Identification of a complex peptidergic neuroendocrine network in the hard tick, Rhipicephalus appendiculatus

Anthropogenic electromagnetic radiation alters the transcription levels of the genes encoding the SIFamide and myoinhibitory peptide and their receptors in Ixodes ricinus synganglion

The research of the influences of man-made electromagnetic fields on tick physiology has been very sparse and long neglected since the pioneer studies published in 1996 and 2000. Once multiple behavioral tests confirmed an attraction and possible perception of electromagnetic fields in ticks, a new interest in this topic erupted in recent years. In this study, qRT-PCR is utilized to determine the changes in the mRNA transcript levels of neuropeptides SIFamide and myoinhibitory peptide (mip and sifa) and their representative receptors (mip-r1 and sifa-r1) in the synganglia of the tick Ixodes ricinus irradiated by 900 MHz radiofrequency electromagnetic field. It was determined that 40 V/m intensity has a significant suppressory effect on the transcript levels of all genes after at least 60 minutes of constant exposure in both sexes. Commonly occurring intensity of radiation in urban areas (2 V/m) produced an elevation in mRNA levels after various timespans in every gene. A significant decrease of transcript abundances was detected in females after one hour of exposure to 2 V/m. Results of this study widen the knowledge of EMF-induced alterations in the neurophysiology of I. ricinus, the most commonly distributed hard tick in Europe.

Effects of Electromagnetic Radiation on Neuropeptide Transcript Levels in the Synganglion of Ixodes ricinus

Anthropogenic electromagnetic radiation is an important environmental factor affecting the functionality of biological systems. Sensitivity to various frequencies of electromagnetic radiation has been detected in ixodid ticks in the past. However, the physiological aspects of radiation effects have not yet been studied in ticks. In the presented experiment, 360 Ixodes ricinus ticks, 180 males and 180 females, were divided into 16 irradiated and 8 control groups. The irradiated groups were exposed to two different intensities of electromagnetic radiation with a frequency of 900 MHz at different lengths of exposure time. RT-PCR was utilized to determine the changes in mRNA levels in tick synganglia after irradiation. Four randomly selected neuropeptide genes were tested-allatotropin (at), FGLa-related allatostatins (fgla/ast), kinin, and arginine-vasopressin-like peptide (avpl). A significant decrease in transcript levels in all female groups exposed to higher intensity radiofrequency radiation for 1 to 3 h was found. After one hour of radiofrequency exposure, a significant downregulation in allatotropin expression in males was detected. A consistent downregulation of the at gene was detected in males irradiated with at a higher intensity. Unfortunately, the specific functions of the studied neuropeptides in ticks are not known yet, so a more comprehensive study is necessary to describe the effects of EMF on observed neuropeptides. This study represents the first report on the effects of the abiotic environment on tick neurophysiology.

Isolation and electrophysiological recording of Ixodes ricinus synganglion neurons

The central nervous system of hard ticks (Ixodidae) consists of a concentrated merged nerve mass known as the synganglion. Although knowledge of tick neurobiology has dramatically improved over the last two decades, this is the first time that isolation and electrophysiological recordings have been carried out on tick neurons from the synganglion. Method: We developed a simple protocol for synganglion neuron isolation and used a whole-cell patch clamp to measure ionic currents induced by acetylcholine, nicotine and muscarine. Relatively large neurons (∼ 25 μm and ∼ 35 μm) were isolated and 1 mM acetylcholine was used to induce strong inward currents of -0.38 ± 0.1 nA and - 1.04 ± 0.1 nA, respectively, with the corresponding cell capacitances being at around 142 pF and 188 pF. In addition, successive application of 1 mM acetylcholine through ∼25 μm and ∼ 35 μm cells for increasing amounts of time resulted in a rapid reduction in current amplitudes. We also found that acetylcholine-evoked currents were associated with a reversible increase in intracellular calcium levels for each neuronal type. In contrast, 1 mM muscarine and nicotine induced a strong and non-reversible increase in intracellular calcium levels. This study serves as a proof of concept for the mechanical isolation of tick synganglion neurons followed by their electrophysiological recording. This approach will aid investigations into the pharmacological properties of tick neurons and provides the tools needed for the identification of drug-targeted sites and effective tick control measures.

Dual SIFamide receptors in Ixodes salivary glands

Salivary glands are vital to tick feeding success and also play a crucial role in tick-borne pathogen transmission. In previous studies of Ixodes scapularis salivary glands, we demonstrated that saliva-producing type II and III acini are innervated by neuropeptidergic axons which release different classes of neuropeptides via their terminals (Šimo et al., 2009b, 2013). Among these, the neuropeptide SIFamide-along with its cognate receptor-were postulated to control the basally located acinar valve via basal epithelial and myoepithelial cells (Vancová et al., 2019). Here, we functionally characterized a second SIFamide receptor (SIFa_R2) from the I. scapularis genome and proved that it senses a low nanomolar level of its corresponding ligand. Insect SIFamide paralogs, SMYamides, also activated the receptor but less effectively compared to SIFamide. Bioinformatic and molecular dynamic analyses suggested that I. scapularis SIFamide receptors are class A GPCRs where the peptide amidated carboxy-terminus is oriented within the receptor binding cavity. The receptor was found to be expressed in Ixodes ricinus salivary glands, synganglia, midguts, trachea, and ovaries, but not in Malpighian tubules. Investigation of the temporal expression patterns suggests that the receptor transcript is highly expressed in unfed I. ricinus female salivary glands and then decreases during feeding. In synganglia, a significant transcript increase was detected in replete ticks. In salivary gland acini, an antibody targeting the second SIFamide receptor recognized basal epithelial cells, myoepithelial cells, and basal granular cells in close proximity to the SIFamide-releasing axon terminals. Immunoreactivity was also detected in specific neurons distributed throughout various I. ricinus synganglion locations. The current findings, alongside previous reports from our group, indicate that the neuropeptide SIFamide acts via two different receptors that regulate distinct or common cell types in the basal region of type II and III acini in I. ricinus salivary glands. The current study investigates the peptidergic regulation of the I. ricinus salivary gland in detail, emphasizing the complexity of this system.

