A pheromone influences larval development in the nematode Caenorhabditis elegans

When stress predicts a shrinking gene pool, trading early reproduction for longevity can increase fitness, even with lower fecundity

BACKGROUND

Stresses like dietary restriction or various toxins increase lifespan in taxa as diverse as yeast, Caenorhabditis elegans, Drosophila and rats, by triggering physiological responses that also tend to delay reproduction. Food odors can reverse the effects of dietary restriction, showing that key mechanisms respond to information, not just resources. Such environmental cues can predict population trends, not just individual prospects for survival and reproduction. When population size is increasing, each offspring produced earlier makes a larger proportional contribution to the gene pool, but the reverse is true when population size is declining.

PRINCIPAL FINDINGS

We show mathematically that natural selection can favor facultative delay in reproduction when environmental cues predict a decrease in total population size, even if lifetime fecundity decreases with delay. We also show that increased reproduction from waiting for better conditions does not increase fitness (proportional representation) when the whole population benefits similarly.

CONCLUSIONS

We conclude that the beneficial effects of stress on longevity (hormesis) in diverse taxa are a side-effect of delaying reproduction in response to environmental cues that population size is likely to decrease. The reversal by food odors of the effects of dietary restriction can be explained as a response to information that population size is less likely to decrease, reducing the chance that delaying reproduction will increase fitness.

Infective juvenile formation in the insect parasitic nematode Steinernema feltiae

Steinernematid nematodes form a developmentally arrested infective juvenile (IJ) stage at the second moult, when conditions inside the insect host are no longer suitable for further reproduction. In a liquid culture micro-assay two environmental cues were shown to influence the formation of Steinernema feltiae IJs. High nematode population density induced IJ formation, suggesting the presence of a nematode pheromone. Bacterial food and soluble nutrients acted competitively to reduce the frequency of IJ formation. Frequency of IJ formation was greatest when lst-stage juveniles were subjected to IJ-inducing conditions. The optimum temperature range for the IJ induction response was 25–30°C. These findings suggest that maximal IJ production in large-scale liquid culture will depend on the correct balance of nematode population density and nutrient availability at the peak of egg hatching.

The endocrine regulation of aging in Caenorhabditis elegans

In recent years, there has been significant growth in our understanding of the regulation of longevity. The most notable change is the identification and detailed description of a number of molecular pathways modulating the rate of aging. A good portion of this new data has come from studies using the genetic model organism Caenorhabditis elegans. In this review, we provide an overview of physiological systems that are involved in the modulation of aging in C. elegans, then discuss the known endocrine signaling systems that are likely to couple these systems together. Finally, we present a working model describing how aging may be regulated as a coordinated system, communicating through endocrine signals.

Biosynthesis of the Caenorhabditis elegans dauer pheromone

To sense its population density and to trigger entry into the stress-resistant dauer larval stage, Caenorhabditis elegans uses the dauer pheromone, which consists of ascaroside derivatives with short, fatty acid-like side chains. Although the dauer pheromone has been studied for 25 years, its biosynthesis is completely uncharacterized. The daf-22 mutant is the only known mutant defective in dauer pheromone production. Here, we show that daf-22 encodes a homolog of human sterol carrier protein SCPx, which catalyzes the final step in peroxisomal fatty acid beta-oxidation. We also show that dhs-28, which encodes a homolog of the human d-bifunctional protein that acts just upstream of SCPx, is also required for pheromone production. Long-term daf-22 and dhs-28 cultures develop dauer-inducing activity by accumulating less active, long-chain fatty acid ascaroside derivatives. Thus, daf-22 and dhs-28 are required for the biosynthesis of the short-chain fatty acid-derived side chains of the dauer pheromone and link dauer pheromone production to metabolic state.

Molecular time-course and the metabolic basis of entry into dauer in Caenorhabditis elegans

When Caenorhabditis elegans senses dauer pheromone (daumone), signaling inadequate growth conditions, it enters the dauer state, which is capable of long-term survival. However, the molecular pathway of dauer entry in C. elegans has remained elusive. To systematically monitor changes in gene expression in dauer paths, we used a DNA microarray containing 22,625 gene probes corresponding to 22,150 unique genes from C. elegans. We employed two different paths: direct exposure to daumone (Path 1) and normal growth media plus liquid culture (Path 2). Our data reveal that entry into dauer is accomplished through the multi-step process, which appears to be compartmentalized in time and according to metabolic flux. That is, a time-course of dauer entry in Path 1 shows that dauer larvae formation begins at post-embryonic stage S4 (48 h) and is complete at S6 (72 h). Our results also suggest the presence of a unique adaptive metabolic control mechanism that requires both stage-specific expression of specific genes and tight regulation of different modes of fuel metabolite utilization to sustain the energy balance in the context of prolonged survival under adverse growth conditions. It is apparent that worms entering dauer stage may rely heavily on carbohydrate-based energy reserves, whereas dauer larvae utilize fat or glyoxylate cycle-based energy sources. We created a comprehensive web-based dauer metabolic database for C. elegans (www.DauerDB.org) that makes it possible to search any gene and compare its relative expression at a specific stage, or evaluate overall patterns of gene expression in both paths. This database can be accessed by the research community and could be widely applicable to other related nematodes as a molecular atlas.

A conserved endocrine mechanism controls the formation of dauer and infective larvae in nematodes

Under harsh environmental conditions, Caenorhabditis elegans larvae undergo arrest and form dauer larvae that can attach to other animals to facilitate dispersal. It has been argued that this phenomenon, called phoresy, represents an intermediate step toward parasitism. Indeed, parasitic nematodes invade their hosts as infective larvae, a stage that shows striking morphological similarities to dauer larvae. Although the molecular regulation of dauer entry in C. elegans involves insulin and TGF-beta signaling, studies of TGF-beta orthologs in parasitic nematodes didn't provide evidence for a common origin of dauer and infective larvae. To identify conserved regulators between Caenorhabditis and parasitic nematodes, we used an evolutionary approach involving Pristionchus pacificus as an intermediate. We show by mutational and pharmacological analysis that Pristionchus and Caenorhabditis share the dafachronic acid-DAF-12 system as the core endocrine module for dauer formation. One dafachronic acid, Delta7-DA, has a conserved role in the mammalian parasite Strongyloides papillosus by controlling entry into the infective stage. Application of Delta7-DA blocks formation of infective larvae and results in free-living animals. Conservation of this small molecule ligand represents a fundamental link between dauer and infective larvae and might provide a general strategy for nematode parasitism.

