Glycoprotein Ib-von Willebrand Factor Interactions Activate Tyrosine Kinases in Human Platelets

Phosphorylation of spleen tyrosine kinase at Y346 negatively regulates ITAM-mediated signaling and function in platelets

Spleen tyrosine kinase (Syk) is expressed in a variety of hemopoietic cells. Upon phosphorylation of the platelet immunoreceptor-based activation motif of the glycoprotein VI (GPVI)/Fc receptor gamma chain collagen receptor, both the tyrosine phosphorylation and activity of Syk are increased leading to downstream signaling events. Although it has been established that the activity of Syk is regulated by tyrosine phosphorylation, the specific roles of individual phosphorylation sites remain to be elucidated. We observed that Syk Y346 in mouse platelets was still phosphorylated when GPVI-induced Syk activity was inhibited. We then generated Syk Y346F mice and analyzed the effect this mutation exerts on platelet responses. Syk Y346F mice bred normally, and their blood cell count was unaltered. We did observe potentiation of GPVI-induced platelet aggregation and ATP secretion as well as increased phosphorylation of other tyrosines on Syk in the Syk Y346F mouse platelets when compared to WT littermates. This phenotype was specific for GPVI-dependent activation, since it was not seen when AYPGKF, a PAR4 agonist, or 2-MeSADP, a purinergic receptor agonist, was used to activate platelets. Despite a clear effect of Syk Y346F on GPVI-mediated signaling and cellular responses, there was no effect of this mutation on hemostasis as measured by tail-bleeding times, although the time to thrombus formation determined using the ferric chloride injury model was reduced. Thus, our results indicate a significant effect of Syk Y346F on platelet activation and responses in vitro and reveal its complex nature manifesting itself by the diversified translation of platelet activation into physiological responses.

Phosphorylation on Syk Y342 is important for both ITAM and hemITAM signaling in platelets

Immune cells express receptors bearing an immune tyrosine activation motif (ITAM) containing two YXXL motifs or hemITAMs containing only one YXXL motif. Phosphorylation of the ITAM/hemITAM is mediated by Src family kinases allowing for the binding and activation of spleen tyrosine kinase (Syk). It is believed that Syk must be phosphorylated on tyrosine residues for activation, and Tyr342, а conserved tyrosine in the interdomain B region, has been shown to be critical for regulating Syk in FcεR1-activated mast cells. Syk is a key mediator of signaling pathways downstream of several platelet pathways including the ITAM bearing glycoprotein VI (GPVI)/Fc receptor gamma chain collagen receptor and the hemITAM containing C-type lectin-like receptor-2 (CLEC-2). Since platelet activation is a crucial step in both hemostasis and thrombosis, we evaluated the importance of Syk Y342 in these processes by producing an Syk Y342F knock-in mouse. When using a CLEC-2 antibody as an agonist, reduced aggregation and secretion were observed in Syk Y342F mouse platelets when compared with control mouse platelets. Platelet reactivity was also reduced in response to the GPVI agonist collagen-related peptide. Signaling initiated by either GPVI or CLEC-2 was also greatly inhibited, including Syk Y519/520 phosphorylation. Hemostasis, as measured by tail bleeding time, was not altered in Syk Y342F mice, but thrombus formation in response to FeCl₃ injury was prolonged in Syk Y342F mice. These data demonstrate that phosphorylation of Y342 on Syk following stimulation of either GPVI or CLEC-2 receptors is important for the ability of Syk to transduce a signal.

Molecular Drivers of Platelet Activation: Unraveling Novel Targets for Anti-Thrombotic and Anti-Thrombo-Inflammatory Therapy

Cardiovascular diseases (CVDs) are the leading cause of death globally-partly a consequence of increased population size and ageing-and are major contributors to reduced quality of life. Platelets play a major role in hemostasis and thrombosis. While platelet activation and aggregation are essential for hemostasis at sites of vascular injury, uncontrolled platelet activation leads to pathological thrombus formation and provokes thrombosis leading to myocardial infarction or stroke. Platelet activation and thrombus formation is a multistage process with different signaling pathways involved to trigger platelet shape change, integrin activation, stable platelet adhesion, aggregation, and degranulation. Apart from thrombotic events, thrombo-inflammation contributes to organ damage and dysfunction in CVDs and is mediated by platelets and inflammatory cells. Therefore, in the past, many efforts have been made to investigate specific signaling pathways in platelets to identify innovative and promising approaches for novel antithrombotic and anti-thrombo-inflammatory strategies that do not interfere with hemostasis. In this review, we focus on some of the most recent data reported on different platelet receptors, including GPIb-vWF interactions, GPVI activation, platelet chemokine receptors, regulation of integrin signaling, and channel homeostasis of NMDAR and PANX1.

Loss of Reelin protects mice against arterial thrombosis by impairing integrin activation and thrombus formation under high shear conditions

Reelin is a secreted glycoprotein and essential for brain development and plasticity. Recent studies provide evidence that Reelin modifies platelet actin cytoskeletal dynamics. In this study we sought to dissect the contribution of Reelin in arterial thrombus formation. Here we analyzed the impact of Reelin in arterial thrombosis ex vivo and in vivo using Reelin deficient (reeler) and wildtype mice. We found that Reelin is secreted upon platelet activation and mediates signaling via glycoprotein (GP)Ib, the amyloid precursor protein (APP) and apolipoprotein E receptor 2 (ApoER2) to induce activation of Akt, extracellular signal-regulated kinase (Erk), SYK and Phospholipase Cγ2. Moreover, our data identifies Reelin as first physiological ligand for platelet APP. Platelets from reeler mice displayed attenuated platelet adhesion and significantly reduced thrombus formation under high shear conditions indicating an important role for Reelin in GPIb-dependent integrin α_IIbβ₃ activation. Accordingly, adhesion to immobilized vWF as well as integrin activation and the phosphorylation of Erk and Akt after GPIb engagement was reduced in Reelin deficient platelets. Defective Reelin signaling translated into protection from arterial thrombosis and cerebral ischemia/reperfusion injury beside normal hemostasis. Furthermore, treatment with an antagonistic antibody specific for Reelin protects wildtype mice from occlusive thrombus formation. Mechanistically, GPIb co-localizes to the major Reelin receptor APP in platelets suggesting that Reelin-induced effects on GPIb signaling are mediated by APP-GPIb interaction. These results indicate that Reelin is an important regulator of GPIb-mediated platelet activation and may represent a new therapeutic target for the prevention and treatment of cardio- and cerebrovascular diseases.

Syk Activity Is Dispensable for Platelet GP1b-IX-V Signaling

The binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein 1b-IX (GP1b-IX) leads to activation of platelets. GP1b was shown to signal via the FcRγ-ITAM (Fc Receptor γ-Immunoreceptor tyrosine-based activation motif) pathway, activating spleen tyrosine kinase (Syk) and other tyrosine kinases. However, there have been conflicting reports regarding the role of Syk in GP1b signaling. In this study, we sought to resolve these conflicting reports and clarify the role of Syk in VWF-induced platelet activation. The inhibition of Syk with the selective Syk inhibitors, OXSI-2 and PRT-060318, did not inhibit VWF-induced platelet adhesion, agglutination, aggregation, or secretion. In contrast, platelets stimulated with the Glycoprotein VI (GPVI) agonist, collagen-related peptide (CRP), failed to cause any aggregation or secretion in presence of the Syk inhibitors. Furthermore, GP1b-induced platelet signaling was unaffected in the presence of Syk inhibitors, but GPVI-induced signaling was abolished under similar conditions. Thus, we conclude that Syk kinase activity does not play any functional role downstream of GP1b-mediated platelet activation.

Metalloproteolytic receptor shedding…platelets "acting their age"

Whilst significant effort has been focused on development of tools and approaches to clinically modulate activation processes that consume platelets, the platelet receptors that initiate activation processes remain untargeted. The modulation of receptor levels is also linked to underlying platelet aging processes which influence normal platelet lifespan and also the functionality and survival of stored platelets that are used in transfusion. In this review, we will focus on platelet adhesion receptors initiating thrombus formation, and discuss how regulation of levels of these receptors impact platelet function and platelet survival.

Distinct pathways regulate Syk protein activation downstream of immune tyrosine activation motif (ITAM) and hemITAM receptors in platelets

Tyrosine kinase pathways are known to play an important role in the activation of platelets. In particular, the GPVI and CLEC-2 receptors are known to activate Syk upon tyrosine phosphorylation of an immune tyrosine activation motif (ITAM) and hemITAM, respectively. However, unlike GPVI, the CLEC-2 receptor contains only one tyrosine motif in the intracellular domain. The mechanisms by which this receptor activates Syk are not completely understood. In this study, we identified a novel signaling mechanism in CLEC-2-mediated Syk activation. CLEC-2-mediated, but not GPVI-mediated, platelet activation and Syk phosphorylation were abolished by inhibition of PI3K, which demonstrates that PI3K regulates Syk downstream of CLEC-2. Ibrutinib, a Tec family kinase inhibitor, also completely abolished CLEC-2-mediated aggregation and Syk phosphorylation in human and murine platelets. Furthermore, embryos lacking both Btk and Tec exhibited cutaneous edema associated with blood-filled vessels in a typical lymphatic pattern similar to CLEC-2 or Syk-deficient embryos. Thus, our data show, for the first time, that PI3K and Tec family kinases play a crucial role in the regulation of platelet activation and Syk phosphorylation downstream of the CLEC-2 receptor.

