Reprogramming chemotaxis responses: sensory neurons define olfactory preferences in C. elegans

Nonlinear integration of sensory and motor inputs by a single neuron in C. elegans

Context is important for sensory integration. Rather than simply considering sensory information independently, the brain integrates this information to inform behavior, however identifying this property at the single-neuron level is not trivial. In Caenorhabditis elegans , the paired interneurons AIBL and AIBR (AIB) have a compartmentalized organization of presynapses along its singular process. Sensory and sensory interneurons primarily synapse along the proximal process, while motor and motor interneurons synapse along the distal process. Since this neuron has graded potentials, the simplest model for AIB integration is simply a convolution of its presynaptic inputs. Through a series of experiments to manipulate sensory and motor input onto AIB, we find that while AIB activity is primarily a convolution of motor inputs, its sensory responses are not integrated independently. Instead, the gain in sensory input is a function of the temporal dynamics of motor input. Sensory information is reinforced when it matches the expected behavioral response. We find this property is also observed in other whole-brain datasets. Context-dependent behavioral responses to sensory input is well-documented. Here, we show this property can be localized to single neurons in the worm nervous system. This integration property likely plays an important role in context-dependent decision-making, as well as the highly variable dynamics of the worm nervous system.

An Extensive Gap Junction Neural Network Modulates Caenorhabditis elegans Aversive Behavior

BACKGROUND/OBJECTIVES

Caenorhabditis elegans rely on sensory perception of environmental cues for survival in their native soil and compost habitats. These cues provide information about nutrient availability, mating partners, or predatory and hazardous beacons. In C. elegans, the two bilaterally-symmetric head sensory neurons termed ASH are the main detectors of aversive nociceptive signals. Through their downstream connections in the nervous system, ASH activation causes the animal to initiate backward locomotion to escape and avoid the harmful stimulus. Modulation of avoidance behavior allows for situation-appropriate sensitivity and response to stimuli. We previously reported a role for gap junctions in the transport of regulatory cGMP to the ASHs where it functions to dampen avoidance responses.

METHODS

Here, we used genetic mutants and a combination of cell-selective rescue and knockdown experiments to identify gap junction proteins (innexins) involved in modulating ASH-mediated nociceptive behavioral responses.

RESULTS

We have characterized six additional C. elegans innexins that have overlapping and distinct roles within this regulatory network: INX-7, INX-15, INX-16, INX-17, UNC-7, and UNC-9.

CONCLUSIONS

This work expands our understanding of the extent to which ASH sensitivity can be tuned in a non-cell-autonomous manner.

Calcium levels in ASER neurons determine behavioral valence by engaging distinct neuronal circuits in C. elegans

The valence of stimuli is shaped by various factors, including environmental cues, internal states, genetic variability, and past experience. However, the mechanisms behind this flexibility remain elusive. In the nematode C. elegans, we found that ethanol, an olfactory stimulus, can elicit opposite chemotaxis responses - attraction vs. aversion - depending on NaCl concentration, demonstrating the role of environmental factors in altering valence. Remarkably, a single chemosensory neuron, ASER, orchestrate this bidirectional ethanol chemotaxis by integrating information from both stimuli - ethanol and NaCl - into its neuronal activity dynamics. Specifically, different calcium dynamics in the ASER neuron differentially activate the signaling molecule CMK-1, thereby engaging different downstream interneurons and leading to opposite chemotaxis directions. Consistently, optogenetic manipulations of the ASER neuron reverse the chemotaxis directions, by altering its calcium dynamics. Our findings reveal a mechanism by which a single neuron integrates multisensory inputs to determine context-dependent behavioral valence, contributing to our current understanding of valence encoding.

Prospects on non-canonical olfaction in the mosquito and other organisms: why co-express?

The Aedes aegypti mosquito utilizes olfaction during the search for humans to bite. The attraction to human body odor is an innate behavior for this disease-vector mosquito. Many well-studied model species have olfactory systems that conform to a particular organization that is sometimes referred to as the 'one-receptor-to-one-neuron' organization because each sensory neuron expresses only a single type of olfactory receptor that imparts the neuron's chemical selectivity. This sensory architecture has become the canon in the field. This review will focus on the recent finding that the olfactory system of Ae. aegypti has a different organization, with multiple olfactory receptors co-expressed in many of its olfactory sensory neurons. We will discuss the canonical organization and how this differs from the non-canonical organization, examine examples of non-canonical olfactory systems in other species, and discuss the possible roles of receptor co-expression in odor coding in the mosquito and other organisms.

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.

Paired C-type lectin receptors mediate specific recognition of divergent oomycete pathogens in C. elegans

Innate immune responses can be triggered upon detection of pathogen- or damage-associated molecular patterns by host receptors that are often present on the surface of immune cells. While invertebrates like Caenorhabditis elegans lack professional immune cells, they still mount pathogen-specific responses. However, the identity of host receptors in the nematode remains poorly understood. Here, we show that C-type lectin receptors mediate species-specific recognition of divergent oomycetes in C. elegans. A CLEC-27/CLEC-35 pair is essential for recognition of the oomycete Myzocytiopsis humicola, while a CLEC-26/CLEC-36 pair is required for detection of Haptoglossa zoospora. Both clec pairs are transcriptionally regulated through a shared promoter by the conserved PRD-like homeodomain transcription factor CEH-37/OTX2 and act in sensory neurons and the anterior intestine to trigger a protective immune response in the epidermis. This system enables redundant tissue sensing of oomycete threats through canonical CLEC receptors and host defense via cross-tissue communication.

Transgenerational and parental impacts of acrylamide exposure on Caenorhabditis elegans: Physiological, behavioral, and genetic mechanisms

Acrylamide is pervasive, and its exposure poses numerous health risks. This study examines both the direct and transgenerational effects of acrylamide toxicity in Caenorhabditis elegans, focusing on physiological and behavioral parameters. Parental exposure to acrylamide compromised several aspects of nematode health, including lifespan, reproductive capacity, body dimensions, and motor and sensory functions. Notably, while exposure to low concentrations of acrylamide did not alter the physiological traits of the offspring-except for their learning and memory-these findings suggest a possible adaptive response to low-level exposure that could be inherited by subsequent generations. Furthermore, continued acrylamide exposure in the offspring intensified both physiological and perceptual toxicity. Detailed analysis revealed dose-dependent alterations in acrylamide's detoxification and metabolic pathways. In particular, it inhibits the gene gst-4, which encodes a crucial enzyme in detoxification, mitigates DNA damage induced by acrylamide, and highlights a potential therapeutic target to reduce its deleterious effects.

