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

Host-Parasite Relationships in Porcine Ascariosis: Anticoagulant Potential of the Third Larval Stage of Ascaris suum as a Possible Survival Mechanism

Simple Summary

Ascaris suum parasitises pigs all over the world causing a disease responsible for producing reductions in weight gains and damages to several organs of the infected animals that incur huge economic losses for the swine industry. While adult worms of this parasite are located in the small intestine of the host, their larval stages migrate through the bloodstream as an evolutionary advantageous strategy within a hostile environment that confronts host responses such as blood clots formation. The aim of this work is to study the ability of A. suum larvae to inhibit blood coagulation as a possible mechanism to control blood clots formation and facilitate their migration. The results showed that these larvae inhibited host blood coagulation and possessed molecules similar to those responsible for inhibiting blood coagulation in pigs. The anticoagulant effect of A. suum larvae could constitute a potential survival mechanism for the parasite. Therefore, developing new control strategies directed at this and similar processes could avoid A. suum larval migration and the establishment of adult worms in their definitive location, which is necessary to confront the damages and economic losses produced by this parasitosis.

Abstract

In order to evade the response of their hosts, helminth parasites have evolved precise and highly regulated mechanisms, including migration strategies of the larval stages. In regard to porcine ascariosis caused by Ascaris suum, its infective third-stage larvae (AsL3) undergo a complex migratory route through the bloodstream of their host before establishing in the small intestine to reach maturation. Despite the benefits attributed to this migration, blood clots formation could compromise larvae survival. The aim of this work was to study the interaction between the cuticle and excretory/secretory antigens of AsL3 and the host coagulation cascade. Larvae were obtained after incubating and hatching A. suum eggs, after which the antigenic extracts were produced. Their ability to disrupt the coagulation cascade was studied using anticoagulation and chromogenic assays, and techniques based on electrophoresis. The obtained results showed that both antigenic extracts possessed anticoagulant potential, being able to inhibit the intrinsic, extrinsic and/or common pathways of the blood coagulation cascade as well as the activated factor X. Moreover, three A. suum serpin proteins were identified as candidates to inhibit this host coagulation factor. To the best of our knowledge, this study shows, for the first time, the anticoagulant potential of the infective larvae of A. suum, which could be used by the parasite as a mechanism to facilitate its invasion and survival in the host.

Apyrase with anti-platelet aggregation activity from the nymph of the camel tick Hyalomma dromedarii

Apyrase is one of the essential platelet aggregation inhibitors in hematophagous arthropods due to its ability to hydrolyze ATP and ADP molecules. Here, an apyrase (TNapyrase) with antiplatelet aggregation activity was purified and characterized from the nymphs of the camel tick Hyalomma dromedarii through anion exchange and gel filtration columns. The homogeneity of TNapyrase was confirmed by native-PAGE, SDS-PAGE as well as with isoelectric focusing. Purified TNapyrase had a molecular mass of 25 kDa and a monomer structure. TNapyrase hydrolyzed various nucleotides in the order of ATP > PPi > ADP > UDP > 6GP. The K_m value was 1.25 mM ATP and its optimum activity reached at pH 8.4. The influence of various ions on TNapyrase activity showed that FeCl₂, FeCl₃ and ZnCl₂ are activators of TNapyrase. EDTA inhibited TNapyrase activity competitively with a single binding site on the molecule and K_i value of 2 mM. Finally, TNapyrase caused 70% inhibition of ADP-stimulated platelets aggregation and is a possible target for antibodies in future tick vaccine studies.

