Insect satiety: sulfakinin localization and the effect of drosulfakinin on protein and carbohydrate ingestion in the blow fly, Phormia regina (Diptera: Calliphoridae)

Identification of sulfakinin receptor regulating feeding behavior and hemolymph trehalose homeostasis in the silkworm, Bombyx mori

Feeding behavior, the most fundamental physiological activity, is controlled by two opposing groups of factors, orexigenic and anorexigenic factors. The sulfakinin family, an insect analogue of the mammalian satiety factor cholecystokinin (CCK), has been shown to suppress food intake in various insects. Nevertheless, the mechanisms through which sulfakinin regulates feeding behavior remain a biological question. This study aimed to elucidate the signaling pathway mediated by the anorexigenic peptide sulfakinin in Bombyx mori. We identified the Bombyx mori neuropeptide G protein-coupled receptor A9 (BNGR-A9) as the receptor for sulfakinin through functional assays. Stimulation with sulfakinin triggered a swift increase in intracellular IP3, Ca2+, and a notable enhancement of ERK1/2 phosphorylation, in a manner sensitive to a Gαq-specific inhibitor. Treatment with synthetic sulfakinin resulted in decreased food consumption and average body weight. Additionally, administering synthetic sulfakinin to silkworms significantly elevated hemolymph trehalose levels, an effect markedly reduced by pre-treatment with BNGR-A9 dsRNA. Consequently, our findings establish the sulfakinin/BNGR-A9 signaling pathway as a critical regulator of feeding behavior and hemolymph trehalose homeostasis in Bombyx mori, highlighting its roles in the negative control of food intake and the positive regulation of energy balance.

Identification and functional characterization of the sulfakinin and sulfakinin receptor in the Chinese white pine beetle Dendroctonus armandi

The sulfakinin (SK) is an important signal molecule. As a neuromodulator, it mediates a variety of behavioral processes and physiological functions in invertebrates through the interaction with G-protein-coupled receptors (GPCRs). However, there is no report on the functional role of SK in the Chinese white pine beetle, Dendroctonus armandi. We have cloned and characterized SK and SKR genes in the D. armandi and carried out bioinformatics predictions on the basis of the deduced amino acid sequences, which are very similar to those from Dendroctonus ponderosa. The expression levels of the two genes were different between male and female adults, and there were significant changes in different developmental stages, tissues, and between starvation and following re-feeding states. Additionally, RNA-interference (RNAi) using double-stranded RNA to knock down SK and SKR reduced the transcription levels of the target genes and increased their body weight. In parallel, injection of SK caused a significant reduction in body weight and increase in mortality of D. armandi and also led to an increase in trehalose and a decrease in glycogen and free fatty acid. The results show that the SK signal pathway plays a positive and significant role in feeding regulation and provides a potential molecular target for the control of this pest.

Expression analysis of peptidergic enteroendocrine cells in the silkworm Bombyx mori

Enteroendocrine cells (ECs) in the insect midgut respond to physiological changes in the intestine by releasing multiple peptides to control food intake, gastrointestinal activity and systemic metabolism. Here, we performed a comprehensive mapping of ECs producing different regulatory peptides in the larval midgut of Bombyx mori. In total, we identified 20 peptide genes expressed in different ECs in specific regions of the midgut. Transcript-specific in situ hybridisation combined with antibody staining revealed approximately 30 subsets of ECs, each producing a unique peptide or a combination of several different peptides. Functional significance of this diversity and specific roles of different enteroendocrine peptides are largely unknown. Results of this study highlight the importance of the midgut as a major endocrine/paracrine source of regulatory molecules in insects and provide important information to clarify functions of ECs during larval feeding and development.

Cholecystokinin/sulfakinin peptide signaling: conserved roles at the intersection between feeding, mating and aggression

Neuropeptides are the most diverse messenger molecules in metazoans and are involved in regulation of daily physiology and a wide array of behaviors. Some neuropeptides and their cognate receptors are structurally and functionally well conserved over evolution in bilaterian animals. Among these are peptides related to gastrin and cholecystokinin (CCK). In mammals, CCK is produced by intestinal endocrine cells and brain neurons, and regulates gall bladder contractions, pancreatic enzyme secretion, gut functions, satiety and food intake. Additionally, CCK plays important roles in neuromodulation in several brain circuits that regulate reward, anxiety, aggression and sexual behavior. In invertebrates, CCK-type peptides (sulfakinins, SKs) are, with a few exceptions, produced by brain neurons only. Common among invertebrates is that SKs mediate satiety and regulate food ingestion by a variety of mechanisms. Also regulation of secretion of digestive enzymes has been reported. Studies of the genetically tractable fly Drosophila have advanced our understanding of SK signaling mechanisms in regulation of satiety and feeding, but also in gustatory sensitivity, locomotor activity, aggression and reproductive behavior. A set of eight SK-expressing brain neurons plays important roles in regulation of these competing behaviors. In males, they integrate internal state and external stimuli to diminish sex drive and increase aggression. The same neurons also diminish sugar gustation, induce satiety and reduce feeding. Although several functional roles of CCK/SK signaling appear conserved between Drosophila and mammals, available data suggest that the underlying mechanisms differ.

Ancient role of sulfakinin/cholecystokinin-type signalling in inhibitory regulation of feeding processes revealed in an echinoderm

Sulfakinin (SK)/cholecystokinin (CCK)-type neuropeptides regulate feeding and digestion in protostomes (e.g. insects) and chordates. Here, we characterised SK/CCK-type signalling for the first time in a non-chordate deuterostome - the starfish Asterias rubens (phylum Echinodermata). In this species, two neuropeptides (ArSK/CCK1, ArSK/CCK2) derived from the precursor protein ArSK/CCKP act as ligands for an SK/CCK-type receptor (ArSK/CCKR) and these peptides/proteins are expressed in the nervous system, digestive system, tube feet, and body wall. Furthermore, ArSK/CCK1 and ArSK/CCK2 cause dose-dependent contraction of cardiac stomach, tube foot, and apical muscle preparations in vitro, and injection of these neuropeptides in vivo triggers cardiac stomach retraction and inhibition of the onset of feeding in A. rubens. Thus, an evolutionarily ancient role of SK/CCK-type neuropeptides as inhibitory regulators of feeding-related processes in the Bilateria has been conserved in the unusual and unique context of the extra-oral feeding behaviour and pentaradial body plan of an echinoderm.

