Identification and characterization of novel salivary thrombin inhibitors from the ixodidae tick, Haemaphysalis longicornis

Abstract

Novel antithrombin molecules were identified from the ixodidae tick, Haemaphysalis longicornis. These molecules, named madanin 1 and 2, are 7-kDa proteins and show no significant similarities to any previously identified proteins. Assays using human plasma showed that madanin 1 and 2 dose-dependently prolonged both activated partial thromboplastin time and prothrombin time, indicating that they inhibit both the intrinsic and extrinsic pathways. Direct binding assay by surface plasmon resonance measurement demonstrated that madanin 1 and 2 specifically interacted with thrombin. Furthermore, it was clearly shown that madanin 1 and 2 inhibited conversion of fibrinogen into fibrin by thrombin, thrombin-catalyzed activation of factor V and factor VIII, and thrombin-induced aggregation of platelets without affecting thrombin amidolytic activity. These results suggest that madanin 1 and 2 bind to the anionbinding exosite 1 on the thrombin molecule, but not to the active cleft, and interfere with the association of fibrinogen, factor V, factor VIII and thrombin receptor on platelets with an anion-binding exosite 1. They appear to be exosite 1-directed competitive inhibitors.Keywords: anticoagulant; Haemaphysalis longicornis; salivary gland; thrombin inhibitor; tick.Thrombin has various physiological functions and plays important roles in hemostasis. For example, in the final step of blood clot formation, thrombin converts soluble fibrinogen into fibrin and subsequently triggers cross-linking between fibrin monomers by activating factor XIII [1]. It also amplifies its own generation by activating nonenzymatic cofactors V and VIII as well as factor XI [2,3]. Conversely, it suppresses its own generation by activating protein C [4], which inactivates factor Va and factor VIIIa together with protein S [5], when bound to the endothelial membrane receptor thrombomodulin. In addition, thrombin induces platelet aggregation via proteolytic activation of G-protein-coupled protease-activated receptors (PARs) [6,7]. Specific interactions of thrombin with these substrates, cofactors, and receptors involve not only the catalytic site and the primary binding pocket, but also secondary recognition sites, termed anion-binding exosite 1 and 2. Anion-binding exosite 1 interacts with negatively charged domains on fibrinogen [8], PARs [6,7,9], and thrombomodulin [10,11]. Anion-binding exosite 2 interacts with heparin [12], promoting inhibition of thrombin by antithrombin III [13] and heparin cofactor II [14]. Furthermore, both exosites are involved in the recognition of factor V and factor VIII by thrombin [15].The salivary glands of blood-sucking animals, such as leeches, insects, and ticks, contain various anticoagulants [16]. These substances inhibit the host hemostatic response so that the blood-sucking organism can feed smoothly on host blood. The best known anticoagulant identified from bloodsucking organisms is hirudin, a highly specific thrombin inhibitor, isolated from the medical leech, Hirudo medicinalis [17]. It interacts with tw...

A Novel Gene Encoding a Thrombin Inhibitory Protein in a cDNA Library from Haemaphysalis longicornis Salivary Gland

“…Furthermore, the sequence contained no cysteine residues, indicating that the protein had no threedimensional structures stabilized by disulfide bonds. These characteristics were similar to the thrombin inhibitory proteins that have been previously isolated from blood feeding arthropods [4,11,21,27].…”

“…APTT is a screening assay for deficiency of coagulation factors associated with the intrinsic pathway of coagulation; on the other hand, PT is an assay for the extrinsic pathway. The above results indicated that chimadanin inhibited the blood coagulation factor that contributed to the common pathway of intrinsic and extrinsic coagulation pathways [11].…”

“…Madanin 1 and 2 were previously identified thrombin inhibitors from H. longicornis [11]. They prevented thrombin from cleaving fibrinogen without inhibiting thrombin amidolytic activity toward a small synthetic substrate.…”

“…This means thrombin has a positive feedback loop for its amplification. It indicates that thrombin inhibition means not only inhibiting the final phase of the cascade but also the earlier phase of coagulation [11,27].…”

“…There are also many kinds of anticoagulant substances identified from hard and soft tick species [12,14]. Within the blood coagulation cascade, several serine protease inhibitors such as factor Xa inhibitors [16,24], a factor VIIa/tissue factor complex inhibitor [7] and thrombin inhibitors [9,11,23] have been described.…”