Identification and expression of short neuropeptide F and its receptors in the tick Ixodes ricinus

In Europe, the tick Ixodes ricinus is the most important vector of numerous pathogens that are transmitted during blood feeding on their vertebrate hosts. To elucidate mechanisms controlling blood intake and associated transmission of pathogens we identified and described expression of short neuropeptide F (sNPF) and its receptors which are known to regulate feeding in insects. Using in situ hybridization (ISH) and immunohistochemistry (IHC) we stained numerous neurons producing sNPF in the central nervous system (CNS; synganglion), while a few peripheral neurons were detected anteriorly to the synganglion, and on the surface of the hindgut and leg muscles. Apparent sNPF expression was also found in enteroendocrine cells individually scattered in anterior lobes of the midgut. In silico analyses and BLAST search for sNPF receptors revealed two putative G protein-coupled receptors (sNPFR1 and sNPFR2) in the I. ricinus genome. Aequorin-based functional assay in CHO cells showed that both receptors were specific and sensitive to sNPF in nanomolar concentrations. Increased expression levels of these receptors in the gut during blood intake suggest that sNPF signaling may be involved in regulation of feeding and digestion processes of I. ricinus.

Putative target sites in synganglion for novel ixodid tick control strategies

Acaricide resistance is a global problem that has impacts worldwide. Tick populations with broad resistance to all commercially available acaricides have been reported. Since resistance selection in ticks and their role in pathogen transmission to animals and humans result in important economic and public health burden, it is essential to develop new strategies for their control (i.e., novel chemical compounds, vaccines, biological control). The synganglion is the tick central nervous system and it is responsible for synthesizing and releasing signaling molecules with different physiological functions. Synganglion proteins are the targets of the majority of available acaricides. In this review we provide an overview of the mode-of-action and resistance mechanisms against neurotoxic acaricides in ticks, as well as putative target sites in synganglion, as a supporting tool to identify new target proteins and to develop new strategies for tick control.

Prevention of tick-borne diseases: challenge to recent medicine

Abstract

Ticks represent important vectors and reservoirs of pathogens, causing a number of diseases in humans and animals, and significant damage to livestock every year. Modern research into protection against ticks and tick-borne diseases focuses mainly on the feeding stage, i.e. the period when ticks take their blood meal from their hosts during which pathogens are transmitted. Physiological functions in ticks, such as food intake, saliva production, reproduction, development, and others are under control of neuropeptides and peptide hormones which may be involved in pathogen transmission that cause Lyme borreliosis or tick-borne encephalitis. According to current knowledge, ticks are not reservoirs or vectors for the spread of COVID-19 disease. The search for new vaccination methods to protect against ticks and their transmissible pathogens is a challenge for current science in view of global changes, including the increasing migration of the human population.

Highlights

• Tick-borne diseases have an increasing incidence due to climate change and increased human migration

• To date, there is no evidence of transmission of coronavirus COVID-19 by tick as a vector

• To date, there are only a few modern, effective, and actively- used vaccines against ticks or tick-borne diseases

• Neuropeptides and their receptors expressed in ticks may be potentially used for vaccine design

Myotropic Activities of Tick Pyrokinin Neuropeptides and Analog in Feeding Tissues of Hard Ticks (Ixodidae)

Neuropeptides regulate many important physiological processes in animals. The G protein-coupled receptors of corresponding small neuropeptide ligands are considered promising targets for controlling arthropod pests. Pyrokinins (PKs) are pleiotropic neuropeptides that, in some insect species, stimulate muscle contraction and modulate pheromone biosynthesis, embryonic diapause, and feeding behavior. However, their function remains unknown in ticks. In this study, we reported the myotropic activity of tick endogenous PKs and a PK agonist analog, PK-PEG₈ (MS[PEG₈]-YFTPRLa), on feeding tissues of two tick species representing the family Ixodidae lineages, namely, Prostriata (Ixodes scapularis) and Metastriata (Rhipicephalus sanguineus). First, we predicted the sequences of two periviscerokinins (PVK), one with a derived ending RNa and five PKs encoded by the CAPA peptide precursor from R. sanguineus and found the encoded PKs were identical to those of R. microplus identified previously. The pharynx-esophagus of both tick species responded with increased contractions to 10 μM of the endogenous PK as well as to PK-PEG₈ but not to the scrambled PK peptide, as expected. A dose-dependent myotropic activity of the PK-PEG₈ was found for both tick species, validating the analog activity previously found in the pyrokinin recombinant receptor assay. In agreement with the tissue activity elicited, we quantified the relative transcript abundance of R. sanguineus PK receptor in unfed female ticks and found it was the highest in the feeding tissues extracted from the capitulum and lowest in the reproductive tissue. This is the first report of the activity of pyrokinins in ticks. These findings strongly indicate the potential role of PKs in regulating tick blood feeding and therefore, making the tick PK receptor a potential target for interference.

Leucokinins: Multifunctional Neuropeptides and Hormones in Insects and Other Invertebrates

Leucokinins (LKs) constitute a neuropeptide family first discovered in a cockroach and later identified in numerous insects and several other invertebrates. The LK receptors are only distantly related to other known receptors. Among insects, there are many examples of species where genes encoding LKs and their receptors are absent. Furthermore, genomics has revealed that LK signaling is lacking in several of the invertebrate phyla and in vertebrates. In insects, the number and complexity of LK-expressing neurons vary, from the simple pattern in the Drosophila larva where the entire CNS has 20 neurons of 3 main types, to cockroaches with about 250 neurons of many different types. Common to all studied insects is the presence or 1-3 pairs of LK-expressing neurosecretory cells in each abdominal neuromere of the ventral nerve cord, that, at least in some insects, regulate secretion in Malpighian tubules. This review summarizes the diverse functional roles of LK signaling in insects, as well as other arthropods and mollusks. These functions include regulation of ion and water homeostasis, feeding, sleep-metabolism interactions, state-dependent memory formation, as well as modulation of gustatory sensitivity and nociception. Other functions are implied by the neuronal distribution of LK, but remain to be investigated.

Enlisting the Ixodes scapularis Embryonic ISE6 Cell Line to Investigate the Neuronal Basis of Tick-Pathogen Interactions

Neuropeptides are small signaling molecules expressed in the tick central nervous system, i.e., the synganglion. The neuronal-like Ixodes scapularis embryonic cell line, ISE6, is an effective tool frequently used for examining tick–pathogen interactions. We detected 37 neuropeptide transcripts in the I. scapularis ISE6 cell line using in silico methods, and six of these neuropeptide genes were used for experimental validation. Among these six neuropeptide genes, the tachykinin-related peptide (TRP) of ISE6 cells varied in transcript expression depending on the infection strain of the tick-borne pathogen, Anaplasma phagocytophilum. The immunocytochemistry of TRP revealed cytoplasmic expression in a prominent ISE6 cell subpopulation. The presence of TRP was also confirmed in A. phagocytophilum-infected ISE6 cells. The in situ hybridization and immunohistochemistry of TRP of I. scapularis synganglion revealed expression in distinct neuronal cells. In addition, TRP immunoreaction was detected in axons exiting the synganglion via peripheral nerves as well as in hemal nerve-associated lateral segmental organs. The characterization of a complete Ixodes neuropeptidome in ISE6 cells may serve as an effective in vitro tool to study how tick-borne pathogens interact with synganglion components that are vital to tick physiology. Therefore, our current study is a potential stepping stone for in vivo experiments to further examine the neuronal basis of tick–pathogen interactions.

