Insights into platelet-based control of coagulation

RNAseq of Gingival Fibroblasts Exposed to PRF Membrane Lysates and PRF Serum

Platelet-rich fibrin (PRF) is prepared by spontaneous coagulation of fractionated blood. When squeezed between two plates, PRF is separated into solid PRF membranes and a liquid exudate, the PRF serum. The question arises regarding how much the overall activity remains in the PRF membranes and what is discarded into the PRF serum. To this end, we have exposed gingival fibroblasts to lysates prepared from PRF membranes and PRF serum, followed by bulk RNA sequencing. A total of 268 up- and 136 down-regulated genes in gingival fibroblasts exposed to PRF membrane lysates were significantly regulated under the premise of a minimum log2 with 2.5-fold change and a minus log10 significance level of two, respectively. PRF serum only caused 62 up- and 32 down-regulated genes under these conditions. Among the 46 commonly up-regulated genes were CXCL1, CXCL5, CXCL6, CXCL8, IL33, IL6, and PTGS2/COX2, stanniocalcin-1-all linked to an inflammatory response. PRF membrane lysates further increased chemokines CCL2, CCL7, CXCL2, CXCL3, and IL1R1, IL1RL1, and IL1RN, as well as the paracrine factors IL11, LIF, IGF1, BMP2, BMP6, FGF2, and CCN2/CTGF, and all hyaluronan synthases. On the other hand, PRF serum increased DKK1. The genes commonly down-regulated by PRF membrane lysates and PRF serum included interferon-induced protein with tetratricopeptide repeats (IFIT1, IFIT2, IFIT3) and odd-skipped-related transcription factors (OSR1 and OSR2), as well as FGF18 and GDF15, respectively. Taken together, PRF membrane lysates, compared to PRF serum, cause a more complex response in gingival fibroblasts, but each increased chemokine expression in gingival fibroblasts.

Fibroblasts and platelets: a face-to-face dialogue at the heart of cardiac fibrosis

Myocardial fibrosis is a feature found in most cardiac diseases and a key element contributing to heart failure and its progression. It has therefore become a subject of particular interest in cardiac research. Mechanisms leading to pathological cardiac remodeling and heart failure are diverse, including effects on cardiac fibroblasts, the main players in cardiac extracellular matrix synthesis, but also on cardiomyocytes, immune cells, endothelial cells, and more recently, platelets. Although transforming growth factor-β (TGF-β) is a primary regulator of fibrosis development, the cellular and molecular mechanisms that trigger its activation after cardiac injury remain poorly understood. Different types of anti-TGF-β drugs have been tested for the treatment of cardiac fibrosis and have been associated with side effects. Therefore, a better understanding of these mechanisms is of great clinical relevance and could allow us to identify new therapeutic targets. Interestingly, it has been shown that platelets infiltrate the myocardium at an early stage after cardiac injury, producing large amounts of cytokines and growth factors. These molecules can directly or indirectly regulate cells involved in the fibrotic response, including cardiac fibroblasts and immune cells. In particular, platelets are known to be a major source of TGF-β1. In this review, we have provided an overview of the classical cellular effectors involved in the pathogenesis of cardiac fibrosis, focusing on the emergent role of platelets, while discussing opportunities for novel therapeutic interventions.

Contractility defects hinder glycoprotein VI-mediated platelet activation and affect platelet functions beyond clot contraction

Background

Active and passive biomechanical properties of platelets contribute substantially to thrombus formation. Actomyosin contractility drives clot contraction required for stabilizing the hemostatic plug. Impaired contractility results in bleeding but is difficult to detect using platelet function tests.

Objectives

To determine how diminished myosin activity affects platelet functions, including and beyond clot contraction.

Methods

Using the myosin IIA-specific pharmacologic inhibitor blebbistatin, we modulated myosin activity in platelets from healthy donors and systematically characterized platelet responses at various levels of inhibition by interrogating distinct platelet functions at each stage of thrombus formation using a range of complementary assays.

Results

Partial myosin IIA inhibition neither affected platelet von Willebrand factor interactions under arterial shear nor platelet spreading and cytoskeletal rearrangements on fibrinogen. However, it impacted stress fiber formation and the nanoarchitecture of cell-matrix adhesions, drastically reducing and limiting traction forces. Higher blebbistatin concentrations impaired platelet adhesion under flow, altered mechanosensing at lamellipodia edges, and eliminated traction forces without affecting platelet spreading, α-granule secretion, or procoagulant platelet formation. Unexpectedly, myosin IIA inhibition reduced calcium influx, dense granule secretion, and platelet aggregation downstream of glycoprotein (GP)VI and limited the redistribution of GPVI on the cell membrane, whereas aggregation induced by adenosine diphosphate or arachidonic acid was unaffected.

Conclusion

Our findings highlight the importance of both active contractile and passive crosslinking roles of myosin IIA in the platelet cytoskeleton. They support the hypothesis that highly contractile platelets are needed for hemostasis and further suggest a supportive role for myosin IIA in GPVI signaling.

4D intravital imaging studies identify platelets as the predominant cellular procoagulant surface in a mouse model of hemostasis

Interplay between platelets, coagulation/fibrinolytic factors, and endothelial cells (ECs) is necessary for effective hemostatic plug formation. This study describes a novel four-dimensional (4D) imaging platform to visualize and quantify hemostatic plug components with high spatiotemporal resolution. Fibrin accumulation following laser-induced endothelial ablation was observed at the EC-platelet plug interface, controlled by the antagonistic balance between fibrin generation and breakdown. Phosphatidylserine (PS) was first detected in close physical proximity to the fibrin ring, followed by exposure across the endothelium. Impaired PS exposure in cyclophilinD -/- mice resulted in a significant reduction in fibrin accumulation. Adoptive transfer and inhibitor studies demonstrated a key role for platelets, but not ECs, in fibrin generation during hemostatic plug formation. Inhibition of fibrinolysis with tranexamic acid (TXA) led to increased fibrin accumulation in WT mice, but not in cyclophilinD -/- mice or WT mice treated with antiplatelet drugs. These studies implicate platelets as the functionally dominant procoagulant surface during hemostatic plug formation. In addition, they suggest that impaired fibrin formation due to reduced platelet procoagulant activity is not reversed by TXA treatment.

Comparative proteomic analysis of ventricular and cisternal cerebrospinal fluid in haemorrhagic stroke patients

BACKGROUND

Analysis of cerebrospinal fluid (CSF) using mass spectrometry is a relatively novel analytical tool, and comparisons of ventricular and cisternal proteomes are yet to be performed. This may have implications for clinical medicine, particularly in demonstrating continuity of the ventricular system with preserved flow in the presence of ventricular blood. Other uses include the identification of novel biomarkers, including for diagnosis of subarachnoid haemorrhage and of aetiology. The primary objective was therefore to characterise and compare the proteomes of ventricular and CSF after haemorrhagic stroke.

METHODS

Paired CSF samples were prospectively collected from the optico-carotid cistern and the frontal horn of the lateral ventricle at the time of craniotomy and clipping in 8 patients with haemorrhagic stroke. Six patients had an aneurysmal subarachnoid haemorrhage (aSAH) from a ruptured saccular aneurysm, one patient had an aSAH after rupture of a mycotic aneurysm and one patient had a spontaneous intracerebral haemorrhage (IPH) with an adjacent unruptured saccular aneurysm. Samples were processed and proteins identified and quantified using data-dependent liquid chromatography tandem mass spectrometry (DDA LC-MSMS).

RESULTS

There was no systematic difference between the cisternal and ventricular proteomes. However, blinded principal component analysis (PCA) of the cisternal and ventricular samples separated patients according to pathophysiology. Additionally CSF D-Dimer levels were not detected in the IPH patient but were reliably measured in aSAH patients.

