The Caenorhabditis elegans unc-64 locus encodes a syntaxin that interacts genetically with synaptobrevin

Rare genetic brain disorders with overlapping neurological and psychiatric phenotypes

Understanding rare genetic brain disorders with overlapping neurological and psychiatric phenotypes is of increasing importance given the potential for developing disease models that could help to understand more common, polygenic disorders. However, the traditional clinical boundaries between neurology and psychiatry result in frequent segregation of these disorders into distinct silos, limiting cross-specialty understanding that could facilitate clinical and biological advances. In this Review, we highlight multiple genetic brain disorders in which neurological and psychiatric phenotypes are observed, but for which in-depth, cross-spectrum clinical phenotyping is rarely undertaken. We describe the combined phenotypes observed in association with genetic variants linked to epilepsy, dystonia, autism spectrum disorder and schizophrenia. We also consider common underlying mechanisms that centre on synaptic plasticity, including changes to synaptic and neuronal structure, calcium handling and the balance of excitatory and inhibitory neuronal activity. Further investigation is needed to better define and replicate these phenotypes in larger cohorts, which would help to gain greater understanding of the pathophysiological mechanisms and identify common therapeutic targets.

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.

ATP6V0C variants impair V-ATPase function causing a neurodevelopmental disorder often associated with epilepsy

The vacuolar H+-ATPase is an enzymatic complex that functions in an ATP-dependent manner to pump protons across membranes and acidify organelles, thereby creating the proton/pH gradient required for membrane trafficking by several different types of transporters. We describe heterozygous point variants in ATP6V0C, encoding the c-subunit in the membrane bound integral domain of the vacuolar H+-ATPase, in 27 patients with neurodevelopmental abnormalities with or without epilepsy. Corpus callosum hypoplasia and cardiac abnormalities were also present in some patients. In silico modelling suggested that the patient variants interfere with the interactions between the ATP6V0C and ATP6V0A subunits during ATP hydrolysis. Consistent with decreased vacuolar H+-ATPase activity, functional analyses conducted in Saccharomyces cerevisiae revealed reduced LysoSensor fluorescence and reduced growth in media containing varying concentrations of CaCl2. Knockdown of ATP6V0C in Drosophila resulted in increased duration of seizure-like behaviour, and the expression of selected patient variants in Caenorhabditis elegans led to reduced growth, motor dysfunction and reduced lifespan. In summary, this study establishes ATP6V0C as an important disease gene, describes the clinical features of the associated neurodevelopmental disorder and provides insight into disease mechanisms.

TOM-1/tomosyn acts with the UNC-6/netrin receptor UNC-5 to inhibit growth cone protrusion in Caenorhabditis elegans

In the polarity/protrusion model of growth cone repulsion from UNC-6/netrin, UNC-6 first polarizes the growth cone of the VD motor neuron axon via the UNC-5 receptor, and then regulates protrusion asymmetrically across the growth cone based on this polarity. UNC-6 stimulates protrusion dorsally through the UNC-40/DCC receptor, and inhibits protrusion ventrally through UNC-5, resulting in net dorsal growth. Previous studies showed that UNC-5 inhibits growth cone protrusion via the flavin monooxygenases and potential destabilization of F-actin, and via UNC-33/CRMP and restriction of microtubule plus-end entry into the growth cone. We show that UNC-5 inhibits protrusion through a third mechanism involving TOM-1/tomosyn. A short isoform of TOM-1 inhibited protrusion downstream of UNC-5, and a long isoform had a pro-protrusive role. TOM-1/tomosyn inhibits formation of the SNARE complex. We show that UNC-64/syntaxin is required for growth cone protrusion, consistent with a role of TOM-1 in inhibiting vesicle fusion. Our results are consistent with a model whereby UNC-5 utilizes TOM-1 to inhibit vesicle fusion, resulting in inhibited growth cone protrusion, possibly by preventing the growth cone plasma membrane addition required for protrusion.

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.

GABA signaling triggered by TMC-1/Tmc delays neuronal aging by inhibiting the PKC pathway in C. elegans

Aging causes functional decline and degeneration of neurons and is a major risk factor of neurodegenerative diseases. To investigate the molecular mechanisms underlying neuronal aging, we developed a new pipeline for neuronal proteomic profiling in young and aged animals. While the overall translational machinery is down-regulated, certain proteins increase expressions upon aging. Among these aging-up-regulated proteins, the conserved channel protein TMC-1/Tmc has an anti-aging function in all neurons tested, and the neuroprotective function of TMC-1 occurs by regulating GABA signaling. Moreover, our results show that metabotropic GABA receptors and G protein GOA-1/Goα are required for the anti-neuronal aging functions of TMC-1 and GABA, and the activation of GABA receptors prevents neuronal aging by inhibiting the PLCβ-PKC pathway. Last, we show that the TMC-1-GABA-PKC signaling axis suppresses neuronal functional decline caused by a pathogenic form of human Tau protein. Together, our findings reveal the neuroprotective function of the TMC-1-GABA-PKC signaling axis in aging and disease conditions.

Cannabinoids activate the insulin pathway to modulate mobilization of cholesterol in C. elegans

The nematode Caenorhabditis elegans requires exogenous cholesterol to survive and its depletion leads to early developmental arrest. Thus, tight regulation of cholesterol storage and distribution within the organism is critical. Previously, we demonstrated that the endocannabinoid (eCB) 2-arachidonoylglycerol (2-AG) plays a key role in C. elegans since it modulates sterol mobilization. However, the mechanism remains unknown. Here we show that mutations in the ocr-2 and osm-9 genes, coding for transient receptors potential V (TRPV) ion channels, dramatically reduce the effect of 2-AG in cholesterol mobilization. Through genetic analysis in combination with the rescue of larval arrest induced by sterol starvation, we found that the insulin/IGF-1signaling (IIS) pathway and UNC-31/CAPS, a calcium-activated regulator of neural dense-core vesicles release, are essential for 2-AG-mediated stimulation of cholesterol mobilization. These findings indicate that 2-AG-dependent cholesterol trafficking requires the release of insulin peptides and signaling through the DAF-2 insulin receptor. These results suggest that 2-AG acts as an endogenous modulator of TRPV signal transduction to control intracellular sterol trafficking through modulation of the IGF-1 signaling pathway

Interspecies complementation identifies a pathway to assemble SNAREs

Unc18 and SNARE proteins form the core of the membrane fusion complex at synapses. To understand the functional interactions within the core machinery, we adopted an "interspecies complementation" approach in Caenorhabditis elegans. Substitutions of individual SNAREs and Unc18 proteins with those from yeast fail to rescue fusion. However, synaptic transmission could be restored in worm-yeast chimeras when two key interfaces were present: an Habc-Unc18 contact site and an Unc18-SNARE motif contact site. A constitutively open form of Unc18 bypasses the requirement for the Habc-Unc18 interface. These data suggest that the Habc domain of syntaxin is required for Unc18 to adopt an open conformation; open Unc18 then templates SNARE complex formation. Finally, we demonstrate that the SNARE and Unc18 machinery in the nematode C. elegans can be replaced by yeast proteins and still carry out synaptic transmission, pointing to the deep evolutionary conservation of these two interfaces.