Molecular cloning and characterization of rhesus monkey platelet glycoprotein Ibα, a major ligand-binding subunit of GPIb-IX-V complex

INTRODUCTION

Through binding to von Willebrand factor (VWF), platelet glycoprotein (GP) Ibα, the major ligand-binding subunit of the GPIb-IX-V complex, initiates platelet adhesion and aggregation in response to exposed VWF or elevated fluid-shear stress. There is little data regarding non-human primate platelet GPIbα. This study cloned and characterized rhesus monkey (Macaca Mullatta) platelet GPIbα.

MATERIALS AND METHODS

DNAMAN software was used for sequence analysis and alignment. N/O-glycosylation sites and 3-D structure modelling were predicted by online OGPET v1.0, NetOGlyc 1.0 Server and SWISS-MODEL, respectively. Platelet function was evaluated by ADP- or ristocetin-induced platelet aggregation.

RESULTS

Rhesus monkey GPIbα contains 2,268 nucleotides with an open reading frame encoding 755 amino acids. Rhesus monkey GPIbα nucleotide and protein sequences share 93.27% and 89.20% homology respectively, with human. Sequences encoding the leucine-rich repeats of rhesus monkey GPIbα share strong similarity with human, whereas PEST sequences and N/O-glycosylated residues vary. The GPIbα-binding residues for thrombin, filamin A and 14-3-3ζ are highly conserved between rhesus monkey and human. Platelet function analysis revealed monkey and human platelets respond similarly to ADP, but rhesus monkey platelets failed to respond to low doses of ristocetin where human platelets achieved 76% aggregation. However, monkey platelets aggregated in response to higher ristocetin doses.

CONCLUSIONS

Monkey GPIbα shares strong homology with human GPIbα, however there are some differences in rhesus monkey platelet activation through GPIbα engagement, which need to be considered when using rhesus monkey platelet to investigate platelet GPIbα 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.

Historical perspective and future directions in platelet research

Platelets are a remarkable mammalian adaptation that are required for human survival by virtue of their ability to prevent and arrest bleeding. Ironically, however, in the past century, the platelets' hemostatic activity became maladaptive for the increasingly large percentage of individuals who develop age-dependent progressive atherosclerosis. As a result, platelets also make a major contribution to ischemic thrombotic vascular disease, the leading cause of death worldwide. In this brief review, I provide historical descriptions of a highly selected group of topics to provide a framework for understanding our current knowledge and the trends that are likely to continue into the future of platelet research. For convenience, I separate the eras of platelet research into the "Descriptive Period" extending from ~1880-1960 and the "Mechanistic Period" encompassing the past ~50 years since 1960. We currently are reaching yet another inflection point, as there is a major shift from a focus on traditional biochemistry and cell and molecular biology to an era of single molecule biophysics, single cell biology, single cell molecular biology, structural biology, computational simulations, and the high-throughput, data-dense techniques collectively named with the "omics postfix". Given the progress made in understanding, diagnosing, and treating many rare and common platelet disorders during the past 50 years, I think it appropriate to consider it a Golden Age of Platelet Research and to recognize all of the investigators who have made important contributions to this remarkable achievement..

A novel interaction between FlnA and Syk regulates platelet ITAM-mediated receptor signaling and function

Filamin A (FlnA) cross-links actin filaments and connects the Von Willebrand factor receptor GPIb-IX-V to the underlying cytoskeleton in platelets. Because FlnA deficiency is embryonic lethal, mice lacking FlnA in platelets were generated by breeding FlnA^loxP/loxP females with GATA1-Cre males. FlnA^loxP/y GATA1-Cre males have a macrothrombocytopenia and increased tail bleeding times. FlnA-null platelets have decreased expression and altered surface distribution of GPIbα because they lack the normal cytoskeletal linkage of GPIbα to underlying actin filaments. This results in ∼70% less platelet coverage on collagen-coated surfaces at shear rates of 1,500/s, compared with wild-type platelets. Unexpectedly, however, immunoreceptor tyrosine-based activation motif (ITAM)- and ITAM-like–mediated signals are severely compromised in FlnA-null platelets. FlnA-null platelets fail to spread and have decreased α-granule secretion, integrin αIIbβ3 activation, and protein tyrosine phosphorylation, particularly that of the protein tyrosine kinase Syk and phospholipase C–γ2, in response to stimulation through the collagen receptor GPVI and the C-type lectin-like receptor 2. This signaling defect was traced to the loss of a novel FlnA–Syk interaction, as Syk binds to FlnA at immunoglobulin-like repeat 5. Our findings reveal that the interaction between FlnA and Syk regulates ITAM- and ITAM-like–containing receptor signaling and platelet function.

Efficient tyrosine phosphorylation of proteins after activation of platelets with thrombin depends on intact glycoprotein Ib

The role of platelet glycoprotein Ib as a thrombin receptor has been often a subject of controversy. We have investigated the role of the thrombin receptors, GPIb and protease-activated receptor (PAR)-1. Tyrosine phosphorylation in whole platelet lysates and in cytoskeletal extracts was evaluated after activation with thrombin and with the thrombin receptor-activating peptide (TRAP). Different experimental approaches were applied including: (i) congenital deficiency of platelet GPIb (Bernard Soulier syndrome, BSS), (ii) antibody to GPIb (AP1), (iii) selective protease cleavage (metalloprotease), and (iv) antibody to (PAR)-1. After activation of control platelets with thrombin or TRAP, multiple proteins became tyrosine phosphorylated in platelet lysates and some of them associated with the cytoskeletal fraction. These effects were absent in BSS platelets. Presence of AP1 or metalloprotease treatment showed an inhibitory effect when platelets were activated with a low concentration of thrombin or TRAP. Blockade of PAR-1 with a specific antibody, SPAN 12, inhibited platelet response to both agonists. This study reinforces the hypothesis that GPIb is the high-affinity receptor for thrombin. The signaling mechanisms occurring through tyrosine phosphorylation of proteins triggered by thrombin seem to be dependent on intact GPIb. Moreover, our results indicate that both receptors, GPIb and PAR-1, are necessary to achieve a full platelet response to thrombin.

Transformation, genomic instability and senescence mediated by platelet/megakaryocyte glycoprotein Ibalpha

GpIbalpha, a subunit of the von Willebrand factor receptor, functions during blood clotting to promote platelet adhesion and activation. GpIbalpha is widely expressed, is positively regulated by c-Myc and is essential for the promotion of c-Myc-mediated chromosomal instability. We now show that GpIbalpha is also a classical oncoprotein in which its deregulated expression leads to transformation, reduced growth factor requirements, increased resistance to apoptosis, and, in primary cells, p53-dependent senescence. Finally, GpIbalpha also promotes double-stranded DNA breaks, and induces profound nuclear dysmorphology, indicating that, in addition to its direct transforming function, it displays genotoxicity at several distinct levels. These findings identify novel functions for GpIbalpha and pathways through which c-Myc mediates transformation and global genomic destabilization.

A role for glycosphingolipid-enriched microdomains in platelet glycoprotein Ib-mediated platelet activation

BACKGROUND

Glycoprotein (GP) Ib, a platelet von Willebrand factor (VWF) receptor, plays a crucial role in thrombosis and hemostasis. As recent reports have suggested that GPIb partially locates in a particular region, designated as glycosphingolipid-enriched microdomains (GEMs), we hypothesized that GEMs play a central role in GPIb-mediated platelet activation.

METHODS

Platelets were stimulated by VWF/botrocetin to activate platelets through GPIb. GEMs and non-GEMs were isolated by sucrose density gradient ultracentrifugation and the location of signaling molecules characterized. The role of GEMs-mediated signaling in platelet behavior was tested by platelet aggregation and by platelet interaction with immobilized VWF under flow conditions when GEMs were disrupted by methyl-beta-cyclodextrin (MbetaCD).

RESULTS

GPIb was partially translocated to GEMs upon VWF/botrocetin stimulation. Immunoprecipitation of GPIb in GEMs and non-GEMs revealed that the tyrosine kinases, Src and Lyn, were associated with GPIb only in GEMs after GPIb-stimulation, and not in non-GEMs. Activation of PLCgamma2 was more intense in GEMs than non-GEMs. Disruption of GEMs by MbetaCD strongly inhibited tyrosine phosphorylation of Syk and PLCgamma2. Functional studies revealed that stable adhesion of platelets to a VWF-coated surface under flow was impaired by GEM disruption by MbetaCD.

CONCLUSION

The combined results suggest that GEMs play an important role in GPIb-mediated platelet activation.

