Coordinated regulation of foraging and metabolism in C. elegans by RFamide neuropeptide signaling

Neuroendocrine Control of Synaptic Transmission by PHAC-1 in C. elegans

A dynamic interplay between fast synaptic signals and slower neuromodulatory signals controls the excitatory/inhibitory (E/I) balance within neuronal circuits. The mechanisms by which neuropeptide signaling is regulated to maintain E/I balance remain uncertain. We designed a genetic screen to isolate genes involved in the peptidergic maintenance of the E/I balance in the C. elegans motor circuit. This screen identified the C. elegans orthologs of the presynaptic phosphoprotein synapsin (snn-1) and the protein phosphatase 1 (PP1) regulatory subunit PHACTR1 (phac-1). We demonstrate that both phac-1 and snn-1 alter the motor behavior of C. elegans, and genetic interactions suggest that SNN-1 contributes to PP1-PHAC-1 holoenzyme signaling. De novo variants of human PHACTR1, associated with early-onset epilepsies [developmental and epileptic encephalopathy 70 (DEE70)], when expressed in C. elegans resulted in constitutive PP1-PHAC-1 holoenzyme activity. Unregulated PP1-PHAC-1 signaling alters the synapsin and actin cytoskeleton and increases neuropeptide release by cholinergic motor neurons, which secondarily affects the presynaptic vesicle cycle. Together, these results clarify the dominant mechanisms of action of the DEE70 alleles and suggest that altered neuropeptide release may alter E/I balance in DEE70.

Modulation by NPY/NPF-like receptor underlies experience-dependent, sexually dimorphic learning

The evolutionary paths taken by each sex within a given species sometimes diverge, resulting in behavioral differences. Given their distinct needs, the mechanism by which each sex learns from a shared experience is still an open question. Here, we reveal sexual dimorphism in learning: C. elegans males do not learn to avoid the pathogenic bacteria PA14 as efficiently and rapidly as hermaphrodites. Notably, neuronal activity following pathogen exposure was dimorphic: hermaphrodites generate robust representations, while males, in line with their behavior, exhibit contrasting representations. Transcriptomic and behavioral analysis revealed that the neuropeptide receptor npr-5, an ortholog of the mammalian NPY/NPF-like receptor, regulates male learning by modulating neuronal activity. Furthermore, we show the dependency of the males' decision-making on their sexual status and demonstrate the role of npr-5 as a modulator of incoming sensory cues. Taken together, these findings illustrate how neuromodulators drive sex-specific behavioral plasticity in response to a shared experience.

Neuropeptide signaling network of Caenorhabditis elegans: from structure to behavior

Neuropeptides are abundant signaling molecules that control neuronal activity and behavior in all animals. Owing in part to its well-defined and compact nervous system, Caenorhabditis elegans has been one of the primary model organisms used to investigate how neuropeptide signaling networks are organized and how these neurochemicals regulate behavior. We here review recent work that has expanded our understanding of the neuropeptidergic signaling network in C. elegans by mapping the evolutionary conservation, the molecular expression, the receptor-ligand interactions, and the system-wide organization of neuropeptide pathways in the C. elegans nervous system. We also describe general insights into neuropeptidergic circuit motifs and the spatiotemporal range of peptidergic transmission that have emerged from in vivo studies on neuropeptide signaling. With efforts ongoing to chart peptide signaling networks in other organisms, the C. elegans neuropeptidergic connectome can serve as a prototype to further understand the organization and the signaling dynamics of these networks at organismal level.

CKR-1 orchestrates two motor states from a single motoneuron in C. elegans

Neuromodulation is pivotal in modifying neuronal properties and motor states. CKR-1, a homolog of the cholecystokinin receptor, modulates robust escape steering and undulation body bending in C. elegans. Nevertheless, the mechanisms through which CKR-1 governs these motor states remain elusive. We elucidate the head motoneuron SMD as the orchestrator of both motor states. This regulation involves two neuropeptides: NLP-12 from DVA enhances undulation body curvature, while NLP-18 from ASI amplifies Ω-turn head curvature. Moreover, synthetic NLP-12 and NLP-18 peptides elicit CKR-1-dependent currents in Xenopus oocytes and Ca2+ transients in SMD neurons. Notably, CKR-1 shows higher sensitivity to NLP-18 compared to NLP-12. In situ patch-clamp recordings reveal CKR-1, NLP-12, and NLP-18 are not essential for neurotransmission at C. elegans neuromuscular junction, suggesting that SMD independently regulates head and body bending. Our studies illustrate that a single motoneuron SMD utilizes a cholecystokinin receptor CKR-1 to integrate two motor states.

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

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

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.

Disexcitation in the ASH/RIM/ADL negative feedback circuit fine-tunes hyperosmotic sensation and avoidance in Caenorhabditis elegans

Sensations, especially nociception, are tightly controlled and regulated by the central and peripheral nervous systems. Osmotic sensation and related physiological and behavioral reactions are essential for animal well-being and survival. In this study, we find that interaction between secondary nociceptive ADL and primary nociceptive ASH neurons upregulates Caenorhabditis elegans avoidance of the mild and medium hyperosmolality of 0.41 and 0.88 Osm but does not affect avoidance of high osmolality of 1.37 and 2.29 Osm. The interaction between ASH and ADL is actualized through a negative feedback circuit consisting of ASH, ADL, and RIM interneurons. In this circuit, hyperosmolality-sensitive ADL augments the ASH hyperosmotic response and animal hyperosmotic avoidance; RIM inhibits ADL and is excited by ASH; thus, ASH exciting RIM reduces ADL augmenting ASH. The neuronal signal integration modality in the circuit is disexcitation. In addition, ASH promotes hyperosmotic avoidance through ASH/RIC/AIY feedforward circuit. Finally, we find that in addition to ASH and ADL, multiple sensory neurons are involved in hyperosmotic sensation and avoidance behavior.

A guide to area-restricted search: a foundational foraging behaviour

Area-restricted search is the capacity to change search effort adaptively in response to resource encounters or expectations, from directional exploration (global, extensive search) to focused exploitation (local, intensive search). This search pattern is used by numerous organisms, from worms and insects to humans, to find various targets, such as food, mates, nests, and other resources. Area-restricted search has been studied for at least 80 years by ecologists, and more recently in the neurological and psychological literature. In general, the conditions promoting this search pattern are: (1) clustered resources; (2) active search (e.g. not a sit-and-wait predator); (3) searcher memory for recent target encounters or expectations; and (4) searcher ignorance about the exact location of targets. Because area-restricted search adapts to resource encounters, the search can be performed at multiple spatial scales. Models and experiments have demonstrated that area-restricted search is superior to alternative search patterns that do not involve a memory of the exact location of the target, such as correlated random walks or Lévy walks/flights. Area-restricted search is triggered by sensory cues whereas concentrated search in the absence of sensory cues is associated with other forms of foraging. Some neural underpinnings of area-restricted search are probably shared across metazoans, suggesting a shared ancestry and a shared solution to a common ecological problem of finding clustered resources. Area-restricted search is also apparent in other domains, such as memory and visual search in humans, which may indicate an exaptation from spatial search to other forms of search. Here, we review these various aspects of area-restricted search, as well as how to identify it, and point to open questions.

