Light and hydrogen peroxide inhibit C. elegans Feeding through gustatory receptor orthologs and pharyngeal neurons

An enteric neuron-expressed variant ionotropic receptor detects ingested salts to regulate salt stress resistance

The detection of internal chemicals by interoceptive chemosensory pathways is critical for regulating metabolism and physiology. The molecular identities of interoceptors, and the functional consequences of chemosensation by specific interoceptive neurons remain to be fully described. The C. elegans pharyngeal neuronal network is anatomically and functionally homologous to the mammalian enteric nervous system. Here, we show that the I3 pharyngeal enteric neuron responds to cations via an I3-specific variant ionotropic receptor (IR) to regulate salt stress tolerance. The GLR-9 IR, located at the gut lumen-exposed sensory end of I3, is necessary and sufficient for salt sensation, establishing a chemosensory function for IRs beyond insects. Salt detection by I3 protects specifically against high salt stress, as glr-9 mutants show reduced tolerance of hypertonic salt but not sugar solutions, with or without prior acclimation. While cholinergic signaling from I3 promotes tolerance to acute high salt stress, peptidergic signaling from I3 during acclimation is essential for resistance to a subsequent high salt challenge. Transcriptomic analyses show that I3 regulates salt tolerance in part via regulating the expression of osmotic stress response genes in distal tissues. Our results describe the mechanisms by which chemosensation mediated by a defined enteric neuron regulates physiological homeostasis in response to a specific abiotic stress.

Blue-shifted ancyromonad channelrhodopsins for multiplex optogenetics

Light-gated ion channels from protists (channelrhodopsins or ChRs) are optogenetic tools widely used for controlling neurons and cardiomyocytes. Multiplex optogenetic applications require spectrally separated molecules that must be found in nature, as they are difficult to engineer without disrupting channel function. Scanning numerous sequence databases, we identified three robust naturally blue-shifted ChRs from ancyromonads. They form a separate branch on the phylogenetic tree and contain residue motifs characteristic of anion ChRs (ACRs). However, only two conduct chloride, whereas the close Nutomonas longa homolog (peak absorption at ~440 nm) generates inward cation currents in mammalian cells under physiological conditions, significantly exceeding those by previously known tools. Measurements of transient absorption changes and pH titration of purified Ancyromonas sigmoides ACR (AnsACR) combined with mutant analysis revealed the roles of the residues in the photoactive site. Both ancyromonad ACRs allowed optogenetic silencing of mouse cortical neurons in brain slices. AnsACR expression in the cholinergic neurons enabled photoinhibition of pharyngeal muscle contraction in live worms. AnsACR could be activated by near-infrared two-photon illumination, which is required to control specific neurons in thick tissue. Our results improved the mechanistic understanding of light-gated channel function and expanded the optogenetic toolkit.

Impact statement

Ancyromonad channelrhodopsins advance our understanding of ionic selectivity and wavelength regulation in light-gated ion channels and also expand the toolkit for all-optical electrophysiology.

The Nematode Pristionchus pacificus Requires the Gβ and Gγ Proteins for Light Adaptation But Not For Light Avoidance

Most organisms can sense and adapt to a wide range of light intensities. Although animals commonly use opsins for light detection, the nematode Pristionchus pacificus lacks conserved photoreceptors. The cyclic GMP signaling pathway and G protein-coupled receptor kinase are essential for light-avoidance behavior in P. pacificus. Although the mechanism of light sensing in P. pacificus has been partially elucidated, it remains unclear whether, and how, P. pacificus adapts to light. Here, we found that prior exposure to light reduced the frequency of light-avoidance behavior in P. pacificus, indicating its ability to adapt to light. To reveal the mechanism of light adaptation in P. pacificus, we used CRISPR/Cas9 genome editing to generate Gβ and Gγ subunit mutants, as these subunits are involved in chemosensory adaptation in the nematode Caenorhabditis elegans. Gβ and Gγ subunit mutants exhibited light-avoidance behavior similar to that of the wild type, but light adaptation was impaired in the Gβ mutants. Similarly, the Gγ and arrestin mutants showed minor abnormalities in light adaptation. These findings suggest that these proteins play a role in sensory adaptation beyond that in chemosensation and could contribute to light response mechanisms in nematodes.

Non-Visual Light Sensing Enhances Behavioral Memory and Drives Gene Expression in C. elegans

Visible light influences a range of physiological processes, yet how animals respond to it independently of the visual system remains largely unknown. Here, we uncover a previously undescribed light-induced transcriptional pathway that modulates behavioral plasticity in C. elegans, a roundworm without eyes. We demonstrate that ambient visible light or controlled-intensity visible-spectrum LED activates an effector gene cyp-14A5 in non-neuronal tissues through the bZIP transcription factors ZIP-2 and CEBP-2. Light induction of cyp-14A5 is more prominent at shorter wavelengths but is independent of the known blue light receptors LITE-1 and GUR-3 in C. elegans. This bZIP-dependent genetic pathway in non-neuronal tissues enhances behavioral adaptability and olfactory memory, suggesting a body-brain communication axis. Furthermore, we use the light-responsive cyp-14A5 promoter to drive ectopic gene expression, causing synthetic light-induced sleep and rapid aging phenotypes in C. elegans. These findings advance our understanding of light-responsive mechanisms outside the visual system and offer a new genetic tool for visible light-inducible gene expression in non-neuronal tissues.

Neuronal PRDX-2-Mediated ROS Signaling Regulates Food Digestion via peripheral UPRmt Activation

All organisms depend on food digestion for survival, yet the brain-gut signaling mechanisms that regulate this process are not fully understood. Here, using an established C. elegans digestion model, we uncover a pathway in which neuronal ROS (free radicals) signal the intestine to suppress digestion. Genetic screening reveals that reducing genes responsible for maintaining ROS balance increases free radicals and decreases digestion. PRDX-2 knockout in olfactory neurons (AWC) elevates ROS and reduces digestive capacity, mediated by the neuropeptide NLP-1 and activation of the mitochondrial unfolded protein response (UPRmt) in the intestine. Additionally, over-expressing nlp-1 or ablating AWC neurons both trigger UPRmt and inhibit digestion. These findings reveal a brain-gut connection in which neuronal PRDX-2-mediated ROS signaling modulates food digestion, highlighting a critical role of free radicals in shutting down digestion to alleviate stress and reduce food consumption.

cGMP-dependent pathway and a GPCR kinase are required for photoresponse in the nematode Pristionchus pacificus

Light sensing is a critical function in most organisms and is mediated by photoreceptor proteins and phototransduction. Although most nematodes lack eyes, some species exhibit phototaxis. In the nematode Caenorhabditis elegans, the unique photoreceptor protein Cel-LITE-1, its downstream G proteins, and cyclic GMP (cGMP)-dependent pathways are required for phototransduction. However, the mechanism of light-sensing in other nematodes remains unknown. To address this question, we used the nematode Pristionchus pacificus, which was established as a satellite model organism for comparison with C. elegans. Similar to C. elegans, illumination with short-wavelength light induces avoidance behavior in P. pacificus. Opsin, cryptochrome/photolyase, and lite-1 were not detected in the P. pacificus genome using orthology and domain prediction-based analyses. To identify the genes related to phototransduction in P. pacificus, we conducted forward genetic screening for light-avoidance behavior and isolated five light-unresponsive mutants. Whole-genome sequencing and genetic mapping revealed that the cGMP-dependent pathway and Ppa-grk-2, which encodes a G protein-coupled receptor kinase (GRK) are required for light avoidance. Although the cGMP-dependent pathway is conserved in C. elegans phototransduction, GRK is not necessary for light avoidance in C. elegans. This suggests similarities and differences in light-sensing mechanisms between the two species. Using a reverse genetic approach, we showed that gamma-aminobutyric acid (GABA) and glutamate were involved in light avoidance. Through reporter analysis and suppression of synapse transmission, we identified candidate photosensory neurons. These findings advance our understanding of the diversity of phototransduction in nematodes even in the absence of eyes.

