Exosite 2-Directed Ligands Attenuate Protein C Activation by the Thrombin-Thrombomodulin Complex.

Thrombin activity, inhibition, and localization are regulated by two exosites that flank the active site. Substrates, cofactors, and inhibitors bind to exosite 1 to promote active site access, whereas exosite 2 interactions hold thrombin on cells, platelets, and proteins. The exosites also serve allosteric roles, whereby ligand binding alters thrombin activity. Previously, we showed that ligands that bind exosite 2 attenuate the exosite 1-mediated interaction of thrombin with fibrin, demonstrating allosteric connection between the exosites. To determine the functional consequences of these inter-exosite interactions, we examined the effect of exosite 2 ligands on thrombin's interaction with thrombomodulin, a key cofactor that binds exosite 1 and redirects thrombin activity to the anticoagulant protein C pathway. Exosite 2-directed ligands, which included the HD22 aptamer, glycoprotein 1bα-derived peptide, and fibrinogen γ'-chain peptide, reduced the level of exosite 1-mediated thrombin binding to the thrombomodulin peptide consisting of the fourth, fifth, and sixth epidermal-like growth factor-like domains, decreasing affinity by >10-fold, and attenuated thrombomodulin-dependent activation of protein C by 60-80%. The ligands had similar effects on thrombin-mediated protein C activation with intact soluble thrombomodulin and with thrombomodulin on the surface of cultured endothelial cells. Their activity was exosite 2-specific because it was attenuated when RA-thrombin, a variant lacking exosite 2, was used in place of thrombin. These results indicate that additional reactions mediated by exosite 1 are amenable to regulation by exosite 2 ligation, providing further evidence of inter-exosite allosteric regulation of thrombin activity.

Arterial thrombosis is accelerated in mice deficient in histidine-rich glycoprotein.

Factor (F) XII, a key component of the contact system, triggers clotting via the intrinsic pathway, and is implicated in propagating thrombosis. Although nucleic acids are potent activators, it is unclear how the contact system is regulated to prevent uncontrolled clotting. Previously, we showed that histidine-rich glycoprotein (HRG) binds FXIIa and attenuates its capacity to trigger coagulation. To investigate the role of HRG as a regulator of the intrinsic pathway, we compared RNA- and DNA-induced thrombin generation in plasma from HRG-deficient and wild-type mice. Thrombin generation was enhanced in plasma from HRG-deficient mice, and accelerated clotting was restored to normal with HRG reconstitution. Although blood loss after tail tip amputation was similar in HRG-deficient and wild-type mice, carotid artery occlusion after FeCl3 injury was accelerated in HRG-deficient mice, and HRG administration abrogated this effect. To confirm that HRG modulates the contact system, we used DNase, RNase, and antisense oligonucleotides to characterize the FeCl3 model. Whereas DNase or FVII knockdown had no effect, carotid occlusion was abrogated with RNase or FXII knockdown, confirming that FeCl3-induced thrombosis is triggered by RNA in a FXII-dependent fashion. Therefore, in a nucleic acid-driven model, HRG inhibits thrombosis by modulating the intrinsic pathway of coagulation.

Selective depletion of factor XI or factor XII with antisense oligonucleotides attenuates catheter thrombosis in rabbits.

Central venous catheter thrombosis can cause venous obstruction and pulmonary embolism. To determine the extent to which catheter thrombosis is triggered by the contact or extrinsic pathway of coagulation, we used antisense oligonucleotides (ASOs) to selectively knock down factor (f)XII, fXI, or high-molecular-weight kininogen (HK), key components of the contact pathway, or fVII, which is essential for the extrinsic pathway. Knockdown of contact pathway components prolonged the activated partial thromboplastin time and decreased target protein activity levels by over 90%, whereas fVII knockdown prolonged the prothrombin time and reduced fVII activity to a similar extent. Using a rabbit model of catheter thrombosis, catheters implanted in the jugular vein were assessed daily until they occluded, up to a maximum of 35 days. Compared with control, fXII and fXI ASO treatment prolonged the time to catheter occlusion by 2.2- and 2.3-fold, respectively. In contrast, both HK and fVII knockdown did not significantly prolong the time to occlusion, and dual treatment with fVII- and fXI-directed ASOs produced a time to occlusion similar to that with the fXI ASO alone. These findings suggest that catheter thrombosis is triggered via the contact pathway and identify fXII and fXI as potential targets to attenuate this complication.

