Defects in synaptic vesicle docking in unc-18 mutants

Presynaptic neurons self-tune by inversely coupling neurotransmitter release with the abundance of CaV2 voltage-gated Ca2+ channels

The abundance of CaV2 voltage-gated calcium channels is linked to presynaptic homeostatic plasticity (PHP), a process that recalibrates synaptic strength to maintain the stability of neural circuits. However, the molecular and cellular mechanisms governing PHP and CaV2 channels are not completely understood. Here, we uncover a previously not described form of PHP in Caenorhabditis elegans, revealing an inverse regulatory relationship between the efficiency of neurotransmitter release and the abundance of UNC-2/CaV2 channels. Gain-of-function unc-2SL(S240L) mutants, which carry a mutation analogous to the one causing familial hemiplegic migraine type 1 in humans, showed markedly reduced channel abundance despite increased channel functionality. Reducing synaptic release in these unc-2SL(S240L) mutants restored channel levels to those observed in wild-type animals. Conversely, loss-of-function unc-2DA(D726A) mutants, which harbor the D726A mutation in the channel pore, exhibited a marked increase in channel abundance. Enhancing synaptic release in unc-2DA mutants reversed this increase in channel levels. Importantly, this homeostatic regulation of UNC-2 channel levels is accompanied by the structural remodeling of the active zone (AZ); specifically, unc-2DA mutants, which exhibit increased channel abundance, showed parallel increases in select AZ proteins. Finally, our forward genetic screen revealed that WWP-1, a HECT family E3 ubiquitin ligase, is a key homeostatic mediator that removes UNC-2 from synapses. These findings highlight a self-tuning PHP regulating UNC-2/CaV2 channel abundance along with AZ reorganization, ensuring synaptic strength and stability.

GABAergic/Glycinergic and Glutamatergic Neurons Mediate Distinct Neurodevelopmental Phenotypes of STXBP1 Encephalopathy

An increasing number of pathogenic variants in presynaptic proteins involved in the synaptic vesicle cycle are being discovered in neurodevelopmental disorders. The clinical features of these synaptic vesicle cycle disorders are diverse, but the most prevalent phenotypes include intellectual disability, epilepsy, movement disorders, cerebral visual impairment, and psychiatric symptoms ( Verhage and Sørensen, 2020; Bonnycastle et al., 2021; John et al., 2021; Melland et al., 2021). Among this growing list of synaptic vesicle cycle disorders, the most frequent is STXBP1 encephalopathy caused by de novo heterozygous pathogenic variants in syntaxin-binding protein 1 (STXBP1, also known as MUNC18-1; Verhage and Sørensen, 2020; John et al., 2021). STXBP1 is an essential protein for presynaptic neurotransmitter release. Its haploinsufficiency is the main disease mechanism and impairs both excitatory and inhibitory neurotransmitter release. However, the disease pathogenesis and cellular origins of the broad spectrum of neurological phenotypes are poorly understood. Here we generate cell type-specific Stxbp1 haploinsufficient male and female mice and show that Stxbp1 haploinsufficiency in GABAergic/glycinergic neurons causes developmental delay, epilepsy, and motor, cognitive, and psychiatric deficits, recapitulating majority of the phenotypes observed in the constitutive Stxbp1 haploinsufficient mice and STXBP1 encephalopathy. In contrast, Stxbp1 haploinsufficiency in glutamatergic neurons results in a small subset of cognitive and seizure phenotypes distinct from those caused by Stxbp1 haploinsufficiency in GABAergic/glycinergic neurons. Thus, the contrasting roles of excitatory and inhibitory signaling reveal GABAergic/glycinergic dysfunction as a key disease mechanism of STXBP1 encephalopathy and suggest the possibility to selectively modulate disease phenotypes by targeting specific neurotransmitter systems.

STXBP1: fast-forward to a brighter future - a patient organization perspective

Syntaxin-binding protein 1 related disorder (STXBP1-RD) is a rare neurologic disorder associated with global neurodevelopmental delay, intellectual disability, early-onset epilepsy, motor abnormalities, and autism. The underlying pathophysiology stems from a de novo mutation in the STXBP1 gene, which codes for the STXBP1 protein. The STXBP1 protein is involved in synaptic vesicle fusion and neurotransmitter release. Pathogenic variants in the STXBP1 gene generally result in haploinsufficiency, an impairment in neurotransmitter release, and subsequent dysfunction in neuronal communication. The STXBP1 Foundation was founded in 2017 to support families of children with STXBP1-RD and accelerate the development of effective therapies and, ultimately, a cure for the disorder. The Foundation initially supported research aimed at better understanding the complex phenotypic presentation of the disease as well as the development of animal and cellular models usable by the research community to more fully characterize STXBP1 function and disease pathogenicity. In 2023, the Foundation embarked on its STXBP1 Fast Forward Strategic Plan, which includes a prospective natural history study and substantive biomarker work to drive forward the development of new precision therapies for STXBP1-RD.

Comparison and agreement between two image analysis tools for quantifying GFP::SNB-1 puncta in fshr-1 mutants of C. elegans

Quantitative imaging of synaptic vesicle localization and abundance using fluorescently labeled synaptic vesicle associated proteins like GFP::SNB-1 is a well-established method for measuring changes in synapse structure at neuromuscular junctions (NMJ) in C. elegans . To date, however, the ability to easily and reproducibly measure key parameters at the NMJ - maximum intensity, size of GFP::SNB-1 puncta, density of puncta - has relied on the use of expensive, customizable software that requires coding skills to modify, precluding widespread access and thus preventing standardization within the field. We carried out a comparative evaluation of a new, open-source Fiji puncta plugin versus traditional Igor-based analysis of GFP::SNB-1 imaging data taken of cholinergic motor neurons in the dorsal nerve cord of loss of function mutants in fshr-1 , which encodes a G protein-coupled receptor known to impact GFP::SNB-1 accumulation. We analyzed images taken on a widefield fluorescence microscope, as well as on a spinning disk confocal microscope. Our data demonstrate strong concordance between the differences in GFP::SNB-1 localization in fshr-1 mutants compared to wild type worms across both analysis platforms (Fiji and Igor), as well as across microscope types (widefield and confocal). These data also agree with previously published observations related to synapse number and GFP::SNB-1 intensity in fshr-1 and wild type worms. Based on these findings, we conclude that the Fiji platform is viable as a method for analyzing synaptic vesicle localization and abundance at cholinergic dorsal nerve cord motor NMJs and expect the Fiji puncta plugin to be of broad utility in imaging across a variety of imaging platforms and synaptic markers.

