The role of high molecular weight kininogen and prothrombin as cofactors in the binding of factor XI A3 domain to the platelet surface

Factor XI: structure, function and therapeutic inhibition

Arterial and venous thromboembolism is a major medical concern that requires therapeutic anticoagulation in various medical fields to prevent its drastic consequences. Despite significant advances in anticoagulant therapy, thrombosis remains a leading cause of morbidity and mortality worldwide. Traditional anticoagulants like heparin and vitamin K antagonists (VKAs) have shown efficacy in preventing and treating thrombosis but come with an inherent risk of bleeding due to their non-specific inhibition of multiple coagulation factors. Subsequent direct oral anticoagulants (DOACs), targeting specific factors such as Xa or thrombin, demonstrated improved safety profiles compared to VKAs, yet bleeding remains a concern. Accordingly, research is focused on developing anticoagulants with improved safety profiles. A safer class of anticoagulants would have broad appeal. The intrinsic pathway of coagulation, involving factor XI (FXI), has attracted attention as a potential target for safer anticoagulants. Preclinical studies and epidemiological data indicate that FXI deficiency or inhibition protects against thrombosis with minimal bleeding. Current research involves evaluating various FXI-directed strategies, and phase 2 studies have shown promising results in orthopedic surgery, atrial fibrillation, end-stage renal disease (ESRD), myocardial infarction, and ischemic stroke. Several agents, such as antisense oligonucleotides, monoclonal antibodies, small synthetic molecules, natural peptides, and aptamers, have been developed to inhibit FXI at different stages, offering potentially safer alternatives to traditional anticoagulants. However, the optimal balance between preventing thrombosis and the risk of bleeding associated with FXI inhibitors requires validation through extensive phase 3 clinical trials using definite clinical endpoints. Several of such trials are currently underway or planned to define the role of FXI inhibitors in clinical practice and determine the most suitable FXI inhibitor for each specific indication. The current review highlights the rationale behind developing FXI inhibitors, presenting the most advanced agents in development, summarizing completed clinical trials, and discussing ongoing research efforts.

How to Capture the Bleeding Phenotype in FXI-Deficient Patients

Factor XI (FXI) is a serine protease involved in the propagation phase of coagulation and in providing clot stability. Several mutations in the F11 gene lead to FXI deficiency, a rare mild bleeding disorder. Current laboratory methods are unable to assess bleeding risk in FXI-deficient patients, because the degree of bleeding tendency does not correlate with plasma FXI activity as measured by routine coagulometric aPTT–based assays. Bleeding manifestations are highly variable among FXI-deficient patients and FXI replacement therapy can be associated with an increased thrombotic risk. A correct evaluation of the patient hemostatic potential is crucial to prevent under- or overtreatment. In recent years, different research groups have investigated the use of global coagulation assays as alternative for studying the role of FXI in hemostasis and identifying the clinical phenotype of FXI deficiency. This brief review article summarizes the main features of coagulation factor XI and its deficiency and resumes the principle axes of research and methods used to investigate FXI functions.

Interplay between platelets and coagulation

Haemostasis stops bleeding at the site of vascular injury and maintains the integrity of blood vessels through clot formation. This regulated physiological process consists of complex interactions between endothelial cells, platelets, von Willebrand factor and coagulation factors. Haemostasis is initiated by a damaged vessel wall, followed with a rapid adhesion, activation and aggregation of platelets to the exposed subendothelial extracellular matrix. At the same time, coagulation factors aggregate on the procoagulant surface of activated platelets to consolidate the platelet plug by forming a mesh of cross-linked fibrin. Platelets and coagulation mutually influence each other and there are strong indications that, thanks to the interplay between platelets and coagulation, haemostasis is far more effective than the two processes separately. Clinically this is relevant because impaired interaction between platelets and coagulation may result in bleeding complications, while excessive platelet-coagulation interaction induces a high thrombotic risk. In this review, platelets, coagulation factors and the complex interaction between them will be discussed in detail.

The dimeric structure of factor XI and zymogen activation

Factor XI (fXI) is a homodimeric zymogen that is converted to a protease with 1 (1/2-fXIa) or 2 (fXIa) active subunits by factor XIIa (fXIIa) or thrombin. It has been proposed that the dimeric structure is required for normal fXI activation. Consistent with this premise, fXI monomers do not reconstitute fXI-deficient mice in a fXIIa-dependent thrombosis model. FXI activation by fXIIa or thrombin is a slow reaction that can be accelerated by polyanions. Phosphate polymers released from platelets (poly-P) can enhance fXI activation by thrombin and promote fXI autoactivation. Poly-P increased initial rates of fXI activation 30- and 3000-fold for fXIIa and thrombin, respectively. FXI monomers were activated more slowly than dimers by fXIIa in the presence of poly-P. However, this defect was not observed when thrombin was the activating protease, nor during fXI autoactivation. The data suggest that fXIIa and thrombin activate fXI by different mechanisms. FXIIa may activate fXI through a trans-activation mechanism in which the protease binds to 1 subunit of the dimer, while activating the other subunit. For activation by thrombin, or during autoactivation, the data support a cis-activation mechanism in which the activating protease binds to and activates the same fXI subunit.