TWISP: a transgenic worm for interrogating signal propagation in Caenorhabditis elegans

Genetically encoded optical indicators and actuators of neural activity allow for all-optical investigations of signaling in the nervous system. But commonly used indicators, actuators, and expression strategies are poorly suited for systematic measurements of signal propagation at brain scale and cellular resolution. Large-scale measurements of the brain require indicators and actuators with compatible excitation spectra to avoid optical crosstalk. They must be highly expressed in every neuron but at the same time avoid lethality and permit the animal to reach adulthood. Their expression must also be compatible with additional fluorescent labels to locate and identify neurons, such as those in the NeuroPAL cell identification system. We present TWISP, a transgenic worm for interrogating signal propagation, that addresses these needs and enables optical measurements of evoked calcium activity at brain scale and cellular resolution in the nervous system of the nematode Caenorhabditis elegans. In every neuron we express a nonconventional optical actuator, the gustatory receptor homolog GUR-3 + PRDX-2, under the control of a drug-inducible system QF + hGR, and a calcium indicator GCAMP6s, in a background with additional fluorophores from the NeuroPAL cell ID system. We show that this combination, but not others tested, avoids optical crosstalk, creates strong expression in the adult, and generates stable transgenic lines for systematic measurements of signal propagation in the worm brain.

Light evokes stereotyped global brain dynamics in Caenorhabditis elegans

Stereotyped oscillations in population neural activity recordings from immobilized Caenorhabditis elegans have garnered interest for their striking low dimensionality and their evocative state-space trajectories or manifolds. Previously these oscillations have been interpreted as intrinsically driven global motor commands. Here we test whether these oscillations are intrinsic. We show that similar oscillations are evoked by high-intensity blue light commonly used for calcium imaging. Oscillations are reduced or absent and have a lower frequency when a longer imaging wavelength is used. Under the original blue light illumination, oscillations are reduced or have a lower frequency in animals that lack GUR-3, an endogenous light- and hydrogen-peroxide-sensitive gustatory receptor. Additional experiments with hydrogen peroxide are consistent with GUR-3's involvement. We therefore propose that blue light evokes global oscillations in part through the creation of reactive oxygen species that activate the hydrogen-peroxide-sensing receptor GUR-3.

Neural signal propagation atlas of Caenorhabditis elegans

Establishing how neural function emerges from network properties is a fundamental problem in neuroscience1. Here, to better understand the relationship between the structure and the function of a nervous system, we systematically measure signal propagation in 23,433 pairs of neurons across the head of the nematode Caenorhabditis elegans by direct optogenetic activation and simultaneous whole-brain calcium imaging. We measure the sign (excitatory or inhibitory), strength, temporal properties and causal direction of signal propagation between these neurons to create a functional atlas. We find that signal propagation differs from model predictions that are based on anatomy. Using mutants, we show that extrasynaptic signalling not visible from anatomy contributes to this difference. We identify many instances of dense-core-vesicle-dependent signalling, including on timescales of less than a second, that evoke acute calcium transients-often where no direct wired connection exists but where relevant neuropeptides and receptors are expressed. We propose that, in such cases, extrasynaptically released neuropeptides serve a similar function to that of classical neurotransmitters. Finally, our measured signal propagation atlas better predicts the neural dynamics of spontaneous activity than do models based on anatomy. We conclude that both synaptic and extrasynaptic signalling drive neural dynamics on short timescales, and that measurements of evoked signal propagation are crucial for interpreting neural function.

Reactive oxygen species drive foraging decisions in Caenorhabditis elegans

Environmental surveillance-mediated behavior integrates multiple cues through complex signaling mechanisms. In Caenorhabditis elegans, neurons coordinate perception and response through evolutionarily conserved molecular signaling cascades to mediate attraction and avoidance behaviors. However, despite lacking eyes, C. elegans was recently reported to perceive and react to the color blue. Here, we provide an explanation for this apparent color perception. We show that internally-generated reactive oxygen species (ROS) occurring in response to light are additive to exogenous sources of ROS, such as bacterial toxins or photosensitizers. Multiple sub-threshold sources of ROS are integrated to coordinate behavioral responses to the environment with internal physiologic cues, independent of color. We further demonstrate that avoidance behavior can be blocked by antioxidants, while ROS is both sufficient and scalable to phenocopy the avoidance response. Moreover, avoidance behavior in response to ROS is plastic and reversible, suggesting it may occur through a post-translation redox modification. Blue light affects C. elegans behavior through ROS generation by endogenous flavins in a process requiring the neuronal gustatory photoreceptor like protein, LITE-1. Our results demonstrate that LITE-1 is also required for ROS-mediated avoidance of pyocyanin and light-activated photosensitizers and this role is mediated through the modification of Cys44. Overall, these findings demonstrate that ROS and LITE-1 are central mediators of C. elegans foraging behavior through integration of multiple inputs, including light.

Fluorescence dynamics of lysosomal-related organelle flashing in the intestinal cells of Caenorhabditis elegans

The biological roles of the autofluorescent lysosome-related organelles ("gut granules") in the intestinal cells of many nematodes, including Caenorhabditis elegans, have been shown to play an important role in metabolic and signaling processes, but they have not been fully characterized. We report here a previously undescribed phenomenon in which the autofluorescence of these granules increased and then decreased in a rapid and dynamic manner that may be associated with nutrient availability. We observed that two distinct types of fluorophores are likely present in the gut granules. One displays a "flashing" phenomenon, in which fluorescence decrease is preceded by a sharp increase in fluorescence intensity that expands into the surrounding area, while the other simply decreases in intensity. Gut granule flashing was observed in the different life stages of C. elegans and was also observed in Steinernema hermaphroditum, an evolutionarily distant nematode. We hypothesize that the "flashing" fluorophore is pH-sensitive, and the fluorescence intensity change results from the fluorophore being released from the lysosome-related organelles into the relatively higher pH environment of the cytosol. The visually spectacular dynamic fluorescence phenomenon we describe might provide a handle on the biochemistry and genetics of these lysosome-related organelles.