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.

Double mutation of open syntaxin and UNC-18 P334A leads to excitatory-inhibitory imbalance and impairs multiple aspects of C. elegans behavior

SNARE and Sec/Munc18 proteins are essential in synaptic vesicle exocytosis. Open form t-SNARE syntaxin and UNC-18 P334A are well-studied exocytosis-enhancing mutants. Here we investigate the interrelationship between the two mutations by generating double mutants in various genetic backgrounds in C. elegans. While each single mutation rescued the motility of CAPS/unc-31 and synaptotagmin/snt-1 mutants significantly, double mutations unexpectedly worsened motility or lost their rescuing effects. Electrophysiological analyses revealed that simultaneous mutations of open syntaxin and gain-of-function P334A UNC-18 induces a strong imbalance of excitatory over inhibitory transmission. In liposome fusion assays performed with mammalian proteins, the enhancement of fusion caused by the two mutations individually was abolished when the two mutations were introduced simultaneously, consistent with what we observed in C. elegans. We conclude that open syntaxin and P334A UNC-18 do not have additive beneficial effects, and this extends to C. elegans' characteristics such as motility, growth, offspring bared, body size, and exocytosis, as well as liposome fusion in vitro. Our results also reveal unexpected differences between the regulation of exocytosis in excitatory versus inhibitory synapses.

Partial inhibition of class III PI3K VPS-34 ameliorates motor aging and prolongs health span

Global increase of life expectancy is rarely accompanied by increased health span, calling for a greater understanding of age-associated behavioral decline. Motor independence is strongly associated with the quality of life of elderly people, yet the regulators for motor aging have not been systematically explored. Here, we designed a fast and efficient genome-wide screening assay in Caenorhabditis elegans and identified 34 consistent genes as potential regulators of motor aging. Among the top hits, we found VPS-34, the class III phosphatidylinositol 3-kinase that phosphorylates phosphatidylinositol (PI) to phosphatidylinositol 3-phosphate (PI(3)P), regulates motor function in aged but not young worms. It primarily functions in aged motor neurons by inhibiting PI(3)P-PI-PI(4)P conversion to reduce neurotransmission at the neuromuscular junction (NMJ). Genetic and pharmacological inhibition of VPS-34 improve neurotransmission and muscle integrity, ameliorating motor aging in both worms and mice. Thus, our genome-wide screening revealed an evolutionarily conserved, actionable target to delay motor aging and prolong health span.

Altered synaptic connectivity in an in vitro human model of STXBP1 encephalopathy

Early infantile developmental and epileptic encephalopathies are devastating conditions, generally of genetic origin, but the pathological mechanisms often remain obscure. A major obstacle in this field of research is the difficulty of studying cortical brain development in humans, at the relevant time period in utero. To address this, we established an in vitro assay to study the impact of gene variants on the developing human brain by using living organotypic cultures of the human subplate and neighbouring cortical regions, prepared from ethically sourced, 14-17 post-conception week brain tissue (www.hdbr.org). We were able to maintain cultures for several months, during which time the gross anatomical structures of the cortical plate, subplate and marginal zone persisted, while neurons continued to develop morphologically and form new synaptic networks. This preparation thus permits the study of genetic manipulations and their downstream effects on an intact developing human cortical network. We focused on STXBP1 haploinsufficiency, which is among the most common genetic causes of developmental and epileptic encephalopathy. This was induced using shRNA interference, leading to impaired synaptic function and a reduced density of glutamatergic synapses. We thereby provide a critical proof-of-principle for how to study the impact of any gene of interest on the development of the human cortex.

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.

SNARE Proteins in Synaptic Vesicle Fusion

Neurotransmitters are stored in small membrane-bound vesicles at synapses; a subset of synaptic vesicles is docked at release sites. Fusion of docked vesicles with the plasma membrane releases neurotransmitters. Membrane fusion at synapses, as well as all trafficking steps of the secretory pathway, is mediated by SNARE proteins. The SNAREs are the minimal fusion machinery. They zipper from N-termini to membrane-anchored C-termini to form a 4-helix bundle that forces the apposed membranes to fuse. At synapses, the SNAREs comprise a single helix from syntaxin and synaptobrevin; SNAP-25 contributes the other two helices to complete the bundle. Unc13 mediates synaptic vesicle docking and converts syntaxin into the permissive "open" configuration. The SM protein, Unc18, is required to initiate and proofread SNARE assembly. The SNAREs are then held in a half-zippered state by synaptotagmin and complexin. Calcium removes the synaptotagmin and complexin block, and the SNAREs drive vesicle fusion. After fusion, NSF and alpha-SNAP unwind the SNAREs and thereby recharge the system for further rounds of fusion. In this chapter, we will describe the discovery of the SNAREs, their relevant structural features, models for their function, and the central role of Unc18. In addition, we will touch upon the regulation of SNARE complex formation by Unc13, complexin, and synaptotagmin.

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.

The Role of Tomosyn in the Regulation of Neurotransmitter Release

Soluble NSF attachment protein receptor (SNARE) proteins play a central role in synaptic vesicle (SV) exocytosis. These proteins include the vesicle-associated SNARE protein (v-SNARE) synaptobrevin and the target membrane-associated SNARE proteins (t-SNAREs) syntaxin and SNAP-25. Together, these proteins drive membrane fusion between synaptic vesicles (SV) and the presynaptic plasma membrane to generate SV exocytosis. In the presynaptic active zone, various proteins may either enhance or inhibit SV exocytosis by acting on the SNAREs. Among the inhibitory proteins, tomosyn, a syntaxin-binding protein, is of particular importance because it plays a critical and evolutionarily conserved role in controlling synaptic transmission. In this chapter, we describe how tomosyn was discovered, how it interacts with SNAREs and other presynaptic regulatory proteins to regulate SV exocytosis and synaptic plasticity, and how its various domains contribute to its synaptic functions.

