More than two decades of research on insect neuropeptide GPCRs: an overview

DeepNeuropePred: A robust and universal tool to predict cleavage sites from neuropeptide precursors by protein language model

Neuropeptides play critical roles in many biological processes such as growth, learning, memory, metabolism, and neuronal differentiation. A few approaches have been reported for predicting neuropeptides that are cleaved from precursor protein sequences. However, these models for cleavage site prediction of precursors were developed using a limited number of neuropeptide precursor datasets and simple precursors representation models. In addition, a universal method for predicting neuropeptide cleavage sites that can be applied to all species is still lacking. In this paper, we proposed a novel deep learning method called DeepNeuropePred, using a combination of pre-trained language model and Convolutional Neural Networks for feature extraction and predicting the neuropeptide cleavage sites from precursors. To demonstrate the model's effectiveness and robustness, we evaluated the performance of DeepNeuropePred and four models from the NeuroPred server in the independent dataset and our model achieved the highest AUC score (0.916), which are 6.9%, 7.8%, 8.8%, and 10.9% higher than Mammalian (0.857), insects (0.850), Mollusc (0.842) and Motif (0.826), respectively. For the convenience of researchers, we provide a web server (http://isyslab.info/NeuroPepV2/deepNeuropePred.jsp).

Functional analysis of adipokinetic hormone signaling in reproductive diapause of Coccinella septempunctata

BACKGROUND

The ladybeetle, Coccinella septempunctata, an important predator, is widely used to control aphids, whiteflies, mites, thrips, and lepidopteran pests. Diapause control technology is key to extending C. septempunctata shelf-life and commercialization. Lipid accumulation is a major feature of reproductive diapause, but the function of AKH signaling as a regulator of lipid mobilization in reproductive diapause remains unclear. This study aimed to identify and characterize AKH and AKHR genes, and clarify their functions in reproductive diapause.

RESULTS

The relative expression levels of CsAKH and CsAKHR were the highest in the head and fat body, respectively, and were significantly decreased under diapause conditions, both in developmental stages and tissues (head, midgut, fat body, and ovary). Furthermore, CsAKH and CsAKHR expression was increased significantly after juvenile hormone (JH) injection, but CsMet silencing significantly inhibited CsAKH and CsAKHR expression, whereas CsMet knockdown blocked the induction effect of JH. CsAKH and CsAKHR knockdown significantly reduced water content, increased lipid storage, and promoted the expression of genes related to lipid synthesis, but significantly blocked ovarian development, and induced forkhead box O (FOXO) gene expression in C. septempunctata under reproduction conditions. By contrast, injection of AKH peptide significantly inhibited FOXO expression, reduced lipid storage, and increased water content in C. septempunctata under diapause conditions.

CONCLUSION

These results indicate that CsAKH and CsAKHR are involved in the regulation of lipid accumulation and ovarian development during diapause in C. septempunctata, and provide a promising target for manipulating C. septempunctata diapause. © 2024 Society of Chemical Industry.

Revisiting the evolution of Family B1 GPCRs and ligands: Insights from mollusca

Family B1 G protein-coupled receptors (GPCRs) are one of the most well studied neuropeptide receptor families since they play a central role in many biological processes including endocrine, gastrointestinal, cardiovascular and reproduction in animals. The genes for these receptors emerged from a common ancestral gene in bilaterian genomes and evolved via gene/genome duplications and deletions in vertebrate and invertebrate genomes. Their existence and function have mostly been characterized in vertebrates and few studies exist in invertebrate species. Recently, an increased interest in molluscs, means a series of genomes have become available, and since they are less modified than insect and nematode genomes, they are ideal to explore the origin and evolution of neuropeptide gene families. This review provides an overview of Family B1 GPCRs and their peptide ligands and incorporates new data obtained from Mollusca genomes and taking a comparative approach challenges existing models on their origin and evolution.

Discovering allatostatin type-C receptor specific agonists

Currently, there is no pesticide available for the selective control of the pine processionary moth (Thaumetopoea pityocampa-specific), and conventional methods typically rely on mechanical techniques such as pheromone traps or broad-spectrum larvicidal chemicals. As climate change increases the range and dispersion capacity of crop and forest pests, outbreaks of the pine processionary occur with greater frequency and significantly impact forestry and public health. Our study is carried out to provide a T. pityocampa-specific pesticide targeting the Allatostatin Type-C Receptor (AlstR-C). We use a combination of computational biology methods, a cell-based screening assay, and in vivo toxicity and side effect assays to identify, for the first time, a series of AlstR-C ligands suitable for use as T. pityocampa-specific insecticides. We further demonstrate that the novel AlstR-C targeted agonists are specific to lepidopteran larvae, with no harmful effects on coleopteran larvae or adults. Overall, our study represents an important initial advance toward an insect GPCR-targeted next-generation pesticide design. Our approach may apply to other invertebrate GPCRs involved in vital metabolic pathways.

Characterization of Neuropeptides from Spodoptera litura and Functional Analysis of NPF in Diet Intake

Neuropeptides are involved in many biological processes in insects. However, it is unclear what role neuropeptides play in Spodoptera litura adaptation to phytochemical flavone. In this study, 63 neuropeptide precursors from 48 gene families were identified in S. litura, including two neuropeptide F genes (NPFs). NPFs played a positive role in feeding regulation in S. litura because knockdown of NPFs decreased larval diet intake. S. litura larvae reduced flavone intake by downregulating NPFs. Conversely, the flavone intake was increased if the larvae were treated with NPF mature peptides. The NPF receptor (NPFR) was susceptible to the fluctuation of NPFs. NPFR mediated NPF signaling by interacting with NPFs to regulate the larval diet intake. In conclusion, this study suggested that NPF signaling regulated diet intake to promote S. litura adaptation to flavone, which contributed to understanding insect adaptation mechanisms to host plants and provide more potential pesticidal targets for pest control.

Evaluating conserved domains and motifs of decapod gonadotropin-releasing hormone G protein-coupled receptor superfamily

G protein-coupled receptors (GPCRs) are an ancient family of signal transducers that are both abundant and consequential in metazoan endocrinology. The evolutionary history and function of the GPCRs of the decapod superfamilies of gonadotropin-releasing hormone (GnRH) are yet to be fully elucidated. As part of which, the use of traditional phylogenetics and the recycling of a diminutive set of mis-annotated databases has proven insufficient. To address this, we have collated and revised eight existing and three novel GPCR repertoires for GnRH of decapod species. We developed a novel bioinformatic workflow that included clustering analysis to capture likely GnRH receptor-like proteins, followed by phylogenetic analysis of the seven transmembrane-spanning domains. A high degree of conservation of the sequences and topology of the domains and motifs allowed the identification of species-specific variation (up to ~70%, especially in the extracellular loops) that is thought to be influential to ligand-binding and function. Given the key functional role of the DRY motif across GPCRs, the classification of receptors based on the variation of this motif can be universally applied to resolve cryptic GPCR families, as was achieved in this work. Our results contribute to the resolution of the evolutionary history of invertebrate GnRH receptors and inform the design of bioassays in their deorphanization and functional annotation.

Single-cell RNA sequencing of mid-to-late stage spider embryos: new insights into spider development

BACKGROUND

The common house spider Parasteatoda tepidariorum represents an emerging new model organism of arthropod evolutionary and developmental (EvoDevo) studies. Recent technical advances have resulted in the first single-cell sequencing (SCS) data on this species allowing deeper insights to be gained into its early development, but mid-to-late stage embryos were not included in these pioneering studies.

RESULTS

Therefore, we performed SCS on mid-to-late stage embryos of Parasteatoda and characterized resulting cell clusters by means of in-silico analysis (comparison of key markers of each cluster with previously published information on these genes). In-silico prediction of the nature of each cluster was then tested/verified by means of additional in-situ hybridization experiments with additional markers of each cluster.

