Identification of SNARE and cell trafficking regulatory proteins in the salivary glands of the lone star tick, Amblyomma americanum (L.)

Analysis of Amblyomma americanum microRNAs in response to Ehrlichia chaffeensis infection and their potential role in vectorial capacity

Background

MicroRNAs (miRNAs) represent a subset of small noncoding RNAs and carry tremendous potential for regulating gene expression at the post-transcriptional level. They play pivotal roles in distinct cellular mechanisms including inhibition of bacterial, parasitic, and viral infections via immune response pathways. Intriguingly, pathogens have developed strategies to manipulate the host's miRNA profile, fostering environments conducive to successful infection. Therefore, changes in an arthropod host's miRNA profile in response to pathogen invasion could be critical in understanding host-pathogen dynamics. Additionally, this area of study could provide insights into discovering new targets for disease control and prevention. The main objective of the present study is to investigate the functional role of differentially expressed miRNAs upon Ehrlichia chaffeensis, a tick-borne pathogen, infection in tick vector, Amblyomma americanum.

Methods

Small RNA libraries from uninfected and E. chaffeensis-infected Am. americanum midgut and salivary gland tissues were prepared using the Illumina Truseq kit. Small RNA sequencing data was analyzed using miRDeep2 and sRNAtoolbox to identify novel and known miRNAs. The differentially expressed miRNAs were validated using a quantitative PCR assay. Furthermore, a miRNA inhibitor approach was used to determine the functional role of selected miRNA candidates.

Results

The sequencing of small RNA libraries generated >147 million raw reads in all four libraries and identified a total of >250 miRNAs across the four libraries. We identified 23 and 14 differentially expressed miRNAs in salivary glands, and midgut tissues infected with E. chaffeensis, respectively. Three differentially expressed miRNAs (miR-87, miR-750, and miR-275) were further characterized to determine their roles in pathogen infection. Inhibition of target miRNAs significantly decreased the E. chaffeensis load in tick tissues, which warrants more in-depth mechanistic studies.

Conclusions

The current study identified known and novel miRNAs and suggests that interfering with these miRNAs may impact the vectorial capacity of ticks to harbor Ehrlichia. This study identified several new miRNAs for future analysis of their functions in tick biology and tick-pathogen interaction studies.

Interference with LTβR signaling by tick saliva facilitates transmission of Lyme disease spirochetes

Lyme spirochetes have coevolved with ticks to optimize transmission to hosts using tick salivary molecules (TSMs) to counteract host defenses. TSMs modulate various molecular events at the tick-host interface. Lymphotoxin-beta receptor (LTβR) is a vital immune receptor and plays protective roles in host immunity against microbial infections. We found that Ltbr knockout mice were more susceptible to Lyme disease spirochetes, suggesting the involvement of LTβR signaling in tick-borne Borrelia infection. Further investigation showed that a 15-kDa TSM protein from Ixodes persulcatus (I. persulcatus salivary protein; IpSAP) functioned as an immunosuppressant to facilitate the transmission and infection of Lyme disease spirochetes. IpSAP directly interacts with LTβR to block its activation, thus inhibiting the downstream signaling and consequently suppressing immunity. IpSAP immunization provided mice with significant protection against I. persulcatus-mediated Borrelia garinii infection. Notably, the immunization showed considerable cross-protection against other Borrelia infections mediated by other ixodid ticks. One of the IpSAP homologs from other ixodid ticks showed similar effects on Lyme spirochete transmission. Together, our findings suggest that LTβR signaling plays an important role in blocking the transmission and pathogenesis of tick-borne Lyme disease spirochetes, and that IpSAP and its homologs are promising candidates for broad-spectrum vaccine development.

Tick feeding modulates the human skin immune landscape to facilitate tick-borne pathogen transmission

During cutaneous tick attachment, the feeding cavity becomes a site of transmission for tick salivary compounds and tick-borne pathogens. However, the immunological consequences of tick feeding for human skin remain unclear. Here, we assessed human skin and blood samples upon tick bite and developed a human skin explant model mimicking Ixodes ricinus bites and tick-borne pathogen infection. Following tick attachment, we observed rapidly occurring patterns of immunomodulation, including increases in neutrophils and cutaneous B and T cells. T cells upregulated tissue residency markers, while lymphocytic cytokine production was impaired. In early stages of Borrelia burgdorferi model infections, we detected strain-specific immune responses and close spatial relationships between macrophages and spirochetes. Preincubation of spirochetes with tick salivary gland extracts hampered accumulation of immune cells and increased spirochete loads. Collectively, we showed that tick feeding exerts profound changes on the skin immune network that interfere with the primary response against tick-borne pathogens.

