Facilitation of synaptic transmission by EGL-30 Gqalpha and EGL-8 PLCbeta: DAG binding to UNC-13 is required to stimulate acetylcholine release

A Systematic RNAi Screen Reveals a Novel Role of a Spindle Assembly Checkpoint Protein BuGZ in Synaptic Transmission in C. elegans

Synaptic vesicles (SV) store various neurotransmitters that are released at the synapse. The molecular mechanisms of biogenesis, exocytosis, and endocytosis for SV, however, remain largely elusive. In this study, using Complex Object Parametric Analysis and Sorter (COPAS) to monitor the fluorescence of synapto-pHluorin (SpH), we performed a whole-genome RNAi screen in C. elegans to identify novel genetic modulators in SV cycling. One hundred seventy six genes that up-regulating SpH fluorescence and 96 genes that down-regulating SpH fluorescence were identified after multi-round screen. Among these genes, B0035.1 (bugz-1) encodes ortholog of mammalian C2H2 zinc-finger protein BuGZ/ZNF207, which is a spindle assembly checkpoint protein essential for mitosis in human cells. Combining electrophysiology, imaging and behavioral assays, we reveal that depletion of BuGZ-1 results in defects in locomotion. We further demonstrate that BuGZ-1 promotes SV recycling by regulating the expression levels of endocytosis-related genes such as rab11.1. Therefore, we have identified a bunch of potential genetic modulators in SV cycling, and revealed an unexpected role of BuGZ-1 in regulating synaptic transmission.

Locomotion Behavior Is Affected by the GαS Pathway and the Two-Pore-Domain K+ Channel TWK-7 Interacting in GABAergic Motor Neurons in Caenorhabditis elegans

Adjusting the efficiency of movement in response to environmental cues is an essential integrative characteristic of adaptive locomotion behavior across species. However, the modulatory molecules and the pathways involved are largely unknown. Recently, we demonstrated that in Caenorhabditis elegans, a loss-of-function of the two-pore-domain potassium (K₂P) channel TWK-7 causes a fast, coordinated, and persistent forward crawling behavior in which five central aspects of stimulated locomotion-velocity, direction, wave parameters, duration, and straightness-are affected. Here, we isolated the reduction-of-function allele cau1 of the C. elegans gene kin-2 in a forward genetic screen and showed that it phenocopies the locomotor activity and locomotion behavior of twk-7(null) animals. Kin-2 encodes the negative regulatory subunit of protein kinase A (KIN-1/PKA). Consistently, we found that other gain-of-function mutants of the Gα_S-KIN-1/PKA pathway resemble kin-2(cau1) and twk-7(null) in locomotion phenotype. Using the powerful genetics of the C. elegans system in combination with cell type-specific approaches and detailed locomotion analyses, we identified TWK-7 as a putative downstream target of the Gα_S-KIN-1/PKA pathway at the level of the γ-aminobutyric acid (GABA)ergic D-type motor neurons. Due to this epistatic interaction, we suggest that KIN-1/PKA and TWK-7 may share a common pathway that is probably involved in the modulation of both locomotor activity and locomotion behavior during forward crawling.

The NCA-1 and NCA-2 Ion Channels Function Downstream of Gq and Rho To Regulate Locomotion in Caenorhabditis elegans

The heterotrimeric G protein G_q positively regulates neuronal activity and synaptic transmission. Previously, the Rho guanine nucleotide exchange factor Trio was identified as a direct effector of G_q that acts in parallel to the canonical G_q effector phospholipase C. Here, we examine how Trio and Rho act to stimulate neuronal activity downstream of G_q in the nematode Caenorhabditis elegans Through two forward genetic screens, we identify the cation channels NCA-1 and NCA-2, orthologs of mammalian NALCN, as downstream targets of the G_q-Rho pathway. By performing genetic epistasis analysis using dominant activating mutations and recessive loss-of-function mutations in the members of this pathway, we show that NCA-1 and NCA-2 act downstream of G_q in a linear pathway. Through cell-specific rescue experiments, we show that function of these channels in head acetylcholine neurons is sufficient for normal locomotion in C. elegans Our results suggest that NCA-1 and NCA-2 are physiologically relevant targets of neuronal G_q-Rho signaling in C. elegans.

A Gustatory Neural Circuit of Caenorhabditis elegans Generates Memory-Dependent Behaviors in Na+ Chemotaxis

Animals show various behaviors in response to environmental chemicals. These behaviors are often plastic depending on previous experiences. Caenorhabditis elegans, which has highly developed chemosensory system with a limited number of sensory neurons, is an ideal model for analyzing the role of each neuron in innate and learned behaviors. Here, we report a new type of memory-dependent behavioral plasticity in Na+ chemotaxis generated by the left member of bilateral gustatory neuron pair ASE (ASEL neuron). When worms were cultivated in the presence of Na+, they showed positive chemotaxis toward Na+, but when cultivated under Na+-free conditions, they showed no preference regarding Na+ concentration. Both channelrhodopsin-2 (ChR2) activation with blue light and up-steps of Na+ concentration activated ASEL only after cultivation with Na+, as judged by increase in intracellular Ca2+ Under cultivation conditions with Na+, photoactivation of ASEL caused activation of its downstream interneurons AIY and AIA, which stimulate forward locomotion, and inhibition of its downstream interneuron AIB, which inhibits the turning/reversal behavior, and overall drove worms toward higher Na+ concentrations. We also found that the Gq signaling pathway and the neurotransmitter glutamate are both involved in the behavioral response generated by ASEL.SIGNIFICANCE STATEMENT Animals have acquired various types of behavioral plasticity during their long evolutionary history. Caenorhabditis elegans prefers odors associated with food, but plastically changes its behavioral response according to previous experience. Here, we report a new type of behavioral response generated by a single gustatory sensory neuron, the ASE-left (ASEL) neuron. ASEL did not respond to photostimulation or upsteps of Na+ concentration when worms were cultivated in Na+-free conditions; however, when worms were cultivated with Na+, ASEL responded and inhibited AIB to avoid turning and stimulated AIY and AIA to promote forward locomotion, which collectively drove worms toward higher Na+ concentrations. Glutamate and the Gq signaling pathway are essential for driving worms toward higher Na+ concentrations.

An extrasynaptic GABAergic signal modulates a pattern of forward movement in Caenorhabditis elegans

As a common neurotransmitter in the nervous system, γ-aminobutyric acid (GABA) modulates locomotory patterns in both vertebrates and invertebrates. However, the signaling mechanisms underlying the behavioral effects of GABAergic modulation are not completely understood. Here, we demonstrate that a GABAergic signal in C. elegans modulates the amplitude of undulatory head bending through extrasynaptic neurotransmission and conserved metabotropic receptors. We show that the GABAergic RME head motor neurons generate undulatory activity patterns that correlate with head bending and the activity of RME causally links with head bending amplitude. The undulatory activity of RME is regulated by a pair of cholinergic head motor neurons SMD, which facilitate head bending, and inhibits SMD to limit head bending. The extrasynaptic neurotransmission between SMD and RME provides a gain control system to set head bending amplitude to a value correlated with optimal efficiency of forward movement.

An extrasynaptic GABAergic signal modulates a pattern of forward movement in Caenorhabditis elegans

As a common neurotransmitter in the nervous system, γ-aminobutyric acid (GABA) modulates locomotory patterns in both vertebrates and invertebrates. However, the signaling mechanisms underlying the behavioral effects of GABAergic modulation are not completely understood. Here, we demonstrate that a GABAergic signal in C. elegans modulates the amplitude of undulatory head bending through extrasynaptic neurotransmission and conserved metabotropic receptors. We show that the GABAergic RME head motor neurons generate undulatory activity patterns that correlate with head bending and the activity of RME causally links with head bending amplitude. The undulatory activity of RME is regulated by a pair of cholinergic head motor neurons SMD, which facilitate head bending, and inhibits SMD to limit head bending. The extrasynaptic neurotransmission between SMD and RME provides a gain control system to set head bending amplitude to a value correlated with optimal efficiency of forward movement.

