GPVI and CLEC-2 in hemostasis and vascular integrity

Emerging Roles for Platelets in Inflammation and Disease

Platelets and their interaction with cells of the immune system contribute through a variety of molecular mechanisms to support hemostasis and inflammation. These simple yet essential cells exert their effects in lymphocytes, monocytes, and neutrophils, both recruiting and modulating their function after activation. Emerging evidence is starting to define the mechanisms that allow platelets to also play pivotal roles in host defense. For example, platelet cell-surface expression of toll-like receptors allows platelets to direct neutrophil activation toward extracellular trap formation and facilitate the elimination of blood pathogens. In addition to these well-known receptors, two of the most recently discovered platelet receptors, C-type lectin receptor 2 (CLEC-2), and TREM-like transcript-1 (TLT-1), have been shown to modulate hemostatic and inflammation-related roles in platelets. This review will discuss the evolution of our understanding of platelet functions from hemostasis to inflammation, and highlight novel mechanisms that platelets use to mediate hemostasis under inflammatory pressure.

Targeting glycoprotein VI and the immunoreceptor tyrosine-based activation motif signaling pathway

Coronary artery thrombosis and ischemic stroke are often initiated by the disruption of an atherosclerotic plaque and consequent intravascular platelet activation. Thus, antiplatelet drugs are central in the treatment and prevention of the initial, and subsequent, vascular events. However, novel pharmacological targets for platelet inhibition remain an important goal of cardiovascular research because of the negative effect of existing antiplatelet drugs on primary hemostasis. One promising target is the platelet collagen receptor glycoprotein VI. Blockade or antibody-mediated depletion of this receptor in circulating platelets is beneficial in experimental models of thrombosis and thrombo-inflammatory diseases, such as stroke, without impairing hemostasis. In this review, we summarize the importance of glycoprotein VI and (hem)immunoreceptor tyrosine-based activation motif signaling in hemostasis, thrombosis, and thrombo-inflammatory processes and discuss the targeting strategies currently under development for inhibiting glycoprotein VI and its signaling.

Targeting GPVI as a novel antithrombotic strategy

While platelet activation is essential to maintain blood vessel patency and minimize loss of blood upon injury, untimely or excessive activity can lead to unwanted platelet activation and aggregation. Resultant thrombosis has the potential to block blood vessels, causing myocardial infarction or stroke. To tackle this major cause of mortality, clinical therapies that target platelet responsiveness (antiplatelet therapy) can successfully reduce cardiovascular events, especially in people at higher risk; however, all current antiplatelet therapies carry an increased probability of bleeding. This review will evaluate new and emerging targets for antithrombotics, focusing particularly on platelet glycoprotein VI, as blockade or depletion of this platelet-specific receptor conveys benefits in experimental models of thrombosis and thromboinflammation without causing major bleeding complications.

The functional role of platelets in the regulation of angiogenesis

Functionally, platelets are primarily recognized as key regulators of thrombosis and hemostasis. Upon vessel injury, the typically quiescent platelet interacts with subendothelial matrix to regulate platelet adhesion, activation and aggregation, with subsequent induction of the coagulation cascade forming a thrombus. Recently, however, newly described roles for platelets in the regulation of angiogenesis have emerged. Platelets possess an armory of pro- and anti-angiogenic proteins, which are actively sequestered and highly organized in α-granule populations. Platelet activation facilitates their release, eliciting potent angiogenic responses through mechanisms that appear to be tightly regulated. In conjunction, the release of platelet-derived phospholipids and microparticles has also earned merit as synergistic regulators of angiogenesis. Consequently, platelets have been functionally implicated in a range of angiogenesis-dependent processes, including physiological roles in wound healing, vascular development and blood/lymphatic vessel separation, whilst facilitating aberrant angiogenesis in a range of diseases including cancer, atherosclerosis and diabetic retinopathy. Whilst the underlying mechanisms are only starting to be elucidated, significant insights have been established, suggesting that platelets represent a promising therapeutic strategy in diseases requiring angiogenic modulation. Moreover, anti-platelet therapies targeting thrombotic complications also exert protective effects in disorders characterized by persistent angiogenesis.

CLEC-2-dependent activation of mouse platelets is weakly inhibited by cAMP but not by cGMP

BACKGROUND

The activation of platelet CLEC-2 by podoplanin on lymphatic endothelial cells (LECs) has a critical role in prevention of mixing of lymphatic and blood vasculatures during embryonic development. Paradoxically, LECs release cAMP and cGMP-elevating agents, prostacyclin (PGI2 ) and nitric oxide (NO), respectively, which are powerful inhibitors of platelet activation. This raises the question of how podoplanin is able to activate CLEC-2 in the presence of the inhibitory cyclic nucleotides.

OBJECTIVES

We investigated the influence of cyclic nucleotides on CLEC-2 signaling in platelets.

METHODS

We used rhodocytin, CLEC-2 monoclonal antibody, LECs and recombinant podoplanin as CLEC-2 agonists on mouse platelets. The effects of the cyclic nucleotide-elevating agents PGI2 , forskolin and the NO-donor GSNO were assessed with light transmission aggregometry, flow cytometry, protein phosphorylation and fluorescent imaging of platelets on LECs.

RESULTS

We show that platelet aggregation induced by CLEC-2 agonists is resistant to GSNO but inhibited by PGI2 . The effect of PGI2 is mediated through decreased phosphorylation of CLEC-2, Syk and PLCγ2. In contrast, adhesion and spreading of platelets on recombinant podoplanin, CLEC-2 antibody and LECs is not affected by PGI2 and GSNO. Consistent with this, CLEC-2 activation of Rac, which is required for platelet spreading, is not altered in the presence of PGI2 .

CONCLUSIONS

The present results demonstrate that platelet adhesion and activation on CLEC-2 ligands or LECs is maintained in the presence of PGI2 and NO.

