Minimum wound size for clotting: flowing blood coagulates on a single collagen fiber presenting tissue factor and von Willebrand factor

It is unknown if a lower size limit exists for human blood coagulation under flow over physiological vessel wall triggers as small as a single collagen fiber. Prior determinations of the smallest sized surface stimuli necessary for clotting of human blood, defined as the patch size threshold, have not deployed whole blood, hemodynamic flow, and platelet adhesive stimuli. For whole blood perfused in microfluidic devices, we report that steady venous flow (wall shear rate, 100 s(-1)) was sufficient to drive platelet deposition on 20 micron long zones of collagen fibers or on a single fiber. With tissue factor (TF)-coated collagen, flowing blood generated robust platelet deposits, platelet-localized thrombin, and fibrin on a single collagen fiber, thus demonstrating the absence of a physiological patch size threshold under venous flow. In contrast, at arterial wall shear rate (1000 s(-1)) with TF present, essentially no platelet or fibrin deposition occurred on 20 micron collagen zones or on a single collagen fiber, demonstrating a patch threshold, which was overcome by pre-coating the collagen with von Willebrand factor (vWF). For venous flows, human blood can clot on one of the smallest biological units of a single collagen fiber presenting TF. For arterial flows, vWF together with TF allows human blood to generate thrombin and fibrin on a patch stimulus as limited as a single collagen fiber. vWF-dependent platelet adhesion represents a particle-based sensing mechanism of micron-scale stimuli that then allows amplification of the molecular components of TF-driven thrombin and fibrin production under arterial flow.

Inhibition of protease-activated receptor 4 impairs platelet procoagulant activity during thrombus formation in human blood

Essentials The platelet thrombin receptor, PAR4, is an emerging anti-thrombotic drug target. We examined the anti-platelet & anti-thrombotic effects of PAR4 inhibition in human blood. PAR4 inhibition impaired platelet procoagulant activity in isolated cells and during thrombosis. Our study shows PAR4 is required for platelet procoagulant function & thrombosis in human blood.

SUMMARY

Background Thrombin-induced platelet activation is important for arterial thrombosis. Thrombin activates human platelets predominantly via protease-activated receptor (PAR)1 and PAR4. PAR1 has higher affinity for thrombin, and the first PAR1 antagonist, vorapaxar, was recently approved for use as an antiplatelet agent. However, vorapaxar is contraindicated in a significant number of patients, owing to adverse bleeding events. Consequently, there is renewed interest in the role of platelet PAR4 in the setting of thrombus formation. Objectives To determine the specific antiplatelet effects of inhibiting PAR4 function during thrombus formation in human whole blood. Methods and Results We developed a rabbit polyclonal antibody against the thrombin cleavage site of PAR4, and showed it to be a highly specific inhibitor of PAR4-mediated platelet function. This function-blocking anti-PAR4 antibody was used to probe for PAR4-dependent platelet functions in human isolated platelets in the absence and presence of concomitant PAR1 inhibition. The anti-PAR4 antibody alone was sufficient to abolish the sustained elevation of cytosolic calcium level and consequent phosphatidylserine exposure induced by thrombin, but did not significantly inhibit integrin αII b β3 activation, α-granule secretion, or aggregation. In accord with these in vitro experiments on isolated platelets, selective inhibition of PAR4, but not of PAR1, impaired thrombin activity (fluorescence resonance energy transfer-based thrombin sensor) and fibrin formation (anti-fibrin antibody) in an ex vivo whole blood flow thrombosis assay. Conclusions These findings demonstrate that PAR4 is required for platelet procoagulant function during thrombus formation in human blood, and suggest PAR4 inhibition as a potential target for the prevention of arterial thrombosis.

Reduced fibrinolytic resistance in patients with factor XI deficiency. Evidence of a thrombin-independent impairment of the thrombin-activatable fibrinolysis inhibitor pathway

Essentials Plasma of factor XI-deficient patients (FXI-dp) displays enhanced fibrinolysis. We investigated the role of thrombin activatable fibrinolysis inhibitor (TAFI) in 18 FXI-dp. FXI-dp generated less activated TAFI (TAFIa) on clotting challenge and were resistant to TAFIa. TAFI activation and TAFIa resistance correlated with bleeding score and bleeding phenotype.

SUMMARY

Background Factor XI (FXI) deficiency, a rare disorder with unpredictable bleeding, has been associated with reduced fibrinolytic resistance as a result of abnormal fibrin density. Objective We investigated the involvement of thrombin-activatable fibrinolysis inhibitor (TAFI) in the increased lysability of FXI-deficient (FXI-def) clots and the role of thrombin. Patients/Methods Eighteen patients with FXI deficiency (1-58%) and 17 matched controls were investigated for fibrinolytic resistance to t-PA, thrombin generation, TAFI activation and response to TAFIa. Results When clotting was induced by 0.5 pm tissue factor (TF), FXI-def plasmas displayed less thrombin and TAFIa generation and shorter lysis time than controls. A 100-fold higher TF concentration (to bypass FXI) abolished the difference in thrombin generation but not in lysis time between patients and controls. Normalization of FXI levels by a FXI concentrate increased thrombin generation but had no effect on the lysis time of FXI-def plasma. Moreover, when clots were induced by purified thrombin and high concentrations of FXa inhibitor, FXI-def plasma still generated less TAFIa and displayed a shorter lysis time than controls. Finally, upon TAFIa addition, the lysis time of FXI-def plasma was prolonged significantly less than that of control plasma, suggesting a TAFIa resistance. TAFIa generation and TAFIa resistance were correlated with the bleeding score, displaying a considerable capacity to discriminate between patients with and without bleeding. Conclusions TAFI pathway impairment, largely caused by a hitherto unknown TAFIa resistance, appears to be one main cause of decreased fibrinolytic resistance in FXI deficiency and might be clinically useful for assessing the bleeding risk of FXI-def patients.

