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.

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.

Nematode ascarosides attenuate mammalian type 2 inflammatory responses

Animal proof-of-concept studies have shown that roundworms have a protective effect against immune-dysregulated disorders, but it has been difficult to study in human trials without individual nematode-derived molecules to develop and test. We discovered that ascarosides, molecules that are secreted by diverse nematodes, suppress asthma in a rodent model via modulation of expression of Il33, a key epithelial cytokine for induction of type 2 immunity, in addition to decreasing memory-type pathogenic Th2 cells and ILC2s and increasing the Il10-expressing subpopulation of interstitial macrophages in the lung. Thus, ascarosides suppress type 2 immune response by affecting both innate and adaptive immunity and could define a potent class of small molecule drugs to treat allergic airway diseases.

Mounting evidence suggests that nematode infection can protect against disorders of immune dysregulation. Administration of live parasites or their excretory/secretory (ES) products has shown therapeutic effects across a wide range of animal models for immune disorders, including asthma. Human clinical trials of live parasite ingestion for the treatment of immune disorders have produced promising results, yet concerns persist regarding the ingestion of pathogenic organisms and the immunogenicity of protein components. Despite extensive efforts to define the active components of ES products, no small molecules with immune regulatory activity have been identified from nematodes. Here we show that an evolutionarily conserved family of nematode pheromones called ascarosides strongly modulates the pulmonary immune response and reduces asthma severity in mice. Screening the inhibitory effects of ascarosides produced by animal-parasitic nematodes on the development of asthma in an ovalbumin (OVA) murine model, we found that administration of nanogram quantities of ascr#7 prevented the development of lung eosinophilia, goblet cell metaplasia, and airway hyperreactivity. Ascr#7 suppressed the production of IL-33 from lung epithelial cells and reduced the number of memory-type pathogenic Th2 cells and ILC2s in the lung, both key drivers of the pathology of asthma. Our findings suggest that the mammalian immune system recognizes ascarosides as an evolutionarily conserved molecular signature of parasitic nematodes. The identification of a nematode-produced small molecule underlying the well-documented immunomodulatory effects of ES products may enable the development of treatment strategies for allergic diseases.

Sex-specific, pdfr-1-dependent modulation of pheromone avoidance by food abundance enables flexibility in C. elegans foraging behavior

To make adaptive feeding and foraging decisions, animals must integrate diverse sensory streams with multiple dimensions of internal state. In C. elegans, foraging and dispersal behaviors are influenced by food abundance, population density, and biological sex, but the neural and genetic mechanisms that integrate these signals are poorly understood. Here, by systematically varying food abundance, we find that chronic avoidance of the population-density pheromone ascr#3 is modulated by food thickness, such that hermaphrodites avoid ascr#3 only when food is scarce. The integration of food and pheromone signals requires the conserved neuropeptide receptor PDFR-1, as pdfr-1 mutant hermaphrodites display strong ascr#3 avoidance, even when food is abundant. Conversely, increasing PDFR-1 signaling inhibits ascr#3 aversion when food is sparse, indicating that this signal encodes information about food abundance. In both wild-type and pdfr-1 hermaphrodites, chronic ascr#3 avoidance requires the ASI sensory neurons. In contrast, PDFR-1 acts in interneurons, suggesting that it modulates processing of the ascr#3 signal. Although a sex-shared mechanism mediates ascr#3 avoidance, food thickness modulates this behavior only in hermaphrodites, indicating that PDFR-1 signaling has distinct functions in the two sexes. Supporting the idea that this mechanism modulates foraging behavior, ascr#3 promotes ASI-dependent dispersal of hermaphrodites from food, an effect that is markedly enhanced when food is scarce. Together, these findings identify a neurogenetic mechanism that sex-specifically integrates population and food abundance, two important dimensions of environmental quality, to optimize foraging decisions. Further, they suggest that modulation of attention to sensory signals could be an ancient, conserved function of pdfr-1.

