Rational design of a potent anticoagulant thrombin

Cytoprotective E-WE thrombin reduces disease severity in a murine model of relapsing-remitting multiple sclerosis

The blood-brain barrier is composed of microvascular endothelial cells, immune cells, and astrocytes that work in concert with the coagulation cascade to control inflammation and immune cell infiltration into the central nervous system. Endothelial cell dysfunction leading to increased permeability and compromised barrier function are hallmarks of neuroinflammatory and autoimmune disorders, including multiple sclerosis (MS). Therapeutic strategies that improve or protect endothelial barrier function may be beneficial in the treatment or prevention of neuroinflammatory diseases. We therefore tested the hypothesis that biasing thrombin toward anticoagulant and cytoprotective activities would provide equivalent or even additive benefit compared with standard-of-care therapeutic strategies, including corticosteroids. In a mouse model of relapsing-remitting MS, treatment with the thrombin mutant, E-WE thrombin, an engineered thrombin mutant with cytoprotective activities that is biased toward anticoagulant and cytoprotective activity, reduced neuroinflammation and extracellular fibrin formation in SJL mice inoculated with proteolipid protein (PLP) peptide. When administered at the onset of detectable disease, E-WE thrombin significantly improved the disease severity of the initial attack as well as the relapse and delayed the onset of relapse to a similar extent as observed with methylprednisolone. Both methylprednisolone and E-WE thrombin reduced demyelination and immune cell recruitment. These results provide rationale for considering engineered forms of thrombin biased toward anticoagulant and cytoprotective activity as a therapeutic strategy and perhaps an effective alternative to high-dose methylprednisolone for the management of acute relapsing MS attacks.NEW & NOTEWORTHY There are limited treatment options for mitigating acute relapsing attacks for patients with multiple sclerosis. We tested the hypothesis that harnessing the cytoprotective activity of the blood coagulation enzyme, thrombin, would provide benefit and protection against relapsing disease in a mouse model of MS. Our results provide rationale for considering engineered forms of thrombin biased toward cytoprotective activity as a therapeutic strategy and perhaps an alternative to steroids for the management of relapsing MS attacks.

Dynamic allostery in thrombin-a review

Thrombin is a serine protease that catalyzes a large number of different reactions including proteolytic cleave of fibrinogen to make the fibrin clot (procoagulant activity), of the protease activated receptors (for cell signaling) and of protein C generating activated protein C (anticoagulant activity). Thrombin has an effector binding site called the anion binding exosite 1 that is allosterically coupled to the active site. In this review, we survey results from thermodynamic characterization of the allosteric coupling as well as hydrogen-deuterium exchange mass spectrometry to reveal which parts of the thrombin structure are changed upon effector binding and/or mutagenesis, and finally NMR spectroscopy to characterize the different timescales of motions elicited by the effectors. We also relate the experimental work to computational network analysis of the thrombin-thrombomodulin complex.

E-WE thrombin, a protein C activator, reduces disease severity and spinal cord inflammation in relapsing-remitting murine experimental autoimmune encephalomyelitis

Objective

Relapses in patients with relapsing-remitting multiple sclerosis (RRMS) are typically treated with high-dose corticosteroids including methylprednisolone. However, high-dose corticosteroids are associated with significant adverse effects, can increase the risk for other morbidities, and often do not impact disease course. Multiple mechanisms are proposed to contribute to acute relapses in RRMS patients, including neuroinflammation, fibrin formation and compromised blood vessel barrier function. The protein C activator, E-WE thrombin is a recombinant therapeutic in clinical development for its antithrombotic and cytoprotective properties, including protection of endothelial cell barrier function. In mice, treatment with E-WE thrombin reduced neuroinflammation and extracellular fibrin formation in myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE). We therefore tested the hypothesis that E-WE thrombin could reduce disease severity in a relapsing-remitting model of EAE.

Methods

Female SJL mice were inoculated with proteolipid protein (PLP) peptide and treated with E-WE thrombin (25 μg/kg; iv) or vehicle at onset of detectable disease. In other experiments, E-WE thrombin was compared to methylprednisolone (100 mg/kg; iv) or the combination of both.

Results

Compared to vehicle, administration of E-WE thrombin significantly improved disease severity of the initial attack and relapse and delayed onset of relapse as effectively as methylprednisolone. Both methylprednisolone and E-WE thrombin reduced demyelination and immune cell recruitment, and the combination of both treatments had an additive effect.

Conclusion

The data presented herein demonstrate that E-WE thrombin is protective in mice with relapsing-remitting EAE, a widely used model of MS. Our data indicate that E-WE thrombin is as effective as high-dose methylprednisolone in improving disease score and may exert additional benefit when administered in combination. Taken together, these data suggest that E-WE thrombin may be an effective alternative to high-dose methylprednisolone for managing acute MS attacks.

Structural insights into the activation of blood coagulation factor XI zymogen by thrombin: A computational molecular dynamics study

Activation of human blood coagulation factor XI zymogen to factor XIa plays a significant role in the upstream coagulation pathway, in which factor XIa activates factor IX zymogen. The mechanistic details of the proteolytic activation of factor XI by the activating enzyme thrombin are not well-understood at atomic level. In this study, we employed a combination of molecular docking and microsecond time-scale molecular dynamics simulations to identify the key regions of interaction between fXI and thrombin. The activating complex between the substrate and enzyme was modeled to represent the initial acylation step of the serine-protease hydrolysis mechanism. The proposed solution structural complex, fIX:fIIa, obtained from 3 microseconds of MD refinement, suggests that the activation of factor XI is mediated by thrombin's anion binding exosite-II interactions with A3 and A4 domains. We predict that the two positively charged arginine residues (Arg409 and Arg413) in the exosite-2 region, the β- and γ-insertion loops of thrombin play an important structural role in the initial activating complex between fXI and thrombin.

