Function and evolution of the serotonin-synthetic bas-1 gene and other aromatic amino acid decarboxylase genes in Caenorhabditis

A neurotransmitter atlas of C. elegans males and hermaphrodites

Mapping neurotransmitter identities to neurons is key to understanding information flow in a nervous system. It also provides valuable entry points for studying the development and plasticity of neuronal identity features. In the C. elegans nervous system, neurotransmitter identities have been largely assigned by expression pattern analysis of neurotransmitter pathway genes that encode neurotransmitter biosynthetic enzymes or transporters. However, many of these assignments have relied on multicopy reporter transgenes that may lack relevant cis-regulatory information and therefore may not provide an accurate picture of neurotransmitter usage. We analyzed the expression patterns of 16 CRISPR/Cas9-engineered knock-in reporter strains for all main types of neurotransmitters in C. elegans (glutamate, acetylcholine, GABA, serotonin, dopamine, tyramine, and octopamine) in both the hermaphrodite and the male. Our analysis reveals novel sites of expression of these neurotransmitter systems within both neurons and glia, as well as non-neural cells. The resulting expression atlas defines neurons that may be exclusively neuropeptidergic, substantially expands the repertoire of neurons capable of co-transmitting multiple neurotransmitters, and identifies novel neurons that uptake monoaminergic neurotransmitters. Furthermore, we also observed unusual co-expression patterns of monoaminergic synthesis pathway genes, suggesting the existence of novel monoaminergic transmitters. Our analysis results in what constitutes the most extensive whole-animal-wide map of neurotransmitter usage to date, paving the way for a better understanding of neuronal communication and neuronal identity specification in C. elegans.

Bifidobacterium animalis subsp. lactis BPL1™ and Its Lipoteichoic Acid Modulate Longevity and Improve Age/Stress-Related Behaviors in Caenorhabditis elegans

Life expectancy has increased globally in recent decades, driving interest in maintaining a healthy life that includes preservation of physical and mental abilities, particularly in elderly people. The gut microbiome becomes increasingly perturbed with aging so the use of probiotics can be a strategy for maintaining a balanced gut microbiome. A previous report showed that Bifidobacterium animalis subsp. lactis BPL1™ induces through its lipoteichoic acid (LTA) fat reduction activities via the insulin/IGF-1 signaling pathway. Here, we have delved into the mechanism of action, eliminating alternative pathways as putative mechanisms. Furthermore, we have identified that BPL1™, its heat treated form (BPL1™ HT) and its LTA prolong longevity in Caenorhabditis elegans (C. elegans) in an insulin/IGF-1-dependent mechanism, and its consumption improves the oxidative stress response, gut permeability and protection against pathogenic infections. Furthermore, positive effects on C. elegans stress-related behaviors and in the Alzheimer's Disease model were found, highlighting the potential of the strain in improving the cognitive functions and proteotoxicity in the nematode. These results indicate the pivotal role of the IGF-1 pathway in the activity of the strain and pave the way for potential applications of BPL1™, BPL1™ HT and its LTA in the field of longevity and age-related markers.

Effect of altered production and storage of dopamine on development and behavior in C. elegans

The nematode, Caenorhabditis elegans, is an advantageous model for studying developmental toxicology due to its homology to humans and well-defined developmental stages. Similarly to humans, C. elegans utilize dopamine as a neurotransmitter to regulate motor behavior. We have previously reported behavioral deficits in a genetic model of C. elegans (OK411) that lack the neurotransmitter transporter necessary for packaging dopamine into synaptic vesicles. Anecdotally, we observed these C. elegans appeared to have a smaller body size, which is supported by prior studies that observed a larger body size in C. elegans that lack the enzyme that catalyzes dopamine synthesis, suggesting a complex regulatory system in which dopamine mediates body size in C. elegans. However, the question of whether body size abnormalities apparent in C. elegans with disruptions to their dopamine system are developmental or purely based on body size remains unanswered. Here, we present data characterizing the effect of gene mutations in dopamine-related proteins on body size, development, and behavior. We analyzed C. elegans that lack the ability to sequester dopamine (OK411), that overproduce dopamine (UA57), and a novel strain (MBIA) generated through crossing OK411 and UA57, which lacks the ability to sequester dopamine into vesicles and additionally endogenously overproduces dopamine. This novel strain was generated to address the hypothesis that an endogenous increase in production of dopamine can rescue deficits caused by a lack of vesicular dopamine sequestration. Compared to wild type, OK411 have shorter body lengths and behavioral deficits in early life stages. In contrast, the MBIA strain have similar body lengths to wild-type by early adulthood and display similar behavior to wild-type by early adulthood. Our data suggests that endogenously increasing the production of dopamine is able to mitigate deficits in C. elegans lacking the ability to package dopamine into synaptic vesicles. These results provide evidence that the dopamine system impacts development, growth, and reproduction in C. elegans.

