Identification of G protein-coupled receptors required for vitellogenin uptake into the oocytes of the red flour beetle, Tribolium castaneum

CAPA Neuropeptide and Its Receptor in Insects: A Mini Review

A neuropeptide, the CAPA, and its cognate receptor have been diversely characterized in different orders of class Insecta. CAPA peptides are synthesized in the abdominal neurohemal system and activate their corresponding receptor, CAPA receptor (CAPA-R), a type of G protein-coupled receptor (GPCR), to initiate cellular signals for diverse physiological functions in insects. Activation of the CAPA-R in Malpighian tubules results in ion-water homeostasis via antidiuresis in the majority of insect species; however, diuresis and myotropic activities are also known to result. Antidiuretic activity of CAPA peptides has been reported from mosquitoes, assassin bugs, spotted wing drosophila, and more; hence, this group of peptides also holds importance as potential targets when it comes to medical and agricultural entomology. GPCRs form a diverse family of cell membrane receptors responsible for signal transduction across the cell membrane in humans as well as in insects. With the advances in knowledge of human GPCRs, their physiological functions in agriculturally important insects have offered an opportunity for designing and implementing GPCR-targeting compounds in integrated pest management programs. In this review, we present a comprehensive view on physiological factors and peptides responsible for the diuresis/anti-diuresis in insects with special reference to the CAPA peptide-receptor interaction. The major focus is on the role of CAPA peptides in fluid and energy homeostasis, stress tolerance, muscle functioning, regulation of reproduction, and diapause-related processes. We end by outlining the significance of insect excretion with respect to the capa-r gene silencing and pest management.

Identification of Histone and N-Terminal Acetyltransferases Required for Reproduction and Embryonic Development of Yellow Fever Mosquito, Aedes aegypti

Histone acetylation levels maintained by histone acetyltransferases (HATs) and histone deacetylases play important roles in maintaining local chromatin accessibility and expression of genes that regulate many biological processes, including development and reproduction. N-terminal acetylation of proteins catalyzed by N-terminal acetyltransferases (NATs) also regulates gene expression. We identified 25 HATs/NATs genes in the yellow fever mosquito, Aedes aegypti, and investigated their function in female reproduction using RNA interference (RNAi). Among the HATs/NATs studied, the knockdown of AANAT1 (Arylamine N-acetyltransferase), NAA40 (N-alpha-acetyltransferase 40), NAA80 (N-alpha-acetyltransferase 80), KAT7 (Histone lysine acetyltransferase 7), ACNAT (Acyl-CoA N-acyltransferase), and MCM3AP (Minichromosome maintenance complex component 3 associated protein) significantly reduced egg laying and caused severe problems in oocyte development compared to that in control insects injected with dsGFP. Gene expression analysis using RT-qPCR revealed that vitellogenin and its receptor genes are downregulated in mosquitoes injected with dsAANAT1, dsNAA40, dsNAA80, dsKAT7, dsACNAT, and dsMCM3AP compared to that in control animals. Also, the knockdown of HATs/NATs genes ATAT1 (Alpha-tubulin N-acetyltransferase 1), AANAT1, TAFIID (Transcription initiation factor TFIID subunit 1), HATB (Histone acetyltransferase type B) and NAT9 (N-acetyltransferase 9) decreased more than 50% egg hatch by blocking embryonic development. These results suggest that the acetylation of proteins, especially histones mediated by NATs and HATs, plays an important role in regulating female reproduction and embryonic development of Ae. aegypti.

Insect Flight and Lipid Metabolism: Beyond the Classic Knowledge

Insects are the most successful animal group by various ecological and evolutionary metrics, including species count, adaptation diversity, biomass, and environmental influence. This book delves into the underlying reasons behind insects' dominance on Earth. Lipids play pivotal roles in insect biology, serving as fuel for physiological processes, signaling molecules, and structural components of biomembranes and providing waterproofing against dehydration, among other functions. The study of insect flight has been instrumental in advancing our understanding of insect metabolism, with the migratory locust (Locusta migratoria) and the tobacco hornworm (Manduca sexta) serving as prominent models. Throughout the 1980s and 1990s, numerous studies shed light on the role of adipokinetic hormone (AKH), a crucial neuropeptide in lipid mobilization, to support the extraordinary energy demands of insect flight. Remarkably, AKH was the first identified peptide hormone in insects. These pioneering works linking lipids and flight laid the groundwork for subsequent research characterizing the physiological roles of other neuroendocrine factors in energy substrate mobilization across diverse insect species. However, in the omics era, one may be surprised by the limited understanding of the complex cascade of events governing lipid supply to insect flight muscles. Thus, this chapter aims to provide a concise overview of the evolutionary significance of insect flight, emphasizing key advancements that expand our classical knowledge in this field. Ultimately, we hope this chapter serves as a modest tribute to the pioneering researchers of one of the most captivating areas in insect biology, inspiring further exploration into the myriad roles of lipids in insect biology.

Receptor tyrosine kinases CAD96CA and FGFR1 function as the cell membrane receptors of insect juvenile hormone

Juvenile hormone (JH) is important to maintain insect larval status; however, its cell membrane receptor has not been identified. Using the lepidopteran insect Helicoverpa armigera (cotton bollworm), a serious agricultural pest, as a model, we determined that receptor tyrosine kinases (RTKs) cadherin 96ca (CAD96CA) and fibroblast growth factor receptor homologue (FGFR1) function as JH cell membrane receptors by their roles in JH-regulated gene expression, larval status maintaining, rapid intracellular calcium increase, phosphorylation of JH intracellular receptor MET1 and cofactor Taiman, and high affinity to JH III. Gene knockout of Cad96ca and Fgfr1 by CRISPR/Cas9 in embryo and knockdown in various insect cells, and overexpression of CAD96CA and FGFR1 in mammalian HEK-293T cells all supported CAD96CA and FGFR1 transmitting JH signal as JH cell membrane receptors.

