Differential activation of "social" and "solitary" variants of the Caenorhabditis elegans G protein-coupled receptor NPR-1 by its cognate ligand AF9

System-wide mapping of peptide-GPCR interactions in C. elegans

Neuropeptides and peptide hormones are ancient, widespread signaling molecules that underpin almost all brain functions. They constitute a broad ligand-receptor network, mainly by binding to G protein-coupled receptors (GPCRs). However, the organization of the peptidergic network and roles of many peptides remain elusive, as our insight into peptide-receptor interactions is limited and many peptide GPCRs are still orphan receptors. Here we report a genome-wide peptide-GPCR interaction map in Caenorhabditis elegans. By reverse pharmacology screening of over 55,384 possible interactions, we identify 461 cognate peptide-GPCR couples that uncover a broad signaling network with specific and complex combinatorial interactions encoded across and within single peptidergic genes. These interactions provide insights into peptide functions and evolution. Combining our dataset with phylogenetic analysis supports peptide-receptor co-evolution and conservation of at least 14 bilaterian peptidergic systems in C. elegans. This resource lays a foundation for system-wide analysis of the peptidergic network.

G protein-coupled receptor kinase-2 (GRK-2) controls exploration through neuropeptide signaling in Caenorhabditis elegans

Animals alter their behavior in manners that depend on environmental conditions as well as their developmental and metabolic states. For example, C. elegans is quiescent during larval molts or during conditions of satiety. By contrast, worms enter an exploration state when removed from food. Sensory perception influences movement quiescence (defined as a lack of body movement), as well as the expression of additional locomotor states in C. elegans that are associated with increased or reduced locomotion activity, such as roaming (exploration behavior) and dwelling (local search). Here we find that movement quiescence is enhanced, and exploration behavior is reduced in G protein-coupled receptor kinase grk-2 mutant animals. grk-2 was previously shown to act in chemosensation, locomotion, and egg-laying behaviors. Using neuron-specific rescuing experiments, we show that GRK-2 acts in multiple ciliated chemosensory neurons to control exploration behavior. grk-2 acts in opposite ways from the cGMP-dependent protein kinase gene egl-4 to control movement quiescence and exploration behavior. Analysis of mutants with defects in ciliated sensory neurons indicates that grk-2 and the cilium-structure mutants act in the same pathway to control exploration behavior. We find that GRK-2 controls exploration behavior in an opposite manner from the neuropeptide receptor NPR-1 and the neuropeptides FLP-1 and FLP-18. Finally, we show that secretion of the FLP-1 neuropeptide is negatively regulated by GRK-2 and that overexpression of FLP-1 reduces exploration behavior. These results define neurons and molecular pathways that modulate movement quiescence and exploration behavior.

Pharmacological and functional similarities of the human neuropeptide Y system in C. elegans challenges phylogenetic views on the FLP/NPR system

BACKGROUND

The neuropeptide Y system affects various processes, among others food intake, and is frequently discussed in the context of targeting obesity. Studies in model organisms are indispensable to enable molecular studies in a physiological context. Although the NPY system is evolutionarily conserved in all bilaterians, in the widely used model Caenorhabditis elegans there is controversy on the existence of NPY orthologous molecules. While the FMRFamide-like peptide (FLP)/Neuropeptide receptor-Resemblance (NPR) system in the nematode was initially suggested to be orthologous to the mammalian NPY system, later global phylogenetic studies indicate that FLP/NPR is protostome-specific.

METHODS

We performed a comprehensive pharmacological study of the FLP/NPR system in transfected cells in vitro, and tested for functional substitution in C. elegans knockout strains. Further, we phenotypically compared different flp loss-of-function strains. Differences between groups were compared by ANOVA and post-hoc testing (Dunnett, Bonferroni).

RESULTS

Our pharmacological analysis of the FLP/NPR system including formerly functionally uncharacterized NPY-like peptides from C. elegans demonstrates that G protein-coupling and ligand requirements for receptor activation are similar to the human NPY system. In vitro and in vivo analyses show cross-reactivity of NPY with the FLP/NPR system manifesting in the ability of the human GPCRs to functionally substitute FLP/NPR signaling in vivo. The high pharmacological/functional similarities enabled us to identify C. elegans FLP-14 as a key molecule in avoidance behavior.

CONCLUSIONS

Our data demonstrate the pharmacological and functional similarities of human NPY and C. elegans NPR systems. This adds a novel perspective to current phylogenetic reconstructions of the neuropeptide Y system. NPY and NPR receptors are pharmacologically so similar that the human receptors can functionally compensate for the C. elegans ones, suggesting orthologous relationships. This is also underlined by the presence of NPY-like peptides and parallels in peptide requirements for receptor activation. Further, the results presented here highlight the potential of this knowledge for physiological as well as molecular studies on neuropeptide GPCRs such as the NPY system in the future.

Neurotransmitter signaling through heterotrimeric G proteins: insights from studies in C. elegans

Neurotransmitters signal via G protein coupled receptors (GPCRs) to modulate activity of neurons and muscles. C. elegans has ∼150 G protein coupled neuropeptide receptor homologs and 28 additional GPCRs for small-molecule neurotransmitters. Genetic studies in C. elegans demonstrate that neurotransmitters diffuse far from their release sites to activate GPCRs on distant cells. Individual receptor types are expressed on limited numbers of cells and thus can provide very specific regulation of an individual neural circuit and behavior. G protein coupled neurotransmitter receptors signal principally via the three types of heterotrimeric G proteins defined by the G alpha subunits Gαo, Gαq, and Gαs. Each of these G alpha proteins is found in all neurons plus some muscles. Gαo and Gαq signaling inhibit and activate neurotransmitter release, respectively. Gαs signaling, like Gαq signaling, promotes neurotransmitter release. Many details of the signaling mechanisms downstream of Gαq and Gαs have been delineated and are consistent with those of their mammalian orthologs. The details of the signaling mechanism downstream of Gαo remain a mystery. Forward genetic screens in C. elegans have identified new molecular components of neural G protein signaling mechanisms, including Regulators of G protein Signaling (RGS proteins) that inhibit signaling, a new Gαq effector (the Trio RhoGEF domain), and the RIC-8 protein that is required for neuronal Gα signaling. A model is presented in which G proteins sum up the variety of neuromodulator signals that impinge on a neuron to calculate its appropriate output level.

