Adipokinetic hormones in the African malaria mosquito, Anopheles gambiae: identification and expression of genes for two peptides and a putative receptor

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.

microRNA-8514-5p regulates adipokinetic hormone/corazonin-related peptide receptor to affect development and reproduction of Plutella xylostella

BACKGROUND

Dicer1 plays a crucial role in regulating the development and reproduction of insects. Knockout of Dicer1 causes pupal deformity, low eclosion and low fecundity in Plutella xylostella, but the mechanism behind this phenomenon is not clear. This study aims to identify differentially-expressed genes and miRNAs in the Dicer1-knockout strain (ΔPxDcr-1) and assess their impact on the reproduction and development of P. xylostella.

RESULTS

The knockout of Dicer1 affected the expression of genes including the adipokinetic hormone/corazonin-related peptide receptor (PxACPR). The expression of PxACPR was upregulated, and the expression of miR-8514-5p was downregulated in ΔPxDcr-1 of P. xylostella. The dual luciferase reporter assay and pull-down assay showed that miR-8514-5p bound to PxACPR in vitro and in vivo. The expression profiles demonstrated a negative correlation between PxACPR mRNA and miR-8514-5p in different developmental stages of the wild-type strain. Both the miR-8514-5p agomir and double-stranded RNA of ACPR (dsPxACPR) injected into the pre-pupae inhibited the mRNA level of PxACPR, causing high mortality and deformity of pupae, and low fecundity and hatching rate, which were consistent with the phenotype of ΔPxDcr-1. The injection of miR-8514-5p antagomir caused a similar phenotype to the injection of miR-8514-5p agomir. Additionally, the injection of miR-8514-5p antagomir significantly rescued the phenotype caused by dsPxACPR.

CONCLUSION

These results indicate that miR-8514-5p affects the development and reproduction of P. xylostella by regulating PxACPR, and the homeostasis of PxACPR expression is essential for the development and reproduction of P. xylostella. © 2024 Society of Chemical Industry.

Endocrine Control of Lipid Metabolism

Lipids are essential in insects and play pleiotropic roles in energy storage, serving as a fuel for energy-driven processes such as reproduction, growth, development, locomotion, flight, starvation response, and diapause induction, maintenance, and termination. Lipids also play fundamental roles in signal transduction, hormone synthesis, forming components of the cell membrane, and thus are essential for maintenance of normal life functions. In insects, the neuroendocrine system serves as a master regulator of most life activities, including growth and development. It is thus important to pay particular attention to the regulation of lipid metabolism through the endocrine system, especially when considering the involvement of peptide hormones in the processes of lipogenesis and lipolysis. In insects, there are several lipogenic and lipolytic hormones that are involved in lipid metabolism such as insulin-like peptides (ILPs), adipokinetic hormone (AKH), 20-hydroxyecdysone (20-HE), juvenile hormone (JH), and serotonin. Other neuropeptides such as diapause hormone-pheromone biosynthesis activating neuropeptide (DH-PBAN), CCHamide-2, short neuropeptide F, and the cytokines Unpaired 1 and 2 may play a role in inducing lipogenesis. On the other hand, neuropeptides such as neuropeptide F, allatostatin-A, corazonin, leukokinin, tachykinins, limostatins, and insulin-like growth factor (ILP6) stimulate lipolysis. This chapter briefly discusses the current knowledge of the endocrine regulation of lipid metabolism in insects that could be utilized to reveal differences between insects and mammalian lipid metabolism which may help understand human diseases associated with dysregulation of lipid metabolism. Physiological similarities of insects to mammals make them valuable model systems for studying human diseases characterized by disrupted lipid metabolism, including conditions like diabetes, obesity, arteriosclerosis, and various metabolic syndromes.

Comparative analysis of adipokinetic hormones and their receptors in Blattodea reveals novel patterns of gene evolution

Adipokinetic hormone (AKH) is a neuropeptide produced in the insect corpora cardiaca that plays an essential role in mobilising carbohydrates and lipids from the fat body to the haemolymph. AKH acts by binding to a rhodopsin-like G protein-coupled receptor (GPCR), the adipokinetic hormone receptor (AKHR). In this study, we tackle AKH ligand and receptor gene evolution as well as the evolutionary origins of AKH gene paralogues from the order Blattodea (termites and cockroaches). Phylogenetic analyses of AKH precursor sequences point to an ancient AKH gene duplication event in the common ancestor of Blaberoidea, yielding a new group of putative decapeptides. In total, 16 different AKH peptides from 90 species were obtained. Two octapeptides and seven putatively novel decapeptides are predicted for the first time. AKH receptor sequences from 18 species, spanning solitary cockroaches and subsocial wood roaches as well as lower and higher termites, were subsequently acquired using classical molecular methods and in silico approaches employing transcriptomic data. Aligned AKHR open reading frames revealed 7 highly conserved transmembrane regions, a typical arrangement for GPCRs. Phylogenetic analyses based on AKHR sequences support accepted relationships among termite, subsocial (Cryptocercus spp.) and solitary cockroach lineages to a large extent, while putative post-translational modification sites do not greatly differ between solitary and subsocial roaches and social termites. Our study provides important information not only for AKH and AKHR functional research but also for further analyses interested in their development as potential candidates for biorational pest control agents against invasive termites and cockroaches.

Multiple endocrine factors regulate nutrient mobilization and storage in Aedes aegypti during a gonadotrophic cycle

Anautogenous mosquitoes must blood feed on a vertebrate host to produce eggs. Each gonadotrophic cycle is subdivided into a sugar-feeding previtellogenic phase that produces primary follicles and a blood meal-activated vitellogenic phase in which large numbers of eggs synchronously mature and are laid. Multiple endocrine factors including juvenile hormone (JH), insulin-like peptides (ILPs), ovary ecdysteroidogenic hormone (OEH), and 20-hydroxyecdysone (20E) coordinate each gonadotrophic cycle. Egg formation also requires nutrients from feeding that are stored in the fat body. Regulation of egg formation is best understood in Aedes aegypti but the role different endocrine factors play in regulating nutrient mobilization and storage remains unclear. In this study, we report that adult female Ae. aegypti maintained triacylglycerol (TAG) stores during the previtellogenic phase of the first gonadotrophic cycle while glycogen stores declined. In contrast, TAG and glycogen stores were rapidly mobilized during the vitellogenic phase and then replenishment. Several genes encoding enzymes with functions in TAG and glycogen metabolism were differentially expressed in the fat body, which suggested regulation was mediated in part at the transcriptional level. Gain of function assays indicated that stored nutrients were primarily mobilized by adipokinetic hormone (AKH) while juvenoids and OEH regulated replenishment. ILP3 further showed evidence of negatively regulating certain lipolytic enzymes. Loss of function assays indicated AKH depends on the AKH receptor (AKHR) for function. Altogether, our results indicate that the opposing activities of different hormones regulate nutrient stores during a gonadotrophic cycle in Ae. aegypti.

Adipokinetic hormone signaling in the malaria vector Anopheles gambiae facilitates Plasmodium falciparum sporogony

An obligatory step in the complex life cycle of the malaria parasite is sporogony, which occurs during the oocyst stage in adult female Anopheles mosquitoes. Sporogony is metabolically demanding, and successful oocyst maturation is dependent on host lipids. In insects, lipid energy reserves are mobilized by adipokinetic hormones (AKHs). We hypothesized that Plasmodium falciparum infection activates Anopheles gambiae AKH signaling and lipid mobilization. We profiled the expression patterns of AKH pathway genes and AgAkh1 peptide levels in An. gambiae during starvation, after blood feeding, and following infection and observed a significant time-dependent up-regulation of AKH pathway genes and peptide levels during infection. Depletion of AgAkh1 and AgAkhR by RNAi reduced salivary gland sporozoite production, while synthetic AgAkh1 peptide supplementation rescued sporozoite numbers. Inoculation of uninfected female mosquitoes with supernatant from P. falciparum-infected midguts activated AKH signaling. Clearly, identifying the parasite molecules mediating AKH signaling in P. falciparum sporogony is paramount.

