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

Serotonin deficiency from constitutive SKN-1 activation drives pathogen apathy

When an organism encounters a pathogen, the host innate immune system activates to defend against pathogen colonization and toxic xenobiotics produced. C. elegans employ multiple defense systems to ensure survival when exposed to Pseudomonas aeruginosa including activation of the cytoprotective transcription factor SKN-1/NRF2. Although wildtype C. elegans quickly learn to avoid pathogens, here we describe a peculiar apathy-like behavior towards PA14 in animals with constitutive activation of SKN-1, whereby animals choose not to leave and continue to feed on the pathogen even when a non-pathogenic and healthspan-promoting food option is available. Although lacking the urgency to escape the infectious environment, animals with constitutive SKN-1 activity are not oblivious to the presence of the pathogen and display the typical pathogen-induced intestinal distension and eventual demise. SKN-1 activation, specifically in neurons and intestinal tissues, orchestrates a unique transcriptional program which leads to defects in serotonin signaling that is required from both neurons and non-neuronal tissues. Serotonin depletion from SKN-1 activation limits pathogen defense capacity, drives the pathogen-associated apathy behaviors and induces a synthetic sensitivity to selective serotonin reuptake inhibitors. Taken together, our work reveals new insights into how animals perceive environmental pathogens and subsequently alter behavior and cellular programs to promote survival.

KEY POINTS

Identify an apathy-like behavioral response for pathogens resulting from the constitutive activation of the cytoprotective transcription factor SKN-1.Uncover the obligate role for serotonin synthesis in both neuronal and non-neuronal cells for the apathy-like state and ability of serotonin treatment to restore normal behaviors.Characterize the timing and tissue specificity of SKN-1 nuclear localization in neurons and intestinal cells in response to pathogen exposure.Define the unique and context-specific transcriptional signatures of animals with constitutive SKN-1 activation when exposed to pathogenic environments.Reveal necessity for both neuronal and non-neuronal serotonin signaling in host survival from pathogen infection.

Aripiprazole Offsets Mutant ATXN3-Induced Motor Dysfunction by Targeting Dopamine D2 and Serotonin 1A and 2A Receptors in C. elegans

The atypical antipsychotic aripiprazole is a Food and Drug Administration-approved drug for the treatment of psychotic, mood, and other psychiatric disorders. Previous drug discovery efforts pinpointed aripiprazole as an effective suppressor of Machado–Joseph disease (MJD) pathogenesis, as its administration resulted in a reduced abundance and aggregation of mutant Ataxin-3 (ATXN3) proteins. Dopamine partial agonism and functional selectivity have been proposed as the main pharmacological mechanism of action of aripiprazole in the treatment of psychosis; however, this mechanism remains to be determined in the context of MJD. Here, we focus on confirming the efficacy of aripiprazole to reduce motor dysfunction in vivo, using a Caenorhabditis elegans (C. elegans) model of MJD, and on unveiling the drug targets required for its positive action against mutant ATXN3 pathogenesis. We employed pharmacogenetics and pharmacological approaches to identify which dopamine and serotonin receptors are critical for aripiprazole-mediated improvements in motor function. We demonstrated that dopamine D₂-like and serotonin 5-HT_1A and 5-HT_2A receptors play important roles in this process. Our findings strengthen the relevance of dopaminergic and serotoninergic signaling modulation against mutant ATXN3-mediated pathogenesis. The identification of aripiprazole’s cellular targets, relevant for MJD and perhaps other neurodegenerative diseases, may pave the way for prospective drug discovery and development campaigns aiming to improve the features of this prototypical compound and reduce side effects not negligible in the case of aripiprazole.