Comparative and Evolutionary Aspects of Gonadotropin-Inhibitory Hormone and FMRFamide-Like Peptide Systems

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that was found in the brain of Japanese quail when investigating the existence of RFamide peptides in birds. GnIH was named because it decreased gonadotropin release from cultured anterior pituitary, which was located in the hypothalamo-hypophysial system. GnIH and GnIH precursor gene related peptides have a characteristic C-terminal LPXRFamide (X = L or Q) motif that is conserved in jawed vertebrates. Orthologous peptides to GnIH are also named RFamide related peptide or LPXRFamide peptide from their structure. A G-protein coupled receptor GPR147 is the primary receptor for GnIH. Similarity-based clustering of neuropeptide precursors in metazoan species indicates that GnIH precursor of vertebrates is evolutionarily related to FMRFamide precursor of mollusk and nematode. FMRFamide peptide is the first RFamide peptide that was identified from the ganglia of the venus clam. In order to infer the evolutionary history of the GnIH-GnIH receptor system we investigate the structural similarities between GnIH and its receptor and well-studied nematode Caenorhabditis elegans (C. elegans) FMRFamide-like peptides (FLPs) and their receptors. We also compare the functions of FLPs of nematode with GnIH of chordates. A multiple sequence alignment and phylogenetic analyses of GnIH, neuropeptide FF (NPFF), a paralogous peptide of GnIH, and FLP precursors have shown that GnIH and NPFF precursors belong to different clades and some FLP precursors have structural similarities to either precursor. The peptide coding regions of FLP precursors in the same clade align well with those of GnIH or NPFF precursors. Alignment of GnIH (LPXRFa) peptides of chordates and FLPs of C. elegans grouped the peptides into five groups according to the last C-terminal amino acid sequences, which were MRFa, LRFa, VRFa, IRFa, and PQRFa. Phylogenetic analysis of receptors suggested that GPR147 has evolutionary relationships with FLP receptors, which regulate reproduction, aggression, locomotion, and feeding. GnIH and some FLPs mediate the effect of stress on reproduction and behavior, which may also be a conserved property of these peptide systems. Future studies are needed to investigate the mechanism of how neuropeptide precursor genes are mutated to evolve new neuropeptides and their inheritance.

Nematode neuropeptides as transgenic nematicides

Plant parasitic nematodes (PPNs) seriously threaten global food security. Conventionally an integrated approach to PPN management has relied heavily on carbamate, organophosphate and fumigant nematicides which are now being withdrawn over environmental health and safety concerns. This progressive withdrawal has left a significant shortcoming in our ability to manage these economically important parasites, and highlights the need for novel and robust control methods. Nematodes can assimilate exogenous peptides through retrograde transport along the chemosensory amphid neurons. Peptides can accumulate within cells of the central nerve ring and can elicit physiological effects when released to interact with receptors on adjoining cells. We have profiled bioactive neuropeptides from the neuropeptide-like protein (NLP) family of PPNs as novel nematicides, and have identified numerous discrete NLPs that negatively impact chemosensation, host invasion and stylet thrusting of the root knot nematode Meloidogyne incognita and the potato cyst nematode Globodera pallida. Transgenic secretion of these peptides from the rhizobacterium, Bacillus subtilis, and the terrestrial microalgae Chlamydomonas reinhardtii reduce tomato infection levels by up to 90% when compared with controls. These data pave the way for the exploitation of nematode neuropeptides as a novel class of plant protective nematicide, using novel non-food transgenic delivery systems which could be deployed on farmer-preferred cultivars.

CRF-like receptor SEB-3 in sex-common interneurons potentiates stress handling and reproductive drive in C. elegans

Environmental conditions can modulate innate behaviours. Although male Caenorhabditis elegans copulation can be perturbed in the presence of stress, the mechanisms underlying its decision to sustain copulation are unclear. Here we describe a mating interference assay, which quantifies the persistence of male C. elegans copulation in noxious blue light. We show that between copulations, the male escapes from blue light illumination at intensities over 370 μW mm⁻². This response is attenuated in mutants with constitutive activation of the corticotropin-releasing factor receptor family homologue SEB-3. We show that activation of this receptor causes sex-common glutamatergic lumbar ganglion interneurons (LUA) to potentiate downstream male-specific reproduction circuits, allowing copulatory behaviours to partially override the light-induced escape responses in the male. SEB-3 activation in LUA also potentiates copulation during mild starvation. We suggest that SEB-3 activation allows C. elegans to acclimate to the environment and thus continue to execute innate behaviours even under non-optimal conditions.

Innate animal behaviours can be negatively regulated by environmental stressors. Jee et al. show that suppression of male C. elegans copulation behaviour by noxious light can be overcome by activation of SEB-3, a homologue of the stress-associated mammalian corticotropin-releasing factor receptor family.

Modulation of Locomotion and Reproduction by FLP Neuropeptides in the Nematode Caenorhabditis elegans

Neuropeptides function in animals to modulate most, if not all, complex behaviors. In invertebrates, neuropeptides can function as the primary neurotransmitter of a neuron, but more generally they co-localize with a small molecule neurotransmitter, as is commonly seen in vertebrates. Because a single neuron can express multiple neuropeptides and because neuropeptides can bind to multiple G protein-coupled receptors, neuropeptide actions increase the complexity by which the neural connectome can be activated or inhibited. Humans are estimated to have 90 plus neuropeptide genes; by contrast, nematodes, a relatively simple organism, have a slightly larger complement of neuropeptide genes. For instance, the nematode Caenorhabditis elegans has over 100 neuropeptide-encoding genes, of which at least 31 genes encode peptides of the FMRFamide family. To understand the function of this large FMRFamide peptide family, we isolated knockouts of different FMRFamide-encoding genes and generated transgenic animals in which the peptides are overexpressed. We assayed these animals on two basic behaviors: locomotion and reproduction. Modulating levels of different neuropeptides have strong as well as subtle effects on these behaviors. These data suggest that neuropeptides play critical roles in C. elegans to fine tune neural circuits controlling locomotion and reproduction.

Different neuropeptides are expressed in different functional subsets of cholinergic excitatory motorneurons in the nematode Ascaris suum

Neuropeptides are known to have dramatic effects on neurons and synapses; however, despite extensive studies of the motorneurons in the parasitic nematode Ascaris suum, their peptide content had not yet been described. We determined the peptide content of single excitatory motorneurons by mass spectrometry and tandem mass spectrometry. There are two subsets of ventral cord excitatory motorneurons, each with neuromuscular output either anterior or posterior to their cell body, mediating forward or backward locomotion, respectively. Strikingly, the two sets of neurons contain different neuropeptides, with AF9 and six novel peptides (As-NLP-21.1-6) in anterior projectors, and the six afp-1 peptides in addition to AF2 in posterior projectors. In situ hybridization confirmed the expression of these peptides, validating the integrity of the dissection technique. This work identifies new components of the functional behavioral circuit, as well as potential targets for antiparasitic drug development.

Neuropeptide receptors NPR-1 and NPR-2 regulate Caenorhabditis elegans avoidance response to the plant stress hormone methyl salicylate

Methyl salicylate (MeSa) is a stress hormone released by plants under attack by pathogens or herbivores . MeSa has been shown to attract predatory insects of herbivores and repel pests. The molecules and neurons underlying animal response to MeSa are not known. Here we found that the nematode Caenorhabditis elegans exhibits a strong avoidance response to MeSa, which requires the activities of two closely related neuropeptide receptors NPR-1 and NPR-2. Molecular analyses suggest that NPR-1 expressed in the RMG inter/motor neurons is required for MeSa avoidance. An NPR-1 ligand FLP-18 is also required. Using a rescuing npr-2 promoter to drive a GFP transgene, we identified that NPR-2 is expressed in multiple sensory and interneurons. Genetic rescue experiments suggest that NPR-2 expressed in the AIZ interneurons is required for MeSa avoidance. We also provide evidence that the AWB sensory neurons might act upstream of RMGs and AIZs to detect MeSa. Our results suggest that NPR-2 has an important role in regulating animal behavior and that NPR-1 and NPR-2 act on distinct interneurons to affect C. elegans avoidance response to MeSa.