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.

Neuronal SNARE complex assembly guided by Munc18-1 and Munc13-1

Neurotransmitter release by Ca²⁺ -triggered synaptic vesicle exocytosis is essential for information transmission in the nervous system. The soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs) syntaxin-1, SNAP-25, and synaptobrevin-2 form the SNARE complex to bring synaptic vesicles and the plasma membranes together and to catalyze membrane fusion. Munc18-1 and Munc13-1 regulate synaptic vesicle priming via orchestrating neuronal SNARE complex assembly. In this review, we summarize recent advances toward the functions and molecular mechanisms of Munc18-1 and Munc13-1 in guiding neuronal SNARE complex assembly, and discuss the functional similarities and differences between Munc18-1 and Munc13-1 in neurons and their homologs in other intracellular membrane trafficking systems.

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.

A de novo nonsense mutation of STXBP1 causes early-onset epileptic encephalopathy

Mutations in syntaxin-binding protein 1, STXBP1 (also known as MUNC18-1), are linked to multiple neurodevelopmental disorders, including severe early-onset epileptic encephalopathies (EOEEs). A de novo nonsense mutation of STXBP1 (c. 863G > A, p. W288X) was found in a patient diagnosed with EOEE at the age of 17 days. The electroencephalogram (EEG) showed sharp waves and spikes, while brain magnetic resonance imaging was normal. We generated a zebrafish EOEE model by overexpressing mutant STXBP1(W288X) and studied the behavioral changes further to understand the mechanism of W288X mutation in epileptogenesis. In addition, effective antiepileptic drugs were screened in the zebrafish model. Zebrafish STXBP1 homologs were highly conserved and prominently expressed in the larval zebrafish brain. The Tg(hSTXBP1^W288X) zebrafish larvae exhibited hyperactivity compared with the wild-type (WT) controls. The expression of STXBP1 decreased during the development course from 1 to 5 days post fertilization. Spontaneous seizures and increased c-fos expression were observed in the mutant zebrafish larvae. The susceptibility of Tg(hSTXBP1^W288X) zebrafish to pentylenetetrazol challenge also dramatically increased. Levetiracetam, clonazepam, and topiramate showed antiepileptic effects in the Tg(hSTXBP1^W288X) larvae to different extents. Our findings in the newly generated mutant line of zebrafish suggested that zebrafish recapitulated clinical phenotypes associated with human STXBP1 mutation, which provided an appropriate in vivo model for epilepsy research.

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.

A perspective on C. elegans neurodevelopment: from early visionaries to a booming neuroscience research

The formation of the nervous system and its striking complexity is a remarkable feat of development. C. elegans served as a unique model to dissect the molecular events in neurodevelopment, from its early visionaries to the current booming neuroscience community. Soon after being introduced as a model, C. elegans was mapped at the level of genes, cells, and synapses, providing the first metazoan with a complete cell lineage, sequenced genome, and connectome. Here, I summarize mechanisms underlying C. elegans neurodevelopment, from the generation and diversification of neural components to their navigation and connectivity. I point out recent noteworthy findings in the fields of glia biology, sex dimorphism and plasticity in neurodevelopment, highlighting how current research connects back to the pioneering studies by Brenner, Sulston and colleagues. Multifaceted investigations in model organisms, connecting genes to cell function and behavior, expand our mechanistic understanding of neurodevelopment while allowing us to formulate emerging questions for future discoveries.

p.His16Arg of STXBP1 (MUNC18-1) Associated With Syntaxin 3B Causes Autosomal Dominant Congenital Nystagmus

Congenital nystagmus (CN) is an ocular movement disorder manifested as involuntary conjugated binocular oscillation and usually occurs in early infancy. The pathological mechanism underlying CN is still poorly understood. We mapped a novel genetic locus 9q33.1-q34.2 in a larger Chinese family with autosomal dominant CN and identified a variant (c.47A>G/p.His16Arg) of STXBP1 by exome sequencing, which fully co-segregated with the nystagmus phenotype in this family and was absent in 571 healthy unrelated individuals. The STXBP1 encodes syntaxin binding protein 1 (also known as MUNC18-1), which plays a pivotal role in neurotransmitter release. In unc-18 (nematode homolog of MUNC18-1) null Caenorhabditis elegans, we found that the p.His16Arg exhibits a compromised ability to rescue the locomotion defect and aldicarb sensitivity, indicating a functional defect in neurotransmitter release. In addition, we also found an enhanced binding of the p.His16Arg mutant to syntaxin 3B, which is a homolog of syntaxin 1A and specifically located in retinal ribbon synapses. We hypothesize that the variant p.His16Arg of STXBP1 is likely to affect neurotransmitter release in the retina, which may be the underlying etiology of CN in this family. Our results provide a new perspective on understanding the molecular mechanism of CN.

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.

Axon growth and synaptic function: A balancing act for axonal regeneration and neuronal circuit formation in CNS trauma and disease

Axons in the adult mammalian central nervous system (CNS) fail to regenerate inside out due to intrinsic and extrinsic neuronal determinants. During CNS development, axon growth, synapse formation, and function are tightly regulated processes allowing immature neurons to effectively grow an axon, navigate toward target areas, form synaptic contacts and become part of information processing networks that control behavior in adulthood. Not only immature neurons are able to precisely control the expression of a plethora of genes necessary for axon extension and pathfinding, synapse formation and function, but also non-neuronal cells such as astrocytes and microglia actively participate in sculpting the nervous system through refinement, consolidation, and elimination of synaptic contacts. Recent evidence indicates that a balancing act between axon regeneration and synaptic function may be crucial for rebuilding functional neuronal circuits after CNS trauma and disease in adulthood. Here, we review the role of classical and new intrinsic and extrinsic neuronal determinants in the context of CNS development, injury, and disease. Moreover, we discuss strategies targeting neuronal and non-neuronal cell behaviors, either alone or in combination, to promote axon regeneration and neuronal circuit formation in adulthood.