CONCLUSIONS

Ventricular CSF is representative of cisternal CSF after aSAH. CSF proteomic PCA analysis can distinguish between haemorrhage types. CSF D-dimer levels may represent a novel diagnostic marker for aSAH. Label free DDA LC-MSMS CSF analysis may inform possible biomarkers.

A sensitive and adaptable method to measure platelet-fibrin clot contraction kinetics

Background

Platelet‐fibrin clot contraction is critical for wound closure and maintenance of vessel patency, yet a molecular understanding of the process has lagged because of a lack of flexible quantitative assay systems capable of assaying multiple samples simultaneously.

Objectives

We devised a sensitive and inexpensive method to assess clot contraction kinetics under multiple conditions.

Methods

Clot contraction was measured using time‐lapse digital photography, automated image processing with customized software, and detailed kinetic analysis using available commercial programs.

Results

Our system was responsive to alterations in platelet counts and calcium, fibrinogen, and thrombin concentrations, and our analysis detected and defined three phases of platelet‐fibrin clot formation: initiation, contraction, and stabilization. Lag time, average contraction velocity, contraction extent, and area under the curve were readily calculated from the data. Using pharmacological agents (blebbistatin and eptifibatide), we confirmed the importance of myosin IIA and the interactions of integrin α_IIbβ₃‐fibrinogen/fibrin in clot contraction. As further proof of our system's utility, we showed how 2‐deoxyglucose affects contraction, demonstrating the importance of platelet bioenergetics, specifically glycolysis.

Conclusions

Our system is an adaptable platform for assessing the effects of multiple conditions and interventions on clot contraction kinetics in a regular laboratory setting, using readily available materials. The automated image processing software we developed will be made freely available for noncommercial uses. This assay system can be used to directly compare and define the effects of different treatments or genetic manipulations on platelet function and should provide a robust tool for future hemostasis/thrombosis research and therapeutic development.

Protein C or Protein S deficiency associates with paradoxically impaired platelet‐dependent thrombus and fibrin formation under flow

Background

Low plasma levels of protein C or protein S are associated with venous thromboembolism rather than myocardial infarction. The high coagulant activity in patients with thrombophilia with a (familial) defect in protein C or S is explained by defective protein C activation, involving thrombomodulin and protein S. This causes increased plasmatic thrombin generation.

Objective

Assess the role of platelets in the thrombus‐ and fibrin‐forming potential in patients with familial protein C or protein S deficiency under high‐shear flow conditions.

Patients/Methods

Whole blood from 23 patients and 15 control subjects was perfused over six glycoprotein VI–dependent microspot surfaces. By real‐time multicolor microscopic imaging, kinetics of platelet thrombus and fibrin formation were characterized in 49 parameters.

Results and Conclusion

Whole‐blood flow perfusion over collagen, collagen‐like peptide, and fibrin surfaces with low or high GPVI dependency indicated an unexpected impairment of platelet activation, thrombus phenotype, and fibrin formation but unchanged platelet adhesion, observed in patients with protein C deficiency and to a lesser extent protein S deficiency, when compared to controls. The defect extended from diminished phosphatidylserine exposure and thrombus contraction to delayed and suppressed fibrin formation. The mechanism was thrombomodulin independent, and may involve negative platelet priming by plasma components.

Platelet-mimicking procoagulant nanoparticles augment hemostasis in animal models of bleeding

Treatment of bleeding disorders using transfusion of donor-derived platelets faces logistical challenges due to their limited availability, high risk of contamination, and short (5 to 7 days) shelf life. These challenges could be potentially addressed by designing platelet mimetics that emulate the adhesion, aggregation, and procoagulant functions of platelets. To this end, we created liposome-based platelet-mimicking procoagulant nanoparticles (PPNs) that can expose the phospholipid phosphatidylserine on their surface in response to plasmin. First, we tested PPNs in vitro using human plasma and demonstrated plasmin-triggered exposure of phosphatidylserine and the resultant assembly of coagulation factors on the PPN surface. We also showed that this phosphatidylserine exposed on the PPN surface could restore and enhance thrombin generation and fibrin formation in human plasma depleted of platelets. In human plasma and whole blood in vitro, PPNs improved fibrin stability and clot robustness in a fibrinolytic environment. We then tested PPNs in vivo in a mouse model of thrombocytopenia where treatment with PPNs reduced blood loss in a manner comparable to treatment with syngeneic platelets. Furthermore, in rat and mouse models of traumatic hemorrhage, treatment with PPNs substantially reduced bleeding and improved survival. No sign of systemic or off-target thrombotic risks was observed in the animal studies. These findings demonstrate the potential of PPNs as a platelet surrogate that should be further investigated for the management of bleeding.

Temporal Roles of Platelet and Coagulation Pathways in Collagen- and Tissue Factor-Induced Thrombus Formation

In hemostasis and thrombosis, the complex process of thrombus formation involves different molecular pathways of platelet and coagulation activation. These pathways are considered as operating together at the same time, but this has not been investigated. The objective of our study was to elucidate the time-dependency of key pathways of thrombus and clot formation, initiated by collagen and tissue factor surfaces, where coagulation is triggered via the extrinsic route. Therefore, we adapted a microfluidics whole-blood assay with the Maastricht flow chamber to acutely block molecular pathways by pharmacological intervention at desired time points. Application of the technique revealed crucial roles of glycoprotein VI (GPVI)-induced platelet signaling via Syk kinase as well as factor VIIa-induced thrombin generation, which were confined to the first minutes of thrombus buildup. A novel anti-GPVI Fab EMF-1 was used for this purpose. In addition, platelet activation with the protease-activating receptors 1/4 (PAR1/4) and integrin αIIbβ3 appeared to be prolongedly active and extended to later stages of thrombus and clot formation. This work thereby revealed a more persistent contribution of thrombin receptor-induced platelet activation than of collagen receptor-induced platelet activation to the thrombotic process.

Platelet Activation Mechanisms and Consequences of Immune Thrombocytopenia

Autoimmune disorders are often associated with low platelet count or thrombocytopenia. In immune-induced thrombocytopenia (IIT), a common mechanism is increased platelet activity, which can have an increased risk of thrombosis. In addition, or alternatively, auto-antibodies suppress platelet formation or augment platelet clearance. Effects of the auto-antibodies are linked to the unique structural and functional characteristics of platelets. Conversely, prior platelet activation may contribute to the innate and adaptive immune responses. Extensive interplay between platelets, coagulation and complement activation processes may aggravate the pathology. Here, we present an overview of the reported molecular causes and consequences of IIT in the most common forms of autoimmune disorders. These include idiopathic thrombocytopenic purpura (ITP), systemic lupus erythematosus (SLE), antiphospholipid syndrome (APS), drug-induced thrombocytopenia (DITP), heparin-induced thrombocytopenia (HIT), COVID-19 vaccine-induced thrombosis with thrombocytopenia (VITT), thrombotic thrombocytopenia purpura (TTP), and hemolysis, the elevated liver enzymes and low platelet (HELLP) syndrome. We focus on the platelet receptors that bind auto-antibodies, the immune complexes, damage-associated molecular patterns (DAMPs) and complement factors. In addition, we review how circulating platelets serve as a reservoir of immunomodulatory molecules. By this update on the molecular mechanisms and the roles of platelets in the pathogenesis of autoimmune diseases, we highlight platelet-based pathways that can predispose for thrombocytopenia and are linked thrombotic or bleeding events.