Platelet immunoreceptor tyrosine-based activation motif (ITAM) signaling and vascular integrity

Platelets are well-known for their critical role in hemostasis, that is, the prevention of blood loss at sites of mechanical vessel injury. Inappropriate platelet activation and adhesion, however, can lead to thrombotic complications, such as myocardial infarction and stroke. To fulfill its role in hemostasis, the platelet is equipped with various G protein-coupled receptors that mediate the response to soluble agonists such as thrombin, ADP, and thromboxane A2. In addition to G protein-coupled receptors, platelets express 3 glycoproteins that belong to the family of immunoreceptor tyrosine-based activation motif receptors: Fc receptor γ chain, which is noncovalently associated with the glycoprotein VI collagen receptor, C-type lectin 2, the receptor for podoplanin, and Fc receptor γII A, a low-affinity receptor for immune complexes. Although both genetic and chemical approaches have documented a critical role for platelet G protein-coupled receptors in hemostasis, the contribution of immunoreceptor tyrosine-based activation motif receptors to this process is less defined. Studies performed during the past decade, however, have identified new roles for platelet immunoreceptor tyrosine-based activation motif signaling in vascular integrity in utero and at sites of inflammation. The purpose of this review is to summarize recent findings on how platelet immunoreceptor tyrosine-based activation motif signaling controls vascular integrity, both in the presence and absence of mechanical injury.

Platelets in lymph vessel development and integrity

Blood platelets have recently been proposed to play a critical role in the development and repair of the lymphatic system. The platelet C-type lectin receptor CLEC-2 and its ligand, the transmembrane protein Podoplanin, which is expressed at high levels on lymphatic endothelial cells (LECs), are required to prevent mixing of the blood and lymphatic vasculatures during mid-gestation. A similar defect is seen in mice deficient in the tyrosine kinase Syk, which plays a vital role in mediating platelet activation by CLEC-2. Furthermore, blood-lymphatic mixing is also present in mice with platelet-/megakaryocyte-specific deletions of CLEC-2 and Syk, suggesting that the phenotype is platelet in origin. The molecular basis of this effect is not known, but it is independent of the major platelet receptors that support hemostasis, including integrin αIIbβ3 (GPIIb-IIIa). Radiation chimeric mice reconstituted with CLEC-2-deficient or Syk-deficient bone marrow exhibit blood-lymphatic mixing in the intestines, illustrating a role for platelets in repair and growth of the lymphatic system. In this review, we describe the events that led to the identification of this novel role of platelets and discuss possible molecular mechanisms and the physiological and pathophysiological significance.

Testing platelet function

Platelet function tests have been traditionally used to aid in the diagnosis and management of patients with bleeding problems. Given the role of platelets in atherothrombosis, several dedicated platelet function instruments are now available that are simple to use and can be used as point-of-care assays. These can provide rapid assessment of platelet function within whole blood without the requirement of sample processing. Some tests can be used to monitor antiplatelet therapy and assess risk of bleeding and thrombosis, although current guidelines advise against this. This article discusses the potential utility of tests/instruments that are available.

Growth factor receptor-bound protein 2 contributes to (hem)immunoreceptor tyrosine-based activation motif-mediated signaling in platelets

RATIONALE

Platelets are anuclear cell fragments derived from bone marrow megakaryocytes (MKs) that safeguard vascular integrity but may also cause pathological vessel occlusion. One major pathway of platelet activation is triggered by 2 receptors that signal through an (hem)immunoreceptor tyrosine-based activation motif (ITAM), the activating collagen receptor glycoprotein (GP) VI and the C-type lectin-like receptor 2 (CLEC-2). Growth factor receptor-bound protein 2 (Grb2) is a ubiquitously expressed adapter molecule involved in signaling processes of numerous receptors in different cell types, but its function in platelets and MKs is unknown.

OBJECTIVE

We tested the hypothesis that Grb2 is a crucial adapter protein in (hem)immunoreceptor tyrosine-based activation motif signaling in platelets.

METHODS AND RESULTS

Here, we show that genetic ablation of Grb2 in MKs and platelets did not interfere with MK differentiation or platelet production. However, Grb2-deficiency severely impaired glycoprotein VI-mediated platelet activation because of defective stabilization of the linker of activated T-cell (LAT) signalosome and activation of downstream signaling proteins that resulted in reduced adhesion, aggregation, and coagulant activity on collagen in vitro. Similarly, CLEC-2-mediated signaling was impaired in Grb2-deficient platelets, whereas the cells responded normally to stimulation of G protein-coupled receptors. In vivo, this selective (hem)immunoreceptor tyrosine-based activation motif signaling defect resulted in prolonged bleeding times but affected arterial thrombus formation only after concomitant treatment with acetylsalicylic acid, indicating that defective glycoprotein VI signaling in the absence of Grb2 can be compensated through thromboxane A2-induced G protein-coupled receptor signaling pathways.

CONCLUSIONS

These results reveal an important contribution of Grb2 in (hem)immunoreceptor tyrosine-based activation motif signaling in platelets in hemostasis and thrombosis by stabilizing the LAT signalosome.

Platelet ITAM signaling

PURPOSE OF REVIEW

G protein-coupled receptors (GPCRs) like PAR1/4 and P2Y12 have long been known for their critical role in hemostasis. In contrast, deficiency in the immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptors glycoprotein (GP)VI or C-type lectin-like receptor (CLEC)-2 is associated with only a mild bleeding diathesis in humans and mice. This review summarizes recent developments on the physiological importance of platelet ITAM signaling as well as the molecular mechanisms facilitating this signaling pathway.

RECENT FINDINGS

Genetic experiments identified a critical role for platelet CLEC-2 signaling in the formation of lymphatic vessels during development. Similarly, signaling by both GPVI and CLEC-2, but not GPCRs, is required for the maintenance of vascular integrity at sites of inflammation in the adult. The molecular mechanisms underlying ITAM signaling in platelets continue to be refined.