Angiopoietin-2 mediates thrombin-induced monocyte adhesion and endothelial permeability

Essentials Mechanism of thrombin-induced inflammation is not fully understood. Thrombin induced monocyte adhesion and barrier loss require Angiopoietin-2 (Ang-2). Ang-2 mediates vessel leakage and monocyte adhesion through SHP-2/p38MAPK pathway. Calcium dependent SHP2/p38MAPK activation regulates Ang-2 expression through a feedback loop.

SUMMARY

Background Thrombin imparts an inflammatory phenotype to the endothelium by promoting increased monocyte adhesion and vascular permeability. However, the molecular players that govern these events are incompletely understood. Objective The aim of this study was to determine whether Angiopoietin-2 (Ang-2) has a role, if any, in regulating inflammatory signals initiated by thrombin. Methods Assessment of vascular leakage by Miles assay was performed by intra-dermal injection on the foot paw. Surface levels of intercellular adhesion molecule-1 (ICAM-1) were determined by flow cytometry. Overexpression, knockdown and phosphorylation of proteins were determined by Western blotting. Results In time-course experiments, thrombin-stimulated Ang-2 up-regulation, peaked prior to the expression of adhesion molecule ICAM-1 in human umbilical vein-derived endothelial cells (HUVECs). Knockdown of Ang-2 blocked both thrombin-induced monocyte adhesion and ICAM-1 expression. In addition, Ang-2(-/-) mice displayed defective vascular leakage when treated with thrombin. Introducing Ang-2 protein in Ang-2(-/-) mice failed to recover a wild-type phenotype. Mechanistically, Ang-2 appears to regulate the thrombin-activated calcium spike that is required for tyrosine phosphatase SHP2 and p38 MAPK activation. Further, down-regulation of SHP2 attenuated both thrombin-induced Ang-2 expression and monocyte adhesion. Down-regulation of the adaptor protein Gab1, a co-activator of SHP2, as well as overexpression of the Gab1 mutant incapable of interacting with SHP2 (YFGab1), inhibited thrombin-mediated effects, including downstream activation of p38 MAPK, which in turn was required for Ang-2 expression. Conclusions The data establish an essential role of the Gab1/SHP2/p38MAPK signaling pathway and Ang-2 in regulating thrombin-induced monocyte adhesion and vascular leakage.

Elucidation of the molecular mechanisms of two nanobodies that inhibit thrombin-activatable fibrinolysis inhibitor activation and activated thrombin-activatable fibrinolysis inhibitor activity

Essentials Thrombin-activatable fibrinolysis inhibitor (TAFI) is a risk factor for cardiovascular disorders. TAFI inhibitory nanobodies represent a promising step in developing profibrinolytic therapeutics. We have solved three crystal structures of TAFI in complex with inhibitory nanobodies. Nanobodies inhibit TAFI through distinct mechanisms and represent novel profibrinolytic leads.

SUMMARY

Background Thrombin-activatable fibrinolysis inhibitor (TAFI) is converted to activated TAFI (TAFIa) by thrombin, plasmin, or the thrombin-thrombomodulin complex (T/TM). TAFIa is antifibrinolytic, and high levels of TAFIa are associated with an increased risk for cardiovascular disorders. TAFI-inhibitory nanobodies represent a promising approach for developing profibrinolytic therapeutics. Objective To elucidate the molecular mechanisms of inhibition of TAFI activation and TAFIa activity by nanobodies with the use of X-ray crystallography and biochemical characterization. Methods and results We selected two nanobodies for cocrystallization with TAFI. VHH-a204 interferes with all TAFI activation modes, whereas VHH-i83 interferes with T/TM-mediated activation and also inhibits TAFIa activity. The 3.05-Å-resolution crystal structure of TAFI-VHH-a204 reveals that the VHH-a204 epitope is localized to the catalytic moiety (CM) in close proximity to the TAFI activation site at Arg92, indicating that VHH-a204 inhibits TAFI activation by steric hindrance. The 2.85-Å-resolution crystal structure of TAFI-VHH-i83 reveals that the VHH-i83 epitope is located close to the presumptive thrombomodulin-binding site in the activation peptide (AP). The structure and supporting biochemical assays suggest that VHH-i83 inhibits TAFIa by bridging the AP to the CM following TAFI activation. In addition, the 3.00-Å-resolution crystal structure of the triple TAFI-VHH-a204-VHH-i83 complex demonstrates that the two nanobodies can simultaneously bind to TAFI. Conclusions This study provides detailed insights into the molecular mechanisms of TAFI inhibition, and reveals a novel mode of TAFIa inhibition. VHH-a204 and VHH-i83 merit further evaluation as potential profibrinolytic therapeutics.

