Glycoprotein Ib-IX-V

Platelet Physiology

Platelets are the smallest blood cells, numbering 150 to 350 × 109/L in healthy individuals. The ability of activated platelets to adhere to an injured vessel wall and form aggregates was first described in the 19th century. Besides their long-established roles in thrombosis and hemostasis, platelets are increasingly recognized as pivotal players in numerous other pathophysiological processes including inflammation and atherogenesis, antimicrobial host defense, and tumor growth and metastasis. Consequently, profound knowledge of platelet structure and function is becoming more important in research and in many fields of modern medicine. This review provides an overview of platelet physiology focusing particularly on the structure, granules, surface glycoproteins, and activation pathways of platelets.

Bernard-Soulier syndrome caused by a novel GP1BB variant and 22q11.2 deletion

Bernard-Soulier syndrome (BSS) is caused by defects in GP1BA, GP1BB, or GP9 genes. Patients with 22q11.2 deletion syndrome (22q11.2DS) are obligate carriers of BSS because GP1BB resides on chromosome 22q11.2. A 15-month-old girl without bleeding symptoms had giant platelets and thrombocytopenia. Physical findings and macrothrombocytopenia suggested 22q11.2DS, which was confirmed by fluorescence in situ hybridization. Flow cytometry showed decreased GPIbα on the platelets. Gene panel testing revealed a novel variant in GP1BB, p.(Val169_Leu172del). These findings confirmed that the patient had BSS. This case suggests that any patient with 22q11.2DS and macrothrombocytopenia should be further tested for BSS.

A Perspective on How Fibrinaloid Microclots and Platelet Pathology May be Applied in Clinical Investigations

Microscopy imaging has enabled us to establish the presence of fibrin(ogen) amyloid (fibrinaloid) microclots in a range of chronic, inflammatory diseases. Microclots may also be induced by a variety of purified substances, often at very low concentrations. These molecules include bacterial inflammagens, serum amyloid A, and the S1 spike protein of severe acute respiratory syndrome coronavirus 2. Here, we explore which of the properties of these microclots might be used to contribute to differential clinical diagnoses and prognoses of the various diseases with which they may be associated. Such properties include distributions in their size and number before and after the addition of exogenous thrombin, their spectral properties, the diameter of the fibers of which they are made, their resistance to proteolysis by various proteases, their cross-seeding ability, and the concentration dependence of their ability to bind small molecules including fluorogenic amyloid stains. Measuring these microclot parameters, together with microscopy imaging itself, along with methodologies like proteomics and imaging flow cytometry, as well as more conventional assays such as those for cytokines, might open up the possibility of a much finer use of these microclot properties in generative methods for a future where personalized medicine will be standard procedures in all clotting pathology disease diagnoses.

Binding Mechanism between Platelet Glycoprotein and Cyclic Peptide Elucidated by McMD-Based Dynamic Docking

The cyclic peptide OS1 (amino acid sequence: CTERMALHNLC), which has a disulfide bond between both termini cysteine residues, inhibits complex formation between the platelet glycoprotein Ibα (GPIbα) and the von Willebrand factor (vWF) by forming a complex with GPIbα. To study the binding mechanism between GPIbα and OS1 and, therefore, the inhibition mechanism of the protein-protein GPIbα-vWF complex, we have applied our multicanonical molecular dynamics (McMD)-based dynamic docking protocol starting from the unbound state of the peptide. Our simulations have reproduced the experimental complex structure, although the top-ranking structure was an intermediary one, where the peptide was bound in the same location as in the experimental structure; however, the β-switch of GPIbα attained a different conformation. Our analysis showed that subsequent refolding of the β-switch results in a more stable binding configuration, although the transition to the native configuration appears to take some time, during which OS1 could dissociate. Our results show that conformational changes in the β-switch are crucial for successful binding of OS1. Furthermore, we identified several allosteric binding sites of GPIbα that might also interfere with vWF binding, and optimization of the peptide to target these allosteric sites might lead to a more effective inhibitor, as these are not dependent on the β-switch conformation.

Force balance ratio is a robust predictor of arterial thrombus stability

Thrombus formation on a damaged vessel wall can lead to the formation of a stable occlusive/subocclusive clot or unstable embolizing thrombus. Both outcomes can cause significant health damage. The mechanisms that regulate maximum thrombus size, its stability, and embolization in both micro- and macrocirculation are poorly understood. To investigate the impact of flow and intrathrombus forces on the stability of homogeneous and heterogeneous platelet thrombi in a wide range of thrombus geometries, critical interplatelet forces, vessel diameters, and hydrodynamic conditions, we took advantage of the recently developed in silico models. To perform analysis of thrombus stability/embolization in arterioles, we used our previously developed particle-based 2D model with a single-platelet resolution. Its results and predictions were further extended to a 3D case and the large spatial scales of arteries using novel particle-based and continuum 3D models. We found a robust quantitative parameter, termed force balance ratio, which quantifies the balance between destabilizing hydrodynamic and stabilizing interplatelet forces. This parameter predicts whether a homogeneous thrombus (or the shell of a heterogeneous thrombus) with a particular value of critical interplatelet forces will embolize under given hydrodynamic conditions. Our simulations also predict that, for a given magnitude of critical interplatelet forces, the longer thrombi are more stable than the shorter ones. Furthermore, the aggregates formed on top of the severe stenosis are more stable than thrombi formed at moderate stenosis. Taken together, our results give new insights into the interplay between critical interplatelet forces, local hydrodynamics, and overall thrombus stability against the flow.

Engineering Platelet Membrane Imitating Nanoparticles for Targeted Therapeutic Delivery

Platelet Membrane Imitating Nanoparticles (PMINs) is a novel drug delivery system that imitates the structure and functionality of platelet membranes. PMINs imitate surface markers of platelets to target specific cells and transport therapeutic cargo. PMINs are engineered by incorporating the drug into the platelet membrane and encapsulating it in a nanoparticle scaffold. This allows PMINs to circulate in the bloodstream and bind to target cells with high specificity, reducing off-target effects and improving therapeutic efficacy. The engineering of PMINs entails several stages, including the separation and purification of platelet membranes, the integration of therapeutic cargo into the membrane, and the encapsulation of the membrane in a nanoparticle scaffold. In addition to being involved in a few pathological conditions including cancer, atherosclerosis, and rheumatoid arthritis, platelets are crucial to the body's physiological processes. This study includes the preparation and characterization of platelet membrane-like nanoparticles and focuses on their most recent advancements in targeted therapy for conditions, including cancer, immunological disorders, atherosclerosis, phototherapy, etc. PMINs are a potential drug delivery system that combines the advantages of platelet membranes with nanoparticles. The capacity to create PMMNs with particular therapeutic cargo and surface markers provides new possibilities for targeted medication administration and might completely change the way that medicine is practiced. Despite the need for more studies to optimize the engineering process and evaluate the effectiveness and safety of PMINs in clinical trials, this technology has a lot of potential.

The Cellular and Protein Arms of Coagulation in Diabetes: Established and Potential Targets for the Reduction of Thrombotic Risk

Diabetes is a metabolic condition with a rising global prevalence and is characterised by abnormally high blood glucose levels. Cardiovascular disease (CVD) accounts for the majority of deaths in diabetes and, despite improvements in therapy, mortality and hospitalisations in this cohort remain disproportionally higher compared to individuals with normal glucose metabolism. One mechanism for increased CVD risk is enhanced thrombosis potential, due to altered function of the cellular and acellular arms of coagulation. Different mechanisms have been identified that mediate disordered blood clot formation and breakdown in diabetes, including dysglycaemia, insulin resistance, and metabolic co-morbidities. Collectively, these induce platelet/endothelial dysfunction and impair the fibrinolytic process, thus creating a prothrombotic milieu. Despite these abnormalities, current antithrombotic therapies are largely similar in diabetes compared to those without this condition, which explains the high proportion of patients experiencing treatment failure while also displaying an increased risk of bleeding events. In this narrative review, we aimed to summarise the physiological functioning of haemostasis followed by the pathological effects of diabetes mellitus on platelets and the fibrin network. Moreover, we carefully reviewed the literature to describe the current and future therapeutic targets to lower the thrombosis risk and improve vascular outcomes in diabetes.

