Feeding-induced changes in allatostatin-A and short neuropeptide F in the antennal lobes affect odor-mediated host seeking in the yellow fever mosquito, Aedes aegypti

Short neuropeptide F in integrated insect physiology

The short neuropeptide F (sNPF) family of peptides is a multifunctional group of neurohormones involved in the regulation of various physiological processes in insects. They have been found in a broad spectrum of species, but the number of isoforms in the precursor molecule varies from one to four. The receptor for sNPF (sNPFR), which belongs to the G protein-coupled receptor family, has been characterized in various insect orders and was shown to be an ortholog of the mammalian prolactin-releasing peptide receptor (PrPR). The sNPF signaling pathway interacts with other neurohormones such as insulin-like peptides, SIFamide, and pigment-dispersing factors (PDFs) to regulate various processes. The main physiological function of sNPF seems to be involved in the regulation of feeding, but the observed effects are species-specific. sNPF is also connected with the regulation of foraging behavior and the olfactory system. The influence of sNPF on feeding and thus energy metabolism may also indirectly affect other vital processes, such as reproduction and development. In addition, these neurohormones are involved in the regulation of locomotor activity and circadian rhythm in insects. This review summarizes the current state of knowledge about the sNPF system in insects.

Evolutionary conserved peptide and glycoprotein hormone-like neuroendocrine systems in C. elegans

Peptides and protein hormones form the largest group of secreted signals that mediate intercellular communication and are central regulators of physiology and behavior in all animals. Phylogenetic analyses and biochemical identifications of peptide-receptor systems reveal a broad evolutionary conservation of these signaling systems at the molecular level. Substantial progress has been made in recent years on characterizing the physiological and putative ancestral roles of many peptide systems through comparative studies in invertebrate models. Several peptides and protein hormones are not only molecularly conserved but also have conserved roles across animal phyla. Here, we focus on functional insights gained in the nematode Caenorhabditis elegans that, with its compact and well-described nervous system, provides a powerful model to dissect neuroendocrine signaling networks involved in the control of physiology and behavior. We summarize recent discoveries on the evolutionary conservation and knowledge on the functions of peptide and protein hormone systems in C. elegans.

Local age-dependent neuromodulation in Rhodnius prolixus antennae

Kissing bugs do not respond to host cues when recently molted and only exhibit robust host-seeking several days after ecdysis. Behavioral plasticity has peripheral correlates in antennal gene expression changes through the week after ecdysis. The mechanisms regulating these peripheral changes are still unknown, but neuropeptide, G-protein coupled receptor, nuclear receptor, and takeout genes likely modulate peripheral sensory physiology. We evaluated their expression in antennal transcriptomes along the first week postecdysis of Rhodnius prolixus 5th instar larvae. Besides, we performed clustering and co-expression analyses to reveal relationships between neuromodulatory (NM) and sensory genes. Significant changes in transcript abundance were detected for 50 NM genes. We identified 73 sensory-related and NM genes that were assigned to nine clusters. According to their expression patterns, clusters were classified into four groups: two including genes up or downregulated immediately after ecdysis; and two with genes with expression altered at day 2. Several NM genes together with sensory genes belong to the first group, suggesting functional interactions. Co-expression network analysis revealed a set of genes that seem to connect with sensory system maturation. Significant expression changes in NM components were described in the antennae of R. prolixus after ecdysis, suggesting that a local NM system acts on antennal physiology. These changes may modify the sensitivity of kissing bugs to host cues during this maturation interval.

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.

Anatomic and neurochemical analysis of the palpal olfactory system in the red flour beetle Tribolium castaneum, HERBST

The paired antennal lobes were long considered the sole primary processing centers of the olfactory pathway in holometabolous insects receiving input from the olfactory sensory neurons of the antennae and mouthparts. In hemimetabolous insects, however, olfactory cues of the antennae and palps are processed separately. For the holometabolous red flour beetle Tribolium castaneum, we could show that primary processing of the palpal and antennal olfactory input also occurs separately and at distinct neuronal centers. While the antennal olfactory sensory neurons project into the antennal lobes, those of the palps project into the paired glomerular lobes and the unpaired gnathal olfactory center. Here we provide an extended analysis of the palpal olfactory pathway by combining scanning electron micrographs with confocal imaging of immunohistochemical staining and reporter expression identifying chemosensory and odorant receptor-expressing neurons in the palpal sensilla. In addition, we extended the anatomical characterization of the gnathal olfactory center by 3D reconstructions and investigated the distribution of several neuromediators. The similarities in the neuromediator repertoire between antennal lobes, glomerular lobes, and gnathal olfactory center underline the role of the latter two as additional primary olfactory processing centers.