DOI:http://dx.doi.org/10.7554/eLife.14197.001

De novo analysis of the transcriptome of Pratylenchus zeae to identify transcripts for proteins required for structural integrity, sensation, locomotion and parasitism

The root lesion nematode Pratylenchus zeae, a migratory endoparasite, is an economically important pest of major crop plants (e.g. cereals, sugarcane). It enters host roots, migrates through root tissues and feeds from cortical cells, and defends itself against biotic and abiotic stresses in the soil and in host tissues. We report de novo sequencing of the P. zeae transcriptome using 454 FLX, and the identification of putative transcripts encoding proteins required for movement, response to stimuli, feeding and parasitism. Sequencing generated 347,443 good quality reads which were assembled into 10,163 contigs and 139,104 singletons: 65% of contigs and 28% of singletons matched sequences of free-living and parasitic nematodes. Three-quarters of the annotated transcripts were common to reference nematodes, mainly representing genes encoding proteins for structural integrity and fundamental biochemical processes. Over 15,000 transcripts were similar to Caenorhabditis elegans genes encoding proteins with roles in mechanical and neural control of movement, responses to chemicals, mechanical and thermal stresses. Notably, 766 transcripts matched parasitism genes employed by both migratory and sedentary endoparasites in host interactions, three of which hybridized to the gland cell region, suggesting that they might be secreted. Conversely, transcripts for effectors reported to be involved in feeding site formation by sedentary endoparasites were conspicuously absent. Transcripts similar to those encoding some secretory-excretory products at the host interface of Brugia malayi, the secretome of Meloidogyne incognita and products of gland cells of Heterodera glycines were also identified. This P. zeae transcriptome provides new information for genome annotation and functional analysis of possible targets for control of pratylenchid nematodes.

Identification of microtubule growth deceleration and its regulation by conserved and novel proteins

Live imaging of microtubule dynamics in Caenorhabditis elegans muscle cells reveals a novel microtubule behavior characterized by an abrupt change in growth rate, named “microtubule growth deceleration.” The conserved protein ZYG-9^TOGp and two novel ORFs, cylc-1 and cylc-2, are involved in the regulation of this novel microtubule behavior.

Microtubules (MTs) are cytoskeletal polymers that participate in diverse cellular functions, including cell division, intracellular trafficking, and templating of cilia and flagella. MTs undergo dynamic instability, alternating between growth and shortening via catastrophe and rescue events. The rates and frequencies of MT dynamic parameters appear to be characteristic for a given cell type. We recently reported that all MT dynamic parameters vary throughout differentiation of a smooth muscle cell type in intact Caenorhabditis elegans. Here we describe local differences in MT dynamics and a novel MT behavior: an abrupt change in growth rate (deceleration) of single MTs occurring in the cell periphery of these cells. MT deceleration occurs where there is a decrease in local soluble tubulin concentration at the cell periphery. This local regulation of tubulin concentration and MT deceleration are dependent on two novel homologues of human cylicin. These novel ORFs, which we name cylc-1 and -2, share sequence homology with stathmins and encode small, very basic proteins containing several KKD/E repeats. The TOG domain–containing protein ZYG-9^TOGp is responsible for the faster polymerization rate within the cell body. Thus we have defined two contributors to the molecular regulation for this novel MT behavior.

Behavioral Plasticity in the C. elegans Mechanosensory Circuit

This review outlines research into the cellular and molecular mechanisms underlying a simple behavior in the soil-dwelling nematode, C. elegans. A tap administered to the side of a petri plate acts as a nonlocalized mechanical stimulus to the worms within. Most adult worms respond to this tap stimulus with backward locomotion, an action known as the tap-withdrawal response. This behavior has been thoroughly characterized and the neural circuit mediating it has been determined. The response habituates following repeated stimulation, and current work is aimed at elucidating the mechanism behind this simple form of nonassociative learning. Changes in cell excitability and the strength of glutamatergic synapses play key roles in mediating this plasticity.

RdgBα reciprocally transfers PA and PI at ER–PM contact sites to maintain PI(4,5)P2 homoeostasis during phospholipase C signalling in Drosophila photoreceptors

Phosphatidylinositol (PI) is the precursor lipid for the synthesis of PI 4,5-bisphosphate [PI(4,5)P2] at the plasma membrane (PM) and is sequentially phosphorylated by the lipid kinases, PI 4-kinase and phosphatidylinositol 4-phosphate (PI4P)-5-kinase. Receptor-mediated hydrolysis of PI(4,5)P2 takes place at the PM but PI resynthesis occurs at the endoplasmic reticulum (ER). Thus PI(4,5)P2 resynthesis requires the reciprocal transport of two key intermediates, phosphatidic acid (PA) and PI between the ER and the PM. PI transfer proteins (PITPs), defined by the presence of the PITP domain, can facilitate lipid transfer between membranes; the PITP domain comprises a hydrophobic cavity with dual specificity but accommodates a single phospholipid molecule. The class II PITP, retinal degeneration type B (RdgB)α is a multi-domain protein and its PITP domain can bind and transfer PI and PA. In Drosophila photoreceptors, a well-defined G-protein-coupled phospholipase Cβ (PLCβ) signalling pathway, phototransduction defects resulting from loss of RdgBα can be rescued by expression of the PITP domain provided it is competent for both PI and PA transfer. We propose that RdgBα proteins maintain PI(4,5)P2 homoeostasis after PLC activation by facilitating the reciprocal transport of PA and PI at ER–PM membrane contact sites.

Caenorhabditis elegans exhibit a coupling between the defecation motor program and directed locomotion

Distinct motor programs can be coupled to refine the repertoire of behavior dynamics. However, mechanisms underlying such coupling are poorly understood. The defecation motor program (DMP) of C. elegans is composed of a succession of body contraction and expulsion steps, performed repeatedly with a period of 50-60 sec. We show that recurring patterns of directed locomotion are executed in tandem with, co-reset, and co-terminate with the DMP cycle. Calcium waves in the intestine and proton signaling were shown to regulate the DMP. We found that genetic manipulations affecting these calcium dynamics regulated the corresponding patterns of directed locomotion. Moreover, we observed the initiation of a recurring locomotion pattern 10 seconds prior to the posterior body contraction, suggesting that the synchronized motor program may initiate prior to the DMP. This study links two multi-step motor programs executed by C. elegans in synchrony, utilizing non-neuronal tissue to drive directed locomotion.

Metabotropic GABA signalling modulates longevity in C. elegans

The nervous system plays an important but poorly understood role in modulating longevity. GABA, a prominent inhibitory neurotransmitter, is best known to regulate nervous system function and behaviour in diverse organisms. Whether GABA signalling affects aging, however, has not been explored. Here we examined mutants lacking each of the major neurotransmitters in C. elegans, and find that deficiency in GABA signalling extends lifespan. This pro-longevity effect is mediated by the metabotropic GABA_B receptor GBB-1, but not ionotropic GABA_A receptors. GBB-1 regulates lifespan through G protein-PLCβ signalling, which transmits longevity signals to the transcription factor DAF-16/FOXO, a key regulator of lifespan. Mammalian GABA_B receptors can functionally substitute for GBB-1 in lifespan control in C. elegans. Our results uncover a new role of GABA signalling in lifespan regulation in C. elegans, raising the possibility that a similar process may occur in other organisms.

