Procoagulant Phosphatidylserine-Exposing Platelets in vitro and in vivo

Artificial Plateletoids Recruit Blood Proteins to Shield Symbiotic Thrombin: a Silk Fibroin/Calcium Interface Medicated Thrombin Generation and Preservation

Breaking the constraints of thrombin during storage and in vivo applications remains challenging because of its low stability and sensitivity to environmental temperature and acidity. Herein, an artificial plateletoid is developed for in situ thrombin generation through a co-incubation approach with plasma in vitro, utilizing a silk fibroin/Ca2+ interface, to enhance the activity and stability of the generated thrombin. Notably, the enzymatic activity of the plateletoid thrombin platform is as high as 30 U g-1, leading to rapid clotting within 55 s, and it persisted for at least 90 days at as high as 37 °C. This considerably lessens the difficulties associated with maintaining the cold chain while storing and shipping thrombin formulations. Additionally, a gastric bleeding model confirmed that the plateletoid platform improved the acid resistance of thrombin by upregulating the pH of the gastric environment (pH 0.8), facilitating oral delivery of thrombin for effective hemorrhage control in highly acidic stomach conditions. This pioneering study addresses the constraints of thrombin in storage and in vivo applications and may provide a basis for further research on biological storage and delivery approaches.

Brief report: Chronic murine schistosomiasis causes aberrant hemostasis

Schistosomiasis afflicts >250 million people worldwide, leading to an annual loss of >3 million disability-adjusted life years. Schistosoma mansoni causes intestinal schistosomiasis with parasite eggs either transversing intestinal tissue or lodging within the liver and other organs, causing intestinal hemorrhage and liver pathology. Large (∼1 cm) adult worms survive for years within blood vessels, but we lack a clear understanding of their impact on hemostasis. We used a chronic mouse model of schistosomiasis to determine the impact on platelet numbers, phenotype and function. Hemostatic function was assessed by platelet phenotyping (flow cytometry and proteomics), whole blood aggregometry, and longitudinal coagulometry. Although platelets from schistosome-infected mice lack elevated surface P-selectin and activated αIIbβ3, unbiased proteomic analysis reveals infection-induced increases in MHC-I, IgM and IgG antibodies, and complement components. Whole blood from schistosome-infected mice spontaneously aggregates in the absence of exogenous agonists. Conversely, prothrombin and activated partial thromboplastin times are prolonged at the chronic stage of infection (10-12 weeks). A mouse model of S. mansoni infection shows wide-ranging changes in hemostatic function which may have clinically relevant implications for populations in endemic regions.

Phosphatidylserine-blocking nanoparticles inhibit thrombosis without increased bleeding in mice

BACKGROUND

Phosphatidylserine (PS) is a procoagulant phospholipid enriched on surfaces of activated vascular cells including platelets, endothelium, monocytes, and microvesicles. As a molecular driver of thrombosis accessible to drug blockade, PS is an attractive pharmacologic target for modulating thrombogenesis, with potentially reduced bleeding risk compared to anticoagulant and antiplatelet therapies.

OBJECTIVES

Test antithrombotic capabilities of a liposomal formulation, Zn-dipicolylamine cyanine-3[22,22]/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (molar ratio, 3:97), designated as DPAL, which we previously described binds selectively to PS-enriched cell surfaces, compared with effects on bleeding, in mouse models.

METHODS

PS-dependent DPAL binding to human and murine platelets was tested in vitro. Thrombosis and bleeding after DPAL intravenous administration were tested in C57Bl/6J mice following FeCl3 carotid arterial injury and tail tip amputation, respectively. Incorporation in hemostatic clots was investigated in the cremaster muscle laser injury model. Toxicity was tested by direct exposure to human endothelial cell cultures.

RESULTS

DPAL bound agonist-stimulated, PS-positive human and murine platelets, blocked by Annexin V or Ano6 deletion, which ablate PS exposure. DPAL prolonged prothrombin time, but did not prevent thrombin-induced fibrinogen receptor activation or aggregation, nor alter blood cell counts including platelets. Following arteriolar laser injury, DPAL bound wound surfaces and edges without destabilizing plugs. DPAL dose-dependently blocked FeCl3-induced arterial thrombosis but did not substantially increase bleeding, or induce endothelial cell death.

CONCLUSION

DPAL reduces thrombogenesis with minimal effects on bleeding in mouse models via selective binding to PS. DPAL may support novel approaches to modulate pathogenic thrombin generation with improved safety profiles in multiple contexts.

Mice with Reduced PAR4 Reactivity show Decreased Venous Thrombosis and Platelet Procoagulant Activity

Background

Hypercoagulation and thrombin generation are major risk factors for venous thrombosis. Sustained thrombin signaling through PAR4 promotes platelet activation, phosphatidylserine exposure, and subsequent thrombin generation. A single-nucleotide polymorphism in PAR4 (rs2227376) changes proline to leucine extracellular loop 3 (P310L), which decreases PAR4 reactivity and is associated with a lower risk for venous thromboembolism (VTE) in a GWAS meta-analysis.

Objective

The goal of this study is to determine the mechanism for the association of rs2227376 with reduced risk for VTE in using mice with a homologous mutation (PAR4-P322L).

Methods

Venous thrombosis was examined using our recently generated PAR4-P322L mice in the inferior vena cava stasis and stenosis models. Coagulation and clot stability was measured using rotational thromboelastometry (ROTEM). Thrombin generating potential was measured in platelet-rich plasma. Phosphatidylserine surface expression and platelet-neutrophil aggregates were analyzed using flow cytometry.

Results

PAR4P/L and PAR4L/L had reduced incidence and size of venous clots at 48 hours. PAR4P/L and PAR4L/L platelets had progressively decreased phosphatidylserine in response to thrombin and convulxin, which led to decreased thrombin generation and decreased PAR4-mediated platelet-neutrophil aggregation.

Conclusions

The leucine allele in extracellular loop 3, PAR4-322L leads to fewer procoagulant platelets and decreased endogenous thrombin potential. This decreased ability to generate thrombin offers a mechanism for PAR4's role in VTE highlighting a key role for PAR4 signaling.

Substrates, Cofactors, and Cellular Targets of Coagulation Factor XIa

Coagulation factor XI (FXI) has increasingly been shown to play an integral role in several physiologic and pathological processes. FXI is among several zymogens within the blood coagulation cascade that are activated by proteolytic cleavage, with FXI converting to the active serine protease form (FXIa). The evolutionary origins of FXI trace back to duplication of the gene that transcribes plasma prekallikrein, a key factor in the plasma kallikrein-kinin system, before further genetic divergence led to FXI playing a unique role in blood coagulation. While FXIa is canonically known for activating the intrinsic pathway of coagulation by catalyzing the conversion of FIX into FIXa, it is promiscuous in nature and has been shown to contribute to thrombin generation independent of FIX. In addition to its role in the intrinsic pathway of coagulation, FXI also interacts with platelets, endothelial cells, and mediates the inflammatory response through activation of FXII and cleavage of high-molecular-weight kininogen to generate bradykinin. In this manuscript, we critically review the current body of knowledge surrounding how FXI navigates the interplay of hemostasis, inflammatory processes, and the immune response and highlight future avenues for research. As FXI continues to be clinically explored as a druggable therapeutic target, understanding how this coagulation factor fits into physiological and disease mechanisms becomes increasingly important.

Dissecting the roles of dynamin and clathrin in platelet pinocytosis

Platelets endocytose many molecules from their environment. However, this process of pinocytosis in platelets is poorly understood. Key endocytic regulators such as dynamin, clathrin, CDC42 and Arf6 are expressed in platelets but their roles in pinocytosis is not known. Stimulated platelets form two subpopulations of pro-aggregatory and procoagulant platelets. The effect of stimulation on pinocytosis is also poorly understood. In this study, washed human platelets were treated with a range of endocytosis inhibitors and stimulated using different activators. The rate of pinocytosis was assessed using pHrodo green, a pH-sensitive 10 kDa dextran. In unstimulated platelets, pHrodo fluorescence increased over time and accumulated as intracellular puncta indicating constituently active pinocytosis. Stimulated platelets (both pro-aggregatory and procoagulant) had an elevated pinocytosis rate compared to unstimulated platelets. Dynamin inhibition blocked pinocytosis in unstimulated, pro-aggregatory and procoagulant platelets indicating that most platelet pinocytosis is dynamin dependent. Although pinocytosis was clathrin-independent in unstimulated and procoagulant populations, clathrin partially contributed to pinocytosis in pro-aggregatory platelets.