How Thrombomodulin Enables W215A/E217A Thrombin to Cleave Protein C but Not Fibrinogen

The W215A/E217A mutant thrombin is called "anticoagulant thrombin" because its activity toward its procoagulant substrate, fibrinogen, is reduced more than 500-fold whereas in the presence of thrombomodulin (TM) its activity toward its anticoagulant substrate, protein C, is reduced less than 10-fold. To understand how these mutations so dramatically alter one activity over the other, we compared the backbone dynamics of wild type thrombin to those of the W215A/E217A mutant thrombin by hydrogen-deuterium exchange coupled to mass spectrometry (HDX-MS). Our results show that the mutations cause the 170s, 180s, and 220s C-terminal β-barrel loops near the sites of mutation to exchange more, suggesting that the structure of this region is disrupted. Far from the mutation sites, residues at the N-terminus of the heavy chain, which need to be buried in the Ile pocket for correct structuring of the catalytic triad, also exchange much more than in wild type thrombin. TM binding causes reduced H/D exchange in these regions and also alters the dynamics of the β-strand that links the TM binding site to the catalytic Asp 102 in both wild type thrombin and in the W215A/E217A mutant thrombin. In contrast, whereas TM binding reduces the dynamics the 170, 180 and 220 s C-terminal β-barrel loops in WT thrombin, this region remains disordered in the W215A/E217A mutant thrombin. Thus, TM partially restores the catalytic activity of W215A/E217A mutant thrombin by allosterically altering its dynamics in a manner similar to that of wild type thrombin.

Thrombin generation and activity in multiple sclerosis

The coagulation cascade and immune system are intricately linked, highly regulated and respond cooperatively in response to injury and infection. Increasingly, evidence of hyper-coagulation has been associated with autoimmune disorders, including multiple sclerosis (MS). The pathophysiology of MS includes immune cell activation and recruitment to the central nervous system (CNS) where they degrade myelin sheaths, leaving neuronal axons exposed to damaging inflammatory mediators. Breakdown of the blood-brain barrier (BBB) facilitates the entry of peripheral immune cells. Evidence of thrombin activity has been identified within the CNS of MS patients and studies using animal models of experimental autoimmune encephalomyelitis (EAE), suggest increased thrombin generation and activity may play a role in the pathogenesis of MS as well as inhibit remyelination processes. Thrombin is a serine protease capable of cleaving multiple substrates, including protease activated receptors (PARs), fibrinogen, and protein C. Cleavage of all three of these substrates represent pathways through which thrombin activity may exert immuno-regulatory effects and regulate permeability of the BBB during MS and EAE. In this review, we summarize evidence that thrombin activity directly, through PARs, and indirectly, through fibrin formation and activation of protein C influences neuro-immune responses associated with MS and EAE pathology.

Host fibrinogen drives antimicrobial function in Staphylococcus aureus peritonitis through bacterial-mediated prothrombin activation

The blood-clotting protein fibrinogen has been implicated in host defense following Staphylococcus aureus infection, but precise mechanisms of host protection and pathogen clearance remain undefined. Peritonitis caused by staphylococci species is a complication for patients with cirrhosis, indwelling catheters, or undergoing peritoneal dialysis. Here, we sought to characterize possible mechanisms of fibrin(ogen)-mediated antimicrobial responses. Wild-type (WT) (Fib+) mice rapidly cleared S. aureus following intraperitoneal infection with elimination of ∼99% of an initial inoculum within 15 min. In contrast, fibrinogen-deficient (Fib-) mice failed to clear the microbe. The genotype-dependent disparity in early clearance resulted in a significant difference in host mortality whereby Fib+ mice uniformly survived whereas Fib- mice exhibited high mortality rates within 24 h. Fibrin(ogen)-mediated bacterial clearance was dependent on (pro)thrombin procoagulant function, supporting a suspected role for fibrin polymerization in this mechanism. Unexpectedly, the primary host initiator of coagulation, tissue factor, was found to be dispensable for this antimicrobial activity. Rather, the bacteria-derived prothrombin activator vWbp was identified as the source of the thrombin-generating potential underlying fibrin(ogen)-dependent bacterial clearance. Mice failed to eliminate S. aureus deficient in vWbp, but clearance of these same microbes in WT mice was restored if active thrombin was administered to the peritoneal cavity. These studies establish that the thrombin/fibrinogen axis is fundamental to host antimicrobial defense, offer a possible explanation for the clinical observation that coagulase-negative staphylococci are a highly prominent infectious agent in peritonitis, and suggest caution against anticoagulants in individuals susceptible to peritoneal infections.

The protein C activator AB002 rapidly interrupts thrombus development in baboons

Although thrombin is a key enzyme in the coagulation cascade and is required for both normal hemostasis and pathologic thrombogenesis, it also participates in its own negative feedback via activation of protein C, which downregulates thrombin generation by enzymatically inactivating factors Va and VIIIa. Our group and others have previously shown that thrombin's procoagulant and anticoagulant activities can be effectively disassociated to varying extents through site-directed mutagenesis. The thrombin mutant W215A/E217A (WE thrombin) has been one of the best characterized constructs with selective activity toward protein C. Although animal studies have demonstrated that WE thrombin acts as an anticoagulant through activated protein C (APC) generation, the observed limited systemic anticoagulation does not fully explain the antithrombotic potency of this or other thrombin mutants. AB002 (E-WE thrombin) is an investigational protein C activator thrombin analog in phase 2 clinical development (clinicaltrials.gov NCT03963895). Here, we demonstrate that this molecule is a potent enzyme that is able to rapidly interrupt arterial-type thrombus propagation at exceedingly low doses (<2 µg/kg, IV), yet without substantial systemic anticoagulation in baboons. We demonstrate that AB002 produces APC on platelet aggregates and competitively inhibits thrombin-activatable fibrinolysis inhibitor (carboxypeptidase B2) activation in vitro, which may contribute to the observed in vivo efficacy. We also describe its safety and activity in a phase 1 first-in-human clinical trial. Together, these results support further clinical evaluation of AB002 as a potentially safe and effective new approach for treating or preventing acute thrombotic and thromboembolic conditions. This trial was registered at www.clinicaltrials.gov as #NCT03453060.

A contradictory action of procoagulant ficin by a fibrinolytic serine protease from Egyptian Ficus carica latex

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We purified a serine protease from Ficus carica latex.

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Serine protease had a high tendency to hydrolyze fibrinogin.

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Serine protease inhibited blood coagulation.

Ficus carica is one of the most popular and edible plants. Its trees emanate latex of high medical importance. The well-studied procoagulant effect of ficin is a hallmark of this latex which protrudes an interesting question of how can the plant control this effect? In the present work, we purified and characterized a serine protease (FPIII) with fibrinolytic activity from F. carica latex and study the anticoagulant character of the latex. FPIII was inhibited by PMSF and its molecular weight was 48 kDa. The optimum pH and temperature of FPIII were detected at 8.5 and 60 °C, respectively. The activation energy of FPIII was 7 kcal/mol and was thermal stable up to 60 °C. FPIII tended to hydrolyze different protein substrates and showed a good catalytic efficiency (K_cat/K_m). The anticoagulant effects and fibrinogenolytic activities of latex crude extract and FPIII were detected, which controls the procoagulant effect of ficin.

