The Caenorhabditis elegans choline transporter CHO-1 sustains acetylcholine synthesis and motor function in an activity-dependent manner

GCY-20 signaling controls suppression of Caenorhabditis elegans egg laying by moderate cold

Organisms sensing environmental cues and internal states and integrating the sensory information to control fecundity are essential for survival and proliferation. The present study finds that a moderate cold temperature of 11°C reduces egg laying in Caenorhabditis elegans. ASEL and AWC neurons sense the cold via GCY-20 signaling and act antagonistically on egg laying through the ASEL and AWC/AIA/HSN circuits. Upon cold stimulation, ASEL and AWC release glutamate to activate and inhibit AIA interneurons by acting on highly and lowly sensitive ionotropic GLR-2 and GLC-3 receptors, respectively. AIA inhibits HSN motor neuron activity via acetylcholinergic ACR-14 receptor signaling and suppresses egg laying. Thus, ASEL and AWC initiate and reduce the cold suppression of egg laying. ASEL's action on AIA and egg laying dominates AWC's action. The biased opposite actions of these neurons on egg laying provide animals with a precise adaptation of reproductive behavior to environmental temperatures.

The wide world of non-mammalian phospholipase D enzymes

Phospholipase D (PLD) hydrolyses phosphatidylcholine (PtdCho) to produce free choline and the critically important lipid signaling molecule phosphatidic acid (PtdOH). Since the initial discovery of PLD activities in plants and bacteria, PLDs have been identified in a diverse range of organisms spanning the taxa. While widespread interest in these proteins grew following the discovery of mammalian isoforms, research into the PLDs of non-mammalian organisms has revealed a fascinating array of functions ranging from roles in microbial pathogenesis, to the stress responses of plants and the developmental patterning of flies. Furthermore, studies in non-mammalian model systems have aided our understanding of the entire PLD superfamily, with translational relevance to human biology and health. Increasingly, the promise for utilization of non-mammalian PLDs in biotechnology is also being recognized, with widespread potential applications ranging from roles in lipid synthesis, to their exploitation for agricultural and pharmaceutical applications.

Vitamin B12 produced by gut bacteria modulates cholinergic signalling

A growing body of evidence indicates that gut microbiota influence brain function and behaviour. However, the molecular basis of how gut bacteria modulate host nervous system function is largely unknown. Here we show that vitamin B12-producing bacteria that colonize the intestine can modulate excitatory cholinergic signalling and behaviour in the host Caenorhabditis elegans. Here we demonstrate that vitamin B12 reduces cholinergic signalling in the nervous system through rewiring of the methionine (Met)/S-adenosylmethionine cycle in the intestine. We identify a conserved metabolic crosstalk between the methionine/S-adenosylmethionine cycle and the choline-oxidation pathway. In addition, we show that metabolic rewiring of these pathways by vitamin B12 reduces cholinergic signalling by limiting the availability of free choline required by neurons to synthesize acetylcholine. Our study reveals a gut-brain communication pathway by which enteric bacteria modulate host behaviour and may affect neurological health.

Biochemical characterization of two novel mutations in the human high-affinity choline transporter 1 identified in a patient with congenital myasthenic syndrome

Congenital myasthenic syndrome (CMS) is a heterogeneous condition associated with 34 different genes, including SLC5A7, which encodes the high-affinity choline transporter 1 (CHT1). CHT1 is expressed in presynaptic neurons of the neuromuscular junction where it uses the inward sodium gradient to reuptake choline. Biallelic CHT1 mutations often lead to neonatal lethality, and less commonly to non-lethal motor weakness and developmental delays. Here, we report detailed biochemical characterization of two novel mutations in CHT1, p.I294T and p.D349N, which we identified in an 11-year-old patient with a history of neonatal respiratory distress, and subsequent hypotonia and global developmental delay. Heterologous expression of each CHT1 mutant in human embryonic kidney cells showed two different mechanisms of reduced protein function. The p.I294T CHT1 mutant transporter function was detectable, but its abundance and half-life were significantly reduced. In contrast, the p.D349N CHT1 mutant was abundantly expressed at the cell membrane, but transporter function was absent. The residual function of the p.I294T CHT1 mutant may explain the non-lethal form of CMS in this patient, and the divergent mechanisms of reduced CHT1 function that we identified may guide future functional studies of the CHT1 myasthenic syndrome. Based on these in vitro studies that provided a diagnosis, treatment with cholinesterase inhibitor together with physical and occupational therapy significantly improved the patient's strength and quality of life.

Presynaptic Congenital Myasthenic Syndromes: Understanding Clinical Phenotypes through In vivo Models

Presynaptic congenital myasthenic syndromes (CMS) are a group of genetic disorders affecting the presynaptic side of the neuromuscular junctions (NMJ). They can result from a dysfunction in acetylcholine (ACh) synthesis or recycling, in its packaging into synaptic vesicles, or its subsequent release into the synaptic cleft. Other proteins involved in presynaptic endplate development and maintenance can also be impaired.Presynaptic CMS usually presents during the prenatal or neonatal period, with a severe phenotype including congenital arthrogryposis, developmental delay, and apnoeic crisis. However, milder phenotypes with proximal muscle weakness and good response to treatment have been described. Finally, many presynaptic genes are expressed in the brain, justifying the presence of additional central nervous system symptoms.Several animal models have been developed to study CMS, providing the opportunity to identify disease mechanisms and test treatment options. In this review, we describe presynaptic CMS phenotypes with a focus on in vivo models, to better understand CMS pathophysiology and define new causative genes.