Structural and mechanistic insights into secretagogin-mediated exocytosis

Secretagogin (SCGN) is a hexa-EF-hand protein that is highly expressed in the pancreas, brain, and gastrointestinal tract. SCGN is known to modulate regulated exocytosis in multiple cell lines and tissues; however, its exact functions and underlying mechanisms remain unclear. Here, we report that SCGN interacts with the plasma membrane SNARE SNAP-25, but not the assembled SNARE complex, in a Ca²⁺-dependent manner. The crystal structure of SCGN in complex with a SNAP-25 fragment reveals that SNAP-25 adopts a helical structure and binds to EF-hands 5 and 6 of SCGN. SCGN strongly inhibits SNARE-mediated vesicle fusion in vitro by binding to SNAP-25. SCGN promotes the plasma membrane localization of SNAP-25, but not Syntaxin-1a, in SCGN-expressing cells. Finally, SCGN controls neuronal growth and brain development in zebrafish, likely via interacting with SNAP-25 or its close homolog, SNAP-23. Our results thus provide insights into the regulation of SNAREs and suggest that aberrant synapse functions underlie multiple neurological disorders caused by SCGN deficiency.

Functional analysis of epilepsy-associated variants in STXBP1/Munc18-1 using humanized Caenorhabditis elegans

OBJECTIVE

Genetic variants in STXBP1, which encodes the conserved exocytosis protein Munc18-1, are associated with a variety of infantile epilepsy syndromes. We aimed to develop an in vivo Caenorhabditis elegans model that could be used to test the pathogenicity of such variants in a cost-effective manner.

METHODS

The CRISPR/Cas9 method was used to introduce a null mutation into the unc-18 gene (the C. elegans orthologue of STXBP1), thereby creating a paralyzed worm strain. We subsequently rescued this strain with transgenes encoding the human STXBP1/Munc18-1 protein (wild-type and eight different epilepsy-associated missense variants). The resulting humanized worm strains were then analyzed via behavioral, electrophysiological, and biochemical approaches.

RESULTS

Transgenic expression of wild-type human STXBP1 protein fully rescued locomotion in both solid and liquid media to the same level as the standard wild-type worm strain, Bristol N2. Six variant strains (E59K, V84D, C180Y, R292H, L341P, R551C) exhibited impaired locomotion, whereas two (P335L, R406H) were no different from worms expressing wild-type STXBP1. Electrophysiological recordings revealed that all eight variant strains displayed less frequent and more irregular pharyngeal pumping in comparison to wild-type STXBP1-expressing strains. Four strains (V84D, C180Y, R292H, P335L) exhibited pentylenetetrazol-induced convulsions in an acute assay of seizure-like activity, in contrast to worms expressing wild-type STXBP1. No differences were seen between wild-type and variant STXBP1 strains in terms of mRNA abundance. However, STXBP1 protein levels were reduced to 20%-30% of wild-type in all variants, suggesting that the mutations result in STXBP1 protein instability.

SIGNIFICANCE

The approach described here is a cost-effective in vivo method for establishing the pathogenicity of genetic variants in STXBP1 and potentially other conserved neuronal proteins. Furthermore, the humanized strains we created could potentially be used in the future for high-throughput drug screens to identify novel therapeutics.

Stxbp1/Munc18-1 haploinsufficiency impairs inhibition and mediates key neurological features of STXBP1 encephalopathy

Mutations in genes encoding synaptic proteins cause many neurodevelopmental disorders, with the majority affecting postsynaptic apparatuses and much fewer in presynaptic proteins. Syntaxin-binding protein 1 (STXBP1, also known as MUNC18-1) is an essential component of the presynaptic neurotransmitter release machinery. De novo heterozygous pathogenic variants in STXBP1 are among the most frequent causes of neurodevelopmental disorders including intellectual disabilities and epilepsies. These disorders, collectively referred to as STXBP1 encephalopathy, encompass a broad spectrum of neurologic and psychiatric features, but the pathogenesis remains elusive. Here we modeled STXBP1 encephalopathy in mice and found that Stxbp1 haploinsufficiency caused cognitive, psychiatric, and motor dysfunctions, as well as cortical hyperexcitability and seizures. Furthermore, Stxbp1 haploinsufficiency reduced cortical inhibitory neurotransmission via distinct mechanisms from parvalbumin-expressing and somatostatin-expressing interneurons. These results demonstrate that Stxbp1 haploinsufficient mice recapitulate cardinal features of STXBP1 encephalopathy and indicate that GABAergic synaptic dysfunction is likely a crucial contributor to disease pathogenesis.

Molecular Modelling and Dynamics Study of nsSNP in STXBP1 Gene in Early Infantile Epileptic Encephalopathy Disease

Early Infantile Epileptic Encephalopathy (known as Ohtahara Syndrome) is one of the most severe and earliest forms of epilepsy, characterized by early seizures onset. It affects newborns and children between two and six years old. Among the genes that have been associated with early infantile epileptic encephalopathy, the STXBP1 gene, which encodes the Syntaxin binding protein1a that is involved in SNARE complex formation, contributes to synaptic vesicles exocytosis. The aim of this study was to identify the most pathogenic polymorphisms of STXBP1 gene and determine their impact on the structure and stability of Stxbp1 protein. The high-risk nonsynonymous single nucleotide polymorphisms (nsSNPs) in the STXBP1 gene were predicted using 13 bioinformatics tools. The conservation analysis was realized by CONSURF web server. The analysis of the impact of the pathogenic SNPs on the structure of Stxbp1 protein was realized using YASARA software, and the molecular dynamics simulation was performed using GROMACS software. Out of 245 nsSNPs, we identified 11 (S42P, H103D R190W, R235G, D238E, L256P, P335S, C354Y, L365V, R406C, and G544D) as deleterious using in silico prediction tools. Conservation analysis results revealed that all these nsSNPs were located in conserved regions. The comparison of the hydrogen and hydrophobic interactions in the wild type Stxbp1 structure and its mutant forms showed that all these nsSNPs affect the protein structure on different levels. The molecular dynamics simulations revealed that the total of nsSNPs affect the protein stability, residual fluctuation, and the compaction at different levels. This study provides helpful information on high risk nsSNPs that may affect the Stxbp1 protein structure and function. Thus, these variants should be taken into consideration during the genetic screening of patients suffering from early infantile epileptic encephalopathy.

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.