Methylmercury Induces Metabolic Alterations in Caenorhabditis elegans: Role for C/EBP Transcription Factor

Methylmercury (MeHg) is a well-known neurotoxicant; however, its role in metabolic diseases has been gaining wider attention. We have previously shown that MeHg causes metabolic alterations in Caenorhabditis elegans, leading to decreased nicotinamide adenine dinucleotide cofactor, mitochondrial dysfunction, and oxidative stress. We were, therefore, interested in whether MeHg also affects nutrient metabolism, particularly lipid homeostasis, which may contribute to the development of metabolic conditions such as obesity or metabolic syndrome (MS). RNA from wild-type worms exposed to MeHg was collected immediately after treatment and used for gene expression analysis by DNA microarray. MeHg differentially regulated 215 genes, 17 genes involved in lipid homeostasis, and 12 genes involved in carbohydrate homeostasis. Of particular interest was cebp-1, the worm ortholog to human C/EBP, a pro-adipogenic transcription factor implicated in MS. MeHg increased the expression of cebp-1 as well as pro-adipogenic transcription factors sbp-1 and nhr-49, triglyceride synthesis enzyme acl-6, and lipid transport proteins vit-2 and vit-6. Concurrent with the altered gene expression, MeHg increased triglyceride levels, lipid storage, and feeding behaviors. Worms expressing mutant cebp-1 were protected from MeHg-induced alterations in lipid content, feeding behaviors, and gene expression, highlighting the importance of this transcription factor in the worm's response to MeHg. Taken together, our data demonstrate that MeHg induces biochemical, metabolic, and behavioral changes in C. elegans that can lead to metabolic dysfunction.

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.

Divergent Nodes of Non-autonomous UPRER Signaling through Serotonergic and Dopaminergic Neurons

In multicellular organisms, neurons integrate a diverse array of external cues to affect downstream changes in organismal health. Specifically, activation of the endoplasmic reticulum (ER) unfolded protein response (UPR^ER) in neurons increases lifespan by preventing age-onset loss of ER proteostasis and driving lipid depletion in a cell non-autonomous manner. The mechanism of this communication is dependent on the release of small clear vesicles from neurons. We find dopaminergic neurons are necessary and sufficient for activation of cell non-autonomous UPR^ER to drive lipid depletion in peripheral tissues, whereas serotonergic neurons are sufficient to drive protein homeostasis in peripheral tissues. These signaling modalities are unique and independent and together coordinate the beneficial effects of neuronal cell non-autonomous ER stress signaling upon health and longevity.

Olfactory specificity regulates lipid metabolism through neuroendocrine signaling in Caenorhabditis elegans

Olfactory and metabolic dysfunctions are intertwined phenomena associated with obesity and neurodegenerative diseases; yet how mechanistically olfaction regulates metabolic homeostasis remains unclear. Specificity of olfactory perception integrates diverse environmental odors and olfactory neurons expressing different receptors. Here, we report that specific but not all olfactory neurons actively regulate fat metabolism without affecting eating behaviors in Caenorhabditis elegans, and identified specific odors that reduce fat mobilization via inhibiting these neurons. Optogenetic activation or inhibition of the responsible olfactory neural circuit promotes the loss or gain of fat storage, respectively. Furthermore, we discovered that FLP-1 neuropeptide released from this olfactory neural circuit signals through peripheral NPR-4/neuropeptide receptor, SGK-1/serum- and glucocorticoid-inducible kinase, and specific isoforms of DAF-16/FOXO transcription factor to regulate fat storage. Our work reveals molecular mechanisms underlying olfactory regulation of fat metabolism, and suggests the association between olfactory perception specificity of each individual and his/her susceptibility to the development of obesity.

Impairment of motor but not anxiety-like behavior caused by the increase of dopamine during development is sustained in zebrafish larvae at later stages