Chemical Equilibrium at the Tick-Host Feeding Interface:A Critical Examination of Biological Relevance in Hematophagous Behavior

Ticks secrete hundreds to thousands of proteins into the feeding site, that presumably all play important functions in the modulation of host defense mechanisms. The current review considers the assumption that tick proteins have functional relevance during feeding. The feeding site may be described as a closed system and could be treated as an ideal equilibrium system, thereby allowing modeling of tick-host interactions in an equilibrium state. In this equilibrium state, the concentration of host and tick proteins and their affinities will determine functional relevance at the tick-host interface. Using this approach, many characterized tick proteins may have functional relevant concentrations and affinities at the feeding site. Conversely, the feeding site is not an ideal closed system, but is dynamic and changing, leading to possible overestimation of tick protein concentration at the feeding site and consequently an overestimation of functional relevance. Ticks have evolved different possible strategies to deal with this dynamic environment and overcome the barrier that equilibrium kinetics poses to tick feeding. Even so, cognisance of the limitations that equilibrium binding place on deductions of functional relevance should serve as an important incentive to determine both the concentration and affinity of tick proteins proposed to be functional at the feeding site.

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.

Amblyomma americanum tick calreticulin binds C1q but does not inhibit activation of the classical complement cascade

In this study we characterized Amblyomma americanum (Aam) tick calreticulin (CRT) homolog in tick feeding physiology. In nature, different tick species can be found feeding on the same animal host. This suggests that different tick species found feeding on the same host can modulate the same host anti-tick defense pathways to successfully feed. From this perspective it's plausible that different tick species can utilize universally conserved proteins such as CRT to regulate and facilitate feeding. CRT is a multi-functional protein found in most taxa that is injected into the vertebrate host during tick feeding. Apart from it's current use as a biomarker for human tick bites, role(s) of this protein in tick feeding physiology have not been elucidated. Here we show that annotated functional CRT amino acid motifs are well conserved in tick CRT. However our data show that despite high amino acid identity levels to functionally characterized CRT homologs in other organisms, AamCRT is apparently functionally different. Pichia pastoris expressed recombinant (r) AamCRT bound C1q, the first component of the classical complement system, but it did not inhibit activation of this pathway. This contrast with reports of other parasite CRT that inhibited activation of the classical complement pathway through sequestration of C1q. Furthermore rAamCRT did not bind factor Xa in contrast to reports of parasite CRT binding factor Xa, an important protease in the blood clotting system. Consistent with this observation, rAamCRT did not affect plasma clotting or platelet aggregation. We discuss our findings in the context of tick feeding physiology.

A 24-48 h fed Amblyomma americanum tick saliva immuno-proteome

BACKGROUND

Multiple tick saliva proteins, the majority of which are unknown, confer tick resistance in repeatedly infested animals. The objective of this study was to identify the 24-48 h fed Amblyomma americanum tick saliva immuno-proteome. The 24-48 h tick-feeding phase is critical to tick parasitism as it precedes important events in tick biology, blood meal feeding and disease agent transmission. Fed male, 24 and 96 h fed female phage display cDNA expression libraries were biopanned using rabbit antibodies to 24 and 48 h fed A. americanum female tick saliva proteins. Biopanned immuno-cDNA libraries were subjected to next generation sequencing, de novo assembly, and bioinformatic analysis.

RESULTS

More than 800 transcripts that code for 24-48 h fed A. americanum immuno-proteins are described. Of the 895 immuno-proteins, 52% (464/895) were provisionally identified based on matches in GenBank. Of these, ~19% (86/464) show high level of identity to other tick hypothetical proteins, and the rest include putative proteases (serine, cysteine, leukotriene A-4 hydrolase, carboxypeptidases, and metalloproteases), protease inhibitors (serine and cysteine protease inhibitors, tick carboxypeptidase inhibitor), and transporters and/or ligand binding proteins (histamine binding/lipocalin, fatty acid binding, calreticulin, hemelipoprotein, IgG binding protein, ferritin, insulin-like growth factor binding proteins, and evasin). Others include enzymes (glutathione transferase, cytochrome oxidase, protein disulfide isomerase), ribosomal proteins, and those of miscellaneous functions (histamine release factor, selenoproteins, tetraspanin, defensin, heat shock proteins).

CONCLUSIONS

Data here demonstrate that A. americanum secretes a complex cocktail of immunogenic tick saliva proteins during the first 24-48 h of feeding. Of significance, previously validated immunogenic tick saliva proteins including AV422 protein, calreticulin, histamine release factor, histamine binding/lipocalins, selenoproteins, and paramyosin were identified in this screen, supporting the specificity of the approach in this study. While descriptive, this study opens opportunities for in-depth tick feeding physiology studies.

