The C. elegans gene odr-7 encodes an olfactory-specific member of the nuclear receptor superfamily

Behavioral evolution: No sex please, we're hermaphrodites

Many 'hard-wired', innate animal behaviors are related to reproduction. So what happens when reproductive systems evolve? New research in nematodes has identified principles underlying the co-evolution of reproductive strategy and sexual behavior, revealing some surprises and raising intriguing new questions.

Neuron cilia restrain glial KCC-3 to a microdomain to regulate multisensory processing

Glia interact with multiple neurons, but it is unclear whether their interactions with each neuron are different. Our interrogation at single-cell resolution reveals that a single glial cell exhibits specificity in its interactions with different contacting neurons. Briefly, C. elegans amphid sheath (AMsh) glia apical-like domains contact 12 neuron-endings. At these ad-neuronal membranes, AMsh glia localize the K/Cl transporter KCC-3 to a microdomain exclusively around the thermosensory AFD neuron to regulate its properties. Glial KCC-3 is transported to ad-neuronal regions, where distal cilia of non-AFD glia-associated chemosensory neurons constrain it to a microdomain at AFD-contacting glial membranes. Aberrant KCC-3 localization impacts both thermosensory (AFD) and chemosensory (non-AFD) neuron properties. Thus, neurons can interact non-synaptically through a shared glial cell by regulating microdomain localization of its cues. As AMsh and glia across species compartmentalize multiple cues like KCC-3, we posit that this may be a broadly conserved glial mechanism that modulates information processing across multimodal circuits.

Evolution remodels olfactory and mating-receptive behaviors in the transition from female to hermaphrodite reproduction

Male/hermaphrodite species have arisen multiple times from a male/female ancestral state in nematodes, providing a model to study behavioral adaptations to different reproductive strategies. Here, we examined the mating behaviors of male/female (gonochoristic) Caenorhabditis species in comparison with male/hermaphrodite (androdiecious) close relatives. We find that females from two species in the Elegans group chemotax to volatile odor from males, but hermaphrodites do not. Females, but not hermaphrodites, also display known mating-receptive behaviors such as sedation when male reproductive structures contact the vulva. Focusing on the male/female species C. nigoni, we show that female chemotaxis to males is limited to adult females approaching adult or near-adult males and relies upon the AWA neuron-specific transcription factor ODR-7, as does male chemotaxis to female odor as previously shown in C. elegans. However, female receptivity during mating contact is odr-7 independent. All C. nigoni female behaviors are suppressed by mating and all are absent in young hermaphrodites from the sister species C. briggsae. However, latent receptivity during mating contact can be uncovered in mutant or aged C. briggsae hermaphrodites that lack self-sperm. These results reveal two mechanistically distinct components of the shift from female to hermaphrodite behavior: the loss of female-specific odr-7-dependent chemotaxis and a sperm-dependent state of reduced receptivity to mating contact. Hermaphrodites from a second androdioecious species, C. tropicalis, recover all female behaviors upon aging, including chemotaxis to males. Regaining mating receptivity after sperm depletion could maximize hermaphrodite fitness across their lifespan.

Induced knockdown of Mg-odr-1 and Mg-odr-3 perturbed the host seeking behavior of Meloidogyne graminicola in rice

Root-knot nematode Meloidogyne graminicola is one of the most destructive plant parasites in upland as well as direct seeded rice. As an integral part of nematode biology, host finding behavior involves perceiving and responding to different chemical cues originating from the rhizosphere. A sustainable management tactic may include retardation of nematode chemoreception that would impair them to detect and discriminate the host stimuli. Deciphering the molecular basis of nematode chemoreception is vital to identify chokepoints for chemical or genetic interventions. However, compared to the well-characterized chemoreception mechanism in model nematode Caenorhabditis elegans, plant nematode chemoreception is yet underexplored. Herein, the full-length cDNA sequences of two chemotaxis-related genes (Mg-odr-1 and Mg-odr-3) were cloned from M. graminicola. Both the genes were markedly upregulated in the early developmental stages of M. graminicola suggesting their involvement in host finding processes. RNAi-induced independent knockdown of Mg-odr-1 and Mg-odr-3 caused behavioral aberration in second-stage juveniles of M. graminicola which in turn perturbed the nematodes' host finding ability and parasitic success inside rice roots. Additionally, nematodes' chemotactic response to different host root exudates, volatile and nonvolatile compounds was affected. Our results demonstrating the role of specific chemosensory genes in modulating M. graminicola host seeking behavior can enrich the existing knowledge of plant nematode chemoreception mechanism, and these genes can be targeted for novel nematicide development or in planta RNAi screens.

Nicotine Motivated Behavior in C. elegans

To maximize the advantages offered by Caenorhabditis elegans as a high-throughput (HTP) model for nicotine dependence studies, utilizing its well-defined neuroconnectome as a robust platform, and to unravel the genetic basis of nicotine-motivated behaviors, we established the nicotine conditioned cue preference (CCP) paradigm. Nicotine CCP enables the assessment of nicotine preference and seeking, revealing a parallel to fundamental aspects of nicotine-dependent behaviors observed in mammals. We demonstrated that nicotine-elicited cue preference in worms is mediated by nicotinic acetylcholine receptors and requires dopamine for CCP development. Subsequently, we pinpointed nAChR subunits associated with nicotine preference and validated human GWAS candidates linked to nicotine dependence involved in nAChRs. Functional validation involves assessing the loss-of-function strain of the CACNA2D3 ortholog and the knock-out (KO) strain of the CACNA2D2 ortholog, closely related to CACNA2D3 and sharing human smoking phenotypes. Our orthogonal approach substantiates the functional conservation of the α2δ subunit of the calcium channel in nicotine-motivated behavior. Nicotine CCP in C. elegans serves as a potent affirmation of the cross-species functional relevance of GWAS candidate genes involved in nicotine seeking associated with tobacco abuse, providing a streamlined yet comprehensive system for investigating intricate behavioral paradigms within a simplified and reliable framework.

Molecular and circuit mechanisms underlying avoidance of rapid cooling stimuli in C. elegans

The mechanisms by which animals respond to rapid changes in temperature are largely unknown. Here, we found that polymodal ASH sensory neurons mediate rapid cooling-evoked avoidance behavior within the physiological temperature range in C. elegans. ASH employs multiple parallel circuits that consist of stimulatory circuits (AIZ, RIA, AVA) and disinhibitory circuits (AIB, RIM) to respond to rapid cooling. In the stimulatory circuit, AIZ, which is activated by ASH, releases glutamate to act on both GLR-3 and GLR-6 receptors in RIA neurons to promote reversal, and ASH also directly or indirectly stimulates AVA to promote reversal. In the disinhibitory circuit, AIB is stimulated by ASH through the GLR-1 receptor, releasing glutamate to act on AVR-14 to suppress RIM activity. RIM, an inter/motor neuron, inhibits rapid cooling-evoked reversal, and the loop activities thus equally stimulate reversal. Our findings elucidate the molecular and circuit mechanisms underlying the acute temperature stimuli-evoked avoidance behavior.