Many neuropsychiatric disorders are associated with both dopaminergic (DAergic) and developmental hypotheses. Since DAergic receptors are expressed in the developing brain, it is possible that alterations in dopamine (DA) signaling may impair brain development and consequent behavior. In our previous study, using a zebrafish model, we showed that an increase of DA during the 3 to 5 days postfertilization (dpf) developmental window (an important window for GABAergic neuronal differentiation) affects the motor behavior of 5 dpf larvae. In this study, we set out to determine whether these behavioral alterations were sustained in larvae at older stages (7 and 14 dpf). To test this hypothesis, we chronically treated zebrafish larvae from 3 to 5 dpf with DA. After washing the drug, we recorded and analyzed the first 5 and 30 min of the motor behavior of 5, 7, and 14 dpf subjects. We analyzed mobile episodes, distance traveled, time mobile, distance traveled per mobile episode, time in movement per mobile episode, and distance traveled per time mobile. We showed, once again, that an increase of DA during the 3 to 5 dpf developmental window reduces the number of movement episodes initiated by 5 dpf larvae. We also detected a decrease of other motor behavior parameters in 5 dpf DA-treated larvae. We observed that these alterations are sustained in the 7 dpf larvae. However, we did not see these general locomotor alterations in the 14 dpf larvae. Moreover, we detected a decrease of distance traveled and an increase of time of locomotion per episode in the first 5 min of behavioral analyses in 14 dpf DA-treated larvae. To test if the alterations in the first 5 min were due to anxiety-like behavior, we used a light/dark preference paradigm. We recorded 5dpf, 7dpf, and 14dpf larvae for 5 min and analyzed time of freezing, preference for light or dark, number of entries to the dark, percentage of time in the light. We observed that 5dpf larvae treated with DA showed more freezing, less passages to the dark, and more time spent in the light as compared to their control counterparts. But 7dpf and 14dpf larvae did not show these alterations. Taken overall, therefore, our results suggest that DA does play a role in the development of zebrafish motor behavior, and, furthermore, that some behaviors are more sensitive than others to the effects of DAergic imbalances during development.

Genetic Silencing of Fatty Acid Desaturases Modulates α-Synuclein Toxicity and Neuronal Loss in Parkinson-Like Models of C. elegans

The molecular basis of Parkinson's disease (PD) is currently unknown. There is increasing evidence that fat metabolism is at the crossroad of key molecular pathways associated with the pathophysiology of PD. Fatty acid desaturases catalyze synthesis of saturated fatty acids from monounsaturated fatty acids thereby mediating several cellular mechanisms that are associated with diseases including cancer and metabolic disorders. The role of desaturases in modulating age-related neurodegenerative manifestations such as PD is poorly understood. Here, we investigated the effect of silencing Δ9 desaturase enzyme encoding fat-5 and fat-7 genes which are known to reduce fat content, on α-synuclein expression, neuronal morphology and dopamine-related behaviors in transgenic PD-like models of Caenorhabditis elegans (C. elegans). The silencing of the fat-5 and fat-7 genes rescued both degeneration of dopamine neurons and deficits in dopamine-dependent behaviors, including basal slowing and ethanol avoidance in worm models of PD. Similarly, silencing of these genes also decreased the formation of protein aggregates in a nematode model of PD expressing α-synuclein in the body wall muscles and rescued deficits in resistance to heat and osmotic stress. On the contrary, silencing of nhr-49 and tub-1 genes that are known to increase total fat content did not alter behavioral and pathological endpoints in the PD worm strains. Interestingly, the genetic manipulation of all four selected genes resulted in differential fat levels in the PD models without having significant effect on the lifespan, further indicating a complex fat homeostasis unique to neurodegenerative pathophysiology. Overall, we provide a comprehensive understanding of how Δ9 desaturase can alter PD-like pathology due to environmental exposures and proteotoxic stress, suggesting new avenues in deciphering the disease etiology and possible therapeutic targets.

Neurotransmitter signaling through heterotrimeric G proteins: insights from studies in C. elegans

Neurotransmitters signal via G protein coupled receptors (GPCRs) to modulate activity of neurons and muscles. C. elegans has ∼150 G protein coupled neuropeptide receptor homologs and 28 additional GPCRs for small-molecule neurotransmitters. Genetic studies in C. elegans demonstrate that neurotransmitters diffuse far from their release sites to activate GPCRs on distant cells. Individual receptor types are expressed on limited numbers of cells and thus can provide very specific regulation of an individual neural circuit and behavior. G protein coupled neurotransmitter receptors signal principally via the three types of heterotrimeric G proteins defined by the G alpha subunits Gαo, Gαq, and Gαs. Each of these G alpha proteins is found in all neurons plus some muscles. Gαo and Gαq signaling inhibit and activate neurotransmitter release, respectively. Gαs signaling, like Gαq signaling, promotes neurotransmitter release. Many details of the signaling mechanisms downstream of Gαq and Gαs have been delineated and are consistent with those of their mammalian orthologs. The details of the signaling mechanism downstream of Gαo remain a mystery. Forward genetic screens in C. elegans have identified new molecular components of neural G protein signaling mechanisms, including Regulators of G protein Signaling (RGS proteins) that inhibit signaling, a new Gαq effector (the Trio RhoGEF domain), and the RIC-8 protein that is required for neuronal Gα signaling. A model is presented in which G proteins sum up the variety of neuromodulator signals that impinge on a neuron to calculate its appropriate output level.