Functional insight and cell-specific expression of the adipokinetic hormone/corazonin-related peptide in the human disease vector mosquito, Aedes aegypti

The adipokinetic hormone/corazonin-related peptide (ACP) is an insect neuropeptide structurally intermediate between corazonin (CRZ) and adipokinetic hormone (AKH). Unlike the AKH and CRZ signaling systems that are widely known for their roles in the mobilization of energy substrates and stress responses, respectively, the main role of ACP and its receptor (ACPR) remains unclear in most arthropods. The current study aimed to localize the distribution of ACP in the nervous system and provide insight into its physiological roles in the disease vector mosquito, Aedes aegypti. Immunohistochemical analysis and fluorescence in situ hybridization localized the ACP peptide and transcript within a number of cells in the central nervous system, including two pairs of laterally positioned neurons in the protocerebrum of the brain and a few ventrally localized neurons within the pro- and mesothoracic regions of the fused thoracic ganglia. Further, extensive ACP-immunoreactive axonal projections with prominent blebs and varicosities were observed traversing the abdominal ganglia. Given the prominent enrichment of ACPR expression within the abdominal ganglia of adult A. aegypti mosquitoes as determined previously, the current results indicate that ACP may function as a neurotransmitter and/or neuromodulator facilitating communication between the brain and posterior regions of the nervous system. In an effort to elucidate a functional role for ACP signaling, biochemical measurement of energy substrates in female mosquitoes revealed a reduction in abdominal fat body in response to ACP that matched the actions of AKH, but interestingly, a corresponding hypertrehalosaemic effect was only found in response to AKH since ACP did not influence circulating carbohydrate levels. Comparatively, both ACP and AKH led to a significant increase in haemolymph carbohydrate levels in male mosquitoes while both peptides had no influence on their glycogen stores. Neither ACP nor AKH influenced circulating or stored lipid levels in both male and female mosquitoes. Collectively, these results reveal ACP signaling in mosquitoes may have complex sex-specific actions, and future research should aim to expand knowledge on the role of this understudied neuropeptide.

A neuroendocrine pathway modulating osmotic stress in Drosophila

Environmental factors challenge the physiological homeostasis in animals, thereby evoking stress responses. Various mechanisms have evolved to counter stress at the organism level, including regulation by neuropeptides. In recent years, much progress has been made on the mechanisms and neuropeptides that regulate responses to metabolic/nutritional stress, as well as those involved in countering osmotic and ionic stresses. Here, we identified a peptidergic pathway that links these types of regulatory functions. We uncover the neuropeptide Corazonin (Crz), previously implicated in responses to metabolic stress, as a neuroendocrine factor that inhibits the release of a diuretic hormone, CAPA, and thereby modulates the tolerance to osmotic and ionic stress. Both knockdown of Crz and acute injections of Crz peptide impact desiccation tolerance and recovery from chill-coma. Mapping of the Crz receptor (CrzR) expression identified three pairs of Capa-expressing neurons (Va neurons) in the ventral nerve cord that mediate these effects of Crz. We show that Crz acts to restore water/ion homeostasis by inhibiting release of CAPA neuropeptides via inhibition of cAMP production in Va neurons. Knockdown of CrzR in Va neurons affects CAPA signaling, and consequently increases tolerance for desiccation, ionic stress and starvation, but delays chill-coma recovery. Optogenetic activation of Va neurons stimulates excretion and simultaneous activation of Crz and CAPA-expressing neurons reduces this response, supporting the inhibitory action of Crz. Thus, Crz inhibits Va neurons to maintain osmotic and ionic homeostasis, which in turn affects stress tolerance. Earlier work demonstrated that systemic Crz signaling restores nutrient levels by promoting food search and feeding. Here we additionally propose that Crz signaling also ensures osmotic homeostasis by inhibiting release of CAPA neuropeptides and suppressing diuresis. Thus, Crz ameliorates stress-associated physiology through systemic modulation of both peptidergic neurosecretory cells and the fat body in Drosophila.

Prediction of neuropeptide precursors and differential expression of adipokinetic hormone/corazonin-related peptide, hugin and corazonin in the brain of malaria vector Nyssorhynchus albimanus during a Plasmodium berghei infection

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We describe precursors that predicted at least sixty neuropeptides in Ny. albimanus.

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At least 16 precursors are encoded in the Ny. albimanus brain.

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Myosuppressin neuropeptide precursor was identified in Ny albimanus.

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acp and hugin transcripts increased in Ny. albimanus brains infected with P. berghei.

Insect neuropeptides, play a central role in the control of many physiological processes. Based on an analysis of Nyssorhynchus albimanus brain transcriptome a neuropeptide precursor database of the mosquito was described. Also, we observed that adipokinetic hormone/corazonin-related peptide (ACP), hugin and corazonin encoding genes were differentially expressed during Plasmodium infection. Transcriptomic data from Ny. albimanus brain identified 29 pre-propeptides deduced from the sequences that allowed the prediction of at least 60 neuropeptides. The predicted peptides include isoforms of allatostatin C, orcokinin, corazonin, adipokinetic hormone (AKH), SIFamide, capa, hugin, pigment-dispersing factor, adipokinetic hormone/corazonin-related peptide (ACP), tachykinin-related peptide, trissin, neuropeptide F, diuretic hormone 31, bursicon, crustacean cardioactive peptide (CCAP), allatotropin, allatostatin A, ecdysis triggering hormone (ETH), diuretic hormone 44 (Dh44), insulin-like peptides (ILPs) and eclosion hormone (EH). The analysis of the genome of An. albimanus and the generated transcriptome, provided evidence for the identification of myosuppressin neuropeptide precursor. A quantitative analysis documented increased expression of precursors encoding ACP peptide, hugin and corazonin in the mosquito brain after Plasmodium berghei infection. This work represents an initial effort to characterize the neuropeptide precursors repertoire of Ny. albimanus and provides information for understanding neuroregulation of the mosquito response during Plasmodium infection.

Functional Analysis of Adipokinetic Hormone Signaling in Bombyx mori

Insect adipokinetic hormones (AKHs) are short peptides produced in the corpora cardiaca and are responsible for mobilizing energy stores from the fat body to the hemolymph. Three related peptides, AKH1, AKH2, and AKH/corazonin-related peptide (ACP) as well as three AKH receptors have been reported in Bombyx mori. AKH1 and AKH2 are specific for the AKHR1 receptor, whereas ACP interacts with the other two AKHRs. To assess the effect of the two silkworm AKHs and ACP in the regulation of energy homeostasis we examined the expression pattern of the three peptides and their receptors as well as their effect on the level of carbohydrates and lipids in the hemolymph. Our results support the hypothesis that only AKH1 and AKH2 peptides together with the AKHR1 receptor are involved in the maintenance of energy homeostasis. Because Bombyx AKHR1 (BmAKHR1) seems to be a true AKHR we generated its mutation. The BmAKHR1 mutant larvae display significantly lower carbohydrate and lipid levels in the hemolymph and reduced sensitivity to starvation. Our study clarifies the role of BmAKHR1 in energy homeostasis.

The Role of Peptide Hormones in Insect Lipid Metabolism

Lipids are the primary storage molecules and an essential source of energy in insects during reproduction, prolonged periods of flight, starvation, and diapause. The coordination center for insect lipid metabolism is the fat body, which is analogous to the vertebrate adipose tissue and liver. The fat body is primarily composed of adipocytes, which accumulate triacylglycerols in intracellular lipid droplets. Genomics and proteomics, together with functional analyses, such as RNA interference and CRISPR/Cas9-targeted genome editing, identified various genes involved in lipid metabolism and elucidated their functions. However, the endocrine control of insect lipid metabolism, in particular the roles of peptide hormones in lipogenesis and lipolysis are relatively less-known topics. In the current review, the neuropeptides that directly or indirectly affect insect lipid metabolism are introduced. The primary lipolytic and lipogenic peptide hormones are adipokinetic hormone and the brain insulin-like peptides (ILP2, ILP3, ILP5). Other neuropeptides, such as insulin-growth factor ILP6, neuropeptide F, allatostatin-A, corazonin, leucokinin, tachykinins and limostatin, might stimulate lipolysis, while diapause hormone-pheromone biosynthesis activating neuropeptide, short neuropeptide F, CCHamide-2, and the cytokines Unpaired 1 and Unpaired 2 might induce lipogenesis. Most of these peptides interact with one another, but mostly with insulin signaling, and therefore affect lipid metabolism indirectly. Peptide hormones are also involved in lipid metabolism during reproduction, flight, diapause, starvation, infections and immunity; these are also highlighted. The review concludes with a discussion of the potential of lipid metabolism-related peptide hormones in pest management.

P-S6K is associated with insect diapause via the ROS/AKT/ S6K/CREB/HIF-1 pathway in the cotton bollworm, Helicoverpa armigera

Diapause is a complex physiological response that allows insects to survive unfavorable environmental conditions, and many signaling pathways participate in regulating this process. However, little is known about TOR signaling in the regulation of diapause. In this study, we found that the TOR pathway-related proteins TOR and Raptor are expressed at low levels in the brains of diapause-destined pupae of Helicoverpa armigera, consistent with a previous report that TOR signaling is associated with development. Interestingly, another TOR signaling-related protein, p-S6K, was increased in the brains of diapause-destined pupae. Our results showed that p-S6K in the brains of diapause-destined pupae can respond to the upstream signals reactive oxygen species (ROS) and AKT and that S6K activates the level of CREB, which binds to the HIF-1α promoter and increases its expression. Previous study has shown that HIF-1α levels elevated by ROS in the brains of diapause-destined pupae cause low mitochondrial activity for insect diapause. Thus, p-S6K in response to ROS/AKT regulates HIF-1α via activating transcription factor CREB for diapause initiation.

