Expression of multiple UNC-13 proteins in the Caenorhabditis elegans nervous system

Low-dose naltrexone extends healthspan and lifespan in C. elegans via SKN-1 activation

As the global aging population rises, finding effective interventions to improve aging health is crucial. Drug repurposing, utilizing existing drugs for new purposes, presents a promising strategy for rapid implementation. We explored naltrexone from the Library of Integrated Network-based Cellular Signatures (LINCS) based on several selection criteria. Low-dose naltrexone (LDN) has gained attention for treating various diseases, yet its impact on longevity remains underexplored. Our study on C. elegans demonstrated that a low dose, but not high dose, of naltrexone extended the healthspan and lifespan. This effect was mediated through SKN-1 (NRF2 in mammals) signaling, influencing innate immune gene expression and upregulating oxidative stress responses. With LDN's low side effects profile, our findings underscore its potential as a geroprotector, suggesting further exploration for promoting healthy aging in humans is warranted.

Expression and function of Caenorhabditis elegans UNCP-18, a paralog of the SM protein UNC-18

Sec1/Munc18 (SM) proteins are important regulators of SNARE complex assembly during exocytosis throughout all major animal tissue types. However, expression of a founding member of the SM family, UNC-18, is mostly restricted to the nervous system of the nematode Caenorhabditis elegans, where it is important for synaptic transmission. Moreover, unc-18 null mutants do not display the lethality phenotype associated with (a) loss of all Drosophila and mouse orthologs of unc-18 and (b) with complete elimination of synaptic transmission in C. elegans. We investigated whether a previously uncharacterized unc-18 paralog, which we named uncp-18, may be able to explain the restricted expression and limited phenotypes of unc-18 null mutants. A reporter allele shows ubiquitous expression of uncp-18. Analysis of uncp-18 null mutants, unc-18 and uncp-18 double null mutants, as well as overexpression of uncp-18 in an unc-18 null mutant background, shows that these 2 genes can functionally compensate for one another and are redundantly required for embryonic viability. Our results indicate that the synaptic transmission defects of unc-18 null mutants cannot necessarily be interpreted as constituting a null phenotype for SM protein function at the synapse.

Muscle-directed mechanosensory feedback activates egg-laying circuit activity and behavior in Caenorhabditis elegans

Mechanosensory feedback of the internal reproductive state drives decisions about when and where to reproduce.1 For instance, stretch in the Drosophila reproductive tract produced by artificial distention or from accumulated eggs regulates the attraction to acetic acid to ensure optimal oviposition.2 How such mechanosensory feedback modulates neural circuits to coordinate reproductive behaviors is incompletely understood. We previously identified a stretch-dependent homeostat that regulates egg laying in Caenorhabditis elegans. Sterilized animals lacking eggs show reduced Ca2+ transient activity in the presynaptic HSN command motoneurons that drive egg-laying behavior, while animals forced to accumulate extra eggs show dramatically increased circuit activity that restores egg laying.3 Interestingly, genetic ablation or electrical silencing of the HSNs delays, but does not abolish, the onset of egg laying,3,4,5 with animals recovering vulval muscle Ca2+ transient activity upon egg accumulation.6 Using an acute gonad microinjection technique to mimic changes in pressure and stretch resulting from germline activity and egg accumulation, we find that injection rapidly stimulates Ca2+ activity in both neurons and muscles of the egg-laying circuit. Injection-induced vulval muscle Ca2+ activity requires L-type Ca2+ channels but is independent of presynaptic input. Conversely, injection-induced neural activity is disrupted in mutants lacking the vulval muscles, suggesting "bottom-up" feedback from muscles to neurons. Direct mechanical prodding activates the vulval muscles, suggesting that they are the proximal targets of the stretch-dependent stimulus. Our results show that egg-laying behavior in C. elegans is regulated by a stretch-dependent homeostat that scales postsynaptic muscle responses with egg accumulation in the uterus.

C. elegans Clarinet/CLA-1 recruits RIMB-1/RIM-binding protein and UNC-13 to orchestrate presynaptic neurotransmitter release

Synaptic transmission requires the coordinated activity of multiple synaptic proteins that are localized at the active zone (AZ). We previously identified a Caenorhabditis elegans protein named Clarinet (CLA-1) based on homology to the AZ proteins Piccolo, Rab3-interactingmolecule (RIM)/UNC-10 and Fife. At the neuromuscular junction (NMJ), cla-1 null mutants exhibit release defects that are greatly exacerbated in cla-1;unc-10 double mutants. To gain insights into the coordinated roles of CLA-1 and UNC-10, we examined the relative contributions of each to the function and organization of the AZ. Using a combination of electrophysiology, electron microscopy, and quantitative fluorescence imaging we explored the functional relationship of CLA-1 to other key AZ proteins including: RIM1, Cav2.1 channels, RIM1-binding protein, and Munc13 (C. elegans UNC-10, UNC-2, RIMB-1 and UNC-13, respectively). Our analyses show that CLA-1 acts in concert with UNC-10 to regulate UNC-2 calcium channel levels at the synapse via recruitment of RIMB-1. In addition, CLA-1 exerts a RIMB-1-independent role in the localization of the priming factor UNC-13. Thus C. elegans CLA-1/UNC-10 exhibit combinatorial effects that have overlapping design principles with other model organisms: RIM/RBP and RIM/ELKS in mouse and Fife/RIM and BRP/RBP in Drosophila. These data support a semiconserved arrangement of AZ scaffolding proteins that are necessary for the localization and activation of the fusion machinery within nanodomains for precise coupling to Ca2+ channels.

Synaptogenesis: unmasking molecular mechanisms using Caenorhabditis elegans

The nematode Caenorhabditis elegans is a research model organism particularly suited to the mechanistic understanding of synapse genesis in the nervous system. Armed with powerful genetics, knowledge of complete connectomics, and modern genomics, studies using C. elegans have unveiled multiple key regulators in the formation of a functional synapse. Importantly, many signaling networks display remarkable conservation throughout animals, underscoring the contributions of C. elegans research to advance the understanding of our brain. In this chapter, we will review up-to-date information of the contribution of C. elegans to the understanding of chemical synapses, from structure to molecules and to synaptic remodeling.

Functional Roles of UNC-13/Munc13 and UNC-18/Munc18 in Neurotransmission

Neurotransmitters are released from synaptic and secretory vesicles following calcium-triggered fusion with the plasma membrane. These exocytotic events are driven by assembly of a ternary SNARE complex between the vesicle SNARE synaptobrevin and the plasma membrane-associated SNAREs syntaxin and SNAP-25. Proteins that affect SNARE complex assembly are therefore important regulators of synaptic strength. In this chapter, we review our current understanding of the roles played by two SNARE interacting proteins: UNC-13/Munc13 and UNC-18/Munc18. We discuss results from both invertebrate and vertebrate model systems, highlighting recent advances, focusing on the current consensus on molecular mechanisms of action and nanoscale organization, and pointing out some unresolved aspects of their functions.