Cholinergic axons regulate type I acini in salivary glands of Ixodes ricinus and Ixodes scapularis ticks

Regulatory factors controlling tick salivary glands (SGs) are direct upstream neural signaling pathways arising from the tick's central nervous system. Here we investigated the cholinergic signaling pathway in the SG of two hard tick species. We reconstructed the organization of the cholinergic gene locus, and then used in situ hybridization to localize mRNA encoding choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) in specific neural cells in the Ixodes synganglion. Immunohistochemical staining revealed that cholinergic axonal projections exclusively reached type I acini in the SG of both Ixodes species. In type I acini, the rich network of cholinergic axons terminate within the basolateral infoldings of the lamellate cells. We also characterized two types (A and B) of muscarinic acetylcholine receptors (mAChRs), which were expressed in Ixodes SG. We pharmacologically assessed mAChR-A to monitor intracellular calcium mobilization upon receptor activation. In vivo injection of vesamicol-a VAChT blocker-at the cholinergic synapse, suppressed forced water uptake by desiccated ticks, while injection of atropine, an mAChR-A antagonist, did not show any effect on water volume uptake. This study has uncovered a novel neurotransmitter signaling pathway in Ixodes SG, and suggests its role in water uptake by type I acini in desiccated ticks.

Differential localization of Hessian fly candidate effectors in resistant and susceptible wheat plants

Hessian fly Mayetiola destructor is a notorious pest of wheat. Previous studies suggest that Hessian fly uses effector-based mechanisms to attack wheat plants during parasitism, but no direct evidence has been reported to support this postulation. Here, we produced recombinant proteins for five Family-1 candidate effectors and antibodies. Indirect immunostaining and western blots were carried out to examine the localization of Hessian fly Family-1 proteins in plant and insect tissues. Confocal images revealed that Family-1 putative effectors were exclusively produced in the basal region of larval salivary glands, which are directly linked to the mandibles' ducts for effector injection. The five Family-1 proteins were detected in infested host plants on western blots. Indirect immunostaining of sectioned host tissues around the feeding site revealed strikingly different localization patterns between resistant and susceptible plants. In susceptible plants, the Family-1 proteins penetrated from the feeding cell into deep tissues, indicative of movement between cells during nutritive cell formation. In contrast, the Hessian fly proteins were primarily limited to the initially attacked cells in resistant plants. The limitation of effectors' spread in resistant plants was likely due to wall strengthening and rapid hypersensitive cell death. Cell death was found in Nicotiana benthamiana in association with hypersensitive reaction triggered by the Family-1 effector SSGP-1A2. Our finding represents a significant progress in visualizing insect effectors in host tissues and mechanisms of plant resistance and susceptibility to gall midge pests.

A Horizontal Gene Transfer Led to the Acquisition of a Fructan Metabolic Pathway in a Gall Midge

Animals are thought to use only glucose polymers (glycogen) as energy reserve, whereas both glucose (starch) and fructose polymers (fructans) are used by microbes and plants. Here, it is reported that the gall midge Mayetiola destructor, and likely other herbivorous animal species, gained the ability to utilize dietary fructans directly as storage polysaccharides by a single horizontal gene transfer (HGT) of bacterial levanase/inulinase gene followed by gene expansion and differentiation. Multiple genes encoding levanases/inulinases have their origin in a single HGT event from a bacterium and they show high expression levels and enzymatic activities in different tissues of the gall midge, including nondigestive fat bodies and eggs, both of which contained significant amounts of fructans. This study provides evidence that animals can also use fructans as energy reserve by incorporating bacterial genes in their genomes.

The Immunomodulatory Effect of IrSPI, a Tick Salivary Gland Serine Protease Inhibitor Involved in Ixodes ricinus Tick Feeding

Ticks are the most important vectors of pathogens affecting both domestic and wild animals worldwide. Hard tick feeding is a slow process—taking up to several days—and necessitates extended control over the host response. The success of the feeding process depends upon injection of tick saliva, which not only controls host hemostasis and wound healing, but also subverts the host immune response to avoid tick rejection that creates a favorable niche for the survival and propagation of diverse tick-borne pathogens. Here, we report on the molecular and biochemical features and functions of an Ixodes ricinus serine protease inhibitor (IrSPI). We characterize IrSPI as a Kunitz elastase inhibitor that is overexpressed in several tick organs—especially salivary glands—during blood-feeding. We also demonstrated that when IrSPI is injected into the host through saliva, it had no impact on tissue factor pathway-induced coagulation, fibrinolysis, endothelial cell angiogenesis or apoptosis, but the protein exhibits immunomodulatory activity. In particular, IrSPI represses proliferation of CD4⁺ T lymphocytes and proinflammatory cytokine secretion from both splenocytes and macrophages. Our study contributes valuable knowledge to tick-host interactions and provides insights that could be further exploited to design anti-tick vaccines targeting this immunomodulator implicated in I. ricinus feeding.

Neural and endocrine regulation of osmoregulatory organs in tick: Recent discoveries and implications

Ticks can survive in harsh and fluctuating vegetated environments for long durations between blood feedings with highly developed osmoregulatory mechanisms. Like the unique life history of hematophagous ticks, osmoregulatory organs and their regulatory mechanisms are significantly different from those in the closely related insect taxa. Over the last ten years, research has uncovered several neuropeptidergic innervations of the primary osmoregulatory organ, the salivary glands: myoinhibitory peptide (MIP), SIFamide, and elevenin. These neuropeptides are thought to be modulators of dopamine's autocrine or paracrine actions controlling the salivary glands, including the activation of fluid transport into the lumen of salivary acini and the pumping and gating action of salivary acini for expelling fluids out into salivary ducts. These actions are through two different dopamine receptors, D1 receptor and invertebrate D1-like dopamine receptor, respectively. Interestingly, MIP and SIFamide are also involved in the control of another important excretory/osmoregulatory organ, the hindgut, where SIFamide is myostimulatory, with MIP having antagonistic effects. FGLamide related allatostatin is also found to have axonal projections located on the surface of the rectum. Investigations of the osmoregulatory mechanisms of these critical vector species will potentially lead to the development of a measure to control tick species.