Molecular and functional characterization of chemosensory genes from the root-knot nematode Meloidogyne graminicola

BACKGROUND

Root-knot nematode Meloidogyne graminicola has emerged as a major threat in rice agroecosystems owing to climate change-induced changes in cultivation practices. Synthetic nematicides are continually being withdrawn from the nematode management toolbox because of their ill effects on the environment. A sustainable strategy would be to develop novel nematicides or resistant plants that would target nematode sensory perception, which is a key step in the host finding biology of plant-parasitic nematodes (PPNs). However, compared to the extensive literature on the free-living nematode Caenorhabditis elegans, negligible research has been performed on PPN chemosensory biology.

RESULTS

The present study characterizes the five chemosensory genes (Mg-odr-7, Mg-tax-4, Mg-tax-4.1, Mg-osm-9, and Mg-ocr-2) from M. graminicola that are putatively associated with nematode host-finding biology. All the genes were highly transcribed in the early life stages, and RNA interference (RNAi)-induced downregulation of each candidate gene perturbed the normal behavioural phenotypes of M. graminicola, as determined by examining the tracking pattern of juveniles on Pluronic gel medium, attraction to and penetration in rice root tip, and developmental progression in rice root. In addition, a detrimental effect on nematode chemotaxis towards different volatile and nonvolatile organic compounds and host root exudates was documented.

CONCLUSION

Our findings enrich the existing literature on PPN chemosensory biology and can supplement future research aimed at identifying a comprehensive chemosensory signal transduction pathway in PPNs.

Specific sensory neurons and insulin-like peptides modulate food type-dependent oogenesis and fertilization in Caenorhabditis elegans

An animal's responses to environmental cues are critical for its reproductive program. Thus, a mechanism that allows the animal to sense and adjust to its environment should make for a more efficient reproductive physiology. Here, we demonstrate that in Caenorhabditis elegans specific sensory neurons influence onset of oogenesis through insulin signaling in response to food-derived cues. The chemosensory neurons ASJ modulate oogenesis onset through the insulin-like peptide (ILP) INS-6. In contrast, other sensory neurons, the olfactory neurons AWA, regulate food type-dependent differences in C. elegans fertilization rates, but not onset of oogenesis. AWA modulates fertilization rates at least partly in parallel to insulin receptor signaling, since the insulin receptor DAF-2 regulates fertilization independently of food type, which requires ILPs other than INS-6. Together our findings suggest that optimal reproduction requires the integration of diverse food-derived inputs through multiple neuronal signals acting on the C. elegans germline.

Transcriptome reveals the toxicity difference of dimethyl disulfide by contact and fumigation on Meloidogyne incognita through calcium channel-mediated oxidative phosphorylation

The prevention and control of root-knot nematode disease has been posing a severe challenge worldwide. Fumigant dimethyl disulfide (DMDS) has excellent biological activity against nematodes. However, DMDS displays significant differences in contact and fumigation toxicity on nematodes. The specific regulatory mechanisms of DMDS on nematodes were investigated by characterizing the ultrastructure of nematodes, examining the physiological and biochemical indicators, and conducting transcriptome high-throughput sequencing. As indicated by the results, DMDS fumigation exhibited the biological activity of against M. incognita 121 times higher than DMDS contact. DMDS contact destroyed nematode body wall cells. Besides, DMDS fumigation destroyed the structure of pseudocoelom. DMDS treatment expedited the oxygen consumption of nematode while inhibiting acetylcholinesterase activity. As indicated by the analysis of vital signaling pathways based on transcriptome, DMDS based on the contact mode penetrated directly into the nematode through the body wall and subsequently affected calcium channels in the body wall and muscle, disrupting their structure; it serves as an uncoupling agent to interfere with ATP synthase. Moreover, DMDS based on the fumigation mode entered the body through the respiratory pathway of olfactory perception-oxygen exchange and subsequently affected calcium channels in the nerve; eventually, DMDS acted on complex IV or complex I.

VISTA: visualizing the spatial transcriptome of the C.elegans nervous system

Summary

Profiling the transcriptomes of single cells without sacrificing spatial information is a major goal of the field of spatial transcriptomics, but current technologies require tradeoffs between single-cell resolution and whole-transcriptome coverage. In one animal species, the nematode worm Caenorhabditis elegans, a comprehensive spatial transcriptome with single-cell resolution is attainable using existing datasets, thanks to the worm's invariant cell lineage and a series of recently generated single cell transcriptomes. Here we present VISTA, which leverages these datasets to provide a visualization of the worm spatial transcriptome, focusing specifically on the nervous system. VISTA allows users to input a query gene and visualize its expression across all neurons in the form of a "spatial heatmap" in which the color of a cell reports the expression level. Underlying gene expression values (in Transcripts Per Million) are displayed when an individual cell is selected. We provide examples of the utility of VISTA for identifying striking new gene expression patterns in specific neurons, and for resolving cellular identities of ambiguous expression patterns generated from in vivo reporter genes. The ability to easily obtain gene-level snapshots of the neuronal spatial transcriptome should facilitate studies on neuron-specific gene expression and regulation and provide a template for the high-resolution spatial transcriptomes the field hopes to obtain for various animal species in the future.

Availability and implementation

VISTA is freely available at the following URL: https://public.tableau.com/app/profile/smu.oit.data.insights/viz/VISTA_16814210566130/VISTA.

Updated knowledge and a proposed nomenclature for nuclear receptors with two DNA binding domains (2DBD-NRs)

Nuclear receptors (NRs) are important transcriptional modulators in metazoans. Typical NRs possess a conserved DNA binding domain (DBD) and a ligand binding domain (LBD). Since we discovered a type of novel NRs each of them has two DBDs and single LBD (2DBD-NRs) more than decade ago, there has been very few studies about 2DBD-NRs. Recently, 2DBD-NRs have been only reported in Platyhelminths and Mollusca and are thought to be specific NRs to lophotrochozoan. In this study, we searched different databases and identified 2DBD-NRs in different animals from both protostomes and deuterostomes. Phylogenetic analysis shows that at least two ancient 2DBD-NR genes were present in the urbilaterian, a common ancestor of protostomes and deuterostomes. 2DBD-NRs underwent gene duplication and loss after the split of different animal phyla, most of them in a certain animal phylum are paralogues, rather than orthologues, like in other animal phyla. Amino acid sequence analysis shows that the conserved motifs in typical NRs are also present in 2DBD-NRs and they are gene specific. From our phylogenetic analysis of 2DBD-NRs and following the rule of Nomenclature System for the Nuclear Receptors, a nomenclature for 2DBD-NRs is proposed.

Amphid sensory neurons of Caenorhabditis elegans orchestrate its survival from infection with broad classes of pathogens

The survival of a host during infection relies on its ability to rapidly sense the invading pathogen and mount an appropriate response. The bacterivorous nematode Caenorhabditis elegans lacks most of the traditional pattern recognition mechanisms. In this study, we hypothesized that the 12 pairs of amphid sensory neurons in the heads of worms provide sensing capability and thus affect survival during infection. We tested animals lacking amphid neurons to three major classes of pathogens, namely-a Gram-negative bacterium Pseudomonas aeruginosa, a Gram-positive bacterium Enterococcus faecalis, and a pathogenic yeast Cryptococcus neoformans By using individual neuronal ablation lines or mutants lacking specific neurons, we demonstrate that some neurons broadly suppress the survival of the host and colonization of all pathogens, whereas other amphid neurons differentially regulate host survival during infection. We also show that the roles of some of these neurons are pathogen-specific, as seen with the AWB odor sensory neurons that promote survival only during infections with P aeruginosa Overall, our study reveals broad and specific roles for amphid neurons during infections.