Creation of a myosin Va-TAP-tagged mouse and identification of potential myosin Va-interacting proteins in the cerebellum

The actin-based motor myosin Va transports numerous cargos, including the smooth endoplasmic reticulum (SER) in cerebellar Purkinje neurons (PNs) and melanosomes in melanocytes. Identifying proteins that interact with this myosin is key to understanding its cellular functions. Toward that end, we used recombineering to insert via homologous recombination a tandem affinity purification (TAP) tag composed of the immunoglobulin G-binding domain of protein A, a tobacco etch virus cleavage site, and a FLAG tag into the mouse MYO5A locus immediately after the initiation codon. Importantly, we provide evidence that the TAP-tagged version of myosin Va (TAP-MyoVa) functions normally in terms of SER transport in PNs and melanosome positioning in melanocytes. Given this and other evidence that TAP-MyoVa is fully functional, we purified it together with associated proteins directly from juvenile mouse cerebella and subjected the samples to mass spectroscopic analyses. As expected, known myosin Va-binding partners like dynein light chain were identified. Importantly, numerous novel interacting proteins were also tentatively identified, including guanine nucleotide-binding protein G(o) subunit alpha (Gnao1), a biomarker for schizophrenia. Consistently, an antibody to Gnao1 immunoprecipitates myosin Va, and Gnao1's localization to PN dendritic spines depends on myosin Va. The mouse model created here should facilitate the identification of novel myosin Va-binding partners, which in turn should advance our understanding of the roles played by this important myosin in vivo.

Defining the Kv2.1-syntaxin molecular interaction identifies a first-in-class small molecule neuroprotectant

The neuronal cell death-promoting loss of cytoplasmic K+ following injury is mediated by an increase in Kv2.1 potassium channels in the plasma membrane. This phenomenon relies on Kv2.1 binding to syntaxin 1A via 9 amino acids within the channel intrinsically disordered C terminus. Preventing this interaction with a cell and blood-brain barrier-permeant peptide is neuroprotective in an in vivo stroke model. Here a rational approach was applied to define the key molecular interactions between syntaxin and Kv2.1, some of which are shared with mammalian uncoordinated-18 (munc18). Armed with this information, we found a small molecule Kv2.1-syntaxin-binding inhibitor (cpd5) that improves cortical neuron survival by suppressing SNARE-dependent enhancement of Kv2.1-mediated currents following excitotoxic injury. We validated that cpd5 selectively displaces Kv2.1-syntaxin-binding peptides from syntaxin and, at higher concentrations, munc18, but without affecting either synaptic or neuronal intrinsic properties in brain tissue slices at neuroprotective concentrations. Collectively, our findings provide insight into the role of syntaxin in neuronal cell death and validate an important target for neuroprotection.

Nanomachinery Organizing Release at Neuronal and Ribbon Synapses

A critical aim in neuroscience is to obtain a comprehensive view of how regulated neurotransmission is achieved. Our current understanding of synapses relies mainly on data from electrophysiological recordings, imaging, and molecular biology. Based on these methodologies, proteins involved in a synaptic vesicle (SV) formation, mobility, and fusion at the active zone (AZ) membrane have been identified. In the last decade, electron tomography (ET) combined with a rapid freezing immobilization of neuronal samples opened a window for understanding the structural machinery with the highest spatial resolution in situ. ET provides significant insights into the molecular architecture of the AZ and the organelles within the presynaptic nerve terminal. The specialized sensory ribbon synapses exhibit a distinct architecture from neuronal synapses due to the presence of the electron-dense synaptic ribbon. However, both synapse types share the filamentous structures, also commonly termed as tethers that are proposed to contribute to different steps of SV recruitment and exocytosis. In this review, we discuss the emerging views on the role of filamentous structures in SV exocytosis gained from ultrastructural studies of excitatory, mainly central neuronal compared to ribbon-type synapses with a focus on inner hair cell (IHC) ribbon synapses. Moreover, we will speculate on the molecular entities that may be involved in filament formation and hence play a crucial role in the SV cycle.

Unc13: a multifunctional synaptic marvel

Nervous systems are built on synaptic connections, and our understanding of these complex compartments has deepened over the past quarter century as the diverse fields of genetics, molecular biology, physiology, and biochemistry each made significant in-roads into synaptic function. On the presynaptic side, an evolutionarily conserved core fusion apparatus constructed from a handful of proteins has emerged, with Unc13 serving as a hub that coordinates nearly every aspect of synaptic transmission. This review briefly highlights recent studies on diverse aspects of Unc13 function including roles in SNARE assembly and quality control, release site building, calcium channel proximity, and short-term synaptic plasticity.

Functional Reconstitution of Intracellular Vesicle Fusion Using Purified SNAREs and Sec1/Munc18 (SM) Proteins

The fusion of intracellular vesicles with target membranes is mediated by two classes of conserved molecules-soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAP receptors or SNAREs) and Sec1/Munc18 (SM) proteins. A conserved function of SM proteins is to recognize their cognate trans-SNARE complexes and accelerate fusion kinetics. Here, we describe a physiologically relevant reconstitution system in which macromolecular crowding agents are included to recapitulate the crowded intracellular environment. Through this system, we elucidate the molecular mechanisms by which SNAREs and SM proteins drive vesicle fusion.

Molecular mechanisms that stabilize short term synaptic plasticity during presynaptic homeostatic plasticity

Presynaptic homeostatic plasticity (PHP) compensates for impaired postsynaptic neurotransmitter receptor function through a rapid, persistent adjustment of neurotransmitter release, an effect that can exceed 200%. An unexplained property of PHP is the preservation of short-term plasticity (STP), thereby stabilizing activity-dependent synaptic information transfer. We demonstrate that the dramatic potentiation of presynaptic release during PHP is achieved while simultaneously maintaining a constant ratio of primed to super-primed synaptic vesicles, thereby preserving STP. Mechanistically, genetic, biochemical and electrophysiological evidence argue that a constant ratio of primed to super-primed synaptic vesicles is achieved by the concerted action of three proteins: Unc18, Syntaxin1A and RIM. Our data support a model based on the regulated availability of Unc18 at the presynaptic active zone, a process that is restrained by Syntaxin1A and facilitated by RIM. As such, regulated vesicle priming/super-priming enables PHP to stabilize both synaptic gain and the activity-dependent transfer of information at a synapse.

