Interaction of linker for activation of T cells with multiple adapter proteins in platelets activated by the glycoprotein VI-selective ligand, convulxin

Analysis of preplatelets and their barbell platelet derivatives by imaging flow cytometry

Circulating large "preplatelets" undergo fission via barbell platelet intermediates into two smaller, mature platelets. In this study, we determine whether preplatelets and/or barbells are equivalent to reticulated/immature platelets by using ImageStream flow cytometry and super-resolution microscopy. Immature platelets, preplatelets, and barbells were quantified in healthy and thrombocytopenic mice, healthy human volunteers, and patients with immune thrombocytopenia or undergoing chemotherapy. Preplatelets and barbells were 1.9% ± 0.18%/1.7% ± 0.48% (n = 6) and 3.3% ± 1.6%/0.5% ± 0.27% (n = 12) of total platelet counts in murine and human whole blood, respectively. Both preplatelets and barbells exhibited high expression of major histocompatibility complex class I with high thiazole orange and Mitotracker fluorescence. Tracking dye experiments confirmed that preplatelets transform into barbells and undergo fission ex vivo to increase platelet counts, with dependence on the cytoskeleton and normal mitochondrial respiration. Samples from antibody-induced thrombocytopenia in mice and patients with immune thrombocytopenia had increased levels of both preplatelets and barbells correlating with immature platelet levels. Furthermore, barbells were absent after chemotherapy in patients. In mice, in vivo biotinylation confirmed that barbells, but not all large platelets, were immature. This study demonstrates that a subpopulation of large platelets are immature preplatelets that can transform into barbells and undergo fission during maturation.

Role of Tyrosine Kinase Syk in Thrombus Stabilisation at High Shear

Understanding the pathways involved in the formation and stability of the core and shell regions of a platelet-rich arterial thrombus may result in new ways to treat arterial thrombosis. The distinguishing feature between these two regions is the absence of fibrin in the shell which indicates that in vitro flow-based assays over thrombogenic surfaces, in the absence of coagulation, can be used to resemble this region. In this study, we have investigated the contribution of Syk tyrosine kinase in the stability of platelet aggregates (or thrombi) formed on collagen or atherosclerotic plaque homogenate at arterial shear (1000 s^-1). We show that post-perfusion of the Syk inhibitor PRT-060318 over preformed thrombi on both surfaces enhances thrombus breakdown and platelet detachment. The resulting loss of thrombus stability led to a reduction in thrombus contractile score which could be detected as early as 3 min after perfusion of the Syk inhibitor. A similar loss of thrombus stability was observed with ticagrelor and indomethacin, inhibitors of platelet adenosine diphosphate (ADP) receptor and thromboxane A₂ (TxA₂), respectively, and in the presence of the Src inhibitor, dasatinib. In contrast, the Btk inhibitor, ibrutinib, causes only a minor decrease in thrombus contractile score. Weak thrombus breakdown is also seen with the blocking GPVI nanobody, Nb21, which indicates, at best, a minor contribution of collagen to the stability of the platelet aggregate. These results show that Syk regulates thrombus stability in the absence of fibrin in human platelets under flow and provide evidence that this involves pathways additional to activation of GPVI by collagen.

SKAP2 as a new regulator of oligodendroglial migration and myelin sheath formation

Oligodendroglial progenitor cells (OPCs) are highly proliferative and migratory cells, which differentiate into complex myelin forming and axon ensheathing mature oligodendrocytes during myelination. Recent studies indicate that the oligodendroglial cell population is heterogeneous on transcriptional and functional level depending on the location in the central nervous system. Here, we compared intrinsic properties of OPC from spinal cord and brain on functional and transcriptional level. Spinal cord OPC demonstrated increased migration as well as differentiation capacity. Moreover, transcriptome analysis revealed differential expression of several genes between both OPC populations. In spinal cord OPC, we confirmed upregulation of SKAP2, a cytoplasmatic adaptor protein known for its implication in cytoskeletal remodeling and migration in other cell types. Recent findings suggest that actin dynamics determine not only oligodendroglial migration, but also differentiation: Whereas actin polymerization is important for process extension, actin destabilization and depolymerization is required for myelin sheath formation. Downregulation or complete lack of SKAP2 in OPC resulted in reduced migration and impaired morphological maturation in oligodendrocytes. In contrast, overexpression of SKAP2 as well as constitutively active SKAP2 increased OPC migration suggesting that SKAP2 function is dependent on activation by phosphorylation. Furthermore, lack of SKAP2 enhanced the positive effect on OPC migration after integrin activation suggesting that SKAP2 acts as modulator of integrin dependent migration. In summary, we demonstrate the presence of intrinsic differences between spinal cord and brain OPC and identified SKAP2 as a new regulator of oligodendroglial migration and sheath formation.

Impact of atrial fibrillation on platelet gene expression

AIMS

Platelets retain cytoplasmic messenger RNA and are capable of protein biosynthesis. Several diseases are known to impact the platelet transcriptome but the effect of non-valvular atrial fibrillation (NVAF) on platelet RNA transcript is essentially unknown. The aim of this study was to evaluate the impact of NVAF on platelet RNA transcript by measuring platelet genes expression in consecutive NVAF patients before and 3-4 months after pulmonary vein isolation (PVI) and compared to normal sinus rhythm controls (NSR).