Sulfakinins influence lipid composition and insulin-like peptides level in oenocytes of Zophobas atratus beetles

Insect sulfakinins are pleiotropic neuropeptides with the homology to vertebrate gastrin/cholecystokinin peptide family. They have been identified in many insect species and affect different metabolic processes. They have a strong influence on feeding and digestion as well as on carbohydrate and lipid processing. Our study reveals that sulfakinins influence fatty acids composition in Zophobas atratus oenocytes and regulate insulin-like peptides (ILPs) level in these cells. Oenocytes are cells responsible for maintenance of the body homeostasis and have an important role in the regulation of intermediary metabolism, especially of lipids. To analyze the lipid composition in oenocytes after sulfakinins injections we used gas chromatography combined with mass spectrometry and for ILPs level determination an immunoenzymatic test was used. Because sulfakinin peptides and their receptors are the main components of sulfakinin signaling, we also analyzed the presence of sulfakinin receptor transcript (SKR2) in insect tissues. We have identified for the first time the sulfakinin receptor transcript (SKR2) in insect oenocytes and found its distribution more widespread in the peripheral tissues (gut, fat body and haemolymph) as well as in the nervous and neuro-endocrine systems (brain, ventral nerve cord, corpora cardiaca/corpora allata CC/CA) of Z. atratus larvae. The presence of sulfakinin receptor transcript (SKR2) in oenocytes suggests that observed effects on oenocytes lipid and ILPs content may result from direction action of these peptides on oenocytes.

Cholecystokinin-like peptide mediates satiety by inhibiting sugar attraction

Feeding is essential for animal survival and reproduction and is regulated by both internal states and external stimuli. However, little is known about how internal states influence the perception of external sensory cues that regulate feeding behavior. Here, we investigated the neuronal and molecular mechanisms behind nutritional state-mediated regulation of gustatory perception in control of feeding behavior in the brown planthopper and Drosophila. We found that feeding increases the expression of the cholecystokinin-like peptide, sulfakinin (SK), and the activity of a set of SK-expressing neurons. Starvation elevates the transcription of the sugar receptor Gr64f and SK negatively regulates the expression of Gr64f in both insects. Interestingly, we found that one of the two known SK receptors, CCKLR-17D3, is expressed by some of Gr64f-expressing neurons in the proboscis and proleg tarsi. Thus, we have identified SK as a neuropeptide signal in a neuronal circuitry that responds to food intake, and regulates feeding behavior by diminishing gustatory receptor gene expression and activity of sweet sensing GRNs. Our findings demonstrate one nutritional state-dependent pathway that modulates sweet perception and thereby feeding behavior, but our experiments cannot exclude further parallel pathways. Importantly, we show that the underlying mechanisms are conserved in the two distantly related insect species.

Food intake is critical for animal survival and reproduction and is regulated both by internal states that signal appetite or satiety, and by external sensory stimuli. It is well known that the internal nutritional state influences the strength of the chemosensory perception of food signals. Thus, both gustatory and olfactory signals of preferred food are strengthened in hungry animals. However, the molecular mechanisms behind satiety-mediated modulation of taste are still not known. We show here that cholecystokinin-like (SK) peptide in brown planthopper and Drosophila signals satiety and inhibits sugar attraction by lowering the activity of sweet-sensing gustatory neurons and transcription of a sugar receptor gene, Gr64f. We show that SK peptide signaling reflects the nutritional state and inhibits feeding behavior. Re-feeding after starvation increases SK peptide expression and spontaneous activity of SK producing neurons. Interestingly, we found that SK peptide negatively regulates the expression of the sweet gustatory receptor and that activation of SK producing neurons inhibits the activity of sweet-sensing gustatory neurons (GRNs). Furthermore, we found that one of the two known SK peptide receptors is expressed in some sweet-sensing GRNs in the proboscis and proleg tarsi. In summary, our findings provide a mechanism that is conserved in distantly related insects and which explains how feeding state modulates sweet perception to regulate feeding behavior. Thus, we have identified a neuropeptide signal and its neuronal circuitry that respond to satiety, and that regulate feeding behavior by inhibiting gustatory receptor gene expression and activity of sweet sensing GRNs.

Trehalose and glucose levels regulate feeding behavior of the phloem-feeding insect, the pea aphid Acyrthosiphon pisum Harris

Trehalose serves multifarious roles in growth and development of insects. In this study, we demonstrated that the high trehalose diet increased the glucose content, and high glucose diet increased the glucose content but decreased the trehalose content of Acyrthosiphon pisum. RNA interference (RNAi) of trehalose-6-phosphate synthase gene (ApTPS) decreased while RNAi of trehalase gene (ApTRE) increased the trehalose and glucose contents. In the electrical penetration graph experiment, RNAi of ApTPS increased the percentage of E2 waveform and decreased the percentage of F and G waveforms. The high trehalose and glucose diets increased the percentage of E2 waveform of A. pisum red biotype. The correlation between feeding behavior and sugar contents indicated that the percentage of E1 and E2 waveforms were increased but np, C, F and G waveforms were decreased in low trehalose and glucose contents. The percentage of np, E1 and E2 waveforms were reduced but C, F and G waveforms were elevated in high trehalose and glucose contents. The results suggest that the A. pisum with high trehalose and glucose contents spent less feeding time during non-probing phase and phloem feeding phase, but had an increased feeding time during probing phase, stylet work phase and xylem feeding phase.

Identification of sulfakinin receptors (SKR) in Tenebrio molitor beetle and the influence of sulfakinins on carbohydrates metabolism

Sulfakinins (SKs) are pleiotropic neuropeptides commonly found in insects, structurally and functionally homologous to the mammalian gastrin/cholecystokinin (CCK) neuropeptides. SKs together with sulfakinin receptors (SKRs) are involved in sulfakinin signaling responsible for variety of biological functions, including food intake or fatty acid metabolism. In the present study, we determined the distribution of SKRs in Tenebrio molitor larvae and characterized the impact of nonsulfated and sulfated SKs on carbohydrates and insulin-like peptides (ILPs) level in beetle hemolymph. Our results indicate the presence of both sulfakinin receptors, SKR1 and SKR2, in the nervous system of T. molitor. The distribution of SKR2 in peripheral tissues was more widespread than SKR1, and their transcripts have been found in fat body, gut and hemolymph. This is also the first evidence for SKRs presence in insect hemocytes indicating immunotropic activity of SKs. Moreover, in the present study, we have demonstrated that SKs regulate ILPs and carbohydrates level in insect hemolymph, and that sulfation is not crucial for peptides activity. Our study confirms the role of SKs in maintaining energy homeostasis in beetles.

