scholarly journals Antiplatelet and Antithrombotic Effects of Epimedium koreanum Nakai

2021 ◽  
Vol 2021 ◽  
pp. 1-11
Author(s):  
Muhammad Irfan ◽  
Tae-Hyung Kwon ◽  
Dong-Ha Lee ◽  
Seung-Bok Hong ◽  
Jae-Wook Oh ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
Light Transmission ◽  
Thrombus Formation ◽  
Calcium Mobilization ◽  
Potential Candidate ◽  
Clot Retraction ◽  
Integrin Αiibβ3 ◽  
Atp Secretion ◽  
Antithrombotic Effects ◽  
Epimedium Koreanum Nakai

Background and Objective. Epimedium koreanum Nakai is a medicinal plant known for its health beneficial effects on impotence, arrhythmia, oxidation, aging, osteoporosis, and cardiovascular diseases. However, there is no report available that shows its effects on platelet functions. Here, we elucidated antiplatelet and antithrombotic effects of ethyl acetate fraction of E. koreanum. Methodology. We analyzed the antiplatelet properties using standard in vitro and in vivo techniques, such as light transmission aggregometry, scanning electron microscopy, intracellular calcium mobilization measurement, dense granule secretion, and flow cytometry to assess integrin αIIbβ3 activation, clot retraction, and Western blot, on washed platelets. The antithrombotic effects of E. koreanum were assessed by arteriovenous- (AV-) shunt model in rats, and its effects on hemostasis were analyzed by tail bleeding assay in mice. Key Results. E. koreanum inhibited platelet aggregation in agonist-stimulated human and rat washed platelets, and it also reduced calcium mobilization, ATP secretion, and TXB2 formation. Fibrinogen binding, fibronectin adhesion, and clot retraction by attenuated integrin αIIbβ3-mediated inside-out and outside-in signaling were also decreased. Reduced phosphorylation of extracellular signal-regulated kinases (ERK), Akt, PLCγ2, and Src was observed. Moreover, the fraction inhibited thrombosis. HPLC results revealed that the fraction predominantly contained icariin. Conclusion and Implications. E. koreanum inhibited platelet aggregation and thrombus formation by attenuating calcium mobilization, ATP secretion, TXB2 formation, and integrin αIIbβ3 activation. Therefore, it may be considered as a potential candidate to treat and prevent platelet-related cardiovascular disorders.

scholarly journals Artocarpesin Prevents Collagen Induced Platelet Aggregation and Clot Retraction Through Cyclic Nucleotides and Dephosphorylation of MAPKs  

2021 ◽  
Author(s):  
Jung-Hae Shin ◽  
Muhammad Irfan ◽  
Yuan Yee Lee ◽  
Man Hee Rhee ◽  
Hyuk-Woo Kwon
Keyword(s):  
Platelet Aggregation ◽  
Cardiovascular Diseases ◽  
Thrombus Formation ◽  
Signaling Molecules ◽  
Serotonin Release ◽  
Calcium Mobilization ◽  
Clot Retraction ◽  
Natural Substances ◽  
Mitogen Activated Protein

Abstract Background The cardiovascular diseases (CVDs) are becoming a critical threat to our lives in these years. It is now widely accepted that platelets play an important role in cardiovascular disease as they have a fundamental role in thrombosis. Therefore, many drugs or natural substances have been developed to treat CVDs. Cudrania tricuspidata (C. tricuspidata) is a regional plant containing various flavonoids and xanthones, and various physiological activities have been reported. Therefore, we evaluated antiplatelet effects using artocarpesin isolated from C. tricuspidata. Methods The in vitro effects of artocarpesin on platelets was assessed using measurement of calcium mobilization and serotonin release, glycoprotein IIb/IIIa activation, clot retraction and phosphorylation of signaling molecules. Results Artocarpesin inhibited human platelet aggregation, calcium mobilization, glycoprotein IIb/IIIa activation and thrombin-induced clot retraction through the regulation of associated signaling molecules such as vasodilator-stimulated phosphoprotein (VASP) and inositol 1, 4, 5-triphosphate receptor I (IP3RI), and on the dephosphorylation of cytosolic phospholipase A2 (cPLA2), mitogen-activated protein kinases p38, JNK and phosphoinositide 3-kinase (PI3K)/Akt. Conclusions This study highlights that artocarpesin has inhibitory effects on platelet activities and thrombus formation and has potential value for preventing platelet-induced cardiovascular diseases.