The Adipokinetic Peptides in Diptera: Structure, Function, and Evolutionary Trends

Nineteen species of various families of the order Diptera and one species from the order Mecoptera are investigated with mass spectrometry for the presence and primary structure of putative adipokinetic hormones (AKHs). Additionally, the peptide structure of putative AKHs in other Diptera are deduced from data mining of publicly available genomic or transcriptomic data. The study aims to demonstrate the structural biodiversity of AKHs in this insect order and also possible evolutionary trends. Sequence analysis of AKHs is achieved by liquid chromatography coupled to mass spectrometry. The corpora cardiaca of almost all dipteran species contain AKH octapeptides, a decapeptide is an exception found only in one species. In general, the dipteran AKHs are order-specific- they are not found in any other insect order with two exceptions only. Four novel AKHs are revealed by mass spectrometry: two in the basal infraorder of Tipulomorpha and two in the brachyceran family Syrphidae. Data mining revealed another four novel AKHs: one in various species of the infraorder Culicumorpha, one in the brachyceran superfamily Asiloidea, one in the family Diopsidae and in a Drosophilidae species, and the last of the novel AKHs is found in yet another Drosophila. In general, there is quite a biodiversity in the lower Diptera, whereas the majority of the cyclorraphan Brachycera produce the octapeptide Phote-HrTH. A hypothetical molecular peptide evolution of dipteran AKHs is suggested to start with an ancestral AKH, such as Glomo-AKH, from which all other AKHs in Diptera to date can evolve via point mutation of one of the base triplets, with one exception.

Recent advances in neuropeptide signaling in Drosophila, from genes to physiology and behavior

This review focuses on neuropeptides and peptide hormones, the largest and most diverse class of neuroactive substances, known in Drosophila and other animals to play roles in almost all aspects of daily life, as w;1;ell as in developmental processes. We provide an update on novel neuropeptides and receptors identified in the last decade, and highlight progress in analysis of neuropeptide signaling in Drosophila. Especially exciting is the huge amount of work published on novel functions of neuropeptides and peptide hormones in Drosophila, largely due to the rapid developments of powerful genetic methods, imaging techniques and innovative assays. We critically discuss the roles of peptides in olfaction, taste, foraging, feeding, clock function/sleep, aggression, mating/reproduction, learning and other behaviors, as well as in regulation of development, growth, metabolic and water homeostasis, stress responses, fecundity, and lifespan. We furthermore provide novel information on neuropeptide distribution and organization of peptidergic systems, as well as the phylogenetic relations between Drosophila neuropeptides and those of other phyla, including mammals. As will be shown, neuropeptide signaling is phylogenetically ancient, and not only are the structures of the peptides, precursors and receptors conserved over evolution, but also many functions of neuropeptide signaling in physiology and behavior.

Adipokinetic Hormone Receptor Mediates Trehalose Homeostasis to Promote Vitellogenin Uptake by Oocytes in Nilaparvata lugens

Adipokinetic hormones (AKHs) are well known to mobilize lipids and carbohydrates for energy-consuming activities in insects. These neuropeptides exert their functions by interacting with AKH receptors (AKHRs) located on the plasma membrane of fat body cells, which regulates energy mobilization by stimulating lipolysis of triacylglycerols (TAG) to diacylglycerols (DAG) and conversion of glycogen into trehalose. Here, we investigated the roles of AKH/AKHR signaling system in trehalose metabolism and vitellogenesis during female reproduction in the brown planthopper, Nilaparvata lugens. Knockdown of AKHR expression by RNA interference (RNAi) resulted in a decrease of the circulating trehalose in hemolymph and significantly increased levels of two trehalases in fat bodies, indicating that the modulation of hemolymph trehalose levels by AKHR may be mediated by regulating trehalose degradation. In addition, adult females that had been injected with double-stranded RNA (dsRNA) for AKHR exhibited delayed oocyte maturation, prolonged pre-oviposition period, as well as decline in egg number and reduction in fecundity. Considering that these phenotypes resulting from AKHR silencing are similar to those of vitellogenin receptor (VgR) RNAi, we further analyzed a possible connection between AKHR and vitellogenesis. Knockdown of AKHR showed no effects on the Vg synthesis in fat bodies, whereas it significantly reduced the levels of VgR in ovaries. With RNAi-females, we observed an increase of Vg accumulation in hemolymph and a decrease of Vg deposition in ovaries. Moreover, the decrease in VgR expression and Vg incorporation by developing oocytes could be partially rescued by injection of trehalose into AKHR RNAi females. The present study has implicated trehalose in the AKH/AKHR signaling-mediated control of reproduction and provided new insight into mechanisms of AKH/AKHR regulation of trehalose metabolism in insect vitellogenesis, oocyte maturation and fecundity.

Adipokinetic Hormone Receptor Mediates Lipid Mobilization to Regulate Starvation Resistance in the Brown Planthopper, Nilaparvata lugens

Lipid storage must be efficiently mobilized to sustain the energy demands during processes of exercise or starvation. In insects, adipokinetic hormone (AKH) and brummer lipase are well-known regulators of lipid mobilization. We recently demonstrated that brummer-dependent lipolysis regulates starvation resistance in the brown planthopper, Nilaparvata lugens, one of the most destructive rice pests. The present work investigated the roles of the AKH signaling system in lipid mobilization during the starvation process in N. lugens. NlAKHR is a typical G protein-coupled receptor (GPCR) and possesses high structure and sequence similarity to other insect AKHRs. Spatial and developmental expression profiles suggested that NlAKH is released from the corpora cardiaca to activate NlAKHR mainly expressed in the fat body. Starvation significantly induced the expression of NlAKH and NlAKHR, indicating a potential role of the AKH signaling system in starvation resistance. To reveal the functions of the AKH signaling system, a double-stranded RNA (dsRNA)-mediated knockdown of NlAKHR and NlAKH peptide injection was performed. The results show NlAKHR silencing decreased the levels of 1,2-diacylglycerol (DAG) in the hemolymph and increased triacylglycerol (TAG) levels in the fat body, whereas NlAKH injection led to a critical accumulation of DAG in the hemolymph and a severe reduction of TAG content in the fat body. Knockdown of NlAKHR resulted in prolonged lifespan and high levels of whole-body TAG, indicating an inability to mobilize TAG reserves during starvation. Conversely, the NlAKH injection reduced the survival and accelerated TAG mobilization during starvation, which further confirms the role of NlAKH in lipolysis. Moreover, NlAKHR silencing caused obesity in N. lugens, whereas NlAKH injection depleted organismal TAG reserves in vivo and produced a slim phenotype. These results indicate that lipid mobilization is regulated by the AKH signaling system, which is essential for adjusting body lipid homeostasis and ensuring energy supplement during starvation in N. lugens.

Lipid metabolism in insect disease vectors

More than a third of the world population is at constant risk of contracting some insect-transmitted disease, such as Dengue fever, Zika virus disease, malaria, Chagas' disease, African trypanosomiasis, and others. Independent of the life cycle of the pathogen causing the disease, the insect vector hematophagous habit is a common and crucial trait for the transmission of all these diseases. This lifestyle is unique, as hematophagous insects feed on blood, a diet that is rich in protein but relatively poor in lipids and carbohydrates, in huge amounts and low frequency. Another unique feature of these insects is that blood meal triggers essential metabolic processes, as molting and oogenesis and, in this way, regulates the expression of various genes that are involved in these events. In this paper, we review current knowledge of the physiology and biochemistry of lipid metabolism in insect disease vectors, comparing with classical models whenever possible. We address lipid digestion and absorption, hemolymphatic transport, and lipid storage by the fat body and ovary. In this context, both de novo fatty acid and triacylglycerol synthesis are discussed, including the related fatty acid activation process and the intracellular lipid binding proteins. As lipids are stored in order to be mobilized later on, e.g. for flight activity or survivorship, lipolysis and β-oxidation are also considered. All these events need to be finely regulated, and the role of hormones in this control is summarized. Finally, we also review information about infection, when vector insect physiology is affected, and there is a crosstalk between its immune system and lipid metabolism. There is not abundant information about lipid metabolism in vector insects, and significant current gaps in the field are indicated, as well as questions to be answered in the future.