Brain-wide bidirectional neuropeptide modulation of individual neuron classes regulates a developmental decision

Secreted neuromodulators, like biogenic amines and neuropeptides, can reconfigure circuit functions both locally and at a distance and establish global brain states that alter circuit outputs over prolonged timescales.1-3 Despite their diversity and ubiquitous presence, many studies on neuromodulation tend to focus on dissecting the function and site of action of individual neuropeptides. Here, we take a different approach by conducting a systems-level investigation of neuropeptide receptor signaling function and cell-type-specific distribution in the context of the Caenorhabditis elegans diapause entry developmental decision. C. elegans diapause entry is controlled by sensory perception of external factors and is regulated by neuropeptide signaling.4-8 We performed a comprehensive functional screen of neuropeptide receptor mutants for pheromone-induced diapause entry phenotypes and integrated these results with published C. elegans single-cell RNA-seq data to reveal that almost all neuron classes expressed at least one receptor with a role in diapause entry.9 Our receptor expression analysis also identified four highly modulated neural hubs with no previously reported roles in diapause entry that are distributed throughout the animal's body, possibly as a means of synchronizing the whole-organism transition into the appropriate larval morph. Furthermore, most neuron classes expressed unique neuropeptide receptor repertoires that have opposing effects on the diapause entry decision. We propose that brain-wide antagonistic neuropeptide modulation of individual neuron classes by distinct neuropeptide receptor subsets could serve as a strategy against overmodulation and that this motif might generalize to other decision-making paradigms in other organisms.

A new use for old drugs: identifying compounds with an anti-obesity effect using a high through-put semi-automated Caenorhabditis elegans screening platform

Obesity is one of the most common global health problems for all age groups with obese people at risk of a variety of associated health complications. Consequently, there is a need to develop new therapies that lower body fat without the side effects. However, obesity is a complex and systemic disease, so that in vitro results are not easily translatable to clinical situations. A promising way to circumnavigate these issues is to reposition already approved drugs for new treatments, enabling a more streamlined drug discovery process due to the availability of pre-existing pharmacological and toxicological datasets. Chemical libraries, such as the Prestwick Chemical Library of 1200 FDA approved drugs, are available for this purpose. We have developed a simple semi-automated whole-organism approach to screening the Prestwick Chemical Library for those compounds which reduce fat content using the model organism Caenorhabditis elegans.

Our whole-organism approach to high-throughput screening identified 9 “lead” compounds that reduced fat within 2 weeks in the model. Further screening and analysis provided 4 “hit” compounds (Midodrine, Vinpocetine, Fenoprofen and Lamivudine) that showed significant promise as drugs to reduce fat levels. The effects of these candidates were found to further reduce fat content in nematodes where an nhr-49/PPAR mutation resulted in “overweight” worms. Upon unblinding the “hit” compounds, they were found to have recently been shown to have anti-obesity effects in mammalian models too. In developing a whole-animal chemical screen to identify pharmacological agents as potential anti-obesity compounds, we demonstrate how chemical libraries can be rapidly and relatively cheaply profiled for active hits. Using the nematode Caenorhabditis elegans thus enables drugs to be assessed for applicability in humans and provides a new incentive to explore drug repurposing as a feasible and efficient way to identify new anti-obesity compounds.

The FMRFamide-like peptide FLP-2 is involved in the modulation of larval development and adult lifespan by regulating the secretion of the insulin-like peptide INS-35 in Caenorhabditis elegans

In the animal kingdom, neuropeptides regulate diverse physiological functions. In invertebrates, FMRFamide and its related peptides, a family of neuropeptides, play an important role as neurotransmitters. The FMRFamide-like peptides (FLPs) are one of the most diverse neuropeptide families and are conserved in nematodes. Our screen for flp genes of the free-living soil nematode Caenorhabditis elegans revealed that the flp-2 gene is involved in larval development. The gene is also conserved in plant-parasitic root-knot nematodes. Our molecular genetic analyses of the C. elegans flp-2 gene demonstrated as follows: 1) the production and secretion of FLP-2, produced in the head neurons, are controlled by environmental factors (growth density and food); 2) the FLP-2 is involved in not only larval development but also adult lifespan by regulating the secretion of one of the insulin-like peptides INS-35, produced in the intestine. These findings provide new insight into the development of new nematicides.

Olfactory system and energy metabolism: a two-way street

Olfactory perception guides daily decisions regarding food consumption, social interactions, and predator avoidance in all mammalian species. Volatile inputs, comprising odorants and pheromones, are relayed to the olfactory bulb (OB) from nasal sensory neurons cells and transferred to secondary processing regions within the brain. Olfaction has recently been shown to shape homeostatic and maladaptive processes of energy intake and expenditure through neuronal circuits involving the medial basal hypothalamus. Reciprocally, gastrointestinal hormones, such as ghrelin and leptin, the secretion of which depends on satiety and adiposity levels, might also influence olfactory sensitivity to alter food-seeking behaviors. Here, in addition to reviewing recent updates on identifying these neuronal networks, we also discuss how bidirectional neurocircuits existing between olfactory and energy processing centers can become dysregulated during obesity.

Transcriptome Analysis of the Marine Nematode Litoditis marina in a Chemically Defined Food Environment with Stearic Acid Supplementation

Stearic acid represents one of the most abundant fatty acids in the Western diet and profoundly regulates health and diseases of animals and human beings. We previously showed that stearic acid supplementation promoted development of the terrestrial model nematode Caenorhabditis elegans in chemically defined CeMM food environment. However, whether stearic acid regulates development of other nematodes remains unknown. Here, we found that dietary supplementation with stearic acid could promote the development of the marine nematode Litoditis marina, belonging to the same family as C. elegans, indicating the conserved roles of stearic acid in developmental regulation. We further employed transcriptome analysis to analyze genome-wide transcriptional signatures of L. marina with dietary stearic acid supplementation. We found that stearic acid might promote development of L. marina via upregulation of the expression of genes involved in aminoacyl-tRNA biosynthesis, translation initiation and elongation, ribosome biogenesis, and transmembrane transport. In addition, we observed that the expression of neuronal signaling-related genes was decreased. This study provided important insights into how a single fatty acid stearic acid regulates development of marine nematode, and further studies with CRISPR genome editing will facilitate demonstrating the molecular mechanisms underlying how a single metabolite regulates animal development and health.

Co-transmission of neuropeptides and monoamines choreograph the C. elegans escape response

Co-localization and co-transmission of neurotransmitters and neuropeptides is a core property of neural signaling across species. While co-transmission can increase the flexibility of cellular communication, understanding the functional impact on neural dynamics and behavior remains a major challenge. Here we examine the role of neuropeptide/monoamine co-transmission in the orchestration of the C. elegans escape response. The tyraminergic RIM neurons, which coordinate distinct motor programs of the escape response, also co-express the neuropeptide encoding gene flp-18. We find that in response to a mechanical stimulus, flp-18 mutants have defects in locomotory arousal and head bending that facilitate the omega turn. We show that the induction of the escape response leads to the release of FLP-18 neuropeptides. FLP-18 modulates the escape response through the activation of the G-protein coupled receptor NPR-5. FLP-18 increases intracellular calcium levels in neck and body wall muscles to promote body bending. Our results show that FLP-18 and tyramine act in different tissues in both a complementary and antagonistic manner to control distinct motor programs during different phases of the C. elegans flight response. Our study reveals basic principles by which co-transmission of monoamines and neuropeptides orchestrate in arousal and behavior in response to stress.

