A Gi-dependent pathway is required for activation of the small GTPase Rap1B in human 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.

Reversible Platelet Integrin αIIbβ3 Activation and Thrombus Instability

Integrin αIIbβ3 activation is essential for platelet aggregation and, accordingly, for hemostasis and arterial thrombosis. The αIIbβ3 integrin is highly expressed on platelets and requires an activation step for binding to fibrinogen, fibrin or von Willebrand factor (VWF). A current model assumes that the process of integrin activation relies on actomyosin force-dependent molecular changes from a bent-closed and extended-closed to an extended-open conformation. In this paper we review the pathways that point to a functional reversibility of platelet αIIbβ3 activation and transient aggregation. Furthermore, we refer to mouse models indicating that genetic defects that lead to reversible platelet aggregation can also cause instable thrombus formation. We discuss the platelet agonists and signaling pathways that lead to a transient binding of ligands to integrin αIIbβ3. Our analysis points to the (autocrine) ADP P2Y₁ and P2Y₁₂ receptor signaling via phosphoinositide 3-kinases and Akt as principal pathways linked to reversible integrin activation. Downstream signaling events by protein kinase C, CalDAG-GEFI and Rap1b have not been linked to transient integrin activation. Insight into the functional reversibility of integrin activation pathways will help to better understand the effects of antiplatelet agents.

G protein-coupled purinergic P2Y receptor oligomerization: Pharmacological changes and dynamic regulation

P2Y receptors (P2YRs) are a δ group of rhodopsin-like G protein-coupled receptors (GPCRs) with many essential functions in physiology and pathology, such as platelet aggregation, immune responses, neuroprotective effects, inflammation, and cellular proliferation. Thus, they are among the most researched therapeutic targets used for the clinical treatment of diseases (e.g., the antithrombotic drug clopidogrel and the dry eye treatment drug diquafosol). GPCRs transmit signals as dimers to increase the diversity of signalling pathways and pharmacological activities. Many studies have frequently confirmed dimerization between P2YRs and other GPCRs due to their functions in cardiovascular and cerebrovascular processes in vivo and in vitro. Recently, some P2YR dimers that dynamically balance physiological functions in the body were shown to be involved in effective signal transduction and exert pathological responses. In this review, we summarize the types, pharmacological changes, and active regulators of P2YR-related dimerization, and delineate new functions and pharmacological activities of P2YR-related dimers, which may be a novel direction to improve the effectiveness of medications.

Endothelial Shear Stress and Platelet FcγRIIa Expression in Intracranial Atherosclerotic Disease

Intracranial atherosclerotic disease (ICAD) has been characterized by the degree of arterial stenosis and downstream hypoperfusion, yet microscopic derangements of endothelial shear stress at the luminal wall may be key determinants of plaque growth, vascular remodeling and thrombosis that culminate in recurrent stroke. Platelet interactions have similarly been a principal focus of treatment, however, the mechanistic basis of anti-platelet strategies is largely extrapolated rather than directly investigated in ICAD. Platelet FcγRIIa expression has been identified as a potent risk factor in cardiovascular disease, as elevated expression markedly increases the risk of recurrent events. Differential activation of the platelet FcγRIIa receptor may also explain the variable response of individual patients to anti-platelet medications. We review existing data on endothelial shear stress and potential interactions with the platelet FcγRIIa receptor that may alter the evolving impact of ICAD, based on local pathophysiology at the site of arterial stenosis. Current methods for quantification of endothelial shear stress and platelet activation are described, including tools that may be readily adapted to the clinical realm for further understanding of ICAD.