VISTA: Visualizing the Spatial Transcriptome of the C. elegans Nervous System

Profiling the transcriptomes of single cells without sacrificing spatial information is a major goal of the field of spatial transcriptomics, but current technologies require tradeoffs between single-cell resolution and whole-transcriptome coverage. In one animal species, the nematode worm C. elegans, a comprehensive spatial transcriptome with single-cell resolution is attainable using existing datasets, thanks to the worm's invariant cell lineage and a series of recently-generated single cell transcriptomes. Here we present VISTA, which leverages these datasets to provide a visualization of the worm spatial transcriptome, focusing specifically on the nervous system. VISTA allows users to input a query gene and visualize its expression across all neurons in the form of a "spatial heatmap" in which the color of a cell reports the expression level. Underlying gene expression values (in Transcripts Per Million) are displayed when an individual cell is selected. We provide examples of the utility of VISTA for identifying striking new gene expression patterns in specific neurons, and for resolving cellular identities of ambiguous expression patterns generated from in vivo reporter genes. The ability to easily obtain gene-level snapshots of the neuronal spatial transcriptome should facilitate studies on neuron-specific gene expression and regulation, and provide a template for the high-resolution spatial transcriptomes the field hopes to obtain for various animal species in the future.

Neural engineering with photons as synaptic transmitters

Neuronal computation is achieved through connections of individual neurons into a larger network. To expand the repertoire of endogenous cellular communication, we developed a synthetic, photon-assisted synaptic transmission (PhAST) system. PhAST is based on luciferases and channelrhodopsins that enable the transmission of a neuronal state across space, using photons as neurotransmitters. PhAST overcomes synaptic barriers and rescues the behavioral deficit of a glutamate mutant with conditional, calcium-triggered photon emission between two neurons of the Caenorhabditis elegans nociceptive avoidance circuit. To demonstrate versatility and flexibility, we generated de novo synaptic transmission between two unconnected cells in a sexually dimorphic neuronal circuit, suppressed endogenous nocifensive response through activation of an anion channelrhodopsin and switched attractive to aversive behavior in an olfactory circuit. Finally, we applied PhAST to dissect the calcium dynamics of the temporal pattern generator in a motor circuit for ovipositioning. In summary, we established photon-based synaptic transmission that facilitates the modification of animal behavior.

Neuron cilia constrain glial regulators to microdomains around distal neurons

Each glia interacts with multiple neurons, but the fundamental logic of whether it interacts with all equally remains unclear. We find that a single sense-organ glia modulates different contacting neurons distinctly. To do so, it partitions regulatory cues into molecular microdomains at specific neuron contact-sites, at its delimited apical membrane. For one glial cue, K/Cl transporter KCC-3, microdomain-localization occurs through a two-step, neuron-dependent process. First, KCC-3 shuttles to glial apical membranes. Second, some contacting neuron cilia repel it, rendering it microdomain-localized around one distal neuron-ending. KCC-3 localization tracks animal aging, and while apical localization is sufficient for contacting neuron function, microdomain-restriction is required for distal neuron properties. Finally, we find the glia regulates its microdomains largely independently. Together, this uncovers that glia modulate cross-modal sensor processing by compartmentalizing regulatory cues into microdomains. Glia across species contact multiple neurons and localize disease-relevant cues like KCC-3. Thus, analogous compartmentalization may broadly drive how glia regulate information processing across neural circuits.

Multiple Receptors Contribute to the Attractive Response of Caenorhabditis elegans to Pathogenic Bacteria

Nematodes feed mainly on bacteria and sense volatile signals through their chemosensory system to distinguish food from pathogens. Although nematodes recognizing bacteria by volatile metabolites are ubiquitous, little is known of the associated molecular mechanism. Here, we show that the antinematode bacterium Paenibacillus polymyxa KM2501-1 exhibits an attractive effect on Caenorhabditis elegans via volatile metabolites, of which furfural acetone (FAc) acts as a broad-spectrum nematode attractant. We show that the attractive response toward FAc requires both the G-protein-coupled receptors STR-2 in AWC neurons and SRA-13 in AWA and AWC neurons. In the downstream olfactory signaling cascades, both the transient receptor potential vanilloid channel and the cyclic nucleotide-gated channel are necessary for FAc sensation. These results indicate that multiple receptors and subsequent signaling cascades contribute to the attractive response of C. elegans to FAc, and FAc is the first reported ligand of SRA-13. Our current work discovers that P. polymyxa KM2501-1 exhibits an attractive effect on nematodes by secreting volatile metabolites, especially FAc and 2-heptanone, broadening our understanding of the interactions between bacterial pathogens and nematodes. IMPORTANCE Nematodes feed on nontoxic bacteria as a food resource and avoid toxic bacteria; they distinguish them through their volatile metabolites. However, the mechanism of how nematodes recognize bacteria by volatile metabolites is not fully understood. Here, the antinematode bacterium Paenibacillus polymyxa KM2501-1 is found to exhibit an attractive effect on Caenorhabditis elegans via volatile metabolites, including FAc. We further reveal that the attractive response of C. elegans toward FAc requires multiple G-protein-coupled receptors and downstream olfactory signaling cascades in AWA and AWC neurons. This study highlights the important role of volatile metabolites in the interaction between nematodes and bacteria and confirms that multiple G-protein-coupled receptors on different olfactory neurons of C. elegans can jointly sense bacterial volatile signals.

Pathogenic bacteria modulate pheromone response to promote mating

Pathogens generate ubiquitous selective pressures and host-pathogen interactions alter social behaviours in many animals1-4. However, very little is known about the neuronal mechanisms underlying pathogen-induced changes in social behaviour. Here we show that in adult Caenorhabditis elegans hermaphrodites, exposure to a bacterial pathogen (Pseudomonas aeruginosa) modulates sensory responses to pheromones by inducing the expression of the chemoreceptor STR-44 to promote mating. Under standard conditions, C. elegans hermaphrodites avoid a mixture of ascaroside pheromones to facilitate dispersal5-13. We find that exposure to the pathogenic Pseudomonas bacteria enables pheromone responses in AWA sensory neurons, which mediate attractive chemotaxis, to suppress the avoidance. Pathogen exposure induces str-44 expression in AWA neurons, a process regulated by a transcription factor zip-5 that also displays a pathogen-induced increase in expression in AWA. STR-44 acts as a pheromone receptor and its function in AWA neurons is required for pathogen-induced AWA pheromone response and suppression of pheromone avoidance. Furthermore, we show that C. elegans hermaphrodites, which reproduce mainly through self-fertilization, increase the rate of mating with males after pathogen exposure and that this increase requires str-44 in AWA neurons. Thus, our results uncover a causal mechanism for pathogen-induced social behaviour plasticity, which can promote genetic diversity and facilitate adaptation of the host animals.