Upregulation of cAMP prevents antibody-mediated thrombus formation in COVID-19

Thromboembolic events are frequently reported in patients infected with the SARS-CoV-2 virus. The exact mechanisms of COVID-19 associated hypercoagulopathy, however, remain elusive. Recently, we observed that platelets (PLTs) from patients with severe COVID-19 infection express high levels of procoagulant markers, which were found to be associated with increased risk for thrombosis. In the current study, we investigated the time course as well as the mechanisms leading to procoagulant PLTs in COVID-19. Our study demonstrates the presence of PLT-reactive IgG antibodies that induce marked changes in PLTs in terms of increased inner-mitochondrial-transmembrane potential (Δψ) depolarization, phosphatidylserine (PS) externalization and P-selectin expression. The IgG-induced procoagulant PLTs and increased thrombus formation was mediated by ligation of PLT Fc gamma RIIA (FcγRIIA). In addition, PLTs´ contents of calcium and cyclic-adenosine-monophosphate (cAMP) were identified to play central role in antibody-induced procoagulant PLT formation. Most importantly, antibody-induced procoagulant events as well as increased thrombus formation in severe COVID-19 were inhibited by Iloprost a clinically approved therapeutic agent that increases the intracellular cAMP levels in PLTs. Our data indicate that upregulation of cAMP could be a potential therapeutic target to prevent antibody-mediated coagulopathy in COVID-19 disease.

Molecular Proteomics and Signalling of Human Platelets in Health and Disease

Platelets are small anucleate blood cells that play vital roles in haemostasis and thrombosis, besides other physiological and pathophysiological processes. These roles are tightly regulated by a complex network of signalling pathways. Mass spectrometry-based proteomic techniques are contributing not only to the identification and quantification of new platelet proteins, but also reveal post-translational modifications of these molecules, such as acetylation, glycosylation and phosphorylation. Moreover, target proteomic analysis of platelets can provide molecular biomarkers for genetic aberrations with established or non-established links to platelet dysfunctions. In this report, we review 67 reports regarding platelet proteomic analysis and signalling on a molecular base. Collectively, these provide detailed insight into the: (i) technical developments and limitations of the assessment of platelet (sub)proteomes; (ii) molecular protein changes upon ageing of platelets; (iii) complexity of platelet signalling pathways and functions in response to collagen, rhodocytin, thrombin, thromboxane A₂ and ADP; (iv) proteomic effects of endothelial-derived mediators such as prostacyclin and the anti-platelet drug aspirin; and (v) molecular protein changes in platelets from patients with congenital disorders or cardiovascular disease. However, sample sizes are still low and the roles of differentially expressed proteins are often unknown. Based on the practical and technical possibilities and limitations, we provide a perspective for further improvements of the platelet proteomic field.

Thermally driven formation of polyphenolic carbonized nanogels with high anticoagulant activity from polysaccharides

We have demonstrated that alginate with negligible anticoagulant activity can be converted into carbonized nanogels with potent anticoagulant activity through a solid-state heating process. The conversion of alginate into graphene-like nanosheet (GNS)-embedded polyphenolic-alginate nanogels (GNS/Alg-NGs) has been carried out through condensation and carbonization processes. The GNS/Alg-NGs exhibit much stronger anticoagulant activity (>520-fold) compared to untreated alginate, mainly because their polyphenolic structures have a high binding affinity [dissociation constant (K_d) = 2.1 × 10^-10 M] toward thrombin. In addition, the thrombin clotting time delay caused by the GNS/Alg-NGs is 10-fold longer than that of natural polyphenolic compounds, such as quercetin, catechin, naringenin, caffeic acid, and ferulic acid. The thrombin- or kaolin-activated thromboelastography of whole-blood coagulation reveals that the GNS/Alg-NGs display a much stronger anticoagulant ability than that of untreated alginate and naturally sulfated polysaccharides (fucoidan). The GNS/Alg-NGs exhibit superior biocompatibility and anticoagulant activity, as observed with an in vivo rat model, revealing their potential as a blood thinner for the treatment of thrombotic disorders.

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.

Platelet calcium signaling by G-protein coupled and ITAM-linked receptors regulating anoctamin-6 and procoagulant activity

Most agonists stimulate platelet Ca²⁺ rises via G-protein coupled receptors (GPCRs) or ITAM-linked receptors (ILRs). Well studied are the GPCRs stimulated by the soluble agonists thrombin (PAR1, PAR4), ADP (P2Y₁, P2Y₁₂), and thromboxane A₂ (TP), signaling via phospholipase (PLC)β isoforms. The platelet ILRs glycoprotein VI (GPVI), C-type lectin-like receptor 2 (CLEC2), and FcγRIIa are stimulated by adhesive ligands or antibody complexes and signal via tyrosine protein kinases and PLCγ isoforms. Marked differences exist between the GPCR- and ILR-induced Ca²⁺ signaling in: (i) dependency of tyrosine phosphorylation; (ii) oscillatory versus continued Ca²⁺ rises by mobilization from the endoplasmic reticulum; and (iii) smaller or larger role of extracellular Ca²⁺ entry via STIM1/ORAI1. Co-stimulation of both types of receptors, especially by thrombin (PAR1/4) and collagen (GPVI), leads to a highly enforced Ca²⁺ rise, involving mitochondrial Ca²⁺ release, which activates the ion and phospholipid channel, anoctamin-6. This highly Ca²⁺-dependent process causes swelling, ballooning, and phosphatidylserine expression, establishing a unique platelet population swinging between vital and necrotic (procoagulant 'zombie' platelets). Additionally, the high Ca²⁺ status of procoagulant platelets induces a set of additional events: (i) Ca²⁺ dependent cleavage of signaling proteins and receptors via calpain and ADAM isoforms; (ii) microvesiculation; (iii) enhanced coagulation factor binding; and (iv) fibrin-coat formation involving transglutaminases. Given the additive roles of GPCR and ILR in Ca²⁺ signal generation, high-throughput screening of biomolecules or small molecules based on Ca²⁺ flux measurements provides a promising way to find new inhibitors interfering with prolonged high Ca²⁺, phosphatidylserine expression, and hence platelet procoagulant activity.

Ethaninidothioic acid (R5421) is not a selective inhibitor of platelet phospholipid scramblase activity

Background and Purpose

Ethaninidothioic acid (R5421) has been used as a scramblase inhibitor to determine the role of phospholipid scrambling across a range of systems including platelet procoagulant activity. The selectivity of R5421 has not been thoroughly studied. Here, we characterised the effects of R5421 on platelet function and its suitability for use as a scramblase inhibitor.

Experimental Approach

Human platelet activation was measured following pretreatment with R5421 and stimulation with a range of agonists. Phosphatidylserine exposure was measured using annexin V binding. Integrin α_IIbβ₃ activation and α‐granule release were measured by flow cytometry. Cytosolic Ca²⁺ signals were measured using Cal520 fluorescence. An in silico ligand‐based screen identified 16 compounds which were tested in these assays.

Key Results

R5421 inhibited A23187‐induced phosphatidylserine exposure in a time‐ and temperature‐dependent manner. R5421 inhibited Ca²⁺ signalling from the PAR1, PAR4 and glycoprotein VI receptors as well as platelet α_IIbβ₃ integrin activation and α‐granule release. R5421 is therefore not a selective inhibitor of platelet scramblase activity. An in silico screen identified the pesticide thiodicarb as similar to R5421. It also inhibited platelet phosphatidylserine exposure, Ca²⁺ signalling from the PAR1 and glycoprotein VI, α_IIbβ₃ activation and α‐granule release. Thiodicarb additionally disrupted Ca²⁺ homeostasis in unstimulated platelets.

