Biosynthesis of the Caenorhabditis elegans dauer pheromone

The nematode-trapping fungus Arthrobotrys oligospora detects prey pheromones via G protein-coupled receptors

The ability to sense prey-derived cues is essential for predatory lifestyles. Under low-nutrient conditions, Arthrobotrys oligospora and other nematode-trapping fungi develop dedicated structures for nematode capture when exposed to nematode-derived cues, including a conserved family of pheromones, the ascarosides. A. oligospora senses ascarosides via conserved MAPK and cAMP-PKA pathways; however, the upstream receptors remain unknown. Here, using genomic, transcriptomic and functional analyses, we identified two families of G protein-coupled receptors (GPCRs) involved in sensing distinct nematode-derived cues. GPCRs homologous to yeast glucose receptors are required for ascaroside sensing, whereas Pth11-like GPCRs contribute to ascaroside-independent nematode sensing. Both GPCR classes activate conserved cAMP-PKA signalling to trigger trap development. This work demonstrates that predatory fungi use multiple GPCRs to sense several distinct nematode-derived cues for prey recognition and to enable a switch to a predatory lifestyle. Identification of these receptors reveals the molecular mechanisms of cross-kingdom communication via conserved pheromones also sensed by plants and animals.

An acyl-CoA thioesterase is essential for the biosynthesis of a key dauer pheromone in C. elegans

Methyl ketone (MK)-ascarosides represent essential components of several pheromones in Caenorhabditis elegans, including the dauer pheromone, which triggers the stress-resistant dauer larval stage, and the male-attracting sex pheromone. Here, we identify an acyl-CoA thioesterase, ACOT-15, that is required for the biosynthesis of MK-ascarosides. We propose a model in which ACOT-15 hydrolyzes the β-keto acyl-CoA side chain of an ascaroside intermediate during β-oxidation, leading to decarboxylation and formation of the MK. Using comparative metabolomics, we identify additional ACOT-15-dependent metabolites, including an unusual piperidyl-modified ascaroside, reminiscent of the alkaloid pelletierine. The β-keto acid generated by ACOT-15 likely couples to 1-piperideine to produce the piperidyl ascaroside, which is much less dauer-inducing than the dauer pheromone, asc-C6-MK (ascr#2, 1). The bacterial food provided influences production of the piperidyl ascaroside by the worm. Our work shows how the biosynthesis of MK- and piperidyl ascarosides intersect and how bacterial food may impact chemical signaling in the worm.

Peripheral peroxisomal β-oxidation engages neuronal serotonin signaling to drive stress-induced aversive memory in C. elegans

Physiological dysfunction confers negative valence to coincidental sensory cues to induce the formation of aversive associative memory. How peripheral tissue stress engages neuromodulatory mechanisms to form aversive memory is poorly understood. Here, we show that in the nematode C. elegans, mitochondrial disruption induces aversive memory through peroxisomal β-oxidation genes in non-neural tissues, including pmp-4/very-long-chain fatty acid transporter, dhs-28/3-hydroxylacyl-CoA dehydrogenase, and daf-22/3-ketoacyl-CoA thiolase. Upregulation of peroxisomal β-oxidation genes under mitochondrial stress requires the nuclear hormone receptor NHR-49. Importantly, the memory-promoting function of peroxisomal β-oxidation is independent of its canonical role in pheromone production. Peripheral signals derived from the peroxisomes target NSM, a critical neuron for memory formation under stress, to upregulate serotonin synthesis and remodel evoked responses to sensory cues. Our genetic, transcriptomic, and metabolomic approaches establish peroxisomal lipid signaling as a crucial mechanism that connects peripheral mitochondrial stress to central serotonin neuromodulation in aversive memory formation.

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

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

CRISPR-Cas9 genome editing in Steinernema entomopathogenic nematodes

Molecular tool development in traditionally non-tractable animals opens new avenues to study gene functions in the relevant ecological context. Entomopathogenic nematodes (EPN) Steinernema and their symbiotic bacteria of Xenorhabdus spp are a valuable experimental system in the laboratory and are applicable in the field to promote agricultural productivity. The infective juvenile (IJ) stage of the nematode packages mutualistic symbiotic bacteria in the intestinal pocket and invades insects that are agricultural pests. The lack of consistent and heritable genetics tools in EPN targeted mutagenesis severely restricted the study of molecular mechanisms underlying both parasitic and mutualistic interactions. Here, I report a protocol for CRISPR-Cas9 based genome-editing that is successful in two EPN species, S. carpocapsae and S. hermaphroditum . I adapted a gonadal microinjection technique in S. carpocapsae , which created on-target modifications of a homologue Sc-dpy-10 (cuticular collagen) by homology-directed repair. A similar delivery approach was used to introduce various alleles in S. hermaphroditum including Sh-dpy-10 and Sh-unc-22 (a muscle gene), resulting in visible and heritable phenotypes of dumpy and twitching, respectively. Using conditionally dominant alleles of Sh-unc-22 as a co-CRISPR marker, I successfully modified a second locus encoding Sh-Daf-22 (a homologue of human sterol carrier protein SCPx), predicted to function as a core enzyme in the biosynthesis of nematode pheromone that is required for IJ development. As a proof of concept, Sh-daf-22 null mutant showed IJ developmental defects in vivo ( in insecta) . This research demonstrates that Steinernema spp are highly tractable for targeted mutagenesis and has great potential in the study of gene functions under controlled laboratory conditions within the relevant context of its ecological niche.

Early pheromone perception remodels neurodevelopment and accelerates neurodegeneration in adult C. elegans

Age-associated neurodegenerative disorders such as Parkinson's and Alzheimer's diseases are mainly caused by protein aggregation. The etiologies of these neurodegenerative diseases share a chemical environment. However, how chemical cues modulate neurodegeneration remains unclear. Here, we found that in Caenorhabditis elegans, exposure to pheromones in the L1 stage accelerates neurodegeneration in adults. Perception of pheromones ascr#3 and ascr#10 is mediated by chemosensory neurons ASK and ASI. ascr#3 perceived by G protein-coupled receptor (GPCR) DAF-38 in ASK activates glutamatergic transmission into AIA interneurons. ascr#10 perceived by GPCR STR-2 in ASI activates the secretion of neuropeptide NLP-1, which binds to the NPR-11 receptor in AIA. Activation of both ASI and ASK is required and sufficient to remodel neurodevelopment via AIA, which triggers insulin-like signaling and inhibits autophagy in adult neurons non-cell-autonomously. Our work reveals how pheromone perception at the early developmental stage modulates neurodegeneration in adults and provides insights into how the external environment impacts neurodegenerative diseases.

Natural genetic variation in the pheromone production of C. elegans

From bacterial quorum sensing to human language, communication is essential for social interactions. Nematodes produce and sense pheromones to communicate among individuals and respond to environmental changes. These signals are encoded by different types and mixtures of ascarosides, whose modular structures further enhance the diversity of this nematode pheromone language. Interspecific and intraspecific differences in this ascaroside pheromone language have been described previously, but the genetic basis and molecular mechanisms underlying the variation remain largely unknown. Here, we analyzed natural variation in the production of 44 ascarosides across 95 wild Caenorhabditis elegans strains using high-performance liquid chromatography coupled to high-resolution mass spectrometry. We discovered wild strains defective in the production of specific subsets of ascarosides (e.g., the aggregation pheromone icas#9) or short- and medium-chain ascarosides, as well as inversely correlated patterns between the production of two major classes of ascarosides. We investigated genetic variants that are significantly associated with the natural differences in the composition of the pheromone bouquet, including rare genetic variants in key enzymes participating in ascaroside biosynthesis, such as the peroxisomal 3-ketoacyl-CoA thiolase, daf-22, and the carboxylesterase cest-3. Genome-wide association mappings revealed genomic loci harboring common variants that affect ascaroside profiles. Our study yields a valuable dataset for investigating the genetic mechanisms underlying the evolution of chemical communication.