Structure-function analysis suggests that the photoreceptor LITE-1 is a light-activated ion channel

Sensation of light is essential for all organisms. The eye-less nematode Caenorhabditis elegans detects UV and blue light to evoke escape behavior. The photosensor LITE-1 absorbs UV photons with an unusually high extinction coefficient, involving essential tryptophans. Here, we modeled the structure and dynamics of LITE-1 using AlphaFold2-multimer and molecular dynamics (MD) simulations and performed mutational and behavioral assays in C. elegans to characterize its function. LITE-1 resembles olfactory and gustatory receptors from insects, recently shown to be tetrameric ion channels. We identified residues required for channel gating, light absorption, and mechanisms of photo-oxidation, involving a likely binding site for the peroxiredoxin PRDX-2. Furthermore, we identified the binding pocket for a putative chromophore. Several residues lining this pocket have previously been established as essential for LITE-1 function. A newly identified critical cysteine pointing into the pocket represents a likely chromophore attachment site. We derived a model for how photon absorption, via a network of tryptophans and other aromatic amino acids, induces an excited state that is transferred to the chromophore. This evokes conformational changes in the protein, possibly leading to a state receptive to oxidation of cysteines and, jointly, to channel gating. Electrophysiological data support the idea that LITE-1 is a photon and H2O2-coincidence detector. Other proteins with similarity to LITE-1, specifically C. elegans GUR-3, likely use a similar mechanism for photon detection. Thus, a common protein fold and assembly, used for chemoreception in insects, possibly by binding of a particular compound, may have evolved into a light-activated ion channel.

LITE-1 mediates behavioral responses to X-rays in Caenorhabditis elegans

Rapid sensory detection of X-ray stimulation has been documented across a wide variety of species, but few studies have explored the underlying molecular mechanisms. Here we report the discovery of an acute behavioral avoidance response in wild type Caenorhabditis elegans to X-ray stimulation. The endogenous C. elegans UV-photoreceptor protein LITE-1 was found to mediate the locomotory avoidance response. Transgenic expression of LITE-1 in C. elegans muscle cells resulted in paralysis and egg ejection responses to X-ray stimulation, demonstrating that ectopic expression of LITE-1 can confer X-ray sensitivity to otherwise X-ray insensitive cells. This work represents the first demonstration of rapid X-ray based genetically targeted (X-genetic) manipulation of cellular electrical activity in intact behaving animals. Our findings suggest that LITE-1 has strong potential for use in this minimally invasive form of neuromodulation to transduce transcranial X-ray signals for precise manipulation of neural activity in mammals, bypassing the need for invasive surgical implants to deliver stimulation.

TWISP: A Transgenic Worm for Interrogating Signal Propagation in C. elegans

Genetically encoded optical indicators and actuators of neural activity allow for all-optical investigations of signaling in the nervous system. But commonly used indicators, actuators and expression strategies are poorly suited for systematic measurements of signal propagation at brain scale and cellular resolution. Large scale measurements of the brain require indicators and actuators with compatible excitation spectra to avoid optical crosstalk. They must be highly expressed in every neuron but at the same time avoid lethality and permit the animal to reach adulthood. And finally, their expression must be compatible with additional fluorescent labels to locate and identify neurons, such as those in the NeuroPAL cell identification system. We present TWISP, a Transgenic Worm for Interrogating Signal Propagation, that address these needs and enables optical measurements of evoked calcium activity at brain scale and cellular resolution in the nervous system of the nematode Caenorhabditis elegans. We express in every neuron a non-conventional optical actuator, the gustatory receptor homolog GUR-3+PRDX-2 under the control of a drug-inducible system QF+hGR, and calcium indicator GCAMP6s, in a background with additional fluorophores of the NeuroPAL cell ID system. We show that this combination, but not others tested, avoids optical-crosstalk, creates strong expression in the adult, and generates stable transgenic lines for systematic measurements of signal propagation in the worm brain.

The head mesodermal cell couples FMRFamide neuropeptide signaling with rhythmic muscle contraction in C. elegans

FMRFamides are evolutionarily conserved neuropeptides that play critical roles in behavior, energy balance, and reproduction. Here, we show that FMRFamide signaling from the nervous system is critical for the rhythmic activation of a single cell of previously unknown function, the head mesodermal cell (hmc) in C. elegans. Behavioral, calcium imaging, and genetic studies reveal that release of the FLP-22 neuropeptide from the AVL neuron in response to pacemaker signaling activates hmc every 50 s through an frpr-17 G protein-coupled receptor (GPCR) and a protein kinase A signaling cascade in hmc. hmc activation results in muscle contraction through coupling by gap junctions composed of UNC-9/Innexin. hmc activation is inhibited by the neuronal release of a second FMRFamide-like neuropeptide, FLP-9, which functions through its GPCR, frpr-21, in hmc. This study reveals a function for two opposing FMRFamide signaling pathways in controlling the rhythmic activation of a target cell through volume transmission.

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.

Microfluidic-Assisted Caenorhabditis elegans Sorting: Current Status and Future Prospects

Caenorhabditis elegans (C. elegans) has been a popular model organism for several decades since its first discovery of the huge research potential for modeling human diseases and genetics. Sorting is an important means of providing stage- or age-synchronized worm populations for many worm-based bioassays. However, conventional manual techniques for C. elegans sorting are tedious and inefficient, and commercial complex object parametric analyzer and sorter is too expensive and bulky for most laboratories. Recently, the development of lab-on-a-chip (microfluidics) technology has greatly facilitated C. elegans studies where large numbers of synchronized worm populations are required and advances of new designs, mechanisms, and automation algorithms. Most previous reviews have focused on the development of microfluidic devices but lacked the summaries and discussion of the biological research demands of C. elegans, and are hard to read for worm researchers. We aim to comprehensively review the up-to-date microfluidic-assisted C. elegans sorting developments from several angles to suit different background researchers, i.e., biologists and engineers. First, we highlighted the microfluidic C. elegans sorting devices' advantages and limitations compared to the conventional commercialized worm sorting tools. Second, to benefit the engineers, we reviewed the current devices from the perspectives of active or passive sorting, sorting strategies, target populations, and sorting criteria. Third, to benefit the biologists, we reviewed the contributions of sorting to biological research. We expect, by providing this comprehensive review, that each researcher from this multidisciplinary community can effectively find the needed information and, in turn, facilitate future research.

Caenorhabditis elegans as a Promising Model Organism in Chronobiology

Circadian rhythms represent an adaptive feature, ubiquitously found in nature, which grants living beings the ability to anticipate daily variations in their environment. They have been found in a multitude of organisms, ranging from bacteria to fungi, plants, and animals. Circadian rhythms are generated by endogenous clocks that can be entrained daily by environmental cycles such as light and temperature. The molecular machinery of circadian clocks includes a transcriptional-translational feedback loop that takes approximately 24 h to complete. Drosophila melanogaster has been a model organism of choice to understand the molecular basis of circadian clocks. However, alternative animal models are also being adopted, each offering their respective experimental advantages. The nematode Caenorhabditis elegans provides an excellent model for genetics and neuro-behavioral studies, which thanks to its ease of use and manipulation, as well as availability of genetic data and mutant strains, is currently used as a novel model for circadian research. Here, we aim to evaluate C. elegans as a model for chronobiological studies, focusing on its strengths and weaknesses while reviewing the available literature. Possible zeitgebers (including light and temperature) are also discussed. Determining the molecular bases and the neural circuitry involved in the central pacemaker of the C. elegans' clock will contribute to the understanding of its circadian system, becoming a novel model organism for the study of diseases due to alterations of the circadian cycle.