Caenorhabditis elegans Neurotoxicity Testing: Novel Applications in the Adverse Outcome Pathway Framework

Neurological hazard assessment of industrial and pesticidal chemicals demands a substantial amount of time and resources. Caenorhabditis elegans is an established model organism in developmental biology and neuroscience. It presents an ideal test system with relatively fewer neurons (302 in hermaphrodites) versus higher-order species, a transparent body, short lifespan, making it easier to perform neurotoxic assessment in a time and cost-effective manner. Yet, no regulatory testing guidelines have been developed for C. elegans in the field of developmental and adult neurotoxicity. Here, we describe a set of morphological and behavioral assessment protocols to examine neurotoxicity in C. elegans with relevance to cholinergic and dopaminergic systems. We discuss the homology of human genes and associated proteins in these two signaling pathways and evaluate the morphological and behavioral endpoints of C. elegans in the context of published adverse outcome pathways of neurodegenerative diseases. We conclude that C. elegans neurotoxicity testing will not only be instrumental to eliminating mammalian testing in neurological hazard assessment but also lead to new knowledge and mechanistic validation in the adverse outcome pathway framework.

Synaptopathies in Developmental and Epileptic Encephalopathies: A Focus on Pre-synaptic Dysfunction

The proper connection between the pre- and post-synaptic nervous cells depends on any element constituting the synapse: the pre- and post-synaptic membranes, the synaptic cleft, and the surrounding glial cells and extracellular matrix. An alteration of the mechanisms regulating the physiological synergy among these synaptic components is defined as “synaptopathy.” Mutations in the genes encoding for proteins involved in neuronal transmission are associated with several neuropsychiatric disorders, but only some of them are associated with Developmental and Epileptic Encephalopathies (DEEs). These conditions include a heterogeneous group of epilepsy syndromes associated with cognitive disturbances/intellectual disability, autistic features, and movement disorders. This review aims to elucidate the pathogenesis of these conditions, focusing on mechanisms affecting the neuronal pre-synaptic terminal and its role in the onset of DEEs, including potential therapeutic approaches.

SNARE Regulatory Proteins in Synaptic Vesicle Fusion and Recycling

Membrane fusion is a universal feature of eukaryotic protein trafficking and is mediated by the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) family. SNARE proteins embedded in opposing membranes spontaneously assemble to drive membrane fusion and cargo exchange in vitro. Evolution has generated a diverse complement of SNARE regulatory proteins (SRPs) that ensure membrane fusion occurs at the right time and place in vivo. While a core set of SNAREs and SRPs are common to all eukaryotic cells, a specialized set of SRPs within neurons confer additional regulation to synaptic vesicle (SV) fusion. Neuronal communication is characterized by precise spatial and temporal control of SNARE dynamics within presynaptic subdomains specialized for neurotransmitter release. Action potential-elicited Ca²⁺ influx at these release sites triggers zippering of SNAREs embedded in the SV and plasma membrane to drive bilayer fusion and release of neurotransmitters that activate downstream targets. Here we discuss current models for how SRPs regulate SNARE dynamics and presynaptic output, emphasizing invertebrate genetic findings that advanced our understanding of SRP regulation of SV cycling.

Tracking Single Molecule Dynamics in the Adult Drosophila Brain

Super-resolution microscopy provides valuable insight for understanding the nanoscale organization within living tissue, although this method is typically restricted to cultured or dissociated cells. Here, we develop a method to track the mobility of individual proteins in ex vivo adult Drosophila melanogaster brains, focusing on a key component of the presynaptic release machinery, syntaxin1A (Sx1a). We show that individual Sx1a dynamics can be reliably tracked within neurons in the whole fly brain, and that the mobility of Sx1a molecules increases following conditional neural stimulation. We then apply this preparation to the problem of general anesthesia, to address how different anesthetics might affect single molecule dynamics in intact brain synapses. We find that propofol, etomidate, and isoflurane significantly impair Sx1a mobility, while ketamine and sevoflurane have little effect. Resolving single molecule dynamics in intact fly brains provides a novel approach to link localized molecular effects with systems-level phenomena such as general anesthesia.

Profiling neural editomes reveals a molecular mechanism to regulate RNA editing during development

Adenosine (A) to inosine (I) RNA editing contributes to transcript diversity and modulates gene expression in a dynamic, cell type–specific manner. During mammalian brain development, editing of specific adenosines increases, whereas the expression of A-to-I editing enzymes remains unchanged, suggesting molecular mechanisms that mediate spatiotemporal regulation of RNA editing exist. Herein, by using a combination of biochemical and genomic approaches, we uncover a molecular mechanism that regulates RNA editing in a neural- and development-specific manner. Comparing editomes during development led to the identification of neural transcripts that were edited only in one life stage. The stage-specific editing is largely regulated by differential gene expression during neural development. Proper expression of nearly one-third of the neurodevelopmentally regulated genes is dependent on adr-2, the sole A-to-I editing enzyme in C. elegans. However, we also identified a subset of neural transcripts that are edited and expressed throughout development. Despite a neural-specific down-regulation of adr-2 during development, the majority of these sites show increased editing in adult neural cells. Biochemical data suggest that ADR-1, a deaminase-deficient member of the adenosine deaminase acting on RNA (ADAR) family, is competing with ADR-2 for binding to specific transcripts early in development. Our data suggest a model in which during neural development, ADR-2 levels overcome ADR-1 repression, resulting in increased ADR-2 binding and editing of specific transcripts. Together, our findings reveal tissue- and development-specific regulation of RNA editing and identify a molecular mechanism that regulates ADAR substrate recognition and editing efficiency.

Open syntaxin overcomes exocytosis defects of diverse mutants in C. elegans

Assembly of SNARE complexes that mediate neurotransmitter release requires opening of a ‘closed’ conformation of UNC-64/syntaxin. Rescue of unc-13/Munc13 mutant phenotypes by overexpressed open UNC-64/syntaxin suggested a specific function of UNC-13/Munc13 in opening UNC-64/ syntaxin. Here, we revisit the effects of open unc-64/syntaxin by generating knockin (KI) worms. The KI animals exhibit enhanced spontaneous and evoked exocytosis compared to WT animals. Unexpectedly, the open syntaxin KI partially suppresses exocytosis defects of various mutants, including snt-1/synaptotagmin, unc-2/P/Q/N-type Ca²⁺ channel alpha-subunit and unc-31/CAPS, in addition to unc-13/Munc13 and unc-10/RIM, and enhanced exocytosis in tom-1/Tomosyn mutants. However, open syntaxin aggravates the defects of unc-18/Munc18 mutants. Correspondingly, open syntaxin partially bypasses the requirement of Munc13 but not Munc18 for liposome fusion. Our results show that facilitating opening of syntaxin enhances exocytosis in a wide range of genetic backgrounds, and may provide a general means to enhance synaptic transmission in normal and disease states.

Opening of the UNC-64/syntaxin closed conformation by UNC-13/Munc13 to form the neuronal SNARE complex is critical for neurotransmitter release. Here the authors show that facilitating the opening of syntaxin enhances exocytosis not only in unc-13 nulls as well as in diverse C. elegans mutants.

