Food and metabolic signalling defects in a Caenorhabditis elegans serotonin-synthesis mutant

Monoaminergic signaling as a target for anthelmintic drug discovery: receptor conservation among the free-living and parasitic nematodes

This review is designed to summarize the information on monoamine-dependent paralysis as a target for anthelmintic development, examine the conservation of monoamine receptors in the genomes of both free-living and parasitic nematodes, and highlight the utility of the Caenorhabditis elegans model system for dissecting the monoaminergic modulation of locomotory decision-making.

Serotonin activates overall feeding by activating two separate neural pathways in Caenorhabditis elegans

Food intake in the nematode Caenorhabditis elegans requires two distinct feeding motions, pharyngeal pumping and isthmus peristalsis. Bacteria, the natural food of C. elegans, activate both feeding motions (Croll, 1978; Horvitz et al., 1982; Chiang et al., 2006). The mechanisms by which bacteria activate the feeding motions are largely unknown. To understand the process, we studied how serotonin, an endogenous pharyngeal pumping activator whose action is triggered by bacteria, activates feeding motions. Here, we show that serotonin, like bacteria, activates overall feeding by activating isthmus peristalsis as well as pharyngeal pumping. During active feeding, the frequencies and the timing of onset of the two motions were distinct, but each isthmus peristalsis was coupled to the preceding pump. We found that serotonin activates the two feeding motions mainly by activating two separate neural pathways in response to bacteria. For activating pumping, the SER-7 serotonin receptor in the MC motor neurons in the feeding organ activated cholinergic transmission from MC to the pharyngeal muscles by activating the Gsα signaling pathway. For activating isthmus peristalsis, SER-7 in the M4 (and possibly M2) motor neuron in the feeding organ activated the G(12)α signaling pathway in a cell-autonomous manner, which presumably activates neurotransmission from M4 to the pharyngeal muscles. Based on our results and previous calcium imaging of pharyngeal muscles (Shimozono et al., 2004), we propose a model that explains how the two feeding motions are separately regulated yet coupled. The feeding organ may have evolved this way to support efficient feeding.

Spatiotemporal localization of D-amino acid oxidase and D-aspartate oxidases during development in Caenorhabditis elegans

Recent investigations have shown that a variety of D-amino acids are present in living organisms and that they possibly play important roles in physiological functions in the body. D-Amino acid oxidase (DAO) and D-aspartate oxidase (DDO) are degradative enzymes stereospecific for D-amino acids. They have been identified in various organisms, including mammals and the nematode Caenorhabditis elegans, although the significance of these enzymes and the relevant functions of D-amino acids remain to be elucidated. In this study, we investigated the spatiotemporal localization of C. elegans DAO and DDOs (DDO-1, DDO-2, and DDO-3) and measured the levels of several D- and L-amino acids in wild-type C. elegans and four mutants in which each gene for DAO and the DDOs was partially deleted and thereby inactivated. Furthermore, several phenotypes of these mutant strains were characterized. The results reported in this study indicate that C. elegans DAO and DDOs are involved in egg-laying events and the early development of C. elegans. In particular, DDOs appear to play important roles in the development and maturation of germ cells. This work provides novel and useful insights into the physiological functions of these enzymes and D-amino acids in multicellular organisms.

Notch signaling is antagonized by SAO-1, a novel GYF-domain protein that interacts with the E3 ubiquitin ligase SEL-10 in Caenorhabditis elegans

Notch signaling pathways can be regulated through a variety of cellular mechanisms, and genetically compromised systems provide useful platforms from which to search for the responsible modulators. The Caenorhabditis elegans gene aph-1 encodes a component of γ-secretase, which is essential for Notch signaling events throughout development. By looking for suppressors of the incompletely penetrant aph-1(zu147) mutation, we identify a new gene, sao-1 (suppressor of aph-one), that negatively regulates aph-1(zu147) activity in the early embryo. The sao-1 gene encodes a novel protein that contains a GYF protein-protein interaction domain and interacts specifically with SEL-10, an Fbw7 component of SCF E3 ubiquitin ligases. We demonstrate that the embryonic lethality of aph-1(zu147) mutants can be suppressed by removing sao-1 activity or by mutations that disrupt the SAO-1-SEL-10 protein interaction. Decreased sao-1 activity also influences Notch signaling events when they are compromised at different molecular steps of the pathway, such as at the level of the Notch receptor GLP-1 or the downstream transcription factor LAG-1. Combined analysis of the SAO-1-SEL-10 protein interaction and comparisons of sao-1 and sel-10 genetic interactions suggest a possible role for SAO-1 as an accessory protein that participates with SEL-10 in downregulation of Notch signaling. This work provides the first mutant analysis of a GYF-domain protein in either C. elegans or Drosophila and introduces a new type of Fbw7-interacting protein that acts in a subset of Fbw7 functions.

Cell excitability necessary for male mating behavior in Caenorhabditis elegans is coordinated by interactions between big current and ether-a-go-go family K(+) channels

Variations in K(+) channel composition allow for differences in cell excitability and, at an organismal level, provide flexibility to behavioral regulation. When the function of a K(+) channel is disrupted, the remaining K(+) channels might incompletely compensate, manifesting as abnormal organismal behavior. In this study, we explored how different K(+) channels interact to regulate the neuromuscular circuitry used by Caenorhabditis elegans males to protract their copulatory spicules from their tail and insert them into the hermaphrodite's vulva during mating. We determined that the big current K(+) channel (BK)/SLO-1 genetically interacts with ether-a-go-go (EAG)/EGL-2 and EAG-related gene/UNC-103 K(+) channels to control spicule protraction. Through rescue experiments, we show that specific slo-1 isoforms affect spicule protraction. Gene expression studies show that slo-1 and egl-2 expression can be upregulated in a calcium/calmodulin-dependent protein kinase II-dependent manner to compensate for the loss of unc-103 and conversely, unc-103 can partially compensate for the loss of SLO-1 function. In conclusion, an interaction between BK and EAG family K(+) channels produces the muscle excitability levels that regulate the timing of spicule protraction and the success of male mating behavior.

Hyperactive neuroendocrine secretion causes size, feeding, and metabolic defects of C. elegans Bardet-Biedl syndrome mutants

Bardet-Biedl syndrome, BBS, is a rare autosomal recessive disorder with clinical presentations including polydactyly, retinopathy, hyperphagia, obesity, short stature, cognitive impairment, and developmental delays. Disruptions of BBS proteins in a variety of organisms impair cilia formation and function and the multi-organ defects of BBS have been attributed to deficiencies in various cilia-associated signaling pathways. In C. elegans, bbs genes are expressed exclusively in the sixty ciliated sensory neurons of these animals and bbs mutants exhibit sensory defects as well as body size, feeding, and metabolic abnormalities. Here we show that in contrast to many other cilia-defective mutants, C. elegans bbs mutants exhibit increased release of dense-core vesicles and organism-wide phenotypes associated with enhanced activities of insulin, neuropeptide, and biogenic amine signaling pathways. We show that the altered body size, feeding, and metabolic abnormalities of bbs mutants can be corrected to wild-type levels by abrogating the enhanced secretion of dense-core vesicles without concomitant correction of ciliary defects. These findings expand the role of BBS proteins to the regulation of dense-core-vesicle exocytosis and suggest that some features of Bardet-Biedl Syndrome may be caused by excessive neuroendocrine secretion.