Identification and functional characterization of a novel F5 mutation (Ala512Val, FVB onn ) associated with activated protein C resistance

Essentials Activated protein C (APC) resistance is a prevalent risk factor for venous thrombosis. A novel missense mutation (Ala512Val - FVBonn ) was characterized in vitro and in silico. FVBonn is a new cause of APC resistance and venous thrombosis. FVBonn expresses additionally enhanced procoagulant activity in the absence of APC.

SUMMARY

Background Activated protein C (APC) resistance is a prevalent risk factor for venous thrombosis. This phenotype is most commonly associated with the factor V Arg506Gln mutation (FV Leiden), which impairs the APC-mediated inactivation of both activated FV (FVa) and activated FVIII (FVIIIa). Objectives Here, we report the identification and characterization of a novel FV mutation (Ala512Val, FVBonn ) in six patients with APC resistance and venous thrombosis or recurrent abortions. Methods FVBonn was expressed in a recombinant system and compared with recombinant wild-type (WT) FV and FV Leiden in several functional assays. Results FVBonn conferred APC resistance to FV-depleted plasma, both in the activated partial thromboplastin time (APTT)-based test (APC sensitivity ratio [APCsr] of 1.98 for FVBonn versus 4.31 for WT FV and 1.59 for FV Leiden) and in the thrombin generation-based test (normalized APCsr of 5.41 for FVBonn versus 1.00 for WT FV and 8.99 for FV Leiden). The APC-mediated inactivation of FVaBonn was slower than that of WT FVa (mainly because of delayed cleavage at Arg506), but was greatly stimulated by protein S. The APC cofactor activity of FVBonn in FVIIIa inactivation was ~ 24% lower than that of WT FV. In line with these findings, an in silico analysis showed that the Ala512Val mutation is located in the same loop as the Arg506 APC cleavage site and might hamper its interaction with APC. Moreover, FVBonn was more procoagulant than WT FV and FV Leiden in the absence of APC, because of an increased activation rate and, possibly, an enhanced interaction with activated FX. Conclusions FVBonn induces hypercoagulability via a combination of increased activation/procoagulant activity, decreased susceptibility to APC-mediated inactivation, and slightly reduced APC cofactor activity.

Fibrin self-assembly is adapted to oxidation

Fibrinogen is extremely susceptible to attack by reactive oxygen species (ROS). Having been suffered an oxidative modification, the fibrinogen molecules, now with altered spatial structure and function of fibrin network, affect hemostasis differently. However, the potential effects of the oxidative stress on the early stages of the fibrin self-assembly process remain unexplored. To clarify the damaging influence of ROS on the knob 'A': hole 'a' and the D:D interactions, the both are operating on the early stages of the fibrin polymerization, we have used a novel approach based on exploration of FXIIIa-mediated self-assembly of the cross-linked fibrin oligomers dissolved in the moderately concentrated urea solutions. The oligomers were composed of monomeric desA fibrin molecules created by cleaving the fibrinopeptides A off the fibrinogen molecules with a thrombin-like enzyme, reptilase. According to the UV-absorbance and fluorescence measurements data, the employed low ozone/fibrinogen ratios have induced only a slight fibrinogen oxidative modification that was accompanied by modest chemical transformations of the aromatic amino acid residues of the protein. Else, a slight consumption of the accessible tyrosine residues has been observed due to intermolecular dityrosine cross-links formation. The set of experimental data gathered with the aid of electrophoresis, elastic light scattering and analytical centrifugation has clearly witnessed that the oxidation can serve as an effective promoter for the observed enhanced self-assembly of the covalently cross-linked oligomers. At urea concentration of 1.20M, the pristine and oxidized fibrin oligomers were found to comprise a heterogeneous set of the double-stranded protofibrils that are cross-linked only by γ-γ dimers and the fibers consisting on average of four strands that are additionally linked by α polymers. The amounts of the oxidized protofibrils and the fibers accumulated in the system were higher than those of the non-oxidized counterparts. Moreover, the γ and α polypeptide chains of the oxidized molecules were more readily crosslinked by the FXIIIa. Upon increasing the urea solution concentration to 4.20M, the cross-linked double-stranded desA fibrin protofibrils have dissociated into the single-stranded fibrin oligomers, whereas the fibers dissociated into both the double-stranded desA fibrin oligomers, the structural integrity of the latter being maintained by means of the intermolecular α polymers, and the single-stranded fibrin oligomers cross-linked only by γ-γ dimers. The data we have obtained in this study indicate that the FXIIIa-mediated process of assembling the cross-linked protofibrils and the fibers constructed from the oxidized monomeric fibrin molecules was facilitated due to the strengthening of D:D interactions. The findings infer that the enhanced longitudinal D:D interactions become more essential in the assembly of soluble protofibrils when the interactions knobs 'A': holes 'a' are injured by oxidation. The new experimental findings presented here could be of help for elucidating the essential adaptive molecular mechanisms capable of mitigating the detrimental action of ROS in the oxidatively damaged fibrin self-assemblage processes.