Parallel pathways for serotonin biosynthesis and metabolism in C. elegans

The neurotransmitter serotonin plays a central role in animal behavior and physiology, and many of its functions are regulated via evolutionarily conserved biosynthesis and degradation pathways. Here we show that in Caenorhabditis elegans, serotonin is abundantly produced in nonneuronal tissues via phenylalanine hydroxylase, in addition to canonical biosynthesis via tryptophan hydroxylase in neurons. Combining CRISPR-Cas9 genome editing, comparative metabolomics and synthesis, we demonstrate that most serotonin in C. elegans is incorporated into N-acetylserotonin-derived glucosides, which are retained in the worm body and further modified via the carboxylesterase CEST-4. Expression patterns of CEST-4 suggest that serotonin or serotonin derivatives are transported between different tissues. Last, we show that bacterial indole production interacts with serotonin metabolism via CEST-4. Our results reveal a parallel pathway for serotonin biosynthesis in nonneuronal cell types and further indicate that serotonin-derived metabolites may serve distinct signaling functions and contribute to previously described serotonin-dependent phenotypes.

Regulation and modulation of biogenic amine neurotransmission in Drosophila and Caenorhabditis elegans

Neurotransmitters are crucial for the relay of signals between neurons and their target. Monoamine neurotransmitters dopamine (DA), serotonin (5-HT), and histamine are found in both invertebrates and mammals and are known to control key physiological aspects in health and disease. Others, such as octopamine (OA) and tyramine (TA), are abundant in invertebrates. TA is expressed in both Caenorhabditis elegans and Drosophila melanogaster and plays important roles in the regulation of essential life functions in each organism. OA and TA are thought to act as the mammalian homologs of epinephrine and norepinephrine respectively, and when triggered, they act in response to the various stressors in the fight-or-flight response. 5-HT regulates a wide range of behaviors in C. elegans including egg-laying, male mating, locomotion, and pharyngeal pumping. 5-HT acts predominantly through its receptors, of which various classes have been described in both flies and worms. The adult brain of Drosophila is composed of approximately 80 serotonergic neurons, which are involved in modulation of circadian rhythm, feeding, aggression, and long-term memory formation. DA is a major monoamine neurotransmitter that mediates a variety of critical organismal functions and is essential for synaptic transmission in invertebrates as it is in mammals, in which it is also a precursor for the synthesis of adrenaline and noradrenaline. In C. elegans and Drosophila as in mammals, DA receptors play critical roles and are generally grouped into two classes, D1-like and D2-like based on their predicted coupling to downstream G proteins. Drosophila uses histamine as a neurotransmitter in photoreceptors as well as a small number of neurons in the CNS. C. elegans does not use histamine as a neurotransmitter. Here, we review the comprehensive set of known amine neurotransmitters found in invertebrates, and discuss their biological and modulatory functions using the vast literature on both Drosophila and C. elegans. We also suggest the potential interactions between aminergic neurotransmitters systems in the modulation of neurophysiological activity and behavior.

Review of the toxicity and potential molecular mechanisms of parental or successive exposure to environmental pollutants in the model organism Caenorhabditis elegans

Environmental pollutants such as heavy metals, nano/microparticles, and organic compounds have been detected in a wide range of environmental media, causing long-term exposure in various organisms and even humans through breathing, contacting, ingestion, and other routes. Long-term exposure to environmental pollutants in organisms or humans promotes exposure of offspring to parental and environmental pollutants, and subsequently results in multiple biological defects in the offspring. This review dialectically summarizes and discusses the existing studies using Caenorhabditis elegans (C. elegans) as a model organism to explore the multi/transgenerational toxicity and potential underlying molecular mechanisms induced by environmental pollutants following parental or successive exposure patterns. Parental and successive exposure to environmental pollutants induces various biological defects in C. elegans across multiple generations, including multi/transgenerational developmental toxicity, neurotoxicity, reproductive toxicity, and metabolic disturbances, which may be transmitted to progeny through reactive oxygen species-induced damage, epigenetic mechanisms, insulin/insulin-like growth factor-1 signaling pathway. This review aims to arouse researchers' interest in the multi/transgenerational toxicity of pollutants and hopes to explore the possible long-term effects of environmental pollutants on organisms and even humans, as well as to provide constructive suggestions for the safety and management of emerging alternatives.

Acetylcholine signaling genes are required for cocaine-stimulated egg laying in Caenorhabditis elegans

The toxicity and addictive liability associated with cocaine abuse are well-known. However, its mode of action is not completely understood, and effective pharmacotherapeutic interventions remain elusive. The cholinergic effects of cocaine on acetylcholine receptors, synthetic enzymes, and degradative enzymes have been the focus of relatively little empirical investigation. Due to its genetic tractability and anatomical simplicity, the egg laying circuit of the hermaphroditic nematode, Caenorhabditis elegans, is a powerful model system to precisely examine the genetic and molecular targets of cocaine in vivo. Here, we report a novel cocaine-induced behavioral phenotype in C. elegans, cocaine-stimulated egg laying. In addition, we present the results of an in vivo candidate suppression screen of synthetic enzymes, receptors, degradative enzymes, and downstream components of the intracellular signaling cascades of the main neurotransmitter systems that control C. elegans egg laying. Our results show that cocaine-stimulated egg laying is dependent on acetylcholine synthesis and synaptic release, functional nicotinic acetylcholine receptors, and the C. elegans acetylcholinesterases.