Neuropeptides and Behaviors: How Small Peptides Regulate Nervous System Function and Behavioral Outputs

One of the reasons that most multicellular animals survive and thrive is because of the adaptable and plastic nature of their nervous systems. For an organism to survive, it is essential for the animal to respond and adapt to environmental changes. This is achieved by sensing external cues and translating them into behaviors through changes in synaptic activity. The nervous system plays a crucial role in constantly evaluating environmental cues and allowing for behavioral plasticity in the organism. Multiple neurotransmitters and neuropeptides have been implicated as key players for integrating sensory information to produce the desired output. Because of its simple nervous system and well-established neuronal connectome, C. elegans acts as an excellent model to understand the mechanisms underlying behavioral plasticity. Here, we critically review how neuropeptides modulate a wide range of behaviors by allowing for changes in neuronal and synaptic signaling. This review will have a specific focus on feeding, mating, sleep, addiction, learning and locomotory behaviors in C. elegans. With a view to understand evolutionary relationships, we explore the functions and associated pathophysiology of C. elegans neuropeptides that are conserved across different phyla. Further, we discuss the mechanisms of neuropeptidergic signaling and how these signals are regulated in different behaviors. Finally, we attempt to provide insight into developing potential therapeutics for neuropeptide-related disorders.

Ascaris suum Informs Extrasynaptic Volume Transmission in Nematodes

Neural circuit synaptic connectivities (the connectome) provide the anatomical foundation for our understanding of nematode nervous system function. However, other nonsynaptic routes of communication are known in invertebrates including extrasynaptic volume transmission (EVT), which enables short- and/or long-range communication in the absence of synaptic connections. Although EVT has been highlighted as a facet of Caenorhabditis elegans neurosignaling, no experimental evidence identifies body cavity fluid (pseudocoelomic fluid; PCF) as a vehicle for either neuropeptide or biogenic amine transmission. In the parasitic nematode Ascaris suum, FMRFamide-like peptides encoded on flp-18 potently stimulate female reproductive organs but are expressed in cells that are anatomically distant from the reproductive organ, with no known synaptic connections to this tissue. Here we investigate nonsynaptic neuropeptide signaling in nematodes mediated by the body cavity fluid. Our data show that (i) A. suum PCF (As-PCF) contains a catalog of neuropeptides including FMRFamide-like peptides and neuropeptide-like proteins, (ii) the A. suum FMRFamide-like peptide As-FLP-18A dominates the As-PCF peptidome, (iii) As-PCF potently modulates nematode reproductive muscle function ex vivo, mirroring the effects of synthetic FLP-18 peptides, (iv) As-PCF activates the C. elegans FLP-18 receptors NPR-4 and -5, (v) As-PCF alters C. elegans behavior, and (vi) FLP-18 and FLP-18 receptors display pan-phylum distribution in nematodes. This study provides the first direct experimental evidence to support an extrasynaptic volume route for neuropeptide transmission in nematodes. These data indicate nonsynaptic signaling within the nematode functional connectome and are particularly pertinent to receptor deorphanization approaches underpinning drug discovery programs for nematode pathogens.

The FMRFamide Neuropeptide FLP-20 Acts as a Systemic Signal for Starvation Responses in Caenorhabditis elegans

Most animals face frequent periods of starvation throughout their entire life and thus need to appropriately adjust their behavior and metabolism during starvation for their survival. Such adaptive responses are regulated by a complex set of systemic signals, including hormones and neuropeptides. While much progress has been made in identifying pathways that regulate nutrient-excessive states, it is still incompletely understood how animals systemically signal their nutrient-deficient states. Here, we showed that the FMRFamide neuropeptide FLP-20 modulates a systemic starvation response in Caenorhabditis elegans. We found that mutation of flp-20 rescued the starvation hypersensitivity of the G protein β-subunit gpb-2 mutants by suppressing excessive autophagy. FLP-20 acted in AIB neurons, where the metabotropic glutamate receptor MGL-2 also functions to modulate a systemic starvation response. Furthermore, FLP-20 modulated starvation-induced fat degradation in a manner dependent on the receptor-type guanylate cyclase GCY-28. Collectively, our results reveal a circuit that senses and signals nutrient-deficient states to modulate a systemic starvation response in multicellular organisms.

CREB mediates the C. elegans dauer polyphenism through direct and cell-autonomous regulation of TGF-β expression

Animals can adapt to dynamic environmental conditions by modulating their developmental programs. Understanding the genetic architecture and molecular mechanisms underlying developmental plasticity in response to changing environments is an important and emerging area of research. Here, we show a novel role of cAMP response element binding protein (CREB)-encoding crh-1 gene in developmental polyphenism of C. elegans. Under conditions that promote normal development in wild-type animals, crh-1 mutants inappropriately form transient pre-dauer (L2d) larvae and express the L2d marker gene. L2d formation in crh-1 mutants is specifically induced by the ascaroside pheromone ascr#5 (asc-ωC3; C3), and crh-1 functions autonomously in the ascr#5-sensing ASI neurons to inhibit L2d formation. Moreover, we find that CRH-1 directly binds upstream of the daf-7 TGF-β locus and promotes its expression in the ASI neurons. Taken together, these results provide new insight into how animals alter their developmental programs in response to environmental changes.

The Prop1-like homeobox gene unc-42 specifies the identity of synaptically connected neurons

Many neuronal identity regulators are expressed in distinct populations of cells in the nervous system, but their function is often analyzed only in specific isolated cellular contexts, thereby potentially leaving overarching themes in gene function undiscovered. We show here that the Caenorhabditis elegans Prop1-like homeobox gene unc-42 is expressed in 15 distinct sensory, inter- and motor neuron classes throughout the entire C. elegans nervous system. Strikingly, all 15 neuron classes expressing unc-42 are synaptically interconnected, prompting us to investigate whether unc-42 controls the functional properties of this circuit and perhaps also the assembly of these neurons into functional circuitry. We found that unc-42 defines the routes of communication between these interconnected neurons by controlling the expression of neurotransmitter pathway genes, neurotransmitter receptors, neuropeptides, and neuropeptide receptors. Anatomical analysis of unc-42 mutant animals reveals defects in axon pathfinding and synaptic connectivity, paralleled by expression defects of molecules involved in axon pathfinding, cell-cell recognition, and synaptic connectivity. We conclude that unc-42 establishes functional circuitry by acting as a terminal selector of functionally connected neuron types. We identify a number of additional transcription factors that are also expressed in synaptically connected neurons and propose that terminal selectors may also function as 'circuit organizer transcription factors' to control the assembly of functional circuitry throughout the nervous system. We hypothesize that such organizational properties of transcription factors may be reflective of not only ontogenetic, but perhaps also phylogenetic trajectories of neuronal circuit establishment.