Neurobiological Basis of Aversion-Resistant Ethanol Seeking in C. elegans

Persistent alcohol seeking despite the risk of aversive consequences is a crucial characteristic of alcohol use disorders (AUDs). Therefore, an improved understanding of the molecular basis of alcohol seeking despite aversive stimuli or punishment in animal models is an important strategy to understand the mechanism that underpins the pathology of AUDs. Aversion-resistant seeking (ARS) is characterized by disruption in control of alcohol use featured by an imbalance between the urge for alcohol and the mediation of aversive stimuli. We exploited C. elegans, a genetically tractable invertebrate, as a model to elucidate genetic components related to this behavior. We assessed the seb-3 neuropeptide system and its transcriptional regulation to progress aversion-resistant ethanol seeking at the system level. Our functional genomic approach preferentially selected molecular components thought to be involved in cholesterol metabolism, and an orthogonal test defined functional roles in ARS through behavioral elucidation. Our findings suggest that fmo-2 (flavin-containing monooxygenase-2) plays a role in the progression of aversion-resistant ethanol seeking in C. elegans.

AWA and ASH Homologous Sensing Genes of Meloidogyne incognita Contribute to the Tomato Infection Process

The AWA neurons of Caenorhabditis elegans mainly perceive volatile attractive odors, while the ASH neurons perceive pH, penetration, nociception, odor tropism, etc. The perceptual neurons of Meloidogyne incognita have been little studied. The number of infestations around and within tomato roots was significantly reduced after RNA interference for high-homology genes in AWA and ASH neurons compared between M. incognita and C. elegans. Through in situ hybridization, we further determined the expression and localization of the homologous genes Mi-odr-10 and Mi-gpa-6 in M. incognita. In this study, we found that M. incognita has neuronal sensing pathways similar to AWA and ASH perception of C. elegans for sensing chemical signals from tomato roots. Silencing the homologous genes in these pathways could affect the nematode perception and infestation of tomato root systems. The results contribute to elucidating the process of the plant host perception of M. incognita.

Thermosensation in Caenorhabditis elegans is linked to ubiquitin-dependent protein turnover via insulin and calcineurin signalling

Organismal physiology and survival are influenced by environmental conditions and linked to protein quality control. Proteome integrity is achieved by maintaining an intricate balance between protein folding and degradation. In Caenorhabditis elegans, acute heat stress determines cell non-autonomous regulation of chaperone levels. However, how the perception of environmental changes, including physiological temperature, affects protein degradation remains largely unexplored. Here, we show that loss-of-function of dyf-1 in Caenorhabditis elegans associated with dysfunctional sensory neurons leads to defects in both temperature perception and thermal adaptation of the ubiquitin/proteasome system centered on thermosensory AFD neurons. Impaired perception of moderate temperature changes worsens ubiquitin-dependent proteolysis in intestinal cells. Brain-gut communication regulating protein turnover is mediated by upregulation of the insulin-like peptide INS-5 and inhibition of the calcineurin-regulated forkhead-box transcription factor DAF-16/FOXO. Our data indicate that perception of ambient temperature and its neuronal integration is important for the control of proteome integrity in complex organisms.

Diverse states and stimuli tune olfactory receptor expression levels to modulate food-seeking behavior

Animals must weigh competing needs and states to generate adaptive behavioral responses to the environment. Sensorimotor circuits are thus tasked with integrating diverse external and internal cues relevant to these needs to generate context-appropriate behaviors. However, the mechanisms that underlie this integration are largely unknown. Here, we show that a wide range of states and stimuli converge upon a single Caenorhabditis elegans olfactory neuron to modulate food-seeking behavior. Using an unbiased ribotagging approach, we find that the expression of olfactory receptor genes in the AWA olfactory neuron is influenced by a wide array of states and stimuli, including feeding state, physiological stress, and recent sensory cues. We identify odorants that activate these state-dependent olfactory receptors and show that altered expression of these receptors influences food-seeking and foraging. Further, we dissect the molecular and neural circuit pathways through which external sensory information and internal nutritional state are integrated by AWA. This reveals a modular organization in which sensory and state-related signals arising from different cell types in the body converge on AWA and independently control chemoreceptor expression. The synthesis of these signals by AWA allows animals to generate sensorimotor responses that reflect the animal's overall state. Our findings suggest a general model in which sensory- and state-dependent transcriptional changes at the sensory periphery modulate animals' sensorimotor responses to meet their ongoing needs and states.

Redundant neural circuits regulate olfactory integration

Olfactory integration is important for survival in a natural habitat. However, how the nervous system processes signals of two odorants present simultaneously to generate a coherent behavioral response is poorly understood. Here, we characterize circuit basis for a form of olfactory integration in Caenorhabditis elegans. We find that the presence of a repulsive odorant, 2-nonanone, that signals threat strongly blocks the attraction of other odorants, such as isoamyl alcohol (IAA) or benzaldehyde, that signal food. Using a forward genetic screen, we found that genes known to regulate the structure and function of sensory neurons, osm-5 and osm-1, played a critical role in the integration process. Loss of these genes mildly reduces the response to the repellent 2-nonanone and disrupts the integration effect. Restoring the function of OSM-5 in either AWB or ASH, two sensory neurons known to mediate 2-nonanone-evoked avoidance, is sufficient to rescue. Sensory neurons AWB and downstream interneurons AVA, AIB, RIM that play critical roles in olfactory sensorimotor response are able to process signals generated by 2-nonanone or IAA or the mixture of the two odorants and contribute to the integration. Thus, our results identify redundant neural circuits that regulate the robust effect of a repulsive odorant to block responses to attractive odorants and uncover the neuronal and cellular basis for this complex olfactory task.

In their natural environment, animals, including humans, encounter complex olfactory stimuli. Thus, how the brain processes multiple sensory cues to generate a coherent behavioral output is critical for the survival of the animal. In the present study, we combined molecular cellular genetics, optical physiology and behavioral analysis to study a common olfactory phenomenon in which the presence of one odorant blocks the response to another. Our results show that the integrated response is regulated by redundant neuronal circuits that engage several interneurons essential for olfactory sensorimotor responses, a mechanism that likely ensures a robust behavioral response to sensory cues representing information critical for survival.

The Basics of Setting up Successful Teaching Labs and Short-Term Projects with C. elegans

Caenorhabditis elegans is an excellent organism for teaching or doing short-term research projects because of the many freely-available comprehensive resources describing its genome and biology, its short-generation time and the ease of working with it in the lab. However, it can be daunting to begin a short-term project or a teaching lab with an organism with which one has little or no experience. Therefore, in this article, we will discuss the minimal equipment, reagents, and protocols needed to begin doing experiments with C. elegans as well as inclusive teaching practices. We will also describe a teaching laboratory for introductory or advanced neuroscience undergraduate courses where students learn to understand the relationship between genetic mutations and behavior through using chemotaxis assays. In addition, instructions for students for the lab module are provided in Subheading "Example of Teaching Lab Module for Students".