Platelet Glycoprotein-Ib (GPIb) May Serve as a Bridge between Type 2 Diabetes Mellitus (T2DM) and Atherosclerosis, Making It a Potential Target for Antiplatelet Agents in T2DM Patients

Type 2 diabetes mellitus (T2DM) is a persistent metabolic condition that contributes to the development of cardiovascular diseases. Numerous studies have provided evidence that individuals with T2DM are at a greater risk of developing cardiovascular diseases, typically two to four times more likely than those without T2DM, mainly due to an increased risk of atherosclerosis. The rupture of an atherosclerotic plaque leading to pathological thrombosis is commonly recognized as a significant factor in advancing cardiovascular diseases caused by TD2M, with platelets inducing the impact of plaque rupture in established atherosclerosis and predisposing to the primary expansion of atherosclerosis. Studies suggest that individuals with T2DM have platelets that display higher baseline activation and reactivity than those without the condition. The expression enhancement of several platelet receptors is known to regulate platelet activation signaling, including platelet glycoprotein-Ib (GPIb). Furthermore, the high expression of platelet GP1b has been reported to increase the risk of platelet adhesion, platelet-leucocyte interaction, and thrombo-inflammatory pathology. However, the study exploring the role of GP1b in promoting platelet activation-induced cardiovascular diseases in T2DM patients is still limited. Therefore, we summarize the important findings regarding pathophysiological continuity between T2DM, platelet GPIb, and atherosclerosis and highlight the potential therapy targeting GPIb as a novel antiplatelet agent for preventing further cardiovascular incidents in TD2M patients.

Shedding Light on the Cell Biology of Platelet-Derived Extracellular Vesicles and Their Biomedical Applications

EVs are membranous subcellular structures originating from various cells, including platelets which consist of biomolecules that can modify the target cell's pathophysiological functions including inflammation, cell communication, coagulation, and metastasis. EVs, which are known to allow the transmission of a wide range of molecules between cells, are gaining popularity in the fields of subcellular treatment, regenerative medicine, and drug delivery. PEVs are the most abundant EVs in circulation, being produced by platelet activation, and are considered to have a significant role in coagulation. PEV cargo is extremely diverse, containing lipids, proteins, nucleic acids, and organelles depending on the condition that induced their release and can regulate a wide range of biological activities. PEVs, unlike platelets, can overcome tissue barriers, allowing platelet-derived contents to be transferred to target cells and organs that platelets cannot reach. Their isolation, characterization, and therapeutic efficacy, on the other hand, are poorly understood. This review summarizes the technical elements of PEV isolation and characterization methods as well as the pathophysiological role of PEVs, including therapeutic potential and translational possibility in diverse disciplines.

Immune-mediated platelet depletion augments Alzheimer's disease neuropathological hallmarks in APP-PS1 mice

In Alzheimer's disease (AD), platelets become dysfunctional and might contribute to amyloid beta deposition. Here, we depleted platelets in one-year-old APP Swedish PS1 dE9 (APP-PS1) transgenic mice for five days, using intraperitoneal injections of an anti-CD42b antibody, and assessed changes in cerebral amyloidosis, plaque-associated neuritic dystrophy and gliosis. In APP-PS1 female mice, platelet depletion shifted amyloid plaque size distribution towards bigger plaques and increased neuritic dystrophy in the hippocampus. In platelet-depleted females, plaque-associated Iba1+ microglia had lower amounts of fibrillar amyloid beta cargo and GFAP+ astrocytic processes showed a higher overlap with thioflavin S+ amyloid plaques. In contrast to the popular hypothesis that platelets foster plaque pathology, our data suggest that platelets might limit plaque growth and attenuate plaque-related neuritic dystrophy at advanced stages of amyloid plaque pathology in APP-PS1 female mice. Whether the changes in amyloid plaque pathology are due to a direct effect on amyloid beta deposition or are a consequence of altered glial function needs to be further elucidated.

Emerging Roles of the Unique Molecular Chaperone Cosmc in the Regulation of Health and Disease

The core-1 β1-3galactosyltransferase-specific chaperone 1 (Cosmc) is a unique molecular chaperone of core-1 β1-3galactosyltransferase(C1GALT1), which typically functions inside the endoplasmic reticulum (ER). Cosmc helps C1GALT1 to fold correctly and maintain activity. It also participates in the synthesis of the T antigen, O-glycan, together with C1GALT1. Cosmc is a multifaceted molecule with a wide range of roles and functions. It involves platelet production and the regulation of immune cell function. Besides that, the loss of function of Cosmc also facilitates the development of several diseases, such as inflammation diseases, immune-mediated diseases, and cancer. It suggests that Cosmc is a critical control point in diseases and that it should be regarded as a potential target for oncotherapy. It is essential to fully comprehend Cosmc's roles, as they may provide critical information about its involvement in disease development and pathogenesis. In this review, we summarize the recent progress in understanding the role of Cosmc in normal development and diseases.

Glycoprotein Ib-regulated micro platelet ghost for biosafe distribution and photothermal oncotherapy

Despite the tremendous theranostics potential of nano-scale drug delivery system (NDDS) in oncology field, their tumor-targeting efficiency and safety remain major challenges due to their proneness of off-target accumulation through widespread vascular endothelial gaps (up to 1 μm). To address this problem, in this research, micro-sized cellular platelet "ghosts" (PGs, 1.32 μm, platelet without inner granules and coagulation) were employed as carriers to ship hollow gold nanoparticles (HGNs, 58.7 nm), forming a hierarchical biosafe system (PG@HGNs) to reduce normal tissue interception and enhance tumor targeting delivery of HGNs for improved photothermal therapy. PGs were prepared by an optimized "swelling-extrusion-elution" method, HGNs were loaded in PGs (PG@HGNs) through a "hypotonic dialysis" method and the safety and biodistribution of the system was evaluated in vitro and in vivo. In in vitro condition that stimulated the tumoral vessel acidic microenvironment (pH = 6.5), PG@HGNs were demonstrated with enhanced membrane fluidity through down-regulation of the glycoprotein Ib expressed on the PGs. This change induced a burst release of nano-sized HGNs which were capable to traverse vascular endothelium layer on a tumor-endothelial cell transwell model, whilst the micro-sized PG carriers were intercepted. In comparison to nano-sized platelet membrane-coated carriers (PM@HGNs), PG@HGNs showed enhanced internalization and cytotoxicity to 4T1 cells. In animal models, PG@HGNs remarkably prolonged circulation most likely due to the presence of "self-recognition" receptor-CD47 of PGs, and effectively reduced normal tissue interception via the micro-scale size effect. These both contributed to the significantly improved tumor targeting efficiency of HGNs. PG@HGNs generated the greater antitumor photothermal efficacy alongside safety in the animals compared to PM@HGNs. Collectively, this study demonstrated the potential of the micro-scale PGs equipped with adjusted membrane GP Ib as biosafe vehicles for HGNs or possibly other nanodrugs. THE STATEMENT OF SIGNIFICANCE: Despite the tremendous theranostics potentials, the safety and tumor-targeting efficiency of nano-scale drug delivery systems (NDDS) are compromised by their undesirable accumulation in normal tissues with widespread vascular endothelial gaps, such as many tumor-targeted NDDSs still accumulated much in liver and/or spleen. Herein, we explored a micro-nano biomimetic cascade delivery system to address the above drawbacks. By forming a hierarchical biosafe system, micro-sized platelet "ghost" (PGs, 1.32 μm) was employed as tumor-targeted delivery carrier to transport hollow gold nanoparticles (HGNs, 58.7 nm). It was demonstrated that this micro-size system could maintain platelet membrane structure thus prolong in vivo circulation, while avoiding extravasation into normal tissues. PG@HGNs could sensitively respond to the acidic microenvironment near tumor vessel via down-regulation of glycoprotein Ib and rapidly release "nano-bullets"-HGNs to further penetrate into the tumor tissues through EPR effect, thus enhancing photothermal efficacy generated by HGNs under NIR irradiation. Collectively, the micro-scaled PGs could be biosafe vehicles for improved tumor-targeted delivery of HGNs or possibly other nanodrugs.