Residues W215, E217 and E192 control the allosteric E*-E equilibrium of thrombin

A pre-existing, allosteric equilibrium between closed (E*) and open (E) conformations of the active site influences the level of activity in the trypsin fold and defines ligand binding according to the mechanism of conformational selection. Using the clotting protease thrombin as a model system, we investigate the molecular determinants of the E*-E equilibrium through rapid kinetics and X-ray structural biology. The equilibrium is controlled by three residues positioned around the active site. W215 on the 215-217 segment defining the west wall of the active site controls the rate of transition from E to E* through hydrophobic interaction with F227. E192 on the opposite 190-193 segment defining the east wall of the active site controls the rate of transition from E* to E through electrostatic repulsion of E217. The side chain of E217 acts as a lever that moves the entire 215-217 segment in the E*-E equilibrium. Removal of this side chain converts binding to the active site to a simple lock-and-key mechanism and freezes the conformation in a state intermediate between E* and E. These findings reveal a simple framework to understand the molecular basis of a key allosteric property of the trypsin fold.

Neuroprotection and vasculoprotection using genetically targeted protease-ligands

Thrombin and activated protein C (APC) are known coagulation factors that exhibit profound effects in brain by acting on the protease activated receptor (PAR). The wild type (WT) proteases appear to impact cell survival powerfully, and therapeutic forms of APC are under development. Engineered recombinant thrombin or APC were designed to separate their procoagulant or anticoagulant effects from their cytoprotective properties. We measured vascular disruption and neuronal degeneration after a standard rodent filament stroke model. For comparison to a robust anticoagulant, we used a GpIIb/IIIa inhibitor, GR144053. During 2 h MCAo both WT murine APC and its mutant, 5A-APC, significantly decreased neuronal death 30 min after reperfusion. During 4 h MCAo, only 5A-APC significantly protected neurons but both WT-APC and 5A-APC exacerbated vascular disruption during 4 h MCAo. Human APC mutants appeared to reduce 24 h neuronal injury significantly when given after 2 h delay after MCAo. In contrast, 24 h vascular damage was worsened by high doses of WT and mutant APCs, although only statistically significantly for high dose 3K3A-APC. Mutated thrombin worsened vascular damage significantly without affecting neuron damage. GR144053 failed to ameliorate vascular disruption or neuronal injury despite significant anticoagulation. Differential effects on neurons and the vasculature were demonstrated using wild-type and mutated proteases. The mutants murine 3K3A-APC and 5A-APC protected neurons in this rodent model but in high doses worsened vascular leakage. Cytoactive effects of plasma proteases may be separated from their coagulation effects. Further studies should explore impact of dose and timing on cytoactive and vasculoactive properties of these drugs.

Enhancing the anticoagulant profile of meizothrombin

Meizothrombin is an active intermediate generated during the proteolytic activation of prothrombin to thrombin in the penultimate step of the coagulation cascade. Structurally, meizothrombin differs from thrombin because it retains the auxiliary Gla domain and two kringles. Functionally, meizothrombin shares with thrombin the ability to cleave procoagulant (fibrinogen), prothrombotic (PAR1) and anticoagulant (protein C) substrates, although its specificity toward fibrinogen and PAR1 is less pronounced. In this study we report information on the structural architecture of meizothrombin resolved by SAXS and single molecule FRET as an elongated arrangement of its individual domains. In addition, we show the properties of a meizothrombin construct analogous to the anticoagulant thrombin mutant W215A/E217A currently in Phase I for the treatment of thrombotic complications and stroke. The findings reveal new structural and functional aspects of meizothrombin that advance our understanding of a key intermediate of the prothrombin activation pathway.

Interplay between conformational selection and zymogen activation

Trypsin-like proteases are synthesized as zymogens and activated through a mechanism that folds the active site for efficient binding and catalysis. Ligand binding to the active site is therefore a valuable source of information on the changes that accompany zymogen activation. Using the physiologically relevant transition of the clotting zymogen prothrombin to the mature protease thrombin, we show that the mechanism of ligand recognition follows selection within a pre-existing ensemble of conformations with the active site accessible (E) or inaccessible (E*) to binding. Prothrombin exists mainly in the E* conformational ensemble and conversion to thrombin produces two dominant changes: a progressive shift toward the E conformational ensemble triggered by removal of the auxiliary domains upon cleavage at R271 and a drastic drop of the rate of ligand dissociation from the active site triggered by cleavage at R320. Together, these effects produce a significant (700-fold) increase in binding affinity. Limited proteolysis reveals how the E*-E equilibrium shifts during prothrombin activation and influences exposure of the sites of cleavage at R271 and R320. These new findings on the molecular underpinnings of prothrombin activation are relevant to other zymogens with modular assembly involved in blood coagulation, complement and fibrinolysis.

Screening cleavage of Factor XIII V34X Activation Peptides by thrombin mutants: A strategy for controlling fibrin architecture

In blood coagulation, thrombin converts fibrinogen into fibrin monomers that polymerize into a clot network. Thrombin also activates Factor XIII by cleaving the R37-G38 peptide bond of the Activation Peptide (AP) segment. The resultant transglutaminase introduces covalent crosslinks into the fibrin clot. A strategy to modify clot architecture would be to design FXIII AP sequences that are easier or more difficult to be thrombin-cleaved thus controlling initiation of crosslinking. To aid in this design process, FXIII V34X (28-41) Activation Peptides were kinetically ranked for cleavage by wild-type thrombin and several anticoagulant mutants. Thrombin-catalyzed hydrolysis of aromatic FXIII F34, W34, and Y34 APs was compared with V34 and L34. Cardioprotective FXIII L34 remained the variant most readily cleaved by wild-type thrombin. The potent anticoagulant thrombins W215A and W215A/E217A (missing a key substrate platform for binding fibrinogen) were best able to hydrolyze FXIII F34 and W34 APs. Thrombin I174A and L99A could effectively accommodate FXIII W34 and Y34 APs yielding kinetic parameters comparable to FXIII AP L34 with wild-type thrombin. None of the aromatic FXIII V34X APs could be hydrolyzed by thrombin Y60aA. FXIII F34 and W34 are promising candidates for FXIII - anticoagulant thrombin systems that could permit FXIII-catalyzed crosslinking in the presence of reduced fibrin formation. By contrast, FXIII Y34 with thrombin (Y60aA or W215A/E217A) could help assure that both fibrin clot formation and protein crosslinking are hindered. Regulating the activation of FXIII is predicted to be a strategy for helping to control fibrin clot architecture and its neighboring environments.