Methuselah-like 2 mediated 20-hydroxyecdysone (20E) signaling regulates molting and fecundity in Nilaparvata lugens (Stål) (Hemiptera: Delphacidae)

BACKGROUND

G protein-coupled receptors (GPCRs) are very promising as the targets of endogenous neuropeptides/neuromodulators that, upon binding to receptors, induce profound changes in insect physiology. The Methuselan/Methuselan-like subfamily of GPCRs is reported to be associated with longevity and stress resistance. A previous study showed the fungicide jingangmycin-induced expression of Mthl2 and enhanced stress resistance in Nilaparvata lugens. However, the other physiological functions of Mthl2 remain unelucidated.

RESULTS

The Mthl2 was highly expressed before molting and decreased after that until the next ecdysis, showing a cyclical pattern related to molting behavior and predominantly distributed in cuticle-producing and reproductive tissues in N. lugens. Silencing Mthl2 by RNAi in nymphs disrupted the synthesis of 20E, caused downregulation of the 20E signaling-related genes, and further affected the transcription of cuticular proteins. Moreover, it led to the malformation of the integument structure and a declined emergence rate, whereas exogenous 20E could rescue the declined emergence rate caused by knockdown of Mthl2. Furthermore, depletion of Mthl2 through RNAi in the N. lugens nymphal stage influenced the development of the ovaries and fecundity in female adults. The soluble protein content in reproductive tissues, the protein and transcript levels of Vitellogenin (Vg) were significantly decreased after silencing of Mthl2, ultimately leading to a decline in the number of offspring with an obviously transgenerational consequence.

CONCLUSION

The current study revealed the physiological functions of Mthl2 in molting and fecundity of N. lugens, which can be used as an RNAi-based insecticide discovery to control this pest. © 2025 Society of Chemical Industry.

Nano-delivery platform with strong protection and efficient delivery: preparation of self-assembled RNA pesticide with dual RNAi targets against Apolygus lucorum

BACKGROUND

RNA pesticide is regarded as the "third revolution in the history of pesticides". However, the double-stranded RNA (dsRNA) is easily degraded in the environment, and its delivery efficiency is not sufficient for pest management. This study aimed to construct a star polycation (SPc)-based delivery platform with strong protection and efficient delivery to develop a self-assembled RNA pesticide with dual RNA interference (RNAi) targets.

RESULTS

The nanocarrier SPc was applied to assemble with dsRNA via electrostatic interaction, hydrogen bond and Van der Waals force, and the self-complexation with SPc formed nanoscale dsRNA/SPc complex. The SPc could protect the dsRNA from the degradation by midgut fluid or RNase A, thus significantly increasing the stability of dsRNA under various environmental conditions. Meanwhile, the SPc was able to improve the translocation of dsRNA across insect cuticle, and increase its plant uptake. Then, dsECR-A and dsTre-1 fragments were individually screened, and the dsECR-A and dsTre-1 fragments with good control effects were co-expressed in pET28-BL21 (DE3) RNase III - system to prepare the dsECR-A + Tre-1/SPc complex. Both topical application and spraying of dsECR-A + Tre-1/SPc complex could effectively control a piercing-sucking agricultural pest Apolygus lucorum. The SPc-loaded dsECR-A + Tre-1 could up-regulate endocytosis-related genes and down-regulate cuticle biosynthesis-related genes, which primarily inhibited insect growth and development.

CONCLUSIONS

Our study comprehensively demonstrated the advantages of SPc-based dsRNA delivery platform, and developed a self-assembled RNA pesticide with dual RNAi targets, which provided a reference for the design of novel RNA pesticides.

The functional assay identified authentic interactions between CAPA peptides and the CAPA receptor isoforms in Bemisia tabaci (Gennadius)

CAPA neuropeptides regulate the diuresis/ antidiuresis process in insects by activating specific cognate receptor, CAPAr. In this study, we characterized the CAPAr gene (BtabCAPAr) in the whitefly, Bemisia tabaci Asia II 1. The two alternatively spliced isoforms of BtabCAPAr gene, BtabCAPAr-1 and BtabCAPAr-2, having six and five exons, respectively, were identified. The BtabCAPAr gene expression was highest in adult whitefly as compared to gene expression in egg, nymphal and pupal stages. Among the three putative CAPA peptides, CAPA-PVK1 and CAPA-PVK2 strongly activated the BtabCAPAr-1 with very low EC50 values of 0.067 nM and 0.053 nM, respectively, in heterologous calcium mobilization assays. None of the peptide activated the alternatively spliced isoform BtabCAPAr-2 that has lost the transmembrane segments 3 and 4. Significant levels of mortality were observed when whiteflies were fed with CAPA-PVK1 at 1.0 μM (50.0%), CAPA-PVK2 at 100.0 nM (43.8%) and CAPA-tryptoPK 1.0 μM (40.0%) at the 96 h after the treatment. This study provides valuable information to design biostable peptides to develop a class of insecticides.

The Role of BmTMED6 in Female Reproduction in Silkworm, Bombyx mori

Transmembrane emp24 domain (TMED) proteins have been extensively studied in mammalian embryonic development, immune regulation, and signal transduction. However, their role in insects, apart from Drosophila melanogaster, remains largely unexplored. Our previous study demonstrated the abundant expression of BmTMED6 across all stages and tissues of the silkworm. In this study, we investigate the function of BmTMED6 in reproduction. We observe significant differences in the expression of BmTMED6 between male and female silkworms, particularly in the head and fatboby, during the larval stage. Furthermore, qRT-PCR and WB analysis reveal substantial variation in BmTMED6 levels in the ovaries during pupal development, suggesting a potential association with silkworm female reproduction. We find that reducing TMED6 expression significantly decreases the number of eggs laid by female moths, leading to an accumulation of unlaid eggs in the abdomen. Moreover, downregulation of BmTMED6 leads to a decrease in the expression of BmDop2R1 and BmDop2R2, while overexpression of BmTMED6 in vitro has the opposite effect. These indicate that BmTMED6 plays a role in oviposition in female moths, potentially through the dopamine signaling pathway. This study provides a new regulatory mechanism for female reproduction in insects.

