from P2Y1-/- mice. Finally, 2MeSADP (2-methyl-thio-ADP)-induced Akt phosphorylation was significantly inhibited in the presence of TGX-221, suggesting a critical role for PI3Kbeta in Gi-mediated signalling. Taken together, our results demonstrate that PI3Kbeta plays an important role in ADP-induced platelet aggregation. Moreover, PI3Kbeta mediates ADP-induced thromboxane A2 generation by regulating ERK phosphorylation.

PI 3-Kinase p110β regulation of platelet integrin α(IIb)β3

Hemopoietic cells express relatively high levels of the type I phosphoinositide (PI) 3-kinase isoforms, with p110δ and γ exhibiting specialized signaling functions in neutrophils, monocytes, mast cells, and lymphocytes. In platelets, p110β appears to be the dominant PI 3-kinase isoform regulating platelet activation, irrespective of the nature of the primary platelet activating stimulus. Based on findings with isoform-selective p110β pharmacological inhibitors and more recently with p110β-deficient platelets, p110β appears to primarily signal downstream of G(i)- and tyrosine kinase-coupled receptors. Functionally, inhibition of p110β kinase function leads to a marked defect in integrin α(IIb)β₃ adhesion and reduced platelet thrombus formation in vivo. This defect in platelet adhesive function is not associated with increased bleeding, suggesting that therapeutic targeting of p110β may represent a safe approach to reduce thrombotic complications in patients with cardiovascular disease.

Non-redundant roles of phosphoinositide 3-kinase isoforms alpha and beta in glycoprotein VI-induced platelet signaling and thrombus formation

Platelets are activated by adhesion to vascular collagen via the immunoglobulin receptor, glycoprotein VI (GPVI). This causes potent signaling toward activation of phospholipase Cgamma2, which bears similarity to the signaling pathway evoked by T- and B-cell receptors. Phosphoinositide 3-kinase (PI3K) plays an important role in collagen-induced platelet activation, because this activity modulates the autocrine effects of secreted ADP. Here, we identified the PI3K isoforms directly downstream of GPVI in human and mouse platelets and determined their role in GPVI-dependent thrombus formation. The targeting of platelet PI3Kalpha or -beta strongly and selectively suppressed GPVI-induced Ca(2+) mobilization and inositol 1,4,5-triphosphate production, thus demonstrating enhancement of phospholipase Cgamma2 by PI3Kalpha/beta. That PI3Kalpha and -beta have a non-redundant function in GPVI-induced platelet activation and thrombus formation was concluded from measurements of: (i) serine phosphorylation of Akt, (ii) dense granule secretion, (iii) intracellular Ca(2+) increases and surface expression of phosphatidylserine under flow, and (iv) thrombus formation, under conditions where PI3Kalpha/beta was blocked or p85alpha was deficient. In contrast, GPVI-induced platelet activation was insensitive to inhibition or deficiency of PI3Kdelta or -gamma. Furthermore, PI3Kalpha/beta, but not PI3Kgamma, contributed to GPVI-induced Rap1b activation and, surprisingly, also to Rap1b-independent platelet activation via GPVI. Together, these findings demonstrate that both PI3Kalpha and -beta isoforms are required for full GPVI-dependent platelet Ca(2+) signaling and thrombus formation, partly independently of Rap1b. This provides a new mechanistic explanation for the anti-thrombotic effect of PI3K inhibition and makes PI3Kalpha an interesting new target for anti-platelet therapy.

Phosphoinositide 3-kinase p110 beta regulates integrin alpha IIb beta 3 avidity and the cellular transmission of contractile forces