CONCLUSIONS

Our data show that SCS data reliably group cells with similar genetic fingerprints into more or less distinct clusters, and thus allows identification of developing cell types on a broader level, such as the distinction of ectodermal, mesodermal and endodermal cell lineages, as well as the identification of distinct developing tissues such as subtypes of nervous tissue cells, the developing heart, or the ventral sulcus (VS). In comparison with recent other SCS studies on the same species, our data represent later developmental stages, and thus provide insights into different stages of developing cell types and tissues such as differentiating neurons and the VS that are only present at these later stages.

Identification of neuropeptides and their G protein-coupled receptors in the predatory stink bug, Arma custos (Hemiptera: Pentatomidae)

The predatory stink bug Arma custos has been selected as an effective biological control agent and has been successfully massly bred and released into fields for the control of a diverse insect pests. As a zoophytophagous generalist, A. custos relies on a complex neuropeptide signaling system to prey on distinct food and adapt to different environments. However, information about neuropeptide signaling genes in A. custos has not been reported to date. In the present study, a total of 57 neuropeptide precursor transcripts and 41 potential neuropeptide G protein-coupled receptor (GPCR) transcripts were found mainly using our sequenced transcriptome data. Furthermore, a number of neuropeptides and their GPCR receptors that were enriched in guts and salivary glands of A. custos were identified, which might play critical roles in feeding and digestion. Our study provides basic information for an in-depth understanding of biological and ecological characteristics of the predatory bug and would aid in the development of better pest management strategies based on the effective utilization and protection of beneficial natural enemies.

Transcriptome analysis reveals salivary gland-specific neuropeptide signaling genes in the predatory stink bug, Picromerus lewisi

Predatory stink bugs derive from phytophagous stink bugs and evolved enhanced predation skills. Neuropeptides are a diverse class of ancient signaling molecules that regulate physiological processes and behavior in animals, including stink bugs. Neuropeptide evolution might be important for the development of predation because neuropeptides can be converted to venoms that impact prey. However, information on neuropeptide signaling genes in predatory stink bugs is lacking. In the present study, neuropeptide signaling genes of Picromerus lewisi, an important predatory stink bug and an effective biological agent, were comprehensively identified by transcriptome analysis, with a total of 59 neuropeptide precursor genes and 58 potential neuropeptide receptor genes found. In addition, several neuropeptides and their receptors enriched in salivary glands of P. lewisi were identified. The present study and subsequent functional research contribute to an in-depth understanding of the biology and behavior of the predatory bugs and can provide basic information for the development of better pest management strategies, possibly including neuropeptide receptors as insecticide targets and salivary gland derived venom toxins as novel killing moleculars.

System-wide mapping of peptide-GPCR interactions in C. elegans

Neuropeptides and peptide hormones are ancient, widespread signaling molecules that underpin almost all brain functions. They constitute a broad ligand-receptor network, mainly by binding to G protein-coupled receptors (GPCRs). However, the organization of the peptidergic network and roles of many peptides remain elusive, as our insight into peptide-receptor interactions is limited and many peptide GPCRs are still orphan receptors. Here we report a genome-wide peptide-GPCR interaction map in Caenorhabditis elegans. By reverse pharmacology screening of over 55,384 possible interactions, we identify 461 cognate peptide-GPCR couples that uncover a broad signaling network with specific and complex combinatorial interactions encoded across and within single peptidergic genes. These interactions provide insights into peptide functions and evolution. Combining our dataset with phylogenetic analysis supports peptide-receptor co-evolution and conservation of at least 14 bilaterian peptidergic systems in C. elegans. This resource lays a foundation for system-wide analysis of the peptidergic network.

Insecticidal Effects of Receptor-Interference Isolated Bioactive Peptides on Fire Ant Colonies

Receptor-interference (Receptor-i) is a novel technology used to identify bioactive peptides as agonists or antagonists against a specific receptor, primarily targeting G-protein-coupled receptors (GPCRs). Using Receptor-i methodology, we targeted the pheromone biosynthesis activating neuropeptide receptor (PBAN-R) of the red imported fire ant (Solenopsis invicta). Based on previous studies, we selected four bioactive peptides cyclized with two cysteines: CVKLGSHFC, CIQQGSHFC, CERVGSHFC, and CMARYMSAC, and we conducted small-scale feeding bioassays, measuring fire ant worker mortality. All peptides reduced ant survival; however, CMARYMSAC (MARY) and CIQQGSHFC (IQQG) were the most effective and were selected for feeding trials against large, fully functional fire ant field colonies containing queen, brood, and up to 8000 workers. At the end of the experiment, day 84, synthetic peptide MARY killed over 80% of the workers and two of four queens. IQQG killed over 70% of the workers and three of four queens. The surviving two MARY queens lost an average of 21% of their starting weight. The surviving IQQG queen lost 31% of its weight. In contrast, control colony queens gained an average of 11% of their starting weight. These results provide proof-of-concept for the Receptor-i technology and will synergize applications to other agricultural and medical pests.

Comparative study of neuropeptide signaling systems in Hemiptera

Numerous physiological processes in insects are tightly regulated by neuropeptides and their receptors. Although they form an ancient signaling system, there is still a great deal of variety in neuropeptides and their receptors among different species within the same order. Neuropeptides and their receptors have been documented in many hemipteran insects, but the differences among them have been poorly characterized. Commercial grapevines worldwide are plagued by the bug Daktulosphaira vitifoliae (Hemiptera: Sternorrhyncha). Here, 33 neuropeptide precursors and 48 putative neuropeptide G protein-coupled receptor (GPCR) genes were identified in D. vitifoliae. Their expression profiles at the probe and feeding stages reflected potential regulatory roles in probe behavior. By comparison, we found that the Releasing Hormone-Related Peptides (GnRHs) system of Sternorrhyncha was differentiated from those of the other 2 suborders in Hemiptera. Independent secondary losses of the adipokinetic hormone/corazonin-related peptide receptor (ACP) and corazonin (CRZ) occurred during the evolution of Sternorrhyncha. Additionally, we discovered that the neuropeptide signaling systems of Sternorrhyncha were very different from those of Heteroptera and Auchenorrhyncha, which was consistent with Sternorrhyncha's phylogenetic position at the base of the order. This research provides more knowledge on neuropeptide systems and sets the groundwork for the creation of novel D. vitifoliae management strategies that specifically target these signaling pathways.

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.

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.

Unravelling the neuropeptidome of the ornate spiny lobster Panulirus ornatus: A focus on peptide hormones and their processing enzymes expressed in the reproductive tissues

Neuropeptides are commonly produced in the neural tissues yet can have effects on far-reaching targets, with varied biological responses. We describe here the neuropeptidome of the ornate spiny lobster, Panulirus ornatus, a species of emerging importance to closed-system aquaculture, with a focus on peptide hormones produced by the reproductive tissues. Transcripts for a precursor to one neuropeptide, adipokinetic hormone/corazonin-related peptide (ACP) were identified in high numbers in the sperm duct of adult spiny lobsters suggesting a role for ACP in the reproduction of this species. Neuropeptide production in the sperm duct may be linked with physiological control of spermatophore production in the male, or alternatively may function in signalling to the female. The enzymes which process nascent neuropeptide precursors into their mature, active forms have seldom been studied in decapods, and never before at the multi-tissue level. We have identified transcripts for multiple members of the proprotein convertase subtisilin/kexin family in the ornate spiny lobster, with some enzymes showing specificity to certain tissues. In addition, other enzyme transcripts involved with neuropeptide processing are identified along with their tissue and life stage expression patterns.