The EGL-4 PKG acts with KIN-29 salt-inducible kinase and protein kinase A to regulate chemoreceptor gene expression and sensory behaviors in Caenorhabditis elegans

The regulation of chemoreceptor (CR) gene expression by environmental signals and internal cues may contribute to the modulation of multiple physiological processes and behavior in Caenorhabditis elegans. We previously showed that KIN-29, a homolog of salt-inducible kinase, acts in sensory neurons to regulate the expression of a subset of CR genes, as well as sensory behaviors. Here we show that the cGMP-dependent protein kinase EGL-4 acts partly in parallel with KIN-29 to regulate CR gene expression. Sensory inputs inhibit both EGL-4 and KIN-29 functions, and KIN-29 function is inhibited in turn by cAMP-dependent protein kinase (PKA) activation. EGL-4 and KIN-29 regulate CR gene expression by antagonizing the gene repression functions of the class II HDAC HDA-4 and the MEF-2 transcription factor, and KIN-29, EGL-4, and PKA target distinct residues in HDA-4 to regulate its function and subcellular localization. While KIN-29 acts primarily via MEF-2/HDA-4 to regulate additional sensory signal-regulated physiological processes and behaviors, EGL-4 acts via both MEF-2-dependent and -independent pathways. Our results suggest that integration of complex sensory inputs via multiple signaling pathways allows animals to precisely regulate sensory gene expression, thereby appropriately modulating physiology and behavior.

Protein-repair and hormone-signaling pathways specify dauer and adult longevity and dauer development in Caenorhabditis elegans

Protein damage that accumulates during aging can be mitigated by a repair methyltransferase, the l-isoaspartyl-O-methyltransferase. In Caenorhabditis elegans, the pcm-1 gene encodes this enzyme. In response to pheromone, we show that pcm-1 mutants form fewer dauer larvae with reduced survival due to loss of the methyltransferase activity. Mutations in daf-2, an insulin/insulin-like growth factor-1-like receptor, and daf-7, a transforming growth factor-beta-like ligand, modulate pcm-1 dauer defects. Additionally, daf-2 and daf-7 mutant dauer larvae live significantly longer than wild type. Although dauer larvae are resistant to many environmental stressors, a proportionately larger decrease in dauer larvae life spans occurred at 25 degrees C compared to 20 degrees C than in adult life span. At 25 degrees C, mutation of the daf-7 or pcm-1 genes does not change adult life span, whereas mutation of the daf-2 gene and overexpression of PCM-1 increases adult life span. Thus, there are both overlapping and distinct mechanisms that specify dauer and adult longevity.

A potent dauer pheromone component in Caenorhabditis elegans that acts synergistically with other components

In the model organism Caenorhabditis elegans, the dauer pheromone is the primary cue for entry into the developmentally arrested, dauer larval stage. The dauer is specialized for survival under harsh environmental conditions and is considered "nonaging" because larvae that exit dauer have a normal life span. C. elegans constitutively secretes the dauer pheromone into its environment, enabling it to sense its population density. Several components of the dauer pheromone have been identified as derivatives of the dideoxy sugar ascarylose, but additional unidentified components of the dauer pheromone contribute to its activity. Here, we show that an ascaroside with a 3-hydroxypropionate side chain is a highly potent component of the dauer pheromone that acts synergistically with previously identified components. Furthermore, we show that the active dauer pheromone components that are produced by C. elegans vary depending on cultivation conditions. Identifying the active components of the dauer pheromone, the conditions under which they are produced, and their mechanisms of action will greatly extend our understanding of how chemosensory cues from the environment can influence such fundamental processes as development, metabolism, and aging in nematodes and in higher organisms.

A starvation-induced noncoding RNA modulates expression of Dicer-regulated genes

Although much has been learned about short noncoding RNAs, long noncoding transcripts are largely uncharacterized. Here, we describe Caenorhabditis elegans rncs-1, a highly base-paired, 800-nucleotide noncoding RNA expressed in hypodermis and intestine. Transcription of rncs-1 is modulated in response to food supply. Although highly double-stranded, we show that rncs-1 RNA is not a substrate for Dicer because of branched structures at its termini. However, rncs-1 RNA inhibits Dicer cleavage of a second dsRNA in vitro, presumably by competition. We validate this observation in vivo by demonstrating that mRNA levels of several Dicer-regulated genes vary with changes in rncs-1 expression. Certain viruses express dsRNA to compete with cellular dsRNA-mediated pathways, and our data suggest that rncs-1 provides a cellular correlate of this phenomenon.

C. elegans dauer formation and the molecular basis of plasticity

Because life is often unpredictable, dynamic, and complex, all animals have evolved remarkable abilities to cope with changes in their external environment and internal physiology. This regulatory plasticity leads to shifts in behavior and metabolism, as well as to changes in development, growth, and reproduction, which is thought to improve the chances of survival and reproductive success. In favorable environments, the nematode Caenorhabditis elegans develops rapidly to reproductive maturity, but in adverse environments, animals arrest at the dauer diapause, a long-lived stress resistant stage. A molecular and genetic analysis of dauer formation has revealed key insights into how sensory and dietary cues are coupled to conserved endocrine pathways, including insulin/IGF, TGF-beta, serotonergic, and steroid hormone signal transduction, which govern the choice between reproduction and survival. These and other pathways reveal a molecular basis for metazoan plasticity in response to extrinsic and intrinsic signals.

Neural and molecular dissection of a C. elegans sensory circuit that regulates fat and feeding

A major challenge in understanding energy balance is deciphering the neural and molecular circuits that govern behavioral, physiological, and metabolic responses of animals to fluctuating environmental conditions. The neurally expressed TGF-beta ligand DAF-7 functions as a gauge of environmental conditions to modulate energy balance in C. elegans. We show that daf-7 signaling regulates fat metabolism and feeding behavior through a compact neural circuit that allows for integration of multiple inputs and the flexibility for differential regulation of outputs. In daf-7 mutants, perception of depleting food resources causes fat accumulation despite reduced feeding rate. This fat accumulation is mediated, in part, through neural metabotropic glutamate signaling and upregulation of peripheral endogenous biosynthetic pathways that direct energetic resources into fat reservoirs. Thus, neural perception of adverse environmental conditions can promote fat accumulation without a concomitant increase in feeding rate.

