Nonredundant Roles of Platelet Glycoprotein VI and Integrin αIIbß3 in Fibrin-Mediated Microthrombus Formation.

OBJECTIVE: Fibrin is considered to strengthen thrombus formation via integrin αIIbß3, but recent findings indicate that fibrin can also act as ligand for platelet glycoprotein VI. Approach and Results: To investigate the thrombus-forming potential of fibrin and the roles of platelet receptors herein, we generated a range of immobilized fibrin surfaces, some of which were cross-linked with factor XIIIa and contained VWF-BP (von Willebrand factor-binding peptide). Multicolor microfluidics assays with whole-blood flowed at high shear rate (1000 s-1) indicated that the fibrin surfaces, regardless of the presence of factor XIIIa or VWF-BP, supported platelet adhesion and activation (P-selectin expression), but only microthrombi were formed consisting of bilayers of platelets. Fibrinogen surfaces produced similar microthrombi. Markedly, tiggering of coagulation with tissue factor or blocking of thrombin no more than moderately affected the fibrin-induced microthrombus formation. Absence of αIIbß3 in Glanzmann thrombasthenia annulled platelet adhesion. Blocking of glycoprotein VI with Fab 9O12 substantially, but incompletely reduced platelet secretion, Ca2+ signaling and aggregation, while inhibition of Syk further reduced these responses. In platelet suspension, glycoprotein VI blockage or Syk inhibition prevented fibrin-induced platelet aggregation. Microthrombi on fibrin surfaces triggered only minimal thrombin generation, in spite of thrombin binding to the fibrin fibers. CONCLUSIONS: Together, these results indicate that fibrin fibers, regardless of their way of formation, act as a consolidating surface in microthrombus formation via nonredundant roles of platelet glycoprotein VI and integrin αIIbß3 through signaling via Syk and low-level Ca2+ rises.

Glycoprotein VI is not a Functional Platelet Receptor for Fibrin Formed in Plasma or Blood.

Glycoprotein VI (GPVI), a platelet collagen receptor, is crucial in mediating atherothrombosis. Besides collagen, injured plaques expose tissue factor (TF) that triggers fibrin formation. Previous studies reported that GPVI also is a platelet receptor for fibrinogen and fibrin. We studied the effect of anti-GPVI antibodies and inhibitors of GPVI signaling kinases (Syk and Btk) on platelet adhesion and aggregate formation onto immobilized fibrinogen and different types of fibrin under arterial flow conditions. Fibrin was prepared from isolated fibrinogen ("pure fibrin"), recombinant fibrinogen ("recombinant fibrin"), or generated more physiologically from endogenous fibrinogen in plasma ("plasma fibrin") or by exposing TF-coated surfaces to flowing blood ("blood fibrin"). Inhibition of GPVI and Syk did not inhibit platelet adhesion and aggregate formation onto fibrinogen. In contrast anti-GPVI antibodies, inhibitors of Syk and Btk and the anti-GPIb antibody 6B4 inhibited platelet aggregate formation onto pure and recombinant fibrin. However, inhibition of GPVI and GPVI signaling did not significantly reduce platelet coverage of plasma fibrin and blood fibrin. Plasma fibrin contained many proteins incorporated during clot formation. Advanced optical imaging revealed plasma fibrin as a spongiform cushion with thicker, knotty, and long fibers and little activation of adhering platelets. Albumin intercalated in plasma fibrin fibers left only little space for platelet attachment. Pure fibrin was different showing a dense mesh of thin fibers with strongly activated platelets. We conclude that fibrin formed in plasma and blood contains plasma proteins shielding GPVI-activating epitopes. Our findings do not support a role of GPVI for platelet activation by physiologic fibrin.

Mincle/Syk Signalling Promotes Intestinal Mucosal Inflammation Through Induction of Macrophage Pyroptosis in Crohn's Disease.