An Insight into Recent Advances on Platelet Function in Health and Disease

Platelets play a variety of roles in vascular biology and are best recognized as primary hemostasis and thrombosis mediators. Platelets have a large number of receptors and secretory molecules that are required for platelet functionality. Upon activation, platelets release multiple substances that have the ability to influence both physiological and pathophysiological processes including inflammation, tissue regeneration and repair, cancer progression, and spreading. The involvement of platelets in the progression and seriousness of a variety of disorders other than thrombosis is still being discovered, especially in the areas of inflammation and the immunological response. This review represents an integrated summary of recent advances on the function of platelets in pathophysiology that connects hemostasis, inflammation, and immunological response in health and disease and suggests that antiplatelet treatment might be used for more than only thrombosis.

Dual-target synergistic antithrombotic mechanism of a Dabigatran etexilate analogue (HY023016)

Thrombin has long been considered a desirable antithrombotic target, but anti-thrombin therapy without anti-platelet therapy has never achieved the ideal effect. HY023016, derived from dabigatran etexilate, exhibited a potent antithrombotic efficacy. In the present study, mechanisms underlying this effect were explored. HY023016 strongly decreased the binding of thrombin to recombinant GPIbα N-terminal sequence which was confirmed by surface plasmon resonance. Flow cytometry revealed that HY023016 selectively decreased the binding of antibody to GPIbα and inhibited the washed human platelet aggregation induced by thrombin. Fluorescence experiment showed that HY023016 remarkably inhibited exosite II by a loss of affinity for the γ'-peptide of fibrinogen. Using intravital microscopy, we observed and recorded the dynamic process of thrombus formation and found that HY023016 effectively prevented thrombus formation in rat arteriovenous shunt thrombosis model. On the basis of these findings, we propose that HY023016 provides a novel insight into the antithrombotic mechanism, which exerts synergistic anticoagulant and antiplatelet effects through thrombin and GPIbα.

Development of antithrombotic peptides based on the molecular interactions between von Willebrand factor and GPIbα

Binding of platelets on vascular endothelia at the damaged site using von Willebrand factor (vWF) as a bridge is of great significance for platelet adhesion and subsequent arterial thrombosis. Molecular...

Imaging of Intracellular and Plasma Membrane Pools of PI(4,5)P2 and PI4P in Human Platelets

Phosphoinositides (PIs) are phosphorylated membrane lipids that have a plethora of roles in the cell, including vesicle trafficking, signaling, and actin reorganization. The most abundant PIs in the cell are phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] and phosphatidylinositol-4-monophosphate (PI4P). The localization and roles of both PI(4,5)P2 and PI4P are well established, is the broadly accepted methodological approach for their immunocytochemical visualization in different cell compartments in several cell lines. However, not much is known about these PIs in platelets (PLTs), the smallest blood cells that detect vessel wall injury, activate, and stop the bleeding. Therefore, we sought to investigate the localization of PI(4,5)P2 and PI4P in resting and activated PLTs by antibody staining. Here, we show that the intracellular pools of PI(4,5)P2 and PI4P can be detected by the established staining protocol, and these pools can be modulated by inhibitors of OCRL phosphatase and PI4KIIIα kinase. However, although resting PLTs readily stain for the plasma membrane (PM) pools of PI(4,5)P2 and PI4P, just a few activated cells were stained with the established protocol. We show that optimized protocol allows for the visualization of PI(4,5)P2 and PI4P at PM in activated PLTs, which could also be modulated by OCRL and PI4KIIIα inhibitors. We conclude that PI(4,5)P2 and PI4P are more sensitive to lipid extraction by permeabilizing agents in activated than in resting human PLTs, which suggests their different roles during PLT activation.

Modeling Thrombus Shell: Linking Adhesion Receptor Properties and Macroscopic Dynamics

Damage to arterial vessel walls leads to the formation of platelet aggregate, which acts as a physical obstacle for bleeding. An arterial thrombus is heterogeneous; it has a dense inner part (core) and an unstable outer part (shell). The thrombus shell is very dynamic, being composed of loosely connected discoid platelets. The mechanisms underlying the observed mobility of the shell and its (patho)physiological implications are unclear. To investigate arterial thrombus mechanics, we developed a novel, to our knowledge, two-dimensional particle-based computational model of microvessel thrombosis. The model considers two types of interplatelet interactions: primary reversible (glycoprotein Ib (GPIb)-mediated) and stronger integrin-mediated interaction, which intensifies with platelet activation. At high shear rates, the former interaction leads to adhesion, and the latter is primarily responsible for stable platelet aggregation. Using a stochastic model of GPIb-mediated interaction, we initially reproduced experimental curves that characterize individual platelet interactions with a von Willebrand factor-coated surface. The addition of the second stabilizing interaction results in thrombus formation. The comparison of thrombus dynamics with experimental data allowed us to estimate the magnitude of critical interplatelet forces in the thrombus shell and the characteristic time of platelet activation. The model predicts moderate dependence of maximal thrombus height on the injury size in the absence of thrombin activity. We demonstrate that the developed stochastic model reproduces the observed highly dynamic behavior of the thrombus shell. The presence of primary stochastic interaction between platelets leads to the properties of thrombus consistent with in vivo findings; it does not grow upstream of the injury site and covers the whole injury from the first seconds of the formation. А simplified model, in which GPIb-mediated interaction is deterministic, does not reproduce these features. Thus, the stochasticity of platelet interactions is critical for thrombus plasticity, suggesting that interaction via a small number of bonds drives the dynamics of arterial thrombus shell.

Actin polymerization regulates glycoprotein Ibα shedding

Glycoprotein (GP) Ibα shedding mediated by ADAM17 (a disintegrin and metalloproteinase 17) plays an important role in negatively regulating platelet function and thrombus formation. However, the mechanism of GPIbα shedding remains elusive. Here, we show that jasplakinolide (an actin-polymerizing peptide)-induced actin polymerization results in GPIbα shedding and impairs platelet function. Thrombin and A23187-induced GPIbα shedding is increased by jasplakinolide; in contrast, GPIbα shedding is reduced by a depolymerization regent (cytochalasin B). We find that actin polymerization activates calpain leading to filamin A hydrolyzation. We further demonstrate that the interaction of filamin A with the cytoplasmic domain of GPIbα plays a critical role in regulating actin polymerization-induced GPIbα shedding. Taken together, these data demonstrate that actin polymerization regulates ADAM17-mediated GPIbα shedding, suggesting a novel strategy to negatively regulate platelet function.

