C. elegans locomotory rate is modulated by the environment through a dopaminergic pathway and by experience through a serotonergic pathway

Clarifying the Functional Role of Serotonin in Meloidogyne graminicola Host Plant Parasitism by Immunolocalization and RNA Interference

Serotonin (5-hydroxytryptamine) is an essential neurotransmitter involved in regulating various behaviors in plant-parasitic nematodes, including locomotion, egg laying, feeding, and mating. However, the functional role of serotonin in root-knot nematode invasion of host plants and the molecular mechanisms underlying feeding behavior remain poorly understood. In this study, we tested the effects of exogenous serotonin and the pharmacological compounds fluoxetine and methiothepin on the feeding behaviors of Meloidogyne graminicola. Our results suggested that M. graminicola possesses an endogenous serotonin signaling pathway and that serotonin plays a crucial role in modulating feeding behaviors in M. graminicola second-stage juveniles. We also identified and cloned the serotonin synthesis enzyme tryptophan hydroxylase (Mg-tph-1) in M. graminicola and investigated the role of endogenous serotonin by generating RNA interference nematodes in Mg-tph-1. Silencing Mg-tph-1 substantially reduced nematode invasion, development, and reproduction. According to the immunostaining results, we speculated that these serotonin immunoreactive cells near the nerve ring in M. graminicola are likely homologous to Caenorhabditis elegans ADFs, NSMs, and RIH serotonergic neurons. Furthermore, we investigated the impact of phytoserotonin on nematode invasion and development in rice by overexpressing OsTDC-3 or supplementing rice plants with tryptamine and found that an increase in phytoserotonin increases nematode pathogenicity. Overall, our study provides insights into the essential role of serotonin in M. graminicola host plant parasitism and proposes that the serotonergic signaling pathway could be a potential target for controlling plant-parasitic nematodes.

Photoaged Nanopolystyrene Affects Neurotransmission to Induce Transgenerational Neurotoxicity in Caenorhabditis elegans

Nanopolystyrene (NPS), a frequently employed nanoplastic, is an emerging environmental contaminant known to cause neurotoxicity in various organisms. However, the potential for transgenerational neurotoxic effects, especially from photoaged NPS (P-NPS), remains underexplored. This study investigated the aging of virgin NPS (V-NPS) under a xenon lamp to simulate natural sunlight exposure, which altered the physicochemical characteristics of the NPS. The parental generation (P0) of Caenorhabditis elegans was exposed to environmental concentrations (0.1-100 μg/L) of V-NPS and P-NPS, with subsequent offspring (F1-F4 generations) cultured under NPS-free conditions. Exposure to 100 μg/L P-NPS resulted in more pronounced deterioration in locomotion behavior in the P0 generation compared to V-NPS; this deterioration persisted into the F1-F2 generations but returned to normal in the F3-F4 generations. Additionally, maternal exposure to P-NPS damaged dopaminergic, glutamatergic, and serotonergic neurons in subsequent generations. Correspondingly, there was a significant decrease in the levels of dopamine, glutamate, and serotonin, associated with reduced expression of neurotransmission-related genes dat-1, eat-4, and tph-1 in the P0 and F1-F2 generations. Further analysis showed that the effects of P-NPS on locomotion behavior were absent in subsequent generations of eat-4(ad572), tph-1(mg280), and dat-1(ok157) mutants, highlighting the pivotal roles of these genes in mediating P-NPS-induced transgenerational neurotoxicity. These findings emphasize the crucial role of neurotransmission in the transgenerational effects of P-NPS on locomotion behavior, providing new insights into the environmental risks associated with exposure to photoaged nanoplastics.

Food preference-based screening method for identification of effectors of substance use disorders using Caenorhabditis elegans

Substance use disorder (SUD) affects over 48 million Americans aged 12 and over. Thus, identifying novel chemicals contributing to SUD will be critical for developing efficient prevention and mitigation strategies. Considering the complexity of the actions and effects of these substances on human behavior, a high-throughput platform using a living organism is ideal. We developed a quick and easy screening assay using Caenorhabditis elegans. C. elegans prefers high-quality food (Escherichia coli HB101) over low-quality food (Bacillus megaterium), with a food preference index of approximately 0.2, defined as the difference in the number of worms at E. coli HB101 and B. megaterium over the total worm number. The food preference index was significantly increased by loperamide, a μ-opioid receptor (MOPR) agonist, and decreased by naloxone, a MOPR antagonist. These changes depended on npr-17, a C. elegans homolog of opioid receptors. In addition, the food preference index was significantly increased by arachidonyl-2'-chloroethylamide, a cannabinoid 1 receptor (CB1R) agonist, and decreased by rimonabant, a CB1R inverse agonist. These changes depended on npr-19, a homolog of CB1R. These results suggest that the conserved opioid and endocannabinoid systems modulate the food preference behaviors of C. elegans. Finally, the humanoid C. elegans strains where npr-17 was replaced with human MOPR and where npr-19 was replaced with human CB1R phenocopied the changes in food preference by the drug treatment. Together, the current results show that this method can be used to rapidly screen the potential effectors of MOPR and CB1R to yield results highly translatable to humans.

Synthesis and Dopaminergic Neuroprotective Activity of the Proposed Structure of Bassiamide A, a Caffeamide Alkaloid Derived from Bassia Indica Wight

Herein, we describe the synthesis of the proposed structure of the caffeamide alkaloid bassiamide A. The amide moiety of bassiamide A was readily formed via an amide coupling reaction between caffeic acid and the known N-(3-aminopropyl)-3-methylbutanamide. However, the spectral data of the synthesized bassiamide A did not agree with that of a previous study. The structure of the synthesized bassiamide A was confirmed using combined two-dimensional NMR analysis. Extended analyses of the bioactivity of the synthesized bassiamide A revealed its efficacy in protecting dopaminergic neurons from MPP+-induced neurotoxicity in Caenorhabditis elegans. Additionally, treatment with bassiamide A notably ameliorated the impaired food-sensing ability and locomotion of Caenorhabditis elegans, suggesting a protective effect on the functionality of dopaminergic neurons.

C. elegans: a prominent platform for modeling and drug screening in neurological disorders

INTRODUCTION

Human neurodevelopmental and neurodegenerative diseases (NDevDs and NDegDs, respectively) encompass a broad spectrum of disorders affecting the nervous system with an increasing incidence. In this context, the nematode C. elegans, has emerged as a benchmark model for biological research, especially in the field of neuroscience.

AREAS COVERED

The authors highlight the numerous advantages of this tiny worm as a model for exploring nervous system pathologies and as a platform for drug discovery. There is a particular focus given to describing the existing models of C. elegans for the study of NDevDs and NDegDs. Specifically, the authors underscore their strong applicability in preclinical drug development. Furthermore, they place particular emphasis on detailing the common techniques employed to explore the nervous system in both healthy and diseased states.

EXPERT OPINION

Drug discovery constitutes a long and expensive process. The incorporation of invertebrate models, such as C. elegans, stands as an exemplary strategy for mitigating costs and expediting timelines. The utilization of C. elegans as a platform to replicate nervous system pathologies and conduct high-throughput automated assays in the initial phases of drug discovery is pivotal for rendering therapeutic options more attainable and cost-effective.

Long-term exposure to 6-PPD quinone at environmentally relevant concentrations causes neurotoxicity by affecting dopaminergic, serotonergic, glutamatergic, and GABAergic neuronal systems in Caenorhabditis elegans

6-PPD quinone (6-PPDQ), an emerging environmental pollutant, is converted based on 6-PPD via ozonation. However, a systematic evaluation on possible neurotoxicity of long-term and low-dose 6-PPDQ exposure and the underlying mechanism remain unknown. In the present work, 0.1-10 μg/L 6-PPDQ was added to treat Caenorhabditis elegans for 4.5 days, with locomotion behavior, neuronal development, sensory perception behavior, neurotransmitter content, and levels of neurotransmission-related genes being the endpoints. 6-PPDQ exposure at 0.1-10 μg/L significantly reduced locomotion behavior, and that at 1-10 μg/L decreased sensory perception behavior in nematodes. Moreover, 6-PPDQ exposure at 10 μg/L notably induced damage to the development of dopaminergic, glutamatergic, serotonergic, and GABAergic neurons. Importantly, nematodes with chronic 6-PPDQ exposure at 10 μg/L were confirmed to suffer obviously decreased dopamine, serotonin, glutamate, dopamine, and GABA contents and altered neurotransmission-related gene expression. Meanwhile, the potential binding sites of 6-PPDQ and neurotransmitter synthesis-related proteins were further shown by molecular docking method. Lastly, Pearson's correlation analysis showed that locomotion behavior and sensory perception behavior were positively correlated with the dopaminergic, serotonergic, glutamatergic, and GABAergic neurotransmission. Consequently, 6-PPDQ exposure disturbed neurotransmitter transmission, while such changed molecular foundation for neurotransmitter transmission was related to 6-PPDQ toxicity induction. The present work sheds new lights on the mechanisms of 6-PPDQ and its possible neurotoxicity to organisms at environmentally relevant concentrations.