Olfactory combinatorial coding supports risk-reward decision making in C. elegans

Olfactory-driven behaviors are essential for animal survival, but mechanisms for decoding olfactory inputs remain poorly understood. We have used whole-network Ca ++ imaging to study olfactory coding in Caenorhabditis elegans. We show that the odorant 1-octanol is encoded combinatorially in the periphery as both an attractant and a repellant. These inputs are integrated centrally, and their relative strengths determine the sensitivity and valence of the behavioral response through modulation of locomotory reversals and speed. The balance of these pathways also dictates the activity of the locomotory command interneurons, which control locomotory reversals. This balance serves as a regulatory node for response modulation, allowing C. elegans to weigh opportunities and hazards in its environment when formulating behavioral responses. Thus, an odorant can be encoded simultaneously as inputs of opposite valence, focusing attention on the integration of these inputs in determining perception, response, and plasticity.

Worms love Coffee too! Characterizing the neural substrates that regulate odor-guided responses to coffee

The coffee industry reaches over 80 billion US dollars in revenue partially due to the numerous chemicals that allow for coffee's highly attractive aroma and overall flavor. Many people integrate coffee into their everyday routine; therefore, understanding the attraction to coffee can facilitate, 1) the characterization of its attractive nature, and, 2) allow further understanding of how humans interpret taste and smell on a molecular and cellular level, from initial sensation to higher processing of these complex neural signals. We report that the model worm, C. elegans , can smell and perform strong attraction behavior using chemotaxis towards various types of coffee odors. In this study, we show that the nematode C. elegans is strongly attracted to various forms of coffee. We have also identified neuronal molecules that mediate this sensory-dependent behavior. Overall, we provide a platform to more thoroughly dissect the mechanisms and neuronal circuits that mediate odor-guided behavior to a complex human-sensed stimulus.

Moderate embryonic delay of paternal mitochondrial elimination impairs mating and cognition and alters behaviors of adult animals

Rapid elimination of paternal mitochondria following fertilization is a conserved event in most animals, but its physiological significance remains unclear. We find that modest delay of paternal mitochondrial elimination (PME) in Caenorhabditis elegans embryos unexpectedly impairs mating and cognition of adult animals and alters their locomotion behaviors. Delayed PME causes decreased adenosine triphosphate (ATP) levels in early embryos, which lead to impaired physiological functions of adult animals through an energy-sensing pathway mediated by an adenosine monophosphate (AMP)-activated protein kinase, AAK-2, and a forkhead box class O (FOXO) transcription factor, DAF-16. Treatment of PME-delayed animals with MK-4, a subtype of vitamin K2 that can improve mitochondrial ATP production, restores ATP levels in early embryos, and rescues physiological defects of adult animals. Our results suggest that moderate PME delay during embryo development adversely affects crucial physiological functions in adults, which could be evolutionarily disadvantageous. These observations provide mechanistic explanations for the need to swiftly remove paternal mitochondria early during embryo development.

Olfactory dysfunction as an early pathogenic indicator in C. elegans models of Alzheimer's and polyglutamine diseases

Neurodegenerative diseases such as Alzheimer's disease and polyglutamine diseases are characterized by abnormal accumulation of misfolded proteins, leading to neuronal dysfunction and subsequent neuron death. However, there is a lack of studies that integrate molecular, morphological, and functional analyses in neurodegenerative models to fully characterize these time-dependent processes. In this study, we used C. elegans models expressing Aβ1-42 and polyglutamine to investigate early neuronal pathogenic features in olfactory neurons. Both models demonstrated significant reductions in odor sensitivity in AWB and AWC chemosensory neurons as early as day 1 of adulthood, while AWA chemosensory neurons showed no such decline, suggesting cell-type-specific early neuronal dysfunction. At the molecular level, Aβ1-42 or Q40 expression caused age-dependent protein aggregation and morphological changes in neurons. By day 6, both models displayed prominent protein aggregates in neuronal cell bodies and neurites. Notably, AWB neurons in both models showed significantly shortened cilia and increased instances of enlarged cilia as early as day 1 of adulthood. Furthermore, AWC neurons expressing Aβ1-42 displayed calcium signaling defects, with significantly reduced responses to odor stimuli on day 1, further supporting early behavioral dysfunction. In contrast, AWA neuron did not exhibit reduced calcium responses, consistent with the absence of detectable decreases in olfactory sensitivity in these neurons. These findings suggest that decreased calcium signaling and dysfunction in specific sensory neuron subtypes are early indicators of neurodegeneration in C. elegans, occurring prior to the formation of visible protein aggregates. We found that the ER unfolded protein response (UPR) is significantly activated in worms expressing Aβ1-42. Activation of the AMPK pathway alleviates olfactory defects and reduces fibrillar Aβ in these worms. This study underscores the use of C. elegans olfactory neurons as a model to elucidate mechanisms of proteostasis in neurodegenerative diseases and highlights the importance of integrated approaches.

UPRER-immunity axis acts as physiological food evaluation system that promotes aversion behavior in sensing low-quality food

To survive in challenging environments, animals must develop a system to assess food quality and adjust their feeding behavior accordingly. However, the mechanisms that regulate this chronic physiological food evaluation system, which monitors specific nutrients from ingested food and influences food-response behavior, are still not fully understood. Here, we established a low-quality food evaluation assay system and found that heat-killed E. coli (HK-E. coli), a low-sugar food, triggers cellular UPRER and immune response. This encourages animals to avoid low-quality food. The physiological system for evaluating low-quality food depends on the UPRER (IRE-1/XBP-1) - Innate immunity (PMK-1/p38 MAPK) axis, particularly its neuronal function, which subsequently regulates feeding behaviors. Moreover, animals can adapt to a low-quality food environment through sugar supplementation, which inhibits the UPRER -PMK-1 regulated stress response by increasing vitamin C biosynthesis. This study reveals the role of the cellular stress response pathway as physiological food evaluation system for assessing nutritional deficiencies in food, thereby enhancing survival in natural environments.

Neural Sequences Underlying Directed Turning in C. elegans

Complex behaviors like navigation rely on sequenced motor outputs that combine to generate effective movement. The brain-wide organization of the circuits that integrate sensory signals to select and execute appropriate motor sequences is not well understood. Here, we characterize the architecture of neural circuits that control C. elegans olfactory navigation. We identify error-correcting turns during navigation and use whole-brain calcium imaging and cell-specific perturbations to determine their neural underpinnings. These turns occur as motor sequences accompanied by neural sequences, in which defined neurons activate in a stereotyped order during each turn. Distinct neurons in this sequence respond to sensory cues, anticipate upcoming turn directions, and drive movement, linking key features of this sensorimotor behavior across time. The neuromodulator tyramine coordinates these sequential brain dynamics. Our results illustrate how neuromodulation can act on a defined neural architecture to generate sequential patterns of activity that link sensory cues to motor actions.