Selection and Comparative Gene Expression of Midgut-Specific Targets for Drosophila suzukii

Spotted-wing drosophila (SWD), Drosophila suzukii, is a destructive and invasive pest that attacks most small fruits and cherries. The current management for SWD involves the use of conventional insecticides. In an effort to develop a biologically based control option, the application of RNA interference (RNAi) has been investigated. To develop an RNAi approach, suitable targets must be identified, and an efficient delivery method must be developed for introducing the double-stranded RNA (dsRNA) in the midgut. In D. suzukii, we previously found that dsRNA nucleases actively degrade dsRNA molecules in the midgut. In this study, we focused on identifying biological targets focused on the midgut membrane. The profile of midgut-specific genes was analyzed and compared with the genes expressed in the whole-body using transcriptome analysis. Differential gene expression analysis revealed that 1921 contigs were upregulated and 1834 contigs were downregulated in the midgut when compared to genes from other body tissues. We chose ten midgut-specifically upregulated genes and empirically confirmed their expressions. We are particularly interested in the midgut membrane proteins, including G protein-coupled receptors (GPCRs) such as diuretic hormone 31 (DH31) receptor, neuropeptide F (NPF) recepror, toll-9, adhesion receptors, methuselah (mth), and gustatory receptor, because insect GPCRs have been offered great potential for next-generation pest management.

Transcriptional Genetically Encoded Calcium Indicators in Drosophila

Knowing which neurons are active during behavior is a crucial step toward understanding how nervous systems work. Neuronal activation is generally accompanied by an increase in intracellular calcium levels. Therefore, intracellular calcium levels are widely used as a proxy for neuronal activity. Many types of synthetic components and bioluminescent or fluorescent proteins that report transient and long-term changes in intracellular calcium levels have been developed over the past 60 years. Calcium indicators that enable imaging of the dynamic activity of a large ensemble of neurons in behaving animals have revolutionized the field of neuroscience. Among these, transcription-based genetically encoded calcium indicators (transcriptional GECIs) have proven easy to use and do not depend on sophisticated imaging systems, offering unique advantages over other types of calcium indicators. Here, we describe the two currently available fly transcriptional GECIs-calcium-dependent nuclear import of LexA (CaLexA) and transcriptional reporter of intracellular calcium (TRIC)-and review studies that have used them. In the accompanying protocol, we present step-by-step details for generating CaLexA- and TRIC-ready flies and for imaging CaLexA and TRIC signals in dissected brains after experimental manipulations of intact free-moving flies.

Bacteriocyte plasticity in pea aphids facing amino acid stress or starvation during development

An important contributing factor to the evolutionary success of insects is nutritional association with microbial symbionts, which provide the host insects with nutrients lacking in their unbalanced diets. These symbionts are often compartmentalized in specialized cells of the host, the bacteriocytes. Even though bacteriocytes were first described more than a century ago, few studies have explored their dynamics throughout the insect life cycle and in response to environmental stressors. Here, we use the Buchnera aphidicola/pea aphid symbiotic system to study how bacteriocytes are regulated in response to nutritional stress throughout aphid development. Using artificial diets, we analyzed the effects of depletion or excess of phenylalanine or leucine, two amino acids essential for aphid growth and whose biosynthetic pathways are shared between the host and the symbiont. Bacteriocytes responded dynamically to those treatments, while other tissues showed no obvious morphological change. Amino acid depletion resulted in an increase in bacteriocyte numbers, with the extent of the increase depending on the amino acid, while excess either caused a decrease (for leucine) or an increase (for phenylalanine). Only a limited impact on survival and fecundity was observed, suggesting that the adjustment in bacteriocyte (and symbiont) numbers is sufficient to withstand these nutritional challenges. We also studied the impact of more extreme conditions by exposing aphids to a 24 h starvation period at the beginning of nymphal development. This led to a dramatic drop in aphid survival and fecundity and a significant developmental delay. Again, bacteriocytes responded dynamically, with a considerable decrease in number and size, correlated with a decrease in the number of symbionts, which were prematurely degraded by the lysosomal system. This study shows how bacteriocyte dynamics is integrated in the physiology of insects and highlights the high plasticity of these cells.

Adipokinetic hormone signaling regulates cytochrome P450-mediated chlorantraniliprole sensitivity in Spodoptera frugiperda (Lepidoptera: Noctuidae)

BACKGROUND

Fall armyworm (FAW, Spodoptera frugiperda) is one of the most destructive and invasive pests worldwide and causes significant economic losses. Intensive and frequent use of insecticides has led to the development of resistance in FAW. Adipokinetic hormone (AKH) have been proven to be involved in insecticide resistance in insects. However, the molecular mechanism underlying chlorantraniliprole resistance mediated by AKH signaling in FAW remains unclear.

RESULTS

The expression of SpfAKHR was highest in male adults and lowest in 1st instar larvae. SpfAKH was expressed the highest in eggs and the lowest in 6th instar larvae. AKH signaling was involved in the sensitivity of FAW to chlorantraniliprole through a toxicological bioassay, and the combination of chlorantraniliprole and bithionol (an inhibitor of key enzymes in the AKH pathway) significantly increased the mortality of FAW. Chlorantraniliprole significantly induced the expression of ten P450s, SpfAKH and SpfAKHR in FAW. RNA interference against SpfAKHR significantly decreased the P450 content, downregulated the expression of three P450 genes (SpfCYP6B50, SpfCYP321A9 and SpfCYP9A58) and inhibited the resistance of FAW to chlorantraniliprole. The topical application of AKH peptide significantly increased the P450 content, upregulated the expression of five P450 genes (SpfCYP321A9, SpfCY321A8, SpfCYP321A10, SpfCYP321A7 and SpfCYP6AB12), and enhanced the survival of FAW against chlorantraniliprole.

CONCLUSIONS

AKH plays an important role in enhancing chlorantraniliprole resistance in FAW by exerting a positive influence on P450 gene expression and P450 content. These results provide valuable insights into insecticide resistance regulation and FAW control strategies. © 2022 Society of Chemical Industry.

Functional characterization of a diuretic hormone receptor associated with desiccation, starvation and temperature tolerance in gypsy moth, Lymantria dispar

Variety of diuretic hormone neuropeptides is known to regulate water and ion balance in invertebrates. By activating their specific neuropeptide, diuretic hormone receptor (DHR) transmits extracellular signals into the cell, and then produces functional cell activity, which plays an important role in regulating physiology and behavior. However, little is known about the function of DHR gene in Lymantria dispar. DHR gene was firstly identified in L. dispar and its physiological functions were investigated using RNA interference (RNAi) technology. The results showed that except for the 6^th instar larvae, the expression levels of DHR gene in the larval stages are higher than that in the egg, pupal and adult stages. The DHR gene is highly expressed in hindgut and midgut tissues. The L. dispar larvae significantly increased their water content and high temperature tolerance after the DHR was silenced, while decreasing excretion and feeding behavior. The physiological function of DHR is associated with desiccation, high temperature and starvation resistance. DHR could contribute to future development of novel insecticide to manage this global forest pest population.

Neuropeptides in Rhipicephalus microplus and other hard ticks

The synganglion is the central nervous system of ticks and, as such, controls tick physiology. It does so through the production and release of signaling molecules, many of which are neuropeptides. These peptides can function as neurotransmitters, neuromodulators and/or neurohormones, although in most cases their functions remain to be established. We identified and performed in silico characterization of neuropeptides present in different life stages and organs of Rhipicephalus microplus, generating transcriptomes from ovary, salivary glands, fat body, midgut and embryo. Annotation of synganglion transcripts led to the identification of 32 functional categories of proteins, of which the most abundant were: secreted, energetic metabolism and oxidant metabolism/detoxification. Neuropeptide precursors are among the sequences over-represented in R. microplus synganglion, with at least 5-fold higher transcription compared with other stages/organs. A total of 52 neuropeptide precursors were identified: ACP, achatin, allatostatins A, CC and CCC, allatotropin, bursicon A/B, calcitonin A and B, CCAP, CCHamide, CCRFamide, CCH/ITP, corazonin, DH31, DH44, eclosion hormone, EFLamide, EFLGGPamide, elevenin, ETH, FMRFamide myosuppressin-like, glycoprotein A2/B5, gonadulin, IGF, inotocin, insulin-like peptides, iPTH, leucokinin, myoinhibitory peptide, NPF 1 and 2, orcokinin, proctolin, pyrokinin/periviscerokinin, relaxin, RYamide, SIFamide, sNPF, sulfakinin, tachykinin and trissin. Several of these neuropeptides have not been previously reported in ticks, as the presence of ETH that was first clearly identified in Parasitiformes, which include ticks and mites. Prediction of the mature neuropeptides from precursor sequences was performed using available information about these peptides from other species, conserved domains and motifs. Almost all neuropeptides identified are also present in other tick species. Characterizing the role of neuropeptides and their respective receptors in tick physiology can aid the evaluation of their potential as drug targets.