SNAREs and developmental disorders

Members of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family mediate membrane fusion processes associated with vesicular trafficking and autophagy. SNAREs mediate core membrane fusion processes essential for all cells, but some SNAREs serve cell/tissue type-specific exocytic/endocytic functions, and are therefore critical for various aspects of embryonic development. Mutations or variants of their encoding genes could give rise to developmental disorders, such as those affecting the nervous system and immune system in humans. Mutations to components in the canonical synaptic vesicle fusion SNARE complex (VAMP2, STX1A/B, and SNAP25) and a key regulator of SNARE complex formation MUNC18-1, produce variant phenotypes of autism, intellectual disability, movement disorders, and epilepsy. STX11 and MUNC18-2 mutations underlie 2 subtypes of familial hemophagocytic lymphohistiocytosis. STX3 mutations contribute to variant microvillus inclusion disease. Chromosomal microdeletions involving STX16 play a role in pseudohypoparathyroidism type IB associated with abnormal imprinting of the GNAS complex locus. In this short review, I discuss these and other SNARE gene mutations and variants that are known to be associated with a variety developmental disorders, with a focus on their underlying cellular and molecular pathological basis deciphered through disease modeling. Possible pathogenic potentials of other SNAREs whose variants could be disease predisposing are also speculated upon.

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

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

Epilepsy-causing STX1B mutations translate altered protein functions into distinct phenotypes in mouse neurons

Syntaxin 1B (STX1B) is a core component of the N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex that is critical for the exocytosis of synaptic vesicles in the presynapse. SNARE-mediated vesicle fusion is assisted by Munc18-1, which recruits STX1B in the auto-inhibited conformation, while Munc13 catalyses the fast and efficient pairing of helices during SNARE complex formation. Mutations within the STX1B gene are associated with epilepsy. Here we analysed three STX1B mutations by biochemical and electrophysiological means. These three paradigmatic mutations cause epilepsy syndromes of different severity, from benign fever-associated seizures in childhood to severe epileptic encephalopathies. An insertion/deletion (K45/RMCIE, L46M) mutation (STX1BInDel), causing mild epilepsy and located in the early helical Habc domain, leads to an unfolded protein unable to sustain neurotransmission. STX1BG226R, causing epileptic encephalopathies, strongly compromises the interaction with Munc18-1 and reduces expression of both proteins, the size of the readily releasable pool of vesicles, and Ca2+-triggered neurotransmitter release when expressed in STX1-null neurons. The mutation STX1BV216E, also causing epileptic encephalopathies, only slightly diminishes Munc18-1 and Munc13 interactions, but leads to enhanced fusogenicity and increased vesicular release probability, also in STX1-null neurons. Even though the synaptic output remained unchanged in excitatory hippocampal STX1B+/− neurons exogenously expressing STX1B mutants, the manifestation of clear and distinct molecular disease mechanisms by these mutants suggest that certain forms of epilepsies can be conceptualized by assigning mutations to structurally sensitive regions of the STX1B−Munc18-1 interface, translating into distinct neurophysiological phenotypes.

SNAREopathies: Diversity in Mechanisms and Symptoms

Neuronal SNAREs and their key regulators together drive synaptic vesicle exocytosis and synaptic transmission as a single integrated membrane fusion machine. Human pathogenic mutations have now been reported for all eight core components, but patients are diagnosed with very different neurodevelopmental syndromes. We propose to unify these syndromes, based on etiology and mechanism, as "SNAREopathies." Here, we review the strikingly diverse clinical phenomenology and disease severity and the also remarkably diverse genetic mechanisms. We argue that disease severity generally scales with functional redundancy and, conversely, that the large effect of mutations in some SNARE genes is the price paid for extensive integration and exceptional specialization. Finally, we discuss how subtle differences in components being rate limiting in different types of neurons helps to explain the main symptoms.

Repression of an activity-dependent autocrine insulin signal is required for sensory neuron development in C. elegans

Nervous system development is instructed by genetic programs and refined by distinct mechanisms that couple neural activity to gene expression. How these processes are integrated remains poorly understood. Here, we report that the regulated release of insulin-like peptides (ILPs) during development of the Caenorhabditis elegans nervous system accomplishes such an integration. We find that the p38 MAP kinase PMK-3, which is required for the differentiation of chemosensory BAG neurons, limits an ILP signal that represses expression of a BAG neuron fate. ILPs are released from BAGs themselves in an activity-dependent manner during development, indicating that ILPs constitute an autocrine signal that regulates the differentiation of BAG neurons. Expression of a specialized neuronal fate is, therefore, coordinately regulated by a genetic program that sets levels of ILP expression during development, and by neural activity, which regulates ILP release. Autocrine signals of this kind might have general and conserved functions as integrators of deterministic genetic programs with activity-dependent mechanisms during neurodevelopment.

Neuroendocrine regulation of fat metabolism by autophagy gene atg-18 in C. elegans dauer larvae

In environments with limited food and high population density, Caenorhabditis elegans larvae may enter the dauer stage, in which metabolism is shifted to fat accumulation to allow larvae to survive for months without food. Mutations in the insulin‐like receptor gene daf‐2 force C. elegans to constitutively form dauer larva at higher temperature. It has been reported that autophagy is required for fat accumulation in daf‐2 dauer larva. However, the mechanism underlying this process remains unknown. Here, we report that autophagy gene atg‐18 acts in a cell nonautonomous manner in neurons and intestinal cells to mediate the influence of daf‐2 signaling on fat metabolism. Moreover, ATG‐18 in chemosensory neurons plays a vital role in this metabolic process. Finally, we report that neuronal ATG‐18 functions through neurotransmitters to control fat storage in daf‐2 dauers, which suggests an essential role of autophagy in the neuroendocrine regulation of fat metabolism by insulin‐like signaling.

Sperm-inherited H3K27me3 impacts offspring transcription and development in C. elegans

Paternal epigenetic inheritance is gaining attention for its growing medical relevance. However, the form in which paternal epigenetic information is transmitted to offspring and how it influences offspring development remain poorly understood. Here we show that in C. elegans, sperm-inherited chromatin states transmitted to the primordial germ cells in offspring influence germline transcription and development. We show that sperm chromosomes inherited lacking the repressive histone modification H3K27me3 are maintained in that state by H3K36me3 antagonism. Inheritance of H3K27me3-lacking sperm chromosomes results in derepression in the germline of somatic genes, especially neuronal genes, predominantly from sperm-inherited alleles. This results in germ cells primed for losing their germ cell identity and adopting a neuronal fate. These data demonstrate that histone modifications are one mechanism through which epigenetic information from a father can shape offspring gene expression and development.

The mechanisms of paternal epigenetic inheritance and its influence on offspring are still poorly understood. Here the authors provide evidence that in C. elegans, sperm-inherited chromatin states influence transcription and cell identity in the germ cells of offspring.

Pharming for Genes in Neurotransmission: Combining Chemical and Genetic Approaches in Caenorhabditis elegans

Synaptic transmission is central to nervous system function. Chemical and genetic screens are valuable approaches to probe synaptic mechanisms in living animals. The nematode Caenorhabditis elegans is a prime system to apply these methods to discover genes and dissect the cellular pathways underlying neurotransmission. Here, we review key approaches to understand neurotransmission and the action of psychiatric drugs in C. elegans. We start with early studies on cholinergic excitatory signaling at the neuromuscular junction, and move into mechanisms mediated by biogenic amines. Finally, we discuss emerging work toward understanding the mechanisms driving synaptic plasticity with a focus on regulation of protein translation.