RNAi-dependent expression of sperm genes in ADL chemosensory neurons is required for olfactory responses in Caenorhabditis elegans

Environmental conditions experienced early in the life of an animal can result in gene expression changes later in its life history. We have previously shown that C. elegans animals that experienced the developmentally arrested and stress resistant dauer stage (postdauers) retain a cellular memory of early-life stress that manifests during adulthood as genome-wide changes in gene expression, chromatin states, and altered life history traits. One consequence of developmental reprogramming in C. elegans postdauer adults is the downregulation of osm-9 TRPV channel gene expression in the ADL chemosensory neurons resulting in reduced avoidance to a pheromone component, ascr#3. This altered response to ascr#3 requires the principal effector of the somatic nuclear RNAi pathway, the Argonaute (AGO) NRDE-3. To investigate the role of the somatic nuclear RNAi pathway in regulating the developmental reprogramming of ADL due to early-life stress, we profiled the mRNA transcriptome of control and postdauer ADL in wild-type and nrde-3 mutant adults. We found 711 differentially expressed (DE) genes between control and postdauer ADL neurons, 90% of which are dependent upon NRDE-3. Additionally, we identified a conserved sequence that is enriched in the upstream regulatory sequences of the NRDE-3-dependent differentially expressed genes. Surprisingly, 214 of the ADL DE genes are considered "germline-expressed", including 21 genes encoding the Major Sperm Proteins and two genes encoding the sperm-specific PP1 phosphatases, GSP-3 and GSP-4. Loss of function mutations in gsp-3 resulted in both aberrant avoidance and attraction behaviors. We also show that an AGO pseudogene, Y49F6A.1 (wago-11), is expressed in ADL and is required for ascr#3 avoidance. Overall, our results suggest that small RNAs and reproductive genes program the ADL mRNA transcriptome during their developmental history and highlight a nexus between neuronal and reproductive networks in calibrating animal neuroplasticity.

A high-throughput behavioral screening platform for measuring chemotaxis by C. elegans

Throughout history, humans have relied on plants as a source of medication, flavoring, and food. Plants synthesize large chemical libraries and release many of these compounds into the rhizosphere and atmosphere where they affect animal and microbe behavior. To survive, nematodes must have evolved the sensory capacity to distinguish plant-made small molecules (SMs) that are harmful and must be avoided from those that are beneficial and should be sought. This ability to classify chemical cues as a function of their value is fundamental to olfaction and represents a capacity shared by many animals, including humans. Here, we present an efficient platform based on multiwell plates, liquid handling instrumentation, inexpensive optical scanners, and bespoke software that can efficiently determine the valence (attraction or repulsion) of single SMs in the model nematode, Caenorhabditis elegans. Using this integrated hardware-wetware-software platform, we screened 90 plant SMs and identified 37 that attracted or repelled wild-type animals but had no effect on mutants defective in chemosensory transduction. Genetic dissection indicates that for at least 10 of these SMs, response valence emerges from the integration of opposing signals, arguing that olfactory valence is often determined by integrating chemosensory signals over multiple lines of information. This study establishes that C. elegans is an effective discovery engine for determining chemotaxis valence and for identifying natural products detected by the chemosensory nervous system.

Understanding the factors regulating host-microbiome interactions using Caenorhabditis elegans

The Human Microbiome Project was a research programme that successfully identified associations between microbial species and healthy or diseased individuals. However, a major challenge identified was the absence of model systems for studying host-microbiome interactions, which would increase our capacity to uncover molecular interactions, understand organ-specificity and discover new microbiome-altering health interventions. Caenorhabditis elegans has been a pioneering model organism for over 70 years but was largely studied in the absence of a microbiome. Recently, ecological sampling of wild nematodes has uncovered a large amount of natural genetic diversity as well as a slew of associated microbiota. The field has now explored the interactions of C. elegans with its associated gut microbiome, a defined and non-random microbial community, highlighting its suitability for dissecting host-microbiome interactions. This core microbiome is being used to study the impact of host genetics, age and stressors on microbiome composition. Furthermore, single microbiome species are being used to dissect molecular interactions between microbes and the animal gut. Being amenable to health altering genetic and non-genetic interventions, C. elegans has emerged as a promising system to generate and test new hypotheses regarding host-microbiome interactions, with the potential to uncover novel paradigms relevant to other systems. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

Exposure to an aversive odor alters Caenorhabditis elegans physiology

Perception of external cues is important for enhancing the fitness and survival of animals. However, the role of odor perception in regulation of longevity and health is incompletely defined. Here, we show that the exposure to an aversive odor 2-nonanone reduces life span, brood size, feeding rate, and increases lipid storage in worms. These effects are restored to normal levels in mutant worms lacking functional olfactory AWB neurons, suggesting a potential role of odor perception in the regulation of animal physiology and longevity.

Comparison of avoidance assay techniques to determine the response to 1-octanol in C. elegans

In C. elegans , avoidance behaviors are vital for the nematode's ability to respond to noxious environmental stimuli, including the odorant 1-octanol. To test avoidance to 1-octanol, researchers expose C. elegans to this odorant and determine the time taken to initiate backward locomotion. However, the 1-octanol avoidance assay is sensitive to sensory adaptation, where the avoidance response is reduced due to overexposure to the odorant. Here, we examined two methods to expose nematodes to 1-octanol, using an eyelash hair or a p10 pipette tip, to compare their susceptibility to cause sensory adaptation.

An efficient behavioral screening platform classifies natural products and other chemical cues according to their chemosensory valence in C. elegans

Throughout history, humans have relied on plants as a source of medication, flavoring, and food. Plants synthesize large chemical libraries and release many of these compounds into the rhizosphere and atmosphere where they affect animal and microbe behavior. To survive, nematodes must have evolved the sensory capacity to distinguish plant-made small molecules (SMs) that are harmful and must be avoided from those that are beneficial and should be sought. This ability to classify chemical cues as a function of their value is fundamental to olfaction, and represents a capacity shared by many animals, including humans. Here, we present an efficient platform based on multi-well plates, liquid handling instrumentation, inexpensive optical scanners, and bespoke software that can efficiently determine the valence (attraction or repulsion) of single SMs in the model nematode, Caenorhabditis elegans. Using this integrated hardware-wetware-software platform, we screened 90 plant SMs and identified 37 that attracted or repelled wild-type animals, but had no effect on mutants defective in chemosensory transduction. Genetic dissection indicates that for at least 10 of these SMs, response valence emerges from the integration of opposing signals, arguing that olfactory valence is often determined by integrating chemosensory signals over multiple lines of information. This study establishes that C. elegans is an effective discovery engine for determining chemotaxis valence and for identifying natural products detected by the chemosensory nervous system.