Nematode Pheromones: Structures and Functions

Pheromones are chemical signals secreted by one individual that can affect the behaviors of other individuals within the same species. Ascaroside is an evolutionarily conserved family of nematode pheromones that play an integral role in the development, lifespan, propagation, and stress response of nematodes. Their general structure comprises the dideoxysugar ascarylose and fatty-acid-like side chains. Ascarosides can vary structurally and functionally according to the lengths of their side chains and how they are derivatized with different moieties. In this review, we mainly describe the chemical structures of ascarosides and their different effects on the development, mating, and aggregation of nematodes, as well as how they are synthesized and regulated. In addition, we discuss their influences on other species in various aspects. This review provides a reference for the functions and structures of ascarosides and enables their better application.

The FMRFamide-like peptide FLP-1 modulates larval development by regulating the production and secretion of the insulin-like peptide DAF-28 in Caenorhabditis elegans

The FMRFamide-like peptides (FLPs) are conserved in both free-living and parasitic nematodes. This molecular genetic study verified the relevance of the flp-1 gene, which is conserved in many nematode species, to the larval development of the free-living soil nematode Caenorhabditis elegans. Using C. elegans as a model, we found that: (1) FLP-1 suppressed larval development, resulting in diapause; (2) the secretion of FLP-1, which is produced in AVK head neurons, was suppressed by the presence of food (Escherichia coli) as an environmental factor to continue larval development; (3) the FLP-1 reduced the production and secretion of DAF-28, which is produced in ASI head neurons and is the predominant insulin-like peptide (INS) present. FLP-1 is conserved in many species of plant-parasitic root-knot nematodes that cause severe damage to crops. Therefore, our findings may provide insight into the development of new nematicides that can disturb their infection and development.

Formation and function of dauer ascarosides in the nematodes Caenorhabditis briggsae and Caenorhabditis elegans

The biosynthetic pathways and functions of ascaroside signaling molecules in the nematode Caenorhabditis elegans have been studied to better understand complex, integrative developmental decision-making. Although it is known that ascarosides play multiple roles in the development and behavior of nematode species other than C. elegans, these parallel pheromone systems have not been well-studied. Here, we show that ascarosides in the nematode Caenorhabditis briggsae are biosynthesized in the same manner as C. elegans and act to induce the alternative developmental pathway that generates the stress-resistant dauer lifestage. We show that ascr#2 is the primary component of crude dauer pheromone in C. briggsae; in contrast, C. elegans dauer pheromone relies on a combination of ascr#2, ascr#3, and several other components. We further demonstrate that Cbr-daf-22, like its C. elegans ortholog Cel-daf-22, is necessary to produce short-chain ascarosides. Moreover, Cbr-daf-22 and Cel-daf-22 mutants produce an ascaroside-independent metabolite that acts antagonistically to crude dauer pheromone and inhibits dauer formation.

Dietary S. maltophilia induces supersized lipid droplets by enhancing lipogenesis and ER-LD contacts in C. elegans

Dietary and symbiotic bacteria can exert powerful influence on metazoan lipid metabolism. Recent studies have emerged that microbiota have a role in animal obesity and related health disorders, but the mechanisms by which bacteria influence lipid storage in their host are unknown. To reduce the complexity of the relationship between gut microbiota and the host, Caenorhabditis elegans (C. elegans) has been chosen as a model organism to study interspecies interaction. Here, we demonstrate that feeding C. elegans with an opportunistic pathogenic bacterium Stenotrophomonas maltophilia (S. maltophilia) retards growth and promotes excessive neutral lipid storage. Gene expression analysis reveals that dietary S. maltophilia induces a lipogenic transcriptional response that includes the SREBP ortholog SBP-1, and fatty acid desaturases FAT-6 and FAT-7. Live imaging and ultrastructural analysis suggest that excess neutral lipid is stored in greatly expanded lipid droplets (LDs), as a result of enhanced endoplasmic reticulum (ER)-LD interaction. We also report that loss of function mutations in dpy-9 in C. elegans confers resistance to S. maltophilia. Dietary S. maltophilia induces supersized LDs by enhancing lipogenesis and ER-LD contacts in C. elegans. This work delineates a new model for understanding microbial regulation of metazoan physiology.

Endocrine pheromones couple fat rationing to dauer diapause through HNF4α nuclear receptors

Developmental diapause is a widespread strategy for animals to survive seasonal starvation and environmental harshness. Diapaused animals often ration body fat to generate a basal level of energy for enduring survival. How diapause and fat rationing are coupled, however, is poorly understood. The nematode Caenorhabditis elegans excretes pheromones to the environment to induce a diapause form called dauer larva. Through saturated forward genetic screens and CRISPR knockout, we found that dauer pheromones feed back to repress the transcription of ACOX-3, MAOC-1, DHS-28, DAF-22 (peroxisomal β-oxidation enzymes dually involved in pheromone synthesis and fat burning), ALH-4 (aldehyde dehydrogenase for pheromone synthesis), PRX-10 and PRX-11 (peroxisome assembly and proliferation factors). Dysfunction of these pheromone enzymes and factors relieves the repression. Surprisingly, transcription is repressed not by pheromones excreted but by pheromones endogenous to each animal. The endogenous pheromones regulate the nuclear translocation of HNF4α family nuclear receptor NHR-79 and its co-receptor NHR-49, and, repress transcription through the two receptors. The feedback repression maintains pheromone homeostasis, increases fat storage, decreases fat burning, and prolongs dauer lifespan. Thus, the exocrine dauer pheromones possess an unexpected endocrine function to mediate a peroxisome-nucleus crosstalk, coupling dauer diapause to fat rationing.

Investigating the molecular mechanisms of learning and memory using Caenorhabditis elegans

Learning is an essential biological process for survival since it facilitates behavioural plasticity in response to environmental changes. This process is mediated by a wide variety of genes, mostly expressed in the nervous system. Many studies have extensively explored the molecular and cellular mechanisms underlying learning and memory. This review will focus on the advances gained through the study of the nematode Caenorhabditis elegans. C. elegans provides an excellent system to study learning because of its genetic tractability, in addition to its invariant, compact nervous system (~300 neurons) that is well-characterised at the structural level. Importantly, despite its compact nature, the nematode nervous system possesses a high level of conservation with mammalian systems. These features allow the study of genes within specific sensory-, inter- and motor neurons, facilitating the interrogation of signalling pathways that mediate learning via defined neural circuits. This review will detail how learning and memory can be studied in C. elegans through behavioural paradigms that target distinct sensory modalities. We will also summarise recent studies describing mechanisms through which key molecular and cellular pathways are proposed to affect associative and non-associative forms of learning.

