Mechanisms of Plasmin-catalyzed Inactivation of Factor VIII

Trauma-induced coagulopathy: What you need to know

ABSTRACT

Trauma-induced coagulopathy (TIC) is a global inflammatory state accompanied by coagulation derangements, acidemia, and hypothermia, which occurs after traumatic injury. It occurs in approximately 25% of severely injured patients, and its incidence is directly related to injury severity. The mechanism of TIC is multifaceted; proposed contributing factors include dysregulation of activated protein C, increased tPA, systemic endothelial activation, decreased fibrinogen, clotting factor consumption, and platelet dysfunction. Effects of TIC include systemic inflammation, coagulation derangements, acidemia, and hypothermia. Trauma-induced coagulopathy may be diagnosed by conventional coagulation tests including platelet count, Clauss assay, international normalized ratio, thrombin time, prothrombin time, and activated partial thromboplastin time; viscoelastic hemostatic assays such as thrombelastography and rotational thrombelastography; or a clinical scoring system known as the Trauma Induced Coagulopathy Clinical Score. Preventing TIC begins in the prehospital phase with early hemorrhage control, blood product resuscitation, and tranexamic acid therapy. Early administration of prothrombin complex concentrate is also being studied in the prehospital environment. The mainstays of TIC treatment include hemorrhage control, blood and component transfusions, and correction of abnormalities such as hypocalcemia, acidosis, and hypothermia.

LEVEL OF EVIDENCE

Therapeutic/Care Management; Level III.

The role of proteolytic cleavage at Arg336 and Arg372 of the A1 domain in factor VIIa/tissue factor-catalyzed reactions of B domain-deleted factor VIII

BACKGROUND

We previously demonstrated that factor (F)VIII was rapidly activated through proteolytic cleavage at Arg372 and Arg740 by activated FVII (FVIIa)/tissue factor (TF) in very early coagulation phase, followed by inactivation by cleavage at Arg336. The influence of the absence of FVIII B domain in this series of FVIIa/TF-catalyzed reaction remains unclear, however.

AIM

To examine the FVIIa/TF-catalyzed reaction of B domain-deleted (BDD-)FVIII.

METHODS AND RESULTS

The FVIII activity (FVIII:C) of commercial full-length (FL-)FVIII and BDD-FVIII increased by ∼1.7-fold within 0.5 min after addition of FVIIa/TF (1 nM/0.1 nM).

FVIII

C decreased to initial levels with inactivation rate constant (k; ∼0.035) within 15 min of FL-FVIII activation, but decreased gradually to initial levels (k; ∼0.017) within 30 min of BDD-FVIII activation. SDS-PAGE analyses demonstrated that the FVIIa/TF-catalyzed cleavage of BDD-FVIII occurred at Arg336 within 0.5 min in parallel with elevation of FVIII:C, but cleavage at Arg372 was not evident. FVIIa/TF-catalyzed activation of both recombinant BDD-FVIII R336A and R372A mutants that were prepared, were similar to that of wild-type (WT) BDD-FVIII. However, FVIII:C returned to initial levels (k; ∼0.046) within 30 min of R336A mutant activation, but little reduction of FVIII:C was observed with R372A mutant (k; ∼0.0046). SDS-PAGE analysis indicated that FVIIa/TF-catalyzed cleavage of WT and R372A mutant was predominant at Arg336, whereas that of R336A mutant was observed at Arg372.

CONCLUSIONS

FVIIa/TF-catalyzed activation of BDD-FVIII was initiated by cleavage at Arg336, and the FVIII B domain appeared to control FVIIa/TF-catalyzed reactions by altering pattern of cleavage at Arg336 and Arg372.