Global characterization of gene expression in the brain of starved immature Rhodnius prolixus

BACKGROUND

Rhodnius prolixus is a vector of Chagas disease and has become a model organism to study physiology, behavior, and pathogen interaction. The publication of its genome allowed initiating a process of comparative characterization of the gene expression profiles of diverse organs exposed to varying conditions. Brain processes control the expression of behavior and, as such, mediate immediate adjustment to a changing environment, allowing organisms to maximize their chances to survive and reproduce. The expression of fundamental behavioral processes like feeding requires fine control in triatomines because they obtain their blood meals from potential predators. Therefore, the characterization of gene expression profiles of key components modulating behavior in brain processes, like those of neuropeptide precursors and their receptors, seems fundamental. Here we study global gene expression profiles in the brain of starved R. prolixus fifth instar nymphs by means of RNA sequencing (RNA-Seq).

RESULTS

The expression of neuromodulatory genes such as those of precursors of neuropeptides, neurohormones, and their receptors; as well as the enzymes involved in the biosynthesis and processing of neuropeptides and biogenic amines were fully characterized. Other important gene targets such as neurotransmitter receptors, nuclear receptors, clock genes, sensory receptors, and takeouts genes were identified and their gene expression analyzed.

CONCLUSION

We propose that the set of neuromodulatory-related genes highly expressed in the brain of starved R. prolixus nymphs deserves functional characterization to allow the subsequent development of tools targeting them for bug control. As the brain is a complex structure that presents functionally specialized areas, future studies should focus on characterizing gene expression profiles in target areas, e.g. mushroom bodies, to complement our current knowledge.

Allatostatin A Signalling: Progress and New Challenges From a Paradigmatic Pleiotropic Invertebrate Neuropeptide Family

Neuropeptides have gained broad attraction in insect neuroscience and physiology, as new genetic tools are increasingly uncovering their wide-ranging pleiotropic functions with high cellular resolution. Allatostatin A (AstA) peptides constitute one of the best studied insect neuropeptide families. In insects and other panarthropods, AstA peptides qualify as brain-gut peptides and have regained attention with the discovery of their role in regulating feeding, growth, activity/sleep and learning. AstA receptor homologs are found throughout the protostomia and group with vertebrate somatostatin/galanin/kisspeptin receptors. In this review, we summarise the current knowledge on the evolution and the pleiotropic and cell-specific non-allatostatic functions of AstA. We speculate about the core functions of AstA signalling, and derive open questions and challengesfor future research on AstA and invertebrate neuropeptides in general.

Bidirectional Microbiome-Gut-Brain-Axis Communication Influences Metabolic Switch-Associated Responses in the Mosquito Anopheles culicifacies

The periodic ingestion of a protein-rich blood meal by adult female mosquitoes causes a drastic metabolic change in their innate physiological status, which is referred to as a ‘metabolic switch’. While understanding the neural circuits for host-seeking is modestly attended, how the gut ‘metabolic switch’ modulates brain functions, and resilience to physiological homeostasis, remains unexplored. Here, through a comparative brain RNA-Seq study, we demonstrate that the protein-rich diet induces the expression of brain transcripts related to mitochondrial function and energy metabolism, possibly causing a shift in the brain’s engagement to manage organismal homeostasis. A dynamic mRNA expression pattern of neuro-signaling and neuro-modulatory genes in both the gut and brain likely establishes an active gut–brain communication. The disruption of this communication through decapitation does not affect the modulation of the neuro-modulator receptor genes in the gut. In parallel, an unusual and paramount shift in the level of neurotransmitters (NTs), from the brain to the gut after blood feeding, further supports the idea of the gut’s ability to serve as a ‘second brain’. After blood-feeding, a moderate enrichment of the gut microbial population, and altered immunity in the gut of histamine receptor-silenced mosquitoes, provide initial evidence that the gut-microbiome plays a crucial role in gut–brain–axis communication. Finally, a comparative metagenomics evaluation of the gut microbiome highlighted that blood-feeding enriches the family members of the Morganellaceae and Pseudomonadaceae bacterial communities. The notable observation of a rapid proliferation of Pseudomonas bacterial sp. and tryptophan enrichment in the gut correlates with the suppression of appetite after blood-feeding. Additionally, altered NTs dynamics of naïve and aseptic mosquitoes provide further evidence that gut-endosymbionts are key modulators for the synthesis of major neuroactive molecules. Our data establish a new conceptual understanding of microbiome–gut–brain–axis communication in mosquitoes.