Platelets, Constant and Cooperative Companions of Sessile and Disseminating Tumor Cells, Crucially Contribute to the Tumor Microenvironment

Although platelets and the coagulation factors are components of the blood system, they become part of and contribute to the tumor microenvironment (TME) not only within a solid tumor mass, but also within a hematogenous micrometastasis on its way through the blood stream to the metastatic niche. The latter basically consists of blood-borne cancer cells which are in close association with platelets. At the site of the primary tumor, the blood components reach the TME via leaky blood vessels, whose permeability is increased by tumor-secreted growth factors, by incomplete angiogenic sprouts or by vasculogenic mimicry (VM) vessels. As a consequence, platelets reach the primary tumor via several cell adhesion molecules (CAMs). Moreover, clotting factor VII from the blood associates with tissue factor (TF) that is abundantly expressed on cancer cells. This extrinsic tenase complex turns on the coagulation cascade, which encompasses the activation of thrombin and conversion of soluble fibrinogen into insoluble fibrin. The presence of platelets and their release of growth factors, as well as fibrin deposition changes the TME of a solid tumor mass substantially, thereby promoting tumor progression. Disseminating cancer cells that circulate in the blood stream also recruit platelets, primarily by direct cell-cell interactions via different receptor-counterreceptor pairs and indirectly by fibrin, which bridges the two cell types via different integrin receptors. These tumor cell-platelet aggregates are hematogenous micrometastases, in which platelets and fibrin constitute a particular TME in favor of the cancer cells. Even at the distant site of settlement, the accompanying platelets help the tumor cell to attach and to grow into metastases. Understanding the close liaison of cancer cells with platelets and coagulation factors that change the TME during tumor progression and spreading will help to curb different steps of the metastatic cascade and may help to reduce tumor-induced thrombosis.

Predictive Value of Kozak Gene Polymorphism for Thrombosis in Patients with Philadelphia-Negative MPNs

Background:

Philadelphia-negative myeloproliferative neoplasms (MPNs) including polycythemia vera (PV), essential thrombocythemia (ET) and myelofibrosis are clonal haematopoietic stem cell disorders characterized by dysregulated proliferation. The arterial and venous thromboses are the major causes of morbidity and mortality in MPNs. The platelet GP Ib-IX-V receptor complex plays an important role in thrombus formation as the Kozak sequence polymorphism of platelet GP Ibα is associated with increased receptor density.

Materials and Methods:

This study was conducted on 286 diagnosed patients with Ph-negative MPNs (94 patients of PV, 102 of ET and 90 of MF). In addition, 107 apparently healthy individuals served as a control group.

Results:

This study revealed that by taking rs2243093 TT as the reference genotype and T as the reference allele; TC, CC, TC+CC genotypes showed lower frequency in ET patients (p= 0.005, 0.007 and 0.001 respectively) and MF patients (p= 0.002, 0.047 and 0.001 respectively) when compared to control groups also, C allele in both groups compared to control (p ≤ 0.001 both). CC genotypes and C allele showed lower frequency in PV patients when compared to control groups (p= 0.032 and 0.026 respectively).

Conclusion:

From this study we could conclude that patients with Philadelphia-negative MPNs carried Kozak gene polymorphism significantly TT genotype in all patients PV, ET, MF patients and TC in ET and MF patients. The platelet glycoprotein Ibα (Kozak) gene could be incorporated into the routine workup to predict venous thrombosis in patients with Ph-negative MPNs specially ET patients.

A novel mutation in GP1BA gene in a family with autosomal dominant Bernard Soulier syndrome variant: A case report

Classic Bernard-Soulier syndrome (BSS) is a rare form of autosomal recessive disorder that is caused by mutations in the GP1BA gene that encode the GPIb-V-IX complex, a receptor of von Willebrand factor. BSS characterized by macrothrombocytopenia and excessive bleeding. The present study reports a single case (18-month Chinese girl) diagnosed with BSS. The patient suffered mild thrombocytopenia, giant platelets and normal platelet aggregation. In addition, mild bleeding and thrombocytopenia were also indicated in thirteen family members, including the proband and her father. Gene sequence analysis identified a monoallelic missense mutation in GP1BA (c.97T>A), which encodes a p.C33R substitution in the N-terminal domain of glycoprotein (GP)Ibα that may disrupt the protein structure. To the best of our knowledge, this dominant variant has not been reported previously. BSS's autosomal dominant inheritance mode is rarely identified and can be easily misdiagnosed as immune thrombocytopenia. For patients with giant platelets, thrombocytopenia and positive family history, next-generation sequencing for inherited thrombocytopenia, especially disorders that are caused by mutations in glycoprotein Ib-IX-V complex, is required.

Molecular Aspects of Pathophysiology of Platelet Receptors

Receptor is a dynamic instrumental surface protein that helps to interact with specific molecules to respond accordingly. Platelet is the smallest in size among the blood components, but it plays many pivotal roles to maintain hemostasis involving its surface receptors. It (platelet) has cell adhesion receptors (e.g., integrins and glycoproteins), leucine-rich repeats receptors (e.g., TLRs, glycoprotein complex, and MMPs), selectins (e.g., CLEC, P-selectin, and CD), tetraspanins (e.g., CD and LAMP), transmembrane receptors (e.g., purinergic—P2Y and P2X1), prostaglandin receptors (e.g., TxA2, PGH2, and PGI2), immunoglobulin superfamily receptors (e.g., FcRγ and FcεR), etc. on its surface. The platelet receptors (e.g., glycoproteins, protease-activated receptors, and GPCRs) during platelet activation are over expressed and their granule contents are secreted (including neurotransmitters, cytokines, and chemokines) into circulation, which are found to be correlated with different physiological conditions. Interestingly, platelets promote metastasis through circulation protecting from cytolysis and endogenous immune surveillance involving several platelets receptors. The updated knowledge about different types of platelet receptors in all probable aspects, including their inter- and intra-signaling mechanisms, are discussed with respect to not only its (platelets) receptor type but also under different pathophysiological conditions.

Zebrafish for thrombocytopoiesis- and hemostasis-related researches and disorders

Platelets play vital roles in hemostasis, inflammation, and vascular biology. Platelets are also active participants in the immune responses. As vertebrates, zebrafish have a highly conserved hematopoietic system in the developmental, cellular, functional, biochemical, and genetic levels with mammals. Thrombocytes in zebrafish are functional homologs of mammalian platelets. Here, we summarized thrombocyte development, function, and related research techniques in zebrafish, and reviewed available zebrafish models of platelet-associated disorders, including congenital amegakaryocytic thrombocytopenia, inherited thrombocytopenia, essential thrombocythemia, and blood coagulation disorders such as gray platelet syndrome. These elegant zebrafish models and methods are crucial for understanding the molecular and genetic mechanisms of thrombocyte development and function, and provide deep insights into related human disease pathophysiology and drug development.

Modulation of Immune Responses by Platelet-Derived ADAM10

Platelets have a crucial function in maintaining hemostasis. However, beyond their role in coagulation and thrombus formation, platelets have been implicated to affect various pathophysiological conditions such as infectious diseases, autoimmune disorders, and cancer. It is well-established that platelets aid local cancer growth by providing growth factors or contributing to cancer angiogenesis. In addition, they promote metastasis, among others by facilitation of tumor cell-extravasation and epithelial-to-mesenchymal-like transition as well as protecting metastasizing cancer cells from immunosurveillance. A variety of membrane-bound and soluble platelet-derived factors are involved in these processes, and many aspects of platelet biology in both health and disease are regulated by platelet-associated metalloproteinases and their inhibitors. Platelets synthesize (i) members of the matrix metalloproteinase (MMP) family and also inhibitors of MMPs such as members of the "tissue inhibitor of metalloproteinases" (TIMP) family as well as (ii) members of the "a disintegrin and metalloproteinase" (ADAM) family including ADAM10. Notably, platelet-associated metalloproteinase activity not only influences functions of platelets themselves: platelets can also induce expression and/or release of metalloproteinases e.g., in leukocytes or cancer cells, and ADAMs are emerging as important components by which platelets directly affect other cell types and function. This review outlines the function of metalloproteinases in platelet biology with a focus on ADAM10 and discusses the role of platelet-derived metalloproteinases in the interaction of platelets with components of the immune system and/or cancer cells.