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

ABSTRACT

Interplay between platelets, coagulation factors, endothelial cells (ECs), and fibrinolytic factors is necessary for effective hemostatic plug formation. This study describes a 4-dimensional (4D) imaging platform to visualize and quantify hemostatic plug components in mice with high spatiotemporal resolution. Fibrin accumulation after laser-induced vascular injury was observed at the platelet plug-EC interface, controlled by the antagonistic balance between fibrin generation and breakdown. We observed less fibrin accumulation in mice expressing low levels of tissue factor or F12-/-mice compared with controls, whereas increased fibrin accumulation, including on the vasculature adjacent to the platelet plug, was observed in plasminogen-deficient mice or wild-type mice treated with tranexamic acid. Phosphatidylserine (PS), a membrane lipid critical for the assembly of coagulation factors, was first detected at the platelet plug-EC interface, followed by exposure across the endothelium. Impaired PS exposure resulted in a significant reduction in fibrin accumulation in cyclophilin D-/-mice. Adoptive transfer studies demonstrated a key role for PS exposure on platelets, and to a lesser degree on ECs, in fibrin accumulation during hemostatic plug formation. Together, these studies suggest that (1) platelets are the functionally dominant procoagulant cellular surface, and (2) plasmin is critical for limiting fibrin accumulation at the site of a forming hemostatic plug.

Hutchinson-Gilford progeria syndrome mice display accelerated arterial thrombus formation and increased platelet reactivity

INTRODUCTION

Hutchinson-Gilford Progeria Syndrome (HGPS) is an ultra-rare premature aging genetic disorder caused by a point mutation in the lamin A gene, LMNA. Children with HGPS display short lifespans and typically die due to myocardial infarction or ischemic stroke, both acute cardiovascular events that are tightly linked to arterial thrombosis. Despite this fact, the effect of the classic HGPS LMNA gene mutation on arterial thrombosis remains unknown.

METHODS

Heterozygous LmnaG609G knock-in (LmnaG609G/+) mice, yielding an equivalent classic mutation observed in HGPS patients (c.1824C>T; pG608G mutation in the human LMNA gene) and corresponding wild-type (WT) control littermates underwent photochemically laser-induced carotid injury to trigger thrombosis. Coagulation and fibrinolytic factors were measured. Furthermore, platelet activation and reactivity were investigated.

RESULTS

LmnaG609G/+ mice displayed accelerated arterial thrombus formation, as underlined by shortened time to occlusion compared to WT littermates. Levels of factors involved in the coagulation and fibrinolytic system were comparable between groups, while LmnaG609G/+ animals showed higher plasma levels of thrombin-antithrombin complex and lower levels of antithrombin. Bone marrow analysis showed larger megakaryocytes in progeric mice. Lastly, enhanced platelet activation upon adenosine diphosphate, collagen-related peptide, and thrombin stimulation was observed in LmnaG609G/+ animals compared to the WT group, indicating a higher platelet reactivity in progeric animals.

CONCLUSIONS

LMNA mutation in HGPS mice accelerates arterial thrombus formation, which is mediated, at least in part, by enhanced platelet reactivity, which consequently augments thrombin generation. Given the wide spectrum of antiplatelet agents available clinically, further investigation is warranted to consider the most suitable antiplatelet regimen for children with HGPS to mitigate disease mortality and morbidity.

Lysophosphatidylcholine induces oxidative stress and calcium-mediated cell death in human blood platelets

Platelets are essential component of circulation that plays a major role in hemostasis and thrombosis. During activation and its demise, platelets release platelet-derived microvesicles, with lysophosphatidylcholine (LPC) being a prominent component in their lipid composition. LPC, an oxidized low-density lipoprotein, is involved in cellular metabolism, but its higher level is implicated in pathologies like atherosclerosis, diabetes, and inflammatory disorders. Despite this, its impact on platelet function remains relatively unexplored. To address this, we studied LPC's effects on washed human platelets. A multimode plate reader was employed to measure reactive oxygen species and intracellular calcium using H2DCF-DA and Fluo-4-AM, respectively. Flow cytometry was utilized to measure phosphatidylserine expression, mitochondrial membrane potential (ΔΨm), and mitochondrial permeability transition pore (mPTP) formation using FITC-Annexin V, JC-1, and CoCl2/calcein-AM, respectively. Additionally, platelet morphology and its ultrastructure were observed via phase contrast and electron microscopy. Sonoclot and light transmission aggregometry were employed to examine fibrin formation and platelet aggregation, respectively. The findings demonstrate that LPC induced oxidative stress and increased intracellular calcium in platelets, resulting in increased phosphatidylserine expression and reduced ΔΨm. LPC triggered caspase-independent platelet death and mPTP opening via cytosolic and mitochondrial calcium, along with microvesiculation and reduced platelet counts. LPC increased the platelet's size, adopting a balloon-shaped morphology, causing membrane fragmentation and releasing its cellular contents, while inducing a pro-coagulant phenotype with increased fibrin formation and reduced integrin αIIbβ3 activation. Conclusively, this study reveals LPC-induced oxidative stress and calcium-mediated platelet death, necrotic in nature with pro-coagulant properties, potentially impacting inflammation and repair mechanisms during vascular injury.

Cancer progression and tumor hypercoagulability: a platelet perspective

Venous thromboembolism, which is common in cancer patients and accompanies or even precedes malignant tumors, is known as cancer-related thrombosis and is an important cause of cancer- associated death. At present, the exact etiology of the elevated incidence of venous thrombosis in cancer patients remains elusive. Platelets play a crucial role in blood coagulation, which is intimately linked to the development of arterial thrombosis. Additionally, platelets contribute to tumor progression and facilitate immune evasion by tumors. Tumor cells can interact with the coagulation system through various mechanisms, such as producing hemostatic proteins, activating platelets, and directly adhering to normal cells. The relationship between platelets and malignant tumors is also significant. In this review article, we will explore these connections.

Fibrinogen binding to activated platelets and its biomimetic thrombus-targeted thrombolytic strategies

Thrombosis is associated with various fatal arteriovenous syndromes including ischemic stroke, myocardial infarction, and pulmonary embolism. However, current clinical thrombolytic treatment strategies still have many problems in targeting and safety to meet the thrombolytic therapy needs. Understanding the molecular mechanism that underlies thrombosis is critical in developing effective thrombolytic strategies. It is well known that platelets play a central role in thrombosis and the binding of fibrinogen to activated platelets is a common pathway in the process of clot formation. Based on this, a concept of biomimetic thrombus-targeted thrombolytic strategy inspired from fibrinogen binding to activated platelets in thrombosis was proposed, which could selectively bind to activated platelets at a thrombus site, thus enabling targeted delivery and local release of thrombolytic agents for effective thrombolysis. In this review, we first summarized the main characteristics of platelets and fibrinogen, and then introduced the classical molecular mechanisms of thrombosis, including platelet adhesion, platelet activation and platelet aggregation through the interactions of activated platelets with fibrinogen. In addition, we highlighted the recent advances in biomimetic thrombus-targeted thrombolytic strategies which inspired from fibrinogen binding to activated platelets in thrombosis. The possible future directions and perspectives in this emerging area are briefly discussed.

Direct red blood cell effect on thrombosis is dependent on the interaction of tissue factor and calcium with membrane phosphatidylserine

BACKGROUND

Prior literature has implicated red blood cells (RBCs) in the initiation of thrombosis and suggests that posttransfusion hypercoagulability may occur secondary to the effects of RBCs. Elevated serum tissue factor is a known sequelae of acute trauma. Phosphatidylserine (PS) is a prothrombotic phospholipid present within the RBC cell membrane. We hypothesized that RBC aggregation is dependent on the interaction between RBC membrane bound (exposed) PS, extracellular calcium, and tissue factor.