Mitochondrial energy state controls AMPK-mediated foraging behavior in C. elegans

Organisms surveil and respond to their environment using behaviors entrained by metabolic cues that reflect food availability. Mitochondria act as metabolic hubs and at the center of mitochondrial energy production is the protonmotive force (PMF), an electrochemical gradient generated by metabolite consumption. The PMF serves as a central integrator of mitochondrial status, but its role in governing metabolic signaling is poorly understood. We used optogenetics to dissipate the PMF in Caenorhabditis elegans tissues to test its role in food-related behaviors. Our data demonstrate that PMF reduction in the intestine is sufficient to initiate locomotor responses to acute food deprivation. This behavioral adaptation requires the cellular energy regulator AMP-activated protein kinase (AMPK) in neurons, not in the intestine, and relies on mitochondrial dynamics and axonal trafficking. Our results highlight a role for intestinal PMF as an internal metabolic cue, and we identify a bottom-up signaling axis through which changes in the PMF trigger AMPK activity in neurons to promote foraging behavior.

SKN-1 is indispensable for protection against Aβ-induced proteotoxicity by a selenopeptide derived from Cordyceps militaris

Oxidative stress (OS) and disruption of proteostasis caused by aggregated proteins are the primary causes of cell death in various diseases. Selenopeptides have shown the potential to control OS and alleviate inflammatory damage, suggesting promising therapeutic applications. However, their potential function in inhibiting proteotoxicity is not yet fully understood. To address this gap in knowledge, this study aimed to investigate the effects and underlying mechanisms of the selenopeptide VPRKL(Se)M on amyloid β protein (Aβ) toxicity in transgenic Caenorhabditis elegans. The results revealed that supplementation with VPRKL(Se)M can alleviate Aβ-induced toxic effects in the transgenic C. elegans model. Moreover, the addition of VPRKL(Se)M inhibited the Aβ aggregates formation, reduced the reactive oxygen species (ROS) levels, and ameliorated the overall proteostasis. Importantly, we found that the inhibitory effects of VPRKL(Se)M on Aβ toxicity and activation of the unfolded protein are dependent on skinhead-1 (SKN-1). These findings suggested that VPRKL(Se)M is a potential bioactive agent for modulating SKN-1, which subsequently improves proteostasis and reduces OS. Collectively, the findings from the current study suggests VPRKL(Se)M may play a critical role in preventing protein disorder and related diseases.

Polyethylene nanoparticles at environmentally relevant concentrations enhances neurotoxicity and accumulation of 6-PPD quinone in Caenorhabditis elegans

The exposure risk of 6-PPD quinone (6-PPDQ) has aroused increasing concern. In the natural environment, 6-PPDQ could interact with other pollutants, posing more severe environmental problems and toxicity to organisms. We here examined the effect of polyethylene nanoplastic (PE-NP) on 6-PPDQ neurotoxicity and the underling mechanisms in Caenorhabditis elegans. In nematodes, PE-NP (1 and 10 μg/L) decreased locomotion behavior, but did not affect development of D-type neurons. Exposure to PE-NP (1 and 10 μg/L) strengthened neurotoxicity of 6-PPDQ (10 μg/L) on the aspect of locomotion and neurodegeneration induction of D-type motor neurons. Exposure to PE-NPs (10 μg/L) caused increase in expressions of mec-4, asp-3, and asp-4 governing neurodegeneration in 10 μg/L 6-PPDQ exposed nematodes. Moreover, exposure to PE-NP (10 μg/L) increased expression of some neuronal genes (daf-7, dbl-1, jnk-1, and mpk-1) in 6-PPDQ exposed nematodes, and RNAi of these genes resulted in susceptibility to neurotoxicity of PE-NP and 6-PPDQ. 6-PPDQ could be adsorbed by PE-NPs, and resuspension of PE-NP and 6-PPDQ after adsorption equilibrium exhibited similar neurotoxicity to co-exposure of PE-NP and 6-PPDQ. In addition, exposure to PE-NP (1 and 10 μg/L) increased 6-PPDQ accumulation in body of nematodes and increased defecation cycle length in 6-PPDQ exposed nematodes. Therefore, 6-PPDQ could be adsorbed on nanoplastics (such as PE-NPs) and enhance both neurotoxicity and accumulation of 6-PPDQ in organisms.

Assessing locomotory rate in response to food for the identification of neuronal and muscular defects in C. elegans

C. elegans is a bacteria-eating soil-dwelling nematode. Typical cultivation of laboratory-reared populations occurs on bacteria-covered solid media, where they move along with sinusoidal undulations. Nematodes decelerate when they encounter food. Dopaminergic and serotonergic neurotransmission regulate this behavior. Here, we describe the procedure for determining food-dependent locomotion rate of fed and fasting nematodes. We detail steps for assay plate preparation, C. elegans synchronization, and assessment of locomotion. The behaviors we describe provide information regarding the animal's physiological neuronal and muscular function. For complete details on the use and execution of this protocol, please refer to Petratou et al. (2023)1 and Sawin et al. (2000).2.

Expression of human Ras-related protein Rab39B variant T168K in Caenorhabditis elegans leads to motor dysfunction and dopaminergic neuron degeneration

Human RAB39B gene is related to familial early-onset Parkinson disease. In early adulthood, men with the RAB39B c.503C > A (Thr168Lys, p. T168K) mutation develop typical tremor, bradykinesia, and alpha-synuclein accumulation. We investigated the pathological mechanism of RAB39B T168K in a Caenorhabditis elegans model. In early adult C. elegans, RAB39B T168K led to dopaminergic neuron degeneration that presented as disrupted dendrites and blunt neuronal cells. Abnormal dopamine secretion was inferred from a decline in motor function and a positive basal slowing phenotype. Dopamine-associated tests confirmed that synthesis and recycling of dopamine were normal. The RAB39B T168K mutation might impair dopamine vesicular transmission from the presynaptic membrane to the synaptic gap in dopaminergic neurons. The release-dependent feedback mechanism in neurotransmitters regulates the balance of receptor activities. Protein-protein interactions network analysis revealed that RAB39B may also function in lysosomal degradation and autophagy. Impaired disposal of misfolded α-synuclein eventually leads to protein aggregation. Thus, like other members of the Rab family, RAB39B may be involved in vesicular transport associated with dopamine secretion and α-synuclein clearance.

A Necessary Role for PKC-2 and TPA-1 in Olfactory Memory and Synaptic AMPAR Trafficking in Caenorhabditis elegans

Protein kinase C (PKC) functions are essential for synaptic plasticity, learning, and memory. However, the roles of specific members of the PKC family in synaptic function, learning, and memory are poorly understood. Here, we investigated the role of individual PKC homologs for synaptic plasticity in Caenorhabditis elegans and found a differential role for pkc-2 and tpa-1, but not pkc-1 and pkc-3 in associative olfactory learning and memory. More specifically we show that PKC-2 is essential for associative learning and TPA-1 for short-term associative memory (STAM). Using endogenous labeling and cell-specific rescues, we show that TPA-1 and PKC-2 are required in AVA for their functions. Previous studies demonstrated that olfactory learning and memory in C. elegans are tied to proper synaptic content and trafficking of AMPA-type ionotropic glutamate receptor homolog GLR-1 in the AVA command interneurons. Therefore, we quantified synaptic content, transport, and delivery of GLR-1 in AVA and showed that loss of pkc-2 and tpa-1 leads to decreased transport and delivery but only a subtle decrease in GLR-1 levels at synapses. AVA-specific expression of both PKC-2 and TPA-1 rescued these defects. Finally, genetic epistasis showed that PKC-2 and TPA-1 likely act in the same pathway to control GLR-1 transport and delivery, while regulating different aspects of olfactory learning and STAM. Thus, our data tie together cell-specific functions of 2 PKCs to neuronal and behavioral outcomes in C. elegans, enabling comparative approaches to understand the evolutionarily conserved role of PKC in synaptic plasticity, learning, and memory.