Molecular Interface of Neuronal Innate Immunity, Synaptic Vesicle Stabilization, and Presynaptic Homeostatic Plasticity

We define a homeostatic function for innate immune signaling within neurons. A genetic analysis of the innate immune signaling genes IMD, IKKβ, Tak1, and Relish demonstrates that each is essential for presynaptic homeostatic plasticity (PHP). Subsequent analyses define how the rapid induction of PHP (occurring in seconds) can be coordinated with the life-long maintenance of PHP, a time course that is conserved from invertebrates to mammals. We define a novel bifurcation of presynaptic innate immune signaling. Tak1 (Map3K) acts locally and is selective for rapid PHP induction. IMD, IKKβ, and Relish are essential for long-term PHP maintenance. We then define how Tak1 controls vesicle release. Tak1 stabilizes the docked vesicle state, which is essential for the homeostatic expansion of the readily releasable vesicle pool. This represents a mechanism for the control of vesicle release, and an interface of innate immune signaling with the vesicle fusion apparatus and homeostatic plasticity.

The N-peptide-binding mode is critical to Munc18-1 function in synaptic exocytosis

Sec1/Munc18 (SM) proteins promote intracellular vesicle fusion by binding to N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). A key SNARE-binding mode of SM proteins involves the N-terminal peptide (N-peptide) motif of syntaxin, a SNARE subunit localized to the target membrane. In in vitro membrane fusion assays, inhibition of N-peptide motif binding previously has been shown to abrogate the stimulatory function of Munc18-1, a SM protein involved in synaptic exocytosis in neurons. The physiological role of the N-peptide-binding mode, however, remains unclear. In this work, we addressed this key question using a "clogged" Munc18-1 protein, in which an ectopic copy of the syntaxin N-peptide motif was directly fused to Munc18-1. We found that the ectopic N-peptide motif blocks the N-peptide-binding pocket of Munc18-1, preventing the latter from binding to the native N-peptide motif on syntaxin-1. In a reconstituted system, we observed that clogged Munc18-1 is defective in promoting SNARE zippering. When introduced into induced neuronal cells (iN cells) derived from human pluripotent stem cells, clogged Munc18-1 failed to mediate synaptic exocytosis. As a result, both spontaneous and evoked synaptic transmission was abolished. These genetic findings provide direct evidence for the crucial role of the N-peptide-binding mode of Munc18-1 in synaptic exocytosis. We suggest that clogged SM proteins will also be instrumental in defining the physiological roles of the N-peptide-binding mode in other vesicle-fusion pathways.

Mechanism-based rescue of Munc18-1 dysfunction in varied encephalopathies by chemical chaperones

Heterozygous de novo mutations in the neuronal protein Munc18-1 are linked to epilepsies, intellectual disability, movement disorders, and neurodegeneration. These devastating diseases have a poor prognosis and no known cure, due to lack of understanding of the underlying disease mechanism. To determine how mutations in Munc18-1 cause disease, we use newly generated S. cerevisiae strains, C. elegans models, and conditional Munc18-1 knockout mouse neurons expressing wild-type or mutant Munc18-1, as well as in vitro studies. We find that at least five disease-linked missense mutations of Munc18-1 result in destabilization and aggregation of the mutant protein. Aggregates of mutant Munc18-1 incorporate wild-type Munc18-1, depleting functional Munc18-1 levels beyond hemizygous levels. We demonstrate that the three chemical chaperones 4-phenylbutyrate, sorbitol, and trehalose reverse the deficits caused by mutations in Munc18-1 in vitro and in vivo in multiple models, offering a novel strategy for the treatment of varied encephalopathies.

Munc18-1 is an evolutionary conserved gene whose mutations are linked to various neurological diseases in human. In order to better understand the exact nature of the mutations, the authors here utilize several model systems to show mutant Munc18-1 can aggregate and deplete functional pool of Wt protein, and that chemical chaperones can reverse the cellular deficits.

SNARE zippering requires activation by SNARE-like peptides in Sec1/Munc18 proteins

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) catalyze membrane fusion by forming coiled-coil bundles between membrane bilayers. The SNARE bundle zippers progressively toward the membranes, pulling the lipid bilayers into close proximity to fuse. In this work, we found that the +1 and +2 layers in the C-terminal domains (CTDs) of SNAREs are dispensable for reconstituted SNARE-mediated fusion reactions. By contrast, all CTD layers are required for fusion reactions activated by the cognate Sec1/Munc18 (SM) protein or a synthetic Vc peptide derived from the vesicular (v-) SNARE, correlating with strong acceleration of fusion kinetics. These results suggest a similar mechanism underlying the stimulatory functions of SM proteins and Vc peptide in SNARE-dependent membrane fusion. Unexpectedly, we identified a conserved SNARE-like peptide (SLP) in SM proteins that structurally and functionally resembles Vc peptide. Like Vc peptide, SLP binds and activates target (t-) SNAREs, accelerating the fusion reaction. Disruption of the t-SNARE-SLP interaction inhibits exocytosis in vivo. Our findings demonstrated that a t-SNARE-SLP intermediate must form before SNAREs can drive efficient vesicle fusion.

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.

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.

Revisiting interaction specificity reveals neuronal and adipocyte Munc18 membrane fusion regulatory proteins differ in their binding interactions with partner SNARE Syntaxins

The efficient delivery of cellular cargo relies on the fusion of cargo-carrying vesicles with the correct membrane at the correct time. These spatiotemporal fusion events occur when SNARE proteins on the vesicle interact with cognate SNARE proteins on the target membrane. Regulatory Munc18 proteins are thought to contribute to SNARE interaction specificity through interaction with the SNARE protein Syntaxin. Neuronal Munc18a interacts with Syntaxin1 but not Syntaxin4, and adipocyte Munc18c interacts with Syntaxin4 but not Syntaxin1. Here we show that this accepted view of specificity needs revision. We find that Munc18c interacts with both Syntaxin4 and Syntaxin1, and appears to bind “non-cognate” Syntaxin1 a little more tightly than Syntaxin4. Munc18a binds Syntaxin1 and Syntaxin4, though it interacts with its cognate Syntaxin1 much more tightly. We also observed that when bound to non-cognate Munc18c, Syntaxin1 captures its neuronal SNARE partners SNAP25 and VAMP2, and Munc18c can bind to pre-formed neuronal SNARE ternary complex. These findings reveal that Munc18a and Munc18c bind Syntaxins differently. Munc18c relies principally on the Syntaxin N-peptide interaction for binding Syntaxin4 or Syntaxin1, whereas Munc18a can bind Syntaxin1 tightly whether or not the Syntaxin1 N-peptide is present. We conclude that Munc18a and Munc18c differ in their binding interactions with Syntaxins: Munc18a has two tight binding modes/sites for Syntaxins as defined previously but Munc18c has just one that requires the N-peptide. These results indicate that the interactions between Munc18 and Syntaxin proteins, and the consequences for in vivo function, are more complex than can be accounted for by binding specificity alone.