Dopamine receptor DOP-1 engages a sleep pathway to modulate swimming in C. elegans

Animals require robust yet flexible programs to support locomotion. Here we report a pathway that connects the D1-like dopamine receptor DOP-1 with a sleep mechanism to modulate swimming in C. elegans. We show that DOP-1 plays a negative role in sustaining swimming behavior. By contrast, a pathway through the D2-like dopamine receptor DOP-3 negatively regulates the initiation of swimming, but its impact fades quickly over a few minutes. We find that DOP-1 and the GPCR kinase (G-protein-coupled receptor kinase-2) function in the sleep interneuron RIS, where DOP-1 modulates the secretion of a sleep neuropeptide FLP-11. We further show that DOP-1 and FLP-11 act in the same pathway to modulate swimming. Together, these results delineate a functional connection between a dopamine receptor and a sleep program to regulate swimming in C. elegans. The temporal transition between DOP-3 and DOP-1 pathways highlights the dynamic nature of neuromodulation for rhythmic movements that persist over time.

Mitochondrial hydrogen peroxide positively regulates neuropeptide secretion during diet-induced activation of the oxidative stress response

Mitochondria play a pivotal role in the generation of signals coupling metabolism with neurotransmitter release, but a role for mitochondrial-produced ROS in regulating neurosecretion has not been described. Here we show that endogenously produced hydrogen peroxide originating from axonal mitochondria (mtH2O2) functions as a signaling cue to selectively regulate the secretion of a FMRFamide-related neuropeptide (FLP-1) from a pair of interneurons (AIY) in C. elegans. We show that pharmacological or genetic manipulations that increase mtH2O2 levels lead to increased FLP-1 secretion that is dependent upon ROS dismutation, mitochondrial calcium influx, and cysteine sulfenylation of the calcium-independent PKC family member PKC-1. mtH2O2-induced FLP-1 secretion activates the oxidative stress response transcription factor SKN-1/Nrf2 in distal tissues and protects animals from ROS-mediated toxicity. mtH2O2 levels in AIY neurons, FLP-1 secretion and SKN-1 activity are rapidly and reversibly regulated by exposing animals to different bacterial food sources. These results reveal a previously unreported role for mtH2O2 in linking diet-induced changes in mitochondrial homeostasis with neuropeptide secretion.

FMRFamide-Like Peptide 22 Influences the Head Movement, Host Finding, and Infection of Heterodera glycines

The FMRFamide-like peptides (FLPs) represent the largest family of nematode neuropeptides and are involved in multiple parasitic activities. The immunoreactivity to FMRFamide within the nervous system of Heterodera glycines, the most economically damaging parasite of soybean [Glycine max L. (Merr)], has been reported in previous research. However, the family of genes encoding FLPs of H. glycines were not identified and functionally characterized. In this study, an FLP encoding gene Hg-flp-22 was cloned from H. glycines, and its functional characterization was uncovered by using in vitro RNA interference and application of synthetic peptides. Bioinformatics analysis showed that flp-22 is widely expressed in multiple nematode species, where they encode the highly conserved KWMRFamide motifs. Quantitative real-time (qRT)-PCR results revealed that Hg-flp-22 was highly expressed in the infective second-stage juveniles (J2s) and adult males. Silencing of Hg-flp-22 resulted in the reduced movement of J2s to the host root and reduced penetration ability, as well as a reduction in their subsequent number of females. Behavior and infection assays demonstrated that application of synthetic peptides Hg-FLP-22b (TPQGKWMRFa) and Hg-FLP-22c (KMAIEGGKWVRFa) significantly increased the head movement frequency and host invasion abilities in H. glycines but not in Meloidogyne incognita. In addition, the number of H. glycines females on the host roots was found to be significantly higher in Hg-FLP-22b treated nematodes than the ddH₂O-treated control J2s. These results presented in this study elucidated that Hg-flp-22 plays a role in regulating locomotion and infection of H. glycines. This suggests the potential of FLP signaling as putative control targets for H. glycines in soybean production.

Phylum-Spanning Neuropeptide GPCR Identification and Prioritization: Shaping Drug Target Discovery Pipelines for Nematode Parasite Control

Nematode parasites undermine human health and global food security. The frontline anthelmintic portfolio used to treat parasitic nematodes is threatened by the escalation of anthelmintic resistance, resulting in a demand for new drug targets for parasite control. Nematode neuropeptide signalling pathways represent an attractive source of novel drug targets which currently remain unexploited. The complexity of the nematode neuropeptidergic system challenges the discovery of new targets for parasite control, however recent advances in parasite 'omics' offers an opportunity for the in silico identification and prioritization of targets to seed anthelmintic discovery pipelines. In this study we employed Hidden Markov Model-based searches to identify ~1059 Caenorhabditis elegans neuropeptide G-protein coupled receptor (Ce-NP-GPCR) encoding gene homologs in the predicted protein datasets of 10 key parasitic nematodes that span several phylogenetic clades and lifestyles. We show that, whilst parasitic nematodes possess a reduced complement of Ce-NP-GPCRs, several receptors are broadly conserved across nematode species. To prioritize the most appealing parasitic nematode NP-GPCR anthelmintic targets, we developed a novel in silico nematode parasite drug target prioritization pipeline that incorporates pan-phylum NP-GPCR conservation, C. elegans-derived reverse genetics phenotype, and parasite life-stage specific expression datasets. Several NP-GPCRs emerge as the most attractive anthelmintic targets for broad spectrum nematode parasite control. Our analyses have also identified the most appropriate targets for species- and life stage- directed chemotherapies; in this context we have identified several NP-GPCRs with macrofilaricidal potential. These data focus functional validation efforts towards the most appealing NP-GPCR targets and, in addition, the prioritization strategy employed here provides a blueprint for parasitic nematode target selection beyond NP-GPCRs.

Simultaneous RNAi Knockdown of Three FMRFamide-Like Peptide Genes, Mi-flp1, Mi-flp12, and Mi-flp18 Provides Resistance to Root-Knot Nematode, Meloidogyne incognita

Root-knot nematode, Meloidogyne incognita, is a devastating sedentary endoparasite that causes considerable damage to agricultural crops worldwide. Modern approaches targeting the physiological processes have confirmed the potential of FMRFamide like peptide (FLPs) family of neuromotor genes for nematode management. Here, we assessed the knock down effect of Mi-flp1, Mi-flp12, and Mi-flp18 of M. incognita and their combinatorial fusion cassette on infection and reproduction. Comparative developmental profiling revealed higher expression of all three FLPs in the infective 2nd stage juveniles (J2s). Further, Mi-flp1 expression in J2s could be localized in the ventral pharyngeal nerves near to metacarpal bulb of the central nervous system. In vitro RNAi silencing of three FLPs and their fusion cassette in M. incognita J2s showed that combinatorial silencing is the most effective and affected nematode host recognition followed by reduced penetration ability and subsequent infection into tomato and adzuki bean roots. Northern blot analysis of J2s soaked in fusion dsRNA revealed the presence of siRNA of all three target FLPs establishing successful processing of fusion gene dsRNA in the J2s. Further, evaluation of the fusion gene cassette is done through host-delivered RNAi in tobacco. Transgenic plants with fusion gene RNA-expressing vector were generated in which transgene integration was confirmed by PCR, qRT-PCR, and Southern blot analysis. Transcript accumulation of three FLPs constituting the fusion gene was reduced in the M. incognita females collected from the transgenic plants that provided additional evidence for successful gene silencing. Evaluation of positive T₁ transgenic lines against M. incognita brought down the disease burden as indicated by various disease parameters that ultimately reduced the nematode multiplication factor (MF) by 85% compared to the wild-type plants. The study establishes the possibility of simultaneous silencing of more than one FLPs gene for effective management of M. incognita.