Ticks and tick-borne pathogens at the cutaneous interface: host defenses, tick countermeasures, and a suitable environment for pathogen establishment

Ticks are unique among hematophagous arthropods by continuous attachment to host skin and blood feeding for days; complexity and diversity of biologically active molecules differentially expressed in saliva of tick species; their ability to modulate the host defenses of pain and itch, hemostasis, inflammation, innate and adaptive immunity, and wound healing; and, the diverse array of infectious agents they transmit. All of these interactions occur at the cutaneous interface in a complex sequence of carefully choreographed host defense responses and tick countermeasures resulting in an environment that facilitates successful blood feeding and establishment of tick-borne infectious agents within the host. Here, we examine diverse patterns of tick attachment to host skin, blood feeding mechanisms, salivary gland transcriptomes, bioactive molecules in tick saliva, timing of pathogen transmission, and host responses to tick bite. Ticks engage and modulate cutaneous and systemic immune defenses involving keratinocytes, natural killer cells, dendritic cells, T cell subpopulations (Th1, Th2, Th17, Treg), B cells, neutrophils, mast cells, basophils, endothelial cells, cytokines, chemokines, complement, and extracellular matrix. A framework is proposed that integrates tick induced changes of skin immune effectors with their ability to respond to tick-borne pathogens. Implications of these changes are addressed. What are the consequences of tick modulation of host cutaneous defenses? Does diversity of salivary gland transcriptomes determine differential modulation of host inflammation and immune defenses and therefore, in part, the clades of pathogens effectively transmitted by different tick species? Do ticks create an immunologically modified cutaneous environment that enhances specific pathogen establishment? Can tick saliva molecules be used to develop vaccines that block pathogen transmission?

A Novel Factor Xa-Inhibiting Peptide from Centipedes Venom

Centipedes have been used as traditional medicine for thousands of years in China. Centipede venoms consist of many biochemical peptides and proteins. Factor Xa (FXa) is a serine endopeptidase that plays the key role in blood coagulation, and has been used as a new target for anti-thrombotic drug development. A novel FXa inhibitor, a natural peptide with the sequence of Thr-Asn-Gly-Tyr-Thr (TNGYT), was isolated from the venom of Scolopendra subspinipesmutilans using a combination of size-exclusion and reverse-phase chromatography. The molecular weight of the TNGYT peptide was 554.3 Da measured by electrospray ionization mass spectrometry. The amino acid sequence of TNGYT was determined by Edman degradation. TNGYT inhibited the activity of FXa in a dose-dependent manner with an IC₅₀ value of 41.14 mg/ml. It prolonged the partial thromboplastin time and prothrombin time in both in vitro and ex vivo assays. It also significantly prolonged whole blood clotting time and bleeding time in mice. This is the first report that an FXa inhibiting peptide was isolated from centipedes venom.

Deorphanization and target validation of cross-tick species conserved novel Amblyomma americanum tick saliva protein

We previously identified a cross-tick species conserved tick feeding stimuli responsive Amblyomma americanum (Aam) AV422 gene. This study demonstrates that AamAV422 belongs to a novel group of arthropod proteins that is characterized by 14 cysteine amino acid residues: C(23)-X7/9-C(33)-X23/24-C(58)-X8-C(67)-X7-C(75)-X23-C(99)-X15-C(115)-X10-C(126)-X24/25/33-C(150)C(151)-X7-C(159)-X8-C(168)-X23/24-C(192)-X9/10-C(202) predicted to form seven disulfide bonds. We show that AamAV422 protein is a ubiquitously expressed protein that is injected into the host within the first 24h of the tick attaching onto the host as revealed by Western blotting analyses of recombinant (r)AamAV422, tick saliva and dissected tick organ protein extracts using antibodies to 24 and 48 h tick saliva proteins. Native AamAV422 is apparently involved with mediating tick anti-hemostasis and anti-complement functions in that rAamAV422 delayed plasma clotting time in a dose responsive manner by up to ≈ 160 s, prevented platelet aggregation by up to ≈ 16% and caused ≈ 24% reduction in production of terminal complement complexes. Target validation analysis revealed that rAamAV422 is a potential candidate for a cocktail or multivalent tick vaccine preparation in that RNA interference (RNAi)-mediated silencing of AamAV422 mRNA caused a statistically significant (≈ 44%) reduction in tick engorgement weights, which is proxy for amounts of ingested blood. We speculate that AamAV422 is a potential target antigen for development of the highly desired universal tick vaccine in that consistent with high conservation among ticks, antibodies to 24h Ixodes scapularis tick saliva proteins specifically bound rAamAV422. We discuss data in this study in the context of advancing the biology of tick feeding physiology and discovery of potential target antigens for tick vaccine development.