METHODS

Human whole blood (WB) was separated into components, including RBCs and platelet-rich plasma (PRP). Whole blood, PRP, and RBCs underwent impedance aggregometry utilizing arachidonic acid (AA), ADP, collagen, calcium, and tissue factor (TF)-based agonists. Red blood cells then underwent impedance aggregometry utilizing combined calcium and TF agonists. Red blood cells were pretreated with Annexin V, a known PS blocking agent, and underwent impedance aggregometry with combined calcium and TF agonists to determine if the mechanism of calcium/TF-induced RBC aggregability is dependent on PS. Red blood cells treated with calcium, TF, calcium+TF, and pre-treated with Annexin V followed by calcium+TF were perfused through an in vitro model of pulmonary microcirculatory flow.

RESULTS

Red blood cell aggregation was significantly higher than that of WB and PRP when utilizing a TF agonist, an effect unique to TF. The combination of calcium and TF demonstrated significantly higher RBC aggregation than either agonist alone. Pretreatment with Annexin V resulted in a significantly reduced aggregability of RBC following treatment with TF + calcium. Red blood cells aged to 42 days did not exhibit significant change in aggregation. Exposure to calcium and TF significantly reduced time to thrombosis of RBCs perfused through a pulmonary microcirculatory model.

CONCLUSION

Treatment with both TF and calcium synergistically induces RBC aggregation. Phosphatidylserine appears to play an integral role in the TF/calcium-based, age-independent RBC aggregation response. Red blood cells treated with TF + calcium exhibit more rapid thrombus formation in an in vitro model of pulmonary microcirculatory perfusion.

Calprotectin Impairs Platelet Survival in Patients With Primary Antiphospholipid Syndrome

OBJECTIVE

While thrombosis and pregnancy loss are the best-known clinical features of antiphospholipid syndrome (APS), many patients also exhibit "extra-criteria" manifestations, such as thrombocytopenia. The mechanisms that drive APS thrombocytopenia are not completely understood, and no clinical biomarkers are available for predicting antiphospholipid antibody (aPL)-mediated thrombocytopenia. Calprotectin is a heterodimer of S100A8 and S100A9 that is abundant in the neutrophil cytoplasm and released upon proinflammatory neutrophil activation. Here, we sought to evaluate the presence, clinical associations, and potential mechanistic roles of circulating calprotectin in a cohort of primary APS and aPL-positive patients.

METHODS

Levels of circulating calprotectin were determined in plasma by the QUANTA Flash chemiluminescent assay. A viability dye-based platelet assay was used to assess the potential impact of calprotectin on aPL-mediated thrombocytopenia.

RESULTS

Circulating calprotectin was measured in 112 patients with primary APS and 30 aPL-positive (without APS criteria manifestations or lupus) patients as compared to patients with lupus (without APS), patients with unprovoked venous thrombosis (without aPL), and healthy controls. Levels of calprotectin were higher in patients with primary APS and aPL-positive patients compared to healthy controls. After adjustment for age and sex, calprotectin level correlated positively with absolute neutrophil count (r = 0.41, P < 0.001), positively with C-reactive protein level (r = 0.34, P = 0.002), and negatively with platelet count (r = -0.24, P = 0.004). Mechanistically, we found that calprotectin provoked aPL-mediated thrombocytopenia by engaging platelet surface toll-like receptor 4 and activating the NLRP3-inflammasome, thereby reducing platelet viability in a caspase-1-dependent manner.

CONCLUSION

These data suggest that calprotectin has the potential to be a functional biomarker and a new therapeutic target for APS thrombocytopenia.

Thioredoxin-related transmembrane protein 1 negatively regulates coagulation and phosphatidylserine exposure

Background

Five secreted platelet protein disulfide isomerases (PDIs) and 1 transmembrane PDI regulate platelet function and thrombosis. Thioredoxin-related transmembrane protein 1 (TMX1) was the first member of the PDI family found to negatively regulate platelet aggregation and platelet accumulation in vivo. The effect of TMX1 on coagulation is unknown.

Objectives

To determine the effect of TMX1 on coagulation.

Methods

TMX1-/- mice were used to study platelet accumulation and fibrin deposition in vivo in the laser-induced thrombosis injury model. Annexin V deposition at the site of vascular injury was studied using conditional TMX1 knockout mice. Annexin V binding to platelets was studied using human platelets, anti-TMX1 antibodies, and TMX1-deficient platelets.

Results

TMX1-/- mice had increased fibrin deposition that was reversed with infusion of recombinant TMX1. Infusion of recombinant TMX1 inhibited platelet accumulation and fibrin deposition in wild-type mice and inhibited fibrin deposition in β3-null mice. Platelet accumulation is absent in β3-null mice, suggesting that TMX1 inhibits coagulation independently of platelets. Annexin V binding was increased in activated human platelets incubated with an anti-TMX1 antibody and mouse platelets lacking TMX1. Addition of recombinant TMX1 decreased annexin V binding to platelets. Annexin V binding was increased at the site of vascular injury in Tie2-Cre/TMX1fl/fl mice deficient in endothelial cell TMX1.

Conclusion

TMX1 decreases coagulation at the site of vascular injury and negatively regulates phosphatidylserine exposure on endothelial cells and platelets.

Extracellular vesicles from activated platelets possess a phospholipid-rich biomolecular profile and enhance prothrombinase activity

BACKGROUND

Extracellular vesicles (EVs), in particular those derived from activated platelets, are associated with a risk of future venous thromboembolism.

OBJECTIVES

To study the biomolecular profile and function characteristics of EVs from control (unstimulated) and activated platelets.

METHODS

Biomolecular profiling of single or very few (1-4) platelet-EVs (control/stimulated) was performed by Raman tweezers microspectroscopy. The effects of such EVs on the coagulation system were comprehensively studied.

RESULTS

Raman tweezers microspectroscopy of platelet-EVs followed by biomolecular component analysis revealed for the first time 3 subsets of EVs: (i) protein rich, (ii) protein/lipid rich, and (iii) lipid rich. EVs from control platelets presented a heterogeneous biomolecular profile, with protein-rich EVs being the main subset (58.7% ± 3.5%). Notably, the protein-rich subset may contain a minor contribution from other extracellular particles, including protein aggregates. In contrast, EVs from activated platelets were more homogeneous, dominated by the protein/lipid-rich subset (>85%), and enriched in phospholipids. Functionally, EVs from activated platelets increased thrombin generation by 52.4% and shortened plasma coagulation time by 34.6% ± 10.0% compared with 18.6% ± 13.9% mediated by EVs from control platelets (P = .015). The increased procoagulant activity was predominantly mediated by phosphatidylserine. Detailed investigation showed that EVs from activated platelets increased the activity of the prothrombinase complex (factor Va:FXa:FII) by more than 6-fold.

CONCLUSION

Our study reports a novel quantitative biomolecular characterization of platelet-EVs possessing a homogenous and phospholipid-enriched profile in response to platelet activation. Such characteristics are accompanied with an increased phosphatidylserine-dependent procoagulant activity. Further investigation of a possible role of platelet-EVs in the pathogenesis of venous thromboembolism is warranted.

Phosphatidylserine accelerates wound healing and reduces necrosis in the rats: Growth factor activation

To examine the effect of topical phosphatidylserine (PS) on wound healing factors and tissue necrosis in in vivo models. Topical PS was applied to evaluate aspects of the wound healing process and growth factors production of vascular endothelial growth factors (VEGF) as well a necrosis reduction in the skin flap of rat models. Moreover, phenytoin (PHT) and cyclosporine A (CsA) were used topically as positive control treatments in wound and necrosis models, respectively. Immunohistochemistry (IHC) VEGF, transforming growth factor-β (TGF-β), fibroblast growth factor (FGF) and histopathology were analysed on the wounds of rats. In the necrosis assessment, necrotic areas were determined on photography taken from the back skin of rats. Results indicated that PS topically enhanced significantly (P < 0.05) numbers of fibroblasts and endothelium while inhibiting the neutrophils and macrophages during the 14 days of wound treatment. Moreover, higher values of collagen deposition and epithelialization scores as well as wound recovery percentage (near 80%) were determined significantly (P < 0.05) in the PS group compared with the control. IHC analysis determined that FGF and VEGF cytokine factors were elevated in the wound site by topical PS. Moreover, the necrotic area was significantly (P < 0.05) improved in the PS group. Our experiment indicated that wound improvement and flap survival values in PS treatments were superior to PHT and CsA control groups, respectively. In conclusion, these findings suggest the potential of PS application in the healing of wounds and control of necrosis development after surgery or skin injuries.