Identification, Functional Characterization, and Pharmacological Analysis of Two Sulfakinin Receptors in the Medically-Important Insect Rhodnius prolixus

The chordate gastrin/cholecystokinin and ecdysozoan sulfakinin (SK)-signaling systems are functionally and structurally homologous. In the present study, we isolated the cDNA sequences encoding the SK receptors in Rhodnius prolixus (Rhopr-SKR-1 and Rhopr-SKR-2). The Rhopr-SKRs have been functionally characterized and their intracellular signaling pathways analysed via a functional receptor assay. Both Rhopr-SKRs are exclusively activated via the two native R. prolixus sulfakinins, Rhopr-SK-1 and Rhopr-SK-2, but not via nonsulfated Rhopr-SK-1. The Rhopr-SKRs are each linked to the intracellular Ca²⁺ second messenger pathway, and not to the cyclic AMP pathway. Spatial transcript expression analyses revealed that each Rhopr-SKR is predominantly expressed in the central nervous system with lower expression throughout peripheral tissues. The critical importance of the SK-signaling pathway in the blood-feeding behaviour of R. prolixus was demonstrated by knockdown of the transcripts for Rhopr-SKs and Rhopr-SKRs, which results in an increase in the mass of blood meal taken. The parasite causing Chagas disease is transmitted to the host after R. prolixus has taken a blood meal, and characterization of the SKRs provides further understanding of the coordination of feeding and satiation, and ultimately the transmission of the parasite.

Function of myosuppressin in regulating digestive function in the two-spotted cricket, Gryllus bimaculatus

Myosuppressin is one of essential peptides controlling biological processes including feeding behavior. Here we identified and characterized the cDNAs that encode myosuppressin precursor and its receptor in the two-spotted cricket Gryllus bimaculatus. The presence of the mature peptide (Grybi-MS) was confirmed by direct measurement of adult brain. RT-PCR revealed the tissue distribution of these transcripts; myosuppressin is expressed predominantly in the brain and central nervous system, whereas its receptor is ubiquitously expressed in the cricket body. To address the function of Grybi-MS, we performed several bioassays to test concerning feeding behavior and digestive function upon exposure to Grybi-MS. Administration of synthetic Grybi-MS resulted in increased feeding motivation, accompanied by an increase in food intake. Meanwhile, the hemolymph lipid and carbohydrate titers were both elevated after Grybi-MS injection. As the intestinal contraction is significantly inhibited by the exposure to Grybi-MS, the upregulating feeding index might be complicated in the cricket body. The current data indicate that Grybi-MS modulates feeding behavior to control the physiological processes in the cricket.

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.

Parasite-altered feeding behavior in insects: integrating functional and mechanistic research frontiers

Research on parasite-altered feeding behavior in insects is contributing to an emerging literature that considers possible adaptive consequences of altered feeding behavior for the host or the parasite. Several recent ecoimmunological studies show that insects can adaptively alter their foraging behavior in response to parasitism. Another body of recent work shows that infection by parasites can change the behavior of insect hosts to benefit the parasite; manipulations of host feeding behavior may be part of this phenomenon. Here, we address both the functional and the underlying physiological frontiers of parasite-altered feeding behavior in order to spur research that better integrates the two. Functional categories of parasite-altered behavior that are adaptive for the host include prophylaxis, therapy and compensation, while host manipulation is adaptive for the parasite. To better understand and distinguish prophylaxis, therapy and compensation, further study of physiological feedbacks affecting host sensory systems is especially needed. For host manipulation in particular, research on mechanisms by which parasites control host feedbacks will be important to integrate with functional approaches. We see this integration as critical to advancing the field of parasite-altered feeding behavior, which may be common in insects and consequential for human and environmental health.

Cloning, localization, and physiological effects of sulfakinin in the kissing bug, Rhodnius prolixus

Sulfakinins (SKs) are a family of multifunctional neuropeptides that have been shown to have myotropic activity on muscles of the digestive system and to function as feeding satiety factors. Here, we confirm via cloning the presence of two sulfakinins (Rhopr-SK-1 and Rhopr-SK-2) in Rhodnius prolixus. Reverse transcriptase quantitative PCR demonstrates that the Rhopr-SK transcript is highly expressed in the central nervous system (CNS) of unfed fifth-instar R. prolixus. Fluorescent in situ hybridization shows transcript expression only in neurons in the brain. Immunohistochemical staining of SK-like peptides was observed in the same neurons in the brain and in processes extending throughout the CNS, as well as over the posterior midgut and anterior hindgut. Rhopr-SK-1 (sulfated form) induces contractions of the hindgut in a dose-dependent manner. Injection Rhopr-SK-1 (sulfated form) significantly decreases the overall weight of the blood meal consumed, suggesting SK's role as a satiety factor in R. prolixus.

Insulin receptor regulates food intake through sulfakinin signaling in the red flour beetle, Tribolium castaneum

Insects obtain energy and nutrients via feeding to support growth and development. The insulin signaling pathway is involved in the regulation of feeding; however, the underlying mechanisms are not fully understood. Here, we show that insulin signaling regulates food intake via crosstalk with neuropeptide sulfakinin in the red flour beetle, Tribolium castaneum. Silencing of the insulin receptor (InR) decreased the food intake in the penultimate and final instar stages, leading to a decrease of weight gain and mortality during larval-pupal metamorphosis. Interestingly, the knockdown of InR co-occurred with an increased expression of sulfakinin (sk), a gene encoding neuropeptide SK functioning as a satiety signal. In parallel, double silencing of sk and InR eliminated the inhibitory effect on food intake as induced by silencing of InR and the larvae died as prepupae. In conclusion, this study shows, for the first time, that the insulin/InR signaling regulates food intake through the sulfakinin signaling pathway in the larval stages of this important model and pest insect, indicating a novel target for pest control.

PHARMACOLOGICAL REGULATION OF DIGESTION IN THE ANAUTOGENOUS FLESH FLY, Sarcophaga crassipalpis, BY SIMPLE INJECTION OF 6-HYDROXYDOPAMINE

Female anautogenous Sarcophaga flesh flies need a protein meal to start large-scale yolk polypeptides (YPs) production and oocyte maturation. Protein meal rapidly elicits a brain-dependent increase in midgut proteolytic activity. Trypsin and chymotrypsin together represent over 80% of protease activity in liver-fed flies. Abdominal injection of 6-hydroxydopamine (6-OHDA) dose-dependently prohibits this increase in proteolytic activity at translational level in a similar way as post liver feeding decapitation. Delayed injection of 6-OHDA later than 6 h post liver meal has no effect. In flesh flies, chemical decapitation by 6-OHDA, by interrupting the brain-gut dopaminergic signaling, can be used as tool for the controlled inhibition of midgut proteolytic activity and subsequent ovarial development. Inhibition of ovarial development is probably indirect due to a deficit in circulating amino acids needed for YPs synthesis.