The evolution and nomenclature of GnRH-type and corazonin-type neuropeptide signaling systems

Gonadotropin-releasing hormone (GnRH) was first discovered in mammals on account of its effect in triggering pituitary release of gonadotropins and the importance of this discovery was recognized forty years ago in the award of the 1977 Nobel Prize for Physiology or Medicine. Investigation of the evolution of GnRH revealed that GnRH-type signaling systems occur throughout the chordates, including agnathans (e.g. lampreys) and urochordates (e.g. sea squirts). Furthermore, the discovery that adipokinetic hormone (AKH) is the ligand for a GnRH-type receptor in the arthropod Drosophila melanogaster provided evidence of the antiquity of GnRH-type signaling. However, the occurrence of other AKH-like peptides in arthropods, which include corazonin and AKH/corazonin-related peptide (ACP), has complicated efforts to reconstruct the evolutionary history of this family of related neuropeptides. Genome/transcriptome sequencing has revealed that both GnRH-type receptors and corazonin-type receptors occur in lophotrochozoan protostomes (annelids, mollusks) and in deuterostomian invertebrates (cephalochordates, hemichordates, echinoderms). Furthermore, peptides that act as ligands for GnRH-type and corazonin-type receptors have been identified in mollusks. However, what has been lacking is experimental evidence that distinct GnRH-type and corazonin-type peptide-receptor signaling pathways occur in deuterostomes. Importantly, we recently reported the identification of two neuropeptides that act as ligands for either a GnRH-type receptor or a corazonin-type receptor in an echinoderm species - the common European starfish Asterias rubens. Discovery of distinct GnRH-type and corazonin-type signaling pathways in this deuterostomian invertebrate has demonstrated for the first time that the evolutionarily origin of these paralogous systems can be traced to the common ancestor of protostomes and deuterostomes. Furthermore, lineage-specific losses of corazonin signaling (in vertebrates, urochordates and nematodes) and duplication of the GnRH signaling system in arthropods (giving rise to the AKH and ACP signaling systems) and quadruplication of the GnRH signaling system in vertebrates (followed by lineage-specific losses or duplications) accounts for the phylogenetic distribution of GnRH/corazonin-type peptide-receptor pathways in extant animals. Informed by these new insights, here we review the history of research on the evolution of GnRH/corazonin-type neuropeptide signaling. Furthermore, we propose a standardized nomenclature for GnRH/corazonin-type neuropeptides wherein peptides are either named "GnRH" or "corazonin", with the exception of the paralogous GnRH-type peptides that have arisen by gene duplication in the arthropod lineage and which are referred to as "AKH" (or red pigment concentrating hormone, "RCPH", in crustaceans) and "ACP".

Ecdysis-related pleiotropic neuropeptides expression during Anopheles albimanus development

OBJECTIVE

To analyze the transcription pattern of neuropeptides in the ontogeny of a malaria vector, the mosquito Anopheles albimanus.

MATERIALS AND METHODS

The transcription pattern of Crustacean CardioActive peptide (CCAP), corazonin, Ecdysis Triggering Hormone (ETH), allatostatin-A, orcokinin, Insulin Like Peptide 2 (ILP2), Insulin Like Peptide 5 (ILP5) and bursicon was evaluated using qPCR on larvae (1st - 4th instar), pupae and adult mosquitoes.

RESULTS

Unlike in other insects, transcripts of CCAP (70.8%), ETH (60.2%) and corazonin (76.5%) were expressed in 4th instar larvae, probably because these three neuropeptides are associated with the beginning of ecdysis. The neuropeptide ILP2 showed higher transcription levels in other stages and orcokinin decreased during the development of the mosquito.

CONCLUSIONS

The CCAP, corazonin and ETH neuropeptidesare potential targets for the design of control strategies aimed at disrupting An. albiamnus larval development.

Distinct physiological, biochemical and morphometric adjustments in the malaria vectors Anopheles gambiae and A. coluzzii as means to survive dry season conditions in Burkina Faso

Aestivation and dispersive migration are the two strategies evoked in the literature to explain the way in which malaria vectors Anopheles coluzzii and A. gambiae survive the harsh climatic conditions of the dry season in sub-Saharan Africa. However, the physiological mechanisms regulating these two strategies are unknown. In the present study, mosquito species were exposed to controlled environmental conditions mimicking the rainy and dry seasons of south western Burkina Faso. Survival strategies were studied through morphometric (wing length), ecophysiological (respiratory gas exchanges), biochemical (cuticular hydrocarbons composition) and molecular (AKH mRNA expression levels) parameters, variations of which are usually considered to be hallmarks of aestivation and dispersion mechanisms in various insects. Our results showed that ecophysiological and morphometric adjustments are made in both species to prevent water losses during the dry season. However, the usual metabolic rate modifications expected as signatures of aestivation and migration were not observed, highlighting specific and original physiological mechanisms sustaining survival in malaria mosquitoes during the dry season. Differences in epicuticular hydrocarbon composition and AKH levels of expression were found between the permanent and temporary A. coluzzii populations, illustrating the great phenotypic plasticity of this mosquito species. Altogether, our work underlines the diverse and complex pattern of changes occurring in the two mosquito species and at the population level to cope with the dry season and highlights potential targets of future control tools.

Molecular identification, transcript expression, and functional deorphanization of the adipokinetic hormone/corazonin-related peptide receptor in the disease vector, Aedes aegypti

The recently discovered adipokinetic hormone/corazonin-related peptide (ACP) is an insect neuropeptide structurally intermediate between corazonin (CRZ) and adipokinetic (AKH) hormones, which all demonstrate homology to the vertebrate gonadotropin-releasing hormone (GnRH). To date, the function of the ACP signaling system remains unclear. In the present study, we molecularly identified the complete open reading frame encoding the Aedes aegypti ACP receptor (ACPR), which spans nine exons and undergoes alternative splicing giving rise to three transcript variants. Only a single variant, AedaeACPR-I, yielding a deduced 577 residue protein, contains all seven transmembrane domains characteristic of rhodopsin-like G protein-coupled receptors. Functional deorphanization of AedaeACPR-I using a heterologous cell culture-based system revealed highly-selective and dose-dependent receptor activation by AedaeACP (EC₅₀ = 10.25 nM). Analysis of the AedaeACPR-I and AedaeACP transcript levels in all post-embryonic developmental stages using quantitative RT-PCR identified enrichment of both transcripts after adult eclosion. Tissue-specific expression profiling in adult mosquitoes reveals expression of the AedaeACPR-I receptor transcript in the central nervous system, including significant enrichment within the abdominal ganglia. Further, the AedaeACP transcript is prominently detected within the brain and thoracic ganglia. Collectively, these results indicate a neuromodulator or neurotransmitter role for ACP and suggest this neuropeptide may function in regulation of post-ecdysis activities.

Adipokinetic hormone receptor gene identification and its role in triacylglycerol mobilization and sexual behavior in the oriental fruit fly (Bactrocera dorsalis)

Energy homeostasis requires continuous compensation for fluctuations in energy expenditure and availability of food resources. In insects, energy mobilization is under control of the adipokinetic hormone (AKH) where it is regulating the nutritional status by supporting the mobilization of lipids. In this study, we characterized the gene coding for the AKH receptor (AKHR) and investigated its function in the oriental fruit fly (Bactrocera dorsalis) that is economically one of the most important pest insects of tropical and subtropical fruit. Bacdo-AKHR is a typical G protein-coupled receptor (GPCR) and phylogenetic analysis confirmed that Bacdo-AKHR is closely related to insect AKHRs from other species. When expressed in Chinese hamster ovary (CHO) cells, Bacdo-AKHR exhibited a high sensitivity and selectivity for AKH peptide (EC₅₀ = 19.3 nM). Using qPCR, the developmental stage and tissue-specific expression profiles demonstrated that Bacdo-AKHR was highly expressed in both the larval and adult stages, and also specifically in the fat body and midgut of the adult with no difference in sex. To investigate the role of AKHR in B. dorsalis, RNAi assays were performed with dsRNA against Bacdo-AKHR in adult flies of both sexes and under starvation and feeding condition. As major results, the knockdown of this gene resulted in triacylglycerol (TAG) accumulation. With RNAi-males, we observed a severe decrease in their sexual courtship activity when starved, but there was a partial rescue in copulation when refed. Also in RNAi-males, the tethered-flight duration declined compared with the control group when starved, which is confirming the dependency on energy metabolism. In RNAi-females, the sexual behavior was not affected, but their fecundity was decreased. Our findings indicate an interesting role of AKHR in the sexual behavior of males specifically. The effects are associated with TAG accumulation, and we also reported that the conserved role of AKH-mediated system in B. dorsalis is nutritional state-dependent. Hence, we provided further understanding on the multiple functions of AKH/AKHR in B. dorsalis.

Comparative physiological plasticity to desiccation in distinct populations of the malarial mosquito Anopheles coluzzii

Background

In West Africa, populations of the malaria vector mosquito, Anopheles coluzzii, are seasonally exposed to strong desiccating conditions during the dry season. Their dynamics strictly follows the pace of the availability of suitable larval development sites (water collections). Accordingly, mosquitoes can reproduce all year long where permanent breeding is possible, or stop reproduction and virtually disappear at the onset of the dry season when surface water dries up, like observed in temporary habitats of dry savannah areas. This highlights the strong adaptive abilities of this mosquito species, which relies at least in part, upon physiological and molecular mechanisms of specific signatures.

Methods

Here, we analysed a range of physiological and molecular responses expressed by geographically different populations of An. coluzzii inhabiting permanent and temporary breeding sites from the north and the south-west of Burkina Faso. Four mosquito colonies, namely (i) Oursi, built from females breeding in permanent habitats of the north; (ii) Déou, from temporary northern habitats; (iii) Soumousso from south-western temporary breeding sites; and (iv) Bama, from permanent habitats of the same south-western zone, were reared in climatic chambers under contrasted environmental conditions, mimicking temperature, relative humidity and light regimen occurring in northern Burkina Faso. Female mosquitoes were analysed for the seasonal variation in their amounts of proteins, triglycerides and free-circulating metabolites. The expression level of genes coding for the adipokinetic (AKH-I) and the AKH/corazonin-related peptides (ACP) were also assessed and compared among populations and environmental conditions.