Bardet-Biedl syndrome, BBS, is a rare human genetic disease caused by mutations in many genes. The BBS phenotype is very complex; it is principally characterized by early-onset obesity, progressive blindness, extra digits on the hands and feet, and renal problems. BBS patients may also suffer from developmental delay, learning disabilities, diabetes, and loss of the sense of smell. This complexity suggests that BBS proteins function in a variety of tissues, causing defects in many organs. A unifying theme for the diverse features of BBS emerged when BBS genes were identified and their protein products were found to function in the cilium, a sensory structure found in many cell types. Since then, the various manifestations of BBS have been attributed to the loss of ciliary function in the corresponding tissues. This notion was also supported by the finding that mutations in several genes required for proper cilia formation and function reproduce some of the features seen in BBS patients. Here, we have further investigated the defects found in Caenorhabditis elegans strains carrying mutations in BBS genes (bbs mutants). We find that not only do they display sensory deficits associated with loss of ciliary function, but they also exhibit increased release of multiple peptide and biogenic amine hormones contained in dense-core vesicles of ciliated sensory neurons. Importantly, limiting this excessive hormonal release without correcting the ciliary defects of bbs mutants was sufficient to restore normal body size, feeding, and metabolism to these mutants. Moreover, we show that although non-bbs ciliary mutations can mimic some of the phenotypes of bbs mutants, these effects can be attributed to distinct spatial and molecular mechanisms. Our findings indicate that C. elegans bbs mutants exhibit features of both ciliary and endocrine defects and suggest that some of the clinical manifestations of human BBS may result from excessive endocrine activity, independently of the loss of ciliary function.

Nictation, a dispersal behavior of the nematode Caenorhabditis elegans, is regulated by IL2 neurons

Many nematodes show a stage-specific behavior called nictation in which a worm stands on its tail and waves its head in three dimensions. Here we show that nictation is a dispersal behavior regulated by a specific set of neurons, the IL2 cells, in C. elegans. We established assays for nictation and showed that cholinergic transmission was required for nictation. Cell type-specific rescue experiments and genetic ablation experiments revealed that the IL2 ciliated head neurons were essential for nictation. Intact cilia in IL2 neurons, but not in other ciliated head neurons, were essential, as the restoration of the corresponding wild-type gene activity in IL2 neurons alone in cilia-defective mutants was sufficient to restore nictation. Optogenetic activation of IL2 neurons induced nictation, suggesting that signals from IL2 neurons are sufficient for nictation. Finally, we demonstrated that nictation is required for transmission of C. elegans to a new niche using flies as artificial carriers, suggesting a role of nictation as a dispersal and survival strategy under harsh conditions.

Lipid droplets as fat storage organelles in Caenorhabditis elegans: Thematic Review Series: Lipid Droplet Synthesis and Metabolism: from Yeast to Man

Lipid droplets are evolutionarily conserved organelles where cellular fat storage and mobilization are exquisitely regulated. Recent studies have defined lipid droplets in C. elegans and explored how they are regulated by genetic and dietary factors. C. elegans offers unique opportunities to visualize lipid droplets at single-cell resolution in live animals. The development of novel microscopy techniques and protein markers for lipid droplets will accelerate studies on how nutritional states and subcellular organization are linked in vivo. Together with powerful tools for genetic and biochemical analysis of metabolic pathways, alteration in lipid droplet abundance, size, and distribution in C. elegans can be readily connected to whole-animal energy homeostasis, behavior, and life span. Therefore, further studies on lipid droplets in C. elegans promise to yield valuable insights that complement our knowledge gained from yeast, Drosophila, and mammalian systems on cellular and organismal fat storage.

STR-33, a novel G protein-coupled receptor that regulates locomotion and egg laying in Caenorhabditis elegans

Despite their predicted functional importance, most G protein-coupled receptors (GPCRs) in Caenorhabditis elegans have remained largely uncharacterized. Here, we focused on one GPCR, STR-33, encoded by the str-33 gene, which was discovered through a ligand-based screening procedure. To characterize STR-33 function, we performed UV-trimethylpsolaren mutagenesis and isolated an str-33-null mutant. The resulting mutant showed hypersinusoidal movement and a hyperactive egg-laying phenotype. Two types of egg laying-related mutations have been characterized: egg laying-deficient (Egl-d) and hyperactive egg laying (Egl-c). The defect responsible for the egg laying-deficient Egl-d phenotype is related to Gα(q) signaling, whereas that responsible for the opposite, hyperactive egg-laying Egl-c phenotype is related to Gα(o) signaling. We found that the hyperactive egg-laying defect of the str-33(ykp001) mutant is dependent on the G protein GOA-1/Gα(o). Endogenous acetylcholine suppressed egg laying in C. elegans via a Gα(o)-signaling pathway by inhibiting serotonin biosynthesis or release from the hermaphrodite-specific neuron. Consistent with this, in vivo expression of the serotonin biosynthetic enzyme, TPH-1, was up-regulated in the str-33(ykp001) mutant. Taken together, these results suggest that the GPCR, STR-33, may be one of the neurotransmitter receptors that regulates locomotion and egg laying in C. elegans.

Partner in fat metabolism: role of KLFs in fat burning and reproductive behavior

The abnormalities caused by excess fat accumulation can result in pathological conditions which are linked to several interrelated diseases, such as cardiovascular disease and obesity. This set of conditions, known as metabolic syndrome, is a global pandemic of enormous medical, economic, and social concern affecting a significant portion of the world’s population. Although genetics, physiology and environmental components play a major role in the onset of disease caused by excessive fat accumulation, little is known about how or to what extent each of these factors contributes to it. The worm, Caenorhabditis elegans offers an opportunity to study disease related to metabolic disorder in a developmental system that provides anatomical and genomic simplicity relative to the vertebrate animals and is an excellent eukaryotic genetic model which enable us to answer the questions concerning fat accumulation which remain unresolved. The stored triglycerides (TG) provide the primary source of energy during periods of food deficiency. In nature, lipid stored as TGs are hydrolyzed into fatty acids which are broken down through β-oxidation to yield acetyl-CoA. Our recent study suggests that a member of C. elegans Krüppel-like factor, klf-3 regulates lipid metabolism by promoting FA β-oxidation and in parallel may contribute in normal reproduction and fecundity. Genetic and epigenetic factors that influence this pathway may have considerable impact on fat related diseases in human. Increasing number of studies suggest the role of mammalian KLFs in adipogenesis. This functional conservation should guide our further effort to explore C. elegans as a legitimate model system for studying the role of KLFs in many pathway components of lipid metabolism.

Dissecting the serotonergic food signal stimulating sensory-mediated aversive behavior in C. elegans

Nutritional state often modulates olfaction and in Caenorhabditis elegans food stimulates aversive responses mediated by the nociceptive ASH sensory neurons. In the present study, we have characterized the role of key serotonergic neurons that differentially modulate aversive behavior in response to changing nutritional status. The serotonergic NSM and ADF neurons play antagonistic roles in food stimulation. NSM 5-HT activates SER-5 on the ASHs and SER-1 on the RIA interneurons and stimulates aversive responses, suggesting that food-dependent serotonergic stimulation involves local changes in 5-HT levels mediated by extrasynaptic 5-HT receptors. In contrast, ADF 5-HT activates SER-1 on the octopaminergic RIC interneurons to inhibit food-stimulation, suggesting neuron-specific stimulatory and inhibitory roles for SER-1 signaling. Both the NSMs and ADFs express INS-1, an insulin-like peptide, that appears to cell autonomously inhibit serotonergic signaling. Food also modulates directional decisions after reversal is complete, through the same serotonergic neurons and receptors involved in the initiation of reversal, and the decision to continue forward or change direction after reversal is dictated entirely by nutritional state. These results highlight the complexity of the "food signal" and serotonergic signaling in the modulation of sensory-mediated aversive behaviors.