The C. elegans nervous system influences organismal lifespan but mechanistic details are poorly understood. Here, Chun et al. show that the neurotransmitter GABA regulates worm lifespan by acting on GABA_B receptors in motor neurons, which activate the transcription factor DAF-16 in the intestine.

A Generative Statistical Algorithm for Automatic Detection of Complex Postures

This paper presents a method for automated detection of complex (non-self-avoiding) postures of the nematode Caenorhabditis elegans and its application to analyses of locomotion defects. Our approach is based on progressively detailed statistical models that enable detection of the head and the body even in cases of severe coilers, where data from traditional trackers is limited. We restrict the input available to the algorithm to a single digitized frame, such that manual initialization is not required and the detection problem becomes embarrassingly parallel. Consequently, the proposed algorithm does not propagate detection errors and naturally integrates in a "big data" workflow used for large-scale analyses. Using this framework, we analyzed the dynamics of postures and locomotion of wild-type animals and mutants that exhibit severe coiling phenotypes. Our approach can readily be extended to additional automated tracking tasks such as tracking pairs of animals (e.g., for mating assays) or different species.

High-Throughput All-Optical Analysis of Synaptic Transmission and Synaptic Vesicle Recycling in Caenorhabditis elegans

Synaptic vesicles (SVs) undergo a cycle of biogenesis and membrane fusion to release transmitter, followed by recycling. How exocytosis and endocytosis are coupled is intensively investigated. We describe an all-optical method for identification of neurotransmission genes that can directly distinguish SV recycling factors in C. elegans, by motoneuron photostimulation and muscular RCaMP Ca²⁺ imaging. We verified our approach on mutants affecting synaptic transmission. Mutation of genes affecting SV recycling (unc-26 synaptojanin, unc-41 stonin, unc-57 endophilin, itsn-1 intersectin, snt-1 synaptotagmin) showed a distinct ‘signature’ of muscle Ca²⁺ dynamics, induced by cholinergic motoneuron photostimulation, i.e. faster rise, and earlier decrease of the signal, reflecting increased synaptic fatigue during ongoing photostimulation. To facilitate high throughput, we measured (3–5 times) ~1000 nematodes for each gene. We explored if this method enables RNAi screening for SV recycling genes. Previous screens for synaptic function genes, based on behavioral or pharmacological assays, allowed no distinction of the stage of the SV cycle in which a protein might act. We generated a strain enabling RNAi specifically only in cholinergic neurons, thus resulting in healthier animals and avoiding lethal phenotypes resulting from knockdown elsewhere. RNAi of control genes resulted in Ca²⁺ measurements that were consistent with results obtained in the respective genomic mutants, albeit to a weaker extent in most cases, and could further be confirmed by opto-electrophysiological measurements for mutants of some of the genes, including synaptojanin. We screened 95 genes that were previously implicated in cholinergic transmission, and several controls. We identified genes that clustered together with known SV recycling genes, exhibiting a similar signature of their Ca²⁺ dynamics. Five of these genes (C27B7.7, erp-1, inx-8, inx-10, spp-10) were further assessed in respective genomic mutants; however, while all showed electrophysiological phenotypes indicative of reduced cholinergic transmission, no obvious SV recycling phenotypes could be uncovered for these genes.

NLP-12 engages different UNC-13 proteins to potentiate tonic and evoked release

A neuropeptide (NLP-12) and its receptor (CKR-2) potentiate tonic and evoked ACh release at Caenorhabditis elegans neuromuscular junctions. Increased evoked release is mediated by a presynaptic pathway (egl-30 Gαq and egl-8 PLCβ) that produces DAG, and by DAG binding to short and long UNC-13 proteins. Potentiation of tonic ACh release persists in mutants deficient for egl-30 Gαq and egl-8 PLCβ and requires DAG binding to UNC-13L (but not UNC-13S). Thus, NLP-12 adjusts tonic and evoked release by distinct mechanisms.

From head to tail it's a 2 way street for neuro-immune communication

Animals need to be able to rapidly and effectively respond to changes in their external and internal environment. To achieve this the nervous and immune systems need to coordinate their responses, integrating multiple cues including presence of potential pathogens, and availability of food. In our recent study (1) we demonstrate that signaling by sensory neurons in the head using the classical neurotransmitter serotonin can negatively regulate the rectal epithelial immune response upon infection of C. elegans with the naturally occurring bacterial pathogen Microbacterium nematophilum (M. nematophilum). The complicated nature of the mammalian brain and immune system has made it difficult to identify the molecular mechanisms mediating these interactions. With its simple, well described, nervous system and a rapidly growing understanding of its immune system, C. elegans has emerged as an excellent model to study the mechanisms by which animals recognize pathogens and coordinate behavioral and cellular immune responses to infection.

Alteration in cellular acetylcholine influences dauer formation in Caenorhabditis elegans

Altered acetylcholine (Ach) homeostasis is associated with loss of viability in flies, developmental defects in mice, and cognitive deficits in human. Here, we assessed the importance of Ach in Caenorhabditis elegans development, focusing on the role of Ach during dauer formation. We found that dauer formation was disturbed in choline acetyltransferase (cha-1) and acetylcholinesterase (ace) mutants defective in Ach biosynthesis and degradation, respectively. When examined the potential role of G-proteins in dauer formation, goa-1 and egl-30 mutant worms, expressing mutated versions of mammalian G_o and G_q homolog, respectively, showed some abnormalities in dauer formation. Using quantitative mass spectrometry, we also found that dauer larvae had lower Ach content than did reproductively grown larvae. In addition, a proteomic analysis of acetylcholinesterase mutant worms, which have excessive levels of Ach, showed differential expression of metabolic genes. Collectively, these results indicate that alterations in Ach release may influence dauer formation in C. elegans. [BMB Reports 2014; 47(2): 80-85]

Spatial and molecular cues for cell outgrowth during C. elegans uterine development

The Caenorhabditis elegans uterine seam cell (utse) is an H-shaped syncytium that connects the uterus to the body wall. Comprising nine nuclei that move outward in a bidirectional manner, this synctium undergoes remarkable shape change during development. Using cell ablation experiments, we show that three surrounding cell types affect utse development: the uterine toroids, the anchor cell and the sex myoblasts. The presence of the anchor cell (AC) nucleus within the utse is necessary for proper utse development and AC invasion genes fos-1, cdh-3, him-4, egl-43, zmp-1 and mig-10 promote utse cell outgrowth. Two types of uterine lumen epithelial cells, uterine toroid 1 (ut1) and uterine toroid 2 (ut2), mediate proper utse outgrowth and we show roles in utse development for two genes expressed in the uterine toroids: the RASEF ortholog rsef-1 and Trio/unc-73. The SM expressed gene unc-53/NAV regulates utse cell shape; ablation of sex myoblasts (SMs), which generate uterine and vulval muscles, cause defects in utse morphology. Our results clarify the nature of the interactions that exist between utse and surrounding tissue, identify new roles for genes involved in cell outgrowth, and present the utse as a new model system for understanding cell shape change and, putatively, diseases associated with cell shape change.