The chemosensitivity of labellar sugar receptor in female Phormia regina is paralleled with ovary maturation: Effects of serotonin

Oogenesis in most adult insects is a nutrient-dependent process involving ingestion of both proteins and carbohydrates that ultimately depends on peripheral input from chemoreceptors. The main goal of this study was to characterize, in the female blowfly Phormia regina, the responsive changes of the labellar chemoreceptors to carbohydrates and proteins in relation to four different stages along the ovarian cycle: (1) immature ovaries, (2) mid-mature ovaries, (3) mature ovaries and ready for egg-laying and (4) post egg-laying ovaries. Then, the possible effects exerted by exogenous serotonin on the chemoreceptor sensitivity profiles were investigated. Our results show that ovary length, width and contraction rate progressively increase from stage 1 to 3, when all these parameters reach their maximum values, before declining in the next stage 4. The sensitivity of the labellar "sugar" chemoreceptors to both sucrose and proteins varies during the ovarian maturation stages, reaching a minimum for sucrose in stage 3, while that to proteins begins. Exogenous 5-HT supply specifically increases the chemoreceptor sensitivity to sugar at the stages 3 and 4, while it does not affect that to proteins. In conclusion, our results provide evidence that in female blowflies the cyclic variations in the sensitivity of the labellar chemosensilla to sugars and proteins are time-related to ovarian development and that during the stages 3 and 4 the responsiveness of the sugar cell to sucrose is under serotonergic control.

The activity of the nonsulfated sulfakinin Zopat-SK-1 in the neck-ligated larvae of the beetle Zophobas atratus

Insect sulfakinins (SKs) are multifunctional neuropeptides structurally and functionally homologous to the mammalian gastrin/cholecystokinin (CCK). It has been proposed that SKs play a role in modulating energy management in insects by interacting with adipokinetic hormone (AKH), the principle hormone controlling insect intermediary metabolism. To exclude head factors (including AKH) that influence the activity of the nonsulfated sulfakinin Zopat-SK-1 in the larvae of the beetle Zophobas atratus, ligature and in vitro bioassays were used. Our study showed that in the neck-ligated larvae, Zopat-SK-1 evoked a much more pronounced glycogenolytic effect in fat body tissue and a significantly higher hypertrahelosemic effect in hemolymph than in larvae without ligation. We found that the concentration of the sugar trehalose increased under hormonal treatment but no changes in glucose levels were observed. Under in vitro conditions, the maximal glycogenolytic effect of Zopat-SK-1 in fat body was observed at 10 pmol of hormone. Ligature and in vitro bioassays indicated that Zopat-SK-1 activity in the Z. atratus larvae is modulated by head signals and/or factors from the gastrointestinal tract. Our data indicate the existence of a brain-gastrointestinal axis that has a role in controlling of energy (carbohydrate) metabolism in the insect body. Moreover, these results, together with immunological evidence of a cholecystokinin-like (sulfakinin) receptor in the Z. atratus fat body, help us to better understand the SK signaling pathways and its physiological role in insect biology.

New metabolic activity of the nonsulfated sulfakinin Zopat-SK-1 in the insect fat body

Insect sulfakinins are multifunctional neuropeptides homologous to vertebrate gastrin/cholecystokin (CCK) neuropeptide hormones. We investigated the action of the nonsulfated sulfakinin Zopat-SK-1 (pETSDDYGHLRFa) on the levels of chosen metabolites in the Zophobas atratus beetle fat body. Samples of fat body were collected 2h and 24h after hormone injection. The administration of 20pmol of Zopat-SK-1 to feeding larvae significantly increased concentrations of lipids and proteins and decreased the content of glycogen in fat body tissue in the 24h experimental group. In contrast, the only increase in total lipid concentration in prepupal fat bodies was observed 24h after Zopat-SK-1 treatment. Simultaneously, changes in the quality and quantity of free sugars in the hemolymph were measured. In larval hemolymph, a marked increase in free sugar concentration and a decrease in glucose content were observed 24h and 2h after Zopat-SK-1 application, respectively. No changes in the prepupal stage were observed. For the first time we show potent metabolic activity of sulfakinin in the fat body tissue of an insect. Our findings imply a physiological function of the nonsulfated form of sulfakinin in energy storage and release processes in fat body tissue of larvae and prepupae was indicated. We suggest a role for sulfakinin signaling in the regulation of energy metabolism in insect tissues.

Sulfakinin is an important regulator of digestive processes in the migratory locust, Locusta migratoria

Sulfakinin (SK) is a sulfated insect neuropeptide that is best known for its function as a satiety factor. It displays structural and functional similarities with the vertebrate peptides gastrin and cholecystokinin. Peptidomic studies in multiple insects, crustaceans and arachnids have revealed the widespread occurrence of SK in the arthropod phylum. Multiple studies in hemi- and holometabolous insects revealed the pleiotropic nature of this neuropeptide: in addition to its activity as a satiety factor, SK was also reported to affect muscle contraction, digestive enzyme release, odor preference, aggression and metabolism. However, the main site of action seems to be the digestive system of insects. In this study, we have investigated whether SK can intervene in the control of nutrient uptake and digestion in the migratory locust (Locusta migratoria). We provide evidence that sulfakinin reduces food uptake in this species. Furthermore, we discovered that SK has very pronounced effects on the main digestive enzyme secreting parts of the locust gut. It effectively reduced digestive enzyme secretion from both the midgut and gastric caeca. SK injection also elicited a reduction in absorbance and proteolytic activity of the gastric caeca contents. The characteristic sulfation of the tyrosine residue is crucial for the observed effects on digestive enzyme secretion. In an attempt to provide potential leads for the development of peptidomimetic compounds based on SK, we also tested two mimetic analogs of the natural peptide ligand in the digestive enzyme secretion assay. These analogs were able to mimic the effect of the natural SK, but their effects were milder. The results of this study provide new insights into the action of SK on the digestive system in (hemimetabolous) insects.