METHODS AND RESULTS

RNA from isolated platelets were reverse transcribed, assayed against 15 genes using real-time qPCR, and expressed as mean cycle threshold (ΔCt) using beta-2-microglobulin as endogenous control. Expression of all evaluated genes, except cathepsin A gene, was significantly lower (higher ΔCt) in 103 NVAF patients compared to 55 NSR controls. Insulin-like growth factor binding protein acid labile subunit gene (IGFALS) had expression more than 16 fold-lower (17.0±2.8 vs 12.5±3.8, P<.001), follow by genes encoding for prostacyclin receptor, and for von Willebrand factor which had fourfold lower expression compared to NSR controls. Gender, type of atrial fibrillation, heart failure, hypertension, prior stroke, diabetes mellitus, and atherosclerosis were associated with different gene expression. Following PVI, expression of four genes significantly increased, particularly IGFALS gene (increased 256-fold) and ADAMT gene increased 16-fold); expression of three genes significantly decreased, and expression of eight genes has not changed.

CONCLUSIONS

Platelets are capable to respond to the circulatory environment of NVAF by altering transcript and changing prothrombotic status. This shows platelet potential for molecular "reprogramming" possibly induced by flow disturbances of NVAF.

Skap2 is required for β2 integrin-mediated neutrophil recruitment and functions

Integrin activation is required for neutrophil functions. Impaired integrin activation on neutrophils is the hallmark of leukocyte adhesion deficiency (LAD) syndrome in humans, characterized by impaired leukocyte recruitment and recurrent infections. The Src kinase-associated phosphoprotein 2 (Skap2) is involved in integrin functions in different leukocyte subtypes. However, the role of Skap2 in β2 integrin activation and neutrophil recruitment is unknown. In this study, we demonstrate the crucial role of Skap2 in regulating actin polymerization and binding of talin-1 and kindlin-3 to the β2 integrin cytoplasmic domain, thereby being indispensable for β2 integrin activation and neutrophil recruitment. The direct interaction of Skap2 with the Wiskott-Aldrich syndrome protein via its SH3 domain is critical for integrin activation and neutrophil recruitment in vivo. Furthermore, Skap2 regulates integrin-mediated outside-in signaling events and neutrophil functions. Thus, Skap2 is essential to activate the β2 integrins, and loss of Skap2 function is sufficient to cause a LAD-like phenotype in mice.

TULA-2 Protein Phosphatase Suppresses Activation of Syk through the GPVI Platelet Receptor for Collagen by Dephosphorylating Tyr(P)346, a Regulatory Site of Syk

Protein-tyrosine phosphatase TULA-2 has been shown to regulate receptor signaling in several cell types, including platelets. Platelets are critical for maintaining vascular integrity; this function is mediated by platelet aggregation in response to recognition of the exposed basement membrane collagen by the GPVI receptor, which is non-covalently associated with the signal-transducing FcRγ polypeptide chain. Our previous studies suggested that TULA-2 plays an important role in negatively regulating signaling through GPVI-FcRγ and indicated that the tyrosine-protein kinase Syk is a key target of the regulatory action of TULA-2 in platelets. However, the molecular basis of the down-regulatory effect of TULA-2 on Syk activation via FcRγ remained unclear. In this study, we demonstrate that suppression of Syk activation by TULA-2 is mediated, to a substantial degree, by dephosphorylation of Tyr(P)³⁴⁶, a regulatory site of Syk, which becomes phosphorylated soon after receptor ligation and plays a critical role in initiating the process that yields fully activated Syk. TULA-2 is capable of dephosphorylating Tyr(P)³⁴⁶ with high efficiency, thus controlling the overall activation of Syk, but is less efficient in dephosphorylating other regulatory sites of this kinase. Therefore, dephosphorylation of Tyr(P)³⁴⁶ may be considered an important "checkpoint" in the regulation of Syk activation process. Putative biological functions of TULA-2-mediated dephosphorylation of Tyr(P)³⁴⁶ may include deactivation of receptor-activated Syk or suppression of Syk activation by suboptimal stimulation.

Heat shock protein 70 regulates platelet integrin activation, granule secretion and aggregation

Molecular chaperones that support protein quality control, including heat shock protein 70 (Hsp70), participate in diverse aspects of cellular and physiological function. Recent studies have reported roles for specific chaperone activities in blood platelets in maintaining hemostasis; however, the functions of Hsp70 in platelet physiology remain uninvestigated. Here we characterize roles for Hsp70 activity in platelet activation and function. In vitro biochemical, microscopy, flow cytometry, and aggregometry assays of platelet function, as well as ex vivo analyses of platelet aggregate formation in whole blood under shear, were carried out under Hsp70-inhibited conditions. Inhibition of platelet Hsp70 blocked platelet aggregation and granule secretion in response to collagen-related peptide (CRP), which engages the immunoreceptor tyrosine-based activation motif-bearing collagen receptor glycoprotein VI (GPVI)-Fc receptor-γ chain complex. Hsp70 inhibition also reduced platelet integrin-αIIbβ3 activation downstream of GPVI, as Hsp70-inhibited platelets showed reduced PAC-1 and fibrinogen binding. Ex vivo, pharmacological inhibition of Hsp70 in human whole blood prevented the formation of platelet aggregates on collagen under shear. Biochemical studies supported a role for Hsp70 in maintaining the assembly of the linker for activation of T cells signalosome, which couples GPVI-initiated signaling to integrin activation, secretion, and platelet function. Together, our results suggest that Hsp70 regulates platelet activation and function by supporting linker for activation of T cells-associated signaling events downstream of platelet GPVI engagement, suggesting a role for Hsp70 in the intracellular organization of signaling systems that mediate platelet secretion, "inside-out" activation of platelet integrin-αIIbβ3, platelet-platelet aggregation, and, ultimately, hemostatic plug and thrombus formation.