Functional neuropeptidomics in invertebrates

Neuropeptides are key messengers in almost all physiological processes. They originate from larger precursors and are extensively processed to become bioactive. Neuropeptidomics aims to comprehensively identify the collection of neuropeptides in an organism, organ, tissue or cell. The neuropeptidome of several invertebrates is thoroughly explored since they are important model organisms (and models for human diseases), disease vectors and pest species. The charting of the neuropeptidome is the first step towards understanding peptidergic signaling. This review will first discuss the latest developments in exploring the neuropeptidome. The physiological roles and modes of action of neuropeptides can be explored in two ways, which are largely orthogonal and therefore complementary. The first way consists of inferring the functions of neuropeptides by a forward approach where neuropeptide profiles are compared under different physiological conditions. Second is the reverse approach were neuropeptide collections are used to screen for receptor-binding. This is followed by localization studies and functional tests. This review will focus on how these different functional screening methods contributed to the field of invertebrate neuropeptidomics and expanded our knowledge of peptidergic signaling. This article is part of a Special Issue entitled: Neuroproteomics: Applications in Neuroscience and Neurology.

Homeostasis in C. elegans sleep is characterized by two behaviorally and genetically distinct mechanisms

Biological homeostasis invokes modulatory responses aimed at stabilizing internal conditions. Using tunable photo- and mechano-stimulation, we identified two distinct categories of homeostatic responses during the sleep-like state of Caenorhabditis elegans (lethargus). In the presence of weak or no stimuli, extended motion caused a subsequent extension of quiescence. The neuropeptide Y receptor homolog, NPR-1, and an inhibitory neuropeptide known to activate it, FLP-18, were required for this process. In the presence of strong stimuli, the correlations between motion and quiescence were disrupted for several minutes but homeostasis manifested as an overall elevation of the time spent in quiescence. This response to strong stimuli required the function of the DAF-16/FOXO transcription factor in neurons, but not that of NPR-1. Conversely, response to weak stimuli did not require the function of DAF-16/FOXO. These findings suggest that routine homeostatic stabilization of sleep may be distinct from homeostatic compensation following a strong disturbance.

DOI:http://dx.doi.org/10.7554/eLife.04380.001

The regenerative properties of sleep are required by all animals, with even the simplest animal, the nematode Caenorhabditis elegans, displaying a sleep-like state called lethargus. During development, nematodes must pass through four larval stages en route to adulthood, and the end of each stage is preceded by a period of lethargus lasting 2 to 3 hr.

Human sleep is divided into distinct stages that recur in a prescribed order throughout the night. Nematodes, on the other hand, simply experience alternating periods of activity and stillness as they sleep. Nevertheless, in both species, any disruptions to sleep automatically lead to adjustments of the rest of the sleep cycle to compensate for the disturbance and to ensure that the organism gets an adequate amount of sleep overall.

To date, it has been assumed that a single mechanism is responsible for adjusting the sleep cycle after any disturbance, regardless of its severity. However, Nagy, Tramm, Sanders et al. now show that this is not the case in C. elegans. Sleeping nematodes that were lightly disturbed by exposing them to light or to vibrations—causing them to briefly increase their activity levels—compensated for the disturbance by lengthening their next inactive period. By contrast, worms that were vigorously agitated by stronger vibrations showed a different response: the alternating pattern of stillness and activity was disrupted for several minutes, followed by an overall increase in the length of time spent in the stillness phase.

Experiments using genetically modified worms revealed that these two responses involve distinct molecular pathways. A signaling molecule called neuropeptide Y affects the response to minor sleep disruptions, whereas a transcription factor called DAF-16/FOXO is involved in the corresponding role after major disruptions. Given that neuropeptide Y has already been implicated in sleep regulation in humans and flies, it is not implausible that similar mechanisms may occur in response to disturbances of our own sleep.

DOI:http://dx.doi.org/10.7554/eLife.04380.002

New insights into the FLPergic complements of parasitic nematodes: Informing deorphanisation approaches

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We report the identification of flp and flp-GPCR gene homologues in parasitic nematodes.

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We provide data to support re-evaluation of the number of flp-genes in nematodes.

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Post BLAST phylogenetic analysis facilitates identification of putative flp-GPCRs in nematode parasites.

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We expose the most highly conserved flp and flp-GPCR genes in key pathogens within phylum Nematoda.

FMRFamide-like peptide (FLP) receptors are appealing as putative anthelmintic targets. To date, 31 flp-encoding genes have been identified in Caenorhabditis elegans and thirteen FLP-activated G-protein coupled receptors (FLP-GPCRs) have been reported. The lack of knowledge on FLPs and FLP-GPCRs in parasites impedes their functional characterisation and chemotherapeutic exploitation. Using homology-based BLAST searches and phylogenetic analyses this study describes the identification of putative flp and flp-GPCR gene homologues in 17 nematode parasites providing the first pan-phylum genome-based overview of the FLPergic complement. These data will facilitate FLP-receptor deorphanisation efforts in the quest for novel control targets for nematode parasites.

The FMRFamide-Like Peptide Family in Nematodes

In the three decades since the FMRFamide peptide was isolated from the mollusk Macrocallista nimbosa, structurally similar peptides sharing a C-terminal RFamide motif have been identified across the animal kingdom. FMRFamide-like peptides (FLPs) represent the largest known family of neuropeptides in invertebrates. In the phylum Nematoda, at least 32 flp-genes are classified, making the FLP system of nematodes unusually complex. The diversity of the nematode FLP complement is most extensively mapped in Caenorhabditis elegans, where over 70 FLPs have been predicted. FLPs have shown to be expressed in the majority of the 302 C. elegans neurons including interneurons, sensory neurons, and motor neurons. The vast expression of FLPs is reflected in the broad functional repertoire of nematode FLP signaling, including neuroendocrine and neuromodulatory effects on locomotory activity, reproduction, feeding, and behavior. In contrast to the many identified nematode FLPs, only few peptides have been assigned a receptor and there is the need to clarify the pathway components and working mechanisms of the FLP signaling network. Here, we review the diversity, distribution, and functions of FLPs in nematodes.

Family of FLP Peptides in Caenorhabditis elegans and Related Nematodes

Neuropeptides regulate all aspects of behavior in multicellular organisms. Because of their ability to act at long distances, neuropeptides can exert their effects beyond the conventional synaptic connections, thereby adding an intricate layer of complexity to the activity of neural networks. In the nematode Caenorhabditis elegans, a large number of neuropeptide genes that are expressed throughout the nervous system have been identified. The actions of these peptides supplement the synaptic connections of the 302 neurons, allowing for fine tuning of neural networks and increasing the ways in which behaviors can be regulated. In this review, we focus on a large family of genes encoding FMRFamide-related peptides (FaRPs). These genes, the flp genes, have been used as a starting point to identifying flp genes throughout Nematoda. Nematodes have the largest family of FaRPs described thus far. The challenges in the future are the elucidation of their functions and the identification of the receptors and signaling pathways through which they function.

Analysis of NPR-1 reveals a circuit mechanism for behavioral quiescence in C. elegans

Animals undergo periods of behavioral quiescence and arousal in response to environmental, circadian, or developmental cues. During larval molts, C. elegans undergoes a period of profound behavioral quiescence termed lethargus. Locomotion quiescence during lethargus was abolished in mutants lacking a neuropeptide receptor (NPR-1) and was reduced in mutants lacking NPR-1 ligands (FLP-18 and FLP-21). Wild-type strains are polymorphic for the npr-1 gene, and their lethargus behavior varies correspondingly. Locomotion quiescence and arousal were mediated by decreased and increased secretion of an arousal neuropeptide (PDF-1) from central neurons. PDF receptors (PDFR-1) expressed in peripheral mechanosensory neurons enhanced touch-evoked calcium transients. Thus, a central circuit stimulates arousal from lethargus by enhancing the sensitivity of peripheral mechanosensory neurons in the body. These results define a circuit mechanism controlling a developmentally programmed form of quiescence.