Cell-Free DNA Modulates Clot Structure and Impairs Fibrinolysis in Sepsis.

OBJECTIVES: Sepsis is characterized by systemic activation of inflammation and coagulation in response to infection. In sepsis, activated neutrophils extrude neutrophil extracellular traps composed of cell-free DNA (CFDNA) that not only trap pathogens but also provide a stimulus for clot formation. Although the effect of CFDNA on coagulation has been extensively studied, much less is known about the impact of CFDNA on fibrinolysis. To address this, we (1) investigated the relationship between CFDNA levels and fibrinolytic activity in sepsis and (2) determined the mechanisms by which CFDNA modulates fibrinolysis. APPROACH AND RESULTS: Plasma was collected from healthy and septic individuals, and CFDNA was quantified. Clot lysis assays were performed in plasma and purified systems, and lysis times were determined by monitoring absorbance. Clot morphology was assessed using scanning electron microscopy. Clots formed in plasma from septic patients containing >5 µg/mL CFDNA were dense in structure and resistant to fibrinolysis, a phenomenon overcome by deoxyribonuclease addition. These effects were recapitulated in control plasma supplemented with CFDNA. In a purified system, CFDNA delayed fibrinolysis but did not alter tissue-type plasminogen activator-induced plasmin generation. Using surface plasmon resonance, CFDNA bound plasmin with a Kd value of 4.2±0.3 µmol/L, and increasing concentrations of CFDNA impaired plasmin-mediated degradation of fibrin clots via the formation of a nonproductive ternary complex between plasmin, CFDNA, and fibrin. CONCLUSIONS: Our studies suggest that the increased levels of CFDNA in sepsis impair fibrinolysis by inhibiting plasmin-mediated fibrin degradation, thereby identifying CFDNA as a potential therapeutic target for sepsis treatment.

Reduced plasminogen binding and delayed activation render γ'-fibrin more resistant to lysis than γA-fibrin.

Fibrin (Fn) clots formed from γ'-fibrinogen (γ'-Fg), a variant with an elongated γ-chain, are resistant to lysis when compared with clots formed from the predominant γA-Fg, a finding previously attributed to differences in clot structure due to delayed thrombin-mediated fibrinopeptide (FP) B release or impaired cross-linking by factor XIIIa. We investigated whether slower lysis of γ'-Fn reflects delayed plasminogen (Pg) binding and/or activation by tissue plasminogen activator (tPA), reduced plasmin-mediated proteolysis of γ'-Fn, and/or altered cross-linking. Clots formed from γ'-Fg lysed more slowly than those formed from γA-Fg when lysis was initiated with tPA/Pg when FPA and FPB were both released, but not when lysis was initiated with plasmin, or when only FPA was released. Pg bound to γ'-Fn with an association rate constant 22% lower than that to γA-Fn, and the lag time for initiation of Pg activation by tPA was longer with γ'-Fn than with γA-Fn. Once initiated, however, Pg activation kinetics were similar. Factor XIIIa had similar effects on clots formed from both Fg isoforms. Therefore, slower lysis of γ'-Fn clots reflects delayed FPB release, which results in delayed binding and activation of Pg. When clots were formed from Fg mixtures containing more than 20% γ'-Fg, the upper limit of the normal level, the delay in lysis was magnified. These data suggest that circulating levels of γ'-Fg modulate the susceptibility of clots to lysis by slowing Pg activation by tPA and provide another example of the intimate connections between coagulation and fibrinolysis.

Zn2+ mediates high affinity binding of heparin to the αC domain of fibrinogen.

