The interaction of thrombomodulin with Ca2+

1H, 15N, 13C backbone and sidechain resonance assignments and secondary structure of mouse NOTCH1 EGF27

NOTCH1 is a transmembrane receptor in metazoans that is linked to a variety of disorders. The receptor contains an extracellular domain (ECD) with 36 tandem epidermal growth factor-like (EGF) repeats. The ECD is responsible for intercellular signaling via protein-ligand interactions with neighboring cells. Each EGF repeat consists of approximately 40 amino acids and 3 conserved disulfide bonds. The Abruptex region (EGF24-29) is critical for NOTCH1 signaling and is known for its missense mutations. Certain EGF repeats are modified with the addition of O-linked glycans and many have calcium binding sites, which give each EGF repeat a unique function. It has been shown that the loss of the O-fucose site of EGF27 alters NOTCH1 activity. To investigate the role of glycosylation in the NOTCH1 signaling pathway, nuclear magnetic resonance spectroscopy has been employed to study the structures of EGF27 and its glycoforms. Here, we report the backbone and sidechain 1H, 15N, and 13C-resonance assignments of the unmodified EGF27 protein and the predicted secondary structure derived from the assigned chemical shifts.

A case of thrombomodulin mutation causing defective thrombin binding with absence of protein C and TAFI activation

Thrombomodulin functions as an anticoagulant through thrombin binding and protein C activation. We herein report the first case of hereditary functional thrombomodulin deficiency presenting with recurrent subcutaneous hemorrhage and old cerebral infarction. The patient had a homozygous substitution of glycine by aspartate at amino acid residue 412 (Gly412Asp) in the thrombin-binding domain of the thrombomodulin gene (designated thrombomodulin-Nagasaki). In vitro assays using a recombinant thrombomodulin with the same mutation as the patient showed a total lack of thrombin binding and activation of protein C and thrombin-activatable fibrinolysis inhibitor (TAFI). Marked clinical and laboratory improvement was obtained with recombinant human soluble thrombomodulin therapy.

The Lectin-Like Domain of Thrombomodulin Inhibits β1 Integrin-Dependent Binding of Human Breast Cancer-Derived Cell Lines to Fibronectin

Thrombomodulin is a molecule with anti-coagulant and anti-inflammatory properties. Recently, thrombomodulin was reported to be able to bind extracellular matrix proteins, such as fibronectin and collagen; however, whether thrombomodulin regulates the binding of human breast cancer-derived cell lines to the extracellular matrix remains unknown. To investigate this, we created an extracellular domain of thrombomodulin, TMD123-Fc, or domain deletion TM-Fc proteins (TM domain 12-Fc, TM domain 23-Fc) and examined their bindings to fibronectin in vitro by ELISA. The lectin-like domain of thrombomodulin was found to be essential for the binding of the extracellular domain of thrombomodulin to fibronectin. Using a V-well cell adhesion assay or flow cytometry analysis with fluorescent beads, we found that both TMD123-Fc and TMD12-Fc inhibited the binding between β1 integrin of human breast cancer-derived cell lines and fibronectin. Furthermore, TMD123-Fc and TMD12-Fc inhibited the binding of activated integrins to fibronectin under shear stress in the presence of Ca²⁺ and Mg²⁺ but not under strong integrin-activation conditions in the presence of Mg²⁺ without Ca²⁺. This suggests that thrombomodulin Fc fusion protein administered exogenously at a relatively early stage of inflammation may be applied to the development of new therapies that inhibit the binding of β1 integrin of breast cancer cell lines to fibronectin.