The omega-3 fatty acid eicosapentaenoic acid is required for normal alcohol response behaviors in C. elegans

Alcohol addiction is a widespread societal problem, for which there are few treatments. There are significant genetic and environmental influences on abuse liability, and understanding these factors will be important for the identification of susceptible individuals and the development of effective pharmacotherapies. In humans, the level of response to alcohol is strongly predictive of subsequent alcohol abuse. Level of response is a combination of counteracting responses to alcohol, the level of sensitivity to the drug and the degree to which tolerance develops during the drug exposure, called acute functional tolerance. We use the simple and well-characterized nervous system of Caenorhabditis elegans to model the acute behavioral effects of ethanol to identify genetic and environmental factors that influence level of response to ethanol. Given the strong molecular conservation between the neurobiological machinery of worms and humans, cellular-level effects of ethanol are likely to be conserved. Increasingly, variation in long-chain polyunsaturated fatty acid levels has been implicated in complex neurobiological phenotypes in humans, and we recently found that fatty acid levels modify ethanol responses in worms. Here, we report that 1) eicosapentaenoic acid, an omega-3 polyunsaturated fatty acid, is required for the development of acute functional tolerance, 2) dietary supplementation of eicosapentaenoic acid is sufficient for acute tolerance, and 3) dietary eicosapentaenoic acid can alter the wild-type response to ethanol. These results suggest that genetic variation influencing long-chain polyunsaturated fatty acid levels may be important abuse liability loci, and that dietary polyunsaturated fatty acids may be an important environmental modulator of the behavioral response to ethanol.

In situ imaging in C. elegans reveals developmental regulation of microtubule dynamics

Microtubules (MTs) are cytoskeletal polymers that undergo dynamic instability, the stochastic transition between growth and shrinkage phases. MT dynamics are required for diverse cellular processes and, while intrinsic to tubulin, are highly regulated. However, little is known about how MT dynamics facilitate or are regulated by tissue biogenesis and differentiation. We imaged MT dynamics in a smooth muscle-like lineage in intact developing Caenorhabditis elegans. All aspects of MT dynamics change significantly as stem-like precursors exit mitosis and, secondarily, as they differentiate. We found that suppression, but not enhancement, of dynamics perturbs differentiated muscle function in vivo. Distinct ensembles of MT-associated proteins are specifically required for tissue biogenesis versus tissue function. A CLASP family MT stabilizer and the depolymerizing kinesin MCAK are differentially required for MT dynamics in the precursor or differentiated cells, respectively. All of these multidimensional phenotypic comparisons were facilitated by a data display method called the diamond graph.

Inhibition of diacylglycerol kinases as a physiological way to promote diacylglycerol signaling

Diacylglycerol is a key regulator of cell physiology, controlling the membrane recruitment and activation of signaling molecules. Accordingly, diacylglycerol generation and metabolism are strictly controlled, allowing for localized regulation of its concentration. While the increased production of diacylglycerol upon receptor triggering is well recognized, the modulation of diacylglycerol metabolism by diacylglycerol kinases (DGKs) is less characterized. Some agonists induce DGK activation and recruitment to the plasma membrane, promoting diacylglycerol metabolism to phosphatidic acid. Conversely, several reports indicate that signaling pathways that selectively inhibits DGK isoforms can enhance cellular diacylglycerol levels and signal transduction. For example, the impairment of DGKθ activity by RhoA binding to the catalytic domain represents a conserved mechanism controlling diacylglycerol signaling from Caenorhabditis elegans motoneurons to mammalian hepatocytes. Similarly, DGKα activity is inhibited in lymphocytes by TCR signaling, thus contributing to a rise in diacylglycerol concentration for downstream signaling. Finally, DGKμ activity is inhibited by ischemia-reperfusion-generated reactive oxygen species in airway endothelial cells, promoting diacylglycerol-mediated ion channel opening and edema. In those systems, DGKs provide a gatekeeper function by blunting diacylglycerol levels or possibly establishing permissive domains for diacylglycerol signaling. In this review, I discuss the possible general relevance of DGK inhibition to enhanced diacylglycerol signaling.

Munc18-1 is a dynamically regulated PKC target during short-term enhancement of transmitter release

Transmitter release at synapses is regulated by preceding neuronal activity, which can give rise to short-term enhancement of release like post-tetanic potentiation (PTP). Diacylglycerol (DAG) and Protein-kinase C (PKC) signaling in the nerve terminal have been widely implicated in the short-term modulation of transmitter release, but the target protein of PKC phosphorylation during short-term enhancement has remained unknown. Here, we use a gene-replacement strategy at the calyx of Held, a large CNS model synapse that expresses robust PTP, to study the molecular mechanisms of PTP. We find that two PKC phosphorylation sites of Munc18-1 are critically important for PTP, which identifies the presynaptic target protein for the action of PKC during PTP. Pharmacological experiments show that a phosphatase normally limits the duration of PTP, and that PTP is initiated by the action of a ‘conventional’ PKC isoform. Thus, a dynamic PKC phosphorylation/de-phosphorylation cycle of Munc18-1 drives short-term enhancement of transmitter release during PTP.

DOI:http://dx.doi.org/10.7554/eLife.01715.001

Brain function depends on the rapid transfer of information from one brain cell to the next at junctions known as synapses. Small packages called vesicles play an important role in this process. The arrival of an electrical action potential at the nerve terminal of the first cell causes some vesicles in the cell to fuse with the cell membrane, and this leads to the neurotransmitters inside the vesicles being released into the synapse. The neurotransmitters then bind to receptors on the second cell, which leads to an electrical signal in the second cell. A protein called Munc18-1 has a central role in the fusion of the vesicle at the cell membrane.

The strength of a synapse—that is, how easily the first brain cell can impact the electrical behaviour of the second—can change, and this ‘synaptic plasticity’ is thought to underlie learning and memory. Long-term changes in synaptic strength require additional receptors to be inserted into the membrane of the second cell. However, synapses can also be temporarily strengthened: the arrival of a burst of action potentials—a tetanus—causes some synapses to increase the amount of neurotransmitter they release in response to any subsequent, single, action potential.

This temporary increase in synaptic strength, which is known as post-tetanic potentiation, requires an enzyme called protein kinase C; the role of this enzyme is to phosphorylate specific target proteins (i.e., to add phosphate groups to them). Now, Genç et al. have genetically modified a mouse synapse in vivo and shown that protein kinase C brings about post-tetanic potentiation by phosphorylating Munc18-1. Furthermore, pharmacological experiments show that proteins called phosphatases, which de-phosphorylate proteins, normally terminate the post-tetanic potentiation after about one minute. Taken together, the study identifies a target protein which is phosphorylated by protein kinase C during post-tetanic potentiation. The study also suggests that in addition to its fundamental role in vesicle fusion, the phosphorylation state of Munc18-1 can change the probability of vesicle fusion in a more subtle way, thereby contributing to synaptic plasticity.

DOI:http://dx.doi.org/10.7554/eLife.01715.002

Distinct DNA binding sites contribute to the TCF transcriptional switch in C. elegans and Drosophila

Regulation of gene expression by signaling pathways often occurs through a transcriptional switch, where the transcription factor responsible for signal-dependent gene activation represses the same targets in the absence of signaling. T-cell factors (TCFs) are transcription factors in the Wnt/ß-catenin pathway, which control numerous cell fate specification events in metazoans. The TCF transcriptional switch is mediated by many co-regulators that contribute to repression or activation of Wnt target genes. It is typically assumed that DNA recognition by TCFs is important for target gene location, but plays no role in the actual switch. TCF/Pangolin (the fly TCF) and some vertebrate TCF isoforms bind DNA through two distinct domains, a High Mobility Group (HMG) domain and a C-clamp, which recognize DNA motifs known as HMG and Helper sites, respectively. Here, we demonstrate that POP-1 (the C. elegans TCF) also activates target genes through HMG and Helper site interactions. Helper sites enhanced the ability of a synthetic enhancer to detect Wnt/ß-catenin signaling in several tissues and revealed an unsuspected role for POP-1 in regulating the C. elegans defecation cycle. Searching for HMG-Helper site clusters allowed the identification of a new POP-1 target gene active in the head muscles and gut. While Helper sites and the C-clamp are essential for activation of worm and fly Wnt targets, they are dispensable for TCF-dependent repression of targets in the absence of Wnt signaling. These data suggest that a fundamental change in TCF-DNA binding contributes to the transcriptional switch that occurs upon Wnt stimulation.