Identification of microRNAs in the Lyme Disease Vector Ixodes scapularis

MicroRNAs (miRNAs) are a class of small non-coding RNAs involved in many biological processes, including the immune pathways that control bacterial, parasitic, and viral infections. Pathogens probably modify host miRNAs to facilitate successful infection, so they might be useful targets for vaccination strategies. There are few data on differentially expressed miRNAs in the black-legged tick Ixodes scapularis after infection with Borrelia burgdorferi, the causative agent of Lyme disease in the United States. Small RNA sequencing and qRT-PCR analysis were used to identify and validate differentially expressed I. scapularis salivary miRNAs. Small RNA-seq yielded 133,465,828 (≥18 nucleotides) and 163,852,135 (≥18 nucleotides) small RNA reads from Borrelia-infected and uninfected salivary glands for downstream analysis using the miRDeep2 algorithm. As such, 254 miRNAs were identified across all datasets, 25 of which were high confidence and 51 low confidence known miRNAs. Further, 23 miRNAs were differentially expressed in uninfected and infected salivary glands: 11 were upregulated and 12 were downregulated upon pathogen infection. Gene ontology and network analysis of target genes of differentially expressed miRNAs predicted roles in metabolic, cellular, development, cellular component biogenesis, and biological regulation processes. Several Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, including sphingolipid metabolism; valine, leucine and isoleucine degradation; lipid transport and metabolism; exosome biogenesis and secretion; and phosphate-containing compound metabolic processes, were predicted as targets of differentially expressed miRNAs. A qRT-PCR assay was utilized to validate the differential expression of miRNAs. This study provides new insights into the miRNAs expressed in I. scapularis salivary glands and paves the way for their functional manipulation to prevent or treat B. burgdorferi infection.

Tick Saliva and Salivary Glands: What Do We Know So Far on Their Role in Arthropod Blood Feeding and Pathogen Transmission

Ticks are blood-sucking arthropods that have developed myriad of strategies to get a blood meal from the vertebrate host. They first attach to the host skin, select a bite site for a blood meal, create a feeding niche at the bite site, secrete plethora of molecules in its saliva and then starts feeding. On the other side, host defenses will try to counter-attack and stop tick feeding at the bite site. In this constant battle between ticks and the host, arthropods successfully pacify the host and completes a blood meal and then replete after full engorgement. In this review, we discuss some of the known and emerging roles for arthropod components such as cement, salivary proteins, lipocalins, HSP70s, OATPs, and extracellular vesicles/exosomes in facilitating successful blood feeding from ticks. In addition, we discuss how tick-borne pathogens modulate(s) these components to infect the vertebrate host. Understanding the biology of arthropod blood feeding and molecular interactions at the tick-host interface during pathogen transmission is very important. This information would eventually lead us in the identification of candidates for the development of transmission-blocking vaccines to prevent diseases caused by medically important vector-borne pathogens.

Room for Two: The Synaptophysin/Synaptobrevin Complex

Synaptic vesicle release is regulated by upwards of 30 proteins at the fusion complex alone, but disruptions in any one of these components can have devastating consequences for neuronal communication. Aberrant molecular responses to calcium signaling at the pre-synaptic terminal dramatically affect vesicle trafficking, docking, fusion, and release. At the organismal level, this is reflected in disorders such as epilepsy, depression, and neurodegeneration. Among the myriad pre-synaptic proteins, perhaps the most functionally mysterious is synaptophysin (SYP). On its own, this vesicular transmembrane protein has been proposed to function as a calcium sensor, a cholesterol-binding protein, and to form ion channels across the phospholipid bilayer. The downstream effects of these functions are largely unknown. The physiological relevance of SYP is readily apparent in its interaction with synaptobrevin (VAMP2), an integral element of the neuronal SNARE complex. SNAREs, soluble NSF attachment protein receptors, comprise a family of proteins essential for vesicle fusion. The complex formed by SYP and VAMP2 is thought to be involved in both trafficking to the pre-synaptic membrane as well as regulation of SNARE complex formation. Recent structural observations specifically implicate the SYP/VAMP2 complex in anchoring the SNARE assembly at the pre-synaptic membrane prior to vesicle fusion. Thus, the SYP/VAMP2 complex appears vital to the form and function of neuronal exocytotic machinery.

