Two secreted cystatins of the soft tick Ornithodoros moubata: differential expression pattern and inhibitory specificity

Tick salivary protein Cystatin: structure, anti-inflammation and molecular mechanism

Ticks are blood-sucking ectoparasites that secrete immunomodulatory substances in saliva to hosts during engorging. Cystatins, a tick salivary protein and natural inhibitor of Cathepsins, are attracting growing interest globally because of the immunosuppressive activities and the feasibility as an antigen for developing anti-tick vaccines. This review outlines the classification and the structure of tick Cystatins, and focuses on the anti-inflammatory effects and molecular mechanisms. Tick Cystatins can be divided into four families based on structures and cystatin 1 and cystatin 2 are the most abundant. They are injected into hosts during blood feeding and effectively mitigate the host inflammatory response. Mechanically, tick Cystatins exert anti-inflammatory properties through the inhibition of TLR-NF-κb, JAK-STAT and p38 MAPK signaling pathways. Further investigations are crucial to confirm the reduction of inflammation in other cell types like neutrophils and mast cells, and fully elucidate the underlying mechanism (like the structural mechanism) to make Cystatin a potential candidate for the development of novel anti-inflammation agents.

Characterization of a Novel Cysteine Protease Inhibitor from Poultry Red Mites: Potential Vaccine for Chickens

Poultry red mite (PRM; Dermanyssus gallinae) is a hazardous, blood-sucking ectoparasite of birds that constitutes a threat to poultry farming worldwide. Acaricides, commonly used in poultry farms to prevent PRMs, are not effective because of the rapid emergence of acaricide-resistant PRMs. However, vaccination may be a promising strategy to control PRM. We identified a novel cystatin-like molecule in PRMs: Dg-Cys. Dg-Cys mRNA expression was detected in the midgut and ovaries, in all stages of life. The PRM nymphs that were artificially fed with the plasma from chickens that were immunized with Dg-Cys in vitro had a significantly reduced reproductive capacity and survival rate. Moreover, combination of Dg-Cys with other antigen candidates, like copper transporter 1 or adipocyte plasma membrane-associated protein, enhanced vaccine efficacies. vaccination and its application as an antigen for cocktail vaccines could be an effective strategy to reduce the damage caused by PRMs in poultry farming.

Immunomodulatory Proteins in Tick Saliva From a Structural Perspective

To feed successfully, ticks must bypass or suppress the host’s defense mechanisms, particularly the immune system. To accomplish this, ticks secrete specialized immunomodulatory proteins into their saliva, just like many other blood-sucking parasites. However, the strategy of ticks is rather unique compared to their counterparts. Ticks’ tendency for gene duplication has led to a diverse arsenal of dozens of closely related proteins from several classes to modulate the immune system’s response. Among these are chemokine-binding proteins, complement pathways inhibitors, ion channels modulators, and numerous poorly characterized proteins whose functions are yet to be uncovered. Studying tick immunomodulatory proteins would not only help to elucidate tick-host relationships but would also provide a rich pool of potential candidates for the development of immunomodulatory intervention drugs and potentially new vaccines. In the present review, we will attempt to summarize novel findings on the salivary immunomodulatory proteins of ticks, focusing on biomolecular targets, structure-activity relationships, and the perspective of their development into therapeutics.

Transcriptomic Analysis of Salivary Glands of Ornithodoros brasiliensis Aragão, 1923, the Agent of a Neotropical Tick-Toxicosis Syndrome in Humans

Tick salivary glands produce and secrete a variety of compounds that modulate host responses and ensure a successful blood meal. Despite great progress made in the identification of ticks salivary compounds in recent years, there is still a paucity of information concerning salivary molecules of Neotropical argasid ticks. Among this group of ticks, considering the number of human cases of parasitism, including severe syndromes and hospitalization, Ornithodoros brasiliensis can be considered one of the major Neotropical argasid species with impact in public health. Here, we describe the transcriptome analysis of O. brasiliensis salivary glands (ObSG). The transcriptome yielded ~14,957 putative contigs. A total of 368 contigs were attributed to secreted proteins (SP), which represent approximately 2.5% of transcripts but ~53% expression coverage transcripts per million. Lipocalins are the major protein family among the most expressed SP, accounting for ~16% of the secretory transcripts and 51% of secretory protein abundance. The most expressed transcript is an ortholog of TSGP4 (tick salivary gland protein 4), a lipocalin first identified in Ornithodoros kalahariensis that functions as a leukotriene C₄ scavenger. A total of 55 lipocalin transcripts were identified in ObSG. Other transcripts potentially involved in tick-host interaction included as: basic/acid tail secretory proteins (second most abundant expressed group), serine protease inhibitors (including Kunitz inhibitors), 5' nucleotidases (tick apyrases), phospholipase A₂, 7 disulfide bond domain, cystatins, and tick antimicrobial peptides. Another abundant group of proteins in ObSG is metalloproteases. Analysis of these major protein groups suggests that several duplication events after speciation were responsible for the abundance of redundant compounds in tick salivary glands. A full mitochondrial genome could be assembled from the transcriptome data and confirmed the close genetic identity of the tick strain sampled in the current study, to a tick strain previously implicated in tick toxicoses. This study provides novel information on the molecular composition of ObSG, a Brazilian endemic tick associated with several human cases of parasitism. These results could be helpful in the understanding of clinical findings observed in bitten patients, and also, could provide more information on the evolution of Neotropical argasids.

Mialostatin, a Novel Midgut Cystatin from Ixodes ricinus Ticks: Crystal Structure and Regulation of Host Blood Digestion

The hard tick Ixodes ricinus is a vector of Lyme disease and tick-borne encephalitis. Host blood protein digestion, essential for tick development and reproduction, occurs in tick midgut digestive cells driven by cathepsin proteases. Little is known about the regulation of the digestive proteolytic machinery of I. ricinus. Here we characterize a novel cystatin-type protease inhibitor, mialostatin, from the I. ricinus midgut. Blood feeding rapidly induced mialostatin expression in the gut, which continued after tick detachment. Recombinant mialostatin inhibited a number of I. ricinus digestive cysteine cathepsins, with the greatest potency observed against cathepsin L isoforms, with which it co-localized in midgut digestive cells. The crystal structure of mialostatin was determined at 1.55 Å to explain its unique inhibitory specificity. Finally, mialostatin effectively blocked in vitro proteolysis of blood proteins by midgut cysteine cathepsins. Mialostatin is likely to be involved in the regulation of gut-associated proteolytic pathways, making midgut cystatins promising targets for tick control strategies.

Sialotranscriptomics of the argasid tick Ornithodoros moubata along the trophogonic cycle

The argasid tick Ornithodoros moubata is the main vector of human relapsing fever (HRF) and African swine fever (ASF) in Africa. Salivary proteins are part of the host-tick interface and play vital roles in the tick feeding process and the host infection by tick-borne pathogens; they represent interesting targets for immune interventions aimed at tick control.

The present work describes the transcriptome profile of salivary glands of O. moubata and assesses the gene expression dynamics along the trophogonic cycle using Illumina sequencing.

De novo transcriptome assembling resulted in 71,194 transcript clusters and 41,011 annotated transcripts, which represent 57.6% of the annotation success. Most salivary gene expression takes place during the first 7 days after feeding (6,287 upregulated transcripts), while a minority of genes (203 upregulated transcripts) are differentially expressed between 7 and 14 days after feeding. The functional protein groups more abundantly overrepresented after blood feeding were lipocalins, proteases (especially metalloproteases), protease inhibitors including the Kunitz/BPTI-family, proteins with phospholipase A2 activity, acid tail proteins, basic tail proteins, vitellogenins, the 7DB family and proteins involved in tick immunity and defence. The complexity and functional redundancy observed in the sialotranscriptome of O. moubata are comparable to those of the sialomes of other argasid and ixodid ticks.