Phosphoinositide (PI) 3-kinase (PI3K) signaling processes play an important role in regulating the adhesive function of integrin alpha(IIb)beta(3), necessary for platelet spreading and sustained platelet aggregation. PI3K inhibitors are effective at reducing platelet aggregation and thrombus formation in vivo and as a consequence are currently being evaluated as novel antithrombotic agents. PI3K regulation of integrin alpha(IIb)beta(3) activation (affinity modulation) primarily occurs downstream of G(i)-coupled and tyrosine kinase-linked receptors linked to the activation of Rap1b, AKT, and phospholipase C. In the present study, we demonstrate an important role for PI3Ks in regulating the avidity (strength of adhesion) of high affinity integrin alpha(IIb)beta(3) bonds, necessary for the cellular transmission of contractile forces. Using knock-out mouse models and isoform-selective PI3K inhibitors, we demonstrate that the Type Ia p110 beta isoform plays a major role in regulating thrombin-stimulated fibrin clot retraction in vitro. Reduced clot retraction induced by PI3K inhibitors was not associated with defects in integrin alpha(IIb)beta(3) activation, actin polymerization, or actomyosin contractility but was associated with a defect in integrin alpha(IIb)beta(3) association with the contractile cytoskeleton. Analysis of integrin alpha(IIb)beta(3) adhesion contacts using total internal reflection fluorescence microscopy revealed an important role for PI3Ks in regulating the stability of high affinity integrin alpha(IIb)beta(3) bonds. These studies demonstrate an important role for PI3K p110 beta in regulating the avidity of high affinity integrin alpha(IIb)beta(3) receptors, necessary for the cellular transmission of contractile forces. These findings may provide new insight into the potential antithrombotic properties of PI3K p110 beta inhibitors.

Genetic evidence for a predominant role of PI3Kβ catalytic activity in ITAM- and integrin-mediated signaling in platelets

Phosphatidylinositol 3-kinase (PI3K) isoforms PI3Kβ and PI3Kγ are implicated in platelet adhesion, activation, and aggregation, but their relative contribution is still unclear or controversial. Here, we report the first comparative functional analysis of platelets from mice expressing a catalytically inactive form of PI3Kβ or PI3Kγ. We demonstrate that both isoforms were similarly required for maximal activation of the small GTPase Rap1b and for complete platelet aggregation upon stimulation of G protein–coupled receptors for adenosine 5′-diphosphate (ADP) or U46619. Their contribution to these events, however, was largely redundant and dispensable. However, PI3Kβ, but not PI3Kγ, enzymatic activity was absolutely required for Akt phosphorylation, Rap1 activation, and platelet aggregation downstream of the immunoreceptor tyrosine-based activation motif (ITAM)–bearing receptor glycoprotein VI (GPVI). Moreover, PI3Kβ was a major essential regulator of platelet adhesion to fibrinogen and of integrin αIIbβ3-mediated spreading. These results provide genetic evidence for a crucial and selective role of PI3Kβ in signaling through GPVI and integrin αIIbβ3.

Role of phosphoinositide 3-kinase beta in glycoprotein VI-mediated Akt activation in platelets

Glycoprotein (GP) VI is a critical platelet collagen receptor. Phosphoinositide 3-kinase (PI3K) plays an important role in GPVI-mediated platelet activation, yet the major PI3K isoforms involved in this process have not been identified. In addition, stimulation of GPVI results in the activation of Akt, a downstream effector of PI3K. Thus, we investigated the contribution of PI3K isoforms to GPVI-mediated platelet activation and Akt activation. A protein kinase C inhibitor GF 109203X or a P2Y(12) receptor antagonist AR-C69931MX partly reduced GPVI-induced Akt phosphorylation. Platelets from mice dosed with clopidogrel also showed partial Akt phosphorylation, indicating that GPVI-mediated Akt phosphorylation is regulated by both secretion-dependent and -independent pathways. In addition, GPVI-induced Akt phosphorylation in the presence of ADP antagonists was completely inhibited by PI3K inhibitor LY294002 and PI3Kbeta inhibitor TGX-221 indicating an essential role of PI3Kbeta in Akt activation directly downstream of GPVI. Moreover, GPVI-mediated platelet aggregation, secretion, and intracellular Ca(2+) mobilization were significantly inhibited by TGX-221, and less strongly inhibited by PI3Kalpha inhibitor PIK75, but were not affected by PI3Kgamma inhibitor AS252424 and PI3Kdelta inhibitor IC87114. Consistently, GPVI-induced integrin alpha(IIb)beta(3) activation of PI3Kgamma(-/-) and PI3Kdelta(-/-) platelets also showed no significant difference compared with wild-type platelets. These results demonstrate that GPVI-induced Akt activation in platelets is dependent in part on G(i) stimulation through P2Y(12) receptor activation by secreted ADP. In addition, a significant portion of GPVI-dependent, ADP-independent Akt activation also exists, and PI3Kbeta plays an essential role in GPVI-mediated platelet aggregation and Akt activation.