Characterization of a novel RNAi yeast insecticide that silences mosquito 5-HT1 receptor genes

G protein-coupled receptors (GPCRs), which regulate numerous intracellular signaling cascades that mediate many essential physiological processes, are attractive yet underexploited insecticide targets. RNA interference (RNAi) technology could facilitate the custom design of environmentally safe pesticides that target GPCRs in select target pests yet are not toxic to non-target species. This study investigates the hypothesis that an RNAi yeast insecticide designed to silence mosquito serotonin receptor 1 (5-HTR1) genes can kill mosquitoes without harming non-target arthropods. 5-HTR.426, a Saccharomyces cerevisiae strain that expresses an shRNA targeting a site specifically conserved in mosquito 5-HTR1 genes, was generated. The yeast can be heat-inactivated and delivered to mosquito larvae as ready-to-use tablets or to adult mosquitoes using attractive targeted sugar baits (ATSBs). The results of laboratory and outdoor semi-field trials demonstrated that consumption of 5-HTR.426 yeast results in highly significant mortality rates in Aedes, Anopheles, and Culex mosquito larvae and adults. Yeast consumption resulted in significant 5-HTR1 silencing and severe neural defects in the mosquito brain but was not found to be toxic to non-target arthropods. These results indicate that RNAi insecticide technology can facilitate selective targeting of GPCRs in intended pests without impacting GPCR activity in non-targeted organisms. In future studies, scaled production of yeast expressing the 5-HTR.426 RNAi insecticide could facilitate field trials to further evaluate this promising new mosquito control intervention.

Egfr signaling promotes juvenile hormone biosynthesis in the German cockroach

BACKGROUND

In insects, an interplay between the activities of distinct hormones, such as juvenile hormone (JH) and 20-hydroxyecdysone (20E), regulates the progression through numerous life history hallmarks. As a crucial endocrine factor, JH is mainly synthesized in the corpora allata (CA) to regulate multiple physiological and developmental processes, including molting, metamorphosis, and reproduction. During the last century, significant progress has been achieved in elucidating the JH signal transduction pathway, while less progress has been made in dissecting the regulatory mechanism of JH biosynthesis. Previous work has shown that receptor tyrosine kinase (RTK) signaling regulates hormone biosynthesis in both insects and mammals. Here, we performed a systematic RNA interference (RNAi) screening to identify RTKs involved in regulating JH biosynthesis in the CA of adult Blattella germanica females.

RESULTS

We found that the epidermal growth factor receptor (Egfr) is required for promoting JH biosynthesis in the CA of adult females. The Egf ligands Vein and Spitz activate Egfr, followed by Ras/Raf/ERK signaling, and finally activation of the downstream transcription factor Pointed (Pnt). Importantly, Pnt induces the transcriptional expression of two key enzyme-encoding genes in the JH biosynthesis pathway: juvenile hormone acid methyltransferase (JHAMT) and methyl farnesoate epoxidase (CYP15A1). Dual-luciferase reporter assay shows that Pnt is able to activate a promoter region of Jhamt. In addition, electrophoretic mobility shift assay confirms that Pnt directly binds to the - 941~ - 886 nt region of the Jhamt promoter.

CONCLUSIONS

This study reveals the detailed molecular mechanism of Egfr signaling in promoting JH biosynthesis in the German cockroach, shedding light on the intricate regulation of JH biosynthesis during insect development.

Periviscerokinin (Cap2b; CAPA) receptor silencing in females of Rhipicephalus microplus reduces survival, weight and reproductive output

BACKGROUND

The cattle fever tick, Rhipicephalus (Boophilus) microplus, is a vector of pathogens causative of babesiosis and anaplasmosis, both highly lethal bovine diseases that affect cattle worldwide. In Ecdysozoa, neuropeptides and their G-protein-coupled receptors play a critical integrative role in the regulation of all physiological processes. However, the physiological activity of many neuropeptides is still unknown in ticks. Periviscerokinins (CAP2b/PVKs) are neuropeptides associated with myotropic and diuretic activities in insects. These peptides have been identified only in a few tick species, such as Ixodes ricinus, Ixodes scapularis and R. microplus, and their cognate receptor only characterized for the last two.

METHODS

Expression of the periviscerokinin receptor (Rhimi-CAP2bR) was investigated throughout the developmental stages of R. microplus and silenced by RNA interference (RNAi) in the females. In a first experiment, three double-stranded (ds) RNAs, named ds680-805, ds956-1109 and ds1102-1200, respectively, were tested in vivo. All three caused phenotypic effects, but only the last one was chosen for subsequent experiments. Resulting RNAi phenotypic variables were compared to those of negative controls, both non-injected and dsRNA beta-lactamase-injected ticks, and to positive controls injected with beta-actin dsRNA. Rhimi-CAP2bR silencing was verified by quantitative reverse-transcriptase PCR in whole females and dissected tissues.

RESULTS

Rhimi-CAP2bR transcript expression was detected throughout all developmental stages. Rhimi-CAP2bR silencing was associated with increased female mortality, decreased weight of surviving females and of egg masses, a delayed egg incubation period and decreased egg hatching (P < 0.05).