C3G contributes to platelet activation and aggregation by regulating major signaling pathways

C3G is a GEF (guanine nucleotide exchange factor) for Rap GTPases, among which the isoform Rap1b is an essential protein in platelet biology. Using transgenic mouse models with platelet-specific overexpression of C3G or mutant C3GΔCat, we have unveiled a new function of C3G in regulating the hemostatic function of platelets through its participation in the thrombin-PKC-Rap1b pathway. C3G also plays important roles in angiogenesis, tumor growth, and metastasis through its regulation of the platelet secretome. In addition, C3G contributes to megakaryopoiesis and thrombopoiesis. Here, we used a platelet-specific C3G-KO mouse model to further support the role of C3G in hemostasis. C3G-KO platelets showed a significant delay in platelet activation and aggregation as a consequence of the defective activation of Rap1, which resulted in decreased thrombus formation in vivo. Additionally, we explored the contribution of C3G-Rap1b to platelet signaling pathways triggered by thrombin, PMA or ADP, in the referenced transgenic mouse model, through the use of a battery of specific inhibitors. We found that platelet C3G is phosphorylated at Tyr504 by a mechanism involving PKC-Src. This phosphorylation was shown to be positively regulated by ERKs through their inhibition of the tyrosine phosphatase Shp2. Moreover, C3G participates in the ADP-P2Y12-PI3K-Rap1b pathway and is a mediator of thrombin-TXA2 activities. However, it inhibits the synthesis of TXA2 through cPLA2 regulation. Taken together, our data reveal the critical role of C3G in the main pathways leading to platelet activation and aggregation through the regulation of Rap1b.

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.

Variation in platelet expression of FcγRIIa after myocardial infarction

FcγRIIa amplifies platelet activation and greater platelet expression of FcγRIIa identifies patients at greater risk of subsequent cardiovascular events. Thus, platelet expression of FcγRIIa may be useful to guide therapy. Because platelet function tests are impacted by preparative procedures and substantial intra-individual variability, we examined the impact of these factors on platelet expression of FcγRIIa in blood from healthy subjects and in patients after myocardial infarction (MI). Platelet expression of FcγRIIa was quantified with the use of flow cytometry. Blood was taken from healthy subjects and 114 patients after a MI in whom platelet expression of FcγRIIa was quantified before discharge and at 6 ± 1 months. Neither anticoagulants nor the antiplatelet agent cangrelor changed platelet expression of FcγRIIa. Intra-individual variation in platelet FcγRIIa expression was 8.5% ± 5% over the course of 1 month in healthy subjects. Platelet FcγRIIa expression was within 20% of the baseline value after 6 months in 71% of patients after MI. In summary, because FcγRIIa is a protein on the surface of platelets, assay conditions and antiplatelet agents do not change expression. Intra-individual variability in platelet expression of FcγRIIa is modest. Accordingly, platelet expression of FcγRIIa is a marker of increased platelet reactivity that can be reliably and repeatedly measured.Clinical Trial Registration: NCT02505217.

Epinephrine restores platelet functions inhibited by ticagrelor: A mechanistic approach

Ticagrelor, an antagonist of the platelet adenosine diphosphate (ADP)-P2Y₁₂ receptor is recommended for patients with acute coronary syndromes. However, ticagrelor exposes to a risk of bleeding, the management of which is challenging because platelet transfusion is ineffective, and no antidote is yet available. We hypothesized that the vasopressor drug epinephrine could counter the antiplatelet effects of ticagrelor and restore platelet functions. We assessed in vitro the efficiency of epinephrine in restoring platelet aggregation inhibited by ticagrelor and investigated the underlying mechanisms. Washed platelet aggregation and secretion were measured upon stimulation by epinephrine alone or in combination with ADP, in the presence or absence of ticagrelor. Mechanistic investigations used P2Y₁ and phosphoinositide 3-kinase (PI3K) inhibitors and included vasodilator-stimulated phosphoprotein (VASP) and Akt phosphorylation assays as well as measurement of Ca²⁺ mobilisation. We found that epinephrine restored ADP-induced platelet aggregation, but not dense granule release. Epinephrine alone failed to induce aggregation whereas it fully induced VASP dephosphorylation and Akt phosphorylation regardless of the presence of ticagrelor. In the presence of ticagrelor, blockage of the P2Y₁ receptor prevented restoration of platelet aggregation by the combination of epinephrine and ADP, as well as intracellular Ca²⁺ mobilisation. In combination with ADP, epinephrine induced platelet aggregation of ticagrelor-treated platelets through inhibition of the cAMP pathway and activation of the PI3K pathway, thus enabling the P2Y₁ receptor signalling and subsequent Ca²⁺ mobilisation. This proof-of-concept study needs to be challenged in vivo for the management of bleeding in ticagrelor-treated patients.