Factor VIII: A Dynamic Modulator of Hemostasis and Thrombosis in Trauma

A trace amount of thrombin cleaves factor VIII (FVIII) into an active form (FVIIIa), which catalyzes FIXa-mediated activation of FX on the activated platelet surface. FVIII rapidly binds to von Willebrand factor (VWF) after secretion and becomes highly concentrated via VWF-platelet interaction at a site of endothelial inflammation or injury. Circulating levels of FVIII and VWF are influenced by age, blood type (nontype O > type O), and metabolic syndromes. In the latter, hypercoagulability is associated with chronic inflammation (known as thrombo-inflammation). In acute stress including trauma, releasable pools of FVIII/VWF are secreted from the Weibel-Palade bodies in the endothelium and then augment local platelet accumulation, thrombin generation, and leukocyte recruitment. Early systemic increases of FVIII/VWF (>200% of normal) levels in trauma result in a lower sensitivity of contact-activated clotting time (activated partial thromboplastin time [aPTT] or viscoelastic coagulation test [VCT]). However, in severely injured patients, multiple serine proteases (FXa plasmin and activated protein C [APC]) are locally activated and may be systemically released. Severity of traumatic injury correlates with prolonged aPTT and elevated activation markers of FXa, plasmin, and APC, culminating in a poor prognosis. In a subset of acute trauma patients, cryoprecipitate that contains fibrinogen, FVIII/VWF, and FXIII is theoretically advantageous over purified fibrinogen concentrate to promote stable clot formation, but comparative efficacy data are lacking. In chronic inflammation or subacute phase of trauma, elevated FVIII/VWF contributes to the pathogenesis of venous thrombosis by enhancing not only thrombin generation but also augmenting inflammatory functions. Future developments in coagulation monitoring specific to trauma patients, and targeted to enhancement or inhibition of FVIII/VWF, are likely to help clinicians gain better control of hemostasis and thromboprophylaxis. The main goal of this narrative is to review the physiological functions and regulations of FVIII and implications of FVIII in coagulation monitoring and thromboembolic complications in major trauma patients.

Tranexamic Acid and Plasminogen/Plasmin Glaring Paradox in COVID-19

Coronavirus disease 2019 (COVID-19) is caused by a severe acute respiratory syndrome, coronavirus type 2 (SARS-CoV-2), leading to acute tissue injury and an overstated immune response. In COVID-19, there are noteworthy changes in the fibrinolytic system with the development of coagulopathy. Therefore, modulation of the fibrinolytic system may affect the course of COVID-19. Tranexamic acid (TXA) is an anti-fibrinolytic drug that reduces the conversion of plasminogen to plasmin, which is necessary for SARS-CoV-2 infectivity. In addition, TXA has anti-inflammatory, anti-platelet, and anti-thrombotic effects, which may attenuate the COVID-19 severity. Thus, in this narrative review, we try to find the beneficial and harmful effects of TXA in COVID-19.

Emicizumab improves the stability and structure of fibrin clot derived from factor VIII-deficient plasma, similar to the addition of factor VIII

INTRODUCTION

Emicizumab is an antifactor (F)IXa/FX bispecific antibody, mimicking FVIIIa cofactor function. Emi prophylaxis effectively reduces bleeding events in patients with haemophilia A. The physical properties of emicizumab-induced fibrin clots remain to be investigated, however.

AIM

We have investigated the stability and structure of emicizumab-induced fibrin clots.

METHODS

Coagulation was initiated by activated partial thromboplastin time (aPTT) trigger and prothrombin time (PT)/aPTT-mixed trigger in FVIII-deficient plasma with various concentrations of emicizumab or recombinant FVIII. The turbidity and stability of fibrin clots were assessed by clot waveform and clot-fibrinolysis waveform analyses, respectively. The resulting fibrin was analysed by scanning electron microscopy (SEM).

RESULTS

Using an aPTT trigger, the turbidity was decreased and the fibrinolysis times were prolonged in the presence of emicizumab dose-dependently. Scanning electron microscopy imaging demonstrated that emicizumab improved the structure of fibrin network with thinner fibres than in its absence. Although emicizumab shortened the aPTT dramatically, the nature of emicizumab-induced fibrin clots did not reflect the hypercoagulable state. Similarly, using a PT/aPTT-mixed trigger that could evaluate potential emicizumab activity, emicizumab improved the stability and structure of fibrin clot in a series of experiments. In this circumstance, fibrin clot properties with emicizumab at 50 and 100 µg/mL appeared to be comparable to those with FVIII at ~12 and ~24-32 IU/dL, respectively.

CONCLUSION

Emicizumab effectively improved fibrin clot stability and structure in FVIII-deficient plasma, and the physical properties of emicizumab-induced fibrin clots were similar to those with FVIII.

Therapeutics targeting the fibrinolytic system

The function of the fibrinolytic system was first identified to dissolve fibrin to maintain vascular patency. Connections between the fibrinolytic system and many other physiological and pathological processes have been well established. Dysregulation of the fibrinolytic system is closely associated with multiple pathological conditions, including thrombosis, inflammation, cancer progression, and neuropathies. Thus, molecules in the fibrinolytic system are potent therapeutic and diagnostic targets. This review summarizes the currently used agents targeting this system and the development of novel therapeutic strategies in experimental studies. Future directions for the development of modulators of the fibrinolytic system are also discussed.