GPVI potentiation of platelet activation by thrombin and adhesion molecules independent of Src kinases and Syk

OBJECTIVE

The present study investigates the role of Src and Syk tyrosine kinases in signaling by G-protein coupled and platelet adhesion receptors.

METHODS AND RESULTS

Using Syk-/- platelets or the Src kinase inhibitor PP2, we demonstrate a critical role for Src and Syk kinases in mediating lamellipodia formation on VWF, collagen, CRP, fibrinogen, and fibronectin. In all cases, the spreading defect was overcome by addition of thrombin. Conversely, platelet aggregation and alphaIIb beta3 activation induced by thrombin was similar to controls, arguing against a functional role for Src and Syk in alphaIIb beta3 activation. Unexpectedly, CRP potentiated integrin alphaIIb beta3 activation and platelet aggregation induced by subthreshold concentrations of thrombin in Syk-/- platelets or in the presence of the Src kinase inhibitor PP2. Potentiation in the presence of PP2 was lost in the absence of FcRgamma-chain or GPVI confirming that it was mediated through the immunoglobulin receptor. Further delineation of this PP2-resistant synergy revealed that PAR4 could trigger the enhanced response in combination with CRP.

CONCLUSIONS

We show that Syk is critical for lamellipodia formation on a range of immobilized proteins but that this can be overcome by addition of thrombin. Further, we reveal a novel role for GPVI in supporting thrombin-induced activation, independent of Syk and Src kinases.

Von Willebrand factor activates endothelial nitric oxide synthase in blood platelets by a glycoprotein Ib-dependent mechanism

BACKGROUND

The molecular regulation of endothelial nitric oxide synthase (eNOS) in blood platelets and the signalling events induced by platelet-derived NO are poorly defined. In particular, the ability of von Willebrand factor (VWF) to stimulate cyclic guanosine monophosphate (cGMP) formation in platelets has produced conflicting data.

OBJECTIVES

To determine the mechanisms leading to eNOS activation and clarify the downstream signaling pathways activated by platelet-derived NO in response to VWF.

METHODS

We used three independent markers of NO signaling, [3H] l-citrulline production, cGMP accrual and immunoblotting of vasodilator-stimulated phosphoprotein (VASP) to examine the NO signaling cascade in response to VWF.

RESULTS

VWF increased NO synthesis and bioavailability, as evidenced by increased [3H] l-citrulline production and cGMP accrual, respectively. VWF-induced eNOS activation was GPIb-IX-dependent and independent of integrin alpha(IIb)beta3. cGMP formation in response to VWF required Ca2+ mobilization, Src family kinases, phosphatidylinositol 3-kinase and phospholipase C, but not protein kinase C. This suggests that a cross-talk between the signaling mechanisms regulates platelet activation and NO synthesis. VWF-induced cGMP accrual was completely blocked by apyrase and indomethacin, demonstrating an essential role for platelet-derived ADP and thromboxane A2 (TxA2). Elevated cGMP levels led to increased VASP phosphorylation at serine239 that was both protein kinase G (PKG)- and protein kinase A (PKA)-dependent.

CONCLUSIONS

We demonstrate that VWF activates eNOS through a specific Ca2+-dependent GPIb receptor-signaling cascade that relies on the generation of platelet-derived ADP and TxA2. Furthermore, we provide the first evidence to suggest that platelet derived-NO/cGMP activates PKA in addition to PKG.

Sequential activation of p38 and ERK pathways by cGMP-dependent protein kinase leading to activation of the platelet integrin alphaIIb beta3

Integrin activation (inside-out signaling) in platelets can be initiated by agonists such as von Willebrand factor (VWF) and thrombin. Here we show that a mitogen-activated protein kinase (MAPK), p38, plays an important role in the activation of integrin alphaIIb beta3 induced by VWF and thrombin. A dominant-negative mutant of p38, p38AF, inhibits alphaIIb beta3 activation induced by VWF binding to its receptor, the platelet glycoprotein Ib-IX (GPIb-IX), and p38 inhibitors diminish platelet aggregation induced by VWF or low-dose thrombin. The inhibitory effect of p38 inhibitor is unlikely to be caused by the previous suggested effect on cyclo-oxygenase, as inhibition also was observed in the presence of high concentrations of cyclo-oxygenase inhibitor, aspirin. VWF or thrombin induces p38 activation, which is inhibited in cGMP-dependent protein kinase (PKG)-knockout mouse platelets and PKG inhibitor-treated human platelets, indicating that activation of p38 is downstream from PKG in the signaling pathway. p38AF or p38 inhibitors diminish PKG-induced phosphorylation of extracellular stimuli-responsive kinase (ERK), which also is important in integrin activation. Thus, p38 plays an important role in mediating PKG-dependent activation of ERK. These data delineate a novel signaling pathway in which platelet agonists sequentially activate PKG, p38, and ERK pathways leading to integrin activation.

Intracellular signaling in platelets

PURPOSE OF REVIEW

Over the past few years, a large portion of platelet research has focused on intracellular signaling events that contribute to stable platelet adhesion and aggregation.

RECENT FINDINGS

Studies of knockout mice have suggested critical roles for several previously unappreciated signaling molecules including phosphatidylinositol 3-kinase, the exchange factor CalDAG-GEFI, and the small GTPase Rap1b. These proteins may function to remodel the platelet cytoskeleton and thereby regulate both adhesion and aggregation. The abundant cytoskeletal protein talin appears to be a key regulator of the platelet integrin alphaIIbbeta3. Recent evidence suggests that talin binding to the cytoplasmic tail of beta3 promotes integrin oligomerization, thereby increasing the binding avidity the alphaIIbbeta3 complex for fibrinogen.

SUMMARY

The identification of platelet signaling pathways not only has clinical implications for diagnosis, but perhaps more importantly for rationale drug design. Aspirin, dipyridamole (Persantine), and thienopyridines (ticlopidine and clopidogrel) are all examples of agents that specifically target discrete platelet signaling pathways. These drugs have already been proven to be beneficial in the treatment of cardiovascular disease. Novel agents that target newly identified signaling pathways hold promise of greater specificity and efficacy.

Platelet GPIb-IX-V-dependent signaling

Although the signaling pathways related to GPIb-IX-V have not been fully elucidated, an accumulating body of evidence suggests that phospholipase C (PLC)gamma2 activation, subsequent Ca++ release and oscillations constitute an essential signal transduction pathway related to GPIb-IX-V. Src family kinases are required for PLCgamma2 activation, while FcR gamma-chain/Fc gammaRIIA may be dispensable for PLCgamma2 activation. Although PI-3K serves to potentiate various signaling events culminating in alpha(IIb)beta3 activation, PI-3K activity may be dispensable for Src-PLCgamma2 activation in GPIb-IX-V-mediated signaling. Glycosphingolipid-enriched microdomains (GEMs) appear to provide platforms for the signal transduction pathway related to GIb-IX-V, as the interaction between GPIb-IX-V and Src or PLCgamma2 tyrosine phosphorylation occurs exclusively in GEMs.

Botrocetin/VWF-induced signaling through GPIb-IX-V produces TxA2 in an alphaIIbbeta3- and aggregation-independent manner

Binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein (GP) Ib-IX-V complex initiates a signaling cascade that causes alphaIIbbeta3 activation and platelet aggregation. Previous work demonstrated that botrocetin (bt)/VWF-mediated agglutination activates alphaIIbbeta3 and elicits adenosine triphosphate (ATP) secretion in a thromboxane A2 (TxA2)- and Ca2+-dependent manner. This agglutination-elicited TxA2 production occurs in the absence of ATP secretion. However, the signaling components and signaling network or pathway activated by GPIb-mediated agglutination to cause TxA2 production have not been identified. Therefore, the focus of this study was to elucidate at least part of the signal transduction network or pathway activated by GPIb-mediated agglutination to cause TxA2 production. The phosphatidylinositol 3-kinase (PI3K) selective inhibitor wortmannin, and mouse platelets deficient in Lyn, Src, Syk, Src homology 2 (SH2) domain-containing leukocyte protein 76 (SLP-76), phospholipase Cgamma2 (PLCgamma2), linker for activation of T cells (LAT), or Fc receptor gamma-chain (FcRgamma-chain) were used for these studies. LAT and FcRgamma-chain were found not to be required for agglutination-driven TxA2 production or activation of alphaIIbbeta3, but were required for granule secretion and aggregation. The results also clearly demonstrate that bt/VWF-mediated agglutination-induced TxA2 production is dependent on signaling apparently initiated by Lyn, enhanced by Src, and propagated through Syk, SLP-76, PI3K, PLCgamma2, and protein kinase C (PKC).