Worm-Based Diagnosis Combining Microfluidics toward Early Cancer Screening

Early cancer diagnosis increases therapy efficiency and saves huge medical costs. Traditional blood-based cancer markers and endoscopy procedures demonstrate limited capability in the diagnosis. Reliable, non-invasive, and cost-effective methods are in high demand across the world. Worm-based diagnosis, utilizing the chemosensory neuronal system of C. elegans, emerges as a non-invasive approach for early cancer diagnosis with high sensitivity. It facilitates effectiveness in large-scale cancer screening for the foreseeable future. Here, we review the progress of a unique route of early cancer diagnosis based on the chemosensory neuronal system of C. elegans. We first introduce the basic procedures of the chemotaxis assay of C. elegans: synchronization, behavior assay, immobilization, and counting. Then, we review the progress of each procedure and the various cancer types for which this method has achieved early diagnosis. For each procedure, we list examples of microfluidics technologies that have improved the automation, throughput, and efficiency of each step or module. Finally, we envision that microfluidics technologies combined with the chemotaxis assay of C. elegans can lead to an automated, cost-effective, non-invasive early cancer screening technology, with the development of more mature microfluidic modules as well as systematic integration of functional modules.

Behavioral and molecular response of the insect parasitic nematode Steinernema carpocapsae to plant volatiles

Entomopathogenic nematodes (EPNs) use the chemical cues emitted by insects and insect-damaged plants to locate their hosts. Steinernema carpocapsae, a species of EPN, is an established biocontrol agent used against insect pests. Despite its promising potential, the molecular mechanisms underlying its ability to detect plant volatiles remain poorly understood. In this study, we investigated the response of S. carpocapsae infective juveniles (IJs) to 8 different plant volatiles. Among these, carvone was found to be the most attractive volatile compound. To understand the molecular basis of the response of IJs to carvone, we used RNA-Seq technology to identify gene expression changes in response to carvone treatment. Transcriptome analysis revealed 721 differentially expressed genes (DEGs) between carvone-treated and control groups, with 403 genes being significantly upregulated and 318 genes downregulated. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that the responsive DEGs to carvone attraction were mainly involved in locomotion, localization, behavior, response to stimulus, and olfactory transduction. We also identified four upregulated genes of chemoreceptor and response to stimulus that were involved in the response of IJs to carvone attraction. Our results provide insights into the potential transcriptional mechanisms underlying the response of S. carpocapsae to carvone, which can be utilized to develop environmentally friendly strategies for attracting EPNs.

G protein-coupled receptor-based thermosensation determines temperature acclimatization of Caenorhabditis elegans

Animals must sense and acclimatize to environmental temperatures for survival, yet their thermosensing mechanisms other than transient receptor potential (TRP) channels remain poorly understood. We identify a trimeric G protein-coupled receptor (GPCR), SRH-40, which confers thermosensitivity in sensory neurons regulating temperature acclimatization in Caenorhabditis elegans. Systematic knockdown of 1000 GPCRs by RNAi reveals GPCRs involved in temperature acclimatization, among which srh-40 is highly expressed in the ADL sensory neuron, a temperature-responsive chemosensory neuron, where TRP channels act as accessorial thermoreceptors. In vivo Ca2+ imaging demonstrates that an srh-40 mutation reduced the temperature sensitivity of ADL, resulting in supranormal temperature acclimatization. Ectopically expressing SRH-40 in a non-warmth-sensing gustatory neuron confers temperature responses. Moreover, temperature-dependent SRH-40 activation is reconstituted in Drosophila S2R+ cells. Overall, SRH-40 may be involved in thermosensory signaling underlying temperature acclimatization. We propose a dual thermosensing machinery through a GPCR and TRP channels in a single sensory neuron.

Heat Shock Proteins Function as Signaling Molecules to Mediate Neuron-Glia Communication During Aging

The nervous system is primarily composed of neurons and glia, and the communication between them plays profound roles in regulating the development and function of the brain. Neuron-glia signal transduction is known to be mediated by secreted or juxtacrine signals through ligand-receptor interactions on the cell membrane. Here, we report a novel mechanism for neuron-glia signal transduction, wherein neurons transmit proteins to glia through extracellular vesicles, activating glial signaling pathways. We find that in the amphid sensory organ of Caenorhabditis elegans, different sensory neurons exhibit varying aging rates. This discrepancy in aging is governed by the crosstalk between neurons and glia. We demonstrate that early-aged neurons can transmit heat shock proteins (HSP) to glia via extracellular vesicles. These neuronal HSPs activate the IRE1-XBP1 pathway, further increasing their expression in glia, forming a positive feedback loop. Ultimately, the activation of the IRE1-XBP-1 pathway leads to the transcriptional regulation of chondroitin synthases to protect glia-embedded neurons from aging-associated functional decline. Therefore, our studies unveil a novel mechanism for neuron-glia communication in the nervous system and provide new insights into our understanding of brain aging.

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.

A cGAL-UAS bipartite expression toolkit for Caenorhabditis elegans sensory neurons

Animals integrate sensory information from the environment and display various behaviors in response to external stimuli. In Caenorhabditis elegans hermaphrodites, 33 types of sensory neurons are responsible for chemosensation, olfaction, and mechanosensation. However, the functional roles of all sensory neurons have not been systematically studied due to the lack of facile genetic accessibility. A bipartite cGAL-UAS system has been previously developed to study tissue- or cell-specific functions in C. elegans. Here, we report a toolkit of new cGAL drivers that can facilitate the analysis of a vast majority of the 60 sensory neurons in C. elegans hermaphrodites. We generated 37 sensory neuronal cGAL drivers that drive cGAL expression by cell-specific regulatory sequences or intersection of two distinct regulatory regions with overlapping expression (split cGAL). Most cGAL-drivers exhibit expression in single types of cells. We also constructed 28 UAS effectors that allow expression of proteins to perturb or interrogate sensory neurons of choice. This cGAL-UAS sensory neuron toolkit provides a genetic platform to systematically study the functions of C. elegans sensory neurons.

Dissecting the genetic landscape of GPCR signaling through phenotypic profiling in C. elegans

G protein-coupled receptors (GPCRs) mediate responses to various extracellular and intracellular cues. However, the large number of GPCR genes and their substantial functional redundancy make it challenging to systematically dissect GPCR functions in vivo. Here, we employ a CRISPR/Cas9-based approach, disrupting 1654 GPCR-encoding genes in 284 strains and mutating 152 neuropeptide-encoding genes in 38 strains in C. elegans. These two mutant libraries enable effective deorphanization of chemoreceptors, and characterization of receptors for neuropeptides in various cellular processes. Mutating a set of closely related GPCRs in a single strain permits the assignment of functions to GPCRs with functional redundancy. Our analyses identify a neuropeptide that interacts with three receptors in hypoxia-evoked locomotory responses, unveil a collection of regulators in pathogen-induced immune responses, and define receptors for the volatile food-related odorants. These results establish our GPCR and neuropeptide mutant libraries as valuable resources for the C. elegans community to expedite studies of GPCR signaling in multiple contexts.