Neuroligin-mediated neurodevelopmental defects are induced by mitochondrial dysfunction and prevented by lutein in C. elegans

Complex-I-deficiency represents the most frequent pathogenetic cause of human mitochondriopathies. Therapeutic options for these neurodevelopmental life-threating disorders do not exist, partly due to the scarcity of appropriate model systems to study them. Caenorhabditis elegans is a genetically tractable model organism widely used to investigate neuronal pathologies. Here, we generate C. elegans models for mitochondriopathies and show that depletion of complex I subunits recapitulates biochemical, cellular and neurodevelopmental aspects of the human diseases. We exploit two models, nuo-5/NDUFS1- and lpd-5/NDUFS4-depleted animals, for a suppressor screening that identifies lutein for its ability to rescue animals’ neurodevelopmental deficits. We uncover overexpression of synaptic neuroligin as an evolutionarily conserved consequence of mitochondrial dysfunction, which we find to mediate an early cholinergic defect in C. elegans. We show lutein exerts its beneficial effects by restoring neuroligin expression independently from its antioxidant activity, thus pointing to a possible novel pathogenetic target for the human disease.

Mitochondrial deficiency causes rare incurable disorders. Here, the authors use C. elegans to study these diseases and find that the natural compound lutein prevents neurodevelopmental deficits, thus pointing to a possible therapeutic target for the human diseases.

Trimethylamine modulates dauer formation, neurodegeneration, and lifespan through tyra-3/daf-11 signaling in Caenorhabditis elegans

In the nematode Caenorhabditis elegans, signals derived from bacteria in the diet, the animal's major nutrient source, can modulate both behavior and healthspan. Here we describe a dual role for trimethylamine (TMA), a human gut flora metabolite, which acts as a nutrient signal and a neurotoxin. TMA and its associated metabolites are produced by the human gut microbiome and have been suggested to serve as risk biomarkers for diabetes and cardiovascular diseases. We demonstrate that the tyramine receptor TYRA-3, a conserved G protein-coupled receptor (GPCR), is required to sense TMA and mediate its responses. TMA activates guanylyl cyclase DAF-11 signaling through TYRA-3 in amphid neurons (ASK) and ciliated neurons (BAG) to mediate food-sensing behavior. Bacterial mutants deficient in TMA production enhance dauer formation, extend lifespan, and are less preferred as a food source. Increased levels of TMA lead to neural damage in models of Parkinson's disease and shorten lifespan. Our results reveal conserved signaling pathways modulated by TMA in C. elegans that are likely to be relevant for its effects in mammalian systems.

Competitive inhibition of the high-affinity choline transporter by tetrahydropyrimidine anthelmintics

The high-affinity choline transporter CHT1 mediates choline uptake, the rate-limiting and regulatory step in acetylcholine synthesis at cholinergic presynaptic terminals. CHT1-medated choline uptake is specifically inhibited by hemicholinium-3, which is a type of choline analog that acts as a competitive inhibitor. Although the substrate choline and the inhibitor hemicholinium-3 are well-established ligands of CHT1, few potent ligands other than choline analogs have been reported. Here we show that tetrahydropyrimidine anthelmintics, known as nicotinic acetylcholine receptor agonists, act as competitive inhibitors of CHT1. A ligand-dependent trafficking assay in cell lines expressing human CHT1 was designed to search for CHT1 ligands from a collection of biologically active compounds. We found that morantel as well as other tetrahydropyrimidines, pyrantel and oxantel, potently inhibits the high-affinity choline uptake activity of CHT1 in a competitive manner similar to the inhibitor hemicholinium-3. They also inhibit the high-affinity choline transporter from the nematode Caenorhabditis elegans. Finally, tetrahydropyrimidines potently inhibit the high-affinity choline uptake in rat brain synaptosomes at a low micromolar level, resulting in the inhibition of acetylcholine synthesis. The rank order of potency in synaptosomes is as follows: morantel > pyarantel > oxantel (K_i = 1.3, 5.7, and 8.3 μM, respectively). Our results reveal that tetrahydropyrimidine anthelmintics are novel CHT1 ligands that inhibit the high-affinity choline uptake for acetylcholine synthesis in cholinergic neurons.

Signal Decoding for Glutamate Modulating Egg Laying Oppositely in Caenorhabditis elegans under Varied Environmental Conditions

Animals' ability to sense environmental cues and to integrate this information to control fecundity is vital for continuing the species lineage. In this study, we observed that the sensory neurons Amphid neuron (ASHs and ADLs) differentially regulate egg-laying behavior in Caenorhabditis elegans under varied environmental conditions via distinct neuronal circuits. Under standard culture conditions, ASHs tonically release a small amount of glutamate and inhibit Hermaphrodite specific motor neuron (HSN) activities and egg laying via a highly sensitive Glutamate receptor (GLR)-5 receptor. In contrast, under Cu²⁺ stimulation, ASHs and ADLs may release a large amount of glutamate and inhibit Amphid interneuron (AIA) interneurons via low-sensitivity Glutamate-gated chloride channel (GLC)-3 receptor, thus removing the inhibitory roles of AIAs on HSN activity and egg laying. However, directly measuring the amount of glutamate released by sensory neurons under different conditions and assaying the binding kinetics of receptors with the neurotransmitter are still required to support this study directly.

