The role of the red cell membrane in thrombin generation

Effect of fibrinogen on blood coagulation detected by optical coherence tomography

Our previous work demonstrated that an optical coherence tomography (OCT) technique and the parameter 1/e light penetration depth (d1/e) were able to characterize the whole blood coagulation process in contrast to existing optical tests that are performed on plasma samples. To evaluate the feasibility of the technique for quantifying the effect of fibrinogen (Fbg) on blood coagulation, a dynamic study of d1/e of blood in various Fbg concentrations was performed in static state. Two groups of blood samples of hematocrit (HCT) in 35, 45, and 55% were reconstituted of red blood cells with: 1) treated plasma with its intrinsic Fbg removed and commercial Fbg added (0-8 g L(-1)); and 2) native plasma with commercial Fbg added (0-8 g L(-1)). The results revealed a typical behavior due to coagulation induced by calcium ions and the clotting time is Fbg concentration-dependent. The clotting time was decreased by the increasing amount of Fbg in both groups. Besides, the blood of lower HCT with various levels of Fbg took shorter time to coagulate than that of higher HCT. Consequently, the OCT method is a useful and promising tool for the detection of blood-coagulation processes induced with different Fbg levels.

Whole blood thrombin generation in Bmal1-deficient mice

The Calibrated Automated Thrombogram (CAT) assay that measures thrombin generation (TG) in platelet-poor and -rich plasma, is increasingly being recognised as a more sensitive tool to determine the overall function of the haemostatic system. We developed a method enabling the measurement of TG in a small aliquot of blood. The objective was to validate this assay in mouse blood and to examine the rate and extent of TG in a mouse model of premature aging. TG was assayed in blood from 20- to 28-week-old brain and muscle ARNT-like protein-1 (Bmal1)-deficient (knockout, KO) mice and wild-type (WT) littermates. Bmal1-KO mice are known to display symptoms of premature aging. TG was initiated by adding calcium, tissue factor and a thrombin specific substrate. After TG, the samples were prepared for scanning electron microscopy (SEM). The intra-assay variations (%) in mouse blood of the endogenous thrombin potential (ETP), peak height, lag time, time-to-peak and velocity index were 10% or less (n=24). We found that Bmal1-KO mice have a significantly (p<0.001) higher ETP (437 ± 7 nM.min; mean ± SD, n=7) when compared with WT mice (ETP=220 ± 45 nM.min; mean ± SD, n=5). The peak heights also differed significantly (p=0.027). By applying SEM we found that Bmal1 deficient mice display a denser fibrin network with smaller pores compared to WT mice. In conclusion, the whole blood TG assay in mice revealed to be reproducible. As a proof-of-principle we have shown that the whole blood TG assay is capable of detecting a prothrombotic phenotype in Bmal1-KO mice.

New insights into the spatiotemporal localization of prothrombinase in vivo

The membrane-dependent interaction of factor Xa (FXa) with factor Va (FVa) forms prothrombinase and drives thrombin formation essential for hemostasis. Activated platelets are considered to provide the primary biological surface to support prothrombinase function. However, the question of how other cell types may cooperate within the biological milieu to affect hemostatic plug formation remains unaddressed. We used confocal fluorescence microscopy to image the distribution of site-specific fluorescent derivatives of FVa and FXa after laser injury in the mouse cremaster arteriole. These proteins bound to the injury site extend beyond the platelet mass to the surrounding endothelium. Although bound FVa and FXa may have been present on the platelet core at the nidus of the injury, bound proteins were not evident on platelets adherent even a small distance from the injury site. Manipulations to drastically reduce adherent platelets yielded a surprisingly modest decrease in bound FXa and FVa with little impact on fibrin formation. Thus, platelets adherent to the site of vascular injury do not play the presumed preeminent role in supporting prothrombinase assembly and thrombin formation. Rather, the damaged/activated endothelium and possibly other blood cells play an unexpectedly important role in providing a procoagulant membrane surface in vivo.

Homocysteine induces procoagulant activity of red blood cells via phosphatidylserine exposure and microparticles generation

Increased homocysteine (Hcy) levels in plasma correlate with the risk of thromboic events. Red blood cells (RBCs), the most abundant blood cells in circulation, also play an active role in the process of thrombus formation. However, the effect of Hcy on procoagulant activity (PCA) of RBCs is unclear. In the present study, RBCs from healthy adults were treated with Hcy (8, 20, 80, 200, 800 μmol/L) for 24 h. Phosphatidylserine (PS) exposure of RBCs and red blood cell-derived microparticles (RMPs) release were detected using Alexa Fluor 488-lactadherin. PCA was assessed by coagulation time and purified clotting complexes testes. We found that Hcy treatment dose dependently enhanced PS exposure and consequent PCA of RBCs. Hcy also elevated the formation of procoagulant RMPs, with statistical significance at 800 μmol/L of Hcy. Moreover, 128 nmol/L lactadherin inhibited about 90% PCA of RBCs and RMPs. Our data suggest that PS exposure and RMPs shedding are key sources for Hcy-induced PCA of RBCs. Lactadherin could be used to modulate the anticoagulant and procoagulant balance in this process.