Conclusion and Implications

R5421 is not a selective inhibitor of platelet scramblase activity. We have identified the pesticide thiodicarb, which had similar effects on platelet function to R5421 as well as additional disruption of Ca²⁺ signalling which may underlie some of thiodicarb's toxicity.

Whole Blood Based Multiparameter Assessment of Thrombus Formation in Standard Microfluidic Devices to Proxy In Vivo Haemostasis and Thrombosis

Microfluidic assays are versatile tests which, using only small amounts of blood, enable high throughput analyses of platelet function in several minutes. In combination with fluorescence microscopy, these flow tests allow real-time visualisation of platelet activation with the possibility of examining combinatorial effects of wall shear rate, coagulation and modulation by endothelial cells. In particular, the ability to use blood and blood cells from healthy subjects or patients makes this technology promising, both for research and (pre)clinical diagnostic purposes. In the present review, we describe how microfluidic devices are used to assess the roles of platelets in thrombosis and haemostasis. We place emphasis on technical aspects and on experimental designs that make the concept of "blood-vessel-component-on-a-chip" an attractive, rapidly developing technology for the study of the complex biological processes of blood coagulability in the presence of flow.

Hemoperfusion leads to impairment in hemostasis and coagulation process in patients with acute pesticide intoxication

Hemoperfusion (HP) is one of the important treatment modalities in extracorporeal therapy for patients with acute intoxication. Its use has declined during the past 20 years despite its efficacy, because of its side effects, especially an increased risk of bleeding. Mechanisms of hemostasis impairment have not been clearly elucidated and studies demonstrating the mechanism are lacking. It is not clear which step of the hemostatic process is impaired during HP, and whether it leads to an increased risk of bleeding. We performed both in vivo and in vitro studies to elucidate the mechanism of impairment in the hemostatic process. In patients with acute pesticide intoxication who underwent HP, the platelet count decreased rapidly during the first 30 minutes from 242.4 ± 57.7 × 10³/μL to 184.8 ± 49.6 × 10³/μL, then gradually decreased even lower to 145.4 ± 61.2 × 10³/μL over time (p < 0.001). As markers of platelet activation, platelet distribution width increased continuously during HP from 41.98 ± 9.28% to 47.69 ± 11.18% (p < 0.05), however, mean platelet volume did not show significant change. In scanning electron microscopy, activated platelets adhered to modified charcoal were observed, and delayed closure time after HP in PFA-100 test suggested platelet dysfunction occurred during HP. To confirm these conflicting results, changes of glycoprotein expression on the platelet surface were evaluated when platelets were exposed to modified charcoal in vitro. Platelet expression of CD61, fibrinogen receptor, significantly decreased from 95.2 ± 0.9% to 73.9 ± 1.6%, while those expressing CD42b, von Willebrand factor receptor, did not show significant change. However, platelet expression of CD49b, collagen receptor, significantly increased from 24.6 ± 0.7% to 51.9 ± 2.3%. Thrombin-antithrombin complex, a marker for thrombin generation, appeared to decrease, however, it was not statistically significant. Fibrin degradation products and d-dimers, markers for fibrinolysis, increased significantly during HP. Taken together, our data suggests that hemoperfusion leads to impairment of platelet aggregation with incomplete platelet activation, which was associated with reduced thrombin generation, accompanied by increased fibrinolysis.

Impact of Deficiency of Intrinsic Coagulation Factors XI and XII on Ex Vivo Thrombus Formation and Clot Lysis

The contributions of coagulation factor XI (FXI) and FXII to human clot formation is not fully known. Patients with deficiency in FXI have a variable mild bleeding risk, whereas FXII deficiency is not associated with bleeding. These phenotypes make FXII and FXI attractive target proteins in anticoagulant therapy. Here, we studied the mechanisms of fibrin clot formation, stability, and fibrinolytic degradation in patients with severe FXI or FXII deficiency. Thrombin generation was triggered in platelet-poor (PPP) and platelet-rich plasma (PRP) with the biological FXII trigger sulfatides. Intrinsic and extrinsic thrombus formation and degradation in whole blood were determined with rotational thromboelastometry (ROTEM). Clot formation under flow was assessed by perfusion of whole blood over collagen microspots with(out) tissue factor (TF). Thrombin generation and clot formation were delayed in FXII- and FXI-deficient patients triggered with sulfatides. In FXI-deficient plasma, this delay was more pronounced in PRP compared to PPP. In whole blood of FXII-deficient patients, clots were smaller but resistance to fibrinolysis was normal. In whole blood of FXI-deficient patients, clot formation was normal but the time to complete fibrinolysis was prolonged. In flow chamber experiments triggered with collagen/TF, platelet coverage was reduced in severe compared with moderate FXI deficiency, and fibrin formation was impaired. We conclude that quantitative defects in FXII and FXI have a substantial impact on contact activation-triggered coagulation. Furthermore, FXI deficiency has a dose-dependent suppressing effect on flow-mediated and platelet/TF-dependent clot formation. These last data highlight the contribution of particularly FXI to hemostasis.

The myeloperoxidase product, hypochlorous acid, reduces thrombus formation under flow and attenuates clot retraction and fibrinolysis in human blood

Hypochlorite (HOCl), a strong oxidant and antimicrobial agent, has been proposed to be associated with hemostatic abnormalities during inflammatory response. However, its complex impact on hemostasis is not completely understood. In this report we studied the effect of clinically relevant (micromolar) HOCl concentrations on thrombus formation under flow, kinetics of platelet-fibrin clot formation, its architecture, retraction, and lysis. We found that HOCl (up to 500 µM) did not affect kinetics of coagulation measured in whole blood. HOCl (500-1000 µM) markedly diminished thrombus formation under flow. Clot retraction rate was reduced by HOCl dose-dependently (50-500 µM). HOCl (125-500 µM) inhibited fibrinolysis in whole blood and in platelet-depleted plasma, dose-dependently. Activity of plasmin was reduced by HOCl at concentrations started from 500 µM. HOCl (up to 500 µM) did not reduce plasminogen binding to fibrin under flow. HOCl (125-500 µM) modulated architecture of fibrin- and platelet-fibrin clots towards structures made of thin and densely packed fibers. Exposure of pure fibrinogen to HOCl (10-1000 µM) resulted in formation of dityrosine and was associated with altered fibrin structure derived from such modified fibrinogen. HOCl-altered fibrin net structure was not related with modulation of platelet procoagulant response, thrombin generation, and factor XIII activity. We conclude that, in human blood, clinically relevant HOCl concentrations may inhibit thrombus formation under flow, clot retraction and fibrinolysis. Fibrinolysis and clot retraction seem to be the most sensitive to HOCl-evoked inhibition. HOCl-modified fibrinogen and altered clot structure associated with it are likely to be primary sources of attenuated fibrinolysis.