Neuronal control of maternal provisioning in response to social cues

Mothers contribute cytoplasmic components to their progeny in a process called maternal provisioning. Provisioning is influenced by the parental environment, but the molecular pathways that transmit environmental cues between generations are not well understood. Here, we show that, in Caenorhabditis elegans, social cues modulate maternal provisioning to regulate gene silencing in offspring. Intergenerational signal transmission depends on a pheromone-sensing neuron and neuronal FMRFamide (Phe-Met-Arg-Phe)-like peptides. Parental FMRFamide-like peptide signaling dampens oxidative stress resistance and promotes the deposition of mRNAs for translational components in progeny, which, in turn, reduces gene silencing. This study identifies a previously unknown pathway for intergenerational communication that links neuronal responses to maternal provisioning. We suggest that loss of social cues in the parental environment represents an adverse environment that stimulates stress responses across generations.

Neuronal perception of the social environment generates an inherited memory that controls the development and generation time of C. elegans

An old and controversial question in biology is whether information perceived by the nervous system of an animal can "cross the Weismann barrier" to alter the phenotypes and fitness of their progeny. Here, we show that such intergenerational transmission of sensory information occurs in the model organism, C. elegans, with a major effect on fitness. Specifically, that perception of social pheromones by chemosensory neurons controls the post-embryonic timing of the development of one tissue, the germline, relative to others in the progeny of an animal. Neuronal perception of the social environment thus intergenerationally controls the generation time of this animal.

Acyl-CoA oxidase ACOX-1 interacts with a peroxin PEX-5 to play roles in larval development of Haemonchus contortus

Hypobiosis (facultative developmental arrest) is the most important life-cycle adaptation ensuring survival of parasitic nematodes under adverse conditions. Little is known about such survival mechanisms, although ascarosides (ascarylose with fatty acid-derived side chains) have been reported to mediate the formation of dauer larvae in the free-living nematode Caenorhabditis elegans. Here, we investigated the role of a key gene acox-1, in the larval development of Haemonchus contortus, one of the most important parasitic nematodes that employ hypobiosis as a routine survival mechanism. In this parasite, acox-1 encodes three proteins (ACOXs) that all show a fatty acid oxidation activity in vitro and in vivo, and interact with a peroxin PEX-5 in peroxisomes. In particular, a peroxisomal targeting signal type1 (PTS1) sequence is required for ACOX-1 to be recognised by PEX-5. Analyses on developmental transcription and tissue expression show that acox-1 is predominantly expressed in the intestine and hypodermis of H. contortus, particularly in the early larval stages in the environment and the arrested fourth larval stage within host animals. Knockdown of acox-1 and pex-5 in parasitic H. contortus shows that these genes play essential roles in the post-embryonic larval development and likely in the facultative arrest of this species. A comprehensive understanding of these genes and the associated β-oxidation cycle of fatty acids should provide novel insights into the developmental regulation of parasitic nematodes, and into the discovery of novel interventions for species of socioeconomic importance.

A Keller-Segel model for C elegans L1 aggregation

We describe a mathematical model for the aggregation of starved first-stage C elegans larvae (L1s). We propose that starved L1s produce and respond chemotactically to two labile diffusible chemical signals, a short-range attractant and a longer range repellent. This model takes the mathematical form of three coupled partial differential equations, one that describes the movement of the worms and one for each of the chemical signals. Numerical solution of these equations produced a pattern of aggregates that resembled that of worm aggregates observed in experiments. We also describe the identification of a sensory receptor gene, srh-2, whose expression is induced under conditions that promote L1 aggregation. Worms whose srh-2 gene has been knocked out form irregularly shaped aggregates. Our model suggests this phenotype may be explained by the mutant worms slowing their movement more quickly than the wild type.

Interaction of Symbiotic Rhizobia and Parasitic Root-Knot Nematodes in Legume Roots: From Molecular Regulation to Field Application

Legumes form two types of root organs in response to signals from microbes, namely, nodules and root galls. In the field, these interactions occur concurrently and often interact with each other. The outcomes of these interactions vary and can depend on natural variation in rhizobia and nematode populations in the soil as well as abiotic conditions. While rhizobia are symbionts that contribute fixed nitrogen to their hosts, parasitic root-knot nematodes (RKN) cause galls as feeding structures that consume plant resources without a contribution to the plant. Yet, the two interactions share similarities, including rhizosphere signaling, repression of host defense responses, activation of host cell division, and differentiation, nutrient exchange, and alteration of root architecture. Rhizobia activate changes in defense and development through Nod factor signaling, with additional functions of effector proteins and exopolysaccharides. RKN inject large numbers of protein effectors into plant cells that directly suppress immune signaling and manipulate developmental pathways. This review examines the molecular control of legume interactions with rhizobia and RKN to elucidate shared and distinct mechanisms of these root-microbe interactions. Many of the molecular pathways targeted by both organisms overlap, yet recent discoveries have singled out differences in the spatial control of expression of developmental regulators that may have enabled activation of cortical cell division during nodulation in legumes. The interaction of legumes with symbionts and parasites highlights the importance of a comprehensive view of root-microbe interactions for future crop management and breeding strategies.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Male pheromones modulate synaptic transmission at the C. elegans neuromuscular junction in a sexually dimorphic manner

The development of functional synapses in the nervous system is important for animal physiology and behaviors, and its disturbance has been linked with many neurodevelopmental disorders. The synaptic transmission efficacy can be modulated by the environment to accommodate external changes, which is crucial for animal reproduction and survival. However, the underlying plasticity of synaptic transmission remains poorly understood. Here we show that in Caenorhabditis elegans, the male environment increases the hermaphrodite cholinergic transmission at the neuromuscular junction (NMJ), which alters hermaphrodites' locomotion velocity and mating efficiency. We identify that the male-specific pheromones mediate this synaptic transmission modulation effect in a developmental stage-dependent manner. Dissection of the sensory circuits reveals that the AWB chemosensory neurons sense those male pheromones and further transduce the information to NMJ using cGMP signaling. Exposure of hermaphrodites to the male pheromones specifically increases the accumulation of presynaptic CaV2 calcium channels and clustering of postsynaptic acetylcholine receptors at cholinergic synapses of NMJ, which potentiates cholinergic synaptic transmission. Thus, our study demonstrates a circuit mechanism for synaptic modulation and behavioral flexibility by sexual dimorphic pheromones.

Chemosensory signal transduction in Caenorhabditis elegans

Chemosensory neurons translate perception of external chemical cues, including odorants, tastants, and pheromones, into information that drives attraction or avoidance motor programs. In the laboratory, robust behavioral assays, coupled with powerful genetic, molecular and optical tools, have made Caenorhabditis elegans an ideal experimental system in which to dissect the contributions of individual genes and neurons to ethologically relevant chemosensory behaviors. Here, we review current knowledge of the neurons, signal transduction molecules and regulatory mechanisms that underlie the response of C. elegans to chemicals, including pheromones. The majority of identified molecules and pathways share remarkable homology with sensory mechanisms in other organisms. With the development of new tools and technologies, we anticipate that continued study of chemosensory signal transduction and processing in C. elegans will yield additional new insights into the mechanisms by which this animal is able to detect and discriminate among thousands of chemical cues with a limited sensory neuron repertoire.