Antithrombotic Effects and Bleeding Time Prolongation with Synthetic Platelet GPIIb/llla Inhibitors in Animal Models of Platelet-Mediated Thrombosis

1994 ◽  
Vol 71 (01) ◽  
pp. 095-102 ◽  
Author(s):  
Désiré Collen ◽  
Hua Rong Lu ◽  
Jean-Marie Stassen ◽  
Ingrid Vreys ◽  
Tsunehiro Yasuda ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
Bleeding Time ◽  
Bolus Injection ◽  
Cell Injury ◽  
Ex Vivo ◽  
Thrombus Formation ◽  
Femoral Vein ◽  
Thrombotic Occlusion ◽  
Antithrombotic Effects

SummaryCyclic Arg-Gly-Asp (RGD) containing synthetic peptides such as L-cysteine, N-(mercaptoacetyl)-D-tyrosyl-L-arginylglycyl-L-a-aspartyl-cyclic (1→5)-sulfide, 5-oxide (G4120) and acetyl-L-cysteinyl-L-asparaginyl-L-prolyl-L-arginyl-glycyl-L-α-aspartyl-[0-methyltyrosyl]-L-arginyl-L-cysteinamide, cyclic 1→9-sulfide (TP9201) bind with high affinity to the platelet GPIIb/IIIa receptor.The relationship between antithrombotic effect, ex vivo platelet aggregation and bleeding time prolongation with both agents was studied in hamsters with a standardized femoral vein endothelial cell injury predisposing to platelet-rich mural thrombosis, and in dogs with a carotid arterial eversion graft inserted in the femoral artery. Intravenous administration of G4120 in hamsters inhibited in vivo thrombus formation with a 50% inhibitory bolus dose (ID50) of approximately 20 μg/kg, ex vivo ADP-induccd platelet aggregation with ID50 of 10 μg/kg, and bolus injection of 1 mg/kg prolonged the bleeding time from 38 ± 9 to 1,100 ± 330 s. Administration of TP9201 in hamsters inhibited in vivo thrombus formation with ID50 of 30 μg/kg, ex vivo platelet aggregation with an ID50 of 50 μg/kg and bolus injection of 1 mg/kg did not prolong the template bleeding time. In the dog eversion graft model, infusion of 100 μg/kg of G4120 over 60 min did not fully inhibit platelet-mediated thrombotic occlusion but was associated with inhibition of ADP-induccd ex vivo platelet aggregation and with prolongation of the template bleeding time from 1.3 ± 0.4 to 12 ± 2 min. Infusion of 300 μg/kg of TP9201 over 60 min completely prevented thrombotic occlusion, inhibited ex vivo platelet aggregation, but was not associated with prolongation of the template bleeding time.TP9201, unlike G4120, inhibits in vivo platelet-mediated thrombus formation without associated prolongation of the template bleeding time.