The many faces of the contact pathway and their role in thrombosis

Understanding inherent differences between thrombosis and hemostasis in the vascular system are critical to developing safe and effective anticoagulants. To this end, constituents of the contact activated and intrinsic pathway of coagulation appear to be involved in pathological thrombus formation, but are not required for normal hemostasis. In addition to coagulation, activation of the contact system is involved in fibrinolytic, inflammatory, and angiogenic processes that can also contribute to the thrombotic environment. This review discusses the role of the contact system in these processes, and highlights the potential of FXII and FXI as safer targets for antithrombotic therapy.

Molecular characterization of FXI deficiency

Factor XI (FXI) deficiency is a rare autosomal bleeding disease associated with genetic defects in the FXI gene. It is a heterogeneous disorder with variable tendency in bleeding and variable causative FXI gene mutations. It is characterized as a cross-reacting material-negative (CRM-) FXI deficiency due to decreased FXI levels or cross-reacting material-positive (CRM+) FXI deficiency due to impaired FXI function. Increasing number of mutations has been reported in FXI mutation database, and most of the mutations are affecting serine protease (SP) domain of the protein. Functional characterization for the mutations helps to better understand the molecular basis of FXI deficiency. Prevalence of the disease is higher in certain populations such as Ashkenazi Jews. The purpose of this review is to give an overview of the molecular basis of congenital FXI deficiency.

Structure and function of factor XI

Factor XI (FXI) is the zymogen of an enzyme (FXIa) that contributes to hemostasis by activating factor IX. Although bleeding associated with FXI deficiency is relatively mild, there has been resurgence of interest in FXI because of studies indicating it makes contributions to thrombosis and other processes associated with dysregulated coagulation. FXI is an unusual dimeric protease, with structural features that distinguish it from vitamin K-dependent coagulation proteases. The recent availability of crystal structures for zymogen FXI and the FXIa catalytic domain have enhanced our understanding of structure-function relationships for this molecule. FXI contains 4 "apple domains" that form a disk structure with extensive interfaces at the base of the catalytic domain. The characterization of the apple disk structure, and its relationship to the catalytic domain, have provided new insight into the mechanism of FXI activation, the interaction of FXIa with the substrate factor IX, and the binding of FXI to platelets. Analyses of missense mutations associated with FXI deficiency have provided additional clues to localization of ligand-binding sites on the protein surface. Together, these data will facilitate efforts to understand the physiology and pathology of this unusual protease, and development of therapeutics to treat thrombotic disorders.

Identification of coagulation factor XI as a ligand for platelet apolipoprotein E receptor 2 (ApoER2)

OBJECTIVE

Factor XI (FXI) promotes hemostasis and thrombosis through enhancement of thrombin generation and has been shown to play a critical role in the formation of occlusive thrombi in arterial injury models. The aim of this study was to investigate the mechanisms governing interactions between FXI and platelets.

METHODS AND RESULTS

Platelet adhesion to immobilized FXI was abrogated in the presence of the low-density lipoprotein (LDL) receptor antagonist, receptor-associated protein (RAP), soluble recombinant apolipoprotein E receptor 2 (ApoER2), or the LDL-binding domain 1 or 2 of ApoER2. FXI supported wild-type murine platelet binding; in contrast, ApoER2-deficient murine platelets did not adhere to FXI. In the presence of shear, platelet aggregates formed on FXI or activated FXI (FXIa) surfaces, whereas the presence of RAP, binding domain 1 of ApoER2, or an anti-GPIb alpha mAb blocked platelet adhesion to FXI or FXIa under shear. Soluble FXI bound to immobilized ApoER2' with an affinity of 61 nmol/L.

CONCLUSIONS

This study has identified apolipoprotein E receptor 2 (ApoER2, LRP8), a member of the LDL receptor family, as a platelet receptor for FXI. The interaction of FXI with other cell types that express ApoER2 remains to be explored.

Cloning of the kininogen gene from Lampetra japonica provides insights into its phylogeny in vertebrates

Kininogens, the precursors of bradykinins, ubiquitously exist in vertebrates, including mammals, birds, amphibians, and fishes. To elucidate the phylogeny of kininogen genes in early vertebrates, we cloned the full-length cDNA of kininogen gene from the liver of Lampetra japonica. The open reading frame of this sequence contained 546 bp and encoded 181 amino acids, including a cystatin domain without the canonical binding site for cysteine proteinases and a bradykinin domain. Our results suggested that in lampreys and most of other vertebrates, there might be only one kininogen gene, which was fused by certain sequences during vertebrate evolution and encoded proteins with more functions; however, another special kininogen gene, only encoding the bradykinin domain with multiple copies in some species, arose only in amphibians for adapting themselves to the unique environment. Using reverse transcription PCR, kininogen mRNA was also detected in lamprey gut, kidney, and leukocyte, but absent in lamprey buccal gland. Our findings may provide insights into the phylogeny of kininogen genes as well as other gene families in vertebrates.