The fluorochrome-to-protein ratio is crucial for the flow cytometric detection of tissue factor on extracellular vesicles

Extracellular vesicles (EVs) have crucial roles in hemostasis and coagulation. They sustain coagulation by exposing phosphatidylserine and initiate clotting by surface expression of tissue factor (TF) under inflammatory conditions. As their relevance as biomarkers of coagulopathy is increasingly recognized, there is a need for the sensitive and reliable detection of TF+ EVs, but their flow cytometric analysis is challenging and has yielded controversial findings for TF expression on EVs in the vascular system. We investigated the effect of different fluorochrome-to-protein (F/P) ratios of anti-TF-fluorochrome conjugates on the flow cytometric detection of TF+ EVs from activated monocytes, mesenchymal stem cells (MSCs), and in COVID-19 plasma. Using a FITC-labeled anti-TF antibody (clone VD8), we show that the percentage of TF+ EVs declined with decreasing F/P ratios. TF was detected on 7.6%, 5.4%, and 1.1% of all EVs derived from activated monocytes at F/P ratios of 7.7:1, 6.6:1, and 5.2:1. A similar decline was observed for EVs from MSCs and for EVs in plasma, whereas the detection of TF on cells remained unaffected by different F/P ratios. We provide clear evidence that next to the antibody clone, the F/P ratio affects the flow cytometric detection of TF+ EVs and should be carefully controlled.

Research into New Molecular Mechanisms in Thrombotic Diseases Paves the Way for Innovative Therapeutic Approaches

Thrombosis is a multifaceted process involving various molecular components, including the coagulation cascade, platelet activation, platelet-endothelial interaction, anticoagulant signaling pathways, inflammatory mediators, genetic factors and the involvement of various cells such as endothelial cells, platelets and leukocytes. A comprehensive understanding of the molecular signaling pathways and cell interactions that play a role in thrombosis is essential for the development of precise therapeutic strategies for the treatment and prevention of thrombotic diseases. Ongoing research in this field is constantly uncovering new molecular players and pathways that offer opportunities for more precise interventions in the clinical setting. These molecular insights into thrombosis form the basis for the development of targeted therapeutic approaches for the treatment and prevention of thrombotic disease. The aim of this review is to provide an overview of the pathogenesis of thrombosis and to explore new therapeutic options.

Heparin-activated procoagulant platelet assay: a flow cytometry-based functional test for heparin-induced thrombocytopenia

BACKGROUND

Functional platelet activation assays are required for the diagnosis of heparin-induced thrombocytopenia (HIT). Due to their sophisticated methodology, they are only available in reference centers.

OBJECTIVES

To evaluate the diagnostic accuracy of the flow cytometry-based heparin-activated procoagulant platelet (HAPP) assay in the laboratory diagnosis of HIT.

METHODS

Procoagulant platelets (PCP), defined by the expression of phosphatidylserine and CD62-P, were evaluated by flow cytometry in platelet-rich plasma from healthy donors after incubation with patient sera in the absence and presence of heparin. A sample was considered positive in HAPP assay, if the following 3 criteria were met: 1) the percentage of PCPs was ≥10.3% after incubation with 0.2 IU/mL heparin, 2) the fold increase in presence of 0.2 IU/mL heparin compared with buffer was ≥1.5, and 3) 100 IU/mL of heparin resulted in ≥50% inhibition of PCP. HAPP assay was validated in a prospective cohort (n = 202) of consecutive specimens submitted to our laboratory for serologic diagnosis of HIT. Heparin-induced platelet activation (HIPA) assay was used as the reference standard.

RESULTS

HIT-positive sera induced PCPs in the presence of 0.2 IU/mL heparin, which was inhibited with 100 IU/mL of heparin. In the prospective validation cohort, there were 15 HIPA+ and 187 HIPA- sera. HAPP was positive in 20 samples in this cohort. Using optimized cut-offs, HAPP assay had a sensitivity of 93.3% and specificity of 96.8%.

CONCLUSION

HAPP assay is promising as a simple and reliable functional assay for HIT; however, further studies are needed to confirm our results in larger cohorts.

A novel GATA1 variant p.G229D causing the defect of procoagulant platelet formation

INTRODUCTION

GATA1 is one of the master transcription factors in hematopoietic lineages development which is crucial for megakaryocytic differentiation and maturation. Previous studies have shown that distinct GATA1 variants are associated with varying severities of macrothrombocytopenia and platelet dysfunction.

OBJECTIVE

To determine the underlying pathological mechanisms of a novel GATA1 variant (c. 686G > A, p. G229D) in a patient with recurrent traumatic muscle hematomas.

METHODS

Comprehensive phenotypic analysis of the patient platelets was performed. Procoagulant platelet formation and function were detected using flow cytometry assay and thrombin generation test (TGT), respectively. The ANO6 expression was measured by qPCR and western blot. The intracellular supramaximal calcium flux was detected by Fluo-5N fluorescent assay.

RESULTS

The patient displayed mild macrothrombocytopenia with defects of platelet granules, aggregation, and integrin αIIbβ3 activation. The percentage of the procoagulant platelet formation of the patient upon the stimulation of thrombin plus collagen was lower than that of the healthy controls (40.9 % vs 49.0 % ± 5.1 %). The patient platelets exhibited a marked reduction of thrombin generation in platelet rich plasma TGT compared to the healthy controls (peak value: ∼70 % of the healthy controls; the endogenous thrombin potential: ∼40 % of the healthy controls). The expression of ANO6 and intracellular calcium flux were impaired, which together with abnormal granules of the patient platelets might contribute to defect of procoagulant platelet function.

CONCLUSIONS

The G229D variant could lead to a novel platelet phenotype characterized by defective procoagulant platelet formation and function, which extended the range of GATA1 variants associated platelet disorders.

Platelet-targeted gene therapy induces immune tolerance in hemophilia and beyond

Blood platelets have unique storage and delivery capabilities. Platelets play fundamental roles in hemostasis, inflammatory reactions, and immune responses. Beyond their functions, platelets have been used as a target for gene therapy. Platelet-targeted gene therapy aims to deliver a sustained expression of neo-protein in vivo by genetically modifying the target cells, resulting in a cure for the disease. Even though there has been substantial progress in the field of gene therapy, the potential development of immune responses to transgene products or vectors remains a significant concern. Of note, multiple preclinical studies using platelet-specific lentiviral gene delivery to hematopoietic stem cells in hemophilia have demonstrated promising results with therapeutic levels of neo-protein that rescue the hemorrhagic bleeding phenotype and induce antigen-specific immune tolerance. Further studies using ovalbumin as a surrogate protein for platelet gene therapy have shown robust antigen-specific immune tolerance induced via peripheral clonal deletions of antigen-specific CD4- and CD8-T effector cells and induction of antigen-specific regulatory T (Treg) cells. This review discusses platelet-targeted gene therapy, focusing on immune tolerance induction.

Unraveling the Multifaceted Roles of Extracellular Vesicles: Insights into Biology, Pharmacology, and Pharmaceutical Applications for Drug Delivery

Extracellular vesicles (EVs) are nanoparticles released from various cell types that have emerged as powerful new therapeutic option for a variety of diseases. EVs are involved in the transmission of biological signals between cells and in the regulation of a variety of biological processes, highlighting them as potential novel targets/platforms for therapeutics intervention and/or delivery. Therefore, it is necessary to investigate new aspects of EVs' biogenesis, biodistribution, metabolism, and excretion as well as safety/compatibility of both unmodified and engineered EVs upon administration in different pharmaceutical dosage forms and delivery systems. In this review, we summarize the current knowledge of essential physiological and pathological roles of EVs in different organs and organ systems. We provide an overview regarding application of EVs as therapeutic targets, therapeutics, and drug delivery platforms. We also explore various approaches implemented over the years to improve the dosage of specific EV products for different administration routes.