Role of insulin and growth hormone/insulin-like growth factor-I signaling in lifespan extension: rodent longevity models for studying aging and calorie restriction

Insulin/insulin-like growth factor-I (IGF-I) pathways are recognized as critical signaling pathways involved in the control of lifespans in lower organisms to mammals. Caloric restriction (CR) reduces plasma concentration of insulin, growth hormone (GH), and IGF-I. CR retards various age-dependent disorders such as nuerodegenerative diseases and extends lifespan in laboratory rodents. These beneficial effects of CR are partly mimicked in spontaneous or genetically engineered rodent models of reduced insulin and GH/IGF-I axis. Most of these long-living rodents show increased insulin sensitivity; however, recent study has revealed that some other rodents show normal or reduced insulin sensitivity. Thus, increased insulin sensitivity might be not prerequisite for lifespan extension in insulin/GH/IGF-I altered longevity rodent models. These results highlighted that, for lifespan extension, the intracellular signaling molecules of insulin/GH/IGF-I pathways might be more important than actual peripheral or systemic insulin action.

Sensory perception and aging in model systems: from the outside in

Sensory systems provide organisms from bacteria to humans with the ability to interact with the world. Numerous senses have evolved that allow animals to detect and decode cues from sources in both their external and internal environments. Recent advances in understanding the central mechanisms by which the brains of simple organisms evaluate different cues and initiate behavioral decisions, coupled with observations that sensory manipulations are capable of altering organismal lifespan, have opened the door for powerful new research into aging. Although direct links between sensory perception and aging have been established only recently, here we discuss these initial discoveries and evaluate the potential for different forms of sensory processing to modulate lifespan across taxa. Harnessing the neurobiology of simple model systems to study the biological impact of sensory experiences will yield insights into the broad influence of sensory perception in mammals and may help uncover new mechanisms of healthy aging.

Regulation of extrasynaptic 5-HT by serotonin reuptake transporter function in 5-HT-absorbing neurons underscores adaptation behavior in Caenorhabditis elegans

Serotonin [5-hydroxytryptamine (5-HT)]-absorbing neurons use serotonin reuptake transporter (SERT) to uptake 5-HT from extracellular space but do not synthesize it. While 5-HT-absorbing neurons have been identified in diverse organisms from Caenorhabditis elegans to humans, their function has not been elucidated. Here, we show that SERT in 5-HT-absorbing neurons controls behavioral response to food deprivation in C. elegans. The AIM and RIH interneurons uptake 5-HT released from chemosensory neurons and secretory neurons. Genetic analyses suggest that 5-HT secreted by both synaptic vesicles and dense core vesicles diffuse readily to the extrasynaptic space adjacent to the AIM and RIH neurons. Loss of mod-5/SERT function blocks the 5-HT absorption. mod-5/SERT mutants have been shown to exhibit exaggerated locomotor response to food deprivation. We found that transgenic expression of MOD-5/SERT in the 5-HT-absorbing neurons fully corrected the exaggerated behavior. Experiments of cell-specific inhibition of synaptic transmission suggest that the synaptic release of 5-HT from the 5-HT-absorbing neurons is not required for this behavioral modulation. Our data point to the role of 5-HT-absorbing neurons as temporal-spatial regulators of extrasynaptic 5-HT. Regulation of extrasynaptic 5-HT levels by 5-HT-absorbing neurons may represent a fundamental mechanism of 5-HT homeostasis, integrating the activity of 5-HT-producing neurons with distant targets in the neural circuits, and could be relevant to some actions of selective serotonin reuptake inhibitors in humans.

Behavioral effects of clozapine: involvement of trace amine pathways in C. elegans and M. musculus

Clozapine is an antipsychotic medication with superior efficacy in treatment refractory schizophrenia. The molecular basis of clozapine's therapeutic profile is not well understood. We studied behavioral effects of clozapine in Caenorhabditis elegans to identify novel pathways that modulate clozapine's biological effects. Clozapine stimulated egg laying in C. elegans in a dose-dependent manner. This effect was clozapine-specific, as it was not observed with exposure to a typical antipsychotic, haloperidol or an atypical antipsychotic, olanzapine. A candidate gene screen of biogenic amine neurotransmitter systems identified signaling pathways that mediate this clozapine-specific effect on egg laying. Specifically, we found that clozapine-induced increase in egg laying requires tyramine biosynthesis. To test the implications of this finding across species, we explored whether trace amine systems modulate clozapine's behavioral effects in mammals by studying trace amine-associated receptor 1 (TAAR1) knockout mice. Clozapine increased prepulse inhibition (PPI) in wild-type mice. This increase in PPI was abrogated in TAAR1 knockout mice, implicating TAAR1 in clozapine-induced PPI enhancement. In transfected mammalian cell lines, we found no TAAR activation by antipsychotics, suggesting that modulation of trace amine signaling in mice does not occur directly at the receptor itself. In summary, we report a heretofore-unknown role for trace amine systems in clozapine-mediated effects across two species: C. elegans and mice.

The robustness of Caenorhabditis elegans male mating behavior depends on the distributed properties of ray sensory neurons and their output through core and male-specific targets

Many evolutionarily significant behaviors, such as mating, involve dynamic interactions with animate targets. This raises the question of what features of neural circuit design are essential to support these complex types of behavior. The Caenorhabditis elegans male uses 18 ray sensilla of the tail to coordinate mate apposition behavior, which facilitates a systematic search of the hermaphrodite surface for the vulva. Precisely how ray neuron types, A and B, robustly endow the male with a high degree of spatial and temporal precision is unknown. We show that the appositional postures that drive the search trajectory reflect the complex interplay of ray neuron type-induced motor outputs. Cell-type-specific ablations reveal that the A-neurons are required for all appositional postures. Their activity is instructive because the A-neurons can induce scanning- and turning-like appositional postures when artificially activated with channel rhodopsin (ChR2). B-neurons are essential only for initiation of the behavior in which they enhance male responsiveness to hermaphrodite contact. When artificially activated using ChR2, A- and B-neurons produce different tail ventral curl postures. However, when coactivated, A-neuron posture dominates, limiting B-neuron contributions to initiation or subsequent postures. Significantly, males lacking the majority of rays retain a high degree of postural control, indicating significant functional resilience in the system. Furthermore, eliminating a large number of male-specific ray neuron targets only partially attenuates tail posture control revealing that gender-common cells make an important contribution to the behavior. Thus, robustness may be a crucial feature of circuits underlying complex behaviors, such as mating, even in simple animals.