Robust regulatory architecture of pan-neuronal gene expression

Pan-neuronally expressed genes, such as genes involved in the synaptic vesicle cycle or in neuropeptide maturation, are critical for proper function of all neurons, but the transcriptional control mechanisms that direct such genes to all neurons of a nervous system remain poorly understood. We show here that six members of the CUT family of homeobox genes control pan-neuronal identity specification in Caenorhabditis elegans. Single CUT mutants show barely any effects on pan-neuronal gene expression or global nervous system function, but such effects become apparent and progressively worsen upon removal of additional CUT family members, indicating a critical role of gene dosage. Overexpression of each individual CUT gene rescued the phenotype of compound mutants, corroborating that gene dosage, rather than the activity of specific members of the gene family, is critical for CUT gene family function. Genome-wide binding profiles, as well as mutation of CUT homeodomain binding sites by CRISPR/Cas9 genome engineering show that CUT genes directly control the expression of pan-neuronal features. Moreover, CUT genes act in conjunction with neuron-type-specific transcription factors to control pan-neuronal gene expression. Our study, therefore, provides a previously missing key insight into how neuronal gene expression programs are specified and reveals a highly buffered and robust mechanism that controls the most critical functional features of all neuronal cell types.

Transcriptional Control of Parallel-Acting Pathways That Remove Specific Presynaptic Proteins in Remodeling Neurons

Synapses are actively dismantled to mediate circuit refinement, but the developmental pathways that regulate synaptic disassembly are largely unknown. We have previously shown that the epithelial sodium channel ENaC/UNC-8 triggers an activity-dependent mechanism that drives the removal of presynaptic proteins liprin-α/SYD-2, Synaptobrevin/SNB-1, RAB-3, and Endophilin/UNC-57 in remodeling GABAergic neurons in Caenorhabditis elegans (Miller-Fleming et al., 2016). Here, we report that the conserved transcription factor Iroquois/IRX-1 regulates UNC-8 expression as well as an additional pathway, independent of UNC-8, that functions in parallel to dismantle functional presynaptic terminals. We show that the additional IRX-1-regulated pathway is selectively required for the removal of the presynaptic proteins, Munc13/UNC-13 and ELKS, which normally mediate synaptic vesicle (SV) fusion and neurotransmitter release. Our findings are notable because they highlight the key role of transcriptional regulation in synapse elimination during development and reveal parallel-acting pathways that coordinate synaptic disassembly by removing specific active zone proteins.SIGNIFICANCE STATEMENT Synaptic pruning is a conserved feature of developing neural circuits but the mechanisms that dismantle the presynaptic apparatus are largely unknown. We have determined that synaptic disassembly is orchestrated by parallel-acting mechanisms that target distinct components of the active zone. Thus, our finding suggests that synaptic disassembly is not accomplished by en masse destruction but depends on mechanisms that dismantle the structure in an organized process.

An hourglass circuit motif transforms a motor program via subcellularly localized muscle calcium signaling and contraction

Neural control of muscle function is fundamental to animal behavior. Many muscles can generate multiple distinct behaviors. Nonetheless, individual muscle cells are generally regarded as the smallest units of motor control. We report that muscle cells can alter behavior by contracting subcellularly. We previously discovered that noxious tastes reverse the net flow of particles through the C. elegans pharynx, a neuromuscular pump, resulting in spitting. We now show that spitting results from the subcellular contraction of the anterior region of the pm3 muscle cell. Subcellularly localized calcium increases accompany this contraction. Spitting is controlled by an ‘hourglass’ circuit motif: parallel neural pathways converge onto a single motor neuron that differentially controls multiple muscles and the critical subcellular muscle compartment. We conclude that subcellular muscle units enable modulatory motor control and propose that subcellular muscle contraction is a fundamental mechanism by which neurons can reshape behavior.

A unique C2 domain at the C terminus of Munc13 promotes synaptic vesicle priming

Neurotransmitter release during synaptic transmission comprises a tightly orchestrated sequence of molecular events, and Munc13-1 is a cornerstone of the fusion machinery. A forward genetic screen for defects in neurotransmitter release in Caenorhabditis elegans identified a mutation in the Munc13-1 ortholog UNC-13 that eliminated its unique and deeply conserved C-terminal module (referred to as HC2M) containing a Ca²⁺-insensitive C2 domain flanked by membrane-binding helices. The HC2M module could be functionally replaced in vivo by protein domains that localize to synaptic vesicles but not to the plasma membrane. HC2M is broadly conserved in other Unc13 family members and is required for efficient synaptic vesicle priming. We propose that the HC2M domain evolved as a vesicle/endosome adaptor and acquired synaptic vesicle specificity in the Unc13ABC protein family.

The M domain in UNC-13 regulates the probability of neurotransmitter release

Synapses exhibit multiple forms of short-term plasticities, which have been attributed to the heterogeneity of neurotransmitter release probability. However, the molecular mechanisms that underlie the differential release states remain to be fully elucidated. The Unc-13 proteins appear to have key roles in synaptic function through multiple regulatory domains. Here, we report that deleting the M domain in Caenorhabditis elegans UNC-13MR leads to a significant increase in release probability, revealing an inhibitory function of this domain. The inhibitory effect of this domain is eliminated when the C1 and C2B domains are absent or activated, suggesting that the M domain inhibits release probability by suppressing the activity of C1 and C2B domains. When fused directly to the MUNC2C fragment of UNC-13, the M domain greatly enhances release probability. Thus, our findings reveal a mechanism by which the UNC-13 M domain regulates synaptic transmission and provides molecular insights into the regulation of release probability.

Disentangling the Roles of RIM and Munc13 in Synaptic Vesicle Localization and Neurotransmission

Efficient neurotransmitter release at the presynaptic terminal requires docking of synaptic vesicles to the active zone membrane and formation of fusion-competent synaptic vesicles near voltage-gated Ca²⁺ channels. Rab3-interacting molecule (RIM) is a critical active zone organizer, as it recruits Ca²⁺ channels and activates synaptic vesicle docking and priming via Munc13-1. However, our knowledge about Munc13-independent contributions of RIM to active zone functions is limited. To identify the functions that are solely mediated by RIM, we used genetic manipulations to control RIM and Munc13-1 activity in cultured hippocampal neurons from mice of either sex and compared synaptic ultrastructure and neurotransmission. We found that RIM modulates synaptic vesicle localization in the proximity of the active zone membrane independent of Munc13-1. In another step, both RIM and Munc13 mediate synaptic vesicle docking and priming. In addition, while the activity of both RIM and Munc13-1 is required for Ca²⁺-evoked release, RIM uniquely controls neurotransmitter release efficiency. However, activity-dependent augmentation of synaptic vesicle pool size relies exclusively on the action of Munc13s. Based on our results, we extend previous findings and propose a refined model in which RIM and Munc13-1 act in overlapping and independent stages of synaptic vesicle localization and release.

SIGNIFICANCE STATEMENT The presynaptic active zone is composed of scaffolding proteins that functionally interact to localize synaptic vesicles to release sites, ensuring neurotransmission. Our current knowledge of the presynaptic active zone function relies on structure-function analysis, which has provided detailed information on the network of interactions and the impact of active zone proteins. Yet, the hierarchical, redundant, or independent cooperation of each active zone protein to synapse functions is not fully understood. Rab3-interacting molecule and Munc13 are the two key functionally interacting active zone proteins. Here, we dissected the distinct actions of Rab3-interacting molecule and Munc13-1 from both ultrastructural and physiological aspects. Our findings provide a more detailed view of how these two presynaptic proteins orchestrate their functions to achieve synaptic transmission.