Outcrossing in Caenorhabditis elegans increases in response to food limitation

Theory predicts that organisms should diversify their offspring when faced with a stressful environment. This prediction has received empirical support across diverse groups of organisms and stressors. For example, when encountered by Caenorhabditis elegans during early development, food limitation (a common environmental stressor) induces the nematodes to arrest in a developmental stage called dauer and to increase their propensity to outcross when they are subsequently provided with food and enabled to develop to maturity. Here we tested whether food limitation first encountered during late development/early adulthood can also induce increased outcrossing propensity in C. elegans. Previously well-fed C. elegans increased their propensity to outcross when challenged with food limitation during the final larval stage of development and into early adulthood, relative to continuously well-fed (control) nematodes. Our results thus support previous research demonstrating that the stress of food limitation can induce increased outcrossing propensity in C. elegans. Furthermore, our results expand on previous work by showing that food limitation can still increase outcrossing propensity even when it is not encountered until late development, and this can occur independently of the developmental and gene expression changes associated with dauer.

Automated multimodal imaging of Caenorhabditis elegans behavior in multi-well plates

Large-scale assays of behavior in model organisms play an important role in genetic screens, drug testing, and the elucidation of gene-behavior relationships. We have developed an automated, high-throughput imaging and analysis method for assaying behaviors of the nematode C. elegans . We use high-resolution optical imaging to longitudinally record the behaviors of 96 animals at a time in multi-well plates, and computer vision software to quantify the animals' locomotor activity, behavioral states, and egg laying events. To demonstrate the capabilities of our system we used it to examine the role of serotonin in C. elegans behavior. We found that egg-laying events are preceded by a period of reduced locomotion, and that this decline in movement requires serotonin signaling. In addition, we identified novel roles of serotonin receptors SER-1 and SER-7 in regulating the effects of serotonin on egg laying across roaming, dwelling, and quiescent locomotor states. Our system will be useful for performing genetic or chemical screens for modulators of behavior.

Serotonin deficiency from constitutive SKN-1 activation drives pathogen apathy

When an organism encounters a pathogen, the host innate immune system activates to defend against pathogen colonization and toxic xenobiotics produced. C. elegans employ multiple defense systems to ensure survival when exposed to Pseudomonas aeruginosa including activation of the cytoprotective transcription factor SKN-1/NRF2. Although wildtype C. elegans quickly learn to avoid pathogens, here we describe a peculiar apathy-like behavior towards PA14 in animals with constitutive activation of SKN-1, whereby animals choose not to leave and continue to feed on the pathogen even when a non-pathogenic and healthspan-promoting food option is available. Although lacking the urgency to escape the infectious environment, animals with constitutive SKN-1 activity are not oblivious to the presence of the pathogen and display the typical pathogen-induced intestinal distension and eventual demise. SKN-1 activation, specifically in neurons and intestinal tissues, orchestrates a unique transcriptional program which leads to defects in serotonin signaling that is required from both neurons and non-neuronal tissues. Serotonin depletion from SKN-1 activation limits pathogen defense capacity, drives the pathogen-associated apathy behaviors and induces a synthetic sensitivity to selective serotonin reuptake inhibitors. Taken together, our work reveals new insights into how animals perceive environmental pathogens and subsequently alter behavior and cellular programs to promote survival.

KEY POINTS

Identify an apathy-like behavioral response for pathogens resulting from the constitutive activation of the cytoprotective transcription factor SKN-1.Uncover the obligate role for serotonin synthesis in both neuronal and non-neuronal cells for the apathy-like state and ability of serotonin treatment to restore normal behaviors.Characterize the timing and tissue specificity of SKN-1 nuclear localization in neurons and intestinal cells in response to pathogen exposure.Define the unique and context-specific transcriptional signatures of animals with constitutive SKN-1 activation when exposed to pathogenic environments.Reveal necessity for both neuronal and non-neuronal serotonin signaling in host survival from pathogen infection.

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.

Neuroendocrine gene expression coupling of interoceptive bacterial food cues to foraging behavior of C. elegans

Animal internal state is modulated by nutrient intake, resulting in behavioral responses to changing food conditions. The neural mechanisms by which internal states are generated and maintained are not well understood. Here, we show that in the nematode Caenorhabditis elegans, distinct cues from bacterial food - interoceptive signals from the ingestion of bacteria and gustatory molecules sensed from nearby bacteria - act antagonistically on the expression of the neuroendocrine TGF-beta ligand DAF-7 from the ASJ pair of sensory neurons to modulate foraging behavior. A positive-feedback loop dependent on the expression of daf-7 from the ASJ neurons acts to promote transitions between roaming and dwelling foraging states and influence the persistence of roaming states. SCD-2, the C. elegans ortholog of mammalian anaplastic lymphoma kinase (ALK), which has been implicated in the central control of metabolism of mammals, functions in the AIA interneurons to regulate foraging behavior and cell-non-autonomously control the expression of DAF-7 from the ASJ neurons. Our data establish how a dynamic neuroendocrine daf-7 expression feedback loop regulated by SCD-2 functions to couple sensing and ingestion of bacterial food to foraging behavior. We further suggest that this neuroendocrine feedback loop underlies previously characterized exploratory behaviors in C. elegans. Our data suggest that the expression of daf-7 from the ASJ neurons contributes to and is correlated with an internal state of 'unmet need' that regulates exploratory foraging behavior in response to bacterial cues in diverse physiological contexts.

Silica Nanoparticles Disclose a Detailed Neurodegeneration Profile throughout the Life Span of a Model Organism

The incidence of age-related neurodegenerative diseases is rising globally. However, the temporal sequence of neurodegeneration throughout adult life is poorly understood. To identify the starting points and schedule of neurodegenerative events, serotonergic and dopaminergic neurons were monitored in the model organism C. elegans, which has a life span of 2-3 weeks. Neural morphology was examined from young to old nematodes that were exposed to silica nanoparticles. Young nematodes showed phenotypes such as dendritic beading of serotonergic and dopaminergic neurons that are normally not seen until late life. During aging, neurodegeneration spreads from specifically susceptible ADF and PDE neurons in young C. elegans to other more resilient neurons, such as dopaminergic CEP in middle-aged worms. Investigation of neurodegenerative hallmarks and animal behavior revealed a temporal correlation with the acceleration of neuromuscular defects, such as internal hatch in 2-day-old C. elegans. Transcriptomics and proteomics of young worms exposed to nano silica showed a change in gene expression concerning the gene ontology groups serotonergic and dopaminergic signaling as well as neuropeptide signaling. Consistent with this, reporter strains for nlp-3, nlp-14 and nlp-21 confirmed premature degeneration of the serotonergic neuron HSN and other neurons in young C. elegans. The results identify young nematodes as a vulnerable age group for nano silica-induced neural defects with a significantly reduced health span. Neurodegeneration of specific neurons impairs signaling by classical neurotransmitters as well as neuropeptides and compromises related neuromuscular behaviors in critical phases of life, such as the reproductive phase.

Neural mechanisms of dopamine function in learning and memory in Caenorhabditis elegans

Research into learning and memory over the past decades has revealed key neurotransmitters that regulate these processes, many of which are evolutionarily conserved across diverse species. The monoamine neurotransmitter dopamine is one example of this, with countless studies demonstrating its importance in regulating behavioural plasticity. However, dopaminergic neural networks in the mammalian brain consist of hundreds or thousands of neurons, and thus cannot be studied at the level of single neurons acting within defined neural circuits. The nematode Caenorhabditis elegans (C. elegans) has an experimentally tractable nervous system with a completely characterized synaptic connectome. This makes it an advantageous system to undertake mechanistic studies into how dopamine encodes lasting yet flexible behavioural plasticity in the nervous system. In this review, we synthesize the research to date exploring the importance of dopaminergic signalling in learning, memory formation, and forgetting, focusing on research in C. elegans. We also explore the potential for dopamine-specific fluorescent biosensors in C. elegans to visualize dopaminergic neural circuits during learning and memory formation in real-time. We propose that the use of these sensors in C. elegans, in combination with optogenetic and other light-based approaches, will further illuminate the detailed spatiotemporal requirements for encoding behavioural plasticity in an accessible experimental system. Understanding the key molecules and circuit mechanisms that regulate learning and forgetting in more compact invertebrate nervous systems may reveal new druggable targets for enhancing memory storage and delaying memory loss in bigger brains.