Transcriptomic analysis of resistance and short-term induction response to pyrethroids, in Anopheles coluzzii legs

BACKGROUND

Insecticide-treated bed nets and indoor residual spraying comprise the major control measures against Anopheles gambiae sl, the dominant vector in sub-Saharan Africa. The primary site of contact with insecticide is through the mosquitoes' legs, which represents the first barrier insecticides have to bypass to reach their neuronal targets. Proteomic changes and leg cuticle modifications have been associated with insecticide resistance that may reduce the rate of penetration of insecticides. Here, we performed a multiple transcriptomic analyses focusing on An. coluzzii legs.

RESULTS

Firstly, leg-specific enrichment analysis identified 359 genes including the pyrethroid-binder SAP2 and 2 other chemosensory proteins, along with 4 ABCG transporters previously shown to be leg enriched. Enrichment of gene families included those involved in detecting chemical stimuli, including gustatory and ionotropic receptors and genes implicated in hydrocarbon-synthesis. Subsequently, we compared transcript expression in the legs of a highly resistant strain (VK7-HR) to both a strain with very similar genetic background which has reverted to susceptibility after several generations without insecticide pressure (VK7-LR) and a lab susceptible population (NG). Two hundred thirty-two differentially expressed genes (73 up-regulated and 159 down-regulated) were identified in the resistant strain when compared to the two susceptible counterparts, indicating an over-expression of phase I detoxification enzymes and cuticular proteins, with decrease in hormone-related metabolic processes in legs from the insecticide resistant population. Finally, we analysed the short-term effect of pyrethroid exposure on An. coluzzii legs, comparing legs of 1 h-deltamethrin-exposed An. coluzzii (VK7-IN) to those of unexposed mosquitoes (VK7-HR) and identified 348 up-regulated genes including those encoding for GPCRs, ABC transporters, odorant-binding proteins and members of the divergent salivary gland protein family.

CONCLUSIONS

The data on An. coluzzii leg-specific transcriptome provides valuable insights into the first line of defense in pyrethroid resistant and short-term deltamethrin-exposed mosquitoes. Our results suggest that xenobiotic detoxification is likely occurring in legs, while the enrichment of sensory proteins, ABCG transporters and cuticular genes is also evident. Constitutive resistance is primarily associated with elevated levels of detoxification and cuticular genes, while short-term insecticide-induced tolerance is linked with overexpression of transporters, GPCRs and GPCR-related genes, sensory/binding and salivary gland proteins.

Neuropeptide F from endocrine cells in Plutella xylostella midgut modulates feeding and synergizes Cry1Ac action

With the wide cultivation of transgenic plants throughout the world and the rising risk of resistance to Bacillus thuringiensis crystal (Cry) toxins, it is essential to design an adaptive resistance management strategy for continued use. Neuropeptide F (NPF) of insects has proven to be valuable for the production of novel-type transgenic plants via its important role in the control of feeding behavior. In this study, the gene encoding NPF was cloned from the diamondback moth, Plutella xylostella, an important agricultural pest. Real-time quantitative reverse transcription-polymerase chain reaction and in situ hybridization showed a relatively high expression of P. xylostella-npf (P. x-npf) in endocrine cells of the midgut of fourth instar larvae, and it was found to participate in P. xylostella feeding behavior and Cry1Ac-induced feeding inhibition. Prokaryotic expression and purification provided structure unfolded P. x-npf from inclusion bodies for diet surface overlay bioassays and the results demonstrated a significant synergistic effect of P. x-npf on Cry1Ac toxicity by increasing intake of noxious food which contains Cry toxins, especially quick death at an early stage of feeding. Our findings provided a potential new way to efficiently control pests by increasing intake of lower dose Cry toxins and a novel hint for the complex Cry toxin mechanism.

Identification and characterization of expression profiles of neuropeptides and their GPCRs in the swimming crab, Portunus trituberculatus

Neuropeptides and their G protein-coupled receptors (GPCRs) regulate multiple physiological processes. Currently, little is known about the identity of native neuropeptides and their receptors in Portunus trituberculatus. This study employed RNA-sequencing and reverse transcription-polymerase chain reaction (RT-PCR) techniques to identify neuropeptides and their receptors that might be involved in regulation of reproductive processes of P. trituberculatus. In the central nervous system transcriptome data, 47 neuropeptide transcripts were identified. In further analyses, the tissue expression profile of 32 putative neuropeptide-encoding transcripts was estimated. Results showed that the 32 transcripts were expressed in the central nervous system and 23 of them were expressed in the ovary. A total of 47 GPCR-encoding transcripts belonging to two classes were identified, including 39 encoding GPCR-A family and eight encoding GPCR-B family. In addition, we assessed the tissue expression profile of 33 GPCRs (27 GPCR-As and six GPCR-Bs) transcripts. These GPCRs were found to be widely expressed in different tissues. Similar to the expression profiles of neuropeptides, 20 of these putative GPCR-encoding transcripts were also detected in the ovary. This is the first study to establish the identify of neuropeptides and their GPCRs in P. trituberculatus, and provide information for further investigations into the effect of neuropeptides on the physiology and behavior of decapod crustaceans.

Identification of G protein-coupled receptors required for vitellogenesis and egg development in an insect with panoistic ovary

G protein-coupled receptors (GPCRs), a superfamily of integral transmembrane proteins regulate a variety of physiological processes in insects. Juvenile hormone (JH) is known to stimulate Vitellogenin (Vg) synthesis in the fat body, secretion into the hemolymph and uptake by developing oocytes. However, the role of GPCRs in JH-dependent insect vitellogenesis and oocyte maturation remains elusive. In the present study, we performed transcriptomic analysis and RNA interference (RNAi) screening in vitellogenic females of the migratory locust Locusta migratoria. Of 22 GPCRs identified in ovarian transcriptome, LGR4, OR-A1, OR-A2, Mthl1, Mthl5 and Smo were most abundant in the ovary. By comparison, mAChR-C expressed at higher levels in the fat body, whereas Oct/TyrR, OARβ, AdoR and ADGRA3 were at higher expression levels in the brain. Our RNAi screening demonstrated that knockdown of six GPCRs resulted in defective phenotypes of Vg accumulation in developing oocytes, accompanied by blocked ovarian development and impaired oocyte maturation. While LGR4 and Oct/TyrR appeared to control Vg synthesis in the fat body, OR-A1, OR-A2, mAChR-C and CirlL regulated Vg transportation and uptake. The findings provide fundamental evidence for deciphering the regulatory mechanisms of GPCRs in JH-stimulated insect reproduction.

