Meizothrombin is an unexpectedly zymogen-like variant of thrombin

Increased Mucosal Thrombin is Associated with Crohn's Disease and Causes Inflammatory Damage through Protease-activated Receptors Activation

BACKGROUND AND AIMS

Thrombin levels in the colon of Crohn's disease patients have recently been found to be elevated 100-fold compared with healthy controls. Our aim was to determine whether and how dysregulated thrombin activity could contribute to local tissue malfunctions associated with Crohn's disease.

METHODS

Thrombin activity was studied in tissues from Crohn's disease patients and healthy controls. Intracolonic administration of thrombin to wild-type or protease-activated receptor-deficient mice was used to assess the effects and mechanisms of local thrombin upregulation. Colitis was induced in rats and mice by the intracolonic administration of trinitrobenzene sulphonic acid.

RESULTS

Active forms of thrombin were increased in Crohn's disease patient tissues. Elevated thrombin expression and activity were associated with intestinal epithelial cells. Increased thrombin activity and expression were also a feature of experimental colitis in rats. Colonic exposure to doses of active thrombin comparable to what is found in inflammatory bowel disease tissues caused mucosal damage and tissue dysfunctions in mice, through a mechanism involving both protease-activated receptors -1 and -4. Intracolonic administration of the thrombin inhibitor dabigatran, as well as inhibition of protease-activated receptor-1, prevented trinitrobenzene sulphonic acid-induced colitis in rodent models.

CONCLUSIONS

Our data demonstrated that increased local thrombin activity, as it occurs in the colon of patients with inflammatory bowel disease, causes mucosal damage and inflammation. Colonic thrombin and protease-activated receptor-1 appear as possible mechanisms involved in mucosal damage and loss of function and therefore represent potential therapeutic targets for treating inflammatory bowel disease.

Progress Curve Analysis of the one stage chromogenic assay for ecarin

Snake venom prothrombin activators such as Ecarin are readily assayed by continuous spectrophotometric monitoring of p-nitroaniline production in a one step assay containing prothrombin and a p-nitroanilide peptide substrate for thrombin. The coupled reactions result in accelerating p-nitroaniline (pNA) production over the course of the assay giving non-linear progress curves, from which initial velocities are not readily obtained. Most studies therefore resort to approximate estimates of activity, based on the absorbance reached at an arbitrary time. A simple kinetic analysis of the coupled reactions shows that the early points of such curves should be fitted by second order polynomials, representing the accelerating reaction rate in μmol pNA/min/min. The first derivative of the polynomial then gives the increasing velocity of pNA production in μmol pNA/min over the time course of the assay. We demonstrate here that, with the substrate S2238, these rates can be converted to absolute thrombin concentrations using the Michaelis-Menten equation, substituted with values for kcat and Km. These thrombin concentrations increase linearly over the time course of the assay allowing the activity to be expressed in units, defined as μmol product/min, most commonly used to report enzyme activity.

Pharmacological Profile of JNJ-64179375: A Novel, Long-Acting Exosite-1 Thrombin Inhibitor

JNJ-64179375 (JNJ-9375) is a recombinant human IgG4 monoclonal antibody engineered to mimic an IgA antibody that was identified in a patient who exhibited markedly prolonged clotting times but without spontaneous bleeding episodes over several years of follow-up. The crystal structure of the JNJ-9375 antigen-binding fragment/thrombin complex showed an almost identical binding mode to that of the patient IgA. In the current study, we characterized the in vitro and in vivo properties of JNJ-9375. Surface plasmon resonance studies demonstrated that JNJ-9375 binds to α-thrombin with high affinity and specificity (K D: 0.8 nM for human thrombin). JNJ-9375 produced concentration-dependent prolongation of in vitro clotting assays in human plasma, including thrombin time (TT), ecarin clotting time, prothrombin time, and activated partial thromboplastin time, with EC2X values of 4.4, 12.4, 172.6, and 202.7 µg/ml, respectively. JNJ-9375 inhibited thrombin-induced platelet aggregation in human plasma with an IC50 value of 52.6 nM (7.8 µg/ml) and produced concentration-dependent prolongation of reaction time tested by thromboelastography. JNJ-9375 pretreatment resulted in dose-dependent reduction in thrombus formation in the rat arteriovenous (AV) shunt model of thrombosis. Robust efficacy was observed at 0.3 mg/kg accompanied by 1.5× of TT. Bleeding was increased at 3 mg/kg in a rat tail transection bleeding model demonstrating a therapeutic index of 10× compared with 1× for apixaban in the same models. Our data suggest that thrombin exosite I inhibition is efficacious against thrombosis in a pretreatment prevention animal model. SIGNIFICANCE STATEMENT: JNJ-9375 is a novel, fully human monoclonal antibody that binds to the exosite I region of thrombin with high affinity and specificity. JNJ-9375 concentration dependently prolonged clotting times and inhibited thrombin-induced platelet aggregation in in vitro assays based on its mechanism of action. In an in vivo rat AV shunt model, JNJ-9375 prevented thrombus formation in a dose-dependent fashion while demonstrating reduced bleeding risk. The present study demonstrated the antithrombotic effects of inhibiting the exosite I region of thrombin when given in a prevention mode in preclinical animal models.