A blend of small molecules regulates both mating and development in Caenorhabditis elegans

In many organisms, population-density sensing and sexual attraction rely on small-molecule-based signalling systems. In the nematode Caenorhabditis elegans, population density is monitored through specific glycosides of the dideoxysugar ascarylose (the 'ascarosides') that promote entry into an alternative larval stage, the non-feeding and highly persistent dauer stage. In addition, adult C. elegans males are attracted to hermaphrodites by a previously unidentified small-molecule signal. Here we show, by means of combinatorial activity-guided fractionation of the C. elegans metabolome, that the mating signal consists of a synergistic blend of three dauer-inducing ascarosides, which we call ascr#2, ascr#3 and ascr#4. This blend of ascarosides acts as a potent male attractant at very low concentrations, whereas at the higher concentrations required for dauer formation the compounds no longer attract males and instead deter hermaphrodites. The ascarosides ascr#2 and ascr#3 carry different, but overlapping, information, as ascr#3 is more potent as a male attractant than ascr#2, whereas ascr#2 is slightly more potent than ascr#3 in promoting dauer formation. We demonstrate that ascr#2, ascr#3 and ascr#4 are strongly synergistic, and that two types of neuron, the amphid single-ciliated sensory neuron type K (ASK) and the male-specific cephalic companion neuron (CEM), are required for male attraction by ascr#3. On the basis of these results, male attraction and dauer formation in C. elegans appear as alternative behavioural responses to a common set of signalling molecules. The ascaroside signalling system thus connects reproductive and developmental pathways and represents a unique example of structure- and concentration-dependent differential activity of signalling molecules.

Notch signalling is required for both dauer maintenance and recovery in C. elegans

The Notch signalling pathway is conserved among higher metazoans and is used repeatedly throughout development to specify distinct cell fates among populations of equipotent cells. Mounting evidence suggests that Notch signalling may also be crucial in neuronal function in postmitotic, differentiated neurons. Here, we demonstrate a novel role for the canonical Notch signalling pathway in postmitotic neurons during a specialised ;diapause-like' post-embryonic developmental stage in C. elegans called dauer. Our data suggest that cell signalling downstream of the developmental decision to enter dauer leads to the activation of Notch-responding genes in postmitotic neurons. Consistent with this, we demonstrate that glp-1, one of the two C. elegans Notch receptors, and its ligand lag-2 are expressed in neurons during the dauer stage, and both genes are required to maintain this stage in a daf-7/TGFbeta dauer constitutive background. Our genetic data also suggest that a second Notch receptor, lin-12, functions upstream of, or in parallel with, insulin-like signalling components in response to replete growth conditions to promote dauer recovery. Based on our findings, cues associated with the onset of dauer ultimately trigger a glp-1-dependent Notch signalling cascade in neurons to maintain this developmental state. Then, as growth conditions improve, activation of the LIN-12 Notch receptor cooperates with the insulin-like signalling pathway to signal recovery from the dauer stage.

Genetic identification of HSD-1, a conserved steroidogenic enzyme that directs larval development inCaenorhabditis elegans

In C. elegans, steroid hormones function in conjunction with insulin/IGF-1-like signaling in promoting reproductive development over entry into the diapausal dauer stage. The NCR-1 and -2 (NPC1-related) intracellular cholesterol transporters function redundantly in preventing dauer arrest,presumably by regulating the availability of substrates for steroid hormone synthesis. We have identified hsd-1 as a new component of this cholesterol trafficking/processing pathway, using an ncr-1 enhancer screen. HSD-1 is orthologous to 3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerases(3β-HSDs), which are key steroidogenic enzymes in vertebrates, and is exclusively expressed in two neuron-like XXX cells that are crucial in preventing dauer arrest, suggesting that it is involved in biosynthesis of dauer-preventing steroid hormones. The hsd-1 null mutant displays defects in inhibiting dauer arrest: it forms dauers in the deletion mutant backgrounds of ncr-1 or daf-28/insulin; as a single mutant,it is hypersensitive to dauer pheromone. We found that hsd-1 defects can be rescued by feeding mutant animals with several steroid intermediates that are either downstream of or in parallel to the 3β-HSD function in the dafachronic acid biosynthetic pathway, suggesting that HSD-1 functions as a 3β-HSD. Interestingly, sterols that rescued hsd-1 defects also bypassed the need for the NCR-1 and/or -2 functions, suggesting that HSD-1-mediated steroid hormone production is an important functional output of the NCR transporters. Finally, we found that the HSD-1-mediated signal activates insulin/IGF-I signaling in a cell non-autonomous fashion, suggesting a novel mechanism for how these two endocrine pathways intersect in directing development.

Metabolite induction of Caenorhabditis elegans dauer larvae arises via transport in the pharynx

Caenorhabditis elegans sense natural chemicals in their environment and use them as cues to regulate their development. This investigation probes the mechanism of sensory trafficking by evaluating the processing of fluorescent derivatives of natural products in C. elegans. Fluorescent analogs of daumone, an ascaroside, and apigenin were prepared by total synthesis and evaluated for their ability to induce entry into a nonaging dauer state. Fluorescent imaging detailed the uptake and localization of every labeled compound at each stage of the C. elegans life cycle. Comparative analyses against natural products that did not induce dauer indicated that dauer-triggering natural products accumulated in the cuticle of the pharnyx. Subsequent transport of these molecules to amphid neurons signaled entry into the dauer state. These studies provide cogent evidence supporting the roles of the glycosylated fatty acid daumone and related ascarosides and the ubiquitous plant flavone apigenin as chemical cues regulating C. elegans development.

Steinernema feltiae: ammonia triggers the emergence of their infective juveniles

Entomopathogenic nematodes complete their life cycles inside dead insects. The emergence of new infective juveniles from the cadaver has been attributed (but never demonstrated) to food depletion or to the accumulation of metabolites from the breakdown of the host's tissues. Here we give evidence that emergence is triggered by ammonia, a product of nematode defecation. We found that the emergence of Steinernemafeltiae infective juveniles from Galleriamellonella cadavers was stimulated by a particular level of ammonia. Emergence was delayed when ammonia in the cadaver was decreased and was prompted when increased. These findings will further improve the understanding of the nematode life cycle. Here we speculate that production of infective juveniles can be mediated by ammonia and work in a manner analogous to that of the dauer recovery inhibiting factor (DRIF) in Caenorhabditiselegans.

Quantitative genetic analysis of life-history traits of Caenorhabditis elegans in stressful environments

BACKGROUND

Organisms live in environments that vary. For life-history traits that vary across environments, fitness will be maximised when the phenotype is appropriately matched to the environmental conditions. For the free-living nematode Caenorhabditis elegans, we have investigated how two major life-history traits, (i) the development of environmentally resistant dauer larvae and (ii) reproduction, respond to environmental stress (high population density and low food availability), and how these traits vary between lines and the genetic basis of this variation.