Morphological hallmarks of dopaminergic neurodegeneration are associated with altered neuron function in Caenorhabditis elegans

Caenorhabditis elegans (C. elegans) is an excellent model system to study neurodegenerative diseases, such as Parkinson's disease, as it enables analysis of both neuron morphology and function in live animals. Multiple structural changes in neurons, such as cephalic dendrite morphological abnormalities, have been considered hallmarks of neurodegeneration in this model, but their relevance to changes in neuron function are not entirely clear. We sought to test whether hallmark morphological changes associated with chemically induced dopaminergic neuron degeneration, such as dendrite blebbing, breakage, and loss, are indicative of neuronal malfunction and result in changes in behavior. We adapted an established dopaminergic neuronal function assay by measuring paralysis in the presence of exogenous dopamine, which revealed clear differences between cat-2 dopamine deficient mutants, wildtype worms, and dat-1 dopamine abundant mutants. Next, we integrated an automated image processing algorithm and a microfluidic device to segregate worm populations by their cephalic dendrite morphologies. We show that nematodes with dopaminergic dendrite degeneration markers, such as blebbing or breakage, paralyze at higher rates in a dopamine solution, providing evidence that dopaminergic neurodegeneration morphologies are correlated with functional neuronal outputs.

Comprehensive Neurotoxicity of Lead Halide Perovskite Nanocrystals in Nematode Caenorhabditis elegans

To date, all-inorganic lead halide perovskite quantum dots have emerged as promising materials for photonic, optoelectronic devices, and biological applications, especially in solar cells, raising numerous concerns about their biosafety. Most of the studies related to the toxicity of perovskite quantum dots (PeQDs) have focused on the potential risks of hybrid perovskites by using zebrafish or human cells. So far, the neurotoxic effects and fundamental mechanisms of PeQDs remain unknown. Herein, a comprehensive methodology is designed to investigate the neurotoxicity of PeQDs by using Caenorhabditis elegans as a model organism. The results show that the accumulation of PeQDs mainly focuses on the alimentary system and head region. Acute exposure to PeQDs results in a decrease in locomotor behaviors and pharyngeal pumping, whereas chronic exposure to PeQDs causes brood decline and shortens lifespan. In addition, some abnormal issues occur in the uterus during reproduction assays, such as vulva protrusion, impaired eggs left in the vulva, and egg hatching inside the mother. Excessive reactive oxygen species formation is also observed. The neurotoxicity of PeQDs is explained by gene expression. This study provides a complete insight into the neurotoxicity of PeQD and encourages the development of novel nontoxic PeQDs.

Sensory neurons couple arousal and foraging decisions in Caenorhabditis elegans

Foraging animals optimize feeding decisions by adjusting both common and rare behavioral patterns. Here, we characterize the relationship between an animal's arousal state and a rare decision to leave a patch of bacterial food. Using long-term tracking and behavioral state classification, we find that food leaving decisions in Caenorhabditis elegans are coupled to arousal states across multiple timescales. Leaving emerges probabilistically over minutes from the high arousal roaming state, but is suppressed during the low arousal dwelling state. Immediately before leaving, animals have a brief acceleration in speed that appears as a characteristic signature of this behavioral motif. Neuromodulatory mutants and optogenetic manipulations that increase roaming have a coupled increase in leaving rates, and similarly acute manipulations that inhibit feeding induce both roaming and leaving. By contrast, inactivating a set of chemosensory neurons that depend on the cGMP-gated transduction channel TAX-4 uncouples roaming and leaving dynamics. In addition, tax-4-expressing sensory neurons promote lawn-leaving behaviors that are elicited by feeding inhibition. Our results indicate that sensory neurons responsive to both internal and external cues play an integrative role in arousal and foraging decisions.

Towards improved screening of toxins for Parkinson's risk

Parkinson's disease (PD) is a chronic, progressive and disabling neurodegenerative disorder. The prevalence of PD has risen considerably over the past decades. A growing body of evidence suggest that exposure to environmental toxins, including pesticides, solvents and heavy metals (collectively called toxins), is at least in part responsible for this rapid growth. It is worrying that the current screening procedures being applied internationally to test for possible neurotoxicity of specific compounds offer inadequate insights into the risk of developing PD in humans. Improved screening procedures are therefore urgently needed. Our review first substantiates current evidence on the relation between exposure to environmental toxins and the risk of developing PD. We subsequently propose to replace the current standard toxin screening by a well-controlled multi-tier toxin screening involving the following steps: in silico studies (tier 1) followed by in vitro tests (tier 2), aiming to prioritize agents with human relevant routes of exposure. More in depth studies can be undertaken in tier 3, with whole-organism (in)vertebrate models. Tier 4 has a dedicated focus on cell loss in the substantia nigra and on the presumed mechanisms of neurotoxicity in rodent models, which are required to confirm or refute the possible neurotoxicity of any individual compound. This improved screening procedure should not only evaluate new pesticides that seek access to the market, but also critically assess all pesticides that are being used today, acknowledging that none of these has ever been proven to be safe from a perspective of PD. Importantly, the improved screening procedures should not just assess the neurotoxic risk of isolated compounds, but should also specifically look at the cumulative risk conveyed by exposure to commonly used combinations of pesticides (cocktails). The worldwide implementation of such an improved screening procedure, would be an essential step for policy makers and governments to recognize PD-related environmental risk factors.

Dissecting the Neuronal Contributions of the Lipid Regulator NHR-49 Function in Lifespan and Behavior in C. elegans

Although the importance of lipid homeostasis in neuronal function is undisputed, how they are regulated within neurons to support their unique function is an area of active study. NHR-49 is a nuclear hormone receptor functionally similar to PPARα, and a major lipid regulator in C. elegans. Although expressed in most tissues, little is known about its roles outside the intestine, the main metabolic organ of C. elegans. Here, using tissue- and neuron-type-specific transgenic strains, we examined the contribution of neuronal NHR-49 to cell-autonomous and non-autonomous nhr-49 mutant phenotypes. We examined lifespan, brood size, early egg-laying, and reduced locomotion on food. We found that lifespan and brood size could be rescued by neuronal NHR-49, and that NHR-49 in cholinergic and serotonergic neurons is sufficient to restore lifespan. For behavioral phenotypes, NHR-49 in serotonergic neurons was sufficient to control egg-laying, whereas no single tissue or neuron type was able to rescue the enhanced on-food slowing behavior. Our study shows that NHR-49 can function in single neuron types to regulate C. elegans physiology and behavior, and provides a platform to further investigate how lipid metabolism in neurons impact neuronal function and overall health of the organism.

TRPM2 as a conserved gatekeeper determines the vulnerability of DA neurons by mediating ROS sensing and calcium dyshomeostasis

Different dopaminergic (DA) neuronal subgroups exhibit distinct vulnerability to stress, while the underlying mechanisms are elusive. Here we report that the transient receptor potential melastatin 2 (TRPM2) channel is preferentially expressed in vulnerable DA neuronal subgroups, which correlates positively with aging in Parkinson's Disease (PD) patients. Overexpression of human TRPM2 in the DA neurons of C. elegans resulted in selective death of ADE but not CEP neurons in aged worms. Mechanistically, TRPM2 activation mediates FZO-1/CED-9-dependent mitochondrial hyperfusion and mitochondrial permeability transition (MPT), leading to ADE death. In mice, TRPM2 knockout reduced vulnerable substantia nigra pars compacta (SNc) DA neuronal death induced by stress. Moreover, the TRPM2-mediated vulnerable DA neuronal death pathway is conserved from C. elegans to toxin-treated mice model and PD patient iPSC-derived DA neurons. The vulnerable SNc DA neuronal loss is the major symptom and cause of PD, and therefore the TRPM2-mediated pathway serves as a promising therapeutic target against PD.