CCK(-like) and receptors: structure and phylogeny in a comparative perspective

Cholecystokinin (CCK) and gastrin are regulatory peptides in vertebrates. Their homologues are widely present in metazoan animals, in form of cionin in tunicates, neuropeptide-like protein 12 in nematodes and sulfakinin (SK) in arthropods. CCK(-like) peptides exert diverse physiological effects through binding their corresponding receptors, which are important members of the hormone-binding G-protein-coupled receptors. In this paper, CCK(-like) peptides and receptors are reviewed in a comparative way at levels of molecular structure, physiological functions and phylogeny. CCK signalling system is widely involved in the regulation of satiety, gastric acid secretion, pancreatic secretion, anxiety and memory processes in vertebrates. Its counterpart SK in arthropods is also found with similar functions on regulation of satiety and gastrointestinal motility. Co-evolution of peptide and receptor has been recognized through metazoans. The CCK(-like) receptors seem to be evolved from a common ancestor based on the phylogenetic analysis, with species-specific events in arthropods. In addition, tetraploidization has been brought up to study the evolution of receptors. There are 2 receptors in chordates and nematodes, whereas, the number of sulfakinin receptor varies in arthropods from 0 to 2. We discussed here that the presence or absence of the SK signalling system is likely to be related to feeding behaviour.

Differential peptidomics highlights adipokinetic hormone as key player in regulating digestion in anautogenous flesh fly, Sarcophaga crassipalpis

Females of anautogenous flesh flies, Sarcophaga crassipalpis, need a protein meal in order to produce their first batch of eggs. This protein meal elicits an increase in midgut proteolytic activity that is under neuropeptidergic regulation. Time series of decapitation and rescue experiments of liver fed flies evidenced the need of a peptide factor released by corpora cardiaca (CC) within 4h post protein feeding in order to assure complete protein digestion. Q-Exactive quantitative differential peptidomics analysis on CC of sugar fed flies and flies 5h post protein feeding respectively, showed a unique consistent decrease in the stored amount of adipokinetic hormone (AKH) ranging between 16% up to 63%. Injection of AKH into liver fed decapitated flies as well as sugar fed intact flies resulted in dose dependent enhanced midgut proteolytic activity up to the level of intact protein fed flies. This suggests a key role of AKH in food depended reproduction.

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.

Development of cell-based bioassay with Sf9 cells expressing TcSKR1 and TcSKR2 and differential activation by sulfated and non-sulfated SK peptides

Insect sulfakinin receptors (SKRs) are G-protein-coupled receptors (GPCRs) that interact with sulfakinins (SKs) to modulate diverse biological processes. One of the indispensable roles of SKs is in the regulation of food intake in insects. In this project we report on the development of a cell-based receptor assay system with insect Sf9 cells, expressing TcSKR1 and TcSKR2 from the red flour beetle Tribolium castaneum, a model and important pest insect in agriculture. In this system, a stable presence of the two TcSKRs was supported by Western blotting. The expressed TcSKRs were coupled to Gαs-protein upon activation and stimulated cAMP accumulation in Sf9 cells. Exposure of the transfected cell lines to sulfated SK (sSK) activated TcSKR1 at 1 nM; the EC50 of sSK to obtain 50% of receptor activation was similar for both receptors. In contrast, μM concentrations of non-sulfated SK were necessary to activate both TcSKRs. In conclusion, this cell-based TcSKR assay system is useful to screen SK-related peptides and mimetics and to better document ligand-receptor structure-activity relationships. Given the importance of SK signaling system in insects, the present study may provide new insights on the development of new methods to control pest insects.

Cholecystokinin-Like Peptide (DSK) in Drosophila, Not Only for Satiety Signaling

Cholecystokinin (CCK) signaling appears well conserved over evolution. In Drosophila, the CCK-like sulfakinins (DSKs) regulate aspects of gut function, satiety and food ingestion, hyperactivity and aggression, as well as escape-related locomotion and synaptic plasticity during neuromuscular junction development. Activity in the DSK-producing neurons is regulated by octopamine. We discuss mechanisms behind CCK function in satiety, aggression, and locomotion in some detail and highlight similarities to mammalian CCK signaling.

Characterization of sulfakinin and sulfakinin receptor and their roles in food intake in the red flour beetle Tribolium castaneum

Sulfakinins (SK) are multifunctional neuropeptides widely found in insects that are structurally and functionally homologous to the mammalian gastrin/cholecystokinin (CCK) neuropeptides. CCK is involved in various biological processes such as the feeding regulation where it induces satiety. In this project we characterized SK and SK receptor (SKR) of an important pest and model beetle insect, the red flour beetle Tribolium castaneum, with the aim to better understand the SK signaling pathway and its function in food intake. The sk gene encoded a SK precursor with 113 amino acids and the skr gene a seven-transmembrane SKR with 554 amino acids. Both genes were expressed in the larval, pupal and adult stages with different expression levels in tested tissues. By RNA interference, sk dsRNA and skr dsRNA reduced the expression of the corresponding target gene by 80-90% and 30-50%, respectively, and stimulated food intake in the larvae. In parallel, we injected insects with a SK analog reducing food intake. In conclusion, the data are discussed in relation to the SK signaling pathway and its physiological-endocrinological role in regulating food intake and potential usage in the control of important pest insects.

Analogs of sulfakinin-related peptides demonstrate reduction in food intake in the red flour beetle, Tribolium castaneum, while putative antagonists increase consumption

The insect sulfakinins (SKs) constitute a family of neuropeptides that display both structural and functional similarities to the mammalian hormones gastrin and cholecystokinin (CCK). As a multifunctional neuropeptide, SKs are involved in muscle contractions as well as food intake regulation in many insects. In the red flour beetle Tribolium castaneum, the action on food intake by a series of synthetic SK analogs and one putative antagonist was investigated by injection in beetle adults. The most remarkable result was that both sulfated and non-sulfated SKs [FDDY(SO3H)GHMRFamide] inhibited food intake by about 70%. Strong activity observed for SK analogs featuring a residue that mimics the acidic nature of Tyr(SO3H) but lack the phenyl ring of Tyr, indicate that aromaticity is not a critical characteristic for this position of the peptide. SK demonstrated considerable tolerance to Ser and Ala substitution in position 8 (basic Arg), as analogs featuring these uncharged substitutions retained almost all of the food intake inhibitory activity. Also, the Phe in position 1 could be replaced by Ser without complete loss of activity. Conversely, substitution of Met by Nle in position 7 led to inactive compounds. Finally, the Caenorhabditis elegans sulfated neuropeptide-like protein-12 (NLP-12), that shares some sequence similarities with the SKs but features a Gln-Phe-amide rather than an Arg-Phe-amide at the C-terminus, elicited increased food intake in T. castaneum, which may indicate an antagonist activity. Co-injection of NLP-12 with nsSK blocked the food intake inhibitory effects of nsSK.

