The contribution of bovine Factor V and Factor Va to the activity of prothrombinase

Clot or Not? Reviewing the Reciprocal Regulation Between Lipids and Blood Clotting

Both hyperlipidemia and thrombosis contribute to the risks of atherosclerotic cardiovascular diseases, which are the leading cause of death and reduced quality of life in survivors worldwide. The accumulation of lipid-rich plaques on arterial walls eventually leads to the rupture or erosion of vulnerable lesions, triggering excessive blood clotting and leading to adverse thrombotic events. Lipoproteins are highly dynamic particles that circulate in blood, carry insoluble lipids, and are associated with proteins, many of which are involved in blood clotting. A growing body of evidence suggests a reciprocal regulatory relationship between blood clotting and lipid metabolism. In this review article, we summarize the observations that lipoproteins and lipids impact the hemostatic system, and the clotting-related proteins influence lipid metabolism. We also highlight the gaps that need to be filled in this area of research.

Membrane procoagulation and N‑terminomics/TAILS profiling in Montreal platelet syndrome kindred with VWF p.V1316M mutation

BACKGROUND

The Montreal platelet syndrome kindred (MPS) with VWF p.V1316M mutation (2B-VWDMPS) is an extremely rare disorder. It has been associated with macrothrombocytopenia, spontaneous platelet clumping, mucocutaneous, and other bleeding, which can be largely prevented by von Willebrand factor (VWF) concentrate infusion. However, supplemental platelet transfusion has been required on occasion, particularly for severe gastrointestinal bleeds. This raised the question of whether a previously uncharacterized platelet dysfunction contributes to bleeding diathesis in 2B-VWDMPS patients. We have previously shown that membrane ballooning, a principal part of the platelet procoagulant membrane dynamics (PMD) after collagen stimulation, is driven by the influx of Na+ and Cl-, followed by the entry of water.

METHODS

We study two members (mother and daughter) of the MPS kindred with severe bleeding phenotype and address this question by coupling quantitative platelet shotgun proteomics and validating biochemical assays, with the systematic analysis of platelet procoagulant membrane dynamics (PMD). Using N-terminomics/TAILS (terminal amine isotopic labeling of substrates), we compare changes in proteolysis between healthy and 2B-VWDMPS platelets.

RESULTS

Here, we report in 2B-VWDMPS platelets, the loss of the transmembrane chloride channel-1 (CLIC1), and reduced chloride ion influx after collagen stimulation. This was associated with diminished membrane ballooning, phosphatidylserine externalization, and membrane thrombin formation, as well as a distinct phenotypic composition of platelets over fibrillar collagen. We also identify processing differences of VWF, fibronectin (FN1), and Crk-like protein (CRKL). 2B-VWDMPS platelets are shown to be basally activated, partially degranulated, and have marked loss of regulatory, cytoskeletal, and contractile proteins.

CONCLUSIONS

This may account for structural disorganization, giant platelet formation, and a weakened hemostatic response.

Longitudinal observations of TFPI levels in paediatric Haemophilia A patients

INTRODUCTION

Inhibition of tissue factor pathway inhibitor (TFPI) is a potential new mode of action to achieve haemostasis in haemophilia A and B patients.

AIM

Knowledge about potential developmental changes of TFPI levels during childhood are a prerequisite to translate adult doses of TFPI inhibitors to doses in paediatric patients.

METHODS

In this study we present longitudinal data for total TFPI concentrations (TFPI-T) and TFPI activity (TFPI-A) from 48 paediatric Haemophilia A patients in the age range from 3 to 18 years (2-12 observations per patient).

RESULTS

TFPI-T and TFPI-A tend to decrease over age during childhood. Lowest values were observed between 12 and <18 years. On average, TFPI-T and TFPI-A were lower in adolescent haemophilia patients than in adult haemophilia patients.

CONCLUSION

In summary, the presented information on TFPI levels in children adds to the current knowledge of developmental haemostasis and it can be helpful in evaluating how children respond to haemophilia treatment including the new class of anti-TFPI compounds.

The Tissue Factor Pathway in Cancer: Overview and Role of Heparan Sulfate Proteoglycans

Historically, the only focus on tissue factor (TF) in clinical pathophysiology has been on its function as the initiation of the extrinsic coagulation cascade. This obsolete vessel-wall TF dogma is now being challenged by the findings that TF circulates throughout the body as a soluble form, a cell-associated protein, and a binding microparticle. Furthermore, it has been observed that TF is expressed by various cell types, including T-lymphocytes and platelets, and that certain pathological situations, such as chronic and acute inflammatory states, and cancer, may increase its expression and activity. Transmembrane G protein-coupled protease-activated receptors can be proteolytically cleaved by the TF:FVIIa complex that develops when TF binds to Factor VII (PARs). The TF:FVIIa complex can activate integrins, receptor tyrosine kinases (RTKs), and PARs in addition to PARs. Cancer cells use these signaling pathways to promote cell division, angiogenesis, metastasis, and the maintenance of cancer stem-like cells. Proteoglycans play a crucial role in the biochemical and mechanical properties of the cellular extracellular matrix, where they control cellular behavior via interacting with transmembrane receptors. For TFPI.fXa complexes, heparan sulfate proteoglycans (HSPGs) may serve as the primary receptor for uptake and degradation. The regulation of TF expression, TF signaling mechanisms, their pathogenic effects, and their therapeutic targeting in cancer are all covered in detail here.

The complement alternative pathway and hemostasis

The complement and hemostatic systems are complex systems, and both involve enzymatic cascades, regulators, and cell components-platelets, endothelial cells, and immune cells. The two systems are ancestrally related and are defense mechanisms that limit infection by pathogens and halt bleeding at the site of vascular injury. Recent research has uncovered multiple functional interactions between complement and hemostasis. On one side, there are proteins considered as complement factors that activate hemostasis, and on the other side, there are coagulation proteins that modulate complement. In addition, complement and coagulation and their regulatory proteins strongly interact each other to modulate endothelial, platelet and leukocyte function and phenotype, creating a potentially devastating amplifying system that must be closely regulated to avoid unwanted damage and\or disseminated thrombosis. In view of its ability to amplify all complement activity through the C3b-dependent amplification loop, the alternative pathway of complement may play a crucial role in this context. In this review, we will focus on available and emerging evidence on the role of the alternative pathway of complement in regulating hemostasis and vice-versa, and on how dysregulation of either system can lead to severe thromboinflammatory events.

Phosphatidylserine Regulation of Coagulation Proteins Factor IXa and Factor VIIIa

Blood coagulation is an intricate process, and it requires precise control of the activities of pro- and anticoagulant factors and sensitive signaling systems to monitor and respond to blood vessel insults. These requirements are fulfilled by phosphatidylserine, a relatively miniscule-sized lipid molecule amid the myriad of large coagulation proteins. This review limelight the role of platelet membrane phosphatidylserine (PS) in regulating a key enzymatic reaction of blood coagulation; conversion of factor X to factor Xa by the enzyme factor IXa and its cofactor factor VIIIa. PS is normally located on the inner leaflet of the resting platelet membrane but appears on the outer leaflet surface of the membrane surface after an injury happens. Human platelet activation leads to exposure of buried PS molecules on the surface of the platelet-derived membranes and the exposed PS binds to discrete and specific sites on factors IXa and VIIIa. PS binding to these sites allosterically regulates both factors IXa and VIIIa. The exposure of PS and its binding to factors IXa/VIIIa is a vital step during clotting. Insufficient exposure or a defective binding of PS to these clotting proteins is responsible for various hematologic diseases which are discussed in this review.