Photoaged microplastics induce neurotoxicity associated with damage to serotonergic, glutamatergic, dopaminergic, and GABAergic neuronal systems in Caenorhabditis elegans

Microplastics (MPs) are ubiquitous environmental contaminants that cause neurotoxicity in various organisms. MPs are typically affected by light irradiation and undergo photoaging. However, the neurotoxic effects of photoaged polystyrene (P-PS) and its underlying mechanisms remain unclear. In this study, locomotion behaviors, neuronal development, neurotransmitter levels, and the expression of neurotransmission-related genes were investigated in Caenorhabditis elegans exposed to P-PS at environment-relevant concentrations (0.1-100 μg/L). The characterization results showed that photoaging accelerated the aging process and changed the physicochemical properties of the MPs. The toxicity results suggested that exposure to 1-100 μg/L P-PS caused more severe neurotoxicity than virgin polystyrene (V-PS) with endpoints of head thrashes, body bends, wavelength, and mean amplitude. Exposure to P-PS also altered the fluorescence intensity and neurodegeneration percentage of serotonergic, glutamatergic, dopaminergic, and aminobutyric acid (GABA) in transgenic nematodes. Similarly, significant reductions in the levels of these neurotransmitters were also observed. Based on Pearson's correlation, locomotion behaviors were negatively correlated with the neurotransmission of serotonin, glutamate, dopamine, and GABA. Further investigation suggested that the expression of neurotransmitter-related genes (e.g., tph-1, eat-4, and unc-46) was significantly altered in the nematodes. Collectively, the neurotoxic effects of P-PS were attributed to abnormal neurotransmission. This study highlights the potential toxicity of MPs photoaged under environmentally relevant conditions.

Simira cordifolia protects against metal induced-toxicity in Caenorhabditis elegans

Simira cordifolia (Hook.f.) Steyerm (Rubiaceae) is a vascular plant used in Northern Colombia as a source of pigments and wood. However, there is a lack of information regarding its pharmacology and toxicity. This research aimed to study the hydroalcoholic extract of Simira cordifolia as a protector against metal-induced toxicity in Caenorhabditis elegans. Preliminary phytochemical screening of the hydroalcoholic extract of S. cordifolia (HAE-Sc) was conducted using HPLC-ESI-QTOF. Wild-type N2 C. elegans larvae were exposed to different concentrations of HAE-Sc evaluating lethality (50-5000 μg/mL), growth, lifespan, resistance to heat stress, and its protective effect against Mercury (Hg)-, Lead (Pb)- and Cadmium (Cd)-induced lethality (50-1000 μg/mL). The main metabolites present in the extract were iridoids, β-carboline-alkaloids and polyphenols. Bioassays demonstrated that HAE-Sc exhibited low toxicity, with significant lethality (4.2% and 9.4%) occurring at 2500-5000 μg/mL. Growth inhibition reached up to 23.3%, while reproduction declined 13% and 17% at concentrations 500 and 1000 μg/mL, respectively. HAE-Sc enhanced the survival rate of the nematode under thermal stress by up to 79.8%, and extended the mean lifespan of worms by over 33% compared to control. The average lifespan was prolonged by 15.3% and 18.5% at 50 and 100 μg/mL HAE-Sc, respectively. The extract (1000 μg/mL) was able to reduce the death of C. elegans in the presence of heavy metals up to 65.9, 96.8% and 87% for Pb, Hg, and Cd, respectively. In summary, S. cordifolia shows potential protective effects in C. elegans against toxicity caused by heavy metals and heat.

Disruption of dopamine metabolism by exposure to 6-PPD quinone in Caenorhabditis elegans

Caenorhabditis elegans is a useful model for examining metabolic processes and related mechanisms. We here examined the effect of exposure to N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ) on dopamine metabolism and underling molecular basis in nematodes. The dopamine content was reduced by 6-PPDQ (1 and 10 μg/L). Meanwhile, dopamine related behaviors (basal slowing response and area restricted searching) were changed by 6-PPDQ (1 and 10 μg/L). Exposure to 6-PPDQ (1 and 10 μg/L) decreased expressions of genes (cat-2 and bas-1) encoding enzymes governing dopamine synthesis and cat-1 encoding dopamine transporter. Development of dopaminergic neurons was also affected by 10 μg/L 6-PPDQ as reflected by decrease in fluorescence intensity, neuronal loss, and defect in dendrite development. Exposure to 6-PPDQ (1 and 10 μg/L) altered expressions of ast-1 and rcat-1 encoding upregulators of cat-2 and bas-1. The dopamine content and expressions of cat-2 and bas-1 were inhibited by RNAi of ast-1 and increased by RNAi of rcat-1 in 6-PPDQ exposed nematodes. Using endpoints of locomotion behavior and brood size, in 6-PPDQ exposed nematodes, the susceptibility to toxicity was caused by RNAi of ast-1, cat-2, bas-1, and cat-1, and the resistance to toxicity was induced by RNAi of rcat-1. Therefore, 6-PPDQ exposure disrupted dopamine metabolism and the altered molecular basis for dopamine metabolism was associated with 6-PPDQ toxicity induction. Moreover, the defects in dopamine related behaviors and toxicity on locomotion and reproduction could be rescued by treatment with 0.1 mM dopamine.

Neuroendocrine Gene Expression Coupling of Interoceptive Bacterial Food Cues to Foraging Behavior of C. elegans

Animal internal state is modulated by nutrient intake, resulting in behavioral responses to changing food conditions. The neural mechanisms by which internal states are generated and maintained are not well understood. Here, we show that in the nematode Caenorhabditis elegans, distinct cues from bacterial food - interoceptive signals from the ingestion of bacteria and gustatory molecules sensed from nearby bacteria - act antagonistically on the expression of the neuroendocrine TGF-beta ligand DAF-7 from the ASJ pair of sensory neurons to modulate foraging behavior. A positive-feedback loop dependent on the expression of daf-7 from the ASJ neurons acts to promote transitions between roaming and dwelling foraging states and influence the persistence of roaming states. SCD-2, the C. elegans ortholog of mammalian Anaplastic Lymphoma Kinase (ALK), which has been implicated in the central control of metabolism of mammals, functions in the AIA interneurons to regulate foraging behavior and cell-non-autonomously control the expression of DAF-7 from the ASJ neurons. Our data establish how a dynamic neuroendocrine daf-7 expression feedback loop regulated by SCD-2 functions to couple sensing and ingestion of bacterial food to foraging behavior. We further suggest that this neuroendocrine feedback loop underlies previously characterized exploratory behaviors in C. elegans. Our data suggest that the expression of daf-7 from the ASJ neurons contributes to and is correlated with an internal state of "unmet need" that regulates exploratory foraging behavior in response to bacterial cues in diverse physiological contexts.

A single neuron in C. elegans orchestrates multiple motor outputs through parallel modes of transmission

Animals generate a wide range of highly coordinated motor outputs, which allows them to execute purposeful behaviors. Individual neurons in the circuits that generate behaviors have a remarkable capacity for flexibility as they exhibit multiple axonal projections, transmitter systems, and modes of neural activity. How these multi-functional properties of neurons enable the generation of adaptive behaviors remains unknown. Here, we show that the HSN neuron in C. elegans evokes multiple motor programs over different timescales to enable a suite of behavioral changes during egg laying. Using HSN activity perturbations and in vivo calcium imaging, we show that HSN acutely increases egg laying and locomotion while also biasing the animals toward low-speed dwelling behavior over minutes. The acute effects of HSN on egg laying and high-speed locomotion are mediated by separate sets of HSN transmitters and different HSN axonal compartments. The long-lasting effects on dwelling are mediated in part by HSN release of serotonin, which is taken up and re-released by NSM, another serotonergic neuron class that directly evokes dwelling. Our results show how the multi-functional properties of a single neuron allow it to induce a coordinated suite of behaviors and also reveal that neurons can borrow serotonin from one another to control behavior.