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Short-term exposure of CuSO₄ evokes hyperactive egg laying

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ASHs inhibit HSNs and egg laying via GLR-5 receptor under no Cu²⁺ treatment

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AIA interneurons suppress HSNs and thus egg laying through ACR-14 signaling

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Under noxious Cu²⁺ treatment, ASHs and ADLs suppress AIAs and augment egg laying

Cholinergic transmission in C. elegans: Functions, diversity, and maturation of ACh-activated ion channels

Acetylcholine is an abundant neurotransmitter in all animals. Effects of acetylcholine are excitatory, inhibitory, or modulatory depending on the receptor and cell type. Research using the nematode C. elegans has made ground-breaking contributions to the mechanistic understanding of cholinergic transmission. Powerful genetic screens for behavioral mutants or for responses to pharmacological reagents identified the core cellular machinery for synaptic transmission. Pharmacological reagents that perturb acetylcholine-mediated processes led to the discovery and also uncovered the composition and regulators of acetylcholine-activated channels and receptors. From a combination of electrophysiological and molecular cellular studies, we have gained a profound understanding of cholinergic signaling at the levels of synapses, neural circuits, and animal behaviors. This review will begin with a historical overview, then cover in-depth current knowledge on acetylcholine-activated ionotropic receptors, mechanisms regulating their functional expression and their functions in regulating locomotion.

Sensory regulated Wnt production from neurons helps make organ development robust to environmental changes in C. elegans

Long-term survival of an animal species depends on development being robust to environmental variations and climate changes. We used C. elegans to study how mechanisms that sense environmental changes trigger adaptive responses that ensure animals develop properly. In water, the nervous system induces an adaptive response that reinforces vulval development through an unknown backup signal for vulval induction. This response involves the heterotrimeric G-protein EGL-30//Gαq acting in motor neurons. It also requires body-wall muscle, which is excited by EGL-30-stimulated synaptic transmission, suggesting a behavioral function of neurons induces backup signal production from muscle. We now report that increased acetylcholine during liquid growth activates an EGL-30-Rho pathway, distinct from the synaptic transmission pathway, that increases Wnt production from motor neurons. We also provide evidence that this neuronal Wnt contributes to EGL-30-stimulated vulval development, with muscle producing a parallel developmental signal. As diverse sensory modalities stimulate motor neurons via acetylcholine, this mechanism enables broad sensory perception to enhance Wnt-dependent development. Thus, sensory perception improves animal fitness by activating distinct neuronal functions that trigger adaptive changes in both behavior and developmental processes.

Dopamine-dependent, swimming-induced paralysis arises as a consequence of loss of function mutations in the RUNX transcription factor RNT-1

Dopamine (DA) is a neurotransmitter with actions across phylogeny that modulate core behaviors such as motor activity, reward, attention, and cognition. Perturbed DA signaling in humans is associated with multiple disorders, including addiction, ADHD, schizophrenia, and Parkinson's disease. The presynaptic DA transporter exerts powerful control on DA signaling by efficient clearance of the neurotransmitter following release. As in vertebrates, Caenorhabditis elegans DAT (DAT-1) constrains DA signaling and loss of function mutations in the dat-1 gene result in slowed crawling on solid media and swimming-induced paralysis (Swip) in water. Previously, we identified a mutant line, vt34, that exhibits robust DA-dependent Swip. vt34 exhibits biochemical and behavioral phenotypes consistent with reduced DAT-1 function though vt34; dat-1 double mutants exhibit an enhanced Swip phenotype, suggesting contributions of the vt34-associated mutation to additional mechanisms that lead to excess DA signaling. SNP mapping and whole genome sequencing of vt34 identified the site of the molecular lesion in the gene B0412.2 that encodes the Runx transcription factor ortholog RNT-1. Unlike dat-1 animals, but similar to other loss of function rnt-1 mutants, vt34 exhibits altered male tail morphology and reduced body size. Deletion mutations in both rnt-1 and the bro-1 gene, which encodes a RNT-1 binding partner also exhibit Swip. Both vt34 and rnt-1 mutations exhibit reduced levels of dat-1 mRNA as well as the tyrosine hydroxylase ortholog cat-2. Although reporter studies indicate that rnt-1 is expressed in DA neurons, its re-expression in DA neurons of vt34 animals fails to fully rescue Swip. Moreover, as shown for vt34, rnt-1 mutation exhibits additivity with dat-1 in generating Swip, as do rnt-1 and bro-1 mutations, and vt34 exhibits altered capacity for acetylcholine signaling at the neuromuscular junction. Together, these findings identify a novel role for rnt-1 in limiting DA neurotransmission and suggest that loss of RNT-1 may disrupt function of both DA neurons and body wall muscle to drive Swip.