A critical role for 14-3-3zeta protein in regulating the VWF binding function of platelet glycoprotein Ib-IX and its therapeutic implications

The platelet receptor for von Willebrand factor (VWF), glycoprotein (GP) Ib-IX, mediates platelet adhesion and activation. The cytoplasmic domains of the GPIb alpha and beta subunits contain binding sites for the phosphorylation-dependent signaling molecule, 14-3-3zeta. Here we show that a novel membrane-permeable inhibitor of 14-3-3zeta-GPIbalpha interaction, MPalphaC, potently inhibited VWF binding to platelets and VWF-mediated platelet adhesion under flow conditions. MPalphaC also inhibited VWF-dependent platelet agglutination induced by ristocetin. Furthermore, activation of the VWF binding function of GPIb-IX induced by GPIbbeta dephosphorylation is diminished by mutagenic disruption of the 14-3-3zeta binding site in the C-terminal domain of GPIbalpha, mimicking MPalphaC-induced inhibition, indicating that the inhibitory effect of MPalphaC is likely to be caused by disruption of 14-3-3zeta binding to GPIbalpha. These data suggest a novel 14-3-3zeta-dependent regulatory mechanism that controls the VWF binding function of GPIb-IX, and also suggest a new type of antiplatelet agent that may be potentially useful in preventing or treating thrombosis.

Platelet adhesion signalling and the regulation of thrombus formation

Platelets perform a central role in haemostasis and thrombosis. They adhere to subendothelial collagens exposed at sites of blood vessel injury via the glycoprotein (GP) Ib-V-IX receptor complex, GPVI and integrin alpha(2)beta(1). These receptors perform distinct functions in the regulation of cell signalling involving non-receptor tyrosine kinases (e.g. Src, Fyn, Lyn, Syk and Btk), adaptor proteins, phospholipase C and lipid kinases such as phosphoinositide 3-kinase. They are also coupled to an increase in cytosolic calcium levels and protein kinase C activation, leading to the secretion of paracrine/autocrine platelet factors and an increase in integrin receptor affinities. Through the binding of plasma fibrinogen and von Willebrand Factor to integrin alpha(IIb)beta(3), a platelet thrombus is formed. Although increasing evidence indicates that each of the adhesion receptors GPIb-V-IX and GPVI and integrins alpha(2)beta(1) and alpha(IIb)beta(3) contribute to the signalling that regulates this process, the individual roles of each are only beginning to be dissected. By contrast, adhesion receptor signalling through platelet endothelial cell adhesion molecule 1 (PECAM-1) is implicated in the inhibition of platelet function and thrombus formation in the healthy circulation. Recent studies indicate that understanding of platelet adhesion signalling mechanisms might enable the development of new strategies to treat and prevent thrombosis.

Collagen-induced generation of platelet-derived microparticles in whole blood is dependent on ADP released from red blood cells and calcium ions

We have evaluated the effects of different anti-coagulants or agonists on the generation of platelet-derived microparticles (PMPs) using flow cytometry. Twenty microg/ml of collagen induced significantly greater PMP formation in whole blood anti-coagulated with argatroban, a selective thrombin inhibitor, as compared with platelet-rich plasma, or whole blood anti-coagulated with citrate. Thus, whole blood kept at the physiological Ca2+ concentration provides an optimal condition for the formation of PMP. Convulxin, a GPVI-selective agonist, also induced PMP formation at the magnitude which far exceeds those of other agonists, such as thrombin receptor-activating peptide, ADP or epinephrine. These findings suggest that GPVI-mediated platelet activation plays a key role in the formation of PMP in the presence of physiological Ca2+ in whole blood. The addition of red blood cells to PRP potentiated PMP formation induced by collagen. Pretreatment of whole blood with the combination of creatine phosphate and creatine phosphokinase reduced PMP formation induced by collagen. Blockade of ADP receptors, P2Y12 with AR-C69931MX and P2Y1 with A3P5P, respectively, further suppressed collagen-induced PMP formation. We conclude that ADP released from red blood cells enhances PMP formation induced by collagen, and that both P2Y12 and P2Y1 contribute to ADP-potentiation of PMP generation induced by collagen.

Signalling through the platelet glycoprotein Ib-V–IX complex

The glycoprotein Ib-V-IX is one of the major adhesive receptors expressed on the surface of circulating platelets. It is composed of four different polypeptides-GPIbalpha, GPIbbeta, GPIX, and GPV-and represents a multifunctional receptor able to interact with a number of ligands, including the adhesive protein von Willebrand factor, the coagulation factors thrombin, factors XI and XII, and the membrane glycoproteins P-selectin and Mac-1. Interaction of GPIb-V-IX with the subendothelial von Willebrand factor is essential for primary haemostasis, as it initiates platelet adhesion to the subendothelial matrix at the sites of vascular injury even under high flow conditions. Upon interaction with von Willebrand factor, GPIb-V-IX initiates transmembrane signalling events for platelet activation, which eventually result in integrin alpha(IIb)beta(3) stimulation and platelet aggregation. The investigation of the biochemical mechanisms for platelet activation by GPIb-V-IX has attracted increasing attention during the last years. This review will describe and discuss recent findings that have provided new insights into the events underlying GPIb-V-IX transmembrane signalling. In particular, it will summarise basic concepts on the structure of this receptor, extracellular ligands, and intracellular interactors potentially involved in transmembrane signalling. The recently suggested role of membrane Fc receptors in GPIb-V-IX-initiated platelet activation will also be discussed, along with the involvement of lipid metabolising enzymes, tyrosine kinases, and the cytoskeleton in the crosstalk between GPIb-V-IX and integrin alpha(IIb)beta(3).

Cleavage of platelet endothelial cell adhesion molecule-1 (PECAM-1) in platelets exposed to high shear stress

Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a 130 kDa transmembrane glycoprotein that belongs to the immunoglobulin superfamily and is expressed on the surface of endothelial cells, platelets, and other blood cells. Although the importance of this adhesion molecule in various cell-cell interactions is established, its functional role in platelets remains to be elucidated. In this study, we examined whether PECAM-1 underwent changes in platelets exposed to high shear stress. Platelet PECAM-1 was cleaved under high shear stress and was released into the extracellular fluid as a fragment with an approximate molecular weight of 118 kDa. The cleavage was inhibited by an anti-VWF MoAb, but not by recombinant VWF A1 domains. These findings suggest that the GPIb-VWF interaction is involved in PECAM-1 cleavage under high shear stress, and that the cleavage is independent of GPIb clustering by VWF multimers. Furthermore, EGTA or calpeptin inhibited PECAM-1 cleavage. This finding provides evidence for the involvement of calpain in PECAM-1 cleavage. Flow-cytometric analysis revealed that PECAM-1 expression on the platelet surface was decreased under high shear stress. This reduction occurred exclusively in a specific population of platelets, which corresponded to platelet-derived microparticles (PMP). In conclusion, PECAM-1 cleavage under high shear stress is closely related to the activation of calpain and the process of PMP formation mediated by the GPIb-VWF interaction.

Signaling through GP Ib-IX-V activates alpha IIb beta 3 independently of other receptors

Platelet adhesion to von Willebrand factor (VWF) activates alpha IIb beta 3, a prerequisite for thrombus formation. However, it is unclear whether the primary VWF receptor, glycoprotein (GP) Ib-IX-V, mediates alpha IIb beta 3 activation directly or through other signaling proteins physically associated with it (eg, FcR gamma-chain), possibly with the contribution of other agonist receptors and of VWF signaling through alpha IIb beta 3. To resolve this question, human and GP Ibalpha transgenic mouse platelets were plated on dimeric VWF A1 domain (dA1VWF), which engages only GP Ib-IX-V, in the presence of inhibitors of other agonist receptors. Platelet adhesion to dA1VWF induced Src kinase-dependent tyrosine phosphorylation of the FcR gamma-chain and the adapter molecule, ADAP, and triggered intracellular Ca(2+) oscillations and alpha IIb beta 3 activation. Inhibition of Ca(2+) oscillations with BAPTA-AM prevented alpha IIb beta 3 activation but not tyrosine phosphorylation. Pharmacologic inhibition of protein kinase C (PKC) or phosphatidylinositol 3-kinase (PI 3-kinase) prevented alpha IIb beta 3 activation but not Ca(2+) oscillations. Inhibition of Src with 2 distinct compounds blocked all responses downstream of GP Ib-IX-V under static or flow conditions. However, dA1VWF-induced responses were reduced only slightly in GP Ibalpha transgenic platelets lacking FcR gamma-chain. These data establish that GP Ib-IX-V itself can signal to activate alpha IIb beta 3, through sequential actions of Src kinases, Ca(2+) oscillations, and PI 3-kinase/PKC.