RESULTS

We found that lines of C. elegans vary in their phenotypic plasticity of dauer larva development, i.e. there is variation in the likelihood of developing into a dauer larva for the same environmental change. There was also variation in how lifetime fecundity and the rate of reproduction changed under conditions of environmental stress. These traits were related, such that lines that are highly plastic for dauer larva development also maintain a high population growth rate when stressed. We identified quantitative trait loci (QTL) on two chromosomes that control the dauer larva development and population size phenotypes. The QTLs affecting the dauer larva development and population size phenotypes on chromosome II are closely linked, but are genetically separable. This chromosome II QTL controlling dauer larva development does not encompass any loci previously identified to control dauer larva development. This chromosome II region contains many predicted 7-transmembrane receptors. Such proteins are often involved in information transduction, which is clearly relevant to the control of dauer larva development.

CONCLUSION

C. elegans alters both its larval development and adult reproductive strategy in response to environmental stress. Together the phenotypic and genotypic data suggest that these two major life-history traits are co-ordinated responses to environmental stress and that they are, at least in part, controlled by the same genomic regions.

Chemical Complexity and the Genetics of Aging

We examine how aging is impacted by various chemical challenges that organisms face and by the molecular mechanisms that have evolved to regulate lifespan in response to them. For example, environmental information, which is detected and processed through sensory systems, can modulate lifespan by providing information about the presence and quality of food as well as presence and density of conspecifics and predators. In addition, the diverse forms of molecular damage that result from constant exposure to damaging chemicals that are generated from the environment and from metabolism pose an informatic and energetic challenge for detoxification systems, which are important in ensuring longevity. Finally, systems of innate immunity are vital for recognizing and combating pathogens but are also seen as of increasing importance in causing the aging process. Integrating ideas of molecular mechanism with context derived from evolutionary considerations will lead to exciting new insights into the evolution of aging.

Genetic analysis of dauer formation in Caenorhabditis briggsae

Molecular changes that underlie evolutionary changes in behavior and physiology are not well understood. Dauer formation in Caenorhabditis elegans is a temperature-sensitive process controlled through a network of signaling pathways associated with sensory neurons and is potentially an excellent system in which to investigate molecular changes in neuronal function during evolution. To begin to investigate the evolution of dauer formation in the genus Caenorhabditis at the molecular level, we isolated dauer-formation mutations in C. briggsae, a species closely related to the model organism C. elegans. We identified mutations in orthologs of C. elegans genes daf-2 (insulin receptor), daf-3 (Smad), and daf-4 (TGF-beta type 2 receptor), as well as genes required for formation of sensory cilia. Phenotypic analyses revealed that functions of these genes are conserved between C. elegans and C. briggsae. Analysis of C. briggsae mutations also revealed a significant difference between the two species in their responses to high temperatures (>26 degrees). C. elegans is strongly induced to form dauers at temperatures above 26 degrees, near the upper limit for growth of C. elegans. In contrast, C. briggsae, which is capable of growth at higher temperatures than C. elegans, lacks this response.

Small-molecule pheromones that control dauer development in Caenorhabditis elegans

In response to high population density or low food supply, the nematode Caenorhabditis elegans enters an alternative larval stage, known as the dauer, that can withstand adverse conditions for prolonged periods. C. elegans senses its population density through a small-molecule signal, traditionally called the dauer pheromone, that it secretes into its surroundings. Here we show that the dauer pheromone consists of several structurally related ascarosides-derivatives of the dideoxysugar ascarylose-and that two of these ascarosides (1 and 2) are roughly two orders of magnitude more potent at inducing dauer formation than a previously reported dauer pheromone component (3) and constitute a physiologically relevant signal. The identification of dauer pheromone components 1 and 2 will facilitate the identification of target receptors and downstream signaling proteins.

Genetic regulation of arrested development in nematodes: are age-1 and daf-gene orthologs present in Dictyocaulus viviparus?

In opposite to the free-living soil nematode Caenorhabditis elegans, the genetic regulation of hypobiosis or inhibited or arrested development in parasitic nematodes is completely unknown. In C. elegans, the daf-genes or the age-1 gene are of major importance in signaling pathways regulating arrested development. To investigate if orthologs of these genes are present in the bovine lungworm Dictyocaulus viviparus, a PCR analysis with gene-specific primer combinations was performed. No orthologs of the age-1 or daf-genes could be identified in D. viviparus. The possible differences in the role of the daf-genes concerning arrested development in parasitic and free-living nematodes will be discussed.

Public and private mechanisms of life extension in Caenorhabditis elegans

Model organisms have been widely used to study the ageing phenomenon in order to learn about human ageing. Although the phylogenetic diversity between vertebrates and some of the most commonly used model systems could hardly be greater, several mechanisms of life extension are public (common characteristic in divergent species) and likely share a common ancestry. Dietary restriction, reduced IGF-signaling and, seemingly, reduced ROS-induced damage are the best known mechanisms for extending longevity in a variety of organisms. In this review, we summarize the knowledge of ageing in the nematode Caenorhabditis elegans and compare the mechanisms of life extension with knowledge from other model organisms.

Generation and modulation of chemosensory behaviors in C. elegans

C. elegans recognizes and discriminates among hundreds of chemical cues using a relatively compact chemosensory nervous system. Chemosensory behaviors are also modulated by prior experience and contextual cues. Because of the facile genetics and genomics possible in this organism, C. elegans provides an excellent system in which to explore the generation of chemosensory behaviors from the level of a single gene to the motor output. This review summarizes the current knowledge on the molecular and neuronal substrates of chemosensory behaviors and chemosensory behavioral plasticity in C. elegans.