Munc18a clusters SNARE-bearing liposomes prior to trans-SNARE zippering

Sec1-Munc18 (SM) proteins co-operate with SNAREs {SNAP [soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein] receptors} to mediate membrane fusion in eukaryotic cells. Studies of Munc18a/Munc18-1/Stxbp1 in neurotransmission suggest that SM proteins accelerate fusion kinetics primarily by activating the partially zippered trans-SNARE complex. However, accumulating evidence has argued for additional roles for SM proteins in earlier steps in the fusion cascade. Here, we investigate the function of Munc18a in reconstituted exocytic reactions mediated by neuronal and non-neuronal SNAREs. We show that Munc18a plays a direct role in promoting proteoliposome clustering, underlying vesicle docking during exocytosis. In the three different fusion reactions examined, Munc18a-dependent clustering requires an intact N-terminal peptide (N-peptide) motif in syntaxin that mediates the binary interaction between syntaxin and Munc18a. Importantly, clustering is preserved under inhibitory conditions that abolish both trans-SNARE complex formation and lipid mixing, indicating that Munc18a promotes membrane clustering in a step that is independent of trans-SNARE zippering and activation.

Epileptic Encephalopathies-Clinical Syndromes and Pathophysiological Concepts

Epileptic encephalopathies account for a large proportion of the intractable early-onset epilepsies and are characterized by frequent seizures and poor developmental outcome. The epileptic encephalopathies can be loosely divided into two related groups of named syndromes. The first comprises epilepsies where continuous EEG changes directly result in cognitive and developmental dysfunction. The second includes patients where cognitive impairment is present at seizure onset and is due to the underlying etiology but the epileptic activity may then worsen the cognitive abilities over time. Recent, large-scale exome studies have begun to establish the genetic architecture of the epileptic encephalopathies, resulting in a re-consideration of the boundaries of these named syndromes. The emergence of this genetic architecture has lead to three main pathophysiological concepts to provide a mechanistic framework for these disorders. In this article, we will review the classic syndromes, the most significant genetic findings, and relate both to the pathophysiological understanding of epileptic encephalopathies.

UNC-18 and Tomosyn Antagonistically Control Synaptic Vesicle Priming Downstream of UNC-13 in Caenorhabditis elegans

Munc18-1/UNC-18 is believed to prime SNARE-mediated membrane fusion, yet the underlying mechanisms remain enigmatic. Here, we examine how potential gain-of-function mutations of Munc18-1/UNC-18 affect locomotory behavior and synaptic transmission, and how Munc18-1-mediated priming is related to Munc13-1/UNC-13 and Tomosyn/TOM-1, positive and negative SNARE regulators, respectively. We show that a Munc18-1(P335A)/UNC-18(P334A) mutation leads to significantly increased locomotory activity and acetylcholine release in Caenorhabditis elegans, as well as enhanced synaptic neurotransmission in cultured mammalian neurons. Importantly, similar to tom-1 null mutants, unc-18(P334A) mutants partially bypass the requirement of UNC-13. Moreover, unc-18(P334A) and tom-1 null mutations confer a strong synergy in suppressing the phenotypes of unc-13 mutants. Through biochemical experiments, we demonstrate that Munc18-1(P335A) exhibits enhanced activity in SNARE complex formation as well as in binding to the preformed SNARE complex, and partially bypasses the Munc13-1 requirement in liposome fusion assays. Our results indicate that Munc18-1/UNC-18 primes vesicle fusion downstream of Munc13-1/UNC-13 by templating SNARE complex assembly and acts antagonistically with Tomosyn/TOM-1.SIGNIFICANCE STATEMENT At presynaptic sites, SNARE-mediated membrane fusion is tightly regulated by several key proteins including Munc18/UNC-18, Munc13/UNC-13, and Tomosyn/TOM-1. However, how these proteins interact with each other to achieve the precise regulation of neurotransmitter release remains largely unclear. Using Caenorhabditis elegans as an in vivo model, we found that a gain-of-function mutant of UNC-18 increases locomotory activity and synaptic acetylcholine release, that it partially bypasses the requirement of UNC-13 for release, and that this bypass is synergistically augmented by the lack of TOM-1. We also elucidated the biochemical basis for the gain-of-function caused by this mutation. Thus, our study provides novel mechanistic insights into how Munc18/UNC-18 primes synaptic vesicle release and how this protein interacts functionally with Munc13/UNC-13 and Tomosyn/TOM-1.

Munc13-1 and Munc18-1 together prevent NSF-dependent de-priming of synaptic vesicles

Synaptic transmission requires a stable pool of release-ready (primed) vesicles. Here we show that two molecules involved in SNARE-complex assembly, Munc13-1 and Munc18-1, together stabilize release-ready vesicles by preventing de-priming. Replacing neuronal Munc18-1 by a non-neuronal isoform Munc18-2 (Munc18-1/2SWAP) supports activity-dependent priming, but primed vesicles fall back into a non-releasable state (de-prime) within seconds. Munc13-1 deficiency produces a similar defect. Inhibitors of N-ethylmaleimide sensitive factor (NSF), N-ethylmaleimide (NEM) or interfering peptides, prevent de-priming in munc18-1/2SWAP or munc13-1 null synapses, but not in CAPS-1/2 null, another priming-deficient mutant. NEM rescues synaptic transmission in munc13-1 null and munc18-1/2SWAP synapses, in acute munc13-1 null slices and even partially in munc13-1/2 double null synapses. Together these data indicate that Munc13-1 and Munc18-1, but not CAPS-1/2, stabilize primed synaptic vesicles by preventing NSF-dependent de-priming.