Chemosensory Neurons Modulate the Response to Oomycete Recognition in Caenorhabditis elegans

Understanding how animals detect and respond to pathogen threats is central to dissecting mechanisms of host immunity. The oomycetes represent a diverse eukaryotic group infecting various hosts from nematodes to humans. We have previously shown that Caenorhabditis elegans mounts a defense response consisting of the induction of chitinase-like (chil) genes in the epidermis to combat infection by its natural oomycete pathogen Myzocytiopsis humicola. We provide here evidence that C. elegans can sense the oomycete by detecting an innocuous extract derived from animals infected with M. humicola. The oomycete recognition response (ORR) leads to changes in the cuticle and reduction in pathogen attachment, thereby increasing animal survival. We also show that TAX-2/TAX-4 function in chemosensory neurons is required for the induction of chil-27 in the epidermis in response to extract exposure. Our findings highlight that neuron-to-epidermis communication may shape responses to oomycete recognition in animal hosts.

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C. elegans senses its natural oomycete pathogen M. humicola without infection

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Exposure to a pathogen extract triggers an oomycete recognition response

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Upon pathogen detection, C. elegans resists infection through changes in the cuticle

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The response involves signaling between sensory neurons and the epidermis

Reduced peroxisomal import triggers peroxisomal retrograde signaling

Maintaining organelle function in the face of stress is known to involve organelle-specific retrograde signaling. Using Caenorhabditis elegans, we present evidence of the existence of such retrograde signaling for peroxisomes, which we define as the peroxisomal retrograde signaling (PRS). Specifically, we show that peroxisomal import stress caused by knockdown of the peroxisomal matrix import receptor prx-5/PEX5 triggers NHR-49/peroxisome proliferator activated receptor alpha (PPARα)- and MDT-15/MED15-dependent upregulation of the peroxisomal Lon protease lonp-2/LONP2 and the peroxisomal catalase ctl-2/CAT. Using proteomic and transcriptomic analyses, we show that proteins involved in peroxisomal lipid metabolism and immunity are also upregulated upon prx-5(RNAi). While the PRS can be triggered by perturbation of peroxisomal β-oxidation, we also observed hallmarks of PRS activation upon infection with Pseudomonas aeruginosa. We propose that the PRS, in addition to a role in lipid metabolism homeostasis, may act as a surveillance mechanism to protect against pathogens.

miR-31-5p regulates cold acclimation of the wood-boring beetle Monochamus alternatus via ascaroside signaling

BACKGROUND

Survival to cold stress in insects living in temperate environments requires the deployment of strategies that lead to physiological changes involved in freeze tolerance or freeze avoidance. These strategies may consist of, for instance, the induction of metabolic depression, accumulation of cryoprotectants, or the production of antifreeze proteins, however, little is known about the way such mechanisms are regulated and the signals involved in their activation. Ascarosides are signaling molecules usually known to regulate nematode behavior and development, whose expression was recently found to relate to thermal plasticity in the Japanese pine sawyer beetle Monochamus alternatus. Accumulating evidence also points to miRNAs as another class of regulators differentially expressed in response to cold stress, which are predicted to target genes involved in cold adaptation of insects. Here, we demonstrate a novel pathway involved in insect cold acclimation, through miRNA-mediated regulation of ascaroside function.

RESULTS

We initially discovered that experimental cold acclimation can enhance the beetle's cold hardiness. Through screening and functional verification, we found miR-31-5p, upregulated under cold stress, significantly contributes to this enhancement. Mechanistically, miR-31-5p promotes production of an ascaroside (asc-C9) in the beetle by negatively targeting the rate-limiting enzyme, acyl-CoA oxidase in peroxisomal β-oxidation cycles. Feeding experiments with synthetic asc-C9 suggests it may serve as a signal to promote cold acclimation through metabolic depression and accumulation of cryoprotectants with specific gene expression patterns.

CONCLUSIONS

Our results point to important roles of miRNA-mediated regulation of ascaroside function in insect cold adaptation. This enhanced cold tolerance may allow higher survival of M. alternatus in winter and be pivotal in shaping its wide distribution range, greatly expanding the threat of pine wilt disease, and thus can also inspire the development of ascaroside-based pest management strategies.

An Untargeted Approach for Revealing Electrophilic Metabolites

Reactive electrophilic intermediates such as coenzyme A esters play central roles in metabolism but are difficult to detect with conventional strategies. Here, we introduce hydroxylamine-based stable isotope labeling to convert reactive electrophilic intermediates into stable derivatives that are easily detectable via LC-MS. In the model system Caenorhabditis elegans, parallel treatment with ¹⁴NH₂OH and ¹⁵NH₂OH revealed >1000 labeled metabolites, e.g., derived from peptide, fatty acid, and ascaroside pheromone biosyntheses. Results from NH₂OH treatment of a pheromone biosynthesis mutant, acox-1.1, suggested upregulation of thioesterase activity, which was confirmed by gene expression analysis. The upregulated thioesterase contributes to the biosynthesis of a specific subset of ascarosides, determining the balance of dispersal and attractive signals. These results demonstrate the utility of NH₂OH labeling for investigating complex biosynthetic networks. Initial results with Aspergillus and human cell lines indicate applicability toward uncovering reactive metabolomes in diverse living systems.

Deep Interrogation of Metabolism Using a Pathway-Targeted Click-Chemistry Approach

Untargeted metabolomics indicates that the number of unidentified small-molecule metabolites may exceed the number of protein-coding genes for many organisms, including humans, by orders of magnitude. Uncovering the underlying metabolic networks is essential for elucidating the physiological and ecological significance of these biogenic small molecules. Here we develop a click-chemistry-based enrichment strategy, DIMEN (deep interrogation of metabolism via enrichment), that we apply to investigate metabolism of the ascarosides, a family of signaling molecules in the model organism C. elegans. Using a single alkyne-modified metabolite and a solid-phase azide resin that installs a diagnostic moiety for MS/MS-based identification, DIMEN uncovered several hundred novel compounds originating from diverse biosynthetic transformations that reveal unexpected intersection with amino acid, carbohydrate, and energy metabolism. Many of the newly discovered transformations could not be identified or detected by conventional LC-MS analyses without enrichment, demonstrating the utility of DIMEN for deeply probing biochemical networks that generate extensive yet uncharacterized structure space.

Small molecule signals mediate social behaviors in C. elegans

The last few decades have seen the structural and functional elucidation of small-molecule chemical signals called ascarosides in C. elegans. Ascarosides mediate several biological processes in worms, ranging from development, to behavior. These signals are modular in their design architecture, with their building blocks derived from metabolic pathways. Behavioral responses are not only concentration dependent, but also are influenced by the current physiological state of the animal. Cellular and circuit-level analyses suggest that these signals constitute a complex communication system, employing both synergistic molecular elements and sex-specific neuronal circuits governing the response. In this review, we discuss research from multiple laboratories, including our own, that detail how these chemical signals govern several different social behaviors in C. elegans. We propose that the ascaroside repertoire represents a link between diverse metabolic and neurobiological life-history traits and governs the survival of C. elegans in its natural environment.