The fibrinolytic system was originally identified to dissolve blood clots, and is shown to have important roles in other pathological processes, including cancer progression, inflammation, and thrombosis. Molecules or therapeutics targeting fibrinolytic system have been successfully used in the clinical treatments of cancer and thrombotic diseases. The clinical studies and experimental models targeting fibrinolytic system are reviewed by Haili Lin at Sanming First Hosipital, Mingdong Huang at Fuzhou University in China, and Peng Xu at A*STAR in Singapore to demonstrate fibrinolytic system as novel therapeutic targets. As an example, the inhibition of fibrinolytic system protein can be used to suppress cancer prolifieration and metastasis. This review also discusses the potential therapeutic effects of inhibitiors of fibrinolytic system on inflammatory disorders.

Application of thrombelastography (TEG) for safety evaluation of tranexamic acid in primary total joint arthroplasty

BACKGROUND

Questions remain, mainly concerning whether tranexamic acid (TXA) is truly safe since all available trials were underpowered to identify clinically important differences. The objective of this study is to evaluate the safety of TXA by using a novel technique-thromboelastography (TEG).

METHODS

A retrospective review was conducted on 359 consecutive patients who underwent primary total hip arthroplasty (THA) or total knee arthroplasty (TKA) and received multiple-dose or single-dose of TXA at a tertiary academic center. TEG parameters, TEG coagulation status, conventional coagulation test parameters, and incidence of thrombotic events were used for safety evaluation.

RESULTS

Compared with single-dose cohort, patients who received multiple-dose of TXA had consistent statistically significant shortened R times on post-operative day 1 (POD1) and POD3 in both THA (POD1: 4.06 ± 0.71 s versus 4.45 ± 1.28 s, P = 0.011; POD3: 4.36 ± 0.83 s versus 5.12 ± 1.64 s, P < 0.0001) and TKA (POD1: 3.90 ± 0.73 s versus 4.29 ± 0.92 s, P = 0.011; POD3: 4.24 ± 0.94 s versus 4.65 ± 1.07 s, P = 0.023), while the K, α-angle, and MA values were similar during the perioperative period. TEG coagulation status analysis indicated that patients were significantly (P = 0.003) more likely with hypercoagulable status during the course of multiple-dose TXA. Conventional coagulation test parameters were similar. Only one patient developed calf vein thrombosis in the multiple-dose cohort.

CONCLUSIONS

Multiple-dose of TXA was associated with aggravated hypercoagulable state when compared with single-dose of TXA, but this prothrombotic state does not provoke thrombosis when combined with appropriate anticoagulant therapy. Therefore, multiple-dose of TXA remains safe and could be recommended for clinical practice. Potential benefits and possible risks should be trade-off when considering increasing the dosage and frequency of TXA on the present basis.

TRIAL REGISTRATION

ChiCTR1800015422 .

Increased urokinase and consumption of α2 -antiplasmin as an explanation for the loss of benefit of tranexamic acid after treatment delay

Essentials Delayed treatment with tranexamic acid results in loss of efficacy and poor outcomes. Increasing urokinase activity may account for adverse effects of late tranexamic acid treatment. Urokinase + tranexamic acid produces plasmin in plasma or blood and disrupts clotting. α2 -Antiplasmin consumption with ongoing fibrinolysis increases plasmin-induced coagulopathy. SUMMARY: Background Tranexamic acid (TXA) is an effective antifibrinolytic agent with a proven safety record. However, large clinical trials show TXA becomes ineffective or harmful if treatment is delayed beyond 3 h. The mechanism is unknown but urokinase plasminogen activator (uPA) has been implicated. Methods Inhibitory mechanisms of TXA were explored in a variety of clot lysis systems using plasma and whole blood. Lysis by tissue plasminogen activator (tPA), uPA and plasmin were investigated. Coagulopathy was investigated using ROTEM and activated partial thromboplastin time (APTT). Results IC50 values for antifibrinolytic activity of TXA varied from < 10 to > 1000 μmol L-1 depending on the system, but good fibrin protection was observed in the presence of tPA, uPA and plasmin. However, in plasma or blood, active plasmin was generated by TXA + uPA (but not tPA) and coagulopathy developed leading to no or poor clot formation. The extent of coagulopathy was sensitive to available α2 -antiplasmin. No clot formed with plasma containing 40% normal α2 -antiplasmin after short incubation with TXA + uPA. Adding purified α2 -antiplasmin progressively restored clotting. Plasmin could be inhibited by aprotinin, IC50 = 530 nmol L-1 , in plasma. Conclusions Tranexamic acid protects fibrin but stimulates uPA activity and slows inhibition of plasmin by α2 -antiplasmin. Plasmin proteolytic activity digests fibrinogen and disrupts coagulation, exacerbated when α2 -antiplasmin is consumed by ongoing fibrinolysis. Additional direct inhibition of plasmin by aprotinin may prevent development of coagulopathy and extend the useful time window of TXA treatment.