Functional expression and characterization of the C. elegans G-protein-coupled FLP-2 Receptor (T19F4.1) in mammalian cells and yeast

Profound neuropeptide diversity characterizes the nematode nervous system, but it has proven challenging to match neuropeptide G protein-coupled receptors (GPCR) with their cognate ligands in heterologous systems. We have expressed the Caenorhabditis elegans GPCR encoded in the locus T19F4.1, previously matched with FMRFamide-like peptides encoded on the flp-2 precursor gene, in mammalian cells and in the yeast Saccharomyces cerevisiae. Pharmacological characterization revealed that the receptor is potently activated by flp-2 peptides in CHO cells (∼10 nM EC50) and in yeast (∼100 nM EC50), signaling through a Gqα pathway in each system. The yeast GPCR expression system provides a robust assay for screening for agonists of the flp-2 receptor and is the target of an ongoing high-throughput screening exercise.

Select Neuropeptides and their G-Protein Coupled Receptors in Caenorhabditis Elegans and Drosophila Melanogaster

The G-protein coupled receptor (GPCR) family is comprised of seven transmembrane domain proteins and play important roles in nerve transmission, locomotion, proliferation and development, sensory perception, metabolism, and neuromodulation. GPCR research has been targeted by drug developers as a consequence of the wide variety of critical physiological functions regulated by this protein family. Neuropeptide GPCRs are the least characterized of the GPCR family as genetic systems to characterize their functions have lagged behind GPCR gene discovery. Drosophila melanogaster and Caenorhabditis elegans are genetic model organisms that have proved useful in characterizing neuropeptide GPCRs. The strength of a genetic approach leads to an appreciation of the behavioral plasticity that can result from subtle alterations in GPCRs or regulatory proteins in the pathways that GPCRs control. Many of these invertebrate neuropeptides, GPCRs, and signaling pathway components serve as models for mammalian counterparts as they have conserved sequences and function. This review provides an overview of the methods to match neuropeptides to their cognate receptor and a state of the art account of neuropeptide GPCRs that have been characterized in D. melanogaster and C. elegans and the behaviors that have been uncovered through genetic manipulation.

Neuropeptide GPCRs in C. elegans

Like most organisms, the nematode Caenorhabditis elegans relies heavily on neuropeptidergic signaling. This tiny animal represents a suitable model system to study neuropeptidergic signaling networks with single cell resolution due to the availability of powerful molecular and genetic tools. The availability of the worm's complete genome sequence allows researchers to browse through it, uncovering putative neuropeptides and their cognate G protein-coupled receptors (GPCRs). Many predictions have been made about the number of C. elegans neuropeptide GPCRs. In this review, we report the state of the art of both verified as well as predicted C. elegans neuropeptide GPCRs. The predicted neuropeptide GPCRs are incorporated into the receptor classification system based on their resemblance to orthologous GPCRs in insects and vertebrates. Appointing the natural ligand(s) to each predicted neuropeptide GPCR (receptor deorphanization) is a crucial step during characterization. The development of deorphanization strategies resulted in a significant increase in the knowledge of neuropeptidergic signaling in C. elegans. Complementary localization and functional studies demonstrate that neuropeptides and their GPCRs represent a rich potential source of behavioral variability in C. elegans. Here, we review all neuropeptidergic signaling pathways that so far have been functionally characterized in C. elegans.

Neuropeptide gene families in Caenorhabditis elegans

Neuropeptides are short sequences ofamino acids that function in all multicellular organisms to communicate information between cells. The first sequence ofa neuropeptide was reported in 1970' and the number of identified neuropeptides remained relatively small until the 1990s when the DNA sequence of multiple genomes revealed treasure troves ofinformation. Byblasting away at the genome, gene families, the sizes ofwhich were previously unknown, could now be determined. This information has led to an exponential increase in the number of putative neuropeptides and their respective gene families. The molecular biology age greatly benefited the neuropeptide field in the nematode Caenorhabditis elegans. Its genome was among the first to be sequenced and this allowed us the opportunity to screen the genome for neuropeptide genes. Initially, the screeningwas slow, as the Genefinder and BLAST programs had difficulty identifying small genes and peptides. However, as the bioinformatics programs improved, the extent of the neuropeptide gene families in C. elegans gradually emerged.

Together or alone?: foraging strategies in Caenorhabditis elegans

A central goal in Life Sciences is to understand how genes encode behaviour and how environmental factors influence the expression of the genes concerned. To reach this goal a combined ecological, molecular biological and physiological approach is required in combination with a suitable model organism. Such an approach allows the elucidation of all parts of the complicated chain of events that lead from induction of gene expression to behaviour, i.e. from environmental stimulus, sensory organs and extracellular and intracellular neuronal signal processing to activation of effector organs. A particularly good model species with which to take this approach is the nematode Caenorhabditis elegans, as it has been described in great detail at the genomic, cellular and behavioural levels. Different strains of C. elegans display prominent behavioural variation in foraging behaviour. Some strains will form social feeding groups when subjected to certain environmental stimuli, while others do not. This variation is due to the existence of just two isoforms of the gene npr-1, namely 215F and 215V. Here, we describe these behavioural variations at the molecular and cellular levels to attempt to determine the environmental inputs that cause aggregation of these small nematodes. As many different stimuli affect aggregation either positively or negatively, aggregation behaviour seems to be displayed when it improves survival chances. However, not much is known about the ecological context in which C. elegans lives. Investigation of the habitats of different strains of C. elegans would help us to understand why and how a specific foraging strategy enhances survival. The relatively well-understood molecular pathways that direct its social feeding behaviour make C. elegans a highly suitable model organism to test ecological and behavioural hypotheses about the mechanisms that differentiate between aggregation and solitary behaviours.

Coevolution of neuropeptidergic signaling systems: from worm to man

Despite the general knowledge and repeated predictions of peptide G protein-coupled receptors following the elucidation of the Caenorhabditis elegans genome in 1998, only a few have been deorphanized so far. This was attributed to the apparent lack of coevolution between (neuro)peptides and their cognate receptors. To resolve this issue, we have used an in silico genomic data mining tool to identify the real putative peptide GPCRs in the C. elegans genome and then made a well-considered selection of orphan peptide GPCRs. To maximize our chances of a successful deorphanization, we adopted a combined reverse pharmacology approach. At this moment, we have successfully uncovered four C. elegans neuropeptide signaling systems that support the theory of receptor-ligand coevolution. All four systems are extremely well conserved within nematodes and show a high degree of similarity with their vertebrate and arthropod counterparts. Our data indicate that these four neuropeptide signaling systems have been well conserved during the course of evolution and that they were already well established prior to the divergence of protostomes and deuterostomes.