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

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

The N-terminal SH2 domain of Syk is required for (hem)ITAM, but not integrin, signaling in mouse platelets

We have used a novel knockin mouse to investigate the effect of disruption of phosphotyrosine binding of the N-terminal SH2 domain of Syk on platelet activation by GPVI, CLEC-2, and integrin αIIbβ3. The Syk(R41Afl/fl) mouse was crossed to a PF4-Cre(+) mouse to induce expression of the Syk mutant in the megakaryocyte/platelet lineage. Syk(R41Afl/fl;PF4-Cre) mice are born at approximately 50% of the expected frequency and have a similar phenotype to Syk(fl/fl;PF4-Cre) mice, including blood-lymphatic mixing and chyloascites. Anastomosis of the venous and lymphatic vasculatures can be seen in the mesenteric circulation accounting for rapid and continuous mixing of the 2 vasculatures. Platelet activation by CLEC-2 and GPVI is abolished in Syk(R41Afl/fl;PF4-Cre) platelets. Syk phosphorylation on Tyr519/20 is blocked in CLEC-2-stimulated platelets, suggesting a model in which binding of Syk via its N-terminal SH2 domain regulates autophosphorylation. In contrast, outside-in signaling by integrin αIIbβ3 is not altered, but it is inhibited in the presence of inhibitors of Src and Syk tyrosine kinases. These results demonstrate that αIIbβ3 regulates Syk through an ITAM-independent pathway in mice and provide novel insight into the course of events underlying Syk activation and hemITAM phosphorylation by CLEC-2.

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.

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.

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.

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.

An acquired defect associated with abnormal signaling of the platelet collagen receptor glycoprotein VI

INTRODUCTION

Ligands acting at the platelet collagen receptor, glycoprotein (GP)VI, induce intracellular FcRγ/Syk-dependent signaling pathways and Syk-dependent or Syk-independent generation of intracellular reactive oxygen species (ROS). Additional signaling-dependent or signaling-independent pathways lead to metalloproteinase-mediated shedding of GPVI.

AIM

Analysis of platelet GPVI expression and signaling in a patient with a collagen-selective defect associated with myelodysplastic syndrome (MDS) uniquely demonstrates divergent pathways leading to ROS generation and Syk phosphorylation in human platelets.

METHODS

Surface expression of GPVI and ligand-induced ROS generation was quantitated by flow cytometry. GPVI shedding and Syk phosphorylation were analyzed by Western blot.

RESULTS

Despite platelet count/size and GPVI surface expression within normal ranges, platelet-rich plasma showed no aggregation in response to collagen or GPVI-selective agonist collagen-related peptide, but aggregated in response to other agonists, consistent with dysfunctional GPVI signaling. We observed rapid GPVI-dependent Syk-independent ROS generation and disulfide-dependent GPVI homodimerization, but not Syk-dependent ROS or ligand-induced shedding. Temporal analysis showed a gradual decline in platelet count and the appearance of ligand-induced phosphorylation of an ∼40-kDa Syk fragment.

CONCLUSIONS

These studies show that GPVI ligation in platelets induces intracellular ROS production independent of either Syk activation or divergent pathways leading to platelet aggregation or ectodomain shedding.

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

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

SKAP-55, SKAP-55-related and ADAP adaptors modulate integrin-mediated immune-cell adhesion

Integrin adhesion is essential for aspects of immune function, including antigen presentation and migration in lymph nodes, germinal centers and sites of inflammation. Antigen receptors on B and T cells generate 'inside-out' signals for increased integrin clustering and adhesion. Although upstream components of B-cell-receptor or T-cell-receptor signaling are needed, the identity of key downstream effectors that mediate integrin adhesion is only just emerging. New candidates include immune-cell-specific adaptor proteins ADAP, SKAP-55 and SKAP-55-related (SKAP-55R). SKAP-55 has recently been identified as an effector in T cells in SKAP-55-deficient mice, whereas SKAP-55R is needed for B-cell adhesion. ADAP is required for SKAP-55 and SKAP-55R protein stability. SKAP-55 and SKAP-55R have unexpectedly specialized roles in T- and B-cell adhesion of the immune system.