SUMMARY

Platelet ITAM signaling plays a key role for the maintenance of vascular integrity in development and the adult. This novel form of hemostasis differs from hemostasis at sites of vascular injury in that it does not depend on major platelet adhesion receptors or GPCR signaling.

RhoG protein regulates glycoprotein VI-Fc receptor γ-chain complex-mediated platelet activation and thrombus formation

We investigated the mechanism of activation and functional role of a hitherto uncharacterized signaling molecule, RhoG, in platelets. We demonstrate for the first time the expression and activation of RhoG in platelets. Platelet aggregation, integrin αIIbβ3 activation, and α-granule and dense granule secretion in response to the glycoprotein VI (GPVI) agonists collagen-related peptide (CRP) and convulxin were significantly inhibited in RhoG-deficient platelets. In contrast, 2-MeSADP- and AYPGKF-induced platelet aggregation and secretion were minimally affected in RhoG-deficient platelets, indicating that the function of RhoG in platelets is GPVI-specific. CRP-induced phosphorylation of Syk, Akt, and ERK, but not SFK (Src family kinase), was significantly reduced in RhoG-deficient platelets. CRP-induced RhoG activation was consistently abolished by a pan-SFK inhibitor but not by Syk or PI3K inhibitors. Interestingly, unlike CRP, platelet aggregation and Syk phosphorylation induced by fucoidan, a CLEC-2 agonist, were unaffected in RhoG-deficient platelets. Finally, RhoG(-/-) mice had a significant delay in time to thrombotic occlusion in cremaster arterioles compared with wild-type littermates, indicating the important in vivo functional role of RhoG in platelets. Our data demonstrate that RhoG is expressed and activated in platelets, plays an important role in GPVI-Fc receptor γ-chain complex-mediated platelet activation, and is critical for thrombus formation in vivo.

Podoplanin maintains high endothelial venule integrity by interacting with platelet CLEC-2

Circulating lymphocytes continuously enter lymph nodes (LNs) for immune surveillance through specialised blood vessels named high endothelial venules (HEVs)^1–5, a process that increases dramatically during immune responses. How HEVs permit lymphocyte transmigration while maintaining vascular integrity is unknown. Here, we report a role for the transmembrane O-glycoprotein podoplanin (PDPN, also known as gp38 and T1α)^6–8 in maintaining HEV barrier function. Mice with postnatal deletion of PDPN lost HEV integrity and exhibited spontaneous bleeding in mucosal LNs, and bleeding in the draining peripheral LN after immunisation. Blocking lymphocyte homing rescued bleeding, indicating that PDPN is required to protect the barrier function of HEVs during lymphocyte trafficking. Further analyses demonstrated that PDPN expressed on fibroblastic reticular cells (FRCs)⁷, which surround HEVs, functions as an activating ligand for platelet C-type lectin-like receptor 2 (CLEC-2)^9,10. Mice lacking FRC PDPN or platelet CLEC-2 exhibited significantly reduced levels of VE-cadherin (VE-cad), which is essential for overall vascular integrity^11,12, on HEVs. Infusion of wild-type (WT) platelets restored HEV integrity in CLEC-2-deficient mice. Activation of CLEC-2 induced release of sphingosine-1-phosphate (S1P)^13,14 from platelets, which promoted expression of VE-cad on HEVs ex vivo. Furthermore, draining peripheral LNs of immunised mice lacking S1P had impaired HEV integrity similar to PDPN- and CLEC-2-deficient mice. These data demonstrate that local S1P release after PDPN-CLEC-2-mediated platelet activation is critical for HEV integrity during immune responses.

An adenine insertion in exon 6 of human GP6 generates a truncated protein associated with a bleeding disorder in four Chilean families

BACKGROUND

Glycoprotein VI (GPVI), 60-65 kDa, is a major collagen receptor on platelet membranes involved in adhesive and signaling responses. Mice lacking GPVI have impaired platelet response to collagen and defective primary adhesion and subsequent thrombus formation. Complete or partial deficiency of GPVI in humans is a rare condition presenting as a mild bleeding disorder. The defect in most of the reported patients is acquired and associated with other diseases. To date, only two patients have been characterized at the molecular level who carry different compound heterozygous mutations in the GP6 gene.

OBJECTIVE

To report four unrelated patients from non-consanguineous families who presented with mucocutaneous bleeding. They had absent platelet aggregation and (14) C-5-HT secretion with collagen, convulxin and collagen-related peptide.

RESULTS

Flow cytometry and immunofluorescence-confocal microscopy showed an absence of GPVI in non-permeabilized platelets. All the patients had an adenine insertion in exon 6 (c.711_712insA), changing the reading frame and generating a premature 'stop codon' in site 242 of the protein. The mutation predicts the synthesis of the truncated protein before the trans-membrane domain, corresponding to a band of ≈49 kDa observed in western blots and in permeabilized platelets by immunofluorescence. Platelet mRNA from all the patients was sequenced and contained the corresponding adenine insertion. Heterozygous relatives had no pathological bleeding, normal response to collagen and convulxin and intermediate membrane expression of GPVI.

CONCLUSIONS

The identification of four unrelated homozygous patients with an identical defect suggests that inherited GPVI deficiency is more frequent than previously suspected, at least in Chile.