Extracellular vesicles from blood plasma: determination of their morphology, size, phenotype and concentration

BACKGROUND

Plasma and other body fluids contain membranous extracellular vesicles (EVs), which are considered to derive from activated or apoptotic cells. EVs participate in physiological and pathological processes and have potential applications in diagnostics or therapeutics. Knowledge on EVs is, however, limited, mainly due to their sub-micrometer size and to intrinsic limitations in methods applied for their characterization.

OBJECTIVES

Our aim was to provide a comprehensive description of EVs from plasma of healthy subjects.

METHODS

Cryo-transmission electron microscopy combined with receptor-specific gold labeling was used to reveal the morphology, size and phenotype of EVs. An original approach based on sedimentation on electron microscopy grids was developed for enumerating EVs. A correlation was performed between conventional flow cytometry and electron microscopy results.

RESULTS

We show that platelet-free plasma samples contain spherical EVs, 30 nm to 1 μm in diameter, tubular EVs, 1-5 μm long, and membrane fragments, 1-8 μm large. We show that only a minority of EVs expose the procoagulant lipid phosphatidylserine, in contrast to the classical theory of EV formation. In addition, the concentrations of the main EV sub-populations are determined after sedimentation on EM grids. Finally, we show that conventional flow cytometry, the main method of EV characterization, detects only about 1% of them.

CONCLUSION

This study brings novel insights on EVs from normal plasma and provides a reference for further studies of EVs in disease situations.

Trauma-Induced Coagulopathy: An Institution's 35 Year Perspective on Practice and Research

INTRODUCTION

Injury is the second leading cause of death worldwide, and as much as 40% of injury-related mortality is attributed to uncontrollable hemorrhage. This persists despite establishment of regionalized trauma systems and advances in the management of severely injured patients. Trauma-induced coagulopathy has been identified as the most common preventable cause of postinjury mortality.

METHODS

A review of the current literature was performed by collecting PUBMED references related to trauma-induced coagulopathy. Data were then critically analyzed and summarized based on the authors' clinical and research perspective, as well as that reported by other institutions and researchers interested in trauma-induced coagulopathy. A particular focus was placed on those aspects of coagulopathy in which agreement among clinical and basic scientists is currently lacking; these include, pathophysiology, the role of blood components and factor therapy, and goal-directed assessment and management.

RESULTS

Trauma-induced coagulopathy has been recognized in approximately one-third of trauma patients. There is a vast range of severity, and the emergence of viscoelastic assays, such as thrombelastography and rotational thromboelastogram, has refined its diagnosis and management, particularly through the establishment of goal-directed massive transfusion protocols. Despite advancements in the diagnosis and management of trauma-induced coagulopathy, much remains to be understood regarding its pathophysiology. The cell-based model of hemostasis has allowed for characterization of endothelial dysfunction, impaired thrombin generation, platelet dysfunction, fibrinolysis, endogenous anticoagulants such as protein-C, and antifibrinolytic proteins. These concepts collectively compose the contemporary, but still partial, understanding of trauma-induced coagulopathy.

CONCLUSION

Trauma-induced coagulopathy is a complex pathophysiological condition, of which some mechanisms have been characterized, but much remains to be understood in order to translate this knowledge into improved outcomes for the injured patient.

Global Thrombosis Test (GTT) can detect major determinants of haemostasis including platelet reactivity, endogenous fibrinolytic and thrombin generating potential

BACKGROUND

Detection of both thrombosis and bleeding risk are essential in clinical cardiology. Thrombin generated by activated platelets and from the extrinsic coagulation pathway is the major determinant of thrombogenesis and hemostasis. Although novel oral anticoagulants further increase the bleeding risk of antiplatelet drugs, platelet function tests do not reliably predict hemorrhagic complications. It seems that in addition to platelet aggregation, true assessment of bleeding risks requires the measurement of both platelet and plasma derived thrombin activity.

OBJECTIVE

To adapt a novel, near-patient test for the assessment of both antithrombotic and anticoagulant effects of oral thrombin inhibitors.

