NKD, a developmentally regulated tachykinin receptor in Drosophila

A brief history of insect neuropeptide and peptide hormone research

This review briefly summarizes 50 years of research on insect neuropeptide and peptide hormone (collectively abbreviated NPH) signaling, starting with the sequencing of proctolin in 1975. The first 25 years, before the sequencing of the Drosophila genome, were characterized by efforts to identify novel NPHs by biochemical means, mapping of their distribution in neurons, neurosecretory cells, and endocrine cells of the intestine. Functional studies of NPHs were predominantly dealing with hormonal aspects of peptides and many employed ex vivo assays. With the annotation of the Drosophila genome, and more specifically of the NPHs and their receptors in Drosophila and other insects, a new era followed. This started with matching of NPH ligands to orphan receptors, and studies to localize NPHs with improved detection methods. Important advances were made with introduction of a rich repertoire of innovative molecular genetic approaches to localize and interfere with expression or function of NPHs and their receptors. These methods enabled cell- or circuit-specific interference with NPH signaling for in vivo assays to determine roles in behavior and physiology, imaging of neuronal activity, and analysis of connectivity in peptidergic circuits. Recent years have seen a dramatic increase in reports on the multiple functions of NPHs in development, physiology and behavior. Importantly, we can now appreciate the pleiotropic functions of NPHs, as well as the functional peptidergic "networks" where state dependent NPH signaling ensures behavioral plasticity and systemic homeostasis.

Another fly diuretic hormone: tachykinins increase fluid and ion transport by adult Drosophila melanogaster Malpighian 'renal' tubules

Insects such as the model organism Drosophila melanogaster must modulate their internal physiology to withstand changes in temperature and availability of water and food. Regulation of the excretory system by peptidergic hormones is one mechanism by which insects maintain their internal homeostasis. Tachykinins are a family of neuropeptides that have been shown to stimulate fluid secretion from the Malpighian 'renal' tubules (MTs) in some insect species, but it is unclear if that is the case in the fruit fly, D. melanogaster. A central objective of the current study was to examine the physiological role of tachykinin signaling in the MTs of adult D. melanogaster. Using the genetic toolbox available in this model organism along with in vitro and whole-animal bioassays, our results indicate that Drosophila tachykinins (DTKs) function as diuretic hormones by binding to the DTK receptor (DTKR) localized in stellate cells of the MTs. Specifically, DTK activates cation and anion transport across the stimulated MTs, which impairs their survival in response to desiccation because of their inability to conserve water. Thus, besides their previously described roles in neuromodulation of pathways controlling locomotion and food search, olfactory processing, aggression, lipid metabolism and metabolic stress, processing of noxious stimuli and hormone release, DTKs also appear to function as bona fide endocrine factors regulating the excretory system and appear essential for the maintenance of hydromineral balance.

Endocrine cybernetics: neuropeptides as molecular switches in behavioural decisions

Plasticity in animal behaviour relies on the ability to integrate external and internal cues from the changing environment and hence modulate activity in synaptic circuits of the brain. This context-dependent neuromodulation is largely based on non-synaptic signalling with neuropeptides. Here, we describe select peptidergic systems in the Drosophila brain that act at different levels of a hierarchy to modulate behaviour and associated physiology. These systems modulate circuits in brain regions, such as the central complex and the mushroom bodies, which supervise specific behaviours. At the top level of the hierarchy there are small numbers of large peptidergic neurons that arborize widely in multiple areas of the brain to orchestrate or modulate global activity in a state and context-dependent manner. At the bottom level local peptidergic neurons provide executive neuromodulation of sensory gain and intrinsically in restricted parts of specific neuronal circuits. The orchestrating neurons receive interoceptive signals that mediate energy and sleep homeostasis, metabolic state and circadian timing, as well as external cues that affect food search, aggression or mating. Some of these cues can be triggers of conflicting behaviours such as mating versus aggression, or sleep versus feeding, and peptidergic neurons participate in circuits, enabling behaviour choices and switches.

G protein-coupled receptors (GPCRs) in rotifers and cladocerans: Potential applications in ecotoxicology, ecophysiology, comparative endocrinology, and pharmacology

The G protein-coupled receptor (GPCR) superfamily plays a fundamental role in both sensory functions and the regulation of homeostasis, and is highly conserved across the eukaryote taxa. Its functional diversity is related to a conserved seven-transmembrane core and invariant set of intracellular signaling mechanisms. The interplay between these properties is key to the evolutionary success of GPCR. As this superfamily originated from a common ancestor, GPCR genes have evolved via lineage-specific duplications through the process of adaptation. Here we summarized information on GPCR gene families in rotifers and cladocerans based on their evolutionary position in aquatic invertebrates and their potential application in ecotoxicology, ecophysiology, comparative endocrinology, and pharmacology. Phylogenetic analyses were conducted to examine the evolutionary significance of GPCR gene families and to provide structural insight on their role in aquatic invertebrates. In particular, most GPCR gene families have undergone sporadic evolutionary processes, but some GPCRs are highly conserved across species despite the dynamics of GPCR evolution. Overall, this review provides a better understanding of GPCR evolution in aquatic invertebrates and expand our knowledge of the potential application of these receptors in various fields.

Multiple tachykinins and their receptors characterized in the gastropod mollusk Pacific abalone: Expression, signaling cascades, and potential role in regulating lipid metabolism

Tachykinin (TK) families, including the first neuropeptide substance P, have been intensively explored in bilaterians. Knowledge of signaling of TK receptors (TKRs) has enabled the comprehension of diverse physiological processes. However, TK signaling systems are largely unknown in Lophotrochozoa. This study identified two TK precursors and two TKR isoforms in the Pacific abalone Haliotis discus hannai (Hdh), and characterized Hdh-TK signaling. Hdh-TK peptides harbored protostomian TK-specific FXGXRamide or unique YXGXRamide motifs at the C-termini. A phylogenetic analysis showed that lophotrochozoan TKRs, including Hdh-TKRs, form a monophyletic group distinct from arthropod TKRs and natalisin receptor groups. Although reporter assays demonstrated that all examined Hdh-TK peptides activate intracellular cAMP accumulation and Ca²⁺ mobilization in Hdh-TKR-expressing mammalian cells, Hdh-TK peptides with N-terminal aromatic residues and C-terminal FXGXRamide motifs were more active than shorter or less aromatic Hdh-TK peptides with a C-terminal YXGXRamide. In addition, we showed that ligand-stimulated Hdh-TKRs mediate ERK1/2 phosphorylation in HEK293 cells and that ERK1/2 phosphorylation is inhibited by PKA and PKC inhibitors. In three-dimensional in silico Hdh-TKR binding modeling, higher docking scores of Hdh-TK peptides were consistent with the lower EC50 values in the reporter assays. The transcripts for Hdh-TK precursors and Hdh-TKR were highly expressed in the neural ganglia, with lower expression levels in peripheral tissues. When abalone were starved for 3 weeks, Hdh-TK1 transcript levels, but not Hdh-TK2, were increased in the cerebral ganglia (CG), intestine, and hepatopancreas, contrasting with the decreased lipid content and transcript levels of sterol regulatory element-binding protein (SREBP). At 24 h post-injection in vivo, the lower dose of Hdh-TK1 mixture increased SREBP transcript levels in the CG and hepatopancreas and accumulative food consumption of abalone. Higher doses of Hdh-TK1 and Hdh-TK2 mixtures decreased the SREBP levels in the CG. When Hdh-TK2-specific siRNA was injected into abalone, intestinal SREBP levels were significantly increased, whereas administration of both Hdh-TK1 and Hdh-TK2 siRNA led to decreased SREBP expression in the CG. Collectively, our results demonstrate the first TK signaling system in gastropod mollusks and suggest a possible role for TK peptides in regulating lipid metabolism in the neural and peripheral tissues of abalone.