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.

Thrombin induces morphological and inflammatory astrocytic responses via activation of PAR1 receptor

Spinal cord injury (SCI) will result in the significant elevation of thrombin production at lesion site via either breakage of blood-spinal cord barrier or upregulated expression within nerve cells. Thrombin-induced activation of the protease activated receptors (PARs) evokes various pathological effects that deteriorate the functional outcomes of the injured cord. The cellular consequences of thrombin action on the astrocytes, as well as the underlying mechanism are not fully elucidated by far. In the present study, SCI model of rats was established by contusion, and primary astrocytes were isolated for culture from newborn rats. The expression levels of thrombin and PAR1 receptor at lesion sites of the spinal cord were determined. The primary astrocytes cultured in vitro were stimulated with different concentration of thrombin, and the resultant morphological changes, inflammatory astrocytic responses, as well as PAR1-activated signal pathway of astrocytes were accordingly examined using various agonists or antagonists of the receptor. Thrombin was found to reverse astrocytic stellation, promote proliferation but inhibit migration of astrocytes. Furthermore, the serine protease was shown to facilitate inflammatory response of astrocytes through regulation of MAPKs/NFκB pathway. Our results have provided the morphological evidence of astrocytic reactivity in response to thrombin stimulation and its neuroinflammatory effects following SCI, which will be indicative for the fundamental insights of thrombin-induced neuropathology.

Understanding the anchoring interaction of coagulation factor Va light chain on zeolites: A molecular dynamics study

HYPOTHESIS

Factor Va (FXa) and Xa (FVa) can assemble on the phosphatidylserine (PS) membrane (of platelet) to form prothrombinase complex and contribute to blood clotting. Very recently, we discovered that Ca-zeoliteacts as a type of reinforced activated inorganic platelet to enable assembly of prothrombinase complex and display an unusual zymogen (prothrombin) activation pattern. Inspired but not constrained by nature, it is of great interest to understand how FVa and FXa assembly on the inorganic surface (e.g., zeolites) and perform their biocatalytic function.

EXPERIMENTS

Given the important role of FVa C1-C2 domains in the assembly and activity of the prothrombinase complex, in this work, molecular dynamics simulations were performed to investigate the binding details of FVa A3-C1-C2 domains on the PS membranes and Ca²⁺-LTA-type (CaA) zeolite surface.

FINDINGS

We found that different from the natural PS membrane, FVa light chain repeatedly exhibits a strong C2 domain anchoring interaction on the CaA zeolite. It mainly arises from the porous surface structure of CaA zeolite and local highly dense solvation water clusters on the CaA zeolite surface restrict the movement of some lysine residues on the C2 domain. The anchoring interaction can be suppressed by reducing the surface negative charge density, so that FVa light chain can change from single-foot (only C2 domain) to double-foot (both C1-C2 domain) adsorption states on the zeolite surface. This double-foot adsorption state is similar to natural PS membrane systems, which may make FVa have higher cofactor activity.

Human coagulation factor IX: a systematic review of its characteristics

Human coagulation factor IX (FIX) is a vitamin K dependent glycoprotein. It is used in therapy of factor IX deficiency, known as haemophilia B. This article explained the coagulation process involved in FIX and the principle of haemophilia B caused by its absence. We searched and aligned the sequence information of IX in the NCBI's EST database to predict its structure and understand the molecular biological basis of its function. Further, by sorting out the development history of FIX, we summarized the general process of its isolation and purification. Finally, the clinical indications and complications of FIX were overviewed. Through a comprehensive description of the characteristics of FIX, we hope to give readers a clear understanding and put forward our views on the future development of the product.

An engineered activated factor V for the prevention and treatment of acute traumatic coagulopathy and bleeding in mice

^superFVa arrests severe bleeding and prevents the development of ATC after trauma.

^superFVa therapy restores functional hemostasis when initiated after onset of ATC caused by traumatic bleeding.

Acute traumatic coagulopathy (ATC) occurs in approximately 30% of patients with trauma and is associated with increased mortality. Excessive generation of activated protein C (APC) and hyperfibrinolysis are believed to be driving forces for ATC. Two mouse models were used to investigate whether an engineered activated FV variant (^superFVa) that is resistant to inactivation by APC and contains a stabilizing A2-A3 domain disulfide bond can reduce traumatic bleeding and normalize hemostasis parameters in ATC. First, ATC was induced by the combination of trauma and shock. ATC was characterized by activated partial thromboplastin time (APTT) prolongation and reductions of factor V (FV), factor VIII (FVIII), and fibrinogen but not factor II and factor X. Administration of ^superFVa normalized the APTT, returned FV and FVIII clotting activity levels to their normal range, and reduced APC and thrombin-antithrombin (TAT) levels, indicating improved hemostasis. Next, a liver laceration model was used where ATC develops as a consequence of severe bleeding. ^superFVa prophylaxis before liver laceration reduced bleeding and prevented APTT prolongation, depletion of FV and FVIII, and excessive generation of APC. Thus, prophylactic administration of ^superFVa prevented the development of ATC. ^superFVa intervention started after the development of ATC stabilized bleeding, reversed prolonged APTT, returned FV and FVIII levels to their normal range, and reduced TAT levels that were increased by ATC. In summary, ^superFVa prevented ATC and traumatic bleeding when administered prophylactically, and ^superFVa stabilized bleeding and reversed abnormal hemostasis parameters when administered while ATC was in progress. Thus, ^superFVa may be an attractive strategy to intercept ATC and mitigate traumatic bleeding.

Decreased platelet surface phosphatidylserine predicts increased bleeding in patients with severe factor VIII deficiency

BACKGROUND

Bleeding phenotypes among individuals with severe factor VIII (FVIII) deficiency are variable, even with routine prophylactic FVIII administration. Activated platelets, in addition to their role in primary hemostasis, play a major role in coagulation by providing a phospholipid surface to which coagulation factors bind.

OBJECTIVES

The aim of this study was to determine whether platelet function is associated with past and/or future bleeding in patients with severe FVIII deficiency on prophylaxis.

PATIENTS/METHODS

Platelet function quantified by platelet surface expression of phosphatidylserine, platelet surface glycoprotein (GP) VI, platelet surface activated GPIIb-IIIa, platelet surface P-selectin, the percentage of coated platelets, and the percentage of platelet-derived microparticles in the presence or absence of in vitro activation by various agonists was assessed in 34 patients.

RESULTS

Decreased platelet surface phosphatidylserine expression (identified by annexin V binding), both in the presence and absence of ADP/thrombin receptor activating peptide, demonstrated a significant association with both prior and subsequent bleeding in any location and specifically with hemarthrosis. No significant difference between patients with and without bleeding was observed in any of the other platelet activity markers.

CONCLUSIONS

Decreased platelet surface phosphatidylserine expression measured by annexin V binding predicts increased bleeding in severe FVIII deficient patients on prophylaxis.