The role of P2Y12 receptor in ischemic stroke of atherosclerotic origin

Atherosclerosis is a chronic and progressive disease of the arterial walls and a leading cause of non-cardioembolic ischemic stroke. P2Y12 is a well-recognized receptor that is expressed on platelets and is a target of thienopyridine-type antiplatelet drugs. In the last few decades, P2Y12 receptor inhibitors, such as clopidogrel, have been applied for the secondary prevention of non-cardioembolic ischemic stroke. Recent clinical studies have suggested that these P2Y12 receptor inhibitors may be more effective than other antiplatelet drugs in patients with ischemic stroke/transient ischemic attack of atherosclerotic origin. Moreover, animal studies have also shown that the P2Y12 receptor may participate in atherogenesis by promoting the proliferation and migration of vascular smooth muscle cells (VSMCs) and endothelial dysfunction, and affecting inflammatory cell activities in addition to amplifying and maintaining ADP-induced platelet activation and platelet aggregation. P2Y12 receptor inhibitors may also exert neuroprotective effects after ischemic stroke. Thus, P2Y12 receptor inhibitors may be a better choice for secondary prevention in patients with atherosclerotic ischemic stroke subtypes because of their triple functions (i.e., their anti-atherosclerotic, anti-platelet aggregation, and neuroprotective activities), and the P2Y12 receptor may also serve as a noval therapeutic target for atherosclerosis. In this review, we summarize the current knowledge on the P2Y12 receptor and its key roles in atherosclerosis and ischemic stroke of atherosclerotic origin.

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.

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.

The Phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) Binder Rasa3 Regulates Phosphoinositide 3-kinase (PI3K)-dependent Integrin αIIbβ3 Outside-in Signaling

The class I PI3K family of lipid kinases plays an important role in integrin α_IIbβ₃ function, thereby supporting thrombus growth and consolidation. Here, we identify Ras/Rap1GAP Rasa3 (GAP1^IP4BP) as a major phosphatidylinositol 3,4,5-trisphosphate-binding protein in human platelets and a key regulator of integrin α_IIbβ₃ outside-in signaling. We demonstrate that cytosolic Rasa3 translocates to the plasma membrane in a PI3K-dependent manner upon activation of human platelets. Expression of wild-type Rasa3 in integrin α_IIbβ₃-expressing CHO cells blocked Rap1 activity and integrin α_IIbβ₃-mediated spreading on fibrinogen. In contrast, Rap1GAP-deficient (P489V) and Ras/Rap1GAP-deficient (R371Q) Rasa3 had no effect. We furthermore show that two Rasa3 mutants (H794L and G125V), which are expressed in different mouse models of thrombocytopenia, lack both Ras and Rap1GAP activity and do not affect integrin α_IIbβ₃-mediated spreading of CHO cells on fibrinogen. Platelets from thrombocytopenic mice expressing GAP-deficient Rasa3 (H794L) show increased spreading on fibrinogen, which in contrast to wild-type platelets is insensitive to PI3K inhibitors. Together, these results support an important role for Rasa3 in PI3K-dependent integrin α_IIbβ₃-mediated outside-in signaling and cell spreading.

Platelets and hemostasis: a new perspective on an old subject

Publisher's Note: This article has a companion Counterpoint by Kapur and Semple. Publisher's Note: Join in the discussion of these articles at Blood Advances Community Conversations.

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.

