Platelet Receptor Glycoprotein Research Articles

Reversible Thrombocytopenia of Functional Platelets after Nose-Horned Viper Envenomation Is Induced by a Snaclec.

Profound and transient thrombocytopenia of functional platelets without bleeding was observed in patients envenomed by Vipera a. ammodytes (Vaa). This condition was rapidly reversed by administration of F(ab)2 fragments of immunoglobulin G targeting the whole venom, leaving platelets fully functional. To investigate the potential role of snake venom C-type lectin-like proteins (snaclecs) in this process, Vaa-snaclecs were isolated from the crude venom using different liquid chromatographies. The purity of the isolated proteins was confirmed by Edman sequencing and mass spectrometry. The antithrombotic effect was investigated by platelet agglutination and aggregation assays and blood coagulation tests. Using flow cytometry, the platelet activation and binding of Vaa-snaclecs to various platelet receptors was analyzed. Antithrombotic efficacy was tested in vivo using a mouse model of vascular injury. Two Vaa-snaclecs were purified from the venom. One of them, Vaa-snaclec-3/2, inhibited ristocetin-induced platelet agglutination. It is a covalent heterodimer of Vaa-snaclec-3 (α-subunit) and Vaa-snaclec-2 (β-subunit). Our results suggest that Vaa-snaclec-3/2 induces platelet agglutination and consequently thrombocytopenia by binding to the platelet receptor glycoprotein Ib. Essentially, no platelet activation was observed in this process. In vivo, Vaa-snaclec-3/2 was able to protect the mouse from ferric chloride-induced carotid artery thrombosis, revealing its applicative potential in interventional angiology and cardiology.

Abstract 28: Patients Randomised to Glenzocimab Suffered Less Haemorrhagic Transformation at 24 Hours Compared to Placebo: AI-Imaging Sub-Analysis of the ACTIMIS Trial

Introduction: ACTIMIS (NCT03803007) was a randomized phase 1b/2a clinical trial evaluating glenzocimab, a monoclonal antibody fragment targeting platelet receptor glycoprotein VI in patients with acute ischemic stroke treated by thrombolysis. Primary analysis demonstrated a reduction in intracranial hemorrhage occurrence stroke-related mortality. In this sub-analysis, volumetric imaging biomarkers were used to assess efficacy of glenzocimab. Methods: In the phase 2a study, patients were randomized (1:1) with 1000mg glenzocimab or placebo. CT or MRI was acquired at baseline with CT at 24 hours and MRI at 7 days for safety and efficacy analysis. Baseline and follow up imaging were processed as post-hoc analysis using AI core lab software (Brainomix, Oxford, UK). Automated output was reviewed for accuracy by an expert clinician (DC) blinded to treatment allocation. Results and Conclusions: Follow up imaging data were available from 103/106 patients (51 glenzocimab, 52 placebo) at 24 hours. Of these, 54 underwent mechanical thrombectomy (MT, 27 glenzocimab, 27 placebo). Day-7 imaging was available for 9 fewer placebo patients and 1 glenzocimab patient. All except 2 patients (1 placebo, 1 glenzocimab) with missing data at Day-7 died during the study. Preliminary analysis showed smaller volume of hemorrhagic transformation (HT) in the glenzocimab group at 24 hours compared to placebo and a trend towards smaller volume of ischemic injury. Exploratory analysis also highlighted an interaction effect between risk of HT following MT and glenzocimab (lower risk in patients treated with glenzocimab). The full results of the imaging sub-analysis of the ACTIMIS study will be presented at the conference and discussed in the context of current literature.

Description of an In Vitro Platelet Function Analyzer-PFA-100™.

This manuscript represents a republication of a manuscript originally published in STH in 1995. This republication is to help celebrate 50 years of publishing for STH. The original abstract follows.A new in vitro system for the detection of platelet dysfunction, PFA-100®, has been developed. It provides a quantitative measure of platelet function in anticoagulated whole blood. The system comprises a microprocessor-controlled instrument and a disposable test cartridge containing a biologically active membrane. The instrument aspirates a blood sample under constant vacuum from the sample reservoir through a capillary and a microscopic aperture cut into the membrane. The membrane is coated with collagen and epinephrine or adenosine 5'-diphosphate. The presence of these biochemical stimuli, and the high shear rates generated under the standardized flow conditions, result in platelet attachment, activation, and aggregation, slowly building a stable platelet plug at the aperture. The time required to obtain full occlusion of the aperture is reported as the "closure time." We have found that impairment of von Willebrand factor, or inhibition of platelet receptors glycoprotein Ib or IIb/IIIa with monoclonal antibodies or peptides, resulted in abnormal closure times. An antifibrinogen antibody, in contrast, failed to show any effect. The test appears to be sensitive to platelet adherence and aggregation abnormalities. The PFA-100® system has potential applications in routine evaluation of platelet function in the clinical setting because of its accuracy, case of operation, and rapid turnaround of results.