Genetic dissection of neuropeptide cell biology at high and low activity in a defined sensory neuron

Neuropeptides are ubiquitous modulators of behavior and physiology. They are packaged in specialized secretory organelles called dense core vesicles (DCVs) that are released upon neural stimulation. Whereas local recycling of synaptic vesicles has been investigated intensively, there are few studies on recycling of DCV proteins. We set up a paradigm to study DCVs in a neuron whose activity we can control. We validate our model by confirming many previous observations on DCV cell biology. We identify a set of genes involved in recycling of DCV proteins. We also find evidence that different mechanisms of DCV priming and exocytosis may operate at high and low neural activity.

Neuropeptides are ubiquitous modulators of behavior and physiology. They are packaged in specialized secretory organelles called dense core vesicles (DCVs) that are released upon neural stimulation. Unlike synaptic vesicles, which can be recycled and refilled close to release sites, DCVs must be replenished by de novo synthesis in the cell body. Here, we dissect DCV cell biology in vivo in a Caenorhabditis elegans sensory neuron whose tonic activity we can control using a natural stimulus. We express fluorescently tagged neuropeptides in the neuron and define parameters that describe their subcellular distribution. We measure these parameters at high and low neural activity in 187 mutants defective in proteins implicated in membrane traffic, neuroendocrine secretion, and neuronal or synaptic activity. Using unsupervised hierarchical clustering methods, we analyze these data and identify 62 groups of genes with similar mutant phenotypes. We explore the function of a subset of these groups. We recapitulate many previous findings, validating our paradigm. We uncover a large battery of proteins involved in recycling DCV membrane proteins, something hitherto poorly explored. We show that the unfolded protein response promotes DCV production, which may contribute to intertissue communication of stress. We also find evidence that different mechanisms of priming and exocytosis may operate at high and low neural activity. Our work provides a defined framework to study DCV biology at different neural activity levels.

Developmental basis for intestinal barrier against the toxicity of graphene oxide

BACKGROUND

Intestinal barrier is crucial for animals against translocation of engineered nanomaterials (ENMs) into secondary targeted organs. However, the molecular mechanisms for the role of intestinal barrier against ENMs toxicity are still largely unclear. The intestine of Caenorhabditis elegans is a powerful in vivo experimental system for the study on intestinal function. In this study, we investigated the molecular basis for intestinal barrier against toxicity and translocation of graphene oxide (GO) using C. elegans as a model animal.

RESULTS

Based on the genetic screen of genes required for the control of intestinal development at different aspects using intestine-specific RNA interference (RNAi) technique, we identified four genes (erm-1, pkc-3, hmp-2 and act-5) required for the function of intestinal barrier against GO toxicity. Under normal conditions, mutation of any of these genes altered the intestinal permeability. With the focus on PKC-3, an atypical protein kinase C, we identified an intestinal signaling cascade of PKC-3-SEC-8-WTS-1, which implies that PKC-3 might regulate intestinal permeability and GO toxicity by affecting the function of SEC-8-mediated exocyst complex and the role of WTS-1 in maintaining integrity of apical intestinal membrane. ISP-1 and SOD-3, two proteins required for the control of oxidative stress, were also identified as downstream targets for PKC-3, and functioned in parallel with WTS-1 in the regulation of GO toxicity.

CONCLUSIONS

Using C. elegans as an in vivo assay system, we found that several developmental genes required for the control of intestinal development regulated both the intestinal permeability and the GO toxicity. With the focus on PKC-3, we raised two intestinal signaling cascades, PKC-3-SEC-8-WTS-1 and PKC-3-ISP-1/SOD-3. Our results will strengthen our understanding the molecular basis for developmental machinery of intestinal barrier against GO toxicity and translocation in animals.

How alternative splicing affects membrane-trafficking dynamics

The cell biology field has outstanding working knowledge of the fundamentals of membrane-trafficking pathways, which are of critical importance in health and disease. Current challenges include understanding how trafficking pathways are fine-tuned for specialized tissue functions in vivo and during development. In parallel, the ENCODE project and numerous genetic studies have revealed that alternative splicing regulates gene expression in tissues and throughout development at a post-transcriptional level. This Review summarizes recent discoveries demonstrating that alternative splicing affects tissue specialization and membrane-trafficking proteins during development, and examines how this regulation is altered in human disease. We first discuss how alternative splicing of clathrin, SNAREs and BAR-domain proteins influences endocytosis, secretion and membrane dynamics, respectively. We then focus on the role of RNA-binding proteins in the regulation of splicing of membrane-trafficking proteins in health and disease. Overall, our aim is to comprehensively summarize how trafficking is molecularly influenced by alternative splicing and identify future directions centered on its physiological relevance.

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.

Syntaxins on granules promote docking of granules via interactions with munc18

SNAREs and SNARE-binding accessory proteins are believed to be central molecular components of neurotransmitter release, although the precise sequence of molecular events corresponding to distinct physiological states is unclear. The mechanism of docking of vesicles to the plasma membrane remains elusive, as the anchoring protein residing on vesicles is unknown. Here I show that targeting small amounts of syntaxin to granules by transmembrane domain alteration leads to a substantial enhancement of syntaxin clustering beneath granules, as well as of morphological granule docking. The effect was abolished without munc18 and strongly reduced by removal of the N-terminal peptide in the syntaxin mutant. Thus, in contrast to the current paradigm, I demonstrate that syntaxin acts from the vesicular membrane, strongly facilitating docking of vesicles, likely via interaction of its N-peptide with munc18. Docking was assayed by quantifying the syntaxin clusters beneath granules, using two-color Total Internal Reflectance Fluorescence microscopy in live PC-12 cells and confirmed by electron microscopy. Hereby, I propose a new model of vesicle docking, wherein munc18 bridges the few syntaxin molecules residing on granules to the syntaxin cluster on the plasma membrane, suggesting that the number of syntaxins on vesicles determines docking and conceivably fusion probability.

AIP limits neurotransmitter release by inhibiting calcium bursts from the ryanodine receptor

Pituitary tumors are frequently associated with mutations in the AIP gene and are sometimes associated with hypersecretion of growth hormone. It is unclear whether other factors besides an enlarged pituitary contribute to the hypersecretion. In a genetic screen for suppressors of reduced neurotransmitter release, we identified a mutation in Caenorhabditis elegans AIPR-1 (AIP-related-1), which causes profound increases in evoked and spontaneous neurotransmitter release, a high frequency of spontaneous calcium transients in motor neurons and an enlarged readily releasable pool of vesicles. Calcium bursts and hypersecretion are reversed by mutations in the ryanodine receptor but not in the voltage-gated calcium channel, indicating that these phenotypes are caused by a leaky ryanodine receptor. AIPR-1 is physically associated with the ryanodine receptor at synapses. Finally, the phenotypes in aipr-1 mutants can be rescued by presynaptic expression of mouse AIP, demonstrating that a conserved function of AIP proteins is to inhibit calcium release from ryanodine receptors.