Rational Design of Protein C Activators

In addition to its procoagulant and proinflammatory functions mediated by cleavage of fibrinogen and PAR1, the trypsin-like protease thrombin activates the anticoagulant protein C in a reaction that requires the cofactor thrombomodulin and the endothelial protein C receptor. Once in the circulation, activated protein C functions as an anticoagulant, anti-inflammatory and regenerative factor. Hence, availability of a protein C activator would afford a therapeutic for patients suffering from thrombotic disorders and a diagnostic tool for monitoring the level of protein C in plasma. Here, we present a fusion protein where thrombin and the EGF456 domain of thrombomodulin are connected through a peptide linker. The fusion protein recapitulates the functional and structural properties of the thrombin-thrombomodulin complex, prolongs the clotting time by generating pharmacological quantities of activated protein C and effectively diagnoses protein C deficiency in human plasma. Notably, these functions do not require exogenous thrombomodulin, unlike other anticoagulant thrombin derivatives engineered to date. These features make the fusion protein an innovative step toward the development of protein C activators of clinical and diagnostic relevance.

Thrombin Cleavage of Plasmodium falciparum Erythrocyte Membrane Protein 1 Inhibits Cytoadherence

Plasmodium falciparum malaria remains one of the most deadly infections worldwide. The pathogenesis of the infection results from the sequestration of infected erythrocytes (IRBC) in vital organs, including the brain, with resulting impairment of blood flow, hypoxia, and lactic acidosis. Sequestration occurs through the adhesion of IRBC to host receptors on microvascular endothelium by Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1), a large family of variant surface antigens, each with up to seven extracellular domains that can bind to multiple host receptors. Consequently, antiadhesive therapies directed at single endothelial adhesion molecules may not be effective. In this study, we demonstrated that the serine protease thrombin, which is pivotal in the activation of the coagulation cascade, cleaved the major parasite adhesin on the surface of IRBC. As a result, adhesion under flow was dramatically reduced, and already adherent IRBC were detached. Thrombin cleavage sites were mapped to the Duffy binding-like δ1 (DBLδ1) domain and interdomains 1 and 2 in the PfEMP1 of the parasite line IT4var19. Furthermore, we observed an inverse correlation between the presence of thrombin and IRBC in cerebral malaria autopsies of children. We investigated a modified (R67A) thrombin and thrombin inhibitor, hirugen, both of which inhibit the binding of substrates to exosite I, thereby reducing its proinflammatory properties. Both approaches reduced the barrier dysfunction induced by thrombin without affecting its proteolytic activity on PfEMP1, raising the possibility that thrombin cleavage of variant PfEMP1 may be exploited as a broadly inhibitory antiadhesive therapy.

Plasmodium falciparum malaria is the third leading cause of mortality due to a pathogen, with 214 million people infected and 438,000 deaths annually. The adhesion of Plasmodium falciparum-infected erythrocytes (IRBC) to microvascular endothelium is a major pathological process in severe malaria. While the recent implementation of artemisinin-based antimalarial therapy for severe malaria improves patient survival by targeting all parasite stages, antiparasite drugs alone may not immediately reverse pathophysiological processes in occluded vessels. Here we show that thrombin, an enzyme intimately involved in the clotting process, cleaves the main parasite adhesin expressed on the surface of IRBC, thereby preventing and reversing the binding of IRBC to endothelial cells. This beneficial effect of thrombin can be achieved by modified thrombins that cause significantly less clotting and vessel leakage while preserving the ability to cleave the parasite protein. Our results provide the basis for using modified thrombins as adjunctive therapy in severe malaria.

Thrombin mutant W215A/E217A treatment improves neurological outcome and attenuates central nervous system damage in experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a neuroinflammatory disease characterized by demyelination and axonal damage of the central nervous system. The pathogenesis of MS has also been linked to vascular inflammation and local activation of the coagulation system, resulting in perivascular fibrin deposition. Treatment of experimental autoimmune encephalomyelitis (EAE), a model of human MS, with antithrombotic and antiinflammatory activated protein C (APC) reduces disease severity. Since recombinant APC (Drotecogin alfa), originally approved for the treatment of severe sepsis, is not available for human MS studies, we tested the hypothesis that pharmacologic activation of endogenous protein C could likewise improve the outcome of EAE. Mice were immunized with murine myelin oligodendrocyte glycoprotein (MOG) peptides and at the onset of EAE symptoms, were treated every other day with either WE thrombin (25 μg/kg; i.v.), a selective recombinant protein C activator thrombin analog, or saline control. Mice were monitored for changes in disease score until euthanized for ex vivo analysis of inflammation. Administration of WE thrombin significantly ameliorated clinical severity of EAE, reduced inflammatory cell infiltration and demyelination, suppressed the activation of macrophages comprising the CD11b + population and reduced accumulation of fibrin (ogen) in the spinal cord. These data suggest that symptomatic MS may respond to a treatment strategy that involves temporal pharmacological enhancement of endogenous APC generation.

WEDGE: an anticoagulant thrombin mutant produced by autoactivation

BACKGROUND

The production of therapeutically relevant proteases typically involves activation of a zymogen precursor by external enzymes, which may raise regulatory issues about availability and purity. Recent studies of thrombin precursors have shown how to engineer constructs that spontaneously convert to the mature protease by autoactivation, without the need for external enzymes.

OBJECTIVES

Autoactivation is an innovative strategy that promises to simplify the production of proteases of therapeutic relevance, but has not been tested in practical applications. The aim of this study was to provide a direct test of this strategy.

METHODS

An autoactivating version of the thrombin mutant W215A/E217A (WE), which is currently in preclinical development as an anticoagulant, was engineered.

RESULTS AND CONCLUSIONS

The autoactivating version of WE can be produced in large quantities, like WE made in BHK cells or Escherichia coli, and retains all significant functional properties in vitro and in vivo. The results serve as proof of principle that autoactivation is an innovative and effective strategy for the production of trypsin-like proteases of therapeutic relevance.

Thrombomodulin is required for the antithrombotic activity of thrombin mutant W215A/E217A in a mouse model of arterial thrombosis

INTRODUCTION

The thrombin mutant W215A/E217A (WE thrombin) has greatly reduced procoagulant activity, but it activates protein C in the presence of thrombomodulin and inhibits binding of platelet glycoprotein Ib to von Willebrand factor and collagen under flow conditions. Both thrombomodulin-dependent protein C activation and inhibition of platelet adhesion could contribute to the antithrombotic activity of WE thrombin.