This transcriptome provides a valuable reference database for ongoing proteomics studies of the salivary glands and saliva of O. moubata aimed at confirming and expanding previous data on the O. moubata sialoproteome.

The soft tick Ornithodoros moubata constitutes an important medical and veterinary problem in Africa because, in addition to being the vector of African swine fever, it transmits human relapsing fever (HRF), a hyper-endemic and lethal, but still neglected, tick-borne disease. Effective control of HRF requires eradicating its vector tick from domestic environments. As chemical acaricide application is ineffective against this tick, development of anti-tick vaccines seems the most promising method for tick control. Salivary proteins play essential functions for tick feeding and survival, which convert them in potential antigen targets for the development of tick vaccines. To know which these proteins are, we obtained the salivary transcriptome of O. moubata females and established, for the first time in a soft tick, the salivary gene transcription dynamics along its trophogonic cycle. Thereby, we have identified numerous genes encoding bioactive proteins essential for tick feeding. This information is essential to drive the selection of candidate antigens for anti-tick vaccine development and evaluate its protective potential in animal immunization trials. These data significantly enlarge the current repertory of known protein-coding sequences from soft tick salivary glands and establish a valuable reference database to improve our knowledge of the O. moubata salivary proteome.

RNA-seq analysis of the salivary glands and midgut of the Argasid tick Ornithodoros rostratus

Ornithodoros rostratus is a South American argasid tick which importance relies on its itchy bite and potential as disease vector. They feed on a wide variety of hosts and secrete different molecules in their saliva and intestinal content that counteract host defences and help to accommodate and metabolize the relatively large quantity of blood upon feeding. The present work describes the transcriptome profile of salivary gland (SG) and midgut (MG) of O. rostratus using Illumina sequencing. A total of 8,031 contigs were assembled and assigned to different functional classes. Secreted proteins were the most abundant in the SG and accounted for ~67% of all expressed transcripts with contigs with identity to lipocalins and acid tail proteins being the most representative. On the other hand, immunity genes were upregulated in MG with a predominance of defensins and lysozymes. Only 10 transcripts in SG and 8 in MG represented ~30% of all RNA expressed in each tissue and one single contig (the acid tail protein ORN-9707) represented ~7% of all expressed contigs in SG. Results highlight the functional difference of each organ and identified the most expressed classes and contigs of O. rostratus SG and MG.

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.

Argasid and ixodid systematics: Implications for soft tick evolution and systematics, with a new argasid species list

The systematics of the genera and subgenera within the soft tick family Argasidae is not adequately resolved. Different classification schemes, reflecting diverse schools of scientific thought that elevated or downgraded groups to genera or subgenera, have been proposed. In the most recent classification scheme, Argas and Ornithodoros are paraphyletic and the placement of various subgenera remains uncertain because molecular data are lacking. Thus, reclassification of the Argasidae is required. This will enable an understanding of soft tick systematics within an evolutionary context. This study addressed that knowledge gap using mitochondrial genome and nuclear (18S and 28S ribosomal RNA) sequence data for representatives of the subgenera Alectorobius, Argas, Chiropterargas, Ogadenus, Ornamentum, Ornithodoros, Navis (subgen. nov.), Pavlovskyella, Persicargas, Proknekalia, Reticulinasus and Secretargas, from the Afrotropical, Nearctic and Palearctic regions. Hard tick species (Ixodidae) and a new representative of Nuttalliella namaqua (Nuttalliellidae), were also sequenced with a total of 83 whole mitochondrial genomes, 18S rRNA and 28S rRNA genes generated. The study confirmed the utility of next-generation sequencing to retrieve systematic markers. Paraphyly of Argas and Ornithodoros was resolved by systematic analysis and a new species list is proposed. This corresponds broadly with the morphological cladistic analysis of Klompen and Oliver (1993). Estimation of divergence times using molecular dating allowed dissection of phylogeographic patterns for argasid evolution. The discovery of cryptic species in the subgenera Chiropterargas, Ogadenus and Ornithodoros, suggests that cryptic speciation is common within the Argasidae. Cryptic speciation has implications for past biological studies of soft ticks. These are discussed in particular for the Ornithodoros (Ornithodoros) moubata and Ornithodoros (Ornithodoros) savignyi groups.

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.

Inducible glutathione S-transferase (IrGST1) from the tick Ixodes ricinus is a haem-binding protein

Blood-feeding parasites are inadvertently exposed to high doses of potentially cytotoxic haem liberated upon host blood digestion. Detoxification of free haem is a special challenge for ticks, which digest haemoglobin intracellularly. Ticks lack a haem catabolic mechanism, mediated by haem oxygenase, and need to dispose of vast majority of acquired haem via its accumulation in haemosomes. The knowledge of individual molecules involved in the maintenance of haem homeostasis in ticks is still rather limited. RNA-seq analyses of the Ixodes ricinus midguts from blood- and serum-fed females identified an abundant transcript of glutathione S-transferase (gst) to be substantially up-regulated in the presence of red blood cells in the diet. Here, we have determined the full sequence of this encoding gene, ir-gst1, and found that it is homologous to the delta-/epsilon-class of GSTs. Phylogenetic analyses across related chelicerates revealed that only one clear IrGST1 orthologue could be found in each available transcriptome from hard and soft ticks. These orthologues create a well-supported clade clearly separated from other ticks' or mites' delta-/epsilon-class GSTs and most likely evolved as an adaptation to tick blood-feeding life style. We have confirmed that IrGST1 expression is induced by dietary haem(oglobin), and not by iron or other components of host blood. Kinetic properties of recombinant IrGST1 were evaluated by model and natural GST substrates. The enzyme was also shown to bind haemin in vitro as evidenced by inhibition assay, VIS spectrophotometry, gel filtration, and affinity chromatography. In the native state, IrGST1 forms a dimer which further polymerises upon binding of excessive amount of haemin molecules. Due to susceptibility of ticks to haem as a signalling molecule, we speculate that the expression of IrGST1 in tick midgut functions as intracellular buffer of labile haem pool to ameliorate its cytotoxic effects upon haemoglobin intracellular hydrolysis.

IrC2/Bf - A yeast and Borrelia responsive component of the complement system from the hard tick Ixodes ricinus

Ticks possess components of a primordial complement system that presumably play a role in the interaction of the tick immune system with tick-borne pathogens and affect their transmission. Here we characterized a novel complement component, tagged as IrC2/Bf, from the hard tick Ixodes ricinus, the principal vector of Lyme disease in Europe. IrC2/Bf is a multi-domain molecule composed of 5-7 CCP modules, varied by alternative splicing, followed by a von Willebrand factor A domain and a C-terminal trypsin-like domain. The primary structure and molecular architecture of IrC2/Bf displays the closest homology to the C3-complement component convertases described in horseshoe crabs. The irc2/bf gene is mainly expressed in the tick fat body associated with the trachea and, as determined by western blotting, the protein is present in low amounts in tick hemolymph. Expression of irc2/bf mRNA was significantly up-regulated in response to the intra-hemocoelic injection of the yeast Candida albicans and all tested Borrelia sp. strains (B. burgdorferi NE5264, B. burgdorferi CB26, B. garinii MSLB, B. afzelii CB43), but was not affected by injection of model Gram-negative and Gram-positive bacteria or the aseptic injection control. In-line with these results, RNAi-mediated silencing of irc2/bf inhibited phagocytosis of B. afzelii and C. albicans but not the other bacteria. Tissue expression profiles, specific responses to microbial challenges, and patterns of phagocytic phenotypes upon RNAi silencing observed for IrC2/Bf match well with the previously reported characteristics of I. ricinus C3-related molecule 1 (IrC3-1). Therefore we presume that IrC2/Bf functions as a convertase in the same complement activation pathway protecting ticks against yeast and Borrelia infection.