Metabolic control of daily locomotor activity mediated by tachykinin in Drosophila

Metabolism influences locomotor behaviors, but the understanding of neural curcuit control for that is limited. Under standard light-dark cycles, Drosophila exhibits bimodal morning (M) and evening (E) locomotor activities that are controlled by clock neurons. Here, we showed that a high-nutrient diet progressively extended M activity but not E activity. Drosophila tachykinin (DTk) and Tachykinin-like receptor at 86C (TkR86C)-mediated signaling was required for the extension of M activity. DTk neurons were anatomically and functionally connected to the posterior dorsal neuron 1s (DN1_ps) in the clock neuronal network. The activation of DTk neurons reduced intracellular Ca²⁺ levels in DN1_ps suggesting an inhibitory connection. The contacts between DN1_ps and DTk neurons increased gradually over time in flies fed a high-sucrose diet, consistent with the locomotor behavior. DN1_ps have been implicated in integrating environmental sensory inputs (e.g., light and temperature) to control daily locomotor behavior. This study revealed that DN1_ps also coordinated nutrient information through DTk signaling to shape daily locomotor behavior.

Lee and colleagues report the effect of a high-sucrose diet on Drosophila locomotor activity via DTk-TkR86C neuropeptide signalling. This signalling pattern appears to involve a circadian element, with pacemaker neuron involvement having a possible time-of-day effect on locomotor behaviour.

Bacterial-mediated RNAi and functional analysis of Natalisin in a moth

The neuropeptide natalisin (NTL) has been determined to play essential roles in reproduction in two Diptera and one Coleoptera species. Whether NTL has similar or even different functions in Lepidoptera remains to be determined. Here, we cloned the NTL transcript in the common cutworm moth Spodoptera litura. This transcript encodes a 438-amino acid protein. Twelve putative Sl-NTL neuropeptides were defined by cleavage sites. These NTL peptides share a DDPFWxxRamide C-terminal motif. The expressions of Sl-NTL is low during the egg and larval stages, which increased to a higher level during the pupal stage, and then reached the maximum during the adult stage. Moreover, the expression pattern during the pupal stage is similar between sexes while during the adult stage, it is dimorphic. To explore the function of Sl-NTL and assess its potential as a target for pest control, we knocked down the expression of Sl-NTL in both sexes by using bacteria-mediated RNAi. This technique significantly down regulated (reduced up to 83%) the expression of Sl-NTL in both sexes. Knocking down Sl-NTL expression did not significantly affect its development, survival and morphology but significantly reduced adults' reproductive behavior (including female calling, male courtship, mating and remating patterns and rates) and reproductive output (offspring gain reduced more than 70%).

Ligand-dependent internalization of Bombyx mori tachykinin-related peptide receptor is regulated by PKC, GRK5 and β-arrestin2/BmKurtz

Tachykinin signaling system is present in both vertebrates and invertebrates, and functions as neuromodulator responsible for the regulation of various physiological processes. In human, the internalization of G protein-coupled receptors has been extensively characterized; however, the insect GPCR internalization has been rarely investigated. Here, we constructed two expression vectors of Bombyx tachykinin-related peptide receptor (BmTKRPR) fused with Enhanced Green Fluorescent Protein (EGFP) at the C-terminal end for direct visualization of receptor expression, localization, and trafficking in cultured mammalian HEK293 and insect Sf21 cells. Our results demonstrated that agonist-activated BmTKRPR underwent rapid internalization in a dose-and time-dependent manner via a clathrin-dependent pathway in both HEK293 and Sf21 cells. Further investigation via RNAi or specific inhibitors, or co-immunoprecipitation demonstrated that agonist-induced BmTKRPR internalization was mediated by PKC, GRK5 and β-arrestin2/BmKurtz. In addition, we also observed that most of the internalized BmTKRP receptors were recycled to the cell surface via early endosomes upon peptide ligand removal. Our study provides the first in-depth information on mechanisms underlying insect TKRP receptor internalization and perhaps aids in the interpretation of the signaling in the regulation of physiological processes.

Tachykinins: Neuropeptides That Are Ancient, Diverse, Widespread and Functionally Pleiotropic

Tachykinins (TKs) are ancient neuropeptides present throughout the bilaterians and are, with some exceptions, characterized by a conserved FX1GX2Ramide carboxy terminus among protostomes and FXGLMamide in deuterostomes. The best-known TK is the vertebrate substance P, which in mammals, together with other TKs, has been implicated in health and disease with important roles in pain, inflammation, cancer, depressive disorder, immune system, gut function, hematopoiesis, sensory processing, and hormone regulation. The invertebrate TKs are also known to have multiple functions in the central nervous system and intestine and these have been investigated in more detail in the fly Drosophila and some other arthropods. Here, we review the protostome and deuterostome organization and evolution of TK precursors, peptides and their receptors, as well as their functions, which appear to be partly conserved across Bilateria. We also outline the distribution of TKs in the brains of representative organisms. In Drosophila, recent studies have revealed roles of TKs in early olfactory processing, neuromodulation in circuits controlling locomotion and food search, nociception, aggression, metabolic stress, and hormone release. TK signaling also regulates lipid metabolism in the Drosophila intestine. In crustaceans, TK is an important neuromodulator in rhythm-generating motor circuits in the stomatogastric nervous system and a presynaptic modulator of photoreceptor cells. Several additional functional roles of invertebrate TKs can be inferred from their distribution in various brain circuits. In addition, there are a few interesting cases where invertebrate TKs are injected into prey animals as vasodilators from salivary glands or paralyzing agents from venom glands. In these cases, the peptides are produced in the glands of the predator with sequences mimicking the prey TKs. Lastly, the TK-signaling system appears to have duplicated in Panarthropoda (comprising arthropods, onychophores, and tardigrades) to give rise to a novel type of peptides, natalisins, with a distinct receptor. The distribution and functions of natalisins are distinct from the TKs. In general, it appears that TKs are widely distributed and act in circuits at short range as neuromodulators or cotransmitters.

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.

Blood-induced differential gene expression in Anopheles dirus evaluated using RNA sequencing

Malaria parasites are transmitted through blood feeding by female Anopheline mosquitoes. Unveiling the blood-feeding process will improve understanding of vector biology. Anopheles dirus (Diptera: Culicidae) is one of the primary malaria vectors in the Greater Mekong Subregion, the epicentre of malaria drug resistance. In this study, differential gene expression between sugar- and blood-fed An. dirus was investigated by RNA sequencing (RNA-seq). A total of 589 transcripts were found to be upregulated and 703 transcripts downregulated as a result of blood feeding. Transcriptional differences were found in genes involved in blood digestion, peritrophic matrix formation, oogenesis and vitellogenesis. The expression levels of several genes were validated by quantitative reverse transcription polymerase chain reaction. The present results provide better understanding of An. dirus biology in relation to its blood feeding.

Characterization of a tachykinin signalling system in the bivalve mollusc Crassostrea gigas

Although tachykinin-like neuropeptides have been identified in molluscs more than two decades ago, knowledge on their function and signalling has so far remained largely elusive. We developed a cell-based assay to address the functionality of the tachykinin G-protein coupled receptor (Cragi-TKR) in the oyster Crassostrea gigas. The oyster tachykinin neuropeptides that are derived from the tachykinin precursor gene Cragi-TK activate the Cragi-TKR in nanomolar concentrations. Receptor activation is sensitive to Ala-substitution of critical Cragi-TK amino acid residues. The Cragi-TKR gene is expressed in a variety of tissues, albeit at higher levels in the visceral ganglia (VG) of the nervous system. Fluctuations of Cragi-TKR expression is in line with a role for TK signalling in C. gigas reproduction. The expression level of the Cragi-TK gene in the VG depends on the nutritional status of the oyster, suggesting a role for TK signalling in the complex regulation of feeding in C. gigas.