The nonspecific binding of heparin to plasma proteins compromises its anticoagulant activity by reducing the amount of heparin available to bind antithrombin. In addition, interaction of heparin with fibrin promotes formation of a ternary heparin-thrombin-fibrin complex that protects fibrin-bound thrombin from inhibition by the heparin-antithrombin complex. Previous studies have shown that heparin binds the E domain of fibrinogen. The current investigation examines the role of Zn(2+) in this interaction because Zn(2+) is released locally by platelets and both heparin and fibrinogen bind the cation, resulting in greater protection from inhibition by antithrombin. Zn(2+) promotes heparin binding to fibrinogen, as determined by chromatography, fluorescence, and surface plasmon resonance. Compared with intact fibrinogen, there is reduced heparin binding to fragment X, a clottable plasmin degradation product of fibrinogen. A monoclonal antibody directed against a portion of the fibrinogen αC domain removed by plasmin attenuates binding of heparin to fibrinogen and a peptide analog of this region binds heparin in a Zn(2+)-dependent fashion. These results indicate that the αC domain of fibrinogen harbors a Zn(2+)-dependent heparin binding site. As a consequence, heparin-catalyzed inhibition of factor Xa by antithrombin is compromised by fibrinogen to a greater extent when Zn(2+) is present. These results reveal the mechanism by which Zn(2+) augments the capacity of fibrinogen to impair the anticoagulant activity of heparin.

Batroxobin binds fibrin with higher affinity and promotes clot expansion to a greater extent than thrombin.

Batroxobin is a thrombin-like serine protease from the venom of Bothrops atrox moojeni that clots fibrinogen. In contrast to thrombin, which releases fibrinopeptide A and B from the NH2-terminal domains of the Aα- and Bß-chains of fibrinogen, respectively, batroxobin only releases fibrinopeptide A. Because the mechanism responsible for these differences is unknown, we compared the interactions of batroxobin and thrombin with the predominant γA/γA isoform of fibrin(ogen) and the γA/γ' variant with an extended γ-chain. Thrombin binds to the γ'-chain and forms a higher affinity interaction with γA/γ'-fibrin(ogen) than γA/γA-fibrin(ogen). In contrast, batroxobin binds both fibrin(ogen) isoforms with similar high affinity (Kd values of about 0.5 µM) even though it does not interact with the γ'-chain. The batroxobin-binding sites on fibrin(ogen) only partially overlap with those of thrombin because thrombin attenuates, but does not abrogate, the interaction of γA/γA-fibrinogen with batroxobin. Furthermore, although both thrombin and batroxobin bind to the central E-region of fibrinogen with a Kd value of 2-5 µM, the α(17-51) and Bß(1-42) regions bind thrombin but not batroxobin. Once bound to fibrin, the capacity of batroxobin to promote fibrin accretion is 18-fold greater than that of thrombin, a finding that may explain the microvascular thrombosis that complicates envenomation by B. atrox moojeni. Therefore, batroxobin binds fibrin(ogen) in a manner distinct from thrombin, which may contribute to its higher affinity interaction, selective fibrinopeptide A release, and prothrombotic properties.

Human platelets contain a pool of free zinc in dense granules.

Background: Activated platelets release procoagulant factors that include Ca2+ and Zn2+. Releasable Ca2+ stores have been identified in platelet dense granules and the dense tubular system, but similar stores of free Zn2+ have not been identified. Objectives: Guided by studies of platelet Ca2+, we employed minimally disruptive methods to identify and localize concentrated free Zn2+ in human platelets. Methods: Resting platelets from normal donors (NDs), patients with gray platelet syndrome (GPS) lacking α-granules, and patients with Hermansky-Pudlak syndrome (HPS) deficient in dense granules were loaded with cell-permeant fluorescent probes specific to free Ca2+ or Zn2+. Ion concentrations were detected in fixed cells as bright puncta via high-resolution confocal microscopy. Ions were also directly detected via transmission electron microscopy and energy dispersive X-ray analysis. Levels of total platelet Ca, Zn, and Mg were measured via inductively coupled plasma optical emission spectroscopy. Results: Fluorescent Zn2+ puncta counts were similar in ND and GPS platelets and markedly lower in HPS platelets, pointing to dense granules as likely reservoirs of free Zn2+. This localization was supported by direct detection of Ca2+, Zn2+, and Na+ in platelet dense granules via transmission electron microscopy and energy dispersive X-ray analysis. Measurements of total platelet Ca, Zn, and Mg via inductively coupled plasma optical emission spectroscopy indicated that free Zn2+ represents a small proportion of total platelet zinc, consistent with the strong affinity of Zn2+ for binding proteins, including several abundant in platelet α-granules. Conclusion: We conclude that normal human platelets contain a pool of free Zn2+ concentrated in dense granules that is available for secretion upon platelet activation and potentially contributes to hemostasis.