CONCLUSIONS

CAP2b/PVKs appear to be associated with the regulation of female feeding, reproduction and survival. Since the Rhimi-CAP2bR loss of function was detrimental to females, the discovery of antagonistic molecules of the CAP2b/PVK signaling system should cause similar effects. Our results point to this signaling system as a promising target for tick control.

Juvenile Hormone Membrane Signaling Enhances its Intracellular Signaling Through Phosphorylation of Met and Hsp83

Juvenile hormone (JH) regulates insect development and reproduction through both intracellular and membrane signaling, and the two pathways might crosstalk with each other. Recent studies have reported that JH membrane signaling induces phosphorylation of the JH intracellular receptor Met, thus enhancing its transcriptional activity. To gain more insights into JH-induced Met phosphorylation, we here performed phosphoproteomics to identify potential phosphorylation sites of Met and its paralog Germ-cell expressed (Gce) in Drosophila Kc cells. In vitro experiments demonstrate that JH-induced phosphorylation sites in the basic helix-loop-helix (bHLH) domain, but not in the Per-Arnt-Sim-B (PAS-B) domain, are required for maximization of Met transcriptional activity. Moreover, phosphoproteomics analysis reveale that JH also induces the phosphorylation of Hsp83, a chaperone protein involved in JH-activated Met nuclear import. The JH-induced Hsp83 phosphorylation at S219 facilitates Hsp83-Met binding, thus promoting Met nuclear import and its transcription. By using proteomics, subcellular distribution, and co-immunoprecipitation approaches, we further characterized 14-3-3 proteins as negative regulators of Met nuclear import through physical interaction with Hsp83. These results show that JH membrane signaling induces phosphorylation of the key components in JH intracellular signaling, such as Met and Hsp83, and consequently facilitating JH intracellular signaling.

Juvenile hormone membrane signaling phosphorylates USP and thus potentiates 20-hydroxyecdysone action in Drosophila

Juvenile hormone (JH) and 20-hydroxyecdysone (20E) coordinately regulate development and metamorphosis in insects. Two JH intracellular receptors, methoprene-tolerant (Met) and germ-cell expressed (Gce), have been identified in the fruit fly Drosophila melanogaster. To investigate JH membrane signaling pathway without the interference from JH intracellular signaling, we characterized phosphoproteome profiles of the Met gce double mutant in the absence or presence of JH in both chronic and acute phases. Functioning through a potential receptor tyrosine kinase and phospholipase C pathway, JH membrane signaling activated protein kinase C (PKC) which phosphorylated ultraspiracle (USP) at Ser35, the PKC phosphorylation site required for the maximal action of 20E through its nuclear receptor complex EcR-USP. The usp^S35A mutant, in which Ser was replaced with Ala at position 35 by genome editing, showed decreased expression of Halloween genes that are responsible for ecdysone biosynthesis and thus attenuated 20E signaling that delayed developmental timing. The usp^S35A mutant also showed lower Yorkie activity that reduced body size. Altogether, JH membrane signaling phosphorylates USP at Ser35 and thus potentiates 20E action that regulates the normal fly development. This study helps better understand the complex JH signaling network.

Neuropeptide Bursicon Influences Reproductive Physiology in Tribolium Castaneum

Bursicon is a neuropeptide belonging to the cystine knot family and is composed of burs and partner of burs (pburs) subunits. It can form heterodimers or homodimers to execute different biological functions. Bursicon heterodimers regulate cuticle sclerotization and wing maturation, whereas bursicon homodimers mediate innate immunity and midgut stem cell proliferation. A recent study has shown that bursicon potentially induces the expression of vitellogenin (Vg) in the black tiger shrimp Penaeus monodon; however, the underlying mechanism remains unknown. In this study, we investigated the role of bursicon in the reproductive physiology of the red flour beetle, Tribolium castaneum. The knockdown of burs, pburs, or its receptor T. castaneum rickets (Tcrk) in 2-day pupae significantly downregulated the expression levels of Vg1, Vg2, and Vg receptor (VgR) genes in females 3- and 5-day post-adult emergence, leading to abnormal oocytes with limited Vg content. The silencing of burs repressed the number of eggs laid and completely inhibited egg hatch, whereas the silencing of pburs dramatically decreased the number of eggs laid, hatch rate, and offspring larval size, and this RNA interference (RNAi) effects persisted to the next generation. Furthermore, the knockdown of burs or pburs downregulated the expression of the insulin/insulin-like signaling/target of rapamycin (TOR) signaling genes encoding insulin receptor (InR), protein kinase B (Akt), TOR, and ribosomal protein S6 kinase (S6K). Most importantly, the injection of recombinant pburs (r-pburs) protein was able to upregulate the expression of Vg, VgR, InR, Akt, TOR, S6K, JH synthesis (JHAMT), Methoprene-tolerant (Met), and Taiman (Tai) in normal females and rescue the expression of Vg and VgR in pburs RNAi females but failed to rescue Vg and VgR in Tcrk knockdown females. We infer that bursicon homodimers influence Vg expression via the receptor Tcrk, possibly by mediating the expression of the juvenile hormone (JH) and IIS/TOR pathway genes, thereby regulating reproduction in T. castaneum.