Alternative UNC13D Promoter Encodes a Functional Munc13-4 Isoform Predominantly Expressed in Lymphocytes and Platelets

Autosomal recessive mutations in genes required for cytotoxicity are causative of a life-threatening, early-onset hyperinflammatory syndrome termed familial hemophagocytic lymphohistiocytosis (FHL). Mutations in UNC13D cause FHL type 3. UNC13D encodes Munc13-4, a member of the Unc13 protein family which control SNARE complex formation and vesicle fusion. We have previously identified FHL3-associated mutations in the first intron of UNC13D which control transcription from an alternative transcriptional start site. Using isoform specific antibodies, we demonstrate that this alternative Munc13-4 isoform with a unique N-terminus is preferentially expressed in human lymphocytes and platelets, as compared to the conventional isoform that was mostly expressed in monocytes and neutrophils. The distinct N-terminal of the two isoforms did not impact on Munc13-4 localization or trafficking to the immunological synapse of cytotoxic T cells. Moreover, ectopic expression of both isoforms efficiently restored exocytosis by FHL3 patient-derived Munc13-4 deficient T cells. Thus, we demonstrate that the conventional and alternative Munc13-4 isoforms have different expression pattern in hematopoietic cell subsets, but display similar localization and contribution to T cell exocytosis. The use of an alternative transcriptional starting site (TSS) in lymphocytes and platelets could be selected for increasing the overall levels of Munc13-4 expression for efficient secretory granule release.

Ferritin is regulated by a neuro-intestinal axis in the nematode Caenorhabditis elegans

Iron is vital for the life of most organisms. However, when dysregulated, iron can catalyze the formation of oxygen (O₂) radicals that can destroy any biological molecule and thus lead to oxidative injury and death. Therefore, iron metabolism must be tightly regulated at all times, as well as coordinated with the metabolism of O₂. However, how is this achieved at the whole animal level is not well understood. Here, we explore this question using the nematode Caenorhabditis elegans. Exposure of worms to O₂ starvation conditions (i.e. hypoxia) induces a major upregulation in levels of the conserved iron-cage protein ferritin 1 (ftn-1) in the intestine, while exposure to 21% O₂ decreases ftn-1 level. This O₂-dependent inhibition is mediated by O₂-sensing neurons that communicate with the intestine through neurotransmitter and neuropeptide signalling, and requires the activity of hydroxylated HIF-1. By contrast, the induction of ftn-1 in hypoxia appears to be HIF-1-independent. This upregulation provides protection against Pseudomonas aeruginosa bacteria and oxidative injury. Taken together, our studies uncover a neuro-intestine axis that coordinates O₂ and iron responses at the whole animal level.

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The expression of ferritin 1 (ftn-1) is tightly regulated by O₂ tension.

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O₂-sensing neurons inhibit the expression of ftn-1 in the intestine at 21% O₂.

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Hydroxylated–HIF–1 inhibits the expression of ftn-1 at 21% O₂.

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ftn-1 is important for protecting against Pseudomonas aeruginosa bacteria.

Gene Cascade Finder: A tool for identification of gene cascades and its application in Caenorhabditis elegans

Obtaining a comprehensive understanding of the gene regulatory networks, or gene cascades, involved in cell fate determination and cell lineage segregation in Caenorhabditis elegans is a long-standing challenge. Although RNA-sequencing (RNA-Seq) is a promising technique to resolve these questions, the bioinformatics tools to identify associated gene cascades from RNA-Seq data remain inadequate. To overcome these limitations, we developed Gene Cascade Finder (GCF) as a novel tool for building gene cascades by comparison of mutant and wild-type RNA-Seq data along with integrated information of protein-protein interactions, expression timing, and domains. Application of GCF to RNA-Seq data confirmed that SPN-4 and MEX-3 regulate the canonical Wnt pathway during embryonic development. Moreover, lin-35, hsp-3, and gpa-12 were found to be involved in MEX-1-dependent neurogenesis, and MEX-3 was found to control the gene cascade promoting neurogenesis through lin-35 and apl-1. Thus, GCF could be a useful tool for building gene cascades from RNA-Seq data.

Intrinsic and extrinsic mechanisms of synapse formation and specificity in C. elegans

Precise neuronal wiring is critical for the function of the nervous system and is ultimately determined at the level of individual synapses. Neurons integrate various intrinsic and extrinsic cues to form synapses onto their correct targets in a stereotyped manner. In the past decades, the nervous system of nematode (Caenorhabditis elegans) has provided the genetic platform to reveal the genetic and molecular mechanisms of synapse formation and specificity. In this review, we will summarize the recent discoveries in synapse formation and specificity in C. elegans.

The presynaptic machinery at the synapse of C. elegans

Synapses are specialized contact sites that mediate information flow between neurons and their targets. Important physical interactions across the synapse are mediated by synaptic adhesion molecules. These adhesions regulate formation of synapses during development and play a role during mature synaptic function. Importantly, genes regulating synaptogenesis and axon regeneration are conserved across the animal phyla. Genetic screens in the nematode Caenorhabditis elegans have identified a number of molecules required for synapse patterning and assembly. C. elegans is able to survive even with its neuronal function severely compromised. This is in comparison with Drosophila and mice where increased complexity makes them less tolerant to impaired function. Although this fact may reflect differences in the function of the homologous proteins in the synapses between these organisms, the most likely interpretation is that many of these components are equally important, but not absolutely essential, for synaptic transmission to support the relatively undemanding life style of laboratory maintained C. elegans. Here, we review research on the major group of synaptic proteins, involved in the presynaptic machinery in C. elegans, showing a strong conservation between higher organisms and highlight how C. elegans can be used as an informative tool for dissecting synaptic components, based on a simple nervous system organization.

Vesicle release site organization at synaptic active zones

Information transfer between nerve cells (neurons) forms the basis of behavior, emotion, and survival. Signal transduction from one neuron to another occurs at synapses, and relies on both electrical and chemical signal propagation. At chemical synapses, incoming electrical action potentials trigger the release of chemical neurotransmitters that are sensed by the connected cell and here reconverted to an electrical signal. The presynaptic conversion of an electrical to a chemical signal is an energy demanding, highly regulated process that relies on a complex, evolutionarily conserved molecular machinery. Here, we review the biophysical characteristics of this process, the current knowledge of the molecules operating in this reaction and genetic specializations that may have evolved to shape inter-neuronal signaling.

The cell non-autonomous function of ATG-18 is essential for neuroendocrine regulation of Caenorhabditis elegans lifespan

Dietary restriction (DR) and reduced insulin growth factor (IGF) signaling extend lifespan in Caenorhabditis elegans and other eukaryotic organisms. Autophagy, an evolutionarily conserved lysosomal degradation pathway, has emerged as a central pathway regulated by various longevity signals including DR and IGF signaling in promoting longevity in a variety of eukaryotic organisms. However, the mechanism remains unclear. Here we show that the autophagy protein ATG-18 acts cell non-autonomously in neuronal and intestinal tissues to maintain C. elegans wildtype lifespan and to respond to DR and IGF-mediated longevity signaling. Moreover, ATG-18 activity in chemosensory neurons that are involved in food detection sufficiently mediates the effect of these longevity pathways. Additionally, ATG-18-mediated cell non-autonomous signaling depends on the release of neurotransmitters and neuropeptides. Interestingly, our data suggest that neuronal and intestinal ATG-18 acts in parallel and converges on unidentified neurons that secrete neuropeptides to regulate C. elegans lifespan through the transcription factor DAF-16/FOXO in response to reduced IGF signaling.