Factor VIII and Factor V Membrane Bound Complexes

The formation of membrane-bound complexes between specific coagulation factors at different cell surfaces is required for effective blood clotting. The most important of these complexes, the intrinsic Tenase and Prothrombinase complexes, are formed on the activated platelet surface during the propagation phase of coagulation. These two complexes are highly specific in their assembly mechanism and function modulated by anionic membranes, thus offering desirable targets for pharmaceutical interventions. Factor V (FV) and factor VIII (FVIII) are highly homologous non-enzymatic proteins. In their active state, FVa and FVIIIa serve as cofactors for the respective serine proteases factor Xa (FXa) and factor IXa (FIXa), significantly increasing their catalytic activity. This is achieved by forming well organized membrane-bound complexes at the phosphatidylserine rich activated platelet membrane in the presence of Ca²⁺ ions. The tenase (FVIIIa/FIXa) complex, catalyzes the proteolytic conversion of FX to FXa. Subsequently the prothrombinase (FVa/FXa) complex catalyzes the conversion of prothrombin to thrombin, required for efficient blood clotting. Although significant knowledge of FV and FVIII biochemistry and regulation has been achieved, the molecular mechanisms of their function are yet to be defined. Understanding the geometric assembly of the tenase and prothrombinase complexes is paramount in defining the structural basis of bleeding and thrombotic disorders. Such knowledge will enable the design of efficient pro- and anticoagulant therapies critical for regulating abnormal hemostasis. In this chapter, we will summarize the findings to date, showing our achievement in the field and outlining the future findings required to grasp the complexity of these proteins.

Red Blood Cell Contribution to Hemostasis

Red Blood Cells (RBCs) have been increasingly recognized to play important roles in hemostasis and the mechanisms by which they do so continue to be elucidated. First and foremost, RBC biomechanics are the principal determinant of viscosity and flow dynamics of blood, which strongly influence all features of hemostasis. Of note, morphologic pathology, such as that found in sickle cell disease, leads to increased risk of thrombotic disease. RBC surface interactions govern signaling between platelets and RBCs and also aid in the conversion of prothrombin to thrombin. Additionally, RBCs generate microparticles which have been shown to reduce clotting time. Finally, blood clot structure and maturation are dependent on the inclusion of RBCs in forming thrombi. Here, we review the above mechanisms of RBC contribution to hemostasis.

An engineered factor Va prevents bleeding induced by direct-acting oral anticoagulants by different mechanisms

Control of bleeding with direct-acting oral anticoagulants (DOACs) remains an unmet clinical need. Activated superFactor V (superFVa) is an engineered activated protein C (APC)-resistant FVa variant with enhanced procoagulant activity resulting from an A2/A3 domain disulfide bond and was studied here for control of DOAC-induced bleeding. SuperFVa reversed bleeding induced by FXa inhibitors (rivaroxaban, apixaban), and the FIIa inhibitor dabigatran in BalbC mice. The blocking anti-protein C and APC [(A)PC] antibody SPC-54 also reduced FXa inhibitor induced bleeding similar to superFVa, whereas dabigatran-induced bleeding was not affected. This indicated that sufficient APC was generated to contribute to bleeding in the presence of FXa inhibitors, but not in the presence of dabigatran, suggesting that mechanisms contributing to bleeding differed for FXa and FIIa inhibitors. Despite different mechanisms contributing to bleeding, superFVa effectively reduced bleeding for all DOACs, indicating the versatility of superFVa's properties that contribute to its universal prohemostatic effects for DOAC associated bleeding. Supported by thrombin generation assays on endothelial cells in normal plasma spiked with DOACs and patient plasma anticoagulated with DOACs, 3 complementary mechanisms were identified by which superFVa achieved DOAC class-independent prohemostatic efficiency. These mechanisms are resistance to inactivation by APC, overcoming the FV activation threshold, and maximizing the efficiency of the prothrombinase complex when the available FXa is increased by FVIIa-based prohemostatics. In summary, it is this versatility of superFVa that delineates it from other prohemostatic agents as a promising class-independent rescue agent in bleeding situations associated with DOACs.

Assembly of alternative prothrombinase by extracellular histones initiate and disseminate intravascular coagulation

Thrombin generation is pivotal to both physiological blood clot formation and pathological development of disseminated intravascular coagulation (DIC). In critical illness, extensive cell damage can release histones into the circulation, which can increase thrombin generation and cause DIC, but the molecular mechanism is not clear. Typically, thrombin is generated by the prothrombinase complex, comprising activated factor X (FXa), activated co-factor V (FVa) and phospholipids to cleave prothrombin in the presence of calcium. In this study, we found that in the presence of extracellular histones, an alternative prothrombinase could form without FVa and phospholipids. Histones directly bind to prothrombin fragments F1 and F2 specifically, to facilitate FXa cleavage of prothrombin to release active thrombin, unlike FVa which requires phospholipid surfaces to anchor the classical prothrombinase complex. In vivo, histone infusion into mice induced DIC, which was significantly abrogated when prothrombin fragments F1+F2 were infused prior to histones, to act as decoy. In a cohort of intensive care unit (ICU) patients with sepsis (n=144), circulating histone levels were significantly elevated in patients with DIC. These data suggest that histone-induced alternative prothrombinase without phospholipid anchorage may disseminate intravascular coagulation, and reveal a new molecular mechanism of thrombin generation and DIC development. In addition, histones significantly reduced the requirement for FXa in the coagulation cascade to enable clot formation in Factor VIII and IX-deficient plasma, as well as in Factor VIII-deficient mice. In conclusion, this study highlights a novel mechanism in coagulation with therapeutic potential in both targeting systemic coagulation activation as well as in correcting coagulation factor deficiency.

Varying effects of tyrosine kinase inhibitors on platelet function-A need for individualized CML treatment to minimize the risk for hemostatic and thrombotic complications?

Since their introduction, tyrosine kinase inhibitors (TKIs, eg, imatinib, nilotinib, dasatinib, bosutinib, ponatinib) have revolutionized the treatment of chronic myeloid leukemia (CML). However, long-term treatment with TKIs is associated with serious adverse events including both bleeding and thromboembolism. Experimental studies have shown that TKIs can cause platelet dysfunction. Herein, we present the first side-by-side investigation comparing the effects of currently used TKIs on platelet function and thrombin generation when used in clinically relevant concentrations. A flow cytometry multiparameter protocol was used to study a range of significant platelet activation events (fibrinogen receptor activation, alpha granule, and lysosomal exocytosis, procoagulant membrane exposure, and mitochondrial permeability changes). In addition, thrombin generation was measured in the presence of TKIs to assess the effects on global hemostasis. Results show that dasatinib generally inhibited platelet function, while bosutinib, nilotinib, and ponatinib showed less consistent effects. In addition to these general trends for each TKI, we observed a large degree of interindividual variability in the effects of the different TKIs. Interindividual variation was also observed when blood from CML patients was studied ex vivo with whole blood platelet aggregometry, free oscillation rheometry (FOR), and flow cytometry. Based on the donor responses in the side-by-side TKI study, a TKI sensitivity map was developed. We propose that such a sensitivity map could potentially become a valuable tool to help in decision-making regarding the choice of suitable TKIs for a CML patient with a history of bleeding or atherothrombotic disease.