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

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

Regulation of dopamine production in the brains during sexual maturation in male honey bees

To explore the physiological mechanisms that underlie age-related dopamine increases during sexual maturation in the brains of male honey bees, we focused on the expression of genes encoding the enzymes tyrosine hydroxylase (Amth) and DOPA decarboxylase (Amddc), which are involved in dopamine biosynthesis in the brain. We hypothesized that juvenile hormone in hemolymph and tyrosine intake from food known as factors enhancing brain dopamine levels might both control the expression of genes related to dopamine production, and we tested this hypothesis in experiments. The brain levels of tyrosine and DOPA, which are precursors of dopamine, decreased as males aged, whereas the dopamine levels increased, suggesting active metabolism of dopamine precursors. The relative expression levels of Amth and Amddc were significantly higher in the brains of 4-day-old males compared with 0-day-old males, and the higher level of Amddc was maintained after 8 days. Topical application of the juvenile hormone analog methoprene enhanced the expression levels of Amth and Amddc in the brains according to the methoprene concentration. Oral intake of tyrosine enhanced the tyrosine, DOPA and dopamine levels in the brain, and activated Amddc expression in the brain, suggesting that tyrosine intake can increase both substrates and enzyme for dopamine biosynthesis. These results support our hypothesis that juvenile hormone and tyrosine intake may enhance the expression levels of genes encoding enzymes involved in dopamine biosynthesis in male honey bee brains during sexual maturation.

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

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

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

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

Social Evolution With Decoupling of Multiple Roles of Biogenic Amines Into Different Phenotypes in Hymenoptera

Convergent evolution of eusociality with the division of reproduction and its plastic transition in Hymenoptera has long attracted the attention of researchers. To explain the evolutionary scenario of the reproductive division of labor, several hypotheses had been proposed. Among these, we focus on the most basic concepts, i.e., the ovarian ground plan hypothesis (OGPH) and the split-function hypothesis (SFH). The OGPH assumes the physiological decoupling of ovarian cycles and behavior into reproductive and non-reproductive individuals, whereas the SFH assumes that the ancestral reproductive function of juvenile hormone (JH) became split into a dual function. Here, we review recent progress in the understanding of the neurohormonal regulation of reproduction and social behavior in eusocial hymenopterans, with an emphasis on biogenic amines. Biogenic amines are key substances involved in the switching of reproductive physiology and modulation of social behaviors. Dopamine has a pivotal role in the formation of reproductive skew irrespective of the social system, whereas octopamine and serotonin contribute largely to non-reproductive social behaviors. These decoupling roles of biogenic amines are seen in the life cycle of a single female in a solitary species, supporting OGPH. JH promotes reproduction with dopamine function in primitively eusocial species, whereas it regulates non-reproductive social behaviors with octopamine function in advanced eusocial species. The signal transduction networks between JH and the biogenic amines have been rewired in advanced eusocial species, which could regulate reproduction in response to various social stimuli independently of JH action.

Developmental regulation of oocyte lipid intake through 'patent' follicular epithelium in Drosophila melanogaster

Epithelia form protective permeability barriers that selectively allow the exchange of material while maintaining tissue integrity under extreme mechanical, chemical, and bacterial loads. Here, we report in the Drosophila follicular epithelium a developmentally regulated and evolutionarily conserved process "patency", wherein a breach is created in the epithelium at tricellular contacts during mid-vitellogenesis. In Drosophila, patency exhibits a strict temporal range potentially delimited by the transcription factor Tramtrack69 and a spatial pattern influenced by the dorsal-anterior signals of the follicular epithelium. Crucial for growth and lipid uptake by the oocyte, patency is also exploited by endosymbionts such as Spiroplasma pulsonii. Our findings reveal an evolutionarily conserved and developmentally regulated non-typical epithelial function in a classic model system.

Regulatory Mechanisms of Vitellogenesis in Insects

Vitellogenesis is pre-requisite to insect egg production and embryonic development after oviposition. During insect vitellogenesis, the yolk protein precursor vitellogenin (Vg) is mainly synthesized in the fat body, transported by the hemolymph through the intercellular spaces (known as patency) in the follicular epithelium to reach the membrane of maturing oocytes, and sequestered into the maturing oocytes via receptor-mediated endocytosis. Insect vitellogenesis is governed by two critical hormones, the sesquiterpenoid juvenile hormone (JH) and the ecdysteriod 20-hydroxyecdysone (20E). JH acts as the principal gonadotropic hormone to stimulate vitellogenesis in basal hemimetabolous and most holometabolous insects. 20E is critical for vitellogenesis in some hymenopterans, lepidopterans and dipterans. Furthermore, microRNA (miRNA) and nutritional (amino acid/Target of Rapamycin and insulin) pathways interplay with JH and 20E signaling cascades to control insect vitellogenesis. Revealing the regulatory mechanisms underlying insect vitellogenesis is critical for understanding insect reproduction and helpful for developing new strategies of insect pest control. Here, we outline the recent research progress in the molecular action of gonadotropic JH and 20E along with the role of miRNA and nutritional sensor in regulating insect vitellogenesis. We highlight the advancements in the regulatory mechanisms of insect vitellogenesis by the coordination of hormone, miRNA and nutritional signaling pathways.

Insulin/IGF signaling and TORC1 promote vitellogenesis via inducing juvenile hormone biosynthesis in the American cockroach

Vitellogenesis, including vitellogenin (Vg) production in the fat body and Vg uptake by maturing oocytes, is of great importance for the successful reproduction of adult females. The endocrinal and nutritional regulation of vitellogenesis differs distinctly in insects. Here, the complex crosstalk between juvenile hormone (JH) and the two nutrient sensors insulin/IGF signaling (IIS) and target of rapamycin complex1 (TORC1), was investigated to elucidate the molecular mechanisms of vitellogenesis regulation in the American cockroach, Periplaneta americana Our data showed that a block of JH biosynthesis or JH action arrested vitellogenesis, in part by inhibiting the expression of doublesex (Dsx), a key transcription factor gene involved in the sex determination cascade. Depletion of IIS or TORC1 blocked both JH biosynthesis and vitellogenesis. Importantly, the JH analog methoprene, but not bovine insulin (to restore IIS) and amino acids (to restore TORC1 activity), restored vitellogenesis in the neck-ligated (IIS-, TORC1- and JH-deficient) and rapamycin-treated (TORC1- and JH-deficient) cockroaches. Combining classic physiology with modern molecular techniques, we have demonstrated that IIS and TORC1 promote vitellogenesis, mainly via inducing JH biosynthesis in the American cockroach.