Identification of novel-vector control target proteins of Aedes sp.: A Systems Network Biology Approach

Aedes is an important vector for various viruses that cause dengue, chikungunya and zika, which affect human health globally. Due to regular outbreaks of these diseases worldwide, there is a need to identify essential vector proteins that are critical for the survival of the vector, which may be targeted to control the spread of vector-borne disease (VBD). In silico computational methods involving comparative proteomics, analysis of orthologous proteins common amongst members of Aedes genus and protein-protein interaction (PPI) pathway were used to identify essential proteins that could act as novel therapeutic candidates. Twenty-three conserved proteins between A. aegypti and A. albopictus were identified from a BLASTP search with an e-value threshold of 0.005, and their PPI networks were constructed in the STRING database. The merged network was analyzed using various Cytoscape plugins viz. ClusterONE, Cytohubba and MCODE. Thirty-one hub proteins were identified from the system's network biology analysis, and detailed data and literature mining were carried out. Twelve novel vector-control target proteins of A. aegypti, having no human homologs, were determined in the present study that can effectively act as potential therapeutic candidates for drug design and vaccine development.

Feeding State-Dependent Modulation of Feeding-Related Motor Patterns

Studies elucidating modulation of microcircuit activity in isolated nervous systems have revealed numerous insights regarding neural circuit flexibility, but this approach limits the link between experimental results and behavioral context. To bridge this gap, we studied feeding behavior-linked modulation of microcircuit activity in the isolated stomatogastric nervous system (STNS) of male Cancer borealis crabs. Specifically, we removed hemolymph from a crab that was unfed for ≥24 h ('unfed' hemolymph) or fed 15 min - 2 h before hemolymph removal ('fed' hemolymph). After feeding, the first significant foregut emptying occurred >1 h later and complete emptying required ≥6 h. We applied the unfed or fed hemolymph to the stomatogastric ganglion (STG) in an isolated STNS preparation from a separate, unfed crab to determine its influence on the VCN (ventral cardiac neuron)-triggered gastric mill (chewing)- and pyloric (filtering of chewed food) rhythms. Unfed hemolymph had little influence on these rhythms, but fed hemolymph from each examined time-point (15 min, 1- or 2 h post-feeding) slowed one or both rhythms without weakening circuit neuron activity. There were also distinct parameter changes associated with each time-point. One change unique to the 1 h time-point (i.e. reduced activity of one circuit neuron during the transition from the gastric mill retraction to protraction phase) suggested the fed hemolymph also enhanced the influence of a projection neuron which innervates the STG from a ganglion isolated from the applied hemolymph. Hemolymph thus provides a feeding state-dependent modulation of the two feeding-related motor patterns in the C. borealis STG.

Expression patterns and functional analysis of the short neuropeptide F and NPF receptor genes in Rhopalosiphum padi

The short neuropeptide F (sNPF) and NPF receptor (NPFR) genes play important roles in many physiological processes. However, information on the survival-related functions of sNPF and NPFR under different stress conditions is lacking in aphids. In this study, we cloned sNPF and NPFR, and investigated the expression levels of these genes in different developmental stages, wing morphs, and stress conditions of the bird cherry-oat aphid (Rhopalosiphum padi L.), an important agricultural pest. The sNPF and NPFR transcript levels varied among developmental stages, and their expression levels in alate females were significantly higher than those in apterous females. In addition, starvation resulted in significantly increased sNPF expression, which then recovered after refeeding. Heat stress and insecticides significantly affected transcription of both genes. sNPF and NPFR knockdown in R. padi using RNA interference revealed optimal interference efficiency at 48 h post-injection. sNPF knockdown significantly decreased adult longevity, 15-d fecundity, and food intake. Additionally, mortality under starvation, insecticides, and heat stress conditions was significantly higher after injection with double-stranded sNPF in R. padi. NPFR knockdown significantly affected food intake and starvation resistance in R. padi. These results strongly indicate that sNPF plays vital roles in food intake, longevity, and reproduction in R. padi, and it can significantly affect the pest's response to stress conditions.