Function of the natalisin receptor in mating of the oriental fruit fly, Bactrocera dorsalis (Hendel) and testing of peptidomimetics

Natalisins (NTLs) are conservative neuropeptides, which are only found in arthropods and are documented to regulate reproductive behaviors in insects. In our previous study, we have confirmed that NTLs regulate the reproductive process in an important agricultural pest, Bactrocera dorsalis (Hendel). Hence, in this study, to further confirm the in vivo function of NTL receptor (NTLR) and assess the potential of NTLR as an insecticide target, RNA interference targeting NTLR mRNA was performed. We found that mating frequencies of both males and females were reduced by RNAi-mediated knockdown of the NTLR transcript, while there was no effect on mating duration. Moreover, we functionally expressed the B. dorsalis NTLR in Chinese Hamster Ovary (CHO) cells and was co-transfected with an aequorin reporter to measure ligand activities. A total of 13 biostable multi-Aib analogs were tested for agonistic and antagonistic activities. While most of these NTL analogs did not show strong activity, one analog (NLFQV[Aib]DPFF[Aib]TRamide) had moderate antagonistic activity. Taken together, we provided evidence for the important roles of NTLR in regulating mating frequencies of both male and female in this fly and also provided in vitro data on mimetic analogs that serve as leading structures for the development of agonists and antagonists to disrupt the NTL signaling pathway.

Molecular characterizations of natalisin and its roles in modulating mating in the oriental fruit fly, Bactrocera dorsalis (Hendel)

Initially, natalisin (NTL) was identified from three holometabolous insect species, Drosophila melanogaster, Tribolium castaneum and Bombyx mori, and was documented to regulate reproductive behaviours in D. melanogaster and T. castaneum. In this study, we report the sequences of the NTL precursor and its receptor (NTLR) from an important agricultural pest, Bactrocera dorsalis (Hendel). NTLR is a typical G-protein coupled receptor and phylogenetic analysis showed that B. dorsalis NTLR was closely related to insect natalisin receptors from other species. A functional assay of NTLR transiently expressed in Chinese hamster ovary cells showed that it was activated by putative natalisin mature peptides in a concentration-dependent manner, with 50% effective concentrations (EC₅₀ ) at nanomolar or micromolar levels. As indicated by quantitative real-time PCR, both NTL and NTLR had the highest expression in the central nervous system of B. dorsalis compared with the other tested tissues. Three pairs of adult brain neurones of B. dorsalis were identified with immunohistochemical antibody staining against D. melanogaster NTL4, and in situ hybridization with specific DNA probes. Moreover, RNA interference mediated by double-stranded RNA injection in adults provided evidence for the important roles of NTL in regulating both male and female mating frequencies in this fly.

Comparative biology of pain: What invertebrates can tell us about how nociception works

The inability to adequately treat chronic pain is a worldwide health care crisis. Pain has both an emotional and a sensory component, and this latter component, nociception, refers specifically to the detection of damaging or potentially damaging stimuli. Nociception represents a critical interaction between an animal and its environment and exhibits considerable evolutionary conservation across species. Using comparative approaches to understand the basic biology of nociception could promote the development of novel therapeutic strategies to treat pain, and studies of nociception in invertebrates can provide especially useful insights toward this goal. Both vertebrates and invertebrates exhibit segregated sensory pathways for nociceptive and nonnociceptive information, injury-induced sensitization to nociceptive and nonnociceptive stimuli, and even similar antinociceptive modulatory processes. In a number of invertebrate species, the central nervous system is understood in considerable detail, and it is often possible to record from and/or manipulate single identifiable neurons through either molecular genetic or physiological approaches. Invertebrates also provide an opportunity to study nociception in an ethologically relevant context that can provide novel insights into the nature of how injury-inducing stimuli produce persistent changes in behavior. Despite these advantages, invertebrates have been underutilized in nociception research. In this review, findings from invertebrate nociception studies are summarized, and proposals for how research using invertebrates can address questions about the fundamental mechanisms of nociception are presented.

Role of a tachykinin-related peptide and its receptor in modulating the olfactory sensitivity in the oriental fruit fly, Bactrocera dorsalis (Hendel)

Insect tachykinin-related peptide (TRP), an ortholog of tachykinin in vertebrates, has been linked with regulation of diverse physiological processes, such as olfactory perception, locomotion, aggression, lipid metabolism and myotropic activity. In this study, we investigated the function of TRP (BdTRP) and its receptor (BdTRPR) in an important agricultural pest, the oriental fruit fly Bactrocera dorsalis. BdTRPR is a typical G-protein coupled-receptor (GPCR), and it could be activated by the putative BdTRP mature peptides with the effective concentrations (EC₅₀) at the nanomolar range when expressed in Chinese hamster ovary cells. Consistent with its role as a neuromodulator, expression of BdTRP was detected in the central nervous system (CNS) of B. dorsalis, specifically in the local interneurons with cell bodies lateral to the antennal lobe. BdTRPR was found in the CNS, midgut and hindgut, but interestingly also in the antennae. To investigate the role of BdTRP and BdTRPR in olfaction behavior, adult flies were subjected to RNA interference, which led to a reduction in the antennal electrophysiological response and sensitivity to ethyl acetate in the Y-tube assay. Taken together, we demonstrate the impact of TRP/TRPR signaling on the modulation of the olfactory sensitivity in B. dorsalis. The result improve our understanding of olfactory processing in this agriculturally important pest insect.

Preliminary Report of a Neurokinin-Like Receptor Gene Sequence for the Nemertean Paranemertes sp

Tachykinins (TKs) are a family of neurotransmitters that function as signaling molecules for such processes as maintaining homeostasis, regulating stress response, and modulating pain. TKs require the expression of at least one of three receptor subtypes: Neurokinin Receptor-1 (NKR-1), Neurokinin Receptor-2 (NKR-2), or Neurokinin Receptor-3 (NKR-3). We have isolated and cloned a portion of a gene coding for a tachykinin-like receptor from the nemertean Paranemertes sp. This 488-bp portion contains a short 101-bp segment that shares 85% similarity to the mouse substance-K receptor in Mus musculus and 83% similarity to the moth neuropeptide receptor A24 in Bombyx mori. Translated homology analysis aligning the coding sequence with the initial cytoplasmic carboxyl terminus of numerous G-protein coupled neuropeptide receptors also revealed 73% similarity to B. mori neuropeptide receptor A24. Our finding is the first report of a sequence amplified from Paranemertes sp. that may code for a small portion of a G-protein-coupled neuropeptide receptor with significant similarity to the TKR family, particularly the NKR-3 receptor isoform. This novel finding may open new avenues into exploring the role of tachykinin and its receptor in nemertean neurophysiology.

Tachykinin acts upstream of autocrine Hedgehog signaling during nociceptive sensitization in Drosophila

Pain signaling in vertebrates is modulated by neuropeptides like Substance P (SP). To determine whether such modulation is conserved and potentially uncover novel interactions between nociceptive signaling pathways we examined SP/Tachykinin signaling in a Drosophila model of tissue damage-induced nociceptive hypersensitivity. Tissue-specific knockdowns and genetic mutant analyses revealed that both Tachykinin and Tachykinin-like receptor (DTKR99D) are required for damage-induced thermal nociceptive sensitization. Electrophysiological recording showed that DTKR99D is required in nociceptive sensory neurons for temperature-dependent increases in firing frequency upon tissue damage. DTKR overexpression caused both behavioral and electrophysiological thermal nociceptive hypersensitivity. Hedgehog, another key regulator of nociceptive sensitization, was produced by nociceptive sensory neurons following tissue damage. Surprisingly, genetic epistasis analysis revealed that DTKR function was upstream of Hedgehog-dependent sensitization in nociceptive sensory neurons. Our results highlight a conserved role for Tachykinin signaling in regulating nociception and the power of Drosophila for genetic dissection of nociception.

DOI:http://dx.doi.org/10.7554/eLife.10735.001

Injured animals from humans to insects become extra sensitive to sensations such as touch and heat. This hypersensitivity is thought to protect areas of injury or inflammation while they heal, but it is not clear how it comes about.