Methoprene-tolerant is essential for embryonic development of the red flour beetle Tribolium castaneum

Insect juvenile hormone (JH) is well known to regulate post-embryonic development and reproduction in concert with ecdysteroids in a variety of insect species. In contrast, our knowledge on the role of JH in embryonic development is limited and inconsistent. Preceding studies indicate that JH biosynthesis or JH signaling genes are dispensable in holometabolous Drosophila melanogaster and Bombyx mori, while essential in hemimetabolous Blattella germanica. In the red flour beetle Tribolium castaneum, we performed functional analyses of key factors in JH signaling, i.e. the JH receptor Methoprene-tolerant (Met) and the early JH-response gene Krüppel homolog 1 (Kr-h1) using parental RNA interference. Knockdown of Met resulted in a significant reduction in hatching rates and survival rates in the first and second larval instars. Meanwhile, knockdown of Kr-h1 caused no significant effect on hatching or survival. The unhatched embryos under Met knockdown developed up to the late embryonic stage, but their body shape was flat and tubby compared with the controls. Attempts to suppress JH biosynthesis by parental RNA interference of JH biosynthetic enzymes were unsuccessful due to insufficient knockdown efficiency. These results suggested that Met but not Kr-h1 is essential for the embryonic development of T. castaneum, although involvement of JH still remains to be examined. Taken together, the function of Met in embryonic development seems to be diverse among insect species.

Mechanisms, Applications, and Challenges of Insect RNA Interference

The RNA interference (RNAi) triggered by short/small interfering RNA (siRNA) was discovered in nematodes and found to function in most living organisms. RNAi has been widely used as a research tool to study gene functions and has shown great potential for the development of novel pest management strategies. RNAi is highly efficient and systemic in coleopterans but highly variable or inefficient in many other insects. Differences in double-stranded RNA (dsRNA) degradation, cellular uptake, inter- and intracellular transports, processing of dsRNA to siRNA, and RNA-induced silencing complex formation influence RNAi efficiency. The basic dsRNA delivery methods include microinjection, feeding, and soaking. To improve dsRNA delivery, various new technologies, including cationic liposome-assisted, nanoparticle-enabled, symbiont-mediated, and plant-mediated deliveries, have been developed. Major challenges to widespread use of RNAi in insect pest management include variable RNAi efficiency among insects, lack of reliable dsRNA delivery methods, off-target and nontarget effects, and potential development of resistance in insect populations.

Transcriptome Analysis of Female and Male Conopomorpha sinensis (Lepidoptera: Gracilariidae) Adults With a Focus on Hormone and Reproduction

Conopomorpha sinensis Bradley is the dominant borer pest of litchi and longan in the Asian-pacific area. Reduction or interference of reproduction and mating of adult moths is one of the most used strategies to control C. sinensis. Insect reproduction is a critical biological process closely related to endocrine control. Conopomorpha sinensis genome and transcriptome information is limited, hampering both our understanding of the molecular mechanisms underlying hormone activity and reproduction and the development of control strategies for this borer pest. To explore the sex differences in gene expression profiles influencing these biological processes, de novo transcriptomes were assembled from female and male adult C. sinensis specimens. This analysis yielded 184,422 unigenes with an average length of 903 bp and 405,961 transcripts after sequencing and assembly. About 45.06, 22.41, 19.53, 34.05, 35.82, 36.42, and 19.85% of the unigenes had significant matches in seven public databases. Subsequently, gene ontology (GO) and kyoto encyclopedia of genes and genomes (KEGG) enrichment analysis revealed comprehensive information about the function of each gene and identified enriched categories and pathways that were associated with the 2,890 female-biased genes and 2,964 male-biased genes. In addition, we identified some important unigenes related to hormone activity and reproduction among the sex-differentially expressed genes (DEGs), including unigenes coding for ecdysone-induced protein 78C, juvenile hormone (JH)-regulated gene fatty acyl-CoA reductase, vitellogenin, etc. Our findings provide a more comprehensive portrait of the sex differences involved in the relationship of two important physiological features-hormone activity and reproduction in C. sinensis and members of the family Gracillariidae.

Juvenile hormone signaling - a mini review

Since it was first postulated by Wigglesworth in 1934, juvenile hormone (JH) is considered a status quo hormone in insects because it prevents metamorphosis that is initiated by the molting hormone 20-hydroxyecdysone (20E). During the last decade, significant advances have been made regarding JH signaling. First, the bHLH-PAS transcription factor Met/Gce was identified as the JH intracellular receptor. In the presence of JH, with the assistance of Hsp83, and through physical association with a bHLH-PAS transcriptional co-activator, Met/Gce enters the nucleus and binds to E-box-like motifs in promoter regions of JH primary-response genes for inducing gene expression. Second, the zinc finger transcription factor Kr-h1 was identified as the anti-metamorphic factor which transduces JH signaling. Via Kr-h1 binding sites, Kr-h1 represses expression of 20E primary-response genes (i.e. Br, E93 and E75) to prevent 20E-induced metamorphosis. Third, through the intracellular signaling, JH promotes different aspects of female reproduction. Nevertheless, this action varies greatly from species to species. Last, a hypothetical JH membrane receptor has been predicted to be either a GPCR or a tyrosine kinase receptor. In future, it will be a great challenge to understand how the JH intracellular receptor Met/Gce and the yet unidentified JH membrane receptor coordinate to regulate metamorphosis and reproduction in insects.