Regulating VEGF signaling in platelet concentrates via specific VEGF sequestering

Platelets contain an abundance of growth factors that mimic the composition of the wound healing milieu, and platelet-derived VEGF in particular can negatively influence wound healing if unregulated. Here, we sought to capture and regulate the activity of VEGF factor from human platelets using poly(ethylene glycol) microspheres. In this communication, we demonstrate that platelet freeze/thaw produced significantly higher levels of Vascular Endothelial Growth Factor (VEGF) than either calcium chloride treatment, protease activated receptor 1 activating peptide (PAR1AP) treatment, or thrombin treatment. PEG microspheres containing a VEGF-binding peptide (VBP), derived from VEGFR2, sequestered VEGF from platelet concentrate, prepared via freeze/thaw, and reduced the bioactivity of platelet concentrate in HUVEC culture, which suggests that VBP microspheres sequestered and reduced the activity of VEGF from patient-derived platelets. Here, we demonstrate the ability of VEGF sequestering microspheres to regulate the activity of VEGF derived from a growth factor-rich autologous human blood product.

Stabilizing interactions between D666-S1787 and T657-Y1792 at the A2-A3 interface support factor VIIIa stability in the blood clotting pathway

Essentials Factor VIIIa (FVIIIa) is unstable due to loss of A2; D666 and Y1792 contribute to its stability. We conducted a study to identify the interactions made at these residues at the A2-A3 interface. We present evidence for stabilizing interactions between D666-S1787 and T657-Y1792 in FVIIIa. A D666C/S1788C variant with a disulfide A2-A3 linkage has a FVIIIa decay rate that is 1% of wild-type.

SUMMARY

Background Factor (F)VIIIa activity and stability depends on the non-covalent association of the A2 subunit with the A1/A3C1C2 dimer, but the interactions that contribute to A2 association are not well defined. Previous work had shown that D666A and Y1792F mutations at the A2-A3 interface resulted in increased FVIIIa decay, suggesting that the residues were involved in bonding interactions important for FVIIIa stability. Objectives Several potential hydrogen bonding partners of D666 and Y1792 across the A2-A3 interface were selected from the low-resolution FVIII crystal structure, and we used mutagenesis and biochemical analysis to examine the bonding interactions occurring at D666 and Y1792. Methods Using a series of stability and functional analyses, we analyzed FVIII variants in which D666 and Y1792 were each swapped with the residues of potential bonding partners. Results and conclusions We present evidence for hydrogen bonds between D666 and S1787 and between Y1792 and T657 that are important for FVIIIa stability. D666S/S1787D and T657Y/Y1792T variants each displayed wild-type (WT)-like FVIIIa stability and performed like WT FVIII in a series of functional analyses, whereas D666S, S1787D, and Y1792T single variants showed increased FVIIIa decay and a T657Y variant had little FVIIIa activity. These results suggest that WT hydrogen bonds are disrupted with the single mutations but maintained in the swap variants. Furthermore, mutation of D666 and S1788 to cysteine resulted in disulfide bond formation across the A2-A3 interface, confirming the close proximity of D666 and S1787, and this covalent attachment of the A2 subunit significantly increased FVIIIa stability.

Procoagulant microparticles promote coagulation in a factor XI-dependent manner in human endotoxemia

Essentials The procoagulant effects of microparticles (MPs) on coagulation in endotoxemia are not known. MPs from endotoxemia volunteers were evaluated for procoagulant activity in a plasma milieu. MPs from endotoxemia volunteers shortened clotting times and enhanced thrombin generation. MP procoagulant effects were mediated in a factor XI-dependent manner.

SUMMARY

Background Human endotoxemia is characterized by acute inflammation and activation of coagulation, as well as increased numbers of circulating microparticles (MPs). Whether these MPs directly promote coagulation and through which pathway their actions are mediated, however, has not been fully explored. Objectives In this study, we aimed to further characterize endotoxin-induced MPs and their procoagulant properties using several approaches. Methods Enumeration and characterization of MPs were performed using a new-generation flow cytometer. Relative contributions of the extrinsic and intrinsic pathways in MP-mediated procoagulant activity were assessed using plasmas deficient in factor (F) VII or FXI or with blocking antibodies to tissue factor (TF) or FXIa. Results Total MPs and platelet MPs were significantly elevated in plasma at 6 h after infusion of endotoxin in healthy human subjects. MPs isolated from plasma following endotoxin infusion also demonstrated increased TF activity in a reconstituted buffer system. When added to recalcified platelet-poor plasma, these MPs also promoted coagulation, as judged by a decreased clotting time with shortening of the lag time and time to peak thrombin using calibrated automated thrombography (CAT). However, the use of FVII-deficient plasma or blocking antibody to TF did not inhibit these procoagulant effects. In contrast, plasma clotting time was prolonged in FXI-deficient plasma and a blocking antibody to FXIa inhibited all MP-mediated parameters in the CAT assay. Conclusions The initiation of coagulation by cellular TF in endotoxemia is in contrast to (and presumably complemented by) the intrinsic pathway-mediated procoagulant effects of circulating MPs.

The Alzheimer's disease peptide β-amyloid promotes thrombin generation through activation of coagulation factor XII

Essentials How the Alzheimer's disease (AD) peptide β-amyloid (Aβ) disrupts neuronal function in the disease is unclear. Factor (F) XII initiates blood clotting via FXI, and thrombosis has been implicated in AD. Aβ triggers FXII-dependent FXI and thrombin activation, evidence of which is seen in AD plasma. Aβ-triggered clotting could contribute to neuronal dysfunction in AD and be a novel therapeutic target.