Coordinated transcriptomics and peptidomics of central nervous system identify neuropeptides and their G protein-coupled receptors in the oriental fruit moth Grapholita molesta

The oriental fruit moth Grapholita molesta is a cosmopolitan pest of orchard, which causes serious economic losses to the fruit production. Neuropeptides and their specific receptors (primarily G protein-coupled receptors, GPCRs) regulate multiple biological functions in insects and represent promising next-generation pest management strategy. Here, we generated a transcriptome of the central nervous system (CNS) of G. molesta. Overall, 57 neuropeptide precursor genes were identified and 128 various mature peptides were predicted from these precursors. Using peptidomic analysis of CNS of G. molesta, we identified total of 28 mature peptides and precursor-related peptides from 16 precursors. A total of 41 neuropeptide GPCR genes belonging to three classes were also identified. These GPCRs and their probable ligands were predicted. Additionally, expression patterns of these 98 genes in various larval tissues were evaluated using quantitative real-time PCR. Taken together, these results will benefit further investigations to determine physiological functions and pharmacological characterization of neuropeptides and their GPCRs in G. molesta; and to develop specific neuropeptide-based agents for this tortricid fruit pest control.

Nemertean, brachiopod and phoronid neuropeptidomics reveals ancestral spiralian signalling systems

Neuropeptides are diverse signaling molecules in animals commonly acting through G-protein coupled receptors (GPCRs). Neuropeptides and their receptors underwent extensive diversification in bilaterians and the relationships of many peptide–receptor systems have been clarified. However, we lack a detailed picture of neuropeptide evolution in lophotrochozoans as in-depth studies only exist for mollusks and annelids. Here, we analyze peptidergic systems in Nemertea, Brachiopoda, and Phoronida. We screened transcriptomes from 13 nemertean, 6 brachiopod, and 4 phoronid species for proneuropeptides and neuropeptide GPCRs. With mass spectrometry from the nemertean Lineus longissimus, we validated several predicted peptides and identified novel ones. Molecular phylogeny combined with peptide-sequence and gene-structure comparisons allowed us to comprehensively map spiralian neuropeptide evolution. We found most mollusk and annelid peptidergic systems also in nemerteans, brachiopods, and phoronids. We uncovered previously hidden relationships including the orthologies of spiralian CCWamides to arthropod agatoxin-like peptides and of mollusk APGWamides to RGWamides from annelids, with ortholog systems in nemerteans, brachiopods, and phoronids. We found that pleurin neuropeptides previously only found in mollusks are also present in nemerteans and brachiopods. We also identified cases of gene family duplications and losses. These include a protostome-specific expansion of RFamide/Wamide signaling, a spiralian expansion of GnRH-related peptides, and duplications of vasopressin/oxytocin before the divergence of brachiopods, phoronids, and nemerteans. This analysis expands our knowledge of peptidergic signaling in spiralians and other protostomes. Our annotated data set of nearly 1,300 proneuropeptide sequences and 600 GPCRs presents a useful resource for further studies of neuropeptide signaling.

Cloning and Expression of the Neuropeptide F and Neuropeptide F Receptor Genes and Their Regulation of Food Intake in the Chinese White Pine Beetle Dendroctonus armandi

Neuropeptide F (NPF) is an important signaling molecule that acts as a neuromodulator to regulate a diversity of physiological and behavioral processes from vertebrates to invertebrates by interaction with NPF receptors, which are G protein-coupled receptors (GPCR). However, nothing is known about NPF in Chinese white pine beetle, Dendroctonus armandi, a destructive pest of natural and coniferous forests in the middle Qinling Mountains of China. We have cloned and characterized cDNAs encoding one NPF precursor and two NPF receptors in D. armandi and made bioinformatics predictions according to the deduced amino acid sequences. They were highly similar to that of Dendroctonus ponderosa. The transcription levels of these genes were different between larvae and adults of sexes, and there were significant differences among the different developmental stages and tissues and between beetles under starvation and following re-feeding states. Additionally, downregulation of NPF and NPFR by injecting dsRNA into beetles reduced their food intake, caused increases of mortality and decreases of body weight, and also resulted in a decrease of glycogen and free fatty acid and an increase of trehalose. These results indicate that the NPF signaling pathway plays a significant positive role in the regulation of food intake and provides a potential target for the sustainable management of this pest.

G protein-coupled receptors as candidates for modulation and activation of the chemical senses in decapod crustaceans

Many studies have characterized class A GPCRs in crustaceans; however, their expression in crustacean chemosensory organs has yet to be detailed. Class A GPCRs comprise several subclasses mediating diverse functions. In this study, using sequence homology, we classified all putative class A GPCRs in two chemosensory organs (antennular lateral flagellum [LF] and walking leg dactyls) and brain of four species of decapod crustaceans (Caribbean spiny lobster Panulirus argus, American lobster Homarus americanus, red-swamp crayfish Procambarus clarkii, and blue crab Callinectes sapidus). We identified 333 putative class A GPCRs– 83 from P. argus, 81 from H. americanus, 102 from P. clarkii, and 67 from C. sapidus–which belong to five distinct subclasses. The numbers of sequences for each subclass in the four decapod species are (in parentheses): opsins (19), small-molecule receptors including biogenic amine receptors (83), neuropeptide receptors (90), leucine-rich repeat-containing GPCRs (LGRs) (24), orphan receptors (117). Most class A GPCRs are predominately expressed in the brain; however, we identified multiple transcripts enriched in the LF and several in the dactyl. In total, we found 55 sequences with higher expression in the chemosensory organs relative to the brain across three decapod species. We also identified novel transcripts enriched in the LF including a metabotropic histamine receptor and numerous orphan receptors. Our work establishes expression patterns for class A GPCRs in the chemosensory organs of crustaceans, providing insight into molecular mechanisms mediating neurotransmission, neuromodulation, and possibly chemoreception.

In silico identification of excretory/secretory proteins and drug targets in monogenean parasites

The Excretory/Secretory (ES) proteins of parasites are involved in invasion and colonization of their hosts. In addition, since ES proteins circulate in the extracellular space, they can be more accessible to drugs than other proteins, which makes ES proteins optimal targets for the development of new and better pharmacological strategies. Monogeneans are a group of parasitic Platyhelminthes that includes some pathogenic species problematic for finfish aquaculture. In the present study, 8297 putative ES proteins from four monogenean species which genomic resources are publicly available were identified and functionally annotated by bioinformatic tools. Additionally, for comparative purposes, ES proteins in other parasitic and free-living platyhelminths were identified. Based on data from the monogenean Gyrodactylus salaris, 15 ES proteins are considered potential drug targets. One of them showed homology to 10 cathepsins with known 3D structure. A docking molecular analysis uncovered that the anthelmintic emodepside shows good affinity to these cathepsins suggesting that emodepside can be experimentally tested as a monogenean's cathepsin inhibitor.

G-Protein Coupled Receptors (GPCRs) in Insects-A Potential Target for New Insecticide Development

G-protein coupled receptors (GPCRs) play important roles in cell biology and insects' physiological processes, toxicological response and the development of insecticide resistance. New information on genome sequences, proteomic and transcriptome analysis and expression patterns of GPCRs in organs such as the central nervous system in different organisms has shown the importance of these signaling regulatory GPCRs and their impact on vital cell functions. Our growing understanding of the role played by GPCRs at the cellular, genome, transcriptome and tissue levels is now being utilized to develop new targets that will sidestep many of the problems currently hindering human disease control and insect pest management. This article reviews recent work on the expression and function of GPCRs in insects, focusing on the molecular complexes governing the insect physiology and development of insecticide resistance and examining the genome information for GPCRs in two medically important insects, mosquitoes and house flies, and their orthologs in the model insect species Drosophila melanogaster. The tissue specific distribution and expression of the insect GPCRs is discussed, along with fresh insights into practical aspects of insect physiology and toxicology that could be fundamental for efforts to develop new, more effective, strategies for pest control and resistance management.