Drosulfakinin activates CCKLR-17D1 and promotes larval locomotion and escape response in Drosophila

Neuropeptides are ubiquitous in both mammals and invertebrates and play essential roles in regulation and modulation of many developmental and physiological processes through activation of G-protein-coupled-receptors (GPCRs). However, the mechanisms by which many of the neuropeptides regulate specific neural function and behaviors remain undefined. Here we investigate the functions of Drosulfakinin (DSK), the Drosophila homolog of vertebrate neuropeptide cholecystokinin (CCK), which is the most abundant neuropeptide in the central nervous system. We provide biochemical evidence that sulfated DSK-1 and DSK-2 activate the CCKLR-17D1 receptor in a cell culture assay. We further examine the role of DSK and CCKLR-17D1 in the regulation of larval locomotion, both in a semi-intact larval preparation and in intact larvae under intense light exposure. Our results suggest that DSK/CCKLR-17D1 signaling promote larval body wall muscle contraction and is necessary for mediating locomotor behavior in stress-induced escape response.

Peptidergic control of food intake and digestion in insects 1This review is part of a virtual symposium on recent advances in understanding a variety of complex regulatory processes in insect physiology and endocrinology, including development, metabolism, cold hardiness, food intake and digestion, and diuresis, through the use of omics technologies in the postgenomic era

Like all heterotrophic organisms, insects require a strict control of food intake and efficient digestion of food into nutrients to maintain homeostasis and to fulfill physiological tasks. Feeding and digestion are steered by both external and internal signals that are transduced by a multitude of regulatory factors, delivered either by neurons innervating the gut or mouthparts, or by midgut endocrine cells. The present review gives an overview of peptide regulators known to control feeding and digestion in insects. We describe the discovery and functional role in these processes for insect allatoregulatory peptides, diuretic hormones, FMRFamide-related peptides, (short) neuropeptide F, proctolin, saliva production stimulating peptides, kinins, and tachykinins. These peptides control either gut myoactivity, food intake, and (or) release of digestive enzymes. Some peptides exert their action at multiple levels, possibly having a biological function that depends on their site of delivery. Many regulatory peptides have been physically extracted from different insect species. However, multiple peptidomics, proteomics, transcriptomics, and genome sequencing projects have led to increased discovery and prediction of peptide (precursor) and receptor sequences. In combination with physiological experiments, these large-scale projects have already led to important steps forward in unraveling the physiology of feeding and digestion in insects.

Effect of serotonin and calcium on the supercontractile muscles of the adult blowfly crop

Bioassays and electrophysiological recordings were conducted to determine the role of serotonin and calcium on the supercontractile pump muscles of the diverticulated crop of adult blowflies. Using in situ crop preparations, serotonin was found to significantly increase the rates of contractions of a specific pump in the crop wall, pump P4. The addition of the serotonin antagonist, mianserin, or calcium free saline, both significantly reduced the contraction rates of this pump. Recordings, using suction electrodes from pump P4, confirm the in situ bioassay data and show that serotonin promotes muscle activity in empty crops in which no pump activity is normally observed. Moreover, our data indicate the crucial role of extracellular calcium ions in crop pump contractile activity. These results provide new information on how the crop of adult dipterans is modulated and suggest that serotonin, possibly supplied by neurons in the thoracico-abdominal neural plexus, may be involved in modulating the pumping of crop contents into the midgut for digestion or triggering antiperistalsis from the foregut in the process known as regurgitation or 'bubbling'.

CRF-like diuretic hormone negatively affects both feeding and reproduction in the desert locust, Schistocerca gregaria

Diuretic hormones (DH) related to the vertebrate Corticotropin Releasing Factor (CRF) have been identified in diverse insect species. In the migratory locust, Locusta migratoria, the CRF-like DH (CRF/DH) is localized in the same neurosecretory cells as the Ovary Maturating Parsin (OMP), a neurohormone that stimulates oocyte growth, vitellogenesis and hemolymph ecdysteroid levels in adult female locusts. In this study, we investigated whether CRF-like DH can influence feeding and reproduction in the desert locust, Schistocerca gregaria. We identified two highly similar S. gregaria CRF-like DH precursor cDNAs, each of which also encodes an OMP isoform. Alignment with other insect CRF-like DH precursors shows relatively high conservation of the CRF/DH sequence while the precursor region corresponding to OMP is not well conserved. Quantitative real-time RT-PCR revealed that the precursor transcripts mainly occur in the central nervous system and their highest expression level was observed in the brain. Injection of locust CRF/DH caused a significantly reduced food intake, while RNAi knockdown stimulated food intake. Therefore, our data indicate that CRF-like DH induces satiety. Furthermore, injection of CRF/DH in adult females retarded oocyte growth and caused lower ecdysteroid titers in hemolymph and ovaries, while RNAi knockdown resulted in opposite effects. The observed effects of CRF/DH may be part of a wider repertoire of neurohormonal activities, constituting an integrating control system that affects food intake and excretion, as well as anabolic processes like oocyte growth and ecdysteroidogenesis, following a meal. Our discussion about the functional relationship between CRF/DH and OMP led to the hypothesis that OMP may possibly act as a monitoring peptide that can elicit negative feedback effects.

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.

Feeding and the rhodopsin family g-protein coupled receptors in nematodes and arthropods

In vertebrates, receptors of the rhodopsin G-protein coupled superfamily (GPCRs) play an important role in the regulation of feeding and energy homeostasis and are activated by peptide hormones produced in the brain-gut axis. These peptides regulate appetite and energy expenditure by promoting or inhibiting food intake. Sequence and function homologs of human GPCRs involved in feeding exist in the nematode roundworm, Caenorhabditis elegans (C. elegans), and the arthropod fruit fly, Drosophila melanogaster (D. melanogaster), suggesting that the mechanisms that regulate food intake emerged early and have been conserved during metazoan radiation. Nematodes and arthropods are the most diverse and successful animal phyla on Earth. They can survive in a vast diversity of environments and have acquired distinct life styles and feeding strategies. The aim of the present review is to investigate if this diversity has affected the evolution of invertebrate GPCRs. Homologs of the C. elegans and D. melanogaster rhodopsin receptors were characterized in the genome of other nematodes and arthropods and receptor evolution compared. With the exception of bombesin receptors (BBR) that are absent from nematodes, a similar gene complement was found. In arthropods, rhodopsin GPCR evolution is characterized by species-specific gene duplications and deletions and in nematodes by gene expansions in species with a free-living stage and gene deletions in representatives of obligate parasitic taxa. Based upon variation in GPCR gene number and potentially divergent functions within phyla we hypothesize that life style and feeding diversity practiced by nematodes and arthropods was one factor that contributed to rhodopsin GPCR gene evolution. Understanding how the regulation of food intake has evolved in invertebrates will contribute to the development of novel drugs to control nematodes and arthropods and the pests and diseases that use them as vectors.