ELKS1 localizes the synaptic vesicle priming protein bMunc13-2 to a specific subset of active zones

Presynaptic active zones (AZs) are unique subcellular structures at neuronal synapses, which contain a network of specific proteins that control synaptic vesicle (SV) tethering, priming, and fusion. Munc13s are core AZ proteins with an essential function in SV priming. In hippocampal neurons, two different Munc13s-Munc13-1 and bMunc13-2-mediate opposite forms of presynaptic short-term plasticity and thus differentially affect neuronal network characteristics. We found that most presynapses of cortical and hippocampal neurons contain only Munc13-1, whereas ∼10% contain both Munc13-1 and bMunc13-2. Whereas the presynaptic recruitment and activation of Munc13-1 depends on Rab3-interacting proteins (RIMs), we demonstrate here that bMunc13-2 is recruited to synapses by the AZ protein ELKS1, but not ELKS2, and that this recruitment determines basal SV priming and short-term plasticity. Thus, synapse-specific interactions of different Munc13 isoforms with ELKS1 or RIMs are key determinants of the molecular and functional heterogeneity of presynaptic AZs.

DLK-1, SEK-3 and PMK-3 Are Required for the Life Extension Induced by Mitochondrial Bioenergetic Disruption in C. elegans

Mitochondrial dysfunction underlies numerous age-related pathologies. In an effort to uncover how the detrimental effects of mitochondrial dysfunction might be alleviated, we examined how the nematode C. elegans not only adapts to disruption of the mitochondrial electron transport chain, but in many instances responds with extended lifespan. Studies have shown various retrograde responses are activated in these animals, including the well-studied ATFS-1-dependent mitochondrial unfolded protein response (UPRmt). Such processes fall under the greater rubric of cellular surveillance mechanisms. Here we identify a novel p38 signaling cascade that is required to extend life when the mitochondrial electron transport chain is disrupted in worms, and which is blocked by disruption of the Mitochondrial-associated Degradation (MAD) pathway. This novel cascade is defined by DLK-1 (MAP3K), SEK-3 (MAP2K), PMK-3 (MAPK) and the reporter gene Ptbb-6::GFP. Inhibition of known mitochondrial retrograde responses does not alter induction of Ptbb-6::GFP, instead induction of this reporter often occurs in counterpoint to activation of SKN-1, which we show is under the control of ATFS-1. In those mitochondrial bioenergetic mutants which activate Ptbb-6::GFP, we find that dlk-1, sek-3 and pmk-3 are all required for their life extension.

Pharyngeal pumping in Caenorhabditis elegans depends on tonic and phasic signaling from the nervous system

Rhythmic movements are ubiquitous in animal locomotion, feeding, and circulatory systems. In some systems, the muscle itself generates rhythmic contractions. In others, rhythms are generated by the nervous system or by interactions between the nervous system and muscles. In the nematode Caenorhabditis elegans, feeding occurs via rhythmic contractions (pumping) of the pharynx, a neuromuscular feeding organ. Here, we use pharmacology, optogenetics, genetics, and electrophysiology to investigate the roles of the nervous system and muscle in generating pharyngeal pumping. Hyperpolarization of the nervous system using a histamine-gated chloride channel abolishes pumping, and optogenetic stimulation of pharyngeal muscle in these animals causes abnormal contractions, demonstrating that normal pumping requires nervous system function. In mutants that pump slowly due to defective nervous system function, tonic muscle stimulation causes rapid pumping, suggesting tonic neurotransmitter release may regulate pumping. However, tonic cholinergic motor neuron stimulation, but not tonic muscle stimulation, triggers pumps that electrophysiologically resemble typical rapid pumps. This suggests that pharyngeal cholinergic motor neurons are normally rhythmically, and not tonically active. These results demonstrate that the pharynx generates a myogenic rhythm in the presence of tonically released acetylcholine, and suggest that the pharyngeal nervous system entrains contraction rate and timing through phasic neurotransmitter release.

Multiple Sensory Inputs Are Extensively Integrated to Modulate Nociception in C. elegans

Sensory inputs are integrated extensively before decision making, with altered multisensory integration being associated with disorders such as autism. We demonstrate that the two C. elegans AIB interneurons function as a biphasic switch, integrating antagonistic, tonic, and acute inputs from three distinct pairs of sensory neurons to modulate nociception. Off food, animals reverse away from a noxious stimulus. In contrast, on food or serotonin, AIB signaling is inhibited and, although animals initiate an aversive response more rapidly, they continue forward after the initial backward locomotion is complete. That is, animals continue to move forward and feed even when presented with a noxious repellant, with AIB inhibition decreasing the repellant concentration evoking a maximal response. These studies demonstrate that the AIBs serve as an integrating hub, receiving inputs from different sensory neurons to modulate locomotory decision making differentially, and highlight the utility of this model to analyze the complexities of multisensory integration.

SIGNIFICANCE STATEMENT

Dysfunctional sensory signaling and perception are associated with a number of disease states, including autism spectrum disorders, schizophrenia, and anxiety. We have used the C. elegans model to examine multisensory integration at the interneuron level to better understand the modulation of this complex, multicomponent process. C. elegans responds to a repulsive odorant by first backing up and then either continuing forward or turning and moving away from the odorant. This decision-making process is modulated extensively by the activity state of the two AIB interneurons, with the AIBs integrating an array of synergistic and antagonistic glutamatergic inputs, from sensory neurons responding directly to the odorant to others responding to a host of additional environmental variables to ultimately fine tune aversive behaviors.

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.

A novel nondevelopmental role of the sax-7/L1CAM cell adhesion molecule in synaptic regulation in Caenorhabditis elegans

The L1CAM family of cell adhesion molecules is a conserved set of single-pass transmembrane proteins that play diverse roles required for proper nervous system development and function. Mutations in L1CAMs can cause the neurological L1 syndrome and are associated with autism and neuropsychiatric disorders. L1CAM expression in the mature nervous system suggests additional functions besides the well-characterized developmental roles. In this study, we demonstrate that the gene encoding the Caenorhabditis elegans L1CAM, sax-7, genetically interacts with gtl-2, as well as with unc-13 and rab-3, genes that function in neurotransmission. These sax-7 genetic interactions result in synthetic phenotypes that are consistent with abnormal synaptic function. Using an inducible sax-7 expression system and pharmacological reagents that interfere with cholinergic transmission, we uncovered a previously uncharacterized nondevelopmental role for sax-7 that impinges on synaptic function.