Bacteria-induced intracellular signalling in platelets

Multiple studies have now shown that various species of bacteria can stimulate platelets; many in a strain and donor-dependent manner. The signalling pathways underlying this platelet activation has been the subject of scrutiny for the last decade. The best-delineated pathway is that in response to Streptococcal species, such as Streptococcus sanguinis (S. sanguinis), Streptococcus gordonii (S. gordonii) and Streptococcus oralis (S. oralis), where a pathway is initiated by the engagement of the low affinity IgG receptor, FcγRIIA. This leads to and involves the tyrosine kinase Syk, the adaptor protein Linker of Activated T Cells (LAT) and subsequently both phospholipase Cγ2 (PLCγ2) and phosphatidylinositol-3-kinase (PI-3-K). Finally, this leads to the expression of the αIIbβ3 integrin, the synthesis and release of thromboxane A2 (T × A2) and the exocytosis of PF4, each of which plays a crucial role in secondary signalling and full platelet activation. Roles for other signalling pathways in Streptococcal-induced platelet activation are less clear, although an ADP-mediated inhibition of adenylyl cyclase, a glycoprotein Ib/IX/V-mediated pathway and perhaps a complement-induced pathway have each been proposed. Platelet activation by Porphyromonas gingivalis (P. gingivalis) at least partially shares the FcγRIIA/Syk/PLCγ2/PI-3-K mechanism utilised by Streptococcal species. However, it has also been suggested that P. gingivalis activates platelets by two additional methods; stimulation of the protease-activated receptors leading to activation of phospholipase Cβ (PLCβ), and the engagement of Toll-like receptors 2 and 4 by released lipopolysaccharide leading to an ill-defined pathway which may involve PI-3-K. Consequently, it appears that bacteria can stimulate platelets by eliciting multiple signalling pathways some of which are common, and some unique, to individual species.

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.

Association analysis of member RAS oncogene family gene polymorphisms with aspirin intolerance in asthmatic patients

Member RAS oncogene family (RAB1A), a member of the RAS oncogene family, cycles between inactive GDP-bound and active GTP-bound forms regulating vesicle transport in exocytosis. Thus, functional alterations of the RAB1A gene may contribute to aspirin intolerance in asthmatic sufferers. To investigate the relationship between single-nucleotide polymorphisms (SNPs) in the RAB1A gene and aspirin-exacerbated respiratory disease (AERD), asthmatics (n=1197) were categorized into AERD and aspirin-tolerant asthma (ATA). All subjects were diagnosed as asthma on the basis of the Global Initiative for Asthma (GINA) guidelines. AERD was defined as asthmatics showing 15% or greater decreases in forced expiratory volume in one second (FEV(1)) or naso-ocular reactions by the oral acetyl salicylic acid (ASA) challenge (OAC) test. In total, eight SNPs were genotyped. Logistic regression analysis identified that the minor allele frequency of +14444 T>G and +41170 C>G was significantly higher in the AERD group (n=181) than in the ATA group (n=1016) (p=0.0003-0.03). Linear regression analysis revealed a strong association between the SNPs and the aspirin-induced decrease in FEV(1) (p=0.0004-0.004). The RAB1A gene may play a role in the development of AERD in asthmatics and the genetic polymorphisms of the gene have the potential to be used as an indicator of this disease.

Phosphorylation of the guanine-nucleotide-exchange factor CalDAG-GEFI by protein kinase A regulates Ca(2+)-dependent activation of platelet Rap1b GTPase