A single chemosensory GPCR is required for a concentration-dependent behavioral switching in C. elegans

Animals detect and discriminate countless environmental chemicals for their well-being and survival. Although a single chemical can trigger opposing behavioral responses depending on its concentration, the mechanisms underlying such a concentration-dependent switching remain poorly understood. Here, we show that C. elegans exhibits either attraction or avoidance of the bacteria-derived volatile chemical dimethyl trisulfide (DMTS) depending on its concentration. This behavioral switching is mediated by two different types of chemosensory neurons, both of which express the DMTS-sensitive seven-transmembrane G protein-coupled receptor (GPCR) SRI-14. These two sensory neurons share downstream interneurons that process and translate DMTS signals via distinct glutamate receptors to generate the appropriate behavioral outcome. Thus, our results present one mechanism by which an animal connects two distinct types of chemosensory neurons detecting a common ligand to alternate downstream circuitry, thus efficiently switching between specific behavioral programs based on ligand concentration.

Cisplatin-induced neurotoxicity involves the disruption of serotonergic neurotransmission

Neurotoxicity is a frequent side effect of cisplatin (CisPt)-based anticancer therapy whose pathophysiology is largely vague. Here, we exploited C. elegans as a 3R-compliant in vivo model to elucidate molecular mechanisms contributing to CisPt-induced neuronal dysfunction. To this end, we monitored the impact of CisPt on various sensory functions as well as pharyngeal neurotransmission by recording electropharyngeograms (EPGs). CisPt neither affected food and odor sensation nor mechano-sensation, which involve dopaminergic and glutaminergic neurotransmission. However, CisPt reduced serotonin-regulated pharyngeal pumping activity independent of changes in the morphology of related neurons. CisPt-mediated alterations in EPGs were fully rescued by addition of serotonin (5-HT) (≤ 2 mM). Moreover, the CisPt-induced pharyngeal injury was prevented by co-incubation with the clinically approved serotonin re-uptake inhibitory drug duloxetine. A protective effect of 5-HT was also observed with respect to CisPt-mediated impairment of another 5-HT-dependent process, the egg laying activity. Importantly, CisPt-induced apoptosis in the gonad and learning disability were not influenced by 5-HT. Using different C. elegans mutants we found that CisPt-mediated (neuro)toxicity is independent of serotonin biosynthesis and re-uptake and likely involves serotonin-receptor subtype 7 (SER-7)-related functions. In conclusion, by measuring EPGs as a surrogate parameter of neuronal dysfunction, we provide first evidence that CisPt-induced neurotoxicity in C. elegans involves 5-HT-dependent neurotransmission and SER-7-mediated signaling mechanisms and can be prevented by the clinically approved antidepressant duloxetine. The data highlight the particular suitability of C. elegans as a 3R-conform in vivo model in molecular (neuro)toxicology and, moreover, for the pre-clinical identification of neuroprotective candidate drugs.

Transcription Factors That Control Behavior-Lessons From C. elegans

Behavior encompasses the physical and chemical response to external and internal stimuli. Neurons, each with their own specific molecular identities, act in concert to perceive and relay these stimuli to drive behavior. Generating behavioral responses requires neurons that have the correct morphological, synaptic, and molecular identities. Transcription factors drive the specific gene expression patterns that define these identities, controlling almost every phenomenon in a cell from development to homeostasis. Therefore, transcription factors play an important role in generating and regulating behavior. Here, we describe the transcription factors, the pathways they regulate, and the neurons that drive chemosensation, mechanosensation, thermosensation, osmolarity sensing, complex, and sex-specific behaviors in the animal model Caenorhabditis elegans. We also discuss the current limitations in our knowledge, particularly our minimal understanding of how transcription factors contribute to the adaptive behavioral responses that are necessary for organismal survival.

Molecular topography of an entire nervous system

We have produced gene expression profiles of all 302 neurons of the C. elegans nervous system that match the single-cell resolution of its anatomy and wiring diagram. Our results suggest that individual neuron classes can be solely identified by combinatorial expression of specific gene families. For example, each neuron class expresses distinct codes of ∼23 neuropeptide genes and ∼36 neuropeptide receptors, delineating a complex and expansive "wireless" signaling network. To demonstrate the utility of this comprehensive gene expression catalog, we used computational approaches to (1) identify cis-regulatory elements for neuron-specific gene expression and (2) reveal adhesion proteins with potential roles in process placement and synaptic specificity. Our expression data are available at https://cengen.org and can be interrogated at the web application CengenApp. We expect that this neuron-specific directory of gene expression will spur investigations of underlying mechanisms that define anatomy, connectivity, and function throughout the C. elegans nervous system.

In silico analysis of the transcriptional regulatory logic of neuronal identity specification throughout the C. elegans nervous system

The generation of the enormous diversity of neuronal cell types in a differentiating nervous system entails the activation of neuron type-specific gene batteries. To examine the regulatory logic that controls the expression of neuron type-specific gene batteries, we interrogate single cell expression profiles of all 118 neuron classes of the Caenorhabditis elegans nervous system for the presence of DNA binding motifs of 136 neuronally expressed C. elegans transcription factors. Using a phylogenetic footprinting pipeline, we identify cis-regulatory motif enrichments among neuron class-specific gene batteries and we identify cognate transcription factors for 117 of the 118 neuron classes. In addition to predicting novel regulators of neuronal identities, our nervous system-wide analysis at single cell resolution supports the hypothesis that many transcription factors directly co-regulate the cohort of effector genes that define a neuron type, thereby corroborating the concept of so-called terminal selectors of neuronal identity. Our analysis provides a blueprint for how individual components of an entire nervous system are genetically specified.

1-Undecene from Pseudomonas aeruginosa is an olfactory signal for flight-or-fight response in Caenorhabditis elegans

Animals possess conserved mechanisms to detect pathogens and to improve survival in their presence by altering their own behavior and physiology. Here, we utilize Caenorhabditis elegans as a model host to ask whether bacterial volatiles constitute microbe-associated molecular patterns. Using gas chromatography-mass spectrometry, we identify six prominent volatiles released by the bacterium Pseudomonas aeruginosa. We show that a specific volatile, 1-undecene, activates nematode odor sensory neurons inducing both flight and fight responses in worms. Using behavioral assays, we show that worms are repelled by 1-undecene and that this aversion response is driven by the detection of this volatile through AWB odor sensory neurons. Furthermore, we find that 1-undecene odor can induce immune effectors specific to P. aeruginosa via AWB neurons and that brief pre-exposure of worms to the odor enhances their survival upon subsequent bacterial infection. These results show that 1-undecene derived from P. aeruginosa serves as a pathogen-associated molecular pattern for the induction of protective responses in C. elegans.

