Serine Protease Inhibitors in Ticks: An Overview of Their Role in Tick Biology and Tick-Borne Pathogen Transmission

Abstract

New tick and tick-borne pathogen control approaches that are both environmentally sustainable and which provide broad protection are urgently needed. Their development, however, will rely on a greater understanding of tick biology, tick-pathogen, and tick-host interactions. The recent advances in new generation technologies to study genomes, transcriptomes, and proteomes has resulted in a plethora of tick biomacromolecular studies. Among these, many enzyme inhibitors have been described, notably serine protease inhibitors (SPIs), whose importance in various tick biological processes is only just beginning to be fully appreciated. Among the multiple active substances secreted during tick feeding, SPIs have been shown to be directly involved in regulation of inflammation, blood clotting, wound healing, vasoconstriction and the modulation of host defense mechanisms. In light of these activities, several SPIs were examined and were experimentally confirmed to facilitate tick pathogen transmission. In addition, to prevent coagulation of the ingested blood meal within the tick alimentary canal, SPIs are also involved in blood digestion and nutrient extraction from the meal. The presence of SPIs in tick hemocytes and their involvement in tick innate immune defenses have also been demonstrated, as well as their implication in hemolymph coagulation and egg development. Considering the involvement of SPIs in multiple crucial aspects of tick-host-pathogen interactions, as well as in various aspects of the tick parasitic lifestyle, these molecules represent highly suitable and attractive targets for the development of effective tick control strategies. Here we review the current knowledge regarding this class of inhibitors in tick biology and tick-borne pathogen transmission, and their potential as targets for future tick control trials.

Keywords: immune responses; tick serine protease inhibitors; tick-borne pathogens; ticks; tick–host interactions.

Figure 1

Representative schematic diagrams of secondary structures of the four types of serine protease inhibitors: serpins, α2 macroglobulins, Kunitz-type inhibitors, and Kazal-type inhibitors. Loops, β-sheets, and α-helices are labeled with blue, yellow, and red, respectively. (A) IRS 2 serpin from the I. ricinus tick showing the three β-sheets A, B, and C (Chmelar et al., ; PDB accession number: 3NDA). The green arrow represents the additional strand (s4A) formed after proteolysis, resulting in Reactive Center Loop cleavage, and insertion of the amino-terminal portion into the A β-sheet. (B) Human α2 macroglobulin complement component 5 in native homodimer conformation (Fredslund et al., ; PDB accession number: 3CU7). (C) Kunitz-type inhibitor Tick-Derived Protease Inhibitor (TDPI) from R. appendiculatus harboring one Kunitz domain composed of β-sheets and an α-helix stabilized by three highly conserved disulphide bridges (Paesen et al., ; PDB accession number: 2UUX). (D) Kazal-type inhibitor Dipetalin from blood-sucking insect Dipetalogaster maximus. harboring one Kazal domain composed of one α-helix with adjacent ß-sheets and loop (Schlott et al., ; PDB accession number: 1KMA).

Figure 2

Schematic representation of a tick during feeding. Several tick serine protease inhibitors identified in different tick species and implicated in both tick biology/physiology and modulation of the vertebrate host responses to tick bite are also indicated. The red arrow represents blood absorption. The blue arrow represents saliva injection. The red star corresponds to a tick-borne pathogen. Note that only half of the digestive tract and a single salivary gland and ovary are represented here. Boophilus microplus subtilisin inhibitors (BmSI); Boophilus microplus Chymotrypsin inhibitor (BmCI); Ixodes ricinus alpha macroglobuline (IrAM); Dermacentor variabilis Kunitz protease inhibitor (DvKPI); Tick α-Macroglobulin (TAM); Boophilus microplus trypsin inhibitor (BmTIs); R. appendiculatus Serpins (RAS); H. longicornis serine proteinase genes (HLSG); Haemaphysalis longicornis midgut Kunitz-type inhibitor (HlMKI); Haemaphysalis longicornis serpin (HLS); Haemaphysalis longicornis chymotrypsin inhibition (HlChI); R. appendiculatus Midgut Serine proteinases (RAMSP); R.B. microplus Serpin (RMS); Amblyomma americanum serpin 6 (AamS6); Amblyomma americanum serpin 19 (AAS19); R.B. microplus Kunitz kallikrein inhibitor (RmKK); I. ricinus contact phase inhibitor (IrCPI); Tick Anticoagulant Protein (TAP); Boophilus microplus Anticoagulant Protein (BmAP); B. microplus gut thrombin inhibitor (BmGTI); Nymph thrombin inhibitor (NTI); Rhipicephalus haemaphysaloides serpin (RHS); I. ricinus immunosuppressive (Iris); I. ricinus serpin (IRS); Ixodes persulcatus immunosuppressive (Ipis); I. ricinus serine protease inhibitor (IrSPI); R. microplus Larval Trypsin Inhibitor (RmLTI).

Figure 3

Schematic overview of the three vertebrate blood coagulation cascade pathways, with indicated tick serine protease inhibitors and their targets. The intrinsic pathway is activated following a trauma and when blood comes into contact with subepithelial cells [contact phase with XII, Prekallikrein (PK), and High Molecular Weight Kininogen molecules (HMWK)], which then results in successive activation of factors XII, XI, and IX. The IXa/VIIIa complex then activates transformation of factor X into factor Xa. The extrinsic pathway is initiated following contact between Tissue Factor (TF) from vessels and circulating factor VII. After injury, they form a complex and catalyze the activation of factor X into factor Xa. Once activated by either pathway, factor Xa complexes with its cofactor Va to form the pro-thrombinase complex, which can then convert prothrombin into thrombin. Thrombin then transforms fibrinogen into fibrin to create a plug. Black arrows represent direct activation, dotted blue lines show positive thrombin feedback, red lines correspond to tSPI inhibition targets.