Modular metabolite assembly in Caenorhabditis elegans depends on carboxylesterases and formation of lysosome-related organelles

Signaling molecules derived from attachment of diverse metabolic building blocks to ascarosides play a central role in the life history of C. elegans and other nematodes; however, many aspects of their biogenesis remain unclear. Using comparative metabolomics, we show that a pathway mediating formation of intestinal lysosome-related organelles (LROs) is required for biosynthesis of most modular ascarosides as well as previously undescribed modular glucosides. Similar to modular ascarosides, the modular glucosides are derived from highly selective assembly of moieties from nucleoside, amino acid, neurotransmitter, and lipid metabolism, suggesting that modular glucosides, like the ascarosides, may serve signaling functions. We further show that carboxylesterases that localize to intestinal organelles are required for the assembly of both modular ascarosides and glucosides via ester and amide linkages. Further exploration of LRO function and carboxylesterase homologs in C. elegans and other animals may reveal additional new compound families and signaling paradigms.

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.

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.

Pheromones and Nutritional Signals Regulate the Developmental Reliance on let-7 Family MicroRNAs in C. elegans

Adverse environmental conditions can affect rates of animal developmental progression and lead to temporary developmental quiescence (diapause), exemplified by the dauer larva stage of the nematode Caenorhabditis elegans (C. elegans). Remarkably, patterns of cell division and temporal cell-fate progression in C. elegans larvae are not affected by changes in developmental trajectory. However, the underlying physiological and gene regulatory mechanisms that ensure robust developmental patterning despite substantial plasticity in developmental progression are largely unknown. Here, we report that diapause-inducing pheromones correct heterochronic developmental cell lineage defects caused by insufficient expression of let-7 family microRNAs in C. elegans. Moreover, two conserved endocrine signaling pathways, DAF-7/TGF-β and DAF-2/Insulin, that confer on the larva diapause and non-diapause alternative developmental trajectories interact with the nuclear hormone receptor, DAF-12, to initiate and regulate a rewiring of the genetic circuitry controlling temporal cell fates. This rewiring includes engagement of certain heterochronic genes, lin-46, lin-4, and nhl-2, that are previously associated with an altered genetic program in post-diapause animals, in combination with a novel ligand-independent DAF-12 activity, to downregulate the critical let-7 family target Hunchback-like-1 (HBL-1). Our results show how pheromone or endocrine signaling pathways can coordinately regulate both developmental progression and cell-fate transitions in C. elegans larvae under stress so that the developmental schedule of cell fates remains unaffected by changes in developmental trajectory.

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.

Linking Genomic and Metabolomic Natural Variation Uncovers Nematode Pheromone Biosynthesis

In the nematodes Caenorhabditis elegans and Pristionchus pacificus, a modular library of small molecules control behavior, lifespan, and development. However, little is known about the final steps of their biosynthesis, in which diverse building blocks from primary metabolism are attached to glycosides of the dideoxysugar ascarylose, the ascarosides. We combine metabolomic analysis of natural isolates of P. pacificus with genome-wide association mapping to identify a putative carboxylesterase, Ppa-uar-1, that is required for attachment of a pyrimidine-derived moiety in the biosynthesis of ubas#1, a major dauer pheromone component. Comparative metabolomic analysis of wild-type and Ppa-uar-1 mutants showed that Ppa-uar-1 is required specifically for the biosynthesis of ubas#1 and related metabolites. Heterologous expression of Ppa-UAR-1 in C. elegans yielded a non-endogenous ascaroside, whose structure confirmed that Ppa-uar-1 is involved in modification of a specific position in ascarosides. Our study demonstrates the utility of natural variation-based approaches for uncovering biosynthetic pathways.

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.

Structural characterization of acyl-CoA oxidases reveals a direct link between pheromone biosynthesis and metabolic state in Caenorhabditis elegans

Caenorhabditis elegans secretes ascarosides as pheromones to communicate with other worms and to coordinate the development and behavior of the population. Peroxisomal β-oxidation cycles shorten the side chains of ascaroside precursors to produce the short-chain ascaroside pheromones. Acyl-CoA oxidases, which catalyze the first step in these β-oxidation cycles, have different side chain-length specificities and enable C. elegans to regulate the production of specific ascaroside pheromones. Here, we determine the crystal structure of the acyl-CoA oxidase 1 (ACOX-1) homodimer and the ACOX-2 homodimer bound to its substrate. Our results provide a molecular basis for the substrate specificities of the acyl-CoA oxidases and reveal why some of these enzymes have a very broad substrate range, whereas others are quite specific. Our results also enable predictions to be made for the roles of uncharacterized acyl-CoA oxidases in C. elegans and in other nematode species. Remarkably, we show that most of the C. elegans acyl-CoA oxidases that participate in ascaroside biosynthesis contain a conserved ATP-binding pocket that lies at the dimer interface, and we identify key residues in this binding pocket. ATP binding induces a structural change that is associated with tighter binding of the FAD cofactor. Mutations that disrupt ATP binding reduce FAD binding and reduce enzyme activity. Thus, ATP may serve as a regulator of acyl-CoA oxidase activity, thereby directly linking ascaroside biosynthesis to ATP concentration and metabolic state.