Plasticity of pheromone-mediated avoidance behavior in C. elegans

Caenorhabditis elegans secretes a complex cocktail of small chemicals collectively called ascaroside pheromones which serves as a chemical language for intra-species communication. Subsets of ascarosides have been shown to mediate a broad spectrum of C. elegans behavior and development, such as gender-specific attraction, repulsion, aggregation, olfactory plasticity, and dauer formation. Recent studies show that specific components of ascarosides elicit a rapid avoidance response that allows animals to avoid predators and escape from unfavorable conditions. Moreover, this avoidance behavior is modulated by external conditions, internal states, and previous experience, indicating that pheromone avoidance behavior is highly plastic. In this review, we describe molecular and circuit mechanisms underlying plasticity in pheromone avoidance behavior which pave a way to better understanding circuit mechanisms underlying behavioral plasticity in higher animals, including humans.

A Large Family of Enzymes Responsible for the Modular Architecture of Nematode Pheromones

The nematode Caenorhabditis elegans produces a broad family of pheromones, known as the ascarosides, that are modified with a variety of groups derived from primary metabolism. These modifications are essential for the diverse activities of the ascarosides in development and various behaviors, including attraction, aggregation, avoidance, and foraging. The mechanism by which these different groups are added to the ascarosides is poorly understood. Here, we identify a family of over 30 enzymes, which are homologous to mammalian carboxylesterase (CES) enzymes, and show that a number of these enzymes are responsible for the selective addition of specific modifications to the ascarosides. Through stable isotope feeding experiments, we demonstrate the in vivo activity of the CES-like enzymes and provide direct evidence that the acyl-CoA synthetase ACS-7, which was previously implicated in the attachment of certain modifications to the ascarosides in C. elegans, instead activates the side chains of certain ascarosides for shortening through β-oxidation. Our data provide a key to the combinatorial logic that gives rise to different modified ascarosides, which should greatly facilitate the exploration of the specific biological functions of these pheromones in the worm.

The peroxisomal fatty acid transporter ABCD1/PMP-4 is required in the C. elegans hypodermis for axonal maintenance: A worm model for adrenoleukodystrophy

Adrenoleukodystrophy is a neurometabolic disorder caused by a defective peroxisomal ABCD1 transporter of very long-chain fatty acids (VLCFAs). Its pathogenesis is incompletely understood. Here we characterize a nematode model of X-ALD with loss of the pmp-4 gene, the worm orthologue of ABCD1. These mutants recapitulate the hallmarks of X-ALD: i) VLCFAs accumulation and impaired mitochondrial redox homeostasis and ii) axonal damage coupled to locomotor dysfunction. Furthermore, we identify a novel role for PMP-4 in modulating lipid droplet dynamics. Importantly, we show that the mitochondria targeted antioxidant MitoQ normalizes lipid droplets size, and prevents axonal degeneration and locomotor disability, highlighting its therapeutic potential. Moreover, PMP-4 acting solely in the hypodermis rescues axonal and locomotion abnormalities, suggesting a myelin-like role for the hypodermis in providing essential peroxisomal functions for the nematode nervous system.

Convergent evolution of small molecule pheromones in Pristionchus nematodes

The small molecules that mediate chemical communication between nematodes-so-called 'nematode-derived-modular-metabolites' (NDMMs)-are of major interest because of their ability to regulate development, behavior, and life-history. Pristionchus pacificus nematodes produce an impressive diversity of structurally complex NDMMs, some of which act as primer pheromones that are capable of triggering irreversible developmental switches. Many of these NDMMs have only ever been found in P. pacificus but no attempts have been made to study their evolution by profiling closely related species. This study brings a comparative perspective to the biochemical study of NDMMs through the systematic MS/MS- and NMR-based analysis of exo-metabolomes from over 30 Pristionchus species. We identified 36 novel compounds and found evidence for the convergent evolution of complex NDMMs in separate branches of the Pristionchus phylogeny. Our results demonstrate that biochemical innovation is a recurrent process in Pristionchus nematodes, a pattern that is probably typical across the animal kingdom.

A Molecular Basis for Reciprocal Regulation between Pheromones and Hormones in Response to Dietary Cues in C. elegans

Under stressful conditions, the early larvae of C. elegans enter dauer diapause, a non-aging period, driven by the seemingly opposite influence of ascaroside pheromones (ASCRs) and steroid hormone dafachronic acids (DAs). However, the molecular basis of how these small molecules engage in competitive crosstalk in coordination with insulin/IGF-1 signaling (IIS) remains elusive. Here we report a novel transcriptional regulatory pathway that seems to operate between the ASCR and DA biosynthesis under ad libitum (AL) feeding conditions or bacterial deprivation (BD). Although expression of the ASCR and DA biosynthetic genes reciprocally inhibit each other, ironically and interestingly, such dietary cue-mediated modulation requires the presence of the competitors. Under BD, induction of ASCR biosynthetic gene expression required DA, while ASCR suppresses the expression of the DA biosynthetic gene daf-36. The negative regulation of DA by ASCR was IIS-dependent, whereas daf-36 regulation appeared to be independent of IIS. These observations suggest that the presence of ASCR determines the IIS-dependency of DA gene expression regardless of dietary conditions. Thus, our work defines a molecular basis for a novel reciprocal gene regulation of pheromones and hormones to cope with stressful conditions during development and aging.

Nutrient Sensing and Response Drive Developmental Progression in Caenorhabditis elegans

In response to nutrient limitation, many animals, including Caenorhabditis elegans, slow or arrest their development. This process requires mechanisms that sense essential nutrients and induce appropriate responses. When faced with nutrient limitation, C. elegans can induce both short and long-term survival strategies, including larval arrest, decreased developmental rate, and dauer formation. To select the most advantageous strategy, information from many different sensors must be integrated into signaling pathways, including target of rapamycin (TOR) and insulin, that regulate developmental progression. Here, how nutrient information is sensed and integrated into developmental decisions that determine developmental rate and progression in C. elegans is reviewed.

Pheromones Modulate Learning by Regulating the Balanced Signals of Two Insulin-like Peptides

Social environment modulates learning through unknown mechanisms. Here, we report that a pheromone mixture that signals overcrowding inhibits C. elegans from learning to avoid pathogenic bacteria. We find that learning depends on the balanced signaling of two insulin-like peptides (ILPs), INS-16 and INS-4, which act respectively in the pheromone-sensing neuron ADL and the bacteria-sensing neuron AWA. Pheromone exposure inhibits learning by disrupting this balance: it activates ADL and increases expression of ins-16, and this cellular effect reduces AWA activity and AWA-expressed ins-4. The activities of the sensory neurons are required for learning and the expression of the ILPs. Interestingly, pheromones also promote the ingestion of pathogenic bacteria while increasing resistance to the pathogen. Thus, the balance of the ILP signals integrates social information into the learning process as part of a coordinated adaptive response that allows consumption of harmful food during times of high population density.