EGL-4/PKG regulates the role of an interneuron in a chemotaxis circuit of C. elegans through mediating integration of sensory signals

Interneurons, innervated by multiple sensory neurons, need to integrate information from these sensory neurons and respond to sensory stimuli adequately. Mechanisms how sensory information is integrated to form responses of interneurons are not fully understood. In Caenorhabditis elegans, loss-of-function mutations of egl-4, which encodes a cGMP-dependent protein kinase (PKG), cause a defect in chemotaxis to odorants. Our genetic and imaging analyses revealed that the response property of AIY interneuron to an odorant is reversed in the egl-4 mutant, while the responses of two upstream olfactory neurons, AWA and AWC, are largely unchanged. Cell- ablation experiments show that AIY in the egl-4 mutant functions to suppress chemotaxis. Furthermore, the reversal of AIY response occurs only in the presence of sensory signals from both AWA and AWC. These results suggest that sensory signals are inadequately integrated in the egl-4 mutant. We also show that egl-4 expression in AWA and another sensory neuron prevents the reversed AIY response and restores chemotaxis in the egl-4 mutants. We propose that EGL-4/PKG, by suppressing aberrant integration of signals from olfactory neurons, converts the response property of an interneuron to olfactory stimuli and maintains the role of the interneuron in the circuit to execute chemotactic behavior.

Multiple olfactory pathways contribute to the lure process of Caenorhabditis elegans by pathogenic bacteria

Chemosensation is indispensable for the survival of Caenorhabditis elegans to discriminate food and pathogenic bacteria in their living environment. Food-like odors emitted by the pathogen Bacillus nematocida B16 for trapping its hosts and an olfactory signaling pathway responsible to sense the attractant 2-heptanone were identified in our previous study. Here, we further explore how the worms recognize the attractive molecules indole and 2-ethyl hexanol, which have different chemical properties and modest nematode-luring ability. We show that the chemotaxis toward indole and 2-ethyl hexanol requires the G protein-coupled receptors encoded by str-193 on AWC and str-7 on AWA. In a further genetic screen for downstream effectors in olfactory signaling cascades, the Gα subunit GSA-1, guanylyl cyclase ODR-1 and DAF-11 and the cGMP-gated channel TAX-2/TAX-4 were found to be necessary for indole sensation, whereas the TRPV channels OSM-9/OCR-2 and the PLC pathway activated by GPA-6 are responsible for the detection of 2-ethyl hexanol. Altogether, our current work further clarifies the distinct olfactory signaling pathways through which C. elegans senses different chemicals and is lured by B. nematocida B16, improving our comprehensive understanding of the mechanisms by which bacterial pathogens effectively infect their hosts.

Signaling by AWC Olfactory Neurons Is Necessary for Caenorhabditis elegans' Response to Prenol, an Odor Associated with Nematode-Infected Insects

Chemosensation plays a role in the behaviors and life cycles of numerous organisms, including nematodes. Many guilds of nematodes exist, ranging from the free-living Caenorhabditis elegans to various parasitic species such as entomopathogenic nematodes (EPNs), which are parasites of insects. Despite ecological differences, previous research has shown that both EPNs and C. elegans respond to prenol (3-methyl-2-buten-1-ol), an odor associated with EPN infections. However, it is unclear how C. elegans responds to prenol. By utilizing natural variation and genetic neuron ablation to investigate the response of C. elegans to prenol, we found that the AWC neurons are involved in the detection of prenol and that several genes (including dcap-1, dcap-2, and clec-39) influence response to this odorant. Furthermore, we identified that the response to prenol is mediated by the canonically proposed pathway required for other AWC-sensed attractants. However, upon testing genetically diverse isolates, we found that the response of some strains to prenol differed from their response to isoamyl alcohol, suggesting that the pathways mediating response to these two odorants may be genetically distinct. Further, evaluations leveraging natural variation and genome wide association revealed specific genes that influence nematode behavior and provide a foundation for future studies to better understand the role of prenol in nematode behavioral ecology.

Reliability of an interneuron response depends on an integrated sensory state

The central nervous system transforms sensory information into representations that are salient to the animal. Here we define the logic of this transformation in a Caenorhabditis elegans integrating interneuron. AIA interneurons receive input from multiple chemosensory neurons that detect attractive odors. We show that reliable AIA responses require the coincidence of two sensory inputs: activation of AWA olfactory neurons that are activated by attractive odors, and inhibition of one or more chemosensory neurons that are inhibited by attractive odors. AWA activates AIA through an electrical synapse, while the disinhibitory pathway acts through glutamatergic chemical synapses. AIA interneurons have bistable electrophysiological properties consistent with their calcium dynamics, suggesting that AIA activation is a stereotyped response to an integrated stimulus. Our results indicate that AIA interneurons combine sensory information using AND-gate logic, requiring coordinated activity from multiple chemosensory neurons. We propose that AIA encodes positive valence based on an integrated sensory state.

Evolution of neuronal anatomy and circuitry in two highly divergent nematode species

The nematodes C. elegans and P. pacificus populate diverse habitats and display distinct patterns of behavior. To understand how their nervous systems have diverged, we undertook a detailed examination of the neuroanatomy of the chemosensory system of P. pacificus. Using independent features such as cell body position, axon projections and lipophilic dye uptake, we have assigned homologies between the amphid neurons, their first-layer interneurons, and several internal receptor neurons of P. pacificus and C. elegans. We found that neuronal number and soma position are highly conserved. However, the morphological elaborations of several amphid cilia are different between them, most notably in the absence of ‘winged’ cilia morphology in P. pacificus. We established a synaptic wiring diagram of amphid sensory neurons and amphid interneurons in P. pacificus and found striking patterns of conservation and divergence in connectivity relative to C. elegans, but very little changes in relative neighborhood of neuronal processes. These findings demonstrate the existence of several constraints in patterning the nervous system and suggest that major substrates for evolutionary novelty lie in the alterations of dendritic structures and synaptic connectivity.

Nerve cells, also called neurons, are responsible both for sensing signals from the environment and for determining how organisms react. This means that the unique features of an animal’s nervous system underpin its characteristic behaviors. Comparing the anatomy of the nervous systems in different animals could therefore yield valuable insights into how structural and behavioral differences emerge over time.

Behavioral variation often occurs even in similar-looking animals. One example is a group of microscopic worms, called nematodes. Although many nematode species exist, their overall body plans are the same, and the worms of each species contain a fixed number of cells. Despite these apparent similarities, different species of nematodes inhabit a variety of environments and may respond differently to the same signals.

The main sensory organs in nematodes are called the amphid sensilla. They are used to detect chemicals, as well as other inputs from the environment such as temperature and pheromones from other nematodes. Although researchers have often speculated that the number of cells in these organs and their arrangement are broadly the same across species, their anatomy had not been studied in detail.

Hong, Riebesell et al. compared the detailed structure and genetic features of the sensory systems in two distantly related species of nematode worms, Pristionchus pacificus and Caenorhabditis elegans. These two species behave in different ways, for example, P. pacificus is usually found in association with different species of beetles, while C. elegans is free-living and usually found on rotting fruit. By comparing the two, Hong, Riebesell et al. wanted to determine whether the diverse behaviors observed in the two species could be determined by differences between their sensory systems.

Experiments using electron microscopy yielded several thousand high resolution images spanning the entire sensory organ. These images were then used to create detailed reconstructions of the sensory nervous system in each worm species, demonstrating that both species had the same number of sensory nerve cells, allowing one-to-one comparisons between them.

Further analysis showed that while the overall structure of the neuronal connections remains the same between the two species, the neurons in P. pacificus made more diverse connections than those in C. elegans. Detailed studies of gene activity also revealed that neurons in each species switched on a slightly different group of genes, possibly indicating that each type of worm processes sensory signals in different ways.