Abstract 006: Glenzocimab Is Associated With Less Haemorrhagic Transformation Using Artificial Intelligence Imaging In Mechanical Thrombectomy Patients

Introduction Lesion volume measurement provides an objective and quantitative assessment of stroke severity and it is often used as a surrogate endpoint of clinical outcome in therapeutic trials. ACTIMIS (NCT03803007) was a randomized phase 1b/2a clinical trial evaluating glenzocimab, a monoclonal antibody fragment targeting platelet receptor glycoprotein VI, versus placebo in patients with acute ischemic stroke treated by thrombolysis alone or associated with mechanical thrombectomy (MT), both subgroups being well balanced. Primary analysis demonstrated a significant reduction in intracranial hemorrhage occurrence stroke‐related mortality and a trend towards reduction in severe disability. In this sub‐analysis, Artificial Intelligence (AI) imaging biomarkers were used to assess efficacy of glenzocimab in the subgroup undergoing MT in the pooled phase 1b (dose escalation) and phase 2a (dose confirmation) studies. Methods In the phase 1b study, patients were randomized to an escalating dose of glenzocimab or placebo (4:1, n=12 per group, 125mg, 250mg, 500mg, 1000mg). In the phase 2 dose confirmation study, patients were randomized (1:1) with 1000mg glenzocimab or placebo. CT scan or MRI was acquired at baseline with CT at 24 hours and MRI at 7 days (CT if MRI not available) for safety and efficacy analysis, respectively. Baseline and follow up imaging were processed using Brainomix software (Oxford, UK). AI output was reviewed for accuracy by an expert clinician (DC) blinded to treatment allocation. Results Of 166 patients in the trial, 81 patients underwent MT and had follow up non‐contrast CT imaging available at 24 hours (48 glenzocimab, 33 placebo) and 72 had Day‐7 imaging (47 glenzocimab, 25 placebo). Day‐7 imaging was available for 8 fewer placebo patients and 1 glenzocimab patient than at 24 hours. All except 2 patients (1 placebo, 1 glenzocimab) with missing data at Day‐7 died during the study. Multivariate regression modelling showed a significant interaction between patients undergoing MT and receiving glenzocimab. Glenzocimab had a greater effect on reducing the risk of haemorrhagic transformation in the MT subgroup (exploratory p=0.001). Presenting acute infarct volume was similar between glenzocimab and placebo arms (mean [SD]; 16.27mL [30.2] vs. 19.62mL [28.98], non‐significant). There was a significantly smaller volume of haemorrhagic transformation in the glenzocimab group at 24 hours compared to placebo (1.83mL [6.59] vs 33.46mL [75.07], exploratory p<0.01) and a trend towards smaller volume of ischemic injury in the glenzocimab group (52.32mL [82.07] vs 62.93mL [79.4]). Similar significant trend at Day‐7 for haemorrhagic transformation was seen but less marked due to the unbalanced loss to follow up at Day‐7 (2.99mL [12.35] vs 4.19mL [10.83], exploratory p<0.05). Conclusion Glenzocimab reduces likelihood of haemorrhagic transformation in patients undergoing MT. Haemorrhage volumes were smaller in the glenzocimab group and encouragingly there was a trend to smaller volumes of ischemic injury. There was a greater drop‐out rate in the placebo group between 24 hours and Day‐7, which resulted in a smaller difference in haemorrhagic volumes at Day‐7. Those results, which are to be further analysed in ACTIMIS overall population of AIS patients, could highlight glenzocimab mechanism in the reperfusion injury mitigation.

Platelet Glycoprotein Receptor Ia-C807T and IIIa-PlA1/PlA2 Genetic Polymorphisms Are Associated With Enhanced Platelet Function in Women With Recurrent Miscarriages.