In blood platelets the small GTPase Rap1b is activated by cytosolic Ca2+ and promotes integrin αIIbβ3 inside-out activation and platelet aggregation. cAMP is the major inhibitor of platelet function and antagonizes Rap1b stimulation through a mechanism that remains unclear. In the present study we demonstrate that the Ca2+-dependent exchange factor for Rap1b, CalDAG-GEFI (calcium and diacylglycerol-regulated guanine-nucleotide-exchange factor I), is a novel substrate for the cAMP-activated PKA (protein kinase A). CalDAG-GEFI phosphorylation occurred in intact platelets treated with the cAMP-increasing agent forskolin and was inhibited by the PKA inhibitor H89. Purified recombinant CalDAG-GEFI was also phosphorylated in vitro by the PKA catalytic subunit. By screening a panel of specific serine to alanine residue mutants, we identified Ser116 and Ser586 as PKA phosphorylation sites in CalDAG-GEFI. In transfected HEK (human embryonic kidney)-293 cells, as well as in platelets, forskolin-induced phosphorylation of CalDAG-GEFI prevented the activation of Rap1b induced by the Ca2+ ionophore A23187. In platelets this effect was associated with the inhibition of aggregation. Moreover, cAMP-mediated inhibition of Rap1b was lost in HEK-293 cells transfected with a double mutant of CalDAG-GEFI unable to be phosphorylated by PKA. The results of the present study demonstrate that phosphorylation of CalDAG-GEFI by PKA affects its activity and represents a novel mechanism for cAMP-mediated inhibition of Rap1b in platelets.

Dysfunction of the PI3 kinase/Rap1/integrin α(IIb)β(3) pathway underlies ex vivo platelet hypoactivity in essential thrombocythemia

Patients with myeloproliferative disorders (MPDs), such as essential thrombocythemia (ET) have increased risk of thrombosis and bleeding, which are major sources of morbidity and mortality. Most MPD patients have a gain of function mutation in Janus kinase 2 (JAK2V617F), but little is known how JAK2V617F affects platelet function. Here, we demonstrate that platelets from ET patients have impaired SFLLRN-mediated fibrinogen binding and have lost the potentiating effect of thrombopoietin (which couples to JAK2) on this pathway. In contrast, SFLLRN-mediated P-selectin expression, ATP secretion, phosphorylation of the PKC substrate pleckstrin, and Ca(2+) mobilization were unaffected in JAK2V617F positive platelets. In addition, thrombopoietin-mediated JAK2 phosphorylation was unchanged, suggesting that signaling pathways activated downstream of JAK2 are impaired. Indeed, we found that platelets from JAK2V617F positive ET patients have significantly reduced phosphorylation of the PI3 kinase substrate Akt, and have reduced activation of Rap1 in response to thrombopoietin, IGF-1,ADP, SFLLRN, and thrombin. This effect was independent of Giα P2Y12 purinergic receptor function as ADP-mediated inhibition of VASP phosphorylation was unchanged. These results demonstrate that the PI3 kinase/Rap1 pathway is intrinsically impaired in platelets from JAK2V617F-positive ET patients, resulting in diminished thrombin and thrombopoietin-mediated integrin α(IIb)β(3) activation.

The P2Y(12) antagonists, 2MeSAMP and cangrelor, inhibit platelet activation through P2Y(12)/G(i)-dependent mechanism

Background

ADP is an important physiological agonist that induces integrin activation and platelet aggregation through its receptors P2Y₁ (Gα_q-coupled) and P2Y₁₂ (Gα_i-coupled). P2Y₁₂ plays a critical role in platelet activation and thrombosis. Adenosine-based P2Y₁₂ antagonists, 2-methylthioadenosine 5′-monophosphate triethylammonium salt hydrate (2MeSAMP) and Cangrelor (AR-C69931MX) have been widely used to demonstrate the role of P2Y₁₂ in platelet function. Cangrelor is being evaluated in clinical trials of thrombotic diseases. However, a recent study reported that both 2MeSAMP and Cangrelor raise intra-platelet cAMP levels and inhibit platelet aggregation through a P2Y₁₂-independent mechanism.