Tyrosine phosphorylation on spleen tyrosine kinase (Syk) is differentially regulated in human and murine platelets by protein kinase C isoforms

Protein kinase C (PKC) isoforms differentially regulate platelet functional responses downstream of glycoprotein VI (GPVI) signaling, but the role of PKCs regulating upstream effectors such as Syk is not known. We investigated the role of PKC on Syk tyrosine phosphorylation using the pan-PKC inhibitor GF109203X (GFX). GPVI-mediated phosphorylation on Syk Tyr-323, Tyr-352, and Tyr-525/526 was rapidly dephosphorylated, but GFX treatment inhibited this dephosphorylation on Tyr-525/526 in human platelets but not in wild type murine platelets. GFX treatment did not affect tyrosine phosphorylation on FcRγ chain or Src family kinases. Phosphorylation of Lat Tyr-191 and PLCγ2 Tyr-759 was also increased upon treatment with GFX. We evaluated whether secreted ADP is required for such dephosphorylation. Exogenous addition of ADP to GFX-treated platelets did not affect tyrosine phosphorylation on Syk. FcγRIIA- or CLEC-2-mediated Syk tyrosine phosphorylation was also potentiated with GFX in human platelets. Because potentiation of Syk phosphorylation is not observed in murine platelets, PKC-deficient mice cannot be used to identify the PKC isoform regulating Syk phosphorylation. We therefore used selective inhibitors of PKC isoforms. Only PKCβ inhibition resulted in Syk hyperphosphorylation similar to that in platelets treated with GFX. This result indicates that PKCβ is the isoform responsible for Syk negative regulation in human platelets. In conclusion, we have elucidated a novel pathway of Syk regulation by PKCβ in human platelets.

Targeted Phosphotyrosine Profiling of Glycoprotein VI Signaling Implicates Oligophrenin-1 in Platelet Filopodia Formation

Objective—

Platelet adhesion to subendothelial collagen is dependent on the integrin α 2 β 1 and glycoprotein VI (GPVI) receptors. The major signaling routes in collagen-dependent platelet activation are outlined; however, crucial detailed knowledge of the actual phosphorylation events mediating them is still limited. Here, we explore phosphotyrosine signaling events downstream of GPVI with site-specific detail.

Approach and Results—

Immunoprecipitations of phosphotyrosine-modified peptides from protein digests of GPVI-activated and resting human platelets were compared by stable isotope-based quantitative mass spectrometry. We surveyed 214 unique phosphotyrosine sites over 2 time points, of which 28 showed a significant increase in phosphorylation on GPVI activation. Among these was Tyr370 of oligophrenin-1 (OPHN1), a Rho GTPase–activating protein. To elucidate the function of OPHN1 in platelets, we performed an array of functional platelet analyses within a small cohort of patients with rare oligophrenia. Because of germline mutations in the OPHN1 gene locus, these patients lack OPHN1 expression entirely and are in essence a human knockout model. Our studies revealed that among other unaltered properties, patients with oligophrenia show normal P-selectin exposure and α IIb β 3 activation in response to GPVI, as well as normal aggregate formation on collagen under shear conditions. Finally, the major difference in OPHN1-deficient platelets turned out to be a significantly reduced collagen-induced filopodia formation.

Conclusions—

In-depth phosphotyrosine screening revealed many novel signaling recipients downstream of GPVI activation uncovering a new level of detail within this important pathway. To illustrate the strength of such data, functional follow-up of OPHN1 in human platelets deficient in this protein showed reduced filopodia formation on collagen, an important parameter of platelet hemostatic function.

Platelet receptors activated via mulitmerization: glycoprotein VI, GPIb-IX-V, and CLEC-2

While very different in structure, GPVI - the major collagen receptor on platelet membranes, the GPIb-IX-V complex - the receptor for von Willebrand factor, and CLEC-2, a novel platelet activation receptor for podoplanin, share several common features in terms of function and platelet activation signal transduction pathways. All employ Src family kinases (SFK), Syk, and other signaling molecules involving tyrosine phosphorylation, similar to those of immunoreceptors for T and B cells. There appear to be overlapping functional roles for these glycoproteins, and in some cases, they can compensate for each other, suggesting a degree of redundancy. New ligands for these receptors are being identified, which broadens their functional relevancy. This is particularly true for CLEC-2, whose functions beyond hemostasis are being explored. The common mode of signaling, clustering, and localization to glycosphingolipid-enriched microdomains (GEMs) suggest that GEMs are central to signaling function by ligand-dependent association of these receptors, SFK, Syk, phosphotyrosine phosphatases, and other signaling molecules.

Immunoreceptor tyrosine-based inhibitory motif (ITIM)-mediated inhibitory signaling is regulated by sequential phosphorylation mediated by distinct nonreceptor tyrosine kinases: a case study involving PECAM-1

The activation state of many blood and vascular cells is tightly controlled by a delicate balance between receptors that contain immunoreceptor tyrosine-based activation motifs (ITAMs) and those that contain immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Precisely how the timing of cellular activation by ITAM-coupled receptors is regulated by ITIM-containing receptors is, however, poorly understood. Using platelet endothelial cell adhesion molecule 1 (PECAM-1) as a prototypical ITIM-bearing receptor, we demonstrate that initiation of inhibitory signaling occurs via a novel, sequential process in which Src family kinases phosphorylate the C-terminal ITIM, thereby enabling phosphorylation of the N-terminal ITIM of PECAM-1 by other Src homology 2 domain-containing nonreceptor tyrosine kinases (NRTKs). NRTKs capable of mediating the second phosphorylation event include C-terminal Src kinase (Csk) and Bruton's tyrosine kinase (Btk). Btk and Csk function downstream of phosphatidylinositol 3-kinase (PI3K) activation during ITAM-dependent platelet activation. In ITAM-activated platelets that were treated with a PI3K inhibitor, PECAM-1 was phosphorylated but did not bind the tandem SH2 domain-containing tyrosine phosphatase SHP-2, indicating that it was not phosphorylated on its N-terminal ITIM. Csk bound to and phosphorylated PECAM-1 more efficiently than did Btk and required its SH2 domain to perform these functions. Additionally, the phosphorylation of the N-terminal ITIM of Siglec-9 by Csk is enhanced by the prior phosphorylation of its C-terminal ITIM, providing evidence that the ITIMs of other dual ITIM-containing receptors are also sequentially phosphorylated. On the basis of these findings, we propose that sequential ITIM phosphorylation provides a general mechanism for precise temporal control over the recruitment and activation of tandem SH2 domain-containing tyrosine phosphatases that dampen ITAM-dependent signals.