The molecular mechanism underlying the generation and maintenance of the readily releasable pool composed of primed synaptic vesicles is only partially known. Here the authors show that in mouse primary neurons, Munc13-1 and Munc18-1 stabilize primed synaptic vesicles by preventing NSF-dependent de-priming.

Molecular regulation of insulin granule biogenesis and exocytosis

Type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by hyperglycemia, insulin resistance and hyperinsulinemia in early disease stages but a relative insulin insufficiency in later stages. Insulin, a peptide hormone, is produced in and secreted from pancreatic β-cells following elevated blood glucose levels. Upon its release, insulin induces the removal of excessive exogenous glucose from the bloodstream primarily by stimulating glucose uptake into insulin-dependent tissues as well as promoting hepatic glycogenesis. Given the increasing prevalence of T2DM worldwide, elucidating the underlying mechanisms and identifying the various players involved in the synthesis and exocytosis of insulin from β-cells is of utmost importance. This review summarizes our current understanding of the route insulin takes through the cell after its synthesis in the endoplasmic reticulum as well as our knowledge of the highly elaborate network that controls insulin release from the β-cell. This network harbors potential targets for anti-diabetic drugs and is regulated by signaling cascades from several endocrine systems.

Autoinhibition of Munc18-1 modulates synaptobrevin binding and helps to enable Munc13-dependent regulation of membrane fusion

Munc18-1 orchestrates SNARE complex assembly together with Munc13-1 to mediate neurotransmitter release. Munc18-1 binds to synaptobrevin, but the relevance of this interaction and its relation to Munc13 function are unclear. NMR experiments now show that Munc18-1 binds specifically and non-specifically to synaptobrevin. Specific binding is inhibited by a L348R mutation in Munc18-1 and enhanced by a D326K mutation designed to disrupt the ‘furled conformation’ of a Munc18-1 loop. Correspondingly, the activity of Munc18-1 in reconstitution assays that require Munc18-1 and Munc13-1 for membrane fusion is stimulated by the D326K mutation and inhibited by the L348R mutation. Moreover, the D326K mutation allows Munc13-1-independent fusion and leads to a gain-of-function in rescue experiments in Caenorhabditis elegans unc-18 nulls. Together with previous studies, our data support a model whereby Munc18-1 acts as a template for SNARE complex assembly, and autoinhibition of synaptobrevin binding contributes to enabling regulation of neurotransmitter release by Munc13-1.

DOI:http://dx.doi.org/10.7554/eLife.24278.001

Nerve cells communicate with other nerve cells by releasing small molecules called neurotransmitters. The neurotransmitters are first packaged inside bubble-like structures called vesicles, which fuse with the membrane of the nerve cell when it is stimulated. Once the vesicle and membrane have fused, the neurotransmitters are released outside the nerve cell and are detected when they bind to proteins on the surface of other nearby nerve cells.

A machinery of different proteins controls membrane fusion. Amongst these proteins are five called Munc18-1, Munc13-1, syntaxin-1, synaptobrevin and SNAP-25. The last three form a tight bundle called SNARE complex that brings the vesicle and cell membrane together and is essential for the two to fuse. Munc18-1 and Munc13-1 orchestrate the assembly of the SNARE complex. Previous studies suggested that Munc18-1 binds to synaptobrevin, providing a template to bring syntaxin-1 and synaptobrevin together and thereby helping the SNARE complex to form. However, the importance of the interaction between Munc18-1 and synaptobrevin was not clearly established, and it was not known how Munc13-1 is involved.

Sitarska, Xu et al. have now measured how mutated versions of Munc18-1 bind to synaptobrevin and tested how the mutations affect membrane fusion. A mutation in Munc18-1 that increased binding to synaptobrevin increased membrane fusion too, while a mutation that decreased binding had the opposite effect and reduced fusion. The results support the idea that Munc18-1 provides a template for the SNARE complex to form. One mutation stimulated Munc18-1 so that Munc13-1 was no longer needed for fusion when the mutant Munc18-1 was tested in fusion assays with artificial membranes. This mutation was designed to perturb the structure of a region of Munc18-1 protein that normally inhibits the binding of synaptobrevin.

These results suggest that by adopting a state where it cannot bind synaptobrevin, Munc18-1 can only be stimulated to form the SNARE complex and trigger release of neurotransmitter when Munc13-1 is present. This provides a way for Munc13-1, which is regulated by many factors, to fine-tune the release of neurotransmitter. Future work will test whether these proteins work in the same way in living animals. This will help us understand how communication between neurons is finely controlled to enable the brain to carry out its many different tasks.

DOI:http://dx.doi.org/10.7554/eLife.24278.002

Epilepsy, Behavioral Abnormalities, and Physiological Comorbidities in Syntaxin-Binding Protein 1 (STXBP1) Mutant Zebrafish

Mutations in the synaptic machinery gene syntaxin-binding protein 1, STXBP1 (also known as MUNC18-1), are linked to childhood epilepsies and other neurodevelopmental disorders. Zebrafish STXBP1 homologs (stxbp1a and stxbp1b) have highly conserved sequence and are prominently expressed in the larval zebrafish brain. To understand the functions of stxbp1a and stxbp1b, we generated loss-of-function mutations using CRISPR/Cas9 gene editing and studied brain electrical activity, behavior, development, heart physiology, metabolism, and survival in larval zebrafish. Homozygous stxbp1a mutants exhibited a profound lack of movement, low electrical brain activity, low heart rate, decreased glucose and mitochondrial metabolism, and early fatality compared to controls. On the other hand, homozygous stxbp1b mutants had spontaneous electrographic seizures, and reduced locomotor activity response to a movement-inducing “dark-flash” visual stimulus, despite showing normal metabolism, heart rate, survival, and baseline locomotor activity. Our findings in these newly generated mutant lines of zebrafish suggest that zebrafish recapitulate clinical phenotypes associated with human syntaxin-binding protein 1 mutations.

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.