Regulation of synaptic nlg-1/neuroligin abundance by the skn-1/Nrf stress response pathway protects against oxidative stress

The Nrf family of transcription factors mediates adaptive responses to stress and longevity, but the identities of the crucial Nrf targets, and the tissues in which they function in multicellular organisms to promote survival, are not known. Here, we use whole transcriptome RNA sequencing to identify 810 genes whose expression is controlled by the SKN-1/Nrf2 negative regulator WDR-23 in the nervous system of Caenorhabditis elegans. Among the genes identified is the synaptic cell adhesion molecule nlg-1/neuroligin. We find that the synaptic abundance of NLG-1 protein increases following pharmacological treatments that generate oxidative stress or by the genetic activation of skn-1. Increasing nlg-1 dosage correlates with increased survival in response to oxidative stress, whereas genetic inactivation of nlg-1 reduces survival and impairs skn-1-mediated stress resistance. We identify a canonical SKN-1 binding site in the nlg-1 promoter that binds to SKN-1 in vitro and is necessary for SKN-1 and toxin-mediated increases in nlg-1 expression in vivo. Together, our results suggest that SKN-1 activation in the nervous system can confer protection to organisms in response to stress by directly regulating nlg-1/neuroligin expression.

Position of UNC-13 in the active zone regulates synaptic vesicle release probability and release kinetics

The presynaptic active zone proteins UNC-13/Munc13s are essential for synaptic vesicle (SV) exocytosis by directly interacting with SV fusion apparatus. An open question is how their association with active zones, hence their position to Ca²⁺ entry sites, regulates SV release. The N-termini of major UNC-13/Munc13 isoforms contain a non-calcium binding C₂A domain that mediates protein homo- or hetero-meric interactions. Here, we show that the C₂A domain of Caenorhabditis elegans UNC-13 regulates release probability of evoked release and its precise active zone localization. Kinetics analysis of SV release supports that the proximity of UNC-13 to Ca²⁺ entry sites, mediated by the C₂A-domain containing N-terminus, is critical for accelerating neurotransmitter release. Additionally, the C₂A domain is specifically required for spontaneous release. These data reveal multiple roles of UNC-13 C₂A domain, and suggest that spontaneous release and the fast phase of evoked release may involve a common pool of SVs at the active zone.

DOI:http://dx.doi.org/10.7554/eLife.01180.001

Neurons are connected to each other by junctions called synapses. When an electrical signal travelling along a neuron arrives at a synapse, it causes the release of bubble-like structures called synaptic vesicles that contain chemicals called neurotransmitters. When released by the vesicles these neurotransmitters bind to receptors on a second neuron and allow the signal to continue on its way through the nervous system.

The release of synaptic vesicles from the neuron depends largely on the number of calcium ions that enter this neuron via structures called ion channels, and also on the rate at which they enter. Vesicles are released in one of three ways: they can be released quickly (within a few milliseconds) in response to the influx of calcium ions; they can be released slowly (over a period of tens or hundreds of milliseconds) in response to the influx; or they can be released at random times that are not related to the influx.

It is known that the sensitivity of certain calcium sensors near the synapse influences the release of the vesicles. It had been thought that the distance between the “active zone” where the calcium ions enter the neuron and the region where the vesicles reside might also influence rate of release, but the molecular mechanism underlying this hypothesis is poorly understood.

Zhou et al. have now shed new light on this question by performing a series of experiments that involved manipulating a protein called UNC-13 – which is known to be involved in the release of vesicles – in neurons from C. elegans, a nematode worm. First it was shown that the precise position of UNC-13 in the active zone depended on a domain within the protein called the C₂A domain. Next it was shown that the distance between the UNC-13 protein and the calcium ion channels strongly influences the quick mode of vesicle release. Finally, Zhou et al. showed that the C₂A domain also had a significant influence on the spontaneous release of vesicles, which suggests that a common fleet of vesicles might be used for both the quick and the spontaneous modes of vesicle release. Zhou et al. also generated mutant worms that mimicked a neurological disease, epileptic seizure, and showed that eliminating the C₂A domain can relieve some of the symptoms associated with the disease.

Many neurological diseases are caused by signals not being transmitted properly at synapses, so in addition to providing insights into the basic mechanism underlying synaptic action, these results could also assist with the development of new strategies for managing neurological diseases.

DOI:http://dx.doi.org/10.7554/eLife.01180.002

Regulation of organismal proteostasis by transcellular chaperone signaling

A major challenge for metazoans is to ensure that different tissues, each expressing distinctive proteomes, are nevertheless well protected at an organismal level from proteotoxic stress. We show that expression of endogenous metastable proteins in muscle cells, which rely on chaperones for proper folding, induces a systemic stress response throughout multiple tissues of C. elegans. Suppression of misfolding in muscle cells can be achieved not only by enhanced expression of HSP90 in muscle cells but as effectively by elevated expression of HSP90 in intestine or neuronal cells. This cell-nonautonomous control of HSP90 expression relies upon transcriptional feedback between somatic tissues that is regulated by the FoxA transcription factor PHA-4. This transcellular chaperone signaling response maintains organismal proteostasis when challenged by a local tissue imbalance in folding and provides the basis for organismal stress-sensing surveillance.

UNC-13L, UNC-13S, and Tomosyn form a protein code for fast and slow neurotransmitter release in Caenorhabditis elegans

Synaptic transmission consists of fast and slow components of neurotransmitter release. Here we show that these components are mediated by distinct exocytic proteins. The Caenorhabditis elegans unc-13 gene is required for SV exocytosis, and encodes long and short isoforms (UNC-13L and S). Fast release was mediated by UNC-13L, whereas slow release required both UNC-13 proteins and was inhibited by Tomosyn. The spatial location of each protein correlated with its effect. Proteins adjacent to the dense projection mediated fast release, while those controlling slow release were more distal or diffuse. Two UNC-13L domains accelerated release. C2A, which binds RIM (a protein associated with calcium channels), anchored UNC-13 at active zones and shortened the latency of release. A calmodulin binding site accelerated release but had little effect on UNC-13’s spatial localization. These results suggest that UNC-13L, UNC-13S, and Tomosyn form a molecular code that dictates the timing of neurotransmitter release.

DOI:http://dx.doi.org/10.7554/eLife.00967.001

Neurons communicate with one another at junctions called synapses. When an electrical signal known as an action potential travels along a neuron and arrives at a synapse, the neuron releases a package of transmitter chemicals into the synapse. These chemicals then diffuse across the gap and bind to receptors on a second neuron, conveying the signal to the target neuron.

The strength of a synapse depends in part on the number of packages, or vesicles, of transmitter chemicals that are available for release. Most synapses contain multiple populations of vesicles: some that are released within a few milliseconds of the arrival of an action potential, and others that are released more slowly. The vesicles that are released rapidly are found close to sites at which calcium ions enter the neuron, whereas the others are located further from these sites. However, little is known about the molecular basis of the differences between fast and slow vesicle release.

Now Hu et al. have studied the proteins involved in these two processes in C. elegans, a nematode worm that is often used in neuroscience because it has a simple nervous system, consisting of just 302 neurons, and a well-characterized genome. Hu et al. showed that the release of synaptic vesicles at the neuromuscular junction between neurons and muscles in C. elegans also has slow and fast components. A long form of UNC-13, which is also found in mammals, promotes fast release of transmitter vesicles. Slow release is mediated by an independent pathway that involves both long and short UNC-13 proteins, as well as a protein called Tomosyn. As in mammals, long UNC-13 is localized to the sites at which calcium ions enter neurons, whereas short UNC-13 is more widely distributed throughout neurons.