Tick extracellular vesicles enable arthropod feeding and promote distinct outcomes of bacterial infection

Extracellular vesicles are thought to facilitate pathogen transmission from arthropods to humans and other animals. Here, we reveal that pathogen spreading from arthropods to the mammalian host is multifaceted. Extracellular vesicles from Ixodes scapularis enable tick feeding and promote infection of the mildly virulent rickettsial agent Anaplasma phagocytophilum through the SNARE proteins Vamp33 and Synaptobrevin 2 and dendritic epidermal T cells. However, extracellular vesicles from the tick Dermacentor andersoni mitigate microbial spreading caused by the lethal pathogen Francisella tularensis. Collectively, we establish that tick extracellular vesicles foster distinct outcomes of bacterial infection and assist in vector feeding by acting on skin immunity. Thus, the biology of arthropods should be taken into consideration when developing strategies to control vector-borne diseases.

Recent advances in understanding tick and rickettsiae interactions

Ticks are haematophagous arthropods with unique molecular mechanisms for digesting host blood meal while acting as vectors for various pathogens of public health significance. The tick's pharmacologically active saliva plays a fundamental role in modulating the host's immune system for several days to weeks, depending on the tick species. The vector tick has also developed sophisticated molecular mechanisms to serve as a competent vector for pathogens, including the spotted fever group (SFG) rickettsiae. Evidence is still inadequate concerning tick-rickettsiae-host interactions and saliva-assisted transmission of the pathogen to the mammalian host. Rickettsia parkeri, of the SFG rickettsia, can cause a milder version of Rocky Mountain spotted fever known as American Boutonneuse fever. The Gulf Coast tick (Amblyomma maculatum) often transmits this pathogenic rickettsia in the USA. This review discusses the knowledge gap concerning tick-rickettsiae-host interactions by highlighting the SFG rickettsia and the Am maculatum model system. Filling this knowledge gap will provide a better understanding of the tick-rickettsiae-host interactions in disease causation, which will be crucial for developing effective methods for preventing tick-borne diseases.

Discovery of Exosomes From Tick Saliva and Salivary Glands Reveals Therapeutic Roles for CXCL12 and IL-8 in Wound Healing at the Tick-Human Skin Interface

Ticks secrete various anti-coagulatory, anti-vasoconstrictory, anti-inflammatory, and anti-platelet aggregation factors in their saliva at the bite site during feeding to evade host immunological surveillance and responses. For the first time, we report successful isolation of exosomes (small membrane-bound extracellular signaling vesicles) from saliva and salivary glands of partially fed or unfed ixodid ticks. Our data showed a novel role of these in vivo exosomes in the inhibition of wound healing via downregulation of C-X-C motif chemokine ligand 12 (CXCL12) and upregulation of interleukin-8 (IL-8). Cryo-electron microscopy (cryo-EM) analysis revealed that tick saliva and salivary glands are composed of heterogeneous populations of in vivo exosomes with sizes ranging from 30 to 200 nm. Enriched amounts of tick CD63 ortholog protein and heat shock protein 70 (HSP70) were evident in these exosomes. Treatment of human skin keratinocytes (HaCaT cells) with exosomes derived from tick saliva/salivary glands or ISE6 cells dramatically delayed cell migration, wound healing, and repair process. Wound healing is a highly dynamic process with several individualized processes including secretion of cytokines. Cytokine array profiling followed by immunoblotting and quantitative-PCR analysis revealed that HaCaT cells treated with exosomes derived from tick saliva/salivary glands or ISE6 cells showed enhanced IL-8 levels and reduced CXCL12 loads. Inhibition of IL-8 or CXCL12 further delayed exosome-mediated cell migration, wound healing, and repair process, suggesting a skin barrier protection role for these chemokines at the tick bite site. In contrast, exogenous treatment of CXCL12 protein completely restored this delay and enhanced the repair process. Taken together, our study provides novel insights on how tick salivary exosomes secreted in saliva can delay wound healing at the bite site to facilitate successful blood feeding.