A robust and miniaturized screening platform to study natural products affecting metabolism and survival in Caenorhabditis elegans

In this study a robust, whole organism screening based on Caenorhabditis elegans is presented for the discovery of natural products (NP) with beneficial effects against obesity and age-related diseases. Several parameters of the elaborated workflow were optimized to be adapted for probing multicomponent mixtures combining knowledge from traditional medicine and NP chemistry by generating optimized small-scale extracts considering scarcity of the natural source, solubility issues, and potential assay interferences. The established miniaturized assay protocol allows for in vivo probing of small amounts of even complex samples (~ 1 mg) to test their ability to increase the nematodes' survival time and the suppression of fat accumulation assessed by Nile red staining as hall marks of "healthy aging". The workflow was applied on 24 herbal and fungal materials traditionally used against symptoms of the metabolic syndrome and revealed promising results for the extracts of Gardenia jasminoides fruits and the sclerotia from Inonotus obliquus. Tested at 100 µg/mL they were able to significantly reduce the Nile red fluorescence and extend the 50% survival rate (DT50) compared to the control groups. This phenotype-directed in vivo approach opens up new horizons for the selection of natural starting materials and the investigation of their active principles as fast drug discovery tool with predictive value for human diseases.

Olfactory specificity regulates lipid metabolism through neuroendocrine signaling in Caenorhabditis elegans

Olfactory and metabolic dysfunctions are intertwined phenomena associated with obesity and neurodegenerative diseases; yet how mechanistically olfaction regulates metabolic homeostasis remains unclear. Specificity of olfactory perception integrates diverse environmental odors and olfactory neurons expressing different receptors. Here, we report that specific but not all olfactory neurons actively regulate fat metabolism without affecting eating behaviors in Caenorhabditis elegans, and identified specific odors that reduce fat mobilization via inhibiting these neurons. Optogenetic activation or inhibition of the responsible olfactory neural circuit promotes the loss or gain of fat storage, respectively. Furthermore, we discovered that FLP-1 neuropeptide released from this olfactory neural circuit signals through peripheral NPR-4/neuropeptide receptor, SGK-1/serum- and glucocorticoid-inducible kinase, and specific isoforms of DAF-16/FOXO transcription factor to regulate fat storage. Our work reveals molecular mechanisms underlying olfactory regulation of fat metabolism, and suggests the association between olfactory perception specificity of each individual and his/her susceptibility to the development of obesity.

Control of Locomotory Behavior of Caenorhabditis elegans by the Immunoglobulin Superfamily Protein RIG-3

Cell surface immunoglobulin superfamily (IgSF) proteins play important roles in the development and function of the nervous system . Here we define the role of a Caenorhabditis elegans IgSF protein, RIG-3, in the function of the AVA command interneuron. This study reveals that RIG-3 regulates the abundance of the glutamate receptor subunit, GLR-1, in the AVA command interneuron and also regulates reversal behavior in C. elegans The mutant strain lacking rig-3 (rig-3 (ok2156)) shows increased reversal frequency during local search behaviors. Genetic and behavioral experiments suggest that RIG-3 functions through GLR-1 to regulate reversal behavior. We also show that the increased reversal frequency seen in rig-3 mutants is dependent on the increase in GLR-1 abundance at synaptic inputs to AVA, suggesting that RIG-3 alters the synaptic strength of incoming synapses through GLR-1 Consistent with the imaging experiments, altered synaptic strength was also reflected in increased calcium transients in rig-3 mutants when compared to wild-type control animals. Our results further suggest that animals lacking rig-3 show increased AVA activity, allowing the release of FLP-18 neuropeptide from AVA, which is an activity-dependent signaling molecule. Finally, we show that FLP-18 functions through the neuropeptide receptor, NPR-5, to modulate reversal behavior in C. elegans.

Molecular and cellular modulators for multisensory integration in C. elegans

In the natural environment, animals often encounter multiple sensory cues that are simultaneously present. The nervous system integrates the relevant sensory information to generate behavioral responses that have adaptive values. However, the neuronal basis and the modulators that regulate integrated behavioral response to multiple sensory cues are not well defined. Here, we address this question using a behavioral decision in C. elegans when the animal is presented with an attractive food source together with a repulsive odorant. We identify specific sensory neurons, interneurons and neuromodulators that orchestrate the decision-making process, suggesting that various states and contexts may modulate the multisensory integration. Among these modulators, we characterize a new function of a conserved TGF-β pathway that regulates the integrated decision by inhibiting the signaling from a set of central neurons. Interestingly, we find that a common set of modulators, including the TGF-β pathway, regulate the integrated response to the pairing of different foods and repellents. Together, our results provide mechanistic insights into the modulatory signals regulating multisensory integration.

The present study characterizes the modulation of a behavioral decision in C. elegans when the worm is presented with a food lawn that is paired with a repulsive smell. We show that multiple specific sensory neurons and interneurons play roles in making the decision. We also identify several modulatory molecules that are essential for the integrated decision when the animal faces a choice between the cues of opposing valence. We further show that many of these factors, which often represent different states and contexts, are common for behavioral decisions that integrate sensory information from different types of foods and repellents. Overall, our results reveal the molecular and cellular basis for integration of simultaneously present attractive and repulsive cues to fine-tune decision-making.

Regulation of Feeding and Metabolism by Neuropeptide F and Short Neuropeptide F in Invertebrates

Numerous neuropeptide systems have been implicated to coordinately control energy homeostasis, both centrally and peripherally. However, the vertebrate neuropeptide Y (NPY) system has emerged as the best described one regarding this biological process. The protostomian ortholog of NPY is neuropeptide F, characterized by an RXRF(Y)amide carboxyterminal motif. A second neuropeptide system is short NPF, characterized by an M/T/L/FRF(W)amide carboxyterminal motif. Although both short and long NPF neuropeptide systems display carboxyterminal sequence similarities, they are evolutionary distant and likely already arose as separate signaling systems in the common ancestor of deuterostomes and protostomes, indicating the functional importance of both. Both NPF and short-NPF systems seem to have roles in the coordination of feeding across bilaterian species, but during chordate evolution, the short NPF system appears to have been lost or evolved into the prolactin releasing peptide signaling system, which regulates feeding and has been suggested to be orthologous to sNPF. Here we review the roles of both NPF and sNPF systems in the regulation of feeding and metabolism in invertebrates.

