Isolation and Structural Charaderization of a Potent Inhibitor of Coagulation Factor Xa from the Leech Haementeria ghilianii

The Origin and Evolution of Antistasin-like Proteins in Leeches (Hirudinida, Clitellata)

Bloodfeeding is employed by many parasitic animals and requires specific innovations for efficient feeding. Some of these innovations are molecular features that are related to the inhibition of hemostasis. For example, bloodfeeding insects, bats, and leeches release proteins with anticoagulatory activity through their salivary secretions. The antistasin-like protein family, composed of serine protease inhibitors with one or more antistasin-like domains, is tightly linked to inhibition of hemostasis in leeches. However, this protein family has been recorded also in non-bloodfeeding invertebrates, such as cnidarians, mollusks, polychaetes, and oligochaetes. The present study aims to 1) root the antistasin-like gene tree and delimit the major orthologous groups, 2) identify potential independent origins of salivary proteins secreted by leeches, and 3) identify major changes in domain and/or motif structure within each orthologous group. Five clades containing leech antistasin-like proteins are distinguishable through rigorous phylogenetic analyses based on nine new transcriptomes and a diverse set of comparative data: the trypsin + leukocyte elastase inhibitors clade, the antistasin clade, the therostasin clade, and two additional, unnamed clades. The antistasin-like gene tree supports multiple origins of leech antistasin-like proteins due to the presence of both leech and non-leech sequences in one of the unnamed clades, but a single origin of factor Xa and trypsin + leukocyte elastase inhibitors. This is further supported by three sequence motifs that are exclusive to antistasins, the trypsin + leukocyte elastase inhibitor clade, and the therostasin clade, respectively. We discuss the implications of our findings for the evolution of this diverse family of leech anticoagulants.

Functional characterisation of Vizottin, the first factor Xa inhibitor purified from the leech Haementeria vizottoi

The strategic position of factor Xa (FXa) in blood coagulation makes it a compelling target for the development of new anticoagulants. Bloodsucking animals have in their salivary glands mixtures of anticoagulants, which could be used for designing novel antithrombotic compounds. Herein, we describe Vizottin, the first FXa inhibitor from the salivary complex of the leech Haementeria vizottoi. Vizottin was purified by gel filtration and reverse-phase chromatography, and shown to have anticoagulant effects in human plasma, prolonging the recalcification time in a dose-dependent manner (IC50 40 nM). Vizottin induced blood incoagulability in FX-deficient plasma, whereas in normal and reconstituted plasma, Vizottin doubled the prothrombin time at 160 nM. This peptide competitively inhibited human FXa (Ki 2 nM) like FXa inhibitors from other leeches, albeit via a distinct mechanism of action. At high concentrations, vizottin inhibited the amidolytic activity of factor VIIa/tissue factor (IC50 96.4 nM). Vizottin inhibited FXa in the prothrombinase complex and Gla-domainless FXa. Moreover, vizottin did not interfere with FX activation induced by RVV-X, a known enzyme that requires the Gla-domain of FX for activation. Competition experiments in the presence of FXa and GGACK-FXa (active site blocked) demonstrated that the inhibition of FXa by vizottin is through binding to the active site rather than an exosite. This novel inhibitor appears to exert its inhibitory effects through direct binding to the active site of FXa in a time-dependent manner, but not involving a tight-binding model. In this context, vizottin is a promising model for designing novel anticoagulants for the treatment of thrombotic diseases.

State-of-the-Art Review : Antithrombotic Agents from Hematophagous Animals

Bloodsucking animals produce anticoagu lantly effective substances that are a challenge to coagu lation studies. In the past 40 years efforts have been fo cused on the isolation and chemical characterization of such agents as well as on the clarification of their modes of action. Following the success in the development of the anticoagulant agent hirudin from medicinal leeches, these naturally occurring anticoagulants were recently in vestigated as a source of antithrombotics for pharmaceu tical use. These polypeptides or miniproteins were shown to be specific inhibitors of certain coagulation factors that block either the formation or the effect of thrombin or are supported by substances that inhibit the aggregation and adhesion of blood platelets and by proteolytic enzymes with fibrinolytic activity. By advances in biotechnology of protein-like substances, especially gene technology, these antithrombotics have been obtained in amounts suf ficient for preclinical and clinical studies. Thus, the in vestigation of the anticoagulant agents from hematopha gous animals offers a new line of research in antithrom botic drugs. Key Words: Bloodsucking animals— Naturally occurring anticoagulants—Fibrinolytics and platelet inhibitors.