A high affinity interaction of plasminogen with fibrin is not essential for efficient activation by tissue-type plasminogen activator.

Fibrin (Fn) enhances plasminogen (Pg) activation by tissue-type plasminogen activator (tPA) by serving as a template onto which Pg and tPA assemble. To explore the contribution of the Pg/Fn interaction to Fn cofactor activity, Pg variants were generated and their affinities for Fn were determined using surface plasmon resonance (SPR). Glu-Pg, Lys-Pg (des(1-77)), and Mini-Pg (lacking kringles 1-4) bound Fn with K(d) values of 3.1, 0.21, and 24.5 µm, respectively, whereas Micro-Pg (lacking all kringles) did not bind. The kinetics of activation of the Pg variants by tPA were then examined in the absence or presence of Fn. Whereas Fn had no effect on Micro-Pg activation, the catalytic efficiencies of Glu-Pg, Lys-Pg, and Mini-Pg activation in the presence of Fn were 300- to 600-fold higher than in its absence. The retention of Fn cofactor activity with Mini-Pg, which has low affinity for Fn, suggests that Mini-Pg binds the tPA-Fn complex more tightly than tPA alone. To explore this possibility, SPR was used to examine the interaction of Mini-Pg with Fn in the absence or presence of tPA. There was 50% more Mini-Pg binding to Fn in the presence of tPA than in its absence, suggesting that formation of the tPA-Fn complex exposes a cryptic site that binds Mini-Pg. Thus, our data (a) indicate that high affinity binding of Pg to Fn is not essential for Fn cofactor activity, and (b) suggest that kringle 5 localizes and stabilizes Pg within the tPA-Fn complex and contributes to its efficient activation.

Mechanism of catheter thrombosis: comparison of the antithrombotic activities of fondaparinux, enoxaparin, and heparin in vitro and in vivo.

In patients undergoing percutaneous coronary intervention, catheter thrombosis is more frequent with fondaparinux than heparin. This study was undertaken to identify the responsible mechanism and to develop strategies for its prevention. Percutaneous coronary intervention catheter segments shortened plasma clotting times from 971 ± 92 to 352 ± 22 seconds. This activity is factor XII (fXII) dependent because it was attenuated with corn trypsin inhibitor and was abolished in fXII-deficient plasma. Heparin and enoxaparin blocked catheter-induced clotting at 0.5 and 2 anti-Xa U/mL, respectively, whereas fondaparinux had no effect. Addition of fondaparinux to bivalirudin or low-dose heparin attenuated catheter-induced clotting more than either agent alone. In a rabbit model of catheter thrombosis, a 70 anti-Xa U/kg intravenous bolus of heparin or enoxaparin prolonged the time to catheter occlusion by 4.6- and 2.5-fold, respectively, compared with saline, whereas the same dose of fondaparinux had no effect. Although 15 anti-Xa U/kg heparin had no effect on its own, when given in conjunction with 70 anti-Xa U/kg fondaparinux, the time to catheter occlusion was prolonged 2.9-fold. These findings indicate that (1) catheters are prothrombotic because they trigger fXII activation, and (2) fondaparinux does not prevent catheter-induced clotting unless supplemented with low-dose heparin or bivalirudin.

Histidine-rich glycoprotein binds factor XIIa with high affinity and inhibits contact-initiated coagulation.

Histidine-rich glycoprotein (HRG) circulates in plasma at a concentration of 2µM and binds plasminogen, fibrinogen, and thrombospondin. Despite these interactions, the physiologic role of HRG is unknown. Previous studies have shown that mice and humans deficient in HRG have shortened plasma clotting times. To better understand this phenomenon, we examined the effect of HRG on clotting tests. HRG prolongs the activated partial thromboplastin time in a concentration-dependent fashion but has no effect on tissue factor-induced clotting, localizing its effect to the contact pathway. Plasma immunodepleted of HRG exhibits a shortened activated partial thromboplastin time that is restored to baseline with HRG replenishment. To explore how HRG affects the contact pathway, we examined its binding to factors XII, XIIa, XI, and XIa. HRG binds factor XIIa with high affinity, an interaction that is enhanced in the presence of Zn²(+), but does not bind factors XII, XI, or XIa. In addition, HRG inhibits autoactivation of factor XII and factor XIIa-mediated activation of factor XI. These results suggest that, by binding to factor XIIa, HRG modulates the intrinsic pathway of coagulation, particularly in the vicinity of a thrombus where platelet release of HRG and Zn²(+) will promote this interaction.