Neuroligin tuning of pharyngeal pumping reveals extrapharyngeal modulation of feeding in Caenorhabditis elegans

The integration of distinct sensory modalities is essential for behavioural decision making. In C aenorhabditis elegans, this process is coordinated by neural circuits that integrate sensory cues from the environment to generate an appropriate behaviour at the appropriate output muscles. Food is a multimodal cue that impacts the microcircuits to modulate feeding and foraging drivers at the level of the pharyngeal and body wall muscle, respectively. When food triggers an upregulation in pharyngeal pumping, it allows the effective ingestion of food. Here, we show that a C elegans mutant in the single gene orthologous to human neuroligins, nlg-1, is defective in food-induced pumping. This was not due to an inability to sense food, as nlg-1 mutants were not defective in chemotaxis towards bacteria. In addition, we found that neuroligin is widely expressed in the nervous system, including AIY, ADE, ALA, URX and HSN neurons. Interestingly, despite the deficit in pharyngeal pumping, neuroligin was not expressed within the pharyngeal neuromuscular network, which suggests an extrapharyngeal regulation of this circuit. We resolved electrophysiologically the neuroligin contribution to the pharyngeal circuit by mimicking food-dependent pumping and found that the nlg-1 phenotype is similar to mutants impaired in GABAergic and/or glutamatergic signalling. We suggest that neuroligin organizes extrapharyngeal circuits that regulate the pharynx. These observations based on the molecular and cellular determinants of feeding are consistent with the emerging role of neuroligin in discretely impacting functional circuits underpinning complex behaviours.

Neuropeptide Signaling Regulates Pheromone-Mediated Gene Expression of a Chemoreceptor Gene in C. elegans

Animals need to be able to alter their developmental and behavioral programs in response to changing environmental conditions. This developmental and behavioral plasticity is mainly mediated by changes in gene expression. The knowledge of the mechanisms by which environmental signals are transduced and integrated to modulate changes in sensory gene expression is limited. Exposure to ascaroside pheromone has been reported to alter the expression of a subset of putative G protein-coupled chemosensory receptor genes in the ASI chemosensory neurons of C. elegans (Kim et al., 2009; Nolan et al., 2002; Peckol et al., 1999). Here we show that ascaroside pheromone reversibly represses expression of the str-3 chemoreceptor gene in the ASI neurons. Repression of str-3 expression can be initiated only at the L1 stage, but expression is restored upon removal of ascarosides at any developmental stage. Pheromone receptors including SRBC-64/66 and SRG-36/37 are required for str-3 repression. Moreover, pheromone-mediated str-3 repression is mediated by FLP-18 neuropeptide signaling via the NPR-1 neuropeptide receptor. These results suggest that environmental signals regulate chemosensory gene expression together with internal neuropeptide signals which, in turn, modulate behavior.

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.

A transcriptomic snapshot of early molecular communication between Pasteuria penetrans and Meloidogyne incognita

BACKGROUND

Southern root-knot nematode Meloidogyne incognita (Kofoid and White, 1919), Chitwood, 1949 is a key pest of agricultural crops. Pasteuria penetrans is a hyperparasitic bacterium capable of suppressing the nematode reproduction, and represents a typical coevolved pathogen-hyperparasite system. Attachment of Pasteuria endospores to the cuticle of second-stage nematode juveniles is the first and pivotal step in the bacterial infection. RNA-Seq was used to understand the early transcriptional response of the root-knot nematode at 8 h post Pasteuria endospore attachment.

RESULTS

A total of 52,485 transcripts were assembled from the high quality (HQ) reads, out of which 582 transcripts were found differentially expressed in the Pasteuria endospore encumbered J2 s, of which 229 were up-regulated and 353 were down-regulated. Pasteuria infection caused a suppression of the protein synthesis machinery of the nematode. Several of the differentially expressed transcripts were putatively involved in nematode innate immunity, signaling, stress responses, endospore attachment process and post-attachment behavioral modification of the juveniles. The expression profiles of fifteen selected transcripts were validated to be true by the qRT PCR. RNAi based silencing of transcripts coding for fructose bisphosphate aldolase and glucosyl transferase caused a reduction in endospore attachment as compared to the controls, whereas, silencing of aspartic protease and ubiquitin coding transcripts resulted in higher incidence of endospore attachment on the nematode cuticle.

CONCLUSIONS

Here we provide evidence of an early transcriptional response by the nematode upon infection by Pasteuria prior to root invasion. We found that adhesion of Pasteuria endospores to the cuticle induced a down-regulated protein response in the nematode. In addition, we show that fructose bisphosphate aldolase, glucosyl transferase, aspartic protease and ubiquitin coding transcripts are involved in modulating the endospore attachment on the nematode cuticle. Our results add new and significant information to the existing knowledge on early molecular interaction between M. incognita and P. penetrans.

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.

FLP-18 Functions through the G-Protein-Coupled Receptors NPR-1 and NPR-4 to Modulate Reversal Length in Caenorhabditis elegans

Animal behavior is critically dependent on the activity of neuropeptides. Reversals, one of the most conspicuous behaviors in Caenorhabditis elegans, plays an important role in determining the navigation strategy of the animal. Our experiments on hermaphrodite C. elegans show the involvement of a neuropeptide FLP-18 in modulating reversal length in these hermaphrodites. We show that FLP-18 controls the reversal length by regulating the activity of AVA interneurons through the G-protein-coupled neuropeptide receptors, NPR-4 and NPR-1. We go on to show that the site of action of these receptors is the AVA interneuron for NPR-4 and the ASE sensory neurons for NPR-1. We further show that mutants in the neuropeptide, flp-18, and its receptors show increased reversal lengths. Consistent with the behavioral data, calcium levels in the AVA neuron of freely reversing C. elegans were significantly higher and persisted for longer durations in flp-18, npr-1, npr-4, and npr-1 npr-4 genetic backgrounds compared with wild-type control animals. Finally, we show that increasing FLP-18 levels through genetic and physiological manipulations causes shorter reversal lengths. Together, our analysis suggests that the FLP-18/NPR-1/NPR-4 signaling is a pivotal point in the regulation of reversal length under varied genetic and environmental conditions.

SIGNIFICANCE STATEMENT In this study, we elucidate the circuit and molecular machinery required for normal reversal behavior in hermaphrodite Caenorhabditis elegans. We delineate the circuit and the neuropeptide receptors required for maintaining reversal length in C. elegans. Our work sheds light on the importance of a single neuropeptide, FLP-18, and how change in levels in this one peptide could allow the animal to change the length of its reversal, thereby modulating how the C. elegans explores its environment. We also go on to show that FLP-18 functions to maintain reversal length through the neuropeptide receptors NPR-4 and NPR-1. Our study will allow for a better understanding of the complete repertoire of behaviors shown by freely moving animals as they explore their environment.