GTP-cyclohydrolase and development in Teladorsagia circumcincta and Dictyocaulus viviparus (Nematoda: Strongylida)

GTP-Cyclohydrolase (GTP-CH) is necessary for the production of tetrahydrobiopterin, a required cofactor for the three aromatic amino acid hydroxylases and nitric oxide synthases. The gene encoding GTP-CH is transcribed at high levels in infective third larval stages of a number of parasitic trichostrongylid nematodes. We explore the potential role of GTP-CH within the processes of nematode development and environmentally-induced hypobiosis. For two species of parasitic nematode that are of major economic and welfare importance to livestock in temperate regions, Teladorsagia circumcincta and Dictyocaulus viviparus, we have demonstrated that each of the pre-parasitic larval stages transcribe high mean levels of cat-4 (the gene encoding GTP-CH). Using quantitative real-time polymerase chain reaction analysis and two different isolates of D. viviparus, only one of which is capable of entering hypobiosis, we have shown that there were only minor differences between these isolates in mean cat-4 transcript levels, both during the parasitic stages and during the earlier environmental life cycle stages (L(1)-L(3)). Taken together, these data indicate that, although both species of nematode produce high levels of cat-4 transcript in pre-parasitic larval stages, GTP-CH levels are unlikely to be involved in the induction of parasite hypobiosis. Alternative roles for GTP-CH in larval development are discussed.

The effects of transient starvation persist through direct interactions between CaMKII and ether-a-go-go K+ channels in C. elegans males

Prolonged nutrient limitation has been extensively studied due to its positive effects on life span. However, less is understood of how brief periods of starvation can have lasting consequences. In this study, we used genetics, biochemistry, pharmacology and behavioral analysis to show that after a limited period of starvation, the synthesis of egl-2-encoded ether-a-go-go (EAG) K+ channels and its C-terminal modifications by unc-43-encoded CaMKII have a perduring effect on C. elegans male sexual behavior. EGL-2 and UNC-43 interactions, induced after food deprivation, maintain reduced excitability in muscles involved in sex. In young adult males, spastic contractions occur in cholinergic-activated sex muscles that lack functional unc-103-encoded ERG-like K+ channels. Promoting EGL-2 and UNC-43 interactions in unc-103 mutant adult males by starving them for a few hours reduce spastic muscle contractions over multiple days. Although transient starvation during early adulthood has a hormetic effect of suppressing mutation-induced muscle contractions, the treatment reduces the ability of young wild-type (WT) males to compete with well-fed cohorts in siring progeny.

Between destiny and disease: genetics and molecular pathways of human central nervous system aging

Aging of the human brain is associated with "normal" functional, structural, and molecular changes that underlie alterations in cognition, memory, mood and motor function, amongst other processes. Normal aging also imposes a robust constraint on the onset of many neurological diseases, ranging from late onset neurodegenerative diseases, such as Alzheimer's (AD) and Parkinson's diseases (PD), to early onset psychiatric disorders, such as bipolar disorder (BPD) and schizophrenia (SCZ). The molecular mechanisms and genetic underpinnings of age-related changes in the brain are understudied, and, while they share some overlap with peripheral mechanisms of aging, many are unique to the largely non-mitotic brain. Hence, understanding mechanisms of brain aging and identifying associated modulators may have profound consequences for the prevention and treatment of age-related impairments and diseases. Here we review current knowledge on age-related functional and structural changes, their molecular and genetic underpinnings, and discuss how these pathways may contribute to the vulnerability to develop age-related neurological diseases. We highlight recent findings from human post-mortem brain microarray studies, which we hypothesize, point to a potential genetically controlled transcriptional program underlying molecular changes and age-gating of neurological diseases. Finally, we discuss the implications of this model for understanding basic mechanisms of brain aging and for the future investigation of therapeutic approaches.

Enhancement of odor avoidance regulated by dopamine signaling in Caenorhabditis elegans

The enhancement of sensory responses after prior exposure to a stimulus is a fundamental mechanism of neural function in animals. Its molecular basis, however, has not been studied in as much depth as the reduction of sensory responses, such as adaptation or habituation. We report here that the avoidance behavior of the nematode Caenorhabditis elegans in response to repellent odors (2-nonanone or 1-octanol) is enhanced rather than reduced after preexposure to the odors. This enhancement effect of preexposure was maintained for at least 1 h after the conditioning. The enhancement of 2-nonanone avoidance was not dependent on the presence or absence of food during conditioning, which generally functions as a strong positive or negative unconditioned stimulus in the animals. These results suggest that the enhancement is acquired as a type of nonassociative learning. In addition, genetic and pharmacological analyses revealed that the enhancement of 2-nonanone avoidance requires dopamine signaling via D(2)-like dopamine receptor DOP-3, which functions in a pair of RIC interneurons to regulate the enhancement. Because dopamine signaling has been tightly linked with food-related information to modulate various behaviors of C. elegans, it may play different role in the regulation of the enhancement of 2-nonanone avoidance. Thus, our data suggest a new genetic and pharmacological paradigm for nonassociative enhancement of neural responses that is regulated by dopamine signaling.

Big ideas for small brains: what can psychiatry learn from worms, flies, bees and fish?

While the research community has accepted the value of rodent models as informative research platforms, there is less awareness of the utility of other small vertebrate and invertebrate animal models. Neuroscience is increasingly turning to smaller, non-rodent models to understand mechanisms related to neuropsychiatric disorders. Although they can never replace clinical research, there is much to be learnt from 'small brains'. In particular, these species can offer flexible genetic 'tool kits' that can be used to explore the expression and function of candidate genes in different brain regions. Very small animals also offer efficiencies with respect to high-throughput screening programs. This review provides a concise overview of the utility of models based on worm, fruit fly, honeybee and zebrafish. Although these species may have small brains, they offer the neuropsychiatric research community opportunities to explore some of the most important research questions in our field.

Functional organization of a neural network for aversive olfactory learning in Caenorhabditis elegans

Many animals use their olfactory systems to learn to avoid dangers, but how neural circuits encode naive and learned olfactory preferences, and switch between those preferences, is poorly understood. Here, we map an olfactory network, from sensory input to motor output, which regulates the learned olfactory aversion of Caenorhabditis elegans for the smell of pathogenic bacteria. Naive animals prefer smells of pathogens but animals trained with pathogens lose this attraction. We find that two different neural circuits subserve these preferences, with one required for the naive preference and the other specifically for the learned preference. Calcium imaging and behavioral analysis reveal that the naive preference reflects the direct transduction of the activity of olfactory sensory neurons into motor response, whereas the learned preference involves modulations to signal transduction to downstream neurons to alter motor response. Thus, two different neural circuits regulate a behavioral switch between naive and learned olfactory preferences.

Insulin/FOXO signaling regulates ovarian prostaglandins critical for reproduction

Abnormalities in insulin/IGF-1 signaling are associated with infertility, but the molecular mechanisms are not well understood. Here we use liquid chromatography with electrospray ionization tandem mass spectrometry to show that the C. elegans insulin/FOXO pathway regulates the metabolism of locally acting lipid hormones called prostaglandins. C. elegans prostaglandins are synthesized without prostaglandin G/H synthase homologs, the targets of nonsteroidal anti-inflammatory drugs. Our results support the model that insulin signaling promotes the conversion of oocyte polyunsaturated fatty acids (PUFAs) into F-series prostaglandins that guide sperm to the fertilization site. Reduction in insulin signaling activates DAF-16/FOXO, which represses the transcription of germline and intestinal genes required to deliver PUFAs to oocytes in lipoprotein complexes. Nutritional and neuroendocrine cues target this mechanism to control prostaglandin metabolism and reproductive output. Prostaglandins may be conserved sperm guidance factors and widespread downstream effectors of insulin actions that influence both reproductive and nonreproductive processes.