Innovative Three-Step Microwave-Promoted Synthesis of N-Propargyltetrahydroquinoline and 1,2,3-Triazole Derivatives as a Potential Factor Xa (FXa) Inhibitors: Drug Design, Synthesis, and Biological Evaluation

The coagulation cascade is the process of the conversion of soluble fibrinogen to insoluble fibrin that terminates in production of a clot. Factor Xa (FXa) is a serine protease involved in the blood coagulation cascade. Moreover, FXa plays a vital role in the enzymatic sequence which ends with the thrombus production. Thrombosis is a common causal pathology for three widespread cardiovascular syndromes: acute coronary syndrome (ACS), venous thromboembolism (VTE), and strokes. In this research a series of N-propargyltetrahydroquinoline and 1,2,3-triazole derivatives as a potential factor Xa (FXa) inhibitor were designed, synthesized, and evaluated for their FXa inhibitor activity, cytotoxicity activity and coagulation parameters. Rational design for the desired novel molecules was performed through protein-ligand complexes selection and ligand clustering. The microwave-assisted synthetic strategy of selected compounds was carried out by using Ullmann-Goldberg, N-propargylation, Mannich addition, Friedel-Crafts, and 1,3-dipolar cycloaddition type reactions under microwave irradiation. The microwave methodology proved to be an efficient way to obtain all novel compounds in high yields (73–93%). Furthermore, a thermochemical analysis, optimization and reactivity indexes such as electronic chemical potential (µ), chemical hardness (η), and electrophilicity (ω) were performed to understand the relationship between the structure and the energetic behavior of all the series. Then, in vitro analysis showed that compounds 27, 29–31, and 34 exhibited inhibitory activity against FXa and the corresponding half maximal inhibitory concentration (IC₅₀) values were calculated. Next, a cell viability assay in HEK293 and HepG2 cell lines, and coagulation parameters (anti FXa, Prothrombin time (PT), activated Partial Thromboplastin Time (aPTT)) of the most active novel molecules were performed to determine the corresponding cytotoxicity and possible action on clotting pathways. The obtained results suggest that compounds 27 and 29 inhibited FXa targeting through coagulation factors in the intrinsic and extrinsic pathways. However, compound 34 may target coagulation FXa mainly by the extrinsic and common pathway. Interestingly, the most active compounds in relation to the inhibition activity against FXa and coagulation parameters did not show toxicity at the performed coagulation assay concentrations. Finally, docking studies confirmed the preferential binding mode of N-propargyltetrahydroquinoline and 1,2,3-triazole derivatives inside the active site of FXa.

Platelet Carcinoembryonic Antigen Cell Adhesion Molecule 5 (CEACAM5) as a Possible Novel Diagnostic Tool for Evaluation of Acute Coronary Syndrome

Background

Acute coronary syndrome (ACS) occurs approximately every 40 seconds, and was an underlying cause of death in 1 out of every 7 deaths. More accurate indicators are needed to distinguish patients with ACS from patients manifesting negative changes in electrocardiogram (ECG) and myocardial enzymes. This study aimed to investigate whether the expression of platelet carcinoembryonic antigen cell adhesion molecule-5 (CEACAM5/CEA/CD66e) could help predict ACS.

Material/Methods

We enrolled 82 participants (mean age 60 years, 33 females and 49 males). The expression of CEA on washed human platelets was assessed using two-color flow cytometry. The CEA levels on platelets and in serum of these 82 consecutive patients were detected using two-color whole-blood flow cytometry analysis and a custom-made Luminex multiplex assay, respectively.

Results

CEA was expressed on the surface of human platelets. The expression of platelet CEA (P<0.01), but not serum CEA (P=0.30), was significantly higher in patients with ACS compared to patients with normal coronary artery. Increased platelet CEA levels could serve as a new independent indicator for ACS (P=0.0003). Platelet CEA testing (P=0.000002), as well as cardiac troponin I (cTnI) (P=0.0005), can diagnose ACS with high sensitivity and specificity, and, combined with cTnI (P<0.0001), can improve the diagnostic value.

Conclusions

Platelet CEA expression was higher in individuals presenting with ACS. Hence, platelet CEA might be a novel and reliable biomarker for ACS. Large-scale studies are needed to confirm this hypothesis.

The Extraordinary Role of Extracellular RNA in Arteriogenesis, the Growth of Collateral Arteries

Arteriogenesis is an intricate process in which increased shear stress in pre-existing arteriolar collaterals induces blood vessel expansion, mediated via endothelial cell activation, leukocyte recruitment and subsequent endothelial and smooth muscle cell proliferation. Extracellular RNA (eRNA), released from stressed cells or damaged tissue under pathological conditions, has recently been discovered to be liberated from endothelial cells in response to increased shear stress and to promote collateral growth. Until now, eRNA has been shown to enhance coagulation and inflammation by inducing cytokine release, leukocyte recruitment, and endothelial permeability, the latter being mediated by vascular endothelial growth factor (VEGF) signaling. In the context of arteriogenesis, however, eRNA has emerged as a transmitter of shear stress into endothelial activation, mediating the sterile inflammatory process essential for collateral remodeling, whereby the stimulatory effects of eRNA on the VEGF signaling axis seem to be pivotal. In addition, eRNA might influence subsequent steps of the arteriogenesis cascade as well. This article provides a comprehensive overview of the beneficial effects of eRNA during arteriogenesis, laying the foundation for further exploration of the connection between the damaging and non-damaging effects of eRNA in the context of cardiovascular occlusive diseases and of sterile inflammation.

Platelet biology of the rapidly failing lung

Acute respiratory distress syndrome (ARDS) is characterized by a rapid-onset respiratory failure with a mortality rate of approximately 40%. This physiologic inflammatory process is mediated by disruption of the alveolar-vascular interface, leading to pulmonary oedema and impaired oxygen exchange, which often warrants mechanical ventilation to increase survival in the acute setting. One of the least understood aspects of ARDS is the role of the platelets in this process. Platelets, which protect vascular integrity, play a pivotal role in the progression and resolution of ARDS. The recent substantiation of the age-old theory that megakaryocytes are found in the lungs has rejuvenated interest in and raised new questions about the importance of platelets for pulmonary function. In addition to primary haemostasis, platelets provide a myriad of inflammatory functions that are poised to aid the innate immune system. This review focuses on the evidence for regulatory roles of platelets in pulmonary inflammation, with an emphasis on two receptors, CLEC-2 and TLT-1. Studies of these receptors identify novel pathways through which platelets may regulate vascular integrity and inflammation in the lungs, thereby influencing the development of ARDS.

Structurally Robust and Functionally Highly Versatile-C-Type Lectin (-Related) Proteins in Snake Venoms

Snake venoms contain an astounding variety of different proteins. Among them are numerous C-type lectin family members, which are grouped into classical Ca²⁺- and sugar-binding lectins and the non-sugar-binding snake venom C-type lectin-related proteins (SV-CLRPs), also called snaclecs. Both groups share the robust C-type lectin domain (CTLD) fold but differ in a long loop, which either contributes to a sugar-binding site or is expanded into a loop-swapping heterodimerization domain between two CLRP subunits. Most C-type lectin (-related) proteins assemble in ordered supramolecular complexes with a high versatility of subunit numbers and geometric arrays. Similarly versatile is their ability to inhibit or block their target molecules as well as to agonistically stimulate or antagonistically blunt a cellular reaction triggered by their target receptor. By utilizing distinct interaction sites differentially, SV-CLRPs target a plethora of molecules, such as distinct coagulation factors and receptors of platelets and endothelial cells that are involved in hemostasis, thrombus formation, inflammation and hematogenous metastasis. Because of their robust structure and their high affinity towards their clinically relevant targets, SV-CLRPs are and will potentially be valuable prototypes to develop new diagnostic and therapeutic tools in medicine, provided that the molecular mechanisms underlying their versatility are disclosed.