Thrombophilic genetic polymorphisms of the platelet glycoproteins Ia (GpIa) and IIIa (GpIIIa) have been associated with an increased risk of recurrent miscarriages. The aim of this study was to investigate the association of genetic polymorphisms GpIa-C807T and GpIIIa-T1565C-PlA1/PlA2 with platelet function in women with unexplained spontaneous recurrent miscarriages. This cross-sectional study comprised 196 unrelated nulliparous Greek women with a history of unexplained recurrent miscarriages. Patients were genotyped for the presence of the GpIa-C807T (rs1126643) and GpIIIa-T1565C-PlA1/PlA2 (rs5918) genetic polymorphisms by pyrosequencing, and the collagen/epinephrine closure time (COL/EPI CT) of the subjects was assessed using the platelet function analyzer (PFA)-100. In the total population of women with recurrent miscarriages, the COL/EPI CT ranged from 70 to 160 seconds (median: 122 seconds, interquartile range (IQR): 102.3-138 seconds). In comparison with the double homozygotes CC/PlA1PlA1 that had the most prolonged CT (mean: 131.9 ± 17.5 seconds), the COL/EPI CT was statistically significantly shorter for the GpIa-807T single carriers (mean: 120.3 ± 20.9 seconds) (p=0.011) (absolute difference: 11.6 seconds, 95% confidence interval (CI): 21.2 to -2.0 seconds;relative difference: -9%, 95% CI: -16% to -2%), and the GpIIIa-PlA2 single carriers also displayed a trend for shorter COL/EPI CT (mean:121.3 ± 23.7 seconds) (p=0.141) (absolute difference: -10.6 seconds, relative difference: -8%), whereas the combined carriers of the GpIa-807T and the GpIIIa-PlA2 alleles exhibited the shortest COL/EPI CT (mean: 104.1 ± 19.7 seconds) (absolute difference: -27.7 seconds, 95% CI: -39.1 to -16.3 seconds; relative difference: -21%, 95% CI: -30% to -12%) (p<0.001). In comparing genotype frequencies in the lower half with those in the upper half of the COL/EPI CT range, the GpIa-807T and the GpIIIa-PlA2 single carriers were associated with higher odds of COL/EPI CT < 122 seconds (odds ratio (OR)=3.4, 95% CI: 1.5 to 7.5, p=0.002, and OR=2.6, 95% CI: 1.0 to 7.2, p=0.053, respectively). The association was strongest for the combined carriers with OR of 15.0 (95% CI: 5.2 to 43.2, p<0.001) for COL/EPI CT below the medianand OR of 35.5 (95% CI: 4.4 to 284.5, p<0.001) for COL/EPI CT < 100 seconds. The GpIa-C807T and GpIIIa-PlA1/PlA2 polymorphisms and more pronouncedly the combined carriers of the risk variants are associated with enhanced platelet reactivity expressed via shorter COL/EPI CT. These findings provide further evidence for the role of platelet-associated genetic thrombophilia in the pathogenesis of recurrent miscarriages and promotethe analysis of platelet function as a diagnostic tool in the evaluation of this disorder.

Trivalent nanobody-based ligands mediate powerful activation of GPVI, CLEC-2, and PEAR1 in human platelets whereas FcγRIIA requires a tetravalent ligand

BackgroundClustering of the receptors glycoprotein receptor VI (GPVI), C-type lectin-like receptor 2 (CLEC-2), low-affinity immunoglobulin γ Fc region receptor II-a (FcγRIIA), and platelet endothelial aggregation receptor 1 (PEAR1) leads to powerful activation of platelets through phosphorylation of tyrosine in their cytosolic tails and initiation of downstream signaling cascades. GPVI, CLEC-2, and FcγRIIA signal through YxxL motifs that activate Syk. PEAR1 signals through a YxxM motif that activates phosphoinositide 3-kinase. Current ligands for these receptors have an undefined valency and show significant batch variation and, for some, uncertain specificity. ObjectivesWe have raised nanobodies against each of these receptors and multimerized them to identify the minimum number of epitopes to achieve robust activation of human platelets. MethodsDivalent and trivalent nanobodies were generated using a flexible glycine-serine linker. Tetravalent nanobodies utilize a mouse Fc domain (IgG2a, which does not bind to FcγRIIA) to dimerize the divalent nanobody. Ligand affinity measurements were determined by surface plasmon resonance. Platelet aggregation, adenosine triphosphate secretion, and protein phosphorylation were analyzed using standardized methods. ResultsMultimerization of the nanobodies led to a stepwise increase in affinity with divalent and higher-order nanobody oligomers having sub-nanomolar affinity. The trivalent nanobodies to GPVI, CLEC-2, and PEAR1 stimulated powerful and robust platelet aggregation, secretion, and protein phosphorylation at low nanomolar concentrations. A tetravalent nanobody was required to activate FcγRIIA with the concentration-response relationship showing a greater variability and reduced sensitivity compared with the other nanobody-based ligands, despite a sub-nanomolar binding affinity. ConclusionThe multivalent nanobodies represent a series of standardized, potent agonists for platelet glycoprotein receptors. They have applications as research tools and in clinical assays.