Trans-cellular introduction of HIV-1 protein Nef induces pathogenic response in Caenorhabditis elegans

BACKGROUND

Caenorhabditis elegans has emerged as a very powerful model for studying the host pathogen interactions. Despite the absence of a naturally occurring viral infection for C. elegans, the model is now being exploited experimentally to study the basic aspects of virus-host interplay. The data generated from recent studies suggests that the virus that infects mammalian cells does infect, replicate and accumulate in C. elegans.

METHODOLOGY/PRINCIPAL FINDINGS

We took advantage of the easy-to-achieve protein introduction in C. elegans and employing the methodology, we administered HIV-1 protein Nef into live worms. Nef is known to be an important protein for exacerbating HIV-1 pathogenesis in host by enhancing viral replication. The deletion of nef from the viral genome has been reported to inhibit its replication in the host, thereby leading to delayed pathogenesis. Our studies, employing Nef introduction into C. elegans, led to creation of an in-vivo model that allowed us to study, whether or not, the protein induces effect in the whole organism. We observed a marked lipodystrophy, effect on neuromuscular function, impaired fertility and reduced longevity in the worms exposed to Nef. The observed effects resemble to those observed in Nef transgenic mice and most interestingly the effects also relate to some of the pathogenic aspects exhibited by human AIDS patients.

CONCLUSIONS/SIGNIFICANCE

Our studies underline the importance of this in vivo model for studying the interactions of Nef with host proteins, which could further be used for identifying possible inhibitors of such interactions.

The regulation of feeding and metabolism in response to food deprivation in Caenorhabditis elegans

This review considers the factors involved in the regulation of feeding and metabolism in response to food deprivation using Caenorhabditis elegans as a model organism. Some of the sensory neurons and interneurons involved in food intake are described, together with an overview of pharyngeal pumping. A number of chemical transmitters control feeding in C. elegans including 5-hydroxytryptamine (5-HT, serotonin), acetylcholine, glutamate, dopamine, octopamine, and tyramine. The roles of these transmitters are modified by neuropeptides, including FMRFamide-like peptides (FLPs), neuropeptide-like protein (NLPs), and insulin-like peptides. The precise effects of many of these neuropeptides have yet to be elucidated but increasingly they are being shown to play a role in feeding and metabolism in C. elegans. The regulation of fat stores is complex and appears to involve the expression of a large number of genes, many with mammalian homologues, suggesting that fat regulatory signalling is conserved across phyla. Finally, a brief comparison is made between C. elegans and mammals where for both, despite their evolutionary distance, classical transmitters and neuropeptides have anorectic or orexigenic properties. Thus, there is a rationale to support the argument that an understanding of the molecular and genetic basis of feeding and fat regulation in C. elegans may contribute to efforts aimed at the identification of targets for the treatment of conditions associated with abnormal metabolism and obesity.

A genetic survey of fluoxetine action on synaptic transmission in Caenorhabditis elegans

Fluoxetine is one of the most commonly prescribed medications for many behavioral and neurological disorders. Fluoxetine acts primarily as an inhibitor of the serotonin reuptake transporter (SERT) to block the removal of serotonin from the synaptic cleft, thereby enhancing serotonin signals. While the effects of fluoxetine on behavior are firmly established, debate is ongoing whether inhibition of serotonin reuptake is a sufficient explanation for its therapeutic action. Here, we provide evidence of two additional aspects of fluoxetine action through genetic analyses in Caenorhabditis elegans. We show that fluoxetine treatment and null mutation in the sole SERT gene mod-5 eliminate serotonin in specific neurons. These neurons do not synthesize serotonin but import extracellular serotonin via MOD-5/SERT. Furthermore, we show that fluoxetine acts independently of MOD-5/SERT to regulate discrete properties of acetylcholine (Ach), gamma-aminobutyric acid (GABA), and glutamate neurotransmission in the locomotory circuit. We identified that two G-protein-coupled 5-HT receptors, SER-7 and SER-5, antagonistically regulate the effects of fluoxetine and that fluoxetine binds to SER-7. Epistatic analyses suggest that SER-7 and SER-5 act upstream of AMPA receptor GLR-1 signaling. Our work provides genetic evidence that fluoxetine may influence neuronal functions and behavior by directly targeting serotonin receptors.

Redox regulation, gene expression and longevity

Lifespan can be lengthened by genetic and environmental modifications. Study of these might provide valuable insights into the mechanism of aging. Low doses of radiation and short-term exposure to heat and high concentrations of oxygen prolong the lifespan of the nematode Caenorhabditis elegans. These might be caused by adaptive responses to harmful environmental conditions. Single-gene mutations have been found to extend lifespan in C. elegans, Drosophila and mice. So far, the best-characterized system is the C. elegans mutant in the daf-2, insulin/IGF-I receptor gene that is the component of the insulin/IGF-I signaling pathway. The mutant animals live twice as long as the wild type. The insulin/IGF-I signaling pathway regulates the activity of DAF-16, a FOXO transcription factor. However, the unified explanation for the function of DAF-16 transcription targets in the lifespan extension is not yet fully established. As both of the Mn superoxide dismutase (MnSOD) isoforms (sod-2 and sod-3) are found to be targets of DAF-16, we attempted to assess their functions in regulating lifespan and oxidative stress responsivity. We show that the double deletions of sod-2 and sod-3 genes induced oxidative-stress sensitivity but do not shorten lifespan in the daf-2 mutant background, indicating that oxidative stress is not necessarily a limiting factor for longevity. Furthermore, the deletion in the sod-3 gene lengthens lifespan in the daf-2 mutant. We conclude that the MnSOD systems in C. elegans fine-tune the insulin/IGF-I-signaling based regulation of longevity by acting not as anti-oxidants but as physiological-redox-signaling modulators.

The Flamingo ortholog FMI-1 controls pioneer-dependent navigation of follower axons in C. elegans

Development of a functional neuronal network during embryogenesis begins with pioneer axons creating a scaffold along which later-outgrowing axons extend. The molecular mechanism used by these follower axons to navigate along pre-existing axons remains poorly understood. We isolated loss-of-function alleles of fmi-1, which caused strong axon navigation defects of pioneer and follower axons in the ventral nerve cord (VNC) of C. elegans. Notably follower axons, which exclusively depend on pioneer axons for correct navigation, frequently separated from the pioneer. fmi-1 is the sole C. elegans ortholog of Drosophila flamingo and vertebrate Celsr genes, and this phenotype defines a new role for this important molecule in follower axon navigation. FMI-1 has a unique and strikingly conserved structure with cadherin and C-terminal G-protein coupled receptor domains and could mediate cell-cell adhesion and signaling functions. We found that follower axon navigation depended on the extracellular but not on the intracellular domain, suggesting that FMI-1 mediates primarily adhesion between pioneer and follower axons. By contrast, pioneer axon navigation required the intracellular domain, suggesting that FMI-1 acts as receptor transducing a signal in this case. Our findings indicate that FMI-1 is a cell-type dependent axon guidance factor with different domain requirements for its different functions in pioneers and followers.