A review of FMRFamide- and RFamide-like peptides in metazoa

Neuropeptides are a diverse class of signalling molecules that are widely employed as neurotransmitters and neuromodulators in animals, both invertebrate and vertebrate. However, despite their fundamental importance to animal physiology and behaviour, they are much less well understood than the small molecule neurotransmitters. The neuropeptides are classified into families according to similarities in their peptide sequence; and on this basis, the FMRFamide and RFamide-like peptides, first discovered in molluscs, are an example of a family that is conserved throughout the animal phyla. In this review, the literature on these neuropeptides has been consolidated with a particular emphasis on allowing a comparison between data sets in phyla as diverse as coelenterates and mammals. The intention is that this focus on the structure and functional aspects of FMRFamide and RFamide-like neuropeptides will inform understanding of conserved principles and distinct properties of signalling across the animal phyla.

Neuronal signaling in schistosomes: current status and prospects for postgenomicsThe present review is one of a series of occasional review articles that have been invited by the Editors and will feature the broad range of disciplines and expertise represented in our Editorial Advisory Board

Parasitic platyhelminths of the genus Schistosoma Weinland, 1858 (Trematoda, Digenea) are the etiological agents of human schistosomiasis, one of the most prevalent and debilitating parasitic diseases worldwide. Praziquantel is the only drug treatment available in most parts of the world and the effectiveness of the drug is threatened by the prospect of drug resistance. There is a pressing need to learn more about the basic biology of this organism and to identify molecular targets for new therapeutic drugs. The nervous system of schistosomes coordinates many activities that are essential for parasite survival, and as such is an attractive target for chemotherapeutic intervention. Until recently, very little was known about the molecular mechanisms of neuronal signaling in these organisms, but this is rapidly changing following the completion of the genome sequence and several recent developments in schistosome transgenesis and gene silencing. Here we review the current status of schistosome neurobiology and discuss prospects for future research as the field moves into a postgenomics era. One of the themes that will emerge from this discussion is that schistosomes have a rich diversity of neurotransmitters and receptors, indicating a more sophisticated system of neuronal communication than might be expected of a parasitic flatworm. Moreover, many of these transmitter receptors share little sequence homology with those of the human host, making them ideally suited for selective drug targeting. Strategies for characterization of these important parasite proteins will be discussed.

Receptor-based discovery strategies for insecticides and parasiticides: a review

Drug discovery is an iterative process with high risks and low chance of success. New genomics technologies allow veterinary medicine and agrochemical companies to validate and functionally screen new receptor-based targets, including neuropeptide G-protein coupled receptors, which were previously not amenable to high throughput screening. However this is just the first step in a long process to translate a mechanistic assay hit into a drug on the market. In addition to effectively eradicating pests on crops and parasites on their host, the molecules must also be safe, cheap to synthesise, formulatable and patentable. This is a costly process in which early attrition of unsuitable molecules is key to any successful program. Although first principle discovery is risky the ultimate benefits are considerable and future genomics resources will help to generate higher quality hits to strengthen the discovery pipeline.

Identification of new members of the (short) neuropeptide F family in locusts and Caenorhabditis elegans

Both the long and short neuropeptides F (NPF) represent important families of invertebrate neuropeptides that have been implicated in the regulation of reproduction and feeding behavior. In the present study, two short NPFs (SNRSPS(L/I)R(L/I)RFamide and SPS(L/I)R(L/I)RFamide) were de novo sequenced by mass spectrometry in two major pest insects, the desert locust Schistocerca gregaria and the African migratory locust Locusta migratoria. They are two of the most widespread peptides in the locust neuroendocrine system. A peptide that was previously reported to accelerate egg development in S. gregaria is shown to represent a truncated form of long NPF. This peptide is most likely derived by a novel processing mechanism involving cleavage at RY. In addition, an NPF peptide from the nematode Caenorhabditis elegans was isolated and sequenced by tandem mass spectrometry.

Neuropeptides

The role of neuropeptides in modulating behavior is slowly being elucidated. With the sequencing of the C. elegans genome, the extent of the neuropeptide genes in C. elegans can be determined. To date, 113 neuropeptide genes encoding over 250 distinct neuropeptides have been identified. Of these, 40 genes encode insulin-like peptides, 31 genes encode FMRFamide-related peptides, and 42 genes encode non-insulin, non-FMRFamide-related neuropeptides. As in other systems, C. elegans neuropeptides are derived from precursor molecules that must be post-translationally processed to yield the active peptides. These precursor molecules contain a single peptide, multiple copies of a single peptide, multiple distinct peptides, or any combination thereof. The neuropeptide genes are expressed extensively throughout the nervous system, including in sensory, motor, and interneurons. In addition, some of the genes are also expressed in non-neuronal tissues, such as the somatic gonad, intestine, and vulval hypodermis. To address the effects of neuropeptides on C. elegans behavior, animals in which the different neuropeptide genes are inactivated or overexpressed are being isolated. In a complementary approach the receptors to which the neuropeptides bind are also being identified and examined. Among the knockout animals analyzed thus far, defects in locomotion, dauer formation, egg laying, ethanol response, and social behavior have been reported. These data suggest that neuropeptides have a modulatory role in many, if not all, behaviors in C. elegans.

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.

Evidence for a role for cyclic AMP in modulating the action of 5-HT and an excitatory neuropeptide, FLP17A, in the pharyngeal muscle of Caenorhabditis elegans

The feeding activity of the nematode Caenorhabditis elegans is regulated by an anatomically well-defined network of 20 enteric neurones that employs small molecule and neuropeptidergic signalling. Two of the most potent excitatory agents are 5-HT and the neuropeptide FLP17A. Here we have examined the role of cAMP in modulating their excitatory actions by pharmacological manipulation of the level of cAMP. Application of the membrane permeable cAMP analogue, dibutyryl-cAMP (1 microM), enhanced the excitatory response to both FLP17A and 5-HT. Furthermore, the adenylyl cyclase activator, forskolin (50 nM), significantly enhanced the excitatory response to both FLP17A and 5-HT. The phosphodiesterase inhibitor, ibudilast (10 microM), enhanced the excitatory response to FLP17A. The protein kinase inhibitor, H-9 dihydrochloride (10 microM) significantly reduced the excitatory response to 5-HT. H-9 dihydrochloride also had a direct effect on pharyngeal activity. The effect of FLP17A and 5-HT on two mutants, egl-8 (loss-of-function phospholipase-Cbeta) and egl-30 (loss-of-function Galphaq) was also investigated. Both these mutants have a lower pharyngeal pumping rate than wild-type which has to be considered when interpreting the effects of these mutations on the excitatory responses to FLP17A and 5HT. However, even taking into consideration the lower basal activity of these mutants, it is clear that the percentage increase in pharyngeal pumping rate induced by FLP17A is greatly reduced in both mutants compared to wild-type. In the case of 5-HT, the effect of the mutant backgrounds on the response was less pronounced. Overall, the data support a role for cAMP in modulating the excitatory action of both FLP17A and 5-HT on C. elegans pharyngeal pumping and furthermore implicate an EGL-30 dependent pathway in the regulation of the response to FLP17A.

