A Kunitz-type proteinase inhibitor from the midgut of the ixodid tick, Haemaphysalis longicornis, and its endogenous target serine proteinase

A longitudinal transcriptomic analysis from unfed to post-engorgement midguts of adult female Ixodes scapularis

The hematophagy behavior has evolved independently several times within the Arthropoda phylum. Interestingly, the process of acquiring a blood meal in ticks is considerably distinct from that observed in other blood-feeding arthropods. Instead of taking seconds to minutes to complete a blood meal, an adult female Ixodes scapularis tick can remain attached to its host for numerous days. During this extended feeding period, the tick undergoes drastic morphological changes. It is well established that the tick midgut plays a pivotal role not only in blood meal digestion but also in pathogen acquisition and transmission. However, our understanding of the underlying molecular mechanisms involved in these events remains limited. To expedite tick research, we conducted a comprehensive longitudinal RNA-sequencing of the tick midgut before, during, and after feeding. By collecting ticks in different feeding stages (unfed, slow feeding, rapid feeding, and early post-detached), we obtained a comprehensive overview of the transcripts present in each stage and the dynamic transcriptional changes that occur between them. This provides valuable insights into tick physiology. Additionally, through unsupervised clustering, we identified transcripts with similar patterns and stage-specific sequences. These findings serve as a foundation for selecting targets in the development of anti-tick control strategies and facilitate a better understanding of how blood feeding and pathogen infection impact tick physiology.

Tick Salivary Kunitz-Type Inhibitors: Targeting Host Hemostasis and Immunity to Mediate Successful Blood Feeding

Kunitz domain-containing proteins are ubiquitous serine protease inhibitors with promising therapeutic potential. They target key proteases involved in major cellular processes such as inflammation or hemostasis through competitive inhibition in a substrate-like manner. Protease inhibitors from the Kunitz superfamily have a low molecular weight (18-24 kDa) and are characterized by the presence of one or more Kunitz motifs consisting of α-helices and antiparallel β-sheets stabilized by three disulfide bonds. Kunitz-type inhibitors are an important fraction of the protease inhibitors found in tick saliva. Their roles in inhibiting and/or suppressing host homeostatic responses continue to be shown to be additive or synergistic with other protease inhibitors such as cystatins or serpins, ultimately mediating successful blood feeding for the tick. In this review, we discuss the biochemical features of tick salivary Kunitz-type protease inhibitors. We focus on their various effects on host hemostasis and immunity at the molecular and cellular level and their potential therapeutic applications. In doing so, we highlight that their pharmacological properties can be exploited for the development of novel therapies and vaccines.

Hemalin from Haemaphysalis flava ticks: cloning, expression and antithrombogenicity

Hemalin, initially described in Haemaphysalis longicornis, is a protein with anticoagulant activity. We retrieved a gene fragment functionally annotated as hemalin from H. flava salivary gland transcriptomic library, but its full-length complementary DNA (cDNA) and antithrombogenicity have not been investigated in the species. Here we cloned the full length of hemalin (Hf-hemalin) by 3'-end rapid-amplification of cDNA ends, and the open reading frame (ORF) of Hf-hemalin was expressed in Escherichia coli. The recombinant protein (rHf-Hemalin) was tested for antithrombogenicity. The full-length of Hf-hemalin was 607 bp with an ORF of423 bp. Protein encoded by Hf-hemalin was predicted to contain 2 Kunitz domains and a signal peptide. The expression of Hf-hemalin in salivary glands, midguts and ovaries was higher in the semi-engorged than the fully engorged. Prokaryotic expression yielded a product of 40 kDa containing a glutathione S-transferase (GST) tag. Incubation of rHf-Hemalin with rat plasma significantly extended prothrombin time and activated partial thromboplastin time compared with normal saline and GST controls. Our data demonstrated that Hemalin from H. flava shared a similar primary structure with that from H. longicornis, and was also anticoagulant. Further investigations are needed to test its feasibility to be an antigen candidate for the development of vaccines against ticks.