An Expanded Role for the RFX Transcription Factor DAF-19, with Dual Functions in Ciliated and Nonciliated Neurons

Regulatory Factor X (RFX) transcription factors (TFs) are best known for activating genes required for ciliogenesis in both vertebrates and invertebrates. In humans, eight RFX TFs have a variety of tissue-specific functions, while in the worm Caenorhabditis elegans, the sole RFX gene, daf-19, encodes a set of nested isoforms. Null alleles of daf-19 confer pleiotropic effects including altered development with a dauer constitutive phenotype, complete absence of cilia and ciliary proteins, and defects in synaptic protein maintenance. We sought to identify RFX/daf-19 target genes associated with neuronal functions other than ciliogenesis using comparative transcriptome analyses at different life stages of the worm. Subsequent characterization of gene expression patterns revealed one set of genes activated in the presence of DAF-19 in ciliated sensory neurons, whose activation requires the daf-19c isoform, also required for ciliogenesis. A second set of genes is downregulated in the presence of DAF-19, primarily in nonsensory neurons. The human orthologs of some of these neuronal genes are associated with human diseases. We report the novel finding that daf-19a is directly or indirectly responsible for downregulation of these neuronal genes in C. elegans by characterizing a new mutation affecting the daf-19a isoform (tm5562) and not associated with ciliogenesis, but which confers synaptic and behavioral defects. Thus, we have identified a new regulatory role for RFX TFs in the nervous system. The new daf-19 candidate target genes we have identified by transcriptomics will serve to uncover the molecular underpinnings of the pleiotropic effects that daf-19 exerts on nervous system function.

FMRFamide-like peptides expand the behavioral repertoire of a densely connected nervous system

Animals, including humans, can adapt to environmental stress through phenotypic plasticity. The free-living nematode Caenorhabditis elegans can adapt to harsh environments by undergoing a whole-animal change, involving exiting reproductive development and entering the stress-resistant dauer larval stage. The dauer is a dispersal stage with dauer-specific behaviors for finding and stowing onto carrier animals, but how dauers acquire these behaviors, despite having a physically limited nervous system of 302 neurons, is poorly understood. We compared dauer and reproductive development using whole-animal RNA sequencing at fine time points and at sufficient depth to measure transcriptional changes within single cells. We detected 8,042 genes differentially expressed during dauer and reproductive development and observed striking up-regulation of neuropeptide genes during dauer entry. We knocked down neuropeptide processing using sbt-1 mutants and demonstrate that neuropeptide signaling promotes the decision to enter dauer rather than reproductive development. We also demonstrate that during dauer neuropeptides modulate the dauer-specific nictation behavior (carrier animal-hitchhiking) and are necessary for switching from repulsion to CO₂ (a carrier animal cue) in nondauers to CO₂ attraction in dauers. We tested individual neuropeptides using CRISPR knockouts and existing strains and demonstrate that the combined effects of flp-10 and flp-17 mimic the effects of sbt-1 on nictation and CO₂ attraction. Through meta-analysis, we discovered similar up-regulation of neuropeptides in the dauer-like infective juveniles of diverse parasitic nematodes, suggesting the antiparasitic target potential of SBT-1. Our findings reveal that, under stress, increased neuropeptide signaling in C. elegans enhances their decision-making accuracy and expands their behavioral repertoire.

Luqin-like RYamide peptides regulate food-evoked responses in C. elegans

Peptide signaling controls many processes involving coordinated actions of multiple organs, such as hormone-mediated appetite regulation. However, the extent to which the mode of action of peptide signaling is conserved in different animals is largely unknown, because many peptides and receptors remain orphan and many undiscovered peptides still exist. Here, we identify two novel Caenorhabditis elegans neuropeptides, LURY-1-1 and LURY-1-2, as endogenous ligands for the neuropeptide receptor-22 (NPR-22). Both peptides derive from the same precursor that is orthologous to invertebrate luqin/arginine-tyrosine-NH2 (RYamide) proneuropeptides. LURY-1 peptides are secreted from two classes of pharyngeal neurons and control food-related processes: feeding, lifespan, egg-laying, and locomotory behavior. We propose that LURY-1 peptides transmit food signals to NPR-22 expressed in feeding pacemaker neurons and a serotonergic neuron. Our results identified a critical role for luqin-like RYamides in feeding-related processes and suggested that peptide-mediated negative feedback is important for satiety regulation in C. elegans.

Opiates Modulate Noxious Chemical Nociception through a Complex Monoaminergic/Peptidergic Cascade

The ability to detect noxious stimuli, process the nociceptive signal, and elicit an appropriate behavioral response is essential for survival. In Caenorhabditis elegans, opioid receptor agonists, such as morphine, mimic serotonin, and suppress the overall withdrawal from noxious stimuli through a pathway requiring the opioid-like receptor, NPR-17. This serotonin- or morphine-dependent modulation can be rescued in npr-17-null animals by the expression of npr-17 or a human κ opioid receptor in the two ASI sensory neurons, with ASI opioid signaling selectively inhibiting ASI neuropeptide release. Serotonergic modulation requires peptides encoded by both nlp-3 and nlp-24, and either nlp-3 or nlp-24 overexpression mimics morphine and suppresses withdrawal. Peptides encoded by nlp-3 act differentially, with only NLP-3.3 mimicking morphine, whereas other nlp-3 peptides antagonize NLP-3.3 modulation. Together, these results demonstrate that opiates modulate nociception in Caenorhabditis elegans through a complex monoaminergic/peptidergic cascade, and suggest that this model may be useful for dissecting opiate signaling in mammals.

SIGNIFICANCE STATEMENT

Opiates are used extensively to treat chronic pain. In Caenorhabditis elegans, opioid receptor agonists suppress the overall withdrawal from noxious chemical stimuli through a pathway requiring an opioid-like receptor and two distinct neuropeptide-encoding genes, with individual peptides from the same gene functioning antagonistically to modulate nociception. Endogenous opioid signaling functions as part of a complex, monoaminergic/peptidergic signaling cascade and appears to selectively inhibit neuropeptide release, mediated by a α-adrenergic-like receptor, from two sensory neurons. Importantly, receptor null animals can be rescued by the expression of the human κ opioid receptor, and injection of human opioid receptor ligands mimics exogenous opiates, highlighting the utility of this model for dissecting opiate signaling in mammals.

Molecular characterization of FMRFamide-like peptides in Meloidogyne graminicola and analysis of their knockdown effect on nematode infectivity

The rice root-knot nematode, Meloidogyne graminicola, seriously impairs the growth and yield of rice which is an important staple food worldwide. The disruption of neuropeptide signalling leading to attenuation in nematode behaviour and thereby perturbed infection, offers an attractive alternative to control nematodes. In this direction, the present study was aimed at mining of putative FMRFamide-like peptides (FLPs) from the transcriptomic dataset of M. graminicola followed by characterization of those FLPs via sequencing of PCR products, qRT-PCR and Southern hybridization analysis. We have characterized nine flp genes (flp-1, flp-3, flp-6, flp-7, flp-11, flp-12, flp-14, flp-16 and flp-18) and a partial neuropeptide receptor gene (flp-18 GPCR) from M. graminicola in the present study. In addition, in situ localization revealed the expression of flp-1 and flp-7 in neurons posterior to the circumpharyngeal nerve ring of M. graminicola. In vitro silencing of nine flp genes and flp-18 GPCR in M. graminicola J2 and their subsequent infection in rice and wheat roots demonstrated the reduced penetration ability of FLP silenced worms which underscores the potential of the FLPergic system as a broad-spectrum target to manage the root-knot nematode problem in rice-wheat cropping system.