Polarized Proteins in Endothelium and Their Contribution to Function

Protein localization in endothelial cells is tightly regulated to create distinct signaling domains within their tight spatial restrictions including luminal membranes, abluminal membranes, and interendothelial junctions, as well as caveolae and calcium signaling domains. Protein localization in endothelial cells is also determined in part by the vascular bed, with differences between arteries and veins and between large and small arteries. Specific protein polarity and localization is essential for endothelial cells in responding to various extracellular stimuli. In this review, we examine protein localization in the endothelium of resistance arteries, with occasional references to other vessels for contrast, and how that polarization contributes to endothelial function and ultimately whole organism physiology. We highlight the protein localization on the luminal surface, discussing important physiological receptors and the glycocalyx. The protein polarization to the abluminal membrane is especially unique in small resistance arteries with the presence of the myoendothelial junction, a signaling microdomain that regulates vasodilation, feedback to smooth muscle cells, and ultimately total peripheral resistance. We also discuss the interendothelial junction, where tight junctions, adherens junctions, and gap junctions all convene and regulate endothelial function. Finally, we address planar cell polarity, or axial polarity, and how this is regulated by mechanosensory signals like blood flow.

F376A/M388A-solulin, a new promising antifibrinolytic for severe haemophilia A

INTRODUCTION

Haemophilia is a major bleeding disorder due to a deficiency of procoagulant factor VIII (type A) or IX (type B). The treatment is substitutive and based on infusion of factor concentrates. Main limitations of this therapy are cost, short factor half-life and the development of inhibitors (up to 30% of severe HA patients). An important aggravating factor of haemophilia is due to a premature fibrinolysis, directing attention to the therapeutic potential of suitable antifibrinolytics. Thrombomodulin (TM) is a key player of the coagulation cascade by activating protein C (an inhibitor of thrombin generation, thus antagonizing coagulation) and of the fibrinolytic cascade by activating thrombin activatable fibrinolysis inhibitor TAFI (thus reducing fibrinolysis). Solulin is a soluble form of TM that shows both capabilities.

AIM

Here, we developed a new generation of solulin variants (F376A-, M388A- and F376A/M388A-solulin) with a decreased ability to activate protein C and a conserved capacity to activate TAFI.

METHODS

We produced and characterized solulin variants in vitro. In addition, F376A/M388A-solulin was tested ex vivo, using blood samples of haemophilic A patients, with thromboelastography.

RESULTS

The solulin variants (F376A, M388A and the double-mutant F376A/M388A) lost their abilities to activate protein C but are still capable to activate TAFI. Thrombelastography showed increased clot firmness and stability, that, as opposed to wild-type solulin, was maintained even at high concentrations of F376A/M388A-solulin (100 nm).

CONCLUSION

In sum, these results open new opportunities for the development of specific medication for haemophilic patients.

Thrombomodulin enhances complement regulation through strong affinity interactions with factor H and C3b-Factor H complex

INTRODUCTION

Coagulation and complement systems are simultaneously activated at sites of tissue injury, leading to thrombin generation and opsonisation with C3b. Thrombomodulin (TM) is a cell-bound regulator of thrombin activation, but can also enhance the regulatory activity of complement factor H (FH), thus accelerating the degradation of C3b into inactive iC3b.

OBJECTIVES

This study sought to determine the biophysical interaction affinities of two recombinant TM analogs with thrombin, FH and C3b in order to analyze their ability to regulate serum complement activity.

METHODS

Surface plasmon resonance (SPR) analysis was used to determine binding affinities of TM analogs with FH and C3b, and compared to thrombin as positive control. The capacity of the two recombinant TM analogs to regulate complement in serum was tested in standard complement hemolytic activity assays.

RESULTS

SPR analysis showed that both TM analogs bind FH and C3b-Factor H with nanomolar and C3b with micromolar affinity; binding affinity for its natural ligand thrombin was several fold higher than for FH. At a physiological relevant concentration, TM inhibits complement hemolytic activity in serum via FH dependent and independent mechanisms.

CONCLUSIONS

TM exhibits significant binding affinity for complement protein FH and C3b-FH complex and its soluble form is capable at physiologically relevant concentrations of inhibiting complement activation in serum.