Opposing chemosensory functions of closely related gustatory receptors

In the fruit fly Drosophila melanogaster, gustatory sensory neurons express taste receptors that are tuned to distinct groups of chemicals, thereby activating neural ensembles that elicit either feeding or avoidance behavior. Members of a family of ligand -gated receptor channels, the Gustatory receptors (Grs), play a central role in these behaviors. In general, closely related, evolutionarily conserved Gr proteins are co-expressed in the same type of taste neurons, tuned to chemically related compounds, and therefore triggering the same behavioral response. Here, we report that members of the Gr28 subfamily are expressed in largely non-overlapping sets of taste neurons in Drosophila larvae, detect chemicals of different valence, and trigger opposing feeding behaviors. We determined the intrinsic properties of Gr28 neurons by expressing the mammalian Vanilloid Receptor 1 (VR1), which is activated by capsaicin, a chemical to which wild-type Drosophila larvae do not respond. When VR1 is expressed in Gr28a neurons, larvae become attracted to capsaicin, consistent with reports showing that Gr28a itself encodes a receptor for nutritious RNA. In contrast, expression of VR1 in two pairs of Gr28b.c neurons triggers avoidance to capsaicin. Moreover, neuronal inactivation experiments show that the Gr28b.c neurons are necessary for avoidance of several bitter compounds. Lastly, behavioral experiments of Gr28 deficient larvae and live Ca2+ imaging studies of Gr28b.c neurons revealed that denatonium benzoate, a synthetic bitter compound that shares structural similarities with natural bitter chemicals, is a ligand for a receptor complex containing a Gr28b.c or Gr28b.a subunit. Thus, the Gr28 proteins, which have been evolutionarily conserved over 260 million years in insects, represent the first taste receptor subfamily in which specific members mediate behavior with opposite valence.

The G protein alpha chaperone and guanine-nucleotide exchange factor RIC-8 regulates cilia morphogenesis in Caenorhabditis elegans sensory neurons

Heterotrimeric G (αβγ) proteins are canonical transducers of G-protein-coupled receptor (GPCR) signaling and play critical roles in communication between cells and their environment. Many GPCRs and heterotrimeric G proteins localize to primary cilia and modulate cilia morphology via mechanisms that are not well understood. Here, we show that RIC-8, a cytosolic guanine nucleotide exchange factor (GEF) and chaperone for Gα protein subunits, shapes cilia membrane morphology in a subset of Caenorhabditis elegans sensory neurons. Consistent with its role in ciliogenesis, C. elegans RIC-8 localizes to cilia in different sensory neuron types. Using domain mutagenesis, we demonstrate that while the GEF function alone is not sufficient, both the GEF and Gα-interacting chaperone motifs of RIC-8 are required for its role in cilia morphogenesis. We identify ODR-3 as the RIC-8 Gα client and demonstrate that RIC-8 functions in the same genetic pathway with another component of the non-canonical G protein signaling AGS-3 to shape cilia morphology. Notably, despite defects in AWC cilia morphology, ags-3 null mutants exhibit normal chemotaxis toward benzaldehyde unlike odr-3 mutant animals. Collectively, our findings describe a novel function for the evolutionarily conserved protein RIC-8 and non-canonical RIC-8-AGS-3-ODR-3 signaling in cilia morphogenesis and uncouple Gα ODR-3 functions in ciliogenesis and olfaction.

Opposing chemosensory functions of closely related gustatory receptors

Most animals have functionally distinct populations of taste cells, expressing receptors that are tuned to compounds of different valence. This organizational feature allows for discrimination between chemicals associated with specific taste modalities and facilitates differentiating between unadulterated foods and foods contaminated with toxic substances. In the fruit fly D. melanogaster , primary sensory neurons express taste receptors that are tuned to distinct groups of chemicals, thereby activating neural ensembles that elicit either feeding or avoidance behavior. Members of a family of ligand gated receptor channels, the Gustatory receptors (Grs), play a central role in these behaviors. In general, closely related, evolutionarily conserved Gr proteins are co-expressed in the same type of taste neurons, tuned to chemically related compounds, and therefore triggering the same behavioral response. Here, we report that members of the Gr28 subfamily are expressed in largely non-overlapping sets of taste neurons in Drosophila larvae, detect chemicals of different valence and trigger opposing feeding behaviors. We determined the intrinsic properties of Gr28 neurons by expressing the mammalian Vanilloid Receptor (VR1), which is activated by capsaicin, a chemical to which wildtype Drosophila larvae do not respond. When VR1 is expressed in Gr28a neurons, larvae become attracted to capsaicin, consistent with reports showing that Gr28a itself encodes a receptor for nutritious RNA. In contrast, expression of VR1 in two pairs of Gr28b.c neurons triggers avoidance to capsaicin. Moreover, neuronal inactivation experiments show that the Gr28b.c neurons are necessary for avoidance of several bitter compounds. Lastly, behavioral experiments of Gr28 deficient larvae and live Ca 2+ imaging studies of Gr28b.c neurons revealed that denatonium benzoate, a synthetic bitter compound that shares structural similarities with natural bitter chemicals, is a ligand for a receptor complex containing a Gr28b.c or Gr28b.a subunit. Thus, the Gr28 proteins, which have been evolutionarily conserved over 260 million years in insects, represent the first taste receptor subfamily in which specific members mediate behavior with opposite valence.

Integration of spatially opposing cues by a single interneuron guides decision-making in C. elegans

The capacity of animals to respond to hazardous stimuli in their surroundings is crucial for their survival. In mammals, complex evaluations of the environment require large numbers and different subtypes of neurons. The nematode C. elegans avoids hazardous chemicals they encounter by reversing their direction of movement. How does the worms' compact nervous system process the spatial information and direct motion change? We show here that a single interneuron, AVA, receives glutamatergic excitatory and inhibitory signals from head and tail sensory neurons, respectively. AVA integrates the spatially distinct and opposing cues, whose output instructs the animal's behavioral decision. We further find that the differential activation of AVA stems from distinct localization of inhibitory and excitatory glutamate-gated receptors along AVA's process and from different threshold sensitivities of the sensory neurons. Our results thus uncover a cellular mechanism that mediates spatial computation of nociceptive cues for efficient decision-making in C. elegans.