Functional Conservation and Divergence of daf-22 Paralogs in Pristionchus pacificus Dauer Development

Small-molecule signaling in nematode dauer formation has emerged as a major model to study chemical communication in development and evolution. Developmental arrest as nonfeeding and stress-resistant dauer larvae represents the major survival and dispersal strategy. Detailed studies in Caenorhabditis elegans and Pristionchus pacificus revealed that small-molecule communication changes rapidly in evolution resulting in extreme structural diversity of small-molecule compounds. In C. elegans, a blend of ascarosides constitutes the dauer pheromone, whereas the P. pacificus dauer pheromone includes additional paratosides and integrates building blocks from diverse primary metabolic pathways. Despite this complexity of small-molecule structures and functions, little is known about the biosynthesis of small molecules in nematodes outside C. elegans Here, we show that the genes encoding enzymes of the peroxisomal β-oxidation pathway involved in small-molecule biosynthesis evolve rapidly, including gene duplications and domain switching. The thiolase daf-22, the most downstream factor in C. elegans peroxisomal β-oxidation, has duplicated in P. pacificus, resulting in Ppa-daf-22.1, which still contains the sterol-carrier-protein (SCP) domain that was lost in C. elegans daf-22, and Ppa-daf-22.2. Using the CRISPR/Cas9 system, we induced mutations in both P. pacificus daf-22 genes and identified an unexpected complexity of functional conservation and divergence. Under well-fed conditions, ascaroside biosynthesis proceeds exclusively via Ppa-daf-22.1 In contrast, starvation conditions induce Ppa-daf-22.2 activity, resulting in the production of a specific subset of ascarosides. Gene expression studies indicate a reciprocal up-regulation of both Ppa-daf-22 genes, which is, however, independent of starvation. Thus, our study reveals an unexpected functional complexity of dauer development and evolution.

Plant basal resistance to nematodes: an update

Most plant-parasitic nematodes are obligate biotrophs feeding on the roots of their hosts. Whereas ectoparasites remain on the root surface and feed on the outer cell layers, endoparasitic nematodes enter the host to parasitize cells around or within the central cylinder. Nematode invasion and feeding causes tissue damage which may, in turn, lead to the activation of host basal defence responses. Hitherto, research interests in plant-nematode interaction have emphasized effector-triggered immunity rather than basal plant defence responses. However, some recent investigations suggest that basal defence pathways are not only activated but also play an important role in determining interaction outcomes. In this review we discuss the major findings and point out future directions to dissect the molecular mechanisms underlying plant basal defence to nematodes further.

Function and Regulation of Lipid Biology in Caenorhabditis elegans Aging

Rapidly expanding aging populations and a concomitant increase in the prevalence of age-related diseases are global health problems today. Over the past three decades, a large body of work has led to the identification of genes and regulatory networks that affect longevity and health span, often benefiting from the tremendous power of genetics in vertebrate and invertebrate model organisms. Interestingly, many of these factors appear linked to lipids, important molecules that participate in cellular signaling, energy metabolism, and structural compartmentalization. Despite the putative link between lipids and longevity, the role of lipids in aging remains poorly understood. Emerging data from the model organism Caenorhabditis elegans suggest that lipid composition may change during aging, as several pathways that influence aging also regulate lipid metabolism enzymes; moreover, some of these enzymes apparently play key roles in the pathways that affect the rate of aging. By understanding how lipid biology is regulated during C. elegans aging, and how it impacts molecular, cellular, and organismal function, we may gain insight into novel ways to delay aging using genetic or pharmacological interventions. In the present review we discuss recent insights into the roles of lipids in C. elegans aging, including regulatory roles played by lipids themselves, the regulation of lipid metabolic enzymes, and the roles of lipid metabolism genes in the pathways that affect aging.