Recombinant thrombomodulin of different domains for pharmaceutical, biomedical, and cell transplantation applications

Thrombomodulin (TM) is a membrane glycoprotein mainly expressed by vascular endothelial cells and is involved in many physiological and pathological processes, such as coagulation, inflammation, cancer development, and embryogenesis. Human TM consists of 557 amino acids divided into five distinct domains: N-terminal lectin-like domain (designated as TMD1); six epidermal growth factor (EGF)-like domain (TMD2); Ser/Thr-rich domain (TMD3); transmembrane domain (TMD4); and cytoplasmic tail domain (TMD5). The different domains are responsible for different biological functions of TM. In the past decades, various domains of TM have been cloned and expressed for TM structural and functional study. Further, recombinant TMs of different domains show promising antithrombotic and anti-inflammatory activity in both rodents and primates and a recombinant soluble TM has been approved for therapeutic application. This review highlights recombinant TMs of diverse structures and their biological functions, as well as the complex interactions of TM with factors involved in the related biological processes. Particularly, recent advances in exploring recombinant TM of different domains for pharmaceutical, biomedical, and cell transplantation applications are summarized.

Thrombomodulin and the vascular endothelium: insights into functional, regulatory, and therapeutic aspects

Thrombomodulin (TM) is a 557-amino acid protein with a broad cell and tissue distribution consistent with its wide-ranging physiological roles. When expressed on the lumenal surface of vascular endothelial cells in both large vessels and capillaries, its primary function is to mediate endothelial thromboresistance. The complete integral membrane-bound protein form displays five distinct functional domains, although shorter soluble (functional) variants comprising the extracellular domains have also been reported in fluids such as serum and urine. TM-mediated binding of thrombin is known to enhance the specificity of the latter serine protease toward both protein C and thrombin activatable fibrinolysis inhibitor (TAFI), increasing their proteolytic activation rate by almost three orders of magnitude with concomitant anticoagulant, antifibrinolytic, and anti-inflammatory benefits to the vascular wall. Recent years have seen an abundance of research into the cellular mechanisms governing endothelial TM production, processing, and regulation (including flow-mediated mechanoregulation)--from transcriptional and posttranscriptional (miRNA) regulation of TM gene expression, to posttranslational processing and release of the expressed protein--facilitating greater exploitation of its therapeutic potential. The goal of the present paper is to comprehensively review the endothelial/TM system from these regulatory perspectives and draw some fresh conclusions. This paper will conclude with a timely examination of the current status of TM's growing therapeutic appeal, from novel strategies to improve the clinical efficacy of recombinant TM analogs for resolution of vascular disorders such as disseminated intravascular coagulation (DIC), to an examination of the complex pleiotropic relationship between statin treatment and TM expression.

Solulin increases clot stability in whole blood from humans and dogs with hemophilia

Solulin is a soluble form of thrombomodulin that is resistant to proteolysis and oxidation. It has been shown to increase the clot lysis time in factor VIII (fVIII)-deficient plasma by an activated thrombin-activatable fibrinolysis inhibitor (TAFIa)-dependent mechanism. In the present study, blood was drawn from humans and dogs with hemophilia, and thromboelastography was used to measure tissue factor-initiated fibrin formation and tissue-plasminogen activator-induced fibrinolysis. The kinetics of TAFI and protein C activation by the thrombin-Solulin complex were determined to describe the relative extent of anticoagulation and antifibrinolysis. In severe hemophilia A, clot stability increased by > 4-fold in the presence of Solulin while minimally affecting clot lysis time. Patients receiving fVIII/fIX prophylaxis showed a similar trend of increased clot stability in the presence of Solulin. The catalytic efficiencies of TAFI and protein C activation by the thrombin-Solulin complex were determined to be 1.53 and 0.02/μM/s, respectively, explaining its preference for antifibrinolysis over anticoagulation at low concentrations. Finally, hemophilic dogs given Solulin had improved clot strength in thromboelastography assays. In conclusion, the antifibrinolytic properties of Solulin are exhibited in hemophilic human (in vitro) and dog (in vivo/ex vivo) blood at low concentrations. Our findings suggest the therapeutic utility of Solulin at a range of very low doses.