Role of phospholipase D in the lifespan of Caenorhabditis elegans

We have previously shown that phospholipase D (PLD) downregulation accelerates cellular senescence, which is widely believed to play an important role in aging, by stimulating reactive oxygen species (ROS) accumulation in human cells. In this study, we examined the role of PLD in aging using the nematode Caenorhabditis elegans. The mRNA level of pld-1 was found to be inversely correlated with aging. RNAi-mediated knockdown of pld-1 expression in nematodes enhanced ROS and lipofuscin accumulation and decreased lifespan, motility, and resistance to stress compared to that in nematodes treated with control RNAi. Pld-1 knockdown repressed the long lifespan of age-1 and akt-1 mutants but did not further reduce the short lifespan of daf-16 mutants, suggesting that PLD functions between AKT-1 and DAF-16. The ROS scavenger N-acetyl-L-cysteine, a PLD effector phosphatidic acid and a possible CK2 activator spermidine attenuated the lifespan shortening and age-related biomarkers triggered by pld-1 knockdown. Pld-1 RNAi downregulated the expression of DAF-16 target genes such as sod-3, dod-11, and mtl-1 in nematodes. In human cells, furthermore, PLD2 downregulation decreased the transcription of FoxO3a target genes (Cu/ZnSOD, MnSOD, catalase, thioredoxin-2, and peroxiredoxin-5), whereas ectopic PLD2 expression elevated the mRNA levels of these antioxidant genes. Taken together, these results indicated that PLD downregulation shortens longevity and induces age-related biomarkers through ROS accumulation by inhibiting the DAF-16/FoxO3a pathway in nematodes.

Choline transporter mutations in severe congenital myasthenic syndrome disrupt transporter localization

The presynaptic, high-affinity choline transporter is a critical determinant of signalling by the neurotransmitter acetylcholine at both central and peripheral cholinergic synapses, including the neuromuscular junction. Here we describe an autosomal recessive presynaptic congenital myasthenic syndrome presenting with a broad clinical phenotype due to homozygous choline transporter missense mutations. The clinical phenotype ranges from the classical presentation of a congenital myasthenic syndrome in one patient (p.Pro210Leu), to severe neurodevelopmental delay with brain atrophy (p.Ser94Arg) and extend the clinical outcomes to a more severe spectrum with infantile lethality (p.Val112Glu). Cells transfected with mutant transporter construct revealed a virtually complete loss of transport activity that was paralleled by a reduction in transporter cell surface expression. Consistent with these findings, studies to determine the impact of gene mutations on the trafficking of the Caenorhabditis elegans choline transporter orthologue revealed deficits in transporter export to axons and nerve terminals. These findings contrast with our previous findings in autosomal dominant distal hereditary motor neuropathy of a dominant-negative frameshift mutation at the C-terminus of choline transporter that was associated with significantly reduced, but not completely abrogated choline transporter function. Together our findings define divergent neuropathological outcomes arising from different classes of choline transporter mutation with distinct disease processes and modes of inheritance. These findings underscore the essential role played by the choline transporter in sustaining acetylcholine neurotransmission at both central and neuromuscular synapses, with important implications for treatment and drug selection.

AGEs/RAGE-Related Neurodegeneration: daf-16 as a Mediator, Insulin as an Ameliorant, and C. elegans as an Expedient Research Model

Advanced glycation end-products (AGEs) are nonenzymatically glycated proteins, lipids, and nucleic acids. These compounds both originate exogenously and are formed endogenously, and they are associated, along with one of their receptors (RAGE), with a variety of pathologies and neurodegeneration. Some of their deleterious effects include affecting insulin signaling and FOXO-related pathways in both receptor-dependent and -independent manners. A potential ameliorating agent for these effects is insulin, which is being studied in several in vivo and in vitro models; one of these models is C. elegans, whose maintenance, genetic malleability, and well-described longevity-related pathways make it an optimal complementary model for assessing these objectives. In the realm of neuroscience, this model is currently being used only for general assessment of neurodegeneration and shortened lifespan. We suggest that characterization of (a) the effects of AGEs/RAGE on specific neurotransmitter systems, (b) the role of the daf-2/daf-16 pathway in these neurodegenerative processes, and

Epidermal Growth Factor Receptor Cell Survival Signaling Requires Phosphatidylcholine Biosynthesis

Identification of pro-cell survival signaling pathways has implications for cancer, cardiovascular, and neurodegenerative disease. We show that the Caenorhabditis elegans epidermal growth factor receptor LET-23 (LET-23 EGFR) has a prosurvival function in counteracting excitotoxicity, and we identify novel molecular players required for this prosurvival signaling. uv1 sensory cells in the C. elegans uterus undergo excitotoxic death in response to activation of the OSM-9/OCR-4 TRPV channel by the endogenous agonist nicotinamide. Activation of LET-23 EGFR can effectively prevent this excitotoxic death. We investigate the roles of signaling pathways known to act downstream of LET-23 EGFR in C. elegans and find that the LET-60 Ras/MAPK pathway, but not the IP₃ receptor pathway, is required for efficient LET-23 EGFR activity in its prosurvival function. However, activation of LET-60 Ras/MAPK pathway does not appear to be sufficient to fully mimic LET-23 EGFR activity. We screen for genes that are required for EGFR prosurvival function and uncover a role for phosphatidylcholine biosynthetic enzymes in EGFR prosurvival function. Finally, we show that exogenous application of phosphatidylcholine is sufficient to prevent some deaths in this excitotoxicity model. Our work implicates regulation of lipid synthesis downstream of EGFR in cell survival and death decisions.