The DNA of cells must be correctly “read” so that the proper genes are expressed. Transcription factors are the primary “DNA readers”, and these proteins bind to specific DNA sequences. Using nematodes as a model system, we investigated the rules of DNA binding for a particular transcription factor, called POP-1, which mediates Wnt signaling, an important cell-cell communication pathway. In addition to its known DNA binding site, we found that POP-1 recognizes additional sequences, termed Helper sites, which are essential for activation of Wnt targets. We used this knowledge to discover that Wnt signaling is active in pacemaker cells in the nematode intestine, which control defecation, a rhythmic behavior with parallels to the vertebrate heartbeat. POP-1 has a dual role in regulating Wnt targets, repressing target genes in the absence of signaling and activating them upon signal stimulation. Surprisingly, we found that Helper sites are only required for activation and not repression, and that this is also the case in the fruit fly Drosophila. This work thus reveals an unexpected complexity in POP-1 DNA binding, which is likely to be relevant for its human counterparts, which play important roles in stem cell biology and cancer.

Concentration memory-dependent synaptic plasticity of a taste circuit regulates salt concentration chemotaxis in Caenorhabditis elegans

It is poorly understood how sensory systems memorize the intensity of sensory stimulus, compare it with a newly sensed stimulus, and regulate the orientation behaviour based on the memory. Here we report that Caenorhabditis elegans memorizes the environmental salt concentration during cultivation and exhibits a strong behavioural preference for this concentration. The right-sided amphid gustatory neuron known as ASER, senses decreases in salt concentration, and this information is transmitted to the postsynaptic AIB interneurons only in the salt concentration range lower than the cultivation concentration. In this range, animals migrate towards higher concentration by promoting turning behaviour upon decreases in salt concentration. These observations provide a mechanism for adjusting the orientation behaviour based on the memory of sensory stimulus using a simple neural circuit.

QuantWorm: a comprehensive software package for Caenorhabditis elegans phenotypic assays

Phenotypic assays are crucial in genetics; however, traditional methods that rely on human observation are unsuitable for quantitative, large-scale experiments. Furthermore, there is an increasing need for comprehensive analyses of multiple phenotypes to provide multidimensional information. Here we developed an automated, high-throughput computer imaging system for quantifying multiple Caenorhabditis elegans phenotypes. Our imaging system is composed of a microscope equipped with a digital camera and a motorized stage connected to a computer running the QuantWorm software package. Currently, the software package contains one data acquisition module and four image analysis programs: WormLifespan, WormLocomotion, WormLength, and WormEgg. The data acquisition module collects images and videos. The WormLifespan software counts the number of moving worms by using two time-lapse images; the WormLocomotion software computes the velocity of moving worms; the WormLength software measures worm body size; and the WormEgg software counts the number of eggs. To evaluate the performance of our software, we compared the results of our software with manual measurements. We then demonstrated the application of the QuantWorm software in a drug assay and a genetic assay. Overall, the QuantWorm software provided accurate measurements at a high speed. Software source code, executable programs, and sample images are available at www.quantworm.org. Our software package has several advantages over current imaging systems for C. elegans. It is an all-in-one package for quantifying multiple phenotypes. The QuantWorm software is written in Java and its source code is freely available, so it does not require use of commercial software or libraries. It can be run on multiple platforms and easily customized to cope with new methods and requirements.

CAPS and Munc13 utilize distinct PIP2-linked mechanisms to promote vesicle exocytosis

Phosphoinositides provide compartment-specific signals for membrane trafficking. Plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP2) is required for Ca(2+)-triggered vesicle exocytosis, but whether vesicles fuse into PIP2-rich membrane domains in live cells and whether PIP2 is metabolized during Ca(2+)-triggered fusion were unknown. Ca(2+)-dependent activator protein in secretion 1 (CAPS-1; CADPS/UNC31) and ubMunc13-2 (UNC13B) are PIP2-binding proteins required for Ca(2+)-triggered vesicle exocytosis in neuroendocrine PC12 cells. These proteins are likely effectors for PIP2, but their localization during exocytosis had not been determined. Using total internal reflection fluorescence microscopy in live cells, we identify PIP2-rich membrane domains at sites of vesicle fusion. CAPS is found to reside on vesicles but depends on plasma membrane PIP2 for its activity. Munc13 is cytoplasmic, but Ca(2+)-dependent translocation to PIP2-rich plasma membrane domains is required for its activity. The results reveal that vesicle fusion into PIP2-rich membrane domains is facilitated by sequential PIP2-dependent activation of CAPS and PIP2-dependent recruitment of Munc13. PIP2 hydrolysis only occurs under strong Ca(2+) influx conditions sufficient to activate phospholipase Cη2 (PLCη2). Such conditions reduce CAPS activity and enhance Munc13 activity, establishing PLCη2 as a Ca(2+)-dependent modulator of exocytosis. These studies provide a direct view of the spatial distribution of PIP2 linked to vesicle exocytosis via regulation of lipid-dependent protein effectors CAPS and Munc13.

Serotonergic chemosensory neurons modify the C. elegans immune response by regulating G-protein signaling in epithelial cells

The nervous and immune systems influence each other, allowing animals to rapidly protect themselves from changes in their internal and external environment. However, the complex nature of these systems in mammals makes it difficult to determine how neuronal signaling influences the immune response. Here we show that serotonin, synthesized in Caenorhabditis elegans chemosensory neurons, modulates the immune response. Serotonin released from these cells acts, directly or indirectly, to regulate G-protein signaling in epithelial cells. Signaling in these cells is required for the immune response to infection by the natural pathogen Microbacterium nematophilum. Here we show that serotonin signaling suppresses the innate immune response and limits the rate of pathogen clearance. We show that C. elegans uses classical neurotransmitters to alter the immune response. Serotonin released from sensory neurons may function to modify the immune system in response to changes in the animal's external environment such as the availability, or quality, of food.

Betaine acts on a ligand-gated ion channel in the nervous system of the nematode C. elegans

Prior to the advent of synthetic nematocides, natural products such as seaweed were used to control nematode infestations. The nematocidal agent in seaweed is betaine, an amino acid that functions as an osmolyte and methyl donor. However, the molecular mechanisms of betaine toxicity are unknown. Here, we identify the betaine transporter SNF-3 and a betaine receptor ACR-23 in the nematode C. elegans. Mutating snf-3 in a sensitized background causes the animals to be hypercontracted and paralyzed, presumably because of excess extracellular betaine. These behavioral defects are suppressed by mutations in acr-23, which encodes a ligand-gated cation channel of the cys-loop family. ACR-23 is activated by betaine and functions in the mechanosensory neurons to maintain basal levels of locomotion. However, overactivation of the receptor by excess betaine or by the allosteric modulator monepantel causes hypercontraction and death of the nematode. Thus, monepantel targets a betaine signaling pathway in nematodes.