The C. elegans neural editome reveals an ADAR target mRNA required for proper chemotaxis

ADAR proteins alter gene expression both by catalyzing adenosine (A) to inosine (I) RNA editing and binding to regulatory elements in target RNAs. Loss of ADARs affects neuronal function in all animals studied to date. Caenorhabditis elegans lacking ADARs exhibit reduced chemotaxis, but the targets responsible for this phenotype remain unknown. To identify critical neural ADAR targets in C. elegans, we performed an unbiased assessment of the effects of ADR-2, the only A-to-I editing enzyme in C. elegans, on the neural transcriptome. Development and implementation of publicly available software, SAILOR, identified 7361 A-to-I editing events across the neural transcriptome. Intersecting the neural editome with adr-2 associated gene expression changes, revealed an edited mRNA, clec-41, whose neural expression is dependent on deamination. Restoring clec-41 expression in adr-2 deficient neural cells rescued the chemotaxis defect, providing the first evidence that neuronal phenotypes of ADAR mutants can be caused by altered gene expression.

DNA is the blueprint that tells each cell in an organism how it should operate. It encodes the instructions to make proteins and other molecules. To make a protein, a section of DNA known as a gene is used as a template to make molecules known as messenger ribonucleic acids (or mRNAs for short). The message in RNA consists of a series of individual letters, known as nucleotides, that tell the cell how much of a protein should be produced (referred to as gene expression) as well as the specific activities of each protein.

The letters in mRNAs can be changed in specific cells and at certain points in development through a process known as RNA editing. This process is essential for animals to grow and develop normally and for the brain to work properly. Errors in RNA editing are found in patients suffering from a variety of neuropathological diseases, including Alzheimer’s disease, depression and brain tumors. Humans have millions of editing sites that are predicted to affect gene expression. However, many studies of RNA editing have only focused on the changes that alter protein activity.

The ADAR proteins carry out a specific type of RNA editing in animals. In a microscopic worm known as Caenorhabditis elegans the loss of an ADAR protein called ADR-2 reduces the ability of the worm to move in response to chemicals, a process known as chemotaxis. Deffit et al. found that loss of ADR-2 affected the expression of over 150 genes in the nervous system of the worm. To identify which letters in the mRNAs were edited in the nervous system, Deffit et al. developed a new publically available software program called SAILOR (software for accurately identifying locations of RNA editing). This program can be used to detect RNA editing in any cell, tissue or organism.

By combining the experimental and computational approaches, Deffit et al. were able to identify a gene that was edited in normal worms and expressed at lower levels in the mutant worms. Increasing the expression of just this one of gene in the mutant worms restored the worms’ ability to move towards a chemical “scent”.

Together, these findings suggest that when studying human neuropathological diseases we should consider the effect of RNA editing on the amount of gene expression as well as protein activity. Future work should investigate the importance of RNA editing in controlling gene expression in other diseases including cancers.

A new platform for long-term tracking and recording of neural activity and simultaneous optogenetic control in freely behaving Caenorhabditis elegans

BACKGROUND

Real-time recording and manipulation of neural activity in freely behaving animals can greatly advance our understanding of how neural circuits regulate behavior. Ca²⁺ imaging and optogenetic manipulation with optical probes are key technologies for this purpose. However, integrating the two optical approaches with behavioral analysis has been technically challenging.

NEW METHOD

Here, we developed a new imaging system, ICaST (Integrated platform for Ca²⁺ imaging, Stimulation, and Tracking), which combines an automatic worm tracking system and a fast-scanning laser confocal microscope, to image neurons of interest in freely behaving C. elegans. We optimized different excitation wavelengths for the concurrent use of channelrhodopsin-2 and G-CaMP, a green fluorescent protein (GFP)-based, genetically encoded Ca²⁺ indicator.

RESULTS

Using ICaST in conjunction with an improved G-CaMP7, we successfully achieved long-term tracking and Ca²⁺ imaging of the AVA backward command interneurons while tracking the head of a moving animal. We also performed all-optical manipulation and simultaneous recording of Ca²⁺ dynamics from GABAergic motor neurons in conjunction with behavior monitoring.

COMPARISON WITH EXISTING METHOD(S)

Our system differs from conventional systems in that it does not require fluorescent markers for tracking and can track any part of the worm's body via bright-field imaging at high magnification. Consequently, this approach enables the long-term imaging of activity from neurons or nerve processes of interest with high spatiotemporal resolution.

CONCLUSION

Our imaging system is a powerful tool for studying the neural circuit mechanisms of C. elegans behavior and has potential for use in other small animals.

An emerging model organism Caenorhabditis elegans for alternative pre-mRNA processing in vivo

A nematode Caenorhabditis elegans is an intron-rich organism and up to 25% of its pre-mRNAs are estimated to be alternatively processed. Its compact genomic organization enables construction of fluorescence splicing reporters with intact genomic sequences and visualization of alternative processing patterns of interest in the transparent living animals with single-cell resolution. Genetic analysis with the reporter worms facilitated identification of trans-acting factors and cis-acting elements, which are highly conserved in mammals. Analysis of unspliced and partially spliced pre-mRNAs in vivo raised models for alternative splicing regulation relying on specific order of intron excision. RNA-seq analysis of splicing factor mutants and CLIP-seq analysis of the factors allow global search for target genes in the whole animal. An mRNA surveillance system is not essential for its viability or fertility, allowing analysis of unproductively spliced noncoding mRNAs. These features offer C. elegans as an ideal model organism for elucidating alternative pre-mRNA processing mechanisms in vivo. Examples of isoform-specific functions of alternatively processed genes are summarized. WIREs RNA 2017, 8:e1428. doi: 10.1002/wrna.1428 For further resources related to this article, please visit the WIREs website.

Distinct Functions of Syntaxin-1 in Neuronal Maintenance, Synaptic Vesicle Docking, and Fusion in Mouse Neurons

Neurotransmitter release requires the formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes by SNARE proteins syntaxin-1 (Stx1), synaptosomal-associated protein 25 (SNAP-25), and synaptobrevin-2 (Syb2). In mammalian systems, loss of SNAP-25 or Syb2 severely impairs neurotransmitter release; however, complete loss of function studies for Stx1 have been elusive due to the functional redundancy between Stx1 isoforms Stx1A and Stx1B and the embryonic lethality of Stx1A/1B double knock-out (DKO) mice. Here, we studied the roles of Stx1 in neuronal maintenance and neurotransmitter release in mice with constitutive or conditional deletion of Stx1B on an Stx1A-null background. Both constitutive and postnatal loss of Stx1 severely compromised neuronal viability in vivo and in vitro, indicating an obligatory role of Stx1 for maintenance of developing and mature neurons. Loss of Munc18-1, a high-affinity binding partner of Stx1, also showed severely impaired neuronal viability, but with a slower time course compared with Stx1A/1B DKO neurons, and exogenous Stx1A or Stx1B expression significantly delayed Munc18-1-dependent lethality. In addition, loss of Stx1 completely abolished fusion-competent vesicles and severely impaired vesicle docking, demonstrating its essential roles in neurotransmission. Putative partial SNARE complex assembly with the SNARE motif mutant Stx1A(AV) (A240V, V244A) was not sufficient to rescue neurotransmission despite full recovery of vesicle docking and neuronal survival. Together, these data suggest that Stx1 has independent functions in neuronal maintenance and neurotransmitter release and complete SNARE complex formation is required for vesicle fusion and priming, whereas partial SNARE complex formation is sufficient for vesicle docking and neuronal maintenance.