GPIb-dependent platelet activation is dependent on Src kinases but not MAP kinase or cGMP-dependent kinase

Glycoprotein Ib-IX-V (GPIb-IX-V) mediates platelet tethering to von Willebrand factor (VWF), recruiting platelets into the thrombus, and activates integrin alphaIIbbeta3 through a pathway that is dependent on Src kinases. In addition, recent reports indicate that activation of alphaIIbbeta3 by VWF is dependent on protein kinase G (PKG) and mitogen-activated protein (MAP) kinases. The present study compares the importance of these signaling pathways in the activation of alphaIIbbeta3 by GPIb-IX-V. In contrast to a recent report, VWF did not promote an increase in cyclic guanosine monophosphate (cGMP), while agents that elevate cGMP, such as the nitrous oxide (NO) donor glyco-SNAP-1 (N-(beta-D-glucopyranosyl)-N2-acetyl-S-nitroso-D,L-penicillaminamide) or the type 5 phosphosdiesterase inhibitor, sildenafil, inhibited rather than promoted activation of alphaIIbbeta3 by GPIb-IX-V and blocked aggregate formation on collagen at an intermediate rate of shear (800 s(-1)). Additionally, sildenafil increased blood flow in a rabbit model of thrombus formation in vivo. A novel inhibitor of the MAP kinase pathway, which is active in plasma, PD184161, had no effect on aggregate formation on collagen under flow conditions, whereas a novel inhibitor of Src kinases, which is also active in plasma, PD173952, blocked this response. These results demonstrate a critical role for Src kinases but not MAP kinases in VWF-dependent platelet activation and demonstrate an inhibitory role for cGMP-elevating agents in regulating this process.

A palmitylated peptide derived from the glycoprotein Ib beta cytoplasmic tail inhibits platelet activation

The platelet receptor GPIb/IX/V mediates a crucial role in hemostasis, yet the signaling mechanisms involved are incompletely understood. The complex consists of four polypeptides GPIb alpha, GPIb beta, GPIX and GPV. We identified an amino acid sequence in the cytoplasmic tail of the GPIb beta subunit between residues R151 and A161 that is highly conserved across species and hypothesized that it has functional importance. To target this motif, we synthesized a corresponding cell-permeable palmitylated peptide (Pal-RRLRARARARA) and investigated its effect on platelet function. Pal-RRLRARARARA completely inhibited low dose thrombin- and ristocetin-induced aggregation in washed platelets but only partially inhibited collagen- and U46619-induced aggregation. Thromboxane production in platelets stimulated with thrombin was significantly reduced by Pal-RRLRARARARA compared with collagen. Activation of the integrin alpha IIb beta 3 in response to thrombin was significantly reduced when platelets were preincubated with Pal-RRLRARARARA. The adhesion of washed platelets to von Willebrand factor (VWF) under static conditions was significantly reduced by Pal-RRLRARARARA. Under conditions of high shear, the velocity of platelets rolling on VWF was significantly increased when platelets are preincubated with Pal-RRLRARARARA. This study defines a novel function for the RRLRARARARA motif of GPIb beta in platelet activation.

PECAM-1 negatively regulates GPIb/V/IX signaling in murine platelets

Platelet adhesion at sites of vascular injury is mediated, in part, by interaction of the platelet plasma membrane glycoprotein (GP) Ib/V/IX complex with von Willebrand Factor (VWF) presented on collagen-exposed surfaces. Recent studies indicate that GPIb/V/IX may be functionally coupled with the Fc receptor gamma (FcR gamma)-chain, which, by virtue of its cytoplasmic immunoreceptor tyrosine-based activation motif, sends activation signals into the cell. Platelet endothelial cell adhesion molecule-1 (PECAM-1) is an inhibitory receptor that has previously been shown to negatively regulate platelet responses to collagen, which transduces activation signals via the GPVI/FcR gamma-chain complex. To determine whether PECAM-1 might similarly regulate signals emanating from GPIb/FcR gamma, we compared activation and aggregation responses to VWF of PECAM-1-positive and PECAM-1-deficient murine platelets. PECAM-1 and the FcR gamma-chain became rapidly tyrosine phosphorylated in platelets following botrocetin-induced VWF binding, but FcR gamma-chain tyrosine phosphorylation was delayed in PECAM-1-positive, versus PECAM-1-deficient, platelets. PECAM-1-deficient platelets were hyperaggregable to VWF, exhibited enhanced spreading and, under conditions of arterial flow, formed markedly larger thrombi on immobilized VWF than did wild-type platelets. Taken together, these data support the notion that engagement of the GPIb complex, in addition to sending activation signals, also initiates a negative feedback loop involving PECAM-1 that controls the rate and extent of platelet activation.

Filamin A binding to the cytoplasmic tail of glycoprotein Ibalpha regulates von Willebrand factor-induced platelet activation

We examined the hypothesis that filamin A binding to the cytoplasmic tail of platelet glycoprotein Ibalpha (GpIbalpha) is regulated by pathologic shear stress and modulates von Willebrand factor (VWF)-induced platelet activation. To begin, we examined filamin binding to GpIbalpha in Chinese hamster ovary cells coexpressing mutant human GpIb-IX and wild-type human filamin A. We observed that many different deletions and truncations N-terminal to GpIbalpha's cytoplasmic domain residue 594 disrupted filamin A binding, but that binding was unaffected by 14 different point mutations in hydrophilic residues between amino acids 557 and 593. To try to narrow GpIbalpha's filamin A-binding domain, we next measured the effect of several cytoplasmic domain peptides on human filamin A binding to a GST-GpIbalpha cytoplasmic domain fusion protein. One peptide (residues 557-575; designated "A4 peptide") inhibited filamin A binding to the GST-GpIbalpha cytoplasmic domain fusion protein and competed with GpIbalpha for binding to filamin A. When the A4 peptide was delivered to intact human platelets using a carrier peptide, we observed the dose-dependent inhibition of VWF-induced platelet aggregation in response to both ristocetin and shear stress. The effect of the A4 peptide on shear-induced platelet aggregation was accompanied by the attenuation of shear-induced filamin A binding to GpIbalpha and diminished shear-dependent protein tyrosine phosphorylation. These results suggest that shear-dependent VWF-induced platelet activation affects filamin A binding to GpIb-IX-V, and that filamin A binding to the cytoplasmic tail of GpIbalpha regulates proaggregatory tyrosine kinase signaling.

Signaling role for phospholipase C gamma 2 in platelet glycoprotein Ib alpha calcium flux and cytoskeletal reorganization. Involvement of a pathway distinct from FcR gamma chain and Fc gamma RIIA

Interaction of the platelet GPIb-V-IX complex with surface immobilized von Willebrand factor (vWf) is required for the capture of circulating platelets and their ensuing activation. In previous work, it was found that GPIb/vWf-mediated platelet adhesion triggers Ca2+ release from intracellular stores, leading to cytoskeletal reorganization and filopodia extension. Despite the potential functional importance of GPIb-induced cytoskeletal changes, the signaling mechanisms regulating this process have remained ill-defined. The studies presented here demonstrate an important role for phospholipase C (PLC)-dependent phosphoinositide turnover for GPIb-dependent cytoskeletal remodeling. This is supported by the findings that the vWf-GPIb interaction induced a small increase in inositol 1,4,5-triphosphate (IP3) and that treating platelets with the IP3 receptor antagonist APB-2 or the PLC inhibitor U73122 blocked cytosolic Ca2+ flux and platelet shape change. Normal shape change was observed in G alpha q-/- mouse platelets, excluding a role for PLC beta isoforms in this process. However, decreased shape change and Ca2+ mobilization were observed in mice lacking PLC gamma 2, demonstrating that this isotype played an important, albeit incomplete, role in GPIb signaling. The signaling pathways utilized by GPIb involved one or more members of the Src kinase family as platelet shape change and Ca2+ flux were inhibited by the Src kinase inhibitors PP1 and PP2. Strikingly, shape change and Ca2+ release occurred independently of immunoreceptor tyrosine-based activation motif (ITAM)-containing receptors, because these platelet responses were normal in human platelets treated with the anti-Fc gamma RIIA blocking monoclonal antibody IV.3 and in mouse platelets deficient in the FcR gamma chain. Taken together, these studies define an important role for PLC gamma 2 in GPIb signaling linked to platelet shape change. Moreover, they demonstrate that GPIb-dependent calcium flux and cytoskeletal reorganization involves a signaling pathway distinct from that utilized by ITAM-containing receptors.

Signaling events underlying thrombus formation

Recent in vivo studies have highlighted the dynamic and complex nature of platelet thrombus growth and the requirement for multiple adhesive receptor-ligand interactions in this process. In particular, the importance of von Willebrand factor (VWF) in promoting both primary adhesion and aggregation under high shear conditions is now well established. In general, the efficiency with which platelets adhere and aggregate at sites of vessel wall injury is dependent on the synergistic action of various adhesive and soluble agonist receptors, with the contribution of each of the individual receptors dependent on the prevailing blood flow conditions. In this review, we will discuss the major platelet adhesive interactions regulating platelet thrombus formation under high shear, with specific focus on the VWF (GPIb and integrin alphaIIbbeta3) and collagen receptors (GPVI and integrin alpha2beta1). We will also discuss the signaling mechanisms utilized by these receptors to induce platelet activation with specific emphasis on the role of cytosolic calcium flux in regulating platelet adhesion dynamics. The role of soluble agonists in promoting thrombus growth will be highlighted and a model to explain the synergistic requirement for adhesive and soluble stimuli for efficient platelet aggregation will be discussed.

