Characterisation of a functional allatotropin receptor in the bumblebee, Bombus terrestris (Hymenoptera, Apidae)

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

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

Uncovering the role of juvenile hormone in ovary development and egg laying in bumble bees

Juvenile hormone (JH) regulates developmental and physiological processes in insects. In bumble bees, the hormone acts as a gonadotropin that mediates ovary development, but the exact physiological pathways involved in ovary activation and subsequent egg laying are poorly understood. In this study, we examine how queen hibernation state, caste, and species impact the gonadotropic effect of JH in bumble bee queens through methoprene (JH analogue) application. We extend previous research by assessing queen egg laying and colony initiation, alongside ovary development. Furthermore, we compared sensitivity of workers of both species to the juvenile hormone's gonadotropic effect. In both bumble bee species, the ovaries of hibernated queens were developed five to six days after breaking diapause, regardless of methoprene treatment. By contrast, methoprene did have a stimulatory effect on ovary development in non-hibernated queens. The dose needed to obtain this effect was higher in B. impatiens. Methoprene did not have gonadotropic effects in callow workers of both species. These results indicate that the physiological effect of exogenous methoprene application varies according to species, caste and hibernation status. Interestingly, despite gonadotropic effects in non-hibernated queens, oviposition was not accelerated by JH. This suggests that JH alone is insufficient to induce egg laying and that an additional stimulus, which is naturally present in hibernated queens, is required. Consequently, our findings indicate that other physiological processes, beyond a rise in JH alone, are required for oviposition and colony initiation.

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 receptor signal transduction and Ca2+ signaling pathways of the allatotropin/orexin system in Hydra

Allatotropin is a pleiotropic peptide originally characterized in insects. The existence of AT neuropeptide signaling was proposed in other invertebrates. In fact, we previously proposed the presence of an AT-like system regulating feeding behavior in Hydra sp. Even in insects, the information about the AT signaling pathway is incomplete. The aim of this study is to analyze the signaling cascade activated by AT in Hydra plagiodesmica using a pharmacological approach. The results show the involvement of Ca²⁺ and IP₃ signaling in the transduction pathway of the peptide. Furthermore, we confirm the existence of a GPCR system involved in this pathway, that would be coupled to a G_q subfamily of Gα protein, which activates a PLC, inducing an increase in IP₃ and cytosolic Ca²⁺. To the best of our knowledge, this work represents the first in vivo approach to study the overall signaling pathway and intracellular events involved in the myoregulatory effect of AT in Hydra sp.

Neuropeptide and microRNA regulators of juvenile hormone production

The sesquiterpenoid juvenile hormone(s) (JHs) of insects are the primary regulators of growth, metamorphosis, and reproduction in most insect species. As a consequence, it is essential that JH production be precisely regulated so that it is present only during appropriate periods necessary for the control of these processes. The presence of JH at inappropriate times results in disruption to metamorphosis and development and, in some cases, to disturbances in female reproduction. Neuropeptides regulate the timing and production of JH by the corpora allata. Allatostatin and allatotropin were the names coined for neuropeptides that serve as inhibitors or stimulators of JH biosynthesis, respectively. Three different allatostatin neuropeptide families are capable of inhibiting juvenile hormone but only one family is utilized for that purpose dependent on the insect studied. The function of allatotropin also varies in different insects. These neuropeptides are pleiotropic in function acting on diverse physiological processes in different insects such as muscle contraction, sleep and neuromodulation. Genome projects and expression studies have assigned individual neuropeptide families to their respective receptors. An understanding of the localization of these receptors is providing clues as to how numerous peptide families might be integrated in regulating physiological functions. In recent years microRNAs have been identified that down-regulate enzymes and transcription factors that are involved in the biosynthesis and action of juvenile hormone.

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

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

Isolation functional characterization of allatotropin receptor from the cotton bollworm, Helicoverpa armigera

Insect allatotropin (AT) plays multi-functions including regulation of juvenile hormone synthesis, growth, development and reproduction. In the present study, the full-length cDNA encoding the AT receptor was cloned from the brain of Helicoverpa armigera (Helar-ATR). The ORF of Helar-ATR exhibited the characteristic seven transmembrane domains of the G protein-coupled receptor (GPCR) and was close to the ATR of Manduca sexta in the phylogenetic tree. The Helar-ATR expressed in vertebrate cell lines can be activated by Helar-AT and each Helar-ATL in a dose-responsive manner, in the following order: Helar-ATLI > Helar-ATLII > Helar-AT > Helar-ATLIII. Helar-ATLI and Helar-ATLII represented the functional ligands to Helar-ATR in vitro, while Helar-AT and Helar-ATLIII behaved as partial agonists. The in vitro functional analysis suggested that the Helar-ATR signal was mainly coupled with elevated levels of Ca²⁺ and independent of cAMP levels. Helar-ATR mRNA in larvae showed the highest level in the brain, followed by the thorax ganglion, abdomen ganglion, fat body and midgut. Helar-ATR mRNA levels in the complex of the brain-thoracic-abdomen ganglion on the 2nd day of the larval stage and during later pupal stages were observed to be relatively higher than in the wandering and early pupal stages.