G-Protein Coupled Receptors (GPCRs): Signaling Pathways, Characterization, and Functions in Insect Physiology and Toxicology

G-protein-coupled receptors (GPCRs) are known to play central roles in the physiology of many organisms. Members of this seven α-helical transmembrane protein family transduce the extracellular signals and regulate intracellular second messengers through coupling to heterotrimeric G-proteins, adenylate cyclase, cAMPs, and protein kinases. As a result of the critical function of GPCRs in cell physiology and biochemistry, they not only play important roles in cell biology and the medicines used to treat a wide range of human diseases but also in insects’ physiological functions. Recent studies have revealed the expression and function of GPCRs in insecticide resistance, improving our understanding of the molecular complexes governing the development of insecticide resistance. This article focuses on the review of G-protein coupled receptor (GPCR) signaling pathways in insect physiology, including insects’ reproduction, growth and development, stress responses, feeding, behaviors, and other physiological processes. Hormones and polypeptides that are involved in insect GPCR regulatory pathways are reviewed. The review also gives a brief introduction of GPCR pathways in organisms in general. At the end of the review, it provides the recent studies on the function of GPCRs in the development of insecticide resistance, focusing in particular on our current knowledge of the expression and function of GPCRs and their downstream regulation pathways and their roles in insecticide resistance and the regulation of resistance P450 gene expression. The latest insights into the exciting technological advances and new techniques for gene expression and functional characterization of the GPCRs in insects are provided.

GPCR-Based Bioactive Peptide Screening Using Phage-Displayed Peptides and an Insect Cell System for Insecticide Discovery

The discovery of new insecticides improves integrated pest management (IPM), but is usually a long high-risk process with a low probability of success. For over two decades, insect neuropeptides (NPs) and their G-protein coupled receptors (GPCRs) have been considered as biological targets for insect pest control, because they are involved in almost all physiological processes associated with insect life stages. A key roadblock to success has been the question of how large volume chemical libraries can be efficiently screened for active compounds. New genomic and proteomic tools have advanced and facilitated the development of new approaches to insecticide discovery. In this study, we report a novel GPCR-based screening technology that uses millions of short peptides randomly generated by bacteriophages, and a method using an insect Sf9 cell expression system. The fire ant is a good model system, since bioactive peptides have been identified for a specific GPCR. The novel small peptides could interfere with the target GPCR-ligand functions. Therefore, we refer to this new mechanism as “receptor interference” (RECEPTORi). The GPCR-based bioactive peptide screening method offers multiple advantages. Libraries of phage-displayed peptides (~10⁹ peptides) are inexpensive. An insect cell-based screening system rapidly leads to target specific GPCR agonists or antagonists in weeks. Delivery of bioactive peptides to target pests can be flexible, such as topical, ingestion, and plant-incorporated protectants. A variety of GPCR targets are available, thus minimizing the development of potential insecticide resistance. This report provides the first proof-of-concept for the development of novel arthropod pest management strategies using neuropeptides, and GPCRs.

Leucokinin and Associated Neuropeptides Regulate Multiple Aspects of Physiology and Behavior in Drosophila

Leucokinins (LKs) constitute a family of neuropeptides identified in numerous insects and many other invertebrates. LKs act on G-protein-coupled receptors that display only distant relations to other known receptors. In adult Drosophila, 26 neurons/neurosecretory cells of three main types express LK. The four brain interneurons are of two types, and these are implicated in several important functions in the fly's behavior and physiology, including feeding, sleep-metabolism interactions, state-dependent memory formation, as well as modulation of gustatory sensitivity and nociception. The 22 neurosecretory cells (abdominal LK neurons, ABLKs) of the abdominal neuromeres co-express LK and a diuretic hormone (DH44), and together, these regulate water and ion homeostasis and associated stress as well as food intake. In Drosophila larvae, LK neurons modulate locomotion, escape responses and aspects of ecdysis behavior. A set of lateral neurosecretory cells, ALKs (anterior LK neurons), in the brain express LK in larvae, but inconsistently so in adults. These ALKs co-express three other neuropeptides and regulate water and ion homeostasis, feeding, and drinking, but the specific role of LK is not yet known. This review summarizes Drosophila data on embryonic lineages of LK neurons, functional roles of individual LK neuron types, interactions with other peptidergic systems, and orchestrating functions of LK.

Molecular Characterization of the β2-like Octopamine Receptor of Helicoverpa armigera

Helicoverpa armigera is a devastating polyphagous and cosmopolitan crop pest. There are reports of this insect being resistant to a variety of pesticides raising concern worldwide. The Octopamine (OA) binding β₂-like receptor (OAR), a GPCR, is widely distributed in the nervous system of the insect and plays essential roles in the physiology and development and thus is an important target for insecticides. Yet, the molecular characterization of the H. armigera OAR (HarmOAR) and rational design of compounds based on this receptor is lacking. As a first step, we performed multiple sequence alignment of all insect OARs, which revealed that the sequences contained all conserved class A GPCR motifs. Phylogenetic studies showed clade-specific variations in the protein sequences primarily arising owing to differences in the ICL3 loop region. Further, a structural model of HarmOAR was built using the inactive human β₂AR as a template. 0.9 µs atomistic simulations revealed conserved inter helical contacts and water molecules of HarmOAR. The detailed binding of octopamine was studied using molecular docking and 0.3 µs atomistic simulations. Twenty-two insecticides active against octopamine receptors of other insects were compiled and docked to HarmOAR followed by rescoring with binding free energies to prioritize them for H. armigera. Our study suggests α-terpineol to be a good candidate as an insecticide or insect repellent for Helicoverpa armigera.

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.

Drosophila reward system - A summary of current knowledge

The fruit fly Drosophila melanogaster brain is the most extensively investigated model of a reward system in insects. Drosophila can discriminate between rewarding and punishing environmental stimuli and consequently undergo associative learning. Functional models, especially those modelling mushroom bodies, are constantly being developed using newly discovered information, adding to the complexity of creating a simple model of the reward system. This review aims to clarify whether its reward system also includes a hedonic component. Neurochemical systems that mediate the 'wanting' component of reward in the Drosophila brain are well documented, however, the systems that mediate the pleasure component of reward in mammals, including those involving the endogenous opioid and endocannabinoid systems, are unlikely to be present in insects. The mushroom body components exhibit differential developmental age and different functional processes. We propose a hypothetical hierarchy of the levels of reinforcement processing in response to particular stimuli, and the parallel processes that take place concurrently. The possible presence of activity-silencing and meta-satiety inducing levels in Drosophila should be further investigated.

Evolution and Potential Function in Molluscs of Neuropeptide and Receptor Homologues of the Insect Allatostatins

The allatostatins (ASTs), AST-A, AST-B and AST-C, have mainly been investigated in insects. They are a large group of small pleotropic alloregulatory neuropeptides that are unrelated in sequence and activate receptors of the rhodopsin G-protein coupled receptor family (GPCRs). The characteristics and functions of the homologue systems in the molluscs (Buccalin, MIP and AST-C-like), the second most diverse group of protostomes after the arthropods, and of high interest for evolutionary studies due to their less rearranged genomes remains to be explored. In the present study their evolution is deciphered in molluscs and putative functions assigned in bivalves through meta-analysis of transcriptomes and experiments. Homologues of the three arthropod AST-type peptide precursors were identified in molluscs and produce a larger number of mature peptides than in insects. The number of putative receptors were also distinct across mollusc species due to lineage and species-specific duplications. Our evolutionary analysis of the receptors identified for the first time in a mollusc, the cephalopod, GALR-like genes, which challenges the accepted paradigm that AST-AR/buccalin-Rs are the orthologues of vertebrate GALRs in protostomes. Tissue transcriptomes revealed the peptides, and their putative receptors have a widespread distribution in bivalves and in the bivalve Mytilus galloprovincialis, elements of the three peptide-receptor systems are highly abundant in the mantle an innate immune barrier tissue. Exposure of M. galloprovincialis to lipopolysaccharide or a marine pathogenic bacterium, Vibrio harveyi, provoked significant modifications in the expression of genes of the peptide precursor and receptors of the AST-C-like system in the mantle suggesting involvement in the immune response. Overall, our study reveals that homologues of the arthropod AST-systems in molluscs are potentially more complex due to the greater number of putative mature peptides and receptor genes. In bivalves they have a broad and varying tissue distribution and abundance, and the elements of the AST-C-like family may have a putative function in the immune response.