A homeostatic gut-to-brain insulin antagonist restrains neuronally stimulated fat loss

In C. elegans mechanisms by which peripheral organs relay internal state information to the nervous system remain unknown, although strong evidence suggests that such signals do exist. Here we report the discovery of a peptide of the ancestral insulin superfamily called INS-7 that functions as an enteroendocrine peptide and is secreted from specialized cells of the intestine. INS-7 secretion increases during fasting, and acts as a bona fide gut-to-brain homeostatic signal that attenuates neuronally induced fat loss during food shortage. INS-7 functions as an antagonist at the canonical DAF-2 receptor in the nervous system, and phylogenetic analysis suggests that INS-7 bears greater resemblance to members of the broad insulin/relaxin superfamily than to conventional mammalian insulin and IGF peptides. The discovery of an endogenous insulin antagonist secreted by specialized intestinal cell with enteroendocrine functions suggests that much remains to be learned about the intestine and its role in directing neuronal functions.

Sink or swim: Does a worm paralysis phenotype hold clues to neurodegenerative disease?

Receiving a neurodegenerative disease (NDD) diagnosis, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, or amyotrophic lateral sclerosis, is devastating, particularly given the limited options for treatment. Advances in genetic technologies have allowed for efficient modeling of NDDs in animals and brought hope for new disease-modifying medications. The complexity of the mammalian brain and the costs and time needed to identify and develop therapeutic leads limits progress. Modeling NDDs in invertebrates, such as the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans, offers orders of magnitude increases in speed of genetic analysis and manipulation, and can be pursued at substantially reduced cost, providing an important, platform complement and inform research with mammalian NDD models. In this review, we describe how our efforts to exploit C. elegans for the study of neural signaling and health led to the discovery of a paralytic phenotype (swimming-induced paralysis) associated with altered dopamine signaling and, surprisingly, to the discovery of a novel gene and pathway whose dysfunction in glial cells triggers neurodegeneration. Research to date on swip-10 and its putative mammalian ortholog MBLAC1, suggests that a tandem analysis will offer insights into NDD mechanisms and insights into novel, disease-modifying therapeutics.

Electric shock causes a fleeing-like persistent behavioral response in the nematode Caenorhabditis elegans

Behavioral persistency reflects internal brain states, which are the foundations of multiple brain functions. However, experimental paradigms enabling genetic analyses of behavioral persistency and its associated brain functions have been limited. Here, we report novel persistent behavioral responses caused by electric stimuli in the nematode Caenorhabditis elegans. When the animals on bacterial food are stimulated by alternating current, their movement speed suddenly increases 2- to 3-fold, persisting for more than 1 minute even after a 5-second stimulation. Genetic analyses reveal that voltage-gated channels in the neurons are required for the response, possibly as the sensors, and neuropeptide signaling regulates the duration of the persistent response. Additional behavioral analyses implicate that the animal's response to electric shock is scalable and has a negative valence. These properties, along with persistence, have been recently regarded as essential features of emotion, suggesting that C. elegans response to electric shock may reflect a form of emotion, akin to fear.

Cytochrome P450 and Epoxide Hydrolase Metabolites in Aβ and tau-induced Neurodegeneration: Insights from Caenorhabditis elegans

This study aims to uncover potent cytochrome P450 (CYP) and epoxide hydrolase (EH) metabolites implicated in Aβ and/or tau-induced neurodegeneration, independent of neuroinflammation, by utilizing Caenorhabditis elegans (C. elegans) as a model organism. Our research reveals that Aβ and/or tau expression in C. elegans disrupts the oxylipin profile, and epoxide hydrolase inhibition alleviates the ensuing neurodegeneration, likely through elevating the epoxy-to-hydroxy ratio of various CYP-EH metabolites. In addition, our results indicated that the Aβ and tau likely affect the CYP-EH metabolism of PUFA through different mechanism. These findings emphasize the intriguing relationship between lipid metabolites and neurodegenerations, in particular, those linked to Aβ and/or tau aggregation. Furthermore, our investigation sheds light on the crucial and captivating role of CYP PUFA metabolites in C. elegans physiology, opening up possibilities for broader implications in mammalian and human contexts.

Identification of the bacterial metabolite aerugine as potential trigger of human dopaminergic neurodegeneration

The causes of nigrostriatal cell death in idiopathic Parkinson's disease are unknown, but exposure to toxic chemicals may play some role. We followed up here on suggestions that bacterial secondary metabolites might be selectively cytotoxic to dopaminergic neurons. Extracts from Streptomyces venezuelae were found to kill human dopaminergic neurons (LUHMES cells). Utilizing this model system as a bioassay, we identified a bacterial metabolite known as aerugine (C10H11NO2S; 2-[4-(hydroxymethyl)-4,5-dihydro-1,3-thiazol-2-yl]phenol) and confirmed this finding by chemical re-synthesis. This 2-hydroxyphenyl-thiazoline compound was previously shown to be a product of a wide-spread biosynthetic cluster also found in the human microbiome and in several pathogens. Aerugine triggered half-maximal dopaminergic neurotoxicity at 3-4 µM. It was less toxic for other neurons (10-20 µM), and non-toxic (at <100 µM) for common human cell lines. Neurotoxicity was completely prevented by several iron chelators, by distinct anti-oxidants and by a caspase inhibitor. In the Caenorhabditis elegans model organism, general survival was not affected by aerugine concentrations up to 100 µM. When transgenic worms, expressing green fluorescent protein only in their dopamine neurons, were exposed to aerugine, specific neurodegeneration was observed. The toxicant also exerted functional dopaminergic toxicity in nematodes as determined by the "basal slowing response" assay. Thus, our research has unveiled a bacterial metabolite with a remarkably selective toxicity toward human dopaminergic neurons in vitro and for the dopaminergic nervous system of Caenorhabditis elegans in vivo. These findings suggest that microbe-derived environmental chemicals should be further investigated for their role in the pathogenesis of Parkinson's disease.

Ethyl caffeate attefnuates Aβ-induced toxicity in Caenorhabditis elegans AD models via the insulin/insulin-like growth factor-1 signaling pathway

The pathogenesis of Alzheimer's disease (AD), a multifactorial progressive neurodegenerative disease associated with aging, is unclear. Ethyl caffeate is a plant polyphenol that has been reported to have neuroprotective effects, but the mechanisms by which it acts are unclear. In this study, for the first time, we investigated the molecular mechanism of its anti-AD properties using the Caernorhabditis elegans model. The results of our experiments showed that ethyl caffeate delayed the paralysis symptoms of CL4176 to a different extent and reduced the exogenous 5-hydroxytryptophan-induced paralysis phenotype. Further studies revealed that ethyl caffeate lowered Aβ plaques and depressed the expression of Aβ monomers and oligomers, but did not influence the mRNA levels of Aβ. Moreover, it was able to bring paraquat-induced ROS levels down to near-standard conditions. Real-time quantitative PCR experiment showed a significant upregulation of the transcript abundance of daf-16, skn-1 and hsf-1, key factors associated with the insulin/insulin-like growth factor 1 (IGF-1) signaling pathway (IIS), and their downstream genes sod-3, gst-4 and hsp-16.2. It was further shown that ethyl caffeate activated the translocation of DAF-16 and SKN-1 from the cytoplasm to the nucleus and enhanced the expression of sod-3::GFP, gst-4::GFP and hsp-16.2::GFP in transgenic nematodes. This meant that the protection against Aβ toxicity by ethyl caffeate may be partly through the IIS signaling pathway. In addition, ethyl caffeate suppressed the aggregation of polyglutamine proteins in AM141, which indicated a potential protective effect against neurodegenerative diseases based on abnormal folding and aggregation of amyloid proteins. Taken together, ethyl caffeate is expected to develop as a potential drug for the management of AD.

Dopaminergic modulation by quercetin: In silico and in vivo evidence using Caenorhabditis elegans as a model

Quercetin is a flavonol widely distributed in plants and has various described biological functions. Several studies have reported on its ability to restore neuronal function in a wide variety of disease models, including animal models of neurodegenerative disorders such as Parkinson's disease. Quercetin per se can act as a neuroprotector/neuromodulator, especially in diseases related to impaired dopaminergic neurotransmission. However, little is known about how quercetin interacts with the dopaminergic machinery. Here we employed the nematode Caenorhabditis elegans to study this putative interaction. After observing behavioral modulation, mutant analysis and gene expression in C. elegans upon exposure to quercetin at a concentration that does not protect against MPTP, we constructed a homology-based dopamine transporter protein model to conduct a docking study. This led to suggestive evidence on how quercetin may act as a dopaminergic modulator by interacting with C. elegans' dopamine transporter and alter the nematode's exploratory behavior. Consistent with this model, quercetin controls C. elegans behavior in a way dependent on the presence of both the dopamine transporter (dat-1), which is up-regulated upon quercetin exposure, and the dopamine receptor 2 (dop-2), which appears to be mandatory for dat-1 up-regulation. Our data propose an interaction with the dopaminergic machinery that may help to establish the effects of quercetin as a neuromodulator.