The G protein alpha Chaperone and Guanine-Nucleotide Exchange Factor RIC-8 Regulates Cilia Morphogenesis in Caenorhabditis elegans Sensory Neurons

Heterotrimeric G (αβγ) proteins are canonical transducers of G-protein-coupled receptor (GPCR) signaling and play critical roles in communication between cells and their environment. Many GPCRs and heterotrimeric G proteins localize to primary cilia and modulate cilia morphology via mechanisms that are not well understood. Here, we show that RIC-8, a cytosolic guanine nucleotide exchange factor (GEF) and chaperone for Gα protein subunits, shapes cilia membrane morphology in a subset of Caenorhabditis elegans sensory neurons. Consistent with its role in ciliogenesis, C. elegans RIC-8 localizes to cilia in different sensory neuron types. Using domain mutagenesis, we demonstrate that while the GEF function alone is not sufficient, both the GEF and Gα-interacting chaperone motifs of RIC-8 are required for its role in cilia morphogenesis. We identify ODR-3 as the RIC-8 Gα client and demonstrate that RIC-8 functions in the same genetic pathway with another component of the non-canonical G protein signaling AGS-3 to shape cilia morphology. Notably, despite severe defects in AWC cilia morphology, ags-3 null mutants exhibit normal chemotaxis toward benzaldehyde unlike odr-3 mutant animals. Collectively, our findings describe a novel function for the evolutionarily conserved protein RIC-8 and non-canonical RIC-8-AGS-3-ODR-3 signaling in cilia morphogenesis and uncouple Gα ODR-3 functions in ciliogenesis and olfaction.

Forkhead transcription factor FKH-8 cooperates with RFX in the direct regulation of sensory cilia in Caenorhabditis elegans

Cilia, either motile or non-motile (a.k.a primary or sensory), are complex evolutionarily conserved eukaryotic structures composed of hundreds of proteins required for their assembly, structure and function that are collectively known as the ciliome. Ciliome gene mutations underlie a group of pleiotropic genetic diseases known as ciliopathies. Proper cilium function requires the tight coregulation of ciliome gene transcription, which is only fragmentarily understood. RFX transcription factors (TF) have an evolutionarily conserved role in the direct activation of ciliome genes both in motile and non-motile cilia cell-types. In vertebrates, FoxJ1 and FoxN4 Forkhead (FKH) TFs work with RFX in the direct activation of ciliome genes, exclusively in motile cilia cell-types. No additional TFs have been described to act together with RFX in primary cilia cell-types in any organism. Here we describe FKH-8, a FKH TF, as a direct regulator of the sensory ciliome genes in Caenorhabditis elegans. FKH-8 is expressed in all ciliated neurons in C. elegans, binds the regulatory regions of ciliome genes, regulates ciliome gene expression, cilium morphology and a wide range of behaviors mediated by sensory ciliated neurons. FKH-8 and DAF-19 (C. elegans RFX) physically interact and synergistically regulate ciliome gene expression. C. elegans FKH-8 function can be replaced by mouse FOXJ1 and FOXN4 but not by other members of other mouse FKH subfamilies. In conclusion, RFX and FKH TF families act jointly as direct regulators of ciliome genes also in sensory ciliated cell types suggesting that this regulatory logic could be an ancient trait predating functional cilia sub-specialization.

Input density tunes Kenyon cell sensory responses in the Drosophila mushroom body

The ability to discriminate sensory stimuli with overlapping features is thought to arise in brain structures called expansion layers, where neurons carrying information about sensory features make combinatorial connections onto a much larger set of cells. For 50 years, expansion coding has been a prime topic of theoretical neuroscience, which seeks to explain how quantitative parameters of the expansion circuit influence sensory sensitivity, discrimination, and generalization. Here, we investigate the developmental events that produce the quantitative parameters of the arthropod expansion layer, called the mushroom body. Using Drosophila melanogaster as a model, we employ genetic and chemical tools to engineer changes to circuit development. These allow us to produce living animals with hypothesis-driven variations on natural expansion layer wiring parameters. We then test the functional and behavioral consequences. By altering the number of expansion layer neurons (Kenyon cells) and their dendritic complexity, we find that input density, but not cell number, tunes neuronal odor selectivity. Simple odor discrimination behavior is maintained when the Kenyon cell number is reduced and augmented by Kenyon cell number expansion. Animals with increased input density to each Kenyon cell show increased overlap in Kenyon cell odor responses and become worse at odor discrimination tasks.

Multigenerational exposure of cadmium trans-generationally impairs locomotive and chemotactic behaviors in Caenorhabditis elegans

Cadmium is a naturally existing heavy metal and a notorious environmental pollutant. While its toxic outcomes and underlying mechanisms remain largely elusive. To explore the behavioral change caused by multigenerational exposure of cadmium to C. elegans, we challenged the C. elegans with cadmium for six generations and observed its impact on animal behaviors. Wild-type worms were randomly divided into two groups, the control and cadmium exposure groups. Locomotive and chemotactic behaviors were observed across six generations. Head thrashing frequency, chemotaxis index, and fold change index were used to evaluate the neurotoxicity of multigenerational cadmium exposure. Multigenerational cadmium exposure can trans-generationally increase the head thrashing frequency of C. elegans during swimming, and impair the chemotactic behaviors to isoamyl alcohol, diacetyl, and 2-nonanone. Our findings proposed a trans-generationally behavioral impact induced by multigenerational cadmium exposure.

Principles for coding associative memories in a compact neural network

A major goal in neuroscience is to elucidate the principles by which memories are stored in a neural network. Here, we have systematically studied how four types of associative memories (short- and long-term memories, each as positive and negative associations) are encoded within the compact neural network of Caenorhabditis elegans worms. Interestingly, sensory neurons were primarily involved in coding short-term, but not long-term, memories, and individual sensory neurons could be assigned to coding either the conditioned stimulus or the experience valence (or both). Moreover, when considering the collective activity of the sensory neurons, the specific training experiences could be decoded. Interneurons integrated the modulated sensory inputs and a simple linear combination model identified the experience-specific modulated communication routes. The widely distributed memory suggests that integrated network plasticity, rather than changes to individual neurons, underlies the fine behavioral plasticity. This comprehensive study reveals basic memory-coding principles and highlights the central roles of sensory neurons in memory formation.

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.