The work of Hu et al. provides a molecular explanation for how the timing of transmitter release is determined. Because the UNC-13 and Tomosyn proteins are evolutionarily conserved, this mechanism is likely to be present in other animals too.

DOI:http://dx.doi.org/10.7554/eLife.00967.002

Neuropeptide gene families in Caenorhabditis elegans

Neuropeptides are short sequences ofamino acids that function in all multicellular organisms to communicate information between cells. The first sequence ofa neuropeptide was reported in 1970' and the number of identified neuropeptides remained relatively small until the 1990s when the DNA sequence of multiple genomes revealed treasure troves ofinformation. Byblasting away at the genome, gene families, the sizes ofwhich were previously unknown, could now be determined. This information has led to an exponential increase in the number of putative neuropeptides and their respective gene families. The molecular biology age greatly benefited the neuropeptide field in the nematode Caenorhabditis elegans. Its genome was among the first to be sequenced and this allowed us the opportunity to screen the genome for neuropeptide genes. Initially, the screeningwas slow, as the Genefinder and BLAST programs had difficulty identifying small genes and peptides. However, as the bioinformatics programs improved, the extent of the neuropeptide gene families in C. elegans gradually emerged.

Endophilin functions as a membrane-bending molecule and is delivered to endocytic zones by exocytosis

Two models have been proposed for endophilin function in synaptic vesicle (SV) endocytosis. The scaffolding model proposes that endophilin's SH3 domain recruits essential endocytic proteins, whereas the membrane-bending model proposes that the BAR domain induces positively curved membranes. We show that mutations disrupting the scaffolding function do not impair endocytosis, whereas those disrupting membrane bending cause significant defects. By anchoring endophilin to the plasma membrane, we show that endophilin acts prior to scission to promote endocytosis. Despite acting at the plasma membrane, the majority of endophilin is targeted to the SV pool. Photoactivation studies suggest that the soluble pool of endophilin at synapses is provided by unbinding from the adjacent SV pool and that the unbinding rate is regulated by exocytosis. Thus, endophilin participates in an association-dissociation cycle with SVs that parallels the cycle of exo- and endocytosis. This endophilin cycle may provide a mechanism for functionally coupling endocytosis and exocytosis.

Research progress on the age-related changes in proteins of the synaptic active zone

Neurotransmitter release during synaptic transmission is mediated by the presynaptic active zone. Multiple protein components at the active zone region interact to regulate docking, priming and fusion of the synaptic vesicles with the presynaptic membrane to maintain normal neurotransmitter release. This review discusses research progress in questions of protein transcript and expression pattern changes at the synaptic active zone related to aging and whether these changes have the effects on learning and memory. We will specifically address normal synaptic structure and proteins; active zone structure and components; active zone functional regulation and age-related changes in active zone proteins.

The microRNA miR-124 controls gene expression in the sensory nervous system of Caenorhabditis elegans

miR-124 is a highly conserved microRNA (miRNA) whose in vivo function is poorly understood. Here, we identify miR-124 targets based on the analysis of the first mir-124 mutant in any organism. We find that miR-124 is expressed in many sensory neurons in Caenorhabditis elegans and onset of expression coincides with neuronal morphogenesis. We analyzed the transcriptome of miR-124 expressing and nonexpressing cells from wild-type and mir-124 mutants. We observe that many targets are co-expressed with and actively repressed by miR-124. These targets are expressed at reduced relative levels in sensory neurons compared to the rest of the animal. Our data from mir-124 mutant animals show that this effect is due to a large extent to the activity of miR-124. Genes with nonconserved target sites show reduced absolute expression levels in sensory neurons. In contrast, absolute expression levels of genes with conserved sites are comparable to control genes, suggesting a tuning function for many of these targets. We conclude that miR-124 contributes to defining cell-type-specific gene activity by repressing a diverse set of co-expressed genes.

Graded synaptic transmission at the Caenorhabditis elegans neuromuscular junction

Most neurotransmission is mediated by action potentials, whereas sensory neurons propagate electrical signals passively and release neurotransmitter in a graded manner. Here, we demonstrate that Caenorhabditis elegans neuromuscular junctions release neurotransmitter in a graded fashion. When motor neurons were depolarized by light-activation of channelrhodopsin-2, the evoked postsynaptic current scaled with the strength of the stimulation. When motor neurons were hyperpolarized by light-activation of halorhodopsin, tonic release of synaptic vesicles was decreased. These data suggest that both evoked and tonic neurotransmitter release is graded in response to membrane potential. Acetylcholine synapses are depressed by high-frequency stimulation, in part due to desensitization of the nicotine-sensitve ACR-16 receptor. By contrast, GABA synapses facilitate before becoming depressed. Graded transmission and plasticity confer a broad dynamic range to these synapses. Graded release precisely transmits stimulation intensity, even hyperpolarizing inputs. Synaptic plasticity alters the balance of excitatory and inhibitory inputs into the muscle in a use-dependent manner.

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.

CAPS and syntaxin dock dense core vesicles to the plasma membrane in neurons

Docking to the plasma membrane prepares vesicles for rapid release. Here, we describe a mechanism for dense core vesicle docking in neurons. In Caenorhabditis elegans motor neurons, dense core vesicles dock at the plasma membrane but are excluded from active zones at synapses. We have found that the calcium-activated protein for secretion (CAPS) protein is required for dense core vesicle docking but not synaptic vesicle docking. In contrast, we see that UNC-13, a docking factor for synaptic vesicles, is not essential for dense core vesicle docking. Both the CAPS and UNC-13 docking pathways converge on syntaxin, a component of the SNARE (soluble N-ethyl-maleimide-sensitive fusion protein attachment receptor) complex. Overexpression of open syntaxin can bypass the requirement for CAPS in dense core vesicle docking. Thus, CAPS likely promotes the open state of syntaxin, which then docks dense core vesicles. CAPS function in dense core vesicle docking parallels UNC-13 in synaptic vesicle docking, which suggests that these related proteins act similarly to promote docking of independent vesicle populations.

An evolutionarily conserved presynaptic protein is required for isoflurane sensitivity in Caenorhabditis elegans

BACKGROUND

Volatile general anesthetics inhibit neurotransmitter release by an unknown mechanism. A mutation in the presynaptic soluble NSF attachment protein receptor (SNARE) protein syntaxin 1A was previously shown to antagonize the anesthetic isoflurane in Caenorhabditis elegans. The mechanism underlying this antagonism may identify presynaptic anesthetic targets relevant to human anesthesia.

METHODS

Sensitivity to isoflurane concentrations in the human clinical range was measured in locomotion assays on adult C. elegans. Sensitivity to the acetylcholinesterase inhibitor aldicarb was used as an assay for the global level of C. elegans neurotransmitter release. Comparisons of isoflurane sensitivity (measured by the EC50) were made by simultaneous curve fitting and F test as described by Waud.