Thrombomodulin and its role in inflammation

The goal is to provide an extensive review of the physiologic role of thrombomodulin (TM) in maintaining vascular homeostasis, with a focus on its anti-inflammatory properties. Data were collected from published research. TM is a transmembrane glycoprotein expressed on the surface of all vascular endothelial cells. Expression of TM is tightly regulated to maintain homeostasis and to ensure a rapid and localized hemostatic and inflammatory response to injury. By virtue of its strategic location, its multidomain structure and complex interactions with thrombin, protein C (PC), thrombin activatable fibrinolysis inhibitor (TAFI), complement components, the Lewis Y antigen, and the cytokine HMGB1, TM exhibits a range of physiologically important anti-inflammatory, anti-coagulant, and anti-fibrinolytic properties. TM is an essential cofactor that impacts on multiple biologic processes. Alterations in expression of TM and its partner proteins may be manifest by inflammatory and thrombotic disorders. Administration of soluble forms of TM holds promise as effective therapies for inflammatory diseases, and infections and malignancies that are complicated by disseminated intravascular coagulation.

Solulin reduces infarct volume and regulates gene-expression in transient middle cerebral artery occlusion in rats

Background

Thrombolysis after acute ischemic stroke has only proven to be beneficial in a subset of patients. The soluble recombinant analogue of human thrombomodulin, Solulin, was studied in an in vivo rat model of acute ischemic stroke.

Methods

Male SD rats were subjected to 2 hrs of transient middle cerebral artery occlusion (tMCAO). Rats treated with Solulin intravenously shortly before reperfusion were compared to rats receiving normal saline i.v. with respect to infarct volumes, neurological deficits and mortality. Gene expression of IL-6, IL-1β, TNF-α, MMP-9, CD11B and GFAP were semiquantitatively analyzed by rtPCR of the penumbra.

Results

24 hrs after reperfusion, rats were neurologically tested, euthanized and infarct volumes determined. Solulin significantly reduced mean total (p = 0.001), cortical (p = 0.002), and basal ganglia (p = 0.036) infarct volumes. Hippocampal infarct volumes (p = 0.191) were not significantly affected. Solulin significantly downregulated the expression of IL-1β (79%; p < 0.001), TNF-α (59%; p = 0.001), IL-6 (47%; p = 0.04), and CD11B (49%; p = 0.001) in the infarcted cortex compared to controls.

Conclusions

Solulin reduced mean total, cortical and basal ganglia infarct volumes and regulated a subset of cytokines and proteases after tMCAO suggesting the potency of this compound for therapeutic interventions.

Thermodynamic compensation upon binding to exosite 1 and the active site of thrombin

Several lines of experimental evidence including amide exchange and NMR suggest that ligands binding to thrombin cause reduced backbone dynamics. Binding of the covalent inhibitor dPhe-Pro-Arg chloromethyl ketone to the active site serine, as well as noncovalent binding of a fragment of the regulatory protein, thrombomodulin, to exosite 1 on the back side of the thrombin molecule both cause reduced dynamics. However, the reduced dynamics do not appear to be accompanied by significant conformational changes. In addition, binding of ligands to the active site does not change the affinity of thrombomodulin fragments binding to exosite 1; however, the thermodynamic coupling between exosite 1 and the active site has not been fully explored. We present isothermal titration calorimetry experiments that probe changes in enthalpy and entropy upon formation of binary ligand complexes. The approach relies on stringent thrombin preparation methods and on the use of dansyl-l-arginine-(3-methyl-1,5-pantanediyl)amide and a DNA aptamer as ligands with ideal thermodynamic signatures for binding to the active site and to exosite 1. Using this approach, the binding thermodynamic signatures of each ligand alone as well as the binding signatures of each ligand when the other binding site was occupied were measured. Different exosite 1 ligands with widely varied thermodynamic signatures cause a similar reduction in ΔH and a concomitantly lower entropy cost upon DAPA binding at the active site. The results suggest a general phenomenon of enthalpy-entropy compensation consistent with reduction of dynamics/increased folding of thrombin upon ligand binding to either the active site or exosite 1.