OMIP-097: High-parameter phenotyping of human platelets by spectral flow cytometry

Using spectral flow cytometry, we developed a 16-color panel for analysis of platelet phenotype and function in human whole blood. The panel contains markers of clinical relevance and follows an optimized protocol for the high-parameter phenotyping of (phosphatidylserine positive) procoagulant platelets. Inclusion of established markers, such as CD62P and PAC-1, allows the subsetting of classic (proinflammatory and proaggregatory) phenotypes, while addition of novel markers, such as TLR9, allows the resolution of platelets with nonclassic functions. Multiple inducible (C3b, CD63, CD107a, CD154, and TLT-1) and constitutive (CD29, CD31, CD32, CD36, CD42a, CD61, and GPVI) markers are also measurable, and we demonstrate the use of automatic gating for platelet analysis. The panel is widely applicable to research and clinical settings and can be readily modified, should users wish to tailor the panel to more specific needs.

Factor VIIa releases phosphatidylserine-enriched extracellular vesicles from endothelial cells by activating acid sphingomyelinase

BACKGROUND

Our recent studies showed that activated factor (F) VII (FVIIa) releases extracellular vesicles (EVs) from the endothelium. FVIIa-released EVs were found to be enriched with phosphatidylserine (PS) and contribute to the hemostatic effect of FVIIa in thrombocytopenia and hemophilia.

OBJECTIVE

To investigate mechanisms by which FVIIa induces EV biogenesis and enriches EVs with PS.

METHODS

FVIIa activation of acid sphingomyelinase (aSMase) was evaluated by its translocation to the cell surface. The role of aSMase in the biogenesis of FVIIa-induced EVs and their enrichment with PS was investigated using specific siRNAs and inhibitors of aSMase and its downstream metabolites. Wild-type and aSMase-/- mice were injected with a control vehicle or FVIIa. EVs released into circulation were quantified by nanoparticle tracking analysis. EVs hemostatic potential was assessed in a murine thrombocytopenia model.

RESULTS

FVIIa activation of aSMase is responsible for both the externalization of PS and the release of EVs in endothelial cells. FVIIa-induced aSMase activation led to ceramide generation and de novo expression of transmembrane protein 16F. Inhibitors of ceramidases, sphingosine kinase, or sphingosine-1-phosphate receptor modulator blocked FVIIa-induced expression of transmembrane protein 16F and PS externalization without interfering with FVIIa release of EVs. In vivo, FVIIa release of EVs was markedly impaired in aSMase-/- mice compared with wild-type mice. Administration of a low dose of FVIIa, sufficient to induce EVs release, corrected bleeding associated with thrombocytopenia in wild-type mice but not in aSMase-/- mice.

CONCLUSION

Our study identifies a novel mechanism by which FVIIa induces PS externalization and releases PS-enriched EVs.

Cryopreservation affects platelet macromolecular composition over time after thawing and differently impacts on cancer cells behavior in vitro

Cryopreservation affects platelets' function, questioning their use for cancer patients. We aimed to investigate the biochemical events that occur over time after thawing to optimize transfusion timing and evaluate the effect of platelet supernatants on tumor cell behavior in vitro. We compared fresh (Fresh-PLT) with Cryopreserved platelets (Cryo-PLT) at 1 h, 3 h and 6 h after thawing. MCF-7 and HL-60 cells were cultured with Fresh- or 1 h Cryo-PLT supernatants to investigate cell proliferation, migration, and PLT-cell adhesion. We noticed a significant impairment of hemostatic activity accompanied by a post-thaw decrease of CD42b+ , which identifies the CD62P--population. FTIR spectroscopy revealed a decrease in the total protein content together with changes in their conformational structure, which identified two sub-groups: 1) Fresh and 1 h Cryo-PLT; 2) 3 h and 6 h cryo-PLT. Extracellular vesicle shedding and phosphatidylserine externalization (PS) increased after thawing. Cryo-PLT supernatants inhibited cell proliferation, impaired MCF-7 cell migration, and reduced ability to adhere to tumor cells. Within the first 3 hours after thawing, irreversible alterations of biomolecular structure occur in Cryo-PLT. Nevertheless, Cryo-PLT should be considered safe for the transfusion of cancer patients because of their insufficient capability to promote cancer cell proliferation, adhesion, or migration.

Extracellular vesicles in venous thromboembolism and pulmonary hypertension

Venous thromboembolism (VTE) is a multifactorial disease, and pulmonary hypertension (PH) is a serious condition characterized by pulmonary vascular remodeling leading with increased pulmonary vascular resistance, ultimately leading to right heart failure and death. Although VTE and PH have distinct primary etiologies, they share some pathophysiologic similarities such as dysfunctional vasculature and thrombosis. In both conditions there is solid evidence that EVs derived from a variety of cell types including platelets, monocytes, endothelial cells and smooth muscle cells contribute to vascular endothelial dysfunction, inflammation, thrombosis, cellular activation and communications. However, the roles and importance of EVs substantially differ between studies depending on experimental conditions and parent cell origins of EVs that modify the nature of their cargo. Numerous studies have confirmed that EVs contribute to the pathophysiology of VTE and PH and increased levels of various EVs in relation with the severity of VTE and PH, confirming its potential pathophysiological role and its utility as a biomarker of disease severity and as potential therapeutic targets.

Genomic communication via circulating extracellular vesicles and long-term health consequences of COVID-19

COVID-19 continues to affect an unprecedented number of people with the emergence of new variants posing a serious challenge to global health. There is an expansion of knowledge in understanding the pathogenesis of Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and the impact of the acute disease on multiple organs. In addition, growing evidence reports that the impact of COVID-19 on different organs persists long after the recovery phase of the disease, leading to long-term consequences of COVID-19. These long-term consequences involve pulmonary as well as extra-pulmonary sequelae of the disease. Noteably, recent research has shown a potential association between COVID-19 and change in the molecular cargo of extracellular vesicles (EVs). EVs are vesicles released by cells and play an important role in cell communication by transfer of bioactive molecules between cells. Emerging evidence shows a strong link between EVs and their molecular cargo, and regulation of metabolism in health and disease. This review focuses on current knowledge about EVs and their potential role in COVID-19 pathogenesis, their current and future implications as tools for biomarker and therapeutic development and their possible effects on long-term impact of COVID-19.

Combined effect of high factor VIII levels and high mean platelet volume on the risk of future incident venous thromboembolism

BACKGROUND

High factor VIII (FVIII) levels and large platelets, as reflected by a high mean platelet volume (MPV), are separately associated with increased risk of venous thromboembolism (VTE). Whether the combination of high FVIII levels and large platelets has a supra-additive effect on VTE risk is unknown.

OBJECTIVES

We aimed to investigate the joint effect of high FVIII levels and large platelets, as reflected by high MPV, on the risk of future incident VTE.

METHODS

A population-based nested case-control study with 365 incident VTE cases and 710 controls was derived from the Tromsø study. FVIII antigen levels and MPV were measured in blood samples drawn at baseline. Odds ratios with 95% CIs were estimated across FVIII tertiles (<85%, 85%-108%, and ≥108%) and within predefined MPV strata (<8.5, 8.5-9.5, and ≥9.5 fL).

RESULTS

VTE risk increased linearly across FVIII tertiles (Ptrend < .001) in models adjusted for age, sex, body mass index, and C-reactive protein. In the combined analysis, participants with FVIII levels in the highest tertile and an MPV of ≥9.5 fL (ie, joint exposure) had an odds ratio for VTE of 2.71 (95% CI, 1.44-5.11) compared with those with FVIII levels in the lowest tertile and an MPV of <8.5 fL (reference). In the joint exposure group, 52% (95% CI, 17%-88%) of VTEs were attributable to the biological interaction between FVIII and MPV.