Now, Im et al. have addressed this question by assessing pain in fruit flies after tissue damage. The experiments used ultraviolet radiation to essentially cause ‘localized sunburn’ to fruit fly larvae. Electrical impulses were then recorded from the larvae’s pain-detecting neurons and the larvae were analyzed for behaviors that indicate pain responses (for example, rolling).

Im et al. found that tissue injury lowers the threshold at which temperature causes pain in fruit fly larvae. Further experiments using mutant flies that lacked genes involved in two signaling pathways showed that a signaling molecule called Tachykinin and its receptor (called DTKR) are needed to regulate the observed threshold lowering. When the genes for either of these proteins were deleted, the larvae no longer showed the pain hypersensitivity following an injury.

Further experiments then uncovered a genetic interaction between Tachykinin signaling and a second signaling pathway that also regulates pain sensitization (called Hedgehog signaling). Im et al. found that Tachykinin acts upstream of Hedgehog in the pain-detecting neurons. Following on from these findings, the biggest outstanding questions are: how, when and where does tissue damage lead to the release of Tachykinin to sensitize neurons? Future studies could also ask whether the genetic interactions between Hedgehog and Tachykinin (or related proteins) are conserved in other animals such as humans and mice.

DOI:http://dx.doi.org/10.7554/eLife.10735.002

Activation of BNGR-A24 by direct interaction with tachykinin-related peptides from the silkworm Bombyx mori leads to the Gq- and Gs-coupled signaling cascades

Tachykinins constitute one of the largest peptide families in the animal kingdom and exert their diverse actions via G protein-coupled receptors (GPCRs). In this study, the Bombyx tachykinin-related peptides (TKRPs) were identified as specific endogenous ligands for the Bombyx neuropeptide GPCR A24 (BNGR-A24) and thus designated BNGR-A24 as BmTKRPR. Using both mammalian cell line HEK293 and insect cell line Sf21, further characterization demonstrated that BmTKRPR was activated, thus resulting in intracellular accumulation of cAMP, Ca(2+) mobilization, and ERK1/2 phosphorylation in a Gs and Gq inhibitor-sensitive manner. Moreover, quantitative reverse transcriptase polymerase chain reaction analysis and dsRNA-mediated knockdown experiments suggested a possible role for BmTKRPR in the regulation of feeding and growth. Our findings enhance the understanding of the Bombyx TKRP system in the regulation of fundamental physiological processes.

Identification and characterization of receptors for ion transport peptide (ITP) and ITP-like (ITPL) in the silkworm Bombyx mori

Ion transport peptide (ITP) and its alternatively spliced variant, ITP-like (ITPL), are insect peptides that belong to the crustacean hyperglycemic hormone family. These peptides modulate the homeostatic mechanisms for regulating energy metabolism, molting, and reproduction and are specifically conserved in ecdysozoans. Many of the details of the molecular mechanisms by which crustacean hyperglycemic hormone family peptides exert pleiotropy remain to be elucidated, including characterization of their receptors. Here we identified three Bombyx mori orphan neuropeptide G protein-coupled receptors (BNGRs), BNGR-A2, -A24, and -A34, as receptors for ITP and ITPL (collectively referred to as ITPs). BNGR-A2 and -A34 and BNGR-A24 respond to recombinant ITPs, respectively, with EC50 values of 1.1-2.6 × 10(-8) M, when expressed in a heterologous expression system. These three candidate BNGRs are expressed at larval B. mori tissues targeted by ITPs, with cGMP elevation observed after exposure to recombinant ITPs. ITPs also increased the cGMP level in B. mori ovary-derived BmN cells via membrane-bound and soluble guanylyl cyclases. The simultaneous knockdown of bngr-A2 and -A34 significantly decreased the response of BmN cells to ITP, whereas knockdown of bngr-A24 led to decreased responses to ITPL. Conversely, transient expression of bngr-A24 potentiated the response of BmN cells to ITPL. An in vitro binding assay showed direct interaction between ITPs and heterologously expressed BNGRs in a ligand-receptor-specific manner. Taken together, these data demonstrate that BNGR-A2 and -A34 are ITP receptors and that BNGR-A24 is an ITPL receptor in B. mori.

Natalisin, a tachykinin-like signaling system, regulates sexual activity and fecundity in insects

An arthropod-specific peptidergic system, the neuropeptide designated here as natalisin and its receptor, was identified and investigated in three holometabolous insect species: Drosophila melanogaster, Tribolium castaneum, and Bombyx mori. In all three species, natalisin expression was observed in 3-4 pairs of the brain neurons: the anterior dorso-lateral interneurons, inferior contralateral interneurons, and small pars intercerebralis neurons. In B. mori, natalisin also was expressed in two additional pairs of contralateral interneurons in the subesophageal ganglion. Natalisin-RNAi and the activation or silencing of the neural activities in the natalisin-specific cells in D. melanogaster induced significant defects in the mating behaviors of both males and females. Knockdown of natalisin expression in T. castaneum resulted in significant reduction in the fecundity. The similarity of the natalisin C-terminal motifs to those of vertebrate tachykinins and of tachykinin-related peptides in arthropods led us to identify the natalisin receptor. A G protein-coupled receptor, previously known as tachykinin receptor 86C (also known as the neurokinin K receptor of D. melanogaster), now has been recognized as a bona fide natalisin receptor. Taken together, the taxonomic distribution pattern of the natalisin gene and the phylogeny of the receptor suggest that natalisin is an ancestral sibling of tachykinin that evolved only in the arthropod lineage.

Peptidergic signaling in Calanus finmarchicus (Crustacea, Copepoda): in silico identification of putative peptide hormones and their receptors using a de novo assembled transcriptome

The copepod Calanus finmarchicus is the most abundant zooplankton species in the North Atlantic. While the life history of this crustacean is well studied, little is known about its peptidergic signaling systems despite the fact that these pathways are undoubtedly important components of its physiological/behavioral control systems. Here we have generated and used a de novo assembled transcriptome for C. finmarchicus (206,041 sequences in total) to identify peptide precursor proteins and receptors. Using known protein queries, 34 transcripts encoding peptide preprohormones and 18 encoding peptide receptors were identified. Using a combination of online software programs and homology to known arthropod isoforms, 148 mature peptides were predicted from the deduced precursors, including members of the allatostatin-A, allatostatin-B, allatostatin-C, bursicon, crustacean cardioactive peptide (CCAP), crustacean hyperglycemic hormone, diuretic hormone 31 (DH31), diuretic hormone 44 (DH44), FMRFamide-like peptide (myosuppressin, neuropeptide F [NPF] and extended FL/IRFamide subfamilies), leucokinin, neuroparsin, orcokinin, orcomyotropin, periviscerokinin, RYamide and tachykinin-related peptide (TRP) families. The identified receptors included ones for allatostatin-A, allatostatin-C, bursicon, CCAP, DH31, DH44, ecdysis-triggering hormone, NPF, short NPF, FMRFamide, insulin-like peptide, leucokinin, periviscerokinin, pigment dispersing hormone, and TRP. Developmental profiling of the identified transcripts in embryos, early nauplii, late nauplii, early copepodites, late copepodites, and adult females was also undertaken, with all showing the highest expression levels in the naupliar and copepodite stages. Collectively, these data radically expand the catalog of known C. finmarchicus peptidergic signaling proteins and provide a foundation for experiments directed at understanding the physiological roles served by them in this species.