Inactivation of thrombomodulin by ionizing radiation in a cell-free system: possible implications for radiation responses in vascular endothelium

Normal tissue radiation injury is associated with loss of vascular thromboresistance, notably because of deficient levels of endothelial thrombomodulin (TM). TM is located on the luminal surface of most endothelial cells and has critical anticoagulant and anti-inflammatory functions. Chemical oxidation of a specific methionine residue (Met388) at the thrombin-binding site in TM reduces its main functional activity, i.e., the ability to activate protein C. We examined whether exposure to ionizing radiation affects TM in a similar manner. Full-length recombinant human TM, a construct of epidermal growth factor-like domains 4-6, which are involved in protein C activation, and a synthetic peptide containing the methionine of interest were exposed to gamma radiation in a cell-free system, i.e., a system not confounded by TM turnover or ectodomain shedding. The influence of radiation on functional activity was assessed with the protein C activation assay; formation of a TM-thrombin complex was assessed with surface plasmon resonance (Biacore), and oxidation of Met388 was assessed by HPLC and confirmed by mass spectroscopy. Exposure to radiation caused a dose-dependent reduction in protein C activation, impaired TM-thrombin complex formation, and oxidation of Met388. These results demonstrate that ionizing radiation adversely affects the TM molecule. Our findings may have relevance to normal tissue toxicity in clinical radiation therapy as well as to the development of radiation syndromes in the non-therapeutic radiation exposure setting.

Thrombomodulin: from haemostasis to inflammation and tumourigenesis

Thrombomodulin (TM), a transmembrane endothelial receptor, participates in coagulation, in inflammation, in cancer and plays a role during embryogenesis. The nucleotide sequence of the TM cDNA allows the structure of this protein to be visualized. The protein starts with a signal peptide, followed by the N-terminal globular domain, six repeats of epidermal growth factor-like sequence, a serine/threonine-rich region, a transmembrane domain and a cytoplasmic domain. High-resolution nuclear magnetic resonance (NMR) spectroscopy was employed to define the exact thrombin-binding region. Residues Y(413)ILDD(417) and D(423)IDE(426) are crucial for binding to thrombin; the two critical amino acids for thrombin binding, Ile(414) and Ile(424), are brought into spatial proximity by beta-sheet structure. There also exist some residues for co-factor activity, namely Asp(349), Glu(357), Tyr(358), Phe(376) and Met(388). The complex transcriptional and post-transcriptional control of TM underline its importance in a wide variety of biological systems and pathophysiological processes.

High levels of protein expression using different mammalian CMV promoters in several cell lines

With the recent completion of the human genome sequencing project, scientists are faced with the daunting challenge of deciphering the function of these newly found genes quickly and efficiently. Equally as important is to produce milligram quantities of the therapeutically relevant gene products as quickly as possible. Mammalian expression systems provide many advantages to aid in this task. Mammalian cell lines have the capacity for proper post-translational modifications including proper protein folding and glycosylation. In response to the needs described above, we investigated the protein expression levels driven by the human CMV in the presence or absence of intron A, the mouse and rat CMV promoters with intron A, and the MPSV promoter in plasmid expression vectors. We evaluated the different promoters using an in-house plasmid vector backbone. The protein expression levels of four genes of interest driven by these promoters were evaluated in HEK293EBNA and CHO-K1 cells. Stable and transient transfected cells were utilized. In general, the full-length human CMV, in the presence of intron A, gave the highest levels of protein expression in transient transfections in both cell lines. However, the MPSV promoter resulted in the highest levels of stable protein expression in CHO-K1 cells. Using the CMV driven constitutive promoters in the presence of intron A, we have been able to generate >10 microg/ml of recombinant protein using transient transfections.

Thrombomodulin

Since its discovery as a critical cofactor in the initiation of the protein C (PC) anticoagulant pathway [1,2], biochemical and structural investigations, combined with in vivo analyses of genetically engineered mice have revealed new, and in part PC- and thrombin-independent aspects of thrombomodulin (TM) function in fibrinolysis and inflammation, and in embryogenesis. This review summarizes more recent structural and functional investigations of TM, gives an overview of the association of TM gene polymorphisms with human disease, and provides a synopsis of what is know about TM function in disease states of thrombosis, stroke, arteriosclerosis, and cancer. Newly emerging aspects of TM function in inflammation and embryogenesis are presented and discussed in detail.