Characterization of Novel Forms of Coagulation Factor XIa: independence of factor XIa subunits in factor IX activation

Factor XI is the zymogen of a dimeric plasma protease, factor XIa, with two active sites. In solution, and during contact activation in plasma, conversion of factor XI to factor XIa proceeds through an intermediate with one active site (1/2-FXIa). Factor XIa and 1/2-FXIa activate the substrate factor IX, with similar kinetic parameters in purified and plasma systems. During hemostasis, factor IX is activated by factors XIa or VIIa, by cleavage of the peptide bonds after Arg145 and Arg180. Factor VIIa cleaves these bonds sequentially, with accumulation of factor IX alpha, an intermediate cleaved after Arg145. Factor XIa also cleaves factor IX preferentially after Arg145, but little intermediate is detected. It has been postulated that the two factor XIa active sites cleave both factor IX peptide bonds prior to releasing factor IX abeta. To test this, we examined cleavage of factor IX by four single active site factor XIa proteases. Little intermediate formation was detected with 1/2-FXIa, factor XIa with one inhibited active site, or a recombinant factor XIa monomer. However, factor IX alpha accumulated during activation by the factor XIa catalytic domain, demonstrating the importance of the factor XIa heavy chain. Fluorescence titration of active site-labeled factor XIa revealed a binding stoichiometry of 1.9 +/- 0.4 mol of factor IX/mol of factor XIa (Kd = 70 +/- 40 nm). The results indicate that two forms of activated factor XI are generated during coagulation, and that each half of a factor XIa dimer behaves as an independent enzyme with respect to factor IX.

Microneme proteins in apicomplexans

Microneme secretion supports several key cellular processes including gliding motility, active cell invasion and migration through cells, biological barriers, and tissues. The modular design of microneme proteins enables these molecules to assist each other in folding and passage through the quality control system, accurately target to the micronemes, oligimerizing with other parasite proteins, and engaging a variety of host receptors for migration and cell invasion. Structural and biochemical analyses of MIC domains is providing new perspectives on how adhesion is regulated and the potentially distinct roles MICs might play in long or short range interactions during parasite attachment and entry. New access to complete genome sequences and ongoing advances in genetic manipulation should provide fertile ground for refining current models and defining exciting new roles for MICs in apicomplexan biology.

A catalytic domain exosite (Cys527-Cys542) in factor XIa mediates binding to a site on activated platelets

The zymogen, factor XI, and the enzyme, factor XIa, interact specifically with functional receptors on the surface of activated platelets. These studies were initiated to identify the molecular subdomain within factor XIa that binds to activated platelets. Both factor XIa (Ki approximately 1.4 nM) and a chimeric factor XIa containing the Apple 3 domain of prekallikrein (Ki approximately 2.7 nM) competed with [125I]factor XIa for binding sites on activated platelets, suggesting that the factor XIa binding site for platelets is not located in the Apple 3 domain which mediates factor XI binding to platelets. The recombinant catalytic domain (Ile370-Val607) inhibited the binding of [125I]factor XIa to the platelets (Ki approximately 3.5 nM), whereas the recombinant factor XI heavy chain did not, demonstrating that the platelet binding site is located in the light chain of factor XIa. A conformationally constrained cyclic peptide (Cys527-Cys542) containing a high-affinity (KD approximately 86 nM) heparin-binding site within the catalytic domain of factor XIa also displaced [125I]factor XIa from the surface of activated platelets (Ki approximately 5.8 nM), whereas a scrambled peptide of identical composition was without effect, suggesting that the binding site in factor XIa that interacts with the platelet surface resides in the catalytic domain near the heparin binding site of factor XIa. These data support the conclusion that a conformational transition accompanies conversion of factor XI to factor XIa that conceals the Apple 3 domain factor XI (zymogen) platelet binding site and exposes the factor XIa (enzyme) platelet binding site within the catalytic domain possibly comprising residues Cys527-Cys542.