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.

Allatoregulatory-like systems and changes in cytosolic Ca2+ modulate feeding behavior in Hydra

Allatotropin (AT) and allatostatin-C (AST-C) are neuropeptides originally characterized by their ability to modulate the secretion of juvenile hormones in insects. Beyond the allatoregulatory function, these neuropeptides are pleiotropic acting as myoregulators not only in insects, but also in other groups of invertebrates. We have previously proposed the existence of AT and AST-C like systems in Hydra sp., a member of the phylum Cnidaria, which is a basal group of Metazoa, sharing a common ancestor with Bilateria. In the present study we analyze the regulatory effects of both peptides on the activity of the hypostome during feeding in Hydra sp. Furthermore, the importance of changes in the cytosolic Ca²⁺ levels involved in the response of the hypostome were analyzed. Physiological assays showed that while the presence of food or treatment with AT stimulates the extrusion of the hypostome, AST-C has an inhibitory effect on the behavior induced by both, food and AT. These facts suggest that both systems participate in the regulatory mechanisms associated with feeding and, as in insects, AST-C and AT may exert opposite effects. The use of thapsigargin (TG) and nifedipine, two compounds that modify the levels of cytosolic Ca²⁺, showed that changes in the levels of this ion are involved in the regulation of the activity of the hypostome. Indeed, these results suggest that the two basic mechanisms operating to increase the cytosolic levels of Ca²⁺ (i.e. the influx from the extracellular space and the release from endoplasmic reticulum) are relevant for the extrusion of the hypostome. Like in insects, the treatment with TG counteracted the effect of AST-C, suggesting that this peptide acts by reducing cytosolic Ca²⁺ levels. Furthermore, nifedipine prevented the myostimulatory effect of AT, showing that the effect of this peptide depends on the influx of Ca²⁺ throughout voltage-gated calcium channels. Altogether, these results suggest that the Allatotropin/Orexin and Allatostatin/Somatostatin regulatory systems could represent an ancestral mechanisms regulating hypostome activity and feeding behavior in Cnidaria.

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.

Gene Expression Dynamics in Major Endocrine Regulatory Pathways along the Transition from Solitary to Social Life in a Bumblebee, Bombus terrestris

Understanding the social evolution leading to insect eusociality requires, among other, a detailed insight into endocrine regulatory mechanisms that have been co-opted from solitary ancestors to play new roles in the complex life histories of eusocial species. Bumblebees represent well-suited models of a relatively primitive social organization standing on the mid-way to highly advanced eusociality and their queens undergo both, a solitary and a social phase, separated by winter diapause. In the present paper, we characterize the gene expression levels of major endocrine regulatory pathways across tissues, sexes, and life-stages of the buff-tailed bumblebee, Bombus terrestris, with special emphasis on critical stages of the queen's transition from solitary to social life. We focused on fundamental genes of three pathways: (1) Forkhead box protein O and insulin/insulin-like signaling, (2) Juvenile hormone (JH) signaling, and (3) Adipokinetic hormone signaling. Virgin queens were distinguished by higher expression of forkhead box protein O and downregulated insulin-like peptides and JH signaling, indicated by low expression of methyl farnesoate epoxidase (MFE) and transcription factor Krüppel homolog 1 (Kr-h1). Diapausing queens showed the expected downregulation of JH signaling in terms of low MFE and vitellogenin (Vg) expressions, but an unexpectedly high expression of Kr-h1. By contrast, reproducing queens revealed an upregulation of MFE and Vg together with insulin signaling. Surprisingly, the insulin growth factor 1 (IGF-1) turned out to be a queen-specific hormone. Workers exhibited an expression pattern of MFE and Vg similar to that of reproducing queens. Males were characterized by high Kr-h1 expression and low Vg level. The tissue comparison unveiled an unexpected resemblance between the fat body and hypopharyngeal glands across all investigated genes, sexes, and life stages.