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

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

Advances in Clinical and Basic Science of Coagulation: Illustrated abstracts of the 9th Chapel Hill Symposium on Hemostasis

This 9th Symposium on Hemostasis is an international scientific meeting held biannually in Chapel Hill, North Carolina. The meeting is in large measure the result of the close friendship between the late Dr. Harold R. Roberts of UNC Chapel Hill and Dr. Ulla Hedner of Novo Nordisk. When Novo Nordisk was developing the hemophilia therapy that would become NovoSeven, they sponsored a series of meetings to understand the basic biology and clinical applications of factor VIIa. The first meeting in Chapel Hill was held April 4-6, 2002 with Dr. Roberts as the organizer. Over the years, the conference emphasis has expanded from discussions of factor VIIa and tissue factor to additional topics in hemostasis and thrombosis. This year's meeting includes presentations by internationally renowned speakers that discuss the state-of-the-art on an array of important topics, including von Willebrand factor, engineering advances, coagulation and disease, tissue factor biology, therapeutic advances, and basic clotting factor biology. Included in this review article are illustrated abstracts provided by our speakers, which highlight the main conclusions of each invited talk. This will be the first meeting without Dr. Roberts in attendance, yet his commitment to excellent science and his focus on turning science to patient care are pervasively reflected in the presentations by our speakers.

The Fragment 1 Region of Prothrombin Facilitates the Favored Binding of Fragment 12 to Zymogen and Enforces Zymogen-like Character in the Proteinase

Thrombin is produced from the C-terminal half of prothrombin following its proteolytic activation. The N-terminal half, released as the propiece Fragment 12 (F12), is composed of an N-terminal γ-carboxyglutamate domain (Gla) followed by two kringles (K1 and K2). The propiece plays essential roles in regulating prothrombin activation and proteinase function. The latter results from the ability of F12 to reversibly bind to the (pro)catalytic domain through K2 with high affinity and highly favorable thermodynamic constants when it is a zymogen in comparison to proteinase. Such discrimination is lost for K2 binding after proteolytic removal of the N-terminal Gla-K1 region of F12. The Ca(2+)-stabilized structure of the Gla domain is not required for F12 to bind the zymogen form more favorably. Enhanced binding to zymogen versus proteinase correlates with the ability of the propiece to enforce zymogen-like character in the proteinase. This is evident in variants of meizothrombin, an intermediate of prothrombin activation that contains the propiece covalently attached. This phenomenon is also independent of the Gla domain. Thus, the presence of K1 in covalent linkage with K2 in the propiece governs the ability of K2 to bind the (pro)catalytic domain in favor of zymogen, thereby enforcing zymogen-like character in the proteinase.

MASP-1 Induced Clotting--The First Model of Prothrombin Activation by MASP-1

Mannan-binding lectin-associated serine protease-1 (MASP-1), a protein of the complement lectin pathway, resembles thrombin in terms of structural features and substrate specificity. Due to its interplay with several coagulation factors, it has the ability to induce fibrin clot formation independent of the usual coagulation activation pathways. We have recently shown that MASP-1 activates prothrombin and identified arginine (R) 155, R271, and R393 as potential cleavage sites. FXa cleaves R320 instead of R393, and thrombin cleaves R155 and R284 in prothrombin. Here we have used three arginine-to-glutamine mutants of prothrombin, R271Q, R320Q, R393Q and the serine-to-alanine active site mutant S525A to investigate in detail the mechanism of MASP-1 mediated prothrombin activation. Prothrombin wildtype and mutants were digested with MASP-1 and the cleavage products were analysed by SDS-PAGE and N-terminal sequencing. A functional clotting assay was performed by thrombelastography. We have found that MASP-1 activates prothrombin via two simultaneous pathways, either cleaving at R271 or R393 first. Both pathways result in the formation of several active alternative thrombin species. Functional studies confirmed that both R393 and R320 are required for prothrombin activation by MASP-1, whereas R155 is not considered to be an important cleavage site in this process. In conclusion, we have described for the first time a detailed model of prothrombin activation by MASP-1.