BACKGROUND: Macrophage-inducible C-type lectin [Mincle] signalling plays a proinflammatory role in different organs such as the brain and liver, but its role in intestinal inflammation, including Crohn's disease [CD], remains unknown. METHODS: The characteristics of Mincle signalling expression in CD patients and experimental colitis were examined. The functional role of Mincle signalling in the intestine was addressed in experimental colitis models in vivo by using Mincle knock-out [Mincle-/-] mice. In addition, neutralising anti-Mincle antibody, downstream spleen tyrosine kinase [Syk] inhibitor, and Mincle pharmacological agonist were used to study the Mincle signalling in intestine. Bone marrow-derived macrophages were collected from mice and used to further verify the effect of Mincle signalling in macrophages. RESULTS: This study has shown that Mincle signalling was significantly elevated in active human CD and experimental colitis, and macrophages were the principal leukocyte subset that upregulate Mincle signalling. Mincle deficiency and Syk pharmacological inhibition ameliorated the colitis by reducing induced macrophage pyroptosis, and activation of Mincle with the agonist aggravated the intestinal inflammation. The ex vivo studies demonstrated that activation of Mincle signalling promoted the release of proinflammatory cytokines, whereas its absence restricted release of proinflammatory cytokines from pyroptosis of macrophages. In addition, Mincle/Syk signalling in macrophages could promote the production of chemokines to recruit neutrophils by activating mitogen-activated protein kinase [MAPK] during intestinal inflammation. CONCLUSIONS: Mincle signalling promotes intestinal mucosal inflammation by inducing macrophage pyroptosis. Modulation of the Mincle/Syk axis emerges as a potential therapeutic strategy to target inflammation and treat CD.

The Expression and Clinical Significance of Spleen Tyrosine Kinase in Patients with Coronary Heart Disease.

BACKGROUND Our research was designed to investigate the relationship of spleen tyrosine kinase (Syk) and inflammatory factors with coronary heart disease (CHD) and the risk factors of CHD. MATERIAL AND METHODS In our study, 226 patients were enrolled, from October 2017 to March 2018. Clinical and biochemical data were collected. We collected samples of peripheral blood monocytes (PBMs) from the enrolled patients. The patients were divided in 4 groups: patients without coronary artery disease (control group), patients with stable angina pectoris (SAP group), patients with non-ST-segment elevation acute coronary syndrome (NSTE-ACS group), and patients with ST-segment elevation acute myocardial infarction group (STEMI group). We detect the protein levels of Syk and inflammatory factors expression by western blot. RESULTS Our results found the protein levels of Syk and inflammatory factors expression in the NSTE-ACS and STEMI groups were higher than those in the SAP and control groups. The protein levels of Syk and inflammatory factors expression in the SAP group were higher than those in the control group. Moreover, there were many risk factors significantly associated with Syk. Besides that, these risk factors were also independent risk factors of CHD. CONCLUSIONS Our results found that the level of Syk was associated with the severity of CHD. From our study, we found that higher levels of Syk and inflammatory factors protein were associated with worse results of the CHD. For the first time, Syk was reported to be a promising therapeutic factor for CHD patients.

Synthetic glycopolymers and natural fucoidans cause human platelet aggregation via PEAR1 and GPIbα.

Fucoidans are sulfated fucose-based polysaccharides that activate platelets and have pro- and anticoagulant effects; thus, they may have therapeutic value. In the present study, we show that 2 synthetic sulfated α-l-fucoside-pendant glycopolymers (with average monomeric units of 13 and 329) and natural fucoidans activate human platelets through a Src- and phosphatidylinositol 3-kinase (PI3K)-dependent and Syk-independent signaling cascade downstream of the platelet endothelial aggregation receptor 1 (PEAR1). Synthetic glycopolymers and natural fucoidan stimulate marked phosphorylation of PEAR1 and Akt, but not Syk. Platelet aggregation and Akt phosphorylation induced by natural fucoidan and synthetic glycopolymers are blocked by a monoclonal antibody to PEAR1. Direct binding of sulfated glycopolymers to epidermal like growth factor (EGF)-like repeat 13 of PEAR1 was shown by avidity-based extracellular protein interaction screen technology. In contrast, synthetic glycopolymers and natural fucoidans activate mouse platelets through a Src- and Syk-dependent pathway regulated by C-type lectin-like receptor 2 (CLEC-2) with only a minor role for PEAR1. Mouse platelets lacking the extracellular domain of GPIbα and human platelets treated with GPIbα-blocking antibodies display a reduced aggregation response to synthetic glycopolymers. We found that synthetic sulfated glycopolymers bind directly to GPIbα, substantiating that GPIbα facilitates the interaction of synthetic glycopolymers with CLEC-2 or PEAR1. Our results establish PEAR1 as the major signaling receptor for natural fucose-based polysaccharides and synthetic glycopolymers in human, but not in mouse, platelets. Sulfated α-l-fucoside-pendant glycopolymers are unique tools for further investigation of the physiological role of PEAR1 in platelets and beyond.

miR-129-2-3p directly targets SYK gene and associates with the risk of ischaemic stroke in a Chinese population.