SUMMARY

Background β-Amyloid (Aβ) is a key pathologic element in Alzheimer's disease (AD), but the mechanisms by which it disrupts neuronal function in vivo are not completely understood. AD is characterized by a prothrombotic state, which could contribute to neuronal dysfunction by affecting cerebral blood flow and inducing inflammation. The plasma protein factor XII triggers clot formation via the intrinsic coagulation cascade, and has been implicated in thrombosis. Objectives To investigate the potential for Aβ to contribute to a prothrombotic state. Methods and results We show that Aβ activates FXII, resulting in FXI activation and thrombin generation in human plasma, thereby establishing Aβ as a possible driver of prothrombotic states. We provide evidence for this process in AD by demonstrating decreased levels of FXI and its inhibitor C1 esterase inhibitor in AD patient plasma, suggesting chronic activation, inhibition and clearance of FXI in AD. Activation of the intrinsic coagulation pathway in AD is further supported by elevated fibrin levels in AD patient plasma. Conclusions The ability of Aβ to promote coagulation via the FXII-driven contact system identifies new mechanisms by which it could contribute to neuronal dysfunction and suggests potential new therapeutic targets in AD.

Platelet-targeting thiol reduction sensor detects thiol isomerase activity on activated platelets in mouse and human blood under flow

Essentials Protein disulfide isomerases may have an essential role in thrombus formation. A platelet-binding sensor (PDI-sAb) was developed to detect thiol reductase activity under flow. Primary human platelet adhesion to collagen at 200 s(-1) was correlated with the PDI-sAb signal. Detected thiol reductase activity was localized in the core of growing thrombi at the site of injury in mice.

SUMMARY

Background Protein disulfide isomerases (PDIs) may regulate thrombus formation in vivo, although the sources and targets of PDIs are not fully understood. Methods and results Using click chemistry to link anti-CD61 and a C-terminal azido disulfide-linked peptide construct with a quenched reporter, we developed a fluorogenic platelet-targeting antibody (PDI-sAb) for thiol reductase activity detection in whole blood under flow conditions. PDI-sAb was highly responsive to various exogenous reducing agents (dithiothreitol, glutathione and recombinant PDI) and detected thiol reductase activity on P-selectin/phosphatidylserine-positive platelets activated with convulxin/PAR1 agonist peptide, a signal partially blocked by PDI inhibitors and antibody. In a microfluidic thrombosis model using 4 μg mL(-1) corn trypsin inhibitor-treated human blood perfused over collagen (wall shear rate = 100 s(-1) ), the PDI-sAb signal increased mostly over the first 200 s, whereas platelets continually accumulated for over 500 s, indicating that primary adhesion to collagen, but not secondary aggregation, was correlated with the PDI-sAb signal. Rutin and the PDI blocking antibody RL90 reduced platelet adhesion and the PDI-sAb signal only when thrombin production was inhibited with PPACK, suggesting limited effects of platelet thiol isomerase activity on platelet aggregation on collagen in the presence of thrombin. With anti-mouse CD41 PDI-sAb used in an arteriolar laser injury model, thiol reductase activity was localized in the core of growing thrombi where platelets displayed P-selectin and were in close proximity to disrupted endothelium. Conclusion PDI-sAb is a sensitive and real-time reporter of platelet- and vascular-derived disulfide reduction that targets clots as they form under flow to reveal spatial gradients.

Bioengineering the Endocrine Pancreas: Intraomental Islet Transplantation Within a Biologic Resorbable Scaffold

Transplantation of pancreatic islets is a therapeutic option to preserve or restore β-cell function. Our study was aimed at developing a clinically applicable protocol for extrahepatic transplantation of pancreatic islets. The potency of islets implanted onto the omentum, using an in situ-generated adherent, resorbable plasma-thrombin biologic scaffold, was evaluated in diabetic rat and nonhuman primate (NHP) models. Intraomental islet engraftment in the biologic scaffold was confirmed by achievement of improved metabolic function and preservation of islet cytoarchitecture, with reconstitution of rich intrainsular vascular networks in both species. Long-term nonfasting normoglycemia and adequate glucose clearance (tolerance tests) were achieved in both intrahepatic and intraomental sites in rats. Intraomental graft recipients displayed lower levels of serum biomarkers of islet distress (e.g., acute serum insulin) and inflammation (e.g., leptin and α2-macroglobulin). Importantly, low-purity (30:70% endocrine:exocrine) syngeneic rat islet preparations displayed function equivalent to that of pure (>95% endocrine) preparations after intraomental biologic scaffold implantation. Moreover, the biologic scaffold sustained allogeneic islet engraftment in immunosuppressed recipients. Collectively, our feasibility/efficacy data, along with the simplicity of the procedure and the safety of the biologic scaffold components, represented sufficient preclinical testing to proceed to a pilot phase I/II clinical trial.

Novel Anthranilamide-Based FXa Inhibitors: Drug Design, Synthesis and Biological Evaluation

Factor Xa (FXa) plays a significant role in the blood coagulation cascade and it has become a promising target for anticoagulation drugs. Three oral direct FXa inhibitors have been approved by the FDA for treating thrombotic diseases. By structure-activity relationship (SAR) analysis upon these FXa inhibitors, a series of novel anthranilamide-based FXa inhibitors were designed and synthesized. According to our study, compounds 1a, 1g and 1s displayed evident FXa inhibitory activity and excellent selectivity over thrombin in in vitro inhibition activities studies. Compounds 1g and 1s also exhibited pronounced anticoagulant activities in in vitro anticoagulant activity studies.