By increasing the affinity of heparin for fibrin, Zn(2+) promotes the formation of a ternary heparin-thrombin-fibrin complex that protects thrombin from inhibition by antithrombin.

Heparin binds fibrin and, by bridging thrombin onto fibrin, promotes the formation of a ternary heparin-thrombin-fibrin complex that protects thrombin from inhibition by antithrombin. Because thrombin binds γ(A)/γ'-fibrin, a variant with an extended γ-chain, with higher affinity than the bulk γ(A)/γ(A)-fibrin, γ(A)/γ'-fibrin affords bound thrombin more protection from inhibition by antithrombin-heparin. We examined the effect of Zn(2+) on heparin-thrombin-fibrin complex formation because Zn(2+) modulates heparin-protein interactions. Zn(2+) increased the affinity of heparin for γ(A)/γ(A)- and γ(A)/γ'-fibrin by 4.3- and 3.7-fold, respectively, but had no effect on the affinity of thrombin for either form of fibrin. In contrast, in the presence of heparin, Zn(2+) increased the affinity of thrombin for γ(A)/γ(A)-fibrin 4-fold (from a K(d) value of 0.8 to 0.2 µM) and slowed the rate of thrombin dissociation from γ(A)/γ(A)-fibrin clots. These findings suggest that Zn(2+) enhances the formation of ternary heparin-thrombin-fibrin complexes with γ(A)/γ(A)-fibrin but does not influence the already high affinity interaction of thrombin with γ(A)/γ'-fibrin. Consistent with this concept, in the presence of Zn(2+), γ(A)/γ(A)-fibrin protected thrombin from inhibition by antithrombin-heparin to a similar extent as γ(A)/γ'-fibrin. Therefore, by enhancing the binding of heparin to fibrin, physiological concentrations of Zn(2+) render fibrin-bound thrombin more protected from inhibition by antithrombin. Because fibrin-bound thrombin can trigger thrombus expansion, these findings help to explain why recurrent thrombosis can occur despite heparin treatment.

Histidine-rich glycoprotein binds fibrin(ogen) with high affinity and competes with thrombin for binding to the gamma'-chain.

Histidine-rich glycoprotein (HRG) is an abundant protein that binds fibrinogen and other plasma proteins in a Zn(2+)-dependent fashion but whose function is unclear. HRG has antimicrobial activity, and its incorporation into fibrin clots facilitates bacterial entrapment and killing and promotes inflammation. Although these findings suggest that HRG contributes to innate immunity and inflammation, little is known about the HRG-fibrin(ogen) interaction. By immunoassay, HRG-fibrinogen complexes were detected in Zn(2+)-supplemented human plasma, a finding consistent with a high affinity interaction. Surface plasmon resonance determinations support this concept and show that in the presence of Zn(2+), HRG binds the predominant γ(A)/γ(A)-fibrinogen and the γ-chain elongated isoform, γ(A)/γ'-fibrinogen, with K(d) values of 9 nm. Likewise, (125)I-labeled HRG binds γ(A)/γ(A)- or γ(A)/γ'-fibrin clots with similar K(d) values when Zn(2+) is present. There are multiple HRG binding sites on fibrin(ogen) because HRG binds immobilized fibrinogen fragment D or E and γ'-peptide, an analog of the COOH terminus of the γ'-chain that mediates the high affinity interaction of thrombin with γ(A)/γ'-fibrin. Thrombin competes with HRG for γ'-peptide binding and displaces (125)I-HRG from γ(A)/γ'-fibrin clots and vice versa. Taken together, these data suggest that (a) HRG circulates in complex with fibrinogen and that the complex persists upon fibrin formation, and (b) by competing with thrombin for γ(A)/γ'-fibrin binding, HRG may modulate coagulation. Therefore, the HRG-fibrin interaction may provide a novel link between coagulation, innate immunity, and inflammation.