Results

Our analysis did not reveal an apparent pattern of physiological and molecular variations strictly correlated with either the larval ecotype or the geographical origin of the mosquitoes. However, specific distinct responses were observed among populations, suggesting that dry season survival may rely on more complex ecological parameters at a micro-habitat scale. Interestingly, the physiological and molecular data support the hypothesis that different aestivation abilities exist among populations of An. coluzzii inhabiting contrasted ecological settings. In particular, the striking metabotypes differentiation and the AKH mRNA expression level observed in females from temporary northern populations may suggest the existence of a “strong” aestivation strategy in these specimens.

Conclusion

Our work provides insights into the physiological and molecular basis of dry and rainy season responses in An. coluzzii, and highlights the important diversity of the mechanisms involved. Such results represent key data for understanding the ecophysiological mechanisms underpinning the strong adaptive potential of this malaria vector species, which undoubtedly contributes to the spreading of mosquito distribution areas in space and time.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1854-1) contains supplementary material, which is available to authorized users.

Mosquito Peptide Hormones: Diversity, Production, and Function

Mosquitoes, like other insects, produce a diversity of peptide hormones that are processed from different precursor proteins and have a range of activities. Early studies relied on purification of bioactive peptides for hormone identification, but more recently genomic data have provided the information needed to more comprehensively identify peptide hormone genes and associated receptors. The first part of this chapter summarizes the known or predicted peptide hormones that are produced by mosquitoes. The second part of this chapter discusses the sources of these molecules and their functions.

Characterization of the adipokinetic hormone receptor of the anautogenous flesh fly, Sarcophaga crassipalpis

Adipokinetic hormone (AKH) is an insect neuropeptide mainly involved in fat body energy mobilization. In flies (Phormia regina, Sarcophaga crassipalpis), bugs (Pyrrhocoris apterus) and cockroaches (Periplaneta americana) AKH was also demonstrated to be involved in the regulation of digestion. This makes AKH an important peptide for anautogenous female flies that need to feed on a supplementary protein meal to initiate vitellogenesis, the large scale synthesis of yolk proteins and their uptake by the developing oocytes. Flesh fly AKH, originally identified as Phormia terraenovae hypertrehalosemic hormone (PhoteHrTH), functions through activation of the AKH receptor (AKHR). This is a G protein-coupled receptor that is the orthologue of the human gonadotropin-releasing hormone receptor. Pharmacological characterization indicated that the receptor can be activated by two related dipteran AKH ligands with an EC50 value in the low nanomolar range, whereas micromolar concentrations of the Tribolium castaneum AKH were needed. Consistent with the energy mobilizing function of AKH, the receptor transcript levels were most abundant in the fat body tissue. Nonetheless, Sarcophaga crassipalpis AKHR transcript levels were also high in the brain, the foregut and the hindgut. Interestingly, the receptor transcript numbers were reduced in almost all measured tissues after protein feeding. These changes may enforce the use of ingested energy carrying molecules prior to stored energy mobilization.

Characterization and pharmacological analysis of two adipokinetic hormone receptor variants of the tsetse fly, Glossina morsitans morsitans

Adipokinetic hormones (AKH) are well known regulators of energy metabolism in insects. These neuropeptides are produced in the corpora cardiaca and perform their hormonal function by interacting with specific G protein-coupled receptors (GPCRs) at the cell membranes of target tissues, mainly the fat body. Here, we investigated the sequences, spatial and temporal distributions, and pharmacology of AKH neuropeptides and receptors in the tsetse fly, Glossina morsitans morsitans. The open reading frames of two splice variants of the Glomo-akh receptor (Glomo-akhr) gene and of the AKH neuropeptide encoding genes, gmmhrth and gmmakh, were cloned. Both tsetse AKHR isoforms show strong sequence conservation when compared to other insect AKHRs. Glomo-AKH prepropeptides also have the typical architecture of AKH precursors. In an in vitro Ca(2+) mobilization assay, Glomo-AKH neuropeptides activated each receptor isoform up to nanomolar concentrations. We identified structural features of tsetse AKH neuropeptides essential for receptor activation in vitro. Gene expression profiles suggest a function for AKH signaling in regulating Glossina energy metabolism, where AKH peptides are released from the corpora cardiaca and activate receptors mainly expressed in the fat body. This analysis of the ligand-receptor coupling, expression, and pharmacology of the two Glomo-AKHR variants facilitates further elucidation of the function of AKH in G. m. morsitans.

Adipokinetic hormone receptor gene identification and its role in triacylglycerol metabolism in the blood-sucking insect Rhodnius prolixus

Adipokinetic hormone (AKH) has been associated with the control of energy metabolism in a large number of arthropod species due to its role on the stimulation of lipid, carbohydrate and amino acid mobilization/release. In the insect Rhodnius prolixus, a vector of Chagas' disease, triacylglycerol (TAG) stores must be mobilized to sustain the metabolic requirements during moments of exercise or starvation. Besides the recent identification of the R. prolixus AKH peptide, other components required for the AKH signaling cascade and its mode of action remain uncharacterized in this insect. In the present study, we identified and investigated the expression profile of the gene encoding the AKH receptor of R. prolixus (RhoprAkhr). This gene is highly conserved in comparison to other sequences already described and its transcript is abundant in the fat body and the flight muscle of the kissing bug. Moreover, RhoprAkhr expression is induced in the fat body at moments of increased TAG mobilization; the knockdown of this gene resulted in TAG accumulation both in fat body and flight muscle after starvation. The inhibition of Rhopr-AKHR transcription as well as the treatment of insects with the peptide Rhopr-AKH in its synthetic form altered the transcript levels of two genes involved in lipid metabolism, the acyl-CoA-binding protein-1 (RhoprAcbp1) and the mitochondrial glycerol-3-phosphate acyltransferase-1 (RhoprGpat1). These results indicate that the AKH receptor is regulated at transcriptional level and is required for TAG mobilization under starvation. In addition to the classical view of AKH as a direct regulator of enzymatic activity, we propose here that AKH signaling may account for the regulation of nutrient metabolism by affecting the expression profile of target genes.

Peptidomics of Neuropeptidergic Tissues of the Tsetse Fly Glossina morsitans morsitans

Neuropeptides and peptide hormones are essential signaling molecules that regulate nearly all physiological processes. The recent release of the tsetse fly genome allowed the construction of a detailed in silico neuropeptide database (International Glossina Genome Consortium, Science 344, 380-386 (2014)), as well as an in-depth mass spectrometric analysis of the most important neuropeptidergic tissues of this medically and economically important insect species. Mass spectrometric confirmation of predicted peptides is a vital step in the functional characterization of neuropeptides, as in vivo peptides can be modified, cleaved, or even mispredicted. Using a nanoscale reversed phase liquid chromatography coupled to a Q Exactive Orbitrap mass spectrometer, we detected 51 putative bioactive neuropeptides encoded by 19 precursors: adipokinetic hormone (AKH) I and II, allatostatin A and B, capability/pyrokinin (capa/PK), corazonin, calcitonin-like diuretic hormone (CT/DH), FMRFamide, hugin, leucokinin, myosuppressin, natalisin, neuropeptide-like precursor (NPLP) 1, orcokinin, pigment dispersing factor (PDF), RYamide, SIFamide, short neuropeptide F (sNPF) and tachykinin. In addition, propeptides, truncated and spacer peptides derived from seven additional precursors were found, and include the precursors of allatostatin C, crustacean cardioactive peptide, corticotropin releasing factor-like diuretic hormone (CRF/DH), ecdysis triggering hormone (ETH), ion transport peptide (ITP), neuropeptide F, and proctolin, respectively. The majority of the identified neuropeptides are present in the central nervous system, with only a limited number of peptides in the corpora cardiaca-corpora allata and midgut. Owing to the large number of identified peptides, this study can be used as a reference for comparative studies in other insects. Graphical Abstract ᅟ.

Hormonal Regulation of Response to Oxidative Stress in Insects-An Update

Insects, like other organisms, must deal with a wide variety of potentially challenging environmental factors during the course of their life. An important example of such a challenge is the phenomenon of oxidative stress. This review summarizes the current knowledge on the role of adipokinetic hormones (AKH) as principal stress responsive hormones in insects involved in activation of anti-oxidative stress response pathways. Emphasis is placed on an analysis of oxidative stress experimentally induced by various stressors and monitored by suitable biomarkers, and on detailed characterization of AKH’s role in the anti-stress reactions. These reactions are characterized by a significant increase of AKH levels in the insect body, and by effective reversal of the markers—disturbed by the stressors—after co-application of the stressor with AKH. A plausible mechanism of AKH action in the anti-oxidative stress response is discussed as well: this probably involves simultaneous employment of both protein kinase C and cyclic adenosine 3′,5′-monophosphate pathways in the presence of extra and intra-cellular Ca²⁺ stores, with the possible involvement of the FoxO transcription factors. The role of other insect hormones in the anti-oxidative defense reactions is also discussed.