Visual object detection biases escape trajectories following acoustic startle in larval zebrafish

In this study, we investigated whether the larval zebrafish is sensitive to the presence of obstacles in its environment. Zebrafish execute fast escape swims when in danger of predation. We posited that collisions with solid objects during escape would be maladaptive to the fish, and therefore, the direction of escape swims should be informed by the locations of barriers. To test this idea, we developed a closed-loop imaging rig outfitted with barriers of various qualities. We show that when larval zebrafish escape in response to a non-directional vibrational stimulus, they use visual scene information to avoid collisions with obstacles. Our study demonstrates that barrier avoidance rate corresponds to the absolute distance of obstacles, as distant barriers outside of collision range elicit less bias than nearby collidable barriers that occupy the same amount of visual field. The computation of barrier avoidance is covert: the fact that fish will avoid barriers during escape cannot be predicted by its routine swimming behavior in the barrier arena. Finally, two-photon laser ablation experiments suggest that excitatory bias is provided to the Mauthner cell ipsilateral to approached barriers, either via direct excitation or a multi-step modulation process. We ultimately propose that zebrafish detect collidable objects via an integrative visual computation that is more complex than retinal occupancy alone, laying a groundwork for understanding how cognitive physical models observed in humans are implemented in an archetypal vertebrate brain. VIDEO ABSTRACT.

Left-right asymmetry in oxidative stress sensing neurons in C. elegans

Perception of oxidative stress in nematodes involves specific neurons expressing antioxidant enzymes. Here, we carefully characterized GFP knock-in lines for C. elegans peroxiredoxin PRDX-2 and thioredoxin TRX-1, and uncovered that left and right I2, PHA and ASJ neurons reproducibly express an asymmetric level of each enzyme. We observed that high-expressing neurons are in most cases associated with a particular side, indicating a directional rather than stochastic type of asymmetry. We propose that the biological relevance of this left-right asymmetry is to fine-tune H ₂ O ₂ or light sensing, which remains to be investigated.

Neuronal temperature perception induces specific defenses that enable C. elegans to cope with the enhanced reactivity of hydrogen peroxide at high temperature

Hydrogen peroxide is the most common reactive chemical that organisms face on the microbial battlefield. The rate with which hydrogen peroxide damages biomolecules required for life increases with temperature, yet little is known about how organisms cope with this temperature-dependent threat. Here, we show that Caenorhabditis elegans nematodes use temperature information perceived by sensory neurons to cope with the temperature-dependent threat of hydrogen peroxide produced by the pathogenic bacterium Enterococcus faecium. These nematodes preemptively induce the expression of specific hydrogen peroxide defenses in response to perception of high temperature by a pair of sensory neurons. These neurons communicate temperature information to target tissues expressing those defenses via an insulin/IGF1 hormone. This is the first example of a multicellular organism inducing their defenses to a chemical when they sense an inherent enhancer of the reactivity of that chemical.

Distinct mechanisms underlie H2O2 sensing in C. elegans head and tail

Environmental oxidative stress threatens cellular integrity and should therefore be avoided by living organisms. Yet, relatively little is known about environmental oxidative stress perception. Here, using microfluidics, we showed that like I2 pharyngeal neurons, the tail phasmid PHA neurons function as oxidative stress sensing neurons in C. elegans, but display different responses to H2O2 and light. We uncovered that different but related receptors, GUR-3 and LITE-1, mediate H2O2 signaling in I2 and PHA neurons. Still, the peroxiredoxin PRDX-2 is essential for both, and might promote H2O2-mediated receptor activation. Our work demonstrates that C. elegans can sense a broad range of oxidative stressors using partially distinct H2O2 signaling pathways in head and tail sensillae, and paves the way for further understanding of how the integration of these inputs translates into the appropriate behavior.

Automatically tracking feeding behavior in populations of foraging C. elegans

Caenorhabditis elegans feeds on bacteria and other small microorganisms which it ingests using its pharynx, a neuromuscular pump. Currently, measuring feeding behavior requires tracking a single animal, indirectly estimating food intake from population-level metrics, or using restrained animals. To enable large throughput feeding measurements of unrestrained, crawling worms on agarose plates at a single worm resolution, we developed an imaging protocol and a complementary image analysis tool called PharaGlow. We image up to 50 unrestrained crawling worms simultaneously and extract locomotion and feeding behaviors. We demonstrate the tool's robustness and high-throughput capabilities by measuring feeding in different use-case scenarios, such as through development, with genetic and chemical perturbations that result in faster and slower pumping, and in the presence or absence of food. Finally, we demonstrate that our tool is capable of long-term imaging by showing behavioral dynamics of mating animals and worms with different genetic backgrounds. The low-resolution fluorescence microscopes required are readily available in C. elegans laboratories, and in combination with our python-based analysis workflow makes this methodology easily accessible. PharaGlow therefore enables the observation and analysis of the temporal dynamics of feeding and locomotory behaviors with high-throughput and precision in a user-friendly system.

Potent anticancer activity of a novel iridium metallodrug via oncosis

Oncosis (from Greek ónkos, meaning "swelling") is a non-apoptotic cell death process related to energy depletion. In contrast to apoptosis, which is the main form of cell death induced by anticancer drugs, oncosis has been relatively less explored but holds potential to overcome drug resistance phenomena. In this study, we report a novel rationally designed mitochondria-targeted iridium(III) complex (OncoIr3) with advantageous properties as a bioimaging agent. OncoIr3 exhibited potent anticancer activity in vitro against cancer cells and displayed low toxicity to normal dividing cells. Flow cytometry and fluorescence-based assays confirmed an apoptosis-independent mechanism involving energy depletion, mitochondrial dysfunction and cellular swelling that matched with the oncotic process. Furthermore, a Caenorhabditis elegans tumoral model was developed to test this compound in vivo, which allowed us to prove a strong oncosis-derived antitumor activity in animals (with a 41% reduction of tumor area). Indeed, OncoIr3 was non-toxic to the nematodes and extended their mean lifespan by 18%. Altogether, these findings might shed new light on the development of anticancer metallodrugs with non-conventional modes of action such as oncosis, which could be of particular interest for the treatment of apoptosis-resistant cancers.

madd-4 plays a critical role in light against Bursaphelenchus xylophilus

Bursaphelenchus xylophilus is a notorious invasive species, causing extensive losses to pine ecosystems globally. Previous studies had shown that the development of B. xylophilus was seriously suppressed by light. However, the mechanism involved in the inhibition is unknown. Here, it is the first report that Bxy-madd-4 is a light-regulated gene, plays a potential role in B. xylophilus in responding to the blue light. Transcriptome sequencing revealed that the expression level of Bxy-madd-4 declined by 86.39% under blue light. The reverse transcription quantitative real-time PCR results were in accord with the transcriptome sequencing, confirming the expression level of Bxy-madd-4 was suppressed by blue light. Bxy-madd-4 promoter::mCherry reporter constructed in Caenorhabditis elegans were utilized to mimic the spatiotemporal expression patterns of Bxy-madd-4. Bxy-madd-4A promoter activity had a strong continuity throughout all development stages in C. elegans. Further RNA interference indicated that only 36.8% of the Bxy-madd-4 dsRNA treated embryos were hatched. Moreover, 71.6% of the hatched nematodes were abnormal, such as particles on the body surface and concave tissues. Our findings contribute towards a better understanding of the mechanism of light against the destructive invasive nematode, providing a promising hint for control of the destructive invasive nematode.