FUNDC2 regulates platelet activation through AKT/GSK-3β/cGMP axis

2018 ◽  
Vol 115 (11) ◽  
pp. 1672-1679 ◽  
Author(s):  
Qi Ma ◽  
Weilin Zhang ◽  
Chongzhuo Zhu ◽  
Junling Liu ◽  
Quan Chen
Keyword(s):  
Platelet Aggregation ◽  
Platelet Activation ◽  
Platelet Rich Plasma ◽  
Thrombus Formation ◽  
Mitochondrial Protein ◽  
Dependent Manner ◽  
Clot Retraction ◽  
Akt Phosphorylation ◽  
Gsk 3Β

Abstract Aims AKT kinase is vital for regulating signal transduction in platelet aggregation. We previously found that mitochondrial protein FUNDC2 mediates phosphoinositide 3-kinase (PI3K)/phosphatidylinositol-3,4,5-trisphosphate (PIP3)-dependent AKT phosphorylation and regulates platelet apoptosis. The aim of this study was to evaluate the role of FUNDC2 in platelet activation and aggregation. Methods and results We demonstrated that FUNDC2 deficiency diminished platelet aggregation in response to a variety of agonists, including adenosine 5′-diphosphate (ADP), collagen, ristocetin/VWF, and thrombin. Consistently, in vivo assays of tail bleeding and thrombus formation showed that FUNDC2-knockout mice displayed deficiency in haemostasis and thrombosis. Mechanistically, FUNDC2 deficiency impairs the phosphorylation of AKT and downstream GSK-3β in a PI3K-dependent manner. Moreover, cGMP also plays an important role in FUNDC2/AKT-mediated platelet activation. This FUNDC2/AKT/GSK-3β/cGMP axis also regulates clot retraction of platelet-rich plasma. Conclusion FUNDC2 positively regulates platelet functions via AKT/GSK-3β/cGMP signalling pathways, which provides new insight for platelet-related diseases.

scholarly journals Characterization of GPVI- or GPVI-CD39-Coated Nanoparticles and Their Impact on In Vitro Thrombus Formation

2021 ◽  
Vol 23 (1) ◽  
pp. 11
Author(s):  
Jeremy A. Nestele ◽  
Anne-Katrin Rohlfing ◽  
Valerie Dicenta ◽  
Alexander Bild ◽  
Daniela Eißler ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
Antithrombotic Therapy ◽  
Light Transmission ◽  
Thrombus Formation ◽  
Bleeding Risk ◽  
Significant Inhibition ◽  
Site Specific ◽  
Glycoprotein Vi ◽  
Coated Nanoparticles

Traditional antithrombotic agents commonly share a therapy-limiting side effect, as they increase the overall systemic bleeding risk. A novel approach for targeted antithrombotic therapy is nanoparticles. In other therapeutic fields, nanoparticles have enabled site-specific delivery with low levels of toxicity and side effects. Here, we paired nanotechnology with an established dimeric glycoprotein VI-Fc (GPVI-Fc) and a GPVI-CD39 fusion protein, thereby combining site-specific delivery and new antithrombotic drugs. Poly(lactic-co-glycolic acid) (PLGA) nanoparticles, NP-BSA, NP-GPVI and NP-GPVI-CD39 were characterized through electron microscopy, atomic force measurements and flow cytometry. Light transmission aggregometry enabled analysis of platelet aggregation. Thrombus formation was observed through flow chamber experiments. NP-GPVI and NP-GPVI-CD39 displayed a characteristic surface coating pattern. Fluorescence properties were identical amongst all samples. NP-GPVI and NP-GPVI-CD39 significantly impaired platelet aggregation. Thrombus formation was significantly impaired by NP-GPVI and was particularly impaired by NP-GPVI-CD39. The receptor-coated nanoparticles NP-GPVI and the bifunctional molecule NP-GPVI-CD39 demonstrated significant inhibition of in vitro thrombus formation. Consequently, the nanoparticle-mediated antithrombotic effect of GPVI-Fc, as well as GPVI-CD39, and an additive impact of CD39 was confirmed. In conclusion, NP-GPVI and NP-GPVI-CD39 may serve as a promising foundation for a novel therapeutic approach regarding targeted antithrombotic therapy.

scholarly journals Antithrombotic effect of a monoclonal antibody to the platelet glycoprotein IIb/IIIa receptor in an experimental animal model