Tick salivary secretion as a source of antihemostatics

Ticks are mostly obligatory blood feeding ectoparasites that have an impact on human and animal health. In addition to direct damage due to feeding, some tick species serve as the vectors for the causative agents of several diseases, such as the spirochetes of the genus Borrelia causing Lyme disease, the virus of tick-borne encephalitis, various Rickettsial pathogens or even protozoan parasites like Babesia spp. Hard ticks are unique among bloodfeeders because of their prolonged feeding period that may last up to two weeks. During such a long period of blood uptake, the host develops a wide range of mechanisms to prevent blood loss. The arthropod ectoparasite, in turn, secretes saliva in the sites of bite that assists blood feeding. Indeed, tick saliva represents a rich source of proteins with potent pharmacologic action that target different mechanisms of coagulation, platelet aggregation and vasoconstriction. Tick adaptation to their vertebrate hosts led to the inclusion of a powerful protein armamentarium in their salivary secretion that has been investigated by high-throughput methods. The resulting knowledge can be exploited for the isolation of novel antihemostatic agents. Here we review the tick salivary antihemostatics and their characterized functions at the molecular and cellular levels.

Transcriptome analysis of the salivary glands of Dermacentor andersoni Stiles (Acari: Ixodidae)

Amongst blood-feeding arthropods, ticks of the family Ixodidae (hard ticks) are vectors and reservoirs of a greater variety of infectious agents than any other ectoparasite. Salivary glands of ixodid ticks secrete a large number of pharmacologically active molecules that not only facilitate feeding but also promote establishment of infectious agents. Genomic, proteomic and immunologic characterization of bioactive salivary gland molecules are, therefore, important as they offer new insights into molecular events occurring at the tick-host interface and they have implications for development of novel control strategies. The present work uses complementary DNA (cDNA) sequence analysis to identify salivary gland transcripts expressed by the Rocky Mountain wood tick, Dermacentor andersoni, a vector of the human pathogens causing Rocky Mountain spotted fever, Colorado tick fever, tularemia, and Powassan encephalitis as well as the veterinary pathogen Anaplasma marginale. Dermacentor andersoni is also capable of inducing tick paralysis. Automated single-pass DNA sequencing was conducted on 1440 randomly selected cDNA clones from the salivary glands of adult female D. andersoni collected during the early stages of feeding (18-24h). Analysis of the expressed sequence tags (ESTs) resulted in 544 singletons and 218 clusters with more than one quality read and attempts were made to assign putative functions to tick genes based on amino acid identity to published protein databases. Approximately 25.6% (195) of the sequences showed limited or no homology to previously identified gene products. A number of novel sequences were identified which presented significant sequence similarity to mammalian genes normally associated with extracellular matrix (ECM), regulation of immune responses, tumor suppression, and wound healing. Several coding sequences possessed various degrees of homology to previously described proteins from other tick species. Preliminary nucleotide variation analysis of these and other tick sequences suggests extensive nucleotide diversity, which has implications for evolution of tick feeding. Intra-species diversity studies can be a promising tool for identifying sequence variations potentially associated with phenotypic traits affecting vector-host-pathogen interactions.