Peptidase inhibitors in tick physiology

Peptidase inhibitors regulate a wide range of physiological processes involved in the interaction between hematophagous parasites and their hosts, including tissue remodeling, the immune response and blood coagulation. In tick physiology, peptidase inhibitors have a crucial role in adaptation to improve parasitism mechanisms, facilitating blood feeding by interfering with defense-related host peptidases. Recently, a larger number of studies on this topic led to the description of several new tick inhibitors displaying interesting novel features, for example a role in pathogen transmission to the host. A comprehensive review discussing these emerging concepts can therefore shed light on peptidase inhibitor functions, their relevance to tick physiology and their potential applications. Here, we summarize and examine the general characteristics, functional diversity and action of tick peptidase inhibitors with known physiological roles in the tick-host-pathogen interaction.

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

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.

Identification and anticoagulant activity of a novel Kunitz-type protein HA11 from the salivary gland of the tick Hyalomma asiaticum

Kunitz/bovine pancreatic trypsin inhibitor proteins are abundant in the salivary glands of ticks and perform multiple functions in blood feeding, including inhibiting blood coagulation, regulating host blood supply and disrupting host angiogenesis. In this study, we identified a novel gene designated HA11 (Hyalomma asiaticum 11 kDa protein) from the salivary gland of the tick H. asiaticum. HA11 is encoded by a gene with an open reading frame of 306 bp that is translated into a deduced 101 amino acid 11 kDa protein that shares 27% sequence identity with a Kunitz-like protease inhibitor precursor in Amblyomma variegatum. Bioinformatic analysis confirmed HA11 as a member of the Kunitz-type family of inhibitors. Real time-PCR detected HA11 mRNA transcripts in tick larvae and nymphae stages, with levels highest in salivary gland tissue, and transcription was induced by blood feeding. HA11 anticoagulant activity was demonstrated by its ability to delay normal clotting of rabbit plasma in an activated partial thromboplastin time assay. Furthermore, RNA interference confirmed that HA11 influences H. asiaticum development and blood feeding, and the recombinant protein exerted low hemolytic activity. These results suggest HA11 is a novel Kunitz-type anticoagulant protein involved in tick blood feeding that may have potential as an anticoagulant drug or vaccine.

Understanding the evolutionary structural variability and target specificity of tick salivary Kunitz peptides using next generation transcriptome data

BACKGROUND

Ticks are blood-sucking arthropods and a primary function of tick salivary proteins is to counteract the host's immune response. Tick salivary Kunitz-domain proteins perform multiple functions within the feeding lesion and have been classified as venoms; thereby, constituting them as one of the important elements in the arms race with the host. The two main mechanisms advocated to explain the functional heterogeneity of tick salivary Kunitz-domain proteins are gene sharing and gene duplication. Both do not, however, elucidate the evolution of the Kunitz family in ticks from a structural dynamic point of view. The Red Queen hypothesis offers a fruitful theoretical framework to give a dynamic explanation for host-parasite interactions. Using the recent salivary gland Ixodes ricinus transcriptome we analyze, for the first time, single Kunitz-domain encoding transcripts by means of computational, structural bioinformatics and phylogenetic approaches to improve our understanding of the structural evolution of this important multigenic protein family.

RESULTS

Organizing the I. ricinus single Kunitz-domain peptides based on their cysteine motif allowed us to specify a putative target and to relate this target specificity to Illumina transcript reads during tick feeding. We observe that several of these Kunitz peptide groups vary in their translated amino acid sequence, secondary structure, antigenicity, and intrinsic disorder, and that the majority of these groups are subject to a purifying (negative) selection. We finalize by describing the evolution and emergence of these Kunitz peptides. The overall interpretation of our analyses discloses a rapidly emerging Kunitz group with a distinct disulfide bond pattern from the I. ricinus salivary gland transcriptome.