Assembly, activation, and physiologic influence of the plasma kallikrein/kinin system

The plasma kallikrein/kinin system that consists of the proteins factor XII, prekallikrein, and high molecular weight kininogen was first recognized as a surface-activated coagulation system arising when blood or plasma interacts with artificial surfaces. Although surface-activated contact activation occurs in vivo when various negatively charged surfaces become exposed, including a developing platelet thrombus, a physiologic, non-injury mechanism for activation, regulation, and function of this system has been elusive. Recent investigations have shown that there is a physiologic pathway for assembly and activation of this system independent of factor XII. Gene deficient mice of the bradykinin B2 receptor and factor XII have been recognized to have reduced risk for arterial thrombosis. This plasma proteolytic system influences arterial thrombosis independent of influencing hemostasis. Thus, the plasma kallikrein/kinin system has two mechanisms for its activation: one that is dependent and another independent of factor XII. Better understanding of this system may lead to insight into mechanisms for arterial thrombosis, independent of hemostasis.

Domain V of β2-Glycoprotein I Binds Factor XI/XIa and Is Cleaved at Lys317-Thr318

The fifth domain (DV) of beta2-glycoprotein I (beta2GPI) is important for binding a number of ligands including phospholipids and factor XI (FXI). Beta2GPI is proteolytically cleaved in DV by plasmin but not by thrombin, VIIa, tissue plasminogen activator, or uPA. Following proteolytic cleavage of DV by plasmin, beta2GPI retains binding to FXI but not to phospholipids. Native beta2GPI, but not cleaved beta2GPI, inhibits activation of FXI by thrombin and factor XIIa, attenuating a positive feedback mechanism for additional thrombin generation. In this report, we have defined the FXI/FXIa binding site on beta2GPI using site-directed mutagenesis. We show that the positively charged residues Lys284, Lys286, and Lys287 in DV are essential for the interaction of beta2GPI with FXI/FXIa. We also demonstrate that FXIa proteolytically cleaves beta2GPI at Lys317-Thr318 in DV. Thus, FXIa cleavage of beta2GPI in vivo during thrombus formation may accelerate FXI activation by decreasing the inhibitory effect of beta2GPI.

Structural interpretation of 42 mutations causing factor XI deficiency using homology modeling

BACKGROUND

Factor (F)XI is important in the consolidation phase of blood coagulation. The structural effects of mutations causing FXI deficiency have not been well described due to the lack of a structure for FXI.

OBJECTIVES

To develop molecular models of the four apple (Ap) and serine protease (SP) domains in FXI in order to assess the structural effects of published FXI mutations in the light of their phenotypes.

METHODS

The Ap domains were modeled using the NMR structure of an adhesin from Eimeria tenella. The SP domain was modeled using the crystal structure of beta-tryptase.

RESULTS

The effect of 42 mutations causing FXI deficiency was analyzed using homology models for the Ap and SP domains in FXI. Protein misfolding was implicated as the likely structural mechanism of disease in six of 14 mutations in the four Ap domains with Type I phenotypes. Likewise, misfolding was implicated in eight of 14 mutations in the SP domain with Type I phenotypes. Unlike other coagulation factor deficiencies, Type II phenotypes based on a catalytically dysfunctional FXI are uncommon. The structural models indicated that two known Type II mutations in the Ap domains could be correlated with functional defects in substrate or cofactor binding, and likewise four Type II mutations in the SP domain would disrupt the active site.

CONCLUSIONS

New FXI disease-causing mutations can now be structurally characterized to complement phenotypic data, and expression studies can be designed to verify the molecular basis of each deficiency.

Identification of a binding site for glycoprotein Ibalpha in the Apple 3 domain of factor XI

Factor XI (FXI) is a homodimeric plasma zymogen that is cleaved at two internal Arg(369)-Ile(370) bonds by thrombin, factor XIIa, or factor XIa. FXI circulates as a complex with the glycoprotein high molecular weight kininogen (HK). FXI binds to specific sites (K(d) = approximately 10 nM, B(max) = approximately 1,500/platelet) on the surface of stimulated platelets, where it is efficiently activated by thrombin. The FXI Apple 3 (A3) domain mediates binding to platelets in the presence of HK and zinc ions (Zn(2+)) or prothrombin and calcium ions. The platelet glycoprotein (GP) Ib-IX-V complex is the receptor for FXI. Using surface plasmon resonance, we determined that FXI binds specifically to glycocalicin, the extracellular domain of GPIbalpha, in a Zn(2+)-dependent fashion (K(d) = approximately 52 nM). We now show that recombinant FXI A3 domain inhibits FXI inbinding to glycocalicin in the presence of Zn(2+), whereas the recombinant FXI A1, A2, or A4 domains have no effect. Experiments with full-length recombinant FXI mutants show that, in the presence of Zn(2+), glycocalicin binds FXI at a heparin-binding site in A3 (Lys(252) and Lys(253)) and not by amino acids previously shown to be required for platelet binding (Ser(248), Arg(250), Lys(255), Phe(260), and Gln(263)). However, binding in the presence of HK and Zn(2+) requires Ser(248), Arg(250), Lys(255), Phe(260), and GLn(263) and not Lys(252) and Lys(253). Thus, binding of FXI to GPIbalpha is mediated by amino acids in the A3 domain in the presence or absence of HK. This interaction is important for the initiation of the consolidation phase of blood coagulation and the generation of thrombin at sites of platelet thrombus formation.