Pharmacological Characterization of the Stick Insect Carausius morosus Allatostatin-C Receptor with Its Endogenous Agonist

G protein-coupled receptors (GPCRs) play a pivotal role in regulating key physiological events in all animal species. Recent advances in collective analysis of genes and proteins revealed numerous potential neuropeptides and GPCRs from insect species, allowing for the characterization of peptide-receptor pairs. In this work, we used fluorescence resonance energy transfer (FRET)-based genetically encoded biosensors in intact mammalian cells to study the pharmacological features of the cognate GPCR of the type-C allatostatin (AST-C) peptide from the stick insect, Carausius morosus. Analysis of multiple downstream pathways revealed that AST-C can activate the human Gi2 protein, and not Gs or Gq, through AST-C receptor (AlstRC). Activated AlstRC recruits β-arrestin2 independent of the Gi protein but stimulates ERK phosphorylation in a Gi protein-dependent manner. Identification of Gαi-, arrestin-, and GRK-like transcripts from C. morosus revealed high evolutionary conservation at the G protein level, while β-arrestins and GRKs displayed less conservation. In conclusion, our study provides experimental and homology-based evidence on the functionality of vertebrate G proteins and downstream signaling biosensors to characterize early signaling steps of an insect GPCR. These results may serve as a scaffold for developing assays to characterize pharmacological and structural aspects of other insect GPCRs and can be used in deorphanization and pesticide studies.

Adipokinetic hormone enhances CarE-mediated chlorpyrifos resistance in the brown planthopper, Nilaparvata lugens

Adipokinetic hormone (AKH), the principal stress-responsive neurohormone in insects, has been implicated in insect responses to insecticides. However, the functionality of AKH and its mode of signalling in insecticide resistance are unknown. Herein, we demonstrated that the enhanced activity of carboxylesterases (CarEs) is involved in the chlorpyrifos resistance in Nilaparvata lugens [brown planthopper (BPH)]. Chlorpyrifos exposure significantly induced the expression of AKH and its receptor AKHR in the susceptible BPH (Sus), and these two AKH signalling genes were over-expressed in the chlorpyrifos-resistant strain (Res) compared to Sus. RNA interference (RNAi) against AKH or AKHR decreased the CarE activity and suppressed the BPH's resistance to chlorpyrifos in Res. Conversely, AKH peptide injection elevated the CarE activity and enhanced the BPH's survival against chlorpyrifos in Sus. Furthermore, five CarE genes were identified to be positively affected by the AKH pathway using RNAi and AKH injection. Among these CarE genes, CarE and Esterase E4-1 were found to be over-expressed in Res compared to Sus, and knockdown of either gene decreased the BPH's resistance to chlorpyrifos. In conclusion, AKH plays a role in enhancing chlorpyrifos resistance in the BPH through positive influence on the expression of CarE genes and CarE enzyme activity.

Neuropeptide signalling systems - An underexplored target for venom drug discovery

Neuropeptides are signalling molecules mainly secreted from neurons that act as neurotransmitters or peptide hormones to affect physiological processes and modulate behaviours. In humans, neuropeptides are implicated in numerous diseases and understanding their role in physiological processes and pathologies is important for therapeutic development. Teasing apart the (patho)physiology of neuropeptides remains difficult due to ligand and receptor promiscuity and the complexity of the signalling pathways. The current approach relies on a pharmacological toolbox of agonists and antagonists displaying high selectivity for independent receptor subtypes, with the caveat that only few selective ligands have been discovered or developed. Animal venoms represent an underexplored source for novel receptor subtype-selective ligands that could aid in dissecting human neuropeptide signalling systems. Multiple endogenous-like neuropeptides as well as peptides acting on neuropeptide receptors are present in venoms. In this review, we summarise current knowledge on neuropeptides and discuss venoms as a source for ligands targeting neuropeptide signalling systems.

Pigment Dispersing Factors and Their Cognate Receptors in a Crustacean Model, With New Insights Into Distinct Neurons and Their Functions

Pigment dispersing factors (PDFs, or PDHs in crustaceans) form a structurally related group of neuropeptides found throughout the Ecdysozoa and were first discovered as pigmentary effector hormones in crustaceans. In insects PDFs fulfill crucial neuromodulatory roles, most notably as output regulators of the circadian system, underscoring their central position in physiological and behavioral organization of arthropods. Intriguingly, decapod crustaceans express multiple isoforms of PDH originating from separate genes, yet their differential functions are still to be determined. Here, we functionally define two PDH receptors in the crab Carcinus maenas and show them to be selectively activated by four PDH isoforms: PDHR 43673 was activated by PDH-1 and PDH-2 at low nanomolar doses whilst PDHR 41189 was activated by PDH-3 and an extended 20 residue e-PDH. Detailed examination of the anatomical distribution of all four peptides and their cognate receptors indicate that they likely perform different functions as secreted hormones and/or neuromodulators, with PDH-1 and its receptor 43,673 implicated in an authentic hormonal axis. PDH-2, PDH-3, and e-PDH were limited to non-neurohemal interneuronal sites in the CNS; PDHR 41189 was largely restricted to the nervous system suggesting a neuromodulatory function. Notably PDH-3 and e-PDH were without chromatophore dispersing activity. This is the first report which functionally defines a PDHR in an endocrine system in a crustacean and to indicate this and other putative roles of this physiologically pivotal peptide group in these organisms. Thus, our findings present opportunities to further examine the endocrine and circadian machinery in this important arthropod phylum.

Role of peptide hormones in insect gut physiology

Nutrient uptake and digestion are essential for optimal growth and development. In insects, these processes are regulated by the gut-brain axis, which is a neurohumoral communication system for maintaining gut homeostasis. The insect gut is a complex organ consisting of three distinct structures, denominated foregut, midgut and hindgut, each with their specific specializations. These specializations are tightly regulated by the interplay of several neuropeptides: a versatile group of signalling molecules involved in a multitude of processes including gut physiology. Neuropeptides take part in the regulation of gut processes ranging from digestive enzyme release to muscle activity and satiety. Some neuropeptide mimetics are a promising strategy for ecological pest management. This review focuses on a selection of neuropeptides that are well-known for their role in gut physiology, and neuropeptides for which the mode of action is yet to be unravelled.

What happens after a blood meal? A transcriptome analysis of the main tissues involved in egg production in Rhodnius prolixus, an insect vector of Chagas disease

The blood-sucking hemipteran Rhodnius prolixus is a vector of Chagas disease, one of the most neglected tropical diseases affecting several million people, mostly in Latin America. The blood meal is an event with a high epidemiological impact since adult mated females feed several times, with each meal resulting in a bout of egg laying, and thereby the production of hundreds of offspring. By means of RNA-Sequencing (RNA-Seq) we have examined how a blood meal influences mRNA expression in the central nervous system (CNS), fat body and ovaries in order to promote egg production, focusing on tissue-specific responses under controlled nutritional conditions. We illustrate the cross talk between reproduction and a) lipids, proteins and trehalose metabolism, b) neuropeptide and neurohormonal signaling, and c) the immune system. Overall, our molecular evaluation confirms and supports previous studies and provides an invaluable molecular resource for future investigations on different tissues involved in successful reproductive events. These analyses serve as a starting point for new investigations, increasing the chances of developing novel strategies for vector population control by translational research, with less impact on the environment and more specificity for a particular organism.

Can BRET-based biosensors be used to characterize G-protein mediated signaling pathways of an insect GPCR, the Schistocerca gregaria CRF-related diuretic hormone receptor?