Dynamics of Insulin Signaling in the Black-Legged Tick, Ixodes scapularis

Insulin-like peptides (ILPs) have been identified in several invertebrates, particularly insects, and work on these ILPs has revealed many roles including regulation of energy homeostasis, growth, development, and lifespan to name a few. However, information on arthropod ILPs outside of insects is sparse. Studies of Ixodid tick ILPs are particularly scarce, despite their importance as vectors of infectious agents, most notably Lyme disease. The recent publication of the genome of the black-legged tick, Ixodes scapularis, has advanced opportunities to study this organism from a molecular standpoint, a resource sorely needed for an organism with challenging life history requirements for study in the laboratory, such as a long life cycle and obligate, prolonged, blood-feeding at each life stage. Through bioinformatics searches of the tick genome and other available I. scapularis databases, we identified four putative ILP sequences. Full-length sequences of these ILP transcripts were confirmed, and quantitative RT-PCR was used to examine expression levels of these ILPs in different life stages, feeding states, and adult tissues. This work serves as an initial characterization of ILP expression in ticks and provides the foundation for further exploration of the roles of ILPs in these important arthropod vectors.

Ultrastructural mapping of salivary gland innervation in the tick Ixodes ricinus

The salivary gland of hard ticks is a highly innervated tissue where multiple intertwined axonal projections enter each individual acini. In the present study, we investigated the ultrastructural architecture of axonal projections within granular salivary gland type II and III acini of Ixodes ricinus female. Using immunogold labeling, we specifically examined the associations of SIFamide neuropeptide, SIFamide receptor (SIFa_R), neuropeptide pigment dispersing factor (PDF), and the invertebrate-specific D1-like dopamine receptor (InvD1L), with acinar cells. In both acini types, SIFamide-positive axons were found to be in direct contact with either basal epithelial cells or a single adlumenal myoepithelial cell in close proximity to the either the acinar duct or its valve, respectively. Accordingly, SIFa_R staining correlated with SIFamide-positive axons in both basal epithelial and myoepithelial cells. Immunoreactivity for both InvD1L and PDF (type II acini exclusively) revealed positive axons radiating along the acinar lumen. These axons were primarily enclosed by the adlumenal myoepithelial cell plasma membrane and interstitial projections of ablumenal epithelial cells. Our study has revealed the detailed ultrastructure of I. ricinus salivary glands, and provides a solid baseline for a comprehensive understanding of the cell-axon interactions and their functions in this essential tick organ.

Molecular characterization of neuropeptide elevenin and two elevenin receptors, IsElevR1 and IsElevR2, from the blacklegged tick, Ixodes scapularis

Understanding salivation in hematophagous arthropod vectors is crucial to developing novel methods to prevent vector-borne disease transmission. The interactions between the tick, host, and pathogens during salivation are highly complex, and are dynamically regulated by the tick central nervous system (synganglion). Recently, tick salivary modulation via neuropeptides was highlighted by mapping neuropeptidergic cells in the synganglion and salivary glands in hard ticks. In this study, we characterized the role of a novel neuropeptide, elevenin (IsElev), and its receptors (IsElevR1 and IsElevR2) in the innervation of the salivary glands from Ixodes scapularis female ticks. Homology-based BLAST searches of the I. scapularis genome and Sequence Read Archive (SRA), followed by gene cloning, identified candidate genes: IsElev, IsElevR1, and IsElevR2. The IsElev candidate contained common elevenin features: a signal peptide immediately before an elevenin precursor and two cysteines. During functional assays, synthetic IsElev efficiently activated both IsElevR1 and IsElevR2, as indicated by elevated calcium mobilization. IsElevR1 (EC50: 0.01 nM) was about 560 times more sensitive to synthetic IsElev than IsElevR2 (EC50: 5.59 nM). Immunoreactivity (IR) for IsElev and IsElevR1 was detected as a complex neuronal projection and several neurons in the synganglion. In salivary glands, IsElev-IR was detected in an axonal projection on the surface of the main salivary duct and in axon terminals within type II/III salivary gland acini, which are colocalized with SIFamide-IR. IsElevR1-IR was detected on the luminal surface of both type II/III acini. IsElev transcript levels were high in the synganglion and reached a peak at day 5 post-blood feeding. Salivary glands expressed IsElevR1, which gradually increased over the course of blood feeding until repletion. Here, we propose that IsElev and IsElevR1, localized in salivary gland acini types II/III, are likely involved in tick salivary secretion in the rapid engorgement phase of tick feeding. In addition, we also provide the evidences for IsElev action on the ovary by showing IsElevR1-IR and IsElevR2-IR on the surface of ovary.

The transcriptome of the lone star tick, Amblyomma americanum, reveals molecular changes in response to infection with the pathogen, Ehrlichia chaffeensis

The lone star tick, Amblyomma americanum, is an obligatory ectoparasite of many vertebrates and the primary vector of Ehrlichia chaffeensis, the causative agent of human monocytic ehrlichiosis. This study aimed to investigate the comparative transcriptomes of A. americanum underlying the processes of pathogen acquisition and of immunity towards the pathogen. Differential expression of the whole body transcripts in six different treatments were compared: females and males that were E. chaffeensis non-exposed, E. chaffeensis-exposed/uninfected, and E. chaffeensis-exposed/infected. The Trinity assembly pipeline produced 140,574 transcripts from trimmed and filtered total raw sequence reads (approximately 117M reads). The gold transcript set of the transcriptome data was established to minimize noise by retaining only transcripts homologous to official peptide sets of Ixodes scapularis and A. americanum ESTs and transcripts covered with high enough frequency from the raw data. Comparison of the gene ontology term enrichment analyses for the six groups tested here revealed an up-regulation of genes for defense responses against the pathogen and for the supply of intracellular Ca⁺⁺ for pathogen proliferation in the pathogen-exposed ticks. Analyses of differential expression, focused on functional subcategories including immune, sialome, neuropeptides, and G protein-coupled receptor, revealed that E. chaffeensis-exposed ticks exhibited an upregulation of transcripts involved in the immune deficiency (IMD) pathway, antimicrobial peptides, Kunitz, an insulin-like peptide, and bursicon receptor over unexposed ones, while transcripts for metalloprotease were down-regulated in general. This study found that ticks exhibit enhanced expression of genes responsible for defense against E. chaffeensis.