Hematopoietic lineage cell specific protein 1 (HS1) is a functionally important signaling molecule in platelet activation

Collagen activates platelets through an intracellular signaling cascade downstream of glycoprotein VI (GPVI). We have investigated the contribution of hematopoietic lineage cell-specific protein 1 (HS1) downstream of GPVI in platelet activation. Stimulation of GPVI leads to tyrosine phosphorylation of HS1, which is blocked by Src-family kinase inhibitors. Coimmunoprecipitation experiments revealed that HS1 associates with Syk and phosphatidylinositol 3-kinases. HS1-null mice displayed increased bleeding times and increased time to occlusion in the FeCl(3) in vivo thrombosis model compared with their wild-type littermates. In addition, aggregation and secretion responses were diminished in HS1-null mouse platelets after stimulation of GPVI and protease-activated receptor 4 (PAR-4) agonists compared with wild-type littermate mouse platelets. Finally, Akt phosphorylation was diminished after GPVI or PAR-4 stimulation in platelets from HS1-null mice compared with their wild-type littermates. These results demonstrate that phosphorylation of the HS1 protein occurs downstream of GPVI stimulation and that HS1 plays a significant functional role in platelet activation downstream of GPVI and PARs.

Roles of the C-terminal tyrosine residues of LAT in GPVI-induced platelet activation: insights into the mechanism of PLC gamma 2 activation

Linker for activation of T cells (LAT) is an adaptor protein required for organization of the signaling machinery downstream of the platelet collagen receptor, the glycoprotein VI (GPVI). Here, we investigated the effect of LAT mutations on specific signaling pathways and on platelet functions in response to GPVI triggering by convulxin (Cvx). Using mice containing tyrosine to phenylalanine mutations of the adaptor, we show the crucial role played by the tyrosine residues at positions 175, 195, and 235 in the phosphorylation of LAT and in the whole pattern of protein tyrosine phosphorylation in response to Cvx. These 3 C-terminal tyrosine residues are important to recruit the tyrosine kinase Fyn, which may be involved in LAT phosphorylation. Efficient phosphoinositide 3-kinase (PI3K) activation requires the 3 C-terminal tyrosine residues of LAT but not its tyrosine 136. Interestingly, single mutation of the tyrosine 136 results in the loss of phospholipase C gamma2 (PLCgamma2) activation without affecting its PI3K-dependent membrane association, and is sufficient to impair platelet responses to Cvx. Thus, activation of PLCgamma2 via GPVI is dependent on 2 complementary events: its interaction with the tyrosine 136 of LAT and its membrane location, which itself requires events mediated by the 3 C-terminal tyrosines of LAT.

Purification and functional characterization of AAV1, a novel P-III metalloproteinase, from Formosan Agkistrodon acutus venom

AAV1, an alkaline glycoprotein (GP), was purified from Agkistrodon acutus venom by two chromatographic steps on successive DEAE-Sephadex A-50 and Superdex 75 FPLC columns. AAV1 on SDS-PAGE under non-reducing conditions migrated as a monomeric and a polymeric forms with apparent molecular mass of 57 and 180 kDa, respectively. Upon reduction, it appeared as a single broad band with a mass of 50.3 kDa corresponding to the size of a typical P-III metalloproteinase acurhagin. The N-terminal sequence of an autoproteolytical 30 kDa-fragment of AAV1 showed a high homology to that of venom proteins with Metalloproteinase, Disintegrin-like, and Cysteine-rich (MDC) domains. Although it was devoid of cleaving activity toward gelatin, fibronectin and prothrombin, AAV1 preferentially digested the Aalpha chain of fibrinogen and followed by the Bbeta chain, leading to the inhibition of fibrinogen-induced platelet aggregation in elastase-treated human platelets. However, the proteolytic activity of AAV1 was completely inactivated by the chelating agent but not serine proteinase inhibitor. Furthermore, AAV1 could concentration-dependently inhibit platelet aggregation and suppress tyrosine phosphorylation of intracellular proteins in collagen- and convulxin-stimulated platelets, respectively. The interaction of MDC domains in AAV1 molecule with platelet GPVI was responsible for the inhibitory effect of AAV1 on collagen- and convulxin-induced platelet aggregation. Taken together, these pieces of evidence suggest that AAV1 from Formosan viper venom belongs to a new member of high-molecular mass metalloproteinase family and functions as a GPVI antagonist.

A global proteomics approach identifies novel phosphorylated signaling proteins in GPVI-activated platelets: involvement of G6f, a novel platelet Grb2-binding membrane adapter

Collagen-related peptide (CRP) stimulates powerful activation of platelets through the glycoprotein VI (GPVI)-FcR gamma-chain complex. We have combined proteomics and traditional biochemistry approaches to study the proteome of CRP-activated platelets, focusing in detail on tyrosine phosphorylation. In two separate approaches, phosphotyrosine immunoprecipitations followed by 1-D-PAGE, and 2-DE, were used for protein separation. Proteins were identified by MS. By following these approaches, 96 proteins were found to undergo PTM in response to CRP in human platelets, including 11 novel platelet proteins such as Dok-1, SPIN90, osteoclast stimulating factor 1, and beta-Pix. Interestingly, the type I transmembrane protein G6f was found to be specifically phosphorylated on Tyr-281 in response to platelet activation by CRP, providing a docking site for the adapter Grb2. G6f tyrosine phoshporylation was also found to take place in response to collagen, although not in response to the G protein-coupled receptor agonists, thrombin and ADP. Further, we also demonstrate for the first time that Grb2 and its homolog Gads are tyrosine-phosphorylated in CRP-stimulated platelets. This study provides new insights into the mechanism of platelet activation through the GPVI collagen receptor, helping to build the basis for the development of new drug targets for thrombotic disease.