Effects of Plasmin on von Willebrand Factor and Platelets: A Narrative Review

Plasmin is the major fibrinolytic protease responsible for dissolving thrombi by cleavage of its primary substrate fibrin. In addition, emerging evidence points to other roles of plasmin: (1) as a back-up for ADAMTS13 in proteolysis of ultra-large von Willebrand factor (VWF) multimers and (2) as an activator of platelets. Although the molecular mechanisms of fibrinolysis are well defined, insights on the effects of plasmin on VWF and platelets are relatively scarce and sometimes conflicting. Hence, this review provides an overview of the literature on the effects of plasmin on VWF multimeric structures, on VWF binding to platelets, and on platelet activation. This information is placed in the context of possible applications of thrombolytic therapy for the condition thrombotic thrombocytopenic purpura.

A putative inhibitory mechanism in the tenase complex responsible for loss of coagulation function in acquired haemophilia A patients with anti-C2 autoantibodies

Acquired haemophilia A (AHA) is caused by the development of factor (F)VIII autoantibodies, demonstrating type 1 or type 2 inhibitory behaviour, and results in more serious haemorrhagic symptoms than in congenital severe HA. The reason(s) for this remains unknown, however. The global coagulation assays, thrombin generation tests and clot waveform analysis, demonstrated that coagulation parameters in patients with AHA-type 2 inhibitor were more significantly depressed than those in patients with moderate HA with similar FVIII activities. Thrombin and intrinsic FXa generation tests were significantly depressed in AHA-type 1 and AHA-type 2 compared to severe HA, and more defective in AHA-type 1 than in AHA-type 2. To investigate these inhibitory mechanism(s), anti-FVIII autoantibodies were purified from AHA plasmas. AHA-type 1 autoantibodies, containing an anti-C2 ESH4-epitope, blocked FVIII(a)-phospholipid binding, whilst AHA-type 2, containing an anti-C2 ESH8-epitope, inhibited thrombin-catalysed FVIII activation. The coagulation function in a reconstituted AHA-model containing exogenous ESH4 or ESH8 was more abnormal than in severe HA. The addition of anti-FIX antibody to FVIII-deficient plasma resulted in lower coagulation function than its absence. These results support the concept that global coagulation might be more suppressed in AHA than in severe HA due to the inhibition of FIXa-dependent FX activation by steric hindrance in the presence of FVIII-anti-C2 autoantibodies. Additionally, AHA-type 1 inhibitors prevented FVIIIa-phospholipid binding, essential for the tenase complex, whilst AHA-type 2 antibodies decreased FXa generation by inhibiting thrombin-catalysed FVIII activation. These two distinct mechanisms might, in part, contribute to and exacerbate the serious haemorrhagic symptoms in AHA.

Presented in abstract form at the 52nd annual meeting of the American Society of Hematology, Orlando, Florida, USA, December 6, 2010.

Mechanisms of human neutrophil elastase-catalysed inactivation of factor VIII(a)

Mechanisms of inflammation and coagulation are linked through various pathways. Human neutrophil elastase (HNE), can bind to activated platelets, might be localised on platelet membranes that provide negatively-charged phospholipid essential for the optimum function of tenase complex. In this study, we examined the effect of HNE on factor (F)VIII. FVIII activity was rapidly diminished in the presence of HNE and was undetectable within 10 minutes. The inactivation rate waŝ8-fold greater than that of activated protein C (APC). This time-dependent inactivation was moderately affected by von Willebrand factor. HNE proteolysed the heavy chain (HCh) of FVIII into two terminal products, A11–358 and A2375–708, by limited proteolysis at Val358, Val374, and Val708. Cleavage at Val708 was much slower than that at Val358 in the >90-kDa A1-A2-B compared to the 90-kDa A1-A2. The 80-kDa light chain (LCh) was proteolysed to 75-kDa product by cleavage at Val1670. HNE-cata- lysed FVIIIa inactivation was markedly slower than that of native FVIII (by ~25-fold), due to delayed cleavage at Val708 in FVIIIa. The inactivation rate mediated by HNE was ~8-fold lower than that by APC. Cleavages at Val358 and Val708 were regulated by the presence of LCh and HCh, respectively. In conclusion, HNE-catalysed FVIII inactivation was associated with the limited-proteolysis that led to A11–358, A2375–708, and A3-C1-C21671–2332, and subsequently to critical cleavage at Val708. HNE-related FVIII(a) reaction might play a role in inactivation of HNE-induced coagulation process, and appeared to depend on the amounts of inactivated FVIII and active FVIIIa which is predominantly resistant to HNE inactivation.