The Essential Role of Tick Salivary Glands and Saliva in Tick Feeding and Pathogen Transmission

As long-term pool feeders, ticks have developed myriad strategies to remain discreetly but solidly attached to their hosts for the duration of their blood meal. The critical biological material that dampens host defenses and facilitates the flow of blood-thus assuring adequate feeding-is tick saliva. Saliva exhibits cytolytic, vasodilator, anticoagulant, anti-inflammatory, and immunosuppressive activity. This essential fluid is secreted by the salivary glands, which also mediate several other biological functions, including secretion of cement and hygroscopic components, as well as the watery component of blood as regards hard ticks. When salivary glands are invaded by tick-borne pathogens, pathogens may be transmitted via saliva, which is injected alternately with blood uptake during the tick bite. Both salivary glands and saliva thus play a key role in transmission of pathogenic microorganisms to vertebrate hosts. During their long co-evolution with ticks and vertebrate hosts, microorganisms have indeed developed various strategies to exploit tick salivary molecules to ensure both acquisition by ticks and transmission, local infection and systemic dissemination within the vertebrate host.

Prediction of G protein-coupled receptor encoding sequences from the synganglion transcriptome of the cattle tick, Rhipicephalus microplus

The cattle tick, Rhipicephalus (Boophilus) microplus, is a pest which causes multiple health complications in cattle. The G protein-coupled receptor (GPCR) super-family presents a candidate target for developing novel tick control methods. However, GPCRs share limited sequence similarity among orthologous family members, and there is no reference genome available for R. microplus. This limits the effectiveness of alignment-dependent methods such as BLAST and Pfam for identifying GPCRs from R. microplus. However, GPCRs share a common structure consisting of seven transmembrane helices. We present an analysis of the R. microplus synganglion transcriptome using a combination of structurally-based and alignment-free methods which supplement the identification of GPCRs by sequence similarity. TMHMM predicts the number of transmembrane helices in a protein sequence. GPCRpred is a support vector machine-based method developed to predict and classify GPCRs using the dipeptide composition of a query amino acid sequence. These two bioinformatic tools were applied to our transcriptome assembly of the cattle tick synganglion. Together, BLAST and Pfam identified 85 unique contigs as encoding partial or full length candidate cattle tick GPCRs. Collectively, TMHMM and GPCRpred identified 27 additional GPCR candidates that BLAST and Pfam missed. This demonstrates that the addition of structurally-based and alignment-free bioinformatic approaches to transcriptome annotation and analysis produces a greater collection of prospective GPCRs than an analysis based solely upon methodologies dependent upon sequence alignment and similarity.

Orcokinin-like immunoreactivity in central neurons innervating the salivary glands and hindgut of ixodid ticks

Orcokinins are conserved neuropeptides within the Arthropoda but their cellular distribution and functions in ticks are unknown. We use an antibody against the highly conserved N-terminal (NFDEIDR) of mature orcokinin peptides to examine their distribution in six ixodid species: Amblyomma variegatum, Dermacentor reticulatus, Hyalomma anatolicum, Ixodes scapularis, Ixodes ricinus and Rhipicephalus appendiculatus. Numerous immunoreactive neurons (~100) were detected in various regions of the synganglion (central nervous system) in all examined tick species. Immunoreactive projections of two prominent groups of efferent neurons in the post-oesophageal region were examined in detail: (1) neurons innervating the salivary glands; (2) neurons innervating the hindgut. Using matrix-assisted laser desorption/ionisation-time-of-flight (MALDI-TOF), we detected orcokinin peaks in extracts of the synganglia and hindguts but not in the salivary glands of I. scapularis females. Our data provide further evidence of the presence of orcokinin in ixodid ticks and establish a morphological basis for functional studies of identified peptidergic neuronal networks.

Invertebrate specific D1-like dopamine receptor in control of salivary glands in the black-legged tick Ixodes scapularis

The control of tick salivary secretion, which plays a crucial role in compromising the host immune system, involves complex neural mechanisms. Dopamine is known to be the most potent activator of salivary secretion, as a paracrine/autocrine factor. We describe the invertebrate-specific D1-like dopamine receptor (InvD1L), which is highly expressed in tick salivary glands. The InvD1L phylogenic clade was found only in invertebrates, suggesting that this receptor was lost in vertebrates during evolution. InvD1L expressed in Chinese hamster ovary (CHO)-K1 cells was activated by dopamine with a median effective dose (EC50 ) of 1.34 μM. Immunohistochemistry using the antibody raised against InvD1L revealed two different types of immunoreactivities: basally located axon terminals that are colocalized with myoinhibitory peptide (MIP) and SIFamide neuropeptides, and longer axon-like processes that are positive only for the InvD1L antibody and extended to the apical parts of the acini. Both structures were closely associated with the myoepithelial cell, as visualized by beta-tubulin antibody, lining the acinar lumen in a web-like fashion. Subcellular localizations of InvD1L in the salivary gland suggest that InvD1L modulates the neuronal activities including MIP/SIFamide varicosities, and leads the contraction of myoepithelial cells and/or of the acinar valve to control the efflux of the luminal content. Combining the previously described D1 receptor with its putative function for activating an influx of fluid through the epithelial cells of acini, we propose that complex control of the tick salivary glands is mediated through two different dopamine receptors, D1 and InvD1L, for different downstream responses of the acinar cells.

Transcriptome of the female synganglion of the black-legged tick Ixodes scapularis (Acari: Ixodidae) with comparison between Illumina and 454 systems

Illumina and 454 pyrosequencing were used to characterize genes from the synganglion of female Ixodes scapularis. GO term searching success for biological processes was similar for samples sequenced by both methods. However, for molecular processes, it was more successful for the Illumina samples than for 454 samples. Functional assignments of transcripts predicting neuropeptides, neuropeptide receptors, neurotransmitter receptors and other genes of interest was done, supported by strong e-values (<-6), and high consensus sequence alignments. Transcripts predicting 15 putative neuropeptide prepropeptides ((allatostatin, allatotropin, bursicon α, corticotropin releasing factor (CRF), CRF-binding protein, eclosion hormone, FMRFamide, glycoprotein A, insulin-like peptide, ion transport peptide, myoinhibitory peptide, inotocin ( =  neurophysin-oxytocin), Neuropeptide F, sulfakinin and SIFamide)) and transcripts predicting receptors for 14 neuropeptides (allatostatin, calcitonin, cardioacceleratory peptide, corazonin, CRF, eclosion hormone, gonadotropin-releasing hormone/AKH-like, insulin-like peptide, neuropeptide F, proctolin, pyrokinin, SIFamide, sulfakinin and tachykinin) are reported. Similar to Dermacentor variabilis, we found transcripts matching pro-protein convertase, essential for converting neuropeptide hormones to their mature form. Additionally, transcripts predicting 6 neurotransmitter/neuromodulator receptors (acetylcholine, GABA, dopamine, glutamate, octopamine and serotonin) and 3 neurotransmitter transporters (GABA transporter, noradrenalin-norepinephrine transporter and Na+-neurotransmitter/symporter) are described. Further, we found transcripts predicting genes for pheromone odorant receptor, gustatory receptor, novel GPCR messages, ecdysone nuclear receptor, JH esterase binding protein, steroidogenic activating protein, chitin synthase, chitinase, and other genes of interest. Also found were transcripts predicting genes for spermatogenesis-associated protein, major sperm protein, spermidine oxidase and spermidine synthase, genes not normally expressed in the female CNS of other invertebrates. The diversity of messages predicting important genes identified in this study offers a valuable resource useful for understanding how the tick synganglion regulates important physiological functions.