Regulation of monoamine levels by typical and atypical antipsychotics in Caenorhabditis elegans mutant for nuclear distribution element genes

Mammalian nuclear distribution genes encode proteins with essential roles in neuronal migration and brain formation during embryogenesis. The implication of human nuclear distribution genes, namely nudC and NDE1 (Nuclear Distribution Element 1)/NDEL1 (Nuclear Distribution Element-Like 1), in psychiatric disorders including schizophrenia and bipolar disorder, has been recently described. The partial loss of NDEL1 expression results in neuronal migration defects, while ndel1 null knockout (KO) leads to early embryonic lethality in mice. On the other hand, loss-of-function of the orthologs of nuclear distribution element genes (nud) in Caenorhabditis elegans renders viable worms and influences behavioral endophenotypes associated with dopaminergic and serotoninergic pathways. In the present work, we evaluated the role of nud genes in monoamine levels at baseline and after the treatment with typical or atypical antipsychotics. Dopamine, serotonin and octopamine levels were significantly lower in homozygous loss-of-function mutant worms KO for nud genes compared with wild-type (WT) C. elegans at baseline. While treatment with antipsychotics determined significant differences in monoamine levels in WT, the nud KO mutant worms appear to respond differently to the treatment. According to the best of our knowledge, we are the first to report the influence of nud genes in the monoamine levels changes in response to antipsychotic drugs, ultimately placing the nuclear distribution genes family at the cornerstone of pathways involved in the modulation of monoamines in response to different classes of antipsychotic drugs.

Biogenic amine neurotransmitters promote eicosanoid production and protein homeostasis

Metazoans use protein homeostasis (proteostasis) pathways to respond to adverse physiological conditions, changing environment, and aging. The nervous system regulates proteostasis in different tissues, but the mechanism is not understood. Here, we show that Caenorhabditis elegans employs biogenic amine neurotransmitters to regulate ubiquitin proteasome system (UPS) proteostasis in epithelia. Mutants for biogenic amine synthesis show decreased poly-ubiquitination and turnover of a GFP-based UPS substrate. Using RNA-seq and mass spectrometry, we found that biogenic amines promote eicosanoid production from poly-unsaturated fats (PUFAs) by regulating expression of cytochrome P450 monooxygenases. Mutants for one of these P450s share the same UPS phenotype observed in biogenic amine mutants. The production of n-6 eicosanoids is required for UPS substrate turnover, whereas accumulation of n-6 eicosanoids accelerates turnover. Our results suggest that sensory neurons secrete biogenic amines to modulate lipid signaling, which in turn activates stress response pathways to maintain UPS proteostasis.

Neuromodulators: an essential part of survival

The coordination between the animal's external environment and internal state requires constant modulation by chemicals known as neuromodulators. Neuromodulators, such as biogenic amines, neuropeptides and cytokines, promote organismal homeostasis. Over the past several decades, Caenorhabditiselegans has grown into a powerful model organism that allows the elucidation of the mechanisms of action of neuromodulators that are conserved across species. In this perspective, we highlight a collection of articles in this issue that describe how neuromodulators optimize C. elegans survival.

Arsenic induces transgenerational behavior disorders in Caenorhabditis elegans and its underlying mechanisms

The present study aimed to identify the effects of arsenic on behaviors in Caenorhabditis elegans (C. elegans) and the transgenerational effects. The synchronized C. elegans (P generation) were exposed to 0, 0.2, 1.0, and 5.0 mM NaAsO₂ and the subsequent generations (F1 and F2) were maintained on fresh nematode growth medium (NGM). The behaviors and growth were recorded at 0, 12, 24, 36, 48, 60, and 72 h post synchronization. The results demonstrated that arsenic affected various indicators regarding the behavior (head thrash, body bend, movement speed, wavelength, amplitude and so on) and in general the effects started to accumulate from 24 h and lasted throughout the exposure. The behavior impairments were transgenerational with varying patterns, amongst the head thrash and body bend responded most sensitively though the responses gradually declined across generations. Arsenic exposure inhibited the growth (body length, body width, and body area) in P C. elegans from 24 h to 60 h, however there was no difference between treatments groups and the control at 72 h. Arsenic led to a dose-dependent degeneration of dopaminergic neurons in C. elegans, and inhibition of BAS-1 and CAT-2 expressions. The expressions of GCS-1, GSS-1, and SKN-1 were induced by arsenic exposure. Overall, chronic arsenic exposure impaired the behaviors and there were transgenerational effects. The head thrash and body bend responded most sensitively. Arsenic induced behavioral disorders might be attributed to degeneration of dopaminergic neurons which was associated with oxidative stress.