Starvation induces cAMP response element-binding protein-dependent gene expression through octopamine-Gq signaling in Caenorhabditis elegans

The nervous system plays a critical role in adaptation to a new environment. In Caenorhabditis elegans, reduced access to food requires both changes in behavior as well as metabolic adaptation for survival, which is postulated to involve the bioamine octopamine. The transcription factor cAMP response element-binding protein (CREB) is generally activated by G-protein-coupled receptors (GPCRs) that activate G alpha(s) and is known to play an important role in long-term changes, including synaptic plasticity. We show that, in C. elegans, the CREB ortholog CRH-1 (CREB homolog family member 1) activates in vivo a cAMP response element-green fluorescent protein fusion reporter in a subset of neurons during starvation. This starvation response is mediated by octopamine via the GPCR SER-3 (serotonin/octopamine receptor family member 3) and is fully dependent on the subsequent activation of the G alpha(q) ortholog EGL-30 (egg-laying defective family member 30). The signaling cascade is only partially dependent on the phospholipase C beta (EGL-8) and is negatively regulated by G alpha(o) [GOA-1 (G-protein, O, alpha subunit family member 1)] and calcium/calmodulin-dependent kinase [UNC-43 (uncoordinated family member 43)]. Nonstarved animals in a liquid environment mediate a similar response that is octopamine independent. The results show that the endogenous octopamine system in C. elegans is activated by starvation and that different environmental stimuli can activate CREB through G alpha(q).

Activation of nicotinic receptors uncouples a developmental timer from the molting timer inC. elegans

C. elegans develops through four larval stages (L1 to L4)separated by molts. The identity of larval stages is mostly determined by stage-specific expression of heterochronic genes, which constitute an intrinsic genetic timer. However, extrinsic cues such as food availability or population density also modulate the developmental timing of C. elegans by mechanisms that remain largely unknown. To investigate a potential role of the nervous system in the temporal regulation of C. elegans development, we pharmacologically manipulated nicotinic neurotransmission, which represents a prominent signaling component in C. elegans nervous system. Exposure to the nicotinic agonist DMPP during post-embryonic development is lethal at the L2/L3 molt. Specifically, it delays cell divisions and differentiation during the L2 stage but does not affect the timing of the molt cycle, hence causing exposure of a defective L3 cuticle to the environment after the L2/L3 molt. Forcing development through a previously uncharacterized L2 diapause resynchronizes these events and suppresses DMPP-induced lethality. Nicotinic acetylcholine receptors (nAChRs)containing the UNC-63 subunit are required, probably in neurons, to trigger the action of DMPP. Using a forward genetic screen, we further demonstrated that the nuclear hormone receptor (NHR) DAF-12 is necessary to implement the developmental effects of DMPP. Therefore, a novel neuroendocrine pathway involving nAChRs and the NHR DAF-12 can control the speed of stage-specific developmental events in C. elegans. Activation of DMPP-sensitive nAChRs during the second larval stage uncouples a molting timer and a developmental timer, thus causing a heterochronic phenotype that is lethal at the subsequent molt.

Staying alive in adversity: transcriptome dynamics in the stress-resistant dauer larva

In response to food depletion and overcrowding, the soil nematode Caenorhabditis elegans can arrest development and form an alternate third larval stage called the dauer. Though nonfeeding, the dauer larva is long lived and stress resistant. Metabolic and transcription rates are lowered but the transcriptome of the dauer is complex. In this study, distribution analysis of transcript profiles generated by Serial Analysis of Gene Expression (SAGE) in dauer larvae and in mixed developmental stages is presented. An inverse relationship was observed between frequency and abundance/copy number of SAGE tag types (transcripts) in both profiles. In the dauer profile, a relatively greater proportion of highly abundant transcripts was counterbalanced by a smaller fraction of low to moderately abundant transcripts. Comparisons of abundant tag counts between the two profiles revealed relative enrichment in the dauer profile of transcripts with predicted or known involvement in ribosome biogenesis and protein synthesis, membrane transport, and immune responses. Translation-coupled mRNA decay is proposed as part of an immune-like stress response in the dauer larva. An influence of genomic region on transcript level may reflect the coordination of transcription and mRNA turnover.

C. elegans STAT cooperates with DAF-7/TGF-beta signaling to repress dauer formation

The DAF-7/TGF-beta pathway in C. elegans interprets environmental signals relayed through amphid neurons and actively inhibits dauer formation during reproductive developmental growth . In metazoans, the STAT pathway interprets external stimuli through regulated tyrosine phosphorylation, nuclear translocation, and gene expression , but its importance for developmental commitment, particularly in conjunction with TGF-beta, remains largely unknown. Here, we report that the nematode STAT ortholog STA-1 accumulated in the nuclei of five head neuron pairs, three of which are amphid neurons involved in dauer formation . Moreover, sta-1 mutants showed a synthetic dauer phenotype with selected TGF-beta mutations. sta-1 deficiency was complemented by reconstitution with wild-type protein, but not with a tyrosine mutant. Canonical TGF-beta signaling involves the DAF-7/TGF-beta ligand activating the DAF-1/DAF-4 receptor pair to regulate the DAF-8/DAF-14 Smads . Interestingly, STA-1 functioned in the absence of DAF-7, DAF-4, and DAF-14, but it required DAF-1 and DAF-8. Additionally, STA-1 expression was induced by TGF-beta in a DAF-3-dependent manner, demonstrating a homeostatic negative feedback loop. These results highlight a role for activated STAT proteins in repression of dauer formation. They also raise the possibility of an unexpected function for DAF-1 and DAF-8 that is independent of their normal upstream activator, DAF-7.

Parastrongyloides trichosuri, a nematode parasite of mammals that is uniquely suited to genetic analysis

Commonly studied nematode parasites have not proven amenable to simple genetic analyses and this has significantly reduced the available research options. We introduce here a nematode parasite of mammals, Parastrongyloides trichosuri, which has features uniquely suited for genetic analysis. This parasite has the capacity to undergo multiple reproductive cycles as a free-living worm and thereby amplify the numbers of its infective L3s in faeces. Culture conditions are presented that permit facile laboratory maintenance of this worm for >90 free-living life cycles (to date) without the need for re-entry into a permissive host. Even after long maintenance as a free-living worm, culture conditions can be manipulated to favour development of infective L3 worms, which remain able to successfully infect their marsupial hosts. The switch to infective L3 development is triggered by a secreted factor contained in culture medium conditioned by multiple generations of free-living worm culture. It is simple to perform single pair crosses with P. trichosuri to carry out Mendelian genetics in the laboratory and this has been done multiple times with sibling pairs to generate highly inbred lines. Lines of worms can readily be cryopreserved and recovered. Over 7000 expressed sequence tags have been produced from cDNAs at different life cycle stages and used to identify single nucleotide polymorphisms and microsatellites as genetic markers. Free-living worms live only a few days on average while the patency of parasitic infections can last for several months. Since we show this is not the result of re-infection, we conclude that parasitic worms have a lifespan capacity at least 20-30 times longer than their free-living counterparts. We discuss how it should be possible to exploit these unique features of P. trichosuri as a model for future studies that explore the genetic basis of longevity and parasitism.