CONCLUSIONS

We propose a model to explain the structural and functional evolution of tick salivary Kunitz peptides that we call target-oriented evolution. Our study reveals that combining analytical approaches (transcriptomes, computational, bioinformatics and phylogenetics) improves our understanding of the biological functions of important salivary gland mediators during tick feeding.

Characterization of a new Kunitz-type serine protease inhibitor from the hard tick Rhipicephalus hemaphysaloides

A new Kunitz-type serine protease inhibitor, Rhipilin-2, was identified in the tick Rhipicephalus hemaphysaloides. The cDNA sequence of Rhipilin-2 is 693 bp, and it encodes a deduced 195 amino acid protein with a size of 22 kDa. Bioinformatic analysis shows that Rhipilin-2 belongs to the Kunitz-type family of inhibitors, containing one Kunitz domain with homology to the tissue factor pathway inhibitor. Using Real time polymerase chain reaction (Real time-PCR), Rhipilin-2 mRNA transcripts were detected in tick salivary glands and midgut. Blood feeding induced transcript expression. The recombinant protein was expressed in insect Sf9 cells and confirmed by immunofluorescence test and Western blot analysis with an anti-His antibody. The purified recombinant Rhipilin-2 inhibited serine protease trypsin and elastase, but not thrombin. The anticoagulant activity of Rhipilin-2 was shown by delaying normal clotting of rabbit plasma in the activated partial thromboplastin time tests. These results indicate that Rhipilin-2 is a novel Kunitz-type serine protease inhibitor involved in tick blood feeding.

Screening and molecular cloning of a protective antigen from the midgut of Haemaphysalis longicornis

Vaccination is considered a promising alternative for controlling tick infestations. Haemaphysalis longicornis midgut proteins separated by SDS-PAGE and transferred to polyvinylidene difluoride (PVDF) membrane were screened for protective value against bites. The western blot demonstrated the immunogenicity of 92 kDa protein (P92). The analysis of the P92 amino acid sequence by LC-MS/MS indicated that it was a H. longicornis paramyosin (Hl-Pmy). The full lenghth cDNA of Hl-Pmy was obtained by rapid amplification of cDNA ends (RACE) which consisted of 2,783 bp with a 161 bp 3' untranslated region. Sequence alignment of tick paramyosin (Pmy) showed that Hl-Pmy shared a high level of conservation among ticks. Comparison with the protective epitope sequence of other invertebrate Pmy, it was calculated that the protective epitope of Hl-Pmy was a peptide (LEEAEGSSETVVEMNKKRDTE) named LEE, which was close to the N-terminal of Hl-Pmy protein. The secondary structure analysis suggested that LEE had non-helical segments within an α-helical structure. These results provide the basis for developing a vaccine against biting H. longicornis ticks.

A four-domain Kunitz-type proteinase inhibitor from Solen grandis is implicated in immune response

Serine proteinase inhibitor (SPI) serves as a negative regulator in immune signal pathway by restraining the activities of serine proteinase (SP) and plays an essential role in the innate immunity. In the present study, a Kunitz-type SPI was identified from the mollusk razor clam Solen grandis (designated as SgKunitz). The full-length cDNA of SgKunitz was of 1284 bp, containing an open reading frame (ORF) of 768 bp. The ORF encoded four Kunitz domains, and their amino acids were well conserved when compared with those in other Kunitz-type SPIs, especially the six cysteines involved in forming of three disulfide bridges in each domain. In addition, the tertiary structure of all the four domains adopted a typical model of Kunitz-type SPI family, indicating SgKunitz was a new member of Kunitz-type SPI superfamily. The mRNA transcripts of SgKunitz were detected in all tested tissues of razor clam, including muscle, mantle, gonad, gill, hepatopancreas and hemocytes, and with the highest expression level in gill. When the razor clams were stimulated by LPS, PGN or β-1, 3-glucan, the expression level of SgKunitz mRNA in hemocytes was significantly up-regulated (P < 0.01), suggesting SgKunitz might involved in the processes of inhibiting the activity of SPs during the immune responses triggered by various pathogens. Furthermore, the recombinant protein of SgKunitz could effectively inhibit the activities of SP trypsin and chymotrypsin in vitro. The present results suggested SgKunitz could serve as an inhibitor of SP involving in the immune response of S. grandis, and provided helpful evidences to understand the regulation mechanism of immune signal pathway in mollusk.