Functional characterization of three G protein-coupled receptors for pigment dispersing factors in Caenorhabditis elegans

Here, we report the identification, cloning, and functional characterization of three Caenorhabditis elegans G protein-coupled pigment dispersing factor (PDF) receptors, which we designated as Ce_PDFR-1a, -b, and -c. They represent three splice isoforms of the same gene (C13B9.4), which share a high degree of similarity with the Drosophila PDF receptor and are distantly related to the mammalian vasoactive intestinal peptide receptors (VPAC2) and calcitonin receptors. In a reverse pharmacological screen, three bioactive C. elegans neuropeptides, which were recently identified as the Drosophila PDF orthologues, were able to activate these receptors in a dose-dependent manner with nanomolar potency (isoforms a and b). Integrated green fluorescent protein reporter constructs reveal the expression of these PDF receptors in all body wall muscle cells and many head and tail neurons involved in the integration of environmental stimuli and the control of locomotion. Using a custom data analysis system, we demonstrate the involvement of this newly discovered neuropeptide signaling system in the regulation of locomotor behavior. Overexpression of PDF-2 phenocopies the locomotor defects of a PDF-1 null mutant, suggesting that they elicit opposite effects on locomotion through the identified PDF receptors. Our findings strengthen the hypothesis that the PDF signaling system, which imposes the circadian clock rhythm on behavior in Drosophila, has been functionally conserved throughout the protostomian evolutionary lineage.

Tyramine and octopamine independently inhibit serotonin-stimulated aversive behaviors in Caenorhabditis elegans through two novel amine receptors

Biogenic amines modulate key behaviors in both vertebrates and invertebrates. In Caenorhabditis elegans, tyramine (TA) and octopamine (OA) inhibit aversive responses to 100%, but not dilute (30%) octanol. TA and OA also abolish food- and serotonin-dependent increases in responses to dilute octanol in wild-type but not tyra-3(ok325) and f14d12.6(ok371) null animals, respectively, suggesting that TA and OA modulated responses to dilute octanol are mediated by separate, previously uncharacterized, G-protein-coupled receptors. TA and OA are high-affinity ligands for TYRA-3 and F14D12.6, respectively, based on their pharmacological characterization after heterologous expression. f14d12.6::gfp is expressed in the ASHs, the neurons responsible for sensitivity to dilute octanol, and the sra-6-dependent expression of F14D12.6 in the ASHs is sufficient to rescue OA sensitivity in f14d12.6(ok371) null animals. In contrast, tyra-3::gfp appears not to be expressed in the ASHs, but instead in other neurons, including the dopaminergic CEP/ADEs. However, although dopamine (DA) also inhibits 5-HT-dependent responses to dilute octanol, TA still inhibits in dop-2; dop-1; dop-3 animals that do not respond to DA and cat-2(tm346) and Pdat-1::ICE animals that lack significant dopaminergic signaling, suggesting that DA is not an intermediate in TA inhibition. Finally, responses to TA and OA selectively desensitize after preexposure to the amines. Our data suggest that although tyraminergic and octopaminergic signaling yield identical phenotypes in these olfactory assays, they act independently through distinct receptors to modulate the ASH-mediated locomotory circuit and that C. elegans is a useful model to study the aminergic modulation of sensory-mediated locomotory behaviors.

FMRFamide-like peptides encoded on the flp-18 precursor gene activate two isoforms of the orphan Caenorhabditis elegans G-protein-coupled receptor Y58G8A.4 heterologously expressed in mammalian cells

Two alternatively spliced variants of an orphan Caenorhabditis elegans G-protein-coupled receptors (GPCRs; Y58G8A.4a and Y58G8A.4b) were cloned and functionally expressed in Chinese hamster ovary (CHO) cells. The Y58G8A.4a and Y58G8A.4b proteins (397 and 433 amino acid residues, respectively) differ both in amino acid sequence and length of the C-terminal tail of the receptor. A calcium mobilization assay was used as a read-out for receptor function. Both receptors were activated, with nanomolar potencies, by putative peptides encoded by the flp-18 precursor gene, leading to their designation as FLP-18R1a (Y58G8A.4a) and FLP-18R1b (Y58G8A.4b). Three Ascaris suum neuropeptides AF3, AF4, and AF20 all sharing the same FLP-18 C-terminal signature, -PGVLRF-NH(2), were also potent agonists. In contrast to other previously reported C. elegans GPCRs expressed in mammalian cells, both FLP-18R1 variants were fully functional at 37 degrees C. However, a 37 to 28 degrees C temperature shift improved their activity, an effect that was more pronounced for FLP-18R1a. Despite differences in the C-terminus, the region implicated in distinct G-protein recognition for many other GPCRs, the same signaling pathways were observed for both Y58G8A.4 isoforms expressed in CHO cells. Gq protein coupling seems to be the main but not the exclusive signaling pathway, because pretreatment of cells with U-73122, a phospholipase inhibitor, attenuated but did not completely abolish the Ca(2+) signal. A weak Gs-mediated receptor activation was also detected as reflected in an agonist-triggered concentration-dependent cAMP increase. The matching of the FLP-18 peptides with their receptor(s) allows for the evaluation of the pharmacology of this system in the worm in vivo.

Classical transmitters and their receptors in flatworms

The flatworm nervous system employs a wide repertoire of neuroactive substances, including small chemical messengers, the so called classical transmitters, and several types of neuropeptides. A large body of research accumulated over four decades has provided a wealth of information on the tissue localization and effects of these substances, their biochemistry and, recently, their molecular modes of action in all major classes of flatworms. This evidence will be reviewed, with particular emphasis on the small (classical) transmitters and the receptors that mediate their effects. One of the themes that will emerge from this discussion is that classical transmitters regulate core activities such as movement, metabolism and transport, and thus are essential for survival of the organism. In addition, the evidence shows that flatworms have multiple neurotransmitter receptors, many with unusual pharmacological features, which make them particularly attractive as drug targets. Understanding the molecular basis of these distinctive properties, and developing new, more specific receptor agonists and antagonists will undoubtedly become a major challenge in future research.

Nematode neuropeptide receptors and their development as anthelmintic screens

This review addresses the potential use of neuropeptide receptors for the discovery of anthelmintic agents, and particularly for the identification of non-peptide ligands. It outlines which nematode neuropeptides are known and have been characterized, the published information on drug discovery around these targets, information about existing high- and low-throughput screening systems and finally the likely safety of neuropeptide mimetics.

Inter-phyla studies on neuropeptides: the potential for broad-spectrum anthelmintic and/or endectocide discovery

Flatworm, nematode and arthropod parasites have proven their ability to develop resistance to currently available chemotherapeutics. The heavy reliance on chemotherapy and the ability of target species to develop resistance has prompted the search for novel drug targets. In view of its importance to parasite/pest survival, the neuromusculature of parasitic helminths and pest arthropod species remains an attractive target for the discovery of novel endectocide targets. Exploitation of the neuropeptidergic system in helminths and arthropods has been hampered by a limited understanding of the functional roles of individual peptides and the structure of endogenous targets, such as receptors. Basic research into these systems has the potential to facilitate target characterization and its offshoots (screen development and drug identification). Of particular interest to parasitologists is the fact that selected neuropeptide families are common to metazoan pest species (nematodes, platyhelminths and arthropods) and fulfil specific roles in the modulation of muscle function in each of the three phyla. This article reviews the inter-phyla activity of two peptide families, the FMRFamide-like peptides and allatostatins, on motor function in helminths and arthropods and discusses the potential of neuropeptide signalling as a target system that could uncover novel endectocidal agents.