Intestinal peroxisomal fatty acid β-oxidation regulates neural serotonin signaling through a feedback mechanism

The ability to coordinate behavioral responses with metabolic status is fundamental to the maintenance of energy homeostasis. In numerous species including Caenorhabditis elegans and mammals, neural serotonin signaling regulates a range of food-related behaviors. However, the mechanisms that integrate metabolic information with serotonergic circuits are poorly characterized. Here, we identify metabolic, molecular, and cellular components of a circuit that links peripheral metabolic state to serotonin-regulated behaviors in C. elegans. We find that blocking the entry of fatty acyl coenzyme As (CoAs) into peroxisomal β-oxidation in the intestine blunts the effects of neural serotonin signaling on feeding and egg-laying behaviors. Comparative genomics and metabolomics revealed that interfering with intestinal peroxisomal β-oxidation results in a modest global transcriptional change but significant changes to the metabolome, including a large number of changes in ascaroside and phospholipid species, some of which affect feeding behavior. We also identify body cavity neurons and an ether-a-go-go (EAG)-related potassium channel that functions in these neurons as key cellular components of the circuitry linking peripheral metabolic signals to regulation of neural serotonin signaling. These data raise the possibility that the effects of serotonin on satiety may have their origins in feedback, homeostatic metabolic responses from the periphery.

Bx-daf-22 Contributes to Mate Attraction in the Gonochoristic Nematode Bursaphelenchus xylophilus

Studying sex communication is necessary to develop new methods to control the population expansion of gonochoristic species Bursaphelenchus xylophilus, the pathogen of pine wilt disease (PWD). Small chemical signals called ascarosides have been reported to attract potential mates. However, they have not been studied in the sex attraction of B. xylophilus. Here, we confirmed the sex attraction of B. xylophilus using a chemotaxis assay. Then, we cloned the downstream ascaroside biosynthetic gene Bx-daf-22 and explored its function in the sex attraction of B. xylophilus through bioinformatics analysis and RNA interference. The secretions of females and males were the sources of sex attraction in B. xylophilus, and the attractiveness of females to males was stronger than that of males to females. Compared with daf-22 of Caenorhabditis elegans, Bx-daf-22 underwent gene duplication events, resulting in Bx-daf-22.1, Bx-daf-22.2, and Bx-daf-22.3. RNA interference revealed that the attractiveness of female secretions to males increased after all three Bx-daf-22 genes or Bx-daf-22.3 had been interfered. However, the reciprocal experiments had no effect on the attractiveness of male secretions to females. Thus, Bx-daf-22 genes, especially Bx-daf-22.3, may be crucial for the effectiveness of female sex attractants. Our studies provide fundamental information to help identify the specific components and signal pathways of sex attractants in B. xylophilus.

Males, Outcrossing, and Sexual Selection in Caenorhabditis Nematodes

Males of Caenorhabditis elegans provide a crucial practical tool in the laboratory, but, as the rarer and more finicky sex, have not enjoyed the same depth of research attention as hermaphrodites. Males, however, have attracted the attention of evolutionary biologists who are exploiting the C. elegans system to test longstanding hypotheses about sexual selection, sexual conflict, transitions in reproductive mode, and genome evolution, as well as to make new discoveries about Caenorhabditis organismal biology. Here, we review the evolutionary concepts and data informed by study of males of C. elegans and other Caenorhabditis We give special attention to the important role of sperm cells as a mediator of inter-male competition and male-female conflict that has led to drastic trait divergence across species, despite exceptional phenotypic conservation in many other morphological features. We discuss the evolutionary forces important in the origins of reproductive mode transitions from males being common (gonochorism: females and males) to rare (androdioecy: hermaphrodites and males) and the factors that modulate male frequency in extant androdioecious populations, including the potential influence of selective interference, host-pathogen coevolution, and mutation accumulation. Further, we summarize the consequences of males being common vs rare for adaptation and for trait divergence, trait degradation, and trait dimorphism between the sexes, as well as for molecular evolution of the genome, at both micro-evolutionary and macro-evolutionary timescales. We conclude that C. elegans male biology remains underexploited and that future studies leveraging its extensive experimental resources are poised to discover novel biology and to inform profound questions about animal function and evolution.

Ascaroside Pheromones: Chemical Biology and Pleiotropic Neuronal Functions

Pheromones are neuronal signals that stimulate conspecific individuals to react to environmental stressors or stimuli. Research on the ascaroside (ascr) pheromones in Caenorhabditis elegans and other nematodes has made great progress since ascr#1 was first isolated and biochemically defined in 2005. In this review, we highlight the current research on the structural diversity, biosynthesis, and pleiotropic neuronal functions of ascr pheromones and their implications in animal physiology. Experimental evidence suggests that ascr biosynthesis starts with conjugation of ascarylose to very long-chain fatty acids that are then processed via peroxisomal β-oxidation to yield diverse ascr pheromones. We also discuss the concentration and stage-dependent pleiotropic neuronal functions of ascr pheromones. These functions include dauer induction, lifespan extension, repulsion, aggregation, mating, foraging and detoxification, among others. These roles are carried out in coordination with three G protein-coupled receptors that function as putative pheromone receptors: SRBC-64/66, SRG-36/37, and DAF-37/38. Pheromone sensing is transmitted in sensory neurons via DAF-16-regulated glutamatergic neurotransmitters. Neuronal peroxisomal fatty acid β-oxidation has important cell-autonomous functions in the regulation of neuroendocrine signaling, including neuroprotection. In the future, translation of our knowledge of nematode ascr pheromones to higher animals might be beneficial, as ascr#1 has some anti-inflammatory effects in mice. To this end, we propose the establishment of pheromics (pheromone omics) as a new subset of integrated disciplinary research area within chemical ecology for system-wide investigation of animal pheromones.

Natural products as chemical tools to dissect complex biology in C. elegans

The search for novel pheromones, hormones, and other types of natural products in the nematode Caenorhabditis elegans has accelerated over the last 10-15 years. Many of these natural products perturb fundamental processes such as developmental progression, metabolism, reproductive and somatic aging, and various behaviors and have thus become essential tools for probing these processes, which are difficult to study in higher organisms. Furthermore, given the similarity between C. elegans and parasitic nematodes, these natural products could potentially be used to manipulate the development and behavior of parasitic nematodes and target the infections caused by them.

Tryptophan Metabolism in Caenorhabditis elegans Links Aggregation Behavior to Nutritional Status

Caenorhabditis elegans uses aggregation pheromones to communicate its nutritional status and recruit fellow members of its species to food sources. These aggregation pheromones include the IC-ascarosides, ascarosides modified with an indole-3-carbonyl (IC) group on the 4'-position of the ascarylose sugar. Nothing is known about the biosynthesis of the IC modification beyond the fact that it is derived from tryptophan. Here, we show that C. elegans produces endogenously several indole-containing metabolites, including indole-3-pyruvic acid (IPA), indole-3-acetic acid (IAA; auxin), and indole-3-carboxylic acid, and that these metabolites are intermediates in the biosynthetic pathway from tryptophan to the IC group. Stable isotope-labeled IPA and IAA are incorporated into the IC-ascarosides. Importantly, we show that flux through the biosynthetic pathway is affected by the activity of the pyruvate dehydrogenase complex (PDC). Knockdown of the PDC by RNA interference leads to an accumulation of upstream metabolites and a reduction in downstream metabolites in the pathway. Our results show that production of aggregation pheromones is linked to PDC activity and that aggregation behavior may reflect a favorable metabolic state in the worm. Lastly, we show that treatment of  C. elegans with indole-containing metabolites in the pathway induces the biosynthesis of the IC-ascarosides. Because the natural environment of C. elegans is rotting plant material, indole-containing metabolites in this environment could potentially stimulate pheromone biosynthesis and aggregation behavior in the worm. Thus, there may be important links between tryptophan metabolism in C. elegans and in plants and bacteria that enable interkingdom signaling.