Regulation of C. elegans longevity by specific gustatory and olfactory neurons

The life span of C. elegans is extended by mutations that inhibit the function of sensory neurons. In this study, we show that specific subsets of sensory neurons influence longevity. We find that certain gustatory neurons inhibit longevity, whereas others promote longevity, most likely by influencing insulin/IGF-1 signaling. Olfactory neurons also influence life span, and they act in a distinct pathway that involves the reproductive system. In addition, we find that a putative chemosensory G protein-coupled receptor that is expressed in some of these sensory neurons inhibits longevity. Together our findings imply that the life span of C. elegans is regulated by environmental cues and that these cues are perceived and integrated in a complex and sophisticated fashion by specific chemosensory neurons.

A defect in the activity of Delta6 and Delta5 desaturases may be a factor predisposing to the development of insulin resistance syndrome

GLUT-4 (glucose transporter) receptor, tumor necrosis factor-alpha (TNF-alpha), interleukins-6 (IL-6), daf-genes and PPARs (peroxisomal proliferation activator receptors) play a role in the development of insulin resistance syndrome and associated conditions. But, the exact interaction between these molecules/factors and the mechanism(s) by which they produce insulin resistance syndrome is not clear. I propose that a defect in the activity of the enzymes Delta6 and Delta5 desaturases that are essential for the formation of long chain metabolites of essential fatty acids, linoleic acid and alpha-linolenic acid, is a factor in the development of insulin resistance syndrome. Long chain polyunsaturated fatty acids (LCPUFAs) increase cell membrane fluidity and enhance the number of insulin receptors and the affinity of insulin to its receptors; suppress TNF-alpha, IL-6, macrophage migration inhibitory factor (MIF) and leptin synthesis; increase the number of GLUT-4 receptors, serve as endogenous ligands of PPARs, modify lipolysis, and regulate the balance between pro- and anti-oxidants, and thus, play a critical role in the pathogenesis of insulin resistance. In the nematode, Caenorhabditis elegans, the protein encoded by daf-2 is 35% identical to the human insulin receptor; daf-7 codes a transforming growth factor-beta (TGF-beta) type signal and daf-16 enhances superoxide dismutase (SOD) expression. Melatonin has anti-oxidant actions similar to daf-16, TGF-beta and SOD. Calorie restriction enhances the activity of Delta6 and Delta5 desaturases, melatonin production, decreases daf-2 signaling, free radical generation, and augments anti-oxidant defenses that may explain the beneficial effect of diet control in the management of obesity, insulin resistance, and type II diabetes mellitus. These evidences suggest that the activities of Delta6 and Delta5 enzymes play a critical role in the expression and regulation of GLUT-4, TNF-alpha, IL-6, MIF, daf-genes, melatonin, and leptin by modulating the synthesis and tissue concentrations of LCPUFAs. Caloric restriction delays ageing by activating Sir 2 deacetylase in yeast, and expression of Sir 2 (SIRT1) in human cells. Both insulin and insulin-like growth factor-1 (IGF-1) attenuated this response. SIRT1 sequesters the proapoptotic factor Bax, prevents stress-induced apoptosis of cells, and thus, prolongs survival. In addition, SIRT1 repressed PPAR-gamma, and overexpression of SIRT1 attenuated adipogenesis, and upregulation of SIRT in differentiated fat cells triggered lipolysis and loss of fat, events that are known to attenuate insulin resistance and prolong life span. It remains to be seen whether LCPUFAs have a regulatory role in SIRT1 expression and control Sir 2 deacetylase activity. Thus, calorie restriction or reduced food intake has a role not only in the pathobiology of insulin resistance, but also in other associated conditions such as obesity, type II diabetes mellitus, ageing, and longevity.

Hookworm larval infectivity, arrest and amphiparatenesis: the Caenorhabditis elegans Daf-c paradigm

Arrested development dramatically alters the life history of some species of soil-transmitted nematodes and elicits profound variations in the epidemiology of the infections they cause. Here, Peter Hotez, John Hawdon and Gerhard Schad show how an understanding of the cellular and molecular bases of arrested development may lead to new approaches for the control of ancylostomiasis and related infections.

The prepared mind of the worm

In C. elegans, dauer pheromone is an indicator of population density and influences pathways that regulate metabolism, development, and aging. In a recent publication in Nature, Paik and coworkers (Jeong at al., 2005) show the purified substance to be a pyran ring conjugated to heptanoic acid, setting the stage for dissecting downstream signaling pathways.

Chemical structure and biological activity of the Caenorhabditis elegans dauer-inducing pheromone

Pheromones are cell type-specific signals used for communication between individuals of the same species. When faced with overcrowding or starvation, Caenorhabditis elegans secrete the pheromone daumone, which facilitates communication between individuals for adaptation to adverse environmental stimuli. Daumone signals C. elegans to enter the dauer stage, an enduring and non-ageing stage of the nematode life cycle with distinctive adaptive features and extended life. Because daumone is a key regulator of chemosensory processes in development and ageing, the chemical identification of daumone is important for elucidating features of the daumone-mediated signalling pathway. Here we report the isolation of natural daumone from C. elegans by large-scale purification, as well as the total chemical synthesis of daumone. We present the stereospecific chemical structure of purified daumone, a fatty acid derivative. We demonstrate that both natural and chemically synthesized daumones equally induce dauer larva formation in C. elegans (N2 strain) and certain dauer mutants, and also result in competition between food and daumone. These results should help to elucidate the daumone-mediated signalling pathway, which might in turn influence ageing and obesity research and the development of antinematodal drugs.

Transcriptional targets of DAF-16 insulin signaling pathway protectC.elegansfrom extreme hypertonic stress

All cells adapt to hypertonic stress by regulating their volume after shrinkage, by accumulating organic osmolytes, and by activating mechanisms that protect against and repair hypertonicity-induced damage. In mammals and nematodes, inhibition of signaling from the DAF-2/IGF-1 insulin receptor activates the DAF-16/FOXO transcription factor, resulting in increased life span and resistance to some types of stress. We tested the hypothesis that inhibition of insulin signaling in Caenorhabditis elegans also increases hypertonic stress resistance. Genetic inhibition of DAF-2 or its downstream target, the AGE-1 phosphatidylinositol 3-kinase, confers striking resistance to a normally lethal hypertonic shock in a DAF-16-dependent manner. However, insulin signaling is not inhibited by or required for adaptation to hypertonic conditions. Microarray studies have identified 263 genes that are transcriptionally upregulated by DAF-16 activation. We identified 14 DAF-16-upregulated genes by RNA interference screening that are required for age- 1 hypertonic stress resistance. These genes encode heat shock proteins, proteins of unknown function, and trehalose synthesis enzymes. Trehalose levels were elevated approximately twofold in age- 1 mutants, but this increase was insufficient to prevent rapid hypertonic shrinkage. However, age- 1 animals unable to synthesize trehalose survive poorly under hypertonic conditions. We conclude that increased expression of proteins that protect eukaryotic cells against environmental stress and/or repair stress-induced molecular damage confers hypertonic stress resistance in C. elegans daf- 2/ age- 1 mutants. Elevated levels of solutes such as trehalose may also function in a cytoprotective manner. Our studies provide novel insights into stress resistance in animal cells and a foundation for new studies aimed at defining molecular mechanisms underlying these essential processes.