Methodology/Principal Findings

The present work, using P2Y₁₂ deficient mice, sought to clarify previous conflicting reports and to elucidate the mechanisms by which 2MeSAMP and Cangrelor inhibit platelet activation and thrombosis. 2MeSAMP and Cangrelor inhibited aggregation and ATP release of wild-type but not P2Y₁₂ deficient platelets. 2MeSAMP and Cangrelor neither raised intracellular cAMP concentrations nor induced phosphorylation of vasodilator-stimulated phosphoprotein (VASP) in washed human or mouse platelets. Furthermore, unlike the activators (PGI₂ and forskolin) of the cAMP pathway, 2MeSAMP and Cangrelor failed to inhibit Ca²⁺ mobilization, Akt phosphorylation, and Rap1b activation in P2Y₁₂ deficient platelets. Importantly, while injection of Cangrelor inhibited thrombus formation in a FeCl₃-induced thrombosis model in wild-type mice, it failed to affect thrombus formation in P2Y₁₂ deficient mice.

Conclusions

These data together demonstrate that 2MeSAMP and Cangrelor inhibit platelet function through the P2Y₁₂-dependent mechanism both in vitro and in vivo.

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.

Platelet signaling

This chapter summarizes current ideas about the intracellular signaling that drives platelet responses to vascular injury. After a brief overview of platelet activation intended to place the signaling pathways into context, the first section considers the early events of platelet activation leading up to integrin activation and platelet aggregation. The focus is on the G protein-mediated events utilized by agonists such as thrombin and ADP, and the tyrosine kinase-based signaling triggered by collagen. The second section considers the events that occur after integrin engagement, some of which are dependent on close physical contact between platelets. A third section addresses the regulatory events that help to avoid unprovoked or excessive platelet activation, after which the final section briefly considers individual variations in platelet reactivity and the role of platelet signaling in the innate immune response and embryonic development.

Rap1-Rac1 circuits potentiate platelet activation

OBJECTIVE

The goal of this study was to investigate the potential crosstalk between Rap1 and Rac1, 2 small GTPases central to platelet activation, particularly downstream of the collagen receptor GPVI.

METHODS AND RESULTS

We compared the activation response of platelets with impaired Rap signaling (double knock-out; deficient in both the guanine nucleotide exchange factor, CalDAG-GEFI, and the Gi-coupled receptor for ADP, P2Y12), to that of wild-type platelets treated with a small-molecule Rac inhibitor, EHT 1864 (wild-type /EHT). We found that Rac1 is sequentially activated downstream of Rap1 on stimulation via GPVI. In return, Rac1 provides important feedback for both CalDAG-GEFI- and P2Y12-dependent activation of Rap1. When analyzing platelet responses controlled by Rac1, we observed (1) impaired lamellipodia formation, clot retraction, and granule release in both double knock-out and EHT 1864-treated wild-type platelets; and (2) reduced calcium store release in EHT 1864-treated wild-type but not double knock-out platelets. Consistent with the latter finding, we identified 2 pools of Rac1, one activated immediately downstream of GPVI and 1 activated downstream of Rap1.

CONCLUSIONS

We demonstrate important crosstalk between Rap1 and Rac1 downstream of GPVI. Whereas Rap1 signaling directly controls sustained Rac1 activation, Rac1 affects CalDAG-GEFI- and P2Y12-dependent Rap1 activation via its role in calcium mobilization and granule/ADP release, respectively.

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.

The kinetics of αIIbβ3 activation determines the size and stability of thrombi in mice: implications for antiplatelet therapy

Two major pathways contribute to Ras-proximate-1-mediated integrin activation in stimulated platelets. Calcium and diacyglycerol-regulated guanine nucleotide exchange factor I (CalDAG-GEFI, RasGRP2) mediates the rapid but reversible activation of integrin αIIbβ3, while the adenosine diphosphate receptor P2Y12, the target for antiplatelet drugs like clopidogrel, facilitates delayed but sustained integrin activation. To establish CalDAG-GEFI as a target for antiplatelet therapy, we compared how each pathway contributes to thrombosis and hemostasis in mice. Ex vivo, thrombus formation at arterial or venous shear rates was markedly reduced in CalDAG-GEFI(-/-) blood, even in the presence of exogenous adenosine diphosphate and thromboxane A(2). In vivo, thrombosis was virtually abolished in arterioles and arteries of CalDAG-GEFI(-/-) mice, while small, hemostatically active thrombi formed in venules. Specific deletion of the C1-like domain of CalDAG-GEFI in circulating platelets also led to protection from thrombus formation at arterial flow conditions, while it only marginally increased blood loss in mice. In comparison, thrombi in the micro- and macrovasculature of clopidogrel-treated wild-type mice grew rapidly and frequently embolized but were hemostatically inactive. Together, these data suggest that inhibition of the catalytic or the C1 regulatory domain in CalDAG-GEFI will provide strong protection from athero-thrombotic complications while maintaining a better safety profile than P2Y12 inhibitors like clopidogrel.