Comparative Analysis of Microfluidics Thrombus Formation in Multiple Genetically Modified Mice: Link to Thrombosis and Hemostasis

Genetically modified mice are indispensable for establishing the roles of platelets in arterial thrombosis and hemostasis. Microfluidics assays using anticoagulated whole blood are commonly used as integrative proxy tests for platelet function in mice. In the present study, we quantified the changes in collagen-dependent thrombus formation for 38 different strains of (genetically) modified mice, all measured with the same microfluidics chamber. The mice included were deficient in platelet receptors, protein kinases or phosphatases, small GTPases or other signaling or scaffold proteins. By standardized re-analysis of high-resolution microscopic images, detailed information was obtained on altered platelet adhesion, aggregation and/or activation. For a subset of 11 mouse strains, these platelet functions were further evaluated in rhodocytin- and laminin-dependent thrombus formation, thus allowing a comparison of glycoprotein VI (GPVI), C-type lectin-like receptor 2 (CLEC2) and integrin α₆β₁ pathways. High homogeneity was found between wild-type mice datasets concerning adhesion and aggregation parameters. Quantitative comparison for the 38 modified mouse strains resulted in a matrix visualizing the impact of the respective (genetic) deficiency on thrombus formation with detailed insight into the type and extent of altered thrombus signatures. Network analysis revealed strong clusters of genes involved in GPVI signaling and Ca²⁺ homeostasis. The majority of mice demonstrating an antithrombotic phenotype in vivo displayed with a larger or smaller reduction in multi-parameter analysis of collagen-dependent thrombus formation in vitro. Remarkably, in only approximately half of the mouse strains that displayed reduced arterial thrombosis in vivo, this was accompanied by impaired hemostasis. This was also reflected by comparing in vitro thrombus formation (by microfluidics) with alterations in in vivo bleeding time. In conclusion, the presently developed multi-parameter analysis of thrombus formation using microfluidics can be used to: (i) determine the severity of platelet abnormalities; (ii) distinguish between altered platelet adhesion, aggregation and activation; and (iii) elucidate both collagen and non-collagen dependent alterations of thrombus formation. This approach may thereby aid in the better understanding and better assessment of genetic variation that affect in vivo arterial thrombosis and hemostasis.

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-RBC interaction mediated by FasL/FasR induces procoagulant activity important for thrombosis

Red blood cells (RBCs) influence rheology, and release ADP, ATP, and nitric oxide, suggesting a role for RBCs in hemostasis and thrombosis. Here, we provide evidence for a significant contribution of RBCs to thrombus formation. Anemic mice showed enhanced occlusion times upon injury of the carotid artery. A small population of RBCs was located to platelet thrombi and enhanced platelet activation by a direct cell contact via the FasL/FasR (CD95) pathway known to induce apoptosis. Activation of platelets in the presence of RBCs led to platelet FasL exposure that activated FasR on RBCs responsible for externalization of phosphatidylserine (PS) on the RBC membrane. Inhibition or genetic deletion of either FasL or FasR resulted in reduced PS exposure of RBCs and platelets, decreased thrombin generation, and reduced thrombus formation in vitro and protection against arterial thrombosis in vivo. Direct cell contacts between platelets and RBCs via FasL/FasR were shown after ligation of the inferior vena cava (IVC) and in surgical specimens of patients after thrombectomy. In a flow restriction model of the IVC, reduced thrombus formation was observed in FasL-/- mice. Taken together, our data reveal a significant contribution of RBCs to thrombosis by the FasL/FasR pathway.

Integrating platelet and coagulation activation in fibrin clot formation

Platelets interact with the coagulation system in a multitude of ways, not only during the phases of thrombus formation, but also in specific areas within a formed thrombus. This review discusses current concepts of platelet control of thrombin generation, fibrin formation and structure, and anticoagulation. Indicated are how combined signalling via the platelet receptors for collagen (glycoprotein VI) and thrombin induces the secretion of (anti)coagulation factors, as well as surface exposure of phosphatidylserine, thereby catalysing thrombin generation. This procoagulant platelet response is also facilitated by the adhesive complexes glycoprotein Ib-V-IX and integrin αIIbβ3. In the buildup of a platelet-fibrin thrombus, the extrinsic, tissue factor-driven coagulation pathway is predominant in early stages, while the intrinsic, factor XII pathway seems to promote at later time points. Already early generation of thrombin enforces platelet responses and stimulates intra-thrombus heterogeneity with patches of loosely aggregated, contracted, and phosphatidylserine-exposing platelets. Fibrin actively formed on the surface of activated platelets supports thrombus growth, but also captures thrombin. The fibrin distribution in a thrombus appears to rely on the local procoagulant trigger and the blood flow rate. Clinical studies support the importance of the platelet-coagulation interplay, by showing beneficial effects of combination therapy in the secondary prevention of cardiovascular disease.

Thrombosis in adult patients with acute leukemia

PURPOSE OF REVIEW

Recent studies indicate that the risk of thrombosis in hematologic patients may be similar or even higher than that found in patients with solid tumors. However, available information about pathogenesis and incidence of thrombosis in acute leukemia is limited. This review focuses on mechanisms underlying thrombosis in acute leukemia and discusses recent literature data.

RECENT FINDINGS

In the last few years, proofs have been provided that leukemic cells release free prothrombotic products, such as micro-vesicles, tissue factors, circulating free DNA and RNA. Furthermore, leukemic blasts can activate the procoagulant population of platelets, which initiate and amplify coagulation, causing thrombosis. In addition to factors produced by acute leukemia itself, others concur to trigger thrombosis. Some drugs, infections and insertion of central venous catheter have been described to increase risk of thrombosis in patients with acute leukemia.

SUMMARY

Thrombosis represents a serious complication in patients affected by myeloid and lymphoid acute leukemia. A proper knowledge of its pathophysiology and of the predisposing risk factors may allow to implement strategies of prevention. Improving prevention of thrombosis appears a major goal in patients whose frequent conditions of thrombocytopenia impede an adequate delivery of anticoagulant therapy.

Platelet interaction with activated endothelium: mechanistic insights from microfluidics

Traditionally, in vitro flow chamber experiments and in vivo arterial thrombosis studies have been proved to be of vital importance to elucidate the mechanisms of platelet thrombus formation after vessel wall injury. In recent years, it has become clear that platelets also act as modulators of inflammatory processes, such as atherosclerosis. A key element herein is the complex cross talk between platelets, the coagulation system, leukocytes, and the activated endothelium. This review provides insight into the platelet-endothelial interface, based on in vitro flow chamber studies and cross referenced with in vivo thrombosis studies. The main mechanisms of platelet interaction with the activated endothelium encompass (1) platelet rolling via interaction of platelet glycoprotein Ib-IX-V with endothelial-released von Willebrand factor with a supporting role for the P-selectin/P-selectin glycoprotein ligand 1 axis, followed by (2) firm platelet adhesion to the endothelium via interaction of platelet α_IIbβ₃ with endothelial α_vβ₃ and intercellular adhesion molecule 1, and (3) a stimulatory role for thrombin, the thrombospondin-1/CD36 axis and cyclooxygenase 1 in subsequent platelet activation and stable thrombus formation. In addition, the molecular mechanisms underlying the stimulatory effect of platelets on leukocyte transendothelial migration, a key mediator of atheroprogression, are discussed. Throughout the review, emphasis is placed on recommendations for setting up, reporting, interpreting, and comparing endothelial-lined flow chamber studies and suggestions for future studies.

Thrombosis in diabetes: a shear flow effect?

Cardiovascular events are the major cause of morbidity and mortality in Type 2 diabetes (T2D). This condition is associated with heightened platelet reactivity, contributing to increased atherothrombotic risk. Indeed, individuals with diabetes respond inadequately to standard antiplatelet therapy. Furthermore, they often experience recurrent events as well as side effects that include excess bleeding. This highlights the need for identification of novel regulators of diabetes-associated thrombosis to target for therapeutic intervention. It is well established that platelet aggregation, a process essential for thrombus formation, is tightly regulated by shear stress; however, the mechanisms underlying shear activation of platelets, particularly in the setting of diabetes, are still poorly understood. This review will address the limitations of current diagnostic systems to assess the importance of shear stress in the regulation of thrombus formation in T2D, and the inability to recapitulate the pro-thrombotic phenotype seen clinically in the setting of T2D. Moreover, we will discuss recent findings utilizing new technologies to define the importance of shear stress in thrombus formation and their potential application to the setting of diabetes. Finally, we will discuss the potential of targeting shear-dependent mechanisms of thrombus formation as a novel therapeutic approach in the setting of T2D.