Extrasynaptic muscarinic acetylcholine receptors on neuronal cell bodies regulate presynaptic function in Caenorhabditis elegans

Acetylcholine (ACh) is a potent neuromodulator in the brain, and its effects on cognition and memory formation are largely performed through muscarinic acetylcholine receptors (mAChRs). mAChRs are often preferentially distributed on specialized membrane regions in neurons, but the significance of mAChR localization in modulating neuronal function is not known. Here we show that the Caenorhabditis elegans homolog of the M1/M3/M5 family of mAChRs, gar-3, is expressed in cholinergic motor neurons, and GAR-3-GFP fusion proteins localize to cell bodies where they are enriched at extrasynaptic regions that are in contact with the basal lamina. The GAR-3 N-terminal extracellular domain is necessary and sufficient for this asymmetric distribution, and mutation of a predicted N-linked glycosylation site within the N-terminus disrupts GAR-3-GFP localization. In transgenic animals expressing GAR-3 variants that are no longer asymmetrically localized, synaptic transmission at neuromuscular junctions is impaired and there is a reduction in the abundance of the presynaptic protein sphingosine kinase at release sites. Finally, GAR-3 can be activated by endogenously produced ACh released from neurons that do not directly contact cholinergic motor neurons. Together, our results suggest that humoral activation of asymmetrically localized mAChRs by ACh is an evolutionarily conserved mechanism by which ACh modulates neuronal function.

Large-scale gene knockdown in C. elegans using dsRNA feeding libraries to generate robust loss-of-function phenotypes

RNA interference by feeding worms bacteria expressing dsRNAs has been a useful tool to assess gene function in C. elegans. While this strategy works well when a small number of genes are targeted for knockdown, large scale feeding screens show variable knockdown efficiencies, which limits their utility. We have deconstructed previously published RNAi knockdown protocols and found that the primary source of the reduced knockdown can be attributed to the loss of dsRNA-encoding plasmids from the bacteria fed to the animals. Based on these observations, we have developed a dsRNA feeding protocol that greatly reduces or eliminates plasmid loss to achieve efficient, high throughput knockdown. We demonstrate that this protocol will produce robust, reproducible knock down of C. elegans genes in multiple tissue types, including neurons, and will permit efficient knockdown in large scale screens. This protocol uses a commercially available dsRNA feeding library and describes all steps needed to duplicate the library and perform dsRNA screens. The protocol does not require the use of any sophisticated equipment, and can therefore be performed by any C. elegans lab.

Reduced sleep-like quiescence in both hyperactive and hypoactive mutants of the Galphaq Gene egl-30 during lethargus in Caenorhabditis elegans

Sleep-like states are characterized by massively reduced behavioral activity. Little is known about genetic control of sleep-like behavior. It is also not clear how general activity levels during wake-like behavior influence activity levels during sleep-like behavior. Mutations that increase wake-like activity are generally believed to also increase activity during sleep-like behavior and mutations that decrease wake-like activity are believed to have decreased activity during sleep-like behavior. We studied sleep-like behavior during lethargus in larvae of Caenorhabditis elegans. We looked through a small set of known mutants with altered activity levels. As expected, mutants with increased activity levels typically showed less sleep-like behavior. Among these hyperactive mutants was a gain-of-function mutant of the conserved heterotrimeric G protein subunit Galphaq gene egl-30. We found, however, that an unusual semidominant hypoactive mutant of egl-30 also had reduced sleep-like behavior. While movement was severely reduced and impaired in the semidominant egl-30 mutant, sleep-like behavior was severely reduced: the semidominant egl-30 mutant lacked prolonged periods of complete immobility, reduced spontaneous neural activity less, and reduced responsiveness to stimulation less. egl-30 is a well-known regulator of behavior. Our results suggest that egl-30 controls not only general activity levels, but also differences between wake-like and sleep-like behavior.

Functional aging in the nervous system contributes to age-dependent motor activity decline in C. elegans

Aging is characterized by a progressive decline in multiple physiological functions (i.e., functional aging). As animals age, they exhibit a gradual loss in motor activity, but the underlying mechanisms remain unclear. Here we approach this question in C. elegans by functionally characterizing its aging nervous system and muscles. We find that motor neurons exhibit a progressive functional decline, beginning in early life. Surprisingly, body-wall muscles, which were previously thought to undergo functional aging, do not manifest such a decline until mid-late life. Notably, motor neurons first develop a deficit in synaptic vesicle fusion followed by that in quantal size and vesicle docking/priming, revealing specific functional deteriorations in synaptic transmission. Pharmacological stimulation of synaptic transmission can improve motor activity in aged animals. These results uncover a critical role for the nervous system in age-dependent motor activity decline in C. elegans and provide insights into how functional aging occurs in this organism.

TGF-β signaling in C. elegans

Transforming Growth Factor-β (TGF-β) superfamily ligands regulate many aspects of cell identity, function, and survival in multicellular animals. Genes encoding five TGF-β family members are present in the genome of C. elegans. Two of the ligands, DBL-1 and DAF-7, signal through a canonical receptor-Smad signaling pathway; while a third ligand, UNC-129, interacts with a noncanonical signaling pathway. No function has yet been associated with the remaining two ligands. Here we summarize these signaling pathways and their biological functions.

Localized sphingolipid signaling at presynaptic terminals is regulated by calcium influx and promotes recruitment of priming factors

Activity-dependent changes in presynaptic function represent a critical mechanism by which synaptic strength is controlled. However, how changes in synaptic activity couple to presynaptic components to control synaptic vesicle release and recycling are poorly understood. Sphingosine kinase (SphK) is a sphingolipid metabolic enzyme whose activity-dependent recruitment to membrane regions within presynaptic terminals promotes neurotransmitter release. Here, we show that synaptic recruitment of SPHK-1, the SphK ortholog in Caenorhabditis elegans, is mediated by presynaptic calcium influx. Quantitative fluorescence imaging of live presynaptic terminals reveals that blocking presynaptic calcium influx reduces synaptic SPHK-1 abundance whereas increasing calcium influx increases SPHK-1 synaptic abundance. CALM-1, the calcium and integrin binding protein ortholog, colocalizes with SPHK-1 at release sites and regulates muscarinic-mediated synaptic SPHK-1 recruitment. We identify two additional sphingolipid metabolic enzymes that are concentrated at presynaptic terminals, and mutants lacking one of these, HYL-1/ceramide synthase, have defects in synaptic transmission and in synaptic vesicle cycling. Finally, we show that SPHK-1 activity is required for the recruitment of the priming protein UNC-13/Munc13 to presynaptic terminals following activation by muscarinic signaling. These findings suggest that calcium-dependent regulation of local S1P metabolism at synapses may be an important mechanism by which synaptic vesicle priming factors are recruited to release sites to promote synaptic transmission.

Monoaminergic orchestration of motor programs in a complex C. elegans behavior

A single monoamine can orchestrate different phases of a compound motor sequence in C. elegans through the synaptic and extra-synaptic activation of distinct classes of receptors.

Monoamines provide chemical codes of behavioral states. However, the neural mechanisms of monoaminergic orchestration of behavior are poorly understood. Touch elicits an escape response in Caenorhabditis elegans where the animal moves backward and turns to change its direction of locomotion. We show that the tyramine receptor SER-2 acts through a Gα_o pathway to inhibit neurotransmitter release from GABAergic motor neurons that synapse onto ventral body wall muscles. Extrasynaptic activation of SER-2 facilitates ventral body wall muscle contraction, contributing to the tight ventral turn that allows the animal to navigate away from a threatening stimulus. Tyramine temporally coordinates the different phases of the escape response through the synaptic activation of the fast-acting ionotropic receptor, LGC-55, and extrasynaptic activation of the slow-acting metabotropic receptor, SER-2. Our studies show, at the level of single cells, how a sensory input recruits the action of a monoamine to change neural circuit properties and orchestrate a compound motor sequence.