Identification of the histidine-rich glycoprotein domains responsible for contact pathway inhibition.

BACKGROUND: Previously, we showed that histidine-rich glycoprotein (HRG) binds factor (F) XIIa with high affinity, inhibits FXII autoactivation and FXIIa-mediated activation of FXI, and attenuates ferric chloride-induced arterial thrombosis in mice. Therefore, HRG downregulates the contact pathway in vitro and in vivo. OBJECTIVE: To identify the domains on HRG responsible for contact pathway inhibition. METHODS: Recombinant HRG domain constructs (N-terminal [N1, N2, and N1N2], proline-rich regions, histidine-rich region [HRR], and C-terminal) were expressed and purified. The affinities of plasma-derived HRG, HRG domain constructs, and synthetic HRR peptides for FXII, FXIIa, ß-FXIIa, and polyphosphate (polyP) were determined using surface plasmon resonance, and their effects on polyP-induced FXII autoactivation, FXIIa-mediated activation of FXI and prekallikrein, the activated partial thromboplastin time (APTT), and thrombin generation were examined. RESULTS: HRG and HRG domain constructs bind FXIIa, but not FXII or ß-FXII. HRR, N1, and N1N2 bind FXIIa with affinities comparable with that of HRG, whereas the remaining domains bind with lower affinity. Synthetic HRR peptides bind FXIIa and polyP with high affinity. HRG and HRR significantly inhibit FXII autoactivation and prolong the APTT. Like HRG, synthetic HRR peptides inhibit FXII autoactivation, attenuate FXIIa-mediated activation of prekallikrein and FXI, prolong the APTT, and attenuate thrombin generation. CONCLUSION: The interaction of HRG with FXIIa and polyP is predominantly mediated by the HRR domain. Like intact HRG, HRR downregulates the contact pathway and contributes to HRG-mediated down regulation of coagulation.

Long range communication between exosites 1 and 2 modulates thrombin function.

Although exosites 1 and 2 regulate thrombin activity by binding substrates and cofactors and by allosterically modulating the active site, it is unclear whether there is direct allosteric linkage between the two exosites. To begin to address this, we first titrated a thrombin variant fluorescently labeled at exosite 1 with exosite 2 ligands, HD22 (a DNA aptamer), gamma'-peptide (an analog of the COOH terminus of the gamma'-chain of fibrinogen) or heparin. Concentration-dependent and saturable changes in fluorescence were elicited, supporting inter-exosite linkage. To explore the functional consequences of this phenomenon, we evaluated the capacity of exosite 2 ligands to inhibit thrombin binding to gamma(A)/gamma(A)-fibrin, an interaction mediated solely by exosite 1. When gamma(A)/gamma(A)-fibrinogen was clotted with thrombin in the presence of HD22, gamma'-peptide, or prothrombin fragment 2 there was a dose-dependent and saturable decrease in thrombin binding to the resultant fibrin clots. Furthermore, HD22 reduced the affinity of thrombin for gamma(A)/gamma(A)-fibrin 6-fold and accelerated the dissociation of thrombin from preformed gamma(A)/gamma(A)-fibrin clots. Similar responses were obtained when surface plasmon resonance was used to monitor the interaction of thrombin with gamma(A)/gamma(A)-fibrinogen or fibrin. There is bidirectional communication between the exosites, because exosite 1 ligands, HD1 (a DNA aptamer) or hirudin-(54-65) (an analog of the COOH terminus of hirudin), inhibited the exosite 2-mediated interaction of thrombin with immobilized gamma'-peptide. These findings provide evidence for long range allosteric linkage between exosites 1 and 2 on thrombin, revealing further complexity to the mechanisms of thrombin regulation.

Multimerin 1 binds factor V and activated factor V with high affinity and inhibits thrombin generation.