Inner Mitochondrial Membrane Disruption Links Apoptotic and Agonist-Initiated Phosphatidylserine Externalization in Platelets

OBJECTIVE

Phosphatidylserine exposure mediates platelet procoagulant function and regulates platelet life span. Apoptotic, necrotic, and integrin-mediated mechanisms have been implicated as intracellular determinants of platelet phosphatidylserine exposure. Here, we investigate (1) the role of mitochondrial events in platelet phosphatidylserine exposure initiated by these distinct stimuli and (2) the cellular interactions of the procoagulant platelet in vitro and in vivo.

APPROACH AND RESULTS

Key mitochondrial events were examined, including cytochrome c release and inner mitochondrial membrane (IMM) disruption. In both ABT-737 (apoptotic) and agonist (necrotic)-treated platelets, phosphatidylserine externalization was temporally correlated with IMM disruption. Agonist stimulation resulted in rapid cyclophilin D-dependent IMM disruption that coincided with phosphatidylserine exposure. ABT-737 treatment caused rapid cytochrome c release, eventually followed by caspase-dependent IMM disruption that again closely coincided with phosphatidylserine exposure. A nonmitochondrial and integrin-mediated mechanism has been implicated in the formation of a novel phosphatidylserine-externalizing platelet subpopulation. Using image cytometry, this subpopulation is demonstrated to be the result of the interaction of an aggregatory platelet and a procoagulant platelet rather than indicative of a novel intracellular mechanism regulating platelet phosphatidylserine externalization. Using electron microscopy, similar interactions between aggregatory and procoagulant platelets are demonstrated in vitro and in vivo within a mesenteric vein hemostatic thrombus.

CONCLUSIONS

Platelet phosphatidylserine externalization is closely associated with the mitochondrial event of IMM disruption identifying a common pathway in phosphatidylserine-externalizing platelets. The limited interaction of procoagulant platelets and integrin-active aggregatory platelets identifies a potential mechanism for procoagulant platelet retention within the hemostatic thrombus.

Platelet populations and priming in hematological diseases

In healthy subjects and patients with hematological diseases, platelet populations can be distinguished with different response spectra in hemostatic and vascular processes. These populations partly overlap, and are less distinct than those of leukocytes. The platelet heterogeneity is linked to structural properties, and is enforced by inequalities in the environment. Contributing factors are variability between megakaryocytes, platelet ageing, and positive or negative priming of platelets during their time in circulation. Within a hemostatic plug or thrombus, platelet heterogeneity is enhanced by unequal exposure to agonists, with populations of contracted platelets in the thrombus core, discoid platelets at the thrombus surface, patches of ballooned and procoagulant platelets forming thrombin, and coated platelets binding fibrin. Several pathophysiological hematological conditions can positively or negatively prime the responsiveness of platelet populations. As a consequence, in vivo and in vitro markers of platelet activation can differ in thrombotic and hematological disorders.

Protein kinase A activation by the anti-cancer drugs ABT-737 and thymoquinone is caspase-3-dependent and correlates with platelet inhibition and apoptosis

Chemotherapy-induced thrombocytopenia is a common bleeding risk in cancer patients and limits chemotherapy dose and frequency. Recent data from mouse and human platelets revealed that activation of protein kinase A/G (PKA/PKG) not only inhibited thrombin/convulxin-induced platelet activation but also prevented the platelet pro-coagulant state. Here we investigated whether or not PKA/PKG activation could attenuate caspase-dependent apoptosis induced by the anti-cancer drugs ABT-737 (the precursor of navitoclax) and thymoquinone (TQ), thereby potentially limiting chemotherapy-induced thrombocytopenia. This is particularly relevant as activation of cyclic nucleotide signalling in combination chemotherapy is an emerging strategy in cancer treatment. However, PKA/PKG-activation, as monitored by phosphorylation of Vasodilator-stimulated phosphoprotein (VASP), did not block caspase-3-dependent platelet apoptosis induced by the compounds. In contrast, both substances induced PKA activation themselves and PKA activation correlated with platelet inhibition and apoptosis. Surprisingly, ABT-737- and TQ-induced VASP-phosphorylation was independent of cAMP levels and neither cyclases nor phosphatases were affected by the drugs. In contrast, however, ABT-737- and TQ-induced PKA activation was blocked by caspase-3 inhibitors. In conclusion, we show that ABT-737 and TQ activate PKA in a caspase-3-dependent manner, which correlates with platelet inhibition and apoptosis and therefore potentially contributes to the bleeding risk in chemotherapy patients.

Development and validation of a high throughput whole blood thrombolysis plate assay

The objective of this work was to develop a high throughput assay for testing in vitro the thrombolytic activity using citrated whole blood samples, and to overcome the limitations of currently available techniques. We successfully developed a method that involves forming halo shaped, tissue factor induced, whole blood clots in 96 well plates, and then precisely measuring the thrombolysis process with a spectrophotometer plate reader. We here describe the implementation of this novel method, which we refer to as halo assay, and its validation with plasmin, urokinase and tissue plasminogen activator at different doses. The resulting data is a highly detailed thrombolysis profile, allowing comparison of different fibrinolytic agents. The time point analysis allows kinetic data to be collected and calculated to determine key parameters such as the activation time and the rate of fibrinolysis. We also assessed the capacity of the model to study the effect of clot maturation time on the fibrinolytic rate, an aspect of thrombosis rather unexplored with currently available methods, but of increasing importance in drug development. This novel thrombolysis assay could be an extremely useful research tool; to study the complex process of thrombolysis, and a valuable translational clinical tool; as a screening device to rapidly identify hypo- or hyper-fibrinolysis.

Matrix Metalloproteinases and Platelet Function

Platelets contain and release several matrix metalloproteinases (MMPs) and their tissue inhibitors of matrix metalloproteinases (TIMPs), including MMP-1, -2, -3, -9, and -14 and TIMP-1, -2, and -4. Although devoid of a nucleus, platelets also synthesize TIMP-2 upon activation. Platelet-released MMPs/TIMPs, as well as MMPs generated by other cells within the cardiovascular system, modulate platelet function in health and disease. In particular, a normal hemostatic platelet response to vessel wall injury may be transformed into pathologic thrombus formation by the release from platelets and/or by the local generation of some MMPs. Moreover, platelets may localize the production of leukocyte-derived MMPs to sites of vascular damage, contributing to atherosclerosis development and complications and to arterial aneurysm formation. Finally, the interaction between platelets and tumor cells is strongly influenced by MMPs/TIMPs. All these mechanisms are emerging as important in atherothrombosis, inflammatory disease, and cancer growth and dissemination. Increasing knowledge of these mechanisms may open the way to novel therapeutic approaches.

Antiplatelet and antithrombotic effects of cordycepin-enriched WIB-801CE from Cordyceps militaris ex vivo, in vivo, and in vitro

BACKGROUND

A species of the fungal genus Cordyceps has been used as a complementary and alternative medicine of traditional Chinese medicine, and its major component cordycepin and cordycepin-enriched WIB-801CE are known to have antiplatelet effects in vitro. However, it is unknown whether they have also endogenous antiplatelet and antithrombotic effects. In this study, to resolve these doubts, we prepared cordycepin-enriched WIB-801CE, an ethanol extract from Cordyceps militaris-hypha, then evaluated its ex vivo, in vivo, and in vitro antiplatelet and antithrombotic effects.