Blood ◽  
1986 ◽  
Vol 68 (3) ◽  
pp. 783-786 ◽  
Author(s):  
BS Coller ◽  
JD Folts ◽  
LE Scudder ◽  
SR Smith
Keyword(s):  
Monoclonal Antibody ◽  
Animal Model ◽  
Platelet Aggregation ◽  
Ex Vivo ◽  
Thrombus Formation ◽  
Platelet Glycoprotein ◽  
Antithrombotic Effect ◽  
Antithrombotic Agent ◽  
Antithrombotic Effects ◽  
Platelet Thrombus

A murine monoclonal antibody directed at the platelet glycoprotein IIb/IIIa complex, which blocks platelet aggregation ex vivo, was tested for its antithrombotic effects in an established animal model of acute platelet thrombus formation in partially stenosed arteries. Infusion of 0.7 to 0.8 mg/kg of the F(ab')2 fragment of the antibody completely blocked new thrombus formation despite multiple provocations, making it the most potent antithrombotic agent tested in this model.

CalDAG-GEFI Is a Key Signal Integrator in Platelet Activation and Thrombus Formation.

Blood ◽  
2004 ◽  
Vol 104 (11) ◽  
pp. 326-326
Author(s):  
Wolfgang Bergmeier ◽  
Jill R. Crittenden ◽  
Crystal L. Piffath ◽  
Denisa D. Wagner ◽  
David E. Housman ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
Platelet Activation ◽  
Knockout Mice ◽  
Thrombus Formation ◽  
Es Cells ◽  
Embryonic Stem ◽  
Premature Stop Codon ◽  
Small Gtpase ◽  
Integrin Αiibβ3

Abstract Inside-out activation of platelet integrin αIIbβ3 is a key step in agonist-induced platelet aggregation. Recent studies suggested the involvement of the small GTPase Rap1b in this process as it is highly expressed in platelets and becomes activated during platelet activation. In cell lines, overexpression of the Rap activator CalDAG-GEFI increased αIIbβ3-dependent adhesion, while overexpression of RapGAP, which inactivates Rap1, reduced αIIbβ3 activity. Here we provide evidence that CalDAG-GEFI is an essential component of this pathway in vivo. To generate CalDAG-GEFI knockout mice, we engineered mouse embryonic stem (ES) cells with a deletion that results in a frameshift mutation and a premature stop codon at the position encoding the 37th amino acid of CalDAG-GEFI. These ES cells were then used to derive chimeric mice that yielded germline transmission of the CalDAG-GEFI mutation. Deficiency of CalDAG-GEFI in mutant mice was confirmed by immunohistochemistry and western blot analysis. CalDAG-GEFI−/− platelets showed impaired Rap1b activation and aggregation in response to various agonists, with aggregation being completely blocked when platelets were activated with ADP, thromboxaneA2 analog, or calcium ionophore. Under physiological flow conditions in vitro and in vivo, CalDAG-GEFI-deficient platelets showed normal tethering to basement membrane components but failed to form thrombi. Mice deficient in CalDAG-GEFI were further characterized by a greatly increased bleeding time as well as by a strong protection against collagen-induced pulmonary thrombosis. In summary, we identified CalDAG-GEFI as a key signal integrator in the cascade leading through Rap1 and integrin αIIbβ3 to platelet aggregation and thrombus formation. The fact that CalDAG-GEFI knockout mice are resistant to collagen-induced thrombosis, and do not undergo spontaneous hemorrhaging, suggests that CalDAG-GEFI may be a promising new target for antithrombotic therapy.

scholarly journals Naringenin Inhibits Platelet Activation and Arterial Thrombosis Through Inhibition of Phosphoinositide 3-Kinase and Cyclic Nucleotide Signaling

2021 ◽  
Vol 12 ◽  
Author(s):  
Manting Huang ◽  
Minzhen Deng ◽  
Wenqiang Nie ◽  
Dezhi Zou ◽  
Huanlin Wu ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
Platelet Activation ◽  
Cyclic Nucleotide ◽  
Ex Vivo ◽  
Thrombus Formation ◽  
Antiplatelet Agent ◽  
Protective Effects ◽  
Clot Retraction ◽  
Vasp Phosphorylation ◽  
Nucleotide Signaling