ADAP is required for normal alphaIIbbeta3 activation by VWF/GP Ib-IX-V and other agonists

Interaction between von Willebrand factor (VWF) and platelet GP Ib-IX-V is required for hemostasis, in part because intracellular signals from VWF/GP Ib-IX-V activate the ligand-binding function of integrin alphaIIbbeta3. Because they also induce tyrosine phosphorylation of the ADAP adapter, we investigated ADAP's role in GP Ib-IX-V signal transduction. Fibrinogen or ligand-mimetic POW-2 Fab binding to alphaIIbbeta3 was stimulated by adhesion of ADAP+/+ murine platelets to dimeric VWF A1A2 but was significantly reduced in ADAP-/- platelets (P<.01). alphaIIbbeta3 activation by ADP or a Par4 thrombin receptor agonist was also decreased in ADAP-/- platelets. ADAP stabilized the expression of another adapter, SKAP-HOM, via interaction with the latter's SH3 domain. However, no abnormalities in alphaIIbbeta3 activation were observed in SKAP-HOM-/- platelets, which express normal ADAP levels, further implicating ADAP as a modulator of alphaIIbbeta3 function. Under shear flow conditions over a combined surface of VWF A1A2 and fibronectin to test interactions involving GP Ib-IX-V and alphaIIbbeta3, respectively, ADAP-/- platelets displayed reduced alphaIIbbeta3-dependent stable adhesion. Furthermore, ADAP-/- mice demonstrated increased rebleeding from tail wounds. These studies establish ADAP as a component of inside-out signaling pathways that couple GP Ib-IX-V and other platelet agonist receptors to alphaIIbbeta3 activation.

Role of the p110delta PI 3-kinase in integrin and ITAM receptor signalling in platelets

We have investigated the function of the p110delta catalytic subunit of phosphoinositide 3-kinase (PI 3-kinase) in platelets using p110delta knock-out (p110delta(-/-)) mice and p110delta knock-in (p110delta(D910A/D910A)) mice, which express a catalytically inactive form of the enzyme. Aggregation to threshold concentrations of the GPVI-specific agonist, CRP, was partially reduced in p110delta(-/-) and p110delta(D910A/D910A) platelets. This inhibition was overcome by higher concentrations of CRP. The degree of inhibition was considerably weaker than that induced by LY294002 and wortmannin, which inhibit all isoforms of PI 3-kinase. p110delta(-/-) platelets showed decreased spreading on fibrinogen- or von Willebrand factor (VWF)-coated surfaces under static conditions, whereas they spread normally on collagen. LY294002 had a more pronounced inhibitory effect on spreading on all three surfaces. Adhesion and aggregate formation of p110delta(-/-) platelets to collagen or fibrinogen/VWF at intermediate/high rates of shear were normal. This study demonstrates a minor role for the p110delta catalytic subunit in mediating platelet activation by the collagen receptor GPVI and integrin alphaIIbeta3. The more pronounced inhibitory effect of LY294002 and wortmannin indicates that other isoforms of PI 3-kinase play a more significant role in signalling by the two platelet glycoprotein receptors.

Docking protein Gab2 positively regulates glycoprotein VI-mediated platelet activation

Gab2, a recently identified docking protein, contains a pleckstrin homology domain and potential binding sites for SH2 and SH3 domain-containing proteins. Gab2 has been shown to support growth, differentiation, and function in a number of haematopoietic cells, although its role in platelets remains to be determined. Here we report that cross-linking of the collagen receptor GPVI by the snake venom toxin convulxin stimulates tyrosine phosphorylation of Gab2. Furthermore, platelet aggregation induced by submaximal concentrations of convulxin is attenuated in the absence of Gab2, although recovery is seen with higher concentrations of the toxin. Consistent with this, tyrosine phosphorylation of Fc receptor gamma-chain, Syk, Btk, and phospholipase Cgamma2 by convulxin is reduced in the absence of Gab2. In comparison, the G protein-coupled receptor agonist, thrombin, does not induce phosphorylation of Gab2 and aggregation is unaltered in the absence of the toxin. These findings provide evidence for a functional role of Gab2 in supporting platelet activation by GPVI.

Primary structure and antiplatelet mechanism of a snake venom metalloproteinase, acurhagin, from Agkistrodon acutus venom

Acurhagin has been characterized as a P-III hemorrhagic metalloproteinase. We herein report the complete sequence of acurhagin by molecular cloning. Analysis of the cDNA-predicted amino acid sequence encoding acurhagin precursor revealed that this mosaic Asn-linked glycoprotein possesses a multidomain structure including a proprotein, a metalloproteinase, a disintegrin-like and a cysteine-rich domains (189/205/102/114 residues), with an overall 87% identity to that of jararhagin, an integrin alpha2beta1-cleaving metalloproteinase. Acurhagin has a Ser-Glu-Cys-Asp sequence in the disintegrin-like domain instead of the typical Arg-Gly-Asp motif. In contrast to inhibiting fibrinogen-integrin alphaIIbbeta3 interaction by disintegrins, acurhagin selectively showed a dose-dependent inhibition on platelet aggregation induced by collagen, and suppression on tyrosine phosphorylation of several signaling proteins in convulxin-stimulated platelets. Although the immobilized acurhagin was shown to bind platelet GPVI and collagen in a primary structure- and steric conformation-dependent manner, respectively, the mechanism of acurhagin under short incubation is mainly through its binding to GPVI and collagen, instead of binding to alpha2beta1, or cleaving platelet membrane glycoproteins. Moreover, the molecular conformation maintained by divalent cations is required for the proteolytic activity of acurhagin toward extracellular matrix fibronectin. Taken together, these results suggest that all the three domains in mature acurhagin may cooperatively contribute to its biological function.