Platelet ITAM signaling is critical for vascular integrity in inflammation

Platelets play a critical role in maintaining vascular integrity during inflammation, but little is known about the underlying molecular mechanisms. Here we report that platelet immunoreceptor tyrosine activation motif (ITAM) signaling, but not GPCR signaling, is critical for the prevention of inflammation-induced hemorrhage. To generate mice with partial or complete defects in these signaling pathways, we developed a protocol for adoptive transfer of genetically and/or chemically inhibited platelets into thrombocytopenic (TP) mice. Unexpectedly, platelets with impaired GPCR signaling, a crucial component of platelet plug formation and hemostasis, were indistinguishable from WT platelets in their ability to prevent hemorrhage at sites of inflammation. In contrast, inhibition of GPVI or genetic deletion of Clec2, the only ITAM receptors expressed on mouse platelets, significantly reduced the ability of platelets to prevent inflammation-induced hemorrhage. Moreover, transfusion of platelets without ITAM receptor function or platelets lacking the adapter protein SLP-76 into TP mice had no significant effect on vascular integrity during inflammation. These results indicate that the control of vascular integrity is a major function of immune-type receptors in platelets, highlighting a potential clinical complication of novel antithrombotic agents directed toward the ITAM signaling pathway.

Fucoidan is a novel platelet agonist for the C-type lectin-like receptor 2 (CLEC-2)

Fucoidan, a sulfated polysaccharide from Fucus vesiculosus, decreases bleeding time and clotting time in hemophilia, possibly through inhibition of tissue factor pathway inhibitor. However, its effect on platelets and the receptor by which fucoidan induces cellular processes has not been elucidated. In this study, we demonstrate that fucoidan induces platelet activation in a concentration-dependent manner. Fucoidan-induced platelet activation was completely abolished by the pan-Src family kinase (SFK) inhibitor, PP2, or when Syk is inhibited. PP2 abolished phosphorylations of Syk and Phospholipase C-γ2. Fucoidan-induced platelet activation had a lag phase, which is reminiscent of platelet activation by collagen and CLEC-2 receptor agonists. Platelet activation by fucoidan was only slightly inhibited in FcRγ-chain null mice, indicating that fucoidan was not acting primarily through GPVI receptor. On the other hand, fucoidan-induced platelet activation was inhibited in platelet-specific CLEC-2 knock-out murine platelets revealing CLEC-2 as a physiological target of fucoidan. Thus, our data show fucoidan as a novel CLEC-2 receptor agonist that activates platelets through a SFK-dependent signaling pathway. Furthermore, the efficacy of fucoidan in hemophilia raises the possibility that decreased bleeding times could be achieved through activation of platelets.

New Fundamentals in Hemostasis

Hemostasis encompasses the tightly regulated processes of blood clotting, platelet activation, and vascular repair. After wounding, the hemostatic system engages a plethora of vascular and extravascular receptors that act in concert with blood components to seal off the damage inflicted to the vasculature and the surrounding tissue. The first important component that contributes to hemostasis is the coagulation system, while the second important component starts with platelet activation, which not only contributes to the hemostatic plug, but also accelerates the coagulation system. Eventually, coagulation and platelet activation are switched off by blood-borne inhibitors and proteolytic feedback loops. This review summarizes new concepts of activation of proteases that regulate coagulation and anticoagulation, to give rise to transient thrombin generation and fibrin clot formation. It further speculates on the (patho)physiological roles of intra- and extravascular receptors that operate in response to these proteases. Furthermore, this review provides a new framework for understanding how signaling and adhesive interactions between endothelial cells, leukocytes, and platelets can regulate thrombus formation and modulate the coagulation process. Now that the key molecular players of coagulation and platelet activation have become clear, and their complex interactions with the vessel wall have been mapped out, we can also better speculate on the causes of thrombosis-related angiopathies.

Bruton's Tyrosine Kinase mediates platelet receptor-induced generation of microparticles: a potential mechanism for amplification of inflammatory responses in rheumatoid arthritis synovial joints

Platelet microparticles (pMPs) are small membrane-coated vesicles that are released from the plasma membrane upon platelet activation. In the joint fluid of patients with rheumatoid arthritis, pMP can interact with and activate fibroblast-like synoviocytes (FLS), which are important effector cells that mediate both immune activation and joint destruction. The signaling process by which engagement of glycoprotein VI (GPVI), a surface glycoprotein receptor for collagen which is expressed on platelets, triggers pMP generation is poorly understood, but has been suggested to involve Spleen Tyrosine Kinase (SYK), best known as an upstream activator of Bruton's Tyrosine Kinase (BTK) in B cells. In this study, we showed that activation of human platelets triggered by convulxin or collagen, specific ligands for GPVI receptor, or alternatively by antibody-mediated cross-linking of another platelet receptor, C type lectin-like receptor 2 (CLEC2), resulted in phosphorylation of BTK and downstream effector, phospholipase Cγ2 (PLCγ2). A potent and selective BTK inhibitor, RN486, inhibited GPVI- or CLEC2-mediated PLCγ2 phosphorylation and pMP production in a dose-dependent manner. BTK is also an essential effector of B cell receptor (BCR)-induced B cell signaling. Consistent with the biology, the IC50s of BTK inhibitors with varying potencies in a BCR-dependent B cell activation marker assay correlated with those in the GPVI-mediated PLCγ2 phosphorylation. In a co-culture system consisting of human primary synovial FLS and activated human platelets, convulxin stimulation resulted in elevated production of pro-inflammatory cytokines, IL-6 and IL-8, an effect which was dose-dependently blocked by RN486. The effects are specific as RN486 abrogated platelet aggregation induced by GPVI ligands but not by other platelet surface receptor agonists. Taken together, our data further support the potential therapeutic utility of BTK inhibitors in RA therapy, by inhibiting GPVI-mediated platelet activation and thus subsequent amplification of inflammation driven by pMP-induced FLS cytokines production.