GPCR annotation, G proteins, and transcriptomics of fire ant (Solenopsis invicta) queen and worker brain: An improved view of signaling in an invasive superorganism

Knowledge of G protein-coupled receptors (GPCRs) and their signaling modalities is crucial to advancing insect endocrinology, specifically in highly successful invasive social insects, such as the red imported fire ant, Solenopsis invicta Buren. In the first published draft genome of S. invicta, emphasis was placed on the annotation of olfactory receptors, and only the number of predicted GPCR genes was reported. Without an organized and curated resource for GPCRs, it will be difficult to test hypotheses on the endocrine role of neuropeptide hormones, or the function of neurotransmitters and neuromodulators. Therefore, we mined the S. invicta genome for GPCRs and found 324 predicted transcripts encoded by 125 predicted loci and improved the annotation of 55 of these loci. Among them are sixteen GPCRs that are currently annotated as "uncharacterized proteins". Further, the phylogenetic analysis of class A neuropeptide receptors presented here and the comparative listing of GPCRs in the hymenopterans S. invicta, Apis mellifera (both eusocial), Nasonia vitripennis (solitary), and the solitary model dipteran Drosophila melanogaster will facilitate comparative endocrinological studies related to social insect evolution and diversity. We compiled the 24 G protein transcripts predicted (15 α, 7 β, and 2 γ) from 12 G protein genes (5 α, 5 β, and 2 γ). Reproductive division of labor is extreme in this ant species, therefore, we compared GPCR and G protein gene expression among worker, mated queen and alate virgin queen ant brain transcriptomes. Transcripts for ten GPCRs and two G proteins were differentially expressed between queen and worker brains. The differentially expressed GPCRs are candidate receptors to explore hypotheses on division of labor in this species.

Ocular Albinism Type 1 Regulates Deltamethrin Tolerance in Lymantria dispar and Drosophila melanogaster

The ocular albinism type 1 (OA1), a pigment cell-specific integral membrane glycoprotein, is a member of the G-protein-coupled receptor (GPCR) superfamily that binds to heterotrimeric G proteins in mammalian cells. We aimed to characterize the physiological functions an insect OA1 from Lymantria dispar (LdOA1) employs in the regulation of insecticide tolerance. In the present study, we investigated the roles of LdOA1 in response to deltamethrin exposure in both L. dispar and Drosophila melanogaster. LdOA1 was expressed at the lowest level during the 4^th instar stage, while LdOA1 was significantly upregulated in the 5^th instar and male stages. Knockdown of LdOA1 by injecting dsRNA of LdOA1 into gypsy moth larvae caused a 4.80-fold higher mortality than in control larvae microinjected with dsRNA of GFP under deltamethrin stress. Nine out of 11 L. dispar CYP genes were significantly downregulated under deltamethrin stress in LdOA1 silenced larvae as compared to control larvae. Moreover, the LdOA1 gene was successfully overexpressed in D. melanogaster using transgenic technique. The deltamethrin contact assay showed that the LdOA1 overexpression in flies significantly enhanced the tolerance to deltamethrin compared to the control flies. Furthermore, the downstream Drosophila CYP genes were upregulated in the LdOA1 overexpression flies, suggesting LdOA1 may play a master switch role in P450-mediated metabolic detoxification. This study is the first report of an insect OA1 gene regulating insecticide tolerance and potentially playing a role in the regulation of downstream cytochrome P450 expression. These results contribute to the future development of novel insecticides targeting insect GPCRs.

An update on ecdysone signaling during insect oogenesis

An overview is presented of the different functions of ecdysone signaling during insect oogenesis. An extensive genetic toolkit allowed analysis with unprecedented temporal and spatial detail in Drosophila where functions were revealed in stem cell proliferation and niche maintenance, germline cyst differentiation and follicle formation, integration of nutrient and lipid signaling, follicle maturation and ovulation. Besides putative autocrine/paracrine signaling, hormonal networks were identified that integrate ecdysone with other endocrine signaling pathways. In other insects, progress in oogenesis has lagged behind although recently RNAi emerged as a new tool to analyze gene function in ovaries in hemimetabolous insects and Tribolium.

Insulin-like peptides of the legume pod borer, Maruca vitrata, and their mediation effects on hemolymph trehalose level, larval development, and adult reproduction

Insulin-like peptides (ILPs) of insects mediate various physiological processes including hemolymph sugar level, immature growth, female reproduction, and lifespan. In target cells of ILPs, insulin/insulin-like growth factor signaling (IIS) is highly conserved in animals. IIS in the legume pod borer, Maruca vitrata (Lepidoptera: Crambidae), is known to be involved in maintaining hemolymph trehalose levels and promoting larval growth. However, ILPs in M. vitrata have not been reported yet. This study predicted two ILP genes of Mv-ILP1 and Mv-ILP2 from transcriptome of M. vitrata. Mv-ILP1 and Mv-ILP2 shared high sequence homologies and domain architecture with Drosophila ILPs. Both ILPs exhibited similar expression patterns in most developmental stages, showing high expression levels in adult stage. In the larval stage, Mv-ILP1 and Mv-IlP2 were expressed mostly in the brain and fat body. However, in the adult stage, both ILP genes were expressed more in the abdomen than those in the head containing the brain. RNA interference (RNAi) of either Mv-ILP1 or Mv-ILP2 during larval stage resulted in significant malfunctioning in regulating hemolymph trehalose titers. RNAi-treated larvae also exhibited significant retardation of larval growth. RNAi treatment in adult stage interfered with the ovarian development of females. These results suggest that Mv-ILP1 and Mv-ILP2 play crucial roles in mediating larval growth and adult reproduction.