Spleen tyrosine kinase (SYK) gene has been identified as novel susceptibility locus for ischaemic stroke (IS) previously. However, regulation of SYK gene remains unknown in IS. In this study, we aimed to identify miRNAs that might be involved in the development of IS by targeting SYK gene. miRNAs were firstly screened by bioinformatics predicting tool. The expression levels of SYK gene were detected by qRT-PCR and western blotting, respectively, after miRNA transfection. Luciferase reporter assay was applied to investigate the direct binding between miRNAs and target gene. miRNA levels were detected by miRNA TaqMan assays in the blood cells of 270 IS patients and 270 control volunteers. Results suggest that SYK gene might be a direct target of miR-129-2-3p. The blood level of miR-129-2-3p was significantly lower in IS patients (P < 0.05), and negatively associated with the risk of IS (adjusted OR: 0.88; 95% CI: 0.80-0.98; P = 0.021) by multivariable logistic regression analysis. The blood levels of SYK gene were significantly higher in IS patients, and miR-129-2-3p expression was negatively correlated with mean platelet volume. In summary, our study suggests that miR-129-2-3p might be involved in the pathogenesis of IS through interrupting SYK expression and the platelet function, and further investigation is needed to explore the underlying mechanism.

An αIIb ß3 antagonist prevents thrombosis without causing Fc receptor γ-chain IIa-mediated thrombocytopenia.

Essentials FcγRIIa-mediated thrombocytopenia is associated with drug-dependent antibodies (DDAbs). We investigated the correlation between αIIb ß3 binding epitopes and induction of DDAbs. An FcγRIIa-transgenic mouse model was used to evaluate thrombocytopenia among anti-thrombotics. An antithrombotic with binding motif toward αIIb ß-propeller domain has less bleeding tendency. SUMMARY: Background Thrombocytopenia, a common side effect of Arg-Gly-Asp-mimetic antiplatelet drugs, is associated with drug-dependent antibodies (DDAbs) that recognize conformation-altered integrin αIIb ß3 . Objective To explore the correlation between αIIb ß3 binding epitopes and induction of DDAb binding to conformation-altered αIIb ß3 , we examined whether two purified disintegrins, TMV-2 and TMV-7, with distinct binding motifs have different effects on induction of αIIb ß3 conformational change and platelet aggregation in the presence of AP2, an IgG1 inhibitory mAb raised against αIIb ß3 . Methods We investigated the possible mechanisms of intrinsic platelet activation of TMV-2 and TMV-7 in the presence of AP2 by examining the signal cascade, tail bleeding time and immune thrombocytopenia in Fc receptor γ-chain IIa (FcγRIIa) transgenic mice. Results TMV-7 has a binding motif that recognizes the αIIb ß-propeller domain of αIIb ß3 , unlike that of TMV-2. TMV-7 neither primed the platelets to bind ligand, nor caused a conformational change of αIIb ß3 as identified with the ligand-induced binding site mAb AP5. In contrast to eptifibatide and TMV-2, cotreatment of TMV-7 with AP2 did not induce FcγRIIa-mediated platelet aggregation and the downstream activation cascade. Both TMV-2 and TMV-7 efficaciously prevented occlusive thrombosis in vivo. Notably, both eptifibatide and TMV-2 caused severe thrombocytopenia mediated by FcγRIIa, prolonged tail bleeding time in vivo, and repressed human whole blood coagulation indexes, whereas TMV-7 did not impair hemostatic capacity. Conclusions TMV-7 shows antiplatelet and antithrombotic activities resulting from a mechanism different from that of all other tested αIIb ß3 antagonists, and may offer advantages as a therapeutic agent with a better safety profile.

The Novel Oral Syk Inhibitor, Bl1002494, Protects Mice From Arterial Thrombosis and Thromboinflammatory Brain Infarction.