Occlusion of anion-binding exosite 2 in meizothrombin explains its impaired ability to activate factor V

The proteolytic conversion of factor V to factor Va is central for amplified flux through the blood coagulation cascade. Heterodimeric factor Va is produced by cleavage at three sites in the middle of factor V by thrombin, yielding an N terminus-derived heavy chain and a C terminus-derived light chain. Here, we show that light chain formation resulting from the C-terminal cleavage is the rate-limiting step in the formation of fully cleaved Va. This rate-limiting step also corresponded to and was sufficient for the ability of cleaved factor V to bind Xa and assemble into the prothrombinase complex. Meizothrombin, the proteinase intermediate in thrombin formation, cleaves factor V more slowly than does thrombin, resulting in a pronounced defect in the formation of the light chain. A ∼100-fold reduced rate of meizothrombin-mediated light chain formation by meizothrombin corresponded to equally slow production of active cofactor and an impaired ability to amplify flux through the coagulation cascade initiated in plasma. We show that this defect arises from the occlusion of anion-binding exosite 2 in the catalytic domain by the covalently retained propiece in meizothrombin. Our findings provide structural insights into the prominent role played by exosite 2 in the rate-limiting step of factor V activation. They also bear on how factor V is converted into a cofactor capable of assembling into prothrombinase.

Quantitative proteomic analysis and antivenom study revealing that neurotoxic phospholipase A2 enzymes, the major toxin class of Russell's viper venom from southern India, shows the least immuno-recognition and neutralization by commercial polyvalent antivenom

The proteome composition of Russell's viper venom (RVV) from southern India (SI) was investigated by 1D-SDS-PAGE of venom followed by tandem mass spectrometry analysis of protein bands. A total of 66 proteins belonging to 14 snake venom protein families were identified by LC-MS/MS analysis against Viperidae (taxid 8689) protein entries from the non-redundant NCBI database. Phospholipase A2 (43.25%) and snaclec (14.57%) represented the most abundant enzymatic and non-enzymatic proteins, respectively. SI RVV was characterized as containing a higher quantity of PLA2 and a lower amount of Kunitz-type serine protease inhibitors, in comparison to RVV from other regions of the Indian subcontinent. The enzymatic activities, pharmacological properties, and clinical manifestations of RV envenomation in SI were well correlated with its proteome composition; however, ATPase, ADPase, and hyaluronidase enzymes were not identified by LC-MS/MS analysis, owing to paucity of the existing database. Neurological symptoms exhibited by RV-bite patients in SI were correlated to the presence of abundant neurotoxic phospholipase A2 enzymes (15.66%) in SI RVV. Neutralization studies, immunological cross-reactivity, and antivenomics studies unequivocally demonstrated the poor recognition and lowest neutralization of PLA2 enzymes by commercial polyvalent antivenom, which is a major concern for the treatment of RV-envenomed patients in SI.

Proteomic analysis reveals geographic variation in venom composition of Russell's Viper in the Indian subcontinent: implications for clinical manifestations post-envenomation and antivenom treatment

INTRODUCTION

The Russell's Viper (RV) (Daboia russelii), a category I medically important snake, is responsible for a significant level of morbidity and mortality in the Indian sub-continent. Areas covered: The current review highlights the variation in RV venom (RVV) composition from different geographical locales on the Indian sub-continent, as revealed by biochemical and proteomic analyses. A comparison of these RVV proteomes revealed significant differences in the number of toxin isoforms and relative toxin abundances, highlighting the impact of geographic location on RVV composition. Antivenom efficacy studies have shown differential neutralization of toxicity and enzymatic activity of different RVV samples from the Indian sub-continent by commercial polyvalent antivenom (PAV). The proteome analysis has provided deeper insights into the variation of RVV composition leading to differences in antivenom efficacy and severity of clinical manifestations post RV-envenomation across the Indian sub-continent. Expert commentary: Variation in RVV antigenicity due to geographical differences and poor recognition of low molecular mass (<20 kDa) RVV toxins by PAV are serious concerns for effective antivenom treatment against RV envenomation. Improvements in immunization protocols that take into account the poorly immunogenic components and geographic variation in RVV composition, can lead to better hospital management of RV bite patients.

Proteomic Analysis and Immuno-Profiling of Eastern India Russell's Viper ( Daboia russelii) Venom: Correlation between RVV Composition and Clinical Manifestations Post RV Bite

The proteomes of Russell's viper venom (RVV) from Burdwan (RVV B) and Nadia (RVV N), the two districts of West Bengal, eastern India (EI), were investigated by gel-filtration chromatography (GFC) followed by tandem mass spectrometry of tryptic fragments of the fractions. A total of 73 and 69 proteins belonging to 15 snake venom protein families were identified in RVV B and RVV N, respectively, by MS/MS search against Viperidae (taxid 8689) protein entries of the nonredundant NCBI database. The minor differences in venom composition of both the EI RV were established unequivocally by their biochemical and pharmacological properties and by SDS-PAGE, gel filtration chromatography, and LC-MS/MS analyses. The composition of EI RVVs was well correlated with published reports on the pathophysiology of RV-envenomed patients from this part of the country. Venom-antivenom cross-reactivity determined by ELISA, Western blotting, and antivenomics approaches demonstrated poor recognition of low molecular mass (<20 kDa) RVV proteins by commercial polyvalent antivenoms, which was substantiated by neutralization of RVV enzymes by antivenom.

TFPIα interacts with FVa and FXa to inhibit prothrombinase during the initiation of coagulation

Tissue factor pathway inhibitor α (TFPIα) inhibits prothrombinase, the thrombin-generating complex of factor Xa (FXa) and factor Va (FVa), during the initiation of coagulation. This inhibition requires binding of a conserved basic region within TFPIα to a conserved acidic region in FXa-activated and platelet-released FVa. In this study, the contribution of interactions between TFPIα and the FXa active site and FVa heavy chain to prothrombinase inhibition were examined to further define the inhibitory biochemistry. Removal of FXa active site binding by mutation or by deletion of the second Kunitz domain (K2) of TFPIα produced 17- or 34-fold weaker prothrombinase inhibition, respectively, establishing that K2 binding to the FXa active site is required for efficient inhibition. Substitution of the TFPIα basic region uncharged residues (Leu252, Ile253, Thr255) with Ala (TFPI-AAKA) produced 5.8-fold decreased inhibition. This finding was confirmed using a basic region peptide (Leu252-Lys261) and Ala substitution peptides, which established that the uncharged residues are required for prothrombinase inhibitory activity but not for binding the FVa acidic region. This suggests that the uncharged residues mediate a secondary interaction with FVa subsequent to acidic region binding. This secondary interaction seems to be with the FVa heavy chain, because the FV Leiden mutation weakened prothrombinase inhibition by TFPIα but did not alter TFPI-AAKA inhibitory activity. Thus, efficient inhibition of prothrombinase by TFPIα requires at least 3 intermolecular interactions: (1) the TFPIα basic region binds the FVa acidic region, (2) K2 binds the FXa active site, and (3) Leu252-Thr255 binds the FVa heavy chain.

Covalently linking heparin to antithrombin enhances prothrombinase inhibition on activated platelets

Factor (F)Xa within the prothrombinase complex is protected from inhibition by unfractionated heparin (UFH), enoxaparin and fondaparinux. We have developed a covalent antithrombin-heparin complex (ATH) with enhanced anticoagulant activity. We have also demonstrated that ATH is superior at inhibiting coagulation factors when assembled on artificial surfaces. The objective of the present study is to determine the ability of ATH vs AT+UFH to inhibit FXa within the prothrombinase complex when the enzyme complex is assembled on the more native platelet system. Discontinuous inhibition assays were performed to determine final k 2-values for inhibition of FXa, FXa within the platelet-prothrombinase, or FXa within prothrombinase devoid of various components. Thrombin generation and plasma clotting was also assayed in the presence of resting/activated platelets ± inhibitors. Protection of FXa was not observed for ATH, whereas a moderate 60% protection was observed for AT+UFH. ATH inhibited platelet-prothrombinase ∼4-fold faster than AT+UFH. Relative to intact prothrombinase, rates for FXa inhibition by AT+UFH in prothrombinase complexes devoid of either prothrombin (II)/activated platelets/FVa were higher. However, inhibition by AT+UFH of prothrombinase devoid of FII yielded slightly lower rates compared to free FXa inhibition. Thrombin generation and plasma clotting was enhanced with activated platelets, while inhibition was better by ATH compared to AT+UFH, thus suggesting an overall enhanced anticoagulant activity of ATH against platelet-bound prothrombinase complexes.