A tachykinin-like neuroendocrine signalling axis couples central serotonin action and nutrient sensing with peripheral lipid metabolism

Serotonin, a central neuromodulator with ancient ties to feeding and metabolism, is a major driver of body fat loss. However, mechanisms by which central serotonin action leads to fat loss remain unknown. Here, we report that the FLP-7 neuropeptide and its cognate receptor, NPR-22, function as the ligand-receptor pair that defines the neuroendocrine axis of serotonergic body fat loss in Caenorhabditis elegans. FLP-7 is secreted as a neuroendocrine peptide in proportion to fluctuations in neural serotonin circuit functions, and its release is regulated from secretory neurons via the nutrient sensor AMPK. FLP-7 acts via the NPR-22/Tachykinin2 receptor in the intestine and drives fat loss via the adipocyte triglyceride lipase ATGL-1. Importantly, this ligand-receptor pair does not alter other serotonin-dependent behaviours including food intake. For global modulators such as serotonin, the use of distinct neuroendocrine peptides for each output may be one means to achieve phenotypic selectivity.

Neuroendocrine modulation sustains the C. elegans forward motor state

Neuromodulators shape neural circuit dynamics. Combining electron microscopy, genetics, transcriptome profiling, calcium imaging, and optogenetics, we discovered a peptidergic neuron that modulates C. elegans motor circuit dynamics. The Six/SO-family homeobox transcription factor UNC-39 governs lineage-specific neurogenesis to give rise to a neuron RID. RID bears the anatomic hallmarks of a specialized endocrine neuron: it harbors near-exclusive dense core vesicles that cluster periodically along the axon, and expresses multiple neuropeptides, including the FMRF-amide-related FLP-14. RID activity increases during forward movement. Ablating RID reduces the sustainability of forward movement, a phenotype partially recapitulated by removing FLP-14. Optogenetic depolarization of RID prolongs forward movement, an effect reduced in the absence of FLP-14. Together, these results establish the role of a neuroendocrine cell RID in sustaining a specific behavioral state in C. elegans.

DOI:http://dx.doi.org/10.7554/eLife.19887.001

Sequestration of ubiquitous dietary derived pigments enables mitochondrial light sensing

Animals alter their physiological states in response to their environment. We show that the introduction of a chlorophyll metabolite, a light-absorbing pigment widely consumed in human diets, to Caenorhabditis elegans results in animals whose fat mass can be modulated by exposure to light, despite the worm consuming the same amount of food. In the presence of the chlorophyll metabolite, exposing the worms to light increased adenosine triphosphate, reduced oxidative damage, and increased median life spans, without an effect on animal reproduction. Mice fed a dietary metabolite of chlorophyll and exposed to light, over several months, showed reductions in systemic inflammation as measured by plasma α-macroglobulin. We propose that dietary chlorophyll metabolites can enable mitochondria to use light as an environmental cue, by absorbing light and transferring the energy to mitochondrial coenzyme Q.

Neuropeptidergic Signaling and Active Feeding State Inhibit Nociception in Caenorhabditis elegans

Food availability and nutritional status are important cues affecting behavioral states. Here we report that, in Caenorhabditis elegans, a cascade of dopamine and neuropeptide signaling acts to inhibit nociception in food-poor environments. In the absence of food, animals show decreased sensitivity and increased adaptation to soluble repellents sensed by the polymodal ASH nociceptors. The effects of food on adaptation are affected by dopamine and neuropeptide signaling; dopamine acts via the DOP-1 receptor to decrease adaptation on food, whereas the neuropeptide receptors NPR-1 and NPR-2 act to increase adaptation off food. NPR-1 and NPR-2 function cell autonomously in the ASH neurons to increase adaptation off food, whereas the DOP-1 receptor controls neuropeptide release from interneurons that modulate ASH activity indirectly. These results indicate that feeding state modulates nociception through the interaction of monoamine and neuropeptide signaling pathways.

Sleep-active neuron specification and sleep induction require FLP-11 neuropeptides to systemically induce sleep

Sleep is an essential behavioral state. It is induced by conserved sleep-active neurons that express GABA. However, little is known about how sleep neuron function is determined and how sleep neurons change physiology and behavior systemically. Here, we investigated sleep in Caenorhabditis elegans, which is induced by the single sleep-active neuron RIS. We found that the transcription factor LIM-6, which specifies GABAergic function, in parallel determines sleep neuron function through the expression of APTF-1, which specifies the expression of FLP-11 neuropeptides. Surprisingly FLP-11, and not GABA, is the major component that determines the sleep-promoting function of RIS. FLP-11 is constantly expressed in RIS. At sleep onset RIS depolarizes and releases FLP-11 to induce a systemic sleep state.

FLP-4 neuropeptide and its receptor in a neuronal circuit regulate preference choice through functions of ASH-2 trithorax complex in Caenorhabditis elegans

Preference choice on food is an important response strategy for animals living in the environment. Using assay system of preference choice on bacterial foods, OP50 and PA14, we identified the involvement of ADL sensory neurons in the control of preference choice in Caenorhabditis elegans. Both genetically silencing and ChR2-mediated activation of ADL sensory neurons significantly affected preference choice. ADL regulated preference choice by inhibiting function of G protein-coupled receptor (GPCR)/SRH-220. ADL sensory neurons might regulate preference choice through peptidergic signals of FLP-4 and NLP-10, and function of FLP-4 or NLP-10 in regulating preference choice was regulated by SRH-220. FLP-4 released from ADL sensory neurons further regulated preference choice through its receptor of NPR-4 in AIB interneurons. In AIB interneurons, NPR-4 was involved in the control of preference choice by activating the functions of ASH-2 trithorax complex consisting of SET-2, ASH-2, and WDR-5, implying the crucial role of molecular machinery of trimethylation of histone H3K4 in the preference choice control. The identified novel neuronal circuit and the underlying molecular mechanisms will strengthen our understanding neuronal basis of preference choice in animals.

Neural Regulatory Pathways of Feeding and Fat in Caenorhabditis elegans

The compact nervous system of Caenorhabditis elegans and its genetic tractability are features that make this organism highly suitable for investigating energy balance in an animal system. Here, we focus on molecular components and organizational principles emerging from the investigation of pathways that largely originate in the nervous system and regulate feeding behavior but also peripheral fat regulation through neuroendocrine signaling. We provide an overview of studies aimed at understanding how C. elegans integrate internal and external cues in feeding behavior. We highlight some of the similarities and differences in energy balance between C. elegans and mammals. We also provide our perspective on unresolved issues, both conceptual and technical, that we believe have hampered critical evaluation of findings relevant to fat regulation in C. elegans.

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.