CONCLUSION

Our results suggest that large platelets, as reflected by high MPV, might play a role in the mechanism by which high FVIII level increases the risk of incident VTE.

Platelet phosphatidylserine is the critical mediator of thrombosis in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a severe immune-mediated prothrombotic disorder caused by antibodies (Ab) reactive to complexes of platelet factor 4 and heparin. Platelets (PLT) and their interaction with different immune cells contribute to prothrombotic conditions in HIT. However, the exact mechanisms and the role of different PLT subpopulations in this prothrombotic environment remain poorly understood. In this study, we observed that HIT patient Ab induce a new PLT population that is characterized by increased P-selectin expression and phosphatidylserine (PS) externalization. Formation of this procoagulant PLT subpopulation was dependent on engagement of PLT Fc-γ-RIIA by HIT Ab and resulted in a significant increase of thrombin generation on the PLT surface. Using an ex vivo thrombosis model and multi-parameter assessment of thrombus formation, we observed that HIT Ab-induced procoagulant PLT propagated formation of large PLT aggregates, leukocyte recruitment and most importantly, fibrin network generation. These prothrombotic conditions were prevented via the upregulation of PLT intracellular cAMP with Iloprost, a clinically approved prostacyclin analogue. Additionally, the functional relevance of P-selectin and PS was dissected. While inhibition of P-selectin did not affect thrombus formation, the specific blockade of PS prevented HIT Ab-mediated thrombin generation and most importantly procoagulant PLT-mediated thrombus formation ex vivo. Taken together, our findings indicate that procoagulant PLT are critical mediators of prothrombotic conditions in HIT. Specific PS targeting could be a promising therapeutic approach to prevent thromboembolic events in HIT patients.

Endothelium dysfunction and thrombosis in COVID-19 with type 2 diabetes

SARS-CoV-2 can directly or indirectly damage endothelial cells. Endothelial injury, especially phosphatidylserine (PS) exposure on the outer membrane of cells, can more easily promote thrombosis. Type 2 diabetes(T2D) patients were more susceptible to COVID-19, they had more severe symptoms, higher risk of thrombotic complications, and longer duration of post-COVID-19 sequelae. This review provided a detailed overview of the mechanisms underlying endothelial dysfunction in T2D patients with COVID-19 (including long COVID), which may be influenced by hyperglycemia, hypoxia, and pro-inflammatory environments. The mechanisms of thrombosis in T2D patients with COVID-19 are also explored, particularly the effects of increased numbers of PS-exposing particles, blood cells, and endothelial cells on hypercoagulability. Given the high risk of thrombosis in T2D patients with COVID-19, early antithrombotic therapy can both minimize the impact of the disease on patients and maximize the chances of improvement, thereby alleviating patient suffering. We provided detailed guidance on antithrombotic drugs and dosages for mild, moderate, and severe patients, emphasizing that the optimal timing of thromboprophylaxis is a critical factor in influencing prognosis. Considering the potential interactions between antidiabetic, anticoagulant, and antiviral drugs, we proposed practical and comprehensive management recommendations to supplement the incomplete efficacy of vaccines in the diabetic population, reduce the incidence of post-COVID-19 sequelae, and improve patient quality of life.

The impact of donor biological variation on the quality and function of cold-stored platelets

BACKGROUND AND OBJECTIVES

Room temperature-stored platelets (RTPs) maximize platelet viability but limit shelf life. The aims of this study were to investigate the impact of donor variability on cold-stored platelets (CSPs) and RTP, to determine whether RTP quality markers are appropriate for CSP.

MATERIALS AND METHODS

Double platelet donations (n = 10) were collected from consented regular male donors stored in 100% plasma. A full blood count, donor age, weight, height and body mass index (BMI) were collected at the time of donation. Platelet donations were split equally into two bags, and assigned to non-agitated CSP or agitated RTP. The quality and function of platelets were assessed throughout the standard 7 days of storage and at expiry (day 8). Non-parametric statistical analyses were used to analyse results given the small sample size.

RESULTS

As expected, there were significant differences between CSP and RTP throughout storage including a reduction in CSP concentration as well as a loss of swirling. Furthermore, a significant increase in CSP exhibiting activation and apoptotic markers was observed. Platelet concentrations were further impacted by donor BMI, and donors with the highest BMI (>29) had the lowest platelet concentration and activation response at the end of CSP storage.

CONCLUSION

Platelet quality and functionality play a vital role in transfusion outcomes; however, blood components are inherently variable. This study demonstrated, for the first time, the specific impact of donor BMI on CSP quality and function and highlights the requirement for novel quality markers for assessing CSPs.

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.

Necroptosis executioner MLKL plays pivotal roles in agonist-induced platelet prothrombotic responses and lytic cell death in a temporal order

Necroptosis is a form of programmed cell death executed by receptor-interacting serine/threonine protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like (MLKL). Platelets are circulating cells that play central roles in haemostasis and pathological thrombosis. In this study we demonstrate seminal contribution of MLKL in transformation of agonist-stimulated platelets to active haemostatic units progressing eventually to necrotic death on a temporal scale, thus attributing a yet unrecognized fundamental role to MLKL in platelet biology. Physiological agonists like thrombin instigated phosphorylation and subsequent oligomerization of MLKL in platelets in a RIPK3-independent but phosphoinositide 3-kinase (PI3K)/AKT-dependent manner. Inhibition of MLKL significantly curbed agonist-induced haemostatic responses in platelets that included platelet aggregation, integrin activation, granule secretion, procoagulant surface generation, rise in intracellular calcium, shedding of extracellular vesicles, platelet-leukocyte interactions and thrombus formation under arterial shear. MLKL inhibition, too, prompted impairment in mitochondrial oxidative phosphorylation and aerobic glycolysis in stimulated platelets, accompanied with disruption in mitochondrial transmembrane potential, augmented proton leak and drop in both mitochondrial calcium as well as ROS. These findings underscore the key role of MLKL in sustaining OXPHOS and aerobic glycolysis that underlie energy-intensive platelet activation responses. Prolonged exposure to thrombin provoked oligomerization and translocation of MLKL to plasma membranes forming focal clusters that led to progressive membrane permeabilization and decline in platelet viability, which was prevented by inhibitors of PI3K/MLKL. In summary, MLKL plays vital role in transitioning of stimulated platelets from relatively quiescent cells to functionally/metabolically active prothrombotic units and their ensuing progression to necroptotic death.

Beyond Macromolecules: Extracellular Vesicles as Regulators of Inflammatory Diseases

Inflammation is the defense mechanism of the immune system against harmful stimuli such as pathogens, toxic compounds, damaged cells, radiation, etc., and is characterized by tissue redness, swelling, heat generation, pain, and loss of tissue functions. Inflammation is essential in the recruitment of immune cells at the site of infection, which not only aids in the elimination of the cause, but also initiates the healing process. However, prolonged inflammation often brings about several chronic inflammatory disorders; hence, a balance between the pro- and anti-inflammatory responses is essential in order to eliminate the cause while producing the least damage to the host. A growing body of evidence indicates that extracellular vesicles (EVs) play a major role in cell-cell communication via the transfer of bioactive molecules in the form of proteins, lipids, DNA, RNAs, miRNAs, etc., between the cells. The present review provides a brief classification of the EVs followed by a detailed description of how EVs contribute to the pathogenesis of various inflammation-associated diseases and their implications as a therapeutic measure. The latter part of the review also highlights how EVs act as a bridging entity in blood coagulation disorders and associated inflammation. The findings illustrated in the present review may open a new therapeutic window to target EV-associated inflammatory responses, thereby minimizing the negative outcomes.