Allatostatin-C antagonizes the synergistic myostimulatory effect of allatotropin and serotonin in Rhodnius prolixus (Stal)

Haematophagous insects can ingest large quantities of blood in a single meal producing a large quantity of urine in the following hours to eliminate the excess of water and mineral ions incorporated. The excretory activity of the Malpighian tubules is facilitated by an increase in haemolymph circulation as a result of the intensification of aorta contractions, combined with an increase of anterior midgut peristaltic waves. We have recently shown that haemolymph circulation during post-prandial diuresis is modulated by the synergistic activity of allatotropin (AT) and serotonin, resulting in an increase in aorta and crop contraction rates. In the present study we describe the antagonistic effect of allatostatin-C (AST-C) on the increase of aorta frequency of contractions induced by serotonin/AT in Rhodnius prolixus. The administration of AST-C counteracted the increase in the frequency induced by the treatment with serotonin/AT, but did not affect the increase in frequency induced by the administration of serotonin alone, suggesting that AST-C is altering the synergism between serotonin and AT. Furthermore, the administration of AST-C during post-prandial diuresis decreases the number of peristaltic waves of the anterior midgut. The AST-C putative receptor is expressed in the hindgut, midgut and dorsal vessel, three critical organs involved in post-prandial diuresis. All together these findings provide evidence that AST-C plays a key role as a myoregulatory and cardioregulatory peptide in R. prolixus.

The pleiotropic allatoregulatory neuropeptides and their receptors: A mini-review

Juvenile hormones (JH) are highly pleiotropic insect hormones essential for post-embryonic development. The circulating JH titer in the hemolymph of insects is influenced by enzymatic degradation, binding to JH carrier proteins, uptake and storage in target organs, but evidently also by rates of production at its site of synthesis, the corpora allata (CA). The multiple processes in which JH is involved alongside the critical significance of JH in insect development emphasize the importance for elucidating the control of JH production. Production of JH in CA cells is regulated by different factors: by neurotransmitters, such as dopamine and glutamate, but also by allatoregulatory neuropeptides originating from the brain and axonally transported to the CA where they bind to their G protein-coupled receptors (GPCRs). Different classes of allatoregulatory peptides exist which have other functions aside from acting as influencers of JH production. These pleiotropic neuropeptides regulate different processes in different insect orders. In this mini-review, we will give an overview of allatotropins and allatostatins, and their recently characterized GPCRs with a view to better understand their modes of action and different action sites.

Molecular cloning and characterization of the allatotropin precursor and receptor in the desert locust, Schistocerca gregaria

Allatotropins (ATs) are pleiotropic neuropeptides initially isolated from the tobacco hornworm, Manduca sexta. In 2008, the first receptor for AT-like peptides (ATR) was characterized in Bombyx mori. Since then, ATRs have also been characterized in M. sexta, Tribolium castaneum, Aedes aegypti and Bombus terrestris. These receptors show sequence similarity to vertebrate orexin (ORX) receptors. When generating an EST-database of the desert locust (Schistocerca gregaria) central nervous system, we found cDNA sequences encoding the Schgr-AT precursor and a fragment of its putative receptor. This receptor cDNA has now been completed and functionally expressed in mammalian cell lines. Activation of this receptor, designated as Schgr-ATR, by Schgr-AT caused an increase in intracellular calcium ions, as well as cyclic AMP (cAMP), with an EC₅₀ value in the nanomolar range. In addition, the transcript distribution of both the Schgr-AT precursor and Schgr-ATR was investigated by means of quantitative real-time PCR. Moreover, we found more evidence for the myotropic and allatostimulatory actions of Schgr-AT in the desert locust. These data are discussed and situated in a broader context by comparison with literature data on AT and ATR in insects.

Signaling Properties and Pharmacological Analysis of Two Sulfakinin Receptors from the Red Flour Beetle, Tribolium castaneum

Sulfakinin is an insect neuropeptide that constitutes an important component of the complex network of hormonal and neural factors that regulate feeding and digestion. The key modulating functions of sulfakinin are mediated by binding and signaling via G-protein coupled receptors. Although a substantial amount of functional data have already been reported on sulfakinins in different insect species, only little information is known regarding the properties of their respective receptors. In this study, we report on the molecular cloning, functional expression and characterization of two sulfakinin receptors in the red flour beetle, Tribolium castaneum. Both receptor open reading frames show extensive sequence similarity with annotated sulfakinin receptors from other insects. Comparison of the sulfakinin receptor sequences with homologous vertebrate cholecystokinin receptors reveals crucial conserved regions for ligand binding and receptor activation. Quantitative reverse transcriptase PCR shows that transcripts of both receptors are primarily expressed in the central nervous system of the beetle. Pharmacological characterization using 29 different peptide ligands clarified the essential requirements for efficient activation of these sulfakinin receptors. Analysis of the signaling pathway in multiple cell lines disclosed that the sulfakinin receptors of T. castaneum can stimulate both the Ca²⁺ and cyclic AMP second messenger pathways. This in depth characterization of two insect sulfakinin receptors may provide useful leads for the further development of receptor ligands with a potential applicability in pest control and crop protection.