Platelet Receptor Glycoprotein VI-Dimer Is Overexpressed in Patients with Atrial Fibrillation at High Risk of Ischemic Stroke.

Introduction Atrial fibrillation (AF) increases the risk of ischemic stroke (IS). We hypothesized that the functional form of platelet receptor glycoprotein (GP) VI, GPVI-dimer, which binds to collagen and fibrin causing platelet activation, is overexpressed in patients with AF who have not had a stroke. Methods A total of 75 inpatients with AF were recruited. None were admitted with or had previously had thrombotic events, including IS or myocardial infarction. Platelet surface expression of total GPVI, GPVI-dimer, and the platelet activation marker P-selectin were quantitated by whole blood flow cytometry. Serum biomarkers were collected in AF patients. Results were compared against patients contemporaneously admitted to hospital with similar age and vascular risk-factor profiles without AF (noAF, n = 30). Results Patients with AF have similar total GPVI surface expression ( p = 0.58) and P-selectin exposure ( p = 0.73) on their platelets compared with noAF patients but demonstrate significantly higher GPVI-dimer expression ( p = 0.02 ). Patients with paroxysmal AF express similar GPVI-dimer levels compared with permanent AF and GPVI-dimer levels were not different between anticoagulated groups. Serum N-terminal pro b-type natriuretic peptide ( p < 0.0001 ) and high sensitivity C-reactive protein ( p < 0.0001 ) were significantly correlated with GPVI-dimer expression in AF platelets. AF was the only vascular risk factor that was independently associated with higher GPVI-dimer expression in the whole population ( p = 0.02 ) . Conclusion GPVI inhibition is being explored in clinical trials as a novel target for IS treatment. As GPVI-dimer is elevated in AF patients' platelets, the exploration of targeted GPVI-dimer inhibition for stroke prevention in patients at high risk of IS due to AF is supported.

Rhomb-I, a P–I metalloproteinase from Lachesis muta rhombeata venom degrades vessel extra cellular matrix components and impairs platelet aggregation

Rhomb-I, a 23-kDa metalloproteinase was isolated from L. m. rhombeata venom. Its dimethylcasein proteolysis was abolished by metal chelators, and slightly enhanced by Ca2+ and Mg2+ ions, but inhibited by Co2+, Zn2+ and α2-macroglobulin. In aqueous solution, rhomb-I autoproteolyzed to a 20- and 11-kDa fragments at 37 °C. The amino acid sequence showed high homology with other snake venom metalloproteinases. Rhomb-I causes hemorrhage that may be ascribed to hydrolysis of essential basement membrane, extracellular matrix and plasma proteins. It preferentially cleaves the α-chains of fibrin (ogen). Rhomb-I inhibited convulxin- and von Willebrand factor (vWF)-induced aggregation on human platelets without significant effect on collagen-stimulated aggregation or other effectors. It digests vWF into a low-molecular-mass multimers of vWF and a rvWF-A1 domain to a 27-kDa fragment as revealed by western blotting with mouse anti-rvWF A1-domain IgG. Incubation of platelets with rhomb-I resulted in adhesion to and cleavage of platelet receptors glycoprotein (GP)Ibα and GPVI to release a 55-kDa soluble form. Both membrane glycoproteins GPIbα that binds vWF, together with GPVI which binds collagen, play a key role in mediating platelet adhesion/activation and can initiate (patho)physiological thrombus formation. Conclusions: rhomb-I is implicated in the pathophysiology of Lachesis envenoming by disrupting vasculature, hemostasis and platelet aggregation through impairing vWF-GPIb axis and blocking GPVI-collagen binding.