Discovery of Alpha-Gal-Containing Antigens in North American Tick Species Believed to Induce Red Meat Allergy

Development of specific IgE antibodies to the oligosaccharide galactose-α-1, 3-galactose (α-gal) following tick bites has been shown to be the source of red meat allergy. In this study, we investigated the presence of α-gal in four tick species: the lone-star tick (Amblyomma americanum), the Gulf-Coast tick (Amblyomma maculatum), the American dog tick (Dermacentor variabilis), and the black-legged tick (Ixodes scapularis) by using a combination of immunoproteomic approach and, carbohydrate analysis. Anti-α-gal antibodies identified α-gal in the salivary glands of both Am. americanum and Ix. scapularis, while Am. maculatum and De. variabilis appeared to lack the carbohydrate. PNGase F treatment confirmed the deglycosylation of N-linked α-gal-containing proteins in tick salivary glands. Immunolocalization of α-gal moieties to the salivary secretory vesicles of the salivary acini also confirmed the secretory nature of α-gal-containing antigens in ticks. Am. americanum ticks were fed on human blood (lacks α-gal) using a silicone membrane system to determine the source of the α-gal. N-linked glycan analysis revealed that Am. americanum and Ix. scapularis have α-gal in their saliva and salivary glands, but Am. maculatum contains no detectable quantity. Consistent with the glycan analysis, salivary samples from Am. americanum and Ix. scapularis stimulated activation of basophils primed with plasma from α-gal allergic subjects. Together, these data support the idea that bites from certain tick species may specifically create a risk for the development of α-gal-specific IgE and hypersensitivity reactions in humans. Alpha-Gal syndrome challenges the current food allergy paradigm and broadens opportunities for future research.

Impact of Subolesin and Cystatin Knockdown by RNA Interference in Adult Female Haemaphysalis longicornis (Acari: Ixodidae) on Blood Engorgement and Reproduction

Currently, multi-antigenic vaccine use is the method of choice for the strategic control of ticks. Therefore, determining the efficacy of combined antigens is a promising avenue of research in the development of anti-tick vaccines. The antigen responsible for blood intake and reproduction has proven suitable as a vaccine antigen. It has been shown to silence Haemaphysalis longicornis salivary cystatin (HlSC-1) and subolesin by RNA interference. Adult unfed female ticks were injected with double-stranded RNA of (A) subolesin, (B) cystatin, (C) subolesin plus cystatin, and (D) injection buffer, then fed alongside normal unfed males up to spontaneous drop-down. The percentage of knockdowns was determined by real-time polymerase chain reaction. Sixty-three percent and 53% knockdown rates were observed in subolesin and cystatin double-stranded RNA-injected ticks respectively, while 32 and 26% knockdown rates of subolesin and cystatin transcript were observed in subolesin plus cystatin double-stranded RNA-injected ticks. Subolesin and/or cystatin knockdown causes a significant (p < 0.05) reduction in tick engorgement, egg mass weight, and egg conversion ratio. Most importantly, combined silencing did not act synergistically, but caused a similarly significant (p < 0.05) reduction in tick engorgement, egg mass weight, and egg conversion ratio. Therefore, the elucidation of multiple antigens may be helpful in the future of vaccines.

Assessment of tick antioxidant responses to exogenous oxidative stressors and insight into the role of catalase in the reproductive fitness of the Gulf Coast tick, Amblyomma maculatum

As obligate blood-sucking ectoparasites, to avoid tissue damage, ticks must neutralize the reactive oxygen species (ROS) generated from uptake and digestion of a bloodmeal. Consequently, ticks utilize a battery of antioxidant molecules, including catalase (CAT), an enzyme that converts hydrogen peroxide (H2 O2 ) into water and oxygen. Here, we investigated the tick antioxidant machinery by exogenous injection of sublethal doses of H2 O2 or paraquat. The relative transcript levels of selected Amblyomma maculatum antioxidant targets in tissues were determined by quantitative reverse transcriptase PCR following treatment. The results showed 2-16-fold increases in target antioxidant gene transcripts, signifying the ability of Am. maculatum to regulate its antioxidant machinery when exposed to increased ROS levels. Next, RNA interference was used to determine the functional role of CAT in haematophagy, redox homeostasis and reproductive fitness. CAT gene silencing was confirmed by transcript depletion within tick tissues; however, CAT knockdown alone did not interfere with tick haematophagy or phenotype, as confirmed by the resulting differential expression of antioxidant genes, thereby indicating an alternative mechanism for ROS control. Interestingly, double stranded RNA of CAT gene (dsCAT) and the CAT inhibitor, 3-aminotriazole, together reduced tick reproductive fitness via a marked reduction in egg mass and larval eclosion rates, highlighting a role for CAT in tick redox-homeostasis, making it a potential target for tick control.