A steady-state competition model describes the modulating effects of thrombomodulin on thrombin inhibition by plasminogen activator inhibitor-1 in the absence and presence of vitronectin

Thrombomodulin (TM) slows down the interaction rate between thrombin and plasminogen activator inhibitor 1 (PAI-1). We now show that the 12-fold reduced inhibition rate in the presence of TM does not result from an altered distribution between PAI-1 cleavage and irreversible complex formation. Surface plasmon resonance (SPR) revealed an over 200-fold reduced affinity of TM for thrombin-VR1tPA as compared to thrombin, demonstrating the importance of the VR1 loop in the interaction of thrombin with both TM and PAI-1. Furthermore, in contrast to ATIII, PAI-1 was not able to bind the thrombin/TM complex demonstrating complete competitive binding between PAI-1 and TM. Kinetic modeling on the inhibitory effect of TM confirms a mechanism that involves complete steric blocking of the thrombin/PAI-1 interaction. Also, it accurately decribes the biphasic inhibition profile resulting from the substantial reduction of the extremely fast rate of reversible Michaelis complex formation, which is essential for efficient inhibition of thrombin by PAI-1. Vitronectin (VN) is shown to partially relieve TM inhibitory action only by vastly increasing the initial rate of interaction between free thrombin and PAI-1. In addition, SPR established that solution-phase PAI-1/VN complexes and non-native VN (extracellular matrix form) bind TM directly via the chondroitin sulphate moiety of TM. Collectively, these results show that VR1 is a subsite of exosite 1 on thrombin's surface, which regulates exclusive binding of either PAI-1 or TM. This competition will be physiologically significant in controlling the mitogenic activity of thrombin during vascular disease.

Ca(2+) binding to both the heavy and light chains of factor VIII is required for cofactor activity

Previously, we demonstrated that Ca(2+) was necessary for the generation of cofactor activity following reconstitution of factor VIII from its isolated light chain (LC) and heavy chain (HC) but that Ca(2+) did not affect HC-LC binding affinity (Wakabayashi et al. (2001) Biochemistry 40, 10293-10300). Titration of EDTA-treated factor VIII with Ca(2+) followed by factor Xa generation assay showed a two-site binding pattern, with indicated high-affinity (K(d) = 8.9 +/- 1.8 microM) and low-affinity (K(d) = 4.0 +/- 0.6 mM) sites. Analysis by equilibrium dialysis using (45)Ca and <400 microM free Ca(2+) verified a high-affinity binding (K(d) = 18.9 +/- 3.7 microM). Preincubation of either HC or LC with 6 mM Ca(2+) followed by reassociation with the untreated complementary chain in the presence of 0.12 mM Ca(2+) failed to generate significant cofactor activity (<0.5 nM min(-1) (nM LC)(-1)). However, pretreatment of both HC and LC with 6 mM Ca(2+) followed by reassociation (at 0.12 mM Ca(2+)) generated high activity (7.5 +/- 0.4 nM min(-1) (nM LC)(-1)). Progress curves for activity regain following factor VIII-Ca(2+) association kinetics fitted well to a series reaction scheme rather than one of simple association (p < 0.0001), suggesting a multistep process which may include a Ca(2+)-dependent conformational change. These results suggest that factor VIII contains two Ca(2+) binding sites with different affinities and that active factor VIII can be reconstituted from HC and LC only when both chains are preactivated by Ca(2+).

Survey of the 1999 surface plasmon resonance biosensor literature

The application of surface plasmon resonance biosensors in life sciences and pharmaceutical research continues to increase. This review provides a comprehensive list of the commercial 1999 SPR biosensor literature and highlights emerging applications that are of general interest to users of the technology. Given the variability in the quality of published biosensor data, we present some general guidelines to help increase confidence in the results reported from biosensor analyses.