A novel type I cystatin of parasite origin with atypical legumain-binding domain

Parasite inhibitors of cysteine peptidases are known to influence a vast range of processes linked to a degradation of either the parasites' own proteins or proteins native to their hosts. We characterise a novel type I cystatin (stefin) found in a sanguinivorous fish parasite Eudiplozoon nipponicum (Platyhelminthes: Monogenea). We have identified a transcript of its coding gene in the transcriptome of adult worms. Its amino acid sequence is similar to other stefins except for containing a legumain-binding domain, which is in this type of cystatins rather unusual. As expected, the recombinant form of E. nipponicum stefin (rEnStef) produced in Escherichia coli inhibits clan CA peptidases - cathepsins L and B of the worm - via the standard papain-binding domain. It also blocks haemoglobinolysis by cysteine peptidases in the worm's excretory-secretory products and soluble extracts. Furthermore, we had confirmed its ability to inhibit clan CD asparaginyl endopeptidase (legumain). The presence of a native EnStef in the excretory-secretory products of adult worms, detected by mass spectrometry, suggests that this protein has an important biological function at the host-parasite interface. We discuss the inhibitor's possible role in the regulation of blood digestion, modulation of antigen presentation, and in the regeneration of host tissues.

A proteomic insight into the midgut proteome of Ornithodoros moubata females reveals novel information on blood digestion in argasid ticks

Background

The argasid tick Ornithodoros moubata is the main African vector of the human relapsing fever agent Borrelia duttoni and the African swine fever virus. Together with saliva, the tick midgut forms part of the host-tick-pathogen interface, and numerous midgut proteins play key functions in the blood digestion-related process and the infection and transmission of pathogens. This work explores the composition of the midgut proteome of unfed and fed O. moubata females with the aim to complete the biological information already obtained from the midgut transcriptome and provide a more robust and comprehensive perspective of this biological system.

Methods

Midgut tissues taken from females before feeding and 48 h after feeding were subjected to LC/MS-MS analysis. After functional characterization and classification of the proteins identified, the differences in the proteome between unfed and fed females were analysed and discussed. Additionally, a detailed analysis of particular groups of proteins that are involved in the processes of nutrient digestion and responses to the oxidative stress was carried out.

Results

1491 non-redundant tick proteins were identified: 1132 of them in the midgut of unfed ticks, 1138 in the midgut of fed ticks, and up to 779 shared by both physiological conditions. Overall, the comparative analysis of the midgut proteomes of O. moubata females before and after feeding did not reveal great differences in the number or class of proteins expressed, enzymatic composition or functional classification.

Conclusions

The hemoglobinolytic system in ixodids and argasids is very similar in spite of the fact that they display very different feeding and reproductive strategies. Although the main source of nutrients in ticks are proteins, lipids and carbohydrates also constitute significant nutritional sources and play an important part in the process of blood digestion. The genes and proteins involved in intracellular transport mechanisms, defensive responses, detoxifying responses and stress responses seem to be closely regulated, highlighting the complexity and importance of these processes in tick biology, which in turn assigns them a great interest as targets for therapeutic and immunological interventions.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2300-8) contains supplementary material, which is available to authorized users.

Salivary Tick Cystatin OmC2 Targets Lysosomal Cathepsins S and C in Human Dendritic Cells

To ensure successful feeding tick saliva contains a number of inhibitory proteins that interfere with the host immune response and help to create a permissive environment for pathogen transmission. Among the potential targets of the salivary cystatins are two host cysteine proteases, cathepsin S, which is essential for antigen- and invariant chain-processing, and cathepsin C (dipeptidyl peptidase 1, DPP1), which plays a critical role in processing and activation of the granule serine proteases. Here, the effect of salivary cystatin OmC2 from Ornithodoros moubata was studied using differentiated MUTZ-3 cells as a model of immature dendritic cells of the host skin. Following internalization, cystatin OmC2 was initially found to inhibit the activity of several cysteine cathepsins, as indicated by the decreased rates of degradation of fluorogenic peptide substrates. To identify targets, affinity chromatography was used to isolate His-tagged cystatin OmC2 together with the bound proteins from MUTZ-3 cells. Cathepsins S and C were identified in these complexes by mass spectrometry and confirmed by immunoblotting. Furthermore, reduced increase in the surface expression of MHC II and CD86, which are associated with the maturation of dendritic cells, was observed. In contrast, human inhibitor cystatin C, which is normally expressed and secreted by dendritic cells, did not affect the expression of CD86. It is proposed that internalization of salivary cystatin OmC2 by the host dendritic cells targets cathepsins S and C, thereby affecting their maturation.

Gut transcriptome analysis on females of Ornithodoros mimon (Acari: Argasidae) and phylogenetic inference of ticks

Abstract Ornithodoros mimon is an argasid tick that parasitizes bats, birds and opossums and is also harmful to humans. Knowledge of the transcripts present in the tick gut helps in understanding the role of vital molecules in the digestion process and parasite-host relationship, while also providing information about the evolution of arthropod hematophagy. Thus, the present study aimed to know and ascertain the main molecules expressed in the gut of argasid after their blood meal, through analysis on the gut transcriptome of engorged females of O. mimon using 454-based RNA sequencing. The gut transcriptome analysis reveals several transcripts associated with hemoglobin digestion, such as serine, cysteine, aspartic proteases and metalloenzymes. The phylogenetic analysis on the peptidases confirmed that most of them are clustered with other tick genes. We recorded the presence a cathepsin O peptidase-coding transcript in ticks. The topology of the phylogenetic inferences, based on transcripts of inferred families of homologues, was similar to that of previous reports based on mitochondrial genome and nuclear rRNA sequences. We deposited 2,213 sequence of O. mimon to the public databases. Our findings may help towards better understanding of important argasid metabolic processes, such as digestion, nutrition and immunity.

Protease Inhibitors in Tick Saliva: The Role of Serpins and Cystatins in Tick-host-Pathogen Interaction

The publication of the first tick sialome (salivary gland transcriptome) heralded a new era of research of tick protease inhibitors, which represent important constituents of the proteins secreted via tick saliva into the host. Three major groups of protease inhibitors are secreted into saliva: Kunitz inhibitors, serpins, and cystatins. Kunitz inhibitors are anti-hemostatic agents and tens of proteins with one or more Kunitz domains are known to block host coagulation and/or platelet aggregation. Serpins and cystatins are also anti-hemostatic effectors, but intriguingly, from the translational perspective, also act as pluripotent modulators of the host immune system. Here we focus especially on this latter aspect of protease inhibition by ticks and describe the current knowledge and data on secreted salivary serpins and cystatins and their role in tick-host-pathogen interaction triad. We also discuss the potential therapeutic use of tick protease inhibitors.

Functional annotation and analysis of the Ornithodoros moubata midgut genes differentially expressed after blood feeding

The argasid tick Ornithodoros moubata is the main vector of the African swine fever and the human relapsing fever in Africa. As part of the host-parasite-pathogen interface, the tick midgut expresses key proteins for tick survival and tick-borne pathogen transmission. Accordingly, midgut proteins are potential targets for the development of new drugs and vaccines aimed at tick control, and obtaining proteomic and transcriptomic data from the O. moubata midgut would facilitate the identification of such target candidates. With this aim, we have assembled and characterized the midgut transcriptome of O. moubata females before and 48h after a blood meal, and identified the genes that are differentially expressed in the midgut after feeding. Overall, 23,863 transcripts were obtained, and of them, 9,164 were identified and annotated. The most represented molecular functions were catalytic and binding activities, and the most represented biological processes were metabolic, cellular and single-organism processes. KEGG analysis of the annotated sequences assigned up to 3,053 of them to 130 active pathways, among which, the top 30 pathways were mostly metabolic routes. Differential expression analysis between unfed and fed ticks detected 8,026 Differentially Expressed Genes (DEGs), 4,093 up-regulated and 3,933 down-regulated, respectively. The biological significance of these DEGs was further investigated using the KEEG, Pfam and GO databases. The functional groups of the genes/proteins predicted to be involved in the processes of blood digestion, nutrient transport and metabolism, and in responses related to defence and oxidative stress are discussed in more detail. This work reports the first midgut transcriptome analysis of an argasid tick species, and provides a wealth of novel molecular information about the argasid machinery involved in blood feeding and digestion. This information represents a starting point for the development of alternative strategies for tick control.