Serotonin: a regulator of neuronal morphology and circuitry

Serotonin is an important neuromodulator associated with a wide range of physiological effects in the central nervous system. The exact mechanisms whereby serotonin influences brain development are not well understood, although studies in invertebrate and vertebrate model organisms are beginning to unravel a regulatory role for serotonin in neuronal morphology and circuit formation. Recent data suggest a developmental window during which altered serotonin levels permanently influence neuronal circuitry, however, the temporal constraints and molecular mechanisms responsible are still under investigation. Growing evidence suggests that alterations in early serotonin signaling contribute to a number of neurodevelopmental and neuropsychiatric disorders. Thus, understanding how altered serotonin signaling affects neuronal morphology and plasticity, and ultimately animal physiology and pathophysiology, will be of great significance.

A developmental timing switch promotes axon outgrowth independent of known guidance receptors

To form functional neuronal connections, axon outgrowth and guidance must be tightly regulated across space as well as time. While a number of genes and pathways have been shown to control spatial features of axon development, very little is known about the in vivo mechanisms that direct the timing of axon initiation and elongation. The Caenorhabditis elegans hermaphrodite specific motor neurons (HSNs) extend a single axon ventrally and then anteriorly during the L4 larval stage. Here we show the lin-4 microRNA promotes HSN axon initiation after cell cycle withdrawal. Axons fail to form in lin-4 mutants, while they grow prematurely in lin-4-overexpressing animals. lin-4 is required to down-regulate two inhibitors of HSN differentiation--the transcriptional regulator LIN-14 and the "stemness" factor LIN-28--and it likely does so through a cell-autonomous mechanism. This developmental switch depends neither on the UNC-40/DCC and SAX-3/Robo receptors nor on the direction of axon growth, demonstrating that it acts independently of ventral guidance signals to control the timing of HSN axon elongation.

Trehalose extends longevity in the nematode Caenorhabditis elegans

Trehalose is a disaccharide of glucose found in diverse organisms and is suggested to act as a stress protectant against heat, cold, desiccation, anoxia, and oxidation. Here, we demonstrate that treatment of Caenorhabditis elegans with trehalose starting from the young-adult stage extended the mean life span by over 30% without any side effects. Surprisingly, trehalose treatment starting even from the old-adult stage shortly thereafter retarded the age-associated decline in survivorship and extended the remaining life span by 60%. Demographic analyses of age-specific mortality rates revealed that trehalose extended the life span by lowering age-independent vulnerability. Moreover, trehalose increased the reproductive span and retarded the age-associated decrease in pharyngeal-pumping rate and the accumulation of lipofuscin autofluorescence. Trehalose also enhanced thermotolerance and reduced polyglutamine aggregation. These results suggest that trehalose suppressed aging by counteracting internal or external stresses that disrupt protein homeostasis. On the other hand, the life span-extending effect of trehalose was abolished in long-lived insulin/IGF-1-like receptor (daf-2) mutants. RNA interference-mediated inactivation of the trehalose-biosynthesis genes trehalose-6-phosphate synthase-1 (tps-1) and tps-2, which are known to be up-regulated in daf-2 mutants, decreased the daf-2 life span. These findings indicate that a reduction in insulin/IGF-1-like signaling extends life span, at least in part, through the aging-suppressor function of trehalose. Trehalose may be a lead compound for potential nutraceutical intervention of the aging process.

UNC-64 and RIC-4, the plasma membrane-associated SNAREs syntaxin and SNAP-25, regulate fat storage in nematode Caenorhabditis elegans

OBJECTIVE

To investigate whether genes required for synaptogenesis and synaptic function are also involved in fat storage control in Caenorhabditis elegans.

METHODS

Fat storage was examined in mutants of genes affecting the synaptogenesis and synaptic function. In addition, the genetic interactions of SNAREs syntaxin/unc-64 and SNAP-25/ric-4 with daf-2, daf-7, nhr-49, sbp-1 and mdt-15 in regulating fat storage were further investigated. The tissue-specific activities of unc-64 and ric-4 were investigated to study the roles of unc-64 and ric-4 in regulating fat storage in the nervous system and/or the intestine.

RESULTS

Mutations of genes required for the formation of presynaptic neurotransmission site did not obviously influence fat storage. However, among the genes required for synaptic function, the plasma membrane-associated SNAREs syntaxin/unc-64 and SNAP-25/ric-4 genes were involved in the fat storage control. Fat storage in the intestinal cells was dramatically increased in unc-64 and ric-4 mutants as revealed by Sudan Black and Nile Red strainings, although the fat droplet size was not significantly changed. Moreover, in both the nervous system and the intestine, expression of unc-64 significantly inhibited the increase in fat storage observed in unc-64 mutant. And expression of ric-4 in the nervous system completely restored fat storage in ric-4 mutant. Genetic interaction assay further indicated that both unc-64 and ric-4 regulated fat storage independently of daf-2 [encoding an insulin-like growth factor-I (IGF-I) receptor], daf-7 [encoding a transforming growth factor-beta (TGF-beta) ligand], and nhr-49 (encoding a nuclear hormone receptor). Besides, mutation of daf-16 did not obviously affect the phenotype of increased fat storage in unc-64 or ric-4 mutant. Furthermore, unc-64 and ric-4 regulated fat storage probably through the ARC105/mdt-15- and SREBP/sbp-1-mediated signaling pathways. In addition, fat storage in unc-64; ric-4 was higher than that in either unc-64 or ric-4 single mutant nematodes, suggesting that unc-64 functions in parallel with ric-4 in regulating fat storage.

CONCLUSION

The plasma membrane-associated SNAREs syntaxin/unc-64 and SNAP-25/ric-4 function in parallel in regulating fat storage in C. elegans, probably through the ARC105/mdt-15- and SREBP/sbp-1-mediated signaling pathways.

MADD-2, a homolog of the Opitz syndrome protein MID1, regulates guidance to the midline through UNC-40 in Caenorhabditis elegans

The body muscles of Caenorhabditis elegans extend plasma membrane extensions called muscle arms to the midline motor axons to form the postsynaptic membrane of the neuromuscular junction. Through a screen for muscle arm development defective (Madd) mutants, we previously discovered that the UNC-40/DCC guidance receptor directs muscle arm extension through the Rho-GEF UNC-73. Here, we describe a gene identified through our mutant screen called madd-2, and show that it functions in an UNC-40 pathway. MADD-2 is a C1-TRIM protein and a homolog of human MID1, mutations in which cause Opitz Syndrome. We demonstrate that MADD-2 functions cell autonomously to direct muscle and axon extensions to the ventral midline of worms. Our results suggest that MADD-2 may enhance UNC-40 pathway activity by facilitating an interaction between UNC-40 and UNC-73. The analogous phenotypes that result from MADD-2 and MID1 mutations suggest that C1-TRIM proteins may have a conserved biological role in midline-oriented developmental events.