Presynaptic modulation of early olfactory processing in Drosophila

Most animals are endowed with an olfactory system that is essential for finding foods, avoiding predators, and locating mating partners. The olfactory system must encode the identity and intensity of behaviorally relevant stimuli in a dynamic environmental landscape. How is olfactory information represented? How is a large dynamic range of odor concentrations represented in the olfactory system? How is this representation modulated to meet the demands of different internal physiological states? Recent studies have found that sensory terminals are important targets for neuromodulation. The emerging evidence suggests that presynaptic inhibition scales with sensory input and thus provides a mechanism to increase dynamic range of odor representation. In addition, presynaptic facilitation could be a mechanism to alter behavioral responses in hungry animals. This review will focus on the GABA(B) (gamma-aminobutyric acid) receptor-mediated presynaptic inhibition, and neuropeptide-mediated presynaptic modulation in Drosophila.

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.

Endocrine regulation of insect diuresis in the early postgenomic era1This 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

Diuresis, the removal of excess metabolic waste through production of primary urine while maintaining homeostasis, is an important biological process that is tightly regulated by endocrine factors. Several hormonal components that act as diuretic or antidiuretic factors in insects have been identified in the last few decades. Physiological mechanisms responsible for ion and water transport across biological membranes have been intensively studied. The large amount of data rapidly accumulating in the genomics era has led to an increased dependence on reverse genetic and physiological approaches, first identifying candidate genes and subsequently deriving functions. In many cases, the reverse approaches have been highly successful, especially in studies of the receptors for diuretic factors, which are mainly G-protein-coupled receptors. This review summarizes research on insect diuretic and antidiuretic endocrine factors, and their receptors. Emphases of the review are given to the genomics of ligands and their receptors, as well as to their implications for evolution and function.

Insulin-producing cells and their regulation in physiology and behavior ofDrosophila1This 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

Insulin-like peptide signaling regulates development, growth, reproduction, metabolism, stress resistance, and life span in a wide spectrum of animals. Not only the peptides, but also their tyrosine kinase receptors and the downstream signaling pathways are conserved over evolution. This review summarizes roles of insulin-like peptides (DILPs) in physiology and behavior of Drosophila melanogaster Meigen, 1830. Seven DILPs (DILP1–7) and one receptor (dInR) have been identified in Drosophila. These DILPs display cell and stage specific expression patterns. In the adult, DILP2, 3, and 5 are expressed in insulin-producing cells (IPCs) among the median neurosecretory cells of the brain, DILP7 in 20 neurons of the abdominal ganglion, and DILP6 in the fat body. The DILPs of the IPCs regulate starvation resistance, responses to oxidative and temperature stress, and carbohydrate and lipid metabolism. Furthermore, the IPCs seem to regulate feeding, locomotor activity, sleep and ethanol sensitivity, but the mechanisms are not elucidated. Insulin also alters the sensitivity in the olfactory system that affects food search behavior, and regulates peptidergic neurons that control aspects of feeding behavior. Finally, the control of insulin production and release by humoral and neuronal factors is discussed. This includes a fat body derived factor and the neurotransmitters GABA, serotonin, octopamine, and two neuropeptides.

Discovery and functional study of a novel crustacean tachykinin neuropeptide

Tachykinin-related peptide (TRP) refers to a large and structurally diverse family of neuropeptides found in vertebrate and invertebrate nervous systems. These peptides have various important physiological functions, from regulating stress in mammals to exciting the pyloric (food filtering) rhythm in the stomatogastric nervous system (STNS) of decapod crustaceans. Here, a novel TRP, which we named CalsTRP (Callinectes sapidus TRP), YPSGFLGMRamide (m/z 1026.52), was identified and de novo sequenced using a multifaceted mass spectrometry-based platform in both the central nervous system (CNS) and STNS of C. sapidus. We also found, using isotopic formaldehyde labeling, that CalsTRP in the C. sapidus brain and commissural ganglion (CoG) was up-regulated after food-intake, suggesting that TRPs in the CNS and STNS are involved in regulating feeding in Callinectes. Using imaging mass spectrometry, we determined that the previously identified CabTRP Ia (APSGFLGMRamide) and CalsTRP were co-localized in the C. sapidus brain. Lastly, our electrophysiological studies show that bath-applied CalsTRP and CabTRP Ia each activates the pyloric and gastric mill rhythms in C. sapidus, as shown previously for pyloric rhythm activation by CabTRP Ia in the crab Cancer borealis. In summary, the newly identified CalsTRP joins CabTRP Ia as a TRP family member in the decapod crustacean nervous system, whose actions include regulating feeding behavior.

Insulin production and signaling in renal tubules of Drosophila is under control of tachykinin-related peptide and regulates stress resistance

The insulin-signaling pathway is evolutionarily conserved in animals and regulates growth, reproduction, metabolic homeostasis, stress resistance and life span. In Drosophila seven insulin-like peptides (DILP1-7) are known, some of which are produced in the brain, others in fat body or intestine. Here we show that DILP5 is expressed in principal cells of the renal tubules of Drosophila and affects survival at stress. Renal (Malpighian) tubules regulate water and ion homeostasis, but also play roles in immune responses and oxidative stress. We investigated the control of DILP5 signaling in the renal tubules by Drosophila tachykinin peptide (DTK) and its receptor DTKR during desiccative, nutritional and oxidative stress. The DILP5 levels in principal cells of the tubules are affected by stress and manipulations of DTKR expression in the same cells. Targeted knockdown of DTKR, DILP5 and the insulin receptor dInR in principal cells or mutation of Dilp5 resulted in increased survival at either stress, whereas over-expression of these components produced the opposite phenotype. Thus, stress seems to induce hormonal release of DTK that acts on the renal tubules to regulate DILP5 signaling. Manipulations of S6 kinase and superoxide dismutase (SOD2) in principal cells also affect survival at stress, suggesting that DILP5 acts locally on tubules, possibly in oxidative stress regulation. Our findings are the first to demonstrate DILP signaling originating in the renal tubules and that this signaling is under control of stress-induced release of peptide hormone.

Biostable multi-Aib analogs of tachykinin-related peptides demonstrate potent oral aphicidal activity in the pea aphid Acyrthosiphon pisum (Hemiptera: Aphidae)

The tachykinin-related peptides (TRPs) are multifunctional neuropeptides found in a variety of arthropod species, including the pea aphid Acyrthosiphon pisum (Hemiptera: Aphidae). Two new biostable TRP analogs containing multiple, sterically hindered Aib residues were synthesized and found to exhibit significantly enhanced resistance to hydrolysis by angiotensin converting enzyme and neprilysin, membrane-bound enzymes that degrade and inactivate natural TRPs. The two biostable analogs were also found to retain significant myostimulatory activity in an isolated cockroach hindgut preparation, the bioassay used to isolate and identify the first members of the TRP family. Indeed one of the analogs (Leuma-TRP-Aib-1) matched the potency and efficacy of the natural, parent TRP peptide in this myotropic bioassay. The two biostable TRP analogs were further fed in solutions of artificial diet to the pea aphid over a period of 3 days and evaluated for antifeedant and aphicidal activity and compared with the effect of treatment with three natural, unmodified TRPs. The two biostable multi-Aib TRP analogs were observed to elicit aphicidal effects within the first 24 h. In contrast natural, unmodified TRPs, including two that are native to the pea aphid, demonstrated little or no activity. The most active analog, double-Aib analog Leuma-TRP-Aib-1 (pEA[Aib]SGFL[Aib]VR-NH(2)), featured aphicidal activity calculated at an LC(50) of 0.0083 nmol/μl (0.0087 μg/μl) and an LT(50) of 1.4 days, matching or exceeding the potency of commercially available aphicides. The mechanism of this activity has yet to be established. The aphicidal activity of the biostable TRP analogs may result from disruption of digestive processes by interfering with gut motility patterns and/or with fluid cycling in the gut; processes shown to be regulated by the TRPs in other insects. These active TRP analogs and/or second generation analogs offer potential as environmentally friendly pest aphid control agents.