Note: An account of this work was presented at the 51st annual meeting of the American Society of Hematology, December 10, 2009, New Orleans, LA, USA.

Bleeding related to disturbed fibrinolysis

The components and reactions of the fibrinolysis system are well understood. The pathway has fewer reactants and interactions than coagulation, but the generation of a complete quantitative model is complicated by the need to work at the solid‐liquid interface of fibrin. Diagnostic tools to detect disease states due to malfunctions in the fibrinolysis pathway are also not so well developed as is the case with coagulation. However, there are clearly a number of inherited or acquired pathologies where hyperfibrinolysis is a serious, potentially life‐threatening problem and a number of antifibrinolytc drugs are available to treat hyperfibrinolysis. These topics will be covered in the following review.

An in vitro study of the effects of t-PA and tranexamic acid on whole blood coagulation and fibrinolysis

AIMS

Acute traumatic coagulopathy is characterised by fibrinolysis and low fibrinogen. It is unclear how much fibrinogenolysis contributes to reduce fibrinogen levels. The study aim was to: investigate in vitro the effects of tissue-plasminogen activator (t-PA) and tranexamic acid (TXA) on coagulation and fibrinolysis.

METHODS

Whole blood was spiked with varying t-PA concentrations. Clauss fibrinogen levels and thrombelastography (TEG, Haemonetics) were performed, including functional fibrinogen level (FLEV). TXA effects were assessed using four TXA concentrations. Recorded parameters from kaolin activated TEG included maximal amplitude (MA), clot strength (G), percentage lysis (LY). Plasmin-antiplasmin complex (PAP), endogenous thrombin potential (ETP), prothrombin fragment 1+2 (PF1+2), factor V and factor VIII levels were all measured.

RESULTS

t-PA induced fibrinolysis: it increased PAP and LY, but decreased MA and G. t-PA induced fibrinogenolysis, with a concentration-dependant decrease in fibrinogen from 2.7 (2.6-3.1) to 0.8 (0.8-0.9) g/L with 60 nM t-PA. FLEV and fibrinogen levels were well correlated. High t-PA doses increased PF1+2, decreased ETP of 19% and FVIII of 63% but not FV. TXA had no effect on plasmin generation as evidenced by no change in PAP. It corrected LY, MA and G and partly protected fibrinogen against fibrinogenolysis: 0.03 mg/mL TXA reduced the fibrinogen fall induced by t-PA 20 nM from 43% to 14%. TXA halved the FVIII fall and increased ETP.

CONCLUSIONS

t-PA induced plasminogen activation and fibrinogenolysis in a concentration-dependant manner. TXA did not affect plasmin activation but reduced fibrinogenolysis. These results suggest that TXA given early in bleeding patients may prevent fibrinogenolysis.

Procoagulant and fibrinolytic activity after polytrauma in rat

The purpose of this study was to determine whether trauma-induced coagulopathy is due to changes in 1) thrombin activity, 2) plasmin activity, and/or 3) factors that stimulate or inhibit thrombin or plasmin. Sprague-Dawley rats were anesthetized with 1-2% isoflurane/100% oxygen, and their left femoral artery and vein were cannulated. Polytrauma included right femur fracture, and damage to the small intestines, the left and medial liver lobes, and right leg skeletal muscle. Rats were then bled 40% of blood volume. Plasma samples were taken before trauma, and at 30, 60, 120, and 240 min. Polytrauma and hemorrhage led to a significant fall in prothrombin levels. However, circulating thrombin activity did not change significantly over time. Antithrombin III and α2 macroglobulin fell significantly by 2 h, then rose by 4 h. Soluble thrombomodulin was significantly elevated over the 4 h. Circulating plasmin activity, plasminogen, and D-dimers were elevated for the entire 4 h. Tissue plasminogen activator (tPA) was elevated at 30 min, then decreased below baseline levels after 1 h. Plasminogen activator inhibitor-1 was significantly elevated at 2-4 h. Neither tissue factor pathway inhibitor nor thrombin activatable fibrinolysis inhibitor changed significantly over time. The levels of prothrombin and plasminogen were 30-100 times higher than their respective active enzymes. Polytrauma and hemorrhage in rats lead to a fibrinolytic coagulopathy, as demonstrated by an elevation in plasmin activity, D-dimers, and tPA. These results are consistent with the observed clinical benefit of tranexamic acid in trauma patients.