OBJECTIVE: Ischemic stroke, which is mainly caused by thromboembolic occlusion of brain arteries, is the second leading cause of death and disability worldwide with limited treatment options. The platelet collagen receptor glycoprotein VI (GPVI) is a key player in arterial thrombosis and a critical determinant of stroke outcome, making its signaling pathway an attractive target for pharmacological intervention. The spleen tyrosine kinase (Syk) is an essential signaling mediator downstream of not only GPVI but also other platelet and immune cell receptors. We sought to assess whether Syk might be an effective antithrombotic target. APPROACH AND RESULTS: We demonstrate that mice lacking Syk in platelets specifically are protected from arterial thrombus formation and ischemic stroke but display unaltered hemostasis. Furthermore, we show that mice treated with the novel, selective, and orally bioavailable Syk inhibitor BI1002494 were protected in a model of arterial thrombosis and had smaller infarct sizes and a significantly better neurological outcome 24 hours after transient middle cerebral artery occlusion, also when BI1002494 was administered therapeutically, that is, after ischemia. CONCLUSIONS: These results provide direct evidence that pharmacological Syk inhibition might provide a safe therapeutic strategy to prevent arterial thrombosis and to limit infarct progression in acute stroke.

Differential Roles of the NADPH-Oxidase 1 and 2 in Platelet Activation and Thrombosis.

OBJECTIVE: Reactive oxygen species (ROS) are known to regulate platelet activation; however, the mechanisms of ROS production during platelet activation remain unclear. Platelets express different isoforms of nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) oxidases (NOXs). Here, we investigated the role of NOX1 and NOX2 in ROS generation and platelet activation using NOX1 and NOX2 knockout mice. APPROACH AND RESULTS: NOX1(-/Y) platelets showed selective defects in G-protein-coupled receptor-mediated platelet activation induced by thrombin and thromboxane A2 analog U46619, but were not affected in platelet activation induced by collagen-related peptide, a glycoprotein VI agonist. In contrast, NOX2(-/-) platelets showed potent inhibition of collagen-related peptide-induced platelet activation, and also showed partial inhibition of thrombin-induced platelet activation. Consistently, production of ROS was inhibited in NOX1(-/Y) platelets stimulated with thrombin, but not collagen-related peptide, whereas NOX2(-/-) platelets showed reduced ROS generation induced by collagen-related peptide or thrombin. Reduced ROS generation in NOX1/2-deficient platelets is associated with impaired activation of Syk and phospholipase Cγ2, but minimally affected mitogen-activated protein kinase pathways. Interestingly, laser-induced arterial thrombosis was impaired but the bleeding time was not affected in NOX2(-/-) mice. Wild-type thrombocytopenic mice injected with NOX2(-/-) platelets also showed defective arterial thrombosis, suggesting an important role for platelet NOX2 in thrombosis in vivo but not hemostasis. CONCLUSIONS: NOX1 and NOX2 play differential roles in different platelet activation pathways and in thrombosis. ROS generated by these enzymes promotes platelet activation via the Syk/phospholipase Cγ2/calcium signaling pathway.

Stability of Syk protein and mRNA in human peripheral blood basophils.

In human basophils, Syk expression is 10-fold lower than most other types of leukocytes. There are indirect studies that suggest that Syk protein is highly unstable (a calculated half-life less than 15 min) in human peripheral blood basophils. Therefore, in these studies, Syk stability was directly examined. Purified basophils were metabolically labeled and a pulse-chase experimental design showed Syk protein to be stable in the time frame of 12 h (95% likelihood that half-life is more than 12 h). However, its synthetic rate was very slow (∼10-fold slower) compared with CD34-derived basophils, which have been shown to express levels of Syk consistent with other mature circulating leukocytes. Syk mRNA expression was found to be 5-30-fold lower than other cell types (CD34-derived basophils, peripheral blood eosinophils, and plasmacytoid dendritic cells). Syk protein and mRNA levels, across cell types, were relatively concordant. Syk mRNA in basophils showed a half-life of 3.5 h, which was greater than that of interleukin-4 or Fc epsilon receptor I-α mRNA (∼2 h), but somewhat shorter than Fc epsilon receptor I-ß mRNA (8 h). A comparison of miR expression between CD34-derived and peripheral blood basophils demonstrated only 1 significant increase, in miR-150 (77-fold). Transfection in human embryonic kidney cells of a stabilized form of miR-150 showed that it modified expression of c-Myb mRNA but not of Syk mRNA or protein. These results suggest that low Syk expression in basophils results, not from protein instability and perhaps not from mRNA stability. Instead, the results point to the transcriptional nature of an important point of regulation.