Constitutive dimerization of glycoprotein VI (GPVI) in resting platelets is essential for binding to collagen and activation in flowing blood

The platelet collagen receptor glycoprotein VI (GPVI) has been suggested to function as a dimer, with increased affinity for collagen. Dissociation constants (K(d)) obtained by measuring recombinant GPVI binding to collagenous substrates showed that GPVI dimers bind with high affinity to tandem GPO (Gly-Pro-Hyp) sequences in collagen, whereas the markedly lower affinity of the monomer for all substrates implies that it is not the collagen-binding form of GPVI. Dimer binding required a high density of immobilized triple-helical (GPO)(10)-containing peptide, suggesting that the dimer binds multiple, discrete peptide helices. Differential inhibition of dimer binding by dimer-specific antibodies, m-Fab-F and 204-11 Fab, suggests that m-Fab-F binds at the collagen-binding site of the dimer, and 204-11 Fab binds to a discrete site. Flow cytometric quantitation indicated that GPVI dimers account for ~29% of total GPVI in resting platelets, whereas activation by either collagen-related peptide or thrombin increases the number of dimers to ~39 and ~44%, respectively. m-Fab-F inhibits both GPVI-dependent static platelet adhesion to collagen and thrombus formation on collagen under low and high shear, indicating that pre-existing dimeric GPVI is required for the initial interaction with collagen because affinity of the monomer is too low to support binding and that interaction through the dimer is essential for platelet activation. These GPVI dimers in resting circulating platelets will enable them to bind injury-exposed subendothelial collagen to initiate platelet activation. The GPVI-specific agonist collagen-related peptide or thrombin further increases the number of dimers, thereby providing a feedback mechanism for reinforcing binding to collagen and platelet activation.

Podoplanin-rich stromal networks induce dendritic cell motility via activation of the C-type lectin receptor CLEC-2

To initiate adaptive immunity, dendritic cells (DCs) move from parenchymal tissues to lymphoid organs by migrating along stromal scaffolds that display the glycoprotein podoplanin (PDPN). PDPN is expressed by lymphatic endothelial and fibroblastic reticular cells and promotes blood-lymph separation during development by activating the C-type lectin receptor, CLEC-2, on platelets. Here, we describe a role for CLEC-2 in the morphodynamic behavior and motility of DCs. CLEC-2 deficiency in DCs impaired their entry into lymphatics and trafficking to and within lymph nodes, thereby reducing T cell priming. CLEC-2 engagement of PDPN was necessary for DCs to spread and migrate along stromal surfaces and sufficient to induce membrane protrusions. CLEC-2 activation triggered cell spreading via downregulation of RhoA activity and myosin light-chain phosphorylation and triggered F-actin-rich protrusions via Vav signaling and Rac1 activation. Thus, activation of CLEC-2 by PDPN rearranges the actin cytoskeleton in DCs to promote efficient motility along stromal surfaces.

Polymorphisms of protein tyrosine phosphatase CD148 influence FcγRIIA-dependent platelet activation and the risk of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is due primarily to IgG antibodies specific to platelet factor 4/heparin complexes (PF4/Hs) that activate platelets via FcγRIIA. CD148 is a protein tyrosine phosphatase that regulates Src kinases and collagen-induced platelet activation. Three polymorphisms affecting CD148 (Q276P, R326Q, and D872E) were studied in HIT patients and 2 control groups, with or without antibodies to PF4/Hs. Heterozygote status for CD148 276P or 326Q alleles was less frequent in HIT patients, suggesting a protective effect of these polymorphisms. Aggregation tests performed with collagen, HIT plasma, and monoclonal antibodies cross-linking FcγRIIA showed consistent hyporesponsiveness of platelets expressing the 276P/326Q alleles. In addition, platelets expressing the 276P/326Q alleles exhibited a greater sensitivity to the Src family kinases inhibitor dasatinib in response to collagen or ALB6 cross-linking FcγRIIA receptors. Moreover, the activatory phosphorylation of Src family kinases was considerably delayed as well as the phosphorylation of Linker for activation of T cells and phospholipase Cγ2, 2 major signaling proteins downstream from FcγRIIA. In conclusion, this study shows that CD148 polymorphisms affect platelet activation and probably exert a protec-tive effect on the risk of HIT in patients with antibodies to PF4/Hs.

Distinct and overlapping functional roles of Src family kinases in mouse platelets

BACKGROUND AND OBJECTIVES

Src family kinases (SFKs) play a critical role in initiating and propagating signals in platelets. The aims of this study were to quantitate SFK members present in platelets and to analyze their contribution to platelet regulation using glycoprotein VI (GPVI) and intregrin αIIbβ3, and in vivo.

METHODS AND RESULTS

Mouse platelets express four SFKs, Fgr, Fyn, Lyn and Src, with Lyn expressed at a considerably higher level than the others. Using mutant mouse models, we demonstrate that platelet activation by collagen-related peptide (CRP) is delayed and then potentiated in the absence of Lyn, but only marginally reduced in the absence of Fyn or Fgr, and unaltered in the absence of Src. Compound deletions of Lyn/Src or Fyn/Lyn, but not of Fyn/Src or Fgr/Lyn, exhibit a greater delay in activation relative to Lyn-deficient platelets. Fibrinogen-adherent platelets show reduced spreading in the absence of Src, potentiation in the absence of Lyn, but no change in the absence of Fyn or Fgr. In mice double-deficient in Lyn/Src or Fgr/Lyn, the inhibitory role of Lyn on spreading on fibrinogen is lost. Lyn is the major SFK-mediating platelet aggregation on collagen at arterial shear and its absence leads to a reduction in thrombus size in a laser injury model.

CONCLUSION

These results demonstrate that SFKs share individual and overlapping roles in regulating platelet activation, with Lyn having a dual role in regulating GPVI signaling and an inhibitory role downstream of αIIbβ3, which requires prior signaling through Src.