METHODS

Ex vivo effects of WIB-801CE on collagen- and ADP-induced platelet aggregation, serotonin release, thromboxane A2 (TXA2) production and its associated activities of enzymes [cyclooxygenase-1 (COX-1), TXA2 synthase (TXAS)], arachidonic acid (AA) release and its associated phosphorylation of phospholipase Cβ3, phospholipase Cγ2 or cytosolic phospholipase A2, mitogen-activated protein kinase (MAPK) [p38 MAPK, extracellular signal-regulated kinase (ERK)], and blood coagulation time in rats were investigated. In vivo effects of WIB-801CE on collagen plus epinephrine-induced acute pulmonary thromboembolism, and tail bleeding time in mice were also inquired. In vitro effects of WIB-801CE on cytotoxicity, and fibrin clot retraction in human platelets, and nitric oxide (NO) production in RAW264.7 cells or free radical scavenging activity were studied.

RESULTS

Cordycepin-enriched WIB-801CE inhibited ex vivo platelet aggregation, TXA2 production, AA release, TXAS activity, serotonin release, and p38 MAPK and ERK2 phosphorylation in collagen- and ADP-activated rat platelets without affecting blood coagulation. Furthermore, WIB-801CE manifested in vivo inhibitory effect on collagen plus epinephrine-induced pulmonary thromboembolism mice model. WIB-801CE inhibited in vitro NO production and fibrin clot retraction, but elevated free radical scavenging activity without affecting cytotoxicity against human platelets.

CONCLUSION

WIB-801CE inhibited collagen- and ADP-induced platelet activation and its associated thrombus formation ex vivo and in vivo. These were resulted from down-regulation of TXA2 production and its related AA release and TXAS activity, and p38MAPK and ERK2 activation. These results suggest that WIB-801CE has therapeutic potential to treat platelet activation-mediated thrombotic diseases in vivo.

Identification of Potential Biomarkers for Rhegmatogenous Retinal Detachment Associated with Choroidal Detachment by Vitreous iTRAQ-Based Proteomic Profiling

Rhegmatogenous retinal detachment associated with choroidal detachment (RRDCD) is a complicated and serious type of rhegmatogenous retinal detachment (RRD). In this study, we identified differentially expressed proteins in the vitreous humors of RRDCD and RRD using isobaric tags for relative and absolute quantitation (iTRAQ) combined with nano-liquid chromatography-electrospray ion trap-mass spectrometry-mass spectrometry (nano-LC-ESI-MS/MS) and bioinformatic analysis. Our result shows that 103 differentially expressed proteins, including 54 up-regulated and 49 down-regulated proteins were identified in RRDCD. Gene ontology (GO) analysis suggested that most of the differentially expressed proteins were extracellular．The Kyoto encyclopedia of genes and genomes (KEGG) pathway analysis suggested that proteins related to complement and coagulation cascades were significantly enriched. iTRAQ-based proteomic profiling reveals that complement and coagulation cascades and inflammation may play important roles in the pathogenesis of RRDCD. This study may provide novel insights into the pathogenesis of RRDCD and offer potential opportunities for the diagnosis and treatment of RRDCD.

Metalloproteolytic receptor shedding…platelets "acting their age"

Whilst significant effort has been focused on development of tools and approaches to clinically modulate activation processes that consume platelets, the platelet receptors that initiate activation processes remain untargeted. The modulation of receptor levels is also linked to underlying platelet aging processes which influence normal platelet lifespan and also the functionality and survival of stored platelets that are used in transfusion. In this review, we will focus on platelet adhesion receptors initiating thrombus formation, and discuss how regulation of levels of these receptors impact platelet function and platelet survival.

Prevention of arterial thrombosis by edoxaban, an oral factor Xa inhibitor in rats: monotherapy and in combination with antiplatelet agents

In addition to platelet aggregation, coagulation activation is considered to be involved in arterial thrombosis. In this study, we determined antithrombotic effects of edoxaban, an oral factor Xa (FXa) inhibitor, as both a monotherapy and in combination with antiplatelet agents in a rat model of arterial thrombosis. We further examined its effects on a procoagulant biomarker and bleeding. Arterial thrombosis was induced by topical application of 15% ferric chloride to rat abdominal aortas. Bleeding time was measured by a tail incision method. Edoxaban, clopidogrel, and aspirin were orally administered 30min, 4h, and 2h before thrombus or bleeding induction. As a biomarker of coagulation activation, plasma thrombin-antithrombin complex (TAT) was measured. Edoxaban dose-dependently prevented arterial thrombosis in a manner comparable to clopidogrel and aspirin. The combination of edoxaban plus clopidogrel or edoxaban plus aspirin significantly potentiated the antithrombotic effects compared with these drugs alone. The combination of edoxaban and clopidogrel was more potent than clopidogrel and aspirin. Plasma TAT concentration was elevated after thrombus induction and suppressed by edoxaban and clopidogrel, but not by aspirin, suggesting P2Y12 receptor-mediated platelet procoagulant activity. Bleeding time was prolonged by the coadministration of edoxaban and clopidogrel, but not by edoxaban and aspirin. In conclusion, the present study demonstrates that the monotherapy with edoxaban and combination therapy with edoxaban plus clopidogrel or edoxaban plus aspirin are promising options for the prevention of arterial thrombosis as effective as the standard antiplatelet agents; however, a combination of edoxaban and clopidogrel increased the risk of bleeding.

Reduced clot retraction rate and altered platelet energy production in patients with asthma

OBJECTIVE

Asthma enhances the risk of pulmonary embolism. The mechanism of this phenomenon is unclear.

METHODS

We evaluated the kinetics of clot formation, clot retraction rate (CRR), clot volume at 40 min, the rate of lactate production (a marker of aerobic glycolysis in platelets in contracting clots), blood eosinophil count (EOS), nitric oxide in exhaled breath (FENO), and spirometry (FEV1) in 50 healthy controls and in 81 allergic asthmatics (41 subjects with steroid-naïve asthma and 40 with steroid-treated asthma).

RESULTS

Thromboelastometry revealed that only steroid-treated asthmatics had slightly activated coagulation. Compared with healthy controls, whole asthmatics demonstrated (p < 0.05) reduced CRR, higher clot volume at 40 minutes, higher FENO, decreased FEV1, elevated EOS, and augmented lactate production in retracting clots. Reduced CRR was observed also in the absence of native plasma. In whole study population (asthmatics and healthy controls), CRR positively correlated with spirometry (rS = 0.668, p = <0.001) and negatively with FENO (rS = -0.543; p < 0.001), EOS (rS = -0.367, p < 0.002), and lactate production (rS = -0.791; p < 0.001). However, in steroid-treated asthmatics, the CRR did not correlate with FENO and EOS. In all study patients lactate production negatively correlated with FEV1 and positively with FENO.

CONCLUSION

Collectively, this data is consistent with the hypothesis that, in asthmatics, reactive nitrogen species produced in the lungs may reduce platelet contractility (and CRR) through the diminution of platelet energy production. CRR inhibition would predispose asthmatics to pulmonary embolism.

Microparticles as biomarkers of lung disease: enumeration in biological fluids using lipid bilayer microspheres

Extracellular vesicles, specifically microparticles (MPs), are rapidly gaining attention for their capacity to act as biomarkers for diagnosis, prognosis, or responsiveness to therapy in lung disease, in keeping with the concept of precision medicine. However, MP analysis by high-sensitivity flow cytometry (FCM) is complicated by a lack of accurate means for MP enumeration. To address this gap, we report here an enhanced FCM MP gating and enumeration technique based on the use of novel engineered lipid bilayer microspheres (LBMs). By comparison of LBM-based MP enumeration with conventional bead- or fluorescent-based FCM enumeration techniques and a gravimetric consumption gold standard, we found LBMs to be superior to commercial bead preparations, showing the smallest fixed bias and limits of agreement in Bland Altman analyses. LBMs had simultaneous capacity to aid FCM enumeration of MPs in plasma, BAL, and cell culture supernatants. LBM enumeration detected differences in MP counts in mice exposed to intraperitoneal lipopolysaccharide or saline. LBMs provided for 1) higher sensitivity for gating MPs populations, 2) reduced background within MP gates, 3) more appropriate size, and 4) an inexpensive alternative amenable to different fluorescent tags. LBM-based MP enumeration was useful for a series of different FCM systems assessed, whereas LBM gating benefited high- but not low-sensitivity FCM systems compared with fluorescence gating. By offering exclusive advantages over current means of gating and enumerating MPs, LBMs are uniquely suited to realizing the potential of MPs as biomarkers in biological lung fluids and facilitating precision medicine in lung disease.