SIGNIFICANCE STATEMENT

Syntaxin-1 (Stx1) is a component of the synaptic vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex and is essential for neurotransmission. We present the first detailed loss-of-function characterization of the two Stx1 isoforms in central mammalian neurons. We show that Stx1 is fundamental for maintenance of developing and mature neurons and also for vesicle docking and neurotransmission. We also demonstrate that neuronal maintenance and neurotransmitter release are regulated by Stx1 through independent functions. Furthermore, we show that SNARE complex formation is required for vesicle fusion, whereas partial SNARE complex formation is sufficient for vesicle docking and neuronal maintenance. Therefore, our work provides insights into differential functions of Stx1 in neuronal maintenance and neurotransmission, with the latter explored further into its functions in vesicle docking and fusion.

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.

Evolutionary Divergence of the C-terminal Domain of Complexin Accounts for Functional Disparities between Vertebrate and Invertebrate Complexins

Complexin is a critical presynaptic protein that regulates both spontaneous and calcium-triggered neurotransmitter release in all synapses. Although the SNARE-binding central helix of complexin is highly conserved and required for all known complexin functions, the remainder of the protein has profoundly diverged across the animal kingdom. Striking disparities in complexin inhibitory activity are observed between vertebrate and invertebrate complexins but little is known about the source of these differences or their relevance to the underlying mechanism of complexin regulation. We found that mouse complexin 1 (mCpx1) failed to inhibit neurotransmitter secretion in Caenorhabditis elegans neuromuscular junctions lacking the worm complexin 1 (CPX-1). This lack of inhibition stemmed from differences in the C-terminal domain (CTD) of mCpx1. Previous studies revealed that the CTD selectively binds to highly curved membranes and directs complexin to synaptic vesicles. Although mouse and worm complexin have similar lipid binding affinity, their last few amino acids differ in both hydrophobicity and in lipid binding conformation, and these differences strongly impacted CPX-1 inhibitory function. Moreover, function was not maintained if a critical amphipathic helix in the worm CPX-1 CTD was replaced with the corresponding mCpx1 amphipathic helix. Invertebrate complexins generally shared more C-terminal similarity with vertebrate complexin 3 and 4 isoforms, and the amphipathic region of mouse complexin 3 significantly restored inhibitory function to worm CPX-1. We hypothesize that the CTD of complexin is essential in conferring an inhibitory function to complexin, and that this inhibitory activity has been attenuated in the vertebrate complexin 1 and 2 isoforms. Thus, evolutionary changes in the complexin CTD differentially shape its synaptic role across phylogeny.

Excitatory neurons sculpt GABAergic neuronal connectivity in the C. elegans motor circuit

Establishing and maintaining the appropriate number of GABA synapses is key for balancing excitation and inhibition in the nervous system, though we have only a limited understanding of the mechanisms controlling GABA circuit connectivity. Here, we show that disrupting cholinergic innervation of GABAergic neurons in the C. elegans motor circuit alters GABAergic neuron synaptic connectivity. These changes are accompanied by reduced frequency and increased amplitude of GABAergic synaptic events. Acute genetic disruption in early development, during the integration of post-embryonic-born GABAergic neurons into the circuit, produces irreversible effects on GABAergic synaptic connectivity that mimic those produced by chronic manipulations. In contrast, acute genetic disruption of cholinergic signaling in the adult circuit does not reproduce these effects. Our findings reveal that GABAergic signaling is regulated by cholinergic neuronal activity, probably through distinct mechanisms in the developing and mature nervous system.

Synergism between soluble guanylate cyclase signaling and neuropeptides extends lifespan in the nematode Caenorhabditis elegans

Oxygen (O₂) homeostasis is important for all aerobic animals. However, the manner by which O₂ sensing and homeostasis contribute to lifespan regulation is poorly understood. Here, we use the nematode Caenorhabditis elegans to address this question. We demonstrate that a loss‐of‐function mutation in the neuropeptide receptor gene npr‐1 and a deletion mutation in the atypical soluble guanylate cyclase gcy‐35 O₂ sensor interact synergistically to extend worm lifespan. The function of npr‐1 and gcy‐35 in the O₂‐sensing neurons AQR, PQR, and URX shortens the lifespan of the worm. By contrast, the activity of the atypical soluble guanylate cyclase O₂ sensor gcy‐33 in these neurons is crucial for lifespan extension. In addition to AQR, PQR, and URX, we show that the O₂‐sensing neuron BAG and the interneuron RIA are also important for the lifespan lengthening. Neuropeptide processing by the proprotein convertase EGL‐3 is essential for lifespan extension, suggesting that the synergistic effect of joint loss of function of gcy‐35 and npr‐1 is mediated through neuropeptide signal transduction. The extended lifespan is regulated by hypoxia and insulin signaling pathways, mediated by the transcription factors HIF‐1 and DAF‐16. Moreover, reactive oxygen species (ROS) appear to play an important function in lifespan lengthening. As HIF‐1 and DAF‐16 activities are modulated by ROS, we speculate that joint loss of function of gcy‐35 and npr‐1 extends lifespan through ROS signaling.

Germ Granules Prevent Accumulation of Somatic Transcripts in the Adult Caenorhabditis elegans Germline

The germ cells of multicellular organisms protect their developmental potential through specialized mechanisms. A shared feature of germ cells from worms to humans is the presence of nonmembrane-bound, ribonucleoprotein organelles called germ granules. Depletion of germ granules in Caenorhabditis elegans (i.e., P granules) leads to sterility and, in some germlines, expression of the neuronal transgene unc-119::gfp and the muscle myosin MYO-3 Thus, P granules are hypothesized to maintain germ cell totipotency by preventing somatic development, although the mechanism by which P granules carry out this function is unknown. In this study, we performed transcriptome and single molecule RNA-FISH analyses of dissected P granule-depleted gonads at different developmental stages. Our results demonstrate that P granules are necessary for adult germ cells to downregulate spermatogenesis RNAs and to prevent the accumulation of numerous soma-specific RNAs. P granule-depleted gonads that express the unc-119::gfp transgene also express many other genes involved in neuronal development and concomitantly lose expression of germ cell fate markers. Finally, we show that removal of either of two critical P-granule components, PGL-1 or GLH-1, is sufficient to cause germ cells to express UNC-119::GFP and MYO-3 and to display RNA accumulation defects similar to those observed after depletion of P granules. Our data identify P granules as critical modulators of the germline transcriptome and guardians of germ cell fate.

Regrowing axons with alternative splicing

The regeneration of axons relies on a previously unknown mechanism that involves the regulation of alternative splicing by CELF proteins.