Defective Expression of Mitochondrial, Vacuolar H+-ATPase and Histone Genes in a C. elegans Model of SMA

Spinal muscular atrophy (SMA) is a severe motor neuron degenerative disease caused by loss-of-function mutations in the survival motor neuron gene SMN1. It is widely posited that defective gene expression underlies SMA. However, the identities of these affected genes remain to be elucidated. By analyzing the transcriptome of a Caenorhabditis elegans SMA model at the pre-symptomatic stage, we found that the expression of numerous nuclear encoded mitochondrial genes and vacuolar H⁺-ATPase genes was significantly down-regulated, while that of histone genes was significantly up-regulated. We previously showed that the uaf-1 gene, encoding key splicing factor U2AF large subunit, could affect the behavior and lifespan of smn-1 mutants. Here, we found that smn-1 and uaf-1 interact to affect the recognition of 3′ and 5′ splice sites in a gene-specific manner. Altogether, our results suggest a functional interaction between smn-1 and uaf-1 in affecting RNA splicing and a potential effect of smn-1 on the expression of mitochondrial and histone genes.

DOPA Decarboxylase Modulates Tau Toxicity

BACKGROUND

The microtubule-associated protein tau accumulates into toxic aggregates in multiple neurodegenerative diseases. We found previously that loss of D₂-family dopamine receptors ameliorated tauopathy in multiple models including a Caenorhabditis elegans model of tauopathy.

METHODS

To better understand how loss of D₂-family dopamine receptors can ameliorate tau toxicity, we screened a collection of C. elegans mutations in dopamine-related genes (n = 45) for changes in tau transgene-induced behavioral defects. These included many genes responsible for dopamine synthesis, metabolism, and signaling downstream of the D₂ receptors.

RESULTS

We identified one dopamine synthesis gene, DOPA decarboxylase (DDC), as a suppressor of tau toxicity in tau transgenic worms. Loss of the C. elegans DDC gene, bas-1, ameliorated the behavioral deficits of tau transgenic worms, reduced phosphorylated and detergent-insoluble tau accumulation, and reduced tau-mediated neuron loss. Loss of function in other genes in the dopamine and serotonin synthesis pathways did not alter tau-induced toxicity; however, their function is required for the suppression of tau toxicity by bas-1. Additional loss of D₂-family dopamine receptors did not synergize with bas-1 suppression of tauopathy phenotypes.

CONCLUSIONS

Loss of the DDC bas-1 reduced tau-induced toxicity in a C. elegans model of tauopathy, while loss of no other dopamine or serotonin synthesis genes tested had this effect. Because loss of activity upstream of DDC could reduce suppression of tau by DDC, this suggests the possibility that loss of DDC suppresses tau via the combined accumulation of dopamine precursor levodopa and serotonin precursor 5-hydroxytryptophan.

Probiotic Strain Bifidobacterium animalis subsp. lactis CECT 8145 Reduces Fat Content and Modulates Lipid Metabolism and Antioxidant Response in Caenorhabditis elegans

Recently, microbial changes in the human gut have been proposed as a possible cause of obesity. Therefore, modulation of microbiota through probiotic supplements is of great interest to support obesity therapeutics. The present study examines the functional effect and metabolic targets of a bacterial strain, Bifidobacterium animalis subsp. lactis CECT 8145, selected from a screening in Caenorhabditis elegans. This strain significantly reduced total lipids (40.5% ± 2.4) and triglycerides (27.6% ± 0.5), exerting antioxidant effects in the nematode (30% ± 2.8 increase in survival vs control); activities were also preserved in a final food matrix (milk). Furthermore, transcriptomic and metabolomic analyses in nematodes fed with strain CECT 8145 revealed modulation of the energy and lipid metabolism, as well as the tryptophan metabolism (satiety), as the main metabolic targets of the probiotic. In conclusion, our study describes for the first time a new B. animalis subsp. lactis strain, CECT 8145, as a promising probiotic for obesity disorders. Furthermore, the data support future studies in obesity murine models.

Cuticle integrity and biogenic amine synthesis in Caenorhabditis elegans require the cofactor tetrahydrobiopterin (BH4)

Tetrahydrobiopterin (BH4) is the natural cofactor of several enzymes widely distributed among eukaryotes, including aromatic amino acid hydroxylases (AAAHs), nitric oxide synthases (NOSs), and alkylglycerol monooxygenase (AGMO). We show here that the nematode Caenorhabditis elegans, which has three AAAH genes and one AGMO gene, contains BH4 and has genes that function in BH4 synthesis and regeneration. Knockout mutants for putative BH4 synthetic enzyme genes lack the predicted enzymatic activities, synthesize no BH4, and have indistinguishable behavioral and neurotransmitter phenotypes, including serotonin and dopamine deficiency. The BH4 regeneration enzymes are not required for steady-state levels of biogenic amines, but become rate limiting in conditions of reduced BH4 synthesis. BH4-deficient mutants also have a fragile cuticle and are generally hypersensitive to exogenous agents, a phenotype that is not due to AAAH deficiency, but rather to dysfunction in the lipid metabolic enzyme AGMO, which is expressed in the epidermis. Loss of AGMO or BH4 synthesis also specifically alters the sensitivity of C. elegans to bacterial pathogens, revealing a cuticular function for AGMO-dependent lipid metabolism in host-pathogen interactions.