Activation of pp125FAK by type 2B recombinant von Willebrand factor binding to platelet GPIb at a high shear rate occurs independently of alpha IIb beta 3 engagement

Shear-induced platelet aggregation (SIPA) involves the sequential interaction of von Willebrand factor (VWF) with both glycoprotein Ib (GPIb) and alphaIIbbeta3 receptors. Type 2B recombinant VWF (2B-rVWF), characterized by an increased affinity for GPIb, induces strong SIPA at a high shear rate (4000 s-1). Despite the increased affinity of 2B-rVWF for GPIb, patients with type 2B von Willebrand disease have a paradoxical bleeding disorder, which is not well understood. The purpose of this study was to determine if SIPA induced by 2B-rVWF was associated with alphaIIbbeta3-dependent platelet activation. To this end, we have addressed the influence of 2B-rVWF (Val553Met substitution) on SIPA-dependent variations of tyrosine protein phosphorylation (P-Tyr) and the effect of alphaIIbbeta3 blockers. At a high shear rate, 2B-rVWF induced a strong SIPA, as shown by a 92.7% +/- 0.4% disappearance of single platelets (DSP) after 4.5 minutes. In these conditions, increased P-Tyr of proteins migrating at positions 64 kd, 72 kd, and 125 kd were observed. The band at 125 kd was identified as pp125FAK using anti-phospho-FAK antibody. This effect, which required a high level of SIPA (> 70% DSP), was observed at 4000 s-1 but not at 200 s-1. Monoclonal antibodies (MoAbs) 6D1 (anti-GPIb) and 328 (anti-VWF A1 domain), completely abolished SIPA and p125FAK phosphorylation mediated by 2B-rVWF. In contrast, neither RGDS peptide nor MoAb 7E3, both known to block alphaIIbbeta3 engagement, had any effect on SIPA and pp125FAK. The size of aggregates formed at a high shear rate in the presence of 2B-rVWF was decreased by genistein, demonstrating the biologic relevance of pp125FAK. These findings provide a unique mechanism whereby the enhanced interaction of 2B-rVWF with GPIb, without engagement of alphaIIbbeta3, is sufficient to induce SIPA but does not lead to stable thrombus formation.

Interaction between von Willebrand factor and glycoprotein Ib activates Src kinase in human platelets: role of phosphoinositide 3-kinase

The binding of von Willebrand factor (VWF) to glycoprotein (GP) Ib-IX-V stimulates transmembrane signaling events that lead to platelet adhesion and aggregation. Recent studies have implied that activation of Src family kinases is involved in GPIb-mediated platelet activation, although the related signal transduction pathway remains poorly defined. This study presents evidence for an important role of Src and GPIb association. In platelet lysates containing Complete, a broad-spectrum protease inhibitor mixture, Src and Lyn dynamically associated with GPIb on VWF-botrocetin stimulation. Cytochalasin D, which inhibits translocation of Src kinases to the cytoskeleton, further increased Src and GPIb association. Similar results were obtained with botrocetin and monomeric A1 domain, instead of intact VWF, with induction of both Src activation and association between GPIb and Src. These findings suggest that ligand binding of GPIb, without receptor clustering, is sufficient to activate Src. Immunoprecipitation studies demonstrated that Src, phosphoinositide 3- kinase (PI 3-kinase), and GPIb form a complex in GPIb-stimulated platelets. When the p85 subunit of PI 3-kinase was immunodepleted, association of Src with GPIb was abrogated. However, wortmannin, a specific PI 3-kinase inhibitor, failed to block complex formation between Src and GPIb. The Src-SH3 domain as a glutathione S-transferase (GST)-fusion protein coprecipitated the p85 subunit of PI 3-kinase and GPIb. These findings taken together suggest that the p85 subunit of PI 3-kinase mediates GPIb-related activation signals and activates Src independently of the enzymatic activity of PI 3- kinase.

von Willebrand factor (VWF)-dependent human platelet activation: porcine VWF utilizes different transmembrane signaling pathways than does thrombin to activate platelets, but both require protein phosphatase function

The interaction between von Willebrand factor (VWF) and glycoprotein (GP) Ib results in platelet agglutination and activation of many signaling intermediates. To determine if VWF-dependent platelet activation requires the participation of pivotal transmembrane signaling pathways, we analyzed VWF-dependent platelet activation profiles following inhibition of several transmembrane signaling intermediates. This was accomplished using porcine VWF, which has been shown to interact with human GPIb independently of shear stress or ristocetin. Platelet alpha (CD62) and lysozomal granule release (CD63), microparticle formation, and platelet agglutination/aggregation were evaluated. The ability of signaling inhibitors to prevent VWF-dependent platelet activation was compared to their ability to inhibit thrombin-dependent activation. The results demonstrate that VWF-dependent platelet activation can occur independently of the activities of protein kinase C (PKC), wortmannin-sensitive phosphatidylinositide 3-kinase, and phospholipase C, as well as independently of elevations in the concentration of intracellular calcium. In sharp contrast, these transmembrane signaling intermediates are required for thrombin-dependent platelet activation. In addition, thrombin-dependent but not VWF-dependent platelet activation was associated with elevations in the concentration of intracellular calcium under the conditions used. The family of signaling intermediates which appeared to be pivotal for both thrombin- and VWF-dependent platelet activation were the protein tyrosine phosphatases and the serine/threonine phosphatases. It is concluded that thrombin-dependent platelet activation relies on the activation of several transmembrane signaling pathways, whereas VWF-dependent platelet activation is dependent upon the activity of protein phosphatases. Inhibition of these phosphatases in vivo may provide a novel therapeutic approach for treating VWF-dependent platelet disorders such as thrombotic thrombocytopenic purpura or arterial thrombosis.

Localization of the adhesion receptor glycoprotein Ib-IX-V complex to lipid rafts is required for platelet adhesion and activation

The platelet glycoprotein (GP) Ib-IX-V complex mediates the attachment of platelets to the blood vessel wall by binding von Willebrand factor (VWF), an interaction that also transmits signals for platelet activation and aggregation. Because the complex is extensively palmitoylated, a modification known to target proteins to lipid rafts, we investigated the role of raft localization in GP Ib-IX-V functions. In unstimulated platelets, a minor portion of the complex localized to Triton-insoluble raft fractions; this portion increased three to sixfold with platelet activation by VWF. Raft-associated GP Ib-IX-V was selectively palmitoylated, with GP Ib-IX-V-associated palmitate increasing in the raft fraction on VWF-mediated activation. The raft fraction was also the site of association between GP Ib-IX-V and the Fc receptor FcgammaRIIA. The importance of this association was demonstrated by the ability of the FcgammaRIIA antibody IV.3 to inhibit shear-induced platelet aggregation. Disruption of rafts by depleting membrane cholesterol impaired several GP Ib-IX-V-dependent platelet fractions: aggregation to VWF under static conditions and under shear stress, tyrosine phosphorylation, and adhesion to a VWF surface. Partial restoration of membrane cholesterol content partially restored shear-induced platelet aggregation and tyrosine phosphorylation. Thus, localization of the GP Ib-IX-V complex within rafts is crucial for both platelet adhesion and postadhesion signaling.

Reconstitution of adhesive properties of human platelets in liposomes carrying both recombinant glycoproteins Ia/IIa and Ib alpha under flow conditions: specific synergy of receptor-ligand interactions

Liposomes carrying both recombinant glycoprotein Ia/IIa (rGPIa/IIa) and Ib alpha (rGPIb alpha) (rGPIa/IIa-Ib alpha-liposomes) instantaneously and irreversibly adhered to the collagen surface in the presence of soluble von Willebrand factor (VWF) at high shear rates, in marked contrast with translocation of liposomes carrying rGPIb alpha alone on the VWF surface. In the absence of soluble VWF, the adhesion of rGPIa/IIa-Ib alpha-liposomes to the collagen surface decreased with increasing shear rates, similar to liposomes carrying rGPIa/IIa alone. While adhesion of liposomes with exofacial rGPIa/IIa and rGPIb alpha densities of 2.17 x 10(3) and 1.00 x 10(4) molecules per particle, respectively, was efficient at high shear rates, reduction in rGPIb alpha density to 5.27 x 10(3) molecules per particle resulted in decreased adhesion even in the presence of soluble VWF. A 50% reduction in the exofacial rGPIa/IIa density resulted in a marked decrease in the adhesive ability of the liposomes at all shear rates tested. The inhibitory effect of antibody against GPIb alpha (GUR83-35) on liposome adhesion was greater at higher shear rates. Further, the anti-GPIa antibody (Gi9) inhibited liposome adhesion more than GUR83-35 at all shear rates tested. These results suggest that the rGPIa/IIa-collagen interaction dominates the adhesion of rGPIa/IIa-Ib alpha-liposomes to the collagen surface at low shear rates, while the rGPIa/IIa-collagen and rGPIb alpha-VWF interaction complements each other, and they synergistically provide the needed functional integration required for liposome adhesion at high shear rates. This study thus has confirmed for the first time the proposed mechanisms of platelet adhesion to the collagen surface under flow conditions using the liposome system.