Cell-free histones and the cell-based model of coagulation

The cell-based model of coagulation remains the basis of our current understanding of clinical hemostasis and thrombosis. Its advancement on the coagulation cascade model has enabled new prohemostatic and anticoagulant treatments to be developed. In the past decade, there has been increasing evidence of the procoagulant properties of extracellular, cell-free histones (CFHs). Although high levels of circulating CFHs released following extensive cell death in acute critical illnesses, such as sepsis and trauma, have been associated with adverse coagulation outcomes, including disseminated intravascular coagulation, new information has also emerged on how its local effects contribute to physiological clot formation. CFHs initiate coagulation by tissue factor exposure, either by destruction of the endovascular barrier or induction of endoluminal tissue factor expression on endothelia and monocytes. CFHs can also bind prothrombin directly, generating thrombin via the alternative prothrombinase pathway. In amplifying and augmenting the procoagulant signal, CFHs activate and aggregate platelets, increase procoagulant material bioavailability through platelet degranulation and Weibel-Palade body exocytosis, activate intrinsic coagulation via platelet polyphosphate release, and induce phosphatidylserine exposure. CFHs also inhibit protein C activation and downregulate thrombomodulin expression to reduce anti-inflammatory and anticoagulant effects. In consolidating clot formation, CFHs augment the fibrin polymer to confer fibrinolytic resistance and integrate neutrophil extracellular traps into the clot structure. Such new information holds the promise of new therapeutic developments, including improved targeting of immunothrombotic pathologies in acute critical illnesses.

Vagus nerve stimulation primes platelets and reduces bleeding in hemophilia A male mice

Deficiency of coagulation factor VIII in hemophilia A disrupts clotting and prolongs bleeding. While the current mainstay of therapy is infusion of factor VIII concentrates, inhibitor antibodies often render these ineffective. Because preclinical evidence shows electrical vagus nerve stimulation accelerates clotting to reduce hemorrhage without precipitating systemic thrombosis, we reasoned it might reduce bleeding in hemophilia A. Using two different male murine hemorrhage and thrombosis models, we show vagus nerve stimulation bypasses the factor VIII deficiency of hemophilia A to decrease bleeding and accelerate clotting. Vagus nerve stimulation targets acetylcholine-producing T lymphocytes in spleen and α7 nicotinic acetylcholine receptors (α7nAChR) on platelets to increase calcium uptake and enhance alpha granule release. Splenectomy or genetic deletion of T cells or α7nAChR abolishes vagal control of platelet activation, thrombus formation, and bleeding in male mice. Vagus nerve stimulation warrants clinical study as a therapy for coagulation disorders and surgical or traumatic bleeding.

Comorbidities and clinical complications associated with SARS-CoV-2 infection: an overview

The novel severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) causes major challenges to the healthcare system. SARS-CoV-2 infection leads to millions of deaths worldwide and the mortality rate is found to be greatly associated with pre-existing clinical conditions. The existing dataset strongly suggests that cardiometabolic diseases including hypertension, coronary artery disease, diabetes and obesity serve as strong comorbidities in coronavirus disease (COVID-19). Studies have also shown the poor outcome of COVID-19 in patients associated with angiotensin-converting enzyme-2 polymorphism, cancer chemotherapy, chronic kidney disease, thyroid disorder, or coagulation dysfunction. A severe complication of COVID-19 is mostly seen in people with compromised medical history. SARS-CoV-2 appears to attack the respiratory system causing pneumonia, acute respiratory distress syndrome, which lead to induction of severe systemic inflammation, multi-organ dysfunction, and death mostly in the patients who are associated with pre-existing comorbidity factors. In this article, we highlighted the key comorbidities and a variety of clinical complications associated with COVID-19 for a better understanding of the etiopathogenesis of COVID-19.

IVIg treatment increases thrombin activation of platelets and thrombin generation in paediatric patients with immune thrombocytopenia

Clinical manifestations and laboratory parameters of haemostasis were investigated in 23 children with newly diagnosed immune thrombocytopenia (ITP) before and after intravenous immunoglobulin (IVIg) treatment. ITP patients with platelet counts of less than 20 × 109 /L and mild bleeding symptoms, graded by a standardized bleeding score (BS), were compared with healthy children with normal platelet counts and children with chemotherapy-related thrombocytopenia. Markers of platelet activation and platelet apoptosis in the absence and presence of platelet activators were analysed by flow cytometry; thrombin generation in plasma was determined. ITP patients at diagnosis presented with increased proportions of platelets expressing CD62P and CD63 and activated caspases, and with decreased thrombin generation. Thrombin-induced activation of platelets was reduced in ITP compared with controls, while increased proportions of platelets with activated caspases were observed. Children with a higher BS had lower proportions of CD62P-expressing platelets compared with those with a lower BS. IVIg treatment increased the number of reticulated platelets, the platelet count to more than 20 × 109 /L and improved bleeding in all patients. Decreased thrombin-induced platelet activation, as well as thrombin generation, were ameliorated. Our results indicate that IVIg treatment helps to counteract diminished platelet function and coagulation in children with newly diagnosed ITP.

Platelet activation and ferroptosis mediated NETosis drives heme induced pulmonary thrombosis

Cell-free heme (CFH) is a product of hemoglobin, myoglobin and hemoprotein degradation, which is a hallmark of pathologies associated with extensive hemolysis and tissue damage. CHF and iron collectively induce cytokine storm, lung injury, respiratory distress and infection susceptibility in the lungs suggesting their key role in the progression of lung disease pathology. We have previously demonstrated that heme-mediated reactive oxygen species (ROS) induces platelet activation and ferroptosis. However, interaction of ferroptotic platelets and neutrophils, the mechanism of action and associated complications remain unclear. In this study, we demonstrate that heme-induced P-selectin expression and Phosphatidylserine (PS) externalization in platelets via ASK-1-inflammasome axis increases platelet-neutrophil aggregates in circulation, resulting in Neutrophil extracellular traps (NET) formation in vitro and in vivo. Further, heme-induced platelet activation in mice increased platelet-neutrophil aggregates and accumulation of NETs in the lungs causing pulmonary damage. Thus, connecting CFH-mediated platelet activation to NETosis and pulmonary thrombosis. As lung infections induce acute respiratory stress, thrombosis and NETosis, we propose that heme -mediated platelet activation and ferroptosis might be crucial in such clinical manifestations. Further, considering the ability of redox modulators and ferroptosis inhibitors like FS-1, Lpx-1 and DFO to inhibit heme-induced ferroptotic platelets-mediated NETosis and pulmonary thrombosis. They could be potential adjuvant therapy to regulate respiratory distress-associated clinical complications.

Impaired Platelet Function and Thrombus Formation in PDE5A-Deficient Mice

Intracellular cyclic GMP (cGMP) inhibits platelet function. Platelet cGMP levels are controlled by phosphodiesterase 5A (PDE5A)-mediated degradation. However, the exact role of PDE5A in platelet function and thrombus formation remains poorly understood. In this study, we characterized the role of PDE5A in platelet activation and function. Platelets were isolated from wild type or PDE5A^-/- mice to measure platelet aggregation, activation, phosphatidylserine exposure (annexin-V binding), reactive oxygen species (ROS) generation, platelet spreading as well as clot retraction. Cytosolic calcium mobilization was measured using Fluo-4 AM by a microplate reader. Western blot was used to measure the phosphorylation of VASP, ERK1/2, p38, JNK, and AKT. FeCl₃-induced arterial thrombosis and venous thrombosis were assessed to evaluate the in vivo hemostatic function and thrombus formation. Additionally, in vitro thrombus formation was assessed in a microfluidic whole-blood perfusion assay. PDE5A-deficient mice presented significantly prolonged tail bleeding time and delayed arterial and venous thrombus formation. PDE5A deficiency significantly inhibited platelet aggregation, ATP release, P-selectin expression, and integrin aIIbb3 activation. In addition, an impaired spreading on collagen or fibrinogen and clot retraction was observed in PDE5A-deficient platelets. Moreover, PDE5A deficiency reduced phosphatidylserine exposure, calcium mobilization, ROS production, and increased intracellular cGMP level along with elevated VASP phosphorylation and reduced phosphorylation of ERK1/2, p38, JNK, and AKT. In conclusion, PDE5A modulates platelet activation and function and thrombus formation, indicating that therapeutically targeting it might be beneficial for the treatment of thrombotic diseases.