Spellbinding Effects of the Acidic COOH-Terminus of Factor Va Heavy Chain on Prothrombinase Activity and Function

Human factor Va (hfVa) is the important regulatory subunit of prothrombinase. Recent modeling data have suggested a critical role for amino acid Arg⁷⁰¹ of hfVa for human prothrombin (hPro) activation by prothrombinase. Furthermore, it has also been demonstrated that hfVa has a different effect than that of bovine fVa on prethrombin-1 activation by prothrombinase. The difference between the two cofactor molecules was also found within the Asn⁷⁰⁰-Arg⁷⁰¹ dipeptide in the human factor V (hfV) molecule, which is replaced by the Asp-Glu sequence in bfV. As a consequence, we produced a recombinant hfV (rhfV) molecule with the substitution ⁷⁰⁰NR⁷⁰¹→DE. rhfV^NR→DE together with the wild-type molecule (rhfV^WT) were expressed in COS7 cells, purified, and tested for their capability to function within prothrombinase. Kinetic studies showed that the K_d of rhfVa^NR→DE for human fXa as well as the k_cat and K_m of prothrombinase made with rhfVa^NR→DE for hPro activation were similar to the values obtained following hPro activation by prothrombinase made with rhfVa^WT. Remarkably, sodium dodecyl sulfate polyacrylamide gel electrophoresis analyses of hPro activation time courses demonstrated that the rate of cleavage of hPro by prothrombinase reconstituted with rhfVa^NR→DE was significantly delayed with substantial accumulation of meizothrombin, and delayed thrombin generation, when compared to activation of hPro by prothrombinase made with rhfVa^WT. These unanticipated results provide significant insights on the role of the carboxyl-terminal end of the heavy chain of hfVa for hPro cleavage and activation by prothrombinase and show that residues ⁷⁰⁰NR⁷⁰¹ regulate at least in part the enzyme-substrate/product interaction during fibrin clot formation.

Peptides derived from MARCKS block coagulation complex assembly on phosphatidylserine

Blood coagulation involves activation of platelets and coagulation factors. At the interface of these two processes resides the lipid phosphatidylserine. Activated platelets expose phosphatidylserine on their outer membrane leaflet and activated clotting factors assemble into enzymatically active complexes on the exposed lipid, ultimately leading to the formation of fibrin. Here, we describe how small peptide and peptidomimetic probes derived from the lipid binding domain of the protein myristoylated alanine-rich C-kinase substrate (MARCKS) bind to phosphatidylserine exposed on activated platelets and thereby inhibit fibrin formation. The MARCKS peptides antagonize the binding of factor Xa to phosphatidylserine and inhibit the enzymatic activity of prothrombinase. In whole blood under flow, the MARCKS peptides colocalize with, and inhibit fibrin cross-linking, of adherent platelets. In vivo, we find that the MARCKS peptides circulate to remote injuries and bind to activated platelets in the inner core of developing thrombi.

Prothrombin Loading of Vascular Smooth Muscle Cell-Derived Exosomes Regulates Coagulation and Calcification

OBJECTIVE

The drug warfarin blocks carboxylation of vitamin K-dependent proteins and acts as an anticoagulant and an accelerant of vascular calcification. The calcification inhibitor MGP (matrix Gla [carboxyglutamic acid] protein), produced by vascular smooth muscle cells (VSMCs), is a key target of warfarin action in promoting calcification; however, it remains unclear whether proteins in the coagulation cascade also play a role in calcification.

APPROACH AND RESULTS

Vascular calcification is initiated by exosomes, and proteomic analysis revealed that VSMC exosomes are loaded with Gla-containing coagulation factors: IX and X, PT (prothrombin), and proteins C and S. Tracing of Alexa488-labeled PT showed that exosome loading occurs by direct binding to externalized phosphatidylserine (PS) on the exosomal surface and by endocytosis and recycling via late endosomes/multivesicular bodies. Notably, the PT Gla domain and a synthetic Gla domain peptide inhibited exosome-mediated VSMC calcification by preventing nucleation site formation on the exosomal surface. PT was deposited in the calcified vasculature, and there was a negative correlation between vascular calcification and the levels of circulating PT. In addition, we found that VSMC exosomes induced thrombogenesis in a tissue factor-dependent and PS-dependent manner.

CONCLUSIONS

Gamma-carboxylated coagulation proteins are potent inhibitors of vascular calcification suggesting warfarin action on these factors also contributes to accelerated calcification in patients receiving this drug. VSMC exosomes link calcification and coagulation acting as novel activators of the extrinsic coagulation pathway and inducers of calcification in the absence of Gla-containing inhibitors.

Reduced Prothrombinase Inhibition by Tissue Factor Pathway Inhibitor Contributes to the Factor V Leiden Hypercoagulable State

Activated factor V (FVa) and factor X (FXa) form prothrombinase, which converts prothrombin to thrombin. The α isoform of tissue factor (TF) pathway inhibitor (TFPI) dampens early procoagulant events, partly by interacting with FV. FV Leiden (FVL) is the most common genetic thrombophilia in Caucasians. Thrombosis risk is particularly elevated in women with FVL taking oral contraceptives, which produce acquired TFPIα deficiency. In mice, FVL combined with 50% reduction in TFPI causes severe thrombosis and perinatal lethality. However, a possible interaction between FVL and TFPIα has not been defined in humans. Here, we examined this interaction using samples from patients with FVL in thrombin generation and fibrin formation assays. In dilute TF- or FXa-initiated reactions, these studies exposed a TFPI-dependent activation threshold for coagulation initiation that was greatly reduced by FVL. The reduced threshold was progressively overcome with higher concentrations of TF or FXa. Plasma assays using anti-TFPI antibodies or a TFPI peptide that binds and inhibits FVa demonstrated that the decreased activation threshold resulted from reduced TFPIα inhibition of prothrombinase. In assays using purified proteins, TFPIα was a 1.7-fold weaker inhibitor of prothrombinase assembled with FVL than with FV. Thus, FVL reduces the threshold for initiating coagulation, and this threshold is further reduced in situations of low TFPIα concentration. Individuals with FVL are likely prone to thrombosis in response to weak procoagulant stimuli that would not initiate blood clot formation in individuals with FV.

Impact of microparticles derived from erythrocytes on fibrinolysis

It has long been known that negatively charged membranes of erythrocyte-derived microparticles display procoagulant activity. However, relatively little is known about the possible fibrinolytic activity of such microparticles. This issue becomes particularly important during red blood cell storage, which significantly increases the number of microparticles. Whole blood was collected from 30 healthy donors. Microparticles were isolated on days 7, 14, 21, and 28 of erythrocyte storage. The effect of microparticles on the fibrinolytic activity of the donor plasma was determined by coagulation and optical (chromogenic substrate) methods. We demonstrated that erythrocyte microparticles had a prominent fibrinolytic activity which cleaves not only fibrin but also chromogenic substrates. Microparticles present fibrinolytic activity mainly due to the presence of plasminogen on them. Microparticles derived from erythrocytes significantly enhance cleavage of the chromogenic substrate by the streptokinase-plasminogen complex, but to a lesser extent accelerate euglobulin clot lysis time. Erythrocyte-derived microparticles display prominent fibrinolytic activity, which significantly decreases during storage of red blood cells.