Juvenile hormone signaling in insect oogenesis

Juvenile hormone (JH) plays a crucial role in insect reproduction, but its molecular mode of action only became clear within the last decade. We here review recent findings revealing the intricate crosstalk between JH and ecdysone signaling with nutrient sensing pathways in Drosophila melanogaster, Aedes aegypti, Tribolium castaneum and Locusta migratoria. The finding for a critical role of ecdysis triggering hormone (ETH) in both molting and ooogenesis now also highlights the importance of an integrated view of development and reproduction. Furthermore, insights from non-model insects, especially so social Hymenoptera and termites, where JH function gradually becomes decoupled from reproduction and plays a role in division of labor, emphasize the need to consider life cycle and life history strategies when studying insect reproductive physiology.

Epigenetic modifications acetylation and deacetylation play important roles in juvenile hormone action

BACKGROUND

Epigenetic modifications including DNA methylation and post-translational modifications of histones are known to regulate gene expression. Antagonistic activities of histone acetyltransferases (HATs) and histone deacetylases (HDACs) mediate transcriptional reprogramming during insect development as shown in Drosophila melanogaster and other insects. Juvenile hormones (JH) play vital roles in the regulation of growth, development, metamorphosis, reproduction and other physiological processes. However, our current understanding of epigenetic regulation of JH action is still limited. Hence, we studied the role of CREB binding protein (CBP, contains HAT domain) and Trichostatin A (TSA, HDAC inhibitor) on JH action.

RESULTS

Exposure of Tribolium castaneum cells (TcA cells) to JH or TSA caused an increase in expression of Kr-h1 (a known JH-response gene) and 31 or 698 other genes respectively. Knockdown of the gene coding for CBP caused a decrease in the expression of 456 genes including Kr-h1. Interestingly, the expression of several genes coding for transcription factors, nuclear receptors, P450 and fatty acid synthase family members that are known to mediate JH action were affected by CBP knockdown or TSA treatment.

CONCLUSIONS

These data suggest that acetylation and deacetylation mediated by HATs and HDACs play an important role in JH action.

De novo characterization of Phenacoccus solenopsis transcriptome and analysis of gene expression profiling during development and hormone biosynthesis

The cotton mealybug Phenacoccus solenopsis is a devastating pest of cotton causing tremendous loss in the yield of crops each year. Widespread physiological and biological studies on P. solenopsis have been carried out, but the lack of genetic information has constrained our understanding of the molecular mechanisms behind its growth and development. To understand and characterize the different developmental stages, RNA-Seq platform was used to execute de-novo transcriptome assembly and differential gene expression profiling for the eggs, first, second, third instar and adult female stages. About 182.67 million reads were assembled into 93,781 unigenes with an average length of 871.4 bp and an N50 length of 1899 bp. These unigenes sequences were annotated and classified by performing NCBI non-redundant (Nr) database, Kyoto Encyclopedia of Genes and Genomes (KEGG) and Clusters of Orthologous Groups (COG), Gene ontology (GO), the Swiss-Prot protein database (Swiss-Prot), and nearest related organism Acyrthosiphon pisum (pea aphid) database. To get more information regarding the process of metamorphosis, we performed a pairwise comparison of four developmental stages and obtained 29,415 differentially expressed genes. Some of the differentially expressed genes were associated with functional protein synthesis, anti-microbial protection, development and hormone biosynthesis. Functional pathway enrichment analysis of differentially expressed genes showed the positive correlation with specific physiological activities of each stage, and these results were confirmed by qRT-PCR experiments. This study gives a valuable genomics resource of P. solenopsis covering all its developmental stages and will promote future studies on biological processes at the molecular level.

Brain gene expression analyses in virgin and mated queens of fire ants reveal mating-independent and socially regulated changes

Transcriptomes of dissected brains from virgin alate and dealate mated queens from polygyne fire ants (Solenopsis invicta) were analyzed and compared. Thirteen genes were upregulated in mated queen brain, and nine were downregulated. While many of the regulated genes were either uncharacterized or noncoding RNAs, those annotated genes included two hexamerin proteins, astakine neuropeptide, serine proteases, and serine protease inhibitors. We found that for select differentially expressed genes in the brain, changes in gene expression were most likely driven by the changes in physiological state (i.e., age, nutritional status, or dominance rank) or in social environment (released from influence of primer pheromone). This was concluded because virgins that dealated after being separated from mated queens showed similar patterns of gene expression in the brain as those of mated queens for hexamerin 1, astakine, and XR_850909. Abaecin (XR_850725), however, appears upregulated only after mating. Therefore, our findings contribute to distinguish how specific gene networks, especially those influenced by queen primer pheromone, are regulated in queen ants. Additionally, to identify brain signaling pathways, we mined the fire ant genome and compiled a list of G-protein-coupled receptors (GPCRs). The expression level of GPCRs and other genes in the "genetic toolkit" in the brains of virgin alates and mated dealate queens is reported.

Molecular cloning and characterization of the SIFamide precursor and receptor in a hymenopteran insect, Bombus terrestris

SIFamides (SIFa) are a family of neuropeptides that are highly conserved among arthropods. In insects, this peptide is mainly expressed in four medial interneurons in the pars intercerebralis and affects sexual behavior, sleep regulation and pupal mortality. Furthermore, an influence on the hatching rate has been observed. The first SIFa receptor (SIFR) was pharmacologically characterized in Drosophila melanogaster and is homologous to the vertebrate gonadotropin-inhibitory hormone (GnIH) receptor (NPFFR). In this study, we pharmacologically characterized the SIFR of the buff-tailed bumblebee Bombus terrestris. We demonstrated an intracellular increase in calcium ions and cyclic AMP (cAMP) upon ligand binding with an EC₅₀ value in the picomolar and nanomolar range, respectively. In addition, we studied the agonistic properties of a range of related and modified peptides. By means of quantitative real time PCR (qPCR), we examined the relative transcript levels of Bomte-SIFa and Bomte-SIFR in a variety of tissues.