Neuropeptidergic control of the hindgut in the black-legged tick Ixodes scapularis

The hindgut, as a part of the tick excretory system, plays an important physiological role in maintaining homoeostases and waste elimination. Immunoreactive projections from the synganglion to the hindgut were found using antibodies against four different neuropeptides: FGLamide related allatostatin, myoinhibitory peptide, SIFamide, and orcokinin. The presence of FGLamide related allatostatin, myoinhibitory peptide and SIFamide in both synganglia (source) and hindgut (target organ) extracts was confirmed by MALDI-TOF. Tissue-specific PCR revealed the expression of four putative FGLamide related allatostatin receptors and an SIFamide receptor in the hindgut. An antibody against Ixodes scapularis SIFamide receptor detected immunoreactive spots in epithelial cells as well as the visceral muscles surrounding the rectal sac, while staining with the antibody against myoinhibitory peptide receptor 1 revealed that the immunoreactivity was only associated with the visceral muscles. In hindgut motility assays, SIFamide activated hindgut motility in a dose-dependent manner. None of other three neuropeptides (FGLamide related allatostatin, myoinhibitory peptide and orcokinin) activated hindgut motility when tested alone. Myoinhibitory peptide antagonised the SIFamide-stimulated hindgut mobility when it was tested in combination with SIFamide.

Autocrine/paracrine dopamine in the salivary glands of the blacklegged tick Ixodes scapularis

Dopamine (DA) is known to be the most potent activator of tick salivary secretion, which is an essential component of successful tick feeding. We examined the quantitative changes of catecholamines using a method coupling high-pressure liquid chromatography with electrochemical detection (HPLC-ECD). We also investigated the levels of catecholamines conjugated to other molecules utilising appropriate methods to hydrolyse the conjugates. Three different biological samples, salivary glands, synganglia, ovaries and haemolymph were compared, and the largest quantity of DA was detected in salivary gland extracts (up to ∼100pg/tick), supporting the hypothesis that autocrine/paracrine dopamine activates salivary secretion. Quantitative changes of catecholamines in the salivary glands over the entire blood feeding duration were examined. The amount of dopamine in the salivary glands increased until the day 5 of feeding, at which the rapid engorgement phase began. We also detected a small but significant amount of norepinephrine in the salivary glands. Interestingly, saliva collected after induction of salivary secretion by the cholinergic agonist pilocarpine contained a large amount of DA sulphate with a trace amount of DA, suggesting a potential biological role of DA sulphate in tick saliva.

Molecular and functional characterization of the first tick CAP2b (periviscerokinin) receptor from Rhipicephalus (Boophilus) microplus (Acari: Ixodidae)

The cDNA of the receptor for CAP(2b)/periviscerokinin (PVK) neuropeptides, designated Rhimi-CAP(2b)-R, was cloned from synganglia of tick Rhipicephalus (Boophilus) microplus. This receptor is the ortholog of the insect CAP(2b)/PVK receptor, as concluded from analyses of the predicted protein sequence, phylogenetics and functional expression. Expression analyses of synganglion, salivary gland, Malpighian tubule, and ovary revealed Rhimi-CAP(2b)-R transcripts. The expression in mammalian cells of the open reading frame of Rhimi-CAP(2b)-R cDNA fused with a hemagglutinin tag at the receptor N-terminus was confirmed by immunocytochemistry. In a calcium bioluminescence assay the recombinant receptor was activated by the tick Ixodes scapularis CAP(2b)/PVK and a PVK analog with EC₅₀s of 64 nM and 249 nM, respectively. Tick pyrokinins were not active. This is the first report on the functional characterization of the CAP(2b)/PVK receptor from any tick species which will now permit the discovery of the physiological roles of these neuropeptides in ticks, as neurohormones, neuromodulators and/or neurotransmitters.

Molecular characterization of tick salivary gland glutaminyl cyclase

Glutaminyl cyclase (QC) catalyzes the cyclization of N-terminal glutamine residues into pyroglutamate. This post-translational modification extends the half-life of peptides and, in some cases, is essential in binding to their cognate receptor. Due to its potential role in the post-translational modification of tick neuropeptides, we report the molecular, biochemical and physiological characterization of salivary gland QC during the prolonged blood feeding of the black-legged tick (Ixodes scapularis) and the gulf-coast tick (Amblyomma maculatum). QC sequences from I. scapularis and A. maculatum showed a high degree of amino acid identity to each other and other arthropods and residues critical for zinc binding/catalysis (D159, E202, and H330) or intermediate stabilization (E201, W207, D248, D305, F325, and W329) are conserved. Analysis of QC transcriptional gene expression kinetics depicts an upregulation during the bloodmeal of adult female ticks prior to fast-feeding phases in both I. scapularis and A. maculatum suggesting a functional link with bloodmeal uptake. QC enzymatic activity was detected in saliva and extracts of tick salivary glands and midguts. Recombinant QC was shown to be catalytically active. Furthermore, knockdown of QC transcript by RNA interference resulted in lower enzymatic activity, and small, unviable egg masses in both studied tick species as well as lower engorged tick weights for I. scapularis. These results suggest that the post-translational modification of neurotransmitters and other bioactive peptides by QC is critical to oviposition and potentially other physiological processes. Moreover, these data suggest that tick-specific QC-modified neurotransmitters/hormones or other relevant parts of this system could potentially be used as novel physiological targets for tick control.

Sex peptides and MIPs can activate the same G protein-coupled receptor

In many animal species, copulation elicits a number of physiological and behavioral changes in the female partner. In Drosophila melanogaster, the main molecular effector of these physiological responses has been identified as sex peptide (SP). The sex peptide receptor (SPR) has been characterized and recently, its activation by Drosophila myoinhibiting peptides (MIPs)-in addition to SP-has been demonstrated. The myoinhibiting peptides are members of a conserved peptide family, also known as B-type allatostatins, which generally feature the C-terminal motif -WX6Wamide.