Citrus flavanoids intake can reduce the risk of cardiovascular diseases. Naringenin, a natural predominant flavonoid abundant in citrus fruits, possesses protective effects against atherothrombotic diseases. As platelet activation plays central roles in atherothrombogenesis, we studied the effects of naringenin on platelet activation, signaling, thrombosis and hemostasis. Naringenin dose-dependently inhibited agonist-induced platelet aggregation in vitro, and exhibited more-potent efficacy on ADP-induced platelet aggregation. It also suppressed platelet aggregation stimulated by ADP ex vivo. Naringenin inhibited ADP-induced platelet α-granule secretion, fibrinogen binding, intracellular calcium mobilization and platelet adhesion on collagen-coated surface. Naringenin also inhibited platelet spreading on fibrinogen and clot retraction, processes mediated by outside-in integrin signaling. Mechanism studies indicated that naringenin suppressed PI3K-mediated signaling and phosphodiesterase activity in platelets, in addition to increasing cGMP levels and VASP phosphorylation at Ser239. Furthermore, naringenin-induced VASP phosphorylation and inhibition of platelet aggregation were reversed by a PKA inhibitor treatment. Interestingly, naringenin inhibited thrombus formation in the (FeCl3)-induced rat carotid arterial thrombus model, but not cause a prolonged bleeding time in mice. This study suggests that naringenin may represent a potential antiplatelet agent targeting PI3K and cyclic nucleotide signaling, with a low bleeding risk.

scholarly journals GPVI inhibition by glenzocimab synergistically inhibits atherosclerotic plaque-induced platelet activation when combined with conventional dual antiplatelet therapy

2021 ◽  
Vol 42 (Supplement_1) ◽  
Author(s):  
F O Alenazy ◽  
M H Harbi ◽  
D P Kavanagh ◽  
J Price ◽  
P Brady ◽  
...  
Keyword(s):  
Myocardial Infarction ◽  
Animal Model ◽  
Platelet Aggregation ◽  
Platelet Activation ◽  
Atherosclerotic Plaque ◽  
Ex Vivo ◽  
Thrombus Formation ◽  
Microvascular Thrombosis ◽  
Antithrombotic Effects ◽  
St Elevation