Patterning of sexually dimorphic neurogenesis in the caenorhabditis elegans ventral cord by Hox and TALE homeodomain transcription factors

BACKGROUND

Reproduction in animals requires development of distinct neurons in each sex. In C. elegans, most ventral cord neurons (VCNs) are present in both sexes, with the exception of six hermaphrodite-specific neurons (VCs) and nine pairs of male-specific neurons (CAs and CPs) that arise from analogous precursor cells. How are the activities of sexual regulators and mediators of neuronal survival, division, and fate coordinated to generate sex-specificity in VCNs?

RESULTS

To address this, we have developed a toolkit of VCN markers that allows us to examine sex-specific neurogenesis, asymmetric fates of daughters of a neuroblast division, and regional specification on the anteroposterior axis. Here, we describe the roles of the Hox transcription factors LIN-39 and MAB-5 in promoting survival, differentiation, and regionalization of VCNs. We also find that the TALE class homeodomain proteins CEH-20 and UNC-62 contribute to specification of neurotransmitter fate in males. Furthermore, we identify that VCN sex is determined during the L1 larval stage.

CONCLUSIONS

These findings, combined with future analyses made possible by the suite of VCN markers described here, will elucidate how Hox-mediated cell fate decisions and sex determination intersect to influence development of neuronal sex differences.

Caenorhabditis elegans male sensory-motor neurons and dopaminergic support cells couple ejaculation and post-ejaculatory behaviors

The circuit structure and function underlying post-coital male behaviors remain poorly understood. Using mutant analysis, laser ablation, optogenetics, and Ca²⁺ imaging, we observed that following C. elegans male copulation, the duration of post-coital lethargy is coupled to cellular events involved in ejaculation. We show that the SPV and SPD spicule-associated sensory neurons and the spicule socket neuronal support cells function with intromission circuit components, including the cholinergic SPC and PCB and the glutamatergic PCA sensory-motor neurons, to coordinate sex muscle contractions with initiation and continuation of sperm movement. Our observations suggest that the SPV and SPD and their associated dopamine-containing socket cells sense the intrauterine environment through cellular endings exposed at the spicule tips and regulate both sperm release into the hermaphrodite and the recovery from post-coital lethargy.

DOI:http://dx.doi.org/10.7554/eLife.02938.001

The nematode worm, C. elegans, is roughly 1 mm long, made up of around 1000 cells and has two sexes: male and hermaphrodite. Hermaphrodite worms produce both eggs and sperm and can self-fertilize to generate around 300 offspring each time. Fertilization by a male, on the other hand, results in three times as many progeny and introduces genetic diversity into the population. However, it also reduces the lifespan of the hermaphrodite.

Mating also incurs a cost for males: it requires a lot of energy, which prevents male works from engaging in other activities, such as feeding, and it also increases their risk of predation. In many species, including C. elegans, the frequency with which a male can mate is limited by a period of reduced mating drive and ability that follows each instance of successful mating. However, the molecular and cellular basis of this ‘refractory period’ remains largely unclear.

Using a range of techniques, LeBoeuf et al. have now identified the circuits that regulate male mating behavior in C. elegans. When male worms were introduced into a Petri dish containing 15 hermaphrodites, most males initiated mating within about 2 min. The length of the refractory period varied between worms, but averaged roughly 12 min. This consisted of a period of disinterest, in which males did not approach hermaphrodites, followed by a period in which males attempted mating but were slower and less efficient, suggesting that the neural circuits controlling mating behaviors had yet to recover completely.

Males with longer refractory periods produced more progeny in their second mating than those with shorter refractory periods, suggesting that the interval also enables males to replenish their sperm levels. Further experiments revealed that a chemical transmitter called dopamine promotes ejaculation and then immediately reduces the worm's activity levels, giving rise to the refractory period.

By enforcing a delay between matings, the refractory period may also increase the likelihood that successive matings will be with different hermaphrodites, helping to maximize the number and diversity of offspring. Some aspects of the neural circuitry that controls the refractory period in C. elegans resemble those seen in mammals, suggesting that insights gained from an animal with 1000 cells could also be relevant to more complex species.

DOI:http://dx.doi.org/10.7554/eLife.02938.002

Dopamine signaling regulates fat content through β-oxidation in Caenorhabditis elegans

The regulation of energy balance involves an intricate interplay between neural mechanisms that respond to internal and external cues of energy demand and food availability. Compelling data have implicated the neurotransmitter dopamine as an important part of body weight regulation. However, the precise mechanisms through which dopamine regulates energy homeostasis remain poorly understood. Here, we investigate mechanisms through which dopamine modulates energy storage. We showed that dopamine signaling regulates fat reservoirs in Caenorhabditis elegans. We found that the fat reducing effects of dopamine were dependent on dopaminergic receptors and a set of fat oxidation enzymes. Our findings reveal an ancient role for dopaminergic regulation of fat and suggest that dopamine signaling elicits this outcome through cascades that ultimately mobilize peripheral fat depots.