Regulation of platelet lifespan by apoptosis

The lifespan of platelets in circulation is brief, close to 10 days in humans and 5 days in mice. Bone marrow residing megakaryocytes produce around 100 billion platelets per day. In a healthy individual, the majority of platelets are not consumed by hemostatic processes, but rather their lifespan is controlled by programmed cell death, a canonical intrinsic apoptosis program. In the last decade, insights from genetically manipulated mouse models and pharmacological developments have helped to define the components of the intrinsic, or mitochondrial, apoptosis pathway that controls platelet lifespan. This review focuses on the molecular regulation of apoptosis in platelet survival, reviews thrombocytopenic conditions linked to enhanced platelet death, examines implications of chemotherapy-induced thrombocytopenia through apoptosis-inducing drugs in cancer therapy as well as discusses ex vivo aging of platelets.

Preclinical models of atherosclerosis. The future of Hybrid PET/MR technology for the early detection of vulnerable plaque

Cardiovascular diseases are the leading cause of death in developed countries. The aetiology is currently multifactorial, thus making them very difficult to prevent. Preclinical models of atherothrombotic diseases, including vulnerable plaque-associated complications, are now providing significant insights into pathologies like atherosclerosis, and in combination with the most recent advances in new non-invasive imaging technologies, they have become essential tools to evaluate new therapeutic strategies, with which can forecast and prevent plaque rupture. Positron emission tomography (PET)/computed tomography imaging is currently used for plaque visualisation in clinical and pre-clinical cardiovascular research, albeit with significant limitations. However, the combination of PET and magnetic resonance imaging (MRI) technologies is still the best option available today, as combined PET/MRI scans provide simultaneous data acquisition together with high quality anatomical information, sensitivity and lower radiation exposure for the patient. The coming years may represent a new era for the implementation of PET/MRI in clinical practice, but first, clinically efficient attenuation correction algorithms and research towards multimodal reagents and safety issues should be validated at the preclinical level.

Getting to the Outer Leaflet: Physiology of Phosphatidylserine Exposure at the Plasma Membrane

Phosphatidylserine (PS) is a major component of membrane bilayers whose change in distribution between inner and outer leaflets is an important physiological signal. Normally, members of the type IV P-type ATPases spend metabolic energy to create an asymmetric distribution of phospholipids between the two leaflets, with PS confined to the cytoplasmic membrane leaflet. On occasion, membrane enzymes, known as scramblases, are activated to facilitate transbilayer migration of lipids, including PS. Recently, two proteins required for such randomization have been identified: TMEM16F, a scramblase regulated by elevated intracellular Ca(2+), and XKR8, a caspase-sensitive protein required for PS exposure in apoptotic cells. Once exposed at the cell surface, PS regulates biochemical reactions involved in blood coagulation, and bone mineralization, and also regulates a variety of cell-cell interactions. Exposed on the surface of apoptotic cells, PS controls their recognition and engulfment by other cells. This process is exploited by parasites to invade their host, and in specialized form is used to maintain photoreceptors in the eye and modify synaptic connections in the brain. This review discusses what is known about the mechanism of PS exposure at the surface of the plasma membrane of cells, how actors in the extracellular milieu sense surface exposed PS, and how this recognition is translated to downstream consequences of PS exposure.

Normal platelet function in platelet concentrates requires non-platelet cells: a comparative in vitro evaluation of leucocyte-rich (type 1a) and leucocyte-poor (type 3b) platelet concentrates

Background

Therapeutic success of platelet-rich plasma (PRP) may vary based on the composition and preparation method. The objective of this study was to evaluate the cellular components of platelet concentrates produced by a leucocyte-rich (LR-PRP) and a leucocyte-poor PRP systems (LP-PRP).

Methods

Parameters evaluated included platelet recovery, platelet concentration, red blood cell (RBC) and white blood cell (WBC) composition, platelet growth factor release and stimulation of human tendon cell proliferation in vitro.

Results

Platelet recoveries were 52% for LP-PRP and 89% for LR-PRP. LR-PRP demonstrated greater reproducibility with a 4.2% coefficient of variation (CV) compared with 19.4% for LP-PRP (p<0.001). LR-PRP demonstrated a greater increase in platelet concentration (7.9-fold) than LP-PRP (2.2-fold; p<0.001). LP-PRP showed 5.0-fold reductions in WBCs, while LR-PRP showed a 4.0-fold increase (p<0.001). LP-PRP reduced RBCs to a haematocrit of 0.25, while LR-PRP reduced haematocrit to 11.8. LP-PRP did not coagulate robustly on reactivation with CaCl₂, and released significantly lower levels of epidermal growth factor (EGF) and transforming growth factor β1 (TGF-β1) than whole blood (p<0.03). LP-PRP also did not stimulate tendon cell proliferation greater than whole blood. In contrast, LR-PRP showed increases in each growth factor on activation with CaCl₂ (p<0.01) and stimulated greater proliferation (p<0.05) compared with whole blood. Forced activation of LP-PRP with exogenous thrombin rescued the coagulation deficiency and induced greater growth factor release than comparable whole blood (p<0.03).

Conclusions

These data suggest that non-platelet cellular components in platelet concentrates are important for proper platelet function, including thrombin generation, growth factor release and clot retraction.

Platelet Control of Fibrin Distribution and Microelasticity in Thrombus Formation Under Flow

OBJECTIVE

Platelet- and fibrin-dependent thrombus formation is regulated by blood flow and exposure of collagen and tissue factor. However, interactions between these blood-borne and vascular components are not well understood.

APPROACH AND RESULTS

Here, we developed a method to assess whole-blood thrombus formation on microspots with defined amounts of collagen and tissue factor, allowing determination of the mechanical properties and intrathrombus composition. Confining the collagen content resulted in diminished platelet deposition and fibrin formation at high shear flow conditions, but this effect was compensated by a larger thrombus size and increased accumulation of fibrin in the luminal regions of the thrombi at the expense of the base regions. These thrombi were more dependent on tissue factor-triggered thrombin generation. Microforce nanoindentation analysis revealed a significantly increased microelasticity of thrombi with luminal-oriented fibrin. At a low shear rate, fibrin fibers tended to luminally cover the thrombi, again resulting in a higher microelasticity. Studies with blood from patients with distinct hemostatic insufficiencies indicated an impairment in the formation of a platelet-fibrin thrombus in the cases of dilutional coagulopathy, thrombocytopenia, Scott syndrome, and hemophilia B.

CONCLUSIONS

Taken together, our data indicate that (1) thrombin increases the platelet thrombus volume; (2) tissue factor drives the formation of fibrin outside of the platelet thrombus; (3) limitation of platelet adhesion redirects fibrin from bottom to top of the thrombus; (4) a lower shear rate promotes thrombus coverage with fibrin; (5) the fibrin distribution pattern determines thrombus microelasticity; and (6) the thrombus-forming process is reduced in patients with diverse hemostatic defects.