Changes to social feeding behaviors are not sufficient for fitness gains of the Caenorhabditis elegans N2 reference strain

The standard reference Caenorhabditis elegans strain, N2, has evolved marked behavioral changes in social feeding behavior since its isolation from the wild. We show that the causal, laboratory-derived mutations in two genes, npr-1 and glb-5, confer large fitness advantages in standard laboratory conditions. Using environmental manipulations that suppress social/solitary behavior differences, we show the fitness advantages of the derived alleles remained unchanged, suggesting selection on these alleles acted through pleiotropic traits. Transcriptomics, developmental timing, and food consumption assays showed that N2 animals mature faster, produce more sperm, and consume more food than a strain containing ancestral alleles of these genes regardless of behavioral strategies. Our data suggest that the pleiotropic effects of glb-5 and npr-1 are a consequence of changes to O₂ -sensing neurons that regulate both aerotaxis and energy homeostasis. Our results demonstrate how pleiotropy can lead to profound behavioral changes in a popular laboratory model.

Why do humans walk on two feet? And what makes us smarter than our ape ancestors? The answers to these questions, and countless others about the particular traits of any number of species, is often said to be natural selection – a process where genes that ensure the survival of a species are favored of others. But it is not always the answer. Other evolutionary forces, such as random changes to the frequency of certain gene variants, restrictions on the development of a certain trait and pleiotropy (where one gene influences other, seemingly unrelated traits) can also cause differences between species.

Designing experiments to test whether a trait difference is due to natural selection or other factors is notoriously difficult. However, the humble nematode worm, Caenorhabditis elegans, has proven to be particularly useful in this respect. One subtype or strain of C. elegans with certain changes to its genes is used internationally as a ‘reference strain’, to ensure results between labs are comparable. This strain, N2, has been bred in the laboratory for hundreds of generations, isolated from its wild counterparts. N2 shows several differences in behavior from the wildtype, including its feeding habits. Wild C. elegans tend to feed together socially, whereas N2 prefers to feed alone.

In 1998 and 2009, researchers – including some involved in the current study – have identified the genetic modifications responsible for this change in behavior. Now, Zhao et al. set out to determine whether this was due to natural selection, and if so, was there a benefit to solitary feeding in laboratory conditions that was driving this genetic change? Zhao et al. found that the genetic changes in the N2 strain gave the worms a considerable advantage in the artificial environment. However, experiments to modify the conditions the animals grew in revealed that the solitary feeding habits were not necessary for the fitness advantage. In other words, the changes in feeding habits were a symptom of the genetic changes that gave N2 a selective advantage, but they were not the cause. In other words, the changes in feeding behavior were not a result of natural selection, but rather of pleiotropy.

The findings highlight that not every change in a trait is down to natural selection and must therefore be put to the test. With declining costs of DNA sequencing, researchers can now easily identify genes and regions of DNA that are likely to be under selection. However, they must be careful before leaping to the conclusion that behavioral differences linked to genetic changes are adaptive. In addition, the findings show that the laboratories relying on N2 as a model organism should be aware that the strain has evolved fundamental differences in its brain connections compared with the wildtype.

Biosynthetic tailoring of existing ascaroside pheromones alters their biological function in C. elegans

Caenorhabditis elegans produces ascaroside pheromones to control its development and behavior. Even minor structural differences in the ascarosides have dramatic consequences for their biological activities. Here, we identify a mechanism that enables C. elegans to dynamically tailor the fatty-acid side chains of the indole-3-carbonyl (IC)-modified ascarosides it has produced. In response to starvation, C. elegans uses the peroxisomal acyl-CoA synthetase ACS-7 to activate the side chains of medium-chain IC-ascarosides for β-oxidation involving the acyl-CoA oxidases ACOX-1.1 and ACOX-3. This pathway rapidly converts a favorable ascaroside pheromone that induces aggregation to an unfavorable one that induces the stress-resistant dauer larval stage. Thus, the pathway allows the worm to respond to changing environmental conditions and alter its chemical message without having to synthesize new ascarosides de novo. We establish a new model for biosynthesis of the IC-ascarosides in which side-chain β-oxidation is critical for controlling the type of IC-ascarosides produced.

Small roundworms such as Caenorhabditis elegans release chemical signals called ascarosides in order to communicate with other worms of the same species. Using the ascarosides, the worm can tell its friends, for example, how crowded the neighborhood is and whether there is enough food. The ascarosides thus help the worms in the population decide whether the neighborhood is good – meaning they should hang around, eat, and make babies – or whether the neighborhood is bad. If so, the worms should develop into a larval stage specialized for dispersal that will allow them to find a better neighborhood.

Roundworms make the ascarosides by attaching a long chemical ‘side chain’ to an ascarylose sugar. Further chemical modifications allow the worms to produce different signals. In general, to signal a good neighborhood, worms attach a structure called an indole group to the ascarosides. To signal a bad neighborhood, worms make the side chain very short. But how does a worm control which ascarosides it makes?

Zhou, Wang et al. now show that C. elegans can change the meaning of its chemical message by modifying the ascarosides that it has already produced instead of making new ones from scratch. Specifically, as their neighborhood runs out of food, C. elegans can use an enzyme called ACS-7 to initiate the shortening of the side chains of indole-ascarosides. The worm can thus change a favorable ascaroside signal that causes the worms to group together into an unfavorable ascaroside signal that causes the worms to enter their dispersal stage.

Although Zhou, Wang et al. have focused on chemical communication in C. elegans, the findings could easily apply to the many other species of roundworm that produce ascarosides. Knowing how worms communicate will help us to understand how worms respond to their environment. This knowledge could potentially be used to interfere with the lifecycles and survival of parasitic worm species that harm health and crops.

Effects of Larval Density on Gene Regulation in Caenorhabditis elegans During Routine L1 Synchronization

Bleaching gravid C. elegans followed by a short period of starvation of the L1 larvae is a routine method performed by worm researchers for generating synchronous populations for experiments. During the process of investigating dietary effects on gene regulation in L1 stage worms by single-worm RNA-Seq, we found that the density of resuspended L1 larvae affects expression of many mRNAs. Specifically, a number of genes related to metabolism and signaling are highly expressed in worms arrested at low density, but are repressed at higher arrest densities. We generated a GFP reporter strain based on one of the most density-dependent genes in our dataset – lips-15 – and confirmed that this reporter was expressed specifically in worms arrested at relatively low density. Finally, we show that conditioned media from high density L1 cultures was able to downregulate lips-15 even in L1 animals arrested at low density, and experiments using daf-22 mutant animals demonstrated that this effect is not mediated by the ascaroside family of signaling pheromones. Together, our data implicate a soluble signaling molecule in density sensing by L1 stage C. elegans, and provide guidance for design of experiments focused on early developmental gene regulation.