Membrane binding by prothrombin mediates its constrained presentation to prothrombinase for cleavage

Long-standing dogma proposes a profound contribution of membrane binding by prothrombin in determining the rate at which it is converted to thrombin by prothrombinase. We have examined the action of prothrombinase on full-length prothrombin variants lacking γ-carboxyglutamate modifications (desGla) with impaired membrane binding. We show an unexpectedly modest decrease in the rate of thrombin formation for desGla prothrombin but with a major effect on the pathway for substrate cleavage. Using desGla prothrombin variants in which the individual cleavage sites have been singly rendered uncleavable, we find that loss of membrane binding and other Gla-dependent functions in the substrate leads to a decrease in the rate of cleavage at Arg(320) and a surprising increase in the rate of cleavage at Arg(271). These compensating effects arise from a loss in the membrane component of exosite-dependent tethering of substrate to prothrombinase and a relaxation in the constrained presentation of the individual cleavage sites for active site docking and catalysis. Loss of constraint is evident as a switch in the pathway for prothrombin cleavage and the intermediate produced but without the expected profound decrease in rate. Extension of these findings to the action of prothrombinase assembled on platelets and endothelial cells on fully carboxylated prothrombin reveals new mechanistic insights into function on physiological membranes. Cell-dependent enzyme function is probably governed by a differential ability to support prothrombin binding and the variable accumulation of intermediates from the two possible pathways of prothrombin activation.

Crystal structure of the prothrombinase complex from the venom of Pseudonaja textilis

The prothrombinase complex, composed of the protease factor (f)Xa and cofactor fVa, efficiently converts prothrombin to thrombin by specific sequential cleavage at 2 sites. How the complex assembles and its mechanism of prothrombin processing are of central importance to human health and disease, because insufficient thrombin generation is the root cause of hemophilia, and excessive thrombin production results in thrombosis. Efforts to determine the crystal structure of the prothrombinase complex have been thwarted by the dependence of complex formation on phospholipid membrane association. Pseutarin C is an intrinsically stable prothrombinase complex preassembled in the venom gland of the Australian Eastern Brown Snake (Pseudonaja textilis). Here we report the crystal structures of the fX-fV complex and of activated fXa from P textilis venom and the derived model of active pseutarin C. Structural analysis supports a single substrate binding channel on fVa, to which prothrombin and the intermediate meizothrombin bind in 2 different orientations, providing insight into the architecture and mechanism of the prothrombinase complex-the molecular engine of blood coagulation.

The transition of prothrombin to thrombin

The proteolytic conversion of prothrombin to thrombin catalyzed by prothrombinase is one of the more extensively studied reactions of blood coagulation. Sophisticated biophysical and biochemical insights into the players of this reaction were developed in the early days of the field. Yet, many basic enzymological questions remained unanswered. I summarize new developments that uncover mechanisms by which high substrate specificity is achieved, and the impact of these strategies on enzymic function. Two principles emerge that deviate from conventional wisdom that has otherwise dominated thinking in the field. (i) Enzymic specificity is dominated by the contribution of exosite binding interactions between substrate and enzyme rather than by specific recognition of sequences flanking the scissile bond. Coupled with the regulation of substrate conformation as a result of the zymogen to proteinase transition, novel mechanistic insights result for numerous aspects of enzyme function. (ii) The transition of zymogen to proteinase following cleavage is not absolute and instead, thrombin can reversibly interconvert between zymogen-like and proteinase-like forms depending on the complement of ligands bound to it. This establishes new paradigms for considering proteinase allostery and how enzyme function may be modulated by ligand binding. These insights into the action of prothrombinase on prothrombin have wide-ranging implications for the understanding of function in blood coagulation.

Ligand binding to anion-binding exosites regulates conformational properties of thrombin

Thrombin participates in coagulation, anticoagulation, and initiation of platelet activation. To fulfill its diverse roles and maintain hemostasis, this serine protease is regulated via the extended active site region and anion-binding exosites (ABEs) I and II. For the current project, amide proton hydrogen-deuterium exchange coupled with MALDI-TOF mass spectrometry was used to characterize ligand binding to individual exosites and to investigate the presence of exosite-active site and exosite-exosite interactions. PAR3(44-56) and PAR1(49-62) were observed to bind to thrombin ABE I and then to exhibit long range effects over to ABE II. By contrast, Hirudin(54-65) focused more on ABE I and did not transmit influences over to ABE II. Although these three ligands were each directed to ABE I, they did not promote the same conformational consequences. D-Phe-Pro-Arg-chloromethyl ketone inhibition at the thrombin active site led to further local and long range consequences to thrombin-ABE I ligand complexes with the autolysis loop often most affected. When Hirudin(54-65) was bound to ABE I, it was still possible to bind GpIbα(269-286) or fibrinogen γ'(410-427) to ABE II. Each ligand exerted its predominant influences on thrombin and also allowed interexosite communication. The results obtained support the proposal that thrombin is a highly dynamic protein. The transmission of ligand-specific local and long range conformational events is proposed to help regulate this multifunctional enzyme.