Comparison of Effects of Anti-thrombin Aptamers HD1 and HD22 on Aggregation of Human Platelets, Thrombin Generation, Fibrin Formation, and Thrombus Formation Under Flow Conditions

HD1 and HD22 are two of the most-studied aptamers binding to thrombin exosite I and exosite, respectively. To complete of their pharmacological profiles, the effects of HD1 and HD22 on thrombin-, ristocetin-, and collagen-induced human platelet aggregation, on thrombin generation and fibrin formation in human plasma, as well as on thrombus formation in human whole blood under flow conditions were assessed. The dissociation constants for HD1 and HD22 complexes with thrombin in simulated plasma ionic buffer were also evaluated. HD1 was more potent than HD22 in terms of inhibiting thrombin-induced platelet aggregation in platelet-rich plasma (PRP; 0.05-3 μM) and in washed platelets (WPs; 0.005-3 μM): approximately 8.31% (±6.99% SD) and 89.53% (±11.38% SD) for HD1 (0.5 μM) and HD22 (0.5 μM), respectively. Neither HD1 nor HD22 (3 μM) did influence platelets aggregation induced by collagen. Both of them inhibited ristocetin-induced aggregation in PRP. Surprisingly, HD1 and HD22 aptamers (3 μM) potentiated ristocetin-induced platelet aggregation in WP. HD1 reduced thrombin generation in a concentration-dependent manner [ETP at 3 μM: 1677.53 ± 55.77 (nM⋅min) vs. control 2271.71 ± 423.66 (nM⋅min)], inhibited fibrin formation (lag time at 3 μM: 33.70 min ± 8.01 min vs. control 7.91 min ± 0.91 min) and reduced thrombus formation under flow conditions [AUC30 at 3 μM: 758.30 ± 344.23 (kPa⋅min) vs. control 1553.84 ± 118.03 (kPa⋅min)]. HD22 (3 μM) also delayed thrombin generation but increased the thrombin peak. HD22 (3 μM) shortened the lag time of fibrin generation (5.40 min ± 0.26 min vs. control 7.58 min ± 1.14 min) but did not modify thrombus formation (3, 15 μM). K d values for the HD1 complex with thrombin was higher (257.8 ± 15.0 nM) than the K d for HD22 (97.6 ± 2.2 nM). In conclusion, HD1 but not HD22 represents a potent anti-thrombotic agent, confirming the major role of exosite I in the action of thrombin. HD22 aptamer blocking exosite II displays weaker anti-platelet and anti-coagulant activity, with surprising activating effects on thrombin and fibrin generation most likely induced by HD22-induced allosteric changes in thrombin dynamic structure.

Platelet Shp2 negatively regulates thrombus stability under high shear stress

Essentials Shp2 negatively regulates thrombus stability under pathological shear rate. Shp2 suppresses TXA2 receptor-mediated platelet dense granule secretion. Through αIIbβ3 outside-in signaling, Shp2 targets calmodulin-dependent activation of Akt. Shp2 may serve to prevent the formation of unwanted occlusive thrombi. SUMMARY: Background Perpetuation is the final phase of thrombus formation; however, its mechanisms and regulation are poorly understood. Objective To investigate the mechanism of Shp2 in platelet function and thrombosis. Methods and results We demonstrate that the platelet-expressed Src homology region 2 domain-containing protein tyrosine phosphatase Shp2 is a negative regulator of thrombus stability under high shear stress. In a ferric chloride-induced mesenteric arteriole thrombosis model, megakaryocyte/platelet-specific Shp2-deficient mice showed less thrombi shedding than wild-type mice, although their occlusion times were comparable. In accordance with this in vivo phenotype, a microfluidic whole-blood perfusion assay revealed that the thrombi formed on collagen surfaces by Shp2-deficient platelets were more stable under high shear rates than those produced by wild-type platelets. Whereas Shp2 deficiency did not alter platelet responsiveness towards thrombin, ADP and collagen stimulation, Shp2-deficient platelets showed increased dense granule secretion when stimulated by the thromboxane A2 analog U46619. Shp2 appears to act downstream of integrin αIIb β3 outside-in signaling, inhibiting the phosphorylation of Akt (Ser473 and Thr308) and dense granule secretion. Calmodulin was also shown to bind both Shp2 and Akt, linking Shp2 to Akt activation. Conclusions Platelet Shp2 negatively regulates thrombus perpetuation under high shear stress. This signaling pathway may constitute an important mechanism for the prevention of unwanted occlusive thrombus formation, without dramatically interfering with hemostasis.

Association of Fibrinogen Receptor (Integrin αIIbβ3) Polymorphism in Sudanese Ischemic Stroke Patients

BACKGROUND AND OBJECTIVE

The fibrinogen receptor the human platelet antigen (HPA1 and HPA3) have an essential role in Atherothrombosis. This study aimed to detect the association of αIIbβ3 polymorphism with ischemic stroke in Sudanese patients and its association with the common risk factors.

METHODOLOGY

This is a case-control study. Fifty atherosclerotic with ischemic stroke Sudanese patients were included in present study and were compared to apparently 50 healthy Sudanese subjects at the same ages. The ages of both groups were >18 years. About 5 mL of venous blood sample was taken from each patient and control. The laboratory analyses were done for HbA1c, lipid profile and DNA genotyping by polymerase chain reaction (PCR) followed by FokI and ScrFI digestion.

RESULTS

The result showed that, the risk factors (TRI.G, HDL, HbA1C, and body mass index were associated with the increased risk of ischemic stroke). None of the cholesterol levels and LDL increased the risk of stroke. The risk of ischemic stroke was higher with B/B genotype in HPA3 (p-value 0.009) and A/B genotype in HPA1 (p-value 0.041) and HPA1 (p-value 0.041).

CONCLUSION

The αIIbβ3 polymorphism were with ischemic stroke in Sudanese patients.

Increased GPIbα shedding from platelets treated with immune thrombocytopenia plasma

Immune thrombocytopenia (ITP) is a heterogeneous autoimmune disease, characterized by accelerated platelet destruction/clearance or decreased platelet production. ADAM17-mediated platelet receptor GPIbα extracellular domain shedding has been shown to be involved in platelet clearance. Whether GPIbα shedding participates in the pathogenesis of ITP remains poorly understood. This study aims to investigate the role of GPIbα shedding in the development of ITP via incubating normal platelets with ITP plasma to mimic ITP in vivo environment. Plasma was isolated from ITP patients or healthy control and incubated with platelets in vitro followed by measuring GPIbα expression by flow cytometry and western blot, ADAM17 expression by western blot, ROS generation and platelet activation by flow cytometry. Compared with control plasma, ITP plasma-treated platelet displayed significantly reduced GPIbα surface expression, increased ADAM17 expression and ROS generation. However, metalloproteinase inhibitor GM6001 blocked the ITP-plasma-induced decrease in GPIbα surface expression, increase in ADAM17 expression and platelet activation. In addition, inhibitors of NADPH oxidase or mitochondria respiration significantly inhibited ROS generation from ITP plasma-treated platelets. Moreover, ROS inhibition or blocking FcγRIIa attenuated the decrease in GPIbα surface expression, platelet activation and ROS generation (for blocking FcγRIIa) in ITP plasma-treated platelets. In conclusion, ITP plasma induces platelet receptor GPIbα extracellular domain shedding, suggesting that it might participate in the pathogenesis of ITP and targeting it might be a novel approach for treating ITP.