Blocking inhibition of prothrombinase by tissue factor pathway inhibitor alpha: a procoagulant property of heparins

Unfractionated heparin (UFH) has procoagulant activity in antithrombin/heparin cofactor II (HCII)-depleted plasma. UFH prevents tissue factor pathway inhibitor alpha (TFPIα) from inhibiting the procoagulant enzyme complex, prothrombinase, providing a possible mechanism for its procoagulant activity. The procoagulant potential of UFH and various low molecular weight heparins (LMWHs) were characterized for TFPIα dependence, using thrombin generation assays performed with antithrombin/HCII-depleted plasma. UFH, the LMWHs enoxaparin and dalteparin, and the low anticoagulant LMWH 2-O, 3-O desulphated heparin (ODSH) all promoted thrombin generation, but fondaparinux did not, and this activity was blocked by a TFPIα antibody. UFH, enoxaparin, and dalteparin were anticoagulant in reactions containing 1-2% normal plasma. In prothrombinase activity assays, UFH, enoxaparin, dalteparin and ODSH blocked prothrombinase inhibition by TFPIα, while again fondaparinux did not. In both the plasma and purified assays, LMWHs displayed greater procoagulant potential than UFH, even when normalized to saccharide concentration. These biochemical data reveal that UFH and LMWHs, but not fondaparinux, block prothrombinase inhibition by TFPIα, thereby producing their paradoxical procoagulant activity observed in the absence of antithrombin/HCII. The findings may help to understand the complex pathophysiology and treatment of patients that are simultaneously bleeding and clotting, such as those with disseminated intravascular coagulation.

Mechanisms Regulating Acquisition of Platelet-Derived Factor V/Va by Megakaryocytes

Factor Va serves as the nonenzymatic protein cofactor for the prothrombinase complex, which converts prothrombin to thrombin in the events leading to formation of a hemostatic plug. Several observations support the concept that platelet-derived factor V/Va is physically and functionally distinct and plays a more important role in thrombin generation at sites of vascular injury as compared to its plasma counterpart. Platelet-derived factor V/Va is generated following endocytosis of the plasma-derived molecule by the platelet precursor cells, megakaryocytes, via a two receptor system consisting of low density lipoprotein (LDL) receptor-related protein-1 (LRP-1) and an unidentified specific "binding site". More recently, it was suggested that a cell surface-expressed β-galactoside binding protein, galectin-8, was involved in factor V endocytosis. Endocytosed factor V is trafficked through the cell and retailored prior to its storage in α-granules. Given the essential role of platelet-derived factor Va in clot formation, understanding the cellular and molecular mechanisms that regulate how platelets acquire this molecule will be important for the treatment of excessive bleeding or clotting.

Expanding the 3-O-Sulfate Proteome--Enhanced Binding of Neuropilin-1 to 3-O-Sulfated Heparan Sulfate Modulates Its Activity

Binding of proteins to heparan sulfate is driven predominantly by electrostatic interactions between positively charged amino acid residues in the protein and negatively charged sulfate groups located at various positions along the polysaccharide chain. Although many heparin/heparan-sulfate-binding proteins have been described, few exhibit preferential binding for heparan sulfates containing relatively rare 3-O-sulfated glucosamine residues. To expand the "3-O-sulfate proteome," affinity matrices were created from Chinese hamster ovary (CHO) cell heparan sulfate engineered in vitro with and without 3-O-sulfate groups. Fractionation of different animal sera yielded several proteins that bound specifically to columns containing 3-O-sulfated heparan sulfate modified by two members of the heparan sulfate 3-O-sulfotransferase superfamily, Hs3st1 and Hs3st2. Neuropilin-1 was analyzed in detail because it has been implicated in angiogenesis and axon guidance. We show that 3-O-sulfation enhanced the binding of neuropilin-1 to heparan sulfate immobilized on plastic plates and to heparan sulfate present on cultured cells. Chemoenzymatically synthesized 3-O-sulfated heparan sulfate dodecamers protected neuropilin-1 from thermal denaturation and inhibited neuropilin-1-dependent, semaphorin-3a-induced growth cone collapse of neurons derived from murine dorsal root ganglia. The effect of 3-O-sulfation was cell autonomous and specific to Hs3st2 based on collapse assays of neurons derived from Hs3st1- and Hs3st2-deficient mice. Finally, 3-O-sulfated heparan sulfate enhanced the inhibition of endothelial cell sprouting by exogenous heparan sulfate. These findings demonstrate a reliable method to identify members of the 3-O-sulfate proteome and that 3-O-sulfation of heparan sulfate can modulate axonal growth cone collapse and endothelial cell sprouting.

Discovery of Novel Nonactive Site Inhibitors of the Prothrombinase Enzyme Complex

The risk of serious bleeding is a major liability of anticoagulant drugs that are active-site competitive inhibitors targeting the Factor Xa (FXa) prothrombin (PT) binding site. The present work identifies several new classes of small molecule anticoagulants that can act as nonactive site inhibitors of the prothrombinase (PTase) complex composed of FXa and Factor Va (FVa). These new classes of anticoagulants were identified, using a novel agnostic computational approach to identify previously unrecognized binding pockets at the FXa-FVa interface. From about three million docking calculations of 281,128 compounds in a conformational ensemble of FXa heavy chains identified by molecular dynamics (MD) simulations, 97 compounds and their structural analogues were selected for experimental validation, through a series of inhibition assays. The compound selection was based on their predicted binding affinities to FXa and their ability to successfully bind to multiple protein conformations while showing selectivity for particular binding sites at the FXa/FVa interface. From these, thirty-one (31) compounds were experimentally identified as nonactive site inhibitors. Concentration-based assays further identified 10 compounds represented by four small-molecule families of inhibitors that achieve dose-independent partial inhibition of PTase activity in a nonactive site-dependent and self-limiting mechanism. Several compounds were identified for their ability to bind to protein conformations only seen during MD, highlighting the importance of accounting for protein flexibility in structure-based drug discovery approaches.

The Dual Regulatory Role of Amino Acids Leu480 and Gln481 of Prothrombin

Prothrombin (FII) is activated to α-thrombin (IIa) by prothrombinase. Prothrombinase is composed of a catalytic subunit, factor Xa (fXa), and a regulatory subunit, factor Va (fVa), assembled on a membrane surface in the presence of divalent metal ions. We constructed, expressed, and purified several mutated recombinant FII (rFII) molecules within the previously determined fVa-dependent binding site for fXa (amino acid region 473-487 of FII). rFII molecules bearing overlapping deletions within this significant region first established the minimal stretch of amino acids required for the fVa-dependent recognition exosite for fXa in prothrombinase within the amino acid sequence Ser(478)-Val(479)-Leu(480)-Gln(481)-Val(482). Single, double, and triple point mutations within this stretch of rFII allowed for the identification of Leu(480) and Gln(481) as the two essential amino acids responsible for the enhanced activation of FII by prothrombinase. Unanticipated results demonstrated that although recombinant wild type α-thrombin and rIIa(S478A) were able to induce clotting and activate factor V and factor VIII with rates similar to the plasma-derived molecule, rIIa(SLQ→AAA) with mutations S478A/L480A/Q481A was deficient in clotting activity and unable to efficiently activate the pro-cofactors. This molecule was also impaired in protein C activation. Similar results were obtained with rIIa(ΔSLQ) (where rIIa(ΔSLQ) is recombinant human α-thrombin with amino acids Ser(478)/Leu(480)/Gln(481) deleted). These data provide new evidence demonstrating that amino acid sequence Leu(480)-Gln(481): 1) is crucial for proper recognition of the fVa-dependent site(s) for fXa within prothrombinase on FII, required for efficient initial cleavage of FII at Arg(320); and 2) is compulsory for appropriate tethering of fV, fVIII, and protein C required for their timely activation by IIa.