Toso, a functional IgM receptor, is regulated by IL-2 in T and NK cells

We find that the cell surface receptor Toso is dramatically downregulated by in vitro stimulation of human T and NK cells with IL-2 in a STAT5-dependent manner. The fact that IL-2 is known to prime NK and T cells for Fas/TNF-mediated activation-induced cell death (AICD) fits nicely with the original and recent descriptions of Toso as an inhibitor of Fas/TNF-induced apoptosis. In support of this possibility, effector memory T cells express markedly lower levels of Toso than those of naive T cells, indicating that activation in vivo correlates with the downregulation of Toso. Moreover, in vitro activation of memory T cells through TCR dramatically downregulates Toso expression compared with that of naive CD4 T cells. However, overexpression of Toso in human NK cells and Jurkat T cells does not inhibit Fas-mediated apoptosis, and, in agreement with other recent reports, Toso clearly functions as an IgM receptor. Unlike CD16, Toso expression by NK cells does not convey cytotoxic potential, but its ligation does trigger intracellular signaling in NK cells. In summary, our data indicate that Toso is a functional IgM receptor that is capable of activating signaling molecules, is regulated by IL-2, and is not inherently an antiapoptotic molecule.

CLEC-2 and Syk in the megakaryocytic/platelet lineage are essential for development

The C-type lectin receptor CLEC-2 signals through a pathway that is critically dependent on the tyrosine kinase Syk. We show that homozygous loss of either protein results in defects in brain vascular and lymphatic development, lung inflation, and perinatal lethality. Furthermore, we find that conditional deletion of Syk in the hematopoietic lineage, or conditional deletion of CLEC-2 or Syk in the megakaryocyte/platelet lineage, also causes defects in brain vascular and lymphatic development, although the mice are viable. In contrast, conditional deletion of Syk in other hematopoietic lineages had no effect on viability or brain vasculature and lymphatic development. We show that platelets, but not platelet releasate, modulate the migration and intercellular adhesion of lymphatic endothelial cells through a pathway that depends on CLEC-2 and Syk. These studies found that megakaryocyte/platelet expression of CLEC-2 and Syk is required for normal brain vasculature and lymphatic development and that platelet CLEC-2 and Syk directly modulate lymphatic endothelial cell behavior in vitro.

Phospholipase Cγ2 is required for basal but not oestrogen deficiency-induced bone resorption

BACKGROUND

Osteoclasts play a critical role in bone resorption under basal conditions, but they also contribute to pathological bone loss during diseases including postmenopausal osteoporosis. Phospholipase Cγ2 (PLCγ2) is an important signalling molecule in diverse haematopoietic lineages. Here, we tested the role of PLCγ2 in basal and ovariectomy-induced bone resorption, as well as in in vitro osteoclast cultures using PLCγ2-deficient (PLCγ2(-/-) ) mice.

MATERIALS AND METHODS

The trabecular architecture of long bone metaphyses was tested by micro-CT and histomorphometric analyses. Postmenopausal osteoporosis was modelled by surgical ovariectomy. Osteoclast development and function, gene expression and PLCγ2 phosphorylation were tested on in vitro osteoclast and macrophage cultures.

RESULTS

  PLCγ2(-/-) mice had significantly higher trabecular bone mass under basal conditions than wild-type mice. PLCγ2 was required for in vitro development and resorptive function of osteoclasts, but not for upregulation of osteoclast-specific gene expression. PLCγ2 was phosphorylated in a Src-family-dependent manner upon macrophage adhesion but not upon stimulation by M-CSF or RANKL. Surprisingly, ovariectomy-induced bone resorption in PLCγ2(-/-) mice was similar to, or even more robust than, that in wild-type animals.

CONCLUSIONS

Our results indicate that PLCγ2 participates in bone resorption under basal conditions, likely because of its role in adhesion receptor signalling during osteoclast development. In contrast, PLCγ2 does not appear to play a major role in ovariectomy-induced bone loss. These results suggest that basal and oestrogen deficiency-induced bone resorption utilizes different signalling pathways and that PLCγ2 may not be a suitable therapeutic target in postmenopausal osteoporosis.

Platelet interaction with the vessel wall

Platelets have attracted a growing interest among basic scientists and clinicians, as they have been shown to play an important role in many physiological and pathophysiological conditions. Beyond hemostasis, platelets participate in wound healing, inflammation, infectious diseases, maintenance of the endothelial barrier function, angiogenesis, and tumor metastasis. Over the last 50 years enormous progress has been made in our understanding of the role of platelets in hemostasis. Platelets circulate in blood in a resting state, but they are able to react immediately upon a vessel wall injury by adhering to the exposed collagen, followed by platelet-platelet interaction to form a plug that effectively seals the injured vessel wall to prevent excessive blood loss. Comparable events will take place on a rupturing atherosclerotic plaque, which may result in a platelet-rich thrombus. This chapter will address the molecular basis of platelet adhesion and aggregation, the regulation of platelet function and the interaction of primary and secondary hemostasis.

Blood platelet biochemistry

Defects in platelet function or formation increase the risk for bleeding or thrombosis, which indicates the crucial role for platelets in maintaining haemostasis in normal life. Upon vascular injury, platelets instantly adhere to the exposed extracellular matrix which results in platelet activation and aggregation and the formation a haemostatic plug that stops bleeding. To prevent excessive platelet aggregate formation that eventually would occlude the vessels, this self-amplifying process nevertheless requires a tight control. This review intends to give a comprehensive overview of the currently established main mechanisms in platelet function.