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.

RGD-ligand mimetic antagonists of integrin alphaIIbbeta3 paradoxically enhance GPVI-induced human platelet activation

BACKGROUND

The integrin alpha(IIb)beta(3) is the major mediator of platelet aggregation and has, therefore, become an important target of antithrombotic therapy. Antagonists of alpha(IIb)beta(3), for example abciximab, tirofiban and eptifibatide, are used in the treatment of acute coronary syndromes. However, in addition to effective blockade of the integrin, binding of can induce conformational changes in the integrin and can also induce integrin clustering. This class effect of RGD-ligand mimetics might, therefore, underlie paradoxical platelet activation and thrombosis previously reported.

OBJECTIVES

To examine the components of signaling pathways and functional responses in platelets that may underlie this phenomenon of paradoxical platelet activation.

METHODS

We assessed the effect of lotrafiban, and other alpha(IIb)beta(3) antagonists including the clinically used drug tirofiban, on tyrosine phosphorylation of key signaling proteins in platelets by immunoblotting and also platelet functional outputs such as cytosolic calcium responses, phosphatidylserine exposure (pro-coagulant activity) and dense granule release.

RESULTS

In all cases, no effect of alpha(IIb)beta(3) antagonists were observed on their own, but these integrin antagonists did lead to a marked potentiation of glycoprotein VI (GPVI)-associated FcR gamma-chain phosphorylation, activation of Src family kinases and Syk kinase. This correlated with increased dense granule secretion, cytosolic calcium response and exposure of phosphatidylserine on the platelet surface. P2Y(12) antagonism abolished the potentiated phosphatidylserine exposure and dense granule secretion but not the cytosolic calcium response.

CONCLUSIONS

These data provide a mechanism for enhancement of platelet activity by alpha(IIb)beta(3) inhibitors, but also reveal a potentially important signaling pathway operating from the integrin to GPVI signaling.

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.

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.

Leukotriene E4-induced pulmonary inflammation is mediated by the P2Y12 receptor

Of the potent lipid inflammatory mediators comprising the cysteinyl leukotrienes (LTs; LTC₄, LTD₄, and LTE₄), only LTE₄ is stable and abundant in vivo. Although LTE₄ shows negligible activity at the type 1 and 2 receptors for cys-LTs (CysLT₁R and CysLT₂R), it is a powerful inducer of mucosal eosinophilia and airway hyperresponsiveness in humans with asthma. We show that the adenosine diphosphate (ADP)–reactive purinergic (P2Y₁₂) receptor is required for LTE₄-mediated pulmonary inflammation. P2Y₁₂ receptor expression permits LTE₄ -induced activation of extracellular signal-regulated kinase in Chinese hamster ovary cells and permits chemokine and prostaglandin D₂ production by LAD2 cells, a human mast cell line. P2Y₁₂ receptor expression by LAD2 cells is required for competition between radiolabeled ADP and unlabeled LTE₄ but not for direct binding of LTE₄, suggesting that P2Y₁₂ complexes with another receptor to recognize LTE₄. Administration of LTE₄ to the airways of sensitized mice potentiates eosinophilia, goblet cell metaplasia, and expression of interleukin-13 in response to low-dose aerosolized allergen. These responses persist in mice lacking both CysLT₁R and CysLT₂R but not in mice lacking P2Y₁₂ receptors. The effects of LTE₄ on P2Y₁₂ in the airway were abrogated by platelet depletion. Thus, the P2Y₁₂ receptor is required for proinflammatory actions of the stable abundant mediator LTE₄ and is a novel potential therapeutic target for asthma.