Conserved molecular switch interactions in modeled cardioactive RF-NH2 peptide receptors: Ligand binding and activation

Peptides may act through G protein-coupled receptors to influence cardiovascular performance; thus, delineating mechanisms involved in signaling is a molecular-based strategy to influence health. Molecular switches, often represented by conserved motifs, maintain a receptor in an inactive state. However, once the switch is broken, the transmembrane regions move and activation occurs. The molecular switches of Drosophila melanogaster myosuppressin (MS) receptors were previously identified to include a unique ionic lock and novel 3-6 lock, as well as transmission and tyrosine toggle switches. In addition to MS, cardioactive ligands structurally related by a C-terminal RF-NH2 include sulfakinin, neuropeptide F (NPF), short NPF, and FMRF-NH2-containing peptide subfamilies. We hypothesized receptor molecular switch motifs were conserved within a RF-NH2 subfamily and across species. Thus, we investigated RF-NH2 receptor (RFa-R) molecular switches in D. melanogaster, Tribolium castaneum, Anopheles gambiae, Rhodnius prolixus, and Bombyx mori. Adipokinetic hormone (AKH), which does not contain a RF-NH2, was also examined. The tyrosine toggle switch and ionic lock showed a higher degree of conservation within a RF-NH2 subfamily than the transmission switch and 3-7 lock. AKH receptor motifs were not representative of a RF-NH2 subfamily. The motifs and interactions of switches in the RFa-Rs were consistent with receptor activation and ligand-specific binding.

Adipokinetic hormone signalling system in the Chagas disease vector, Rhodnius prolixus

Neuropeptides and their G protein-coupled receptors are widespread throughout Metazoa and in several cases, clear orthologues can be identified in both protostomes and deuterostomes. One such neuropeptide is the insect adipokinetic hormone (AKH), which is related to the mammalian gonadotropin-releasing hormone. AKH has been studied extensively and is known to mobilize lipid, carbohydrates and proline for energy-consuming activities such as flight. In order to determine the possible roles for this signalling system in Rhodnius prolixus, we isolated the cDNA sequences encoding R. prolixus AKH (Rhopr-AKH) and its receptor (Rhopr-AKHR). We also examined their spatial expression pattern using quantitative PCR. Our expression analysis indicates that Rhopr-AKH is only expressed in the corpus cardiacum of fifth-instars and adults. Rhopr-AKHR, by contrast, is expressed in several peripheral tissues including the fat body. The expression of the receptor in the fat body suggests that AKH is involved in lipid mobilization, which was confirmed by knockdown of Rhopr-AKHR via RNA interference. Adult males that had been injected with double-stranded RNA (dsRNA) for Rhopr-AKHR exhibited increased lipid content in the fat body and decreased lipid levels in the haemolymph. Moreover, injection of Rhopr-AKH in Rhopr-AKHR dsRNA-treated males failed to elevate haemolymph lipid levels, confirming that this is indeed the receptor for Rhopr-AKH.

Molecular characterization, tissue distribution, and ultrastructural localization of adipokinetic hormones in the CNS of the firebug Pyrrhocoris apterus (Heteroptera, Insecta)

Adipokinetic hormones (AKHs) are a group of insect metabolic neurohormones, synthesized and released from an endocrine retrocerebral gland, the corpus cardiacum (CC). Small amounts of AKH have also been identified in the brain, although their role in this organ is not clear. To address this gap in the knowledge about insect brain biology, we studied the nucleotide sequence, tissue distribution, and subcellular localization of AKHs in the brain and CC of the firebug Pyrrhocoris apterus. This insect expresses two AKHs; the octapeptides Pyrap-AKH and Peram-CAH-II, the presence of which was documented in the both studied organs. In situ hybridization and quantitative reverse-transcription (q-RT)-PCR revealed the expression of the genes encoding for both AKHs not only in the CC, but also in brain. Electron microscopy analysis of the brain revealed the presence of these hormones in specialized secretory granules localized predominantly in the cellular bodies of neurons. The hormones might be transported from the granules into the axons, where they could play a role in neuronal signaling. Under acute stress induced by the injection of 3μmol KCl, the level of AKHs in the brain increased to a greater extent than that in the CC. These results might indicate an enhanced role of brain-derived AKHs in defence reaction under acute stress situations.

Reprint of "The distribution and physiological effects of three evolutionarily and sequence-related neuropeptides in Rhodnius prolixus: Adipokinetic hormone, corazonin and adipokinetic hormone/corazonin-related peptide"

We have examined the distribution and physiological effects of three evolutionarily and sequence-related neuropeptides in Rhodnius prolixus. These neuropeptides, adipokinetic hormone (RhoprAKH), corazonin (CRZ) and adipokinetic hormone/corazonin-related peptide (RhoprACP) are present in distinct, non-overlapping neuronal subsets in the central nervous system (CNS), as determined by immunohistochemistry. Corazonin-like immunoreactive cell bodies are present in the brain and ventral nerve cord, whereas ACP-like immunoreactive cell bodies are only present in the brain, and AKH-like immunoreactive cell bodies only present in the corpus cardiacum (CC). The immunoreactivity to ACP, CRZ and AKH in R. prolixus suggests that ACP and CRZ are released within the CNS, and that CRZ and AKH are released as neurohormones from the CC. Injection of RhoprAKH into adult males elevated haemolymph lipid levels, but injection of CRZ or RhoprACP failed to have any effect on haemolymph lipid levels. Corazonin stimulated an increase in heart-beat frequency in vitro, but RhoprAKH and RhoprACP failed to do so. Thus, although all three neuropeptides share sequence similarity, the AKH and CRZ receptors only respond to their own ligand.

Identification and characterization of seminal fluid proteins in the Asian tiger mosquito, Aedes albopictus

The Asian tiger mosquito (Aedes albopictus) is an important vector for pathogens that affect human health, including the viruses that cause dengue and Chikungunya fevers. It is also one of the world's fastest-spreading invasive species. For these reasons, it is crucial to identify strategies for controlling the reproduction and spread of this mosquito. During mating, seminal fluid proteins (Sfps) are transferred from male mosquitoes to females, and these Sfps modulate female behavior and physiology in ways that influence reproduction. Despite the importance of Sfps on female reproductive behavior in mosquitoes and other insects, the identity of Sfps in Ae. albopictus has not previously been reported. We used transcriptomics and proteomics to identify 198 Sfps in Ae. albopictus. We discuss possible functions of these Sfps in relation to Ae. albopictus reproduction-related biology. We additionally compare the sequences of these Sfps with proteins (including reported Sfps) in several other species, including Ae. aegypti. While only 72 (36.4%) of Ae. albopictus Sfps have putative orthologs in Ae. aegypti, suggesting low conservation of the complement of Sfps in these species, we find no evidence for an elevated rate of evolution or positive selection in the Sfps that are shared between the two Aedes species, suggesting high sequence conservation of those shared Sfps. Our results provide a foundation for future studies to investigate the roles of individual Sfps on feeding and reproduction in this mosquito. Functional analysis of these Sfps could inform strategies for managing the rate of pathogen transmission by Ae. albopictus.

The distribution and physiological effects of three evolutionarily and sequence-related neuropeptides in Rhodnius prolixus: Adipokinetic hormone, corazonin and adipokinetic hormone/corazonin-related peptide

We have examined the distribution and physiological effects of three evolutionarily and sequence-related neuropeptides in Rhodnius prolixus. These neuropeptides, adipokinetic hormone (RhoprAKH), corazonin (CRZ) and adipokinetic hormone/corazonin-related peptide (RhoprACP) are present in distinct, non-overlapping neuronal subsets in the central nervous system (CNS), as determined by immunohistochemistry. Corazonin-like immunoreactive cell bodies are present in the brain and ventral nerve cord, whereas ACP-like immunoreactive cell bodies are only present in the brain, and AKH-like immunoreactive cell bodies only present in the corpus cardiacum (CC). The immunoreactivity to ACP, CRZ and AKH in R. prolixus suggests that ACP and CRZ are released within the CNS, and that CRZ and AKH are released as neurohormones from the CC. Injection of RhoprAKH into adult males elevated haemolymph lipid levels, but injection of CRZ or RhoprACP failed to have any effect on haemolymph lipid levels. Corazonin stimulated an increase in heart-beat frequency in vitro, but RhoprAKH and RhoprACP failed to do so. Thus, although all three neuropeptides share sequence similarity, the AKH and CRZ receptors only respond to their own ligand.