Serotonin control of thermotaxis memory behavior in nematode Caenorhabditis elegans

Caenorhabditis elegans is as an ideal model system for the study of mechanisms underlying learning and memory. In the present study, we employed C. elegans assay system of thermotaxis memory to investigate the possible role of serotonin neurotransmitter in memory control. Our data showed that both mutations of tph-1, bas-1, and cat-4 genes, required for serotonin synthesis, and mutations of mod-5 gene, encoding a serotonin reuptake transporter, resulted in deficits in thermotaxis memory behavior. Exogenous treatment with serotonin effectively recovered the deficits in thermotaxis memory of tph-1 and bas-1 mutants to the level of wild-type N2. Neuron-specific activity assay of TPH-1 suggests that serotonin might regulate the thermotaxis memory behavior by release from the ADF sensory neurons. Ablation of ADF sensory neurons by expressing a cell-death activator gene egl-1 decreased the thermotaxis memory, whereas activation of ADF neurons by expression of a constitutively active protein kinase C homologue (pkc-1(gf)) increased the thermotaxis memory and rescued the deficits in thermotaxis memory in tph-1 mutants. Moreover, serotonin released from the ADF sensory neurons might act through the G-protein-coupled serotonin receptors of SER-4 and SER-7 to regulate the thermotaxis memory behavior. Genetic analysis implies that serotonin might further target the insulin signaling pathway to regulate the thermotaxis memory behavior. Thus, our results suggest the possible crucial role of serotonin and ADF sensory neurons in thermotaxis memory control in C. elegans.

A cocoa peptide protects Caenorhabditis elegans from oxidative stress and β-amyloid peptide toxicity

Background

Cocoa and cocoa-based products contain different compounds with beneficial properties for human health. Polyphenols are the most frequently studied, and display antioxidant properties. Moreover, protein content is a very interesting source of antioxidant bioactive peptides, which can be used therapeutically for the prevention of age-related diseases.

Methodology/Principal Findings

A bioactive peptide, 13L (DNYDNSAGKWWVT), was obtained from a hydrolyzed cocoa by-product by chromatography. The in vitro inhibition of prolyl endopeptidase (PEP) was used as screening method to select the suitable fraction for peptide identification. Functional analysis of 13L peptide was achieved using the transgenic Caenorhabditis elegans strain CL4176 expressing the human Aβ_1–42 peptide as a pre-clinical in vivo model for Alzheimer's disease. Among the peptides isolated, peptide 13L (1 µg/mL) showed the highest antioxidant activity (P≤0.001) in the wild-type strain (N2). Furthermore, 13L produced a significant delay in body paralysis in strain CL4176, especially in the 24–47 h period after Aβ_1–42 peptide induction (P≤0.0001). This observation is in accordance with the reduction of Aβ deposits in CL4176 by western blot. Finally, transcriptomic analysis in wild-type nematodes treated with 13L revealed modulation of the proteosomal and synaptic functions as the main metabolic targets of the peptide.

Conclusions/Significance

These findings suggest that the cocoa 13L peptide has antioxidant activity and may reduce Aβ deposition in a C. elegans model of Alzheimer's disease; and therefore has a putative therapeutic potential for prevention of age-related diseases. Further studies in murine models and humans will be essential to analyze the effectiveness of the 13L peptide in higher animals.

The SWI/SNF chromatin remodeling complex selectively affects multiple aspects of serotonergic neuron differentiation

Regulatory programs that control the specification of serotonergic neurons have been investigated by genetic mutant screens in the nematode Caenorhabditis elegans. Loss of a previously uncloned gene, ham-3, affects migration and serotonin antibody staining of the hermaphrodite-specific neuron (HSN) pair. We characterize these defects here in more detail, showing that the defects in serotonin antibody staining are paralleled by a loss of the transcription of all genes involved in serotonin synthesis and transport. This loss is specific to the HSN class as other serotonergic neurons appear to differentiate normally in ham-3 null mutants. Besides failing to migrate appropriately, the HSNs also display axon pathfinding defects in ham-3 mutants. However, the HSNs are still generated and express a subset of their terminal differentiation features in ham-3 null mutants, demonstrating that ham-3 is a specific regulator of select features of the HSNs. We show that ham-3 codes for the C. elegans ortholog of human BAF60, Drosophila Bap60, and yeast Swp73/Rsc6, which are subunits of the yeast SWI/SNF and vertebrate BAF chromatin remodeling complex. We show that the effect of ham-3 on serotonergic fate can be explained by ham-3 regulating the expression of the Spalt/SALL-type Zn finger transcription factor sem-4, a previously identified regulator of serotonin expression in HSNs and of the ham-2 Zn transcription factor, a previously identified regulator of HSN migration and axon outgrowth. Our findings provide the first evidence for the involvement of the BAF complex in the acquisition of terminal neuronal identity and constitute genetic proof by germline knockout that a BAF complex component can have cell-type-specific roles during development.