Nematode surface functionalization with hydrogel sheaths tailored in situ

Engineering the surfaces of biological organisms allows the introduction of novel functions and enhances their native functions. However, studies on surface engineering remained limited to unicellular organisms. Herein, nematode surfaces are engineered through in situ hydrogelation mediated by horseradish peroxidase (HRP) anchored to nematode cuticles. With this method, hydrogel sheaths of approximately 10-μm thickness are fabricated from a variety of polysaccharides, proteins, and synthetic polymers. Caenorhabditis elegans and Anisakis simplex coated with a hydrogel sheath showed a negligible decrease in viability, chemotaxis and locomotion. Hydrogel sheaths containing UV-absorbable groups and catalase functioned as shields to protect nematodes from UV and hydrogen peroxide, respectively. The results also showed that hydrogel sheaths containing glucose oxidase have the potential to be used as living drug delivery systems for cancer therapy. The nematode functionalization method developed in this study has the potential to impact a wide range of fields from agriculture to medicine.

A reversible mitochondrial complex I thiol switch mediates hypoxic avoidance behavior in C. elegans

C. elegans react to metabolic distress caused by mismatches in oxygen and energy status via distinct behavioral responses. At the molecular level, these responses are coordinated by under-characterized, redox-sensitive processes, thought to initiate in mitochondria. Complex I of the electron transport chain is a major site of reactive oxygen species (ROS) production and is canonically associated with oxidative damage following hypoxic exposure. Here, we use a combination of optogenetics and CRISPR/Cas9-mediated genome editing to exert spatiotemporal control over ROS production. We demonstrate a photo-locomotory remodeling of avoidance behavior by local ROS production due to the reversible oxidation of a single thiol on the complex I subunit NDUF-2.1. Reversible thiol oxidation at this site is necessary and sufficient for the behavioral response to hypoxia, does not respond to ROS produced at more distal sites, and protects against lethal hypoxic exposure. Molecular modeling suggests that oxidation at this thiol residue alters the ability for NDUF-2.1 to coordinate electron transfer to coenzyme Q by destabilizing the Q-binding pocket, causing decreased complex I activity. Overall, site-specific ROS production regulates behavioral responses and these findings provide a mechanistic target to suppress the detrimental effects of hypoxia.

Association of Two Opposing Responses Results in the Emergence of a Novel Conditioned Response

Recent studies examining association of opposing responses, contrasting emotional valences, or counter motivational states have begun to elucidate how learning and memory processes can translate to clinical therapies for trauma or addiction. In the current study, association of opposing responses is tested in C. elegans. Due to its relatively simple and well-described nervous system, it was hypothesized that association of two oppositional stimuli presented in a delayed conditioning protocol would strengthen the behavioral response to the first stimulus (alpha conditioning). To test this, C. elegans were exposed to a tone vibration stimulus (to activate a mechanosensory-driven locomotor reversal response) paired with a blue light (to activate a forward locomotor response) at a 2-s delay. After five pairings, behavior was measured following a tone-alone stimulus. Worms that received stimulus pairing did not show an enhanced response to the first presented stimulus (tone vibration) but rather showed a marked increase in time spent in pause (cessation of movement), a new behavioral response (beta conditioning). This increase in pause behavior was accompanied by changes in measures of both backward and forward locomotion. Understanding the dynamics of conditioned behavior resulting from pairing of oppositional responses could provide further insight into how learning processes occur and may be applied.

The enteric nervous system of the C. elegans pharynx is specified by the Sine oculis-like homeobox gene ceh-34

Overarching themes in the terminal differentiation of the enteric nervous system, an autonomously acting unit of animal nervous systems, have so far eluded discovery. We describe here the overall regulatory logic of enteric nervous system differentiation of the nematode Caenorhabditis elegans that resides within the foregut (pharynx) of the worm. A C. elegans homolog of the Drosophila Sine oculis homeobox gene, ceh-34, is expressed in all 14 classes of interconnected pharyngeal neurons from their birth throughout their life time, but in no other neuron type of the entire animal. Constitutive and temporally controlled ceh-34 removal shows that ceh-34 is required to initiate and maintain the neuron type-specific terminal differentiation program of all pharyngeal neuron classes, including their circuit assembly. Through additional genetic loss of function analysis, we show that within each pharyngeal neuron class, ceh-34 cooperates with different homeodomain transcription factors to individuate distinct pharyngeal neuron classes. Our analysis underscores the critical role of homeobox genes in neuronal identity specification and links them to the control of neuronal circuit assembly of the enteric nervous system. Together with the pharyngeal nervous system simplicity as well as its specification by a Sine oculis homolog, our findings invite speculations about the early evolution of nervous systems.

Optophysiology: Illuminating cell physiology with optogenetics

Optogenetics combines light and genetics to enable precise control of living cells, tissues, and organisms with tailored functions. Optogenetics has the advantages of noninvasiveness, rapid responsiveness, tunable reversibility, and superior spatiotemporal resolution. Following the initial discovery of microbial opsins as light-actuated ion channels, a plethora of naturally occurring or engineered photoreceptors or photosensitive domains that respond to light at varying wavelengths has ushered in the next chapter of optogenetics. Through protein engineering and synthetic biology approaches, genetically-encoded photoswitches can be modularly engineered into protein scaffolds or host cells to control a myriad of biological processes, as well as to enable behavioral control and disease intervention in vivo. Here, we summarize these optogenetic tools on the basis of their fundamental photochemical properties to better inform the chemical basis and design principles. We also highlight exemplary applications of opsin-free optogenetics in dissecting cellular physiology (designated "optophysiology"), and describe the current progress, as well as future trends, in wireless optogenetics, which enables remote interrogation of physiological processes with minimal invasiveness. This review is anticipated to spark novel thoughts on engineering next-generation optogenetic tools and devices that promise to accelerate both basic and translational studies.