Role of adipokinetic hormone in stimulation of salivary gland activities: the fire bug Pyrrhocoris apterus L. (Heteroptera) as a model species

The effect of adipokinetic hormone (Pyrap-AKH) in stimulating the function of insect salivary glands (SGs) in extra-oral digestive processes was studied in the firebug, Pyrrhocoris apterus L. (Heteroptera). The analyses were performed on samples of SGs and extracts of linden seeds, a natural source of the bug's food. The SGs from 3-day old P. apterus females (when the food ingestion culminates), primarily contained polygalacturonase (PG) enzyme activity, whereas the level of lipase, peptidase, amylase and α-glucosidase was negligible. The transcription of PG mRNA and enzymatic activity were significantly increased in SGs after Pyrap-AKH treatment. The piercing and sucking of linden seeds by the bugs stimulated the intrinsic enzymatic cocktail of seeds (lipase, peptidase, amylase, glucosidase), and moreover the activity of these enzymes was significantly enhanced when the seeds were fed on by the Pyrap-AKH treated bugs. Similarly, a significant increase in PG activity was recorded in linden seeds fed on by hormonally-treated bugs or when injected by SG extract from hormonally treated ones as compared to untreated controls. The mechanism of AKH action in SGs is unknown, but likely involves cAMP (and excludes cGMP) as a second messenger, since the content of this compound doubled in SGs after Pyrap-AKH treatment. This new and as yet undescribed function of AKH in SGs is compared with the effect of this hormone on digestive processes in the midgut elucidated earlier.

Adipokinetic hormone exerts its anti-oxidative effects using a conserved signal-transduction mechanism involving both PKC and cAMP by mobilizing extra- and intracellular Ca2+ stores

The involvement of members of the adipokinetic hormone (AKH) family in regulation of response to oxidative stress (OS) has been reported recently. However, despite these neuropeptides being the best studied family of insect hormones, their precise signaling pathways in their OS responsive role remain to be elucidated. In this study, we have used an in vitro assay to determine the importance of extra and intra-cellular Ca(2+) stores as well as the involvement of protein kinase C (PKC) and cyclic adenosine 3',5'-monophosphate (cAMP) pathways by which AKH exerts its anti-oxidative effects. Lipid peroxidation product (4-HNE) was significantly enhanced and membrane fluidity reduced in microsomal fractions of isolated brains (CNS) of Pyrrhocoris apterus when treated with hydrogen peroxide (H2O2), whereas these biomarkers of OS were reduced to control levels when H2O2 was co-treated with Pyrap-AKH. The effects of mitigation of OS in isolated CNS by AKH were negated when these treatments were conducted in the presence of Ca(2+) channel inhibitors (CdCl2 and thapsigargin). Presence of either bisindolylmaliemide or chelyrythrine chloride (inhibitors of PKC) in the incubating medium also compromised the anti-oxidative function of AKH. However, supplementing the medium with either phorbol myristate acetate (PMA, an activator of PKC) or forskolin (an activator of cAMP) restored the protective effects of exogenous AKH treatment by reducing 4-HNE levels and increasing membrane fluidity to control levels. Taken together, our results strongly implicate the importance of both PKC and cAMP pathways in AKHs' anti-oxidative action by mobilizing both extra and intra-cellular stores of Ca(2+).

Anopheles gambiae, Anoga-HrTH hormone, free and bound structure--a nuclear magnetic resonance experiment

The spread of malaria by the female mosquito, Anopheles gambiae, is dependent, amongst other things, on its ability to fly. This in turn, is dependent on the adipokinetic hormone, Anoga-HrTH (pGlu-Leu-Thr-Phe-Thr-Pro-Ala-Trp-NH2). No crystal structure of this important neuropeptide is available and hence NMR restrained molecular dynamics was used to investigate its conformational space in aqueous solution and when bound to a membrane surface. The results showed that Anoga-HrTH has an almost cyclic conformation that is stabilized by a hydrogen bond between the C-terminus and Thr3. Upon docking of the agonist to its receptor, this H-bond is broken and the molecule adopts a more extended structure. Preliminary AKHR docking calculations give the free energy of binding to be -47.30 kJ/mol. There is a close correspondence between the structure of the docked ligand and literature structure-activity studies. Information about the 3D structure and binding mode of Anoga-HrTH to its receptor is vital for the design of suitable mimetics which can act as insecticides.

Unique roles of glucagon and glucagon-like peptides: Parallels in understanding the functions of adipokinetic hormones in stress responses in insects

Glucagon is conventionally regarded as a hormone, counter regulatory in function to insulin and plays a critical anti-hypoglycemic role by maintaining glucose homeostasis in both animals and humans. Glucagon performs this function by increasing hepatic glucose output to the blood by stimulating glycogenolysis and gluconeogenesis in response to starvation. Additionally it plays a homeostatic role by decreasing glycogenesis and glycolysis in tandem to try and maintain optimal glucose levels. To perform this action, it also increases energy expenditure which is contrary to what one would expect and has actions which are unique and not entirely in agreement with its role in protection from hypoglycemia. Interestingly, glucagon-like peptides (GLP-1 and GLP-2) from the major fragment of proglucagon (in non-mammalian vertebrates, as well as in mammals) may also modulate response to stress in addition to their other physiological actions. These unique modes of action occur in response to psychological, metabolic and other stress situations and mirror the role of adipokinetic hormones (AKHs) in insects which perform a similar function. The findings on the anti-stress roles of glucagon and glucagon-like peptides in mammalian and non-mammalian vertebrates may throw light on the multiple stress responsive mechanisms which operate in a concerted manner under regulation by AKH in insects thus functioning as a stress responsive hormone while also maintaining organismal homeostasis.

Knockdown of the adipokinetic hormone receptor increases feeding frequency in the two-spotted cricket Gryllus bimaculatus

Adipokinetic hormone (AKH) is a peptide hormone that regulates the nutritional state in insects by supporting the mobilization of lipids. In the present study, we manipulated AKH signaling to evaluate how metabolic state regulates feeding in an orthopteran insect, the two-spotted cricket, Gryllus bimaculatus. This was accomplished by RNA interference (RNAi) targeting the receptor gene for AKH [G. bimaculatus AKHR (GrybiAKHR)]. We found that the knockdown of GrybiAKHR by AKHR-double-stranded RNA treatment decreased the levels of 1,2-diacylglycerol and trehalose in the hemolymph, whereas it increased the level of triacylglycerol in the fat body. In addition, the knockdown of GrybiAKHR enhanced starvation resistance and increased food intake. Furthermore, direct observation of GrybiAKHR(RNAi) crickets revealed that the knockdown of GrybiAKHR increased feeding frequency but did not alter meal duration, whereas locomotor activity decreased. The increased frequency of feeding by GrybiAKHR(RNAi) crickets eventually resulted in an increase of food intake. These data demonstrate that the regulation of the metabolic state by AKH signaling affects feeding frequency, probably through nutritional control.

Adipokinetic hormones (AKHs) of sphingid Lepidoptera, including the identification of a second M. sexta AKH

The adipokinetic hormones (AKHs) from the corpora cardiaca (CC) of representative species from all three subfamilies of the Sphingidae (hawkmoths) were investigated using matrix-assisted laser desorption-ionization time-of-flight (MALDI-TOF) and liquid chromatography electrospray ion trap mass spectrometry (LC-ESI MS), including a re-examination of the AKH complement of the tobacco hawkmoth, Manduca sexta. In addition to larvae and adults of M. sexta (subfamily: Sphinginae), adults from the following subfamilies were examined: Macroglossinae (large elephant hawkmoth, Deilephila elpenor), Smerinthinae (poplar hawkmoth, Laothoe populi and eyed hawkmoth, Smerinthus ocellata), and Sphinginae (death's head hawkmoth, Acherontia atropos). All moths are shown to have the nonapeptide Manse-AKH (pELTFTSSWGamide) [corrected] in their CC, together with a second AKH, which, on the basis of mass ions ([M+Na](+), [M+K](+)) and partial sequence analysis is identical in all species examined. The structure of this AKH was extracted from the CC [corrected] of adult M. sexta and shown, by ESI-collision-induced dissociation (CID) tandem mass spectrometry (MS/MS), to be a novel decapeptide AKH with a sequence of pELTFSSWGQamide. [corrected]. The new peptide has been code named Manse-AKH-II. Sequence confirmation was obtained from identical MS studies with synthetic Manse-AKH-II and with the native peptide. Manse-AKH-II has significant lipid-mobilizing activity when injected at low dose (5pmol) into newly emerged adult M. sexta. The potential implications of a second AKH, in M. sexta in particular, are discussed in relation to putative receptor(s).