Abstract Introduction Aspirin and a potent platelet P2Y12 inhibitor, such as prasugrel or ticagrelor, are not always sufficient to prevent thrombus formation in patients with ST-elevation MI (STEMI), leading to “slow flow” or “no reflow” effects after stenting. GPIIb/IIIa inhibitors, such as eptifibatide, may help in this setting, but are not used routinely due to their bleeding risk. GPVI has critical roles in thrombosis and a minimal role in haemostasis. Here we tested whether depletion of GPVI has effects on thrombus formation after MI in an animal model and investigated the effects of a novel platelet GPVI inhibitor, glenzocimab (a Fab fragment of a monoclonal antibody), on platelet activation and thrombus formation when combined with aspirin and ticagrelor. Methods We used intravital microscopy in a murine model of ST-elevation myocardial infarction and ischaemia-reperfusion injury to investigate microvascular thrombosis. We investigated the antithrombotic effects of adding glenzocimab (previously known as ACT017) to blood from healthy donors and 20 patients with ACS treated with aspirin and ticagrelor. We compared the effect of glenzocimab with the GPIIb/IIIa inhibitor eptifibatide ex-vivo. We stimulated platelets with collagen and atherosclerotic plaque material that was sourced from patients undergoing carotid endarterectomy. We investigated effects on platelet aggregation, spreading, signalling, adhesion, thrombin generation, thrombus formation and clot stability ex vivo. Results Genetic depletion of GPVI in an animal model of myocardial infarction reduced microvascular thrombosis. Ex vivo, aspirin and ticagrelor partially inhibited atherosclerotic plaque-induced platelet aggregation (assessed by multiple electrode aggregometry) by 48% compared to control (34±3 vs. 65±4 U; P<0.001; Figure 1). Atherosclerotic plaque-induced platelet aggregation, adhesion, secretion and activation were critically dependent on platelet GPVI activation and were potently inhibited by glenzocimab. Glenzocimab alone reduced atherosclerotic plaque-induced platelet aggregation by 75% compared to control (16±4 vs. 65±4 U; P<0.001; Figure 1) and by over 95% when combined with aspirin and ticagrelor (3±1 vs 65±4 U; P<0.001; Figure 1). Furthermore, glenzocimab provided multiple synergistic antithrombotic effects when added to the blood of aspirin and ticagrelor-treated patients with ACS ex vivo. Glenzocimab and the GPIIb/IIIa inhibitor, eptifibatide, had many similar antithrombotic effects but glenzocimab had less effect on mechanisms of general haemostasis compared to eptifibatide, as assessed by ROTEM (Figure 2). Conclusions The addition of glenzocimab to aspirin and ticagrelor provides synergistic inhibition of multiple critical mechanisms of atherothrombosis. Glenzocimab and the GPIIb/IIIa inhibitor, eptifibatide, share many similar antithrombotic effects, although glenzocimab has less impact on mechanisms involved in haemostasis compared to eptifibatide. FUNDunding Acknowledgement Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Academy of Medical Sciences UK Clinical Lecturer Starter GrantRoyal Embassy of Saudi Arabia

Antithrombotic Effect of a Recombinant von Willebrand Factor, VCL, on Nitrogen Laser-Induced Thrombus Formation in Guinea Pig Mesenteric Arteries

1995 ◽  
Vol 73 (02) ◽  
pp. 318-323 ◽  
Author(s):  
K Azzam ◽  
L I Garfinkel ◽  
C Bal dit Sollier ◽  
M Cisse Thiam ◽  
L Drouet
Keyword(s):  
Platelet Aggregation ◽  
Bleeding Time ◽  
Ex Vivo ◽  
Thrombus Formation ◽  
Mesenteric Arteries ◽  
Antithrombotic Effects ◽  
Von Willebrand ◽  
Willebrand Factor

SummaryTo assess the antithrombotic effectiveness of blocking the platelet glycoprotein (GP) Ib/IX receptor for von Willebrand factor (vWF), the antiaggregating and antithrombotic effects were studied in guinea pigs using a recombinant fragment of vWF, Leu 504-Lys 728 with a single intrachain disulfide bond linking residues Cys 509-Cys 695. The inhibitory effect of this peptide, named VCL, was tested in vitro on ristocetin- and botrocetin-induced platelet aggregation and compared to the ADP-induced platelet aggregation. In vivo, the antithrombotic effect of VCL was tested in a model of laser-injured mesentery small arteries and correlated to the ex vivo ristocetin-induced platelet aggregation. In this model of laser-induced thrombus formation, five mesenteric arteries were studied in each animal, and the number of recurrent thrombi during 15 min, the time to visualization and time to formation of first thrombus were recorded.In vitro, VCL totally abolished ristocetin- and botrocetin-induced platelet aggregation, but had no effect on ADP-induced platelet aggregation. Ex vivo, VCL (0.5 to 2 mg/kg) administered as a bolus i. v. injection inhibits ristocetin-induced platelet aggregation with a duration of action exceeding 1 h. The maximum inhibition was observed 5 min after injection of VCL and was dose related. The same doses of VCL had no significant effect on platelet count and bleeding time. In vivo, VCL (0.5 to 2 mg/kg) had no effect on the appearance of the thrombi formed but produced dose-dependent inhibition of the mean number of recurrent thrombi (the maximal effect was obtained at 5 min following i. v. injection of the highest dose: 0.8 ± 0.2 thrombi versus 4 ± 0.4 thrombi in controls). The three doses of VCL increased the time in which the first thrombus in a concentration-dependent manner was formed. However, the time to visualize the first thrombus was only prolonged in the higher dose-treated group.These in-vivo studies confirm that VCL induces immediate, potent, and transient antithrombotic effects. Most importantly, this inhibition was achieved without inducing thrombocytopenia nor prolongation of the bleeding time.