Transcriptomic, peptidomic, and mass spectrometry imaging analysis of the brain in the ant Cataglyphis nodus

Behavioral flexibility is an important cornerstone for the ecological success of animals. Social Cataglyphis nodus ants with their age-related polyethism characterized by age-related behavioral phenotypes represent a prime example for behavioral flexibility. We propose neuropeptides as powerful candidates for the flexible modulation of age-related behavioral transitions in individual ants. As the neuropeptidome of C. nodus was unknown, we collected a comprehensive peptidomic data set obtained by transcriptome analysis of the ants' central nervous system combined with brain extract analysis by Q-Exactive Orbitrap mass spectrometry (MS) and direct tissue profiling of different regions of the brain by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS. In total, we identified 71 peptides with likely bioactive function, encoded on 49 neuropeptide-, neuropeptide-like, and protein hormone prepropeptide genes, including a novel neuropeptide-like gene (fliktin). We next characterized the spatial distribution of a subset of peptides encoded on 16 precursor proteins with high resolution by MALDI MS imaging (MALDI MSI) on 14 µm brain sections. The accuracy of our MSI data were confirmed by matching the immunostaining patterns for tachykinins with MSI ion images from consecutive brain sections. Our data provide a solid framework for future research into spatially resolved qualitative and quantitative peptidomic changes associated with stage-specific behavioral transitions and the functional role of neuropeptides in Cataglyphis ants.

Regulation of the antennal transcriptome of the dengue vector, Aedes aegypti, during the first gonotrophic cycle

Background

In the light of dengue being the fastest growing transmissible disease, there is a dire need to identify the mechanisms regulating the behaviour of the main vector Aedes aegypti. Disease transmission requires the female mosquito to acquire the pathogen from a blood meal during one gonotrophic cycle, and to pass it on in the next, and the capacity of the vector to maintain the disease relies on a sustained mosquito population.

Results

Using a comprehensive transcriptomic approach, we provide insight into the regulation of the odour-mediated host- and oviposition-seeking behaviours throughout the first gonotrophic cycle. We provide clear evidence that the age and state of the female affects antennal transcription differentially. Notably, the temporal- and state-dependent patterns of differential transcript abundance of chemosensory and neuromodulatory genes extends across families, and appears to be linked to concerted differential modulation by subsets of transcription factors.

Conclusions

By identifying these regulatory pathways, we provide a substrate for future studies targeting subsets of genes across disparate families involved in generating key vector behaviours, with the goal to develop novel vector control tools.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07336-w.

Modulation of odour-guided behaviour in mosquitoes

Mosquitoes are emerging as model systems with which to study innate behaviours through neuroethology and functional genomics. Decades of work on these disease vectors have provided a solid behavioural framework describing the distinct repertoire of predominantly odour-mediated behaviours of female mosquitoes, and their dependence on life stage (intrinsic factors) and environmental cues (extrinsic factors). The purpose of this review is to provide an overview of how intrinsic factors, including adult maturation, age, nutritional status, and infection, affect the attraction to plants and feeding on plant fluids, host seeking, blood feeding, supplemental feeding behaviours, pre-oviposition behaviour, and oviposition in female mosquitoes. With the technological advancements in the recent two decades, we have gained a better understanding of which volatile organic compounds are used by mosquitoes to recognise and discriminate among various fitness-enhancing resources, and characterised their neural and molecular correlates. In this review, we present the state of the art of the peripheral olfactory system as described by the neural physiology, functional genomics, and genetics underlying the demonstrated changes in the behavioural repertoire in female mosquitoes. The review is meant as a summary introduction to the current conceptual thinking in the field.