Transcriptional activation of antioxidants may compensate for selenoprotein deficiencies in Amblyomma maculatum (Acari: Ixodidae) injected with selK- or selM-dsRNA

The Gulf-Coast tick, Amblyomma maculatum, possesses an elaborate set of selenoproteins, which prevent the deleterious effects from oxidative stress that would otherwise occur during feeding. In the current work, we examined the role of selenoprotein K (SelK) and selenoprotein M (SelM) in feeding A. maculatum by bioinformatics, transcriptional gene expression, RNA interference and antioxidant assays. The transcriptional expression of SelK did not vary significantly in salivary glands or midguts throughout the bloodmeal. However, there was a 58-fold increase in transcript levels of SelM in tick midguts. Ticks injected with selK-dsRNA or selM-dsRNA did not reveal any observable differences in egg viability but oviposition was reduced. Surprisingly, salivary antioxidant activity was higher in selenoprotein knockouts compared with controls, which is probably the result of compensatory transcriptional expression of genes involved in combating reactive oxygen species. In fact, quantitative real-time PCR data suggest that the transcriptional expression of catalase increased in ticks injected with selM-double-stranded RNA. Additionally, the transcriptional expression of selN decreased ∼90% in both SelK/SelM knockdowns. These data indicate that SelK and SelM are salivary antioxidants but are not essential for tick survival or reproduction and are compensated by other antioxidant systems.

An insight into the microbiome of the Amblyomma maculatum (Acari: Ixodidae)

The aim of this study was to survey the bacterial diversity of Amblyomma maculatum Koch, 1844, and characterize its infection with Rickettsia parkeri. Pyrosequencing of the bacterial 16S rRNA was used to determine the total bacterial population in A. maculatum. Pyrosequencing analysis identified Rickettsia in A. maculatum midguts, salivary glands, and saliva, which indicates successful trafficking in the arthropod vector. The identity of Rickettsia spp. was determined based on sequencing the rickettsial outer membrane protein A (rompA) gene. The sequence homology search revealed the presence of R. parkeri, Rickettsia amblyommii, and Rickettsia endosymbiont ofA. maculatum in midgut tissues, whereas the only rickettsia detected in salivary glands was R. parkeri, suggesting it is unique in its ability to migrate from midgut to salivary glands, and colonize this tissue before dissemination to the host. Owing to its importance as an emerging infectious disease, the R. parkeri pathogen burden was quantified by a rompB-based quantitative polymerase chain reaction (qPCR) assay and the diagnostic effectiveness of using R. parkeri polyclonal antibodies in tick tissues was tested. Together, these data indicate that field-collected A. maculatum had a R. parkeri infection rate of 12-32%. This study provides an insight into the A. maculatum microbiome and confirms the presence of R. parkeri, which will serve as the basis for future tick and microbiome interaction 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.

Molecular characterization and functional significance of the Vti family of SNARE proteins in tick salivary glands