Molecular and structural characterization of novel cystatins from the taiga tick Ixodes persulcatus

Cystatins are cysteine peptidase inhibitors that in ticks mediate processes such as blood feeding and digestion. The ixodid tick Ixodes persulcatus is endemic to the Eurasia, where it is the principal vector of Lyme borreliosis. To date, no I. persulcatus cystatin has been characterized. In the present work, we describe three novel cystatins from I. persulcatus, named JpIpcys2a, JpIpcys2b and JpIpcys2c. In addition, the potential of tick cystatins as cross-protective antigens was evaluated by vaccination of hamsters using BrBmcys2c, a cystatin from Rhipicephalus microplus, against I. persulcatus infestation. Sequence analysis showed that motifs that are characteristic of cystatins type 2 are fully conserved in JpIpcys2b, while mutations are present in both JpIpcys2a and JpIpcys2c. Protein-protein docking simulations further revealed that JpIpcys2a, JpIpcys2b and JpIpcys2c showed conserved binding sites to human cathepsins L, all of them covering the active site cleft. Cystatin transcripts were detected in different I. persulcatus tissues and instars, showing their ubiquitous expression during I. persulcatus development. Serological analysis showed that although hamsters immunized with BrBmcys2c developed a humoral immune response, this response was not adequate to protect against a heterologous challenge with I. persulcatus adult ticks. The lack of cross-protection provided by BrBmcys2c immunization is perhaps linked to the fact that cystatins cluster into multigene protein families that are expressed differentially and exhibit functional redundancy. How to target such small proteins that are secreted in low quantities remains a challenge in the development of suitable anti-tick vaccine antigens.

RNA-seq analyses of the midgut from blood- and serum-fed Ixodes ricinus ticks

Adult females of the genus Ixodes imbibe blood meals exceeding about 100 times their own weight within 7‒9 days. During this period, ticks internalise components of host blood by endocytic digest cells that line the tick midgut epithelium. Using RNA-seq, we aimed to characterise the midgut transcriptome composition in adult Ixodes ricinus females during early and late phase of engorgement. To address specific adaptations to the haemoglobin-rich diet, we compared the midgut transcriptomes of genetically homogenous female siblings fed either bovine blood or haemoglobin-depleted serum. We noted that tick gut transcriptomes are subject to substantial temporal-dependent expression changes between day 3 and day 8 of feeding. In contrast, the number of transcripts significantly affected by the presence or absence of host red blood cells was low. Transcripts relevant to the processes associated with blood-meal digestion were analysed and involvement of selected encoded proteins in the tick midgut physiology discussed. A total of 7215 novel sequences from I. ricinus were deposited in public databases as an additional outcome of this study. Our results broaden the current knowledge of tick digestive system and may lead to the discovery of potential molecular targets for efficient tick control.

Acquisition of exogenous haem is essential for tick reproduction

Haem and iron homeostasis in most eukaryotic cells is based on a balanced flux between haem biosynthesis and haem oxygenase-mediated degradation. Unlike most eukaryotes, ticks possess an incomplete haem biosynthetic pathway and, together with other (non-haematophagous) mites, lack a gene encoding haem oxygenase. We demonstrated, by membrane feeding, that ticks do not acquire bioavailable iron from haemoglobin-derived haem. However, ticks require dietary haemoglobin as an exogenous source of haem since, feeding with haemoglobin-depleted serum led to aborted embryogenesis. Supplementation of serum with haemoglobin fully restored egg fertility. Surprisingly, haemoglobin could be completely substituted by serum proteins for the provision of amino-acids in vitellogenesis. Acquired haem is distributed by haemolymph carrier protein(s) and sequestered by vitellins in the developing oocytes. This work extends, substantially, current knowledge of haem auxotrophy in ticks and underscores the importance of haem and iron metabolism as rational targets for anti-tick interventions.

DOI:http://dx.doi.org/10.7554/eLife.12318.001

Ticks are small blood-feeding parasites that transmit a range of diseases through their bites, including Lyme disease and encephalitis in humans. Like other blood-feeders, ticks acquire essential nutrients from their host in order to develop and reproduce.

Iron and haem (the iron-containing part of haemoglobin) are essential for the metabolism of every breathing animal on Earth. Most organisms obtain iron by degrading haem and, reciprocally, most of the iron in cells is used to make haem. However, an initial search of existing genome databases revealed that ticks lack the genes required to make the proteins that make and degrade haem.

Perner et al. wanted to find out if ticks can steal haem from the host and use it for their own development. To achieve this, Perner et al. exploited a method of tick membrane feeding that simulates natural feeding on a host by using a silicone imitation of a skin and cow smell extracts (“l´odeur de vache”). Ticks were fed either a haemoglobin-rich (whole blood) or a haemoglobin-poor (serum) diet. This experiment revealed that ticks can develop normally without haemoglobin, but female ticks fed a haemoglobin-poor diet lay sterile eggs out of which no offspring can hatch.

Further investigation showed that haemoglobin is vitally important as a source of haem but not as a source of the amino acids needed to produce the vitellin proteins that nourish embryos. As ticks are not armed with the ability to degrade haem, they do not acquire iron from the host haem but rather from a serum transferrin, a major iron transporter protein found in mammalian blood. Further experiments revealed that ticks have evolved proteins that can transport and store haem and so make the obtained haem available across the whole tick body.

Overall, Perner et al.’s findings suggest that targeting the mechanisms by which ticks metabolise haem and iron could lead to the design of new “anti-tick” strategies.

DOI:http://dx.doi.org/10.7554/eLife.12318.002

Midgut proteome of an argasid tick, Ornithodoros erraticus: a comparison between unfed and engorged females

Background

The argasid tick Ornithodoros erraticus is the vector of African swine fever virus and of several Borrelia species that cause human relapsing fever in the Iberian Peninsula. The tick midgut is part of the ectoparasite-host interface and expresses proteins that are vital for the survival of the tick. Midgut proteins are therefore potential targets for drug and/or vaccine design aimed at the development of new strategies for tick control. Thus, the aim of this work was the characterization of the proteome of the O. erraticus midgut before and after a blood meal trying to elucidate the induced changes upon blood feeding.

Methods

Midgut tissues from unfed and engorged O. erraticus females were dissected and proteins were fractionated by centrifugation and SDS-PAGE, and the corresponding gel pieces analysed by LC–MS/MS. The identified proteins were classified according to their Protein Class and Molecular Function and the differences between fed and unfed specimens were analysed.

Results

Overall 555 tick proteins were identified: 414 in the midgut of the unfed specimens and 376 in the fed specimens, of which 235 were present in both groups. The proteins with catalytic, binding and structural functions were the most numerous and abundant, consistent with their role in the intracellular processing of the blood meal. The analysis of some groups of proteins putatively involved directly in blood meal digestion, including protein digestion (peptidase activity), iron metabolism, enzymes involved in oxidative stress and detoxification and membrane traffic and transport proteins, detected some differences between the fed and unfed ticks

Conclusions

This work reports for the first time the collection and analysis of the midgut proteome of an argasid tick species and provides molecular information about the argasid machinery involved in blood digestion. This information represents a starting point for the identification and selection of new targets for the development of alternative control strategies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1148-z) contains supplementary material, which is available to authorized users.