A homolog of FHM2 is involved in modulation of excitatory neurotransmission by serotonin in C. elegans

The C. elegans eat-6 gene encodes a Na(+), K(+)-ATPase alpha subunit and is a homolog of the familial hemiplegic migraine candidate gene FHM2. Migraine is the most common neurological disorder linked to serotonergic dysfunction. We sought to study the pathophysiological mechanisms of migraine and their relation to serotonin (5-HT) signaling using C. elegans as a genetic model. In C. elegans, exogenous 5-HT inhibits paralysis induced by the acetylcholinesterase inhibitor aldicarb. We found that the eat-6(ad467) mutation or RNAi of eat-6 increases aldicarb sensitivity and causes complete resistance to 5-HT treatment, indicating that EAT-6 is a component of the pathway that couples 5-HT signaling and ACh neurotransmission. While a postsynaptic role of EAT-6 at the bodywall NMJs has been well established, we found that EAT-6 may in addition regulate presynaptic ACh neurotransmission. We show that eat-6 is expressed in ventral cord ACh motor neurons, and that cell-specific RNAi of eat-6 in the ACh neurons leads to hypersensitivity to aldicarb. Electron microscopy showed an increased number of synaptic vesicles in the ACh neurons in the eat-6(ad467) mutant. Genetic analyses suggest that EAT-6 interacts with EGL-30 Galphaq, EGL-8 phospholipase C and SLO-1 BK channel signaling to modulate ACh neurotransmission and that either reduced or excessive EAT-6 function may lead to increased ACh neurotransmission. Study of the interaction between eat-6 and 5-HT receptors revealed both stimulatory and inhibitory 5-HT inputs to the NMJs. We show that the inhibitory and stimulatory 5-HT signals arise from distinct 5-HT neurons. The role of eat-6 in modulation of excitatory neurotransmission by 5-HT may provide a genetic explanation for the therapeutic effects of the drugs targeting 5-HT receptors in the treatment of migraine patients.

Hypoxia activates a latent circuit for processing gustatory information in C. elegans

Dedicated neuronal circuits enable animals to engage in specific behavioral responses to environmental stimuli. We found that hypoxic stress enhanced gustatory sensory perception via previously unknown circuitry in Caenorhabditis elegans. The hypoxia-inducible transcription factor HIF-1 upregulated serotonin (5-HT) expression in specific sensory neurons that are not normally required for chemosensation. 5-HT subsequently promoted hypoxia-enhanced sensory perception by signaling through the metabotropic G protein-coupled receptor SER-7 in an unusual peripheral neuron, the M4 motor neuron. M4 relayed this information back into the CNS via the FMRFamide-related neuropeptide FLP-21 and its cognate receptor, NPR-1. Thus, physiological detection of hypoxia results in the activation of an additional, previously unrecognized circuit for processing sensory information that is not required for sensory processing under normoxic conditions.

Genes required for cellular UNC-6/netrin localization in Caenorhabditis elegans

UNC-6/Netrin is an evolutionarily conserved, secretory axon guidance molecule. In Caenorhabditis elegans, UNC-6 provides positional information to the axons of developing neurons, probably by establishing a concentration gradient from the ventral to the dorsal side of the animal. Although the proper localization of UNC-6 is important for accurate neuronal network formation, little is known about how its localization is regulated. Here, to examine the localization mechanism for UNC-6, we generated C. elegans expressing UNC-6 tagged with the fluorescent protein Venus and identified 13 genes, which are involved in the cellular localization of VenusUNC-6. For example, in unc-51, unc-14, and unc-104 mutants, the neurons showed an abnormal accumulation of VenusUNC-6 in the cell body and less than normal level of VenusUNC-6 in the axon. An aberrant accumulation of VenusUNC-6 in muscle cells was seen in unc-18 and unc-68 mutants. unc-51, unc-14, and unc-104 mutants also showed defects in the guidance of dorso-ventral axons, suggesting that the abnormal localization of UNC-6 disturbed the positional information it provides. We propose that these genes regulate the process of UNC-6 secretion: expression, maturation, sorting, transport, or exocytosis. Our findings provide novel insight into the localization mechanism of the axon guidance molecule UNC-6/Netrin.

Something worth dyeing for: molecular tools for the dissection of lipid metabolism in Caenorhabditis elegans

The nematode Caenorhabditis elegans (C. elegans) has during the last decade emerged as an invaluable eukaryotic model organism to understand the metabolic and neuro-endocrine regulation of lipid accumulation. The fundamental pathways of food intake, digestion, metabolism, and signalling are evolutionary conserved between mammals and worms making C. elegans a genetically and metabolically extremely tractable model to decipher new regulatory mechanisms of lipid storage and to understand how nutritional and genetic perturbations can lead to obesity and other metabolic diseases. Besides providing an overview of the most important regulatory mechanisms of lipid accumulation in C. elegans, we also critically assess the current methodologies to monitor lipid storage and content as various methods differ in their applicability, consistency, and simplicity.

Genetic and dietary regulation of lipid droplet expansion in Caenorhabditis elegans

Dietary fat accumulates in lipid droplets or endolysosomal compartments that undergo selective expansion under normal or pathophysiological conditions. We find that genetic defects in a peroxisomal beta-oxidation pathway cause size expansion in lipid droplets that are distinct from the lysosome-related organelles in Caenorhabditis elegans. Expansion of lipid droplets is accompanied by an increase in triglycerides (TAG) that are resistant to fasting- or TAG lipase-triggered lipolysis. Nevertheless, in mutant animals, a diet poor in vaccenic acid reduced the TAG level and lipid droplet size. Our results implicate peroxisomal dysfunction in pathologic lipid droplet expansion in animals and illustrate how dietary factors modulate the phenotype of such genetic defects.

Neuroligin-deficient mutants of C. elegans have sensory processing deficits and are hypersensitive to oxidative stress and mercury toxicity

Neuroligins are postsynaptic cell adhesion proteins that bind specifically to presynaptic membrane proteins called neurexins. Mutations in human neuroligin genes are associated with autism spectrum disorders in some families. The nematode Caenorhabditis elegans has a single neuroligin gene (nlg-1), and approximately a sixth of C. elegans neurons, including some sensory neurons, interneurons and a subset of cholinergic motor neurons, express a neuroligin transcriptional reporter. Neuroligin-deficient mutants of C. elegans are viable, and they do not appear deficient in any major motor functions. However, neuroligin mutants are defective in a subset of sensory behaviors and sensory processing, and are hypersensitive to oxidative stress and mercury compounds; the behavioral deficits are strikingly similar to traits frequently associated with autism spectrum disorders. Our results suggest a possible link between genetic defects in synapse formation or function, and sensitivity to environmental factors in the development of autism spectrum disorders.

Tissue-specific activities of an immune signaling module regulate physiological responses to pathogenic and nutritional bacteria in C. elegans

Microbes represent both an essential source of nutrition and a potential source of lethal infection to the nematode Caenorhabditis elegans. Immunity in C. elegans requires a signaling module comprised of orthologs of the mammalian Toll-interleukin-1 receptor (TIR) domain protein SARM, the mitogen-activated protein kinase kinase kinase (MAPKKK) ASK1, and MAPKK MKK3, which activates p38 MAPK. We determined that the SARM-ASK1-MKK3 module has dual tissue-specific roles in the C. elegans response to pathogens--in the cell-autonomous regulation of innate immunity and the neuroendocrine regulation of serotonin-dependent aversive behavior. SARM-ASK1-MKK3 signaling in the sensory nervous system also regulates egg-laying behavior that is dependent on bacteria provided as a nutrient source. Our data demonstrate that these physiological responses to bacteria share a common mechanism of signaling through the SARM-ASK1-MKK3 module and suggest the co-option of ancestral immune signaling pathways in the evolution of physiological responses to microbial pathogens and nutrients.