Lateral clustering of platelet GP Ib-IX complexes leads to up-regulation of the adhesive function of integrin alpha IIbbeta 3

Binding of von Willebrand factor (VWF) to GP Ib-IX mediates initial platelet adhesion and increases the subsequent adhesive function of alpha(IIb)beta(3). Because these responses are promoted most effectively by large VWF multimers, we hypothesized that receptor clustering modulates GP Ib-IX function. To test this, GP IX was fused at its cytoplasmic tail to tandem repeats of FKBP, and GP Ib-IX(FKBP)(2) and alpha(IIb)beta(3) were expressed in Chinese hamster ovary cells. Under flow conditions at wall shear rates of up to 2000 s(-1), GP Ib-IX(FKBP)(2) mediated cell tethering to immobilized VWF, just as in platelets. Conditional oligomerization of GP Ib-IX(FKBP)(2) by AP20187, a cell-permeable FKBP dimerizer, caused a decrease in cell translocation velocities on VWF (p < 0.001). Moreover, clustering of GP Ib-IX(FKBP)(2) by AP20187 led to an increase in alpha(IIb)beta(3) function, manifested under static conditions by increased cell adhesion to fibrinogen (p < 0.01) and under flow by increased stable cell adhesion to VWF (p < 0.04). Clustering of GP Ib-IX(FKBP)(2) also stimulated rapid tyrosine phosphorylation of ectopically expressed Syk, a putative downstream effector of GP Ib-IX in platelets. These studies establish that GP Ib-IX oligomerization, per se, affects the interaction of this receptor with VWF and its ability to influence the adhesive function of alpha(IIb)beta(3). By extrapolation, GP Ib-IX clustering in platelets may promote thrombus formation.

Identification and characterisation of a platelet GPIb/V/IX-like complex on human breast cancers: implications for the metastatic process

The glycoprotein (GP) Ib /V/IX receptor complex is an important adhesion molecule, originally thought to be unique to the megakaryocytic lineage. Recent evidence now indicates that GPIb /V/IX may be more widely expressed. In this study we report the presence of all subunits of the complex on four breast cancer cell lines, and 51 / 80 primary breast tumours. The surface expression of GPIb /V/IX was confirmed by flow cytometry, and by immunoprecipitation of biotin surface-labelled tumour cells. Western blotting of cell lysates under reducing conditions revealed that tumour cell-GPIb alpha had a relative molecular weight of 95 kDa as compared to 135 kDa on platelets. Despite the discrepant protein size, molecular analyses on the tumour cell-GPIb alpha subunit using RT-PCR and DNA sequencing revealed 100% sequence homology to platelet GPIb alpha. Tumour cell-GPIb /V/IX was capable of binding human von Willebrand factor (vWf), and this binding caused aggregation of tumour cells in suspension. Tumour cells bound to immobilised vWf in the presence of EDTA and demonstrated prominent filapodial extensions indicative of cytoskeletal reorganisation. Furthermore, in a modified Boyden chamber assay, prior exposure to vWf or a GPIb alpha monoclonal antibody, AK2, enhanced cell migration. The presence of a functional GPIb /V/IX-like complex in tumour cells suggests that this complex may participate in the process of haematogenous breast cancer metastasis.

Platelet activation by von Willebrand factor requires coordinated signaling through thromboxane A2 and Fc gamma IIA receptor

Interaction of von Willebrand Factor with glycoprotein Ib-IX-V induces platelet activation through a still poorly defined mechanism. Previous studies have suggested a possible role for the low affinity receptor for immunoglobulin, Fc gamma RIIA, in GPIb-IX-V signaling. Here we show that binding of vWF to platelets induces the tyrosine phosphorylation of Fc gamma RIIA by a Src kinase. Treatment of platelets with the anti-Fc gamma RIIA monoclonal antibody IV.3 specifically inhibits vWF-induced but not thrombin-induced pleckstrin phosphorylation and serotonin secretion. Moreover, vWF fails to induce pleckstrin phosphorylation in mouse platelets, lacking Fc gamma RIIA, and serotonin secretion is impaired. Pleckstrin phosphorylation and serotonin secretion in human platelets stimulated with vWF are blocked by the cyclooxygenase inhibitor acetylsalicylic acid. However, release of arachidonic acid and synthesis of TxA(2) induced by vWF are not affected by the anti-Fc gamma RIIA monoclonal antibody IV.3. Similarly, vWF-induced tyrosine phosphorylation of Fc gamma RIIA, as well as of Syk and PLC gamma 2, occurs normally in aspirinized platelets. Inhibition of the tyrosine kinase Syk by piceatannol does not affect vWF-induced tyrosine phosphorylation of Fc gamma RIIA but prevents phosphorylation of PLC gamma 2. Pleckstrin phosphorylation and platelet secretion induced by vWF, but not by thrombin, are also inhibited by piceatannol. Pleckstrin phosphorylation is also sensitive to the phosphatidylinositol 3-kinase inhibitor wortmannin. These results indicate that PLC gamma 2 plays a central role in platelet activation by vWF and that the stimulation of this enzyme requires coordinated signals through endogenous TxA(2) and Fc gamma RIIA.

IL-1 induced chemokine production through the association of Syk with TNF receptor-associated factor-6 in nasal fibroblast lines

The fibroblasts stimulated by cytokines released the chemokine and recruited the infiltrating cells, including eosinophils, that play a key role in the pathogenesis of airway disease. We established the human fibroblast lines showing high Syk expression and the lines showing low Syk expression from pieces of nasal polyp. IL-1 induces the interaction of TNFR-associated factor (TRAF) 6 with IL-1R-associated kinase, which is rapidly recruited to the IL-1R after IL-1 induction, whereas TRAF2 participates in TNF-alpha-signaling. In the present study, we found that Syk played a different role in IL-1- and TNF-alpha-induced chemokine production through a signaling complex involving Syk and TRAF6. Overexpression of wild-type Syk by gene transfer enhanced RANTES production from nasal fibroblasts stimulated with IL-1. The decrease of Syk expression by the administration of Syk antisense inhibited RANTES production in response to IL-1. However, the change of Syk expression did not affect RANTES production by TNF-alpha stimulation. We concluded that Syk is required for the IL-1-induced chemokine production through the association with TRAF-6 in fibroblasts of nasal polyps.

Aggretin, a heterodimeric C-type lectin from Calloselasma rhodostoma (Malayan pit viper), stimulates platelets by binding to α2β1 integrin and glycoprotein Ib, activating Syk and phospholipase Cγ 2, but does not involve the glycoprotein VI/Fc receptor γ chain collagen receptor

Aggretin, a potent platelet activator, was isolated from Calloselasma rhodostoma venom, and 30-amino acid N-terminal sequences of both subunits were determined. Aggretin belongs to the heterodimeric snake C-type lectin family and is thought to activate platelets by binding to platelet glycoprotein alpha(2)beta(1). We now show that binding to glycoprotein (GP) Ib is also required. Aggretin-induced platelet activation was inhibited by a monoclonal antibody to GPIb as well as by antibodies to alpha(2)beta(1). Binding of both of these platelet receptors to aggretin was confirmed by affinity chromatography. No binding of other major platelet membrane glycoproteins, in particular GPVI, to aggretin was detected. Aggretin also activates platelets from Fc receptor gamma chain (Fcgamma)-deficient mice to a greater extent than those from normal control mice, showing that it does not use the GPVI/Fcgamma pathway. Platelets from Fcgamma-deficient mice expressed fibrinogen receptors normally in response to collagen, although they did not aggregate, indicating that these platelets may partly compensate via other receptors including alpha(2)beta(1) or GPIb for the lack of the Fcgamma pathway. Signaling by aggretin involves a dose-dependent lag phase followed by rapid tyrosine phosphorylation of a number of proteins. Among these are p72(SYK), p125(FAK), and PLCgamma2, whereas, in comparison with collagen and convulxin, the Fcgamma subunit neither is phosphorylated nor coprecipitates with p72(SYK). This supports an independent, GPIb- and integrin-based pathway for activation of p72(SYK) not involving the Fcgamma receptor.