scholarly journals cAMP- and cGMP-elevating agents inhibit GPIbα-mediated aggregation but not GPIbα-stimulated Syk activation in human platelets

2019 ◽  
Vol 17 (1) ◽  
Author(s):  
Stephanie Makhoul ◽  
Katharina Trabold ◽  
Stepan Gambaryan ◽  
Stefan Tenzer ◽  
Daniele Pillitteri ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
Tyrosine Phosphorylation ◽  
Intracellular Signaling ◽  
Light Transmission ◽  
Thrombus Formation ◽  
Coagulation Factors ◽  
Human Platelets ◽  
Induce Platelet Aggregation ◽  
Akt Phosphorylation ◽  
Camp And Cgmp

Abstract Background The glycoprotein (GP) Ib-IX-V complex is a unique platelet plasma membrane receptor, which is essential for platelet adhesion and thrombus formation. GPIbα, part of the GPIb-IX-V complex, has several physiological ligands such as von Willebrand factor (vWF), thrombospondin and distinct coagulation factors, which trigger platelet activation. Despite having an important role, intracellular GPIb-IX-V signaling and its regulation by other pathways are not well defined. Our aim was to establish the intracellular signaling response of selective GPIbα activation in human platelets, in particular the role of the tyrosine kinase Syk and its regulation by cAMP/PKA and cGMP/PKG pathways, respectively. We addressed this using echicetin beads (EB), which selectively bind to GPIbα and induce platelet aggregation. Methods Purified echicetin from snake Echis carinatus venom was validated by mass spectrometry. Washed human platelets were incubated with EB, in the presence or absence of echicetin monomers (EM), Src family kinase (SFK) inhibitors, Syk inhibitors and the cAMP- and cGMP-elevating agents iloprost and riociguat, respectively. Platelet aggregation was analyzed by light transmission aggregometry, protein phosphorylation by immunoblotting. Intracellular messengers inositolmonophosphate (InsP1) and Ca2+i were measured by ELISA and Fluo-3 AM/FACS, respectively. Results EB-induced platelet aggregation was dependent on integrin αIIbβ3 and secondary mediators ADP and TxA2, and was antagonized by EM. EB stimulated Syk tyrosine phosphorylation at Y352, which was SFK-dependent and Syk-independent, whereas Y525/526 phosphorylation was SFK-dependent and partially Syk-dependent. Furthermore, phosphorylation of both Syk Y352 and Y525/526 was completely integrin αIIbβ3-independent but, in the case of Y525/526, was partially ADP/TxA2-dependent. Syk activation, observed as Y352/ Y525/Y526 phosphorylation, led to the phosphorylation of direct substrates (LAT Y191, PLCγ2 Y759) and additional targets (Akt S473). PKA/PKG pathways inhibited EB-induced platelet aggregation and Akt phosphorylation but, surprisingly, enhanced Syk and LAT/PLCγ2 tyrosine phosphorylation. A similar PKA/PKG effect was confirmed with convulxin−/GPVI-stimulated platelets. EB-induced InsP1 accumulation/InsP3 production and Ca2+-release were Syk-dependent, but only partially inhibited by PKA/PKG pathways. Conclusion EB and EM are specific agonists and antagonists, respectively, of GPIbα-mediated Syk activation leading to platelet aggregation. The cAMP/PKA and cGMP/PKG pathways do not inhibit but enhance GPIbα−/GPVI-initiated, SFK-dependent Syk activation, but strongly inhibit further downstream responses including aggregation. These data establish an important intracellular regulatory network induced by GPIbα. Graphical abstract

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