CELF RNA binding proteins promote axon regeneration in C. elegans and mammals through alternative splicing of Syntaxins

Axon injury triggers dramatic changes in gene expression. While transcriptional regulation of injury-induced gene expression is widely studied, less is known about the roles of RNA binding proteins (RBPs) in post-transcriptional regulation during axon regeneration. In C. elegans the CELF (CUGBP and Etr-3 Like Factor) family RBP UNC-75 is required for axon regeneration. Using crosslinking immunoprecipitation coupled with deep sequencing (CLIP-seq) we identify a set of genes involved in synaptic transmission as mRNA targets of UNC-75. In particular, we show that UNC-75 regulates alternative splicing of two mRNA isoforms of the SNARE Syntaxin/unc-64. In C. elegans mutants lacking unc-75 or its targets, regenerating axons form growth cones, yet are deficient in extension. Extending these findings to mammalian axon regeneration, we show that mouse Celf2 expression is upregulated after peripheral nerve injury and that Celf2 mutant mice are defective in axon regeneration. Further, mRNAs for several Syntaxins show CELF2 dependent regulation. Our data delineate a post-transcriptional regulatory pathway with a conserved role in regenerative axon extension.

DOI:http://dx.doi.org/10.7554/eLife.16072.001

Nerve cells or neurons carry information around the body along projections known as axons. An injury or trauma, such as a stroke, can damage the axons and lead to permanent disability because the damaged axons fail to regenerate over long distances.

Axon damage triggers large changes in the activity of many genes that promote regeneration. When a gene is active, its DNA is copied to make molecules of messenger RNA (mRNA), which are then used as templates to make proteins. Many mRNAs undergo a process called alternative splicing, in which different combinations of mRNA sections may be removed from the final molecule. This enables a single gene to produce more than one type of protein.

Recent studies point to an important role for so-called RNA binding proteins in regulating the alternative splicing process. An RNA binding protein called UNC-75 in a worm known as Caenorhabditis elegans has previously been shown to be involved in axon regeneration, but it was not clear how UNC-75 acts on neurons. Here, Chen et al. combined a technique called CLIP-seq (Cross-linking ImmunoPrecipitation-deep sequencing) with genetic testing to identify the mRNAs that UNC-75 regulates during axon regeneration.

The experiments found a set of C. elegans genes required for information to pass between neurons whose mRNAs are also targeted by UNC-75. Many of these genes are also required for axon regeneration. Chen et al. studied one of the mRNA targets – which encodes a protein called syntaxin – in more detail and found that the syntaxin mRNA is required for regenerating axons over long distances. UNC-75 alternatively splices this mRNA to produce a particular form of syntaxin that is mainly found in neurons. Mutant worms that lack either UNC-75 or syntaxin are unable to properly regenerate axons over long distances.

Further experiments show that a mouse protein known as CELF2 that is equivalent to worm UNC-75 plays a similar role in regenerating axons. Moreover, mouse CELF2 restores the ability of worm neurons that lack UNC-75 to regenerate. Like worm UNC-75, the mouse protein is also involved in alternative splicing of syntaxin. The next step is to examine the other mRNA targets of UNC-75 to find out what role they play in axon regeneration and other processes in neurons.

DOI:http://dx.doi.org/10.7554/eLife.16072.002

Synaptic activity regulates the abundance and binding of complexin

Nervous system function relies on precise chemical communication between neurons at specialized junctions known as synapses. Complexin (CPX) is one of a small number of cytoplasmic proteins that are indispensable in controlling neurotransmitter release through SNARE and synaptic vesicle interactions. However, the mechanisms that recruit and stabilize CPX are poorly understood. The mobility of CPX tagged with photoactivatable green fluorescent protein (pGFP) was quantified in vivo using Caenorhabditis elegans. Although pGFP escaped the synapse within seconds, CPX-pGFP displayed both fast and slow decay components, requiring minutes for complete exchange of the synaptic pool. The longer synaptic residence time of CPX arose from both synaptic vesicle and SNARE interactions, and surprisingly, CPX mobility depended on synaptic activity. Moreover, mouse CPX-GFP reversibly dispersed out of hippocampal presynaptic terminals during stimulation, and blockade of vesicle fusion prevented CPX dispersion. Hence, synaptic CPX can rapidly redistribute and this exchange is influenced by neuronal activity, potentially contributing to use-dependent plasticity.

FET proteins regulate lifespan and neuronal integrity

The FET protein family includes FUS, EWS and TAF15 proteins, all of which have been linked to amyotrophic lateral sclerosis, a fatal neurodegenerative disease affecting motor neurons. Here, we show that a reduction of FET proteins in the nematode Caenorhabditis elegans causes synaptic dysfunction accompanied by impaired motor phenotypes. FET proteins are also involved in the regulation of lifespan and stress resistance, acting partially through the insulin/IGF-signalling pathway. We propose that FET proteins are involved in the maintenance of lifespan, cellular stress resistance and neuronal integrity.

Pioneer Axon Navigation Is Controlled by AEX-3, a Guanine Nucleotide Exchange Factor for RAB-3 in Caenorhabditis elegans

Precise and accurate axon tract formation is an essential aspect of brain development. This is achieved by the migration of early outgrowing axons (pioneers) allowing later outgrowing axons (followers) to extend toward their targets in the embryo. In Caenorhabditis elegans the AVG neuron pioneers the right axon tract of the ventral nerve cord, the major longitudinal axon tract. AVG is essential for the guidance of follower axons and hence organization of the ventral nerve cord. In an enhancer screen for AVG axon guidance defects in a nid-1/Nidogen mutant background, we isolated an allele of aex-3 aex-3 mutant animals show highly penetrant AVG axon navigation defects. These defects are dependent on a mutation in nid-1/Nidogen, a basement membrane component. Our data suggest that AEX-3 activates RAB-3 in the context of AVG axon navigation. aex-3 genetically acts together with known players of vesicular exocytosis: unc-64/Syntaxin, unc-31/CAPS, and ida-1/IA-2. Furthermore our genetic interaction data suggest that AEX-3 and the UNC-6/Netrin receptor UNC-5 act in the same pathway, suggesting AEX-3 might regulate the trafficking and/or insertion of UNC-5 at the growth cone to mediate the proper guidance of the AVG axon.

An Inquiry-Based Approach to Study the Synapse: Student-Driven Experiments Using C. elegans

Inquiry-based instruction has been well demonstrated to enhance long term retention and to improve application and synthesis of knowledge. Here we describe an inquiry-based teaching module that trains undergraduates as scientists who pose questions, design and execute hypothesis-driven experiments, analyze data and communicate their research findings. Before students design their research projects, they learn and practice several research techniques with the model organism, Caenorhabditis elegans. This nematode is an ideal choice for experimentation in an undergraduate lab due to its powerful genetics, ease and low cost of maintenance, and amenability for undergraduate training. Students are challenged to characterize an instructor-assigned "mystery mutant" C. elegans strain. The "mystery mutant" strain has a defect in cholinergic synaptic transmission. Students are well poised to experimentally test how the mutation impacts synaptic transmission. For example, students design experiments that address questions including: Does the effected gene influence acetylcholine neurotransmitter release? Does it inhibit postsynaptic cholinergic receptors? Students must apply their understanding of the synapse while using their recently acquired research skills (including aldicarb and levamisole assays) to successfully design, execute and analyze their experiments. Students prepare an experimental plan and a timeline for proposed experiments. Undergraduates work collaboratively in pairs and share their research findings in oral and written formats. Modifications to suit instructor-specific goals and courses with limited or no lab time are provided. Students have anonymously reported their surprise regarding how much can be learned from a worm and feelings of satisfaction from conducting research experiments of their own design.