The PAPS transporter PST-1 is required for heparan sulfation and is essential for viability and neural development in C. elegans

Sulfations of sugars, such as heparan sulfates (HS), or tyrosines require the universal sulfate donor 3'-phospho-adenosine-5'-phosphosulfate (PAPS) to be transported from the cytosol into the Golgi. Metazoan genomes encode two putative PAPS transporters (PAPST1 and PAPST2), which have been shown in vitro to preferentially transport PAPS across membranes. We have identified the C. elegans orthologs of PAPST1 and PAPST2 and named them pst-1 and pst-2, respectively. We show that pst-1 is essential for viability in C. elegans, functions non-redundantly with pst-2, and can act non-autonomously to mediate essential functions. Additionally, pst-1 is required for specific aspects of nervous system development rather than for formation of the major neuronal ganglia or fascicles. Neuronal defects correlate with reduced complexity of HS modification patterns, as measured by direct biochemical analysis. Our results suggest that pst-1 functions in metazoans to establish the complex HS modification patterns that are required for the development of neuronal connectivity.

Molecular and sensory basis of a food related two-state behavior in C. elegans

Most animals display multiple behavioral states and control the time allocation to each of their activity phases depending on their environment. Here we develop a new quantitative method to analyze Caenorhabditis elegans behavioral states. We show that the dwelling and roaming two-state behavior of C. elegans is tightly controlled by the concentration of food in the environment of the animal. Sensory perception through the amphid neurons is necessary to extend roaming phases while internal metabolic perception of food nutritional value is needed to induce dwelling. Our analysis also shows that the proportion of time spent in each state is modulated by past nutritional experiences of the animal. This two-state behavior is regulated through serotonin as well as insulin and TGF-beta signaling pathways. We propose a model where food nutritional value is assessed through internal metabolic signaling. Biogenic amines signaling could allow the worm to adapt to fast changes in the environment when peptide transcriptional pathways may mediate slower adaptive changes.

Characterization of the effects of methylmercury on Caenorhabditis elegans

The rising prevalence of methylmercury (MeHg) in seafood and in the global environment provides an impetus for delineating the mechanism of the toxicity of MeHg. Deleterious effects of MeHg have been widely observed in humans and in other mammals, the most striking of which occur in the nervous system. Here we test the model organism, Caenorhabditis elegans (C. elegans), for MeHg toxicity. The simple, well-defined anatomy of the C. elegans nervous system and its ready visualization with green fluorescent protein (GFP) markers facilitated our study of the effects of methylmercuric chloride (MeHgCl) on neural development. Although MeHgCl was lethal to C. elegans, induced a developmental delay, and decreased pharyngeal pumping, other traits including lifespan, brood size, swimming rate, and nervous system morphology were not obviously perturbed in animals that survived MeHgCl exposure. Despite the limited effects of MeHgCl on C. elegans development and behavior, intracellular mercury (Hg) concentrations ( Collapse

Key Words

• caenorhabditis elegans
• methylmercury

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MESH Headings

• Animals
• Caenorhabditis elegans/drug effects
• Caenorhabditis elegans/embryology
• Caenorhabditis elegans/growth & development
• Caenorhabditis elegans/metabolism
• Dose-Response Relationship, Drug
• Drug Resistance
• Environmental Pollutants/metabolism
• Environmental Pollutants/toxicity
• Larva/drug effects
• Larva/metabolism
• Longevity/drug effects
• Methylmercury Compounds/metabolism
• Methylmercury Compounds/toxicity
• Nervous System/drug effects
• Nervous System/embryology
• Nervous System/growth & development
• Nervous System/metabolism
• Neurogenesis/drug effects
• Neurons/drug effects
• Neurons/metabolism
• Pharyngeal Muscles/drug effects
• Pharyngeal Muscles/growth & development
• Reproduction/drug effects
• Time Factors

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Grants

• T32 ES007028 NIEHS NIH HHS
• R01 ES007331 NIEHS NIH HHS
• R01 NS026115 NINDS NIH HHS
• ES 007028 NIEHS NIH HHS
• 10563 PHS HHS
• NS 26115 NINDS NIH HHS

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Affiliation(s)

Kirsten J Helmcke Pharmacology Department, Vanderbilt University Medical Center, Nashville, TN 37232-0414, USA
Tore Syversen
David M Miller
Michael Aschner

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Collaborators Collapse

The neural network for chemotaxis to tastants in Caenorhabditis elegans is specialized for temporal differentiation

Chemotaxis in Caenorhabditis elegans depends critically on the rate of change of attractant concentration computed as the worm moves through its environment. This computation depends, in turn, on the neuron class ASE, a left-right pair of pair of chemosensory neurons that is functionally asymmetric such that the left neuron is an on-cell, whereas the right neuron is an off-cell. To determine whether this coding strategy is a general feature of chemosensation in C. elegans, we imaged calcium responses in all chemosensory neurons known or in a position to contribute to chemotaxis to tastants in this organism. This survey revealed one new class of on-cells (ADF) and one new class of off-cells (ASH). Thus, the ASE class is unique in having both an on-cell and an off-cell. We also found that the newly characterized on-cells and off-cells promote runs and turns, respectively, mirroring the pattern reported previously for ASEL and ASER. Our results suggest that the C. elegans chemotaxis network is specialized for the temporal differentiation of chemosensory inputs, as required for chemotaxis.

Investigating bacterial sources of toxicity as an environmental contributor to dopaminergic neurodegeneration

Parkinson disease (PD) involves progressive neurodegeneration, including loss of dopamine (DA) neurons from the substantia nigra. Select genes associated with rare familial forms of PD function in cellular pathways, such as the ubiquitin-proteasome system (UPS), involved in protein degradation. The misfolding and accumulation of proteins, such as α-synuclein, into inclusions termed Lewy Bodies represents a clinical hallmark of PD. Given the predominance of sporadic PD among patient populations, environmental toxins may induce the disease, although their nature is largely unknown. Thus, an unmet challenge surrounds the discovery of causal or contributory neurotoxic factors that could account for the prevalence of sporadic PD. Bacteria within the order Actinomycetales are renowned for their robust production of secondary metabolites and might represent unidentified sources of environmental exposures. Among these, the aerobic genera, Streptomyces, produce natural proteasome inhibitors that block protein degradation and may potentially damage DA neurons. Here we demonstrate that a metabolite produced by a common soil bacterium, S. venezuelae, caused DA neurodegeneration in the nematode, Caenorhabditis elegans, which increased as animals aged. This metabolite, which disrupts UPS function, caused gradual degeneration of all neuronal classes examined, however DA neurons were particularly vulnerable to exposure. The presence of DA exacerbated toxicity because neurodegeneration was attenuated in mutant nematodes depleted for tyrosine hydroxylase (TH), the rate-limiting enzyme in DA production. Strikingly, this factor caused dose-dependent death of human SH-SY5Y neuroblastoma cells, a dopaminergic line. Efforts to purify the toxic activity revealed that it is a highly stable, lipophilic, and chemically unique small molecule. Evidence of a robust neurotoxic factor that selectively impacts neuronal survival in a progressive yet moderate manner is consistent with the etiology of age-associated neurodegenerative diseases. Collectively, these data suggest the potential for exposures to the metabolites of specific common soil bacteria to possibly represent a contributory environmental component to PD.