The pathogenesis of traumatic coagulopathy

Over the last 10 years, the management of major haemorrhage in trauma patients has changed radically. This is mainly due to the recognition that many patients who are bleeding when they come in to the emergency department have an established coagulopathy before the haemodilution effects of fluid resuscitation. This has led to the use of new terminology: acute traumatic coagulopathy, acute coagulopathy of trauma shock or trauma-induced coagulopathy. The recognition of acute traumatic coagulopathy is important, because we now understand that its presence is a prognostic indicator, as it is associated with poor clinical outcome. This has driven a change in clinical management, so that the previous approach of maintaining an adequate circulating volume and oxygen carrying capacity before, as a secondary event, dealing with coagulopathy, has changed to haemostatic resuscitation as early as possible. While there is as yet no universally accepted assay or definition, many experts use prolongation of the prothrombin time to indicate that there is, indeed, a coagulopathy. Hypoxia, acidosis and hypothermia and hormonal, immunological and cytokine production, alongside consumption and blood loss, and the dilutional effects of resuscitation may occur to varying extents depending on the type of tissue damaged, the type and extent of injury, predisposing to, or amplifying, activation of coagulation, platelets, fibrinolysis. These are discussed in detail within the article.

Tranexamic acid: less bleeding and less thrombosis?

The early administration of tranexamic acid (TXA) to bleeding trauma patients reduces all-cause mortality without increasing the risk of vascular occlusive events. Indeed, the risk of arterial thrombosis appears to be reduced with TXA. In this commentary we hypothesize that TXA has an antithrombotic effect and explore potential mechanisms. These include inhibition of the inflammatory effects of plasmin, effects on platelets and effects on factors V and VIII. If proven, these antithrombotic effects would have major implications for the systemic use of TXA in surgical patients, where TXA has been clearly shown to reduce bleeding.

Sequences flanking Arg336 in factor VIIIa modulate factor Xa-catalyzed cleavage rates at this site and cofactor function

Factor (F)VIII can be activated to FVIIIa by FXa following cleavages at Arg(372), Arg(740), and Arg(1689). FXa also cleaves FVIII/FVIIIa at Arg(336) and Arg(562) resulting in inactivation of the cofactor. These inactivating cleavages occur on a slower time scale than the activating ones. We assessed the contributions to cleavage rate and cofactor function of residues flanking Arg(336), the primary site yielding FVIII(a) inactivation, following replacement of these residues with those flanking the faster-reacting Arg(740) and Arg(372) sites and the slower-reacting Arg(562) site. Replacing P4-P3' residues flanking Arg(336) with those from Arg(372) or Arg(740) resulted in ∼4-6-fold increases in rates of FXa-catalyzed inactivation of FVIIIa, which paralleled the rates of proteolysis at Arg(336). Examination of partial sequence replacements showed a predominant contribution of prime residues flanking the scissile bonds to the enhanced rates. Conversely, replacement of this sequence with residues flanking the slow-reacting Arg(562) site yielded inactivation and cleavage rates that were ∼40% that of the WT values. The capacity for FXa to activate FVIII variants where cleavage at Arg(336) was accelerated due to flanking sequence replacement showed marked reductions in peak activity, whereas reducing the cleavage rate at this site enhanced peak activity. Furthermore, plasma-based thrombin generation assays employing the variants revealed significant reductions in multiple parameter values with acceleration of Arg(336) cleavage suggesting increased down-regulation of FXase. Overall, these results are consistent with a model of competition for activating and inactivating cleavages catalyzed by FXa that is modulated in large part by sequences flanking the scissile bonds.

Mass spectrometry-assisted study reveals that lysine residues 1967 and 1968 have opposite contribution to stability of activated factor VIII

The A2 domain rapidly dissociates from activated factor VIII (FVIIIa) resulting in a dampening of the activity of the activated factor X-generating complex. The amino acid residues that affect A2 domain dissociation are therefore critical for FVIII cofactor function. We have now employed chemical footprinting in conjunction with mass spectrometry to identify lysine residues that contribute to the stability of activated FVIII. We hypothesized that lysine residues, which are buried in FVIII and surface-exposed in dissociated activated FVIII (dis-FVIIIa), may contribute to interdomain interactions. Mass spectrometry analysis revealed that residues Lys(1967) and Lys(1968) of region Thr(1964)-Tyr(1971) are buried in FVIII and exposed to the surface in dis-FVIIIa. This result, combined with the observation that the FVIII variant K1967I is associated with hemophilia A, suggests that these residues contribute to the stability of activated FVIII. Kinetic analysis revealed that the FVIII variants K1967A and K1967I exhibit an almost normal cofactor activity. However, these variants also showed an increased loss in cofactor activity over time compared with that of FVIII WT. Remarkably, the cofactor activity of a K1968A variant was enhanced and sustained for a prolonged time relative to that of FVIII WT. Surface plasmon resonance analysis demonstrated that A2 domain dissociation from activated FVIII was reduced for K1968A and enhanced for K1967A. In conclusion, mass spectrometry analysis combined with site-directed mutagenesis studies revealed that the lysine couple Lys(1967)-Lys(1968) within region Thr(1964)-Tyr(1971) has an opposite contribution to the stability of FVIIIa.