Are Caenorhabditis elegans receptors useful targets for drug discovery: pharmacological comparison of tyramine receptors with high identity from C. elegans (TYRA-2) and Brugia malayi (Bm4)

The biogenic amine, tyramine (TA), modulates a number of key processes in nematodes and a number of TA-specific receptors have been identified. In the present study, we have identified a putative TA receptor (Bm4) in the recently completed Brugia malayi genome and compared its pharmacology to its putative Caenorhabditis elegans orthologue, TYRA-2, under identical expression and assay conditions. TYRA-2 and Bm4 are the most closely related C. elegans and B. malayi BA receptors and differ by only 14aa in the TM regions directly involved in ligand binding. Membranes from HEK-293 cells stably expressing Bm4 exhibited specific, saturable, high affinity, [(3)H]LSD and [(3)H]TA binding with K(d)s of 18.1+/-0.93 and 15.1+/-0.2 nM, respectively. More importantly, both TYRA-2 and Bm4 TA exhibited similar rank orders of potencies for a number of potential tyraminergic ligands. However, some significant differences were noted. For example, chloropromazine exhibited an order of magnitude higher affinity for Bm4 than TYRA-2 (pK(i)s of 7.6+/-0.2 and 6.49+/-0.1, respectively). In contrast, TYRA-2 had significantly higher affinity for phentolamine than Bm4. These results highlight the utility of the nearly completed B. malayi genome and the importance of using receptors from individual parasitic nematodes for drug discovery.

Ligand screening system using fusion proteins of G protein-coupled receptors with G protein alpha subunits

G protein-coupled receptors (GPCRs) constitute one of the largest families of genes in the human genome, and are the largest targets for drug development. Although a large number of GPCR genes have recently been identified, ligands have not yet been identified for many of them. Various assay systems have been employed to identify ligands for orphan GPCRs, but there is still no simple and general method to screen for ligands of such GPCRs, particularly of G(i)-coupled receptors. We have examined whether fusion proteins of GPCRs with G protein alpha subunit (Galpha) could be utilized for ligand screening and showed that the fusion proteins provide an effective method for the purpose. This article focuses on the followings: (1) characterization of GPCR genes and GPCRs, (2) identification of ligands for orphan GPCRs, (3) characterization of GPCR-Galpha fusion proteins, and (4) identification of ligands for orphan GPCRs using GPCR-Galpha fusion proteins.

Neuropeptidergic signaling in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans joins the menagerie of behavioral model systems next to the fruit fly Drosophila melanogaster, the marine snail Aplysia californica and the mouse. In contrast to Aplysia, which contains 20,000 neurons having cell bodies of hundreds of microns in diameter, C. elegans harbors only 302 tiny neurons from which the cell lineage is completely described, as is the case for all the other somatic cells. As such, this nervous system appears at first sight incommensurable with those of higher organisms, although genome-wide comparison of predicted C. elegans genes with their counterparts in vertebrates revealed many parallels. Together with its short lifespan and ease of cultivation, suitability for high-throughput genetic screenings and genome-wide RNA interference approaches, access to an advanced genetic toolkit and cell-ablation techniques, it seems that this tiny transparent organism of only 1mm in length has nothing to hide. Recently, highly exciting developments have occurred within the field of neuropeptidergic signaling in C. elegans, not only because of the availability of a sequenced genome since 1998, but especially because of state of the art post genomic technologies, that allow for molecular characterization of the signaling molecules. Here, we will focus on endogenous, bioactive (neuro)peptides and mainly discuss biosynthesis, peptide sequence information, localization and G-protein coupled receptors of the three major peptide families in C. elegans.

Identification of a platyhelminth neuropeptide receptor

We report the characterisation of the first neuropeptide receptor from the phylum Platyhelminthes, an early-diverging phylum which includes a number of important human and veterinary parasites. The G protein-coupled receptor (GPCR) was identified from the model flatworm Girardia tigrina (Tricladida: Dugesiidae) based on the presence of motifs widely conserved amongst GPCRs. In two different assays utilising heterologous expression in Chinese hamster ovary cells, the Girardia GPCR was most potently activated by neuropeptides from the FMRFamide-like peptide class. The most potent platyhelminth neuropeptide in both assays was GYIRFamide, a FMRFamide-like peptide known to be present in G. tigrina. There was no activation by neuropeptide Fs, another class of flatworm neuropeptides. Also active were FMRFamide-like peptides derived from other phyla but not known to be present in any platyhelminth. Most potent among these were nematode neuropeptides encoded by the Caenorhabditis elegans flp-1 gene which share a PNFLRFamide carboxy terminal motif. The ability of nematode peptides to stimulate a platyhelminth receptor demonstrates a degree of structural conservation between FMRFamide-like peptide receptors from these two distinct, distant phyla which contain parasitic worms.

Molecular signaling involved in regulating feeding and other mitivated behaviors

The metabolic and nutritional status of an organism influences multiple behaviors in addition to food intake. When an organism is hungry, it employs behaviors that help it locate and ingest food while suppressing behaviors that are not associated with this goal. Alternatively, when an organism is satiated, food-seeking behaviors are repressed so that the animal can direct itself to other goal-oriented tasks such as reproductive behaviors. Studies in both vertebrate and invertebrate model systems have revealed that food-deprived and -satiated behaviors are differentially executed and integrated via common molecular signaling mechanisms. This article discusses cellular and molecular mechanisms for how insulin, neuropeptide Y (NPY), and serotonin utilize common signaling pathways to integrate feeding and metabolic state with other motivated behaviors. Insulin, NPY, and serotonin are three of the most well-studied molecules implicated in regulating such behaviors. Overall, insulin signaling allows an organism to coordinate proper behavioral output with changes in metabolism, NPY activates behaviors required for locating and ingesting food, and serotonin modulates behaviors performed when an organism is satiated. These three molecules work to ensure that the proper behaviors are executed in response to the feeding state of an organism.

Characterization of an RFamide-related peptide orphan GPCR in C. elegans

We cloned and characterized an orphan FMRFamide-related peptide (FaRP) GPCR in Caenorhabditis elegans. We synthesized numerous structurally different FaRPs that were found in the C. elegans genome by bioinformatic analysis and used them to screen cells expressing the C26F1.6 receptor. Two peptides ending in M(orL)VRFamide elicited a calcium response in receptor-expressing mammalian Chinese hamster ovary cells. The response was dose-dependent and appeared to be very specific; that is, none of the other FaRPs were active, not even closely related peptides also ending in M(orL)VRFamide, which are encoded by the same peptide precursor. Pharmacological profiling with a truncated series of the most active peptide revealed that the full peptide sequence is necessary for receptor activation.

The FLP-side of nematodes

The central role of FMRFamide-like peptides (FLPs) in nematode motor and sensory capabilities makes FLP signalling an appealing target for new parasiticides. Accumulating evidence has revealed an astounding level of FLP sequence conservation and diversity in the phylum Nematoda, and preliminary work has begun to identify the nematode FLP receptor complement in Caenorhabditis elegans, with a view to investigating their basic biology and therapeutic potential. However, much work is needed to clarify the functional aspects of FLP signalling and how these peptides exert their effects at the organismal level. Here, we summarize our current knowledge of nematode FLP signalling.