A Multiscale Model for Shear-Mediated Platelet Adhesion Dynamics: Correlating In Silico with In Vitro Results.

Platelet adhesion to blood vessel walls is a key initial event in thrombus formation in both vascular disease processes and prosthetic cardiovascular devices. We extended a deformable multiscale model (MSM) of flowing platelets, incorporating Dissipative Particle Dynamics (DPD) and Coarse-Grained Molecular Dynamics (CGMD) describing molecular-scale intraplatelet constituents and their interaction with surrounding flow, to predict platelet adhesion dynamics under physiological flow shear stresses. Binding of platelet glycoprotein receptor Ibα (GPIbα) to von Willebrand factor (vWF) on the blood vessel wall was modeled by a molecular-level hybrid force field and validated with in vitro microchannel experiments of flowing platelets at 30 dyne/cm2. High frame rate videos of flipping platelets were analyzed with a Semi-Unsupervised Learning System (SULS) machine learning-guided imaging approach to segment platelet geometries and quantify adhesion dynamics parameters. In silico flipping dynamics followed in vitro measurements at 15 and 45dyne/cm2 with high fidelity, predicting GPIbα-vWF bonding and debonding processes, distribution of bonds strength, and providing a biomechanical insight into initiation of the complex platelet adhesion process. The adhesion model and simulation framework can be further integrated with our established MSMs of platelet activation and aggregation to simulate initial mural thrombus formation on blood vessel walls.

Antiphospholipid antibodies and the risk of thrombosis in myeloproliferative neoplasms.

The morbidity and mortality of BCR-ABL-negative myeloproliferative neoplasia (MPN) patients is highly dependent on thrombosis that may be affected by antiphospholipid antibodies (aPLA) and lupus anticoagulant. Our aim was to evaluate the association of the aPLA together with platelet receptor glycoprotein (GP) Ia/IIa c.807C>T CT/TT genotypes and thrombotic complications in patients with MPNs. The study included 108 patients with BCR-ABL-negative MPN with data of previous thrombosis. Two different screening and one confirmatory test for the lupus anticoagulant were performed. Thrombotic complications were present in 59 (54.6%) subjects. aPLA were more frequently found in MPN patients with thrombosis vs no thrombosis (25.4 and 6.1%; p = 0.007). MPN patients with arterial thrombosis were more frequently positive for aPLA vs no arterial thrombosis (38.8 and 11.9%; p = 0.001). aPLA were more frequently found in patients with cerebrovascular events vs other arterial thrombotic complications or no thrombosis, respectively (39.3, 6.1, and 12.9%; p < 0.001). MPN patients with thrombosis were more frequently positive with aPLA and had platelet receptor GP Ia/IIa c.807C>T CT/TT genotypes compared to MPN patients without thrombosis (18.6 and 2.0%; p = 0.006). aPLA alone or with coexistence with platelet receptor GP Ia/IIa c.807C>T CT/TT polymorphism could be associated with thrombotic complications in patients with MPN.

Experimental validation of computerised models of clustering of platelet glycoprotein receptors that signal via tandem SH2 domain proteins.

The clustering of platelet glycoprotein receptors with cytosolic YxxL and YxxM motifs, including GPVI, CLEC-2 and PEAR1, triggers activation via phosphorylation of the conserved tyrosine residues and recruitment of the tandem SH2 (Src homology 2) domain effector proteins, Syk and PI 3-kinase. We have modelled the clustering of these receptors with monovalent, divalent and tetravalent soluble ligands and with transmembrane ligands based on the law of mass action using ordinary differential equations and agent-based modelling. The models were experimentally evaluated in platelets and transfected cell lines using monovalent and multivalent ligands, including novel nanobody-based divalent and tetravalent ligands, by fluorescence correlation spectroscopy. Ligand valency, receptor number, receptor dimerisation, receptor phosphorylation and a cytosolic tandem SH2 domain protein act in synergy to drive receptor clustering. Threshold concentrations of a CLEC-2-blocking antibody and Syk inhibitor act in synergy to block platelet aggregation. This offers a strategy for countering the effect of avidity of multivalent ligands and in limiting off-target effects.

DNA Polymorphisms in Pregnant Women with Sticky Platelet Syndrome.