How the nervous system controls complex behaviors has intrigued neurobiologists for decades. There are many examples where sequential motor patterns of specific behaviors have been described in great detail. However, the neural mechanisms that orchestrate a full behavioral sequence are poorly understood. Gentle touch to the head of the roundworm C. elegans elicits an escape response in which the animal quickly moves backward. The reversal is followed by a deep turn that allows the animal to change its direction of locomotion and move away from the threatening stimulus. We found that the neurotransmitter tyramine controls the initial reversal phase of the escape response through the activation of a fast-acting ion channel and the later turning phase through the activation of a slow-acting G-protein coupled receptor (GPCR). We show that this tyramine GPCR is expressed in neurons that make contacts with the ventral muscles of the animal. Activation of this receptor facilitates the contraction of ventral muscles and thus allows the animal to turn and resume locomotion in the opposite direction during its escape. Our studies show how a single neurotransmitter coordinates sequential phases of a complex behavior through the activation of distinct classes of receptors.

A neuronal signaling pathway of CaMKII and Gqα regulates experience-dependent transcription of tph-1

Dynamic serotonin biosynthesis is important for serotonin function; however, the mechanisms that underlie experience-dependent transcriptional regulation of the rate-limiting serotonin biosynthetic enzyme tryptophan hydroxylase (TPH) are poorly understood. Here, we characterize the molecular and cellular mechanisms that regulate increased transcription of Caenorhabditis elegans tph-1 in a pair of serotonergic neurons ADF during an aversive experience with pathogenic bacteria, a common environmental peril for worms. Training with pathogenic bacteria induces a learned aversion to the smell of the pathogen, a behavioral plasticity that depends on the serotonin signal from ADF neurons. We demonstrate that pathogen training increases ADF neuronal activity. While activating ADF increases tph-1 transcription, inhibiting ADF activity abolishes the training effect on tph-1, demonstrating the dependence of tph-1 transcriptional regulation on ADF neural activity. At the molecular level, the C. elegans homolog of CaMKII, UNC-43, functions cell-autonomously in ADF neurons to generate training-dependent enhancement in neuronal activity and tph-1 transcription, and this cell-autonomous function of UNC-43 is required for learning. Furthermore, selective expression of an activated form of UNC-43 in ADF neurons is sufficient to increase ADF activity and tph-1 transcription, mimicking the training effect. Upstream of ADF, the Gqα protein EGL-30 facilitates training-dependent induction of tph-1 by functional regulation of olfactory sensory neurons, which underscores the importance of sensory experience. Together, our work elucidates the molecular and cellular mechanisms whereby experience modulates tph-1 transcription.

Endocannabinoid-Goα signalling inhibits axon regeneration in Caenorhabditis elegans by antagonizing Gqα-PKC-JNK signalling

The ability of neurons to regenerate their axons after injury is determined by a balance between cellular pathways that promote and those that inhibit regeneration. In Caenorhabditis elegans, axon regeneration is positively regulated by the c-Jun N-terminal kinase mitogen activated protein kinase pathway, which is activated by growth factor-receptor tyrosine kinase signalling. Here we show that fatty acid amide hydrolase-1, an enzyme involved in the degradation of the endocannabinoid anandamide (arachidonoyl ethanolamide), regulates the axon regeneration response of γ-aminobutyric acid neurons after laser axotomy. Exogenous arachidonoyl ethanolamide inhibits axon regeneration via the Goα subunit GOA-1, which antagonizes the Gqα subunit EGL-30. We further demonstrate that protein kinase C functions downstream of Gqα and activates the MLK-1-MEK-1-KGB-1 c-Jun N-terminal kinase pathway by phosphorylating MLK-1. Our results show that arachidonoyl ethanolamide induction of a G protein signal transduction pathway has a role in the inhibition of post-development axon regeneration.

The F-box protein MEC-15 (FBXW9) promotes synaptic transmission in GABAergic motor neurons in C. elegans

Ubiquitination controls the activity of many proteins and has been implicated in almost every aspect of neuronal cell biology. Characterizing the precise function of ubiquitin ligases, the enzymes that catalyze ubiquitination of target proteins, is key to understanding distinct functions of ubiquitination. F-box proteins are the variable subunits of the large family of SCF ubiquitin ligases and are responsible for binding and recognizing specific ubiquitination targets. Here, we investigated the function of the F-box protein MEC-15 (FBXW9), one of a small number of F-box proteins evolutionarily conserved from C. elegans to mammals. mec-15 is widely expressed in the nervous system including GABAergic and cholinergic motor neurons. Electrophysiological and behavioral analyses indicate that GABAergic synaptic transmission is reduced in mec-15 mutants while cholinergic transmission appears normal. In the absence of MEC-15, the abundance of the synaptic vesicle protein SNB-1 (synaptobrevin) is reduced at synapses and increased in cell bodies of GABAergic motor neurons, suggesting that MEC-15 affects the trafficking of SNB-1 between cell bodies and synapses and may promote GABA release by regulating the abundance of SNB-1 at synapses.

miR-786 regulation of a fatty-acid elongase contributes to rhythmic calcium-wave initiation in C. elegans

BACKGROUND

Rhythmic behaviors are ubiquitous phenomena in animals. In C. elegans, defecation is an ultradian rhythmic behavior: every ∼50 s a calcium wave initiating in the posterior intestinal cells triggers the defecation motor program that comprises three sequential muscle contractions. Oscillatory calcium signaling is central to the periodicity of defecation. The posteriormost intestinal cells function as the pacemaker for this rhythmic behavior, although it is unclear how the supremacy of these cells for calcium-wave initiation is controlled.

RESULTS

We describe how the loss of the mir-240/786 microRNA cluster, which results in arrhythmic defecation, causes ectopic intestinal calcium-wave initiation. mir-240/786 expression in the intestine is restricted to the posterior cells that function as the defecation pacemaker. Genetic data indicate that mir-240/786 functions upstream of the inositol 1,4,5-trisphosphate (IP(3)) receptor. Through rescue analysis, it was determined that miR-786 functions to regulate defecation. Furthermore, we identified elo-2, a fatty-acid elongase with a known role in defecation cycling, as a direct target for miR-786. We propose that the regulation of palmitate levels through repression of elo-2 activity is the likely mechanistic link to defecation.

CONCLUSIONS

Together, these data indicate that miR-786 confers pacemaker status on posterior intestinal cells for the control of calcium-wave initiation through the regulation of elo-2 and, subsequently, palmitate levels. We propose that a difference in fatty-acid composition in the posterior intestinal cells may alter the activities of membrane proteins, such as IP(3)-receptor or TRPM channels, that control pacemaker activity in the C. elegans intestine.

Genetic interactions between UNC-17/VAChT and a novel transmembrane protein in Caenorhabditis elegans

The unc-17 gene encodes the vesicular acetylcholine transporter (VAChT) in Caenorhabditis elegans. unc-17 reduction-of-function mutants are small, slow growing, and uncoordinated. Several independent unc-17 alleles are associated with a glycine-to-arginine substitution (G347R), which introduces a positive charge in the ninth transmembrane domain (TMD) of UNC-17. To identify proteins that interact with UNC-17/VAChT, we screened for mutations that suppress the uncoordinated phenotype of UNC-17(G347R) mutants. We identified several dominant allele-specific suppressors, including mutations in the sup-1 locus. The sup-1 gene encodes a single-pass transmembrane protein that is expressed in a subset of neurons and in body muscles. Two independent suppressor alleles of sup-1 are associated with a glycine-to-glutamic acid substitution (G84E), resulting in a negative charge in the SUP-1 TMD. A sup-1 null mutant has no obvious deficits in cholinergic neurotransmission and does not suppress unc-17 mutant phenotypes. Bimolecular fluorescence complementation (BiFC) analysis demonstrated close association of SUP-1 and UNC-17 in synapse-rich regions of the cholinergic nervous system, including the nerve ring and dorsal nerve cords. These observations suggest that UNC-17 and SUP-1 are in close proximity at synapses. We propose that electrostatic interactions between the UNC-17(G347R) and SUP-1(G84E) TMDs alter the conformation of the mutant UNC-17 protein, thereby restoring UNC-17 function; this is similar to the interaction between UNC-17/VAChT and synaptobrevin.