Shear stimulated red blood cell microparticles: Effect on clot structure, flow and fibrinolysis

BACKGROUND

Microparticles (MPs) have activity in thrombus promotion and generation. Erythrocyte microparticles (ErMPs) have been reported to accelerate fibrinolysis in the absence of permeation. We hypothesized that shear induced ErMPs would affect fibrin structure of clots and change flow with implications for fibrinolysis.

OBJECTIVE

To determine the effect of ErMPs on clot structure and fibrinolysis.

METHODS

Plasma with elevated ErMPs was isolated from whole blood or from washed red blood cells (RBCs) resuspended in platelet free plasma (PFP) after high shear. Dynamic light scattering (DLS) provided size distribution of ErMPs from sheared samples and unsheared PFP controls. Clots were formed by recalcification for flow/lysis experiments and examined by confocal microscopy and SEM. Flow rates through clots and time-to-lysis were recorded. A cellular automata model showed the effect of ErMPs on fibrin polymerization and clot structure.

RESULTS

Coverage of fibrin increased by 41% in clots formed from plasma of sheared RBCs in PFP over controls. Flow rate decreased by 46.7% under a pressure gradient of 10 mmHg/cm with reduction in time to lysis from 5.7 ± 0.7 min to 12.2 ± 1.1 min (p < 0.01). Particle size of ErMPs from sheared samples (200 nm) was comparable to endogenous microparticles.

CONCLUSIONS

ErMPs alter the fibrin network in a thrombus and affect hydraulic permeability resulting in decelerated delivery of fibrinolytic drugs.

Coagulation potential and the integrated omics of extracellular vesicles from COVID-19 positive patient plasma

Extracellular vesicles (EVs) participate in cell-to-cell communication and contribute toward homeostasis under physiological conditions. But EVs can also contribute toward a wide array of pathophysiology like cancer, sepsis, sickle cell disease, and thrombotic disorders. COVID-19 infected patients are at an increased risk of aberrant coagulation, consistent with elevated circulating levels of ultra-high molecular weight VWF multimers, D-dimer and procoagulant EVs. The role of EVs in COVID-19 related hemostasis may depend on cells of origin, vesicular cargo and size, however this is not well defined. We hypothesized that the procoagulant potential of EV isolates from COVID-19 (+) patient plasmas could be defined by thrombin generation assays. Here we isolated small EVs (SEVs) and large EVs (LEVs) from hospitalized COVID-19 (+) patient (n = 21) and healthy donor (n = 20) plasmas. EVs were characterized by flow cytometry, Transmission electron microscopy, nanoparticle tracking analysis, plasma thrombin generation and a multi-omics approach to define coagulation potential. These data were consistent with differences in EV metabolite, lipid, and protein content when compared to healthy donor plasma isolated SEVs and LEVs. Taken together, the effect of EVs on plasma procoagulant potential as defined by thrombin generation and supported by multi-omics is enhanced in COVID-19. Further, we observe that this effect is driven both by EV size and phosphatidyl serine.

Immunothrombosis and the Role of Platelets in Venous Thromboembolic Diseases

Venous thromboembolism (VTE) is the third leading cardiovascular cause of death and is conventionally treated with anticoagulants that directly antagonize coagulation. However, recent data have demonstrated that also platelets play a crucial role in VTE pathophysiology. In the current review, we outline how platelets are involved during all stages of experimental venous thrombosis. Platelets mediate initiation of the disease by attaching to the vessel wall upon which they mediate leukocyte recruitment. This process is referred to as immunothrombosis, and within this novel concept inflammatory cells such as leukocytes and platelets directly drive the progression of VTE. In addition to their involvement in immunothrombosis, activated platelets can directly drive venous thrombosis by supporting coagulation and secreting procoagulant factors. Furthermore, fibrinolysis and vessel resolution are (partly) mediated by platelets. Finally, we summarize how conventional antiplatelet therapy can prevent experimental venous thrombosis and impacts (recurrent) VTE in humans.

Platelets and Sera from Donors of Convalescent Plasma after Mild COVID-19 Show No Procoagulant Phenotype

Coronavirus disease-2019 (COVID-19) is associated with increased thromboembolic complications. Long-term alteration in the coagulation system after acute COVID-19 infection is still a subject of research. Furthermore, the effect of sera from convalescent subjects on platelets is not known. In this study, we investigated platelet phenotype, coagulation, and fibrinolysis in COVID-19 convalescent plasma (CCP) donors and analyzed convalescent sera-induced effects on platelets. We investigated CCP donors who had a history of mild COVID-19 infection and donors who did not have COVID-19 were used as controls. We analyzed phosphatidylserine (PS) externalization, CD62p expression, and glycoprotein VI (GPVI) shedding both in platelet-rich plasma (PRP) and after incubation of washed healthy platelets with donors' sera using flow cytometry. Coagulation and fibrinolysis systems were assessed with thromboelastometry. Forty-seven CCP donors (22 males, 25 females; mean age (±SD): 41.4 ± 13.7 years) with a history of mild COVID-19 infection were included. Median duration after acute COVID-19 infection was 97 days (range, 34–401). We did not find an increased PS externalization, CD62p expression, or GPVI shedding in platelets from CCP donors. Sera from CCP donors did not induce PS externalization or GPVI shedding in healthy platelets. Sera-induced CD62p expression was slightly, albeit statistically significantly, lower in CCP donors than in plasma donors without a history of COVID-19. One patient showed increased maximum clot firmness and prolonged lysis time in thromboelastometry. Our findings suggest that procoagulant platelet phenotype is not present after mild COVID-19. Furthermore, CCP sera do not affect the activation status of platelets.

Update on platelet procoagulant mechanisms in health and in bleeding disorders

Platelet procoagulant mechanisms are emerging to be complex and important to achieving haemostasis. The mechanisms include the release of procoagulant molecules from platelet storage granules, and strong agonist-induced expression of procoagulant phospholipids on the outer platelet membrane for tenase and prothrombinase assembly. The release of dense granule polyphosphate is important to platelet procoagulant function as it promotes the activation of factors XII, XI and V, inhibits tissue factor pathway inhibitor and fibrinolysis, and strengthens fibrin clots. Platelet procoagulant function also involves the release of partially activated factor V from platelets. Scott syndrome has provided important insights on the mechanisms that regulate procoagulant phospholipids expression on the external platelet membrane, which require strong agonist stimulation that increase cystolic calcium levels, mitochondrial calcium uptake, the loss of flippase function and activation of the transmembrane scramblase protein anoctamin 6. There have been advances in the methods used to directly and indirectly assess platelet procoagulant function in health and disease. Assessments of thrombin generation with platelet rich plasma samples has provided new insights on how platelet procoagulant function is altered in inherited platelet disorders, and how platelets influence the bleeding phenotype of a number of severe coagulation factor deficiencies. Several therapies, including desmopressin and recombinant factor VIIa, improve thrombin generation by platelets. There is growing interest in targeting platelet procoagulant function for therapeutic benefit. This review highlights recent advances in our understanding of platelet-dependent procoagulant mechanisms in health and in bleeding disorders.

The regulatory role of exosomes in venous thromboembolism

Exosomes are nanoscale endocytic vesicles, 30-150 nm in diameter, secreted by most cells. They mainly originate from multivesicular bodies formed by intracellular invagination of lysosomal microparticles, and released into the extracellular matrix after fusion of multivesicular bodies with cell membrane. Studies have shown that exosomes contain a variety of active molecules, such as proteins, lipids and RNAs (such as mRNA, miRNA, lncRNA, circRNA, etc.), which regulate the behavior of recipient cells and serve as circulating biomarkers of diseases, including thrombosis. Therefore, exosome research is important for the diagnosis, treatment, therapeutic monitoring, and prognosis of thrombosis in that it can reveal the counts, surface marker expression, protein, and miRNA cargo involved. Recent studies have shown that exosomes can be used as therapeutic vectors for tissue regeneration and as alternative vectors for drug delivery. In this review, we summarize the physiological and biochemical characteristics, isolation, and identification of exosomes. Moreover, we focus on the role of exosomes in thrombosis, specifically venous thromboembolism, and their potential clinical applications, including as biomarkers and therapeutic vectors for thrombosis.