Platelet glycoprotein VI: its structure and function

Glycoprotein (GP) VI is a platelet membrane protein with a molecular weight of 62 kDa that was identified as a physiological collagen receptor from studies of patients deficient in this protein. GPVI-deficient platelets lacked specifically collagen-induced aggregation and the ability to form thrombi on a collagen surface under flow conditions, suggesting that GPVI makes an indispensable contribution to collagen-induced platelet activation. On the platelet surface, GPVI is present as a complex with the Fc receptor (FcR) gamma-chain, probably composed of two GPVI molecules and one FcR gamma-chain dimer. GPVI must form such a dimeric complex to exhibit high affinity binding to collagen. The GPVI-induced activation mechanism is initiated by tyrosine phosphorylation of the immunoreceptor tyrosine-based activation motif (ITAM) of the FcR gamma-chain, and then this signal is transduced to many related proteins, mainly by tyrosine phosphorylation. GPVI is widely recognized as a requisite factor for the formation of platelet aggregates on a collagen surface under blood flow. However, individuals with GPVI-deficient or null platelets do not exhibit any strong bleeding tendency. Analyzing this apparent dichotomy should provide us with a more precise understanding of the mechanism of thrombus formation.

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.

PRAM-1 is required for optimal integrin-dependent neutrophil function

PML-retinoic acid receptor alpha (RARalpha) regulated adaptor molecule 1 (PRAM-1) is an intracellular adaptor molecule that is upregulated during the induced granulocytic differentiation of promyelocytic leukemic cells and during normal human myelopoiesis. This report describes the generation of PRAM-1-deficient mice and an analysis of the function of this adaptor in neutrophil differentiation and mature neutrophil function. We demonstrate here that neutrophil differentiation is not impaired in PRAM-1-deficient mice and that PRAM-1-deficient neutrophils function normally following engagement of Fcgamma receptors. In contrast, mature PRAM-1-null neutrophils exhibit significant defects in adhesion-dependent reactive oxygen intermediate production and degranulation. Surprisingly, other integrin-dependent responses, such as cell spreading and activation of several signaling pathways, are normal. Together, these findings demonstrate the uncoupling of key integrin-dependent responses in the absence of PRAM-1 and show this adaptor to be critical for select integrin functions in neutrophils.

Glycoproteins VI and Ib-IX-V stimulate tyrosine phosphorylation of tyrosine kinase Syk and phospholipase Cgamma2 at distinct sites

Glycoproteins GPVI and GPIb-IX-V stimulate robust tyrosine phosphorylation of Syk and PLCg2 (phospholipase Cg2) in washed platelets, but only the former stimulates pronounced activation of phospholipase. Using phospho-specific antibodies, we demonstrate that GPVI, but not GPIb-IX-V, stimulates significant tyrosine phosphorylation of Syk at the autophosphorylation site pY525/526, a marker of Syk activity. In addition, GPVI stimulates tyrosine phosphorylation of PLCg2 at Tyr753 and Tyr759, whereas GPIb-IX-V only induces significant phosphorylation at Tyr753. Both receptors stimulate tyrosine phosphorylation of Btk at the regulatory Tyr223 and Tyr551. Syk and Btk phosphorylate peptides from PLCg2 containing Tyr753 and Tyr759 respectively, suggesting that they may stimulate phosphorylation at these sites in phospholipase. Studies using PLCg2-deficient platelets demonstrated that phospholipase is not required for the activation of integrin aIIbb3 by GPIb-IX-V. Our results demonstrate fundamental differences between GPVI and GPIb-IX-V in the regulation of tyrosine phosphorylation of Syk and PLCg2 consistent with the functional impairment of phospholipase in signalling by GPIb-IX-V.

Transmembrane adaptor proteins in membrane microdomains: important regulators of immunoreceptor signaling

Membrane microdomains enriched in glycosphingolipids, cholesterol, glycosylphosphatidylinositol-anchored proteins and Src-family kinases (lipid rafts, GEMs) appear to play many important roles, especially in immunoreceptor signaling. Most transmembrane proteins are excluded from these specialized areas of membranes, notable exceptions being several palmitoylated proteins such as the T cell coreceptors CD4 and CD8, and several recently described transmembrane adaptor proteins, LAT, non-T cell activation linker (NTAL)/linker for activation of B cells (LAB), phosphoprotein associated with GEMs (PAG)/Csk-binding protein (Cbp) and LIME. All these molecules possess a very short N-terminal extracellular peptide (4-17 amino acids), transmembrane segment followed by a palmitoylation motif (CxxC) and cytoplasmic domain containing up to 10 tyrosine residues potentially phosphorylated by the Src- or Syk-family kinases. Tyrosine-phosphorylated transmembrane adaptors bind (directly via SH2 domains or indirectly) other signaling molecules such as several cytoplasmic adaptors and enzymes. LAT is indispensable for TCR signaling (and participates also at signal transduction initiated by some other receptors), NTAL/LAB appears to play a LAT-like role in signaling initiated by BCR and some Fc-receptors; PAG/Cbp cooperates with Csk, the cytoplasmic tyrosine kinase negatively regulating Src-family kinases. Additional transmembrane adaptors exist (TRIM, SIT, LAX) that are however not palmitoylated and therefore excluded from the lipid rafts; structurally and functionally, the zeta-chain family proteins tightly associated with immunoreceptors and activating NK-receptors may be also considered as transmembrane adaptors.