Cholinergic genetics of visual attention: Human and mouse choline transporter capacity variants influence distractibility

The basal forebrain cholinergic projection system to the cortex mediates essential aspects of visual attention performance, including the detection of cues and the response to performance challenges (top-down control of attention). Higher levels of top-down control are mediated via elevated levels of cholinergic neuromodulation. The neuronal choline transporter (CHT) strongly influences the synthesis and release of acetylcholine (ACh). As the capacity of the CHT to import choline into the neuron is a major, presynaptic determinant of cholinergic neuromodulation, we hypothesize that genetically-imposed CHT capacity variation impacts the balance of bottom-up versus top-down control of visual attention. Following a brief review of the cognitive concepts relevant for this hypothesis, we describe the key results from our research in mice and humans that possess genetically-imposed changes in choline uptake capacity. CHT subcapacity is associated with poor top-down attentional control and attenuated (cholinergic) activation of right frontal regions. Conversely, mice overexpressing the CHT, and humans expressing a CHT variant hypothesized to enhance choline transporter function, are relatively resistant to challenges of visual attention performance. Genetic or environmental modulation of CHT expression and function may be associated with vulnerabilities for cognitive disorders.

Choline on the Move: Perspectives on the Molecular Physiology and Pharmacology of the Presynaptic Choline Transporter

Genetic, biochemical, physiological, and pharmacological approaches have advanced our understanding of cholinergic biology for over 100 years. High-affinity choline uptake (HACU) was one of the last features of cholinergic signaling to be defined at a molecular level, achieved through the cloning of the choline transporter (CHT, SLC5A7). In retrospect, the molecular era of CHT studies initiated with the identification of hemicholinium-3 (HC-3), a potent, competitive CHT antagonist, though it would take another 30 years before HC-3, in radiolabeled form, was used by Joseph Coyle's laboratory to identify and monitor the dynamics of CHT proteins. Though HC-3 studies provided important insights into CHT distribution and regulation, another 15 years would pass before the structure of CHT genes and proteins were identified, a full decade after the cloning of most other neurotransmitter-associated transporters. The availability of CHT gene and protein probes propelled the development of cell and animal models as well as efforts to gain insights into how human CHT gene variation affects the risk for brain and neuromuscular disorders. Most recently, our group has pursued a broadening of CHT pharmacology, elucidating novel chemical structures that may serve to advance cholinergic diagnostics and medication development. Here we provide a short review of the transformation that has occurred in HACU research and how such advances may promote the development of novel therapeutics.

Synthesis and structure-activity relationships of a series of 4-methoxy-3-(piperidin-4-yl)oxy benzamides as novel inhibitors of the presynaptic choline transporter

The synthesis and SAR of 4-methoxy-3-(piperidin-4-yl) benzamides identified after a high-throughput screen of the MLPCN library is reported. SAR was explored around the 3-piperidine substituent as well as the amide functionality of the reported compounds. Starting from the initial lead compounds, 1-7, iterative medicinal chemistry efforts led to the identification of ML352 (10m). ML352 represents a potent and selective inhibitor of CHT based on a drug-like scaffold.

Functional regulation of phospholipase D expression in cancer and inflammation

Phospholipase D (PLD) regulates downstream effectors by generating phosphatidic acid. Growing links of dysregulation of PLD to human disease have spurred interest in therapeutics that target its function. Aberrant PLD expression has been identified in multiple facets of complex pathological states, including cancer and inflammatory diseases. Thus, it is important to understand how the signaling network of PLD expression is regulated and contributes to progression of these diseases. Interestingly, small molecule PLD inhibitors can suppress PLD expression as well as enzymatic activity of PLD and have been shown to be effective in pathological mice models, suggesting the potential for use of PLD inhibitors as therapeutics against cancer and inflammation. Here, we summarize recent scientific developments regarding the regulation of PLD expression and its role in cancer and inflammatory processes.

An open-source analytical platform for analysis of C. elegans swimming-induced paralysis

BACKGROUND

The nematode Caenhorhabditis elegans offers great power for the identification and characterization of genes that regulate behavior. In support of this effort, analytical methods are required that provide dimensional analyses of subcomponents of behavior. Previously, we demonstrated that loss of the presynaptic dopamine (DA) transporter, dat-1, evokes DA-dependent Swimming-Induced Paralysis (Swip) (Mcdonald et al., 2007), a behavior compatible with forward genetic screens (Hardaway et al., 2012).

NEW METHOD

Here, we detail the development and implementation of SwimR, a set of tools that provide for an automated, kinetic analysis of C. elegans Swip. SwimR relies on open source programs that can be freely implemented and modified.

RESULTS

We show that SwimR can display time-dependent alterations of swimming behavior induced by drug-treatment, illustrating this capacity with the dat-1 blocker and tricyclic antidepressant imipramine (IMI). We demonstrate the capacity of SwimR to extract multiple kinetic parameters that are impractical to obtain in manual assays.

COMPARISON WITH EXISTING METHODS

Standard measurements of C. elegans swimming utilizes manual assessments of the number of animals exhibiting swimming versus paralysis. Our approach deconstructs the time course and rates of movement in an automated fashion, offering a significant increase in the information that can be obtained from swimming behavior.