Rhipicephalus microplus and Ixodes ovatus cystatins in tick blood digestion and evasion of host immune response

BACKGROUND

Cystatins are a group of cysteine protease inhibitors responsible for physiological proteolysis regulation and present in a wide range of organisms. Studies about this class of inhibitors in parasites have contributed to clarify their roles in important physiological processes, like blood digestion and modulation of host immune response during blood feeding. Thus, cystatins are a subject of research on the development of new parasite control methods. Additionally, the characterization of proteins shared by different parasite species represents a valuable strategy to find potential targets in multi-species control methods. However, cystatin functions in ticks remain undetermined, especially in Rhipicephalus microplus and Ixodes ovatus, two species that affect livestock and human health, respectively.

METHODS

Here we report the inhibitory profile of two R. microplus (BrBmcys2b and BrBmcys2c) and one I. ovatus (JpIocys2a) cystatins to commercial cathepsins B, C, and L. The presence of native cystatins in R. microplus tissues was analyzed using sera against recombinant BrBmcys2b and BrBmcys2c. Also, a peptide from JpIocys2a was synthesized for rabbit immunization, and this serum was used to analyze the cross antigenicity between R. microplus and I. ovatus cystatins.

RESULTS

Enzymatic inhibition profile of tick cystatins shows a distinct modulation for cathepsins related to tick blood digestion and evasion of host immune response. Furthermore, BrBmcys2b was detected in saliva and different tissues along tick stages, while BrBmcys2c was detected mainly in gut from partially engorged R. microplus females, demonstrating a distinct pattern of cystatin expression, secretion and traffic between tick tissues. Moreover, phylogenetic analysis suggests that JpIocys2a belongs to the group of tick gut secreted cystatins. Finally, cross-antigenicity assays revealed that antibodies against the JpIocys2a peptide recognize native and recombinant R. microplus cystatins.

CONCLUSION

The presence of these proteins in different tissues and their ability to differentially inhibit cathepsins suggest distinct roles for JpIocys2a, BrBmcys2b, and BrBmcys2c in blood digestion, egg and larvae development, and modulation of host immune response in tick physiology. The cross-antigenicity between native and recombinant cystatins supports further experiments using JpIocys2a, BrBmcys2b, and BrBmcys2c as vaccine antigens.

IrFC - An Ixodes ricinus injury-responsive molecule related to Limulus Factor C

Limulus Clotting Factor C is a multi-domain serine protease that triggers horseshoe crab hemolymph clotting in the presence of trace amounts of bacterial lipopolysaccharides. Here we describe and functionally characterize an homologous molecule, designated as IrFC, from the hard tick Ixodes ricinus. Tick Factor C consists of an N-terminal cysteine-rich domain, four complement control protein (sushi) modules, an LCCL domain, a truncated C-lectin domain and a C-terminal trypsin-type domain. Developmental expression profiling by quantitative real-time PCR revealed that the irfc mRNA is expressed in all stages including eggs. In tissues dissected from adult I. ricinus females, the irfc mRNA is present mainly in tick hemocytes and accordingly, indirect immunofluorescence microscopy localized IrFC intracellularly, in tick hemocytes. Irfc mRNA levels were markedly increased upon injection of sterile saline, or different microbes, demonstrating that the irfc gene transcription occurs in response to injury. This indicates a possible role of IrFC in hemolymph clotting and/or wound healing, although these defense mechanisms have not been yet definitely demonstrated in ticks. RNAi silencing of irfc expression resulted in a significant reduction in phagocytic activity of tick hemocytes against the Gram-negative bacteria Chryseobacterium indologenes and Escherichia coli, but not against the yeast, Candida albicans. This result suggests that IrFC plays a role in the tick primordial complement system and as such possibly mediates transmission of tick-borne pathogens.

A ray of venom: Combined proteomic and transcriptomic investigation of fish venom composition using barb tissue from the blue-spotted stingray (Neotrygon kuhlii)

Fish venoms remain almost completely unstudied despite the large number of species. In part this is due to the inherent nature of fish venoms, in that they are highly sensitive to heat, pH, lyophilisation, storage and repeated freeze-thawing. They are also heavily contaminated with mucus, which makes proteomic study difficult. Here we describe a novel protein-handling protocol to remove mucus contamination, utilising ammonium sulphate and acetone precipitation. We validated this approach using barb venom gland tissue protein extract from the blue-spotted stingray Neotrygon kuhlii. We analysed the protein extract using 1D and 2D gels with LC-MS/MS sequencing. Protein annotation was underpinned by a venom gland transcriptome. The composition of our N. kuhlii venom sample revealed a variety of protein types that are completely novel to animal venom systems. Notably, none of the detected proteins exhibited similarity to the few toxin components previously characterised from fish venoms, including those found in other stingrays. Putative venom toxins identified here included cystatin, peroxiredoxin and galectin. Our study represents the first combined survey of gene and protein composition from the venom apparatus of any fish and our novel protein handling method will aid the future characterisation of toxins from other unstudied venomous fish lineages.

BIOLOGICAL SIGNIFICANCE

These results show an efficient manner for removing mucus from fish venoms. These results are the first insights into the evolution of proteins present on stingrayvenom barbs.

Proteomic analysis of cattle tick Rhipicephalus (Boophilus) microplus saliva: a comparison between partially and fully engorged females

The cattle tick Rhipicephalus (Boophilus) microplus is one of the most harmful parasites affecting bovines. Similarly to other hematophagous ectoparasites, R. microplus saliva contains a collection of bioactive compounds that inhibit host defenses against tick feeding activity. Thus, the study of tick salivary components offers opportunities for the development of immunological based tick control methods and medicinal applications. So far, only a few proteins have been identified in cattle tick saliva. The aim of this work was to identify proteins present in R. microplus female tick saliva at different feeding stages. Proteomic analysis of R. microplus saliva allowed identifying peptides corresponding to 187 and 68 tick and bovine proteins, respectively. Our data confirm that (i) R. microplus saliva is complex, and (ii) that there are remarkable differences in saliva composition between partially engorged and fully engorged female ticks. R. microplus saliva is rich mainly in (i) hemelipoproteins and other transporter proteins, (ii) secreted cross-tick species conserved proteins, (iii) lipocalins, (iv) peptidase inhibitors, (v) antimicrobial peptides, (vii) glycine-rich proteins, (viii) housekeeping proteins and (ix) host proteins. This investigation represents the first proteomic study about R. microplus saliva, and reports the most comprehensive Ixodidae tick saliva proteome published to date. Our results improve the understanding of tick salivary modulators of host defense to tick feeding, and provide novel information on the tick-host relationship.