Epinephrine induces intracellular Ca2+mobilization in thrombin-desensitized platelets: a role for GPIb-IX-V

In this work we have investigated the ability of epinephrine to trigger the release of intracellular Ca2+ in thrombin-desensitized platelets. Addition of thrombin to platelets in the presence of extracellular EGTA caused a rapid and transient release of Ca2+ from intracellular stores and rendered platelets unresponsive to a second addition of the same agonist. Although epinephrine alone had no effect on intracellular Ca2+ mobilization, its addition to thrombin-desensitized platelets was associated to a rapid and evident secondary release of intracellular Ca2+. This effect of epinephrine was not observed when platelets were desensitized with other agonists able to induce phospholipase C activation, including convulxin, U46619, and ADP. Although the platelet receptor for epinephrine is coupled to the Gi family member Gz, no secondary Ca2+ release was seen in thrombin-desensitized platelets upon stimulation of other Gi-coupled receptors, including the P2Y12 receptor and the CXCR4. Addition of hirudin to thrombin-desensitized platelets prevented epinephrine-promoted secondary release of Ca2+, indicating that thrombin, rather than epinephrine itself, is actually responsible for this event as a consequence of thrombin receptors resensitization. Studies with platelets stimulated with specific PAR1- and PAR4- activating peptides proved that neither one of these thrombin receptors were involved in the secondary epinephrine-assisted Ca2+ release. Moreover, we found that thrombin was still able to induce a reduced, but evident release of Ca2+ from internal stores in PAR1- and PAR4-desensitized platelets, which could be followed by a secondary Ca2+ release upon subsequent addition of epinephrine. Importantly, both the primary and the secondary Ca2+ release induced by thrombin and epinephrine in PAR1- and PAR4-desensitized platelets were abrogated upon cleavage of GPIbalpha by the metalloproteinase mocarhagin. These results demonstrate a direct role of thrombin binding to GPIb-IX-V in the mobilization of Ca2+ from intracellular stores, and reveal that epinephrine can restore this process in desensitized platelets, thus prolonging the effect of thrombin stimulation.

Phosphoinositide 3-kinase signalling in the vascular system

Phosphoinositide 3-kinases (PI3Ks) are protein and lipid kinases activated by different classes of membrane receptors, including G-protein coupled and tyrosine kinase receptors. Several lines of evidence have uncovered specific roles for distinct PI3K isoforms in the vascular system in both physiology and disease. The present review will summarize and discuss the most recent advances regarding PI3K-Akt signalling in endothelial cells, vascular smooth muscle cells, platelets, and inflammatory cells involved in the atherosclerotic process. Of interest, the development of novel isoform-selective PI3K inhibitor drugs offers a unique opportunity to selectively and differentially target PI3K-driven pathways in the vascular system and may give rise to new strategies for the treatment of cardiovascular diseases.

The Gi-coupled P2Y12 receptor regulates diacylglycerol-mediated signaling in human platelets

Stimulation of G(q)-coupled receptors activates phospholipase C and is supposed to promote both intracellular Ca(2+) mobilization and protein kinase C (PKC) activation. We found that ADP-induced phosphorylation of pleckstrin, the main platelet substrate for PKC, was completely inhibited not only by an antagonist of the G(q)-coupled P2Y1 receptor but also upon blockade of the G(i)-coupled P2Y12 receptor. The role of G(i) on PKC regulation required stimulation of phosphatidylinositol 3-kinase rather than inhibition of adenylyl cyclase. P2Y12 antagonists also inhibited pleckstrin phosphorylation, Rap1b activation, and platelet aggregation induced upon G(q) stimulation by the thromboxane A(2) analogue U46619. Importantly, activation of phospholipase C and intracellular Ca(2+) mobilization occurred normally. Phorbol 12-myristate 13-acetate overcame the inhibitory effect of P2Y12 receptor blockade on PKC activation but not on Rap1b activation and platelet aggregation. By contrast, inhibition of diacylglycerol kinase restored both PKC and Rap1b activity and caused platelet aggregation. Stimulation of P2Y12 receptor or direct inhibition of diacylglycerol kinase potentiated the effect of membrane-permeable sn-1,2-dioctanoylglycerol on platelet aggregation and pleckstrin phosphorylation, in association with inhibition of its phosphorylation to phosphatidic acid. These results reveal a novel and unexpected role of the G(i)-coupled P2Y12 receptor in the regulation of diacylglycerol-mediated events in activated platelets.