Multiple doublesex-related genes specify critical cell fates in a C. elegans male neural circuit

Background

In most animal species, males and females exhibit differences in behavior and morphology that relate to their respective roles in reproduction. DM (Doublesex/MAB-3) domain transcription factors are phylogenetically conserved regulators of sexual development. They are thought to establish sexual traits by sex-specifically modifying the activity of general developmental programs. However, there are few examples where the details of these interactions are known, particularly in the nervous system.

Methodology/Principal Findings

In this study, we show that two C. elegans DM domain genes, dmd-3 and mab-23, regulate sensory and muscle cell development in a male neural circuit required for mating. Using genetic approaches, we show that in the circuit sensory neurons, dmd-3 and mab-23 establish the correct pattern of dopaminergic (DA) and cholinergic (ACh) fate. We find that the ETS-domain transcription factor gene ast-1, a non-sex-specific, phylogenetically conserved activator of dopamine biosynthesis gene transcription, is broadly expressed in the circuit sensory neuron population. However, dmd-3 and mab-23 repress its activity in most cells, promoting ACh fate instead. A subset of neurons, preferentially exposed to a TGF-beta ligand, escape this repression because signal transduction pathway activity in these cells blocks dmd-3/mab-23 function, allowing DA fate to be established. Through optogenetic and pharmacological approaches, we show that the sensory and muscle cell characteristics controlled by dmd-3 and mab-23 are crucial for circuit function.

Conclusions/Significance

In the C. elegans male, DM domain genes dmd-3 and mab-23 regulate expression of cell sub-type characteristics that are critical for mating success. In particular, these factors limit the number of DA neurons in the male nervous system by sex-specifically regulating a phylogenetically conserved dopamine biosynthesis gene transcription factor. Homologous interactions between vertebrate counterparts could regulate sex differences in neuron sub-type populations in the brain.

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.

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.

A comparison of experience-dependent locomotory behaviors and biogenic amine neurons in nematode relatives of Caenorhabditis elegans

Background

Survival of an animal depends on its ability to match its responses to environmental conditions. To generate an optimal behavioral output, the nervous system must process sensory information and generate a directed motor output in response to stimuli. The nervous system should also store information about experiences to use in the future. The diverse group of free-living nematodes provides an excellent system to study macro- and microevolution of molecular, morphological and behavioral character states associated with such nervous system function. We asked whether an adaptive behavior would vary among bacterivorous nematodes and whether differences in the neurotransmitter systems known to regulate the behavior in one species would reflect differences seen in the adaptive behavior among those species. Caenorhabditis elegans worms slow in the presence of food; this 'basal' slowing is triggered by dopaminergic mechanosensory neurons that detect bacteria. Starved worms slow more dramatically; this 'enhanced' slowing is regulated by serotonin.

Results

We examined seven nematode species with known phylogenetic relationship to C. elegans for locomotory behaviors modulated by food (E. coli), and by the worm's recent history of feeding (being well-fed or starved). We found that locomotory behavior in some species was modulated by food and recent feeding experience in a manner similar to C. elegans, but not all the species tested exhibited these food-modulated behaviors. We also found that some worms had different responses to bacteria other than E. coli. Using histochemical and immunological staining, we found that dopaminergic neurons were very similar among all species. For instance, we saw likely homologs of four bilateral pairs of dopaminergic cephalic and deirid neurons known from C. elegans in all seven species examined. In contrast, there was greater variation in the patterns of serotonergic neurons. The presence of presumptive homologs of dopaminergic and serotonergic neurons in a given species did not correlate with the observed differences in locomotory behaviors.

Conclusions

This study demonstrates that behaviors can differ significantly between species that appear morphologically very similar, and therefore it is important to consider factors, such as ecology of a species in the wild, when formulating hypotheses about the adaptive significance of a behavior. Our results suggest that evolutionary changes in locomotory behaviors are less likely to be caused by changes in neurotransmitter expression of neurons. Such changes could be caused either by subtle changes in neural circuitry or in the function of the signal transduction pathways mediating these behaviors.

Evolution of neuronal patterning in free-living rhabditid nematodes I: Sex-specific serotonin-containing neurons

As a first step toward understanding the evolution of neuronal patterning and function in a group of simple animals, we have examined serotonin-containing neurons in 17 species of free-living rhabditid nematodes and compared them with identified neurons of Caenorhabditis elegans. We found many serotonin-immunoreactive (serotonin-IR) neurons that are likely homologs of those in C. elegans; this paper focuses on sex-specific neurons such as the egg laying hermaphrodite-specific neurons (HSNs), VCs, and male CAs, CPs, and ray sensory neurons known to function in mating. These cells vary in number and position in the species examined but are consistent with a current molecularly based phylogeny. Two groups (Oscheius and Pristionchus) appear independently to have lost a serotonin-IR HSN. Oscheius furthermore has no serotonin-IR innervation of the vulval region, in contrast to every other species we examined. We also saw variation in the location of somas of putative HSN, consistent with evolutionary changes in HSN migration. In C. elegans, the HSN soma migrates during embryogenesis from the tail to the central body, where it innervates its major postsynaptic targets, the vulval muscles. For other species, we observed putative HSN homologs along the anterior-posterior axis from the head to the tail, but typically HSNs were located near the vulva, which also varies in anterior-posterior position among the species we examined. The varying positions of the HSN somas in other species are reminiscent of phenotypes seen in various C. elegans mutants with altered HSN migration, suggesting possible mechanisms for the evolutionary differences we observed.