Sequence characterization and immunogenicity of cystatins from the cattle tick Rhipicephalus (Boophilus) microplus

Various classes of endopeptidases and their inhibitors facilitate blood feeding and digestion in ticks. Cystatins, a family of tight-binding and reversible inhibitors of cysteine endopeptidases, have recently been found in several tick tissues. Moreover, vaccine trials using tick cystatins have been found to induce protective immune responses against tick infestation. However, the mode of action of tick cystatins is still poorly understood, limiting the elucidation of their physiological role. Against this background, we have investigated sequence characteristics and immunogenic properties of 5 putative cystatins from Rhipicephalus (Boophilus) microplus from Brazil and Uruguay. The similarity of the deduced amino acid sequences among cystatins from the Brazilian tick strain was 27-42%, all of which had a secretory signal peptide. The cystatin motif (QxVxG), a glycine in the N-terminal region, and the PW motif in the second hairpin loop in the C-terminal region are highly conserved in all 5 cystatins identified in this study. Four cysteine residues in the C terminus characteristic of type 2 cystatins are also present. qRT-PCR revealed differential expression patterns among the 5 cystatins identified, as well as variation in mRNA transcripts present in egg, larva, gut, salivary glands, ovary, and fat body tissues. One R. microplus cystatin showed 97-100% amino acid similarity between Brazilian and Uruguayan isolates. Furthermore, by in silico analysis, antigenic amino acid regions from R. microplus cystatins showed high degrees of homology (54-92%) among Rhipicephalus spp. cystatins. Three Brazilian R. microplus cystatins were expressed in Escherichia coli, and immunogenicity of the recombinant proteins were determined by vaccinating mice. Western blotting using mice sera indicated cross-reactivity between the cystatins, suggesting shared epitopes. The present characterization of Rhipicephalus spp. cystatins represents an empirical approach in an effort to evaluate the physiological role of cystatins in a larger context of targeting them for use in future tick control strategies.

New insights into the machinery of blood digestion by ticks

Blood-protein digestion is a key physiological process providing essential nutrients for ticks and is a prerequisite for the transmission of tick-borne pathogens. Recently, substantial progress has been made in determining the proteolytic machinery in tick gut tissue, which is based on a dynamic multienzyme network capable of processing a vast amount of host blood. In this article we summarize our current knowledge of the molecular mechanisms of tick hematophagy and their similarities to those of Platyhelminthes, nematodes, and Plasmodium. Future research perspectives, including the potential for rational control of ticks and transmitted diseases, are also discussed.

Bioinformatics and expression analyses of the Ixodes scapularis tick cystatin family

The cystatins are inhibitors of papain- and legumain-like cysteine proteinases, classified in MEROPS subfamilies I25A-I25C. This study shows that 84 % (42/50) of tick cystatins are putatively extracellular in subfamily I25B and the rest are putatively intracellular in subfamily I25A. On the neighbor joining phylogeny guide tree, subfamily I25A members cluster together, while subfamily I25B cystatins segregate among prostriata or metastriata ticks. Two Ixodes scapularis cystatins, AAY66864 and ISCW011771 that show 50-71 % amino acid identity to metastriata tick cystatins may be linked to pathways that are common to all ticks, while ISCW000447 100 % conserved in I. ricinus is important among prostriata ticks. Likewise metastriata tick cystatins, Dermacentor variabilis-ACF35512, Rhipicephalus microplus-ACX53850, A. americanum-AEO36092, R. sanguineus-ACX53922, D. variabilis-ACF35514, R. sanguineus-ACX54033 and A. maculatum-AEO35155 that show 73-86 % amino acid identity may be essential to metastriata tick physiology. RT-PCR expression analyses revealed that I. scapularis cystatins were constitutively expressed in the salivary glands, midguts and other tissues of unfed ticks and ticks that were fed for 24-120 h, except for ISCW017861 that are restricted to the 24 h feeding time point. On the basis of mRNA expression patterns, I. scapularis cystatins, ISCW017861, ISCW011771, ISCW002215 and ISCW0024528 that are highly expressed at 24 h are likely involved in regulating early stage tick feeding events such as tick attachment onto host skin and creation of the feeding lesion. Similarly, ISCW018602, ISCW018603 and ISCW000447 that show 2-3 fold transcript increase by 120 h of feeding are likely associated with blood meal up take, while those that maintain steady state expression levels (ISCW018600, ISCW018601 and ISCW018604) during feeding may not be associated with tick feeding regulation. We discuss our findings in the context of advancing our knowledge of tick molecular biology.

Cysteine peptidases and their inhibitors in Tetranychus urticae: a comparative genomic approach

Background

Cysteine peptidases in the two-spotted spider mite Tetranychus urticae are involved in essential physiological processes, including proteolytic digestion. Cystatins and thyropins are inhibitors of cysteine peptidases that modulate their activity, although their function in this species has yet to be investigated. Comparative genomic analyses are powerful tools to obtain advanced knowledge into the presence and evolution of both, peptidases and their inhibitors, and could aid to elucidate issues concerning the function of these proteins.

Results

We have performed a genomic comparative analysis of cysteine peptidases and their inhibitors in T. urticae and representative species of different arthropod taxonomic groups. The results indicate: i) clade-specific proliferations are common to C1A papain-like peptidases and for the I25B cystatin family of inhibitors, whereas the C1A inhibitors thyropins are evolutionarily more conserved among arthropod clades; ii) an unprecedented extensive expansion for C13 legumain-like peptidases is found in T. urticae; iii) a sequence-structure analysis of the spider mite cystatins suggests that diversification may be related to an expansion of their inhibitory range; and iv) an in silico transcriptomic analysis shows that most cathepsin B and L cysteine peptidases, legumains and several members of the cystatin family are expressed at a higher rate in T. urticae feeding stages than in embryos.

Conclusion

Comparative genomics has provided valuable insights on the spider mite cysteine peptidases and their inhibitors. Mite-specific proliferations of C1A and C13 peptidase and I25 cystatin families and their over-expression in feeding stages of mites fit with a putative role in mite’s feeding and could have a key role in its broad host feeding range.

Characterization of gut-associated cathepsin D hemoglobinase from tick Ixodes ricinus (IrCD1)

To identify the gut-associated tick aspartic hemoglobinase, this work focuses on the functional diversity of multiple Ixodes ricinus cathepsin D forms (IrCDs). Out of three encoding genes representing Ixodes scapularis genome paralogs, IrCD1 is the most distinct enzyme with a shortened propeptide region and a unique pattern of predicted post-translational modifications. IrCD1 gene transcription is induced by tick feeding and is restricted to the gut tissue. The hemoglobinolytic role of IrCD1 was further supported by immunolocalization of IrCD1 in the vesicles of tick gut cells. Properties of recombinantly expressed rIrCD1 are consistent with the endo-lysosomal environment because the zymogen is autoactivated and remains optimally active in acidic conditions. Hemoglobin cleavage pattern of rIrCD1 is identical to that produced by the native enzyme. The preference for hydrophobic residues at the P1 and P1' position was confirmed by screening a novel synthetic tetradecapeptidyl substrate library. Outside the S1-S1' regions, rIrCD1 tolerates most amino acids but displays a preference for tyrosine at P3 and alanine at P2'. Further analysis of the cleavage site location within the peptide substrate indicated that IrCD1 is a true endopeptidase. The role in hemoglobinolysis was verified with RNAi knockdown of IrCD1 that decreased gut extract cathepsin D activity by >90%. IrCD1 was newly characterized as a unique hemoglobinolytic cathepsin D contributing to the complex intestinal proteolytic network of mainly cysteine peptidases in ticks.

The role of cystatins in tick physiology and blood feeding

Ticks, as obligate hematophagous ectoparasites, impact greatly on animal and human health because they transmit various pathogens worldwide. Over the last decade, several cystatins from different hard and soft ticks were identified and biochemically analyzed for their role in the physiology and blood feeding lifestyle of ticks. All these cystatins are potent inhibitors of papain-like cysteine proteases, but not of legumain. Tick cystatins were either detected in the salivary glands and/or the midgut, key tick organs responsible for blood digestion and the expression of pharmacologically potent salivary proteins for blood feeding. For example, the transcription of two cystatins named HlSC-1 and Sialostatin L2 was highly upregulated in these tick tissues during feeding. Vaccinating hosts against Sialostatin L2 and Om-cystatin 2 as well as silencing of a cystatin gene from Amblyomma americanum significantly inhibited the feeding ability of ticks. Additionally, Om-cystatin 2 and Sialostatin L possessed strong host immunosuppressive properties by inhibiting dendritic cell maturation due to their interaction with cathepsin S. These two cystatins, together with Sialostatin L2 are the first tick cystatins with resolved three-dimensional structure. Sialostatin L, furthermore, showed preventive properties against autoimmune diseases. In the case of the cystatin Hlcyst-2, experimental evidence showed its role in tick innate immunity, since increased Hlcyst-2 transcript levels were detected in Babesia gibsoni-infected larval ticks and the protein inhibited Babesia growth. Other cystatins, such as Hlcyst-1 or Om-cystatin 2 are assumed to be involved in regulating blood digestion. Only for Bmcystatin was a role in tick embryogenesis suggested. Finally, all the biochemically analyzed tick cystatins are powerful protease inhibitors, and some may be novel antigens for developing anti-tick vaccines and drugs of medical importance due to their stringent target specificity.