G protein-coupled receptors (GPCRs) are membrane-bound receptors that are considered prime candidates for the development of novel insect pest management strategies. However, the molecular signaling properties of insect GPCRs remain poorly understood. In fact, most studies on insect GPCR signaling are limited to analysis of fluctuations in the secondary messenger molecules calcium (Ca²⁺) and/or cyclic adenosine monophosphate (cAMP). In the current study, we characterized a corticotropin-releasing factor-related diuretic hormone (CRF-DH) receptor of the desert locust, Schistocerca gregaria. This Schgr-CRF-DHR is mainly expressed in the nervous system and in brain-associated endocrine organs. The neuropeptide Schgr-CRF-DH induced Ca²⁺-dependent aequorin-based bioluminescent responses in CHO cells co-expressing this receptor with the promiscuous Gα₁₆ protein. Furthermore, when co-expressed with the cAMP-dependent bioluminescence resonance energy transfer (BRET)-based CAMYEL biosensor in HEK293T cells, this receptor elicited dose-dependent agonist-induced responses with an EC₅₀ in the nanomolar range (4.02 nM). In addition, we tested if vertebrate BRET-based G protein biosensors, can also be used to detect direct Gα protein subunit activation by an insect GPCR. Therefore, we analyzed ten different human BRET-based G protein biosensors, representing members of all four Gα protein subfamilies; Gα_s, Gα_i/o, Gα_q/11 and Gα_12/13. Our data demonstrate that stimulation of Schgr-CRF-DHR by Schgr-CRF-DH can dose-dependently activate Gα_i/o and Gα_s biosensors, while no significant effects were observed with the Gα_q/11 and Gα_12/13 biosensors. Our study paves the way for future biosensor-based studies to analyze the signaling properties of insect GPCRs in both fundamental science and applied research contexts.

Transcriptomic characterisation of neuropeptides and their putative cognate G protein-coupled receptors during late embryo and stage-1 juvenile development of the Aotearoa-New Zealand crayfish, Paranephrops zealandicus

We de novo assembled a transcriptome for early life-stages of the Aotearoa-New Zealand crayfish, Paranephrops zealandicus, establishing the first genetic resource for this under-developed aquaculture species and for the Paranephrops genus. Mining of this transcriptome for neuropeptides and their putative cognate G protein-coupled receptors (GPCRs) yielded a comprehensive catalogue of neuropeptides, but few putative neuropeptide GPCRs. Of the neuropeptides commonly identified from decapod transcriptomes, only crustacean female sex hormone and insulin-like peptide were absent from our trinity de novo transcriptome assembly, and also RNA-sequence reads. We identified 63 putative neuropeptide precursors from 43 families, predicted to yield 122 active peptides. Transcripts encoding 26 putative neuropeptide GPCRs were identified but were often incomplete. Putative GPCRs for 15 of the neuropeptides identified here were absent from our transcriptome and RNAseq reads. These data highlight the diverse neuropeptide systems already present at the early development life stages sampled here for P. zealandicus.

Identification and expression profiling of neuropeptides and neuropeptide receptor genes in Atrijuglans hetaohei

Atrijuglans hetaohei Yang (Lepidoptera: Gelechioidea), is one of the major pests that can seriously damage the walnut fruits. Neuropeptides and their receptors regulate most physiological functions in insects and represent new targets for the development of control agents. To identify the neuropeptides and their receptors from A. hetaohei, we sequenced and analyzed its head transcriptomic data, identified 32 neuropeptides and 39 neuropeptide receptor genes. Sequence comparisons and phylogenetic analyses suggest that A. hetaohei neuropeptides and receptor genes have high homology with those in Bombyx mori, Chilo suppressalis, Plutella xylostella and Helicoverpa armigera. Moreover, gene expression patterns revealed that neuropeptide genes such as AKH1, CP, MS and PTTH were expressed specifically in male head, while CAP3, DH, NPLP1, PBAN and SIF showed higher expression in the female head. Bur showed abdomen biased expression in both male and female. Neuropeptide receptor genes such as A8, A11, A15 and LGR were highly expressed in male head, whereas A24 and LGR2 were preferentially expressed in female head. This is the first sequencing, identification and expression analyses of neuropeptides and neuropeptide receptor genes from A. hetaohei. Our results could provide a powerful background that will facilitate the further investigations using transcriptomics to determine neuropeptides and their receptors presence, functions, and indicates potential targets in A. hetaohei for a novel pest management strategy.

Neuropeptides and G-Protein Coupled Receptors (GPCRs) in the Red Palm Weevil Rhynchophorus ferrugineus Olivier (Coleoptera: Dryophthoridae)

The red palm weevil Rhynchophorus ferrugineus is a devastating, invasive pest that causes serious damages to palm trees, and its invasiveness depends on its strong ability of physiological and behavioral adaptability. Neuropeptides and their receptors regulate physiology and behavior of insects, but these protein partners have not been identified from many insects. Here, we systematically identified neuropeptide precursors and the corresponding receptors in the red palm weevil, and analyzed their tissue expression patterns under control conditions and after pathogen infection. A total of 43 putative neuropeptide precursors were identified, including an extra myosuppressin peptide was identified with amino acid substitutions at two conserved sites. Forty-four putative neuropeptide receptors belonging to three classes were also identified, in which neuropeptide F receptors and insulin receptors were expanded compared to those in other insects. Based on qRT-PCR analyses, genes coding for several neuropeptide precursors and receptors were highly expressed in tissues other than the nervous system, suggesting that these neuropeptides and receptors play other roles in addition to neuro-reception. Some of the neuropeptides and receptors, like the tachykinin-related peptide and receptor, were significantly induced by pathogen infection, especially sensitive to Bacillus thuringiensis and Metarhizium anisopliae. Systemic identification and initial characterization of neuropeptides and their receptors in the red palm weevil provide a framework for further studies to reveal the functions of these ligand- and receptor-couples in regulating physiology, behavior, and immunity in this important insect pest species.

Identification and Characterization of Neuropeptides and Their G Protein-Coupled Receptors (GPCRs) in the Cowpea Aphid Aphis craccivora

Neuropeptides are the most abundant and diverse signal molecules in insects. They act as neurohormones and neuromodulators to regulate the physiology and behavior of insects. The majority of neuropeptides initiate downstream signaling pathways through binding to G protein-coupled receptors (GPCRs) on the cell surface. In this study, RNA-seq technology and bioinformatics were used to search for genes encoding neuropeptides and their GPCRs in the cowpea aphid Aphis craccivora. And the expression of these genes at different developmental stages of A. craccivora was analyzed by quantitative real-time PCR (qRT-PCR). A total of 40 candidate genes encoding neuropeptide precursors were identified from the transcriptome data, which is roughly equivalent to the number of neuropeptide genes that have been reported in other insects. On this basis, software analysis combined with homologous prediction estimated that there could be more than 60 mature neuropeptides with biological activity. In addition, 46 neuropeptide GPCRs were obtained, of which 40 belong to rhodopsin-like receptors (A-family GPCRs), including 21 families of neuropeptide receptors and 7 orphan receptors, and 6 belong to secretin-like receptors (B-family GPCRs), including receptors for diuretic hormone 31, diuretic hormone 44 and pigment-dispersing factor (PDF). Compared with holometabolous insects such as Drosophila melanogaster, the coding genes for sulfakinin, corazonin, arginine vasopressin-like peptide (AVLP), and trissin and the corresponding receptors were not found in A. craccivora. It is speculated that A. craccivora likely lacks the above neuropeptide signaling pathways, which is consistent with Acyrthosiphon pisum and that the loss of these pathways may be a common feature of aphids. In addition, expression profiling revealed neuropeptide genes and their GPCR genes that are differentially expressed at different developmental stages and in different wing morphs. This study will help to deepen our understanding of the neuropeptide signaling systems in aphids, thus laying the foundation for the development of new methods for aphid control targeting these signaling systems.