RESULTS

Expression of a truncated syntaxin fragment (residues 1-106) antagonized isoflurane sensitivity in C. elegans. This portion of syntaxin interacts with the presynaptic protein UNC-13, suggesting the hypothesis that truncated syntaxin binds to UNC-13 and antagonizes an inhibitory effect of isoflurane on UNC-13 function. Consistent with this hypothesis, overexpression of UNC-13 suppressed the isoflurane resistance of the truncated syntaxins, and unc-13 loss-of-function mutants were highly isoflurane resistant. Normal anesthetic sensitivity was restored by full-length UNC-13, by a shortened form of UNC-13 lacking a C2 domain, but not by a membrane-targeted UNC-13 that might bypass isoflurane inhibition of membrane translocation of UNC-13. Isoflurane was found to inhibit synaptic localization of UNC-13.

CONCLUSIONS

These data show that UNC-13, an evolutionarily conserved protein that promotes neurotransmitter release, is necessary for isoflurane sensitivity in C. elegans and suggest that its vertebrate homologs may be a component of the general anesthetic mechanism.

UNC-31 (CAPS) is required for dense-core vesicle but not synaptic vesicle exocytosis in Caenorhabditis elegans

Previous studies indicated that CAPS (calcium-dependent activator protein for secretion) functions as an essential component for the Ca2+-dependent exocytosis of dense-core vesicles in neuroendocrine cells. However, recent mouse knock-out studies suggested an alternative role in the vesicular uptake or storage of catecholamines. To genetically assess the functional role of CAPS, we characterized the sole Caenorhabditis elegans CAPS ortholog UNC-31 (uncoordinated family member) and determined its role in dense-core vesicle-mediated peptide secretion and in synaptic vesicle recycling. Novel assays for dense-core vesicle exocytosis were developed by expressing a prepro-atrial natriuretic factor-green fluorescent protein fusion protein in C. elegans. unc-31 mutants exhibited reduced peptide release in vivo and lacked evoked peptide release in cultured neurons. In contrast, cultured neurons from unc-31 mutants exhibited normal stimulated synaptic vesicle recycling measured by FM4-64 [N-(3-triethylammoniumpropyl)-4-(6-(4-diethylamino)phenyl)hexatrienyl)pyridinium dibromide] dye uptake. Conversely, UNC-13, which exhibits sequence homology to CAPS/UNC-31, was found to be essential for synaptic vesicle but not dense-core vesicle exocytosis. These findings indicate that CAPS/UNC-31 function is not restricted to catecholaminergic vesicles but is generally required for and specific to dense-core vesicle exocytosis. Our results suggest that CAPS/UNC-31 and UNC-13 serve parallel and dedicated roles in dense-core vesicle and synaptic vesicle exocytosis, respectively, in the C. elegans nervous system.

Synaptic neurotransmission protein UNC-13 affects RNA interference in neurons

Caenorhabditis elegans UNC-13 is an integral component of the synaptic vesicle cycle, functioning in the priming step. A recent yeast two-hybrid screen against UNC-13 identified three interacting proteins that are thought to function in pathways other than neurotransmitter release. One such protein, ERI-1, negatively regulates exogenous RNA interference in the nervous system and other tissues. This study investigates a role for UNC-13 in RNAi through analysis of RNAi penetrance in unc-13 and eri-1 mutant strains. Feeding these strains double stranded RNA corresponding to a neuronally expressed GFP reporter resulted in a significant reduction of GFP in double mutants compared to GFP expression in eri-1 mutants, indicating that UNC-13 functions in conjunction with ERI-1 in RNAi. There is no evidence for altered neurotransmission in eri-1 mutants.

UNC-13 and UNC-10/rim localize synaptic vesicles to specific membrane domains

Synaptic vesicles undergo a maturation step, termed priming, in which they become competent to fuse with the plasma membrane. To morphologically define the site of vesicle priming and identify fusion-competent synaptic vesicles, we combined a rapid physical-fixation technique with immunogold staining and high-resolution morphometric analysis at Caenorhabditis elegans neuromuscular junctions. In these presynaptic terminals, a subset of synaptic vesicles contact the plasma membrane within approximately 100 nm of a presynaptic dense projection. UNC-13, a protein required for vesicle priming, localizes to this same region of the plasma membrane. In an unc-13 null mutant, few synaptic vesicles contact the plasma membrane, suggesting that membrane-contacting synaptic vesicles represent the morphological correlates of primed vesicles. Interestingly, a subpopulation of membrane-contacting vesicles, located within 30 nm of a dense projection, are unperturbed in unc-13 mutants. We show that UNC-10/Rim, a protein implicated in presynaptic plasticity, localizes to dense projections and that loss of UNC-10/Rim causes an UNC-13-independent reduction in membrane-contacting synaptic vesicles within 30 nm of the dense projections. Our data together identify a discrete domain for vesicle priming within 100 nm of dense projections and further suggest that UNC-10/Rim and UNC-13 separately contribute to the membrane localization of synaptic vesicles within this domain.

The genetics of synapse formation and function in Caenorhabditis elegans

The aim of this review is to introduce the reader to Caenorhabditis elegans as a model system, especially with respect to studies of synapse formation and function. We begin by giving a short description of the structure of the nervous system of C. elegans. As most of the findings that are reviewed here have emerged from studies of neuromuscular junctions (NMJs), two prominent NMJs of C. elegans will be outlined briefly. In addition, we summarize new findings that have added to our understanding of NMJs during the last few years.

Antagonistic Regulation of Synaptic Vesicle Priming by Tomosyn and UNC-13

Priming of synaptic vesicles (SVs) is essential for synaptic transmission. UNC-13 proteins are required for priming. Current models propose that UNC-13 stabilizes the open conformation of Syntaxin, in which the SNARE helix is available for interactions with Synaptobrevin and SNAP-25. Here we show that Tomosyn inhibits SV priming. Tomosyn contains a SNARE motif, which forms an inhibitory SNARE complex with Syntaxin and SNAP-25. Mutants lacking Tomosyn have increased synaptic transmission, an increased pool of primed vesicles, and increased abundance of UNC-13 at synapses. Behavioral, imaging, and electrophysiological studies suggest that SV priming was reconstituted in unc-13 mutants by expressing a constitutively open mutant Syntaxin, or by mutations eliminating Tomosyn. Thus, priming is modulated by the balance between Tomosyn and UNC-13, perhaps by regulating the availability of open-Syntaxin. Even when priming was restored, synaptic transmission remained defective in unc-13 mutants, suggesting that UNC-13 is also required for other aspects of secretion.

Molecular organization of the presynaptic active zone

The exocytosis of neurotransmitter-filled synaptic vesicles is under tight temporal and spatial control in presynaptic nerve terminals. The fusion of synaptic vesicles is restricted to a specialized area of the presynaptic plasma membrane: the active zone. The protein network that constitutes the cytomatrix at the active zone (CAZ) is involved in the organization of docking and priming of synaptic vesicles and in mediating use-dependent changes in release during short-term and long-term synaptic plasticity. To date, five protein families whose members are highly enriched at active zones (Munc13s, RIMs, ELKS proteins, Piccolo and Bassoon, and the liprins-alpha), have been characterized. These multidomain proteins are instrumental for the diverse functions performed by the presynaptic active zone.