Phosphatidylinositol-3-kinase-gamma is integral to homing functions of progenitor cells

Endothelial progenitor cells (EPCs) and hematopoietic progenitor cells are recruited to ischemic regions, improving neovascularization. beta1 and beta2 integrins play a crucial role for progenitor cell homing to ischemic tissues. Integrin activity is regulated by chemokines and their respective G protein-coupled receptors. The phosphatidylinositol-3-kinase catalytic subunit gamma (PI3Kgamma) is the PI3K isoform that selectively transduces signals from G protein-coupled receptors. Here, we investigated the role of PI3Kgamma as a signaling intermediate in the chemokine-induced integrin-dependent homing functions of progenitor cells. A pharmacological PI3Kgamma inhibitor significantly reduced chemokine-induced chemotaxis and stromal cell-derived factor (SDF)1alpha-induced transmigration of human EPCs. Moreover, the PI3Kgamma inhibitor significantly reduced SDF1alpha-induced adhesion of EPCs to intercellular adhesion molecule-1 and human umbilical vein endothelial cell monolayers. These findings were corroborated with Lin(-) bone marrow-derived progenitor cells from PI3Kgamma-deficient mice that displayed reduced SDF1alpha-induced migration and intercellular adhesion molecule-1 adhesion as compared with wild-type cells. Pharmacological inhibition or genetic ablation of PI3Kgamma reduced SDF1alpha-induced integrin activation in human EPCs and in murine Lin(-) BM-derived progenitor cells, respectively. In vivo, the homing of PI3Kgamma-deficient Lin(-) progenitor cells to ischemic muscles after intravenous infusion in the model of hindlimb ischemia and their neovascularization-promoting capacity was reduced as compared with wild-type cells. In conclusion, PI3Kgamma is integral to the integrin-dependent homing of progenitor cells.

Identification of a unique co-operative phosphoinositide 3-kinase signaling mechanism regulating integrin alpha IIb beta 3 adhesive function in platelets

Phosphoinositide (PI) 3-kinases play an important role in regulating the adhesive function of a variety of cell types through affinity modulation of integrins. Two type I PI 3-kinase isoforms (p110 beta and p110 gamma) have been implicated in G(i)-dependent integrin alpha(IIb)beta(3) regulation in platelets, however, the mechanisms by which they coordinate their signaling function remains unknown. By employing isoform-selective PI 3-kinase inhibitors and knock-out mouse models we have identified a unique mechanism of PI 3-kinase signaling co-operativity in platelets. We demonstrate that p110 beta is primarily responsible for G(i)-dependent phosphatidylinositol 3,4-bisphosphate (PI(3,4)P(2)) production in ADP-stimulated platelets and is linked to the activation of Rap1b and AKT. In contrast, defective integrin alpha(IIb)beta(3) activation in p110 gamma(-/-) platelets was not associated with alterations in the levels of PI(3,4)P(2) or active Rap1b/AKT. Analysis of the effects of active site pharmacological inhibitors confirmed that p110 gamma principally regulated integrin alpha(IIb)beta(3) activation through a non-catalytic signaling mechanism. Inhibition of the kinase function of PI 3-kinases, combined with deletion of p110 gamma, led to a major reduction in integrin alpha(IIb)beta(3) activation, resulting in a profound defect in platelet aggregation, hemostatic plug formation, and arterial thrombosis. These studies demonstrate a kinase-independent signaling function for p110 gamma in platelets. Moreover, they demonstrate that the combined catalytic and non-catalytic signaling function of p110 beta and p110 gamma is critical for P2Y(12)/G(i)-dependent integrin alpha(IIb)beta(3) regulation. These findings have potentially important implications for the rationale design of novel antiplatelet therapies targeting PI 3-kinase signaling pathways.