MATERIALS AND METHODS

To assess the role of thrombomodulin, we administered WE thrombin to thrombomodulin-deficient (TM(Pro/Pro)) mice and measured the time to occlusive thrombus formation in the carotid artery after photochemical injury of the endothelium.

RESULTS AND CONCLUSIONS

Doses of WE thrombin ≥10μg/kg prolonged the thrombosis time of wild-type mice (>1.6-fold), while doses ≥100μg/kg only slightly prolonged the thrombosis time of TM(Pro/Pro) mice. We conclude that thrombomodulin plays a predominate role in mediating the antithrombotic effect of WE thrombin in the arterial circulation of mice after endothelial injury. Thrombomodulin-independent effects may occur only when high doses of WE thrombin are administered.

Crystal structures of prethrombin-2 reveal alternative conformations under identical solution conditions and the mechanism of zymogen activation

Prethrombin-2 is the immediate zymogen precursor of the clotting enzyme thrombin, which is generated upon cleavage at R15 and separation of the A chain and catalytic B chain. The X-ray structure of prethrombin-2 determined in the free form at 1.9 Å resolution shows the 215-217 segment collapsed into the active site and occluding 49% of the volume available for substrate binding. Remarkably, some of the crystals harvested from the same crystallization well, under identical solution conditions, diffract to 2.2 Å resolution in the same space group but produce a structure in which the 215-217 segment moves >5 Å and occludes 24% of the volume available for substrate binding. The two alternative conformations of prethrombin-2 have the side chain of W215 relocating >9 Å within the active site and are relevant to the allosteric E*-E equilibrium of the mature enzyme. Another unanticipated feature of prethrombin-2 bears on the mechanism of prothrombin activation. R15 is found buried within the protein in ionic interactions with E14e, D14l, and E18, thereby making its exposure to solvent necessary for proteolytic attack and conversion to thrombin. On the basis of this structural observation, we constructed the E14eA/D14lA/E18A triple mutant to reduce the level of electrostatic coupling with R15 and promote zymogen activation. The mutation causes prethrombin-2 to spontaneously convert to thrombin, without the need for the snake venom ecarin or the physiological prothrombinase complex.

Design of Factor XIII V34X activation peptides to control ability to interact with thrombin mutants

Thrombin helps to activate Factor XIII (FXIII) by hydrolyzing the R37-G38 peptide bond. The resultant transglutaminase introduces cross-links into the fibrin clot. With the development of therapeutic coagulation factors, there is a need to better understand interactions involving FXIII. Such knowledge will help predict ability to activate FXIII and thus ability to promote/hinder the generation of transglutaminase activity. Kinetic parameters have been determined for a series of thrombin species hydrolyzing the FXIII (28-41) V34X activation peptides (V34, V34L, V34F, and V34P). The V34P substitution introduces PAR4 character into the FXIII, and the V34F exhibits important similarities to the cardioprotective V34L. FXIII activation peptides containing V34, V34L, or V34P could each be accommodated by alanine mutants of thrombin lacking either the W60d or Y60a residue in the 60-insertion loop. By contrast, FXIII V34F AP could be cleaved by thrombin W60dA but not by Y60aA. FXIII V34P is highly reliant on the thrombin W215 platform for its strong substrate properties whereas FXIII V34F AP becomes the first segment that can maintain its K(m) upon loss of the critical thrombin W215 residue. Interestingly, FXIII V34F AP could also be readily accommodated by thrombin L99A and E217A. Hydrolysis of FXIII V34F AP by thrombin W217A/E217A (WE) was similar to that of FXIII V34L AP whereas WE could not effectively cleave FXIII V34P AP. FXIII V34F and V34P AP show promise for designing FXIII activation systems that are either tolerant of or greatly hindered by the presence of anticoagulant thrombins.

Thrombin mutant W215A/E217A treatment improves neurological outcome and reduces cerebral infarct size in a mouse model of ischemic stroke

BACKGROUND AND PURPOSE

Treatment of ischemic stroke by activation of endogenous plasminogen using tissue plasminogen activator is limited by bleeding side effects. In mice, treatment of experimental ischemic stroke with activated protein C improves outcomes; however, activated protein C also has bleeding side effects. In contrast, activation of endogenous protein C using thrombin mutant W215A/E217A (WE) is antithrombotic without hemostasis impairment in primates. Therefore, we investigated the outcome of WE-treated experimental ischemic stroke in mice.

METHODS

The middle cerebral artery was occluded with a filament for 60 minutes to induce ischemic stroke. Vehicle, recombinant WE, or tissue plasminogen activator was administered during middle cerebral artery occlusion or 2 hours after middle cerebral artery occlusion. Neurological performance was scored daily. Intracranial bleeding and cerebral infarct size, defined by 2,3,5-triphenyltetrazolium chloride exclusion, were determined on autopsy. Hemostasis was evaluated using tail bleeding tests.

RESULTS

WE improved neurological performance scores, increased laser Doppler flowmetry-monitored post-middle cerebral artery occlusion reperfusion of the parietal cortex, and reduced 2,3,5-triphenyltetrazolium chloride-defined cerebral infarct size versus vehicle controls. However, unlike tissue plasminogen activator, WE did not increase tail bleeding or intracranial hemorrhage.

CONCLUSIONS

WE treatment is neuroprotective without hemostasis impairment in experimental acute ischemic stroke in mice and thus may provide an alternative to tissue plasminogen activator for stroke treatment.

Rigidification of the autolysis loop enhances Na(+) binding to thrombin

Binding of Na(+) to thrombin ensures high activity toward physiological substrates and optimizes the procoagulant and prothrombotic roles of the enzyme in vivo. Under physiological conditions of pH and temperature, the binding affinity of Na(+) is weak due to large heat capacity and enthalpy changes associated with binding, and the K(d)=80 mM ensures only 64% saturation of the site at the concentration of Na(+) in the blood (140 mM). Residues controlling Na(+) binding and activation have been identified. Yet, attempts to improve the interaction of Na(+) with thrombin and possibly increase catalytic activity under physiological conditions have so far been unsuccessful. Here we report how replacement of the flexible autolysis loop of human thrombin with the homologous rigid domain of the murine enzyme results in a drastic (up to 10-fold) increase in Na(+) affinity and a significant improvement in the catalytic activity of the enzyme. Rigidification of the autolysis loop abolishes the heat capacity change associated with Na(+) binding observed in the wild-type and also increases the stability of thrombin. These findings have general relevance to protein engineering studies of clotting proteases and trypsin-like enzymes.