Acyl-CoA Oxidases Fine-Tune the Production of Ascaroside Pheromones with Specific Side Chain Lengths

Caenorhabditis elegans produces a complex mixture of ascaroside pheromones to control its development and behavior. Acyl-CoA oxidases, which participate in β-oxidation cycles that shorten the side chains of the ascarosides, regulate the mixture of pheromones produced. Here, we use CRISPR-Cas9 to make specific nonsense and missense mutations in acox genes and determine the effect of these mutations on ascaroside production in vivo. Ascaroside production in acox-1.1 deletion and nonsense strains, as well as a strain with a missense mutation in a catalytic residue, confirms the central importance of ACOX-1.1 in ascaroside biosynthesis and suggests that ACOX-1.1 functions in part by facilitating the activity of other acyl-CoA oxidases. Ascaroside production in an acox-1.1 strain with a missense mutation in an ATP-binding site at the ACOX-1.1 dimer interface suggests that ATP binding is important for the enzyme to function in ascaroside biosynthesis in vivo. Ascaroside production in strains with deletion, nonsense, and missense mutations in other acox genes demonstrates that ACOX-1.1 works with ACOX-1.3 in processing ascarosides with 7-carbon side chains, ACOX-1.4 in processing ascarosides with 9- and 11-carbon side chains, and ACOX-3 in processing ascarosides with 13- and 15-carbon side chains. It also shows that ACOX-1.2, but not ACOX-1.1, processes ascarosides with 5-carbon ω-side chains. By modeling the ACOX structures, we uncover characteristics of the enzyme active sites that govern substrate preferences. Our work demonstrates the role of specific acyl-CoA oxidases in controlling the length of ascaroside side chains and thus in determining the mixture of pheromones produced by C. elegans.

Low-level shear stress promotes migration of liver cancer stem cells via the FAK-ERK1/2 signalling pathway

Cancer stem cells (CSCs) are a small subpopulation of tumour cells that have been proposed to be responsible for cancer initiation, chemotherapy resistance and cancer recurrence. Shear stress activated cellular signalling is involved in cellular migration, proliferation and differentiation. However, little is known about the effects of shear stress on the migration of liver cancer stem cells (LCSCs). Here, we studied the effects of shear stress that are generated from a parallel plated flow chamber system, on LCSC migration and the activation of focal adhesion kinase (FAK) and extracellular signal regulated kinase1/2 (ERK1/2), using transwell assay and western blot, respectively. We found that 2 dyne/cm² shear stress loading for 6 h promotes LCSC migration and activation of the FAK and ERK1/2 signalling pathways, whereas treatment with the FAK phosphorylation inhibitor PF573228 or the ERK1/2 phosphorylation inhibitor PD98059 suppressed the shear stress-promoted migration, indicating the involvement of FAK and ERK1/2 activation in shear stress-induced LCSC migration. Additionally, atomic force microscopy (AFM) analysis showed that shear stress lowers LCSC stiffness via the FAK and ERK1/2 pathways, suggesting that the mechanism by which shear stress promotes LCSC migration might partially be responsible for the decrease in cell stiffness. Further experiments focused on the role of the actin cytoskeleton, demonstrating that the F-actin filaments in LCSCs are less well-defined after shear stress treatment, providing an explanation for the reduction in cell stiffness and the promotion of cell migration. Overall, our study demonstrates that shear stress promotes LCSC migration through the activation of the FAK-ERK1/2 signalling pathways, which further results in a reduction of organized actin and softer cell bodies.

Comparative Ascaroside Profiling of Caenorhabditis Exometabolomes Reveals Species-Specific (ω) and (ω - 2)-Hydroxylation Downstream of Peroxisomal β-Oxidation

Chemical communication in nematodes such as the model organism Caenorhabditis elegans is modulated by a variety of glycosides based on the dideoxysugar l-ascarylose. Comparative ascaroside profiling of nematode exometabolome extracts using a GC-EIMS screen reveals that several basic components including ascr#1 (asc-C7), ascr#2 (asc-C6-MK), ascr#3 (asc-ΔC9), ascr#5 (asc-ωC3), and ascr#10 (asc-C9) are highly conserved among the Caenorhabditis. Three novel side chain hydroxylated ascaroside derivatives were exclusively detected in the distantly related C. nigoni and C. afra. Molecular structures of these species-specific putative signaling molecules were elucidated by NMR spectroscopy and confirmed by total synthesis and chemical correlations. Biological activities were evaluated using attraction assays. The identification of (ω)- and (ω - 2)-hydroxyacyl ascarosides demonstrates how GC-EIMS-based ascaroside profiling facilitates the detection of novel ascaroside components and exemplifies how species-specific hydroxylation of ascaroside aglycones downstream of peroxisomal β-oxidation increases the structural diversity of this highly conserved class of nematode signaling molecules.

Metabolomic "Dark Matter" Dependent on Peroxisomal β-Oxidation in Caenorhabditis elegans

Peroxisomal β-oxidation (pβo) is a highly conserved fat metabolism pathway involved in the biosynthesis of diverse signaling molecules in animals and plants. In Caenorhabditis elegans, pβo is required for the biosynthesis of the ascarosides, signaling molecules that control development, lifespan, and behavior in this model organism. Via comparative mass spectrometric analysis of pβo mutants and wildtype, we show that pβo in C. elegans and the satellite model P. pacificus contributes to life stage-specific biosynthesis of several hundred previously unknown metabolites. The pβo-dependent portion of the metabolome is unexpectedly diverse, e.g., intersecting with nucleoside and neurotransmitter metabolism. Cell type-specific restoration of pβo in pβo-defective mutants further revealed that pβo-dependent submetabolomes differ between tissues. These results suggest that interactions of fat, nucleoside, and other primary metabolism pathways can generate structural diversity reminiscent of that arising from combinatorial strategies in microbial natural product biosynthesis.

Lipid and Carbohydrate Metabolism in Caenorhabditis elegans

Lipid and carbohydrate metabolism are highly conserved processes that affect nearly all aspects of organismal biology. Caenorhabditis elegans eat bacteria, which consist of lipids, carbohydrates, and proteins that are broken down during digestion into fatty acids, simple sugars, and amino acid precursors. With these nutrients, C. elegans synthesizes a wide range of metabolites that are required for development and behavior. In this review, we outline lipid and carbohydrate structures as well as biosynthesis and breakdown pathways that have been characterized in C. elegans We bring attention to functional studies using mutant strains that reveal physiological roles for specific lipids and carbohydrates during development, aging, and adaptation to changing environmental conditions.

An oxytocin-dependent social interaction between larvae and adult C. elegans

Oxytocin has a conserved role in regulating animal social behaviour including parental-offspring interactions. Recently an oxytocin-like neuropeptide, nematocin, and its cognate receptors have been identified in the nematode Caenorhabditis elegans. We provide evidence for a pheromone signal produced by C. elegans larvae that modifies the behaviour of adult animals in an oxytocin-dependent manner increasing their probability of leaving a food patch which the larvae are populating. This increase is positively correlated to the size of the larval population but cannot be explained by food depletion nor is it modulated by biogenic amines, which suggest it is not an aversive behaviour. Moreover, the food-leaving behaviour is conspecific and pheromone dependent: C. elegans adults respond more strongly to C. elegans larvae compared to other nematode species and this effect is absent in C. elegans daf-22 larvae which are pheromone deficient. Neurotransmitter receptors previously implicated in C. elegans foraging decisions NPR-1 and TYRA-3, for NPY-like neuropeptides and tyramine respectively, do not appear to be involved in oxytocin-dependent adult food-leaving. We conclude oxytocin signals within a novel neural circuit that regulates parental-offspring social behaviour in C. elegans and that this provides evidence for evolutionary conservation of molecular components of a parental decision making behaviour.