Tachykinin-related peptides and their receptors in invertebrates: a current view

Members of the tachykinin peptide family have been well conserved during evolution and are mainly expressed in the central nervous system and in the intestine of both vertebrates and invertebrates. In these animals, they act as multifunctional messengers that exert their biological effects by specifically interacting with a subfamily of structurally related G protein-coupled receptors. Despite the identification of multiple tachykinin-related peptides (TKRPs) in species belonging to the insects, crustaceans, mollusks and echiuroid worms, only five invertebrate receptors harboring profound sequence similarities to mammalian receptors for tachykinins have been functionally characterized to date. Three of these have been cloned from dipteran insect species, i.e. NKD (neurokinin receptor from Drosophila), DTKR (Drosophila tachykinin receptor) and STKR (tachykinin-related peptide receptor from the stable fly, Stomoxys calcitrans). In addition, two receptors from non-insect species, present in echiuroid worms and mollusks, respectively have been identified as well. In this brief review, we will survey some recent findings and insights into the signaling properties of invertebrate tachykinin-related peptides via their respective receptors. In this context, we will also point out the necessity to take into account differences in signaling mechanisms induced by distinct TKRP isoforms in insects.

Presynaptic peptidergic modulation of olfactory receptor neurons in Drosophila

The role of classical neurotransmitters in the transfer and processing of olfactory information is well established in many organisms. Neuropeptide action, however, is largely unexplored in any peripheral olfactory system. A subpopulation of local interneurons (LNs) in the Drosophila antannal lobe is peptidergic, expressing Drosophila tachykinins (DTKs). We show here that olfactory receptor neurons (ORNs) express the DTK receptor (DTKR). Using two-photon microscopy, we found that DTK applied to the antennal lobe suppresses presynaptic calcium and synaptic transmission in the ORNs. Furthermore, reduction of DTKR expression in ORNs by targeted RNA interference eliminates presynaptic suppression and alters olfactory behaviors. We detect opposite behavioral phenotypes after reduction and over expression of DTKR in ORNs. Our findings suggest a presynaptic inhibitory feedback to ORNs from peptidergic LNs in the antennal lobe.

Characterization and distribution of NKD, a receptor for Drosophila tachykinin-related peptide 6

Neuropeptides related to vertebrate tachykinins have been identified in Drosophila and are referred to as drosotachykinins, or DTKs. Two Drosophila G protein-coupled receptors, designated NKD (neurokinin receptor from Drosophila; CG6515) and DTKR (Drosophila tachykinin receptor; CG7887), display sequence similarities to mammalian tachykinin receptors. Whereas DTKR was shown to be activated by DTKs [Birse RT, Johnson EC, Taghert PH, Nässel DR. Widely distributed Drosophila G-protein-coupled receptor (CG7887) is activated by endogenous tachykinin-related peptides. J Neurobiol 2006;66:33-46; Poels J, Verlinden H, Fichna J, Van Loy T, Franssens V, Studzian K, et al. Functional comparison of two evolutionary conserved insect neurokinin-like receptors. Peptides 2007;28:103-8] and was localized by immunocytochemistry in Drosophila central nervous system (CNS), agonist-dependent activation and distribution of NKD have not yet been investigated in depth. In the present study, we have challenged NKD-expressing mammalian and insect cells with a library of Drosophila neuropeptides and discovered DTK-6 as a specific agonist that can induce a calcium response in these cells. In addition, we have produced antisera to sequences from NKD protein to analyze receptor distribution. We found that NKD is less abundantly distributed in the central nervous system than DTKR, and only NKD was found in the intestine. In fact, the two receptors are distributed in mutually exclusive patterns in the CNS. The combined distribution of the receptors in brain neuropils corresponds well with the distribution of DTKs. Most interestingly, NKD appears to be activated only by DTK-6, known to possess an Ala-substitution in an otherwise conserved C-terminal core motif. Our findings suggest that NKD and DTKR provide substrates for two functionally and spatially separated peptide signaling systems.

Neuropeptide receptor transcriptome reveals unidentified neuroendocrine pathways

Neuropeptides are an important class of molecules involved in diverse aspects of metazoan development and homeostasis. Insects are ideal model systems to investigate neuropeptide functions, and the major focus of insect neuropeptide research in the last decade has been on the identification of their receptors. Despite these vigorous efforts, receptors for some key neuropeptides in insect development such as prothoracicotropic hormone, eclosion hormone and allatotropin (AT), remain undefined. In this paper, we report the comprehensive cloning of neuropeptide G protein-coupled receptors from the silkworm, Bombyx mori, and systematic analyses of their expression. Based on the expression patterns of orphan receptors, we identified the long-sought receptor for AT, which is thought to stimulate juvenile hormone biosynthesis in the corpora allata (CA). Surprisingly, however, the AT receptor was not highly expressed in the CA, but instead was predominantly transcribed in the corpora cardiaca (CC), an organ adjacent to the CA. Indeed, by using a reverse-physiological approach, we purified and characterized novel allatoregulatory peptides produced in AT receptor-expressing CC cells, which may indirectly mediate AT activity on the CA. All of the above findings confirm the effectiveness of a systematic analysis of the receptor transcriptome, not only in characterizing orphan receptors, but also in identifying novel players and hidden mechanisms in important biological processes. This work illustrates how using a combinatorial approach employing bioinformatic, molecular, biochemical and physiological methods can help solve recalcitrant problems in neuropeptide research.

SAGA-mediated H2B deubiquitination controls the development of neuronal connectivity in the Drosophila visual system

Nonstop, which has previously been shown to have homology to ubiquitin proteases, is required for proper termination of axons R1-R6 in the optic lobe of the developing Drosophila eye. Herein, we establish that Nonstop actually functions as an ubiquitin protease to control the levels of ubiquitinated histone H2B in flies. We further establish that Nonstop is the functional homolog of yeast Ubp8, and can substitute for Ubp8 function in yeast cells. In yeast, Ubp8 activity requires Sgf11. We show that in Drosophila, loss of Sgf11 function causes similar photoreceptor axon-targeting defects as loss of Nonstop. Ubp8 and Sgf11 are components of the yeast SAGA complex, suggesting that Nonstop function might be mediated through the Drosophila SAGA complex. Indeed, we find that Nonstop does associate with SAGA components in flies, and mutants in other SAGA subunits display nonstop phenotypes, indicating that SAGA complex is required for accurate axon guidance in the optic lobe. Candidate genes regulated by SAGA that may be required for correct axon targeting were identified by microarray analysis of gene expression in SAGA mutants.

A genome-wide inventory of neurohormone GPCRs in the red flour beetle Tribolium castaneum

Insect neurohormones (biogenic amines, neuropeptides, and protein hormones) and their G protein-coupled receptors (GPCRs) play a central role in the control of behavior, reproduction, development, feeding and many other physiological processes. The recent completion of several insect genome projects has enabled us to obtain a complete inventory of neurohormone GPCRs in these insects and, by a comparative genomics approach, to analyze the evolution of these proteins. The red flour beetle Tribolium castaneum is the latest addition to the list of insects with a sequenced genome and the first coleopteran (beetle) to be sequenced. Coleoptera is the largest insect order and about 30% of all animal species living on earth are coleopterans. Some coleopterans are severe agricultural pests, which is also true for T. castaneum, a global pest for stored grain and other dried commodities for human consumption. In addition, T. castaneum is a model for insect development. Here, we have investigated the presence of neurohormone GPCRs in Tribolium and compared them with those from the fruit fly Drosophila melanogaster (Diptera) and the honey bee Apis mellifera (Hymenoptera). We found 20 biogenic amine GPCRs in Tribolium (21 in Drosophila; 19 in the honey bee), 48 neuropeptide GPCRs (45 in Drosophila; 35 in the honey bee), and 4 protein hormone GPCRs (4 in Drosophila; 2 in the honey bee). Furthermore, we identified the likely ligands for 45 of these 72 Tribolium GPCRs. A highly interesting finding in Tribolium was the occurrence of a vasopressin GPCR and a vasopressin peptide. So far, the vasopressin/GPCR couple has not been detected in any other insect with a sequenced genome (D. melanogaster and six other Drosophila species, Anopheles gambiae, Aedes aegypti, Bombyx mori, and A. mellifera). Tribolium lives in very dry environments. Vasopressin in mammals is the major neurohormone steering water reabsorption in the kidneys. Its presence in Tribolium, therefore, might be related to the animal's need to effectively control water reabsorption. Other striking differences between Tribolium and the other two insects are the absence of the allatostatin-A, kinin, and corazonin neuropeptide/receptor couples and the duplications of other hormonal systems. Our survey of 340 million years of insect neurohormone GPCR evolution shows that neuropeptide/receptor couples can easily duplicate or disappear during insect evolution. It also shows that Drosophila is not a good representative of all insects, because several of the hormonal systems that we now find in Tribolium do not exist in Drosophila.