C. elegans is not a robust model organism for the magnetic sense

Magnetoreception is defined as the ability to sense and use the Earth's magnetic field, for example to orient and direct movements. The receptors and sensory mechanisms underlying behavioral responses to magnetic fields remain unclear. A previous study described magnetoreception in the nematode Caenorhabditis elegans, which requires the activity of a single pair of sensory neurons. These results suggest C. elegans as a tractable model organism for facilitating the search for magnetoreceptors and signaling pathways. The finding is controversial, however, as an attempt to replicate the experiment in a different laboratory was unsuccessful. We here independently test the magnetic sense of C. elegans, closely replicating the assays developed in the original publication. We find that C. elegans show no directional preference in magnetic fields of both natural and higher intensity, suggesting that magnetotactic behavior in the worm is not robustly evoked in a laboratory setting. Given the lack of a robust magnetic response under controlled conditions, we conclude that C. elegans is not a suitable model organism to study the mechanism of the magnetic sense.

Functional imaging and quantification of multineuronal olfactory responses in C. elegans

Many animals perceive odorant molecules by collecting information from ensembles of olfactory neurons, where each neuron uses receptors that are tuned to recognize certain odorant molecules with different binding affinity. Olfactory systems are able, in principle, to detect and discriminate diverse odorants using combinatorial coding strategies. We have combined microfluidics and multineuronal imaging to study the ensemble-level olfactory representations at the sensory periphery of the nematode Caenorhabditis elegans. The collective activity of C. elegans chemosensory neurons reveals high-dimensional representations of olfactory information across a broad space of odorant molecules. We reveal diverse tuning properties and dose-response curves across chemosensory neurons and across odorants. We describe the unique contribution of each sensory neuron to an ensemble-level code for volatile odorants. We show that a natural stimuli, a set of nematode pheromones, are also encoded by the sensory ensemble. The integrated activity of the C. elegans chemosensory neurons contains sufficient information to robustly encode the intensity and identity of diverse chemical stimuli.

Investigation into the communication between unheated and heat-stressed Caenorhabditis elegans via volatile stress signals

Our research group has recently found that radiation-induced airborne stress signals can be used for communication among Caenorhabditis elegans (C. elegans). This paper addresses the question of whether heat stress can also induce the emission of airborne stress signals to alert neighboring C. elegans and elicit their subsequent stress response. Here, we report that heat-stressed C. elegans produces volatile stress signals that trigger an increase in radiation resistance in neighboring unheated C. elegans. When several loss-of-function mutations affecting thermosensory neuron (AFD), heat shock factor-1, HSP-4, and small heat-shock proteins were used to test heat-stressed C. elegans, we found that the production of volatile stress signals was blocked, demonstrating that the heat shock response and ER pathway are involved in controlling the production of volatile stress signals. Our data further indicated that mutations affecting the DNA damage response (DDR) also inhibited the increase in radiation resistance in neighboring unheated C. elegans that might have received volatile stress signals, indicating that the DDR might contribute to radioadaptive responses induction by volatile stress signals. In addition, the regulatory pattern of signal production and action was preliminarily clarified. Together, the results of this study demonstrated that heat-stressed nematodes communicate with unheated nematodes via volatile stress signals.

Characterization of the Doublesex/MAB-3 transcription factor DMD-9 in Caenorhabditis elegans

DMD-9 is a Caenorhabditis elegans Doublesex/MAB-3 Domain transcription factor (TF) of unknown function. Single-cell transcriptomics has revealed that dmd-9 is highly expressed in specific head sensory neurons, with lower levels detected in non-neuronal tissues (uterine cells and sperm). Here, we characterized endogenous dmd-9 expression and function in hermaphrodites and males to identify potential sexually dimorphic roles. In addition, we dissected the trans- and cis-regulatory mechanisms that control DMD-9 expression in neurons. Our results show that of the 22 neuronal cell fate reporters we assessed in DMD-9-expressing neurons, only the neuropeptide-encoding flp-19 gene is cell-autonomously regulated by DMD-9. Further, we did not identify defects in behaviors mediated by DMD-9 expressing neurons in dmd-9 mutants. We found that dmd-9 expression in neurons is regulated by 4 neuronal fate regulatory TFs: ETS-5, EGL-13, CHE-1, and TTX-1. In conclusion, our study characterized the DMD-9 expression pattern and regulatory logic for its control. The lack of detectable phenotypes in dmd-9 mutant animals suggests that other proteins compensate for its loss.

Hacking brain development to test models of sensory coding

Animals can discriminate myriad sensory stimuli but can also generalize from learned experience. You can probably distinguish the favorite teas of your colleagues while still recognizing that all tea pales in comparison to coffee. Tradeoffs between detection, discrimination, and generalization are inherent at every layer of sensory processing. During development, specific quantitative parameters are wired into perceptual circuits and set the playing field on which plasticity mechanisms play out. A primary goal of systems neuroscience is to understand how material properties of a circuit define the logical operations-computations--that it makes, and what good these computations are for survival. A cardinal method in biology-and the mechanism of evolution--is to change a unit or variable within a system and ask how this affects organismal function. Here, we make use of our knowledge of developmental wiring mechanisms to modify hard-wired circuit parameters in the Drosophila melanogaster mushroom body and assess the functional and behavioral consequences. By altering the number of expansion layer neurons (Kenyon cells) and their dendritic complexity, we find that input number, but not cell number, tunes odor selectivity. Simple odor discrimination performance is maintained when Kenyon cell number is reduced and augmented by Kenyon cell expansion.

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

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

Neurophysiological basis of stress-induced aversive memory in the nematode Caenorhabditis elegans

Physiological stress induces aversive memory formation and profoundly impacts animal behavior. In C. elegans, concurrent mitochondrial disruption induces aversion to the bacteria that the animal inherently prefers, offering an experimental paradigm for studying the neural basis of aversive memory. We find that, under mitochondrial stress, octopamine secreted from the RIC modulatory neuron targets the AIY interneuron through the SER-6 receptor to trigger learned bacterial aversion. RIC responds to systemic mitochondrial stress by increasing octopamine synthesis and acts in the formation of aversive memory. AIY integrates sensory information, acts downstream of RIC, and is important for the retrieval of aversive memory. Systemic mitochondrial dysfunction induces RIC responses to bacterial cues that parallel stress induction, suggesting that physiological stress activates latent communication between RIC and the sensory neurons. These findings provide insights into the circuit and neuromodulatory mechanisms underlying stress-induced aversive memory.

Neuron-glia interaction at the receptor level affects olfactory perception in adult Drosophila

Some types of glia play an active role in neuronal signaling by modifying their activity although little is known about their role in sensory information signaling at the receptor level. In this research, we report a functional role for the glia that surround the soma of the olfactory receptor neurons (OSNs) in adult Drosophila. Specific genetic modifications have been targeted to this cell type to obtain live individuals who are tested for olfactory preference and display changes both increasing and reducing sensitivity. A closer look at the antenna by Ca²⁺ imaging shows that odor activates the OSNs, which subsequently produce an opposite and smaller effect in the glia that partially counterbalances neuronal activation. Therefore, these glia may play a dual role in preventing excessive activation of the OSNs at high odorant concentrations and tuning the chemosensory window for the individual according to the network structure in the receptor organ.