CONCLUSIONS

The SwimR platform is a powerful tool for the deconstruction of worm thrashing behavior in the context of both genetic and pharmacological manipulations that can be used to segregate pathways that underlie nematode swimming mechanics.

The SLC6 transporters: perspectives on structure, functions, regulation, and models for transporter dysfunction

The human SLC6 family is composed of approximately 20 structurally related symporters (co-transporters) that use the transmembrane electrochemical gradient to actively import their substrates into cells. Approximately half of the substrates of these transporters are amino acids, with others transporting biogenic amines and/or closely related compounds, such as nutrients and compatible osmolytes. In this short review, five leaders in the field discuss a number of currently important research themes that involve SLC6 transporters, highlighting the integrative role they play across a wide spectrum of different functions. The first essay, by Gary Rudnick, describes the molecular mechanism of their coupled transport which is being progressively better understood based on new crystal structures, functional studies, and modeling. Next, the question of multiple levels of transporter regulation is discussed by Reinhard Krämer, in the context of osmoregulation and stress response by the related bacterial betaine transporter BetP. The role of selected members of the human SLC6 family that function as nutrient amino acid transporters is then reviewed by François Verrey. He discusses how some of these transporters mediate the active uptake of (essential) amino acids into epithelial cells of the gut and the kidney tubule to support systemic amino acid requirements, whereas others are expressed in specific cells to support their specialized metabolism and/or growth. The most extensively studied members of the human SLC6 family are neurotransmitter reuptake transporters, many of which are important drug targets for the treatment of neuropsychiatric disorders. Randy Blakely discusses the role of posttranscriptional modifications of these proteins in regulating transporter subcellular localization and activity state. Finally, Dennis Murphy reviews how natural gene variants and mouse genetic models display consistent behavioral alterations that relate to altered extracellular neurotransmitter levels.

Choline transporter hemizygosity results in diminished basal extracellular dopamine levels in nucleus accumbens and blunts dopamine elevations following cocaine or nicotine

Dopamine (DA) signaling in the central nervous system mediates the addictive capacities of multiple commonly abused substances, including cocaine, amphetamine, heroin and nicotine. The firing of DA neurons residing in the ventral tegmental area (VTA), and the release of DA by the projections of these neurons in the nucleus accumbens (NAc), is under tight control by cholinergic signaling mediated by nicotinic acetylcholine (ACh) receptors (nAChRs). The capacity for cholinergic signaling is dictated by the availability and activity of the presynaptic, high-affinity, choline transporter (CHT, SLC5A7) that acquires choline in an activity-dependent matter to sustain ACh synthesis. Here, we present evidence that a constitutive loss of CHT expression, mediated by genetic elimination of one copy of the Slc5a7 gene in mice (CHT+/-), leads to a significant reduction in basal extracellular DA levels in the NAc, as measured by in vivo microdialysis. Moreover, CHT heterozygosity results in blunted DA elevations following systemic nicotine or cocaine administration. These findings reinforce a critical role of ACh signaling capacity in both tonic and drug-modulated DA signaling and argue that genetically imposed reductions in CHT that lead to diminished DA signaling may lead to poor responses to reinforcing stimuli, possibly contributing to disorders linked to perturbed cholinergic signaling including depression and attention-deficit hyperactivity disorder (ADHD).

Forward genetic analysis to identify determinants of dopamine signaling in Caenorhabditis elegans using swimming-induced paralysis

Disrupted dopamine (DA) signaling is believed to contribute to the core features of multiple neuropsychiatric and neurodegenerative disorders. Essential features of DA neurotransmission are conserved in the nematode Caenorhabditis elegans, providing us with an opportunity to implement forward genetic approaches that may reveal novel, in vivo regulators of DA signaling. Previously, we identified a robust phenotype, termed Swimming-induced paralysis (Swip), that emerges in animals deficient in the plasma membrane DA transporter. Here, we report the use and quantitative analysis of Swip in the identification of mutant genes that control DA signaling. Two lines captured in our screen (vt21 and vt22) bear novel dat-1 alleles that disrupt expression and surface trafficking of transporter proteins in vitro and in vivo. Two additional lines, vt25 and vt29, lack transporter mutations but exhibit genetic, biochemical, and behavioral phenotypes consistent with distinct perturbations of DA signaling. Our studies validate the utility of the Swip screen, demonstrate the functional relevance of DA transporter structural elements, and reveal novel genomic loci that encode regulators of DA signaling.

Emerging findings from studies of phospholipase D in model organisms (and a short update on phosphatidic acid effectors)

Phospholipase D (PLD) catalyses the hydrolysis of phosphatidylcholine to generate phosphatidic acid and choline. Historically, much PLD work has been conducted in mammalian settings although genes encoding enzymes of this family have been identified in all eukaryotic organisms. Recently, important insights on PLD function are emerging from work in yeast, but much less is known about PLD in other organisms. In this review we will summarize what is known about phospholipase D in several model organisms, including C. elegans, D. discoideum, D. rerio and D. melanogaster. In the cases where knockouts are available (C. elegans, Dictyostelium and Drosophila) the PLD gene(s) appear not to be essential for viability, but several studies are beginning to identify pathways where this activity has a role. Given that the proteins in model organisms are very similar to their mammalian counterparts, we expect that future studies in model organisms will complement and extend ongoing work in mammalian settings. At the end of this review we will also provide a short update on phosphatidic acid targets, a topic last reviewed in 2006.