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.

c-Cbl negatively regulates platelet activation by glycoprotein VI

BACKGROUND

The adapter protein c-Cbl has emerged as having a potential role in negative regulation of immune receptor signaling. The major platelet-signaling receptor for collagen, glycoprotein VI (GpVI), is associated with the Fc receptor (FcR) gamma-chain, and signals through a similar pathway to immune receptors. c-Cbl is tyrosine-phosphorylated in response to stimulation of GpVI, whereas phosphorylation of c-Cbl in thrombin-activated platelets is dependent on fibrinogen binding to the integrin GpIIb/IIIa.

OBJECTIVE

To investigate the role of c-Cbl in platelet signaling.

METHODS

Murine platelets lacking functional c-Cbl or Src family kinases were analyzed.

RESULTS

Phosphorylation of c-Cbl through GpVI is reduced in murine platelets deficient in the Src-family kinases Fyn and Lyn, demonstrating that they lie upstream of c-Cbl phosphorylation. Phosphorylation of several proteins of the GpVI-signaling pathway, including the FcR gamma-chain, Syk and phospholipase Cgamma2 (PLCgamma2), is increased in the absence of c-Cbl. In line with this, aggregation is potentiated in response to the GpVI-specific collagen-related peptide (CRP) after a slight delay. A delay in potentiation is also seen in response to stimulation by thrombin.

CONCLUSIONS

These observations demonstrate that c-Cbl negatively regulates platelet responses to GpVI agonists and to thrombin, with the latter effect possibly being mediated downstream of GpIIb/IIIa. c-Cbl may play a physiological role in helping to prevent unwanted platelet activation in vivo.

The tyrosine phosphatase 1B regulates linker for activation of T-cell phosphorylation and platelet aggregation upon FcgammaRIIa cross-linking

Human platelets express the receptor for immunoglobulin G, FcgammaRIIa, that triggers cell aggregation upon interaction with immune complexes. Here, we report that the rapid tyrosine phosphorylation of the Linker for Activation of T-cell (LAT) in human platelets stimulated by FcgammaRIIa cross-linking was followed by its complete dephosphorylation in an alphaIIb/beta3 integrin-dependent manner. Concomitant to LAT dephosphorylation, the protein tyrosine phosphatase 1B (PTP1B) was activated through a mechanism involving its proteolysis by calpains downstream of integrins. Both PTP1B and LAT were associated with the actin cytoskeleton complex formed during platelet aggregation. Moreover, phospho-LAT appeared as a good substrate of activated PTP1B in vitro and these two proteins interacted upon platelet activation by FcgammaRIIa cross-linking. The permeant substrate-trapping PTP1B (TAT-PTP1B D181A) partly inhibited LAT dephosphorylation in human platelets, strongly suggesting that this tyrosine phosphatase was involved in this regulatory pathway. Using a pharmacological inhibitor, we provide evidence that PTP1B activation and LAT dephosphorylation processes were required for irreversible platelet aggregation. Altogether, our results demonstrate that PTP1B plays an important role in the integrin-mediated dephosphorylation of LAT in human platelets and is involved in the control of irreversible aggregation upon FcgammaRIIa stimulation.

Delineation of the region in the glycoprotein VI tail required for association with the Fc receptor gamma-chain

The glycoprotein VI (GPVI).Fc receptor gamma-chain (FcRgamma-chain) complex is the major activation receptor for collagen on platelets. GPVI cross-linking mediates activation through tyrosine phosphorylation of an ITAM (immunoreceptor tyrosine-based activation motif) in the FcR gamma-chain by Src family kinases. It has been previously shown that a transmembrane arginine and the cytoplasmic domain of GPVI are required for association with the FcR gamma-chain in immortalized cell lines. In this study, we have delineated the regions in the GPVI tail that promote binding to FcR gamma-chain and mediate functional responses to the snake venom convulxin by reconstitution of mutant forms of GPVI in RBL-2H3 cells. Sequential truncation of the cytoplasmic tail of GPVI revealed a major role for the basic region and a minor role for the juxtamembrane six amino acids in the association with FcR gamma-chain and functional responses to convulxin. Analysis of selective deletions in the GPVI tail supported this conclusion. In addition, we show that the proline-rich domain is required for optimal Ca2+ release, whereas it is dispensable for FcR gamma-chain association.