Plasticity and modulation of olfactory circuits in insects

Olfactory circuits change structurally and physiologically during development and adult life. This allows insects to respond to olfactory cues in an appropriate and adaptive way according to their physiological and behavioral state, and to adapt to their specific abiotic and biotic natural environment. We highlight here findings on olfactory plasticity and modulation in various model and non-model insects with an emphasis on moths and social Hymenoptera. Different categories of plasticity occur in the olfactory systems of insects. One type relates to the reproductive or feeding state, as well as to adult age. Another type of plasticity is context-dependent and includes influences of the immediate sensory and abiotic environment, but also environmental conditions during postembryonic development, periods of adult behavioral maturation, and short- and long-term sensory experience. Finally, plasticity in olfactory circuits is linked to associative learning and memory formation. The vast majority of the available literature summarized here deals with plasticity in primary and secondary olfactory brain centers, but also peripheral modulation is treated. The described molecular, physiological, and structural neuronal changes occur under the influence of neuromodulators such as biogenic amines, neuropeptides, and hormones, but the mechanisms through which they act are only beginning to be analyzed.

Sugar Feeding Patterns for Aedes aegypti and Culex quinquefasciatus (Diptera: Culicidae) Mosquitoes in South Texas

Effective mosquito surveillance and management depend on a thorough understanding of the biology and feeding patterns unique to species and sex. Given that a propensity to sugar feed is necessary for some mosquito surveillance and newer control strategies, we sought to document the amount of total sugar in wild Aedes aegypti (L.) and Culex quinquefasciatus (Say) captured from five different locations in the Lower Rio Grande Valley (LRGV) of South Texas over 2 yr. We used Biogents Sentinel 2 (BGS2) traps in year 1 and aspirators, BGS2, and CDC resting traps in years 2 and 3 to collect adult mosquitoes. The hot anthrone test was used to quantify total sugar content in each mosquito. Additionally, the cold and hot anthrone tests were used to distinguish fructose content from total sugars for mosquitoes captured in 2019. Overall, Ae. aegypti females had significantly lower total sugar content than Ae. aegypti males as well as both sexes of Cx. quinquefasciatus. However, the percentage of Ae. aegypti positive for fructose consumption was four to eightfold higher than Ae. aegypti previously reported in other regions. The difference between locations was significant for males of both species, but not for females. Seasonality and trapping method also revealed significant differences in sugar content of captured mosquitoes. Our results reinforce that sugar feeding in female Ae. aegypti is less than Cx. quinquefasciatus, although not absent. This study provides necessary data to evaluate the potential effectiveness of sugar baits in surveillance and control of both Ae. aegypti and Cx. quinquefasciatus mosquitoes.

Small-Molecule Agonists of Ae. aegypti Neuropeptide Y Receptor Block Mosquito Biting

Female Aedes aegypti mosquitoes bite humans to obtain blood to develop their eggs. Remarkably, their strong attraction to humans is suppressed for days after the blood meal by an unknown mechanism. We investigated a role for neuropeptide Y (NPY)-related signaling in long-term behavioral suppression and discovered that drugs targeting human NPY receptors modulate mosquito host-seeking. In a screen of all 49 predicted Ae. aegypti peptide receptors, we identified NPY-like receptor 7 (NPYLR7) as the sole target of these drugs. To obtain small-molecule agonists selective for NPYLR7, we performed a high-throughput cell-based assay of 265,211 compounds and isolated six highly selective NPYLR7 agonists that inhibit mosquito attraction to humans. NPYLR7 CRISPR-Cas9 null mutants are defective in behavioral suppression and resistant to these drugs. Finally, we show that these drugs can inhibit biting and blood-feeding on a live host, suggesting a novel approach to control infectious disease transmission by controlling mosquito behavior. VIDEO ABSTRACT.

Mosquito Host-Seeking Regulation: Targets for Behavioral Control

Female Aedes aegypti mosquitoes require protein from blood to develop eggs. They have evolved a strong innate drive to find and bite humans and engorge on their blood. Decades of research have revealed that attraction to hosts is suppressed for days after blood-feeding. During this time, females coordinate complex physiological changes, allowing them to utilize blood protein to develop eggs: clearing excess fluid, digesting protein, and egg maturation. How do mechanosensation, nutrient consumption, and reproductive pathways combine to produce the full expression of host-seeking suppression? Understanding mechanisms of endogenous host-seeking suppression may allow them to be 'weaponized' against mosquitoes through exogenous activation and developed as tools for vector control. Recent work allows unprecedented genetic and pharmacological access to characterize and disrupt this behavioral cycle.