Identification of a thrombomodulin interaction site on thrombin-activatable fibrinolysis inhibitor that mediates accelerated activation by thrombin

BACKGROUND

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a human plasma zymogen that provides a molecular connection between coagulation and fibrinolysis. TAFI is activated through proteolytic cleavage by thrombin, thrombin in complex with the endothelial cell cofactor thrombomodulin (TM) or plasmin. Evidence from several studies suggests that TM and TAFI make direct contact at sites remote from the activating cleavage site to facilitate acceleration of thrombin-mediated TAFI activation. The elements of TAFI structure that allow accelerated activation of thrombin by TM are incompletely defined.

OBJECTIVES

To identify TM interaction regions on TAFI that mediate acceleration of activation by thrombin and therefore indicate TM binding sites on TAFI.

METHODS

We mutated selected surface-exposed charged residues on TAFI to alanine in order to identify sites that mediate acceleration of activation by TM. The kinetics of activation of the mutants by thrombin in the presence or absence of TM, as well as their thermal stabilities and antifibrinolytic potentials, were determined.

RESULTS

TAFI variants R15A, E28A, K59A, D75A/E77A/D78A, E99A and E106A all exhibited moderately reduced catalytic efficiencies of activation by thrombin-TM. TAFI variants R377A and, particularly, R12A and R12A/R15A exhibited severely reduced activation by thrombin-TM that was not explained by differences in activation by thrombin alone.

CONCLUSIONS

We have identified R12 as a critical residue for the activation of TAFI by thrombin-TM, extending a previous report that identified a role for this residue. R12 is likely to directly bind to TM while another key residue, R377, may affect the thrombin-TAFI interaction specifically in the presence of TM.

The influence of corticosteroids on hemostasis in healthy subjects

BACKGROUND

Corticosteroids have been associated with an increased risk of venous thromboembolism in patients treated for inflammatory diseases. It is unclear whether the thrombotic risk is induced by the inflammation of the underlying inflammatory diseases or whether corticosteroids are prothrombotic as well. Considering the widespread use of corticosteroids in clinical practise, it is critical to know whether corticosteroids enhance coagulation.

OBJECTIVE

To investigate whether a 10-day prednisolone burst therapy activates hemostasis in healthy individuals.

METHODS

Healthy subjects received either 0.5 mg kg(-1) day(-1) of oral prednisolone or placebo. Venous blood was collected at baseline, day 1 and day 10 and tested for thrombin-antithrombin complexes (TATc), D-dimer, plasmin-alpha2-antiplasmin complexes (PAPc), plasminogen-activator inhibitor type-1 (PAI-1), von Willebrand factor (VWF) and thrombin generation (peak thrombin, velocity index and endogenous thrombin potential [ETP]).

RESULTS

Fifteen subjects received prednisolone and 16 placebo (median age 29 vs. 22 years, female subjects 33% vs. 56%, respectively). Peak thrombin and velocity index were higher in the placebo group at baseline. After 10 days of treatment, peak thrombin, velocity index, PAI-1 and VWF increased in the oral prednisolone group as compared with the placebo group (15.8 [SD 16.3] vs. -0.1 [SD 16.1], 41.2 [SD 41.3] vs. -2.3 [SD 42.7], 18.0 [IQR 8.0-37.0] vs. 0.5 [IQR -18.5-13.0], 4.0 [IQR -1.0-12.0] vs. 0.0 [IQR -2.5-1.5], respectively). No changes were observed for TATc, ETP, PAPc and D-dimer.

CONCLUSIONS

Oral prednisolone induces a procoagulant state in healthy subjects, suggesting that corticosteroid treatment may increase the thromboembolic risk in patients with inflammatory diseases.

Structure-function relationships in thrombin-activatable fibrinolysis inhibitor

Thrombin-activatable fibrinolysis inhibitor (TAFI) is an important regulator in the balance of coagulation and fibrinolysis. TAFI is a metallocarboxypeptidase that circulates in plasma as zymogen. Activated TAFI (TAFIa) cleaves C-terminal lysine or arginine residues from peptide substrates. The removal of C-terminal lysine residues from partially degraded fibrin leads to reduced plasmin formation and thus attenuation of fibrinolysis. TAFI also plays a role in inflammatory processes via the removal of C-terminal arginine or lysine residues from bradykinin, thrombin-cleaved osteopontin, C3a, C5a and chemerin. TAFI has been studied extensively over the past three decades and recent publications provide a wealth of information, including crystal structures, mutants and structural data obtained with antibodies and peptides. In this review, we combined and compared available data on structure/function relationships of TAFI.

Allosterism-based simultaneous, dual anticoagulant and antiplatelet action: allosteric inhibitor targeting the glycoprotein Ibα-binding and heparin-binding site of thrombin

BACKGROUND

Allosteric inhibition is a promising approach for developing a new group of anticoagulants with potentially reduced bleeding consequences. Recently, we designed sulfated β-O4 lignin (SbO4L) as an allosteric inhibitor that targets exosite 2 of thrombin to reduce fibrinogen cleavage through allostery and compete with glycoprotein Ibα to reduce platelet activation.