Interaction between platelets and endothelium: from pathophysiology to new therapeutic options

Platelets were traditionally considered to purely have a role in the maintenance of haemostasis. Recently their role in vasomotor function, inflammation and atherosclerosis has been very well-recognized. Endothelium which was originally considered as a simple passive barrier, it is now viewed as an organ whose normal functioning is crucial for maintaining vascular health. When endothelial balance is disturbed, vascular disease initiates. Platelet interactions with endothelium have an important contribution in this process. Low-grade inflammation, endothelial dysfunction, and platelet hyper-reactivity are all independently associated with an increased risk of cardiovascular events. Older antiplatelet agents like aspirin and clopidogrel and newer more potent agents like prasugrel and ticagrelor have been proven effective in all the clinical spectrum of coronary artery disease patients. Current antiplatelet medications and especially newer generation P2Y12 inhibitor ticagrelor, offer clinical benefits not only due to their well-recognized antithrombotic effect, but also via the attenuation of platelet inflammatory action, impediment of P2Y12 activation effects in other cells and through other complex and sometimes undefined pathways. Future research is expected to better define platelet-endothelium interactions and the multiple impact of current antiplatelet therapy on them.

Snake venom components in medicine: From the symbolic rod of Asclepius to tangible medical research and application

Both mythologically and logically, snakes have always fascinated man. Snakes have attracted both awe and fear not only because of the elegant movement of their limbless bodies, but also because of the potency of their deadly venoms. Practically, in 2017, the world health organization (WHO) listed snake envenomation as a high priority neglected disease, as snakes inflict up to 2.7 million poisonous bites, around 100.000 casualties, and about three times as many invalidities on man. The venoms of poisonous snakes are a cocktail of potent compounds which specifically and avidly target numerous essential molecules with high efficacy. The individual effects of all venom toxins integrate into lethal dysfunctions of almost any organ system. It is this efficacy and specificity of each venom component, which after analysis of its structure and activity may serve as a potential lead structure for chemical imitation. Such toxin mimetics may help in influencing a specific body function pharmaceutically for the sake of man's health. In this review article, we will give some examples of snake venom components which have spurred the development of novel pharmaceutical compounds. Moreover, we will provide examples where such snake toxin-derived mimetics are in clinical use, trials, or consideration for further pharmaceutical exploitation, especially in the fields of hemostasis, thrombosis, coagulation, and metastasis. Thus, it becomes clear why a snake captured its symbolic place at the Asclepius rod with good reason still nowadays.

Platelet communication with the vascular wall: role of platelet-derived microparticles and non-coding RNAs

Platelets play an important role in vascular homeostasis through their interaction with circulating blood cells as well as the vascular wall. Platelet-mediated communication with other cells can take the form of direct cell-cell interactions via membrane receptors or indirectly through the release of different soluble factors stored in their granules as well as through the release of microparticles. The latter carry different proteins and RNAs which are transferred to the target cells. The aim of this review is to discuss the role of platelet-derived factors, adhesion molecules as well as RNAs as mediators of the cross-talk between platelets and the vessel wall.

Regulation of platelet activation and thrombus formation by reactive oxygen species

Reactive oxygen species (ROS) are generated within activated platelets and play an important role in regulating platelet responses to collagen and collagen-mediated thrombus formation. As a major collagen receptor, platelet-specific glycoprotein (GP)VI is a member of the immunoglobulin (Ig) superfamily, with two extracellular Ig domains, a mucin domain, a transmembrane domain and a cytoplasmic tail. GPVI forms a functional complex with the Fc receptor γ-chain (FcRγ) that, following receptor dimerization, signals via an intracellular immunoreceptor tyrosine-based activation motif (ITAM), leading to rapid activation of Src family kinase signaling pathways. Our previous studies demonstrated that an unpaired thiol in the cytoplasmic tail of GPVI undergoes rapid oxidation to form GPVI homodimers in response to ligand binding, indicating an oxidative submembranous environment in platelets after GPVI stimulation. Using a redox-sensitive fluorescent dye (H2DCF-DA) in a flow cytometric assay to measure changes in intracellular ROS, we showed generation of ROS downstream of GPVI consists of two distinct phases: an initial Syk-independent burst followed by additional Syk-dependent generation. In this review, we will discuss recent findings on the regulation of platelet function by ROS, focusing on GPVI-dependent platelet activation and thrombus formation.

Anfibatide, a novel GPIb complex antagonist, inhibits platelet adhesion and thrombus formation in vitro and in vivo in murine models of thrombosis

Platelet adhesion and aggregation at the sites of vascular injury are key events for thrombosis and haemostasis. It has been well demonstrated that interaction between glycoprotein (GP) Ib and von Willebrand factor (VWF) initiates platelet adhesion and contributes to platelet aggregation, particularly at high shear. GPIb has long been suggested as a desirable antithrombotic target, but anti-GPIb therapy has never been successfully developed. Here, we evaluated the antithrombotic potential of Anfibatide, a novel snake venom-derived GPIb antagonist. We found Anfibatide inhibited washed murine platelet aggregation induced by ristocetin and recombinant murine VWF. It also blocked botrocetin-induced binding of murine plasma VWF to recombinant human GPIb . Interestingly, Anfibatide did not inhibit botrocetin- induced aggregation of platelet-rich plasma, indicating that its binding site may differ from other snake venom-derived GPIb antagonists. Anfibatide strongly inhibited platelet adhesion, aggregation, and thrombus formation in perfusion chambers at high shear conditions and efficiently dissolved preformed thrombi. Anfibatide also inhibited thrombus growth at low shear conditions, though less than at high shear. Using intravital microscopy, we found that Anfibatide markedly inhibited thrombosis in laser-injured cremaster vessels and prevented vessel occlusion in FeCl3-injured mesenteric vessels. Importantly, Anfibatide further inhibited residual thrombosis in VWF-deficient mice, suggesting that Anfibatide has additional antithrombotic effect beyond its inhibitory role in GPIb-VWF interaction. Anfibatide did not significantly cause platelet activation, prolong tail bleeding time, or cause bleeding diathesis in mice. Thus, consistent with the data from an ongoing clinical trial, the data from this study suggests that Anfibatide is a potent and safe antithrombotic agent.

Structural origins of hemostasis and adaptive immunity

BACKGROUND

Adaptive immunity in jawless fishes is performed by a unique set of proteins termed variable lymphocyte receptors (VLRs). Here we compare the crystallographic structures of VLRs and the human primary hemostasis receptor, glycoprotein (GP) Ib. It has been estimated jawless fish vertebrates diverged from jawed vertebrates 500 million years ago. Identifying structural similarities provides insights into the origins of primary hemostasis and the unique adaptive immunity of jawless fishes.

METHODS

Three-dimensional structures obtained from crystallographic data and primary sequences alignments are compared. The results focus on overall domain arrangement to include the structural roles of leucine-rich repeats (LRRs), disulfide bond, and disulfide loop arrangements.

RESULTS

The crystal structures of human GPIb (GPIbαN) and jawless fish VLRs are made up of three common segments each. The N-terminal cap and the C-terminal cap are characterized by disulfide bonds conserved in both GPIbαN and VLRs. The body of each molecule consists of LRRs which varies depending on the number of LRRs present in each molecule. The stacking of the LRRs results in the formation of a concave surface which serves as a motif to build ligand-binding specificity with the flanking regions.

CONCLUSION

A comparison of VLR and GPIb structures reveals a phylogenetic trail of cellular differentiation contributing to mammalian hemostasis and jawless fish adaptive immunity. The results provide a structural basis to explain some of the interrelationships between hemostasis and immunity in vertebrates and potentially identifies a common ancestral motif linking hemostasis and immunity.