Insulin-Producing Cells in the Drosophila Brain also Express Satiety-Inducing Cholecystokinin-Like Peptide, Drosulfakinin

Regulation of meal size and assessing the nutritional value of food are two important aspects of feeding behavior. The mechanisms that regulate these two aspects have not been fully elucidated in Drosophila. Diminished signaling with insulin-like peptides Drosophila insulin-like peptides (DILPs) affects food intake in flies, but it is not clear what signal(s) mediates satiety. Here we investigate the role of DILPs and drosulfakinins (DSKs), cholecystokinin-like peptides, as satiety signals in Drosophila. We show that DSKs and DILPs are co-expressed in insulin-producing cells (IPCs) of the brain. Next we analyzed the effects of diminishing DSKs or DILPs employing the Gal4-UAS system by (1) diminishing DSK-levels without directly affecting DILP levels by targeted Dsk-RNAi, either in all DSK-producing cells (DPCs) or only in the IPCs or (2) expressing a hyperpolarizing potassium channel to inactivate either all the DPCs or only the IPCs, affecting release of both peptides. The transgenic flies were assayed for feeding and food choice, resistance to starvation, and for levels of Dilp and Dsk transcripts in brains of fed and starved animals. Diminishment of DSK in the IPCs alone is sufficient to cause defective regulation of food intake and food choice, indicating that DSK functions as a hormonal satiety signal in Drosophila. Quantification of Dsk and Dilp transcript levels reveals that knockdown of either peptide type affects the transcript levels of the other, suggesting a possible feedback regulation between the two signaling pathways. In summary, DSK and DILPs released from the IPCs regulate feeding, food choice and metabolic homeostasis in Drosophila in a coordinated fashion.

The CCK(-like) receptor in the animal kingdom: functions, evolution and structures

In this review, the cholecystokinin (CCK)(-like) receptors throughout the animal kingdom are compared on the level of physiological functions, evolutionary basis and molecular structure. In vertebrates, the CCK receptor is an important member of the G-protein coupled receptors as it is involved in the regulation of many physiological functions like satiety, gastrointestinal motility, gastric acid secretion, gall bladder contraction, pancreatic secretion, panic, anxiety and memory and learning processes. A homolog for this receptor is also found in nematodes and arthropods, called CK receptor and sulfakinin (SK) receptor, respectively. These receptors seem to have evolved from a common ancestor which is probably still closely related to the nematode CK receptor. The SK receptor is more closely related to the CCK receptor and seems to have similar functions. A molecular 3D-model for the CCK receptor type 1 has been built together with the docking of the natural ligands for the CCK and SK receptors in the CCK receptor type 1. These molecular models can help to study ligand-receptor interactions, that can in turn be useful in the development of new CCK(-like) receptor agonists and antagonists with beneficial health effects in humans or potential for pest control.

The leucokinin pathway and its neurons regulate meal size in Drosophila

BACKGROUND

Total food intake is a function of meal size and meal frequency, and adjustments to these parameters allow animals to maintain a stable energy balance in changing environmental conditions. The physiological mechanisms that regulate meal size have been studied in blowflies but have not been previously examined in Drosophila.

RESULTS

Here we show that mutations in the leucokinin neuropeptide (leuc) and leucokinin receptor (lkr) genes cause phenotypes in which Drosophila adults have an increase in meal size and a compensatory reduction in meal frequency. Because mutant flies take larger but fewer meals, their caloric intake is the same as that of wild-type flies. The expression patterns of the leuc and lkr genes identify small groups of brain neurons that regulate this behavior. Leuc-containing presynaptic terminals are found close to Lkr neurons in the brain and ventral ganglia, suggesting that they deliver Leuc peptide to these neurons. Lkr neurons innervate the foregut. Flies in which Leuc or Lkr neurons are ablated have defects identical to those of leucokinin pathway mutants.

CONCLUSIONS

Our data suggest that the increase in meal size in leuc and lkr mutants is due to a meal termination defect, perhaps arising from impaired communication of gut distension signals to the brain. Leucokinin and the leucokinin receptor are homologous to vertebrate tachykinin and its receptor, and injection of tachykinins reduces food consumption. Our results suggest that the roles of the tachykinin system in regulating food intake might be evolutionarily conserved between insects and vertebrates.

Neuropeptides associated with the regulation of feeding in insects

The stomatogastric nervous system plays a pivotal role in feeding behaviour. Central to this system is the frontal ganglion, which is responsible for foregut motor activity, and hence the passage of food through the gut. Many insect peptides, which exhibit myoactivity on the visceral muscles of the gut in vitro, have been detected in the stomatogastric nervous system by immunochemical or mass spectrometric techniques. This localisation of myoactive peptides, particularly in the frontal ganglion, implies roles for these peptides in the neural control and modulation of feeding in insects. Insect sulfakinins, tachykinins, allatotropin and proctolin have all been shown to stimulate the foregut muscles, whereas myosuppressins, myoinhibitory peptides and allatostatins all inhibited spontaneous contractions of the foregut in a variety of insects. Some of these peptides, when injected, inhibited feeding in vivo. Both the A-type and B-type allatostatins suppressed feeding activity when injected into the cockroach, Blattella germanica and the Manduca sexta C-type allatostatin and allatotropin inhibited feeding when injected into the larvae of two noctuid moths, Lacanobia oleracea and Spodoptera frugiperda, respectively. Injection of sulfakinins into the fly Phormia regina, the locust Schistocera gregaria and the cockroach B. germanica also suppressed feeding, whereas silencing the sulfakinin gene through the injection of double stranded RNA resulted in an increase in food consumption in the cricket Gryllus bimaculatus. The regulation of feeding in insects is clearly very complex, and involves the interaction of a number of mechanisms, one of which is the release, either centrally or locally, of neuropeptides. However, the role of neuropeptides, their mechanisms of action, interactions with each other, and their release are still poorly understood. It is also unclear why insects possess such a number of different peptides, some with multiples copies or homologues, which stimulate or inhibit gut motility, and how their release, sometimes from the same neurone, is regulated. These neuropeptides may also act at sites other than visceral muscles, such as centrally through the brain or on gut stretch receptors.