Exocytosis involves membrane fusion between secretory vesicles and the plasma membrane. The Soluble N-ethylmaleimide-sensitive factor attachment proteins (SNAPs) and their receptor proteins (SNAREs) interact to fuse vesicles with the membrane and trigger the release of their sialosecretome out of the tick salivary gland cells. In this study, we examined the functional significance of the Vti family of SNARE proteins of blood-feeding Amblyomma maculatum and Amblyomma americanum. Vti1A and Vti1B have been implicated in multiple functional roles in vesicle transport. QRT-PCR studies demonstrated that the highest transcriptional expression of vti1a and vti1b genes occurs in unfed salivary glands, suggesting that elevated secretory vesicle formation occurs prior to feeding but continues at low rates after blood feeding commences. Vti1A and Vti1B localize to the secretory vesicles in unfed tick salivary glands in immunofluorescence microscopy studies. Knockdown of vti1a and vti1b by RNA interference resulted in a significant decrease in the engorged tick weight compared to the control during prolonged blood-feeding on the host. RNA interference of vti1a or vti1b impaired oviposition and none of the ticks produced eggs masses. Surprisingly, the double knockdown did not produce a strong phenotype and ticks fed normally on the host and produced egg masses, suggesting a compensatory mechanism exists within the secretory system which may have been activated in the double knockdown. These results suggest an important functional role of the Vti family of SNARE proteins in tick blood feeding and ultimately oviposition. Understanding the basic functions of the Vti family of SNARE proteins in salivary glands may lead to better ways to prevent tick attachment and transmission of tick-borne diseases.

Transcriptomic analysis of the salivary glands of an invasive whitefly

BACKGROUND

Some species of the whitefly Bemisia tabaci complex cause tremendous losses to crops worldwide through feeding directly and virus transmission indirectly. The primary salivary glands of whiteflies are critical for their feeding and virus transmission. However, partly due to their tiny size, research on whitefly salivary glands is limited and our knowledge on these glands is scarce.

METHODOLOGY/PRINCIPAL FINDINGS

We sequenced the transcriptome of the primary salivary glands of the Mediterranean species of B. tabaci complex using an effective cDNA amplification method in combination with short read sequencing (Illumina). In a single run, we obtained 13,615 unigenes. The quantity of the unigenes obtained from the salivary glands of the whitefly is at least four folds of the salivary gland genes from other plant-sucking insects. To reveal the functions of the primary glands, sequence similarity search and comparisons with the whole transcriptome of the whitefly were performed. The results demonstrated that the genes related to metabolism and transport were significantly enriched in the primary salivary glands. Furthermore, we found that a number of highly expressed genes in the salivary glands might be involved in secretory protein processing, secretion and virus transmission. To identify potential proteins of whitefly saliva, the translated unigenes were put into secretory protein prediction. Finally, 295 genes were predicted to encode secretory proteins and some of them might play important roles in whitefly feeding.

CONCLUSIONS/SIGNIFICANCE

The combined method of cDNA amplification, Illumina sequencing and de novo assembly is suitable for transcriptomic analysis of tiny organs in insects. Through analysis of the transcriptome, genomic features of the primary salivary glands were dissected and biologically important proteins, especially secreted proteins, were predicted. Our findings provide substantial sequence information for the primary salivary glands of whiteflies and will be the basis for future studies on whitefly-plant interactions and virus transmission.

Laboratory-infected Ehrlichia chaffeensis female adult Amblyomma americanum salivary glands reveal differential gene expression

Ticks are efficient ectoparasites that are able to steal blood, a rich source of nutrients, from their vertebrate hosts. The nymphal developmental stage of ticks plays an important role for pathogen transmission to human and other animal hosts. In this article, we describe a bloodmeal-based sex differentiation tool to generate adult female ticks infected with Ehrlichia chaffeensis to investigate vector-pathogen interactions (functional genomics and gene expression studies). We demonstrate that there is a correlation between the uptake of blood during nymph attachment and the molting into male or female adult ticks. The data obtained from the bloodmeal experiments suggest that nymphs that molt into females presumably imbibe more blood than those that become male during the nymphal stage. The natural low E. chaffeensis infection rate in female adult Amblyomma americanum (L.) is a major limiting factor to investigate Ehrlichia-Amblyomma interactions. To generate Ehrlichia-infected female adult ticks, we inoculated obligate E. chaffeensis (Arkansas strain) infected DH82 cells into heavier engorged nymphs (> 12 mg) and allowed them to molt. Freshly molted adults were used to test the E. chaffeensis infection rate. E. chaffeensis genomic DNA was extracted from individual unfed and partially blood fed tick midgut and salivary gland tissues. The tissue samples were tested for the presence of E. chaffeensis using the nested polymerase chain reaction process. Polymerase chain reaction-amplified fragments were detected in unfed and partially fed tissues, demonstrating successful E. chaffeensis infection of tick tissues. This method was used to successfully show differential expression of selected tick genes in E. chaffeensis-infected midguts and salivary glands.