How the Linker Connecting the Two Kringles Influences Activation and Conformational Plasticity of Prothrombin

A flexible linker (Lnk2) composed of 26 amino acids connects kringle-1 to kringle-2 in the coagulation factor prothrombin. Recent studies point to Lnk2 as a key determinant of the structure and function of this zymogen. Using a combination of mutagenesis, structural biology, and single molecule spectroscopy, we show how Lnk2 influences activation and conformational plasticity of prothrombin. Scrambling the sequence of Lnk2 is inconsequential on activation, and so is extension by as many as 22 residues. On the other hand, below a critical length of 15 residues, the rate of prothrombin activation increases (10-fold) in the absence of cofactor Va and decreases (3-fold) in the presence of cofactor. Furthermore, activation by prothrombinase takes place without preference along the prethrombin-2 (cleavage at Arg(271) first) or meizothrombin (cleavage at Arg(320) first) pathways. Notably, these transitions in the rate and pathway of activation require the presence of phospholipids, pointing to an important physiological role for Lnk2 when prothrombin is anchored to the membrane. Two new crystal structures of prothrombin lacking 22 (ProTΔ146-167) or 14 (ProTΔ154-167) residues of Lnk2 document striking conformational rearrangements of domains located across this linker. FRET measurements of freely diffusing single molecules prove that these structural transitions are genuine properties of the zymogen in solution. These findings support a molecular model of prothrombin activation where Lnk2 presents the sites of cleavage at Arg(271) and Arg(320) to factor Xa in different orientations by pivoting the C-terminal kringle-2/protease domain pair on the N-terminal Gla domain/kringle-1 pair anchored to the membrane.

Tissue Factor Pathway Inhibitor: Multiple Anticoagulant Activities for a Single Protein

Tissue factor (TF) pathway inhibitor (TFPI) is an anticoagulant protein that inhibits early phases of the procoagulant response. Alternatively spliced isoforms of TFPI are differentially expressed by endothelial cells and human platelets and plasma. The TFPIβ isoform localizes to the endothelium surface where it is a potent inhibitor of TF-factor VIIa complexes that initiate blood coagulation. The TFPIα isoform is present in platelets. TFPIα contains a stretch of 9 amino acids nearly identical to those found in the B-domain of factor V that are well conserved in mammals. These amino acids provide exosite binding to activated factor V, which allows for TFPIα to inhibit prothrombinase during the initiation phase of blood coagulation. Endogenous inhibition at this point in the coagulation cascade was only recently recognized and has provided a biochemical rationale to explain the pathophysiological mechanisms underlying several clinical disorders. These include the east Texas bleeding disorder that is caused by production of an altered form of factor V with high affinity for TFPI and a paradoxical procoagulant effect of heparins. In addition, these findings have led to ideas for pharmacological targeting of TFPI that may reduce bleeding in hemophilia patients.

Lyso-Sulfatide Binds Factor Xa and Inhibits Thrombin Generation by the Prothrombinase Complex

Blood coagulation reactions are strongly influenced by phospholipids, but little is known about the influence of sphingolipids on coagulation mechanisms. Lysosulfatide (lyso-SF) (sulfogalactosyl sphingosine) prolonged factor Xa (fXa) 1-stage plasma clotting assays, showing it had robust anticoagulant activity. In studies using purified clotting factors, lyso-SF inhibited >90% of prothrombin (II) activation for reaction mixtures containing fXa/factor Va (fVa)/II, and also inhibited II activation generation by fXa/ phospholipids and by Gla-domainless-fXa/fVa/phospholipids. When lyso-SF analogs were tested, results showed that N-acetyl-sulfatide was not anticoagulant, implying that the free amine group was essential for the anticoagulant effects of lyso-SF. Lyso-SF did not inhibit fXa enzymatic hydrolysis of small peptide substrates, showing it did not directly inhibit the fXa activity. In surface plasmon resonance studies, lyso-SF bound to immobilized inactivated fXa as well as inactivated Gla-domainless-fXa. Confirming this lyso-SF:fXa interaction, fluorescence studies showed that fluorescently-labeled-fXa in solution bound to lyso-SF. Thus, lyso-SF is an anticoagulant lipid that inhibits fXa when this enzyme is bound to either phospholipids or to fVa. Mechanisms for inhibition of procoagulant activity are likely to involve lyso-SF binding to fXa domain(s) that are distinct from the fXa Gla domain. This suggests that certain sphingolipids, including lyso-SF and some of its analogs, may down-regulate fXa activity without inhibiting the enzyme's active site or binding to the fXa Gla domain.

Viscoelasticity and Ultrastructure in Coagulation and Inflammation: Two Diverse Techniques, One Conclusion

The process of blood clotting has been studied for centuries. A synopsis of current knowledge pertaining to haemostasis and the blood components, including platelets and fibrin networks which are closely involved in coagulation, are discussed. Special emphasis is placed on tissue factor (TF), calcium and thrombin since these components have been implicated in both the coagulation process and inflammation. Analysis of platelets and fibrin morphology indicate that calcium, tissue factor and thrombin at concentrations used during viscoelastic analysis (with thromboelastography or TEG) bring about alterations in platelet and fibrin network ultrastructure, which is similar to that seen in inflammation. Scanning electron microscopy indicated that, when investigating platelet structure in disease, addition of TF, calcium or thrombin will mask disease-induced alterations associated with platelet activation. Therefore, washed platelets without any additives is preferred for morphological analysis. Furthermore, morphological and viscoelastic analysis confirmed that thrombin activation is the preferred method of fibrin activation when investigating fibrin network ultrastructure.

Platelet-rich plasma stimulated by pulse electric fields: Platelet activation, procoagulant markers, growth factor release and cell proliferation

Therapeutic use of activated platelet-rich plasma (PRP) has been explored for wound healing, hemostasis and antimicrobial wound applications. Pulse electric field (PEF) stimulation may provide more consistent platelet activation and avoid complications associated with the addition of bovine thrombin, the current state of the art ex vivo activator of therapeutic PRP. The aim of this study was to compare the ability of PEF, bovine thrombin and thrombin receptor activating peptide (TRAP) to activate human PRP, release growth factors and induce cell proliferation in vitro. Human PRP was prepared in the Harvest SmartPreP2 System and treated with vehicle, PEF, bovine thrombin, TRAP or Triton X-100. Platelet activation and procoagulant markers and microparticle generation were measured by flow cytometry. Released growth factors were measured by ELISA. The releasates were tested for their ability to stimulate proliferation of human epithelial cells in culture. PEF produced more platelet-derived microparticles, P-selectin-positive particles and procoagulant annexin V-positive particles than bovine thrombin or TRAP. These differences were associated with higher levels of released epidermal growth factor after PEF than after bovine thrombin or TRAP but similar levels of platelet-derived, vascular-endothelial, and basic fibroblast growth factors, and platelet factor 4. Supernatant from PEF-treated platelets significantly increased cell proliferation compared to plasma. In conclusion, PEF treatment of fresh PRP results in generation of microparticles, exposure of prothrombotic platelet surfaces, differential release of growth factors compared to bovine thrombin and TRAP and significant cell proliferation. These results, together with PEF's inherent advantages, suggest that PEF may be a superior alternative to bovine thrombin activation of PRP for therapeutic applications.