Multimerin 1 (MMRN1) is a polymeric, factor V (FV) binding protein that is stored in platelet and endothelial cell secretion granules but is undetectable in normal plasma. In human platelet alpha-granules, FV is stored complexed to MMRN1, predominantly by noncovalent binding interactions. The FV binding site for MMRN1 is located in the light chain, where it overlaps the C1 and C2 domain membrane binding sites essential for activated FV (FVa) procoagulant function. Surface plasmon resonance (SPR), circular dichroism (CD) and thrombin generation assays were used to study the binding of FV and FVa to MMRN1, and the functional consequences. FV and FVa bound MMRN1 with high affinities (K(D): 2 and 7 nM, respectively). FV dissociated more slowly from MMRN1 than FVa in SPR experiments, and CD analyses suggested greater conformational changes in mixtures of FV and MMRN1 than in mixtures of FVa and MMRN1. SPR analyses indicated that soluble phosphatidylserine (1,2-Dicaproylsn-glycero-3-phospho-L-serine) competitively inhibited both FV-MMRN1 and FVa-MMRN1 binding. Furthermore, exogenous MMRN1 delayed and reduced thrombin generation by plasma and platelets, and it reduced thrombin generation by preformed FVa. Exogenous MMRN1 also delayed FV activation, triggered by adding tissue factor to plasma, or by adding purified thrombin or factor Xa to purified FV. The high affinity binding of FV to MMRN1 may facilitate the costorage of the two proteins in platelet alpha-granules. As a consequence, MMRN1 release during platelet activation may limit platelet dependent thrombin generation in vivo.

Mechanistic Basis for the Differential Effects of Rivaroxaban and Apixaban on Global Tests of Coagulation.

Rivaroxaban and apixaban are both small molecules that reversibly inhibit factor Xa. Compared with rivaroxaban, apixaban has minimal effects on the prothrombin time and activated partial thromboplastin time. To investigate this phenomenon, we used a factor Xa-directed substrate in a buffer system. Although rivaroxaban and apixaban inhibited factor Xa with similar K i values at equilibrium, kinetic measurements revealed that rivaroxaban inhibited factor Xa up to 4-fold faster than apixaban ( p < 0.001). Using a discontinuous chromogenic assay to monitor thrombin production by prothrombinase in a purified system, rivaroxaban was 4-fold more potent than apixaban (K i values of 0.7 ± 0.3 and 2.9 ± 0.5 nM, respectively; p = 0.02). Likewise, in thrombin generation assays in plasma, rivaroxaban prolonged the lag time and suppressed endogenous thrombin potential to a greater extent than apixaban. To characterize how the two inhibitors differ in recognizing factor Xa, inhibition of prothrombinase was monitored in real-time using a fluorescent probe for thrombin. The data were fit using a mixed-inhibition model and the individual association and dissociation rate constants were determined. The association rates for the binding of rivaroxaban to either free factor Xa or factor Xa incorporated into the prothrombinase complex were 10- and 1,193-fold faster than those for apixaban, respectively, whereas dissociation rates were about 3-fold faster. Collectively, these findings suggest that rivaroxaban and apixaban differ in their capacity to inhibit factor Xa and provide a plausible explanation for the observation that rivaroxaban has a greater effect on global tests of coagulation than apixaban.

Selection and characterization of a DNA aptamer inhibiting coagulation factor XIa.

Factor XIa (FXIa) is a serine protease that catalyzes the activation of Factor IX (FIX) in the blood coagulation cascade. FXIa and its precursor FXI are emergent therapeutic targets for the development of safer anticoagulant agents. Here, we sought a novel DNA-based agent to inhibit FXIa. Towards this goal, an 80 base, single-stranded DNA aptamer library (containing a 40 base randomized core) was screened for FXIa-binding candidates, using ten rounds of positive and negative selection. After selection, 6 of 89 different sequences inhibited FXIa-mediated chromogenic substrate S2366 cleavage. The most active anti-FXIa aptamer had a hypervariable central sequence 5'-AACCTATCGGACTATTGTTAGTGATTTTTATAGTGT-3' and was designated Factor ELeven Inhibitory APtamer (FELIAP). FELIAP, but not a scrambled aptamer control (SCRAPT), competitively inhibited FXIa-catalyzed S2366 cleavage, FIX activation, and complex formation with antithrombin. No effect of FELIAP on FXI activation was observed. FELIAP inhibited plasma clotting and thrombin generation assays to a significantly greater extent than SCRAPT. Immobilized FELIAP bound FXIa with strong affinity and an equilibrium binding constant (KD) in the low nanomolar range determined using surface plasmon resonance. FELIAP is the first FXIa-inhibitory aptamer to be described and constitutes a lead compound to develop related aptamers for in vivo use.