The molecular and neural regulation of ultraviolet light phototaxis and its food-associated learning behavioral plasticity in C. elegans

Ultraviolet light is quite toxic to all the animals and evoke the avoidance behavior of UV. The soil nematode Caenorhabditis elegans senses UV and is known to avoid UV by using four sensory neurons. However, it is not clear what signaling molecules act for UV avoidance in the neuronal pathway constituted of four sensory neurons. In addition, it is not clear whether this harmful environmental signal can be associated with other benefit signals such as food. In this study, by using newly developed assay system, we found that C. elegans can associate UV and food and changes behavioral strategy against harmful UV signal. This is the first indication that C. elegans shows associate learning with UV and food. Using our assay system, we also found that glutamate is used as a transmitter in both the UV avoidance and UV associate learning neural circuits. However, one sensory neuron showed a significant role for associative learning, compared to a complimentary role in four sensory neurons for direct associative learning, and different sets of glutamate receptors seemed to be acting for UV avoidance and UV associate learning. These findings suggest that a distinct neuronal network is used for UV learning compared to that for direct avoidance behavior of UV.

Modulation of sensory perception by hydrogen peroxide enables Caenorhabditis elegans to find a niche that provides both food and protection from hydrogen peroxide

Hydrogen peroxide (H2O2) is the most common chemical threat that organisms face. Here, we show that H2O2 alters the bacterial food preference of Caenorhabditis elegans, enabling the nematodes to find a safe environment with food. H2O2 induces the nematodes to leave food patches of laboratory and microbiome bacteria when those bacterial communities have insufficient H2O2-degrading capacity. The nematode's behavior is directed by H2O2-sensing neurons that promote escape from H2O2 and by bacteria-sensing neurons that promote attraction to bacteria. However, the input for H2O2-sensing neurons is removed by bacterial H2O2-degrading enzymes and the bacteria-sensing neurons' perception of bacteria is prevented by H2O2. The resulting cross-attenuation provides a general mechanism that ensures the nematode's behavior is faithful to the lethal threat of hydrogen peroxide, increasing the nematode's chances of finding a niche that provides both food and protection from hydrogen peroxide.

Mimicking human riboflavin responsive neuromuscular disorders by silencing flad-1 gene in C. elegans: Alteration of vitamin transport and cholinergic transmission

Riboflavin (Rf), or vitamin B2, is the precursor of FMN and FAD, redox cofactors of several dehydrogenases involved in energy metabolism, redox balance and other cell regulatory processes. FAD synthase, coded by FLAD1 gene in humans, is the last enzyme in the pathway converting Rf into FAD. Mutations in FLAD1 gene are responsible for neuromuscular disorders, in some cases treatable with Rf. In order to mimic these disorders, the Caenorhabditis elegans (C. elegans) gene orthologue of FLAD1 (flad‐1) was silenced in a model strain hypersensitive to RNA interference in nervous system. Silencing flad‐1 resulted in a significant decrease in total flavin content, paralleled by a decrease in the level of the FAD‐dependent ETFDH protein and by a secondary transcriptional down‐regulation of the Rf transporter 1 (rft‐1) possibly responsible for the total flavin content decrease. Conversely an increased ETFDH mRNA content was found. These biochemical changes were accompanied by significant phenotypical changes, including impairments of fertility and locomotion due to altered cholinergic transmission, as indicated by the increased sensitivity to aldicarb. A proposal is made that neuronal acetylcholine production/release is affected by alteration of Rf homeostasis. Rf supplementation restored flavin content, increased rft‐1 transcript levels and eliminated locomotion defects. In this aspect, C. elegans could provide a low‐cost animal model to elucidate the molecular rationale for Rf therapy in human Rf responsive neuromuscular disorders and to screen other molecules with therapeutic potential.

Base excision repair causes age-dependent accumulation of single-stranded DNA breaks that contribute to Parkinson disease pathology

Aging, genomic stress, and mitochondrial dysfunction are risk factors for neurodegenerative pathologies, such as Parkinson disease (PD). Although genomic instability is associated with aging and mitochondrial impairment, the underlying mechanisms are poorly understood. Here, we show that base excision repair generates genomic stress, promoting age-related neurodegeneration in a Caenorhabditis elegans PD model. A physiological level of NTH-1 DNA glycosylase mediates mitochondrial and nuclear genomic instability, which promote degeneration of dopaminergic neurons in older nematodes. Conversely, NTH-1 deficiency protects against α-synuclein-induced neurotoxicity, maintaining neuronal function with age. This apparent paradox is caused by modulation of mitochondrial transcription in NTH-1-deficient cells, and this modulation activates LMD-3, JNK-1, and SKN-1 and induces mitohormesis. The dependance of neuroprotection on mitochondrial transcription highlights the integration of BER and transcription regulation during physiological aging. Finally, whole-exome sequencing of genomic DNA from patients with idiopathic PD suggests that base excision repair might modulate susceptibility to PD in humans.

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Incomplete base excision repair is a source of genomic stress during aging

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The NTH-1 DNA glycosylase is a key mediator of age-dependent genomic instability

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Compromised NTH-1 activity promotes neuroprotection in PD nematodes

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NTH-1 deficiency triggers LMD-3/JNK-1/SKN-1-dependent mitohormetic response

C. elegans does a spit take

Eating can turn into spitting because individual parts of a muscle cell are able to contract in different ways.

An hourglass circuit motif transforms a motor program via subcellularly localized muscle calcium signaling and contraction

Neural control of muscle function is fundamental to animal behavior. Many muscles can generate multiple distinct behaviors. Nonetheless, individual muscle cells are generally regarded as the smallest units of motor control. We report that muscle cells can alter behavior by contracting subcellularly. We previously discovered that noxious tastes reverse the net flow of particles through the C. elegans pharynx, a neuromuscular pump, resulting in spitting. We now show that spitting results from the subcellular contraction of the anterior region of the pm3 muscle cell. Subcellularly localized calcium increases accompany this contraction. Spitting is controlled by an ‘hourglass’ circuit motif: parallel neural pathways converge onto a single motor neuron that differentially controls multiple muscles and the critical subcellular muscle compartment. We conclude that subcellular muscle units enable modulatory motor control and propose that subcellular muscle contraction is a fundamental mechanism by which neurons can reshape behavior.

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

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

Use of C. elegans as a 3R-compliant in vivo model for the chemoprevention of cisplatin-induced neurotoxicity

Anticancer therapeutics can provoke severe side effects that impair the patient's quality of life. A frequent dose-limiting side effect of platinum-based anticancer therapy is neurotoxicity. Its pathophysiology is poorly understood, and effective preventive or therapeutic measures are missing. Therefore, elucidation of the molecular mechanism of platinating drug-induced neurotoxicity and the development of preventive strategies is urgently needed. To this end, we aim to use C. elegans as a 3R-compliant in vivo model. The 3R principles were conceived for animal welfare in science concerning animal experiments, which should be replaced, reduced or refined. We can analytically demonstrate dose-dependent uptake of cisplatin (CisPt) in C. elegans, as well as genotoxic and cytotoxic effects based on DNA adduct formation (i.e., 1,2-GpG intrastrand crosslinks), induction of apoptosis, and developmental toxicity. Measuring the impairment of pharyngeal pumping as a marker of neurotoxicity, we found that especially CisPt reduces the pumping frequency at concentrations where basal and touch-provoked movement were not yet affected. CisPt causes glutathione (GSH) depletion and RNAi-mediated knockdown of the glutamate-cysteine ligase GCS-1 aggravates the CisPt-induced inhibition of pharyngeal pumping. Moreover, N-acetylcysteine (NAC) mitigated CisPt-triggered toxicity, indicating that GSH depletion contributes to the CisPt-induced pharyngeal damage. In addition to NAC, amifostine (WR1065) also protected the pharynx of C. elegans from the toxic effects of CisPt. Measuring pharyngeal activity by the electrophysiological recording of neurotransmission in the pharynx, we confirmed that CisPt is neurotoxic in C. elegans and that NAC is neuroprotective in the nematode. The data support the hypothesis that monitoring the pharyngeal activity of C. elegans is a useful surrogate marker of CisPt-induced neurotoxicity. In addition, a low GSH pool reduces the resistance of neurons to CisPt treatment, and both NAC and WR1065 are capable of attenuating platinum-induced neurotoxicity during post-incubation in C. elegans. Overall, we propose C. elegans as a 3R-compliant in vivo model to study the molecular mechanisms of platinum-induced neurotoxicity and to explore novel neuroprotective therapeutic strategies to alleviate respective side effects of platinum-based cancer therapy.