Functional characterization of the adipokinetic hormone in the pea aphid, Acyrthosiphon pisum

Aphids are important plant phloem-sucking pests and detailed knowledge about the hormonal control of their metabolism can potentially contribute to the development of methods for their management. The insect metabolism is predominantly controlled by neuropeptides belonging to the adipokinetic hormone/red pigment-concentrating hormone family (AKH/RPCH). The main goal of this study was to obtain the sequence of AKH transcripts and analyze its expression in all polyphenic female forms of the pea aphid, Acyrthosiphon pisum. The neuropeptide is expressed in the brain of all female forms and in the ovaries of the both (wingless and winged) parthenogenetic forms. The form of active Acypi-AKH decapeptide was confirmed by the LC/MS and +ESI tandem mass spectrometry. The highest relative amount of Acypi-AKH was recorded in winged virginoparae. Furthermore, a potential role of this hormone when directly applied to the aphid was studied as well. Interestingly, no significant increase of trehalose in the wingless virginoparae after application of synthetic Acypi-AKH was detected. Yet this treatment did affect the level of protective polyol (mannitol) and furthermore led to increased activity of the detoxification enzyme glutathione S-transferase. The possible physiological function of AKH in A. pisum under the stress conditions is discussed.

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

This review focuses on the state of the art on neuropeptide receptors in insects. Most of these receptors are G protein-coupled receptors (GPCRs) and are involved in the regulation of virtually all physiological processes during an insect's life. More than 20 years ago a milestone in invertebrate endocrinology was achieved with the characterization of the first insect neuropeptide receptor, i.e., the Drosophila tachykinin-like receptor. However, it took until the release of the Drosophila genome in 2000 that research on neuropeptide receptors boosted. In the last decade a plethora of genomic information of other insect species also became available, leading to a better insight in the functions and evolution of the neuropeptide signaling systems and their intracellular pathways. It became clear that some of these systems are conserved among all insect species, indicating that they fulfill crucial roles in their physiological processes. Meanwhile, other signaling systems seem to be lost in several insect orders or species, suggesting that their actions were superfluous in those insects, or that other neuropeptides have taken over their functions. It is striking that the deorphanization of neuropeptide GPCRs gets much attention, but the subsequent unraveling of the intracellular pathways they elicit, or their physiological functions are often hardly examined. Especially in insects besides Drosophila this information is scarce if not absent. And although great progress made in characterizing neuropeptide signaling systems, even in Drosophila several predicted neuropeptide receptors remain orphan, awaiting for their endogenous ligand to be determined. The present review gives a précis of the insect neuropeptide receptor research of the last two decades. But it has to be emphasized that the work done so far is only the tip of the iceberg and our comprehensive understanding of these important signaling systems will still increase substantially in the coming years.

RNA interference unveils functions of the hypertrehalosemic hormone on cyclic fluctuation of hemolymph trehalose and oviposition in the virgin female Blattella germanica

Hypertrehalosemic hormone (HTH) is a neuropeptide within the adipokinetic hormone (AKH) family that induces a release of trehalose from fat body into hemolymph in a number of insects. In this study, we first showed that female adult German cockroach, Blattella germanica, displayed a cyclic fluctuation of hemolymph trehalose levels correlated to the maturation of oocytes in the reproductive cycle. After cloning the HTH cDNA from the German cockroach (Blage-HTH), expression studies indicated that Blage-HTH mRNA showed the cyclic changes during the first reproductive cycle, where peak values occurred in 8-day-old virgin female cockroaches, which were going to produce oothecae. The functions of Blage-HTH were studied using RNA interference (RNAi) to knockdown its expression. Adult virgin females of B. germanica injected with Blage-HTH dsRNA increased hemolymph trehalose levels in the late period of vitellogenesis more slowly than control. Furthermore, RNAi of Blage-HTH delayed oviposition time and some (10%) individuals did not produce the first ootheca until 15 days after eclosion, whereas the control group produced ootheca before 9 days in all cases.

Genome-wide profiling of diel and circadian gene expression in the malaria vector Anopheles gambiae

Anopheles gambiae, the primary African vector of malaria parasites, exhibits numerous rhythmic behaviors including flight activity, swarming, mating, host seeking, egg laying, and sugar feeding. However, little work has been performed to elucidate the molecular basis for these daily rhythms. To study how gene expression is regulated globally by diel and circadian mechanisms, we have undertaken a DNA microarray analysis of An. gambiae under light/dark cycle (LD) and constant dark (DD) conditions. Adult mated, non-blood-fed female mosquitoes were collected every 4 h for 48 h, and samples were processed with DNA microarrays. Using a cosine wave-fitting algorithm, we identified 1,293 and 600 rhythmic genes with a period length of 20-28 h in the head and body, respectively, under LD conditions, representing 9.7 and 4.5% of the An. gambiae gene set. A majority of these genes was specific to heads or bodies. Examination of mosquitoes under DD conditions revealed that rhythmic programming of the transcriptome is dependent on an interaction between the endogenous clock and extrinsic regulation by the LD cycle. A subset of genes, including the canonical clock components, was expressed rhythmically under both environmental conditions. A majority of genes had peak expression clustered around the day/night transitions, anticipating dawn and dusk. Genes cover diverse biological processes such as transcription/translation, metabolism, detoxification, olfaction, vision, cuticle regulation, and immunity, and include rate-limiting steps in the pathways. This study highlights the fundamental roles that both the circadian clock and light play in the physiology of this important insect vector and suggests targets for intervention.

Open conformation of adipokinetic hormone receptor from the malaria mosquito facilitates hormone binding

Insect flight requires rapid mobilization of energy reserves during flight, which is mediated and regulated by hormonal control via adipokinetic hormones. The structure of the G-protein receptors to which these hormones bind, are crucial in understanding many of the physiological processes in which they play a central role. To date no 3D structure of an insect G-protein coupled receptor (GPCR) is available. Here, the first models of the 3D structures of a GPCR from the malaria mosquito are presented. Homology modeling of the receptor identified from the genome of Anopheles gambiae was used to construct two models of the receptor. The 7 transmembrane helical bundles of these two models are based on the crystal structures of beta2-adrenergic receptor and rhodopsin. The flexible loop regions were modeled using high temperature simulated annealing and constrained molecular dynamic simulations. The two receptor models differ in a number of critical features, the most important of which is that the rhodopsin-based model has a 'closed' structure while the beta2-based structure is 'open'. The 'open' conformation provides easy access of the hormone to the binding pocket. Docking calculations with the insect adipokinetic hormones, AKH-1 (pGlu-Leu-Thr-Phe-Thr-Pro-Ala-Trp-NH(2)) from the malaria mosquito and Del-CC (pGlu-Lys-Asn-Phe-Ser-Pro-Asn-Trp-Gly-Asn-NH(2)) from the blister beetle showed that while the binding motif of the two is similar, AKH-1 has more than 30 times higher affinity than Del-CC, which strongly suggests that the binding is specific, and that the correct binding site was identified. Using these models it is possible to design antagonists, which block the binding site and are thus species-specific insecticides.

Neuropeptide physiology in insects

In a search for more environmentally benign alternatives to chemical pesticides, insect neuropeptides have been suggested as ideal candidates. Neuropeptides are neuromodulators and/or neurohormones that regulate most major physiological and behavioral processes in insects. The major neuropeptide structures have been identified through peptide purification in insects (peptidomics) and insect genome projects. Neuropeptide receptors have been identified and characterized in Drosophila and similar receptors are being targeted in other insects considered to be economically detrimental pests in agriculture and forestry. Defining neuropeptide action in different insect systems has been more challenging and as a consequence, identifying unique targets for potential pest control is also a challenge. In this chapter, neuropeptide biosynthesis as well as select physiological processes are examined with a view to pest control targets. The application of molecular techniques to transform insects with neuropeptide or neuropeptide receptor genes, or knockout genes to identify potential pest control targets, is a relatively new area that offers promise to insect control. Insect immune systems may also be manipulated through neuropeptides which may aid in compromising the insects ability to defend against foreign invasion.

Evolution of GnRH: diving deeper

Gonadotropin-releasing hormone (GnRH) plays a central role in vertebrate reproduction. The evolutionary origin of this neuropeptide and its receptor is not obvious, but the advent of genomics makes it possible to examine the roots of GnRH and delve deeper into its ancestral relationships. New peptide sequences identified in invertebrates from annelids to tunicates reveal GnRH-like peptides of 10-12 amino acids. Structural conservation suggests homology between the 15 known invertebrate peptides and the 15 known vertebrate GnRHs. The functions of the invertebrate GnRH-like peptides are not necessarily related to reproduction. We suggest that structurally related families of invertebrate peptides including corazonin and adipokinetic hormone (AKH) form a superfamily of neuropeptides with the GnRH family. GnRH receptors have also been identified in invertebrates from annelids to tunicates suggesting that the origin of GnRH and its receptor extends deep in evolution to the origin of bilaterian animals. To resolve the relationship of invertebrate and vertebrate receptors, we conducted large-scale phylogenetic analysis using maximum likelihood. The data support a superfamily that includes GnRH, AKH and corazonin receptors derived from both published sequences and unpublished gene model predictions. Closely related to the GnRHR superfamily is the vasopressin/oxytocin superfamily of receptors. Phylogenetic analysis suggests a shared ancestry with deep roots. A functional role for GnRH in vertebrates or invertebrates leads to questions about the evolutionary origin of the pituitary. Our analysis suggests a functioning pituitary was the result of genomic duplications in early vertebrates.