A New Hope: A Hermaphroditic Nematode Enables Analysis of a Recent Whole Genome Duplication Event

Whole genome duplication (WGD) is often considered a major driver of evolution that leads to phenotypic novelties. However, the importance of WGD for evolution is still controversial because most documented WGD events occurred anciently and few experimental systems amenable to genetic analysis are available. Here, we report a recent WGD event in the hermaphroditic nematode Allodiplogaster sudhausi and present a comparison with a gonochoristic (male/female) sister species that did not undergo WGD. Self-fertilizing reproduction of A. sudhausi makes it amenable to functional analysis and an ideal system to study WGD events. We document WGD in A. sudhausi through karyotype analysis and whole genome sequencing, the latter of which allowed us to 1) identify functional bias in retention of protein domains and metabolic pathways, 2) show most duplicate genes are under evolutionary constraint, 3) show a link between sequence and expression divergence, and 4) characterize differentially expressed duplicates. We additionally show WGD is associated with increased body size and an abundance of repeat elements (36% of the genome), including a recent expansion of the DNA-hAT/Ac transposon family. Finally, we demonstrate the use of CRISPR/Cas9 to generate mutant knockouts, whereby two WGD-derived duplicate genes display functional redundancy in that they both need to be knocked out to generate a phenotype. Together, we present a novel experimental system that is convenient for examining and characterizing WGD-derived genes both computationally and functionally.

Developmental history modulates adult olfactory behavioral preferences via regulation of chemoreceptor expression in Caenorhabditiselegans

Developmental experiences play critical roles in shaping adult physiology and behavior. We and others previously showed that adult Caenorhabditiselegans which transiently experienced dauer arrest during development (postdauer) exhibit distinct gene expression profiles as compared to control adults which bypassed the dauer stage. In particular, the expression patterns of subsets of chemoreceptor genes are markedly altered in postdauer adults. Whether altered chemoreceptor levels drive behavioral plasticity in postdauer adults is unknown. Here, we show that postdauer adults exhibit enhanced attraction to a panel of food-related attractive volatile odorants including the bacterially produced chemical diacetyl. Diacetyl-evoked responses in the AWA olfactory neuron pair are increased in both dauer larvae and postdauer adults, and we find that these increased responses are correlated with upregulation of the diacetyl receptor ODR-10 in AWA likely via both transcriptional and posttranscriptional mechanisms. We show that transcriptional upregulation of odr-10 expression in dauer larvae is in part mediated by the DAF-16 FOXO transcription factor. Via transcriptional profiling of sorted populations of AWA neurons from control and postdauer animals, we further show that the expression of a subset of additional chemoreceptor genes in AWA is regulated similarly to odr-10 in postdauer animals. Our results suggest that developmental experiences may be encoded at the level of olfactory receptor regulation, and provide a simple mechanism by which C. elegans is able to precisely modulate its behavioral preferences as a function of its current and past experiences.

Multisensory Integration in Caenorhabditis elegans in Comparison to Mammals

Multisensory integration refers to sensory inputs from different sensory modalities being processed simultaneously to produce a unitary output. Surrounded by stimuli from multiple modalities, animals utilize multisensory integration to form a coherent and robust representation of the complex environment. Even though multisensory integration is fundamentally essential for animal life, our understanding of the underlying mechanisms, especially at the molecular, synaptic and circuit levels, remains poorly understood. The study of sensory perception in Caenorhabditis elegans has begun to fill this gap. We have gained a considerable amount of insight into the general principles of sensory neurobiology owing to C. elegans’ highly sensitive perceptions, relatively simple nervous system, ample genetic tools and completely mapped neural connectome. Many interesting paradigms of multisensory integration have been characterized in C. elegans, for which input convergence occurs at the sensory neuron or the interneuron level. In this narrative review, we describe some representative cases of multisensory integration in C. elegans, summarize the underlying mechanisms and compare them with those in mammalian systems. Despite the differences, we believe C. elegans is able to provide unique insights into how processing and integrating multisensory inputs can generate flexible and adaptive behaviors. With the emergence of whole brain imaging, the ability of C. elegans to monitor nearly the entire nervous system may be crucial for understanding the function of the brain as a whole.

Dissection of a sensorimotor circuit underlying pathogen aversion in C. elegans

BACKGROUND

Altering animal behavior to reduce pathogen exposure is a key line of defense against pathogen attack. In Caenorhabditis elegans, alterations in intestinal physiology caused by pathogen colonization and sensation of microbial metabolites may lead to activation of pathogen aversive behaviors ranging from aversive reflexes to learned avoidance. However, the neural circuitry between chemosensory neurons that sense pathogenic bacterial cues and the motor neurons responsible for avoidance-associated locomotion remains unknown.

RESULTS

Using C. elegans, we found that backward locomotion was a component of learned pathogen avoidance, as animals pre-exposed to Pseudomonas aeruginosa or Enterococcus faecalis showed reflexive aversion to drops of the bacteria driven by chemosensory neurons, including the olfactory AWB neurons. This response also involved intestinal distention and, for E. faecalis, required expression of TRPM channels in the intestine and excretory system. Additionally, we uncovered a circuit composed of olfactory neurons, interneurons, and motor neurons that controls the backward locomotion crucial for learned reflexive aversion to pathogenic bacteria, learned avoidance, and the repulsive odor 2-nonanone.

CONCLUSIONS

Using whole-brain simulation and functional assays, we uncovered a novel sensorimotor circuit governing learned reflexive aversion. The discovery of a complete sensorimotor circuit for reflexive aversion demonstrates the utility of using the C. elegans connectome and computational modeling in uncovering new neuronal regulators of behavior.

Positive interaction between ASH and ASK sensory neurons accelerates nociception and inhibits behavioral adaptation

Central and peripheral sensory neurons tightly regulate nociception and avoidance behavior. The peripheral modulation of nociception provides more veridical and instantaneous information for animals to achieve rapid, more fine-tuned and concentrated behavioral responses. In this study, we find that positive interaction between ASH and ASK sensory neurons is essential for the fast-rising phase of ASH Ca²⁺ responses to noxious copper ions and inhibits the adaption of avoiding Cu²⁺. We reveal the underlying neuronal circuit mechanism. ASK accelerates the ASH Ca²⁺ responses by transferring cGMP through gap junctions. ASH excites ASK via a disinhibitory neuronal circuit composed of ASH, AIA, and ASK. Avoidance adaptation depends on the slope rate of the rising phase of ASH Ca²⁺ responses. Thus, in addition to amplitude, sensory kinetics is significant for sensations and behaviors, especially for sensory and behavioral adaptations.