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.

The evolution of platelet-directed pharmacotherapy

The evolution of platelet directed pharmacotherapy in the prevention and treatment of patients with thrombotic disorders is based soundly on a rapidly expanding knowledge of platelet biology. Traditionally viewed, throughout most of its relatively brief history in medicine, as an anucleate, passive contributor to hemostasis, a more contemporary perspective acknowledges platelets as complex, multidimensional cells that participate actively in coagulation, vascular repair, angiogenesis and thrombosis within the micro and the macro-circulatory systems. Herein, we consider platelet-directed pharmacotherapy from these fundamental, biology-based exemplars--megakaryocytes, signal transduction and the platelet--coagulation protease interface. We also highlight the emerging biopharmacology platform of oligonucleotide platelet adhesion antagonists and their complementary antidotes.

A signaling pathway contributing to platelet storage lesion development: targeting PI3-kinase-dependent Rap1 activation slows storage-induced platelet deterioration

BACKGROUND

The term platelet storage lesion (PSL) describes the structural and biochemical changes in platelets (PLTs) during storage. These are typified by alterations of morphologic features and PLT metabolism leading to reduced functionality and hence reduced viability for transfusion. While the manifestations of the storage lesion are well characterized, the biochemical pathways involved in the initiation of this process are unknown.

STUDY DESIGN AND METHODS

A complementary proteomic approach has recently been applied to analyze changes in the PLT proteome during storage. By employing stringent proteomic criteria, 12 proteins were identified as significantly and consistently changing in relative concentration over a 7-day storage period. Microscopy, Western blot analysis, flow cytometry, and PLT functionality analyses were used to unravel the involvement of a subset of these 12 proteins, which are connected through integrin signaling in one potential signaling pathway underlying storage lesion development.

RESULTS

Microscopic analysis revealed changes in localization of glycoprotein IIIa, Rap1, and talin during storage. Rap1 activation was observed to correlate with expression of the PLT activation marker CD62P. PLTs incubated for 7 days with the PI3-kinase inhibitor LY294002 showed diminished Rap1 activation as well as a moderate reduction in integrin alphaIIbbeta3 activation and release of alpha-granules. Furthermore, this inhibitor seemed to improve PLT integrity and quality during storage as several in vitro probes showed a deceleration of PLT activation.

CONCLUSION

These results provide the first evidence for a signaling pathway mediating PSL in which PI3-kinase-dependent Rap1 activation leads to integrin alphaIIbbeta3 activation and PLT degranulation.

G-protein-coupled receptors as signaling targets for antiplatelet therapy

Platelet G protein-coupled receptors (GPCRs) initiate and reinforce platelet activation and thrombus formation. The clinical utility of antagonists of the P2Y(12) receptor for ADP suggests that other GPCRs and their intracellular signaling pathways may represent viable targets for novel antiplatelet agents. For example, thrombin stimulation of platelets is mediated by 2 protease-activated receptors (PARs), PAR-1 and PAR-4. Signaling downstream of PAR-1 or PAR-4 activates phospholipase C and protein kinase C and causes autoamplification by production of thromboxane A(2), release of ADP, and generation of more thrombin. In addition to ADP receptors, thrombin and thromboxane A(2) receptors and their downstream effectors-including phosphoinositol-3 kinase, Rap1b, talin, and kindlin-are promising targets for new antiplatelet agents. The mechanistic rationale and available clinical data for drugs targeting disruption of these signaling pathways are discussed. The identification and development of new agents directed against specific platelet signaling pathways may offer an advantage in preventing thrombotic events while minimizing bleeding risk.