Behavioral Mechanisms That Depend on Dopamine and Serotonin in Caenorhabditis elegans Interact With the Antipsychotics Risperidone and Aripiprazole

The neurotransmitters dopamine and serotonin participate in specific behavioral neuromuscular mechanisms in the nematode Caenorhabditis elegans. Dopamine is involved in the gentle touch response and serotonin in the pharyngeal pumping rate. In its genome, the worm presents genes encoding dopamine and serotonin receptors orthologous to those of human genes. Risperidone and aripiprazole are a class of drugs known as atypical antipsychotics commonly used to treat schizophrenia, bipolar disorder, and irritability associated with autism. Risperidone is an antagonist of the dopamine D2 and serotonin 5-HT2A receptors. Aripiprazole functions as a partial agonist of the dopamine D2 receptor and as a partial agonist and antagonist of 5-HT1A and 5-HT2A serotonin receptors, respectively. Our results show that risperidone and aripiprazole alter the touch response and pharyngeal pumping in wild-type worm animals. Furthermore, in the presence of the drugs, both behaviors change to varying degrees in dopamine (dop-1, dop-2, and dop-3), serotonin (ser-1), and tyramine (ser-2) receptor-deficient mutants. This variation in response reveals specific targets for these antipsychotics in the nematode. Interestingly, their effect on behavior persisted to some extent in successive generations, indicating that they might induce epigenetic changes throughout development. Sodium butyrate, a histone deacetylase inhibitor, eliminated the consecutive generation effect of both drugs. In addition, these transgenerational effects were also abolished after the dauer stage. These observations suggest that risperidone and aripiprazole, in addition to interacting with specific receptors impairing the function of the nervous system of the nematode, may lead to the deposition of long-lasting epigenetic marks.

Metabolic effects of manganese in the nematode Caenorhabditis elegans through DAergic pathway and transcription factors activation

Manganese (Mn) is an essential trace element for physiological functions since it acts as an enzymatic co-factor. Nevertheless, overexposure to Mn has been associated with a pathologic condition called manganism. Furthermore, Mn has been reported to affect lipid metabolism by mechanisms which have yet to be established. Herein, we used the nematode Caenorhabditis elegans to examine Mn's effects on the dopaminergic (DAergic) system and determine which transcription factors that regulate with lipid metabolism are affected by it. Worms were exposed to Mn for four hours in the presence of bacteria and in a liquid medium (85 mM NaCl). Mn increased fat storage as evidenced both by Oil Red O accumulation and triglyceride levels. In addition, metabolic activity was reduced as a reflection of decreased oxygen consumption caused by Mn. Mn also affected feeding behavior as evidenced by decreased pharyngeal pumping rate. DAergic neurons viability were not altered by Mn, however the dopamine levels were significantly reduced following Mn exposure. Furthermore, the expression of sbp-1 transcription factor and let-363 protein kinase responsible for lipid accumulation control was increased and decreased, respectively, by Mn. Altogether, our data suggest that Mn increases the fat storage in C. elegans, secondary to DAergic system alterations, under the control of SBP-1 and LET-363 proteins.

Impaired Dopamine-Dependent Locomotory Behavior of C. elegans Neuroligin Mutants Depends on the Catechol-O-Methyltransferase COMT-4