Factor XI deficiency

Factor XI (FXI) deficiency leads to an injury-related bleeding diathesis, which is notable for the variability in the bleeding tendency and the lack of a clear relationship between bleeding and FXI coagulant activity. Bleeding in this disorder occurs especially in areas of high fibrinolytic activity. Although a rare disorder, the frequency of FXI deficiency is high in certain populations, notably persons of Ashkenazi descent and the Basque population of Southern France. In these populations, five mutations of the FXI gene have been identified and a founder effect has been confirmed for three of these. This paper reviews the role of FXI in coagulation and documents factors known to modify the bleeding tendency. Treatment of surgical bleeding in patients with FXI deficiency is reviewed with emphasis on the combined use of recombinant activated factor VII (rFVIIa; NovoSeven(R), Novo Nordisk, Bagsvaerd, Denmark) and the antifibrinolytic agent, tranexamic acid.

Thrombin activation of factor XI on activated platelets requires the interaction of factor XI and platelet glycoprotein Ib alpha with thrombin anion-binding exosites I and II, respectively

Activation of factor XI (FXI) by thrombin on stimulated platelets plays a physiological role in hemostasis, providing additional thrombin generation required in cases of severe hemostatic challenge. Using a collection of 53 thrombin mutants, we identified 16 mutants with <50% of the wild-type thrombin FXI-activating activity in the presence of dextran sulfate. These mutants mapped to anion-binding exosite (ABE) I, ABE-II, the Na+-binding site, and the 50-insertion loop. Only the ABE-II mutants showed reduced binding to dextran sulfate-linked agarose. Selected thrombin mutants in ABE-I (R68A, R70A, and R73A), ABE-II (R98A, R245A, and K248A), the 50-insertion loop (W50A), and the Na+-binding site (E229A and R233A) with <10% of the wild-type activity also showed a markedly reduced ability to activate FXI in the presence of stimulated platelets. The ABE-I, 50-insertion loop, and Na+-binding site mutants had impaired binding to FXI, but normal binding to glycocalicin, the soluble form of glycoprotein Ibalpha (GPIb alpha). In contrast, the ABE-II mutants were defective in binding to glycocalicin, but displayed normal binding to FXI. Our data support a quaternary complex model of thrombin activation of FXI on stimulated platelets. Thrombin bound to one GPIb alpha molecule, via ABE-II on its posterior surface, is properly oriented for its activation of FXI bound to a neighboring GPI alpha molecule, via ABE-I on its anterior surface. GPIb alpha plays a critical role in the co-localization of thrombin and FXI and the resultant efficient activation of FXI.

The relative priority of prekallikrein and factors XI/XIa assembly on cultured endothelial cells

Investigations determined the relative preference of prekallikrein (PK) or factor XI/XIa (FXI/FXIa) binding to endothelial cells (HUVECs). In microtiter plates, biotinylated high molecular weight kininogen (biotin-HK) or biotin-FXI binding to HUVEC monolayers or their matrix proteins, but not fibronectin-coated plastic microtiter plate wells, was specifically blocked by antibodies to each of the receptors of HK, uPAR, gC1qR, or cytokeratin 1. Fluorescein isothiocyanate (FITC)-PK specifically bound to HUVEC suspensions without added Zn2+, whereas FITC-FXI or -FXIa binding to HUVEC suspensions required 10 microM added Zn2+ to support specific binding. Plasma concentrations of FXI did not block FITC-PK binding to HUVECs in the absence or presence of 10 microM Zn2+. In the absence of HK, the level of FITC-FXI or -FXIa binding was half that seen in its presence. At physiologic concentrations, PK (450 nM) abolished FITC-FXI or -FXIa binding to HUVEC suspensions in the absence or presence of HK in the presence of 10 microM Zn2+. Released Zn2+ from 2-8 x 10(8) collagen-activated platelets/ml supported biotin-FXI binding to HUVEC monolayers, but platelet activation was not necessary to support biotin-PK binding to HUVECs. At physiologic concentrations, PK also abolished FXI binding to HUVECs in the presence of activated platelets, but FXI did not influence PK binding. PK in the presence or absence of HK preferentially bound to HUVECs over FXI or FXIa. Elevated Zn2+ concentrations are required for FXI but not PK binding, but the presence of physiologic concentrations of PK and HK also prevented FXI binding. PK preferential binding to endothelial cells contributes to their anticoagulant nature.