Are Caenorhabditis elegans receptors useful targets for drug discovery: pharmacological comparison of tyramine receptors with high identity from C. elegans (TYRA-2) and Brugia malayi (Bm4)

The biogenic amine, tyramine (TA), modulates a number of key processes in nematodes and a number of TA-specific receptors have been identified. In the present study, we have identified a putative TA receptor (Bm4) in the recently completed Brugia malayi genome and compared its pharmacology to its putative Caenorhabditis elegans orthologue, TYRA-2, under identical expression and assay conditions. TYRA-2 and Bm4 are the most closely related C. elegans and B. malayi BA receptors and differ by only 14aa in the TM regions directly involved in ligand binding. Membranes from HEK-293 cells stably expressing Bm4 exhibited specific, saturable, high affinity, [(3)H]LSD and [(3)H]TA binding with K(d)s of 18.1+/-0.93 and 15.1+/-0.2 nM, respectively. More importantly, both TYRA-2 and Bm4 TA exhibited similar rank orders of potencies for a number of potential tyraminergic ligands. However, some significant differences were noted. For example, chloropromazine exhibited an order of magnitude higher affinity for Bm4 than TYRA-2 (pK(i)s of 7.6+/-0.2 and 6.49+/-0.1, respectively). In contrast, TYRA-2 had significantly higher affinity for phentolamine than Bm4. These results highlight the utility of the nearly completed B. malayi genome and the importance of using receptors from individual parasitic nematodes for drug discovery.

Serotonin targets the DAF-16/FOXO signaling pathway to modulate stress responses

Stress response is a fundamental form of behavioral and physiological plasticity. Here we describe how serotonin (5HT) governs stress behavior by regulating DAF-2 insulin/IGF-1 receptor signaling to the DAF-16/FOXO transcription factor at the nexus of development, metabolism, immunity, and stress responses in C. elegans. Serotonin-deficient tph-1 mutants, like daf-2 mutants, exhibit DAF-16 nuclear accumulation and constitutive physiological stress states. Exogenous 5HT and fluoxetine (Prozac) prevented DAF-16 nuclear accumulation in wild-type animals under stresses. Genetic analyses imply that DAF-2 is a downstream target of 5HT signaling and that distinct serotonergic neurons act through distinct 5HT receptors to influence distinct DAF-16-mediated stress responses. We suggest that modulation of FOXO by 5HT represents an ancient feature of stress physiology and that the C. elegans is a genetically tractable model that can be used to delineate the molecular mechanisms and drug actions linking 5HT, neuroendocrine signaling, immunity, and mitochondrial function.

Dopamine Signaling Architecture in Caenorhabditis elegans

AIMS

In this review, we highlight the identification and analysis of molecules orchestrating dopamine (DA) signaling in the nematode Caenorhabditis elegans, focusing on recent characterizations of DA transporters and receptors.

METHODS

We illustrate the isolation and characterization of molecules important for C. elegans DA synthesis, packaging, reuptake and signaling and examine how mutations in these proteins are being exploited through in vitro and in vivo paradigms to yield novel insights of protein structure, DA signaling pathways and DA-supported behaviors.

RESULTS

DA signaling in the worm, as in man, arises by synaptic and nonsynaptic release from a small number of cells that exert modulatory control over a larger network underlying C. elegans behavior.

CONCLUSIONS

The C. elegans model system offers unique opportunities to elucidate ill-defined pathways that support DA release, inactivation, and signaling in addition to clarifying mechanisms of DA-mediated behavioral plasticity. Further use of the model offers prospects for the identification of novel genes and proteins whose study may yield benefits for DA-supported neural disorders in man.

Tyramine Functions Independently of Octopamine in the Caenorhabditis elegans Nervous System

Octopamine biosynthesis requires tyrosine decarboxylase to convert tyrosine into tyramine and tyramine beta-hydroxylase to convert tyramine into octopamine. We identified and characterized a Caenorhabditis elegans tyrosine decarboxylase gene, tdc-1, and a tyramine beta-hydroxylase gene, tbh-1. The TBH-1 protein is expressed in a subset of TDC-1-expressing cells, indicating that C. elegans has tyraminergic cells that are distinct from its octopaminergic cells. tdc-1 mutants have behavioral defects not shared by tbh-1 mutants. We show that tyramine plays a specific role in the inhibition of egg laying, the modulation of reversal behavior, and the suppression of head oscillations in response to anterior touch. We propose a model for the neural circuit that coordinates locomotion and head oscillations in response to anterior touch. Our findings establish tyramine as a neurotransmitter in C. elegans, and we suggest that tyramine is a genuine neurotransmitter in other invertebrates and possibly in vertebrates as well.