Similar mechanisms regulate protein exocytosis from the salivary glands of ixodid and argasid ticks

Tissue-Specific Signatures in the Transcriptional Response to Anaplasma phagocytophilum Infection of Ixodes scapularis and Ixodes ricinus Tick Cell Lines

Anaplasma phagocytophilum are transmitted by Ixodes spp. ticks and have become one of the most common and relevant tick-borne pathogens due to their impact on human and animal health. Recent results have increased our understanding of the molecular interactions between Ixodes scapularis and A. phagocytophilum through the demonstration of tissue-specific molecular pathways that ensure pathogen infection, development and transmission by ticks. However, little is known about the Ixodes ricinus genes and proteins involved in the response to A. phagocytophilum infection. The tick species I. scapularis and I. ricinus are evolutionarily closely related and therefore similar responses are expected in A. phagocytophilum-infected cells. However, differences may exist between I. scapularis ISE6 and I. ricinus IRE/CTVM20 tick cells associated with tissue-specific signatures of these cell lines. To address this hypothesis, the transcriptional response to A. phagocytophilum infection was characterized by RNA sequencing and compared between I. scapularis ISE6 and I. ricinus IRE/CTVM20 tick cell lines. The transcriptional response to infection of I. scapularis ISE6 cells resembled that of tick hemocytes while the response in I. ricinus IRE/CTVM20 cells was more closely related to that reported previously in infected tick midguts. The inhibition of cell apoptosis by A. phagocytophilum appears to be a key adaptation mechanism to facilitate infection of both vertebrate and tick cells and was used to investigate further the tissue-specific response of tick cell lines to pathogen infection. The results supported a role for the intrinsic pathway in the inhibition of cell apoptosis by A. phagocytophilum infection of I. scapularis ISE6 cells. In contrast, the results in I. ricinus IRE/CTVM20 cells were similar to those obtained in tick midguts and suggested a role for the JAK/STAT pathway in the inhibition of apoptosis in tick cells infected with A. phagocytophilum. Nevertheless, tick cell lines were derived from embryonated eggs and may contain various cell populations with different morphology and behavior that could affect transcriptional response to infection. These results suggested tissue-specific signatures in I. scapularis ISE6 and I. ricinus IRE/CTVM20 tick cell line response to A. phagocytophilum infection that support their use as models for the study of tick-pathogen interactions.

Morpho-histochemical characterization of the salivary glands of semi-engorged Amblyomma triste (Koch, 1844) (Acari: Ixodidae) female ticks

This study presents the morphological and physiological characterization of the salivary glands of semi-engorged Amblyomma triste females. Unfed individuals were placed on New Zealand White rabbits for feeding and the females, after 4 days, were collected, dissected and the salivary glands were submitted to the application of histological (hematoxylin–eosin technique) and histochemical tests for the detection of protein (bromophenol blue technique, polysaccharides (periodic acid–Schiff technique), lipid (Nile blue technique) and calcium (von Kossa technique). The histological results show that the glandular tissue is composed by a system of ducts and three types of acini (I, II and III). The acini I are formed by a large central cell surrounded by several smaller agranular peripheral cells. Acini II are formed by cells a, c1, c2, c3 and c5, which are full of secretion granules. Acini III are constituted by cells d, e and f; the former two contain secretion granules, the latter is agranular. The glandular histochemical composition was also verified. Data obtained here will certainly help in the understanding of the cellular morphology and of the general physiology of these organs in this specie, providing important information for the creation of scientific bases which will contribute for the development of more specific and efficient methods of control.

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.

RNA interference-mediated depletion of N-ethylmaleimide sensitive fusion protein and synaptosomal associated protein of 25 kDa results in the inhibition of blood feeding of the Gulf Coast tick, Amblyomma maculatum

The signalling pathways in tick salivary glands that control 'sialo-secretome' secretion at the tick-host interface remain elusive; however, this complex process is essential for successful feeding and manipulation of the host haemostatic response. Exocytosis of the sialo-secretome in the salivary glands requires a core of soluble N-ethylmaleimide-sensitive fusion (NSF) attachment proteins (SNAPs) and receptor proteins (SNAREs). SNAREs have been identified as the key components in regulating the sialo-secretome in the salivary gland cells. In this study, we utilized RNA interference to investigate the functional role of two Amblyomma maculatum SNARE complex proteins, AmNSF and AmSNAP-25, in the tick salivary glands during extended blood feeding on the vertebrate host. Knock-down of AmNSF and AmSNAP-25 resulted in death, impaired feeding on the host, lack of engorgement and inhibited oviposition in ticks. Depletion also led to important morphological changes in the collapse of the Golgi apparatus in the salivary gland cells. Our results imply a functional significance of AmNSF and AMSNAP-25 in prolonged tick feeding, and survival on the host. Further characterization of the factors that regulate exocytosis may lead to novel approaches to prevent tick-borne diseases.

Two novel neuropeptides in innervation of the salivary glands of the black-legged tick, Ixodes scapularis: myoinhibitory peptide and SIFamide

The peptidergic signaling system is an ancient cell-cell communication mechanism that is involved in numerous behavioral and physiological events in multicellular organisms. We identified two novel neuropeptides in the neuronal projections innervating the salivary glands of the black-legged tick, Ixodes scapularis (Say, 1821). Myoinhibitory peptide (MIP) and SIFamide immunoreactivities were colocalized in the protocerebral cells and their projections terminating on specific cells of salivary gland acini (types II and III). Immunoreactive substances were identified by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) analysis: a 1,321.6-Da peptide with the sequence typical for MIP (ASDWNRLSGMWamide) and a 1,395.7-Da SIFamide (AYRKPPFNGSIFamide), which are highly conserved among arthropods. Genes encoding these peptides were identified in the available Ixodes genome and expressed sequence tag (EST) database. In addition, the cDNA encoding the MIP prepropeptide was isolated by rapid amplification of cDNA ends (RACE). In this report, we describe the anatomical structure of specific central neurons innervating salivary gland acini and identify different neuropeptides and their precursors expressed by these neurons. Our data provide evidence for neural control of salivary gland by MIP and SIFamide from the synganglion, thus leading a basis for functional studies of these two distinct classes of neuropeptides.