Sticky platelet syndrome (SPS) is a thrombophilia caused by the increased aggregability of platelets in response to the addition of low concentrations of epinephrine (EPI) and/or adenosine diphosphate (ADP). Some of the single nucleotide polymorphisms (SNP), alleles and haplotypes of platelet glycoprotein receptors were proved to have a role in the etiology of thrombotic episodes When comparing SPS and the control group, in VEGFA rs3025039, the p value for both CC vs. TT and CT vs. TT analyses was &lt;0.001. Interestingly, no minor TT genotype was present in the SPS group, suggesting the thrombotic pathogenesis of recurrent spontaneous abortions (RSA) in these patients. Moreover, we found a significant difference in the presence of AT containing a risky A allele and TT genotype of ALPP rs13026692 (p = 0.034) in SPS patients when compared with the controls. Additionally, we detected a decreased frequency of the GG (CC) genotype of FOXP3 rs3761548 in patients with SPS and RSA when compared with the control group (p value for the CC (GG) vs. AA (TT) 0.021). This might indicate an evolutionary protective mechanism of the A (T) allele in the SPS group against thrombotic complications in pregnancy. These results can be used for antithrombotic management in such pregnant patients.

Improving treatment for acute ischemic stroke-Clot busting innovation in the pipeline.

Acute ischemic stroke is a consequence of disrupted blood flow to the brain, caused by thrombosis—the pathological formation of occlusive clots within blood vessels, which can embolize distally to downstream tissues and microvasculature. The highest priority of stroke treatment is the rapid removal of occlusive clots and restoration of tissue perfusion. Intravenous thrombolysis is the pharmacological standard-of-care for the dissolution of blood clots, wherein thrombolytic drugs are administered to restore vessel patency. While the introduction of recombinant tissue-plasminogen activator (rtPA) in 1996 demonstrated the benefit of acute thrombolysis for clot removal, this was countered by severe limitations in terms of patient eligibility, lytic efficacy, rethrombosis and safety implications. Development of safer and efficacious treatment strategies to improve clot lysis has not significantly progressed over many decades, due to the challenge of maintaining the necessary efficacy-safety balance for these therapies. As such, rtPA has remained the sole approved acute therapeutic for ischemic stroke for over 25 years. Attempts to improve thrombolysis with coadministration of adjunct antithrombotics has demonstrated benefit in coronary vessels, but remain contraindicated for stroke, given all currently approved antithrombotics adversely impact hemostasis, causing bleeding. This Perspective provides a brief history of stroke drug development, as well as an overview of several groups of emerging drugs which have the potential to improve thrombolytic strategies in the future. These include inhibitors of the platelet receptor glycoprotein VI and the signaling enzyme PI3-Kinase, novel anticoagulants derived from hematophagous creatures, and proteolysis-targeting chimeras.

Platelet depletion does not alter long-term functional outcome after cerebral ischaemia in mice

Platelets are key mediators of thrombus formation and inflammation during the acute phase of ischaemic stroke. Particularly, the platelet glycoprotein (GP) receptors GPIbα and GPVI have been shown to mediate platelet adhesion and activation in the ischaemic brain. GPIbα and GPVI blockade could reduce infarct volumes and improve functional outcome in mouse models of acute ischaemic stroke, without concomitantly increasing intracerebral haemorrhage. However, the functional role of platelets during long-term stroke recovery has not been elucidated so far.Thus, we here examined the impact of platelet depletion on post-stroke recovery after transient middle cerebral artery occlusion (tMCAO) in adult male mice. Platelet depleting antibodies or isotype control were applied from day 3–28 after tMCAO in mice matched for infarct size. Long-term functional recovery was assessed over the course of 28 days by behavioural testing encompassing motor and sensorimotorical functions, as well as anxiety-like or spontaneous behaviour. Whole brain flow cytometry and light sheet fluorescent microscopy were used to identify resident and infiltrated immune cell types, and to determine the effects of platelet depletion on the cerebral vascular architecture, respectively.We found that delayed platelet depletion does not improve long-term functional outcome in the tMCAO stroke model. Immune cell abundance, the extent of thrombosis and the organisation of the cerebral vasculature were also comparable between platelet-depleted and control mice. Our study demonstrates that, despite their critical role in the acute stroke setting, platelets appear to contribute only marginally to tissue reorganisation and functional recovery at later stroke stages.

Antithrombotic Effects of Fostamatinib in Combination with Conventional Antiplatelet Drugs.