Glycoprotein Ib-IX-V Complex Transmits Cytoskeletal Forces That Enhance Platelet Adhesion

Platelets bind to exposed vascular matrix at a wound site through a highly specialized surface receptor, glycoprotein (GP) Ib-IX-V complex, which recognizes von Willebrand factor (VWF) in the matrix. GPIb-IX-V is a catch bond for it becomes more stable as force is applied to it. After attaching to the wound site, platelets generate cytoskeletal forces to compact and reinforce the hemostatic plug. Here, we evaluated the role of the GPIb-IX-V complex in the transmission of cytoskeletal forces. We used arrays of flexible, silicone nanoposts to measure the contractility of individual platelets on VWF. We found that a significant proportion of cytoskeletal forces were transmitted to VWF through GPIb-IX-V, an unexpected finding given the widely held notion that platelet forces are transmitted exclusively through its integrins. In particular, we found that the interaction between GPIbα and the A1 domain of VWF mediates this force transmission. We also demonstrate that the binding interaction between GPIbα and filamin A is involved in force transmission. Furthermore, our studies suggest that cytoskeletal forces acting through GPIbα are involved in maintaining platelet adhesion when external forces are absent. Thus, the GPIb-IX-V/VWF bond is able to transmit force, and uses this force to strengthen the bond through a catch-bond mechanism. This finding expands our understanding of how platelets attach to sites of vascular injury, describing a new, to the best of our knowledge, mechanism in which the catch bonds of GPIb-IX-V/VWF can be supported by internal forces produced by cytoskeletal tension.

A Brazilian case of Bernard-Soulier syndrome with two distinct founder mutations

Bernard-Soulier syndrome (BSS) is a rare bleeding disorder of autosomal recessive inheritance characterized by macrothrombocytopenia. We report the case of a 14-year-old girl diagnosed with BSS who is a fourth-generation Brazilian of Japanese descent and has a compound heterozygote mutation as the responsible gene. The compound heterozygosity would have occurred from the global and long-term racial migration that brought about an accidental encounter of two rare mutant alleles of different origins.

Effects of vessel size, cell sedimentation and haematocrit on the adhesion of leukocytes and platelets from flowing blood

BACKGROUND

Leukocytes and platelets typically fulfil their functions through adhesion to the walls of vessels with different size, haematocrit and shear rate.

OBJECTIVE

We aimed to investigate differential effects of these variables on leukocyte and platelet adhesion.

METHODS

Blood with varying haematocrit was perfused at a range of wall shear rates through capillaries of depth 100 or 300 µm coated with P-selectin or collagen.

RESULTS

Adhesion of leukocytes was much more efficient in the smaller capillaries, but was equal on the upper and lower surfaces and showed nearly identical shear rate dependence for either size of vessel. Platelets also adhered more efficiently in the smaller vessels (although the effect of size was not so great), and equally on upper and lower surfaces, but their adhesion was much less sensitive to increasing shear rate. In previous studies using vertically-orientated capillaries, leukocyte adhesion increased with increasing haematocrit (Am. J. Physiol.285 (2003), H229-H240). Here, in horizontal 100 µm capillaries, leukocyte adhesion was highly efficient at haematocrit of 10% but restricted to the lower surface. Adhesion decreased initially as haematocrit was increased to 30% and then increased slightly again at 40% haematocrit. Increasing haematocrit supported a monotonic increase in platelet adhesion in the horizontal capillaries.

CONCLUSIONS

Platelets adhere efficiently over a wider range of sizes and shear rates, and at high haematocrit. Leukocytes adhere better in smaller vessels and at low haematocrit in horizontal vessels. The different behaviours may represent 'rheological adaptation' to functions in inflammation vs. haemostasis.

Structural basis for the specific inhibition of glycoprotein Ibα shedding by an inhibitory antibody

Ectodomain shedding of glycoprotein (GP) Ibα is thought to mediate the clearance of activated, aged or damaged platelets. A monoclonal antibody, 5G6, has been developed recently to specifically bind to the GPIbα shedding cleavage site and to inhibit its shedding. However, the molecular mechanism underlying antigen recognition and inhibitory specificity is not clear. To elucidate the structural basis for 5G6 binding to GPIbα, we determined the crystal structure of 5G6 Fab fragment in complex with its epitope peptide KL10 (GPIbα residues 461–470, KLRGVLQGHL), to 2.4-Å resolution. Key residues in both 5G6 and KL10 were mutated to validate their effects in antibody binding by using isothermal titration calorimetry. The 5G6 Fab-KL10 peptide complex structure confirmed the direct association of 5G6 with its target GPIbα residues and elucidated the molecular basis underlying its binding specificity and high affinity. The similar binding properties of 5G6 Fab fragment to GPIbα on human platelets as those to KL10 suggests that such an interaction may not be affected by the plasma membrane or nearby GPIbβ. This structural information may facilitate further antibody optimization and humanization.

Comparison of the gene encoding, and the predicted amino acid composition of, platelet membrane receptor subunit glycoprotein Ibα in members of the family Felidae

BACKGROUND

There is minimal information regarding platelet receptors in the family Felidae. Comparative studies assist with identifying amino acids critical for protein structure and function.

OBJECTIVE

The purpose of the study was to compare the gene encoding, and the predicted amino acid composition of, platelet membrane receptor subunit GPIbα in Felidae family members.

METHODS

Genomic DNA samples isolated from whole blood of 13 domestic cats and 50 big cats representing 8 different species were subjected to PCR using primers designed to flank the coding region of GPIbα in overlapping fashion. PCR products were separated via electrophoresis on agarose gels, and extracted products were submitted for sequencing. DNA sequences were used to predict the length and amino acid composition of the protein.

RESULTS

Varying protein lengths were predicted in Felidae family members which were primarily due to polymorphisms in the variable number of tandem repeats region encoding the macroglycopeptide region of GPIbα. Other areas of the gene and predicted amino acid compositions were fairly conserved when compared to human sequences and between Felidae family members.

CONCLUSION

Various polymorphisms within GPIbα, including length variants encoding the macroglycopeptide region, were identified in members of the family Felidae. More studies are needed to determine if a correlation exists between various polymorphisms and predisposition for hemorrhage or thrombosis as suggested in people.

Comparison of common platelet receptors between the chacma baboon (Papio ursinus) and human for use in pre-clinical human-targeted anti-platelet studies

Anti-platelet agents play a central part in the treatment and prevention of acute thrombotic events. Discriminating animal models are needed for the development of novel agents. The chacma baboon has been extensively used as a model to evaluate anti-platelet agents. However, limited data exist to prove the translatability of this species to humans. We aimed to determine the suitability of the chacma baboon in preclinical human targeted GPIIb/IIIa, GPIbα and P2Y12 studies. Light-transmission platelet aggregometry (LTA), whole blood impedance aggregometry, receptor number quantification and genomic DNA sequencing were performed. Baboon ADP and arachidonic acid-induced LTA aggregation results differed significantly from human values, even at increased concentrations. LTA ristocetin-induced agglutination was comparable between species, but baboon platelets needed twice the concentration of ristocetin to elicit a similar response. Citrated baboon blood had significantly less aggregation than humans when evaluated with impedance aggregometry. However, hirudinised baboon whole blood gave similar aggregation as humans at the same agonist concentrations. GPIIb, GPIIIa and GPIbα numbers were significantly more on the baboon platelets. None of the amino acids deemed vital for receptor function, ligand binding or receptor inhibition, were radically different between the species. However, a conservative change in a calcium-binding region of GPIIb may render the baboon platelets more sensitive to calcium-binding agents. The chacma baboon may be used for the evaluation of human-targeted GPIIb/IIIa-, GPIbα- and P2Y12-inhibiting agents. However, the best anticoagulant, optimal agonist concentrations, increase in receptor number and sequence differences must be considered for any future studies.

Platelets and Their Interactions with Other Immune Cells

Platelets are anucleate blood cells, long known to be critically involved in hemostasis and thrombosis. In addition to their role in blood clots, increasing evidence reveals significant roles for platelets in inflammation and immunity. However, the notion that platelets represent immune cells is not broadly recognized in the field of Physiology. This article reviews the role of platelets in inflammation and immune responses, and highlights their interactions with other immune cells, including examples of major functional consequences of these interactions.