Factor XI apple domains and protein dimerization

The coagulation protease zymogen factor (F)XI is a disulfide bond-linked homodimer, a configuration that is necessary for protein secretion and function. The non-catalytic portion of the FXI polypeptide contains four repeats called apple domains (A1-A4). It is clear that FXI A4 plays a key role in dimer formation, however, the importance of other apple domains to this process has not been examined. We prepared recombinant FXI molecules in which apple domains were exchanged with those of the structurally homologous monomeric protein prekallikrein (PK). As expected, FXI/PK chimeras containing FXI A4 are dimers, while those with PK A4 are monomers. FXI A4 contains cysteine at position 321 that forms the interchain disulfide bond, while Cys321 in PK is unavailable for interchain bond formation because it is paired with Cys326. FXI/PK chimeras containing PK A4 were modified by changing Cys326 to glycine, leaving Cys321 unpaired (PKA4-Gly326). FXI with a PK A4 domain is a monomer, however, introducing PKA4-Gly326 results in a disulfide bond-linked dimer. This indicates that dimer formation can occur in the absence of FXI A4. In proteins containing PKA4-Gly326, replacing FXI A3 with PK A3 partially interferes with dimer formation, while substitution of A2, or A2 and A3 prevents dimer formation. PKA4-Gly326 cannot induce the native PK molecule to dimerize. The data indicate that FXI A2 and A3 make contributions to dimer formation. As these domains are involved in activities that require dimeric protein, it seems reasonable that they stabilize this conformation.

Glycosaminoglycans affect the interaction of human plasma kallikrein with plasminogen, factor XII and inhibitors

Human plasma kallikrein, a serine proteinase, plays a key role in intrinsic blood clotting, in the kallikrein-kinin system, and in fibrinolysis. The proteolytic enzymes involved in these processes are usually controlled by specific inhibitors and may be influenced by several factors including glycosaminoglycans, as recently demonstrated by our group. The aim of the present study was to investigate the effect of glycosaminoglycans (30 to 250 micro/ml) on kallikrein activity on plasminogen and factor XII and on the inhibition of kallikrein by the plasma proteins C1-inhibitor and antithrombin. Almost all available glycosaminoglycans (heparin, heparan sulfate, bovine and tuna dermatan sulfate, chondroitin 4- and 6-sulfates) reduced (1.2 to 3.0 times) the catalytic efficiency of kallikrein (in a nanomolar range) on the hydrolysis of plasminogen (0.3 to 1.8 microM) and increased (1.9 to 7.7 times) the enzyme efficiency in factor XII (0.1 to 10 microM) activation. On the other hand, heparin, heparan sulfate, and bovine and tuna dermatan sulfate improved (1.2 to 3.4 times) kallikrein inhibition by antithrombin (1.4 microM), while chondroitin 4- and 6-sulfates reduced it (1.3 times). Heparin and heparan sulfate increased (1.4 times) the enzyme inhibition by the C1-inhibitor (150 nM).

RETRACTED: The glycoprotein Ib-IX-V complex mediates localization of factor XI to lipid rafts on the platelet membrane

Factor XI binds to activated platelets where it is efficiently activated by thrombin. The factor XI receptor is the platelet membrane glycoprotein (GP) Ib-IX-V complex (Baglia, F. A., Badellino, K. O., Li, C. Q., Lopez, J. A., and Walsh, P. N. (2002) J. Biol. Chem. 277, 1662-1668), a significant fraction of which exists within lipid rafts on stimulated platelets (Shrimpton, C. N., Borthakur, G., Larrucea, S., Cruz, M. A., Dong, J. F., and Lopez, J. A. (2002) J. Exp. Med. 196, 1057-1066). Lipid rafts are membrane microdomains enriched in cholesterol and sphingolipids implicated in localizing membrane ligands and in cellular signaling. We now show that factor XI was localized to lipid rafts in activated platelets ( approximately 8% of total bound) but not in resting platelets. Optimal binding of factor XI to membrane rafts required prothrombin (and Ca2+) or high molecular weight kininogen (and Zn2+), which are required for factor XI binding to platelets. An antibody to GPIb (SZ-2) that disrupts factor XI binding to the GPIb-IX-V complex also disrupted factor XI-raft association. The isolated recombinant Apple 3 domain of factor XI, which mediates factor XI binding to platelets, also completely displaces factor XI from membrane rafts. To investigate the physiological relevance of the factor XI-raft association, the structural integrity of lipid rafts was disrupted by cholesterol depletion utilizing methyl-beta-cyclodextrin. Cholesterol depletion completely prevented FXI binding to lipid rafts, and initial rates of factor XI activation by thrombin on activated platelets were inhibited >85%. We conclude that factor XI is localized to GPIb in membrane rafts and that this association is important for promoting the activation of factor XI by thrombin on the platelet surface.