Protein mapping of the salivary complex from a hematophagous leech

The salivary complex of leeches contains many components able to modulate physiological mechanisms, such as coagulation and fibrinolysis, and it is composed by the salivary glands and proboscis, encompassing two different proteomes. The bidimensional electrophoretic pattern of the salivary complex from the Haementeria depressa leech revealed a total of 352 spots, 103 in common with the muscular tissue and 249 exclusive from the salivary complex as detected by silver staining; these spots showed isoelectric points from 3.5 to 9.5 and covered an apparent molecular weight range from 10 to 105 kDa. The following isoforms of proteins were identified by mass spectrometry analysis: antiplatelet protein, myohemerythrin and carbonic anhydrase. Since the leeches were not fed for about 2-3 months to stimulate the secretion of proteins that facilitates the blood metabolism, these most abundant proteins in the salivary complex excised from leeches, are expected to play a role during feeding and might have some anti-hemostatic properties. Furthermore, by zymography, a gelatinolytic and a fibrinolytic protein were identified.

Factor Xa (FXa) inhibitor from the nymphs of the camel tick Hyalomma dromedarii

An inhibitor of factor Xa (FXa) was isolated from the nymphs of the camel tick Hyalomma dromedarii by a combination of chromatography on DEAE-cellulose and Sephacryl S-300 columns. The isolated nymphal FXa inhibitor turned out to be a homogenous preparation of a single polypeptide chain (15 kDa) as judged by both the native and denatured SDS-PAGE. Its pI value ranged from 7.7 to 7.9. The inhibitor is a potent anticoagulant since it prolonged both the activated partial thromboplastin time (APTT) and the prothrombin time (PT) of the camel plasma in a concentration-dependent manner. Its activity was threefold lower toward thrombin than FXa, but it did not inhibit any of the proteases; trypsin, alpha-chymotrypsin, papain, pepsin and subtilisin. The inhibitor binds at two sites on FXa uncompetitively with an inhibition constant (K(i)) value of 134 nM.

Structure of the complex of the antistasin-type inhibitor bdellastasin with trypsin and modelling of the bdellastasin-microplasmin system

The serine proteinase plasmin is, together with tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA), involved in the dissolution of blood clots in a fibrin-dependent manner. Moreover, plasmin plays a key role in a variety of other activation cascades such as the activation of metalloproteinases, and has also been implicated in wound healing, pathogen invasion, cancer invasion and metastasis. The leech-derived (Hirudo medicinalis) antistasin-type inhibitor bdellastasin represents a specific inhibitor of trypsin and plasmin and thus offers a unique opportunity to evaluate the concept of plasmin inhibition. The complexes formed between bdellastasin and bovine as well as porcine beta-trypsin have been crystallised in a monoclinic and a tetragonal crystal form, containing six molecules and one molecule per asymmetric unit, respectively. Both structures have been solved and refined to 3.3 A and 2.8 A resolution. Bdellastasin turns out to have an antistasin-like fold exhibiting a bis-domainal structure like the tissue kallikrein inhibitor hirustasin. The interaction between bdellastasin and trypsin is restricted to the C-terminal subdomain of bdellastasin, particularly to its primary binding loop, comprising residues Asp30-Glu38. The reactive site of bdellastasin differs from other antistasin-type inhibitors of trypsin-like proteinases, exhibiting a lysine residue instead of an arginine residue at P1. A model of the bdellastasin-microplasmin complex has been created based on the X-ray structures. Our modelling studies indicate that both trypsin and microplasmin recognise bdellastasin by interactions which are characteristic for canonically binding proteinase inhibitors. On the basis of our three-dimensional structures, and in comparison with the tissue-kallikrein-bound and free hirustasin and the antistasin structures, we postulate that the binding of the inhibitors toward trypsin and plasmin is accompanied by a switch of the primary binding loop segment P5-P3. Moreover, in the factor Xa inhibitor antistasin, the core of the molecule would prevent an equivalent rotation of the P3 residue, making exosite interactions of antistasin with factor Xa imperative. Furthermore, Arg32 of antistasin would clash with Arg175 of plasmin, thus impairing a favourable antistasin-plasmin interaction and explaining its specificity.

The 1.2 A crystal structure of hirustasin reveals the intrinsic flexibility of a family of highly disulphide-bridged inhibitors

BACKGROUND

Leech-derived inhibitors have a prominent role in the development of new antithrombotic drugs, because some of them are able to block the blood coagulation cascade. Hirustasin, a serine protease inhibitor from the leech Hirudo medicinalis, binds specifically to tissue kallikrein and possesses structural similarity with antistasin, a potent factor Xa inhibitor from Haementeria officinalis. Although the 2.4 A structure of the hirustasin-kallikrein complex is known, classical methods such as molecular replacement were not successful in solving the structure of free hirustasin.