New antithrombotic medications with less effect on haemostasis are needed for the long-term treatment of acute coronary syndromes (ACS). The platelet receptor glycoprotein VI (GPVI) is critical in atherothrombosis, mediating platelet activation at atherosclerotic plaque. The inhibition of spleen tyrosine kinase (Syk) has been shown to block GPVI-mediated platelet function. The aim of our study was to investigate if the Syk inhibitor fostamatinib could be repurposed as an antiplatelet drug, either alone or in combination with conventional antiplatelet therapy. The effect of the active metabolite of fostamatinib (R406) was assessed on platelet activation and function induced by atherosclerotic plaque and a range of agonists in the presence and absence of the commonly used antiplatelet agents aspirin and ticagrelor. The effects were determined ex vivo using blood from healthy volunteers and aspirin- and ticagrelor-treated patients with ACS. Fostamatinib was also assessed in murine models of thrombosis. R406 mildly inhibited platelet responses induced by atherosclerotic plaque homogenate, likely due to GPVI inhibition. The anti-GPVI effects of R406 were amplified by the commonly-used antiplatelet medications aspirin and ticagrelor; however, the effects of R406 were concentration-dependent and diminished in the presence of plasma proteins, which may explain why fostamatinib did not significantly inhibit thrombosis in murine models. For the first time, we demonstrate that the Syk inhibitor R406 provides mild inhibition of platelet responses induced by atherosclerotic plaque and that this is mildly amplified by aspirin and ticagrelor.

Thrombocytopenia and splenic platelet-directed immune responses after IV ChAdOx1 nCov-19 administration

AbstractVaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are based on a range of novel platforms, with adenovirus-based approaches (like ChAdOx1 nCov-19) being one of them. Recently, a novel complication of SARS-CoV-2–targeted adenovirus vaccines has emerged: immune thrombocytopenia, either isolated, or accompanied by thrombosis (then termed VITT). This complication is characterized by low platelet counts, and in the case of VITT, also by platelet-activating platelet factor 4 antibodies reminiscent of heparin-induced thrombocytopenia, leading to a prothrombotic state with clot formation at unusual anatomic sites. Here, we detected antiplatelet antibodies targeting platelet glycoprotein receptors in 30% of patients with proven VITT (n = 27) and 42% of patients with isolated thrombocytopenia after ChAdOx1 nCov-19 vaccination (n = 26), indicating broad antiplatelet autoimmunity in these clinical entities. We use in vitro and in vivo models to characterize possible mechanisms of these platelet-targeted autoimmune responses leading to thrombocytopenia. We show that IV but not intramuscular injection of ChAdOx1 nCov-19 triggers platelet-adenovirus aggregate formation and platelet activation in mice. After IV injection, these aggregates are phagocytosed by macrophages in the spleen, and platelet remnants are found in the marginal zone and follicles. This is followed by a pronounced B-cell response with the emergence of circulating antibodies binding to platelets. Our work contributes to the understanding of platelet-associated complications after ChAdOx1 nCov-19 administration and highlights accidental IV injection as a potential mechanism of platelet-targeted autoimmunity. Hence, preventing IV injection when administering adenovirus-based vaccines could be a potential measure against platelet-associated pathologies after vaccination.

Downregulation of ADAM17 in pediatric immune thrombocytopenia impairs proplatelet formation

BackgroundImmune thrombocytopenia (ITP) is the most common etiology of acquired thrombocytopenia diseases in children. ITP is characterized by the immune-mediated decreased formation and excessive destruction of platelets. The pathogenesis and management of pediatric ITP are distinct from adult ITP. A disintegrin and metalloproteinase 17 (ADAM17) mediates the shedding of platelet receptor glycoprotein Ib α (GPIb α) in extracellular domain, functioning in the platelet activation and clearance. Our study aims to probe the roles and mechanisms of ADAM17 in pediatric ITP.MethodsThe differently expressed ADAM17 in megakaryocytes was obtained from children with ITP through the next-generation RNA-Sequence. Hematoxylin-eosin and Giemsa staining were performed for cell morphology identification. Flow cytometry was applied to assess autoantibodies against platelets, subtypes of lymphocytes, the surface expression level of ADAM17 and polyploidization of megakaryocytes, as well as the full-length GP Ib α.ResultsADAM17 was significantly downregulated in megakaryocytes and platelets in children with ITP. Higher values of PDW and positive autoantibodies presence were observed in children with ITP. Loss of ADAM17 in mice led to defects in proplatelet formation and significantly elevated expression of phosphorylated myosin light chain (p-MLC) in megakaryocytes.ConclusionsOur study indicated that the downregulation of ADAM17 might be an innate cause of inefficient platelet production in pediatric ITP.