The Dunce cAMP phosphodiesterase PDE-4 negatively regulates G alpha(s)-dependent and G alpha(s)-independent cAMP pools in the Caenorhabditis elegans synaptic signaling network

Forward genetic screens for mutations that rescue the paralysis of ric-8 (Synembryn) reduction-of-function mutations frequently reveal mutations that cause hyperactivation of one or more components of the G alpha(s) pathway. Here, we report that one of these mutations strongly reduces the function of the Dunce cAMP phosphodiesterase PDE-4 by disrupting a conserved active site residue. Loss of function and neural overexpression of PDE-4 have profound and opposite effects on locomotion rate, but drug-response assays suggest that loss of PDE-4 function does not affect steady-state acetylcholine release or reception. Our genetic analysis suggests that PDE-4 regulates both G alpha(s)-dependent and G alpha(s)-independent cAMP pools in the neurons controlling locomotion rate. By immunostaining, PDE-4 is strongly expressed throughout the nervous system, where it localizes to small regions at the outside boundaries of synaptic vesicle clusters as well as intersynaptic regions. The synaptic subregions containing PDE-4 are distinct from those containing active zones, as indicated by costaining with an antibody against the long form of UNC-13. This highly focal subsynaptic localization suggests that PDE-4 may exert its effects by spatially regulating intrasynaptic cAMP pools.

Redundant localization mechanisms of RIM and ELKS in Caenorhabditis elegans

Active zone proteins play a fundamental role in regulating neurotransmitter release and defining release sites. The functional roles of active zone components are beginning to be elucidated; however, the mechanisms of active zone protein localization are unknown. Studies have shown that glutamine, leucine, lysine, and serine-rich protein (ELKS), a recently defined member of the active zone complex, acts to localize the active zone protein Rab3a-interacting molecule (RIM) and regulates synaptic transmission in cultured neurons. Here, we test the function of ELKS in vivo. Like mammalian ELKS, Caenorhabditis elegans ELKS is an active zone protein that directly interacts with the postsynaptic density-25/Discs large/zona occludens (PDZ) domain of RIM. However, RIM protein localizes in the absence of ELKS and vice versa. In addition, elks mutants exhibit neither the behavioral nor the physiological defects associated with unc-10 RIM mutants, indicating that ELKS is not a critical component of the C. elegans release machinery. Interestingly, expression of the soluble PDZ domain of RIM disrupts ELKS active zone targeting, suggesting a tight association between the two proteins in vivo. RIM truncations containing only the PDZ and C2A domains target to release sites in an ELKS-dependent manner. Together, these data identify ELKS as a new member of the C. elegans active zone complex, define the role of ELKS in synaptic transmission, and characterize the relationship between ELKS and RIM in vivo. Furthermore, they demonstrate that multiple different protein-protein interactions redundantly anchor both ELKS and RIM to active zones and implicate novel proteins in the formation of the active zone.

Overlapping and distinct functions for a Caenorhabditis elegans SIR2 and DAF-16/FOXO

The conserved SIR2 protein regulates life span in both yeast and worms: in both organisms overexpression of SIR2 can extend life span and in Caenorhabditis elegans this life span extension is dependent on the forkhead transcription factor, DAF-16. Here, we have done extensive genetic analysis with sir-2.1(ok434), a null mutant of C. elegans sir-2.1, the closest homolog to yeast Sir2p and human SIRT1 to further elucidate its function in life span regulation. sir-2.1(ok434) mutants show a slight decrease in life span as well as sensitivity to various stresses. Our genetic analysis suggests that sir-2.1 is required for life span extension by caloric restriction, independent of the insulin/IGF-1 signaling pathway. Importantly, analysis with unc-13 mutants indicates that sir-2.1 and daf-16 have overlapping and distinct roles in life span regulation. Our expression analysis shows that sir-2.1 has overlapping and distinct expression pattern compared with daf-16, consistent with the results from our genetic data. Our data defines a central role for C. elegans SIR2 in regulation of life span by caloric restriction and demonstrates that sir-2.1 and daf-16 have both overlapping and distinct functions in regulation of C. elegans life span.

UNC-13 Interaction with Syntaxin Is Required for Synaptic Transmission

Neurotransmitter secretion at synapses is controlled by several processes-morphological docking of vesicles at release sites, priming of docked vesicles to make them fusion competent, and calcium-dependent fusion of vesicles with the plasma membrane . In worms, flies, and mice, mutants lacking UNC-13 have defects in vesicle priming . Current models propose that UNC-13 primes vesicles by stabilizing Syntaxin's "open" conformation by directly interacting with its amino-terminal regulatory domain . However, the functional significance of the UNC-13/Syntaxin interaction has not been tested directly. A truncated protein containing the Munc homology domains (MHD1 and MHD2) and the carboxy-terminal C2 domain partially rescued both the behavioral and secretion defects of unc-13 mutants in C. elegans. A double mutation in MHD2 (F1000A/K1002A) disrupts the UNC-13/Syntaxin interaction. The rate of endogenous synaptic events and the amplitude of nerve-evoked excitatory post-synaptic currents (EPSCs) were both significantly reduced in UNC-13S(F1000A/K1002A). However, the pool of primed (i.e., fusion-competent) vesicles was normal. These results suggest that the UNC-13/Syntaxin interaction is conserved in C. elegans and that, contrary to current models, the UNC-13/Syntaxin interaction is required for nerve-evoked vesicle fusion rather than synaptic-vesicle priming. Thus, UNC-13 may regulate multiple steps of the synaptic-vesicle cycle.

Systematic analysis of genes required for synapse structure and function

Chemical synapses are complex structures that mediate rapid intercellular signalling in the nervous system. Proteomic studies suggest that several hundred proteins will be found at synaptic specializations. Here we describe a systematic screen to identify genes required for the function or development of Caenorhabditis elegans neuromuscular junctions. A total of 185 genes were identified in an RNA interference screen for decreased acetylcholine secretion; 132 of these genes had not previously been implicated in synaptic transmission. Functional profiles for these genes were determined by comparing secretion defects observed after RNA interference under a variety of conditions. Hierarchical clustering identified groups of functionally related genes, including those involved in the synaptic vesicle cycle, neuropeptide signalling and responsiveness to phorbol esters. Twenty-four genes encoded proteins that were localized to presynaptic specializations. Loss-of-function mutations in 12 genes caused defects in presynaptic structure.

Genetic suppression of intronic +1G mutations by compensatory U1 snRNA changes in Caenorhabditis elegans

Mutations to the canonical +1G of introns, which are commonly found in many human inherited disease alleles, invariably result in aberrant splicing. Here we report genetic findings in C. elegans that aberrant splicing due to +1G mutations can be suppressed by U1 snRNA mutations. An intronic +1G-to-U mutation, e936, in the C. elegans unc-73 gene causes aberrant splicing and loss of gene function. We previously showed that mutation of the sup-39 gene promotes splicing at the mutant splice donor in e936 mutants. We demonstrate here that sup-39 is a U1 snRNA gene; suppressor mutations in sup-39 are compensatory substitutions in the 5' end, which enhance recognition of the mutant splice donor. sup-6(st19) is an allele-specific suppressor of unc-13(e309), which contains an intronic +1G-to-A transition. The e309 mutation activates a cryptic splice site, and sup-6(st19) restores splicing to the mutant splice donor. sup-6 also encodes a U1 snRNA and the mutant contains a compensatory substitution at its 5' end. This is the first demonstration that U1 snRNAs can act to suppress the effects of mutations to the invariant +1G of introns. These findings are suggestive of a potential treatment of certain alleles of inherited human genetic diseases.