RESULTS

Ab initio real/reciprocal space iteration has been used to solve the structure of free hirustasin using either 1.4 A room temperature data or 1.2 A low temperature diffraction data. The structure was also solved independently from a single pseudo-symmetric gold derivative using maximum likelihood methods. A comparison of the free and complexed structures reveals that binding to kallikrein causes a hinge-bending motion between the two hirustasin subdomains. This movement is accompanied by the isomerisation of a cis proline to the trans conformation and a movement of the P3, P4 and P5 residues so that they can interact with the cognate protease.

CONCLUSIONS

The inhibitors from this protein family are fairly flexible despite being highly cross-linked by disulphide bridges. This intrinsic flexibility is necessary to adopt a conformation that is recognised by the protease and to achieve an optimal fit, such observations illustrate the pitfalls of designing inhibitors based on static lock-and-key models. This work illustrates the potential of new methods of structure solution that require less or even no prior phase information.

A new structural class of serine protease inhibitors revealed by the structure of the hirustasin-kallikrein complex

BACKGROUND

Hirustasin belongs to a class of serine protease inhibitors characterized by a well conserved pattern of cysteine residues. Unlike the closely related inhibitors, antistasin/ghilanten and guamerin, which are selective for coagulation factor Xa or neutrophil elastase, hirustasin binds specifically to tissue kallikrein. The conservation of the pattern of cysteine residues and the significant sequence homology suggest that these related inhibitors possess a similar three-dimensional structure to hirustasin.

RESULTS

The crystal structure of the complex between tissue kallikrein and hirustasin was analyzed at 2.4 resolution. Hirustasin folds into a brick-like structure that is dominated by five disulfide bridges and is sparse in secondary structural elements. The cysteine residues are connected in an abab cdecde pattern that causes the polypeptide chain to fold into two similar motifs. As a hydrophobic core is absent from hirustasin the disulfide bridges maintain the tertiary structure and present the primary binding loop to the active site of the protease. The general structural topography and disulfide connectivity of hirustasin has not previously been described.

CONCLUSIONS

The crystal structure of the kallikrein-hirustasin complex reveals that hirustasin differs from other serine protease inhibitors in its conformation and its disulfide bond connectivity, making it the prototype for a new class of inhibitor. The disulfide pattern shows that the structure consists of two domains, but only the C-terminal domain interacts with the protease. The disulfide pattern of the N-terminal domain is related to the pattern found in other proteins. Kallikrein recognizes hirustasin by the formation of an antiparallel beta sheet between the protease and the inhibitor. The P1 arginine binds in a deep negatively charged pocket of the enzyme. An additional pocket at the periphery of the active site accommodates the sidechain of the P4 valine.

Recombinant hirustasin: production in yeast, crystallization, and interaction with serine proteases

A synthetic gene coding for the 55-amino acid protein hirustasin, a novel tissue kallikrein inhibitor from the leech Hirudo medicinalis, was generated by polymerase chain reaction using overlapping oligonucleotides, fused to the yeast alpha-factor leader sequence and expressed in Saccharomyces cerevisiae. Recombinant hirustasin was secreted mainly as incompletely processed fusion protein, but could be processed in vitro using a soluble variant of the yeast yscF protease. The processed hirustasin was purified to better than 97% purity. N-terminal sequence analysis and electrospray ionization mass spectrometry confirmed a correctly processed N-terminus and the expected amino acid sequence and molecular mass. The biological activity of recombinant hirustasin was identical to that of the authentic leech protein. Crystallized hirustasin alone and in complex with tissue kallikrein diffracted beyond 1.4 A and 2.4 A, respectively. In order to define the reactive site of the inhibitor, the interaction of hirustasin with kallikrein, chymotrypsin, and trypsin was investigated by monitoring complex formation in solution as well as proteolytic cleavage of the inhibitor. During incubation with high, nearly equimolar concentration of tissue kallikrein, hirustasin was cleaved mainly at the peptide bond between Arg 30 and Ile 31, the putative reactive site, to yield a modified inhibitor. In the corresponding complex with chymotrypsin, mainly uncleaved hirustasin was found and cleaved hirustasin species accumulated only slowly. Incubation with trypsin led to several proteolytic cleavages in hirustasin with the primary scissile peptide bond located between Arg 30 and Ile 31. Hirustasin appears to fall into the class of protease inhibitors displaying temporary inhibition.