Fat body-specific vitellogenin expression regulates host-seeking behaviour in the mosquito Aedes albopictus

The high vector competence of mosquitoes is intrinsically linked to their reproductive strategy because females need a vertebrate blood meal to develop large batches of eggs. However, the molecular mechanisms and pathways regulating mosquito host-seeking behaviour are largely unknown. Here, we test whether host-seeking behaviour may be linked to the female’s energy reserves, with low energy levels triggering the search for a nutrient-rich blood meal. Our results demonstrate that sugar feeding delays host-seeking behaviour in the invasive tiger mosquito Aedes albopictus, but the levels of energy reserves do not correlate with changes in host-seeking behaviour. Using tissue-specific gene expression analyses, we show for the first time, to our knowledge, that sugar feeding alone induces a transient up-regulation of several vitellogenesis-related genes in the female fat body, resembling the transcriptional response after a blood meal. Specifically, high expression levels of a vitellogenin gene (Vg-2) correlated with the lowest host-seeking activity of sugar-fed females. Knocking down the Vg-2 gene via RNA interference (RNAi) restored host-seeking behaviour in these females, firmly establishing that Vg-2 gene expression has a pivotal role in regulating host-seeking behaviour in young Ae. albopictus females. The identification of a molecular mechanism regulating host-seeking behaviour in mosquitoes could pave the way for novel vector control strategies aiming to reduce the biting activity of mosquitoes. From an evolutionary perspective, this is the first demonstration of vitellogenin genes controlling feeding-related behaviours in nonsocial insects, while vitellogenins are known to regulate caste-specific foraging and brood-care behaviours in eusocial insects. Hence, this work confirms the key role of vitellogenin in controlling feeding-related behaviours in distantly related insect orders, suggesting that this function could be more ubiquitous than previously thought.

Sugar feeding induces a transient up-regulation of the vitellogenin gene in the abdominal fat body of the mosquito Aedes albopictus, reducing host-seeking behaviour. This suggests that nutritional-related behaviours co-opt reproductive regulatory pathways, even in non-social insects.

Mosquitoes on a Diet Reduce Those Pesky Bites

Targeting the internal regulation of mosquito's human-seeking capacity provides a novel means for vector control. Mosquitoes bite us to obtain blood in order to develop their eggs. Professor Vosshall and colleagues (Cell 2019;176:687-701) have exploited the modulatory pathway to control this hunger for blood by designing drugs based on human diet-suppression pharmaceuticals.

Regulation of Feeding and Metabolism by Neuropeptide F and Short Neuropeptide F in Invertebrates

Numerous neuropeptide systems have been implicated to coordinately control energy homeostasis, both centrally and peripherally. However, the vertebrate neuropeptide Y (NPY) system has emerged as the best described one regarding this biological process. The protostomian ortholog of NPY is neuropeptide F, characterized by an RXRF(Y)amide carboxyterminal motif. A second neuropeptide system is short NPF, characterized by an M/T/L/FRF(W)amide carboxyterminal motif. Although both short and long NPF neuropeptide systems display carboxyterminal sequence similarities, they are evolutionary distant and likely already arose as separate signaling systems in the common ancestor of deuterostomes and protostomes, indicating the functional importance of both. Both NPF and short-NPF systems seem to have roles in the coordination of feeding across bilaterian species, but during chordate evolution, the short NPF system appears to have been lost or evolved into the prolactin releasing peptide signaling system, which regulates feeding and has been suggested to be orthologous to sNPF. Here we review the roles of both NPF and sNPF systems in the regulation of feeding and metabolism in invertebrates.