C. elegans discriminates colors to guide foraging

Color detection is used by animals of diverse phyla to navigate colorful natural environments and is thought to require evolutionarily conserved opsin photoreceptor genes. We report that Caenorhabditis elegans roundworms can discriminate between colors despite the fact that they lack eyes and opsins. Specifically, we found that white light guides C. elegans foraging decisions away from a blue-pigment toxin secreted by harmful bacteria. These foraging decisions are guided by specific blue-to-amber ratios of light. The color specificity of color-dependent foraging varies notably among wild C. elegans strains, which indicates that color discrimination is ecologically important. We identified two evolutionarily conserved cellular stress response genes required for opsin-independent, color-dependent foraging by C. elegans, and we speculate that cellular stress response pathways can mediate spectral discrimination by photosensitive cells and organisms-even by those lacking opsins.

Regulation of photosensation by hydrogen peroxide and antioxidants in C. elegans

The eyeless C. elegans exhibits robust phototaxis behavior in response to short-wavelength light, particularly UV light. C. elegans senses light through LITE-1, a unique photoreceptor protein that belongs to the invertebrate taste receptor family. However, it remains unclear how LITE-1 is regulated. Here, we performed a forward genetic screen for genes that when mutated suppress LITE-1 function. One group of lite-1 suppressors are the genes required for producing the two primary antioxidants thioredoxin and glutathione, suggesting that oxidization of LITE-1 inhibits its function. Indeed, the oxidant hydrogen peroxide (H₂O₂) suppresses phototaxis behavior and inhibits the photoresponse in photoreceptor neurons, whereas other sensory behaviors are relatively less vulnerable to H₂O₂. Conversely, antioxidants can rescue the phenotype of lite-1 suppressor mutants and promote the photoresponse. As UV light illumination generates H₂O₂, we propose that upon light activation of LITE-1, light-produced H₂O₂ then deactivates LITE-1 to terminate the photoresponse, while antioxidants may promote LITE-1’s recovery from its inactive state. Our studies provide a potential mechanism by which H₂O₂ and antioxidants act synergistically to regulate photosensation in C. elegans.

The nematode C. elegans possesses a unique photoreceptor protein, LITE-1, which mediates a light-avoidance behavior upon light exposure. C. elegans avoids short-wavelength light, particularly UV light, providing a potential mechanism by which worms escape from the dangerous UV rays in the sunlight. However, it is not clear how LITE-1 is regulated. Here, we performed a genetic screen to identify genes regulating LITE-1. We uncovered six genes that when mutated suppress LITE-1 function. All these genes are involved in regenerating cellular antioxidants that function to clear reactive oxygen species, particularly hydrogen peroxide (H₂O₂), suggesting that the function of LITE-1 is vulnerable to H₂O₂. Indeed, we show that H₂O₂ exposure inhibits LITE-1 function, while antioxidants promote it. Notably, other sensory functions are relatively less sensitive to H₂O₂. As UV light illumination is known to generate H₂O₂ within the cell, this provides a potential mechanism to turn off LITE-1. Our results uncover a potential mechanism of LITE-1 regulation, where antioxidants and oxidants act to promote and suppress LITE-1 function, respectively.

A putative origin of the insect chemosensory receptor superfamily in the last common eukaryotic ancestor

The insect chemosensory repertoires of Odorant Receptors (ORs) and Gustatory Receptors (GRs) together represent one of the largest families of ligand-gated ion channels. Previous analyses have identified homologous 'Gustatory Receptor-Like' (GRL) proteins across Animalia, but the evolutionary origin of this novel class of ion channels is unknown. We describe a survey of unicellular eukaryotic genomes for GRLs, identifying several candidates in fungi, protists and algae that contain many structural features characteristic of animal GRLs. The existence of these proteins in unicellular eukaryotes, together with ab initio protein structure predictions, provide evidence for homology between GRLs and a family of uncharacterized plant proteins containing the DUF3537 domain. Together, our analyses suggest an origin of this protein superfamily in the last common eukaryotic ancestor.

Social Chemical Communication Determines Recovery From L1 Arrest via DAF-16 Activation

In a population, chemical communication determines the response of animals to changing environmental conditions, what leads to an enhanced resistance against stressors. In response to starvation, the nematode Caenorhabditis elegans arrest post-embryonic development at the first larval stage (L1) right after hatching. As arrested L1 larvae, C. elegans become more resistant to diverse stresses, allowing them to survive for several weeks expecting to encounter more favorable conditions. L1 arrested at high densities display an enhanced resistance to starvation, dependent on soluble compounds released beyond hatching and the first day of arrest. Here, we show that this chemical communication also influences recovery after prolonged periods in L1 arrest. Animals at high density recovered faster than animals at low density. We found that the density effect on survival depends on the final effector of the insulin signaling pathway, the transcription factor DAF-16. Moreover, DAF-16 activation was higher at high density, consistent with a lower expression of the insulin-like peptide DAF-28 in the neurons. The improved recovery of animals after arrest at high density depended on soluble compounds present in the media of arrested L1s. In an effort to find the nature of these compounds, we investigated the disaccharide trehalose as putative signaling molecule, since its production is enhanced during L1 arrest and it is able to activate DAF-16. We detected the presence of trehalose in the medium of arrested L1 larvae at a low concentration. The addition of this concentration of trehalose to animals arrested at low density was enough to rescue DAF-28 production and DAF-16 activation to the levels of animals arrested at high density. However, despite activating DAF-16, trehalose was not capable of reversing survival and recovery phenotypes, suggesting the participation of additional signaling molecules. With all, here we describe a molecular mechanism underlying social communication that allows C. elegans to maintain arrested L1 larvae ready to quickly recover as soon as they encounter nutrient sources.