Neural control of salivary glands in ixodid ticks

Studies of tick salivary glands (SGs) and their components have produced a number of interesting discoveries over the last four decades. However, the precise neural and physiological mechanisms controlling SG secretion remain enigmatic. Major studies of SG control have identified and characterized many pharmacological and biological compounds that activate salivary secretion, including dopamine (DA), octopamine, γ-aminobutyric acid (GABA), ergot alkaloids, pilocarpine (PC), and their pharmacological relatives. Specifically, DA has shown the most robust activities in various tick species, and its effect on downstream actions in the SGs has been extensively studied. Our recent work on a SG dopamine receptor has aided new interpretations of previous pharmacological studies and provided new concepts for SG control mechanisms. Furthermore, our recent studies have suggested that multiple neuropeptides are involved in SG control. Myoinhibitory peptide (MIP) and SIFamide have been identified in the neural projections reaching the basal cells of acini types II and III. Pigment-dispersing factor (PDF)-immunoreactive neural projections reach type II acini, and RFamide- and tachykinin-immunoreactive projections reach the SG ducts, but the chemical nature of the latter three immunoreactive substances are unidentified yet. Here, we briefly review previous pharmacological studies and provide a revised summary of SG control mechanisms in ticks.

Expression, localization, and functional role for synaptotagmins in pancreatic acinar cells

Secretagogue-induced changes in intracellular Ca(2+) play a pivotal role in secretion in pancreatic acini yet the molecules that respond to Ca(2+) are uncertain. Zymogen granule (ZG) exocytosis is regulated by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes. In nerve and endocrine cells, Ca(2+)-stimulated exocytosis is regulated by the SNARE-associated family of proteins termed synaptotagmins. This study examined a potential role for synaptotagmins in acinar secretion. RT-PCR revealed that synaptotagmin isoforms 1, 3, 6, and 7 are present in isolated acini. Immunoblotting and immunofluorescence using three different antibodies demonstrated synaptotagmin 1 immunoreactivity in apical cytoplasm and ZG fractions of acini, where it colocalized with vesicle-associated membrane protein 2. Synaptotagmin 3 immunoreactivity was detected in membrane fractions and colocalized with an endolysosomal marker. A potential functional role for synaptotagmin 1 in secretion was indicated by results that introduction of synaptotagmin 1 C2AB domain into permeabilized acini inhibited Ca(2+)-dependent exocytosis by 35%. In contrast, constructs of synaptotagmin 3 had no effect. Confirmation of these findings was achieved by incubating intact acini with an antibody specific to the intraluminal domain of synaptotagmin 1, which is externalized following exocytosis. Externalized synaptotagmin 1 was detected exclusively along the apical membrane. Treatment with CCK-8 (100 pM, 5 min) enhanced immunoreactivity by fourfold, demonstrating that synaptotagmin is inserted into the apical membrane during ZG fusion. Collectively, these data indicate that acini express synaptotagmin 1 and support that it plays a functional role in secretion whereas synaptotagmin 3 has an alternative role in endolysosomal membrane trafficking.

Gene expression in male tick salivary glands is affected by feeding in the presence of females

In this study, we have compared gene expression in the salivary glands of male Dermacentor andersoni ticks fed in the presence or absence of females to that in unfed males. We have established that the patterns of expression are different using arbitrarily primed polymerase chain reaction following reverse transcription of RNA (RAP-PCR) and differential display. Our results indicate a significant difference in salivary gland gene expression between fed and unfed males and also between males fed in the presence of females versus males fed in the absence of females. This difference in expression was partially confirmed by hybridization of probes made from mRNA from the three groups of ticks against the cloned gene fragments. Gene fragments for septin, utrophin, eukaryotic chain release factor 3, and elongation factor 2 were identified in this study, though the majority of fragments were not similar to anything in the databases.

Tick salivary glands: function, physiology and future

The salivary glands are the organs of osmoregulation in ticks and, as such, are critical to the biological success of ticks both during the extended period off the host and also during the feeding period on the host. Absorption of water vapour from unsaturated air into hygroscopic fluid produced by the salivary glands permit the tick to remain hydrated and viable during the many months between blood-meals. When feeding, the tick is able to return about 70% of the fluid and ion content of the blood-meal into the host by salivation into the feeding site. This saliva also contains many bioactive protein and lipid components that aid acquisition of the blood-meal. The salivary glands are the site of pathogen development and the saliva the route of transmission. The importance of the multifunctional salivary glands to tick survival and vector competency makes the glands a potential target for intervention. Here we review the cell biology of tick salivary glands and discuss the application of new approaches such as expressed sequence tag projects and RNA interference to this important area in the field of tick and tick-borne pathogen research.