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.

Distinct Mechanisms Underlie Quiescence during Two Caenorhabditis elegans Sleep-Like States

Electrophysiological recordings have enabled identification of physiologically distinct yet behaviorally similar states of mammalian sleep. In contrast, sleep in nonmammals has generally been identified behaviorally and therefore regarded as a physiologically uniform state characterized by quiescence of feeding and locomotion, reduced responsiveness, and rapid reversibility. The nematode Caenorhabditis elegans displays sleep-like quiescent behavior under two conditions: developmentally timed quiescence (DTQ) occurs during larval transitions, and stress-induced quiescence (SIQ) occurs in response to exposure to cellular stressors. Behaviorally, DTQ and SIQ appear identical. Here, we use optogenetic manipulations of neuronal and muscular activity, pharmacology, and genetic perturbations to uncover circuit and molecular mechanisms of DTQ and SIQ. We find that locomotion quiescence induced by DTQ- and SIQ-associated neuropeptides occurs via their action on the nervous system, although their neuronal target(s) and/or molecular mechanisms likely differ. Feeding quiescence during DTQ results from a loss of pharyngeal muscle excitability, whereas feeding quiescence during SIQ results from a loss of excitability in the nervous system. Together these results indicate that, as in mammals, quiescence is subserved by different mechanisms during distinct sleep-like states in C. elegans.

Conformational states of syntaxin-1 govern the necessity of N-peptide binding in exocytosis of PC12 cells and Caenorhabditis elegans

Syntaxin-1 is the central SNARE protein for neuronal exocytosis. It interacts with Munc18-1 through its cytoplasmic domains, including the N-terminal peptide (N-peptide). Here we examine the role of the N-peptide binding in two conformational states ("closed" vs. "open") of syntaxin-1 using PC12 cells and Caenorhabditis elegans. We show that expression of "closed" syntaxin-1A carrying N-terminal single point mutations (D3R, L8A) that perturb interaction with the hydrophobic pocket of Munc18-1 rescues impaired secretion in syntaxin-1-depleted PC12 cells and the lethality and lethargy of unc-64 (C. elegans orthologue of syntaxin-1)-null mutants. Conversely, expression of the "open" syntaxin-1A harboring the same mutations fails to rescue the impairments. Biochemically, the L8A mutation alone slightly weakens the binding between "closed" syntaxin-1A and Munc18-1, whereas the same mutation in the "open" syntaxin-1A disrupts it. Our results reveal a striking interplay between the syntaxin-1 N-peptide and the conformational state of the protein. We propose that the N-peptide plays a critical role in intracellular trafficking of syntaxin-1, which is dependent on the conformational state of this protein. Surprisingly, however, the N-peptide binding mode seems dispensable for SNARE-mediated exocytosis per se, as long as the protein is trafficked to the plasma membrane.

Natural Variation in plep-1 Causes Male-Male Copulatory Behavior in C. elegans

In sexual species, gametes have to find and recognize one another. Signaling is thus central to sexual reproduction and involves a rapidly evolving interplay of shared and divergent interests [1-4]. Among Caenorhabditis nematodes, three species have evolved self-fertilization, changing the balance of intersexual relations [5]. Males in these androdioecious species are rare, and the evolutionary interests of hermaphrodites dominate. Signaling has shifted accordingly, with females losing behavioral responses to males [6, 7] and males losing competitive abilities [8, 9]. Males in these species also show variable same-sex and autocopulatory mating behaviors [6, 10]. These behaviors could have evolved by relaxed selection on male function, accumulation of sexually antagonistic alleles that benefit hermaphrodites and harm males [5, 11], or neither of these, because androdioecy also reduces the ability of populations to respond to selection [12-14]. We have identified the genetic cause of a male-male mating behavior exhibited by geographically dispersed C. elegans isolates, wherein males mate with and deposit copulatory plugs on one another's excretory pores. We find a single locus of major effect that is explained by segregation of a loss-of-function mutation in an uncharacterized gene, plep-1, expressed in the excretory cell in both sexes. Males homozygous for the plep-1 mutation have excretory pores that are attractive or receptive to copulatory behavior of other males. Excretory pore plugs are injurious and hermaphrodite activity is compromised in plep-1 mutants, so the allele might be unconditionally deleterious, persisting in the population because the species' androdioecious mating system limits the reach of selection.

Syntaxin 1B is important for mouse postnatal survival and proper synaptic function at the mouse neuromuscular junctions

STX1 is a major neuronal syntaxin protein located at the plasma membrane of the neuronal tissues. Rodent STX1 has two highly similar paralogs, STX1A and STX1B, that are thought to be functionally redundant. Interestingly, some studies have shown that the distribution patterns of STX1A and STX1B at the central and peripheral nervous systems only partially overlapped, implying that there might be differential functions between these paralogs. In the current study, we generated an STX1B knockout (KO) mouse line and studied the impact of STX1B removal in neurons of several brain regions and the neuromuscular junction (NMJ). We found that either complete removal of STX1B or selective removal of it from forebrain excitatory neurons in mice caused premature death. Autaptic hippocampal and striatal cultures derived from STX1B KO mice still maintained efficient neurotransmission compared with neurons from STX1B wild-type and heterozygous mice. Interestingly, examining high-density cerebellar cultures revealed a decrease in the spontaneous GABAergic transmission frequency, which was most likely due to a lower number of neurons in the STX1B KO cultures, suggesting that STX1B is essential for neuronal survival in vitro. Moreover, our study also demonstrated that although STX1B is dispensable for the formation of the mouse NMJ, it is required to maintain the efficiency of neurotransmission at the nerve-muscle synapse.

Mechanosensation circuitry in Caenorhabditis elegans: A focus on gentle touch

Forward or reverse movement in Caenorhabditis elegans is the result of sequential contraction of muscle cells arranged along the body. In larvae, muscle cells are innervated by distinct classes of motorneurons. B motorneurons regulate forward movement and A motorneurons regulate backward movement. Ablation of the D motor neurons results in animals that are uncoordinated in either direction, which suggests that D motorneurons regulate the interaction between the two circuits. C. elegans locomotion is dictated by inputs from interneurons that regulate the activity of motorneurons which coordinate muscle contraction to facilitate forward or backwards movement. As C. elegans moves through the environment, sensory neurons interpret chemical and mechanical information which is relayed to the motor neurons that control locomotory direction. A mechanosensory input known as light nose touch can be simulated in the laboratory by touching the nose of the animal with a human eyebrow hair. The recoil reaction that follows from light nose touch appears to be primarily mediated by glutamate release from the polymodal sensory neuron ASH. Numerous glutamate receptor types are found in different neurons and interneurons which suggest that several pathways may regulate the aversive response. Based on the phenotypes of mutants in which neuropeptide processing is abolished, neuropeptides play a role in circuit regulation. The light touch response is also regulated by transient receptor channel proteins and degenerin/epithelial sodium channels which modulate the activity of sensory neurons involved in the nose touch response.