Plasmin-induced procoagulant effects in the blood coagulation: a crucial role of coagulation factors V and VIII

Plasminogen activators provide effective treatment for patients with acute myocardial infarction. However, paradoxical elevation of thrombin activity associated with failure of clot lysis and recurrent thrombosis has been reported. Generation of thrombin in these circumstances appears to be owing to plasmin (Plm)-induced activation of factor (F) XII. Plm catalyzes proteolysis of several coagulant factors, but the roles of these factors on Plm-mediated procoagulant activity remain to be determined. Recently developed global coagulation assays were used in this investigation. Rotational thromboelastometry using whole blood, clot waveform analysis and thrombin generation tests using plasma, showed that Plm (> or =125 nmol/l) shortened the clotting times in similar dose-dependent manners. In particular, the thrombin generation test, which was unaffected by products of fibrinolysis, revealed the enhanced coagulation with an approximately two-fold increase of peak level of thrombin generation. Studies using alpha2-antiplasmin-deficient plasma revealed that much lower dose of Plm (> or =16 nmol/l) actually contributed to enhancing thrombin generation. The shortening of clotting time could be observed even in the presence of corn trypsin inhibitor, supporting that Plm exerted the procoagulant activity independently of FXII. In addition, using specific coagulation-deficient plasmas, the clot waveform analysis showed that Plm did not shorten the clotting time in only FV-deficient or FVIII-deficient plasma in prothrombin time-based or activated partial thromboplastin time-based assay, respectively. Our results indicated that Plm did possess procoagulant activity in the blood coagulation, and this effect was likely attributed by multicoagulation factors, dependent on FV and/or FVIII.

Engineering kunitz domain 1 (KD1) of human tissue factor pathway inhibitor-2 to selectively inhibit fibrinolysis: properties of KD1-L17R variant

Tissue factor pathway inhibitor-2 (TFPI-2) inhibits factor XIa, plasma kallikrein, and factor VIIa/tissue factor; accordingly, it has been proposed for use as an anticoagulant. Full-length TFPI-2 or its isolated first Kunitz domain (KD1) also inhibits plasmin; therefore, it has been proposed for use as an antifibrinolytic agent. However, the anticoagulant properties of TFPI-2 or KD1 would diminish its antifibrinolytic function. In this study, structure-based investigations and analysis of the serine protease profiles revealed that coagulation enzymes prefer a hydrophobic residue at the P2' position in their substrates/inhibitors, whereas plasmin prefers a positively charged arginine residue at the corresponding position in its substrates/inhibitors. Based upon this observation, we changed the P2' residue Leu-17 in KD1 to Arg (KD1-L17R) and compared its inhibitory properties with wild-type KD1 (KD1-WT). Both WT and KD1-L17R were expressed in Escherichia coli, folded, and purified to homogeneity. N-terminal sequences and mass spectra confirmed proper expression of KD1-WT and KD1-L17R. Compared with KD1-WT, the KD1-L17R did not inhibit factor XIa, plasma kallikrein, or factor VIIa/tissue factor. Furthermore, KD1-L17R inhibited plasmin with ∼6-fold increased affinity and effectively prevented plasma clot fibrinolysis induced by tissue plasminogen activator. Similarly, in a mouse liver laceration bleeding model, KD1-L17R was ∼8-fold more effective than KD1-WT in preventing blood loss. Importantly, in this bleeding model, KD1-L17R was equally or more effective than aprotinin or tranexamic acid, which have been used as antifibrinolytic agents to prevent blood loss during major surgery/trauma. Furthermore, as compared with aprotinin, renal toxicity was not observed with KD1-L17R.

Mechanisms of factor VIIa-catalyzed activation of factor VIII

BACKGROUND

Factor (F)VIIa, complexed with tissue factor (TF), is a primary trigger of blood coagulation, and has extremely restricted substrate specificity. The complex catalyzes limited proteolysis of FVIII, but these mechanisms are poorly understood.

OBJECTIVES

In the present study, we investigated the precise mechanisms of FVIIa/TF-catalyzed FVIII activation.