Neurexins and neuroligins are neuronal membrane adhesion molecules that have been involved in neuropsychiatric and neurodevelopmental disorders. The nrx-1 and nlg-1 genes of Caenorhabditis elegans encode NRX-1 and NLG-1, orthologue proteins of human neurexins and neuroligins, respectively. Dopaminergic and serotoninergic signalling control the locomotory rate of the nematode. When well-fed animals are transferred to a plate with food (bacterial lawn), they reduce the locomotory rate. This behavior, which depends on dopamine, is known as basal slowing response (BSR). Alternatively, when food-deprived animals are moved to a plate with a bacterial lawn, further decrease their locomotory rate. This behavior, known as enhanced slowing response (ESR), is serotonin dependent. C. elegans nlg-1-deficient mutants are impaired in BSR and ESR. Here we report that nrx-1-deficient mutants were defective in ESR, but not in BSR. The nrx-1;nlg-1 double mutant was impaired in both behaviors. Interestingly, the nlg-1 mutants upregulate the expression of comt-4 which encodes an enzyme with putative catechol-O-methyltransferase activity involved in dopamine degradation. Our study also shows that comt-4(RNAi) in nlg-1-deficient mutants rescues the wild type phenotypes of BSR and ESR. On the other hand, comt-4(RNAi) in nlg-1-deficient mutants also recovers, at least partially, the gentle touch response and the pharyngeal pumping rate that were impaired in these mutants. These latter behaviors are dopamine and serotonin dependent, respectively. Based on these results we propose a model for the neuroligin function in modulating the dopamine-dependent locomotory behavior in the nematode.

Behavioral Phenotyping and Pathological Indicators of Parkinson's Disease in C. elegans Models

Parkinson's disease (PD) is a neurodegenerative disorder with symptoms that progressively worsen with age. Pathologically, PD is characterized by the aggregation of α-synuclein in cells of the substantia nigra in the brain and loss of dopaminergic neurons. This pathology is associated with impaired movement and reduced cognitive function. The etiology of PD can be attributed to a combination of environmental and genetic factors. A popular animal model, the nematode roundworm Caenorhabditis elegans, has been frequently used to study the role of genetic and environmental factors in the molecular pathology and behavioral phenotypes associated with PD. The current review summarizes cellular markers and behavioral phenotypes in transgenic and toxin-induced PD models of C. elegans.

Catecholaminergic System of Invertebrates: Comparative and Evolutionary Aspects in Comparison With the Octopaminergic System

In this review we examined the catecholaminergic system of invertebrates, starting from protists and getting to chordates. Different techniques used by numerous researchers revealed, in most examined phyla, the presence of catecholamines dopamine, noradrenaline, and adrenaline or of the enzymes involved in their synthesis. The catecholamines are generally linked to the nervous system and they can act as neurotransmitters, neuromodulators, and hormones; moreover they play a very important role as regards the response to a large number of stress situations. Nevertheless, in some invertebrate phyla belonging to Protostoma, the monoamine octopamine is the main biogenic amine. The presence of catecholamines in some protists suggests a role as intracellular or interorganismal signaling molecules and an ancient origin of their synthetic pathways. The catecholamines appear also involved in the regulation of bioluminescence and in the control of larval development and metamorphosis in some marine invertebrate phyla.

Lifespan Extension Induced by Caffeine in Caenorhabditis elegans is Partially Dependent on Adenosine Signaling

Caffeine is a widely used psychoactive substance. Studies have shown that caffeine may play a protective role in aging-associated disorders. However, the mechanisms by which caffeine modulates aging are not yet clear. In this study, we have shown that caffeine increases Caenorhabditis elegans lifespan, delays its larval development, reduces reproduction and body length. These phenotypes were partly reversed by worm's exposure to adenosine, which suggest a putative common target. Moreover, they were dependent on a functional insulin/IGF-1-like pathway. Our results may shed light on new genetic determinants of aging.

Neural Regulatory Pathways of Feeding and Fat in Caenorhabditis elegans

The compact nervous system of Caenorhabditis elegans and its genetic tractability are features that make this organism highly suitable for investigating energy balance in an animal system. Here, we focus on molecular components and organizational principles emerging from the investigation of pathways that largely originate in the nervous system and regulate feeding behavior but also peripheral fat regulation through neuroendocrine signaling. We provide an overview of studies aimed at understanding how C. elegans integrate internal and external cues in feeding behavior. We highlight some of the similarities and differences in energy balance between C. elegans and mammals. We also provide our perspective on unresolved issues, both conceptual and technical, that we believe have hampered critical evaluation of findings relevant to fat regulation in C. elegans.