The conserved role of the AKT/GSK3 axis in cell survival and glycogen metabolism in Rhipicephalus (Boophilus) microplus embryo tick cell line BME26

BACKGROUND

Tick embryogenesis is a metabolically intensive process developed under tightly controlled conditions and whose components are poorly understood.

METHODS

In order to characterize the role of AKT (protein kinase B) in glycogen metabolism and cell viability, glycogen determination, identification and cloning of an AKT from Rhipicephalus microplus were carried out, in parallel with experiments using RNA interference (RNAi) and chemical inhibition.

RESULTS

A decrease in glycogen content was observed when AKT was chemically inhibited by 10-DEBC treatment, while GSK3 inhibition by alsterpaullone had an opposing effect. RmAKT ORF is 1584-bp long and encodes a polypeptide chain of 60.1 kDa. Phylogenetic and sequence analyses showed significant differences between vertebrate and tick AKTs. Either AKT or GSK3 knocked down cells showed a 70% reduction in target transcript levels, but decrease in AKT also reduced glycogen content, cell viability and altered cell membrane permeability. However, the GSK3 reduction promoted an increase in glycogen content. Additionally, either GSK3 inhibition or gene silencing had a protective effect on BME26 viability after exposure to ultraviolet radiation. R. microplus AKT and GSK3 were widely expressed during embryo development. Taken together, our data support an antagonistic role for AKT and GSK3, and strongly suggest that such a signaling axis is conserved in tick embryos, with AKT located upstream of GSK3.

GENERAL SIGNIFICANCE

The AKT/GSK3 axis is conserved in tick in a way that integrates glycogen metabolism and cell survival, and exhibits phylogenic differences that could be important for the development of novel control methods.

Receptors for the neuropeptides, myoinhibitory peptide and SIFamide, in control of the salivary glands of the blacklegged tick Ixodes scapularis

Tick salivary glands are important organs that enable the hematophagous feeding of the tick. We previously described the innervation of the salivary gland acini types II and III by a pair of protocerebral salivary gland neurons that produce both myoinhibitory peptide (MIP) and SIFamide (Šimo et al., 2009b). In this study we identified authentic receptors expressed in the salivary glands for these neuropeptides. Homology-based searches for these receptors in the Ixodes scapularis genome sequence were followed by gene cloning and functional expression of the receptors. Both receptors were activated by low nanomolar concentrations of their respective ligands. The temporal expression patterns of the two ligands and their respective receptors suggest that the SIFamide signaling system pre-exists in unfed salivary glands, while the MIP system is activated upon initiation of feeding. Immunoreactivity for the SIFamide receptor in the salivary gland was detected in acini types II and III, surrounding the acinar valve and extending to the basal region of the acinar lumen. The location of the SIFamide receptor in the salivary glands suggests three potential target cell types and their probable functions: myoepithelial cell that may function in the contraction of the acini and/or the control of the valve; large, basally located dopaminergic granular cells for regulation of paracrine dopamine; and neck cells that may be involved in the control of the acinar duct and its valve.

Cardioacceleratory function of the neurohormone CCAP in the mosquito Anopheles gambiae

Crustacean cardioactive peptide (CCAP) is a highly conserved arthropod neurohormone that is involved in ecdysis, hormone release and the modulation of muscle contractions. Here, we determined the CCAP gene structure in the malaria mosquito Anopheles gambiae, assessed the developmental expression of CCAP and its receptor and determined the role that CCAP plays in regulating mosquito cardiac function. RACE sequencing revealed that the A. gambiae CCAP gene encodes a neuropeptide that shares 100% amino acid identity with all sequenced CCAP peptides, with the exception of Daphnia pulex. Quantitative RT-PCR showed that expression of CCAP and the CCAP receptor displays a bimodal distribution, with peak mRNA levels in second instar larvae and pupae. Injection of CCAP revealed that augmenting hemocoelic CCAP levels in adult mosquitoes increases the anterograde and retrograde heart contraction rates by up to 28%, and increases intracardiac hemolymph flow velocities by up to 33%. Partial CCAP knockdown by RNAi had the opposite effect, decreasing the mosquito heart rate by 6%. Quantitative RT-PCR experiments showed that CCAP mRNA is enriched in the head region, and immunohistochemical experiments in newly eclosed mosquitoes detected CCAP in abdominal neurons and projections, some of which innervated the heart, but failed to detect CCAP in the abdomens of older mosquitoes. Instead, in older mosquitoes CCAP was detected in the pars lateralis, the subesophageal ganglion and the corpora cardiaca. In conclusion, CCAP has a potent effect on mosquito circulatory physiology, and thus heart physiology in this dipteran insect is under partial neuronal control.

Homology of insect corpora allata and vertebrate adenohypophysis?

Animal species of various phyla possess neuroendocrine glands whose hormonal products regulate developmental and physiological mechanisms and directly impact behavior. Two examples, the corpora allata of insects and the vertebrate adenohypophysis have previously been regarded as analogous tissues that evolved independently from diffuse epidermal nerve nets of early metazoans. More recent developmental and functional studies accumulated evidence suggesting that the bilaterian nervous systems including its modern parts (e.g. pallium or cortex and mushroom bodies) and its neuroendocrine appendages (that are considered to be more ancient structures) possess a single evolutionary origin. The corpora allata of insects and the vertebrate adenohypophysis share a number of characteristics in respect of morphology, control of hormone release by RFamides, metabolites produced by closely related cytochrome P450 enzymes and gene expression during embryonic development. This review incorporates latest findings into an extensive description of similarities between insect corpora allata and vertebrate adenohypophysis that should encourage further studies about the onto- and phylogenetic origin of these neuroendocrine glands.

In silico cloning of genes encoding neuropeptides, neurohormones and their putative G-protein coupled receptors in a spider mite

The genome of the spider mite was prospected for the presence of genes coding neuropeptides, neurohormones and their putative G-protein coupled receptors. Fifty one candidate genes were found to encode neuropeptides or neurohormones. These include all known insect neuropeptides and neurohormones, with the exception of sulfakinin, corazonin, neuroparsin and PTTH. True orthologs of adipokinetic hormone (AKH) were neither found, but there are three genes encoding peptides similar in structure to both AKH and the AKH-corazonin-related peptide. We were also unable to identify the precursors for pigment dispersing factor (PDF) or the recently discovered trissin. However, the spider mite probably does have such genes, as we found their putative receptors. A novel arthropod neuropeptide gene was identified that shows similarity to previously described molluscan neuropeptide genes and was called EFLamide. A total of 65 putative neuropeptide GPCR genes were also identified, of these 58 belong to the A-family and 7 to the B-family. Phylogenetic analysis showed that 50 of them are closely related to insect GPCRs, which allowed the identification of their putative ligand in 39 cases with varying degrees of certainty. Other spider mite GPCRs however have no identifiable orthologs in the genomes of the four holometabolous insect species best analyzed. Whereas some of the latter have orthologs in hemimetabolous insect species, crustaceans or ticks, for others such arthropod homologs are currently unknown.