Redesigning allosteric activation in an enzyme

Enzyme activation by monovalent cations is widely documented in plants and the animal world. In type II enzymes, activation entails two steps: binding of the monovalent cation to its allosteric site and transduction of this event into enhanced catalytic activity. The effect has exquisite specificity for either Na(+) or K(+), the most abundant cations present in physiological environments. Enzymes requiring K(+) such as kinases and molecular chaperones are not activated as well or at all by the larger cation Cs(+) or the smaller cations Na(+) and Li(+). Enzymes requiring Na(+) such as β-galactosidase and clotting proteases are not activated as well by Li(+), or the larger cations K(+), Rb(+), and Cs(+). Efforts to switch specificity between Na(+) and K(+) in this large class of enzymes and completely redesign the mechanism of allosteric transduction leading to enhanced catalytic activity have so far been unsuccessful. Here we show how mutagenesis of two loops defining the Na(+) binding site of thrombin, a Na(+)-activated clotting protease, generates a construct that is most active in the presence of K(+) toward synthetic and physiological substrates. The effect is the result of a higher binding affinity and more efficient allosteric transduction of binding into enhanced catalytic activity for K(+) compared to Na(+), which represents a complete reversal of the properties of wild type. In addition, the construct features altered specificity toward physiological substrates resulting in a significant anticoagulant profile. The findings are relevant to all Na(+)-activated proteases involved in blood coagulation and the complement system.

The development of inflammatory joint disease is attenuated in mice expressing the anticoagulant prothrombin mutant W215A/E217A

Thrombin is a positive mediator of thrombus formation through the proteolytic activation of protease-activated receptors (PARs), fibrinogen, factor XI (fXI), and other substrates, and a negative regulator through activation of protein C, a natural anticoagulant with anti-inflammatory/cytoprotective properties. Protease-engineering studies have established that 2 active-site substitutions, W215A and E217A (fII(WE)), result in dramatically reduced catalytic efficiency with procoagulant substrates while largely preserving thrombomodulin (TM)-dependent protein C activation. To explore the hypothesis that a prothrombin variant favoring antithrombotic pathways would be compatible with development but limit inflammatory processes in vivo, we generated mice carrying the fII(WE) mutations within the endogenous prothrombin gene. Unlike fII-null embryos, fII(WE/WE) mice uniformly developed to term. Nevertheless, these mice ultimately succumbed to spontaneous bleeding events shortly after birth. Heterozygous fII(WT/WE) mice were viable and fertile despite a shift toward an antithrombotic phenotype exemplified by prolonged tail-bleeding times and times-to-occlusion after FeCl₃ vessel injury. More interestingly, prothrombin(WE) expression significantly ameliorated the development of inflammatory joint disease in mice challenged with collagen-induced arthritis (CIA). The administration of active recombinant thrombin(WE) also suppressed the development of CIA in wild-type mice. These studies provide a proof-of-principle that pro/thrombin variants engineered with altered substrate specificity may offer therapeutic opportunities for limiting inflammatory disease processes.

Autosomal-recessive posterior microphthalmos is caused by mutations in PRSS56, a gene encoding a trypsin-like serine protease

Posterior microphthalmos (MCOP) is a rare isolated developmental anomaly of the eye characterized by extreme hyperopia due to short axial length. The population of the Faroe Islands shows a high prevalence of an autosomal-recessive form (arMCOP) of the disease. Based on published linkage data, we refined the position of the disease locus (MCOP6) in an interval of 250 kb in chromosome 2q37.1 in two large Faroese families. We detected three different mutations in PRSS56. Patients of the Faroese families were either homozygous for c.926G>C (p.Trp309Ser) or compound heterozygous for c.926G>C and c.526C>G (p.Arg176Gly), whereas a homozygous 1 bp duplication (c.1066dupC) was identified in five patients with arMCOP from a consanguineous Tunisian family. In one patient with MCOP from the Faroe Islands and in another one from Turkey, no PRSS56 mutation was detected, suggesting nonallelic heterogeneity of the trait. Using RT-PCR, PRSS56 transcripts were detected in samples derived from the human adult retina, cornea, sclera, and optic nerve. The expression of the mouse ortholog could be first detected in the eye at E17 and was maintained into adulthood. The predicted PRSS56 protein is a 603 amino acid long secreted trypsin-like serine peptidase. The c.1066dupC is likely to result in a functional null allele, whereas the two point mutations predict the replacement of evolutionary conserved and functionally important residues. Molecular modeling of the p.Trp309Ser mutant suggests that both the affinity and reactivity of the enzyme toward in vivo protein substrates are likely to be substantially reduced.

Evidence of the E*-E equilibrium from rapid kinetics of Na+ binding to activated protein C and factor Xa

Na(+) binding to thrombin enhances the procoagulant and prothrombotic functions of the enzyme and obeys a mechanism that produces two kinetic phases: one fast (in the microsecond time scale) due to Na(+) binding to the low activity form E to produce the high activity form E:Na(+) and another considerably slower (in the millisecond time scale) that reflects a pre-equilibrium between E and the inactive form E*. In this study, we demonstrate that this mechanism also exists in other Na(+)-activated clotting proteases like factor Xa and activated protein C. These findings, along with recent structural data, suggest that the E*-E equilibrium is a general feature of the trypsin fold.

Pro-coagulant Activity of Phenolic Acids Isolated from Blumea Riparia

The effects of extracts of the aerial part of Blumea riparia DC. and their phenolic acids on hemostasis were evaluated. The EtOAc fraction showed significantly reduced blood clotting time (CT) and tail bleeding time of transection (BT) of mice in vivo. This fraction contained vanillic acid (1), syringic acid (2), p-coumaric acid (3), caffeic acid (4), and protocatechuic acid (5). Compound 1 reduced prothrombin time (PT), and strengthened mice uterine contractions. Compound 3 reduced CT and the activated partial thromboplastin time (APTT). Compound 5 reduced CT and increased the frequency of mice uterine contraction in a dose-dependent manner. Compound 2 reduced APTT. Compound 4 remarkably strengthened uterine contraction. Taken together, these data suggest that compounds 1, 3, and 5 possess procoagulant activity which jointly synergize blood coagulation via different mechanisms.