OBJECTIVE

To assess: (i) the antithrombotic potential of a novel approach of simultaneous exosite 2-dependent allosteric inhibition of thrombin and competitive inhibition of platelet activation; and (ii) the promise of SbO4L as the first-in-class antithrombotic agent.

METHODS

A combination of whole blood thromboelastography, hemostasis analysis, mouse arterial thrombosis models and mouse tail bleeding studies were used to assess antithrombotic potential.

RESULTS AND CONCLUSIONS

SbO4L extended the clot initiation time, and reduced maximal clot strength, platelet contractile force, and the clot elastic modulus, suggesting dual anticoagulant and antiplatelet effects. These effects were comparable to those observed with enoxaparin. A dose of 1 mg of SbO4L per mouse prevented occlusion in 100% of arteries, and lower doses resulted in a proportionally reduced response. Likewise, the time to occlusion increased by ~ 70% with a 0.5-mg dose in the mouse Rose Bengal thrombosis model. Finally, tail bleeding studies demonstrated that SbO4L does not increase bleeding propensity. In comparison, a 0.3-mg dose of enoxaparin increased the bleeding time and blood volume loss. Overall, this study highlights the promise of the allosteric inhibition approach, and presents SbO4L as a novel anticoagulant with potentially reduced bleeding side effects.

Reduced plasma fibrin clot permeability and susceptibility to lysis are associated with increased risk of postthrombotic syndrome

BACKGROUND

The postthrombotic syndrome (PTS) is a severe complication of deep vein thrombosis (DVT). Reduced plasma clot permeability and lysability have been linked to DVT and residual vein obstruction. OBJECTIVES We investigated whether altered fibrin clot properties are associated with the occurrence of PTS.

PATIENTS AND METHODS

Plasma fibrin clot permeability (Ks ) and lysability were investigated in a cohort of 197 consecutive patients aged 18 to 65 years recruited 3 months following the first-ever DVT. Patients with severe thrombophilia or comorbidities known to adversely affect clot phenotype were ineligible.

RESULTS

During a 1-year follow-up PTS developed in 48 (24%) patients, who were characterized by lower Ks , prolonged fibrin clot lysis time (CLT) and slower release of D-dimer from clots (D-Drate ), together with higher plasma D-dimer, C-reactive protein and thrombin-activatable fibrinolysis inhibitor (TAFI). No PTS-associated differences in fibrinogen, thrombin generation, factor VIII, other fibrinolysis proteins and the quality of anticoagulation were observed. Ks (r = -0.71), CLT (r = 0.45), D-Drate (r = -0.30) and TAFI activity (r = 0.38) were associated with the Villalta scale (all P < 0.05). Recurrent VTE occurred also more commonly in PTS patients during follow-up and the 26 (13.2%) patients had lower Ks , longer CLT and lower D-Drate (all P < 0.05). A multivariate model adjusted for age, body mass index, fibrinogen and glucose showed that independent predictors of PTS were idiopathic DVT, plasma D-dimer, Ks , D-Drate , tissue plasminogen activator and TAFI activity.

CONCLUSIONS

This study demonstrates that formation of more compact fibrin clots displaying impaired susceptibility to lysis predisposes to PTS.

Activated factor VII-antithrombin complex predicts mortality in patients with stable coronary artery disease: a cohort study

BACKGROUND

Plasma concentration of activated factor VII (FVIIa)-antithrombin (AT) complex has been proposed as an indicator of intravascular exposure of tissue factor.

OBJECTIVES

The aims of this observational study were to evaluate (i) FVIIa-AT plasma concentration in subjects with or without coronary artery disease (CAD) and (ii) its association with mortality in a prospective cohort of patients with CAD.

METHODS

FVIIa-AT levels were measured by elisa in 686 subjects with (n = 546) or without (n = 140) angiographically proven CAD. Subjects with acute coronary syndromes and those taking anticoagulant drugs at the time of enrollment were excluded. CAD patients were followed for total and cardiovascular mortality.

RESULTS

There was no difference in FVIIa-AT levels between CAD (84.8 with 95% confidence interval [CI] 80.6-88.2 pmol L(-1) ) and CAD-free subjects (83.9 with 95% CI 76.7-92.8 pmol L(-1) ). Within the CAD population, during a 64-month median follow-up, patients with FVIIa-AT levels higher than the median value at baseline (≥ 79 pmol L(-1) ) had a two-fold greater risk of both total and cardiovascular mortality. Results were confirmed after adjustment for sex, age, the other predictors of mortality (hazard ratio for total mortality: 2.05 with 95% CI 1.22-3.45, hazard ratio for cardiovascular mortality 1.94 with 95% CI 1.01-3.73, with a slight improvement of C-statistic over traditional risk factors), FVIIa levels, drug therapy at discharge, and even patients using all the usual medications for CAD treatment. High FVIIa-AT levels also correlated with increased thrombin generation.

CONCLUSIONS

This preliminary study suggests that plasma concentration of FVIIa-AT is a thrombophilic marker of total and cardiovascular mortality risk in patients with clinically stable CAD.