A novel tachykinin-related peptide receptor of Octopus vulgaris- evolutionary aspects of invertebrate tachykinin and tachykinin-related peptide

The tachykinin (TK) and tachykinin-related peptide (TKRP) family represent one of the largest peptide families in the animal kingdom and exert their actions via a subfamily of structurally related G-protein-coupled receptors. In this study, we have identified a novel TKRP receptor from the Octopus heart, oct-TKRPR. oct-TKRPR includes domains and motifs typical of G-protein-coupled receptors. Xenopus oocytes that expressed oct-TKRPR, like TK and TKRP receptors, elicited an induction of membrane chloride currents coupled to the inositol phosphate/calcium pathway in response to Octopus TKRPs (oct-TKRP I-VII) with moderate ligand selectivity. Substance P and Octopus salivary gland-specific TK, oct-TK-I, completely failed to activate oct-TKRPR, whereas a Substance P analog containing a C-terminal Arg-NH2 exhibited equipotent activation of oct-TKRPs. These functional analyses prove that oct-TKRPs, but not oct-TK-I, serve as endogenous functional ligands through oct-TKRPR, although both of the family peptides were identified in a single species, and the importance of C-terminal Arg-NH2 in the specific recognition of TKRPs by TKRPR is conserved through evolutionary lineages of Octopus. Southern blotting of RT-PCR products revealed that the oct-TKRPR mRNA was widely distributed in the central and peripheral nervous systems plus several peripheral tissues. These results suggest multiple physiologic functions of oct-TKRPs as neuropeptides both in the Octopus central nervous system and in peripheral tissues. This is the first report on functional discrimination between invertebrate TKRPs and salivary gland-specific TKs.

A review of neurohormone GPCRs present in the fruitfly Drosophila melanogaster and the honey bee Apis mellifera

G protein-coupled receptor (GPCR) genes are large gene families in every animal, sometimes making up to 1-2% of the animal's genome. Of all insect GPCRs, the neurohormone (neuropeptide, protein hormone, biogenic amine) GPCRs are especially important, because they, together with their ligands, occupy a high hierarchic position in the physiology of insects and steer crucial processes such as development, reproduction, and behavior. In this paper, we give a review of our current knowledge on Drosophila melanogaster GPCRs and use this information to annotate the neurohormone GPCR genes present in the recently sequenced genome from the honey bee Apis mellifera. We found 35 neuropeptide receptor genes in the honey bee (44 in Drosophila) and two genes, coding for leucine-rich repeats-containing protein hormone GPCRs (4 in Drosophila). In addition, the honey bee has 19 biogenic amine receptor genes (21 in Drosophila). The larger numbers of neurohormone receptors in Drosophila are probably due to gene duplications that occurred during recent evolution of the fly. Our analyses also yielded the likely ligands for 40 of the 56 honey bee neurohormone GPCRs identified in this study. In addition, we made some interesting observations on neurohormone GPCR evolution and the evolution and co-evolution of their ligands. For neuropeptide and protein hormone GPCRs, there appears to be a general co-evolution between receptors and their ligands. This is in contrast to biogenic amine GPCRs, where evolutionarily unrelated GPCRs often bind to the same biogenic amine, suggesting frequent ligand exchanges ("ligand hops") during GPCR evolution.

Functional comparison of two evolutionary conserved insect neurokinin-like receptors

Tachykinins are multifunctional neuropeptides that have been identified in vertebrates as well as invertebrates. The C-terminal FXGXRa-motif constitutes the consensus active core region of invertebrate tachykinins. In Drosophila, two putative G protein-coupled tachykinin receptors have been cloned: DTKR and NKD. This study focuses on the functional characterization of DTKR, the Drosophila ortholog of the stable fly's tachykinin receptor (STKR). Tachykinins containing an alanine residue instead of the highly conserved glycine (FXAXRa) display partial agonism on STKR-mediated Ca(2+)-responses, but not on cAMP-responses. STKR therefore seems to differentiate between a number of tachykinins. Gly- and Ala-containing tachykinins are both encoded in the Drosophila tachykinin precursor, thus raising the question of whether DTKR can also distinguish between these two tachykinin types. DTKR was activated by all Drosophila tachykinins and inhibited by tachykinin antagonists. Ala-containing analogs did not produce the remarkable activation behavior previously observed with STKR, suggesting different mechanisms of discerning ligands and/or activating effector pathways for STKR and DTKR.

Characterization of tachykinin-related peptides from different insect species on Drosophila tachykinin receptor-expressing cell line

Drosophila tachykinin receptor, a neurokinin receptor cloned from the fruit fly Drosophila melanogaster, is a G-protein-coupled receptor, which upon activation by a peptide agonist induces a transient increase in the concentration of intracellular calcium. The functional assay based on aequorin-derived luminescence triggered by receptor-mediated changes in Ca(2+) levels was used to examine and compare the effect of tachykinin-related peptides from different species. Among the endogenous Drosophila peptides, Drm-TK I induced the strongest calcium response. The most potent tachykinin-related peptides from Leucophaea maderae, Locusta migratoria, and Calliphora vomitoria, were partial agonists at the Drosophila tachykinin receptor.

Tachykinin-related peptides modulate odor perception and locomotor activity in Drosophila

The invertebrate tachykinin-related peptides (TKRPs) constitute a conserved family, structurally related to the mammalian tachykinins, including members such as substance P and neurokinins A and B. Although their expression has been documented in the brains of insects and mammals, their neural functions remain largely unknown, particularly in behavior. Here, we have studied the role of TKRPs in Drosophila. We have analyzed the olfactory perception and the locomotor activity of individuals in which TKRPs are eliminated in the nervous system specifically, by using RNAi constructs to silence gene expression. The perception of specific odorants and concentrations is modified towards a loss of sensitivity, thus resulting in a significant change of the behavioral response towards indifference. In locomotion assays, the TKRP-deficient flies show hyperactivity. We conclude that these peptides are modulators of olfactory perception and locomotion activity in agreement with their abundant expression in the olfactory lobes and central complex. In these brain centers, TKRPs seem to enhance the regulatory inhibition of the neurons in which they are expressed.

Widely distributed Drosophila G-protein-coupled receptor (CG7887) is activated by endogenous tachykinin-related peptides

Neuropeptides related to vertebrate tachykinins have been identified in Drosophila. Two Drosophila G-protein-coupled receptors (GPCRs), designated NKD (CG6515) and DTKR (CG7887), cloned earlier, display sequence similarities to mammalian tachykinin receptors. However, they were not characterized with the endogenous Drosophila tachykinins (DTKs). The present study characterizes one of these receptors, DTKR. We determined that HEK-293 cells transfected with DTKR displayed dose-dependent increases in both intracellular calcium and cyclic AMP levels in response to the different DTK peptides. DTK peptides also induced internalization of DTKR-green fluorescent protein (GFP) fusion constructs in HEK-293 cells. We generated specific antireceptor antisera and showed that DTKR is widely distributed in the adult brain and more scarcely in the larval CNS. The distribution of the receptor in brain neuropils corresponds well with the distribution of its ligands, the DTKs. Our findings suggest that DTKR is a DTK receptor in Drosophila and that this ligand-receptor system plays multiple functional roles.