RESULTS

FVIII activity increased ~4-fold within 30 s in the presence of FVIIa/TF, and then decreased to initial levels within 20 min. FVIIa (0.1 nM), at concentrations present physiologically in plasma, activated FVIII in the presence of TF, and this activation was more rapid than that induced by thrombin. The heavy chain (HCh) of FVIII was proteolyzed at Arg(740) and Arg(372) more rapidly by FVIIa/TF than by thrombin, consistent with the enhanced activation of FVIII. Cleavage at Arg(336) was evident at ~1 min, whilst little cleavage of the light chain (LCh) was observed. Cleavage of the HCh by FVIIa/TF was governed by the presence of the LCh. FVIII bound to Glu-Gly-Arg-active-site-modified FVIIa (K(d), ~0.8 nM) with a higher affinity for the HCh than for the LCh (K(d), 5.9 and 18.9 nm). Binding to the A2 domain was particularly evident. Von Willebrand factor (VWF) modestly inhibited FVIIa/TF-catalyzed FVIII activation, in keeping with the concept that VWF could moderate FVIIa/TF-mediated reactions.

CONCLUSIONS

The results demonstrated that this activation mechanism was distinct from those mediated by thrombin, and indicated that FVIIa/TF functions through a 'priming' mechanism for the activation of FVIII in the initiation phase of coagulation.

Activated factor X cleaves factor VIII at arginine 562, limiting its cofactor efficiency

BACKGROUND

Factor VIII (FVIII) and its activated form (FVIIIa) are subject to proteolysis that dampens their cofactor function. Among the proteases that attack FVIII (activated factor X (FXa), activated protein C (APC) and plasmin), only APC cleaves within the FVIII A2 domain at R562 to fully abolish FVIII activity.

OBJECTIVES

We investigated the possible involvement of the FXa cleavage at R562 within the A2 domain in the process of FVIII inactivation.

METHODS

An antibody (GMA012/R8B12) that recognizes the carboxy-terminus extremity of the A2 domain (A2C) was used to evaluate FXa action. A molecule mutated at R562 was also generated to assess the functional role of this particular residue.

RESULTS AND CONCLUSIONS

The appearance of the A2C domain as a function of time evidenced the identical cleavage within the A2 domain of FVIII and FVIIIa by FXa. This cleavage required phospholipids and occurred within minutes. In contrast, the isolated A2 domain was not cleaved by FXa. Von Willebrand factor and activated FIX inhibited the cleavage in a dose-dependent manner. Mutation R562K increased both the FVIII specific activity and the generation of FXa due to an increase in FVIII catalytic efficiency. Moreover, A2C fragment could not be identified from FVIII-R562K cleavage. In summary, this study defines a new mechanism for A2 domain-mediated FVIII degradation by FXa and implicates the bisecting of the A2 domain at R562.

Identification of plasmin-interactive sites in the light chain of factor VIII responsible for proteolytic cleavage at Lys36

We have recently reported that plasmin likely associates with the factor VIII light chain to proteolyze at Lys36 within the A1 domain. In this study, we determined that the rate of plasmin-catalyzed inactivation on the forms of factor VIIIa containing A1-(1-336) and 1722A3C1C2, reflecting Lys36 cleavage, was reduced by approximately 60%, compared with those containing 1649A3C1C2 and 1690A3C1C2. SDS-PAGE analysis revealed that Lys36 cleavage of factor VIIIa with 1722A3C1C2 was markedly slower than those with 1649A3C1C2 and 1690A3C1C2. Surface plasmon resonance-based assays, using active site-modified anhydro-plasmin (Ah-plasmin) showed that 1722A3C1C2 bound to Ah-plasmin with an approximately 3-fold lower affinity than 1649A3C1C2 or 1690A3C1C2 (Kd, 176, 68.2, and 60.3 nM, respectively). Recombinant A3 bound to Ah-plasmin (Kd, 44.2 nM), whereas C2 failed to bind, confirming the presence of a plasmin-binding site within N terminus of A3. Furthermore, the Glu-Gly-Arg active site-modified factor IXa also blocked 1722A3C1C2 binding to Ah-plasmin by approximately 95%, supporting the presence of another plasmin-binding site overlapping the factor IXa-binding site in A3. In keeping with a major contribution of the lysine-binding sites in plasmin for interaction with the factor VIII light chain, analysis of the A3 sequence revealed two regions involving clustered lysine residues in 1690-1705 and 1804-1818. Two peptides based on these regions blocked 1649A3C1C2 binding to Ah-plasmin by approximately 60% and plasmin-catalyzed Lys36 cleavage of factor VIIIa with A1-(1-336) by approximately 80%. Our findings indicate that an extended surface, centered on residues 1690-1705 and 1804-1818 within the A3 domain, contributes to a unique plasmin-interactive site that promotes plasmin docking during cofactor inactivation by cleavage at Lys36.