The different effects of structurally related sulfakinins on Drosophila melanogaster odor preference and locomotion suggest involvement of distinct mechanisms

Sulfakinins are myoactive peptides and antifeedant factors. Naturally occurring drosulfakinin I (DSK I; FDDYGHMRFNH(2)) and drosulfakinin II (DSK II; GGDDQFDDYGHMRFNH(2)) contain sulfated or nonsulfated tyrosine. We discovered sDSK II and nsDSK II influenced Drosophila melanogaster larval odor preference. However, sDSK I, nsDSK I, MRFNH(2), and saline did not influence odor preference. We discovered sDSK I and nsDSK I influenced larval locomotion. However, sDSK II, nsDSK II, MRFNH(2), and saline did not influence locomotion. Our novel data suggest distinct mechanisms underlie the effects of DSK I and DSK II peptides on odor preference and locomotion, parameters important to many facets of animal survival.

Discovery of a cholecystokinin-gastrin-like signaling system in nematodes

Members of the cholecystokinin (CCK)/gastrin family of peptides, including the arthropod sulfakinins, and their cognate receptors, play an important role in the regulation of feeding behavior and energy homeostasis. Despite many efforts after the discovery of CCK/gastrin immunoreactivity in nematodes 23 yr ago, the identity of these nematode CCK/gastrin-related peptides has remained a mystery ever since. The Caenorhabditis elegans genome contains two genes with high identity to the mammalian CCK receptors and their invertebrate counterparts, the sulfakinin receptors. By using the potential C. elegans CCK receptors as a fishing hook, we have isolated and identified two CCK-like neuropeptides encoded by neuropeptide-like protein-12 (nlp-12) as the endogenous ligands of these receptors. The neuropeptide-like protein-12 peptides have a very limited neuronal expression pattern, seem to occur in vivo in the unsulfated form, and react specifically with a human CCK-8 antibody. Both receptors and ligands share a high degree of structural similarity with their vertebrate and arthropod counterparts, and also display similar biological activities with respect to digestive enzyme secretion and fat storage. Our data indicate that the gastrin-CCK signaling system was already well established before the divergence of protostomes and deuterostomes.

Clonidine, octopaminergic receptor agonist, reduces protein feeding in the blow fly, Phormia regina (Meigen)

Results in this study are consistent with those of Murdock and his colleagues who clearly demonstrated that clonidine, an agonist of octopaminergic receptors in some insects, significantly increases sucrose feeding. Their studies, however, did not examine the effect of clonidine on protein feeding. Injection of a 20 microg/microl/fly dose of clonidine significantly reduces protein feeding in both sexes of Phormia regina, instead of stimulating feeding as is observed with carbohydrate feeding. The manner in which the flies are fed prior to starvation and the method of testing influences the amounts of diet consumed. It is proposed that the biogenic amines influence the state of hunger (i.e., protein versus carbohydrates) while other chemicals and neural mechanisms (i.e., such as sulfakinins and stretch receptors, respectively) affect satiety.

RNA interference suggests sulfakinins as satiety effectors in the cricket Gryllus bimaculatus

In the Mediterranean field cricket, Gryllus bimaculatus, the action of sulfakinin (SK) gene expression on food intake, food transport in the gut and carbohydrate digestion (alpha-amylase activity) was investigated by using the RNA interference (RNAi) method. Injection of SK double-stranded (ds) RNA into the abdomen of female adults and last instar larvae led to a systemic silencing of the SK gene, as was shown by RT-PCR studies. In adults, suppression of SK gene expression was effective from the first day after injection up to at least the third day. Treatment of the adult crickets by injection or feeding of dsRNA led to a stimulation of the food intake. Assuming that the gene silencing is followed by a depletion of the SK in tissues and/or haemolymph implies an inhibitiory role of the native SK peptides on food intake. The alpha-amylase activity in vitro in the midgut tissue and in the secretions of adult females was not affected by silencing the SK gene.

Structure of the sulfakinin cDNA and gene expression from the Mediterranean field cricket Gryllus bimaculatus

The sulfakinins are multifunctional insect neuropeptides displaying sequence similarities with the gastrin/ cholecystokinin (CCK) peptide family. In vertebrates, the peptides gastrin and CCK are involved in the regulation of digestion and food-intake. In this study sulfakinin cDNA was cloned and sequenced from the Mediterranean field cricket Gryllus bimaculatus. The cDNA encodes two peptides flanked by endoproteolytic processing sites, designated GrybiSKI (QSDDYGHMRFG) and GrybiSKII (EPFDDYGHMRFG). The peptides include the characteristic amino acid Tyr, which is potentially sulphated, and a Gly, as a recognition site for amidation yeilding the common C-terminal amino acid sequence of the sulfakinin peptide family. RT-PCR studies indicate an expression of the gene restricted to the brain, with a constant level of expression throughout the last larval stage, but showing an age-dependent decrease of expression in adult females.

Identification and cardiotropic actions of sulfakinin peptides in the American lobster Homarus americanus

In arthropods, a group of peptides possessing a–Y(SO3H)GHM/LRFamide carboxy-terminal motif have been collectively termed the sulfakinins. Sulfakinin isoforms have been identified from numerous insect species. In contrast, members of this peptide family have thus far been isolated from just two crustaceans, the penaeid shrimp Penaeus monodon and Litopenaeus vannamei. Here, we report the identification of a cDNA encoding prepro-sulfakinin from the American lobster Homarus americanus. Two sulfakinin-like sequences were identified within the open-reading frame of the cDNA. Based on modifications predicted by peptide modeling programs, and on homology to the known isoforms of sulfakinin, particularly those from shrimp, the mature H. americanus sulfakinins were hypothesized to be pEFDEY(SO3H)GHMRFamide (Hoa-SK I) and GGGEY(SO3H)DDY(SO3H)GHLRFamide (Hoa-SK II). Hoa-SK I is identical to one of the previously identified shrimp sulfakinins, while Hoa-SK II is a novel isoform. Exogenous application of either synthetic Hoa-SK I or Hoa-SK II to the isolated lobster heart increased both the frequency and amplitude of spontaneous heart contractions. In preparations in which spontaneous contractions were irregular, both peptides increased the regularity of the heartbeat. Our study provides the first molecular characterization of a sulfakinin-encoding cDNA from a crustacean, as well as the first demonstration of bioactivity for native sulfakinins in this group of arthropods.