Tachykinin-related peptide precursors in two cockroach species

Tachykinins and tachykinin-related peptides (TKRPs) play major roles in signaling in the nervous system and intestine of both invertebrates and vertebrates. Here we have identified cDNAs encoding precursors of multiple TKRPs from the cockroaches Leucophaea maderae and Periplaneta americana. All nine LemTKRPs that had been chemically isolated in earlier experiments could be identified on the precursor of L. maderae. Four previously unidentified LemTKRPs were found in addition on the precursor. The P. americana cDNA displayed an open reading frame very similar to that of L. maderae with 13 different TKRPs. MALDI-TOF mass spectra from tissues of both species confirms the presence of all the TKRPs encoded on the precursor plus two additional peptides that are cleavage products of the N-terminally extended TKRPs. A tissue-specific distribution of TKRPs was observed in earlier experiments at isolation from brain and midgut of L. maderae. Our data do not suggest a differential gene expression but a different efficacy in processing of LemTKRP-2 and Lem/PeaTKRP-3 in the brain and intestine, respectively. This results in a gut-specific accumulation of these extended peptides, whereas in the brain their cleavage products, LemTKRP-1 and LemTKRP-3(11-19), are most abundant. Mass spectrometric analysis demonstrated the occurrence of the different TKRPs in single glomeruli of the tritocerebrum and in cells of the optical lobe.

Insect Neuropeptide and Peptide Hormone Receptors: Current Knowledge and Future Directions

Peptides form a very versatile class of extracellular messenger molecules that function as chemical communication signals between the cells of an organism. Molecular diversity is created at different levels of the peptide synthesis scheme. Peptide messengers exert their biological functions via specific signal-transducing membrane receptors. The evolutionary origin of several peptide precursor and receptor gene families precedes the divergence of the important animal Phyla. In this chapter, current knowledge is reviewed with respect to the analysis of peptide receptors from insects, incorporating many recent data that result from the sequencing of different insect genomes. Therefore, detailed information is provided on six different peptide receptor families belonging to two distinct receptor categories (i.e., the heptahelical and the single transmembrane receptors). In addition, the remaining problems, the emerging concepts, and the future prospects in this area of research are discussed.

The distribution and activity of tachykinin-related peptides in the blood-feeding bug, Rhodnius prolixus

The invertebrate tachykinin-related peptides (TRPs) with the conserved C-terminal sequence FX1GX2Ramide shows sequence similarity to the vertebrate tachykinins after which they are named, and are hypothesized to be ancestrally related. In this study a polyclonal antiserum generated against a locust tachykinin (LomTK I), was used to demonstrate the presence and describe the distribution of LomTK-like immnoreactivity in the CNS and gut of Rhodnius prolixus. Reverse phase high performance liquid chromatography (RP-HPLC) was used in combination with a sensitive radioimmunoassay (RIA) to demonstrate picomolar amounts of immunoreactive material in the CNS, and femptomolar amounts associated with the hindgut. Furthermore, the results from CNS extracts separated by RP-HPLC, suggest that at least two tachykinin isoforms exist in R. prolixus. A hindgut contraction assay was developed to quantify the myotropic effects of selected LomTKs on R. prolixus hindgut contraction. Both LomTK I and II caused an increase in the frequency of hindgut contractions with EC50 values of 3.6x10(-8)M and 3.8x10(-8)M for LomTK I and II, respectively.

Pharmacology of stomoxytachykinin receptor depends on second messenger system

STKR is a neurokinin receptor derived from the stable fly, Stomoxys calcitrans. Insect tachykinin-related peptides, also referred to as "insectatachykinins", produce dose-dependent calcium and cyclic AMP responses in cultured Drosophila melanogaster Schneider 2 (S2) cells that were stably transfected with the cloned STKR cDNA. Pronounced differences in pharmacology were observed between agonist-induced calcium and cyclic AMP responses. The results indicate that the pharmacological properties of STKR depend on its coupling to a unique second messenger system. Therefore, a model postulating the existence of multiple active receptor conformations is proposed. This article presents the first evidence that an insect peptide receptor with dual coupling properties to second messenger systems can display agonist-dependent functional differences.

APSGFLGMRamide is a unique tachykinin-related peptide in crustaceans

We report here the identification of a tachykinin-related peptide (TRP) in crustaceans. Direct MALDI-TOF MS with brain slices of the crayfish Procambarus clarkii indicated the presence of a peptide having an MS number of around 934. Quadrupole orthogonal acceleration time-of-flight (Q-TOF) MS/MS analysis implied the sequence to be APSGFLGMRamide, identical to that of CabTRP Ia, isolated previously from the crab Cancer borealis, and Pev-tachykinin, from the shrimp Penaeus vannamei. The peptide has been shown to be myoactive in the cockroach hindgut, but the structure of its precursor protein had not been elucidated. A cDNA encoding crayfish preproTRP was cloned, revealing that the 225-residue protein contains seven identical copies of the peptide APSGFLGMRamide. This is unique because TRPs identified in other invertebrates were known to exist in several related forms in each species. The conserved structure of TRP in crustaceans was confirmed by cloning preproTRP from the spiny lobster Panulirus interruptus. RT-PCR and Northern blotting analyses suggested that the crayfish preproTRP mRNA is expressed throughout the nervous system, and in situ hybridization studies of the brain revealed that the transcript predominantly localizes to cell clusters 11 (dorsal lateral cells) and 9 (ventral lateral cells).

Identification of Drosophila neuropeptide receptors by G protein-coupled receptors-beta-arrestin2 interactions

Activation of G protein-coupled receptors (GPCR) leads to the recruitment of beta-arrestins. By tagging the beta-arrestin molecule with a green fluorescent protein, we can visualize the activation of GPCRs in living cells. We have used this approach to de-orphan and study 11 GPCRs for neuropeptide receptors in Drosophila melanogaster. Here we verify the identities of ligands for several recently de-orphaned receptors, including the receptors for the Drosophila neuropeptides proctolin (CG6986), neuropeptide F (CG1147), corazonin (CG10698), dFMRF-amide (CG2114), and allatostatin C (CG7285 and CG13702). We also de-orphan CG6515 and CG7887 by showing these two suspected tachykinin receptor family members respond specifically to a Drosophila tachykinin neuropeptide. Additionally, the translocation assay was used to de-orphan three Drosophila receptors. We show that CG14484, encoding a receptor related to vertebrate bombesin receptors, responds specifically to allatostatin B. Furthermore, the pair of paralogous receptors CG8985 and CG13803 responds specifically to the FMRF-amide-related peptide dromyosuppressin. To corroborate the findings on orphan receptors obtained by the translocation assay, we show that dromyosuppressin also stimulated GTPgammaS binding and inhibited cAMP by CG8985 and CG13803. Together these observations demonstrate the beta-arrestin-green fluorescent protein translocation assay is an important tool in the repertoire of strategies for ligand identification of novel G protein-coupled receptors.

Diuretic action of the peptide locustatachykinin I: cellular localisation and effects on fluid secretion in Malpighian tubules of locusts

In insects primary urine is produced by the Malpighian tubules under hormonal control. Here we have analysed the effects of the peptide locustatachykinin I (Lom-TK-I) on secretion in isolated Malphigian tubules. We also mapped the distribution of Lom-TK immunoreactivity in the gut in comparison with Locusta diuretic hormone (Lom-DH) and serotonin, two other factors that are active on locust tubules. Lom-TK-I produces an immediate, potent and long-lasting stimulation of fluid secretion. Furthermore, we show that Lom-TK-I acts synergistically with Lom-DH on fluid secretion and demonstrate that Lom-TKs are co-localised with Lom-DH in endocrine cells of the midgut ampullae. Thus, the two peptides might be released together to act synergistically on fluid secretion. Also serotonin and Lom-DH act synergistically and we can demonstrate a plexus of serotonin-containing axon processes over the midgut.