These results shed new light on how nervous systems in related species can change over time without any change in neuron count. In the future, a better understanding of these changes could link the evolution of the nervous system to the emergence of different behaviors, in both simple and more complex organisms.

Irrational behavior in C. elegans arises from asymmetric modulatory effects within single sensory neurons

C. elegans worms exhibit a natural chemotaxis towards food cues. This provides a potential platform to study the interactions between stimulus valence and innate behavioral preferences. Here we perform a comprehensive set of choice assays to measure worms' relative preference towards various attractants. Surprisingly, we find that when facing a combination of choices, worms' preferences do not always follow value-based hierarchy. In fact, the innate chemotaxis behavior in worms robustly violates key rationality paradigms of transitivity, independence of irrelevant alternatives and regularity. These violations arise due to asymmetric modulatory effects between the presented options. Functional analysis of the entire chemosensory system at a single-neuron resolution, coupled with analyses of mutants, defective in individual neurons, reveals that these asymmetric effects originate in specific sensory neurons.

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.

Identification of Odor Blend Used by Caenorhabditis elegans for Pathogen Recognition

Animals have evolved specialized pathways to detect appropriate food sources and avoid harmful ones. Caenorhabditis elegans can distinguish among the odors of various species of bacteria, its major food source, but little is known about what specific chemical cue or combination of chemical cues C. elegans uses to detect and recognize different microbes. Here, we examine the strong innate attraction of C. elegans for the odor of the pathogenic bacterium, Serratia marcescens. This initial attraction likely facilitates ingestion and infection of the C. elegans host. Using solid-phase microextraction and gas chromatography coupled with mass spectrometry, we identify 5 volatile odors released by S. marcescens and identify those that are attractive to C. elegans. We use genetic methods to show that the amphid chemosensory neuron, AWCON, senses both S. marcescens-released 2-butanone and acetone and drives attraction to S. marcescens. In C. elegans, pairing a single odorant with food deprivation results in a reduced attractive response for that specific odor. We find that pairing the natural odor of S. marcescens with food deprivation results in a reduced attraction for the natural odor of S. marcescens and a similar reduced attraction for the synthetic blend of acetone and 2-butanone. This result indicates that only 2 odorants represent the more complex odor bouquet of S. marcescens. Although bacterial-released volatiles have long been known to be attractive to C. elegans, this study defines for the first time specific volatile cues that represent a particular bacterium to C. elegans.

SRD-1 in AWA neurons is the receptor for female volatile sex pheromones in C. elegans males

Pheromones are critical cues for attracting mating partners for successful reproduction. Sexually mature Caenorhabditis remanei virgin females and self-sperm-depleted Caenorhabditis elegans hermaphrodites produce volatile sex pheromones to attract adult males of both species from afar. The chemoresponsive receptor in males has remained unknown. Here, we show that the male chemotactic behavior requires amphid sensory neurons (AWA neurons) and the G-protein-coupled receptor SRD-1. SRD-1 expression in AWA neurons is sexually dimorphic, with the levels being high in males but undetectable in hermaphrodites. Notably, srd-1 mutant males lack the chemotactic response and pheromone-induced excitation of AWA neurons, both of which can be restored in males and hermaphrodites by AWA-specific srd-1 expression, and ectopic expression of srd-1 in AWB neurons in srd-1 mutants results in a repulsive behavioral response in both sexes. Furthermore, we show that the C-terminal region of SRD-1 confers species-specific differences in the ability to perceive sex pheromones between C. elegans and C. remanei These findings offer an excellent model for dissecting how a single G-protein-coupled receptor expressed in a dimorphic neural system contributes to sex-specific behaviors in animals.

daf-16/FOXO isoform b in AIY neurons is involved in low preference for Bifidobacterium infantis in Caenorhabditis elegans

The neural and molecular mechanisms underlying food preference have been poorly understood. We previously showed that Bifidobacterium infantis (B. infantis), a well-known probiotic bacterium, extends the lifespan of Caenorhabditis elegans (C. elegans) compared with a standard food, Escherichia coli (E. coli) OP50. In this study, we characterized C. elegans behavior against B. infantis and examined the neural and molecular mechanisms governing that behavior. The majority of the wild-type animals were outside of the B. infantis lawn 10 min after transfer. Although worms did not prefer B. infantis compared to E. coli OP50, they preferred the B. infantis lawn over a lawn containing M9 buffer alone, in which there was no food. Mutant analyses suggested that leaving the B. infantis lawn required daf-16/FOXO. Isoform-specific mutant phenotypes suggested that daf-16 isoform b seemed to be associated with leaving. Genetic rescue experiments demonstrated that the function of daf-16b in AIY interneurons was involved in leaving the B. infantis lawn. The daf-18/PTEN mutants were also defective in leaving. In conclusion, C. elegans showed a low preference for B. infantis, and daf-16b in AIY interneurons and daf-18 had roles in leaving B. infantis.

Single olfactory receptors set odor detection thresholds

In many species, survival depends on olfaction, yet the mechanisms that underlie olfactory sensitivity are not well understood. Here we examine how a conserved subset of olfactory receptors, the trace amine-associated receptors (TAARs), determine odor detection thresholds of mice to amines. We find that deleting all TAARs, or even single TAARs, results in significant odor detection deficits. This finding is not limited to TAARs, as the deletion of a canonical odorant receptor reduced behavioral sensitivity to its preferred ligand. Remarkably, behavioral threshold is set solely by the most sensitive receptor, with no contribution from other highly sensitive receptors. In addition, increasing the number of sensory neurons (and glomeruli) expressing a threshold-determining TAAR does not improve detection, indicating that sensitivity is not limited by the typical complement of sensory neurons. Our findings demonstrate that olfactory thresholds are set by the single highest affinity receptor and suggest that TAARs are evolutionarily conserved because they determine the sensitivity to a class of biologically relevant chemicals.

Odorous chemicals broadly activate subsets of olfactory receptors in the nose, but how individual receptors contribute to behavioral sensitivity is not clear. Here, the authors demonstrate that detection thresholds in mice are set solely by the highest affinity receptor for a given odorant.

Odor-dependent temporal dynamics in Caenorhabitis elegans adaptation and aversive learning behavior

Animals sense an enormous number of cues in their environments, and, over time, can form learned associations and memories with some of these. The nervous system remarkably maintains the specificity of learning and memory to each of the cues. Here we asked whether the nematode Caenorhabditis elegans adjusts the temporal dynamics of adaptation and aversive learning depending on the specific odor sensed. C. elegans senses a multitude of odors, and adaptation and learned associations to many of these odors requires activity of the cGMP-dependent protein kinase EGL-4 in the AWC sensory neuron. We identified a panel of 17 attractive odors, some of which have not been tested before, and determined that the majority of these odors require the AWC primary sensory neuron for sensation. We then devised a novel assay to assess odor behavior over time for a single population of animals. We used this assay to evaluate the temporal dynamics of adaptation and aversive learning to 13 odors and find that behavior change occurs early in some odors and later in others. We then examined EGL-4 localization in early-trending and late-trending odors over time. We found that the timing of these behavior changes correlated with the timing of nuclear accumulation of EGL-4 in the AWC neuron suggesting that temporal changes in behavior may be mediated by aversive learning mechanisms. We demonstrate that temporal dynamics of adaptation and aversive learning in C. elegans can be used as a model to study the timing of memory formation to different sensory cues.