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 (<or=3 ng Hg/mg protein) in MeHgCl-treated nematodes approached levels that are highly toxic to mammals. If MeHgCl reaches these concentrations throughout the animal, this finding indicates that C. elegans cells, particularly neurons, may be less sensitive to MeHgCl toxicity than mammalian cells. We propose, therefore, that C. elegans should be a useful model for discovering intrinsic mechanisms that confer resistance to MeHgCl exposure.

A network of G-protein signaling pathways control neuronal activity in C. elegans

The Caenorhabditis elegans neuromuscular junction (NMJ) is one of the best studied synapses in any organism. A variety of genetic screens have identified genes required both for the essential steps of neurotransmitter release from motorneurons as well as the signaling pathways that regulate rates of neurotransmitter release. A number of these regulatory genes encode proteins that converge to regulate neurotransmitter release. In other cases genes are known to regulate signaling at the NMJ but how they act remains unknown. Many of the proteins that regulate activity at the NMJ participate in a network of heterotrimeric G-protein signaling pathways controlling the release of synaptic vesicles and/or dense-core vesicles (DCVs). At least four heterotrimeric G-proteins (Galphaq, Galpha12, Galphao, and Galphas) act within the motorneurons to control the activity of the NMJ. The Galphaq, Galpha12, and Galphao pathways converge to control production and destruction of the lipid-bound second messenger diacylglycerol (DAG) at sites of neurotransmitter release. DAG acts via at least two effectors, MUNC13 and PKC, to control the release of both neurotransmitters and neuropeptides from motorneurons. The Galphas pathway converges with the other three heterotrimeric G-protein pathways downstream of DAG to regulate neuropeptide release. Released neurotransmitters and neuropeptides then act to control contraction of the body-wall muscles to control locomotion. The lipids and proteins involved in these networks are conserved between C. elegans and mammals. Thus, the C. elegans NMJ acts as a model synapse to understand how neuronal activity in the human brain is regulated.

Identification of major classes of cholinergic neurons in the nematode Caenorhabditis elegans

The neurotransmitter acetylcholine (ACh) is specifically synthesized by the enzyme choline acetyltransferase (ChAT). Subsequently, it is loaded into synaptic vesicles by a specific vesicular acetylcholine transporter (VAChT). We have generated antibodies that recognize ChAT or VAChT in a model organism, the nematode Caenorhabditis elegans, in order to examine the subcellular and cellular distributions of these cholinergic proteins. ChAT and VAChT are found in the same neurons, including more than one-third of the 302 total neurons present in the adult hermaphrodite. VAChT is found in synaptic regions, whereas ChAT appears to exist in two forms in neurons, a synapse-enriched form and a more evenly distributed possibly cytosolic form. We have used antibodies to identify the cholinergic neurons in the body of larval and adult hermaphrodites. All of the classes of putative excitatory motor neurons in the ventral nerve cord appear to be cholinergic: the DA and DB neurons in the first larval stage and the AS, DA, DB, VA, VB, and VC neurons in the adult. In addition, several interneurons with somas in the tail and processes in the tail or body are cholinergic; sensory neurons are generally not cholinergic. Description of the normal pattern of cholinergic proteins and neurons will improve our understanding of the role of cholinergic neurons in the behavior and development of this model organism.

Vigorous motor activity in Caenorhabditis elegans requires efficient clearance of dopamine mediated by synaptic localization of the dopamine transporter DAT-1

The catecholamine dopamine (DA) functions as a powerful modulatory neurotransmitter in both invertebrates and vertebrates. As in man, DA neurons in the nematode Caenorhabditis elegans express a cocaine-sensitive transporter (DAT-1), presumably to regulate synaptic DA signaling and limit DA spillover to extrasynaptic sites, although evidence supporting this is currently lacking. In this report, we describe and validate a novel and readily quantifiable phenotype, swimming-induced paralysis (SWIP) that emerges in DAT-1-deficient nematodes when animals exert maximal physical activity in water. We verify the dependence of SWIP on DA biosynthesis, vesicular packaging, synaptic release, and on the DA receptor DOP-3. Using DAT-1 specific antibodies and GFP::DAT-1 fusions, we demonstrate a synaptic enrichment of DAT-1 that is achieved independently of synaptic targeting of the vesicular monoamine transporter (VMAT). Importantly, dat-1 deletions and point mutations that disrupt DA uptake in cultured C. elegans neurons and/or impact DAT-1 synaptic localization in vivo generate SWIP. SWIP assays, along with in vivo imaging of wild-type and mutant GFP::DAT-1 fusions identify a distal COOH terminal segment of the transporter as essential for efficient somatic export, synaptic localization and in vivo DA clearance. Our studies provide the first description of behavioral perturbations arising from altered trafficking of DATs in vivo in any organism and support a model whereby endogenous DA actions in C. elegans are tightly regulated by synaptic DAT-1.