The role of the leech in medical therapeutics

Leeching is considered by many to be a discredited medical relic of the past. This view is not justified, since leeches still play an important part in modern medicine, as in microsurgery and in the treatment of patients with post-phlebitic syndrome. Hirudin, the potent thrombin inhibitor of leech saliva, has been cloned and is used in the treatment of cardiological and hematological disorders. In our search for other antihemostatic factors in Hirudo medicinalis saliva, we found inhibitors of platelet aggregation induced by thrombin, collagen, adenosine 5'-diphosphate, epinephrine, platelet-activating factor and arachidonic acid. We purified apyrase (adenosine 5'-triphosphate diphosphohydrolase), which is a non-specific inhibitor of platelet aggregation by virtue of its action on adenosine 5'-diphosphate. We isolated and characterized the platelet-activating factor antagonist and also identified and recovered an inhibitor of coagulation factor Xa from leech saliva. This report summarizes our findings and those of other investigators, as well as the experience of one of us (A.E.) in leech therapy.

Mutational analysis of antistasin, an inhibitor of blood coagulation factor Xa derived from the Mexican leech Haementeria officinalis

Antistasin is a Factor Xa inhibitor that is present in the salivary glands of the Mexican leech Haementeria officinalis. The antistasin protein consists of 119 amino acids, of which residues 1-55 (domain I) are 56% similar to residues 56-110 (domain II). Of the nine C-terminal amino acids (residues 111-119; domain III), four are positively charged. The reactive site for Factor Xa is located in domain I. In this study we assessed the role of separate domains and of individual amino acids in the reactive site for the inhibition of Factor Xa. A series of mutants was constructed and expressed in Chinese hamster ovary (CHO) cells. In vitro chromogenic assays for Factor Xa show that domain I is sufficient for inhibition of Factor Xa. Domains II and III neither contain any intrinsic Factor Xa inhibitory activity, nor contribute to the activity of domain I. Furthermore, domain II does not become a Factor Xa inhibitor by partially adaptating its sequence towards that of the reactive site in domain I. Mutation of the cysteine at position 33 is not crucial for Factor Xa inhibition, suggesting a relatively rigid reactive site loop structure.

Isolation and characterization of hirustasin, an antistasin-type serine-proteinase inhibitor from the medical leech Hirudo medicinalis

Antistasin, a potent inhibitor of the blood coagulation factor Xa, is the prototype of a novel family of serine-proteinase inhibitors. We have now isolated, sequenced and characterized an antistasin-type inhibitor from the medical leech Hirudo medicinalis. Hirustasin (Hirudo antistasin) was purified to apparent homogeneity by cation-exchange and affinity chromatography. Amino acid sequencing of the 55 amino acid protein (M(r) 5866) revealed that hirustasin is the only antistasin-type protein known to consist of one domain only; 27% and 32% sequence identity was found to the first and second domains of antistasin, respectively, and a nearly exact conservation of the spacing of the ten cysteine residues. Hirustasin is the first inhibitor of tissue kallikrein identified in leeches, and is also a tight-binding inhibitor of trypsin, chymotrypsin and neutrophil cathepsin G. However, despite the high similarity to antistasin, particularly in the vicinity of the putative reactive-site peptide bond, hirustasin neither inhibits blood coagulation in vitro nor amidolytic activity of isolated factor Xa. Thus, structural elements other than the reactive site sequence significantly influence the specificity of antistasin-type proteinase inhibitors.

A potent inhibitor of platelet activating factor from the saliva of the leech Hirudo medicinalis

Leech saliva is shown to contain protein platelet aggregation inhibitors and a range of selective low molecular weight (LMW) aggregation inhibitors. Gel filtration on Bio-Gel P-2 (cut-off kDa) yields a protein fraction (Fr. I) and three LMW fractions. Fr. I inhibits aggregation induced by collagen, ADP, epinephrine and arachidonic acid. Of all the fractions, only one, Fr. II (LMW) specifically inhibits aggregation induced by platelet activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine). Fr. II also inhibits thrombin-induced platelet aggregation. Fr. III inhibits aggregation induced by ADP, epinephrine and arachidonic acid, and Fr. IV only that induced by arachidonic acid. Fr. II also inhibits PAF- and thrombin-induced thromboxane generation in platelets, but does not inhibit arachidonic acid-induced thromboxane generation. Efforts to separate the anti-PAF from the anti-thrombin activity have been unsuccessful. The inhibition may therefore be due to a single inhibitor, though it may also be due to several inhibitors. Fr. II also inhibits superoxide anion production in formyl Met-Leu-Phe (fMLP)- and ionophore 23187- stimulated neutrophils. This may be due to the inhibition of the effects of PAF generated within the cell. Preliminary results suggest that the Fr. II inhibitor(s) is (are) amphipathic. The interaction of platelets with PAF and their interaction with the inhibitor(s) are mutually exclusive, and the inhibition may be competitive.