Chemotaxis of Caenorhabditis elegans toward volatile organic compounds from Stropharia rugosoannulata induced by amino acids

A variety of natural substances including both volatile organic compounds and water-soluble compounds play a significant role in the chemotactic behaviors of the model nematode Caenorhabditis elegans. We observed chemotactic behaviors of C. elegans with respect to response to attractants produced by nematode parasitic fungus Stropharia rugosoannulata, which were partially induced by specific amino acids. The results of gas chromatography-mass spectrometer analysis suggested that 1-octen-3-ol was produced and benzaldehyde concentrations increased when L-phenylalanine was added to water agar plate. Similarly, the addition of L-tryptophan to the medium induced the production of benzaldehyde, 1-octen-3-ol and indole. The presence of L-phenylalanine and L-tryptophan increased the attraction of C. elegans to S. rugosoannulata. With attraction increased, nematode mortality increased more than 6 times higher.

Molecular characterization and gene expression patterns of retinoid receptors, in normal and regenerating tissues of the sea cucumber, Holothuria glaberrima

Retinoic acid receptors (RAR) and retinoid X receptors (RXR) are ligand-mediated transcription factors that synchronize intricate signaling networks in metazoans. Dimer formation between these two nuclear receptors mediates the recruitment of co-regulatory complexes coordinating the progression of signaling cascades during developmental and regenerative events. In the present study we identified and characterized the receptors for retinoic acid in the sea cucumber Holothuria glaberrima; a model system capable of regenerative organogenesis during adulthood. Molecular characterizations revealed the presence of three isoforms of RAR and two of RXR as a consequence of alternative splicing events. Various analyses including: primary structure sequencing, phylogenetic analysis, protein domain prediction, and multiple sequence alignment further confirmed their identity. Semiquantitative reverse transcription PCR analysis of each receptor isoform herein identified showed that the retinoid receptors are expressed in all tissues sampled: the mesenteries, respiratory trees, muscles, gonads, and the digestive tract. During regenerative organogenesis two of the receptors (RAR-L and RXR-T) showed differential expression in the posterior segment while RAR-S is differentially expressed in the anterior segment of the intestine. This work presents the first description of the components relaying the signaling for retinoic acid within this model system.

Morphological Diversity of C. elegans Sensory Cilia Instructed by the Differential Expression of an Immunoglobulin Domain Protein

Cilia on dendritic endings of sensory neurons organize distinct types of sensory machinery [1]. Ciliated endings display neuron-type-specific patterns of membrane elaborations [1-3], but it is not well understood how such neuron-type-specific morphologies are generated and whether they are coupled to the specification of other identity aspects of a terminally differentiated sensory neuron. In the course of a genome-wide analysis of members of a small family of immunoglobulin domain proteins, we found that OIG-8, a previously uncharacterized transmembrane protein with a single immunoglobulin (Ig) domain, instructs the distinct, neuron-type-specific elaboration of ciliated endings of different olfactory neuron types in the nematode C. elegans. OIG-8 protein localizes to ciliated endings of these sensory neurons, and is transcribed at different levels in distinct olfactory neuron types. oig-8 expression levels correlate with the extent of sensory cilia growth and branching patterns. Loss of oig-8 leads to a reduction in the branching patterns of cilia, whereas raising the levels of oig-8 results in an increase in elaborations. Levels of OIG-8 expression are controlled by the specific combination of a terminal selector type of transcription factors that also specify other identity features of distinct olfactory neuron types.

Isoamyl alcohol odor promotes longevity and stress tolerance via DAF-16 in Caenorhabditis elegans

The possibility that odor plays a role in lifespan regulation through effects on the nervous system is indicated by research on Caenorhabditis elegans. In fact, ablation of AWA and AWC, which are suggested as olfactory neurons, has been shown to extend lifespan via DAF-16, a homolog of FoxO. However, the effects of odor stimuli on the lifespan still remain unclear. Thus, we here aimed to clarify the effect of attractive and repulsive odors on longevity and stress tolerance in C. elegans and to analyze the pathways thereof. We used isoamyl alcohol as an attractive odor, and acetic acid as a repellent component, as identified by chemotaxis assay. We found that isoamyl alcohol stimulus promoted longevity in a DAF-16-dependent manner. On the other hand, acetic acid stimulus promoted thermotolerance through mechanisms independent of DAF-16. Above all, our results indicate that odor stimuli affect the lifespan and stress tolerance of C. elegans, with attractive and repulsive odors exerting their effects through different mechanisms, and that longevity is induced by both activation and inactivation of olfactory neurons.

Nematophagous fungus Arthrobotrys oligospora mimics olfactory cues of sex and food to lure its nematode prey

To study the molecular basis for predator-prey coevolution, we investigated how Caenorhabditis elegans responds to the predatory fungus Arthrobotrys oligospora. C. elegans and other nematodes were attracted to volatile compounds produced by A. oligospora. Gas-chromatographic mass-spectral analyses of A. oligospora-derived volatile metabolites identified several odors mimicking food cues attractive to nematodes. One compound, methyl 3-methyl-2-butenoate (MMB) additionally triggered strong sex- and stage-specific attraction in several Caenorhabditis species. Furthermore, when MMB is present, it interferes with nematode mating, suggesting that MMB might mimic sex pheromone in Caenorhabditis species. Forward genetic screening suggests that multiple receptors are involved in sensing MMB. Response to fungal odors involves the olfactory neuron AWCs. Single-cell RNA-seq revealed the GPCRs expressed in AWC. We propose that A. oligospora likely evolved the means to use olfactory mimicry to attract its nematode prey through the olfactory neurons in C. elegans and related species.

DOI:http://dx.doi.org/10.7554/eLife.20023.001

Altered Sensory Code Drives Juvenile-to-Adult Behavioral Maturation in Caenorhabditis elegans

Adults perform better than juveniles in food-seeking tasks. Using the nematode Caenorhabditis elegans to probe the neural mechanisms underlying behavioral maturation, we found that adults and juveniles require different combinations of sensory neurons to generate age-specific food-seeking behavior. We first show that adults and juveniles differ in their response to and preference for food-associated odors, and we analyze genetic mutants to map the neuronal circuits required for those behavioral responses. We developed a novel device to trap juveniles and record their neuronal activity. Activity measurements revealed that adult and juvenile AWA sensory neurons respond to the addition of diacetyl stimulus, whereas AWB, ASK, and AWC sensory neurons encode its removal specifically in adults. Further, we show that reducing neurotransmission from the additional AWB, ASK, and AWC sensory neurons transforms odor preferences from an adult to a juvenile-like state. We also show that AWB and ASK neurons drive behavioral changes exclusively in adults, providing more evidence that age-specific circuits drive age-specific behavior. Collectively, our results show that an odor-evoked sensory code is modified during the juvenile-to-adult transition in animal development to drive age-appropriate behavior. We suggest that this altered sensory code specifically enables adults to extract additional stimulus features and generate robust behavior.