Cysteine proteases from bloodfeeding arthropod ectoparasites

Cysteine proteases have been discovered in various bloodfeeding ectoparasites. Here, we assemble the available information about the function of these peptidases and reveal their role in hematophagy and parasite development. While most of the data shed light on key proteolytic events that play a role in arthropod physiology, we also report on the association of cysteine proteases with arthropod vectorial capacity. With emphasis on ticks, specifically Ixodes ricinus, we finally propose a model about the contribution of cysteine peptidases to blood digestion and how their concerted action with other tick midgut proteases leads to the absorbance of nutrients by the midgut epithelial cells.

Dynamics of digestive proteolytic system during blood feeding of the hard tick Ixodes ricinus

Background

Ticks are vectors of a wide variety of pathogens causing severe diseases in humans and domestic animals. Intestinal digestion of the host blood is an essential process of tick physiology and also a limiting factor for pathogen transmission since the tick gut represents the primary site for pathogen infection and proliferation. Using the model tick Ixodes ricinus, the European Lyme disease vector, we have previously demonstrated by genetic and biochemical analyses that host blood is degraded in the tick gut by a network of acidic peptidases of the aspartic and cysteine classes.

Results

This study reveals the digestive machinery of the I. ricinus during the course of blood-feeding on the host. The dynamic profiling of concentrations, activities and mRNA expressions of the major digestive enzymes demonstrates that the de novo synthesis of peptidases triggers the dramatic increase of the hemoglobinolytic activity along the feeding period. Overall hemoglobinolysis, as well as the activity of digestive peptidases are negligible at the early stage of feeding, but increase dramatically towards the end of the slow feeding period, reaching maxima in fully fed ticks. This finding contradicts the established opinion that blood digestion is reduced at the end of engorgement. Furthermore, we show that the digestive proteolysis is localized intracellularly throughout the whole duration of feeding.

Conclusions

Results suggest that the egressing proteolytic system in the early stage of feeding and digestion is a potential target for efficient impairment, most likely by blocking its components via antibodies present in the host blood. Therefore, digestive enzymes are promising candidates for development of novel 'anti-tick' vaccines capable of tick control and even transmission of tick-borne pathogens.

Characterization of Hlcyst-3 as a member of cystatins from the tick Haemaphysalis longicornis

The gene encoding cystatin from the tick Haemaphysalis longicornis has been reported previously. In the study reported here, we characterized a member of cystatins and designated it as Hlcyst-3 (H. longicornis cystatin-3). Its full-length cDNA is 602 bp, and it encodes a putative 129 amino acid protein with an obvious signal peptide. Sequence analysis revealed that it has significant homology with the known secreted cystatin. The recombinant protein was expressed in a GST-fused soluble form in Escherichia coli and was purified by affinity chromatography. The inhibitory activity of the recombinant protein against papain and cathepsin L was identified by fluorogenic substrate analysis. Real-time PCR revealed that Hlcyst-3 was mostly expressed in the tick midgut.

Crystal structure and functional characterization of an immunomodulatory salivary cystatin from the soft tick Ornithodoros moubata

The saliva of blood-feeding parasites is a rich source of peptidase inhibitors that help to overcome the host's defence during host-parasite interactions. Using proteomic analysis, the cystatin OmC2 was demonstrated in the saliva of the soft tick Ornithodoros moubata, an important disease vector transmitting African swine fever virus and the spirochaete Borrelia duttoni. A structural, biochemical and biological characterization of this peptidase inhibitor was undertaken in the present study. Recombinant OmC2 was screened against a panel of physiologically relevant peptidases and was found to be an effective broad-specificity inhibitor of cysteine cathepsins, including endopeptidases (cathepsins L and S) and exopeptidases (cathepsins B, C and H). The crystal structure of OmC2 was determined at a resolution of 2.45 A (1 A=0.1 nm) and was used to describe the structure-inhibitory activity relationship. The biological impact of OmC2 was demonstrated both in vitro and in vivo. OmC2 affected the function of antigen-presenting mouse dendritic cells by reducing the production of the pro-inflammatory cytokines tumour necrosis factor alpha and interleukin-12, and proliferation of antigen-specific CD4+ T-cells. This suggests that OmC2 may suppress the host's adaptive immune response. Immunization of mice with OmC2 significantly suppressed the survival of O. moubata in infestation experiments. We conclude that OmC2 is a promising target for the development of a novel anti-tick vaccine to control O. moubata populations and combat the spread of associated diseases.

Hlcyst-1 and Hlcyst-2 are potential inhibitors of HlCPL-A in the midgut of the ixodid tick Haemaphysalis longicornis

Although the actions of cysteine proteases are controlled in part by endogenous tight-binding cysteine protease inhibitors from the cystatin superfamily, regulatory mechanisms used by ticks to control protease activities are unknown. We report here the interaction of 2 endogenous midgut cysteine protease inhibitors, Hlcyst-1 and Hlcyst-2, with an endogenous midgut cysteine protease, HlCPL-A in Haemaphysalis longicornis. In vitro inhibition assays demonstrated that the hydrolytic activity of HlCPL-A was inhibited by Hlcyst-1 and Hlcyst-2 in dose dependent manner. Immunofluorescent studies revealed that Hlcyst-1 and Hlcyst-2 are co-localized with HlCPL-A in the epithelial cells of the midgut. The hemoglobin degradation activity of HlCPL-A was dose-dependently inhibited by Hlcyst-1 and Hlcyst-2. These results strongly indicate that, Hlcyst-1 and Hlcyst-2 are possible inhibitor of HlCPL-A and play a key role in regulatory mechanisms of hemoglobin degradation process in ticks.

Hemoglobin digestion in blood-feeding ticks: mapping a multipeptidase pathway by functional proteomics

Hemoglobin digestion is an essential process for blood-feeding parasites. Using chemical tools, we deconvoluted the intracellular hemoglobinolytic cascade in the tick Ixodes ricinus, a vector of Lyme disease and tick-borne encephalitis. In tick gut tissue, a network of peptidases was demonstrated through imaging with specific activity-based probes and activity profiling with peptidic substrates and inhibitors. This peptidase network is induced upon blood feeding and degrades hemoglobin at acidic pH. Selective inhibitors were applied to dissect the roles of the individual peptidases and to determine the peptidase-specific cleavage map of the hemoglobin molecule. The degradation pathway is initiated by endopeptidases of aspartic and cysteine class (cathepsin D supported by cathepsin L and legumain) and is continued by cysteine amino- and carboxy-dipeptidases (cathepsins C and B). The identified enzymes are potential targets to developing novel anti-tick vaccines.