Identification of a thrombin-binding region in the sixth epidermal growth factor-like repeat of human thrombomodulin

The interaction of thrombin with a 28-residue peptide corresponding to the N-terminal subdomain of the sixth EGF-like repeat of human thrombomodulin plus the junction between the fifth and the sixth EGF-like domains was characterized in solution by use of NMR spectroscopy, particularly differential resonance perturbations and transferred nuclear Overhauser effects (transferred NOEs). The EGF-like thrombomodulin fragment, or hTM422-449, is conformationally flexible in the absence of thrombin. Upon addition of thrombin, differential resonance perturbations and transferred NOEs are observed for the thrombomodulin peptide, suggesting specific and rapidly reversible binding and structuring of hTM422-449 in complex with thrombin. Residue-specific analysis of the differential line broadening, resonance shifts, and transferred NOEs identified regions of hTM422-449 responding to thrombin binding as the N-terminal residues Thr422-Ile424 and residues His438-Ile447 corresponding to the central beta-hairpin, or B-loop, of the consensus EGF-like repeat. The formation of the beta-hairpin is supported by the pattern of transferred NOEs bringing the two beta-strands together and characterizing a type I beta-turn. Docking of the thrombomodulin peptide to the anion-binding exosite I of thrombin revealed structural details capturing binding contacts identified so far as essential for the thrombin-thrombomodulin interaction. Definition of specific interactions between thrombin and a minimal fragment of the sixth EGF-like domain of human TM may lead to the discovery of new peptidomimetic molecules as modulators of blood coagulation.

Elements of the primary structure of thrombomodulin required for efficient thrombin-activable fibrinolysis inhibitor activation

Deletion and point mutants of soluble thrombomodulin were used to compare and contrast elements of primary structure required for the activation of thrombin-activable fibrinolysis inhibitor (TAFI) and protein C. The smallest mutant capable of efficiently promoting TAFI activation contained residues including the c-loop of epidermal growth factor-3 (EGF3) through EGF6. This mutant is 13 residues longer than the smallest mutant that functioned well with protein C; the latter consisted of residues from the interdomain loop connecting EGF3 and EGF4 through EGF6. Alanine point mutants showed no loss of function in protein C activation for mutations within the c-loop of EGF3. In TAFI activation, however, alanine mutations cause a 50% reduction at Tyr-337, 67% reductions at Asp-338 and Leu-339, and 90% or greater reductions at Val-340, Asp-341, and Glu-343. A mutation at Asp-349 in the peptide connecting EGF3 to EGF4 eliminated activity against both TAFI and protein C. Oxidation of Met-388 in the peptide connecting EGF5 to EGF6 reduced the rate of protein C activation by 80% but marginally, if at all, affected the rate of TAFI activation. Mutation at Phe-376 severely reduced protein C activation but only marginally influenced that of TAFI. A Q387P mutation, however, severely reduced both activities. TAFI activation was shown to be Ca(2+)-dependent. The response, unlike that of protein C, was monotonic and was half-maximal at 0.25 mm Ca(2+). Like protein C activation, TAFI activation was eliminated by a monoclonal antibody directed at the thrombin-binding domain (EGF5) but was not affected by one directed at EGF2. Thus, elements of structure in the thrombin-binding domain are needed for the activation of both protein C and TAFI, but more of the primary structure is needed for TAFI activation. In addition, some residues are needed for one of the reactions but not the other.

Structural basis for the anticoagulant activity of the thrombin-thrombomodulin complex

The serine proteinase alpha-thrombin causes blood clotting through proteolytic cleavage of fibrinogen and protease-activated receptors and amplifies its own generation by activating the essential clotting factors V and VIII. Thrombomodulin, a transmembrane thrombin receptor with six contiguous epidermal growth factor-like domains (TME1-6), profoundly alters the substrate specificity of thrombin from pro- to anticoagulant by activating protein C. Activated protein C then deactivates the coagulation cascade by degrading activated factors V and VIII. The thrombin-thrombomodulin complex inhibits fibrinolysis by activating the procarboxypeptidase thrombin-activatable fibrinolysis inhibitor. Here we present the 2.3 A crystal structure of human alpha-thrombin bound to the smallest thrombomodulin fragment required for full protein-C co-factor activity, TME456. The Y-shaped thrombomodulin fragment binds to thrombin's anion-binding exosite-I, preventing binding of procoagulant substrates. Thrombomodulin binding does not seem to induce marked allosteric structural rearrangements at the thrombin active site. Rather, docking of a protein C model to thrombin-TME456 indicates that TME45 may bind substrates in such a manner that their zymogen-activation cleavage sites are presented optimally to the unaltered thrombin active site.