Protein Recognition by Phase Transition of Aptamer-Linked Polythiophene Single Nanowire

A novel protein recognition platform is developed using aptamer-linked polythiophene nanowires. As the aptamer functionalized poly (3-methylthiophene) nanowire is treated by the specific protein, resonance Raman and photoluminescence signals are simultaneously enhanced. Statistical analyses deliver the capability of a single conjugated polymer nanowire with phase-transition characteristics in response to selectivity and concentration.

Thrombin-Responsive Gated Silica Mesoporous Nanoparticles As Coagulation Regulators

The possibility of achieving sophisticated actions in complex biological environments using gated nanoparticles is an exciting prospect with much potential. We herein describe new gated mesoporous silica nanoparticles (MSN) loaded with an anticoagulant drug and capped with a peptide containing a thrombin-specific cleavage site. When the coagulation cascade was triggered, active thrombin degraded the capping peptidic sequence and induced the release of anticoagulant drugs to delay the clotting process. The thrombin-dependent response was assessed and a significant increase in coagulation time in plasma from 2.6 min to 5 min was found. This work broadens the application of gated silica nanoparticles and demonstrates their ability to act as controllers in a complex scenario such as hemostasis.

Anticoagulant and antithrombotic activities of modified xylofucan sulfate from the brown alga Punctaria plantaginea

Selectively and totally sulfated (1 → 3)-linked linear homofucans bearing ∼ 20 monosaccharide residues on average have been prepared from the branched xylofucan sulfate isolated from the brown alga Punctaria plantaginea. Anticoagulant and antithrombotic properties of the parent biopolymer and its derivatives were assessed in vitro. Highly sulfated linear fucan derivatives were shown to inhibit clot formation in APTT assay and ristocetin induced platelets aggregation, while the partially sulfated analogs were inactive. In the experiments with purified proteins, fucan derivatives with degree of sulfation of ∼ 2.0 were found to enhance thrombin and factor Xa inhibition by antithrombin III. The effect of sulfated fucans on thrombin inhibition, which was similar to those of heparinoid Clexane(®) (enoxaparin) and of a fucoidan from the brown alga Saccharina latissima studied previously, can be explained by the multicenter interaction and formation of a ternary complex thrombin-antithrombin III-polysaccharide. The possibility of such complexation was confirmed by computer docking study.

Discovery and characterization of an antibody directed against exosite I of thrombin

ESSENTIALS: An IgA paraprotein with anti-thrombin activity was not associated with a severe bleeding phenotype. This observation challenges the paradigm that anticoagulant therapy necessarily increases bleeding risk. Characterization of the antibody showed that it specifically binds to thrombin exosite I. A therapeutic drug with the properties of this antibody might be an antithrombotic that doesn't cause bleeding.

BACKGROUND

We report the case of a 54-year-old female who presented with a traumatic subdural hemorrhage. Coagulation tests were markedly prolonged due to the presence of an anti-thrombin IgA paraprotein at 3 g L(-1) . The patient made a complete recovery and has had no abnormal bleeding during a 7-year follow-up, despite the persistence of the paraprotein.

OBJECTIVES

To determine how the paraprotein prolonged clotting tests by defining its target and its epitope.

METHODS

The paraprotein was purified and added to normal pooled plasma for in vitro clotting assays. Binding studies were conducted to determine the affinity of the IgA for thrombin. The Fab was isolated and crystallized with thrombin.

RESULTS

The purified IgA was sufficient to confer the patient's in vitro coagulation profile in normal pooled plasma, and was found to bind specifically and with high affinity to thrombin. A crystal structure of the Fab fragment in complex with thrombin revealed an exosite I interaction involving CDRH3 of the antibody.

CONCLUSIONS

Although the patient originally presented with a subdural bleed, the hematoma resolved without intervention, and no other bleeding event occurred during the subsequent 7 years. During this period, the patient's IgA paraprotein levels have remained constant at 3 g L(-1) , suggesting that the presence of a high-affinity, exosite I-directed antibody is consistent with normal hemostasis. A therapeutic derivative of this antibody might therefore permit antithrombotic dose escalation without an associated increase in the risk of bleeding.

Study of the Differential Activity of Thrombin Inhibitors Using Docking, QSAR, Molecular Dynamics, and MM-GBSA

Non-peptidic thrombin inhibitors (TIs; 177 compounds) with diverse groups at motifs P1 (such as oxyguanidine, amidinohydrazone, amidine, amidinopiperidine), P2 (such as cyanofluorophenylacetamide, 2-(2-chloro-6-fluorophenyl)acetamide), and P3 (such as phenylethyl, arylsulfonate groups) were studied using molecular modeling to analyze their interactions with S1, S2, and S3 subsites of the thrombin binding site. Firstly, a protocol combining docking and three dimensional quantitative structure-activity relationship was performed. We described the orientations and preferred active conformations of the studied inhibitors, and derived a predictive CoMSIA model including steric, donor hydrogen bond, and acceptor hydrogen bond fields. Secondly, the dynamic behaviors of some selected TIs (compounds 26, 133, 147, 149, 162, and 177 in this manuscript) that contain different molecular features and different activities were analyzed by creating the solvated models and using molecular dynamics (MD) simulations. We used the conformational structures derived from MD to accomplish binding free energetic calculations using MM-GBSA. With this analysis, we theorized about the effect of van der Waals contacts, electrostatic interactions and solvation in the potency of TIs. In general, the contents reported in this article help to understand the physical and chemical characteristics of thrombin-inhibitor complexes.