Engineering thrombin for selective specificity toward protein C and PAR1

Thrombin elicits functional responses critical to blood homeostasis by interacting with diverse physiological substrates. Ala-scanning mutagenesis of 97 residues covering 53% of the solvent accessible surface area of the enzyme identifies Trp(215) as the single most important determinant of thrombin specificity. Saturation mutagenesis of Trp(215) produces constructs featuring k(cat)/K(m) values for the hydrolysis of fibrinogen, protease-activated receptor PAR1, and protein C that span five orders of magnitude. Importantly, the effect of Trp(215) replacement is context dependent. Mutant W215E is 10-fold more specific for protein C than fibrinogen and PAR1, which represents a striking shift in specificity relative to wild-type that is 100-fold more specific for fibrinogen and PAR1 than protein C. However, when the W215E mutation is combined with deletion of nine residues in the autolysis loop, which by itself shifts the specificity of the enzyme from fibrinogen and PAR1 to protein C, the resulting construct features significant activity only toward PAR1. These findings demonstrate that thrombin can be re-engineered for selective specificity toward protein C and PAR1. Mutations of Trp(215) provide important reagents for dissecting the multiple functional roles of thrombin in the blood and for clinical applications.

Mutant N143P reveals how Na+ activates thrombin

The molecular mechanism of thrombin activation by Na(+) remains elusive. Its kinetic formulation requires extension of the classical Botts-Morales theory for the action of a modifier on an enzyme to correctly account for the contribution of the E*, E, and E:Na(+) forms. The extended scheme establishes that analysis of k(cat) unequivocally identifies allosteric transduction of Na(+) binding into enhanced catalytic activity. The thrombin mutant N143P features no Na(+)-dependent enhancement of k(cat) yet binds Na(+) with an affinity comparable to that of wild type. Crystal structures of the mutant in the presence and absence of Na(+) confirm that Pro(143) abrogates the important H-bond between the backbone N atom of residue 143 and the carbonyl O atom of Glu(192), which in turn controls the orientation of the Glu(192)-Gly(193) peptide bond and the correct architecture of the oxyanion hole. We conclude that Na(+) activates thrombin by securing the correct orientation of the Glu(192)-Gly(193) peptide bond, which is likely flipped in the absence of cation. Absolute conservation of the 143-192 H-bond in trypsin-like proteases and the importance of the oxyanion hole in protease function suggest that this mechanism of Na(+) activation is present in all Na(+)-activated trypsin-like proteases.

Serine proteases

Over one third of all known proteolytic enzymes are serine proteases. Among these, the trypsins underwent the most predominant genetic expansion yielding the enzymes responsible for digestion, blood coagulation, fibrinolysis, development, fertilization, apoptosis, and immunity. The success of this expansion resides in a highly efficient fold that couples catalysis and regulatory interactions. Added complexity comes from the recent observation of a significant conformational plasticity of the trypsin fold. A new paradigm emerges where two forms of the protease, E* and E, are in allosteric equilibrium and determine biological activity and specificity.

Mechanism of the anticoagulant activity of thrombin mutant W215A/E217A

The thrombin mutant W215A/E217A (WE) is a potent anticoagulant both in vitro and in vivo. Previous x-ray structural studies have shown that WE assumes a partially collapsed conformation that is similar to the inactive E* form, which explains its drastically reduced activity toward substrate. Whether this collapsed conformation is genuine, rather than the result of crystal packing or the mutation introduced in the critical 215-217 beta-strand, and whether binding of thrombomodulin to exosite I can allosterically shift the E* form to the active E form to restore activity toward protein C are issues of considerable mechanistic importance to improve the design of an anticoagulant thrombin mutant for therapeutic applications. Here we present four crystal structures of WE in the human and murine forms that confirm the collapsed conformation reported previously under different experimental conditions and crystal packing. We also present structures of human and murine WE bound to exosite I with a fragment of the platelet receptor PAR1, which is unable to shift WE to the E form. These structural findings, along with kinetic and calorimetry data, indicate that WE is strongly stabilized in the E* form and explain why binding of ligands to exosite I has only a modest effect on the E*-E equilibrium for this mutant. The E* --> E transition requires the combined binding of thrombomodulin and protein C and restores activity of the mutant WE in the anticoagulant pathway.

Stabilization of the E* form turns thrombin into an anticoagulant

Previous studies have shown that deletion of nine residues in the autolysis loop of thrombin produces a mutant with an anticoagulant propensity of potential clinical relevance, but the molecular origin of the effect has remained unresolved. The x-ray crystal structure of this mutant solved in the free form at 1.55 A resolution reveals an inactive conformation that is practically identical (root mean square deviation of 0.154 A) to the recently identified E* form. The side chain of Trp(215) collapses into the active site by shifting > 10 A from its position in the active E form, and the oxyanion hole is disrupted by a flip of the Glu(192)-Gly(193) peptide bond. This finding confirms the existence of the inactive form E* in essentially the same incarnation as first identified in the structure of the thrombin mutant D102N. In addition, it demonstrates that the anticoagulant profile often caused by a mutation of the thrombin scaffold finds its likely molecular origin in the stabilization of the inactive E* form that is selectively shifted to the active E form upon thrombomodulin and protein C binding.

Protection of vascular barrier integrity by activated protein C in murine models depends on protease-activated receptor-1

Protease activated receptor-1 (PAR1) mediates barrier protective signalling of activated protein C (APC) in human endothelial cells in vitro and may contribute to APC's beneficial effects in patients with severe sepsis. Mouse models are of key importance for translational research but species differences may limit conclusions for the human system. We analysed whether mouse APC can cleave, activate and induce signalling through murine PAR1 and tested in newly established mouse models if long-term infusion of APC prevents from vascular leakage. Cell surface immunoassays demonstrated efficient cleavage of endogenous murine endothelial PAR1 by either murine or human APC. Pharmacological concentrations of APC of either species had powerful barrier protective effects on cultured murine endothelial cells that required PAR1 cleavage. Vascular endothelial growth factor-mediated hyperpermeability in the skin was reduced by either endogenously generated as well as directly infused recombinant mouse APC in wild-type mice. However APC did not significantly alter the vascular barrier function in PAR1-deficient mice. In endotoxin-challenged mice, infused APC significantly prevented from pulmonary fluid accumulation in the wild-type mice but not in mice lacking PAR1. Our results directly show that murine APC cleaves and signals through PAR1 in mouse endothelial cells. APC reduces vascular permeability in mouse models and PAR1 plays a major role in mediating these effects. Our data in vitro and in vivo support the paradigm that PAR1 contributes to protective effects of APC on vascular barrier integrity in sepsis.