Evolution, expansion and expression of the Kunitz/BPTI gene family associated with long-term blood feeding in Ixodes Scapularis

Background

Recent studies of the tick saliva transcriptome have revealed the profound role of salivary proteins in blood feeding. Kunitz/BPTI proteins are abundant in the salivary glands of ticks and perform multiple functions in blood feeding, such as inhibiting blood coagulation, regulating host blood supply and disrupting host angiogenesis. However, Kunitz/BPTI proteins in soft and hard ticks have different functions and molecular mechanisms. How these differences emerged and whether they are associated with the evolution of long-term blood feeding in hard ticks remain unknown.

Results

In this study, the evolution, expansion and expression of Kunitz/BPTI family in Ixodes scapularis were investigated. Single- and multi-domain Kunitz/BPTI proteins have similar gene structures. Single-domain proteins were classified into three groups (groups I, II and III) based on their cysteine patterns. Group I represents the ancestral branch of the Kunitz/BPTI family, and members of this group function as serine protease inhibitors. The group I domain was used as a module to create multi-domain proteins in hard ticks after the split between hard and soft ticks. However, groups II and III, which evolved from group I, are only present and expanded in the genus Ixodes. These lineage-specific expanded genes exhibit significantly higher expression during long-term blood feeding in Ixodes scapularis. Interestingly, functional site analysis suggested that group II proteins lost the ability to inhibit serine proteases and evolved a new function of modulating ion channels. Finally, evolutionary analyses revealed that the expansion and diversification of the Kunitz/BPTI family in the genus Ixodes were driven by positive selection.

Conclusions

These results suggest that the differences in the Kunitz/BPTI family between soft and hard ticks may be linked to the evolution of long-term blood feeding in hard ticks. In Ixodes, the lineage-specific expanded genes (Group II and III) lost the ancient function of inhibiting serine proteases and evolved new functions to adapt to long-term blood feeding. Therefore, these genes may play a profound role in the long-term blood feeding of hard ticks. Based our analysis, we propose that the six genes identified in our study may be candidate target genes for tick control.

Characterization of Kunitz-type protease inhibitor purified from hemolymph of Galleria mellonella larvae

We characterized a Kunitz-type protease inhibitor (Gm KTPI) obtained from the hemolymph of Galleria mellonella larvae immunized with Escherichia coli. The structural analysis of the cloned cDNA showed that it consists of 56 residues derived from the precursor of 75 amino acids. The peptide was constitutively produced in the fat bodies, but not in the midgut nor the integument of larvae. In our analysis of stage-dependent expression, its transcript was detected within the midgut, the fat bodies and the integument of the prepupae, which undergo tissue remodeling. The inhibition assays showed that Gm KTPI was capable of inhibiting only the trypsin-like activity of the larval midgut extracts. Furthermore, it was determined that Gm KTPI induced the activation of extracellular signal-regulated kinase (ERK) in the fat bodies and integument cells, and this kinase is known to perform a central role in cell proliferation signaling. Its effect on ERK activation was also verified in a control experiment using a human endothelial cell culture. Collectively, it was suggested that Gm KTPI might be responsible for the protection of other tissues against proteolytic attack by trypsin-like protease(s) from larval midgut during metamorphosis, and might play a role in the proliferation of cells in the fat body and integument.