Morphological hallmarks of dopaminergic neurodegeneration are associated with altered neuron function in Caenorhabditis elegans

Caenorhabditis elegans (C. elegans) is an excellent model system to study neurodegenerative diseases, such as Parkinson's disease, as it enables analysis of both neuron morphology and function in live animals. Multiple structural changes in neurons, such as cephalic dendrite morphological abnormalities, have been considered hallmarks of neurodegeneration in this model, but their relevance to changes in neuron function are not entirely clear. We sought to test whether hallmark morphological changes associated with chemically induced dopaminergic neuron degeneration, such as dendrite blebbing, breakage, and loss, are indicative of neuronal malfunction and result in changes in behavior. We adapted an established dopaminergic neuronal function assay by measuring paralysis in the presence of exogenous dopamine, which revealed clear differences between cat-2 dopamine deficient mutants, wildtype worms, and dat-1 dopamine abundant mutants. Next, we integrated an automated image processing algorithm and a microfluidic device to segregate worm populations by their cephalic dendrite morphologies. We show that nematodes with dopaminergic dendrite degeneration markers, such as blebbing or breakage, paralyze at higher rates in a dopamine solution, providing evidence that dopaminergic neurodegeneration morphologies are correlated with functional neuronal outputs.

Caenorhabditis elegans foraging patterns follow a simple rule of thumb

Rules of thumb are behavioral algorithms that approximate optimal behavior while lowering cognitive and sensory costs. One way to reduce these costs is by simplifying the representation of the environment: While the theoretically optimal behavior may depend on many environmental variables, a rule of thumb may use a smaller set of variables that performs reasonably well. Experimental proof of this simplification requires an exhaustive mapping of all relevant combinations of several environmental parameters, which we performed for Caenorhabditis elegans foraging by covering systematically combinations of food density (across 4 orders of magnitude) and food type (across 12 bacterial strains). We found that worms' response is dominated by a single environmental variable: food density measured as number of bacteria per unit surface. They disregard other factors such as biomass content or bacterial strain. We also measured experimentally the impact on fitness of each type of food, determining that the rule is near-optimal and therefore constitutes a rule of thumb that leverages the most informative environmental variable. These results set the stage for further investigations into the underlying genetic and neural mechanisms governing this simplification process, and into its role in the evolution of decision-making strategies.

A proton-inhibited DEG/ENaC ion channel maintains neuronal ionstasis and promotes neuronal survival under stress

The nervous system participates in the initiation and modulation of systemic stress. Ionstasis is of utmost importance for neuronal function. Imbalance in neuronal sodium homeostasis is associated with pathologies of the nervous system. However, the effects of stress on neuronal Na⁺ homeostasis, excitability, and survival remain unclear. We report that the DEG/ENaC family member DEL-4 assembles into a proton-inactivated sodium channel. DEL-4 operates at the neuronal membrane and synapse to modulate Caenorhabditis elegans locomotion. Heat stress and starvation alter DEL-4 expression, which in turn alters the expression and activity of key stress-response transcription factors and triggers appropriate motor adaptations. Similar to heat stress and starvation, DEL-4 deficiency causes hyperpolarization of dopaminergic neurons and affects neurotransmission. Using humanized models of neurodegenerative diseases in C. elegans, we showed that DEL-4 promotes neuronal survival. Our findings provide insights into the molecular mechanisms by which sodium channels promote neuronal function and adaptation under stress.

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.

A Reliable Method Based on Liquid Chromatography-Tandem Mass Spectrometry for the Simultaneous Quantification of Neurotransmitters in Caenorhabditis elegans

Neurotransmitters like dopamine (DA), serotonin (SRT), γ-aminobutyric acid (GABA) and acetylcholine (ACh) are messenger molecules that play a pivotal role in transmitting excitation between neurons across chemical synapses, thus enabling complex processes in the central nervous system (CNS). Balance in neurotransmitter homeostasis is essential, and altered neurotransmitter levels are associated with various neurological disorders, e.g., loss of dopaminergic neurons (Parkinson's disease) or altered ACh synthesis (Alzheimer's disease). Therefore, it is crucial to possess adequate tools to assess precise neurotransmitter levels, and to apply targeted therapies. An established in vivo model to study neurotoxicity is the model organism Caenorhabditis elegans (C. elegans), as its neurons have been well characterized and functionally are analogous to mammals. We have developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method including a sample preparation assuring neurotransmitter stability, which allows a simultaneous neurotransmitter quantification of DA, SRT, GABA and ACh in C. elegans, but can easily be applied to other matrices. LC-MS/MS combined with isotope-labeled standards is the tool of choice, due to its otherwise unattainable sensitivity and specificity. Using C. elegans together with our analytically validated and verified method provides a powerful tool to evaluate mechanisms of neurotoxicity, and furthermore to identify possible therapeutic approaches.

Dissecting the functional organization of the C. elegans serotonergic system at whole-brain scale

Serotonin influences many aspects of animal behavior. But how serotonin acts on its diverse receptors across the brain to modulate global activity and behavior is unknown. Here, we examine how serotonin release in C. elegans alters brain-wide activity to induce foraging behaviors, like slow locomotion and increased feeding. Comprehensive genetic analyses identify three core serotonin receptors (MOD-1, SER-4, and LGC-50) that induce slow locomotion upon serotonin release and others (SER-1, SER-5, and SER-7) that interact with them to modulate this behavior. SER-4 induces behavioral responses to sudden increases in serotonin release, whereas MOD-1 induces responses to persistent release. Whole-brain imaging reveals widespread serotonin-associated brain dynamics, spanning many behavioral networks. We map all sites of serotonin receptor expression in the connectome, which, together with synaptic connectivity, helps predict which neurons show serotonin-associated activity. These results reveal how serotonin acts at defined sites across a connectome to modulate brain-wide activity and behavior.

The recurrent pathogenic Pro890Leu substitution in CLTC causes a generalized defect in synaptic transmission in Caenorhabditis elegans

De novo CLTC mutations underlie a spectrum of early-onset neurodevelopmental phenotypes having developmental delay/intellectual disability (ID), epilepsy, and movement disorders (MD) as major clinical features. CLTC encodes the widely expressed heavy polypeptide of clathrin, a major component of the coated vesicles mediating endocytosis, intracellular trafficking, and synaptic vesicle recycling. The underlying pathogenic mechanism is largely unknown. Here, we assessed the functional impact of the recurrent c.2669C > T (p.P890L) substitution, which is associated with a relatively mild ID/MD phenotype. Primary fibroblasts endogenously expressing the mutated protein show reduced transferrin uptake compared to fibroblast lines obtained from three unrelated healthy donors, suggesting defective clathrin-mediated endocytosis. In vitro studies also reveal a block in cell cycle transition from G0/G1 to the S phase in patient's cells compared to control cells. To demonstrate the causative role of the p.P890L substitution, the pathogenic missense change was introduced at the orthologous position of the Caenorhabditis elegans gene, chc-1 (p.P892L), via CRISPR/Cas9. The resulting homozygous gene-edited strain displays resistance to aldicarb and hypersensitivity to PTZ, indicating defective release of acetylcholine and GABA by ventral cord motor neurons. Consistently, mutant animals show synaptic vesicle depletion at the sublateral nerve cords, and slightly defective dopamine signaling, highlighting a generalized deficit in synaptic transmission. This defective release of neurotransmitters is associated with their secondary accumulation at the presynaptic membrane. Automated analysis of C. elegans locomotion indicates that chc-1 mutants move slower than their isogenic controls and display defective synaptic plasticity. Phenotypic profiling of chc-1 (+/P892L) heterozygous animals and transgenic overexpression experiments document a mild dominant-negative behavior for the mutant allele. Finally, a more severe phenotype resembling that of chc-1 null mutants is observed in animals harboring the c.3146 T > C substitution (p.L1049P), homologs of the pathogenic c.3140 T > C (p.L1047P) change associated with a severe epileptic phenotype. Overall, our findings provide novel insights into disease mechanisms and genotype-phenotype correlations of CLTC-related disorders.