Role of the A chain in thrombin function

The A chain of thrombin is covalently linked to the catalytic B chain but is separate from any known epitope for substrate recognition. In this study we present the results of the Ala replacement of 12 charged residues controlling the stability of the A chain and its interaction with the B chain. Residues Arg4 and Glu8 play a significant role in substrate recognition, even though they are located > 20 A away from residues of the catalytic triad, the primary specificity pocket and the Na+ site. The R4A mutation causes significant perturbation of Na+ binding, fibrinogen clotting and PAR1 cleavage, but modest reduction of protein C activation in the presence of thrombomodulin. These findings challenge our current paradigm of thrombin structure-function relations focused exclusively on the properties of the catalytic B chain, and explain why certain naturally occurring mutations of the A chain cause serious bleeding.

Structural identification of the pathway of long-range communication in an allosteric enzyme

Allostery is a common mechanism of regulation of enzyme activity and specificity, and its signatures are readily identified from functional studies. For many allosteric systems, structural evidence exists of long-range communication among protein domains, but rarely has this communication been traced to a detailed pathway. The thrombin mutant D102N is stabilized in a self-inhibited conformation where access to the active site is occluded by a collapse of the entire 215-219 beta-strand. Binding of a fragment of the protease activated receptor PAR1 to exosite I, 30-A away from the active site region, causes a large conformational change that corrects the position of the 215-219 beta-strand and restores access to the active site. The crystal structure of the thrombin-PAR1 complex, solved at 2.2-A resolution, reveals the details of this long-range allosteric communication in terms of a network of polar interactions.

Thrombin

Thrombin is a Na+-activated, allosteric serine protease that plays opposing functional roles in blood coagulation. Binding of Na+ is the major driving force behind the procoagulant, prothrombotic and signaling functions of the enzyme, but is dispensable for cleavage of the anticoagulant protein C. The anticoagulant function of thrombin is under the allosteric control of the cofactor thrombomodulin. Much has been learned on the mechanism of Na+ binding and recognition of natural substrates by thrombin. Recent structural advances have shed light on the remarkable molecular plasticity of this enzyme and the molecular underpinnings of thrombin allostery mediated by binding to exosite I and the Na+ site. This review summarizes our current understanding of the molecular basis of thrombin function and allosteric regulation. The basic information emerging from recent structural, mutagenesis and kinetic investigation of this important enzyme is that thrombin exists in three forms, E*, E and E:Na+, that interconvert under the influence of ligand binding to distinct domains. The transition between the Na+ -free slow from E and the Na+ -bound fast form E:Na+ involves the structure of the enzyme as a whole, and so does the interconversion between the two Na+ -free forms E* and E. E* is most likely an inactive form of thrombin, unable to interact with Na + and substrate. The complexity of thrombin function and regulation has gained this enzyme pre-eminence as the prototypic allosteric serine protease. Thrombin is now looked upon as a model system for the quantitative analysis of biologically important enzymes.

Thrombin-activated thrombelastography for evaluation of thrombin interaction with thrombin inhibitors

For intravenous anticoagulation, heparin has been the mainstay drug, but its use may be contraindicated in heparin-induced thrombocytopenia and thrombosis. Heparin alternatives including direct thrombin inhibitors are available, but clotting assays (e.g. partial thromboplastin time) measure the time required to form fibrin gel when only a small amount of thrombin is generated. It was hypothesized that the extent of thrombin inhibition varies among inhibitors, and thrombin-activated thrombelastography would provide useful data on therapeutic responses to thrombin inhibitors. Thrombin was added (0-100 nmol/l final concentration) to nonrecalcified whole blood to evaluate clot formation on thrombelastography. Effects of direct thrombin inhibitors (argatroban 3.75 microg/ml, bivalirudin 15 microg/ml, and lepirudin 3.0 microg/ml), and heparin cofactor II activator and dermatan disulfate (20 microg/ml) were evaluated in the presence of 100 nmol/l thrombin. The interactions of thrombin and respective inhibitors were also compared by fluorogenic thrombin substrate cleavage. Increasing concentrations of thrombin progressively shortened the lag time and increased viscoelasticity on thrombelastography. Only 20 nmol/l thrombin caused instantaneous clotting, but maximal viscoelastic force was obtained at 50-100 nmol/l thrombin. All thrombin inhibitors prolonged the lag time (lepirudin > bivalirudin > argatroban = dermatan disulfate), but full recovery of thrombelastography viscoelasticity was observed with argatroban and bivalirudin. Lepirudin abrogated clotting, and dermatan disulfate suppressed clot development on thrombelastography. Thrombin substrate cleavage was observed only for bivalirudin, and heparin cofactor II without dermatan disulfate. The modified thrombelastography technique using nonrecalcified whole blood may be useful in evaluating the extent and reversibility of thrombin blockade with direct or indirect thrombin inhibitors.

The effects of vasoactive agents, platelet agonists and anticoagulation on thrombelastography

BACKGROUND

Platelet activation is a critical step in primary hemostasis and clot formation. We tested a hypothesis that platelet stimulating effects of vasoactive agents or platelet agonists could be shown using thrombelastography (TEG) as faster onset or increased clot strength. We further examined if TEG could be modified to evaluate activated platelets as a reversal of anticoagulation in the presence of partial thrombin inhibition.

METHODS

Blood samples were obtained from 126 non-cardiac surgical patients. Effects of vasoactive agents on TEG and aggregometry were examined using epinephrine, norepinephrine, vasopressin, desmopressin acetate, milrinone and olprinone (Experiment I). Platelet agonists (epinephrine, ADP and collagen) were separately tested on TEG (Experiment II). Effects of platelet agonists (ADP and collagen) on TEG under anticoagulation in the absence or presence of abciximab were studied (Experiment III). We also tested antiplatelet effects of milrinone and olprinone in the presence of anticoagulants on TEG (Experiment IV).

RESULTS

Neither vasoactive agents nor platelet agonists affected TEG or aggregometry results except for milrinone and olprinone on aggregometry (Experiment I, II). Platelet agonists facilitated clotting in the presence of anticoagulants (Experiment III). Abciximab-treated platelets still exhibited procoagulant effects in the presence of heparin, while not in the presence of argatroban (Experiment III). Platelet inhibition on the modified TEG was more extensive with milrinone than olprinone, and it was dose dependent (Experiment IV).

CONCLUSION

Modified TEG using heparin or argatroban might delineate the procoagulant effects of platelets by adding platelet specific agonist.