Ornatins: potent glycoprotein IIb-IIIa antagonists and platelet aggregation inhibitors from the leech Placobdella ornata

The purification and characterization of six isoforms of ornatin, potent glycoprotein IIb-IIIa (GP IIb-IIIa) antagonists and platelet aggregation inhibitors are described. These isoforms were purified from whole leech homogenates of the leech Placobdella ornata, a North American leech commonly known as the turtle leech, by trichloroacetic acid precipitation, Sephadex G-50 size exclusion chromatography, GP IIb-IIIa affinity chromatography, and C18 reverse-phase HPLC. Each of the five completely sequenced isoforms, which range from 41 to 52 residues in length, contains the Arg-Gly-Asp (RGD) sequence, a common recognition sequence in adhesion proteins, as well as 6 cysteine residues; the positions of both of these features are conserved in the primary sequences. The amino acid sequences of ornatin isoforms B, C, D, and E are highly conserved, whereas ornatin A2 and A3 are less similar and lack 9 residues at the N-terminus. The ornatins are approximately 40% identical with decorsin, a GP IIb-IIIa antagonist isolated from the leech Macrobdella decora [Seymour, J. L., Henzel, W. J., Nevins, B., Stults, J. T. & Lazarus, R. A. (1990) J. Biol. Chem. 265, 10143-10147]; furthermore, the RGD sequence and 5 out of 6 cysteine residues are maintained in the same relative positions in both decorsin and ornatin. The ornatin isoforms do not exhibit significant similarity to any members of the snake-venom-derived family of GP IIb-IIIa antagonists [Dennis, M. S., Henzel, W. J., Pitti, R. M., Lipari, M. T., Napier, M. A., Deisher, T. A., Bunting, S. & Lazarus, R. A. (1990) Proc. Natl Acad. Sci. USA 87, 2471-2475] except in the RGD region of these proteins. The ornatin isoforms inhibit the binding of GP IIb-IIIa to immobilized fibrinogen with IC50 values ranging over 2.9-5.3 nM; ornatin isoforms A2, C, and E inhibit ADP-induced human platelet aggregation with IC50 values of about 130, 280, and 440 nM, respectively.

Purification and characterization of recombinant antistasin: a leech-derived inhibitor of coagulation factor Xa

Antistasin (ATS) is a selective, tight-binding inhibitor of blood coagulation Factor Xa originally isolated from the salivary glands of the Mexican leech Haementeria officinalis. In order to provide sufficient quantities of ATS to further investigate the role of Factor Xa in blood coagulation, a recombinant version of ATS has been produced in an insect baculovirus host-vector system. In this study, we describe the purification and in vitro and in vivo characterization of a single recombinant antistasin (rATS) isoform. The purified protein constitutes a minor isoform relative to the more abundant ATS isoforms present in leech salivary gland extracts. In vitro, rATS inhibits purified human Factor Xa stoichiometrically, prolongs plasma-based clotting assays at nanomolar concentrations, and like native ATS, is cleaved at a single position by Factor Xa during the course of inhibition. An initial evaluation of the in vivo efficacy of rATS was addressed utilizing a rhesus monkey model of mild disseminated intravascular coagulation. rATS was shown to fully suppress thromboplastin-induced fibrinopeptide A generation in a dose-dependent fashion. The availability of rATS should provide a valuable tool for the critical evaluation of the specific role played by Factor Xa in coagulation.

Amino acid sequence of ghilanten: anticoagulant-antimetastatic principle of the South American leech, Haementeria ghilianii

This study reports the amino acid sequence of ghilanten, an anticoagulant-antimetastatic principle of the hematophagous leech, Haementeria ghilianii. Ghilanten consists of 119 amino acids with twenty cysteines and a consensus sequence for heparin-binding at its carboxyl-terminus. Arginine-34 represents the reactive residue involved in the active-site inhibition of trypsin and Factor Xa. Immunoreactivity data suggest that heterogeneity among ghilantens is due in part to amino acid substitutions at their carboxyltermini.