New defensins from hard and soft ticks: similarities, differences, and phylogenetic analyses

Despite the importance of ticks as vectors of disease very little is known about their immune system. Antimicrobial peptides, including defensins (phylogenetically ancient antibacterial peptides) are major components of innate immunity in ticks that have been shown to provide protection against gram-negative and gram-positive bacteria, fungi, viruses and protozoan parasites. With the aim of studying the evolution of the genes involved in tick defense, we identified the preprodefensin genes from four Ornithodoros tick species (O. papillipes: isoforms A, B, and D; O. tartakovskyi and O. puertoricensis: isoforms A and B; O. rostratus: isoform A) and from two Dermacentor tick species (D. reticulatus and D. marginatus: one isoform) not previously described. Phylogenetic analyses revealed that Ornithodoros defensin isoforms (A, B, C, and D) form 4 separate clades, while hard tick defensins are divided into several branches based on particular tick species.

The Toxicogenomic Multiverse: Convergent Recruitment of Proteins Into Animal Venoms

Throughout evolution, numerous proteins have been convergently recruited into the venoms of various animals, including centipedes, cephalopods, cone snails, fish, insects (several independent venom systems), platypus, scorpions, shrews, spiders, toxicoferan reptiles (lizards and snakes), and sea anemones. The protein scaffolds utilized convergently have included AVIT/colipase/prokineticin, CAP, chitinase, cystatin, defensins, hyaluronidase, Kunitz, lectin, lipocalin, natriuretic peptide, peptidase S1, phospholipase A2, sphingomyelinase D, and SPRY. Many of these same venom protein types have also been convergently recruited for use in the hematophagous gland secretions of invertebrates (e.g., fleas, leeches, kissing bugs, mosquitoes, and ticks) and vertebrates (e.g., vampire bats). Here, we discuss a number of overarching structural, functional, and evolutionary generalities of the protein families from which these toxins have been frequently recruited and propose a revised and expanded working definition for venom. Given the large number of striking similarities between the protein compositions of conventional venoms and hematophagous secretions, we argue that the latter should also fall under the same definition.

IrAM-An alpha2-macroglobulin from the hard tick Ixodes ricinus: characterization and function in phagocytosis of a potential pathogen Chryseobacterium indologenes

The universal protease inhibitors of the alpha(2)-macroglobulin (alpha(2)M) family are evolutionarily conserved constituents of innate immunity, presumably because they guard organisms against undesired proteolytic attacks of a different origin. Here, we determined the primary structure of alpha(2)-macroglobulin from the hard tick Ixodes ricinus (IrAM) by sequencing of overlapping PCR products. Predicted disulfide and glycosylation patterns, post-translational cleavage and alternative splicing within its 'bait region' demonstrate that IrAM is closely related to the alpha(2)-macroglobulin from the soft tick Ornithodoros moubata. The IrAM message is expressed in all tick developmental stages and tissues, except for the gut, and the protein was detected to be mainly present in the hemolymph. Silencing of IrAM by dsRNA interference markedly reduced the phagocytosis of a potential pathogen, Chryseobacterium indologenes, by tick hemocytes both in vitro and in vivo. In contrast, phagocytosis of the Lyme disease spirochete Borrelia burgdorferi or a commensal bacteria Staphylococcus xylosus was not affected by the IrAM knock-down. Similar results were obtained upon deactivation of all thioester proteins in tick hemolymph by methylamine. We have further demonstrated that phagocytosis of C. indologenes is dependent on an active metalloprotease secreted by the bacteria. These data indicate that interaction of tick alpha(2)-macroglobulin with a protease of an invading pathogen is linked with cellular immune response.

The role of saliva in tick feeding

When attempting to feed on their hosts, ticks face the problem of host hemostasis (the vertebrate mechanisms that prevent blood loss), inflammation (that can produce itching or pain and thus initiate defensive behavior on their hosts) and adaptive immunity (by way of both cellular and humoral responses). Against these barriers, ticks evolved a complex and sophisticated pharmacological armamentarium, consisting of bioactive lipids and proteins, to assist blood feeding. Recent progress in transcriptome research has uncovered that hard ticks have hundreds of different proteins expressed in their salivary glands, the majority of which have no known function, and include many novel protein families (e.g., their primary structure is unique to ticks). This review will address the vertebrate mechanisms of these barriers as a guide to identify the possible targets of these large numbers of known salivary proteins with unknown function. We additionally provide a supplemental Table that catalogues over 3,500 putative salivary proteins from various tick species, which might assist the scientific community in the process of functional identification of these unique proteins. This supplemental file is accessble fromhttp://exon.niaid.nih.gov/transcriptome/tick_review/Sup-Table-1.xls.gz.

An insight into the salivary transcriptome and proteome of the soft tick and vector of epizootic bovine abortion, Ornithodoros coriaceus

The salivary glands of blood-sucking arthropods contain a redundant 'magic potion' that counteracts their vertebrate host's hemostasis, inflammation, and immunity. We here describe the salivary transcriptome and proteomics (sialome) of the soft tick Ornithodoros coriaceus. The resulting analysis helps to consolidate the classification of common proteins found in both soft and hard ticks, such as the lipocalins, Kunitz, cystatin, basic tail, hebraein, defensin, TIL domain, metalloprotease, 5'-nucleotidase/apyrase, and phospholipase families, and also to identify protein families uniquely found in the Argasidae, such as the adrenomedullin/CGRP peptides, 7DB, 7 kDa, and the RGD-containing single-Kunitz proteins. Additionally, we found a protein belonging to the cytotoxin protein family that has so far only been identified in hard ticks. Three other unique families common only to the Ornithodoros genus were discovered. Edman degradation, 2D and 1D-PAGE of salivary gland homogenates followed by tryptic digestion and HPLC MS/MS of results confirms the presence of several proteins. These results indicate that each genus of hematophagous arthropods studied to date evolved unique protein families that assist blood feeding, thus characterizing potentially new pharmacologically active components or antimicrobial agents.

Exploring the mialome of ticks: an annotated catalogue of midgut transcripts from the hard tick, Dermacentor variabilis (Acari: Ixodidae)

Background

Ticks are obligate blood feeders. The midgut is the first major region of the body where blood and microbes ingested with the blood meal come in contact with the tick's internal tissues. Little is known about protein expression in the digestive tract of ticks. In this study, for analysis of global gene expression during tick attachment and feeding, we generated and sequenced 1,679 random transcripts (ESTs) from cDNA libraries from the midguts of female ticks at varying stages of feeding.

Results

Sequence analysis of the 1,679 ESTs resulted in the identification of 835 distinct transcripts, from these, a total of 82 transcripts were identified as proteins putatively directly involved in blood meal digestion, including enzymes involved in oxidative stress reduction/antimicrobial activity/detoxification, peptidase inhibitors, protein digestion (cysteine-, aspartic-, serine-, and metallo-peptidases), cell, protein and lipid binding including mucins and iron/heme metabolism and transport. A lectin-like protein with a high match to lectins in other tick species, allergen-like proteins and surface antigens important in pathogen recognition and/or antimicrobial activity were also found. Furthermore, midguts collected from the 6-day-fed ticks expressed twice as many transcripts involved in bloodmeal processing as midguts from unfed/2-day-fed ticks.

Conclusion

This tissue-specific transcriptome analysis provides an opportunity to examine the global expression of transcripts in the tick midgut and to compare the gut response to host attachment versus blood feeding and digestion. In contrast to those in salivary glands of other Ixodid ticks, most proteins in the D. variabilis midgut cDNA library were intracellular. Of the total ESTs associated with a function, an unusually large number of transcripts were associated with peptidases, cell, lipid and protein binding, and oxidative stress or detoxification. Presumably, this is consistent with their role in intracellular processing of the blood meal and response to microbial infections. The presence of many proteins with similar functions is consistent with the hypothesis that gene duplication contributed to the successful adaptation of ticks to hematophagy. Furthermore, these transcripts may be useful to scientists investigating the role of the tick midgut in blood-meal digestion, antimicrobial activity or the transmission of tick-borne pathogens.