Attenuation of Dopaminergic Neurodegeneration in a C. elegans Parkinson's Model through Regulation of Xanthine Dehydrogenase (XDH-1) Expression by the RNA Editase, ADR-2

Differential RNA editing by adenosine deaminases that act on RNA (ADARs) has been implicated in several neurological disorders, including Parkinson's disease (PD). Here, we report results of a RNAi screen of genes differentially regulated in adr-2 mutants, normally encoding the only catalytically active ADAR in Caenorhabditis elegans, ADR-2. Subsequent analysis of candidate genes that alter the misfolding of human α-synuclein (α-syn) and dopaminergic neurodegeneration, two PD pathologies, reveal that reduced expression of xdh-1, the ortholog of human xanthine dehydrogenase (XDH), is protective against α-synuclein-induced dopaminergic neurodegeneration. Further, RNAi experiments show that WHT-2, the worm ortholog of the human ABCG2 transporter and a predicted interactor of XDH-1, is the rate-limiting factor in the ADR-2, XDH-1, WHT-2 system for dopaminergic neuroprotection. In silico structural modeling of WHT-2 indicates that the editing of one nucleotide in the wht-2 mRNA leads to the substitution of threonine with alanine at residue 124 in the WHT-2 protein, changing hydrogen bonds in this region. Thus, we propose a model where wht-2 is edited by ADR-2, which promotes optimal export of uric acid, a known substrate of WHT-2 and a product of XDH-1 activity. In the absence of editing, uric acid export is limited, provoking a reduction in xdh-1 transcription to limit uric acid production and maintain cellular homeostasis. As a result, elevation of uric acid is protective against dopaminergic neuronal cell death. In turn, increased levels of uric acid are associated with a decrease in ROS production. Further, downregulation of xdh-1 is protective against PD pathologies because decreased levels of XDH-1 correlate to a concomitant reduction in xanthine oxidase (XO), the form of the protein whose by-product is superoxide anion. These data indicate that modifying specific targets of RNA editing may represent a promising therapeutic strategy for PD.

A proprioceptive feedback circuit drives Caenorhabditis elegans locomotor adaptation through dopamine signaling

An animal adapts its motor behavior to navigate the external environment. This adaptation depends on proprioception, which provides feedback on an animal's body postures. How proprioception mechanisms interact with motor circuits and contribute to locomotor adaptation remains unclear. Here, we describe and characterize proprioception-mediated homeostatic control of undulatory movement in the roundworm Caenorhabditis elegans. We found that the worm responds to optogenetically or mechanically induced decreases in midbody bending amplitude by increasing its anterior amplitude. Conversely, it responds to increased midbody amplitude by decreasing the anterior amplitude. Using genetics, microfluidic and optogenetic perturbation response analyses, and optical neurophysiology, we elucidated the neural circuit underlying this compensatory postural response. The dopaminergic PDE neurons proprioceptively sense midbody bending and signal to AVK interneurons via the D2-like dopamine receptor DOP-3. The FMRFamide-like neuropeptide FLP-1, released by AVK, regulates SMB head motor neurons to modulate anterior bending. We propose that this homeostatic behavioral control optimizes locomotor efficiency. Our findings demonstrate a mechanism in which proprioception works with dopamine and neuropeptide signaling to mediate motor control, a motif that may be conserved in other animals.

The Pesticide Chlordecone Promotes Parkinsonism-like Neurodegeneration with Tau Lesions in Midbrain Cultures and C. elegans Worms

Chlordecone (CLD) is an organochlorine pesticide (OCP) that is currently banned but still contaminates ecosystems in the French Caribbean. Because OCPs are known to increase the risk of Parkinson's disease (PD), we tested whether chronic low-level intoxication with CLD could reproduce certain key characteristics of Parkinsonism-like neurodegeneration. For that, we used culture systems of mouse midbrain dopamine (DA) neurons and glial cells, together with the nematode C. elegans as an in vivo model organism. We established that CLD kills cultured DA neurons in a concentration- and time-dependent manner while exerting no direct proinflammatory effects on glial cells. DA cell loss was not impacted by the degree of maturation of the culture. The use of fluorogenic probes revealed that CLD neurotoxicity was the consequence of oxidative stress-mediated insults and mitochondrial disturbances. In C. elegans worms, CLD exposure caused a progressive loss of DA neurons associated with locomotor deficits secondary to alterations in food perception. L-DOPA, a molecule used for PD treatment, corrected these deficits. Cholinergic and serotoninergic neuronal cells were also affected by CLD in C. elegans, although to a lesser extent than DA neurons. Noticeably, CLD also promoted the phosphorylation of the aggregation-prone protein tau (but not of α-synuclein) both in midbrain cell cultures and in a transgenic C. elegans strain expressing a human form of tau in neurons. In summary, our data suggest that CLD is more likely to promote atypical forms of Parkinsonism characterized by tau pathology than classical synucleinopathy-associated PD.

Synthesis of a natural quinoline alkaloid isolated from the deep-sea-derived fungus and its potential as a therapeutic for Parkinson's disease

2-(Quinoline-8-carboxamido)benzoic acid (2-QBA; 1) is a natural quinoline alkaloid isolated from the deep-sea-derived fungus Aspergillus sp. SCSIO06786. Alkaloid 1 was synthesized by an amidation reaction of 8-quinolinecaroxylic acid with methyl anthranilate, followed by hydrolysis. The neuroprotective properties of 1 were evaluated using a Caenorhabditis elegans Parkinson's disease model, which revealed that 1 significantly ameliorated 1-methyl-4-phenylpyridinium (MPP⁺)-induced dopaminergic neurodegeneration in a dose-dependent manner. MPP⁺-induced behavioral defects in worms, including impaired locomotion and basal slowing ability, were restored by treatment with 1. We further demonstrated that treatment with 1 modulates the formation of neurotoxic α-synuclein oligomers by suppressing α-synuclein expressions and enhancing proteasome activity. These results suggest that 1 is a promising therapeutic candidate for the treatment of Parkinson's disease.

High-fidelity encoding of mechanostimuli by tactile food-sensing neurons requires an ensemble of ion channels

The nematode C. elegans uses mechanosensitive neurons to detect bacteria, which are food for worms. These neurons release dopamine to suppress foraging and promote dwelling. Through a screen of genes highly expressed in dopaminergic food-sensing neurons, we identify a K2P-family potassium channel-TWK-2-that damps their activity. Strikingly, loss of TWK-2 restores mechanosensation to neurons lacking the NOMPC-like channel transient receptor potential 4 (TRP-4), which was thought to be the primary mechanoreceptor for tactile food sensing. The alternate mechanoreceptor mechanism uncovered by TWK-2 mutation requires three Deg/ENaC channel subunits: ASIC-1, DEL-3, and UNC-8. Analysis of cell-physiological responses to mechanostimuli indicates that TRP and Deg/ENaC channels work together to set the range of analog encoding of stimulus intensity and to improve signal-to-noise characteristics and temporal fidelity of food-sensing neurons. We conclude that a specialized mechanosensory modality-tactile food sensing-emerges from coordination of distinct force-sensing mechanisms housed in one type of sensory neuron.

Dithianon exposure induces dopaminergic neurotoxicity in Caenorhabditis elegans

Dithianon is a conventional broad-spectrum protectant fungicide widely used in agriculture, but its potential neurotoxic risk to animals remains largely unknown. In this study, neurotoxic effects of Dithianon and its underlying cellular and molecular mechanisms were investigated using the nematode, Caenorhabditis elegans, as a model system. Upon chronic exposure of C. elegans to Dithianon, dopaminergic neurons were found to be vulnerable, with significant degeneration in terms of structure and function in a concentration-dependent manner. In examining toxicity mechanisms, we observed significant Dithianon-induced increases in oxidative stress and mitochondrial fragmentation, both of which are often associated with cellular stress. The present study suggests that Dithianon exposure causes dopaminergic neurotoxicity in C. elegans, by inducing oxidative stress and mitochondrial dysfunction. These findings contribute to a better understanding of Dithianon's neurotoxic potential.