Systematic Analysis of Transmitter Coexpression Reveals Organizing Principles of Local Interneuron Heterogeneity

Broad neuronal classes are surprisingly heterogeneous across many parameters, and subclasses often exhibit partially overlapping traits including transmitter coexpression. However, the extent to which transmitter coexpression occurs in predictable, consistent patterns is unknown. Here, we demonstrate that pairwise coexpression of GABA and multiple neuropeptide families by olfactory local interneurons (LNs) of the moth Manduca sexta is highly heterogeneous, with a single LN capable of expressing neuropeptides from at least four peptide families and few instances in which neuropeptides are consistently coexpressed. Using computational modeling, we demonstrate that observed coexpression patterns cannot be explained by independent probabilities of expression of each neuropeptide. Our analyses point to three organizing principles that, once taken into consideration, allow replication of overall coexpression structure: (1) peptidergic neurons are highly likely to coexpress GABA; (2) expression probability of allatotropin depends on myoinhibitory peptide expression; and (3) the all-or-none coexpression patterns of tachykinin neurons with several other neuropeptides. For other peptide pairs, the presence of one peptide was not predictive of the presence of the other, and coexpression probability could be replicated by independent probabilities. The stochastic nature of these coexpression patterns highlights the heterogeneity of transmitter content among LNs and argues against clear-cut definition of subpopulation types based on the presence of single neuropeptides. Furthermore, the receptors for all neuropeptides and GABA were expressed within each population of principal neuron type in the antennal lobe (AL). Thus, activation of any given LN results in a dynamic cocktail of modulators that have the potential to influence every level of olfactory processing within the AL.

Functional characterization of mosquito short neuropeptide F receptors

Mosquito blood feeding transiently inhibits sugar- and host seeking through neuropeptide signaling. Short neuropeptide F (sNPF) is one of the neuromodulators involved in this regulation. Here, we identified the genes for the sNPF precursor and the sNPF receptor in the southern house mosquito, Culex quinquefasciatus. Comparative analyses are made with the genes of the sNPF precursor and receptor from two other important vectors, Aedes aegypti and Anopheles coluzzii. We functionally characterized the receptors in all three species using endogenous neuropeptides, and quantified their transcript expression following a blood meal and a sugar meal. Our analysis reveals several Cx. quinquefasciatus-specific duplications of the sNPF-3 isoform on the sNPF precursor, which are not reflected in the precursors of the other two species. In contrast, the structure of the sNPF receptors is highly conserved within mosquitoes, and a putative ligand binding region is proposed and discussed. Reflecting the high structural conservation, the sNPF receptor sensitivity to endogenous sNPF isoforms is conserved across mosquito species. Using quantitative real time PCR, we demonstrate that transcript abundance of the sNPF receptor and precursor is regulated following feeding, only in Cx. quinquefasciatus. We discuss our findings in relation to previous work on sNPF signaling and its role in feeding regulation.

Functional characterization of the dual allatostatin-A receptors in mosquitoes

The neuropeptide allatostatin-A (AstA) and its cognate receptors (AstARs) are involved in the modulation of feeding behavior, which in hematophagous insects includes the regulation of the disease vector-related behaviors, host seeking and blood feeding. In mosquitoes and other dipterans, there are two copies of AstAR, contrasting with the single copy found in other insects. In this study, we identified and cloned the dual AstAR system of two important disease vectors Aedes aegypti and Culex quinquefasciatus, and compared them with those previously described, including those in Anopheles coluzzii and Drosophila melanogaster. Phylogenetic analysis of the AstARs revealed that the mosquito AstAR1s has retained a similar amino acid sequence as the AstARs from non-dipteran insect species. Intron analysis revealed that the number of introns accumulated in the AstAR2s is similar to that in other insects, and that introns are conserved within the receptor types, but that only the final two introns are conserved across AstAR1s and 2s. We functionally characterized the dual AstARs in An. coluzzii, Ae. aegypti and Cx. quinquefasciatus by stably expressing the receptors in a Chinese hamster oocyte cell line (CHO) also stably expressing a promiscuous G-protein (G16), and challenged them with the endogenous isoforms of AstA from the three mosquito species. In the culicine mosquitoes, Ae. aegypti and Cx. quinquefasciatus, the AstARs demonstrated differential sensitivity to AstA, with the AstAR2s displaying a higher sensitivity than the AstAR1s, suggesting a divergence of functional roles for these AstARs. In contrast, both An. coluzzii AstARs demonstrated a similar sensitivity to the AstA ligands. We discuss our findings in the light of AstA acting as a regulator of blood feeding in mosquitoes. A better understanding of the regulation of host seeking and blood feeding in vector mosquitoes will lead to the rational development of novel approaches for vector control.