RNAi-mediated gene silencing to assess the role of synaptobrevin and cystatin in tick blood feeding

In addition to being the conduit for pathogens into hosts, tick saliva contains a broad array of secretory products that facilitate prolonged tick attachment and blood feeding. Proteins found in tick saliva modulate host hemostasis and immune responses. However, it is not clear whether ticks manipulate the immune responses of their hosts by disrupting the antigen-processing pathways of the hosts. Protein secretion into tick saliva from the salivary glands is due to exocytosis of vesicular membrane-bound granular material regulated by SNARE complex proteins. Proteins associated with vesicles (v-SNAREs) are essential components of the exocytotic process. In this study, we assessed the functional significance of synaptobrevin, a SNARE protein, and cystatin, a cysteine protease inhibitor to blood feeding success, in the lone star tick, Amblyomma americanum, using in vivo RNA interference. In separate experiments, tick salivary cystatin and synpatobrevin genes were silenced by injecting adult ticks with 500 ng of dsRNA complementing each gene sequence. Silencing was demonstrated by reduced transcript in midguts and salivary glands. Additionally, disrupting expression of cystatin and synaptobrevin by RNAi reduced the ability of ticks to feed successfully, as demonstrated by feeding inhibition and reduced engorgement weights. Moreover, normal ticks exposed to a rabbit previously exposed to cystatin-silenced ticks exhibited significant resistance to tick feeding. Based on these findings, ticks appear to skillfully evade the host immune system by secreting cystatin, which disrupts normal antigen processing in antigen-presenting cells of hosts.

Amblyomma americanum salivary gland homolog of nSec1 is essential for saliva protein secretion

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins assemble in tight core complexes which promote fusion of carrier vesicles with target compartments. Members of this class of proteins are expressed in all eukaryotic cells and distributed in distinct subcellular compartments. All vesicle transport mechanisms known to date have an essential requirement for a member of the Sec1 protein family, including the nSec1 in regulated exocytosis. A homolog of nSec1 was cloned and sequenced from the salivary glands of partially fed female ticks. Double-stranded RNA was used to specifically reduce the amount of nSec1 mRNA and protein in female adult tick salivary glands. This reduction was accompanied by a decrease in anticoagulant protein release by the glands and by abnormalities in feeding by dsRNA treated ticks. We report the efficacy of double-stranded RNA-mediated interference in "knocking down" nSec1 both in vivo and in vitro in tick salivary glands and the applicability of this technique for studying the mechanism of exocytosis in tick salivary glands.

Amblyomma americanum salivary glands: double-stranded RNA-mediated gene silencing of synaptobrevin homologue and inhibition of PGE2 stimulated protein secretion

Protein secretion into the saliva from the tick salivary glands is due to exocytosis of vesicular membrane bound granular material regulated by SNARE complex proteins after salivary gland stimulation by PGE2 [Insect Biochem. Mol. Biol. 32 (2002) 1711]. Proteins associated with vesicles (v-SNAREs) are essential components of the exocytotic process. Synaptobrevin is a key v-SNARE in all secreting cells studied to date. A vesicle-associated synaptobrevin cDNA fragment homologue from the salivary glands of partially fed lone star tick females was cloned and sequenced. Double-stranded (ds) RNA interference (RNAi) is an effective method to silence specific gene expression. The functional role of synaptobrevin in protein secretion in partially fed tick salivary glands was studied with an in vitro RNAi method. Incubation of isolated salivary glands with double-stranded RNA (dsRNA) transcribed from a tick salivary gland synaptobrevin cDNA fragment resulted in decreased expression of the transcript, a reduction in the level of synaptobrevin protein and inhibition of PGE2 stimulated anticoagulant protein secretion by isolated salivary glands. We demonstrate the applicability of RNAi for studying individual steps in the mechanism of PGE2 stimulated exocytosis in the salivary glands of ixodid ticks.