Efficacy and safety of a bridging strategy that uses intravenous platelet glycoprotein receptor inhibitors for patients undergoing surgery after coronary stent implantation: a meta-analysis

BackgroundCurrent guidelines indicate we can consider a bridging strategy that uses intravenous, reversible glycoprotein inhibitors for patients that required surgery following recent stent implantation. However, no strong clinical evidence exists that demonstrates the efficacy and safety of this treatment. Therefore, in this study, the efficacy and safety of a bridging strategy that uses intravenous platelet glycoprotein receptor inhibitors will be evaluated.MethodsA meta-analysis was performed on preoperative bridging studies in patients undergoing coronary stent surgery. The primary outcome was the success rate of no major adverse cardiovascular events (MACE). The secondary outcomes were the success rate of no reoperations to stop bleeding.ResultsA total of 10 studies that included 382 patients were used in this meta-analysis. For the primary endpoint, the success rate was 97.7% (95% CI 94.4–98.0%) for glycoprotein IIb/IIIa inhibitors, 98.8% (95% CI 96.0–100%) for tirofiban (6 studies) and 95.8% (95% CI 90.4–99.4%) for eptifibatide (4 studies). For secondary endpoints, the success rate was 98.0% (95% CI 94.8–99.9%) for glycoprotein IIb/IIIa inhibitors, 99.7% (95% CI 97.1–100%) for tirofiban (5 studies), and 95.3% (95% CI 88.5–99.4%) for eptifibatide (4 studies).ConclusionThe results of this study showed that the use of intravenous platelet glycoprotein IIb/IIIa inhibitors as a bridging strategy might be safe and effective for patients undergoing coronary stent implantation that require surgery soon after.

Structural insights into collagen binding by platelet receptor glycoprotein VI

AbstractGlycoprotein VI (GPVI) mediates collagen-induced platelet activation after vascular damage and is an important contributor to the onset of thrombosis, heart attack, and stroke. Animal models of thrombosis have identified GPVI as a promising target for antithrombotic therapy. Although for many years the crystal structure of GPVI has been known, the essential details of its interaction with collagen have remained elusive. Here, we present crystal structures of the GPVI ectodomain bound to triple-helical collagen peptides, which reveal a collagen-binding site across the β-sheet of the D1 domain. Mutagenesis and binding studies confirm the observed binding site and identify Trp76, Arg38, and Glu40 as essential residues for binding to fibrillar collagens and collagen-related peptides (CRPs). GPVI binds a site on collagen comprising two collagen chains with the core formed by the sequence motif OGPOGP. Potent GPVI-binding peptides from Toolkit-III all contain OGPOGP; weaker binding peptides frequently contain a partial motif varying at either terminus. Alanine-scanning of peptide III-30 also identified two AGPOGP motifs that contribute to GPVI binding, but steric hindrance between GPVI molecules restricts the maximum binding capacity. We further show that no cooperative interactions could occur between two GPVI monomers binding to a stretch of (GPO)5 and that binding of ≥2 GPVI molecules to a fibril-embedded helix requires non-overlapping OGPOGP motifs. Our structure confirms the previously suggested similarity in collagen binding between GPVI and leukocyte-associated immunoglobulin-like receptor 1 (LAIR-1) but also indicates significant differences that may be exploited for the development of receptor-specific therapeutics.

Hydrogen-deuterium exchange of plasma-derived von Willebrand factor reveals similar dynamics to isolated A1 domain and autoinhibitory module

Von Willebrand factor (VWF) is a large multimeric, multidomain glycoprotein that circulates in the bloodstream and is essential for mediating platelet adhesion at sites of vascular injury through its A1 domain. Our lab has recently provided detailed characterizations of the autoinhibitory module (AIM), the discontinuous flanking regions that cooperatively shield the A1 domain and regulate its binding to cognate platelet receptor glycoprotein (GP) Ibα. While our recent analyses have described the mechanical and biochemical properties of AIM-A1 as an independent functional unit, the impact of the neighboring D’D3 and A2 domains on AIM dynamics remain unclear.