Factor XI, but not prekallikrein, blocks high molecular weight kininogen binding to human umbilical vein endothelial cells

Previous studies on the interaction of high molecular weight kininogen (HK) with endothelial cells have reported a large number of binding sites (106-107 sites/cell) with differing relative affinities (KD = 7-130 nm) and have implicated various receptors or receptor complexes. In this study, we examined the binding of HK to human umbilical vein endothelial cells (HUVEC) with a novel assay system utilizing HUVEC immobilized on microcarrier beads, which eliminates the detection of the high affinity binding sites found nonspecifically in conventional microtiter well assays. We report that HK binds to 8.5 x 104 high affinity (KD = 21 nm) sites per HUVEC, i.e. 10-100-fold fewer than previously reported. Although HK binding is unaffected by the presence of a physiological concentration of prekallikrein, factor XI abrogates HK binding to HUVEC in a concentration-dependent manner. Disruption of the naturally occurring complex between factor XI and HK by the addition of a 31-amino acid peptide mimicking the factor XI-binding site on HK restored HK binding to HUVEC. Furthermore, HK inhibited thrombin-stimulated von Willebrand factor release by HUVEC but not thrombin receptor activation peptide (SFLLRN-amide)-stimulated von Willebrand factor release. Factor XI restored the ability of thrombin to stimulate von Willebrand factor release in the presence of low HK concentrations. These results suggest that free HK, or HK in complex with prekallikrein but not in complex with factor XI, interacts with the endothelium and can maintain endothelial cell quiescence by preventing endothelial stimulation by thrombin.

The factor IX gamma-carboxyglutamic acid (Gla) domain is involved in interactions between factor IX and factor XIa

During hemostasis, factor IX is activated to factor IXabeta by factor VIIa and factor XIa. The glutamic acid-rich gamma-carboxyglutamic acid (Gla) domain of factor IX is involved in phospholipid binding and is required for activation by factor VIIa. In contrast, activation by factor XIa is not phospholipid-dependent, raising questions about the importance of the Gla for this reaction. We examined binding of factors IX and IXabeta to factor XIa by surface plasmon resonance. Plasma factors IX and IXabeta bind to factor XIa with K(d) values of 120 +/- 11 nm and 110 +/- 8 nm, respectively. Recombinant factor IX bound to factor XIa with a K(d) of 107 nm, whereas factor IX with a factor VII Gla domain (rFIX/VII-Gla) and factor IX expressed in the presence of warfarin (rFIX-desgamma) did not bind. An anti-factor IX Gla monoclonal antibody was a potent inhibitor of factor IX binding to factor XIa (K(i) 34 nm) and activation by factor XIa (K(i) 33 nm). In activated partial thromboplastin time clotting assays, the specific activities of plasma and recombinant factor IX were comparable (200 and 150 units/mg), whereas rFIX/VII-Gla activity was low (<2 units/mg). In contrast, recombinant factor IXabeta and activated rFIX/VIIa-Gla had similar activities (80 and 60% of plasma factor IXabeta), indicating that both proteases activate factor X and that the poor activity of zymogen rFIX/VII-Gla was caused by a specific defect in activation by factor XIa. The data demonstrate that factor XIa binds with comparable affinity to factors IX and IXabeta and that the interactions are dependent on the factor IX Gla domain.

Characterization of the H-kininogen-binding site on factor XI: a comparison of factor XI and plasma prekallikrein

Factor XI (FXI), the zymogen of the blood coagulation protease FXIa, and the structurally homologous protein plasma prekallikrein circulate in plasma in noncovalent complexes with H-kininogen (HK). HK binds to the heavy chains of FXI and of prekallikrein. Each chain contains four apple domains (F1-F4 for FXI and P1-P4 for prekallikrein). Previous studies indicated that the HK-binding site on FXI is located in F1, whereas the major HK-binding site on prekallikrein is in P2. To determine the contribution of each FXI apple domain to HK-FXI complex formation, we examined binding of recombinant single apple domain-tissue plasminogen activator fusion proteins to HK. The order of affinity from highest to lowest is F2 F4 > F1 F3. Monoclonal antibodies against F2 are superior to F4 or F1 antibodies as inhibitors of HK binding to FXI. Antibody alphaP2, raised against prekallikrein, cross-reacts with FXI F2 and inhibits FXI-HK binding with an IC(50) of 8 nm. HK binding to a platelet-specific FXI variant lacking the N-terminal half of F2 is reduced > 5-fold compared with full-length FXI. A chimeric FXI molecule in which F2 is replaced by P2 is cleaved within P2 during activation by factor XIIa, resulting in greatly reduced HK binding capacity. In contrast, wild-type FXI is not cleaved within F2, and its binding capacity for HK is unaffected by factor XIIa. Our data show that HK binding to FXI involves multiple apple domains, with F2 being most important. The findings demonstrate a similarity in mechanism for FXI and prekallikrein binding to HK.