Pharmacology of neurotransmitter transport into secretory vesicles

Many neuropsychiatric disorders appear to involve a disturbance of chemical neurotransmission, and the mechanism of available therapeutic agents supports this impression. Postsynaptic receptors have received considerable attention as drug targets, but some of the most successful agents influence presynaptic processes, in particular neurotransmitter reuptake. The pharmacological potential of many other presynaptic elements, and in particular the machinery responsible for loading transmitter into vesicles, has received only limited attention. The similarity of vesicular transporters to bacterial drug resistance proteins and the increasing evidence for regulation of vesicle filling and recycling suggest that the pharmacological potential of vesicular transporters has been underestimated. In this review, we discuss the pharmacological effects of psychostimulants and therapeutic agents on transmitter release.

Amyolid precursor protein mediates presynaptic localization and activity of the high-affinity choline transporter

The key pathological features of Alzheimer's disease include synaptic dysfunction, profound changes in the cholinergic system, and deposition of beta-amyloid peptides generated by proteolytic processing of the amyloid-beta precursor protein (APP). However, the pathways linking APP with synaptic activity and cholinergic neuronal function are poorly understood. We report here that APP is essential in regulating the presynaptic expression and activity of the high-affinity choline transporter (CHT), a molecule that mediates the rate-limiting step of cholinergic synaptic transmission in both the neuromuscular junction and central cholinergic neurons. Loss of APP leads to aberrant localization of CHT at the neuromuscular synapses and reduced CHT activity at cholinergic projections. At the cellular level, we show that APP and CHT can be found in Rab5-positive endosomal compartments and that APP affects CHT endocytosis. Furthermore, we demonstrate that APP interacts with CHT through the C-terminal domain, providing support for a specific and direct regulation of CHT by APP through protein-protein interactions. These results identify a physiological activity of APP in cholinergic neurons, and our data indicate that deregulation of APP function may contribute to cholinergic impairment and AD pathogenesis.

Choline transport and de novo choline synthesis support acetylcholine biosynthesis in Caenorhabditis elegans cholinergic neurons

The cho-1 gene in Caenorhabditis elegans encodes a high-affinity plasma-membrane choline transporter believed to be rate limiting for acetylcholine (ACh) synthesis in cholinergic nerve terminals. We found that CHO-1 is expressed in most, but not all cholinergic neurons in C. elegans. cho-1 null mutants are viable and exhibit mild deficits in cholinergic behavior; they are slightly resistant to the acetylcholinesterase inhibitor aldicarb, and they exhibit reduced swimming rates in liquid. cho-1 mutants also fail to sustain swimming behavior; over a 33-min time course, cho-1 mutants slow down or stop swimming, whereas wild-type animals sustain the initial rate of swimming over the duration of the experiment. A functional CHO-1GFP fusion protein rescues these cho-1 mutant phenotypes and is enriched at cholinergic synapses. Although cho-1 mutants clearly exhibit defects in cholinergic behaviors, the loss of cho-1 function has surprisingly mild effects on cholinergic neurotransmission. However, reducing endogenous choline synthesis strongly enhances the phenotype of cho-1 mutants, giving rise to a synthetic uncoordinated phenotype. Our results indicate that both choline transport and de novo synthesis provide choline for ACh synthesis in C. elegans cholinergic neurons.

Cell-specific microarray profiling experiments reveal a comprehensive picture of gene expression in the C. elegans nervous system

BACKGROUND

With its fully sequenced genome and simple, well-defined nervous system, the nematode Caenorhabditis elegans offers a unique opportunity to correlate gene expression with neuronal differentiation. The lineal origin, cellular morphology and synaptic connectivity of each of the 302 neurons are known. In many instances, specific behaviors can be attributed to particular neurons or circuits. Here we describe microarray-based methods that monitor gene expression in C. elegans neurons and, thereby, link comprehensive profiles of neuronal transcription to key developmental and functional properties of the nervous system.

RESULTS

We employed complementary microarray-based strategies to profile gene expression in the embryonic and larval nervous systems. In the MAPCeL (Microarray Profiling C. elegans cells) method, we used fluorescence activated cell sorting (FACS) to isolate GFP-tagged embryonic neurons for microarray analysis. To profile the larval nervous system, we used the mRNA-tagging technique in which an epitope-labeled mRNA binding protein (FLAG-PAB-1) was transgenically expressed in neurons for immunoprecipitation of cell-specific transcripts. These combined approaches identified approximately 2,500 mRNAs that are highly enriched in either the embryonic or larval C. elegans nervous system. These data are validated in part by the detection of gene classes (for example, transcription factors, ion channels, synaptic vesicle components) with established roles in neuronal development or function. Of particular interest are 19 conserved transcripts of unknown function that are also expressed in the mammalian brain. In addition to utilizing these profiling approaches to define stage-specific gene expression, we also applied the mRNA-tagging method to fingerprint a specific neuron type, the A-class group of cholinergic motor neurons, during early larval development. A comparison of these data to a MAPCeL profile of embryonic A-class motor neurons identified genes with common functions in both types of A-class motor neurons as well as transcripts with roles specific to each motor neuron type.

CONCLUSION

We describe microarray-based strategies for generating expression profiles of embryonic and larval C. elegans neurons. These methods can be applied to particular neurons at specific developmental stages and, therefore, provide an unprecedented opportunity to obtain spatially and temporally defined snapshots of gene expression in a simple model nervous system.