NMR analysis of Caenorhabditis elegans FLP-18 neuropeptides: implications for NPR-1 activation

Phe-Met-Arg-Phe-NH2 (FMRFamide)-like peptides (FLPs) are the largest neuropeptide family in animals, particularly invertebrates. FLPs are characterized by a C-N-terminal gradient of decreasing amino acid conservation. Neuropeptide receptor 1 (NPR-1) is a G-protein coupled receptor (GPCR), which has been shown to be a strong regulator of foraging behavior and aggregation responses in Caenorhabditis elegans. Recently, ligands for NPR-1 were identified as neuropeptides coded by the precursor genes flp-18 and flp-21 in C. elegans. The flp-18 gene encodes eight FLPs including DFDGAMPGVLRF-NH2 and EMPGVLRF-NH2. These peptides exhibit considerably different activities on NPR-1, with the longer one showing a lower potency. We have used nuclear magnetic resonance and biological activity to investigate structural features that may explain these activity differences. Our data demonstrate that long-range electrostatic interactions exist between N-terminal aspartates and the C-terminal penultimate arginine as well as N-terminal hydrogen-bonding interactions that form transient loops within DFDGAMPGVLRF-NH2. We hypothesize that these loops, along with peptide charge, diminish the activity of this peptide on NPR-1 relative to that of EMPGVLRF-NH2. These results provide some insight into the large amino acid diversity in FLPs.

FMRFamide related peptide ligands activate the Caenorhabditis elegans orphan GPCR Y59H11AL.1

G-protein coupled receptors (GPCRs) are ancient molecules that can sense environmental and physiological signals. Currently, the majority of the predicted Caenorhabditis elegans GPCRs are orphan. Here, we describe the characterization of such an orphan C. elegans GPCR, which is categorized in the tachykinin-like group of receptors. Since the C. elegans genome predicts only one tachykinin-like peptide (SFDRMGGTEFGLM), which could not activate the receptor, we hypothesized that one or some of the numerous FMRFamide related peptides (FaRPs) could be the cognate ligands for this receptor. This hypothesis was based on the suggestion that RFamides may be ancestral neuropeptides, from which a lot of the amidated neuropeptides, including tachykinins, derived. Indeed, we found that the orphan receptor encoded by the Y59H11AL.1 gene is activated by several C. elegans neuropeptides, including SPMERSAMVRFamide. These peptides activate the receptor in a concentration-dependent way.

Analysis of FMRFamide-like peptide (FLP) diversity in phylum Nematoda

This study reports a series of systematic BLAST searches of nematode ESTs on the Genbank database, using search strings derived from known nematode FLPs (those encoded by Caenorhabditis elegans flp genes as well as those isolated from other nematodes including Ascaris suum), as well as query sequences representative of theoretical FLPs. Over 1000 putative FLP-encoding ESTs were identified from multiple nematode species. A total of 969 ESTs representing sequelogs of the 23 known C. elegans flp genes were identified in 32 species, from clades I, III, IV and V. Numerical analysis of EST numbers suggests that flp-1, flp-11 and flp-14 are amongst the most highly expressed flp genes. Speculative BLAST searches were performed using theoretical FLP C-termini as queries, in an attempt to identify putative novel FLP sequences in the EST database. These searches yielded eight multi-species sequelogs encoding FLPs with novel signatures that are believed to identify distinct flp genes. These novel genes encode 25 distinct previously unidentified FLPs, and raise the current total of known nematode flp genes to 31. Additionally, software-based analyses of the presence of signal peptides were performed, with signal peptides being identified on at least one member of each group of flp ESTs, further confirming their status as secreted peptides. The data reveal that nematode FLPs encompass the most complex neuropeptide family known within the metazoa. Moreover, individual FLPs and FLP motifs are highly conserved across the nematodes with little evidence for inter-clade or inter-lifestyle variation, supporting their fundamental role in free-living and parasitic species.

Neuromuscular function in plant parasitic nematodes: a target for novel control strategies?

Over the last decade the need for new strategies and compounds to control parasitic helminths has become increasingly urgent. The neuromuscular systems of these worms have been espoused as potential sources of target molecules for new drugs which may address this need. One facet of helminth neuromuscular biology which has garnered considerable research interest is that of neuropeptidergic neurotransmission, particularly regarding parasites of humans and animals, as well as free-living nematode model species. This research interest has been piqued by the fact that neuropeptides have been demonstrated to be fundamentally important to nematode biology and thus may be of utility in this search for new drug targets. This review focuses on the neuropeptide biology of plant parasitic nematodes, a subject which has been comparatively neglected despite the fact that the search for alternative control measures also extends to these economically important parasites. We focus on the FMRFamide-like peptide (FLP) neuropeptides and the complexity and distribution of this peptide family in plant parasitic nematodes. Possible roles for FLPs in plant parasitic nematode behaviour, as elucidated by a combination of molecular imaging techniques and RNA interference (RNAi), are discussed. We propose that disruption of FLP neurosignalling in plant parasitic nematodes represents a novel form of pest control and speculate as to how this may be achieved.

The ever-expanding neuropeptide gene families in the nematode Caenorhabditis elegans

Neuropeptides act as chemical signals in the nervous system to modulate behaviour. With the ongoing EST projects and DNA sequence determination of different genomes, the identification of neuropeptide genes has been made easier. Despite the relatively 'simple' repertoire of behaviours in the nematode Caenorhabditis elegans, this worm contains a surprisingly large and diverse set of neuropeptide genes. At least 109 genes encoding over 250 potential neuropeptides have been identified in C. elegans; all genes are likely to be expressed and many, if not all, of the predicted peptides are produced. The predicted peptides include: 38 insulin-like peptides, several of which are involved in development and reproductive growth, and over 70 FMRFamide-related peptides, some of which are involved in locomotion, reproduction, and social behaviour. Many of the C. elegans peptides are identical or highly similar to those isolated or predicted in parasitic nematodes, such as Ascaris suum, Haemonchus contortus, Ancylostoma caninum, Heterodera glycines and Meloidogyne arenaria, suggesting that the function of these peptides is similar across species. The challenge for the future is to determine the function of all the genes and individual peptides and to identify the receptors through which the peptides signal.

Role of a FMRFamide-like family of neuropeptides in the pharyngeal nervous system of Caenorhabditis elegans

The nervous system of C. elegans has a remarkable abundance of flp genes encoding FMRFamide-like (FLP) neuropeptides. To provide insight into the physiological relevance of this neuropeptide diversity, we have tested more than 30 FLPs (encoded by 23 flps) for bioactivity on C. elegans pharynx. Eleven flp genes encode peptides that inhibit pharyngeal activity, while eight flp genes encode peptides that are excitatory. Three potent peptides (inhibitory, FLP-13A, APEASPFIRFamide; excitatory, FLP-17A, KSAFVRFamide; excitatory, FLP-17B, KSQYIRFamide) are encoded by flp genes, which, according to reporter gene constructs, are expressed in pharyngeal motoneurons. Thus, they may act through receptors localized on the pharyngeal muscle. The two other potent peptides, FLP-8 (excitatory AF1, KNEFIRFamide,) and FLP-11A (inhibitory, AMRNALVRFamide), appear to be expressed in extrapharyngeal neurons and are therefore likely to act either indirectly or as neurohormones. Intriguingly, a single neuron can express peptides that have potent but opposing biological activity in the pharynx. Only five flp genes encode neuropeptides that have no observable effect on the pharynx, but none of these have shown reporter gene expression in the pharyngeal nervous system. To examine the roles of multiple peptides produced from single precursors, a comparison was made between the bioactivity of different neuropeptides for five flp genes (flp-3, flp-13, flp-14, flp-17, and flp-18). For all but one gene (flp-14), the effects of peptides encoded by the same gene were similar. Overall, this study demonstrates the impressive neurochemical complexity of the simple circuit that regulates feeding in the nematode, C. elegans.