Factor Xa dimerization competes with prothrombinase complex formation on platelet-like membrane surfaces

Exposure of phosphatidylserine (PS) molecules on activated platelet membrane surface is a crucial event in blood coagulation. Binding of PS to specific sites on factor Xa (fXa) and factor Va (fVa) promotes their assembly into a complex that enhances proteolysis of prothrombin by approximately 10⁵. Recent studies demonstrate that both soluble PS and PS-containing model membranes promote formation of inactive fXa dimers at 5 mM Ca²⁺. In the present study, we show how competition between fXa dimerization and prothrombinase formation depends on Ca²⁺ and lipid membrane concentrations. We used homo-FRET measurements between fluorescein-E-G-R-chloromethylketone (CK)-Xa [fXa irreversibly inactivated by alkylation of the active site histidine residue with FEGR (FEGR-fXa)] and prothrombinase activity measurements to reveal the balance between fXa dimer formation and fXa-fVa complex formation. Changes in FEGR-fXa dimer homo-FRET with addition of fVa to model-membrane-bound FEGR-fXa unambiguously demonstrated that formation of the FEGR-fXa-fVa complex dissociated the dimer. Quantitative global analysis according to a model for protein interaction equilibria on a surface provided an estimate of a surface constant for fXa dimer dissociation (K(fXa×fXa)(d, σ)) approximately 10-fold lower than K(fXa×fVa)(d,σ) for fXa-fVa complex. Experiments performed using activated platelet-derived microparticles (MPs) showed that competition between fXa dimerization and fXa-fVa complex formation was even more prominent on MPs. In summary, at Ca²⁺ concentrations found in the maturing platelet plug (2-5 mM), fVa can compete fXa off of inactive fXa dimers to significantly amplify thrombin production, both because it releases dimer inhibition and because of its well-known cofactor activity. This suggests a hitherto unanticipated mechanism by which PS-exposing platelet membranes can regulate amplification and propagation of blood coagulation.

2013 scientific sessions Sol Sherry distinguished lecture in thrombosis: polyphosphate: a novel modulator of hemostasis and thrombosis

Polyphosphate is a highly anionic, linear polymer of inorganic phosphates that is found throughout biology, including in many infectious microorganisms. Recently, polyphosphate was discovered to be stored in a subset of the secretory granules of human platelets and mast cells, and to be secreted on activation of these cells. Work from our laboratory and others has now shown that polyphosphate is a novel, potent modulator of the blood clotting and complement systems that likely plays roles in hemostasis, thrombosis, inflammation, and host responses to pathogens. Therapeutics targeting polyphosphate may have the potential to limit thrombosis with fewer hemorrhagic complications than conventional anticoagulant drugs that target essential proteases of the blood clotting cascade.

Ca2+ switches the effect of PS-containing membranes on Factor Xa from activating to inhibiting: implications for initiation of blood coagulation

Calcium (Ca2+) plays a pivotal role in cellular and organismal physiology. The Ca2+ ion has an intermediate protein-binding affinity and thus it can serve as an on/off switch in the regulation of different biochemical processes. The serum level of ionized Ca2+ is regulated with normal ionized Ca2+ being in the range 1.10-1.3 mM. Hypocalcaemia (free Ca2+<1.1 mM) in critically ill patients is commonly accompanied by haemostatic abnormalities, ranging from isolated thrombocytopenia to complex defects such as disseminated intravascular coagulation, commonly thought to be due to insufficient functioning of anticoagulation pathways. A small amount of fXa (Factor Xa) produced by Factor VIIa and exposed tissue factor is key to initiating blood coagulation by producing enough thrombin to induce the later stages of coagulation. fXa must bind to PS (phosphatidylserine)-containing membranes to produce thrombin at a physiologically significant rate. In the present study, we show that overall fXa activity on PS-containing membranes is sharply regulated by a 'Ca2+ switch' centred at 1.16 mM, below which fXa is active and above which fXa forms inactive dimers on PS-exposing membranes. Our data lead to a mathematical model that predicts the variation of fXa activity as a function of both Ca2+ and membrane concentrations. Because the critical Ca2+ concentration is at the lower end of the normal plasma ionized Ca2+ concentration range, we propose a new regulatory mechanism by which local Ca2+ concentration switches fXa from an intrinsically active form to a form requiring its cofactor [fVa (Factor Va)] to achieve significant activity.

Polyphosphate accelerates factor V activation by factor XIa

Factor Va enhances the rate of prothrombin activation by factor Xa by four to five orders of magnitude. Production of initiating levels of factor Va from its precursor, factor V, is a critical event early in haemostasis, as factor V exhibits negligible cofactor activity. While thrombin is the most potent physiological back-activator of factor V, the first prothrombinase complexes require a source of factor Va prior to thrombin generation. A recent study by Whelihan et al. (J Thromb Haemost 2010; 8:1532-1539) identified factor XIa as a candidate for the initial thrombin-independent activation of factor V, although this reaction was slow and required relatively high concentrations of factors V and XIa. Activated platelets secrete polyphosphate, which we previously showed to be potently procoagulant. We now report that polyphosphate greatly accelerates factor V activation by factor XIa, and that this is supported by polyphosphate polymers of the size secreted by activated human platelets. This finding provides additional evidence that factor XIa-mediated generation of factor Va may contribute to the initiation of haemostasis.

An engineered factor Va prevents bleeding induced by anticoagulant wt activated protein C

OBJECTIVE

An increased risk of bleeding is observed in patients receiving activated protein C (APC), which may be a limiting factor for the application of novel APC therapies. Since APC's therapeutic effects often require its cytoprotective activities on cells but not APC's anticoagulant activities, an agent that specifically antagonizes APC's anticoagulant effects but not its cytoprotective effects could provide an effective means to control concerns for risk of bleeding. We hypothesized that superFVa, an engineered activated FVa-variant that restores hemostasis in hemophilia could reduce APC-induced bleeding.

APPROACH AND RESULTS

SuperFVa was engineered with mutations of the APC cleavage sites (Arg506/306/679Gln) and a disulfide bond (Cys609-Cys1691) between the A2 and A3 domains, which augment its biological activity and cause high resistance to APC. SuperFVa normalized APC-prolonged clotting times and restored APC-suppressed thrombin generation in human and murine plasma at concentrations where wild-type (wt) FVa did not show effects. Following intravenous injection of APC into BALB/c mice, addition to whole blood ex vivo of superFVa but not wt-FVa significantly normalized whole blood clotting. Blood loss following tail clip or liver laceration was significantly reduced when superFVa was administered intravenously to BALB/c mice prior to intravenous APC-treatment. Furthermore, superFVa abolished mortality (∼50%) associated with excessive bleeding following liver laceration in mice treated with APC.

CONCLUSIONS

Our results provide proof of concept that superFVa is effective in preventing APC-induced bleeding and may provide therapeutic benefits as a prohemostatic agent in various situations where bleeding is a serious risk.