Cyclic nucleotide crosstalk in salivary glands from partially fed Dermacentor variabilis (Say)

Enzyme immunosorbent assays were used to measure cyclic nucleotide concentrations in homogenates of salivary glands from partially fed female Dermacentor variabilis. The adenylyl cyclase activator forskolin (100 microM) increased homogenate cGMP concentrations greater than three-fold over controls. Competitive inhibition of nitric oxide synthase with 1mM l-NMMA, an l-arginine analog, demonstrated that crosstalk occurs downstream of nitric oxide synthesis. Forskolin-stimulated synthesis of cGMP was diminished 58% by the soluble guanylyl cyclase inhibitor ODQ (2 microM). The protein kinase A selective inhibitor Rp-cAMPS (50 microM) inhibited forskolin-stimulated cGMP by 49%. Whole glands treated with 10 microM dopamine increased cGMP levels two-fold in the presence of 1mM IBMX. Treatment of whole salivary glands with equimolar concentrations of 8-Br-cAMP and 8-Br-cGMP produced no greater fluid uptake than in glands treated with 8-Br-cGMP alone, suggesting that cAMP and cGMP share a downstream target. The protein kinase G-selective inhibitor Rp-8-pCPT-cGMPS (100 microM) impeded 10mM 8-Bromo-cGMP-stimulated gland weight increases. Pretreatment with verapamil, a Ca(2+) channel blocker, attenuated cyclic nucleotide-stimulated fluid uptake indicating that whole gland fluid changes are dependent on extracellular Ca(2+). Together, our data suggest that cGMP production is mediated in part by cAMP-dependent activation of soluble guanylyl cyclase. Experiments measuring changes in whole salivary gland weight support the hypothesis that cAMP and cGMP signaling cascades have a common target and that cyclic nucleotide-stimulated fluid movement is dependent on Ca(2+) influx.

Blocking the secretion of saliva by silencing the HlYkt6 gene in the tick Haemaphysalis longicornis

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) have been identified as the key components of the protein complexes that facilitate vesicle traffic, of which Ykt6 (from Saccharomyces cerevisiae, v-SNARE) is proved to be a multifunctional protein in the membrane fusion. In the present study, a tick homologue of Ykt6 (HlYkt6, predicted 22.6 kDa), was isolated from the ixodid tick Haemaphysalis longicornis. RT-PCR and Western blot analysis indicated that the gene and the encoded protein were expressed ubiquitously in different tissues of the partially fed adult tick. Silencing of the HlYkt6 gene resulted in a significant decrease of the engorged body weight (82.9 +/- 26.8 mg vs. 232.17 +/- 59.1 mg in the PBS-injected control group and 178.7 +/- 57.0 mg in the GFP dsRNA-injected control group) and high mortality of replete ticks (100% in tested group vs. 4.8% in the PBS and 20.4% in GFP dsRNA-injected control groups). Disruption of HlYkt6 mRNA led to the suppression of saliva secretion, and a lower anticoagulant activity of the released liquid from the glands (APTT time: 25.25 +/- 1.50 s) than that of the control groups (39.25 +/- 0.50 s in the PBS-treated group and 40.0 +/- 1.41 s in the GFP dsRNA-treated group). These results suggest the vital role of the HlYkt6 protein in the exocytosis of saliva proteins, the feeding and survival of ticks.

Ornithodoros savignyi: soft tick apyrase belongs to the 5'-nucleotidase family

Salivary apyrases are nucleotide-metabolising enzymes that blood-feeding parasites utilise for modulation of extracellular nucleotides to prevent platelet activation and aggregation. In this study a 5'-nucleotidase specific degenerate primer was used to identify homologous transcripts from Ornithodoros savignyi salivary gland cDNA. Two 5'-nucleotidase isoforms that share significant sequence identity to putative apyrases from Rhipicephalus appendiculatus and Ixodes scapularis were identified. Structure prediction showed a tertiary structure similar to periplasmic ecto-5'-nucleotidase from Escherichia coli, with high conservation of functional residues. The O. savignyi 5'-nucleotidase isoform I was recombinantly expressed in Pichia pastoris. Cross-reactivity was demonstrated with polyclonal anti-apyrase antisera produced against O. savignyi apyrase. Subsequent Edman sequencing and MS/MS analysis of purified O. savignyi apyrase identified peptide sequence fragments that shared sequence identity with both newly identified 5'-nucleotidase isoforms. It was concluded that wild-type apyrase is a mixture of the isoforms identified from the salivary glands of O. savignyi. These results represent the first confirmation of a soft (argasid) tick apyrase that belongs to the 5'-nucleotidase family of enzymes.

Tick anti-hemostatics: targets for future vaccines and therapeutics

For ticks, a significant obstacle in obtaining a blood meal is counteracting the hemostatic system of the host. To this end, ticks have developed a broad array of anti-hemostatics, which is reflected in the presence of structurally related tick proteins with different functions. Disruption of blood flow which blocks successful tick feeding makes anti-hemostatics attractive targets for anti-tick vaccines. Moreover, the limited number of drugs currently available for a range of important cardio-vascular diseases makes ticks a potential source of novel therapeutics. This review aims to summarize the key features of tick anti-hemostatics, their structures, mode of action and possible future application as vaccines and novel therapeutic agents.