Murine GPVI stimulates weak integrin activation in PLCgamma2-/- platelets: involvement of PLCgamma1 and PI3-kinase

Collagen stimulates platelet activation through a tyrosine kinase-based pathway downstream of the glycoprotein VI (GPVI)-Fc receptor (FcR) gamma-chain complex. Genetic ablation of FcR gamma-chain results in a complete inhibition of aggregation to collagen. In contrast, a steady increase in light transmission is induced by collagen in phospholipase Cgamma2-deficient (PLCgamma2-/-) platelets in a Born aggregometer, indicating a weak level of activation. This increase is inhibited partially in the presence of an alpha2beta1-blocking antibody or an alphaIIbbeta3 antagonist and completely by a combination of the 2 inhibitors. It is also abolished by the Src kinase inhibitor PP1 and reduced in the presence of the phosphatidylinositol (PI) 3-kinase inhibitor wortmannin. The GPVI-specific agonists convulxin and collagen-related peptide (CRP) also stimulate weak aggregation in PLCgamma2-/- platelets, which is inhibited by wortmannin and PP1. Collagen and CRP stimulate tyrosine phosphorylation of PLCgamma1 at its regulatory site, Tyr 783, in murine but not in human platelets through a Src kinase-dependent pathway. Adhesion of PLCgamma2-/- platelets to a collagen monolayer is severely reduced at a shear rate of 800 s-1, relative to controls, whereas it is abolished in FcR gamma-chain-/- platelets. These results provide strong evidence that engagement of GPVI stimulates limited integrin activation in PLCgamma2-/- platelets via PLCgamma1 and PI3-kinase.

Platelet-collagen interaction: is GPVI the central receptor?

At sites of vascular injury, platelets come into contact with subendothelial collagen, which triggers their activation and the formation of a hemostatic plug. Besides glycoprotein Ib (GPIb) and alphaIIbbeta3 integrin, which indirectly interact with collagen via von Willebrand factor (VWF), several collagen receptors have been identified on platelets, most notably alpha2beta1 integrin and the immunoglobulin (Ig) superfamily member GPVI. Within the last few years, major advances have been made in understanding platelet-collagen interactions including the molecular cloning of GPVI, the generation of mouse strains lacking individual collagen receptors, and the development of collagen receptor-specific antibodies and synthetic peptides. It is now recognized that platelet adhesion to collagen requires prior activation of integrins through "inside-out" signals generated by GPVI and reinforced by released second-wave mediators adenosine diphosphate (ADP) and thromboxane A2. These developments have led to revision of the original "2-site, 2-step" model, which now places GPVI in a central position in the complex processes of platelet tethering, activation, adhesion, aggregation, degranulation, and procoagulant activity on collagen. This review discusses these recent developments and proposes possible mechanisms for how GPVI acts in concert with other receptors and signaling pathways to initiate hemostasis and arterial thrombosis.

Fps/Fes and Fer non-receptor protein-tyrosine kinases regulate collagen- and ADP-induced platelet aggregation

Fps/Fes and Fer proto-oncoproteins are structurally related non-receptor protein-tyrosine kinases implicated in signaling downstream from cytokines, growth factors and immune receptors. We show that Fps/Fes and Fer are expressed in human and mouse platelets, and are activated following stimulation with collagen and collagen-related peptide (CRP), suggesting a role in GPVI receptor signaling. Fer was also activated following stimulation with thrombin and a protease-activated receptor4 (PAR4)-activating peptide, suggesting a role in signaling downstream from the G protein-coupled PAR4. There were no detectable perturbations in CRP-induced activation of Syk, PLCgamma2, cortactin, Erk, Jnk, Akt or p38 in platelets from mice lacking Fps/Fes, Fer, or both kinases. Platelets lacking Fps/Fes, from a targeted fps/fes null strain of mice, showed increased rates and amplitudes of collagen-induced aggregation, relative to wild-type platelets. P-Selectin expression was also elevated on the surface of Fps/Fes-null platelets in response to CRP. Fer-deficient platelets, from mice targeted with a kinase-inactivating mutation, disaggregated more rapidly than wild-type platelets in response to ADP. This report provides the first evidence that Fps/Fes and Fer are expressed in platelets and become activated downstream from the GPVI collagen receptor, and that Fer is activated downstream from a G-protein coupled receptor. Furthermore, using targeted mouse models we show that deficiency in Fps/Fes or Fer resulted in disregulated platelet aggregation and disaggregation, demonstrating a role for these kinases in regulating platelet functions.

Characterization of promoter elements directing Mona/Gads molecular adapter expression in T and myelomonocytic cells: involvement of the AML-1 transcription factor

Monocytic adaptor (Mona, also called Gads) is a molecular adaptor implicated in T cell activation and macrophage differentiation. The objective of this study was to identify elements regulating specific expression of Mona/Gads in human T cell and myelomonocytic cell lines. We first confirmed that the -2000 to +150 genomic region relative to the Mona gene transcription start site is sufficient to direct specific reporter gene expression in T cell lines, Jurkat, and MOLT-4 and in the immature myeloid cell lines, KG1a and RC2A. Deletion analysis and electrophoresis mobility shift assay identified several cis regulatory elements: overlapping initiator sequences, one interferon response factor-2 (IRF-2)-binding site at position -154, one GC box recognized by Sp1 and Sp3 at position -52, and two acute myeloid leukemia (AML)-1 binding sites at positions -70 and -13. Site-directed mutagenesis experiments indicated a key role of AML-1 for driving Mona expression in T cells and myeloid cells, and involvement of Sp1/Sp3 and IRF-2 transcription factors to modulate Mona expression in a cell-specific manner.