Adaptation of ticks to a blood-feeding environment: evolution from a functional perspective

Ticks had to adapt to an existing and complex vertebrate hemostatic system from being free-living scavengers. A large array of anti-hemostatic mechanisms evolved during this process and includes blood coagulation as well as platelet aggregation inhibitors. Several questions regarding tick evolution exist. What was the nature of the ancestral tick? When did ticks evolve blood-feeding capabilities? How did these capabilities evolve? Did host specificity influence the adaptation of ticks to a blood-feeding environment? What are the implications of tick evolution for future research into tick biology and vaccine development? We investigate these questions in the light of recent research into protein superfamilies from tick saliva. Our conclusions are that the main tick families adapted independently to a blood-feeding environment. This is supported by major differences observed in all processes involved with blood-feeding for hard and soft ticks. Gene duplication events played a major role in the evolution of novel protein functions involved in tick-host interactions. This occurred during the late Cretaceous and was stimulated by the radiation of birds and placental mammals, which provided numerous new niches for ticks to adapt to a new lifestyle. Independent adaptation of the main tick families to a blood-feeding environment has several implications for future tick research in terms of tick genome projects and vaccine development.

Inhibition mechanisms of hematophagous invertebrate compounds acting on the host blood coagulation and platelet aggregation pathways

To facilitate blood feeding, hematophagous invertebrates have evolved a sophisticated array of physiological compounds that counteract homeostatic systems and inflammatory reactions of the vertebrate host. For this reason, hematophagous invertebrates possess a variety of anticoagulation components that are inhibitors of coagulant factors or antagonists of the platelet receptor. The examination of kinetic data and the crystal structure analysis have exposed the inhibition mechanisms for many of these anticoagulant reagents. Here, we attempt to classify the antihemostatic molecules and to focus on the kinetic approaches that have been instrumental in defining these mechanisms.

A novel family of anticoagulants from the saliva of Ixodes scapularis

Using biochemical and molecular approaches, we have identified a 9.8 kDa protein in the saliva of Ixodes scapularis that inhibits the intrinsic pathway of coagulation. The 9.8 kDa anticoagulant protein was purified by reverse-phase HPLC and its N-terminal amino acid sequence determined. The N-terminal sequence showed homology with Salp14, an immuno-dominant antigen present in the saliva of engorging I. scapularis nymphs. Recombinant Salp14 expressed in Escherichia coli prolonged the activated partial thromboplastin time (APTT) of human plasma in a dose-dependent manner and was a specific inhibitor of factor Xa. A cDNA encoding a 9.3 kDa protein, Salp9Pac, was subsequently isolated from an I. scapularis salivary gland cDNA library. Salp9Pac showed 93% identity to the N-terminal sequence of the anticoagulant purified by HPLC. These data indicate that the anticoagulant protein purified by HPLC, Salp9Pac and Salp14 are members of a family of novel coagulation protease inhibitors present in tick saliva. While recombinant Salp9Pac did not show biological activity in the assays tested currently, it is likely to be mechanistically different from its paralogues. This raises the possibility that ticks may enhance their adaptive ability to cope with a wide spectrum of proteases, by transcribing such structurally related anticoagulant proteins with different functions.

Factor Xa inhibition in the prevention of venous thromboembolism and treatment of patients with venous thromboembolism

Venous thromboembolism (VTE) is a life-threatening complication following orthopedic surgery. Selective factor Xa inhibition is a new antithrombotic approach designed to avoid difficulties associated with heparins and other current anticoagulants. Several antifactor Xa compounds are in early investigation, but fondaparinux (Arixtra; NV Organon, Oss, The Netherlands; Sanofi-Synthelabo, Paris, France) is the first and most advanced compound in the development of a new class of synthetic antithrombotic agents--the selective factor Xa inhibitors. Fondaparinux has a highly favorable pharmacokinetic profile; four large phase 3 trials comparing subcutaneous fondaparinux 2.5 mg once daily with the low molecular weight heparin (LMWH) enoxaparin in doses approved by regulatory bodies showed that fondaparinux reduced the overall risk of VTE in major orthopedic surgery by > 50% without increasing clinically relevant bleeding. Fondaparinux also appears to be a very promising candidate for the treatment of patients with existing VTE.