RETRACTED: Factor XI binding to the platelet glycoprotein Ib-IX-V complex promotes factor XI activation by thrombin

Factor XI binds to high affinity sites on the surface of stimulated platelets where it is efficiently activated by thrombin. Here, we provide evidence that the factor XI binding site on platelets is in the glycoprotein (GP) Ibalpha subunit of the GP Ib-IX-V complex as follows. 1) Bernard-Soulier platelets, lacking the complex, are deficient in factor XI binding; 2) two GP Ibalpha ligands, SZ-2 (a monoclonal antibody) and bovine von Willebrand factor, inhibit factor XI binding to platelets; 3) by surface plasmon resonance, factor XI bound specifically to glycocalicin (the extracellular domain of GP Ibalpha) in Zn(2+)-dependent fashion (K(d)( app) approximately 52 nm). We then investigated whether glycocalicin could promote factor XI activation by thrombin, another GP Ibalpha ligand. In the presence of high molecular weight kininogen (45 nm), Zn(2+) and Ca(2+) ions, thrombin activated factor XI in the presence of glycocalicin at rates comparable with those seen in the presence of dextran sulfate (1 microg/ml). With higher high molecular weight kininogen concentrations (360 nm), the rate of thrombin-catalyzed factor XI activation in the presence of glycocalicin was comparable with that on activated platelets. Thus, factor XI binds to the GP Ib-IX-V complex, promoting its activation by thrombin.

Survey of the year 2000 commercial optical biosensor literature

We have compiled a comprehensive list of the articles published in the year 2000 that describe work employing commercial optical biosensors. Selected reviews of interest for the general biosensor user are highlighted. Emerging applications in areas of drug discovery, clinical support, food and environment monitoring, and cell membrane biology are emphasized. In addition, the experimental design and data processing steps necessary to achieve high-quality biosensor data are described and examples of well-performed kinetic analysis are provided.

Defective binding of factor XI-N248 to activated human platelets

Variants of factor XI containing Gln226 to Arg (Q226 to R) and Ser248 to Asn (S248 to N) substitutions were first identified in an African American family with a history of excessive bleeding. The substitutions have recently been identified in unrelated individuals, suggesting they are relatively common. Both amino acids are located in the third apple domain of factor XI, an area implicated in binding interactions with factor IX and activated platelets. Recombinant factor XI-R226 and factor XI-N248 were compared with wild-type factor XI in assays for factor IX activation or platelet binding. Factor XI-R226 activates factor IX with a Michaelis-Menten constant (K(m)) about 5-fold greater than wild-type protein. The catalytic efficiency of factor IX activation is similar to wild-type protein, however, due to an increase in the turnover number (k(cat)) for the reaction. Iodinated factor XI-N248 binds to activated platelets with a dissociation constant (K(d)) more than 5-fold higher than wild-type protein (55 nM and 10 nM, respectively). Activation of factor XI-N248 by thrombin in the presence of activated platelets is slower and does not progress to the same extent as activation of the wild-type protein under similar conditions. Factor XI-N248 activates factor IX normally in a purified protein system and has relatively normal activity in activated partial thromboplastin time (aPTT) assays. Factor XI-N248 is the first factor XI variant described with a clear functional difference compared with wild-type protein. Importantly, the defect in platelet binding would not be detected by routine clinical evaluation with an aPTT assay.

Model for a factor IX activation complex on blood platelets: dimeric conformation of factor XIa is essential

Human coagulation factor XI (FXI) is a plasma serine protease composed of 2 identical 80-kd polypeptides connected by a disulfide bond. This dimeric structure is unique among blood coagulation enzymes. The hypothesis was tested that dimeric conformation is required for normal FXI function by generating a monomeric version of FXI (FXI/PKA4) and comparing it to wild-type FXI in assays requiring factor IX activation by activated FXI (FXIa). FXI/PKA4 was made by replacing the FXI A4 domain with the A4 domain from prekallikrein (PK). A dimeric version of FXI/PKA4 (FXI/PKA4-Gly326) was prepared as a control. Activated FXI/PKA4 and FXI/PKA4-Gly326 activate factor IX with kinetic parameters similar to those of FXIa. In kaolin-triggered plasma clotting assays containing purified phospholipid, FXI/PKA4 and FXI/PKA4-Gly326 have coagulant activity similar to FXI. The surface of activated platelets is likely to be a physiologic site for reactions involving FXI/FXIa. In competition binding assays FXI/PKA4, FXI/PKA4-Gly326, and FXI have similar affinities for activated platelets (K(i) = 12-16 nM). In clotting assays in which phospholipid is replaced by activated platelets, the dimeric proteins FXI and FXI/PKA4-Gly326 promote coagulation similarly; however, monomeric FXI/PKA4 has greatly reduced activity. Western immunoblot analysis confirmed that activated monomeric FXI/PKA4 activates factor IX poorly in the presence of activated platelets. These findings demonstrate the importance of the dimeric state to FXI activity and suggest a novel model for factor IX activation in which FXIa binds to activated platelets by one chain of the dimer, while binding to factor IX through the other.