Role of Fc receptor gamma-chain in platelet glycoprotein Ib-mediated signaling

Interaction between von Willebrand factor (vWF) and glycoprotein Ib (GPIb) stimulates tyrosine kinases and subsequent tyrosine phosphorylation events in human platelets. This study found that the combination of vWF and botrocetin, by interacting with GPIb, induced tyrosine phosphorylation of Fc receptor gamma-chain (FcR gamma-chain), Syk, linker for activation of T cells (LAT), and phospholipase C gamma2 (PLCgamma2). Pretreatment of platelets with 10 microM PP1 completely inhibited these tyrosine phosphorylation events. On GPIb stimulation, Src and Lyn formed a complex with FcR gamma-chain and Syk, suggesting that Src and Lyn are involved in FcR gamma-chain tyrosine phosphorylation and downstream signals. In spite of the PLCgamma2 tyrosine phosphorylation, however, there was no intracellular calcium release and inositol 1,4,5-trisphosphate production. In Brij 35 lysates, FcR gamma-chain was found to constitutively associate with GPIb. The number of GPIb expressed on FcR gamma-chain-deficient platelets was comparable to that of the wild-type, as assessed by flow cytometry. However, tyrosine phosphorylation of Syk, LAT, and PLCgamma2 in response to vWF plus botrocetin was significantly suppressed, suggesting that FcR gamma-chain mediates activation signals related to GPIb. Compared with the aggregation response of wild-type platelets, that of FcR gamma-chain-deficient platelets in response to vWF plus botrocetin was impaired, implying that FcR gamma-chain is required for the full activation of platelets mediated by GPIb. (Blood. 2001;97:3836-3845)

Dynamic aspects of platelet adhesion under flow

1. Cell-cell and cell-matrix adhesive interactions are critical for a wide range of physiological processes, including embryogenesis, inflammation, immunity and haemostasis. 2. The ability of circulating blood cells, such as platelets and leucocytes, to adhere to sites of vascular injury is complicated by the presence of blood flow, which imposes hydrodynamic forces on adhesion contacts. 3. To overcome this problem, platelets and leucocytes have evolved specific adhesion receptors with unique biomechanical properties that enable these cells to adhere to the vessel wall under flow conditions. 4. Platelet adhesion in the normal circulation appears to be a multiple-step process involving an initial reversible interaction between the platelet adhesion receptor glycoprotein Ib-IX-V and the vascular adhesion protein von Willebrand factor. Once tethered to the vessel wall, platelets form irreversible adhesion contacts through the binding of one or more platelet integrins to specific subendothelial matrix proteins. 5. There is now a wealth of evidence demonstrating that these receptors not only mediate platelet adhesion, but also transduce signals leading to platelet activation. 6. In the present review, we will briefly discuss the current understanding of the specific roles of individual platelet receptors in supporting the haemostatic function of platelets and discuss mechanisms by which these receptors induce platelet activation.

Echicetin, a GPIb-binding snake C-type lectin from Echis carinatus, also contains a binding site for IgMkappa responsible for platelet agglutination in plasma and inducing signal transduction

Echicetin, a heterodimeric snake C-type lectin from Echis carinatus, is known to bind specifically to platelet glycoprotein (GP)Ib. We now show that, in addition, it agglutinates platelets in plasma and induces platelet signal transduction. The agglutination is caused by binding to a specific protein in plasma. The protein was isolated from plasma and shown to cause platelet agglutination when added to washed platelets in the presence of echicetin. It was identified as immunoglobulin Mkappa (IgMkappa) by peptide sequencing and dot blotting with specific heavy and light chain anti-immunoglobulin reagents. Platelet agglutination by clustering echicetin with IgMkappa induced P-selectin expression and activation of GPIIb/IIIa as well as tyrosine phosphorylation of several signal transduction molecules, including p53/56(LYN), p64, p72(SYK), p70 to p90, and p120. However, neither ethylenediaminetetraacetic acid nor specific inhibition of GPIIb/IIIa affected platelet agglutination or activation by echicetin. Platelet agglutination and induction of signal transduction could also be produced by cross-linking biotinylated echicetin with avidin. These data indicate that clustering of GPIb alone is sufficient to activate platelets. In vivo, echicetin probably activates platelets rather than inhibits platelet activation, as previously proposed, accounting for the observed induction of thrombocytopenia.

Platelet GP Ib/IX/V complex: physiological role

Platelets play an essential role in primary hemostasis and in thrombotic events, particularly in arterial vessels, as rheological conditions originate closer interactions between platelets and endothelium than lower shear rates. In response to vascular injury, platelets adhere to the subendothelial matrix by membrane receptors potentiating the generation of thrombin, become activated, and a series of biochemical processes induce platelet aggregation and liberation of intracellular metabolic products to the extracelular medium. Among platelet receptors, glycoprotein (GP) Ib/IX/V complex is peculiar, as it binds adhesive proteins, mainly von Willebrand factor (vWF), and thrombin, the main platelet agonist. Platelet adhesion and subsequent aggregation under conditions of high shear flow, essentially relies upon this receptor's capacity of binding to the subendothelial matrix, initiating signal transduction. Two proteins associated to GP Ib/IX/V, actin-binding protein (ABP) 280 and 14-3-3zeta, are potential mediators of signal transduction by the complex, but their specific contribution in this process is not yet fully understood. Additionally, two proteins implicated in signal transduction by immune stimuli, FcgammaRIIA and FcR gamma-chain, associate with GPIb/IX/V complex, and increasing data indicate a potential role in GPIbalpha mediated signal transduction.

The GPIb thrombin-binding site is essential for thrombin-induced platelet procoagulant activity

The role of the platelet glycoprotein (GP) Ib-V-IX receptor in thrombin activation of platelets has remained controversial although good evidence suggests that blocking this receptor affects platelet responses to this agonist. The mechanism of expression of procoagulant activity in response to platelet agonists is also still obscure. Here, the binding site for thrombin on GPIb is shown to have a key role in the exposure of negatively charged phospholipids on the platelet surface and thrombin generation, in response to thrombin, which also requires protease-activated receptor-1, GPIIb-IIIa, and platelet-platelet contact. Von Willebrand factor binding to GPIb is not essential to initiate development of platelet procoagulant activity. Inhibition of fibrinogen binding to GPIIb-IIIa also failed to block platelet procoagulant activity. Both heparin and low molecular weight heparin block thrombin-induced platelet procoagulant activity, which may account for part of their clinical efficacy. This study demonstrates a new, critical role for platelet GPIb in hemostasis, showing that platelet activation and coagulation are tightly interwoven, which may have implications for alternative therapies for thrombotic diseases.

Phosphoinositide 3-kinase forms a complex with platelet membrane glycoprotein Ib-IX-V complex and 14-3-3ζ

The binding of von Willebrand factor (vWF) to glycoprotein (GP) Ib-IX-V stimulates transmembrane signaling events that lead to platelet adhesion and aggregation. Recent studies have revealed that the signaling protein 14-3-3ζ binds directly to the cytoplasmic domain of GP Ib. In this study, the dynamic association of 14-3-3ζ with GP Ib-IX, the phosphoinositide 3-kinase (PI 3-kinase), or both, was investigated in resting, thrombin, or vWF and botrocetin-stimulated platelets by analysis of discrete subcellular fractions. Results of this study demonstrate maximal coimmunoprecipitation of 14-3-3ζ with GP Ib-IX in the nonstimulated cytosolic fraction and in the actin cytoskeletal fraction of thrombin- or vWF-stimulated human platelets. Immunoprecipitated 14-3-3ζ or GP Ib from cytosolic fractions contained PI 3-kinase enzyme activity and an 85-kd polypeptide recognized by antibodies to the p85 subunit of PI 3-kinase. After platelet activation, the level of association between these species decreased in the cytosolic fraction. However, increased complex formation between 14-3-3ζ and GP Ib-IX and between PI 3-kinase and GP Ib-IX was detected in actin cytoskeletal fractions derived from thrombin- or vWF-stimulated platelets. Recombinant glutathione S-transferase-14-3-3ζ fusion protein (14-3-3ζ–GST) inhibited affinity-captured PI 3-kinase enzyme activity up to 70% at 2 μmol/L 14-3-3ζ–GST. However, increasing concentrations up to 5 μmol/L 14-3-3ζ–GST resulted in the 3-fold enhancement of PI 3-kinase enzyme activity. We propose that the association between PI 3-kinase and 14-3-3ζ with GP Ib-IX serves to promote the rapid translocation of these signaling proteins to the activated cytoskeleton, thereby regulating the formation of 3-position phosphoinositide-signaling molecules in this subcellular compartment.

Differential activation and redistribution of c-Src and Fyn in platelets, assessed by MoAb specific for C-terminal tyrosine-dephosphorylated c-Src and Fyn

Tyrosine kinases, c-Src and Fyn, in their active form, have their C-terminal tyrosine residue dephosphorylated. In this study, we used clone 28, a monoclonal antibody (MoAb) that recognizes dephosphorylated C-terminal tyrosine of c-Src and Fyn, to investigate the mode of activation and mobilization of these kinases. Independently of integrin alphaIIbbeta3 signaling, the Fyn activity increased by 8.3-fold 5 s after stimulation with 20 microM TRAP (thrombin receptor agonist peptide), while that of c-Src increased only by 2.9-fold 15 s after stimulation. Both c-Src and Fyn translocated to the Triton-insoluble cytoskeletal fraction in an aggregation-dependent manner. Five minutes after TRAP-stimulation, 85% of Fyn translocated to the cytoskeleton, while only about 20% of c-Src was recovered in this fraction. The Triton-insoluble fraction was further fractionated by RIPA (radioimmunoprecipitation assay) buffer containing 0.1% SDS. While active c-Src was predominantly present in the Triton-insoluble/RIPA-insoluble fraction, clone 28-negative c-Src was present in the Triton-insoluble/RIPA-soluble fraction. On the other hand, Fyn was present only in the Triton-insoluble/RIPA-insoluble fraction. These findings suggest that the mode of activation and redistribution into the cytoskeleton differs between c-Src and Fyn, and that clone 28 provides a useful tool for investigating the activation and mobilization of Src family tyrosine kinases.