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.

Synganglion transcriptome and developmental global gene expression in adult females of the American dog tick, Dermacentor variabilis (Acari: Ixodidae)

454 Pyrosequencing was used to characterize the expressed genes from the synganglion and associated neurosecretory organs of unfed and partially fed virgin and mated replete females of the American dog tick, Dermacentor variabilis. A total of 14,881 contiguous sequences (contigs) was assembled, with an average size of 229 bp. Gene ontology terms for Level 2 biological processes were assigned to 4366 contigs. Seven acetylcholinesterases, a muscarinic acetylcholine (ACh) receptor, two nicotinic ACh receptor β-subunits, two ACh unc-18 regulators, two dopamine receptors, two gamma aminobutyric acid (GABA) receptors, two GABA transporters, two norepinephrine transporters and an octopamine receptor are described. Microarrays were conducted to examine global gene expression and quantitative real-time polymerase chain reaction was used to verify expression of selected neuropeptides. Hierarchical clustering of all differentially expressed transcripts grouped part-fed and replete ticks as being more similar in terms of differentially expressed genes with unfed ticks as the outgroup. Nine putative neuropeptides (allatostatin, bursicon-β, preprocorazonin, glycoprotein hormone α, insulin-like peptide, three orcokinins, preprosulphakinin) and a gonadotropin releasing hormone receptor were differentially expressed, and their developmental expression and role in reproduction was investigated. The presence of eclosion hormone, corazonin and bursicon in the synganglion, which in insects regulate behaviour and cuticle development associated with moulting, suggest that this system may be used in ticks to regulate blood feeding, cuticle expansion and development related to female reproduction; adult ticks do not moult.

Evidence for D1 dopamine receptor activation by a paracrine signal of dopamine in tick salivary glands

Ticks that feed on vertebrate hosts use their salivary secretion, which contains various bioactive components, to manipulate the host's responses. The mechanisms controlling the tick salivary gland in this dynamic process are not well understood. We identified the tick D1 receptor activated by dopamine, a potent inducer of the salivary secretion of ticks. Temporal and spatial expression patterns examined by immunohistochemistry and reverse transcription polymerase chain reaction suggest that the dopamine produced in the basal cells of salivary gland acini is secreted into the lumen and activates the D1 receptors on the luminal surface of the cells lining the acini. Therefore, we propose a paracrine function of dopamine that is mediated by the D1 receptor in the salivary gland at an early phase of feeding. The molecular and pharmacological characterization of the D1 receptor in this study provides the foundation for understanding the functions of dopamine in the blood-feeding of ticks.

Transcriptome analysis of the synganglion from the brown dog tick, Rhipicephalus sanguineus

Tick control strategies rely heavily on chemicals (acaricides), most of which target the central nervous system. With increasing resistance, new acaricides are urgently needed but knowledge of tick neurobiology is surprisingly limited, notably the number of neural-specific gene sequences. One thousand and eight expressed sequence tags (ESTs) were obtained from a normalized cDNA library from Rhipicephalus sanguineus synganglia. Putative functional identities were assigned to 44% whereas 34% were unknown/novel sequences. Of particular interest were ESTs encoding a chitinase-like enzyme, an acetylcholinesterase and four transmembrane receptors including two glutamate-gated chloride channel receptors, a leucokinin-like receptor and a nicotinic acetylcholine receptor alpha-subunit. This study highlights the benefits of using both neural tissues and normalized libraries in an EST-approach for identifying potential acaricide targets expressed as rare transcripts.

MIPs are ancestral ligands for the sex peptide receptor

Upon mating, females of many animal species undergo dramatic changes in their behavior. In Drosophila melanogaster, postmating behaviors are triggered by sex peptide (SP), which is produced in the male seminal fluid and transferred to female during copulation. SP modulates female behaviors via sex peptide receptor (SPR) located in a small subset of internal sensory neurons that innervate the female uterus and project to the CNS. Although required for postmating responses only in these female sensory neurons, SPR is expressed broadly in the CNS of both sexes. Moreover, SPR is also encoded in the genomes of insects that lack obvious SP orthologs. These observations suggest that SPR may have additional ligands and functions. Here, we identify myoinhibitory peptides (MIPs) as a second family of SPR ligands that is conserved across a wide range of invertebrate species. MIPs are potent agonists for Drosophila, Aedes, and Aplysia SPRs in vitro, yet are unable to trigger postmating responses in vivo. In contrast to SP, MIPs are not produced in male reproductive organs, and are not required for postmating behaviors in Drosophila females. We conclude that MIPs are evolutionarily conserved ligands for SPR, which are likely to mediate functions other than the regulation of female reproductive behaviors.

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.

Neuropeptide signaling sequences identified by pyrosequencing of the American dog tick synganglion transcriptome during blood feeding and reproduction

Ticks are important vectors of numerous pathogens that impact human and animal health. The tick central nervous system represents an understudied area in tick biology and no tick synganglion-specific transcriptome has been described to date. Here we characterize whole or partial cDNA sequences of fourteen putative neuropeptides (allatostatin, insulin-like peptide, ion-transport peptide, sulfakinin, bursicon alpha/beta, eclosion hormone, glycoprotein hormone alpha/beta, corazonin, four orcokinins) and five neuropeptide receptors (gonadotropin receptor, leucokinin-like receptor, sulfakinin receptor, calcitonin receptor, pyrokinin receptor) translated from cDNA synthesized from the synganglion of unfed, partially fed and replete female American dog ticks, Dermacentor variabilis. Their homology to the same neuropeptides in other taxa is discussed. Many of these neuropeptides such as an allatostatin, insulin-like peptide, eclosion hormone, bursicon alpha and beta and glycoprotein hormone alpha and beta have not been previously described in the Chelicerata. An insulin-receptor substrate protein was also found indicating that an insulin signaling network is present in ticks. A putative type-2 proprotein processing convertase was also sequenced that may be involved in cleavage at monobasic and dibasic endoproteolytic cleavage sites in prohormones. The possible physiological role of the proteins discovered in adult tick blood feeding and reproduction will be discussed.