Novel platelet activation receptor CLEC-2: from discovery to prospects

C-type lectin-like receptor 2 (CLEC-2) has been identified as a receptor for the platelet activating snake venom rhodocytin. CLEC-2 elicits powerful platelet activation signals in conjunction with Src, Syk kinases, and phospholipase Cγ2, similar to the collagen receptor glycoprotein (GP) VI/FcRγ-chain complex. In contrast to GPVI/FcRγ, which initiates platelet activation through the tandem YxxL motif immunoreceptor tyrosine-based activation motif (ITAM), CLEC-2 signals via the single YxxL motif hemi-ITAM. The endogenous ligand of CLEC-2 has been identified as podoplanin, which is expressed on the surface of tumour cells and facilitates tumour metastasis by inducing platelet activation. Studies of CLEC-2-deficient mice have revealed several physiological roles of CLEC-2. Podoplanin is also expressed in lymphatic endothelial cells as well as several other cells, including type I alveolar cells and kidney podocytes, but is absent from vascular endothelial cells. In the developmental stages, when the primary lymph sac is derived from the cardinal vein, podoplanin activates platelets in lymphatic endothelial cells by binding to CLEC-2, which facilitates blood/lymphatic vessel separation. Moreover, CLEC-2 is involved in thrombus stabilisation under flow conditions in part through homophilic interactions. However, the absence of CLEC-2 does not significantly increase bleeding tendency. CLEC-2 may be a good target protein for novel anti-platelet drugs or anti-metastatic drugs having therapeutic and preventive effects on arterial thrombosis and cancer, the primary causes of mortality in developed countries. In this article, we review the mechanisms of signal transduction, structure, expression, and function of CLEC-2.

Platelet adhesion and activation mechanisms in arterial thrombosis and ischaemic stroke

Platelet adhesion, activation and aggregation on the exposed subendothelial extracellular matrix (ECM) are essential for haemostasis, but may also lead to occlusion of diseased vessels. Binding of the glycoprotein (GP)Ib-V-IX complex to immobilised von Willebrand factor (VWF) initiates adhesion of flowing platelets to the ECM, and thereby enables the collagen receptor GPVI to interact with its ligand and to mediate platelet activation. This process is reinforced by locally produced thrombin and platelet-derived secondary mediators, such as adenosine diphosphate (ADP) and thromboxane A(2) (TxA(2)). Together, these events promote a shift of β1 and β3 integrins from a low to a high affinity state for their ligands through 'inside-out' signalling allowing firm platelet adhesion and aggregation. Formed platelet aggregates are stabilised by fibrin formation and signalling events between adjacent platelets involving multiple platelet receptors, such as the newly discovered C-type lectin-like receptor 2 (CLEC-2). While occlusive thrombus formation is the principal pathogenic event in myocardial infarction, the situation is more complex in ischaemic stroke where infarct development often progresses despite sustained early reperfusion of previously occluded major intracranial arteries, a process referred to as 'reperfusion injury'. Increasing experimental evidence now suggests that early platelet adhesion and activation events, orchestrate a 'thrombo-inflammatory' cascade in this setting, whereas platelet aggregation and thrombus formation are not required. This review summarises recent developments in understanding the principal platelet adhesion receptor systems with a focus on their involvement in arterial thrombosis and ischaemic stroke models.

Tetraspanin microdomains: fine-tuning platelet function

Platelets are crucial for preventing excessive blood loss at sites of injury by plugging holes in damaged blood vessels through thrombus formation. Platelet thrombi can, however, cause heart attack or stroke by blocking diseased vessels upon rupture of atherosclerotic plaques. Current anti-platelet therapy is not effective in all patients and carries a risk of bleeding. As such, a major goal in platelet research is to identify new drug targets to specifically inhibit platelets in disease processes. Tetraspanins are potential candidates because of their capacity to regulate other proteins in microdomains, and their defined roles in cell adhesion and signalling. In the last 6 years, analyses of tetraspanin-deficient mice have suggested that tetraspanins are indeed important for fine-tuning platelet responses. The future characterization of novel regulatory mechanisms in tetraspanin microdomains may lead to new drug targets for the prevention and treatment of heart attack and stroke.

Transmembrane collagen receptors

Collagen, the most abundant protein in animals, is a key component of extracellular matrices. Not only do collagens provide essential structural support for connective tissues, but they are also intimately involved in controlling a spectrum of cellular functions such as growth, differentiation, and morphogenesis. All collagens possess triple-helical regions through which they interact with a host of other proteins including cell surface receptors. A structurally diverse group of transmembrane receptors mediates the recognition of the collagen triple helix: integrins, discoidin domain receptors, glycoprotein VI, and leukocyte-associated immunoglobulin-like receptor-1. These collagen receptors regulate a wide range of behaviors including cell adhesion and migration, hemostasis, and immune function. Here these collagen receptors are discussed in terms of their molecular basis of collagen recognition, their signaling and developmental functions, and their roles in disease.

Platelets at work in primary hemostasis

When platelet numbers are low or when their function is disabled, the risk of bleeding is high, which on the one hand indicates that in normal life vascular damage is a rather common event and that hence the role of platelets in maintaining a normal hemostasis is a continuously ongoing physiological process. Upon vascular injury, platelets instantly adhere to the exposed extracellular matrix resulting in platelet activation and aggregation to form a hemostatic plug. This self-amplifying mechanism nevertheless requires a tight control to prevent uncontrolled platelet aggregate formation that eventually would occlude the vessel. Therefore endothelial cells produce inhibitory compounds such as prostacyclin and nitric oxide that limit the growth of the platelet thrombus to the damaged area. With this review, we intend to give an integrated survey of the platelet response to vascular injury in normal hemostasis.

Platelet adhesion to collagen

Platelets play a central role in maintaining hemostasis mainly by binding to subendothelial collagen exposed upon vascular injury, thereby initiating thrombus formation. Platelets can bind directly to the exposed collagen through two major receptors i.e. the integrin a2b1 and glycoprotein (GP) VI. However, under high shear conditions the GPIb-V-IX receptor complex and its main ligand von Willebrand Factor are additionally needed for firm platelet adhesion to the vessel wall. In this review, we summarize the current knowledge on the individual roles and structure-function relationships of these main platelet adhesion receptors.