METHODS

The point-of-care Global Thrombosis Test (GTT), which measures platelet reactivity to shear-activation in native blood, was used. Thrombin, generated from activated platelets (procoagulant activity) plays a pivotal role in GTT measurement. In order to assess endogenous thrombin potential, in a separate blood sample thrombin generation was induced by microparticles formed during hypotonic hemolysis. Thus two blood samples were tested to measure simultaneously platelet reactivity (occlusion time, OT) and hemolysis (microparticles)-induced endogenous thrombin potential (OT-H).

RESULTS

In healthy subjects (n=32), OT measured in native blood was reduced in hemolysed blood (100% vs. 43 ± 4%; OT vs. OT-H respectively). Shortening of OT in hemolysed blood (OT-H) was dose-dependently inhibited by the in vitro added thrombin inhibitor argatroban. In patients receiving dabigatran (n=27), OT and, to a lesser extent, OT-H was prolonged, compared to healthy volunteers. Intra-assay variation of OT-H was low (4.5%), but interindividual variation was great, both in healthy subjects (61%) and in patients on dabigatran (65%). Thrombin inhibitors argatroban, heparin (in vitro) and dabigatran (in vivo) all prolonged both OT and OT-H. There was no correlation between the measured OT and OT-H data.

CONCLUSIONS

Microparticles shed from erythrocytes during hypotonic lysis of native blood considerably shortened OT. In a direct proportion to the applied concentrations, various thrombin inhibitors prolonged both OT (antithrombotic effect) and to a lesser extent, OT-H (anticoagulant effect). Further large studies are required to evaluate the usefulness of this technique in a clinical setting, in assessing the anticoagulant and antithrombotic effects of medication and relating GTT results with observed thrombotic and bleeding events.

Hemostatic abnormalities in sickle cell disease

PURPOSE OF REVIEW

Although it has long been recognized that sickle cell disease (SCD) and other hemoglobinopathies are associated with a state of chronic hyperactivation of coagulation, the study of the epidemiology of venous thromboembolic (VTE) complications in SCD is only now beginning to evolve. In parallel, mechanistic studies of the hypercoagulable state in humans and mouse models implicate an increasingly important causative role of hemolysis.

RECENT FINDINGS

The case for SCD as a thrombophilic state has been strengthened by the recent literature. In an attempt to better understand the underlying mechanism(s), global assays of coagulation (thromboelastography and thrombin generation assays) have been utilized by several groups, but thus far, the results have been inconsistent, probably because of the technical differences. However, global assays continue to support the case for an important role of peripheral blood cells and their derived microparticles in promoting coagulation activation.

SUMMARY

VTE is an underappreciated and potentially morbid complication of SCD. The mechanisms underlying this hypercoagulable state are complex. A greater understanding of these pathways may lead to the rational selection of therapies that not only prevent thrombosis, but also impact on many of the other vaso-occlusive complications of SCD.

A simplified mathematical model for thrombin generation

A new phenomenological mathematical model based directly on laboratory data for thrombin generation and having a patient-specific character is described. A set of the solved equations for cell-based models of blood coagulation that can reproduce the temporal evolution of thrombin generation is proposed; such equations are appropriate for use in computational fluid dynamic (CFD) simulations. The initial values for the reaction rates are either taken from already existing model or experimental data, or they can obtained from simple reasoning under certain assumptions; it is shown that coefficients can be adjusted in order to fit a range of different thrombin generation curves as derived from thrombin generation assays. The behaviour of the model for different platelet concentration seems to be in good agreement with reported experimental data. It is shown that the reduced set of equations used represents to a good approximation a low-order model of the detailed mechanism and thus it can represent a cost-effective and-case specific mathematical model of coagulation reactions up to thrombin generation.

Brodde M. State of the art in platelet function testing

Platelets perform many functions in hemostasis but also in other areas of physiology and pathology. Therefore, it is obvious that many different function tests have been developed, each one conceived and standardized for a special purpose. This review will summarize the different fields in which platelet function testing is currently in use; diagnostics of patients with bleeding disorders, monitoring patients' response to anti-platelet therapy, monitoring in transfusion medicine (blood donors, platelet concentrates, and after transfusion), and monitoring in perioperative medicine to predict bleeding tendency. The second part of the review outlines different methods for platelet function testing, spanning bleeding time, and platelet counting as well as determining platelet adhesion, platelet secretion, platelet aggregation, platelet morphology, platelet signal transduction, platelet procoagulant activity, platelet apoptosis, platelet proteomics, and molecular biology.