Lipophilic regulator of a developmental switch in Caenorhabditis elegans

Abstract In Caenorhabditis elegans, the decision to develop into a reproductive adult or arrest as a dauer larva is influenced by multiple pathways including insulin-like and transforming growth factor beta (TGFbeta)-like signalling pathways. It has been proposed that lipophilic hormones act downstream of these pathways to regulate dauer formation. One likely target for such a hormone is DAF-12, an orphan nuclear hormone receptor that mediates these developmental decisions and also influences adult lifespan. In order to find lipophilic hormones we have generated lipophilic extracts from mass cultures of C. elegans and shown that they rescue the dauer constitutive phenotype of class 1 daf-2 insulin signalling mutants and the TGFbeta signalling mutant daf-7. These extracts are also able to rescue the lethal dauer phenotype of daf-9 mutants, which lack a P450 steroid hydroxylase thought to be involved in the synthesis of the DAF-12 ligand; extracts, however, have no effect on a DAF-12 ligand binding domain mutant that is predicted to be ligand insensitive. The production of this hormone appears to be DAF-9 dependent as extracts from a daf-9;daf-12 double mutant do not exhibit this activity. Preliminary fractionation of the lipophilic extracts shows that the activity is hydrophobic with some polar properties, consistent with a small lipophilic hormone. We propose that the dauer rescuing activity is a hormone synthesized by DAF-9 that acts through DAF-12.

Akt phosphorylation and kinase activity are down-regulated during hibernation in the 13-lined ground squirrel

Hibernation in mammals is a reversible state of suspended animation associated with tolerance to an otherwise lethal reduction of core body temperature and metabolism. An integral aspect of hibernation is tolerance to a profound decrease of cerebral perfusion. Identification of regulatory mechanisms that control hibernation in ground squirrels can guide efforts to develop improved treatment for stroke and brain trauma. In this study, we show in multiple tissues that S473 phosphorylation of Akt (Protein kinase B), a phosphatidylinositol-3 kinase-regulated serine/threonine kinase, was significantly reduced (P<0.001) as was its kinase activity (P=0.023) in the 13-lined ground squirrel, Spermophilus tridecemlineatus, during hibernation. T308 phosphorylation of Akt was relatively preserved. Brain immunohistochemical staining confirmed these results. In hibernating animals, reduction of immunoreactive phospho (S473)-Akt was noted throughout the brain. Akt is a key molecule in the insulin/insulin-like growth factor signal transduction pathway, which plays a critical role in the balance between survival and apoptosis. The data presented here raise the possibility that down-regulation of Akt phosphorylation plays a regulatory role in hibernation. This would resemble dauer larva formation in Caenorhabditis elegans where Akt inhibition is associated with energy conservation, fat storage, expression of antioxidant enzymes and growth arrest.

Host immune status affects maturation time in two nematode species--but not as predicted by a simple life-history model

In theory, the age at which maturation occurs in parasitic nematodes is inversely related to pre-maturational mortality rate, and cross-species data on mammalian nematodes are consistent with this prediction. Immunity is a major source of parasite mortality and parasites stand to gain sizeable fitness benefits through short-term adjustments of maturation time in response to variation in immune-mediated mortality. The effects of thymus-dependent immune responses on maturation in the nematode parasites Strongyloides ratti and Nippostrongylus brasiliensis were investigated using congenitally thymus-deficient (nude) rats. As compared with worms in normal rats, reproductive maturity of parasites (presence of eggs in utero) in nude rats occurred later in S. ratti but earlier in N. brasiliensis. Immune-mediated differences in maturation time were not associated with differences in worm length. Thymus-dependent immunity had no effect on prematurational mortality. Results are discussed in relation to theoretical expectations and possible explanations for the observed patterns in parasite maturation.

Expression and function of conserved nuclear receptor genes in Caenorhabditis elegans

The Caenorhabditis elegans genome encodes 284 nuclear receptor (NR) genes. Among these 284 NR genes are 15 genes conserved among the Metazoa. Here, we analyze the expression and function of eight heretofore uncharacterized conserved C. elegans NR genes. Reporter gene analysis demonstrates that these genes have distinct expression patterns and that a majority of the C. elegans cell types express a conserved NR gene. RNA interference with NR gene function resulted in visible phenotypes for three of the genes, revealing functions in various processes during postembryonic development. Five of the conserved NR genes are orthologs of NR genes that function during molting and metamorphosis in insects. Functional studies confirm a role for most of these 'ecdysone cascade' NR orthologs during the continuous growth and dauer molts. Transcript levels for these genes fluctuate in a reiterated pattern during the molting cycles, reminiscent of the expression hierarchy observed in the insect ecdysone response. Together, these analyses provide a foundation for further dissecting the role of NRs in nematode development as well as for evaluating conservation of NR functions among the Metazoa.

Tissue-specific activities of C. elegans DAF-16 in the regulation of lifespan

In C. elegans, the transcription factor DAF-16 promotes longevity in response to reduced insulin/IGF-1 signaling or germline ablation. In this study, we have asked how different tissues interact to specify the lifespan of the animal. We find that several tissues act as signaling centers. In particular, DAF-16 activity in the intestine, which is also the animal's adipose tissue, completely restores the longevity of daf-16(-) germline-deficient animals, and increases the lifespans of daf-16(-) insulin/IGF-1-pathway mutants substantially. Our findings indicate that DAF-16 may control two types of downstream signals: DAF-16 activity in signaling cells upregulates DAF-16 in specific responding tissues, possibly via regulation of insulin-like peptides, and also evokes DAF-16-independent responses. We suggest that this network of tissue interactions and feedback regulation allows the tissues to equilibrate and fine-tune their expression of downstream genes, which, in turn, coordinates their rates of aging within the animal.