The connectome of the Caenorhabditis elegans pharynx

Detailed anatomical maps of individual organs and entire animals have served as invaluable entry points for ensuing dissection of their evolution, development, and function. The pharynx of the nematode Caenorhabditis elegans is a simple neuromuscular organ with a self-contained, autonomously acting nervous system, composed of 20 neurons that fall into 14 anatomically distinct types. Using serial electron micrograph (EM) reconstruction, we re-evaluate here the connectome of the pharyngeal nervous system, providing a novel and more detailed view of its structure and predicted function. Contrasting the previous classification of pharyngeal neurons into distinct inter- and motor neuron classes, we provide evidence that most pharyngeal neurons are also likely sensory neurons and most, if not all, pharyngeal neurons also classify as motor neurons. Together with the extensive cross-connectivity among pharyngeal neurons, which is more widespread than previously realized, the sensory-motor characteristics of most neurons define a shallow network architecture of the pharyngeal connectome. Network analysis reveals that the patterns of neuronal connections are organized into putative computational modules that reflect the known functional domains of the pharynx. Compared with the somatic nervous system, pharyngeal neurons both physically associate with a larger fraction of their neighbors and create synapses with a greater proportion of their neighbors. We speculate that the overall architecture of the pharyngeal nervous system may be reminiscent of the architecture of ancestral, primitive nervous systems.

Synchronization of circadian locomotor activity behavior in Caernorhabditis elegans: Interactions between light and temperature

Circadian rhythms are driven by an endogenous clock which is synchronized by daily environmental cycles (known as zeitgebers). Although the circadian responses of C. elegans to light have been recently reported, the mechanisms and pathways involved in their synchronization are still unknown. Here we present, by means of a novel behavioral approach, a complete characterization of C. elegans circadian synchronization to light and temperature cycles. Moreover, we screened mutant strains in search of defects of photic and thermal responses in order to study their putative pathways. We show that the wild-type strain is able to synchronize to combined cycles of light and temperature, with the best performance achieved under an optimal combination and phase-relationship of zeitgebers (high temperature in the dark phase and low temperature in the light phase). A lower responsiveness for the mutant strains MT21793 (lite-1/gur3 ko) and IK597 (gcy 8, 18 and 23 ko) was found in response to light and temperature, respectively. However, both mutants were still able to synchronize to a combined cycle of both stimuli. Our results shed light on the response of C. elegans to different zeitgebers as well as their possible synchronization pathways, the molecular components involved in these pathways, and their relative strength.

Cross talk between calcium and ROS regulate the UVA-induced melanin response in human melanocytes

Exposure to high doses of solar long wavelength ultraviolet radiation (UVA) damages human skin via reactive oxygen species (ROS). Whether physiological UVA doses also generate ROS that has an effect on the skin remains unknown. We previously showed that in human epidermal melanocytes UVA activates a G-protein coupled signaling pathway that leads to calcium mobilization and increased melanin. Here, we report that ROS generated by the UVA phototransduction pathway are critical cellular messengers required to augment melanin. Using simultaneous UVA exposure and live-cell imaging of primary human melanocytes, we found that physiological doses of UVA generate two spatiotemporally distinct sources of ROS: one upstream of the G-protein activation that potentiates calcium responses, and another source downstream of calcium, in the mitochondria (ROSmito ). UVA-evoked signaling led to mitochondrial calcium uptake via mitochondrial calcium uniporter to promote ROSmito production leading to melanin synthesis. Our findings reveal a novel mechanism in which ROS function as signaling messengers necessary for melanin production, thus having a protective role in the UVA-induced skin response.

C. elegans: a sensible model for sensory biology

From Sydney Brenner's backyard to hundreds of labs across the globe, inspiring six Nobel Prize winners along the way, Caenorhabditis elegans research has come far in the past half century. The journey is not over. The virtues of C. elegans research are numerous and have been recounted extensively. Here, we focus on the remarkable progress made in sensory neurobiology research in C. elegans. This nematode continues to amaze researchers as we are still adding new discoveries to the already rich repertoire of sensory capabilities of this deceptively simple animal. Worms possess the sense of taste, smell, touch, light, temperature and proprioception, each of which is being studied in genetic, molecular, cellular and systems-level detail. This impressive organism can even detect less commonly recognized sensory cues such as magnetic fields and humidity.

Evolutionary History of Major Chemosensory Gene Families across Panarthropoda

Chemosensory perception is a fundamental biological process of particular relevance in basic and applied arthropod research. However, apart from insects, there is little knowledge of specific molecules involved in this system, which is restricted to a few taxa with uneven phylogenetic sampling across lineages. From an evolutionary perspective, onychophorans (velvet worms) and tardigrades (water bears) are of special interest since they represent the closest living relatives of arthropods, altogether comprising the Panarthropoda. To get insights into the evolutionary origin and diversification of the chemosensory gene repertoire in panarthropods, we sequenced the antenna- and head-specific transcriptomes of the velvet worm Euperipatoides rowelli and analyzed members of all major chemosensory families in representative genomes of onychophorans, tardigrades, and arthropods. Our results suggest that the NPC2 gene family was the only family encoding soluble proteins in the panarthropod ancestor and that onychophorans might have lost many arthropod-like chemoreceptors, including the highly conserved IR25a receptor of protostomes. On the other hand, the eutardigrade genomes lack genes encoding the DEG-ENaC and CD36-sensory neuron membrane proteins, the chemosensory members of which have been retained in arthropods; these losses might be related to lineage-specific adaptive strategies of tardigrades to survive extreme environmental conditions. Although the results of this study need to be further substantiated by an increased taxon sampling, our findings shed light on the diversification of chemosensory gene families in Panarthropoda and contribute to a better understanding of the evolution of animal chemical senses.

The metabolome as a link in the genotype-phenotype map for peroxide resistance in the fruit fly, Drosophila melanogaster

BACKGROUND

Genetic association studies that seek to explain the inheritance of complex traits typically fail to explain a majority of the heritability of the trait under study. Thus, we are left with a gap in the map from genotype to phenotype. Several approaches have been used to fill this gap, including those that attempt to map endophenotype such as the transcriptome, proteome or metabolome, that underlie complex traits. Here we used metabolomics to explore the nature of genetic variation for hydrogen peroxide (H2O2) resistance in the sequenced inbred Drosophila Genetic Reference Panel (DGRP).

RESULTS

We first studied genetic variation for H2O2 resistance in 179 DGRP lines and along with identifying the insulin signaling modulator u-shaped and several regulators of feeding behavior, we estimate that a substantial amount of phenotypic variation can be explained by a polygenic model of genetic variation. We then profiled a portion of the aqueous metabolome in subsets of eight 'high resistance' lines and eight 'low resistance' lines. We used these lines to represent collections of genotypes that were either resistant or sensitive to the stressor, effectively modeling a discrete trait. Across the range of genotypes in both populations, flies exhibited surprising consistency in their metabolomic signature of resistance. Importantly, the resistance phenotype of these flies was more easily distinguished by their metabolome profiles than by their genotypes. Furthermore, we found a metabolic response to H2O2 in sensitive, but not in resistant genotypes. Metabolomic data further implicated at least two pathways, glycogen and folate metabolism, as determinants of sensitivity to H2O2. We also discovered a confounding effect of feeding behavior on assays involving supplemented food.

CONCLUSIONS

This work suggests that the metabolome can be a point of convergence for genetic variation influencing complex traits, and can efficiently elucidate mechanisms underlying trait variation.