G-proteins: Fighting infection on two fronts

Animals have evolved multiple strategies for coping with the presence of pathogenic microbes. The best characterized is the immune response where animals activate their physical and cellular defenses to respond to invading microorganisms. However, behavioral changes can also be triggered by exposure to microbes and play an important role in defending many species, including humans, from pathogen attack. In our recent study we demonstrate that, following infection, C. elegans uses the same G-protein signaling pathway in neurons and epithelial cells to coordinate avoidance behaviors and immune responses. Coordination of these responses allows animals to mount an immune response to the immediate threat while simultaneously taking action to remove the pathogen, however, the complicated nature of the mammalian brain and immune system has made it difficult to identify the molecular mechanisms mediating these interactions. With its simple, well described, nervous system and a rapidly growing understanding of its immune system, C. elegans has emerged as an excellent model to study the mechanisms by which animals recognize pathogens and coordinate behavioral and immune responses to infection.

Neuronally expressed Ras-family GTPase Di-Ras modulates synaptic activity in Caenorhabditis elegans

Ras-family GTPases regulate a wide variety of cellular functions including cell growth and differentiation. Di-Ras, which belongs to a distinct subfamily of Ras-family GTPases, is expressed predominantly in brain, but the role of Di-Ras in nervous systems remains totally unknown. Here, we report that the Caenorhabditis elegans Di-Ras homologue drn-1 is expressed specifically in neuronal cells and involved in synaptic function at neuromuscular junctions. Loss of function of drn-1 conferred resistance to the acetylcholinesterase inhibitor aldicarb and partially suppressed the aldicarb-hypersensitive phenotypes of heterotrimeric G-protein mutants, in which acetylcholine release is up-regulated. drn-1 mutants displayed no apparent defects in the axonal distribution of the membrane-bound second messenger diacylglycerol (DAG), which is a key stimulator of acetylcholine release. Finally, we have identified EPAC-1, a C. elegans Epac homologue, as a binding partner for DRN-1. Deletion mutants of epac-1 displayed an aldicarb-resistant phenotype as drn-1 mutants. Genetic analysis of drn-1 and epac-1 showed that they acted in the same pathway to control acetylcholine release. Furthermore, DRN-1 and EPAC-1 were co-immunoprecipitated. These findings suggest that DRN-1 may function cooperatively with EPAC-1 to modulate synaptic activity in C. elegans.

Recruitment of sphingosine kinase to presynaptic terminals by a conserved muscarinic signaling pathway promotes neurotransmitter release

Sphingolipids are potent lipid second messengers that regulate cell differentiation, migration, survival, and secretion, and alterations in sphingolipid signaling have been implicated in a variety of diseases. However, how sphingolipid levels are regulated, particularly in the nervous system, remains poorly understood. Here, we show that the generation of sphingosine-1-phosphate by sphingosine kinase (SphK) promotes neurotransmitter release. Electrophysiological, imaging, and behavioral analyses of Caenorhabditis elegans mutants lacking sphingosine kinase sphk-1 indicate that neuronal development is normal, but there is a significant defect in neurotransmitter release from neuromuscular junctions. SPHK-1 localizes to discrete, nonvesicular regions within presynaptic terminals, and this localization is critical for synaptic function. Muscarinic agonists cause a rapid increase in presynaptic SPHK-1 abundance, whereas reduction of endogenous acetylcholine production results in a rapid decrease in presynaptic SPHK-1 abundance. Muscarinic regulation of presynaptic SPHK-1 abundance is mediated by a conserved presynaptic signaling pathway composed of the muscarinic acetylcholine receptor GAR-3, the heterotrimeric G protein Gαq, and its effector, Trio RhoGEF. SPHK-1 activity is required for the effects of muscarinic signaling on synaptic transmission. This study shows that SPHK-1 promotes neurotransmitter release in vivo and identifies a novel muscarinic pathway that regulates SphK abundance at presynaptic terminals.

PKC-2 phosphorylation of UNC-18 Ser322 in AFD neurons regulates temperature dependency of locomotion

Diacylglycerol (DAG)/protein kinase C (PKC) signaling plays an integral role in the regulation of neuronal function. This is certainly true in Caenorhabditis elegans and in particular for thermosensory signaling and behavior. Downstream molecular targets for transduction of this signaling cascade remain, however, virtually uncharacterized. We investigated whether PKC phosphorylation of Munc18-1, an essential protein in vesicle trafficking and exocytosis, was the downstream effector for DAG regulation of thermosensory behavior. We demonstrate here that the C. elegans ortholog of Munc18-1, UNC-18, was phosphorylated in vitro at Ser322. Transgenic rescue of unc-18-null worms with Ser322 phosphomutants displayed altered thermosensitivity. C. elegans expresses three DAG-regulated PKCs, and blocking UNC-18 Ser322 phosphorylation was phenocopied only by deletion of calcium-activated PKC-2. Expression of nonphosphorylatable UNC-18 S322A, either pan-neuronally or specifically in AFD thermosensory neurons, converted wild-type worms to a pkc-2-null phenotype. These data demonstrate that an individual DAG-dependent thermosensory behavior of an organism is effected specifically by the downstream PKC-2 phosphorylation of UNC-18 on Ser322 in AFD neurons.

Gαo and Gαq regulate the expression of daf-7, a TGFβ-like gene, in Caenorhabditis elegans

Caenorhabditis elegans enter an alternate developmental stage called dauer in unfavorable conditions such as starvation, overcrowding, or high temperature. Several evolutionarily conserved signaling pathways control dauer formation. DAF-7/TGFβ and serotonin, important ligands in these signaling pathways, affect not only dauer formation, but also the expression of one another. The heterotrimeric G proteins GOA-1 (Gα_o) and EGL-30 (Gα_q) mediate serotonin signaling as well as serotonin biosynthesis in C. elegans. It is not known whether GOA-1 or EGL-30 also affect dauer formation and/or daf-7 expression, which are both modulated in part by serotonin. The purpose of this study is to better understand the relationship between proteins important for neuronal signaling and developmental plasticity in both C. elegans and humans. Using promoter-GFP transgenic worms, it was determined that both goa-1 and egl-30 regulate daf-7 expression during larval development. In addition, the normal daf-7 response to high temperature or starvation was altered in goa-1 and egl-30 mutants. Despite the effect of goa-1 and egl-30 mutations on daf-7 expression in various environmental conditions, there was no effect of the mutations on dauer formation. This paper provides evidence that while goa-1 and egl-30 are important for normal daf-7 expression, mutations in these genes are not sufficient to disrupt dauer formation.

Compartmentalized calcium dynamics in a C. elegans interneuron encode head movement

The confinement of neuronal activity to specific subcellular regions is a mechanism for expanding the computational properties of neurons. Although the circuit organization underlying compartmentalized activity has been studied in several systems, its cellular basis is still unknown. Here we characterize compartmentalized activity in Caenorhabditis elegans RIA interneurons, which have multiple reciprocal connections to head motor neurons and receive input from sensory pathways. We show that RIA spatially encodes head movement on a subcellular scale through axonal compartmentalization. This subcellular axonal activity is dependent on acetylcholine release from head motor neurons and is simultaneously present and additive with glutamate-dependent globally synchronized activity evoked by sensory inputs. Postsynaptically, the muscarinic acetylcholine receptor GAR-3 acts in RIA to compartmentalize axonal activity through the mobilization of intracellular calcium stores. The compartmentalized activity functions independently of the synchronized activity to modulate locomotory behaviour.