SDF-9, a protein tyrosine phosphatase-like molecule, regulates the L3/dauer developmental decision through hormonal signaling in C. elegans

The dauer larva of the nematode Caenorhabditis elegans is a good model system for investigating the regulation of developmental fates by environmental cues. Here we show that SDF-9, a protein tyrosine phosphatase-like molecule, is involved in the regulation of dauer larva formation. The dauer larva of sdf-9 mutants is different from a normal dauer larva but resembles the dauer-like larva of daf-9 and daf-12 dauer-constitutive mutants. Like these mutants, the dauer-constitutive phenotypes of sdf-9 mutants were greatly enhanced by cholesterol deprivation. Epistasis analyses, together with the relationship between sdf-9 mutations and daf-9 expression, suggested that SDF-9 increases the activity of DAF-9 or helps the execution of the DAF-9 function. SDF-9 was expressed in two head cells in which DAF-9 is expressed. By their position and by genetic mosaic experiments, we identified these cells as XXXL/R cells, which are known as embryonic hypodermal cells and whose function at later stages is unknown. Killing of the sdf-9-expressing cells in the wild-type first-stage larva induced formation of the dauer-like larva. Since this study on SDF-9 and former studies on DAF-9 showed that the functions of these proteins are related to those of steroids, XXXL/R cells seem to play a key role in the metabolism or function of a steroid hormone(s) that acts in dauer regulation.

Molecular evolution and quantitative variation for chemosensory behaviour in the nematode genus Caenorhabditis

Caenorhabditis elegans is a model organism in biology, yet despite the tremendous information generated from genetic, genomic and functional analyses, C. elegans has rarely been used to address questions in ecological genetics. Here, we analyse genetic variation for chemosensory behaviour, an ecologically important trait that is also genetically well characterized, at both the phenotypic and molecular levels within three species of the genus Caenorhabditis. We show that the G-protein ODR-3 plays an important role in chemosensory avoidance behaviour and identify orthologues of odr-3 in C. briggsae and C. remanei. Both quantitative genetic analysis of chemosensory behaviour and molecular population genetic analysis of odr-3 show that there is little genetic variation among a worldwide collection of isolates of the primarily selfing C. elegans, whereas there is substantially more variation within a single population of the outcrossing C. remanei. Although there are a large number of substitutions at silent sites within odr-3 among the three species, molecular evolution at the protein level is extremely conserved, suggesting that odr-3 plays an important role in cell signalling during chemosensation and/or neuronal cilia development in C. remanei and in C. briggsae as it does in C. elegans. Our results suggest that C. remanei may be a more suitable subject for ecological and evolutionary genetic studies than C. elegans.

daf-28 encodes a C. elegans insulin superfamily member that is regulated by environmental cues and acts in the DAF-2 signaling pathway

In Caenorhabditis elegans, the decision to enter a developmentally arrested dauer larval stage is triggered by a combination of signals from sensory neurons in response to environmental cues, which include a dauer pheromone. These sensory inputs are coupled to the parallel DAF-2/insulin receptor-like and DAF-7/TGFbeta-like signaling pathways. Although sensory inputs have been shown to physiologically regulate DAF-7/TGFbeta expression, no such regulation of insulin-like ligands in the DAF-2 pathway has been reported. We show here that daf-28 encodes an insulin-like protein, which when mutated causes dauer arrest and down-regulation of DAF-2/IR signaling. A daf-28GFP fusion gene is expressed in ASI and ASJ, two sensory neurons that regulate dauer arrest. daf-28GFP expression in ASI and ASJ is down-regulated under dauer-inducing conditions and in mutants of DAF-11/guanylyl cyclase, a predicted component of the dauer-pheromone-sensing pathway. Thus, daf-28 expression in sensory neurons is regulated by the environmental cues that normally trigger dauer arrest. Among the 38 C. elegans insulin genes, daf-28 is so far the only insulin mutant to affect dauer arrest. daf-28 was revealed from this functional redundancy by a dominant-negative allele that disrupts a probable proteolytic processing site required for insulin maturation. This DAF-28 mutant is likely to be poisonous to wild-type DAF-28 and other insulins.

The DAF-7 TGF-beta signaling pathway regulates chemosensory receptor gene expression in C. elegans

Regulation of chemoreceptor gene expression in response to environmental or developmental cues provides a mechanism by which animals can alter their sensory responses. Here we demonstrate a role for the daf-7 TGF-beta pathway in the regulation of expression of a subset of chemoreceptor genes in Caenorhabditis elegans. We describe a novel role of this pathway in maintaining receptor gene expression in the adult and show that the DAF-4 type II TGF-beta receptor functions cell-autonomously to modulate chemoreceptor expression. We also find that the alteration of receptor gene expression in the ASI chemosensory neurons by environmental signals, such as levels of a constitutively produced pheromone, may be mediated via a DAF-7-independent pathway. Receptor gene expression in the ASI and ASH sensory neurons appears to be regulated via distinct mechanisms. Our results suggest that the expression of individual chemoreceptor genes in C. elegans is subject to multiple modes of regulation, thereby ensuring that animals exhibit the responses most appropriate for their developmental stage and environmental conditions.

A Caenorhabditis elegans pheromone antagonizes volatile anesthetic action through a go-coupled pathway

Volatile anesthetics (VAs) disrupt nervous system function by an ill-defined mechanism with no known specific antagonists. During the course of characterizing the response of the nematode C. elegans to VAs, we discovered that a C. elegans pheromone antagonizes the VA halothane. Acute exposure to pheromone rendered wild-type C. elegans resistant to clinical concentrations of halothane, increasing the EC(50) from 0.43 +/- 0.03 to 0.90 +/- 0.02. C. elegans mutants that disrupt the function of sensory neurons required for the action of the previously characterized dauer pheromone blocked pheromone-induced resistance (Pir) to halothane. Pheromone preparations from loss-of-function mutants of daf-22, a gene required for dauer pheromone production, lacked the halothane-resistance activity, suggesting that dauer and Pir pheromone are identical. However, the pathways for pheromone's effects on dauer formation and VA action were not identical. Not all mutations that alter dauer formation affected the Pir phenotype. Further, mutations in genes not known to be involved in dauer formation completely blocked Pir, including those altering signaling through the G proteins Goalpha and Gqalpha. A model in which sensory neurons transduce the pheromone activity through antagonistic Go and Gq pathways, modulating VA action against neurotransmitter release machinery, is proposed.